biomass as renewable feedstock in standard refinery units. feasibility, opportunities and challenges

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Cite this: Energy Environ. Sci., 2012, 5, 7393

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Biomass as renewable feedstock in standard refinery units. Feasibility,opportunities and challenges

Juan Antonio Melero,*a Jose Iglesiasb and Alicia Garciaa

Received 30th January 2012, Accepted 30th March 2012

DOI: 10.1039/c2ee21231e

Within the present contribution we highlight the feasibility of standard refinery units for the production

of biofuels from different biomass-derived feedstock. The energy densification of biomass, as well as it’s

logistics and incorporation within the refinery supply chain is thoroughly discussed. Likewise, special

attention is focused on the catalytic cracking and hydrotreating of triglyceride-rich biomass feedstock,

which is probably the most suitable one for co-processing in conventional refinery conversion units.

However, the opportunities of other highly oxygenated feedstocks such as pyrolysis oils and sugars are

also discussed. Conversion of different feedstocks into conventional liquid fuels by coupling of aqueous

phase reforming (APR) with catalytic systems typical of standard petroleum refineries is also evaluated.

Thus, here we review the chemistry, catalysis and challenges involved in the production of biofuels from

biomass in conventional refineries.

1. Introduction

One of the most important challenges to face in this 21st century

is the reduction of global warming whilst satisfying growing

aDepartment of Chemical and Environmental Technology, ESCET,Universidad Rey Juan Carlos, C/Tulipan s/n, E28933 Mostoles, Spain.E-mail: [email protected] of Chemical and Energy Technology, ESCET, UniversidadRey Juan Carlos, C/Tulipan s/n, E28933 Mostoles, Spain

Juan Antonio Melero

Juan Antonio Melero studied

chemistry in Complutense

University of Madrid (1988–

1993) and received his PhD in

1998 working on the synthesis

and applications of zeolitic

materials for redox and acid-

catalyzed reactions. He holds

a position as Full Professor in

Chemical Engineering at the

Department of Chemical and

Environmental Technology of

Rey Juan Carlos University in

Madrid. He has written around

75 high-impact scientific articles

and several book chapters

focused on the synthesis and characterization of porous materials

and their catalytic application in refining, fine chemistry and

environmental catalysis. Currently, he is leader–researcher of

several projects related to the processing of biomass feedstock.

This journal is ª The Royal Society of Chemistry 2012

energy demands. Nowadays, most fuels and energy come from

fossil energy resources, but environmental concerns together

with the depletion of crude oil resources, and consequent

increasing prices of this raw material, are becoming important

driving forces encouraging the search for new feedstocks, as

alternative to crude oil, to meet the increasing energy demand.

Many different possibilities have been reported in the literature.

However, such a substitution involves important requirements,

like the renewable nature of the raw material to ensure

Jose Iglesias

Jose Iglesias was born in Bena-

vente (Spain) in 1976. He

received his M. Eng. from Uni-

versidad Complutense, Madrid,

(1999), and his PhD in Chem-

ical Engineering from Uni-

versidad Rey Juan Carlos

(2005) working in the develop-

ment of new catalysts for asym-

metric oxidation reactions.

Since 2008 he is associate

professor in the Department of

Chemical and Energy Tech-

nology in Universidad Rey Juan

Carlos. His main research

interests are focused on the

rational design of heterogeneous catalytic systems for green and

fine chemistry, biofuels production and selective oxidation.

Energy Environ. Sci., 2012, 5, 7393–7420 | 7393

http://dx.doi.org/10.1039/c2ee21231e





http://dx.doi.org/10.1039/C2EE21231E

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http://pubs.rsc.org/en/journals/journal/EE?issueid=EE005006

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a sustainable supply. Nevertheless, a very high availability seems

to be another one of the most important requirements, bearing in

mind the high quantities of feedstock needed to ensure a full

coverage of the current demand of automotive fuels. Biomass is

one of the few resources showing high potential to meet the

challenges of sustainable and green energy, and hence its use is

expected to grow in the foreseeable future. Indeed, several

governments have implemented mandatory legislation in order

to increase gross domestic energy from renewable resources,

especially biomass. The U.S. Department of Energy set ambi-

tious goals to derive 20% of the transportation fuel from biomass

by 2030. The European Union has set a mandatory target of 20%

for renewable energy’s share of energy consumption by 2020 as

well as a mandatory minimum target of 10% of renewable energy

sources in transport. Fortunately, the worldwide production

capabilities for renewable and sustainable biomass production

are, in comparison, enormous and thus it exceeds the current and

foreseeable energy demand.

To help reach the ambitious goals established by the United

States and European Union, a system similar to a petroleum

refinery called a ‘‘biorefinery’’ has been proposed in the future.1–5

According to the National Renewable Energy Laboratory

(NREL), a biorefinery is a facility that integrates biomass

conversion processes and equipment to produce fuels, power,

and chemicals from biomass. The biorefinery concept is analo-

gous to today’s petroleum refineries, which produce multiple

fuels and products from petroleum. Industrial biorefineries have

been identified as the most promising route to the creation of

a new domestic bio-based industry. By producing multiple

products, a biorefinery can take advantage of the differences in

biomass components and intermediates and maximize the value

derived from the biomass feedstock. The main goal of a bio-

refinery is to produce high-value low-volume (HVLV) chemicals

products and low-value high-volume (LVHV) biofuels using

different unit operations, while generating electricity and process

heat for its own use and perhaps enough for sale. The operations

must be designed to maximize the mass and energy efficiency and

minimizing the waste streams. The high-value products enhance

profitability, the high-volume fuel helps meet national energy

Alicia Garcia

Alicia Garc�ıa studied Chemical

Engineering in Complutense

University of Madrid (1993–

1998) and received her PhD in

2005 in Rey Juan Carlos

University (Madrid). She has

occupied several teaching posi-

tions in the Department of

Chemical and Environmental

Technologies, starting in 1999

as teaching assistant, becoming

assistant professor in 2001 and

lecturer since 2008. Her main

research lines are focused on

developing new heterogeneous

catalytic systems for green and

fine chemistry, feedstock recycling of plastic wastes and more

recently biodiesel and hydrogen production.

7394 | Energy Environ. Sci., 2012, 5, 7393–7420

needs, and the power production reduces costs and avoids

greenhouse-gas emissions. Fig. 1 illustrates the elements of

a biorefinery in which biomass feedstock are used to produce

various useful products such as fuel, power, and chemicals using

biological, chemical, and thermochemical conversion processes.

Nevertheless, the chemical and energy integration of biomass

transformations in biorefineries is still in the early beginnings

and, in a short- to medium-term, biorefinery development will

likely incorporate existing petroleum refinery infrastructure to

circumvent high capital costs. Hence, one promising alternative

for the production of biofuels is the co-processing of biomass in

conventional oil refineries.6 This alternative involves the co-

feeding of biomass-derived feedstock with typical petroleum

feedstock in conventional refining units. This strategy has

significant advantages as compared with conventional processes

of biofuels production, such as installations that are already built

and hence their use would require little capital investment.

Furthermore, the production process does not require either

secondary reagents or yield by-products, which should have

a market share. Finally, a wide range of biofuels could be

obtained, not only in the range of gasoline and diesel, but also in

the range of LPG, kerosene or fuel oil. All these advantages have

boosted the investigation on this possibility, and some oil

companies have already developed industrial processes for

biomass transformation into fuels.7,9

The purpose of this review is to identify economically attrac-

tive opportunities for biofuels production using petroleum

refinery processes and the challenges for the future integration of

biomass as feedstock for refineries. The study will be focused on

the production of fuels and hydrogen from a high variety of

biomass, including triglyceride7,8 to lignocellulosic-based feed-

stock.6 Many different opportunities for integrating bio-

renewable feeds and products in existing petroleum refineries as

Fig. 1 Main elements in future biorefineries.



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well as new potential refining approaches will be reviewed and

discussed.

2. Biomass feedstock

2.1 Diversity and availability

Biomass is one of the most promising candidates that fulfil the

requirements to substitute crude oil as the main source for

chemical energy, since it is readily available in high quantities,

and it is renewable.10 Biomass feedstock can be grouped into two

wide categories: oleaginous feedstock and carbohydrates.

Triglyceride molecules are the main component of oleaginous

feedstock. These molecules consist of glycerine coupled in ester

form to long alkyl-chain fatty acids, ranging their length between

C8 and C20 (but 16, 18 and 20 carbons are the most common).

The empirical formula of these compounds could be assigned

that corresponding to oleyl triglyceride (C57H114O6) since this is

the most abundant fatty acid in nature. However, triglyceride

molecules display one of the very highest H/C atomic ratios

combined with a low oxygen content, considering the different

chemicals present in biomass feedstock.11 Another important

fraction of oleaginous feedstock is that formed by free fatty acids

(FFA), which could account for most of the composition of

certain materials. The FFA fraction finds its origin in the

hydrolysis of triglyceride molecules and thus, they are usually

present in processed and low-grade oleaginous materials.

Currently, the main use of oleaginous feedstock as crude-oil-

substituting raw materials is the production of biodiesel by

reacting the substrates with methanol. Oils and fats sources

include a wide variety of vegetable and animal raw materials.

Soybean, palm, rapeseed and sunflower oil are the most impor-

tant in terms of worldwide production. However, other sources

of vegetable oils for biofuel conversion can be found in waste

streams of different industries, such as the food industry, animal

waste rendering and many other sources; these last types are

much more stable supplies, and their transformation into fuels

does not create competition with other important sectors. A

source of triglycerides which is currently receiving much atten-

tion can also be found in algae.12–14 However, unlike vegetable

oils and animal fats, which can be easily produced in large scale

with already-built infrastructure, algae oleaginous biomass

production is only on its beginning, and much effort has still to

be invested in order to design and create effective production

processes based on algae.

Carbohydrates are molecules formed of carbon, hydrogen and

oxygen and these are, by far, the most abundant component

found in biomass. There are different families of carbohydrates,

which could be divided into two groups, mono- and poly-

saccharides. The first group is the less abundant in nature, but it

can be found in different plants like sugarcane or sugar beet,

which are quite plentiful in certain areas. Its use in energy

processes is usually assigned to ethanol production by fermen-

tation. Simple monosaccharides include 6-carbon sugars

(C6H12O6) like glucose, galactose and mannose, and 5-carbon

sugars (C5H10O5) like xylose and arabinose. Polysaccharides are

now awakening much interest as source for monosaccharides,

because the former can be turned into simple sugars by hydro-

lysis, though not always easily. Polysaccharides include a whole


collection of different substances, like starch, cellulose or hemi-

cellulose. Starch (C6H10O5)n is a polysaccharide composed of

a-glucose molecules linked through a-1,4 bondings with

branches formed as a result of a-1,4 linkages. Starch is mostly

produced from several crops like cereals (wheat and corn) and

tubers. Lignocellulose is the most common form in which poly-

saccharides are present in nature. It is not a substance itself but

a mixture of three major components: cellulose (40–50 wt%) and

hemicellulose (25–40 wt%), which are different forms of poly-

saccharides, and lignin (10–25 wt%). Though cellulose

(C6H10O5)n is, like starch, a polysaccharide of glucose, it differs

from the first in the configuration of the link between adjacent

hexose units, b-linkages forming cellobiose, and the absence of

branches to lead a linear polymer of D-glucopyranose. This linear

configuration facilitates the interaction of cellulose polymer

chains through hydrogen bondings leading to the formation of

rigid bundles of cellulose chains. In this way, the hydrolysis of

starch can be easily promoted by enzymes or by acids, while

cellulose is much more difficult to be hydrolyzed, mainly due to

its crystallinity and the relatively hindered access to the ether

bondings between the monomeric units. On the other hand,

hemicellulose is a relatively amorphous component, easier to be

broken down with chemicals and/or heat than cellulose. Finally,

lignin is the glue that provides the overall rigidity to the structure

of plants and trees and its composition is very complex, though it

could be described as a kind of highly condensed polymer (M z10 000) of coniferyl alcohol, so that, it is not a source for simple

sugars. The empirical formula describing the composition of

lignin is C9H10O2(OCH3)n, where n varies from 0.94 for softwood

to 1.40 for hardwood, all along 1.18 for grasses.15 Lignin is a by-

product of little to no commercial value in the pulp and paper

industry, except for its use as a low grade fuel in Kraft pulping,

the dominant pulping process. Large amounts of cellulosic

biomass can be produced via dedicated crops like perennial

herbaceous plant species, or short rotation woody crops. Other

sources of lignocellulose biomass are wastes and residues, like

straw from agriculture, wood waste from the pulp and paper

industry and forestry residues.

The overall biomass composition is estimated to consist of

roughly 75% carbohydrates (sugars), 20% lignin and 5% of other

substances in minor amounts such as oils, fats, proteins,

terpenes, alkaloids, terpenoids and waxes. Fortunately, the

worldwide production capabilities for renewable and sustainable

biomass production are, though quite difficult to estimate,16,17

enormous in quantity. For instance, in the U.S. over 408 million

dry tons of non-used lignocellulosic materials can be harvested

from forest, agriculture and urban and industrial activities.18

Similarly, large lignocellulosic production capacity is available in

Europe, which could yearly produce around 190 millions of dry

tones of lignocellulose biomass.19 With regards to the production

of waste lipids and fats, including rendered fats,20,21 and yellow

and brown grease,22–24 the potential production is much lower,

being of 4.9 and 4.4 millions of tones the estimated yearly

potential for the US and EU25, respectively.

Fig. 2 displays the C, H, O mass composition of several

biomass-derived feedstocks, such as lipids, cellulose and lignin,

together with pyrolysis bio-oil and crude oil, used as reference. In

general terms, biomass feedstocks display, in molar terms, much

lower carbon and hydrogen content than conventional crude oil.



Fig. 2 Ternary diagram showing the mass composition of usual

biomass-derived raw materials.

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Nevertheless, one of the main differences between the renewable

biomass-derived feedstock and crude oil, is the large amount of

oxygen present in the former, which is one of the causes of the

much lower energy density of these raw materials. Only lipids,

with rather small oxygen content, display a composition close to

that showed by crude oil, though their relatively low availability

makes it necessary to also count on the use of lignocellulosic

feedstock for the substitution of the fossil raw materials. Unlike

lipids, lignocellulosic materials display a high oxygen content,

making its removal necessary to enhance the energy density of

the products. In this way, the overall challenge with biomass

conversion is how to remove efficiently oxygen from the biomass

feedstock and produce a molecule that has a high energy density

and good combustion properties as current petroleum derived

fuels (eqn (1)). Catalytic cracking and hydrotreating are very

effective at removing oxygen from the biomass-derived feed-

stock. However, oxygen is not always removed through the

optimal pathway, and undesired products such as coke or

oxygenated by-products are usually formed during the process.

CxHyOz / aCx�b�d�eHy�2cOz�2b�c�d + bCO2

+ cH2O + dCO + eC (1)

In this sense, Chen et al.25 have defined the effective hydrogen

index (H/C)eff as the amount of hydrogen in the fuel which is

available for energy production, where H, C, O, N and S

correspond to the moles of hydrogen, carbon, oxygen, nitrogen

and sulfur present in the feed (eqn (2)).

ðH=CÞeff ¼ H � 2O� 3N � 2S

C(2)

This index is clearly lower than 1 for highly oxygenated

biomass feedstock, which means that this feedstock is mainly

formed by hydrogen-deficient molecules. In the case of a mixture

of hydrocarbons the (H/C)eff index ranges from 2 (liquid alkanes)

to 1 (for benzene). In contrast, triglyceride-based biomass show


hydrogen index of ca. 1.5, a much more energy-dense feedstock

and closer to that showed by hydrocarbons.

2.2 Conditioning and energy-densification

Apart from the composition, there are also other important

differences between crude oil and biomass feedstock which

explain the different energy densities between both kinds of raw

materials. Thus, most of the biomass-derived materials display

a rather low density (80–100 kg m�3 for grasses, to 150–200 kg

m�3 for woody materials), and thus, these are characterised by

a poor energy content. This is not a trouble affecting oleaginous

feedstock because it is the concentrated form of a processed

biomass raw material, whereas lignocellulose is usually found in

the form it appears in nature. However, properties of ligno-

cellulosic materials can be enhanced by biomass densifica-

tion.26,27 This can be achieved by means of physical procedures,

like different mechanical compressing techniques, mainly adap-

ted from other applications, including pellet mills, briquette

presses or screw extruders, among others. Compacting is prob-

ably the simplest way to enhance the energy-density of these

renewable materials. However, other important transformations,

involving chemical modifications, could be considered for a pre-

treatment step focused on the enhancement of, not only the

physical density of the materials, but mostly of the energy density

of the biomass feedstock. Several of the treatment steps are

dedicated to reduce the moisture content of lignocellulose, one of

the reasons of the low energy density of these raw-materials.28

These transformations not necessarily preserve most of the initial

properties of the biomass feedstock, as steam explosion29 or

torrefaction30,31 do, but they can also modify the form in which

the raw materials are presented.

2.2.1 Lignocellulose to bio-oils. Bio-oils can be considered as

an energy-dense form of biomass, produced by a pyrolysis

treatment.32 Bio-oils are produced by direct thermal decompo-

sition of biomass feedstock in the absence of oxygen, or at least in

presence of significantly less oxygen than required for complete

combustion. The gaseous product (mainly carbon oxides,

methane and some higher hydrocarbons) is formed together with

a solid carbonaceous residue and a liquid phase (bio-oil).32,33 The

properties of bio-oils, as well as their composition, depend both

on the specific starting feedstock and the conversion conditions.34

From a compositional point of view, bio-oils consist of two

phases: an aqueous phase, comprising 15–30 wt% of the total

bio-oil, in which several low molecular weight oxygenated

organics are dissolved (acetic acid, methanol, acetone.), and

a non-aqueous phase (35–50 wt%) comprising different oxygen-

containing structures (aliphatic alcohols, carbonyls, acids,

phenols, sugars, hydroxyaldehydes, hydroxyketones.), and

aromatic hydrocarbons (benzene, toluene, indene, naphtha-

lene.).34 Yields of bio-oil from biomass vary in the range�60 to

95%, depending on the initial feedstock: lower for high-lignin

content lignocellulosic biomass, since the presence of lignin

depresses the production of liquids; and higher for more cellu-

losic materials.35 Due to the high oxygen content, bio-oil is

characterised by a low heating value, though this depends on the

initial composition of the starting material, since a high lignin

feedstock leads to bio-oils with higher heating values, because of



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the lower oxygen content of this substance (see Fig. 2). Bio-oil is

rather immiscible with hydrocarbon fuels because of the high

polarity of oxygenated compounds, it is chemically unstable, and

it displays low volatility, high viscosity and corrosiveness.36

Nevertheless, the liquid nature of this intermediate makes easier

to handle biomass-derived feedstock than on their initial form, as

solids. However, bearing in mind all the properties of bio-oils,

they cannot be directly used as a replacement for diesel and

gasoline fuels, but can be considered as an energy-dense form of

lignocellulosic biomass, showing suitable properties to be fed to

a refinery production facility.

2.2.2 Lignocellulose to platform molecules. An even more

desirable option to that showed by pyrolysis conversion of

lignocellulosic biomass into bio-oils is the controlled chemical

transformation of these feedstocks into simple, well-defined

molecules (Fig. 3), called platform molecules, easily separated

from non-interesting substances.37 In this sense, the depolymeri-

sation of lignocellulosic materials into simple sugars and their

transformation into versatile, valuable molecules able to be fed

to a refining process could be considered as the most advanced

way to provide energy-dense biomass-derived refinery-process-

able feedstock. In this approach, lignocellulosic materials have to

be separated into their constituents (lignin, cellulose and hemi-

cellulose) and depolymerised to the corresponding building

blocks. The building blocks of lignin are aromatic alcohols, but

controlled lignin depolymerisation is rather difficult on a tech-

nical scale and this problem has not yet been properly solved.

Controlled cellulose depolymerisation results in glucose whereas

the hemicelluloses are depolymerised to a mixture of different

sugars, mostly pentoses. These simple sugars are the key mole-

cules for the production several important platform molecules,

useful for the production of chemicals and fuels. Conversion of

biomass into functionalized, targeted platform molecules is

unique to hydrolysis based methods and it allows the production

of a wide range of fuel components. Among different platform

molecules, furfural (2-furaldehyde), 5-hydroxymethylfurfural (5-

HMF), levulinic acid (LA) and gamma-valerolactone (GVL) are

of special interest. Fig. 3 schematizes the production of platform

molecules from sugar-based raw materials. Starting from the

hydrolysis of cellulose and hemicellulose into simple sugars, these

are then transformed into 5-HMF or furfural (depending if

Fig. 3 Platform chemical molecules derived from cellulose materials and

reactions involved in their production.


starting from an hexose or a pentose, respectively) by dehydra-

tion. If hydration progresses from HMF, levulinic acid (a sugar-

derived g-keto acid platform molecule) is then produced, though

at the expense of losing a carbon atom in the form of formic acid.

Levulinic acid, on the other hand, can be transformed into GVL

by hydrogenolysis.38 Among all these chemicals, HMF and LA

are probably the most interesting ones as platform molecules,

displaying a higher functionalization degree and thus a higher

reactivity. HMF also displays several advantages over LA, such

as a higher carbon economy (if its preparation from an hexose

sugar is considered) or the good versatility to be transformed not

only into biofuels, but also into several interesting chemical

intermediates. Nevertheless, HMF is quite unstable under several

conditions, and it easily evolves to levulinic acid by acid-

promoted hydrolysis, LA being a much more stable chemical.

However, both types of platform molecules have been used in the

preparation of several chemicals useful as fuel and fuel additives.

Dehydration of sugars to furan compounds: furfural and 5-

HMF. Furfural is obtained by dehydration of C5 sugars like

xylose in a well-developed industrial process.39 Most of them

make use of concentrated sulfuric acid as catalyst, which is

extremely corrosive and highly toxic and suffers from serious

drawbacks concerning homogeneous catalytic processes, such as

difficult separation and recycling of the mineral acid and product

contamination. Other major drawbacks of these processes are

extensive side reactions, resulting in loss of furfural yield due to

long residence times, and the need for significant waste disposal.

Several attempts have been made to develop heterogeneous

catalytic processes for the transformation of pentosans/pentoses

into furfural offering environmental and economic benefits.

Unfortunately, the catalytic performances achieved up to now

have been unsatisfactory for industrial implementation.

Dehydration of hexoses in acid media leads to the formation of

5-(hydroxymethyl)furfural (5-HMF). This process has been

carried out using a great variety of different catalytic systems:

homogeneous organic acids (p-toluenesulfonic acid, H2SO4 and

HCl) as well as heterogeneous catalysts (ionic exchange resins,

H-form zeolites, vanadyl phosphate and ZrO2) and in presence of

different solvents. Reaction conditions range from temperatures

between 100 and 200 �C using conventional heating (reaction

times up to 48 hours depending on the catalytic system) as well as

microwave (shorter reaction times in the range of minutes).40 In

principle, solid acid catalysts are more desirable for this reaction

and display several advantages in comparison with liquid acid

catalysts. These heterogeneous catalysts make separation from

the product and recycling easier; they also allow working under

higher temperatures, thus shortening the reaction time and

avoiding 5-HMF decomposition due to prolonged reaction time;

and finally the adjustment of their surface acidity might allow

controlling 5-HMF selectivity.

Several reaction media have been used in the dehydration of

hexoses. The water medium is a suitable candidate from an

ecological point of view but unfortunately, 5-HMF undergoes

several reaction under aqueous acid conditions to form undesired

side products such as levulinic and formic acids or even self-

condenses to form both soluble polymers and insoluble humins.

In order to minimize these secondary reactions and increase the

yield towards 5-HMF, the use of high-boiling organic solvents



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has been described in literature. For instance, pure dime-

thylsulfoxide (DMSO) in presence of acid resins gave a 5-HMF

yield over 90%, starting from fructose (in water media the yields

are hardly over 60%).41 Likewise, biphasic systems (water–

organic solvent) have also been used in the synthesis of 5-HMF

with the purpose of solving the low solubility of sugars in organic

solvents whilst the continuous extraction of the evolving 5-HMF

from the aqueous phase prevents its degradation. These systems

have been deeply studied by Dumesic and coworkers in order to

improve the selectivity of HMF formation from fructose.42,43

Both homogeneous and heterogeneous catalysts have been

assayed with this biphasic system, including HCl, H2SO4,

H3PO4, ion-exchange resins and niobium phosphate catalysts.

For instance, fructose and glucose were dehydrated to 5-HMF

with selectivities of 89% and 53%, respectively, with high sugar

conversions in a system comprised of a reactive aqueous phase

modified with DMSO, combined with an organic extracting

phase (mixture of methyl isobutyl ketone (MIBK) and 2-

butanol) at 130 �C.Another relatively new catalytic system to produce 5-HMF

involves the use of ionic liquids (ILs). Several works have been

reported in literature combining ILs with homogeneous and

heterogeneous catalysts.44 However, the main drawbacks asso-

ciated with the use of ILs are the need for purification after

recycling, potential sensitivity to moisture and impurities, as well

as the cost for commercial applications.

The dehydration process is more efficient and selective for 5-

HMF when starting from fructose than from glucose. Thus, the

most efficient method for the preparation of 5-HMF is the acid-

catalyzed dehydration of fructose, which can be obtained by

acid-hydrolysis of sucrose and inulin or, as alternative, by means

of glucose isomerization. However, glucose is more abundant

and readily available and hence more appealing feedstock for the

production of 5-HMF. Thus, there is an important incentive to

transform glucose into 5-HMF with high yields. In this sense,

several works have described a new strategy based on the

combination of a basic catalyst (Al/Mg hydrotalcite) and an acid

catalyst (Amberlyst-15).45,46 The basic catalyst is responsible for

the isomerization of glucose into fructose, whereas the acid

catalyst promotes the subsequent dehydration to 5-HMF. The

authors reported a 5-HMF selectivity of 76% with a glucose

conversion of 60% using this mixture of catalytic systems.

Finally, the direct use of polyssacharides, cellulose and

lignocellulose as feedstock for the production of 5-HMF is

more appealing from a commercial point of view. Processing

these highly functionalized polysaccharides, that are inexpen-

sive and abundant, eliminates the need for simple carbohydrate

molecules. This approach has been poorly described in the

literature but there are some works that deserve to be

mentioned. Chheda and co-workers43 achieved good selectivities

for 5-HMF at high conversions from sucrose, starch, cellobiose

and xylan, using a mineral acid as catalyst and a biphasic

reactor. More recently, McNeff et al.47 have described the

continuous production of 5-hydroxymetylfurfural from simple

and complex carbohydrates using a fixed bed porous metal

oxide-based catalytic process (ZrO2 and TiO2) and using methyl

isobutyl ketone as solvent. For instance, they obtained a cellu-

lose conversion of 87% and 5-HMF selectivity of 35% using this

catalytic system.


Nevertheless, the large-scale production of 5-HMF from C6

sugars is still far from the industrial implementation and some

challenges must be addressed in the future: utilization of inex-

pensive and high available glucose as a sugar feedstock and the

implementation of solid acid catalysts highly resistant and

selective in appropriate solvent systems.

Synthesis of 5-HMF derivates: levulinic acid and g-valero-

lactone. Levulinic acid (LA, 4-oxopentanoic acid) is an impor-

tant biomass derivative that can be obtained by hydrolysis of

lignocellulosic wastes, such as paper mill sludge, urban waste

paper, and agricultural residues, through the Biofine process.48,49

This process starts by treating the biomass feedstock with

sulfuric acid (1.5–3 wt%) in an initial plug-flow reactor where the

hydrolysis of carbohydrates to intermediates (HMF) takes place

at 483–493 K and 25 bar with a short residence time (12 s) to

minimize the formation of degradation products. Subsequently,

in a second reactor, the intermediates are converted into levulinic

acid and formic acid at 463–473 K and 14 bar, with a residence

time around 20 min. These conditions have been optimized to

remove formic acid, as well as to remove the furfural arising from

dehydration of the C5 sugars present in biomass. Yields towards

levulinic acid are close to 70–80% which corresponds to 50% of

the C6 sugars initial mass, being the rest collected as formic acid

(20%) and a solid insoluble residue (humins). Levulinic acid can

be subsequently converted to g-valerolactone (GVL) by catalytic

hydrogenation. This reaction is carried out at relatively low

temperatures (373–543 K) and high pressures (50–150 bars) and

using both homogeneous and heterogeneous catalysts. The

reduction usually uses external hydrogen, but the achievement of

hydrogen from formic acid, produced as by-product together

with levulinic acid is a promising alternative. Recently, several

works report a simple process for the production of GVL, which

integrates hydrolysis/dehydration of carbohydrates to form LA

and the subsequent hydrogenation to GVL in a single step.50

2.3 Logistics and refinery supply chains

In 2011, the total world crude oil consumption aroused to

89.0 million barrels per day.51 Considering the substitution of

even a small fraction of this huge amount of energy resources by

biomass-derived feedstock involves the use and feeding of large

amounts of renewable raw-materials to refineries. Although the

availability of biomass seems not to be a problem, at least in

a worldwide perspective, there are other practical troubles. Thus,

unlike crude oil, which is collected in a concentrated manner

from large deposits, biomass is produced in a low-energy-density

form. Enhancing the energy density of biomass, as already

stated, can be overcome, but there is still a critical issue, from

a practical point of view, in the incorporation of biomass-derived

feedstock into a fuel-producing scheme of a refinery: How to

supply enough quantities of this renewable resource to a plant so

demanding of raw-materials as a refinery.52–54 Thus, very large

harvesting areas are needed to provide useful quantities of the

raw material containing an appreciable amount of energy. In

addition, several other features, differing in biomass logistics

from those corresponding to other industries, have to be

considered.55,56 Some of the most important characteristics

determining a logistic chain for biomass are the discontinuous



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production (determined by the harvesting season, weather

effects, frequency, etc.), the variety of biomass feedstock to be

supplied to the refinery (involving composition and quality

variation, etc.),57,58 the transportation chains,59–61 the storage of

biomass feedstock (capacity, conditions, etc.)55 and/or the

required pretreatments to accomplish energy densification,62,63

among many others. All these considerations are very important

in the design of a logistic system for biomass feedstock, but the

efficient and already-built infrastructure for the production of

the oleaginous feedstock64 largely simplifies its case in compari-

son to lignocellulosic feedstock.

Several studies have dealt with the optimization of supply

chain systems for the provision of lignocellulosic materials. Most

of them involve the insertion of economic parameters, together

with the rest previously described, in mathematical models with

the aim to build an efficient logistic system by optimising the cost

structure of the same.55,65,66 For this purpose, several alternatives

have been proposed, including the use of small-sized bio-

refineries66 instead of large-capacity crude oil refining facilities,

which seems not to be a feasible option in the foreseeable future.

The location of these facilities close to biomass producing areas

would reduce the transportation costs. However, lower produc-

tion capacities would have to be compensated by building a large

number of biorefineries. As alternative, the building of satellite

storage locations (SSLs) has also been proposed for temporary

storage and loading of biomass, which would collect the renew-

able feedstock from the producing areas before being trans-

ported to refineries.67,68 However, the most interesting option

seems to be the consequence of combining the use of a similar

structure based on the use of SSL with energy densification

treatments,69 like those described in the previous section, to

condition the starting lignocellulosic biomass feedstock, saving

transportation costs from SSL to refineries. In this way, the

supply of biomass-derived feedstock to conventional refineries

can be accomplished as pre-treated streams which can be directly

processed into the already-built infrastructure of a conventional

refinery, by using existing catalytic technology.70 Fig. 4 displays

Fig. 4 Combined supply chain structure for co-feeding biomass and

crude oil as starting raw-materials for conventional refineries.


the logistic infrastructure for collecting and pretreatment steps to

condition biomass feedstock and their insertion in a refinery

production process as raw materials, together with other

biomass-derived substances like oleaginous feedstock and crude

oil.

3. Processing biomass feedstock in conventionalrefineries

Fig. 5 briefly outlines the possibilities for the production of

biofuels in standard refinery units, which will be further dis-

cussed in the review. Thus, the properties of the starting biomass

raw material, mainly described by the effective hydrogen index,

not only conditions the energy density of the starting feedstock,

but also induce the distinct chemistry involved in the catalytic

process, resulting in a different product distribution. Different oil

companies are already investigating the possibility for develo-

ping some new industrial processes in which biomass-derived

feedstock is treated in conventional refineries. Thus, Neste Oil

has developed a hydrotreating process (NExBTL technology),

which allows flexible use of any vegetable or waste oil in the

production of renewable diesel fuel.71 Recently, a US Depart-

ment of Energy funded collaboration between UOP, the

National Renewable Energy Laboratory, and the Pacific

Northwest National Laboratory completed an evaluation of the

economics of biofuels integration in petroleum refineries.9 Many

economically attractive opportunities were identified for inte-

grating biorenewable feeds and products in existing or new

refining operations,72 particularly for two feedstocks: vegetable

oils/greases to produce green diesel, gasoline or chemicals; and

pyrolysis oil to produce green gasoline.

3.1 FCC and hydrotreating units as the core of conventional

refineries

Several options are available for converting biomass–derived

feedstocks into biofuels in a petroleum refinery: (1) Thermal

Fig. 5 Integration of biomass feedstock in conventional refinery

processes.



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(visbreaking and cocker units) and catalytic (FCC unit) cracking

(2) hydrotreating and (3) hydrocracking.

Cracking reactions in a conventional refinery can be carried

out in presence of catalyst (FCC unit) and in its absence (thermal

units). Thermal units are not considered of interest for the

production of biofuels since the resulting organic liquid product

contains a high content of oxygenated compounds, indepen-

dently from the composition of biomass feedstock, reducing its

interest as fuel transport. In contrast, catalytic cracking is faster

and more selective than thermal cracking and it allows working

under milder reaction conditions, minimizing the yield towards

by-products such as gases, coke and heavy fractions while

maximizing the production of the liquid fraction suitable for use

as transport fuel, showing an effective hydrogen index in the

range from 1 to 2. Fluid catalytic cracking (FCC) is the most

widely used process for the conversion of crude oil into gasoline

and other hydrocarbons because of its flexibility to changing the

feedstock and product demands. The feedstock for the catalytic

cracking process at the petroleum refinery has traditionally been

gas oil with an average molecular weight in the range of 200–600

or higher, though processed bio-oils and triglycerides are being

studied as co-feeds. The reactions occurring in the FCC process

include cracking reactions (cracking of alkanes, alkenes, nap-

thene and alkyl aromatics to lighter products), hydrogen trans-

fer, isomerisation, and coking reactions.73 A variety of process

configurations and catalysts have been developed for the FCC

process. FCC catalysts usually contain mixtures of a Y-zeolite

within a silica–alumina matrix, a binder, a clay, and some

additives.

Hydrotreatment is an indispensable unit operation in

conventional refineries and the hydrotreatment catalysts are,

together with the cracking and three-way exhaust gas catalysts,

the most important industrial catalysts. The Mo and W transi-

tion metal sulfides promoted with Co and/or Ni metal sulfides

have been the active components of hydrotreatment catalysts

from the early beginnings of the process. The objective of

hydrotreating in a petroleum refinery is to remove sulfur

(hydrodesulfurization, HDS), nitrogen (hydrodenitrogenation,

HDN), metals (hydrodemetalation, HDM) and oxygen (hydro-

deoxygenation, HDO) from the heavy gas oil feedstock.

Hydrogen is necessary to perform these transformations and it is

added with the heavy gas oil feed. Typical reaction conditions

employed are temperatures of 300–450 �C, pressures of 35–170bar H2 and liquid hourly space velocities (LHSVs) of 0.2–10 h�1.

Hydrogen based processes are typically more expensive than

cracking because these require hydrogen. Likewise, its

consumption is higher when biomass feedstock is processed, due

to its higher oxygen content in comparison to conventional fuel

sources. On the contrary, hydrotreating processes display

a higher selectivity towards the liquid fraction, minimizing gas

and coke production as compared with cracking units, and also

providing high energy-dense products such as green diesel, which

shows a high (H/C)eff index.

Though cracking and hydrotreating units are already built-up

in conventional refineries, as previously noted, most of the

available biomass feedstock is present in the form of sugars and

polymers. These raw materials display a very low effective

hydrogen index and thus, these have to be processed in more

complicated units than those previously described. The energy


upgrading of these chemicals also makes necessary the provision

of large quantities of hydrogen. Fortunately, recent develop-

ments in processing sugar-rich biomass allows production of the

required hydrogen from the same sugar feedstock (APR process)

for the transformation of the own biomass raw material into

hydrocarbons, leading to more energy-dense products.

3.2 Oleaginous biomass feedstock processing

Oleaginous raw materials, such as fats and vegetable oils are

primarily water insoluble, hydrophobic substances that are

comprised almost completely of triglycerides and small amounts

of mono- and diglycerides.74 Triglycerides can be easily con-

verted into liquid transportation fuels because of their low

oxygen content.6 A variety of feedstocks for the production of

biofuel from triglyceride based agriculture-derived fats and oils

can be divided into four different types: crude vegetable oil (palm

oil, rapeseed, soybean), used vegetable oil (waste cooking oil),

animal fats (lard, tallow) and non-edible oil (castor oil, tall oil,

Jatropha curcas, Cynnara cardunculos.).3 Waste oils and fats is

one of the most economical choices to produce biofuel. Large

quantities of these feedstock are available throughout the world,

especially in developed countries,75 though not in quantity

enough to completely replace crude oil.76

This feedstock possesses similar properties (density, viscosity,

hydrogen/carbon ratio.) to those found in vacuum or hydro-

treated gas oil usually fed to the refinery conversion units.

Indeed, co-processing these renewable feedstocks together with

conventional feeds, can lead to a lower content in metals (such as

nickel or vanadium) and heteroatoms (such as sulfur or nitrogen)

in the final products because of the lower content of these

impurities on the composition of the renewable feedstock.

3.2.1 Catalytic cracking of oleaginous feedstock. Catalytic

cracking of vegetable oils, animal fats, and waste oleaginous

feedstock can be used to produce automotive fuels.6 A huge

amount of different studies have been reported dealing with the

catalytic cracking of oleaginous feedstock over different acid

catalysts: zeolites (H-ZSM-5, H-Y, H-mordenite.),77–88

aluminium-containing mesostructured materials (Al-MCM-41,

Al-SBA-15),3,84,88–93 amorphous materials (aluminosilicates,

pillared clays, alumina.).81,82,84,94,95

Typical products obtained from the catalytic cracking of

oleaginous feedstock include gaseous products (hydrocarbons

C1–C5, CO, CO2), organic liquid products (OLP), water and

coke.96 The organic liquid product is composed of hydrocarbons

corresponding to the gasoline, kerosene, and diesel boiling point

ranges. The oxygen initially present in the feedstock is removed

as water, easily isolated, CO and CO2. Therefore, there is no

remarkable presence of oxygenated hydrocarbons in the final

organic cracking products. A general reaction pathway of the

catalytic cracking of a triglyceride molecule over an acid catalyst

is proposed in Fig. 6.96 Once the triglyceride molecule has been

primarily decomposed to less bulky compounds such as free fatty

acids, ketones, aldehydes and esters, their transformation into

different products starts by breaking the C–O and C–C bonds by

b-scission reactions. This transformation follows two competi-

tive routes: (i): decarboxylation and decarbonylation reactions

followed by C–C bond cleavage of the resulting hydrocarbon



Fig. 6 Chemical reactions taking place in the thermal and catalytic

cracking of triglycerides.

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radicals or (ii): C–C bond cleavage within the hydrocarbon

section of the oxygenated hydrocarbon molecule followed by

decarboxylation and decarbonylation of the resulting short chain

molecule.83 The occurrence of these different reaction routes

depends on the presence of double bonds in the initial oxygen-

ated hydrocarbon. Whereas C–C bond breaking in a and

b position is favoured in presence of unsaturated hydrocarbon

molecules, decarboxylation and decarbonylation reactions take

place before C–C bond cleavage for saturated oxygenated

hydrocarbons, since the least endothermic bonding in a saturated

hydrocarbon chain is the one associated with the b position of the

carbonyl group.97 Different subsequent cracking reactions finally

yield CO, CO2 and water, as the main oxygenated compounds,

and a mixture of hydrocarbons produced by different reactions

such as b-scission, hydrogen transfer, isomerization, cyclization,

or aromatization, some of them possible because there is an acid

catalyst present in the reaction system. Furthermore, coke is

formed by means of polymerization reactions.98

The choice of the catalyst is crucial in processes driven by

heterogeneous catalysts, due to its role in controlling the yield

and selectivity towards the different products present in the

biofuel. The properties of catalysts are governed by acidity, pore

shape and size. Furthermore, the reaction conditions (tempera-

ture, pressure, space velocity, presence of steam, type of reactor,

etc.) and the nature of feedstock also exert a significant influence.

Generally, the presence of zeolite catalysts increases the yield

towards the OLP fraction whereas amorphous catalysts

predominantly produce a higher amount of gases.82,84,99 Co-

feeding steam during the reaction helps to increase both the olefin

yield as well as the durability of the catalyst, since the presence of

steam diminishes the coke formation and thus the catalyst

deactivation.81 The use of a fluidized bed usually reduces the

selectivity towards the OLP fraction in contrast to that found for

a fixed bed. This fact arises from the shorter contact time in

a fluidized bed which diminishes the extent of the oligomerization

reactions taking place between C2–C5 olefins.100 From all the

different studies, it can be concluded that most of the triglyceride

feedstock is converted (>80%), leading to an OLP fraction

mainly composed of a high amount of aromatics (�50%) with

a null presence of oxygenated hydrocarbons.82,86,89,100 The


different authors have shown that the initial stages in the trans-

formation of the triglyceride molecule are thermally driven

processes, which are followed by the subsequent secondary

cracking reactions (hydrogen transfer, isomerization, oligo-

merization, b-scission, aromatization) in which the acid catalyst

plays a crucial role.89

Table 1 summarizes the most relevant works dealing with the

catalytic cracking of triglyceride molecules indicating the type of

feedstock, the reaction conditions, and the catalyst. Typically

these studies have been performed in fixed bed reactors, in

a temperature range between 300 and 500 �C, with liquid space

velocities ranging from 2 to 4 h�1. Although the cracking of

vegetable oils into liquid fuels has been much studied, the

cracking of triglycerides molecules under realistic FCC condi-

tions is scarcely described in the literature. However, certain

number of authors have performed studies about the processing

of vegetable oils93,101–107 and animal fats104,106,107 under conditions

which try to simulate the operating conditions of the FCC unit.

In these studies, the reaction system employed is usually based in

a riser reactor and a FCC catalyst. After the catalytic cracking

reactions, conversion is usually over 75%.85,96,107,108 Furthermore,

there are no remarkable amounts of oxygenated hydrocarbons in

the final cracking products, since almost all the initial oxygen

present in the triglyceride molecule is transferred to water or

carbon gases (CO and CO2).101,104,107

Co-processing renewable raw materials in an FCC refining

unit has also been studied by several research groups, demon-

strating the technical feasibility of co-processing several mixtures

of vegetable oils (palm, rapeseed, soybean or sunflower oils),

waste cooking oil and animal fats and vacuum gasoil under FCC

conditions.104,113–117 Not only the operation conditions registered,

but also the final products obtained after the catalytic cracking

reactions, are perfectly compatible with the conditions and

products usually related to the FCC unit. However, there is

a strong effect of the feedstock composition in the cracking

products’ distribution.

The main differences in the processability of the typical feed-

stock (oil derived) and its mixtures with vegetable oils and animal

fats are the production of oxygenated compounds coming from

the presence of oxygen in the starting triglycerides. However, the

usual conditions of the FCC unit are severe enough to decom-

pose the heavy oxygenated hydrocarbons by means of decar-

boxylation, decarbonylation or dehydration reactions to yield

a mixture of different hydrocarbons. Nevertheless, beyond the

production of non-valuable oxygenated compounds (which are

easily separated from the valuable ones), it is possible to find

several differences between the cracking of a hydrocarbon and

a vegetable oil in FCC conditions. Triglyceride-based biomass

enhances the gas production and always reduces the yield

towards the liquid fraction, especially liquid cycle oil (LCO) and

mainly towards decanted oil (DO)104 (Fig. 7A). These results are

associated with the higher crackability of vegetable oils and

animal fats in comparison with the petrol feedstock, since the

later present a lower concentration of aromatic rings, which tend

to be refractory and more difficult to be cracked. Hence, the

gasoline content in the organic liquid product (GLN + LCO +

DO) is always enhanced as the percentage of vegetable oil is

increased in the initial feedstock.113,116 Bormann et al.114 indicate

that the percentage of gasoline in the liquid products rises from



Table 1 Catalysts and reaction conditions used in the catalytic cracking of several oleaginous feedstock

Catalyst Reaction conditions Feedstock References

SiO2 & silicalite MCM-41 SBA-15 Fixed bed reactor T ¼ 375–500 �C.WVSH: 1.8–15.4 h�1

Rapeseed oil, palm oil 82, 84, 88 and 91

Al2O3 gamma bauxite Fixed bed reactor T ¼ 400–500 �CWVSH: 0.6–15.4 h�1

Rapeseed oil, coconut oil 84 and 94

SiO2–Al2O3 Al-MCM-41 Al-SBA-15 Fixed bed and raiser reactors. T:200–600 �C. WVSH: 1.8–15.4 h�1

Rapeseed oil, palm oil, canola oil 84, 90, 108 and 109

H-ZSM-5 Fixed bed reactor T: 340–500 �CWVSH: 1–4 h�1

Rapeseed oil, corn oil, WCO 78, 79, 82, 83, 85–88, 91 and 109–112

H-Mordenite H-Y Fixed bed reactor T: 200–600 �CWVSH: 1.8–3.6 h�1

Rapeseed oil, canola oil 82 and 109

FCC catalyst Fixed & fluidized bed, micro riser &FCC pilot plant T: 380–585 �Ccontact t: 50 ms �30 min

Rapeseed oil, cottonseed oil, palmoil, soybean oil, Calopris pocera

101–105

Fig. 7 Reaction results from the catalytic cracking of palm oil–VGO

mixtures. Data adapted from ref. 104. Reaction conditions: Temperature

¼ 565 �C; Catalyst to oil mass ratio ¼ 4.Publ

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60.3% to 61.1%, when using rapeseed oil instead of vacuum

gasoil in cracking experiments. Similar results were obtained by

Carlos deMedeiros et al.,116 when cracking soybean oil instead of

the typical vacuum gasoil, achieving a yield to gasoline in OLP

8.6 points higher than under normal feeding conditions. This

better crackability of triglyceride-based biomass is also clearly

confirmed by the gasoline distribution obtained by Melero

et al.104 The medium (MN; 90–140 �C) and heavy (HN; 140–

221 �C) naphtha yields are reduced 4.7% and 6.8%, respectively,

in the catalytic cracking of palm oil in comparison to vacuum gas

oil cracking, whereas light naphtha (LN; C5–90�C) production is

favoured when increasing the amount of renewable feedstock. In

addition, the decanted oil fraction in the catalytic cracking of

triglyceride-based materials is produced by means of polymeri-

sation reactions, in contrast to the petrol feedstock, which is the

remaining unconverted fraction.

Another important difference found when co-processing

vegetable oil in an FCC unit is the higher yield produced for

aromatic compounds (Fig. 7B). These aromatic compounds are

mainly monoaromatics, which would finally be incorporated to

the gasoline (GLN) fraction (raising its octane number) and


diaromatic compounds. The presence of renewable feedstock

reduces the yield towards polyaromatic compounds, since these

are not present in the starting raw-material. In this way, the high

aromaticity of the liquid fraction when cracking renewable raw

materials comes from the formation of olefins, which have a big

trend to end as aromatic compounds,101 during the removal of

hydrogen from the triglyceride molecules. This transformation is

rather important and several authors, like Adjaye et al.,118 have

described liquid products with more than a 95 wt% of aromatics,

for instance by treating rapeseed oil in presence of ZSM-5. In the

same way, if excessive hydrogen elimination from hydrocarbons

continues, an increase in coke formation is produced.101,104

Therefore, coke production is enhanced with the increase of

triglyceride-based biomass in the feedstock.104,113,116,119

A recent report by Universal Oil Products (UOP) Corporation

discussed how biofuels might be economically produced in

a FCC unit,7,72 starting from vegetable oils and greases to

produce gasoline and liquid cycle oil (LCO). Nevertheless,

a pretreatment unit to remove catalytic poisons such as alkali

metals and other problematic components, such as water and

solids, has been considered as a crucial step. Then, the pretreated

feed can be introduced as a co-feed with vacuum gas oil (VGO) to

yield gasoline and other products. A modified catalytic cracking

process was also proposed to yield high-value products such as

ethylene and propylene (severe conditions to maximize olefin

production).

3.2.2 Hydrotreating of oleaginous feedstock. A different

option to the catalytic cracking in the use oleaginous feedstock in

conventional crude-oil refinery plants is their hydrotreatment to

produce straight chain alkanes ranging from C12 to C18. These

alkanes have high cetane numbers (80–100) and good fuel

properties,120 which can be used in producing diesel-like fuels.

The advantages of hydrotreating over transesterification are that

the former is compatible with current treatment infrastructure,

products are fully usable in existing engines, and there is some

flexibility with respect to the feedstock.121 Hydrotreating,

because of the need for hydrogen, is much more expensive than

catalytic cracking, but the products achieved through this

pathway (green diesel) are pure hydrocarbons indistinguishable

from their petroleum counterparts. In fact, the high cetane

number displayed by green diesel makes it possible to meet the

current specifications for petroleum-derived diesel-like fuels. In



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this sense, several studies reveal a meaningful decrease of

hydrocarbons, carbon monoxide and nitrogen oxides in diesel

engine emissions when using green and conventional diesel

mixtures instead of petroleum-derived diesel as fuels.121–123

A general reaction pathway for hydrotreating vegetable oils is

shown in Fig. 8. The total hydrogenation of triglycerides leads to

the formation of n-alkanes and propane as main reaction prod-

ucts, and water, CO and CO2 as by-products. This process

involves several reactions which can be lumped into two different

reaction pathways:124,125 hydrodecarboxylation (HDC) and

hydrodeoxygenation (HDO). The former involves the loss of

a carbon atom, coming from the carboxylic group at the fatty

acid chain. In this way, the main n-alkanes obtained as products

display one less carbon atom than the original fatty acid alkyl

chain, this carbon being lost as carbon monoxide or dioxide.

Nevertheless, hydrodeoxygenation leads to the formation of long

n-alkanes showing the same number of carbon atoms as the

original fatty acid alkyl chain as well as propane, coming from

the hydrodeoxygenation of glycerine. The proportion of hydro-

deoxygenation and hydrodecarboxylation pathways at the total

conversion of triglycerides to hydrocarbons is decisive for the

overall hydrogen consumption of the process. Based on the

stoichiometry of the hydrodeoxygenation mechanism, the mass

balance indicates the requirement for 12 moles of hydrogen per

mole of triglyceride plus an additional mole of H2 per double

bond present in the fatty acid alkyl chains, which is the faster

transformation, and thus occurs in first term when hydrotreating

triglycerides.6 Hence, the total hydrodeoxygenation of rapeseed

oil (4 double bonds per mole) involves the requirement for 16

moles of hydrogen per mole of triglyceride, leading to a mixture

of water (six moles), propane (one mole) and a mixture of n-C18

and n-C22 (three moles) –rapeseed oil fatty acid profile is mainly

composed of oleic, linoleic and linolenic acids (C18 z 35 wt%)

and erucic acid (C22 z 40 wt%). Nevertheless, only 3 moles of

hydrogen are needed to process a mole of triglyceride by the

hydrodecarboxylating mechanism, plus the additional H2 mole-

cule required for reducing each double bond, leading to

Fig. 8 Proposed mechanism for triglyceride hydrotreating reactions.


a hydrogen consumption of 7 moles per mole of rapeseed oil.

This fact suggests that hydrodecarboxylation should be favoured

over hydrodeoxygenation to reduce hydrogen consumption,

a critical issue in the profitability of hydrotreating units.

However, the mere presence of CO2 in the reaction system leads

to the existence of methanation or at least the partial reduction of

the same to CO, as well as the water gas shift reaction,126 though

to a minor extent. Hence, these reactions should be considered

when evaluating the process economy, since they lead to the

consumption/production of hydrogen. The extent of these reac-

tions depends on the catalyst and reaction conditions. Therefore,

if all CO2 was to be converted into methane, or its trans-

formation into carbon monoxide and subsequent methanation,

12 additional moles of hydrogen should be added to the three

already considered from the HDC transformation – plus the

hydrogen consumed in reducing hydrocarbon bonds. Conse-

quently, rapeseed oil hydrodecarboxylation would need 19 moles

of hydrogen per mole of triglyceride, leading to n-C17 and n-C21

as the main products (three moles), water (six moles), propane

(one mole) and methane (three moles). Hence, considering

hydrogen consumption, the HDO pathway may be more

attractive than HDC, though some authors have reported

experimental results contrary to these calculations.127 Neverthe-

less, such a low difference between H2 consumption by both

mechanisms and the similarities observed in the final hydro-

carbon products do not allow the determination of which is the

best option, depending on the process and on the catalyst used in

the hydrotreating unit.124

Because of the similarity between hydrodesulfurization and

hydrodeoxygenation, oxygen removal from triglycerides by

HDO seems to be a rather easy task to implement on conven-

tional refinery hydrotreating units used for hydrodesulfurization

of petroleum-derived streams.128 Hydrodesulfurization (HDS)

usually co-exists with HDO and hydrodenitrogenation (HDN)

for the removal of sulfur, oxygen and nitrogen, respectively. The

most important industrial catalysts used for HDS conventionally

involve alumina-supported molybdenum and tungsten sulfides as

the main catalytic species, usually promoted with cobalt and/or

nickel.129 However, Co–Mo and Ni–Mo are the most widespread

catalysts in hydrotreating units, due to their higher catalytic

activity.130 These catalysts are employed because of their high

resistance against sulfur poisoning, in contrast with noble metals,

which display a much higher hydrogenating activity but a lower

resistance against deactivation with sulfur. Nevertheless,

important different behaviors are found for the HDS catalysts

when hydrotreating triglyceride-containing feedstock (HDO

process) in comparison with petroleum-derived streams (HDS

process). These differences are associated with the nature of the

feedstock and the final form of the heteroatoms to be removed

(oxygen and sulfur, respectively), even though the reaction

pathways and mechanisms are rather similar in HDS and HDO.

Thus, hydrodeoxygenation leads to the formation of water

(H2O) whereas hydrodesulfurization leads to the formation of

H2S, both of them interacting with the surface of sulfided cata-

lysts.131 It is well known that H2S promote the acid catalytic

activity of these catalysts,132 enhancing the reaction rate of the

acid-driven transformations. On the contrary, the interaction of

water with hydrogenating sites leads to the inhibition of the

hydrogenation reactions.133 Sxenol et al.134 described the



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hydrogenation of model aliphatic methyl esters in the presence of

alumina-supported sulfided CoMo and NiMo catalysts as

a sequence of three reaction pathways: the formation of alcohols

which evolve towards hydrocarbons by dehydration; the de-

esterification between the alcohol and the carboxylic acid func-

tionality; and the hydrogenation of the carboxylic acid towards

hydrocarbon, either passing through the alcohol or not. Thus,

dehydration, hydrolysis and hydrogenation reactions are present

at the same time when hydrotreating these triglyceride-contai-

ning feedstocks, the two first reactions being promoted by acid

catalysis whereas the last one is driven by the hydrogenating

activity. In fact, some decarboxylating activity134 has also been

found to be present, which could not be avoided, at least under

the employed reaction conditions. This last reaction is also

driven by the acid sites of these catalysts, whose presence is

associated with sulfhydryl acid groups.131 A proof of this

behavior is the low hydrodecarboxylating activity of the same

catalysts when used as non-sulfided metal oxides.134

The HDO reactions carried out in the presence of H2S might

avoid the deactivation caused by the evolving water molecules,

but the enhancement of acid activity modifies the behaviour of

the catalyst leading to a meaningful increase of the extent of

hydrodecarboxylation reactions.131,132 On the contrary, hydro-

genolysis and hydrogenation reactions are due to the presence of

sulfur vacancies associated with molybdenum atoms.135

The reaction pathways observed when treating more realistic

feedstocks like sunflower136 or waste cooking oils137,138 were

essentially the same to those previously described for model

compounds, so that the catalytic performance of conventional

hydrodesulfurization catalysts seems not to be affected by the

nature of the feedstock. However, increasing the reaction

temperature leads to a much higher extent of the removal of

oxygen, though this enhancement of triglyceride conversion is

accompanied by a much higher rate in HDC, isomerization and

even hydrocracking reactions, which are more important when

NiMo/g-Al2O3 was used as catalyst.139–141 Thus, though the

triglycerides conversion increases with the operation tempera-

ture, the yield towards green diesel decreases. A proof of this fact

is the decreasing of the cetane number in the final product or the

increase of the bromine index when operating at high tempera-

ture (�400 �C). In this way, it seems preferable to operate at

lower temperatures and recirculate the heavy fraction and the

residue to maximize the yield towards biodiesel.

In order to increase the catalytic performance of hydro-

desulfurization catalysts in triglyceride HDO treatments, some

modifications of conventional catalysts have been assayed. The

main improvements lie in the modification of the catalytic

support, mainly tackled through the enhancement of the support

surface area142 or the assay of different supports like silica or

silica alumina mixed oxides.143,144 However, the conversion of

triglycerides was lower than that achieved over conventional

hydrodesulfurization catalysts. Hence, further research is needed

to fully understand the influence of the catalyst-related proper-

ties on the triglyceride HDO process.

The use of conventional HDS catalyst for treating mixed

blends composed by triglyceride-containing oils and petroleum-

derived oils is now gaing more and more interest by petroleum

refineries. This strategy is readily applicable in conventional

refineries without the needing to implement great modifications


on existing hydrotreating units. Several advantages have been

found when treating the mixture of renewable and conventional

raw-materials in comparison to the single processing of both

feedstocks. Thus, Huber et al.145 reported the treatment of

sunflower oil together with heavy vacuum oil (HVO) in presence

of a conventional sulfided NiMo/g-Al2O3 catalyst. Interestingly,

the treatment of the mixture led to a higher amount of straight

alkyl chains in the range C15–C18 than if treating pure sunflower

oil, as a consequence of the dilution of free fatty acids (FFA),

which inhibit polymerization and hydrocracking reactions.146

Moreover, since hydrodesulfurization is a much slower reaction

than alkane production from the vegetable oils, the use of feed-

stock mixtures does not affect the rate of desulfuration. Similar

results have also been obtained using CoMo/g-Al2O3 catalyst in

the hydrotreatment of cottonseed oil.147 Besides this fact, quality

enhancement of several properties of the final products can also

be found, like the cetane index,148 which shows higher values

compared to those achieved when treating only petroleum frac-

tions. In this way, the direct application of the existing infra-

structure in petroleum refineries for treating petroleum streams

and vegetable oils mixtures makes possible extensive industrial

application in the near future. However, blending vegetable oils

with VGO dilutes the latter. Consequently, contact time has to be

adjusted to maintain high rates of conversion of sulfur and

nitrogen. This change may cause the catalysts to deactivate faster

and decrease the catalyst cycle length.72

Therefore, the hydrotreatment of triglycerides for the

production of hydrocarbons on an industrial scale can be

implemented either by HDO of triglycerides in stand-alone units

or by co-processing of triglyceride feedstock with crude-oil-

derived fractions (Fig. 9). Co-processing offers the advantage of

low implementation cost due to the possibility of using the

existing hydrotreatment equipment. However, several potential

risks have been identified, including: (i) the need for a pretreat-

ment reactor to remove contaminants from the vegetable oil; (ii)

revamping the gas recycling systems to deal with the deoxygen-

ation products (CO2, CO, H2O); (iii) the competition of HDO

reactions with HDS leading to lower desulfurization efficiency;

and (iv) the cold-flow properties of the combined diesel product

may limit the quantity of vegetable oil that can be processed as n-

paraffin – the primary product from hydrotreating vegetable oil –

and will impact the cloud point.7 After considering all the

potential issues, it seems to be more cost-effective to build

a dedicated unit specifically designed for vegetable oil processing

(Fig. 10) than using a hydrotreatment unit for co-processing

renewable and conventional feedstock.

Many companies investigate and plan to apply hydrotreat-

ment technologies for the production of renewable diesel fuel

from vegetable oils and animal fats. Some of these developments

consist on modified gasoil hydrotreating processes to which

a blend of gasoil and triglyceride-containing mixtures is fed.149–151

However, industrial experience has been gained in building

trygliceride hydrotreatment stand-alone units. Thus, Neste Oil

OYJ, a Finnish oil refining company, has licensed a new process –

the NExBTL process – for the production of green diesel from

pure vegetable oils and fats.126,152 NexBTL renewable diesel is

currently produced at three renewable diesel plants in: Porvoo

(Finland) with a total capacity of 190 000 metric tons per year, at

the world largest renewable diesel refinery in Singapore (800 000



Fig. 9 Option for co-processing triglycerides with crude oil-derived

fractions in a hydrotreating unit.

Fig. 10 UOP/ENI Ecofining process flow diagram.

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metric tons per year) and at the Europe largest renewable diesel

refinery in Rotterdam, Netherlands (800 000 metric tons per

year).71 A similar process is that licensed by UOP/ENI – the

Ecofining process153,154 – which involves the same two different

reaction stages as in the NExBTL process. Thus, the two distinct

stages comprise the hydrodeoxygenation step in which trigly-

ceride-based biomass and hydrogen are fed to. Obviously, not

only HDO reactions occur during this step, but also HDC and

subsequent methanation takes place in this first reaction. The

second stage involves the hydroisomerisation of the deoxygenated

product to improve its cold properties. Light ends can be used to

produce hydrogen, which is then recycled to the reaction stages to

increase the profitability of the processes.

Essentially, both processes, NExBTL and Ecofining, are rather

similar and the final products coming from the process are the

same: light naphtha and propane and green diesel. This last is the

major component, comprising more than 80 wt% of the final


products. The final properties of this green diesel are rather

similar to those achieved through the Fischer–Tropsch liquid fuel

from syngas. Table 2 lists the properties calculated for diesel-like

fuels, including green diesel and its blend with conventional

diesel fuel. These fuels are featured by a high cetane number

(involving very good engine efficiency), low oxygen content

(makes green diesel more similar to petroleum derived fuels), and

very low sulfur content. The boiling range of green diesel is rather

similar to that of conventional diesel fuel, preventing vapor-

ization problems in the combustion chamber. Besides, green

diesel displays very high stability in contrast with biodiesel.

Finally, the low specific gravity of this fuel makes possible to

upgrade to low value and highly dense refinery streams.

The economical and environmental impact of the Eco-

fining153,154 and the NExBTL155 processes in conventional

petroleum refineries has been studied by means of lifecycle

assessments. The results of these studies indicate that these

processes are competitive with biodiesel when operating

moderately sized units. However, the profitability of the

processes depends on the differential price between crude

petroleum and renewable plant oils. From an environmental

point of view, the production processes for green diesel are

clearly superior to conventional diesel fuel production, both in

terms of energy consumption and greenhouse gas emissions.

Nevertheless, differences with conventional diesel fuels can

become negligible depending on the starting triglyceride-con-

taining oil or even the farming practices.

As an alternative to these NExBTL and Ecofining

processes,153–155 a process for producing an isoparaffinic product

useful as jet fuel from renewable feedstocks – the Bio-Synfining�process – has been also developed by the Syntroleum Corpora-

tion.156–158 Syntroleum’s Bio-Synfining� process is capable of

processing a wide range of renewable feedstock including vege-

table oils, fats and greases into a broad slate of synthetic ultra-

clean fuels, including jet fuel. The patent-pending process is

schematically represented in Fig. 11. This process catalytically

converts the fatty acids into paraffinic hydrocarbon fractions

containing virtually no residual oxygenates. The fatty acid chains

are first hydrogenated and deoxygenated into straight chain

paraffins, which are then hydrocracked into shorter straight and

branched paraffins (iso-paraffins). The hydrocracker product

consists of a broad distribution of mainly C3–C18 hydrocarbons

that may be fractionated into LPG, naphtha and jet fuel frac-

tions. In a first step, triglycerides are converted into a LPG,

a naphtha fraction and a kerosene or gas oil fraction, whose

relative amounts depend on the conditions used in the hydro-

cracker. The steam cracking of the renewable naphtha in a pilot

plant revealed that high olefin yields are obtained, being this

option useful as an alternative route for the production of light

olefins.159

3.2.3 Catalytic deoxygenation of oleaginous feedstock.

Although, previous studies on the hydroprocessing of trigly-

ceride-based feedstocks have forwarded considerable informa-

tion, there are still some weak points that would benefit from

more attention. From an economics and environmental view-

point, minimizing unnecessary hydrogen consumption is

important. The amount of hydrogen consumed for triglyceride

conversion by the HDO route (breaking the C–O linkage with no



Table 2 Properties of different diesel-like fuels obtained through different techniques. Data from ref. 6, 123, 149 and 150

Property Diesel fuel Biodiesel Green diesel Blenda

Cetane number 53 50 70–90 57.8Oxygen content (wt%) 0 11 0 0Sulfur content (wt ppm) <10 <10 <1 4.7Distillation/�C 180–360 340–355 265–320 249–341Lower heating value/MJ kg�1 35.7 38 44 36.5Cloud point/�C �5 �5 �10 to 20 �4.1Stability Baseline Low Baseline BaselineSpecific gravity/kg m�3 835 883 780 827

a Effluent coming from the HDS unit when co-feeding VGO and palm oil (10 wt%).

Fig. 11 Bio-Synfining process flow diagram.

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release of CO/CO2) is considerable higher than that by the

decarboxylation route (breaking of a C–C bond with release of

CO or CO2). However, as previously mentioned, the total

hydrogen consumed by the decarboxylation route could theo-

retically exceed the HDO route through secondary reactions such

as water gas shift and methanation.124 The extent of hydrogen

consumption via the different mechanism is expected to be

sensitive to the process conditions and catalysts, and thus, more

studies need to minimize unnecessary hydrogen consumption

should be undertaken.

Recently, Murzin et al. have investigated the catalytic deoxy-

genation of long-chain free fatty acids and derivatives by the

decarboxylation route under low hydrogen pressures or without

hydrogen.126,160–164Awide variety of supported noble metals were

used as catalysts for catalytic deoxygenation of stearic acid under

He.160 Reaction experiments indicate that supported metal

carbon-based catalysts display a much higher selectivity towards

decarboxylated products than analogue catalysts based on

different supports. Considering the active metal species, palla-

dium and platinum display a much better catalytic performance

than other metals like ruthenium, rhodium or iridium. However,

palladium mainly drives decarboxylation reactions, whereas

platinum produces decarbonylation. Apart, from deoxygenation

reactions, other reactions, such as hydrogenation, dehydroge-

nation, cyclization, ketonization, dimerization and cracking were

observed to various extents depending on the catalysts. Never-

theless, the preparation conditions used for these Pd/C materials


exert a dramatic influence on their catalytic behavior, being the

particle size of the final supported metal species one of the most

important parameter conditioning the catalytic activity.161

Moreover, the Pd/C catalyst studied deactivated rapidly,

apparently because of coking.161,164 However, the FFA deoxy-

genation reactions carried out under low H2 partial pressures

might avoid catalyst deactivation and with and enhancement of

the catalyst lifetime and deoxygenation rate while maintaining

high selectivity to the H2-neutral decarboxylation pathway.165,166

Apart from FFA, these catalysts have also been used for

treating different feedstock such as alkyl esters and triglycerides.

Independent of the feedstock, the major products were hydro-

carbons – both saturated and unsaturated – formed by an alkyl

or alkenyl chain with one carbon atom less than the original fatty

acid chain.162 However, FFA deoxygenation seems to be rather

effective and fast, being more difficult than the deoxygenation of

alkyl esters163 and even more complicated in the case of triglyc-

erides. These differences seem to be caused by a different reaction

mechanism. Thus, whilst the deoxygenation of free fatty acids

follows a decarboxylating pathway, in the case of fatty acid alkyl

esters it mostly proceeds via decarbonylation,164 though both

types of reactions coexists in both cases,126 and the dominant one

can be tuned depending on the catalyst and the reaction

conditions.

Based on the results obtained in the deoxygenation of

tristearin, triolein and soybean oil over Ni/C, Pd/C and Pt/C

catalysts, Morgan et al. suggested a generalized scheme for

triglycerides deoxygenation (Fig. 12).167 The formation of

hydrocarbons from triglyceride can proceed via different path-

ways. In the bottom route, liberation of a fatty acid occurs via

a b-elimination process, the co-products being an unsaturated

glycol difatty ester. The acid subsequently undergoes de-

carboxylation in a separate step to give an alkane containing one

carbon less than the acid. In the case of the unsaturated glycol

difatty ester formed from the triglyceride, further reaction by b-

elimination is not possible and other pathways must be involved

to explain the formation of additional hydrocarbons. Alternative

pathways for hydrocarbon formation involve either scission of

the C–C bond between the ester carbonyl carbon and a carbon of

the hydrocarbon chain, or scission between the b and g carbon

atoms. These processes may lead to alkanes or terminal alkenes

depending on whether hydrogen abstraction occurs during bond

scission.

In order to avoid Pd/C catalyst under decarboxylation

conditions, the modification of the catalytic support has been



Fig. 12 Suggested general reaction scheme for triglyceride

deoxygenation.167

Fig. 13 Process flow diagram of Avjet Biotech’s Red Wolf Refining

System.175

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investigated for different authors. Ping et al. have reported the

synthesis, characterization and application of well-dispersed

palladium nanoparticles supported on a mesocellular silica

support in the catalytic decarboxylation of stearic acid.168,169 The

different catalysts synthesized were active in the batch decar-

boxylation of stearic acid under nitrogen atmosphere at 300 �C,reaching conversions of 85–90% after 6 h with complete selec-

tivity to the decarboxylation product n-heptadecane. However,

lower conversions (<15%) were observed in the decarboxylation

of ethyl stearate. After one use, the catalysts activity was

dramatically reduced, achieving less than 5% conversion to n-

heptadecane in the second use, as a consequence of carbonaceous

deposition. Nevertheless, contrary to previous assumptions, the

majority of these carbonaceous deposits were found not to be

traditional coke, but instead residual reactants, solvent and

product. Detrimental coke formation was verified to be absent,

as extraction of surface-deposited organic species yields nearly

complete recovery of the total surface area, pore volume and

active palladium surface area. Moreover, regenerated catalyst

exhibits a significant recovery of decarboxylating activity.

Berenblyum et al. have studied the catalytic decarboxylation

of stearic acid over palladium supported over acid solids.127,170

Palladium compounds were deposited onto WO3/ZrO2 support

(3 : 7) followed by the reduction of these compounds into

metals.170 It was found that the reaction proceeds quite effec-

tively on the catalyst with a palladium loading of 0.5% in an inert

atmosphere. However, it has been proved hydrogen promotes the

reaction, although the stoichiometry of the decarboxylation

reaction does not require the introduction of hydrogen from an

external source. The conversion of stearic acid has been also

carried out in the presence of palladium on alumina, being

heptadecane the main product of the reaction at a relatively low

H2 pressure (10 atm or less).127

High activity and selectivity toward diesel-like hydrocarbons

from both model compound (methyl octanoate) and real vege-

table oil feedstock (methyl stearate) can be successfully achieved

with 1%Pt/g-Al2O3.171 When feeding methyl octanoate,

a mixture of C7 alkane and alkenes, which results from decar-

bonylation or decarboxylation of the ester, acid and other 1-

oxygen-containing compounds are dominant products. C8

products are also formed but in much smaller quantities.

Nevertheless, if the reaction is carried out under hydrogen

atmosphere, formation of heavy products are much suppressed

compared to the reaction carried out under inert gas. Regarding

to methyl stearate, its decarboxylation produces C17 with


high selectivity, though both the conversion and selectivity

towards paraffin are higher under hydrogen than under an inert

gas.

Since the precious metals are rare and expensive, some tran-

sition metal oxide and metal catalysts have been studied for the

catalytic decarboxylation process. Hydrotalcites with various

MgO contents172 and Ni-containing hydrotalcites173 have been

tested as catalysts in the decarboxylation of oleic acid instead of

precious metals. Hydrotalcite is a layered double hydroxide

composed of MgO and MgAl2O4 and is used as absorbents for

CO2 and catalyst supports. Hydrotalcites showed decarboxyl-

ation activity without using hydrogen and could produce pure

hydrocarbon streams. However, selectivity of heptadecene, the

product obtained by direct decarboxylation of oleic acid, was not

very high, implying that the cracking and decarboxylation

occurred simultaneously during the reaction over hydrotalcites.

Ni catalysts supported on MgO–Al2O3 have also shown catalytic

activity for decarboxylation of oleic acid, but heptadecene

selectivity was lower than 15%.174

The Red Wolf Refining System, developed by Avjet Biotech

Inc. is already harnessing this technology to convert the fatty

acids coming from the hydrolytic conversion of triglycerides to

aviation biofuels on an industrial scale.175 As depicted in Fig. 13,

the process comprises the following steps: (1) thermal hydrolysis

of a lipidic biomass to yield a product stream comprising free

fatty acids and a by-product stream comprising glycerol; (2)

catalytic deoxygenation of the free fatty acid stream to form

a product stream comprising n-alkanes; and (3) reforming step of

the n-alkane stream to form a product stream comprising

a mixture of hydrocarbon compounds selected from the group

consisting of n-alkanes, isoalkanes, aromatics and cycloalkanes.

Hydrogen consumption during the process is minimal. Most of

the hydrogen consumed is used to saturate any unsaturated fatty

acids during the deoxygenation process (a necessary step in

making drop-in fuels). Likewise, this hydrogen is not actually

consumed, but becomes part of the fuel molecule, increasing the

energy density of the fuel. In contrast, hydrogenation processes

where the hydrogen is used to deoxygenate the fat/oil/lipid result

in more water than energy-dense fuel.



Fig. 14 Reactions and mechanisms taking place in the thermal and

catalytic cracking of bio-oils.

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3.3 Lignocellulosic based feedstock processing

The use of lignocellulose as a refinery feedstock is a promising

alternative to conventional crude oil because of the large abun-

dance and variety of this raw material.176 Lignocellulosic-derived

feedstock accounts for more than 95% of the total biomass

produced by the whole planet every year. There are a huge

variety of lignocellulosic materials which could be considered as

feedstock for biorefineries:177 agricultural residues (i.e. corn

stover, cereals straw and bagasse.), woody materials including

hardwood (poplar, aspen.) and softwood (pine), herbaceous

materials (switchgrass, sorghum) and wastes from paper

industry, among others. Despite only certain lignocellulose

sources seem to lead to an optimal combination of biomass-

starting raw materials as the entrance point to a refinery, both

from an environmental and energy point of view, any kind of

lignocellulose source can be used as substitutes for crude oil in

a biorefinery.10 Furthermore, the pseudo-homogeneous compo-

sition of cellulosic materials, polysaccharides, are all built from

simple sugars, which facilitates their processing, making easier

their inclusion as starting raw material of a continuously oper-

ating production process. The use of lignocellulosic biomass in

a refinery can be accomplished in different ways, however, it

seems rather difficult to establish a biomass supply chain con-

sisting of the lignocellulosic material in the form it is collected, as

it was previously established, due to the low energy density these

feedstocks show. Quite effective options are the transformation

lignocellulosic biomass into a fluid by pyrolytic treatment,

leading to the production of a liquid called bio-oil, or by de-

polymerising the constituents of lignocellulosic biomass into

simple sugars and subsequently transforming them into platform

molecules.

3.3.1 Bio-oils to hydrocarbon fuels. Bio-oil upgrading to

hydrocarbon fuels can be accomplished by means of different

procedures,178 like the treatment of bio-oils in presence of zeolite

catalysts, which allows a reduction the oxygen content and

improvement of the thermal stability of the feedstock.179Thermal

treatment at 350–500 �C and atmospheric pressure leads to the

production of hydrocarbons (both aromatic and aliphatic),

water-soluble organics, water, oil soluble organics, gases (CO2,

CO, light alkanes) and coke. The chemical pathway through

which these chemicals are produced is very complex,35 and

several types of reactions take place simultaneously, including

dehydration, cracking, polymerization, deoxygenation and

aromatization reactions. Nonetheless, a poor hydrocarbon yield

and high production rates of coke limit the usefulness of zeolite

upgrading.

Adjaye and Bakhshi180 explored the possibility to feed bio-oil

to a FCC unit by means of a deep study of the reactions taking

place when feeding different bio-oil model compounds to a fixed-

bed reactor loading a HZSM-5-based catalyst. These authors

concluded that the bio-oil became separated into volatile and

non-volatile fractions in the early stages of the reaction while

reaching the reaction temperature. The heavy fraction (which

accounts for 37 wt% of the total bio-oil) undergoes a cracking

pathway, yielding volatile compounds and a solid residue

produced by means of polymerization reactions and coke

formation. The resultant volatile fraction suffers deoxygenation


(both through dehydration, leading to water formation, and

through decarbonylation and decarboxylation reactions,

producing carbon oxides), secondary cracking (leading to

hydrocarbon gases), oligomerization, olefin formation, hydrogen

transfer, cyclation, disproportionation, alkylation and isomeri-

sation reactions to form the hydrocarbon-rich product. Some of

the forming hydrocarbons also polymerize and condense to form

more residue. Other types of catalysts181–184 have also been tested

in the upgrading of bio-oils, including HZSM-5, H-Y, morde-

nite, silicalite and silica alumina. Acid zeolite-based catalysts

have revealed to be highly effective in the conversion of bio-oil to

hydrocarbons, and thus the HZSM-5 catalyst produced the

highest amount (34 wt% of feed) of organic liquid products

among the tested catalysts, while providing the lowest coke

formation. On the other hand, the less acidic silica-alumina

catalyst minimizes char formation, whereas the H-Y catalyst

leads to a minimum tar formation and maximum production of

aliphatic hydrocarbon. From these experiments, authors have

proposed several reaction pathways for bio-oil when treated in

presence of acid zeolites (Fig. 14). Two different reaction stages

were proposed in the reaction mechanism, consisting of an initial

thermal step followed by a thermocatalytic one. The former leads

to the separation of bio-oil into light and heavy organic

compounds, while inducing char formation. The thermocatalytic

step produces coke, tar, gas, water and the desired organic

distillate fraction. Deoxygenation, cracking, cyclation, aromati-

zation, isomerization and polymerization reactions take place

during this step. This double stage transformation allows sepa-

ration of the different reaction steps in different reactors by using

an installation consisting of two reaction units. Thus, in a first

empty reactor the thermal transformations can be accomplished,

whereas in a second reactor, loaded with the catalyst, the ther-

mocatalytic transformations are promoted. In this way, a longer

catalyst life is achieved, since the coke formed during the first

stage is not deposited on the surface of the catalyst,185 avoiding

its poisoning.

Similar studies, but feeding model bio-oil compounds (alde-

hydes, ketones, acids, alcohols, phenols and their mixtures)

instead of real bio-oil186–188 revealed differences between the

reactivity of the assayed families of chemicals. However, most of

them produced large amounts of coke, and caused catalyst

deactivation. In this way, the previous stabilization of the bio-oil,

for instance through a specific thermal step,189 was required

before being treated in presence of the acid catalyst. Co-feeding



Fig. 15 Pretreatment options for bio-oil in its co-feeding with vacuum

gas oil to FCC units.

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methanol (�70 wt%) resulted to be another option to minimize

coke deposition on the catalyst particles.190

Model bio-oil compounds, such as acetic acid, hydroxyacetone

and phenol, added to a standard gasoil FCC feed, lead to

increasing overall raw material conversion, reduced coke yields

(except for phenol) and higher fuel gas, LPG and gasoline

production, when treated in presence of an industrial FCC

equilibrium catalyst (E-CAT).191 However, the bio-oil model

compounds suppress the effect of the ZSM-5 catalyst additive,

suggesting a preferential interaction of such compounds with the

ZSM-5 additive, leading to its inactivation.

The direct use of bio-oils in refineries, either as the feedstock of

FCC unites or mixed with conventional petroleum streams, is

conditioned by the high concentration of water, oxygen and

metals, particularly potassium and calcium. Metals can be

removed by retention in guard beds by ion exchange. On the

contrary, the low thermal stability, high water content and very

high oxygen content of bio-oils make difficult their blending with

common refinery intermediate streams such as VGO. A

maximum amount of 10 wt% of oxygenated compounds, referred

to gasoil, has been estimated to be possibly fed to a FCC unit

without major problems,191 though the amount of benzene in the

product stream can be rather high due to the presence of

phenolics in the feed stream. A different drawback of bio-oils to

be treated in standard refinery units, but nonetheless highly

important, lies in its high acid number which makes the standard

refinery vessels unsuitable to treat such acid feedstocks. The

industry standard supports acid numbers on the streams to be

treated below 1.5 mg KOH g�1. Bio-oils can probably be pro-

cessed using 317 stainless steel cladding, which is not standard in

refinery units. Therefore bio-oils would require to be pre-pro-

cessed in 317 stainless steel-made pre-conditioning units to

reduce their acid number before being fed to typical refinery

units.72 Since the FCC is the biggest unit and the heart of most

refineries, much development work is required before such an

approach would become viable. Hence, the direct feeding of bio-

oils into standard refinery (see Fig. 15a) does not appear as

a straightforward task.

A different option to tackle the insertion of bio-oils as feed-

stock in a conventional refinery is by first hydrotreating this

biomass-derived raw material, prior to feeding to the FCC unit

(Fig. 15b). In fact, a hydrotreatment conditioning step is rather

usual, depending on the feedstock, in conventional refineries,

prior to the catalytic cracking treatment. Thus, acid numbers in

bio-oils can be reduced by hydrodeoxygenation (HDO) together

with the oxygen content. This treatment allows stabilizing of the

bio-oil as well as increasing its energy density, leading to a stream

able to be blended with a petroleum-derived feedstock. The

reactions involved when hydrotreating bio-oils are rather similar

to those taking place in petroleum fractions. Furimsky192 estab-

lished the following order of reactivity of oxygen-containing

groups: alcohols > ketones > alkyl ethers > carboxylic acids zm- and p-alkyl–substituted phenols z naphthol > phenol >

diaryl ethers z o-alkyl-substituted phenols z alkylfurans >

benzofurans > dibenzofurans. Bearing in mind that the most

abundant compounds in bio-oils are phenols, acids and esters,

their reactivities are the most relevant. Phenols, which may

account for up to 25 wt% of liquids obtained by pyrolysis of

lignocellulosic materials, especially if lignin is abundant, are


refractory oxygenates. The overall mechanism for the HDO of

o-substituted phenols can be split into two main HDO reactions:

direct HDO and HDO via hydrogenated phenol, both taking

place at the same time.193 In the latter case, H2O elimination may

result in the formation of intermediate methylcyclohexene

species, which are quickly hydrogenated. The formation of

cyclohexene, alkyl-cyclohexenes and methyl-cyclopentanes can

also be present, though these are only minor products. Guaiacol

(GUA) and its derivatives are also interesting compounds, since

they are present in high quantities in bio-oils. The hydrotreat-

ment of guaiacol has also been deeply studied132,194 to enhance

the stability of bio-oil, since these compounds are quite unstable.

The proposed mechanism through which this compound is

converted considers the hydrogenolysis of the methoxy group of

guaiacol to catechol and methane as the first stage, followed by

the dehydroxylation of catechol in the second stage to produce

phenol. At the same time coke can be formed from both guaiacol

and catechol.

HDO of bio-oils has been carried out at moderate tempera-

tures (300–600 �C) under high H2 pressure in presence of

heterogeneous catalysts. Most of these studies have been focused

on the use of conventional hydrotreating catalysts like sulfided

Co–Mo- and Ni–Mo.132,194 However, as previously stated in the

case of hydrotreatment of oleaginous feedstock, these catalysts

have been developed for hydrodesulfurization (HDS), and thus,

in this case too, sulfur must be added to avoid catalyst deacti-

vation. Elliott and co-workers developed a two-step bio-oil

hydrotreating process based on the use of Al2O3-supported,

sulfided Co–Mo and Ni–Mo catalysts.195,196 The first step con-

sisted on a low-temperature (270 �C, 136 atm H2) catalytic

treatment to promote the hydrogenation of the thermally

unstable bio-oil compounds, which would otherwise undergo

thermal decomposition to form coke and plug the reactor. The

second step consisted of a catalytic hydrogenation but at a higher

temperature (400 �C, 136 atm H2). During this process, 20–30%

of the total carbon in the starting bio-oil is converted into

gaseous compounds, decreasing the overall liquid yield. Catalyst

stability and formation of gums in the lines were identified as

points of major uncertainty of the process, and future work is

needed to develop improved hydrotreating catalysts. However,

treating bio-oil at higher pressures (142–178 atm) and longer



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contact times,72 allowed these troubles to be overcome, since

hydrodeoxygenation prevails, though this approach is unlikely to

be commercially viable because of the high hydrogen require-

ment and the high capital cost of the hydrotreatment step.

Different types of catalysts, like Pt/SiO2-Al2O3,197 vanadium

nitride,198 and ruthenium/C,199 have also been used for hydro-

deoxygenating bio-oils. These catalysts do not need to co-feed

sulfur together with the feedstock to avoid catalyst deactivation,

and besides, they display a much higher hydrogenating capability

than sulfided metal oxides. Nevertheless, these catalysts are also

known to drive decarboxylation reactions, which could not be

interesting due to the lower atom efficiency of this reaction

pathway in comparison to HDO, though much more efficient in

terms of hydrogen consumption. Furthermore, their cost is also

much higher than that corresponding to the sulfided metal

oxides.

Hydrodeoxygenation, as a conditioning step to pre-treat bio-

oils before being fed to a FCC unit, has been tested by different

authors (Fig. 15c). For instance, de Miguel Mercader et al.199

hydrotreated bio-oils in batch conditions (230–240 �C; 290 bar

H2; 5 wt% Ru/C catalyst), looking for the required product

properties necessary for successful FCC co-processing (misci-

bility with FCC feed and good yield structure: little gas/coke

production and good boiling range liquid yields). The product

resulted in three well-separated phases consisting of: gases

(mainly CO2, from the decarboxylation reactions, and CH4,

coming from the methanation of carbon oxides); an aqueous

phase; and an oil phase. Increasing the reaction temperature does

not led to meaningful variations with regards to the yield towards

the aqueous and oil phases, though some compounds (hydro-

deoxygenated sugars) were transferred from the aqueous to the

oil phase, increasing the carbon recovery in the oil product (up to

70 wt% of the carbon in the staring bio-oil). The resultant oil

fraction was, despite the relatively high oxygen content (from 17

to 28 wt%, on dry basis), miscible with the long residue (20 wt%),

being co-processable, at least in a lab scale catalytic cracking unit

(MAT reactor). The assays yielded near normal FCC gasoline

(44–46 wt%) and Light Cycle Oil (23–25 wt%) products without

an excessive increase of undesired coke and dry gas formation, as

compared to the base feed. Samolada et al.200 reported a similar

process consisting of a two-step procedure in which bio-oil is

hydrotreated before being fed to the catalytic cracker. The

thermal hydrotreatment produced the liquid products able to be

upgraded in a refinery. The heavy liquid product of this process,

mixed with light cycle oil (LCO) (15/85 w/w), was considered as

a potential FCC feedstock. Commercially available cracking

catalysts were found to have an acceptable performance in the

transformation of this mixed feedstock, leading to a bio-gasoline

similar to that obtained solely from VGO but in lower yields

(�20 wt%). A similar study to this previous one, accomplished by

Lappas et al.,146 allowed the reduction of the oxygen content

below 6.5 wt% for a hydrodeoxygenation conversion around

85%. These results in the HDO step allowed the accomplishment

of the separation of the hydrotreated bio-oil by distillation into

two different fractions: the light and the heavy ones. The former

mainly contains hydrocarbons lying in the gasoline and diesel

ranges and thus, it could be directly blended with the corre-

sponding petroleum fractions. The heavy fraction was instead

quite similar to conventional VGO, being processable as co-feed


with vacuum gas-oils (VGO) in the catalytic cracker. This

process was developed by Viba oil.146 The co-processing of VGO

with the heavy fraction showed that the presence of the bio-oil

fraction favours gasoline and diesel production but increases the

coke yield. However, depending on the concentration of biomass

liquids, it was shown that this option is technically feasible for

FCC units running with good quality feedstock, that is the FCC

unit with excess coke burning capacity.

Fogassy and co-workers increased the amount of HDO-oil to

20 wt% in the feeding stream, composed of VGO, which was co-

fed to a fixed-bed reactor,201 simulating FCC conditions, and to

a lab-scale FCC reactor.202 In both cases, an industrial equili-

brium FCC catalyst was tested, whereas in the FCC reactor HY

and HZSM-5 zeolites were also used, for comparison purposes.

In both cases, the results were almost the same when treating the

mixed HDO/VGO feeds, no differences relating to the gasoline

fraction were observed in comparison to those achieved when

VGO was treated. Nevertheless, the higher amount of oxygen in

the mixed feedstock when adding HDO-bio-oil leads to a higher

hydrogen consumption during dehydroxylation, resulting in

a poor hydrogen reaction media which finally leads to a product

richer in aromatics and olefins, as well as producing higher

amounts of coke and not totally converting the phenolic fraction.

This phenolic fraction mainly comes from lignin polymers, which

undergo cracking reactions in the external surface of the catalyst

followed by deoxygenation. Since oxygenated compounds

strongly interact with acid sites, large amounts of coke are also

produced, leading to the blockage of the catalyst pores, hindering

the access to the zeolite catalytic sites. In this way, the structural

parameters of the zeolite catalysts have to be tuned in order to

maximize the phenolics’ conversion whilst keeping the coke

deposition low.

Hence, the best approach for the processing of bio-oils in

refining units, is that schematized in Fig. 15d, which is composed

of three different stages: the first one is a hydrogenating step in

which the feed is conditioned to preserve the activity of the

catalyst in the second reaction stage, the hydrodeoxygenation.

The aim of HDO is thus, to extend the catalyst life in the third

stage, the FCC. This strategy allows a better catalytic perfor-

mance of the HDO and FCC, and a good quality of the final

products, though the hydrogen requirements are much higher.

3.3.2 Sugar derivatives as refining raw materials. Fuels from

sugars can be obtained by conventional fermentation, mainly

resulting in the formation of bioethanol, an excellent alternative

to gasoline. However, this conversion involves drawbacks,

especially in view of atom economy, the low energy density of

ethanol and its relative low boiling point. Alternatively, via

a number of direct chemical catalytic conversion processes, fuel-

type molecules could be synthesized by controlled trans-

formation of sugars into several important platform molecules

and subsequent conversions (dehydration, reforming, hydroge-

nation, hydrogenolysis, aldol condensations and olygomeriza-

tion) (see Fig. 16). However, these processes involve the

controlled degradation of polymers and the development of

highly selective transformations of sugars to target molecules in

aqueous solutions. Approaches to solve these challenges may

have to involve novel catalytic materials as well as novel reaction

systems. Moreover, the conversion of renewable feedstock with



Fig. 17 Preparation of oxygenated bio-fuels and bio-fuel additives from

cellulose-derived platform molecules.

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heterogeneous catalysts provides new challenges in inorganic

catalyst research widely used in the oil industry.203 These unique

challenges include the need to convert selectively highly func-

tionalized molecules and to develop catalytic liquid–solid inter-

faces, in which the liquid phase is commonly aqueous, to control

phase effects and to develop novel reaction systems.

Sugars to oxygenated fuels. One interesting alternative to use

sugar-derived platform molecules is their transformation into

second-generation biofuels, keeping the original carbon skeleton

of the platform molecules but enhancing the energy density and

fuel properties of the same. This way to produce biofuels from

lignocellulosic materials is superior, for instance, to the

production of ethanol by fermentation, in the sense of the

preserved atom efficiency avoids the intrinsic energy loss while

helping to reduce the CO2 emissions. Among the different

possibilities, much effort has been centred on the transformation

of these platform molecules into oxygenated fuels (Fig. 17).

One of the easiest ways to prepare a biofuel from sugar

derivatives is that patented by Avantium, which have recently

described two procedures to take advantage of the hydroxyl

group in 5-HMF by means of etherification and esterification

reactions. In particular, final chemicals consist of ethers204 and

esters205 and these are prepared from a glucose-rich raw material

and an alcohol or an organic acid, depending on the final desired

type of HMF derivative. Both reactions can be either accom-

plished in presence of an acid catalyst, which, if strong enough, it

is able to promote the interetherification between two 5-HMF

molecules,206,207 or by reaction with stoichiometric reactants such

as anhydrides206 or acyl chlorides.208,209 The resultant compounds

resulting from both the etherification and esterification reactions,

usually called furanics, display high energy density, making them

interesting compounds to be used as biofuels. Thus, the energy

density of ethoxymethylfurfural (EMF), a chemical liable to be

produced by etherification between 5-HMF and ethanol,

contains 8.7 kWh L�1. This is as good as regular gasoline

(8.8 kWh L�1), as good as diesel (9.7 kWh L�1) and significantly

more energy-dense than ethanol (6.1 kWh L�1).

Fig. 16 Transformations required for the conversion of platform

molecules into bio-fuels and fuel additives.


A different option in the preparation of biofuels from 5-HMF

is its conversion by hydrogenation/hydrogenolysis, yielding 2,5-

dimethylfuran (DMF) and 2-methylfuran (2-MF).210 For this

purpose hydrogenation has to be performed in presence of Cu-

Ru/C catalysts40,210–214 to drive the transformation towards the

reduction of the formyl and the hydroxyl groups, instead of the

reducing the furan heterocycle to tetrahydrofuran, which can be

easily accomplished in presence of Pt, Pd, or Ni.214,215 The

resultant products display high octane number, limited oxygen

content, high efficient hydrogen index and thus high energy

density. Physically, they are miscible with gasoline, all these

properties being good reasons to use these furans as biofuels.

However, initial estimates about the cost for the production of

DMF and the low performance of the production process make

it, at the moment, non-viable for commercial applications.215

Levulinic acid (LA) constitutes another good starting point for

the production of oxygenated biofuels216,217 (Fig. 17), for instance

methyltetrahydrofuran (MTHF), a gasoline-soluble fuel

extender.218 MTHF can be blended up to 30% with gasoline

without modification of current internal combustion engines.

Though the heating value of MTHF is lower than that of gaso-

line, this drawback is compensated by the higher specific gravity

of the former, leading to very similar mileage for both fuels.

Though the direct conversion of levulinic acid to MTHF is

possible,219 improved MTHF yields can be achieved through

indirect routes, for instance by means of firstly hydrogenating

LA to 1,4-pentanediol, followed by subsequent dehydration to

MTHF,216 with a total consumption of 3 moles of external H2 per

mole of LA.220 Nevertheless, this is not the only way to take

advantage of LA for the synthesis of biofuels. Esters of this

platform molecule,221–223 produced from either methanol or

ethanol, have significant potential as blending components in

diesel formulations.224,225 LA esters display similar properties to

fatty acid methyl esters (FAME) that are used in some low-sulfur

diesel formulations, whereas they lack their principal drawbacks

– mainly related to cold flow properties and gum formation. In

this way, the formulation of FAME and alkyl levulinates

mixtures alleviates these troubles, which makes these chemicals

acceptable diluents for biodiesel fuels, even if they contain a high

saturated fatty acid content.225 In this sense, the formulation of



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ethyl levulinate (EL) with conventional diesel fuel, has been

studied,226 demonstrating several advantages can be attained

when using the mixture in contrast to the use of conventional

fuel. Thus, these mixtures lead to a low-smoke diesel formulation

(the high oxygen content of EL, 33%, helps to provide clean

burning), with low sulfur emissions (EL does not supply sulfur to

the mixture) but improved lubricity in comparison to low sulfur

diesel (because of the excellent lubricity provided by EL).

Furthermore, this oxygenated additive does not lead to a signif-

icant decrease of the engine efficiency, and thus, a similar mileage

per volume can be achieved with the diesel/EL mixture compared

with conventional diesel. The synthesis of alkyl levulinates

usually takes place starting from cellulose undergoing a multistep

process. However, a dual catalytic system for the one-step

synthesis of methyl levulinate from cellulose has been recently

reported.227 In this work, the combination of two homogeneous

catalysts, metal triflates (Lewis catalyst) and sulfonic acids

(Br€onsted acids) yielded up to 75% methyl levulinate.

As a different option, GVL can also be converted into a huge

variety of chemicals and fuels, though its use as an oxygenated

fuel or as a fuel additive itself has been scarcely reported.38,228

Nonetheless, blending GVL with gasoline leads to a composition

displaying similar properties to those achieved from gasoline–

ethanol mixtures, though the lower vapour pressure of GVL also

provides improved performance. GVL can be prepared by

hydrogenation of LA, either by using H2 or by means of

hydrogen transfer from formic acid, in the presence of noble

metal-based catalysts,50,229–231 especially ruthenium-based cata-

lysts. Likewise, GVL can be hydrogenated to valeric acid fol-

lowed by esterificationwith alcohols to yield alkyl valerate esters –

valeric biofuels232 (Fig. 17). Gasoline blended with 10 and 20%

of ethylvalerate (EV) largely complies with the European gasoline

specification and even EV blends show an enhancement of some

gasoline properties – increasing the octane number and lowering

aromatics, olefins and sulfur contents. Likewise, EV also offers

the advantages of a higher energy density and lower blending

volatility than ethanol. Heavier esters, such as butyl and pentyl-

valerates, showed polarity, volatility, and ignition properties that

are suitable to be mixed with diesel, which could be another

possibility for their use as biofuels.

Sugars to hydrocarbon fuels: APR and related processes. A

very interesting alternative to the production of oxygenated

biofuels from sugar-derived platform molecules is the direct

transformation of these compounds into a mixture of hydro-

carbons. This option involves numerous advantages, like the

highest stability of the final fuel products, their higher intrinsic

energy density and the perfect compatibility with conventional

fuels, such as gasoline or diesel fuels. However, transforming

highly functionalized compounds, such as HMF, LA or furfural,

into hydrocarbons, involves severe treatments directed towards

the removal of the oxygen atoms from the starting molecules.

This objective involves the use of dehydration reaction pathways

to remove hydroxyl functionalities, which finally leads to C]C

bonding, that have to be reduced by hydrogenation to enhance

the chemical stability of the final products. As an alternative

option, hydrogenolysis and hydrogenation pathways can also be

employed to remove oxygen, A second important difficulty,

apart from the severe treatments when directly transforming


sugar-derived platform molecules into automotive fuels, lies in

the small size of these sugar-derived compounds. Thus, platform

molecules like furfural, HMF or LA display five and six atom

carbon-skeletons, whereas conventional automotive fuels are

composed of chemicals containing up to 10 carbon atoms, in the

case of gasoline, and up to 20 carbon atoms for diesel fuels. In

this way, not only the removal of oxygen from the starting

molecules is needed, but also the oligomerization of the raw

materials is mandatory to achieve fuel products showing carbon

chain lengths in the range of those showed by conventional

hydrocarbon fuels.

The process developed by Dumesic and co-workers233–235 for

the conversion of sugar-derived compounds is, up to now, the

most successful in performing the required transformation for

converting sugar-derivatives into hydrocarbons. This process has

attracted much attention during the last years, as it is inferred

from the huge amount of references reported on the matter. This

process is based on a reaction pathway in which the carbohy-

drate-derived compounds undergo condensation reactions to

form C–C bonds whereas oxygen is removed by dehydration,

hydrogenation and hydrogenolysis reactions, which are con-

ducted in aqueous phase. An important drawback of this tech-

nology is the need for large quantities of hydrogen, which

involves high operation costs which have to be compensated

through higher prices for the obtained fuel products, making less

favorable the competition of these renewable feedstock-derived

fuels in comparison to those obtained from petroleum.

One alternative for the production of hydrogen to be used in

such a hydrogen demanding process as the biomass to hydro-

carbon fuels, is the use of the same biomass from which derived

platform molecules are produced, as a substrate for the

production of hydrogen. This option can be accomplished by

means of a process named Aqueous Phase Reforming

(APR).234–239 This procedure allows obtaining large quantities of

hydrogen from several reactions conducted in the liquid phase,

taking advantage of the favoured water gas shift reaction under

these conditions. This procedure allows the production of a H2

stream in a single step, with very low formation of carbon

monoxide, in contrast to conventional processes used for the

production of hydrogen from non-renewable sources of hydro-

carbons. The starting point of aqueous phase reforming is the

availability of oxygenated hydrocarbons, which can be obtained

from a huge variety of biomass feedstock. Biomass resources

such as ethanol,240,241 ethylene glycol,242 polyol compounds (such

as glycerol,243–247 sugars248 or sorbitol249), cellulose (either as

a pure compound250 or coming from waste paper or wood251),

woody bio-oil252 or even waste-water,253 have been reported to be

transformed through APR under moderate temperature

conditions.

The reactions taking place in hydrogen production through

APR involve the cleavage of C–C, C–H and O–H bonds. Carbon

monoxide is simultaneously produced together with hydrogen,

though CO is readily removed from the surface of the catalysts

by its transformation into CO2 through the water gas shift

reaction.254 This reaction allows the increase of the formation of

hydrogen while reducing the amount of CO present in the

product stream, producing a ‘cleaner’ hydrogen stream.

Depending on the starting oxygenated hydrocarbon, the reaction

mechanism is more or less complicated, but in any case, these can



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be simplified as C–C and C–H bonds cleavage reactions

accompanied by dehydration, hydrogenation and dehydrogena-

tion reactions, as is shown in Fig. 18.

Bearing in mind the complex and multiple reaction pathways

taking place during aqueous-phase reforming of oxygenated

hydrocarbons to hydrogen, discerning requirements are needed

for the catalyst to be used in APR transformation. First, as

already mentioned, the catalyst must promote C–C, C–H and

O–H bonds cleavage. These reactions are needed because they

lead to the formation of hydrogen and carbon monoxide.

Second, the water gas shift must be promoted because the

evolved carbon monoxide from the previous step depresses the

activity of the catalysts, and thus hydrogen production via bond

cleavage is reduced. Moreover, the water gas shift increases the

yield towards hydrogen.

Simultaneously to these determining factors, the catalyst

should present very low activity in C–O cleavage and CO/CO2

methanation reactions, because both of them lead to hydrogen

consumption. Bearing in mind all these considerations, several

metal species such as Pd, Pt, Ru, Rh or Ir,254 or their mixtures,

could promote C–C, C–H and O–H bond cleavage reactions.

Most of the reported catalysts used in APR involve one or more

of these expensive metal species, though it has been shown that Pt

and Pd had higher selectivity towards H2 production. However,

their high price is a very important drawback to extend their use

to an industrial scale, though their intrinsic catalytic activity and

Fig. 18 Main types of reactions taking place in the aqueous phase

reforming of oxygenated compounds.


selectivity can be enhanced by using bimetallic catalysts incor-

porating Ni, Co or Fe species,254 or by selecting the proper

catalyst support.255

Alternatively, Sn-promoted Ni Raney� catalysts have

revealed to be an interesting option233,256 to the noble-metal

based catalysts. This inexpensive catalyst displays several

advantages over other APR catalysts such as Pd/C or Pt/Al2O3,

its lower price being the most important one from an economic

point of view. Furthermore, Ni–Sn Raney catalysts display

a very high resistance against deactivation, being reusable for

several days on stream.256 An important feature of this catalyst

over analogous Ni-Raney catalysts lies in the effect of the tin

species, which promotes hydrogen-producing reactions to reac-

tion rates comparable to those achieved with Pt/Al2O3 catalysts,

whilst depressing the intrinsic catalytic activity of Ni in the

methanation of CO and CO2.256 Optimal reaction conditions for

this catalytic system usually involve mild temperatures and low

pressures, so that the required equipment to perform the aqueous

phase reforming of biomass-derived compounds is not greatly

demanding, being possible to conduct these transformations in

conventional hydrotreating refinery systems.

A key feature of the APR process, which makes it even more

interesting, is that, while hydrogen evolves from the reaction

media, intermediate oxygenated hydrocarbons are produced too.

These chemicals include alcohols, acids, ketones and aldehydes,

though the exact composition depends on both the APR condi-

tions and the starting biomass-derived substrate. One possibility

to take advantage of these compounds is their transformation, by

means of different pathways (condensation, hydro-

deoxygenation, dehydration, oligomerization.), into larger

alkanes which can be used as fuels (diesel, gasolines or kerosene).

This is the origin of the process licensed by Virent Energy

Systems, the Bioforming process,238 whose main steps are

depicted in Fig. 19.257–259

Thus, Virent’s BioForming process combines the APR

procedure for hydrogen production with several hydrogenation,

dehydration and base-catalyzed condensation reactions to

prepare saturated hydrocarbons, suitable for the formulation of

liquid fuels. The most impressive fact of this process is the nature

of the raw material, since this can be accomplished starting from

renewable biomass derived carbohydrates such as sugars.

However, since the reaction routes to achieve the alkane fraction

as the main product are different to those required for maxi-

mizing the hydrogen production, a different catalyst is required

for the production of alkanes in the BioForming process. Thus,

not only hydrogenation, C–C and C–O bond cleavage are

required (in contrast to those preferred for hydrogen produc-

tion), but dehydration reactions are also needed, so that the

employed catalysts should present a bifunctional activity. On one

hand, the metal species are needed to promote the already-

described reactions for H2 production, on the other hand, acid

activity is needed to drive the dehydrogenation reactions.

Finally, preliminary economic analysis suggests that, since

approximately 90% of the total energy contained in the starting

carbohydrate and H2 feed is transferred to the final products, this

technology may be competitive at crude oils prices greater than

60 $/bbl.

As in the case of the APR-evolving light oxygenated hydro-

carbons, other compounds, such as furfural or HMF, can also be



Fig. 19 Virent Bio-forming Process for the transformation of glucose-

based biomass into hydrocarbon fuels.

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used for the production of hydrocarbon fuels directly in the

range of gasoline, but also in the range of jet fuel or even diesel

fuel. However, in these cases, oligomerization reactions are

needed to increase the molecular weight of the carbon skeleton

from the starting furfural or HMF to the final hydrocarbonated

fuels. These oligomerization reactions can be accomplished in

many different ways,260 though the cross-condensation of either

furfural (self-aldol condensation), or HMF with other carbonyl

compounds are the most-reported one.261–263 These reactions are

conducted in presence of base catalysts, which promote several

aldol-condensations increasing the size of the resulting

compound carbon chain. On the other hand, the dehydration,

hydrogenation and hydrogenolysis reactions take place through

aqueous-phase dehydration/hydrogenation (APD/H) reactions,

involving the use of bi-functional metal-acid catalysts. A large

variety of different combinations of active species for aldol-

condensations, dehydrations and hydrogenation–hydrogenolysis

reactions, has been proposed. Thus, from the homogeneous

water-soluble NaOH to solid base catalysts, such as Mg–Al

oxides, have been described to be effective catalysts in the

promotion of C–C bond formation. In a similar way to APR, Pd,

Pt, Ni, Ru, Rh or Ir,254 and their mixtures and combinations,

have been described as active species in hydrogenation–hydro-

genolysis reactions, whereas, different solids, including solid

acids like alumina, silica-alumina, several metal oxides or

carbon, have been proposed as supports.264,265 Among the

reported results in hydrocarbon production from furfural/HMF,

the most promising results have been achieved when using

bifunctional catalysts. These catalytic systems promote the


production of liquid hydrocarbon fuels with targeted chain

lengths (C9–C15 for HMF and C8–C13 for furfural). Some

examples of these bi-functional catalysts are the Pt/NbPO4

catalyst reported by Serrano-Ruiz,266 which leads to an overall

carbon yield of 60%, or the more complex, less expensive Pd/

MgO–ZrO2 catalyst,267 which provides an overall carbon yield of

about 80%.

The bulk chemicals produced during condensation reactions

lead to the separation of an organic layer from the aqueous

reaction media because of the low polarity of the forming

compounds. This fact involves a great advantage, since the

separation of the products from the starting raw materials and

reaction media becomes energetically non-demanding. Further-

more, the use of organic solvents and biphasic systems allows the

enhancement of this spontaneous separation, making the sepa-

ration and product recovering from the reaction media even

easier, as well as the reaction equilibrium is shifted towards the

formation of higher quantities of products. Nevertheless, bearing

in mind that two different liquid layers are produced, and that

most of the catalysts used for catalyzing the condensation reac-

tions are heterogeneous in nature, at least three different phases

coexist inside the reactor. Furthermore, if oxygen removal by

hydrogenolysis–hydrogenation reactions is considered too, the

complexity of the reaction system is even higher, since a fourth

phase has to be added to the three already-considered ones,

because hydrogen plays the central role in APD/H. This high

complexity must be considered in the design of both the catalytic

systems and the reaction equipment in order to control the mass

transfer between the different phases taking place during the

transformation of furans into hydrocarbons.267

Levulinic acid, like HMF, is the other starting raw-material

useful as an intermediate in the transformation of sugars into

hydrocarbons, since the de-oxygenation of LA can lead to

energy-dense chemicals easily upgradable to hydrocarbon fuels.

One of these options passes through the obtention of GVL by

hydrogenolysis of LA,215 followed by lactone ring opening by

acid isomerisation, leading to different isomers of pentenoic acid.

Two different options have been described for taking advantage

of these pentenoic acid isomers: the decarboxylation route268 and

the hydrogenation/ketone condensation to form larger hydro-

carbon structures (see Fig. 20). The decarboxylation route,

applied to pentenoic acids, leads to the formation of a mixture of

butene isomers. In this transformation, acid catalysts, such as

silica-alumina, can be used to promote the decarboxylation

reaction, even starting from GVL,268 a catalytic activity also

found when treating other lactones. Nevertheless, the resultant

olefins can then be processed in an alkylation unit, for instance in

a butamer process, using the same procedure used to transform

butenes into gasolines in a conventional refinery fed with crude

oil. Alternatively, the use of a strong solid acid catalyst, such as

HZSM-5 or a sulfonic acid-functionalized resin, as suggested by

Dumesic and co-workers,269 can promote the oligomerization of

the resultant butenes into larger i-alkanes, allowing the produc-

tion of both gasoline and jet fuel, though at a substantial lower

production rate than the commercial process. Anyhow, the real

advantage of this option is the low H2 consumption, which

makes the decarboxylation pathway economically desirable. A

proof of this is that the techno-economic analysis of this option

(GVL decarboxylation followed by oligomerization of butenes)



Fig. 20 Conversion of GVL into hydrocarbon fuels.

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reveals it as a cost-effective route to hydrocarbon fuels from

lignocellulosic biomass.270 The second option to take advantage

of GVL as key molecule in the transformation of LA into

hydrocarbon fuels is the production of pentanoic acid (PA) (by

hydrogenation of pentenoic acids coming from GVL isomeriza-

tion). Later, PA is submitted to base-catalyzed ketone oligo-

merization. The hydrogenation of pentenoic to pentanoic acids

can be accomplished in presence of a water stable Pd/Nb2O5

catalyst, whereas ketonization can be either accomplished with

Ce0.5Zr0.5O2271,272 or with the same Pd/Nb2O5

273 catalyst used for

hydrogenation, but with longer contact times, compared to the

basic catalyst. The resultant 5-nonanone spontaneously sepa-

rates from water, and it is subsequently hydrogenated into the

corresponding alcohol. The C9 alcohol can be transformed into

n-nonane or nonene by hydrogenation/dehydration. The last

part of the process is the isomerisation of the resultant C9

hydrocarbons to achieve the required properties for the desired

fuel.

4. Conclusions and final remarks

There are potential analogies between the 20th century petroleum

refinery and the 21st century biorefinery. In the beginning, the

petroleum refinery made few products and incorporated little

chemical and energy integration. Development of the petroleum

refinery took considerable effort to become the highly efficient

and integrated system that it is nowadays, and most of the

breakthroughs that allowed this remarkable transformation

involved enhanced catalytic technologies. Current biorefineries,

which are still in their tender infancy, produce relatively few

chemicals and fuels and most of the processes involve little

chemical and energy integration. In analogy to the history of

petrol industry, with the development and updating of different

chemical and biochemical conversion technologies, the bio-

refinery can also become an efficient and highly integrated system

to meet the chemical and energy requirements of the 21st century.

In order to materialize this system, serious efforts must be

addressed to the development of new infrastructures dealing with

the setting-up and optimisation of new logistic chain systems able

to provide huge amounts of biomass-derived feedstock to cen-

tralised transformation centres like conventional refineries. This

is a crucial step in the insertion of biomass raw-materials into the

production schemes of current refineries. Furthermore, efficient

biochemical processes, integrating different conversion technol-

ogies and industrial scaling-up has to be considered in the feeding

of biomass-derived feedstock to the current units in refineries.


Nevertheless, a significant percentage of the chemical conversion

technologies available in a petroleum refinery can also be used in

biomass transformation into valuable fuels and chemicals. In this

review different economically attractive options have been

identified for the integration of biorenewable feedstock and

biofuels in petroleum refineries.

Low quality vegetable oils and greases are likely to be promi-

sing in a short-to-medium term to yield green diesel and jet-fuel

by means of hydroprocessing of triglyceride-based feedstock. In

fact, there are several countries where these fuels are already

commercialised. Moreover, these are synthesized using

commercially available refining technology, though their

production is limited to a small fraction of liquid transport fuels,

mainly due to the limited availability of the feedstock. However,

some aspects of these hydrotreatments still need more attention.

From an economical and environmental point of view, mini-

mizing unnecessary hydrogen consumption is crucial and further

insights must be addressed. In this sense, the development of

active and stable catalysts for hydrogen-free catalytic deoxy-

genation is presented as a good alternative to the expensive

hydrodeoxygenation–hydrodecarboxylation conversion routes.

Likewise, to ensure optimal co-processing of triglycerides with

petroleum fractions in existing refinery units, it is critical to fully

understand the effect of triglycerides (feed and conversion

products, especially water) on the processing of the petroleum

fraction. In this sense, the use of a separate modular unit where

processing conditions are optimized for the triglyceride based

feedstock is an attractive approach.

Pyrolysis oil processing requires larger efforts in commercial

development, since the commercial production of this substance

is still in its infancy, apart from the numerous drawbacks found

in its upgrading to biofuels. Due to the poor quality of the bio-

oils, the conventional hydrotreating catalysts are expected to

have a considerably lower catalyst life in bio-oil upgrading

operations than that observed with petroleum feedstock. While

the current generation commercial catalysts are excellent

hydroprocessing catalysts, they are optimized for petroleum

feedstock. Since the bio-oils have significantly different proper-

ties than petroleum feedstock, it would be worthwhile to dedicate

efforts to developing catalysts specifically designed for upgrading

bio-oils. From a widespread commercial application viewpoint,

an ideal catalyst for bio-oil upgrading should have the attributes

as follows: high activity for deoxygenation; ability to withstand

large quantities of coke and/or minimize coke formation; high

tolerance to water and poisons; and high availability with

a competitive cost. We honestly think that, in the long term, the

huge potential volume of pyrolysis oil coming from large amount

of available lignocellulosic wastes might replace shortages in

petroleum fuel and thus, developing this technology would be

one of the milestones, in the medium term, in substituting crude

oil as energy source.

Ethanol, the main biomass-derived fuel used today, is widely

used in refineries for the formulation of gasoline. However,

ethanol suffers from important limitations as a fuel (e.g., low

energy density, high solubility in water, etc.) that can be over-

come by designing strategies to convert non-edible lignocellulosic

biomass into liquid hydrocarbon fuels chemically similar to those

currently used in internal combustion engines. The present

review has described the main routes available to carry out such



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deep chemical transformation with particular emphasis on those

pathways involving aqueous-phase catalytic reactions. These

routes offer the opportunity to selectively carry out a variety of

reactions to achieve the deep chemical transformations and C–C

coupling reactions required when converting sugars into liquid

hydrocarbons. Nonetheless, aqueous-phase routes require

lignocellulosic biomass to be subjected to pretreatment/hydro-

lysis steps to yield water-soluble sugars (and platform chemicals

derived from them). Unfortunately, the economic trans-

formation of lignocellulosic biomass into soluble sugar and

platform molecules represents a major challenge as a conse-

quence of the complex nature of the biomass and the presence of

non-cellulose components. As described in this review, aqueous-

phase routes for the transformation of sugars and derivatives

into hydrocarbons is carried out with multi-step processes and

requires the use of fossil fuel-based hydrogen sources. In this

sense, the insertion of lignocellulosic derived platform molecules

as raw materials in the supply chain of conventional refineries

seems not to be probable in the near future and further

improvements in the existing technology for their transformation

into fuels have to be firstly addressed. However, it must also be

pointed out that most of these catalytic transformations might be

achieved by using conventional heterogeneous catalysts, though

their improvement and the development of more selective cata-

lysts remains a key technical barrier which seems to be

unavoidable for the future.

Most of the biomass conversion processes carried out in

a refinery need a high amount of hydrogen in order to remove

oxygen and yield high energy density fuels. It is likely that, in the

future, hydrogenwouldhave tobeproducedbymeansof renewable

energy sources such as the sun, wind, or biomass. Hopefully, the

transformation of carbohydrates towards hydrogen using APR

processes might be a good alternative to current H2 sources,

supplying renewable hydrogen in future refineries.

Although biomass valorization can be performed on current

commercially available petroleum-based technology, it should be

considered that petroleum and biomass feedstocks are very

different from a chemical point of view. It seems that heteroge-

neous catalysis, which has made it possible to convert efficiently

petroleum-derived resources to fuels, will also be able to provide

the necessary technology to get similar fuels starting from

biomass feedstock. However, it is most likely that applying the

‘‘old’’ catalyst technology, developed for petroleum refining, to

renewable biomass-derived substrates will not be enough.

Intensified efforts would have to be applied in the development

of new heterogeneous catalytic materials, based on a specific

design considering the nature of the renewable feedstock, a fact

which also constitutes new catalytic opportunities.

Acknowledgements

Financial support from the Spanish Ministry of Economy and

Competitiveness and from the Regional Government of Madrid

through the projects CTQ2011-28216-C02–01 and S2009-

ENE1743, respectively, is kindly acknowledged.

Notes and references

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biomass as renewable feedstock in standard refinery units. feasibility, opportunities and challenges

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