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Effects of headspace fraction and aqueous alkalinityon subcritical hydrothermal gasification of cellulose

Ryan Dolan, Sudong Yin, Zhongchao Tan*

Department of Mechanical & Manufacturing Engineering, Centre for Environmental Engineering Research & Education,

Schulich School of Engineering, University of Calgary, 2500 University Dr. N.W. Calgary, AB, Canada T2N 1N4

a r t i c l e i n f o

Article history:

Received 14 January 2010

Accepted 2 April 2010

Available online 13 May 2010

Keywords:

Hydrothermal conversion

Hydrothermal gasification

Cellulose

Headspace fraction

Biomass

* Corresponding author at: Department of MeEducation, Schulich School of Engineering, Uþ1 403 220 2698.

E-mail address: tanz@ucalgary.ca (Z. Tan0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.04.029

a b s t r a c t

In order to better understand the pathways of hydrothermal gasification of cellulose, the

effect of headspace fraction and alkalinity on the hydrothermal gasification of cellulose has

been studied at 315 �C in the presence of Pt/Al2O3 as catalyst. It was found that regardless

of alkalinity the headspace fraction had a large impact on gasification yield, with larger

headspace fractions resulting in considerably more gas product. Without the addition of

sodium carbonate, the effect of headspace fraction became more pronounced, with gas

increasing by approximately a factor of forty from the lowest to highest headspace frac-

tion. On the other hand, for the same residence time the addition of sodium carbonate co-

catalyst dampened the magnitude of the effect, to a factor of 2.5 and 1.5, for 50 and 100 mM

sodium carbonate solutions, respectively. These results indicated that the headspace

fraction affected the phase behaviour, and that this altered the pathway of the cellulose

decomposition. While furfural alcohol was the major product obtained with a 49% head-

space fraction, it was effectively suppressed by using 78% or greater headspace fractions.

Based on the effects of phase behaviour and previous literature, the reduced effect

occurring upon the addition of sodium carbonate may relate to catalysis of the Lobry de-

bruyn Van Eckenstein transform to produce lactic acid rather than intermediates

proceeding through glycolaldehyde.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction Furthermore, the unique properties of hot compressed water

Cellulose is the most globally abundant biopolymer,

accounting for 1.5� 1012 tonnes of annually available biomass

[1]. As such, there has been considerable interest in recent

years in the development of new technologies for the utili-

zation of cellulose as a feedstock for the production of fine

chemicals [2e5] and hydrogen [6e15]. Amongst these tech-

nologies, hydrothermal conversion is of growing interest since

pre-drying of feedstocks is avoided and the water gas shift can

be performed in situ using platinum group metal catalysts.

chanical & Manufacturinniversity of Calgary, MEB

).ssor T. Nejat Veziroglu. P

can be tuned to control decomposition pathways.

Upon hydrolysis to glucose, the decomposition of cellulose

in hot compressed water is known to follow the same path of

glucose decomposition, forming a large number of organic

acids, aldehydes, ketones, furfurals, and phenolic structures

as intermediates [16e18]. Kabyemela et al. have investigated

the kinetics of the decomposition of glucose [18], the mono-

mer of cellulose, in subcritical water at short residence times

and proposed basic non-catalytic pathways for its initial

decomposition as presented in Fig. 1. These pathways have

g Engineering, Centre for Environmental Engineering Research &305, 2500 University Dr. N.W. Calgary, AB, Canada T2N 1N4. Tel.:

ublished by Elsevier Ltd. All rights reserved.

Fig. 1 e Basic chemical pathways known from previous literature on the decomposition of glucose at 350 �C in hot water

based on original kinetics by Kabyemela [18], and extended based on Ref. [25,27,36]. Ox: Oxidation, eCO: Decarbonylation,

eH2O: Dehydration, DH2: Hydrogenation.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 6 6 0 0e6 6 1 0 6601

formed the backbone of current understanding on the

decomposition of cellulosic biomass in hot compressed water

thus far. While it is known that gas products originate from

short chain aldehydes and acids [16] which acid/aldehydes the

gas is primarily produced from, and through which interme-

diates are of great interest. More work is required on under-

standing which pathways contribute to the gas formation.

Since each chemical intermediate has a different rate of

gasification, understanding which pathways generate more

easily gasified intermediates is an important research goal.

These pathways are quite sensitive to solution pH. For

example, it is well known that under acidic conditions,

furfural alcohol (HMF) becomes a major intermediate, while

under basic conditions HMF formation is suppressed and

organic acids such as lactic and acetic becomemore dominant

[19]. In addition, solution pH is known to not only affect the

decomposition of initial intermediates, but also affects the

decomposition pathways of these intermediates. For example,

under acidic conditions, decarbonylation of lactic acid to

acetaldehyde is preferred whereas its dehydration to acrylic

acid is favored under basic conditions [20].

For hydrothermal gasification, phase behaviour also plays

an important role in mediating the balance between ionic and

free radical reactions in hot compressed water. High-density

phases restrict the diffusion of free radicals through the

solution, while low density conditions enhance the diffusion.

Previous research comparing subcritical and supercritical

gasification has shown that supercritical, free radical condi-

tions enhance gasification [17]. Less work has been devoted to

understanding the effects of phase behaviour in subcritical

water on cellulose gasification. Recently, Azadi et al. studied

the effect of the altering the headspace fraction (percentage of

the reactor volume occupied by the headspace) of the reactor

between 63% and 88% and found that it can have a drastic

impact on gas yield at 350 �C, with higher headspace fractions

(higher fractions of vapor) generating almost five fold more

gas in the presence of Raney nickel [21]. The authors attrib-

uted the effect primarily to a decreased partial pressure of

hydrogen in the solution due to the enlarged headspace, and

reduced hydrogenation reactions occurring as a result.

Besides hydrogenation, we expect that phase behaviour

will also affect the decomposition pathways based onwhether

the intermediates degrade in a high-density liquid phase, or in

a low density vapor. Effects of phase behaviour have been

found in the hydrothermal gasification of cellulose in previous

research [21e23]. It has been found that low density phases

(free radical conditions) promote the formation of gas, while

ionic conditions inhibit gas production. The effect of phase

behaviour also affects the decomposition of chemical inter-

mediates in hydrothermal gasification. For example, while

glycolaldehyde can be produced nearly selectively at very low

densities (supercritical conditions), furfural alcohol is a major

product under typical subcritical conditions [23]. In this paper

we seek to understand the effect of subcritical phase behav-

iour in terms of how it controls the gasification pathways of

cellulose.

In the present study, the effect of headspace fraction is

studied to determine if the altered phase behaviour of the

solution affects decomposition pathways, and how these may

lead to gasification. The effects of sodium carbonate concen-

tration and headspace fraction at 315 �C are studied. Sodium

carbonate concentrations were studied between 0 and 1 M

concentration in the presence of 1% by weight, 5% Pt/Al2O3 as

metal catalyst. Headspace fractions between 49 and 93% of the

reactor volume were studied at sodium carbonate concen-

trations of 0, 50, 100 and 500 mM. Finally, the relationship

between the headspace fraction and changes in the liquid

phase composition are discussed in terms of the balance

between free radical and ionic reaction pathways that

mediate the decomposition of cellulose.

2. Experimental

Experiments were performed in a batch 69 mL reactor con-

structed from stainless steel 316 tubing (w17% Cr, w13 Ni,

Fig. 2 e Effect of Sodium Carbonate Concentration on gas

yield in the hydrothermal gasification of 1 g cellulose with

0.01 g 5% Pt/Al2O3. For above experiments, a 15 min

residence time was used and a headspace fraction of 63.8%

was used.

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w3% Mb, w2% Mg, <0.03% S) which was heated in a muffle

furnace preheated to the set point temperature. The reactor

was filledwith 5, 10, 15, 25 or 35mL sodium carbonate solution

containing 4% by weight microgranular cellulose (Sigma) and

0.04% by weight 5% Pt/Al2O3 (Sigma, powder). The 5, 10, 15, 25

or 35mL slurries permitted study of headspace fractions of 93,

86, 78, 64 and 49% respectively.

Prior to heating, the reactor headspace was filled and

evacuated with argon several times to purge the headspace of

molecular oxygen. Finally, the headspace was filled with

15 psi of argon to allow for sufficient pressures for gas

sampling in cases where little gas was produced. The heating

profile of the tube reactor was determined using a type K

thermocouple to follow a first order differential equation of

the form:

dTdt

¼ hðToven � TreactorÞ

where the coefficient h ¼ 0.047 min�1. The preheating time

was defined as the time it took for the reactor to reach within

2 �C of the set point temperature (315 �C for all experiments),

which was approximately 60 min. After the preheating time,

the reactor remained in the oven for the required residence

time, and finally the reactor was removed from the oven and

submerged into a tap water bath to quench further reactions.

The reactor was fully cooled to room temperature within

14 min, at which point the headspace was analyzed by micro-

GC (Varian 4900), equipped with dual carrier gases (argon and

helium) andmolsieve and ppU columns. Next, the gas product

was analyzed by connecting the valve of the reactor to

a micro-GC sampling system. The pressure inside the reactor

was recorded, and knowing the additional pressure generated

in the headspace as well as the gas composition, the quantity

of each gas produced was determined. For determination of

the amount of gas produced, the ideal gas law was used,

where the pressure (measured), headspace volume, and

temperature were known. Gases quantified in these experi-

ments were carbon monoxide, hydrogen, carbon dioxide,

methane, oxygen, nitrogen, and ethane. Argon was one of the

carrier gases used for the micro-GC, and as such was not

detected by the micro-GC.

Finally, the remaining liquid solution was filtered using #1

Whatman filter paper, the pH recorded (Extech pH meter

EC500), and 1 mL of the remaining liquid solution was injected

by syringe into a GC-FID (Varian 330 suitable for direct injec-

tion of the aqueous phase and detection of organic acids. The

injection mode was split with a split ratio of 20. The flow

rate of helium and hydrogen and air was 25, 30 and 300

mL/min respectively. The column dimensions were

L � ID � OD ¼ 25 m � 0.25 mm � 0.39 mm. The column was

a CP7717 column with a stationary phase of CP-Wax58 (FFAP)

CB and film thickness 0.2 mm. Calibration was performed with

certified standards in aqueous solution (Sigma). For experi-

ments where sodium carbonate was used as co-catalyst, the

final pH of the remaining solution was first adjusted below 4

using hydrochloric acid to ensure that acids which had reac-

ted with sodium carbonate in the solution would be converted

from sodium salts to the corresponding free acid for analysis.

In these experiments, the effect of headspace fraction was

measured at 0, 50, 100 and 500 mmol concentrations of

sodium carbonate, all in the presence of 1% by weight of

cellulose, and 5% Pt/Al2O3 a residence time of 15min. To avoid

altering the concentrations of either cellulose or the Pt/Al2O3

catalyst, the weight of these chemicals was scaled accord-

ingly, and the amount of gas produced per gram of cellulose

has been reported. The actual volume fraction of liquid versus

vapor was not determined, since in our experiments pressure

was recorded after experiments, (not during), and we were

therefore unsure of the additional vapor pressure that would

be generated from cellulose decomposition products and the

produced gas. Thus, what is reported herein is the headspace

fraction before conversion, the vapor fraction generated

during the experiment.

Experiments were performed in replicate to ensure

repeatability. Discrepancy was no more than 20, 16 and 10

percent for kinetic (Fig. 3), headspace fraction (Fig. 5), and

alkalinity experimental data (Fig. 2), respectively. Thus, the

presented results seemed repeatable.

3. Results

3.1. Effects of alkalinity

As can be seen in Fig. 2 below, 3 distinct regionswere observed

when studying the effects of sodium carbonate concentration

on the gas yield in the presence of Pt/Al2O3. Hydrogen yield

increased from about 0.3 mmol to 1.5 mmol with the use of

0.075 MNa2CO3 solution. Carbon dioxide also increased in this

region, from about 1.5 to 3.3 mmol. The rates of increase in

hydrogen and carbon dioxide in the first region of Fig. 2 were

determined to be 16 and 22.6 mmol per M sodium carbonate

respectively and the ratio of increase of carbon dioxide to

hydrogen is therefore slightly greater than 1.4. The additional

gas that is generated in region 1 therefore appears to be

produced in a ratio of approximately 3:2, carbon dioxide to

hydrogen. In the second region of alkalinity, between 75 and

150mMNa2CO3, hydrogen and carbon dioxide both decreased.

Fig. 3 e Effect of Residence time on the gas generated using

a 62.8% headspace fraction with 0.01 g Pt/Al2O3 and four

different levels of sodium carbonate. (A): 0e100 mM

Na2CO3. (B): 100e500 mM Na2CO3).

Fig. 4 e Relationship between methane and acetic acid in

the liquid phase at residence times between 0 and 60 min

for 0 and 50 (top) and 100 and 500 (bottom) mM sodium

carbonate solutions.

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The rate of decrease of hydrogen was small however in

comparison to the rate of decrease of carbon dioxide being

4 mmol and 20 mmol per M Na2CO3, respectively. Thus the

ratio of the rate of carbon dioxide decrease to hydrogen

decrease was about 5. Finally, in the third region hydrogen

remained relatively constant while carbon dioxide greatly

decreased, and was nearly absent in the headspace when 1 M

sodium carbonate concentration was used. Also in Fig. 2, the

measured final pH of the resulting liquid solution is shown in

relation to the hydrogen and carbon dioxide yield. It would

appear that the peak in hydrogen and carbon dioxide yields

occurred at approximately the same final pH where the solu-

tion changed from basic to acidic conditions. The highest gas

yield (when studying the effects of alkalinity) occurred at the

conditions where the cellulose was degraded to intermediates

under basic conditions, but then experienced either dilute

basic or neutral conditions after initial decomposition.

Based on the effects of alkalinity at a residence time of

15 min in Fig. 2, the effects of residence time on gas yield with

4 different concentrations of sodium carbonate were studied

at 0, 50, 100 and 500 mmol Na2CO3. Without the addition of

sodium carbonate, the hydrogen yield reached a maximum of

about 0.7 mmol and about 3.2 mmol carbon dioxide. With the

use of 50 mmol and 100 mmol Na2CO3 solutions, hydrogen

yield was considerably enhanced in comparison to the case

where no sodium carbonate was used with maximum yields

of 2.3 and 3mmol per gram cellulose respectively. However, in

50 mmol sodium carbonate the ratio of carbon dioxide

appeared to increase at a ratio of 2:1 whereas in 100 mmol

sodium carbonate 1 mmol of hydrogen was produced per

mmol carbon dioxide. The rate of increase in a 100 mmol

solution also appeared to be nearly linear in both hydrogen

and carbon dioxide between 0 and 45 min. In the 500 mM

solution, a large increase in hydrogen was observed at the

longest residence time.

The only other gas produced in relevant quantities was

methane, which wasmost effectively produced in the 100mM

sodium carbonate solution. For 0, 50 and 100 mM sodium

carbonate solution, the concentrations of acetic acid found in

the liquid product were compared to the produced methane

and can be found in Fig. 4. Yields of both methane and acetic

acid were most significant in 50 and 100 mM solutions.

Particularly in the 50 mM solution, a clear relationship

Fig. 5 e Effect of the headspace fraction of the reactor when

using 0.04 g Pt/Al2O3 as catalyst with a 15 min residence

time. (A): 0e100 mM Na2CO3. (B): 100e500 mM Na2CO3).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 6 6 0 0e6 6 1 06604

between acetic acid and methane was present. In 100 mM

Na2CO3 solution a similar relationship was observed.

However, at 60 min acetic acid once again increased, along

with a corresponding decrease in methane. After the acetic

acid was no longer present in the solution, methane yields

remained constant. Acetic acid concentrations increased with

50 mM solution, and to an even greater degree when using

100 mM, where considerably more methane was produced.

For the use of both 0 and 500mMsolutions,methane yieldwas

less than 0.1 mmol maximum at 60 min. The effects of resi-

dence time on acetic acid concentration in 500 mM sodium

carbonate solution could not be determined, but methane

yields were low.

3.2. Effect of headspace fraction

As can be seen by Fig. 5, without the addition of sodium

carbonate, increasing the headspace fraction from 49.3% to

92.8% increased the gas yield from 0.25 to 6.3 mmol per gram

cellulose. The effect was most distinct at very high headspace

fractions. In all three of the 0, 50 and 100 mmol Na2CO3 solu-

tions, carbon dioxide increased as the fraction of water vapor

increased, but decreased with a 92.8% headspace fraction.

Hydrogen however, consistently increased for all four levels of

alkalinity as the headspace fraction increased, and most

sharplywhen the 92.8%headspace fractionwas tested. In 0, 50

and 100 mM solutions, this phase behaviour had seemingly

similar qualitative effects. However, the magnitude of the

effect appeared to be dampened with increasing concentra-

tions of sodium carbonate. For comparison, the ratio of

increase in hydrogen gas from 49.3% to 92.8% headspace

fraction for the 0, 50 and 100 mmol sodium carbonate solu-

tions was 40, 2.5 and 1.5 respectively. At 500 mM Na2CO3,

a qualitatively different effect was observed for carbon

dioxide, with carbon dioxide increasing even at very high

headspace fractions. For the 85.5% headspace fraction at 0 M

Na2CO3, an additional test was donewhere the headspacewas

evacuated with a vacuum pump prior to the experiment. In

this case, a very similar amount of hydrogen, 2.4 rather than

2.3 mmol was produced, while carbon dioxide increased

slightly from 4.6 to 5.4 mmol/g cellulose.

Fig. 6 shows GC-FID results for 49.3% and 78.3% headspace

fractions with 0 and 0.5 M sodium carbonate concentrations

as well as the liquid phase when using a 92.8% headspace

fraction and no sodium carbonate. Since the concentration of

cellulose in the solutions was the same in all cases, the

chromatogram results can be interpreted directly without

scaling. In general, chemicals detected in the solution at

higher headspace fractions were of decreased concentration.

This is not unreasonable given that a larger fraction of

chemicalswas gasified under these conditions. A considerably

different distribution of products was observed in the liquid

phase. In comparison to the 49.3% headspace fraction test,

very little furfural alcohol was found in the 78.3% headspace

fraction test, (or in the 92.8% headspace fraction test). The

proportion of levulinic acid to furfural alcohol became larger

at a 78% headspace fraction and the intermediate at a reten-

tion time of 4.1 min become relatively larger in terms of

relative abundance. This peak could not be identified. In

a separate experiment however, a solution of dihydroxyace-

tone (0.5 g dihydroxyacetone, 25mLwater) was degradedwith

a 63.8% headspace fraction for 15minwithout Na2CO3. GC-FID

analysis of the liquid phase revealed that this unknown

intermediate (having a residence time of 4.1 min) was a major

decomposition product of dihydroxyacetone. Thuswe assume

it is an intermediate generated through the same pathway. At

the lowest slurry loading (92.8% headspace fraction), even the

peak at 4.1 min retention time appeared to be reduced, and

butyric and acetic acid became the largest identified peaks in

the solution. Butyric acid was detected in most of the slurries

(even at a 49.3% headspace fraction) although its relative

abundance was very small in comparison to furfural alcohol

in that case.

4. Discussion

4.1. Alkalinity

The effect of alkali salts on hydrothermal conversion has been

investigated previously in Ref. [24e26]. The two main effects

that have been noted have been catalysis of thewater gas shift

[27], and a reduction in the hydrothermal char mass fraction

[7,26]. Regarding the water gas shift reaction (shown below),

carbon monoxide was not observed regardless of the alkali

Fig. 6 e GC-FID chromatograms resulting from analysis of the aqueous phase after tests for 0 and 0.5 M Na2CO3 and 49.3%,

78.3% and 92.8% headspace fractions. Also shown is the GC-FID chromatogram obtained from the liquid phase using no

sodium carbonate and a 92.8% headspace fraction.

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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 6 6 0 0e6 6 1 06606

salts in our experiments, likely due to water gas shift catalysis

(shown below) by the Pt/Al2O3 catalyst.

COþH2O/CO2 þH2

Regarding hydrothermal char, recent nuclear magnetic

resonance studies on hydrothermal chars generated from

glucose show that they primarily consist of cross linked

furfural alcohol. The mechanism appears complex however,

and levulinic acid, formic acid and dihydroxyacetonemay also

enhance the char formation [28,29]. Thus, the predictions by

other researchers relating the reduction in char with alkali

salts to the reduction in furfural alcohol productions seem to

be supported and it is probable that this can explain the

increase in gas yield associated with experiments in region 1

of Fig. 2. A considerable reduction in hydrothermal char was

observed as sodium carbonate increased from 0 to 75 mmol

concentration. These residues were not quantified since

sodiumbicarbonatewas expected to be present in the residual

solids and this would prevent any attempt at obtaining an

accurate mass balance [30]. However, visual confirmation has

been included in Fig. 7 for discussion purposes. Even with the

addition of small amounts of sodium carbonate, the hydrox-

ymethylfurfural in the liquid phase decreased. For example,

upon the addition of sodium carbonate, HMF decreased from

21mmol/g cellulose to 0.3mmol in 0 M and 50mmol solutions

respectively at a 15 min residence time. To determine if this

was due to an increase in HMF decomposition or simply

suppression of HMF production, 0.2 g of HMFwas decomposed

with and without the presence of 100 mmol Na2CO3 in 20 mL

slurries (corresponding to a 71% headspace fraction). In

neither case was a measureable amount of gas produced.

Thus, in our experimental conditions it seems that pathways

acting through the decomposition of furfural alcohol were

relatively insignificant contributors to gas production. Thus,

Fig. 7 e Effect of sodium carbonate on the residual solids

formed. Without the addition of sodium carbonate (left)

considerable solid residues, which were black in color,

were formed. With 100 mmol sodium carbonate, the

residues were suppressed considerably.

gas produced under our conditions appears to have mainly

been generated through either the erythrose/glycolaldehyde

pathway, or through the glyceraldehyde/dihydroxyacetone

pathway as seen in Fig. 1.

The linear increase in both hydrogen and carbon dioxide in

region 1 of Fig. 2 shows the increase in hydrogen was also

related to the formation of carbon dioxide. The ratio of the

slopes for increasing hydrogen and carbon dioxide in Fig. 2

suggests that 3 mmol of carbon dioxide was generated in

this region for 2 mmol of hydrogen. In region 2, the hydrogen

and carbon dioxide yields both decreased with a ratio of 1:5

respectively. Also in region 2, the final pH of the solutions

changed from acidic to basic, thus it seems probably that the

mechanism of the decomposition changed in this region.

Moreover, it suggests that intermediates produced under

(initially) basic conditions, were more susceptible (at least in

terms of gas yield), to acidic decomposition. Lactic acid, which

is an intermediate produced primarily under basic conditions,

should be susceptible to an acid catalyzed decarbonylation,

which would produce hydrogen after the water gas shift. On

the other hand, under more basic conditions, lactic acid is

known to undergo dehydration to acrylic acid, followed by

hydrogenation to propionic acid, the net reaction for which is

shown below.

C3H6O3 þH2/C3H6O2 þH2O

The sharp decrease in carbon dioxide in this region is likely

a combination of less carbon dioxide being produced

(assuming it is tied to the production of hydrogen) as well as

more sodium bicarbonate being formed during cooling. In

region 3, considerably less carbon dioxide was observed. The

decrease in carbon dioxide in region 3 does not necessarily

infer considerably lower amounts of carbon dioxide were

produced. Rather, it implies that it was consumed to produce

sodium bicarbonate residues as has been found recently by

other researchers [7,24]. The carbon dioxide readings for

a 60 min residence time were taken after cooling for an

additional 45 min, indicating that at 15 min thermodynamic

equilibrium was not obtained between carbon dioxide and

sodium carbonate. For 100 mM solution, the amount of

hydrogen evolved increased with an increase in residence

time before slightly decreasing at 60 min. This may indicate

that hydrogenation reactions occurred to some degree under

these conditions. For example, while the acid catalyzed

decomposition of lactic acid should produce acetic acid, its

base catalyzed decomposition should produce acrylic acid.

Acrylic acid is known to consume gaseous hydrogen under

hydrothermal conditions to produce propionic acid.

Regarding the formation of methane in Fig. 4, decarboxyl-

ation of acetic acid to produce methane and carbon dioxide

appeared to be a likely source for the producedmethane under

50 mM conditions. Considerably more acetic acid, as well as

methane, was detectedwhen using 100mMsodium carbonate

solution. In the casewheremore sodium carbonatewas added

methane decreased to levels similar to those obtainedwithout

its addition. The high yields of acetic acid and methane in

100mM solutionmay be a result of the enhanced formation of

lactic acid (a base catalyzed reaction), combined with a final

pH near neutral conditions. This would favor the formation of

acetic acid, rather than propionic acid, (as shown in Fig. 1)

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since decarbonylation and dehydration are the acid and base

catalyzed pathways (respectively) for lactic acid decomposi-

tion [20,31]. Nonetheless, it does not appear necessary to

evoke methanation as the primary mechanism for methane

generation under our experimental conditions.

4.2. Headspace fraction

The liquid vapor equilibrium was not calculated in these

experiments since the vapor pressure from the produced gas

and intermediates is expected to shift the equilibrium.

However, the effect of the headspace fraction clearly estab-

lishes that as the state of water shifted from liquid to vapor,

hydrogen increased considerably, particularly at the 92.8%

headspace fraction. As mentioned by Azadi et al., we would

expect that higher headspace fractions should reduce partial

pressures of hydrogen in the liquid phase [21]. Certainly,

hydrogenation is a possible reaction under the hydrothermal

conditions utilized in our experiments and may have

contributed to the slight decrease in hydrogen yield at 60 min

in Fig. 3.

As can be seen by the effects of residence times for 0, 50

and 100 mM sodium carbonate concentrations in Fig. 3,

hydrogen either decreased very slightly or remained constant

at residence times longer than 30 min, although since only

two experiments were done it is not possible to determine

whether the decrease was statistically significant. Compara-

tively, at 60 min with 500 mM Na2CO3 slurry, the gas yield

continued to increase.

In non-catalytic kinetics set forth by Kabyemela [18] for the

decomposition of glucose in subcritical water (350 �C) thereare at least three different pathways that contribute signifi-

cantly to the decomposition products of glucose which are

shown in Fig. 1. The first, through glycolaldehyde is known

from previous literature to occur under low density free

radical conditions, such as supercritical water [2]. The last

pathway, through furfural alcohol is known to occur through

primarily ionic, acidic conditions and is preferred at slow

heating rates where glucose can degrade in high density,

liquid water [23]. Finally, the conversion of glucose to lactic

acid, proceeding through pyruvaldehyde is known to

primarily occur through basic conditions. As can be seen in

Fig. 6, at a 49.3% headspace fraction the main product was

furfural alcohol (21 mM). However, as the headspace fraction

increased, concentrations of furfural alcohol decreased

considerably. At low liquid loadings only very small amounts

of hydroxymethylfurfural were detected in the liquid phase.

Accompanying the decrease in hydroxymethylfurfural,

a larger number of organic acids were observed. Decomposi-

tion of hydroxymethylfurfural has previously been observed

to occur under free radical conditions yielding primary levu-

linic acid [16]. With a 78.3% headspace fraction the ratio of

levulinic acid to HMF was considerably higher than at lower

headspace fractions. While the gas yield was slightly

enhanced by a 78.3% headspace fraction, the effect was rela-

tively mild in comparison to the increase in gas obtained

when using a 85.5% or 92.8% fraction. With a 92.8% headspace

fraction, where the best gasification results were obtained,

a number of peaks including butyric, lactic, dihydroxyacetone

and acetic acid were found. With a 92.8% headspace fraction,

the peak at 4.1 min RT was also small relative to other peaks

indicating that this peak was suppressed with the use of high

headspace fractions.

The above indicates that a shift in mechanism likely

occurred based on the fraction of vapor and liquid in the

reactor (headspace fraction). We were unable to determine

erythrose in our solution, but under the assumption that the

low density favored direct fragmentation [2], we would expect

erythrose and glycolaldehyde to be produced. The majority of

acids produced, such as acetic, propionic and levulinic can be

readily explained from either the decomposition of lactic acid

or HMF [31,32]. Regarding the presence of butyric acid

however, we are unaware of any explanations in the litera-

ture. One possibility would be that erythrose underwent both

dehydration/hydrogenation at the alcohol functional group,

as well as oxidation of its aldehyde functional group. The

oxidation of aldehydes to corresponding acids appears to

occur readily in hot compressed water without molecular

oxygen given the conversion of acetaldehyde to acetic acid

during lactic acid decomposition [31]. This is merely specu-

lative however, and kinetic study on the decomposition of

both erythrose and glycolaldehyde would be useful. To the

authors knowledge no work has studied the decomposition of

erythrose in hot compressed water and little work has been

done regarding glycolaldehyde decomposition.

Furthermore, we found that high vapor/liquid fractions

greatly enhanced the production of gas. Given the effects of

density on the fragmentation of glucose, this may occur

through glycolaldehyde, although it could not be quantified in

our experiments, nor could erythrose. Under our experi-

mental conditions, little gas can be attributed to hydrox-

ymethylfurfural since its decomposition produced almost no

gas at headspace fractions of 78.3 and 63.8% and sodium

carbonate concentrations of 0 and 100 mmol. This result is

congruent with non-catalytic results from Ref. [33], where it

was found that almost no gaswas produced from5-HMF. Non-

catalytic results from Ref. [34] also indicate that below 350 �Clittle gas can be produced from HMF. To the authors knowl-

edge there is no evidence for HMF decomposing to a consid-

erable amount of gas under subcritical conditions, although

we are unaware of any information obtained under catalytic

conditions.

Recent literature has investigated the gasification of gly-

colaldehyde and suggests it proceeds through a C1 interme-

diate [35]. Given the vastly greater kinetic parameters for C1

compounds (formaldehyde and formic acid) relative to C2

compounds (acetaldehyde and acetic acid), it seems probable

that this could be the reason for the enhanced gasification

under the low density conditions studied [33,36,37]. A recent

review on hydrothermal conversion of cellulosic biomass by

Arai et al. includes glycolic acid as the decomposition product

of glycolaldehyde, although we were unable to find published

experimental validation of this path but assume it is forth-

coming [32]. However, if glycolaldehyde decomposition

proceeds through glycolic acid, then further information can

be gleamed from the known decomposition pathways of lactic

acid based on the structural similarities of lactic and glycolic

acid. Theoretically, glycolic acid should undergo decarbon-

ylation in the gas phase to produce formaldehyde and carbon

monoxide, particularly under neutral or acidic conditions [38].

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 6 6 0 0e6 6 1 06608

Formaldehyde could either go on to generate hydrogen and

carbon monoxide, or undergo oxidation (similar to the

formation of acetic acid from lactic acid through acetalde-

hyde) to produce formic acid, which is also easily gasified

under hydrothermal conditions [33]. Stoichiometrically, we

would expect such a reaction pathway to produce 1.5mmol H2

for every 1 mmol CO2. At the highest headspace fraction,

without sodium carbonate, this ratio was 1.7:1. In comparison,

other headspace fraction at 0 M sodium carbonate produced

no higher than a 1:5 ratio of hydrogen to carbon dioxide. In the

presence of sodium carbonate, the effect of headspace frac-

tion was qualitatively still present but was found to be

dampened considerably. Instead, as can be seen in Fig. 6,

larger yields of dihydroxyacetone and lactic acid were found

in the product. This could be due to catalysis of the Lobry de-

Bruyn Van Ekenstein transform since the transformation is

known to be based catalyzed. It has been proposed that

erythrose and glycolaldehyde can be generated directly from

the first intermediate in the isomerization of glucose to fruc-

tose, while glyceraldehyde can only be generated from the 1,3

enediol, which would require the (Lobry de-Bruyn Van Eken-

stein) isomerization [18]. Alternatively, it may infer that gly-

colaldehyde was still produced and that its decomposition

Fig. 8 e Hydrothermal Gasification Pathways

occurred via a different mechanism than that studied by

Huber et al in Ref. [35]. For example, Kishida et al. [39] studied

the decomposition of glycolaldehyde with sodium hydroxide

and found that at high concentrations lactic acid became an

important product, whereas at lower concentrations of

sodium hydroxide, formic and acetic acid increased. These

pathway, which are of relevance to the hydrothermal gasifi-

cation of cellulose are shown in Fig. 8.

Finally, given the effect of the phase behaviour on the

apparent formation of furfural alcohol, we would like to

discuss the effects of heating speed. Effects of heating speed

have been observed by other researchers. Fang et al. found

that low heating speeds suppressed the formation of solid

residues, and attributed phase behaviour to this reduction in

hydrothermal char [22]. The effect has also been investigated

by Sinag et al. where it was found that heating rate affected

the liquid product distribution as well, with higher yields of

furfurals occurring at low heating rates [23]. As temperature is

increased in the reactor, the liquid fraction decreases and the

vapor phase fraction increases. In terms of phase behaviour,

this should resemble the effect of headspace fraction. There-

fore, our results appear to be congruent with Sinag et al. [23]

regarding product distribution. Additionally, given the strong

of Cellulose based on [5,18,31,32,38e40].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 6 6 0 0e6 6 1 0 6609

evidence for furfural alcohol in hydrothermal char [28,29], and

its apparent suppression under low density conditions, it

seems reasonable that the decrease in char associated with

higher heating rates might be related to both suppression of

furfural alcohol production, and an increased proportion of

intermediates proceeding through glycolaldehyde and eryth-

rose since these intermediaries are produced primarily under

free radical conditions.

Themetal catalyst used in these experiments, Pt/Al2O3, did

not appear to fundamentally alter the production of initial

intermediates, such as HMF, lactic acid, or acetic acid, at the

concentrations used. The expected product shift obtained

with the addition of sodium carbonate was still present. Thus,

we assume its role was mainly restricted to the reforming of

short chain acids/aldehydes generated from cellulose

decomposition. However, the interaction of sodium carbonate

and the platinummetal (in terms of chemical promotion), has

not been considered. Study of these effects may also be rele-

vant to the effects of sodium carbonate, and this should be

considered in further study.

5. Conclusions

In this paper the effects of alkalinity and headspace fraction

in the presence of Pt/Al2O3 were investigated. A strong rela-

tionship between the headspace fraction and the gas yield

was determined, which was most distinct without the addi-

tion of sodium carbonate. The effect of headspace fraction in

subcritical water appears to be attributed primarily to the

high fraction of water vapor, and the free radical reactions

promoted therein due to the lower water density. Not only

was a considerable increase in hydrogen found at high

headspace fractions, but the liquid phase composition was

found to change considerably. Furfural alcohol concentra-

tions in the solution decreased with increasing headspace

fraction. The effect of headspace fraction on gas yield was

qualitatively similar in the presence of 50 and 100 mmol

sodium carbonate, but considerably dampened in compar-

ison to the case where no sodium carbonate was added.

Considerable increases in hydrogen and a decrease in carbon

dioxide were observed at the highest headspace fraction

(93%). At this condition, primary products in the liquid phase

were found to be butyric and acetic acids. Although glyco-

laldehyde may play a more important role as intermediate

when lower density conditions and higher vapor fractions are

present, the possibility of enhanced free radical decomposi-

tion of intermediates in the glyceraldehyde and dihydroxy-

acetone pathway, such as lactic acid is also a possibility, and

future research regarding the sensitivity of model compounds

such as lactic acid and glycolaldehyde to phase behaviour

would be useful.

Regarding the effects of alkalinity, yields of hydrogen

initially increasedwith increasing sodium carbonate, but then

peaked and decreased slightly before remaining relatively

constant. In the first region, the increase in gas product could

be attributed to a decrease in hydrothermal char as has been

noted by previous research. However, a peak in both hydrogen

and carbon dioxide yield appeared to occur at the location

where the pH changed from acidic to basic conditions. This

suggests that the products, which were generated under basic

conditions initially, were slightly more susceptible to acidic

decomposition than alkaline or neutral conditions, at least as

it pertains to gas yield. The primary products identified in the

liquid phase at high concentrations of sodium carbonate were

found to be dihydroxyacetone, lactic and acetic acid. Lactic

acid is most likely formed fromdihydroxyacetone through the

pyruvaldehyde intermediate, and acetic acid through the

decarbonylation of lactic acid followed by oxidation of the

resulting acetaldehyde.

Acknowledgements

The authors would like to acknowledge financial support from

Alberta Agricultural Research Institute and NSERC.

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