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Decomposition characteristics of softwood lignophenol under hydrothermal conditions

Hiroshi Nonaka*, Masamitsu Funaoka

Graduate School of Bioresources, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan

a r t i c l e i n f o

Article history:

Received 30 October 2009

Received in revised form

5 November 2010

Accepted 22 December 2010

Available online 22 January 2011

Keywords:

Lignophenol

Phase separation system

Lignin

Softwood

Hydrothermal decomposition

Monophenol

* Corresponding author. Tel.: þ81 59 231 952E-mail addresses: nonaka@bio.mie-u.ac.j

0961-9534/$ e see front matter ª 2010 Elsevdoi:10.1016/j.biombioe.2010.12.040

a b s t r a c t

Lignophenol is a novel lignin-based functional polymer. Hydrothermal decomposition of

ligno-p-cresol derived from softwood (Douglas fir) was studied from 300 to 400 �C as

a candidate technology for modification of the molecular structure and the recovery of

monophenols. Ligno-p-cresol was promptly depolymerized probably by hydrolysis of aryl

ether linkages and the half was converted to ether-soluble compounds at 300 �C for 10 min.

With increasing temperature and reaction time, the yields of char and gas were increased,

indicating the acceleration of carbonization and decomposition of monophenols. The

monophenols obtained were mainly p-cresol and guaiacol. Their yields only exceed 10% at

365 �C and 60 min probably due to the inefficient pyrolysis of 1,1-bis aryl structures.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction promising lignin-based products on the market. One reason is

Cellulose, hemicellulose and lignin are three major compo­

nents of plants. Cellulose and hemicellulose are polymers of

sugars, and lignin is an aromatic polymer produced by dehy­

drogenative polymerization of few monomeric precursors.

Lignin forms interpenetrating polymer network structure

with cellulose and hemicellulose in woody tissues, therefore,

it is impossible to extract most lignin simply by organic

solvents. Tremendous efforts have been made to isolate lignin

from plants for the purpose of analyzing the structure of

native lignin or utilizing lignin as an alternative of fossil

resources. However, we have not seen any successful tech­

nology to isolate native lignin in a high yield and any

0; fax: þ81 59 231 9591. p (H. Nonaka), funaoka@bier Ltd. All rights reserved

that every industrial lignin e.g. kraft lignin on paper making

and hydrolysis lignin on saccharification of woody materials,

suffers structural changes under isolation conditions, result­

ing in the formation of lignin with unexpectable and uncon­

trollable molecular structure.

“Phase separation system” [1e3] realized isolation of lignin

from plants as a useful polymer in a high yield. This novel

lignin-based polymer is named “lignophenol” by Funaoka. In

this system, wood meals are treated with a phenol derivative

and a concentrated acid under atmospheric temperature.

Crystalline cellulose is swollen and hydrolyzed by the

concentrated acid. Non-crystalline cellulose and hemi­

cellulose are more easily hydrolyzed and dissolved by acid. In

io.mie-u.ac.jp (M. Funaoka). .

1608 b i om a s s and b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 6 0 7e1 6 1 1

contrast, lignin is phenolated with the surrounding phenol

derivative at the benzyl (Ca) positions under the acidic

condition, thereby making condensation reactions between

lignin molecules avoid. The lignin becomes relatively linear

polymer due to the cleavage of a-aryl-ether bond. Based on the

fact that the rate of hydrolysis of a-aryl-ether bond is much

faster than that of b-aryl-ether bond [4], the reaction

temperature and time is designed so that the hydrolysis of

b-aryl-ether bond could be almost inhibited in the phase

separation system.

Lignin in softwood species consists of guaiacyl propane

units originated from dehydrogenative polymerization of

coniferyl alcohol. Although several kinds of linkages are

known between the monomeric units, the half is b-aryl-ether

bond [5,6], whose formation is typically followed by nucleo­

philic attack of water or a phenolic group to Ca position. This

representative structure of softwood lignin is converted

through the phase separation system as demonstrated in

Fig. 1. The major structures of softwood lignophenol are

b-aryl-ether bond and 1,1-bis-aryl structure derived from the

grafted phenol derivative. This strong information enables us

to control and modify its structure and properties, which has

resulted in novel applications as functional polymers for these

several years [7e10].

Since lignin is the only promising renewable source of

aromatic compounds, it is also important to develop a tech­

nology to convert lignin-based polymers to simple mono-

phenols for chemical industries after used as a functional

polymer. The purpose of this study is to examine the adapt­

ability of hydrothermal decomposition to selective and efficient

conversion of softwood lingophenol into monomeric phenols

and to obtain fundamental information on what are key reac­

tions and how to control them. The motive is stimulated by the

possibility of highly selective hydrolysis of b-aryl-ether bond in

concurrence with pyrolytic cleavage of 1,1-bis-aryl structure

into phenolic units.

2. Materials and methods

2.1. Synthesis of ligno-p-cresol

Ligno-p-cresol was isolated from extractive-free wood meals

of Douglas fir (Psedotsuga menziesii) through the two step

process II of the phase separation system [3,8] as a repre­

sentative of softwood lignophenol. Seventy two percent of

4- O-5 -5

5- 5

1,1-bis aryl

OHR = H, Aryl

RO O H+

1 OM e 26

OH35 OMe4

O( H) H3C p-c re so l

Representative structure as a phenol of native lignin derivative

Fig. 1 e Change of the major structure of softwood lignin using c

system.

sulfuric acid was added to wood meals sorbing p-cresol. The

mixture was stirred vigorously for 1 h at room temperature,

and then poured into excess water with vigorous stirring. The

precipitate was washed and extracted with acetone. The

acetone solution was added drop-wise to an excess amount

of cold diethyl ether with vigorous stirring. Ligno-p-cresol is

collected by centrifugation and dried.

2.2. Hydrothermal decomposition of ligno-p-cresol

Small stainless steel batch-type reactors with a volume of

8.5 mL were used for hydrothermal reactions. After 0.1 g of

ligno-p-cresol and 3.4 g of distilled water are added to

a reactor, the reactor was immersed in a molten salt bath for

heating to desired temperature. For experiments at 400 �C, a muffle furnace was used for heating. The reactor was

transferred to a water bath to stop the reaction. The product

was washed with acetone, and then filtrated. The acetone-

insoluble residue was weighed after oven drying. The filtrate

was subjected to evaporation of acetone, and then was

extracted by diethyl ether using a separating funnel with

salting-out technique. Ether-insoluble compounds were

collected by deionized water and weighed after oven drying.

The ether solution was partly oven-dried to obtain the yield of

ether-soluble products.

2.3. Analyses of the products

Acetone and n-butyl phenol were added to the remaining

ether solution, and then was subjected to GC-FID analysis

(Shimadzu GC-17A) to determine monophenols. A non-polar

capillary column (Quadrex crosslinked methyl silicone

column, Length: 50 m, Id: 0.25 mm, Thickness: 0.25 mm) was

used with helium as a carrier gas. The column temperature

was increased from 100 �C to 270 �C at a rate of 4 �C/min. The

injector and the detector were maintained at 250 �C. The

added n-butyl phenol was used as an internal standard to

calculate the yields of monophenols. The molecular weight

distributions of the ether-insoluble and ether-soluble prod­

ucts after the decomposition at 300 �C and 10 min were

measured by GPC (Shimadzu Class LC-10 system) equipped

with four columns (Shodex KF804, KF803, KF802 and KF801 in

series) using THF as an eluent. FT-IR spectra of some samples

were obtained by KBr method. The KBr disks were prepared by

a mini-hand press kit (Shimadzu MHP-1).

structure

OH

CH3

O( H)

OH

O

OM e

OM e

-aryl ether bond

Representative structure of ligno-p-cresol

oncentrated acid and p-cresol through the phase separation

1609 b i om a s s and b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 6 0 7e1 6 1 1

Fig. 2 e Yield of each fraction after the treatment of

softwood ligno-p-cresol under hydrothermal conditions.

300 , 10 min

365 , 10 min

365 , 60 min

Ligno-p -cresol

4400 3400 2400 1400 400

Wave number (cm-1

)

Fig. 4 e Comparison of FT-IR spectra between ligno-p­

cresol and ether-insoluble products obtained at each

experimental condition.

3. Results and discussion

3.1. Characterization of the products

Fig. 2 shows the proportions of acetone-insoluble, ether-insol­

uble and ether-soluble products for each experimental condi­

tion. Ligno-p-cresol is originally acetone-soluble and ether-

insoluble compoundsbecause it is purified as suchusing acetone

and ether in the phase separation system. After the treatment at

300 �C for 10 min, the half changed to ether-soluble compounds.

By GPC analyses, we confirmed that the molecular weights of

ether-insoluble and ether-soluble products are transformed to

higher and significantly lower than that of the raw ligno-p­

cresol, respectively (Fig. 3). First, a little amount of a-aryl-ether

linkage remaining in ligno-p-cresol must be cleaved. Moreover,

to account for the low molecular weight of ether-soluble prod­

ucts it is natural to consider that b-aryl-ether bonds were partly

cleaved in this condition. Since p-cresol is not introduced at all

benzyl positions, typically about 25wt% (0.65 mol/C9) for soft­

wood ligno-p-cresol [3], simultaneous polymerization could be

due to condensation reaction at the most reactive benzyl posi­

tions even at low temperature such as 300 �C. With increasing temperature and prolonging reaction time,

acetone-insoluble and undetected fractions increased. Such

conditions promote pyrolysis, resulting in polymerization via

radical coupling of pyrolyzed products and further carbon­

ization. In reality, the colors of the ether-insoluble products

obtained at 365 �C were nearly black. FT-IR analyses demon­

strate that ether-insoluble products at 300 �C almost maintain

the original structure of ligno-p-cresol, whereas those at

Mw 8000 2000 228 108

Res

pons

e

Ligno-p-cresol

Ether-insoluble

Ether-soluble

20 25 30 35 40 45

Time (min)

Fig. 3 e Gel permeation chromatograms of each fraction for

softwood ligno-p-cresol treated at 300 �C for 10 min.

Fig. 5 e GC chromatograms of ether-soluble products after

hydrothermal treatments at (a) 300 �C, 10 min, (b) 365 �C, 10 min, (a) 365 �C, 60 min.

1610 b i om a s s and b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 6 0 7e1 6 1 1

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0 60 120

Reaction time (min)

0.09

p-cresol guaiacol p-cresol, 365 guaiacol, 365

300 C

400 C

Yie

ld (

g/g-

dry

ligno

-p -c

reso

l)

Fig. 6 e Change of the yields of p-cresol and guaiacol from

softwood ligno-p-cresol at 365 �C with reaction time and

the yields at other temperatures (300 and 400 �C).

365 �C only show broad adsorption suggesting they are char­

red (Fig. 4). The increase of the undetected fraction, probably

gas such as CO2 or volatile organic matters, could be explained

by the acceleration of carbonization and further decomposi­

tion of the produced monophenols.

3.2. Production of monophenols

Fig. 5 shows GC chromatograms of ether-soluble products

after hydrothermal treatments. The monophenols obtained

were mainly p-cresol and guaiacol (2-methoxyphenol). Fig. 6

shows the yields of p-cresol and guaiacol with reaction time.

Their total yield only exceeds 10% at 365 �C and 60 min.

Therefore, it is likely that the pyrolysis of 1,1-bis-aryl structure

is a rate-limiting step. At 365 �C, the yield of p-cresol seems to

reach the equilibrium yield around 8%, while the yield of

guaiacol gradually decreases after 10 min with reaction time.

RO

O( H)

OH

O

OMe

OM e

CH3

4- O- 5 -5

5-5

OH

+H2O

Hy droly sis of -ary l-ethe r

Ligno-p-c reso l

OH

CH3

O( H)

OH

O

OM e

OM e

O( H)

OH

O

OM e

OM e +

+H+

-ROH

Fast hy dr ol ysis of -a ry l-ethe r

Poly me rized residu e at lower te mp erature th ro ugh cond ensation

Fig. 7 e Proposed mechanism on the decomposition of sof

This is because guaiacol is easily converted to catechol under

hydrothermal conditions [11]. At 400 �C and 60 min, we could

not get the higher yield of p-cresol and guaiacol than 365 �C. Since it is reported that supercritical water treatment of

guaiacol at 673 K and 30 MPa gives 25% unknown derivatives

and a small amount of solid residue [11], this result is similarly

suggesting the promotion of the decomposition of mono-

phenols and the participation of monophenols in char

formation [11e13] at higher temperature. The yield of p-cresol

was always larger than that of guaiacol. It is because about

half of the guaiacyl units are originally connected to another

monomeric unit via 4-O-5, b-5 and b-b bonds.

3.3. Decomposition mechanism

Decomposition mechanism under hydrothermal conditions is

tentatively proposed for softwood ligno-p-cresol as shown in

Fig. 7. Softwood ligno-p-cresol has two pathways: hydrolysis

or polymerization. At lower hydrothermal temperature like

300 �C, hydrolysis mainly takes place to give low molecular

weight products. A part of lignophenol could be probably

polymerized via reactive benzyl positions without p-cresol

because every benzyl position in lignin can not be phenolated

by p-cresol. With increasing temperature and reaction time,

pyrolysis would be promoted to give many radical species,

resulting in further polymerization and the following

carbonization. From a view point of low yields of p-cresol, the

pyrolysis of 1,1-bis-aryl structure is a rate-limiting step.

Therefore, it would be the most important to find a reaction

condition and catalysts for accelerating the cleavage of 1,1­

bis-aryl structure. Continuous extraction of producing

monophenols from the reactor would also be effective if the

formation of monophenols is achieving equilibrium.

OH

OH

OH

OH

OM e

OMe Inefficient py rolysi s of 1,1-bis ar yl stru ctur e CH3

OH OM e

OH OM e

Cha r

OH OH

Undetecte d pr od ucts

Poly me rized residu e via radical couplin g

Particip ation of sm all mo lecule s

twood ligno-p-cresol under hydrothermal conditions.

OH

p-creso l

gu ai ac ol

catechol

1611 b i om a s s and b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 6 0 7e1 6 1 1

4. Conclusions

Softwood lignophenol was depolymerized probably by hydro­

lysis under hydrothermal conditions. However, the maximum

yield of monophenols only reaches 10% due to the slow pyrol­

ysis of 1,1-bis-aryl structure. Higher temperature and reaction

time simultaneously leads to polymerization, carbonization

and decomposition of monophenols. To maximize the yield of

monophenols from lignophenol, it would be the most impor­

tant to find a reaction condition and catalysts for accelerating

the cleavage of 1,1-bis-aryl structure. Since softwood ligno­

phenol almost keeps the structure of native softwood lignin

with the exception of the grafted phenol, our data should be

informative to hydrothermal decomposition of softwood.

Acknowledgments

This work was supported by SORST (Solution Oriented

Research for Science and Technology) grant from Japan

Science and Technology Corporation (JST).

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