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					                     The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
C-033 (O)                                                               21-23 November 2006, Bangkok, Thailand

        Wood Pyrolysis Mechanism on Molecular Basis for Selective Conversion into Liquid Fuels

                                                 Haruo Kawamoto1 and Shiro Saka1,*

                                 Graduate School of Energy Science, Kyoto University, Kyoto, Japan

Abstract: Wood pyrolysis is the fundamental basis of various thermochemical conversion processes (gasification, fast pyrolysis,
carbonization and so on). Liquid bio-fuels could be obtained via two thermochemical processes; one is a direct thermochemical
process including fast pyrolysis, and the other is a two-step process consisting of gasification and subsequent conversion with
catalyst. However, commercialization of these processes is still difficult. Low product selectivity in these pyrolysis and gasification
processes causes some operational problems such as deactivation of catalyst by tarry by-product in the two step process as well as
low quality of the product bio-fuel. So, in these processes, improving the product selectivity is very important. In our laboratory, this
is approached by clarifying the wood pyrolysis mechanism on molecular basis, especially the key pyrolysis reactions which govern the
product selectivity between gaseous, liquid and solid carbonized product. Wood is a composite material consisting of cellulose,
hemicellulose and lignin. So, our research started from the study of the pyrolysis of theses constituent polymers separately, and then,
wood pyrolysis mechanism is approached with these results. Characteristic wood pyrolysis is observed at a relatively low temperature
of 150-400oC compared with those of fossil resources, oil and coal. So, this temperature range is especially important to understand
and control the wood pyrolysis. In this paper, our recent results are summarized.

Keywords: Wood, Pyrolysis, Mechanism on Molecular Basis, Controlled Pyrolysis, Liquid Bio-fuel, Cellulose, Lignin

                                                       1. INTRODUCTION

   Pyrolysis is conducted at high temperature under limited amount of air (oxygen). Under these conditions, wood is converted into
various products including gaseous, liquid (tar and wood vinegar) and solid carbonized substances. Traditionally, wood pyrolysis has
been used as carbonization and dry distillation processes for producing wood charcoal and some low molecular weight (MW)
chemicals such as acetic acid and creosote. Heating condition is known to affect the product selectivity significantly; at high
temperature (>600oC) selectivity of the gaseous products increases (gasification); quick heating and quick quenching of the volatile
products between 400-600oC substantially increase the liquid product formation (fast pyrolysis). Therefore, gasification and fast
pyrolysis, which give combustible gas and bio-oil, respectively, has been studied extensively to convert woody biomass to fuels and
useful chemicals.
   Liquid biofuel can be produced form woody biomass by the methods based on these thermochemical processes (Fig. 1). One is a
direct process which includes fast pyrolysis process, and the other is a two-stage process which consists of the wood gasification to
produce synthesis gas (H2 and CO) and subsequent conversion with catalyst. The latter method is already commercialized based on
natural gas, and hence the gasification is the key technology in the two-stage process based on woody biomass. However,
commercialization of these processes is still difficult even today. We think that the low product selectivity in wood pyrolysis is a
main drawbak and this causes some troubles in operating the system as well as decreasing the yield of desired product. For example,
tary by-product in the product gas cases deactivation of the catalyst through coking in the two-stage process. Tar problem is also
significant in power generation based on wood gasification. In the research of the 21COE program, fundamental research (wood
pyrolysis mechanism on molecular basis) is conducted to establish liquid biofuel production from woody biomass. We especially
focus on the key reactions which govern the product selectivity between gaseous, liquid and solid carbonized products. Such
fundamental information would provide new thermochemical process with higher selectivity between these types of products.


                                                                                   Catalytic
                                                          Gasification
                                                                                  conversion
                                 Thermochemical
                                     process                         Indirect
                                                                    (Two step)
                     Woody                                                                           Liquid
                     biomass                                                                         biofuel

                                    Controlled                           Direct
                                    pyrolysis


                                   Fig. 1 Liquid biofuel production via thermochemical processes

  Characteristic feature of the wood pyrolysis is the lower temperature process than fossil resources such as oil, coal
and natural gas. Pyrolysis of these fossil resources started at the temperature higher than 500 oC, while characteristic
wood pyrolysis reactions occur at 150-400oC [1]. These differences would be derived from the difference in their
chemical structures. Wood constituent polymers have various functional groups such as acetal, hydroxyl and aldehyde
groups, while oil and natural gas have only C-C and C-H bonds, which requires high temperature for their homolytic
cleavage. So, wood pyrolysis reactions at 150-400oC is especially focused in this study as well as the role of functional
groups in pyrolysis. In this paper, our recent results are summarized.


Corresponding author: saka@energy.kyoto-u.ac.jp


                                                                                                                                      1
                    The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
C-033 (O)                                                              21-23 November 2006, Bangkok, Thailand


                                                         2. CELLULOSE

   Cellulose pyrolysis has been studied most extensively among the wood constituent polymers. In 1918, Pictet reported
levoglucosan (1,6-anhydro--D-glucopyranose) as a major pyrolysis product from cellulose and also reported the ring-opening
polymerization of levoglucosan occurring at 240oC [2]. After this, many efforts have been conducted in 1950s-1970s about cellulose
pyrolysis relating to the levoglucosan intermediate, which include the formation mechanism (ionic or homolytic) and effects of
crystalline structure and inorganic substances in cellulose on the formation of levoglucosan and other pyrolysis products [3,4].
Thermal analysis including thermogravimetry (TG) and differential thermal analysis (DTA) give important information about the
volatilization [4]; volatilization of pure cellulose occurs at narrow temperature range of around 350oC as an endothermic process, and
this characteristic behavior is substantially modified in the presence of inorganic substances. Effects of inorganic substances have
also been studied relating to the fire retardants of wood and cellulosic materials [5]. Many kinetic studies based on thermogravimetry
have been conducted to explain the volatilization processes, and several kinetic models including Broido-Shafizadeh model have
been proposed [3]. As for low MW products, extensive studies have been conducted with pyrolysis-gas chromatography (GC)
techniques [6]. However, the mechanism related to the product selectivity is still unknown. Under these circumstances, our research
started from the pyrolysis behaviors of levoglucosan as the major pyrolysis intermediate from cellulose.

2.1 Pyrolysis behavior of levoglucosan
    Poduct formation were found to occur in a stepwise manor: levoglucosan → polysaccharide → carbonized product [7].
Furthermore, pyrolysis of levoglucosan in sulfolane (tetramethylene sulfone), an aprotic polar solvent, carbonization was effectively
inhibited through suppressing the polymerization reactivity in the homogeneous solution.        From these results, the ring-opening
polymerization of levoglucosan has been proposed as a key reaction between low MW and solid carbonized products.

2.2 Mechanism of cellulose pyrolysis
   Based on the results of levoglucosan, pyrolysis conditions in sulfolane were applied to cellulose. Cellulose was completely
converted into the soluble low MW products without any solid carbonized product formation at 200-330oC [8]. These results
strongly suggest that inhibition of the levoglucosan polymerization is quite effective to controll the pyrolysis into low MW products.
   Furthermore, during pyrolysis, residues in sulfolane were usually obtained as colorless substances, although cellulose pyrolysis in
nitrogen or dioctylphthalate (a poor solvent for levoglucosan) gave dark colored carbonized substances [8]. These results are
interesting in relation to the pyrolysis mechanism of crystalline cellulose. Infra-red (IR) and microscopic analyses under cross-polars
conditions indicated that the residue in sulfolane is crystalline cellulose itself, although the residues obtained in nitrogen or
dioctylphthalate showed the strong peaks at 1700cm-1 appearing in relation to carbonization and the decreased birefringence
showing the decreased crystallinity. Furthermore, in the X-ray diffraction analysis, changes in the crystallite size and crystallinity
were very small during pyrolysis in sulfolane and suddenly decreased and disappeared just before complete disappearance of the
residues [9]. Changes of the crystallite size and crystallinity were also very small during cellulose pyrolysis in nitrogen or
dioctylphthalate before complete disappearance of the X-ray diffraction signals of the samples. These results indicate that crystalline
cellulose pyrolysis is very heterogeneous. From these lines of evidence, pyrolysis mechanism of cellulose is proposed as shown in
Fig. 2 [7-9]. Cellulose molecules in the ordered regions would be stabilized through intermolecular interactions such as hydrogen
bonding and hydrophobic interaction, and hence, pyrolysis is expected to start from the surface molecules which can be associated
with the solvent, sulfolane. Ring-opening polymerization (a reversible reaction) is a key reaction between low MW product and char.
From levoglucosan, low MW products such as levoglucosenone and furfural with lower boiling points are preferably formed through
their easy removal from the heated zone, while unvolatile polysaccharides stay in the heated zone to be pyrolyzed further into
carbonized substances. With these pyrolysis reactions, which proceed from surface to the inner molecules successively, cellulose is
converted to the carbonized products with releasing the low MW products in nitrogen or dioctylphthalate. Selective
depolymerization in sulfolane would be established through inhibiting the polymerization reaction of levoglucosan through its
effective solubilization by sulfolane which is expected to penetrate in the disordered region of cellulose during the swelling process.


                                      “Inner”
                                     molecules      “Surface” molecule



                                                        Levoglucosan                 Volatile low MW
                                                                                       products

                                                                Polymerization

                                        Stable
                                        Stable         Polysaccharides
                                                               Carbonization
                                                   Carbonized substances

                                                       Carbonized layer
                                                           formation
                                       Cellulose molecule
                                                 Pyrolysis progression


                                         Fig. 2 A proposed mechanism of cellulose pyrolysis




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                    The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
C-033 (O)                                                              21-23 November 2006, Bangkok, Thailand


2.3 Controlled pyrolysis into low MW chemicals
   Even in sulfolane, the obtained products are the complex mixture of low MW compounds including levoglucosan, lev
oglucosenone, furufural and 5-hydroxymethyl furfural (5-HMF). Use of catalyst in sulfolane was effective to control th
e products into specific chemicals. Cellulose pyrolysis in sulfolane containing sulfuric acid or polyphosphoric acid at 2
00-280oC gave levoglucosenone and furfural in optimized yields of 42.2 and 26.9 mol%, respectively [10]. Sulfuric aci
d, a strong acid [pKa: -5.2, 1.99 (secondary)], tended to give furfural, while polyphosphoric acid, a weak acid [pKa: 2.1
5, 7.20 (secondary), 12.38 (tertially)], gave levoglucosenone more selectively. From the detailed study with the interme
diates, the pathways were indicated including the conversion from levoglucosenone to furfural and interconversion betw
een levoglucosan and its furanose isomers to give an equilibrium mixture of 3:1 under such acidic conditions (Fig. 3). F
urthermore, the conversion from levoglucosenone to furfural was found to require water. This information is very usefu
l for further control of the pyrolysis reaction. Even under the similar pyrolysis conditions (0.1 wt% sulfuric acid / sulfol
ane / 200oC / 6min), product selectivity significantly varies depending on the water content in the pyrolysis system; nitr
ogen flow conditions at atmospheric or slightly reduced pressure to remove the product water substantially increases the
 levoglucosenone yield form 24.8 to 42.2 mol% with the decreased yield (furfural + 5-HMF) from 11.8 to 3.1 mol%; on
the other hand, steam distillation conditions significantly improve the yields (furfural + 5-HMF) from 11.8 to 35.7 mo
l% with decreasing the levoglucosenone yield from 24.8 to 8.5 mol%.
                                                                 HO
                                                          O                    O
                                                    OH            HO
                                                                           O
                                                     OH                             ?
                                               OH         OH                   OH
                                                                  1,6-Anhydro--
                                                                  D-glucofuranose
                                                          O                O
                                                    OH                                    O
                                                                                                   CHO
                               Cellulose             O                 O

                                                                           O
                                               OH        OH
                                                                              H2O
                                                                Levoglucosenone         Furfural



   Fig. 3 Catalytic conversion of cellulose in sulfolane to levoglucosenone and furfural in the presence of polyphosphoric acid and
                                                            sulfuric acid, respectively

                                                               3. LIGNIN

   Lignin is a heterogeneous polymer consisting of the phenyl propanes linked through several ether-types (-ether, -ether etc.) and
condensed (C-C)-types (-aryl, biphenyl, - etc.) of the linkages. Thus, understanding the pyrolysis behaviors of these
substructures is quite important for understanding the overall behavior of lignin in wood.
   Lignin pyrolysis have been studied with isolated lignin such as milled wood lignin (MWL) and Kraft lignin as well as model
compounds. Thermogravimetric analysis showed that the volatilization starts from 150oC and the major volatilization occurs
between 300 and 400oC with relatively large amount of solid carbonized residue [1]. Volatile products from lignin have been studied
with TG-IR, pyrolysis-GC/MS techniques which include aromatic monomers, methanol, formaldehyde, formic acid, CO and CO2
[11]. Kinetic models were also studied to explain these volatilization processes [11]. As for model compound study, Domberg’s
group reported the decomposition behaviors of several model dimers including -ether, biphenyl and pinoresinol types based on their
thermogravimetric analysis [11]. Many studies were conducted with very simplified model compounds such as phenethyl phenyl
ether with no aromatic substituent groups [11]. However, there are very few papers which describe the pyrolytic reactions of lignin
model dimers with guaiacyl or syringyl type of the aromatic ring.
   Under these circumstances, two approaches have been conducted in our research; one is with lignin model dimers which gives the
details of the pyrolytic reactions occurring in substructures along with their relative reactivities; the other is with the milled wood
lignin (MWL) isolated from wood sample which gives the pyrolysis behaviors of lignin macromolecule including condensation,
depolymerization and carbonization.

3.1 Pyrolysis pathways of model dimers
   Figure 4 shows the pyrolytic pathways of ether-types and condensed-types of model dimers with guaiacyl type of aromatic rings
identified in 1min pyrolysis in nitrogen at 400oC [12]. Since ether linkages [-ether (48), -ether (16-21), 5-O-4 (4)] and C-C
linkages [-aryl (10), biphenyl (9.5-11), - (2)](number in the parenthesis: number of the linkage in 100 phenyl propane units) are
reported in spruce MWL [13], the model dimers 1-8 give information about most of the major linkages in softwood lignin.
   Reactivity and pyrolysis pathway were quite different depending on the model compound structure; phenolic compounds are much
more reactive than the corresponding non-phenolic compounds; condensed types are more stable for depolymerization, while
phenolic ether types are easily depolymerized. Alfa-ether linkages are easily cleaved at 400oC in both phenolic and non-phenolic
forms (1 and 2). For -ether types (3 and 4), two types of the reactions were observed: -ether cleavage to form cinnamyl alcohols
(b) and C-elimination to form enol ethers (c). In the phenolic form (3), reaction b was substantially enhanced and the -ether
cleavage also occurred from the enol ether. The C-C cleavage (d) and C-elimination (e) to form stilbens were observed in -aryl
types (5 and 6), and reaction d was substantially enhanced in the phenolic form 5.


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                     The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
C-033 (O)                                                               21-23 November 2006, Bangkok, Thailand


           -Ether type:                                                              -Aryl type:
              -Ether cleavage              Depolymerization                              C-C Cleavage (reaction d)          Depolymerization
                         OCH3                                                             C-Elimination (reaction e)
                    O                                                                           [Phenolic form (5): reactivity in e: enahnced]
                                        a
                                                      HO                                                                     O
                                                              OCH3
                        OCH3                                                                                                                 +
                   OR                                                                     HO
                                                                                                                  d                   OCH3                OCH3
                  1: R = H                                                              HO               OCH3                    OR                  OR
                  2: R = CH3                                                                                          H2O
                                                                                                         OR
                                                                                                  OCH3            e              H3CO        OR
                                                                                             OR
           -Ether type:                                                                   5: R = H           HCHO
               -Ether cleavage (reaction b)         Depolymerization                      6: R = CH3
               C-Elimination (reaction c)
                     [Phenolic form (3): reactivity in c: enhanced ]                                                        RO        OCH3
                                                HO                                                                          Stilbene 10 (R = H)


                                                              +      HO
                                                                                      Aryl-aryl type: relatively stable
                 HO                b                 OCH3                OCH3
               HO                               OR
                 O                           Coniferyl alcohol                                                       CH3   CH3
                          OCH3
                                    c           (9) (R = H)
                                                                                                                                             7: R = H
                        OCH3                                                                              H3CO                    OCH3
                  OR                                                                                                  OR    OR               8: R = CH3
                                 HCHO           HC O
                  3: R = H
                                                HC OCH
                  4: R = CH3                          3


                                                      OCH3
                                                 OR


                                                 Fig. 4 Pyrolysis pathways of lignin model dimers

3.2 Model dimer vs. isolated lignin from wood
   Pyrolysis behavior of MWL isolated from Japanese cedar (Cryptomeria japonica) wood was studied in 1 min pyrolysis in nitrogen
at 200-400oC by analyzing the products fractionated into dioxane-soluble and insoluble fractions [14]. As a result, MWL was
converted to the dioxane-insoluble products at the lowest temperature of 200oC. This indicates the condensation reaction to form
larger molecule occurring at this temperature. By increasing the temperature at 300-350oC, substantial depolymerization started.
UV spectrum of the depolymerized products indicated that -ether structure, the abundant ether structure in lignin, is cleaved at this
temperature. Above 350-400oC, a strong peak at 1510cm-1 in IR spectra, which is assigned to the aromatic nuclei in lignin, decreased
and finally disappeared. This observation indicates the disappearance of the original aromatic structure in lignin probably due to
polycyclic aromatic hydrocarbon (PAH) formation related to the carbonization. Figure 5 summarizes these pyrolysis behaviors of
MWL as the temperatures at which condensation, depolymerization and carbonization started to be observed. The temperatures at
which the model dimers became reactive are also included. Pyrolysis pathways of model dimers 1-6 at this temperature range were
similar to those shown in Fig. 4.
                                                                      Temperature (oC)
                                                200            250          300         350          400

                                                     Condensation                             Depolymerization
                                MWL
                                                                                                              Carboization
                                                                                                              (PAH formation)


                                                      - Ether (ph)                           - Ether (non-ph)
                                Model                                 - Ether (ph)                            - Ether (non-ph)
                                dimer
                                                                      - Aryl (ph)                          - Aryl (non-ph)
                                                                                                           Aryl - Aryl

                                ph: phenolic, non-ph: non-phenolic

     Fig. 5 Comparison of the results between MWL and model dimers (temperature: MWL and model dimer became reactive)

  Comparison between pyrolysis behavior of MWL and model dimer reactivity gives useful information to understand the role of
substructures of lignin in lignin pyrolysis. For example, phenolic structure is expected to be closely related to the condensation
behavior of lignin. At the temperature where carbonization occurres, all of the model dimers were reactive, and this indicates the
importance of aromatic moiety commonly included in the dimers. These results are also important to establish the controlled



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                    The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
C-033 (O)                                                              21-23 November 2006, Bangkok, Thailand

pyrolysis to enhance the product selectivity. If we could know the key reactions leading to condensation, depolymerization and
carbonization, respectively, modification of these reactions would lead to the improvement of the yield of the specific type of the
product, namely selective depolymerization or carbonization. In the following section, some results related to the condensation
mechanism and depolymerization mechanism are presented.

3.3 Condensation mechanism at relatively low pyrolysis temperature
   Polymerization behaviors of several model compounds including guaiacol (2-methoxyphenol), veratrol (1,2-dimethoxybenzene),
creosol (2-methoxy-4-methylphenol) and their derivatives with side chain such as–C(OH)CH3 and -CH=CH2 were studied under the
heating conditions (in air / 250oC / 2h) [15]. Polymerization reactivity and the structures of dimeric products indicate three types of
pathways as possible condensation mechanisms of lignin, which include vinyl polymerization, ionic polymerization via
quinonemethide intermediate and radical coupling between radicals derived from phenoxy radicals. As model compounds for the
structures which is suggested to be formed through primary pyrolysis reactions from model dimer study, stilbene 10 (from -aryl
structure) was very stable, while coniferyl alcohol (9) and 4-O-methyl coniferyl alcohol (from -ether structure) were very reactive
for polymerization. Heating of coniferyl alcohol (9) with excess amount of creosol gave their coupling product formed through
ionic pathway via quinonemethide types of intermediates.

3.4 Depolymerization (ether cleavage) mechanism
   As already mentioned, pyrolytic depolymerization of lignin is closely related to the cleavage of the ether structures. Readtivities
of the ether types of model dimers are quite different as shown in Fig. 5; -ether (phenolic (ph)) (200oC), -ether (non-phenolic
(non-ph)) (350oC), -ether (ph) (250oC) and -ether (non-ph) (400oC), (temperature at which ether linkage in dimer started to be
cleaved). These different reactivities suggest different cleavage mechanisms for these dimers.
   Model dimers or trimers with C-O-phenyl groups having several substituent groups at the p-positions were useful to identify the
mechanism of the -ether cleavage [ionic vs. homolytic (radical) mechanisms] [16]. If the cleavage proceeds in an ionic mechanism,
the reactivity is expected to be positively related with the order of the Hammett substituent constant p: -COMe (0.502) > -Cl (0.227)
> -H (0) > -OMe (-0.268). Contrary to this, if the cleavage proceeds in a homolytic mechanism, the reactivity is expected to be
positively related with the BDE (bond dissociation energy) value (kcal/mol): -OMe (3.9) > -Cl (1.1) > -H (0) >-COMe (-0.6)
[BDE (kcal/mol) = 63.5 (anisol) –BDE (p-sustituted anisol)]. From the relationships between reactivity and p or BDE value,
phenolic -ether linkage is indicated to proceed in an ionic mechanism, while non-phenolic -ether linkage is indicated to proceed in
a homolytic mechanism as shown in Figs. 6 and 7.

                       ArO                          Homolytic
                                  O                                           O                   -Scission                   O
                                                (rate-determining)
                                         OCH3                                         OCH3                                          OCH3

                                 OCH3                                       OCH3                                          OCH3
                                                        ArO
                              OCH3                                       OCH3                                          OCH3
                                                                     Benzyl radical

                         Fig. 6 Cleavage mechanism of the non-phenolic -,-diether types of model trimers

                                 CH3                               CH3                                    CH3
                          ArO                                        O
                                     O          Ionic                             Homolytic
                                         OCH3                             OCH3
                                                                                                                        O
                                     OCH3                            OCH3                                  OCH3              OCH3
                                OH               ArOH          O                                      O

                                                              Quinonemethide
                                                                                              CH3
                                                                                       ArO
                                                                                                  O
                                                                                                          OCH3
                                                                                                                        H-Abstraction
                                                                                                  OCH3
                                                                                             OH

                                                                                                                       CH3
                                                                              ArO
                                                                                  ・




                                                                                                             ArO
                                                                                                                        O
                                                                                                                             OCH3

                                                                                                                        OCH3
                                                                                                                   O

                             Fig. 7 Cleavage mechanism of the phenolic -,-diether types of model trimers

   As for the ,-diether types of model compounds, -ether cleavage was found to be a rate-determining step for the -ether
cleavage. In the non-phenolic ,-diether types, -ether linkage is suggested to be cleaved via -session type of reaction from the
benzyl radical formed through initial homolytic -ether cleavage (a rate-determining step) (Fig. 6). The phenolic ,-diether types
of model compounds showed somewhat complex substituent effects on their reactivities, which indicate the change in the mechanism
of -ether cleavage from ionic to radical. These observations strongly indicate the radical chain reaction occurring with the ionic -
ether cleavage mechanism as shown in Fig. 7. Much easier cleavage of the -ether bond in quinonemethide intermediate (cleavage
temperature: 250oC) than the original -ether bond (cleavage temperature: 400oC) was clearly shown from the results of the phenolic



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                     The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
C-033 (O)                                                               21-23 November 2006, Bangkok, Thailand


,-diether types of model compounds. Higher reactivity of the phenolic -ether type of dimer 3 than corresponding non-phenolic
type 4 (Fig. 5) would be due to this quinonemethide intermediate formation occurring only in the phenolic form.

   Relating to the -ether cleavage reactivity, deoxygenation of the C-OH in model dimer 3 substantially reduced the reactivity
(cleavage temperature: increasing from 250 to 400oC) [17]. This is an unexpected result since C-OH is not directly involved in the
quinonmethide formation. Study with the model dimers having variously p-substituted phenoxy groups at C revealed that the direct
homolytic C-O bond cleavage (not via quinonemethide) occurs in the C-deoxy type [18]. These results indicate that C-OH is very
important in the formation of quinonemethide. Role of the C-OH in quinonemethide formation is considered by introducing the
stabilization mechanism of the transition state by intramolecular hydrogen bondings between C- and C- hydroxyl groups. Under
pyrolysis conditions, stabilization of the ionic products as like the reaction in solvent is not expected, and hence, such intramolecular
hydrogen bond would be effective to promote the ionic reaction. These results also indicate that quinonemethide formation is very
dependent on the environmental factors.
   Phenolic types of dimers are the models for the end groups of lignin endwise structure. So, if end -ether structure could be
cleaved at 250oC, which is the cleaving temperature of the phenolic -ether types of dimer 3, most of the ether linkages in the
endwise structure would be cleaved through chain reaction at this temperature. Contrary to this, substantial ether cleavage
(depolymerization) of MWL required heating up to 350oC (Fig. 5). These conflicting results are conceivable in assuming that
quinonemethide formation is not so effective in MWL pyrolysis.

                                                              4. WOOD

   Wood gasification is a two stage process which includes the primary pyrolysis to form primary volatiles and char and their
subsequent reactions with gasifying agents such as oxygen and steam. So, we started from the identification of the primary pyrolysis
focusing on the wood constituent polymers to understand the overall gasification mechanism on molecular basis [19]. As a result,
pyrolysis behaviors including secondary char formation and volatile compositions were quite different between wood
polysaccharides and lignin. Furthermore, strong interactions were found to exist between wood polysaccharides and lignin [20],
although wood constituent polymers are believed to proceed independently without significant interactions. Levoglucosan, the
primary product from cellulose, was effectively stabilized against its ring-opening polymerization and thermal degradation, and a
mechanism for this stabilization has been proposed, which includes CH- interaction between levoglucosan and aromatic substances
from lignin [21].

                                                         5. CONCLUSION

   Some of the pyrolysis mechanisms of woody biomass relating to the product selectivity were clarified. Although further study is n
ecessary to establish the liquid biofuel production system based on thermochemical conversion, these results will give useful informa
tion to understand and control of the product selectivity in wood pyroloysis and gasification.

                                                   6. ACKNOWLEDGMENTS

  Part of this research was supported by a Grant-in-Aid for Scientific Research C(2)(N0. 11660164 and No. 16580132) and 21st
COE program “Establishment of Sustainable Energy System” from Ministry of Education, Culture, Sports, Science and Technology,
Japan.

                                                         7. REFERENCES

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[3]    Antal Jr., M. J. Adv. Solar Energy 1983, 1, 61-111.
[4]    Shafizadeh, F. J. Anal. Appl. Pyrolysis 1982, 3, 283-305.
[5]    Kandola, B. K.; Horrocks, A. R.; Price, D.; Coleman, G. V. J.M.S.-Rev. Macromol. Chem. Phys., 1996, C36(4), 721-794.
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posted:10/4/2012
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