Chemical Structures and Primary Pyrolysis Characteristics of Lignins Obtained from Different Preparation Methods

986 J. Jpn. Inst. Energy, Vol. Journal 93, No. of 10, the 2014 Japan Institute of Energy, 93, 986-994(2014) Special Articles: Biomass 特集 : バイオマス Chemical Structures and Primary Pyrolysis Characteristics of Lignins Obtained from Different Preparation Methods Huamei YANG 1, Srinivas APPARI 2, Shinji KUDO 2, Jun-ichiro HAYASHI 2 3, Satoshi KUMAGAI 4, and Koyo NORINAGA 2 (Received March 14, 2014) This work aims at investigating correlations between primary pyrolysis characteristics of lignin and chemical structure of lignin feedstock. Three different types of lignin samples were prepared through enzymatic hydrolysis, organosolv extraction, and Klason procedure. Analysis by FT-IR and solid state 13 C-NMR revealed that the lignin samples exhibited different contents of aromatic carbons, connection carbons, methoxyl carbons, and aliphatic side chains. The three lignin samples were pyrolyzed in a two-stage-tubular reactor at 650, and pyrolysis products were analyzed with gas chromatographs on-line. More than fifty compounds including inorganic gases, light hydrocarbons (LHs), aromatic hydrocarbons (AHs), phenol derivatives and light non-phenolic oxy-compounds (NPOCs) were gaschromatographically separable and quantified. The influence of the lignin structures on the pyrolysis characteristics was studied, and the correlation of product distribution and lignin chemical structures was examined. The total carbon selectivity into char and tar was increased with increasing lignin aromaticity. Methoxyl group and aliphatic substituents likely contributed for enhancing char formation, while hydrogen in lignin enhanced tar formation. Yields of LHs and NPOCs were increased with increasing aliphatic carbons of the lignin samples. AHs were formed from gas-phase recombination of LHs such as olefins, diolefines and alkynes, rather than directly from aromatic structures in the original lignin likely because of high energy required to cleavage carbon-oxygen bond existed in major structural units such as syringols or guaiacols. 3 種類の異なる調製法で得られたリグニンを試料とし, これらの化学構造と急速熱分解特性との関係を実験的に調査した リ グニン試料は, 酵素加水分解処理, オルガノソルブ処理, およびクラソン法により得た 試料の化学構造を元素分析, 固体 13 C-NMR および FT-IR 測定により調べるとともに, 熱分解 GC,GC/MS を用いて 650 における急速熱分解生成物のうち 50 種 類以上を定性, 定量した 本実験範囲内で得られた主要な知見は次のとおりである (1) リグニン試料の構造は調製法に大きく依存し, 例えば, 試料に含まれる全炭素中芳香族炭素の割合 (C ar/ % ) は, クラソン法 ( K L, C ar=90), 酵素加水分解処理 (ENL,C ar= 7 3 ), オルガノソルブ処理 ( O R L, C ar=57) の順で大きい (2) 熱分解におけるタールやチャーといった重質成分への転換率と C ar には正の相関関係が認められる タールとチャーへの個々の 転換率に着目すると, タールへの転換率は ENL が最も大きく, チャーへの転換率は ENL が最も小さい これは, 使用した試 料中で水素含有量が最も多い ENL の場合, 水素による熱分解生成ラジカルの安定化が最も顕著となり, 熱分解時の重合反 応が抑制されたためと考えられる また,ORL の C ar は ENL よりも小さいにもかかわらず, チャーへの転換率が大きい理由は, 芳香族がメトキシ基や脂肪族鎖に最も多く置換されている ORL の場合, 熱分解過程においてこれらの置換基の分解によって 活性なフラグメントがより多く生成し, これらの再結合による重合が促進されたためと推察した (3) 熱分解におけるベンゼン, トルエン, ナフタレンなどの芳香族炭化水素 (A Hs) への転換率と C ar には負の相関関係がある こ れに対し,AHs の収率と熱分解における低級炭化水素や非フェノール性含酸素化合物への転換率の合計値には正の相関が ある これは, リグニン構造中の芳香族成分は, 熱分解において生成する AHs の直接の前駆体ではないことを示唆する Key Words Lignin, Pyrolysis, Chemical structure, Product analysis 1 Interdisciplinary Graduate School of Engineering Sciences, Kyushu University Kasuga, 816-8580, Japan 2 Institute for Materials Chemistry and Engineering, Kyushu University Kasuga, 816-8580, Japan 3 Research and Education Centre of Carbon Resources, Kyushu University Kasuga, 816-8580, Japan 4 Organization for Cooperation with Industry and Regional Community, Saga University Honjo, Saga 840-8502, Japan This study was partly presented in the 1st Asian Conference on Biomass Science, Jan 14, 2014, Kochi, Japan

J. Jpn. Inst. Energy, Vol. 93, No. 10, 2014 987 1. Introduction Fast pyrolysis is considered as a promising and viable technique to convert biomass into gas, liquid and solid fuel, and chemicals 1) 2). Various biomass and cellulose have been studied to find the reaction chemistry of pyrolysis 3) ~ 7). Besides cellulose, lignin is another abundant component in biomass, and occupies 18-40 wt% of the dry wood 8). Its pyrolysis kinetics and chemistry are important for the advancement of biomass conversion process. However, the structure of lignin is complex, consisting of three units (coumaryl, coniferyl and sinapyl alcohols) through eight types of linkages 2) 9), which exhibit different pyrolysis chemistries. Investigating the correlation of pyrolysis products and lignin structure is thus significant in characterizing the lignin pyrolysis behavior. In this study, three different lignins, enzymatic hydrolysis lignin (ENL), organic extracted lignin (ORL), and Klason lignin (KL) were studied to establish a correlation between the product distribution and lignin structure. Phenols are known to be primary products in lignin pyrolysis. Guaiacols, syringols, and phenols are the main products with various side chains such as -C=C, -C-OH and -C=O. A series of free-radical reactions have been proposed to represent the cleavage of inter-unit linkages and the formation of phenolic compounds during lignin pyrolysis 1) 9) ~ 12). Jiang et al. 13) found that the yield of phenolic compounds from lignin pyrolysis by Py-GC/MS at 600 was around 17 wt%. Char yield of lignin pyrolysis is 30-40 wt%. The other compounds should be inorganic compounds (IGs), light hydrocarbons (LHs), light organic nonphenolic oxy-compounds (NPOCs), aromatic hydrocarbons (AHs), and heavy tar. Deeper understanding of the pyrolysis mechanism of lignin and biomass necessitates the quantitative information of light volatile products, which are particularly important in gasification/pyrolysis regarded as promising conversion routes for biomass and lignins. In contrast with phenols, limited literatures paid attention on the light volatile products, especially LHs and NPOCs. The aim of this work is to analyze the thermal degradation products especially LHs and NPOCs formed during the pyrolysis of different lignin samples, and to investigate the effects of lignin structure on the products distribution. Three lignin samples (ENL, ORL, and KL) were pyrolyzed with twostage-tubular reactor at 650, and pyrolysis products were quantified by directly connected gas chromatograph (GC) at the reactor downstream. Detailed structures of the three lignin samples were analyzed by Fourier Transform Infrared Spectroscopy (FT-IR) and solid state 13 C nuclear magnetic resonance ( 13 C-NMR) to gain insight into correlations between the structure of lignin and the composition of volatile compounds. 2. Experimental 2.1 Sample Three types of lignin (ENL, ORL, and KL) were employed in this study. Samples used in this experiment were smaller than 150 μm of particle size and dried under vacuum overnight. Elemental compositions of ENL, ORL and KL are listed in Table 1. ENL was prepared from empty fruit branches (EFB) by removal of hemicellulose and cellulose. Hemicellulose was removed by hydrothermal treatment. After filtration and washing, a pretreated solid was then hydrolyzed to obtain lignin residue. Filtration and washing of the lignin residue yielded the lignin used in this research and named as ENL. The organosolve lignin named as ORL was obtained by extracting the hydrothermally treated EFB with ethanol. The lignin contents of ENL and ORL were 96 and 97 wt%, respectively. KL used in this research was obtained by an established method of National Renewable Energy Laboratory, U.S 14). The procedure was as following: Pulverized samples finer than 0.5 mm were dried in a vacuum-drying oven at 60 for 24 h. To remove the wax in the sample, 5.0 g of the sample was treated with ethanol using Soxhlet extractor at 80 for 8 h. The extracted sample was then treated with 150 mg of 72 wt% H 2SO 4 at 30 for 1h. Subsequently, 42 ml of H 2O was added and treated in an autoclave at 121 for 1 h. After cooling, the reaction products were filtered using a GP 16 glass filter with hot water washing. The solid residue on the filter was dried to obtain the KL. Table 1 Elemental compositions of ENL, ORL and KL wt % Samples (dry ash free basis) molar ratio C H O a N O/C H/C ENL 63.3 5.9 28.9 1.9 0.34 1.12 ORL 62.0 5.7 31.6 0.8 0.38 1.09 KL 62.9 5.3 31.4 0.4 0.37 1.01 a by difference

988 J. Jpn. Inst. Energy, Vol. 93, No. 10, 2014 2.2 FT-IR analysis The FT-IR spectra of the lignin samples were recorded on a FT-IR spectrometer (PerkinElmer Spectrum Two) with universal attenuated total reflectance (UATR) accessories. The UATR employs a DiComp TM crystal, which is composed of a diamond ATR with a ZnCe focusing element. The scan was conducted in the range from 4000 to 400 cm -1 at a resolution of 1 cm -1. 2.3 NMR analysis Solid-state 13 C-NMR spectra for the lignin samples were recorded with the DEPTH2 technique 15) at 100.53 MHz in a JEOL ECA 400 spectrometer. The repetition time was 60.03 s. The sweep width and the acquisition time were 400 ppm and 25.47 ms, respectively. Magic angle spinning was performed at 15 khz in the commercial probe (JEOL 4 mm CPMAS). 2.4 Analysis of product from fast pyrolysis A two-stage-tubular reactor (TSTR) connected to a GC was used to investigate the pyrolysis behavior of lignin. This reactor was made-up of quartz and divided into two zones by a quartz filter, which reduces the interaction of char and volatile. The detailed information about the reactor can be found elsewhere 4). Pyrolysis temperature was kept at 650 and residence time of the volatile in the second zone was 0.6 s. Compositions of the volatile were measured directly by two GCs equipped with TCD and FID. To detect the products in detail, three different columns were employed, including a 4 m long packed column with 60/80 mesh Gasukuropak 54 (GL Sciences, Co. Ltd.), a 25 m long, 0.25 mm i.d. capillary column (PoraBOND Q, d f = 0.25 μm, Agilent), and a 60 m long, 0.25 mm i.d. capillary column (TC-1701, d f = 0.25 μm, GL Sciences, Co. Ltd.). Inorganic gaseous products such as CO, CO 2 and H 2O were detected by TCD, while the other products were detected by FID. Undetectable compounds were here defined as 'tar' which was condensed at the reactor downstream or captured by GC column packing. at 300 ; the chromatographic separation was performed with a 60 m long, 0.25 mm i.d. TC-1701 (GL Sciences, Co. Ltd.) capillary column; the oven temperature was programmed from 40 (5 min) to 250 (20 min) with a 4 /min heating rate; the mass spectra were obtained from m/z=50 to 650 amu. The peak identifications were based on the matching of the mass fragmentation patterns with those included in the National Institute of Standards and Technology (NIST) library. 3. Results and discussion 3.1 Sample characteristics 3.1.1 Elemental analysis The elemental compositions of lignin samples are given in Table 1. There are no apparent differences in the elemental compositions with formulas being CH1.12O 0.34, CH 1.09O 0.38 and CH 1.01O 0.37 for ENL, ORL, and KL, respectively. Among the three lignins, ENL had the highest carbon content, while ORL had the lowest carbon content and the highest oxygen content, indicating that more oxygen functional groups were present in ORL. 3.1.2 Structure analysis. The FT-IR spectra of the three lignin samples are shown in Fig. 1. The spectra were similar with each other but noticeable differences were observed in the region of 3000-2800 cm -1, which were attributed to C-H of aliphatic carbons and methoxyl groups. Two absorption bands (2920 cm -1 and 2850 cm -1 ) were obviously observed in ORL, while ENL and KL had a broad band between 3000 and 2800 cm -1. It indicated that there were more methylene and methoxyl groups existing in ORL. A carbonyl group was characterized by the band at near 1710 cm -1. This group was richer in ORL and ENL than in KL. The conjugated carbonyl stretching vibration at 1658 cm -1 solely appeared in ENL. Bonds in region of 1300-1000 cm -1 were mainly attributed to the C-O vibration. Obviously, ENL and ORL contained more C-O structures than KL. In the region of 1000-400 cm -1, ENL and ORL showed more complex substituents of aromatic ring than KL. 2.5 Curie-point pyrolysis and GC/MS analysis Samples were also pyrolyzed in a Curie-point pyrolyzer (JCI-22) to analyze the products with GC/MS (Clarus SQ 8 S; PerkinElmer). It enables us to characterize the possible compounds included in the tar that were undetectable by the analytical pyrolysis experiments mentioned in section 2.4. Pyrolysis temperature was set at 650 with holding time of 5 s. Products from lignin pyrolysis were analyzed in a GC/ MS with helium as a carrier gas. The GC/MS conditions were set as following: the injector temperature was kept Fig. 1 FTIR spectra of ENL, ORL, and KL

J. Jpn. Inst. Energy, Vol. 93, No. 10, 2014 989 Solid state 13 C-NMR spectra are shown in Fig. 2. ENL and ORL had similar NMR spectra, while KL showed a very different pattern from others. The relative intensities of the carbons with different chemical shifts were evaluated by the spectral deconvolution and are listed in Table 2. ENL and ORL exhibited much more aliphatic carbons of alkylsubstituents (<50 ppm), methoxyl (57 ppm) and connection carbons (70-100 ppm). The content of aliphatic carbons was reduced in the order of ORL>ENL>KL. Chemical shifts of aromatic carbons are between 100 ppm and 150 ppm. Listed as Table 2, KL had much more aromatic carbons with aromaticity of 90 % than ENL and ORL of which aromaticities were 73 % and 57 %, respectively. Peaks at 100-125 ppm are attributed to the aryl carbon in aryl-h and their intensities were decreased in the order of KL>ENL>ORL. Peaks at 125-140 ppm are due to the aryl carbon in aryl-c and peaks at 140-150 ppm are due to the aryl carbon in aryl-o-c. These peaks were much Fig. 2 Solid 13 C-NMR spectra of ENL, ORL and KL Table 2 Carbon distributions characterized by the 13 C-NMR spectral deconvolution for ENL, ORL and KL ENL ORL KL Aromatic Carbon 100-150 ppm Ar-C 47.7 32.5 52.2 Ar-O 8.3 13.2 5.6 Ar-H 17.3 11.6 32.5 Total 73.3 57.3 90.4 Aliphatic Carbon 10-100 ppm Alkyl C-C 12.1 15.9 0.0 Methoxyl 7.3 9.8 4.6 Aliphatic C-O- 8.4 13.4 5.0 Total 27.8 39.1 9.6 C=O 1.9 3.7 0.0 more abundant in ENL and ORL than in KL, indicating that more alkoxyl and alkyl substituents on aryl structures existed in ENL and ORL. The peak at around 172 ppm shows that carbonyl group reduced as the following order: ORL>ENL>KL, which is consistent with FT-IR results. 3.3 Product distribution of lignin pyrolysis The identified fifty-one products of lignin pyrolysis in TSTR are listed in Table 3, including four IGs (CO, CO2, H 2 and H 2O), twenty LHs (including C 1-C 6 hydrocarbons), eleven NPOCs (alcohols, ketones, acids, aldehydes and furans), eight AHs and eight phenols. Yields, carbon selectivity and oxygen selectivity of different groups, char and tar are shown in Fig. 3. Fig. 3 shows that carbon in lignin was mainly converted into char, tar and LHs and most of oxygen was released in the form of IGs as well as NPOCs and phenols. Char, tar, and IGs were major products of lignin pyrolysis. KL produced the most char and IGs, followed by ORL, while ENL produced much more tar than ORL and KL. Curie-point pyrolyzer-gc/ms was employed to analyze the possible compositions of tar. The three lignin samples were pyrolyzed at 650 in JCI-22 Curie-point pyrolyzer. As shown in Fig. 4, besides phenol, guaiacol and syringol, more alkyl-phenols were produced from ENL than ORL and KL, even though the chemical structure of ORL had more substituents than ENL. The possible reason for this was the higher hydrogen content in ENL, which will be discussed in section 3.4. Yields of pyrolysis products from ENL, ORL and KL at 650 are listed in Table 3. There are apparent differences in the distribution of LHs, NPOCs, and AHs produced from the three lignins. As shown in Table 3, twenty LHs are quantified from pyrolysis of ENL, ORL, and KL, including alkanes, C 2- C 6 olefins, diolefins and alkynes. ORL produced more LHs than ENL and KL. The yield of LHs from KL was the lowest. Methane, ethene, and propene were three primary LHs produced from lignin. The yields of methane, ethane, and ethene were not so much different among the three lignins. Methane has been widely studied as one of the major gaseous products 9) 16) 17). It was thought to be formed through the demethylation of methoxyl, fragmentation of the side chains and the rupture of aromatic rings. The yields of C 3-C 4 alkanes, olefins, diolefins and alkynes from KL pyrolysis were much lower than those from ENL pyrolysis and ORL pyrolysis. C 2-C 3 LHs can be formed from propyl groups, while C 4-C 6 LHs were possibly formed through the combinations of C 1-C 3 LHs. More intriguing is that six olefins, five diolefins and four alkynes, especially propadiene, 1,3-butadiene, cyclopentadiene, 1,4-pentadiene,

990 J. Jpn. Inst. Energy, Vol. 93, No. 10, 2014 Table 3 Yield (wt% dry lignin) of products from pyrolysis of ENL, ORL and KL at 650 Compounds ENL ORL KL Compounds ENL ORL KL IGs CO 6.49 6.15 9.75 H 2O 8.40 9.22 9.74 CO 2 6.50 10.06 6.97 IGs total yield 21.39 25.43 26.46 LHs Methane 4.80 4.60 4.54 1-Butene-3-yne 0.03 0.09 0.02 Ethyne 0.13 0.08 0.10 1,3-Butadiene 0.46 0.48 0.12 Ethene 1.61 1.76 1.30 Butane 0.40 0.44 0.07 Ethane 0.52 0.53 0.47 But-1-ene 0.38 0.43 0.06 Propane 0.10 0.15 0.06 But-2-ene 0.04 0.05 0.01 Propene 1.18 1.14 0.38 Cyclopentadiene 0.23 0.25 0.07 1-Propyne 0.16 0.18 0.07 1,4-Pentadiene 0.18 0.21 0.06 Propadiene 0.88 0.77 0.20 1,2-Pentadiene 0.07 0.09 0.03 Iso-butene 0.69 0.75 0.37 1-Hexen-3-yne 0.16 0.19 0.06 Iso-butane 0.07 0.08 0.07 1-Hexene 0.18 0.22 0.04 LHs total yield 12.31 12.50 8.11 AHs Benzene 0.74 0.68 0.59 o-xylene 0.07 0.11 0.05 Toluene 0.47 0.49 0.41 Stylene 0.15 0.21 0.07 Ethyl-benzene 0.11 0.14 0.03 Naphthalene 0.08 0.15 0.07 p-xylene+m-xylene 0.13 0.22 0.04 Methylnaphthalene 0.07 0.08 0.16 AHs total yield 1.82 2.07 1.41 NPOCs Methanol 2.28 3.29 2.90 Furan 0.05 0.07 0.03 Acetaldehyde 0.20 0.28 0.82 Acetone 0.15 0.07 0.09 Ethanol 0.08 0.18 0.07 Acetic acid 2.20 4.28 0.39 Acrylaldehyde 0.11 0.11 0.29 But-3-en-2-one 0.04 0.05 0.02 Hydroxyacetone 0.47 0.38 0.32 2-Methylfuran 0.08 0.08 0.03 Furfural 0.74 0.68 0.59 NPOCs total yield 6.40 9.45 5.54 Phenols Phenol 2.08 4.53 0.82 2,3-Dimethylphenol 0.24 0.24 0.21 O-cresol 0.37 0.55 0.26 2-Methoxyphenol 0.28 0.31 0.4 2,6-Dimethylphenol 0.08 0.12 0.07 Catechol 0.69 0.16 0.96 p-cresol 0.57 0.58 0.42 2,6-Dimethoxyphenol 0.98 0.26 1.61 Phenols total yield 5.29 6.74 4.75 Char 29.67 34.4 41.00 Tar (undetectable compounds) 23.45 9.74 12.77 Fig. 3 Yield, carbon selectivity and oxygen selectivity of different groups, char, and tar from ENL, ORL and KL with TSTR

J. Jpn. Inst. Energy, Vol. 93, No. 10, 2014 991 Fig. 4 GC/MS total ion chromatograph of Curie-point pyrolysis of ENL, ORL and KL at 650 and 1-hexen-3-yne were detected in the lignin pyrolysis. So far, these compounds were paid less attention possibly because of their low yield and the difficulty to detect and quantify. Their yields reduced in order of ORL>ENL>KL. As discussed above, KL had fewer substituents on the aromatic structure and side chains than ENL and ORL, which was the possible reason for the low yield of LHs, especially the low yield of olefins, diolefins and alkynes from KL. Eleven NPOCs were quantified and are shown in Table 3. As shown in Fig. 3 and Table 3, similarly to LHs, ORL produced the highest yield of NPOCs, while the yield of NPOCs from KL was the lowest. Methanol and acetic acid were two main light products produced from lignin pyrolysis, while yields of other NPOCs were very low and less than 0.1wt%. Those compounds exhibited the diversity of side chains in lignin. Oxygen containing compounds would be produced at low temperature, and possibly converted into olefins, diolefins or alkynes through deoxidation. Yields of methanol and ethanol were both changed in the order of ORL>KL>ENL. Note that although ENL had more methoxyl group than KL, yield of methanol from ENL was lower than that from KL. At this temperature, methanol was mainly formed from the -CH2OH group in γ-position rather than aromatic methoxyl groups during lignin pyrolysis 9), which could be confirmed by the formation of ethyl and vinyl phenols 13) 16. Acetic acid showed the highest yield from ORL, followed by ENL, and KL yielded much lower acetic acid than ENL. Acetic acid was mainly formed from the carboxyl group which showed the highest intensity in ORL and the lowest intensity in KL. In contrast, yields of acetaldehyde and acrylaldehyde were reduced in the order of KL>ORL>ENL. Aldehyde could be converted to CO 9) 18), which was the possible reason for the high yield of CO from KL. ENL yielded more furans and ketones than KL and ORL. Hydroxyl acetone was most possibly formed from the breakage of C Ar-C α. Furfural and furan were likely formed from the pyrolysis of the cellulose and hemicellulose components survived even after the lignin preparations. As shown in Table 3, although lignin contains inherently aromatic structures, total yields of AHs were very low, just 1.8 wt%, 2.1 wt% and 1.4 wt% for ENL, ORL and KL, respectively. The yield of AHs from ORL was the highest. In detail, yields of alkyl-benzene changed in order of ORL>ENL>KL. Benzene, toluene and styrene were three main AHs. Yield of benzene from ENL was the highest while that from KL was the lowest. Yields of toluene were almost independent of the lignin samples. Yield of styrene showed a big difference among the samples. KL had a lower yield of styrene than ENL and ORL. 3.4 Correlation of products and lignin structure. As characterized by FT-IR and 13 C-NMR, three lignins exhibited very different structure, and the carbon distribution changed in order of: Aromaticity: ORL <ENL <KL Aliphatic carbon: Aliphatic C-C: ORL >ENL >KL Aliphatic C-O-: ORL >ENL >KL Methoxy carbon: ORL >ENL >KL Carboxyl carbon: ORL >ENL >KL. Comparison of carbon distribution (Table 2) with the carbon selectivity (Fig. 3) of the decomposition products would offer additional insights on the impact of original chemical structure of lignin on the primary pyrolysis

992 J. Jpn. Inst. Energy, Vol. 93, No. 10, 2014 Fig. 5 Correlation of pyrolysis products and lignin chemical structure Fig. 6 An example pathway from lignols to products during lignin pyrolysis characteristics. As shown in Fig. 5(a), the total carbon selectivity of char and tar increased with the lignin aromaticity. The formation of tar and char was affected by various factors, such as hydrogen content, and oxygen functional groups (such as methoxyl substituent and C β-o-c, C α-o-c) 16) 19) 20). High aromaticity indicated a stable chemical structure such as KL, and preferred to produce char rather than tar. But ENL showed higher aromaticity than ORL, and produced less char and more tar than ORL. The possible reasons were: (1) ORL contained more methoxyl substituents than ENL. Methoxyl homolysis causes the radical induced rearrangement to form a key intermediate (o-quinonethide) for coking 20). (2) As Fig. 6 shows, connection substituents are easy to break and provide radicals to polymerize and form char. ORL contained more connection substituents than ENL. (3) Hydrogen consumes radicals and terminates radical reactions as shown in Fig. 6, which could suppress charring reactions and enhance the formation of tar. Table 1 showed ENL had higher hydrogen content than ORL, which led to produce more tar from ENL than from ORL. At the same time, Curie-point pyrolyzer-gc/ms results showed that much more alkyl-phenols were obtained from ENL, while ORL had higher content of non-alkyl phenols. NMR showed that ORL contained more aliphatic carbon oxygen bond. As shown in Fig. 6, radical (*) was formed after the cleavage of aliphatic C-O-. The low bond dissociation energy of radical ( =422 kj /mol, =39 kj/mol) 1) made it easy to get non-alkyl phenols as well as light fragments such as C 3. Carbon selectivity of LHs and NPOCs increased with aliphatic carbon content as shown in Fig. 5(b). Side chains have significant effects on the pyrolysis behavior and light volatile products of lignin. As Fig. 5(c) shows, carbon selectivity of AHs was very low for the three lignins and decreased with increased aromaticity. Removal oxygen from aromatic carbon to form AHs was difficult at 650 because of the high bond dissociation energy as below (kj/ mol) 1) :

J. Jpn. Inst. Energy, Vol. 93, No. 10, 2014 993 Comparison of the carbon selectivity of AHs with LHs and NPOCs is shown in Fig. 5(d). The carbon selectivity of AHs increased with the increased carbon selectivity of LHs and NPOCs. It indicated that AHs were produced through the recombination of light volatile compounds rather than the originally occuring aromatic structure of lignin during lignin pyrolysis. As mentioned above, six olefins, five diolefins and four alkynes were quantified from lignin pyrolysis. These compounds were identified as important intermediates in the secondary reaction of lignin pyrolysis. The recombination and aromatization of olefins, diolefins and alkynes such as R(1)- R(6) have been investigated by pyrolysis of light hydrocarbons 21) and cellulose 4). It could be thus considered that these reactions would contibute to the formation of AHs in the fast pyrolysis of lignin even with short residence time of volatile in gas phase. and CH4, six olefins, five diolefins and four alkynes were also quantified, including propadiene, 1,3-butadiene, 1-butene- 3-yne, cyclopentadiene, 1,4-pentadiene, and 1-hexen-3-yne. (3) High aromaticity of lignin preferred the formation of char and tar. The presence of methoxyl group and aliphatic substituents enhanced the char formation by providing the active fragments with high tendency of polymerization, while the presence of hydrogen in lignin was likely to suppress the char formation by stabilizing the active fragments and promote the tar formation. (4) The yields of LHs and NPOCs were strongly influenced by aliphatic substituents. Carbon selectivity of AHs reduced with increased aromaticity of the original lignin and increased with increased carbon selectivity of LHs and NPOCs. LHs and NPOCs played key roles in the formation of AHs. AHs were possibly formed from the recombination of olefins, diolefins, and alkynes rather than natural aromatic structure. Acknowledgement This study was in part financially supported by Grant in-aid for Young Scientist (A) (Grant Number; 23686112) and MOST-JST, Strategic International Collaborative Research Program, SICORP. Huamei Yang is grateful to the China Scholarship Council (File Number: 201206420006). 4. Conclusions Experimental study was conducted to analyze quantitatively the pyrolysis products of differently prepared lignins and investigated the effects of chemical structure on the products distribution of lignin fast pyrolysis at 650. The following conclusions were drawn from the present research. (1) The solid state 13 C-NMR results indicated that ORL and ENL had more methoxyl group and aliphatic C-C and C-O substituents than KL, while KL showed more condensed structure with high aromaticity and less substitution. (2) KL and ORL were found to yield more char than ENL, while ENL produced more tar. Totally fiftyone volatile compounds were quantified and compared to examine the correlations of products and lignin structure. Light compounds, including IGs, LHs, and NPOCs were accounted about 40-47 wt% of lignin. Besides CO, CO 2, H 2O, References 1)Hu, J.; Shen, D. K.; Xiao, R.; Wu, S. L.; Zhang, H. Y., Energ Fuel, 27, 285-293 (2013) 2)Cho, J. M.; Chu, S.; Dauenhauer, P. J.; Huber, G. W., Green Chem, 14, 428-439 (2012) 3)White, J. E.; Catallo, W. J.; Legendre, B. L., J Anal Appl Pyrol, 91, 1-33 (2011) 4)Norinaga, K.; Shoji, T.; Kudo, S.; Hayashi, J., Fuel, 103, 141-150 (2013) 5)Zhang, L. H.; Xu, C. B.; Champagne, P., Energ Convers Manage, 51, 969-982 (2010) 6)Bai, X. L.; Johnston, P.; Brown, R. C., J Anal Appl Pyrol, 99, 130-136 (2013) 7)Shaik, S. M.; Koh, C. Y.; Sharratt, P. N.; Tan, R. B. H., Thermochimica Acta, 566, 1-9 (2013) 8)Amen-Chen, C.; Pakdel, H.; Roy, C., Bioresource Technol, 79, 277-299 (2001) 9)Shen, D. K.; Gu, S.; Luo, K. H.; Wang, S. R.; Fang, M. X., Bioresource Technol, 101, 6136-6146 (2010) 10)Chu, S.; Subrahmanyam, A. V.; Huber, G. W., Green Chem, 15, 125-136 (2013) 11)Beste, A.; Buchanan, A. C., J PHYS CHEM A, 116, 12242-12248 (2012) 12)Dorrestijn, E.; Mulder, P., Journal of the Chemical Society-

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