Py-GC/MS study of lignin pyrolysis and effect of catalysts on product distribution

: Fast pyrolysis is one of the most promising methods to convert lignin into fuels and chemicals. In the present study, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was used to evaluate vapor phase product distribution of lignin fast pyrolysis. During the non-catalytic pyrolysis process, lignin was pyrolyzed at 400 ° C, 500 ° C and 600 ° C respectively, finding that the highest yield of aromatic hydrocarbons was obtained at 600 ° C. Catalytic pyrolysis experiments were also conducted to investigate the effects of catalyst pore structure and acidity on the product distributions. Five different catalysts (HZSM-5, MCM-41, TiO 2 , ZrO 2 and Mg(Al)O) were applied to lignin catalytic pyrolysis, and the catalytic performance was estimated by analyzing the pyrolytic products. The catalysts were characterized by using X-ray diffraction (XRD), BET, and NH 3 (CO 2 ) temperature programmed desorption. Based on these characterizations, discussion was carried out to explain the formation of the produc distributions. Among the five catalysts, HZSM-5 exhibited the best performance on the formation of aromatic hydrocarbons. Citation: Si Z, Wang C G, Bi K, Zhang X H, Yu C L, Dong R J, et al. Py-GC/MS study of lignin pyrolysis and effect of catalysts on product distribution. Int J Agric & Biol Eng, 2017; 10(5): 214–225.


Introduction
Due to the depletion of fossil resources and concerns over carbon emission, lignocellulosic biomass has hemicelluloses and lignin) is a relatively simple thermochemical process which is usually conducted at a temperature of 400°C-600°C in the absence of oxygen [1] , and has the advantages of high conversion efficiency and environmentally friendly [2] .
One of the primary advantages of fast pyrolysis is that solid biomass can be directly converted to liquid fuels, i.e. pyrolysis oil.
However, high oxygen content (20%-40%) of pyrolysis oil leads to undesirable properties such as low energy density, high viscosity and corrosion, thermal and chemical instability. Thus, catalytic upgrading was used to improve the qualities of pyrolysis oil. Catalytic pyrolysis was proved to be an effective way to improve the bio-oil quality. Aho et al. [3] carried out catalytic pyrolysis of pine wood with Hβ, HY, HZSM-5 and Mordenite, finding that acids and alcohols were decreased in the presence of HZSM-5. Zhang et al. [4] studied the effect of HZSM-5 on fast pyrolysis of corncob in a fluidized bed, proposing that the existence of catalyst could decrease 25% oxygen content of pyrolysis oil, while increase the yields of gas, water and coke. On account of the complexities of pyrolysis process and productions, some researchers turned to investigate the catalytic pyrolysis of individual component of lignocellulosic biomass to further understand the pyrolysis mechanisms. Carlson et al. [5] tested several catalysts including ZSM-5, silicalite, β, Y-zeolite and silica-alumina on the pyrolysis of cellulose and obtained the highest yield of aromatics in the presence of ZSM-5.
Karanjkar et al. [6] obtained a similar aromatic yield of 39.5% C from catalytic pyrolysis of cellulose using ZSM-5 as catalyst. Zhu et al. [7] investigated the effect of HZSM-5 and M/HZSM-5 (M = Fe, Zn) on xylan pyrolysis, revealing that catalytic pyrolysis reduced oxygenates yield and increased hydrocarbons yield.
Kim et al. [8] presented that mesoporous Y zeolite with larger quantity and stronger acidity could also reduce the oxygenates content and increase the aromatics to a larger extent from xylan pyrolysis. Guo et al. [9] found that USY catalyst performed the best effect on deoxygenation than HZSM-5 and Hβ in xylan pyrolysis.
A representative lignin structure showing three primary units is illustrated in Figure 1 [10] . The difference with cellulose and hemicellulose, lignin is a cross-linked amorphous biopolymer of three primary units including guaiacol (G), syringyl (S) and p-hydroxyphenol (H) units which are bonded with C-O-C and C-C bonds [11] . Additionally, the content of each unit is varied with plant types. For example, conifer wood (softwood) contains 90%-95% G unit, 0-1% S unit and 0.5%-3.4% H unit [12] . The aromatic rings existed in the primary units are conducive to the immediate conversion of lignin to aromatic products [13] .
However, the wide distribution of lignin pyrolysis productions caused by the complex structure [14,15] results in receiving relatively less attention compared to cellulose and hemicellulose. Hence, the understanding of pyrolysis process and the effect of catalyst is essential.
Thring et al. [16] investigated the effect of HZSM-5 in the catalytic pyrolysis of lignin in a fixed bed reactor and obtained the highest toluene yield (44 wt.%) at 650°C.
The deactivation of zeolite catalyst during lignin pyrolysis was likely caused by simple phenols produced from the deconstruction of the lignin polymer [17] . The role of shape selectivity in lignin pyrolysis was studied by Yu et al. [18] , suggesting that ZSM-5 produced the highest aromatic yield, while β zeolite was suited to convert bulky oxygenates. Adhikari et al. [19]  under He flow and subsequent exposed to a 40% CO 2 /He stream for 60 min. Flushing with He at 80°C for 3 h was applied to remove the adsorbed CO 2 , and TPD analysis was carried out from 80°C to 600°C at a heating rate of 10°C/min. Institute of Standards and Technology (NIST) mass spectral library. Since the chromatographic peak area of a compound is considered linear with its quantity, a semi-quantitative estimation was used in this study that the yield and content of compounds could be revealed by peak area and peak area%, respectively.

Characterization of lignin
Lignin structure could directly affect pyrolysis product distribution, so that FT-IR was firstly used to investigate the bond structure of lignin. The FT-IR spectrum result is shown in Figure 2 and the corresponding assignments of bonds are given in Table 1 according to previous researches. The broad band around 3415 cm -1 was caused by O-H stretching vibration.
The bond at 2930 cm -1 and 1454 cm -1 were assigned respectively to C-H stretching vibration and C-H bending vibration attributable to the methyl and methylene groups.
The peak at 1130 cm -1 had lower intensity than that at 1270 cm -1 indicating lower syringyl (S) unit content.
Moreover, strong signals at 1515 cm -1 and 1032 cm -1 also indicated a predominance of guaiacol (G) units in this lignin, consisting with the C-H out-of-plane deformation bands at 854 cm -1 and 817 cm -1 typical of G rings.
FT-IR results suggested that the lignin used here was mainly composed of G type rings.

Characterization of catalysts
Since the crystal structure of catalysts played a role on the catalytic pyrolysis process of lignin [14] , XRD patterns of five catalysts used in this study are shown in consists of two channels of 10-membered rings and zigzag channels [22] . The pattern of TiO 2 exhibited strong peaks at 2θ=24° and 48° corresponding to the anatase phase with tetragonal system [23] . The peaks were in good agreement with the standard spectrum (JCPDS NO.:  cations were well dispersed in the structure of MgO without formation of spinel species [25] . The XRD pattern of MCM-41 in Figure 3b exhibited a characteristic intense (100) peak at 2θ=2.25° and two higher order (110) and (200)     The porosity of the five catalysts is illustrated in Figure 6, and the surface area and total pore volume are listed in Table 2. Among the five catalysts, only HZSM-5 showed a dominant peak at 0.43 nm and another peak at 0.58 nm, indicating a microporous structure.
MCM-41 showed a pore size distribution mainly centered at 3.04 nm corresponding to mesoporous structure.
MCM-41 had the largest BET surface area and total pore volume among the five zeolite catalysts (1067.66 m 2 /g for surface area and 1.03 m 3 /g for total pore volume). Both

Effect of temperature on lignin pyrolysis
The products of lignin fast pyrolysis included The product distribution for non-catalytic pyrolysis of lignin at 400°C, 500°C and 600°C is shown in Table 3 and Figure 7. It is found that the total peak area increased significantly from 3.66×10 9 at 400°C to 1.68×10 10 at 500°C and then slightly decreased to  [30] .

Production distribution from catalytic pyrolysis of lignin
To investigate the performances of catalysts on the product distribution of lignin pyrolysis, catalytic pyrolysis of lignin with zeolites (HZSM-5, MCM-41) and metal oxides (TiO 2 , ZrO 2 , Mg(Al)O) were conducted.
Since the aromatic hydrocarbons were produced at 600°C, the catalytic pyrolysis was conducted at the same temperature. Table 4 and Figure 8 show the product distribution from catalytic pyrolysis of lignin with HZSM-5 and MCM-41. In the first column of the Table   4, non-catalytic pyrolysis is listed for comparison of the catalysts effect on the product distribution of the volatile liquid fraction. It is found that the total amount was both decreased with the two zeolite catalysts. In the presence of HZSM-5 catalysts, the amount of alkoxyls decreased from 6.13×10 9 to 5.35×10 9 , and aromatic hydrocarbons significantly increased from 2.53×10 7 to 2.24×10 8 . Previous studies showed that HZSM-5 zeolite contained Bronsted acid sites that were favorable for the formation of aromatic hydrocarbons [31][32][33] . Li et al. [34] also obtained similar results and demonstrated that as the acidity of HZSM-5 increased, the yields of aromatic hydrocarbons increased. In addition to acidity, pore structure could also affect the catalytic process significantly due to shape selectivity [35] . However, most oxygenates derived from lignin pyrolysis had a larger dimension than the pore size of HZSM-5, resulting that oxygenates could only be converted at the external surface, in which only existed a small fraction of acid sites [18] . This might be the reason for the relatively low yield of aromatic hydrocarbons with which could not be detected in our system.  Due to the much lower surface area, ZrO 2 could lead to high yield of oxygenats and be less effective on deoxygenation than TiO 2 [37] . This result was similar to Kaewpengkrow et al. [38] who studied the effect of catalyst resulted in an obvious reduction in the yield of volatile liquid fraction. The peak area of alkoxyls sharply decreased from 6.13×10 9 to 1.34×10 9 , and that of polyaromatics also decreased from 6.12×10 9 to 9.65×10 8 .
Auta et al. [39] also obtained the similar results with MgO catalyst that the yields of liquid decreased while the yields of gas and char increased. Wang et al. [40] also investigated the catalytic pyrolysis with base catalyst (CaO). The residue yield was much higher than the non-catalytic run at 600°C, which meant to the lower yield of liquid products.