Catalytic pyrolysis of lignin over HZSM-5 catalysts: Effect of various parameters on the production of aromatic hydrocarbon

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Catalytic pyrolysis of lignin over HZSM-5 catalysts: Effect of various parameters on the production of aromatic hydrocarbon Jae-Young Kim a , Jae Hoon Lee a , Jeesu Park a , Jeong Kwon Kim b , Donghwan An c , In Kyu Song b , Joon Weon Choi d,∗ a Department of Forest Sciences and Research Institute for Agriculture and Life Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-921, South Korea b School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-921, South Korea c Department of agricultural economics and rural development, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-921, South Korea d Graduate School of International Agricultural Technology and Institute of Green-Bio Science and Technology, Seoul National University, Pyeongchang, Gangwon-do 232-916, South Korea

a r t i c l e

i n f o

Article history: Received 9 March 2015 Received in revised form 15 June 2015 Accepted 15 June 2015 Available online xxx Keywords: Lignin Catalytic pyrolysis HZSM-5 Si/Al Aromatic hydrocarbon Lignin model compounds

a b s t r a c t The purpose of this study was to investigate (1) the effect of various parameters on catalytic pyrolysis of lignin over HZSM-5 catalysts and (2) the origin of aromatic hydrocarbon (AH) from lignin pyrolysis products. Lignin polymer extracted from poplar wood was pyrolyzed using pyrolysis-GC/MS with HZSM-5 catalysts composed of different Si/Al mole ratios (30–280). Benzene, toluene, ethylbenzene, and p-xylene were produced as the main AH species, and other phenolics as well as fatty acids (FA) were also detected as lignin pyrolysis products. FA was not common lignin pyrolysis products, which might be derived from suberin moieties accumulated together with recovered lignin during lignin separation process. The acidity of HZSM-5 was inversely proportional to Si/Al mole ratio and the yield of AH proportionally increased with increasing catalyst acidity. To trace the origin of AH from lignin pyrolysis products, several lignin model compounds were pyrolyzed with HZSM-5 (Si/Al:30). These results revealed that FA showed the largest contribution to AH production, followed by 4-allylsyringol, coniferaldehyde, and acetoguaiacone. With respect to pyrolysis temperature, AH yield gradually increased with increasing pyrolysis temperature in our investigated range (500–700 ◦ C). In addition, AH yield also increased up to 35.7 mg/g of lignin at a high catalyst to lignin ratio (2:1). Other lignin sources obtained from an industrial pulping/bioethanol plant were also pyrolyzed with HZSM-5 (Si/Al:30) and these results were comparatively studied. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Due to depletion of fossil fuel and increases in energy demand and costs, lignocellulosic biomass has received great attention as an alternative energy resource, because it could be converted to liquid fuel and chemicals via biological/thermochemical conversion processes. Lignin, a lignocellulosic biomass constituent (10–30 wt%), has been gradually generated as a byproduct in bioethanol production and the pulp/paper industries. A prior study suggested that lignin could be potentially used as a carbon source for production of epoxides, adhesives, polymers, phenolic resins, and polyolefins [1]. However, its effective utilization to high value-added products is limited, because of its structural complexity. In fact, it has been

∗ Corresponding author. Fax: +82 33 339 5830. E-mail address: [email protected] (J.W. Choi).

estimated that less than 2% of lignin byproducts from the paper industry are used for specialty products [2]. Pyrolysis is a thermochemical conversion technology, which gives high yields of bio-oil of up to 75 wt% and it can be used directly in a variety of applications [3]. Lignin pyrolysis has been proposed and developed for the efficient utilization of lignin byproduct as a liquid fuel or value-added chemical resource. However, due to the structural complexity of lignin, various kinds of phenols were produced during lignin pyrolysis, resulting in a wide compositional distribution of bio-oil. In addition, some process problems such as feeder blocking, bed agglomeration and subsequent defluidization are still challenging [4]. Recent developments in the field of lignin pyrolysis have led to a renewed interest in application of various catalysts (e.g., inorganic metal, transition metal, noble metal, and zeolite) to avoid these drawbacks [5–8]. Among various catalysts, HZSM-5, one of the representative zeolites, has been known as excellent deoxygenation catalyst for production of low oxygen

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containing bio-oil and value-added aromatic hydrocarbons (AHs), such as benzene, toluene, and p-xylene [9,10]. Therefore, several studies have focused on the catalytic pyrolysis of lignin using HZSM-5 catalyst using pyrolysis-GC/MS. Yu et al. [11] studied the role of shape selectivity of zeolite catalysts during catalytic pyrolysis of lignin. Ma et al. [12] determined the influence of catalyst framework, acidity, and pore size on the distribution of lignin pyrolysis products. Furthermore, several studies investigated the effect of various parameters (sample preparation method, pyrolysis temperature, catalyst type, catalyst amount, and lignin source) on the composition of AH using HZSM-5 catalyst [13–16]. However, very few studies focused on the effect of Si/Al mole ratio of HZSM-5 have been performed. Ben and Ragauskas [17] reported that the influence of Si/Al mole ratio of HZSM-5 on the physicochemical properties of lignin pyrolysis oil using 13 C NMR, 31 P NMR, and 2D-HSQC NMR technique, but GC/MS information was not provided. In addition, there is a lack of research dealing with lignin model compounds in catalytic pyrolysis. It is very important to investigate the origin of AH from lignin pyrolysis products for fundamental development of the field of lignin pyrolysis. Several studies have investigated conversion mechanism of lignin model compounds such as phenol, guaiacol, and eugenol over HZSM-5 [18–20], but there is still insufficient data on this topic. In this study, milled wood lignin (ML), one of the representative native lignin sources, extracted from poplar wood was pyrolyzed using pyroprobe pyrolysis-GC/MS with different Si/Al mole ratios of HZSM-5 (30–280). The effect of other parameters such as pyrolysis temperature, catalyst amount, and lignin sources (Organosolv lignin: OL, Asian lignin: AL, and Inbicon lignin: IL) on the composition of AH was also studied. Furthermore, the origin of AH from lignin pyrolysis products was additionally investigated using several lignin model compounds. 2. Materials and methods 2.1. Feedstock Milled wood lignin (ML) was extracted from poplar wood (Populus alba × Prosopis glandulosa) according to the Bjorkman method and the yield of lignin was 5.5 wt% based on the dry weight of poplar wood xylem [21,22]. Organosolv lignin (OL) was also prepared from poplar wood using a 1:1 w/w water-ethanol mixture with 5 wt% NaOH at 160 ◦ C for 4 h [23]. Asian lignin (AL) generated from the soda pulping process of wheat straw and also sarkanda grass which was purchased from Granit Research and Development SA, and used without further purification. Inbicon lignin (IL) produced from a wheat straw ethanol plant and was provided in pellet form by Inbicon A/S, DONG Energy (Denmark). It was ground using a cutting mill pulverisette (FRITSCH, Germany) to less than 0.50 mm and used without further purification. 2.2. Catalyst preparation Commercially available zeolite (ZSM-5 (X), X is Si/Al mole ratio = 30, 50, 80, 100, and 280) was obtained from Zeolyst International. In order to convert as purchased NH3 -form into H-form (HZSM-5), all catalysts were calcined at 550 ◦ C for 5 h under an air atmosphere. 2.3. Catalyst characteristics Acid properties of HZSM-5 (X) (X is Si/Al mole ratio = 30, 50, 80, 100, and 280) catalysts were determined by NH3 -TPD measurements. Each catalyst (50 mg) was charged into a quartz reactor of the TPD apparatus (BEL Japan, BELCAT B). Each catalyst was prepared at 200 ◦ C for 3 h with a stream of helium

Table 1 Physical characteristics of HZSM-5 catalysts. HZSM-5 (X) (X = Si/Al mole ratio)

Acidity (mmol/g of cat.)

SBET (m2 /g)

HZSM-5 (30) HZSM-5 (50) HZSM-5 (80) HZSM-5 (100) HZSM-5 (280)

0.129 0.112 0.074 0.037 0.024

227.4 311.6 308.2 297.6 328.1

(50 mL/min) to remove the any physisorbed organic molecules. Ammonia (50 mL/min) was then introduced into the reactor at 50 ◦ C for 30 min to saturate the acid sites of the catalyst. Physisorbed ammonia was removed at 150 ◦ C for 1 h under a flow of helium (50 mL/min). After cooling the sample, the furnace temperature was increased from 50 to 650 ◦ C at a heating rate of 5 ◦ C/min under a flow of helium (30 mL/min). The desorbed ammonia was detected using a thermal conductivity detector (TCD). The Brunauer–Emmett–Teller (BET) surface areas were measured with an ASAP 2010 analyzer (Micrometrics). The catalyst characteristics are given in Table 1. 2.4. Catalytic pyrolysis In this study, a coil-type CDS Pyroprobe 5000 (CDS Analytical Inc., Oxford, PA, USA) was used for catalytic pyrolysis. Sample preparation was as follows: 1.00 ± 0.05 mg of ML was introduced to a quartz tube with different catalyst: lignin ratio of HZSM-5 catalyst (0.6:1, 1:1, 1.4:1, and 2:1) via a layer method [15]. The yield of char was calculated gravimetrically. In the model compound study, 0.10 mg of authentic compound was pyrolyzed with 1.0 mg of HZSM-5 (30) at 600 ◦ C. For quantitative analysis, 2 ␮L internal standard (IS; 1.3 mg of fluoranthene/mL of methanol) was also introduced to the prepared sample. The sample was pyrolyzed at a desirable temperature (500, 550, 600, 650, and 700 ◦ C) at 10 ◦ C/ms and maintained for 20 s in an inert atmosphere (>99.9% He). During pyrolysis, the pyrolyzer interface was maintained at 250 ◦ C to prevent recondensation between lignin pyrolysis products. Other lignin sources (OL, AL, and IL) were also pyrolyzed at 600 ◦ C for 20 s with 1.0 mg of HZSM-5 (30) for comparative study. Blank pyrolysis (catalyst without lignin) was carried out to detect any contaminant in pyroprobe system, but no peaks could be observed in GC chromatogram. 2.5. GC–MS/FID (gas chromatography–mass spectroscopy/flame ionization detector) The released volatile products were on-line transferred to a GC–MS/FID (Agilent Technologies 7890A/Agilent Technologies 5975A) equipped with an DB-5 capillary column (30 m × 0.25 mm ID × 0.25 ␮m film thickness) with a split ratio of 1:100. In this system, the separated products were split equally (1:1) to (1) FID and (2) MS system. The gas chromatograph injector and detector temperatures were set at 250 ◦ C and 300 ◦ C, respectively. The oven temperature program began at 50 ◦ C for 5 min, followed by a heating rate of 3 ◦ C/min to 280 ◦ C, and then maintained for 10 min. The separated compounds were introduced into the ionization source of a quadrupole MS and the ionization was achieved with electron impact (EI) mode at 70 eV. The mass spectrometer was scanned from m/z = 50 to m/z = 550. The MS source temperature and MS quad temperature was 230 ◦ C and 150 ◦ C, respectively. Identification of each compound was based on the NIST MS Search 2.0 (NIST/EPA/NIH Mass Spectral Library; NIST 02) and proper reference [24]. For quantitative analysis, 23 types of authentic compounds (benzene, toluene, ethylbenzene, p-xylene, phenol,

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J.-Y. Kim et al. / Journal of Analytical and Applied Pyrolysis xxx (2015) xxx–xxx Table 2 List of identified lignin pyrolysis products by GC/MS analysis. Peak no.

Compounds

RTa

Type

RFb

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Benzene Toluene Ethylbenzene p-Xylene m-Xylene o-Ethyltoluene Phenol 1,2,3-TrimethyIbenzene Indene o-Cresol p-Cresol Guaiacol 2,5-Dimethylphenol Naphthalene Creosol 3-Methoxycatechol 4-Ethylguaiacol 4-Methylcatechol 2-Methylnaphthalene 4-Vinylguaiacol Syringol Vanillin cis-Isoeugenol 1,5-Dimethylnaphthalene 2,7-Dimethylnaphthalene 4-Methylsyringol trans-Isoeugenol Acetoguaiacone 4-Vinylsyringol cis-4-propenylsyringol 4-Propylsyringol trans-4-propenylsyringol Syringylaldehyde 4-Allylsyringol Acetosyringone Coniferaldehyde Syringyl acetone Propiosyringone 2-Methylphenanthrene n-Hexadecanoic acid Sinapaldehyde Octadecanoic acid

4.2 6.5 10.1 10.6 11.6 15.2 15.9 16.7 19.3 19.6 20.8 21.4 24.5 26.4 26.5 29.6 30.5 30.9 31.5 32.1 33.8 35.9 36.2 36.3 36.4 37.8 38.0 39.5 42.5 43.8 44.1 45.7 46.2 47.7 48.8 48.9 49.9 51.9 54.9 56.7 57.1 62.9

AH AH AH AH AH AH H/C6P AH AH H/AP H/AP G/C6P G/AP AH G/AP S/C6P G/AP G/AP AH G/AP S/C6P G/OP G/AP AH AH S/AP G/AP G/OP S/AP S/AP S/AP S/AP S/OP S/AP S/OP G/OP S/OP S/OP AH FA S/OP FA

1.70 1.50 1.60 1.50 1.50 1.46 1.74 1.56 1.51 1.64 1.64 1.50 1.64 1.51 1.49 2.23 1.20 3.31 1.51 1.02 2.36 0.92 4.51 1.51 1.51 1.47 4.51 1.00 1.80 1.47 1.47 1.47 2.75 1.47 2.75 2.51 2.75 2.75 1.51 1.00 0.97 1.00

a b

RT means retention time. RF means response factor.

1,2,3-trimethylbenzene, indene, p-cresol, guaiacol, naphthalene, creosol (4-methylguaiacol), 3-methoxycatechol, 4-ethylguaiacol, 4-methylcatechol, 4-vinylguaiacol, syringol, vanillin, isoeugenol, 4methylsyringol, 4-vinylsyringol, syringaldehyde, coniferaldehyde, and sinapaldehyde) purchased from Sigma–Aldrich were injected into the GC-FID. The response factor (RF) between each authentic compound and the IS was determined on the basis of GC-FID information. The RF of commercially unavailable compounds was assumed to be 1.0 or was estimated based on structural similarity to authentic compounds (Table 2). The yield of each compound was calculated by following equation:



The yield of compound mg/goflignin = RF ×



Compound peakarea (FID ) IS amount (␮g) × IS peak area (FID ) Lignin (mg)

3. Results and discussion 3.1. Catalyst characteristics The NH3 -TPD analysis was used to investigate acid properties of HZSM-5 catalysts and the resulting patterns are presented in Fig. S1. On the basis of the NH3 -TPD result, the acid properties of the

3

catalysts were determined (Table 1). As confirmed in Table 1, the number of acid sites of HZSM-5 gradually decreased with increasing Si/Al mole ratio (0.129 mmol/g of catalyst for HZSM-5 (30) and 0.024 mmol/g of catalyst for HZSM-5 (280)). This finding is in agreement with the finding of Benito et al. [25] who showed the number of acid sites of HZSM-5 was inversely proportional to Si/Al mole ratio. Meanwhile, previous study [26] reported that the BET surface areas of zeolites increased with increasing catalyst acidity but the BET surface areas of the catalysts used in this study did not show notable trends (308.2–328.1 m2 /g of catalyst) except for HZSM-5 (30) (227.4 m2 /g of catalyst). 3.2. Catalytic pyrolysis 3.2.1. Classification of lignin pyrolysis products Catalytic pyrolysis of lignin was performed using a coil-type pyroprobe and the released volatile products were on-line transferred to a GC/MS instrument. A total of 42 pyrolysis products were identified and are listed in Table 2. To better understand the effects of parameters on the distribution of lignin pyrolysis products, GCdetectable compounds were classified into several categories based on their structural characteristics (Fig. 1). Firstly, lignin pyrolysis products were divided to hydroxyphenyl (H), guaiacyl (G), and syringyl (S) types. These groups were additionally classified to three categories (C6P: having no side chain at C4, AP: having a saturated or unsaturated aliphatic side chain at C4, and OP: having an oxygenated functional group in side chain) on the basis of their side chain type (Fig. 1). Several kinds of aromatic hydrocarbon(AH) such as benzene, toluene, xylene, and naphthalene were produced from catalytic pyrolysis of lignin. Interestingly, some fatty acid compounds (FA) including n-hexadecanoic acid and 9-octadecenoic acid were also detected as pyrolysis products of lignin. Blank test (pyrolysis without lignin) was carried out, but no peak could be observed, meaning that FA compounds were one of the pyrolysis products of lignin. These FA compounds were common in suberin of biomass which presents aliphatic domains composed of branched polyester moieties along with aromatic structure [27]. During lignin separation process, these FA compounds might have precipitated also, resulting in accumulation into recovered lignin [28]. Previous studies also observed FA compounds as one of lignin derived products in thermochemical conversion of lignin (pyrolysis and catalytic depolymerization) [29–31], so catalytic conversion of FA compounds were also investigated together with other phenolics in Section 3.2.2.1. 3.2.2. Effect of the Si/Al mole ratio on AH production The yield of classified lignin pyrolysis products as a function of the Si/Al mole ratio is described in Fig. 2. As shown in Fig. 2, the yield of lignin pyrolysis products (GC detectable) increased upon applying HZSM-5 due to enhanced catalytic cracking. In addition, it gradually increased up to Si/Al mole ratio of 80 and maintained similar level after. This was because HZSM-5 having higher Si/Al mole ratio favors degrading aromatic CC bonds as well as preventing recondensation between lignin pyrolysis products [17]. Aho et al. [32] also found that zeolite having higher Si/Al mole ratio produced more organic compounds, which supported our results. The yield of classified lignin pyrolysis products as a function of catalyst acidity is presented in Fig. 3 to better understand the effect of catalyst acidity on the distribution of volatiles. Previous studies reported that the capability of HZSM-5 for converting lignin pyrolysis products to AH increases with catalyst acidity [11,33]. Also, in this study (Fig. 3 (a)), the yield of AH proportionally increased from 0.76 wt% (Si/Al mole ratio of 280) to 2.62 wt% (Si/Al mole ratio of 30) with increasing catalyst acidity (R2 = 0.9851). The largest yield of AH could be obtained with HZSM-5 (30) despite of its lowest surface area, meaning that catalyst acidity was main factor in pro-

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Fig. 1. Classification lignin pyrolysis products on the basis of their structures.

duction of AH. In addition to AH, other lignin pyrolysis products showed increasing or decreasing trends with increasing catalyst acidity (Fig. 3). In particular, as shown in Fig. 3(b) and (c), the yields of G/AP, G/OP, S/AP, and S/OP compounds for HZSM-5 (280) showed 29.5, 41.9, 20.5, and 30.2% decreased values, respectively, as compared with those for HZSM-5 (30). From these results, it could be assumed that AH was mostly derived by catalytic conversion of G/AP, G/OP, S/AP, and S/OP compounds via side chain cracking coupled with deoxygenation over HZSM-5 catalyst. Meanwhile, the yield of char as a function of catalyst acidity ranged from 79.5 to 89.5 wt% with no specific trends (Fig. S2). 3.2.2.1. Lignin model compounds study. Fig. 3 indicates that AH was possibly produced via catalytic conversion of lignin pyrolysis products. In particular, the yield of G/AP, G/OP, S/AP, and S/OP compounds showed significant difference between catalytic (HZSM-5

(30)) and non-catalytic condition, so we compared them in-depth. Table 3 shows the increasing or decreasing ratio of main G/AP, G/OP, S/AP, and S/OP compounds with HZSM-5 (30) versus no catalyst condition. 4-methylsyringol, cis/trans-4-propenylsyringol, 4-allylsyringol, and acetosyringone showed increasing trends at HZSM-5 (30) as compared with no catalyst. This meant that these compounds were released from lignin and/or produced from other lignin pyrolysis products via secondary conversion under HZSM5 (30). As shown in Table 3, 4-methylcatechol showed a major decreasing trend, followed by coniferaldehyde and acetoguaiacone, except for FA. To prove the contribution of lignin pyrolysis products to the production of AH, several model compounds (0.1 mg) were also pyrolyzed at 600 ◦ C with 1.0 mg of HZSM-5 (30) and this result is given in Table 4. The yield of AH produced from pyrolysis of model compounds ranged from 6.6 to 27.8 wt%, which meant that almost all of the model compound remained in its original form or was

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acid considerably yielded AH over HZSM-5 (30) (27.8 and 18.3 wt%, respectively).

Fig. 2. The yield of classified lignin pyrolysis products as a function of Si/Al mole ratio of HZSM.

Table 3 Increase of decrease ratio of main lignin pyrolysis products at HZSM-5 (30) versus no catalyst condition. Lignin pyrolysis products

Type

Increase or decrease ratio at HZSM-5 (30) (%) vs. No cat. condition

Creosol 4-Methylcatechol 4-Vinylguaiacol trans-Isoeugenol Vanillin Acetoguaiacone Coniferaldehyde 4-Methylsyringol 4-Vinylsyringol cis-4-propenylsyringol trans-4-propenylsyringol 4-Allylsyringol Syringylaldehyde Acetosyringone Sinapaldehyde n-Hexadecanoic acid Octadecanoic acid

G/AP G/AP G/AP G/AP G/OP G/OP G/OP S/AP S/AP S/AP S/AP S/AP S/OP S/OP S/OP FA FA

−21.1 −58.2 −10.3 −15.1 −28.2 −40.2 −44.1 +20.9 −8.0 +4.4 +18.1 +12.5 −0.5 +13.9 −23.3 −100.0 −100.0

transformed to coke [19]. In the model compound study, both 4methylcatechol and coniferaldehyde demonstrated major decrease trends within the HZSM-5 (30) condition (Table 3) by producing 15.4 wt% of AH, which was second to 4-allylsyringol (21.7 wt%) in AH production except for FA (18.3 wt% and 27.8 wt%). In contrast, acetoguaiacone showed a relatively low AH production (6.7 wt%) despite its significant decreasing ratio within HZSM-5 condition (−40.2%) (Table 3). This result could be explained by acetoguaiacone being decomposed to guaiacol via deoxygenation followed by side chain cleavage over HZSM-5 (30) rather than contributing to AH production. Indeed, in GC/MS analysis, the peak of guaiacol was not detected during pyrolysis of acetoguaiacone (without catalyst), but this peak (guaiacol) could be observed under HZSM-5 (30) condition (Fig. S3). Interestingly, 4-methylsyringol and 4-allylsyringol also contributed to production of AH (10.4 and 21.7 wt%, respectively) even though these showed increasing trends within HZSM-5 (30) condition (Table 3). Meanwhile, FAs such as n-hexadecanoic acid and octadecanoic acid perfectly disappeared with HZSM-5 (30) catalyst (Table 3). Previous study proposed that these long chain FAs could be thermally degraded to olefins, which subsequently are converted to AH via oligomerization, cyclization, and aromatization over zeolite catalyst [34]. The result of the current study supported previous research, because n-hexadecanoic acid and octadecanoic

3.2.3. Effect of the temperature on AH production To investigate the effect of pyrolysis temperature on the production of AH, ML was pyrolyzed at a temperature range between 500 and 700 ◦ C (50 ◦ C interval) with 1.0 mg of HZSM-5 (30) (Fig. 4). The yield of total volatiles increased from 125.1 mg/g of lignin for 500 ◦ C up to 149.0 mg/g of lignin for 600 ◦ C, and then gradually decreased at higher temperatures due to further decomposition of volatiles to non-condensable gaseous products. The yield of AH also gradually increased up to 700 ◦ C (29.5 mg/g of lignin). The finding of the current study is consistent with that of Wang et al. [14] who showed that the yield of AH continuously increased up to 800 ◦ C. This trend could be explained by the thermal degradation of lignin pyrolysis products to low molecular compounds (e.g., H2 , C1 –C3 compounds, or olefin), which was enhanced at high temperature, leading to increasing conversion possibility of low molecular compounds to AH [35]. Another possible explanation for this is that the pore size of HZSM-5 increases with temperature, sufficiently allowing the penetration of lignin pyrolysis products [36]. Similarly, Ma et al. [12] suggested that large molecules could possibly enter the pore, and then are further converted to AH, because the energetic barrier for large molecules to diffuse into the pores would be decreased at high temperature. However, some researchers have indicated that lignin pyrolysis products can be converted to AH only at the external surface of zeolite catalyst, because almost all lignin derived phenols are too large to pass through the inter pores of HZSM-5 [37,38]. Therefore, it is still not well understood how lignin pyrolysis products are converted to AH within HZSM-5 catalyst framework. The yield of char showed gradual decrease trend until 600 ◦ C and increased after 600 ◦ C. The reason is unclear but it might be due to enhanced coking reactions at high temperature region (Fig. S2). 3.2.4. Effect of catalyst amount on the production of AH The effect of catalyst amount on the production of AH was investigated with varying HZSM-5 (30) amounts at 600 ◦ C and the result is presented in Fig. 5. With increasing catalyst, the contact possibility between lignin pyrolysis products and the acid site of catalyst would be increased. As a result, the yield of classified lignin pyrolysis products, except for AH, showed declining trends with increasing catalyst amount due to improvement of catalytic conversion of volatiles to AH or non-condensable gaseous products (Fig. 5). Consequently, the yield of AH consistently increased from 21.5 (0.6:1 catalyst:lignin ratio) to 35.7 mg/g of lignin (2:1 catalyst:lignin ratio), which was in agreement with previous studies [10,15,39,40]. Meanwhile, as expected, the yield of char increased gradually because coking reaction was also prompted with increasing catalyst amount (Fig S2). 3.3. Production of AH from various lignin sources In addition to ML, three different technical lignins (OL, AL, and IL) were pyrolyzed at 600 ◦ C with 1.0 mg of HZSM-5 (30) (Fig. 6). The resultant pyrolysis products are listed in Table S1. Due to the different parent structures, their pyrolysis products showed different distributions. The chemical composition, as well as structural characteristics of the differing lignins, was investigated in prior studies [22,41,42]. Briefly, AL and OL were mostly composed of lignin moieties (94.9 and 93.5 wt%, respectively), whereas IL contained other components such as carbohydrate (10.5 wt%), ash (12.1 wt%), and unknown (8.1 wt%). AL and IL generated from herbaceous biomass, whereas ML and OL was extracted from hardwood; so the compositional ratio between pyrolysis products varied significantly depending on the lignin source.

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Table 4 The yield of AH produced from pyrolysis of model compounds with HZSM-5 (30). Model compounds

Creosol 4-Methylcatechol 4-Vinylguaiacol Vanillin Acetoguaiacone Coniferaldehyde 4-Methylsyringol 4-Allylsyringol Syringylaldehyde Sinapaldehyde n-Hexadecanoic acid Octadecanoic acid a b

Type

G/AP G/AP G/AP G/OP G/OP G/OP S/AP S/AP S/OP S/OP FA FA

Yield (wt%)a

Benzene

Toluene

Ethylbenzene

Xyleneb

Other AH

Sum

1.2 1.4 0.6 0.9 0.4 1.2 0.8 2.1 0.8 0.5 3.5 2.4

3.5 4.2 1.6 2.8 1.3 2.9 2.4 5.0 2.4 1.9 8.5 6.0

0.2 0.5 0.1 0.4 0.1 0.2 0.1 0.3 0.5 0.1 1.7 0.8

5.0 4.4 2.6 4.4 2.7 4.3 3.8 7.3 3.3 4.8 9.8 7.0

3.1 4.8 1.7 2.1 2.1 6.8 3.2 7.0 2.0 2.1 4.3 2.1

13.0 15.4 6.6 10.7 6.7 15.4 10.4 21.7 9.0 9.3 27.8 18.3

Calculated on the basis of the weight of model compound. Sum of p-Xylene and m-Xylene.

As shown in Fig. 6, the yield of total lignin pyrolysis products was the highest in ML, followed by OL, AL, and IL. ML and OL showed similar S- and G-type product ratios, but the yield of AH was higher in OL (31.7 mg/g of lignin) than ML (26.2 mg/g of lignin). AL yielded a relatively high H- and G-type products ratio than ML, because it was produced from the pulping process of herbaceous crops (e.g. wheat straw and sarkanda grass). The yield of AH from AL was 29.4 mg/g of lignin, which was lower compared with the result of another study

(ca. 60.0 mg/g of lignin) [13]. This difference might result from different pyrolysis conditions (e.g. temperature, sample preparation method, and catalyst type). Meanwhile, IL produced the smallest lignin pyrolysis products, because it contained relatively higher other components than other lignins. Nevertheless, IL produced similar levels of AH (27.6 mg/g of lignin) in comparison with others. This indicated that other IL components, especially carbohydrate, contributed to the production of AH over HZSM-5 catalyst. Indeed,

Fig. 3. The yield of classified lignin pyrolysis products as a function of catalyst acidity.

Please cite this article in press as: J.-Y. Kim, et al., Catalytic pyrolysis of lignin over HZSM-5 catalysts: Effect of various parameters on the production of aromatic hydrocarbon, J. Anal. Appl. Pyrol. (2015), http://dx.doi.org/10.1016/j.jaap.2015.06.007

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7

posed upon application of HZSM-5. These long chain FAs were potentially converted to AH as mentioned above [34]. 4. Conclusions

Fig. 4. The yield of classified lignin pyrolysis products as a function of pyrolysis temperature.

Representative native lignin (ML) was pyrolyzed with HZSM-5 catalysts composed of different Si/Al mole ratios (30–280) for production of aromatic hydrocarbon (AH). The yield of AH increased from 0.76 wt% (Si/Al mole ratio of 280) to 2.62 wt% (Si/Al mole ratio of 30) in proportion with increasing catalyst acidity. Other process parameters including high temperature, and high catalyst amount had positive effects on increasing AH yield. In this study, a maximum 35.7 mg AH per g of lignin could be obtained from ML upon applying 2:1 catalyst:lignin ratio of HZSM-5 (30) at 600 ◦ C. Among lignin samples pyrolysis products, FAs (n-hexadecanoic acid, and octadecanoic acid) significantly contributed to AH production, whereas other phenolics such as 4-vinylguaiacol, acetoguaiacone, 4-methylsyringol, and syringaldehyde showed relatively lower contribution. In addition to ML, OL, AL, and IL were also pyrolyzed with HZSM-5 (30), resulting in AH production (27.6–31.7 mg/g of lignin), and the other pyrolysis products were significantly varied depending on the lignin source. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation (NRF), funded by the Ministry of Education, Science and Technology (Project No. 2013R1A2A2A01011112), Republic of Korea. We also thank to Inbicon A/S, DONG Energy (Denmark) for providing lignin feedstock (Inbicon lignin). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jaap.2015.06.007

Fig. 5. The yield of classified lignin pyrolysis products as a function of catalyst amount.

Fig. 6. The yield of classified lignin pyrolysis products with different lignin sources.

it is well-known that pyrolysis products of IL derived from carbohydrate (e.g., furfuryl alcohol and 3-methyl-1,2-cyclopentanedione) could be easily converted to AH [43]. In addition, several long chain FAs (e.g., n-hexadecanoic acid, 9-octadecenoic acid, methyl ester, 9octadecenoic acid, octadecanoic acid, and 1-nonadecene) were also detected in the pyrolysis of IL, and almost of them were decom-

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