Fast Pyrolysis of Cellulose, Hemicellulose, and Lignin: Effect of

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Fast Pyrolysis of Cellulose, Hemicellulose, and Lignin: Effect of Operating Temperature on Bio-oil Yield and Composition and Insights into the Intrinsic Pyrolysis Chemistry Khursheed B. Ansari,†,∥ Jyotsna S. Arora,† Jia Wei Chew,† Paul J. Dauenhauer,‡ and Samir H. Mushrif*,§ †

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459 Department of Chemical Engineering and Materials Science, University of Minnesota, Amundson Hall, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States § Department of Chemical and Materials Engineering, University of Alberta, 9211-116, Street NW, Edmonton, Alberta T6G 1H9, Canada

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‡

S Supporting Information *

ABSTRACT: Fast pyrolysis of biomass produces bio-oil as a dominant product. However, the yield and composition of bio-oil are governed by numerous pyrolysis reactions which are difficult to understand because of the multiphase decomposition phenomena with convoluted chemistry and transport effects at millisecond time scales. In this work, thin-film pyrolysis experiments of biopolymers present in the biomass (i.e., cellulose (∼50 μm), hemicellulose (using xylan as a model biopolymer, ∼12 μm), and lignin (∼10 μm)) were performed over 200−550 °C, to investigate underlying thermal decomposition reactions, based on the product distribution obtained under reaction-controlled operating conditions. Experimental yields of noncondensable gases, bio-oil, and char at different operating temperatures and in the absence of transport limitations were obtained for each biopolymer. Cellulose- and xylan-derived bio-oil comprised of anhydrosugars, furans, and light oxygenates, in addition to pyrans in cellulosic bio-oil and phenols in xylan-derived bio-oil. Lignin pyrolysis bio-oil contained methoxyphenols, phenolic aldehydes/ketones, low-molecular-weight phenols, and light oxygenates. With an increase in the operating temperature, the anhydrosugars, furans (especially HMF and furfural), and pyrans of cellulosic and xylan bio-oils showed further degradation to form light oxygenates and furanic compounds. In the case of lignin, monolignols, initially formed at lower temperatures, further reacted to form low-molecular-weight phenols and light oxygenates with an increase in the operating temperature. In addition, based on the change in bio-oil yield and composition with temperatures, a reaction network/map was proposed for designing the molecular simulation studies of pyrolysis chemistry and developing detailed and accurate kinetics necessary for the bottom-up design of a pyrolysis reactor.

1. INTRODUCTION

wt %), hemicellulose (25−35 wt %), and lignin (16−33 wt %).3 Fast pyrolysis thermally decomposes these biopolymers and converts them into non-condensable gases, condensable

The global rise in energy demand and liquid fuels, combined with increasing levels of environmental pollution, is driving humankind to search for alternative resources. Alongside fossil fuels, lignocellulosic biomass is the only abundant and low-cost resource for carbon-based materials. Biomass can be converted into renewable fuels and chemicals using a thermochemical conversion route such as fast pyrolysis.1,2 The majority of biomass comprises three biopolymers, viz., cellulose (40−50 © XXXX American Chemical Society

Special Issue: Biorenewable Energy and Chemicals Received: Revised: Accepted: Published: A

February 16, 2019 May 8, 2019 June 3, 2019 June 3, 2019 DOI: 10.1021/acs.iecr.9b00920 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research bio-oils, and carbonaceous solid materials.4−6 Bio-oils are the most desired product from fast pyrolysis for their ability to substitute for existing fossil fuels and as a chemical feedstock suitable for upgrading to liquid hydrocarbon fuels.7−9 1.1. Bio-oil Yield and Composition: Challenges and Opportunity. The overall yield and composition of bio-oil are affected by the design and operating temperature of the reactor, as well as the composition and structure of the biomass particles. The operating temperature that gives the maximum yield of bio-oil is between 400 and 600 °C, depending on the characteristics of the biomass feedstock.10,11 In addition to the yield, the composition of bio-oil is crucial, since it governs biooil stability and quality and determines its suitability for upgrading to a hydrocarbon fuel. Bio-oil derived from lignocellulosic biomass fast pyrolysis typically consists of anhydrosugars, pyrans, furans, light oxygenates, hydrocarbons, and phenolic compounds.6,12,13 The percentage of each of these compounds in the bio-oil, and thus its composition, can be correlated to the individual contributions from cellulose, hemicellulose, and lignin contents of the biomass, which undergo multiple pyrolysis reactions (such as depolymerization, dehydration, reformation, repolymerization, etc.) within the operating temperature range.14,15 However, it is difficult to understand fundamental pyrolysis reaction chemistry due to the multiphase nature of the thermal decomposition of biomass occurring at short time scales with inter-related reaction chemistry and transport limitations. Thus, the systematic design and operation of a pyrolysis reactor to obtain the desired composition of bio-oil remain a challenge and require a systematic study of biomass thermal decomposition chemistry. The understanding of biomass pyrolysis chemistry has progressed from a global decomposition reaction model to component-specific reaction mechanisms (especially forming bio-oil compounds),16−23 where many reaction mechanisms/ pathways have been proposed using kinetic studies,19,20,24,25 isotope studies,26,27 and first-principles calculations.28,29 Despite numerous reports, detailed knowledge of the intrinsic chemistry of biomass decomposition (which determines the bio-oil yield and composition) for varying pyrolysis operating conditions (i.e., operating temperature) remains to be determined. In addition, the millisecond decomposition of biomass makes it difficult to study in a conventional pyrolysis system (i.e., thermogravimetric analysis, or TGA) and comparable analytical instruments. Alternative pyrolysis systems such as CDS Pyroprobe,30 Frontier Micropyrolyzer,31−33 wire-mesh reactor,34,35 and PHASR (Pulsed-Heated Analysis of Solid Reactions) reactor13 heat biomass samples much faster (heating rate ∼200−12000 °C/s) than the conventional method (TGA, heating rate ∼3−4 °C/s).13 Recently, Dauenhauer and co-workers used micrometer thick (≤10 μm) films of cellulose and performed fast pyrolysis experiments in both a micropyrolyzer and a PHASR reactor. These experimental systems enabled sufficiently high heat and mass transport rates to yield isothermal and reaction-controlled pyrolysis conditions. Fundamental knowledge of the intrinsic reaction chemistry of biopolymer pyrolysis can be obtained from the product distribution of reaction-controlled pyrolysis.18,33 The literature investigating biomass pyrolysis chemistry (based on the pyrolysis product distribution at different temperatures) shows that cellulose thin-film pyrolysis at 450 °C produces anhydrosugar, pyrans, and furans via glycosidic

bond cleavage and dehydration reactions.36 It is also shown that cellulose can form furans and small oxygenates directly rather than through intermediates such as glucose.18 A thinfilm of α-cyclodextrin (as a surrogate for cellulose) pyrolyzed in a PHASR reactor showed two distinct kinetic regimes, viz., end-chain and mid-chain decomposition pathways, for temperatures below and above 467 °C, respectively. End-chain scission exhibited glycosidic bond cleavage (or primary reaction), while mid-chain scission showed secondary decomposition reactions producing furans, pyrans, and ring fragmentation compounds.20 In an another study, a cellulose thin-film (∼85 μm) pyrolyzed at different pressures (0.004−1 bar) underwent dehydration reactions, forming cellobiosan and oligosaccharides as products, which further fragmented to result in secondary decomposition products (i.e., linear oxygenates).37 Lignin thin-film (size ∼115 μm) pyrolysis at different pressures (0.004−1 bar) showed a reduced yield of char (by 13 wt %) and an increased production of heavy sugars (i.e., cellobiosan and cellotriosan).38 Co-pyrolysis of 13Clabeled glucose with levoglucosan under reaction-controlled operating conditions showed that glucose undergoes a retroaldol reaction with fructose (as an intermediate) to form glyceraldehyde and grob fragmentation after tautomerization to form acetol.27 Glucose thin-film (size ≤10 μm) pyrolyzed at different temperatures decomposed via dehydration reactions, leading to anhydrosugars, furans, and pyrans formation, followed by ring-opening/closing and fragmentation reactions to result in light oxygenates.39 Biomass undergoes primary and secondary decomposition reactions within the operating temperature range (i.e., 300− 500 °C). Primary reactions convert cellulose into its monomer and/or oligosaccharides, while secondary reactions and/or vapor-phase reactions form furans, light oxygenates, and noncondensable gases. Thus, most of the studies investigating pyrolysis chemistry used cellulose or its surrogates (i.e., cellotetraose, cellohexaose, cyclodextrin, and methylcellobiose) as model compounds.20,21,23 Further, it is shown that the decomposition chemistry of a biopolymer (i.e., cellulose) is likely to be different than that of its own monomer (i.e., glucose).40 However, unlike cellulose, the decomposition chemistries of hemicellulose and xylan are yet to be fully discovered. Also, the thermal decomposition of hemicellulose and lignin using a thin-film pyrolysis approach has been rarely studied experimentally. Since biomass also contains a significant percentage of hemicellulose (25−35 wt %) and lignin (16−33 wt %),3 understanding hemicellulose and lignin thermal decomposition reactions is equally important. Fast pyrolysis of cellulose, hemicellulose (or xylan), and lignin under reaction-controlled operating conditions (or thinfilm pyrolysis) enables their thermal decomposition without transport limitations, and thus the transport effect on the pyrolysis reactions would be unlikely. This is because of the lower time scale of transport (i.e., heat transfer and diffusion) compared to the time scale of pyrolysis reactions.18 Hence, fast pyrolysis can provide an understanding of their intrinsic pyrolysis chemistry. 1.2. Scope of This Work. In this work, we performed thinfilm pyrolysis experiments on cellulose, hemicellulose (using xylan as a model compound), and lignin at different temperatures to thermally decompose them in the reactioncontrolled regime in order to systematically evaluate the effect of operating temperature on pyrolysis product distribution. B

DOI: 10.1021/acs.iecr.9b00920 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

micropyrolyzer was 3−5 orders of magnitude faster than traditional heating rates in pyrolysis techniques.18 Xylan (or hemicellulose) was the least thermally stable compound and thus tended to decompose quickly compared to cellulose and lignin.41−43 Hence, xylan was pyrolyzed between 200 and 500 °C, followed by cellulose (300−500 °C) and lignin (350−550 °C). Identification and quantification of pyrolysis volatile products were conducted using a gas chromatograph (Agilent, model 7890B)/mass spectrometer (model 5977B MSD) connected in-line with the pyrolyzer. The detection of condensable pyrolysis volatile compounds and non-condensable gases was done using Agilent J&W DB-5 and Agilent J&W HP-PLOT-Q GC columns, respectively, with a maximum operating temperature of 320 °C and having the same dimensions (i.e., 30 m × 320 μm × 1.5 μm, length × internal diameter × film). Initially, the oven temperature was set to 35 °C, and then a ramp of 3.5 °C/min was provided to reach a final oven temperature up to 250 °C during analysis. A sample split time (∼2 min) was also set in the GC program to separate non-condensable gases and condensable volatile products (especially forming bio-oil) into two separate columns (i.e., HP-PLOT-Q for non-condensable gases and J&W DB-5 for condensable volatile compounds) in order to achieve their analysis simultaneously. The details of the thin-film pyrolysis experimental procedure and product characterization are reported elsewhere.39 The yields of cellulose/xylan/lignin-derived bio-oil and noncondensable gases were obtained by summing the yields of condensable pyrolysis products and the yields of carbon dioxide/carbon monoxide, respectively. Further, the quantification of individual pyrolysis products (forming bio-oil and non-condensable gases) was performed using calibration of the standards with average error in linear relations of sample weight versus GC area as 0.9819, 0.9907, and 0.9879 for cellulose, xylan, and lignin samples, respectively. Cellulose, xylan, and lignin thin-film pyrolysis experiments were conducted in triplicate, and the average values (product yields, % weight basis) are shown in section 3 with experimental error. The standard deviation in individual product yields ranged between 1% and 4%.

Pyrolysis experiments were performed in a micropyrolyzer, and the products were analyzed with an inline gas chromatography/mass spectrometry (GC/MS) system. The effect of pyrolysis temperature on the overall product yield and on the yields of individual bio-oil compounds were investigated for cellulose, xylan, and lignin samples, individually. In addition, based on the pyrolysis product distributions obtained from thin-film experiments, the decomposition reactions of cellulose, xylan, and lignin at different temperatures were characterized, and detailed reaction maps were proposed.

2. EXPERIMENTAL METHODOLOGY 2.1. Materials and Biopolymers Thin-Film Preparation. Cellulose (product no.: 435236), xylan (product no.: X0078.14), and lignin (product no.: 370959) were purchased from Sigma-Aldrich, Singapore. The lignin used in the present work was isolated using the Kraft process and then purified to 99.5%. The structure of lignin was confirmed using FTIR analysis. Cellulose thin-films were prepared by taking 1.0% (weight basis) of cellulose in deionized (DI) water. Cellulose did not dissolve in DI water, which resulted in a suspension. 25 μL of 1.0 wt % cellulose suspension was transferred into the pyrolysis crucible. The water was removed using roomtemperature evacuation, leaving behind a micrometer-sized film of cellulose.18,33,39 A similar procedure was repeated for preparing xylan (to mimic hemicellulose) and lignin thin-films. However, unlike cellulose, 1.0 wt % of hemicellulose and lignin completely dissolved in the DI water, resulting in a clear solution. The thickness of the cellulose, xylan, and lignin films was measured using a digital microscope (Leica, model DVM6) as shown in Figure 1. Image analysis showed that

3. RESULTS AND DISCUSSION A summary of identified products in cellulose, xylan, and lignin pyrolysis is provided in Table T1 in the Supporting Information. In section 3.1, we discuss the overall product yield (viz, percentages of non-condensable gases, bio-oil, and char) and the individual yields of bio-oil components (viz/, anhydrosugars, pyrans, furans, light oxygenates, and phenolic compounds) obtained from cellulose, xylan, and lignin thinfilm pyrolysis. The effect of operating temperature on the yields of individual bio-oil components, non-condensable gases, and char is described in sections 3.2, 3.3, and 3.4, respectively. Based on the thin-film pyrolysis experimental data, insights into the thermal decomposition reactions of cellulose, xylan, and lignin are drawn, and detailed reaction maps are proposed in sections 3.5, 3.6, and 3.7, respectively. 3.1. Thin-Film Pyrolysis of Cellulose, Xylan, and Lignin: Overall Product Distribution and Bio-oil Composition. 3.1.1. Overall Product Distribution and Biooil Composition in Cellulose Thin-Film Pyrolysis. Cellulose thin-film pyrolysis within 300−500 °C produced noncondensable gases (0.1−7 wt %), bio-oil (61−85 wt %), and char (8−33 wt %) as products (cf. Table 1). The non-

Figure 1. Digital microscope images of [A] cellulose thin-film, [B] xylan thin-film, and [C, D] lignin thin-film.

cellulose, xylan, and lignin thin-films were ∼50, 12−13, and 10−12 μm thick, respectively. The thickness of xylan and lignin films was ∼10 μm, and that of cellulose was ∼50 μm, indicating a reaction-controlled pyrolysis regime.33 2.2. Pyrolysis Experiments. Thin-films of cellulose, xylan, and lignin were pyrolyzed in a micropyrolyzer (PY-3030S, Frontier Laboratories Ltd., Japan). The weight of each biopolymer sample used for the thin-film pyrolysis experiments was 250 μg. The heating rate of the thin-films in a C


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Industrial & Engineering Chemistry Research Table 1. Overall Product Distribution of Cellulose ThinFilm Pyrolysis

Table 3. Overall Product Distribution of Xylan Thin-Film Pyrolysis

pyrolysis product distribution (wt %) temp (°C) 300 350 400 450 500

gases 0.12 2.07 5.35 6.41 7.04

± ± ± ± ±

bio-oil

0.01 0.013 0.07 0.04 0.07

61.16 69.47 76.23 82.81 84.99

± ± ± ± ±

product distribution (wt %) char

0.22 0.88 0.10 0.10 0.10

32.58 25.53 17.12 11.13 8.79

± ± ± ± ±

temp (°C) 0.48 0.29 0.20 0.90 0.12

200 250 300 350 400 500

condensable gases, albeit lower in percentage, increased with an increase in the pyrolysis temperature. The condensable volatile products of cellulose thin-film pyrolysis were grouped into bio-oil. The yield of cellulosic bio-oil increased while char yield decreased with an increase in the temperature. The total yield of pyrolysis products (viz., yields of non-condensable gases, bio-oil, and char) was measured between 94 and 100 wt %, consistent with prior experiments.33 Chemical compounds within bio-oil derived from cellulose thin-film pyrolysis can be categorized into anhydrosugars, pyrans, furans, and light oxygenates (cf. Table 2).33 At 300 °C,

pyrans

furans

light oxygenates

300 350 400 450 500

76.92 56.59 50.57 44.98 40.09

12.32 7.19 6.02 4.52 4.15

10.75 15.92 15.27 16.81 15.55

0 19.90 27.86 33.64 40.19

0.02 0.01 0.10 0.02

± ± ± ± ± ±

0.30 0.10 0.03 0.13 0.06 0.12

char 46.78 39.73 33.88 29.47 23.81 16.25

± ± ± ± ± ±

0.67 0.72 0.28 0.35 0.68 1.05

bio-oil compounds (wt %)a temp (°C)

anhydrosugars

furans

light oxygenates

phenolic compounds

200 250 300 350 400 500

27.14 28.30 27.31 22.39 13.22 5.62

58.11 41.04 37.75 36.25 29.60 27.51

1.77 22.18 29.20 38.52 55.84 66.87

12.98 8.48 5.75 2.85 1.34 0

a

Values shown in the table are average values for bio-oil compounds from triplicate experiments.

with ∼13 wt % phenol and minor light oxygenates (1.7 wt %). Beyond 200 °C, anhydrosugars, furans, and phenolic compounds in the bio-oil decreased by 22, 30, and 13 wt %, respectively, but the light oxygenates increased significantly (∼64 wt %). This indicates that anhydrosugars/furans/ phenolic compounds and/or their intermediates apparently contributed to the formation of light oxygenates via secondary decomposition reactions during xylan thin-film pyrolysis.44 Unlike cellulose, xylan thin-film pyrolysis did not produce any pyrans in bio-oil; instead, it resulted in the formation of phenolic compounds. 3.1.3. Overall Product Distribution and Bio-oil Compounds in Lignin Thin-Film Pyrolysis. Thin-film pyrolysis of lignin over 350−550 °C produced 0−4.7 wt % noncondensable gases, 27−55 wt % bio-oil, and 20−53 wt % char (cf. Table 5). Similar to xylan thin-film pyrolysis, the total

bio-oil compounds (wt %)a anhydrosugars

± ± ± ±

bio-oil 34.24 37.59 40.20 44.67 49.62 55.38

Table 4. Bio-oil Compounds of Xylan Thin-Film Pyrolysis

Table 2. Bio-oil Compounds of Cellulose Thin-Film Pyrolysis temp (°C)

gases 0 0 0.13 1.02 3.77 4.40

a

Values shown in the table are average values for bio-oil compounds from triplicate experiments.

anhydrosugars (∼77 wt %) occupied a major percentage of the bio-oil, while furans and pyrans comprised ∼10−12 wt % (of bio-oil), with negligible light oxygenate. With an increase in the temperature (from 300 to 500 °C), anhydrosugars decreased by ∼37 wt %, and the light oxygenates increased by ∼40 wt %. Pyrans also decreased (∼8 wt %), but furans did not change significantly with a similar increase in pyrolysis temperature. The trends of bio-oil compounds, especially anhydrosugars and light oxygenates, remain consistent with the literature report of cellulose pyrolysis conducted under similar operating conditions.33 3.1.2. Overall Product Distribution and Bio-oil Compounds in Xylan Thin-Film Pyrolysis. Thermal decomposition of xylan under reaction-controlled pyrolysis conditions (or thin-film pyrolysis) resulted in 0−4 wt % non-condensable gases, 34−56 wt % bio-oil, and 16−46 wt % char (cf. Table 3). The total yield of pyrolysis product was low (∼76−80 wt %) due to few unidentified pyrolysis volatile compounds. Similar to cellulose thin-film pyrolysis, non-condensable gases derived from xylan also remained low but showed an increasing trend with the temperature. Bio-oil yield also increased by ∼22 wt %, and char yield decreased by 30 wt %, over 200−500 °C (cf. Table 3). Xylan thin-film pyrolysis bio-oil was composed of anhydrosugars, furans, light oxygenates, and phenolic compounds (cf. Table 4). At 200 °C, the bio-oil composition was dominated by anhydrosugars and furans (collectively ∼85 wt %), along

Table 5. Overall Product Distribution of Lignin Thin-Film Pyrolysis product distribution (wt %) temp (°C) 350 400 450 500 550

gases 0 1.24 3.79 4.23 4.69

± ± ± ±

0.01 0.05 0.12 0.03

bio-oil 26.93 29.27 34.62 43.83 55.35

± ± ± ± ±

0.16 0.29 0.15 0.51 0.18

char 53.05 45.72 35.66 29.58 20.08

± ± ± ± ±

1.81 0.42 1.12 0.92 0.15

yields of lignin-derived pyrolysis products were up to 75−81 wt %. Non-condensable gases still remained low but increased with increasing temperature. The yield of bio-oil increased (∼29 wt %) and that of char yield decreased (∼33 wt %) during lignin decomposition over 350−550 °C. The yields of non-condensable gases, bio-oil, and char obtained in the present work agreed with the literature.38 Lignin-derived bio-oil consisted of methoxyphenols, phenolic aldehydes/ketones, low-molecular-weight phenols, and D


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Industrial & Engineering Chemistry Research light oxygenates (cf. Table 6). At 350 °C, methoxyphenols and phenolic aldehydes/ketones together occupied ∼86 wt % of

with other pyrolysis products suggested that the marginal decrease in AGF yield (1.98 wt %) was compensated by the collective increase in furfural and formic acid yields (cf. Appendix 1, part [C], in the Supporting Information). Further, the decrease in LGO yield (1.6 wt %) up to 400 °C was apparently in agreement with an increase in 5-methylfurfural yield, and beyond that the LGO yield decreased marginally (0.21 wt %) and contributed to furfural and formaldehyde formation (cf. Appendix 1, parts [D] and [E], in the Supporting Information). Unlike LGA, AGF, and LGO, the decrease in DAGP yield (1.38 wt %) over 300−500 °C could not be compared with the increase in the yields of other pyrolysis products. Nevertheless, our analysis of experimental data suggests that, with an increase in operating temperature, the anhydrosugars and/or their intermediates further decomposed (possibly via secondary reactions) and contributed to the formation of furans and light oxygenates. Pyrans. The pyrans in cellulose thin-film-derived bio-oil comprised of 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4one (DHMDHP) and 1,5-anhydro-4-deoxy-D-glycerohex-1-en3-ulose (ADGH).33 Both DHMDHP and ADGH decreased with the increase in the pyrolysis temperature, as shown in Figure 2B. Comparison of the decrease in DHMDHP yield (∼1.92 wt %) with other pyrolysis products over 300−400 °C indicated that DHMDHP apparently contributed to the collective increase in 1,2-cyclopentanedione and carbon dioxide yields, while, between 400 and 500 °C, DHMDHP decreased marginally (∼0.51 wt %) and formed 2,5-dimethylfuran (cf. Appendix 1, parts [F] and [G], in the Supporting Information). Similarly, between 300 and 400 °C, ADGH yield decreased (1.5 wt %) and contributed to 2,3-butanedione and acetic acid formation. However, at a higher temperature (around 500 °C), the decrease in ADGH yield agreed with the collective increase in the yields of 2-methylfuran and carbon dioxide (cf. Appendix 1, parts [H] and [I], in the Supporting Information). Thus, pyrans (viz. ADGH and DHMDHP) formed during cellulose thin-film pyrolysis also converted into furans, light oxygenates, and non-condensable gases with an increase in the operating temperature. Furanic Compounds. Furanic compounds in cellulosederived bio-oil comprised hydroxymethylfurfural (HMF), furfural, 2-furanmethanol, 2(5H)-furanone, 5-methylfurfural, furan, and 2,5-dimethylfuran, as shown in Figure 2C. Among furanic compounds, HMF yield was the highest at 300 °C, but it showed a decreasing trend with an increase in the pyrolysis temperature, while yields of other furans (viz., furfural, 2furanmethanol, 2(5H)-furanone, 5-methylfurfural, furan, and 2,3-dimethylfuran, ≤3 wt %) increased with an increase in the temperature. The decrease in HMF yield with temperature suggested its further decomposition into other furanic compounds and non-condensable gases.22,47 In particular, the decrease in HMF yield between 300 and 400 °C agreed well with the collective increase in 2-furanmethanol and carbon monoxide yields (cf. Appendix 1, part [J] in the Supporting Information). However, beyond 400 °C, the decrease in the HMF yield contributed to 5-methylfurfural formation (cf. Appendix 1, part [K] in the Supporting Information). Unlike HMF, the yield of furfural continued to increase over 300−500 °C, possibly because of the conversion of anhydrosugars (viz., AGF and LGO) or its intermediates into furfural (via secondary pyrolysis reactions), as discussed earlier. Light Oxygenates. The light oxygenates of cellulosic bio-oil comprised of methyl glyoxal (MG), glycolaldehyde (GA),

Table 6. Bio-oil Compounds of Lignin Thin-Film Pyrolysis bio-oil compounds (wt %)a temp (°C)

methoxyphenols

phenolic aldehydes/ketones

low mol wt phenols

light oxygenates

350 400 450 500 550

30.90 27.72 27.85 24.69 20.23

55.40 47.94 39.37 32.45 25.10

13.16 23.02 30.01 35.57 45.17

0.53 1.33 2.77 7.30 9.5

a

Values shown in table are average values for bio-oil compounds from triplicate experiments.

bio-oil, followed by low-molecular-weight phenols (13 wt %) and light oxygenates (0.5 wt %). With an increase in the temperature, methoxyphenols and phenolic aldehydes/ketones decreased by ∼10 and 30 wt %, respectively, and the percentage of low-molecular-weight phenols (viz., phenol, cresol, 2-methylphenol, and 2,3-dimethylphenol) increased significantly (∼32 wt %) in the bio-oil from lignin pyrolysis. Interestingly, light oxygenates, despite an increasing trend with temperature, remained low in percentage. This indicates that the lignin-derived monophenols (viz. methoxyphenols and phenolic aldehyde/ketones) possibly converted into lowmolecular-weight phenols (viz., phenol, cresol, and methyl phenol) via multiple pyrolysis reactions45,46 instead of forming light oxygenates. The results for lignin-derived bio-oil compounds differed from previous literature due to the different operating conditions of the pyrolysis (i.e., the thickness of lignin samples used in the present work was much smaller than that used in the literature).38 3.2. Effect of Temperature on Individual Bio-oil Components, Non-condensable Gases, and Char in Cellulose Pyrolysis. 3.2.1. Effect of Temperature on the Yield of Bio-oil Compounds. Anhydrosugars. Anhydrosugars in cellulose thin-film-derived bio-oil comprised of levoglucosan (LGA), levoglucosenone (LGO), dianhydroglucopyranose (DAGP), and 1,6-anhydroglucofuranose (AGF), as shown in Figure 2A. All anhydrosugars showed a decreasing trend over 300−500 °C. The reduction in the yields of LGA, LGO, DAGP, and AGF was 7.9, 1.83, 1.38, and 1.98 wt %, respectively. The reduction in the experimentally observed yields of anhydrosugars and the increase in other pyrolysis products suggested that the decrease in LGA yield apparently contributed to the light oxygenates formation (i.e., methyl glyoxal, hydroxyacetone, formic acid, and glycolaldehyde) during cellulose thin-film pyrolysis. In particular, the decrease in LGA yield (by 3.8 wt %) between 300 and 400 °C agreed with the collective increase in methyl glyoxal and hydroxyacetone yields, while, at a higher temperature (i.e., 400−500 °C), LGA further reduced (by ∼4.1 wt %) and apparently converted into methyl glyoxal, formic acid, and glycolaldehyde (cf. Appendix 1, parts [A] and [B], in the Supporting Information). LGA is known to be thermally unstable in the presence of other pyrolysis volatile compounds and/or intermediates and can undergo further decomposition (via secondary pyrolysis reactions) to form other anhydrosugars, pyrans, and light oxygenates.19 Mechanistic details of LGA conversion into the aforementioned light oxygenates are described in section 3.5. A similar comparison of AGF yield E


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Figure 2. Yields of individual bio-oil compounds, non-condensable gases, and char at different operating temperatures during cellulose thin-film pyrolysis. Nomenclature shown in the figure for pyrolysis products: DAGP, dianhydroglucopyranose; AGF, 1,6-anhydroglucofuranose; LGA, levoglucosan; LGO, levoglucosenone; ADGH, 1,5-anhydro-4-deoxy-D-glycerohex-1-en-3-ulose; DHMDHP, 2,3-dihydro-3,5-dihydroxy-6-methyl4H-pyran-4-one; CPHM, 2-hydroxy-3-methyl-2-cyclopenten-1-one; and HMF, hydroxymethylfurfural.

major light oxygenates such as MG, HAA, GA, and FA were apparently formed due to the secondary reactions of anhydrosugars (viz., LGA and AGF) during cellulose thinfilm pyrolysis. Mechanistic insights into the formation of minor light oxygenates such as glyoxal, acetaldehyde, and CPHM with temperature could not be obtained because of their low yields. The percentage of light oxygenates in the cellulosic biooil at 500 °C became high, indicating the possibility of more secondary reactions at higher temperatures. 3.2.2. Effect of Temperature on the Yield of Noncondensable Gases and Char. Cellulose thin-film pyrolysis produced carbon monoxide (CO) and carbon dioxide (CO2) as non-condensable gases. Both CO and CO2 showed an increasing trend in their yields with an increase in the operating temperature, as can be seen in Figure 2F. The

formic acid (FA), hydroxyacetone (HAA), 2,3-butanedione (BD), glyoxal, formaldehyde, acetaldehyde, acetic acid, 1,2cyclopentanedione (CPD), and 2-hydroxy-3-methyl-2-cyclopenten-1-one (CPMH), as shown in Figure 2D,E. Among the light oxygenates, the percentages of MG, GA, and FA were higher, and hence they are referred to as the major light oxygenates, while HAA, BD, glyoxal, formaldehyde, acetaldehyde, acetic acid, CPD, and CPMH were lower in percentage (≤1.5%) and hence are designated as minor light oxygenates. All light oxygenates showed an increasing trend with an increase in the temperature and were apparently formed because of the secondary decomposition reactions of anhydrosugar/pyrans/furanic compounds and/or their intermediates, as described earlier.22,39 Comparison between the experimental yields of light oxygenates and anhydrosugars suggests that the F


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Figure 3. Yields of individual bio-oil compounds, non-condensable gases, and char at different operating temperatures during xylan thin-film pyrolysis.

formation of CO and CO2 can be correlated with both primary and secondary decomposition reactions of pyrolysis volatile compounds/intermediates during cellulose pyrolysis.19,48 Our experimental data suggest that CO apparently formed from furans, while pyrans/anhydrosugars contributed to CO2 formation (as discussed in Appendix 1, parts [F], [I], and [J], in the Supporting Information). The literature also shows only CO and CO2 as non-condensable gases during the pyrolysis of cellulose under similar operating conditions, and no formation of CxHy gases was observed.18,33 The yield of char obtained during cellulose thin-film pyrolysis was the highest at low temperature (∼300 °C), but with an increase in temperature, the char yield decreased (cf. Figure 2F). Char yield mainly competes with the yield of bio-

oil during fast pyrolysis, and since bio-oil yield increased with temperature, char yield was reduced.33,39 3.3. Effect of Temperature on the Yield of Individual Bio-oil Components, Non-condensable Gases, and Char in Xylan Pyrolysis. 3.3.1. Effect of Temperature on the Yield of Bio-oil Compounds. Anhydrosugars. Thin-film pyrolysis of xylan produced LGA and 3,4-altrosan as anhydrosugars in the bio-oil. The yield of LGA remained the highest at 200 °C, while no 3,4-altrosan was observed at that temperature, as shown in Figure 3A. The comparison of experimental yields of xylan-derived anhydrosugars with those of other pyrolysis products suggested that the decrease in LGA yield (∼3.01 wt %) from 200 to 300 °C apparently agreed with an increase in 3,4-altrosan yield, while, beyond 300 °C, LGA further decreased (3.5 wt %) and apparently converted into 1G


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Figure 4. Yields of individual bio-oil compounds, non-condensable gases, and char at different operating temperatures during lignin thin-film pyrolysis.

oxygenates and non-condensable gases. However, unlike cellulose, thin-film pyrolysis of xylan did not produce any pyrans (i.e., ADGH and DHMDHP) in the bio-oil, indicating that the pyrolysis reactions resulting in ADGH and DHMDHP formations were not feasible during xylan thermal decomposition.49 Furanic Compounds. The furanic compounds in xylanderived bio-oil comprised HMF, furfural, acetylfuran, 2furanmethanol, 5-methyl-2(3H)-furanone, furan, and 2methylfuran, as shown in Figure 3B,C. Among furanic compounds, HMF, furfural, acetylfuran, and 2-furanmethanol were higher in percentage and were referred to as the major furans, while 2(3H)-furanone, 5-methylfuran, and 2-methylfuran were low in percentage and hence were designated as

hydroxy-2-butanone and acetic acid (cf. Appendix 2, parts [A] and [B], in the Supporting Information). This indicated that, during xylan pyrolysis too, LGA remained thermally unstable (or reacted with its neighboring pyrolysis volatile compounds) and further decomposed into other anhydrosugars and light oxygenates. Further, the yield of 3,4-altrosan beyond 350 °C also decreased, and when compared with the yields of other products of pyrolysis, the decrease in 3,4-altrosan yield (∼4.3 wt %) apparently was in agreement with the collective increase in hydroxyacetone, acetaldehyde, and carbon dioxide (cf. Appendix 2, part [C], in the Supporting Information). This indicated that, with an increase in the operating temperature, the anhydrosugars in xylan thin-film-derived bio-oil underwent decomposition and contributed to the formation of light H


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Industrial & Engineering Chemistry Research minor furans. At 200 °C, HMF and furfural were the highest in yield; however, with an increase in the temperature, their yields decreased. Comparison of the decrease in HMF yield (∼2.2 wt %) between 200 and 300 °C with the increase in yields of other pyrolysis products suggested that HMF apparently contributed to 5-methyl-2(3H)-furanone and carbon dioxide formation, whereas, at higher temperature (between 300 and 500 °C), HMF yield decreased (∼4.4 wt %) while the yield of other furans like acetylfuran increased (cf. Appendix 2, parts [D] and [E], in the Supporting Information).The above observation suggests that the xylan-derived pyrolysis intermediates, leading to HMF formation, kinetically favor acetylfuran formation at elevated temperature.50 A similar comparison of the decrease in the furfural yield over 200−400 °C with other pyrolysis product yields indicated that furfural was likely converted into methyl glyoxal and hydroxy acetaldehyde, and beyond 400 °C, furfural yield decreased slightly (0.37 wt %) in conjunction with the increase in 5methyl-2(3H)-furanone yield (cf. Appendix 2, parts [F] and [G], of the Supporting Information). 2-Furanmethanol continued to form from xylan up to 400 °C, and beyond that its yield decreased (∼1.42 wt %), which contributed to an increase in 2-methylfuran yield (∼1.4 wt %) (cf. Appendix 2, part [H], in the Supporting Information). Thus, the major furanic compounds (such as HMF, furfural, and 2-furanmethanol) experienced secondary decomposition reactions at elevated temperatures (∼500 °C) and resulted in the formation of other furanic compounds, light oxygenates, and non-condensable gases. With an increase in the pyrolysis temperature, HMF and furfural can further decompose and contribute to the formation of other furans and light oxygenates.22 More insights into HMF and furfural conversions into other furanic compounds and/or light oxygenates are presented in section 3.6. Among minor furans, 5-methyl2(3H)-furanone increased up to 300 °C and then decreased, while furan and 2-methylfuran showed an increasing trend with an increase in the pyrolysis temperature over 200−500 °C. Light Oxygenates. Light oxygenates of xylan-derived bio-oil were categorized into major light oxygenates (viz., 3methylcyclopentane-1,2-dione, MG, HAA, 2-cyclohexanol, and 1-hydroxy-2-butanone) and minor light oxygenates (viz., formaldehyde, acetaldehyde, 2,3-butanedione, 1,2-cyclopentanedione, 5-ethyl-1,3-dioxane-5-methanol, acetic acid, and acetaldehyde, hydroxy-), as shown in Figure 3D,E. Except for 3-methylcyclopentane-1,2-dione, all major light oxygenates increased with an increase in the pyrolysis temperature. The increase in MG, HAA, 2-cyclohexanol, and 1-hydroxy-2butanone yields over 250−500 °C was 6.5, 5.2, 4.6, and 3.3 wt %, respectively. 3-Methylcyclopentane-1,2-dione initially increased up to 250 °C, and then it decreased with an increase in the temperature (cf. Figure 3D). Major light oxygenates such as MG, HAA, and 1-hydroxy-2-butanone were apparently formed due to the secondary decomposition of anhydrosugars and/or furanic compounds during xylan thin-film pyrolysis, as discussed earlier. Further, 3-methylcyclopentane-1,2-dione continued to increase up to 250 °C, and beyond that it decreased (∼2.78 wt %) and apparently contributed to the 2cyclohexen-1-ol formation (cf. Appendix 2, part [I], in the Supporting Information). All minor light oxygenates showed the increasing trend with an increase in temperature, except for 5-ethyl-1,3-dioxane-5-methanol, which remained relatively stable up to 400 °C and then decreased slightly at 500 °C (cf. Figure 3E).

Phenolic Compounds. Phloroglucinol was identified as the only phenolic compound in the bio-oil of xylan thin-film pyrolysis. The yield of phloroglucinol (∼4.5 wt %) remained the highest at 200 °C, as shown in Figure 3F. However, with an increase in the operating temperature, phloroglucinol decreased, and at 500 °C, no phloroglucinol was observed. However, the decrease in phloroglucinol yield with temperature could not be compared with other components of xylan thin-film pyrolysis bio-oil. 3.3.2. Effect of Temperature on the Yield of Noncondensable Gases and Char. Xylan thin-film pyrolysis produced CO2 as the only non-condensable gas. The yield of CO2, despite being low in percentage, increased with an increase in the pyrolysis temperature, as shown in Figure 3F. Anhydrosugars and furanic compounds apparently contributed to CO2 formation during xylan thin-film pyrolysis, as discussed previously. The yield of char derived from xylan thin-film pyrolysis remained the highest at 200 °C, but it decreased with an increase in the pyrolysis temperature (cf. Figure 3F). 3.4. Effect of Temperature on the Yield of Individual Bio-oil Components, Non-condensable Gases, and Char in Lignin Pyrolysis. 3.4.1. Effect of Temperature on the Yield of Bio-oil Compounds. Methoxyphenols. Methoxyphenols in lignin thin-film pyrolysis bio-oil was composed of 4ethyl-2-methoxyphenol, 2-methoxy-4-vinylphenol, 2-(4methoxyphenyl)ethanol, 4-methoxy-3-methylphenol, 2methoxyphenol, creosol, eugenol, isoeugenol, and guaiacylacetone, as shown in Figure 4A,B. Among the methoxyphenols, 4-ethyl-2-methoxyphenol, 2-methoxy-4-vinylphenol, 4-methoxy-3-methylphenol, and 2-(4-methoxyphenyl)ethanol were higher in percentage and hence referred to as the major methoxyphenols, while 2-methoxyphenol, creosol, eugenol, isoeugenol, and guaiacylacetone were less than 1 wt % and hence designated as minor methoxyphenols. At 350 °C, the yield of 2-methoxy-4-vinylphenol was the highest; however, with an increase in the temperature (ranging from 350 to 550 °C), the 2-methoxy-4-vinylphenol yield decreased (∼1.3 wt %). Comparison of the decrease in 2-methoxy-4-vinylphenol yield with the yields of other pyrolysis products over 350−450 °C suggested that this phenol apparently converted into 4ethyl-2-methoxyphenol (cf. Appendix 3, part [A], in the Supporting Information). Beyond 450 °C, the 2-methoxy-4vinylphenol yield decreased marginally (∼0.45 wt %) and could not be compared with the increases in the yields of other products of lignin thin-film pyrolysis. The yield of 4-ethyl-2methoxyphenol continued to increase beyond 450 °C, indicating that it may also form directly from lignin at a higher temperature. Further, the yields of 4-methoxy-3methylphenol and 2-(4-methoxyphenyl)ethanol continued to increase (by 2.2 and 0.7 wt %, respectively) with increases in the temperature. During lignin pyrolysis, methoxyphenols could further convert into low-molecular-weight phenols (viz., cresol and phenol) and light oxygenates at a higher temperature (∼500 °C).51 Phenolic Aldehydes/Ketones. Phenolic aldehydes/ketones in the bio-oil of lignin pyrolysis composed of vanillin, apocynin, and 2,3-dihydrobenzofuran, as shown in Figure 4C. With an increase in the pyrolysis temperature, the yields of vanillin and apocynin decreased, while the 2,3-dihydrobenzofuran yield remained relatively stable. A comparison of the decrease in vanillin yield (∼1.6 wt %) with the yields of other lignin-derived products over 350−550 °C suggested that vanillin apparently contributed to cresol, 2-methylphenol, and I


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Figure 5. Reaction map of cellulose-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products: LGA, levoglucosan; LGO, levoglucosenone; AGF, 1,6-anhydroglucofuranose; DAGP, dianhydroglucopyranose; ADGH, 1,5-anhydro-4-deoxy-Dglycerohex-1-en-3-ulose; DHMDHP, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one; HMF, 5-hydroxymethylfurfural; CPHM, 2-hydroxy-3methyl-2-cyclopenten-1-one; CO, carbon monoxide; and CO2, carbon dioxide.

pyrolysis volatile compounds are more facile at higher temperatures. 3.4.2. Effect of Temperature on the Yield of Noncondensable Gases and Char. Similar to the xylan thin-film pyrolysis, the only non-condensable gas derived from lignin was CO2. The yield of CO2 increased with an increase in the temperature (cf. Figure 4F). The formation of CO2 during lignin thin-film pyrolysis is discussed in section 3.7. The yield of char obtained during lignin thin-film pyrolysis remained high at 350 °C, but it decreased with further increases in the pyrolysis temperature. 3.5. Reaction Map of Cellulose Thin-Film Pyrolysis. A reaction map for cellulose thin-film pyrolysis (reactioncontrolled regime)-derived products over a range of 300− 500 °C is proposed, as shown in Figure 5. Thin-film pyrolysis of cellulose at 300 °C showed the formation of anhydrosugars (viz., LGA, LGO, DAGP, and AGF), pyrans (viz., ADGH and DHMDHP), furans (HMF), and char. LGA was observed as the dominant anhydrosugar during cellulose thin-film pyrolysis. Cellulose apparently converted into LGA via a concerted mechanism (53.9 kcal/mol) rather than ionic (90.3 kcal/mol) and free-radical (94.5 kcal/mol) pathways.21,53 The concerted mechanism includes glycosidic bond cleavage (preferably via the end-scission mechanism, since the operating temperature was less than 467 °C)20 followed by a dehydration reaction.22 The LGA formed could further dehydrate (68.7 kcal/mol) into LGO.29 However, our results suggested that LGO formed directly from cellulose instead of following the LGA intermediate path, because the experimental yield of LGO did not match with the decrease in LGA yield. Also, the direct formation of LGO from cellulose (cellobiose as the model compound) via a concerted mechanism requires a slightly

carbon dioxide formation (cf. Appendix 3, parts [B] and [C], in the Supporting Information). Vanillin and/or its intermediate thermally converted into methylphenols and/or cresol.51 Further, the reduction in the yield of apocynin (∼5 wt %) between 350 and 550 °C closely matched with the increase in the yield of 2,3-dimethylphenol (cf. Appendix 3, part [D], in the Supporting Information). This indicates that phenolic aldehydes/ketones (viz., vanillin and apocynin) contribute to the formation of low-molecular-weight phenols and non-condensable gases during lignin thin-film (or reactioncontrolled) fast pyrolysis. Low-Molecular-Weight Phenols. Low-molecular-weight phenols of lignin thin-film pyrolysis comprised of phenol, cresol, 2-methylphenol, and 2,3-dimethylphenol, as shown in Figure 4D. At 350 °C, all low-molecular-weight phenols remained at less than 2 wt %; however, with an increase in the operating temperature, the yields of phenol, cresol, 2methylphenol, and 2,3-dimethylphenol increased by 6, 4, 4.8, and 6.5 wt %, respectively. All low-molecular-weight phenols were apparently formed due to the secondary pyrolysis reactions among lignin-derived methoxyphenols, phenolic aldehyde/ketones, and/or their intermediates.46 Light Oxygenates. Methanol and butyl glycol were observed as the only light oxygenates during lignin thin-film pyrolysis, as shown in Figure 4E. The yield of butyl glycol increased with an increase in the pyrolysis temperature. Methanol formation was not observed up to 450 °C, but beyond that the yield of methanol increased with temperature. The formation of light oxygenates during lignin thermal treatment is reported elsewhere, too.52 The higher yields of methanol and butyl glycol around 550 °C suggest that the secondary decomposition reactions among lignin-derived J


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Figure 6. Reaction map of xylan-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products: LGA, levoglucosan; HMF, 5-hydroxymethylfurfural; and CO2, carbon dioxide.

form 2-furanmethanol, 5-methylfurfural, and carbon monoxide. Also, between 400 and 500 °C, methylfurans (i.e., 2methylfuran and 2,5-dimethylfuran) are produced due to the secondary reactions among pyrans (i.e., ADGH and DHMDHP) of cellulose thin-film pyrolysis along with CO2. 2,5(H)-Furanone formed directly from cellulose, possibly via ring-opening, dehydration, and enol-keto tautomerization reactions, over 400−500 °C.22 Beyond 350 °C, light oxygenates were formed because of the secondary decomposition reactions (viz., ring opening and fragmentation) of anhydrosugars, pyrans, and furans during cellulose thin-film pyrolysis. Particularly, between 300 and 500 °C, light oxygenates such as MG, HAA, FA, and GL were formed via secondary decomposition reactions of LGA, as discussed earlier.57 In a similar temperature range, formic acid formed from AGF and/or its intermediate. 1,2-Cyclopentanedione, 2,3-butanedione, and acetic acid were also produced from the secondary reactions of pyrans (i.e., ADGH and DHMDHP) up to 400 °C. Glyoxal, acetaldehyde, and CPHM were formed between 400 and 500 °C. Non-condensable gases (viz., CO and CO2) were formed through various competing pathways during cellulose thin-film pyrolysis, as discussed earlier, while char was also formed from cellulose between 300 and 500 °C. Thus, in brief, thermal decomposition of cellulose (∼300 °C) under the reaction-controlled operating conditions was initiated with a concerted pathway of glyosidic bond cleavage followed by dehydration reactions. With an increase in the operating temperature, cellulose-derived anhydrosugars, furans, pyrans, and/or their intermediates apparently experienced ring-opening, cyclic grob fragmentation, decarbonylation, deoxygenation, retro-Diels−Alder, and dehydration reactions to yield other furanic compounds, light oxygenates, and noncondensable gases as the products. 3.6. Reaction Map of Xylan Thin-Film Pyrolysis. A reaction map of xylan thin-film pyrolysis products obtained between 200 and 500 °C is shown in Figure 6. At 200 °C, xylan formed furans (i.e., HMF and furfural), anhydrosugar

lower barrier (62.5 kcal/mol) than that of the LGA intermediate path (68.7 kcal/mol).29,54 At a similar temperature (300 °C), cellulose (or glucose, as a surrogate for cellulose) can convert into DAGP and AGF via dehydration reactions after overcoming rate-limiting barriers of 71 and 55 kcal/mol, respectively (Figure S1 in the Supporting Information). 55 The formation of pyrans (viz., ADGH and DHMDHP) from cellulose (or cellobiose) proceeded via a multistep mechanism of glycosylation, ring opening/closing, and dehydration (rate-limiting barrier, 59 kcal/mol) instead of a concerted mechanism (64 kcal/mol).20,56 Cellulose (or cellobiose), via a concerted mechanism of glycosidic bond cleavage and dehydration reactions, can also form furans (i.e., HMF) around 300 °C.55 The HMF formed can further degrade into other furanic compounds (viz., 2-furanmethanol and 5-methylfurfural) via decarbonylation (62 kcal/mol)22 and deoxygenation reactions. HMF can also form furfural via a deformylation reaction (62 kcal/mol);22 however, our results suggested that furfural was formed from one of the anhydrosugars (i.e., AGF) and/or their intermediates, instead of via the HMF path, during cellulose thin-film pyrolysis. With an increase in the operating temperature from 300 to 500 °C, LGA can undergo secondary decomposition reactions (i.e., retro-Diels−Alder, decarbonylation, and dehydration)57 to form light oxygenates such as MG, HAA, FA, and GL. In a similar temperature range (300−500 °C), another anhydrosugar like AGF and/or its intermediate was converted into (i) formic acid via cyclic grob fragmentation (52.5 kcal/mol) and retro-aldol (36.5 kcal/mol) reactions and (ii) furfural via ringopening (32−41 kcal/mol), cyclic grob fragmentation (52.5 kcal/mol), and dehydration (38−57.5 kcal/mol) reactions.22 We have also shown previously that the anhydrosugars can convert into furanic compounds during pyrolysis of glucose, which is a monomeric unit of cellulose.39 The cellulose-derived LGO also converted into furans (furfural and 5-methylfurfural) and light oxygenates (i.e., formaldehyde) over 300−500 °C. Major furans such as HMF undergo decarbonylation (62 kcal/mol)22 and deoxygenation reactions beyond 300 °C to K


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Figure 7. Reaction map of lignin-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products: CO2, carbon dioxide.

xylan was also converted into char over 200−500 °C, as depicted in the reaction map (cf. Figure 6). In brief, xylan, under reaction-controlled pyrolysis conditions, apparently experienced depolymerization, rearrangement, and dehydration reactions at low temperature (∼200 °C). At the same time, the xylan-derived C5 intermediate may also yield C6 species (i.e., LGA, HMF etc.). With an increase in the operating temperature, xylan-derived cyclic compounds (i.e., C5 and C6 species) underwent multiple pyrolysis reactions (viz., rearrangement, retro-Diels−Alder, fragmentation, oxidization, hydrodeoxygenation, and reduction) to yield various experimentally observed pyrolysis products. 3.7. Reaction Map of Lignin Thin-Film Pyrolysis. The thermal decomposition of lignin at different temperatures was characterized and is presented as a reaction map (cf. Figure 7). At 350 °C, lignin primarily depolymerized and dehydrated into monolignols (i.e., p-curamryl/coniferyl/sinapyl alcohols),61 which apparently became intermediates for 2-methyl-4-vinylphenol, vanillin, apocynin, eugenol, isoeugenol, 4-hydroxy-3methoxyphenyl acetone, 4-methoxy-3-methylphenyl creosol, pcresol, and 2-methyoxyphenol formation. In particular, ligninderived coniferyl alcohol (as an intermediate) was apparently converted into 2-methyl-4-vinylphenol via a deformylation reaction61,62 and into vanillin via oxidation and retro-aldol reactions.63,64 Lignin-derived coniferyl alcohol (as an intermediate) also was converted into eugenol and/or isoeugenol after cleavage of its hydroxyl group, followed by a hydrogenation reaction.62 At the same time, the eugenol-forming intermediate also underwent retro-aldol and alkylation reactions to yield creosol (or 4-methylguaiacol) as a product.65 Apocynin and 2-methoxyphenol were formed via dehydration, breaking of the ether linkage (i.e., β-O-4 linkage) within the lignin compound, and rearrangement reaction.46 Creosol (or 4methylguaiacol) was also produced directly from lignin at low temperatures (∼350 °C).

(LGA), a phenolic compound (phloroglucinol), and char. Xylan apparently depolymerized and underwent rearrangement and dehydration reactions to yield furfural as a product.50,58 The xylan-derived C5 pyrolysis intermediate needed one additional carbon atom and/or compound to yield C6 products such as HMF, levoglucosan, phloroglucinol, and 3-methylcyclopentane-1,2-dione which were observed experimentally. We suggest that the required additional carbon was provided in situ during the xylan thin-film pyrolysis to finally yield the aforementioned products. With an increase in the operating temperature from 200 to 300 °C, LGA and/or its intermediates (i.e., C6 species) could convert into 3,4-altrosan via a rearrangement reaction, while, beyond 300 °C, LGA further decomposed into 1-hydroxy-2-butanone and acetic acid via retro-Diels−Alder and fragmentation reactions.57 Between 300 and 500 °C 3,4-altrosan also underwent a fragmentation reaction and formed linear oxygenates such as hydroxyacetone, acetaldehyde, and light gases (i.e., carbon dioxide). Xylanderived HMF, over 200−300 °C, could undergo oxidization and hydrodeoxygenation reactions to yield furanone compounds and non-condensable gas (i.e., CO2).59 Similarly, between 200 and 400 °C, furfural and/or its intermediates underwent retro-aldol condensation reactions (or ring-opening and fragmentation reaction) to form linear oxygenates like methyl glyoxal and hydroxy acetaldehyde.50 In a similar temperature range (200−400 °C), xylan also converted to 2furanmethanol and 2-methylfuran, possibly because of depolymerization, hydrogenation, and hydrodeoxygenation reactions.58 2-Furanmethanol could also be reduced to 2methylfuran over 400−500 °C.60 Between 300 and 500 °C, xylan also converted directly into furanic compounds (viz., acetylfuran and furan) and linear/cyclic oxygenates (i.e., 2,3butanedione and cyclopentanone). In a similar temperature range (i.e., 300−500 °C), 3-methylcyclopentane-1,2-dione could also hydrodeoxygenate into 2-cyclohexan-1-ol. Further, L


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ketones at 350 °C, and with an increase in the operating temperature, 2-methyl-4-vinylphenol, vanillin, apocynin, and eugenol/isoeugenol were apparently converted into lowmolecular-weight phenols. Thus, we present a comprehensive study of cellulose, xylan (or hemicellulose), and lignin thin-film pyrolysis for the first time. The reaction maps of all biopolymers developed in the present work would be useful to capture the overall intrinsic chemistry of pyrolysis and serve as an input to design quantum mechanical calculations investigating pyrolysis reactions of cellulose, hemicellulose (or xylan), and lignin. The information on the intrinsic pyrolysis reactions would also provide an assistance in building detailed kinetic models. In addition, the coupling of such kinetic models with transport processes will enable us to study the crucial interplay between reactions and transport and will aid in optimizing the operating conditions to get the desired yield and composition of bio-oil.

With an increase in the operating temperature (350−450 °C), 2-methoxy-4-vinylphenol was apparently hydrogenated to form 2-methoxy-4-ethylphenol. In a similar temperature range (i.e., 350−450 °C), apocynin was converted into 2,3dimethylphenol and carbon dioxide, while vanillin produced cresol and carbon dioxide via hydrodeoxygenation and decarbonylation reactions.66 Eugenol/isoeugenol and/or their intermediates were apparently converted into 2-(4-methoxyphenyl)ethanol over 350−450 °C. Between 450 and 550 °C, vanillin formed 2-methylphenol via similar retro-aldol, hydrodeoxygenation, and decarbonylation reactions.66 Lignin, over 350−550 °C, was also converted directly into phenol via multiple pyrolysis reactions involving the formation of a guaiacol intermediate followed by its dealkylation and hydrodeoxygenation reactions.46,51 Another product, viz., 4hydroxy-3-methoxyphenyl acetone, also formed directly from lignin between 350 and 550 °C, possibly via depolymerization to yield coniferyl alcohol (as an intermediate) followed by its hydrogenation and reduction. In a similar temperature range (i.e., 350−550 °C), 2,3-dihydrobenzofuran also formed from lignin via an o-quinonemethide intermediate with an allylmethyl group which underwent cyclization reaction followed by hydrogen abstraction.67,68 Light oxygenates (such as butyl glycol and methanol) and char were also formed from lignin between 350 and 550 °C. In brief, lignin thermal decomposition was apparently initiated with depolymerization reactions (or possibly cleavage of ether bonds), followed by dehydration to form monolignols. With an increase in the operating temperature, lignin-derived monolignols (i.e., 2-methyl-4-vinylphenol, vanillin, apocynin, eugenol/isoeugenol) underwent multiple pyrolysis reactions such as deformylation, rearrangement, oxidation, retro-aldol reaction, hydrogenation, alkylation/dealkylation, hydrodeoxygenation, reduction, and decarbonylation to yield various experimentally observed pyrolysis products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00920. Table T1 summarizing the products identified through GC retention time and MS during cellulose, xylan, and lignin thin-film pyrolysis; comparison between experimental yields of pyrolysis volatile products, especially forming bio-oil, of cellulose, xylan (or hemicellulose), and lignin thin-film pyrolysis is provided in Appendix 1, Appendix 2, and Appendix 3, respectively (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +17804924872. ORCID

Khursheed B. Ansari: 0000-0002-6440-3170 Jyotsna S. Arora: 0000-0001-9575-251X Jia Wei Chew: 0000-0002-6603-1649 Paul J. Dauenhauer: 0000-0001-5810-1953 Samir H. Mushrif: 0000-0002-0002-9634

4. CONCLUSIONS In this work, thin-films of cellulose, xylan (or hemicellulose), and lignin were pyrolyzed under reaction-controlled operating conditions to investigate the effect of operating temperature on the overall product distribution and on the yield of individual bio-oil components. Cellulosic bio-oil compounds like LGA, AGF, LGO, HMF, ADGH, and DHMDHP underwent further decomposition to form other furans, light oxygenates, and noncondensable gases with an increase in the operating temperature. Cellulose thermal decomposition was apparently initiated by glycosidic bond cleavage, followed by dehydration reactions, to yield anhydrosugars, furans, and pyrans. With an increase in the operating temperature, the ring-opening, fragmentation, retro-Diels−Alder, dehydration, decarbonylation, and deoxygenation reactions among cellulose-derived anhydrosugars/furans/pyrans and/or their intermediates become more facile. Up to 300 °C, xylan-derived bio-oil compounds like LGA and HMF were apparently converted into 3,4-altrosan and 5-methyl-2(3H)-furanone, while, at still higher temperatures, furfural and 3,4-altrosan were converted into light oxygenates and non-condensable gases. Thus, xylan initially underwent depolymerization, rearrangement, and dehydration reactions, while, with increasing pyrolysis temperature, retro-Diels−Alder, fragmentation, oxidization, hydrodeoxygenation, and reduction reactions among xylan-derived pyrolysis volatiles became facile. Lignin thin-film pyrolysis biooil remained rich in methoxyphenols and phenolic aldehydes/

Present Address ∥

K.B.A.: Department of Chemical Engineering, Aligarh Muslim University, AMU Campus, Aligarh 201002 Uttar Pradesh, India

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the Ministry of Education, Singapore, under the Academic Research Fund (AcRF) Tier-2 grant (Grant No. T2-1-082).

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Fast Pyrolysis of Cellulose, Hemicellulose, and Lignin: Effect of

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