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

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Kinetics, Catalysis, and Reaction Engineering

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 Hemant Mushrif Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00920 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 6, 2019

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Industrial & Engineering Chemistry Research

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†1, 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, USA

§ Department

of Chemical and Materials Engineering, University of Alberta, 9211-116 St NW,

Edmonton, Alberta T6G 1H9, Canada

*Corresponding author Email: [email protected] Tel: +17804924872

1

Current Affiliation: Department of Chemical Engineering, Aligarh Muslim University, AMU Campus, Aligarh 201002 Uttar Pradesh, India

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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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 timescales. In this work, thin-film pyrolysis experiments of all biopolymers present in the biomass (i.e. cellulose (~ 50 µm), hemicellulose (using xylan as a model biopolymer) (~ 12 µm), and lignin (~ 10 µm)) are performed over 200 ⁰C – 550 ⁰C, 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. While, in 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.

Keywords: Cellulose, Hemicellulose, Lignin, Thin-film fast pyrolysis, Bio-oil yield and composition, Reaction Map


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1. Introduction The global rise in energy demand and liquid fuels, combined with increasing levels of environmental pollution is driving mankind to search for alternative resources. Alongside fossil fuel, 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 is comprised of three biopolymers viz. cellulose (40 – 50 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 bio-oil, and carbonaceous solid material.4-6 Bio-oils are the most desired product from fast pyrolysis for their ability to substitute for existing fossil fuel and as a chemical feedstock suitable for upgrading to liquid hydrocarbon fuel.7-9 1.1 Bio-oil yield and composition: Challenges and opportunity The overall yield and composition of bio-oil is affected by the design and operating temperature of the reactor, as well as the composition and structure of biomass particles. The operating temperature for the maximum yield of bio-oil is between 400 °C – 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 bio-oil 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 timescales with inter-related reaction chemistry and transport limitations. Thus, systematic design and operation of a pyrolysis reactor to obtain the desired composition of biooil remains a challenge and requires 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 studies19-20,

24-25

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, 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 Pyroprobe30, Frontier Micropyrolyzer31-33, Wire-mesh reactor34-35, and PHASR (Pulsed-Heated Analysis of Solid Reactions) reactor13 heat biomass samples much faster ((heating rate ~ 200 – 12000 oC/sec) than the conventional method (TGA, heating rate ~ 3 – 4 oC/sec).13 Recently, Dauenhauer and co-workers, used micrometer thick (≤ 10 µm) films of cellulose and performed fast pyrolysis experiments in both micropyrolyzer and PHASR (Pulse-heated Analysis of Solid Reactions) reactor. These experimental systems enabled sufficiently high heat and mass transport rates to yield isothermal and reactioncontrolled 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) show that the cellulose thin-film pyrolysis at 450 °C, produced anhydrosugar, pyrans, and furans via glycosidic bond cleavage and dehydration reactions.36 It is also shown that cellulose can directly form furans and small oxygenates rather than through intermediates such as glucose.18 Thin-film 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 temperature 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, cellulose thin-film (~ 85 µm) pyrolyzed at different pressures (0.004 bar to 1 bar) underwent dehydration reactions forming cellobiosan and oligosaccharides as products which further fragment to result into secondary decomposition products (i.e. linear oxygenates).37 Lignin thin-film (size ~ 115 µm) pyrolysis at different pressures (0.004 – 1 bar) showed reduced yield of char (by 13 wt%) and increased the production of heavy sugars (i.e. cellobiosan and cellotriosan).38 Co-pyrolysis of 13C labelled glucose with levoglucosan under reaction-controlled operating condition showed that glucose undergoes retro-aldol reaction with fructose (as an intermediate) to form glyceraldehyde, and Grob fragmentation after tautomerization to form acetol.27 Glucose thin-film (size ≤ 10 µm) 4 ACS Paragon Plus Environment

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pyrolyzed at different temperatures decomposed via dehydration reactions leading to anhydrosugars, furans, and pyrans formation and 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 (i.e. 300 °C – 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 non-condensable gases. Thus, most of the studies investigating pyrolysis chemistry used cellulose or its surrogates (i.e., cellotetraose, cellohexaose, cyclodextrin, and methyl-cellobiose) as model compounds.20-21, 23 Further. It is shown that the decomposition chemistry of biopolymer (i.e. cellulose) is likely to be different than 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 experimental thermal decomposition of hemicellulose and lignin using thin-film pyrolysis approach is rarely studied. 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 thin-film pyrolysis) enables their thermal decomposition without transport limitations and thus 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, it can provide understanding of their intrinsic pyrolysis chemistry. 1.2 Scope of this work In this work, we performed thin-film pyrolysis experiments of cellulose, hemicellulose (using xylan as model compound), and lignin at different temperatures to thermally decompose them in the reaction-controlled regime to systematically evaluate the effect of operating temperature on pyrolysis product distribution. Pyrolysis experiments were performed in a micropyrolyzer, and the products were analyzed with an inline gas chromatograph mass spectrometer (GCMS) system. The effect of pyrolysis temperature on 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. 5 ACS Paragon Plus Environment


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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 Kraft process and then it is 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 the deionized (DI) water. Cellulose did not dissolve in DI water and resulted in a suspension. 25 μL of 1.0 wt % cellulose suspension was transferred into the pyrolysis crucible. The water was removed using room-temperature 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 and result 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 Figures 1A, 1B, and 1[C/D], respectively. The image analysis showed that cellulose, xylan, and lignin thin-films were of ~ 50 μm, 12 −13 μm, and 10 – 12 μm, respectively. The thickness of xylan and lignin films was ~ 10 μm and that of cellulose was ~ 50 μm, resulting in 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 thinfilm pyrolysis experiments was 250 µg. The heating rate of the thin-films in a micropyrolyzer was three-to-five 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 °C – 500 °C, followed by cellulose (300 °C – 500 °C), and lignin (350 °C – 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 maximum operating temperature of 320 ⁰C and having same dimensions (i.e., 30 m×320 µm×1.5 µm; length×internal diameter×film). Initially, the oven temperature was set to 35 ⁰C, and then a ramp of 3.5 ⁰C/min was provided to reach final oven temperature up to 250 ⁰C during analysis. A sample split time (~ 2 min) was also set in the GC program to separate non-condensable gases and condensable volatile 6 ACS Paragon Plus Environment

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products (especially forming bio-oil) into two separate columns (i.e., HP-PLOT-Q for noncondensable gases and J&W DB-5 for condensable volatile compounds) and simulatenaeously achieve their analysis. The details of the thin-film pyrolysis experimental procedure and product characterization is reported elsewhere.39 The yields of cellulose/xylan/lignin derived bio-oil and non-condensable 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 GS 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%. 3. Results and Discussion The summary of identified products in cellulose, xylan, and lignin pyrolysis is provided in Table T1 in the Supporting Information. In section 3.1, we discussed the overall product yield (viz, percentages of non-condensable gases, bio-oil, and char) and the individual yields of biooil components (viz, anhydrosugars, pyrans, furans, light oxygenates, and phenolic compounds) obtained from cellulose, xylan, and lignin thin-film pyrolysis. The effect of operating temperature on the yields of individual bio-oil components, non-condensable gases, and char was 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 were drawn, and detailed reaction maps were 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 bio-oil composition in cellulose thin-film pyrolysis Cellulose thin-film pyrolysis within 300 °C – 500 °C produced non-condensable gases (0.1 – 7 wt%), bio-oil (61 – 85 wt%), and char (8 – 33 wt%) as products (cf. Table 1[A]). The non-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-



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condensable gases, bio-oil, and char) was measured between 94 – 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 1[B]).33 At 300 °C, anhydrosugars (~ 77 wt%) occupied a major percentage of the bio-oil, while, furans and pyrans were ~ 10 – 12 wt% (of bio-oil) with negligible light oxygenate. With an increase in the temperature (from 300 °C 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 2[A]). 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 °C – 500 °C (cf. Table 2[A]). Xylan thin-film pyrolysis bio-oil was comprised of anhydrosugars, furans, light oxygenates, and phenolic compounds (cf. Table 2[B]). At 200 °C, the bio-oil composition was dominated by anhydrosugars and furans (collectively ~ 85 wt%), along 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 wt%, 30 wt%, 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 thinfilm pyrolysis.44 Unlike cellulose, xylan thin-film pyrolysis did not produce any pyrans in biooil; 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 °C – 550 °C, produced 0 – 4.7 wt% noncondensable gases, 27 – 55 wt% bio-oil, and 20 – 53 wt% char (cf. Table 3[A]). Similar to xylan thin-film pyrolysis, the total yields of lignin-derived pyrolysis products were up to 75 – 8 ACS Paragon Plus Environment

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81 wt%. Non-condensable gases still remained low but increased with an increase in the temperature. The yield of bio-oil increased (~ 29 wt%) and char yield decreased (~ 33 wt%) during lignin decomposition over 350 °C – 550 °C. The yields of non-condensable gases, biooil, 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 light oxygenates (cf. Table 3[B]). At 350 °C, methoxyphenols and phenolic aldehydes/ketones together occupied ~ 86 wt% of 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 wt% and 30 wt%, respectively, and the percentage of low molecular weight phenols (viz. phenol, cresol, 2methylphenol, 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 low molecular weight phenols (viz. phenol, cresol, and methyl phenol) via multiple pyrolysis reactions,45-46 instead of forming light oxygenates. The lignin-derived bio-oil compounds differed from previous literature due to the different operating conditions of pyrolysis (i.e., thickness of lignin samples used in the present work was much smaller than that of lignin 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, 6anhydroglucofuranose (AGF), as shown in Figure 2[A]. All anhydrosugars showed a decreasing trend over 300 °C – 500 °C. The reduction in the yields of LGA, LGO, DAGP, and AGF was 7.9 wt%, 1.83 wt%, 1.38 wt%, 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%) in between 300 °C – 400 °C agreed with the collective increase in methyl glyoxal and hydroxyacetone yields, while, at a higher temperature (i.e. 400 °C – 500 °C), LGA further reduced (by ~ 4.1 wt%) and apparently converted into methyl glyoxal, formic acid, and glycolaldehyde (cf. 9 ACS Paragon Plus Environment


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Appendix 1[A] and 1[B] of 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 later in section 3.5. A similar comparison of AGF yield with other pyrolysis products suggested that the marginal decrease in AGF yield (1.98 wt%) was compensated with the collective increase in furfural and formic acid yields (cf. Appendix 1[C] of 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 LGO yield decreased marginally (0.21 wt%) and contributed to furfural and formaldehyde formation (cf. Appendix 1[D] and 1[E] of the supporting information). Unlike LGA, AGF, and LGO, the decrease in DAGP yield (1.38 wt%) over 300 °C – 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 furans and light oxygenates formation. Pyrans: The pyrans in cellulose thin-film derived bio-oil comprised of 2,3-dihydro-3,5dihydroxy-6-methyl-4HPyran-4-one (DHMDHP) and 1,5-anhydro-4-deoxy-D-glycerohex-1en-3-ulose (ADGH).33 Both DHMDHP and ADGH decreased with the increase in the pyrolysis temperature, as shown in Figure 2[B]. The comparison of decrease in DHMDHP yield (~ 1.92 wt%) with other pyrolysis products, over 300 °C – 400 °C, indicated that DHMDHP apparently contributed for the collective increase in 1, 2-cyclopentanedione and carbon dioxide yields, while, in between 400 °C – 500 °C, DHMDHP decreased marginally (~ 0.51 wt%) and formed 2, 5-dimethylfuran (cf. Appendix 1[F] and 1[G] of the supporting information). Similarly, in between 300 °C – 400 °C, ADGH yield decreased (1.5 wt%) and contributed for 2, 3butanedione 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[H] and 1[I] of 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 cellulose-derived bio-oil comprised of hydroxymethylfurfural (HMF), furfural, 2-furanmethanol, 2(5H)-furanone, 5-methylfurfural, furan, and 2, 5-dimethylfuran, as shown in Figure 2[C]. Amongst furanic compounds, HMF yield was the highest at 300 °C but it showed a decreasing trend with increase in the pyrolysis 10 ACS Paragon Plus Environment

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temperature, while, yields of other furans (viz. furfural, 2-furanmethanol, 2(5H), furanone, 5methylfurfural, 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 °C – 400 °C agreed well with the collective increase in 2furanmethanol and carbon monoxide yields. However, beyond 400 °C, HMF yield decrease contributed to 5-methylfurfural formation. Unlike HMF, the yield of furfural continued to increase over 300 °C – 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), formic acid (FA), hydroxyacetone (HAA), 2,3butanedione (BD), glyoxal, formaldehyde, acetaldehyde, acetic acid, 1,2-cyclopentandione (CPD), and 2-hydroxy-3-methyl-2-cyclopenten-1-one (CPMH), as shown in Figures 2[D] and 2[E]. Amongst light oxygenates, the percentage of MG, GA, and FA were higher, and hence they were referred as major light oxygenates, while, HAA, BD, glyoxal, formaldehyde, acetaldehyde, acetic acid, CPD, and CPMH were lower in percentage (≤ 1.5 %) and hence were 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 The comparison between experimental yields of light oxygenates and anhydrosugars experiments suggested that the 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 thin-film 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 bio-oil 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 non-condensable 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 shown in Figure 2[F]. The 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 11 ACS Paragon Plus Environment


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data suggested that CO apparently formed from furans, while, pyrans/anhydrosugars contributed to CO2 formation (as discussed in Appendix 1[F], 1[I], and 1[J]). The literature also shows only CO and CO2 as non-condensable gases during the pyrolysis of cellulose under the 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, char yield decreased (cf. Figure 2[F]). Char yield mainly competes with the yield of bio-oil during fast pyrolysis and since biooil yield increased with temperature, it resulted in a reduction in char yield.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, 4altrosan was observed at that temperature, as shown in Figure 3[A]. The comparison of experimental yields of xylan-derived anhydrosugars with other pyrolysis products suggested that the decrease in LGA yield (~ 3.01 wt%) from 200 °C 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 1-hydroxy-2-butanone and acetic acid (cf. Appendix 2[A] and 2[B] of the supporting information). This indicated that during xylan pyrolysis too, LGA remained thermally unstable (or reacted with its neighbouring 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 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[C] of 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 light oxygenates and non-condensable gases formation. 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 xylan derived bio-oil comprised of HMF, furfural, acetylfuran, 2-furanmethanol, 2(3H)furanone, 5-methyl, furan, and 2methylfuran, as shown in Figure 3[B] and 3[C]. Amongst furanic compounds, HMF, furfural, 12 ACS Paragon Plus Environment

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acetylfuran, and 2-furanmethanol were higher in percentage and were referred as major furans, while, 2(3H)furanone, 5-methyl, furan, and 2-methylfuran were low in percentage hence were designated as minor furans. At 200 °C, HMF and furfural were the highest in yield, however, with an increase in the temperature, their yields decreased. The comparison of decrease in HMF yield (~ 2.2 wt%) in between 200 °C – 300 °C with the increase in yields of other pyrolysis products suggested that HMF apparently contributed for 2(3H)-furanone, 5-methyl and carbon dioxide formation, while, at higher temperature (in between 300 °C – 500 °C), HMF yield decreased (~ 4.4 wt%), while, the yield of other furans like acetylfuran increased (cf. Appendix 2[D] and 2[E] of the supporting information).The above observation suggests that the xylanderived pyrolysis intermediates, leading to HMF formation, kinetically favour acetylfuran formation at elevated temperature.50 A similar comparison of decrease in the furfural yield over 200 °C – 400 °C with other pyrolysis product yields indicated that furfural likely converted into methyl glyoxal and hydroxy acetaldehyde, and beyond 400 °C furfural yield decreased slightly (0.37 wt%) and matched with the increase in 2(3H)-furanone, 5-methyl yield (cf. Appendix 2[F] and 2[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%) and contributed for an increase in 2-methylfuran yield (~ 1.4 wt%) (cf. Appendix 2[H] of 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 decomposed and contributed 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. Amongst minor furans, 2(3H), furanone, 5-methyl increased up to 300 °C and then decreased, while furan and 2-methylfuran showed an increasing trend over 200 °C – 500 °C, with an increase in the pyrolysis temperature. Light oxygenates: Light oxygenates of xylan-derived bio-oil were categorized into major light oxygenates (viz. 3-methylcyclopentane, 1,2-dione, MG, HAA, 2-cyclohexanol, 1hydroxy-2-butanone, and) and minor light oxygenates (viz. formaldehyde, acetaldehyde, 2,3butanedione, 1,2-cyclopentanedione, 1,3-dioxane-5-methanol, 5-ethyl-, acetic acid and acetaldehyde, hydroxy-), as shown in Figures 3[D] and 3[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-2-butanone yields over 250 °C – 500 °C was 6.5 wt%, 5.2 wt%, 4.6 wt%, and 3.3 wt%, respectively. 3-methylcyclopentane, 1,213 ACS Paragon Plus Environment


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dione initially increased up to 250 °C, and then it decreased with an increase in the temperature (cf. Figure 3[D]). 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, beyond that it decreased (~ 2.78 wt%) and apparently contributed to the 2-cyclohexen-1-ol formation (cf. Appendix 2[I] of the supporting information). All minor light oxygenates showed the increasing trend with an increase in temperature, except, 1,3-dioxane-5-methanol, 5-ethyl, which remained relatively stable up to 400 °C, and then decreased slightly at 500 °C (cf. Figure 3[E]). 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 3[F]. 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 non-condensable gases and char Xylan thin-film pyrolysis produced CO2 as the only non-condensable gas. The yield of CO2, despite low in percentage, increased with an increase in the pyrolysis temperature, as shown in Figure 3[F]. 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 3[F]). 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 comprised of 4ethyl-2-methoxyphenol, 2-methoxy-4-vinylphenol, 2-(4-methoxyphenyl)ethanol, 4-methoxy3-methylphenol, 2-methoxyphenol, creosol, eugenol, isoeugenol, and guaiacylacetone, as shown in Figures 4[A] and 4[B]. Amongst methoxyphenols, 4-ethyl-2-methoxyphenol, 2methoxy-4-vinylphenol, 4-methoxy-3-methylphenol, and 2-(4-methoxyphenyl)ethanol were higher in percentage and hence referred to as 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 (350 °C – 550 °C), 2-methoxy-4-vinylphenol 14 ACS Paragon Plus Environment

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yield decreased (~ 1.3 wt%). The comparison of the decrease in 2-methoxy-4-vinylphenol yield with other pyrolysis products, over 350 °C – 450 °C, suggested that it apparently converted into 4-ethyl-2-methoxyphenol (cf. Appendix 3[A] of the supporting information). Beyond 450 °C, 2-methoxy-4-vinylphenol yield decreased marginally (~ 0.45 wt%) and could not be compared with the increase in the yield of other products of lignin thin-film pyrolysis. The yield of 4-ethyl-2-methoxyphenol continued to increase beyond 450 °C, indicating 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 wt%, and 0.7 wt%, respectively) with increase 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 comprised of vanillin, apocynin, and 2, 3-dihydrobenzofuran, as shown in Figure 4[C]. With an increase in the pyrolysis temperature, the yields of vanillin and apocynin decreased, while, 2,3-dihydorbenzofuran yield remained relatively stable. The comparison of the decrease in vanillin yield (~ 1.6 wt%) with other lignin-derived products, over 350 °C – 550 °C, suggested that vanillin apparently contributed to cresol, 2-methylphenol, and carbon dioxide formation (cf. appendix 3[B] and 3[C] of 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%), in between 350 °C – 550 °C, closely matched with the increase in the yields of 2, 3-dimethylphenol (cf. Appendix 3[D] of the supporting information). This indicated that phenolic aldehydes/ketones (viz. vanillin and apocynin) contributed to the formation of low molecular weight phenols and non-condensable gases during lignin thin-film (or reaction-controlled) 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 4[D]. At 350 °C, all low molecular weight phenols remained less than 2 wt%, however, with an increase in the operating temperature, the yields of phenol, cresol, 2-methylphenol and 2,3-dimethylphenol increased by 6 wt%, 4 wt%, 4.8 wt%, and 6.5 wt%, respectively. All low molecular weight phenols were apparently formed due to the secondary pyrolysis reactions amongst

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 4[E]. The yield of butyl glycol 15 ACS Paragon Plus Environment


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increased with an increase in the pyrolysis temperature, however, 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 suggested that the secondary decomposition reactions amongst lignin-derived pyrolysis volatile compounds were more facile at higher temperatures. 3.4.2 Effect of temperature on the yield of non-condensable 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 4[F]). 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 increase in the pyrolysis temperature. 3.5 Reaction map of cellulose thin-film pyrolysis A reaction map for cellulose thin-film pyrolysis (reaction-controlled 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

Concerted mechanism include glycosidic bond

cleavage (preferably via the end-scission mechanism, since the operating temperature was less than 467 °C)20 followed by 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 concerted mechanism requires slightly lower barrier (62.5 kcal/mole) than that of LGA intermediate path (68.7 kcal/mol).29, 54 At the 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 kcal/mole and 55 kcal/mol, respectively (Figure S1 of 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/mole) instead of a concerted mechanism (64 kcal/mol).20,

56

Cellulose (or 16

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cellobiose) via 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/mole)22 and deoxygenation reactions. HMF can also form furfural via deformylation reaction (62 kcal/mol)22, however, our result suggested that furfural was formed from one of the anhydrosugars (i.e. AGF) and/or their intermediates, instead of HMF path, during cellulose thin-film pyrolysis. With an increase in the operating temperature from 300 °C 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 the similar temperature range (300 °C – 500 °C), another anhydrosugar like AGF and/or its intermediate converted into (i) formic acid via cyclic grob fragmentation (52.5 kcal/mole) and retro aldol (36.5 kcal/mole) reactions and (ii) furfural via ring opening (32 – 41 kcal/mole), cyclic grob fragmentation (52.5 kcal/mole), and dehydration (38 – 57.5 kcal/mole) reactions.22 We have also shown previously that the anhydrosugars can convert into furanic compounds during glucose pyrolysis 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 °C to 500 °C. Major furan such as HMF undergo decarbonylation (62 kcal/mole)22 and deoxygenation reactions beyond 300 °C and formed 2-furanmethanol, 5-methylfurfural, and carbon monoxide. Also, in between 400 °C – 500 °C, methylfurans (i.e. 2-methylfuran and 2, 5-dimethylfuran) were produced due to the secondary reactions amongst 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 °C – 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, in between 300 °C – 500 °C, light oxygenates such as MG, HAA, FA, and GL were formed via secondary decomposition reactions of LGA, as discussed earlier.57 In the 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 °C – 500 °C. The non-condensable gases, viz. CO and CO2, were formed through various competing pathways during cellulose thin-film



Page 18 of 38

pyrolysis, as discussed earlier, while, char was also formed from cellulose in between 300 °C – 500 °C. Thus, in brief, thermal decomposition of cellulose (~ 300 °C) under the reactioncontrolled 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 non-condensable 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 °C – 500 °C is shown in Figure 6. At 200 °C, Xylan formed furans (i.e. HMF and furfural), anhydrosugar (LGA), a phenolic compound (phloroglucinol) and char. Xylan apparently depolymerized, undergo rearrangement and dehydration reactions to yield furfural as a product.50, 58 Xylanderived 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 °C 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 Diel-Alder and fragmentation reactions.57 In between 300 °C to 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). Xylan derived HMF, over 200 °C to 300 °C, could undergo oxidization and hydrodeoxygenation reactions to yield furanone compounds and non-condensable gas (i.e. CO2).59 Similarly, in between 200 °C – 400 °C, furfural and/or its intermediates undergo retro-aldol condensation reactions (or ring opening and fragmentation reaction) to form linear oxygenates like methyl glyoxal and hydroxy acetaldehyde.50 In the similar temperature range (200 °C – 400 °C), xylan also converted to 2furanmethanol and 2-methylfuran possibly because of depolymerization, hydrogenation, and hydrodeoxygenation reactions.58 2-furanmethanol could also reduce to 2-methylfuran over 400 °C – 500 °C.60 In between 300 °C – 500 °C, xylan also converted directly into furanic compound (viz. acetylfuran and furan) and linear/cyclic oxygenates (i.e. 2, 3-butanedione and 18 ACS Paragon Plus Environment

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– 500 °C), 3-

cyclopentanone). In the similar temperature range (i.e. 300 °C

methylcyclopentane, 1,2-dione could also hydrodeoxygenate into 2-cyclohexane-1-ol. Further, xylan also converted into char over 200 °C – 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, 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 Diel-Alder and 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 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 an intermediate for 2-methy-4-vinylyphenol, vanillin, apocynin, eugenol, isoeugenol, 4-hydroxy-3-methoxyphenylactone,

4-methoxy-3-methylphenol

creosol,

p-cresol,

2-

methyoxyphenol formation. In particular, lignin-derived coniferyl alcohol (as an intermediate) apparently converted into 2-methy-4-vinylyphenol via deformylation reaction,61-62 and into vanillin via oxidation and retro-aldol reaction.63-64 Lignin-derived coniferyl alcohol (as an intermediate) also converted into eugenol and/or isoeugenol after cleavage of its hydroxyl group, followed by the hydrogenation reaction.62 At the same time, the eugenol forming intermediate also underwent retro-aldol and alkylation reactions to yield creosol (or 4methylguaiacol) 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 4-methylguaiacol) was also produced directly from lignin at low temperatures (~ 350 °C). With an increase in the operating temperature (350 °C – 450 °C), 2-methoxy-4vinylphenol apparently hydrogenated to form 2-methoxy-4-ethylphenol. In the similar temperature range (i.e. 350 °C – 450 °C), apocynin converted into 2,3-dimethylphenol and carbon dioxide, while, vanillin produced cresol and carbon dioxide via hydrodeoxygenation and decarbonylation reactions.66 Eugenol/isoeugenol and/or their intermediates apparently converted into 2-(4-methoxyphenyl)ethanol, over 350 °C – 450 °C. In between 450 °C – 550 °C, vanillin formed 2-methylphenol via similar retro-aldol, hydrodeoxygenation, and 19 ACS Paragon Plus Environment


Page 20 of 38

decarbonylation reactions.66 Lignin, over 350 °C – 550 °C, also converted directly into phenol via multiple pyrolysis reactions involving the formation of guaiacol intermediate followed by its dealkylation and hydrodeoxygenation reactions.46, 51 Another product viz. 4-Hydroxy-3methoxyphenyl acetone also formed directly from lignin in between 350 °C – 550 °C possibly via depolymerization to yield coniferyl alcohol (as an intermediate) followed by its hydrogenation and reduction. In the similar temperature range (i.e. 350 °C – 550 °C), 2,3dihydrobenzofuran also formed from lignin via an o-quinonemethide intermediate with an allylmethyl group which undergo cyclization reaction followed by hydrogen abstraction to produce 2,3-dihydrobenzofuran.67-68 Light oxygenates (such as butyl glycol and methanol) and char were also formed from lignin in between 350 °C – 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. for 2-methy-4vinylyphenol, vanillin, apocynin, eugenol/isoeugenol) undergo 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. 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 the increase in the operating temperature, the ring opening and fragmentation, retro Diels-Alder, dehydration, decarbonylation,

and

deoxygenation

reactions

amongst

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 apparently converted into 3, 4-altrosan and 2(3H) furanone, 5-methyl, while, at still higher temperature, furfural and 3,4-altrosan converted into light oxygenates and non-condensable gases. Thus, xylan initially underwent depolymerization, rearrangement, and dehydration reactions, while, with increasing pyrolysis temperature, retro Diel-Alder and fragmentation, oxidization, hydrodeoxygenation, and 20 ACS Paragon Plus Environment

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reduction amongst xylan-derived pyrolysis volatile become facile. Lignin thin-film pyrolysis bio-oil remained rich in methoxyphenols and phenolic aldehydes/ketones at 350 °C, and with increase in the operating temperature 2-methy-4-vinylyphenol, vanillin, apocynin, eugenol/isoeugenol apparently converted into low molecular 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 of reactions and transport and will aid in optimizing the operating conditions to get the desired yield and composition of bio-oil.

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

Supporting Information The supporting information is provided for summary of the products identified through gas chromatography (GC) retention time, mass spectroscopy (MS) during cellulose, xylan, and lignin thin-film pyrolysis (Table T1). The comparison between experimental yields of pyrolysis volatile products, especially forming bio-oil, of cellulose, Xylan (or hemicellulose), and lignin thin-film pyrolysis is provided as Appendix 1, Appendix 2, and Appendix 3.

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Page 27 of 38 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures and Table Legends List of Figures Figure 1: Digital microscope images of [A] cellulose thin-film; [B] xylan thin-film; and [C and D] lignin thin-film Figure 2: Yields of individual bio-oil compounds, non-condensable gases, and char at different operating temperatures during cellulose thin-film pyrolysis Figure 3: Yields of individual bio-oil compounds, non-condensable gases, and char at different operating temperatures during xylan thin-film pyrolysis Figure 4: Yields of individual bio-oil compounds, non-condensable gases, and char at different operating temperatures during lignin thin-film pyrolysis Figure 5: Reaction map of cellulose-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products is LGA: Levoglucosan; LGO: Levoglucosenone; AGF: 1, 6-anhydroglucofuranose; DAGP: Dianhydroglucopyranose; ADGH: 1,5anhydro-4-deoxy-D-glycero-hex-1-en-3-ulose;

DHMDHP:

2,3-dihydro-3,5-

dihydroxy-6-methyl-4H-Pyran-4-one; HMF: 5-Hydroxymethylfurfural; CPHM: 2hydroxy-3-methyl-2-cyclopenten-1-one; CO: Carbon monoxide; and CO2: Carbon dioxide Figure 6: Reaction map of Xylan-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products is LGA:

Levoglucosan; HMF: 5-

Hydroxymethylfurfural; and CO2: Carbon dioxide Figure 7: Reaction map of lignin-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products is CO2: Carbon dioxide List of Tables Table 1: [A] Overall product distribution of cellulose thin-film pyrolysis [B] Bio-oil compounds of cellulose thin-film pyrolysis Table 2: [A] Product distribution of xylan thin-film pyrolysis [B] Bio-oil compounds of xylan thin-film pyrolysis Table 3: [A] Product distribution of lignin thin-film pyrolysis [B] Bio-oil compounds of lignin thin-film pyrolysis



Page 28 of 38

[A]

[B]

50 µm 12.7 µm

Cellulose thin-film = ~ 50 µm

Hemicellulose (or Xylan) thin-film = 12 - 13 µm

[C]

[D]

(11.93 µm)

Lignin thin-film = 10 – 12 µm Figure 1: Digital microscope images of [A] cellulose thin-film; [B] xylan thin-film; and [C and D] lignin thin-film


[A] Anhydrosugars

6

40

3

20

15

2

10 1

0 300

350

400 450 Temperature (ᵒC)

DAGP

AGF

LGO

3 2

5

1

0

0

550

250

8

3

7

2.5

6

5

2

4 1.5

3

1

2

0.5

1

0

300

Furfural 5-methylfurfural HMF

400 450 500 Temperature (ᵒC) ADGH DHMDHP

350 400 450 Temperature (ᵒC) 2-furanmethanol 2,5-dimethylfuran

500

550

8 6 4 2 0

550

250

2(5H), furanone furan

300

350

Methyl Glyoxal Hydroxyacetone

400 450 Temeprature (ᵒC) Glycolaldehyde 2,3-butanedione

500

550

Formic acid

[F] Gases and Char

6

[E] Minor Light Oxygenates

2

350

[D] Major Light Oxygenates

10

0

250

300

LGA

[C] Furans

3.5

Yield (wt% basis)

500

4

Yield (wt% basis)

250

Yield (wt% basis)

25

Yield (wt% basis)

4

5

Yield (wt% basis)

Yield (wt% basis)

30

[B] Pyrans

6

35

5

40

35

5 Yield (wt% basis)

1.6 Yield (wt% basis)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2 0.8 0.4

30 4

25

3

20 15

2

10 1

5

0 250

300 CPHM Glyoxal Acetaldehyde

350

400 450 500 550 Temeprature (ᵒC) 1,2-cyclopentanedione Acetic acid Formaldehyde

Yield (wt% basis)

Page 29 of 38

0

0 250

300

350

Carbon dioxide

400 450 Temeprature (ᵒC) Carbon monoxide

500

550 Char

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 is DAGP: dianhydroglucopyranose, AGF: 1,6anhydroglucofuranose, LGA: levoglucosan, LGO: levoglucosenone, ADGH: 1,5anhydro-4-deoxy-D-glycerohex-1-en-3-ulose, DHMDHP: 2,3-dihydro-3,5dihydroxy-6-methyl-4HPyran-4-one, CPHM: 2-hydroxy-3-methyl-2-cyclopenten1-one, and HMF: Hydroxymethylfurfural



[A] Anhydrosugars

8 6 4

10 8

6 4

2

2 0

0 150

200

250

300 350 400 Temperature (ᵒC)

Levoglucosan

450

500

150

550

200

250

Furfural

3,4-Altrosan

[C] Minor furans

HMF

8 Yield (wt% basis)

2

1

300 350 400 Temperature (ᵒC) Acetyl furan

450

500

550

2-furanmethanol

[D] Major Light Oxygenates

6 4 2 0

0 150

200

250


2(3H)-furanone, 5-methyl

Furan

450

500

150

550

2-methylfuran

[E] Minor Light Oxygenates

200

300 350 400 450 500 Temperature (ᵒC) Methyl glyoxal Hydroxyacetone 2-cyclohexanol 1-hydroxy-2-butanone 3-methylcyclopentane,1,2-dione

Yield (wt% basis)

2

1

550

[F] Phenols, Gases, and Char

5

3

250

50

4

40

3

30

2

20

1

10

Yield (wt% basis)

Yield (wt% basis)

[B] Major Furans

12 Yield (wt% basis)

Yield (wt% basis)

10

Yield (wt% basis)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

0 150

200

250

300 350 400 450 500 Temperature (ᵒC) Formaldehyde Acetaldehyde 2,3-butandedione 1,2-cyclopentanedione 1,3-dioxane-5-methanol, 5-ethyl Acetic acid Acetaldehyde, hydroxy-

550

0

0

150

200

250


Phloroglucinol

450

Carbon dioxide

500

550

Char

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


Page 31 of 38

[B] Minor Methoxyphenols

0.8

[A] Major Methoxyphenols Yield (wt% basis)

Yield (wt% basis)

5 4

3 2

0.6

0.4

0.2 1 0

0 300

350

400

450 500 550 600 Temperature (ᵒC) 4-ethyl,2-methoxyphenol 2-methoxy-4-vinylphenol

4-methoxy-3-methylphenol

300

350

8

[C] Phenolic Aldehydes/ketones

6

4

400

2-methoxyphenol Creosol

2-(4-methoxyphenyl)ethanol

Yield (wt% basis)

Yield (wt% basis)

8

2

450 500 Temperature (ᵒC) eugenol Guaiacylacetone

550

600

Isoeugenol

[D] Low Mol. Wt. Phenols

6

4

2

0

0 300

350

400

Vanillin

6

450 500 Temperature (ᵒC) Apocynin

550

600

300

2,3-dihydrobenzofuran

350

5

[E] Light Oxygenates

4

2

400

2,3-dimethylphenol

Yield (wt% basis)

Yield (wt% basis)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


450 500 Temperature (ᵒC) p-cresol

Phenol

550

600

2-methylphenol

[F] Gas and Char

50

4

40 3

30

2

20

1

10

0

0 300

350

400

450

500

550

600

0 300

Temperature (ᵒC) Butyl glycol

Methanol

350

400

450 500 Temperature (ᵒC)

Carbon dioxide

550

600

Char

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



400 °C - 500 °C 2(5H), Furanone

1,2-cyclopentadione 300 °C - 400 °C

400 °C - 500 °C

CO2

2-furanmethanol

DHMDHP

400 - 500 °C

400 °C - 500 °C

2,5-dimethylfuran

2,3-butanedione

ADGH

300 °C

300 °C

300 °C - 400 °C

300 °C

DAGP

300 °C - 400 °C

Acetic acid

300 °C - 400 °C

HMF

CO

300 °C

400 °C - 500 °C Glycolaldehyde

400 °C - 500 °C 2-methylfuran

300 °C - 400 °C

Cellulose

300 °C

400 °C - 500 °C

Acetaldehyde

Furan

300 °C - 500 °C

Methyl glyoxal

Formic acid 300 °C - 400 °C

300 °C - 500 °C CPHM

Hydroxyacetone

400 °C - 500 °C

400 °C - 500 °C

Glyoxal

300 °C - 400 °C LGA

400 - 500 °C

300 °C - 500 °C

Char

300 °C

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 43 44 45 46

Page 32 of 38

Furfural

AGF 500 °C 300 °C - 400 °C

300 °C

Formaldehyde

400 °C - 500 °C

400 °C - 500 °C LGO

300 °C - 400 °C

5-methylfurfural

Figure 5: Reaction map of cellulose-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products is LGA: Levoglucosan; LGO: Levoglucosenone; AGF: 1, 6-anhydroglucofuranose; DAGP: Dianhydroglucopyranose; ADGH: 1,5-anhydro4-deoxy-D-glycero-hex-1-en-3-ulose; DHMDHP: 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-Pyran-4-one; HMF: 5Hydroxymethylfurfural; CPHM: 2-hydroxy-3-methyl-2-cyclopenten-1-one; CO: Carbon monoxide; and CO2: Carbon dioxide


Page 33 of 38 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 43 44 45 46


300 – 500 °C

200 - 300 °C

CO2

200 – 300 °C 200 – 300 °C

200 – 300 °C

200 °C

HMF

Acetic acid

2(3H)-furanone, 5-methyl

300 – 500 °C 1-hydroxy-2-butanone

Hydroxyacetone

300 – 500 °C

300 – 500 °C

3,4-altrosan

LGA

300 – 500 °C

200 °C

200 – 300 °C

200 °C

Xylan

Furfural

Acetylfuran

200 – 400 °C

Methyl glyoxal

300 – 500 °C

Acetaldehyde

200 – 300 °C

200 – 400 °C

300 – 500 °C

Hydroxy Acetaldehyde

200 °C Phloroglucinol 200 – 500 °C

Char

200 – 300 °C

200 – 400 °C 1,2-cyclopentanedione 2-methylfuran

Furan

400 – 500 °C

2-furanmethanol

3-methylcyclopentane, 1,2-dione

200 – 500 °C 300 – 500 °C

200 °C

200 – 400 °C

2-cyclohexene 1-ol

200 – 500 °C

2,3-butanedione

300 – 500 °C

1,3-Dioxane-5-methanol, 5-ethyl-

Figure 6: Reaction map of Xylan-derived products during thin-film pyrolysis. Nomenclature shown in the figure for products is LGA: Levoglucosan; HMF: 5-Hydroxymethylfurfural; and CO2: Carbon dioxide



350 – 550 °C 350 °C

Apocynin

CO2 350 – 450 °C

2,3-dimethylphenol

350 °C

350 – 550 °C

Phenol, 4-methoxy-3-methyl

2,3-dihydrobenzofuran 350 °C

350 °C

Creosol

350 – 450 °C 350 °C

Butyl glycol 350 °C

Lignin

350 °C

p-Cresol

2-methoxy-4-vinylphenol

350 °C

Eugenol/Isoeugenol 350 – 450 °C

450 – 550 °C

2-(4-Methoxyphenyl)ethanol

2-methylphenol

Vanillin

350 – 550 °C

2-methoxyphenol

450 – 550 °C

350 – 550 °C

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 43 44 45 46

Page 34 of 38

350 – 550 °C

450 – 550 °C 350 – 550 °C

350 – 450 °C Phenol, 4-ethyl-2-methoxy-

Phenol

Char

4-Hydroxy-3-methoxyphenyl acetone

350 – 550 °C

350 – 550 °C

Methanol

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


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Table 1[A]: Overall product distribution of cellulose thin-film pyrolysis Pyrolysis product distribution Temperature (ºC) Gases (wt %)

Bio-oil (wt %)

Char (wt %)

300

0.12 ± 0.01

61.16 ± 0.22

32.58 ± 0.48

350

2.07 ± 0.013

69.47 ± 0.88

25.53 ± 0.29

400

5.35 ± 0.07

76.23 ± 0.10

17.12 ± 0.20

450

6.41 ± 0.04

82.81 ± 0.10

11.13 ± 0.90

500

7.04 ± 0.07

84.99 ± 0.10

8.79 ± 0.12

Table 1[B]: Bio-oil compounds of cellulose thin-film pyrolysis Bio-oil Compounds (wt %)* Temperature (ºC) Anhydrosugars

Pyrans

Furans

Light Oxygenates

300

76.92

12.32

10.75

0

350

56.59

7.19

15.92

19.90

400

50.57

6.02

15.27

27.86

450

44.98

4.52

16.81

33.64

500

40.09

4.15

15.55

40.19

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



Page 36 of 38

Table 2[A]: Product distribution of xylan thin-film pyrolysis Product Distribution Temperature (ºC) Gases (wt %)

Bio-oil (wt %)

Char (wt %)

200

0

34.24 ± 0.30

46.78 ± 0.67

250

0

37.59 ± 0.10

39.73 ± 0.72

300

0.13 ± 0.02

40.20 ± 0.03

33.88 ± 0.28

350

1.02 ± 0.01

44.67 ± 0.13

29.47 ± 0.35

400

3.77 ± 0.10

49.62 ± 0.06

23.81 ± 0.68

500

4.40 ± 0.02

55.38 ± 0.12

16.25 ± 1.05

Table 2[B]: Bio-oil compounds of xylan thin-film pyrolysis Bio-oil Compounds (wt %)

Temperature (ºC)

Anhydrosugars

Furans

Light Oxygenates

Phenolic compounds

200

27.14

58.11

1.77

12.98

250

28.30

41.04

22.18

8.48

300

27.31

37.75

29.20

5.75

350

22.39

36.25

38.52

2.85

400

13.22

29.60

55.84

1.34

500

5.62

27.51

66.87

0



Page 37 of 38 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3[A]: Product distribution of lignin thin-film pyrolysis Product Distribution Temperature (ºC) Gases (wt %)

Bio-oil (wt %)

Char (wt %)

350

0

26.93 ± 0.16

53.05 ± 1.81

400

1.24 ± 0.01

29.27 ± 0.29

45.72 ± 0.42

450

3.79 ± 0.05

34.62 ± 0.15

35.66 ± 1.12

500

4.23 ± 0.12

43.83 ± 0.51

29.58 ± 0.92

550

4.69 ± 0.03

55.35 ± 0.18

20.08 ± 0.15

Table 3[B]: Bio-oil compounds of lignin thin-film pyrolysis Bio-oil Compounds (wt %)* Temperature (ºC)

Methoxyphenols

Phenolic Aldehydes/Ketones

Low mol. Wt. Phenols

Light Oxygenates

350

30.90

55.40

13.16

0.53

400

27.72

47.94

23.02

1.33

450

27.85

39.37

30.01

2.77

500

24.69

32.45

35.57

7.30

550

20.23

25.10

45.17

9.5



Industrial & Engineering Chemistry Research 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 43 44 45 46

Page 38 of 38

Graphical Abstract

Hemicellulose Lignin

Product 6

Thin-film (10 – 50 µm) Cellulose

Product 1

Flat Surface Cellulose

Xylan

Reaction-Controlled Fast Pyrolysis

Product 2

Product 5

Reactant Product 4

Product 3

Reaction Map


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

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