Elsevier

Combustion and Flame

Volume 206, August 2019, Pages 177-188
Combustion and Flame

Mechanism insight into the fast pyrolysis of xylose, xylobiose and xylan by combined theoretical and experimental approaches

https://doi.org/10.1016/j.combustflame.2019.04.052Get rights and content

Abstract

The fast pyrolysis mechanism of xylose, xylobiose and xylan was investigated with combined fast pyrolysis experiments and quantum chemistry calculations. The results indicate that acyclic d-xylose is an important intermediate for the formation of the major pyrolytic products in xylose pyrolysis, i.e., 1,4-anhydro-d-xylopyranose (ADX), furfural (FF) and hydroxyacetaldehyde (HAA). ADX derives from the successive cyclization of d-xylose which is the only source for ADX formation. 3-Deoxy-xylose is the most important intermediate for FF formation. HAA results from the Csingle bondC bond scission by retro-aldol and cyclic Grob fragment. In addition, the five-membered intermediate derived from the ring-contraction reaction can transform into FF with high selectivity. The dehydrated xylose is also another source for HAA formation. The fast pyrolysis mechanism of xylobiose is similar to that of xylose, and thus similar pyrolytic product distribution can be obtained from them. However, the pyrolytic products of xylan differ significantly from those of xylose and xylobiose, which is ascribed to its high degree of polymerization and the branches in it.

Introduction

Biomass is an important renewable resource and has gained extensive attention to combat serious environmental pollution and energy shortage. Fast pyrolysis is a promising technique for the clean and efficient utilization of biomass [1], based on which selective pyrolysis techniques are proposed to convert solid biomass materials into high-grade liquid fuels or various valuable chemicals [2,3]. In addition, fast pyrolysis is also one of the stages of combustion process. A full and clear understanding of the pyrolysis mechanism at atomic/molecular level is the foundation for the advancement of pyrolysis and combustion techniques at lab or even industrial scales. Hence, it is essential to deepen the insight into the fast pyrolysis characteristics and mechanisms of biomass, which is also the aim of this study.

Hemicellulose is one of the three main constituents of biomass, accounting for 20–40 wt% [4]. Compared with the other two main constituents of biomass, i.e., cellulose and lignin, less attention has been paid to the pyrolysis characteristics and mechanism of hemicellulose [4], resulting in the lack of a clear and full understanding on the chemical reactions in hemicellulose pyrolysis. Xylan is the predominant hemicellulose polysaccharide in hardwoods and herbaceous biomass, and usually employed as the model component of hemicellulose in pyrolysis experiments [5]. The pyrolytic liquid products from xylan mainly contain carboxylic acids, furans, aldehydes, ketones, cyclopentanone and anhydrosugars, etc. Among these products, acetic acid (AA) is a typical and most abundant carboxylic acid product, mainly derived from the acetyl groups and uronic units [6], [7], [8]. Furfural (FF) is an important furan derivative. Shen et al. [7] proposed that the xylose unit in xylan firstly formed acyclic intermediates and then generated FF through dehydration and cyclization. Wang et al. [6] regarded that acyclic d-xylose was essential for the formation of FF. In regard to the anhydrosugar products, Shen et al. [7] detected 1,4-anhydro-d-xylopyranose (ADX) in the fast pyrolysis of O-acetyl-4-O-methylglucurono-xylan and put forward its formation pathway through a diradical intermediate. However, recent studies revealed that holocellulose mainly decomposed through concerted reactions rather than radical reactions [9]. Except for xylan, monosaccharides (xylose, arabinose, mannose, galactose) were also used as the model compounds of hemicellulose to investigate the pyrolysis characteristics [10,11], whereas, relevant studies are less reported as compared with xylan.

Quantum chemistry approaches have also been applied to the mechanism study of hemicellulose pyrolysis. Xylose, O-acetyl-xylose, mannose and xylobiose were taken as the model compounds of hemicellulose in most of the quantum chemistry investigations [12–15]. The formation of AA from the O-acetyl group, which was determined in the pyrolysis experiments, was also justified by the computational investigation [6,13]. Based on the computational results, Wang et al. [6] reported the decomposition pathways of xylose into three products, 4-hydroxydihydrofuran-2(3H)-one, hydroxyacetone (HA) and FF, and concluded that FF formation pathway was the most favorable route with the lowest energy barrier. In addition, Wang et al. [15] put forward that retro-aldol was easy to take place and important for the formation of hydroxyacetaldehyde (HAA) and aldehyde.

According to previous studies, different postulated decomposition channels of xylan have been put forward based on the experimental results, and fundamental chemical reactions of some model compounds (xylose, O-acetyl xylose, etc.) have also been investigated based on quantum chemistry approaches. However, the pyrolysis mechanisms of hemicellulose/xylan in these studies are still limited and piecemeal. Zhou et al. [16] developed a mechanistic model of fast pyrolysis of xylan, which broadens our knowledge of hemicellulose pyrolysis mechanism. However, the proposed pathways in the mechanistic model did not include all major products especially the anhydro xyloses and dianhydro xyloses, because the structures of the anhydro xyloses and dianhydro xyloses still remained unclear. Although significant advancement has been achieved, great effort is still needed to systematically reveal the pyrolysis mechanisms of hemicellulose/xylan.

In the present work, xylose, xylobiose and xylan were all selected as the model compounds of hemicellulose. Analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were conducted to gain their pyrolysis characteristics and product distribution. Quantum chemistry calculation was performed to investigate the detailed decomposition pathways of the three model compounds. The experimental and computational results were combined to give a deep insight into the pyrolysis of xylose, xylobiose and xylan. The influence of the degree of polymerization (DP) and branches was analyzed in details. The macroscopic observations were proved to be linked to microscopic mechanism study. On this basis, possible measures are discussed for the selective pyrolysis of xylan/hemicellulose at lab and industrial scale.

Section snippets

Experimental methods

Commercial xylose (ultra pure, >99.0%, Aladdin), xylobiose (HPLC, >98%, TCI) and xylan (from birch wood, xylose residues ≥90%, Sigma) were used in the fast pyrolysis experiments without any further purification.

Analytical Py-GC/MS experiments were performed by using the CDS Pyroprobe 5200 HP pyrolyzer and the Perkin Elmer GC/MS (Clarus 560) instrument. The detailed experimental procedure has been reported in our previous work [17]. To be brief, the raw material (exact 0.20 mg) was pyrolyzed at

Fast pyrolysis results

Typical total ion chromatograms from fast pyrolysis of xylose, xylobiose and xylan at 400°C are shown in Fig. 1. Chromatograms under other pyrolysis temperatures for the three materials are illustrated in the Supplementary materials (Figs. S1–S3). Seven major peaks among the pyrolytic products in these chromatograms are numbered, namely (1) HAA, (2) HA, (3) 1-hydroxy-2-butanone (HB), (4) FF, (5) Dianhydro xylose-I (DAX-I), (6) Dianhydro xylose-II (DAX-II), (7) ADX. All the detected products are

Conclusions

In the present study, quantum chemistry calculations and Py-GC/MS experiments were combined to investigate the fast pyrolysis mechanism of xylose, xylobiose and xylan systematically. The following conclusions can be obtained.

Xylose tends to initially undergo ring-opening reaction to form acyclic d-xylose, which plays a vital role in the formation of some major products. d-Xylose is the sole source of ADX and the main source of FF and HAA. d-Xylose forms ADX through successive cyclization

Acknowledgment

The authors thank the National Natural Science Foundation of China (51576064, 51776070), Beijing Nova Program (Z171100001117064), Beijing Natural Science Foundation (3172030), Grants from Fok Ying Tung Education Foundation (161051) and Fundamental Research Funds for the Central Universities (2019JG002, 2018QN057, 2017ZZD05) for financial support.

Declarations of interest: none

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