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Cellulose is one of the major components of biomass. The study on its pyrolysis process will be beneficial to the in-depth understanding of biomass pyrolysis mechanism. In this work, in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT) combined with two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) was first used to characterize the evolution process of the functional groups in cellulose during pyrolysis. The results showed that the degradation of carbon skeleton was prior to the dehydration of free hydroxyls after the destruction of hydrogen bond networks during pyrolysis. The thermal stability of C O in cellulose followed by the order of glycosidic bond

The in-depth study of the cleavage behaviors of linkages in biomass is important for the better understanding of biomass pyrolysis mechanism. This study aimed to clarify the pyrolysis mechanism of the typical ether linkages in biomass including β-1,4-glycosidic bond, α-O-4 bond and methoxyl using cellobiose, benzylphenyl ether and guaiacol as the model compounds. Combining the detection of the key intermediates, especially radicals by SVUV-PIMS and the evaluation of the reaction pathways by density functional theory (DFT) quantum chemical calculations, it was found that the concerted cleavage of β-1,4-glycosidic bond was more kinetically favorable than homolytic cleavage and heterolytic cleavage, and the ring opening of cellobiose via the breakage of C1′-O was likely to occur before the cleavage of glycosidic bond. The α-O-4 bond in benzylphenyl ether was mainly cleaved by Cα-O homolysis, and it was easy for the formed radicals to recombine with each other to yield phenolic dimers. In the initial evolution process of methoxyl in guaiacol, homolytic demethylation was the most important unimolecular reaction, while demethoxylation and radical-induced rearrangement reactions were difficult to occur due to their high energy barriers. In the presence of the formed methyl radicals from homolytic demethylation reaction, these two reactions would occur since their energy barriers were significantly reduced at this condition. [ABSTRACT FROM AUTHOR]

Metal modification on hierarchical HZSM-5 produced by alkaline desilication was adopted to improve aromatics production from catalytic pyrolysis of cellulose. The influence of alkali concentration in the desilication on catalytic performance of zeolites was first studied. Mild alkaline treatment preserved the micropore structure and acid sites of zeolites when introducing mesopores, while further increase of alkali concentration destroyed the micropores and acidity. The HZSM-5 desilicated by 0.3 M NaOH solution had the best catalytic performance, which increased the aromatics production and BTX selectivity. Then, metal (Fe and Ga) modification on the optimal zeolite was used to further improve the catalytic performance. Fe modification showed negative effect on aromatics production while Ga modification had the positive. Both Fe and Ga modification increased the selectivity of monoaromatics at the expense of polyaromatics. Furthermore, Fe modification mainly enhanced the production of BTX, while Ga modification promoted the formation of both BTX and alkylbenzenes. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The catalytic performance of HZSM-5 was improved by CLD modification. • CLD modification reduced the pore opening size and external acid sites of HZSM-5. • CLD modification increased the yield of aromatics while decreased that of cokes. • The distribution of aromatics was optimized after CLD modification. Abstract The catalytic pyrolysis of biomass is a promising technology for producing high-quality fuels and value-added chemicals. However, the development of this technology has been hindered by high coke formation and low target product yield. In this study, chemical liquid deposition (CLD) was adopted to improve the catalytic performance of HZSM-5 in the catalytic pyrolysis of biomass. The changes of the characteristics, including crystallographic structure, porosity and acidity, of the HZSM-5 zeolite after modification were analyzed by XRD, N 2 adsorption-desorption, FTIR and NH 3 -TPD. The results showed that CLD modification reduced the mesopore volume, pore opening size and the number of external acid sites without changing the micropore structure. Py-GC/MS (a pyrolyzer equipped with a gas chromatography-mass spectrometer) and kinetic analysis were used to analyze the catalytic performance of the parent and CLD-modified zeolites. It was found that the production of aromatics increased first and later decreased, reaching optimal levels at a 4% deposition of SiO 2 , while the selectivity of monoaromatics increased continuously. Additionally, CLD modification raised the energy barriers for the char/coke formation reactions and could inhibit its production. The deposition amount of 4% showed the best char/coke inhibitory effects for catalytic pyrolysis of cellulose, while the char/coke formation decreased continuously with the increase of the deposited amount of SiO 2 for catalytic pyrolysis of lignin. [ABSTRACT FROM AUTHOR]

The past decades have seen increasing interest in developing pyrolysis pathways to produce biofuels and bio-based chemicals from lignocellulosic biomass. Pyrolysis is a key stage in other thermochemical conversion processes, such as combustion and gasification. Understanding the reaction mechanisms of biomass pyrolysis will facilitate the process optimization and reactor design of commercial-scale biorefineries. However, the multiscale complexity of the biomass structures and reactions involved in pyrolysis make it challenging to elucidate the mechanism. This article provides a broad review of the state-of-art biomass pyrolysis research. Considering the complexity of the biomass structure, the pyrolysis characteristics of its three major individual components (cellulose, hemicellulose and lignin) are discussed in detail. Recently developed experimental technologies, such as Py-GC–MS/FID, TG-MS/TG-FTIR, in situ spectroscopy, 2D-PCIS, isotopic labeling method, in situ EPR and PIMS have been employed for biomass pyrolysis research, including online monitoring of the evolution of key intermediate products and the qualitative and quantitative measurement of the pyrolysis products. Based on experimental results, many macroscopic kinetic modeling methods with comprehensive mechanism schemes, such as the distributed activation energy model (DAEM), isoconversional method, detailed lumped kinetic model, kinetic Monte Carlo model, have been developed to simulate the mass loss behavior during biomass pyrolysis and to predict the resulting product distribution. Combined with molecular simulations of the elemental reaction routes, an in-depth understanding of the biomass pyrolysis mechanism may be obtained. Aiming to further improve the quality of pyrolysis products, the effects of various catalytic methods and feedstock pretreatment technologies on the pyrolysis behavior are also reviewed. At last, a brief conclusion for the challenge and perspectives of biomass pyrolysis is provided. [ABSTRACT FROM AUTHOR]

The influence of torrefaction on cellulose structural characteristics and the resulting pyrolysis behavior was investigated in this study. Torrefaction reduced O/C ratio in cellulose and increased its high heating value. The crystallinity of cellulose increased slightly first and then decreased sharply with the increase of torrefaction temperature, which could be ascribed to competitive degradation between crystalline region and amorphous region, as indicated by 13 C CP/MAS NMR analysis. Besides, the cleavage of β-1,4-glycosidic bond and the dehydration of hydroxyl were the major reactions occurring in torrefaction. Avrami-Erofeev model was found to be the most suitable kinetic reaction model for explaining the thermogravimetric weight loss during the pyrolysis of the raw and torrefied cellulose. A distributed activation energy model based on Avrami-Erofeev model was subsequently used to reveal the pyrolytic kinetics. It was found that the changes in cellulose structure influenced the kinetic parameters greatly. Torrefaction also changed pyrolytic product distribution. The yields of furfural, alicyclic ketones and anhydrosugars increased while that of 5-hydroxymethyl-furfural decreased as torrefaction temperature increased. [ABSTRACT FROM AUTHOR]

The effects of torrefaction on hemicellulose characteristics and its pyrolysis behaviors were studied in detail. The oxygen content decreased significantly after torrefaction, leading to the increase of high heating value. Two-dimensional perturbation correlation analysis based on diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed to characterize the structural evolutions. It was found the dehydration of hydroxyls and the dissociation of branches were the main reactions at low torrefaction temperature. When the temperature further increased, the depolymerization of hemicellulose and the fragmentation of monosaccharide residues occurred. The distributed activation energy model with double Gaussian functions based on reaction-order model was used to investigate the pyrolysis kinetics. The results showed that torrefaction enhanced the activation energy for degradation reactions while lowered that for condensation reactions, and increased the devolatilization contribution of condensation reactions. Besides, torrefaction decreased the yields of typical pyrolytic products, such as acids, furans, alicyclic ketones and so on. [ABSTRACT FROM AUTHOR]

• The structural differences of biomass had significant influence on pyrolysis mechanism. • Cleavage of glycosidic bond and dissociation of O-acetyl group in softwood hemicellulose were prone to occur. • The breakage of α-ether bond of hemicellulose-lignin bonds mainly occurred via Cα-O concerted cleavage. Biomass pyrolysis involves complex structural changes and numerous chemical reactions. At present, most studies on actual biomass pyrolysis focus on the macroscopic thermal degradation process, while the studies on microscopic pyrolysis rules are still very limited. This work provides a new insight into the pyrolysis mechanism of three types of actual biomass, namely hardwood (eucalyptus saligna), softwood (pinus sylvestris) and straw (corn straw), by combining in-situ characterization of the evolution of biomass structures with density functional theory (DFT) calculations of pyrolysis pathways. Firstly, the evolution of various characteristic structures during biomass pyrolysis was monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT). It was found that the thermal stability of the same functional group in different biomass samples are not exactly the same, and the structural differences in biomass would lead to different pyrolysis behaviors. Then, considering the great structural difference of hemicellulose in different biomass, the key initial pyrolysis reaction paths including the cleavage of glycosidic bonds, the dissociation of side chains and the opening of sugar ring of hemicellulose were studied by DFT calculations. It was found that the cleavage of glycosidic bonds and the dissociation of O-acetyl side chains of glucomannan in softwood hemicellulose are more advantageous than those of xylan in hardwood and straw hemicellulose. In addition, the cleavage mechanism of hemicellulose-lignin bond (LC), a characteristic connection structure between different components of actual biomass, was investigated. The result showed that the cleavage of Cα-O mainly occurred by concerted mechanism for α-ether bond of LC. [ABSTRACT FROM AUTHOR]

The influence mechanism of torrefaction on biomass pyrolysis was studied in detail. 13 C CP/MAS NMR was employed to characterize the evolution of functional groups during torrefaction, indicating that hemicellulose was extensively deacetylated and depolymerized; the crystalline structure of cellulose was destroyed; and the aryl ether linkages and propyl side branches in lignin were cleaved. A distributed activation energy model with three combined Gaussian functions was introduced to analyze the pyrolysis kinetics of biomass. The mean activation energies for three parallel reactions remained unchanged after torrefaction, while the weighing factors proved that their contributions to devolatilization largely changed. Acetic acid yield significantly decreased after torrefaction, with reduction of the yields of 5-hydroxymethylfurfural and levoglucosan because of the depolymerization of carbohydrates during torrefaction. Due to the dissociation of propyl branches and the demethylation of methoxyls in lignin, pyrolysis of torrefied biomass yielded more guaiacol and catechol and less phenols with C4-propyl groups. [ABSTRACT FROM AUTHOR]

The pyrolysis mechanism of hemicellulose has been investigated using two minimally damaged hemicellulose polymers isolated from two agricultural straw samples. The obtained hemicelluloses have been characterized by multiple methods, and the results showed that they were mainly composed of l -arabino-4-O-methyl- d -glucurono- d -xylan. Their O-acetyl groups and high degrees of polymerization and branching were well preserved. Their pyrolyses were subsequently investigated by TG-FTIR and Py-GC/MS. The evolutions of four typical volatile components and the distributions of eight product species were scrutinized. A DG-DAEM kinetic model was applied to quantify the contributions of two major pyrolytic routes for devolatilization during hemicellulose pyrolysis. A mean activation energy of 150 kJ/mol for the formation of volatiles was derived. The thermal stability of each bond in four typical fragments of hemicellulose was assessed by DFT study, and the deduced decomposition pathways were in agreement with experimental analysis. [ABSTRACT FROM AUTHOR]

This study proposed a sequential and coupling method to determine the comprehensive kinetic models for pyrolysis of cellulose, hemicellulose and lignin. Isoconversional method was employed to find the correlation between activation energy and conversion rate. Cellulose pyrolysis could be interpreted by one-step global reaction model, while pyrolysis of hemicellulose and lignin could be divided into three stages and two stages, respectively, in which competitive parallel reactions occurred. Coats-Redfern method and Kissinger method were subsequently used to obtain the reaction model f(α). Particularly, a new mean reaction model f′(α) was introduced to deal with the complex competitive parallel reactions. It was proposed that pyrolysis of cellulose followed Avrami-Erofeev (m = 2) nucleation model, while pyrolysis of hemicellulose and lignin could be described by reaction-order model. The other kinetic parameters and the contribution of each parallel reaction to devolatilization were further analyzed based on the modified distributed activation energy models. And the detailed kinetic models for pyrolysis of biomass components were finally obtained. [ABSTRACT FROM AUTHOR]