The Catalytic Pyrolysis Mechanism of Cellulose On ZSM-5: Based On Py-GC/MS And Density Functional Theory

Cellulose is one of the main components of terrestrial biomass. In this study, a zeolite catalyst (ZSM-5) was used to catalyze the pyrolysis of cellulose. The components produced during pyrolysis were tested by Py-GC/MS, and the pyrolysis mechanism was analyzed by density functional theory (DFT). The results showed that furans and sugars were the primary pyrolysis products. After catalytic pyrolysis, the furfural content was signicantly lower than that of non-catalytic pyrolysis. However, the yield of aromatic hydrocarbons increased signicantly, especially benzene, which increased from almost zero to 13.04%. DFT further explored the specic reaction pathway of catalytic pyrolysis. It was found that under the catalysis of ZSM-5, the catalyst directly participated in most cellulose pyrolysis reactions. Also, the electrophilicity of acid sites in the reaction system played an important role. Therefore, the catalysis of the molecular sieve can signicantly reduce the energy barrier of each path. In generating aromatic hydrocarbons, the decarbonylation reaction and dehydration reaction of furfural can be completely catalyzed, thereby increasing the reaction rate of generating aromatic hydrocarbons. Some deoxygenation steps occurred during the reaction, which made the catalytic pyrolysis reaction easier to develop, and improved the bio-oil quality.


Introduction
In order to achieve national energy security and energy self-su ciency, countries are seeking new energy sources to replace petroleum-based energy Wang  This research aims to understand the role of the catalyst and reveal the mechanisms of the pyrolysis of cellulose catalysized by ZSM- 5 Wang et al. 2013Cao et al. 2021 . In order to thoroughly investigate the distribution of the product and further analyze the product and chemical structure, in this paper, noncatalytic fast pyrolysis of cellulose and fast catalytic pyrolysis of cellulose (ZSM-5 catalyst) were carried out on Py-GC/MS. Besides, according to the product distribution, the formation mechanisms of the leading products of cellulose catalytic pyrolysis (such as 5-hydroxymethyl furfural and levoglucan, etc.) are proposed. The M06-2X method was used to verify and analyze the catalytic reaction path at the calculation level of 6-31G(d). Ultimately, the mechanisms of the rapid pyrolysis of cellulose catalyzed by the ZSM-5 catalyst are revealed by combining experimental research and simulation analysis, which provides references and ideas for future cellulose catalytic development pyrolysis technology.

Experimental materials and instruments
Microcrystalline cellulose Avicel PH105 was purchased from Food Machinery and Chemical Corporation (FMC), which is mainly extracted from ber-rich plants. Chemical Data Systems (CDS) Pyroprobe 5200 was used to connect with GC/MS (Trace DSQII), and Py-GC/MS system was used to separate and identify pyrolysis volatiles, and also display the distribution of products. In this experiment, ZSM-5 (SiO 2 /Al 2 O=3:36), a molecular sieve, was used as the catalyst, which was provided by Nankai University Catalyst Factory. The ZSM-5 molecular sieve catalyst was added to cellulose for catalytic pyrolysis test at the pyrolysis temperature of 823 °K, carrier gas ow rate of 0.2 L/min, and mass ratio of 10:1 (raw material: catalyst).

Computational models and methods
It is very imperative to select suitable model compounds through DFT research. As β-D-glucopyranose is the basic unit of cellulose polysaccharide, it can be used as a model compound to study the catalytic pyrolysis of cellulose Liu et al. 2011).
The catalyst's chemical reaction is relatively complicated, and traditional experimental methods cannot explain how the catalyst works in the pyrolysis reaction, while the DFT method can elucidate the speci c reaction process. However, to study these extremely complex catalytic processes, appropriate simpli cations need to be carried out in accordance with the characteristics of the raw materials. The ZSM-5 molecular sieve catalyst selected in this work is a tetrahedral model based on SiO 4 . Adjacent tetrahedrons are connected to each other by oxygen atoms sharing vertices, thereby forming a threedimensional crystal structure. When the +4 valence Si is replaced with the +3 valence Al, the Al atom's molecular sieve framework is negatively charged. A positively charged proton (H + ) must be introduced to maintain the electrical neutrality of the system. The cluster model's size in this study was represented by 3T, which contains three SiO 4 tetrahedra (Fig1). Since the Si-O bond needs to be cut when constructing the cluster model, dangling bonds are generated on the boundary, and hydrogen atoms were used for saturation. The molecular structure of D-glucose is shown in (Fig2b). Each hydroxyl group on the sugar ring is numbered counterclockwise according to its relative position to ring O. The small cluster model

Results And Discussion
Py-GC/MS catalytic pyrolysis analysis of cellulose at 823 K Table 1 lists the typical products of Py-GC/MS cellulose pyrolysis under the activity of ZSM-5. The catalytic pyrolysis products of cellulose on the catalyst ZSM-5 can be divided into the following compounds: furans, aromatic hydrocarbons, nitrogen-containing substances, and aldehydes, ketones, acids, hydrocarbons and small molecules. The results of Py-GC/MS catalytic pyrolysis of cellulose catalyzed by ZSM-5 indicate that the bio-oil mainly contains acids, benzenes, and furans. Where 4methoxy-2-methylbutyl cyclopropane carboxylic acid, benzene, furfural, and levoglucan were the main products by the peak area percentages of 13.39%, 13.05%, 12.81%, and 11.24%, respectively. The Py-GC/MS partial pyrolysis test products of cellulose pyrolysis without catalyst are listed in the supplementary materials (Table S1). After adding the catalyst ZSM-5, the cellulose pyrolysis products contain benzene and other hydrocarbons. Also, hydrocarbons, such as benzene, have increased dramatically, indicating that a large number of hydrodeoxygenation reactions occurred in cellulose pyrolysis under the catalysis of the zeolite catalyst ZSM-5, which improves the quality of bio-oil Zhou et al. 2014 . Also, the content of carbohydrates and furans was relatively reduced slightly, as cellulose might become pyrolyzed more intensely under the effect of ZSM-5, and carbohydrates and furans were further pyrolyzed into other substances with small molecules. Based on the above facts, we further explored the catalytic pyrolysis mechanisms of cellulose under the action of ZSM-5 based on DFT quantum chemical calculations to better understand the catalytic pyrolysis mechanism of terrestrial biomass Gumidyala et al. 2016 . Besides, the corresponding product yields (for example, benzenes and furans) can be increased through speci c reaction paths, which can be used as an essential means of producing chemical raw materials, which helps future studies. is path 1, and the reaction of reactant (R) to furfural (P 2 ) is path 2-1. However, this article mainly studies the mechanisms of catalytic pyrolysis. To reduce the simulation calculation error, we choose a calculation method that takes the weak interaction force between molecules into the functional-M06-2X. Then, based on the 6-31g(d) level and the temperature, the non-catalytic Gibbs free energy without catalyst participating in the reaction was calculated. The transition states of the elementary reactions are listed in the supplementary information Table 2.
Path 1 (yellow) is a typical path where D-fructose (DF) participates. Glucose rst undergoes a ringopening reaction to form acyclic D-glucose (IM 1 ) with the activation free energy of 8.9 kcal/mol. This reaction involves a coordinated transition state, TS 1 ( Table 2). IM 1 is re-isomerized to DF (IM 2 ), with the activation free energy of 72.8 kcal/mol. This reaction involves a coordinated transition state, TS 2 ( Table   2), in which the 2-hydroxy H was transferred to the 1-carbonyl group, and the other H in the C2 position is transferred to the C1 position. Later, DF formed β-D-fructofuranose (IM 3 ) by forming a transition state TS 3 in the form of a 2,5-acetal ring ( Table 2). The quaternary transition state TS4 dehydrates IM3 at 2-OH+1-H (hydroxyl at C2 position and hydrogen at C1 position) to form intermediate IM 4 (ΔEa = 89.9 kcal/mol), which is the decisive rate of the reaction path step. IM 4 undergoes one-step of electro-ring dehydration to form IM5 directly. The reaction is a six-membered transition state TS 5 (Table 2), where 3-OH and 1-H (OH) are dehydrated, and C2=C3 bonds and C1=O bonds are formed. The reaction is thermodynamically favorable (ΔG = −25.2 kcal/mol) and requires relatively low activation energy (ΔEa = 34 kcal/mol).
Path 2-1 is the most advantageous way for 3-deoxy-glucosone (3-DG) to participate in FF formation. As shown in Fig3, IM 1 is rst dehydrated at the 3-OH+2-H site to form the enol isomer IM 6 of 3-DG (IM 1 -TS 7 , ΔEa = 63 kcal/mol). Then IM 6 undergoes enol-ketone tautomerism through the quaternary transition state TS 8 (IM 6 -TS 8 , ΔEa = 70.1 kcal/mol) into 3-DG (IM 7 ). Both of these reactions are thermodynamically favorable (ΔG = −18.2 and -0.5 kcal/mol). Similar to DF, 3-DG forms a 2,5-acetal ring through hemiacetal reaction to form a ve-membered intermediate IM 8 . The formation of IM 8 involves the quaternary transition state TS 9 , the structure of which is shown in Table 2. In Path 2-1 (blue), IM 8 is dehydrated between 4-OH and 6-H (OH) and formaldehyde to form an intermediate product IM 9 with a high activation free energy of 84.3 kcal/mol. This is the decisive step of path 2-1. The reaction involves the sixmembered transition state TS 10 , the structure of which is shown in Table 2. Finally, IM 9 is dehydrated at the 2-OH + 3-H site to form FF (P 2 ). The acyclic D-glucose produced by the ring-opening reaction of β-Dglucopyranose, and DF and 3-DG as important intermediates, are essential for developing both 5-HMF and FF.
After DF and 3-DG were formed in Path 1 and Path 2-1, respectively, the ve-membered intermediate could quickly form 5-HMF and FF through a continuous dehydration reaction. Since the boiling point of 5-HMF is 114~116℃, but the experimental temperature is 550℃. Therefore, under rapid high-temperature pyrolysis, 5-HMF will be further degraded. Degradation products include furans (5-methylfurfural, furfural, 2-methylfuran, etc.) and small molecules (CO, CO 2 , H 2 O, formic acid, formaldehyde, acetic acid, etc.). Therefore, we consider the reaction of P 1 to directly remove formaldehyde through transition state TS 12 to obtain P 2 as path 2-2 (red). This process of eliminating formaldehyde requires activation-free energy of 85.5 kcal/mol. According to our experimental results, after adding the zeolite catalyst, new aromatic products (benzene, toluene, p-xylene, etc.) appeared in the product. Some researchers have proposed that furan compounds will undergo DA cycloaddition reaction with ole ns, and furfural will undergo decarbonylation to produce carbon monoxide and furan Cheng and Huber 2012 . For this reason, we propose a conversion path from furfural (P 2 ) to benzene (P 3 ) on this basis, as shown in Fig3, path 3 (orange). Furfural rst removes free carbon monoxide through the transition state TS 13 to form the furan intermediate IM 10 . This is the crucial step of the path, which requires activation free energy of 88.2 kcal/mol. During the reaction, IM 10 and short-chain ethylene undergo a DA reaction (IM 10 -TS 14 , ΔEa=70.1 kcal/mol) to obtain intermediate IM 11 . Under the reaction of water molecules, the C4-O ether bond of IM 11 together with the C2=C3 double bond are disturbed. Then, a new C3=C4 double bond is generated, and a C1-C2-O ether bond is generated to obtain a ternary epoxy structure. Finally, a hydrogen bond is formed between the hydrogen of the water molecule and the epoxy, resulting in the IM12 complex. This reaction requires activation free energy of 74.2 kcal/mol. However, the IM 12 intermediate has a shorter residence time and is quickly isomerized to IM 13 . IM 13 continuously removes two molecules of water to obtain the product benzene (P 3 ).
The energy consumed by the non-catalytic reaction path was generally at a high level. The easiest way to reduce the reaction energy barrier is to add a catalyst, and the experimental results also con rm the yield gap of typical products in the pyrolysis reaction after adding the catalyst (Table 3). Next, by calculating the reaction's energy after adding the catalyst, we will explain the changes in the product yield from a theoretical point of view. Possible reaction pathways of typical products of cellulose catalytic pyrolysis In the catalytic reaction system, the reactant and the catalyst interact. To express the reaction energy more accurately and reduce the calculation error, we put the reactant and the catalyst in the same system for simulation. Therefore, add the pre x C_ when naming each compound. When the compound is close to the acidic site of the catalyst, a complex is formed, where this adsorption process has certain adsorption energy. Since adsorption has little effect on product selectivity, this article ignores the effect of adsorption.  Table 4, the 3-OH of R was the most active (-40.2 kcal/mol), followed by the 4-OH position. Also, the active-H of the catalyst was an acidic site. Considering all together, we choose the 4-OH site on R to react with the catalyst rst. Therefore, a catalytic pyrolysis path from D-glucose to 5-HMF was designed, as shown in Fig4. There are two paths for non-catalytic pyrolysis to generate P 2 . We choose the shortest path (path 2-2', green) to calculate catalytic pyrolysis's energy; P 1 removes the terminal free methylene group. The active H of the catalyst promotes the breaking of the C5-C6 single bond, the methylene group is then separated from the furan ring, and then the active hydrogen is added to the C4=C5 double bond. The hydroxyl group of -CH 2 OH removes a hydrogen atom to neutralize the catalyst to maintain charge neutrality. The transition state C_TS 7 is shown in Table 5. This formaldehyde removal process requires activation free energy of 35.3 kcal/mol, which is 50.2 kcal/mol lower than that of non-catalysis.
In the process where furfural reacts with ethylene to convert to benzene (path 3', purple), the catalyst has the same effect, and the catalytic reaction is induced by active hydrogen. The free 1-carbonyl group of furfural removes CO under the catalysis of active hydrogen. First, active-H attacks the C1-C2 positions to promote single bond cleavage. Subsequently, the reactive H undergoes an electrophilic attack to produce the intermediate furan (IM 6 ). At the same time, 1-H is returned to the catalyst and forms free CO. The second step Diels-Alder (DA) reaction, is the key step for forming benzene ring. Since the addition reaction is bimolecular, the transition state C_TS 9 with a roof-shaped spatial geometry is formed during the addition process. A double bond forms, so active-H cannot participate in the reaction, but active H affects the electronic structure around the reactant and can also reduce the energy required for the reaction to a certain extent, which is consistent with the report by Patet et al. Patet et al. 2017 . The third step is that active-H promotes the breaking of the ether bond and the free water molecules in the reaction process, and the oxygen on the ring form a hydrogen bond, preparing for the water addition reaction in the fourth step. Finally, two molecules of water are continuously removed to produce benzene (P 3 ). Table 6 lists the activation energy and reaction heat of the reaction process. Fig7 is the potential energy curve of the noncatalytic and catalytic pathways for forming a benzene ring. We found that in non-catalysis, the decisive step is the rst step of CO removal reaction. In catalysis, the nal step is the DA reaction, and except for the DA reaction, the energy barrier of other elementary reactions is greatly reduced. This is all due to the excellent catalytic effect of the zeolite catalyst. It also shows that the effect of zeolite catalyst on the DA reaction is not favorable. Since the zeolite catalyst could not signi cantly reduce the energy required for DA reaction. We will further explore high-e ciency catalysts to reduce the DA reaction's energy barrier in the future study. Fig8 shows the potential energy curves of different catalytic pathways. In the catalytic reaction, each step of the reaction's activation free energy is greatly reduced, which is very bene cial to the reaction in terms of kinetics. The reaction heat of the non-catalyzed reaction is in the supporting information Table S1, and the reaction heat of the catalyzed reaction is listed in Table 6. The thermodynamic energy of most elementary reactions is negative, representing an exothermic process, which is also thermodynamically bene cial. We also found that most of the transition states involved in non-catalytic reactions are fourmembered ring transition states. Still, the eight-membered transition state dominates the transition states of catalytic reactions, and there is also the ten-membered transition state C_TS 6 . Also, C_TS 6 has the lowest activation energy of transition state structure in all dehydration reactions, which is only 5.3 kcal/mol. If the transition state ring formed at the position where the chemical reaction occurs is larger, the activation energy required for the reaction is less. This is consistent with our previous research results hydroxymethyl furfural is the easiest to produce from glucose, followed by furfural, and nally benzene.
In the experiment, cellulose was rst decomposed into single-molecule glucose, and then the glucose had 5-hydroxymethylfurfural, furfural, and benzene according to the di culty of the reaction. Moreover, benzene can only be produced when the catalyst is present. In the non-catalytic pyrolysis experiment, the yield of furfural reached 25.34%, while after adding the catalyst, it decreased to 12.81%. However, the content of benzene increased from almost zero to 13.04%. Therefore, it can be inferred that furfural participates in the production of benzene, showing that the experimental and theoretical results are consistent.
To study the Diels-Alder cycloaddition reaction of furan and ethylene on the Brønsted-acidic molecular sieve, Patet et al. used electronic structure calculations and microscopic kinetic models, and the subsequent dehydration reaction of furan and ethylene Patet et al. 2017 . The electronic structure calculation showed that the acid strength of the Brønsted-acidic zeolite changes with the changes of Al, Ga, Fe, and B metal substituents. For these four modi ed Brønsted-acidic zeolites, the catalytic effect of the DA reaction is not apparent. However, for dehydration reactions, Brønsted-acidic follows the general principle of acid catalysis for dehydration reactions and plays an essential role in catalyzing these reactions. Our calculation results also showed that the activation energy of the DA reaction catalyzed by zeolite was 45.4 kcal/mol, which is only 9.4 kcal/mol lower than the uncatalyzed activation energy.
However, the dehydration reaction's activation energy is reduced by about 40-70 kcal/mol. Therefore, we suggest that steric hindrance may make the DA cycloaddition reaction more complex, so the change in acid strength shows an extensive role in catalysis.

Conclusion
In this study, density functional theory was used to study the Py-GC/MS experiment of fast pyrolysis of cellulose and its thermal decomposition mechanism. Combining the experimental results, this paper proposes several reaction pathways for the catalytic pyrolysis reaction mechanism of cellulose.
Compared with the results of cellulose monomolecular pyrolysis, under the catalysis of the zeolite catalyst ZSM-5, many deoxygenation reactions occurred during the cellulose pyrolysis process. The role of hydrogen bonds improves the quality of bio-oil, and effectively reduces the reaction's energy barrier, which makes the pyrolysis reaction of cellulose easier to occur. According to the degree of di culty of the reaction (the energy barrier of the speed-determining step and the heat of reaction of the reaction process), glucose sequentially generates 5-hydroxymethylfurfural, furfural and benzene, and the benzene ring can only be formed when the zeolite catalyst is present. The relatively low content of carbohydrates and furans may be due to the higher intensity of cellulose's catalytic pyrolysis. For example, furan compounds will be further decomposed into other small molecules. Moreover, ZSM-5 directly participates in most of the pyrolysis reaction of cellulose, where the electrophilicity of acidic sites in the reaction system plays an important role. Therefore, the catalyst's addition signi cantly reduces the energy barrier of each path in the pyrolysis reaction. The dehydration reaction can be fully catalyzed, the activation energy of the reaction is reduced, and the formation of aromatic compounds were promoted. However, due to the steric hindrance of the Diels-Alder cycloaddition reaction, the energy barrier of the ratedetermining step of converting furan compounds to aromatics is relatively small. The catalyst can lower the reaction energy barrier; the more active sites of the catalyst, the faster the reaction rate, which provides useful guidance to select a suitable catalyst for the dehydration and aromatization of furan compounds in order to produce aromatic compounds in experiments.

Declarations
I hereby declare that all the information contained in this resume is in accordance with facts or truths to my knowledge. I take full responsibility for the correctness of the said information.

Fund
The authors are grateful to the Key Research and Development Project of Jiangsu Province (BE2019009-4).
This study was funded by the Jiangsu Province "333 project" (BRA2019277). This research work is supported by the high-performance computing platform of Jiangsu University.

Con icts of interest/Competing interests
The authors declare that they have no con ict of interest.

Availability of data and materials
The datasets used and analyzed during this article are availability from the corresponding author on reasonable requests.

Code availability
All calculations were performed on the Gaussian 09W program suite.
ChemDraw Pro 14.0 and Origin 2017 software are used for mechanism visualization and data mapping, respectively.

Authors' contributions
Xiaoxue Cheng wrote the paper and produced the charts.
Ding Jiang supervised the calculation method of the paper and the review of the calculation data.
Zhen Xia was in charge of the Py-GC/MS experiment.
Bahram Barati and Karthickeyan Viswanathan are responsible for the correction of English grammar.
Yamin Hu,Lili Qian and Zhixia He supervised the experiment.
Shuang Wang guides the general idea of the article.
Hongping Li directs the design idea of calculation path.

Compliance with Ethical Standards
This paper does not contain any studies with human participants or animals performed by any of the authors.