Molecular Simulation Study Of Pyrolysis Mechanism Of Cellulose

Faced by the increasingly severe problems of global energy shortage and environmental pollution, people begin to pay more and more attentions to biomass energy that is renewable and clean. Biomass liquefaction technology that generates bio-oil is considered to be the most promising technology in biomass energy utilizing field, because there are many advantages of bio-oil, such as easy storage and transportation, and high energy density. Cellulose is the most abundant renewable resource in nature, and as the main composition of biomass, its pyrolytic behavior reflects the law of biomass pyrolysis in a large part. So the study on the mechanism of cellulose pyrolysis contributes to understanding bio-oil performance and its formation mechanism.The early studies on mechanism of cellulose pyrolysis mainly focused on experimental studies to explore the change in output of tar, gases and coke in different conditions based on B-S Model. Also, the thermogravimetric analyses were performed to confirm pyrolytic characteristics of cellulose and the kinetic models of cellulose pyrolysis were constructed to obtain kinetic parameters. However, there are few researches about chemical reactions, formation mechanisms of main products and evolutionary processes of intermediates during cellulose pyrolysis. In order to understand the cellulose pyrolysis mechanism from the molecular level, the pyrolytic process of cellulose and formation mechanism of main products have been investigated by using molecular dynamics simulation and quantum chemistry theory methods in this paper. The main work and results involve:(1) The pyrolytic process of cellulose chain with 10 monomers was simulated by molecular dynamic method in AMBER force fileds. The cleavage of the chemical bonds was only considered in simulations and simulations were performed from 293 K to 1273 K. Simulation results showed that the thermal decomposition process of cellulose was mainly divided into three stages: low temperature stage(﹤550 K), middle temperature stage(550 800 K), high temperature stage(ï¹¥800 K). In low temperature stage, there is a few cleavage of C-OH bonds resulting in dehydration. In middle temperature stage, which is main pyrolytic stage of cellulose, a large number of chemical bonds rupture. At about 600 K, cellulose monomer gets to be formed by glucoside bond rupture, at the same time, pyranoid ring is opened and all kinds of molecular fragments are formed. In high temperature stage, molecular fragments are be further broken down into smaller molecular fragments. Based on related experimental results and analysis, possible formation pathways of major products through reaction of all kinds of molecular fragment have been analyzed.(2) For understanding the dehydration mechanism in initial pyrolysis of cellulose, dehydration reaction mechanism of the glycerine as a model was investigated using quantum chemistry theory methods. Six possible dehydration pathways were designed and and three of them were dehydration with addition of metal ions Li⁺. Two intermediates IMa and IMb can be formed when addition of metal ions Li⁺, and IM_a can be formed via coordination Li⁺ and O(5) and IMb can be formed via coordination Li⁺ and O(6). The calculation results show that each reaction except the reactions of formation of IM_a and IM_b is endothermic processes and dehydration reaction of the glycerine can take place when temperature of reaction exceeds 400 K. Compared to 1-2- dehydration reaction, 1-3- dehydration reaction is liable to take place, in which activation energy is about 233.75 kJ/mol. Addition of metal ions Li⁺ would be in favor of dehydration of the glycerine, and activation energy for 1-2-dehydration of the glycerine with addition of metal ions Li⁺ is about 201.95 kJ/mol, and that for 1-3-dehydration is about 202.14 kJ/mol.(3) For understanding of the formation mechanism of levoglucosan in pyrolysis of cellulose, pyrolysis mechanism of cellobiose as a model was investigated using quantum chemistry theory methods. The calculation results show that Mulliken population of glycosidic bond C1-O23 is smallest in cellobiose, which means that glycosidic bond is weaker bond and more easy to break. In pathway 1, the free radicals IM1a and IM1b can be formed by homolysis of glycosidic bond of cellobiose, and the reaction is endothermic with energies of 321.26kJ/mol. Free radical IM1a may react further to produce levoglucosan via transition state TS1a with an energy barrier of 202.72 kJ/mol. In pathway 2, levoglucosan P1 and glucopyranose P2 can be formed via transition state TS₂ in pyrolysis of cellobiose with a higher energy barrier of 377.54 kJ/mol. The analysis above shows that pathway 1 is the more likely reaction channel in pyrolysis of cellobiose. Addition of H⁺ would be in favor of breakage of glycosidic bond, and intermediate IM₃ , produced via rupture of glycosidic bond of cellobiose with addition of H⁺, can hardly transform to levoglucosan.(4) In order to understand the cellulose pyrolysis mechanism, especially the formation of small molecules such as glycolicaldehyde, acetol and furfural, the pyrolysis processes ofβ-D-glucopyranose are investigated using quantum chemistry theory methods. Four possible reaction pathways ofβ-D-glucopyranose pyrolysis were proposed based on related experimental results. In pathway 1, small molecules such as glycolicaldehyde, acetol, and CO are generated by ring-opening and fragmentation of glucopyranose; in pathway 2, 5-hydroxymethylfurfural is generated by ring-opening of glucopyranose and multiple dehydration; in pathway 3, levoglucosan is generated by dehydration between hydroxy of C(1) and hydroxy of C(6) of glucopyranose; in pathway 4, glucopyranose undergoes dehydration between hydroxyl of C(3) and hydrogen atom of C(4) to form 3,4-anhydroaltrose. The calculation results show that all reactions are endothermic and can take place spontaneously when reaction temperature exceeds 550 K. The changes of Gibbs free energies and the activation energies of rate-determining steps in pathways 1 & 2 are less than that in pathways 3 & 4. The activation energy of rate-determining step in pathway 1 is 297.02 kJ/mol and the activation energy of rate-determining step in pathway 2 is 284.49 kJ/mol. Based on thermodynamics and kinetic analysis, reaction pathways 1 & 2 are major pyrolysis reaction channels and the major products ofβ-D-glucopyranose pyrolysis are low molecular weight compounds such as glycolicaldehyde, 5-hydroxymethylfurfural, acetol and CO.(5) In order to understand formation mechanisms of CO and CO₂ in cellulose pyrolysis, the pyrolysis of 2,3,4-hydroxyl-butyraldehyde and 2,3,4-hydroxyl-butyricacid as model compounds was investigated by using quantum chemistry theory methods. The calculation results show that the energy barrier of decarbonylation of saturated butyraldehyde R1 is 293.95 kJ/mol and the energy barrier of decarbonylation of unsaturated crotonaldehyde generated after dehydration of R₁ increases, which illustrates that dehydration goes against the release of CO. Saturated butyricacid R2 is not easy to decarboxylate for the high energy barrier of decarboxylation, 311.69 kJ/mol, while energy barrier of decarboxylation of unsaturated crotonic acid generated after dehydration of R2 decreases obviously, which indicates that dehydration is in favour of the release of CO₂ and the release of CO₂ is related to the internal dehydration of cellulose. The decarbonylation is endothermic reaction and the absorbed heat decrease slightly with the increasing temperature, while the decarboxylation is exothermic reaction and the liberatedheat increase slightly with the increasing temperature. All reactions are spontaneous thermodynamically and can take place more easily with increasing temperature. (6) The possible pyrolysis processes of levoglucosan, the major product of cellulose pyrolysis, are investigated using quantum chemistry theory methods. The calculation results show that formation of chain intermediate via ring-opening reaction of levoglucosan as follow: at first, two hemiacetal linkages C(1)-O(7) and C(6)-O(8) of levoglucosan rupture to produce intermediate IM1 via transition state TS1 with an energy barrier of 296.53 kJ/mol, after that IM1 isomerizes further to form IM2 via TS2 with a energy barrier of 234..09 kJ/mol. Four possible reaction pathways of IM2 pyrolysis were proposed. In pathway 2, glycolaldehyde and 2-hydroxyl-3-keto-butyraldehyde are generated through breakage of C(2)-C(3) of IM2, in which there are lower energy barriers. In pathway 3, C(3)-C(4) bond of IM2 rupture to produce acetol and 2-hydroxyl-malondialdehyde with an energy barrier of 248.70 kJ/mol. Pathways 1 & 4 involve decarbonylation reactions, which have higher energy barriers, and are not easy to occur. Comparing energy barriers of all reaction pathways, it is easy to know that reaction pathways 2 & 3 are kinetically favorable in pyrolysis of IM₂.