Molecular Simulation And Mechanism Study Of Key Catalytic Processes Of Bio-Based Chemicals

With the increasingly severe environmental and resource issues and the continuous intensification of the greenhouse effect,bio-based chemicals,which can replace petroleum-based products,obtained through efficient conversion or preparation processes using biomass resources as raw materials have attracted increasing attention from researchers and industry.However,many bio-based products face the problems of low catalytic conversion efficiency and complex conversion steps.The key of solving these problems depends on the construction of high-efficiency catalysts for the key transformation process and the development of new production routes,and the fundamental foundation comes from the deep understanding and analysis of the catalytic mechanism.Combining the molecular simulation as the main research methods with the chemical experiments,this research focuses on the two key points of the important transformation processes of bio-based products,which are（1）the control method of the catalyst acidity based on the known reason of the difference of catalyst acidity and（2）the development of high-efficiency catalysts of the key processes and new conversion pathways of bio-based chemicals.Here,the processes of dehydration of lactic acid（LA）to acrylic acid（AA）and the preparation of para-xylene（PX）from 2,5-dimethylfuran（DMF）and ethylene are chosen as the representative key processes of bio-based chemical conversion.The relationship between the morphology and the acidity of hydroxyapatite（HAP）as catalyst was explored,and based on this,a method to regulate the acidity by regulating the morphology of HAP was proposed.The possible adsorption structure and energy of LA on（100）and（001）surfaces of HAP were studied by calculation.The results show that the adsorption structures of LA on HAP surface can be divided into two types:monodentate adsorption and bidentate chelation adsorption according to the differences of oxygen-containing functional groups in LA.The average adsorption energy（1.02 e V）of different LA adsorption structures on（100）surface is significantly lower than that on（001）surface（1.46 e V）,which means the（100）surface has weaker acidity than（001）surface.Additionally,HAP with different morphologies were prepared by the hydrothermal method and characterized.The results show that the area ratios of the two surfaces(S₁₀₀/S₀₀₁)in HAP with different morphologies are 17.35 and 4.5 respectively.Among them,HAP with longer particle size and more exposed（100）surface shows weaker acidity in the acid characterization of NH₃-TPD,which is consistent with the results of the above adsorption energy calculation.For the process of preparing PX from DMF and ethylene,the mechanism of SBA-15 supported tungsten oxide（WOx/SiO₂）in the catalytic process was explored,and compared on the typical Lewis acid（L-acid）site model and the catalytic site cluster model of different metals with similar structure.It is found that WOx/SiO₂ system has no obvious catalytic effect on the Diels alder addition step of DMF and ethylene（only reducing 0.02 e V）,but has an obvious catalytic effect on the dehydration of cycloaddition products.The C-O bond breaking and two-step proton transfer energy barriers are reduced to 0.25 e V,0.7 e V,and 0.15 e V respectively.The rate-limiting step changes from C-O bond breaking in the dehydration step to D-A addition.This mechanism is different from the known conclusion that the rate-limiting step under L acid is the dehydration step,which is reported for the first time in L acid.Based on the analysis of the main reaction mechanism of preparing PX from DMF and ethylene catalyzed by WOx/SiO₂,the energy changes of DMF hydrolysis amplitude reaction and water molecule decomposition at the catalytic site were further explored.The surface occupation state of the catalyst and the selectivity of main and by-products were also simulated on the self-coded Kinetic Mente Carlo（KMC）simulation tool.It is found that the water molecules produced by the formation of PX from DMF and ethylene are more likely to decompose（19.80 kcal/mol）at the catalytic site than desorption（23.28kcal/mol）,and the original catalytic site can be transformed from L acid to Bronsted acid（B acid）.The py-FTIR characterization results of catalyst before and after water treatment verified the transformation from L acid to B acid in the presence of water.KMC simulation shows that the selectivity of HDO decreases from 48%at 450k（about 177°C）to 35%at 600k（about 327°C）,and the selectivity of PX increases from 52%at 450k to 65%at 600k.The experimental results were also consistent with the simulation results that the selectivity of PX increases from 62%（0.5 h）and 53%（6 h）at 200°C to 68%（0.5 h）and 73%（6 h）at 300°C respectively,The selectivity of HDO decreases from 32%（0.5 h）and 42%（6 h）at 200°C to 25%（0.5 h）and 19%（6 h）at300°C,respectively.Through the understanding of the catalytic mechanism and the evaluation of the side reaction energy,the main and side reactions were further calculated around the high-efficiency catalyst Sn PO.The main reaction mechanism at sites with different acid types were analyzed and verified by experiments.The main reaction mechanism of the L acid site（Sn site）is consistent with WOx/SiO₂,which mainly reduces the C-O bond breaking energy barrier in the dehydration step（about 40 kcal/mol）,and the D-A reaction becomes the rate-limiting step.B acid site（P-OH）can simultaneously catalyze the hydrolysis of DMF to HDO and the subsequent evolution of HDO.The maximum activation energy barriers of DMF hydrolysis forward and reverse reactions are 22.08 kcal/mol and 22.14kcal/mol,respectively,which means the DMF hydrolysis is reversible and have energy advantages over the main reaction（D-A addition energy barrier 27.7kcal/mol）.HDO produced by hydrolysis can produce enol intermediate and further produce by-product MCP and polymer under the catalysis of P-OH;O¹⁸isotope labeling also show that DMF and HDO could be transformed into each other under the given reaction conditions.Under the catalytic condition of a small amount of P-OH,the selectivity of preparing DMF from HDO can reach91.2%and the conversion of HDO can reach 90%.The PX selectivity can reach80%and the HDO conversion is close to 100%with HDO and ethylene as raw materials under the catalysis of Sn PO.Based on this,a new bio-based PX synthesis route with HDO as raw material is proposed for the first time,which has fewer steps than the traditional route to get a simple operation.In summary,the relationship between the structure of hydroxyapatite and acidity based on DFT calculations is analyzed first,which can provide an idea of a method to adjust the acidity of hydroxyapatite by changing the morphology of hydroxyapatite.Secondly,analyzes the difference of the mechanism in catalyzing DMF and ethylene to prepare PX between traditional zeolite-based catalysts and WOX/SiO₂ and Sn PO.It was discovered that WOX/SiO₂ and Sn PO as L acid catalysts main catalytic dehydration step which is similar to B acid in the catalytic process;Finally,DMF hydrolysis which is the main side reaction was evaluated.The evolution process and energy change of the reactions under the P-OH were pointed out.The reversibility of the hydrolysis of DMF to HDO was verified by both calculation and experiment,and a new route for the synthesis of PX using HDO as a raw material is proposed for the first time based on the reversibility.