SnO₂-Based Acid-Base Amphoteric Catalysts:Design,Modification And Catalysis Of Biomass Sugar Converting Into Methyl Lactate

The research and development of renewable biomass energy is an effective measure to guarantee energy supply and reduce dependence on fossil energy.Converting biomass into platform compounds and further applied in food,medicine,energy,materials and other fields,which is an important means to realize the high-value utilization of biomass resources.As the most abundant component of biomass,biomass sugar are being widely studied on how to use simple process to convert it into high value-added chemicals such as methyl lactate（MLA）,which can be used in cosmetics,food additives,medicine,coating and other fields.The production of MLA now is esterification of lactic acid,produced by biologic fermentation,with methanol under sulfuric acid or other catalysts.This method has high requirements for microorganisms,and lactic acid is prone to dehydration,carbonization and other side reactions,and sulfuric acid corrodes the equipment seriously.Biomass sugar can be directly converted into methyl lactate under relatively mild conditions,which fully conforms to the spirit of green chemistry.The core of this method’s application scale expansion is to design a highly efficient,green and stable catalyst.In this study,Zn-doped SnO₂ and In-doped SnO₂/SBA-15 were designed as two acid-base heterogeneous catalysts,the main contents included the optimization and characterization of preparation conditions of the catalysts,the exploration of optimal reaction conditions for the preparation of methyl lactate by glucose catalytic conversion,and the investigation of the performance of the catalysts for reuse.The catalytic activity of the catalyst was investigated by calculating the conversion rate of biomass sugar and the yield of sugar to methyl lactate.The main research contents are summarized as follows:（1）Zn doped SnO₂ acid-base amphoteric catalyst was prepared by coprecipitation.XPS characterization confirmed that Zn²⁺was successfully doped into the SnO₂ lattice,Py-IR results showed that Zn²⁺was doped to produce more Lewis acid sites and Br?nsted acid sites,and NH3-TPD showed that Zn²⁺was doped to introduce basic sites.The catalyst with the best acid-base strength and content can be obtained by changing the Zn²⁺doping amount and calcination condition.XRD and BET results showed that the Zn²⁺doping inhibitied the growth of the nanocrystalline and increased the specific surface area of the catalyst.Catalyst prepared by Zn²⁺molar ratio of 20%and calcined at 500℃for4 h has the largest specific surface area and the most suitable acid-base active site strength and content,namely the weak acid sites and total acid sites content,the strong base sites and total content base sites simultaneously reach the maximum,at this time the catalyst has the best performance.When fructose,glucose and sucrose were used as the reaction substrates under the optimal reaction conditions,the methyl lactate yields were 60.3%,56.2%,and 72.6%,respectively.With glucose as the substrate,the yield of methyl lactate did not decrease significantly after repeated use of the catalyst for 8 times.（2）In-doped SnO₂/SBA-15 acid-base amphoteric solid catalyst was prepared by one-pot method.Small-angle X-ray diffraction and TEM results showed that the catalyst has a two-dimensional hexagonal pore structure,and EDX characterization showed that the active components were uniformly loaded in the SBA-15 pore.The catalyst with the best acid-base strength and content can be obtained by changing the In³⁺doping amount,active component loading content and calcination condition.When the molar ratio of In³⁺was 20%and the loading content of the active component was 8wt%,the specific surface area of 0.2IS/SBA-15（8%）prepared was 727 m²/g,the weak acid sites and total acid sites content,the strong base sites and total content base sites simultaneously reach the maximum,at this time the catalyst has the best performance.When fructose,glucose and sucrose were used as the reaction substrates under the optimal reaction conditions,the methyl lactate yields were 64.2%,61.7%,and 73.8%,respectively.The catalyst has good stability and its activity did not change obviously after 8 times of reuse.