Studies On Fast Synthesis Of Sn-Beta Zeolites For Catalytic Conversion Of Sugars To Alkyl Lactates

With the consumption of unrenewable resources such as coal and petroleum,and the problem of environmental pollution related to the use of fossil resources,the development and utilization of clean and cheap renewable resources are attracting the attention of worldwide researchers.Biomass is produced by plants through photosynthesis with sunlight,water and carbon dioxide,which is an abundant,cheap and renewable resource with neutral carbon emission.Carbohydrates are the main components of plant biomass.Production of liquid fuels and chemicals from carbohydrates is an important way to utilize renewable resources.Lactic acid and alkyl lactates are one of important platform compounds produced from carbohydrates,which are widely used in food,medicine,cosmetics and chemical industries.In order to overcome the disadvantages of the present fermentation methods for production of lactic acid and alkyl lactates,researchers have developed a chemical catalytic conversion method for preparation of lactic acid and its esters from carbohydrates in recent years.With cheap,abundant glucose and sucrose as raw materials,production of alkyl lactates through catalytic method is a complex process with multiple steps.In order to improve the yield of alkyl lactates,a variety of catalysts are used to catalyze this reaction.Among the solid catalysts reported in literature,Sn-Beta zeolite hydrothermally synthesized in fluoride medium is considered to be the best catalyst for the conversion of carbohydrates to alkyl lactates.However,it is difficult to synthesize Sn-Beta zeolite by the hydrothermal method.Sn species in the gel greatly inhibit the nucleation and growth of zeolite.Therefore,it needs long crystallization time(usually ten to twenty days),which greatly limits the application of Sn-Beta zeolite.Based on the research of literature and the previous work of our group,fast synthesis of Sn-Beta zeolite for conversion of carbohydrates to alkyl lactates was developed in this thesis.First,pure Si-Beta zeolite was synthesized by the hydrothermal method.Then,Sn precursor and Si-Beta zeolite were ground and calcined,and Sn was incorporated into the framework sites to obtain Sn-Beta zeolite.This method greatly shortened the time for preparing Sn-Beta zeolites.The resultsshowed that the successful introduction of Sn was closely related to the crystallinity of parent Si-Beta zeolites.There were more internal silanol defects in Si-Beta zeolites with low crystallinity,which is advantageous for Sn incorporation into the framework sites.Otherwise,it is difficult for Sn incorporation into the framework of Si-Beta.For Si-Beta zeolites with high crystallinity,alkali treatment was used to desilicate and creat the interal silanols for Sn incoporation,and then Sn-Beta zeolite was prepared successfully.The physicochemical properties of the prepared Sn-Beta zeolites were characterized by XRD,SEM,N2 physisorption,UV-vis,and FT-IR of pyridine and deuterated acetonitrile adsorption.Sn-Beta zeolites prepared from Si-Beta with different crystallinity were used in catalytic conversion of glucose to methyl lactate.The maximum yield of methyl lactate of 48% was obtained at 160 oC for 10 h,which is higher than over Sn-Beta-F(35%)and Sn-Beta-P(22%).Finally,the recycling of Sn-Beta in the conversion of glucose to methyl lactate was studies.It was found that the yield of methyl lactate was improved from 37% to 52% after six runs.The reason for the increase of the yield is probably that the active Sn sites became more easily accessible due to the leach of the extraframework Sn species.