Seed-assisted Hydrothermal Synthesis Of Sn-Beta For Conversion Of Glucose To Methyl Lactate

Lactic acid and its derivatives are important platform chemicals that can be applied to a wide range of industries including food,medicine,cosmetics and chemical industries.Their most common application is to produce polylactic acid,a biodegradable plastic,which has received more attention due to its environmental protection characteristics.So,the demand for lactic acid has greatly increased.At present,microbial fermentation is mainly used to prepare lactic acid and alkyl lactate in industry,but this method has many disadvantages such as high cost and low space-time yield.More and more researchers are beginning to explore chemical methods with simple operation,low cost and high yield.Metal-containing zeolites are versatile solid Lewis acid catalysts,which are widely used in the conversion of biomass sugar*BEA type tin silicate zeolite,namely Sn-Beta,shows excellent catalytic performance for conversion of monosaccharides and disaccharides to lactic acid or alkyl lactate.Sn-Beta is prepared by the hydrothermal synthesis method in the fluorine system,which has the advantages of higher hydrophobicity,higher thermal stability and higher selectivity of alkyl lactate.However,since the catalytic performance of Sn-Beta in the conversion of sugar to alkyl lactate highly depends on the synthesis parameters,it is essential to study the influence of synthesis parameters on the properties and catalytic performance of Sn-Beta.In this work,Sn-Beta was prepared by seed-assisted hydrothermal synthesis method in the fluorine system.The effects of the synthesis parameters such as the water amount of the synthesis gel and the crystallization time on the synthesis,property and catalytic performance of Sn-Beta in the conversion of glucose to methyl lactate(MLA)were investigated in detailed.The crystallization rate was accelerated remarkably by reducing the water amount of the synthesis gel.Sn-Beta was almost fully-crystallized at one day with nH2O/nSiO2 of 4.5,but seven days were needed with nH2O/nSiO2 of 7.5.However,Sn-Beta synthesized from nH2O/nSiO2 of 4.5 has more extraframework SnO2 due to the fast formation of*BEA structure and the slow incorporation of Sn4+into the framework site,which resulted in the lower activity of MLA formation.More Sn4+species were incorporated into the framework sites and thus more Lewis acid sites were generated with increasing the crystallization time.At the same time,the ratio of open to closed Sn sites(RO/C)increased,which led to the decrease in the activity of MLA formation due to that the open Sn site with a proximal silanol is unfavorable for the conversion of glucose to MLA.During recycling study,the yield of MLA increased gradually over the fully-crystallized Sn-Beta catalyst.Characterization results of the recovered catalyst indicate that the structure preserved well,but the value of RO/C reduced due to the change of the state and microenvironment of Sn species,which result in the increase of MLA yield.