Synthesis Of Alkyl Lactate From Biomass-derived Sugar Over Post-synthesized Sn-Beta Zeolite Promoted By Metal Oxide

Lactic acid and alkyl lactate are widely used in the food,cosmetic and chemical industries.As a raw material for the production of biodegradable plastic polylactic acid,the demanding for LA and alkyl lactate is increasing gradually.Traditionally,lactic acid and alkyl lactate are prepared by microbial fermentation using glucose or sucrose as raw material.Chemocatalytic conversion of sugars to lactic acid and alkyl lactate is a new developed method and attracts considerable attention in recent years.Alkyl lactate can be obtained by chemocatalytic conversion of sugars in methanol.Among variety of catalysts,Sn-Beta zeolite exhibits excellent selectivity for alkyl lactate.Sn-Beta zeolite can be prepared by hydrothermal synthesis method and post-synthesis method.Hydrothermal synthesis of Sn-Beta zeolite is often carried out in a fluoride medium,and the obtained sample has large crystal size and few defects.However,since the ionic radius of Sn⁴⁺is larger than that of Si⁴⁺,it is difficult to incorporate Sn⁴⁺into the framework and long time is needed to crystallize Sn-Beta zeolite.The post-synthesis method contains two steps of dealumination and Sn incorporation.This method is time-saving and without toxic fluoride.But the obtained Sn-Beta zeolite often has a large amount of silanol defects,which leads to a lower selectivity for the conversion of glucose to alkyl lactate than the hydrothermally synthesized counterpart.In order to improve the selectivity of post-synthesized Sn-Beta zeolite for the conversion of glucose to alkyl lactate,we investigated the effect of different metal oxides as co-catalysts on the catalytic performance of Sn-Beta in the conversion of glucose.It is found that WO₃ greatly promoted the conversion of glucose to methyl lactate?MLA?.The yield of methyl lactate increased from 25%over Sn-Beta?0.2 g?to 52%over WO₃?0.1 g?and Sn-Beta?0.1 g?at 160 ^oC for 5 h and 3.1 wt.%of glucose concentration.Methyl lactate yield of 38%was attained even at glucose concentration of 10 wt.%.The role of WO₃ in each step of glucose conversion to methyl lactate was investigated,it is found that the binary catalyst of WO₃ and Sn-Beta can reduce the activation energy of the retro-aldol of fructose to C₃ products.,which is the rate determining step of the whole reaction.Thus,the formation of methyl lactate was facilitated.Characterization results indicate fine WO₃ particles adsorbed on surface of Sn-Beta in reaction media and decreased the silanol defects of Sn-Beta,which promotes retro-aldol of fructose.Finally,the reusability of WO₃promoted Sn-Beta catalyst was investigated,the yield of methyl lactate was kept after six runs,proving the recyclability of the binary catalyst.