| Due to the increasing of environmental pollution and global warming,biomass resources,as a clean and renewable "green resource",attract extensive attention.Glucose and xylose are the main monomers of cellulose and hemicellulose,which are abundant in nature.Lactic acid(LA)and its ester prepared from glucose and xylose are widely used in food,medicine,fine chemical industry and other fields.Nowadays,the production of LA and lactate from glucose and xylose is mainly by microbial fermentation,but this method has many problems such as strict technology,complicated operation steps and high recovery cost.Therefore,people try to convert glucose and xylose into LA and lactate by chemocatalysis.Sn-Beta zeolite is an excellent catalyst for the conversion of glucose and xylose because of its strong Lewis acidity and strong C=O activation ability.However,Sn-Beta zeolite prepared by post synthesis method has more silanol defects and poor selectivity to alkyl lactate;However,hydrothermal synthesis of Sn-Beta in fluoride media also has some problems,such as long synthesis time and so on.In order to solve these problems,a series of alkali(earth)metal ions modified Me-Sn-Beta zeolites were prepared by introducing a small amount of alkali(earth)metal salts NaCl,KCl,MgCl2,CaCl2 and BaCl2 into the synthesis gel.The introduction of a small amount of alkali(earth)metal ions significantly promoted the crystallization of Sn-Beta.Among them,the promotion effect of Mg2+ was the most significant.When the nSi/nSn of the gel was 100,the total crystallization time was shortened from 15 d to 5 d with the promotion of Mg2+(nMg/nSn).In Me-Sn-Beta zeolites,Sn4+ is introduced into the framework to generate strong L acid site.Alkali(earth)metal cations replace the protons of silanols,reduce the amount of silanols and produce L basic sites.In the conversion of glucose to methyl lactate(MLA),the selectivity of MLA was significantly improved.At 160℃ for 5 h,the yield of ML A increased from 30%(Sn-Beta)to～50%(Me-Sn-Beta).The step-by-step reaction showed that Me-Sn-Beta significantly promoted the rate-determining step of glucose conversion to MLA,that is fructose retro-aldol reaction,and thus improved the yield of MLA.The Sn content of Mg-Sn-Beta zeolite(nMg/nSn=1.0)was optimized,With the increase of Sn content,L acid amount of Mg-Sn-Beta increased.In the reaction of glucose to MLA,the yield of MLA increased with the increase of Sn content.Mg-Sn-Beta zeolite with different Mg content(nSi/nSn=100)were synthesized to catalyze the conversion of xylose to MLA.With the increase of Mg content,the Sn content of the synthesized Mg-Sn-Beta zeolite gradually decreased,the amount of L acid sites decreased and the L basic sites increased.In the catalytic conversion of xylose to MLA,the MLA yield first increased and then decreased under the synergistic catalysis of L acid-base sites,with the increase of Mg content.The highest MLA yield(48%)was obtained over 2Mg-Sn-Beta(nMg/nSn=2).The reaction results with the intermediate glycolaldehyde as the substrate showed that the L basic sites were beneficial for the aldol condensation,which promoted the formation of MLA.When 2Mg-Sn-Beta was used to catalyze the conversion of glucose xylose mixture,a higher MLA yield(52%)was obtained. |
