Synthesis And Catalytic Biomass Conversion Studies Of Tin-doped Hierarchical Porous Stannosilicate Zeolite

The synthesis of lactic acid or alkyl lactates is a process of great appeal in the areas of renewable biomass conversion and green chemistry: these versatile products find applications in the chemical-,food-,pharmaceutical-and cosmetic industries.The most common commercial processes to produce lactic acid are based on carbohydrate fermentation.Drawbacks of such processes include low productivities and the need for expensive and energy-inefficient separation and purification steps.Since the literature reported that stannosilicate zeolites catalyzed the formation of lactate showed good catalytic activity and selectivity.The research of the stannosilicate zeolites in this aspect has attracted wide attention of researchers.Although the currently synthesize stannosilicate zeolites shows good catalytic performance and application prospects,the zeolites cannot effectively catalyze the conversion of bulky molecules,which are too large to access the active sites located inside the micropores.The catalytic performance of the hierarchical porous stannosilicate zeolites has been significantly improved over that of traditional stannosilicate zeolite.However,most of the currently known hierarchical porous stannosilicate zeolites have a "cavity" structure,this discontinuous pore structure limits the mass transfer capacity inside the zeolites,so it is of great significance to prepare hierarchical porous stannosilicate zeolites with a continuous threedimensional network structure.Hierarchical porous stannosilicate zeolites(HP-Sn-ZSM-5)were synthesized by hard template method in the alkaline solution of tetrapropylammonium hydroxide with anion exchange resin beads as the shape oriented template and stannic silicate as the precursor.Through the processes of hydrolysis,ion exchange,hydrothermal treatment and calcination,the prepared microbead catalysts can replicate the macroscopic morphology and pore structure of IRA-900 anion exchange resin.The morphology and structure of the synthesized samples were characterized by SEM,TEM and N2 adsorption/desorption tests.Characterization results show that the material is regular beads with a relatively uniform particle size,about 0.3mm ? 0.9mm.Ultraviolet-visible spectroscopy indicated that tetrahedral tin was mainly present in the bead sample.Two biomass transformation were as model reactions to study the catalytic performance of the hierarchical porous stannosilicate zeolites(HP-Sn-ZSM-5).HP-SnZSM-5 proved to show activity for the reactions.Under the optimized experimental conditions,for the conversion of 1,3-dihydroxyacetone to lactates,the yield of methyl lactate approached ca.65% in methanol for 6 h at 90 oC,HP-Sn-ZSM-5 has a higher yield of methyl lactate as a catalyst compared to microporous or mesoporous materials.The high catalytic properties were mainly attributed to the enhancement of the access to the catalytic tin sites through the continuous large-mesopore channels;For the conversion of glucose to lactates,the yield of methyl lactate approached ca.42% in methanol for 10 h at 160 °C.The high catalytic properties were mainly attributed to the enhancement of the access to the catalytic tin sites through the continuous large-mesopore channels.In addition,the leaching and reusability tests showed the catalyst was robust and can be reused up to 4 times without significant loss of activity and selectivity,pointing out that the tin species incorporated in the framework possessed high stability.Notably,such stannosilicate beads favored to the extremely direct separation from the reaction system only with a few seconds and did not require any filtration or centrifugation.The feature is very promising for industrial applications.