Synthesis Of Tin-incorporated Mesoporous SiO₂ Beads And Their Catalytic Performances

The development of heteroatom-incorporated molecular sieves effectively overcomes the low activity of pure silica molecular sieves,making it much broader for the practical application of catalytic field.Especially,heteroatom-incorporated molecular sieves with Lewis acid sites have a certain ability to accept electrons,and can activate the substrate and change the reaction process.Up to now,however,most of heteroatomincorporated molecular sieves are in the powder format and generally facing the problem that high-speed centrifugation or special filtration is required during the separation of catalyst from the reaction system,although they have a high catalytic performance in the catalytic reaction.In industry,such problem may lead to a lot of energy waste and drain on manpower and material resources.Therefore,it is very necessary to develop a novel catalyst with good catalytic performance,easy separation,good stability and high reusability.In this paper,tin-incorporated mesoporous SiO2 beads(Sn-MesoSB)were synthesized with anion-exchange resin beads as morphology-directing templates and stannosilicate species as precursor in the alkaline solution of tetrapropylammonium hydroxide via the hard template method.By the processes of hydrolysis,ion exchange,hydrothermal treatment and calcination,the catalysts in bead format replicated perfectly the macroscopic morphology and pore structure of anion-exchange resin IRA-900.The morphology and structure of the synthesized samples were characterized using SEM and TEM images and N2 adsorption/desorption measurement.The characterization results showed the materials are regular beads with a rather uniform size of ca.0.3 mm ~ 0.9 mm and continuous large-mesopore channels of about 32 nm.UV-vis spectra demonstrated that the existence of tetrahedral tin species is mainly in the bead samples.The Baeyer-Villiger oxidation and biomass transformation were as model reactions to study the catalytic performance of the tin-incorporated mesoporous SiO2 beads.SnMesoSB proved to show high activity and selectivity for the reactions.Under the optimized experimental conditions,for the Baeyer-Villiger oxidation,the conversion of adamantanone approached ca.100% in dioxane for 3 h at 90 °C(selectivity up to 99%);For the biomass transformation,the conversion of 1,3-dihydroxyacetone reached 87% and the selectivity of methyl lactate was more than 99%.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 5 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.