Microstructure Control Of Mesoporous CrO_x/SiO₂ Catalyst And Its Application In Propane Dehydrogenation Reaction

Propylene has been used in the production of various industrial chemicals,and as one of the most important raw material in chemical industry.Propylene was traditionally produced from petroleum by steam cracking,catalytic cracking and olefin double decomposition.However,traditional production technologies have been limited due to the shortage of global oil resources,and these production processes consumed too much energy and production cost.Therefore,it is particularly important to explore the method of propane dehydrogenation to propylene with high economic benefits,which meets the green and sustainable economic development needs.Currently,chrome-based catalysts are one of the most important catalysts used in industrial dehydrogenation of propane due to low price and low production cost.Compared with the traditional mesoporous silica,the hierarchical porous silica prepared by emulsion synthesis has larger specific surface area,pore diameter and pore volume,which are more conducive to the dispersion of metal active sites when used as support of catalystsbecause of its less internal diffusion resistance.In addition,organosilane can also modifythe structure of silica by co-condensation and obtain the effect of widening pore size.In this paper,we chose chromium as the metal active species and adopted hierarchical porous silica synthesized by emulsion as the support.We explore the influence of the ratio of water phase to oil phase in the emulsion system on the catalytic performance and the influence of3-methyleneoxypropyl trimethoxy silane?MPTS?on the catalytic performance.Firstly,a series of S-4Cr-SiO₂ catalystswith different morphologies and structures were prepared by adjustingthe concentration of ethanol?the concentration of the water phase?,including irregular hollow silica nanospheres containing wormhole mesoporous pores and hierarchical mesoporou silica nano-spheres containing radial pores.Ethanol was introduced into the synthesis system as assistant surfactant to transform the emulsion system into a micro-emulsion system,leading to different catalytic properties.The experimental results showed that the S0.5-4Cr-SiO₂ sample possesses largest radial channels when the volume ratio of ethanol to ethyl ether was0.5,and it presents the best dispersion of active sites.So then it showed the optimal catalytic performance on oxidative dehydrogenation of propane?ODHP?reaction.In addition,S0.5-4Cr-SiO₂ catalyst shows good long-term stability.Secondly,the method of adjusting the concentration of ethyl ether?the concentration of the oil phase?in the synthesis system fabricate a series catalysts of R-7Cr-SiO₂with different morphologies and structures.The change of ethyl ether concentration changed the silica from smooth nanosphere to flocky nanosphere.The experimental results showed that the R2-7Cr-SiO₂ sample possesses largest radial channels when the volume ratio of ethyl ether to ethanol was 2,it presents the optimal catalytic performance on OPDH reaction with the most uniform dispersion of active sites.In addition,the catalysts prepared by methanol and isopropanol instead of ethanol was used to compare with R2-7Cr-SiO₂ sample,and the catalyst prepared by impregnation was used to compared with R2-7Cr-SiO₂ sample.And the results show that ethanol is the best co-solvent and the catalytic performance of the sample prepard by one-step synthesis method is more active than the impregnation method.Finally,by introducing MPTS as a modifier,and a series of catalysts with different morphologies and structures were prepared by adjusting the concentration of MPTS and different methods of introduction.After modified by premix method,the catalyst gradually changed from smooth nanospheres to red blood cells like nanoparticles,and the pore size and pore volume increased significantly.By comparing the method of adding tetraethoxysilane?TEOS?and MPTS simultaneously with premix method,The experimental results show that 20%is the best modified concentration,and the premix method shows preferable catalytic performance.