Effect Of Hydroxyl Groups On The Surface Of SiO₂ Support On Olefins Cross-substitution Activity Over WO_x/SiO₂ Catalyst

Three domestic SiO₂ carriers(S-SiO₂,M-SiO₂,L-SiO₂)were used to prepare supported WO_x/SiO₂ catalysts.The surface properties of the three carriers were used as the research object,using nitrogen physical adsorption,X-ray diffraction,and Raman spectroscopy,Transmission Electron Microscopy,in suit Fourier transform infrared spectroscopy characterized the structure and properties of the three carriers and catalysts,and focused on the dispersion of the WO_x active species.The study found that the WO_x active species has a higher content of ortho silanol and isolated The S-SiO₂ carrier with terminal silanols has the best dispersibility,According to the Raman characterization results,the number of active sites in the reaction of the support with higher o-silicon-hydroxyl structure and isolated terminal silicon-hydroxyl structure is also significantly more than that of the tungsten-based catalysts prepared by the other two supports.The tungsten-based catalysts prepared on the other two supports indicate that the distribution of hydroxyl groups in the surface properties of the supports is a key factor affecting the dispersion characteristics of the active species of the WO_x/SiO₂ catalyst.The olefin exchange reaction performance of the three catalysts was investigated at 400?,and it was confirmed that the WO_x/S-SiO₂ catalyst exhibited significantly excellent catalytic activity and good propylene selectivity.This indicates that the highly dispersed characteristics of WO_x active species are better when supported.It is conducive to the generation of effective active sites,thereby improving the activity of the catalyst.And by changing the calcination temperature of the S-SiO₂ carrier,the distribution of silanol on the surface of the carrier can be changed.Studies have found that the carrier calcined at 550?has the best silanol distribution,and the tungsten-based catalyst prepared from the S-SiO₂ carrier calcined at this temperature has the highest reactivity.The results show that when different temperatures are used for calcination,different types of silanols on the surface will be transformed,which in turn affects the distribution of silanols on the surface of the carrier and the reaction activity of the prepared catalyst.Two tungsten-based catalysts were prepared using SBA-15 and USY molecular sieves as carriers,and compared with the tungsten-based catalyst prepared by domestic carrier S-SiO₂.It was found that the catalytic activity prepared by S-SiO₂ was significantly stronger than the above-mentioned two molecular sieves.Supported catalysts,this study can provide theoretical guidance for the selection of high-quality supports for the domestic production of high-efficiency olefin exchange reaction WO_x/SiO₂ catalysts.The catalyst characterization and activity evaluation of tungsten-based catalysts prepared with S-SiO₂supports with different W loadings showed that when the catalyst W loading was 8%,the catalyst had the best surface dispersion and catalytic activity.The catalyst activity was evaluated at different space velocities(WHSV=1,2 h^-1)using a tungsten-based catalyst with 8%loading.The results showed that the reaction activity of the catalyst was the best when the mass space velocity(WHSV)was 1 h^-1.High space velocity will lead to too short residence time of the reaction gas on the catalyst surface,which will affect the reaction activity of the catalyst.According to the literature,a silica carrier I-SiO₂ with high specific surface area was prepared,and a tungsten-based catalyst was prepared using the carrier.Through characterization and evaluation of the catalyst,it is found that its surface dispersion and catalytic activity are worse than those of the tungsten-based catalyst prepared by S-SiO₂ with a small total specific surface area,which proves that the specific surface area of the carrier does not affect the catalyst in the olefin exchange reaction.