Preparation Of Vanadium-supported Mesoporous Silica Catalyst And Its Application In Direct Dehydrogenation Of Propane

As an important raw material for chemical industry,propylene has been widely used in the production of various propylene derivatives,such as polypropylene,acrylonitrile and propylene oxide.The demand for propylene is constantly growing,and traditional propylene production methods such as steam cracking and catalytic cracking of oil cannot meet the current market demand.Therefore,it is very important to develop high efficiency catalyst for propane dehydrogenation to propylene?PHD?.At present,the application of chromium and platinum catalysts in propane dehydrogenation reaction has been industrialized,but the environmental pollution of chromium catalysts and the high price of platinum catalysts still prompt researchers to explore new and more effective catalysts.Vanadium-based catalyst as an important propane dehydrogenation catalyst can solve the above industrial problems.In contrast,environmentally friendly vanadium based catalysts are affordable and efficient,which have great potential in future industrial applications.In order to better disperse the active centers of vanadium oxides,this project is committed to finding a kind of silica material with uniform pore diameters and high specific surface area as a carrier.We designed and prepared vanadium-based mesoporous silica catalyst,and studied its catalytic performance in propane dehydrogenation reaction under the following reaction conditions:Reaction temperature:600^oC;Catalyst mass of 0.2g;Composition of feed gas C₃H₈/Ar?molar ratio?=1:9,total flow rate of feed gas of 20mL/min.First of all,this project uses sol-gel method to synthesize high yield VO_X-doped mesoporous silica catalyst nV-SiO₂-0.12CTAB with different V/Si mass ratio using cetyltrimethylammonium bromide?CTAB?as template?n indicates mass ratio,ranging from 3 to10;0.12CTAB indicates the molar ratio of CTAB to silica is 0.12?.After that,the V/Si mass ratio?n=7?was controlled in this study,and VO_X-doped mesoporous silica catalysts?7V-SiO₂-mX,X indicates different types of template agent,m indicates the molar ratio of template agent to silica?with different structures and properties were obtained by changing the type and quality of the template agent.Cetyl trimethyl ammonium bromide?CTAB?,cetyl dimethyl ammonium p-toluene sulfonate?CTATOS?and cetyl dimethyl benzylammonium chloride?BCDAC,Bn in short?were selected as the main template agents.Thecatalystsincluding7V-SiO₂-0.12CTAB,7V-SiO₂-0.06CTAB+0.06CTATOS and 7V-SiO₂-0.12Bn were mainly used for comparative study to explore the influence of the catalyst structure on catalytic performance.In the last part,a simpler method was used to prepare vanadium doped silica nanowire?nV-SiO₂ nanowire?under non-stirring conditions,and its catalytic properties were observed and analyzed.The results shows that the vanadium-doped mesoporous silica catalysts synthesized by sol-gel method made VO_X species well dispersed on the carrier,and its isolated and oligomeric vanadium species are the main catalytic active centers.At the appropriate V/SiO₂ mass ratio,the catalysts obtain a suitable pore volume and pore diameter with large specific surface area.When the V/SiO₂ mass ratio n increases to 9,the 9V-SiO₂-0.12CTAB catalyst shows the optimal catalytic activity and good catalytic regeneration performance.After 6h reaction at 600^oC,the propane conversion decreases from 64%to 35%,and the propylene selectivity remains at about 90%.In the six regeneration cycles,the catalyst activity can be restored through oxidative regeneration.However,when the mass ratio of V/SiO₂ is excessive?n=10?,the pore diameters of the catalyst decreases and results in a highly polymerized VO_X species,which greatly reduced the catalytic performance.Secondly,it was found in the research that 7V-SiO₂-0.12CTAB,7V-SiO₂-0.06CTAB+0.06CTATOS and 7V-SiO₂-0.12Bn catalysts showed different pore channel characteristics,namely wormhole,radial and cavernous pore channels respectively by changing different template agents.Among them,compared with the catalyst?7V-SiO₂-0.12CTAB?synthesized with the single surfactant CTAB,7V-SiO₂-0.06CTAB+0.06CTATOS catalyst has the effect of pore expanding by the addition of CTATOS.It greatly improves the propylene selectivity from 90%to 95%in the PDH reaction.By referring to the comparative study of 7V-SiO₂-0.12Bn catalyst,it was found that the hydrophilic end macromolecular toluene sulfonate?TOS^-?played a key role in pore expanding,rather than simply increasing the molecular weight of the surfactant.The 7V-SiO₂-0.06CTAB+0.06CTATOS catalyst showed excellent catalytic activity.We further optimized its vanadium loading amounts,and found that 9V-SiO₂-0.06 CTAB+0.06CTATOS catalyst could obtain higher propylene selectivity about 97%,which is expected to be further studied and applied in the future industrial production.In the last part,it was found in the preparation and research of nV-SiO₂ nanowire catalyst that 3V-SiO₂ nanowire catalyst with a low vanadium loading amounts could obtain high catalytic activity.In the 6h propane dehydrogenation reaction,the propane conversion rate decreased from 58%to 34%,and the propylene selectivity remained around 87%.The catalyst also has certain research value in the future.