One-step Preparation Of Vanadium-supported Mesoporous Silica Bulk Catalyst And Its Application In Propane Dehydrogenation

Propene is essential petrochemical intermediates,which is widely used as a feedstock in the production of a variety of chemicals.Propene is mainly produced by steam cracking and fluid catalytic cracking?FCC?in conventional processes but its yield cannot meet the market requirements.Besides these two approaches,catalytic dehydrogenation?DH?is an attractive alternative with the rapidly growing demand for propene and its derivatives.Non-oxidative propane dehydrogenation?PDH?has been realized in commercial applications.V-based catalysts are reasonably priced and environmentally friendly,which are also one of significant DH catalysts.They have been used mainly in oxidative dehydrogenation reactions,while a few studies on PDH have also demonstrated attractive catalytic performance.Although oxidative dehydrogenation has many advantages,such as avoiding thermodynamic equilibrium limitations,decreasing carbon deposition and reacting at low temperature,the process unavoidably generates by-products due to deep oxidation of propene.These reasons influence the selectivity of catalysts and limit further industrial development.Given that mesoporous silica monolith has superior textural properties.Our works design and prepare Vanadium-doped mesoporous silica catalysts by a situ one-step synthesis method,where the prepared catalysts are applied to PDH.Note that the preparation process of the catalysts is pollution-free,and there is no loss of the VOx precursor.This topic studies the structure and catalytic performance of the catalysts in three major aspects:V/Si mass ratios,gelation and calcination temperatures,and VOx precursors.The catalysts with different V/Si mass ratios?nVOx-SiO₂,n for V/Si mass ratio and the range from 1/24 to 1/3?are mainly studied the influences of different VO_X contents on the catalysts.The catalysts are prepared with different gelation and calcination temperatures?GT-VOx-SiO₂-CT,GT for gelation temperature?unit:^°C?and the range from 30 to 90;CT for calcination temperature?unit:^°C?and the range from 550 to 650?,where the influences of changes of two main temperature during preparation process are investigated on catalyst properties.In addition,the catalysts with different V precursors?VOx-SiO₂-m,m for different types of precursors?mainly use the ammonium vanadate(NH₄VO₃,V⁵⁺),vanadyl sulfate(VOSO₄,V⁴⁺)and vanadium acetylacetonate(V?AcAc?₃,V³⁺)as vanadium precursors,and are studied their effects on the catalysts.In the work,VOx-SiO₂ catalysts are prepared in situ one-step synthesis method.These results indicate that vanadium species have good dispersion on mesoporous silica,and that both monomeric and low-polymerized vanadium species are the main active sites for PDH reaction.VOx-SiO₂ catalysts with moderate vanadium doping display a mesoporous structure with the high specific surface area and large pore volume,whereas excessively high vanadium doping will disrupt the mesoporous structure due to the formation of bulk V₂O₅ crystallites.Catalytic tests have shown that the catalyst with a V/Si mass ratio of 1:12 exhibits optimal catalytic activity and stability for PDH reaction,achieving a superior propane conversion that decreases from 64%to 30%and stable propylene selectivity of⁹0%over 21 h at 580^°C.In regeneration behavior,it also exhibits good long-term catalytic stability.Considering that the actual reaction temperature is at 580^°C for PDH,the catalyst calcined at the same temperature shows better catalytic stability.In addition,the increase of gelation temperature will obviously shorten gelation process time.The structural and performance results for the catalysts reveal that gelation temperature has only a slight effect on the surface acidity,textural properties and vanadium species dispersion of VO_X-SiO₂ catalysts within a reasonable temperature range?30-60^°C?.The60-VO_X-SiO₂-580 catalyst,with a gelation temperature of 60^°C,a calcination temperature of 580^°C,obviously shortens gelation process time by half compared to the catalyst gelled at 30^°C and exhibits superior catalytic performance for PDH.Note that the catalyst investigated in this study exhibits excellent long-term stability.Its catalytic activity can be fully recovered and propene selectivity has a slight increase over the eight reaction-regeneration cycles for PDH.Finally,the catalyst using NH₄VO₃ as vanadium precursor has higher activity,but the catalyst using V?AcAc?₃ as vanadium precursor shows better stability.These results may provide new insight into industrial applications of vanadium-doped porous silica materials.