Study On Preparation Of VO_x Based SiO₂ Catalysts And Their Performance In Direct Dehydrogenation Of Propane To Propylene

Propylene is a key basic chemical in the petrochemical industry,which is widely used in organic synthesis and polymer synthesis.Propane dehydrogenation（PDH）,as the most direct and selective method to produce propylene,has attracted extensive attention.In the process of PDH,the activation of propane C-H bond is an important step to determine the catalytic performance of the catalyst.Pt and Cr-based catalysts have good advantages in activating C-H bond,but their disadvantages such as high price and environmental pollution are particularly obvious.Recently,supported vanadium oxide（VOx）catalysts have become one of the most concerned non-noble metal catalysts because of their low price and good regeneration stability.This work mainly explores the effect of vanadium species on the catalytic performance of Si O₂supported vanadium-based catalysts.Firstly,vanadium was introduced into hexagonal mesoporous Si O₂（HMS）,and the supported vanadium-based HMS catalyst（V-HMS）was prepared and introduced into PDH reaction.The results showed that the catalytic performance showed a volcanic trend with the increase of vanadium loading.Considering the catalytic performance and stability comprehensively,8V-HMS catalyst is determined as the optimal catalyst.Through in-depth exploration,it is found that the space-time yield of propylene can reach 0.73 kg·H^-1·kg_cat^-1,and has excellent regeneration performance..According to TEM and TG-DTA,it was speculated that the catalyst deactivation is only caused by carbon deposition.The VOx species of the prepared catalysts were explored by UV-vis DRS,NH₃-TPD,H₂-TPR,and XPS.Combined with the degree of polymerization and catalytic performance of VOx,it was speculated that oligomeric VOx species have higher intrinsic propane dehydrogenation activity and stability than VOx with high polymerization.The reaction mechanism was studied by in-situ DRIFTS.It was speculated that V-O-C₃H₅intermediate containing C=C bond might be produced in the process of propane dehydrogenation,and a reaction mechanism of non-oxidative propane dehydrogenation over V-HMS catalyst was proposed.In order to further improve the performance of vanadium based catalyst PDH,a series of supported vanadium-based x V/S-1 catalysts were prepared by using Silicalite-1,a silicon molecular zeolite,as the carrier.When the as-prepared catalysts were introduced into PDH reaction,it was found that 5.5V/S-1 catalyst had the best reaction performance,the space-time yield of propylene was 1.7 times to that of8V-HMS catalyst,and it also had good regeneration stability.Combined with XPS characterization,it is found that there is the the highest proportion of V⁴⁺was found on5.5V/S-1.According to UV-vis DRS,Raman,H₂-TPR and DRIFTS,it is found that the hydroxyl nest on the outer surface of Silicalite-1 can disperse VOx species effectively and promote their reduction.On this basis,the internal and external surface migration mechanism of vanadium active species is proposed.In terms of reaction mechanism,with the assistance of in-situ DRIFTS,it is found that hydroxyl groups on the surface of Silicalite-1 participate in the PDH process.The interaction of these hydroxyl groups with a high proportion of V⁴⁺and a high proportion of isolated vox species in the sample is an important reason to promote the activity of 5.5V/S-1 catalyst.