Investigation Of Converting Ethylene And 1-butene To Propene Via Metathesis Reaction Over W-based Catalyst

Propene,as a basic organic petroleum chemical raw material with various applications,its market demand has been increasing year by year due to the rapid growth of polypropylene demand in recent years.Therefore,researchers are encouraged to develop more rapid and effective processes for increasing propylene production.Methanol to olefins?MTO?and Fischer-Tropsch synthesis to olefins?FTO?are two effective solutions but their products are C₂-C₄ mixtures of olefins.Olefin metathesis is an efficient approach to enhance propene selectivity by using the metathesis of 1-butene and ethylene.Supported WO_x catalysts were extensively investigated for conversion of 1-butene and ethylene into propene by metathesis reaction by using fixied-bed reactor to evaluated their catalytic performance,by combining the characterization techniques such as BET,TEM,XRD,UV-vis DRS,Raman,XPS,pyridine-IR,H₂-TPR,NH₃-TPD and ATR-IR to employed the chemphysical properties,coordination,molecular bonds,reducibility and acidity.Different supported materials?SiO₂,?-Al₂O₃,TiO₂,SBA-15 and SiC?were used to study the role of supports in catalytic performance for metathesis reaction,suggested that more Br?nsted acid sites and bigger specific surface area of catalyst are beneficial for this reaction.Higher calcination temperature will sintering the well-dispersed WO_x species at lower heat environment to crystalline WO₃.By compared the catalytic perfoamance of unsupported WO₃ reference,pure SBA-15 support and supported W/SBA-15 catalysts with different W content,we observed that crystalline WO₃ was short of metathesis activity.In addition,pure SBA-15 exhibited the better double-bond isomerization activity owing to the surface Si-OH could act as weak Br?nsted acid site.The reaction activity and propene selectivity first increased and then slowly decreased with the addition of tungsten oxide,and reached maxima at 15W/SBA-15.Prior to the metathesis reaction,different pretreated atmospheres such as oxidizing gas?O₂ and H₂O?,reducing gas?H₂?and inert gas?N₂?were used to adjust the initially state of WO_x species.The results illustrated that H₂ pretreatment significantly shorten induced period,and water vapor pretreatment led to complete loss of metathesis activity but not affect the isomerization activity,this is due to the sintering of high dispersed WO_x species to form inactive crystalline WO₃ and the suppressed reduction of WO_x species.Noted that the catalytic performance partially recovered after re-calcined in air at 550°C like the preparation procedure for fresh catalyst.The influences of WHSV and reaction temperature are also investigated to get the optimal catalytic performance.The increase of WHSV could reduce the residence time of reactants with active sites and resulting in shorter insufficient time to reach the thermodynamics equilibrium.The optimal catalytic performance is obtained over the reaction temperature of 500°C.According to characterization and performance results,we proposed that the initially formed W-carbene active sites were originated from the partially reduced isolated tetrahedral WO_x species which contained W=O or W-OH bond in W⁵⁺species as corresponding Lewis or Br?nsted acid sites.Based on above-mentioned conclusions,we aimed to use the one-step hydrothermal method to prepare W-MCM-41 catalyst so that the active sites can be highly dispersed on the surface of the support.The BET results shown that W-MCM-41?50?obtained with the stirring time of 5 h has a large specific surface area and regular and ordered two-dimensional straight channels.The stirring time prolongs to form irregular fissure macropores,and carbon deposition is easier.The number of active sites reaches a maximum value in W-MCM-41?30?and was not covered by the aggregated WO_x which led to the optimal catalytic performance.The excessive W content can destroy the ordered pore structure of the support.The presence of upstream isomerization catalyst could just only slightly improved the reaction performance,indicating that the isomerized and metathesis sites contained in W-MCM-41 were sufficient to achieve the reaction thermodynamic equilibrium and the process of 1-butene isomerization was extremely short.