Studies On Selective Oxidation Of Ethane And Ethylene Over SBA-15-Supported Molybdenum Oxide Catalysts

This dissertation focuses on the studies of selective oxidation of C₂H₆ and C₂H₄ over SBA-15-supported molybdenum oxide catalysts by O₂.The catalysts have been characterized in detail and the structure-reactivity relationships have been investigated. The reaction pathways and the mechanisms for C₂H₆ and C₂H₄ selective oxidation by O₂ have also been studied.The conversion of C₂H₆ and the conversion rate of C₂H₆ per Mo atom over MoO_x/SBA-15 with higher Mo contents(≥15.4 wt%) are much higher than those over the catalysts with lower Mo contents(≤6.6 wt%).The catalysts with lower Mo contents favor the formation of CH₃CHO(Its selectivity can be higher than 40%), while those with higher Mo contents can give HCHO with higher selectivity(HCHO selectivity can be higher than 30%).The highest single-pass yield of CH₃CHO(0.95%) can be obtained over the 9.6 wt%MoO_x/SBA-15,while the highest single-pass yield of HCHO(4.2%) and of total aldehyde(CH₃CHO+HCHO,5.0%) can be obtained over the 20.1 wt%MoO_x/SBA-15.The trend in the change of conversion rates of CH₄ per Mo atom over the MoO_x/SBA-15 with changing Mo content is similar to that observed in C₂H₆ selective oxidation.Our characterizations suggest that monomeric MoO_x species exist mainly in the catalysts with Mo content≤4.9 wt%,and that an increase in Mo content from 4.9 to 9.5 wt%causes the formation of MoO_x oligomers or small polymeric MoO_x clusters in SBA-15 mesoporous channels.For the samples with Mo contents between 15.4 and 23.5 wt%,the MoO_x nano- particles with -10-20 nm are the dominant molybdenum species as well as small amount of crystalline MoO₃.The correlation of catalytic performances and catalyst structures reveals that the polymeric MoO_x species are more active than the monomeric ones.The catalysts with highly dispersed MoO_x species favor the formation of CH₃CHO,whereas those containing polymeric MoO_x species show higher HCHO formation rates.The kinetic studies over the 20.1 wt%MoO_x/SBA-15 suggest that HCHO in C₂H₆ oxidation is mainly produced through the consecutive oxidation of C₂H₄.This is a new reaction to our best knowledge.Detail investigations on the selective oxidation of C2H4 show that the catalysts with higher Mo contents(～15.4-20.1 wt%) favor the selective oxidation of C₂H₄ to HCHO.Based on these results,we have designed a double-layer catalytic reaction system for C₂H₆ selective oxidation,which comprises Dy₂O₃-Li⁺-MgO-Cl^-(an C₂H₆ ODH catalyst) as up-stream catalyst and 20.1 wt% MoO_x/SBA-15 as down-stream catalyst.A HCHO single-pass yield of 14.2%can be achieved using this system.The dissertation have preliminarily studied the natures of structure-reactivity relationships and reaction mechanism over 4.9 wt%and 20.1 wt%MoO_x/SBA-15 catalysts.The results suggest that the available lattice oxygen in 20.1 wt%MoO_x/ SBA-15 is much more in amounts,easier to be reduced,better reactivities,and perhaps easier to transfer than that in 4.9 wt%MoO_x/SBA-15.On the other hand,the surface acidity of MoO_x nanoparticals are stronger than that of isolated MoO_x species.In situ FTIR spectroscopic studies suggest that ethylene dialkoxide(-OCH₂CH₂O-) is an intermediate over polymeric Mo species while ethoxide(-OC₂H₅) might be an intermediate over the monomeric Mo species.