On The Catalytic Performances And Mechanism Of Propane Dehydrogenation Over Metal Oxides

Propylene is an important feedstock for the manufacture of various products,such as polypropylene,propylene oxide,and acrylonitrile.Propylene is primarily produced by fluid catalytic cracking（FCC）and steam cracking techniques as a by-product of ethane.However,the flourishing development of shale gas has resulted in a shift from oil-based naphtha to shale-based ethane for producing ethene,leading to a decrease of propylene supply.Propane dehydrogenation（PDH）has became increasingly significant as a propylene on-purpose production technology,which is profitable becacause of lage extent in price between propane and propylene and high puity of alkene.Chromiumoxide-or platinum-based catalysts are widely used in the commercial PDH process.But the high cost of Pt and heavy pollution caused by Cr restrict the further development of the PDH industry.This thesis describes investigations on of catalytic consequences of ZnO-and VOx-based catalysts with the hope to shed more light on understanding reaction mechanism.This thesis starts with a catalyst comprising ZnO modified by trace Pt（0.1 wt%）supported on Al₂O₃.Compared to metallic platinum and chromium oxide,zinc oxide is an inexpensive and low-toxic alternative for direct dehydrogenation of propane（PDH）.However,in addition to the limited activity,conventional zinc-based catalysts suffer serious deactivation due to ZnO reduction and/or carbon deposition.After adding 0.1 wt% Pt,enhancing activity and stability of ZnO/Al₂O₃ were observed.At 600 °C and atmospheric pressure with 3 h-1 propane WHSV,18.4 mmol/h/gcat C3H6（corresponding to 35% C3H8 conversion）and 97% C3H6 selectivity were achieved over the PtZnAl catalyst and the activity only decreased 15% after four-hour PDH reaction.Comparing to the 0.5% Pt/Al₂O₃ catalyst,which contains similar amount of Pt as commercial Ptbased catalyst,this catalyst exhibited comparable activity with much less Pt.Moreover,it can remain stable over 20 hours with approximately 100% propylene selectivity at 550 °C.Additionly,we have provided evidence indicating that Pt serves as a unique promoter to optimize the catalytic performance over ZnO/Al₂O₃ catalyst.It is likely that ZnO was modified as a stronger Lewis acid by Pt because of the electron interaction,thus easier C-H breaking and accelerated H2 desorption were achieved.This thesis also investigates on the nature of valence state and mechanistic aspects of PDH over the VO_x/Al₂O₃ catalysts.Supported VOx catalysts are promising for propane dehydrogenation（PDH）due to relatively superior activity and stable performance after regeneration.However,the nature of active sites and reaction mechanism during PDH over VOx based catalysts remains elusive.Different fraction of V5+/V4+/V3+ ion on the catalyst surface was obtained by reducing the sample with various vanadium densities.Specifically,the catalyst with higher VOx density is easier to be reduced,leading to higher proportion of V3+ ions on the surface.The apparent activation energy declines along with increasing vanadium loading,while TOF over the catalyst（V loading at most 9%）enhances constantly.Therefore,we consider that various active sites exist over the VO_x/Al₂O₃ catalyst and V3+ is more active for propane dehydrogenation than the other V sites with different oxidation state.Same immediate was detected by in situ DRIFTS during both propane dehydrogenation and propylene hydrogenation,indicating they share reverse reaction mechanism.In combination with previous studies,a new intermediate（i.e.,propenyl-vanadium V-C3H5）was proposed,instead of carbonyl compound or carbon material.Propane activation over the catalyst surface involves the subtraction of a hydrogen atom,then the formed vanadium propyl quickly transformed to propenyl-vanadium because of instability,as observed by IR spectroscopic technique.At elevated temperature,propylene was formed from propenyl-vanadium or propyl-vanadium,then desorbing from the catalyst surface,and coke deposition was produced at the same time,leading to catalyst deactivation.