Modified Activated Carbon For Direct Dehydrogenation Of Propane

Propylene,an important chemical raw material,could be utilized to produce general materials such as polypropylene,acrolein,isopropanol and it ranked second in the petrochemical industry.So far,propane dehydrogenation process has realized industrial production.Despite of well catalytic performance Pt-based and Cr-based catalysts,they each have corresponding limitation mainly because of sky-high cost of Pt raw materials and environmental-threatening and rapidly-inactivated Cr-based catalysts.Therefore,researchers are looking for catalysts that are environmentally friendly.highly active,and highly economical.Carbon materials have prominent features such as hicrarchical porous structure,high spetific surface area,rich surface fiunctional groups,and environmental friencdliiness.They have been widely used as catalyst support or catalysts in the fields of electrocat-alysis and alkane dehydrogenation.Among them,the most widely used carbon materials are carbon nanotubes,mesoporous carbon materials,nanodiamonds.and carbon fibers.However,there are few reports on the use of activated carbon for propane dehydrogenation reactions.Compared with other carbon materials,activated carbon has merits of simpler preparation process,lower cost and easy accessibility.In spite of relatively lower energy consumption of the oxidative dehydrogenation reaction,the direct dehydrogenation reaction is easier to control.Meanwhile,carbon materials can avoid the occurrence of side reactions such as oxidative combustion and deep oxidation of products.In this paper=modified activated carbon is used as a catalyst to explore the optimal reaction conditions,the catalytic activity of the catalyst in the process of propane dehydrogenation.Its influencing factors and active sites are discussed.The main content of the paper is as follows:(1)Concentrated hydrochloric acid is used to remove purchased commercial activated carbon impurities,and it is directly used for direct dehydrogenation of propane after drying.In order to s.elect the optimal reaction conditions,the effects of reaction temperatrure and space velocity on the catalytic performance of activated carbon were explored by changing the catalyst reaction conditions.Activated carbon shows excellent catalytic perfomance for direct dehydrogenation of propane when the reaction tempera.ture is 600?:the total feed volume is 20ml/min,and the feed ratio is C3H8:Ar=1:19:but it shows relatively low propylene selectivity under these conditions.(2)In view of the deficiencies of the activated carbon catalyst in the propane dehydrogenation reaction in the above experiments,four different phosphorus precursors were used to modify the activated carbon,and their catalytic performance of propane dehydrogenation was investigated.Among the modified catalysts,triethyl phosphate(TEP)modified activated carbon(PT-AC)showcases the best catalytic activity:and propylene selectivity has also been improved to a certain extent.To further investigate this phenomenon:the TEP concentration was changed to explore its catalytic performance.The experimental results show that when the TEP impregnation concentration is 0.2wt%,the catalytic performance is optimal.As the initial conversion rate reach 50.61%:the selectivity always maintained at about 90%.After multiple analysisit was found that phosphorus doping can remove a certain amount of electrophilic groups(such as carboxyli c acid groups)on the surface of the catalyst,reduce the occumence of side reactionsand improve propylene selectivity0 TEP doping concentration affects the number of active sites on the catalyst surface.and a certain amouunt of P=O groups may also show catalytic activity of dehydro g enation.(3)The carbon source of the commercial activated carbon used in the above experiments is unknown.In order to explore the differences on catalytic properties of activated carbons with different carbon sources:four certain sources of activated carbon were used for direct dehydrogenation of propane,namely coconut shell activated carbon.nut shell activated carbon.wood activated carbon0Coal-based activated carbon.It was found that the first two types of biomass activated carbon display higher catalytic activity.and the selectivity exhibit slowly rising trend during initial two hours of the reaction,then maintain stable during subsequent time.In view of this phenomenon,ammonium molybdate was used to dope-modified the husk activated carbon.The activity test showed that the addition of molybdenum significantly increased the propylene selectivity of the activated carbon catalyst maintaining stable at all times.Therefore,it can be seen that the doping of molybdenum attects the number of carbonyl and carboxylic acid groups on the surface of the activated carbon catalyst,resulting in the imqprovement of catalytic performance for the dehydrogenation reaction.