A Study On Preparation Of Confined Metal Catalysts Within Heteroatomic Zeolites And Performance Of Propane Dehydrogenation

Propylene is one of the most important alkenes in chemical industry.It is used as raw material for the production of a large number of chemicals,including polypropylene(PP),acrylonitrile(AN),acrylic acid(AA),acetone and propylene oxide(PO),etc.Steam cracking and fluid catalytic cracking(FCC)of naphtha,light diesel oil and other petroleum by-products are the most common way to obtain propylene.However,the factors including the traditional process of high energy demand,Poor propylene selectivity,reduced oil reserves and rising oil prices have prompted the petrochemical industry to seek cheap and plentiful feedstocks and more efficient conversion technologies.China is rich in shale gas(mainly methane,ethane and propane)resources.At present,in addition to the technology of direct propylene production from syngas(FTP)or indirect propylene production through methanol intermediates(MTP),the process of producing propylene with high added value by dehydrogenation of cheaper propane has attracted extensive attention of researchers.In this thesis,with the purpose of preparing propane dehydrogenation catalysts of with high activity,selectivity and stability,noble and non-noble metals have been confined within the channels of zeolite carriers,which are used for direct dehydrogenation of propane.The combination of noble metal Pt with heteroatoms(Sn or Ge)in zeolites results in high performance which is difficult to achieved with traditional Pt catalysts.The unique application value of germanosilicates in the preparation of propane dehydrogenation catalyst is discovered for the first time.Moreover,in this research process,we also find that Ge-rich skeleton of germanosilicates can be reduced by hydrogen treatment under high temperature,and obtain several materials with different pore and crystalline structures.In the first part,the*BEA zeolite with high Sn content is synthesized by dissolution-recrystallization method with the deeply dealuminated Beta(DA-Beta)as the silicon source.The influence of reduction temperatures on the preparation of Sn@Sn-Beta catalyst is investigated.Finally,the optimal preparation conditions of the catalyst are determined.However,due to the limited amount of metal Sn extracted from skeleton and the Lewis acidity of Sn-Beta itself,the final catalytic activity and product selectivity of this catalyst in propane dehydrogenation(PDH)are not ideal except for its excellent stability,that is,less than 20%of propane conversion and less than 90%of propene selectivity at the reaction temperature of 600?.In order to improve the catalytic performance,a small amount of Pt is introduced into the support Sn-Beta for cooperative catalysis.The catalyst precursor Pt-Sn-Beta-P is prepared by wet impregnation using Sn-Beta with high Sn content as carrier and H2Pt Cl6 as platinum source.Then we study the influence of different pretreatment processes by calcination-reduction and direct reduction,respectively.The results show that the Pt particles agglomerate obviously after calcination,and the subsequent hydrogen reduction leads to the formation of a large number of Ptx Sny alloys which are unfavorable for the PDH reaction.On the contrary,the catalyst prepared by direct reduction has better catalytic performance.Namely,the precursor Pt-Sn-Beta-P is pre-reduced by hydrogen before beginning of the catalytic reaction,and the resultant catalyst Pt@Sn-Beta-R is subsequently used for the propane dehydrogenation.The catalyst is designed with the benefit of using highly dispersed skeleton Sn sites to anchor the guest Pt,and Pt subnanometric clusters(single atoms and ultrasmall clusters)encapsulated inside the pores of Sn-Beta are generated.The catalyst shows 47%of propane conversion,99%of propene selectivity and a stability period of up to 250 hours at 500?.And meanwhile,the effect of Pt and Sn contents on the catalytic performance is investigated,and an optimal ratio is determined.Then the possible reaction paths of propane dehydrogenation on this catalyst are proposed.Aiming at the result that the catalyst precursor Pt-Sn-Beta-P prepared by the above impregnation method is not resistant to calcination,we propose to load Pt on Sn-Beta by ion-exchange method in the second part.Specifically,the Sn-Beta zeolite containing templates(TEAOH)and Pt(NO3)2 are used as carriers and platinum sources,respectively.The Pt species are replaced into zeolite pores by ion-exchange,and the remaining templates are removed by subsequent air calcination.The Pt-Sn-Beta-ie-CR catalyst is then obtained by H2 pre-reduction treatment before the catalytic reaction and finally used for propane dehydrogenation.The TEA⁺cations are directionally displaced by Pt²⁺,which fall in zeolite pores.Under the aegis of the residual templates,even after the air calcination at high temperature the Pt particles do not grow up.The Ptx Sny alloys which are unfavorable to the reaction will not be formed in the subsequent reduction process,and finally a highly dispersed Pt clusters confined within Sn-Beta are formed.The catalyst obtains an about 35%of stable propane conversion at less Pt loading(0.22wt.%)at 500?,a 99%of propene selectivity,and most importantly,a considerable improvement in stability in comparison to the materials prepared by impregnation and calcination-reduction.We also investigate the cycle life of the catalyst.After the removal of carbon deposition by air calcination,the catalytic performance of the catalyst is basically restored,which gives the catalyst a good prospect for practical application.In the third part,Ge-UTL with different Si/Ge ratios and stabilized crystalline structure is prepared by high temperature pickling under the protection of structure-directing agents and using Si atoms in their own skeletons to isomorphously replace Ge atoms.Using the obtained Ge-UTL with stable structure as the carrier and H2Pt Cl6 as the platinum source,we prepare the catalyst precursor Pt-Ge-UTL-P by wet impregnation.Before the catalytic reaction,the precursor is pre-reduced by hydrogen to obtain the Pt@Ge-UTL catalyst.The special building units of double four-membered rings and high Ge content of Ge-UTL cooperate synergistically to highly disperse the Pt sites,and finally to generate almost atomically dispersed Pt restricted in Ge-UTL zeolites,so as to better improve the catalytic performance of propane dehydrogenation.The catalyst obtains a 54%of propane conversion,99%of propene selectivity at 500^oC and a continued steady catalytic activity over 4200 hours under different propane partial pressures,space velocities and reaction temperatures.The effect of pore size on the preparation and catalytic performance of catalysts is also investigated.Starting from the parent UTL,we prepared two materials(ECNU-15 and ECNU-14)with decreasing average pore sizes in turn,and the structures of which consist of double/single four-membered rings and only single four-membered rings,respectively.The existence of double four rings are necessary for the preparation of highly dispersed metals by impregnation and the stabilization of Pt species during the reaction.At the same time,we also discover for the first time that the Ge-rich skeleton of UTL can be reduced controllably through high-temperature hydrogen treatment,and the materials with two different pore structures,namely,IPC-2 and IPC-6 are obtained in a simple and green way.