Synthesis Of PtSn Catalyst Based On A Composite Of Titanium-silicon Zeolite Enwrapped With Mesoporous SBA-16 Material And Its Catalytic Performance For The Propane Dehydrogenation To Produce Propylene

Due to the wide application of it to produce chemical products such as acrylonitrile,cumene and propylene oxide,propylene has become a very important chemical raw material.Traditional steam cracking and catalytic cracking methods can not meet the increaced global demand to propylene.And with the discovery of large number of shale gas in the word,the direct dehydrogenation of propane to propylene has received much attention recently.Pt-based catalyst is a highly active,environmentally friendly catalyst.And the addition of suitable amount of Sn improves the activity and stability of the catalyst,making the PtSn supported catalyst more widely used.The propane dehydrogenation reaction is an strong endothermic process,which takes place at the two-phase boundary among gas(C3H8)-solid(catalyst)over 550℃.Agglomeration of Pt and carbon deposition are the main reasons for catalyst deactivation at high temperature.Therefore,for Pt-based catalysts,the selection and modification of the suitable carrier is particularly importantNew catalysts were designed and synthesized based on two ideas in this thesis work.One is to design mesoporous and microporous composite hierarchical pore catalysts.This pore structure combines the advantages of mesopore and micropore:it accelerates the diffusion rate of propane and propylene inside the catalyst.It also adds acidity to the mesoporous material of pure silicon oxide.Secondly,sheet-like TS-1 with different silicon-titanium ratios were prepared and the best silicon to titanium ratio was studied.The structure and physicochemical properties of as-prepared titanium-silicon oxide support and PtSn based catalysts were characterized by means of XRD,BET,SEM,TEM,FT-IR,Py-IR,XPS,XRF,NH3-TPD,UV-Vis and Raman.The main research contents and conclusions are as follows1.Spherical TS-1 with an average diameter of lum were prepared by simple hydrothermal synthesis method.And the composite carriers TS-1@SBA-16 with different ratios were also prepared by one-step hydrothermal synthesis.After loading PtSn,it exhibited higher initial activity and stability in propane dehydrogenation than TS-1 or SBA-16 alone.At the same time,the role of hydrogen in the reaction was preliminary studied,and the presence or absence of hydrogen and the effect of different hydrogen addition times on the reaction were compared.The optimum TS-1 amount(10%)was investigated and determined,and excellent cycle performance was exhibited in the propane dehydrogenation reaction of 4 cycles(7 h each time),and good stability was maintained in the long-term test(36 h),showing higher application value.Finally,compared with Al2O3 and ZSM-5 support,the catalytic activity of the prepared composite material supproted catalyst was significantly improved.Under the premise of determining the optimal TS-1 amount,the size of TS-1 zeolite was further changed,and a series of 100-1000 nm TS-1 zeolites were prepared and enwrapped in SBA-16.In the absence of hydrogen,the compostie TS-1@SBA-16 with smaller size showed the best initial activity and stability,indicating that the size effect is also applicable in composite materials.2.Using urea as an auxiliary,a series of sheet-like TS-1 with different ratios of Si/Ti were prepared by hydrothermal method.As the Ti content increases,the crystallinity,morphology and specific surface area of the molecular sieve remain basically unchanged.However,the proportion of Ti species outside the skeleton gradully becomes larger,which leads to an increasingly obvious acidity and electronic effect of the carrier.Excessive acidity will reduce catalyst stability.Moreover,under the same Si/Ti,the sheet-like TS-1 zeolite has a larger external specific surface area and exposes more acid sites than the 1 um spherical TS-1,which is more advantageous for the dispersion of the active metal PtSn and the synergistic activation of propane.The flake with TS-1 also accelerates the mass transfer of reactants and products inside the zeolite,reduces the occurrence of side reactions,and exhibits better catalytic activity than spherical TS-1.