Study On The Catalytic Performance Of Titanium-modified Meso-microporous Zeolite (Alumina) Supported-PtSn Catalysts For The Dehydrogenation Of Propane To Propylene

Propylene is an important basic petrochemical materials and it could be utilized to make plentiful downstream products.The direct dehydrogenation of propane has attracted sustained attention and investigation because of its great potential in meeting the growing demand for propylene in recent years.The abundant natural gas?especially,the shale gas?reserves will further promote the prosperity of PDH industry in future.Platinum-or chromium oxide-based catalysts are most widely used in commercial propane dehydrogenation?PDH?to produce propylene processes and they have been extensively studied.Considered the request for green chemistry,the later system achieved less attention because of its toxicity.In contrast,much investigation has been devoted to the Pt-based catalysts due to its good prospect in industrial application.For further promoting the sustained and steady growth of propylene production,a great deal of effort has been devoted to explore high activity of PtSn-based catalysts to activate the C-H bonds of propane.Because the dehydrogenation of propane is endothermic and requires a high temperature?about 863K?,deep dehydrogenation is also happened with the by-product of deposited coke.To solve this problem,improvements on catalysts are still needed to reduce the side reactions and increase the selectivity of propylene.As is known to us,the catalytic activity of catalyst is significantly affected by the interactions between active sites and support in heterogeneous catalysis.For the traditional PtSn-based catalysts,improvement on the type or nature of support is an efficient method for achieving better catalytic results.In this dissertation work,we prepared a series of mesoporous Ti-KIT-6,hierarchical TS-1,3D mesoporous Ti-Si-spheres and the aluminas with different morphologies supported PtSn catalysts.The relationships among their structural and physic-chemical properties and catalytic performance are further investigated.The following main conclusions are achieved in the thesis work:?1?Titanium-doped KIT-6 composite materials were hydrothermally prepared by a simple one-pot method.After modification with titanium,the BET surface area of the sample can keep above 800 m²/g,which was higher than that of pure silicic KIT-6 and the ordered mesoporous structure was also preserved in long-range.Moreover,titanium was highly dispersed inside the silica and bound to silica via forming Ti-O-Si bond without large accumulation.New metallic sites were generated on Ti-KIT-6 and cooperate with Pt-Sn metal centers.The PtSn/Ti-KIT-6 catalysts were synthesized by incipient-wetness impregnation method.Pt-Sn nanoparticles were uniformly distributed on the surface of support with a size diameter of⁷.3 nm.The titanium-modified PtSn/Ti-KIT-6 catalyst showed a very high conversion of propane?54%?and kept above 51%after 30h.However,the PtSn/KIT-6 catalyst shows a relative low conversion of propane?48.7%?and a significant decrease in activity with the extension of reaction time.The origins of different catalytic results were analyzed by means of XPS.For the PtSn/KIT-6 catalyst,metallic tin takes a proportion of 44%and many alloyed particles of Pt-Sn formed,which is poisonous to the active sites of platinum permanently.In contrast,the tin oxide on PtSn/Ti-KIT-6 possesses a higher proportion of 85%and serves as an auxiliary to cooperate with platinum in PDH reaction.Due to the strong metal support interaction,the catalytic performance is significantly improved.?2?TS-1 zeolite was utilized as support to prepare PtSn-based catalysts.The TS-1zeolite could be synthesized by hydrothermal method simply with typical MFI structure,numerous micro pores but no strong acid sites.Therefore,the reactants are easier to transfer and side-reactions such as coking or cracking could be avoid largely.The grain sizes of TS-1 zeolites were systematically controlled with a series of different crystal diameters?100 nm,201 nm,350 nm,650 nm,1.10?m and 5.72?m?by changing the synthesis conditions.The size of support has distinct influence on the activity of catalysts in the PDH process.A quadratic function is built to correlate the support size and initial conversion of propane.The TS-1 zeolites with smaller size exhibit hierarchical porosity.The diminished size of zeolite grain is accompanied by the generation of mesopore structure between different crystals and shortening of micropores in each single crystal.When tested in the PDH reaction,the minimum TS-1?100nm?supported catalyst showed the best catalytic performance because of its hierarchical porosity.The origin is further discussed based on the Weisz-Prater criterion,which certifies that the smaller sample could increase the capacity for mass transport and diffusion.The reduction of intercrystalline diffusion resistance is beneficial to the transference of both reactant and product molecules.The spent catalysts were continuously regenerated and exhibited a better cycle activity,which made it possible for commercial application.?3?For the catalysts utilized in the PDH reaction,its ideal supports should have smaller particle sizes and larger pore diameters.Here we synthesized three kinds of 3D meosoporous Ti-Si-spheres with different diameters.The added Ti-species are stable in the form of small titanium particles or doped in the skeleton of mesoporous silica by forming the Ti-O-Si bond.Their corresponding catalysts exhibit extreme high surface area and ordered mesoporous structure and show better catalytic performance in the PDH reaction.Among them,PtSn/MS catalyst with lowest diameter?⁴0 nm?shows the highest initial propane conversion of 55.6%,which confirms our previous conclusions.?4??2?TS-1 zeolite was utilized as support to prepare PtSn-based catalysts.The TS-1 zeolite could be synthesized by a simplely hydrothermal method with typical MFI structure,numerous micro pores but no strong acid sites.Therefore,the reactants are easier to transfer and side-reactions such as coking or cracking could be avoided largely.The grain sizes of TS-1 zeolites were systematically controlled with a series of different crystal diameters?100 nm,201 nm,350 nm,650 nm,1.10?m and 5.72?m?by changing the synthesis conditions.The size of support has distinct influence on the activity of catalysts in the PDH process.A quadratic function was built to correlate the support size and initial conversion of propane.The TS-1 zeolites with smaller size exhibit hierarchical porosity.The decreasing of zeolite grain size is accompanied by the generation of mesopore structure among different crystals and shortens the transfered length of micropores inside a crystal grain.When tested in the PDH reaction,the minimum-sized TS-1?100nm?supported catalyst showed the best catalytic performance because of its hierarchical porosity.The origin is further discussed based on the Weisz-Prater criterion,which certifies that the sample with small size could increase the capacity for mass transport and diffusion.The reduction of intercrystalline diffusion resistance is beneficial to the transference of both reactant and product molecules.The spent catalysts were continuously regenerated and exhibited a better cycle activity,which made it possible for commercial application.?3?For the catalysts utilized in the PDH reaction,its ideal supports should have smaller particle sizes and larger pore diameters.Here we synthesized three kinds of 3D meosoporous Ti-Si-spheres with different diameters.The added Ti-species are stable in the form of small titania particles or doped in the skeleton of mesoporous silica by forming the Ti-O-Si bond.Their corresponding catalysts exhibit extreme high surface area and ordered mesoporous structure and show better catalytic performance in the PDH reaction.Among them,PtSn/MS catalyst with lowest diameter?⁴0 nm?shows the highest initial propane conversion of 55.6%,which confirms our previous conclusions.?4?Mesoporous alumina was achieved by cheap raw materials.The hydrothermal system for the synthesis of alumina was precisely controlled in different ph values.The final morphologies of different alumina show a tendency of“growing”from small alumina rods to large alumina sheets.Moreover,numerous mesopores are formd on the surface of the sheet-like alumina.Therefore,the reactants are easier to transfer and side-reactions such as coking or cracking could be avoided largely.After calcined at different temperatures,alumina was achieved with different phase??-,?-,?-,and?-Al₂O₃?.The catalytic performance of PtSn/Al₂O₃ catalysts with different phases and morphologies are further evaluated in the PDH reactions.The sheet-like PtSn/?-Al₂O₃ catalyst showed the best catalytic performance and the origin is ascribed to the proper surface acidity and multiple mesopores in the support.The catalysts with Pt-nanoparticles in sizes of 6.9 and0.9 nm were further prepared.The former shows a higher activity for the dehydrogenation of propane due to its proper size.The deposited carbons on the used catalysts were analyzed by HAADF-STEM and Raman measurements.No large deposited coke particles could be observed and the species of deposited coke are mainly ordered graphite crystal.