Preparation And Performance Of Vanadium Catalyst Based On Phosphate Mesoporous Molecular Sieve

Mesoporous materials,which generally possess well-ordered pore structure,uniform pore size distribution and large specific surface area,are favorable for the adsorption,separation and shape-selective catalysis of reaction molecules.In addition,the amorphous wall structure facilitates the introduction of transition metals to construct the active centers needed for catalytic reactions.Based on those advantages above,mesoporous materials have become the most popular supports for the oxidative dehydrogenation of propane?ODPH?in recent years.However,to date,there are few reports about the mesoporous phosphate molecular sieves used as catalyst supports for the reaction of ODPH,which is mainly due to the poorly stability and unordered mesopores of the samples prepared by traditional hydrothermal methods.Based on the ordered mesoporous phosphate molecular sieves with chemical composition of Ca Zr PO by evaporation-induced self-assembly?EISA?prepared before this paper,we used a triblock copolymer F127 as template agent to prepare another ordered mesoporous phosphate molecular sieve Na Zr PO in this paper.During the experiment,the effect of Na/?Na+Zr?ratio and the introduction of transition metal elements on the orderliness of mesoporous structures was investigated.Then the thermal stability,hydrothermal stability,acid and base stability of Na Zr PO were investigated as well.Based on the workings above,two series of vanadium-based mesoporous catalysts were separately synthesized by impregnation loading and direct doping,and finally the oxidative dehydrogenation of propane to propylene was used as a model reaction.The experiment results and conclusion show as follow:?when the ratio of Na/?Na+Zr?attained 0.1,the sample Na_0.1Zr_0.9PO possess two-dimensional hexagonal ordered mesoporous structure,p6mm space group and amorphous pore wall.The specific surface area,average pore size and the specific pore volume of the sample mentioned above can separately reach 200m²/g,7nm and0.4cm³/g.However,with the increasing of Na content,the pore wall of Na Zr PO would be thinner and the order of the mesopores would decrease at the same time.?A series of transition metal-doped mesoporous phosphate molecular sieves M_0.1Zr_0.9PO?M=Zn,Cu,Co,Ni,Fe?were prepared by replacing Na with others transition metals in this paper.Besides the specific surface area,average pore diameter and pore volume of M_0.1Zr_0.9PO respectively range from 132 to 212 m²/g,6.8 to 7.7nm and 0.22 to 0.41 cm³/g.However,only Zn_0.1Zr_0.9PO and Cu_0.1Zr_0.9PO possess ordered mesostructure among those samples.?After treated at 800^oC for 3 h,treated in boiling water for 240 h,treated in an alkaline solution with p H=11 for 24 h or treated in an acidic solution with p H=2 for36h,sample Na_0.1Zr_0.9PO still retain ordered mesoporous structure,which reveal it has excellent thermal stability,hydrothermal stability,acid and base stability.?The vanadium-based catalyst samples prepared by doping method show better catalytic performance than vanadium-based catalyst prepared by traditional impregnation method.Under the same reaction conditions?550^oC,alkoxy ratio=1:1?,the propane conversion and propylene yield on the doped sample V_0.15-Na Zr PO with7.81 wt%of vanadium can reach 39.5%and 10.7%,which show the best catalytic performance among all vanadium-based catalyst in this paper.In comparison,the propane conversion and propylene yield on the loaded sample 6V/Na Zr PO with 5.75wt%of vanadium can respectively reach 33.5%and 8.4%;In addition,the propane conversion and propylene yield on the doped sample V_0.10-Na Zr PO with 5.87 wt%of vanadium can respectively reach 40.3%and 9.8%.And the experiment result also show that doped catalyst V-Na Zr PO have longer lifetime than those supported ones.?The characterization result reveal the polymerization state of vanadium species is the main factors causing the different catalytic performance of vanadium-based mesoporous phosphates:doped catalyst V-Na Zr PO have more oligomeric vanadium species than the supported catalyst V/Na Zr PO at the same density of vanadium,which means there are more highly active reaction sites in the doped catalyst.