Synthesis And Characterization Of A Number Of Porous Solid Acid Catalytic Material

Solid acids have been widely used in heterogeneous catalysis.Among them zeolite molecular sieves and zirconia-type superacids are two kinds of important solid acids with high performance in catalytic reactions,such as isomerization,alkylation and cracking.However,both of these solid acids suffer some drawbacks.Zeolite molecular sieves have unique shape selectivity.However,their applications are sometimes limited by their small micropores which affect the molecular diffusion and mass transfer.As two kinds of important zirconia-type acids,sulfated zirconia and tungstated zirconia prepared with conventional methods have low surface area and wide pore size distribution.In order to overcome the above disadvantages of molecular sieves and zirconia-type acids,new methods were explored to prepare hollow zeolite microspheres with a mesoporous shell and sulfated zirconia and tungstated zirconia with large surface area and narrow pore size distribution.The main results are the follows.Hollow microspheres of ZSM-5 with a mesoporous shell have been synthesized through formation of amorphous hollow SiO₂/Al₂O₃ microspheres by sol-gel process in multiple oil-water-oil emulsions and transformation of the amorphous species into zeolite by water-organic vapor-phase transport treatment.The morphology of the amorphous and zeolite spheres observed by scanning electron microscopy shows no significant change whereas the molar ratio of Si/Al increases from 6 to 20 during the transformation.The structural feature of zeolite was characterized by X-ray diffraction and ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance. Transmission electron microscopy and N₂ adsorption-desorption isotherms indicate that uniform mesopores in the shell of zeolite spheres arise from the interstices among zeolite crystallites.Sulfated zirconia was prepared by the sol-gel method.N₂ adsorption-desorption experiments indicate existing of uniform mesopores.TEM images further prove that the mesopores exist in the crystalline and do not arise from the interstices among SZ crystallites.NH₃-TPD and IR-Py measurements show that sulfated zirconia has strong acidity.Slow hydrolysis rate is helpful for obtaining samples with large surface area and narrower pore size distribution.Mesoporous tungstated zirconia was synthesized via a combined process of solvent evaporation induced self-assembly and sol-gel process.The structural features of samples were characterized by XRD,TEM,and NH₃-TPD.The results show that calcination temperature and the molar ration of W:Zr have great influence on the crystallization of tetragonal tungstated zirconia.With a molar ratio of W:Zr=1:10 tetragonal tungstated zirconia with large surface area and narrow pore size distribution can be obtained at low temperature.