The Effect Of Support On Two-Step Conversion Of Methane Over Supported Platinum Catalysts

Well-dispersed and stable nanoclusters supported on mesoporous materials and zeolite have been prepared by vacuum impregnation technique. The pore structure and the surface electronic properties of supports and the state of Pt dispersed on the support surface have been investigated by modern characterization techniques, such as XRD, N2 adsorption-desorption, NH3-TPD and transmission electron microscopy (TEM) methods. Meanwhile, it was studied that the influence of the support effects in different catalysts on the catalytic reaction of methane.First of all, the acidity of supports was changed by ion exchange to study the influence of electronic properties of platinum particles on two-step isothermal conversion of methane to higher hydrocarbons. The catalytic results showed that the activity of catalyst increased with the increase of support acidity and the selectivity of C2-C6 products varied with support acidity. Moreover, the distribution and the reactivity of surfacecarbonaceous species chemisorbed on catalysts, generated from the methane decomposition at first step were characterized by temperature programmed hydrogenating (TPH). The formation of C a species, which is known to give C2+ hydrocarbons on hydrogenation, and reactivity of surface carbon are enhanced on acidic support compared to neutral supports.Secondly, mesoporous molecular sieves MCM-41 were synthesized by hydrothermal method. The template in MCM-41 can be removed by extraction with solutions of an acid or salt in ethanol. When extracting with acidic ethanol, ion exchange of sodium ions for protons was achieved simultaneously. As a result, the supports had different acidities. On the other hand, with the increase of Si/Al ratio of materials, the pore diameter was reducing gradually and the surface area was increasing obviously.Finally, a series of MCM-41 mesoporous materials with different Si/Al ratio were used as supports for Pt-supported catalysts. Thesis catalysts showed the obviously different catalytic behaviors for methane conversion to higher alkanes through two-step procedure. The results of experiments suggested:1. Pure silica MCM-41 had the largest surface area and smaller pore diameter, which was better for the higher dispersion of Pt particles with smaller particle size. As a result, the catalyst Pt/MCM-41 presented a superior activity for C2+ formation.2. The total amount of methane converted to C2+ hydrocarbon was increasing with the increase of support acidity. The change ofcatalytic properties is due to electron deficient nature of platinum particles on acidic support from the interaction of metal and support.3. By studying the distribution and hydrogenation reactivity of surface carbonaceous species chemisorbed on catalysts, it was found that the acidity of MCM-41 would strengthen the hydrogenation reactivity of carbonaceous species and promote the production of C a & C .