The Hydrogenolysis of Simple Alkanes Over Supported Platinum and Nickel Catalysts

[摘要] A series of supported platinum and nickel catalysts were prepared and extensively characterised by the Glasgow-Hull-ICI group. The catalysts were prepared using impregnation, co-crystallisation and metal-vapour deposition methods using silica, alumina, tungstia and molybdena as supports. The seven catalysts that were used in this study were prepared by the impregnation method. They were as follows: Pt/Al2O3, NiAl2O3; Pt/SiO2, Ni/SiO2; Pt/MoO3, NiMoO3; Ni/WO3. To complete the characterisation of the nickel catalysts, carbon monoxide chemisorption and high resolution transmission electron microscopy (HRTEM) experiments were carried out. From the carbon monoxide chemisorption experiments the nickel catalyst dispersions were found to lie in the order: Ni/MoO3(47%) > Ni/SiO2 (38%) > Ni/WO3 (26%) Ni/Al2O3 (10%). The HRTEM studies were unsuccessful in producing particle size distributions for the nickel catalysts due to the low loadings, fairly high dispersions and the difficulty of contrasting the nickel particles from the support. The hydrogenolysis of ethane, propane, n-butane and iso-butane was carried out over the series of catalysts. The metal function of the catalysts was the determining factor for the selectivity of the hydrogenolysis reactions; the platinum catalysts showed a high selectivity towards single hydrogenolysis whereas the nickel catalysts strongly favoured multiple hydrogenolysis. The isomerisation reactions of n-butane and isobutane over the nickel and platinum catalysts were strongly influenced by the metal and the support. The isomerisation selectivities for the reactions of both n- butane and iso-butane over the platinum catalysts were found to lie in the order. Pt/MoO3 > Pt/SiO2 > Pt/Al2O3 For the nickel catlaysts the isomerisation selectivities were found to lie in the order, Ni/MoO3 > Ni/WO3 > Ni/Al2O3 > Ni/SiO2 The multiple hydrogenolysis activities of the platinum catalysts followed the same order as the isomerisation selectivities, suggesting a relation between isomerisation and multiple hydrogenolysis. The high activation energies for the isomerisation reactions of n-butane and iso-butane over the Ni/MoO3 catalyst and n-butane over the Pt/MoO3 catalyst are consistent with reactions taking place directly on the support. The similarity between the hydrogenolysis and isomerisation activation energies for the reactions of n-butane and iso-butane over the Pt/Al2O3 and Pt/SiO2 catalysts and iso-butane over the Pt/MoO3 catalyst suggests that these isomerisation reactions are metal-only catalysed rearrangements. The hydrogen pressure rate coefficients (m) were calculated for the reactions of ethane, ethene, propane and propene over the Pt/Al2O3 and Ni/Al2O2 catalysts. The positive order with respect to hydrogen pressure for the reaction of propene over the Pt/Al2O3 catalyst was accompanied by an increase of multiple hydrogenolysis with increasing hydrogen pressure. This suggests a different reaction mechanism for alkene hydrogenolysis over platinum catalysts than that operative in the hydrogenolysis of alkanes. The hydrogenolysis of ethane, propane, n-butane and iso-butane was carried out over the MoO3 support. The MoO3 was shown to readily isomerise n-butane to iso-butane but not iso-butane to n-butane.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry