Alkylation to form trimethylpentanes from isobutane and 1-butene catalyzed by solid superacids of sulfated metal oxides Original Research Article

The gas-phase alkylation of 1-butene with isobutane was carried out over 0.2 g of superacids of sulfated metal oxides, SO4/Fe2O3, SO4/Al2O3, SO4/TiO2, SO4/SnO2, and SO4/ZrO2, at 0°C with a 10 : 1 ratio of isobutane to 1-butene in a closed recirculation reactor. The products were pentanes (C5), hexanes (C6), heptanes (C7), and octanes (C8). The highest acid strength of the catalysts was linearly related to the total yield of C8 paraffins, 2,2,3-, 2,2,4-, 2,3,3-, and 2,3,4-trimethylpentane (TMP); SO4/SnO2 gave the highest yield of TMP. Six kinds of SO4/ZrO2 catalysts with various surface acidities and catalytic activities were prepared. They were examined as regards the alkylation and the pyridine TPD. Their yields of C8 paraffins were also linearly related to the peak areas between 400 and 600°C in the TPD profiles. The reaction over the SO4/ZrO2 catalysts pretreated in vacuum at various temperatures showed a decrease of yield of C5–C7 with increase of the temperature, indicating the cracking of C8 to C5–C7 hydrocarbons on medium-strong Bronsted acid sites. SO4/ZrO2 was modified by the addition of Fe, Mn, or Pt and the alkylation was examined, but the effect of adding metals was not marked. The activities of SO4/ZrO2 largely varied with the pre-adsorption time of isobutane onto the catalyst surface. It was proved from the above relationship between the catalyst acidities and the yields of C8 paraffins that the first intermediate species was a t-butyl cation formed on the superacidic Lewis site by abstraction of H− from isobutane, followed by the alkylation with butenes to form TMP, though the reaction has so far been considered to be the alkylation of isobutane with olefins via the protonated olefins as an intermediate.