Acidic Cs+, NH4+, and K+Salts of 12-Tungstophosphoric Acid as Solid Catalysts for Isobutane/2-butene Alkylation

The caesium, ammonium, and potassium salts of 12-tungstophosphoric acid (HPW) have been prepared with different stoichiometries (1≤ x ≤3), and their catalytic behavior measured for the liquid phase alkylation of isobutane with 2-butene at 80°C. The salts with a cation content of 2.5≤ x ≤3, and that of Cs+withx=2, showed a much higher surface area than the parent acid, which is partially due to the formation of micropores. The micropores presented a fairly homogeneous distribution, with an average diameter in the range of 6–11 Å, depending on the type of cation and cation content. The activity of the heteropoly acid and their monovalent type B salts for isobutane alkylation could be correlated with the surface acidity of the solids. Thus, the initial (1 min TOS) olefin conversion went through a maximum for the NH4 +and K+salts with a cation content ofx=2.5, and in the range of 2≤ x ≤2.5 in the case of the Cs+compounds. (NH4)2.5PW showed a very high initial cracking activity (ca. 72 wt% C5–C7in C5+), suggesting the presence of surface acid sites of a higher acid strength in the ammonium salt, as compared to the caesium and potassium salts with similar composition. In all cases the desired trimethylpentanes (TMPs) were the predominant compounds in the C8fraction formed in the initial reaction stages. The deactivation rate also depended on the nature of the cation exchanged in the heteropoly salts.