Effects of acidity in the isomerization of n-butenes on mixed tin-antimony oxide catalysts. A 13C NMR study

Abstract The role played by acidic sites in the isomerization of n -butenes adsorbed on mixed tin-antimony oxide catalysts is illustrated by the influence of adsorbed acetic acid on the various kinetic constants. However, both acidic and basic sites are found to play a role either in the stabilization or in the transformation of n -butenes in the adsorbed phase. A cyclic-type intermediate explains adequately all the experimental results. It can bear either a partial negative charge (most of the studied tin-antimony oxide catalysts belong to this group) or a partial positive charge (for the 30.0, 40.0 at.% Sb catalysts and those modified by acetic acid adsorption) as shown by the relative amounts of cis - and trans -2-butenes which are formed in the early stages of the 1-butene isomerization reaction. The presence of strong Bronsted acid sites on the 30.0 at.% Sb catalyst is suspected from its higher polymerization activity for n -butenes.