Alkane isomerization over sulfated zirconia and other solid acids

Some solid acids, including sulfated zirconia and certain industrial isomerization catalysts, catalyze two types of n-butane isomerizations, avoiding primary carbenium ions or carbonium ions: (1) an internal rearrangement of the C atoms in n-butane and (2) skeletal isomerization of n-butane to iso-butane. No superacid sites are required for these reactions. The skeletal isomerization is an intermolecular reaction, involving a C8 intermediate. Easily accessible Brønsted acid sites and small amounts of olefin are crucial. Spectroscopic examination of the acid sites on sulfated zirconia shows that they are not stronger than the acid sites in zeolites such as HY. The butane isomerization rate is suppressed by CO, even when no CO is adsorbed on Lewis sites; formation of oxocarbenium ions is likely. The decisive role of Brønsted acid sites is demonstrated by results on deuterated catalysts.