Isomerization of n-Heptane over Pd-Loaded Silico-Alumino-Phosphate Molecular Sieves

Isomerization of n-heptane was studied on a series of bifunctional SAPO-based catalysts with contents of 0.1 wt% Pd. From results on temperature programmed desorption of NH 3 it is deduced that the different molecular sieve structures contain nearly identical amounts of silicon substituting for phosphorus in the framework (0.3 to 0.5 silicon atoms per unit cell). Best activities and selectivities for branched heptane isomers are achieved with SAPO-11 and SAPO-31. SAPO-17 and SAPO-5 show substantially lower activities. With SAPO-5, there is a high cracking selectivity which is assumed to be caused by the reduced accessibility of parts of the bridged hydroxyl groups within the molecular sieve framework. Different locations of these acid sites are evidenced by infrared OH vibration spectra recorded after adsorption of n-heptane.