Acidity of SAPO and CoAPO molecular sieves and their activity in the hydroisomerization of n-heptane

SAPO and CoAPO molecular sieves with AFI and AEL structure were synthesized. The incorporation of Co(2+), the acidity and the accessibility of the active sites were investigated by temperature-programmed desorption of ammonia, infrared spectroscopy of the OH stretching vibrations and adsorption of probe molecules (pyridine, benzene and various octane isomers). The substitution of cobalt in AlPO(4)-5 and AlPO(4)-11 leads to a considerable increase in the strength of Bronsted acid sites compared with SAPO molecular sieves. In the isomerization of n-heptane over palladium-impregnated catalysts it could be shown that the higher acidity of CoAPO compared with SAPO molecular sieves is reflected in a higher activity and a slightly higher selectivity to dibranched isomers. (C) 1999 Elsevier Science B.V. All rights reserved.