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 Co2+, the acidity and the accessibility of the activ e sites were investigated by temperature-programmed desorption of ammonia, infrared spectroscopy of the OH stretching vibrations and adsorption of pro be molecules (pyridine, benzene and various octane isomers). The substituti on of cobalt in AlPO4-5 and AlPO4-11 leads to a considerable increase in th e strength of Bronsted acid sites compared with SAPO molecular sieves. In t he isomerization of n-heptane over palladium-impregnated catalysts it could be shown that the higher acidity of CoAPO compared with SAPO molecular sie ves is reflected in a higher activity and a slightly higher selectivity to dibranched isomers. (C) 1999 Elsevier Science B.V. All rights reserved.