Substitution of Al in the AlPO4-5 and AlPO4-11 frameworks by Si and Fe: a study by Mössbauer, magic-angle-spinning nuclear magnetic resonance and electron paramagnetic resonance spectroscopies and chemical probes

Abstract

Substitution of Al by Fe and Si in the AlPO₄-5 and AIPO₄-11 frameworks has been investigated by a number of experimental methods such as electron paramagnetic resonance (EPR), Mössbauer and magic-angle-spinning nuclear magnetic resonance (MASNMR) spectroscopies and temperature-programmed desorption of ammonia. There are indications that Fe^III replaces Al^III and the extent of substitution increases with time of reaction. The FAPO-n formed, however, is contaminated with an amorphous phase containing Fe^III. Silicon replaces phosphorus in the framework only to a limited extent and in those cases when the synthesis gel had a higher amount of silicon, the product was contaminated with an amorphous silica-containing phase. No evidence for silicon removing a phosphorus–aluminium pair has been found. The conclusions are also supported by the results of acidity measurements and the alkylation of toluene with methanol on these molecular sieve samples.