SUBSTITUTION OF AL IN THE ALPO4-5 AND ALPO4-11 FRAMEWORKS BY SI AND FE - A STUDY BY MOSSBAUER, 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), Mossbauer 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 tor 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.