Phosphatation of zeolite H-ZSM-5 : A combined microscopy and spectroscopy study

Abstract

A variety of phosphated zeolite H-ZSM-5 samples are investigated by using a combination of Fourier transfer infrared (FTIR) spectroscopy, single pulse 27Al, 29Si, 31P, 1H-31P cross polarization (CP), 27Al-31P CP, and 27Al 3Q magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, scanning transmission X-ray microscopy (STXM) and N2 physisorption. This approach leads to insights into the physicochemical processes that take place during phosphatation. Direct phosphatation of H-ZSM-5 promotes zeolite aggregation, as phosphorus does not penetrate deep into the zeolite material and is mostly found on and close to the outer surface of the zeolite, acting as a glue. Phosphatation of pre-steamed H-ZSM-5 gives rise to the formation of a crystalline tridymite AlPO4 phase, which is found in the mesopores of dealuminated H-ZSM-5. Framework aluminum species interacting with phosphorus are not affected by hydrothermal treatment. Dealuminated H-ZSM-5, containing AlPO4, retains relatively more framework Al atoms and acid sites during hydrothermal treatment than directly phosphated H-ZSM-5. Tried and tested: The effect of phosphorus modification of zeolite H-ZSM-5 greatly depends on the applied phosphatation route. By using a multi-pronged spectroscopic approach it is established that phosphatation of H-ZSM-5 promotes zeolite aggregation, external phosphorus deposits, and causes no dealumination. Phosphatation of pre-steamed H-ZSM-5 gives rise to the formation of an extra-framework crystalline tridymite AlPO4 phase.