Molybdenum mordenite

Abstract

Vapour phase adsorption of molybdenum pentachloride was used to successfully prepare MoH-mordenite and MoNa-mordenite with loadings of up to 7 wt% Mo from H-mordenite and Na-mordenite respectively. Adsorption was performed either in a static cell or in a flow system using a nitrogen carrier gas to carry MoCl5 vapour over a fixed bed of the zeolite. The uptake of MoCl5 was followed by gravimetry, electron spin resonance and infra-red spectroscopy. The MoCl5 loaded zeolites were activated by heating in vacuum to produce vapour phase ion exchanged Mo-zeolites. Decomposition of MoCl5 in H-mordenite results in loss of HCl at 100 °C as observed by temperature programmed desorption mass spectrometry, with concurrent loss of zeolitic protons. In the case of MoCl5-Na-mordenite, the complex is far more stable, and on decomposition at 300 °C much of the chloride remains occluded within the zeolite pores to maintain charge balance. X-ray crystallinity, pore volume measurements and scanning electron micrographs were taken to monitor zeolite integrity. Mo-loadings were determined by atomic absorption, energy dispersive analysis by x-rays and x-ray photoelectron spectroscopy. The composition of the parent materials was determined by x-ray fluorescence, atomic absorbtion and solid state nuclear magnetic resonance. The resultant catalysts were extensively characterised by electron spin resonance, infra-red spectroscopy, energy dispersive analysis by x-rays, x-ray photoelectron spectroscopy and x-ray diffraction. Little surface enrichment of the crystallites with Mo was observed, with loading being within the internal pores of the zeolite. The thermal stability of the Mo-loaded zeolites was investigated using infra-red spectroscopy and x-ray diffraction, and found to be slightly lower than that of the parent materials. Catalytic activity was tested using metathesis of propene and hydrogenation of simple olefins as test reactions. The results were compared to those obtained for conventional molybdena-alumina catalysts, and found to exhibit comparable reaction rates but different product distributions due to residual acidity. Two different forms of superoxide species were observed on exposure of the reduced catalysts to molecular oxygen, one of these exhibiting a weak covalent type interaction with the support and having no counterpart in conventional molybdena systems. The superoxide species are compared with those formed on Mo-mordenite prepared by aqueous ion exchange.