Catalytic activity and selectivity in the conversion of methanol to light olefins

The object of this paper is to explore the factors which influence the conversion of methanol to light olefins (ethylene, propylene and butenes)_ over various ZSM5 zeolites. Samples containing different SiO2/Al2O3 ratios, crystalline fractions and ions such as nickel, phosphorus, boron and molybdenum have been studied. The surface acidity of these samples has been characterised by the temperature-programmed desorption (TPD) of NH3. The TPD spectra of these samples are characterised by four maxima around 400, 470, 570 and 675 K respectively. It is observed that catalysts wherein the strong acid sites, responsible for the TPD peak at 675 K, have been suppressed or eliminated (by incorporation of elements such as molybdenum, phosphorus, boron or nickel) exhibit higher selectivity for C2-C4 olefins. A similar effect is also observed when the methanol reaction is carried out over a catalyst covered with pyridine molecules. It is suggested that when elements such as phosphorus are incorporated into ZSM5 lattice, even though the tortuosity in the zeolite channels is increased, the observed higher selectivity for C2-C4 olefins is due more to an alteration of the surface acidity than to any steric effect.