Catalytic conversion of methanol to low molecular weight hydrocarbons

Abstract

The recent demands on the avialable energy have stimulated the search for alternatives to oil. Methanol, because of its abundance and the availability of technology to produce it from coal, is projected as an alternative source for producing low molecular weight olefins. Utilizing Chabazite ion exchanged with ammonium and rare earth chlorides, methanol is converted to ethylene, propylene and propane with carbon yields of 70 to 90% at reaction temperatures of 633 to 723 K and pressures from 1 to 18 atimospheres. Carbon disulfide in the feed at concentrations less than 2000 ppm increases the operating time between regenerations form two hours to twenty hours. At carbon disulfide concentrations of 3000 ppm or greater, the catalyst goes through three stages. The first stage is that of a dehydrogentation catalyst that produces carbon monoxide and hydrogen. The second stage produces ethylene and propylene, and finally the third stage is a dehydration catalyst that produces dimethyl ether. Experiments with 64% water and 36% methanol by mole, indicated that water enhances the catalytic activity in producing olefins. Increase in pressure increases the production of propylene and propane at the expense of ethylene. X-ray diffraction studies, using Cu-K radiation, show no permanent structural changes after a long use. No permanent deactivation was observed even though the catalyst was overheated once, and had been deactivated and regenerated as many as 21 times. The ammonium exchange coupled with the water at high temperature suggest the formation of an ultrastable zeolite. Ethylene yields increase as the temperature increases from 633°K to 723°K. The product distributions and the selectivities obtained at the above mentioned temperature, range suggest that the decomposition of methanol is occurring on two different sites, which differ slightly in acidity. The first type of sites are responsible for complete dehydration to dimethyl ether and the second type of sites are active in formation of olefins. The increase in the dimethyl ether yields with time on stream at all temperatures and the corresponding decrease in the olefin yields suggest that as the time increases, either the sites of first type are converted to sites of type two or are lost due to coke "buildup...