Transition metal clusters in zeolites

Abstract

The adsorption and decomposition of Mo(CO)6 in zeolites X, Y, KL, Na-mordenite and Na-ZSM5 had been followed gravimetrically and spectroscopically. Mo(CO)6 was readily adsorbed into the pore structures of faujasites and zeolite KL but not into Na-mordenite and Na-ZSM5. Infrared studies showed that the adsorption produced a common adsorbed Mo(CO)6 species whose Oh symmetry was lowered. Subsequent activation in-vacuo at 100°C caused loss of approximately two carbonyl ligands per molybdenum which could be reversed by addition of carbon monoxide. Heating at or above 200°C caused complete and permanent decarbonylation. Zerovalent molybdenum clusters were produced within the pore structures of alkali metal-exchanged zeolites but for HY and LaY, oxidised molybdenum ions were also produced. Further uptake of Mo(CO)6 occurred on subsequent exposure of faujasite samples containing molybdenum to the vapour at room temperature. By repeated adsorption and decomposition of Mo(CO)6, zeolite sample containing up to 157 molybdenum per unit cell had been prepared. The carbon monoxide hydrogenation activity and selectivity of molybdenum-zeolite catalysts varied with zeolite structure and exchanged cation. Molybdenum-alkali metal-exchange zeolite catalysts were more selective to alkenes and liquefied petroleum gas range hydrocarbons (C2-C4) than molybdenum-HY and molybdenum-LaY catalysts. Using a combination of gas chromatography and infrared spectroscopy, a mechanism was postulated for the formation of hydrocarbons.