The promotional effect of hydrogen on the catalytic properties of rare-earth zeolites

Abstract

Under high hydrogen pressure, zeolites X and Y exchanged with Ce, La, or mixed rareearth ions showed sustained catalytic activity in cracking n-hexadecane. The carbon number distribution of the cracked product agrees well with that obtained with hydrocracking catalysts containing a weak hydrogenation function, such as sulfided nickel or tungsten. The promotional effect of molecular hydrogen was further confirmed by comparable runs made with helium and nitrogen. In either case, the conversion was lower and the activity declines with time-on-stream. Reversing the gas from helium or nitrogen to hydrogen was found to reversibly restore the catalyst to its steady-state activity in hydrogen. Using cyclohexene as a diagnostic tool, rare-earth-exchanged zeolite X was shown to possess only cracking, hydrogen transfer, and isomerization activities which are characteristics of an acid catalyst. No hydrogenation activity was observed. It is reasoned that rare-earth cations in zeolites behave similarly to supported sulfided nickel or tungsten as a weak hydrogenation function. They cannot hydrogenate olefinic hydrocarbons but can hydrogenate diolefin intermediates formed in the hydrogen transfer process and thus prevent catalyst fouling.