Analysis of nature of adsorption of hydrocarbons on rare earth-loaded HZSM-5 catalysts by pulse-injection method

The formation of aromatic products (benzene, toluene, and xylenes) in the c racking of paraffinic hydrocarbons, such as n-butane, was suppressed remark ably by using rare earth-loaded HZSM-5 catalysts, resulting in improved yie lds of light olefins (ethylene and propylene). Nature of adsorption of hydr ocarbons on rare earth-loaded HZSM-5 was investigated by pulse-injection me thod. When a propylene pulse was injected on HZSM-5 catalyst, only trace am ounts of the desorption were detected. About 65% of the injected pulse, how ever, was desorbed from lanthanum loaded HZSM-5 (at 623 K). When ethylene a nd ethane were used as the injection pulse, it was found that adsorption of ethylene on lanthanum-loaded HZSM-5 were suppressed. On the other hand, it was found that on HZSM-5 catalyst without rare earth loading, ethylene was more adsorptive than ethane. From these experimental results, it was deduc ed that reduced formation of aromatics in the cracking of hydrocarbons is h ighly attributed to the inhibition of olefin adsorption by loaded rare eart h metal oxides.