Catalytic cracking of n-butane over rare earth-loaded HZSM-5 catalysts

The catalytic performance of HZSM-5 modified with rare earth-loading was investigated for cracking of n-butane into light olefins. The formation of aromatic products such as benzene, toluene, and xylenes was suppressed remarkably by loading with rare earths, and as a consequence, yields of light olefins (ethylene and propylene) were improved. Moreover, it was found that para isomer distribution was increased among the xylene isomers. Analyses of NH3-TPD and CO2-TPD proved that basic sites were generated anew on the catalyst surface by rare earth-loading, but the acidic character of the catalyst was hardly affected. SICl4 treatment on HZSM-5 also improved the para isomer distribution of xylenes, but it did not affect olefin and aromatics selectivities. This result indicated that inactivation of the acid sites on the external surface err controlling the pore-opening size contributed little to the inhibition of aromatic formation although part of the loaded rare earth may have deactivated the acid sites on the external surfaces. The inhibition of aromatic formation during the cracking reaction was found to be stimulated by prior steaming of the catalyst, and the analysis of H2O-TPD proved that surface OK groups were generated on the rare earth loaded catalysts, it was concluded that the basic sites and the surface OH groups played important roles for lowering the formation of aromatics.