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

The catalytic performance of HZSM-5 modified with rare earth-loading was in vestigated for cracking of n-butane into light olefins. The formation of ar omatic products such as benzene, toluene, and xylenes was suppressed remark ably by loading with rare earths, and as a consequence, yields of light ole fins (ethylene and propylene) were improved. Moreover, it was found that pa ra isomer distribution was increased among the xylene isomers. Analyses of NH3-TPD and CO2-TPD proved that basic sites were generated anew on the cata lyst surface by rare earth-loading, but the acidic character of the catalys t was hardly affected. SICl4 treatment on HZSM-5 also improved the para iso mer distribution of xylenes, but it did not affect olefin and aromatics sel ectivities. This result indicated that inactivation of the acid sites on th e external surface err controlling the pore-opening size contributed little to the inhibition of aromatic formation although part of the loaded rare e arth may have deactivated the acid sites on the external surfaces. The inhi bition of aromatic formation during the cracking reaction was found to be s timulated by prior steaming of the catalyst, and the analysis of H2O-TPD pr oved that surface OK groups were generated on the rare earth loaded catalys ts, it was concluded that the basic sites and the surface OH groups played important roles for lowering the formation of aromatics.