Cracking of n-heptane over Brønsted acid sites and Lewis acid Ga sites in ZSM-5 zeolite

A series of gallium-contg. ZSM-5 zeolites prepd. by wet impregnation, ion-exchange and chem. vapor deposition (CVD) methods are compared in the cracking of n-heptane. Impregnation results in the dispersion of some of the gallium oxide clusters into the zeolite pore network as charge-compensating Ga species after calcination. Redn. of impregnated Ga/HZSM-5 catalysts leads to complete transformation of the oxidic Ga precursors to charge-compensating Ga+ and GaH2+ species. A small amt. of divalent GaH2+ species can be stabilized; however, with increasing Ga/Al ratio monovalent cations dominate. While a model Ga/HZSM-5 catalyst prepd. by CVD of Ga(CH3)3 contg. mainly charge-compensating Ga cations displays high selectivity to dehydrogenated products (olefins, toluene and coke), catalysts with a lower Ga/Al ratio display improved activity with a product mixt. resulting from contributions of Ga sites (dehydrogenation, aromatization, olefin cracking) and of Bronsted acid sites (protolytic cracking, olefin cracking). The synergy between Ga dehydrogenation sites and Bronsted acid sites is proposed to improve the dehydrogenation rate: the high acidity of the zeolitic proton facilitates hydrogen recombination and concomitant removal of product olefin from the Ga active sites. Ion-exchanged Ga/HZSM-5 catalyst which combines a difficult to reduce gallium oxide phase and high Bronsted acidity has the highest activity with relatively weak coke formation