Type and catalytic activity of surface acid sites of medium and large pore zeolites Their deactivation with bulky organophosphorus compounds

Abstract

External surface acid sites of medium and large pore zeolites have been probed by deactivation with bulky organophosphorus compounds followed by examination of the deactivated catalysts with 31P NMR. Results show that HZSM-5 ( SiO 2 Al 2O 3 = 70 ); and zeolite beta ( SiO 2 Al 2O 3 = 37 ) have different levels of external surface acidity. The total acid sites and the strong acid sites (>HBr) on the external surface of a HZSM-5 sample have been determined to be 15.5% and 2.7%, respectively, of the bulk acid sites. The total and the strong acid sites on the external surface of a zeolite beta sample have been determined to be 17.5% and 10.6%, respectively. Lewis acid site has not been observed on the external surface for any of the zeolites investigated. The acid strength has been related to catalytic activity both in the cracking of 1,3,5-tri- tert.-butylbenzene catalyzed on the external surface of the catalyst, and in the polymerization of propylene catalyzed both on the external surface and inside the pores of the catalyst. In the cracking of 1,3,5-tri- tert.-butylbenzene, using either ZSM-5 or zeolite beta, only the strong acid sites are effective. In the propylene polymerization, also using ZSM-5 or zeolite beta, a major reduction in rate of lube formation (formation of higher molecular weight polymers) and an increase in lube viscosity index (more linear polymer structure) result from the deactivation of the strong external surface acid sites. The effect of participation of the weaker external surface acid sites is minor.