TEMPERATURE-PROGRAMMED DESORPTION DIFFUSION OF AMMONIA IN MOLECULAR-SIEVES .5. ZSM-5 ZEOLITE

Some ZSM-5 zeolite samples with an SiO2/Al2O3 ratio of 10.3 have been analysed by temperature-programmed desorption (TPD) of ammonia to inve stigate how the rate-determining step of the desorption-diffusion proc ess depends on some experimental parameters and on the physicochemical characteristics of the zeolite. By comparison with previous results o n a ZSM-5 sample with SiO2/Al2O3 = 33.6, a shift of the probable rate- determining step from intracrystalline diffusion of the desorbed gaseo us base to desorption of the latter without appreciable readsorption h as been noticed. On the other hand, both the change of the temperature ramp from 10 to 2 K/min and the substitution of Na+ for H+ by ion exc hange shift the rate-determining step to desorption with free readsorp tion of the desorbed base.