1H and 129Xe NMR investigation of the microporous structure of dealuminated H-mordenite probed by methane and xenon

Abstract

The dynamic behavior of methane and xenon adsorbed in H-mordenite (H-MOR) has been investigated by 1H and 129Xe NMR in a wide range of temperatures (4.2–290 K). Previous results for Na-MOR (Si/Al=7.5) showed that methane adsorbs in both the main channels and the side-pockets of mordenite and that adsorption of methane in the side-pockets is favored relative to adsorption in the main channels. When Na-MOR is converted to its protonated form (H-MOR; Si/Al=8.6), the rotational barrier of methane in the side-pocket increases from 2.0 to 5.0 kJ mol −1, whereas for methane in the main channel, the rotational barrier decreases slightly from 1.0 to 0.53 kJ mol −1. This indicates that methane molecules in the side-pockets interact with acidic Brønsted sites via some sort of “unique” hydrogen bonding. For dealuminated H-MOR (Si/Al=40), the temperature dependence of the 1H spin-lattice relaxation time, T 1, for adsorbed methane indicates that dealumination leads to an enlargement of the pore sizes of both the main channel and side-pocket. This effect is more pronounced for the side-pocket than for the main channel. This observation is consistent with literature results indicating that dealumination opens side-pockets from adjacent channels, thereby forming a secondary pore structure connecting the main channels. 129Xe NMR spectra were also measured for both Na- and H-MOR. Chemical exchange between the two possible locations, i.e. main channel and side-pocket, takes place so that only one broad resonance is observed at room temperature in H-MOR ( E a=18±2 kJ mol −1). The exchange process is slower in Na-MOR ( E a=21±2 kJ mol −1). From this observation and the 1H T 1 results, it is concluded that Na + cations restrict the side-pockets more efficiently than H +, and it is suggested that Brønsted acid sites are located near the opening of the side-pockets.