On the possible nature of sites responsible for the enhancement of cracking activity of HZSM-5 zeolites dealuminated under mild steaming conditions

Effect of acidity and diffusibility on coke deactivation over nano-sized HZSM-5 zeolites

“…The increases of acidity and catalytic activity of steamed catalyst were first reported by Lago et al [17]. A similar phenomenon was reported by other researchers [18][19][20]. In the case of the BC i B series samples, however, the amount of strong acid sites of sample BC 450 B is higher than that of BC 400 B, but both of them are lower than that of the untreated sample A, as can be seen from Fig.…”

“…Datka et al [18] pointed out that the increase of the acid strength of Si-OH-Al groups under mild steaming dealumination conditions should be attributed to the removal of the less acidic hydroxyls from the framework of the zeolite. Biaglow et al [19] and Zholobenko et al [20], on the other hand, suggested that the enhanced catalytic activity of the zeolites steamed under mild conditions was not due to the high acidity produced by superacidic hydroxyls. Instead, the polarization of a paraffin molecule by a Lewis acid site created by dealumination may be the reason for the high catalytic activity of the steamed zeolites.…”

“…It is generally agreed that coke deposits occur prior to strong acid sites, which is related to Al species in the framework of the zeolite. Steaming is a very popular dealumination technique, which can modify the acidity and catalytic performance of zeolites [14][15][16][17][18][19][20][21]. Scherzer et al [14] first found that steaming led to dealumination, which decreased the acidity and increased the stability of zeolitic catalysts.…”

“…A number of studies have been done to suppress the formation of coke during aromatization [10][11][12][13][14][15][16][17][18][19][20]. The crystal size of ZSM-5 zeolites has a great effect on catalytic properties.…”

Difference of ZSM-5 zeolites synthesized with various templates

“…Masuda et al [30] had reported that accessibility to acid sites instead of acid strength is the major influencing factor on the catalytic activity. The coexistence of intra-and extra framework aluminum can enhance the catalytic activity as has been shown [31][32][33]. The weak template effect of inorganic templates may cause the coexistence of intra-and extra framework aluminum in the samples, which may be the reason for higher catalytic activity of the samples ZSM-5 with inorganic templates.…”