Intrinsic Catalytic Cracking Activity of Hexane over H−ZSM-5, H−β and H−Y Zeolites

Abstract

Mono-and bimolecular intrinsic rate constants for the conversion of n-hexane over H-Y, H-ZSM-5, and H-β zeolites were measured at 623 K by combining conventional catalytic techniques with gravimetric adsorption measurements. Since H-zeolites are catalytically active, their adsorption properties under reaction conditions could not be directly obtained. Rather, adsorption experiments were carried out on the H-zeolites at temperatures below the onset of catalytic activity (450-530 K). Thermodynamic adsorption constants for the H forms at 623 K were then obtained by extrapolation. In addition, the catalytically inactive Na forms were chosen to characterize the overall temperature dependence of n-hexane adsorption on ZSM-5, Y, and β zeolites in the range 450-623 K. For the cracking experiments, monomolecular conditions were ensured by choosing a low hexane pressure (<0.57 kPa) and short contact times. The intrinsic activity for mono-and bimolecular n-hexane cracking over the zeolites, after taking into account the amounts of adsorbed hydrocarbon, was in the order H-ZSM-5 > H-β > dealuminated H-Y . H-Y.

Observation of a compensation relation for monomolecular alkane cracking by zeolites: the dominant role of reactant sorption

“…This analysis of the cracking reactions is in good agreement with those previously made for the monomolecular cracking pathway for n-hexane [25,28,31].…”

“…Within the experimental uncertainties, the product distributions are identical for all zeolites and are consistent with those previously reported for monomolecular cracking of n-hexane, which are also given in Table 2 [25,31]. For the monomolecular pathway, the primary product of protolysis gives unity molar ratio of methane versus pentene, C 2 versus C 4 (either ethylene plus butane or ethane and butene), propane versus propene, and hydrogen versus hexene.…”

“…In the absence of steaming, the apparent rate constants increase in the order of structure types Y < MOR < MFI in agreement with previous studies [28,31]. The apparent rate constant of FAU (CDHY) was approximately two times lower than that of H-MOR, which in turn was approximately two times lower than MFI.…”

Adsorption and desorption kinetics of hydrocarbons in FCC catalysts studied using a tapered element oscillating microbalance (TEOM). Part 1: experimental measurements

“…where K H is the Henry constant (and thus equals the gradient of the isotherm plots at low coverage Lercher, 1997; Kotrel et al, 1999;Denayer et al, 1997). It can be seen that there is an increase in adsorption strength with chain length.…”

Solid Acid Catalysts for Endothermic Fuels

“…Likewise, structural isomerization of hydrocarbons has been shown to proceed via the same reaction intermediates as cracking [143,144], but under low temperature isomerization conditions, the reactive carbenium ions accept a hydride and desorb before C-C scission can occur. In a previous study, H-ZSM-5 was shown to be two orders of magnitude more active compared to H-Y for n-hexane cracking at 623 K and an n-hexane partial pressure of 5.3 kPa [145]. In addition, unlike in larger-pore catalysts, where isomerized products can easily diffuse out of the solid framework, in H-ZSM-5 branched alkanes preferentially adsorb at the intersections and undergo a jump mechanism for diffusion.…”