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

Abstract

Monomolecular cracking of alkanes was investigated over zeolites of different structure types and after different treatments. Large deviations in apparent rates of reaction and apparent kinetic parameters were detected for zeolites of different structure types; smaller changes were observed within a structure type. The Si/Al ratio and different crystal sizes had no effect on turnover frequency; however, steaming of mordenite and ZSM-5 enhances the rate of reaction by a factor of 2-5 due to the formation of Lewis acid sites with enhanced heat of adsorption, although the number of Brønsted sites decreased. The apparent activation energies and preexponential factors showed a linear compensation relation in a Constable plot. Using the Langmuir-Hinshelwood kinetics formalism, the apparent kinetic parameters include enthalpy and entropy of adsorption and surface reaction terms. If the enthalpy and entropy of adsorption are linearly related, which is experimentally observed for unsteamed and steamed mordenite, then the apparent compensation effect implies that the observed differences in rate are dominated by differences in adsorption of the reactant in the pores of the zeolites, rather than by differences in acid strength. Zeolites with smaller pores show enhanced adsorption, and consequently the rate of reaction increases in the order H-Y < H-MOR < H-ZSM-5.  2004 Elsevier Inc. All rights reserved.

Understanding cage effects in the n-alkane conversion on zeolites

“…For 97%, the enthalpy (kJ/mol) and temperature × entropy (kJ/mol) of topologies with attractive van der Waals interactions obey the relationship T S ads n-C 25 = −3.6 × 10 −3 ( H ads n-C 25 ) 2 − 0.62 H ads n-C 25 − 89.6 (curved line in figure). Interestingly, this second-order relationship is in stark contrast with the usually reported linear (compensation) relation between H ads and T S ads [21,40]. a consideration of the complete pore topology yields more robust correlations than one limited only to window size.…”

IRMS-TPD of ammonia: Direct and individual measurement of Brønsted acidity in zeolites and its relationship with the catalytic cracking activity

“…On the other hand, recent studies by van Bokhoven and co-workers [30,31] arrived at a different conclusion, that the cracking activity is primarily controlled not by the strength of acid sites, but rather by the differences in the adsorption of the reactant in the pores of the zeolites. Preceding that study, Miller and Kung and co-workers [20,[32][33][34] studied the cracking mechanism and active sites on zeolites.…”

Quasi-homogeneous catalytic cracking of JP-10 over high hydrocarbon dispersible nanozeolites

“…There were several factors related to the catalytic performances of HDZs, including acid properties [42,43] and textual structures (pore and surface area). As we discussed before, both HDZ-O and HDZ-W have the similar acid properties and amounts.…”