Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane

Abstract

The effects of zeolite structure on the kinetics of n-butane monomolecular cracking and dehydrogenation are investigated for eight zeolites differing in the topology of channels and cages. Monte Carlo simulations are used to calculate enthalpy and entropy changes for adsorption (ΔHads-H+ and ΔSads-H+) of gas-phase alkanes onto Brønsted protons. These parameters are used to extract intrinsic activation enthalpies (ΔHint‡), entropies (ΔSint‡), and rate coefficients (kint) from measured data. As ΔSads-H+ decreases (i.e., as confinement increases), ΔHint‡ and ΔSint‡ for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for ΔSint‡, indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state), ΔHint‡ is relatively constant, while ΔSint‡ increases as ΔSads-H+ decreases because less entropy is lost upon protonation of the alkane. Concurrently, selectivities to terminal cracking and dehydrogenation decrease relative to central cracking because ΔSint‡ decreases for the former reactions. Depending on channel topology, changes in the measured rate coefficients (kapp) with confinement are driven by changes in kint or by changes in the adsorption equilibrium constant (Kads-H+). Values of ΔSint‡ and ΔHint‡ are positively correlated, consistent with weaker interactions between the zeolite and transition state and with the greater freedom of movement of product fragments within more spacious pores. These results differ from earlier reports that ΔHint‡ and ΔSint‡ are structure-insensitive and that kapp is dominated by Kads-H+. They also suggest that ΔSads-H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.

Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

“…In addition, from the same work 34 it was inferred that higher confinement, holding constant other structural parameters, promotes a higher rate and selectivity to central cracking.…”

“…34, to compute the Henry coefficient (K H ) and the enthalpy of adsorption (ΔH ads ; corresponding to the ensemble average for adsorption of alkane anywhere within the zeolite pores) for linear alkanes (propane through n-hexane) moving from the gas phase into one-dimensional zeolites. Each of these zeolites has one Al atom per unit cell.…”

“…Recent studies of the effects of the zeolite structure on the kinetics of these reactions can be found elsewhere. 34,51 An analysis of the dependence of the ratio of equilibrium constants for formation of different reactant states, and of the configurations that n-butane adopts in each of these adsorbed states, on zeolite structural parameters provides insight on the mechanisms by which structural parameters influence adsorption, as discussed below.…”

“…Apparent rate parameters determined by combining CBMC methodology with quantum mechanics calculations, or by using QM to determine rate parameters directly, would enable better evaluation of zeolite candidates. Empirical relationships could also be derived from additional experimental work similar to that employed in ref 34.…”

“…[30][31][32] It is essential to model adsorption at catalytically active sites accurately in order to properly relate the structure of a zeolite to its activity and selectivity, as has been shown recently for the monomolecular cracking and dehydrogenation of alkanes. 33,34 In this work, the effects of channel and cage topology on adsorption thermodynamics for linear alkanes (C3-C6) adsorbed at Brønsted protons within zeolites at 773 K are investigated systematically using Monte Carlo simulations. An advantage of using Monte Carlo simulations is that the simulation accounts for the contributions of rotation and translation to the entropy, discussed above, without the need to choose distances for translation or the degree of free rotation.…”