Adsorption Thermodynamics and Intrinsic Activation Parameters for Monomolecular Cracking of n-Alkanes on Brønsted Acid Sites in Zeolites

Abstract

Experimental measurements of the rate coefficient (k app ) and apparent enthalpies and entropies of activation (ΔH app and ΔS app ) for alkane cracking catalyzed by acidic zeolites can be used to characterize the effects of zeolite structure and alkane size on the intrinsic enthalpy and entropy of activation, ΔH int ⧧ and ΔS int ⧧ . To determine ΔH int ⧧ and ΔS int ⧧ , enthalpies and entropies of adsorption, ΔH ads-H + and ΔS ads-H + , must be determined for alkane molecules moving from the gas phase to Brønsted acid sites at reaction temperatures (>673 K). Experimental values of ΔH app and ΔS app must also be properly defined in terms of ΔH ads-H + and ΔS ads-H + . We report here a method for determining ΔH ads-H + and ΔS ads-H + in which the adsorption site is represented by a fixed volume that includes the proton. Values of ΔH ads-H + and ΔS ads-H + obtained from Monte Carlo simulations are in good agreement with values obtained from experimental data measured at 300−400 K. An important feature of the simulations, however, is their ability to account for the redistribution of alkane adsorbed at protons in different locations with increasing temperature. Values of ΔH int ⧧ and ΔS int ⧧ for the cracking of propane through n-hexane, determined from measured values of k app and ΔH app and simulated values of ΔH ads-H + and ΔS ads-H + , agree well with values obtained independently from quantum mechanics/molecular mechanics calculations. Application of our method of analysis reveals that the observed increase in k app with increasing n-alkane size is due primarily to a decrease in ΔH int ⧧ with increasing chain length and that ΔS int ⧧ is independent of chain length.

Impact of Zeolite Structure on Entropic–Enthalpic Contributions to Alkane Monomolecular Cracking: An IR Operando Study

“…[4,12,16,28] However the small valueso fa ctivation entropyo btained here are not in favour of al ate transition state as often suggested. [7,25] On the other hand, they do not confirmt he predictions that the alkane chain length and zeolite topology primarily influence the intrinsic activation energy and not the activation entropy. [7,25] On the other hand, they do not confirmt he predictions that the alkane chain length and zeolite topology primarily influence the intrinsic activation energy and not the activation entropy.…”

“…Until recently,t hese adsorption parameters were either estimated by extrapolationo fl ow-temperature measurements, [4,14,17,18] or by molecular simulations. [4,12,16,28] However,s tate of the art simu-lations predict as trong influence of temperature on the adsorptione ntropy, D ads S, [7,20,25,[29][30][31] which was assumed to be negligible in previous studies. On this ground, the variations in apparent rate coefficientsw ere initially accounted by changes in adsorption parameters, K8 ads , [9-11, 14, 15, 27] and more recently by changes in the pre-exponential factor of the intrinsic rate constant, and hence, activation entropy DS°.…”

“…On this ground, the variations in apparent rate coefficientsw ere initially accounted by changes in adsorption parameters, K8 ads , [9-11, 14, 15, 27] and more recently by changes in the pre-exponential factor of the intrinsic rate constant, and hence, activation entropy DS°. [7,25,31,32] Ak ey to resolve these discrepancies is the simultaneous determination of active site coverage and apparent rates at reactionc onditions. This temperature dependence has al arge influence on the estimates of alkane coverage and hence the estimates of intrinsic rates.…”

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

“…Each of these zeolites has one Al atom per unit cell. 33,37,39 Several million Widom insertions were carried out to ensure statistically accurate ensemble averages. To avoid insertions occurring within inaccessible pore space, regions of the zeolite that are inaccessible to methane were first identified using Zeo++.…”

“…[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.…”