Physisorption and chemisorption of alkanes and alkenes in H-FAU: a combined ab initio–statistical thermodynamics study

Abstract

The sorption in H-FAU zeolite of C4-C12 n-alkanes, and linear and branched C2-C8 alkenes has been quantified up to 800 K by combining QM-Pot(MP2//B3LYP) with statistical thermodynamics calculations. The physisorption strength increases linearly with increasing carbon number by 8.5 kJ mol(-1) and does not depend on the detailed alkane or alkene structure. Van der Waals interactions are dominant in physisorption, but alkenes are additionally stabilized by 20 kJ mol(-1) by formation of a pi-complex. Protonation of an alkene leads to the formation of alkoxides, which are more stable than the physisorbed species. As for physisorption a linear relation between the chemisorption energy and the carbon number is obtained. Protonation energies are independent of the carbon number but depend on the type of CC double bond being protonated. The relative stability difference between the secondary and tertiary alkoxides is 15 kJ mol(-1) in favor of the former. Both physisorption and chemisorption are accompanied with entropy losses which increase linearly with the carbon number. A typical compensation effect is obtained: the stronger the stabilization of the sorbed species the more pronounced the entropy loss. For temperatures ranging from 0 to 800 K, all of the derived linear relations expressing the physisorption and/or chemisorption enthalpy and entropy of the alkanes and the alkanes as function of the carbon number are independent of temperature. A good agreement between calculated and experimental values for alkanes is obtained at 500 K.

Reaction path analysis for 1-butanol dehydration in H-ZSM-5 zeolite: Ab initio and microkinetic modeling

“…A more accurate estimation of the entropic contributions could be obtained by accounting for anharmonicities by detailed scanning of the potential energy surface [60,61], but this would require significant computational efforts for large systems. Another approach to treat the low lying modes is the use of a frequency cutoff [ 44,56,57,62]. De Moor et al [62] studied the entropy contributions of these frequencies for alkanes and alkenes in FAU zeolite and suggested the replacement of these spurious frequencies with 50 cm À1 .…”

“…Another approach to treat the low lying modes is the use of a frequency cutoff [ 44,56,57,62]. De Moor et al [62] studied the entropy contributions of these frequencies for alkanes and alkenes in FAU zeolite and suggested the replacement of these spurious frequencies with 50 cm À1 . Therefore, in order to obtain consistent results, the spurious imaginary frequencies and low-lying frequencies were replaced by normal modes of 50 cm À1 [62].…”

Efficient Approach for the Computational Study of Alcohol and Nitrile Adsorption in H-ZSM-5

“…The strong hydrogen bond interactions between the alcohol or nitrile and the zeolite justify the application of the "immobile adsorbate" method, in which only harmonic frequencies are considered. 17,18,25 The vibrational analysis also supplies the necessary thermochemistry data to allow the calculation of theoretical adsorption enthalpies at 400 K, the temperature at which the experimental reference data were obtained. 14 The adsorption enthalpy was calculated by subtracting the enthalpies of the empty zeolite and the gas phase adsorbate from the enthalpy of the adsorption complex…”

Combined Density Functional Theory and Monte Carlo Analysis of Monomolecular Cracking of Light Alkanes Over H-ZSM-5

“…As far as enthalpies are concerned, this approximation is justified because their sensitivity to variations in the harmonic frequency values is weak. 55,56 However, for the calculation of adsorption entropies this treatment usually overestimates the entropy loss during adsorption. 55−57 Moreover, the latter approach uses the information of only one point on the potential energy surface.…”