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 π-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.