Physisorption and Chemisorption of Linear Alkenes in Zeolites: A Combined QM-Pot(MP2//B3LYP:GULP)-Statistical Thermodynamics Study

Physisorption and chemisorption of C2-C8 linear alkenes in H-FAU, H-BEA, H-MOR, and H-ZSM-5 have been quantified up to 800 K by combining QM-Pot(MP2//B3LYP:GULP) with statistical thermodynamics calculations. The influence of the zeolite topology and the alkene CC double bond position on the alkene sorption thermodynamics is addressed on the basis of linear variations of sorption enthalpies and entropies as a function of the carbon number. The physisorption strength and entropy losses increase in the order H-FAU < H-BEA < H-MOR < H-ZSM-5. Higher physisorption strength is computed for 2-alkenes (H-MOR) and 2-, 3-, and 4-alkenes (H-ZSM-5) as compared with 1-alkenes. Protonation of physisorbed alkenes leads to significantly more stable alkoxides. In contrast to the physisorption, higher chemisorption strength does not lead to larger chemisorption entropy losses. Also, the intrinsic stability of the alkoxides, i.e., relative to gas phase H-2 and graphite, only depends on the carbon number and not on the detailed alkoxide structure in H-FAU, H-BEA, and H-MOR. In the narrower pores of H-ZSM-5, the 3- and 4-alkoxides are however more stable than the 2-alkoxides.