Adsorption of n-alkanes has been studied in the industrially relevant zeolites H-FAU, H-BEA, H-MOR, and H-ZSM-5 combining QM−Pot(MP2//B3LYP) with statistical thermodynamics calculations and assuming a mobile adsorbate. In H-ZSM-5, adsorption at the intersection site with the hydrocarbon chain extending in the straight channel (SC+I) as well as in the zigzag channel (ZC+I) has been studied. In addition, differential heats of adsorption and adsorption isotherms at temperatures from 301 to 400 K of all C3−C6 n-alkane in H-ZSM-5 have been measured simultaneously via calorimetry and gravimetry. Calculated adsorption enthalpies are independent of temperature and are virtually identical to the adsorption energies. The adsorption strength increases in the order H-FAU < H-BEA < H-MOR < H-ZSM-5 (SC+I) < H-ZSM-5 (ZC+I) and varies linearly with the carbon number. As compared to experimental values, the calculated adsorption strength is overestimated by some 2 kJ mol−1/CH2 in FAU up to some 4 kJ mol−1/CH2 in H-ZSM-5 suggesting that the QM−Pot(MP2//B3LYP) calculations overestimate van der Waals stabilizing interactions and a correction term has been proposed. Adsorption entropy losses are independent of temperature and increase in the order H-FAU < H-BEA < H-MOR < H-ZSM-5 (SC+I) < H-ZSM-5 (ZC+I), according to the pore size of the zeolites. The calculated adsorption entropies agree nicely with available experimental results in all zeolites. QM−Pot(MP2//B3LYP) calculated adsorption equilibrium coefficients (using the corrected adsorption enthalpies) correspond relatively well to experimentally determined values. Comparison of relative turnover frequencies with relative adsorption equilibrium coefficients indicates that the variation of the equilibrium coefficient with the carbon number or with the zeolite can only partly explain the observed reactivity differences in monomolecular cracking of n-alkanes. In agreement with experimental observations, our results indicate that the difference in reactivity of the n-alkanes for monomolecular cracking in a given zeolite mainly originates from a difference in intrinsic monomolecular cracking rate coefficients.

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.

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 H2 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.

An efficient procedure for normal-mode analysis of extended systems, such as zeolites, is developed and illustrated for the physisorption and chemisorption of n-octane and isobutene in H-ZSM-22 and H-FAU using periodic DFT calculations employing the Vienna Ab Initio Simulation Package. Physisorption and chemisorption entropies resulting from partial Hessian vibrational analysis (PHVA) differ at most 10 J mol(-1) K(-1) from those resulting from full Hessian vibrational analysis, even for PHVA schemes in which only a very limited number of atoms are considered free. To acquire a well-conditioned Hessian, much tighter optimization criteria than commonly used for electronic energy calculations in zeolites are required, i.e., at least an energy cutoff of 400 eV, maximum force of 0.02 eV/Å, and self-consistent field loop convergence criteria of 10(-8) eV. For loosely bonded complexes the mobile adsorbate method is applied, in which frequency contributions originating from translational or rotational motions of the adsorbate are removed from the total partition function and replaced by free translational and/or rotational contributions. The frequencies corresponding with these translational and rotational modes can be selected unambiguously based on a mobile block Hessian-PHVA calculation, allowing the prediction of physisorption entropies within an accuracy of 10-15 J mol(-1) K(-1) as compared to experimental values. The approach presented in this study is useful for studies on other extended catalytic systems.

Kinetics and thermodynamics of isobutene protonation in H-FAU, H-MOR., H-ZSM-5, and H-ZSM-22 have been studied in a temperature range of 300-800 K, combining PW91-D//PW91 periodic density functional :theory calculations with statistical thermodynamics. At temperatures relevant for industrial zeolite-catalyzed processes (500-800 K), the tert-butyl carbenium ions more stable than the tert-butoxy in H-MOR, H-ZSM-5, and H-ZSM-22. Entropy contributions govern the standard Gibbs free energy stability of the chemisorbed intermediates. Due to the absence of a C-O covalent bond, formation of the tert-butyl carbenium ion is accompanied by a lower entropy :Loss and, consequently, has a higher stability than the tert-butoxy in H-MOB., H-ZSM-5, and H-ZSM-22. At 800 K, the protonation toward tert-butoxy in H-FAU, H-MOB, and H-ZSM-5 and to the tert-butyl carbenium ion in H-ZSM-22 is 5 to 7 orders of magnitude faster than the protonation toward isobutoxy. Among the four zeolites, the lowest activation energy is found in H-ZSM-22

The DFT parametrized zeolite force field in the QM-Pot program is extended with carbon−carbon, carbon−hydrogen, and alkoxy bond describing parameters. The extended force field has been combined with B3LYP and with MP2 as the high-level quantum mechanical (QM) method to simulate the physisorption and chemisorption of ethene, isobutene, 1-butene, 1-pentene, and 1-octene in H-FAU (Si/AlF = 95) and for physisorption of 1-pentene, n-pentane, 1-octene, and n-octane in all silica FAU. The new parametrization predicts more stable chemisorption complexes than physisorbed π complexes, but with smaller chemisorption energies which are more reliable as shown by comparison with experimental results and with accurate hybrid MP2:DFT calculations. An embedded cluster size study shows that, due to the importance of the stabilizing van der Waals part in the MM contribution of the cluster, QM-Pot(MP2//B3LYP) calculations yield more reliable physisorption and chemisorption energies of hydrocarbons in zeolites than QM-Pot(B3LYP). The QM-Pot(MP2//B3LYP) results are in good agreement with available experimental data. In H-FAU, the H+···alkane interaction was found to contribute at most 7 kJ/mol to the total physisorption energy of n-alkanes while the H+···π interaction contributes 20−25 kJ/mol to the total physisorption energy of alkenes. For n-alkene physisorption in H-FAU, a linear increase of both the physisorption and chemisorption energies of 8.7 kJ/mol per C-atom is found. The protonation energy of n-alkenes in H-FAU was found to be independent of the C-number and amounts to −50 kJ/mol for the formation of secondary alkoxides. The formation of tertiary alkoxides in H-FAU suffers slightly from steric constraints imposed by the zeolite framework.