Adsorption of C2−C8 n-Alkanes in Zeolites

Abstract

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.

Catalytic cracking of n-alkane naphtha: The impact of olefin addition and active sites differentiation

“…Just as an example, in the work of Eder et al [29] and De Moor et al [53], adsorption was carried out at p  10 -3 mbar (9.87·10 -7 atm) of (propane to n-hexane) and T  400 K. In an analogous way in the work of Dunne et al [54], Ramachandran et al [28] and Ferreira et al [55], among others, adsorption was always performed at p  9.87·10 -5 atm and T  423 K. From the work presented here, it is clear that by working under those experimental conditions it should be very difficult to unequivocally conclude about the homogeneity or heterogeneity of the surface for adsorption sites. Then, one should either follow the methodology presented here or to perform adsorption measurements under the appropriate range of pressures and temperatures to ascertain the homo or heterogeneity of the zeolite surface for catalytic cracking.…”

Location of MTBE and toluene in the channel system of the zeolite mordenite: Adsorption and host–guest interactions

“…Eqs. (3) and (4)) are not very different from each other (see Table 6); however, the latter equation was employed since it has often been used for fitting volatile organic compounds adsorption on zeolites [29]. The fitted saturation capacity of MOR (see Table 6) is higher for toluene than for MTBE.…”

Proton exchange reactions of C2–C4 alkanes sorbed in ZSM-5 zeolite

“…Entropies of activation are negative and have values between 40.6 (ethane) and 47.6 (n-butane/1) cal K -1 mol -1 for the proton exchange reaction in ZSM-5. From the Arrhenius equation, the entropy term contributes to the prefactor [35] and, thus, it slows down the proton exchange rate. Apparent Gibbs energies of activation at 500 K are in the range of 30.3 (ethane) to 40.6 (i-butane/3) kcal/mol.…”

“…De Moor et al[35] using QM-Pot(MP2//B3LYP) have shown that adsorption enthalpies of n-alkanes (C2-C8) in H-FAU, H-BEA,…”