Adsorption and cracking of propane by zeolites of different pore size

Abstract A hybrid QM:QM method (MP2:(PBE+D2) + ΔCCSD(T)) is employed that reaches chemical accuracy (±4 kJ mol−1) for periodic systems. Three mechanisms are considered and evidence is produced that cracking proceeds via protonation of a C C bond at the Bronsted acid site. For H-FER, H-MFI, and H-CHA, the deviations between predictions (uncertainty ± 4 kJ mol−1) and experiment (error ± 5 kJ mol−1) are between −3 and +8 kJ mol−1. The predicted apparent enthalpy barriers for H-FER, H-MFI, H-CHA, and H-FAU at 650 K are 153, 153, 160, and 182 kJ mol−1, respectively. Their variation with the pore size is dominated by the adsorption enthalpies (−55, −47, −40, and −29 kJ mol−1, respectively), whereas the intrinsic enthalpy barriers vary within a narrow range of 10 kJ mol−1 only. Routine PBE+D2 calculations show large errors, −16 to −20 and −40 to −60 kJ mol−1 for adsorption enthalpies and enthalpy barriers, respectively.