(207f) Consequences of Entropy for Reactions within Constrained Zeolite Environments

Brønsted acid-catalyzed hydrocarbon reactions (e.g., cracking, alkylation, hydride transfer) require the formation of cationic transition states and the separation of charge into ion-pairs. In these reactions, the enthalpic demands of charge separation at transition states are compensated by concomitant entropy gains as a result of rotational and vibrational modes that become available as covalently-bound or physisorbed intermediates become ion-pairs. Monomolecular reactions of alkanes on zeolitic acids (H-FER, H-MFI, H-MOR) are used to show how entropic considerations lead to turnover rate differences with alkane structure and spatial constraints. Transition states involved in dehydrogenation of linear alkanes (C3H8, n-C4H10) are higher in enthalpy and entropy relative to those for cracking, yet the opposite trend is observed for branched alkanes (i-C4H10). Relative enthalpy differences for the cationic transition states formed in cracking and dehydrogenation steps are commensurate with differences in stability for equivalent structures in the gas phase, consistent with thermochemical relations for acid catalysis. Zeolite channel structures that confine transition states to lesser extents, such as shallow eight-membered ring (8-MR) pockets in H-MOR via partial confinement effects absent in larger 12-MR channels, result in entropy gains that preferentially stabilize ion-pairs at later and looser transition states. Entropy gains at the transition state are also largely responsible for the >100-fold increase in monomolecular cracking turnover rates of linear C3-C6 alkanes on H-MFI with alkane size. These preeminent effects of entropy on the free energy of ion-pair transition states appear to be a ubiquitous feature of acid catalysis within confined structures.