(495a) Confinement Effects and Catalysis by Acid Sites and Metal Clusters

The location of active sites within zeolite voids strongly influences turnover rates and selectivities for acid-catalyzed carbonylation reactions at low temperatures and dehydrogenation and cracking of alkanes at higher temperatures.  Specifically, small eight-member ring structures in zeolites minimize the free energies of required carbocationic transition states for both reactions.  Partial confinement within shallow pockets in MOR structures leads to higher entropy transition states with lower activation free energies at the high temperatures required for cracking and dehydrogenation of alkanes.  In contrast, more effective enthalpic stabilization via van der Waals contacts within these small voids leads to more stable transition states at the low temperatures of carbonylation catalysis. The consequences of these entropy-enthalpy trade-offs  for transition state stabilization depend on the lateness of the transition states along the reaction coordinates for competing pathways, which lead, in turn, to strong and predictable effects of confinement on selectivities. 

The preferential encapsulation of metal and oxide clusters elements within small-pore zeolites can be achieved using hydrothermal synthetic protocols that synchronize the colloidal precipitation of metal hydroxides with the nucleation of the zeolite nanostructures through the use of ligands that stabilize metal precursors at the conditions required for zeolite crystallization.  These methods lead to clusters that closely resemble in size and uniformity the nanometer-sized zeolite cages that contain them and to surfaces devoid of synthetic debris and exhibiting high catalytic turnover rates.   Encapsulation within small voids protects clusters against sintering and coalescence, leading to unprecedented stability during thermal treatment, and also prevents titration of cluster surfaces by larger organosulfur compounds during catalysis.  The intracrystalline location of these clusters also selectively sieves reactants based on molecular size and catalyzes hydrogenation of small alkenes and dehydrogenation of small alkanols but not the corresponding reactions of their respective larger homologs