Abstract:
Molecular sized pores are not only critical for ion exchange and sorption, but they also provide a unique chemical and steric environment for catalysis. Regular dimensions allow stability of ground and transition states of reacting molecules better than that of larger pore oxide and organic porous materials. This enhances interaction strength and lowers the standard free energies of transition states in a highly selective manner. These properties are analog to qualities that are critical for the high activity of enzymes, for the local constraints and for the local chemical environment at active sites.
Many reactions in petroleum chemistry, such as cracking or isomerization, occur under conditions where the concentration of reacting molecules is low and excellent models to understand reactive interactions under such conditions have been developed. The search for more efficient reactions at lower temperatures, such as eliminations, carbon-carbon bond formation, and the presence of the liquid phase induce complex ordering of reactants, intermediates and products, enabling a subtle way to direct sorption and catalysis. The ordering in protic solvents, such as water, especially leads to new chemistry as acid zeolites transform into a polar oxide environment with hydrated hydronium ions as the stable active site.
The lecture will address the chemical consequences of such an environment and compare it with the environment created in molecular organic frameworks. We will show how water and protic solvent molecules self-organize in this environment and how they impact the thermodynamic state of the sorbed and reacting molecules. It will be shown that the interactions can be designed and controlled via direct synthesis (changing pore sizes and concentration of sites), as well as via the addition of cations, oxidic clusters or organic fragments. Such interactions will be compared to interactions reacting molecules have with coordination compounds and enzyme sites. As examples for catalytic transformations, the lecture will compare elimination reactions of alcohols, alkylation of aromatic molecules and oligomerization of olefins. Experimental methods to define the state of the reacting molecules, combined with detailed kinetic analysis and theory, will be used to explain the principal contributions of the interactions and the confinement to determine reaction rates. We will discuss how reaction rates and pathways can be tailored using the space available for a transition state and the chemical constituents around the active site.
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