(222f) Consequences of Composition and Acid Strength for Catalysis On Solid Acids
Keggin polyoxometalate (POM) clusters and acid form zeolites were used to develop rigorous relations between catalyst structure and function to determine the roles of acid strength and solvation in stabilizing transition states and reactive intermediates in alkanol dehydration and alkene isomerization reactions. Keggin POM clusters with W-addenda atoms are Brønsted acids well suited to build such relations by combined experimental and theoretical studies because their uniform active sites and structures are unchanged over diverse compositions. Keggin clusters have acid strengths, defined as their deprotonation energies (DPE), which vary widely based on their central atom (P, Si, Al, Co) and number of charge-balancing protons. Kinetic and isotopic experiments and ab initio calculations led to the formulation of mechanism-based rate expressions needed to measure rate constants of elementary steps from kinetic experiments. In all cases, measured values decreased exponentially with increasing DPE, suggesting that stronger acids lead to more stable ion-pairs at transition states and lower activation barriers. Sensitivities to DPE varied among different reactions, yet activation barriers always changed less than the corresponding change in DPE. Specifically, activation barriers for isomerization reactions were more sensitive to DPE than either 1-butanol or 2-butanol dehydration, whose similar dependences to DPE demonstrated that difficult reactions are not more sensitive to acid strength. In agreement with this conclusion, the effects of DPE on 2-methylpentene (2MP) isomerization rate constants were similar for multiple products with varying rearrangement difficulty; as a result, 2MP selectivities were influenced by acid strength only because facile reactions approach equilibrium more quickly than difficult reactions. Thermochemical cycles of activation barriers that include properties of reactants (proton affinity, adsorption energies), catalysts (DPE), and transition states (electrostatic interactions, charge distribution) demonstrated that the effects of DPE on reactivity are caused by changes in the stabilities of reacting intermediates and transition states with acid strength. Electrostatic interactions at ion-pair transition states, a ubiquitous feature of Brønsted acid catalysis, partially recover the energy required for deprotonation and attenuate activation barriers of all reactions to DPE. Compensation of DPE by these electrostatic interactions is most effective, and the effects of DPE on activation barriers are weakest, when organic cations have localized charges like those at butanol dehydration transition states compared to diffusely charged isomerization transition states. The effects of DPE on rates of methanol dehydration to dimethyl ether (DME) are determined by the prevalent intermediate at active sites under different reaction conditions. Protonated dimer intermediates are ion-pairs whose stabilities change similar to DME formation transition states with DPE, while stabilities of weakly charged monomer intermediates are much less sensitive to DPE. The similar effects of DPE on protonated dimer and DME formation transition state stabilities cause rates to be less sensitive to acid strength when protonated dimer intermediates are prevalent and indicate the role of reacting intermediates and their charges in determining the effects of DPE. Rates of methanol dehydration on zeolites vary less with pore geometry when active sites are occupied by protonated dimers than by monomers and are also closer to DPE correlations developed for Keggin POM. These results arise from greater solvation of transition states and dimers via van der Waals forces than for monomers because both CH3OH reactant molecules are confined within zeolite pores. Effective solvation of reactants at the transition state in zeolites reduces activation barriers to the greatest extent when monomers occupy active sites and causes differences among POM and zeolites that are unrelated to DPE. This study provides a complete and consistent description of the ways in which reactions sense acid strength and confinement separately and prescribes a framework to rigorously analyze their effects. Specifically, charge distributions in the relevant intermediates and transition states determine the effects of acid strength and the relative stabilization of these species by van der Waals forces determine the effects of confinement.