Robert M Rioux

Department of Chemical Engineering

Department of Chemistry

The Pennsylvania State University

University Park, PA 16802-4400

Quantitative measures of intrinsic adsorption energies, activation barriers and catalytic turnover rates are required to understand the origin of solvent effects in heterogeneous catalysis.  Typically, differential solvation of initial (or final) states relative to transition states leads to changes in thermodynamic or kinetic driving forces for individual elementary reactions.  Most of our understanding of solvent effects on the energetics and therefore kinetics is derived from homogeneous reactions, and a scientific understanding of the impact of solvent effects in heterogeneously-catalyzed reactions is not as well-developed.  Liquid-phase isothermal titration calorimetry will probe the interaction of acid site titrants and alkanol substrates (for eventual catalytic dehydration) in solvents and solvent mixtures of different character to quantify the thermodynamic description of adsorption from the solvent phase.  The influence of the nature of solvent interaction with the acid site and reactive intermediates will be rigorously quantified for the catalytic kinetics of alkanol dehydration.  Three solvents – water, n-heptane and acetonitrile-water solutions – chosen based on their characteristic interactions with protons confined within a medium-pore zeolite (mordenite framework inverted, MFI) will be studied and their impact on the observed dehydration kinetics and reaction mechanism quantified through kinetic modeling.  A more subtle influence of solvent on the thermodynamics of catalytic dehydration will be explored using a mixed solvent system, water-acetonitrile.  Since solvent-proton interactions lead to the formation of a specific acid, tuning the mixed solvent composition may alter the nature and number of acid sites available for catalytic turnover.  The impact of solvent identity and specific interactions with the acid and reactive intermediates will be quantified through measures of reaction order, activation energies and transition state activation parameters and derivation of appropriate reaction mechanisms that capture concentration and temperature-dependent behavior.  Thermochemical Born-Haber cycles to rationalize the influence of solvent on apparent thermodynamics will be constructed utilizing additional information from gas-phase adsorption calorimetry, equilibrium adsorption isotherms and solution thermodynamics.  Reaction rates quantified per available acid site and selectivity for alkanol (cyclohexanol, n-propanol and isopropanol) dehydration will be measured under conditions of variable intrapore solvent concentration and structure, from solvent-free (gas-phase) to the incipient intrapore condensation.  Reaction rates will be rigorously defined utilizing non-ideal thermodynamic formalisms for each intrapore environment examined.  Apparent differences in acid site availability based on the solvent type and structure included in zeolite pores will be probed by liquid-/gas- phase calorimetry and in-situ titration during catalytic turnover.

The results from this proposal will impact our understanding of solvent effects in acid-catalyzed heterogeneous reactions.  A quantitative approach based on isothermal titration calorimetry for assessing the impact of solvent identity on the energetics and availability of adsorption sites has been developed and applicable to related disciplines.  The extent to which thermodynamically non-idealities influence the kinetics of elementary steps in a catalytic cycle will increase our understanding of how solvent impacts intrinsic kinetic barriers and how solvent selection influences catalytic activity and selectivity.  The proposed condensed-phase chemistries to be studied are motivated by biomass valorization, but the proposed methods and reported results will be applicable to catalytic chemistries in non-aqueous condensed-phase systems, such as plastic upcycling and catalysis for fine and bulk chemical production.