(340an) Solvent Effects in Liquid-Phase Catalytic Reactions

My research at the Pennsylvania State University focuses on understanding the impact of solvent effects during acid-catalyzed heterogeneous reactions. Isothermal titration calorimetry (ITC) has provided a novel method to determine thermodynamic and kinetic parameters of model liquid-phase reactions.

The adsorption of pyridine on Brønsted acid sites (BAS) of zeolites is selected as a model acid-base interaction probed in ITC to examine the influence of solvent in acid-catalyzed heterogeneous reaction. ITC experiments demonstrate the choice of solvent strongly affects the adsorption thermodynamics of pyridine on H-ZSM-5 zeolite and the availability of acid sites in solvated zeolites. Differences in measured (or apparent) enthalpies are caused by the different extent to which the initial state and the final state are solvated, and not differences in intrinsic acidity of intrapore protons.

A comparison of liquid- and gas-phase calorimetry results for pyridine adsorption on H-ZSM-5 will be shown for two solvents-water and heptane. The comparison of the calorimetry results in the two phases demonstrates the impact of non-idealities on acid-base reactions within the voids of zeolites depends on their Si/Al ratios. Differences in the measured enthalpy from ITC in different solvents can be explained by developing Born-Haber cycles for pyridine adsorption on zeolites with different Si/Al ratios. We will further discuss the relationship between the gas- and liquid-phase basicity of amines and enthalpies of adsorption on H-ZSM-5 in the presence of water.

We demonstrate the ability to obtain kinetic parameters from ITC thermograms. The complexation of Pd(NH₃)₄Cl₂ with dimethylglyoxime has been selected as candidate reaction for kinetic ITC (kinITC) study. The shape of the thermogram monitored by ITC is influenced by reaction conditions such as temperature and concentration. We examine the reaction order and mechanism of the candidate reaction to assist collaborators in distinguishing the kinetic signal from the instrument response function.

Overview of Research Interests

My research interests focus on understanding the impact of solvent effects during liquid-phase reactions. Liquid-phase isothermal titration calorimetry (ITC) has been utilized as a novel tool to determine both thermodynamic and kinetic parameters in liquid-phase reactions. The use of ITC to inform our understanding on solvent effects can be extended to increase catalytic activity and selectivity of industrial liquid-phase reactions such as biomass derived reactions, active pharmaceutical ingredients (API) synthesis and chemical recycling of plastic waste.

Key words: Catalysis, Solvent Effects, Thermodynamics, Kinetic Modeling, Zeolites, Acid-Base Interactions.

Technique: ITC, HPLC, TGA, DSC, Micromeritics 3 Flex, UV-Vis, ICP-OES, FTIR, Bruker solid- and solution-state NMR spectrometers, Microcalorimeters.

Software: Wolfram Mathematica, MATLAB, Aspen Plus, Aspen HYSYS, Aspen Energy Analyzer, AutoCAD, Smart Plant 3D (SP3D), MNova.

Current Research:

(1)Solvent effects on elementary reactions in solid-acid catalyzed reactions: Acid-base interactions in zeolites.

Biomass-derived molecules are characterized by high oxygen content and boiling points necessitating their conversion to value-added products by catalytic routes be conducted in the liquid phase which often necessitates the use of a solvent. Solvents introduce thermodynamic non-idealities that if accounted for can be used to rigorously compare the behavior of different catalysts in the condensed phase or in the vapor-phase (where we generally assume ideality). Our understanding of the extent to which non-idealities thermodynamically influence the intrinsic kinetic barries will provide guidelines to increase catalytic activity and selectivity by selecting suitable solvent(s) or mixtures thereof.

We examine the influence of solvent in acid-catalyzed heterogeneous reactions using a model acid-base interaction (the adsorption of pyridine on Brønsted acid sites (BAS) of zeolites) which is probed by ITC. ITC results demonstrate the identity of solvent, and the composition of acetonitrile-water mixtures strongly affects (i) adsorption thermodynamics of pyridine on H-ZSM-5 and (ii) the availability of acid sites in solvated zeolites.

We further examine the impact of solvent addition on the gas-phase adsorption thermodynamics of pyridine on BAS using calorimetry. The comparison of the calorimetry results in the gas- and liquid- phase for two solvents – water and heptane demonstrates the influence of non-idealities on acid-base reactions on zeolites depends on the Si/Al ratio (i.e., hydrophobicity) of zeolites.

We demonstrate the ability using liquid- and gas-phase calorimetry to construct thermodynamic Born-Haber cycle for the adsorption of pyridine on solvated BAS of zeolites with different Si/Al ratio. Born-Haber analysis rationalizes the influence of solvent on apparent thermodynamics by examining solvent-zeolite, pyridine-zeolite, and pyridine-solvent interactions separately.

The development of a quantitative understanding of solvent effects on acid-base interactions in zeolites increases our understanding on how non-idealities thermodynamically influence intrinsic kinetic barriers.

(2)Kinetic ITC: Obtaining thermodynamic and kinetic parameters via ITC.

Kinetic ITC (kinITC) has greatly expanded the application of ITC data by providing kinetic information from the ITC thermogram. We selected the complexation of Pd(NH₃)₄Cl₂ with dimethylglyoxime as a candidate reaction to be probed in ITC for rigorous development of kinITC. In many applications, the rate constant (k_on) is critical to quantify than the overall equilibrium constant. The shape of the heat signals is found to be influenced by reaction conditions (e.g., temperature and concentration). We examine the reaction order and mechanisms of the complexation reaction. We provide kinetic parameters (e.g., k_on and k_off) to assist collaborators in distinguishing the kinetic signal from the instrument response function. The ability to obtain both thermodynamic and kinetic data via ITC provides important methods to probe diverse system such as ligand binding and adsorption process.

Future Research

We plan to extend the use of ITC and our understanding on solvent effects in acid-catalyzed heterogeneous reactions to increase the reactivity and selectivity of industrial liquid-phase reactions.