Solvent Effects on Acid-Base Interactions in Zeolites
Restricted (Penn State Only)
- Author:
- Mu, Yanyu
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 28, 2022
- Committee Members:
- Xueyi Zhang, Major Field Member
Robert Rioux, Chair & Dissertation Advisor
Michael Janik, Major Field Member
Seong H. Kim, Professor in Charge/Director of Graduate Studies
Stephen Chmely, Outside Unit & Field Member - Keywords:
- Solvent effects
zeolite catalysis - Abstract:
- Zeolites are important solid acid catalysts in both gas- and liquid-phase reactions. The topology of zeolites and the type of acid sites (i.e., Brønsted vs. Lewis vs. Silanol groups) influence catalytic activity of zeolites. In liquid-phase zeolite-catalyzed reactions, the choice of solvent influences reaction rate and selectivity in comprehensive ways including (1) tuning the composition of reactants and solvents inside zeolite pores; (2) solvating active sites, reactants, transition states and products to different extent, therefore influencing the free energy of kinetically relevant steps, etc. In order to select suitable solvent and zeolites in liquid-phase reactions, it is crucial to understand how the choice of solvent and zeolite topology influence the apparent acidity of Brønsted acid sites (BAS) of zeolites. This dissertation presents our work about investigating the dependence of apparent acidity of zeolites on the choice of solvent and Si/Al ratio by measuring the liquid-phase adsorption thermograms of basic molecules on BAS of zeolites using isothermal titration calorimetry (ITC). Chapter 2 presents the characterization of examined zeolites in order to know the quantity and type of acid sites in examined zeolites. In this Chapter, liquid-phase adsorption enthalpies of pyridine on three H-ZSM-5 zeolites were measured in ITC at 25 °C. Measured enthalpies depend on the choice of solvent and Si/Al ratio of zeolites. Gas-phase adsorption enthalpy of pyridine on H-ZSM-5 zeolites is not dependent on Si/Al ratio of zeolites. In Chapter 3, we developed Born-Haber cycles to correlate gas-phase adsorption enthalpy and liquid-phase adsorption enthalpy of pyridine using water and heptane as respective solvent. Developed Born-Haber cycles quantitively prove the difference in liquid-phase adsorption enthalpy is caused by different extent of solvating initial state and final state. The extent of solvating initial state and final state is influenced by the choice of solvent and Si/Al ratio of zeolites. Acetonitrile (ACN)-water mixtures are widely used solvent mixtures. In Chapter 4, we demonstrated the influence of ACN-water composition on apparent acidity and availability of H-ZSM-5 zeolites in liquid-phase adsorption of pyridine on H-ZSM-5 zeolites in water. Due to the microheterogeneity and preferential solvation of BAS in ACN-water mixtures, the apparent acidity and availability of BAS are decreased in ACN-water mixtures compared to respective pure solvent. Chapter 5 investigates the use of different basic molecules to characterize the apparent acidity of H-ZSM-5 zeolites. Molecules with different basicity could characterize BAS in different solvation environment by water molecules. In this chapter, we discussed the strength and limitations using ITC to measure heterogeneous interactions involving porous zeolites. Chapter 6 proposes to improve our understanding of both solvent and topology effects on apparent acidity of zeolites by combining with spectroscopy and computational studies. We also propose to apply our conclusions in this dissertation to liquid-phase reactions such as cyclohexanol dehydration.