Spectroscopic Technique Development for Understanding Solvent Effects in Liquid Phase Reactions

Meeting Program – April 2018

Nicholas Gould – Student Speaker

Advisor: Bingjun Xu
Department of Chemical and Biomolecular Engineering
University of Delaware

Abstract – Biomass conversion reactions are frequently conducted in a solvent, due to the highly oxygenated nature of the feedstock.1,2 Thus, heterogeneous catalytic active sites exist at a solid-liquid interface, where the solvent can modify surface and adsorbate energetics. Even when the solvent does not play a direct role in the reaction mechanism, it can stabilize or destabilize adsorbates, intermediates, and transition states, often leading to markedly different rates and selectivities between solvent choices.3–5 However, solvent effects are poorly understood because catalyst characterization techniques, such as probe molecule adsorption in FTIR, are most often conducted under vacuum or in vapor phase.6,7 Further, most studies on solvent effects focus on screening solvents via catalytic activity testing, where multiple factors that can influence reactivity exist simultaneously: competitive adsorption, stabilization of reactants and transition states, and phase equilibria differences. Thus, there is currently a need for experimental techniques capable of extracting fundamental thermodynamic properties of solvents in simple systems, with the end goal of decoupling the effects of solvent in catalytic activity tests.8

Attenuated total reflection (ATR) fourier transform infrared spectroscopy (FTIR) was used to characterize zeolites with probe molecules in the presence of solvent. The ATR-FTIR was further developed into a quantitative technique, with a procedure for determining extinction coefficients for adsorbed pyridine on zeolites in the presence of solvent.9 This allowed for quantitative comparisons of the effect of solvent on probe molecule uptake and protonation in zeolite pores. Ongoing applications of the ATR-FTIR cell include adsorption isotherms, diffusion measurements, and temperature programmed desorption (TPD) in porous materials in liquid phase. Further, the effect of solvent on charge stabilization in zeolite pores was studied using a homemade TPD set up under back pressurized, flowing solvent. Preliminary pyridine desorption temperatures from an H/ZSM-5 sample reveal that the ability of a solvent to stabilize pyridinium ions decreases in the order: water > acetonitrile > alkane ≈ vacuum.


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  9. N. S. Gould and B. Xu, J. Catal., 2017, accepted.