Abstract
Solvent selection is a pressing challenge in developing efficient and selective liquid phase catalytic processes, as predictive understanding of the solvent effect remains lacking. In this work, an attenuated total reflection infrared spectroscopy technique is developed to quantitatively measure adsorption isotherms on porous materials in solvent and decouple the thermodynamic contributions of van der Waals interactions within zeolite pore walls from those of pore-phase proton transfer. While both the pore diameter and the solvent identity dramatically impact the confinement (adsorption) step, the solvent identity plays a dominant role in proton-transfer. Combined computational and experimental investigations show increasingly favorable pore-phase proton transfer to pyridine in the order: water < acetonitrile < 1,4 – dioxane. Equilibrium methods unaffected by mass transfer limitations are outlined for quantitatively estimating fundamental thermodynamic values using statistical thermodynamics.
Highlights
Solvent selection is a pressing challenge in developing efficient and selective liquid phase catalytic processes, as predictive understanding of the solvent effect remains lacking
Even for a solvent that is generally considered as inert, i.e., not directly involved in the molecular transformations, solvation impacts the energetics of adsorbates, intermediates, and transition states (TSs), which could in turn lead to modified rates and selectivities[6,7,8,9,10]
The pore-phase pyridine concentration sharply decreases in samples with increasing pore diameters from 7 to 15 Å in siliceous microporous materials, and the confinement effect is minimal in a 25 Å pore Si/MCM-41
Summary
Impact of pore size on the adsorption of pyridine. The pore-phase pyridine concentration sharply decreases in samples with increasing pore diameters from 7 to 15 Å in siliceous microporous materials, and the confinement effect is minimal in a 25 Å pore Si/MCM-41. ATR-FTIR spectroscopy is used to measure pyridine adsorption isotherms in a variety of zeolites and mesoporous materials in liquid water (Fig. 2a). At a constant bulk liquid-phase concentration of pyridine, the equilibrium concentration of pyridine in the siliceous materials is shown to dramatically decrease with increasing pore diameter. The pore-phase pyridine concentration nears the value for pure liquid pyridine (≈12.4 M) at the concentrated end of the Si/ZSM5 isotherm, meaning that in Si/ZSM-5, the pores are almost
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