Effect of liquid water on acid sites of NaY: An in situ liquid phase spectroscopic study

Abstract

Liquid phase catalyst characterization is experimentally challenging, but it deserves consideration because solvent choice can affect catalyst structure and acidity. Thus, in vacuo spectroscopy of adsorbed probe molecules such as pyridine, while valuable, may provide incomplete information on available catalytic sites in liquid phase reactions. To this end, an Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR-FTIR) cell was used to detect pyridine adsorbed on sodium exchanged faujasite (NaY) in liquid water. The slow accumulation of pyridinium on NaY in the presence of liquid water was observed, suggesting water promotes the protonation of pyridine on NaY, a Lewis acidic material. Transmission FTIR and ATR-FTIR experiments revealed that pyridinium concentration increased with increasing amount of water in NaY, making the liquid phase technique more apt than the established vacuum technique for observing the effect of water on the acid sites of NaY. However, the Brønsted catalyzed ring opening of 2,5-dimethylfuran (DMF) to 2,5-hexanedione is not active on NaY in water. The lack of activity is attributed to the low basicity of DMF. This is consistent with the observation that acetonitrile, a weaker base than pyridine, was not protonated on NaY in water, while 2,6-dimethylpyridine, a stronger base than pyridine, was readily protonated. Thus, although NaY is capable of forming pyridinium in water, NaY may not be active with respect to Brønsted catalyzed pathways unless a reactant is sufficiently basic. Results presented in this work suggest that probe molecule choice can lead to different interpretations of the nature of acid sites in the presence of adsorbed, and especially liquid, water.