Istvan Halasz
PQ Corporation, R&D Center, USA
Title: Quantum Chemistry Aided FTIR Spectroscopy to Distinguish Internal and External Brønsted Acidic Sites of Zeolite Nano-Crystals
Biography
Biography: Istvan Halasz
Abstract
Microporous nano-crystals of zeolites are key ingredients in more than 50% of heterogeneous catalysts, which contribute to manufacturing the majority of chemical products. Their Brønsted acidic hydroxyl groups (BA-OH) play pivotal role in many reactions. The presence of BA-OH sites on the crystallite surface bears of special interest, due to diffusion limitations inside of their micropores. Yet distinction of the internal and external BA-OH sites has been ambiguous. Here we show that clear distinction can be made by Fourier Transform Infrared (FTIR) spectroscopy when both diffuse reflectance (DR) and transmission (TR) sampling techniques are employed. Different laboratories use these techniques interchangeably for characterizing solids. To our surprise, we observed very different BA-OH spectra on some zeolites when measured by these two methods. Since physical and chemical differences do not generate such large spectral deviations, we conjectured that emphasized vibrational intensities of the surface and bulk BA-OH groups cause the differences when DR or TR technique is used, respectively. To prove our point we performed selective pyridine adsorption experiments on Chabazite (H-CHA) and H-SAPO-34 zeolites and also computed the density functional theory (DFT) based FTIR spectra of their four geometrically different BA-OH groups in the bulk and on the surface, which confirmed the conjectured vibrational differences. These zeolites are key ingredients of new catalysts for diesel-automobile exhaust control and for the methanol to olefin, MTO, technology.