Probing Brønsted Acidity of Protonic Zeolites with Variable-Temperature Infrared Spectroscopy

Abstract

Most industrial applications of zeolites as solid-acid catalysts rely on their high Brønsted acidity, which affects both catalytic activity and selectivity, and hence the convenience to find an accurate experimental technique for measuring the acid strength. The enthalpy change, Δ 0 , involved in the hydrogen bonding interaction between a weak base (such as carbon monoxide) and the Brønsted acid [Si(OH)Al] hydroxyl groups should correlate directly with the zeolite acid strength. However, on account of simplicity, the bathochromic shift of the O-H stretching frequency, Δ (OH), is usually measured by IR spectroscopy at a (fixed) low temperature instead of Δ 0 and correlated with the acid strength for ranking the zeolite acidity. Herein, the use of variable-temperature IR spectroscopy to determine simultaneously Δ 0 and Δ (OH) is demonstrated, followed by a review of recent experimental results showing that the practice of ranking the acid strength by the corresponding O-H frequency shift probed by a weak base could be misleading; and that can be so much the case of zeolites showing a wide range of structure types.K e y w o r d s: Brønsted acidity, infrared spectroscopy, zeolites.

Non-Linear Enthalpy-Entropy Correlation for Nitrogen Adsorption in Zeolites

“…It is thus clear that, after recording IR absorption and equilibrium pressure over a temperature range, a van’t Hoff plot of Equation (4) or (5) gives direct access to the corresponding values of Δ H 0 and Δ S 0 which characterize the thermodynamics of the gas-solid adsorption process. Details of the assumptions made, and hence on the applicability of Equations (4) and (5) can be found elsewhere [ 19 , 20 ].…”

Ranking the Bronsted Acid Strength of Protonic Zeolites with VTIR Spectroscopy --- An Overview of Current Research

“…Taken together with zeolite topology and Si : Al ratio, the strength of their (catalytically active) Br.nsted acid sites is a major factor determining the catalytic performance of protonic zeolites in terms of both, catalytic activity and selectivity; and therefore, the convenience to have a reliable method to quantify the relative Br.nsted acidity of protonic zeolites [ 6,7]. To that purpose both, IR and solidstate NMR spectroscopy stand out from the repertoire of instrumental techniques most frequently used; and the same applies when it comes to explore the surface chemistry of both, zeolites and microporous aluminosilicates in a broader context [8][9][10][11][12][13][14][15][16][17].…”

Structure and Stability of Gas Adsorption Complexes in Periodic Porous Solids as Studied by VTIR Spectroscopy: An Overview

“…For such a purpose, classical infrared (IR) spectroscopy can give valuable structural information derived from analysis of the wavenumber shifts undergone by meaningful vibrational modes of the adsorbed molecule, and from the (relative) intensity of the corresponding IR absorption bands, but determination of the gas-solid interaction energy is out of reach unless a complementary technique (such as adsorption calorimetry) is also used. Nevertheless, the recently implemented variable-temperature IR (VTIR) spectroscopic method [37][38][39] facilitates direct access to both, structural characterization of the adsorption complex and (simultaneous) measurement of the standard enthalpy change (∆H 0 ) involved in the gas adsorption process, as will be shown below by reviewing several enlightening case studies. The versatility of zeolites (both, natural and synthetic) for practical usage stems, to a large extent, from their large number of structure types, which is well over one hundred, and continues to grow by adding new synthetic members.…”