Zeolites and Intrazeolite Chemistry: Insights from Infrared Spectroscopy

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

Variable-temperature infrared (VTIR) spectroscopy is an instrumental technique that enables structural characterization of gas-solid adsorption complexes by analysis of meaningful vibrational modes, and simultaneous determination of the standard enthalpy change (ΔH0) involved in the gas adsorption process, which allows one to quantify the stability of the corresponding complex. This is achieved by a van’t Hoff analysis of a set of IR spectra recorded over a sufficiently large temperature range. Herein, the use of this versatile spectroscopic technique is demonstrated by reviewing its application to the study of carbon monoxide, carbon dioxide and dinitrogen adsorption on several (alkaline) zeolites, which can be regarded as the archetype of periodic porous solids.

Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research

The current state of the art in the application of variable-temperature IR (VTIR) spectroscopy to the study of (i) adsorption sites in zeolites, including dual cation sites; (ii) the structure of adsorption complexes and (iii) gas-solid interaction energy is reviewed. The main focus is placed on the potential use of zeolites for gas separation, purification and transport, but possible extension to the field of heterogeneous catalysis is also envisaged. A critical comparison with classical IR spectroscopy and adsorption calorimetry shows that the main merits of VTIR spectroscopy are (i) its ability to provide simultaneously the spectroscopic signature of the adsorption complex and the standard enthalpy change involved in the adsorption process; and (ii) the enhanced potential of VTIR to be site specific in favorable cases.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Thermodynamics of reversible gas adsorption on alkali-metal exchanged zeolites—the interplay of infrared spectroscopy and theoretical calculations

Detailed understanding of weak solid-gas interactions giving rise to reversible gas adsorption on zeolites and related materials is relevant to both, fundamental studies on gas adsorption and potential improvement on a number of (adsorption based) technological processes. Combination of variable-temperature infrared spectroscopy with theoretical calculations constitutes a fruitful approach towards both of these aims. Such an approach is demonstrated here (mainly) by reviewing recent studies on hydrogen and carbon monoxide adsorption (at a low temperature) on alkali-metal exchanged ferrierite. However, the methodology discussed, which involves the interplay of experimental measurements and theoretical calculations at the periodic DFT level, should be equally valid for many other gas-solid systems. Specific aspects considered are the identification of gas adsorption complexes and thermodynamic studies related to standard adsorption enthalpy and entropy.

Variable temperature infrared spectroscopy: A convenient tool for studying the thermodynamics of weak solid–gas interactions

This tutorial review describes the use of variable temperature infrared spectroscopy of adsorbed species (VTIR), a recent method for studying the thermodynamics of weak solid-gas interactions. Examples show how a fundamental relationship of thermodynamics (the van't Hoff equation, used long since in several fields of physical chemistry) can describe equilibrium processes at the solid-gas interface. The VTIR method is fully exploited by measuring absorbance of an IR band, temperature and pressure over a wide temperature range: an estimation of the interaction energy is, however, possible even ignoring the equilibrium pressure. Precise thermodynamic characterization of solid-gas interactions is required in several fields: on the applied side, gas sensing, separation and storage, which involve such areas as work-place security, air pollution control and the energy sector; regarding fundamental knowledge, weak solid-gas interactions are relevant to a number of fields, including hydrogen bonding, coordination chemistry and surface phenomena in a broad sense. Infrared (IR) spectroscopy of (gas) molecules adsorbed on a solid is frequently used to characterize both, the adsorbed species and the adsorbing centres at the solid surface. The potential of the technique can be greatly enhanced by obtaining IR spectra over a temperature range, and simultaneously measuring IR absorbance, temperature and equilibrium pressure. When this is done, variable temperature infrared (VTIR) spectroscopy can be used not only for a more detailed surface characterization, but also for precise studies on the thermodynamics of solid-gas interactions. Furthermore, when weak interactions are concerned, the technique shows favourable features compared to adsorption calorimetry, or to other classical methods. The potential of the VTIR method is highlighted by reviewing recently reported studies on dihydrogen, dinitrogen and carbon monoxide adsorption on zeolites. To facilitate understanding, an outline of the basis of the method is also given, together with an appraisal of the critical points involved in its practical use.