Abstract
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.
Highlights
The foregoing discussion of selected case studies should help to appreciate how, appreciate how, for that purpose, variable-temperature IR (VTIR) spectroscopy has a clear edge over more classical methods for that purpose, VTIR spectroscopy has a clear edge over more classical methods such as adsorption such as adsorption calorimetry or the determination of the isosteric heat of adsorption: first, because calorimetry or the determination of the isosteric heat of adsorption: first, because VTIR can probe
VTIR can probe simultaneously the structure of the gas adsorption complex and the gas-solid interaction energy; secondly, because of the potential of VTIR spectroscopy to give site-specific information, which could not be available from calorimetry
Full exploitation of the VTIR method does depend, on (i) the intrinsic details of the gas-solid system being simultaneously the structure of the gas adsorption complex and the gas-solid interaction energy; secondly, because of the potential of VTIR spectroscopy to give site-specific information, which could not be available from calorimetry
Summary
Owing to the regular layout of channel systems and adsorption sites that facilitates the tailored design of gas adsorption properties, periodic porous solids such as zeolites, metal-organic frameworks (MOFs) and related materials are increasingly being investigated as (potentially) improved adsorbents in a wide range of industrial gas separation and purification processes [1,2,3,4,5,6,7,8,9,10,11,12,13]; among them, CO2 capture from the flue gas of power stations burning fossil fuels, sweetening of natural gas, purification of syngas, petrol desulfurization and hydrogen separation from steam reforming of hydrocarbons, to quote only some examples In several of such processes, the gas adsorbent units are commonly operated in a transient mode that involves alternating adsorption-desorption cycles referred to as pressure swing (PSA) or temperature swing adsorption (TSA), depending on the strategy being used for adsorbent regeneration. Due attention is given to adsorption sites involving more than one zeolite extra-framework cation, which can be highly relevant to both gas separation and heterogeneous catalysis alike
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