Gas separation and gas storage using porous materials
CECAM-HQ-EPFL, Lausanne, Switzerland
Main porous solids currently being studied for adsorption of hydrogen, methane and carbon dioxide include activated coal and templated (synthetic) carbons, zeolites and related materials, metal organic frameworks (MOFs) and covalent organic frameworks (COFs). Altogether, COFs and MOFs are the materials stirring greater interest, by virtue of their large variety of structure types and chemical composition as well as low density and very high surface area [1-4], which are desirable properties for gas storage and delivery. Besides synthesis, a first task to be accomplished when studying porous solids for gas adsorption is precise characterization of structure, pore size and shape, topology and nature of the gas adsorbing centres (if localized adsorption is under consideration). For that, standard crystallography is usually insufficient, and several other techniques (mainly spectroscopic) have to be concurrently applied. Computational methods are also very helpful in many instances [5-7], and even better results can usually be obtained by combining both types of approach [8-10].
Hydrogen storage in porous solids was recently review by several authors [11-15], showing how despite the effort made by many research groups, we are still far from having an adsorbent capable of retaining enough hydrogen for on-board practical use at a temperature near to ambient. However, substantial progress was made towards understanding of the conditions needed. On the experimental side, vibrational spectroscopy [16-18] and inelastic neutron scattering [19,20] are among the techniques giving more precise information, since both of them allow not only a characterization of the hydrogen adsorption complex to be performed, but also the adsorption enthalpy (and entropy in the case of IR spectroscopy) to be simultaneously determined. On the computational side a main problem is that weak (dispersion) forces constitute a very substantial part of the gas-solid interaction energy, and that calls for very demanding calculation methods [21,22]. Studies (both theoretical and experimental) on methane adsorption on porous carbons [23,24], MOFs and COFS [25-29] were recently reported by several research groups. At a difference with hydrogen, the U.S. Department of Energy (DOE) storage target for methane has been reached (or even surpassed) with some MOFs [28,29], which showed an adsorption enthalpy of about 30 kJ mol-1. These promising results suggest that there should be room for improvement in the performance of methane adsorbents for on-board methane delivery. Regarding carbon dioxide, there is currently very active research on porous solids which could favourable compete with liquid amines for CCS. Zeolites and ZIFs [30,31], amine-functionalized mesoporous silica , and MOFs  are among the materials receiving greater attention. Besides adsorption capacity, chemical stability over a large number of adsorption-desorption cycles, and favourable thermodynamics, are essential properties of such materials; and there seems to be a fair chance for further improvement on those properties. Finally, it is worth noting that reversible gas-solid adsorption, as required for all of the cases considered above, relies on a relatively small standard adsorption enthalpy, a substantial part of which usually comes from weak intermolecular forces that pose very demanding requirements on theoretical calculations [34-37]. Discussion of these matters is intended to constitute an important part of the workshop.
Claudio Marcelo Zicovich-Wilson (Universidad Autónoma del Estado de Morelos - Facultad de Ciencias) - Organiser & speaker
Carlos Otero Arean (University of the Balearic Islands - Inorganic Chemistry Department) - Organiser
Loredana Valenzano (Michigan Technological University) - Organiser & speaker