Adsorption of carbon monoxide, dinitrogen, and carbon dioxide on the porous metal−organic framework Mg-MOF-74 was investigated by means of a combined methodology comprising variable-temperature infrared spectroscopy and ab initio periodic DFT-D calculations using the CRYSTAL code. Both CO and N2 were found to form nearly linear (Mg2+···CO and Mg2+···NN) adsorption complexes, in contrast with CO2, which forms an angular Mg2+···OCO complex. From IR spectra recorded at a variable-temperature, the standard adsorption enthalpy (ΔH 0) was found to be −29, −21, and −47 kJ mol−1 for CO, N2, and CO2, respectively. Calculated values of ΔH 0, including an empirical correction for dispersion forces, resulted to be in a reasonably good agreement with those experimentally obtained. Calculations also showed the very significant role played by dispersion forces, which account for about one-half of the adsorption enthalpy for each of the three adsorbates, CO, N2, and CO2. The results are discussed in the broader context of the adsorption of the same gases on other MOFs and also on zeolites.

Storage and transport of hydrogen constitutes a key enabling technology for the advent of a hydrogen-based energy transition. Main research trends on hydrogen storage materials, including metal hydrides, porous adsorbents and hydrogen clathrates, are reviewed with a focus on recent developments and an appraisal of the challenges ahead. .

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.

Zeolites are tecto-aluminosilicates which can be described by the general formula M';Lz [(A10,)r(Si02),jr-.zH20, where M can be a metal cation or a proton. The Si/AI ratio in synthetic zeolites varies considerably; limiting extremes being 1 : 1 (lower ratio for zeolite X) to near infinity: 1 (in silicalites). This provides a means to modulate the ionicity of the material, which increases with decreasing Si/Al ratio. The framework of every zeolite is constructed from tetrahedral building blocks, TO,, where T is a tetrahedrally coordinated atom (i.e. Si, Al), as depicted in Scheme 1. An isolated SiO, group would carry a formal charge of -4, but in a solid having an O/T ratio of 2 (as found for all zeolites) the SiO, unit is neutral, because each oxygen atom is part of a bridge between two T atoms. However, the net formal charge of the AIO, units is -1, so that the zeolite framework is negatively charged.

~~ Nanosized ZSM5 zeolites with microcrystal dimensions in the 20-120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route.Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 addducts with silanols (both internal and external), Br~nsted-acid groups (both framework and extraframework), Na+ ions, and Lewis A13+ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.Silicon-rich pentasil zeolites represent a series of mediumpore molecular sieves that have been extensively used as adsorbents and catalysts. ZSM5 (a representative member of the series) has a three-dimensional channel system where straight channels run along the [OlO] direction and have openings of 0.54 nm x 0.56 nm, defined by 10-membered rings of tetrahedra. Intersecting these channels, at right angles, there is a second type of sinusoidal channels running along [loo] direction with openings of 0.51 nm x 0.55 nm.'72 The basic features of this structure are independent of the Si : A1 ratio and are also shown by silicalite, the silica endmember of the ZSM5 compositional series.3P5The combination of high acidity, shape selectivity, thermal stability and low coke formation and ageing rate are the main merits of pentasil zeolites in catalytic ~s a g e .~,~-" Some of these features (e.g. the acidic behaviour and catalytic activity) rely on the nature and distribution of protonic and electronaccepting acid sites.The protonic structure of H-ZSM5 and related zeolites has been the subject of a number of studies using mainly IR spectroscopy,20-28 MAS NMR,29-32 adsorption ~a l o r i m e t r y ,~' .~~ temperature-programmed desorption and thermogravimetric a n a l y~i s .~*~~-~~ Two types of framework hydroxy groups showing IR stretching frequencies at 3720-3750 and 3600-3620 cm-' are generally observed and assigned to silanol groups and to bridged Si(0H)Al hydroxy groups, respectively. 2 3-2 8 . 3 7 -3 8 Th e latter are responsible for the highly acidic properties (Brarnsted sites) of the pentasil zeolites. In addition, a weaker absorption band is sometimes observed at ca. 3670 cm-' and tentatively assigned to O H groups located on extraframework aluminium species.26 However, in spite of the amount of work already done, the nature of the hydroxy groups in H-ZSM5 and the corres...