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

The thermodynamics of dinitrogen adsorption in faujasite-type zeolites, Na-Y, Ca-Y and Sr-Y, were investigated by means of variable-temperature infrared spectroscopy, a technique that affords determination of the standard adsorption enthalpy (Δ⁰) and entropy (Δ⁰) from an analysis of the IR spectra recorded over a range of temperatures. The results obtained, taken together with previously reported values for N₂ adsorption on protonic zeolites, revealed a non-linear correlation between Δ⁰ and Δ⁰. Implications of such a correlation for gas separation and purification by adsorption in porous solids are highlighted.

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.

A rapid microwave-assisted synthesis of a sodium-cadmium metal-organic framework having improved performance as a CO2 adsorbent for CCS.

We report on a facile and rapid microwave-assisted method for preparing a sodium-cadmium metal-organic framework (having coordinatively unsaturated sodium ions) that considerably shortens the conventional synthesis time from 5 days to 1 hour. The obtained (Na,Cd)-MOF showed an excellent volumetric CO2 adsorption capacity (5.2 mmol cm(-3) at 298 K and 1 bar) and better CO2 adsorption properties than those shown by the same metal-organic framework when synthesized following a more conventional procedure.

Measuring the Brønsted acid strength of zeolites--does it correlate with the O-H frequency shift probed by a weak base?

Brønsted-acid zeolites are currently being used as catalysts in a wide range of technological processes, spanning from the petrochemical industry to biomass upgrade, methanol to olefin conversion and the production of fine chemicals. For most of the involved chemical processes, acid strength is a key factor determining catalytic performance, and hence there is a need to evaluate it correctly. Based on simplicity, the magnitude of the red shift of the O-H stretching frequency, Δν(OH), when the Brønsted-acid hydroxyl group of protonic zeolites interacts with an adsorbed weak base (such as carbon monoxide or dinitrogen) is frequently used for ranking acid strength.

Variable-temperature IR spectroscopic and theoretical studies on CO2 adsorbed in zeolite K-FER.

Adsorption of CO(2) in K-FER zeolite is investigated by a combination of variable-temperature IR spectroscopy and periodic DFT calculations augmented for description of dispersion interactions. Calculated adsorption enthalpies for CO(2) adsorption complexes on single extra-framework K(+) sites and on dual-cation sites where CO(2) interacts simultaneously with two extra-framework K(+) cations (-40 and -44 kJ mol(-1), respectively) are in excellent agreement with experimental values. The analysis of effects on the frequency of the asymmetric CO(2) stretching mode ν(3) shows that polarization of CO(2) by the K(+) cation leads to an increase in ν(3), while the interaction of CO(2) with the zeolite framework leads to a decrease in ν(3).

Ammonia-solvated ammonium species in the NH(4)-ZSM-5 zeolite.

Interaction of gaseous ammonia with a NH(4)-ZSM-5 zeolite (Si/Al=11.5) was studied by means of infrared (IR) spectroscopy both at constant ambient temperature and in the temperature range 373-573 K. H-bonding of NH(3) molecules to the NH(4) (+) species takes place.

Thermodynamics of hydrogen adsorption on metal-organic frameworks.

Interaction between adsorbed hydrogen and the coordinatively unsaturated Mg(2+) and Co(2+) cationic centres in Mg-MOF-74 and Co-MOF-74, respectively, was studied by means of variable-temperature infrared (VTIR) spectroscopy. Perturbation of the H(2) molecule by the cationic adsorbing centre renders the H--H stretching mode IR-active at 4088 and 4043 cm(-1) for Mg-MOF-74 and Co-MOF-74, respectively. Simultaneous measurement of integrated IR absorbance and hydrogen equilibrium pressure for spectra taken over the temperature range of 79-95 K allowed standard adsorption enthalpy and entropy to be determined.

Thermodynamics of carbon dioxide adsorption on the protonic zeolite H-ZSM-5.

Adsorption of carbon dioxide on H-ZSM-5 zeolite (Si:Al=11.5:1) was studied by means of variable-temperature FT-IR spectroscopy, in the temperature range of 310-365 K. The adsorbed CO(2) molecules interact with the zeolite Brønsted-acid OH groups bringing about a characteristic red-shift of the O-H stretching band from 3610 cm(-1) to 3480 cm(-1).

Variable-temperature infrared spectroscopy studies on the thermodynamics of CO adsorption on the zeolite Ca-Y.

Variable temperature FT-IR spectroscopy (in the range of 298-380 K) is used to study the thermodynamics of formation of Ca(2+)...

Materials for hydrogen storage: current research trends and perspectives.

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. .