Many industrial applications of protonic zeolites as solid-acid heterogeneous catalysts rely on the strength or their Brønsted acidity, which (together with zeolite topology) affects both, catalytic activity and selectivity. Therefore, the convenience to have an accurate (and simple) experimental technique for measuring Brønsted acid strength. The enthalpy change, ΔH0, corresponding to the hydrogen bonding interaction of a weak base (such as CO or dinitrogen) with their Brønsted acid [Si(OH)Al] hydroxyl groups should directly correlate with the zeolite acid strength. Nevertheless, because of simplicity, the bathochromic shift of the O–H stretching frequency, Δν(OH), is usually measured by IR spectroscopy at a low temperature, and correlated with acid strength, for ranking zeolite acidity. Herein the use of variable-temperature IR (VTIR) spectroscopy to determine simultaneously ΔH0 and Δν(OH) is demonstrated; followed by an abridged overview showing that direct correlation between Δν(OH) and Brønsted acid strength can be misleading when ranking zeolite acidity.

Many industrial applications of protonic zeolites as solid-acid heterogeneous catalysts rely on the strength or their Br nsted acidity, which (together with zeolite topology) affects both, catalytic activity and selectivity. Therefore, the convenience to have an accurate (and simple) experimental technique for measuring Br nsted acid strength. The enthalpy change, Delta H0, corresponding to the hydrogen bonding interaction of a weak base (such as CO or dinitrogen) with their Br nsted acid [Si(OH)Al] hydroxyl groups should directly correlate with the zeolite acid strength. Nevertheless, because of simplicity, the bathochromic shift of the O-H stretching frequency, Deltaν(OH), is usually measured by IR spectroscopy at a low temperature, and correlated with acid strength, for ranking zeolite acidity. Herein the use of variable-temperature IR (VTIR) spectroscopy to determine simultaneously Delta H0 and Deltaν(OH) is demonstrated; followed by an abridged overview showing that direct correlation between Deltaν(OH) and Br.nsted acid strength can be misleading when ranking zeolite acidity. Keywords: VTIR spectroscopy, br nsted acidity, H-zeolites

Protonic gallosilicates H-[Ga]-ZSM-5 were synthesized, following a hydrothermal procedure, from gels having Si/Ga ratios of 25, 50 and 75. Likewise, for comparison, protonic zeolites H-[Al]-ZSM-5 having Si/Al ratios of 25 and 50 were also prepared. Bronsted acidity of the structural Si(OH)Ga groups in the gallosilicates was studied by means of IR spectroscopy at a variable temperature (VTIR) using CO and N-2 as probe molecules. This instrumental technique enables one simultaneous measurement of the bathochromic shift of the stretching O-H mode, Delta nu((OH)) of the Bronsted acid group interacting (through hydrogen bonding) with the probe molecule and the corresponding standard enthalpy change, Delta H-0, in the adsorption process. The results obtained clearly showed that the gallosilicates are distinctively less acidic than the aluminosilicates, whichever acidity indicator is used: Delta nu((OH)) either or Delta H-0. Nevertheless, no change of Bronsted acid strength was found when changing the Si/Ga ratio.

Abstract Detailed analysis of recently reported variable‐temperature IR (VTIR) spectra of carbon monoxide adsorbed in alkaline zeolites shows how, not only the corresponding values of standard adsorption enthalpy (ΔH0 ) and entropy (ΔS0 ) can be obtained, but also the thermodynamic values of molar entropy and enthalpy which characterize the adsorbed gas phase. In addition, it is shown that the so obtained molar entropy data can lead to new insights into soft molecular modes, which would be hardly accessible by conventional IR spectroscopic techniques.

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 (ΔH0) and entropy (ΔS0) 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 ΔH0 and ΔS0. Implications of such a correlation for gas separation and purification by adsorption in porous solids are highlighted.

Most industrial applications of zeolites as solid-acid catalysts rely on their high Brønsted acidity, which affects both catalytic activity and selectivity, and hence the convenience to find an accurate experimental technique for measuring the acid strength. The enthalpy change, ΔH0, involved in the hydrogen bonding interaction between a weak base (such as carbon monoxide) and the Brønsted acid [Si(OH)Al] hydroxyl groups should correlate directly with the zeolite acid strength. However, on account of simplicity, the bathochromic shift of the O–H stretching frequency, Δv(OH), is usually measured by IR spectroscopy at a (fixed) low temperature in-stead of ΔH0 and correlated with the acid strength for ranking the zeolite acidity. Herein, the use of variable-temperature IR spectroscopy to determine simultaneously ΔH0 and Δv(OH) is demonstrated, followed by a review of recent experimental results showing that the practice of ranking the acid strength by the corresponding O–H frequency shift probed by a weak base could be misleading; and that can be so much the case of zeolites showing a wide range of structure types.

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.

This paper replies to two of the most common questions that students usually pose to their teacher during a general chemistry course, i.e. how many chemical elements are in the Periodic Table and how many could be in our Universe. Reply to the former question can be easily found either in the literature or in any updated chemistry book. More interestingly, this communication shows that the latter question may be (simply) answered by making reference to the Bohr’s atomic model that, notwithstanding its well-known limits, allows teachers to demonstrate that (for a hydrogenoid atom) 137 is the highest possible value for Z, as predicted by quantum electrodynamics, a much more complicated theory, usually taught in Physics advanced courses.

Zeolites are often used as the host material for holding and organizing adsorbed molecules and supramolecular species inside their void channels and cages, in order to exploit space confinement and host–guest interaction for engineering composite materials having novel electronic and optical properties. That endeavour would benefit from improved knowledge about the type and strength of the zeolite adsorption sites. To this end, variable temperature infrared spectroscopy (a technique capable of giving simultaneous information on the type and strength of gas-adsorption complexes) was used herein to characterize the zeolite K-L by means of adsorbed CO. Two types of cationic adsorption sites (termed D′ and D″) were found on the wall of the zeolite main channel; formation of the corresponding CO adsorption complexes was found to involve a standard enthalpy change of −23.4 and −26.7kJmol−1, respectively.

Density functional theory (DFT) calculations have played a pivotal role in identifying and understanding different coordination modes of carbon monoxide adsorbed in zeolites: Previous studies combining IR spectroscopic measurements and DFT have firmly established that an adsorbed CO molecule can interact either with a single cation (single-site interaction), or with two or more cations simultaneously (dual-site or multiple-site interaction). However, one aspect that has been scarcely addressed so far is the dependence of the DFT equilibrium structures on the choice of the functional. With the ongoing development of DFT, exemplified by the more widespread use of dispersion-corrected DFT, this question becomes increasingly relevant. The present study investigates whether the inclusion of an empirical dispersion correction leads to qualitatively different predictions in comparison with dispersion-uncorrected DFT, taking CO adsorbed in sodium-exchanged chabazite having two different Si/Al ratios (Si/Al = 11:1 and Si/Al = 2:1) as a model system. Equilibrium structures obtained with the PBE functional and with the dispersion-corrected PBE-D functional are compared, revealing a tendency of dispersion-corrected DFT to favour a stronger interaction of CO with dual sites. This is indicated by a short contact between the oxygen atom of the CO molecule (already coordinated through its carbon atom to a primary Na+ cation) and a secondary Na+ cation. In addition to these qualitative findings, the quantitative agreement of calculated adsorption enthalpies and C–O stretching frequencies with experimental values obtained from variable-temperature IR spectroscopy is evaluated. While neither functional is particularly successful in predicting accurate adsorption enthalpies, the range of C–O stretching frequency values delivered by the PBE-D functional shows a better agreement with the experimental measurements.

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. Moreover, the newly prepared material was found to display high selectivity for adsorption of carbon dioxide over nitrogen, and good regenerability and stability during repeated CO2 adsorption-desorption cycles, which are the required properties for any adsorbent intended for carbon dioxide capture and sequestration (CSS) from the post-combustion flue gas of fossil fuelled power stations.

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. Nevertheless, the enthalpy change, ΔH(0), involved in that hydrogen-bonding interaction should be a better indicator; and in fact Δν(OH) and ΔH(0) are often found to correlate among themselves, but, as shown herein, that is not always the case. We report on experimental determination of the interaction (at a low temperature) of carbon monoxide and dinitrogen with the protonic zeolites H-MCM-22 and H-MCM-56 (which have the MWW structure type) showing that the standard enthalpy of formation of OH···CO and OH···NN hydrogen-bonded complexes is distinctively smaller than the corresponding values reported in the literature for H-ZSM-5 and H-FER, and yet the corresponding Δν(OH) values are significantly larger for the zeolites having the MWW structure type (DFT calculations are also shown for H-MCM-22). These rather unexpected results should alert the reader to the risk of using the O-H frequency shift probed by an adsorbed weak base as a general indicator for ranking zeolite Brønsted acidity.

Many technological processes, mainly in the energy sector, require separation of carbon dioxide from gas mixtures. For that purpose medium and large pore zeolites can be used, provided that the differential CO2 adsorption energy allows thermodynamic separation under equilibrium conditions. Hence the convenience to have a precise knowledge (at the molecular level) about the factors that control CO2 interaction with zeolites, and faujasite-type zeolites in particular, are most relevant in this context on account of their relatively high adsorption capacity. We report on a detailed spectroscopic, calorimetric and theoretical study on the effect of composition on equilibrium CO2 adsorption in alkali-metal exchanged faujasite-type zeolites, which, by combining experimental results with calculations performed at the DFT/CC level on a periodic model of the zeolite yields fine details on the CO2 adsorption complexes and corresponding gas-solid interaction energy. The results obtained are discussed in the broader context of other literature reports; showing, in particular, how the DFT/CC computational approach gives interaction energy values that are in better agreement with experimental data than those obtained using some other computational methods, which show larger limitations to account properly for dispersion interactions. We found out that: (i) dispersion interactions account for about 50% of the overall adsorption enthalpy of CO2 molecules in FAU zeolites, (ii) a very low (experimentally non-detectable) population of sites III and III was found for FAU zeolite with Si/A1 ratio 2.55:1 and all CO2 molecules are adsorbed on sites II and are tilted toward the zeolite wall due to the stabilizing effect of dispersion interactions between CO2 and zeolite, and (iii) minor heterogeneity of adsorption sites present in the FAU samples originating from differences in the number and geometry of Al atoms in the 6R of sites II. (C) 2013 Elsevier B.V. All rights reserved.

•Adsorption calorimetry and variable temperature IR spectroscopy is used to study adsorption of CO2 in the protonic zeolite H-MCM-22.•By simultaneously recording IR absorbance over a temperature range, temperature and equilibrium pressure, standard adsorption enthalpy and entropy of CO2 was determined.•The results are discussed in the broader context of carbon dioxide capture from the flue gas of fossil fuel fired power stations.

Mesoporous MCM-41 type silica spheres having a sub-micrometer size were synthesized following an adaptation of Stöber's method. This parent material was then functionalized with 3-aminopropyl triethoxysilane and with 3-propanonitrile triethoxysilane, followed by oxidation of the cyano-group to the corresponding carboxy-group. After proper characterization, the samples were loaded with cisplatin and subjected to in vitro tests in order to obtain the corresponding drug release profile. The carboxy-functionalized MCM-41 sample was found to show a release kinetics that should facilitate controlled drug delivery over a significantly larger time period (about 140 h) than both, unmodified MCM-41 and amino-functionalized MCM-41 samples.

Porous copolymers of divinylbenzene (DVB) and acrylic acid (AA) having DVB:AA ratios of 6:4, 8:2 and 9:1 were prepared following a distillation-precipitation method, using toluene as the porogenic agent. The materials thus obtained, which showed specific surface area in the range of 380–600 m2 g−1 and pore volume in the range of 0.14–0.18 cm3 g−1, were investigated as possible adsorbents for CO2 capture from the flue gas of coal-fired power stations. For that purpose, the isosteric heat of adsorption (and CO2 adsorption capacity) was analysed from N2 and CO2 adsorption equilibrium isotherms obtained over a temperature range. For CO2, q st resulted to be in the range of 27–31 kJ mol−1 (the highest value corresponding to the 6:4 sample), while for N2 a value of q st ≈ 12 kJ mol−1 was obtained. Equilibrium adsorption capacity for CO2 (at ambient temperature and pressure) showed the value of about 1.35 mmol g−1. These results are discussed in the broader context of corresponding literature data for CO2 capture using protonic zeolites.

The preceding comments by O. Cairon, Phys. Chem. Chem. Phys., 2012, 14, DOI: 10.1039/c2cp40963, question several aspects of our perspective article on the nature of cationic adsorption sites in alkaline zeolites (P. Nachtigall, M. R. Delgado, D. Nachtigallova and C. O. Arean, Phys. Chem. Chem. Phys., 2012, 14, 1552). Questioning spans from experimental details, through methodology and to the very concept of dual and multiple cation sites in zeolites. While acknowledging that questioning constitutes an excellent method to foster understanding, we hope that the answers given herein will help to clarify the relevant points under discussion.

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