C. Busco

Calorimetric and IR spectroscopic study of the interaction of NH3 with variously prepared defective silicalites ☆: Comparison with ab initio computational data

Abstract Nature, population and acidic properties of the hydroxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicalites have been characterised by the adsorption of NH 3 , monitored through the combined use of microcalorimetry and IR spectroscopy. The experimental data (enthalpy of adsorption, stretching frequency of the O–H bonds perturbed by the interaction with NH 3 ) have been compared with ab initio computational results (binding energy, red shift of the stretching frequency) and were found to be in good agreement. The presence of rings of mutually interacting silanols has been postulated (on the basis of both experimental and computational results) in the case of chemically very pure, defective silicalites. The dimension of the rings depends on the preparation conditions. A perfect silicalite sample, taken as hydrophobic reference material, was found to be virtually unreactive towards NH 3 , in that it interacts very scarcely with ammonia, showing a heat of adsorption much lower than the heat of liquefaction of NH 3 itself.

Coordination chemistry of Ca sites at the surface of nanosized hydroxyapatite: interaction with H2O and CO

The affinity towards water of a selection of well-defined, nanostructured hydroxyapatite (HA) samples was investigated by H2O vapour adsorption microcalorimetry and infrared (IR) spectroscopy. A large hydrophilicity of all investigated materials was confirmed. The surface features of hydrated HA were investigated on the as-synthesized samples pre-treated in mild conditions at T=303 K, whereas dehydrated HA features were characterized on samples activated at T=573 K. The relatively large hydrophilicity of the hydrated surface (−ΔadsH∼100–50 kJ mol−1) was due to the interaction of water with the highly polarized H2O molecules strongly coordinated to the surface Ca2+ cations. At the dehydrated surface, exposing coordinatively unsaturated (cus) Ca2+ cations, H2O was still molecularly adsorbed but more strongly (−ΔadsH∼120–90 kJ mol−1). The use of CO adsorption to quantify the Lewis acidic strength of HA surface sites revealed only a moderate strength of cus Ca2+ cations, as confirmed by both microcalorimetric and IR spectroscopic measurements and ab initio calculations. This result implies that the large HA/H2O interaction energy is due to the interplay between cus Ca2+ sites and nearby hydrophilic PO4 groups, not revealed by the CO probe. The lower density of cus Ca2+ cations at the 573 K activated HA surface with respect to the pristine one did not affect the whole hydrophilicity of the surface, as the polarizing effect of Ca sites is so strong to extend up to the fourth hydrated layer, as confirmed by both high-coverage microcalorimetric and IR spectroscopic data. No specific effects due to the investigated specimen preparation method and/or different morphology were observed.

Carbon monoxide adsorption on alkali and proton-exchanged chabazite: an ab-initio periodic study using the CRYSTAL code

Ab initio periodic calculations based on local Gaussian basis sets as coded in the CRYSTAL program have been performed to investigate the structure, the binding energy and the vibrational features of carbon monoxide adsorbed on H+, Li+, Na+ and K+-exchanged chabazite (Si/Al = 11/1, i.e. one Al atom per unit cell). The hybrid B3LYP functional has been adopted for all calculations with a polarized double-zeta quality basis set. The B3LYP binding energies (BSSE corrected) are 16.0, 24.6, 20.4 and 5.1 kJ/mol for H+, Li+, Na+ and K+-exchanged chabazite, respectively. Corresponding CO hypsochromic stretching frequency shifts are 47, 68, 43 and 33 cm−1, respectively. Comparison with the case of CO interacting with bare alkali cations and the available experimental data on a variety of zeolites has also been addressed.

Entrapping Molecules in Zeolites Nanocavities: A Thermodynamic and Ab-Initio Study

Adsorption enthalpies of Ar, N2, CO, H2O, CH3CN and NH3 on H-BEA and H-MFI zeolites and on Silicalite, have been measured calorimetrically at 303K in order to assess the energetic features of dispersive forces interactions (confinement effects), H-bonding interactions with surface silanols and specific interactions with Lewis and Brønsted acidic sites. The adsorption of the molecular probes with model clusters mimicking surface silanols, Lewis and Bronsted sites has been simulated at ab-initio level. The combined use of the two different approaches allowed to discriminate among the different processes contributing to the measured (-ΔadsH). Whereas CO and N2 single out contributions from Lewis and Br{\o}nsted acidic sites, Ar is only sensitive to confinement effects. For H2O, CH3CN and NH3 the adsorption on Brønsted sites is competitive with the adsorption on Lewis sites. The energy of interaction of H2O with all considered zeolites is surprisingly higher than expected on the basis of -ΔadsH vs PA correlation.

Surface Structure, Hydration and Cationic Sites of Nanohydroxyapatite

In this work, we studied the surface/water interaction properties of a pure calcium hydroxyapatite (HA) and their modifications as a consequence of the partial Ca2+/Mg2+ (MHA) substitution by means of IR spectroscopy and microcalorimetry of adsorbed water. IR data indicated that water molecules in direct contact with the surface of HA are coordinated to surface cations and experience H-bond significantly stronger than in liquid water. The heats of adsorption associated to such interactions are very high, being twice-triple the heat of liquefaction of water. Interestingly, water experiences H-bond higher than in its bulk liquid state also in the second layer. Finally the entering in the material of Mg2+ ions was shown to significantly affect the affinity of the material toward water and the properties of its hydration layers.

Structural incorporation of carbon and nitrogen into B-SSZ-13 : a spectroscopic and computational studies

Abstract As prepared B-SSZ-13 exhibits [B(OSi) 4 ] units in T d -like geometry, upon template burning, the break of a B-O-Si bond results in [B(OSi) 3 ] units in D 3h -like geometry, testified by the appearance of the IR fingerprint at 1390 cm –1 and by the evolution of the NEXAFS spectrum. Interaction with CH 3 OH at room temperature results in a rich reactivity with both [B(OSi) 3 ] units and adjacent SiOH species. A gentle thermal treatment at 373 K in NH 3 atmosphere results in the formation of -NH 2 species bonded directly to boron or to silicon, stable at ambient atmosphere. This finding is of relevance because it represents the insertion of basic species inside a zeolitic framework, and thus the achievement of a microporous molecular sieve acting as a Bronsted base.