Gabriele Ricchiardi

Role of Exposed Metal Sites in Hydrogen Storage in MOFs

The role of exposed metal sites in increasing the H2 storage performances in metal-organic frameworks (MOFs) has been investigated by means of IR spectrometry. Three MOFs have been considered: MOF-5, with unexposed metal sites, and HKUST-1 and CPO-27-Ni, with exposed Cu(2+) and Ni(2+), respectively. The onset temperature of spectroscopic features associated with adsorbed H2 correlates with the adsorption enthalpy obtained by the VTIR method and with the shift experienced by the H-H stretching frequency. This relationship can be ascribed to the different nature and accessibility of the metal sites. On the basis of a pure energetic evaluation, it was observed that the best performance was shown by CPO-27-Ni that exhibits also an initial adsorption enthalpy of -13.5 kJ mol(-1), the highest yet observed for a MOF. Unfortunately, upon comparison of the hydrogen amounts stored at high pressure, the hydrogen capacities in these conditions are mostly dependent on the surface area and total pore volume of the material. This means that if control of MOF surface area can benefit the total stored amounts, only the presence of a great number of strong adsorption sites can make the (P, T) storage conditions more economically favorable. These observations lead to the prediction that efficient H2 storage by physisorption can be obtained by increasing the surface density of strong adsorption sites.

Electronic and vibrational properties of a MOF-5 metal–organic framework: ZnO quantum dot behaviour

UV-Vis DRS and photoluminescence (PL) spectroscopy, combined with excitation selective Raman spectroscopy, allow us to understand the main optical and vibrational properties of a metal-organic MOF-5 framework. A O(2-)Zn(2+)[rightward arrow] O(-)Zn(+) ligand to metal charge transfer transition (LMCT) at 350 nm, testifies that the Zn(4)O(13) cluster behaves as a ZnO quantum dot (QD). The organic part acts as a photon antenna able to efficiently transfer the energy to the inorganic ZnO-like QD part, where an intense emission at 525 nm occurs.

Cu(I)-ZSM-5 zeolites prepared by reaction of H-ZSM-5 with gaseous CuCl: Spectroscopic characterization and reactivity towards carbon monoxide and nitric oxide☆

Abstract The strongly acidic Bronsted sites of H-ZSM-5 can be quantitatively exchanged with monovalent copper ions by reaction with CuCl at 573 K, as evidenced by the disappearance of the characteristic IR bands of bridged OH groups. Characterization of the Cu-ZSM-5 samples prepared following this route by means of UV-Vis-NIR (diffuse reflectance) and photoluminescence spectroscopies confirms that the protons are substituted by Cu+ ions, which are isolated and located in a few, structurally well defined sites easily accessible to ligand molecules. These Cu+ ions are highly coordinatively unsaturated and can form Cu+ (CO)n (n=1, 2 or 3) carbonylic and Cu+ (NO)n (n=1 or 2) nitrosylic complexes upon dosage of carbon monoxide or nitric oxide at 77 K. The Cu+ (NO)2 dinytrosylic complexes are unstable at room temperature and evolve with formation of nitrous oxide, NO2− and oxidized CuIINO species. This behaviour strongly supports the hypothesis that a redox mechanism is operating in the nitric oxide decomposition reaction leading to nitrogen and oxygen.

Structural characterization of Ti centres in Ti-silicalite and reaction mechanisms in cyclohexanone ammoximation

Abstract The main results obtained by means of many physical methods (IR, Raman, UV-Vis and XAFS spectroscopies) concerning the structure of the Ti centre in titanium silicalite and the reaction intermediates in the ammoximation of cyclohexanone are concisely reviewed. The Ti is in tetrahedral coordination in vacuo and expands its coordination sphere upon interaction with adsorbates. In the presence of H2O and H2O/H2O2 solutions one of the SiOTi bridges is hydrolyzed with formation of (SiO)3L2TiOH (LH2O) and (SiO)3L2TiOOH species, respectively. When NH3 is dosed on (SiO)3L2TiOOH structures (SiO)3L2TiOO−NH4+ species are formed. These species are thought to play an important role in the ammoximation reaction. The geometries of the peroxidic species (open or bridged) are discussed also on the basis of ab initio calculations.

On the role of acidity in catalytic oxidation

Abstract The role of the catalyst surface acid-base properties on the heterogeneously-catalyzed oxidation reaction mechanisms is discussed. Acid-base properties depend on the covalent/ionic character of the metal-oxygen bonds and are involved in some steps of the oxidation reactions, such as the activation of the CH hydrocarbon bonds, the step associated with the evolution of alkoxide species and the desorption/overoxidation of the partial oxidation products. Thus they participate with the cation redox properties in determining the selective/unselective catalyst behavior.

IR studies of CO and NO adsorbed on well characterized oxide single microcrystals

Abstract A systematic investigation of the surface morphology and of the vibrational properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, α-Cr 2 O 3 , α-Al 2 O 3 , MgAl 2 O 4 and other spinels, TiO 2 , ZrO 2 and other oxides of a similar structure) with regular crystalline habit and exposing thermodynamically stable and neutral faces, is presented with the aim to elucidate the spectroscopic manifestations of CO and NO adsorbed on well defined crystallographic positions. In particular the structure of CO and NO adsorbed on the cationic sites of extended faces of these model solids is presented and discussed with the aim of elucidating the nature of the Me x+ ··· CO/NO bond (Me x+ = non transition metal ion or transition metal ion). When non transition metal ions are involved, the molecule-cation interaction is predominantly electrostatic. This leads to an increase of the CO stretching frequency, which is roughly proportional to the polarizing field. On the contrary, when transition metal ions are involved, beside the predominant electrostatic interactions, a small contribution to the bond stability comes also from d-π overlap forces, which, although not very important from the energetic point of view, greatly influence the static and dynamic dipoles localized on the adsorbed molecules. Consequently, the strength of the dipole-dipole interactions occurring in the ordered adlayers of CO and NO adsorbed on transition and non transition metal oxide surfaces are resulted remarkably different. On these well defined surfaces, the effects influencing the half-width (FWHM) of the CO and NO stretching peaks have also been considered. It has been calculated that the FWHM is a very sensitive parameter of the surface perfection. In a few cases (ZnO, α-Cr 2 O 3 , etc.) FWHM values comprised in the 1.5–3.7 range have been obtained, which are indicative of a single-crystal quality of the exposed faces. These spectroscopic results were compared with those obtained with quantum calculations. Finally the activity towards CO and NO of perfect, low index faces and of more defective situations (like those associated with edges, steps and corners) are compared, in order to have a better insight on the role of surface defectivity in catalytic reactions.

IR study of ethene and propene oligomerization on H-ZSM-5: hydrogen-bonded precursor formation, initiation and propagation mechanisms and structure of the entrapped oligomers

The oligomerization reaction of ethene and propene on H-ZSM-5 has been studied by fast FTIR spectroscopy. Oligomerization proceeds through: (i) formation of short-lived hydrogen-bonded precursors by interaction of the alkene with the internal acidic Bronsted sites, (ii) a protonation step and (iii) a chain-growth step. The relative strength of the hydrogen bonds in the ethene–OH and propene–OH π-complexes (precursors) is estimated on the basis of the downward shift of both the ν(OH) and ν(CC) frequencies (–389 and –11 cm–1 for ethene and –539 and –19 cm –1 for propene). For both molecules, the protonation of the precursors is the rate-determining step of the oligomerization process. The chain-growth mechanism and the structure of the entrapped oligomers are discussed on the basis of computer graphic and molecular dynamics simulations. Mainly linear of low branched products are formed whose length and structure is essentially determined by the steric hindrance imposed by the zeolitic framework.

Formation of CuI—N2 adducts at 298 and 77 K in CuI-ZSM-5: an FTIR investigation

CuI-exchanged ZSM-5 contacted with N2 at room temperature or at 77 K exhibits a narrow IR band at 2295 cm–1. On the basis of 14N2/14N2 and 14N2/14N15N/15N2 isotopic substitution experiments this peak is assigned to the N—N stretching vibration of CuI—N2 end-on dinitrogen complexes formed on isolated CuI sites inside the zeolite channels. The problem of the location of the CuI ions and the role of surface heterogeneity in determining the FWHM (ca. 9 cm–1 at N2 coverage θ= 1) of the 2295 cm–1 band are discussed. Some hypotheses concerning the bonding structure are made by comparison with the spectra of the purely electrostatic M+–N2 and M+–CO adducts (where M+ is an alkali-metal ion) and of the CuI—CO complex formed on the same support.

Theoretical maximal storage of hydrogen in zeolitic frameworks

Physisorption and encapsulation of molecular hydrogen in tailored microporous materials are two of the options for hydrogen storage. Among these materials, zeolites have been widely investigated. In these materials, the attained storage capacities vary widely with structure and composition, leading to the expectation that materials with improved binding sites, together with lighter frameworks, may represent efficient storage materials. In this work, we address the problem of the determination of the maximum amount of molecular hydrogen which could, in principle, be stored in a given zeolitic framework, as limited by the size, structure and flexibility of its pore system. To this end, the progressive filling with H2 of 12 purely siliceous models of common zeolite frameworks has been simulated by means of classical molecular mechanics. By monitoring the variation of cell parameters upon progressive filling of the pores, conclusions are drawn regarding the maximum storage capacity of each framework and, more generally, on framework flexibility. The flexible non-pentasils RHO, FAU, KFI, LTA and CHA display the highest maximal capacities, ranging between 2.86-2.65 mass%, well below the targets set for automotive applications but still in an interesting range. The predicted maximal storage capacities correlate well with experimental results obtained at low temperature. The technique is easily extendable to any other microporous structure, and it can provide a method for the screening of hypothetical new materials for hydrogen storage applications.

The role of Al in the structure and reactivity of iron centers in Fe-ZSM-5-based catalysts: a statistically based infrared study

Abstract A statistical analysis of the infrared data obtained upon absorption of NO on a large number of Al-free Fe-silicalite and Fe-ZSM-5 samples highlighted the influence of Al on the formation and stabilization of extraframework iron species and shed more light on the superior activity of Fe-ZSM-5-based catalysts. It was concluded that Al favors the dispersion of extraframework iron species and that isolated Fe II species with one or two Al atoms in the immediate vicinity are the active sites in partial oxidation reactions. The structure of the α -oxygen species adsorbed on iron centers is also briefly discussed.