23 research outputs found
Theoretical maximal storage of hydrogen in zeolitic frameworks
No full textPhysisorption 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 H-2 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
Interaction of H2 with Alkali-Metal-Exchanged Zeolites: a Quantum Mechanical Study
No full textRecently, zeolites have been proposed as media for hydrogen by means of molecular adsorption. The interaction of a dihydrogen molecule on alkali metal ions in high-silica zeolites has been theoretically studied in cluster and periodic models at the B3-LYP level of theory. The cluster models have been obtained by embedding Li+, Na+, and K+ in aluminosilicate rings of different sizes (Sin-1AlOnH2n, 4 <= n <= 6). The structure of Li, Na, and K-exchanged chabazite with Si/Al = 11:1 has been adopted as the periodic model. In both cases, the hydrogen molecule interacts side-on with the cations, forming T-shaped complexes. The results have been compared with similar data obtained for bare cations and previous experimental studies. Furthermore, the necessity of employing correlated methods for a proper description of the interaction has been verified at the MP2 level
A complete spectroscopic and adsorptive study of a thermally robust pyrazolato-based PCP containing cubic octanuclear Ni(II) clusters
No full textThe pyrazolato-based porous coordination polymer (PCP) [Ni8(μ4-OH)4(μ4-OH2)2(μ4-PBP)6]
(NiPBP, H2PBP = 4,4’-bis(1H-pyrazol-4-yl)biphenyl) is built upon octametallic hydroxo clusters
reciprocally connected by organic spacers. This material is a very promising candidate for gas
adsorption applications, owing to its remarkable thermal stability (up to 400 °C in air) and its high
void volume (70%) [1]. NiPBP was selected as a proof-of-concept material to demonstrate how an
optimized set of laboratory and synchrotron techniques can concur to create a comprehensive and
coherent picture, relating (average and local) structural features to adsorptive properties. To this
aim, the response of NiPBP toward different gases, retrieved by gas adsorption measurements (N2
at 77 K, in the low pressure region; H2 at 77 K, in the high pressure region), was explained in terms
of local-level details. In particular the electronic structure of the material was deeply investigated
combining laboratory UV-Vis spectroscopy, X-ray absorption near edge structure (XANES) and
resonant inelastic X-ray scattering (RIXS) [2]
Stability and Reactivity of Grafted Cr(CO)(3) Species on MOF Linkers: A Computational Study
No full textThe possibility to modulate Cr(CO)(3) properties by grafting it onto metal-organic framework (MOF) linkers of different natures has been investigated using density functional methods. MOF linkers were modeled using clusters constituted by benzene rings doubly substituted in the para position. The effect of the electron-donor or electron-acceptor nature of benzene substituents on the stability of the (eta(6)-arene)Cr(CO)(3) adduct and on the shift of the CO bands has been considered. Different electron-donor (-NH2, -CH3, -OH, -COONa) and electron-acceptor (-F, -COOH, -CN, -CF3) substituents have been used and the results compared with the bare benzene. C6H4(COOZnOH)(2) and C6H4(Zn4O13C6H5)(2) clusters have also been adopted as models of the MOF-5 benzene rings, The possibility of modulating the stability and the reactivity of Cr(CO)(3) species by grafting them to MOFs with different organic linkers was verified. In particular, this study indicates that electron-acceptor (e.g., C6H4(COOH)(2)) substituted MOF linkers facilitate the substitution of CO by incoming molecules, whereas the use of electron-donor ones (e.g., C6H4(OH)(2)) would improve the stability of the Cr(CO)(3) adduct and the ring acidity. Furthermore, an almost linear dependence of the Cr(CO)(3) binding energies on the calculated structural and vibrational features of the tricarbonyl was found, suggesting that the stability of the Cr(CO)(3) adduct can be inferred experimentally from vibrational and diffraction data. In the end, on the basis of the results obtained, it was possible to successfully explain the experimental shift of the CO IR stretching features of grafted Cr(CO)(3) on the UiO-66, CPO-27-Ni, and MOF-5 aromatic linkers and on the benzene rings of poly(ethylstyrene-co-divinylbenzene). The sign of the Delta(v) over tilde (CO) shift with respect to C6H6Cr(CO)(3) has been found to be strongly dependent on higher/lower electron density on the ring
FTIR spectroscopy and thermodynamics of hydrogen adsorbed in a cross-linked polymer
No full textThe adsorption of H-2 in a cross-linked poly(styrene-co-divinylbenzene) (St-DVB) microporous polymer ( BET surface area 920 m(2) g(-1)) is studied by volumetric and gravimetric methods, FTIR spectroscopy at variable temperature ( 300-14 K) and ab initio calculations. At 77 K the polymer reversibly stores up to 1.3 mass% H-2 at a pressure of 1 bar and 1.8 mass% at 10 bar. The adsorption process involves the speci. c interaction of H-2 with the structural phenyl rings through weak dispersive forces. The interacting molecules become IR active and give rise to vibrational and rotational-vibrational manifestations which are affected by the temperature, the contact time and the H-2 equilibrium pressure. The spectra of the H-2/St-DVB system reported here represent the first IR evidence of the adsorption of hydrogen on unsaturated molecules. The adsorption enthalpy is evaluated by the VTIR ( variable temperature IR spectroscopy) method ( C. Otero Arean et al., Phys. Chem. Chem. Phys., 2007, DOI: 10.1039/b615535a) and compared with the results of ab initio calculations for the H-2/benzene interaction and with literature data
Structure-activity relationships of simple molecules adsorbed on MOF materials: in situ experiments vs. theory
No full textMetallorganic Frameworks (MOFs, also known as “Coordination Polymers”) are crystalline nanoporous materials comprised of metal containing clusters connected three-dimensionally by poly-functional organic ligands. The ligands act as spacers, creating an open porous three-dimensional structure, with very high pore volume and surface area.1,2 This hybrid architecture opens the possibility to design and synthesize a great variety of new porous materials, which are in principle able to display novel functionalities that are potentially exploitable for a number of applications in catalysis, ion-exchange, non linear optics, as sensors, in gas separation and/or storage.3-5
The key step for most of the foreseen applications is the solvent removal treatment, that allows make accessible the large pore volume to the desired molecules. For several cases (among all MOF-5, HKUST-1, CPO-27-Ni, UiO-66), the combined use of XRD, EXAFS, XANES, UV-Vis, IR and Raman techniques, supported by ab initio calculations, allowed us to obtain a complete understanding of the structural, electronic and vibrational properties of MOF materials. The adoption of in situ experimental set-ups guarantees the possibility to follow the evolution of such properties along the solvent removal process and the successive interaction with increasing amount of desired adsorbate (H2, N2, CO, NO, CO2, etc...).6-2
Hydrogen adsorption by delta and epsilon crystalline phases of syndiotactic polystirene aerogels
No full textThe H2 uptake from s-PS samples exhibiting different crystalline phases and different
morphologies has been studied by gravimetric measurements at 77 K in the hydrogen pressure range from
0 up to 1.7 MPa and compared with molecular simulations relative to s-PS crystals. Gravimetric experiments
show that the molecular hydrogen sorption is strongly dependent on the sample morphology and is maximum
for low-density polymer aerogels. However, independently of the morphology, theH2 uptake is minimum for
the dense β and γ crystalline phases, intermediate for the channel-shaped nanoporous ε phase, and maximum
for the cavity-shaped nanoporous δ phase. In particular, although the two nanoporous crystalline phases
present essentially the same density (0.98 g/cm3), the hydrogen uptake from the δ phase is roughly double with
respect to the uptake from the ε phase, both for powders and for aerogels. Infrared measurements and
molecular simulations well agree with these quantitative sorption data and clearly indicate that, for both low
and high pressure, the hydrogen molecules are preferentially adsorbed into the nanoporous crystalline
phases. In particular, molecular simulations indicate that the maximum average hydrogen uptake is of nearly
3 molecules per cavity of the δ phase and of nearly 3.5 molecules per unit height of the channels of the ε phase
Design of high surface area poly(ionic liquid)s to convert carbon dioxide into ethylene carbonate
Get PDFA series of porous poly(ionic liquid)s (PILs) were synthesized using an innovative method which involves the synthesis of non-ionic co-polymers such as divinylbenzene and vinylimidazole, followed by an alkylation step to introduce the ionic liquid functionality in the polymeric matrix. This synthetic strategy allowed us to obtain tunable imidazolium type PILs having simultaneously high surface area and exposed ionic moieties. A set of PILs was obtained by changing systematically the alkyl chains, the anions and the cross-linking degree. This approach allowed us to elucidate the effect of each synthetic variable on the catalytic performances of PILs towards the carbon dioxide cycloaddition reaction under very mild conditions (room temperature and low pressure). Finally, in situ FTIR spectroscopy allowed us to establish a relationship between the structure of PILs and their catalytic properties. © The Royal Society of Chemistry 2015
The role of surfaces in hydrogen storage
No full textThis review deals with the main materials employed so far for hydrogen storage and it is specifically focused on the role of surface phenomena in their storage performance. Surface properties are relevant in all classes of materials: when dihydrogen is stored in the form of hydrides, the structure, texture and reactivity of the surfaces have large influence on the kinetics of charge/discharge cycles. In the storage of molecular hydrogen, surface-molecule interactions are responsible for the storage properties of the materials
Spectroscopic and adsorptive studies of a thermally robust pyrazolato-based PCP
Get PDFThe pyrazolato-based PCP [Ni8(OH)4(OH2)2(PBP)6] (NiPBP, H2PBP = 4,4’-bis(1H-pyrazol-4-yl)biphenyl), whose 3-D architecture is built upon octametallic hydroxo clusters reciprocally connected by the organic spaces, is a very promising candidate for gas adsorption applications, owing to its remarkable thermal stability (up to 400 °C in air) and its high void volume (70%). As such, NiPBP was selected as a proof-of-concept material to demonstrate how an optimized set of solid state techniques can concur to create a comprehensive and coherent picture, relating (average and local) structural features to adsorptive properties. To this aim, the response of NiPBP toward different gases, retrieved by gas adsorption measurements (N2 at 77 K, in the low pressure region; H2 at 77 K, in the high pressure region), was explained in terms of local-level details, as emerged by coupling electronic, X-ray (absorption and emission), and variable temperature IR spectroscopy
