Ettore Fois

On the unusual stability of Maya blue paint: molecular dynamics simulations

Abstract Classical molecular dynamics simulations of the indigo-containing clay palygorskite (Maya blue) have been carried out at different temperatures in order to get insight into the high stability of this paint. After several tens of picoseconds the molecules set into stable sites along the channels of the clay, where they remain for relatively long times even at high temperature (573 K). The hydrogen bonds between indigo carbonyl and structural water are seemingly the most important guest–host interactions, however, there is some evidence that they are not essential in keeping the organic dye trapped in the stable sites. Therefore, van der Waals interactions are expected to be crucial in stabilizing Maya blue, allowing to fit the bulky indigo molecules into the palygorskite channels. Furthermore, we successfully tested the possibility that a direct interaction between indigo and clay octahedral cations (not mediated by structural water) could also play an important role in trapping the indigo molecules.

Molecular dynamics studies on zeolites. II: A simple model for silicates applied to anhydrous natrolite

Abstract A simple model potential in two forms (harmonic and anharmonic) is proposed to be used in molecular dynamics simulations of silicate frameworks. This model is applied to the calculation of structural and vibrational properties of anhydrous natrolite, and the results are compared to experimental data. Despite their crudeness, the proposed models succeed in representing satisfactorily the main features of the silicate framework structure and dynamics.

High-pressure behavior of bikitaite: An integrated theoretical and experimental approach

Abstract Pressure-induced structural modifications in the zeolite bikitaite are studied by means of in situ synchrotron X-ray powder diffraction and ab initio molecular dynamics. The experimental cell parameters were refined up to 9 GPa, at which pressure we found reductions of 4.5, 4.5, 6.3, and 15% in a, b, c, and V, respectively. Minor variations were observed for the cell angles. Complete X-ray amorphization is not achieved in the investigated P range, moreover the P-induced effects on the bikitaite structure are completely reversible. Because it was possible to extract only the cell parameters from the powder patterns, the atomic coordinates at 5.7 and 9.0 GPa were obtained by means of Car-Parrinello simulations using the unit-cell parameters experimentally determined at these pressures. Analysis of the computational results for increasing pressures showed that the volume contraction is essentially due to rotations of the tetrahedra; the 8-ring channels become more circular; the pyroxene chain becomes more corrugated in the b-c plane; and the mean Li-O bond distances and coordination polyhedral volumes decrease with increasing pressure without significant distortion of the internal angles. The peculiar aspect of the bikitaite structure, i.e., the presence in the channels of a “floating” one-dimensional water chain, is only partially maintained at high pressure; the compression brings framework O atoms close enough to water hydrogen atoms to allow the formation of host-guest hydrogen bonds, without, however, destroying the one-dimensional chain.

TS-1 from First Principles†

First principles studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO(2) are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi(95)O(192)] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Tis per unit cell [Ti(2)Si(94)O(192)] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO(4) tetrahedra, is discussed. By using cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Tis occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites.

The “template” effect of the extra-framework content on zeolite compression: The case of yugawaralite

Abstract The microscopic behavior of the Ca-zeolite yugawaralite has been studied by ab initio molecular dynamics simulations adopting experimental cell parameters obtained at pressures up to ~9 GPa. Pressure-induced volume contraction occurs via rotations of quasi-rigid TO4 tetrahedra that reduce the size of the channels in which the extra-framework species are located. Such rotations are governed by deformation of the coordination polyhedron of Ca, which is made up of water and framework O atoms. Contraction of the Ca-H2O distances is favored at moderate pressure; at higher pressure the shortening of Ca-framework O atom distances becomes prevalent. The hydrogen bond network plays a fundamental role in the overall response to pressure. Our results indicate that the high-P-induced deformation of the framework structure is strictly correlated to the extra-framework species that act as “templates” in the compression process.

Host–Guest Interactions and Orientation of Dyes in the One-Dimensional Channels of Zeolite L

A combined experimental and modeling study of methylacridine (MeAcr(+)) dye-zeolite L composites unravels the microscopic origin of their functional properties. The anisotropic orientation of the cationic dye inside the ZL channel is unambiguously determined and understood. The most stable orientation of MeAcr(+), which features both its long and short molecular axes nearly perpendicular to the channel axis, is mainly determined by dye-ZL electrostatic interactions but also depends on the cosolvent water. In ZL, MeAcr(+) is not hydrogen bonded to water or ZL framework oxygens and is hydrophobically solvated by water molecules. These findings further support the hypothesis that the cosolvent can importantly influence properties of dye-zeolite composites. Of relevance for a deeper comprehension of the physical chemistry of these hybrids is the observation that trivial energy transfer processes (self-absorption) are often playing a significant role in the optical properties of the composites.

On the collective properties of water molecules in one-dimensional zeolitic channels

The collective properties of water molecules hosted in the one-dimensional non-crossing channels of three zeolites are analyzed and discussed. In particular, we present new ab initio molecular dynamics simulations on the behaviour of water in the synthetic zeolite Na-ABW. New IR and thermogravimetric data are also presented and the comparison of these data with those obtained in the study of strictly related zeolites (bikitaite and Li-ABW) shows that moderate differences in the chemical composition and/or in the topology of the zeolite framework lead to quite different dynamical properties of the guest molecules. According to the balance between host–guest and guest–guest interactions, the behaviour of water in these channels ranges from that of a one-dimensional solid wire of hydrogen bonded H2Os to that of a chain of independent water molecules hydrogen bonded to the framework oxygens.

Molecular dynamics studies on zeolites. Part 5.—Discussion of the structural changes of silicalite

Molecular dynamics simulations of silicalite, with and without sorbed methane molecules, at different temperatures, have been performed using a simple model potential. The results for sorption energetics and diffusion of methane were satisfactory, but the structural properties of the silicalite framework and their dependence on temperature and sorbate loading were reproduced only qualitatively. Suggestions for improvement of model potentials for zeolites are presented.

Dipolar host/guest interactions and geometrical confinement at the basis of the stability of one-dimensional ice in zeolite bikitaite

One-dimensional chains of hydrogen-bonded water molecules are hosted in the noncrossing channels of bikitaite (Li2[Al2Si4O12]⋅2H2O), a rare natural lithium zeolite [C. S. Hurlbut, Jr, Am. Mineral. 42, 792 (1957); Stahl et al., Zeolites 9, 303 (1989)]. We use first-principles methods to study such a mineral with different (Si, Al) ordering in the zeolite framework. The one-dimensional polymer of nondiffusing water molecules, one chain per channel, is arranged in such a way as to develop a permanent polarization parallel to the zeolite channel. We found that the one-dimensional and entropically unfavored water chain structure is stabilized by dipolar host/guest interactions between the dipolar water chain and the (reversed sign) dipolar framework. A ditrigonal distortion of the hexagonal arrangement of tetrahedra in the aluminosilicate bikitaite structure is probably at the origin of the net polarization of the framework, and is induced by the small-sized Li+ cations.

Dehydration dynamics of bikitaite: Part II. Ab initio molecular dynamics study

Abstract High-temperature behavior and the process of thermal dehydration in the natural zeolite bikitaite have been studied by ab initio molecular dynamics simulations, and favorably compared with the X-ray powder diffraction data presented in Part I of this work (Ferro et al. 2003). The microscopic dynamical behavior of the extraframework species (water molecules and Li cations) has been characterized as a function of temperature. Two regimes have been detected, and the transition is characterized by the breaking of the one-dimensional water chain typical of bikitaite at room temperature. The elementary steps for the diffusion of water inside the bikitaite channels have been studied by means of a rare-events-sampling technique (Bluemoon Ensemble). The activation free-energy for a site-to-site water jump has been calculated and a mechanism for the dehydration process is proposed.