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 TiO2 are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi95O192] 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 Ti's per unit cell [Ti2Si94O192] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO4 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 Ti's 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

Water in acid boralites: Hydration effects on framework B sites

Properties and behavior of protonated boron-containing zeolites at different hydration degree have been investigated by means of periodic DFT approaches. Geometry optimization and room-temperature Car-Parrinello molecular dynamics results, in line with experimental findings, indicate that the BO3-bound silanolic acid site typical of dry boralites should convert to a solvated H3O+ hydrogen bonded to tetrahedral BO4 at moderate water content. By increasing the water loading, the tetrahedral structure of the B site is stabilized and the physicochemical properties of the water molecules solvating the acid proton gradually approach the liquid-phase ones. A relevant role of structural and vibrational properties of the zeolite framework in the water-induced trigonal-to-tetrahedral transition at the B site is highlighted by simulation results

Dynamical disorder in hydroxo- and nitrite-sodalitecages investigated by first principles simulations

First principles computer simulations for two synthetic sodium sodalites, nitrite-sodalite and hydroxo-sodalite dihydrate show that long-range electrostatic host-guest interactions affect the motion of the hosted anions. These results are in agreement with the literature experimental data. They provide clearer information than the experimental data and are able to explain at the microscopic level the dynamical disorder experimentally observed. Dynamical disorder is present in many zeolites, both natural and synthetic. Our results indicate that the vibrational motion of the framework directly influences the dynamical behaviour of the hosted species

Structure and dynamics of a Bronsted acid site in a zeolite: An ab initio study of hydrogen sodalite

We present the results of ab initio molecular dynamics simulations on the structural and dynamical properties of the Bronsted acid site in a zeolitic cage. The protonated sodalite H[AlSi11O24] is the case studied, and the sodium sodalite is the reference unprotonated zeolite. The comparison between the geometrical parameters calculated for these two sodalites shows that the formation of an O-H bond gives rise to a significant local distortion of the framework geometry. Moreover, an equilibrium between partially ionic and covalent structures in the T-O bonds of the acid site is deduced from our calculations. We have found that the OH bond oscillates in and out of the plane of the 6-T-ring. The analysis of the simulated vibrational spectra has allowed us to assign the bands relative to the O-H in-plane and out-of-plane bending modes and confirms that these bands overlap with the framework modes

Electronic spectra of Ti(IV) in zeolites: an ab initio approach

Charge transfer in Ti–zeolites: Model titanium–zeolites containing a single Ti(IV) tetrahedral species were built and their electronic excitation spectra were calculated (see picture) via time-dependent density functional theory (TDDFT). A multiple-band profile was obtained in each case, thus highlighting that the features observed in the UV/Vis spectra are not a direct evidence of chemically different Ti(IV) sites

Influence of silanols condensation on surface properties of micelle-templated silicas: A modelling study

The dehydration of model mesoporous silicas has been studied via molecular dynamics simulations. By progressively dehydrating the system, initially characterized by an even distribution of surface silanols, patches with hydrophobic or hydrophilic character are formed on the pores surface. It is found that the local concentration of silanols is strictly correlated to the local structure of pores surface, namely hydrophobic and hydrophilic patches are located in high- and low-curvature surface regions, respectively, in line with experimental evidences. Simulations suggest that the microscopic origin of both the uneven Si-OH distribution and the pores surface deformation should be related to the dehydroxylation process, which is governed by interactions among the silica matrix