A Combination of the Monte Carlo Method and Molecular Mechanics Calculations:  A Novel Way To Study the Ti(IV) Distribution in Titanium Silicalite-1

Abstract

A combination of the Metropolis Monte Carlo method and molecular mechanics calculations is used to study the Ti(IV) distribution in titanium silicalite-1 (TS-1). Calculations are carried out in which the Ti atoms are placed at crystallographically different T-sites rather than at symmetry-related T-positions. In this way, also the effect of already incorporated Ti atoms on the accommodation of new Ti atoms can be studied. It is shown that the Ti atoms are distributed over all crystallographically different lattice positions rather than located at one preferred T-site. The distribution, however, is not random: T2 and T12 are clearly favored. Modeling of the Ti distribution at loadings above the experimentally determined maximum limit of 2/2 Ti atoms per unit cell seems questionable. The framework symmetry is related to both the location of the Ti atoms and the Ti loading. There are much less Ti atoms present as neighbors than can be expected from a random substitution, which might indicate that Loewenstein's rule is dependent not only on electrostatic effects but also on framework deformations.

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

“…It has also been hypothesized that Sn can form paired sites across six-membered rings in Beta topology, which may enhance the Lewis acidity compared with other framework topologies (13,39). Computational studies (42)(43)(44)(45)(46)(47)(48)(49), as well as experimental investigations using neutron diffraction (50-52) and synchrotron radiation X-ray diffraction (XRD) (53, 54) of Ti-MFI, have not shown consistent results on Ti siting. Deka et al (49) proposed that these experimental discrepancies are due mainly to kinetic effects caused by differences in synthesis conditions between research groups.…”

Ti Location in the MFI Framework of Ti−Silicalite-1: A Neutron Powder Diffraction Study

“…If w > n the new distribution is accepted, otherwise the system returns to the previous Ti, Si distribution. Following this approach, Njo et al 33 have found T2 and T12 as preferential sites for Ti substitution, while T8 (suggested as preferential site from MD calculation in ref 31) is among the less populated sites. It should also be noted that the lattice expansion ∆a, ∆b, and ∆c computed by MD calculation 31 for Ti insertion in sites T2 and T12 (suggested as preferential sites in ref 33) are not compatible with the experimental values reported by the group of Perego.…”

“…Following this approach, Njo et al 33 have found T2 and T12 as preferential sites for Ti substitution, while T8 (suggested as preferential site from MD calculation in ref 31) is among the less populated sites. It should also be noted that the lattice expansion ∆a, ∆b, and ∆c computed by MD calculation 31 for Ti insertion in sites T2 and T12 (suggested as preferential sites in ref 33) are not compatible with the experimental values reported by the group of Perego. 11 Finally, the most recent results obtained by Sauer and coworkers 34 highlight how all previous computational results, performed by simulating the zeolite structure in a vacuum (i.e.…”

“…For these reasons the most interesting speculations concerning Ti distribution are to date based on computational chemistry results. [26][27][28][29][30][31][32][33][34] The first theoretical contribution to the debate (Jentys and Catlow 26 ) used defect energy minimization and quantum mechanical cluster computation to study the isomorphous Ti substitution in monoclinic MFI. The obtained distances are very close to those experimentally found by EXAFS experiments on well-manufactured TS-1 samples.…”

Oxidación selectiva de hidrocarburos sobre tamices moleculares de poro grande conteniendo titanio.

“…Cálculos realizados por Njo y col. 159 sobre la incorporación de Ti en la estructura MFI concluyen que el Ti se distribuye en todas las posiciones de red existiendo dos posiciones preferentes y que, además, la presencia de Ti en la red condiciona la incorporación de nuevos átomos de Ti, es decir, que la incorporación de Ti no se produce de forma aleatoria.…”