Interaction of Ti-Zeolites with Water. A Periodic ab Initio Study

Abstract

The interaction of water molecules with Ti sites in Ti-containing zeolites has been studied by means of the ab initio periodic Hartree-Fock program CRYSTAL. The zeolitic part of the system has been represented by a perfect crystalline chabazite framework. The Ti content and distribution within the model structure have been designed so as to keep the maximum symmetry. Water physisorption has been accomplished by adding one (two) water molecule to each symmetry equivalent Ti-site; the geometry has been fully optimized; the calculated adsorption energy is -34.4 ( -43.6) kJ mol -1 . The stability and electronic structure of the hydrated sites are shown to be largely dependent on the framework flexibility. The hydrolysis of the Ti-O-Si bridge to yield Ti-OH and Si-OH has also been studied. This reaction is endothermic within the present model (∆E ) 35.3 kJ mol -1 ). The activation energy was estimated in about 84 kJ mol -1 .

Bathochromic Effects in Electronic Excitation Spectra of Hydrated Ti Zeolites: A Theoretical Characterization

“…Sodalite (its all-silica form has the unit cell formula [Si 12 O 24 ]) is characterized by a maximum pore opening formed by 6-MRs and one crystallographic T site. [9] The model titanium-sodalite (Ti-SOD) has the chemical formula [TiSi 11 O 24 ], and its cubic cell parameter (a = 8.9751 ) is taken from a low Zn-content sodalite [27] . The chemical formula per unit cell of triclinic SSZ-35 [9,22] is (Si 16 O 32 ).…”

“…Smaller titanium zeolites have been studied by ab initio methods by taking into account periodicity. [11,12,[14][15][16] However, to the best of our knowledge, very few modelling studies devoted to electronic transition properties have been published so far. [17][18][19] Recently a mixed approach was presented, where the titanium zeolite structure was optimized with a periodic DFT approach.…”

“…However the hydrolysis-and-inversion systems are less stable than the system in which water is bonded at the titanium site as a ligand. [11,16] Relative energies and significant geometric parameters of the six optimized [TiSi 17 O 36 ]·H 2 O systems are reported in Table 1, while graphical representations can be found in the Supporting Information ( Figure S2). …”

“…It seems that, when titanium is already penta-coordinated from the 12-MR side, another water ligand from a different cage can approach titanium at coordination distance only if it is solvated by at least one other water molecule, indicating that solvent cooperative effects play a relevant role in titanium-shell expansion. [11,14] For titanium to become hexa-coordinated, a sixth ligand should be activated, by hydrogen bonding or solvation, for example, since the Lewis acidity of titanium is lowered when it is penta-coordinated. [11,14] Even though the details of such behaviour may seem to be peculiar to the offretite channel's system, the lowering of the Lewis acid character of zeolitic titanium with increasing coordination appears to be more general.…”

“…[11,14] For titanium to become hexa-coordinated, a sixth ligand should be activated, by hydrogen bonding or solvation, for example, since the Lewis acidity of titanium is lowered when it is penta-coordinated. [11,14] Even though the details of such behaviour may seem to be peculiar to the offretite channel's system, the lowering of the Lewis acid character of zeolitic titanium with increasing coordination appears to be more general. Indeed, hexa-coordinated titanium centres were detected in highly hydrophilic samples of titanium-beta zeolite, [6] while in hydrophobic samples only penta-coordinated titanium-sites were found, indicating that water accessibility affects titanium coordination.…”