Formation of Active Sites in TS-1 by Hydrolysis and Inversion

Abstract

The interaction of water molecules with Ti sites in titanium silicalite has been studied using a hybrid quantum mechanical/molecular mechanical approach. We have examined the structure and stability of a number of structurally different sites, many of which are based on the hydrolysis and inversion of tetrahedral sites in the zeolitic framework. The hydration of all considered Ti centers is found to be exothermic, which is in good agreement with experiment and previous theoretical work. Under such conditions, the most stable configuration considered is the bis(aquo) tripodal model, indicating that hydrolysis and inversion of tetrahedral sites also helps to increase the reactivity of the Ti site toward water. Our results further show that the insertion of one water molecule increases the coordination of the Ti center from four to five in the tetrapodal, tripodal, bipodal, and titanyl configurations and from five to six in the 2MR structure. In the presence of two water molecules, however, only one of the molecules binds directly to the Ti center; thus, the coordination remains the same as the mono(aquo) complex. In the bis(aquo) complexes, there is also a strong hydrogen-bonding network between the hydrogen of the first water molecule and the oxygen of the second water molecule. We find that the local structural features of the resulting series of tetra- and tripodal Ti moieties are in good agreement with data from EXAFS studies.

Zeolite (In)Stability under Aqueous or Steaming Conditions

“…The study showed that while hydrolysis of the first TiOSi bond is energetically favorable, further hydrolysis steps are highly endothermic. [93] However, in the case of tin, hydrolysis of the first SnO(Si) bond was found to be mildly endothermic (20-40 kJ mol −1 with reaction barriers between 75 and 100 kJ mol −1 ) as reported by two recent computational works employing periodic DFT calculations. [94] Another experimental study focusing on partial hydrolysis of titano-and stanno-silicates was reported by Yakimov et al They followed the time evolution of the 119 Sn MAS NMR spectra of a dehydrated Sn-BEA sample exposed to gas-phase water (4 H 2 O per Sn) at ambient temperature and reported that penta-coordinated tin gradually transformed into the hexa-coordinated state at long contact times (from several hours up to a month, depending on the strength of the particular tin Lewis acid site).…”

“…The study showed that while hydrolysis of the first TiOSi bond is energetically favorable, further hydrolysis steps are highly endothermic. [ 93 ] However, in the case of tin, hydrolysis of the first SnO(Si) bond was found to be mildly endothermic (20–40 kJ mol −1 with reaction barriers between 75 and 100 kJ mol −1 ) as reported by two recent computational works employing periodic DFT calculations. [ 94 ] Another experimental study focusing on partial hydrolysis of titano‐ and stanno‐silicates was reported by Yakimov et al.…”

New trends in tailoring active sites in zeolite-based catalysts

“…Computational studies showed that the formation of the first Ti-OH group stabilized the metal ion in the framework, while further hydrolysis of Ti-O-Si bonds was energetically unfavourable. 28 There is theoretical and experimental evidence for the formation of 'open' sites also for Sn-and Zr-containing zeolites. 29…”

Advances in theory and their application within the field of zeolite chemistry

“…The most detailed study addressing these problems was reported by To et al, 440,441 who used QM/MM techniques to model Ti substitutional centers in silicalite. The most detailed study addressing these problems was reported by To et al, 440,441 who used QM/MM techniques to model Ti substitutional centers in silicalite.…”