Titanium silicate molecular sieves active sites

After the submission of this contribution, a remarkable review authored by Ratnasamy, Srinivas and Knozinger has appeared on the investigation, by means of both experimental and theoretical methods, of the active sites and reactive intermediates in titanium silicate molecular sieves [126]. 

Active Sites and Reactive Intermediates in Titanium Silicate Molecular Sieves... 

The turnover numbers (TON) of phenol are 62.4 and 105.4 with a H2O2 efficiency of 28.9 wt.% and 48.9 wt.% for the Zr-Sil-2 samples A and B, respectively. A significant difference in the product distribution between these two runs is also observed. The catechol (CAT) to hydroquinone (HQ) ratios are 0.9 and 1.7 for Zr-Sil-2 (A) and (B) samples, respectively. A CAT/HQ ratio of 0.9 to 1.3 has been reported for titanium and vanadium silicate molecular sieves (TS-2 and VS-2) [13]. The samples synthesized using Zr(acac)4 show a nearly two fold activity in the reaction probably due to the smaller particle size. These results indicate that in the case of Zr-Sil-2 samples synthesized using ZrCU, the Zr " ions are well dispersed within the channels of the MEL structure while in the samples synthesized using Zr(acac)4, the hydroxylation occurs at the external surface as well, where a part of Zr species may be located. For small submicron crystals (<1 pm), external surface sites could be a significant fraction of the total surface area. If the external surface sites are catalytically either the same or more active than the intracrystalline active sites, then the shape selectivity of a zeolite could... 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

The insertion of Ti in the zeolite framework was accompanied by a significant decrease in A1 content (Table 1). However, there was no stoichiometric process between A1 removal and Ti insertion. Moreover, it was found that the treatment of Ig of an aluminum containing beta zeolite with a 75 ml of 3 x 10 M oxalic acid solution decreased the Si/Al ratio from its original value of 30 to 85 due to A1 extraction. Attempts to incorporate Ti into other zeolites like ZSM-12 and mordenite were not successful. Interestingly, the extraction of A1 from these zeolite structures was also unsuccessful with oxalic acid solutions with comparable concentrations. However, preliminary data show that siliceous mesoporous molecular sieves (MCM-41 and HMS) treated similarly with ammonium titanyl oxalate solutions exhibit good epoxidation activity. It is inferred that the presence of framework cations that can be extracted by oxalate species and/or the presence of defect sites in the parent zeolite is a requisite for the subsequent incorporation of titanium. 

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