Ti-silicalite

Role of the organic feed XRD powder patterns and FT-IR spectra confirmed that pure MFI-type Ti-silicalite (TS-1) was obtained [18-23], with a surface area of 530 m g". FT-IR spectra of adsorbed pyridine (Fig. 39.2) showed the presence only of weak Bronsted and Lewis sites [24,25], as confirmed by the complete evacuation from the surface at 373 K. 

An environmentally friendly synthesis of 1,2-methylenedioxybenzene (MDB) can be efficiently carried out in the gas phase, by feeding pyrocatechol (PYC) and formaldehyde acetals and using a catalyst containing weak acid sites and redox sites. The Ti-silicalite (TS-1) was identified as the most active and selective catalyst, indicating the role of well-dispersed octahedrally-coordinated Ti" ions in comparison with some model catalysts. 

It is instructive to compare the properties of metal peroxo and alkyl (or hydro) peroxo groups for the case of Ti because experimental structures of both types are known [117, 119-121] and Ti compounds are catalysts for such important processes as Sharpless epoxidation [22] and epoxidation over Ti-silicalites [122], where alkyl and hydro peroxo intermediates, respectively, are assumed to act as oxygen donors. Actually, the known Ti(t 2-02) complexes are not active in epoxidation. [121-124] However, there is evidence [123] that (TPP)Ti(02) (TPP = tetraphenylporphyrin) becomes active in epoxidation of cyclohexene when transformed to the cis-hydroxo(alkyl peroxo) complex (TPP)Ti(OH)(OOR) although the latter has never been isolated. 

Let us recall that by the sol-gel method one can obtain very efficiently very well-defined systems such as Ti silicalite, which can be considered as a single site system where titanium is tetracoordinated in a zeolitic matrix and undergoes epoxidation of propylene or hydroxylahon of benzene to phenol. Bear in mind that it took industry more than 20 years to realize such an industrial processes (Dow-BASF process) [1]). 

Titanium Alkoxides Silica-supported titanium(IV) alkoxides and Ti-silicalite are industrial epoxidation catalysts [53-56] and have been applied in deperoxidation reactions [57]. Computational and EXAFS data [53, 54] as well as spectroscopic investigations on the surface species [58] have indicated that the dominant active surface species is a four-coordinate trisUoxy complex [(=SiO)3TiOH] [59] whose coordination shell expands to six-coordinate during catalysis [60]. 

A number of heterogeneous systems have been developed for oxidation reactions using H2O2 as oxygen source . In 1981, Taramasso, Notari and collaborators at Enichem opened new perspectives in this field with the discovery of the Ti-silicalite (TS-1) ° , a new synthetic zeolite of the ZSM family. In the TS-1 zeolite, titanium atoms are located in vicariant positions in the place of Si atoms in the crystalline framework . The remarkable reactivity of TS-1 is likely ascribable to the site-isolation of tetrahedral Ti(IV) in a hydrophobic environment. TS-1 has proved to be an efficient catalyst for the epoxidation of unfunctionalized short-chain olefins, especially terminal ones (equation 28). In addition, polyunsaturated compounds are mainly converted into the mono epoxides (equation 29). 

Formic acid, methyl formate, and CO were detected when photoreduction was performed in Ti silicalite molecular sieve using methanol as electron donor.173 Mechanistic studies with labeled compounds indicated, however, that CO originates from secondary photolysis of formic acid, whereas methyl formate emerges mainly from the Tishchenko reaction of formaldehyde, the initial oxidation product of methanol. 

Notari summarizes the science and technology of catalysis by molecular sieves incorporating framework metal ions. The most important example in technology is Ti silicalite, a selective oxidation catalyst. Because the catalysts are crystalline materials, their structures are among the most well-known of any industrial catalysts, and catalytic sites such as Ti cations are identified. The work summarized in this chapter, almost all of it performed in just the preceding few years, gives an... 

Oxidation Phenol (H2O2) Catechol, hydroquinone Ti silicalite (TS1)... 

Reprinted from U. Romano, A. Esposito, F. Maspero, C. Neri, and M.G. Clerici, Selective Oxidations with Ti-Silicalite, Chim. Ind. (Milan), 1990,72, 610, with permission from La Chimica e rindustria. 

Silicalite-1 is the totally siliceous form of the zeolite MFl (ZSM-5), another silica polymorph. It belongs to the P2iln = Clh monoclinic space group (n. 14) with Z = 96. It transforms into an orthorhombic structure, belonging to the Pnma = Dlt space group (n. 62) between 350 and 363 K. Substituted silicalites such as ZSM-5 zeolite (see below) and Ti-silicalite adopt the orthorhombic structure even at room temperature, the transition temperature being strongly shifted to lower temperatures. 

The common means of introducing redox catalytic activity in zeolites is by the substitution of framework atoms such as Si, A1 or P with redox-active metal cations. This has been accomplished by two different methods (1) hydrothermal synthesis and (2) post-synthesis modification. Irrespective of the method of preparation, with the notable exception of titanium silicalites, these redox metals in the framework are susceptible to leaching due to the solvolysis of M-O bonds [77]. Even the Ti silicalites suffer from leaching under basic conditions [76a]. 

Figure 13. (a, b) Schematic representation of the oxidation pathways using redox molecular sieves (a) homolytic free radical autoxidation and (b) heterolytic oxygen transfer, (c) Oxidation of styrene to styrene oxide and transformation to 2-phenylacetaldehyde using a bifunctional Ti-silicalite catalyst. 

Redox silicalites with elements other than titanium have found catalytic applications in various organic oxidations. V-ZSM-11 is reported to catalyze alkane oxidation and is found to oxidize even primary C-H bonds [98]. Detailed studies indicate that V silicalites are better than Ti silicalites for alkane oxidation, as it is found that molecular oxygen can act as the oxidant in the former case [99]. V-ZSM-5 shows selectivity toward alcohol oxidation in allyl and methallyl alcohols... 

Both types of molecular sieve catalysts, i.e., those containing exchangeable metal cations and those with metal ions isomorphously substituted into the framework are difficult to tailor with respect to activity and selectivity, and with the exception of Ti silicalite [161] none of the catalysts is used in a commercial process. 

S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F. Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini, and G. J. Vlaic, /. Phys. Chem., 98, 4125 (1994). XAFS Study of Ti-Silicalite Structure of Framework Ti(lV) in the Presence and Absence of Reactive Molecules (H2O, NH3) and Comparison with Ultraviolet-Visible and IR Results. 

Table 3 compares our results on the oxidation of toluene over the Sn-silicalite samples. The Sn-samples are active in this reaction (39.4, 36.4 and 34.2 mol % H2O2 efficiency in 24 h for samples with Si/Sn ratios of 70). Both the hydroxylation of the aromatic nucleus to give cresols and the oxidation of the methyl substitutent to give benzyl alcohol and benzaldehyde take place simultaneously on the Sn-silicalites. Based on the product distribution, it can be seen that the rate of the oxidation of the methyl substituent is about 6 times faster than the rate of aromatic hydroxylation on all the samples. After 24 h, the concentration of benzaldehyde is the highest in the product. In this respect, the Sn-silicalite molecular sieves are more similar to the V-silicalites, VS-2 than the Ti-silicalites, TS-1 or TS-2 [16]. 

In the literature there has been much debate regarding the role of the lattice or extralattice Ti in Ti silicalite for a variety of oxidation reactions. In order to have a more precise idea of the role of the lattice or surface Ti and more specifically of the role of the coordination sphere of Ti, a series of monopodal and tripodal titanium surface complexes (i. e., =SiOTi(OR)3 and ( SiOIsTiOR) were derived by the reaction of the Ti alkyl (Structure 1) and hydride species with water, oxygen, methanol, and tert-butanol. The resulting complexes were then used in the epoxidation of 1-octene by tert-butyl hydroperoxide. Tripodal complexes, especially (=SiO)3Ti( Bu), were found to be significantly more active and more selective for the epoxidation of 1-octene than their monopodal counterparts [22]. 

As an example. Table 6.2 compares the performance of Ti-MCM-41 [9] with a mesoporous silica-grafted Ti and a microporous Ti silicalite (TS-1) for propene epoxidation with EBHP. With TS-1, the pore diameter is 5.5 A, which imposes restrictions and limits the size of the molecules to be oxidized the diffusion of the oxidant is inhibited, and hence the conversion attained is less than with the mesoporous materials. The difference between the mesoporous catalyst and TS-1 is even greater when the epoxidation of propene is carried out with CHP. The affinity for propene is increased by conferring hydrophobic properties to the surface, by means of silylation. 

C. Lamberti, S. Bordiga, A. Zecchina, G. Artioli, G. Marra, G. Spano, Ti location in the MFl framework of Ti-silicalite-1 A neutron powder diffraction study, /. Am. Chem. Soc. 

G. Tozzola, M. A. Mantegazza, G. Ranghino, G. Petrini, S. Bordiga, G. Ricchiardi, C. Lamberti, R. Zullian, A. Zecchina, On the structure of the active site of Ti-silicalite in reactions with hydrogen peroxide A vibrational and computational study, /. Catal. 179 (1998) 64. 

Titanium silicalite has been reeognized as an efficient redox catalyst in a number of industrial processes. Enichem has a process in which Ti silicalite catalyzed the conversion of eyelohexanone by ammonia and hydrogen peroxide to cyclohexnone oxime . The meehanism appears to involve the Ti 0x0 species. A number of proeedures are available from literature to make the Ti silicalite catalyst and it will be easy to incorporate eleetron mediators such as Ru or Pd into the silicalite matrix. The performance of such composite... 

We prepared composite catalysts containing Ru in Ti silicalite (molar composition Si02 0.025TiO2 0.001 Ru). The catalyst was suspended in 3 mL of a mixture (1 4 weight ratio) of cyclohexanol and cyclohexane in a 100 mL steel autoclave and stirred imder oxygen at 150 for 24 hours. Cyclohexane was converted to cyclohexanone to an extent of 15 - 20%. The reaction was highly selective for cyclohexanone. Products other than unreacted cyclohexane and cyclohexanol were not observed. More than the calculated amount of oxygen was consumed. The experimental set up was similar to the one described by Clerici... 

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