Framework Ti species

A similar development in this direction is the synthesis of a mixed-phase material containing both micro- and mesopores (Ti-MMM-1) (223). This material was synthesized by the addition of organic templates for mesopores (cetyltrimethylammonium bromide, CTABr) and micropores (tetrapropylammo-nium bromide, TPABr) at staggered times and the variation of the temperature of a single reaction mixture. Ti-MMM-1 is more selective (for oxidation of cyclohexane and of n-octane) than either Ti-MCM-41 or TS-1. The powder X-ray diffraction pattern indicates that the material contains both MCM-41 and MFI structures. The mixed phase contains framework Ti species and more atomic order within its walls than Ti-doped MCM-41. 

The hydroxylation of phenol on TS-1 is normally operated in a slurry reactor, at temperatures close to 100°C, with total consumption of the oxidant. The selectivities on phenol and H2O2 are generally in the ranges 90-95% and 80-90%, respectively. The hydroquinone to catechol ratio is well in excess of the statistical value of 1 2, owing to lower steric requirements for / -hydro-xylation and the faster diffusion of the p-substituted product (Table 2). Yields and kinetics are strictly related to the content of lattice Ti. It should be emphasized that any extra-framework Ti species, present as impurities on TS-1, are the major source of unproductive side reactions, such as H2O2 decomposition and unselective radical chain oxidations. 

Characterization is a crucial step preliminary to any catalytic study, since the selectivity of the catalyst is strictly related to the position of Ti, in atomic dispersion, within the crystal lattice. Extra-framework Ti species, such as Ti02 particles and amorphous Ti-siUcates, indeed, promote H2O2 decomposition and radical chain oxidations. Normally, a combination of different techniques is necessary for reUable characterization, for example, UV-Vis, IR and Raman spectroscopies, XRD, EXAFS, XANES, TEM and SEM [8, 20]. Table 18.1 illustrates the main structural features of TS-1 and other Ti-zeoUtes relevant to this review. 

Several authors used diffuse reflectance UV-Visible spectroscopy to probe the local environment of Ti sites [136-138,140,141,143,147]. Sayari et al. [137,141,147] found that both Ti-HMS and Ti-MCM-41 exhibit an absorption band at 220 ran with no indication of a band at 330 ran characteristic of anatase [148]. There was however a weak shoulder at 270 nm, particularly for Ti-iich samples. Conna et al. [140] also found a band at 205-220 nm and a shoulder at 270 nm. The absence of anatase was also confirmed by the absence of the characteristic 140 cm Raman band [138,149]. The band at 220 nm was attributed to isolated framework Ti species in interaction with water molecules [150]. The 270 nm band was assigned to partially condensed hexacoordinated Ti species belonging to a silicon rich amorphous phase [155]. 

EPR Coordination of Ti qualitative estimation of framework and extra-framework Ti species [52,55]... 

Independent of the lattice symmetry, a linear dependence of the lattice parameters (determined by the least-squares fit to the interplanar spacing of selected reflections in the XRD pattern [45] or by the more accurate full-profile fitting analysis [46]) on the Ti content has been found (Fig. 5). The equation relating the unit cell volume to the Ti content (Table 4) is particularly usefiil for determining the real framework composition directly from XRD analysis by comparing this with the Ti content resulting from elemental analysis, the amount of possible extra-framework Ti species can be estimated [46]. 

Tuel et al. performed an EPR investigation of TS-1, after reduction of Ti(IV) to the paramagnetic Ti(III) [56], compared to a Ti(III)-exchanged H-ZSM-5. The authors concluded that Ti exists in tetrahedral coordination in TS-1, completely different from that in the exchanged sample, typical of Ti(III) in distorted octahedral coordination. The same approach has been proposed as a qualitative tool for discriminating between framework and extra-framework Ti species [52]. 

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

In an attempt to quantify the relationship between the TiOOH groups and the yield of propene oxide from the extinction coefficients of the latter s 1409-and 1493-cm-1 bands, it was determined that 0.6 mol of the epoxide formed per mole of framework Ti center in the molecular sieve. That is, at least 60% of all framework Ti (80% of the surface-exposed Ti) is converted to TiOOH upon reaction with H202. The consumption of the TiOOH species during the oxygen insertion into propene was also independently confirmed by the loss in intensity of its LMCT band at 360 nm when the catalyst was brought in contact with propene at room temperature (Fig. 50). 

A difference in the relationship between UCV and degree of substitution x has recently been reported in connection with the use of tetramethyl orthosilicate (TMOS) as the silicon source instead of TEOS. The Ti content can be increased up to a value of 0.05 without evidence for extra-framework titanium species in the UV-visible spectra. However the UCV increases do not follow the relationship of Eq. (5) and the maximum value obtained, 5390 A3 at x = 0.05, is the... 

As a totally different post-synthesis method that is firmly based on the structural charaderistics of MWW, reversible strudural conversion between 3D MWW silicate and its corresponding 2D lamellar precursor MWW (P) has been developed to construd more active Ti species within the framework [24, 70], Figure 4.7 illustrates the strategy of this post-synthesis method of reversible strudural conversion. First, highly siliceous MWW is prepared from hydrothermally synthesized MWW boro-silicate by the combination of caldnation and acid treatment. Second, the MWW silicalite is treated with an aqueous solution of HM or PI and a Ti source. A reversible structure conversion from MWW into the corresponding lamellar precursor occurs as a result of Si-O-Si bond hydrolysis catalyzed by OH, which is supplied by basic amine molecules. This is accompanied by the intercalation of the amine molecules. 

---