Titanium silicates acidity

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

Acid catalysis by titanium silicate molecular sieves another area characterized by recent major progress. Whereas only two categories of acid-catalyzed reactions (the Beckmann rearrangement and MTBE synthesis) were included in the review by Notari in 1996 (33), the list has grown significantly since then. In view of the presence of weak Lewis acid sites on the surfaces of these catalysts, they can be used for reactions that require such weak acidity. 

Acidity, 27 284, 285 catalytic performance, 30 121 crystalline titanium silicates, 41 319-320 estimating, 37 166 heteropoly compounds, 41 139-150 ion exchange and, zeolites, 31 5-6 sulfate-supported metal oxides, 37 186-187 surface, monolayer dispersion, 37 34-35 tin-antimony oxide, 30 114-115, 125-1256 Acids, see also specific compounds adsorption of, on oxide surfaces, 25 243-245... 

Another important point is that, when prepared from pure raw materials, titanium silicates do not have an appreciable acidic character, as demonstrated by the high yields that can be obtained even in applications with acid-sensitive products like propylene oxide. In contrast, mixed oxides of titanium and silicon have been described as being strongly acidic (Tanabe et al., 1981), The reasons for the difference are not clear and deserve further attention. 

Octahedral coordination of Tiiv is also present in the titanium silicates ETS-4 and ETS-10. The structure of these materials is reported to be similar to that of zorite, and they can be described as microporous crystals with uniform pores similar in dimensions to classical small- and large-pore zeolites. In ETS-4 and ETS-10, there are two monovalent cations or one divalent cation for each Tilv ion (Kuznicki, 1989, 1990 Kuznicki et al., 1991a, 1991b, 1991c, 1993 Deeba et al., 1994). A recent report of the synthesis of ETS-10 with tetramethyl-ammonium chloride indicates a ratio of monovalent cations to Tilv of 1.6 (Valtchev et al., 1994). The acidic properties of these materials have not been reported. A material modified by the addition of Al3+ has been obtained, ETAS-10, which, after exchange with NH4 salts, exhibits acidic properties but these are due to the presence of Al3+ and not to the Tilv (Deeba et al., 1994). 

The high-temperature synthesis from strongly alkaline suspensions of salts of Tilv and Silv produces crystalline microporous materials in which Tiiv is present in octahedral coordination. These materials do not exhibit the catalytic properties typical of the other titanium silicates in which TiIV is in tetrahedral coordination (Kuznicki, 1989, 1990 Kuznicki et al., 1991a, 1991b, 1991c, 1993 Deeba et at., 1994). The acidic properties of these materials have been discussed (Section II.B). 

Glycols undergo oxidation with H202 and titanium silicates, but it is also possible that some of the reactions observed proceed as noncatalytic reactions once the primary oxidation products are formed. Ethylene glycol is oxidized to glycolic acid ... 

Investigation of mechanisms of reactions catalyzed by titanium silicates has been limited to oxidation reactions with H202 as the oxidant, as described below. As was previously discussed, elements different from titanium and silicon in the catalyst materials change their properties. Catalytic activity of doubly substituted materials such as Ti-beta, H[Al,Ti]-MFI and -MEL, and H[Fe,Ti]-MFI and -MEL is considered separately because the acidic properties associated with the added element affect the composition of the reaction products. 

Acidity in crystalline titanium silicates has been observed only when a titanium-containing zeolite interacts with H2C>2, but this is due to the formation of peroxo compounds, as discussed below. 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

Recently, Kuma et al. have reported that the M-rotor treated with aluminum sulfate showed better performance for dehumidification than the S-rotor treated sulfuric acid[3,4], Dinnage et al. have reported that the titanium silicate-impregnated ceramic sheet shows better dehumidification behavior than the aluminum silicate-impregnated sheet[S]. 

The synthesis of titanium silicates by basic and acidic hydrolysis in an organic-water media were carried out at 80-150 C in an autoclave with stirring (150-200 rpm) at autogenic pressure (2.5-4.5 at.). As silicon sources tetraethoxysilane and silicic acid have been used. As metal sources the metal-organic compounds alcoholates, stearates, acetylacetonates, acetates or mineral salts have been used. Hexamethylenetetramine, N,N-dimethylocteIamine, monoethanolamine were used as the structure-directing agents. 

There exist several indirect methods for the spectrophotometric determination of silicon. After extraction of molybdosilicic acid, the Mo has been determined with phenylfluorone [45], or 2-amino-4-chlorobenzenethiol (e = 2.0-10 ) [46]. When silicic acid is added to a solution which contains hydrofluorotitanic acid and H2O2, a yellow titanium peroxide complex is formed. Chloranilic acid has also been used for determination of silicon [47],... 

When TS-1 was used as the catalyst the selectivity to the triol product was very low and only became significant at extended reaction times. When the homogeneous stoichiometric oxidant meta-chloro-perbenzoic acid was used it is apparent that the selectivity to the triol was significant even at short reaction times. The low triol selectivity observed with TS-1 is considered to be due the hydrophobic nature of the titanium silicate framework. 

Employing a silicon micro reactor [channel dimensions = 500 or 1000 pm (width) x 250 pm (depth)], wall-coated with the acidic zeolite titanium silicate-1 (TS-1, Si Ti ratio = 17) (83) (3 pm), Gavrilidis and co-workers [52] demonstrated a facile method for the epoxidation of 1-pentene (84) (Scheme 6.23). Using H202 (85) (0.18 M, 30wt%) as the oxidant and 84 (0.90 M) in MeOH, the effect of reactant residence time on the formation of epoxypentane (86) was evaluated at room temperature. The authors observed increased productivity within the 500 pm reaction channel compared with the 1000 pm channel, a feature that is attributed to an increase in the surface-to-volume ratio and thus a higher effective catalyst loading. 

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