Silica-supported titania

High conversion levels of TCE were achieved in a bench scale flat plate fluidised bed photoreactor with a silica supported titania catalyst [204], A supported catalyst was used as the titania fluidisation characteristics were considered to be poor. It was found that the stoichiometric reaction 34 required the simultaneous presence of oxygen, water vapour and TCE. In order to maximise the titania threshold of 350 to 400 nm a single 4-W fluorescent UV source was used. At low concentrations of TCE the oxidation rate was independent of the water concentration whereas the rate of oxidation of the TCE was inhibited by water vapour when the concentrations of the pollutant had increased. Without any water vapour the photooxidation activity of the catalyst rapidly declines. In the presence of water, however, at high TCE concentrations there was a marked deactivation with the photocatalyst. 

One of the first fluidized bed photocatalytic reactors was presented by Dibble and Raupp (1992), who used silica-supported titania catalysts in order to degrade TCE with an AQE of 13%. Here, the UV sources in this bench-scale reactor were located externally to the reactor. Catalyst loss was prevented in this laboratory-scale reactor by introducing a second glass frit located at the reactor outlet. 

Shell subsequently developed a heterogeneous, silica-supported titania catalyst [11,12] which forms the basis of the commercial process for the epoxidation of propylene with ethylbenzene hydroperoxide. The co-product alcohol is dehydrated, in a separate step, to styrene. Ti(IV)Si02 was the first truly heterogeneous epoxidation catalyst useful for continuous operation in the liquid phase. 

J. Marugan, J. Agnado, W. Gernjak, S. Malato, Solar photocatalytic degradation of dichloroacetic acid with silica-supported titania at pilot-plant scale . Catalysis Today, 129, 59-68, (2007). 

The classical Ti-Si02 catalyst was initially prepared from TiCh and pyrogenic Si02 in 1969 [52]. Almost 30 years later, Maier et al. [53] synthesized calcinated xerogels via a sol-gel process with TEOS and various Ti-cyclopentadienyl complexes (entry 1, Table 5.3). In 1995, Baiker [54] demonstrated that sol-gel prepared titania-silica mixed aerogels showed better catalytic behavior in epoxidation of different bulky olefins than Ti zeolites [55] and silica supported titania described at that time [56]. The most common oxidant was cumene peroxide. The drying method, the titanium content and the calcination temperature were the most important parameters. Aerogels dried by semicontinuous extraction with supercritical CO2 at low temperature were found to be more efficient (entry 2, Table 5.3). In 2001 Baiker described the preparation of a series of titania-silica mixed... 

Researchers have discussed the need of crystalline titania in stabilizing active surface vanadia species for the SCR reaction (8, 18). Handy et al. (8) showed that titania-silica mixed gels can be prepared to give either "titania-like or "silica-like" behavior. Only on supports that contain crystalline titania domains can vanadia be dispersed to give high SCR activities. Even when silica-supported titania is used as a support, Jehng and Wachs (16) found an enhanced activity when bulk anatase particles are present. Tliese authors suggested that interactions between surface vanadia species and bulk titania... 

Styrene Monomer) process (215) has operated continuously using a silica-supported titania. The coproduct is styrene, which is positive for the process... 

The thermograms of Cu reduction in silica-, alumina-, titania- and zirconia-supported catalysts show only one pe the maximum of which is reported in Table 3. The amount of hydrogen consumed by the r uction corresponds, within experimental error, to the theoretical amount required for the reaction ... 

The method outUned above was initially investigated for the introduction of isolated Ti(IV) sites onto a sihca substrate for use in selective oxidation catalysis. Since the development of a silica-supported Ti(lV) epoxida-tion catalyst by Shell in the 1970s, titania-sihca materials have attracted considerable attention [135,136]. Many other titania-sihca materials have been studied in this context including, but not hmited to, TSl and TS2 (titanium-substituted molecular sieves), Ti-/i (titanium-substituted zeolite). 

Some non-silica sol-gel materials have also been developed to immobilize bioactive molecules for the construction of biosensors and to synthesize new catalysts for the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix to improve the immobilization of tyrosinase for detection of trace phenols. Titania is another kind of non-silica material easily obtained from the sol-gel process [34, 35], Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-mimetic silica support. In this biosilicification process precipitation was catalyzed by the R5 peptide, the repeat unit of the silaffin, which was identified from the diatom Cylindrotheca fusiformis. During the enzyme immobilization in biosilicification the reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5 peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] developed a biosil method for the encapsulation of plant and animal cells. 

Storsaeter S., Totdal B., Walmsley J.C., Tanem B.S., and Holmen A. 2005. Characterisation of alumina-, silica- and titania-supported cobalt Fischer-Tropsch catalysts. 7. Catal. 236 139-52. 

The authors postulated that on these materials complete site isolation is not mandatory in order to have active and selective titania-silica epoxidation catalysts . The 100% selectivity of the dinuclear, silica-supported... 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Whereas the effect of water on deactivation and on the overall activity of the FTS varies with the support, similar effects of water on the selectivity is reported for all catalysts, to a certain degree independent of the support, promoter and conditions. The effect can be summarized as an increase in C5 + selectivity, a decrease in methane selectivity, and in some instances a weak enhancement of the C02 selectivity is observed. Fig. 4 illustrates the effect on the C5 + and methane selectivity of adding water to cobalt catalysts supported on alumina, silica and titania, and both unpromoted and Re-promoted catalysts are shown. At the outset these selectivities are strong functions of the conversion, the C5 + selectivity increasing and the methane decreasing with increasing conversion, as illustrated by the trendlines in the figures. The points for methane are below, and C5 + -selectivity is above the line when water is added. Similar results were reported by many authors for alumina-supported catalysts,16-19 23 30 silica-supported catalysts,30 37 46-48 and titania-supported catalysts.19 30... 

Extended studies demonstrated that Pt [31-73] and, to a lesser degree, Pd [107,121,127,217-225], Rh [131,226], and Ir [227] are the most suitable active metals. With one exception (colloidal Pt [111,125,228]) all these metals have been used in supported form. The most used supports are alumina, carbon, silica, and titania [56,226], and more recently zeolites [106,229-230],... 

Common catalyst compositions include oxides of chromium or molybdenum, or cobalt and nickel metals, supported on silica, alumina, titania, zirconia, or activated carbon. 

Y. Y. Huang, B. Y. Zhao, and Y. C. Xie, A novel way to prepare silica supported sulfated titania,... 

Heterogeneous catalysts that exhibit good characteristics are silica-supported mixed Mo-V heteropoly acids and their Pd salts,1317 Pd on titania,1318 supported H3PMO12O40, and heteropoly acids and salts with Pd(OAc)21320 or PdCl2.1321... 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Several solid supports have been employed for the attachment of o-iodosobenzoic acid, including silica gel, titania and nylon [89]. Two polymer-supported o-iodoxybenzoic acid reagents have recently been reported. The first was obtained by attaching a carboxymethyloxy derivative of f-butyl o-iodo-benzoate to an aminopropylated silica gel and oxidation with oxone [90]. The second involved chloromethylated polystyrene which was coupled with methyl 5-hydroxy-2-iodobenzoate and eventually oxidized by Bu4NS05H/MeS03H [91]. Some of these polymeric reagents appear in Scheme 31. 

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