Silica-titania aerogels

Synthesis of Monolithic Titania-Silica Aerogel for PCO Reactions... 

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Fig. 3. Photocatalytic oxidation test of titania-silica aerogels, (a) TCE (air 400 seem, TCE 113 ppm), (b) isopropanol (air 200 seem, isopropanol 268 ppm).

Titania-silica aerogels possess very high surface areas (600 to 1000 m2/g) and large pore volumes (1 to 4 cm3/g), and thereby have attracted considerable interest for photocatalysis. A number of studies have shown that titania-silica intimate mixtures exhibit enhanced UV photocatalytic activity compared with pure titania [180-183],... 

Malinowska, B., Walendziewski, J., Robert, D., Weber, J.V., Stolarski, M. 2003b. The study of photocatalytic activities of titania and titania-silica aerogels. Appl Catal B Environ 46 441-451. 

R. Hutter, D. C. M. Dutoit, T. Mallat, M. Schneider, A. Baiker, Novel mesoporous titania-silica aerogels highly active for the selective epoxidation of cyclic olefins, Chem. Commun. (1995) 163. 

R. Hutter, T. Mallet, D. Dutoit, A. Baiker, Titania-silica aerogels with superior catalytic performance in olefin epoxidation compared to large pore Ti-molecular sieves. Top. in Catal. 3 (1996) 421. 

Epoxidation of cycloalkenones over amorphous titania-silica aerogels... 

Most of the earlier studies described the oxidation of simple (electron-rich) cycloalkenes, such as cyclohexene euid cyclododecene. Here we report the catalytic behaviour of titania-silica aerogels in the oxidation of cycloalkenones. The model reactions are the epoxidation of a- and P-isophorone, depicted in scheme 1. 

Preliminary experiments revealed that the selectivity of titania-silica aerogel in the epoxidation of P-isophorone was moderate. The selectivity related to the olefin converted was below 90 % at low temperature, and dropped rapidly at 80 °C or above. The most important side reactions were the formation of 3,5,5-trimethyl-2-cyclohexene-4-hydroxy-l-one (2) by ring opening of the epoxide (1), and the isomerization of P- to a-isophorone (3), as shown in Scheme 2. Epoxidation of 2 and 3, and the oxidation at the OH group of 2 to a dicarbonyl compound were slow and the amounts of these by-products were usually aroimd 1 % or less. 

The epoxidation of two cycloalkenones, a- and P-isophorone, with alkyl hydroperoxides demonstrates that active and selective titania-silica aerogels can be prepared by the sol-gel method combined with extraction of the solvent with supercritical COg at low temperature. The key factors for obtaining high activity in the epoxidation of bulky cyclic olefins are the high Ti-distribution in the silica matrix, the mesoporous structure and high surface area. 

Hutter and co-workers made mesoporous amorphous titania-silica aerogels by the sol-gel method, including supercritical carbon dioxide extraction in the workup.267 These were used with cumenehydroperoxide to epoxidize 1-hexene in 95% selectivity and cyclohexene in 100% selectivity. The authors state that their catalysts are superior to titanium-/ and titanium MCM 41 for the oxidation of bulky compounds such as limonene (4.55). 

Investigation of the adsorption and desorption of NH3 on a titania-silica aerogel. 

The application of PulseTA allows not only the amount of strongly adsorbed adsorbate to be determined, but also quantification of the desorption process and characterization of the adsorption strength and kind of active sites. Such an example is presented in Figure 24, depicting the adsorption of ammonia on titania-silica aerogels. 

Figure 24. A) Adsorption of NH3 on modified titania-silica aerogels at 50 °C followed by B) desorption of ammonia during heating with a rate of 10 K/min.

Aerogels of titania, titania-silica, and titania-vanadia were prepared with titanium butoxide, silicon ethoxide, and vanadium triisopropoxide as precursors. The titania and titania-silica aerogels were then used as supports for vanadia, introduced by the incipient wetness impregnation of vanadium triisopropoxide and the subsequent calcination at 773 K. 

Cao S, Yao N, Yeung KL (2008) Synthesis of freestanding silica and titania-silica aerogels with ordered and disordered mesopores. J Sol-Gel Sci and Tech 46 323-333... 

Figure 20.22. Left Change of the average size of pores (calculated from SAXS data) of mixed titania-silica aerogels upon first sintering stages. Right simplified model of the evolution of sono-aerogels upon first densification steps. Adapted with permission from [30],...

Titania is often combined with other oxides to form binary systems. Cao et al. [248,249] and Yoda et al. [250,251] have investigated the applicability of titania and titania-silica aerogels as potential oxidation catalysts for removal organics from air (VOCs) or water. The concept is to combine the gas adsorption properties of silica with the photocatalytic activity of titania in a mixed catalysis system. Bismuth-molybdenum-titanium xerogel and aerogels are promising catalysts for the oxidation of butadiene to ftiran, where reported activities and selectivities are comparable to state-of-the-art industrial catalysts [252]. 

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