Pore size titania

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Fig. 7. The effect of preparation on the pore size distribution (a), titanium dispersion (b), and the activity for epoxidation of cyclohexene (c) of titania—siUca containing 10 wt % titania and calcined in air at 673 K. Sample A, low-temperature aerogel Sample B, high-temperature aerogel Sample C, aerogel.

The thermal stability of mesoporous frameworks substantially increases with an increase in the wall thickness and pore size, which can be varied even for the same template by changing the processing conditions. Ozin et al.55 developed a way to prepare crystalline titania films with a 2D-hexagonal architecture by replacement of ethanol in the Pluronic-containing precursor solution with more hydrophobic butanol-1. The latter promotes phase separation at low surfactant-to-titania ratios, resulting in thicker pore walls, which are more compatible with the crystal growth during subsequent calcination. 

The pore size of porous titania can be up to 2000 A. Titania is used for the purification of proteins and as a support for bound enzymes. The purification of /1-lactoglobulin from cheese whey, of protease from pineapple, /5-lactamase, and amylase can be achieved with titania. The latter two purifications are impossible on alumina. Titania is also used as a support in peptide synthesis. The separation of plasmid DNA is shown in Figure 3.24. 

The smaller and more uniform the primary particles, and the weaker the agglomerates in the sol are, the smaller the pore size and the sharper its distribution in the membrane will be. The thickness of the layer L, increases linearly with the square root of the dipping time. The process is quantitatively described by Leenaars and Burggraaf (1985). The rate of membrane deposition increases with the slip concentration or with decreasing pore size of the support as shown below. This has been experimentally confirmed for alumina and titania (Leenaars and Burggraaf 198S, Uhlhom et al. 1989). 

Figure 2B. Typical examples of a pore size distribution for (a) y-alumina membranes desorption branch (b) anatase titania membranes desorption branch.

Characteristic microstructural properties of TiOj membranes produced in this way are given in Table 2.5. Mean pore diameters of 4-5 nm were obtained after heat treatment at T < 500°C. The pore size distribution was narrow in this case and the particle size in the membrane layer was about 5 nm. Anderson et al. (1988) discuss sol/gel chemistry and the formation of nonsupported titania membranes using the colloidal suspension synthesis of the type mentioned above. The particle size in the colloidal dispersion increased with the H/Ti ratio from 80 nm (H /Ti = 0.4, minimum gelling volume) to 140 nm (H /Ti " — 1.0). The membranes, thus prepared, had microstructural characteristics similar to those reported in Table 2.5 and are composed mainly of 20 nm anatase particles. Considerable problems were encountered in membrane synthesis with the polymeric gel route. Anderson et al. (1988) report that clear polymeric sols without precipitates could be produced using initial water concentrations up to 16 mole per mole Ti. Transparent gels could be obtained only when the molar ratio of H2O to Ti is < 4. Gels with up to 12 wt.% T1O2 could be produced provided a low pH is used (H /Ti + < 0.025). 

Fabrication of titania nanotube arrays via anodic oxidation of titanium foil in fluoride based solutions was first reported in 2001 by Gong and co-workers [58]. Further studies focused on precise control and extension of the nanotube morphology [21], length and pore size [22], and wall thickness [3]. Electrolyte composition plays a critical role in determining the resultant nanotube array architecture and, potentially, its chemical composition. Electrolyte composition determines both the rate of nanotube array formation, as well as the rate at which the resultant oxide is dissolved. In most cases, a fluoride ion containing electrolyte is needed for nanotube array formation. In an effort to shift the band gap of the titania... 

Fig. 5.6 FESEM images of 10 V titania nanotube arrays anodized at (a) 5°C, and (b) SOX. The pore size is nearly 22 nm for all samples. In (a) the average wall thickness is 34 nm, and in (b) is 9 nm.

Figure 2. Pore size distributions in titania-sulfate aerogels before (a) and after calcination (b).

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. 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

Parameters of the porous structure of titania samples (pores volume Vs, specific surface area Ssp) were calculated using BET theory [34] from the adsorption isotherms of methanol. The average pore diameter (Dp) values were estimated from the differential curves of pore size distribution. 

---