Hydrothermally stable, preparation

The preparation and properties of hydrothermally stable y-alumina membranes... 

The intention of this chapter is to provide an overview of the research performed on silica membranes and membrane materials during the project. Due to the fact that subjects like the preparation of tubular supports and a hydrothermally stable intermediate layer were given a higher priority, most of the results are preliminary and much more research will be needed to get a clear picture of the behaviour of (doped) silica membranes under SASRA conditions. Therefore the results provided should be seen as the result of preliminary work and might serve as a basis for future research. 

The excellent separation properties of silica membranes prepared at temperatures as high as 825°C enables their use for high temperature applications, such as the dehydrogenation of H2S (chapter 8). Unfortunately no hydrothermal stability of the prepared layers could be tested because the mesoporous intermediate layer was not hydrothermally stable, but an indication of the hydrothermal stability of the unsupported material could be obtained from the specific surface area and XRD measurements. These measurements did not show any structural change in the material during SASRA treatment, which is a very hopeful result for the operation of real, supported, membranes at high temperatures and high pressures. 

For the application of microporous silica membranes in steam-containing environments it is of major importance that the silica membranes will be tested on hydrothermally stable supports. Silica membranes should be prepared on the basis of the results of the specific surface area measurements described in chapter 6. Unsupported silica membrane material of which the specific surface area does not change under SASRA conditions is most promising. An example is silica fired at 825°C (chapter 5). The need of doping the silica with foreign ions or atoms is currently uncertain. 

Mixed micelles can be exploited as templates for preparation of nanoporous materials. Oligomeric alkyl-ethylene oxide surfactant (Polyoxyethylene (2) cetyl ether, B52), and ionic surfactant (cetyltrimethylammonium bromide, CTAB) form mixed micelles that self-assemble into well-ordered hexagonal and bimodal mesostructures. These systems can be used as templates to synthesize hydrothermally stable organized periodic mesoporous organosilicas (PMOs). The X-ray diffraction (XRD), TEM, BET, NMR and hydrothermal studies have been used to investigate the effects of B52 on the formation of various PMOs. The addition of B52 in the surfactant solution improves... 

Transition aluminas are good catalyst supports because they are inexpensive and have good physical properties. They are mechanically stable, stable at relatively high temperatures even under hydrothermal conditions, ie, in the presence of steam, and easily formed in processes such as extmsion into shapes that have good physical strength such as cylinders. Transition aluminas can be prepared with a wide range of surface areas, pore volumes, and pore size distributions. 

An example of such thermal dealumination is the formation of ultra-stable Y zeolites (USY zeolites). McDaniel and Maher (6) reported the preparation of two types of ultrastable Y zeolites (a) one type prepared by the hydrothermal... 

Theoretical calculations [43] based on first principles molecular dynamics discussed in Sect. 3.2.6 have suggested that Mg Al LDHs are most stable for n = 3 (i.e. x = 0.25) and indeed many minerals, including hydrotalcite itself, have this stoichiometry [4]. It has been reported that the synthesis of LDHs (with benzoate or terephthalate anions in the interlayers) from solutions containing Mg/Al = 2, leads to LDHs having the same composition when the synthesis is carried out at moderate temperatures but LDHs with Mg/Al = 3 (plus AlOOH) when the reaction is carried out under hydrothermal conditions [44]. It was proposed that the latter ratio represents the thermodynamically most favorable product. A similar observation has been reported [45] for solutions with Ni VPe = 2, where hydrothermal preparation led to segregation of an LDH with Ni VPe = 3 and Ni Fe 204. An attempt to synthesize a Co sAl LDH resulted in partial oxidation of the Co and formation of a Co o.yCo o.s LDH with complete migration of Al " from the layers to generate interlayer aluminum oxy-species [46]. 

Substitution of silicon by other atoms like Ti or Al was reported to improve the thermal and hydrothermal stability to some extent [6]. It was also reported that improved hydrothermal stability could be achieved by adjusting the gel pH several times during crystallization process [7], Post-synthesis silylation technique has also been reported to enhance the hydrothermal stability of mesoporous materials by increasing the hydrophobicity of the samples [8,9]. However, it is most desirable to develop a method for preparing hydrother-mally stable mesoporous material by direct synthesis route. 

Fig. 1 also shows the XRD patterns of the samples after hydrothermal treatment at 100°C for 4 days. It can be seen that samples prepared with additional TPA are quite stable to hydrothermal treatment. In contrast, the sample without TPA collapsed after such treatment. The nitrogen adsorption-desorption isotherms of calcined and hydrothermally treated MCM-... 

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