Crystalline structure collapse

Most of the microporous and mesoporous compounds require the use of structure-directing molecules under hydro(solvo)thermal conditions [14, 15, 171, 172]. A serious handicap is the application of high-temperature calcination to develop their porosity. It usually results in inferior textural and acidic properties, and even full structural collapse occurs in the case of open frameworks, (proto) zeolites containing small-crystalline domains, and mesostructures. These materials can show very interesting properties if their structure could be fully maintained. A principal question is, is there any alternative to calcination. There is a manifested interest to find alternatives to calcination to show the potential of new structures. 

Figure 1 A generalized diagram of the structure of the cetostearyl alcohol gel found in topical and vaginal creams. The bilayers are formed principally of cetostearyl alcohol. The hydrophilic poly(oxyethylene) chains attached to the 5-carbon sorbitan rings in Polysorbate 60 retard water drainage from the interlamellar space and keep the lamellae from collapsing into a dense crystalline structure.

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

Solid-state cellulose can also be noncrystalline, sometimes called amorphous. Intermediate situations are also likely to be important but not well characterized. One example, nematic ordered cellulose has been described [230]. In most treatments that produce amorphous cellulose, the whole fiber is severely degraded. For example, decrystallization can be effected by ball milling, which leaves the cellulose as a fine dust. In this case, some crystalline structure can be recreated by placing the sample in a humid environment. Another approach uses phosphoric acid, which can dissolve the cellulose. Precipitation by dilution with water results in a material with very little crystallinity. There is some chance that the chain may adopt a different shape (a collapsed, sixfold helix) after phosphoric acid treatment. This was concluded because the cellulose stains blue with iodine (see Figure 5.12), similar to the sixfold amylose helix in the starch-iodine complex. 

The thennal and hydrothermal stability of a sample with Si/Al = 26 was investigated in our laboratory by heating batches for 3 h in dry air, or in pure water vapor in the temperature range 550 to 850 °C. In dry air, the extent of dealumination increases with the treatment temperature but the crystallinity and the pore structure are preserved. However, under hydrothermal conditions, extensive dealumination takes place even at 550 °C, and the structure collapses above 650 °C. Another investigation of the thermal and hydrothermal stability of ion-exchanged Al-MCM-41 (Si/Al = 39) was carried out by Ryoo et al. [127] using dry or water vapor saturated O2 for 2 h at different temperatures. Under both sets of conditions, the stability was found to depend on the nature of the counter cations in the following order Y Ca > Na = as-prepared Al-MCM-41 > pure silica MCM-41. 

Thin alumina membranes (Anodise ) with two different nominal pore diameters (20 nm and 200 nm) were obtained commercially (Whatman International Ltd., Maidstone, Kent, UK). These two types of membranes, designated subsequently as A20 and A200 respectively, had a thickness of 50 pm and an overall diameter of either 25 of 47 mm. X-ray diffraction analysis showed that the alumina in the as received membranes was almost amorphous. Subsequent thermal treatment up to 950 °C produced a crystalline structure (y-alumina), without any significant collapse in the porous structure. Here we describe, in general, the characterisation of the untreated membranes. 

Crystalline NASICONs were developed in order to realize high ionic conductivities. Unfortunately NASICON glasses, due to reasons mentioned earlier (structural collapse), do not exhibit high ionic conductivities. But the study of a.c. conductivities of the NASICON glasses have provided some interesting insights, particularly with regard to the validity of the different forms of power laws, as noted in chapter 7. 

According to N2 adsorption, the microporous character of the modified zeolites was largely retained and these samples did not possess any appreciable mesopore volume (Table 1). This finding was in agreement with their unaffected crystallinity. By contrast, the values of surface area and pore volume for the sample with a dealumination level of 64% were severely affected, probably due to the collapse of the crystalline structure and to the blockage of the zeolite pore system to a very large extent [15]. As mentioned above, the structure collapse was also revealed by XRD analysis. On the other hand, it is seen from Table 1 that as the framework Si/Al ratio increased the unit cell size of AHFS-treated samples significantly contracted due to the smaller size of silicon atoms [16]. 

A structure similar to that of LaY zeolites has been suggested for CeY zeolites (3,7). However, structural x-ray studies of CeX zeolites have shown (9) that the presence of trivalent and especially tetravalent- cerium results in a considerable distortion of the 6-membered ring. Such a distortion could explain the lower hydrothermal stability and stronger dealumination of CeY zeolites vs LaY zeolites. Indeed, the loss in crystallinity and surface area observed for steamed CeY zeolites as compared to corresponding LaY zeolites (Table I) is indicative of significant structural collapse. Furthermore, the paramagnetism of trivalent cerium ions and the local framework distortions, caused primarily by tetravalent cerium ions, will result in a deterioration of spectral resolution. 

The phase transformations occurring during sintering of the catalysts are indicated in Fig. 1. Table III shows the relationship between crystalline phase transformations in the standard catalyst, the corresponding pore structure collapse, and the loss in activity upon sintering. Data on the catalysts prepared from silica sols are given in Table IV. Electron micrographs of the sols used for the preparation of catalysts 1, 2, and 3 are shown in Fig. 2. 

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