Structure, dried gels

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

The resins can be divided into two groups having major structural differences gel and macroreticular . In the case of gel type resins if the beads are totally dry, then the polymeric matrix collapses and the polystyrene chains will be as close as atomic forces allow. Therefore, swelling ability of the reactants is a prerequisite for catalysis by gel resins. Gel resins are characterized by a divinyl benzene content that is generally below 12%. 

The Raman spectra (0-1400 cm l) shown in Fig re 6 illustrate the structural changes which accompany the consolidation of silica gels. The 1100°C sample is fully dense, whereas the 50 and 600°C samples have high surface areas (1050 and 890 m2/g), respectively. The important features of the Raman spectra attributable to siloxane bond formation are the broad band at about 430 cm 1 and the sharp bands at 490 and 608 cm 1(which in the literature have been ascribed to defects denoted as D1 and D2, respectively). The D2 band is absent in the dried gel. It appears at about 200°C and becomes very intense at intermediate temperatures, 600-800°C. Its relative intensity in the fully consolidated gel is low and comparable to that in conventional vitreous silica. By comparison the intensities of the 430 and 490 cm 1 bands are much more constant. Both bands are present at each temperature, and the relative intensity of the 430 cm 1 band increases only slightly with respect to D1 as the temperature is increased. Figure 7 shows that in addition to elevated temperatures the relative intensity of D2 also decreases upon exposure to water vapor. 

In both case (gel and dried gels) the advantage or a scheme based on the protcfilament is the high degree of resultant structural flexibility. Interf ila.ment contacts of the physical reticulation can occur by filament juxtaposition, by a fusion of all protofilaments of the two incoming filaments (say n protofilaments) to form an n-start helical fusion zone of reticulation or by an int erf ilament exchange of just one or iP.ore protofilaments. 

Silica sol is a stable dispersion of fine particles, while gel has a three-dimensional continuous structure. Si02 content in sol range between 10-50%, while that in dry sdica gels is between 96.5 to 99.6%. Density of dry gels is 2.22 g/cm3 and sols 2.20 to 2.30 g/cm. Weight loss in sols at 105°C is between... 

Decreasing the degree of crosslinking will increase the water uptake for a mass of dry gel, though compromises in the efficiency will result. The effect of crosslinks on the separation of vitamin B-12, a nonionic solute of molecular weight 1355, is shown in Fig. 4 [16]. As the crosslink density decreases, the polymer chain length between crosslinks increases, yielding a looser structure which vitamin B-12 can more easily penetrate. The behavior fits well with the prediction from Flory excluded volume theory [16] ... 

Recently, various techniques that produce highly oriented linear polyethylene with a ultra high modulus (hereafter, referred to as UHMPE) have been developed. In this section, we will examine the structure of the UHMPE that was prepared by highly drawing a dried gel [69]. Even if bulk polyethylene is uniaxially highly drawn by a normal method at a temperature between the Tg and Tm, the phase structure is essentially similar to the undrawn sample. That is, it involves three phases of the crystalline and two noncrystalline phases, although the mass fraction and detailed content of each phase are somewhat different. However, UHMPE samples may have a particular phase structure. 

Fourier transform infra-red techniques were performed on a block of hydrated gel, similar in form to that shown in Figure 2. The resulting spectra are shown in Figure 9. Spectrum B is after the water background had been subtracted. A comparison of this spectrum with that of A in Figure 8 indicates no real structural difference between the bulk gel and a dried gel film. 

The next stage in processing ceramic films from solutions is the development of structure. There are two types of structure development to consider (1) the structure of the primary aggregate or polymer, and (2) the structure that develops upon gelation. It is possible to control both t5T)es of structure development through the chemistry of the precursor solutions. The control of structure is essential to the final microstructure of the dried gel. 

It is known from Table 1 that gel time increases with the increase of water volume for hydrolysis. Transparent gel body can be obtained under 10 ml or 20 ml of water, whereas it would become opaque and show light white when water volume increases to 40 ml. The structure of gel body will become loosen and a little solution will bleed under the following drying, gel can not be formed when water volume rises up to 50 ml, white... 

From the viewpoint of materials processing, the pore and chemical structures are affected by three kinds of processing conditions one is concerned to the parameters of sol-gel based synthesis (hydrolysis and condensation) route[2-5], another is related to the aging and drying conditions of the wet gels[S-10], and the third is in close relationship to the calcination conditions of the dried gels[l 1],... 

Monolithic dry gels with a hierarchical network structure and an interconnected, multilevel pore system were obtained after drying (Fig. 2). Even within this small pH regime from pH = 6 to 2, the macroscopic gel morphology can to a large extent be controlled. In addition, the choice of glycol has a decisive influence on the architeture of the network. 

In 1990, Xu et al. first reported the transformation of a dry aluminosilicate gel to crystalline MFI by contact with vapors of water and volatile amines, which was named dry gel conversion (DGC).[99] Since then, this method has been extensively studied and a large number of microporous materials with new compositions and structures were prepared. Generally, DGC can be divided into vapor-phase transport (VPT) and steam-assisted conversion (SAC) according to the volatility of the SDAs. For volatile SDAs such as ethylenediamine, a mixture of water and SDA was poured into the bottom of the autoclave and then a dry gel, which does not contain any SDAs, was placed over the liquid surface. Water and SDAs were vaporized at elevated temperature (150 200 °C), reached the dry gel, and initiated the crystallization, which was called VPT. Less volatile SDAs such as tetrapropylammonium hydroxide were usually involved in the dry gel. Only water steam is supplied during the reaction, which was called SAC. 

Depending on the method used for membrane processing, colloidal or polymeric, two main structures of gel layers before drying and sintering can be described according to the literature ... 

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