Hydrothermal Treatments

A suspension containing the recently precipitated LDH, or a suspension of the oxides in a solution of the anion to be intercalated, is submitted to hydrothermal treatment for varying periods of time. The hydrotalcite-type phase is formed, and the small crystallites existing in the suspension are converted into larger crystals, while amorphous precipitates crystallise. 

The hydrothermal treatment can be carried out above 100°C in an autoclave, aiming to accelerate the reaction, or below 100°C in this case it is known as ageing. 

Hydrothermal treatment enhances the crystallinity of the solid, which shows PXRD diagrams with sharper and more intense diffraction maxima. The crystallite size and the particle size increase, decreasing the specific surface area. The infrared (IR) spectra show sharper and more intense absorption bands, due to formation of better ordered, undistorted, solids [118]. 

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Because the chemical reaction is faster at higher temperature, aging can be accelerated by hydrothermal treatment, which increases the rate of the condensation reaction (8). 

Calcium Silicates. Cements aie hydiated at elevated tempeiatuies foi the commercial manufacture of concrete products. Using low pressure steam curing or hydrothermal treatment above 100°C at pressures above atmospheric, the products formed from calcium siUcates are often the same as the hydrates formed from their oxide constituents. Hence lime and siUca ate ftequendy used in various proportions with or without Portland cement in the manufacture of calcium siUcate hydrate products. Some of these compounds are Hsted in Table 6. 

Hydrothermal Treatment Process Processes which involve the use of hydrolysis or that use oxidation/reduction in an aqueous matrix at elevated temperatures. 

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

Zeolites with lower UCS are initially less active than the conventional rare earth exchanged zeolites (Figure 3-5). However, the lower UCS zeolites tend to retain a greater fraction of their activity under severe thermal and hydrothermal treatments, hence the name ultrastable Y. 

In zeolite synthesis (ref. 2) an aqueous mixture containing a silicon source, an aluminum source, an alkali source (usually NaOH) is autoclaved and subjected to hydrothermal treatment. Hydrated Na-ions are then filling the pore system in the as-synthesized zeolite. In the case of relatively high Si/Al zeolites an organic template is required which is usually a tetraalkylammonium compound, applied as the bromide or the hydroxide. 

This solution of lOO ml was transferred to a 250 ml Teflon container held in a stainless-steel vessel. After the vessel was tightly sealed, it was heated at 120 200°C for 5 h. After hydrothermal treatment, the TiOi particles were separated in a centrifuge at 10,000 rpm for 10 min and were then washed in distilled water. The particles were dried at 105 C for 12 h and were then calcined at 300 700 C for 3 h. 

The silicalite-alumina membrane was prepared after adding a solution containing the silicalite precursor (i e silica + template) to the above-mentioned porous tube (hereafter called support) and a specific hydrothermal treatment performed [8], under the chosen conditions no material is formed in the absence of the porous support. The tube is then calcined at 673 K for removing the template. 

The tube has been first characterised just after the hydrothermal treatment step (i.e. before the calcination) The N2 isotherm is typical of macroporous materials (Figure 3, curve 1) and the tube is gas-tight. 

Figure 41.6 illustrates the typical capabilities in tailoring Si-TUD-1 pore diameter using different hydrothermal treatment times (10). The pore diameter can be varied from 4 to 19 nm by increasing the hydrothermal heating time from 0 to 48 hours. The sample with 48 hours of hydrothermal treatment still had an appreciable surface area of400 m /g. 

The binders are silica, lime, slag, or cement. The balls are somewhat dried, if necessary, and then cured in steam autoclaves. During the hydrothermal treatment lime and silica react to form hydrosilicate gels, which act as binders. 

The size of Ti-Beta particles was determined with the high resolution transmission electron microscope (HRTEM). After 28 hours of hydrothermal treatment they grew to... 

Dealuminated samples were obtained by hydrothermal treatment of calcined MCM-22 (Si/Al = 15) at different temperatures (673, 773, 873 K) for 2-24 h under a saturated flow of a nitrogen/steam mixture (flow rate of 200 ml min"1). These steamed samples were further treated with 6N HNO3 solution at 353 K for 4 h in order to remove the extra-framework aluminum species. 

X-ray diffraction patterns were recorded on a Philips PW1820 diffractometer with Cu-Ka radiation (X = 0.154 nm). The collected sample was indexed very well as cubic a-Mn203 bixbyite (JCPDS 41-1442, la-3, a = 0.941 nm) (Fig. 1). The morphologies were visualized by scanning electron microscopy (SEM) (Fig. 1). The abundant well-defined hexagonal-like plates with the sizes from several hundred nanometers to a few micrometers were formed during hydrothermal treatment, which kept initial shape after 700 °C-calcination (Fig. 1). The hexagonal plates are about 50 nm thick with smooth surfaces. 

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