Hydrothermal treatment conditions

Calcined and steamed FAU samples also have complex hydroxyl IR spectra. Figure 4.25 shows the difference between an ammonium ion-exchanged FAU before and after steaming and calcination. The very simple, easily interpretable hydroxyl spectrum of the ammonium exchanged FAU sample is transformed into a complex series of overlapping hydroxyl bands due to contributions from framework and non-framework aluminum atoms in the zeolite resulting from the hydrothermal treatment conditions [101]. 

One can see that increase in hydrothermal treatment temperature results in appreciable decrease in specific surface area (from 188 to 16,5 m /g) and an increase in average pore radius from 4.5 to 42.8 nm). These changes are accompanied by a decrease in pore volume from 0.389 to 0.305 cm /g. This is of course result of very strong transformation of support under hydrothermal treatment conditions (water steam pressure ca 1.5 MPa). In the next step alumina support DN-550 was impregnated with TEOS and the obtained support sample was submitted to hydrothermal treatment. The obtained results are presented in Fig. 3. We can see that supporting of alumina with TEOS (compare samples DN and DN + T) gave increase in... 

This study was performed using three samples (a, b and c) synthesized under different hydrothermal treatment conditions (1 day at 100°C, 1 day at 80°C and 6 days at 60°C respectively). The pore diameters of these compoimds are respectively 8.0, 4.7 and 4.5 nm. The materi were calcined at 550, 600, 700, 800, 900 and 1000°C and the variation of specifie surface area was monitored as a function of this calcination temperature. A large pore MCM-41 (sample d) with 7.5 nm pore size is also reported in Fig. 3 for con jarison. 

From Fig. 3, it appears that the specific surface area of the disordered DWM materials remains very high till 800°C. Beyond this tenperature, for sample B for instance, its value decreases from 800 to 20 mVg if calcination ten rature is raised from 800 to 1000°C. Whatever the hydrothermal treatment conditions of the synthesis are, the thermal resistance of the materials is similar. Even at 1000°C, the recovered materials exhibit a type IV isotherm, characteristic of mesoporous compounds. A part of mesoporosity is thus maintained with quite a narrow pore size distribution, but the maximum adsorbed volume is sharply reduced. This is in accordance with the very broken appearance of the particles observed by SEM. Thermal stability of these disordered materials is, however, very superior to MCM-41, whose structure does not resist beyond 600°C. This behaviour can be related to the different preparation method that affords compounds with a different structure and also thicker walls. 

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. 

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 presence of water, or water vapour, affects the chemistry of thermal modification and heat transfer within the wood (Burmester, 1981). Under dry treatment conditions, the wood is dried prior to thermal modification, or water is removed by the use of an open system, or a recirculating system equipped with a condenser. In closed systems, water evaporated from the wood remains as high-pressure steam during the process. Steam can also be injected into the reactor to act as a heat-transfer medium, and can additionally act as an inert blanket to limit oxidative processes. Such steam treatment processes are referred to as hygrothermal treatments. Where the wood is heated in water, this is known as a hydrothermal process. Hydrothermal treatments have been extensively studied as a... 

Conditions which promote multi-domainic goethites are high ionic strength (either [KOH] or salt) and also low synthesis temperature (<40°C). In alkaline solutions, multi-domainic character decreases and domain width increases as Al substitution increases to Al/(Fe-i-Al) of 0.15, whereas at Al/( Al-nFe) >0.15 single domain crystals result (Schulze Schwertmaim, 1984 Mann et al., 1985). Multidomainic goethites can recrystallize to single domain crystals as a result of hydrothermal treatment at 125-180 °C (Fig. 4.9) (Schwertmann et al., 1985). 

Various methods have been developed to maximize p-xylene formation. A mild hydrothermal treatment of H-ZSM-5 and the incorporation of Pt greatly reduce disproportionation activities and, consequently, increase para selectivity.355 Suppressing the activity of the external surface can also lead to enhanced para selectivity. By judicious choice of reaction conditions and organometallic reagents... 

The addition of water-soluble polymers such as polyethylene oxide (PEO) or polyvinyl alcohol (PVA) into the synthetic mixture of the C TMAX-HN03-TE0S-H20 system (n = 16 or 18 X = Br or Cl) under shear flow is found to promote uniformity and elongation of rope-like mesoporous silica. The millimeter-scaled mesoporous silica ropes are found to possess a three-level hierarchical structure. The addition of water-soluble polymer does not affect the physical properties of the silica ropes. Moreover, further hydrothermal treatment of the acid-made material under basic ammonia conditions effectively promotes reconstruction of the silica nanochannels while maintaining the rope-like morphology. As a result, a notable enhancement in both thermal and hydrothermal stability is found. 

Such ammonia hydrothermal treatment is also applicable to mesoporous silica materials synthesized under different conditions, e.g., different acid source or temperature. This and other interesting issues will be reported later. 

Hydrothermal Stability Test. Hydrothermal stability of the synthesized samples was investigated by mixing about 0.2 g of the calcined sample with 20 g deionised water and heating in a closed bottle at 100°C under static condition for different time periods. After hydrothermal treatment the solid was filtered, washed with deionised water and dried at 70°C for overnight. Hydrothermal stability was followed by measuring the XRD peak intensities. 

The reaction of pure silica MCM-48 with dimethyldichlorosilane and subsequent hydrolysis results in hydrophobic materials with still a high number of anchoring sites for subsequent deposition of vanadium oxide structures. The Molecular Designed Dispersion of VO(acac)2 on these silylated samples results in a V-loading of 1.2 mmol/g. Spectroscopic studies evidence that all V is present as tetrahedral Vv oxide structures, and that the larger fraction of these species is present as isolated species. These final catalysts are extremely stable in hydrothermal conditions. They can withstand easily hydrothermal treatments at 160°C and 6.1 atm pressure without significant loss in crystallinity or porosity. Also, the leaching of the V in aqueous conditions is reduced with at least a factor 4. 

However, they found that the more completely the surface was dehydroxylated, the longer the time required for rehydroxylation. A surface dehydroxylated at 1173 K for 10 h required several years in water at ambient temperature to become fully rehydroxylated. When the same silica sample was hydrothermally treated in boiling water, 60 h sufficed to obtain full rehydroxylation. Caution is necessary however in using hydrothermal treatment, since some silicas tend to undergo drastic changes in structure and reduction in surface area under these conditions. 

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