Transient alumina phases

In a later work of similar nature, Chatterjee etal. [170] prepared a boehmite sol from an aluminum salt. Boehmite particles were precipitated from an aluminum nitrate solution heated to 80 -90 C by addition of an ammonia solution into it. The precipitate was washed and peptized with glacial acetic acid to obtain a colloidal sol. The W/O emulsion was prepared by addition and dispersion of the sol (as the water phase) to an organic liquid, 1,1,1 trichloroethane (TCE) or n-heptane (the oil phase) containing 2-2.5 vol % of Span 80, a non-ionic surfactant. The volume ratio of water phase oil phase was about 1 4, and a suitable sol viscosity range was -10-20 mPa.s. The sol droplets were converted to gel droplets by addition of triethylamine (TEA), a base. The addition of TEA was monitored so that the pH of the system reached a value of 8-9 for completion of ion extraction for this purpose, the volume ratio of the sol to the base was adjusted at 1 0.4. The gel microspheres were washed with acetone and methanol, dried at 100°C and calcined up to 1200"C/1 h, when a-Al203 crystallized from transient alumina phases. The pathway, in brief, was boehmite —> y 5 —> 0 —> a-Al203. Under tailored conditions, it was possible to obtain hollow microspheres (Fig. 4.2). 

Comparison of the methanation activity in Fig. 28 with the H2S concentration transients in Fig. 29 indicates a prolonged adsorption of H2S during the 13-ppb deactivation with the H2S level reaching a steady state at about the same time as the methanation activity. The difference between the sulfur fed to the reactor and that in the effluent represents the sulfur that was adsorbed on the Ni surface, since the alumina support and the quartz reactor did not adsorb sulfur. Time integration of the amount of sulfur adsorbed, obtained from the effluent H2S concentration data, gives the total amount of sulfur adsorbed at equilibrium with the gas-phase H2S concentration. 

We have found a zeolite synthesis reaction which is at the boundary between the five-ring structure and the four- and six-ring structure production. The product depends on the organocation used. A second feature of this synthesis reaction is that no amorphous solid is involved or produced. Tliis reaction derives silica and all of its alumina from the low silica zeolite. Cubic P. This zeolite, organic-free, had been encountered as a transient phase in the discovery of SSZ-13 (J ). 

The oxidation of TiAI3 results in the exclusive formation of external alumina [62] and the parabolic rate constants are essentially the same as those observed for NiAI [88]. However,TiAl3 is a line compound and, as such, is difficult to prepare by melting and casting as a single phase material. It has been found [88] that excess A1 results in rapid transient oxidation prior to the attainment of the slow steady-state rate. 

The morphology of alumina scales on NiAl depends on the oxidation temperature and oxidation time. At lower temperatures (<1400 K). the metastable oxide phases 7-and 0-Al2O3 are formed initially. These metastable phases transform into a-AFO, during a transient time which decreases with higher temperature (> 1200 K) [24—26J. 

By applying transient experiments to the dehydration of ethanol on y-alumina, it has been shown that model 1 and 2 can describe the experimentally observed increase of the reaction rate. The introduction of a new parameter in model 1 (p, the ratio of the number of sites of type two by the number of sites of type one, enables a better description of the drastic increase of ethylene directly after stopping the ethanol feed. The parallel behaviour of surface intermediates, a sharp decrease followed by a slow consumption, is quantitatively depicted by model 1, whereas model 2 gives only a qualitative description. Periodic experimentation confirms that model 1 is able to describe the gas phase and the surface more adequately than model 2. 

To summarize, transient kinetic experiments are an estabHshed and valuable tool in the investigation of heterogeneously catalyzed gas-phase reactions. For liquid-phase systems, transient studies are much rarer than for gas-phase systems. It is probably related to slower dynamics and the fact that the intrinsic kinetic phenomena can be obscured by mass transfer effects and catalyst deactivation. As an illustration, we will consider three-phase continuous enantioselective hydrogenation of ethylbenzoylformate (Fig. 7.13) leading to two products over a platinum catalyst on an alumina support [1]. 

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