Calcination-rehydration effect

The conversion of the mixed metal oxides into LDHs has been variously referred to as regeneration, reconstruction, restoration, rehydration or the calcination-rehydration process , structural memory effect or simply memory effect . This method is usually employed when large guests are intercalated. It also avoids the competitive intercalation of inorganic anions arising from the metal salts. The procedure is more complicated than coprecipitation or ion-exchange methods, however, and amorphous phases are often produced simultaneously. 

In the case of CoMo/A, which presents the same molybdate entities as in the non-promoted sample, their behaviour upon calcination is identical, and AlMoa evolves into an aluminium molybdate phase, and re-appears upon transfer to air. In the case of CoMo/Z, the calcination generally performed at 500°C to eliminate the ammonium counterions induces a decomposition of the cobaltomolybdate entity. Upon rehydration, the so freed molybdenum atoms associate with aluminium as in the impregnation of AHM to give AlMoe. Thus, the calcination-rehydration steps have a levelling effect on the structure of the promoted catalyst the same entities are finally identified whatever the support or the synthesis route. 

This study enables us to draw a parallel between the behaviours of alumina and HY zeolite upon impregnation with oxomolybdate entities. In some conditions, when the buffer effect of the alumina is avoided, both supports see extraction of aluminium atoms and their inclusion in an Anderson-type heteropolyanion, AIMo6024H6. This entity is well dispersed on the alumina support whereas it appears as a bulk compound located in the macropores of the HY zeolite. However, it is still unknown whether the formation of this species is to be favoured. For promoted samples, a significant difference exists between both supports, since CoMo6024H6 can be preserved upon impregnation on a zeolite. Yet, the preparation of the catalysts has to be controlled the final calcination-rehydration steps tend to level the various synthesis and lead to the presence of AIMo6024H6. ... 

Ordinarily the chromium binds to the silica by reacting with hydroxyls on a fully hydrated surface, because chromium is impregnated aqueously onto the silica and then calcined. However, a different catalyst results if the chromium attaches instead to a surface already dehydrated by calcining. A large promotional effect, particularly on the termination rate, is obtained (76). To do this the silica is first dehydrated at 900°C, for example, then impregnated with chromium anhydrously so that the surface is not rehydrated. A secondary calcining step at some lower temperature such as 300-600°C then fixes the chromium to the silica. The method is especially effective if the support also contains titania. 

The relationship of Lewis and Brpnsted acid site concentrations on H—Y zeolite was explored further in a study by Ward (156) of the effect of added water. At low calcination temperatures (<500°C) only a small increase in the Brpnsted acid site concentration occurred upon addition of water to the sample. Rehydration of samples dehydroxylated by calcination above 600°C resulted in a threefold increase in the amount of Brpnsted-bound pyridine. However, no discreet hydroxyl bands were present in the infrared spectrum after rehydration. Thus, the hydroxyl groups reformed upon hydration must be in locations different from those present in the original H—Y zeolite, which gave rise to discreet OH bands at 3650 and 3550 cm-1. 

In view of the uncertainties inherent in Hammett indicator determinations of surface acidity by visual means, a study was made of the spectral behavior of dyes adsorbed on several silica-alumina catalysts and silica gel (62). The effects of catalyst water content, dye concentration, catalyst composition and pretreatment on the spectra of the adsorbed dyes were examined. The Hammett indicator dyes employed and their corresponding pKA values are summarized in Table II. Reference spectra were determined for the base-form of the dye in iso-octane or methylene chloride solution and for the acid-form in an aqueous sulfuric acid or ethanolic-hydrogen chloride solution. Dyes were adsorbed from isooctane solutions onto thin plates of optically transparent catalysts which were installed in evacuated cells of design similar to that shown in Fig. 4. The catalysts samples were routinely pretreated by calcination in oxygen at 500° to remove any organic contaminants, followed by evacuation at this temperature. To examine the effect of variable water content on the spectra the samples were rehydrated in an atmosphere of wateir vapor for 24 hr after pretreatment and subsequently evacuated at some lower temperature. Dye solutions were introduced through a side arm. These solutions were suitably dilute so that the absorbance due to dissolved dye was either below the limits of detection or, at... 

Reconstructed Hydrotalcite. When calcined hydrotalcite (Mg0-Al203) is rehydrated in water or in flowing nitrogen saturated with water, hydrotalcite structure is reconstructed. This phenomenon is called memory effect. The reconstructed materials contain OH in the interlayers. The reconstructed materials are very useful catalysts for aldol condensation, Knoevenagel condensation, Michel addition, and cyanoethylation of alcohols (22-24). The OH ions in the interlayer are believed to be the active sites for these reactions. 

The Claisen-Schmit condensation of 2-hydroxyacetophenones and benzaldehyde derivatives proceed at 423 K using calcined and rehydrated hydrotalcites as the catalyst, the latter being more effective (78,79). Nanocrystalline MgO (80) and mesopous silica SBA 15 having propylamino groups (81) are also effective for this reaction. 

A related method uses what is sometimes referred to as the memory effect of LDH. This technique consists of heating an LDH containing a thermally labile anion. The resulting amorphous oxide is then rehydrated in the presence of the desired replacement anion to form a new LDH. Two factors of importance in this method are the choice of starting material and the temperature of calcination. The starting material must contain a thermally labile anion, and the temperature of calcination must be controlled so as to avoid excessive heating that would result in the formation of spinel, which is resistant to rehydration. 

There are a number of techniques that have been successfully applied to synthesize modified hydrotalcites (48). The most commonly method used is the co-precipitation of two metal salts in alkaline solution at a constant pH value of about 10. Another method uses the classical ion exchange process in which the guest anions are exchanged with the anions in the interlayer spaces of preformed layered double hydroxides to produce specific anion intercalated modified hydrotalcites. StiU another method is a lattice reconstruction after heating, i.e., calcination, which is based on the structural memory effect of these materials, due to which the original structure is reproduced after rehydration. 

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