Hydrotalcite, catalyst preparation effect

Effect of Catalyst Preparation on the Aromatization of n-Hexane over Pt Clusters Supported on Hydrotalcite... 

Although hydrotalcites are relahvely stable (up to circa 500 °C), they are also of potential applicahon as precursors of mixed metal oxide catalysts, for example Reference [66]. Dehydrahon-rehydration equilibria account for the switching between hydrotalcites and mixed/supported metal oxides, which is somehmes termed the memory effect [67-69]. Recent advances have seen attempts to prepare highly dispersed LDH systems, such as those dispersed within mesoporous carbon [70]. Owing to widespread interest in their application, hydrotalcite catalysts have been the subject of a number of reviews, for example References [71-75]. Other layered-based systems have also attracted attention for application in catalysis, for example Reference [76]. 

Nishida K, Atake 1, Li D, Shishido T, Oumi Y, Sano T, Takeira K (2008) Effects of noble metal-doping on Cu/ZnO/Al203 catalysts for water-gas shift reaction. Catalyst preparation by adopting memory effect of hydrotalcite. Appl Catal A 337 48-57... 

Selectivity to ethene achieved about 94-97% (58% conversion, 450°C) on the VMoWNb-0 catalyst. The effect of modifying appeared to be much more poorly under conditions for ODH of propane. The VMoWNb-0 catalysts exhibited high efficiency in oxidative dehydrogenation of ethylbenzene into styrene and dehydrocyclization of -octane (to ethylbenzene and styrene) in the presence of oxygen or CO2. The yield of styrene from ethylbenzene was close to 80% at the 97-98% selectivity (500°C). In a scanty studied reaction of n-octane dehydrocyclization the yields of styrene and ethylbenzene reached 23-28% at the total selectivity of 90%. The efficiency in ODH of several alcohols over V-Mo-W-Nb-Mg-Al-0 catalysts prepared using complex hydrotalcite-like layered hydroxosalts (LDHs) as precursors has been determined. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

If catalysts are prepared by coprecipitation, the relative solubilities of the precipitates and the possibility for the formation of defined mixed phases are essential. If one of the components is much more soluble than the other, there is a possibility that sequential precipitation occurs. This leads to concentration gradients in the product and less intimate mixing of the components. If this effect is not compensated by adsorption or occlusion of the more soluble component, the precipitation should be carried out at high supersaturation in order to exceed the solubility product for both components simultaneously. Precipitation of the less soluble product will proceed slightly faster, and the initially formed particles can act as nucleation sites for the more soluble precipitate which forms by heterogeneous precipitation. The problem is less crucial if both components form a defined, insoluble species. This is for instance the case for the coprecipitation of nickel and aluminum which can form defined compounds of the hydrotalcite type (see the extensive review by Cavani et al. [9] and the summary by Andrew [10]). 

The key parameters in the preparation of active and stable catalysts for N2O decomposition under conditions of industrial interest were examined using mixed oxides obtained form hydrotalcite-type (HT) precursors. The relationship between activity and composition of the catalysts was studied, with attention focused on the nature and amount of metal ions present in the HT precursor (Rh/Mg/Al or Rh/Co/Al). An activation procedure was developed which makes it possible to prepare catalysts, active and stable even under the severe reaction conditions of industrial interest. The relationship between active phase and composition of the bulk matrix was also investigated. Moreover, the flexible structure of the HT precursor made it possible to investigate the use of other active elements (Pd or La) to develop synergetic effects, without any interactions due to side phase segregation, or lack of homogeneity. 

High-concentration NjO has been decomposed catalytically with four metal oxide systems which contain s, p, ti-block elements. In all cases, the decomposition reaction can be treated as first order with respect to partial pressure of N O, considering that decomposition product O2 exhibits no or little inhibition effect on the reaction. Optimal preparation conditions for the catalysts have been explored. In particular, chemical interaction between surface metals and carriers has been investigated. The catalysts have been characterized by various analytical techniques and general material issues have also been addressed. Among the four systems studied, Ru/AljOj and Mg-Co mixed oxides give best catalytic performance. For example, approximately 6 moles of NjO per kg of the hydroxide/hydrotalcite can be decomposed at 350°C within an hour. 

In this work, we studied the deactivation of Mg-Al mixed oxides in the gas phase oligomerization of acetone. We prepared and characterized calcined Mg-Al hydrotalcites with Mg/Al atomic ratios of 1-9. The effect of composition on both the surface and catalytic properties and the catalyst deactivation was investigated by combining several characterization methods with catalytic data. 

Synthetic layered hydroxides with hydrotalcite-like structures are used as heterogeneous catalysts for base-catalyzed reactions. One of their major drawbacks for technological application is their low stability during hydrothermal and/or thermal treatments. Recent attempts to incorporate large, preferably multicharged anions into their interlayer spaces introduced new routes for preparation of more stable materials. The effect of intercalated polymetalates or transition metal complex anions in the fi amework of layered hydroxides is twofold They increase the gallery heights and thermal stability on one hand and introduce additional Lewis or Bronsted acidic sites in a basic framework on the other hand. 

Ni- and Rh-based structured catalysts were prepared by electrosynthesis of hydrotalcite-type compounds on a FeCrAlY foam, followed by calcination. The effect of the electrosynthesis conditions on both the properties of the film coating and the catalytic activity was investigated. The coating growth and the morphology of the final catalytic layer were controlled by the apphed potential and deposition time. By tuning the deposition parameters, it was possible to prepare Ni- and Rh-based catalysts which were very active in the steam reforming and catalytic partial oxidation of methane. 

Co-precipitation methodology is the most cotmnon synthesis techniqne to obtain LDHs, however nucleation and the kinetics growth can not be controlled easily. An alternative method for the synthesis of nanomaterials is reverse microemnlsion (water-in-oil) in which an aqueous phase is dispersed into an oil phase stabihzed by a surfactant film. Microemulsions can be used as nanoreactors leading to homogeneous nanomaterials with a narrow particle size and better textural properties [3]. In this work, Mn and binary Mn-Cu, Mn-Co hydrotalcite-like precursors synthesized in reverse microemulsion and the effects of preparation methods on the performance of catalysts for deep oxidation of VOCs have been studied. 

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