Hydrotalcite applications

The development of novel materials for applications such as catalysis has been a very important area of work, with several classes of materials being developed and applied (for example, zeolites and their mesoporous analogues, hydrotalcites, hydroxyapatites, clays). 

Mg4Al2(0H)12C03 3 0, is commonly written however, these minerals are generally nonstoichiometric by nature and can include some amounts of alternative elements in their compositions. They function similarly to the zeolites but exist in layered structures and have a different trapping mechanism. In addition to their performance enhancement, the hydrotalcite minerals are compatible with PVC and can be used effectively in clear PVC applications as well as the pigmented formulations. 

Recently, Taillefer et al. reported an Fe/Cu cooperative catalysis in the assembly of N-aryl heterocycles by C—N bond formation [90]. Similarly, Wakharkar and coworkers described the N-arylation of various amines with aryl halides in the presence of Cu—Fe hydrotalcite [91]. Interestingly, Correa and Bolm developed a novel and promising ligand-assisted iron-catalyzed N-arylation of nitrogen nucleophiles without any Cu co-catalysts (Scheme 6.19) [92]. Differently substituted aryl iodides and bromides react with various amides and N-heterocycles. The new catalyst system consists of a mixture of inexpensive FeCl3 and N,N -dimethylethylenediamine (dmeda). Clearly, this research established a useful starting point for numerous future applications of iron-catalyzed arylation reactions. 

Cavani, F., Trifiro, F. and Vaccari, A. Hydrotalcite-type anionic clays preparation, properties and applications, Catal. Today, 1991, 11, 173-301. 

This section describes the applications of some important crystalline inorganic ion exchangers, such as the hydrotalcites, titanates, and zirconium phosphates [3,87-111],... 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Hydrotalcite (HT) is a clay mineral which on crushing becomes a fine powder, similar to talc. It is a hy-droxycarbonate of Mg and A1, of the general formula Mg6Al2(0H)i6C03 - 4H20, and occurs in nature in foliated and controlled plates and/or fibrous masses. Anionic clays of the HT type have been of importance in catalysis since 1970. Their interesting properties for catalytic applications are ... 

Cavani F, Trifiro F, Vaccari A. Hydrotalcite-type anionic clays Preparation, properties and applications. Catalysis Today. 1991 11 (2) 173—301. 

The main purpose of this chapter is to review the applications of thermal analysis and calorimetric techniques to the study of catalysts such as oxides, heteropolyanions, hydrotalcites, layered silicates and microporous or mesoporous molecular sieves. A brief summary of studies that made use of calorimetry to characterize crystaUine or amorphous oxides and related materials is also presented. 

Hydrotalcites are of interest because of their ability to function as base catalysts, for example References [57-61]. In a very topical application, IFP have developed a biofuels process using Al, Zn and Ti mixed oxides [62]. The process is being commercialized by Axens. Figueras and coworkers [63]) have studied Mg-Al... 

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]. 

Table 6 shows an example of the cyclic steady state performance of the SERP concept using an admixture of a SMR catalyst (noble metal on alumina) and a CO2 chemisorbent (K2CO3 promoted hydrotalcite) in the reactor [20]. The reactor temperature was 490°C. The feed H20 CH4 ratio was 6 1. The concept can directly produce 95% H2 product (dry basis) with a CH4 to H2 conversion of 73%. The trace impurities in the product gas contained less than 40 ppm COx- The corresponding product gas composition (thermodynamic limit) of a SMR reactor operated without the CO2 chemisorbent will be -67.2% H2, 15.7% CH4, 15.9% CO2, and 1-2% CO (dry basis), and the CH4 to H2 conversion will be only 52%. Thus, the SERP concept may be attractive for direct production of a CO free H2 stream for fuel cell applications. 

The thermal decomposition of hydrotalcites also is intriguing. They decompose on calcination to nanocrystalline spinel-like phases, and eventually, to crystalline spinels. This pathway initially produces spinels which are hydrated in their large surfaces and poorly ordered in their cation distributions. These materials are very reactive and analogous to precursors sought in ceramic synthesis. The thermochemical study of the evolution of MgAl204 from nitrate precursors (McHale et al. 1998) may in fact have encountered hydrotalcite as an intermediate. Further study of the thermal evolution of hydrotalcite nanomaterials may be applicable to both earth and materials sciences. 

Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies. Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process. The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.F23,i24] shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300° C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400°C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. 

Hydrotalcite, Mg6Al2(OH) 16CO34H2O, a synthetic clay mineral with a layered structure of hexagonal platelets, acts as HCl scavengers in PVC stabilization [141]. Together with mixed-metal Ca/Zn systems hydrotalcite constitutes an environmentally friendly, non-toxic, heavy-metal free method of PVC stabilization in flexible, rigid and transparent PVC applications [142]. 

Many papers dealing with applications, and variations in synthesis parameters of hydrotalcite-like compounds can be found in the open literature, but it is beyond the scope to present an overview here. On concluding this paragraph, it should be mentioned that hydrotalcite-like compounds are often used as catalyst precursors in the catalyst industry, because of the properties stated above. 

Reduction of HAS activity in applications in flame and weather resistant polymers and coatings containing halogenated FR is solved by means of HAS having reduced basicity (Sect. 3.3.2). The danger of antagonism in blends HAS/FR is reduced by addition of hydrotalcite [270]. terf-HAS 34 may be used for stabilization of PVC in combination with the conventional HS (salts of carboxylic acids, organotin compounds) [271]. 

Solid catalysts active in MPVO reactions have surface basicity or Lewis acidity. They include, amongst others, alumina, zirconia, magnesium oxide, and magnesium phosphates. More recent developments include the chemical anchoring of catalytically active co-ordination complexes, and the application of hydrotalcites, mesoporous materials (MCM-41), and zeolites. Anchoring of co-ordination compounds might open the route to true heterogeneous enantioselective MPVO reactions. As a result of their inherent shape-selectivity zeolites uniquely afford remarkable stereoselectivity in MPVO reactions. 

---