Supported molten salt catalyst

S.6.2.2 Early Work on Supported Molten Salt and Ionic Liquid Catalyst Systems 5.6.2.2.1 High-temperature supported molten salt catalysts... 

The first supported molten salt catalyst systems date from 1914, where BASF filed a patent on a silica-supported V20s-alkali pyrosul te sulfur dioxide oxidation catalyst [48], which even today - as a slightly modified catalyst system - is still the preferred catalyst for sulfuric acid production [49]. However, it took many years to realize in the 1940s [50,51], that the catalyst system actually was a molten salt SLP-type system which is best described by a mixture of vanadium alkah sulfate/hydrogensulfate/pyrosulfate complexes at reaction conditions in the temperature range 400-600 °C with the vanadium complexes playing a key role in the catalytic reaction [49]. 

Giakoumelou, L, Caraba, R., PSrvulescu, V., et al. (2002). First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts, Cattd. Letters, 78, pp. 209-214. 

Christodoulakis, A. and Boghosian, S. (2003). Molecular Structure of Supported Molten Salt Catalysts for SO2 Oxidation, J. Catal, 215, pp. 139-150. 

Finally, we mention supported molten metal catalysis (SMMC), in which molten metal catalysts are dispersed as nanodroplets or as thin film on the surface of porous supports. Supported salt melts provide a well-defined volume, accessible to few reactant components, with a surface that is dynamically restructuring to give access to metal cations. The supported molten salt forms a thin layer on the top of the support that is stable up to high temperatures (600 °C). Usually, the whole surface is covered, but micro- and small meso-pores are preferentially filled. Such catalysts possess very interesting properties for the oxidative dehydrogenation of light alkanes [138]. 

The removal of soot from diesel exhaust gas is preferably done catalytically. Fuel additives and supported molten salts are promising catalyst for this application. NO in the exhaust gas can be used to increase the soot oxidation rate. 

In the literature terms such as supported molten salt (SMS) catalysts, supported ionic liquid catalysts (SILC) and supported ionic liquid-phase (SILP) catalysts, have been used somewhat indiscriminately to describe catalyst systems containing a catalytic ionic phase. In this section vye will use the terms molten salt or ionic liquid to indicate the melting point of the fluid phase in the systems. Furthermore, we will distinguish between the terms SILC and SILP. SILP is used when the ionic liquid is performing mainly as an immobihzing solvent for the catalytic components. SILC is used in cases where the ionic hquid itself, ionic hquid ions or ionic liquid-like fragments are behaving as the catalytic species. 

Development and applications of supported ionic salt catalyst systems can generally be divided into periods, which are closely related to the liquid temperature range of the ionic salts used (Fig. 5.6-3). Since low-mdting ionic liquids have only been prepared re tively recently, most applications using these supported molten salt... 

Fig. 5.6-4 Schematic illustration of a supported ionic liquid fuel cell containing the Wacker oxidation system (SMSEC supported molten salt electro-catalyst) for co-generatlon of acetaldehyde and electricity from ethanol [55],...

One possible way to achieve a uniform surface is by coating the solid support material with a thin liquid film, thereby defining the material properties by the liquid s properties. Such supported liquid phase (SLP) materials date back a 100 years ago till 1914, when BASF introduced a silica-supported V205-alkah/pyrosulfate SO2 oxidation catalyst for sulfuric acid production (see Figure 1.1) [3]. This catalyst, which is stiU the standard system for sulfuric acid production today, can be described as a supported molten salt, as it consists of a mixture of vanadium alkali sulfate/hydrogensulfate/pyrosulfate complexes that are present under reaction conditions (400-600 °C) [4],... 

Rao, V., Datta, R. (1988). Development of a supported molten-salt Wacker catalyst for the oxidation of ethylene to acetaldehyde. Journal of Catalysis, 114(2), 377—387. 

This catalyst may also be referred to as the molten salt catalyst, as referred to by Idles et al, according to the catalyst materials of this type reported to date. It is well known that some of the transition metal oxides, alkaline, and alkaline-earth metal oxides promote carbon oxidation. These oxides are solid and immobile at room temperature but become mobile on the surfaces of soot and support materials on a micrometer scale above certain temperatures, the melting point, or so-called Tamman temperature. In such a mobile state, the catalyst can maintain contact with the soot while the soot surfaces are continually excavated by oxidation. 

Giakoumelu, L, Parvuleseu, V. and Boghosian, S. (2004). Oxidation of Sulfur Dioxide over Supported Solid V205/Si02 and Supported Molten Salt V205-Cs2S04/SiC>2 Catalysts Molecular... 

In SL-PC, a catalyst is supported on a solid matrix in the form of the film of a nonvolatile liquid phase adsorbed on the solid. The catalytic film can be, for example, a molten salt or a molten oxide (e.g., Deacon s catalyst (CUCI2/KCI) used to oxidize HCl with oxygen for the chlorination of ethylene in the synthesis of vinyl chloride. Figure 6.1 V2O5 for the oxidation of sulphurous to sulphuric anhydride). Alternately, it can be a liquid phase (e.g., ethylene glycol, PPh3, butyl benzyl phthalate, etc.) that contains a soluble catalytic species such as a metal complex. 

Both oxidations are highly exothermic and carried out almost exclusively in tubular reactors cooled by a molten salt.1024 Supported vanadium oxide with additives to improve activity, selectivity, and stability usually serves as the catalyst.970 990 1025 Because of its more favorable stoichiometry (no carbon is lost in oxidation), most new plants use o-xylene as the starting material. 

Zeolite-supported Ni and KNiCa catalysts were prepared by molten-salt method [2], which were designated as Ni/ZSI and KNiCa/ZSI, respectively, hereinafter. The support was a high siliceous ZSM-5 zeolite (UOP S-115) mixed with an alumina. 

The use of a Supported Liquid Phase Catalyst offers an opportunity for continuous removal of soot from diesel exhaust gas. Molten salt mixtures show a higher activity compared to solid metal oxides. This high activity can be ascribed to a better contact between the liquid catalyst and the soot. The contact between soot and catalyst remains intact during oxidation. 

In a later work, both the CuCl/KCl molten salt Wacker oxidation system and a [Bu4N][SnCl3] system (melting point 60 °C) was applied to the electrocatalytic generation of acetaldehyde from ethanol by co-generation of electricity in a fuel cell [56]. In the cell set-up, porous carbon electrodes supported with an ionic liquid catalyst electrolyte were separated by a proton conducting membrane (Fig. 5.6-4), and current efficiency and product selectivity up to 87% and 83%, respectively, were reported at 90 °C. 

In addition to the Wacker oxidation catalysts, supported eutectic molten salt CuCl/KCl-based catalyst systems have also been examined for other processes including, for example, production of synthesis gas from methanol for the use as on-board hydrogen production in vehicles [57] and quantitative combustion of chlorinated hydrocarbons to COx and HCI/CI2 at ambient pressure (200-500 °C) with silica-based systems [58,59]. 

Similar results have been recently reported with iV-alkylp3rridinium oriV,iV-dialkylimidazolium salts as supports of the Wilkinson catalyst in the 1-alkene hydrosilylation by triethoxysilane. The catalytic system [RhCl(PPh3)2]/ionic liquid (molten salts) can be recycled at least 10 times without noticeable decreases in the activity and selectivity (122). 

Molten salt method was previously applied for the synthesis of zirconia stabilized with yttrium and solid solutions were obtained [4]. The surface area of the product was found to be about 130 m /g, Such an yttria-stabilized zirconia was used as a catalytic support for hydrotreating catalysts. 

Molten salt chemistry can be a source of new kind of interactions between an active phase and a support. We have shown here that the reactivity of a mixture of precursor salts in molten nitrates leads in a one-step process to the synthesis of new solids with excellent textural properties. These solids present all the characteristics for catalytic applications [8], they can be used either as catalysts by themselves or as supports since free space remains at the surface of the solid [9]. Moreover, the preparation method may involve several species and mixed phases may be grafted at the surface of zirconia [10]. This work gives an exemple of the fascinating potential of molten salt medium for the preparation of catalysts. 

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