High-temperature supported molten salt catalysts

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

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 

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]. 

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]. 

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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]. 

As catalysts for this reaction, inorganic acids having low volatility and high stability at the temperatures necessary may be used. Phosphoric acid or phosphates, boric acid, boric anhydride or borates may be used either in the molten state, supported on inert carriers, or as the solid salts.108 The use of excess amounts of carbon monoxide and high pressure are specified. Thus, when one volume of methyl chloride and 8 volumes of carbon monoxide are passed, as a mixture, over sodium metaphosphate on pumice at 700° to 800° C. a conversion to 10 or 15 per cent of acetyl chloride is obtained. The use of pressure enables much higher yields to be obtained. 

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