Highly Efficient and Selective CH Activation in Gallium-Containing Molten-Salt Systems

Abstract

in an ionic liquid or molten salt medium with very high selectivities (up to 98 % of gaseous products) to form isobutane. During the reaction, hydrogen and anionic chlorogallate species of Ga I and Ga III are formed. The latter can be electrochemically reduced to Ga 0 in a subsequent step. The chlorine liberated in the regeneration of Ga 0 can be used for the synthesis of the chloromethane feedstock from methane, thus making this new reaction a potential key step in a fundamentally new process chain for increasing the value of methane in a closed chlorine/chloride cycle. Rational and efficient conversion of methane to more useful higher hydrocarbons is a very important topic in natural gas utilization. Today, methane activation is mostly carried out indirectly by transformation of methane to syngas (a mixture of CO and hydrogen), which then is converted into higher hydrocarbons (olefins, gasoline, and waxes) by Fischer–Tropsch synthesis. Alternatively, syngas can be used to synthesize methanol, which later can be converted into hydrocarbons using methanol-to-olefins (MTO) or methanolto-gasoline (MTG) catalysts. [1–4] In both routes, a great part of the overall cost stems from production of syngas. [5] Another problem of Fischer–Tropsch synthesis and methanol conversion processes is that it is not possible to produce a single product with high selectivity, as distributions of products with different chain lengths are obtained. A true alternative for indirect methane conversion is the use of chloromethane as the activated intermediate. Chloromethane is obtained both from radical chlorination of methane and from oxyhydrochlorination [6] of methane. In 1988, Taylor and Noceti published a process for the production of gasoline from chloromethane in an MTG-like process. [7] The byproduct HCl of the MTG-like step can be recycled back to the production of chloromethane by oxyhydrochlorination, thus making the overall process attractive for technical use. In the following years, different types of zeolite catalysts were applied in this kind of chloromethane transformation, showing different performance with respect to catalytic activity, coke formation, and the distribution of hydrocarbons produced. [8–12] Recently, the application of SAPO-34-type molecular sieve catalysts was found to be promising (SAPO = silicon aluminum phosphates). With this type of catalyst, light alkenes (ethene, propene, butenes) can be produced at temperatures of 350–5008C in 70–80 % selectivity in a temperature-dependent mix in which the maximum selectivity for one individual product hardly reaches 45 %. [13] Only minor amounts of light alkanes are obtained, while 15–20 % of the products are heavier than C5. Herein, we describe a new, highly efficient and selective