Alkane activation at ambient temperature - Unusual selectivities, C-C, C-H bond scission vs C-C bond coupling

Availability and low cost of light alkanes (C1-C4 range) make them interesting as feedstock for commercial fuels and chemicals. However, direct conversion of lower alkanes for such purposes is a challenging problem [1]. Two commonly attempted solutions to the problem are to use oxygen and/or selective catalysts for efficient conversions. However, in the case of light alkanes (C1-C3) higher temperatures, in the range 550-850°C, are required to get appreciable alkane conversions. Activation of C-C and C-H bonds at higher temperatures, even in the presence of heterogeneous catalyst systems, tend to be initiated by homogeneous splitting of bonds, creation of radicals and radical chain reactions leading to products. High temperature alkane conversions have inherent difficulties, viz. (i) they lead to extensive, endothermic C-C and C-H bond cleavage causing formation of cracking products or coke, respectively (ii) cause loss of catalyst activity due to sintering and (iii) favor non-selective combustion of alkanes.
Alkane activation at lower temperatures, even though challenging, is very interesting from a commercial point of view. Gaseous plasma, generated by dielectric barrier discharge and which consists of energetic electrons, is reported to be able to activate hydrocarbons [2]. It is possible to generate a cold plasma at ambient conditions (atm pressure) in a micro-reactor [3]. This presentation highlights the oxidative conversion of light alkanes, C1-C3 range, in the presence of cold plasma in a microreactor. Thus, C-C and C-H bond activation at lower temperatures and its influence on products selectivities are discussed. The employed plasma microreactor consists of a Pyrex rectangular chip with microchannels dimension of 30 mm length x 5 mm width and a channel depth of 500 mm (Fig. 1).

The temperature of the gas was firstly determined by the optical spectrum emission [4] recorded during alkanes activation and additionally monitored by inserting a thermocouple inside the micro channel outside the discharge area. Under the conditions used (1 atm, 3 W power input), the temperature was below 40ºC.

In the case of C1-C3 range alkanes and under the same conditions conversion levels of 22 mol% for propane, 15 mol% for ethane and 10 mol% for methane was observed. Most remarkably, in all the experiments, high selectivity to products with higher molecular weight than the starting hydrocarbon (coupling products) were also observed (Fig. 2). In fact, in the presence of a cold plasma, alkane molecules can be directly activated/converted via collisions with energized electrons that produce radicals for eg., C3H7• in the case of propane due to cleavage of C-H bonds (C3H8 + e- → C3H7• + HΙ + e-) [2]. This can initiate radical chain reactions and their propagation and/or termination reactions take place at ambient temperature. The presence of products containing more C atoms than the starting alkane requires C-C bond formation/coupling under the conditions present in the microreactor. Typically formation of C-C bonds is an exothermic process and therefore favored at lower temperatures. Coupling reactions between radicals also imply a decrease in entropy because the number of molecules/radicals decreases. Thus, in our studies direct homologation of alkanes is observed.

This represents an interesting novelty to directly upgrade cheap low molecular weight alkanes to commercially useful fuels and/or feedstock materials for the chemical industry.