Progress in the oxidative dehydrogenation of light alkanes to light olefins on metal-free catalysts

Abstract

Light alkenes are the key building blocks in chemical industry. Dehydrogenation of light alkanes to light olefins offers an important route for the value-added utilization of alkanes and the diversification in olefin supplies. Featured with the free coking and no thermodynamic restriction, oxidative dehydrogenation of alkanes to olefins has attracted much attention. The early reported metal oxide catalysts usually have good activities, but tend to deeply oxidize olefins to CO2, leading to poor yield of olefin products. Later on, metal-free catalysts start to catch the eye. As a new estabilished catalytic system, metal-free boron-based catalysts show distinctive reaction characteristics of the high selectivity of olefins and the inhibition of deep oxidation reactions, as compared to metal-based catalysts. Hexagonal boron nitride ( h -BN) was first proved to be a highly active catalyst in the oxidative dehydrogenation reaction. Afterwards, some borides (e.g., SiB6, CB4) and supported boron catalysts have also been reported. The boron-based catalysts exhibit high activities and selevtivities to olefins with negligible formations of CO2, which provides a new sight in the selective cleavage of C–H bonds. In the combination of spectroscopic and kinetic analysis, the BO x species on the catalyst surface was determined as the active sites. The unique catalysis of boron-based catalysts updates the traditional perception on the alkane activation and has becoming a new hotspot worldwide. Besides, other metal-free catalysts, nanostructured carbon-based materials including carbon nanotubes, carbon nanofibers and nano-diamonds have also been shown a great potential in the dehydrogenation of light alkanes. The carbonyl/quinone groups are verified to be the active sites for dehydrogenation of light alkanes; the carboxyl, carboxylic anhydride and lactone groups could act as deep oxidative sites and cause the reduction of olefin selectivity. Moreover, doping of heteroatoms (B, P and N) into carbon matrix is effective for the improvement of olefin selectivity. This review timely highlights the recent developments of metal-free catalysts in the oxidative dehydrogenation of light alkanes, particulary focusing on the newly established boron-based catalytic system. At the end, we propose the perspective research directions of the dehydrogenation of light alkanes.