Oxidative dehydrogenation (ODH) of light alkanes catalyzed by metal oxides is considered to be a thermodynamically favorable process for olefin production. The strong interaction between the unoccupied d-orbital of metal atom and the π-electrons of olefins, however, leads to deep oxidation of olefin to CO2, especially at elevated temperatures. The challenge lies in the development of selective and low-temperature active catalysts to avoid such unwanted deep oxidation. Here, we report unambiguous evidence on properly prepared mesoporous silica-supported boron oxide catalysts showing high selectivity for ODH of propane. The catalysts are active at a temperature as low as 405 °C, showing a propane conversion of 2.8% and a propene selectivity of 84.1% (C2–3 =: 94.6%). Upon raising the temperature to 450 °C, a propane conversion of 14.8% can be achieved, with a selectivity of 73.3% toward propene or 87.4% for both propene and ethene (C2–3 =). Both experimental and theoretical studies indicate tricoordinated boroxol and hydroxylated linear boron species are the active sites for the ODH of propane. In addition, the oxophilicity of boron sites is responsible for suppressing deep oxidation by eliminating the alkoxyl species, leading to high selectivity toward olefin products.

Metal-free boron- and carbon-based catalysts for the oxidative dehydrogenation of light alkanes is reviewed from the preparation methods, characterization, catalytic performance and mechanistic issues.

Metal-free boron-based catalytic systems are growing to be promising choices in the oxidative dehydrogenation (ODH) of light alkanes to olefins. However, the ambiguity in the understanding of the mechanism has impeded the improvement of novel catalytic systems. Herein, by using density functional theory (DFT), we mapped a complex reaction network for the B 2 O 3catalyzed ODH of propane, which displayed a typical feature of interplay between the on-surface and off-surface channels through the whole reaction from the initiation stage to the termination stage. The results showed that the interplay between the channels in the two regimes was necessary in two aspects: On one hand, to guarantee high selectivity for olefin products, the gaseous channels need the intervention of the surface sites to eliminate the oxygenated intermediates, for example, alkoxyl radicals, that would otherwise evolve into deep oxidation products. On the other hand, to maintain the high conversion of propane, the catalyst surface needs gaseous radicals to regenerate reactive >BO• species. The mechanism also well explained the catalytic role of trace water and addressed the surface dynamical restructuring, thus constituting a plausible comprehensive understanding of the ODH of propane catalyzed by an oxygenated boron system.

Hexagonal boron nitride (h-BN) has lately received great attention in the oxidative dehydrogenation (ODH) reaction of propane to propylene for its extraordinary olefin selectivity in contrast to metal oxides. However, high crystallinity of commercial h-BN and elusive cognition of active sites hindered the enhancement of utilization efficiency. Herein, four kinds of plasmas (N 2 , O 2 , H 2 , Ar) were accordingly employed to regulate the local chemical environment of h-BN. N 2 -treated BN exhibited a remarkable activity, i.e., 26.0 % propane conversion with 89.4 % selectivity toward olefins at 520 8C. Spectroscopy demonstrated that "three-boron center" N-defects in the catalyst played a pivotal role in facilitating the conversion of propane. While the sintering effect of the "BO x " species in O 2 -treated BN, led to the suppressed catalytic performance (12.4 % conversion at 520 8C).

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Ordered macroporous materials with rapid mass transport and enhanced active site accessibility are essential for achieving improved catalytic activity. In this study, boron phosphate crystals with a three-dimensionally interconnected ordered macroporous structure and a robust framework were fabricated and used as stable and selective catalysts in the oxidative dehydrogenation (ODH) of propane. Due to the improved mass diffusion and higher number of exposed active sites in the ordered macroporous structure, the catalyst exhibited a remarkable olefin productivity of ~16 golefin gcat-1 h-1 , which is up to 2-100 times higher than that of ODH catalysts reported to date. The selectivity for olefins was 91.5% (propene: 82.5%, ethene: 9.0%) at 515 °C, with a propane conversion of 14.3%. At the same time, the selectivity for the unwanted deep-oxidized CO2 product remained less than 1.0%. The tri-coordinated surface boron species were identified as the active catalytic sites for the ODH of propane. This study provides a route for preparing a new type of metal-free catalyst with stable structure against oxidation and remarkable catalytic activity, which may represent a potential candidate to promote the industrialization of the ODH process.

Boron-based catalysts show excellent performance in oxidative dehydrogenation (ODH) of light alkanes to alkenes with high selectivity and extremely good antioxidation properties. However, the anti-deep-oxidation mechanism remains unclear. Herein, we chose h-BN and B2O3 as representative boron-based catalysts to investigate their reactions with two important intermediates in the light alkane ODH, Et· (evolving to ethene) and EtO· (evolving to ethene or CO x ), to elucidate the origin of the antioxidation of alkanes. The density functional theory calculations reveal that surface boron sites could eliminate alkoxy in their vicinity, resulting in exceptional inhibition of alkane deep-oxidation. The analysis of the electronic and geometric structures of key stationary points showed that the oxophilicity of B determined the low deep-oxidation of alkanes, and the homoleptic coordination of B with all three ligating atoms being O moderately enhanced its oxophilicity. This work represents a novel conceptual advance in the mechanistic understanding of alkane ODH.

Direct catalytic propane dehydrogenation (PDH) to obtain propylene is a more economical and environmentally friendly route for propylene production. In particular, alumina-supported Cr2O3 catalysts can have better potential applications if the acidic properties could be tuned. Herein, a series of rod-shaped porous alumina were prepared through a hydrothermal route, followed by calcination. It was found that the acidity of the synthesized alumina was generally lower than that of the commercial alumina and could be adjusted well by varying the calcination temperature. Such alumina materials were used as supports for active Cr2O3, and the obtained catalysts could enhance the resistance to coke formation associated with similar activity in PDH reaction compared to the commercial alumina. The amount of coke deposited on a self-made catalyst (Cr-Al-800) was 3.6%, which was much lower than that deposited on the reference catalyst (15.7%). The lower acidity of the catalyst inhibited the side reactions and coke formation during the PDH process, which was beneficial for its high activity and superior anti-coking properties.