| Highly efficient conversion of light paraffins (C2-C6) to corresponding olefins would accelerate the use of untraditional gas as a complement feedstock for fossil fuel, which is widely portrayed as the landmark events in the 21st century. Oxydehydrogenation (ODH) is a promising pathway by one-step reaction without thermodynamic limitations. However, a serious bottleneck for the ODH process is the lack of highly selective catalysts mediating the oxidative extent of substrate, further preventing overoxidation and CO2 emissions. Theoretically, a maximum yield of olefin about 35% could be achieved only if the activation capacity of catalyst toward paraffin is comparable to that of olefin, but none of catalysts known is more active for paraffin than for olefin. Likewise, most metal-based catalysts reported in literatures gave an ethylene or propene yields lower than 20% due to overoxidation.Nanocarbons as metal-free catalysts are the promising candidates for the conventional metal-based catalysts since their cost-effective, high activity, outstanding stability, coking resistance and mass-production. Numerous studies about ODH reaction revealed that the nucleophilic oxygen species (O2-) and ketonic carbonyl groups on the carbon surface are selectively responsible for the deprotonation of C-H bonds, while electrophilic oxygen species (O2-, O22-) engage in the unselective rupture of C-C and C=C bonds, resulting in a total oxidation to carbon dioxides. To avoid the ineradicable overoxidation in ODH reaction, ultimately allow us to exploit earth’s alkane resources more efficiently and cleanly, we employed these three scenarios to design and optimize high efficient catalyst for ODH of light alkanes, as following:(1) Selective enrichment of ketonic carbonyl groups:the A1500 with no disordered carbon debris was used as the stable substrate; and then CrO3-HOAc and O2 were used as the selective oxidizing system, respectively; finally, the oxidized carbon nanotubes containing 93% of epoxy and carbonyl groups was obtained (oAl 500), which is twice more than that of carbon nanotubes oxidized by nitric acid (oCNTs). The alkene selectivity of oA1500 is 3-fold of oCNTs at isoconversion, and the oA1500 catalyst showed outstanding stability. Based on this kind of model catalyst, turnover frequency (TOF) in oxidative dehydrogenation of propane is successfully correlated with ketonic carbonyl groups (C=O) and it is calculated to be 1.5 h-1, which reveals the inherent relations between structure-activity of carbocatalysis in ODH reaction of light alkanes.(2) Heteroatoms shielding toward non-selective defects and electrophilic oxygen species: the oxidation-resistance of heteroatom modified CNTs was enhanced obviously; the carbocatalysis was extended to the complicated C5 system, and the selectivity towards ODH of isopentane was improved to 2-fold. Likewise, it uncovered the intrinsic difference between phosphorous and boric modification:Boron selectively blocked the "zigzag" faces and provided a platform-distribution on the enhancements mentioned above, whereas excessive phosphorus unselectively covered all the surface of CNTs and presented a volcanic distribution on the enhancements mentioned above.(3) Creation of novel nucleophilic active sites:it was found that boron nitrides were capable of acting as metal-free catalysts for alkane ODH reactions. For ethane ODH, the maximum yield of ethylene reached above 47%, which is much higher than the theoretical maximum value 35%. When the conversion of ethane was less than 10%, the ethylene yield can be reached 100%. The superior catalytic performance of boron nitrides outperformed the previous catalysts reported in literature, which provides the firm foundation for the unimplemented industrial application of alkane ODH. This work implies that the periodic table of elements can serve as the powerful tools for designing new materials and catalysts, which could be extended to all the synthesis and application fields of chemical and functional materials. |
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