Yuebing Xu,Wenda Yu,Hao Zhang,Jian Xin,Xiaohui He,Bing Liu,Feng Jiang,Xiaohao Liu
Various Co-based SAPO-34 catalysts were prepared using different methods, including ion exchange (IE), incipient-wetness impregnation (IWI), and solid-phase grinding (SPG), to correlate the chemical states of Co species with the C–H and C–C bond scissions in ethane dehydrogenation. The IE-prepared Co/SAPO-34 led to stable, unreducible, and isolated exchanged Co sites anchored on the zeolite framework with a structure of −AlF–O–Co–O– and showed the highest selectivity to ethylene of close to 98% at 600 °C, which suggests that these Co sites favors suppressing the C–C bond scission in ethane. In comparison, the IWI- and SPG-prepared Co/SAPO-34 catalysts, especially for those with a high Co loading, inevitably give Co oxide clusters that are easily reduced into metallic Co. Together with catalytic results, characterizations, and DFT calculations, it is confirmed that the reduced Co clusters, especially for those outside SAPO-34 channels without the confinement effect, favor both C–H and C–C bond scission, boosting the conversion of ethane into CH4 or/and coke; however, the ionic-state −Co–O– species can smoothly terminate the ethane dehydrogenation for the ethylene product due to relatively high energy barriers for both C–H and C–C bond scission, avoiding a deep dehydrogenation and C–C cracking. As expected, the unreducible −Co–O– sites are very stable in the title reaction without deanchoring from the zeolite framework in a 100 h cyclic test. This study not only demonstrates the stable −Mδ+–Oδ− structure favorable for suppressing C–C bond scission but also highlights a catalyst-constructing strategy for Co-based and similar metal-based catalysts for dehydrogenation of other light alkanes.
Direct conversion of CH 4 to high valued chemicals under mild conditions is attractive but full of challenges. Here, we present a ternary composite that consists of Ni-doped TiO 2 nanocubes (NCs) and CNTs as a new kind of catalyst (denoted as CNTs-Ni@TiO 2 NCs) for CH 4 photocatalytic conversion to C1 oxygenated products at 30 °C. Using H 2 O 2 as an oxidant, a high C1 oxygenated products yield and selectivity of 1.43 mol/(mol Ni ·h) and 95.6% can be reached in 2 h at 30 °C. The superior photocatalytic performance resulted from the synergistic effects of TiO 2 NCs, Ni species, and CNTs such as enhanced visible light absorption , improved electron/hole separation and charge transfer property, and lowered C-H bond cleavage energy required for CH 4 activation, were carefully studied. Good reusability of the catalyst was studied by recycle experiments. Possible reaction mechanism based on the radical routes were carefully discussed by combining in-situ Fourier transform infrared spectroscopy , X-ray photoelectron spectroscopy, electron spin resonance spectroscopy, radical scavenger experiments and density functional theory calculation.
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