Isolated skeleton Co-centered zeolite catalyst facilitates efficient ethane dehydrogenation

On November 18, 2022, Beijing time, researcher Wang Liang of Xiao Harvest Research Group of Zhejiang University, Xiao Jianping, researcher of Dalian Institute of Chemistry, Chinese Academy of Sciences, and Associate Professor Liu Xi of Shanghai Jiao Tong University published a research result entitled “Rivet of cobalt in siliceous zeolite for catalytic ethane dehydrogenation” in the journal Chem.

In this study, the spontaneous dispersion of metals was assisted by mechanical ball milling to construct zeolite zeolite catalytic materials with isolated skeleton Co centers. The catalyst realizes the process of ethane anaerobic dehydrogenation to ethylene at ultra-high air speeds, and exhibits excellent resistance to carbon deposition and sintering. It provides a new way for the design and preparation of high-efficiency non-noble metal catalysts for ethane to ethylene.

Wang Liang, Xiao Jianping and Liu Xi are the corresponding authors of the paper; Liu Lu, Li Huan and Zhou Hang are co-first authors.

Ethylene is one of the most important chemical products of modern industry and is currently mainly produced by the steam cracking route of naphtha. In recent years, with the increase of shale gas production, the ethane dehydrogenation to ethylene route has attracted much attention. Ethane anaerobic dehydrogenation is a strong endothermic process that typically requires higher temperatures to activate C-H, and strong adsorption of olefin products on metal catalysts often causes deep dehydrogenation to form coke, resulting in catalyst inactivation. Although Pt-based catalysts are currently the most widely used commercial catalysts, they still have the disadvantages of high cost and insufficient stability. Therefore, it is of great significance to use non-noble metals to achieve efficient dehydrogenation of ethane and maintain stable catalytic performance.

In this work, the researchers used Silicalite-1 zeolite as a carrier to embed Co into the zeolite skeleton by mechanical ball mill-assisted spontaneous dispersion. Under the conditions of high mass airspeed (6.5-109 h-1) and high ethane partial pressure (1.0 bar), the catalyst was still able to achieve a conversion close to the thermodynamic equilibrium limit, and obtained a space-time yield of ethylene of 13.4 KgC2H4 Kgcat-1 h-1. A series of characterization and in situ tests proved that there was an isolated Co center in the zeolite skeleton, and the existing SiOx-O-Coδ+ structure could effectively stabilize Co species and avoid sintering during dehydrogenation. Theoretical calculations show that H2CoS-1, as the active site of activated ethane C-H, can effectively avoid further splitting of C-C and bring high ethylene selectivity. At the same time, it promotes the rapid desorption of ethylene products to avoid deep dehydrogenation to form coke, thus showing excellent stability.

Figure 1: Schematic diagram of the process of preparing Co/S-1 by mechanical ball milling. Co/S-1-aw is obtained by pickling Co/S-1.

Figure 2: STEM characterization of Co/S-1.

Using scanning transmission electron microscopy high-angle annular darkfield mode (HAADF-STEM) and integrated differential phase contrast mode (STEM-iDPC), the research team simultaneously performed atomic-resolved imaging of zeolite crystal structure and the presence of heterogeneous monatoms at low electron doses. Using this technique, STEM-iDPC images are provided in zeolite[010]Clear image of the 10-MR micropores in the direction (Figures 2A and B), the atomic structure of S-1 zeolite can be observed, unlike the widely studied structure of zeolite micropores containing metal nanoclusters, the zeolite micropores of Co/S-1 are clean. The corresponding HAADF-STEM image (Figure 2E) also shows that there are heavier element single atoms in the Co/S-1 skeleton, which, combined with EDS results, are considered to be the location of Co atoms (Figure 2F).

Figure 3: Ethane-catalyzed dehydrogenation test for Co/S-1.

Figure 4: Carbon deposition studies of Co/S-1 and Co/S-1-aw.

A series of ethane anaerobic dehydrogenation tests show that Co/S-1 has catalytic dehydrogenation ability close to the thermodynamic equilibrium limit at high partial pressure and high air speed, and is better than Pt-based catalysts in activity and stability, and achieves the highest temporal and spatial yield of ethylene in ethane anaerobic catalytic dehydrogenation (Figure 3). At the same time, after removing the external cobalt oxide particles, the catalyst that retains only the isolated Co center inside the zeolite has almost no tendency to inactivate observed in the life test up to 102 h, with excellent stability (Figure 4).

Figure 5: H2CoS-1 ethane dehydrogenation reaction path simulation.

The team also carried out detailed mechanistic studies of the reaction pathway. H2CoS-1 is used as the active site to activate ethane C-H bonds, and the dissociation of the first C-H bond of ethane to form adsorbed ethyl requires overcoming the energy barrier of 1.53 eV. It is easier for the ethyl group to further dissociate the C-H bond (1.36 eV) than the C-C bond (1.64 eV), and after two C-H bond dissociations, the hydrogen atoms produced are adsorbed on Co and O, respectively, and the ethylene in the molecular sieve pores exhibits repulsion characteristics, resulting in ethylene desorption is a strong exothermic process. After ethylene desorption, H-H is coupled to form hydrogen, and the energy barrier of this process is only 0.74 eV. In addition, H-H coupling can also occur before ethyl dehydrogenation (as shown in the blue reaction path in Figure 5), and the two H atoms connected to the molecular sieve backbone O are spatially far apart when the ethyl group occupies the Co site, so that the H-H coupling process needs to overcome an energy barrier of up to 2.28 eV. In general, the release of hydrogen from H-H coupling after two consecutive dehydrogenations of ethane was the most favorable reaction mechanism (red path), and the low energy barrier reflected the high intrinsic activity of ethane dehydrogenation catalyzed by the H2CoS-1 site.

The work was supported by Beijing Synchrotron Radiation Light Source, Shanghai Synchrotron Radiation Light Source, National Natural Science Foundation of China and Key Research and Development Program of the Ministry of Science and Technology. (Source: Science Network)

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