Activation of C–H bonds has received considerable attention in the past decade as this provides an efficient and atom-economy strategy to access a plethora of value-added chemicals from easily available substrates [1]. Nevertheless, the high stability of C–H bonds poses significant challenge for selective functionalization. For instance, the selective oxidation of C–H bonds in hydrocarbons has proved to be high useful in synthesis of alcohol/ketone feedstocks in both areas of C1 chemistry [2] and organic synthesis [3, 4]. However, given the generally high dissociation energy of C–H bonds and possible over-oxidation, it remains challenging to realize both high reactivity and selectivity.

Recently, a team led by Profs. Tao Zhang and Aiqin Wang at the Dalian Institute of Chemical Physics (DICP) has reported an interesting work on the selective oxidation of C–H bonds in aromatic and aliphatic hydrocarbons [5]. The reactions proceeded at room temperature, by using an excess or a stoichiometric amount of tert-butyl hydroperoxide (TBHP) as an oxidant and an atomically dispersed (single-atom catalyst) Fe-N-C as the catalyst. As given in Table 1, a wide scope of substrates, such as various aromatic hydrocarbons with an electron-donating group (–OMe) or electron-withdrawing group (–NO₂), heterocyclic substrates and an aliphatic hydrocarbon (cyclohexane) could be smoothly transformed into the corresponding ketones with > 98% selectivity at high conversions. In fact, the performance of the heterogeneous Fe-N-C catalyst is well comparable to some homogeneous catalysts such as [Cu((R, R)-BPBP)]⁺ complex [6] in terms of both activity and selectivity, but it offers additional advantages such as excellent reusability and water solvent-compatibility.

表 1 (Table 1)

Table 1 Selective oxidation of hydrocarbons in water with Fe-N-C-700 as catalyst. Entry Substrate Product Conv. (%) Selec. (%) 1 99 99 2 99 99 3 95 98 4 79 94 5 99 80 6 98 99 7 99 99 8 85 99 9 62 99 (a/b=1:2.6) Reaction conditions: 10.0 mg of catalyst, 0.5 mmol of substrates, room temperature for 7 h, 500 μL of TBHP as a 70 wt % aqueous solution (7 equiv) diluted with 6.5 mL of H2O.

Table 1 Selective oxidation of hydrocarbons in water with Fe-N-C-700 as catalyst.

A more attractive feature of this work lies in the establishment of structure-performance relationship, which is a long-standing interest in the area of heterogeneous catalysis. The authors made in-depth characterizations of the catalyst using a variety of advanced techniques including aberration-corrected HAADF-STEM, XPS, XAS, ESR, and M ssbauer spectroscopy. Thus, the coordination sphere of the Fe atom (which is actually Fe³⁺) included only nitrogen atoms so as to form Fe-N_x (x = 4, 5, and 6) moieties. Significantly, the catalytic activity is closely correlated to the specific structure of the Fe-N_x. As shown in Fig. 1, the most reactive moiety for the selective C-H oxidation is ascribed to the medium-spin Fe^Ⅲ-N₅ structure, which is at least 1 order of magnitude more reactive than the high-spin and low-spin Fe^Ⅲ-N₆ structures and 3 times more reactive than the Fe^Ⅱ-N₄ structure. The identification of the Fe-N₅ active site will serve to provide guidelines to design more active catalysts in this system by maximizing the number of the Fe-N₅ moiety since it only accounts for 18% of the total Fe species in this work.

Fig. 1

Fig. 1. Schematic diagram of Fe-N-C single-atom catalyst for C–H bond selective oxidation.

The Fe-N_x-C catalyst in this work can be viewed as a single-atom catalyst (SAC) if the surrounding N_x atoms are considered as a robust multidentate ligand. Single-atom catalysis has recently become a very hot topic since it bodes well to maximize the metal utilization efficiency, offer unique selectivity, and bridge the heterogeneous and homogeneous catalysis due to the uniform active sites [7-10]. In this work, the Fe-N₅ active site is reminiscent of the structure of a hemoglobin molecule [11] which is responsible for the adsorption and activation of oxygen in living cells (Fig. 1). In this context, the Fe-N₅ structure provides a good example to bridge mononuclear enzyme catalyst and heterogeneous single-atom catalyst. However, a tiny change in the micro-environmental surrounding may greatly affect the reactivity and selectivity, as exemplified by the poor capability of the heterogeneous Fe-N₅ sites for activation of molecular oxygen. It is believed that this work will open a door of ample space for the design of robust heterogeneous single-atom catalysts to mimic the high activity and selectivity of enzymes as well as molecular catalysts.