Reductive-coupling of nitro compounds and alcohols is a sustainable route for constructing C = N bonds in organonitrogen chemicals, yet challenging due to the inertness of α-C sp3 -H bond in alcohols and the vulnerability of C = N bonds towards hydrogenation. Here, we report the surprising catalytic activity of ultrafine alkaline-earth metal oxide MgO nanoclusters (0.9 ± 0.3 nm) that efficiently activate α-C sp3 -H bonds, facilitating the transfer hydrogenation and synthesis of value-added chemicals bearing C = N bonds with high to excellent yields (86–99%). Controlled experiments and characterizations showed the crucial role of oxygen vacancies (O v ) and local Mg environment (Mg-O bond) in MgO for substrate adsorption and activation via electronic interactions between substrate’s negatively charged oxygen atoms and O v sites in MgO nanoclusters. Theoretical calculation further confirmed that O v significantly lowered the energy barrier of the hydrogen atom transfer from α-C sp3 -H in ethanol to the nitro group in nitrobenzene (29.3 vs . 52.9 kcal/mol), which is the rate-determining step with the highest energy barrier in reductive-coupling reactions. Our method not only provides an efficient and sustainable pathway for synthesizing organonitrogen chemicals with C = N bonds but also inspires the exploration of main group element catalysts as alternatives to transition metal and noble metal catalysts for organic transformations.
Siyang Li,Shujuan Zhan,Jingxiang Sun,Lingai Yao,Jinzhu Zhu,Jinxi Feng,Ya Xiong,Shuanghong Tian
Highly reactive nanoclusters of metal oxides are extremely difficult to synthesize due to their thermodynamic instability. For the first time, MgO nanoclusters supported on sewage sludge-derived hierarchical porous carbon (MgO nanoclusters/SC) were successfully prepared by grafting 1.31 ± 0.03 mmol g−1 acidic groups on the SC, then dynamically controlling the chemisorption of Mg2+ on the modified SC within 20 s, and finally transforming the fixed Mg2+ into MgO nanoclusters with a size of less than 2.4 nm via thermal treatment. Moreover, the cluster size could be facilely tailored by controlling the dynamic adsorption time and initial aqueous Mg2+ concentration. The resulting five MgO nanoclusters/SC-20 s samples with 0.43–1.12% (mass percentage) Mg have a turnover frequency of 0.038 towards the catalytic ozonation of 4-chlorophenol, much higher than those of the bigger nanoclusters and nanocrystals (i.e. MgO/SC-20 s with 1.43% Mg, MgO/SC-3 min with 0.85–2.0% Mg). The pseudo-first-order reaction constant of MgO nanoclusters/SC with 1.12% Mg is 0.0126 min−1, 6.0 and 4.1 times that of SC and bulky MgO/SC, respectively. The presence of MgO nanoclusters/SC also enhanced the removal of cephalexin, florfenicol and sulfamethoxazole by 54.3%, 45.5%, and 57.4%, respectively. The high activity can be ascribed to the rich and highly under-coordinated Mg in the smaller MgO nanoclusters and the unblocked hierarchical pores of the SC support. Finally, DFT calculations revealed at the atomic level that the under-coordinated Mg in MgO nanoclusters could directly or indirectly activate ozone through the protonated hydroxyl or just hydroxyl bonded to the under-coordinated Mg. Except for (MgO)2, the smaller the nanocluster, the higher the activity. This work shed a light on the design and preparation of highly reactive nanoclusters of metal oxide catalysts for catalytic ozonation of refractory organic pollutants.
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