Upgrading of Biomass-Derived Oxygen-Containing Chemicals for Nitrogen-Containing Chemicals: Moving from Nanoparticles to Single Metal Sites and Metal Oxides.

Highly valued N-containing compounds are important and widely applied in the agricultural, pharmaceutical, and material industries. Oxidative or reductive coupling of oxygen-containing chemicals with ammonia or amines is one of the most promising routes to access N-containing compounds. Compared with carbonyl compounds (ketones and aldehydes), starting from alcohols for the synthesis of N-containing compounds is much more challenging. The synthesis of N-containing compounds with alcohols as the raw materials starts with the activation of alcohols via dehydrogenation or oxidation to produce aldehydes, which then couple with nitrogen sources (e.g., amines, NH3) to form imine intermediates. Imine intermediates can then be reduced to amines, oxidized to nitriles, or preserved. Integrating biomass alcohols, such as ethanol, into the N-containing compound supply chain can reduce the carbon footprint and enhance the economic competitiveness of biorefineries; however, activating the unreactive aliphatic α-Csp3-H is challenging due to its high bond dissociation energy (94-96 kcal mol-1). Another problem lies in the selective synthesis of chemicals bearing C═N bonds, including imines and N-heterocycles. Therefore, activating aliphatic biomass-derived alcohols for reductive or oxidative amination to N-containing compounds, especially unsaturated ones, remains a long-standing challenge. To address this issue, we adopted an adsorption-oriented catalyst design strategy. The electron-deficient active center is proposed to selectively adsorb and activate electron-rich nitro and hydroxyl groups while leaving the less polar C═N intermediate inert. This work pioneers a cost-effective pathway for upgrading biomass and synthesizing sustainable N-containing compounds. In this Account, we summarize our recent efforts to design novel catalysts for reductive or oxidative coupling of biomass-derived oxygen-containing chemicals and nitrogen sources, targeting N-containing compounds, especially imines and N-heterocycles. We start by presenting cases of positively charged single-atom catalysts for the construction of amines via base-free hydrogen transfer and extend their use to imine synthesis. Imines with lower polarity and more steric hindrance at the C═N moiety desorb from single-atom sites readily, thereby retaining excellent selectivity. We further highlight that reducible metal oxides are also active in reductive coupling after oxygen vacancies are introduced, acting as positively charged active centers for adsorption. Structurally diverse imines and N-heterocycles can be obtained with satisfactory yields. A nonreducible metal oxide also showed comparable activity if oxygen vacancies were introduced via size engineering. With the excellent catalytic performance of single metal sites and defective metal oxides in the activation of alcohols, we also show that novel routes for the oxidative coupling of alcohols were developed for the synthesis of nitriles with C≡N bonds and amides over metal single atoms and metal oxides. These efforts enabled the controllable synthesis of a broad range of N-containing compounds, including reductive-coupling products of imines and N-heterocycles bearing C═N bonds, as well as oxidative-coupling products of nitriles bearing C≡N bonds from alcohols.