We report the synthesis of sustainable and reusable non-noble transition-metal (cobalt) nanocatalysts containing N-, O-, and S-tridoped carbon nanotube (Co@NOSC) composites. The expensive and benign carrageenan served as the source of carbon, oxygen, and sulfur, whereas urea served as the nitrogen source. The material was prepared via direct mixing of precursors and freeze-drying followed by carbonization under nitrogen at 900 °C. Co@NOSC catalysts comprising a Co inner core and outer electron-rich heteroatom-doped carbon shell were thoroughly characterized using various techniques, namely, TEM, HRTEM, STEM elemental mapping, XPS, BET, and ICP-MS. The utility of the Co@NOSC catalyst was explored for base-free selective oxidative esterification of alcohols to the corresponding esters under mild reaction conditions; excellent conversions (up to 97%) and selectivities (up to 99%) were discerned. Furthermore, the substrate scope was explored for the cross-esterification of benzyl alcohol with long-chain alcohols (up to 98%) and lactonization of diols (up to 68%). The heterogeneous nature and stability of the catalyst facilitated by its ease of separation for long-term performance and recycling studies showed that the catalyst was robust and remained active even after six recycling experiments. EPR measurements were performed to deduce the reaction mechanism in the presence of POBN (α-(4-pyridyl-1-oxide)-N-tert-butylnitrone) as a spin-trapping agent, which confirmed the formation of ˙CH₂OH radicals and H˙ radicals, wherein the solvent plays an active role in a nonconventional manner. A plausible mechanism was proposed for the oxidative esterification of alcohols on the basis of EPR findings. The presence of a cobalt core along with cobalt oxide and the electron-rich N-, O-, and S-doped carbon shell displayed synergistic effects to afford good to excellent yields of products.

中文翻译：

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钴基N，O和S掺杂的碳可作为高效的多功能可持续催化剂，用于醇的无碱选择性氧化酯化

我们报告了可持续的和可重复使用的非贵族过渡金属（钴）纳米催化剂的合成，其中包含N，O和S掺杂的碳纳米管（Co @ NOSC）复合材料。昂贵而良性的角叉菜胶用作碳，氧和硫的来源，而尿素用作氮源。该材料是通过直接将前体混合并冷冻干燥，然后在氮气中于900°C碳化。使用各种技术，包括TEM，HRTEM，STEM元素图谱，XPS，BET和ICP-MS，对包含Co内核和富含外电子的杂原子掺杂碳壳的Co @ NOSC催化剂进行了全面表征。在温和的反应条件下，探索了Co @ NOSC催化剂在醇的无碱选择性氧化酯化成相应的酯中的用途。可以看到出色的转化率（高达97％）和选择性（高达99％）。此外，探索了底物范围，用于苯甲醇与长链醇的交酯化（最高98％）和二醇的内酯化（最高68％）。催化剂易于分离以实现长期性能和循环利用研究，促进了催化剂的非均质性质和稳定性，表明该催化剂坚固耐用，即使经过六次循环实验仍保持活性。进行EPR测量以推断在POBN（α-（4-吡啶基-1-氧化物）-N-叔-butylnitrone）作为自旋捕获剂，这证实CH的形成₂ OH自由基和H自由基，其中所述溶剂中起着非常规的方式积极的作用。根据EPR的发现，提出了一种可行的醇氧化氧化机理。钴核以及氧化钴和富电子的N，O和S掺杂的碳壳的存在表现出协同效应，从而提供了良好的产品良率。