Cobalt Nanoparticles Apically Encapsulated by Nitrogen‐doped Carbon Nanotubes for Oxidative Dehydrogenation and Transfer Hydrogenation of N‐Heterocycles

Abstract

It is important to develop a highly active and stable transition‐metal catalyst with dual‐functional properties in the reversible transformations between various saturated and unsaturated N‐heterocycles. Herein, we prepared the cobalt nanoparticles (Co NPs) apically encapsulated by the N‐doped carbon nanotubes catalyst (Co@NCNTs) via a multiple pyrolysis of low‐cost dicyandiamide and cobalt (II) acetylacetonate. The catalyst shows excellent activity and recyclability towards the oxidative dehydrogenation (ODH) and the catalytic transfer hydrogenation (CTH) for various N‐heterocycles. The structure of outer N‐doped carbon nanotubes (NCNTs) can protect Co NPs from aggregation and leaching. Moreover, the encapsulated Co NPs and the NCNTs may generate a synergistic effect. Both of them facilitate the high performance. The poisoning tests with KSCN were to clarify the different active sites for ODH and CTH reactions: the Co NPs could modify the NCNTs through electrons redistribution, therefore the NCNTs could directly activate O2 in ODH. The encapsulated Co NPs is enhanced by the doped N atoms which is good for the H2 activation in CTH. What's more, the mechanisms of ODH and CTH reactions were also proposed. This work provides a facile and low‐cost method to design catalysts, which are dual‐functional, highly active and stable, for industrial applications.

Catalytically Active Co−Nx Species Stabilized on Nitrogen‐doped Porous Carbon for Efficient Hydrogenation and Dehydrogenation of N‐heteroarenes

“…The best KCC‐1@Co−N−C‐800 catalyst has a high TOF value of 8.24 h −1 , which is 3–24 times higher than the values reported in the transition‐metal‐based catalysts towards the base/base‐free catalytic transfer hydrogenation or hydrogenation for quinoline (Table S2), such as homogeneous Co(BF 4 ) 2 ⋅ 6H 2 O, Mn(CO) 5 Br, and [Co II (L1)(CH 3 CN)](BF 4 ) 2 catalysts and heterogeneous Co@NCNTs‐800, Co/Melamine‐2@C‐700, ISAS−Co/OPNC, Co@OMNC‐700, CoOx@CN, Co@NGS‐800 and Co 0 /ZnCl 2 catalysts . A scale‐up experiment was performed to ascertain the practicality of the KCC‐1@Co−N−C‐800 catalyst.…”

“…After confirming the optimized reaction conditions, the ODH reactions of other saturated N‐heterocycles are tested to confirm the extensive applicability of KCC‐1@Co−N−C‐800. 1,2,3,4‐tetrahydroisoquinoline and 1‐methyl‐1,2,3,4‐tetrahydroisoquinoline were converted into the corresponding products with high conversions (Table , entries 2–3). Moreover, the indoline and 2‐methylindoline were also dehydrogenated with high conversions (Table , entries 4–5).…”

“…In the presence of electron‐donating groups (such as −CH 3 , −OCH 3 , and −OH) in the quinoline derivatives, the substrates were converted into products with excellent conversions and selectivities (Table , entries 3–6). A conversion of 98.2 % for phthalazine was obtained (Table , entry 7). Furthermore, unsaturated N‐heterocycles with electron‐withdrawing groups (such as −Br and −NO 2 ) were also applicable for CTH, conversions of 76.8 % for 4‐bromoisoquinoline and 97.9 % for 2‐methyl‐6‐nitroquinoline were observed (Table , entries 8–9).…”

“…A conversion of 98.2 % for phthalazine was obtained (Table , entry 7). Furthermore, unsaturated N‐heterocycles with electron‐withdrawing groups (such as −Br and −NO 2 ) were also applicable for CTH, conversions of 76.8 % for 4‐bromoisoquinoline and 97.9 % for 2‐methyl‐6‐nitroquinoline were observed (Table , entries 8–9). The KCC‐1@Co−N−C‐800 catalyst was also reused for six cycles without any obvious decrease in the activity (Figure b).…”

A decennary update on applications of metal nanoparticles (MNPs) in the synthesis of nitrogen- and oxygen-containing heterocyclic scaffolds

“…CoNPs encapsulated in nitrogen-doped CNTs Co@NCTs-800 catalyzed the oxidative dehydrogenation of partially saturated heterocycles such as 1,2,3,4-tetrahydro quinolines (36e), 1,2,3,4tetrahydroisoquinoline (40b), and 2,3-dihydroindole (161c, Scheme 78) in excellent conversions into 35e/39b/88e, respectively, with methanol as the solvent and potassium carbonate as the base. 161 The ne tuning of the reaction conditions in toluene using the same catalyst and higher equivalents of formic acid also enabled the formylation of 36e/40b/161c (Scheme 78) in excellent conversions into 36f/40c/88f, respectively. Under the latter conditions, the catalytic transfer hydrogenation of heteroaromatics such as quinolines (35a), isoquinoline (39c), and phthalazine (163) (Scheme 79) was achieved successfully.…”