Chitosan-derived N-doped carbon catalysts with a metallic core for the oxidative dehydrogenation of NH–NH bonds

Sustainable metal-encased (Ni–Co/Fe/Cu)@N-doped-C catalysts were prepared from bio-waste and used for the oxidative dehydrogenation reaction. A unique combination of bimetals, in situ N doping, and porous carbon surfaces resulted in the formation of the effective “three-in-one” catalysts. These N-doped graphene-like carbon shells with bimetals were synthesized via the complexation of metal salts with chitosan and the subsequent pyrolysis at 700 °C. A well-developed thin-layer structure with large lateral dimensions could be obtained by using Ni–Fe as the precursor. Importantly, the Ni–Fe@N-doped-C catalyst was found to be superior for the dehydrogenation of hydrazobenzene under additive/oxidant-free conditions compared to the conventional and other synthesized catalysts. Characterizations by TEM and XPS accompanied by BET analysis revealed that the enhanced catalytic properties of the catalysts arose from their bimetals and could be attributed to the graphitic shell structure and graphitic N species, respectively.

were purchased from the standard chemical suppliers such as Sigma Aldrich and Duksan. All the chemicals were reagent grade and used as received without further purification.

Procedures:
i) General procedure for the synthesis of catalysts: The chitosan supported bimetallic materials were synthesized according to the following typical procedure: In a 500mL round-bottomed flask provided with a magnetic stir bar, 1.5 gm of chitosan was suspended into 100 mL of water. 250 mg of each NiCl 2 .6H 2 O and metal precursors (Co/Fe/Cu) were added into above suspension solution. The pH was maintained around 9 by adding 25% aqueous ammonia solution and the mixture was stirred continuously for 24 h.
Hereafter, the obtained solid was repeatedly washed with water to remove excess impurities and reactants. The solid was then separated by centrifugation process (500 rpm, 5 min) and dried in vacuum at 60 o C overnight. Later, the dried sample was transferred into a tubular furnace for pyrolysis. The furnace then flushed with argon and sample was heated to 700 o C temperature with a temperature gradient of 25 °C/min and the same temperature was held for 2h. After that, the furnace was cooled down to rt. Argon was purged through the furnace constantly during the whole process. The obtained black material was washed with water and ethanol to remove the impurities. The catalysts named as Ni-M@N-doped-C (M=Co/Fe/Cu) were obtained after drying the samples at 60 o C in an oven and stored in a screw-capped vial at room temperature. For the preparation of monometallic catalysts, same procedure was followed using single metal precursors.
ii) General procedure for the synthesis of azobenzene: Typically, a calculated amount of hydrazobenzene (1 mmol), Ni-Fe@N-doped-C (5 wt%) in ethanol (3 mL) were placed in a 10 mL glass vial, and K 2 CO 3 (1 mmol) was added to the mixture under air atmospheric with a magnetic stirring to initiate the reaction at 30 °C for 12 h. After the reaction was completed, the catalyst was separated by external magnet and crude mass was subjected for column chromatography. The obtained product was characterized over NMR spectroscopy and observed data found consistent with the literature report 1 . After each cycle, the catalyst was isolated from the solution by external magnet, washed three times with water and acetone, dried under vacuum to remove the residual solvent and then reused for another reaction cycle.

Additional experiments
Large scale reaction: The mixture of hydrazobenzene (1.84g, 10 mmol), K 2 CO 3 (1.38g, 10 mmol), Ni-Fe@N-doped-C (5 wt%) in ethanol (21 mL) were placed in a 50 mL round bottle flask, and mixture stirred under air atmospheric at 30 °C for 12 h. After the reaction was completed, the catalyst was separated 6 by external magnet and crude mass was subjected for column chromatography. The yield of obtained azobenzene was observed in 91% (1.66 g). Figure S4. Gram-scale synthesis of azobenzene over Ni-Fe@N-doped-C. Figure S4. Separation of the Ni-Fe@N-doped-C catalyst (a,b) after reaction and (c) separation by using permanent magnet.