Catalysis Science & Technology View Article Online PAPER View Journal | View Issue High catalytic activity of mesoporous Co–N/C catalysts for aerobic oxidative synthesis of nitriles† Cite this: Catal. Sci. Technol.,2016, 6,5746 Sensen Shang,ab Lianyue Wang,a Wen Dai,a Bo Chen,ab Ying Lva and Shuang Gao*a A high-efficiency and atom-economic synthetic strategy for nitriles by aerobic ammoxidation of alcohols is developed using a novel mesoporous cobalt-coordinated nitrogen-doped carbon catalyst (meso-Co–N/C) fabricated from a cobalt-coordinating polymer, which manifests superior activity towards the target reac- tion. The catalytic system features a broad substrate scope for various benzylic, allylic as well as heterocy- clic alcohols, providing good to excellent yields of the target products with high selectivities, albeit with 0.5 mol% Co catalyst loading. 11,11′-Bis(dipyrido[3,2-a:2′,3′-c]phenazinyl) (bidppz) with extreme thermostability is selected as a robust ligand bridge between cobalt ions, resulting in the homogeneous distribution of active sites at the atomic or subnanoscale level and high catalyst yield. Silica colloid or ordered mesoporous silica SBA-15 is employed to realize the mesoporous structure. The unprecedented performance of the meso- Received 27th January 2016, Co–N/C catalyst is attributed to its high Brunauer–Emmett–Teller (BET) surface area (up to 680 m2 g−1)with Accepted 14th April 2016 a well-controlled mesoporous structure and homogeneous distribution of active sites. Kinetic analysis demonstrates that the catalytic oxidation of benzyl alcohol to benzaldehyde is the turnover-limiting step DOI: 10.1039/c6cy00195e and that the apparent activation energy for benzonitrile synthesis is 61.5 kJ mol−1 and cationic species www.rsc.org/catalysis are involved in the reaction. Introduction neous ones is prevailing due to the ease of their separation and recycling. Notwithstanding, only a few heterogeneous examples – Nitriles are a very important class of compounds in chemistry have been reported.5h k as well as biology, playing a major role in the elaboration and Recently, carbon materials benefiting from special electronic composition of pharmaceuticals, agrochemicals and fine properties have been widely applied in many catalytic transfor- chemicals.1 Representative general synthetic strategies for nitrile mationsinmodernorganicchemistry.6 Furthermore, the intro- synthesis including Sandmeyer's reaction,2 transition metal- duction of heteroatoms (B, N, P, S, etc.) and non-noble transi- Published on 15 April 2016. Downloaded by Dalian Institute of Chemical Physics, CAS 12/2/2020 12:37:29 PM. catalyzed cross-coupling of aryl halides3 or direct C–H tion metals (Fe, Co, etc.) would be crucial for improving the functionalization of arenes4 commonly need highly toxic cya- catalytic efficiency of the materials by tuning their electronic nides and produce large amounts of inorganic salts as waste, properties and generating active sites.7 Typically, metal– which limit their application from a green chemistry perspec- nitrogen-doped carbon materials (M–N/C) were prepared from tive.Hence,thedevelopmentofnewenvironmentallyfriendly direct pyrolyzation of coordinating compound precursors and atom-economic procedures for nitrile synthesis is a very im- containing Fe or Co salts and small molecular nitrogen- portant subject in modern organic synthesis. In this respect, containing ligands adsorbed on carbon black. However, the ran- aerobic ammoxidation directly from renewable feedstock alco- dom location on carbon black and poor thermostability of these hols and aqueous ammonia would be an alternative approach coordinating compounds undoubtedly result in the agglomera- to generating nitriles which produce water as the only by-prod- tion of active metal species during the subsequent pyrolyzation, uct. Many achievements have been made in converting alcohols which largely decreases atomic catalytic efficiency. Moreover, to nitriles under both homogeneous and heterogeneous sys- the traditional method for preparing M–N/C materials fre- tems.5 From an economic and environmental viewpoint, the quently fails in controlling the specific surface area and porous careful design of heterogeneous catalysts instead of homoge- structure, leading to inadequate exposure of the active sites and relatively poor transport properties. a Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical In this contribution, we developed a novel mesoporous Physics, Chinese Academy of Sciences, Dalian 116023, China. cobalt–nitrogen-doped carbon material (meso-Co–N/C) derived E-mail: [email protected]; Fax: +86 0411 84379728; Tel: +86 0411 84379728 ′ ′ ′ b University of the Chinese Academy of Sciences, Beijing 100049, China from a Co-bidppz (11,11 -Bis(dipyrido[3,2-a:2 ,3 -c]phenazinyl)) † Electronic supplementary information (ESI) available: Additional characteriza- coordinating polymer using template synthesis. The Co-bidppz tion. See DOI: 10.1039/c6cy00195e polymer consists of alternating chains of N-containing ligands 5746 | Catal. Sci. Technol.,2016,6,5746–5753 This journal is © The Royal Society of Chemistry 2016 View Article Online Catalysis Science & Technology Paper and cobalt ions. The existence of a robust ligand bridge between thermally treated under flowing nitrogen at 700–900 °C and fi- cobalt ions efficiently confines the cobalt ions at the molecular nally etched in hydrogen fluoride to remove the templates, level, resulting in the homogeneous distribution of active sites resulting in the meso-Co–N/C catalysts. at the atomic or subnanoscale level, which is responsible for the Note that the extraordinarily high catalyst yield was achieved excellent catalytic performance. Silica colloid or ordered meso- owing to the extreme thermostability of the Co-bidppz polymer, porous silica SBA-15 is employed to realize the mesoporous as was exhibited in the thermogravimetric analysis (TGA) curve structure. It was worth noting that up to 80% catalyst yield was of the Co-bidppz/template composites (weight loss of only 10% achieved resulting from the extreme thermostability of the Co- from 25 °Cto900°C, Fig. S1†). For comparison, a Co–N/C cata- bidppz polymer. The as-fabricated mesoporous catalyst exhibits lyst was prepared by direct pyrolysis of the Co-bidppz polymer unprecedented superior activity towards the aerobic oxidative without using any template. synthesisofnitriles.Tothebestofourknowledge,thisisthe Exploratory catalytic experiments with different catalytic first time that such mesoporous functional carbon materials materials were performed for the aerobic oxidative synthesis have been applied as catalysts for the target reaction. The pro- of benzonitrile (2a) from benzyl alcohol (1a) and aqueous am- posed catalytic system features a broad substrate scope for vari- monia using molecular oxygen. Typically, this model reaction ous benzylic, allylic as well as heterocyclic alcohols, providing was performed at 130 °C using simply 5 bar dioxygen. Blank good to excellent yields of the target products with high selectiv- runs (without any catalysts) gave essentially no activity under ities. The relationship between the physicochemical properties identical conditions (Table 1, entry 1). The Co-bidppz poly- and catalytic behavior of the catalyst is discussed. The catalyst is mer exhibited hardly any conversion of benzyl alcohol, readily separated and reused five times without significant suggesting that cobalt ions coordinated to the bidppz ligand changes in activity, albeit with 0.5 mol% Co catalyst loading. were inactive for alcohol oxidation under the investigated con- ditions (Table 1, entry 2). The Co–N/C (800) catalyst obtained Results and discussion by direct pyrolysis of the Co-bidppz polymer at −800 °C with- out using any template afforded only 10% yield of 2a (Table 1, Fig. 1 depicts the chemical structure of bidppz and the synthetic entry 3). However, to our delight, the prepared meso-Co–N/C – process for the meso-Co N/C catalysts (the details of the fabrica- catalysts using silica colloid as a hard template showed excel- † – tion process are in the ESI ). The readily rotatable C Csingle lent activity for the model reaction, and meso-Co–N/C (800) bond linking two dppz moieties in the bidppz molecule exhibited the best catalytic performance to give 2a in 95% yield (marked in red, Fig. 1a) resulted in numerous configurations of at 130 °C (Table 1, entries 4–6), such a substantial performance bidppz and a network structure of Co-bidppz, which can be uti- improvement ensuring that the presence of porous structures lized as a self-supporting catalyst excluding the requirement of was very essential for the reaction. Upon employing ordered a carbon support. Moreover, nearly 20 wt% concentration of sta- mesoporous silica SBA-15 instead of silica colloid as a hard tem- ble pyridinic nitrogen of bidppz also allows for a high degree of plate, meso-Co–N/C (800, SBA-15) was also found to be an excel- – Co N coordination at high pyrolysis temperatures, benefiting lent catalyst producing 2a in 93% yield (Table 1, entry 7). the overall catalytic activity. To synthesize the meso-Co–N/C cat- alysts, various template materials were first dispersed in the Co- Table 1 Aerobic oxidative synthesis of benzonitrile over different bidppz polymer under vigorous stirring before evaporation of catalystsa Published on 15 April 2016. Downloaded by Dalian Institute of Chemical Physics, CAS 12/2/2020 12:37:29 PM. DMF. The obtained Co-bidppz/template composites were then Entry Catalyst Conv.b (%) Yieldb (%) 1 — <1 <1 2 Co-bidppz <10 3 Co-N/C (800) 19 10 4 Meso-Co-N/C (700) 93 90 5 Meso-Co-N/C (800) 95 95 6 Meso-Co-N/C (900) 93 89 7 Meso-Co-N/C (800, SBA-15) 93 93 8 Meso-Co-N/C (800, H+)2615 9c Meso-Co-N/C (800) 52 52 10d Meso-Co-N/C (800) 88 84 11e Meso-Co–N/C (800) 70 61 12f Meso-Co–N/C (800) 90 84 a Reaction conditions (unless specified otherwise): 1a (0.5 mmol), catalyst (0.5 mol% Co), 3.7 equiv. aq. NH3 (25–28% NH3 basis), 5 bar ° b O2,1mlt-amyl alcohol, 130 C, 18 h.