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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 reaction. The catalytic system features a broad substrate scope for various benzylic, allylic as well as heterocyclic 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

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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 conditions (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 without 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 catalysts, 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. Conversions and yields were Fig. 1 Schematic illustration of the synthetic process for Co–N/C and determined by GC (using biphenyl as standard) and confirmed by meso-Co–N/C catalysts: chemical structures of (a) bidppz and (b) the GC–MS. In the case of lower yields, benzaldehyde was detected as a corresponding Co-bidppz polymer. minor product. c DMF. d 1,4-Dioxane. e THF. f 5 Bar air.

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To unveil whether the transition metal acts as the activity site or it only facilitates the formation of active nitrogen– carbon functional sites, we examined the catalytic efficiency of meso-Co–N–C (800) after reducing the Co content under identical conditions. The as-synthesized meso-Co–N/C (800) was immersed in aqua regia to reduce the surviving Co species (from 0.44 wt% to 0.09 wt%) and the resulting material is denoted as meso-Co–N/C (800, H+). Meso-Co–N/C (800, H+) showed a sharp performance decline in the conversion of benzyl alcohol, demonstrating that the Co species may serve as active sites to catalyze the reaction (Table 1, entry 8). Besides t-amyl alcohol, a series of solvents including N,N- dimethylformamide (DMF), 1,4-dioxane and THF were also examined for the reaction, but the yield of benzonitrile (2a) all de- creased to some extent, suggesting that t-amyl alcohol was the best solvent for the target reaction among the investigated solvents (Table 1, entries 9–11). The conversion under 5 bar air proceeded with a comparable result to that under 5 bar pure oxygen (Table 1, entry 12). Furthermore, the meso-Co–N/C (800) catalyst could be reused at least five times with a slight decrease in activity, which was probably due to physical loss during catalyst recovery and recycling (Fig. 2). The results were in accor- dance with the inductively coupled plasma (ICP) mass analysis experiments, in which only traces of Co (less than 0.4% of the total cobalt) were detected in the solution collected by hot filtration after the reaction. The surface morphology of meso-Co–N/C (800) was deter- Fig. 3 (a) SEM image, (b) TEM image and (c) large-area SEM image along mined by SEM (Fig. 3a), which clearly indicated the introduction with the corresponding C-Ka, N-Ka, Co-Ka and O-Ka elemental maps of the meso-Co–N/C (800) catalyst; d) TEM image of the meso-Co–N/C of spherical pores into the resulting material. A representative (800, SBA-15) catalyst; e) SEM image of the Co–N/C (800) catalyst. transmission electron microscopy (TEM) image of meso-Co–N/C (800) is displayed in Fig. 3b. The interconnected vesicle-like frameworks of the catalyst were obviously observed, and the pore sizes had a range from several to dozens of nanometers. How- (XRD) pattern also further proved the absence of a discernible ever, we hardly observed any metal-containing nanoparticles by metallic Co phase and CoOx species (Fig. S3†). The selected-area careful HRTEM inspection because the etching agent (hydrogen electron diffraction (SAED) image revealed the multicrystalline fluoride) dissolves both the inorganic templates and metal/metal structure of the sample (Fig. S2b†). On the other hand, the Published on 15 April 2016. Downloaded by Dalian Institute of Chemical Physics, CAS 12/2/2020 12:37:29 PM. oxide nanoparticles (Fig. S2a†), although there was 0.43 at% Co energy-dispersive X-ray (EDX) spectrum clearly proved the pres- species which survived from acid leaching, as indicated by X-ray ence of cobalt (Fig. S2c†). Given that the Co-bidppz polymer con- photoelectron spectroscopy (XPS) analysis. The X-ray diffraction sists of alternating chains of the ligand and cobalt, which hin- ders the agglomeration of cobalt species during the subsequent heat treatment, it was reasonable to speculate that the active cobalt species that survived from acid leaching may exist at the atomic or subnanoscale level. In addition, element mapping (Fig. 3c) revealed that C, N, Co and O species were homoge- neously distributed throughout the whole mesoporous structure, undoubtedly signifying that the active cobalt species were uniformly distributed in the N-doped carbon matrix. Fig. 3d shows the well-defined linear array of the mesoporous structure of meso-Co–N/C (800, SBA-15), reflecting the successful introduction of SBA-15-type mesoporosity. For comparison, bulk aggre- gates were present in Co–N/C (800) prepared without using any template, and the smooth surface implied their poor porosity (Fig. 3e). From a significant performance improvement in the Fig. 2 Meso-Co–N/C (800)-catalyzed oxidative synthesis of benzonitrile. Reaction conditions: 1a (0.5 mmol), catalyst (0.5 mol% Co), 3.7 equiv. aq. aerobic ammoxidation of alcohols achieved by the mesoporous

NH3 (25–28% NH3 basis), 5 bar O2,1mlt-amyl alcohol, 130 °C, 18 h. catalysts, it could be deduced that the microstructure of the cata- Yields were determined by GC. lysts played pivotal roles in the target reaction. Notably, the

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mesoporous catalysts of discrepant morphologies prepared via Table 2 Surface areas, micropore surface areas, mesopore surface areas – different templates at the same pyrolysis temperature presented and chemical compositions of all Co N/C samples parallel activities towards the target reaction, which clearly dem- Content (wt%) SBET SMicro SMeso − − − onstrated that the aerobic ammoxidation of alcohols was not Sample (m2 g 1) (m2 g 1) (m2 g 1) Ca Na Ha Cob very sensitive to special pore structures. Co-N/C (800) 227 160 67 63.2 9.1 1.2 5.70 To uncover the underlying factors affecting the catalytic activ- Meso-Co–N/C (700) 506 82 424 62.3 12.1 1.3 0.27 ity, the textural properties of all Co–N/C samples were investi- Meso-Co–N/C (800) 680 157 523 69.8 12.3 1.2 0.44 – – gated by N2 adsorption desorption at 77 K (Fig. 4), and the re- Meso-Co N/C (900) 641 139 502 74.7 9.8 1.1 0.59 – sults are summarized in Table 2. The remarkable hysteresis loop Meso-Co N/C (800, 649 7 642 65.9 10.4 1.0 0.28 SBA-15) indicated the mesoporous nature of meso-Co–N/C (800) (Fig. 4a). The mesopore size distribution was centered at 12.6 nm a Measured by elemental analysis. b Measured by ICP. according to the Barrett–Joyner–Halenda (BJH) model (inset in Fig. 4a). The BET surface areas of meso-Co–N/C (700), (800), − − − (900) and (800, SBA-15) were 506 m2 g 1, 680 m2 g 1, 641 m2 g 1 − and 649 m2 g 1, respectively (Table 2). Notably, a large portion of the surface areas of the templated catalysts comprised meso- pores, and the corresponding mesoporous surface areas were − − − − 424 m2 g 1, 523 m2 g 1, 502 m2 g 1 and 642 m2 g 1,respectively (especially the meso-Co–N/C (800, SBA-15), nearly 99% of its total surface area is mesoporous, in spite of its non-uniform pore size). On the other hand, it was microporosity that dominated in the Co–N/C (800) catalyst, of which the mesoporous surface area − was only 67 m2 g 1, despite its total BET surface area of 2 −1 227 m g . Therefore, it could be speculated that the meso- Fig. 5 XPS spectra of (a) N 1s and (b) Co 2p3/2 in meso-Co–N/C (800). porous structures exert overwhelming influence on the excellent performance of the meso-Co–N/C catalysts, which was attributed to the easy access to active sites and improved masstransport properties. In addition, the chemical compositions of all Co–N/C 401.0 eV might be assigned to nitrogen in a graphitic samples were also characterized by elemental analysis and in- structure.8a,9 Moreover, the ratio of graphitic nitrogen to ductively coupled plasma atomic emission spectroscopy (ICP- pyridinic nitrogen in the meso-Co–N/C catalysts increased when AES). Co, C, N and H elements were mainly detected, and their the pyrolysis temperature was enhanced (e.g.,1.0for700°C, weight contents are shown in Table 2. The N contents were up 1.3 for 800 °C and 1.7 for 900 °C), implying a higher graphitiza- to 12.3%, which could assist in stronger coupling between metal tion degree at higher temperatures (Fig. S5†). The high- – species and N-doped carbon, and thus lead to higher catalytic resolution Co 2p3/2 spectrum of meso-Co N/C (800) can be activity. Meanwhile, the quantities of Co in the samples were less deconvoluted into two peaks with binding energies of 781.6 and than 0.6% because of acid leaching. 779.8 eV (Fig. 5b), which correspond to nitrogen- and oxygen- Published on 15 April 2016. Downloaded by Dalian Institute of Chemical Physics, CAS 12/2/2020 12:37:29 PM. XPS analysis was performed to analyze the chemical states of coordinated metals, respectively,10 while the weak shakeup satel- cobalt and nitrogen in the meso-Co–N/C samples (Fig. 5 and lite at around 786.0 eV from the main spin–orbit component S4–S6†). The high-resolution N 1s spectrum of the meso-Co–N/C was attributed to the surface hydroxyl species (i.e. Co–OH) (800) catalyst was fitted with only two different signals having (Fig. 5b).11 Previous studies have revealed that the cobalt species binding energies of 398.6 and 401.0 eV (Fig. 5a). The peak at stabilized by nitrogen were of vital importance in determining the binding energy of 398.6 eV could be attributed to pyridinic the activity of catalysts in oxidation reactions.7b,12 Therefore, the N, which should also include a contribution from nitrogen surviving cobalt species, which may be stabilized and activated bound to the metal (Me–N), due to the small difference between by nitrogen coordination and were uniformly distributed in the binding energies of N–Me and pyridinic N.8 The peak at the N-doped carbon matrix at the atomic or subnanoscale level, combined with easily accessible mesoporous structure, were highly beneficial for the catalytic performance. After the excellent activity of the meso-Co–N/C catalysts in the model reaction has been demonstrated, it was then ex- tended to the transformation of a series of structurally diverse benzylic alcohols under similar reaction conditions. As can be seen from Table 3, various benzylic alcohols were selectively converted into the corresponding benzonitriles in good to excellent yields, albeit with 0.5 mol% Co catalyst loading. The de- Fig. 4 N2 sorption isotherms of (a) meso-Co–N/C (800), (b) meso-Co–N/C (800, SBA-15) and (c) Co–N/C (800) catalysts. Insets show the pore size sired nitriles were obtained in >96% yields when benzylic alco- distributions of the corresponding samples obtained using the BJH method. hols substituted with strongly electron-donating groups such as

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Table 3 Catalytic aerobic oxidative synthesis of structurally diverse ni- Table 3 (continued) triles over the meso-Co–N/C (800) catalysta

Entry Alcohol Product Yieldb (%) Entry Alcohol Product Yieldb (%) 18 12d 1 95 1r 2r

1a 2a a Reaction conditions (unless specified otherwise): 1 (0.5 mmol), 35 mg – – 2 96 meso-Co N/C (800) catalyst (0.5 mol% Co), 3.7 equiv. aq. NH3 (25 28% b NH3 basis), 5 bar O2,1mlt-amyl alcohol, 130 °C, 18 h. Determined by GC and confirmed by GC–MS. c 24 h. d 70 mg catalyst, 1 ml 2 : 1 1b 2b water : n-heptane, 140 °C, 24 h. 3 >99

1c 2c p-OMe were employed (Table 3, entries 2–5). The different 4 >99 substituted positions on the phenyl ring had a marked influence on the reactivity of the reaction. As an example, the methyl group at the ortho position showed a little lower activity towards 1d 2d 5 >99 the desired nitrile in comparison with those in the para and meta positions (Table 3, entries 6–8). Apart from electron- 1e 2e donating groups, benzylic alcohols substituted with various − − − − 6 >99 electron-withdrawing groups, such as F, Cl, Br, NO2 and – – CF3, were selectively transformed (Table 3, entries 9 14). Nota- 1f 2f bly, even more sensitive allylic alcohols such as cinnamyl alco- 7 >99 hol could be converted into the corresponding nitrile in 90% yield (Table 3, entry 15). It is worth noting that heterocycles 1g 2g such as pyridine-2-methanol and thiophene-2-methanol could 8 97 also be used as substrates in the title reaction and afforded the 1h 2h heterocyclic nitriles in up to 88% yield (Table 3, entries 16 9 94 and 17). Attempts were made to oxidize the challenging aliphatic alcohols. Due to the aliphatic alcohols being inactive, 1i 2i a lower yield of the product was observed (Table 3, entry 18). 10 95 After the reaction, a trace amount of benzaldehyde was detected by GC–MS, which clearly exhibited that benzaldehyde 1j 2j was most likely to be the reaction intermediate. To gain more 11 >99

Published on 15 April 2016. Downloaded by Dalian Institute of Chemical Physics, CAS 12/2/2020 12:37:29 PM. insights into the reaction pathway, a kinetic study of the transformation of benzyl alcohol was carried out, and it was ob- 2k 1k served that benzaldehyde was detected along with the con- 12 97 sumption of the starting benzyl alcohol (Fig. 6a). The plot for the product can be well fitted with pseudo first-order reaction 1l 2l 13 >99

1m 2m 14 82c

1n 2n 15 90

1o 2o 16 85 Fig. 6 Kinetic study for the catalytic aerobic oxidative synthesis of 1p 2p benzonitrile over meso-Co–N/C (800) employing (a) benzyl alcohol 17 88 and (b) benzaldehyde as substrates. Reaction conditions: substrate (0.5

mmol), 35 mg meso-Co–N/C (800) (0.5 mol% Co), 3.7 equiv. aq. NH3 1q 2q (25–28% NH3 basis), 5 bar O2,1mlt-amyl alcohol, 130 °C. Yields were determined by GC using biphenyl as an internal standard.

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Fig. 7 (a) Time-on-stream course of conversion at different temperatures. Reaction conditions: benzyl alcohol (2 mmol), 35 mg meso-Co–N/C

(800), 3.7 equiv. aq. NH3 (25–28% NH3 basis), 5 bar O2,1mlt-amyl alcohol. (b) Arrhenius plot for the catalytic aerobic oxidative synthesis of benzonitrile from benzyl alcohol. Yields were determined by GC using biphenyl as an internal standard. (c) Hammett plot for the aerobic ammoxidation

of the para-substituted benzyl alcohol; logĲkX/kH) was obtained from the ratio of conversion with a reaction time of 1.5 h.

kinetics in the range of 0 to 2 h, and the corresponding rate − constant was 0.29 h 1.Notably,whenbenzaldehydewas employed as the substrate under the standard conditions as − shown in Fig. 6b, the corresponding rate constant was 2.9 h 1, Scheme 2 Gram-scale reactions: 10 mmol benzyl alcohol, 2.8 ml aq. 10-fold higher than that for the whole reaction. These results NH3 (25–28% NH3 basis), 430 mg catalyst (0.3 mol% Co), 5 bar O2,9ml clearly indicated that benzaldehyde is the reaction intermediate t-amyl alcohol, 130 °C, 36 h. Yields were determined by GC. and the rate of the whole reaction depends on the catalytic oxidation of benzyl alcohol to benzaldehyde. The apparent activation energy, Ea, was determined from the Arrhenius plot in the Conclusions temperature range of 110–140 °C and the value was 61.5 kJ − mol 1 (Fig. 7a and b). Finally, the relative rates for the aerobic In summary, the meso-Co–N/C catalyst derived from a Co-

ammoxidation of para-substituted benzyl alcohols (CF3,Cl,H, bidppz polymer demonstrates superior catalytic activity in Me, OMe) were investigated (Fig. 7c). A reasonable linear rela- the aerobic oxidative synthesis of nitriles from alcohols apply-

tionship between logĲkX/kH)andtheBrown–Okamoto constant ing molecular oxygen and aq. ammonia. A series of benzylic σ+ was established, and the resulting reaction constant ρ was alcohols, allylic alcohols and also heterocyclic alcohols could −1.2, indicating that the intermediate cationic species were be converted to their corresponding nitriles in good to excel- involved in the reaction. lent yields. The outstanding catalytic performance of the pre- All the above-mentioned experimental evidence indicates pared catalysts is strongly associated with their well-defined that the present meso-Co–N/C-catalyzed transformation pos- mesoporous structures, high mesoporous surface area and sibly proceeds through the following sequence of reactions homogeneous distribution of numerous active cobalt species. Published on 15 April 2016. Downloaded by Dalian Institute of Chemical Physics, CAS 12/2/2020 12:37:29 PM. (Scheme 1): 1) aerobic oxidative dehydrogenation of an alco- Moreover, the as-prepared catalyst is easily recycled and can hol to an aldehyde, which had been proven to be the be reused at least five times without a significant loss of cata- turnover-limiting step; 2) dehydrative condensation of the lytic activity. This work would provide an alternative, highly aldehyde and ammonia to an aldimine via a hemiaminal efficient and environmentally benign methodology for the intermediate; 3) aerobic oxidative dehydrogenation of the synthesis of nitriles. aldimine to a nitrile. Notably, this unstable imine is immedi- ately oxidized to give nitrile as the final product. In this reac- Experimental tion sequence, water is produced as the only by-product. Materials In a final set of experiments, we examined the synthetic ′ Ĳ ĳ ′ ′ utility of our catalyst system under scale-up conditions. We 11,11 -Bis dipyrido 3,2-a:2 ,3 -c]phenazinyl) (bidppz) was pur- performed gram-scale reactions for benzyl alcohol as shown chased from Jinan Henghua Sci. & Tec. Co., Ltd. All starting in Scheme 2, and we obtained the corresponding benzonitrile materials and solvents were obtained from commercial sup- product in a yield of 92%, albeit employing a very low pliers and used without further purification. amount of catalyst (0.3 mol% Co). Catalyst preparation Synthesis of SBA-15 (ref. 13). A solution of P123 : 2 M HCl :

TEOS : H2O = 2 : 60 : 4.25 : 15 (mass ratio) was stirred for 12 h at 40 °C and then hydrothermally treated at 110 °C for 24 h. Scheme 1 Possible reaction pathway for the meso-Co–N/C-catalyzed The obtained solid was thoroughly washed with water, dried oxidative synthesis of benzylic nitriles. under vacuum at 100 °C overnight and calcined at 550 °C for

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8 h to remove the template (P123: triblock copolymer Procedure for the synthesis of nitriles EO20PO70EO20, Mw = 5800; TEOS: tetraethyl orthosilicate). The magnetic stirring bar and corresponding alcohol were – Synthesis of meso-Co N/C catalysts. The typical meso- transferred into a glass vial and then the solvent was added. Co–N/C catalysts were prepared as follows: to synthesize the Co- The catalyst was added followed by the addition of aq. NH3. bidppz polymer, 270 mg (0.49 mmol) of bidppz and 123.2 mg Then, the vial was fitted with a septum, cap and needle. The Ĳ · (0.49 mmol) of Co OAc)2 4H2O were added to 40 mL of DMF un- reaction vials were placed into an autoclave and the autoclave ° der vigorous stirring. The mixture was refluxed at 160 Cfor1h. was pressurized to 5 bar molecular oxygen. The autoclave was Then, 400 mg of the template material was added to the mixture placed in an oil bath to get the required reaction temperature. ° under vigorous stirring. After evaporation of DMF at 180 C, the The reaction mixture was stirred for the required time and after obtained Co-bidppz/template composites were then pyrolyzed completion of the reaction, the autoclave was cooled to room ° −1 under flowing nitrogen with a heating ramp rate of 5 Cmin temperature. The remaining oxygen was discharged and the – ° to the desired temperatures (700 900 C) and the temperature samples were removed from the autoclave. To the individual – was maintained for 2 h. Finally, 220 mg of meso-Co N/C (x)cat- vials, biphenyl as a standard was added and the reaction prod- alysts, where x indicated the pyrolysis temperature, were uct was diluted with t-amyl alcohol followed by filtration and obtained after removal of the templates by HF (10 wt%) etching then analysed by GC and GC–mass spectrometry (GC–MS). for 24 h at room temperature. Qualitative and quantitative analyses of all products were car- – Synthesis of Co N/C (800) by direct pyrolysis of Co-bidppz ried out using GC and GC–MS. polymer. In a typical synthesis, 270 mg (0.49 mmol) of bidppz Ĳ · and 123 mg (0.49 mmol) of Co OAc)2 4H2O were added to 40 Recovery and reuse of meso-Co–N/C mL of DMF under vigorous stirring. The mixture was refluxed The catalyst recycling experiments were carried out using at 160 °C for 1 h. After evaporation of DMF at 180 °C, the benzyl alcohol as the model substrate applying standard pro- obtained Co-bidppz polymers were then directly pyrolyzed cedures under the following reaction conditions: 0.5 mmol under flowing nitrogen with a heating ramp rate of 5 °C − benzyl alcohol, 35 mg meso-Co–N/C (800), 140 μl aq. NH , min 1 to 800 °C and the temperature was maintained for 2 h. 3 130 °C, 18 h, 5 bar O ,1mlt-amyl alcohol. After completion Synthesis of meso-Co–N/C (800, H+). The prepared meso- 2 of the reaction, in each run, the catalyst was separated by Co–N/C (800) catalyst was dispersed in aqua regia for 24 h. The centrifugation, washed with methanol and calcined at 400 °C solid was washed several times with water, collected by centrifu- under N for 2 h. Then, the catalyst was used for the next gationandfinallydriedinanovenat80°Covernight.The 2 run. Alternatively, after completion of the reaction, the reac- obtained material was denoted as meso-Co–N/C (800, H+). tion solution was carefully decanted, then the fresh solvent, Catalyst characterization substrate and ammonia were added and the reaction was performed. Conversions and yields were determined by GC Thermogravimetric analysis (TGA) of the samples was analysis using biphenyl as a standard. conducted by using a Perkin-Elmer TGA-2 thermogravimetric ° analyzer in N2 from room temperature to 900 C at a rate of − Procedure for gram-scale reaction 10 °C min 1. The Co loadings of the catalysts were measured To a 300 ml Teflon-fitted autoclave, the magnetic stirring bar by inductively coupled plasma atomic emission spectroscopy

Published on 15 April 2016. Downloaded by Dalian Institute of Chemical Physics, CAS 12/2/2020 12:37:29 PM. and corresponding alcohol were transferred and then the sol- (ICP-AES) using a Perkin-Elmer OPTIMA 3300DV. The detec- vent was added. The meso-Co–N/C (800) catalyst was added tion limit was 0.10 ppm. The X-ray powder diffraction (XRD) followed by the addition of aq. NH . The autoclave was pres- patterns of the samples were collected using a Rigaku/Max-3A 3 surized to 5 bar molecular oxygen. The autoclave was placed X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å), in an oil bath to get the required reaction temperature. The with the operation voltage and current maintained at 40 kV reaction mixtures were stirred for the required time and after and 200 mA, respectively. N2 sorption isotherms were mea- completion of the reaction, the autoclave was cooled to room sured at 77 K using a QuadraSorb SI4 station, and the sam- temperature. Biphenyl as a standard was added and the reac- ples were degassed at 300 °C for 6 h before the measure- tion product was diluted with t-amyl alcohol followed by fil- ments. The pore size distribution (PSD) plot was recorded tration and then analysed by GC. from the adsorption branch of the isotherm based on the Barrett–Joyner–Halenda (BJH) model. Scanning electron microscopy (SEM) images were collected using a JSM-7800F Acknowledgements microscope operating at an accelerating voltage of 20 kV. We gratefully acknowledge financial support from the National Transmission electron microscopy (TEM) images were ac- Natural Science Foundation of China (21403219, 21273225). quired with a JEM-2100 microscope. Surface compositions were determined by X-ray photoelectron spectroscopy (XPS) Notes and references using a Thermo Scientific ESCALAB 250Xi instrument with an Al Kα radiation anode (hv = 1486.6 eV), and the C 1s line 1(a) A. J. Fatiadi, In Preparation and Synthetic Applications of (284.8 eV) was used as the reference to correct the binding Cyano Compounds, ed. S. Patai and Z. Rappaport, Wiley, New energies (BE). York, 1983; (b) P. Magnus, D. A. Scott and M. R. Fielding,

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