Solvent-Dependent, Formic Acid-Mediated, Selective Reduction and

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Solvent-Dependent, Formic Acid-Mediated, Selective Reduction and Reductive N‑Formylation of N‑Heterocyclic Arenes with Sustainable Cobalt-Embedded N‑Doped Porous Carbon Catalyst Ashish Kumar Kar† and Rajendra Srivastava*,† †

Department of Chemistry, Indian Institute of Technology Ropar, Nangal Road, Rupnagar 140001, Punjab, India

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S Supporting Information *

ABSTRACT: The usefulness of formic acid is demonstrated here as a sustainable source of H2 and H2 + CO for the selective reduction and reductive N-formylation, respectively, of N-heterocyclic arenes. We synthesized a cobaltembedded porous N-doped carbon catalyst for this purpose. The formation of the Co-embedded porous N-doped carbon framework is confirmed using powder X-ray diffraction, N2-sorption, Raman spectrometry, transmission electron microscopy, and XPS measurements. The catalyst exhibits a formic acid-mediated selective reduction of N-arenes in the fused aromatic heterocyclic ring system in water, whereas the catalyst exclusively produces a reductive N-formylated product in toluene. A very low amount of surface Co (1.35 wt %)-embedded N-doped carbon framework provides 92.1% quinoline conversion with 87% selectivity of 1,2,3,4-tetrahydroquinoline in transfer hydrogenation (with TOF of 16.8 h−1) using formic acid as the H2 source. Moreover, the catalyst exclusively catalyzes the formation of N-formyltetrahydroquinoline with 98% yield using formic acid as the H2 + CO source in the reductive N-formylation reaction (with TOF of 13.4 h−1). The structure−activity relationship is established using quinoline adsorption, CO2-temperature program desorption, control reactions, and other spectroscopic measurements. After five recycles, 6.8 ppm of Co is lost from the catalyst. Only a marginal loss [selective reduction: quinoline conversion (fresh catalyst = 92.1%; after five recycles = 87.4%); reductive N-formylation: (fresh catalyst = 98.0%; after five recycles = 92.4%)] in the catalyst activity is observed. Moreover, the catalyst framework remains stable after five recycles. Further, the heterogeneity and the catalytic efficiency of the process is confirmed from hot filtration and KSCN poisoning tests, respectively. Owing to the development of a stable and recyclable porous catalyst in this study, and also its success in the synthesis of pharmaceutically important synthetic intermediates, we expect to fulfill the requirements for its commercial implementation in various industrial processes. KEYWORDS: Selective reduction, Reductive N-formylation, N-doped porous carbon, Cobalt catalyst, Solvent-dependent activity

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INTRODUCTION Selective reduction of N-arenes in the fused aromatic heterocyclic ring system is very interesting because hydrogenated N-heterocycles constitute an integral part of many pharmaceutically important drugs (examples: diclofensine, nomifensine, oxamniquine), anti-HIV drugs, and lipid-controlling compounds (examples: phenanthroline and quinoxaline) and naturally occurring alkaloids.1−4 Additionally, reduced Nformylated heterocycles are among another class of important synthetic intermediates widely used in the pharmaceutical industries.5 The most general approach for the selective reduction of N-arenes is with the use of abundant gas, H2, whereas, reductive N-formylation is known to take place in the presence of H2 + CO to produce reductive N-formylated heterocycles.6,7 However, these reactions are accomplished using catalysts containing costly noble metals (examples: Pt, Pd, Au, Ru, Rh, Ir).8−13 Moreover, high H2 pressure and special reaction setup are required for these reactions which are seldom industrially encouraging. The industrial production of © XXXX American Chemical Society

H2 by steam reforming of wood produces a large amount of CO2 along with H2 which is a serious environmental concern. Therefore, many researchers are developing alternative strategies for the production of H2 which is not only a source of reduction in chemical industries but also a prominent source for the alternative energy generation devices such as fuel cells. The most interesting environmentally benign and sustainable method for the production of H2 is the visible light-assisted splitting of water for which significant research is being carried out throughout the world.14,15 The selection of a suitable hydrogen donor is crucial for the selective reduction of N-arenes in fused heterocyclic systems because the hydrogenation of an N-heterocyclic ring is difficult due to the delocalization of strong pi bonds in the heterocyclic ring.16 For this kind of reduction, a process known as transfer Received: April 25, 2019 Revised: July 1, 2019 Published: July 3, 2019 A

DOI: 10.1021/acssuschemeng.9b02307 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Stepwise Synthesis Protocol Adopted for Preparation of Co@C-Nx Materials

product,26 where the nitrogen species present in the support matrix provide optimum interaction with the reactants/ products26 and (d) strong interaction with the active metal species so that it resists leaching of metal species in the reaction medium, especially in the presence of formic acid as a hydrogen donor. Considering the above facts, for this purpose, nitrogen-containing porous carbon support materials seems to be ideal. Our group has developed a wide range of carbon nitride (C3N4) and its composite materials for conventional catalysis and photocatalysis.27 Although suitable for photocatalysis, C3N4 appears to be inferior in terms of surface area and porosity and, hence, not suitable for this purpose. We also recently reported the Cu/CuOx-supported carbon materials, derived from Cu-based metal organic frameworks, for the selective C-N coupling and oxidation reactions that exhibit better surface area than carbon nitride materials.28 Efforts have been made by researchers to develop metal nanoparticleembedded N-containing porous carbon materials via different synthesis strategies for photocatalysis,29 electrocatalysis,30 selective reductions of N-arenes,31 and biofuel upgrdation.32,33 Considering these facts related to the design of a heterogeneous catalyst for the selective reduction and reductive N-formylation of N-heterocyclic arenes, we focus this study on the development of a cobalt-embedded N-doped porous carbon material. We demonstrate this material as a stable heterogeneous catalyst for the above-mentioned reactions using formic acid as a hydrogen donor and/or formylating source. In the presence of melamine as a rich source of nitrogen, o-phenylenediamine was polymerized over colloidal silica as a hard template for generating porosity, taking CoCl2 as a cobalt precursor to embed Co into the high surface area porous N-doped carbon network. This catalyst exhibits solvent-dependent catalytic activity in the selective reduction of N-arenes and in the reductive N-formylation of N-heterocyclic compounds using formic acid as the source of hydrogen and the formylating agent, respectively.

hydrogenation can be adopted where low-cost and amply available alcohols and acids can be chosen as the hydrogen donors.17 Although alcohol (such as isopropanol) is most often used as the hydrogen donor, it can give rise to parallel reactions, such as condensation and esterification, that significantly bring down the selectivity for the desired reduced product.18 Formic acid, on the other hand, is another economical hydrogen donor that can be used for the transfer hydrogenation.19 Interestingly, formic acid can be readily produced from the biomass as a green and sustainable hydrogen donor.20 Another advantage of using formic acid is that HCOOH acts as a formylating agent as well as a hydrogen source.13 Two catalytic decomposition pathways (decarboxylation and dehydration) are possible from formic acid. In the decarboxylation pathway, H2 and CO2 are produced, whereas in the dehydration pathway, H2O and CO are formed.21,22 Therefore, for the selective reduction of N-arenes, it is imperative to optimize a reaction parameter that facilitates only one cleavage pathway (i.e., H2 + CO2-mediated pathway) and to develop a robust catalyst that serves the purpose. If both pathways occur in parallel, then both products (reduced Nheterocycles and reductive N-formylated heterocycles) form. Many homogeneous catalysts have been shown to facilitate only the decarboxylation pathway avoiding that of dehydration.23 Moreover, several heterogeneous catalysts based on noble metals (Pd, Pt, Ru, etc.) have also been developed for this purpose.24 Suitable support is required for the dispersion of active metals, and this is another important feature that needs to be considered. Since the acidity of formic acid, unfortunately, triggers the leaching of active metals from the support,25 a worthy support material to the active catalyst is crucial for the selective reduction and reductive N-formylation of N-arenes. For this purpose, the support should afford (a) large surface area for the better dispersion of the active metals, (b) high porosity and surface area for the easy access and diffusion of large reactant of product molecules, and (c) optimum binding of reactant and product on the support matrix. Catalytic supports with enhanced basicities are known to increase the reaction rate and selectivity of the desired N-reduced

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RESULTS AND DISCUSSION In this study, Co@C-N materials were prepared by selecting ophenylenediamine as a C and N source, melamine (as an B


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ACS Sustainable Chemistry & Engineering additional N source), CoCl2 as a Co source, colloidal silica as a hard template, and ammonium peroxydisulfate (APS) as a polymerizing initiator. The proposed pathway for the formation of Co@C-N materials is illustrated in Scheme 1. Initially, polymerization of o-phenylenediamine (OPD) took place on the surface of colloidal silica nanoparticles with the help of ammonium persulfate (APS). o-Phenylenediamine was polymerized by the oxidative polymerization process using APS as the polymer initiator and/or oxidant.34 During this process, melamine units were entrapped in the polymers produced by OPD and form silica−polymer nanocomposites. The role of melamine is to increase the nitrogen content and introduce suitable types of nitrogen environments to stabilize the Co species (discussed later). Co was entrapped in the nitrogen-rich sites of the carbon−nitrogen polymer framework. Upon carbonization in Ar/H2 (95:5) at the desired temperature, surface pyridine, pyrrole, and quaternary nitrogen doping were achieved, and Co was embedded preferentially on these sites. The aim of this synthesis was to prevent the formation of CoOx and restrict the oxidation of embedded Co in the C-N matrix; therefore, Ar/H2 (95% Ar + 5% H2) was used. After carbonization, nanocomposites were treated with HF (24% aq solution) to remove the colloidal silica template. After removal of the template, the material was further treated with an HCl solution (0.1 M) to remove loosely bound and unstable Co species and then again carbonized at the same temperature to obtain Co@C-N materials. Materials carbonized at 700, 800, and 900 °C are represented as Co@C-N700, Co@C-N800, and Co@C-N900, respectively. Various Co@C-N materials and Co@OPD800 prepared in this study exhibit broad XRD reflections in the range of 20− 30° (2θ) with a peak maximum at 26.9° corresponding to the (002) diffraction plane of graphitic carbon (Figure S1). In addition, two weak intensity diffraction peaks at 44.4° and 51.6° (2θ) are observed, and those can be assigned to (111) and (222) planes of embedded Co metal nanoparticles (PDF Card No. 15-0806). N2 adsorption−desorption experiments confirmed that various Co@C-N materials and Co@OPD800 displayed a type IV isotherm and H2 hysteresis (Figure 1a). For example, Co@C-N800 exhibits a sloping steep increase in the N2adsorption in the range of 0.43−0.73 (P/P0), confirming the presence of mesopores in the material. Co@C-N800 displays uniform mesopore size distribution in the range of 5−8 nm in the BJH pore size distribution. BJH analysis confirms that uniform mesopores with pore size distribution in the range of 6−10 nm are present in these materials. In this study, colloidal silica (with an average diameter of 12 nm) is used as a hard template; therefore, pore sizes observed for these materials are lower than 12 nm. Textural properties determined from the N2-sorption experiments are summarized in Table 1. With the increase in the carbonization temperature from 700 to 800 °C, surface area and pore volume are increased. However, with further increase in the carbonization temperature to 900 °C, textural properties are decreased. Large surface area and pore volume are important parameters for catalysis; therefore, it is anticipated that Co@C-N800 would display better activity. Co@OPD800 exhibits better surface area and pore volume than Co@C-N800 which confirms that the entrapment of melamine units in the framework results in a decrease in the porosity and surface area of Co@C-N materials. Co@C-N catalysts and Co@OPD800 exhibit G and D bands in the range of 1000−2000 cm−1 of Raman spectra (Figure

Figure 1. (a) N2-sorption isotherms (inset shows BJH pore size distribution). (b) Raman spectra of various Co@C-N materials and Co@OPD800 prepared at different carbonization temperatures.

Table 1. Textural Properties of Various Catalysts Prepared in This Study Catalyst

Surface area (m2/g)

Pore diameter (nm)

Pore volume (cm3g−1)

External surface area (m2/g)

Co@C-N800 Co@C-N900 Co@C-N700 Co@OPD800

439 336 191 542

6.9 8.1 6.1 7.6

1.89 2.36 0.78 2.18

343 304 109 426

1b). The Raman spectrum shows that G bands are sharper than D bands and appear at a higher wavenumber and confirms the presence of graphitic carbon in the materials. In addition, several vibration peaks are observed in the range of 300−700 cm −1 corresponding to Co-N/Co-O species in these materials.31 With the increase in carbonization temperature from 700 to 800 °C, these peaks get sharper which confirms the stronger Co-N/Co-O interaction in Co@C-N800. However, with further increase in the temperature to 900 °C, less intense peaks in the range of 300−700 cm−1 are observed which confirms the weak Co-N/Co-O interaction in [email protected] Moreover, very weak vibration peaks are observed in the range of 300−700 cm−1 in Co@OPD800 confirming that melamine units provide the required nitrogen environment for the strong entrapment of Co metal nanoparticles in Co@C-N800. The surface chemical composition and the oxidation state of Co species present in Co@C-N800 and Co@OPD800 were determined using XPS. The high-resolution N 1s peak is deconvoluted into pyridinic N, pyrollic N, graphitic N, and pyridine N-oxide species (Figure 2).26 In Co@C-N800, pyridinic N and pyrollic N are the dominant species, whereas C


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ACS Sustainable Chemistry & Engineering

Figure 2. High-resolution X-ray photoelectron spectra of N and Co species present in (a, b) Co@C-N800 and (c, d) Co@OPD800.

Figure 3. (a) TEM image, (b) HR-TEM image, and (c) HAADF image and their corresponding elemental mapping is presented in (d−f) for Co@ C-N800.

condensation of melamine and OPD units. The Co 2p3/2 spectrum of Co@OPD800 is deconvoluted to Co(III)-N/O and Co(II)-N/O species. In this case, a higher concentration of Co(III)-N/O species is observed compared to Co(II)-N/O species. Moreover, the concentration of Co-N/O species is higher in Co@C-N800 than Co@OPD800. C 1s spectra are deconvoluted to designate various surface carbon species present in the sample (Figure S2).35 Both catalysts exhibit a higher concentration of CC species than other C-O/C-N species designated in their high-resolution XPS spectra. Furthermore, a lower intensity C-N species is observed in the deconvoluted spectrum of Co@OPD800 than Co@C-N800 confirming that melamine addition is required for higher N

in Co@OPD800, pyridinic N and graphitic N are the dominant species (Figure 2). Further, more concentration of Co-N species is identified in Co@C-N 800 than Co@OPD 800 confirming that a higher amount of Co is entrapped in Co@ C-N800 than Co@OPD800 (Table S1). Further high-resolution Co 2p3/2 spectra are deconvoluted to demonstrate the different Co species present in the catalysts.31,35 The Co 2p3/2 spectrum of Co@C-N800 is deconvoluted to Co(0), Co(III)-N/O, and Co(II)-N/O species. In this case, the highest concentration of Co(II)-N/O and the lowest concentration of Co (0) species are observed. Co-N/O species are observed because Co ions can be coordinated to N/O sites present in an N-doped carbon framework obtained by the coD


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ACS Sustainable Chemistry & Engineering Table 2. Influence of Catalysts and Solvent in Transfer Hydrogenation of Quinoline to THQa

Entry

Catalyst

Reaction medium

Hydrogen source

Conv. (%)

Product selectivity (THQ/FTHQ) selectivity (%)

THQ GC yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16b

None Co@C-N800 C-N800 Co@C-N700 Co@C-N800 Co@C-N900 Co/AC800 Co@OPD800 Co/C-N800 Co@C-N800 (nontemplated) Co@C-N800 Co@C-N800 Co@C-N800 Co@C-N800 Co@C-N800 Co@C-N800

water water water water water water water water water water ethanol cyclohexane THF toluene water water

HCOOH none HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH H2 (10 bar) HCOOH

0 0 0 61.3 92.1 86.4 11.5 69.2 47.3 31.4 9.3 40.8 31.9 84.2 39.4 87.4

ND ND ND 80/20 87/13 85/15 73/29 76/24 83/17 81/19 89/11 76/24 86/14 1/99 >99 84/16

0 0 0 49.0 80.1 73.4 8.4 52.6 39.3 25.4 8.3 31.0 27.4 0.8 39.4 73.4

a Reaction condition: quinoline (0.5 mmol), catalyst (20 mg), HCOOH (7.5 mmol), H2O (10 mL), time (6 h), temperature (130 °C). bAfter fifth reuse. GC yield = (quinoline conversion × THQ selectivity)/100.

Table 2). Moreover, no reduction product was obtained in the absence of formic acid but in the presence of Co@C-N800 (entry 2, Table 2). Furthermore, no product was observed when C-N800 (cobalt free) was used as a catalyst in the presence of HCOOH (entry 3, Table 2). The Co incorporation in the catalyst was required for this reaction. All Co@C-N catalysts prepared at different carbonization temperatures were active for this reaction (entries 4−6, Table 2). Carbonization temperature played a very important role in achieving high quinoline conversion and THQ selectivity. Catalyst (Co@C-N800) carbonized at 800 °C exhibited the highest quinoline conversion and THQ selectivity (compare entries 4−6, Table 2). Co/AC800 prepared for the comparative study exhibited a very low activity which confirms that Co incorporation in the nitrogen-containing support is very important for this transfer hydrogenation to take place (compare entries 5 and 7, Table 2). Moreover, Co@OPD800 exhibited lower activity than Co@C-N800 (compare entries 5 and 8, Table 2). Co attached to suitable N environments is required for the high activity. Such environments are provided only when melamine and o-phenylenediamine were cocondensed together at a high carbonization temperature of 800 °C. Melamine involvement in the synthesis of a C-N framework increases the content of nitrogen in the catalyst which results in the effective entrapment of the stable cobalt species in the catalyst. During the heat treatment process at a temperature higher than 450 °C, the melamine present in the polymer turns to graphitic carbon nitride (g-C3N4) and the poly (o-PD) forms carbon−nitrogen sheets in the interlayer of graphitic nitride. At higher carbonization temperature, the graphitic nitride decomposes and integrates with carbon− nitrogen sheets while maintaining its mesoporosity and high surface area due to the presence of colloidal silica as a template.37 During the heat treatment process, g-C3N4 decomposes to NH3 and carbon species. Furthermore, NH3 decomposes to free radicals such as NH2, NH, N, and H

doping in the Co@C-N800 sample. XPS analysis confirms that different Co species present in these samples would impart different reactivity in the appointed catalytic reactions. Transmission electron microscopy analysis was carried out for the highly active sample Co@C-N800 (detailed activity is discussed below in the Catalytic Activity section of the Supporting Infomation). Sheet-like morphology of the Ndoped carbon matrix is visible in the TEM image (Figure 3a). The high-resolution TEM image shows the lattice fringes corresponding to Co nanoparticles embedded in the N-doped carbon framework (Figure 3b). Interplanar spacing of 0.205 and 0.364 nm corresponding to Co (111) and C (002) planes, respectively, are observed that match well with the XRD data discussed above.36 The HAADF image and corresponding elemental maps clearly demonstrate the homogeneous distribution of Co species in the N-doped porous carbon framework (Figure 3c−f). Quinoline is an interesting reactant that can produce two important products, 1,2,3,4-tetrahydroquinoline (THQ) and N-formyltetrahydroquinoline (FTHQ), when quinoline is reacted with formic acid.13 FTHQ can be converted to THQ just by hydrolysis in a basic ethanol/water medium.7 However, both THQ and FTHQ are important synthetic intermediates, and their syntheses are very important. In this study, Co@C-N catalysts have been developed that exhibit excellent activity in the selective production of THQ in water using HCOOH as the hydrogen source. Moreover, using the same catalyst, quinoline can be converted to FTHQ in toluene using HCOOH as the hydrogen and formylating source. The detailed discussions pertaining to both these reactions are provided below. Co@C-N catalysts prepared in this study were investigated in the catalytic transfer hydrogenation (CTH) of a model substrate, quinoline, using formic acid as a hydrogen source. Catalytic activity data presented in Table 2 clearly show that this reaction did not occur in the absence of a catalyst (entry 1, E


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Figure 4. Influence of (a) HCOOH concentration, (b) reaction temperature, and (c) catalyst amount in catalytic transfer hydrogenation of quinoline.

Table 3. Catalytic Transfer Hydrogenation of Various N-Heterocycles over Co@C-N800 Using HCOOHa

Reaction condition: Substrate (0.5 mmol), Co@C-N800 catalyst (20 mg), HCOOH (7.5 mmol), H2O (10 mL), time (6 h), temperature (130 °C). Data obtained after 4 h of the reaction. cData obtained after 6 h of the reaction. dData obtained after 8 h of the reaction.

a

b

responsible for the higher activity of Co@C-N800 than Co@ OPD 800 . Two more materials were prepared for the comparative study. Both Co@C-N800 (nontemplated) and Co/C-N800 exhibited lower catalytic activity than Co@C-N800 (compare entries 5 and 9−10, Table 2). Furthermore, Co leaching was observed when Co/C-N800 was used, whereas no leaching was observed when Co@CN800 was used (photographs are presented in Figure S3). Comparative catalytic

atoms, and such radicals have the capability to etch a carbon framework and generate porosity in the material.26 Moreover, these free radical nitrogen species can replace the oxygencontaining species present in the carbon framework and produce surface pyridine, pyrrole, and quaternary nitrogen doping.26 XPS analysis confirms that higher content of pyridinic and pyrrolic nitrogen in Co@C-N800 than Co@ OPD800 stabilizes the large content of Co species which is F


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nitrogen in indole are delocalized over the five-membered ring, and they are not available for binding to the catalyst surface. Therefore, indole exhibits lower activity than quinoline and other N-heterocycles investigated in this study. These catalytic data suggests that the present catalytic protocol is widely applicable for a wide range of N-heterocyclic compounds to afford amines as selective products. Quinoline is expected to adsorb on the catalyst surface for this reaction. Quinoline being a UV−visible active molecule, its adsorption can be monitored using a UV−visible spectrophotometer. First, 20 ppm of ethanolic quinoline solution (10 mL) was taken. Then, 20 mg of Co@C-N800 catalyst was added, and the resultant solution was stirred. At the desired time interval, the supernatant was withdrawn, and the UV−visible spectrum was recorded. Figure S4 shows the UV−visible absorption spectra recorded for the reaction mixture withdrawn at regular time intervals (from 0 to 2 h). With the increase in the equilibration time, the quinoline concentration in the solution was decreased, and quinoline attained good adsorption after 1.5 h. Therefore, the adsorption experiments were carried out using various catalysts prepared at different temperatures for 1.5 h. A UV−visible study confirms that Co@C-N800 adsorbs the maximum amount of quinoline which is consistent with the catalytic activity data obtained for the catalytic transfer hydrogenation reaction (entries 4−6, Table 2). This study shows that quinoline adsorption is expected to be one of the important steps in the catalytic reaction for the catalyst to exhibit high catalytic activity. Formic acid also needs to be adsorbed on the catalyst site for the decomposition of formic acid to H2 + CO2. Since formic acid is an acidic molecule, the optimum basicity of the catalyst surface is required for the efficient adsorption of HCOOH. Therefore, a CO2-temperature-programmed desorption (CO2TPD) measurement was carried out to study the surface basicity of the catalysts (Figure S5). CO2-TPD measurements confirmed that a different amount of CO2 was desorbed from the samples at different temperatures. CO2 desorption in the range of 50−150 °C can be attributed to physisorbed CO2 from the sample. CO2 desorption in the range of 150−350 and 350−550 °C can be attributed to CO2 desorption from the weak basic sites and strong basic sites, respectively.39 CO2 desorbed from various samples follow the order Co@C-N800 > Co@C-N900 > Co@C-N900 > Co@OPD800. Moreover, higher basic strength is obtained for Co@C-N800 which further confirms that HCOOH is expected to absorb more on Co@CN800 than other samples prepared in this study and provides a higher amount of surface Co-H sites for the transfer hydrogenation reaction.36 This study confirms that the highest basicity and basic strength of Co@C-N800 are responsible for the high HCOOH adsorption which leads to produce H2 molecules on the catalyst surface in water (as discussed in the Introduction) that are dissociatively adsorbed on the Co sites and form Co-H species. Quinoline adsorption on the neighboring sites would be helpful for efficient transfer hydrogenation to produce desired THQ in high yield (Scheme 2). It may be noted that when a similar reaction was performed using H2 in a pressure reactor at 10 bar pressure for 6 h, lower THQ yield was obtained in 6 h (entry 15, Table 2). However, in this case, only THQ was obtained as the product. The in situ generation of H2 from HCOOH adsorbed on the catalyst surface is more efficient because it immediately adsorbed on the Co sites and results in the formation of THQ in high yield.

activity data presented in Table S2 suggests that the present catalyst exhibited better catalytic activity (compare TOF, Table S2) than various other catalysts reported in the literature, especially in water and base free conditions. Reaction parameters were optimized by varying the formic acid concentration, reaction temperature, solvent, and catalyst amount. With the increase in the HCOOH amount from 5 equiv (with respect to quinoline) to 20 equiv, the quinoline conversion was increased, whereas the THQ selectivity was decreased. The highest yield of THQ was obtained with the highest THQ selectivity when 15 equiv of HCOOH was chosen with respect to quinoline (Figure 4a). With the increase in temperature from 110 to 150 °C, quinoline conversion was increased from 67% to 98%; however, the selectivity for the desired THQ was decreased from 92% to 67% (Figure 4b). The highest yield of THQ was obtained with the highest THQ selectivity at 130 °C; therefore, this temperature was selected as the optimum temperature. This experimental data indicates that with an increase in the temperature, conversion of formic acid to CO + H2O increases. This also provides evidence that if we want to get FTHQ, then we have to perform the reaction at a higher temperature. When the reaction was performed in other polar solvents such as ethanol, then very low quinoline conversion was obtained; however, THQ selectivity similar to the reaction performed in water was obtained (compare entries 5 and 11, Table 2). Moreover, when similar reactions with other solvents such as toluene, cyclohexane, and THF were performed, lower THQ selectivities were observed (compare entries 5 and 12−14, Table 2). In toluene, a selective N-formyl product (FTHQ) was obtained with 84.2% quinoline conversion (compare entries 5 and 14, Table 2). It is documented in the literature that formic acid decomposes favorably to H2O and CO in the gas phase, whereas to H2 and CO2 in the aqueous phase.22,38 Therefore, water plays an important role to achieve the desired decomposition pathway leading to H2 for the transfer hydrogenation reaction. Water is known to reduce the activation energy barrier for both the decarboxylation and dehydration reactions, but water reduces the activation energy to a larger extent for decarboxylation and favors the formation of more H2.38 It is also documented in the literature that water acts as a homogeneous catalyst and participates in the bond-breaking and bond-forming processes leading to the desired H2 as a product for the transfer hydrogenation reaction.38 Due to these reasons, water provided better THQ yield and selectivity. Furthermore, the amount of catalyst with respect to 0.5 mmol of quinoline was optimized. With an increase in the catalyst amount from 10 to 30 mg, catalytic activity was increased (Figure 4c). If TOF is considered, then 10 mg would be the best amount of the catalyst (TOF (h−1): 20.8 for 10 mg, 16.8 for 20 mg, and 11.9 for 30 mg catalyst). However, to achieve a higher yield of the product, 20 mg was chosen as the optimum catalyst amount for further study. The catalytic activity of Co@C-N800 was also successfully demonstrated for other N-heterocyclic compounds to produce amine as the selective product (Table 3). Pyridine afforded pyrimidine at higher selectivity (96%) and yield (85%) in 4 h than quinoline which afforded 80% yield of THQ in 6 h (Table 3). Furthermore, acridine produced only a marginally lower product yield (77%) in 6 h than quinoline (80%) (Table 3). However, indole afforded lower reactant conversion and amine selectivity even after a prolonged reaction duration (Table 3). The lone pair electrons of G


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from TGA analysis for fresh and recycled catalysts were determined to be 7.9% and 7.7%, respectively, confirming the retention of Co species in the catalyst after five recycles. Furthermore, a hot filtration experiment was also carried out using Co@C-N800 in which the reaction was conducted for 2 to 6 h under the optimized condition, and the quinoline conversion of 37% and THQ selectivity of 92% were obtained (Figure 5b). After 2 h, the catalyst was removed from the reaction mixture, and the reaction was continued for another 4 h under the same reaction condition. No appreciable increase in quinoline conversion was observed. This experiment suggests that the catalysis is purely heterogeneous in nature. Moreover, only 1.8 ppm of Co species was detected in the reaction mixture after the reaction. Based on these two experiments, it is suggested that a formic acid-resistant recyclable catalyst has been developed for efficient transfer hydrogenation. Furthermore, to confirm that Co species is responsible for the catalytic activity, KSCN was added to the reaction mixture after 2 h. SCN− is known to make a strong complex with Co.36 Under the experimental condition, in the presence of KSCN after 2 h of the reaction, the quinoline conversion was increased from 37% to 49%, whereas it was increased to 92% in the case when no KSCN was added (Figure 5c). These experiments suggest that Co present in the C-N800 framework was blocked by coordinating with SCN− and hindered the formation of active Co-H species required for the reaction, and hence, low activity was observed. The high porosity of C-N800 framework provides easy accessibility of SCN− species to reach the Co sites and form the Co-SCN

Scheme 2. Plausible Mechanism for Catalytic Transfer Hydrogenation of Quinoline in Water

Co@C-N800 was efficiently reused for five cycles in quinoline for THQ transformation. No appreciable decrease in the quinoline conversion and THQ selectivity was observed (Figure 5a; compare entries 5 and 16 in Table 2). The cobalt contents in the reaction mixture after each cycle were measured by ICP analysis. Only a marginal loss in the Co content (6.8 ppm) was observed after five cycles. The reused catalyst was subjected to XRD and TGA analysis. The XRD pattern of the recycled catalyst was very similar to that of the fresh catalyst (Figure S6). Moreover, the Co contents obtained

Figure 5. (a) Recycling of Co@C-N800.(b) Progress of reaction after the removal of catalyst after 2 h during the hot filtration test. (c) Poisoning test of Co@C-N800 with the addition of KSCN after 2 h of reaction in the catalytic transfer hydrogenation of quinoline using formic acid as the hydrogen donor. H


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(Figure S7). With the increase in the HCOOH amount from 5 equiv (with respect to quinoline) to 20 equiv, the quinoline conversion was increased, whereas the FTHQ selectivity remained constant (>99%) (Figure S7a). An excellent quinoline conversion (98%) was obtained when 15 equiv of HCOOH was chosen with respect to quinoline. With an increase in the temperature from 110 to 150 °C, quinoline conversion was increased from 78.3% to 99.2%; however, the selectivity of the desired FTHQ remained constant (>99%) (Figure S7b). Excellent (98%) quinoline conversion and FTHQ selectivity (99%) were obtained at 140 °C; therefore, this temperature was selected as the optimum temperature. This experimental data indicates that with an increase in temperature conversion of formic acid to CO and H2 increases which produces a higher yield of FTHQ at a higher temperature. Moreover, the amount of the catalyst with respect to 0.5 mmol of quinoline was optimized (Figure S7c). With an increase in the catalyst amount from 10 to 30 mg, catalytic activity was increased. If TOF is considered, then 10 mg would be the best amount of the catalyst (TOF (h−1): 16.1 for 10 mg, 13.4 for 20 mg, and 9.11 for 30 mg catalyst). However, to achieve a higher yield of the product, 20 mg was chosen as the optimum catalyst amount for further study. The kinetic experiments were conducted at the optimum reaction condition (Figure S8). At the beginning of the reaction, the rate (TOF) was very high, and with the progress of the reaction, the rate was successively decreased. The TOF values (in h−1) calculated after 15 min, 30 min, 1, 2, 4, 6, and 8 h were 113.7, 65.6, 42.9, 30.0, 21.1, 16.0, and 13.4, respectively. In the transformation of quinoline to FTHQ using formic acid as the H2 and CO source, both FTHQ and THQ were formed. THQ concentration was the maximum after 15 min of the reaction. A progressive decrease in the THQ concentration was observed, and THQ disappeared after 2 h of the reaction. Due to the presence of an excessive amount of formic acid, THQ and FTHQ were simultaneously formed in the beginning, and only FTHQ was observed in the reaction mixture after 2 h of the reaction as a selective product. The catalytic activity of Co@C-N800 was also successfully demonstrated for other N-heterocyclic compounds for the reductive N-formylation. Pyridine afforded N-formylated piperidine in very high yield (99%) in 5 h compared with quinoline which afforded 98% yield of FTHQ in 8 h (entries 1 and 2, Table 5). Under a similar reaction condition, piperidine afforded >99% yield of N-formylated piperidine in just 4 h (entry 3, Table 5). Even acridine afforded very good yield, but it was somewhat lower than quinoline (compare entries 1 and 4, Table 5). However, indole and carbazole afforded lower product yield in longer reaction times (compare entries 1 and 4, Table 5). These catalytic data suggest that the present catalytic protocol is widely applicable for a wide range of Nheterocyclic compounds to afford N-formylated product in high yield. This study confirms that quinoline and formic acid adsorb on the basic N-doped porous carbon material. Based on the catalytic results obtained in toluene, we can say that formic acid decomposed to H2 and CO, and then, H2 is dissociatively adsorbed on Co sites. Then, transfer hydrogenation and formylation took place to produce the reductive N-formylated product (Scheme S1). In order to provide the evidence for CO poisoning of the catalyst, two control experiments were conducted. First, the autoclave containing all the reaction mixture was flushed with CO, and then, the reaction was

complexes, and such Co-SCN complexes are not the active species for this reaction. Therefore, the catalytic activity was significantly reduced. The influence of solvent during the quinoline to THQ conversion process confirmed that toluene efficiently produced N-formyltetrahydroquinoline (FTHQ) under the same reaction. This provides evidence that this catalyst can also be used to produce FTHQ in high yield, and therefore, a detailed investigation was made in toluene. Catalytic activity data presented in Table 4 clearly show that this reaction did not Table 4. Reductive N-Formylation of Quinolines over Various Catalysts Prepared in This Studya

Entry

Catalyst

Conv. (%)

FTHQ select. (%)

1 2b 3 4 5 6 7 8 9 10 11c

none Co@C-N800 C-N800 Co@C-N700 Co@C-N800 Co@C-N900 Co@OPD800 Co/C-N800 Co@C-N800 (nontemplated) Co/AC800 Co@C-N800

0 0 0 67.3 98.0 91.1 73.2 58.4 38.1 14.7 92.4

0 0 0 >99 >99 >99 >99 >99 >99 >99 >99

a

Reaction condition: quinoline (0.5 mmol), catalyst (20 mg), HCOOH (7.5 mmol), toluene (2 mL), time (8 h), temperature (140 °C). bReaction was conducted in the absence of HCOOH. c After fifth reuse.

occur in the absence of a catalyst (entry 1, Table 4). Moreover, no FTHQ was obtained when formic acid was not added to the reaction mixture in the presence of Co@C-N800 (entry 2, Table 4). Furthermore, no product was observed when C-N800 (without Co) was used as a catalyst in the presence of HCOOH (entry 3, Table 4). This clearly shows that Co incorporation in the catalyst is required for this reaction. All Co@C-N catalysts prepared at different carbonization temperatures were active for this reaction (entries 4−6, Table 4). Carbonization temperature plays a very important role in achieving high conversion and FTHQ selectivity. Catalyst (Co@C-N800) carbonized at 800 °C exhibited the best catalytic activity and FTHQ selectivity. Co/AC800 exhibited a very low activity which clearly shows that Co incorporation in the nitrogen-containing support is very important for this reductive N-formylation to take place (compare entries 5 and 7, Table 4). Moreover, Co@OPD800 exhibited lower activity than Co@C-N800 (compare entries 5 and 8, Table 4). Co embedded to suitable N environments (pyridinic and pyrrolic N) is required for the high activity. Co@C-N800 (nontemplated) and Co/C-N800 exhibited lower catalytic activity than Co@C-N800 (compare entries 5 and 9−10, Table 4). Comparative catalytic activity data presented in Table S3 suggests that the present catalyst exhibited better catalytic activity (compare TOF, Table S3) than various other catalysts reported in the literature. Reaction parameters were optimized by varying the formic acid concentration, reaction temperature, and catalyst amount I


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Co@C-N800 surface was doped with pyridine, pyrrole, and quaternary nitrogen which were helpful in the strong entrapment of Co species in the C-N framework in the form of highly active Co(0) and Co(II)-N/O species, which was also confirmed from the Raman analysis. XPS analysis showed that only 1.35 wt % of Co was embedded in Co@C-N800. HRTEM analysis confirmed that very small sizes of Co species were embedded in C-N framework in the highly dispersed state with interplanner spacing of 0.205 and 0.364 nm corresponding to Co (111) and C (200) planes. Catalytic activity confirmed that HCOOH was selectively decomposed to H2 + CO2 in water that led to the reduction of quinoline to produce THQ as a selective product (selectivity = 87%) with a yield of 80%, whereas the catalyst followed both decomposition pathways in toluene and produced H2 + CO which led to the exclusive formation of the reductive N-formylated heterocyclic compound FTHQ with excellent yield (98%). Optimum basicity of the C-N framework and highly dispersed Co sites present in the C-N framework facilitated the efficient adsorption of reactants in the close vicinity of Co-H sites so that effective hydrogen transfer takes place. Moreover, in the presence of H2 + CO, H-transfer and N-formylation took place to produce N-formylated heterocyclic compounds. A recycling study confirmed that only 6.8 ppm of Co was leached after five cycles and provided a THQ yield of 73.4% which was just 8% lower than the yield obtained using the fresh catalyst. Furthermore, a hot filtration test confirmed that only 1.5 ppm of Co was leached to the reaction mixture, and no significant reaction took place from these leached Co species. Furthermore, a KSCN poisoning test confirmed that the reactions were occurring on the heterogeneous support. Co@ C-N800 was highly stable and recyclable even for FTHQ production and provided a FTHQ yield of 92.4% after five cycles. The synthesis strategy reported here can be extended to a wide range of active transition metal-embedded C/N-doped carbon materials for their unique application as catalysts, especially to replace the conventional noble metals catalysts in similar catalytic applications.

Table 5. Reductive N-Formylation of Various NHeterocycles over Co@C-N800a

a

Reaction condition: substrate (0.5 mmol), Co@C-N800 (20 mg), HCOOH (15 equiv), toluene (2 mL) at 140 °C. bData obtained after 4 h of the reaction. cData obtained after 5 h of the reaction. dData obtained after 8 h of the reaction. eData obtained after 10 h of the reaction. fData obtained after 12 h of the reaction.

conducted for 8 h. No significant change in the catalytic activity (FTHQ GC yield = 97.4%) was observed. In another control experiment, the reaction was conducted in the presence of 2 bar CO pressure. After the reaction, the catalyst was removed and reused in the next cycle. The catalytic activity of the recycled catalyst was almost similar to that of the fresh catalyst (FTHQ GC yield = 98.3%). These control reactions clearly suggest that cobalt was not poisoned in the presence of CO. This result is consistent with our earlier reports in which we have demonstrated the electrocatalytic oxidation of methanol over NiCo2O4/NiCuCo2O4 modified catalysts.40,41 Catalysts were not deactivated much in the presence of CO which was an intermediate product of methanol oxidation.40,41

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EXPERIMENTAL SECTION

Catalyst Synthesis. Catalysts were prepared according to the recipe known for the synthesis of Co-embedded N-doped carbon materials but with several modifications.30 o-Phenylenediamine (OPD) (3 g), melamine (3 g), and CoCl2·6H2O (1 g) were added to 10 mL of 1 M HCl aqueous solution and stirred for 30 min. Then, colloidal silica (15 g) was added into the reaction mixture followed by the addition of 100 mL of 1 M HCl aqueous solution with vigorous stirring for 1 h under an ice bath (2−3 °C). After that, 50 mL of 1 M HCl containing ammonium peroxydisulfate (APS) (2 g) was added dropwise into the above solution and stirred for 24 h at 2−3 °C. Finally, the solution was dried in a rotary evaporator to obtain a cobalt and melamine co-doped poly (OPD). Then, the resultant material was ground and carbonized at different carbonizing temperatures (700, 800, and 900 °C) for 2 h with a heating ramp of 10 °C min−1 in an Ar/H2 (95% Ar + 5% H2) atmosphere to get Co@SiO2-C-N700, Co@SiO2-C-N800, and Co@SiO2-C-N900. Finally, silica and an unstable cobalt species were removed by washing with 24% HF solution, followed by washing with a 0.1 M HCl solution and subsequently carbonized at a similar carbonizing temperature for 30 min. The catalysts are designated as Co@C-N700, Co@C-N800, and Co@C-N900. For comparison, the C-N material was also prepared in the absence of melamine by following the procedure discussed above, except for the addition of melamine in the initial step, and the product is designated as Co@OPD800. Co@C-N800 was also prepared in the absence of colloidal silica, and the resultant material is designated as

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CONCLUSIONS Cobalt-embedded porous N-doped carbon material was successfully prepared by the polymerization of o-phenylenediamine (as a C and N source) and melamine (as an additional N source) with the help of APS in the presence of colloidal silica as a hard template and CoCl2 as a Co source. Controlled carbonization of the resultant material in a H2/Ar atmosphere in the temperature range of 700−900 °C produced Coembedded porous N-doped carbon materials. Material carbonized at 800 °C (Co@C-N800) exhibited the best activity due to the highest surface area of 439 m2/g and the highest pore volume of 1.89 mL/g. XPS analysis confirmed that the J


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antagonists with potent activity against HIV-1. Bioorg. Med. Chem. Lett. 2010, 20, 2186−2190. (5) Sridharan, V.; Suryavanshi, P. A.; Menéndez, J. C. Advances in the chemistry of tetrahydroquinolines. Chem. Rev. 2011, 111, 7157− 7259. (6) Murahashi, S. − I.; Imada, Y.; Hirai, Y. Rhodium catalyzed hydrogenation of nitrogen heteroaromatics under water gas shift conditions. Selective synthesis of 1,2,3,4-tetrahydroquinolines and Nformyl-1,2,3,4-tetrahydroisoquinolines. Tetrahedron Lett. 1987, 28, 77−80. (7) Chen, F.; Sahoo, B.; Kreyenschulte, C.; Lund, H.; Zeng, M.; He, L.; Junge, K.; Beller, M. Selective cobalt nanoparticles for catalytic transfer hydrogenation of N-heteroarenes. Chem. Sci. 2017, 8, 6239− 6246. (8) Kuwano, R.; Ikeda, R.; Hirasada, K. Catalytic asymmetric hydrogenation of quinoline carbocycles: unusual chemoselectivity in the hydrogenation of quinolines. Chem. Commun. 2015, 51, 7558− 7561. (9) Zhang, S.; Xia, Z.; Ni, T.; Zhang, H.; Wu, C.; Qu, Y. Tuning chemical compositions of bimetallic AuPd catalysts for selective catalytic hydrogenation of halogenated quinolines. J. Mater. Chem. A 2017, 5, 3260−3266. (10) Ren, Y.; Wang, Y.; Li, X.; Zhang, Z.; Chi, Q. Selective hydrogenation of quinolines into 1, 2, 3, 4-tetrahydroquinolines over a nitrogen-doped carbon-supported Pd catalyst. New J. Chem. 2018, 42, 16694−16702. (11) Jiang, H.; Xu, J.; Sun, B. Selective hydrogenation of aromatic compounds using modified iridium nanoparticles. Appl. Organometal Chem. 2018, 32, No. e4260. (12) Yu, X.; Nie, R.; Zhang, H.; Lu, X.; Zhou, D.; Xia, Q. Ordered mesoporous N-doped carbon supported Ru for selective adsorption and hydrogenation of quinoline. Microporous Mesoporous Mater. 2018, 256, 10−17. (13) Zhang, J.-F.; Zhong, R.; Zhou, Q.; Hong, X.; Huang, S.; Cui, H.-Z.; Hou, X.-F. Recyclable silica-supported iridium catalysts for selective reductive transformation of quinolines with formic acid in water. ChemCatChem 2017, 9, 2496−2505. (14) Yuan, Y. − P.; Yin, L. − S.; Cao, S. − W.; Gu, L. − N.; Xu, G. − S.; Du, P.; Chai, H.; Liao, Y. − S.; Xue, C. Microwave-assisted heating synthesis: a general and rapid strategy for large-scale production of highly crystalline g-C3N4 with enhanced photocatalytic H2 production. Green Chem. 2014, 16, 4663−4668. (15) Xie, G.; Zhang, K.; Guo, B.; Liu, Q.; Fang, L.; Gong, J. R. Graphene-based materials for hydrogen generation from light-driven water splitting. Adv. Mater. 2013, 25, 3820−383. (16) Wang, D.- S.; Chen, Q. − A.; Lu, S. − M.; Zhou, Y. − G. Asymmetric hydrogenation of hetero arenes and arenes. Chem. Rev. 2012, 112, 2557−2590. (17) Wang, D.; Astruc, D. The golden age of transfer hydrogenation. Chem. Rev. 2015, 115, 6621−6686. (18) Ren, D.; Wan, X.; Jin, F.; Song, Z.; Liu, Y.; Huo, Z. Selective hydrogenation of levulinate esters to 1, 4-pentanediol using a ternary skeletal CuAlZn catalyst. Green Chem. 2016, 18, 5999−6003. (19) Grasemann, M.; Laurenczy, G. Formic acid as a hydrogen source−recent developments and future trends. Energy Environ. Sci. 2012, 5, 8171−8181. (20) Reichert, J.; Brunner, B.; Jess, A.; Wasserscheid, P.; Albert, J. Biomass oxidation to formic acid in aqueous media using polyoxometalate catalysts−boosting FA selectivity by in-situ extraction. Energy Environ. Sci. 2015, 8, 2985−2990. (21) Wang, X.; Meng, Q.; Gao, L.; Jin, Z.; Ge, J.; Liu, C.; Xing, W. Recent progress in hydrogen production from formic acid decomposition. Int. J. Hydrogen Energy 2018, 43, 7055−7071. (22) Yu, J.; Savage, P. E. Decomposition of formic acid under hydrothermal conditions. Ind. Eng. Chem. Res. 1998, 37, 2−10. (23) Johnson, T. C.; Morris, D. J.; Wills, M. Hydrogen generation from formic acid and alcohols using homogeneous catalysts. Chem. Soc. Rev. 2010, 39, 81−88.

Co@C-N800 (nontemplated) by following the above-mentioned procedure. Moreover, cobalt was loaded after the C-N material is prepared, and this material is designated as Co/C-N800. C-N800 was initially prepared using OPD, melamine, and colloidal silica in a similar way as Co@C-N800. Then, CoCl2·6H2O was impregnated on C-N800 and carbonized at 800 °C in an Ar/H2 atmosphere for 2 h. For comparative purposes, Co-embedded activated carbon (AC) (designated as Co/AC800) was prepared by the wet-impregnation method. Initially, CoCl2·6H2O was impregnated on an AC support, and the resultant solution was dried and subjected to carbonization at 800 °C in an Ar/H2 atmosphere for 2 h. Catalyst Characterization. The details of the instruments and characterization techniques used in this study are provided in the Supporting Information. Catalytic Reactions. Procedures for the selective reduction and reductive N-formylation of N-heterocyclic arene such as quinoline in the presence of formic acid are provided in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02307.

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Details for materials characterization and procedure of catalytic reactions and procedure for quinoline adsorption test. Figures S1−S7: XRD, C 1s XPS, photographs of reaction mixture, UV−vis spectra of quinoline adsorption, CO2 TPD profiles, XRD of fresh and recycled catalyst, and influence of various parameters in N-formylation of quinoline reaction. Scheme S1: plausible mechanism for the reductive N-formlylation of quinoline in toluene. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-1881-242175. Fax: +91-1881-223395. ORCID

Rajendra Srivastava: 0000-0003-2271-5376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to SERB-DST, New Delhi, for funding (EMR/2016/001408) and Jeol India for HRTEM analysis. The authors appreciate the support provided by CIFIIT Guwahati for Raman analysis and IIT Kharagpur for XPS analysis.

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REFERENCES

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Solvent-Dependent, Formic Acid-Mediated, Selective Reduction and

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