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Applied Catalysis A: General 403 (2011) 104– 111 Contents lists available at ScienceDirect Applied Catalysis A: General jo u r n al hom epage: www.elsev ier .com Nitroge rtin catalyz Zhijun H , Gu a Beijing Nation y, Chin b Graduate Uni a r t i c l Article history: Received 11 M Received in re Accepted 20 Ju Available onlin Keywords: Heterogeneou Nanomaterials Surface chemi Copper Arylation ts we can b tal na opart ined ction e cat ance o activ lytic ent su 1. Introduction Graphite (CDG) [3–5 ing block fo oxidized gr are located tate the abs [6–8]. Such catalytic m (g-C3N4) is ered structu in building by theoreti very high t is suitable C3N4 (g-C3N shown prom alysts [17– [22,23]. Ho for catalysi mesostruct template m ∗ Correspon E-mail add involves the pyrolysis of nitrogen-rich precursors, which incor- porating triazine rings or generating them through condensation 0926-860X/$ – doi:10.1016/j. oxide (GO) [1,2] and chemically derived graphene ] have been extensively explored as versatile build- r nanomaterials. Layered structures of GO are partially aphitic sheets where epoxide and hydroxyl groups on the basal planes. These functional groups facili- orption and anchoring of ions, molecules and clusters graphitic sheets are promising stabilizing agents for etal nanoparticles [9–11]. Nitrogen-rich carbon nitride another type of graphene-like two-dimension (2D) lay- re [12–14]. Carbon nitride exhibits various possibilities up lattice with exciting new properties, as predicted cal study predicts [15,16]. Most carbon nitride has a hermal and chemical stability. Its electronic structure for catalyzing many chemical reactions. Graphitic- 4) and some incompletely condensed precursors have ising applications in heterogeneous metal-free cat- 19], photocatalysts [18,20,21], and catalyst supports wever, poor porosity and lacking of mesostructure s limit their applications. To enhance its performance, ured graphitic-C3N4 was synthesized through hard- ethods [24–26]. The synthesis of g-C3N4 generally ding authors. Tel.: +86 10 62634920; fax: +86 10 62559373. resses: lifb@iccas.ac.cn (F. Li), yuangq@iccas.ac.cn (G. Yuan). and polymerization [26–29]. The thermal treatment results in poor porosity in the bulk materials. However, incompletely condensed fragment could be obtained under controlled synthesis conditions [30]. In this work, nitrogen-rich copolymeric fragment sheets were synthesized through controlled solution condensation of melamine and cyanuric chloride. The resulting layered structures can be well-dispersed in some polar solvents by sonication like graphite oxide. Furthermore, the presence of nitrogen in the lattice improves its electronic structure for the interaction with active catalytic centers, such as ions, metal complexes, and nanoparticles. Well- dispersed fragment sheets in water were used as stabilizing agents for “bottom-up” synthesis of supporting metal nanoparticles and the resulting nanocomposite was proven to be an active and recy- clable catalyst for arylation of N-heterocycles. In contrast to numerous efficient homogeneous Pd and Cu catalyzed C–N coupling reactions [31,32], few really recyclable catalysts for C–N coupling under heterogeneous conditions have been developed. Many supports have been attempted specifi- cally for the coupling of unsaturated N-heterocycles with aryl halides. Majoral and co-workers reported an iminopyridine lig- and grafted onto phosphorus dendrimers for supporting copper (I), which catalyzed the coupling of aryl iodides or vinyl bromides with pyrazole at 25–80 ◦C [33]. Ligands or copper complexes sup- ported over silica, the apatite support, magnetic nanoparticles have been designed for the coupling of N-heterocycles with aryl halides see front matter © 2011 Elsevier B.V. All rights reserved. apcata.2011.06.019 n-rich copolymeric microsheets suppo ing arylation of N-heterocycles uanga,b, Fengbo Lia,∗, Bingfeng Chena,b, Fei Xuea,b al Laboratory of Molecular Science, Laboratory of New Materials, Institute of Chemistr versity of Chinese Academy of Sciences, Beijing 100049, PR China e i n f o ay 2011 vised form 15 June 2011 ne 2011 e 25 June 2011 s catalysis stry a b s t r a c t Nitrogen-rich copolymeric microshee and cyanuric chloride. The materials supports or stabilizing agents for me their interaction with the copper nan ticles over these supports were obta characterized by powder X-ray diffra photoelectron spectroscopy (XPS). Th tion to evaluate the catalytic perform yield of coupling products and retain phology transforming during the cata microsheets were proven to be excell / locate /apcata g copper nanoparticles for ochang Chena,b, Guoqing Yuana,∗ ese Academy of Sciences, Beijing 100190, PR China re synthesized through incomplete condensation of melamine e well-dispersed in polar solvent and perform excellently as noparticles. The presence of nitrogen in the lattice improves icle precursor (Cu(II) ions). Monodispersed copper nanopar- after reducing the absorbed precursors. The materials were (XRD), transmission electron microscopy (TEM), and X-ray alytic C–N coupling reaction was selected as the model reac- f the supported nanoparticles. The catalysts show above 90% ity after a five-run recycling test. Surface chemistry and mor- process was further investigated. Nitrogen-rich copolymeric pports and stabilizing agents for metal nanoparticles. © 2011 Elsevier B.V. All rights reserved. Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104– 111 105 [34–36]. Cu or Cu2O nanoparticles are another type of active hetero- geneous catalyst for the C–N coupling reaction [37,38]. In this study, nitrogen-rich copolymeric sheets were synthesized and applied as a support or stabilizing agent for copper nanoparticles. The catalytic C–N couplin the model ported copp 2. Experim 2.1. Chemic Cu(II) carbonate, dimethylfor Sinopharm from powd cesium carb purchased f 2.2. Nitroge nanoparticle Melamin and subseq then the m ride (9.22 g, stirring. Aft to room tem ture was co 50 mL DMF temperatur additional 1 remained a nitrogen at ature and t solution. Th and dichlor twice. The s 50 mg of sonication f and the me was treated hydrate wa trifugation, The proces uum at 100 determined 2.3. Catalys X-ray p with an ES tific using 3 × 10−9 mb line at 284.6 software su facturer wa Transmi 2010 TEM w diffraction ( The copper coupled pl Perkin Elme 1H NMR 400 MHz NM a and b) TEM images of the as-synthesized nitrogen-rich copolymeric eets; (c) powder X-ray diffraction pattern of the sample. o TMS (ı 0.0 ppm). Chemical shifts (ı) were reported as part llion (ppm). 13C NMR spectra were referenced to CDCl3 (ı pm, the middle peak). Mass spectra were obtained with e electron spray ionization on Waters Micromass ZQ4000 ent. Melting points were determined through a Buchi ent. Elemental analysis data were obtained with FLASH 2. g of N-heterocycles with aryl bromide was selected as reaction to evaluate the catalytic performance of sup- er nanoparticles. ental als and materials acetate, CuI, sodium carbonate, potassium triethylamine, dimethylsulfoxidem (DMSO), N,N- mamide (DMF) and pyridine were purchased from Chemical Reagent Co. Ltd. All solvents were distilled er CaH2 before using. Cyanuric chloride, melamine, onate, all the N-heterocycles and aryl bromine were rom Aldrich. All the reagents were used as received. n-rich copolymeric microsheets supporting copper s e (12.61 g, 0.1 mol) was suspended in 100 mL dry DMF uently a pyridine solution (48.3 mL, 0.6 mol) was added, ixture was cooled to 0 ◦C. A solution of cyanuric chlo- 0.05 mol) in 50 mL DMF was then added dropwise with er 6 h at 0 ◦C, the reaction temperature was increased perature and stirred for an additional 24 h. The mix- oled to 0 ◦C and cyanuric chloride (9.22 g, 0.05 mol) in was then added drop wise with stirring. The reaction e was increased to room temperature and stirred for an 2 h. Finally, the system was heated to 80 ◦C for 12 h, and t 100 ◦C for 12 h. All above process was carried out under mosphere. The flask was then cooled to room temper- he precipitates were collected by decanting out DMF e solid product was washed with methanol, acetone, omethane subsequently and this process was repeated olid was dried under vacuum at 100 ◦C. the support was dispersed in 50 mL deionized water by or 1 h. Copper acetate was used as the metal precursors tal load was 5 wt%. After introducing Cu(II), the mixture under sonication for additional 1 h, and then hydrazine s added. The resulting materials were collected by cen- washed with methanol, acetone, and dichloromethane. s was repeated twice. The solid was dried under vac- ◦C and stored under nitrogen. The molar copper content by XPS [Cu/(Cu + N + C + O)] is 7.66%. t characterization hotoelectron spectroscopy (XPS) data were obtained CALab220i-XL electron spectrometer from VG Scien- 300 W Al K� radiation. The base pressure was about ar. The binding energies were referenced to the C 1s eV from adventitious carbon. Eclipse V2.1 data analysis pplied by the VG ESCA-Lab200I-XL instrument manu- s applied in manipulation of the acquired spectra. ssion electron microscopy (TEM) was obtained by a JEOL ith an accelerating voltage of 200 kV. Powder X- ray XRD) data were recorded by using Rigaku D/max-2500. content in the catalyst was determined by inductively asma atomic emission spectroscopy (ICP-OES) using r Optima 5300dv. and 13C NMR spectra were recorded on a Varian-Unity R Spectrophotometer. Spectra were referenced inter- Fig. 1. ( microsh nally t per mi 77.16 p positiv instrum instrum EA111 106 Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104– 111 Scheme 1. Di by sonication 2.4. Genera N-heterocyc In an ov with respec (1 mmol), b atmosphere catalyst wa washed wi temperatur washed wit over anhyd to get the c column chr terization d Information 3. Results 3.1. Synthe microsheets Nitrogen der X-ray (TEM). Fig reflection spacing = 0. phous broa spacing valu patterns as spacing = 0. oligomeric (1 0 0) peak TEM image clearly discr by the pow Asymme nents relat 1: 2� = 26.9 spacing = 0. The presenc (0 0 2) (d (0 to determin be discusse that the nit substrate is a) TEM image of nitrogen-rich copolymeric microsheets supporting copper ticles; (b) Cu 2p envelop e in XPS spectrum of supported copper nanoparti- ding energy of Cu 2p3/2 is labeled as 932.4 eV. spersing nitrogen-rich copolymeric microsheets in deionized water and introducing of copper nanoparticles. l procedure for catalytic C–N coupling of les with aryl bromide en dried 10 mL round-bottom flask, catalyst (10 mol% t to aryl bromine), aryl halide (1 mmol), N-heterocycles ase (2 mmol), DMSO (2 mL) were stirred under argon at 120 ◦C. After the completion of the reaction, the s precipitated with dichloromethane, filtered off and th methanol followed by acetone and dried at room e. The filtrate was diluted with dichloromethane and h saturated NaCl solution. The organic layer was dried rous MgSO4 and concentrated under reduced pressure rude product. Then the crude product was purified by omatography to afford pure product. Detailed charac- ata of C–N coupling products are provided in Supporting . and discussion sis and characterization of nitrogen-rich copolymeric supporting copper nanoparticles -rich copolymeric sheets were characterized by pow- diffraction and transmission electron microscopy . 1c is the powder XRD pattern and a strong peak is clearly identified at 2� = 26.00–29.00◦ (d- 330–0.312 nm), with which there appears an amor- d peak. The sharp peak is attributed to the interplaner- e for the (0 0 2) planes, which shows the similar layered a graphite [39]. In addition, a peak at 2� = 19.66◦ (d- Fig. 2. ( nanopar cles, bin 4512 nm) indicates the slight possibility of forming or polymeric crystals, which might be attributed to the of graphite-like C3N4 microcrystal [40–43]. Based on s (Fig. 1a and b), an array of layered structure can be iminated. This confirms the layered structure described der XRD pattern. trical (0 0 2) line is divided into three compo- ed to polymorphism. They are identified as Peak- 8◦ (d-spacing = 0.3302 nm), Peak-2: 2� = 27.58◦ (d- 3231 nm), Peak-3: 2� = 28.54◦ (d-spacing = 0.3125 nm). e of nitrogen in the lattice decreases d-spacing value of 0 2) for graphite is 0.340 nm) [44–47]. XPS was applied e the chemical composition. The detailed data would d in the following sections. Elemental analysis indicates rogen content is higher than 60 wt% and this proves the really ‘nitrogen-rich’. Nitrogen deionized w into the sol Then light the resultin XPS (Fig. 2b in a monod copper nan are located ded into ga nanoparticl (XPS). The that suppor -rich copolymeric sheets could be well dispersed in ater through sonication. Cu(II) ions are introduced ution and the reducing agent is added after sonication. floccule is formed (Scheme 1). After post- treatment, g materials were characterized by TEM (Fig. 2a) and ). TEM image shows that copper nanoparticles are kept ispersed state and particle size is under 10 nm. Most oparticles have a filigree shape. Some nanoparticles at the surface of the support and some are embed- ps between layers. Chemical states of supported copper es were measured by X-ray photoelectron spectroscopy binding energy of Cu 2p3/2 is 932.4 eV. This indicates ted copper species are kept in a zero-valent chemical Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104– 111 107 Fig. 3. (a) Cou version with r bromide and i state. Herei enriched m 3.2. Catalyt Catalytic reaction to nanocompo of reaction is Cs2CO3. T perature of chelating ab CuI catalyze 75.7% (entr coupling re serious afte sible. Unsu low catalyt and activat activity. Fifteen s system has ity decrease The activity Table 2). T pling of benzene bromide and imidazole, the curve of imidazole con- eaction time; (b) five-run test of recyclability for coupling of benzene midazole. n, monodispersed copper nanoparticles over nitrogen- icrosheets were obtained. ic results for arylation of N-heterocycles arylation of N-heterocycles was selected as a model evaluate the catalytic activity of the as-synthesized site. Table 1 shows the results of screening experiment conditions. DMSO is the best solvent. The best base he target product yield reaches 92% at reaction tem- 120 ◦C. Nitrogen-rich copolymeric microsheets have ility with copper species. They can act as the ligand for d coupling reaction [48] and the yield of this process is y 8). This is an acceptable result for CuI-catalyzed C–N action [49,50]. However, leaching out of the CuI is very r reaction and the recycling of the catalyst is not pos- pported reduced copper specie (entry 10) shows very ic activity. Copper nanoparticles over graphite oxide ed carbon were tested and exhibited middle catalytic ubstrates, as shown in Table 2, were tested. The reaction high activity for some simple substrates, but the activ- s sharply when using benzoimidazole as N-heterocycle. of different aryl halide is ArI > ArBr > ArCl (entry 1 of he alteration of substituting groups of aryl bromide Fig. 4. (a) TEM supported cop shows relat effect plays the electron Catalytic ticles are benzene br increases g hour initiat 20 h runnin five-run re zole (Fig. 3b of dichloro by acetone activity slig than 50% (y image of the used catalyst; (b) Cu 2p envelop e in XPS spectrum of per nanoparticles, binding energy of Cu 2p3/2 is labeled as 931.8 eV. ively little influence on the catalytic activity. The steric a governing role on the activity of coupling reaction and ic effect of substituting group is minute. stability and recyclability of supported metal nanopar- further investigated through the model reaction of omide with imidazole. The conversion of imidazole radually with reaction running (Fig. 3a). There is a five- ing stage. The curve reaches a plateau above 90% after g. The recyclability of the catalysts was studied in a cycling for coupling of benzene bromide with imida- ). After each run, the catalysts were isolated by adding methane, filtered off and washed with water followed and dried at room temperature for the next run. The htly dropped run by run, and sharply decreased to less ield) in the fifth run. In our experiments, unsupported 108 Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104– 111 Table 1 Screening experiments of reaction conditions. . Entry Catalyst Reaction temperature (◦C) Solvent Base Isolated yield (%) 1 Cu-NP 120 DMSO NEt3 No product 2 Cu-NP 120 DMSO K2CO3 37.2 3 Cu-NP 120 DMSO K3PO4 64.8 4 Cu-NP 120 DMF Cs2CO3 76.1 5 Cu-NP 120 DMSO Cs2CO3 92.0 6 Cu-NP 110 DMSO Cs2CO3 63.5 7 Cu-NP 90 DMSO Cs2CO3 29.7 8 CuI 120 DMSO Cs2CO3 75.7 9 Cu(II) acetate 120 DMSO Cs2CO3 15.1 10 Cu(II) acetate/NH2NH2a 120 DMSO Cs2CO3 32.2 11 Cu-GOb 120 DMSO Cs2CO3 56.3 12 Cu-ACc 120 DMSO Cs2CO3 48.6 Cu-NP: nitrogen-rich copolymeric microsheets supporting copper nanoparticles. a Cu(II) acetate was in situ reduced by NH2NH2. b Copper nanoparticles supported over graphite oxide (GO). c Copper nanoparticles supported over activated carbon (AC). reduced copper species cannot be recycled. Copper nanoparticles over graphite oxide and activated carbon show sharply activity decrease in the second run. 3.3. Surface chemistry and morphology transformation during catalytic arylation of N-heterocycles Surface chemistry and morphology of the supported catalysts have subtle relation with several aspects of performance of the cat- alysts. Greater insight into the catalytic behavior of the supported nanoparticl istry and m N-heterocy TEM ima state of cop (Fig. 2a) rev after catalytic arylation of N-heterocycles. Further inspection of Fig. 4a shows that copper nanoparticles transfer to the support surface and aggregate to a certain extent. The main copper nanopar- ticles have the size of around 10 nm. This indicates that copper nanoparticles are catalytic sites and their involvement in the cat- alytic process remodels their morphology. However, the copper nanoparticles are kept in a well-dispersed state. The exact cop- per load amount of the fresh catalyst and the used catalyst are determined by ICP-OES and distinctly leaching of metal has not been observed. Binding energy of Cu 3d of the used catalyst is V (Fi sh ca urfa 3 2− rmin N-he es can be gained through interpreting surface chem- orphology transformation during catalytic arylation of cles. ge of the used catalyst (Fig. 4a) shows the dispersing per nanoparticles. Comparison with the fresh catalyst eals that the particles grow bigger and lose their shape 931.8 e the fre ticles’ s Br−, CO transfo tion of Scheme 2. Defective graphitic structure from direct condensation o 3/2 g. 4b), which has −0.6 eV difference from Cu species of talyst (Fig. 2b). This illustrates that the copper nanopar- ces have adsorbed or chelated with some anions such as in reaction mixture. Surface chemistry and morphology g of the supported nanoparticles during catalytic aryla- terocycles passivate their catalytic activity and directly f melamine and cyanuric chloride. Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104– 111 109 Table 2 Results of catalytic C–N coupling of various substrates. Entry Substrate Het-NH Product Isolated yield (%) 1 Br 2 3 4 5 6 7 8 9 10 11 12 13 Reaction cond (2 mL) were st lead to the the main re in the 5th r We drew of nitrogen catalyst. Fir up the latt ideal mode cyanuric ch defects dist 3, and 4. Su interplaner (0 0 2) peak lattice leads N HN N HN N HN Br NHN Br N HN Br NHN Br N HN Br NPh2 N HN Br NPh2 NHN O Br N HN O Br NHN Br N HN Br NHN Br N HN itions: I