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Research Article pubs.acs.org/journal/ascecg

N‑Formylation of Amines with CO2 and H2 Using Pd−Au Bimetallic Catalysts Supported on Polyaniline-Functionalized Carbon Nanotubes Pengpeng Ju,†,#,§ Jinzhu Chen,*,†,§,‡ Aibing Chen,*,# Limin Chen,‡ and Yifeng Yu# †

College of Chemistry and Materials Science, Jinan University, No. 601 Huangpu Avenue West, Tianhe District, Guangzhou 510632, P.R. China # College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, No. 70 Yuhua Road, Shijiazhuang 050018, P.R. China § Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 2 Nengyuan Road, Wushan, Tianhe District, Guangzhou 510640, P.R. China ‡ Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control. School of Environment and Energy, South China University of Technology, 382 Zhonghuan Road East, Guangzhou Higher Education Mega Centre, Panyu District, 510006, P.R. China S Supporting Information *

ABSTRACT: Bimetallic Pd−Au catalyst was prepared by depositing the Pd−Au alloy nanoparticles on polyanilinefunctionalized carbon nanotubes (PANI-CNT) and the resulting Pd−Au/PANI-CNT catalyst exhibited excellent catalytic activity for the N-formylation of pyrrolidine using CO2/H2. The structural and electronic properties of the Pd− Au/PANI-CNT was characterized by X-ray powder diffraction (XRD), nitrogen adsorption−desorption, transmission electron microscopy (TEM), high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS). Under optimized conditions, an N-formylpyrrolidine yield of 98.3% was obtained from pyrrolidine and CO2/H2 at 125 °C by using Pd−Au/PANICNT with a Pd/Au molar ratio of 1:1. Our research further reveals that Pd atoms should be the true active sites for the hydrogenation reaction and the N-formylation reaction might occur mainly over Pd atoms or over the interface between Pd atoms and Au atoms for the bimetallic Pd−Au/PANI-CNT catalyst. The enhanced catalytic performance of bimetallic Pd−Au/PANI-CNT is mainly related to beneficial interactions between Pd atoms and Au atoms, leading to changes of the electronic properties of the formed bimetallic Pd−Au nanoparticles. KEYWORDS: Bimetallic catalyst, Carbon dioxide, Electronic property, Formylation, Hydrogenation

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H2 system.31−38 Formamides are typically important intermediates with widespread applications in organic synthesis and pharmaceutical compound synthesis.39−44 For example, formamides are useful reagents in Vilsmeier formylation reaction,45 important precursors in the preparation of formamidines, isocyanides, and heterocycles, and intermediates for the production of methylated amines from amines; they also serve as Lewis base catalysts in hydrosilylation and alkylation of carbonyl compounds.46,47 Moreover, N,N-dimethylformamide served as important industrial solvent as well as versatile reagent for synthetic transformation.48,49

INTRODUCTION

As a renewable, abundant, low-cost, and nontoxic C1 resource, the transformation of carbon dioxide (CO2) into value-added compounds has attracted wide attention in both the fields of green and synthetic chemistry.1−7 Moreover, the utilization of CO2 as a carbon source for fine chemical synthesis is one of the primary methods to reduce the amount of CO2 in the atmosphere.8−12 Recent research reported catalytic N-formylation of amines to formamides with CO2 and hydrogen (H2) as carbonylation reagent.13−15 In addition to H2, hydrosilane, hydrosiloxane, and polymethylhydrosiloxane (PMHS) were investigated as reducing agents for N-formylation of amines with CO2.16−30 Generally, CO2/hydrosilane and CO2/hydrosiloxane systems allow diverse amine scope and mild reaction conditions for formamide synthesis when compared with CO2/ © 2017 American Chemical Society

Received: November 28, 2016 Revised: January 18, 2017 Published: February 7, 2017 2516

DOI: 10.1021/acssuschemeng.6b02865 ACS Sustainable Chem. Eng. 2017, 5, 2516−2528

Research Article

ACS Sustainable Chemistry & Engineering The N-formylation reactions with CO2/H2 as carbonylation reagents were reported by mainly using homogeneous ruthenium-complex catalysts such as Ru-pincer-type complexes14 RuCl2(PMe3)4 [PMe3 = trimethylphosphine],16,17 RuCl 2 (dppe) 2 [dppe = 1,2-bis(diphenylphosphino)ethane],18−20 RuCl2(dppbz)2 [dppbz = 1,2-bis(diphenylphosphino)benzene],21 [HRu3(CO)11]−,22 PdCl2/KHCO3,23 Pt2(μ-dppm)3 [dppm = bis(diphenylphosphino)methane],24,25 (PPh 3 ) 2 (CO)IrCl, 2 6 molybdenum-silyl complex, 2 7 (PPh3)3CuCl,26 Fe(BF4)2·6H2O-phosphine ligand,28,29 and Co(BF4)2·6H2O-phosphine ligand.30 In the case of CO2/ hydrosilane- and CO2/hydrosiloxane-based homogeneous carbonylation systems, the investigated catalytic systems involved both transition metal complexes such as Cudiphosphine complexes-PMHS,31,32 chelating bis(tzNHC) [tz = 1,2,3-triazol-5-ylidene, NHC = N-heterocyclic carbene)] rhodium complexes-Ph2SiH2,33 Ir(H) (CF3SO3) (NSiN) (coe) [NSiN = fac-coordinated bis(pyridine-2-yloxy)methylsilyl, coe = cis-cyclooctene]-HSiMe(OSiMe3)2,34 and organocatalytic systems including imidazolium-based ionic liquids: PhSiH3,35 1,3,2-diazaphospholene-Ph2SiH2,36 NHC-PMHS,37 and thiazolium carbine-PMHS.38 Homogeneous ruthenium complex catalysts evidently show excellent catalytic performance for the N-formylation of amines with CO2. Although a modification of heterogeneous catalyst Ru/Al2O3 with phosphine ligand dppe was recently reported for N-formylation of 3-methoxypropylamine with supercritical CO2 and H2, the formation of a Ru-dppe species was suggested as the “true” homogeneous catalyst for this transformation. Notably, as far as we known, very limited heterogeneous catalysts including Pd/Al2O3-NR-RD (NR, nanorod; RD, reductive deposition),50 Ir/HSA-TiO2 (HSA, high surface area),51 and Cu/ZnO were reported for N-formylation of amines with CO2.52 In addition to N-formylation of amines with CO2, recent research reported reductive methylation of amines to give Nmethyl amines or N,N-dimethyl amines by using CO2 and a reducing agent. The investigated catalytic systems included [Pt] Karstedt’s catalyst-HCOOH,53 Pt-MoOx/TiO2-H2,54 Au/ Al 2 O 3 -VS-H 2 , 55 Pd/CuZrO x -H 2 , 56 [Cu(O t Bu)-(IMes)]PhSiH 3,57 ZnCl2 -IPr-PhSiH3 ,58 N-heterocyclic carbenesPhSiH3,59 and B(C6F5)3-PhSiH3.60 The reported catalytic systems for N-formylation (Table S1), N-methylation, and N,N-dimethylation (Table S2) of various amines with CO2 were systematically compared in the Supporting Information. Besides CO2/H2, CO was also investigated as the carbonylation reagent of amines for the synthesis of amides. For example, Dyson and co-workers reported Pd-promoted oxidative carbonylation of C(sp3)−H bonds with CO and amines to give substituted phenyl amides.61 In industry, N,Ndimethylformamide (DMF) is obtained by dimethylamine carbonylation with CO using CH3ONa catalyst in methanol.62 Compared with the toxic CO, CO2 is abundant, cheap, and safe. However, due to kinetic inertness and thermodynamic stability, reductive amination of CO2 to give amide remains a great challenge although highly attractive. Pd and Au were reported respectively as heterogeneous catalysts for CO2 reduction.50,55,56 Notably, Pd−Au bimetallic catalysts have recently attracted widespread attention and are the most widely investigated bimetallic catalyst systems.63−67 Generally, Pd−Au bimetallic catalyst shows easy accessibility as well as broad catalytic scope in oxidation, coupling, hydrogenation, hydrogenolysis, and so on.68−74 Typically, Pd−Au

bimetallic catalysts were commercially used in the industrial production of vinyl acetate and extensively investigated in various reactions such as oxidation of alcohols to aldehydes, low-temperature CO oxidation, Suzuki coupling reaction, direct H2O2 synthesis from H2 and O2, acetylene trimerization, hydrodechlorination of chlorine-containing pollutants, hydrodesulfurization of sulfur-containing compounds, hydrogenation of hydrocarbon, catalytic reduction of 4-nitrophenol with an excess amount of NaBH4, electrochemical hydrogen generation, electrooxidation of formic acid, and electrocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid.65,75 Among these reactions, Pd−Au bimetallic catalyst often exhibit remarkably enhanced catalytic performances when compared with either of the constituent metals. The enhancement of the catalytic performance for Pd−Au bimetallic catalyst is sometimes caused by synergetic effect, alloy effect, ensemble effect, and ligand effect, which is often closely correlated with its alloy composition and controllable morphology.76−84 Previously, a polyaniline (PANI) immobilized bimetallic Pd− Au catalyst was reported for benzyl alcohol oxidation.85 Vanadium dioxide (VO2) supported on PANI-functionalized carbon nanotubes (PANI-CNT) was examined for 5-hydroxymethylfurfural transformation into 2,5-diformylfuran.86 Palladium nanoparticles deposited on PANI-CNT were investigated in selective hydrogenation of phenol to cyclohexanone.87−89 Moreover, Pt−Ru bimetallic catalysts supported on PANI-CNT were reported for electrochemical oxidation of methanol.90,91 Therefore, in this research, we demonstrated the preparation of bimetallic Pd−Au catalysts deposited on composite PANI-CNT owing to stabilization effect of PANI toward nanoparticles and reversible acid/base chemistry, redox property as well as nonsolubility of PANI in most organic solvents. The bimetallic Pd−Au/PANI-CNT exhibits a narrow size distribution of Pd− Au nanoparticles with an average nanoparticle size of 3.0 nm over PANI-CNT. Moreover, heterogeneous catalyst Pd−Au/ PANI-CNT shows outstanding catalytic activity toward the Nformylation of pyrrolidine with the corresponding N-formylpyrrolidine yield of 98.3% at 125 °C using CO2/H2 (Scheme 1). The Pd−Au bimetallic catalyst, containing Pd/Au in a molar ratio of 1:1, is significantly more active than pure Pd/PANICNT and pure Au/PANI-CNT catalysts, indicating a strong alloy effect. Our research further reveals that Pd atoms should be the true active sites for the hydrogenation reaction and the N-formylation reaction might occur mainly over Pd atoms or over the interface between Pd and Au in the phase of alloyed Scheme 1. N-Formylation of Pyrrolidine to NFormylpyrrolidine over Pd−Au/PANI-CNT, Pd/PANICNT, and Au/PANI-CNT Catalysts

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1-Formyl-4-methylpiperazine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.09 (s, 1H), 3.62−3.50 (m, 2H), 3.44−3.32 (m, 2H), 2.42−2.33 (m, 4H), 2.30 (s, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.76, 55.30, 54.07, 46.03, 45.52, 39.76. 3,4-Dihydroisoquinoline-2(1H)-carbaldehyde. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.23 (s, 1H), 7.36−6.95 (m, 4H), 4.62 (s, 2H), 3.72 (t, J = 5.9 Hz, 2H), 3.00−2.71 (m, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.14, 161.59, 134.43, 133.55, 132.17, 131.67, 129.26, 129.00, 127.27, 126.88, 126.80, 126.72, 126.64, 125.98, 47.56, 43.53, 42.47, 38.30, 29.77, 27.84. 1,4-Diformylpiperazine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.02 (s, 1H), 3.50 (dd, J = 15.9, 10.4 Hz, 2H), 3.32 (dd, J = 15.1, 9.9 Hz, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.90, 160.78, 45.98, 44.86, 40.40, 39.35. N-n-Butylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.04 (s, 1H), 6.74 (s, 1H), 3.18 (dd, J = 13.2, 6.6 Hz, 2H), 1.53−1.35 (m, 2H), 1.26 (dq, J = 14.5, 7.2 Hz, 2H), 0.83 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 165.38, 161.89, 41.79, 37.91, 32.99, 31.31, 19.90, 19.42, 13.50, 13.45. N-n-Octylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.14 (s, 1H), 5.85 (s, 1H), 3.53−3.04 (m, 2H), 1.50 (d, J = 6.6 Hz, 2H), 1.27 (d, J = 10.8 Hz, 10H), 0.99−0.75 (m, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 164.89, 161.40, 41.96, 38.34, 31.85, 31.31, 29.61, 29.25, 26.95, 26.49, 22.72, 14.16. N-n-Propylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.15 (s, 1H), 5.92 (s, 1H), 3.21 (dd, J = 13.5, 6.7 Hz, 2H), 1.55 (dt, J = 14.5, 7.3 Hz, 2H), 0.92 (td, J = 7.4, 2.6 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 165.20, 161.66, 43.86, 40.05, 24.42, 22.81, 11.37, 10.97. N-Benzylformamide. 1H NMR (400 MHz, 25 °C, CD3CN) δ 8.17 (s, 1H), 7.44−7.21 (m, 5H), 4.37 (d, J = 6.3 Hz, 2H). 13C NMR (101 MHz, 25 °C, CD3CN) δ 165.7, 162.22, 140.27, 140.03, 129.49, 128.36, 128.12, 128.07, 45.83, 42.12. N,N-Diethylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 7.95 (s, 1H), 3.26 (q, J = 7.2 Hz, 2H), 3.18 (q, J = 7.2 Hz, 2H), 1.09 (t, J = 7.2 Hz, 3H), 1.03 (t, J = 7.2 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.06, 41.73, 36.46, 14.79, 12.66. N,N-Di-n-butylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.02 (s, 1H), 3.38−2.97 (m, 4H), 1.50 (m, 4H), 1.38−1.19 (m, 4H), 0.92 (dd, J = 9.5, 4.6 Hz, 6H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.01, 46.55, 41.25, 30.23, 28.78, 19.68, 19.17, 13.28, 13.13. N-Cyclohexylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.05 (s, 1H), 6.17 (s, 1H), 3.97−2.99 (m, 1H), 1.97−1.42 (m, 5H), 1.41−1.01 (m, 5H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 163.94, 160.13, 50.65, 46.22, 34.04, 32.31, 24.97, 24.56, 24.34. N-tert-Butylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.33−7.82 (m, 1H), 1.30 (d, J = 15.5 Hz, 10H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 163.22, 160.58, 51.26, 50.33, 30.84, 28.91. N-Phenylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.68 (s, 1H), 8.36 (s, 1H), 7.60−7.48 (m, 1H), 7.43−7.28 (m, 2H), 7.24− 7.05 (m, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.95, 159.30, 137.03, 136.95, 129.86, 129.21, 125.39, 124.92, 120.15, 118.92.

bimetallic nanoparticles in Pd−Au/PANI-CNT catalyst. The enhanced catalytic performance of alloyed bimetallic Pd−Au/ PANI-CNT is mainly related to beneficial interactions between Pd and Au, leading to the changes of the electronic properties of the bimetallic Pd−Au nanoparticles.

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EXPERIMENTAL SECTION

Materials and Catalyst Preparation. Information on chemical materials and the preparations of PANI, PANI/CNT, acid-treated CNTs, and reduced graphene oxide (RGO) is provided in the Supporting Information. Synthesis of Various Pd−Au Bimetallic Catalysts. In a typical synthesis of Pd−Au/PANI-CNT, an aqueous solution containing HAuCl4 (13.6 mg, 1 × 10−3 M), H2PdCl4 (PdCl2 5.8 mg in 2−3 drops of hydrochloric acid, 1 × 10−3 M), and poly(vinyl alcohol) (PVA 30 mg, molar ratio of PVA monomer/(Au + Pd) = 10:1) was prepared. The mixture was stirred vigorously at 0 °C (ice−water bath) for 0.5 h. A freshly prepared aqueous solution of NaBH4 (12.4 mg, 0.1 M, 3.3 mL, molar ratio of NaBH4/(Au + Pd) = 5:1) was quickly added to give a dark brown solution. The PANI-CNT (200 mg) was then added to the above colloidal solution under vigorous stirring for 4 h. The resulting Pd−Au/PANI-CNT was filtered and washed thoroughly with deionized water, further dried under vacuum for 12 h at 80 °C, and then reactivated at 200 °C in H2 (3 MPa) for 2 h. Pd−Au/PANI, Pd−Au/CNT, Pd−Au/RGO were obtained with the same procedure used for Pd−Au/PANI-CNT except that the support PANI-CNT (200 mg) was replaced by PANI (200 mg), acidtreated CNTs (200 mg), and RGO (200 mg), respectively. Synthesis of Au/PANI-CNT and Pd/PANI-CNT. In a typical procedure for Au/PANI-CNT preparation, an aqueous solution of HAuCl4 (1 × 10−3 M, 300 mL) with poly(vinyl alcohol) (30 mg, molar ratio PVA monomer/Au = 10:1) was prepared. The mixture was stirred vigorously at 0 °C for 0.5 h. A freshly prepared aqueous solution of NaBH4 (12.4 mg, 0.1 M, 3.3 mL, molar ratio NaBH4/(Au + Pd) = 5:1) was quickly added to give a dark brown solution. The PANI-CNT (200 mg) was then added to the above colloidal solution under vigorous stirring for 4 h. The resulting Au/PANI-CNT was filtered and washed thoroughly with deionized water, further dried under vacuum for 12 h at 80 °C, and then reactivated at 200 °C in H2 (3 MPa) for 2 h.92 Pd/PANI-CNT was obtained with the same procedure used for Au/ PANI-CNT except that HAuCl4 (1 × 10−3 M, 300 mL) was replaced by H2PdCl4 (1 × 10−3 M, 300 mL). Experimental Details. In a typical N-formylation experiment, a 25 mL-stainless steel autoclave reactor was continuously loaded with a magnetic stir bar, Au−Pd/PANI-CNT catalyst (50 mg, Pd 1.6 wt %, Au 3.0 wt %, 0.008 mol % Pd relative to pyrrolidine, Pd/Au molar ratio = 1), 1,4-dioxane solvent (5.0 mL), and pyrrolidine (71 mg, 1.0 mmol). After flushing the reactor three times with CO2, the CO2 pressure was maintained at 3.5 MPa at ambient temperature in the reactor. H2 was then introduced into the reactor up to the desired partial pressure of 3.5 MPa. The stainless steel was stirred at 125 °C for 48 h. After the reaction was halted, the autoclave reactor was quickly cooled down to ambient temperature. The reaction mixtures were analyzed by gas chromatography (GC Fuli 9790II) equipped with a capillary column (KB-5, 0.32 mm × 30 m) and a flame ionization detector (FID) with nitrogen as the carrier gas. N-Formylpyrrolidine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.08 (s, 1H), 3.33 (s, 2H), 3.23 (s, 2H), 1.74 (s, 4H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.57, 45.73, 42.80, 24.59, 23.92. N-Formylmorpholine. 1H NMR (400 MHz, 25 °C, CD3CN) δ 7.98 (s, 1H), 3.65−3.58 (m, 2H), 3.58−3.53 (m, 2H), 3.46−3.41 (m, 2H), 3.37−3.32 (m, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.43, 66.78, 65.93, 45.32, 40.10. 1-Formyl-4-ethylpiperazine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 7.99 (s, 1H), 3.74−3.60 (m, 2H), 3.58−3.46 (m, 2H), 2.71 (ddd, J = 14.5, 8.8, 4.5 Hz, 6H), 1.15 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.72, 51.99, 51.95, 50.90, 43.83, 38.21, 10.21.

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RESULTS AND DISCUSSION In this research, bimetallic Pd−Au/PANI-CNT catalyst was prepared and systematically characterized by X-ray powder diffraction (XRD), nitrogen adsorption−desorption, transmission electron microscopy (TEM), high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS). For comparison purpose, bimetallic Pd−Au/PANICNT, pure Pd/PANI-CNT, and pure Au/PANI-CNT catalysts were investigated for the N-formylation of pyrrolidine to Nformylpyrrolidine using CO2/H2 as the carbonylation reagent. In addition, to probe the effect of support on the N-formylation reaction, bimetallic Pd−Au catalysts loaded on various supports including PANI, CNT, PANI-CNT, and reduced graphene oxide (RGO) were prepared, the resulting bimetallic catalysts 2518


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ACS Sustainable Chemistry & Engineering were examined for the N-formylation of pyrrolidine. In order to further understand the relationship between the structure and catalytic activity of bimetallic catalyst, Pd−Au/PANI-CNT with various Pd/Au molar ratio were synthesized, analyzed with XPS, and applied for the N-formylation reaction. The XRD patterns of CNT, PANI-CNT, RGO, and supported Pd, Au, and Pd−Au catalysts are shown in Figure 1. Both CNT and RGO exhibit two characteristic peaks. The

Figure 2 shows nitrogen adsorption−desorption isotherms of CNT, RGO, PANI-CNT, and various supported Pd, Au, and

Figure 2. Nitrogen adsorption−desorption isotherms and pore-size distribution curves of CNT, RGO, PANI-CNT, and various supported Pd, Au, and Pd−Au catalysts.

Pd−Au catalysts with an inset image of pore size distribution patterns. The resulting surface area, pore diameter, and pore volume of these materials are shown in Table 1. CNT has a Table 1. Textural Properties of CNT, RGO, PANI-CNT, and various supported Pd, Au, and Pd−Au Catalysts sample CNT PANI-CNT Pd−Au/CNT (Pd/Au = 1) Au/PANI-CNT Pd/PANI-CNT Pd−Au/PANI-CNT (Pd/Au = 1) RGO Pd−Au/RGO (Pd/Au = 1)

Figure 1. XRD patterns of CNT, RGO, PANI-CNT, and various supported Pd, Au, Pd−Au catalysts.

diffraction peak with high intensity at 26.1° corresponds to the characteristic carbon (002) plane; meanwhile, the other peak with low intensity at 43.1° matches the characteristic peak of the (100) packing of graphitic structure.93−95 After functionalization of CNT with PANI, the XRD patterns of the obtained PANI/CNT were almost the same as CNT; however, decreased intensities of the two characteristic diffraction peaks were observed.96 For Au/PANI-CNT, the characteristic reflection peaks at 38.4° and 43.9° were attributed to the (110) and (200) planes, respectively, for the metallic Au, indicating the presence of metallic Au nanoparticles.97 In the case of Pd/ PANI-CNT, the diffraction peaks in the XRD patterns at 2θ values of 39.9°, 43.7°, and 68.3° corresponded to the (111), (200), and (220) planes, respectively, for the face-centered cubic (fcc) crystalline Pd.87,95 The XRD patterns of bimetallic samples in Figure 1, including Pd−Au/PANI-CNT, Pd−Au/ PANI, Pd−Au/CNT, and Pd−Au/RGO, exhibit characteristic peaks corresponding to both crystalline Pd and Au, which confirms the presence of bimetallic nanoparticles in the samples. In addition, a comparison of characteristic peaks for (110) planes of metallic Au among Au/PANI-CNT and Pd− Au/PANI-CNT shows a slight shift toward a lower diffraction angle, indicating the formation of Pd−Au alloyed structure.98,99 Moreover, the diffraction peak for the (200) plane of Pd−Au/ PANI-CNT appears between 43.7° and 43.9°, further indicating the presence of Pd−Au alloyed structure.100 Our HAADF-STEM analysis also proves the existence of alloyed bimetallic nanoparticles.

surface area [m2 g−1]a

average pore diameter [nm]b

pore volume [cm3 g−1]a

148.62 100.48 79.40

20.72 18.05 8.53

2.70 2.45 0.60

93.81 42.41 76.47

15.31 16.02 16.13

2.00 1.42 1.80

34.75 55.51

4.00 3.97

0.44 0.58

a

Obtained by BET method. bPore size distribution curves were calculated using the adsorption branch of the isotherms and the density functional theory (DFT) method; pore sizes were obtained from the peak positions of the distribution curves.

high Brunauer−Emmett−Teller (BET) surface area up to 148.62 m2 g−1 and a large pore volume of 2.70 cm3 g−1. Moreover, the hysteresis loop at 0.9 < P/P0 < 1.0 suggests the presence of macropores. After coating CNT with PANI polymer, PANI-CNT exhibits significantly decreased surface area to 100.48 m2 g−1. After the loading of Au, Pd, and bimetallic Pd−Au nanoparticles over the support PANI-CNT, the BET surface area of the resulting Au/PANI-CNT, Pd/ PANI-CNT, and Pd−Au/PANI-CNT samples further declined if compared with their PANI-CNT support. A similar tendency was also observed in a comparison of Pd−Au/CNT with CNT. The TEM images with the corresponding histogram of bimetallic Pd−Au catalysts over various supports are shown in Figure 3. The TEM image of Pd−Au/PANI confirms that Pd− Au bimetallic nanoparticles with average particle size around 3.0 nm and narrow distribution are dispersed onto the PANI support (Figure 3a). For Pd−Au/PANI-CNT, the bimetallic Pd−Au nanoparticles with mean particle size around 3.0 nm are 2519


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Figure 4. (a−d) HAADF-STEM and (e) EDX spectrum of Pd−Au/ PANI-CNT (Pd/Au = 1).

4e), a uniform and well-dispersed alloy nanoparticle consists of both Pd and Au elements. Quantitative analysis of the averaged line scan within the EDX resolution limitation indicates a ratio of 30 wt % Pd to 70 wt % Au based on the Pd-L and Au-L lines. The EDX quantification indicates a Pd/Au molar ratio of 0.8, which is closer to the Pd/Au molar ratio of 1 obtained from XPS analysis. The surface composition of alloy Pd−Au/PANI-CNT and the oxidation states of surface elements for both Pd and Au were investigated with XPS. Figure 5a shows the scan survey XPS spectrum of Pd−Au/PANI-CNT with different Pd/Au molar ratio, indicating the existence of both Pd and Au for Pd− Au/PANI-CNT. For a typical Pd−Au/PANI-CNT (Pd/Au = 1), the high resolution Pd 3d XPS shows two main peaks at the binding energies of 335.5 and 340.8 eV assigned to Pd 3d5/2 and Pd 3d3/2 (Figure 5b), respectively, suggesting the presence of metallic Pd(0).95 While, the other two peaks at the binding energies of 341.8 and 337.1 eV with very low intensity were indexed to Pd 3d3/2 and Pd 3d5/2 (Figure 5b), respectively, demonstrating the presence of Pd(II) species.101 Pd(0) was the major phase (90.5%) for Pd species on the surface of Pd−Au/ PANI-CNT (Pd/Au = 1) based on the integration areas. In the case of Au 4f XPS of Pd−Au/PANI-CNT (Pd/Au = 1), two XPS peaks in Au 4f spectra were assigned as Au 4f7/2 and 4f5/ 2 with the binding energies of 84.0 and 87.7 eV (Figure 5c), respectively, suggesting the existence of metallic Au(0).102 The

Figure 3. Typical TEM micrographs of (a) Pd−Au/PANI (Pd/Au = 1), (b−e) Pd−Au/PANI-CNT (Pd/Au = 1), (f and g) recovered Pd− Au/PANI-CNT (Pd/Au = 1), (h and i) Pd−Au/CNT (Pd/Au = 1), and (j and k) Pd−Au/RGO (Pd/Au = 1) with the corresponding histogram of bimetallic Pd−Au nanoparticles.

observed to be evenly distributed on the PANI-CNT tube wall surface without obvious agglomeration due to the stabilization role of PANI polymer (Figure 3b−e). However, the recovered Pd−Au/PANI-CNT catalyst after four-time recycling shows significant aggregation of Pd−Au bimetallic nanoparticles over PANI-CNT support with an average nanoparticle size of 10 nm (Figure 3f and g). For Pd−Au/CNT and Pd−Au/RGO, the average particle sizes are 3.2 nm for Pd−Au/CNT (Figure 3h and i) and 3.9 nm for Pd−Au/RGO (Figure 3j and k), confirming that Pd−Au nanoparticles on PANI and PANICNT support have a smaller average particle size. This result indicates that the deposition of Pd−Au nanoparticles onto polymer PANI restrains their aggregation. HAADF-STEM imaging of Pd−Au/PANI-CNT shows a homogeneous distribution of the Pd and Au nanoparticles as green and red spots, respectively, over the PANI-CNT support (Figure 4a−d), suggesting the formation of alloyed Pd−Au bimetallic nanoparticles. The presence of Pd and Au nanoparticles was further demonstrated by energy dispersive X-ray (EDX) analysis. As shown in the STEM-EDX line scan (Figure 2520


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Figure 5. (a) XPS scan survey of Pd−Au/PANI-CNT with various Pd/Au molar ratio, (b) Pd 3d XPS spectra and (c) Au 4f XPS spectra of alloy Pd−Au/PANI-CNT with a Pd/Au molar ratio of 1, and XPS spectra of Pd−Au/PANI-CNT with various Pd/Au molar ratios in the (d) Pd 3d and (e) Au 4f region.

Table 2. XPS Binding Energies of Pd−Au/PANI-CNT with Various Pd/Au Molar Ratios binding energy [eV] 0

0

sample

Pd 3d3/2

Pd 3d5/2

Pd 3d3/2

PdII 3d5/2

Pd/PANI-CNT Pd−Au/PANI-CNT (Pd/Au = 2.5) Pd−Au/PANI-CNT (Pd/Au = 1) Pd−Au/PANI-CNT (Pd/Au = 0.5) Au/PANI-CNT

341.5 341.1 340.8 340.7

336.3 335.8 335.5 335.4

342.8 342.3 341.8 341.7

337.4 337.2 337.1 337.0

XPS analysis of Pd−Au/PANI-CNT therefore confirms the formation Pd−Au bimetallic nanoparticles in the sample. To further understand the interaction between Pd and Au in the Pd−Au/PANI-CNT, pure Pd/PANI-CNT, pure Au/PANICNT, and bimetallic Pd−Au/PANI-CNT with different Pd/Au

II

Au0 4f5/2

Au0 4f7/2

87.5 87.7 87.8 87.9

83.7 84.0 84.1 84.2

molar ratios were investigated with XPS analysis. Figure 5d and e exhibits the high-resolution XPS spectral regions of the Pd 3d and Au 4f, respectively, taken from the surface of the bimetallic Pd−Au catalysts with different Pd/Au molar ratio. The resulting results of curve fitting of XPS spectra were listed in 2521


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ACS Sustainable Chemistry & Engineering Table 2. In the case of Pd 3d XPS spectra analysis for Pd−Au bimetallic nanoparticles, a negative shift of the peak position of Pd0 3d with increasing Au content was evidently observed (Figure 5d). The maximum negative shifts of 0.8 eV for Pd0 3d3/2 and 0.9 eV for Pd0 3d5/2, respectively, relative to the binding energy of pure Pd nanoparticles were observed for the Pd/Au molar ratio of 0.5 mixture (Table 2). For the analysis of Au 4f XPS spectra, similarly, a negative shift of the peak position of Au0 4f with increasing Pd content was observed (Figure 5e). The maximum negative shifts of 0.4 eV for Au0 4f5/2 and 0.5 eV for Au0 4f7/2 relative to the binding energy of pure Au nanoparticles were observed for the Pd/Au molar ratio of 2.5 mixture (Table 2). The observed binding energy shift can be explained by the changes in the electronic structure upon alloying, indicating an electron transfer from the Au atoms to the Pd atoms.103−106 Previously, Yan and co-workers prepared heteronuclear Pd−Au nanoparticles immobilized in the functionalized ionic liquid [C2OHmim][NTf2].100 The Pd− Au nanoparticles were reported as effective catalysts for dehalogenation reactions, the author attributed the activity of the Pd−Au catalysts to charge transfer from Pd to Au and this result is in line with our observations. The obtained Pd−Au/PANI-CNT catalyst was investigated as heterogeneous catalyst for N-formylation using CO2/H2 as carbonylation reagent. Initially, the N-formylation of pyrrolidine was investigated on Pd, Au, and bimetallic Pd−Au catalysts to optimize the reaction conditions. Pure Pd catalyst Pd/PANI-CNT showed moderate activity toward pyrrolidine N-formylation with N-formylpyrrolidine yield 50.0% (Table 3, entry 1). While pure Au catalyst Au/PANI-CNT exhibited significantly lower activity to pyrrolidine N-formylation (Table 3, entry 7). To elucidate the role of Pd/Au molar ratio in the bimetallic catalyst on its catalytic performance, Pd−Au/PANICNT with different Pd/Au molar ratio was further investigated for pyrrolidine N-formylation. After mixing Pd with Au, Nformylpyrrolidine yield increased as a decreasing Pd/Au molar ratio in Pd−Au/PANI-CNT, producing a maximum of 98.3% yield at Pd/Au molar ratio of 1:1, as shown in Table 3 (entries 1−4). The enhanced catalytic performance of Pd−Au/PANICNT is mainly related to the changes of the electronic or geometrical properties of the bimetallic Pd−Au nanoparticles induced by the alloying of Au to Pd.107 Therefore, in the case of bimetallic Pd−Au/PANI-CNT catalyst, Pd atoms should be the true active sites for the hydrogenation reaction and the Nformylation reaction might occur mainly over Pd atoms or over the interface between Pd and Au in the alloyed nanoparticles. Moreover, the catalytic performance of Pd−Au/PANI-CNT should be closely associated with its structural properties and electronic configuration, which can be altered by the alloy composition. The beneficial interactions with Au evidently promote the activity of Pd on the catalytic hydrogenation, which is presumably achieved by a combination of electronic interactions between the Pd and Au, a hybridization of the valence between Pd and Au bands, and the significant dilution of Pd with Au.108 A further decreasing the molar fraction of Pd in Pd−Au/PANI-CNT, however, results in a decreased Nformylpyrrolidine yield (Table 3, entries 4−7). This observation is mainly due to the significant decrease of active site of Pd by diluting and alloying with Au, which leads to a decreased catalytic activity of the Pd−Au/PANI-CNT. Therefore, the catalytic activity of the Pd−Au/PANI-CNT on pyrrolidine Nformylation reveals a volcano type correlation with its molar ratio of Pd/Au (Table 3, entries 1−7). Previously, Pd−Au

Table 3. Pd, Au, and Pd−Au Bimetallic Catalyst-Promoted N-Formylation of Pyrrolidine Using CO2/H2 under Various Reaction Conditionsa

entry

catalyst (mg)

1

Pd/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Au/PANI-CNT (50) Pd−Au/PANI (50) Pd−Au/CNT (50) Pd−Au/RGO (50) Pd−Au/PANI-CNT (10) Pd−Au/PANI-CNT (20) Pd−Au/PANI-CNT (30) Pd−Au/PANI-CNT (40) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50) Pd−Au/PANI-CNT (50)

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

CO2 [MPa]

H2 [MPa]

yieldc [%]

3.5

3.5

50.0

3

3.5

3.5

57.1

2.5

3.5

3.5

81.1

1

3.5

3.5

98.3

0.5

3.5

3.5

26.1

0.33

3.5

3.5

5.8

3.5

3.5

3.9

1 1 1 1

3.5 3.5 3.5 3.5

3.5 3.5 3.5 3.5

8.2 16.0 68.0 5.0

1

3.5

3.5

18.4

1

3.5

3.5

60.7

1

3.5

3.5

80.0

1

1.5

1.5

58.1

1

2.0

2.0

71.1

1

2.5

2.5

82.8

1

3.0

3.0

83.3

1

4.0

4.0

98.6

1

2.0

2.0

71.1

1

2.0

3.0

90.6

1

2.0

4.0

93.9

1

2.0

5.0

94.1

1

2.0

6.0

93.6

Pd/Au molar ratiob

a Reaction conditions: pyrrolidine (71 mg, 1.0 mmol), catalyst (10−50 mg), PCO2 (1.5−4.0 MPa), PH2 (1.5−4.0 MPa), 1,4-dioxane (5 mL), T (125 °C), t (48 h). bThe molar ratio of Pd/Au in Pd−Au/PANI-CNT was determined by XPS analysis. cN-Formylpyrrolidine yields were determined by GC-FID based on pyrrolidine using biphenyl as the standard material.

catalysts were investigated for CO oxidation,109 cyclohexene hydrogenation,109 selective oxidation of benzyl alcohol85 and oxidation of alkyl aromatics,92 the catalytic performance of the bimetallic catalyst shows a correlation with the Pd/Au molar ratio. It was suggested that the ensemble effect on the bimetallic Pd−Au plays a key role on the catalytic reaction, in which, with increasing surface Au coverage and continuous dilution of surface Pd atoms by Au atoms, contiguous Pd ensembles 2522


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the temperature from 90 to 125 °C for 48 h. Then the Nformylpyrrolidine yield slightly increased to 98.5% with a raised reaction temperature to 150 °C. The influence of reaction time showed an increased N-formylpyrrolidine yield with reaction time at all reaction temperatures examined (Figure 6). Therefore, under the optimized reaction conditions, an Nformylpyrrolidine yield of 98.3% was obtained from Nformylation of pyrrolidine by using Pd−Au/PANI-CNT with Pd/Au molar ratio of 1:1 under CO2 pressure 3.5 MPa and H2 pressure 3.5 MPa at 125 °C for 48 h in 1,4-dioxane. Notably, Pd−Au/PANI-CNT shows a comparable or even higher catalytic activity on pyrrolidine formylation than the reported heterogeneous catalyst Pd/Al2O3-NR-RD (84.0% yield)50 and homogeneous catalysts such as Ru-pincer-type complexes (99% yield), 16,17 [HRu 3 (CO) 11 ] − (TON 98), 22 and [Ir(H) (CF3SO3) (NSiN) (coe)] (4.0% yield).34 To investigate the reusability of Pd−Au/PANI-CNT, a fourcycle experiment was examined. The Pd−Au/PANI-CNT catalyst was recovered by centrifugation or filtration, washed with ethanol and water, and reused for the next run. However, the obtained N-formylpyrrolidine yields reduced from 98.3 to 50.6% (Figure S2, Supporting Information). The content of palladium metal in Pd−Au/PANI-CNT slightly decreased from 1.6 to 1.1 wt % after the four recycles based on ICP analysis. TEM analysis confirmed that the recovered Pd−Au/PANICNT catalyst shows significant aggregation of Pd−Au bimetallic nanoparticles over PANI-CNT support with a mean nanoparticle size around 10 nm (Figure 3f and g). In sharp contrast, the fresh Pd−Au/PANI-CNT has an average Pd−Au nanoparticle size around 3.0 nm with narrow distribution (Figure 3b−e). Therefore, the partial loss of catalytic activity for recovered Pd−Au/PANI-CNT can presumably be related to aggregation of Pd−Au bimetallic nanoparticles. To probe the scope and limitations of the Pd−Au/PANICNT, we further examined the N-formylation of several amines with CO2 and H2. Generally, cyclic secondary amines such as pyrrolidine, morpholine, 1-ethylpiperazine, 1-methylpiperazine, and 1,2,3,4-tetrahydroisoquinoline were formylated to the corresponding amides in excellent to high yields (Table 4, entries 1−5). Notably, the formylation of cyclic diamine piperazine was also achieved in moderate yields (Table 4, entry 6). In addition, primary amines (n-butylamine, n-octylamine, and phenylmethylamine, Table 4, entries 7−9) and secondary amine (diethylamine and di-n-butylamine, Table 4, entries 10, 11) were converted into the corresponding formylated products from moderate to low yields. However, the N-npropylformamide was obtained in very low yield from the npropylamine (Table 4, entry 12) if compared with N-nbutylformamide and N-n-octylformamide (Table 4, entries 7, 8). As expected, the formylation of cyclohexylamine led to a reduced N-cyclohexylformamide yield to 3.4% presumably due to the effect of steric hindrance (Table 4, entry 13). This result was further proved by the formylation of tert-butylamine and trace amount of desired N-tert-butylformamide product was observed under the investigated conditions (Table 4, entry 14). Finally, the N-formylation of aromatic aniline with CO2 and H2 was also examined. Aniline was converted to N-phenylformamide in a negligible yield if compared with the investigated aliphatic amines (Table 4, entry 15), which can presumably be related to a much lower pKa value of aniline than those of other amines.

gradually disappear, while isolated Pd ensembles accordingly form. The effect of catalyst support on pyrrolidine N-formylation revealed that neither PANI nor CNT was efficient support for Pd−Au catalyst and the desired N-formylpyrrolidine yields were very low (Table 3, entries 8 and 9). Meanwhile, Pd−Au/ RGO provided moderate N-formylpyrrolidine yield of 68.0% for pyrrolidine N-formylation (Table 3, entry 10). Evidently, composite material PANI-CNT was the most effective support for bimetallic Pd−Au catalyst giving a quantitative yield of desired product (Table 3, entry 4). The influence of catalyst loading levels on the pyrrolidine N-formylation indicated that, as expected, both pyrrolidine conversion and N-formylpyrrolidine yield increased with the Pd−Au/PANI-CNT loading amount under the investigated conditions (Table 3, entries 4, 11−14). As for carbonylation reagent, raising the pressure of CO2/H2 (Table 3, entries 15−18, 4) or increasing the pressure ratio of H2 to CO2 (Table 3, entries 20−24) can efficiently promote N-formylation of pyrrolidine under the investigated conditions. However, a further increased CO2/H2 pressure from 3.5/3.5 to 4.0/4.0 MPa led to very limited effects on Nformylpyrrolidine yield (Table 3, entries 4, 19). In addition, when the N-formylation of pyrrolidine was performed under CO atmosphere of 3.5 MPa, the Nformylpyrrolidine yield was only 17.4% (Table S3, Supporting Information). If the pyrrolidine N-formylation was carried out under a mixture atmosphere of CO (3.5 MPa) and H2 (3.5 MPa), the N-formylpyrrolidine yield was around 24.5% (Table S3, Supporting Information). Moreover, CO was undetected by GC-MS analysis after pyrrolidine N-formylation using CO2/H2 with Pd−Au/PANI-CNT as the catalyst under the examined reaction conditions (Table 3, entry 4). The above control experiments thus indicated that CO is not supposed to be the reaction intermediate for the carbonylation reagent CO2/H2 in pyrrolidine N-formylation. The temperature effects on N-formylation of pyrrolidine to N-formylpyrrolidine revealed that the temperature significantly promoted pyrrolidine N-formylation (Figure 6). N-Formylpyrrolidine yields dramatically enhanced from 54.5% to 98.3% with

Figure 6. N-Formylation of pyrrolidine to N-formylpyrrolidine with CO2 and H2 using Pd−Au/PANI-CNT catalyst as a function of reaction temperature and time. Reaction conditions: Pd−Au/PANICNT (50 mg, Pd 1.6 wt %, Au 3.0 wt %, 0.008 mol % Pd relative to pyrrolidine, Pd/Au molar ratio = 1), pyrrolidine (71 mg, 1.0 mmol), 1,4-dioxane (5.0 mL), PCO2 (3.5 MPa), PH2 (3.5 MPa), 90−125 °C, 3− 48 h. N-Formylpyrrolidine yields were determined by GC-FID based on pyrrolidine using biphenyl as the standard material. 2523


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ACS Sustainable Chemistry & Engineering Table 4. N-Formylation of Various Amines with CO2/H2 Using Pd−Au/PANI-CNTa

Scheme 2. Proposed Mechanism for N-Formylation of Pyrrolidine over Pd−Au Bimetallic Catalysts

thermal condensation of the pyrrolidine to the formate yields N-formylpyrrolidine. The step of heterolytic cleavage of H2 molecule is presumably controlled by the pKa value of the amine reagent; while, the condensation step is governed by the effect of steric hindrance of the amine used.13,16

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CONCLUSIONS In summary, we have prepared polyaniline-functionalized carbon nanotubes (PANI-CNT) supported bimetallic, and monometallic Pd and Au catalysts and explored their electronic properties as well as their catalytic performance in the Nformylation of pyrrolidine using CO2/H2. The bimetallic Pd− Au nanoparticles exhibited a narrow size distribution with a mean nanoparticle size around 3.0 nm and were homogeneously distributed over PANI-CNT owing to efficient stabilization effect of the support. The Pd−Au/PANI-CNT with the Pd/Au molar ratio of 1:1 shows outstanding catalytic performance for the N-formylation of pyrrolidine with the corresponding N-formylpyrrolidine yield 98.3% at 125 °C, which is significantly higher than Pd/PANI-CNT and Au/ PANI-CNT. Our research further reveals that Pd atoms should be the true active sites for the hydrogenation reaction and the N-formylation reaction might occur mainly over Pd atoms or over the interface between Pd atoms and Au atoms in the bimetallic Pd−Au/PANI-CNT catalyst. The enhanced catalytic performance of alloyed bimetallic Pd−Au/PANI-CNT is mainly related to beneficial interactions between Pd and Au, leading to the changes of the electronic properties of the bimetallic Pd−Au nanoparticles.

a

Reaction conditions: amine (1.0 mmol), Pd−Au/PANI-CNT (50 mg, Pd 1.6 wt %, Au 3.0 wt %, 0.008 mol % Pd relative to amine, Pd/ Au molar ratio = 1), 1,4-dioxane (5.0 mL), PCO2 (3.5 MPa), PH2 (3.5 MPa), T (125 °C), t (48 h). bAmine conversion was determined by GC-FID using biphenyl as the standard material. cGC yield of the amide was obtained with biphenyl as the standard material and based on the amine used; meanwhile, the isolated yield in the brackets was determined by flash column chromatography.

Scheme 2 illustrates the proposed step sequence of pyrrolidine formylation over Pd−Au bimetallic catalysts. Previous investigation of density functional theory on the surface electronic properties of Pd−Au bimetallic nanoparticles demonstrated that the surface Pd atoms carry positive charge; while, Au atoms possess negative charge (Scheme 2, 1).110 In the initial stage, H2 is adsorbed and activated over the surface Pd atoms (Scheme 2, 2). In the catalytic system, the presence of organic base pyrrolidine promotes partially heterolytic cleavage of H2 molecule leading to the formation of Pd− Hδ−···Hδ+−N species (Scheme 2, 3).111 While, the electron transfer from the surface Au atom to the Pd atom in the bimetallic Pd−Au catalysts improves the electron density of Pd atom and further enhances the reactivity of Pd−H metalhydride bond. Subsequently, a CO2 molecule attacks Pd−Hδ−··· Hδ+−N species and forms a formate anion by insertion into Pd−H bond (Scheme 2, 4 and 5).112 Meanwhile, pyrrolidine, again, functions as an organic base to thermodynamically stabilize the in situ formed formic acid product by the formation of ammonium formate salt (Scheme 2, 6). Finally, a

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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.6b02865. Materials and resources, synthesis of PANI, PANI/CNT, acid-treated CNT, GO, and RGO, methods for catalyst characterization and product analysis, comparison of catalytic systems for N-formylation, N-methylation, and N,N-dimethylation, various carbonylation reagents for Nformylpyrrolidine synthesis, XPS analysis of Pd−Au/ 2524


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PANI-CNT with various Pd/Au molar ratio, recycling of the Pd−Au/PANI-CNT, and 1H and 13C {1H} NMR spectra of N-formylation products investigated in this research (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected]. Tel.: (+86)-20-85222191. Fax: (+86)-20-8522-0223 (J.C.). *E-mail address: [email protected] (A.C.). ORCID

Jinzhu Chen: 0000-0002-6475-1431 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from National Natural Science Foundation of China (21172219, 91645119, and 21472189), Natural Science Foundation of Guangdong Province, China (2015A030312007), Jinan Double Hundred Talents Plan, and Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control.

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