Sulfonated multiwalled carbon nanotubes (MWCNTs)

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Sulfonated multiwalled carbon nanotubes (MWCNTs) as a new, efficient, and recyclable heterogeneous nanocatalyst for the synthesis of amines M. M. Doroodmand, S. Sobhani, and A. Ashoori

Abstract: Sulfonated multiwalled carbon nanotubes (MWCNTs) were synthesized by chemical vapor deposition (CVD) as a new and facile one-pot method using acetylene (as the CNT precursor), thiophene (as the sulfur precursor), and ferrocene (for in situ liberation of metal nanoparticles as the CNT nanocatalyst). A low catalytic amount of the resulting sulfonated MWCNTs with a turnover number (TON) up to 980 and a turnover frequency (TOF) up to 11 160 h–1 was utilized as a new and recyclable heterogeneous nanocatalyst for the efficient one-pot synthesis of various amines (secondary and tertiary) by direct reductive amination of aldehydes and ketones using NaBH4. The catalyst was easily isolated from the reaction mixture by simple filtration and reused at least five times without significant degradation in activity. Key words: multiwalled carbon nanotubes, nanocatalyst, sulfonic acid, amine, reductive amination. Résumé : On a réalisé la synthèse de nanotubes de carbone sulfonés à parois multiples (NTCPM) par une nouvelle méthode monotope simple de dépôt de vapeurs chimiques (DVC) implication l’acétylène comme précurseur des nanotubes de carbone, le thiophène comme précurseur du soufre et le ferrocène (pour la libération in situ des particules métalliques agissant comme nanocatalyseur dans la formation de nanotubes de carbones). On a utilisé une faible quantité catalytique des nanotubes de carbone sulfonés à parois multiples (NTCPM) ainsi obtenu, avec des nombres de renouvellement allant jusqu’à 980 et des fréquences de renouvellement allant jusqu’à 11 160 h–1, comme nouveau catalyseur hétérogène recyclable pour la synthèse monotope efficace de diverses amines (secondaires et tertiaires) par amination réductrice directe d’aldéhydes et de cétones à l’aide de NaBH4. On peut facilement isoler le catalyseur du mélange réactionnel, par simple filtration, et l’utiliser à nouveau au moins cinq fois sans dégradation significative de son activité. Mots‐clés : nanotubes de carbone sulfonés à parois multiples (NTCPM), nanocatalyseur, acide sulfonique, amination réductrice. [Traduit par la Rédaction]

Introduction Amines are of high importance in organic chemistry because of their presence in high quantities in biological and natural molecules, such as alkaloids, amino acids, nucleic acids, dyes, and fine chemicals.1–4 Synthetic methodologies for amines vary depending on the location of the amine in the original source and the type of the amine. Often, there are multiple processes used for the synthesis of a particular amine or family of amines.5–8 Among these processes, direct reductive amination is the most practical and convenient approach, since it allows the conversion of carbonyl functionality to structurally diverse secondary and tertiary amines.9–12 This reaction offers compelling advantages over amine synthesis, including brevity, wide availability of commercial substrates, synthesis of an alkylated amine of higher order and generally mild reaction conditions. Several reagents, which

affect reductive amination, have been recently developed.13–18 However, most of the reagents may have one or more drawbacks such as hard reaction conditions, a tedious workup procedure, and the use of unrecyclable or unstable catalysts, which would eventually produce toxic waste. Therefore, the development of a new method to overcome these shortcomings remains an ongoing challenge for the synthesis of these significant scaffolds. Heterogeneous catalysts for the synthesis of fine chemicals have attracted considerable attention from both the environmental and economical points of view because they offer several advantages in organic synthesis, e.g., simplification of reaction procedures, easy separation of products, long catalytic life, ease of catalyst recyclability, and thermal stability.19 Carbon nanotubes (CNTs) appear to be one of the most promising nanomaterial supports for different functional

Received 26 October 2011. Accepted 8 June 2012. Published at www.nrcresearchpress.com/cjc on 2 August 2012. M.M. Doroodmand. Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran; Nanotechnology Research Institute, Shiraz University, Shiraz 71454, Iran. S. Sobhani and A. Ashoori. Department of Chemistry, College of Sciences, Birjand University, Birjand 414, Iran. Corresponding author: S. Sobhani (e-mail: [email protected] and [email protected]). Can. J. Chem. 90: 701–707 (2012)

doi:10.1139/V2012-049

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Fig. 1. X-ray diffraction (XRD) spectra of sulfonated multiwalled carbon nanotubes (MWCNTs) (gray) and MWCNTs (black).

groups and nanocatalysts because of their properties such as their unique surface structure, small diameter, large active surface area, inherent size and hollow geometry, basal and edge plane sites, desirable chemical stability, high electric conductivity, and excellent mechanical and thermal properties.20–23 On the other hand, the functionalization of CNTs decreases the hydrophobic character of the CNTs, caused by strong intertubular van der Waals forces, and modifies the physical and chemical properties of the CNTs. Sulfonic acid is one of the ideal grafting functional groups to the CNTs surface. Acid-assisted thermal decomposition and electrochemical modification and reduction are the typical synthetic routes for the preparation of sulfonated CNTs.24 Generally, acid-assisted thermal decomposition involves refluxing the CNTs with concentrated acid under high temperatures. Such aggressive treatments may lead to severe damage to the CNTs backbone.25 The electrochemical modification route allows mild operational conditions and its utility is only restricted to electrically contacted CNTs on the confined electrode surface.26,27 In the case of aryl-functionalized CNTs, synthesized by conventional chemical reduction methods, the procedure is usually time-consuming because it involves especially complex processes or multiple steps.28,29 The most popular and widely used method for CNTs synthesis is chemical vapor deposition (CVD). It is because of its low setup cost, high production yield, ease of scale up, ability to harness plenty of hydrocarbons in each state (solid, liquid, or gas), ability to enable the use of various substrates, one-pot covalent functionalization, and ease of CNTs growth either in different forms (powder, thin or thick films, aligned or entangled, or straight or coiled nanotubes) or in a desired architecture of nanotubes on predefined sites of a patterned substrate.30 Because of the need for a clean and green recovery of the heterogeneous catalyst, especially acid catalysts, we wish to introduce a new method for the one-pot synthesis of sulfonated multiwalled carbon nanotubes (MWCNTs) by CVD using acetylene, thiophene, and ferrocene as C, S, and Fe precursors, respectively. We also studied the utility of the synthesized sulfonated MWCNTs as a new and recyclable

heterogeneous catalyst for one-pot synthesis of different amines.

Results and discussion For the synthesis of sulfonated MWCNTs, acetylene (as the CNT precursor), thiophene (as the sulfur precursor), and ferrocene (for in situ liberation of metal nanoparticles as the CNT nanocatalyst) were mixed with hydrogen and argon in a furnace at ~1300 °C. The sulfur-doped carbon nanostructure was produced and purified directly from any amorphous carbon via oxygen purging and the introduction of hydrogen peroxide into the setup. This process was carried out in a second furnace at ~500 °C. Oxygen converts sulfurous species and amorphous carbon to oxidized sulfurous compounds and carbon dioxide, respectively. During the purification process, sulfonated MWCNTs were produced in situ by further oxidation of sulfur-doped carbon nanostructures by a hydrogen peroxide aerosol. The oxidation process was followed by UV and MW irradiations. These irradiations simply provide the necessary energy to cleave the C–C bond, which leads to defect creation in the CNT matrix. The synthesized sulfonated MWCNTs were then characterized by various methods including X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and FT-IR. The amount of sulfonic acid loaded on the surface of the MWCNTs was determined by thermogravimetric analysis (TGA) and confirmed further by back titration. XRD analysis According to the XRD pattern (Fig. 1), the strong peaks positioned at 2q = 28°, 42°, and 48° are related to C (002), C (100), and C (101), respectively. The peaks appearing at 2q = 12°, 30°, 56°, 58°, and 66° belong to S (001), S (110), S (111), and S (004), respectively. The XRD pattern reveals the presence of sulfur atoms in the MWCNT matrix. Raman spectra A significant change in the defect of the Raman spectra of sulfonated MWCNTs compared with that of the MWCNTs, Published by NRC Research Press

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Fig. 2. Raman spectra of sulfonated multiwalled carbon nanotubes (MWCNTs) (black) and MWCNTs (gray).

Fig. 3. Electronic microscopic images including (A) SEM, (B and D) TEM, and (C) EDX analysis of sulfonated MWCNTs.

shows the presence of sulfur atoms in the CNT matrix (Fig. 2). SEM and TEM The size and structure of the sulfonated MWCNTs were determined using SEM and TEM. According to the SEM (Fig. 3A) and TEM (Figs. 3B and 3D), it is clear that the synthesized sulfonated MWCNTs have a nanodimension ranging from ~10 to 30 nm.

FT-IR spectroscopy The appearance of peaks at 1130 and 1531 cm–1 in the FTIR spectrum of sulfonated MWCNTs (Fig. 4A) is ascertained to the C–S (in which carbon is available in the MWCNTs backbone) and S=O bonds, respectively. These two peaks are not observed in the spectrum of MWCNTs (Fig. 4B). This observation indicates the existence of C–SO3H in the sulfonated MWCNTs. Two bands positioned at 3436 and 1635 cm–1 belong to the O–H stretching and bending Published by NRC Research Press

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Fig. 4. FT-IR spectra of (A) sulfonated multiwalled carbon nanotubes (MWCNTs) and (B) MWCNTs.

Can. J. Chem. Vol. 90, 2012 Fig. 6. Nitrogen adosorption isotherm of sulfonated multiwalled carbon nanotubes (MWCNTs, black) and MWCNTs (gray).

Scheme 1.

Fig. 5. Thermogram of sulfonated multiwalled carbon nanotubes (MWCNTs, gray) and MWCNTs (black).

vibrations, respectively. The OH group was produced during the activation of the MWCNTs and the formation of sulfonate functional groups. TGA The thermal behavior of sulfonated MWCNTs and pure MWCNTs is shown in Fig. 5. A significant decrease in the weight percentage of the sulfonated MWCNTs at ~170 °C is related to the desorption of the sulfonate functional group from the catalyst surface. A sharp decrease in the weight percentage at ~620 °C is due to the decomposition of the MWCNTs. According to the thermogram, the amount of sulfonic acid functionalized on the MWCNTs is ~8.29% (w/w). This result is in agreement with back-titration analysis. Nitrogen adsorption isotherm N2 adsorption isotherms of the sulfonated MWCNTs and pure MWCNTs at 25 °C by using a homemade TGA system are shown in Fig. 6. According to the N2 adsorption isotherms, a significant increase (up to ~591 m2 g–1) in the active surface area of the MWCNTs was observed owing to the defects of the CNT matrix during the sulfonation process.

Catalytic activity of sulfonated MWCNTs in the one-pot synthesis of amines As a part of our continuing research on the development of new applications of solid acids as heterogeneous catalysts in organic reactions,31–34 we used sulfonated MWCNTs as a heterogeneous catalyst for the one-pot synthesis of amines by direct reductive amination of aldehydes and ketones (Scheme 1 and Table 1). As shown in Table 1, substituted anilines or aliphatic (primary and secondary) amines underwent direct reductive amination with benzaldehyde and produced the corresponding secondary and tertiary amines in 75%–98% yields (Table 1, entries 1–8). In addition, the reaction of aniline with cyclopentanone and cyclohexanone gave high yields of the corresponding secondary amines (Table 1, entries 9 and 10). In these transformations, turnover number (TON) and turnover frequency (TOF) were in the range of 750–980 and 830 – 11 160 h–1, respectively, which demonstrates the high efficiency of sulfonated MWCNTs for the synthesis of a wide range of amines. After performing the reductive amination of benzaldehyde with aniline under the conditions described in Table 1, EtOH was added to the reaction mixture and the catalyst was filtered off and reused for another reaction. This process was carried out for five runs without any noticeable reduction in the catalytic activity of the catalyst. The average isolated yield for at least five successive runs was 94.6%, which clearly demonstrates the practical reusability of the catalyst (Fig. 7).

Conclusion In conclusion, sulfonated MWCNTs were synthesized via a new and facile one-pot method by CVD using acetylene (as Published by NRC Research Press

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Table 1. Sulfonated multiwalled carbon nanotubes (MWCNTs) catalyzed direct reductive amination of aldehydes and ketones under solventfree conditions at room temperature. Entry 1 2 3 4 5 6 7 8 9 10

Amine Aniline 3-Hydroxyaniline 2-Methoxyaniline 4-Methoxyaniline 4-Methylaniline n-Butylamine Dibutylamine Piperidine Aniline Aniline

Carbonyl compound Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Cyclopentanone Cyclohexanone

Time (min) 30 60 60 45 15 5 15 5 30 30

Yield (%)a 98 83 96 78 95 93 94 75 95 94

TONb 980 830 960 780 950 930 940 750 950 940

TOF (h–1)c 1960 830 960 1040 3800 11160 3760 9000 1900 1880

dC–H (ppm) 4.57 4.34 4.43 4.34 4.35 3.82 3.98 4.04 3.70–3.83 3.25–3.31

dC–H (ppm) (ref.) 4.30 (35) 4.24 (36) 4.30 (35) 4.22 (35) 4.38 (37) 3.82 (38) 3.53 (35) 3.53 (38) 3.72–3.81 (39) 3.16–3.25 (39)

a

Isolated yields. All of the products are known compounds and were identified by their physical and spectral data compared with reference samples. Turnover No. (TON) = mol product / mol catalyst. c Turnover frequency (TOF) = TON/h. b

Fig. 7. Recyclability of sulfonated multiwalled carbon nanotubes (MWCNTs) for the synthesis of benzyl phenylamine.

the CNT precursor), thiophene (as the sulfur precursor), and ferrocene (for in situ liberation of metal nanoparticles as the CNT nano catalyst). The synthesized sulfonated MWCNTs were applied as a new recyclable heterogeneous catalyst for the direct reductive amination of benzaldehyde and ketones with aromatic or aliphatic amines under solvent-free conditions at room temperature. Use of a small amount (0.1 mol %) of sulfonated MWCNTs as a water-tolerant and thermally stable catalyst, short reaction times, high yields, and recyclability of the catalyst without loss of reactivity for at least five reaction cycles are the remarkable properties of this work. Further investigations on the development of organic transformations using this new heterogeneous catalyst are ongoing in our laboratory.

Experimental section Synthesis of sulfonated MWCNTs Acetylene gas with a flow rate of ~50 mL min–1 was bubbled into a solution containing ferrocene (0.30 g) in benzene–thiophene (25 mL, volume ratio, 1:10). This solution was mixed with hydrogen and argon (flow rates = 0.5 and 800 mL min–1, respectively). The resulting reaction mixture was introduced into a quartz tube and passed through a

80 cm tubing furnace with temperature of 1300 °C. The sulfur-doped carbon nanostructures were then directly purified from any amorphous carbon via oxygen purging and introducing hydrogen peroxide into the setup followed by online activation using UV and microwave (MW) irradiations. General procedure for the reductive amination of aldehydes and ketones An aldehyde or a ketone (5.0 mmol) was ground with a mixture of an amine (5.0 mmol) and sulfonated MWCNTs (0.1 mol %) in a mortar by a pestle at room temperature. NaBH4 (5.5 mmol) was added to the resulting mixture, which was ground for an appropriate time (Table 1). EtOH (30 mL) was then added to the reaction mixture and the catalyst was filtered, washed by EtOH (2 × 20 mL), dried, and recycled for the similar reaction. The filtrate solvent was removed under reduced pressure and the resulting crude product was purified by column chromatography and eluted with n-hexane– EtOAc (20:1–2:1). N-Benzylaniline Yellow oil. 1H NMR (400 MHz, CDCl3, ppm) d: 4.57 (s, 2H), 6.73–6.80 (m, 3H), 7.24–7.37 (m, 7H). Published by NRC Research Press

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3-(Benzylamino)phenol Brown oil. 1H NMR (400 MHz, CDCl3, ppm) d: 4.34 (s, 2H), 6.15 (s, 1H), 6.22 (d, J = 7.6 Hz, 1H), 6.27 (d, J = 8.4 Hz, 1H), 7.053 (t, J = 8 Hz, 1H), 7.35–7.39 (m, 5H).


N-Benzyl-2-methoxyaniline Colorless oil. 1H NMR (400 MHz, CDCl3, ppm) d: 3.92 (s, 3H), 4.43 (s, 2H), 4.71 (bs, 1H), 6.68 (d, J = 8 Hz, 1H), 6.77 (t, J = 7.6 Hz, 1H), 6.87 (d, J = 7.2 Hz, 1H), 6.92 (t, J = 7.6 Hz, 1H), 7.33–7.48 (m, 5H). N-Benzyl-4-methoxyaniline Orange solid, mp 50–52 °C; crystallization solvent, n-hexane– EtOAc (20:1). 1H NMR (400 MHz, CDCl3, ppm) d: 3.79 (s, 3H), 4.34 (s, 2H), 6.66 (d, J = 6.8 Hz, 2H), 6.84 (d, J = 6.8 Hz, 2H), 7.34 (d, J = 6.8 Hz, 1H), 7.39 (d, J = 7.6 Hz, 1H), 7.42–7.44 (m, 3H). N-Benzyl-4-methylaniline Orange solid, mp 56–58 °C; crystallization solvent, n-hexane– EtOAc (20:1). 1H NMR (400 MHz, CDCl3, ppm) d: 2.29 (s, 3H), 3.95 (bs, 1H), 4.35 (s, 2H), 6.61 (d, J = 8.0 Hz, 2H), 7.04 (d, J = 8.0 Hz, 2H), 7.31 (d, J = 7.2 Hz, 1H), 7.38 (d, J = 7.6 Hz, 1H), 7.40–7.43 (m, 3H). N-Benzylbutan-1-amine Colorless oil. 1H NMR (400 MHz, CDCl3, ppm) d: 0.94 (t, J = 7.2 Hz, 3H), 1.30–1.42 (m, 2H), 1.67–1.77 (m, 2H), 2.67–2.78 (m, 2H), 3.55 (bs, 1H), 3.82 (s, 2H), 7.31–7.35 (m, 2H), 7.39–7.45 (m, 3H). N-Benzyl-di(n-butyl)amine Colorless oil. 1H NMR (400 MHz, CDCl3, ppm) d: 0.99 (t, J = 7 Hz, 6H), 1.27–1.40 (m, 4H), 1.65–1.74 (m, 4H), 2.64 (t, J = 8.4 Hz, 4H), 3.98 (s, 2H), 7.28–7.44 (m, 5H).

Can. J. Chem. Vol. 90, 2012

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N-Benzylpiperidine Yellow oil. 1H NMR (400 MHz, CDCl3, ppm) d: 1.25– 1.28 (m, 1H), 1.58–1.71 (m, 3H), 2.16–2.25 (m, 2H), 2.63– 2.70 (m, 2H), 2.92–2.97 (m, 2H), 4.04 (s, 2H), 7.41–7.43 (m, 5H).

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N-Cyclopentylaniline Yellow oil. 1H NMR (400 MHz, CDCl3, ppm) d: 1.49– 1.80 (m, 6H), 2.01–2.12 (m, 2H), 3.70–3.83 (m, 1H), 6.63 (d, J = 8.0 Hz, 2H), 6.70 (t, J = 6.4 Hz, 1H), 7.19 (t, J = 7.6 Hz, 2H).

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N-Cyclohexylaniline Yellow oil. 1H NMR (400 MHz, CDCl3, ppm) d: 1.17– 1.43 (m, 5H), 1.66–1.70 (m, 1H), 1.77–1.82 (m, 2H), 2.07– 2.10 (m, 2H), 3.25–3.31 (m, 1H), 6.62 (d, J = 7.6 Hz, 2H), 6.68 (t, J = 7.2 Hz, 1H), 7.18 (t, J = 7.6 Hz, 2H).

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Acknowledgments

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We are thankful to the Research Councils of the Shiraz and Birjand Universities for their support of this work.

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