Catalytic Activity of Green and Recyclable Nanometric Tin Oxide

Article pubs.acs.org/IECR

Catalytic Activity of Green and Recyclable Nanometric Tin Oxide-Doped Silica Nanospheres in the Synthesis of Imines G. Gnana kumar,*,† C. Joseph Kirubaharan,† Ae Rhan Kim,‡ and Dong Jin Yoo*,¶ †

Department of Physical Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu, India Division of Chemical Engineering, Chonbuk National University, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea ¶ Specialized Graduate School of Hydrogen and Fuel Cells Engineering, Hydrogen and Fuel Cell Research Center, Chonbuk National University, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea ‡

ABSTRACT: Nanometric tin oxide particles were effectively doped over silica nanospheres without any free zones via sol−gel technique in an aqueous medium. X-ray diffraction pattern of the prepared composite specifies the formation of amorphous and cassiterite character of the silica and tin oxide particles, respectively. High thermal stability of the prepared materials substantiated ceramic properties of the prepared nanomaterials and has an influence on decreasing the reaction temperature of organic reactions. High surface area associated with prompt porosity values and their solubility in variety of solvents favors the catalytic activity of the prepared nanomaterials in imine formation reactions. The ease of recovery, excellent product yield, and intrinsic stability of the catalyst make this protocol economical and sustainable.

1. INTRODUCTION The enthralling function of catalysts in organic synthesis is immensely attractive due to its simple and effective conversion of multistep reactions into one pot.1−3 Though homogeneous catalysts are most active, with many attractive properties such as high chemo- and regioselectivity and high activities, their limitations such as cumbersome product purification, tedious catalyst recovery process, and deactivation fade the dream of their utilization in organic synthesis at the commercial level.4,5 On the other side, excellent stability and easier handling and separation from the reaction mixture of heterogeneous catalysts improve their competence in organic synthesis.6 Recently, their inferior catalytic performances compared to their counterpart homogeneous catalysts, due to their reduced contacts between catalyst and substrates, has been improved by bringing the catalysts at nanometric level; that is, high surface area enhances the contact between them.7,8 Numerous efforts have been made on various nanoparticles with respect to their catalytic activities to overcome the barriers of conventional organic reactions.9−12 Among the nanomaterials studied, tin oxide and silica nanoparticles are well-known for their large band gap (Eg = 3.62 eV at 300 K) and stability in organic solvents, respectively. Major benefits could be achieved in focus of existing developing paths; that is, bare nanomaterials to composite nanomaterials will capitalize the research effort on catalysis of organic reactions. It assists this research effort to satisfy the requirements of catalytic activity of nanomaterials by synergistically combining the mentioned nanomaterials of interest. Control over the doping, narrow size distribution, stabilization, yield, and textural properties of nanometric heterogeneous catalysts compared to their larger particle size counterparts are the essential parameters and could not be finely tuned by the reported methods. Hence, a simple sol−gel method has been proposed to prepare tin oxide-doped silica nanospheres with effective control over the mentioned parameters. The carbon−nitrogen bond-forming reaction is one of the significant transformations in organic synthesis. In particular, © 2012 American Chemical Society

imines are an important family of organic materials and have been extensively applied in a variety of biological fields such as lipoxygenase inhibition and anti-inflammatory and anti-cancer behavior and also in the configuration of optically active α-alkyl aldehydes, in the preparation of secondary amines by hydrogenation, in nucleophilic addition with organometallic reagents, and in cycloaddition reactions.13−16 Conventionally, imines are synthesized via the addition of amines and carbonyl compounds with azeotropic distillation.17 Condensation reactions in the presence of Lewis acids and oxidation of amines have also been reported for imine synthesis.18 Direct conversion of activated alcohols into imines and amines by use of stoichiometric amounts of MnO2 (oxidizing agent) and polymer-supported cyanoborohydride (reducing agent) has also been reported.19 However, the need for oxidants in stoichiometric amounts prevents the large-scale preparation of imine compounds. The formation of byproducts impedes the catalytic dehydrogenation methods employed in imine synthesis that use ruthenium complexes with dioxygen, iodosylbenzene, or quinone as oxidant.20 With the development of greener and more sustainable pathways for organic reactions and nanomaterials, tin oxide-doped silica nanospheres have been proposed for the preparation of imines in a one-pot reaction under a relatively lower temperature associated with the inclination of byproducts and enormous recyclability.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetraethyl ortho silicate [Si(OC2H5)4, TEOS], ammonium hydroxide, ethanol, and tin chloride (SnCl4·5H2O) were derived from Aldrich and were analytically pure. Received: Revised: Accepted: Published: 15626

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2.2. Synthesis of Tin Oxide−silica Composite. Bare silica particles were synthesized according to the procedure described elsewhere.21 The freshly prepared silica particles were pretreated with ammonium hydroxide (NH4OH) solution and washed with deionized water several times. The pretreated silica particles were redispersed in NH4OH, mixed with 40 mL of deionized water, and stirred. To the mixture, tin chloride (SnCl4·5H2O) (4.5 wt % in ethanol solution) and 25 mL of deionized water were added. After the mixture was stirred for 4 h, the product was aged at room temperature for 48 h and then the resulting product was filtered, washed with deionized water, and dried at ambient temperature. 2.3. Imine Formation Reaction. Imine formation reaction was performed under reflux at atmospheric pressure equipped with a Teflon-coated magnet-stirring bar. An appropriate amount of bare silica or tin oxide-doped silica nanospheres was added into the mixture of aldehyde (1.0 mmol) and amine (1.0 mmol) and stirred magnetically under toluene reflux/solvent-free conditions. The formation of imine product was monitored by thinlayer chromatography (TLC). The resulting reaction mixture was allowed to cool to room temperature and the catalyst was recovered by use of ethyl acetate and washed several times with water and sodium metabisulfate. The obtained catalyst was dried at 80 °C and used for recyclability studies.

2.4. Characterizations. Conventional transmission electron micrographs (TEM) of the prepared nanomaterials were recorded on a JEOL JEM-2010 instrument equipped with the energy-dispersive X-ray spectroscopy (EDX) attachment. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Amicus spectrometer (Kratos Analytical Ltd. Manchester, England). X-ray diffraction (XRD) patterns of the prepared nanomaterials were obtained on a D-Max-3A, Rigaku XRD diffractometer in the 2θ range from 10° to 90° with Cu Kα radiation. Textural properties of the prepared materials were characterized by using a Brunauer−Emmett−Teller (BET) surface area analyzer (Belsorp). Thermal gravimetric analysis (TGA) was carried out on a PerkinElmer instrument and the measurements were carried out under a nitrogen atmosphere with a heating rate of 10 °C·min−1 from 30 to 800 °C. 1H NMR data of products were acquired on a Bruker 300 MHz NMR spectrometer with CDCl3 as the solvent. 2.5. Analytical Data. (E)-N-(4-Methoxybenzylidene)aniline. White solid; mp = 59 °C. Molecular formula C14H13NO. 1 H NMR (300 MHz, CDCl3) δ 8.37 (s, CHN), 6.96−7.86 (m, ArH), 3.86 (s, OCH3). Entry 1, (E)-N-benzylideneaniline. White solid; mp = 54 °C. Molecular formula C13H11N. 1H NMR (300 MHz, CDCl3) δ 8.42 (s, CHN), 7.11−7.90 (m, ArH). Entry 2, (E)-N-benzylidene-4-methylaniline. White solid; mp = 34 °C. Molecular formula C14H13N. 1H NMR (300 MHz, CDCl3) δ 8.60 (s, CHN), 7.04−7.68 (m, ArH), 2.32 (s, CH3). Entry 3, (E)-N-benzylidene-2-methylaniline. White solid; mp = 60 °C. Molecular formula C14H13N. 1H NMR (300 MHz, CDCl3) δ 8.66 (s, CHN), 6.99−7.83 (m, ArH), 2.34 (s, CH3). Entry 4, (E)-N-benzylidene-3-methylaniline. White solid; mp = 60 °C. Molecular formula C14H13N. 1H NMR (300 MHz, CDCl3) δ 8.33 (s, CHN), 6.89−7.92 (m, ArH), 2.36 (s, CH3). Entry 5, (E)-N-(4-methoxybenzylidene)-4-methylaniline. White solid; mp = 83 °C. Molecular formula C15H15NO. 1H NMR (300 MHz, CDCl3) δ 8.51 (s, CHN), 6.98−7.71 (m, ArH), 3.83 (s, OCH3), 2.34 (s, CH3). Entry 6, (E)-N-(4-methoxybenzylidene)-3-methylaniline. White solid; mp = 82 °C. Molecular formula C15H15NO. 1 H NMR (300 MHz, CDCl3) δ 8.33 (s, CHN), 6.92−7.82 (m, ArH), 3.79 (s, OCH3), 2.35 (s, CH3).

Figure 1. TEM image of prepared bare silica nanoparticles.

Figure 2. TEM images of tin oxide-doped silica nanocomposites at different magnifications. 15627

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Figure 3. XPS spectra of (a) bare silica nanoparticles and (b) tin oxide-doped silica nanocomposites.

Entry 7, (E)-N-(4-methoxybenzylidene)-2-methylaniline. White solid; mp = 81 °C. Molecular formula C15H15NO. 1 H NMR (300 MHz, CDCl3) δ 8.20 (s, CHN), 6.81−7.96 (m, ArH), 3.79 (s, OCH3), 2.33 (s, CH3). Entry 8, (E)-N-(4-methoxybenzylidene)-4-chloroaniline. White solid; mp = 93 °C. Molecular formula C14H12ClNO. 1 H NMR (300 MHz, CDCl3) δ 8.42 (s, CHN), 6.92−7.81 (m, ArH), 3.86 (s, OCH3).

3. RESULTS AND DISCUSSION 3.1. Morphological Properties. Figure 1 depicts morphological images of the prepared bare silica particles. The monodispersed spherical silica nanoparticles with the smooth surface were obtained under base-catalyzed reaction and are in the range of 200−220 nm size. Hydrolysis of tetraethyl orthosilicate (TEOS) under alkaline conditions produced the Si(OH)4 tetrahedron. Then the dispersed material is subjected to the condensation (polymerization) reaction, which yielded threedimensional monodispersed silica nanospheres. Ammonium hydroxide played a dual role: it facilitates hydrolysis as well as condensation rates, which results in faster kinetics.21,22 Figure 2exhibits morphological images of the prepared tin oxide−silica composite. A rough morphology was observed for the prepared composite under basic condition.23,24 From Figure 2a−d, it is clear that 3−4 nm sized tin oxide particles were homogeneously distributed over the 200−220 nm sized silica particles. Though the inclusion of tin oxide particles has not altered the shape of silica particles, the size of the silica particles in

Figure 4. XRD spectra of (a) bare silica nanoparticles and (b) tin oxide-doped silica nanocomposites.

the composite was slightly increased. Figure 2 e,f depicts highly magnified images of prepared particles which ensured the homogeneous distribution of 3−4 nm sized tin oxide particles. From the images it is clear that tin oxide particles were uniformly doped over silica nanospheres, without any free bare zones. Energy-dispersive X-ray spectroscopy analysis shows that the Si/Sn ratio maintained in the prepared composite is 10:1. 15628

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Figure 6. Thermogravimetric analysis of (a) bare silica nanoparticles and (b) tin oxide-doped silica nanocomposites.

Figure 5. Adsorption−desorption isotherms of (a) bare silica nanoparticles and (b) tin oxide-doped silica nanocomposites.

Table 1. Physicochemical Properties of Prepared Samples sample

surface area (m2/g)

total pore volume (cm3/g)

mean pore diameter (nm)

silica tin oxide-doped silica

25 118

0.2230 0.3203

37.994 10.858

3.2. Structural Properties. Figure 3 depicts XPS spectra of the prepared bare silica and tin oxide−silica composite. The peaks observed for the bare silica particles (Figure 3a) are assigned as follows: a highly intensified peak observed at 101.75 eV is attributed to the Si 2p spectrum of the prepared silica particles. The peak found at 531.84 eV confirms the O 1s spectrum and is assigned to the hydroxyl (OH) groups present over the surface of silica. In addition, a weak C 1s peak was also observed at 282.4 eV in pure silica (Figure 3a) and is attributed to incomplete hydrolysis of the alkoxide precursor used for the synthesis of silica particles. Thus the observed XPS spectrum clearly demonstrated the formation of pure silica particles.22,25−27

Figure 7. Isolated yield of (E)-N-(4-methoxybenzylidene)aniline versus concentration of the catalyst (1:1 ratio has been maintained for anisaldehyde and aniline reactants) under (a) toluene reflux and (b) solvent-free conditions. 15629

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Table 2. Isolated Yields of Various Imines Catalyzed by Prepared Nanoparticles

Figure 3b shows the XPS spectrum of the prepared silica−tin oxide composite. In addition to the peaks of silica nanoparticles, two new peaks were observed for the silica−tin oxide composite. The peaks observed at 487.5 and 495.9 eV represent the 3d5 and 3d3 of tin oxide particles, respectively. The chemical shift (1.1−1.2 eV) in Sn3d5 peak position from SnO2-coated silica to the standard SnO2 (486.4 eV) indicates the interaction of SiO2 with SnO2. Figure 4 depicts X-ray diffraction patterns of the synthesized silica and tin oxide−silica composite. A broad band observed for the silica particles at 2θ = 22° indicates the amorphous character of the prepared silica nanoparticles (Figure 4a). Whereas strong crystalline peaks observed for the silica−tin oxide composite indicate the high crystalline character of the prepared composite materials (Figure 4b). The bands observed at 32°, 40°, 52°, 58°, and 68° were assigned to the (101), (111), (211), (002), and (301) diffraction planes (Figure 4b), respectively.23,28

The crystallite size of tin oxide particles calculated from X-ray diffraction analysis via Scherrer’s equation is well matched with the obtained TEM images. 3.3. Textural Properties. Nitrogen adsorption−desorption isotherms of the prepared samples at room temperature are obtained by using BET surface area analyzer Belsorp and are given in Figure 5. The surface area, pore volume, and pore size of the prepared nanometric materials were estimated at a relative pressure of 0.99 and were calculated by using the Brunauer− Emmett−Teller (BET) equation. The surface area of bare silica is found to be 25 m2·g−1, and the inclusion of nanometric tin oxide particles increases the surface area of the corresponding composite to 118 m2·g−1. However, the mean pore diameter of the composite has decreased in comparison with bare silica particles (Table 1) 3.4. Thermal Properties. Representative TGA spectra of the prepared samples are displayed in Figure 6. A very low 15630

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particle-catalyzed reactions (Table 2 and Figure 7). Though the same yield increment trend was observed for the tin oxidedoped silica nanocomposites under solvent-free conditions, the yield increment of tin oxide-doped silica nanocomposites over bare silica nanoparticles was nominal in most of the imine derivatives as compared with the solvent conditions (Table 2) and may be attributed to the lower dissolution tendencies of the nanocomposites with the reactants. Lifespan of the catalysts in organic reactions is a vital factor that can determine the competence of the prepared nanometric catalysts. This significant issue has been clarified by a number of performed experiments. After the completion of a fresh reaction under solvent/solvent-free conditions, the catalyst was recovered by using ethyl acetate, washed several times with water, and dried at 80 °C. Then the used catalyst has been tested with fresh aldehyde and amine under identical conditions. As an astonishing fact, a very slight decrease in the yield was observed for the imine formation up to five cycles, which guaranteed the life span of the used catalysts (Figure 8). The products obtained

percentile weight loss, that is, less than 11% of the total weight loss, was observed for the bare silica nanoparticles (Figure 6a). A first main weight loss observed at around 100 °C is attributed to the evaporation of weakly adsorbed water molecules, and the weight loss is about 7.1%. The second weight loss observed at 200−350 °C is ascribed to the condensation of silanol groups, and the weight loss observed in this stage is 8.32%. In the third stage, a weight loss of 10.85% was incurred at around 700−800 °C, which may be due to the loss of water molecules via condensation of silanol groups. The water molecules produced from self-condensation reaction are evaporated, resulting in the continuous weight loss of the produced nanometric silica particles. The tin oxide−silica composite has also exhibited a similar thermal weight loss (Figure 6b). However, a sudden decrease in the thermal stability at 200 °C is attributed to the higher number of water molecules evaporation. It indicates that the included tin oxide nanoparticles effectively increased the hygroscopic characteristics of the silica particles. However, a very low weight percentile loss obtained for the composite even at higher temperatures ensures the thermal stability of the prepared composite and has been achieved via the ceramic properties of the materials. The observed thermal integrities of the prepared materials played a vital role in reducing the temperature needed for the imine formation reaction. 3.5. Catalytic Studies. In general, an increasing demand of tailor-made catalytic species is hampered by their separation and recyclability problems. To effectively tackle these problems, tin oxide-coated silica nanospheres have been explored. The imine formation reactions were carried out under the influence of solvent and in solvent-free conditions. To optimize the reaction conditions, anisaldehyde or p-methoxybenzaldehyde has been chosen as a representative substrate. It has been reacted with aniline under solvent (toluene reflux)/solvent-free conditions for the synthesis of (E)-N-(4-methoxybenzylidene)aniline. The results are qualitatively evaluated by using TLC. For better yields, the mole ratio of 1:1 has been maintained for the anisaldehyde and aniline reactants. For the optimization of catalyst concentration, the mentioned model reaction was carried out with different concentrations of bare silica and tin oxide-loaded silica nanospheres under solvent and solvent-free conditions. Inclusion of nanometric catalysts increases the yield of (E)-N(4-methoxybenzylidene)aniline under solvent (Figure 7a) and solvent-free conditions (Figure 7b). An increase in the concentration of catalysts increases the yield of the corresponding products and is purely attributed to the increase in the number of active sites. However, the yield of imine formation has become constant at 28 wt % of catalyst. Hence, 28 wt % has been chosen as an optimum concentration of the catalysts and has been used for further studies. To evaluate catalytic activity of the prepared catalysts among other aldehyde and amine derivatives, relative experiments were performed under solvent and solvent-free conditions and the results are given in Table 2. All the imine derivatives were obtained under toluene reflux conditions at 30 min. However, the time and temperature were varied for the preparation of different imine derivatives under solvent-free conditions (Table 2). Compared with the bare silica particles, tin oxide−silica composites exhibited higher yield and is purely attributed to the high surface area of the prepared composites (Table 2). An increase in the surface area increases the contact between catalyst and organic substrates, which results in higher yield. Under the solvent conditions, tin oxide-doped silica nanocomposites exhibited a higher yield increment than that of conventional and bare silica

Figure 8. Recyclability of the prepared tin oxide-doped silica nancomposites in the formation of (E)-N-(4-methoxybenzylidene)aniline under (■) toluene reflux and (●) solvent-free conditions.

after the reaction were pure, which negates the requirement of further purification. Thus the use of low-cost and easily prepared catalysts effectively satisfies most of the constructive organic reaction requirements such as minimal reaction time, high yield, and minimal amount of catalyst utilization and makes its application wide open.

4. CONCLUSION Homogeneously doped nanometric tin oxide particles over silica nanospheres were prepared and their morphological, structural, and thermal properties were characterized. It has been proven that the tin oxide-doped silica nanospheres are active catalysts for the preparation of imine compounds under solvent/ solvent-free conditions. The catalysts could be easily recovered by simple extraction and were recycled in successive runs, which make them potential contenders for commercially available catalysts. 15631

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(17) Taguchi, K.; Westheimer, F. H. Catalysis by molecular sieves in the preparation of ketimines and enamines. J. Org. Chem. 1971, 36, 1570. (18) Billman, J. H.; Tai, K. M. Reduction of Schiff bases. II. Benzhydrylamines and structurally related compounds. J. Org. Chem. 1958, 23, 535. (19) Blackburn, L.; Taylor, R. J. K. In situ oxidation-imine formation−reduction routes from alcohols to amines. Org. Lett. 2001, 3, 1637. (20) Tillack, A.; Hollmann, D.; Michalik, D.; Beller, M. A novel ruthenium-catalyzed amination of primary and secondary alcohols. Tetrahedron Lett. 2006, 47, 8881. (21) Kumar, G. G.; Senthilarasu, S.; Lee, D. N.; Kim, A. R.; Kim, P.; Nahm, K. S.; Lee, S. H.; Elizabeth, R.N . Synthesis and characterization of aligned Sio2 nanosphere arrays: Spray method. Synth. Met. 2008, 158, 684. (22) Stober, W.; Fink, A. Controlled growth of monodisperse silica spheres in the micron size range. J. Colloid Interface Sci. 1968, 26, 62. (23) Li, Z.; Shen, W.; Wang, Z.; Xiang, X.; Zu, X.; Wei, Q.; Wang, L. Direct formation of SiO2/SnO2 composite nanoparticles with high surface area and high thermal stability by sol−gel-hydrothermal process. J. Sol−Gel Sci.Technol. 2009, 49, 196. (24) Wang, Y. D.; Ma, C. L.; Li, H. D.; Zhang, S. Synthesis and characterization of the composite of SnO2 nanoparticles coated on SiO2 microspheres. Mater. Chem. Phys. 2008, 107, 248. (25) Carriere, C.; Brion, D.; Escard, J.; Deville, J. P. X-ray photoelectron study of some silicon−oxygen compounds. J. Electron Spectrosc. Relat. Phenom. 1977, 10, 85. (26) Atrens, A. ESCA studies of nitrogen containing stainless steels. Appl. Phys. A: Mater. Sci. Process. 1990, 51, 411. (27) Angelis, B. A. D.; Rizzo, C.; Contarini, S.; Howlett, S. P. XPS study on the dispersion of carbon additives in silicon carbide powders. Appl. Surf. Sci. 1991, 51, 177. (28) Hai, S.; Yan, C.; Yu, H.; Xiao, G.; Wang, D. Preparation and near-infrared absorption of nano-SiO2/SiO2 assemblies with doping and without doping. J. Alloys Compd. 2009, 488, 370.

ACKNOWLEDGMENTS This research work was supported by Department of Science and Technology−SERC, New Delhi, Fast Track Project for Young Scientist Grant No. SR/FT/CS-113/2010(G). This work was supported by Basic Science Research Program through the NRF funded by the Ministry of Education, Science and Technology (2011-0010538). This work was supported by Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration in 2012 (Grants No. C0031452).

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AUTHOR INFORMATION

Corresponding Author

* (G.G.K.) E-mail: [email protected]. Telephone: +919585752997 (D.J.Y.) E-mail: [email protected]. Fax: +82-63270-2306. Notes

The authors declare no competing financial interest.

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dx.doi.org/10.1021/ie302301p | Ind. Eng. Chem. Res. 2012, 51, 15626−15632