The immobilization of iron(iii) aminopyridine complex on MCM-41: its preparation and characterization

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1 Universiti Sains Malaysia From the SelectedWorks of Eng-Poh Ng 2013 The immobilization of iron(iii) aminopyridine complex on MCM-41: its preparation and characterization Eng-Poh Ng, Dr. Available at:

2 DI /s x The immobilization of iron(iii) aminopyridine complex on MCM-41: its preparation and characterization Farook Adam Chien-Wen Kueh Eng-Poh Ng Ó Springer Science+Business Media New York 2013 Abstract 2-Aminopyridinyl iron(iii) complex was grafted on chlorosilane modified mesoporous MCM-41 to give MCM-Py-Fe(III). The immobilization was confirmed by FT-IR, 13 C, 29 Si, 15 N CP/MAS NMR, nitrogen adsorption desorption study and elemental analysis. The powder XRD and TEM microscopy studies of the hybrid material confirmed the retention of the well-ordered honeycomb hexagonal structure in MCM-Py-Fe(III). The specific surface area of MCM-Py-Fe(III) was found to be 455 m 2 g -1 and it had a pore volume of 0.27 cm 3 g -1 with an average pore diameter of 26.7 Å. The 13 C CP/MAS NMR of MCM-Py-Fe(III) showed chemical shifts in the range of ppm, which was assigned to the aromatic carbons in the pyridine ring. The 15 N CP/MAS NMR showed the presence of chemical shifts at 550, 426 and 401 ppm for the three nitrogen atoms in the catalyst. Keywords Mesoporous MCM-41 Grafting 2-aminopyridine Iron nitrate 15 N solid state NMR 1 Introduction The merging of both organic and inorganic compounds to form hybrid materials will result in desirable physical properties within a single composite. Inorganic materials offer a wide range of properties, viz. thermal stability, electronic, magnetic and mechanical properties. n the other hand, organic molecules provide large polarizability, F. Adam (&) C.-W. Kueh E.-P. Ng School of Chemical Sciences, University Sains Malaysia, Penang, Malaysia farook@usm.my; farookdr@gmail.com functionality and structural diversity. Particularly, heterogeneous catalysts have been developed by the combination of inorganic and organic compounds, providing novel materials to catalysis chemistry. The most obvious advantage of these inorganic organic hybrids is that it can overcome the limitations of homogeneous catalysts such as the difficulty in catalyst separation, reusability and selectivity [1]. Thus, these hybrids offer the practical advantage by combining the properties of both the inorganic and organic properties to obtain innovative novel catalysts for multifunctional use [2, 3]. Since the discovery of M41S family of mesoporous molecular sieves by Mobil scientists in 1992, MCM-41 has been aptly selected as a promising support for heterogeneous catalysts. This is due to its unique textural characteristics of the unidimensional pore system, high surface areas and high chemical and thermal stability [4 7]. Furthermore, the presence of silanol groups on the surface of MCM-41 enables the adoption of varying methods for covalently attaching organic compounds which makes it more fascinating [4, 8]. A feasible strategy has now emerged for the grafting of chelating ligand complexes onto well-ordered MCM-41. Some literatures reported the preparation of metal ligand complexes before immobilization onto silica support; some researchers incorporated metals first onto mesoporous MCM-41 before anchoring the organic ligands. There are also papers reporting the synthesis of ligands covalently bonded to MCM-41 first before the metal complexation process. In this regard, the incorporation of L-prolinium nitrate onto a-fe 2 3 -MCM-41 was reported by Rostamizadeh et al. [9]. This catalyst was used in the one-pot oxidative cyclization of isatoic anhydride, benzylhalide and ammonium acetate. Their method of preparation involved the loading of Fe 2 3 to MCM-41 first, followed

3 by the anchoring of L-prolinium nitrate. The resulting magnetic nanocatalyst gave good yields of 4(3H)-quinazolinone derivatives under oxidant and solvent free condition. Molinari et al. used a different method to prepare MCM- 41-Fe(III)-porphyrin catalyst [10]. The porphyrin-iron(iii) complex was first synthesized before it was supported onto MCM-41. This photocatalyst was used in the oxidation of 1,4-pentanediol which gave good regioselectivity to aldehyde. However, the characterizations using 13 C, 29 Si and 15 N solid state MAS NMR were not reported in these works despite both the ligands have carbon and nitrogen atoms. In this work, we report the preparation of MCM-41-Py- Fe(III) hybrid complex by heterogenizing 2-aminopyridine onto MCM-41, which was subsequently metalated with Fe 3?, as shown in Scheme 1. The characterization of the material was carried out using various spectroscopic techniques, including 13 C, 15 N and 29 Si solid state MAS NMR. We believe this is the first report on the use of 15 N solid state MAS NMR to characterize mesoporous hybrid MCM- 41 materials. 2 Experimental 2.1 Chemicals The following chemicals were purchased and used without further purification. Hexadecyltrimethylammonium bromide (CTAB, Aldrich, 98 %), 3-chloropropyltriethoxysilane (CPTES, Aldrich, 95 %), sodium hydroxide (NaH, System, 99 %), nitric acid (Merck, 69 %), ethanol (Merck, 99 %), acetic acid (System, 99.5 %), 2-aminopyridine (Merck, [98 %), acetonitrile (QRëC, 99 %) and Fe(N 3 ) 3 9H 2 (Benson, 99 %). 2.2 Experimental procedures The preparation of rice husk ash silica source The synthetic procedures were carried out according to the literatures [11, 12]. Rice husk was washed with copious amount of water and rinsed with distilled water. After drying at room temperature, the rice husk was stirred with 1.0 M nitric acid at room temperature for 24 h to remove all the metal ions. The acid treated rice husk was washed until neutral and dried at room temperature. This rice husk was calcined in a muffle furnace at 600 C for 6 h to get pure white silica as the final product. This was used for the preparation of sodium silicate Synthesis of MCM-41 Sodium silicate (Na 2 Si 3 ) solution was prepared by combining rice husk ash (6.00 g) with NaH (2.00 g) and H 2 (40.0 ml). The resulting mixture was labeled as solution A which was heated with stirring for 2 h at 80 C. Another solution B was prepared by mixing CTAB (6.00 g) with H 2 (35.0 ml) at 80 C until a clear solution was obtained. Solutions A and B were mixed together in a polypropylene bottle and stirred for 15 min. The mixture was then placed in an air oven for hydrothermal treatment at 100 C for 24 h. The gel mixture was then cooled to room temperature and the ph of the reaction mixture was adjusted to ph 10.2 by drop wise addition of 25 wt% acetic acid with vigorous stirring. The mixture was further hydrothermally treated for an additional 48 h. The final white solid product was filtered, washed, dried and calcined at 550 C in air for 10 h to obtain template free MCM-41 [12] Preparation of MCM-PrCl Scheme 1 The stepwise preparation of MCM-Py-Fe(III) from MCM- 41 A 1.0 g sample of MCM-41 was refluxed with 12.5 mmol 3-chloropropyltriethoxysilane (CPTES) in toluene (30 ml) for 24 h to produce MCM-PrCl [13]. The modified solid

4 was washed in soxhlet apparatus with diethyl etherdichloromethane mixture (1:1) for 12 h [14] Immobilization of MCM-PrCl with 2-aminopyridine 100 MCM-41 MCM-PrCl MCM-Py MCM-Py-Fe (III) MCM-PrCl was refluxed with 2-aminopyridine (12.5 mmol) in acetonitrile at 90 C for 24 h to produce MCM-Py [3]. The excess 2-aminopyridine was removed by soxhlet extraction with ethanol before drying in an oven at 100 C for 12 h. Intensity (a.u.) Preparation of MCM-Py-Fe(III) complex A sample of MCM-Py (1.2 g) was refluxed with 12.5 mmol of Fe(N 3 ) 3 9H 2 in ethanol (20 ml) at 60 C for 24 h. The solid was filtered off and washed with ethanol till the washing was colorless [5]. It was then dried in an oven for 24 h. The resulting hybrid was denoted as MCM- Py-Fe(III). 2.3 Characterization of MCM-41 and modified samples The TEM images were obtained from a Philips CM12 instrument (Eindhoven, Netherlands) equipped with an analyzer and Docu Version 3.2 image processing software (Munster, Germany). The SEM images and EDX elemental analysis were recorded using Leo Supra 50 VP FESEM, equipped with xford INCA 400 energy Dispersive X-ray microanalysis System (Carl Ziess SMT, berkochen, Germany; xford Instruments Analytical, Bucks, UK). The X-ray diffraction patterns were recorded on a Siemens Diffractometer D5000, Kristalloflex (Voltage of 40 kv and current 30 ma) using CuKa.(k = nm) radiation. The 13 C, 15 N and 29 Si MAS NMR were recorded on a Bruker AV 400 WB Solid State NMR machine. 3 Results and discussions 3.1 The X-ray diffraction pattern of MCM-41 and modified samples The powder XRD pattern of MCM-41, MCM-PrCl, MCM-Py and MCM-Py-Fe(III) is shown in Fig. 1. MCM- 41 produced an XRD pattern of four well resolved diffraction peaks, due to (100), (110), (200) and (210) lattice planes. After the modification steps, shifting of the (100), (110) and (200) reflections to higher angles were clearly observed. This can be explained by the presence of anchored organic moieties and incorporation of metal inside the pore channels of MCM-41, causing a decrease in the pore size [4]. Nevertheless, all the characteristic peaks were still retained after each step of the modification, indicating that the long-range order of mesoporous 3 hexagonal channels were still preserved after the modification. 3.2 FT-IR spectra analysis of MCM-41 and modified samples 6 2-Theta ( o ) Figure 2 shows the FT-IR spectra of MCM-PrCl, MCM-Py and MCM-Py-Fe(III). In the spectrum of MCM-PrCl, the CH 2 vibrations of the propyl groups were observed at 2,956 and 2,869 cm -1. After the immobilization of chelating ligand, the spectrum of MCM-Py showed a vibration band at 3,247 cm -1 corresponding to the secondary N H group. 9 (a) (b) (c) (d) Fig. 1 The XRD pattern of a MCM-41, b MCM-PrCl, c MCM-Py, d MCM-Py-Fe(III) Transmission (%) MCM-PrCl MCM-Py MCM-Py-Fe(lll) Wavelength (cm -1 ) Fig. 2 The FT-IR spectra of a MCM-PrCl, b MCM-Py, c MCM-Py- Fe(III) 793 (a) (b) (c)

5 The C H stretching band of aliphatic and aromatic ring were observed at 2,934 and 3,101 cm -1. The aromatic C=C vibrations were assigned at 1,631 and 1,487 cm -1. The stretching vibration at 1,449 cm -1 was attributed to the new C N bond formed between the 2-amino pyridine with the 3-chloropropylated MCM-41 [4]. Upon complexation with Fe(N 3 ) 3 9H 2, the C H stretching bands for the aliphatic and aromatic ring were shifted to a lower frequency at 2,924 and 3,012 cm -1. The vibration frequency of C N had shifted to a lower wavelength of 1,453 cm -1. All the spectra show bands at 1,240, 1,080 and 793 cm -1, which correspond to the Si C stretching vibration and the vibration frequencies of the Si Si mesoporous network. The band at 874 cm -1 was attributed to the Fe vibration after the complexation with iron(iii) nitrate [15]. 3.3 Elemental analysis (EDX) The elemental analysis of MCM-PrCl, MCM-Py and MCM-PyFe(III) are tabulated in Table 1. The reduction of chlorine content from 8.2 to 3.0 % showed the successful immobilization of the chelating ligand, 2-aminopyridine. The nitrogen content detected in MCM-Py showed the ligand s presence in the MCM-41 framework, which was further confirmed by 13 C and 15 N solid state NMR. After the complexation of 2-aminopyridine with Fe(N 3 ) 3 9H 2, the hybrid complex MCM-Py-Fe(III) showed 5.1 % iron content. Table 1 Elemental analysis of MCM-PrCl, MCM-Py, MCM-Py- Fe(III) MCM-PrCl MCM-Py MCM-Py-Fe(III) C Si Cl N Fe Nitrogen sorption analysis The surface properties of all the modified solids, i.e. MCM- 41, MCM-PrCl, MCM-Py, MCM-Py-Fe(III) are tabulated in Table 2. The specific surface area and total pore volume of the as-synthesized MCM-41 were found to be 796 m 2 g -1 and 0.29 cm 3 g -1. Modification of MCM-41 with CPTES resulted in a decrease in the surface area to 673 m 2 g -1, as well as a reduction in pore diameter from 3.97 to 3.74 nm. This is due to the anchoring of CPTES onto the mesoporous wall. The complexation of 2-aminopyridine with Fe(N 3 ) 3 9H 2 further reduced the surface area and pore diameter to 455 m 2 g -1 and 2.67 nm respectively. The consecutive reduction of pore diameter at each step of modification is due to the incorporation of organic moieties and the complexation of metal ions in the silica matrix. 3.5 The 29 Si CP/MAS NMR analysis of MCM-Py-Fe(III) Figure 3 shows the 29 Si CP/MAS NMR of MCM-Py- Fe(III). From the spectrum, it is evident that the organofunctionalized moieties are present in MCM-41-Py-Fe(III). The spectrum exhibited signals at -106 and -97 ppm, which were assigned to Q 4 (Si(Si) 4 ) and Q 3 (Si(Si) 3 H) silicon centers [17, 18]. The T 3 and T 2 signals of the organosiloxane units were observed at -61 and -55 ppm respectively. This shows that the organic ligands have been covalently linked to the MCM-41 silica structure. 3.6 The 13 C CP/MAS NMR analysis of MCM-Py-Fe(III) The 13 C CP/MAS NMR spectrum of MCM-Py-Fe(III) is shown in Fig. 4. The strong resonance at 10, 26 and 48 ppm is due to the carbons of CH 2 CH 2 CH 2 fragment in the CPTES linker [19]. The successful immobilization of chelating ligand was further confirmed from the detection of aromatic peaks at 155, 147, 143, 115 and 113 ppm assigned by C4, C5, C6, C7 and C8 respectively. Table 2 Surface properties of MCM-41, MCM-PrCl and MCM-Py-Fe(III) Samples d 100 spacing (Å) b Unit cell parameter a 0 (Å) c BET surface area (m 2 g -1 ) a Pore volume, V p (cm 3 g -1 ) a Pore wall thickness (Å) c Pore diameter, W d (Å) c MCM MCM-PrCl MCM-Py-Fe(III) a The values obtained from N 2 sorption studies b The values obtained from XRD studies c The values were calculated from formulae in [16]

6 2 Si Si N1 NH N3 Fe(N 3 ) 3.9H 2 N N Fig. 3 The 29 Si CP/MAS NMR of MCM-Py-Fe(III) at 8 k Hz 3.7 The 15 N CP/MAS NMR analysis of MCM-Py and MCM-Py-Fe(III) In order to investigate the chemical interaction of Fe(N 3 ) 3 onto MCM-Py, 15 N CP/MAS NMR spectroscopy was carried out and the spectra of MCM-Py-Fe(III) are shown in Fig. 5. There are two resonances observed at 426 and 401 ppm for N1 and N2 [20]. The resonance at 550 ppm was assigned to the nitrogen, N3 of the nitrate ion. Fig. 5 The 15 N CP/MAS NMR of MCM-Py-Fe(III) at 12 khz. Run time = 48 h 3.8 Electron microscopy studies (TEM and SEM) The hexagonal periodicity of the mesophase of MCM-41 samples were investigated by TEM analysis. In Fig. 6, the Fig. 4 The 13 C CP/MAS NMR of MCM-Py-Fe(III) at 8 k Hz 2 Si Si C2 C1 C3 NH C8 Fe(N 3 ) 3.9H 2 C4 N C5 C6 C7

7 TEM images of MCM-41 and modified samples clearly showed the existence of arrays of highly ordered hexagonal structure. Despite the steps of immobilization and complexation with Fe 3?, the honeycomb structure still remained intact. Therefore, these images are in line with the XRD results discussed earlier, where all the three diffraction peaks of (100), (110), (200) planes could be clearly seen. Fig. 6 TEM morphology of a MCM-41, b MCM-PrCl, c MCM-Py, d MCM-Py-Fe(III) Fig. 7 SEM morphology of a MCM-41, b MCM-PrCl, c MCM-Py, d MCM-Py-Fe(III)

8 The SEM analysis revealed that the morphology of all the materials remained unchanged (Fig. 7). The samples have agglomerated particles and they did not have regular macroscopic ordering. Thus, this observation indicated that the immobilization of the organic ligand and its complexation with iron(iii) did not affect the macro-organization of the mesoporous material. However, distinct spots can be seen in Fig. 7d indicating the presence of the Fe(III) ions. 4 Conclusions Iron(III) species have been successfully coordinated on 2-aminopyridine tethered on MCM-41 through the surface modification of CPTES linker. The XRD, IR, EDX and NMR spectroscopy data evidenced the successful attachment of the organometallic group on the surface of MCM- 41. In addition, the continuous decrease in the specific surface area and BET volume also indicated the successful immobilization of the chelating ligand onto the MCM-41 surface. All the steps of modification and complexation retained the well-ordered hexagonal structures, which showed the tailoring of silica surface did not cause any disintegration. Acknowledgments The authors would like to thank Universiti Sains Malaysia for the Postgraduate Research Grant Scheme (PRGS) (1001/PKIMIA/844073) and RU research Grant (No. PKIMIA/ ) which partly supported this work. CWK would also like to thank the National Science Fellowship (NSF) of Malaysia for the scholarship provided. 2. Y. Li, B. Yan, Sol. State Sci. 11, (2009) 3. Sujandi, E.A. Prasetyanto, D.-S. Han, S.-C. Lee, S.-E. Park, Catal. Today 141, (2009) 4. Y. Luo, J. Lin, Micropor. Mesopor. Mater. 86, (2005) 5. M. Masteri-Farahni, F. Farzaneh, M. Ghandi, J. Mol. Catal. A: Chem. 248, (2006) 6. T. Shamim, D. Choudhary, S. Mahajan, R. Gupta, S. Paul, Catal. Commun. 10, (2009) 7. S. Tangestaninejad, M. Moghadam, V. Mirkhani, I. Mohammadpoor-Baltork, K. Ghani, Catal. Commun. 10, (2009) 8. A. Sakthivel, W. Sun, G. Raudaschl-Sieber, A.S.T. Chiang, M. Hanzlik, F.E. Kuhn, Catal. Commun. 7, (2006) 9. S. Rostamizadeh, M. Nojavan, R. Aryan, E. Isapoor, M. Azad, J. Mol. Catal. A Chem. (2013). doi: /j.molcata A. Molinari, A. Maldotti, A. Bratoycic, G. Magnacca, Catal. Today 161, (2011) 11. F. Adam, H. sman, K.M. Hello, J. Colloid Interface Sci. 331, (2009) 12. E.-P. Ng, H. Nur, K.-L. Wong, M.N.M. Muhid, H. Hamdan, Appl. Catal. A Gen. 323, (2007) 13. M. Masteri-Farahani, F. Farzaneh, M. Ghandi, J. Mol. Catal. 243, (2006) 14. D. Brunel, Micropor. Mesopor. Mater. 27, (1999) 15. F. Adam, J. Andas, I.A. Rahman, Chem. Eng. J. 165, (2010) 16. M. Kruk, M. Jaroniec, Y. Sakamoto,. Terasaki, R. Ryoo, C.H. Ko, J. Phys. Chem. B 104, 294 (2000) 17. S. Udaykumar, M.-K. Lee, H.-L. Shim, S.-W. Park, D.-W. Park, Catal. Commun. 10, (2009) 18. Sujandi, E.A. Prasetyanto, S.-C. Lee, S.-E. Park, Micropor. Mesopor. Mater. 118, (2009) 19. C. Venkatesan, A.P. Singh, J. Catal. 227, (2004) 20. G.C. Levy, R.L. Lichter, Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy (Wiley, New York, 1979) References 1. T. Soundiressane, S. Selvakumar, S. Menage,. Hamelin, M. Fontecave, A.P. Singh, J. Mol. Catal. A: Chem. 270, (2007)