Iron impregnated SBA-15, a mild and efficient catalyst for the catalytic hydride transfer reduction of aromatic nitro compounds

Abstract

Ordered hexagonal mesoporous Fe-SBA-15 with a large pore diameter was synthesized by a simple wet impregnation method using an aqueous solution of iron nitrate as a precursor. The incorporation of iron into the channels of mesoporous silica was confirmed and its textural and morphological properties were studied by various characterization techniques such as XRD, N₂ adsorption–desorption isotherms, FTIR and DRS UV-vis spectroscopy, XPS and SEM. The catalytic activity of Fe-SBA-15 was studied for the catalytic hydride transfer reaction of aromatic nitro compounds into amino compounds. The regio-selectivity of the reduction of different nitro substituted compounds with Fe-SBA-15 was discussed.

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Introduction

Aromatic amines are important compounds that are used in large industrial processes for the manufacture of dye stuffs, pharmaceuticals, agrochemicals, polymers etc. On account of environmental constraints, the catalytic hydride transfer reduction (CHTR) reaction is an easy, safe and cheap option for the reduction of aromatic nitro compounds and involves no complex synthetic routes. CHTR can usually be carried out either in vapor phase or in liquid phase processes. The applications of CHTR for the reductive cleavage and reduction of organic compounds, in general, are centered on the use of expensive catalysts such as Pd/C, Pt/C, Ru/C or Raney-Ni as well as systems like Raney-Ni/hydrazine, Pd–C/triethyl ammonium formate or formic acid.^1–5 Pt, Pd and Rh metal supported alumina also has tremendous catalytic activity and a long life time. But the methods adopted for the synthesis of these catalysts are very often complex and lengthy.^6,7

A supported metal oxide system has received great attention in industrial applications because of its extensive utility in the chemical industry compared to conventional metal oxides. It provides active sites for the dispersion of metal oxides and enhances their catalytic activity.^8–11 Recently several transition metal based mesoporous silica and alumina catalysts have been used for many organic transformation reactions like the oxidation of anthracene (Cr/SBA-15),¹² cyclohexene epoxidation (Ti/SBA-15 and Fe/SBA-16),^13,14 dibenzoylation of biphenyl (Al/SBA-15)¹⁵ oxidation of cyclic olefins (Ti-SBA-12 and Ti-SBA-16),¹⁶ selective oxidation of styrene (Ti-SBA-15),¹⁷ partial oxidation of methane (Fe/V-MCM-41),¹⁸ oxidation of diphenylmethane (Co/MCM-41),¹⁹ tertiary butylation of phenol (Ga, Fe, Al and B/MCM-41/MCM-48),²⁰ oxidation of tetralin to tetralone (Co doped mesoporous alumina)²¹ etc. SBA-15, an ordered mesoporous material with a high surface area and uniform hexagonal channels has major applications in the field of catalysis. It shows better access and openness to metals and a better transportation of reactants and products.^22,23

Supported Pt containing MCM-41, Pt–Al-MCM-41 and Pt–La-MCM-41 mesoporous materials were synthesized by a one step approach and were used as catalysts for nitrobenzene reduction.²⁴ The reaction was done in an autoclave, but hydrogenation with a high pressure inflammable hydrogen source was one of the major disadvantages of this study. The hydrogenation of nitrobenzene was carried out over Ru/SBA-15 in a vertical down flow reactor at high temperature.²⁵ Even though the conversion and selectivity was higher, the cost of the materials and the high reaction temperature made industrial level applications impossible. Even though the dispersion of nickel on SBA-15 is low in Ni-SBA-15,²⁶ its hydrophilic character makes it high yielding in the hydrogenation of vapor phase nitrobenzene with and without water co-feeding. But the high reaction temperature and the need for explosive H₂ gas generates environmentally hazardous substances. In the literature, nickel based catalysts have been specifically used as catalysts for the CHTR reaction.²⁷ Selvam and coworkers^28,29 have made iron and cobalt incorporated hexagonal mesoporous aluminophosphate molecular sieves for the reduction of nitro arenes with a high yield (>90%) up to the sixth run. Ni impregnated titania³⁰ has been used for the reduction of nitrobenzene in an autoclave under high pressure and temperature. Recently, a mesoporous nickel–alumina mixed oxide was reported as a promising catalyst for the hydride transfer reduction of nitrobenzene.³¹ However, the low conversion of nitrobenzene was a drawback of this catalyst. Recently many nanocatalysts have been used for the reduction of nitrobenzene by different methods including photochemical reduction, direct reduction using NaBH₄, etc.^32–39 Although these nanocatalysts showed good efficiency for nitrobenzene reduction, the cost of the materials synthesis was very high, limiting their practical applications.

In the mesoporous silica family, SBA-15 synthesized using P123 as a triblock copolymer under strongly acidic conditions exhibits a larger pore size, thicker pore walls and higher thermal stability than M41S materials. For the Friedel–Crafts alkylation reaction, Fe₂O₃/SBA-15 was synthesized by a chemical vapor infiltration method using ferrocene as a concomitant metal and carbon precursor with SBA-15 as a template.⁴⁰ Physical vapor infiltration is another technique for the doping of Fe on SBA-15 and the resulting catalyst has great catalytic activity towards the styrene oxidation reaction.⁴¹ Davidson et al.⁴² reported Fe/SBA-15 as catalyst, for the identification and location of iron species in the silica framework and its interests in Fenton reaction. Zhang et al.⁴³ presented Fe doped mesoporous silica SBA-15 as a humidity sensing material. The magnetic properties of a nanocomposite Fe doped SBA-15 material were studied by Wang et al.⁴⁴ The ferromagnetic nature of Fe/SBA-15 and the improvement of its magnetic properties with the Fe content on SBA-15 were studied by Li and coworkers.⁴⁵ The incorporation of Fe into the framework of SBA-15 by direct synthesis methods, prepared in highly acidic media, has also been reported.⁴⁶ Chemical and safety hazards caused by the use of toxic, corrosive and explosive precursor gases, as well as low surface selectivity and low deposition rate are some of the main drawbacks of chemical and physical infiltration methods.

In the present work, a highly ordered Fe substituted mesoporous SBA-15 material was prepared by a simple wet impregnation method, which is a convenient, eco-friendly and effective procedure for doping. The properties of SBA-15 and Fe-SBA-15 were examined by X-ray diffraction (XRD), BET surface area, pore size distribution, Fourier Transform Infrared Spectroscopy (FTIR), DRS UV-visible spectroscopy and scanning electron microscopy (SEM). Fe-SBA-15 has been used as a catalyst for the regio-selective hydride transfer reduction of aromatic nitro compounds. The advantages of the present method are the easy to handle hydride source and the use of a small amount of catalyst, which resulted in a good conversion of aromatic nitro compounds. The low cost of the synthesis method and the good activity of the Fe-SBA-15 material for the CHTR reaction make it a superior catalyst for industrial usage.

Experimental section

Chemicals

Reagents such as pluronic acid P123 (MW 5800), hydrochloric acid, n-butanol, tetraethylorthosilicate (TEOS), Fe(NO₃)₃·6H₂O for the synthesis of the materials and all of the chemicals for the catalytic hydrogenation of nitrobenzene and substituted nitrobenzene were received from the Sigma Aldrich chemical company and were used without further purification.

Synthesis of Fe-SBA-15

Tetraethyl orthosilicate (TEOS) and ferric nitrate were used as silica and iron sources, respectively, for the synthesis of Fe-SBA-15. The mesoporous SBA-15 support was synthesized by the procedure previously reported by Zhao et al.⁴⁷ In a typical synthesis of mesoporous SBA-15, 4 g of block copolymer surfactant P123 (EO₂₀PO₇₀EO₂₀) was dispersed in doubly distilled water (40 g) and 2 M aqueous HCl (120 mL) was added with stirring at ambient temperature 35 °C for 3 h. Then, 4 g of TEOS was added to the above solution and stirred at 40 °C for 3 h. The resultant gel was allowed to stand for silica condensation under static hydrothermal conditions at 100 °C for 48 h in a Teflon Parr reactor. The white colored solid product was filtered off, washed several times with warm distilled water, and dried at 100 °C overnight. The as-synthesized solid product was calcined at 540 °C in air for 24 h to remove the surfactant template and yielded SBA-15 as a white powder.

The SBA-15 was soaked in 0.1 mol L⁻¹ HCl for 24 h, filtered, washed with deionized water, and dried in an oven at 100 °C for 8 h for activation. A sample of 1 g of SBA-15 was dispersed in 1 mol L⁻¹ of aqueous Fe(NO₃)₃ solution by ultrasonication for 2 h. The obtained material was filtered and dried in an oven at 100 °C and calcined in a muffle furnace at 750 °C. The final product was denoted as Fe-SBA-15.

Analytical techniques

X-ray powder diffraction was conducted on a Philips Xpert diffractometer with Cu Kα (1.5404 Å) radiation. Scans were made in the 2θ range between 0.5 and 10° with a scan rate of 2° min⁻¹ (low angle diffraction). BET surface area and pore size distributions were measured by N₂ adsorption–desorption using a Micromeritics Gemini V-2380 surface area analyzer. Prior to the analysis, samples were degassed at 300 °C for 1 h. IR spectra were recorded by a KBr pellet method on a Perkin-Elmer Spectrum One FTIR spectrometer instrument. The UV-vis diffuse reflectance was measured for the samples at room temperature in air on a Shimadzu UV-2600 UV-visible spectrophotometer over the range 200 to 800 nm. The morphology of Fe-SBA-15 was studied using a JEOL Model JSM-6390LV scanning electron microscope-energy dispersive spectrometer (HRSEM) using an acceleration voltage of 5.0 to 30 kV. XPS measurements were taken in a custom built ambient pressure photoelectron spectrometer with a VG SCIENTA R3000 analyser and a monochromator (VG SCIENTA MX650) at room temperature and 1 × 10⁻⁹ mbar pressure. The hydride transfer reduction products were analyzed by an Agilent 7820A gas chromatograph with a 30 m HP-5 column.

Hydrogenation reaction of nitrobenzene

The catalytic hydrogenation reactions of nitrobenzene and substituted nitrobenzene were carried out in a 100 mL round bottom flask equipped with a reflux condenser. This was kept in a silicon oil bath at a known temperature and was agitated using a magnetic stirrer. The reaction mixture contained 15 mmol of substrate,15 mmol of base and 10 mg of Fe-SBA-15 in 15 mL solvent. After the completion of the reaction, the product was filtered out from the reaction mixture and used for analysis in a gas chromatograph equipped with an FID detector and N₂ gas as the carrier.

Results and discussion

X-ray diffraction

The low angle XRD patterns in Fig. 1 have narrow peaks at 0.8°, 1.5° and 1.7° for both calcined SBA-15 and Fe-SBA-15, which indicate they have hexagonal symmetry with the diffraction planes (100), (110), and (200), respectively. This reveals the highly ordered mesoporous structure of SBA-15 and Fe-SBA-15.

Fig. 1 Low angle XRD patterns of (a) SBA-15 and (b) Fe-SBA-15.

During synthesis, due to the specific interactions between the non-ionic surfactant and metal ions, the material becomes highly ordered and mesoporous in nature. The surfactant acts as a structure directing agent and after calcination it is removed from the material. The highly ordered structure confirms the effectiveness of the simple wet impregnation method for the synthesis of Fe-SBA-15.

N₂ adsorption–desorption isotherms

N₂ adsorption–desorption isotherms were obtained for the calcined mesoporous SBA-15 and Fe-SBA-15 materials. They show typical type IV isotherms which have sharp capillary condensation at high relative pressure and H1 hysteresis loops as shown in Fig. 2b. This indicates the 2D hexagonal symmetry of the mesoporous materials, with large pore diameters and narrow ranges of size. The BET surface area (S_BET) of SBA-15 and Fe-SBA-15 was found to be 689 m² g⁻¹ and 281 m² g⁻¹, respectively. The decrease in the S_BJH (622 m² g⁻¹ for SBA-15 and 201 m² g⁻¹ for Fe-SBA-15) and S_BET surface area of Fe-SBA-15 compared to SBA-15 is due to the doping of Fe on the pores of mesoporous silica SBA-15. Fig. 2a shows the corresponding BET pore diameters (D_BET) of 7.6 nm and 4.4 nm for SBA-15 and Fe-SBA-15, respectively. The other textural parameters of SBA-15 and Fe-SBA-15 are shown in Table 1. Both samples have a high Langmuir surface area.

Fig. 2 (a) Pore size distribution curves for SBA-15 and Fe-SBA-15. (b) N₂ adsorption isotherms for SBA-15 and Fe-SBA-15.

Table 1 Textural properties of SBA-15 and Fe-SBA-15

Sample	DBET (nm)	DBJH (nm)	SBET (m^2 g^−1)	SBJH (m^2 g^−1)	SLangmuir (m^2 g^−1)
SBA-15	7.6	6.9	689	622	1076
Fe-SBA-15	4.4	4	281	201	432

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Fourier transform infrared spectroscopy

The FTIR spectra of SBA-15 and Fe-SBA-15 are shown in Fig. 3, the broad peak around 3445 cm⁻¹ indicates the O–H bond vibrations due to the presence of the silanol group. An O–H deformation vibration was observed near 1630 cm⁻¹. The adsorbed water molecules on the surface of the materials also show a peak at the same wave number.

Fig. 3 FTIR spectra of SBA-15 and Fe-SBA-15.

The remaining peaks at 471 cm⁻¹, 800 cm⁻¹, 1136 cm⁻¹ correspond to the rocking, bending and asymmetric stretching of Si–O–Si bonds. The absence of C–H vibrations at 2850–3000 cm⁻¹ indicates the efficient removal of the surfactant. The absence of a peak at 1385 cm⁻¹ shows the complete removal of nitrate ions. The FTIR studies of Fe-SBA-15 confirm that there was no change in functional group after the doping of iron on SBA-15.

DRS UV-vis spectroscopy

Diffuse reflectance spectroscopy is a well known method for the identification and characterization of metal ion frameworks and it was carried out for both SBA-15 and Fe-SBA-15. The absorbance band of SBA-15 is around 200–400 nm as shown in Fig. 4. The strong absorption band observed below 300 nm in Fe-SBA-15 is due to the ligand to metal charge transfer that involved the isolated tetra coordinated Fe³⁺ (t₁ → t₂ and t₁ → e). The absorbance band around 300–700 nm observed in Fe-SBA-15 is for the iron oligomer or is due to aggregated iron oxide. The small broadening in the 490–500 nm range is assigned to the oxygen to metal charge transfer (Fe³⁺).

Fig. 4 DRS UV-vis spectra of SBA-15 and Fe-SBA-15.

X-ray photoelectron spectroscopy

The XPS wide scan of Fe-SBA-15 in Fig. 5 shows that it mainly contains Fe, Si and O. The absence of a carbon impurity peak around 284 eV confirmed the effective calcination of the material. The O1s main peak at 534 eV can be observed at high binding energy, which is due to hydroxyl groups or chemisorbed water molecules on the surface of Fe-SBA-15. The small peaks around 705–735 eV can be assigned to 2p_3/2 and 2p_1/2 of Fe³⁺ species.

Fig. 5 XPS spectrum of Fe-SBA-15.

Scanning electron microscopy

The morphology of Fe-SBA-15 is shown in Fig. 6. The images reveal that Fe-SBA-15 consists of rope like structures which are similar to SBA-15. The needle-like crystalline shape confirmed that there was no change in the morphology of SBA-15 after impregnation. The weight percentage of Fe in Fe-SBA-15 is 17.3% as shown in ESI S6.†

Fig. 6 (a) Low resolution and (b) high resolution SEM images of Fe-SBA-15.

Hydride transfer reduction reaction

The liquid phase hydride transfer reduction reaction was carried out with nitrobenzene and substituted nitrobenzene. NaOH was used as a base and promoter to stabilize adsorbed nitrobenzene on the surface of Fe-SBA-15. One attractive feature of this reaction is the hydride source; 2-propanol is easy to handle compared to molecular hydrogen and other explosive gases, and gives a very good conversion of nitrobenzene in each of the reactions. The reduction reaction failed in the absence of the base and the catalyst. In the presence of mesoporous Fe-SBA-15 which has a high surface area, the substituted nitrobenzene was also converted to the corresponding amines. The high conversion percentages of nitrobenzene and substituted nitrobenzene with small amounts of catalyst are shown in Table 2. The reaction conditions were optimized with respect to time, temperature and catalyst amount.

Table 2 Catalytic hydride transfer reduction reaction of nitro compounds with Fe-SBA-15. Reaction conditions: 15 mmol substrate, 15 mmol NaOH (promoter), 15 mL 2-propanol (hydride source) and 10 mg catalyst at 356 K

Substrate	Reaction time (h)	Conversion (%)	Product
	7	78
	7	90
	7	72
	7	58.7
	7	74
	7	63

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The effect of catalyst concentration on the hydrogenation reaction was investigated and results are presented in Fig. 7. As the catalyst amount increased, the conversion also increased due to the proportional increase in the number of active sites. However 0.01 and 0.05 g showed nearly the same conversion of nitrobenzene and beyond that, there was an insignificant increase in the conversion. Therefore, 0.01 g Fe-SBA-15 was taken as the amount of catalyst for further studies.

Fig. 7 Optimization of the amount of catalyst for the conversion of nitrobenzene. Reaction conditions: 15 mmol substrate, 15 mmol NaOH, 15 mL 2-propanol at 356 K.

The suitability of NaOH as a promoter for the catalytic hydride transfer reaction was analyzed with different amounts of catalyst. Fig. 8 shows the conversion percentage of nitrobenzene was higher with 0.01 g of Fe-SBA-15 than with 0.05 g. The increase in the diffusion effect with an increase in catalyst amount was confirmed from the catalyst amount optimization figures. The reaction carried out with the base KOH, resulting in only a low percentage of nitrobenzene conversion as shown in Fig. 9. These two optimization studies confirmed that the hydride transfer reduction reaction was more successful with NaOH as the base. Using a very small amount of the catalyst itself gave a higher conversion of nitrobenzene with NaOH compared to with KOH.

Fig. 8 Conversion of nitrobenzene with 0.01 g and 0.05 g Fe-SBA-15. Reaction conditions: 15 mmol substrate, 15 mmol NaOH, 15 mL 2-propanol at 356 K.

Fig. 9 Conversion of nitrobenzene with 0.01 g and 0.05 g Fe-SBA-15. Reaction conditions: 15 mmol substrate, 15 mmol KOH, 15 mL 2-propanol at 356 K.

The hydride transfer properties of 2-propanol were analyzed by the reduction of nitrobenzene. The reaction was carried out with primary, secondary and tertiary alcohols and the results are shown in Fig. 10. Maximum conversion (76.2%) was obtained for 2-propanol with 0.01 g of Fe-SBA-15 and a NaOH promoter at 356 K for 7 h. Temperature optimization studies were done using 2-propanol and NaOH with the same amount of catalyst. Fig. 11 shows that more conversion of nitrobenzene occurs under reflux conditions for the solvent. An insignificant reaction rate was observed at room temperature. As the temperature of the reaction increased, the conversion of the reactant reached a maximum under reflux conditions. From these studies, it was confirmed that using 2-propanol with NaOH at 356 K were the optimal conditions for the hydride transfer reduction reaction of nitrobenzene. The simple reaction procedure and good amount of conversion compared to the literature makes the use of this catalyst possible on an industrial scale.

Fig. 10 Conversion of nitrobenzene with different hydride sources.

Fig. 11 Conversion of nitrobenzene at different temperatures with Fe-SBA-15. Reaction conditions: 15 mmol substrate, 15 mmol NaOH and 15 mL 2-propanol with 10 mg catalyst.

The regio-selectivity of the reaction was checked by employing substituted nitrobenzenes as substrates under optimal conditions. Compared to the catalysts such as mesoporous Ni–Al₂O₃, and Pt–Pb bimetallic³¹ nanoparticles, Fe-SBA-15 has superior activity towards the nitro group conversion to amino group. From Table 2 it can be seen that the conversion of 1-chloro nitrobenzene to 1-chloroaniline was 90% and m-dinitrobenzene to m-nitroaniline was 72%. The high conversion of chloro substituted nitrobenzene is due to the activated aromatic ring (+R effect). Meanwhile the nitro group had a deactivated aromatic ring and therefore lower conversion. The regio-selectivity of Fe-SBA-15 was also seen here. The selective conversion of one NO₂ group to NH₂ with no trace amount of diaminobenzene was observed. The electron withdrawing nature of the nitro group attached to the benzene ring was more favourable for selective reduction. In this case the nitro group can easily be adsorbed and desorbed from the surface of the catalyst after product formation. Table 2 shows the conversion rates of bromo and chloro substituted nitrobenzene as 74% and 90%, respectively. The electron donating groups, CH₃ (58%) and OH (63%) reduced the reduction of the nitro group in substituted nitrobenzene. The conversion depends on the adsorption of nitro groups on the active sites of Fe-SBA-15. So the regio-selectivity of the prepared Fe-SBA-15 catalyst was once again confirmed. No reduction of the aromatic ring occurred.

After the reduction reaction, the Fe-SBA-15 catalyst was separated from the reaction mixture by simple filtration and washed two or three times with acetone followed by activation at 540 °C for 8 h. For the second reaction after recycling, nearly 58% nitrobenzene conversion was attained. The catalytic hydride transfer reduction of nitrobenzene carried out with the supporting material SBA-15 alone showed only 10% nitrobenzene conversion. The conversion of nitrobenzene with recycled Fe-SBA-15 and SBA-15 shows the significance of Fe-SBA-15 as a catalyst for the reaction.

The mechanism of reduction mainly depends on the adsorption of nitro groups on the active sites of Fe-SBA-15 (Fig. 12). The promoter NaOH helps the stabilization of nitro groups for adsorption on the active sites of catalyst. An adsorbed nitro group takes hydrogen from the hydride source, 2-propanol, and forms a nitroso intermediate. This could be followed by two consecutive hydride transfer steps to form the desired aniline. Nitroso and hydroxylamine intermediates were formed during this reaction, which are eventually converted to aniline. The high surface area of the prepared material helps for efficient adsorption and the presence of Fe(II) on the surface makes it easy to form the intermediates and thereby the products.

Fig. 12 Mechanism of the CHTR reaction with Fe-SBA-15.

Conclusion

The hexagonally symmetrical mesoporous Fe-SBA-15 was prepared by a simple wet impregnation method. The highly ordered mesoporous structures of SBA-15 and Fe-SBA-15 were confirmed by XRD. The N₂ adsorption–desorption isotherm studies demonstrated the typical type IV isotherms and high surface area of the materials. The functional groups, charge transfers and the oxidation state of the iron were analyzed by FTIR, DRS UV-vis spectra and XPS analysis, respectively. The hexagonal morphology of Fe-SBA-15 was verified by HRSEM. Fe-SBA-15 is found to be a mild and efficient catalyst for the reduction of aromatic nitro compounds to aromatic amines. The regio-selectivity of Fe doped SBA-15 was explored by the catalytic hydride transfer reduction of substituted nitrobenzene.

Acknowledgements

We express our thanks to DST-SR/S1/OC-06/2011 (G) dated 29-06-2011 project for financial assistance.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46303f

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