Translate this page into:
Iron-functionalized nanoporous silica type SBA-15: Synthesis, characterization and application in alkene epoxidation in presence of hydrogen peroxide
⁎Corresponding author. Tel.: +98 21 61112614; fax: +98 21 66405141. abadiei@khayam.ut.ac.ir (Alireza Badiei)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Fe(III)salophen complex on a SBA-15 support functionalized with (3-aminopropyl)triethoxysilane as a linker. It has been synthesized and characterized by XRD, adsorption–desorption of nitrogen, SEM, FT-IR and UV–Vis. The formation of metal-salophen complex with the amino groups as connectors to the SBA-15 surface was confirmed. This material was successfully used as a heterogeneous catalyst for the epoxidation of alkenes and the effects of reaction time, different solvents and amount of catalyst on catalytic activity were investigated. This catalyst gave suitable and comparable yield and percentage conversion values. It is also stable and can be recycled and reused in the epoxidation of alkenes.
Keywords
SBA-15
Nanoporous silica
Epoxidation of alkenes
Heterogeneous catalyst
Fe(III)salophen complex
1 Introduction
Schiff base ligands are readily synthesized from the condensation of an aldehyde or a ketone with an amine, and can coordinate with almost all kinds of metals to form stable complexes suitable for numerous applications such as chemical analysis (Baran, 2000), absorption and transport of oxygen (Velde et al., 2000) for use in the treatment of cancer (Wang et al., 2001), in pesticides (Zhu et al., 2000) and as catalysts (Zhang et al., 2006; Wen et al., 2006; Mao et al., 2006). Some of the most well-known Schiff base complexes are salen and salophen complexes that have been shown to be very effective catalysts. Kochi’s discovery of the effectiveness of Mn-salen complexes as epoxidation catalysts (Srinivasan et al., 1986), and their subsequent development in asymmetric catalysis, most notably by Jacobsen (Zhang et al., 1990) and Katsuki (Irie et al., 1990), represents a landmark achievement in catalytic oxidation chemistry for the enantioselective epoxidation of alkenes. There has been a great deal of interest in the catalytic partial oxidation of alkenes to produce epoxides as flexible intermediates and precursors to many useful chemical products including biologically active compounds, which are of wide use in the pharmaceutical and agricultural fields, as well as in the electronic industry. Therefore, the design of new catalysts for the enantioselective epoxidation of alkenes by researchers becomes an important strategy (Pui and Mahy, 2007; Rezaeifard et al., 2010; Sheikhshoaie et al., 2009; Morris et al., 2001).
Despite of all these advantages, these catalysts have important defects. For example, they are not recoverable and their separation from the reaction mixture is so difficult or even impossible. Compared with homogeneous catalysts, heterogeneous systems have potential benefits, including ease of separation from reagents and reaction products, as well as ease of catalyst recycling and purification of products. Therefore, heterogenization of catalysts is extremely important, while combining the advantages of homogeneous catalysts (such as high activity and high velocity, for example) with the advantages of heterogeneous catalysts (such as easy catalyst separation, long catalytic life, easy catalyst regenerability, thermal stability and recycling potential) provides an ideal method to obtain highly efficient catalysts (Collman et al., 2004).
For these reasons, immobilization of chiral metal-salen complexes onto various kinds of supports has received considerable attention in recent years (Song and Lee, 2002; McMorn and Hutchings, 2004; Kantam et al., 2006). Chiral metal-salen homogeneous catalysts have been immobilized onto different supports such as mesoporous silica. These materials have large surface areas, controllable pore sizes of 2–50 nm, high hydrothermal stability and abundant silanol groups on the surface, allowing ready material diffusion and easy functionalization of organic groups for grafting to catalytic active species. Thus, chiral metal-salen homogenous catalysts have been applied widely in the immobilization of chiral salen complexes (Lou et al., 2007; Yu et al., 2006).
Surface functionalized mesoporous materials have emerged as one of the most important research areas in the field of advanced functional materials. They have found wide-spread application in catalysis (Wan and Zhao, 2007; Nozaki et al., 2002) and other areas. Of these materials, MCM-41 (Ajaikumar and Pandurangan, 2008; Serrano et al., 2008) and SBA-15 are used most widely; with SBA-15 in particular being an ideal candidate because of its thick walls that can be easily functionalized using simple silanol chemistry (Kesanli and Lin, 2004). In similar studies, silica-supported complexes such as Mn(III)salophen (Mirkhani et al., 2006) and Ru(III)salophen (Hatefi et al., 2010) were used as heterogeneous catalysts in the presence of NaIO4 for the epoxidation of alkenes with high yields.
As use of the environmentally benign oxidant H2O2 as well as cheaper salts such as FeCl3.6H2O is more sustainable, we recently attempted to immobilize Fe(III)salophen complex on a functionalized SBA-15 support for the first time as a heterogeneous catalyst in the epoxidation of alkenes with high selectivity. This study showed that this mesoporous material could effectively catalyze the epoxidation of alkenes, and that this heterogeneous catalyst is stable and recoverable. Another advantage of our work is the use of an environmentally benign oxidant such as hydrogen peroxide.
2 Materials and methods
2.1 Reagents
Salicylaldehyde, o-phenylenediamine, iron(III) chloride hexahydrate, amphiphilic triblock copolymer (P123), tetraethylorthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES), hydrochloric acid, hydrogen peroxide, methanol, absolute ethanol, acetonitrile, chloroform, dichloromethane and alkenes were purchased from Merck and Aldrich companies.
2.2 Apparatus
Ultraviolet–visible (UV–Vis) spectra were recorded on a Perkin Elmer Lambda 25. Fourier-transform-infrared (FT-IR) spectra were obtained from potassium bromide pellets in the range 400–4000 cm−1 with a Nicolet-Impact 400D spectrometer. Scanning electron micrographs (SEM) of the catalyst and silica were taken using a SEM Philips XL 30 instrument. Gas chromatography (GC) experiments were performed with a Shimadzu GC-16A instrument using a 2-m column packed with silicon DC-200 or Carbowax 20 m. N2 adsorption–desorption isotherms were measured using a BELSORP mini-II.
2.3 Preparation of catalyst
2.3.1 Preparation of mesoporous SBA-15
The mesoporous SBA-15 was synthesized by following a procedure similar to that reported by Zhao et al. (1998). In a typical synthesis, triblock poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (P123) (2.0 g) was dissolved in a mixture of deionized water (15 mL) and HCl 2 mol/L (60 mL) under stirring. Then, tetraethyl orthosilicate (TEOS) (4.25 g) was added dropwise to the solution at 40 °C. After being stirred continuously for 24 h, the mixture was transferred to a Teflon-lined stainless-steel autoclave and placed in an oven at 100 °C for 48 h. The precipitate was in turn removed by filtration, washed with deionized water and acetone and dried at 100 °C for 3 h. The powder obtained was heated to 550 °C and calcinated at this temperature in air for 6 h.
2.3.2 Functionalization of SBA-15 with 3-aminopropyl triethylsilane (APTES)
SBA-15 surface functionalization with APTES was carried out using a post-synthesis method as reported by Pallavi et al. (2007). In brief, calcined SBA-15 (2 g) was first suspended in toluene (30 ml). Then, 2 mmol of APTES was added per gram of mesoporous silica support and the reaction was heated under reflux for 2 h. The white solid of the functionalized SBA-15 (NH2-SBA) was then removed by filtration, washed with toluene and dried under vacuum (Scheme 1).
2.3.3 Synthesis of the salophen ligand
The synthesis of the salophen was performed according to the method described in the literature (Khandar et al., 2007). 1,2-Phenylenediamine (5.0 mmol) in ethanol (25 mL) was added to salicylaldehyde (10 mmol) in ethanol (25 mL) in a 100-mL two-necked flask. After refluxing for 45 min, the mixture was cooled to room temperature. The precipitate was removed by filtration and recrystallized from ethanol. The yield of the product was approximately 92%, and the melting point of this pure orange product was 168 °C.
2.3.4 Synthesis of [Fe(salophen)Cl].H2O complex
The iron(III) complex was synthesized by refluxing ethanolic solutions of equimolar quantities of iron(III) chloride hexahydrate with salophen for 2 h. The dark red–brown product was filtered and washed with a small portion of ethanol (EP 1 949 899 A1). The yield of the product was approximately 74%, and the melting point of this complex was >300 °C.
2.3.5 Preparation of heterogeneous catalyst
The supported catalyst was obtained by adding a solution of [Fe(salophen)Cl]. H2O in DMF to the suspension of functionalized SBA-15 in dry toluene and stirring for 24 h (Scheme 2). The resultant powder was removed by filtration, washed with dry toluene and dried at 80 °C in an oven overnight.
2.4 Catalytic experiments
Alkene (0.5 mmol), hydrogen peroxide 30% (1.5 mL) (as oxidant) and Fe-SBA (600 mg) (as catalyst) in solvent (3 mL) at room temperature were reacted (Scheme 3). Various solvents such as absolute ethanol, methanol, dichloromethane, acetonitrile and a mixture of acetonitrile/H2O (1:1) and different amounts of catalyst at room temperature were tested (Hatefi et al., 2010).
3 Results and discussion
3.1 Characterization methods
3.1.1 Low angle XRD patterns
The low angle XRD patterns of prepared SBA-15 (Fig. 1a) shows a single intensive reflection (1 0 0) and two additional peaks corresponding to the higher ordering (1 1 0) and (2 0 0) reflections, which is associated with a two-dimensional hexagonal structure similar to the typical SBA-15 materials (Zhao et al., 1998). The intensity of reflection at 2θ angle around 0.8° decreases after immobilizations (Fig. 1) shows that the coupling agents and iron complex causes the reduction of the peak intensity of diffraction. This is probably due to the difference in the scattering contrast of the pores and the walls, and to the irregular immobilization of iron complexes on the nano channels.
3.1.2 SEM images
Fig. 2 illustrates the SEM images of SBA-15 and Fe-SBA. The SEM image (Fig. 2b) shows a dominant lengthy rod-like morphology for Fe-SBA, in a bundle arrangement with a diameter of approximately 1–2 μm. The same morphology is observed for SBA-15 (Fig. 2a), indicating that the morphology was maintained without change during the surface modification.
3.1.3 N2 adsorption–desorption
The N2 adsorption–desorption isotherms of all samples (Fig. 3) exhibit type-IV isotherms with an obvious H1-type hysteresis loop, revealing strong evidence of mesoporous cylindrical or rod-like pores. These results indicate that the mesoporous texture of the materials was preserved during the surface modification, which was due to the loading of the functional group in SBA-15 and the complex in NH2-SBA. A considerable decrease in height of the capillary condensation step about the final product (Fig. 3) is observed with respect to decrease in average pore diameter, pore volume and surface area (see Table 1). It can be concluded that iron complex functionalization takes place inside the channels.
| Materials | BET surface area (m2/g) | Total pore volume (cm3/g) | Average pore diameter (nm) |
|---|---|---|---|
| SBA-15 | 805 | 0.72 | 6.3 |
| NH2-SBA | 288 | 0.62 | 5.4 |
| Fe-SBA | 251 | 0.60 | 4.8 |
3.1.4 Thermogravimetry analysis
Thermo gravimetric (TG) analysis of the samples is presented in Fig. 4. The amount of decomposition of organic species and salophen complex was determined by TG analysis. For NH2-SBA shows one weight loss around 330 °C that is due to the loss of organic functional group. The TG curve of Fe-SBA shows two weight losses. The first weight loss at around 100 °C is attributed to the loss of physically adsorbed water, representing approximately 0.18 g/g of the sample. The second weight loss is concentrated in the range of 290–350 °C, resulting from the decomposition of the salophen Fe complex during hydrothermal treatment. The weight losses of the Fe (III)salophen complex are 0.11 g/g of the sample.
3.1.5 FT-IR and UV–Vis
In the IR spectrum of the H2-salophen ligand, a broad band characteristic of the OH group at 3000–3500 cm−1 is observed. The retention of this band in the IR spectrum of the complex is indicative of the fact that the equatorial ligand is coordinated by a water molecule. The ν(C⚌N) band of H2-salophen at 1613 cm−1 also shifts to 1604 cm−1 in [Fe(salophen)Cl].H2O corresponding to the loss of the intramolecular hydrogen bonding and the formation of a new chelate ring between the imine nitrogens and Fe(III) ion (Amirnasr et al., 2001). In the FT-IR spectrum of SBA-NH2, the doublets within the range 3300–3400 cm−1, assigned to the asymmetric and symmetric stretching vibrations of NH2 groups, respectively, are hidden by the OH peak related to adsorb H2O. The peaks at 2905 and 2850 cm−1 are assigned to the C–H stretching vibrations of the methylene groups while the peaks around 1593 and 1404 cm−1 correspond to the N–H bending vibration of NH2. Upon coordination of [Fe(salophen)Cl].H2O to NH2-SBA, a characteristic band at 1661 cm−1 was observed that is assigned to ν(C⚌N). This shift to upper wave number is probably attributed to coordination of complex with lone electron pair of the nitrogen group of NH2 as a sigma donor ligand. This observation provides sufficient evidence for the successful coordination of [Fe(salophen)Cl].H2O to NH2-SBA (Fig. 5).
The UV–Vis spectrum (>250 nm) of the H2-salophen (Fig. 6a) in chloroform consists of relatively intense bands centered at 271 and 335 nm, assigned to π → π∗ and n → π∗ transitions, respectively (Estiu et al., 1996). After complexation with Fe(III) (Fig. 6b), several transitions appear in the UV region that are assigned to ligand-centered (π → π∗) transitions (Rezvani and Crutchley, 1994; Sallivan et al., 1978; Naklicki and Crutchley, 1989; Hadadzadeh et al., 2006). Moreover, an intense ligand-to-metal charge transfer band (πCl → dFe) appears at 298 nm in the spectrum of the [Fe(salophen)Cl].H2O complex. The UV–Vis spectra in the diffuse reflectance mode (Fig. 6c) confirm the presence of Fe(salophen)Cl on the silica support. The electronic spectrum of [Fe(salophen)Cl].H2O showed two bands at 298 and 365 nm that appear near to those in the diffuse reflectance spectrum. This exhibits the presence of Fe(salophen)Cl on NH2-SBA.
3.2 Catalytic activity
In order optimize conditions for the reaction, the epoxidation of cyclooctene was studied and different factors including the effects of reaction time, solvents and amount of catalyst were investigated. The epoxidation of cyclooctene using hydrogen peroxide as an oxidant did not proceed under mild reflux conditions in the absence of a catalyst.
Initially, in order to choose a suitable solvent for the epoxidation reaction, different solvents were investigated in the presence of a supported catalyst and hydrogen peroxide. Among the solvents listed (Table 2), a mixture of acetonitrile/water in an equilibrium ratio was chosen as the optimum reaction solvent because this gave the highest epoxidation yield.
| Row | Solvent | Epoxide yield |
|---|---|---|
| 1 | CH3CN | 91 |
| 2 | CH3CN/H2O (1:1)b | 100 |
| 3 | CHCl3 | 30 |
| 4 | CH2Cl2 | 22 |
| 5 | CH3OH | 64 |
| 6 | CH3CH2OH | 58 |
Next, the reaction was carried out in the presence of a catalyst, hydrogen peroxide, in the acetonitrile/water mixture over different reaction times. It was found that this reaction requires 5.5 h for completion at room temperature (Fig. 7).
Finally, different amounts of catalyst were used in the epoxidation of cyclooctene. The results show that full epoxidation of the starting material was obtained using 600 mg of immobilized [Fe(salophen)Cl] as a heterogeneous catalyst (Fig. 8).
During a reaction time of 5.5 h, cyclooctene converted completely to the epoxide product under the optimized conditions at room temperature.
In this study, various alkenes, listed (Table 3), were epoxidized in the presence of supported [Fe(salophen)Cl] at room temperature under optimized conditions. Characterizations of the products were achieved by comparing their chromatograms with the chromatogram of the standards.
3.3 Catalyst reuse and stability
The reusability and stability of the supported Fe(salophen)Cl were investigated in the repeated epoxidation reactions. Cyclooctene was chosen as a model for studying catalyst reuse and stability in the epoxidation reaction. After each experiment, the catalyst was separated from the reaction mixture by filtration, washed with methanol and diethyl ether and dried under air carefully before use in the subsequent run. After five consecutive uses of the catalyst, the epoxide yield was 81%. The filtrates were used for the determination of iron leaching by atomic absorption spectroscopy. The results showed that after the two next runs, no iron was detected in the reaction mixture (Table 4).
| Run | Epoxide yield (%) | Time (h) | Fe leached (%) |
|---|---|---|---|
| 1 | 100 | 5.5 | 3 |
| 2 | 95 | 5.5 | 1 |
| 3 | 92 | 5.5 | 0 |
| 4 | 85 | 5.5 | 0 |
| 5 | 81 | 5.5 | 0 |
The Fe(III)salophen catalyst gave comparable or even higher yield and percentage conversion values than the homogeneous catalysts. Also in comparison with the newly similar catalysts such as silica-supported Mn(III)salophen (Mirkhani et al., 2006), silica-supported Ru(III)salophen (Hatefi et al., 2010) and polystyrene-supported Ru(III)salophen (Hatefi et al., 2009) with the existing ones, catalytic activity in some cases is higher and in the other ones is closed to their results. It is noticeable that we could use the environmentally benign oxidant H2O2 and cheaper salts such as FeCl3.6H2O.
4 Conclusion
Many Schiff base complexes have catalytic properties. [Fe(salophen)Cl].H2O has some of the best characteristics for use as an effective catalyst. This complex was immobilized onto functionalized SBA-15 via an amine group. The bonding of the support and metal-salophen was so strong that [Fe(salophen)Cl] was not eluted with water or common organic solvents. The Fe(III)salophen catalyst gave comparable or even higher yield and percentage conversion values than the homogeneous catalysts. This heterogeneous Fe(III)salophen catalyst is stable and can be recycled and reused in the epoxidation of alkenes.
Acknowledgment
We are grateful to the University of Tehran and the International Centre for Science, High Technology and Environmental Science for financial support.
References
- Reaction of benzaldehyde with various aliphatic glycols in the presence of hydrophobic Al-MCM-41: a convenient synthesis of cyclic acetals. J. Mol. Catal. A: Chem.. 2008;290:35-43.
- [Google Scholar]
- Synthesis and spectroscopic characterization of [CoIII(salophen)(amine)2]ClO4 (amine = morpholine, pyrrolidine, and piperidine) complexes. The crystal structures of [CoIII(salophen)(morpholine)2]ClO4 and [CoIII(salophen)(pyrrolidine)2]ClO4. Polyhedron. 2001;20:695-702.
- [Google Scholar]
- Oxovanadium(IV) and oxovanadium(V) complexes relevant to biological systems. J. Inorg. Biochem.. 2000;80:1-2.
- [Google Scholar]
- Donor ligand effect on the nature of the oxygenating species in Mn(III)(salen)-catalyzed epoxidation of olefins: experimental evidence for multiple active oxidants. J. Inorg. Chem.. 2004;43:2672-2679.
- [Google Scholar]
- Quantum chemical calculations of the structures and electronic properties of N,N′-bis(3,5-dibromosalicylidene)-1,2-diaminobenzene and its cobalt(II) complex. Origin of the redox activity of the cobalt complex. Inorg. Chem.. 1996;35:263-266. European Patent Application, EP 1 949 899 A1.
- [Google Scholar]
- Synthesis, structure, spectroscopic, magnetic and electrochemical studies of NiII phen-dione complex. Inorg. Chim. Acta. 2006;359:2154-2158.
- [Google Scholar]
- Ru(salophen)Cl supported on polystyrene-bound imidazole: an efficient and robust heterogeneous catalyst for epoxidation of alkenes with sodium periodate. Appl. Catal., A. 2009;370:66-71.
- [Google Scholar]
- Silica supported Ru(salophen)Cl: an efficient and robust heterogeneous catalyst for epoxidation of alkenes with sodium periodate. Polyhedron. 2010;29:2953-2958.
- [Google Scholar]
- Catalytic asymmetric epoxidation of unfunctionalized olefins. Tetrahedron Lett.. 1990;31:7345-7348.
- [Google Scholar]
- Preparation of alumina supported copper nanoparticles and their application in the synthesis of 1,2,3-triazoles. J. Mol. Catal. A: Chem.. 2006;256:273-277.
- [Google Scholar]
- Silica-bound imidazole as a heterogeneous axial ligand for Mn(salophen)Cl: efficient, recoverable and recyclable catalyst for epoxidation of alkenes and hydroxylation of alkanes with sodium periodate. Chem. Commun 2004:548-552.
- [Google Scholar]
- Synthesis, characterization, electrochemical and spectroscopic investigation of cobalt(III) Schiff base complexes with axial amine ligands: the layered crystal structure of [CoIII(salophen)(4-picoline)2]ClO4.CH2Cl2. Inorg. Chim. Acta. 2007;360:3255-3264.
- [Google Scholar]
- Covalently anchored chiral Mn(III) salen-containing ionic species on mesoporous materials as effective catalysts for asymmetric epoxidation of unfunctionalized olefins. J. Catal.. 2007;249:102-110.
- [Google Scholar]
- Novel Schiff base complexes as catalysts in aerobic selective oxidation of β-isophorone. J. Mol. Catal. A: Chem.. 2006;258:178-184.
- [Google Scholar]
- Heterogeneous enantioselective catalysts: strategies for the immobilisation of homogeneous catalysts. Chem. Soc. Rev.. 2004;33:108-122.
- [Google Scholar]
- Silica-bound imidazole as a heterogeneous axial ligand for Mn(salophen)Cl: efficient, recoverable and recyclable catalyst for epoxidation of alkenes and hydroxylation of alkanes with sodium periodate. Appl. Catal., A. 2006;313:122-129.
- [Google Scholar]
- Enhanced activity of enantioselective (salen)Mn(III) epoxidation catalysts through supramolecular complexation. J. Mol. Catal. A: Chem.. 2001;174:15-20.
- [Google Scholar]
- Coordination chemistry of neutral phenylcyanamide ligands bound to pentaammineruthenium(II) Inorg. Chem.. 1989;28:4226-4229.
- [Google Scholar]
- Synthesis, characterization, and catalytic performance of single-site Iron(III) centers on the surface of SBA-15 silica. J. Am. Chem. Soc.. 2002;124:13194-13203.
- [Google Scholar]
- Pallavi, S., Sridevi, N., Prabune, A., Ramasamy, V., 2007. In: Xu, R., Gao, Z., Chen, J., Yan, W. (Eds.), Zeolites to Porous MOF Materials. Proceedings of the 40th Anniversary of International Zeolite Conference.
- Synthesis, characterization and catalytic activity of halo-methyl-bis(salicylaldehyde)ethylenediamine cobalt(II) complexes. Polyhedron. 2007;26:3143-3152.
- [Google Scholar]
- Synthesis, characterization and pronounced epoxidation activity of cis-dioxo-molybdenum(VI) tridentate Schiff base complexes using tert-butyl hydroperoxide. Polyhedron. 2010;29:2703-2709.
- [Google Scholar]
- Dependence of the Ru(III)-cyanamide chromophore on inner-sphere coordination: comparison of cis-[Ru(bpy)2(pcyd)2]+ and [Ru(NH3)5(pcyd)]2+ complexes. Inorg. Chem. 1994;33:170-174.
- [Google Scholar]
- Mixed phosphine 2,2′-bipyridine complexes of ruthenium. Inorg. Chem.. 1978;17:3334-3341.
- [Google Scholar]
- A comparison of methods for the heterogenization of the chiral Jacobsen catalyst on mesostructured SBA-15 supports. Appl. Catal., A. 2008;335:172-179.
- [Google Scholar]
- A novel tridentate Schiff base dioxo-molybdenum(VI) complex: synthesis, crystal structure and catalytic performance in green oxidation of sulfides by urea hydrogen peroxide. Polyhedron. 2009;28:733-738.
- [Google Scholar]
- Epoxidation of olefins with cationic (salen)manganese(III) complexes. The modulation of catalytic activity by substituents. J. Am. Chem. Soc.. 1986;108:2309-2320.
- [Google Scholar]
- Biocatalytic and biomimetic oxidations with vanadium. J. Inorg. Biochem.. 2000;80:81-89.
- [Google Scholar]
- On the controllable soft-templating approach to mesoporous silicates. Chem. Rev.. 2007;107:2821-2860.
- [Google Scholar]
- Synthesis, spectroscopic and antimicrobial studies of binuclear transition metal complexes with tetradentate Schiff base. Transition Met. Chem.. 2001;26:307-310.
- [Google Scholar]
- Asymmetric pinacol coupling reaction catalyzed by dipeptide-type Schiff bases. J. Mol. Catal. A: Chem.. 2006;245:242-247.
- [Google Scholar]
- Highly enantioselective epoxidation of olefins by Mn(III) salen complex immobilized on MCM-48. Catal. Commun.. 2006;7:170-172.
- [Google Scholar]
- Enantioselective epoxidation of unfunctionalized olefins catalyzed by salen manganese complexes. J. Am. Chem. Soc.. 1990;112:2081-2803.
- [Google Scholar]
- Enantioselective epoxidation of olefins catalyzed by the Mn(salen) catalysts immobilized in the nanopores of mesoporous materials. J. Catal.. 2006;238:369-381.
- [Google Scholar]
- Triblock copolymer syntheses of mesoporous silica with periodic 50–300 Angstrom pores. Science. 1998;279:548-552.
- [Google Scholar]
- The Schiff Base N-salicylidene-O, S-dimethylthiophosphorylimine and its metal complexes: synthesis, characterization and insecticidal activity studies. Synth. React. Inorg. Met.-Org. Chem.. 2000;30:625-636.
- [Google Scholar]