Nanohybrid Copper(II) Schiff Base Complex Immobilized into Mesoporous Silica as Efficient Biomimetic Catalyst

Nanohybrid Copper(II) Schiff Base Complex Immobilized into Mesoporous Silica as Efficient Biomimetic Catalyst
Mihaela MURESEANU1, Irina GEORGESCU2, Mihaela FILIP3, Gabriela CARJA4, Teodora RADU5, 6, Viorica PARVULESCU*3,
1University of Craiova, Department of Chemistry, 107 I Calea București, Craiova, Romania
2“Babes Bolyai“ University, Faculty of Chemistry and Chemical Engineering, 11 Arany Janos Street, 400028, Cluj-Napoca, Romania
3”Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Splaiul Independentei 202, 060021, Bucharest, Romania
4Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, 71 D. Mangeron, Iasi, Romania
5National Institute for Research and Development of Isotopic and Molecular Technologies,
65-103 Donath Street, 400293 Cluj Napoca, Romania
6Institute of Interdisciplinary Research in Bio-Nano-Sciences, “Babes Bolyai” University,
400271, Cluj-Napoca, Romania
Abstract: A novel nanostructured catalyst for cyclohexene oxidation by H2O2 and also as an artificial superoxide dismutase (SOD) enzyme was obtained by immobilization of a CuII complex with a Schiff base ligand derived from 2-(3-bromopropoxy) benzaldehyde and 2-aminobenzoic acid. Mesoporous SBA-15 silica was synthesized and functionalized with 3-aminopropyl-triethoxysilane (APTES) by a post-synthesis method and used afterwards for the covalent grafting of the copper complex. The physical-chemical properties of the catalysts were investigated by XRD, N2 adsorption-desorption, FTIR, XPS and TGA analyses. We present evidence that the properties of the CuII based complexes and the specific properties of the SBA-15 mesoporous support both beneficially contributed to establish the efficiency either of alkene catalytic oxidation or of the superoxide radical dismutation. The best results for cyclohexene oxidation were 75% conversion, 45% selectivity for 2-cyclohexen-1-ol and an initial turnover frequency of 130 h-1. The leaching effect of copper was not observed and cyclohexene oxidation yields could be maintained during five cycles. Furthermore, this new hybrid material was proved to be an efficient biomimetic catalyst.
Key words: SBA-15, Schiff bases, CuII complexes, Cyclohexene oxidation, Superoxide dismutase activity
1. Introduction
The selective oxidation of hydrocarbons to oxygen-containing compounds is of great interest both in industry and in academic research.1,2 In particular, allylic oxidation of olefins into unsaturated ketones and alcohols is important in organic chemistry and chemical industry.3,4
Among the metal complexes used as oxidation catalyst, copper based complexes have been extensively used for oxidation of various alkene substrates with different oxidants.5-7
Heterogeneous systems are a more convenable alternative to conventional homogeneous catalysts due to their attractive features such as stability, easy separation, handling and recovery, as well as overcoming the problem of metal leaching. They have been achieved by immobilization of the homogeneous catalysts on different supports such as zeolites8-10, polymers11, layered compounds12, titanium-doped AlPO4-based microporous zeotypes (so-called TAPO materials)13 and ordered mesoporous carbon.14
Nanoarchitectonic of ordered mesoporous solids, like MCM-41 and SBA-15 are potential formulations for heterogenization of valuable homogeneous catalysts due to their large surface area that could be easy modified by using the surface Si-OH groups. CuII Schiff base complexes heterogeneized into mesoporous silica have been used for cyclohexene oxidation reactions.15-17
The natural SOD enzymes, which catalyze the dismutation of the free radical superoxide, are a class of metalloenzymes which contain Cu/Zn, Fe, or Mn complex as active sites. For a biomimetic heterogeneous catalyst to be performant in superoxide scavenging it is important that the support allows a good accessibility and a good interaction between its surface and the active metal ions. Some metal complexes have shown to possess biomimetic SOD activity when they were immobilized into different supports.18-21
There are not any reports on heterogeneous biomimetic oxidation catalysts based on CuII complex with a Schiff base ligand derived from 2-(3 bromopropoxy) benzaldehyde and 2-aminobenzoic acid. Therefore, in this paper we focused on the following aims: (i) to synthesize a new Schiff base ligand and its CuII complex; (ii) to immobilize the complex on an amino-functionalized mesoporous SBA-15 silica support; (iii) to evaluate the potential of the thus prepared heterogeneous system as recyclable catalysts for the peroxidative cyclohexene oxidation and as artificial superoxide dismutase (SOD) enzyme.
Results and discussion
Characterization of CuII complex/SBA-15 catalytic system
A new nanohybrid oxidation catalyst was obtained by covalent grafting into amino-functionalized SBA-15 mesoporous silica of a CuII complex with a Schiff base ligand (Scheme 1). The nanohybrid was synthesized in three steps: (i) post synthesis functionalization of the mesoporous SBA-15 silica support with APTES, (ii) synthesis of 2-(3-bromopropoxy)benzaldehyde ligand and its CuII complex and (iii) complex grafting onto the amino-functionalized silica supports.
2.1.1. Elemental and thermal analysis
The elemental analysis of the CuIIBrAld complex (Table 1) showed a N/Cu molar ratio of 2/1 which indicates that CuII has a N2O2 Schiff base ligand environment and two coordinated H2O molecules, as shown in Scheme 1.
The amount of functional groups grafted on the SBA-15 surface was determined by thermogravimetric analysis. The weight loss due to the aminopropyl moiety was of 6.43% which corresponds to 1.1 mmoles/g. The amount of immobilized complex was determined from TG curve by subtracting the amount of the aminopropyl group previously grafted on the mesoporous silica surface and compared with the elemental analysis results (C%, N% and H%) in order to calculate the complex immobilization yield and the N to Cu molar ratio (N/Cu). The copper content was determined by AAS after sample dissolution in 10% HF (Table 1). The Schiff base ligand loading was 0.18 mmoles/g, the copper content 0.09 mmoles/g and the complex immobilization yield 75%. After immobilization, the N/Cu molar ratio became 4/1 because the two Br atoms of each complex molecule reacted with two aminopropyl groups of the functionalized support for the covalent grafting. The disappearance of the C-Br bond in the FTIR spectra of the immobilized complex supports this affirmation and the structure presented in Scheme 1 indicates that the homogeneous copper complex remained intact after immobilization into the SBA-15 matrix and the complex geometry did not change during the immobilization process.
The TGA profile of CuII-BrAld/SBA-15-NH2 is presented in Figure 1. In the case of modified silicas, the loss of physisorbed water (below 150°C) was about 3-5% in comparison with SBA-15 that exhibited a first mass loss (about 10 wt%) between 20 and 150°C and a second weight loss (6 wt%) observed between 150 and 800°C that can be ascribed to the condensation of the surface silanol groups. Based on the TGA profiles, we can consider that the degradation process of the anchored organic moieties occurred between 150 and 650°C (exothermic process). The mass loss in this range was about 19 wt% for CuII-BrAld/SBA-15-NH2 sample.
2.1.2. Powder X-ray diffraction
The SBA-15 structure for both unmodified and grafted samples was confirmed by the XRD measurements (Figure 2). The SBA-15 material exhibited a strong (100) reflection peak (at 2θ = 0.89°) and two smaller diffraction peaks (110 and 200) at 1.47o and 1.67o, which can be indexed as a hexagonal lattice with d- spacing values of 99.52, 60.41 and 53.32 Å, respectively. The unit cell parameter, ao, of 115.05 Å is characteristic of a well ordered SBA-15 type material.22 No significant changes were observed after functionalization with CuII-BrAld complex.
The (100) peak relative intensity decreased in the case of the CuII-BrAld/ SBA-15-NH2 as compared with the pure SBA-15 silica proving that functionalization mainly occurred inside the mesopore channels, as already reported for other hybrid materials.23
2.1.3. Nitrogen adsorption-desorption isotherms
CuII-BrAld/SBA-15-NH2 displayed an irreversible nitrogen adsorption-desorption isotherm of type IV, similar to that obtained for pure SBA-15 (Figure 3). The H1 hysteresis loop, with a sharp desorption step at about 0.70 p/p0 is characteristic of well-structured mesoporous materials. The main textural properties of solids are listed in Table 2.
The grafted species were located not only on the outer surface, but also inside the mesopores. Consequently, the specific surface area and the mesopore volume for CuII-BrAld/SBA-15-NH2 strongly decreased after CuII-BrAld complex immobilization.
2.1.4. FTIR spectroscopy analysis
FTIR spectra of the BrAld ligand, CuII-BrAld complex, SBA-15 support and the hybrid CuII-BrAld/SBA-15-NH2 illustrated in Figure 4 were recorded in the range of 4000-400 cm-1. The characteristic bands of the Schiff base (BrAld) obtained by the condensation of 2-(3-bromopropoxy) benzaldehyde with 2-aminobenzoic acid are as follows: ((CH=O) at 1687 cm-1, ((CH=C) at 1599 cm-1, (asim(COO-) at 1490 cm-1, (sim(COO-) at 1387 cm-1, (asim(Ar-O-C) at 1243 cm-1, (sim(Ar-O-C) at 1106 cm-1, ((C-H)aromatic at 758 cm-1 and ((C-Br) at 652 cm-1. In the CuII-BrAld spectra the new band at 1608 cm-1 corresponds to the valence vibration of the azomethine (C=N) group and the band corresponding to the carbonyl group disappeared, proving the Schiff base formation. Furthermore, the bands corresponding to the symmetric and asymmetric vibrations of the Ar-O-C groups were slightly shifted compared with the ligand spectrum, suggesting that the etheric O was not involved in copper coordination. Instead, the carboxylic O was coordinated to the metal ion since the valence vibrations (asim(COO-) at 1554 cm-1 and (sim(COO-) at 1378 cm-1 were shifted compared with BrAld ligand. The broad band at (3450 cm-1 could be assigned to the OH groups from the H2O molecules coordinated to the central metallic ion, as the elemental analysis confirmed. The valence vibration of C-Br bond was at 660 cm-1. In the FTIR spectrum of the free complex, the bands corresponding to the valence vibrations of the (Cu-O)carboxyl and Cu-N bonds at 520 cm-1 and 424 cm-1 respectively, suggest the involvement of carboxylic O and N of the azomethine group in CuII coordination. In the FTIR spectrum of SBA-15 silica, the large broad band at 3400 cm-1 is attributed to O-H bond stretching of the surface silanols groups and to the remaining adsorbed water molecules. The absorption band at 1630 cm-1 was determined by deformational vibrations of adsorbed water molecules. The siloxane, -(Si-O)n-, peak appeared centered at 1100 cm-1 and the Si-O bond stretching of silanol groups was detected at 970 cm-1.
The most obvious changes in the FTIR spectrum of CuII-BrAld/SBA-15-NH2 material in comparison with the free complex and the SBA-15 support are: the disappearance of the characteristic band at 660 cm-1 which corresponds to υ (C-Br) and the appearance of new bands at 2964 cm-1 which corresponds to the asymmetric stretching of υ (C-H) from the aminopropyl of the sililating agent. In the FTIR spectra of the immobilized complex, apart from the bands in the overlapping regions of the silica backbone, the other bands of the complex were all clearly present (Figure 4), indicating the successful immobilization of the complex onto SBA-15 by covalent grafting of both Br atoms from the two ligands present in the copper complex molecule to the –NH2 groups of the functionalized SBA-15 silica.
XPS analysis provided further evidence for the chemical structure and composition of the catalyst surface. A XPS spectrum of CuII-BrAld/SBA-15-NH2 sample is shown in Figure 5(a). The Si2p peak position for the copper complex immobilized into SBA-15 silica was in agreement with SiO2-type material. As expected, the spectra showed peaks for N1s at 400.2 eV assigned to amines and imines (Figure 5(b)). The spectra concerning Cu2p core level excitation are presented in Figure 5(c). The sample shows two main peaks centered at ~933.8 eV and 964.2 eV associated with Cu 2p3/2 and 2p1/2, respectively, being accompanied by shake up satellite peaks. It was proved that satellite bands were generated by an electron transfer from a ligand orbital to 3d orbital of Cu, confirming that the d level was filled.24-26 This transition is a np(ligand)→3d(metal) transition27, which is impossible for Cu+ and Cu0 species that have filled levels, being mainly a characteristic of bivalent copper.26 Furthermore, the Cu2p3/2 peak is typical of copper (II) complexes in a mixed N, O coordination sphere.28
2.1.6. SEM and TEM microscopy
SEM image of CuII-BrAld/SBA-15-NH2 showed many ropelike domains with a relatively uniform length of 3μm (Figure 6b). These domains were aggregated to a wheat-like macrostructure similar to earlier reported morphologies of parent SBA-15 (Figure 6a), indicating that the immobilization procedure did not influence the morphology of the pure siliceous SBA-15, apart from a shortening of the ropelike domains.
Figure 7 shows some representative TEM images for SBA-15 and CuII-BrAld/SBA-15-NH2 materials. The hexagonal mesoporous structure characteristic of SBA-15 was observed in all samples, confirming the results obtained by XRD.
2.2. Catalytic activity
2.2.1. Catalytic performances of the CuII complex/SBA-15
The oxidation of cyclohexene with H2O2 in acetonitrile, under air atmosphere, was chosen as a test reaction to determine the catalytic performances of the studied SBA-15 supported CuII Schiff base complex. The optimization of the CH oxidation was previously done29 and the best operation parameters were: 5h reaction run at 600C with 0.03 mmoles catalyst, 10 mL solvent and a 2.2/1 H2O2/C6H10 molar ratio. The joint effects arising from SBA-15 silica surface and acetonitrile solvent would be interesting from the catalytic point of view. The reaction did not proceed in the absence of the catalyst and the CH conversion on pure SBA-15 was very low. The results presented in Table 3 also show that the catalytic performance of the immobilized complexes was better than that of free complexes. Higher catalytic activity of the silica-supported catalysts relative to the homogeneous counterpart is attributed to the fine distribution of the isolated catalytic site on the SBA-15 surface. The optimal cyclohexene conversion was 75%, selectivity for 2-cyclohexen-1-ol was 45% and initial turnover frequency was 130 h-1. The main products obtained during cyclohexene oxidation were cyclohexene oxide (I), 2-cyclohexen-1-ol (II) and 2-cyclohexen-1-one (III) as shown in Scheme 2. According to GC–MS analyses, the products mixture was composed of species formed by oxidation of both double bond and allylic C–H. The preponderance of compounds II and III indicates that the reaction proceeded mainly via the allylic oxidation pathway as opposed to the direct oxidation of the double bond. These results are in agreement with previously reported results concerning salicylaldimine complexes of Cu(II) and Co(II) immobilized on silica supports (MCM-41, SBA-15 and Davisil 710) as catalysts for cyclohexene oxidation using hydrogen peroxide as an oxidant under an oxygen atmosphere.15 It was observed that the cooper catalysts generally produced almost equal amounts of alcohol and ketone and that SBA-15 led to higher levels of alcohol, in comparison with MCM-41 that tends to favor ketone.
In the present reaction system, the CH oxidation was accompanied by the side-reaction of self-decomposition of H2O2. The effective utilization of H2O2 was found to be 61% for CuII-BrAld/ SBA-15-NH2 catalyst, indicating participation of the oxidant agent to other inefficient secondary reactions beside the tested oxidation reaction.
By comparing the CH conversion and selectivity for allylic oxidation compounds with other heterogeneous copper complexes, this new catalyst presents both high conversion and good selectivity. When Cu(Salen) complex intercalated α-zirconium phosphate was tested for the oxidation of CH using tert-butylhydroxide as oxidant, under optimized conditions, the maximum conversion was 26.71% with cyclohexanone as the major product (49.80%).30 M41S type mesoporous silicate spherical particles containing copper nanospecies were used as catalyst for CH oxidation with H2O2 when the maximum conversion was of 30% and the allylic oxidation was the main reaction.31 Heterogeneous Cu(II) complexes with unsubstituted and tertiary-butyl substituted salycilaldimine ligands were immobilized on silica supports and tested for cyclohexene oxidation using hydrogen peroxide as oxidant under an oxygen atmosphere.15 The maximum CH conversion was of 84% and 2-cyclohexen-1-ol and 2-cyclohexen-1-one prevailed as allylic oxidation products.
2.2.2. Stability of CuII complex/SBA-15 in the process of cyclohexene epoxidation
The most significant advantage of this new hybrid CuII-BrAld/SBA-15-NH2 heterogeneous catalyst is its stability. The reusability under the optimum reaction conditions was further investigated by measurements of initial reaction rates and conversions over five cycles. It was proved that the catalyst was still active during the fifth run, with a slight decrease of the initial TOF (Table 4). AAS measurement results revealed that the recovered catalyst had almost the same copper content as the fresh catalyst (Cu, 0.55%). This further indicates that the obtained SBA-15 hosted copper complex was stable and the leaching of the metal was very poor. The catalytically active copper complex was stabilized by covalent grafting onto the mesoporous silica surface and the nature of the active species in the oxidizing medium was not changed very much. By using SBA-15 silica characterized by larger pores than other mesoporous or microporous supports, the limitation in the substrate diffusion with an increase in number of cycles due to the adsorption of the reaction products on the support was partially eliminated. Both CH conversion and TOF values were however slightly diminished after the repeated use, probably due to a change in the morphology of the mesoporous material or in the catalytically active center surrounding.
2.3. Superoxide dismutase (SOD) activity of CuII-BrAld/ SBA-15-NH2 hybrids
CuII-BrAld complex free or immobilized onto the amino functionalized SBA-15 support was tested for SOD activity by measuring inhibition of the NBT reduction. The inhibition (%) of the artificial SOD concentrations was calculated. All materials, except the silica support, displayed catalytic activity in the dismutation reaction of superoxide radical anions. For the free CuII-BrAld complex, the concentration for 50% inhibition of the NBT reduction was 33 µM. After immobilization, the SOD activity changed and the 50% inhibition concentration value was 24 μM for CuII-BrAld/ SBA-15-NH2. Its activity is comparable with and even better than other biomimetic copper complexes immobilized into different supports.18-21
A possible explanation for increased SOD activities of the immobilized complexes is that the silica matrix probably protects the complex that mimics the active center of the natural SOD enzyme, allowing better dispersion of the metallic sites and accessibility of the substrate to these sites. Furthermore, the heterogeneous catalyst could be more easily recovered and reused compared with the free complex.
3. Experimental
3.1. Materials and equipment
Poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyleneglycol) (Pluronic P123, Mav = 5800), tetraethylorthosilicate (TEOS, 98%) and 3-aminopropyltriethoxisilane (APTES, 99%) were purchased from BASF and Aldrich. All chemicals were used without further purification. CuCl2∙2H2O, NH4OH, 2-aminobenzoic acid (Sigma-Aldrich) for the catalyst synthesis, cyclohexene (Aldrich) as substrates, H2O2 (Merck) as oxidant, riboflavin, L-methionine and nitro blue tetrazolium (NBT) for the biochemical test of superoxide dismutase activity, solvents such methanol, ethanol, dimethylformamide (DMF), toluene, diethyl ether, dichloromethane and acetonitrile (Fluka and Aldrich) were used as such in this study.
Powder X-ray diffraction (XRD) measurements were performed on a Bruker AXS D8 diffractometer by using Cu Kα radiation (λ= 0.154 nm) over a 2θ range from 1° to 6°. N2 adsorption–desorption isotherms were measured at -196oC with a Micromeritics ASAP 2010 instrument. The samples were previously degassed under vacuum at 50oC for 12 h. Specific surface area was calculated by the BET method and the mesopore volume was determined from the isotherm at the end of capillary condensation. The pores size distribution was obtained from the desorption branch using the BJH method and the Harkins-Jura standard isotherm. The FTIR spectra were recorded using a Bruker Alpha spectrometer. XPS measurements were performed with a SPECS PHOIBOS 150 MCD instrument equipped with monochromatized Al Kα radiation (1486.69 eV) at 14 kV and 20 mA and a pressure lower than 10−9 mbar. The binding energy scale was charge referenced to the C 1s photoelectron peak at 285 eV. The spectra deconvolution was accomplished with Casa XPS (Casa Software Ltd., UK). The copper content was determined by flame atomic absorption spectrometry (AAS) on a Spectra AA-220 Varian Spectrometer with an air acetylene flame. C, H, and N contents were evaluated by combustion on a Fisons EA1108 elemental analysis apparatus. The thermogravimetric analysis (TG/DTA) was carried out in a Netzsch TG 209C thermobalance in nitrogen flow.
3.2. Synthesis procedures
3.2.1. Preparation of functionalized SBA-15 silica
SBA-15 material was synthesized as described in literature.32 Thus, 1.5 g of Pluronic P123 was dispersed in 15 g of H2O and 45 g of 2 M HCl with stirring at 40oC for 4 h. Then, 3.15 g of TEOS was added and the obtained homogeneous solution was stirred at 40°C for 24 h. The resulting gel was hydrothermally treated in a Teflon-lined autoclave at 100°C for 2 days. The solid was centrifuged, filtered, washed with deionized water and dried in air at room temperature. The as-synthesized SBA-15 was calcined at 550°C during 8 h under air flow.
The organic-inorganic hybrid material was obtained by a post-grafting procedure according to a previously described procedure.33 1g of SBA-15 silica, freshly activated at 130°C overnight under vacuum and 1 mL of APTES were added to 50 mL of dry toluene. After stirring the solution at reflux of toluene for 2 h, the ethanol released was distilled off and the mixture was kept under reflux for 90 min. The NH2-functionalized mesoporous silica (referred as NH2-SBA-15) was filtered and washed with toluene, ethylic alcohol and then with diethyl ether. It was then submitted to a continuous extraction run overnight in a Soxhlet apparatus using diethyl ether/dichloromethane (v/v, 1/1) at 100 °C and dried at 130°C overnight.
3.2.2. Synthesis of the CuII complex
The 2-(3-bromopropoxy)benzaldehyde ligand and its CuII complex were synthesized as described in literature34, and denoted BrAld and CuII-BrAld, respectively. The suspension of 2-aminobenzoic acid (2.048 moles) in absolute ethanol (10 mL) was added to a solution of 2-(3-bromopropoxy)benzaldehyde (2.05 moles) in absolute ethanol (10 mL) and was kept under continuous stirring at RT for 2h. Thereafter, 10 mL of an ethanolic solution of CuCl2∙2H2O (1.02 moles) was added, the pH was adjusted to 8 by adding dropwise a NH4OH solution and the mixture was refluxed for one hour. The obtained green precipitate was collected by filtration, washed with water (2×10 mL), ethanol (2×10 mL), diethyl ether (2×10 mL) and dried at room temperature for 24 h.
3.2.3. CuII Complex/SBA-15 hybrid catalyst
The complex CuII-BrAld (0.1g) and 0,5 g of activated SBA-15-NH2 (3 h at 120 °C under high vacuum) were reacted for 48 h in 10 mL DMF at reflux, with magnetic stirring and under a N2 atmosphere (Scheme 1). The resulting modified mesoporous silica (referred as CuII-BrAld/SBA-15-NH2) was filtered off and washed with toluene (2×30 mL), ethanol (2×30 mL) and diethyl ether (2×30 mL). Finally, the product was dried at 110 °C under vacuum for 4 h.
3.3. Catalytic oxidation of cyclohexene
The oxidation of cyclohexene (CH) was done in the liquid phase over CuII-BrAld/SBA-15-NH2 catalyst as follows: 2.26 mmoles of CH, 0.03 mmoles of catalyst and 10 mL of acetonitrile were added successively to a temperature controlled two-necked round-bottom flask with a reflux condenser, at 60oC, for different times. When the temperature reached the set value, the hydrogen peroxide (30% H2O2) was added drop wise. After the reaction ran for the desired time, the reaction mixure was cooled, the products were filtered and they were analyzed using a Thermo DSQ II system with gas chromatograph GC-Focus, mass spectrometer DSQ II and a Thermo TR-5MS 30 m x 0.25 ID x 0,25 μm film capillary column.
The conversion, selectivity and turnover frequency (TOF) were calculated as follows:
Conversion (%) = 100 × (C0- Ct)/C0;
Product selectivity (%) = [(product formed (moles)/total product detected (moles)] × 100;
TOF = Substrate converted (moles)/[Copper in catalyst (moles) × reaction time (h)].
where C0=initial concentration (moles); Ct=final concentration (moles).
H2O2 consumption was determined by an iodometric titration after the reactions. The H2O2 efficiency was calculated as the percentage of this reactive converted to oxidized products. The persistence of the catalytic activity was checked in the oxidation of CH for 5 consecutive runs.
3.4. Catalysis of superoxide dismutation
The SOD activity of the free or immobilized CuII complex as well as of the SBA-15 support was assayed by measuring the inhibition of NBT photoreduction, using the Beachamp-Fridovich reaction35 as we describe elsewhere.20,21 The quantity of enzyme inhibiting the reaction by 50% (in μM) for each sample was determined from the inhibition curves of NBT photoreduction by increasing the concentration of free or immobilized complex. Samples without catalysts were used to give a background visible absorbance value. Native SOD (7.46 U) from bovine erythrocytes was used as positive control.
4. Conclusions
A novel catalyst based on CuII complex with a Schiff base ligand derived from 2-(3-bromopropoxy) benzaldehyde and 2-aminobenzoic acid supported on SBA-15 matrices was prepared and tested in the process of oxidation of cyclohexene with 30% H2O2 or the dismutation reaction of the superoxide radical anions.
The immobilization of CuII complex on the SBA-15 support beneficially contributes to the catalytic performances in comparison with its homogeneous analogues. Furthermore, CuII complex/SBA-15 catalyst is easily recyclable and can be reused at least five times with no significant loss of the catalytic activity and selectivity.
The obtained results can pave the way for the development of new hybrid materials by the immobilization of metal complexes into mesoporous silica supports that could be used either as efficient alkenes oxidation catalysts or as artificial enzymes with superoxide scavenging activity.
Acknowledgment
The authors gratefully acknowledge the financial support from the Romanian National Authority for Scientific Research, CNCS-UEFISCDI; project number PN-II-IDPCE 75/2013
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Figure 1. TG-DTA curves for CuII-BrAld/SBA-15-NH2


Figure 2. XRD patterns of (a) SBA-15 and (b) CuII-BrAld/SBA-15-NH2


Figure 3. N2 adsorption-desorption isotherms of SBA-15 (1) and CuII-BrAld/SBA-15-NH2 (2)






Figure 4. FTIR spectra of (a) BrAld and (b) CuII-BrAld free complex (1), SBA-15 (2), CuII-BrAld/SBA-15-NH2 (3)








Figure 5. Wide-scan XPS spectrum of the CuII-BrAld/SBA-15-NH2 sample (a), high-resolution XPS spectra of N1s (b) and Cu2p 3/2 (c)





Figure 6. SEM images of SBA-15 (a) and CuII-BrAld/SBA-15-NH2 (b)





Figure 7. TEM images of SBA-15 (a) and CuII-BrAld/SBA-15-NH2 (b)
Compound
Analytical dataa (%)
Complex amountb
(mg g-1)
Cu
amount
(mg g-1)
Cu/N
molar ratio


C
H
N
Cu




CuIIBrAld, C34H34O8N2Br2Cu
49,54
(49,66)
4,15
(4,14)
3,40
(3,41)
7,52
(7,73)
-

1/2

CuII-BrAld/SBA-15-NH2
7.92
(7.84)
0.69
(0.65)
0.51
(0,53)
0.56
(0.57)
119.07
5.72
1/4

aCalculated values are shown in parenthesis; for the immobilized complex, the C%, H% and N% were calculated only for the ligand amount corresponding to the Cu% determined by AAS, considering a metal to ligand ratio of ½ and subtracting the aminopropyl group previously grafted.
bDetermined by TGA after subtraction of the aminopropyl moiety previously grafted on the silica support.
Table 1. Elemental analysis of CuII-Schiff base complex free or SBA-15-supported
Sample
a0
(nm)a
SBET(m2g-1)b
Vp (mL g-1)c
Dp (nm)d
Wall thickness (nm)e

SBA-15
11.3
600
1.1
7.8
3.5

CuII-BrAld/ SBA-15-NH2
11.2
287
0.46
6
5.2

aa0 = 2d100/
; bSBET- specific surface area; cVp- mesopore volume; dDp= pore diameter; eWall thickness = a0–D.
Table 2. Textural properties of calcined and modified SBA-15
Sample
CH conversion (%)
TOF (h-1)
Selectivity (%)




I
II
III

CuII-BrAld
11
12
68
22
10

CuII-BrAld/ SBA-15-NH2
75
130
15
45
40

Reaction conditions: catalyst (0.03 mmoles), substrate (2.26 mmoles), ACN (10 mL), H2O2 (4.75 mmoles), 5h, 600C,
Formed products: cyclohexene oxide (I), 2-cyclohexen-1-ol (II) and 2-cyclohexen-1-one (III)
Table 3. Catalytic performances of the copper complex
No. of cycle
CH conversion %
Initial TOF (h-1)

1
75
130

2
74
129

3
73
127

4
72
126

5
70
125

Table 4. Reusability of CuII-BrAld/SBA-15-NH2 catalyst


Scheme 1. CuII-BrAld/SBA-15-NH2 catalytic system


Scheme 2. Possible process for the oxidation of cyclohexene