Highly ordered Nanomaterial Functionalized Copper Schiff Base Framework: Synthesis, Characterization, and Hydrogen Peroxide Decomposition Performance

Abstract

An immobilized copper Schiff base tridentate complex was prepared in three steps from SBA-15 supports. The immobilized copper nanocatalyst (heterogeneous catalyst) was characterized by Fourier transform infrared spectroscopy (FT-IR), cross polarization magic angle spinning (CP-MAS), 13-carbon nuclear magnetic resonance (¹³C-NMR), atomic absorption spectroscopy (AAS), thermogravimetric analysis (TGA), and N₂-physisorption. Moreover, morphological and structural features of the immobilized nanocatalyst were analyzed using transmission electron microscopy (TEM) and X-ray powder diffraction spectrometry (PXRD). After characterizing the nanocatalyst, the catalytic activity was determined in hydrogen peroxide (H₂O₂) decomposition. The high decomposition yield of H₂O₂ was obtained for low-loaded copper content materials at pH 7 and at room temperature. Furthermore, the nanocatalyst exhibited high activity and stability under the investigated conditions, and could be recovered and reused for at least five consecutive times without any significant loss in activity. No copper leaching was detected during the reaction by AAS measurements.

1. Introduction

The increase on environmental legislation as well as the aim to switch towards a more sustainable chemical industry (reducing industrial wastes) constitute big challenges for current chemical research [1]. Green Chemistry unifies concepts that include the design of novel and more environmentally friendly experimental methods in order to replace traditional, unsustainable chemistries.

Based on these premises, one innovative way that provides an elegant route to raise environmentally friendly processes is the use of immobilized catalysts as an alternative to homogeneous catalysts. These catalysts offer environmentally acceptable routes for the production of different chemicals. Reactions are normally cleaner with these types of catalysts, and can be carried out under mild reaction conditions. The insolubility of the support material offers many advantages over homogeneous catalysts, including an increasingly efficient recovery and reuse after reaction in any laboratory or industrial research. Immobilized catalysts also show more advantages over their homogeneous counterparts that include lower toxicity, easier handling, better reaction activity and selectivity, and a lower waste production [2,3,4].

Periodic mesoporous organosilicas (PMOs) are among the most promising supports to immobilize homogeneous catalysts. They exhibit highly-ordered porous and size-controlled structures, large specific surface areas, and they also present major accessibility and potential to be functionalized with different functional chemical groups on their surface [5,6,7,8,9,10,11]. Due to their remarkable properties, these catalysts have found a significant variety of applications in catalysis, e.g., gas adsorption [12,13,14,15,16], energy conversion [17,18], organo-optoelectronics [19,20,21,22], energy storage [23,24], gas sensors [25,26], and drug delivery [27,28,29]. A common and very effective procedure for the synthesis of PMOs relates to the co-condensation of silica with precursors such as tetraalkoxysilane and trialkoxyorganosilane in the presence of a structure-directing agent (templating agent), which determines the structure features of the resulting materials. Consequently, organic groups can easily be incorporated in the pores of these materials via co-condensation [30,31,32,33].

Since the discovery of the first class of these mesoporous materials (MCM type [34]), all prepared using a surfactant template, this method has been extended to various types of mesoporous materials by different research groups. Extensive studies have been conducted on their synthesis, characterization, and applications [35,36,37]. In literature, different catalytic systems have been employed where hydrogen peroxide decomposition was carried out using 17.8 g·L⁻¹ of H₂O₂ and 2.5 g·L⁻¹ of bimetallic magnetic carbon xerogels with cobalt and iron microparticles at 50 °C and pH = 3 for 2 h. A maximum yield of 65% H₂O₂ decomposition was reported [38]. Bourlinos et al. studied the constant rate of hydrogen peroxide decomposition using Fe-doped carbon dots as a nanocatalyst, which was found to be 0.7 × 10⁻¹ min⁻¹ (with 10 mL 0.02 M of peroxide solution and 10 mg of Fe-doped carbon dots). An Fe(III)-functionalized carbon dot nanocatalyst was designed for this purpose (carbon dots (CDs), carbon-based materials) [39]. Tamami et al. studied a system based on a crosslinked polyacrylamide anchored Schiff base (1.9 mg copper content on the polymer support). The system could catalyze the decomposition of H₂O₂ (20 mL, 0.1 M) after 2 h with 100% conversion [40]. Additionally, Demetgül et al. reported an effective H₂O₂ decomposition over 4,6-diacetylresorcinol-chitosan-Cu(II) materials [41]. The reaction conditions involved 0.1 mmol catalyst Cu(II), 25 °C, 10 mL of a dissolution 3.5 × 10⁻² M H₂O₂ and pH 6.86 for a reaction time of 60 min (90% conversion).

For these reasons, and taking into account our recent results related to the design of immobilized catalysts for organic transformations such as oxidation reactions and carbon-carbon bond formations [42,43,44], we report herein the synthesis of a novel Schiff base tridentate copper complex immobilized on mesoporous nanomaterials, Cu-L@SBA-15. The synthesized Cu-L@SBA-15 was characterized by techniques including transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), cross polarization magic angle spinning (CP-MAS), 13-carbon nuclear magnetic resonance (¹³C-NMR), X-ray powder diffraction spectrometry (PXRD), atomic absorption spectroscopy (AAS) and thermogravimetric analysis (TGA). Moreover, the catalytic performance of Cu-L@SBA-15 was investigated in hydrogen peroxide decomposition. Cu-L@SBA-15 revealed a strong catalytic activity towards the decomposition of H₂O₂ under the investigated conditions.

2. Results and Discussion

2.1. Morphology and Characterization of Immobilized Materials

Schiff bases synthesized from primary amine and carbonyl compounds have received significant attention in biological and coordination chemistry. Schiff base ligands are able to easily attach metal ions and to stabilize them in order to create highly stable coordination compounds [45,46,47,48,49,50]. Due to the biological essential activity of copper, its coordination chemistry has received significant attention. Particularly, a large number of Schiff base copper complexes have been developed to investigate their biological and catalytic potential [51,52,53,54]. The synthesis of a novel immobilized copper Schiff base complex was accomplished in three steps (Scheme 1). An SBA-15 support was prepared from pluronic P123 (EO₂₀PO₇₀EO₂₀, EO = ethylene oxide, PO = propylene oxide, Mn = 5800, Aldrich, Darmstadt, Germany) and tetraethoxysilane under acidic conditions. The resulting SBA-15 material was functionalized with N-(2-Aminoethyl)-3-(trimethoxysilyl) propylamine followed by condensation with pyridine-2 carbaldehyde to afford the corresponding immobilized Schiff base (L2@SBA-15). Finally, a complexation with copper acetate afforded the immobilized copper Schiff base nanocatalyst.

FT-IR was utilized to prove the incorporation of the Schiff base and copper species into the SBA-15 frameworks. FT-IR spectra of L2@SBA-15 and Cu-L@SBA-15 are shown in Figure 1. In the corresponding L2@SBA-15 spectra, Si-O-Si bands are located at ca. 800 and 1100 cm⁻¹, and are attributed to the symmetric and asymmetric Si-O-Si stretching of SBA-15, respectively. The silica network of SBA-15 was reported to exhibit a wide absorption band at 3430 cm⁻¹, attributed to the -OH stretching vibrations of silanol groups. The sharp stretching band at 2870 cm⁻¹ is attributed to asymmetric and symmetric C-H stretching in the propyl chain. Stretching bands in the 1300–1600 cm⁻¹ range are assigned to the vibrational modes of C=N, C=C, and C-H groups of the Schiff base moiety. N-H stretching could be observed at 3200 cm⁻¹. Similarly, the aforementioned bands were broadened and overlapped in the spectrum of Cu-L@SBA-15 due to the chelation of copper ions with the Schiff base. These results confirmed the successful functionalization of SBA-15 with a Schiff base and copper ions.

More data supporting the formation of SBA-15 immobilized Schiff base copper complexes were obtained by solid-state ¹³C-NMR spectrum of Cu-L@SBA-15 (Figure 2). ¹³C-NMR spectra results of Cu-L@SBA-15 revealed that carbon resonances of N-(2-aminoethyl)-3-aminopropyl-chain could be observed at 10 (CH₂-Si), 20 (-CH₂-), and 40–55 (CH₂-NH-CH₂-CH₂-NH₂) ppm. In the aryl region, a series of resonance signals found between 150 and 170 ppm could be attributed to pyridine rings. The peaks at 170–180 ppm could also be assigned to imine (C=N) groups of Schiff base moieties.

The surface properties of the material were investigated by N₂-physisorption (Figure 3). Isotherms showed a hysteresis loop between (0.6–0.8) P/P₀ that indicates the mesoporosity of the sample with a bottle-neck geometry. The isotherm is of type IV with a type H1 hysteresis according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The Brunauer–Emmett-Teller (BET) surface area and pore volume of Cu-L@SBA-15 were 346 m²·g⁻¹ and 0.61 cm³·g⁻¹, respectively, with a mean pore diameter of 8.55 nm.

Further information about the thermal stability of Cu-L@SBA-15 could be extracted from TGA analysis data (Figure 4). Analyses were conducted from room temperature to 800 °C. Up to 200 °C, there is a relatively constant weight decrease (4.51%) due to the loss of water and organic solvent residues from the pore channels. The weight loss (57.73%) between 200 °C and 500 °C is generally attributed to the thermal dissociation of the functional organic moieties in the materials after grafting. The total 0.5 mol % weight loss of material amounts to 62.24%. This result indicates that Cu-L@SBA-15 has good thermal stability up to 400 °C. According to TGA analysis, the amount of copper Schiff base complex incorporated into the SBA-15 materials was 0.41 mmol·g⁻¹.

Figure 5 shows the XRD pattern of Cu-L@SBA-15, which exhibits three distinctive peaks in the low-angle region. One intense signal corresponding to the d₁₀₀ diffraction peak is accompanied by two weaker peaks (d₁₁₀, d₂₀₀), suggesting that the two-dimensional hexagonal (P6mm) pore structure is preserved after the introduction of the Schiff base copper complex.

Further confirmation of the highly ordered mesostructure of the resulting material comes from TEM analysis. TEM images show a two-dimensional (2D) hexagonal arrangement of mesopores (Figure 6) which support the obtained results from PXRD analysis. Interestingly, TEM images also exhibit the presence of minor quantities of Cu nanoparticles (black dots, Figure 6) which were found to be present in very low concentrations, supporting the stable formation of the Schiff complex.

2.2. Catalytic Tests

The catalytic decomposition of hydrogen peroxide to oxygen and water has been extensively investigated by a variety of transition metal compounds and also by peroxidase enzymes found in many living media [55,56]. The decomposition rate of H₂O₂ is an important indicator to evaluate the catalytic activity of Cu-L@SBA-15. After the full characterization of Cu-L@SBA-15, the catalytic performance of the immobilized copper complex was examined by the decomposition of hydrogen peroxide under heterogeneous conditions. Different amounts of Cu-L@SBA-15 were used to investigate the effect of the variation in the amount of catalyst on the catalytic H₂O₂ decomposition performance. Plots of H₂O₂ decomposition at different amounts of Cu-L@SBA-15 nanocatalyst as a function of reaction time are given in Figure 7. Reactions were carried out in aqueous phosphate buffer, pH 7, at room temperature. Almost quantitative H₂O₂ decomposition was obtained using only 0.5 mol % of the supported copper nanocatalyst (Cu-L@SBA-15) in 50 min (Figure 7). Potassium permanganate (KMnO₄) was employed as an oxidizing agent to determine the amount of hydrogen peroxide in the solution. Blank runs (in the absence of the catalyst, and those performed only with the SBA-15 support and L1 and L2 systems) provided negligible decomposition activities under the investigated conditions. The hydrogen peroxide decomposition catalyzed by Cu-L@SBA-15 was kinetically monitored by an analytical titration of the residual H₂O₂ with KMnO4 solutions (0.05 M), standardized with sodium oxalate (primary standard). Hydrogen peroxide reduced the potassium permanganate to a colorless product. The chosen concentration of H₂O₂ was 0.05 M at pH 7. Furthermore, the studied range of the catalyst was from 0.001 mmol to 0.005 mmol Cu(II) at a constant concentration of H₂O₂, pH, and temperature. The decomposition was completed after 50 min (98% yield). The minimum amount of catalyst to ensure an optimum decomposition yield was found to be 0.005 mmol.

2.3. Catalyst Reusability

The possibility of reuse of the immobilized catalyst is one of the most important features which determine the applicability of heterogeneous catalysts. To determine the stability and the reusability properties, the catalyst was separated from the reaction mixture after the first run, washed thoroughly with water and ethanol, and dried before its reuse in another reaction run. The Cu-based catalyst could be successfully reused five times without any appreciable loss in its catalytic activity. Filtrates were tested to determine any catalyst leaching by atomic absorption spectrometry. AAS results indicated that no leaching was observed during the first five consecutive runs. However, it was observed that after five runs, some copper was leached from the SBA-15 support (4 ppm, ca. 12%). Deactivation took place mostly due to the observed copper leaching from the support.

3. Experimental Section

3.1. Materials

All chemicals used in this study were purchased from Merck (Darmstadt, Germany) and were used as received without further purification, unless otherwise stated. Solid-state ¹³C-NMR cross-polarization (CP) and magic angle spinning (MAS) spectra were recorded in a Bruker 300 MHz Ultrashield spectrometer (Rheinstetten, Germany). Nitrogen physisorption measurements were carried out at 77 K using an ASAP 2000 volumetric adsorption analyzer from Micromeritics (Micromeritics, Norcross, GA, USA). Samples were outgassed for 24 h at 100 °C under vacuum (10⁻⁴ mbar) and subsequently analyzed. Powder X-ray diffraction patterns were recorded on a Bruker-AXS diffractometer using Cu Kα radiation (λ = 1.5409 Å) (Rheinstetten, Germany). XPS measurements (Berlin, Germany) were performed in an ultra-high vacuum (UHV) multipurpose surface analysis system operating at pressures <10⁻¹⁰ mbar using a conventional X-ray source (XR-50, Specs, Mg-Kα, 1253.6 eV) in a “stop-and-go” mode to reduce potential damage due to sample irradiation. Powdered samples were deposited on a sample holder using double-sided adhesive tape, and subsequently released under vacuum (<10⁻⁶ mbar) overnight. TEM images were collected in a transmission electron microscope (Hitachi 200 kV electron beam energy, Tokyo, Japan) and TGA analysis was performed at a rate of 10 °C·min⁻¹ in a temperature range of 25 to 800 °C under N₂ atmosphere using a NETZSCH STA 409 PC/PG Instrument (Selb, Germany). Metal content in the materials was determined using a Philips PU 9200 Atomic Absorption Spectrometer (AAS) (Almelo, The Netherlands). Samples were digested in HNO₃ and subsequently analyzed by AAS.

3.2. Preparation of L1@SBA-15

Synthesized SBA-15 was activated by refluxing with 6 M HCl for 12 h. It was filtered, washed with deionized water until neutral washings, and then dried in a vacuum oven at 60 °C for 10 h. Activated SBA-15 (2.0 g) was suspended in 50 mL of dry toluene. Then, 0.445 g (2 mmol) of N-(2-Aminoethyl)-3-(trimethoxysilyl) propylamine was added dropwise to the suspension. The mixture was subsequently refluxed in toluene with a continuous removal of water using a Dean-Stark trap for 24 h. The slurry was filtered and the resulting mesoporous material (L1@SBA-15) was washed with an excess of hot toluene and ethanol in order to remove unreacted diamino silane precursor, and finally dried in a vacuum oven at 60 °C for 10 h to furnish L1@SBA-15 with a N loading ca. 0.55 mmol·g⁻¹ (determined by TGA analysis).

3.3. Preparation of L2@SBA-15

For this preparation method, 1 mmol (0.107 g) of pyridine-2-carbaldehyde was added to the suspension of 1.0 g L1@SBA-15 in methanol (50 mL). The mixture was stirred under refluxed conditions for 24 h. The resulting yellow-colored solid was filtered, washed with an excess of methanol, and dried under vacuum conditions at 60 °C for 10 h to obtain L2@SBA-15 with a N loading of ca. 0.48 mmol·g⁻¹ (determined by TGA analysis).

3.4. Preparation of Cu-L@SBA-15

To the stirred suspension of SG-SB1 (4.6 g) in methanol (50 mL), 0.145 g (0.8 mmol) of copper (II) acetate hydrate were added at 60 °C for 24 h, resulting in a green-colored solid material. After that, the solid phase was separated, washed with methanol, and dried under vacuum conditions for 24 h. The green solid was filtered and washed with 3 × 10 mL of methanol and subsequently dried in an oven at 60 °C overnight to furnish the corresponding nanocatalyst Cu-L@SBA-15 with a Cu loading of ca. 0.41 ± 0.01 mmol·g⁻¹ (determined by TGA and AAS analysis).

3.5. General Procedure for Hydrogen Peroxide Decomposition

In a flask with a magnetic stirrer containing 50 mL of 0.05 M hydrogen peroxide solution in aqueous phosphate buffer pH7, 0.025 mmol of Cu-L@SBA-15 catalyst (62.5 mg) was added at room temperature. At constant pH, the decomposition of H₂O₂ was studied. The pH of the solutions was adjusted to 7 to obtain a neutral media, by using the buffer solution according to the reported literature. The extent of hydrogen peroxide decomposed at different intervals of time (intervals of 10 min from 0 to 60 min) was subsequently determined by taking 5 mL reaction mixture aliquots (filtered before analysis) and titrating them using 0.05 M KMnO₄ in the presence of 0.05 M H₂SO₄. The permanganate ion has a deep purple color, which disappeared after being consumed. The indication of the equivalence point occurred when the titrated solution had a light purple/pink color due to the slight excess of permanganate. To study the reusability of the catalyst, the immobilized copper catalyst was separated from the reaction mixture by simple filtration, then washed with water, and dried at vacuum conditions to be subsequently reused for the following run.

4. Conclusions

The design and synthesis of a highly-ordered mesoporous Schiff base tridentate copper complex (Cu-L@SBA-15) has been reported. The structure and physicochemical properties of Cu-L@SBA-15 were characterized by different analytical techniques such as FT-IR, XRD, TEM, CP-MAS, ¹³C-NMR, AAS, TGA, and N₂-physisorption. The evaluated catalyst performance represents an environmentally benign and atom-economical process for hydrogen peroxide decomposition at room temperature, with further potential applications in several organic transformations. A complete decomposition of hydrogen peroxide takes place within 50 min using only 0.005 mmol of the catalyst. The studied catalyst could also be successfully recycled and reused without any significant loss of catalytic activity for five consecutive runs. Despite the high efficiency, stability, and reusability of the catalyst as well as low catalytic amounts required and a fast reaction rate under mild reaction conditions, the synthetic steps to design the reported catalytic system can be certainly improved from the green chemistry standpoint, with ongoing investigations in our group aimed to further advance the design of similar catalytic nanomaterials with better green credentials.