High catalytic activity of magnetic Fe3O4@SiO2-Schiff base-Co(II) nanocatalyst for aerobic oxidation of alkenes and alcohols and DFT study

Abstract

A new heterogeneous nanocatalyst (Fe 3 O 4 @SiO 2-Schiff base-Co(II)) was successfully fabricated applying silica-coated magnetite nanoparticles as a suitable and efficient support for covalent anchoring of a cobalt(II) Schiff base complex. This catalyst was characterized by FT-IR, XRD, SEM, TEM, EDX, ICP-OES, CHN, DRS, TGA, VSM techniques, and QM study. According to theoretical calculations, the Schiff base-Co(II) complex at high spin state with square planar geometry is more stable than low spin state. Also, Schiff base-Co(II)/imidazole complex has distorted square pyramidal geometry. Evaluating the activity of the nanocatalyst was performed for the oxidizing of different olefins (styrene, norbornene, amethyl styrene, cyclohexene, and cyclooctene) and alcohols (benzyl alcohol, benzhydrol, n-butanol, noctanol and n-heptanol). In these reactions, the influence of two various oxidants, molecular oxygen and tert-butyl hydroperoxide (TBHP), were examined. The results exhibited that the nanocatalyst was considerably effective for the aerobic epoxidation of norbornene, cyclohexene, and cyclooctene with 100% conversion and selectivity within 1 h. Also, by oxidizing of benzyl alcohol and benzhydrol, benzoic acid (conversion: 94%, selectivity: 91%) and benzophenone (conversion and selectivity: 100%) were achieved, respectively. The results suggested that the nanocatalyst has a remarkable role in oxidation reaction of olefins and alcohols. Other promising advantages of the catalyst were easy magnetic separation and recyclability for several times without significant loss of catalytic efficiency.

Co(II) Schiff Base Complex Supported on Nano‐Silica for the Aerobic Oxidation of Cyclohexene: Reaction Pathways and Overoxidation on the Experimental and Calculated Mechanism

“…The o‐vanillin (Figure 1(a)) exhibits one characteristic infrared band of C=O (1639 cm −1 ), while SiO 2 ⋅nH 2 O (Figure 1(b)) exhibits four characteristic peaks at 1629 cm −1 (H−O−H), 1050∼1095 cm −1 (Si−O−Si), 802 and 466 cm −1 (Si−O). Due to the absorption spectra of SiO 2 ⋅nH 2 O overlap with stretching vibrations of C=N (1614∼1618 cm −1 ), C−O (1197∼1205 cm −1 ), Co−O (536 cm −1 ), only a part of infrared absorption of the cobalt complex was obtained [11] . The aliphatic stretching and deformation vibrations of C−H (2939 and 1467 cm −1 ), weak stretching vibrations of benzene ring (1504 cm −1 ) and partial stretching vibrations of C=N (1618∼1614 cm −1 ), confirm the presence of the Schiff‐base in nano−SiO 2 [12] .…”

“…The o-vanillin (Figure 1 cobalt complex was obtained. [11] The aliphatic stretching and deformation vibrations of CÀ H (2939 and 1467 cm À 1 ), weak stretching vibrations of benzene ring (1504 cm À 1 ) and partial stretching vibrations of C=N (1618 ∼ 1614 cm À 1 ), confirm the presence of the Schiff-base in nanoÀ SiO 2 . [12] The conjugative effect of C=N and the benzene ring was weakened by CoÀ N coordinate bond, absorption of benzene ring (1504 cm À 1 ) was obviously enhanced, while the vibration band of C=N shifted slightly (1618 ∼ 1614 cm À 1 ).…”

Solvent‐free oxidation of straight‐chain aliphatic primary alcohols by polymer‐grafted vanadium complexes

“…Thoroughly optimized reaction conditions were employed for the catalytic oxidation of a number of aliphatic primary alcohols in the presence of 4–6 , and results are shown in Figure 10. Although aliphatic alcohols are relatively hard to oxidize, catalysts 4–6 exhibit excellent substrate conversion(%) in comparison with the contemporary catalytic systems [ 26b,43 ] for the listed alcohols.…”

Soft synthesis and characterization of goethite-based nanocomposites as promising cyclooctene oxidation catalysts

“…67,68 Total conversion of cyclooctene to the corresponding epoxide was recently reported for catalysts consisting of Co(II) complexes with Schiff base ligands supported on silica-coated magnetite tested under reux conditions. 69 However, the synthesis of such catalysts is difficult and expensive, and the energy consumption for heating the reaction mixture is high. Another expensive catalyst that allowed reaching 85% conversion of cyclooctene with 92% selectivity for epoxide at a lower temperature (e.g., 35 C) is a Co(II) coordinated metal-organic framework.…”