Novel Synthesis of Cu-Schiff Base Complex@Metal-Organic Framework MIL-101 via a Mild Method: A Comparative Study for Rapid Catalytic Effects

Abstract

The use of metal complex immobilized/decorated porous materials as catalysts has found various applications. As such, finding a new and mild method for synthesis of metal complex immobilized over porous material is of great interest. Immobilized porous materials for styrene oxidation were reported in this work. Immobilized porous material of Cu-Schiff base complex @MIL-101 were described, in which immobilized Cu-Schiff base complex within super cage of a metal-organic framework (MOF)-based porous material, chromium (III) terephthalate MIL-101. They were systematically characterized by using elemental analysis, powder X-ray diffraction, fourier transform infrared spectroscopy, N₂ absorption-desorption, and so on, also used as catalyst for the selective oxidation of styrene to benzaldehyde. Comparatively, the immobilized heterogeneous catalyst of Cu-Schiff base complex@MIL-101 acted as an efficient heterostructure catalyst in the oxidation of styrene to benzaldehyde up to six cycles, and showed superior activity for styrene oxidation over MIL-101.

Keywords: MIL-101; catalysis; immobilization; metal-organic frameworks; styrene.

Figure 1

Schematic illustration of the encapsulation of Cu‐Schiff base complex within the MIL‐101 cage: A) scheme representation of the mesoporous cage of MIL‐101; A1) perspective view of the mesoporous cage of =MIL‐101 with hexagonal window; B) diffusion of salicylaldehyde into the MIL‐101; C) immobilization of bis salicylaldehyde Ethylenediamine Schiff Base in MIL‐101; D) Cu‐Schiff complex@MIL‐101(x) heterogeneous catalysts via in‐situ reaction.

Figure 2

XRD patterns the Cu‐Schiff complex@MIL‐101(x) catalysts: a) the simulated XRD powder patterns of MIL‐101 in reference 12; b) the synthesized MIL‐101; c) x=2 : 1; d) x=4 : 1; e) x=8 : 1; f) Cu‐Schiff base complex.

Scheme 1

Reaction Scheme for Oxidation of Styrene to Benzaldehyde

Figure 3

(A) Decrease in the absorption maximum of benzoquinone at 246 nm catalyzed by Cu‐ Schiff complex@MIL‐101(2) with increasing reaction time. [Styrene]=1×10⁻⁴ mol L⁻¹, [Cu‐Schiff base complex@MIL‐101(2)]/[ Styrene]=0.04, pH=7.6 (Tris−HCl buffer), T=30 °C. (B) Typical plot of At versus time for oxidation of styrene catalyzed by Cu‐Schiff base complex@MIL‐101(2)@MIL‐101(2) using experimental data in Figure 3A, and nonlinear fitting of the experimental data: dots represent the experimental data, and the line shows the fitting results

Figure 4

Infrared spectra of the Cu‐Schiff base complex@MIL‐101(x) catalysts: a) bare MIL‐101; b) x=2 : 1; c) x=4 : 1; d) x=8 : 1; e) Cu‐Schiff base complex.

Figure 5

Nitrogen adsorption isotherms 77 K of Cu‐Schiff base complex@MIL‐101(x) catalysts; a) bare MIL‐101, b) x=2 : 1, c) x=4 : 1, d) x=8 : 1.

Figure 6

SEM images for: a) MIL‐101, b) Cu‐Schiff base complex@MIL‐101(2), c) Cu‐Schiff base complex@MIL‐101(4), d) Cu‐Schiff base complex@MIL‐101(8).

Figure 7

reaction rate of MIL‐101 and Cu‐Schiff base complex@MIL‐101(4) at different cycles

Figure 8

PXRD of patterns for Cu‐Schiff complex)@MIL‐101(4) after catalytic cycles.