Monometallic and bimetallic Cu-Ag MOF/MCM-41 composites: structural characterization and catalytic activity

Abstract

Monometallic and bimetallic MOF/MCM-41 composites (Cu, Ag and Cu-Ag) were synthesized via a solvothermal method. The synthesized composites were characterized by XRD, FTIR, SEM, EDX and BET surface area measurements. The acidity was determined through two techniques; potentiometric titration with n-butyl amine for determining the strength and the total number of acid sites and FTIR spectra of chemisorbed pyridine on the surface of MOFs for determining the type of acid sites (Brønsted and/or Lewis). All the prepared MOFs showed Lewis-acid sites and the higher acidity was observed for the bimetallic Cu-Ag MOF/MCM-41 composite. The catalytic activity was examined on the synthesis of 1-amidoalkyl-2-naphthol via the reaction of benzaldehyde, 2-naphthol and benzamide. The best yield (92.86%) was obtained in the least time (10 min) with a molar ratio 1.2 : 1.2 : 1.7 of benzaldehyde : β-naphthol : benzamide and 0.1 g bimetallic Cu-Ag MOF/MCM-41 composite under solvent-free conditions at 130 °C. Reuse of the catalysts showed that they could be used at least four times without any reduction in the catalytic activity.

Fig. 1. (a) Low and high-angle XRD of MCM-41 and (b) low-angle and (c) high-angle XRD for the prepared catalysts.

Fig. 2. FTIR spectra of the prepared catalysts.

Fig. 3. SEM images of ((a) Cu-MOF/MCM-41, (b) Ag-MOF/MCM-41, (c) Cu–Ag MOF/MCM-41 and (d) Cu–Ag MOF) and EDX analysis of ((e) Cu-MOF/MCM-41 and (f) Cu–Ag MOF).

Fig. 4. XPS spectra of Cu–Ag MOF/MCM-41 composite: (a) the survey scan, (b) C 1s, (c) O 1s, (d) Si 2p, (e) Ag 3d and (f) Cu 2p.

Fig. 5. (a) N₂ adsorption–desorption isotherms and (b) pore size distribution of the prepared composites.

Fig. 6. Potentiometric titration curves of n-butyl amine in acetonitrile for the prepared catalysts.

Fig. 7. FTIR spectra of adsorbed pyridine on (a) Ag-MOF/MCM-41, (b) Cu-MOF/MCM-41, (c) Cu–Ag MOF/MCM-41 and (d) Cu–Ag MOF.

Fig. 8. Effect of the MOF weight on the synthesis of 1-amidoalkyl-2-naphthol.

Fig. 9. Effect of the molar ratio of reactants on the synthesis of 1-amidoalkyl-2-naphthol.

Fig. 10. Effect of the MOF type on the synthesis of 1-amidoalkyl-2-naphthol.

Scheme 1. A possible mechanism for the synthesis of 1-amidoalkyl-2-naphthol.

Fig. 11. Reuse of the Cu–Ag MOF/MCM-41 composite.

Fig. 12. (a) XRD, (b) FTIR and (c) SEM analysis of reused Cu–Ag MOF/MCM-41 composite after catalytic reaction.