The role of PMA in enhancing the surface acidity and catalytic activity of a bimetallic Cr-Mg-MOF and its applications for synthesis of coumarin and dihydropyrimidinone derivatives

Abstract

In the present study, a bimetallic Cr-Mg-MOF was successfully synthesized by the solvothermal method and then modified by loading different amounts of phosphomolybdic acid (PMA) using a simple wet impregnation technique. The morphological and structural properties of the prepared samples were investigated using X-ray diffraction, TEM, SEM, BET and FTIR spectroscopy. Importantly, Mg doping not only caused the Cr-Mg-MOF to have a higher surface area than MIL-101 (Cr) or MOF-74 (Mg), but the strategy of doping metal ions can be an effective way to improve the adsorption performance of MOFs. The surface acidity and the acid strength of the samples were determined using potentiometric titration and the FTIR of pyridine adsorption. The incorporation of PMA crystals gradually enhances both the surface acidity and the acid strength of the PMA/Cr-Mg-MOF catalysts up to 75 wt%. The catalytic performances of the prepared catalysts were tested in two acid-catalyzed organic transformations, namely, 7-hydroxy-4-methyl coumarin and 3,4-dihydropyrimidinone. In the two reactions, the catalytic activity attains the maximum value at 75 wt% PMA loading. The PMA catalysts supported on Cr-Mg-MOF are potentially promising heterogeneous catalysts for acid-catalyzed organic transformations in environmentally friendly processes, to replace the use of conventional homogeneous PMA catalysts.

Fig. 1. XRD pattern of pure and modified Cr–Mg-MOF by different wt% PMA.

Fig. 2. FTIR spectra of (a) mixed Cr–Mg-MOF; (b) 10 wt% PMA/Cr–Mg-MOF; (c) 50 wt% PMA/Cr–Mg-MOF; (d) 75 wt% PMA/Cr–Mg-MOF; (e) 90 wt% PMA/Cr–Mg-MOF and (f) pure phosphomolybdic acid.

Fig. 3. TEM images of (A) mixed Cr–Mg-MOF; (B) 25 wt% PMA/Cr–Mg-MOF; (C) 50 wt% PMA/Cr–Mg-MOF and (D) 90 wt% PMA/Cr–Mg-MOF.

Fig. 4. SEM images of (A) mixed Cr–Mg-MOF; (B) 25 wt% PMA/Cr–Mg-MOF; (C) 50 wt% PMA/Cr–Mg-MOF and (D) 90 wt% PMA/Cr–Mg-MOF.

Fig. 5. Adsorption–desorption isotherms of nitrogen at −196 °C on (a) mixed Cr–Mg-MOF; (b) 10 wt% PMA/Cr–Mg-MOF; (c) 25 wt% PMA/Cr–Mg-MOF; (d) 50 wt% PMA/Cr–Mg-MOF; (e) 75 wt% PMA/Cr–Mg-MOF; (f) 90 wt% PMA/Cr–Mg-MOF.

Fig. 6. Pore volume distribution for (a) mixed Cr–Mg-MOF; (b) 10 wt% PMA/Cr–Mg-MOF; (c) 25 wt% PMA/Cr–Mg-MOF; (d) 50 wt% PMA/Cr–Mg-MOF; (e) 75 wt% PMA/Cr–Mg-MOF; (f) 90 wt% PMA/Cr–Mg-MOF.

Fig. 7. Potentiometric titration curves for (a) mixed Cr–Mg-MOF; (b) 10 wt% PMA/Cr–Mg-MOF; (c) 25 wt% PMA/Cr–Mg-MOF; (d) 50 wt% PMA/Cr–Mg-MOF; (e) 75 wt% PMA/Cr–Mg-MOF; (f) 90 wt% PMA/Cr–Mg-MOF.

Fig. 8. Total number of acid sites and initial potential vs. weight percent of PMA over Cr–Mg-MOF.

Fig. 9. FT-IR spectra of chemisorbed pyridine on (a) mixed Cr–Mg-MOF; (b) 25 wt% PMA/Cr–Mg-MOF; (c) 50 wt% PMA/Cr–Mg-MOF; (d) 75 wt% PMA/Cr–Mg-MOF and (e) 90 wt% PMA/Cr–Mg-MOF.

Fig. 10. Effect of molar ratio of (ethyl acetoacetate : resorcinol) on the catalytic activity over 75 wt% PMA/Cr–Mg-MOF.

Fig. 11. The relationship between PMA content and percentage yield of 7-hydroxy-4-methylcoumarin.

Scheme 1. Reaction mechanism for the synthesis of 7-hydroxy-4-methyl coumarin catalyzed by PMA/Cr–Mg-MOF.

Fig. 12. Reusability of 75 wt% PMA/Cr–Mg-MOF in the synthesis of 7-hydroxy-4-methylcoumarin.

Fig. 13. Effect of molar ratio of (benzaldehyde : urea : ethyl acetoacetate) on the catalytic activity over 75 wt% PMA/Cr–Mg-MOF.

Fig. 14. The relationship between PMA content and percentage yield of 3,4-dihydropyrimidinones.

Fig. 15. Reusability of 75 wt% PMA/Cr–Mg-MOF in the synthesis of 3,4-dihydropyrimidinones.

Fig. 16. Effect of reuse on the structure of 75 wt% PMA/Cr–Mg-MOF catalyst.