Ag-PMA supported on MCM-41: Surface Acidity and Catalytic Activity

Herein Ag exchanged phosphomolybdic catalysts were synthesized by ion-exchange method and characterized using numerous techniques such as XRD, TEM, FTIR, and N₂ adsorption-desorption measurements. The surface acidity of the as-synthesized materials was examined through FTIR spectra of chemisorbed pyridine and non-aqueous potentiometric titration of n-butylamine. XRD analysis displayed the weakness of the amorphous peak related to MCM-41 and the appearance of well-defined diffraction lines of the heteropoly acid by the addition of Ag_xPMA to MCM-41, indicating that the Ag salts of PMA@MCM-41 have good crystallinity compared with MCM-41. TEM images displayed the homogeneous distribution of Ag_xPMA inside and outside the pores of MCM-41 and maintained the hexagonal morphology of MCM-41. All the catalysts exhibited both Lewis and Brønsted acid sites; the Lewis acidity increased with increasing silver contents while the Brønsted acidity decreased as confirmed by FTIR spectra of chemisorbed pyridine. The catalytic activity of the prepared samples was studied through the preparation of 7-hydroxy-4-methyl coumarin and hydroquinone diacetate. The catalytic activity and the surface acidity decreased by loading an extra amount of silver ions due to the decrease in the protons of PMA. Reusability of the prepared catalysts showed that the catalyst could be reused numerous times without losing its catalytic performance. [Figure not available: see fulltext.]