Ag nanoparticles in A4 zeolite as efficient catalysts for the 4-nitrophenol reduction

In the present work, the synthesis of silver species such as nanoparticles (NPs), cations and clusters finely dispersed in A4 zeolite (Ag 0.1 wt%) has been successfully obtained by microwave irradiation and ion-exchange method. The prepared samples were analysed by SEM, HR-TEM, XRD, XPS, UV–Vis spectroscopy, ICP-OES and evaluated in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Physicochemical characterization of samples revealed that ion-exchange method resulted in the main formation of Ag NPs (c.a. 5 nm) while the microwave irradiation promoted the formation of Ag NPs (c.a. 3 nm), Ag cations and clusters as well. The crystalline structure of microporous A4 zeolite was not altered. The sample prepared under microwave irradiation manifested a higher catalytic performance (by 7.5 times in TOF value) in comparison with the reference sample obtained via ion-exchange method. The catalytic activity of the sample prepared by microwave irradiation (TOF value of 396 min⁻¹/μmol-metal) was superior from reference systems reported in the literature. The later confirmed that used synthesis conditions influence the formation of highly active Ag species stabilized on the microporous structure of A4 zeolite. The high capacity for H₂ storage and efficient sorption for the nitroaromatics on A4 zeolite resulted in a promotional effect for the catalytic reduction of 4-NP. Undoubtedly, the obtained results revealed that the selection in the conditions applied for microwave-assisted synthesis and the components of the catalyst (Ag and A4 zeolite) offers an ultra-fast method for the synthesis of highly active catalysts.