Microwave-assisted synthesis of ternary Li-M-Al LDHs (M = Mg, Co, Ni, Cu, Zn, and Cd) and examining their use in phenol oxidation

Abstract

It remained challenging to synthesize Li and Al-containing ternary Li-M-Al layered double hydroxides (LDH) because LiI and other metal ions (MII or MIV) prefer different atomic positions, consequently making traditional methods such as co-precipitation or hydrothermal method fail. In the present work, several transition (Co, Ni, Cu) and non-transition (Mg, Zn, Cd) metal ions incorporated ternary Li-M-Al LDHs were synthesized by reacting LiAlH4 and different divalent metal nitrates under microwave-assisted hydrolysis reaction. The layer arrangement formation was verified from powder X-ray diffraction and SEM images, while the presence of three cations in all these samples was confirmed from ICP-AES measurements. FTIR and Raman spectra of these samples confirmed the nitrate anion's (majorly) presence in the interlayer space and the fingerprint bands due to the hydroxide units. The thermal traces contained a two-step dehydroxylation, suggestive of the existence of three cations in the hydroxide layers. Isothermal magnetization of ternary LDH containing Co, Ni, and Cu showed paramagnetic behavior at room temperature. On calcination, α-LiAlO2 and other metal oxides were formed. All the ternary Li-M-Al LDHs (M = Mg, Co, Ni, Cu, Zn, and Cd) were examined as catalysts for the oxidative degradation of phenol, a well-known toxic pollutant. Only Li-Cu-Al LDH displayed high catalytic activity for phenol oxidation. p-benzoquinone formed from phenol initially (60 min) and degraded completely into oxalic acid, maleic acid, and fumaric acid after 120 min, as determined by UV–visible and GC-HRMS measurements.