Efficient synthesis of Co2+–Co3+ hydrotalcite-like compounds via NaOH precipitation in methanol–water under mild alkaline conditions

Abstract

Divalent/trivalent cobalt hydrotalcite-like compounds (Co2+–Co3+ HTlc) were synthesized via precipitation under mild alkaline conditions by using NaOH as precipitant and methanol–water as solvent. The influences of synthetic parameters including CH3OH:H2O volume ratio, precipitation pH, and hydrothermal treatment on the HTlc formation were investigated. The crystalline phase, morphology, and chemical composition of the synthesized samples were characterized by XRD, FTIR, SEM, TG–MS, and iodometric titration. The formation of Co2+–Co3+ HTlc was strongly affected by the methanol content and a single Co2+–Co3+ HTlc phase was obtained by raising the CH3OH:H2O ratio to 4:1–8:1. It was suggested that the presence of methanol increased the concentration of dissolved oxygen and promoted the oxidation of Co2+ into Co3+, leading to complete transformation of α-Co(OH)2 into Co2+–Co3+ HTlc. Moreover, single-phased Co2+–Co3+ HTlc was available at pH between 10 and 12, and they were hydrothermally stable by treatment at 60 °C for 24 h. Under the synthetic conditions, complete precipitation of cobalt ions was achieved. This highlights that the developed synthesis process is an efficiency way for quantitative synthesis of cobalt HTlc.