Preparation of CuZn-Doped MgAl-Layered Double Hydroxide Catalysts through the Memory Effect of Hydrotalcite for Effective Hydrogenation of CO₂ to Methanol

In this work, a synthesis procedure was developed to introduce transition metal ions into a MgAl-layered double hydroxide (MgAl-LDH, or hydrotalcite) by utilizing the unique “memory effect” property of calcined LDH. This technique allows our MgAl-LDH to retain its pristine flowerlike morphology, which prevents stacking of nanosheets and is fully accessible for reactants. Briefly, calcined MgAl-LDH (or MgAl-layered double oxide, MgAl-LDO) was submerged in dilute monometal or mixed transition metal nitrate solutions, allowing ion exchange of M 2+ cations and the rehydration of MgAl-LDO back into LDH through the memory effect. The product was centrifuged and dried, allowing pre-selected transition metals (e.g., Cu, Zn, Ni, and Co) to occupy some of the octahedral sites of the LDH phase in which Mg 2+ ions were previously located. A CuZn-doped MgAl-LDH was then carefully reduced under a hydrogen atmosphere, precipitating tiny Cu nanoparticles out of the reformed LDH nanosheets, and was subsequently used in a series of CO 2 hydrogenation experiments. The bifunctional material, which comprises a CO 2 -sorbent LDH support with active metal cocatalysts, has achieved high specific activity per copper loading and high methanol selectivity, while maintaining a stable performance with over 70% methanol selectivity over 40 h at 280 °C. The excellent performance of this catalyst is attributed to the easily accessible active sites (Cu 0 nanoparticles) located on the surface of nanostructured petals for the gaseous reactants. The performance was found to be remarkably better than that of a well-established commercial Cu-based catalyst for methanol synthesis. Future potential development could entail the functionalization of other transition metals for the utilization of CO 2 .