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 M2+ 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 Mg2+ 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 CO2 hydrogenation experiments. The bifunctional material, which comprises a CO2-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 (Cu0 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 CO2.