Layered Double Hydroxide (LDH)-Derived Mixed Metal Oxides (MMOs): A Systematic Crystal-Chemical Approach to Investigating the Chemical Composition and its Effect on High Temperature CO₂ capture.

A systematic study was conducted to establish a chemically meaningful relationship between the chemical composition of precursor, layered double hydroxide (LDH) and the CO₂ uptake capacities of the derived mixed-metal oxides (MMOs) sorbent. A simple crystal-chemical approach based on Vegard's law is proposed as a better characterization technique for LDHs, using lattice parameter a to find the true composition, x. Close values of a for all the resultant LDHs (3.051 - 3.056 Å) suggests a thermodynamic preference to produce LDHs having a composition that is close to that of mineral Hydrotalcite (3.054 Å). Solid state ²⁷Al spectra reveal the presence of aluminium based amorphous impurity phases in bulk LDHs. Due to the employed synthesis method and complex solution chemistry of the unitary hydroxides involved, it proved difficult to get the desired composition and to control the presence of impurities. The presence of remnant sodium has a significant effect on the CO₂ uptake capacities of the resultant MMOs. Overall, the choice of synthesis method, purity of the samples, presence of impurities and chemical efficiency of the whole synthesis process plays a major role on developing LDH based MMOs sorbents for CO₂ capture.