Exploring an alternative route for meixnerite synthesis. The impact of the gaseous environment on the reconstruction of the lamellar structure and the catalytic performances

Abstract

This work investigates the possibility to obtain the meixnerite structure Mg6Al2(OH)18·4H2O in carbonate-free environment by direct co-precipitation of Mg and Al nitrates using only NaOH solution as precipitation agent as well as the physico-chemical and catalytic performances of the mixed oxide (MO) obtained by thermal decomposition of this layered material at 460°C under nitrogen flow. The influence played by different gaseous environments, air or nitrogen, during the hydration of the above mentioned mixed oxide on the structural characteristics and catalytic performances of the derived reconstructed layered double hydroxide (LDH) materials (RLDH-A and RLDH-N, respectively) is also scrutinized.The characterization of the solids was performed by chemical and elemental analyses, XRD, DRIFTS, N2 adsorption–desorption isotherms and adsorption of molecules with different pKa in order to determine their basicity. The catalytic activity for cyanoethylation was discussed in relation with the structural properties of the catalytic materials.The results showed that the co-precipitation method employed leads to the formation of an LDH containing also NO3− and CO32− anions besides HO−. Also, low amounts of C and N (<1%) were still present in the derived mixed oxide as well as in the reconstructed layered materials. The use of different gaseous environments, air or nitrogen, during the reconstruction leads to different structural properties which affected the catalytic activity of the solids. It has been noticed that the presence of carbonate anion in the structure of parent LDH-materials as well as that of carbonate low amounts in the LDH-derived mixed oxide improved the ability of the latter to recreate the lamellar structure through memory effect. However this fact is not accompanied by an increase of catalytic activity since the presence of carbonate anions decreases the basicity of the solid. Among the layered materials, the sample reconstructed under nitrogen (RLDH-N) which has a low crystallinity, and hence a larger number of defects, has also the highest number of HO− base sites and is the most active for catalytic cyanoethylation.