Continuous synthesis of doped layered double hydroxides in a meso-scale flow reactor

Abstract

Layered double hydroxides are a class of low-cost structured nanomaterials with many potential applications in environmental catalysis and sustainable technologies. Their large-scale use is hindered by the challenge of reproducible synthesis at scale. Here we report a general, readily scalable process for the reproducible synthesis of transition metal doped hydrotalcites using a two-step process: co-precipitation in a mm-scale (meso-scale) continuous flow reactor, followed by aging. We have shown that co-precipitation in flow at a residence time close to the micromixing time affords good control of particle formation. Reproducible synthesis allowed us, for the first time, to investigate the formation of the pore morphology of hydrotalcites and their thermal stability as a function of metal doping. The obtained samples exhibited surface areas (80–150 m2 g−1) higher than those typically attained in batch syntheses, with very low standard deviation between the samples, a high degree of crystallinity and small crystallite sizes, in the range of 9.5–11.9 nm, depending on composition. A systematic characterization allowed us to elucidate the mechanism of the pore morphology formation: the crystallites were found to agglomerate into disk-like platelets, whereas the pore structure of the hydrotalcites is formed by agglomeration of the platelets.