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 applica-tions in environmental catalysis and sustainable technologies. Their large-scale use is hindered by the chal-lenge of reproducible synthesis at scale. Here we report a general, readily scalable process for the repro-ducible 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. Re-producible 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 sur-face 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.