Continuous-flow hydrogenation of cinnamaldehyde over catalysts derived from modified CoAl4 layered double hydroxides incorporating Mn, Ni, Cu and Zn ions

Series of CoxM1-xAl4 layered triple hydroxides, LTHs were prepared as catalyst precursors for hydrogenation of biomass compound cinnamaldehyde under flow and batch reaction conditions. To this end, we have successfully extended the eco-friendly family of Al-rich layered double hydroxides (LDHs) by synthesizing MnAl4-LDHs, furthermore Co3Al-, Co2Al-LDHs were also exploited. Both the magnetic Co-rich and Al-rich reduced forms behaved as robust, efficient and reusable catalysts with significantly different selectivities correlated with their acid-base properties. Al-rich solids showed increased acidity and lower basicity resulting in high hydrocinnamaldehyde selectivity (~75%), while the Co-rich catalysts attested enhanced hydrocinnamyl alcohol production (~80% selectivity) due to their low acidity and high basicity (mapped by NH3/CO2 temperature programmed desorption tests). Incorporation of Mn, Ni, Cu and Zn ions generally decreased the total acidity of Al-rich catalysts, while their Lewis basicity largely increased: the highest was found for Mn, resulting in activity similar to that of Co-rich catalysts.