Catalytic behavior of Li-Al-LDH prepared via mechanochemical and co-precipitation routes for cyanoethylation reaction

Abstract

It was attempted to get Li-LDH-like compounds containing Li+ and Al3+ in the cationic layer by mechanochemical and co-precipitation methods, starting from nitrates as precursor salts, using for the pH-adjustment either NaOH as inorganic alkali or tetra methyl ammonium hydroxide (TMAH) as an organic one. Using Li+, Al3+ and organic alkali (TMAH), a pure LDH phase is obtained only by co-precipitation method whereas by the mechanochemical method a mixture of pure LDH phase and gibbsite is obtained. This is a step forward in the synthesis of Li-Al-LDH, since the pure phase was usually prepared only from LiOH precursor. The physico-chemical properties of the synthesized solids have been determined using XRD, DRIFT, BET, TG-DTA, and the irreversible adsorption of organic molecules with different pKa values. The catalytic tests performed for the 1,4-addition of ethanol yielding 3-ethoxypropionitrile showed that the gibbsite-unpurified samples led to decreased values of conversion, while higher conversion rates were obtained for the pure LDH phase which had an increased basicity. However, for all investigated solids the selectivity to 3-ethoxypropionitrile product is over 99.9 %. A straight correlation between the number of basic sites and their catalytic activity was evidenced.