Abstract
Morphological modification of bulk layered double hydroxides (LDHs) into surface-engineered, porous materials enabled effective, base-catalyzed liquid phase Knoevenagel condensation in water. The increase in the number of the accessible basic sites on LDH engineering led to the enhancement of the reaction rate, whereas the significant changes in the relative strength of the basic sites were found to cause remarkable differences in the chemoselectivities. Thus, porous hydrocalumite (CaAl-LDH) catalysts allowed long-term and highly selective Knoevenagel condensation using reasonably mild reaction conditions. The advantages of heterogeneous nature and the recyclability of the hydrocalumites were also strengthened by advanced surface treatment. These key factors (basic sites, basic nature, and heterogeneity) can be fine-tuned by selecting the appropriate sacrificial template (cetyl trimethyl ammonium bromide, multiwalled carbon nanotubes, and Santa Barbara Amorphous-15), which regulates the morphology of the composites.
Highlights
Liquid phase Knoevenagel condensation reaction of aldehydes/ ketones with compounds of activated methylene groups is a key process to produce a variety of intermediates and fine chemicals such as substituted alkenes [1,2], therapeutic drugs [3,4], and functional polymers [5,6] for various industrial synthetic processes
A second layered double hydroxides (LDHs) phase with somewhat smaller interlayer distance appeared in the case of cetyl trimethyl ammonium bromide (CTAB)-containing composite
The used templates did not have any characteristic reflections in the 2q range studied, except for multiwalled carbon nanotubes (MWCNTs): this Bragg reflection was indexed to
Summary
Liquid phase Knoevenagel condensation reaction of aldehydes/ ketones with compounds of activated methylene groups is a key process to produce a variety of intermediates and fine chemicals such as substituted alkenes [1,2], therapeutic drugs [3,4], and functional polymers [5,6] for various industrial synthetic processes. Condensation catalysts are generally based on soluble Brønsted bases [7e9] and have been applied widely in industries. In spite of their well-known disadvantages such as unsatisfactory yields, circuitous (or often impossible) reusability, harsh reaction conditions, and high-cost implications, these catalysts are still in use in most Knoevenagel condensation reactions.
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