Abstract
Layered double hydroxides (LDH) or their derived mixed oxides present marked acid-base properties useful in catalysis, but they are generally agglomerated, inducing weak accessibility to the active sites. In the search for improving dispersion and accessibility of the active sites and for controlling the hydrophilic/hydrophobic balance in the catalysts, nanocomposite materials appear among the most attractive. In this study, a series of nanocomposites composed of LDH and reduced graphene oxide (rGO), were successfully obtained by direct coprecipitation and investigated as base catalysts for the Claisen–Schmidt condensation reaction between acetophenone and benzaldehyde. After activation, the LDH-rGO nanocomposites exhibited improved catalytic properties compared to bare LDH. Moreover, they reveal great versatility to tune the selectivity through their composition and the nature or the absence of solvent. This is due to the enhanced basicity of the nanocomposites as the LDH content increases which is assigned to the higher dispersion of the nanoplatelets in comparison to bulk LDH. Lewis-type basic sites of higher strength and accessibility are thus created. The nature of the solvent mainly acts through its acidity able to poison the basic sites of the nanocatalysts.
Highlights
The design of hybrid materials with multifunctional properties through the combination of various building blocks is a growing topic in material science
Considerable attention is given to the development of nanohybrid materials composed of layered double hydroxides (LDH) and carbon based materials [1] such as carbon nanotubes (CNT), carbon nanofibers (CNF), multiwalled carbon nanotubes (MWCNT), graphene (G) and chemically modified graphene such as graphene oxide (GO) [2]
It has been demonstrated that LDH/reduced graphene oxide (rGO) nanomaterials present Lewis-type basic properties which are highly tunable
Summary
The design of hybrid materials with multifunctional properties through the combination of various building blocks is a growing topic in material science. The catalysts obtained by self-assembly of LDHs and (nano)carbon supports exhibit higher mechanical strength, surface area, thermal conductivity and chemical reactivity than the corresponding constituents. LDH/carbon fibers (CF) nanocomposites obtained by growing MgAl LDHs in situ on CF were efficient catalysts for the transesterification reaction of glycerol with diethyl carbonate into glycerol carbonate [3]. Winter et al [5] and, more recently, Álvarez et al [6] investigated the catalytic activity of MgAl-LDH/CNF nanocomposites. Their specific activities were more than four and hundred times higher in the acetone self-condensation and the glycerol transesterification with diethyl carbonate, respectively, than the unsupported
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