This review surveys recent advances in the applications of layered double hydroxides (LDHs) in heterogeneous catalysis. By virtue of the flexible tunability and uniform distribution of metal cations in the brucite-like layers and the facile exchangeability of intercalated anions, LDHs-both as directly prepared or after thermal treatment and/or reduction-have found many applications as stable and recyclable heterogeneous catalysts or catalyst supports for a variety of reactions with high industrial and academic importance. A major challenge in this rapidly growing field is to simultaneously improve the activity, selectivity and stability of these LDH-based materials by developing ways of tailoring the electronic structure of the catalysts and supports. Therefore, this Review article is mainly focused on the most recent developments in smart design strategies for LDH materials and the potential catalytic applications of the resulting materials.

Catalytic applications of layered double hydroxides: recent advances and perspectives

Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO2 are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO2 supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites.

https://science.sciencemag.org/content/sci/331/6014/195.full.pdf

Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles

Layered double hydroxides (LDHs) with their highly flexible and tunable chemical composition and physical properties have attracted tremendous attention in recent years. LDHs have found widespread application as catalysts, anion exchange materials, fire retardants, and nano-fillers in polymer nanocomposites. The ability to exfoliate LDHs into ultrathin nanosheets enables a range of new opportunities for multifunctional materials. In this review we summarize the current available LDH exfoliation methods. In particular, we highlight recent developments for the direct synthesis of single-layer LDH nanosheets, as well as the emerging applications of LDH nanosheets in catalyzing oxygen evolution reactions and preparing light emitting devices, supercapacitors, and flame retardant nanocomposites.

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Preparation of two dimensional layered double hydroxide nanosheets and their applications

Hydrotalcite-like anionic clays (HTs; or layered double hydroxides, LDHs) can be used as precursors to mixed oxides, but their catalytic potential reaches much further. This review demonstrates that HTs can be successfully applied in a broad spectrum of organic reactions, with advantages such as improved activity, selectivity, metal dispersion, less waste production, and an improved recuperation of immobilized catalysts. HTs can be used in the as-synthesized form or after different pretreatments. The HT as such is a solid base or, depending on the elemental composition of its octahedral layers, may have redox properties. Specific metals can be incorporated either as a cation in the octahedral layer or as an anion via exchange. The anion-exchange strategy is especially successful in the context of heterogenizing homogeneous catalysts. A particular advantage for base catalysis is that the number and strength of the basic sites can be tuned precisely to a specific reaction. HTs are excellent materials to des...

Hydrotalcite-like anionic clays in catalytic organic reactions

Hydrotalcites offer unique basic properties that make them very attractive for catalytic applications. It is of primary interest to make use of accurate tools for probing the basicity of hydrotalcite-based catalysts for the purpose of 1) fundamental understanding of base-catalysed processes with hydrotalcites and 2) optimisation of the catalytic performance achieved in reactions of industrial interest. Techniques based on probe molecules, titration techniques and test reactions along with physicochemical characterisation are overviewed in the first part of this review. The aim is to provide the tools for understanding how series of parameters involved in the preparation of hydrotalcite-based catalytic materials can be employed to control and adapt the basic properties of the catalyst towards the basicity demanded by each target chemical reaction. An overview of recent and significant achievements in that perspective is presented in the second part of the paper.

Exploring, tuning, and exploiting the basicity of hydrotalcites for applications in heterogeneous catalysis.

Selective 1,4-additions (Michael reactions) on methyl vinyl ketone, methyl acrylate, simple and substituted chalcones by donors such as nitroalkane, malononitrile, diethylmalonate, cyanoacetamide and thiols were catalysed by solid base, modified Mg-Al hydrotalcite as catalyst in quantitative yields in liquid phase under mild reaction conditions. Products of undesirable side reactions resulting from 1,2-addition, polymerisation and bis-addition are not observed. The work-up procedure is simplified by simple filtration with the use of solid bases.

The first example of Michael addition catalysed by modified Mg–Al hydrotalcite

The present invention relates to a process for the preparation of esters from alcohols using acetic acid as acetylating agent and clays as catalysts, which comprises the preparation of esters in a single step from the reaction of aliphatic, acyclic, cyclic, heterocyclic, α,β-unsaturated and aromatic alcohols with carbon atoms in the range of C 1 to C 10 with acetic acid in a molar ratio of 1:3 to 11 using reusable natural montmorillonite/metal ion-exchanged clay catalysts in the solvent medium of aliphatic, aromatic, or chlorinated hydrocarbons at 30-140° C. for a period in the range of 0.02 to 3.0 hrs, and recovering the corresponding esters by simple work-up procedure.

Process for the production of esters from alcohols using acetic acid as acetylating and clays as catalysts

An improved process for the oxidation of toluene to obtain benzaldehyde and benzyl alcohol with high selectivities using a Co/Mn/Br− composite catalytic system in liquid phase is described. A protocol for recovery and reuse of the composite catalyst is developed. The use of low concentrations of composite catalytic systems aimed at minimizing corrosion of the reaction, and higher concentrations of toluene affording higher productivity and recyclability of the catalyst giving high turnover number, are the remarkable achievements of the present methodology. Investigation into the recycle, aging and spectroscopic studies of the catalytic system improves the understanding of the process, chemistry and mechanism of the reaction. As the market demand for each product fluctuates, the dynamic system developed here to meet changing demands is very important to obtain one of the products in excess quantities with a change of the ratio of Br−/Cl−.

An Improved Process for Selective Liquid-Phase Air Oxidation of Toluene

The production of hydrogen by the relay of electrons from I − to H + in an acidic, aqueous medium and the consumption of hydrogen by reductive N acylation open up enormous opportunities in hydrogen chemistry.

Kottapalli Koteswara Rao

Papers

The first example of Michael addition catalysed by modified Mg–Al hydrotalcite

Process for the production of esters from alcohols using acetic acid as acetylating and clays as catalysts

An Improved Process for Selective Liquid-Phase Air Oxidation of Toluene

Hydrogen Processing by FeIII‐Exchanged Montmorillonite: A Unique Geochemical Protocol

High substrate selectivity in montmorillonite in the hydrogenation of olefins