Thangaraj Baskaran

Progress on layered hydrotalcite (HT) materials as potential support and catalytic materials

Mixed metal oxides derived from layered hydrotalcite (HT) materials are extensively used as a potential catalyst and catalytic support in various organic transformations. Numerous such materials have been developed by varying framework divalent and trivalent metal ions, and interlayer anions. The intercalation of anions present in HT facilitates the design of high surface area materials possessing exposed active sites, which enhances the catalytic and adsorption properties. These HT materials have also been used as drug carriers, and in enzyme encapsulation owing to their flexible accommodation of organic anions in the interlayer of the HT structure. The present review is focused on a recent development in the preparation of various HT materials using different metals and intercalating ions and their importance in the various catalytic processes, such as base catalyzed alkylation, isomerization, condensation and esterification. Effectiveness of transition metals containing HT for redox catalytic processes, viz. oxidation of alcohol, alkyl aromatics, phenol and reduction of nitro compounds have been discussed. Further, the importance of the introduction of a hard anion like silicate into the interlayer space of the HT and its applications in various organic reactions has also been covered in this review.

Silicate anion intercalated cobalt-aluminium hydrotalcite (CoAl-HT-Si): a potential catalyst for alcohol oxidation

Silicate intercalated CoAl hydrotalcite materials were synthesized (CoAl-HT-Si) and systematically characterized. The intercalated hydrotalcite materials lead to the formation of solid solutions of cobalt silicate and cobalt spinel structures, which have higher surface area and pore volume than pure CoAl-HT. The XPS studies revealed the presence of cobalt in divalent and trivalent oxidation states on the surface. The resultant materials were found to be promising catalysts for oxidation of various alcohols.

Silicate anion-stabilized layered magnesium–aluminium hydrotalcite

Layered magnesium–aluminum hydrotalcite (HT) were completely intercalated and stabilized with silicate anions. The silicate anion-stabilized HT retained the layered structure and possessed a high surface area of 530–540 m2 g−1 with needle-like particles 60 nm × 4 nm in dimension. The findings of FT-IR, powder XRD, TGA, DSC and 29Si MAS-NMR revealed that silicate anions were intercalated and coated on the surface of layered HTs with the formation of a solid solution of magnesium silicate and HT. The resultant silicate-anion-intercalated materials are found to be promising catalysts for the synthesis of 1-phenoxy-2-propanol using phenol with propylene oxide.

An environmentally friendly route for grafting of molybdenum carbonyl onto a diaminosilane-modified SBA-15 molecular sieve and its catalytic behaviour in olefin epoxidation

A simple route has been established for grafting molybdenum carbonyl onto the surface of a diaminosilane-modified SBA-15 molecular sieve. The successful grafting of the molybdenum carbonyl species onto diamine-functionalized SBA-15 was evident from FT-IR studies. The resultant molybdenum-carbonyl-complex-grafted SBA-15 (SBA-DA-Mo) materials show promising activity for the epoxidation of various olefins with good conversion (90–95%) and the formation of epoxide as the major product. The catalytic activity remains constant over several runs.

Synthesis and Heterogenization of Siloxane Functionalized Cobalt Complex: Potential Catalyst for Oxidation of Alcohols

Cobalticinium complex containing siloxane moiety on ligand center was first time synthesized, characterized and heterogenized on the surface of SBA-15 molecular sieves. FT-IR and 29Si-MAS-NMR spectrum evidence the successful grafting of the cobalt complex on the surface of SBA-15. N2 sorption isotherm of cobalticinium complex grafted sample showed the decrease in surface area, pore volume, further confirms the presence of complex inside the channel of mesoporous SBA-15. The heterogenized complex exhibited promising activity for oxidation of alcohols with good selectivity of ketone. Further the catalytic activity remains intact after two recycles.Graphical AbstractCobalticinium complex possessing siloxane functionality was prepared first time and heterogenized on SBA-15 surface. The resultant materials showed as promising recyclable heterogeneous catalyst for oxidation of alcohols.