Fine Crystalline Mg-Al Hydrotalcites as Catalysts for Baeyer-Villiger Oxidation of Cyclohexanone with H2O2

Robert Karcz,Joanna Kryściak-Czerwenka,Dorota Duraczyńska,Alicja Michalik,Ewa M Serwicka,Bogna D Napruszewska

doi:10.3390/catal11121493

Abstract

The catalytic activity of Mg-Al hydrotalcite (HT) materials in base-catalyzed reactions is known to be promoted by the low crystallinity of the HT solid. In the present work, two routes enabling the preparation of finely crystalline Mg-Al HT materials were explored: (1) the inverse microemulsion technique, and (2) co-precipitation in the presence of starch. Carbonate, chloride and bromide forms of HT were prepared, examined with X-ray diffraction, scanning electron microscopy/energy dispersive X-ray spectroscopy and infrared spectroscopy, and used as catalysts in the Baeyer–Villiger oxidation of cyclohexanone to ε-caprolactone with a H2O2/acetonitrile system. The bromide forms proved significantly less active than the chlorides and carbonates, as they promoted nonselective consumption of H2O2. The fine crystalline materials were more active than the more crystalline HT references obtained by conventional co-precipitation. Catalysts prepared by inverse microemulsion were less crystalline and more active than the starch-templated ones, but suffered stronger deactivation by the acidic reaction environment. Alkalization of the reaction medium with NaHCO3 stabilized the HT materials and increased the ε-caprolactone yield, which became comparable for both types of fine crystalline catalysts—thus pointing to the synthesis involving a simple and cheap starch templating approach as being a particularly attractive one.

Highlights

Hydrotalcite (HT) is a naturally occurring layered mineral, with the formulaMg6 Al2 (OH)16 CO3 · 4H2 O
The HT materials were synthesized with chloride, bromide or carbonate anions in the interlayer
The choice of bromide and chloride was the consequence of using cetyltrimethylammonium bromide (CTABr) results of thesurfactants elemental in analysis showedcarried that inout all synthesized materials, the Mg/Al ratio and

Summary

Introduction

Hydrotalcite (HT) is a naturally occurring layered mineral, with the formulaMg6 Al2 (OH) CO3 · 4H2 O. Hydrotalcite (HT) is a naturally occurring layered mineral, with the formula. It is structurally related to another layered mineral, brucite (Mg(OH)2 ), composed of stacked layers of edge-sharing octahedra, with Mg2+ in the center and OH- groups in the corner positions. In HT, one Mg2+ is substituted with Al3+ , which results in a net positive charge of the layer, so that electroneutrality is ensured by the presence of anions in the interlayer. In natural HT, this layer charge is compensated for by carbonate anions. HT and HT-like compounds, with the general formula (M2+ 1−x M3+ x (OH)2 )x+ (An− x/n )·mH2 O, where M2+ and M3+ are the layer forming di- and trivalent cations, and An− is the interlayer anion, can be synthesized in the laboratory [2]

Methods

Results

Conclusion