Layered Double Hydroxides: Structure, Synthesis and Catalytic Applications

The objective of the work studied here was to relate the structure of the conventional Layered Double Hydroxide (LDH), hydrotalcite and several transition metal doped hydrotalcites to their function once calcined into catalysts for use in the production of biodiesel.

Attention was paid to the preparation of the LDHs. Three preparative methods were investigated, using sodium hydroxide and carbonate, using ammonia, and using urea as precipitating agents. The properties of the resultant LDHs and those of the mixed oxides produced on calcination were shown to be relatively independent of the synthesis method. The importance of ensuring that sodium salts were removed from the catalyst precursors before use, when using the first synthetic method, was shown. Evidence was provided which showed the significant effect on activity of the calcined catalysts when sodium was present. The importance of thorough washing of the LDH precipitates was clearly demonstrated.

The calcination process was also studied and materials were subjected to two methods of calcination, “feedback-controlled” microwave heating and conventional heating in a furnace. This method of controlled microwave calcination may offer promise in the production of optimised mixed metal oxide and other catalysts.

A copper-substituted hydrotalcite was subjected to calcination under feedback-controlled microwave heating, in which microwave power is continuously modulated to generate a defined sample temperature programme or constant sample temperature. The results showed that microwave calcination resulted in enhanced crystallinity of the resultant oxides and spinel phase formed at high temperature. In addition, an additional phase, Cu2MgO3, was detected following microwave calcination, which was not formed at any temperature (up to 1000 oC) under conventional heating. The concentrations and strengths of surface basic sites were significantly higher for materials calcined using microwaves than using conventional heating. Catalytic activities in the base-catalysed transesterification of tributyrin with methanol were also higher. Microwave calcination under feedback-control, while allowing control of material bulk temperature during calcination and preventing major 6 temperature excursions, may allow quite large but highly localised temperature variation, for instance as water is released during dehydroxylation, which are beneficial in developing surface defects and surface basicity.

Other LDHs were studied incorporating the transition metals, cobalt, nickel and iron. All three showed some activity but basicity appeared to be enhanced by doping hydrotalcite with copper (II) and cobalt (II) particularly. Whether this is because of the presence of these ions on the catalyst surface and their behaviour as Lewis bases, or whether their presence leads to surface defect sites that show electron donating abilities, is not clear.

The effect of microwave calcination on these other substituted LDHs seems to be variable and not always as pronounced as it is with the copper containing LDH. It seems likely that the extent to which microwaves are effective depends on their capacity to couple with the metals in the structures and the fact that this varies between metals perhaps explains why the different LDHs show different behaviours.

Although not the most active material, perhaps the most interesting material formed upon calcination was the mixed oxides of Mg5.82Al1.12Fe0.88(CO3)(OH)16.4H2O, possessing both acidic and basic sites. All other LDHs studied possessed basic sites only. This could be very useful for the production of biodiesel from waste oil containing free fatty acids (FFAs) which require the presence of an acid catalyst for pre-esterification of these free acids.