Microwave calcination of Cu/Mg/Al hydrotalcite catalyst precursor

A copper-substituted hydrotalcite (Cu1.4Mg4.4Al2.2(CO3)1.1(OH)16) has been 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 show that microwave calcination results in enhanced crystallinity of the resultant oxides and spinel phase formed at high temperature, compared to conventional calcination. In addition, an additional phase, Cu2MgO3, is detected following microwave calcination, at a bulk temperature very much lower than previously reported for copper-containing hydrotalcite. The concentrations and strengths of surface basic sites are significantly higher for materials calcined using microwaves than using conventional heating. Catalytic activities in the base-catalysed transesterification of glyceryl tributyrate with methanol are also higher. We suggest that microwave calcination under feedback-control, while allowing control of material bulk temperature during calcination and preventing major 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.