Thermal decomposition of the synthetic hydrotalcite woodallite

Abstract

The thermal stability and thermal decomposition pathways for synthetic woodallite have been determined using thermogravimetry in conjunction with evolved gas mass spectrometry. Chemical analysis showed the formula of the synthesised woodallite to be Mg6.28Cr1.72Cl(OH)16(CO3)0.36 •8.3H2O and X-ray diffraction confirms the layered LDH structure. Dehydration of the woodallite occurred at 65 degrees Celsius. Dehydroxylation occurred at 302 and 338 degrees Celsius. Both steps were associated with the loss of carbonate. Hydrogen chloride gas was evolved over a wide temperature range centred on 507 degrees Celsius. The products of the thermal decomposition were MgO and a spinel MgCr2O4. Experimentally it was found to be difficult to eliminate CO2 from inclusion in the interlayer during the synthesis of the woodallite compound and in this way the synthesised woodallite resembled the natural mineral.