The Structure and Thermal Properties of Solid Ternary Compounds Forming with Ca2+, Al3+ and Heptagluconate Ions

In the present work, the structure and thermal stability of Ca–Al mixed-metal compounds, relevant in the Bayer process as intermediates, have been investigated. X-ray diffraction (XRD) measurements revealed the amorphous morphology of the compounds, which was corroborated by SEM-EDX measurements. The results of ICP-OES and UV-Vis experiments suggested the formation of three possible ternary calcium aluminum heptagluconate (Ca-Al-Hpgl) compounds, with the formulae of CaAlHpgl(OH)40, Ca2AlHpgl2(OH)50 and Ca3Al2Hpgl3(OH)90. Additional IR and Raman experiments revealed the centrally symmetric arrangement of heptagluconate around the metal ion. The increased thermal stability was demonstrated by thermal analysis of the solids and confirmed our findings.


S1 Crystallinity and morphology of the ternary samples
Figure S1 X-ray diffractograms of the ternary precipitates.

S2 The effect of metal coordination on the infrared and Raman spectra of sodium heptagluconate
The information provided by infrared and Raman spectra of the same sample complement each other, since the general selection rules differ from each other. The infrared intensities are proportional to the square of the transitional dipole moment, while the Raman intensities are determined by the change in the polarizability tensor during the transition.
The vibrational normal modes of the D-heptagluconate ion can be classified from that point of view. The normal modes dominated by the displacement of the hydrogen atoms in the O-H groups produce high transitional dipole moment, but hardly influence the polarizability tensor of the molecule. Their intensities are high in the absorption spectrum, and they are practically invisible in the Raman spectrum.
On the other hand, the normal modes, dominated by the combinations of the stretchings of C-C bonds alter the polarizability tensor, but produce negligible change in the dipole moment. So, their intensities are low in the infrared and relatively high in the Raman spectrum [1].
The normal modes, dominated by the carboxylate group, have both effects, since the delocalized 4π electron system strongly shifted to the direction of the oxygen atoms, but prone to further polarization, so can be seen in both spectra. According to the above considerations, the infrared spectra of the samples, provide information mainly on the coordination properties of the sugar, while the Raman spectra on the conformation of the backbone.

Figure S3
The observable difference between the O-H stretching range and in that of the COO-group, presented on the infrared and Raman spectrum of sample CaAl-Hpgl-3.

Figure S4
Fourier Self-Deconvolution performed on the IR and Raman spectra of sample CaAl-Hpgl-8 Peak fitting was performed with every spectrum. The best was achieved by the following strategy. The initial number of fitted peaks and their relative intensities were determined by the result of the Fourier Self-Deconvolution. Gaussian functions were fitted in the first round. The residual curve were examined and the number of peaks was reduced when less peaks seemed to be appropriate to describe the fine structure of the spectrum. When the residual curve contained obvious peak-like features and it was impossible to describe the fine structure of the spectrum further peaks were added accordingly. The fitted functions were changed to mixture of Gaussian and Lorentzian functions, and their ratios were also fitted in the final round. The process was finished when the residual curve showed noise-like character.   Figure S5. Thermograms of the commercial heptagluconic acid sodium-, and calcium salts from Sigma. Aldrich. Figure S6. Thermograms of the commercial heptagluconate sodium-, and calcium salts from Sigma. Aldrich. Figure S7. XRD diffractograms of the heat-treated CaAl-Hpgl-7 sample. The inorganic CaO and Al2O3 compounds were identified, consequently, the mineralization process was confirmed.