In this chapter MgAl-LDHs will be synthesised using two methods: the co-precipitation method using magnesium nitrate and aluminium nitrate in the presence of sodium bicarbonate at constant pH 10 and the co-hydration method as demonstrated by Greenwell et al.123 These materials will be characterised using various experimental techniques, such as PXRD and ICP, and then compared to each other prior to their calcination to yield MMOs which will similarly be analysed, compared and contrasted.
3.2.1 Nomenclature of samples prepared
The LDH and MMO materials prepared in this thesis are described by:
a) Initially their method of preparation – CoP for co-precipitation and CoH for co-hydration;
b) Whether they are in the LDH form or have been calcined to give the MMO form;
c) Mg:Al stoichiometry during preparation, also known as the R-value.
Thus, by way of example, a mixed metal oxide produced via co-hydration with an R-value of 2 would be denoted as CoH-MMO-2. A list of samples prepared, their preparation method and target compounds are shown in Table 3.1.
Table 3.1 Sample ID, synthesis method and target compounds of the materials synthesised in this thesis, where
3.2.2 Synthesis of layered double hydroxides via co-precipitation
The carbonate LDH samples prepared were composed of the theoretical formula [Mg2+ 1-xAl3+x(OH)2]x+(CO32-)x/2.yH2O. Co-precipitated LDHs were prepared using stoichiometric ratios of Mg/Al (denoted R-value) to give R-values of 1-6. Reagents were used as supplied: magnesium nitrate hexahydrate (Sigma-Aldrich, ACS 99 %), aluminium nitrate nonahydrate (Sigma-Aldrich,
ACS 98 %), deionised water (Purelab 7000, 18.2 MΩ), sodium bicarbonate (Sigma-Aldrich, ACS 99.7 %), NaOH(aq) (Fisher Scientific, AR).
An example preparation for R-value 2 is given below, for exact masses of each chemical used in each R-value preparation see Table 3.2. For synthesis of the target compound ([Mg0.66Al0.33(OH)2](CO3)0.165.yH2O (R-value 2), magnesium nitrate hexahydrate, Mg(NO3)2.6H2O a colourless crystalline solid (2.3120g, 9.0 mmol) and aluminium nitrate nonahydrate, Al (NO3)3.9H2O a colourless crystalline solid, (1.6880g, 4.5 mmol) were readily dissolved in 100 ml deionised water. This metal nitrate solution was then added drop wise over a period of 1 hour to a stirred 100 ml excess (CO32- : Al3+ = 10:1) sodium bicarbonate (3.770g, 44.9 mmol) solution at 65
°C. Instantaneous precipitation of a white solid was observed. Simultaneous addition of 1.5 mol dm-3 NaOH(aq) was used to maintain pH 10 ± 0.1, measured using a Jenway 3510 pH meter. The resulting reaction mixture, a white suspension, was then aged in situ at 65 °C for 5 hours, while stirring. After aging, the suspension was filtered under vacuum and the white crystalline solid product was washed on a Buchner vacuum funnel with 1 L of hot deionised water to remove any remaining Na+ ions. The white powder product sample was then dried overnight under air in an oven at 80 °C until there was no further mass loss.
Table 3.2 Masses, moles and ratios of the reactants used for each R-value LDH co-precipitation preparation.
Sample
Mass / concentration per 100 mL solution (g / m mol dm-3) Ratio Mg (NO3)2.6 H2O Al(NO3)3.9H2O NaHCO3 Mg:Al CO3
2-:Al
CoP-LDH1 1.6240, 6.3 2.3760, 6.3 5.320, 63.3 1 10
CoP-LDH2 2.3120, 9.0 1.6880, 4.5 3.770, 44.9 2 10
CoP-LDH3 2.6884, 10.5 1.3112, 3.5 2.930, 34.9 3 10
CoP-LDH4 2.9290, 11.4 1.0712,2.9 2.400, 28.6 4 10
CoP-LDH5 3.0944, 12.1 0.9056, 2.4 2.028, 24.1 5 10
CoP-LDH6 3.2152, 12.5 0.7844, 2.1 1.756, 20.9 6 10
The powder was finely ground using an agate pestle and mortar prior to analysis and use in catalysis (Chapter 5). Samples were analysed using powder X-ray diffraction, thermogravimetric
analysis, scanning electron microscopy, inductively coupled plasma optical emission spectroscopy, surface area analysis, Hammett basicity measurements, elemental analysis and Fourier transform infra-red spectroscopy (Section 2.2).
3.2.3 Synthesis of layered double hydroxides by co-hydration
Co-hydrated LDHs were prepared following a synthesis method developed by Greenwell et al.,123 composed of the theoretical formula [Mg2+1-xAl3+x(OH)2]x+(OA)x/2.yH2O, where OA represents the adipate dianion. Co-hydrated LDHs were prepared using stoichiometric ratios of Mg/Al to give R-values of 1-6, and adipic acid (Figure 3.2), which was previously used by Greenwell123 to generate the required counter anions. Reagents were used as supplied: CP5 aluminium oxide (BASF), adipic acid (Sigma, 99 %), magnesium oxide (Sigma-Aldrich, ACS 98 %) and deionised water (Purelab 7000, 18.2 MΩ). In this current procedure CP5 aluminium oxide (BASF, CP = catalyst precursor particle size 5 μm, 6.7 % mass loss on ignition) was used instead of the CP3 (Alcoa, catalyst precursor particle size 3 μm, 5.8 % mass loss on ignition) stated in the article due to CP3 grade no longer being available from the supplier. The CP5 aluminium oxide will therefore require a longer hydration time in comparison to that described by Greenwell,123 owing to the hydration time for larger alumina particles increasing with their lower surface area per unit weight.186
Figure 3.2 Structure of adipic acid, used in the preparation of LDHs via co-hydration in this thesis.
An example preparation for R-value 2 is given below, for exact masses of each chemical used in each R-value preparation see Table 3.3). For synthesis of the target compound [Mg0.66Al0.33(OH)2](OA)0.165.yH2O (R-value 2), aluminium oxide (1.0100g, 19.8 mmol), a white powder, was added to 250 mL of deionised water at 65 °C followed by adipic acid (1.737g, 16.9
mmol), a white crystalline solid, after 10 minutes and finally magnesium oxide (1.4900g, 37.0 mmol), a white powder, after a further 50 minutes to give an overall 1% slurry weight based on the metal oxides. The reaction mixture was then covered to prevent water loss and aged at 65 °C for 5 hours, while stirring, producing an LDH product which was filtered under vacuum. The white powder sample was then dried overnight under air in an oven at 80 °C until there was no further mass loss, followed by fine grinding using an agate pestle and mortar prior to analysis and use in catalysis (Chapter 5). Samples were analysed using powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, inductively coupled plasma optical emission spectroscopy, surface area analysis, Hammett basicity measurements, elemental analysis and Fourier transform infra-red spectroscopy (Section 2.2).
Table 3.3 Masses, moles and ratios of the reactants used for each R-value LDH co-hydration preparation.
Sample
Mass per 100 mL solution, concentration (g, m mol dm-3) Ratio
MgO Al2O3 Adipic acid Mg/Al COO-:Al
CoH-LDH1 1.0610, 26.3 1.4386, 28.2 2.474, 16.9 1 1.2
CoH-LDH2 1.4900, 37.0 1.0100, 19.8 1.737, 11.9 2 1.2
CoH-LDH3 1.7220, 42.7 0.7775, 15.3 1.337, 9.2 3 1.2
CoH-LDH4 1.8993, 47.1 0.6006, 11.8 1.033, 7.1 4 1.2
CoH-LDH5 1.9952, 49.5 0.5047, 9.9 0.868, 5.9 5 1.2
CoH-LDH6 2.0647, 51.2 0.4353, 8.5 0.749, 5.1 6 1.2