Synthesis of modified hydrocalumite : a novel katoite/portlandite precursor method

Abstract

A novel method of synthesising calcium/aluminium layered double hydroxides (LDHs) was developed, which makes use of a katoite/portlandite mixture as a precursor. The precursor mixture is synthesised by reacting CaO and Al(OH)3 in water. The synthesis has no expensive heating steps and does not produce any effluent, making the synthesis a ―green‖ method. (Van der Westhuizen, 2011) The purpose of this investigation was to launch an exploratory study on the effectiveness and limitations of the novel synthesis method when synthesising intercalated calcium/aluminium LDHs. The novel synthesis method was compared to two traditional synthesis methods: co-precipitation and reconstruction. The novel synthesis method is equally as effective as the reconstruction method for the synthesis of organically modified Ca/Al LDHs. In most intercalations, the precursor method yielded more crystalline products than the reconstruction method. The co-precipitation method with nitrate salts was the least effective, since nitrate was more stable in the interlayer than the majority of the organic anions. For this reason, a Ca/Al-nitrate phase was the dominant crystalline species in these samples. Since the novel synthesis method produces organically modified LDHs of equal or higher quality than the traditional synthesis methods, it is a feasible method for producing LDHs. Since the method is also cheaper and more environmentally friendly than traditional synthesis methods, it is the preferential method for producing organically modified Ca/Al LDHs on a large scale. When the precursor mixture was hydrated in a reaction vessel that was left open to air from 2 h to 24 h, at temperatures ranging from room temperature to 80 °C, the maximum conversion to hydrocalumite was 15 wt.%. This means that if the reactor is left open to air, there will only be 15 wt.% carbonate contamination from air in the form of hydrocalumite. If a sealed vessel is used, the conversion will be drastically less, since there is a negligible amount of carbon dioxide in the small volume of air trapped in the vessel. For this reason, it is not necessary to synthesise intercalated hydrocalumites in an inert atmosphere, unless 100 % purity is needed. Chloride-intercalated Ca/Al LDHs form readily at temperatures from room temperature to 60 °C when aged for 2 h to 24 h. A maximum conversion of 64 wt.% occurs when the sample is aged for 12 h at 40 °C. In general, if the sample is aged for extended periods or at elevated temperatures, conversion decreases. A decrease in conversion is always accompanied by an increase in amorphous material, indicating that a more stable, amorphous phase forms if the LDH is aged for too long or at temperatures that are too high. Since no general trends could be found when intercalating chloride, further tests should be performed to determine what other factors influence the ability of the precursor to form an intercalated hydrocalumite. In general, the conversion of the precursor to a benzoate-intercalated LDH increases with increasing reaction time and decreases with increasing temperature. Benzoate-intercalated LDH did not form at a temperature of 60 °C at any of the aging times tested, indicating that Ca/Al-benzoate is not stable at elevated temperatures. When the samples were aged for 24 h, there are two new primary peaks present on the XRD patterns. For this reason, there is either a new configuration of benzoate in the interlayer or a new crystalline symmetry present in the samples. As the temperature increased, a smaller d-spacing spacing was favoured. Instead of using a katoite/portlandite mixture, a calcium source in the form of calcium oxide [CaO] or calcium hydroxide [Ca(OH)2] can be added to pure katoite prior to intercalating with an anion. Using an extra calcium source in combination with katoite, instead of the precursor mixture, renders similar results. The CaO converts rapidly to Ca(OH)2; therefore, both calcium sources are equally effective. Contrary to popular belief, an intercalated LDH material does not necessarily have to be synthesised at a high pH. All the weak acids with low solubility tested were able to intercalate at a pH as low as 5. This is not the case for soluble acids. Both strong and weak soluble acids tend to break down the layered material as soon as the pH drops below 10, as reported in literature. However, even for organic acids, it is recommended to keep the pH above 10, in order to maximise retention of layered material. The only benefit to intercalating organic acids at a lower pH is that the precursor and carbonate-intercalated LDH are broken down to a larger extent, making the final product purer. When a katoite/portlandite precursor material is titrated with different acids, the top-down titration curves display two distinct plateaus for most acids. However, titrations with anthranilic acid shows only one distinct plateau, and titration with OH-substituted benzoic acid rings tend to exhibit an extra plateau. This is because the OH group dissociates at a high pH. There are several factors affecting intercalation that have not been studied, which may have a large impact on the degree of intercalation. It is, therefore, recommended that the following factors also be investigated:  Mixing efficiency  Boiled vs. non-boiled water  Size of the anion In order to determine the success of the intercalation experiments, the most important factor to test would be the properties of the intercalated LDH materials in their final applications.