Preparing Main Group Metal Clusters from Organoaluminium Reagents; New Possibilities in Alkali-Activated Polymer Crosslinking
The reactions of carboxylic acids with organoaluminium reagents were studied, which led to the formation of novel aluminium compounds. The reactions of orthofunctionalised derivatives of benzoic acid with trivalent aluminium organyls AlR3, led to the formation of different Al-based molecular clusters, depending on the nature of R, the reaction stoichiometry and the character of the benzoic acid derivative. The obtained compounds were characterised in the solid state by X-ray diffraction methods and two main motifs were observed. When the acid and AlR3 reacted in a one-to-two stoichiometry the obtained products, [iBu4Al2(μ-O2CC6H4-2-μ- O)]2, [(Me2Al)2(μ-O2CC6H4-2-μ-NH)]2, [(iBu2Al)2(μ-O2CC6H4-2-μ-NH)]2, [(Me2Al)2(μ- O2CC6H4-2-μ-NMe)]2 and [(iBu2Al)2(μ-O2CC6H4-2-μ-NMe)]2, consisted of a central distorted 12-membered macrocycle, formed by two [Al-O-C-O-Al-X] units (X= O,N) and was found to be dimeric. The reaction between anthranilic acid derivatives and AlR3 could also take place in a one-to-one ratio. For anthranilic acid and Nmethylanthranilic acid the obtained crystals only allowed a qualitative analysis and showed the structure of the products, [MeAl(μ-O2CC6H4-2-μ-NH)]4, [iBuAl(μ-O2CC6H4- 2-μ-NMe)]4 to be tetrameric and each consisting of a distorted 16-membered ring formed by four [O-C-O-Al] units. With the reaction of N-phenylanthranilic acid it was possible to isolate a structural analogous product [iBuAl(μ-O2CC6H4-2-μ-NPh)]4 which could be fully characterised by x-ray crystallography and NMR spectroscopy. Where the quantity and quality of the obtained product was sufficient, the solution behaviour of the compounds was elucidated by multinuclear and multidimensional NMR spectroscopic techniques. The 27Al NMR showed that the aforementioned aggregates are maintained in solution, which for the 12-membered [Al-O-C-O-Al-N] macrocycle of [(iBu2Al)2(μ-O2CC6H4-2-μ-NH)]2 was confirmed by a NOESY spectrum.
The second part of this project focused on the preliminary studies towards the application of aluminium compounds in the crosslinking of guar and carboxymethyl hydroxypropyl guar, which are common additives in hydraulic fracturing. Different commercially available aluminium compounds were tested for their general ability to crosslink the aforementioned polysaccharides, yielding promising results for aluminium lactate, aluminium acetylacetonate and aluminium isopropoxide. For the system comprising aluminium lactate in combination with CMHPG, rheological studies were carried out to determine the viscosity, the viscoelasticity, the shear recovery and the stability towards high temperatures. These sought to evaluate the crosslinking properties of the aluminium additive and to optimise the required conditions of the different system components. Finally, it was possible to obtain first proof-of-concept data suggesting that synthetically obtained aluminium compounds such as [Me2Al(μ- O2CPh)]2 and Al[MeC(CH2O)3]2(AlMe2)3 can be employed for the crosslinking of guar and CMHPG.