Influence of hydrothermal synthesis conditions on the morphology and sorption properties of porous aluminosilicates with kaolinite and halloysite structures

The influence of hydrothermal synthesis conditions on the crystallization of aluminosilicates of the kaolin group – kaolinite and halloysite with various morphologies – was studied. Unique studies using focused ion beam-scanning microscopy (FIB-SEM) to determine the morphology and microstructure of the samples were carried out. The samples were studied by X-ray diffraction, low-temperature nitrogen adsorption, thermal analysis coupled with mass spectrometry, and IR absorption spectroscopy. The sorption ability of the samples with respect to the cationic dye methylene blue was also studied. The synthesis conditions were determined, which made it possible to obtain aluminosilicates with a certain morphology that included the thin nanolayers of particles, spherical particles, nanotubes, nanosponges or plates, as well as the conditions for their transformation. It was shown that in the range of 200 to 220 °C, primarily aluminosilicates with halloysite structure (Al2Si2O5(OH)4.nH2O) crystallized. Depending on the synthesis conditions, halloysite particles had spherical, tubular, or nanospongy morphologies. An increase in temperature to 350 °C favoured the crystallization of kaolinite Al2Si2O5(OH)4 with a predominant platy morphology of particles. For the first time, kaolinite group aluminosilicates with a nanosponged morphology, were obtained and were characterized by a high specific surface area (300–350 m2/g) and a high sorption capacity with respect to methylene blue (more than 100 mg/g after two hours of treatment). It is shown that the porous-textural characteristics and sorption properties of aluminosilicates of the kaolinite group are largely determined by the synthesis conditions and the morphology of the particles.