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

Highlights

• Aluminosilicates with kaolinite and halloysite structures with various morphologies have been synthesized.

• The unique studies using focused ion beam-scanning microscopy have been carried out.

• One-stage hydrothermal synthesis of the aluminosilicate nanosponges is described for the first time.

• Sorption properties of kaolinite and halloysite nanoparticles strongly depend on their morphology.

Abstract

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.

Introduction

The minerals of the kaolin group include kaolinite, halloysite, dickite and nacrite. These minerals are polymorphic modifications of aluminium silicate Al₂Si₂O₅(OH)₄, which correspond to SiO₂ contents of 46.54%, Al₂O₃ contents of 39.5%, and H₂O contents of 13.96% respectively (Malferrari et al., 2015). Minerals of this group are characterized by a structure consisting of two-layer packets containing one octahedral and one tetrahedral sheet, and have the chemical formula Al₂Si₂O₅(OH)₄^.nH₂O, where n = 0 (typical for all representatives of the group) and n = 2 (typical for the hydrated form of halloysite). Hydrogen bonds between the surface hydroxyl groups of the octahedral sheet and the basal oxygen atoms of the tetrahedral silicon‑oxygen sheet play an important role in the interaction of layers of a 1:1 type structure (Malferrari et al., 2015; Pasbakhsh and Churchman, 2015).

The minerals of the kaolin group are interesting in that they can form various morphologies. Halloysite in nature can form up to 10 different morphologies, including tubes, spheres, fibres, cylinders, etc. (Pasbakhsh and Churchman, 2015; Yuan et al., 2015; Joussein, 2016)). Platy morphology is the most characteristic of kaolinite, although in some cases the formation of spherical particles is also possible (Huertas et al., 1993, Huertas et al., 1997; Bauluz Lázaro, 2015). Presumably, the spheroidal morphology of both kaolinite and halloysite is related to the saturation state of their solutions (Tomura et al., 1985; Bauluz Lázaro, 2015). In addition, these minerals, which have the same chemical composition, can significantly differ in properties. It was shown in (Zhao et al., 2013) that nanotubular halloysite has a greater sorption ability with respect to both cationic (rhodamine) and anionic dyes (chrome azurol) than platy kaolin. It was suggested that this difference is due both to morphological features and the fact that halloysite nanotubes have different compositions on the outer and inner surfaces of the layer (i.e. negatively charged SiO₂ and positively charged Al(OH)₃), which allows it to efficiently sorb uncharged ions (Zhao et al., 2013).

The properties of other characteristic morphologies of minerals of the kaolin group (e.g. spherical) have hardly been studied, possibly because researchers are primarily focused on the nanotubular form of kaolinite, as potentially more promising.

Kaolin, with the most common platy morphology, is widely used as a raw material for the production of ceramics, refractory products, silumin, glass, ultramarine and aluminium salts, in the paper industry and is also used as a filler in many rubber goods (Bauluz Lázaro, 2015). Halloysite in nature is usually present in kaolin clays as an impurity of kaolinite. Moreover, halloysite, as already noted, belongs to one of the three representatives of layered silicates along with chrysotile and imogolite and is capable of forming a nanotube morphology. Recently, researchers' interest in compounds capable of forming nanotubes has been steadily growing. This is because the unique mesoporous and macroporous lumens of nanotubular silicates makes them promising carriers for the loading and controlled release of various guests (Kırımlıoğlu et al., 2015; Tan et al., 2015; Yuan et al., 2015; Aguzzi et al., 2016).

The use of synthetic materials instead of natural raw materials is preferable in view of their chemical purity and the possibility of varying their properties even at the object synthesis stage. Many attempts have been made to synthesize analogues of kaolin minerals from aluminosilicate gels or dilute solutions or by experimentally changing geological materials (e.g., volcanic glasses, feldspars, clays, etc.) (Harder, 1970; Kittrick, 1970; La Iglesia and Van Oosterwyck-Gastuche, 1978; Van Oosterwyck-Gastuche and La Iglesia, 1978). The preparation of both platy and spherical forms of kaolin and halloysite has been described (Tomura et al., 1983, Tomura et al., 1985; Joussein et al., 2005; Zsirka et al., 2015), although no convincing evidence has been obtained for producing synthetic nanotubular halloysite.

In general, much work has been devoted to the synthesis of aluminosilicates of the kaolin group (for example,(La Iglesia and Van Oosterwyck-Gastuche, 1978; Tomura et al., 1983; Huertas et al., 1993, Huertas et al., 1999; Miyawaki, 1994; Fialips et al., 2000; Iriarte et al., 2005), however, the optimal conditions for the production of aluminosilicates with a certain set of properties and morphologies have not yet been established. In addition, an important problem is the identification of various polymorphic forms of kaolinite that have an almost identical X-ray diffraction pattern. A complex study involving various physicochemical methods, namely, IR spectroscopy, Raman spectroscopy, X-ray diffraction and electron microscopy, is needed to solve this problem.

Directed hydrothermal synthesis conditions make it possible to obtain aluminosilicates with specified characteristics, such as morphology (nanolayers, nanotubes, spheres, plates etc.), chemical compositions, particle sizes, surface properties, and microstructural and porous-texture characteristics (Golubeva, 2016).

This work studied the synthesis of aluminosilicates of the kaolin group in the Al₂O₃-2SiO₂-nH₂O system, the formation of various morphologies of aluminosilicates, the kinetics of their hydrothermal crystallization and mutual transformation under hydrothermal conditions, and sorption capacity of aluminosilicate nanoparticles of the kaolin group with different morphologies with respect to methyl blue dye. Methylene blue is a common cationic dye used to study the properties of sorbents used in wastewater treatment (for example (Ghosh and Bhattacharyya, 2002; Joseph et al., 2019)), and is also a model substance that simulates the toxicants of medium molecular weights used to assess the properties of medical sorbents, enterosorbents in particular (Reshetnikov, 2003; Golubeva et al., 2018). Studying the sorption capacity of materials towards methylene blue dye makes it possible to assess their potential for solving problems of both ecology and medicine.