Functional Kaolinite.

The world resources of all clays are extremely large. Among the various types of clays, the world mine production of kaolin in 2016 was 37.0 Mt, the largest mined clay. Kaolin is traditionally used in ceramics, refractories and as paper coating and filling. But kaolin, as it is demonstrated in this paper, has a bright potential for use in non-traditional, high value-added, applications. This is particularly true for its principal component: the mineral species kaolinite which has a chemical structure allowing its functionalization, leading to a variety of potential applications. Kaolinite is a layered 1 : 1 clay mineral, the layer being made of two different sheets, a tetrahedral silica sheet and an octahedral alumina sheet. Large dipole-dipole interactions, in addition to a network of H-bonds, link the siloxane surface of a layer to the aluminol surface of another layer, making intercalation of guest species in kaolinite challenging. There is however a limited number of molecular units (molecules or salts) that can directly intercalate in kaolinite to form "pre-intercalates". Once intercalated these molecular units can be exchanged by a large number and variety of guests, providing access to the interlayer space of kaolinite, and to its reactive aluminol internal surfaces. The intercalation of molecules of pharmacological interest showed the potential of kaolinite to act as a slow-releasing agent for drugs, and the intercalation of polymers resulted in the creation of intercalated nanocomposites. The intercalation of ionic liquids gave materials with ionic conductivity properties in the solid-state. Intercalates are however unstable in water. One needed to make these organo-inorgano nanohybrid materials resistant to hydrolysis and more thermally stable. The network of aluminol groups on the internal surfaces of kaolinite offers the opportunity to design and create controlled organo-inorgano nanohybrid materials, taking advantage of their reactivity, in particular with hydroxyl groups of organic compounds, to form Al-O-C bonds. A functional, two-dimensional, spatially restricted, environment can be created with controlled nanoarchitecture. The grafting of organic groups on the aluminol internal carpets has allowed applications in catalysis, in sensing, in heavy metals adsorption, in exfoliated nanocomposite, in luminescence, and in structural modifications to form nanoscrolls or nanorolls. This paper shows how the future of the use of kaolinite will shift from its traditional uses in ceramics, tiles and paper coating to more sophisticated, high value-added, uses. In particular, research should amplify in the years to come to design an efficient and cost-effective method to produce kaolinite with nanotubular morphology. One can foresee also that efficient, easy-to-use, electrochemical devices based on modified kaolinite, will be created to quantify selectively a variety of pollutants in waste waters.

Characterization and Applications of Kaolinite Robustly Grafted by an Ionic Liquid with Naphthyl Functionality.

Functionalization of the kaolinite (K) interlayer space is challenging. In this work, a new kaolinite-based nanohybridmaterial (K-NI) was successfully synthesized by grafting on the interlayer aluminol surfaces the ionic liquid, 1-(1-methylnaphthyl)-3-(2-hydroxyethyl) imidazolium chloride (NI), using a guest displacement strategy. A substantial increase of the basal spacing (10.8 Å) was obtained. This is a grafted derivative of kaolinite possessing one of the largest d-values. Washing in water for several days and other vigorous treatments such as sonication showed a minor effect on the integrity of the material. FTIR and 13C NMR confirmed the conservation of the structure of the ionic liquid after the grafting. Thermal analysis confirmed the presence of grafted material and was used to estimate the abundance of the grafted ionic liquid (0.44 mole per mole of kaolinite structural formula, (Al2Si2O5(OH)4)). By using cyclic voltammetry, the permeability of a film of K-NI for the bulky ferricyanide ions was demonstrated. The accumulation of nitrophenolate anions was effective (maximum capacity of 190 µmol/g), but was less important than what was expected due to the steric hindrance of the bulky grafted NI. Although the presence of chloride anions reduced the adsorption capacity, the affinity of the modified kaolinite interlayer space for the nitrophenolate anions was demonstrated.

Synthesis and catalytic application of palladium nanoparticles supported on kaolinite-based nanohybrid materials.

Palladium nanoparticles (PdNPs) were deposited on the surface of the modified clay mineral, kaolinite. To improve compatibility, abundance and control of the size of the nanoparticles, kaolinite was modified by the grafting of an amino alcohol (triethanolamine (TEA)) and an ionic liquid (1-(2-hydroxyethyl)-3-methylimidazolium (ImIL)). Characterization techniques (XRD, TGA, solid state (13)C NMR and FTIR spectroscopy) confirmed the effective grafting of these compounds on the internal surface of kaolinite. After the synthesis of PdNPs onto clay particles, TEM allowed the visualization of abundant PdNPs with sizes ranging from 4 to 6 nm, uniformly distributed onto the platelets of modified kaolinite. Unmodified clay showed low abundance and random distribution of the nanoparticles. The catalysts obtained were effective for the catalytic reduction of 4-nitrophenol (4-NP), the material with TEA being the most effective. These materials have exhibited excellent performance during the Heck and particularly the Suzuki-Miyaura coupling reactions, with reaction yields up to 100%. These catalysts showed a very slight loss in activity for three consecutive catalytic cycles (less than 10% decrease of the activity compared to the first cycle). This was an evidence that the prior grafting modification of kaolinite helps in significantly improving the quality of the synthesized NPs and also promotes their strong attachment onto the clay mineral surface.

PdNP Decoration of Halloysite Lumen via Selective Grafting of Ionic Liquid onto the Aluminol Surfaces and Catalytic Application.

The synthesis of selectively deposited palladium nanoparticles (PdNPs) inside tubular halloysite lumens is reported. This specific localization was directed by the selective modification of the aluminol surfaces of the clay mineral through stable Al-O-C bonds. An ionic liquid (1-(2-hydroxyethyl)-3-methylimidazolium) was grafted onto halloysite following the guest displacement method (generally used for kaolinite) using halloysite-DMSO preintercalate. The characterization of this clay nanohybrid material (XRD, NMR, TGA) showed characteristics reminiscent of similar materials synthesized from kaolinite. The grafting on halloysite lumens was also effective without using the DMSO preintercalate. The presence of these new functionalities in halloysite directs the synthesis of uniform PdNPs with size ranging between 3 and 6 nm located exclusively in the lumens. This results from the selective adsorption of PdNPs precursors in functionalized lumens through an anion exchange mechanism followed by in situ reduction. In contrast, the unmodified clay mineral displayed nanoparticles both inside and outside the tubes. These catalysts showed significant catalytic activity for the reduction of 4-nitrophenol (4-NP). The most efficient catalysts were recycled up to three times without reducing significantly the catalytic activities.

Intercalation of a block co-polymer in kaolinite.

Intercalates of kaolinite/Polyethylene-block-poly (ethylene glycol) were successfully prepared by melt intercalation of the polymer with a kaolinite: dimethyl sulfoxide pre-intercalate. Powder X-ray diffraction gives an 11.05 Å d-spacing which remains after washing the material with water. Thermal gravimetric analysis and nuclear magnetic resonance confirm the presence of the polymer in the prepared material. Perturbation of the kaolinite hydroxyl stretch and the polymer's CH2 bending modes in the materials' infrared spectrum suggest strong interaction between the aluminol clay sheet and the ethylene glycol units in the polymer. Dipolar dephasing studies show both polymer blocks experience a significant loss in mobility in the prepared material, indicative of intercalation. The presence of highly hydrophobic polyethylene inside kaolinite is expected to drastically modify the nature of the interlayer space, and could be of interest for applications where retention or sequestration of hydrophobic species is required.

Ionic liquid-kaolinite nanohybrid materials for the amperometric detection of trace levels of iodide.

A nanohybrid kaolinite material with anion exchange properties was obtained by the grafting of an ionic liquid in the interlayer spaces of kaolinite. It was used successfully for the amperometric detection of iodide in aqueous solution and tap water after a pre-concentration step, with a detection limit of 1.5 × 10(-7) M. This electrode has an excellent stability. Its selectivity was tested in the presence of several anions.

Preparation and Characterization of Guar-Montmorillonite Nanocomposites.

Polymer-clay nanocomposites are highly sought-after materials, mainly due to their applicability in a variety of avenues. From the standpoint of the preparation of these nanocomposites, however, organic compatibility with clay and adherence to "green chemistry" concepts and principles can be limiting factors. As such, the objective was to prepare a biopolymer-modified clay nanocomposite using a simple and environmentally friendly method of preparation, whereby pre-treatment of the clay for organic compatibility was bypassed. Novel montmorillonite nanocomposites were prepared using neutral guar gum and cationic guar gum. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the formation of intercalated structures. A monolayer configuration of cationic guar within the interlayer space was indicated by XRD results, while treatment with neutral guar gum resulted in the observance of both monolayer and bilayer configurations. Additionally, TEM results indicated partial exfoliation. Results attributed from 13C cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy (CP/MAS NMR) of the nanocomposites indicated peaks corresponding to the guar constituent, confirming the adsorption of the biopolymer. Inductively coupled plasma emission spectrometry (ICP-ES) results indicated the exchange of cations present in neutral guar gum with the sodium cations of montmorillonite, in the case of the neutral guar nanocomposites.

Single kaolinite nanometer layers prepared by an in situ polymerization-exfoliation process in the presence of ionic liquids.

A simple chemical route for the exfoliation of kaolinite in the presence of polymerizable ionic liquids and the resulting obtainment of exfoliated nanocomposites is reported. The exfoliation was achieved using three different ionic liquids structurally bearing a vinyl group: 1-methyl-3-(4-vinylbenzyl)imidazolium chloride salt (IL_1), 1-methyl-1-(4-vinylbenzyl)pyrrolidinium chloride (IL_2), and 1-methyl-3-vinyl imidazolium iodide (IL_3) and a urea-kaolinite intercalate as precursor. The reaction was done in one step by an in situ polymerization-exfoliation process. (13)C CP/MAS NMR spectra confirmed the spontaneous polymerization of the ionic liquid during the exfoliation process to afford atactic polystyrene derivatives in the case of IL_1 and IL_2. The amount of organic material in the exfoliated nanocomposite was close to 30% as shown by thermal gravimetric analysis. This amount is small in comparison to the amount obtained when the exfoliation was done using sodium polyacrylate (Letaief and Detellier, Langmuir2009, 25, 10975). XRD as well as SEM analysis confirmed a total exfoliation of the kaolinite when the reaction was done using urea kaolinite, whereas a microcomposite, made predominantly of kaolinite platelet aggregates dispersed in the polymeric matrix, was formed when dimethylsulfoxide kaolinite was used as the precursor.

Nanoporous polymer--clay hybrid membranes for gas separation.

Nanohybrid organo-inorgano clay mineral-polydimethylsiloxane (PDMS) membranes were prepared by the reaction of pure and/or modified natural clay minerals (Sepiolite and montmorillonite) with PDMS in hexane, followed by evaporation of the solvent at 70 degrees C. The membranes were characterized by means of XRD, SEM, ATD-TG and solid state (29)Si magic angle spinning (MAS) and cross-polarization (CP) CP/MAS NMR. The morphology of the membranes depends on the content loading of clay mineral. For low content, the membrane composition is homogeneous, with well dispersed nanoparticles of clay into the polymer matrix, whereas for higher clay content, the membranes are constituted also of a mixture of well dispersed nanoparticles into the polymer, but in the presence of agglomerations of small clay particles. Quantitative (29)Si MAS NMR demonstrated a strong correlation between the clay content of the membrane and the average length of the PDMS chain, indicating that the nanohybrid material is made of clay particles covalently linked to the PDMS structure. This is particularly the case for Sepiolite with has a high density of Q(2) silanol sites. The separation performances of the prepared membranes were tested for CO(2)/CH(4) and O(2)/N(2) mixtures. The observed separation factors showed an increase of the selectivity in the case of CO(2)/CH(4) in comparison with membranes made from PDMS alone under the same conditions.

Clay-polymer nanocomposite material from the delamination of kaolinite in the presence of sodium polyacrylate.

A chemical route for the delamination of kaolinite in a polymeric matrix is reported in this work. The strategy that was used is based on mixing polyelectrolytes of opposite charges, an organic polyanion, polyacrylate, with an inorganic polycation resulting from the modification of the internal surfaces of kaolinite. The delamination was carried out by the reaction of sodium polyacrylate (PANa) with kaolinite whose internal aluminol surfaces were previously grafted with triethanolamine and subsequently quaternized with iodomethane (TOIM-K) to form an extended lamellar inorganic polycation. X-ray diffraction as well as scanning electron microscopy (SEM) confirmed the complete delamination of the kaolinite particles. 13C CP/MAS NMR showed the removal of the ammonium groups resulting from hydrolysis of the internal surfaces once exposed, and 29Si CP/MAS NMR spectra were in agreement with the retention of the 1:1 aluminosilicate kaolinite layers structures. From the thermogravimetry (TG) data, the respective percentages in mass of PA and kaolinite in the delaminated nanocomposite could be estimated to be 61% and 39%, respectively, in the conditions of the particular experiment. The procedure was repeated several times to show the reproducibility of the delamination. The interlayer functionalization of kaolinite was crucial for the success of the delamination procedure. SEM pictures show that some individual kaolinite platelets fold and form curved structures.

Intercalation of cyclic imides in kaolinite.

The intercalation of two cyclic imides, succinimide and glutarimide, in the interlayer spaces of kaolinite was obtained from a "soft guest-displacement method" by displacing previously intercalated guest molecules. The dimethyl sulfoxide (DMSO)-kaolinite preintercalate was particularly efficient for that purpose. The intercalation exchange was done from a concentrated aqueous solution of the cyclic imides, at ambient temperature, in a relatively short time. Complete displacement of DMSO by the cyclic imides was confirmed by the results of several independent characterizations, including XRD, TG/DTA, FTIR, and (13)C MAS NMR analyses including dipolar dephasing experiments. The imide intercalates are two dimensionally constrained in the kaolinite interlayer spaces, and are structurally organized in a flattened configuration with their cycle roughly parallel to the ab plane of the kaolinite layers. Elemental analysis gives the following compositions: Al(2)Si(2)O(5)(OH)(4)(C(4)H(5)NO(2))(0.65) and Al(2)Si(2)O(5)(OH)(4)(C(5)H(7)NO(2))(0.49), respectively for succinimide and glutarimide. The results of the TG/DTA analyses showed enhanced thermal stabilities of the imide intercalates compared with the starting materials. The intercalation process from the aqueous solution is reversible: in prolonged contact with water, the imide molecules are released, resulting in the rebuilding of the kaolinite structure. These results demonstrate the potential use of kaolinite as a slow-releasing agent for molecules structurally related to the cyclic imides of this study.

Functionalized nanohybrid materials obtained from the interlayer grafting of aminoalcohols on kaolinite.

New robust functionalized nanohybrid materials were prepared by the interlayer covalent attachment of aminoalcohols to the octahedral sheets of kaolinite.

Reactivity of ionic liquids with kaolinite: melt intersalation of ethyl pyridinium chloride in an urea-kaolinite pre-intercalate.

This paper reports a new route for the intercalation of an ionic liquid, namely 1-ethyl pyridinium chloride, into the interlamellar spaces of kaolinite. The intercalation was achieved using a kaolinite-urea intercalate as a starting material. The results of the XRD, FTIR, and TGA analyses confirmed the intercalation of ethyl pyridinium in the interlamellar spaces of kaolinite. 13C CP/MAS spectra indicated the complete displacement of urea by ethyl pyridinium. 29Si and 27Al NMR spectra of the starting materials and the products are also discussed as well as the results of the elemental analysis of the produced nanohybrid material.

Multinuclear magnetic resonance spectroscopy studies of the structuration of the tunnels of sepiolite in the presence of intracrystalline pyridine molecules.

Sepiolite, a naturally occurring magnesiosilicate nanoporous clay mineral with a tunnel structure, contains two types of water molecules in the structure: zeolitic water trapped inside the tunnels and structural water molecules which interact strongly with magnesium cations. The zeolitic water was removed by heating the sepiolite to 120 degrees C. The partially dehydrated sepiolite absorbed pyridine vapor to produce an intercalated material where the pyridine takes the place of the zeolitic water. 1H solid-state MAS NMR spectroscopy showed that there is isotopic H/D exchange between pyridine-d5 and the remaining structural water molecules of the sepiolite framework. The exchange takes place at room temperature over several days. Wide line solid state 2H NMR of the sequestered pyridine-d5 showed that two populations of pyridine molecules coexist in the material: one very mobile physisorbed population, which can be removed by heating at 90 degrees C, and a population due to pyridine trapped in the tunnels. Except for small in-plane librations, the trapped pyridine was shown to be held rigidly by the sepiolite. When the pyridine intercalated material is heated at 400 degrees C the structural water and some of the pyridine is lost. The remaining pyridine takes the place of the structural water to produce a new inorganic-organic nanohybrid material with the pyridine bound to the terminal Mg(II) in the structure. The pyridine in this material as well as the intercalated material was characterized by slow-spinning 15N and 13C CP/MAS NMR spectroscopy. The 15N NMR was shown to be a very sensitive probe to characterize the various types of pyridine. The data indicate that pyridine molecules in the inorganic-organic nanohybrid material are directly bound to magnesium cations exposed in the tunnels of sepiolite.

Reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in water.

The reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in deuterated water were studied by 1H NMR longitudinal relaxation rates. It is shown that the relaxation is purely dipolar in the non-extreme narrowing regime. The distance between the geminal protons could be determined from the NMR data, giving good agreement with the values generally used in correlation time calculations. The correlation times show an Arrhenius behaviour in good agreement with previously reported data from 13C measurements for a similar uncomplexed calixarene. The Arrhenius energies of activation are identical for the uncomplexed and the complexed calixarenes, suggesting a reorientational motion strongly dependent on the structure of the water cage around the complex. This is also in agreement with a complexation of the La(III) cation in the second sphere of solvation of the sulfonate groups, as shown by molecular dynamics simulations.