Helena Pálková

Near-infrared spectroscopic analysis of acid-treated organo-clays

The potential use of near-infrared (NIR) spectroscopy as a characterization tool for organo-clays would be a great asset but little work has been done in this regard because the application of NIR to clay mineral studies is a relatively new phenomenon. The purpose of this study was to use NIR spectroscopy to investigate the effect of alkylammonium cations on the acid dissolution of a high-charge montmorillonite (SAz-1). Detailed analysis of the spectra of Li+-, TMA+- (tetramethylammonium), and HDTMA+- (hexadecyltrimethylammonium) saturated SAz-1 montmorillonite in the NIR region was achieved by comparing the first overtone (2ν) and combination (ν+δ) bands of XH groups (X = O, C) with the fundamental stretching (ν) and bending (δ) vibrations observed in the mid-infrared (MIR) region. Comprehensive analysis of the vibrational modes of CH3-N, CH3-C, and -CH2-C groups of TMA+ and HDTMA+ cations detected in the MIR and NIR regions was also performed. Both MIR and NIR spectra demonstrated that exchange of Li+ by TMA+ only slightly improved the resistance of SAz-1 layers to dissolution in 6 M HCl at 80°C, while exchange by the larger HDTMA+ cations almost completely protected the montmorillonite layers from acid attack. Use of NIR spectra in reaching these conclusions was crucial. Only in the NIR region could the creation of SiOH groups be monitored, which is an important indicator of the acidification of the montmorillonite surface. The OH-overtone region in the spectra of Li-SAz-1 and TMA-SAz-1 revealed that the SiOH band near 7315 cm−1 increases in intensity with enhanced acid treatment. In contrast, no SiOH groups were identified in the NIR spectra of HDTMA-SAz-1 treated in HCl, indicating that HDTMA+ completely covers the inner and outer surfaces of the montmorillonite and hinders access ofprotons to the Si-O− bonds created upon acid treatment.

Acid dissolution of reduced-charge Li- and Ni-montmorillonites

Reduced-charge samples were prepared from Li+- and Ni2+-saturated SAz-1 montmorillonite by heating at 150 and 300°C for 24 h. X-ray diffraction analysis showed interstratification of non-swelling and swelling interlayers in LiS150, while fully expandable interlayers were found in NiS150. Heating at 300°C caused collapse of the interlayers in LiS300 in contrast to NiS300, for which some expandable layers were interstratified in the pseudo-pyrophyllite structure. The infrared (IR) spectra of heated Li-SAz showed new OH-stretching and overtone bands near 3670 cm−1 and 7170 cm−1 (1395 nm), respectively, indicating creation of local trioctahedral domains containing Li(I) in the previously vacant octahedra. No evidence of OH groups in trioctahedral coordination was found in the spectra of heated Ni-SAz. Nickel is supposed to be trapped in the hexagonal holes of the tetrahedral sheets. Reduction of the layer charge substantially affected the extent of the dissolution of SAz-1 montmorillonite in HCl. The MIR and NIR spectra of unheated Li- and Ni-SAz showed a substantial degradation of their structure after acid dissolution. New bands observed at 3744 cm−1 and 7314 cm−1 (1367 nm) were assigned to the vibrations of Si-OH groups formed upon acid treatment. These bands are a means of checking the extent of acid attack on smectites, even in cases when no differences are observed in the 1300−400 cm−1 spectral region (traditionally used to monitor this process). Both the IR spectra and solution analysis revealed that development of non-swelling interlayers in heated montmorillonites substantially reduced dissolution of these samples. The results obtained confirmed that acid attack of the smectite structure occurred at both interlayer surfaces and edges. If the accessibility of the layers for protons is low due to non-swelling interlayers, the dissolution was slower and took place mainly from the particle edges.

Alterations of the surface and morphology of tetraalkyl-ammonium modified montmorillonites upon acid treatment

The effect of short alkyl chain cations on the modification of the structure, surface and textural properties of organo-montmorillonites upon their acid treatment was investigated. Samples prepared from Ca-SAz montmorillonite and tetramethylammonium (Me(4)N(+)-), tetraethylammonium (Et(4)N(+)-), tetrapropylammonium (Pr(4)N(+)-) and tetrabutylammonium (Bu(4)N(+)-) salts were treated in 6 M HCl at 80 °C for 2-8 h and analyzed by different methods. Acid treatment of organo-montmorillonites caused gradual release of Al and Mg from the octahedral sheets and destruction of their layered structure. The extent of the changes depended significantly on the size of organo-cation. While large plate-like particles of Ca-SAz and Me(4)N-SAz were disintegrated during acid treatment and smaller fine grains were created, the morphology of Bu(4)N-SAz was modified only slightly. Pore size analysis showed generation of pore network upon organo-montmorillonites dissolution. After longer acid attack, pore volume increased and pore size distribution curves were shifted to pores with diameter above 25 Å. The surface area of acid-treated samples increased due to destruction of the montmorillonite layers and formation of the SiO(2)-rich reaction product. The highest value 475 m(2)/g was observed for Me(4)N-SAz treated 4 h. Surface area of Et(4)N-SAz, Pr(4)-SAz and Bu(4)N-SAz was 441, 419 and 293 m(2)/g, respectively, after 8 h treatment. Similar decomposition level was observed for Ca-SAz and Me(4)N-SAz, and less destruction was found for Et(4)N-SAz, Pr(4)-SAz and very low for Bu(4)N-SAz. Though Bu(4)N(+) is short alkyl chain cation, its size is large enough to cover the inner and outer surfaces of montmorillonite and thus to protect the clay layers from acid attack.

The effect of acid treatment on the structure and surface acidity of tetraalkylammonium-montmorillonites.

The effect of tetrabutylammonium (Bu4N+) and tetrapentylammonium (Pe4N+) cations on the modification of the organo-montmorillonite structure upon acid-treatment was investigated. Samples were treated with HCl for various times (2-12 h). Structural changes were followed by MAS NMR spectroscopy. The 29Si MAS NMR spectra of initial Na-saturated form (Na-SAz) showed gradual decrease of the intensity of the resonance assigned to SiO4 cross-linked in the tetrahedral sheets Q3(0Al) while signals arising from the reaction product Q31OH and Q4(0Al) became more pronounced upon acid treatment. The Q3(0Al) signal almost completely disappeared for Na-SAz treated for 8 h on contrary to Bu4N-SAz and Pe4N-SAz showing signal of relatively high intensity even after 12 h. The 27Al MAS NMR measurement proved that more than one half of Al remained in the reaction product of Bu4N-SAz and Pe4N-SAz after 8 h treatment, while Al content dropped below 5% for Na-SAz. Formation of acid sites was investigated via pyridine adsorption. Only physically adsorbed and H-bonded pyridine was detected for acid-untreated samples. In contrast, the IR spectra of the samples partially decomposed in HCl revealed bands of pyridine adsorbed on Brønsted acid sites. Strongly bonded pyridine was able to bear up heating even at 230°C.

IR spectroscopy of clay minerals and clay nanocomposites

Recent applications of infrared (IR) spectroscopy in research of clays and clay minerals are reviewed. After a brief description of the structures of clay minerals and basic principles of IR spectroscopy, the selected most interesting papers published in this area in 2007–2009 are discussed. The potential of both middle-IR and near-IR spectroscopy and different sampling techniques used in the investigation of clay minerals occurring on Earth and Mars is presented, including the utilisation of clay materials in the industry and in protection of the environment. Finally, the theoretical studies of the vibrational properties of the clay minerals are considered.

The effect of layer charge and exchangeable cations on sorption of biphenyl on montmorillonites

Montmorillonies separated from the bentonites SAz-1 (Cheto, AZ, USA), and Cressfield (New South Wales, Australia) were used as starting materials. Reduced charge montmorillonites (RCMs) were prepared from these chemically different and Li-saturated montmorillonites via heating at temperatures in the range of 120–300°C. The residual exchangeable Li+ cations were then replaced with tetramethylammonium (TMA+) or hexadecyltrimethylammonium (HDTMA+) cations and the ability of the modified montmorillonites to adsorb biphenyl was investigated. Lower adsorption was observed for Li-montmorillonites than for the organoclays. The extent of adsorption was dependent on both the layer charge of montmorillonite and the size of alkylammonium cations. HDTMA-forms prepared from unheated Li-montmorillonites adsorbed biphenyl better than the organoclays prepared from RCMs. In contrast, the TMA-samples prepared from the Li-montmorillonites that were not heated showed low uptake of biphenyl probably due to high content of TMA+ cations. Reduction of the layer charge, resulting in lower content of TMA+ cations, increased sorption efficiency of both TMA-montmorillonites. The best adsorbents of biphenyl were HDTMA-SAz-1 prepared from the unheated Li-SAz-1 and TMA-Cressfield prepared from the Li-form heated at 180°C. These samples removed about 80% of biphenyl from its aqueous solutions

Spectroscopic study of water adsorption on Li+, TMA+ and HDTMA+ exchanged montmorillonite

The potential of IR and NMR spectroscopy in characterization the interaction of water with natural and organically modified montmorillonites was introduced. Organoclays were prepared from Li-saturated montmorillonite (Li-S) and tetramethylammonium (TMA) or hexadecyltrimethylammonium (HDTMA) salts. The influence of organic cation size on the water vapour uptake was examined and a comparative study with natural clay mineral was provided. The near-IR spectra confirmed the reduced water content in TMA-S and HDTMA-S. After exposure of the samples to water vapour under various relative humidities (RH) the H2O content was determined. According to the adsorption isotherms the amount of water decreased in order Li-S>TMA-S>HDTMA-S. The intensities of the 2νOH and [Formula: see text] bands, corresponding to the vibrations of H2O, gradually increased in hydrated samples. The (13)C MAS NMR and near-IR of hydrated organoclays confirmed the presence of H2O close to the cations headgroup. NMR signals of inner -CH2- groups in HDTMA-S were also affected by hydration: the intensity of disordered gauche conformers (31.1 ppm) overtook the intensity of ordered all-trans conformers (33.0 ppm).

NEAR-INFRARED STUDY OF WATER ADSORPTION ON HOMO-IONIC FORMS OF MONTMORILLONITE

Polycarboxylate superplasticizer (PCE) is a widely used water-reducing agent that can reduce significantly the water demand of concrete, which reduces the porosity and enhances the strength and durability of the concrete. (The PCE consists of a single backbone with many long PEO side chains.) Generally, aggregate occupies >70 wt.% of concrete; clay minerals are ubiquitous in nature and are difficult to avoid in mined aggregates. Clay minerals in aggregate often render the PCE ineffective and give rise to rapid loss of the fluidity of the concrete; this phenomenon is referred to as ‘poor clay tolerance of PCE.’ Though the poor clay tolerance of PCE is known widely, the relationship between the clay tolerance and the molecular structure of the PCE, in particular the effect of the side-chain structures, on clay tolerance is not understood completely. The objective of the present study was to determine the effect of different grafting densities of polyethylene oxide (PEO) side chains on the clay tolerance of PCE. The raw materials included mainly PCE, which was synthesized using acrylic acid and isopentenol polyoxyethylene ether, and a natural montmorillonite (Mnt), one of the most common clay minerals. The loss of fluidity of the cement paste was tested to assess the clay tolerance; total organic carbon was used to measure the amount of PCE adsorbed; X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, and thermogravimetric analysis were used to investigate the microstructure of the intercalated Mnt. The results showed that preventing the superficially adsorbed PCE from being intercalated into Mnt was of great importance in terms of the improvement in clay tolerance of PCE, which increased with greater grafting density of PEO in the side chain of the PCE. The results also suggested the possibility that polymers which intercalate preferentially into the Mnt could improve significantly the clay tolerance of the PCE system.Polycarboxylate superplasticizer (PCE) is a widely used water-reducing agent that can reduce significantly the water demand of concrete, which reduces the porosity and enhances the strength and durability of the concrete. (The PCE consists of a single backbone with many long PEO side chains.) Generally, aggregate occupies >70 wt.% of concrete; clay minerals are ubiquitous in nature and are difficult to avoid in mined aggregates. Clay minerals in aggregate often render the PCE ineffective and give rise to rapid loss of the fluidity of the concrete; this phenomenon is referred to as ‘poor clay tolerance of PCE.’ Though the poor clay tolerance of PCE is known widely, the relationship between the clay tolerance and the molecular structure of the PCE, in particular the effect of the side-chain structures, on clay tolerance is not understood completely. The objective of the present study was to determine the effect of different grafting densities of polyethylene oxide (PEO) side chains on the clay tolerance of PCE. The raw materials included mainly PCE, which was synthesized using acrylic acid and isopentenol polyoxyethylene ether, and a natural montmorillonite (Mnt), one of the most common clay minerals. The loss of fluidity of the cement paste was tested to assess the clay tolerance; total organic carbon was used to measure the amount of PCE adsorbed; X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, and thermogravimetric analysis were used to investigate the microstructure of the intercalated Mnt. The results showed that preventing the superficially adsorbed PCE from being intercalated into Mnt was of great importance in terms of the improvement in clay tolerance of PCE, which increased with greater grafting density of PEO in the side chain of the PCE. The results also suggested the possibility that polymers which intercalate preferentially into the Mnt could improve significantly the clay tolerance of the PCE system.

PDDA-Montmorillonite Composites Loaded with Ru Nanoparticles: Synthesis, Characterization, and Catalytic Properties in Hydrogenation of 2-Butanone

The effect of synthesis parameters on the physicochemical properties of clay/ polydiallyldimethylammonium (PDDA)/Ru composites and their applicability in hydrogenation of 2-butanone under very mild conditions (room temperature, atmospheric pressure, and aqueous solution) was studied. Three synthetic procedures were employed, differing in the order of addition of components and the stage at which metallic Ru species were generated. The materials were characterized with XRD (X-ray diffraction), XRF (X-ray fluorescence), EDS (energy-dispersive spectroscopy), AFM (atomic force microscopy), TEM/HRTEM (transmission electron microscopy/high resolution transmission electron microscopy), and TG/DSC (thermal gravimetry/differential scanning microscopy techniques. The study revealed that the method of composite preparation affects its structural and thermal properties, and controls the distribution and size of Ru particles. All catalysts are active in hydrogenation of 2-butanone. For best catalytic performance (100% conversion within 30 min) both the size of Ru particles and the load of polymer had to be optimized. Superior catalytic properties were obtained over the composite with intermediate crystal size and intermediate PDDA load, prepared by generation of metallic Ru species in the polymer solution prior to intercalation. This method offers an easy way of controlling the crystal size by modification of Ru/PDDA ratio.

Influence of Grain Size Distribution on Hardness and Fracture Toughness of Si3N4/SiC Nanocomposites

Si3N4/SiC nanocomposite material with yttria as sintering additive was prepared by hot pressing method. SiC nanoinclusions were produced by in situ reaction between SiO2 and C during the sintering process. The homogenous microstructure of hot pressed samples contained fine Si3N4 grains with average diameter of 200 nm. The post-sintering treatment at 1750°C for 26 and 70 hours significantly changed the microstructure of Si3N4/SiC composites. The difference between the average grain size of hot pressed sample and sample treated for 70 hours is approx. 300 %. In sake of this fact the mechanical properties (KIC, HV1) were not changed significantly. The largest differences between the mechanical properties of hot pressed and annealed samples were only 8 %.