Cation-exchanged Bentonite Research Articles

Adsorption studies of hydrogen and ethylene on cation-exchanged bentonite

AbstractThe adsorption of C2H4 and H2 gases by bentonite from the Ünye region, Turkey both as raw (B) and as K+-, Li+-, Ag+- and Mg2+-exchanged forms, was investigated using automated volumetric equipment and pressures up to 100 kPa at 273 K and 77 K, respectively. X-ray powder diffraction (XRD) and specific surface area measurement (BET) methods were employed to characterize the bentonite samples. The C2H4and H2gas adsorption capacities of the original and modified forms were in the ranges 1.817–0.201 mmol g−1and 0.522–0.388 mmol g−1, respectively. The influence of salt modifications on the gas adsorption properties of bentonite is discussed.

Inhibition of corrosion-driven organic coating disbondment on galvanised steel by smart release group II and Zn(II)-exchanged bentonite pigments

Bentonite pigments exchanged with either zinc or group II cations are characterised as inhibitors of corrosion-driven cathodic disbondment of model polyvinylbutyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanised steel. An in situ scanning Kelvin probe (SKP) technique is used to quantify rates of coating delamination as a function of pigment volume fraction ( ϕ pt) and draw up a ranking order of inhibitor efficiency. Group II cation-exchanged bentonites show a moderate degree of inhibition, where rates of coating disbondment are reduced by up to 60–70% compared to the unpigmented case. In contrast, bentonite pigments containing exchangeable Zn 2+ ions are markedly more effective, and no delamination is observed over periods of up to 24 h when ϕ pt ≥ 0.1. The efficiency of in-coating Zn 2+ is attributed to the ability to block underfilm cathodic oxygen reduction by reinforcing a pre-existing zinc (hydr)oxide layer.

Characterization of the cation-exchanged bentonites by XRPD, ATR, DTA/TG analyses and BET measurement

X-ray powder diffraction (XRPD) analysis demonstrated that the main montmorillonite component of Unye bentonite was more effective in the Pb NaCl (Sigma, 99.5%), CaCl2·6H2O (Merck, 99.0%), CoCl2·6H2O (Merck, 98.0%), CuCl2·2H2O (Merck, 99.9%), NiCl2·6H2 O( Merck, 99.9%), FeCl3·6H2O (Merck, 98.0%), AlCl3·6H2O (Merck, 99%), PbCl2 (Merck, 99.9%) and ZnCl2 (Merck, 99.9%) were used for preparing the cation-exchanged Unye bentonite samples.

Several textural properties of compacted and cation-exchanged bentonite

Abstract One of the principal applications for bentonite is in drilling muds. Moreover it is widely used as a suspending and stabilizing agent, and as an adsorbent or clarifying agent, in many industries. Recently the bentonites have been proposed as engineered barriers for radioactive waste repository because these materials are supposed to build up a better impermeable zone around wastes by swelling. For these reasons, a textural characterization of bentonites in the laboratory is very important. The aim in this study was to estimate several textural properties of compacted and cation-exchanged bentonite by using Hg-porosimetry, N 2 -adsorption, water vapour adsorption, scanning electron microscopy (SEM) observations and environmental scanning electron microscopy-digital images analysis measurements. For that, bulk samples were mechanically compressed at atmospheric conditions by using a uniaxial system at four different pressures (21, 35, 49, and 63 MPa) in order to obtain four physical densities. On the other hand, the bulk samples of bentonite were treated separately with four concentrated solutions (1N concentration) of sodium, potassium, magnesium and calcium chlorides in order to obtain a homoionic interlayer cation in the clay phase. The results showed that the macro-porosity (porous size>50 nm) and eventually the mesoporosity (porous size 2–50 nm) are affected by the uniaxial compaction. In this case, a transformation of the shape of the macro-pores network from tube to crack was observed. On the other hand, the swelling potential and water content are governed by the relative humidity and by the nature of interlayer cation.

Effect of cation exchanged bentonites (CEBs) on the aggregation of a cresol based dye

The effect of raw and cation exchanged bentonite (CEB) samples on the aggregation of a cresol based dye, bromocresol green, was studied by spectrophotometric method. The changes in the absorption spectra were explained in terms of the layer charge of the clay surfaces which controls the stability of the dye aggregates.

Swelling–shrinkage measurements of bentonite using coupled environmental scanning electron microscopy and digital image analysis

The swelling clays have been proposed as engineered barriers in geological disposal systems for waste because these materials are assumed to build a better impermeable zone around wastes by swelling. However, the swelling potential of soils is also considered a prevalent cause of damage to buildings and constructions. For these reasons, it is fundamental to investigate the physicochemical and mechanical behavior of swelling clays. In the current study, the swelling–shrinkage potential (aggregates scale) was estimated using an environmental scanning electron microscope (ESEM) coupled with a digital image analysis (DIA) program (Visilog). In fact, the isolated aggregates of raw and cation-exchanged bentonite were directly observed at different relative humidities in an ESEM chamber. Then the “Visilog” software was used to estimate the percent augmentation of the aggregate surface as a function of time and as a function of relative humidity. This estimation allows for the calculation of the swelling–shrinkage potential (%) of bentonite. Finally, a kinetic model of first order was tested to fit the kinetic experimental data of swelling–shrinkage potential. The results show that ESEM–DIA coupling can be a powerful method of estimating the swelling–shrinkage potential of expansive clays. In addition, the exponential models fit well with the kinetic experimental data.

Correlation of hydraulic conductivity of clay–sand compacted specimens with clay properties

Hydraulic conductivity of compacted specimens consisting of 1 of 16 clay samples and siliceous sand/natural soils was measured using permeameters with flexible wall cells. Clay samples studied were eight natural bentonites, one purified montmorillonite, five standard clay samples, one synthetic hectorite, and two cation-exchanged bentonites. The clay content to whole solid was fixed at 11.5% w/w, and the mixtures were rammed in a mold to form disk compactions. A higher hydraulic conductivity was observed with increase in the Ca/Na ratio of clay. Hydraulic conductivity was also affected by bentonite content of the clays and the mineral content. A relation between the hydraulic conductivity and the characteristic index properties of the clays such as chemical concentration, methylene blue adsorption capacity (MB), and so on were elucidated, and then a polynomial expression was derived to reproduce measured hydraulic conductivity data. This relation was then utilized to estimate hydraulic conductivity of blended bentonites from characteristic properties of the component bentonites.

Photodimerisation of acenaphthylene in a clay microenvironment.

The photochemical dimerisation of acenaphthylene in the presence of various cation-exchanged bentonite clays has been studied. While the cis-dimer is the predominant product when smaller cations are present in the clay interlayer, the presence of heavier atoms in the clay favors the trans-dimer. Synthetic anionic hydrotalcite clays are also found to be efficient for the efficient dimerisation of acenaphthylene.

Photodimerisation of enones in a clay microenvironment

The photodimerisation of 2-cyclohexenone, 2-cyclopentenone and their 3-methyl derivatives is studied in the confined environment of clay interlayers. In the photolysis of 2-cyclohexenone, a remarkable regioselectivity is observed in favour of the head-to-head dimer when carried out in cation-exchanged bentonite clays while the dimerisation of 2-cyclopentenone favours the formation of the head-to-tail dimer. The results are explained on the basis of the ground state pre-organisation of enones and also the complexing ability of charge-balancing cations with the carbonyl oxygen of enone. In addition to the observed regioselectivity, other advantages of employing clays in photochemical studies are highlighted.

Conversion of cyclohexanol to dicyclohexyl ether catalyzed by cation-exchanged bentonite clays

Bentonite clays modified by exchanging Al 3+, Co 2+ and its Na + form were used in etherification of cyclohexanol to dicyclohexyl ether. Product yields obtained for metal cation exchanged clays followed the order Al 3+ > Co 2+ > Na +. Brønsted acidity of the solid catalysts is suggested to play the key role in the studied reaction.

Near-infrared photoacoustic FTIR spectroscopy of clay minerals and coal

Near-infrared (NIR) photoacoustic spectra of kaolinite, cation-exchanged bentonites and coal have been obtained by reducing the velocity of the moving mirror in a commercial rapid-scan Fourier transform infrared (FTIR) spectrometer so that the characteristic photoacoustic cell resonance no longer falls in the NIR spectral region. The observed NIR bands of kaolinite are attributable to combinations of OH-stretching and AIOH-bending vibrations, and to overtones of OH-stretching modes. Bentonites give rise to bands due to combination of OH-stretching with AlOH- or HOH-bending, as well as overtones of OH-stretching vibrations. Spectra of two separated macerals of sub-bituminous coal show continuous absorption which extends beyond 10,000 cm −1 and arises from low-energy electronic transitions.

ChemInform Abstract: SHEET SILICATES: BROAD SPECTRUM CATALYSTS FOR ORGANIC SYNTHESIS

Abstract Although montmorillonites have been found to have little catalytic activity in their natural Na+ form, various cation-exchanged montmorillonites have proved to be effective catalysts for a wide variety of organic reactions. Alkenes can be induced to add (a) water, (b) alcohols, (c) thiols and (d) carboxylic acids to furnish ethers, thioethers and esters in the presence of these cation-exchanged sheet silicates, but amines failed to undergo addition with alkenes under similar conditions. The number of products obtained in each reaction depended upon the ease of rearrangement of the carbocation intermediates. High yields were obtained in those cases where a single carbocation intermediate was involved. The cation-exchanged sheet silicates also catalysed a variety of elimination reactions. Water was eliminated from alcohols to produce ethers, ammonia was eliminated from amines to make secondary amines and hydrogen sulphide was eliminated from thiols to give dialkyl sulphides. Cation-exchanged bentonites were also effective catalysts in the production of dioxan from ethylene oxide and in the preparation of acetals with orthoformates. Analysis of the products from the reactions suggested that the cation-exchanged montmorillonites reacted, in nearly all cases, as acidic heterogeneous catalysts.

Sheet silicates: broad spectrum catalysts for organic synthesis

Although montmorillonites have been found to have little catalytic activity in their natural Na + form, various cation-exchanged montmorillonites have proved to be effective catalysts for a wide variety of organic reactions. Alkenes can be induced to add (a) water, (b) alcohols, (c) thiols and (d) carboxylic acids to furnish ethers, thioethers and esters in the presence of these cation-exchanged sheet silicates, but amines failed to undergo addition with alkenes under similar conditions. The number of products obtained in each reaction depended upon the ease of rearrangement of the carbocation intermediates. High yields were obtained in those cases where a single carbocation intermediate was involved. The cation-exchanged sheet silicates also catalysed a variety of elimination reactions. Water was eliminated from alcohols to produce ethers, ammonia was eliminated from amines to make secondary amines and hydrogen sulphide was eliminated from thiols to give dialkyl sulphides. Cation-exchanged bentonites were also effective catalysts in the production of dioxan from ethylene oxide and in the preparation of acetals with orthoformates. Analysis of the products from the reactions suggested that the cation-exchanged montmorillonites reacted, in nearly all cases, as acidic heterogeneous catalysts.

Organic reactions in a clay microenvironment

AbstractAlthough natural Na-bentonite has little catalytic activity, various cation-exchanged bentonites have proved to be effective catalysts for a wide variety of organic reactions. In the presence of these catalysts alkenes can be induced to add (a) water, to yield branched-chain symmetrical ethers; (b) alcohols, to give a variety of ethers; (c) thiols, to yield thio-ethers; (d) carboxylic acids, to give esters. The number of products obtained in each reaction depends on the ease of rearrangement of the carbocation intermediates. High yields are obtained where a single carbocation intermediate is formed, A variety of elimination reactions is also catalysed by these sheet silicates. Water is eliminated from alcohols to produce ethers, ammonia is eliminated from amines to make secondary amines, and hydrogen sulphide is eliminated from thiols to give dialkyl sulphides. In most cases the ion-exchanged bentonites react as acidic heterogeneous catalysts.