Klaus Beneke

Intercalation and exchange reactions of clay minerals and non-clay layer compounds

AbstractPresently, a large variety of layered materials are synthesized that are able to intercalate neutral guest molecules or to exchange inorganic and organic ions for interlayer ions. Several of these materials are also found as minerals.The intracrystalline reactivity of a few selected compounds will be described and compared to clay minerals:- intercalation into crystalline silicic acids;- reactions of phosphates, arsenates, and sulfates;- reactions of titanates, niobates, and molybdates with long chain alkylammonium ions, and- anion exchange properties of double hydroxides. A general conclusion is that the non-clay minerals in many ways behave like clay minerals, but there is no doubt that the reactivity of clay minerals and the variety of their reactions cannot be exceeded by any other material.

Anionic surfactants between double metal hydroxide layers

Abstract Surfactant films between double metal hydroxide layers are prepared by exchanging interlayer anions of layered double metal hydroxides {M1−xIIMxIII(OH)2}x+xX− · zH2O. Zinc chromium {Zn2Cr(OH)6}+(NO)3− · 2H2O is used as an example of this group of layered materials. The nitrate ions are exchanged by alkyl sulfate ions CnCH2nC+1SO4− (nC = 6, 8, …, 18) and dodecyl glycol ether sulfate ions C12H25(OCH2CH2)mSO4− (m = 0, 1, 2, 4). In equilibrium with the surfactant solutions, monolayers of surfactant ions are formed between the zinc chromium hydroxide layers. The chain axes are perpendicular to the hyroxide sheets [V1(90°) structure]. After being washed and dried, the materials contain surfactant monolayers with the chains tilted about 56° to the hydroxide sheet [V1(56°) structure]. These materials take up long-chain alkanols CnAH2nA+1OH (nA = 6, 8, …, 18) into the interlayer regions. Bilayers are formed consisting of surfactant ions and alkanol molecules. For most combinations of nC and nA, the distance between the hydroxide sheets is determined by pairs of sulfate ions and alkanol molecules that are perpendicular to the hydroxide sheets and shortened by one, two, or three kinks. At extreme differences between nC and nA the pairs are tilted (56–60°), or other arrangements occur. Small organic molecules (water, some diols, N-methyl formamide, dimethyl sulfoxide) are intercalated with maintenance of the 56° chain orientation. In particular cases and if the alkyl chains are not too long, some guest molecules associate forming larger clusters, causing a considerable change in the monolayer structure.

Cation exchange reactions of the mica-like potassium niobate K4Nb6O17

Abstract The potassium ions in K 4 Nb 6 O 17 ·zH 2 O are exchangeably bound. The cation exchange is not very pronounced for inorganic cations but clearly established for organic cations such as alkylammonium ions and trimethylalkylammonium ions with long alkyl chains. The structure of these organic derivatives prove the layer character of K 4 Nb 6 O 17 . The long chain compounds form bilayers between the niobate sheets and are arranged in gauche -block structures. The gauche -blocks can change their dimensions by intercalation of small polar organic molecules.

Silylation of a crystalline silicic acid: an MAS NMR and porosity study

Tailor-made adsorbents and supports are attracting increasing interest for a wide range of advanced applications in the field of separation techniques and biotechnology. Silica gel silylated with chlorosilanes or alkoxysilanes is a well-established material in chromatography, biotechnology, and affinity separation processes. The surface of the silica gels is disordered, and complete surface coverage with silanes is not always achieved. Crystalline silicic acids from layered potassium silicates, similar to the minerals kenyaite or magadiite, are microcrystalline and their surfaces show a higher degree of regularity. They were modified with different alkyl methoxysilanes to evaluate the possibility of surface silylation. Binding of the silane molecules on surface silanol groups of the crystalline silicic acids was investigated by 29Si MAS NMR spectroscopy. The total surface area was accessible to the silylating agents and almost all surface silanol groups reacted with these molecules. The derivatives contained micropores (between the layers and, possibly, within the layers) and mesopores.

Gas adsorption by a crystalline silicic acid

Crystalline silicic acids are prepared from alkali layer silicates by exchanging protons for the alkali ions. The acid H2Si20O41 · xH2O (parent material K2Si20O41 · xH2O) exhibits some outstanding gas adsorption properties which are related to the layer structure and the interlamellar microporosity. The external surface, about 20 m2 g−1, is estimated from nitrogen adsorption data after blocking the micropores. Slit-shaped ultramicropores (with diameters similiar to that of the nitrogen molecule) between the layers are widened to supermicropores near the crystal edges. During an adsorption run the nitrogen molecules penetrate more deeply into the ultramicropores. Nitrogen molecules strongly adsorbed in the ultramicropores are not desorbed at 77 K. Additional amounts of nitrogen are adsorbed by widening of the slit-shaped micropores at the crystal edges when pressure increases. This process proceeds slowly and is reversible.

Ludwig: The Bioengineer [Retrospectroscope]

On the basis of the strict exclusion of the vis vitalis, the demand was raised by Carl Ludwig, Helmholtz, Du Bois-Reymond, and Brucke for a physiology which was causal-analytical and physically and chemically experimental. If, out of these four investigators, we pick Ludwig as the actual founder of modern physiology, the grounds for this must be justified specifically. That modern physiology is not to be contemplated without the works of the three great students of Johannes Muller is explicitly emphasized. However, Carl Ludwig occupies a special position for physiology. [1]

Selective liquid Sorption and wetting of pillared montmorillonites

Abstract Pillared montmorillonite was used as an adsorbent for binary mixtures of hydrocarbons and alcohols (benzene/n-heptane; ethanol/cyclohexane; ethanol/toluene; propanol/ toluene). The adsorbent was prepared from the Na-form of a bentonite from Milos by reaction with polyhydroxoaluminium solutions. The adsorption behaviour of the pillared montmorillonite was characterized by the adsorption excess isotherms and the heats of immersion. Sorption capacities and wetting properties differed significantly from those of the Na-montmorillonite. Benzene was adsorbed preferentially from mixtures with n-heptane. Ethanol and propanol were also preferentially adsorbed from mixtures with cyclohexane and toluene but the curves showed azeotropic points at molar fractions of alcohols of 0.7-0.9. The calorimetric data revealed the partially hydrophobic character of the pillared montmorillonite. The heat of wetting of benzene or toluene on the pillared samples was distinctly higher than that of ethanol whereas ethanol gave the highest wetting enthalpies on Na-montmorillonite.

Adsorption properties of crystalline silicas: (II) Adsorption of anionic surfactants and delamination

Lamellar crystalline silicas (crystalline silicic acids, chemical composition SiO2·xH2O; examples: H4Si14O30·xH2O, H4Si20O42·xH2O) are distinguished from the amorphous forms by their layered structure and exceptional adsorption properties. One outstanding example is the reaction with anionic surfactants. Several types of crystalline silicas (typical H4Si20O42·xH2O) can intercalate ionic pairs consisting of surfactant anion and gegen ion into the interlayer space. The saturation value of SDS adsorption is 0.475 mmol SDS/g H4Si20O42·3H2O. The acid H4Si14O30·xH2O adsorbs anionic surfactants at the external surfaces only (saturation value 0.04 mmol/g H4Si14O30·0.8 H2O). When anionic surfactants are adsorbed in the interlayer space, the layer separation increases to such an extent that the crystals disarticulate in a fan-like manner or delaminate into thinner packets of layers or smaller aggregates. Washing-out the SDS ionic pairs or drying reconstitutes the parallel layer orientation and leads to re-aggregation of the packets and fragments.

Interlamellar adsorption of alcohols 4. Adsorption properties of crystalline silicas

Abstract Crystalline silicas (crystalline silicic acids) are intracrystalline-reactive layered materials. Two types were studied: H4Si20O42·xH2O and H4Si14O30·xH2O. They show a pronounced preference of methanol and ethanol from mixtures with water, n-butanol, and toluene. The surface excess isotherms are U-shaped with long linear sections extending from x1 = 0.2 or 0.3 to x1 = 1 (x1 molar fraction of methanol or ethanol). Thus, at x1 ≥ 0.2 or 0.3, only methanol or ethanol is adsorbed between the layers. The preference of H4Si20O42 for methanol and ethanol from mixtures with water is as high as in silicalite and NaZSM-5. With longer alcohols (propanol … n-octanol), the isotherms become S-shaped, and molecules of component 2 (water, n-butanol, toluene) are also adsorbed in the interlayer spaces. Adsorption from mixtures of n-pentanol (or n-hexanol) and n-butanol produces unusual surface excess isotherms. The cause is formation of paraffin-type bilayers of pentanol or hexanol which, at a certain molar fraction of butanol, collapse to monolayers of flat-lying alcohol molecules. When the alcohol molecules lie flat in the interlayer space (basal spacing less than 2.3 nm), the volume of the adsorption phase, Vs, comprises the interlamellar volume Vint (determined by X-ray powder techniques) and the volume Vex of a few layers of alcohol molecules adsorbed at the external surface. The ratio Vs/Vint varies between 1.2 and 1.5. When the layer separation becomes larger (paraffin-type bilayers of alcohols), Vint exceeds Vs. This behaviour is also observed for hydrophobized clay minerals in the presence of polar/unpolar liquid mixtures.

Ludwig: The Physiologist [Retrospectroscope]

The thought reproduced in the above epigraph is taken from an article by Thurau et al. [1], who attribute it to Arthur Schopenhauer (17881860), an outstanding philosopher and author of the far-reaching piece Die Welt als Wille und Vorstellung (The World as Will and Representation). In German, it would perhaps read as etwas denken, das niemand vorher gedacht hat, während etwas sehen, was jeder sieht. We could not assert whether Schopenhauer really said that, but it should not be at all surprising if it were, because it sounds simple, perhaps even naïve, and very deep, indeed. It fits perfectly to Carl Friedrich Wilhelm Ludwigs personality (18161895), whom we will look at as physiologist in this second note. Yes, second note—because in the first one [2], we looked at him as bioengineer. A third and last Retrospectroscope column completing this series will deal with his wonderful and always humble and generous activities as teacher.