Chemical Reactions on Clays

Synchrotron SAXS/WAXD and rheological studies of clay suspensions in silicone fluid

Suspensions of two commercial smectite clays, montmorillonite KSF and montmorillonite K10, in a low-viscosity silicone oil (Dow Corning 245 Fluid) were studied by simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) techniques and rheological measurements. In the 0.5% (w/v) KSF clay suspension and two K10 clay suspensions (0.5% and 1.0%), WAXD profiles below 2theta=10.0 degrees did not display any characteristic reflection peaks associated with the chosen montmorillonite clays, while corresponding SAXS profiles exhibited distinct scattering maxima, indicating that both clays were delaminated by the silicone oil. In spite of the large increase in viscosity, the clay suspensions exhibited no gel characteristics. Dynamic rheological experiments indicated that the clay/silicone oil suspensions exhibited the behavior of viscoelasticity, which could be influenced by the type and the concentration of the clay. For the K10 clay suspensions, the frequency-dependent loss modulus (G") was greater in magnitude than the storage modulus (G') in the concentration range from 0.5 to 12.0%. The increase in the clay concentration shifted the crossover point between G' and G" into the accessible frequency range, indicating that the system became more elastic. In contrast, the KSF clay suspension exhibited lower G' and G" values, indicating a weaker viscoelastic response. The larger viscoelasticity response in the K10 clay suspension may be due to the acid treatment generating a higher concentration of silanol groups on the clay surface.

A novel montmorillonite-enwrapped scandium as a heterogeneous catalyst for Michael reaction

Treatment of a Na+-exchanged montmorillonite with an aqueous Sc(OTf)3 solution afforded a robust Sc3+ aqua complex enwrapped by the silicate layers of the montmorillonites (Sc3+-mont). The resulting Sc3+-mont showed outstanding catalytic activities for the Michael reaction of 1,3-dicarbonyls under aqueous and solvent-free conditions. Moreover, this heterogeneous catalyst had an advantage of a strikingly simple workup procedure over the Lewis acids reported to date and was reusable without any appreciable losses in its activity and selectivity.

Hydrolysis of tert-butyl methyl ether (MTBE) in dilute aqueous acid

tert-Butyl methyl ether (MTBE) is generally considered to be resistant to chemical transformation in aqueous solution. This lack of reactivity has led to concerns of the long-term impacts of MTBE in groundwater. Although hydrolysis in the presence of strong acids has been recognized as a mechanism for MTBE transformation, it has been discounted as a significant reaction under environmental conditions. In this study, we have examined the fate of MTBE and other ether oxygenates under moderately acidic conditions (> or=pH 1). The results demonstrate that MTBE is sensitive to acid-catalyzed hydrolysis reaction that generates tert-butyl alcohol (TBA) and methanol as products. The reaction is first-order with respect to the concentration of MTBE and hydronium ion with a second-order rate constant of about 0.9 x 10(-2) M(-1) h(-1) at 26 degrees C. Commercially available acidic ion-exchange resins were also shown to catalyze the hydrolysis of MTBE at near neutral pH. Pseudo-first-order rate constants were observed to be as high as 0.03 h(-1) at 25 degrees C and 0.12 h(-1) at 35 degrees C. These findings are discussed in terms of their possible implications for the treatment and environmental fate of MTBE and other gasoline oxygenates.

The influence of prebiotic-type organic molecules on the crystallization of Al and Mg hydroxides

It is now well accepted that clays could have concentrated prebiotic organic molecules, protected them from UV radiation and served as templates and catalysts in their prebiotic evolution. A complementary question is: How did prebiotic organics in the oceans, in ground water, or in hydrothermal solutions affect the formation and inorganic evolution of oxides, hydroxides, and clay minerals? In this study predominantly amorphous Al oxyhydroxides (Al gels) and crystalline Mg hydroxyoxides were synthesized, and then crystallized and recrystallized respectively, to Al and Mg hydroxides via wet and dry (w/d) cycling using both water and organic solutions. The products that resulted were examined using IR spectroscopy and X-ray diffraction (XRD). XRD scans of the products formed by w/d cycling of the Al gels with either water or 0.1 M aqueous solutions of methanol or formaldehyde showed that bayerite (alpha Al2O3) was the major phase formed. The acetonitrile treated sample exhibited the most defined XRD peaks, and no crystalline phase could be observed by XRD of the 0.1 M formamide solution treated sample. Cycling the Mg hydroxyoxide with water, or 0.1 M solutions of methanol, formamide, formaldehyde, or acetonitrile resulted in the formation of brucite (Mg(OH)2) (in varying amounts) and of three unidentified phases. One unidentified phase, 'phase II', was observed in the formaldehyde cycled sample (and tentatively identified in the methanol and formamide cycled samples), 'phase III' in the formamide and formaldehyde cycled sample, and 'phase IV' in only the formaldehyde. XRD peaks with a spacing of approximately 11.5 Angstrom (assigned to phase III) suggest intercalation of formamide and formaldehyde into the interlayer spaces of the brucite. Phosphate treatment, prior to w/d cycling with water, and also with the above mentioned organics, while totally preventing subsequent formation of any crystalline Al hydroxide, enhanced the formation of Mg phases, shown by XRD data. Formation of brucite was impeded only by w/d cycling using concentrated methanol solution, but even these effects were reversible. The XRD scans of the products resulting from the aqueous and organic solution cycling treatments of the Mg starting materials showed peaks due to brucite and three unidentified phases. Only the brucite is evident in oriented sample XRD scans of the Mg starting material when treated with methanol, formamide, and acetonitrile. In random powder sample XRD scans of the Mg starting material treated with: 1) methanol--the unidentified phase 11 is evident, 2) formamide--phase II and III are seen, and 3) formaldehyde--phases II, III, and IV are evident.

Influence of clay minerals on the hydrolysis of carbamate pesticides

Using batch experiments, we investigated the influence of clay minerals (montmorillonite, beidellite, illite, and vermiculite) on the hydrolysis of five carbamate pesticides: carbosulfan, carbofuran, aldicarb, pirimicarb, and chlorpropham. Compared to the other minerals, montmorillonite had the strongest influence on the hydrolysis of these carbamates. Montmorillonite enhanced the hydrolysis of carbosulfan and aldicarb. In contrast, the hydrolysis of chlorpropham was inhibited by montmorillonite, probably because of its strong adsorption on montmorillonite. The hydrolysis of pirimicarb was not affected by montmorillonite. The presence of organic substances, phosphate, and fluoride in suspensions decreased the catalytic activity of montmorillonite. Surface acidity of montmorillonite and/or formation of surface chelates are probably the key factors of surface catalysis in the case of the hydrolysis of carbosulfan.

Intersalation Reactions of Trisdiimine Metal Complexes with Montmorillonite Clay: A New Approach

Intersalation reactions of the complexes [Ni(L-L)(3)]SO(4) and [Ni(L-L)(3)]Cl(2) (L-L=1,10-phenanthroline (phen), 2,2'-bipyridyl (bpy)), with Na-montmorillonite carried out under different dynamic conditions such as ultrasonic irradiation, refluxing, autoclaving, and vigorous stirring showed that under ultrasonic irradiation, maximum adsorption of the metal complexes occurred within a period of 30 min. Metal complexes containing the phen ligand showed higher adsorption than those containing bpy. Maximum adsorption up to about three times the cation exchange capacity (CEC) of the clay was observed. Aggregated (basal spacing d(001) 12.5 Å) Na-montmorillonite, in treatment with a higher amount (>CEC) of metal complexes, formed monolayered (basal spacing d(001) 17.6 Å) species, while with predispersed clay, bilayered (basal spacing d(001)>28 Å) or pseudo-trilayer (basal spacing d(001)>32 Å) species were formed. The bilayered species were thermally stable up to 250 degrees C and above this temperature monolayered species were formed. The thermal stability of the intersalated species was higher than that of the intercalated ones. X-ray diffraction, UV-visible, IR-spectroscopy, thermal analyses, and surface area measurements were used for characterization of the products. Copyright 2001 Academic Press.

Chemoenzymatic synthesis of homochiral (R)- and (S)-karahanaenol from (R)-limonene

Terpinolene oxide, a monoterpene belonging to the p-menthane group, is easily derived from naturally abundant (R)-limonene. It was isomerized with montmorillonite clay catalyst to karahanaenone (2,2, 5-trimethylcyclohept-4-en-1-one) by ring enlargement. The enantiomers of the corresponding alcohol, karahanaenol (2,2, 5-trimethylcyclohept-4-en-1- ol), known for their individual organoleptic properties, were resolved through Pseudomonas cepacia lipase mediated enantiospecific alcoholysis of its acetate derivative.

Clay-catalyzed Conversion of 2,2-Disubstituted Malononitriles to 2-Oxazolines: Towards Unnatural Amino Acids

Acidic clay catalyzed reaction of malononitrile derivatives with an excess of 1,2-aminoalcohol furnishes mono-oxazolines selectively; a simple strategy to prepare 2,2-dialkyl-3-aminopropionic acid derivatives is presented.