Infrared spectra of adsorbed pyridine have been used to obtain qualitative information on the nature of the interaction of pyridine with different rhodium catalysts supported on palygorskite and silica. Based on these data, qualitative definitions of the adsorption sites of these catalysts have been deduced. The catalysts were prepared with natural palygorskite, and palygorskite dehydrated in vacuo at 150°C and 400°C. In this way, catalysts were obtained that had different water contents and, therefore, different acidities. Lewis-bound pyridine was detected by infrared spectroscopy from room temperature to 500°C. The greatest acidity was found in a catalyst prepared with the palygorskite support dehydrated at 150°C before preparation of the catalyst. With this catalyst, strongly hydrogen-bound pyridine was observed when evacuation was carried out at temperatures between 150°C and 300°C. Catalysts prepared with the palygorskite support pretreated at 400°C did not exhibit strongly hydrogen-bound pyridine, and Lewis acidity decreased significantly. As expected, hydrogen-bound pyridine was also detected for rhodium supported on silica. However, it was desorbed at temperatures below 150°C. The results of the acidity studies follow the same pattern as those for 1 -hexene double-bond migration under hydrogénation reaction conditions.

Observation of quadrupolar splittings for 2H and 17O nuclei from D2O solvent molecules in suspensions of the Ecca Gum BP bentonite points unambiguously to the ordering of water molecules with respect to a static magnetic field. The application of the magnetic field apparently orients the clay platelets as tactoids. Some water molecules adhere to the platelets and are oriented. The magnitude of the quadrupolar splitting depends strongly on the nature of the interstitial cations. The sign of the splitting changes as the ratio of divalent to monovalent cation increases, reflecting a switch in the microdynamics of water molecules next to the plates. With the entry of divalent cations, the clay becomes more cohesive due to the vertical electrostatic attraction of the counterions to the charged plates. More of these hydrated divalent ions compared with monovalent ions condense onto the charged sheet, thereby increasing metal coordination to those water molecules in contact with the clay surface. The dominant reorientation mode of the water molecules switches from rotation of water molecules around hydrogen bonds to the charged sheets to rotation around the electrostatic bond to the divalent metal cation. The effect is spectacular and important in understanding the chemical properties of cation-exchanged montmorillonites.

The effect of the surface acidity of montmorillonite and kaolinite on the hydrolysis rate constants of different agricultural chemicals was studied at variable moisture contents. Ethyl acetate, cyclohexene oxide, isopropyl bromide, 1-(4-methoxyphenyl)-2,3-epoxypropane, and N-methyl p-tolyl carbamate were chosen as representatives of classes of chemicals that exhibit acid-catalyzed, base-catalyzed, and neutral hydrolysis. In addition to being commercially available in pure forms, these chemicals have well-characterized homogeneous kinetics. The presence of montmorillonite or kaolinite in water suspensions had a small effect on the hydrolysis rate constants (kh), whereas, the addition of moisture to oven-dried clays up to the limits of sorbed water resulted in an increase in the rate of hydrolysis of the epoxide by a factor of 10.

The increase in the hydrolysis rate constant suggests that the surface pH of montmorillonite or kaolinite might be 1–2 pH units lower than the bulk pH. The kh value for the carbamate on Na-montmorillonite surface (bulk pH = 8.5) is 6.4 × 10−8/sec which is equivalent to the rate constant at pH values ≤7. The hydrolysis rate constant of the epoxide was reduced by a factor of 4 when the moisture content exceeded the limit of sorbed water. The addition of humic acid to the clay minerals resulted in about a 40% reduction of the epoxide hydrolysis rate constant.

A series of reduced-charge (RC) hectorites were prepared by multiple heat (250°C) treatments of Mg-saturated hectorites (SHCa-1 ). Cation exchange capacity (CEC) measurements and alkylammonium exchange indicated that a decrease in layer charge occurred with each Mg-250 treatment. Chemical analyses showed that decreases in structural Li and increases in structural Mg contents coincided with charge reduction. Fluorescence measurements of adsorbed quinoline indicated that the hectorite surface was acidified during charge reduction; hydroxyl group deprotonation is a possible source for the acidity. Fourier transform infrared spectra (FTIR) indicated that the Mg-250 treatment induced the loss of structural Li and shifted the SiO stretch band to a position similar to that in talc. The relative intensities of the OH and SiO stretch bands in FTIR spectra suggest that some of the hydroxyl groups in hectorite were lost, possibly by deprotonation. However, thermogravimetric data (TG) reveal no significant difference in the hydroxyl contents of the hectorites.

The FTIR spectra, CEC, layer charge, chemical, and TG data all supported the view that Mg substitution for octahedral Li occurred which resulted in a more “talc-like” structure. Charge reduction in smectites is evidently a general phenomenon and can be induced by heat treatment with the proper exchangeable cation. The ability to reduce the charge of hectorites makes it possible to prepare a series of clays which vary in charge but lack structural Fe. Such RC smectites should be suitable for expandable clay mineral studies which utilize spectroscopic techniques that are sensitive to Fe content.

Redox properties of iron-bearing mineral surfaces may play an important role in controlling the transport and transformation of pollutants into ground waters. Suspensions of seven iron-bearing minerals were reacted with pH and redox indicators under anaerobic conditions at the pH of the natural suspension. The responses of the indicators to the mineral surfaces were monitored by UV-visible spectroscopy using a scattered transmission technique. The Hammett surface acidity function (Hs) and the surface redox potential (Ehs) of these iron-bearing minerals were measured. These measured values were used to calculate Eh values for the seven minerals: goethite = +293 mV; chlorite = +290 mV; hematite = +290 mV; almandite = +282 mV; ferruginous smectite = +275 mV; pyrite = +235 mV; and Na-vermiculite = +223 mV. Calculated surface redox potentials of minerals are different from their potentials measured by platinum electrode in bulk suspensions. UV-visible spectroscopy provides a quick and non-destructive way of monitoring organic probe response at the mineral surface.

Inverse gas chromatography at infinite dilution, employing alkanes and alkenes as probes, has been used to characterize the surface properties of a series of smectites of varying chemical composition. The results of this study show that the acidic centers and the interlayer distances have a great influence on the specific interaction of the smectite surface with π-bonds of alkenes. High values of the specific interaction parameter, ɛπ, are caused by the existence of strong acidic centers that are connected with interlayer cations as well as with the chemical structure of the mineral sheets. On the other hand, alkanes, whose interaction with the smectites is predominantly dispersive, are unaffected by changes in the clays’ composition and/or structure.

The nature and number of acid sites per unit weight on a series of materials obtained by interaction of a montmorillonite with zirconium or cerium hydrogenphosphates precipitated in situ by reaction between their precursors have been investigated.

The quantitative determination of the surface acidity has been carried out by three different methods: titration with triethanolamine in aqueous media; TG analysis of the samples after n-butylamine treatment and vacuum desorption; and chemisorption of NH3 at 239.8 K. Additional information about the nature of the surface acid sites has been obtained from the IR spectra of the samples with bases adsorbed.

Results show that the acid site density on the montmorillonite-cerium or zirconium phosphate cross-linked compounds is greater than on the parent montmorillonite and increases as the content in tetravalent metal phosphate rises throughout the different series. Also the number of acid sites for the cerium phosphate-montmorillonite materials is lower than for zirconium ones and the characteristics obtained depend on the bases used for their evaluation.

The presence of two IR adsorption bands at 1400 and 3145 cm−1, assigned to the NH4+ ion, and the absence of the 1170–1361 cm−1 bands, characteristic of the NH3 adsorbed on a Lewis site, strongly suggest the Brönsted character of the acidity of these compounds.

The Hançılı bentonite from Turkey shows significant changes in surface area, micro- and mesoporosity, surface acidity and acid strength with heating from 100 to 900°C for 2 h. The specific surface area (S) and specific micro-mesopore volume (V) of the original and heated samples were evaluated from N2 adsorption and desorption data, respectively, by standard methods. The adsorption of n-butylamine from the solution in cyclohexane on the samples was used to determine the total surface acidity (nm) and the adsorption equilibrium constant (K) was taken as a measure of the acid strength. S, V and nm having initial values of 98 m2g−1, 0.080 cm3g−1 and 4.8 × 10−4 mol g−1, respectively, stayed approximately constant as the temperature increased to 450°C and then decreased almost in parallel with each other, reaching their minimum or zero at 900°C. The total surface acidity, in general, declines with increasing temperature. The most acidic sites, however, increase with heating, and especially at dehydration and dehydroxylation. Acid strength reaches its maximum during the dehydroxylation phase at ∼600°C. It was concluded that the total surface acidity does not necessarily parallel the strength of the most acid sites.

Suspensions of thirty six bentonites were equilibrated for 24 h and then adjusted to pH 3. After an hour, pH titrations up to pH 12 at varying ionic strengths were performed within 4.5 h. The titration data were recalculated into proton affinity distributions (PAD) using the condensed approximation. In spite of widely varying chemical compositions, all bentonites showed a protonation reaction between pK 4 and 5, which could be assigned to aluminol groups at the edge surfaces of the smectites. The average pK is 4.73±80.31. A second prominent peak in the PAD at pK = 10.43±80.13 could be assigned to exchangeable Mg2+ in the interlayer space. The possibility to constrain both pK values at a fixed average value will decrease the number of parameters in surface complexation modelling and thus enhance its convergence.