Abstract--Adsorption of neutral organic molecules and the monovalent organic cations methylene blue (MB) and crystal violet (CV) to sepiolite was determined experimentally and investigated by an adsorption model. The largest amounts of MB and CV adsorbed were about 4-fold of the cation exchange capacity (CEC) of sepiolite. Consequently, it was proposed that most of the above described adsorption was to neutral sites of the clay. The adsorption model considered combines the Gouy-Chapman solution and specific binding in a closed system. The model was extended by allowing cation adsorption to neutral sites of the clay, in addition to adsorption to negatively charged sites and adsorption to neutral complexes formed from 1 cation adsorbed to a negative surface site. The amount of available neutral sites was determined from the adsorption of the neutral molecule Triton-X 100 (TX100). The model could adequately simulate the adsorption of the neutral molecules TX100 and crown ether 15-crown-5 (15C5) as well as the organic cations. Due to aggregation of MB molecules in solution, their adsorption was somewhat less than that of CV at the larger added concentrations. A consideration of the molecular dimensions of TX100, MB and CV suggested that their adsorption was mostly to external sites of the clay and that their entry to the sepiolite channels was largely excluded. This interpretation is supported by infrared spectroscopy (IR) measurements, which show large perturbations of the peak corresponding to vibrations of external Si-OH groups of the clay and confirm complete occupancy of external sites by MB and CV.

The characterization, pillaring ability and some catalytic properties of a saponite from the Vicálvaro deposit of Tolsa, located in Madrid have been studied. This mineral is associated with sepiolite in the same lacustrine environment. The raw saponite presents, as special characteristics, high specific surface area (>200 m2/g) and a high percentage of tetrahedral Al (90% of the total Al in the sample). Natural saponite gives rise to high conversions when used as a catalyst in the reaction of alkylation of benzene with 1-dodecene. Increased catalytic performances are obtained over the alumina-pillared saponite derivatives. These materials have also been tested in the cracking of gas-oil (microactivity test), giving good conversions and high selectivity towards the gasoline fraction, especially the pillared forms.

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