Generation of hydrophobic zeolite X by exchange with octadecylammonium ion

It is well known that zeolites are a hydrophylic class of materials. It has been proposed that this hydrophylic nature is not due to the Si-O Si linkage, but rather to Si OH or cationic sites associated with tetrahedrally bound aluminium [1, 2]. This theory is supported by the decrease in water sorption capacity that is observed as aluminium sites are extracted from mordenite [1]. Zeolites can, therefore, be made hydrophobic by de-aluminization, but with a concomitant loss of sorption capacity for popular species. Hydrophobic zeolites prepared without de-aluminization could be useful in several applications, including the production of novel gas chromatographic columns and the prevention of particle aggregation in the aqueous slurries. The latter application was proposed for zeolites made hydrophobic by reaction of free OH sites with organosilanes [3, 4]. In addition, the modification of surface properties might make these zeolites useful as lightweight fillers with improved compactibility with hydrophobic polymeric materials. We have investigated the possibility of making the surface of zeolites 13X hydrophobic cationic exchange of long-chain alkylammonium ions (specifically n-octadecylammonium ion) for sodium ion. The alkyl end of the ion would then impart a degree of hydrophobicity to the zeolite surface. Although short chain (i.e. 1 to 6 carbon) alkylammonium ions have been exchanged into zeolites to study exchange rates and distributions as a function of ion size [5, 6], we believe this is the first use of such an exchange with long alkyl chains for the purpose of imparting surface hydrophobicity. A relative measure of hydrophobicity both with and without exchange of different alkylammonium salts was provided by the water contact angle. The effect of exchange with n-hexyl and noctadecylammonium ions could then be compared. Zeolite 13X was obtained from Union Carbide Corporation, Linde Division, as a powder without additives or binders. In order to provide a flat surface for contact angle measurements, 0.38 g powder was taken directly from its container and placed in a Perkin-Elmer KBr die (Model 186-0025) and pressed at 10 000 lb (45 000 N) for 15 sec. This corresponds to 49000psi (3.4 × 108Nm 2) across the die face. The resulting pellets were right cylinders with a 13 mm diameter and 2.5 mm thickness. Exchange was done in a 70 mm × 50 mm crystallizing dish. A solution of the appropiate salt in 100ml ethanol (AAPER Alcohol and Chemical Co, Shelbyville, Kentucky, used as-received) was added to the dish and three pellets were placed in the solution on a wire-mesh platinum basket suspended from the sides of the dish by platinum wires. The solution was stirred with a magnetic stir bar and a watch glass was placed over the dish to retard solvent evaporation. Pellets were exchanged in this way for ~ 40 h. Ethanol was used because of the good solubility of the amines and salts used. Each set of three pellets exchanged together was first weighed to ensure that each exchange solution contained the same concentration of ammonium salt per gram zeolite. The two salts used for exchange were prepared as the chlorides by acidification of the respective amines. For the octadecyl salt, 1.75 g n-octadecylamine (from K and K Laboratories, Plainview, New York, used as-received) was dissolved in 200 ml ethanol and acidified with 1 ml concentrated HC1 (Fisher, reagent grade, used as-received). This represents a 1.8:1 molar excess of HC1. The solvent was stripped off and the residue was verified as the salt by melting point. This procedure was modified slightly for the hexylamine, as addition of the water in the acid led to formation of waters of hydration with unknown stoichiometry. Anhydrous HCI was bubbled through an ethanolic solution of hexylamine (Eastman Organic Chemicals, used as-received) and the solvent stripped off. The resulting salt was stored in a desiccator to prevent the uptake of water. Contact angle measurements were made with a Ram6-Hart goniometer. A measure of the hydrophobicity of a surface can be obtained from the angle a drop of water makes with the flat surface. Unfortunately, the angle measured is dependent on several experimental variables, principally the roughness of the surface [7]. The relationship between the true contact angle, 0, and the measured angle, 0', is given by: