The Preparation and Characterization of an X-Type Zeolite An Experiment in Solid-state Chemistry Kenneth J. Balkus, Jr., and Kieu T. Ly University of Texas at Dallas, Richardson, TX 75083-0688 Topics in solid-state chemistry are often neglected in undergraduate laboratories partly because there are relatively few experiments that capture the attention and imagination of students. The advent of high-Tc superconductors that provide dramatic yet simple experiments (1) has increased interest in solid-state materials. We have been part of a movement to introduce solid-state chemistry topics in the advanced undergraduate labs. We report an undergraduate experiment based on the syntheiis and charactrrrzat~ono f m X-twe -. zmlite (21. The synthetic X-type zeolite has the faujasite structure (3) shown below. Faujasite is the naturally occurring, but rare, mineral. The general formula is M,i,(A102),(Si02)~9~.n where n = 77-96 and x = valence of M. The faujasite structure wnsists of ten cages connected by hexagonal prisms or double six-ring (D6R)units. The P cages are oRen called sodalite cages because the structure of the zeolite sodalite (3) contains eight of these truncated octahedra connected through the four rings. The vertices in Figure 1 represent either tetrahedral A1 or Si atoms with the 0 atoms omitted f ~ r ~ c l a r i t yThis . structure transcribes a sphere of about 12 Awhich we designate a s the supercage. The 12 ring apertures to the a or supercage are about 7.4 A in diameter. These dimensions allow only certain molecules to be adsorbed by the zeolite. I n this experiment we will probe the pore size by monitoring the adsorption of various molecules. If we view zeolite as a micropourous silicate in which the Si4+has been partially substitued with A13t then we realize that for every A13+there must be an equal number of positive charges added for balance. The cations that compensate for the cage
prism
Figure 1. Structure of zeolite X.
positive trivalent charge on Al can be virtually any electropositive element, from protons to lanthanides. These cations can he located throughout the structure. I n fact, there are more cation sites than cations. In this experiment we will prepare the sodium form ofan X-tvoezenlite.Thesodium ionscan then brexchanged with othk; metal ions such a s cobalt(I1). We have found the X-type zeolite can be prepared during a typical lab period and later characterized by infrared spectroscopy, ion exchange, and adsorption. Preparation of NaX Many variables can influence zeolite synthesis: temperature, pH, crystallization time, order of mixing, amount of Si, Al, Na, and HzO. Zeolite NaX is a metastable phase, meaning that other types of zeolites such a s P, A, or sodalite may form if this recipe is not followed carehlly. The procedure detailed below should produce highly crystalline NaX within a normal lab period (2-4 h) without detectable contamination from other phases. The first procedure is to prepare sodium silicate and sodium aluminate separately. The sources of silicon and aluminum can vary, but we have found that silica gel (Aldrich)and aluminumisopropoxide (Aldrich)are suitable starting materials. Since this experiment requires working under alkaline conditions, plasticware should be used. I'olypropylene bottles shouldbe used for the rnstallizatiun berausr 1hc.v are generdly autoclavnble. Sote: Polyethylene bottles $11 crick and leak.) The sodium silicate is prepared by adding 3.0 g of silica gel, 2.4 g of sodium hydroxide, and 6 mL of deionized water to a 250-mL plastic beaker. This mixture is swirled until the solids are completely dissolved. The sodium aluminate solution is prepared simultaneously by adding 6.9 g of aluminum isopropoxide, 2.4 g of sodium hydroxide, and 9 mL of deionized water to a 250-mL plastic beaker. The mixture is stirred with gentle heating (below 80 ' C ) in a water bathuntil the solids are dissolved (about 10 min) and a clear gel is formed. A watch glass placed over the beaker should prevent loss ofwntcr. (Note: The isopropanol furmed during i h e hydrolysis of aluminum isopropoxide will evaporate or form droplets a t the surface. There is no need to separate the alcohol from the gel.) When both solutions are a t room temperature, the aluminate solution is added to the silicate solution. An additional 27 mL of water is added
Volume 68 Number 10 October 1991
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