'Magic angle' reveals zeolite reactions.

'Magic angle' reveals zeolite reactions

Strange things happen in a zeolite. A labyrinth of molecule-wide corridors zigzags through this crystal, and within such narrow confines, chemical reactions become skewed. Chemists have learned to harness zeolites' reaction-altering properties to synthesize compounds, but they have yet to determine how the reactants yield the products.

Using a technique called magic-angle-spinning nuclear magnetic resonance, researchers at the University of Cambridge in England have solved some of the mystery lurking in these crooked corridors. In the May 18 NATURE, Jacek Klinowski and Michael Anderson describe the first clear look into a zeolite reaction.

Zeolite's networks alter chemical reactions through a process of "shape selection": The cramped quarters prevent some bulky molecules from getting in and others from getting out. The zeolite studied by the Cambridge group, ZSM-5, exerts its shape-selecting influence in a way that's especially useful for making gasoline. In a reaction converting methanol to gasoline, ZSM-5 favors products with high octane ratings. This zeolite-catalyzed reaction provides one-third of all gasoline produced in oil-scarce New Zealand, notes George Kerr of Lawrenceville, N.J., a retired chemist with the Mobil Oil Corp.

Scientists didn't understand the mechanism of the widely used reaction until Anderson and Klinowski unveiled it with a powerful technique -- a variation on nuclear magnetic resonance (NMR) spectroscopy. In NMR, atoms of sample molecules exposed to a specific radio frequency give off identifying signals as the strength of an external magnetic field changes. Each atom's "fingerprint" is largely determined by the types of atoms nearby. For instance, a carbon atom at the end of a carbon chain will create a signature different from that of a carbon atom situated between two other carbons in a ring.

NMR generally doesn't work in zeolites and other solids because of various obscurring effects that arise when solid molecules hold a fixed orientation with respect to an applied magnetic field. For each atomic bond, the obscuring effects are minimized when the bond sits at what chemists call a "magin angle" of 54.44[deg.] in relation to the magnetic field. That's where the magic angle spinning comes in. By spinning a zeolite sample around an axis point at the magic angle from the magnetic field, Klinowski says, he can cancel out enough of the damaging effects to get a clear signature for each molecule.

With this method, the researchers discerned 29 different players in the gasoline-making reaction. The distribution of compounds they observed within the zeolite was very different from the combination of products appearing at the end of the reaction. For the first time, Klinowski and Anderson detected carbon monoxide as an important reaction intermediate. They also found some compounds that got stuck in the zeolite passages, including several isomers of tetramethylbenzenes -- chemicals as bulky as their name.

Kerr suggests the Cambridge study will lead to better zeolite-mediated reactions. Chemists today are challenged with a wide choice of zeolites to use for each given reaction. "If you know the mechanism," Kerr says, "you might be able to make better choices between the many zeolites available."