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Analytical Chemistry

Lattice Mismatches Limit Zeolites

Chemists apply microscopy and spectroscopy methods to probe crystal defects that obstruct the diffusion of molecules through catalyst pores

by Mitch Jacoby
September 28, 2009 | APPEARED IN VOLUME 87, ISSUE 39

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Credit: Utrecht U
Molecules diffusing though neighboring regions of a zeolite lattice (pink single arrows) can be hampered by dead ends that arise from complete misalignments (green arrows) and minor mismatches (pink double arrows).
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Credit: Utrecht U
Molecules diffusing though neighboring regions of a zeolite lattice (pink single arrows) can be hampered by dead ends that arise from complete misalignments (green arrows) and minor mismatches (pink double arrows).

Imagine running deep into a maze through twisting, turning passages and slamming into a dead end—and then having to back up to get out. That's what happens much of the time to molecules as they diffuse through the pores and channels of zeolite crystals, according to Lukasz Karwacki, Marianne H. F. Kox, and Bert M. Weckhuysen of Utrecht University, in the Netherlands, and coworkers (Nat. Mater., DOI: 10.1038/nmat2530). Zeolites are microporous crystalline aluminosilicates that are widely used as catalysts in petroleum refining. Diffusion barriers that obstruct the flow of reactant and product molecules through the crystals' internal passageways limit the throughput and overall usefulness of zeolites. Yet until now the nature of those barriers had not been studied in detail. By applying a battery of microscopy and spectroscopy methods to probe the internal architectures of several ZSM-5 zeolite crystals, the researchers found that the barriers result from a variety of mismatches among lattice subunits. In some cases, would-be interconnecting channels in neighboring lattice regions are dead ends due to a 90° misalignment, the team says. In other cases, diffusion is hampered by lattice mismatches as small as 0.5°.

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