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Physical Chemistry

Intrazeolitic chemistry creates new molecules

Chemists use crystallography to spy unusual compounds that form when gases interact with silver in zeolite pores

by Stephen K. Ritter
March 14, 2016 | A version of this story appeared in Volume 94, Issue 11

CORRECTION: On March 18, 2016, the image captions in this story were updated to correct the quantum dot composition in the top image (Ag4Br4) and the reduced form of the benzene mimic in the bottom image (C3H3O3+).
Credit: J. Phys. Chem. C
This image depicts formation of Ag4Br4 quantum dots within the pores of a silver-laced zeolite treated with a methanolic KBr solution.
An extended array of zeolite pores filled with Ag4Cl4 clusters.
Credit: J. Phys. Chem. C
This image depicts formation of Ag4Br4 quantum dots within the pores of a silver-laced zeolite treated with a methanolic KBr solution.

For the past several decades, Karl Seff of the University of Hawaii and his coworkers have been conducting intrazeolitic chemistry in which they use X-ray crystallography to watch what happens inside the tiny pores of zeolites as they are heated or chemically treated. Put another way, Seff says, “it’s doing chemistry in a 1-nm test tube.” The research has paid off with observations that could be helpful in understanding how the industrially important catalysts and sorbents work, as well as with the discovery of a few unexpected new molecules (J. Phys. Chem. C 2016, DOI: 10.1021/acs.jpcc.5b11490). The Seff team’s work has centered on the synthetic sodium aluminosilicates known as zeolites A and X in which some sodium ions are replaced by silver ions. In one observation, the researchers noticed that the crystallinity of the material waxes and wanes depending on the dynamics of oxygen atoms in the zeolite framework and the presence of O2. Furthermore, the team observed formation of quantum-dot-like silver halide clusters and octahedral Ag6 clusters within the pores under various conditions. Seff notes that Ag6 is the smallest possible single crystal of silver that can form. When passing ammonia through the silver-exchanged zeolites, the researchers observed formation of unprecedented bent N3H5 and cyclic N3H3. And with methanol, they saw cyclic C3H3O33+—a new molecule that is electronically equivalent to benzene. Seff cautions that this menagerie of molecules may only be spotted within the pores of zeolites and be impossible to isolate outside those confines.

Structure of cyclic C3H3O33+ and a model of how its dimer packs inside a zeolite pore.
Credit: J. Phys. Chem. C
When treated with methanol, a silver-containing zeolite generates the benzene mimic C3H3O33+. When reduced to C3H3O3+, the molecule forms a dimer that neatly fits into a zeolite pore (right).


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