Into the Void: Porous crystals could do more chemistry

Industrial chemistry worth billions of dollars unfolds within the pores of crystal catalysts, and it’s the size of those pores that determines what particular reactions can occur. Now, chemists have devised a new approach that creates crystalline material with some of the largest pores yet.

RICH PORES. A new crystal with large pores, as depicted in this diagonal slice, could lead to a wide range of industrial catalysts. Zou

Most methods for making porous crystals include a catch: Achieving larger pore sizes comes at the expense of the ordered structure. For many applications, it’s important to know the precise position of every atom, which is possible in crystals but not in disordered structures, notes Xiaodong Zou, a physicist and chemist at Stockholm University.

Zou, chemist Michael O’Keeffe of Arizona State University in Tempe, and their colleagues had been synthesizing porous crystals made with germanium oxide. The basic units of these molecular structures contain a germanium atom bound to four or six oxygen atoms to make a tetrahedron or an octahedron, respectively.

Unlike basic units of silicon oxide and other metal oxides in widely used porous crystalline materials, the germanium-oxide units form secondary structures that contain six tetrahedra and four octahedra. These secondary structures assemble into the large-pore architecture.

This provides “one more level of complexity and one more level of scale” than crystals typically have, O’Keeffe says. He and his colleagues describe the new structure in the Sept. 29 Nature.

The germanium-oxide crystal has two large-pore networks within it. Each one resembles a helical tunnel, with additional tunnels branching off in different directions. The tunnels are 1 nanometer wide at their narrowest point and just over 2 nm across at their widest, a size range that could accommodate larger molecules than most porous crystals can.

This work “illustrates that very large-pore material can be rationally designed” by linking secondary structures, comments Thomas Pinnavaia, an inorganic chemist at Michigan State University in East Lansing. The secondary structures “are unique, and the resulting hierarchical structure they form is unique,” he says.

Large-pore crystals could be a boon to oil refining, notes Pinnavaia. The catalytic crystals now used to convert crude oil to gasoline exclude the largest oil molecules. These high–molecular weight components end up in asphalts and roof shingles, but larger-pore catalysts could convert them into fuels, which are more valuable. “This … is a step in that direction,” Pinnavaia says.

The tunnels of the two networks in the germanium-oxide crystal corkscrew in opposite directions, one with a left-handed twist and one with a right-handed twist. The researchers have made a version of the crystal with one of these networks blocked off. In theory, such a crystal could be used to make drug molecules that are themselves either right- or left-handed, says Galen Stucky, a synthetic-materials chemist at the University of California, Santa Barbara. Often, a drug’s function depends on the molecule’s handedness.

Aimee Cunningham is the biomedical writer. She has a master’s degree in science journalism from New York University.

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