Issue Date: September 14, 2009
Thin As Thin Can Be
Zeolite crystals measuring just a single unit cell in thickness can be prepared by a new synthesis based on a novel organic structure-directing agent. The technique yields 2-nm-thick stable catalyst crystals that outperform conventional bulk samples of the same type of zeolite in commercial catalytic processes.
Zeolites are microporous crystalline aluminosilicates. Because of properties such as strong solid-state acidity and structural uniformity, they are commonly used as catalysts in petroleum refining and fine chemicals synthesis. The crystals’ pore dimensions and interconnectivity impart catalytic selectivity by allowing only reactant molecules with the right sizes and shapes to pass through the pores en route to catalytically active sites. But although the pore dimensions promote selectivity by limiting access to select molecules, they hamper catalytic activity by restricting throughput.
To improve diffusion through zeolites, researchers have tried to make the crystals thinner to reduce the path lengths that reactants and products must traverse. Most of the methods, however, require postsynthesis treatment of zeolites and have not yielded crystals thinner than 5 nm.
Now, chemists Ryong Ryoo, Minkee Choi, and Kyungsu Na of the Korea Advanced Institute of Science & Technology (KAIST), in Daejeon, and coworkers in Japan and Sweden have devised a method that directly produces sheets of zeolite ZSM-5 (also known as MFI) just 2 nm thick, which corresponds to the thickness of a single unit cell. They report their work in Nature (2009, 461, 246).
The procedure exploits the unusual behavior of a long structure-directing agent (SDA)—a segmented cationic surfactant composed of a long-chain alkyl tail (C22) and a head group consisting of two short alkyl segments (C6) separated by two quaternary ammonium ions.
In typical syntheses, short SDAs guide the inorganic frame precursors to form cavities that encapsulate the SDAs. When the SDAs are later removed, they leave hollow pores and the zeolite framework.
In contrast, in the new study using the long SDA, the head groups alone direct formation of the pores and crystallization of the zeolite. The long hydrophobic tails serve a separate role: They limit the region in which crystallization can occur. Thus, only ultrathin crystals form.
In tests comparing ultrathin ZSM-5 to the bulk zeolite, the team found the nanoscale catalyst to be more active in polymer-cracking reactions and production of aromatic ketones and other fine chemicals. Similarly, in tests converting methanol to gasoline, the group found that the interiors of the ultrathin crystals’ pores remain free from carbon buildup and are significantly longer lived than the conventional form of ZSM-5.
Avelino Corma, who is a leading authority on zeolites and is based at Polytechnic University of Valencia, in Spain, predicts that the ultrathin catalysts and the associated synthetic strategy will play an important role in oil refining, cracking, and other key petrochemical processes. Commenting in the same issue of Nature, Corma adds that this study “certainly enlarges the number of possibilities for zeolites in future applications.”
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