In the April 18 issue of the journal, a research team headed by K. Yamamoto describes for the first time incorporating organic materials into the structure of zeolites, silicate-based inorganic crystalline materials that contain pores approximately the size of small molecules.
Replacing portions of the zeolite structure with the right organic groups could create new sites for catalytic activity that would allow the materials to promote a broad range of novel reactions with potentially improved selectivity, says a Georgia Institute of Technology researcher who authored a commentary article to be published in the same issue.
"Making catalysts that are as active as enzymes and as selective as enzymes has been a long-standing goal of the scientific community, but it is very difficult to do because enzymes are made up of complex sequences of amino acids that give them very specific structures and active reaction sites," said Christopher Jones, an assistant professor in Georgia Tech's School of Chemical Engineering. "This development is interesting because now the actual framework of a zeolite can be made up of organic groups, in the same way that enzymes are made up of organic groups."
The engineered structures can't yet approach the versatility and efficiency of natural enzymes because their catalytic activity comes only from the metal ions that are part of their structure. While chemical engineers have had success altering the chemical activity by varying the metal groups in the zeolites, incorporating organic structural components should open up a broad range of new possibilities.
"Zeolites are not great enzyme mimics at this point because they are inorganic materials," Jones added. "But every time you can incorporate more organic species into them, you move them closer to being true enzyme mimics."
In 1998, Jones published a paper in Nature that described using covalent bonding to attach organic groups to zeolite structures. Though it was a significant advance in catalysis, the process could only be used with certain zeolite materials - and the organic groups tended to clog the tiny pores where the chemical reactions take place.
Incorporating organic groups directly into the zeolite structure should eliminate those disadvantages, Jones said.
The methyl and methylene groups the Japanese scientists incorporated into their first-generation zeolite structure aren't catalytically active, so replacing them with active groups will be high on future research agendas. "To make an organic-active site, they would have to change the organic group to have a chemical functionality that would promote a catalytic reaction," Jones noted.
But simply incorporating the organic materials into the complex zeolite structures could significantly change the way they work.
"All of the zeolite structures have different dynamics and different behaviors that are dictated by the size of the pores and their flexibility upon heating," Jones explained. "Those are all characteristics that will be changed by replacing inorganic groups in the lattice with organic ones. The dynamics in these silicate structures will probably be strongly affected, providing a future area of research."
Beyond their use as catalysts, zeolites can also be used as adsorbents. For that application, the methyl groups used by the Japanese researchers could facilitate the separation of chemicals that are now difficult to separate. Jones predicts that may provide the nearest term application for the new organic zeolites.
"If you can tailor the organic groups inside the micropores of a solid such that they have a specific affinity for another type of molecule, you might be able to selectively adsorb that molecule and induce a separation," Jones noted. "Catalysis is probably a least a generation off with these materials because the organic groups that they have incorporated are not catalytically active. But adsorptive applications could occur with this first generation of materials because the organic groups are already suitable."
The specific adsorption ability may also help improve the catalytic efficiency of the metal-based reaction that have already made zeolites useful. Before these catalytic reactions can occur, the molecules must be adsorbed into the pores of the zeolite. If the organic groups can facilitate that process, they could improve the activity of existing zeolite materials.
Zeolites were first used as catalysts industrially in 1962. Since then, research has focused on understanding their structure and function, and altering their chemical properties. The chemical activity of zeolites usually comes from the aluminum in the silicate lattice, which induces a negative charge on the oxide framework. That charge is balanced by positively charged sodium, potassium or hydrogen ions in the pores. The cations provide zeolites with their ion-exchange capacity.
In their Science article, the Yamamoto group reports using its new process to produce aluminosilicate and pure silicate zeolite structures in which up to 30 percent of the silicon atoms are functionalized with methyl or methylene groups. That represents a significant step forward, Jones said.
"Enzymes are Nature's catalysts, promoting all the important reactions that occur in the body," he concluded. "With this work, chemical engineers are slowly moving zeolite materials closer to true enzyme mimics."
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