Researchers Invent Innovative Technique to Synthesize Chiral Molecular Sieves

Mark Davis (Photo by Seth Hansen for Caltech)

In a finding that means a lot to the specialty chemicals industries such as pharmaceuticals, a research team including scientists from the Rice University as well as the California Institute of Technology has successfully accomplished a long-pending analysis of producing right-handed and left-handed versions of a molecular sieve, the highly used industrial, solid materials.

Every year, chemical plants use molecular sieves containing zeolites, or silicate minerals, to produce millions of tons of different products such as gasoline and diesel fuel, as well as to obtain purified oxygen from air. Regardless of their omnipresent nature, the use of molecular sieves at present is naturally restricted as they lack “chirality,” that is, handedness. Similar to a man’s right and left hands, certain molecules exist as mirror opposites, specifically molecules produced by living cells.

Chirality is particularly important in biology,” stated biophysicist Michael Deem, co-author of a paper that describes the discovery that will be published online in the Proceedings of the National Academy of Sciences Early Edition this week.

DNA is right-handed. Amino acids are mostly left-handed. Sugars are right-handed, and so on. Most industrial and organic chemistry that humans can easily perform synthetically is not chiral. When chiral compounds are produced, most often they are made in an equal mix of right- and left-handed versions, or enantiomers. Because the proteins in our bodies are chiral, they can react very differently to a right or left enantiomer of a drug. For example, one enantiomer of thalidomide reduces nausea and the other causes birth defects.

Michael Deem, Professor of Bioengineering, Rice University

One such industrial example in which chirality plays a vital role is polylactic acid (PLA), which is the elementary unit of biodegradable plastic. Athletes have a good understanding of lactic acid, which is a natural compound that causes “burning” of muscles while carrying out strenuous exercises. However, plants, animals, as well as humans produce left-handed lactic acid, and it is this enantiomer that is biodegradable. As bacteria cannot digest the right-handed enantiomer of PLA, biodegradable plastic mandates the use of enantiomerically pure PLA, which cannot be easily produced by using prevalent industrial chemical techniques.

Research has been going on for over 30 years to make chiral molecular sieves because it was assumed that chiral molecular sieves would be able to perform chiral catalysis and separation, and allow for new ways to make chiral products in bulk at lower costs. We present the first convincing evidence of a chiral molecular sieve, and show that these materials can perform chiral catalysis and separations. With our co-authors at Rice, we have demonstrated a methodology that can in principle produce many different chiral molecular sieves, each with unique properties.

Mark Davis, Chemical Engineer, Caltech

Molecular sieves resemble Swiss cheese when observed at the microscopic level as they have interconnected pores. All molecular sieves termed zeolites are made of aluminum, oxygen, and silicon. Although the shape and size of their pores vary, their size does not exceed 2 nm. These pores render the sieves very helpful to chemists since molecules of only a specific shape and size can pass through them. Moreover, the pores can also function as catalytic reaction chambers in order to spur the production of particular chemical products.

Deem, who is a John W. Cox Professor of Bioengineering and a professor of physics and astronomy, has spent over 15 years in developing computational techniques for identifying and designing zeolites. In his lab, he has not only computationally identified the best zeolites for eliminating carbon dioxide from power plant exhaust but also developed a database containing nearly 2.6 million potentially synthesizable zeolites. Deem stated that the study of the chiral molecular sieve along with Davis’ team was the result of a 2013 research in which he, Rice visiting research scholar Frits Daeyaert, and other collaborators developed a computational technique for identifying small organic molecules that can be potentially applied for synthesizing zeolites that have tailored characteristics such as chirality.

Deem and Daeyaert designed the chiral organic molecules for this research at Rice, where chemical assistance was given by Davis and Caltech co-author Joel Schmidt. While Davis’ colleagues from Caltech used the chiral organic molecules to induce chiral molecular sieve synthesis, the Caltech team carried out catalytic and adsorption investigations to ensure the chirality of not only molecular sieve samples but also the catalytic and adsorption products produced using these samples. Deem stated that the chiral molecular sieve samples were synthesized as the right-handed or left-handed version of a man-made molecular sieve that had earlier been synthesized only as equal mixtures of left and right versions.

The chiral organic molecule designed for the investigations at Caltech was one among a dozen such molecules synthesized in a yearlong computational study at Rice.

Several of the other candidates may also prove useful, and we’ve begun working with our partners to synthesize some of these other candidates with an eye toward producing additional chiral molecular sieves, each with slightly different properties,” stated Deem.

He added that zeolites and chiral molecular sieves similar to those discovered in this research and those that the team is still searching for can be highly useful in synthesizing enantiomerically pure drugs, biodegradable plastics, and other compounds that cannot be easily produced by means of nonchiral chemistry.

Right now, there are specialty chemicals that are chiral, but more and more in the future, there will be bulk commodities that will be made from chiral materials. Chiral synthesis, particularly on hydrocarbon-based chemicals, is one of the things that chiral zeolites could make economically feasible.

Michael Deem, Professor of Bioengineering, Rice University

Stephen Brand and Marat Orazov from Caltech, and Yanhang Ma and Osamu Terasaki from the ShanghaiTech University are the additional co-authors of the study. Chevron, the Department of Energy, and ShanghaiTech supported the study.

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