By Lynn Yarris, LCYarris@lbl.gov
Researchers in LBL's Structural Biology Division (SBD), led by chemist Heinz Frei, have developed a technique in which red light is used to convert abundant hydrocarbons into valuable chemical compounds at no cost to the environment.
A central theme of current catalysis research is the redesigning of the industrial processes used to manufacture plastics and other synthetic materials.
Frei explains, "Many current synthesizing processes in industry create problems for the environment. They also waste energy. Industry needs new processes that are product specific, environmentally benign, and energy efficient."
Most synthetic materials are made through the oxidation of small hydrocarbons, one of Nature's most plentiful raw materials. For the past several years, Frei has been exploring the use of red and near-infrared radiation to initiate hydrocarbon oxidation in a tightly controlled fashion. Because red and near-infrared photons are the least expensive to artificially produce, this photochemical technique is economically competitive with other catalytic methods.
In earlier work, Frei and coworkers achieved oxidation reactions in a "cryogenic matrix environment," where potentially reactive molecules were immobilized in a frozen gas and irradiated with red and near-infrared photons. These photons are substantially lower in energy than the blue or ultraviolet light most often used in photochemistry.
"The use of low energy photons gave us access to low energy reaction pathways," Frei says. "This gave us product specificity because we were below the dissociation energies of the reaction partners. Excitation above dissociation energies often leads to unwanted byproducts."
Now, working with Fritz Blatter and Hai Sun, Frei has developed a new technique for selective hydrocarbon oxidation that not only permits the use of red and near-infrared photons, but also enables photochemical reactions to take place at room temperature. What makes this possible is the immobilization of reactants not in a cryogenic matrix, but in molecular-sized cages of inert solid materials called zeolites.
Zeolites are alumino silicates used widely in the petrochemical industry as catalysts. They feature unique pore structures inside which smaller molecules can be imprisoned. Using a synthetic version of a natural zeolite called faujasite, Frei and his colleagues successfully held and photo-oxidized small alkene molecules into industrially important chemical building blocks and intermediaries such as hydroperoxides, carbonyls, and epoxides. This was accomplished without the customary release of carbon dioxide, the chief contributor to global warming and the greenhouse effect.
To photo-oxidize their alkene molecules, Frei and Blatter loaded them as a gas into zeolite pellets and added oxygen. Reactant molecules paired with one another and were held together by the zeolite cages in close contact so that irradiation with red light triggered oxidation.
Says Frei, "Zeolites offer an ideal environment for the formation of new complexes at high concentrations. Excitation of these complexes with light allows us to access reaction surfaces and, in some cases, products that are not attainable through conventional thermal activation."
Another advantage in using zeolties, Frei points out, is that their inherently large electrostatic fields act to stabilize the excited charge-transfer energy states of hydrocarbon-oxygen complexes. This allows zeolite photochemistry to be performed with light from a conventional tungsten lamp rather than a laser.
Since practically all of the polymers from which come today's synthetic materials are manufactured from chemical building blocks that are themselves produced by oxidation of hydrocarbons, Frei's red-light photochemistry research has far-reaching industrial potential. He and his colleagues are now studying the application of their technique to organic compounds derived from hydrocarbons.