"THE COMBINATION OF significant solvent replacement [up to 80%] with CO₂, much higher turnover rates, moderate reaction pressures, and facile catalyst separation makes these processes both environmentally and economically appealing," Busch concluded.

One of the promising developments in green catalysis has been the preparation of tetraamido macrocyclic ligand (TAML) iron(III) activators by Carnegie Mellon University chemistry professor Terrence J. Collins. In 1999, Collins received a Presidential Green Chemistry Challenge Award for demonstrating that TAML activators significantly increase the oxidizing ability of hydrogen peroxide and that the TAML/H₂O₂ system can be used in a variety of commercial applications.

In two talks in Orlando, Collins described his group's ongoing efforts to study the properties of TAML activators and elaborated on several applications being developed. Areas under investigation include replacement of chlorine or chlorine dioxide in wood pulp processing, paper and textile mill effluent decolorization, laundry applications, decontamination of chemical and biological warfare agents, and oxidation of sulfur contaminants in petroleum refining.

"TAML activators are water soluble, and different examples allow access to a broad pH range [1 to 13]," Collins noted. "The activators are relatively easy to synthesize, and they function well in nanomolar to low-micromolar concentrations." The activators can be readily modified to achieve a desired selectivity, he added, and for most applications minimal capital costs will be needed for their implementation. Initial lab tests indicate that the TAML activators have low toxicity, but complete toxicology studies to check environmental persistence and bioaccumulation are still needed, Collins said.

COLLINS REPORTED that his group has most recently used TAML activators for the rapid total destruction of chlorophenols [Science, 296, 270 and 326 (2002)]. Chlorophenols, which are recognized by EPA as persistent environmental pollutants, are commonly used in pesticides, wood preservatives, and personal care products. They are also a by-product of wood pulp bleaching.

Bacteria and fungi can be used to break down chlorophenols, Collins noted, but that process takes days. In addition, when peroxidase enzymes are used by microorganisms to digest chlorophenols, toxic chlorinated organics such as dioxins and dibenzofurans are formed.

Several chemical methods to degrade chlorophenols are already known, Collins added. For example, he pointed to "seminal work" by chemist Bernard Meunier of the National Center for Scientific Research, in Toulouse, France, on the use of H₂O₂ with an iron phthalocyanine catalyst. However, Meunier's system requires a larger amount of catalyst and a cosolvent, Collins said, and the chlorophenol destruction is much less complete.

Less than 10 µM of a TAML iron activator can promote 0.5 M H₂O₂ to degrade millimolar aqueous solutions of pentachlorophenol or 2,4,6-trichlorophenol to nonhazardous products, Collins said. The process, which takes about 10 minutes under ambient conditions, converts nearly 99% of the chlorophenols to a mixture of CO, CO₂, and HCl along with biodegradable chlorinated and nonchlorinated C₁ to C₄ organic acids. After further treatment, most of the remaining small chlorinated organics are degraded and no measurable amounts of dioxins are observed.

"TAML activators have been developed around a different design concept compared to what nature uses in producing enzymes," Collins told C&EN. "Nature designs around the big idea that any desired reaction can be sped up in the presence of myriad simultaneously occurring reactions. In contrast, we have followed the central design concept that reactions we do not want to occur in an oxidation process can be slowed down without suppressing the rate of the targeted reactions. After 20 years of catalyst design work, we are learning that this approach has considerable merit for solving problems in green oxidation chemistry."

The theme of several sessions in I&EC's program was developing greener technologies to help lessen the environmental impact of the production and use of chemicals for agriculture and agriculture-based technologies. The sessions were part of the first effort at an ACS meeting "to bring green chemistry and agricultural practices on the same page," according to co-organizer William M. Nelson, a researcher at the Illinois Waste Management & Research Center, Champaign.

"One of the important areas under the umbrella of green chemistry is going to be how researchers, government agencies, and industry work together to develop sustainable agriculture to provide food and renewable resources for a growing population," noted Nelson, whose research includes microbial degradation of hydrocarbons in waste streams.

Although there have been some great technology ideas derived from environmentally benign chemistry over the years, he added, often these ideas fall flat economically when it comes to industry's bottom line. But persistent work and continued innovations are now helping to diffuse these greener technologies into industry, Nelson said.

BLACK WATER The Tarawera River in New Zealand, flowing into the Pacific Ocean, is discolored by residual lignin discharged in treated wastewater from a paper mill. In a field trial, Collins and coworkers are using a TAML activator to promote H₂O₂ to further degrade the lignin, resulting in a 50% reduction of color in the wastewater.

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Copyright © 2002 American Chemical Society