Issue Date: July 12, 2004
Why do chemical companies and academic researchers make the effort to develop greener products and processes? This rhetorical question, asked recently by American Chemical Society President Charles P. Casey, has a long answer: technology breakthroughs, competitive advantage, corporate culture, internal expertise, and public pressure. These are all drivers for innovation, he said. Ultimately, the desire to be a good corporate citizen and to make a commitment to sustainability also is an important part of the motivation, he added.
Indeed, all those influences are at the heart of the technologies developed by this year's winners of the Presidential Green Chemistry Challenge Awards.
In the ninth edition of these popular annual awards, four companies and a pair of individuals were honored during a ceremony held on June 28 at the National Academy of Sciences in Washington, D.C. The ceremony took place on the eve of the 8th Annual Green Chemistry & Engineering Conference, which this year had a theme of "The Business Imperative for Sustainability." The conference featured talks by the award winners along with plenary sessions and case studies on the incentives and barriers to adopting greener technologies.
Casey commented during the awards ceremony that the green chemistry technologies being recognized are "remarkable for their innovative approaches to pollution prevention." One challenge that the chemical profession faces is making the connections between chemistry and daily living, he said. This year's winners do just that with their impact on everyday consumer products, such as pharmaceuticals, paper, detergents, and plastics. "Green chemistry is making a real difference in our science and engineering and is making major societal contributions in the U.S. and around the world," he noted.
Besides Casey, several other dignitaries representing ACS, the Environmental Protection Agency, the National Academies, and the White House were on hand to welcome guests to the ceremony and congratulate the award winners. Susan B. Hazen, principal deputy assistant administrator of EPA's Office of Prevention, Pesticides & Toxic Substances, said that the Green Chemistry Awards program has helped "catapult environmental protection to a new level by designing and creating manufacturing processes that are more efficient and less polluting."
The awards were established by EPA in 1995 as a competitive effort to help the agency implement the provisions of the Pollution Prevention Act of 1990, Hazen explained. They are administered by the Green Chemistry Program in the Office of Pollution Prevention & Toxics and are supported by some 20 partners from industry, government, academia, and other organizations, including ACS and its Green Chemistry Institute.
"EPA saw the awards program as a wonderful opportunity to carry out not only the letter of the law but the spirit of that law," Hazen said. "What we needed was precisely the goal of the Presidential Green Chemistry Challenge: the creation of scientific and technical innovations that would eliminate pollution before it's created."
Award nominations are solicited in five categories: academic, small business, alternative synthetic pathways, alternative reaction conditions, and designing safer chemicals. The work that is described in the nomination must have been carried out or demonstrated in the U.S. within the preceding five-year period. An independent panel, appointed by ACS, judges the nominations and selects the award winners.
The results of the program "have been absolutely incredible," Hazen added. Since the program's inception, EPA's tracking of the impact of the winning technologies shows them to have prevented 460 million lb of hazardous substances from being produced, saved more than 440 million gal of process water, and prevented 170 million lb of carbon dioxide emissions.
Chemical engineering professor Charles A. Eckert and chemistry professor Charles L. Liotta of Georgia Institute of Technology were selected as Green Chemistry Award winners in the academic category for their work using carbon dioxide and water to develop environmentally benign "tunable" solvent systems. The Eckert-Liotta team has used supercritical CO2, near-critical water, and CO2 in "gas-expanded" organic liquids as replacement solvents in a variety of chemical reactions and separations.
The researchers use these solvent systems to create homogeneous conditions for improved reaction rates and selectivity while maintaining the advantages of heterogeneous product isolation and catalyst recycling. Overall, their replacement solvents help eliminate waste and provide economic advantages for industrial processes.
Eckert and Liotta began working together about 15 years ago. Eckert was already an authority on supercritical fluids, while Liotta was a leader in phase-transfer catalysis. The pair focused on studying the phase equilibria and kinetics of solvent and cosolvent mixtures, observing how small changes in temperature, pressure, and composition could help facilitate reaction chemistry.
One of their goals has been to overturn the traditional approach of treating reactions and separations separately, Eckert noted. The same solvent is generally used for both steps, he said, but it's usually optimized only for the reaction. Separations typically make up 60 to 80% of the cost of industrial processes, he added, and the separation process almost always has a large environmental impact because of the volume of organic solvent needed.
Much of the team's chemistry is reviewed in an upcoming invited feature article in the Journal of Physical Chemistry A. One example is the first phase-transfer catalysis reaction in supercritical CO2 (35–75 °C and 100–150 bar). This reaction involved nucleophilic displacement of chlorine on benzyl chloride with bromide or cyanide ion using tetraheptylammonium bromide as the catalyst, Eckert related. In supercritical CO2, the quaternary ammonium salt creates an interfacial layer between the solid bromide or cyanide salt and the benzyl chloride dissolved in the supercritical CO2, allowing the reaction to take place.
In near-critical water, Eckert and Liotta have run a number of acid- and base-catalyzed reactions, such as Friedel-Crafts acylations and aldol condensations. Under near-critical conditions (275 °C and 60 bar), water's highly structured network of hydrogen bonding comes apart, and the density and dielectric constant decrease, Eckert explained. The dielectric constant remains high enough to dissolve inorganic salts, but it's low enough to dissolve organic substrates. Once a reaction is complete, the vessel is cooled, dissipating the pressure and causing phase separation so the product can be isolated by decanting the water.
In addition, the dissociation of water under near-critical conditions leads to a high concentration of hydronium and hydroxide ions, which can serve as self-neutralizing catalysts. That means some reactions can be run without an added catalyst. This is an advantage in the Friedel-Crafts reaction, for example, which typically requires a stoichiometric amount or an excess of an acid catalyst, such as AlCl3. Once the reaction is neutralized, there is a lot of waste salt generated--as much as 10 lb of salt per pound of product, Eckert said.
Another example of Eckert and Liotta's work is using CO2 as an "antisolvent" to control phase equilibrium for facilitating crystallization of polymers or separating racemic mixtures. Still another example is homogeneous enzyme catalysis of hydrophobic substrates in a water-dimethyl ether miscible mixture. The product separation and enzyme recycling is facilitated by shifting the phase equilibrium of the mixture with water, ether, or CO2.
Eckert and Liotta are veteran chemists, each with 40-year academic careers and a history of cultivating industrial and government research partnerships to help facilitate technology transfer. The pair codirect chemistry and chemical engineering students and share lab space, an effort reinforced by Georgia Tech's strong support of collaborative and interdisciplinary research, Liotta said.
"We believe the chemical problems that face us now require multidisciplinary solutions," Liotta emphasized. "The most important thing we have done is to educate people, and these people are better educated by working on multidisciplinary teams rather than being off doing chemistry by themselves."
Jeneil Biosurfactant Co., Saukville, Wis., received the award in the small-business category for commercial development of a series of rhamnolipid biosurfactant products. Rhamnolipids, natural glycolipids produced by bacteria in soil and on plants, contain a sugar group and a long, branched fatty acid tail. Their high surface-active properties and low toxicity are making them attractive supplements or alternatives to alkylphenol ethoxylates and other synthetic surfactants used in the petroleum industry, agriculture, personal care products, and other applications.
Jeneil's biosurfactants are actually a mixture of two molecules, noted N. R. Gandhi, the company's president and principal scientist. One compound has a single rhamnose ring while the second compound has two rhamnose rings, he explained. The surface-active properties of the compounds arise from the hydrophilic sugar rings and the carboxylic acid functional groups, which are separated from each other by a long, hydrophobic hydrocarbon chain (a hydroxydecanoylhydroxydecanoate group).
Although rhamnolipids have been studied for 50 years, Gandhi said, Jeneil was the first company to find a suitable pathway for commercial production of the compounds. Since 1998, Jeneil has been licensing selected patents on glycolipids from several universities to develop potential applications.
Jeneil started by isolating efficient strains of Pseudomonas aeruginosa from soil and engineering large-scale fermentation and postfermentation recovery systems, Gandhi noted. During the recovery phase, the fermentation broth is sterilized and centrifuged to isolate the biosurfactants, which are then further purified. The postfermentation processing required extensive pilot testing, in part to perfect material recovery systems to prevent any discharge of biosurfactant from the production facility, he added.
Toxicity studies show that rhamnolipids, while slightly irritating to skin and eyes like other surfactants, are considerably less toxic than other surfactants to honey bees and to Daphnia, a water flea commonly used in toxicity studies. Based on these studies and other data, Jeneil was granted a federal exemption earlier this year from the requirement to establish a maximum permissible level for rhamnolipid residues on food when it's used as a fungicide.
The biosurfactant is sold as aqueous solutions of various concentrations and purity levels, Gandhi said. It can be substituted for or used in conjunction with synthetic surfactants, such as nonylphenol ethoxylate, a common surfactant that has come under scrutiny for its potential environmental toxicity and is being phased out in some countries.
"The potential range of viable applications is exceeding our ability to develop them independently," Gandhi said. "To capture the potential benefits of these exceptional molecules, we are now working with other companies that have expertise in focused application areas."
In agricultural applications, rhamnolipids are used as wetting and dispersing agents to aid application of all types of agricultural chemicals. Jeneil's Zonix formulation, for example, also has antifungal properties against spore-producing plant pathogens that cause downy mildew in fruit crops, late blight in potatoes, and other plant diseases.
The Reco product line is currently being used to clean and recover petrochemicals from storage tanks. The rhamnolipids, mixed with water and hydrocarbons, create an emulsion that is pumped from the tank. When the emulsion breaks, the water and oil layers can be separated from any residual solids, with the hydrocarbons being of sufficient quality to allow their reintroduction into the refinery stream. Traditionally, these tanks have been cleaned manually and the sludge stored in pits.
In other applications in use or being developed, the rhamnolipid biosurfactants are used as emulsifiers in cosmetics and shampoos, as surface bonding agents in asphalt and concrete, to aid wetting and penetration of chemicals for leather tanning and dyeing, and in remediation processes such as soil washing to remove hydrocarbons and heavy metals.
Bristol-Myers Squibb was recognized in the alternative synthetic pathways category for its "Paclitaxel Greenness Project," which has culminated with a plant-cell fermentation process to produce its anticancer drug Taxol (paclitaxel). The new system is a more sustainable approach to generate large quantities of paclitaxel. It replaces the original method of extracting the drug from tree bark and the follow-up method of synthesizing the drug from an intermediate.
Paclitaxel is a natural product that was first isolated from the bark of the Pacific yew tree, Taxus brevifolia, explained Jonathan C. Walker, director of Chemical Development Labs, Technical Operations. The discovery of paclitaxel came about during a National Cancer Institute (NCI) program in the 1960s to screen plants for compounds with antitumor activity, Walker said. Clinical trials in the 1980s explored the efficacy of paclitaxel for treating cancer, and in 1991 Bristol-Myers Squibb was selected by NCI to be its partner for commercial development of paclitaxel.
Initially, the drug was obtained by extracting it from bark stripped from yew trees. The bark contains very little paclitaxel, however, and procuring the bark ended up killing the trees, Walker noted. Because the trees take 200 years to mature, extracting paclitaxel from yew bark was not economically or ecologically feasible, he said. The total synthesis of paclitaxel, which requires 40 steps and has a low yield, also was not practical on a large scale, he added.
"For these reasons, Bristol-Myers Squibb undertook an intensive research program to develop a more sustainable way to make paclitaxel," Walker said. In 1995, the company developed a semisynthetic route to make the drug. The synthesis starts with 10-deacetylbaccatin III, a compound that contains most of the structural complexity of paclitaxel and can be extracted from the leaves and twigs of the European yew, T. baccata. The compound can be isolated without harm to the trees, which are cultivated in Europe.
"This was a successful and important synthesis because it enabled us to produce sufficient quantities of paclitaxel for additional clinical trials and to meet commercial requirements," Walker said.
Bristol-Myers Squibb's process to convert 10-deacetylbaccatin III to paclitaxel is still a complex synthesis, he pointed out. It includes 11 synthetic steps and seven isolation steps, requiring 13 solvents along with 13 organic reagents and other materials.
In the end, the company realized that the semisynthesis still wasn't the most sustainable process to make paclitaxel, Walker said. Bristol-Myers Squibb then started to look at developing an even more sustainable process, which led to the new plant-cell fermentation method.
Bristol-Myers Squibb created the new process using technology developed by Phyton Inc., Ithaca, N.Y. The fermentation process starts with clusters of cells from the needles of the Chinese yew, T. chinensis. These clusters, called calluses, produce paclitaxel. They are cultivated initially in petri dishes and later in a large-scale fermentor. The paclitaxel is isolated from the fermentation broth and is purified by chromatography and crystallization.
Compared with the semisynthesis, the cell-culture approach provides a more reliable year-round source of paclitaxel and requires no chemical transformations. The purification steps still require the use of organic solvents, such as dichloromethane, acetonitrile, and isopropanol, Walker said, but the total number of different solvents needed has been reduced from 15 to five. The new process also saves a considerable amount of energy by eliminating five cryogenic or low-temperature reaction steps and six drying steps.
"Going to a greener process certainly gives us a very competitive product," Walker concluded. "But going green doesn't mean you have to go more expensive. Certainly, there is potential with plant-cell fermentation for further cost reduction by developing more productive Taxus cell lines."
Since 2002, Taxol has been produced solely from the cell-culture method. It's now used to treat ovarian, breast, and lung cancer, and it has been prescribed to more than 1 million patients.
Specialty chemical company Buckman Laboratories International, Memphis, was honored in the category of alternative reaction conditions for its Optimyze enzyme technology to improve processing of recycled paper. Optimyze is an esterase liquid formulation added during the paper-recycling process to help degrade and remove chemical buildup that can foul processing equipment and lead to lower quality products. The enzyme formulation is inherently safer to transport and handle than organic solvents and surfactants currently used for most paper recycling, and it provides economic benefits by boosting paper production.
Paper is a resource that can be recycled numerous times, and currently about 50% of the paper and paperboard used in the U.S. is collected and reused, according to Philip M. Hoekstra, Buckman's director of applications. Recyclable papers contain adhesives, plastics, inks, and other additives that form sticky, hydrophobic agglomerates during the papermaking process, he explained. These "stickies" tend to build up on processing equipment, so production has to stop periodically for cleaning, which is often carried out using an organic solvent, such as mineral spirits.
For recycling, the paper--made up of newspapers, cardboard, or mixed office waste--is mixed with water to form a slurry, Hoekstra said. Sometimes surfactants or other chemicals are added to help separate ink and some of the stickies. The slurry is then sprayed onto a porous plastic screen moving at about 60 mph, where water is removed, forming the sheet of paper. The sheet next passes through rollers to press out excess water, followed by a pass through a series of steam-heated dryers. Over time, stickies tend to build up into small blobs on the plastic screen or felt roller pads all along the production line, which can lead to spots or holes on the finished paper.
A major component of stickies is poly(vinyl acetate) and related compounds from adhesives, Hoekstra noted. The company found a way to battle the stickies by teaming with enzyme-producer Novozymes to pick an appropriate enzyme that could degrade the polymers. The esterase in Optimyze catalyzes hydrolysis of the vinyl acetate materials, converting them to water-soluble vinyl alcohols and acetic acid that are removed in the process water.
Buckman researchers developed a screening method to test a couple dozen enzymes before selecting the Optimyze esterase, which is produced by Novozymes in a fermentation process using an engineered bacterium. Buckman then developed a stable liquid formulation that can be used at several different points in the paper-recycling process.
Eliminating stickies is a significant problem, Hoekstra said. "The biggest savings in using Optimyze is better efficiency and increased production from reduced downtime," he noted. This cost to the paper industry is more than $500 million annually for major recycled paper grades.
In a paper mill producing more than 1,000 tons of paper per day, for example, Optimyze reduced chemical use by 600,000 lb per year and increased production by 6%. Overall, the mill had a $1 million benefit over the year by using Buckman's product, he said.?
Optimyze has been on the market for just over a year and is now being used in more than 40 paper mills worldwide that produce some 2 million tons of tissue, paper towels, paperboard, and corrugated cartons, Hoekstra said. The company plans to continue its exploration of enzymes for other applications, he added. "There are not many other companies utilizing enzymes in the paper industry at this point," he said. "We expect to use enzymes a lot more in the future."
In the designing safer chemicals category, Engelhard Corp. received the Green Chemistry Award for developing a diverse range of azo pigments that are less toxic and have a lower environmental impact during production than traditional colorants. The pigments, sold under the Rightfit brand, are used in plastic containers and other niche applications. They contain calcium, strontium, and barium metal counterions rather than traditional lead, chromium, and cadmium ions in inorganic pigments, and they are produced in an aqueous process rather than in hydrocarbon solvents typically used to make organic pigments.
Historically, pigments containing lead molybdate (PbMoO4), lead chromate (PbCrO4), cadmium sulfoselenide (CdSe/CdS), and related heavy-metal-based compounds have served as the pigments of choice for the red, orange, and yellow color market, according to Amrit P. Bindra, Engelhard's manager of color research and development. The blue and green color needs are met by phthalocyanine blue and green pigments at reasonable cost and don't pose environmental concerns, he added.
When EPA began regulating heavy metals in the 1970s, color formulators started turning to organic pigments such as dichlorobenzidines, isoindolinones, and quinacridones, Bindra explained. Although these organic pigments meet performance requirements, he said, they are relatively expensive and their manufacture requires large volumes of organic solvent. In addition, polyphosphoric acid is sometimes used to make some of these compounds, which leads to environmentally harmful phosphates in the plant effluent. Some of the pigments also contain polychlorinated aromatics.
In the mid-1990s, Engelhard began a program to develop environmentally friendlier and lower cost azo pigments to meet new EPA requirements, Bindra said. "Commonly available azo pigments have limited heat stability and are not suitable for most plastic applications," he noted. "But we identified appropriate substituents on the starting amine and other reactants that were conducive to heat stability and were reactive in aqueous systems."
The azo compounds contain a –N=N– bridge that links two aromatic groups, such as a substituted phenyl and a substituted naphthyl. The compounds generally are synthesized by reaction of aniline or other amines with nitrous acid, followed by coupling to a second nucleophilic aromatic compound. The azo compounds are highly colored because the nitrogen bridge extends the conjugation of the aromatic ring systems, resulting in strong absorption in the visible region.
Because of their solubility, azo dyes undergo N=N cleavage, resulting in generation of aromatic amines that in some cases are toxic, Bindra noted. The azo pigments, on the other hand, are virtually insoluble and are unlikely to undergo N=N cleavage. Therefore, they have a lower demonstrated toxicity, he said. Engelhard scientists engineered the Rightfit pigments by placing carboxylic acid or sulfonic acid substituents on the aromatic groups on both sides of the –N=N– bridge, Bindra added, so that even if cleavage did occur, the cleaved components would not be bioaccumulative.
The choice of alkyl, alkoxy, or halogen substituent groups on the aromatic rings in addition to the acid groups leads to azo pigments of nearly every color shade, he said. Using another approach, desired shades can be obtained by producing mixed crystals of two or more pigments. "These pigments provide brilliant colors with high color strength and good heat stability for applications in plastics," he noted.
Engelhard has reduced its production of heavy-metal pigments by more than 80%, from 6.5 million lb in 1995 to 1.2 million lb in 2002, Bindra said. Production of the heavy-metal pigments is expected to be phased out later this year.
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