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July 1,  2002
Volume 80, Number 26
CENEAR 80 26 pp. 26-30
ISSN 0009-2347

SWEET DEAL Confectionery wrap is one application for Cargill Dow's NatureWorks polylactic acid resin, the first synthetic polymer produced from an annually renewable resource--corn-derived dextrose.
CARGILL DOW PHOTO

STEPHEN K. RITTER , C&EN WASHINGTON

Who could argue that saving more than 50,000 gal of organic solvent per ton of product isn't a good deal? Or creating a family of commodity polymers that are derived from dextrose supplied by corn? How about eliminating millions of pounds of toxic arsenic and chromium compounds used to pressure-treat wood each year in the U.S.? These are the benefits of some of the environmentally friendly innovations that are being recognized with 2002 Presidential Green Chemistry Challenge Awards.

The seventh annual presentation of these awards was made last week to one individual and four companies at the National Academy of Sciences in Washington, D.C. The ceremony took place on the eve of the sixth annual Green Chemistry & Engineering Conference, a three-day meeting that featured talks by the five award winners and plenary and technical sessions on advances in green chemistry and engineering research, education, and policy issues.

Several dignitaries representing the Environmental Protection Agency, the National Academies, the American Chemical Society, and the White House made brief remarks to welcome guests to the ceremony and congratulated the award winners. These included National Academy of Sciences President Bruce M. Alberts and presidential science adviser John H. Marburger III.

Marburger read a message from President George W. Bush recognizing the award winners for their achievements to enhance the quality of human health and the environment. "Leaders in industry, academia, and government must work in partnership to use the principles of green chemistry to achieve clean water, land, and clear skies that benefit our environment and economy," Bush's letter stated. "Through a sustained commitment to these endeavors and by building on the success of breakthroughs such as those honored today, we can ensure America's economic and environmental vitality."

ACS PRESIDENT Eli M. Pearce noted in his remarks that chemistry has played a vital role in improving the welfare of humankind, but that the benefits have not come without consequences. "Green chemistry provides unique opportunities to change the way we operate within the chemical enterprise," Pearce said. "Green chemistry highlights chemistry's role as the enabling science that advances humanity through its discoveries and processes. When [this year's] winning technologies are measured by the three pillars of the sustainability paradigm--economic, environmental, and social concerns--they demonstrate that sustainability really can be achieved."

ACS Board Chair Nina I. McClelland added that education is central to the continued implementation of green chemistry. Students from elementary school on up, practicing chemists, and individuals in other disciplines--such as economists, engineers, biologists, and policymakers--must become familiar with the principles of green chemistry, she said. "The public should also be made aware that the chemical products and technologies we rely on to support longer, high-quality lives are dependent upon basic and applied research, development, and production practiced in an environmentally responsible manner."?

The Presidential Green Chemistry Challenge Awards were established in 1995 as a competitive effort to promote chemical products and manufacturing processes that prevent pollution yet are economically viable. The awards program is administered by EPA's Green Chemistry Program in the Office of Pollution Prevention & Toxics. EPA operates the awards program with some 20 partners from industry, government, academia, and other organizations, including ACS and its Green Chemistry Institute.

Award nominations are solicited in five categories: academic, small business, alternative synthetic pathways, alternative reaction conditions, and design of safer chemicals. The work described in the nomination must have been carried out or demonstrated in the U.S. within the preceding five years. An independent panel selected by ACS judges the nominations and selects the award winners.

The Kenneth G. Hancock Memorial Scholarship was also presented at the ceremony by McClelland. The annual scholarship, open to undergraduate and graduate students, consists of a one-time $1,000 cash award. It is named in honor of Hancock, an early proponent of green chemistry who died unexpectedly in 1993 during his tenure as director of the Chemistry Division of the National Science Foundation.

The 2002 Hancock Scholarship was presented to Bianca R. Sculimbrene, a graduate student at Boston College working in the group of associate chemistry professor Scott J. Miller. Sculimbrene was selected for her work on screening a library of small-peptide catalysts to identify a pentapeptide that enantioselectively phosphorylates a derivative of myo-inositol, an important biologically active compound [J. Am. Chem. Soc., 123, 10125 (2001); C&EN, Oct. 8, 2001, page 9].

The catalyst leads to the preparation of D-myo-inositol-L-phosphate in a highly efficient manner without the aid of an enzyme, Sculimbrene notes. The new synthesis reduces the reliance on elaborate protecting-group strategies, chiral auxiliaries, and laborious purification steps, she adds, and should be useful as a model to develop additional selective, efficient phosphorylation methods that use enzyme mimics.

FAVORABLE INTERACTIONS Styranec (from left), Beckman, and Sarbu with the apparatus they used
to study poly(ether-carbonate) copolymers (shown below) and other CO2-philes that increase the usefulness of supercritical CO2 as a solvent.
Eric J. Beckman, a chemical engineering professor at the University of Pittsburgh, was selected as the Green Chemistry Award winner in the academic category for his group's design and preparation of the first nonfluorinated copolymer "CO2-philes," additives that make carbon dioxide a more useful solvent.

 CARBON DIOXIDE has been investigated extensively as a nonflammable, environmentally benign, inexpensive solvent--both as a liquid and in its supercritical state. In the 1980s, supercritical CO2 was being hailed as a medium with solvent properties similar to those of n-alkanes, Beckman notes, leading many researchers to believe that the chemical industry could use CO2 as a simple drop-in replacement for a wide variety of organic solvents.

Early on, however, researchers began to note some shortcomings of supercritical CO2, such as the poor solubility of some compounds and the need for high process pressures. It turns out that the initial solubility studies had inflated CO2's solvent power value by as much as 20%, Beckman says, meaning pure supercritical CO2 "is actually a rather feeble solvent" compared to alkanes.

"Water and CO2 are our two most popular benign solvents," Beckman observes, "yet our knowledge of water's behavior as a solvent still dwarfs that of CO2. If we could remedy that, then CO2 could be green from the environmental perspective and green from the economic perspective as well."

Many research groups, including Beckman's, began working toward that goal by designing CO2-philic materials that help CO2 dissolve otherwise insoluble or poorly soluble polar, ionic, organometallic, or high-molecular-weight compounds. The most effective CO2-philes have been polysiloxanes or fluorocarbons such as fluoroacrylate and fluoroether polymers.

These solubilizers, used in amounts of a few weight percent, have opened up some applications for CO2, Beckman says, including heterogeneous polymerization, protein extraction, and homogeneous catalysis. But the high cost of preparing and recycling CO2-philes and concern over the long-term environmental persistence of fluorocarbons have limited their commercial use, he adds.

A couple of years ago, Beckman, postdoc Traian Sarbu, and graduate student Thomas J. Styranec began to look at the thermodynamic behavior of CO2 to develop some simple rules for designing less expensive nonfluorinated copolymers that could serve as CO2-philes [Nature, 405, 129 and 165 (2000); Ind. Eng. Chem. Res., 39, 4678 (2000)]. Sarbu is now a postdoc in the group of chemistry professor Krzysztof Matyjaszewski at Carnegie Mellon University, and Styranec is a staff engineer at Dow Chemical.

According to these rules, one of the monomers should lead to a polymer that has a low glass-transition temperature so that it is highly flexible. This monomer also should give a poly-mer that interacts only weakly with other polymer chains. The second monomer should contain a Lewis base functional group, such as carbonyl, that creates sites in the polymer for favorable interactions with CO2, which is a Lewis acid.

CLEAN MACHINE SC Fluids' Arroyo uses supercritical CO2 to remove photoresists and etching residues during the patterning of semiconductor wafers. At bottom are cross sections of a partially processed wafer (left) and a completely cleaned wafer.
SC FLUIDS PHOTOS
BECKMAN'S GROUP subsequently designed and synthesized a series of economical nonfluorinated copolymers that readily dissolve in CO2. The copolymers--poly(ether-carbonate)s--are prepared from CO2 and propylene oxide, ethylene oxide, or other related compounds. They are miscible in CO2 at lower pressures than fluoroether polymers--about 100 bar versus 200 bar--yet are biodegradable and can be prepared at 1% of the cost. The group has extended its model to make acetate-functional polyethers, poly(vinyl ester)s, and functional silicones with Lewis base groups on side chains.

Several companies have acquired options to license the Pittsburgh patents on CO2-philes, Beckman adds. He hopes to use the copolymers to make chelating agents, catalyst ligands, and other compounds. Potential applications include dyeing and cleaning of fibers and textiles, polymerization and polymer processing, purification and crystallization of pharmaceuticals, and general chemical synthesis.

Also on the CO2 theme, SC Fluids, based in Nashua, N.H., was selected for the award in the small-business category for the commercial development of SCORR (Supercritical CO2 Resist Remover). SCORR is a process that uses supercritical CO2 rather than wet chemical treatment to remove photoresist masks as well as post-etching and other treatment residues during the patterning of semiconductor wafers.

SCORR was initially developed at Los Alamos National Laboratory (LANL) following a request from Hewlett-Packard (now Agilent Technologies). A Cooperative Research & Development Agreement between LANL and SC Fluids led to the development of SC Fluids' Arroyo System, a machine that automates the process. Arroyo is now being evaluated in the semiconductor industry.

The fabrication of integrated circuits relies on multiple-step photolithography to define the shape and pattern of individual circuit features. Current chip manufacturing primarily uses wet chemical processing involving hydroxylamines, mineral acids, and organic solvents at various stages. In the process, a copious amount of ultrapure deionized water is needed for rinsing. Chip fabrication plants thus consume thousands of gallons of chemicals and generate some 4 million gal of wastewater in a typical day. A large amount of isopropyl or other alcohol also is used as a drying solvent.

Arroyo uses supercritical CO2 with small amounts of benign cosolvents, such as 1 wt % propylene carbonate, according to Laura B. Rothman, SC Fluids' vice president for technology. The semiconductor device to be stripped or cleaned is first "soaked" for a few minutes in the supercritical CO2/cosolvent mixture to soften and loosen the photoresist. Pressure pulsing then creates turbulence to dislodge and remove the resist. A final "rinse" with pure supercritical CO2 removes any remaining debris and residual propylene carbonate. A drying step is not needed, and the inexpensive CO2 and propylene carbonate can be recycled.

The name Arroyo derives from SC Fluids' staff working with researchers at LANL in New Mexico. "We were influenced by the Southwest," Rothman says. "Arroyo is by definition 'a deep gully cut by an intermittent stream; a dry gulch.' We creatively interpreted this as a 'dry wash' for naming our machine."

Overall, SCORR simplifies and streamlines the integrated circuit manufacturing process, resulting in large cost savings and a lower burden on the environment, Rothman notes. The technical merit of using supercritical CO2 is equally important to the environmental benefits, she adds. Supercritical CO2's very low surface tension and gaslike viscosity will be able to continue doing the job of cleaning wafer surfaces as deep or narrow architectures become ever smaller, Rothman says. Existing wet chemical stripping technology, however, will eventually be limited by the physical properties of the liquids.

SC Fluids has teamed with Air Products & Chemicals, ATMI, and IBM to complete the development of Arroyo (C&EN, Sept. 17, 2001, page 10). Air Products will bring its experience in the supply, delivery, and storage of bulk gases to the semiconductor industry. As part of their agreement, Air Products recently installed one of SC Fluids' cleaning machines at its R&D facility in Allentown, Pa. ATMI, a supplier of specialty materials and equipment to the semiconductor industry, brings its expertise in chemistry to processing semiconductor materials. IBM will help to speed development of supercritical CO2wafer cleaning processes needed to manufacture smaller, more complex semiconductors.

 
PFIZER PHOTO
PROCESS TEAM Taber (left) and Colberg spearheaded Pfizer's improved synthesis of the antidepressant sertraline. The new process reduces the solvent requirement by thousands of gallons per ton of product and eliminates several hundred metric tons of TiO2, HCl, and NaOH waste per year.

FOR DRAMATICALLY improving the manufacturing process for sertraline, the active ingredient in its antidepressant drug Zoloft, Pfizer has been selected to receive the Green Chemistry Award in the category of alternative synthetic pathways. Sertraline is a selective serotonin reuptake inhibitor used for treatment of major depression, panic disorder, obsessive-compulsive disorder, and posttraumatic stress disorder. Approved for use in the U.S. in the early 1990s, sertraline is the most prescribed drug of its class, with 2001 worldwide sales of nearly $2.4 billion.

The key improvement in the sertraline synthesis was reducing a three-step sequence in the original process to a single step (C&EN, April 22, page 30). Overall, the process changes reduce the solvent requirement to 6,000 gal from 60,000 gal per ton of sertraline, according to Pfizer. On an annual basis, the changes eliminate 440 metric tons of titanium dioxide-methylamine hydrochloride salt waste, 150 metric tons of 35% hydrochloric acid waste, and 100 metric tons of 50% sodium hydroxide waste.

In the original synthesis, the carbonyl group of the dichlorophenyltetralone starting material is converted to an imine using methylamine in tetrahydrofuran or toluene. This step uses titanium tetrachloride as a dehydrating agent to drive the reaction equilibrium toward the imine. The imine product is isolated and then hydrogenated to form amine isomers (cis:trans, 6:1) using hydrogen and a palladium/carbon catalyst in tetrahydrofuran.

The mixture of amine isomers is crystallized to selectively isolate the racemic cis hydrochloride salt, which is resolved via d-mandelic acid in ethanol to obtain the needed (S,S)-cis isomer. In the final step, sertraline mandelate is converted to the hydrochloride salt in ethyl acetate.

In the new process, developed by Pfizer manufacturing chemists Geraldine Taber, Juan Colberg, and David Pfisterer, the first three reaction steps are carried out without isolating the intermediates. The team chose to use the more environmentally benign ethanol as the only solvent, which eliminates the need to use, distill, and recover the other solvents. The unreacted methylamine can be recovered by distillation.

Titanium tetrachloride is no longer needed to drive the first step since the imine has poor solubility in ethanol and precipitates from the reaction mixture. In addition, switching to palladium on calcium carbonate as the catalyst provides better cis amine selectivity (cis:trans, 18:1). In total, the overall yield has been nearly doubled to 37% and raw materials for the synthesis--implemented in the U.S. in 1998 and in Europe in 2000--have been cut by 60, 45, and 20%, respectively, for methylamine, dichlorophenyltetralone, and d-mandelic acid.

SELECTED AS the winner in the category of alternative reaction conditions is Cargill Dow, which is being recognized for its novel synthesis of polylactic acid (PLA) starting from corn-derived dextrose. Cargill Dow's NatureWorks brand PLA is the first synthetic polymer class to be produced from annually renewable resources. NatureWorks polymers are expected "to bridge the gap in performance between conventional synthetic fibers and natural fibers" used in clothing and carpeting, the company says, and compete head-to-head with plastics for food packaging and for agricultural or gardening applications.

Polylactic acid is a biodegradable aliphatic polyester that can be prepared by condensation of lactic acid or by the ring-opening polymerization of the cyclic lactide dimer, according to Patrick R. Gruber, Cargill Dow's vice president and chief technology officer. The synthesis of PLA has been known for many years, Gruber notes, but previous attempts at large-scale production were hampered by high production costs related to purification of the lactide. In addition, the condensation route is complicated by the need to remove trace amounts of water, which limits the final molecular weight.

In the early 1990s, Gruber and coworkers at Cargill developed the improved route to PLA that makes it competitive with commodity petroleum-based polymers. The key advances include synthesizing both the lactide and PLA in the melt rather than using a solvent and using vacuum distillation in the purification steps. In 1997, Cargill Dow was formed as a 50-50 joint venture between Cargill and Dow Chemical to develop the new process commercially.

NatureWorks production starts with the bacterial fermentation of dextrose from corn to produce lactic acid. The lactic acid is purified and then condensed in a continuous process to make low-molecular-weight PLA (Mn = 5,000), Gruber explains. This material is next depolymerized in the melt using tin octoate as the catalyst to form a mixture of lactide stereoisomers.

One unique aspect of the Cargill Dow process is separation of the stereoisomers during a set of purification steps, Gruber points out. The separation allows the tin octoate ring-opening polymerization of the molten lactide using selected compositions of the stereoisomers. The L-lactide is the primary component, he notes, and varying the amount of the D-lactide is important for controlling the physical properties of the high-molecular-weight PLA product (Mn = 60,000 to 150,000). Residual lactic acid and lactide are removed under vacuum and recycled throughout the process, which has better than 90% yield.

Cargill Dow's PLA requires 20 to 50% less fossil-fuel resources than comparable plastics, Gruber says, and PLA is biodegradable or readily hydrolyzed back into lactic acid for recycling. "The idea of creating a more sustainable business model is to establish a new industrial system where society can go on forever without depleting Earth's natural resources, without compromising people, and helping to create a better quality of life," Gruber observes. "We take this idea very seriously and believe we are developing a system that accomplishes that."

NatureWorks looks and processes like polystyrene and has the stiffness and tensile strength of polyethylene terephthalate, Gruber says. Like cellophane, PLA films stay folded when twisted, making them suitable for candy wrappers. PLA also is a good barrier to flavors and aromas and resists greases, fats, and oils for food packaging. Carpet makers will find the polymer attractive, he adds, because both the face fiber and backing can be made of PLA, facilitating recycling.

The first world-scale NatureWorks PLA plant, located in Blair, Neb., became operational earlier this year and has an annual capacity of 140,000 metric tons (C&EN, May 20, page 13). Cargill Dow expects to sell up to 50,000 metric tons of PLA in 2002, and to achieve a global capacity of 500,000 metric tons by 2006. Future plants will be able to use various other biomass feedstocks, such as wheat, rice, or agricultural wastes, Gruber says, depending on what the most economical resource is in a given geographical area.

Chemical Specialties Inc. (CSI) received the Green Chemistry Award in the category of designing safer chemicals for developing alkaline copper quaternary (ACQ) compounds to replace the widely used chromated copper arsenate (CCA) as a preservative for pressure-treated wood. Sold under the company's Preserve brand, the compounds are much less toxic than CCA yet work just as effectively to protect wood from dry rot, termites, marine borers, and other pests.

ACQ Preserve is a 2:1 combination of a proprietary alkaline copper(II) carbonate complex and a quaternary ammonium chloride, notes David Fowlie, CSI's vice president for sales and business development. On the U.S. market since 1996, it was created for customers who wanted an arsenic- and chromium-free wood preservative.

CCA is a mixture of chromic acid, arsenic acid, and copper oxide that has been used for wood treatment since the 1940s. More than 95% of the 7 billion board feet of pressure-treated wood used in the U.S. each year--a $4 billion industry--is preserved with CCA. That translates to some 40 million lb of inorganic arsenic compounds and 64 million lb of chromium(VI) compounds, Fowlie says, which are both known human carcinogens.

 
UNDER PRESSURE Lumber is loaded into a chamber at a pressure-treatment plant where CSI's alkaline copper quaternary compound--a replacement for chromated copper arsenate--is used as a wood preservative.
CHEMICAL SPECIALTIES INC. PHOTO

EPA HAS NOTED that the health risks posed by CCA's use in treated wood are low. But there has been growing demand in the past two years by consumer groups to halt its use in such applications as playground equipment, picnic tables, decks, fencing, and landscaping timbers. There has been a corresponding increase in the demand for ACQ Preserve formulations during the same period, Fowlie says.

In February, treated-wood producers and EPA reached a voluntary agreement to phase out CCA for treating wood for consumer applications--about 80% of the total--by the end of 2003. Industrial uses of CCA for utility poles, highway construction, aquaculture, marine pilings, boats, and some other uses will continue. EPA does not believe there is any reason to remove or replace existing CCA-treated structures or that removing any surrounding soil is necessary.

Replacing CCA with ACQs is an important pollution-prevention advance, the company notes, because when fully implemented the switch could eliminate the industrial use of more than 90% of all arsenic compounds in the U.S. The changeover not only would benefit consumers and users of pressure-treated wood, but should provide safer conditions for the industry's production workers.

In 2001, CSI sold more than 1 million lb of ACQ Preserve for wood treatment. This year, the company's production capacity will increase to more than 50 million lb, if new capacity comes onstream as expected in the third quarter. Other producers of CCA are Arch Chemicals and Osmose. Arch Chemicals is switching from CCA to a copper boron azole compound, while Osmose is turning to ACQ through a licensing agreement with CSI.

"Our market leadership position with ACQ is based on our ongoing commitment to produce advanced preservation technology to meet the changing demands of the industry and the consumer market for treated-wood products," Fowlie says. CSI is working with customers, distributors, and end users of treated-wood products, he adds, "to ensure a smooth transition to the new-generation ACQ preservatives."

GREEN CHALLENGE



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