SCIENCE/TECHNOLOGY
'Green' Technology Presents Challenge to Chemists • Symposium showcases progress toward alternative chemical syntheses that are environmentally benign by design Deborah L. Illman, C&EN Washington
CHICAGO
T
he economic prosperity of the U.S. "demands a robust chemical industry. Environmental preservation demands new advances in environmentally benign, 'green' technologies. Central to both," observes National Science Foundation chemistry director Kenneth G. Hancock, "is the ingenuity of the synthetic chemist." The synthetic chemist, Hancock explains, not only must create new products but also find new ways to design chemical syntheses with their environmental ramifications in mind. As the Chicago national meeting of the American Chemical Society demonstrated, these approaches to synthesis are well under way. Hancock addressed a symposium on alternative synthetic design for pollution prevention, sponsored by the Division of Environmental Chemistry in collaboration with the Division of Organic Chemistry. Organized by Carol A. Farris and Paul T. Anastas of the Environmental Protection Agency's Office of Pollution Prevention & Toxics (OPPT), the symposium showcased about a dozen papers on recent research efforts to develop synthesis routes that check pollution at its source. NSF and EPA joined forces to accelerate research in environmentally benign synthesis through a cooperative agreement signed earlier this year. Programs 26
SEPTEMBER 6,1993 C&EN
under way in both organizations have embraced the adage, "an ounce of prevention is worth a pound of cure." Hancock stresses that preventing pollution will require a change in the design of synthesis pathways used to manufacture chemical products. Green synthesis routes of the future "will require making choices about reactants, solvents, and r e action conditions to reduce both resource consumption and waste production," he predicts. Anastas agrees. "A chemist who puts pencil to paper to design a synthetic s e quence for a chemical product is also intrinsically making decisions about whether that sequence will use or generate hazardous substances that require treatment, recycling, transportation, or disposal. By putting forethought into the design, the chemist may be able to r e duce or eliminate the toxic substances used or generated in the process as well as their associated costs," he says. "Most organic chemists have been trained to identify reaction pathways that provide the highest yields as one of the fundamental evaluation criteria
for a reaction scheme," observes Anastas. "This evaluation criterion has slighted the potential problems associated with hazardous feedstocks, solvents, catalysts, by-products, and impurities. With the rising costs of waste treatment, waste disposal, compliance with regulations, and liability insurance, chemists will increasingly need to consider the broader impacts of a given synthetic method beyond that of maximum yield," he asserts. Hancock emphasizes that the goal of developing environmentally benign pathways of chemical synthesis offers a new intellectual challenge for organic chemists. He describes some of the intriguing research opportunities waiting to be explored: aqueous solvent-based reactions, ambient-temperature reactions, just-intime in-situ generation of toxic interme diates, chiral catalysts, artificial enzymes, and built-in recyclability. Results of these and future research investigations to identify practical alternatives to synthesis pathways will affect both academe and industry, predicts Anastas. As new synthesis routes are de-
In Epling's lab, graduate student Wang harnesses the energy of visible light in an environmentally benign alternative to oxidation reactions that involve toxic or hazardous compounds
veloped, the results will find their way into the scientific literature and into chemistry texts. 'The ultimate goal of the Alternate Synthetic Pathway Initiative [at EPA] is to train future generations of chemists in new ways of assessing the desirability of synthetic methodologies that encompass all of the costs, as well as the benefits, associated with the chosen synthetic route/' he says. Harnessing the energy of visible light to bring about desired chemical transformations without using or generating toxic compounds is the aim of two different groups reporting at the symposium. Gary A. Epling, professor of chemistry at the University of Connecticut, Storrs, and graduate student Qingxi Wang are exploring the use of nontoxic food dyes as catalysts in oxidation reactions that previously could only be carried out with toxic compounds of metals such as cadmium, lead, mercury, nickel, and chromium. In another approach, chemistry professor George A. Kraus and coworkers at Iowa State University, Ames, are developing a photochemical alternative to certain Friedel-Crafts reactions, with the aim of eliminating the use of toxic Lewis acids such as aluminum chloride and of avoiding the toxic solvents commonly employed in this synthesis. Both strategies use visible light as the reagent that provides the driving force for chemical transformations. Epling says his group has chosen to work on environmentally benign alternatives to oxidation reactions because of the high toxicity of conventional reagents and the widespread applications of these reactions. His strategy utilizes a dye to absorb visible light and then transfer an electron efficiently and selectively to bring about a desired reaction: for example, cleavage of dithianes to aldehydes and ketones; benzyl ethers to alcohols; and oxathianes to the corresponding carbonyl and thioalcohol. Epling reports that "these transformations have been accomplished in high yield," and that "the method appears to offer a practical alternative to the use of toxic heavy metals." Visible light, rather than ultraviolet light, is used for many reasons, says Epling. Intense sources of visible light are readily available and inexpensive. In addition, the high energy of ultraviolet light is absorbed by many chromophores, leading to a host of unwanted reactions in complex substrates. The
lower energy of visible light makes posDye, light used in 'green' oxidations sible a higher reacHStion selectivity, with RÏ S Ο (CH2)n (CH2)n higher yields and Dye Χ HSminimal by-prodR2Sucts. Dithianes A 120-watt spotlight, available at R—OH + CH=0 CH 2 —O—R Dye hardware stores for about $5.00, proBenzyl ethers vides enough energy HSRl S to react 20 g of mateH Λν X J "ïïyT R i F*2 rial in 12 to 24 hours HOof illumination, says R! and R = Alkyl groups η = 1,0 Epling. He speculates 1,3-Oxathianes that with "sources With light as reagent and dye as catalyst, deprotection that are more powerof organic functional groups proceeds under neutral ful, or by using a soconditions without the use of heavy metals or chemi lar collector for suncal oxidants. Reaction at top shows deprotection of light, it should be dithianes. Shown in the middle, the benzyl ether pro feasible to perform tecting group is often used to protect an alcohol dur reactions on the kiloing organic synthesis. The usual ways to remove this gram scale, particublocking group—catalytic hydrogénation or alkalilarly if improvements metal reduction—involve conditions that may result in additional, unwanted reductions in the alcohol of efficiency result molecule. Using visible light and a dye catalyst, from additional reEpling has reported achieving excellent yields of the search." alcohol. The bottom reaction shows 1,3-oxathianes, Epling's group is used in stereo-controlled synthesis routes, and which exploring other dye often are deprotected by oxidative methods that incatalysts that might volve mercuric chloride in acetic acid, mercuric chloserve in these reacride with alkaline ethanolic water, or silver nitrate tions. "Several good with N-chlorosuccinimide. In addition to the carbonyl product, Epling obtains the nonoxidized thioalcohol, alcandidates have lowing chiral starting material to be recovered while emerged," Epling avoiding the generation of toxic pollutants says, "some of which have sufficiently low toxicity that they have been used as food colorants, in cos- theoretical framework for photochemimetics, and in medicines." Eosin (yellow), cal reactions is well developed, and erythrosin (red), and methylene blue are modifications for the use of visible light examples of dyes that have worked well, are often straightforward, he says. And he notes. precedent has been set for scale up of Using just the common spotlight as a photochemical reactions—some are alsource, Epling has removed the dithio ready used on a large scale in industry. protecting group from carbonyl comKraus is targeting a photochemical alpounds in quantities on the order of 15 ternative to the Friedel-Crafts reaction, g. After six to 10 hours of illumination, which is used for making a number of "very high isolated yields (86 to 97%) commercial products, including diazof aldehydes and ketones were ob- epam, zomepirac, acebutolol, ibuprofen, tained in the 16 cases examined," he and doxepin. The Friedel-Crafts reaction says. Dithio acetals and ketals are used involves the use of Lewis acids such as to protect carbonyls in organic synthe- aluminum chloride and tin tetrachloride, sis, but their utility has been somewhat as well as corrosive, air-sensitive, and diminished by the sometimes problem- toxic acid chlorides, and is often carried atic deprotection step—which Epling out in solvents such as nitrobenzene, manages under neutral conditions and carbon disulfide, or halogenated hydrowithout heavy metals or toxic oxidants. carbons. Kraus points out that the idea of usKraus' photochemical alternative exing light as a reagent to develop a new ploits the reaction between a quinone generation of alternative synthesis and an aldehyde, initiated by a simple pathways has many advantages. The sunlamp. "A variety of substituted al/TV
Λ,
2
SEPTEMBER 6,1993 C&EN
27
SCIENCE/TECHNOLOGY
centage of consumed D-glucose into the common pathway of aromatic amino acid biosynthesis, and then siphon the flow of carbon away from aromatic amino acids and into the synthesis of the desired industrial chemicals. Friedel-Crafts: Another alternative to the use of benzene as a starting material was described by Orville L. Chapman, professor in the department of chemistry and biochemistry at the University of California, Los Angeles. He outlined research opportunities relating to the Kraus: UCLA styrene process, which converts equilibrium mixed xylenes to styrènes in a single step. Existing styrene processes use benzene and ethylene as raw Kraus and coworkers find that a variety of substituted aldehydes and quimaterials and involve catalytic alkylanones can be used successfully in this photochemical alternative to the Friedeltion and subsequent dehydrogenation, Crafts reaction Chapman says. "The UCLA styrene process could eliminate 13 billion lb per year of benzene from the global economy/' he claims, adding, "xylenes have a clear edge as a raw material for styrene manufacture." An industrial perspective on efforts to eliminate by-products and process X = CI, OCH3, CHO, Z = OCH3,CI, X,Z = CI,OCH 3 ,CH 3 , waste was provided at the symposium F, C0 2 CH 3 , CH3 CN,CH 3 ,C 6 H 5 CN, C 6 H 5 , SCH3 by Leo E. Manzer, associate director in Du Pont's central science and engineerThere appear to be no restrictions for functional groups meta and para to the ing laboratory in Wilmington, Del. He formyl group of the benzaldehyde. Ortho groups that are compatible with the reaction conditions include alkoxy groups, alkyl groups, esters, and halogens. points out that "hazardous and toxic Many substituted benzoquinones react with aromatic and aliphatic aldehydes materials such as HCN, HF, HC1, Cl2, according to this scheme acrylonitrile, formaldehyde, ethylene oxide, sulfuric acid, and phosgene . . . are essential building reagents in the dehydes and quinones can be used suc- products. Frost says using genetically chemical industry since they often conengineered microbes and D-glucose as tain functionality or reactivity required cessfully" in this scheme, he says. Chemistry professor John W. Frost of starting materials provides not only a for further chemical reactions. Future Purdue University looks to biotechnolo- more benign synthesis route but may business practices must avoid or minigy for a new route to industrial chemi- also improve the long-term, global com- mize the inventory and transportation of these materials." cals. Frost has been working to coax mi- petitiveness of U.S. industry. For example, methylisocyanate, familcrobes to convert D-glucose into industriFrost has developed technology emally important compounds, with the ploying genetically engineered microbes iar as a result of the tragic accident ingoal of replacing benzene as a starting to catalyze the synthesis of hydroqui- volving its release at Bhopal, India, in material (C&EN, Dec. 14,1992, page 23). none, benzoquinone, catechol, and, most 1984, was produced by the phosgenation Benzene is still used in the manufac- recently, adipic acid that is used in the of methylamine. Out of concern over the ture of a variety of chemicals even production of nylon. About 1.75 billion use and storage of the toxic material, Du though it is a carcinogen and must be lb of adipic acid are produced annually Pont developed a proprietary catalytic derived from nonrenewable fossil fu- in the U.S. In addition to requiring ben- oxidative dehydrogenation reaction proels. Frost points out that about 12 bil- zene as a starting material, manufacture cess that makes methylisocyanate and lion lb of benzene are produced in the of adipic acid also generates nitrous ox- converts it in situ to an agrochemical ide gas that contributes to global warm- product. In that way, the potential for U.S. each year. exposure is greatly reduced. "This trend Frost further notes that some 98% of ing and ozone depletion. Genes essential to the conversion of in in-situ manufacture and derivatizaall organic chemicals are currently manD-glucose into hydroquinone, benzoqui- tion is clearly the way of the future for ufactured from petroleum feedstocks, which puts the U.S. chemical industry at none, catechol, and adipic acid were iso- hazardous chemicals," says Manzer. Wayne L. Gladfelter, of the departa competitive disadvantage. D-Glucose is lated from microbes and then expressed abundant and inexpensive in the U.S. in Escherichia coli that were genetically ment of chemistry at the University of because it can be derived from numer- engineered to overexpress certain en- Minnesota, Minneapolis, is conducting ous agricultural products as well as zymes. The strategy is to make the or- research on the mechanism of highwaste streams from processing food ganism direct the largest possible per- pressure synthesis of carbamates as an
Alternative to Friedel-Crafts avoids toxic acids, solvents
28
SEPTEMBER 6,1993 C&EN
Engineered bacteria produce industrial chemicals from D-glucose alternative to the use of phosgene in synthesizing isocyanates and polycarbonates. His group is conducting insitu spectroscopic, kinetic, and mechanistic studies of the catalytic conversion of nitroaromatics and methanol to methyl N-arylcarbamates using a ruthenium catalyst. Another group is exploring the use of carbon dioxide in place of phosgene in the synthesis of urethanes and isocyanates. Dennis P. Riley, senior fellow and manager of process research at Monsanto, St. Louis, and colleagues have discovered new chemistry utilizing the reaction of carbon dioxide with a primary or secondary amine in the presence of base to generate a carbamate anion. "The control of the reactivity of this carbamate anion ... allows us to generate in quantitative yields either urethanes or isocyanates directly," say the researchers. The reactions are accomplished rapidly under relatively mild conditions: temperatures from 0 to 80 °C and pressures of 1 to 10 atm of carbon dioxide. Michael K. Stern, fellow at Monsanto Corporate Research in St. Louis, described work on halide-free routes for the production of aromatic amines. "One of the oldest practiced industrial chemical reactions is the activation of aromatic C—H bonds by chlorine oxidation. The resulting chlorobenzenes are employed in a variety of commercial processes for production of substituted aromatic amines. Since neither chlorine atom ultimately resides in the final product, the ratio of pounds of byproducts produced per pound of product generated in these processes is highly unfavorable," says Stern. "In addition, these processes typically generate aqueous waste streams which contain high levels of inorganic salts that are difficult and expensive to treat." Stern uses nucleophilic aromatic substitution for hydrogen to generate intermediates for manufacturing 4-arrrinodiphenylamine, eliminating the need for halogen oxidation (C&EN, Nov. 30,1992, page 26). Recent work in his laboratory has focused on direct formation of aromatic amide bonds by a similar approach. Stern says the reaction of benzamide and nitrobenzene generates 4-nitrobenzanilide in high yield under mild conditions. It represents "the first example of the direct formation of aromatic amide bonds via nucleophilic aromatic substitution for hydrogen, and represents a new route for the amination of nitrobenzene."
An alternative to the use of highly toxic and potentially explosive fluorinating reagents such as HF, FCIO3, and CF3OF was described by Guido P. Pez, chief scientist, inorganic and organofluorine chemistry, Air Products & HsOaPO^A^X^^ Chemicals, AUentown, Pa. The reagent, called SelectOH fluor, or F-TEDA-BF4, is 1ÇP0 3 H 2 chloromethyl-4-fluoro-l ,4OH diazonia[2.2.2]bicyclooctane bis(tetrafluoroborate). First C0 2 H synthesized by R. E. Banks at the University of Manchester Institute of Science & Technology in the U.K., H 2 0 3 PO the reagent was further deOH veloped at Air Products & Hydroquinone C0 2 H Chemicals. "Selectfluor ... provides pharmaceutical manufacX5s turers with a powerful tool for developing and proDHQ ducing high-performance Benzoquinone Benzene fluorinecontaining drugs/' .OH notes Pez. "Only four years ^ C0 2 H after the compound was OH discovered, pharmaceutical Catechol companies are beginning to OH H0 2 C use the reagent to manufacture existing drugs, and they are exploring its use C0 2 H for many new compounds Adipic acid now in development/' L-Phenylalanine L-Tyrosine Pez says the reagent fluL-Tryptophan orinates rapidly under mild Using genetically engineered organisms as synconditions with high effithetic catalysts to convert D-glucose into indusciency and selectivity, and trially important chemicals presents two chalrequires no special equiplenges: directing the largest possible percentage ment or handling techof the consumed D-glucose into the common niques. Soluble in many solpathway of aromatic amino acid synthesis, and vents and compatible with assembling new biosynthetic pathways inside common reactor materials, the organism to siphon carbon flow away from the reagent offers the addithose amino acids and into the synthesis of the tional advantage of being desired industrial chemicals. To synthesize hy"degradable into managedroquinone and benzoquinone, 3-dehydroable waste products/' he quinate (DHQ) is siphoned from the common pathway by the action of quinic acid dehydroadds. genase. Catechol and adipic acid synthesis rely So far, several compaon siphoning off 3-dehydroshikimate (DHS) nies are developing medicinal compounds such as fluorosteroid anti-inflammatory drugs, fluoronucleoside antiviral hazardous solvents in chemical reacagents, and nonsteroidal anti-inflamma- tions with environmentally friendly tory agents, says Pez. Fluoroaromatics ones. James M. Tanko and Joseph F. used as agrochemical intermediates may Blackert from the department of chemalso be generated in this way. istry at Virginia Polytechnic Institute & Another group reporting at the sym- State University, Blacksburg, are exposium is looking for ways to replace ploring supercritical fluids as a medi-
1
SEPTEMBER 6,1993 C&EN 2 9
SCIENCE/TECHNOLOGY
Route to aromatic amines is environmentally safer ο
/
— NH2 Benzamide H2
H 2 N—(\ V
/ > — NH 2 '/
-ι Catalyst
PPD
Base
\ N
—' Nitrobenzene
/—\ x
Π Η
/7~^\
—'
/ = \
H2N-/X / > — N 0 2 + (x 2 \_J/ \v
ο
/ ^
Cleaner routes found to rubber antiozonants
\=J
pj
CH 3 OH, NH 3
NHo
pNA
p-Nitroaniline (PNA) and /?-phenylenediamine (PPD) are synthesized by Stern and coworkers using nucleophilic aromatic substitution for hydrogen instead of a pathway using chlorine oxidation. Benzamide and nitrobenzene react in the presence of base under aerobic conditions to give 4-nitrobenzanilide in high yield. Further treatment with methanolic ammonia gives PNA and regenerates benzamide um in which to carry out free-radical reactions, replacing toxic solvents such as benzene and environmentally dam aging chlorofluorocarbons for that pur pose. "Supercritical fluids offer unique op portunities as a medium for probing solvent effects. Relatively minor chang es in temperature and/or pressure can be used to 'dial up' solvent properties such as viscosity and Hildebrand sol vent parameter without changing the molecular functionality of the solvent. Such flexibility is impossible with con ventional liquid solvents/' say the re searchers. The researchers report that free-radi cal brominations can be conducted in high yield in supercritical carbon diox ide solvent. Reaction of toluene with bromine in supercritical carbon dioxide yields benzyl bromide as the major re action product. In an effort to help chemists identify theoretical reaction pathways that are environmentally safer, J. Dirk Nies of Chemical Information Services, Rockville, Md., has teamed up with Anastas and Stephen C. DeVito of OPPT to evaluate software tools for synthesis design. On hand at the symposium to summarize recent results of the project was Joseph Breen, chief of the industri al chemical branch of OPPT. In various forms, such computer software has been under development around the country for about the past 25 years. Its purpose is to help chemists
identify new syntheses for target mole cules from the myriad potential routes and to suggest novel chemical reactions that might be investigated. Most of these software tools are ret30
SEPTEMBER 6,1993 C&EN
C HICAGO
rosynthetic—that is, they generate syn theses for target molecules by working backwards from the target to candidate starting materials. Other programs are synthetic—they identify side reactions, by-products, and the effects of varying conditions on reaction outcomes. "But none of them was built with the goals of environmentally benign synthesis in mind," says Nies. Out of about 20 software tools exam ined by Nies and colleagues, three pro grams appear to be most useful for pro viding theoretical alternative synthesis pathways in support of EPA's pollution prevention initiatives: Cameo, which op erates synthetically, and Syngen and Lhasa, which both operate retrosynthetically. Nies and colleagues find that "the Cameo, Lhasa, and Syngen computer programs appear useful for providing theoretical alternative synthetic path ways to target molecules. The user, how ever, will have to decide from consider ation of the health and environmental hazards of the starting reagents which pathways are environmentally safer." Applying retrosynthetic and synthet ic programs in sequence "in theory permits optimal routes to be identified, and their associated conditions, by products, estimated costs, and potential hazards to be compared," say the in vestigators. Nies looks toward the day when com puter-assisted synthesis design tools will include features to make them function as true expert systems in support of pollution prevention goals—a sort of "one-stop shopping" for the design of environmentally benign chemical syn theses. Π
Progress on developing new and better routes to certain important rubber pro cessing chemicals has been made by chemists from two different groups. As a class, these chemicals are the N-2-alkyl-N'phenyl-p-phenylenediamine antiozonants that are used to protect rubber tires and other rubber goods from degradation by ozone while in service. The usual way of making such unsymmetrical phenylenediamines is re ductive condensation of p-nitroso-, p-nitro-, or p-aminodiphenylamine with the appropriate methyl alkyl ketone. But production of these intermediates has been costly and energy intensive or has involved using chlorinated organic com pounds. Speaking to the Division of Organic Chemistry, technician Brian K. Cheng of Monsanto, St. Louis, described reaction of azobenzene with aniline to give a quantitative yield of p-phenylaminoazobenzene. This result established the abil ity of the azo group to set up reactivity of a benzene ring toward nucleophilic aromatic substitution. Cheng's cowork ers in the project were chemists Michael K. Stern and Frederick D. Hileman. Although Cheng did not go beyond the mechanistic implications of this reac tion, it is clear that treatment of the p-phenylaminoazobenzene product with a reductant such as sodium dithionite would yield p-aminodiphenylamine and aniline. The p-aminodiphenylamine could then be used for alkylation to an unsymmetrical phenylenediamine antiozonant, while the aniline would be re cycled for additional reaction with azo benzene. The Monsanto group earlier reported a relatively clean, simple way to make p-nitrosodiphenylamine [/. Am. Chem. Soc, 114, 9237 (1992)]. Reaction of ani line with nitrobenzene and tetramethylammonium hydroxide produces that antiozonant intermediate together with small amounts of other products. To an audience in the Division of Poly mer Chemistry, meanwhile, research
Two economical routes to rubber antiozonants developed H
ft
V-N==N—ft
\
+ C6H5NH2.
^
C
6
H
5
K H ^ N = N ^ )
^
^
C6H| i5NH2
+
4 Azobenzene
HO-
OH
HJ^
C6H5N
)—NH2
CH3
H3C—C"~~"Cn2
OH -!2L
C 6 H 5 N^^V=C
C
'—CH3
—
CH3 H C6H5N
CH3
-N-C—CH 2 —C—CH 3< H H H
[H]
/V-4-Methyï-2-pentyl-iV 'phenyl-p-phenylenediamine
Hydroquînone
chemist Joseph A. Kuczkowski of Goodyear Tire & Rubber, Akron, Ohio, described making unsymmetrical phenylenediamines from hydroquînone, which Goodyear produces. This project resulted in a relatively clean, low-energy synthesis. And observations made along the way suggested why alkylarylphenylenediamines have always made such good rubber antiozonants. Working with Goodyear chemists Kirkwood S. Cottman and Fredric H. Hoppstock, Kuczkowski began by reacting hydroquînone with aniline to get p-hydroxydiphenylamine. They oxidized this with a proprietary agent to N-phenyl-p-qumoneimine. They treated the qumoneimine with 2-amino-4-methylpentane to get N-4-methyl-2-pentyl-ATphenylquinonediimine, which they hydrogenated to the unsymmetrical phenylenediamine. This method thus uses an uncommon alkylamine to aminate a quinonetype ketone. The usual process calls for amination of the commercially available methyl isobutyl ketone (MIBK) by an aromatic amine. So to commercialize the new route, Goodyear will have to make the amine, probably by reductive amination of MIBK. During work toward the unsymmetrical phenylenediamine form of the antiozonant, the Goodyear researchers noted a reversible oxidation to the quinonediimine form. This observation led them to realize how the additive functions in rubber. In service, the additive is oxidized to the quinonediimine. But the diamine is the preferred, low-energy form—or, as Kuczkowski put it, a "thermodynamic sink/' Thus, the quinodiimine continuously abstracts hydrogen atoms from the rubber hydrocarbon, regenerating itself as the diamine antiozonant. And the additive
N
N-Phenyl-pphenylenediamine
I
NH2 C 6 H 5 NH 2
// ^
C6H5N—(/
Diazonium group provokes novel reaction co2cH3 OCH2CH2OH
C0 2 CH 3 OCH2CH2OH
HNQ2
NiCN
H2N C0 2 CH 3
C0 2 CH 3 Aniline derivative
Diazonium salt
therefore protects the rubber much longer than would be expected. Though not related to production of rubber processing chemicals, observations reported by chemists at the University of Mississippi show the power of groups other than the nirro group to activate an aromatic nucleus toward nucleophilic substitution. In that sense, their findings are analogous to those of the Monsanto group. Chemistry professor Charles A. Panetta told the Division of Organic Chem-
C0 2 CH 3 Quinonediazide ketal
istry that an attempt to convert a hydroxyethoxyaniline to a hydroxyethoxybenzonirrile via diazotization and nickel cyanide resulted instead in a quinonediazide ketal. He and his coworkers, chemistry professor Norman E. Heimer and graduate student Zheng Fang, suggest that the strongly alkaline conditions combined with activation by the diazonium group caused the hydroxyethoxy chain to snake around and attack the benzene ring. Stephen Stinson
Catalytic process cuts isobutene from C4 streams
CHICAGO A process for selective separation of isobutene from C4 streams via catalytic reaction has been developed by researchers at Texaco Chemical Co. John F. Knifton, Texaco honorary fellow, told the Division of Petroleum Chemistry that the process uses solid acid catalysts and involves the formation of intermediate glycol ethers. C4 streams issuing from steam or catalytic cracking operations contain desirable butènes that are currently separated
using several processes. Among these processes are selective polymerization of butènes, addition reactions with alcohols or water, selective extraction with acid solvents, and physical adsorption. For isobutene separation three processes dominate: extraction using a mineral acid, conversion of other stream constituents to isobutene through dehydrogenation (terf-butyl alcohol), and cracking (methyl ferf-butyl ether). Texaco's new process separates isobutene from mixed C4 hydrocarbon streams by initial etherification of the C4 mixture with a suitable diol to give the corresponding glycol mono tert-butyl ethers as intermediates. This is followed by de-etherification at higher temperaSEPTEMBER 6,1993 C&EN
31
SCIENCE/TECHNOLOGY tures to yield pure isobutene and regen erate glycol. Among the advantages to this approach are that the catalyst is a solid, no solvents are involved, and be cause of catalyst selectivity there are no equilibrium limitations. A typical C4 stream that would be treated in this way is the refinery stream often designated as ^affinate-l." This stream would contain, say, 12.7% isobu tene, 12.2% 1-butene, 13.4% ris-2-butene, 17.5% fraws-2-butene, 10.8% isobutane, and 31.6% π-butane, the remainder being miscellaneous C4 compounds. Classes of solid acid catalysts that would be suitable for use in the process ing include acidified montmorillonite clays and heteropoly acids dispersed on Group IV oxides. Knifton notes that a Japanese group has proposed a similar process using sulfonic acid organic res ins as catalysts. He believes the Texaco process works better because the Texaco catalysts have higher thermal stability, particularly at temperatures higher than 120 °C—the preferred temperature for de-etherification. In a typical pilot test of the Texaco
process, Raffinate-1 feedstock and 1,2propylene glycol were fed separately to a 50-cc plug-flow reactor charged with 12-tungstophosphoric acid on titania as the catalyst. Etherification to generate propylene glycol terf-butyl ethers oc curred at temperatures between 60 and 120 °C. All effluent products appeared in two layers. Data indicated up to a 44% conversion of the isobutene fraction to propylene glycol mono tert-butyl ethers. The principal product is l-butoxy-2-propanol with smaller quantities of 2-butoxy-1-propanol. The other C4 olefins do not undergo significant etherification. Montmorillonite clays also function as catalysts but are generally less effective than the heteropoly acids. The heavier glycol phase was separated from the lighter Raffinate-1 phase and passed, separately, through a similar 50-cc reactor charged with fresh catalyst. Deetherification of the glycolferf-butylether intermediates to produce pure isobutene plus glycol to be recycled occurs at higher reaction temperatures over the same classes of solid acid catalysts. Joseph Haggin
Quest for commercial polyvinylamine advances
CHICAGO For some years, chemists have been try ing to find a way to turn polyvinylamine (PVAm) into a commercial product. Re searchers at Air Products & Chemicals report progress in that quest. Beckoning chemists in their decadeslong interest is the potential of a product with the high density of very polar,
Lewis-basic amino groups, which could have useful interactions with other sub stances. Thus, PVAm could find uses in water treatment, papermaking, textile finishes, personal care products, adhesives, coatings, and oil field chemicals. The trick is to produce it easily and inex pensively. As research chemist Robert K. Pinschmidt Jr. told the Division of Polymer ic Materials: Science & Engineering, the key to success thus far has been making and working with N-vinylformamide monomer, CH 2 =CHNHCHO. The cor
Low-cost monomer technology opens route to polyvinylamine ^NHCHO H3CCHO + 2 H2NCHO — • H3CHC.
Δ
• 2 H 2 C=CH—NHCHO
NHCHO Ethylideneformamide Polymerization
^
,
x
^C^-ÇHJJJNHCHO Polyvinylformamide
32
SEPTEMBER 6,1993 C&EN
H + orOH~
.
/V-Vinylformamide /
\
(CH.-ÇH)^ NH2 Polyvinylamine
responding polyvinylformamide is readi ly hydrolyzed to PVAm in acidic or basic solution. Working with Richard J. Badesso and Dennis J. Sagl, Pinschmidt makes N-vi nylformamide by pyrolysis of ethylideneformamide, CH 3 CH(NHCHO) 2 , which in turn comes from acid-catalyzed condensation of formamide with acetaldehyde. The researchers have developed a series of solution and emulsion poly merizations of N-vinylformamide that give molecular weights of 2 million to 4 million, 300,000 to 500,000, and 50,000. These differing molecular weights yield a range of materials to try in different applications. Some of the situations met in making PVAm give a preview of the interesting properties that await users. For example, acid-catalyzed hydrolysis of polyvinylformamide proceeds to only 65% com pletion because of the intense positive charge building on the resin chains. Base-catalyzed hydrolysis, on the other hand, can be 100% complete. But because the formula weight of formate ions liberated (45) is comparable to the formula weight of vinylamine repeat ing units (43), large amounts of formate salts remain in intimate association with the resin. The Air Products team has succeeded in removing formates by dialysis or by precipitation from butanol solutions of the resin. Other, cheaper, more convenient ways to pro duce resins free of formates are in progress, Pinschmidt says. Some past efforts of other research ers give an idea of the fervor with which PVAm has been pursued. Poly merization of N-vinylphthalimide fol lowed by hydrazinolysis succeeded, but at great expense. N-Vinylacetamide was cheaper, but hydrolysis conditions were severe. By contrast, N-vinylformamide is quite tractable. It is a liquid with negli gible vapor pressure at room tempera ture that distills at 84 °C under 10 mm Hg, which corresponds to an atmo spheric boiling point of 210 °C. It can be stored for months at room temper ature. Its LD50 (the dose that is lethal to 50% of a population in an acute tox icity test) of 1444 mg per kg in rats in dicates low toxicity. Such azo com pounds as azobis(isobutyronitrile) are best as initiators, whereas persulfates generate acidity that could lead to ex plosions. Stephen Stinson
Fluorocarbon-tipped polyethylene synthesized
Polymerization process yields new polymers
C10F21I + H 2 C=CH(CH 2 ) 4 CH==CH 2 n-Perf luorodecyl iodide
C10F2i(CH2)6CH==CH2
1,7-Octadiene WCR
CHICAGO
CioF 2 i(CH 2 ) e CH=CH(CH 2 ) e C 10 F 21 + H2C = C H 2 |
What would polyethylene be like if chemists could actually make it? Real-life polymerization of ethylene gives chains that sprout branches in great numbers and varying lengths. Even so-called linear low-density polyethylene is only relatively linear in that the branches are short and of controlled distribution. In Chicago, chemists from Dow Chemical and the University of Florida, Gainesville, told the Division of Polymer Chemistry that they can now make defect-free polyethylene by metathesis polymerization of dienes, followed by reduction of residual double bonds. On another note, what would chemists get if they linked blocks of perfect polyethylene with blocks of polytetrafluoroethylene? Researchers from Du Pont told the Polymer Chemistry Division that when they tackled the problem with metathesis polymerization, they got a resin that processes as easily as polyethylene but has a no-stick surface like Du Ponfs Teflon fluorocarbon resin. Researchers have used olefin metathesis for decades as a source of both commercial chemicals and resins and of products of academic interest. Metathesis means transposition of parts. In terms of olefins, it means that two nine-carbon molecules of, for example, 1-nonene react to form a 16-carbon molecule of 8-hexadecene and a twocarbon molecule of ethylene. And when the starting material is a diolefin such as 1,9-decadiene, the products are
WCR WCR = Tungsten carbene p-CH3C6H4S02NHNH2 CIQF2I-(-CH2CH2-)-C1OF21
« )
CioFg^CHJeCHicHiCHgJ^CH^CHiCH^C^Fg!
Fluorocarbon-tipped polyethylene
polyoctenylene, an unsaturated resin with eight-carbon repeating units, and ethylene split out. This is the approach described by chemist Stephen F. Hahn of Dow. Working with organic chemistry professor Kenneth B. Wagener and postdoctoral fellow John E. O'Gara from the University of Florida, Hahn first made polyoctenylene. The metathesis polymerization catalyst was a molybdenum carbene originally invented by organic chemistry professor Richard R. Schrock of the Massachusetts Institute of Technology. Next the Florida chemists reduced the double bonds of polyoctenylene with p-toluenesulfonylhydrazide, a source of the hydrogen donor diimide, NH = NH. This yielded defect-free polyethylene. They prepared the resin in four molecular weight ranges from 6000 to 40,000 with polydispersities between 2.0 and 2.6. Polydispersity is the ratio of the weight-average molecular weight to the number-average molecular weight. Values close to 1.0 indicate a narrow spectrum of chain lengths. Melting points between 130 and 132 °C for all molecular weight ranges and high calorimetric
Metathesis polymerization yields defect-free polyethylene H2C=CH-(CH2^CH=CH2 J ^
i c H - ( C H 2 )-CH=|= L
1,9-Decadiene
6
+
Jη
Polyoctenylene p-CH 3 C 6 H 4 S0 2 NHNH 2
—JCH 2 -^CH 2 |-CH 2 4— 6 L Jn MoCR = Molybdenum carbene
Polyethylene
H 2 C=CH 2 Î
heats of melting indicated high degrees of crystallinity. The work at Du Pont focused on fashioning polyethylene chains of molecular weights varying from 5000 to 200,000 with perfluorodecyl (-C10F21) end-groups attached. The resulting resins had melting points, of about 128 °C, compared with 327 °C for polytetrafluoroethylene (PTFE), which meant that the new resins could be processed more easily. And yet the surfaces of the fluorine-tipped resins were even less wettable than PTFE itself. Du Pont chemist Steve J. McLain, who described the work, ascribed these surface properties to a higher concentration of fluoropolymer at the surface, and in particular to a high concentration of very unwettable trifluoromethyl (-CF3) groups. Working with Bryan Sauer and Lawrence Firment, McLain measured wettability toward hexadecane by contact angle, the angle that a bead of liquid makes with a surface. If there is no wetting, the bead stands straight up on the surface at a 90° angle. The Du Pont scientists began resin syntheses by coupling n-perfluorodecyl iodide with 1,7-octadiene to get 1-octadecene with the last 10 carbon atoms fluorinated. Treatment of this monoolefin with a tungsten carbene metathesis catalyst of the Schrock type furnished 17-tetratriacontane with carbon atoms C-l to C-10 and C-25 to C-34 fluorinated. Metathesis copolymerization of that olefin with cyclododecene yielded an unsaturated version of the final polymer. By hydrogenating the resin with p-toluenesulfonylhydrazide, the team reached its ultimate goal—the synthesis of fluorocarbon-tipped polyethylene. Stephen Stinson SEPTEMBER 6,1993 C&EN 33
