The University of CaliforniaâLos Angeles Styrene Process - ACS

Chapter 9

The University of California—Los Angeles Styrene Process

Downloaded by STANFORD UNIV GREEN LIBR on August 18, 2012 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0577.ch009

Orville L. Chapman Department of Chemistry and Biochemistry, University of California—Los Angeles, Los Angeles, CA 90024-1569

Styrene manufacture utilizes approximately 13 billion pounds of benzene each year. Existing technology alkylates benzene with ethylene and then dehydrogenates the ethylbenzene. Environmental considerations dictate that we should replace benzene, and the UCLA styrene process can. This process uses only mixed xylenes, which are more environmentally friendly than benzene. The UCLA styrene process converts equilibrium mixed xylenes, the cheapest aromatic source available, to styrene in a single high-temperature step. The mechanism of this remarkable oxidative rearrangement is complex, but the overall process is very simple.

The existing styrene process uses two starting materials, two steps, and two catalysts, and rests on roughly one hundred million dollars in research and development. Thefirststep alkylates benzene with ethylene using an acid catalyst. The second step dehydrogenates ethyl benzene to styrene using a dehydrogenation catalyst. Both steps give high yields with very low production of by-products. Research and development and capital investment have long ago been recouped, and the process makes money. The only problem is a nagging fear that the chemical industry is going to have to do without benzene. In human livers, the P-450 enzyme system converts benzene to benzene epoxide, which is a carcinogen. Concern about the safety of benzene and the scale on which it is used-about 13 billion lbs/year in the United States-has fostered sporadic attempts to find an economically satisfactory replacement process.

0097-6156/94/0577-0114$08.00/0 © 1994 American Chemical Society

In Benign by Design; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

9. CHAPMAN

The UCLA Styrene Process


Ο

Catalyst

a

+ H C=CH 2

CH CH 2

115

^^xCH^CI^

2

Catalyst

3

^w^CH=CH

— -

2

Ο

Alternatives to benzene are indeed few, and attention has focused primarily on toluene. Innes and Swift have summarized and critically reviewed the work with toluene (7). No really satisfactory alternative process exists. As strange as it may seem, no one has published work on xylenes as an alternative to benzene in styrene production even though the xylenes have eight carbons as does styrene.

σ

CH

PbO

3

»-

HCPh=CHPh

Olefin PhCH=CHPh+ H C=CH 2

CH=CH

2

2

Metathesis The UCLA styrene process derivesfromour investigation of the mechanism of the transformation of the isomeric diazomethyltoluenes to benzocyclobutane and styrene in low yield (2-5). An alternative process for a major industrial commodity chemical thus began with a purely academic investigation of reaction mechanism. The mechanism of these intriguing rearrangements begins with nitrogen expulsion giving the tolylmethylenes. The tolylmethylenes interconvert through the

+ Ν, HaC

Tolyldiazomethane

0 r U

S

h t

Tolylmethylene


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BENIGN BY DESIGN


methylcycloheptatetraenes ultimately yielding benzocyclobutane and styrene. Using low-temperature matrix-isolation spectroscopy with infrared, ultraviolet, and electron spin resonance spectroscopy as probes and guided by our earlier characterization of 1,2,4,6-cycloheptatetraene (6-7), we observed and characterized each of the intermediates in Scheme 1 (8-9). At the suggestion of industrial friends, we turned our attention to the possibility of making styrene the sole product of the

Scheme 1 process. We needed two things to make styrene. First, we had to convert benzocyclobutane to styrene, and second, we had to find an economically satisfactory route to the tolylmethylenes. In point of fact, the second problem proved to be the limiting factor. The conversion of benzocyclobutane to styrene was, in fact, a solved problem (2-5). Baron and DeCamp had reported a 94% yield at 930 degrees Centigrade (5).


9. CHAPMAN



a

ÇH CH

2

117

930° C

2

We were interested in knowing whether the mechanism that we had elucidated at low temperature using photochemical interconversion applied also in the high-temperature domain. Scheme 2 shows two possible mechanisms, which carbon-13 labeling can distinguish. The standard diradical mechanism predicts equal labeling in the alpha and beta carbons of styrene; the mechanism based on our earlier studies predicts equal labeling in the ortho and beta positions of styrene. Mechanism 1

Scheme 2 The labeling results showed beta (48 %), ortho (30 %), alpha (14 %), meta

(4 %), and para (4 %), These results are clearly consistent with our mechanism (Mechanism 2) as the dominant mechanism, but some minor processes must also intervene. The alpha label could come from the diradical mechanism, but neither mechanism that we considered (Scheme 2) accounts for the minor amounts of meta and para labels. As a control, we checked the possibility that styrene itself can rearrange under the conditions. In fact, to our great surprise we found that betalabeled styrene gave alpha-labeled styrene in 4% yield (5).


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This process accounts for at least some of the observed alpha label. Interconversion of /?ora-tolylmethylene and /?ara-xylylene and meia-xylylene could, however, introduce label into the meta and para positions of the styrene. These interconversions would occur in a loop that might exist in our mechanism.



9. CHAPMAN


119

We could easily test for para-xylylene and meta-xylylene conversion to styrene, which would imply conversion of the xylylenes to the corresponding tolylmethylenes. Pyrolysis of [2.2]/?ara-cyclophane gives /rara-xylylene and by inference [2.2]m£to-cyclophane pyrolysis should give meta-xylylene. At 930° C, both cyclophanes ought to give styrene if the xylylenes convert to the tolylmethylenes. In fact, high-temperature pyrolysis of both cyclophanes does give styrene (5). The principle of microscopic reversibility suggests that the xylylenes interconvert with the tolylmethylenes. Thermodynamics favor the xylylenes over the tolylmethylenes, and orr/io-tolylmethylene certainly goes to ortho-xylylcne (10). The loop mechanism with the interconversion of the xylylenes and the tolylmethylenes explains the minor labels in the meta and para positions of styrene (5). The idea that /rara-tolylmethylene and para-xylylene interconvert suggested that we might use para-xyltne as a precursor to styrene because Union Carbide Corporation has a patent that describes the thermal conversion of para-xylene to a high-temperature polymer presumably via para-xylylene. In fact, thermolysis of /wa-xylylene at high temperature does give styrene (77). In addition, meta and ortho xylenes also give styrene (77). Commercial, equilibrium-mixed xylenes, which contain some ethylbenzene, also give styrene (77). The equilibrium mixed xylenes are the cheapest aromatic available and are environmentally much safer than benzene. An alternative to benzene and toluene as raw materials for styrene manufacture now exists. The process involves a single-step conversion that does not use a catalyst and gives 40% per pass yields of styrene. It uses a single, cheaper starting material that is environmentally safer than benzene. Physiological oxidation of the isomeric xylenes involves the methyl groups and does not make the arene oxide, which is the source of problems. The UCLA styrene process can probably use existing styrene plants, but a considerable development looms before this is a true commercial process. In fairness, the current styrene process development has invested roughly 10^timesthe dollars that we have spent in developing the UCLA styrene process. This study presents an interesting case of the United States research strategy in action. The National Science Foundation funded basic research directed toward solving an intriguing mechanistic problem, and the Environmental Protection Agency seeing promise in the results funded the development of the equilibrium mixed xylenes as a safer alternative to benzene. The fate of the UCLA styrene process rests on industry interest and on the political decision to take benzene out the global environment. If the decision is made to eliminate benzene, the UCLA styrene process offers an alternative. The simple fact that an alternative exists makes the political decision easier to make. Removing 13 billion pounds of benzene from global commerce can make a difference. Acknowledgment This work was supported by the National Science Foundation and by the Environmental Protection Agency.


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Literature Cited 1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11.

Innes, R. Α.; Swift, H. E. CHEMTECH 1981, 244-248. Cava, M. P.; Deana, A. A. J. Am. Chem. Soc. 1959, 81, 4266-4268. Baron, W. J.; DeCamp, M. R. Tetrahedron Lett. 1973, 4225-4228. Chapman, O. L.; Tsou, U.-P. Ε. J. Am. Chem. Soc. 1984, 106, 7974-7976. Chapman, O. L.; Tsou, U.-P. E.; Johnson, J. W. J. Am. Chem. Soc. 1987, 109, 553-559. West. P. R.; Chapman, O. L.; LeRoux, J.-P. J. Am. Chem. Soc. 1982, 104, 1779-1782. McMahon, R. J.; Abelt, C. J.; Chapman, O. L.; Johnson, J. W.; Kreil, C.L.; LeRoux, J.-P.; Mooring, A. M.; West, P. R. J. Am. Chem. Soc. 1987, 24562469. Chapman, O. L.; McMahon, R. J.; West, P. R. J. Am. Chem. Soc. 1984, 106, 7973-7974. Chapman, O. L.; Johnson, J. W.; McMahon, R. J.; West, P. R. J. Am. Chem. Soc. 1988, 110, 501-509. McMahon, R. J.; Chapman, O. L. J. Am. Chem. Soc. 1987, 109, 683-692. Chapman, O. L.; Tsou, U.-P. E.; U.S. Patent 4,554,782, October 1, 1985.

RECEIVED September 13, 1994


The University of CaliforniaâLos Angeles Styrene Process - ACS

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The University of CaliforniaâLos Angeles Styrene Process - ACS