Ethylene: lhe Organic Chemical 1ndustry"S Most Important Building Block Edited by
W. Conrad Fernelius Harold Wittcoff Robert E. Varnerin How many pounds of organic chemicals do we produce yearly in the US.? The answer is 250 billion pounds per year-more than 1000 pounds for every person in the U.S. I t is hard for even a chemist to visualize how each of us in this (:c,untryuses a half-ton ol chemicals yearly. We'd cwtainly he overwhelmed if someone dumped our share on uur front lawn. In fact, of course, those chemkals do not appear on our front lawn except perhaps as fertilizer and weed killers. But they do appear in the clothes we wear, the homes we live in, the furnishings in those homes, the offices and plants we work in, the food we eat, our automobiles and airplanes, the medical equipment that is used to make us well, the sports equipment that gives us recreational pleasure, and, of course, in the vehicles that go to the moon and that monitor the planets. In other words, the more than 1000 pounds of organic chemicals produced for each of us yearly provides the basis for modern technolow. More ;l;m Y(Pooforganic chemicals come from petroltwn and natural gas. and i t ~ian amazing fact that prnl t~callyall of these chemicals are based on seven raw materials: ethylene, propylene, Ca unsaturates (hutenes and hutadiene), henzene, toluene, xylene, and methane. It is indeed a compliment to the chemist and chemical engineer that our sophisticated chemical technology is based on these seven simple materials. The most important is ethylene, and in 1977, 25 billion pounds were produced. Note that this is 10% of the 250 billion pound total. From that quantity of ethylene, 85 billion more pounds of chemicals and polymers are produced. Thus, if we add together the amount of ethylene and the amount of chemicals we prepare from ethylene we get 110 billion pounds-44% of all the organic chemicals manufactured. The Source of Ethylene Ethylene does not occur as such in petroleum or natural gas. I t is made by the so-called cracking o6hydrocarbons such as ethane. orooane. and butane which are found in both nil and nntural gas'and higher hydrocnrhms and cvcloaliphstic hydrocarbons which compris~~ the fraction of petroleum knuwn as naphtha. When these hydrocarbons are pyrolyzed-usually in the presence of steam to reduce the partial pressure of the reactant and prevent carhmization-ethylene, propylene. and [he l)utt!nrs result. Thus, cracking hreaks carhon-mrhun bonds and also brines about dehydroeenation. This thermal or steam cracking is Lot to he conFoseiwith catalytic cracking whose maior Duroose is to break the large molecules found in petroleum into smaller ones useful for gasoline. The mechanism for thermal cracking has been described by Wiseman.' Ethylene is always the major product of cracking and the amount of ethvlene produced relative to the amount of propylene and the butehes from various feedstocks is shown in Table 1. The Chemistry of Ethylene Ethylene undergoes three distinct types of reactions. Ethylene, first of all, can he polymerized and oligomerized. Secondly, in chemistry that classical textbooks would consider heretical, one of the ethylenic hydrogens of ethylene can he
. . .
' Wiseman,P., J. CHEM. EDUC., 54,154 (1977).
Table 1. Amounts of Unsaturates Produced by Cracklng Various Feedstocks Millions of Pounds of Product Produced from Sufficient Feedstock to Give One Billion Pounds of Ethylene Med. Range
Prcducts
Ethane
Prooane
+Butane
Naohtha or Gas Oil
Ethylene Propylene Butenes
1000
1000 400 101
1000 432 255
1000 462 26 1
36 35
replaced by a functional group. An example is the replacement of a hydrogen with an acetoxy group to yield vinyl acetate. Thirdly, ethylene undergoes classical reactions such as the Friedel-Crafts condensation with henzene to provide ethyl"
.... ..- . >
Ethylene may he polymerized in the presence 01a peruride cataly4 at temperatures of 3U0°(: and pressures of' 15,000 psi to orovide so-mlled low density. .polyethylene. It was t h ~rei a c h n that thermodynamicists predicted could never happen! This means that although one dare not areue with thermodynamics, one should at times have the temerity to argue with thermodynamicists about the adeauacv . . of the data for the applicathn of thermodynamics. Low density polyethylene is the polymer produced in largest volume and is familiar to us in the form of film which is its largest application. Low density polyethylene is, in addition, an excellent insulator; and its first application, a highly valuable one during World War 11, was as an insulator for high freauencv radar cables. High density polyethylene evolved I)el.alae chemisrsafter World War I1 wrre trvinr! . .. to learn how to condense ethvlene into gasoline-sizemoleculrs. Inswad the cawlya systems they were using provided H rx,lvethvlene with a higher density than . . the polyethylene then-known, a higher mole&lar weight, and much less branching. Thus, the product was stiffer and had greater tensile strength. In addition i t could he made under mild conditions such as 60°F and 300 psi. A breakthrough announced recently is a process for making low density polyethylene a t these mild conditions. High density polyethvlene's first aoolication. ~ e r h a ounbelievable s but still true. waifor hula hoops. We knbw higf;density polyethylene today in the forms of bottles used for bleach. for lahoratorv.eauio.. ment, and for molded parts. Inherent in the discovery of high density polyethylene was a Nohel Prize for the German chemist, Karl Ziegler, whose catalyst system comprised titanium tri- or tetrachloride with an aluminum trialkyl. He shared the Nohel Prize with Gulio Natta who showed that Ziegler's catalyst system was widely applicable to unsaturated materials and that, quite unexpectedly, the catalyst system provided so-called stereoregular polymers, i.e., polymers whose chiral centers have regularly recurring patterns of R and S character. Nature had always known how to do this. Natural rubber. for examnle. . . is a stereoregular polymer. The ability to duplicate nature's s.ynthetic orowcss excited wide attention. There are other catalvst systems very useful for providing high density polyethylene, primarily oxides of molybdenum or chromium supported on ~
Volume 56, Number 6, June 1979 1 385
silica. hut these do not have the wide a. ~.~ l i c a h i l i of t v the Ziegler catalysts. Olieomerization of ethylene starts with aluminum triethyl. ~ e a c c o nwith more ethyiene leads to aluminum trialkyls &h 4-22 carhon atoms in the ally1 groups. Reaction of the aluminum trialkyl with water leads to fatty alcohols whereas treatment with more ethylene a t high temperatures gives a-olefins. Ethylene can also he oligomerized directly to a-olefins with a nickel catalyst. The C12 and Clq alcohols and a-olefins are of ereatest value. for these. when sulfated and suli~nated,bvcome surface a t t i w ayrnti for use in detergents which ;art, rendilv hicdeerarlahle and which are tderant to h a d water in low concentrations of phosphate builders. There are three i m ~ o r t a ncom~ounds t that result from the replacement of a hydrogen atom of ethylene. In one, the UTackerreaction, ethylene is converted directly to acetaldehyde in the presence of a palladium chloride-cupric chloride catalyst with oxygen, water, and hydrochloric acid. O2.PdCI*IC"C12/HCI CH2=CH2 CHGHO acetaldehyde Conceivahly the hydrogen is replaced by a hydroxyl group to give vinyl alcohol which rearranges to acetaldehyde. Although this is the net result, it is not the mechanism. Figure 1shows a proposed mechanism involving a a-bonded complex between ethylene's double hond and the palladium atom. Oxidation of acetaldehyde to acetic acid is a major hut declining use of acetaldehyde. In a dramatic new reaction, methanol is carhonylated to yield acetic acid in very pure form.
Acetic acid can also he made by the oxidation of butane, a cheap starting material. But oxidations tend to give mixtures of products which require expensive separation. Thus the methanol carhonylation technique is now the preferred way to make acetic acid. Acetic acid is in turn converted to acetic anhydride. This is done by pyrolyzing the acetic acid to ketene and reacting the ketene with more acetic acid to yield the anhydride. One of the major uses for acetic anhydride is in the production of cellulose acetate used in films, packaging, and molding applications. Another important reaction of acetaldehyde which has been made obsolete by newer chemistry is its conversion to n-hntanol by the aldol reaction. ZCHBCHO~CH~-CHOH-CH~-CH -H*0
dCH3-CH2-CH1-CHzOH HZ n-butanol A cheaper way to make alcohols is hy the so-called 0x0 reaction in which an olefin is treated with CO and hydrogen. This method is more economical because the olefin is cheaper than a derivative of it like acetaldehyde. However, in the past this has led to a mixture of isomers as the following equation with propylene demonstrates.
H,. co
CH,--CH=CH,
CO(CO),
CH,-CH2-CH,-CHO n-butyraldehyde
CHO isohutyraldehyde The aldehydes, of course, can he reduced to the corresponding alcohols. Today, however, catalysts have been discovered, one of which is a rhodium carhonyl complex with an organophosphine ligand, which give very high yields of the linear ~roduct
386 1 Journal of Chemical Education
r BONDED
COMPLEX
4
CH,CHO
H P
+ Pd + 4 HCI
Figure 1. Mechanism for acetaldehyde formation.
Thus the oxo reaction on propylene is the preferred way to make n-hutanol. Another reaction in which an ethylenic hydrogen is replaced with a functional group involves treatment with acetic acid and oxygen in the presence of the same catalyst ~.system used for acetaldehyde to give vinyl acetate. CH2=CH2
OZ.CH~CVOH,P~CISIC~CII
CH3COzCH-CH2 vinyl acetate A hydrogen is replaced by an acetoxy group. This too is called a Wacker reaction although the reaction differs from acetaldehyde formation in that the HC1 is omitted so that the catalyst system is heterogeneous rather than homogeneous. Although the reaction goes with a homogeneous catalyst, corrosion is extensive, and it is for this reason that the HC1 is omitted. The biggest use for vinyl acetate is to make polymers-poly(viny1 acetate) and its copolymers-which end up as coatings such as the latex paints for our walls and as adhesives. The replacement of a chlorine atom in ethylene leads to vinyl chloride. This is a two-step reaction. CH*=CHz
CI? -HCI 4 C H p C I C H 2 C I dCH2=CHCI
ethylene vinyl dichloride chloride Very interesting is the reaction known as an oxychlorination CH2=CH2
CUCI~,KCI.AIZO~ or SiOn HCI.03. 2S0°C
-HCI
CH2CICHzCI+CHFCHCI
Here ethylene dichioride is made not by adding chlorine to the double hond of ethvlene hut bv reactine HC1 and oxveen with ethylene in the of cupric chloide and pota&ikn chloride supported on alumina or silica. The problem with chlorine addition arises in the second step where a mole of HCI is s ~ l iout. t HCI is a hiehlv corrosive material which cannot he Shipped and for whici ;use must usually he found on-site. Such a use is the oxychlorination reaction. Most vinyl chloride plants make half the necessary ethylene dichloride by direct chlorination and half hy oxychlorination. T h e higgest use of vinyl chloride is for the manufacture of polymers and copolymers. Vinyl polymers are all around us in the form of upholstery fabric, clothing, flooring, luggage, siding for homes, electrical insulation, and resistant coatings. The major so-called classical reaction of ethylene is direct oxidation to ethylene oxide which in turn is reacted with water to give ethylene glycol. H,O
0 2
CH,=CHI
4
A=
CH,-CH,
\/
* CHPH-CH,OH ethylene glycol
ethylene oxide Ethylene glycol has two major uses. One is as antifreeze and the other is for reaction with terephthalic acid to provide the fiher-forming polyester known a s Dacron. Recently, a plant
has gone on-stream to prepare ethylene glycol by the following very interesting reaction
.
T e O , RX. ~ i +
CH,=CHI
CH,COOH. 0,
ETHYLBENZENE
ethylene glycol diacetate 2CH,COOH + HOCH,CH,OH acetic ethylene acid glycol Here a complex catalyst system is used and ethylene glycol diacetate results. This reaction of course is reminiscent of the Wacker reaction for vinyl acetate and indeed pyrolysis of the diacetnk yields vinyl ac&te. Hydrolysis of the acetoxy p0ups leads toethyltmeglycul :md aceticacid whi~hcnn be recycled. 11\11if this The nvw nlmt issnid to be ha5,inesrart-~~~~difticult\. reaction goes it will cause a marked decrease in the production of ethvlene oxide. The following equation shows the reaction of benzene with ethylene to provide ethylbenzene in a classical FriedelLCrafts reaction. Dehydrogenation or cracking leads to styrene whose hieeest oolvstvrene and stvrene ." use is in the ~roductionof . . copolymers. ~~
-
H--TH,
-H,
H=TH
ethyl benzene styrene We find these in foams used for packaging, decorative purDoses, and insulation as well as in the throwaway items such as tumblers. Styrene copolymers with butadiene and styrene can be molded into automobile parts and extruded into pipe. The synthesis of styrene certainly is straightforward and seems to bee" the auestion of whether there mieht he other methods for its manufacture. The chemical industry is characterized hv the fact that there are usuallv several wavs to make large volume chemicals, and styrene is no exception. Recentlv a plant has cone on-stream to manufacture stvrene as a copr&ct ofpropylene oxide according to rhe ren;tims t said to have cnoaciry for d(Njrni1lion in Picure 2. This ~ l a n is pounds per year of propylene oxide and one billion pounds per year of styrene. Since total styrene consumption in the U S . in 1977 was 6.8 billion pounds, the new process is significant. Two advantages are associated with it. First of all two products are prepared in one set of equipment, probably allowing for decreased capital'costs. Naturally there must be markets for the two materials in anoroximatelv the volume ratios a t which they are produced. SkEondly thedehydration of the phenylethylcarbinol is asimpler reaction than the cracking if ethylbenzene and, correspondingly. yields are higher. A simple reaction of ethylene is hvdration over a ohosphoric . acid catalyst to provide eihanol. Hz0
PHENYLMETHYLCARBINOL
I
CHFCHZ -+CH3-CHzOH Ethanol Ethanol usage is decreasing for a t least two reasons. One is that export of ethanol is beriming less as countries around thc world set up to manufactnre it. The second is that ethanul fmnerly was used to manuf;tcture acvtldehyde by oxidation. ~ l ~ in there tigures Iwcawc the US. Hurmu "l'lere a d c ~ r lcounting of the Census has decided that u.hrn a chemical or pdymrr i~ p.xkaged or removed from one reaction site to another by, for example, a ni~elineit must be counted as a finished ~roduct.Thus in our firmres wk fount ethylene,ethylbenzene, styrene;and polystyrene sepa&tely even though theonly product in that sequence that finds an end use is the polystyrene. Kirk-Othmer,"Encyclopedia of Chemical Technology," Vols. 1-22 plus supplement, Interscience Publishers, New York, 1963-71.
TH=THI
STYRENE
~
-
+ CH =TH
PROPYLENE OXIDE
ETHYLBENZENE HYDROPEROXIDE
Figure 2. New Styrene synthesis. Today, however, practically all acetaldehyde is made by the Wacker reaction discussed above. In the above discussion we have covered the major chemistry of ethylene in terms of pounds of product produced. Tahle 2 shows the major chemicals and polymers produced from ethylene and the volume in 1977. The chemicals and polymers listed in Tahle 2 amount to about 74 billion pounds. If we combine this figure with 25 billion pounds of ethylene from which the materials were produced we end u p with 99 billion pounds, or 90% of the 110 billion pounds of chemicals and polymers that we initially indicated were made from eth~lene.~ The remaining 10% accounts for a vast amount of chemistry not even mentioned in this brief article. Indeed there is a large body of chemistry associated with each of the chemicals that has been discussed. The "Encyclopedia of Chemical Technology""~ an excellent source of additional information. Ethylene is the organic chemical industry's most important building block. The chemistry appears simple but in fact it is sophisticated, largely hecausiof heavy reliance on catalysis. It is chemistry that affects the lives of almost everyone in the world dailv. Thus in the world's most nrimitive societies women 11111 the clothes they wash in d strewn in polyethylene haskrts. And in xwhisticated sncieties...uolveth\.le~~e urovidcs " " insulation for sophisticated electronic devices that 'take astronauts to the moon. Table 2. Chemicals and Polymers from Ethylene Chemical
Millions of Pounds-1977
1) Ethylene dichloride 2) Styrene 3) Ethylbenzene 4) Vinyl chloride 5) Ethylene oxide
10.480
5.820 7.300 5.810 4.420
6) Elhylene glycol 7) Acetic acida 8) Ethanol 9) Acetic anhydride' 10) Acetaldehyde 11) Vinyl acetate
3.470 2.580 1,300 1.500
892 1,480
Tdal Polymer
46.052
Millions of Pounds-1977
Palyethyiene-low density Polystyrene and copolymers Poly(vinylchloride)and copolymers 4) Styrene-butadieneelastomers 5) Terephthalate polyester 6) Polyelhylene-high density 7) Poly(viny1acetate) 1) 2) 3)
6.470 4.630 6.250 3.130 3,640 3,650 526' Total
?R?86
~*saUsasidamacaNsanny*~*~~IIImsle~diit~mox~~~m~mmmm,~oIMOH
'
and CO. 1975 tigua
Volume 56, Number 6, June 1979 / 387
