real world of indu~t rial c hemift ry
edited by
K. E. KOLB Bradley University Peoria. IL 61625
HAROLO W~TTCOFF Chem Systems. International. Ltd. 26 St. James Square London SWlY 4JH. Enqland
The SHOP Process An Example of Industrial Creativity Bryan Reuben South Bank Polytechnic. Borough Road, London SEl OAA, England Harold Wlttcoff Chem Systems International Ltd.. 28 St. James's Square, London S W l Y 4JH, England The Shell Higher Olefins Process (SHOP) is probably the most remarkable industrial chemical process to have been developed in the past decade. I t involves four reactionsoligomerization, linear hydroformylation, double bond isomerization, and metathesis-reactions which have scarcely found their way into college chemistry textbooks. At the same time i t has satisfied an important industrial need. The new process involves the manipulation of older reactions with aparticular application in mind; it is this missionrelated chemistry that typifies creative thinking in industry. Themain aimof the process is the manufacture of primary Cll-Cls fatty alcohols. These are valuable for the manufacture of alcohol sulfate detergents. Such detergents have been known for a long time hut were used only to a limited extent in heavy duty detergents because of their cost. They are desired now because they hiodegrade more quickly than the conventional linear alkylbenzene sulfonates and are less toxic to fish. The traditional route to straight-chain fatty alcohols involves the alcoholysis of a vegetable oil-a fatty acid triglyceride--to its methyl ester, which is then converted to a primary alcohol hy hydrogenolysis. The production of lauric . acid from coconut oil is an example: ~
-
CHpOOC C I I H Z ~
I CHaOH CH OOC CIIHZ3 I CH200C CnHz3
CH20H
I I
CHOH
+ 3CllHz3COOCHB
CH20H
Cat. 3C~H250H+ 3CHsOH
The chemical industry does not like agricultural feedstocks because supplies are unreliahle and fluctuate with factors such as the weather and attack by pests. As the detergent industry grew, various routes to straight-chain intermediates were devised based on ~etrochemicalfeedstocks. All these suffered from the sameproblem; they led to mixtures of products that spanned a range of molecular wei~hts.Once thd compounds in the detergent range had been kxtracted, byproducts of lower and higher chain length remained, of which few found a ready market. The SHOP process solves this problem by producing only alcohols of the chain length nreferred for surfactants. A simplified flowsheet for the SHOP process is shown in the firure. We shall describe the individual reactions before showcug how they integrate into the overall process. The starting material is ethylene (ethene), which is readily available from the steam cracking of hydrocarbons, aprocess that is the keystone of the petrochemical industry and that is carried out in huge plants each producing at least one billion pounds per year of ethylene. Ollgornerlzatlon Ethylene may he polymerized to polyethylene by ZieglerNatta polymerization, which involves the use of a titanium tetrachloride/aluminum trialkyl catalyst. A related process gives ethylene oligomers. Thus stoichiometric amounts of aluminum triethyl react with ethylene to give trialkylaluminum compounds. Air oxidation gives aluminum alkoxides, which may he hydrolyzed to straight-chain fatty alcohols or pyrolyzed to olefins:
Volume 65 Number 7 July 1988
605
straight-chain fatty alcohols
linear a-olefins
Isomerlzatlon Catalysts are well known that will shift double bonds from the a-position to random positions in a molecule to give socalled internal olefins. Basic catalysts such as potassium hydroxide are effective; Shell probably uses magnesium oxide granules. Metathesis
This was the basis of the Alfol process, developed in 1960 as a route to straight-chain intermediates containing even numbers of carbon atoms. Shell devised a catalytic process, with a homogeneous catalyst consisting of nickel chloride and the potassium salt of o-diphenylphosphinobenzoic acid in 1.4hutanediol. Under the conditions of operation x , y, and z lie between 1and 11 for the Alfol product and between 1and 19 for the SHOP product. Linear Hydroformylation The conventional hydroformylation reaction has been used hy the chemical industry since 1940. a-Olefins react with carbon monoxide and hydrogen at 150 "C and 200 bar in the presence of a hydrocarbon-soluble dicobalt octacarbonyl complex, CO~(CO)S. The product is a mixture of the straieht chain and branched chain aldehydes, and these can easil; he reduced to the corresponding &ohols. For examnle. ~ r o ~ v l e eives n e iso- and n-hutanol in the molar ratio of 3
CH&H=CHz
corn? eat
CHZCHCHB+ CH3CH2CHzCH0 I
The reaction was widely studied, and academic chemists devised mechanisms to explain the reaction and the formation of the less desirable branched-chain product. T o the industrial chemist, however, the reaction was less than satisfactory because the straight-chain alcohol was usually the premium product. Why is this so? Because the alcohols are converted to acetates for use as solvents for protective coatings. The straight chain molecules are hetter because, in conceptual terms, they fit more readily between polymer chains; that is, they achieve molecular nearness more readily, and thus they solvate hetter. In only a few applications (Koch acids, gasoline) are branched chains an advantage. Catalysts were developed-organophosphine (primarily triphenylphosphine) ligand-modified rhodium chloride-that give n-hutyraldehyde to isohutyraldehyde ratios of about 10:l. The exact ratio depends on the ligand. One experimenproduces a ratio tal ligand (CsH5)2PCH2CH2(P=O)(C6H5)2 of 33.8:l. The process is described as linear hydroformylation. The phosphine ligands increase the activity of the catalyst, and, berause they are bulky, the steric requirements of the lignnds facilitate anti-Markownikoff addition to yield a higher ~ r o ~ o r r i oofn the straight chain structure. Furrhermore, the newer cktalysts make possible the use of lower temperatures and pressures. 606
Journal o f Chemical Education
Olefin metathesis is a unique reaction discovered by Phillips Petroleum. Its original purpose was to convert cheap propylene to higher valued ethylene and 2-butene. Two molecules of propylene react in the presence of various catalysts including Lewis acids and salts of tungsten, molybdenum, or rhenium. The reaction was used for this purpose only on a limited commercial scale but ARCO, who has surplus ethylene, has announced that it will use the reverse reaction to alleviate a propylene shortage. 2-Butene is made in the ARCO process hy dimerization of ethylene. The dimerization is itself noteworthy. Unlike propylene, ethylene is difficult to dimerize, and only the development of a special catalyst made it possible. The metathesis of 2-hutene with ethylene gives propylene. As will he seen, the metathesis reaction is of general applicability to nonconjugated olefins without other functional groups. Putting It Together The figureshows how these four steps were integrated into a complete pnxess. The initial oligomerization reaction pro-
Shell higher olefin process (SHOP)
vides a range of n-olefins with even numbers i f carbon atoms from C4 to Ca. These are fed to a distillation column and split into three fractions, a light C4-C8 fraction, the desired C l d 1 4 fraction, and a heavy Cl,j-C40 fraction. The Cia-CI~ fraction is fed to a hydroformylation reactor where it is converted t o straight-chain aldehydes and then to the corresponding alcohols. The bulky ligands make i t difficult for the cobalt carbonyl complex to approach the &carbon atom; hence production of the branched chain alcohol is inhibited. For example:
C13 primary alcohol
(l-decene)
These alcohols are the desired endproducts of the reaction. For reasons that will become apparent, a cobalt catalyst with ligands is used, in spite of the fact that rhodium gives a higher yield of straight-chain products. The light and heavy fractions are fed to an isomerization reactor where n-olefins are isomerized to internal olefins. For example: CH3CH2CH=CHz
--
CH3CH4HCH3
CH3(CHz)17CH=CH2 CH~(CHZ)~CH=CHZ(CHZ)~CH~ The internal olefinspass to the metathesis reactor where the short- and long-chain internal olefins disproportionate. For example, the C4 and Czo internal olefins demonstrated above will give two molecules of a C12 internal olefin:
Of course, a mixture of internal olefins results, but the point is the eao left bv the earlier removal of Clo-Cia olefins ~-that ~-~~~~ -~ has been p k t k l y filied. The products of the metathesis reaction are fed to the fractionating column, and the ClO-C14 internal olefins are separated and passed to the hydroformylation reactor. I t is ~
~
here that the advantage of the cobalt rather than the rhodium catalyst becomesapparent. The cobalt catalyst causes the double bond in these internal olefins to migrate to the a nosition for hvdroformvlation. The rhodium catalvst cannot ho this. By recycling to exhaustion, all of the ethylene can be converted to Cn-Cis straight-chain primary alcohols. If desired, either a-olefins or internal olefins can be produced instead. The original purpose of the SHOP process was to give detergent intermediates in the Cll-Cis range. It can also be programmed, however, to give alcohols in the C7-Cl3 range for use in plasticizers. Poly(viny1chloride) (PVC) is normally a rigid material, but, when plasticized with the phthalates of Cv-C>e . ."alcohols. it nives a soft flexible material used. for example, in PVC IeaGer and sheeting. The traditional'oxo alcohols used for this ouroose are branched-chain. but the straight-chain alcohol; confer lower volatile lossand extractabilitv and nreater softenina vower for the same reason that hutylacetaie (seeabove) isahetter solvent than isobutvl acetate. Althouah this application is important. its vruf.. itability is low. A newer application is the production of a-olefins for use in linear low-density polyethylene. Ethylene is polymerized as in the production of high density polyethylene but with a comonomer such as l-butene. The pendant -CHz-CHJ groups decrease the crystallinity of the polymer to the level that exists in low-densitv This nrocess has - nolvethvlene. .. " supplemented the production of low-density polyethylene, which involves high vressures and temveratures. Althouah l-butene is the most important monomer for linear lowdensity polyethylene, l-hexene and l-octene are useful in specialty polymers, and these a-olefins also may be produced by the SHOP process in the quantities required. The difference between industrial and academic chemistry is that the latter aims to observe and interpret new reactions. In industrial chemistry, the additional constraint is that less useful reactions have to be modified in some way to eive more useful nroducts. In fulfilline this reauirement. t h e inventors of thk SHOP process reassembl& existing knowledge to achieve a novel result. This is the best example in industrial chemistry of such creativity, and the process that resulted is so versatile that i t will vroduce any desired chain length of fatty alcohol, a-olefin, or internal olefin.
Volume 65
Number 7
July 1988
607
