Chemicals from Propylene and Butylene

Transcription

1 Chapter 10 Chemicals from Propylene and Butylene As we learned in Chapter 8, the official production of propylene is usually about half that of ethylene, only because a large part of the propylene is used by petroleum refineries internally to alkylate gasolines. This captive use is not reported. Of the propylene used for chemical manufacture, nearly 40% is polymerized to polypropylene, to be discussed in a later chapter. Of the remaining amount of propylene, seven chemicals from the top 50 are manufactured. These are listed in Table Their industrial manufacturing methods are summarized in Fig Note that four of these chemicals, cumene, phenol, acetone, and bisphenol A, are also derived from a second basic organic chemical, benzene. 1. ACRYLONITRILE (2-PROPENONITRILE) CH 2 =CH-C=N Table 10.1 Propylene Derivatives in the Top 50 Acrylonitrile Propylene Oxide Cumene Phenol Acetone Bisphenol A ft-butyraldehyde

2 propylene acrylonitrile propylene oxide styrene cumene phenol acetone bisphenol A fl-butyraldehyde isobutyraldehyde Figure 10.1 Synthesis of propylene derivatives in the top 50 chemicals. Acrylonitrile and other three-carbon analogs containing a double bond have a common name derived from the word acrid, meaning strong and disagreeable, in regard to the odor of most of these chemicals. Compounds in this family are given here with their common names. acrylonitrile acrylic acid

3 acrolein acrylamide Acrylonitrile was made completely from acetylene in 1960 by reaction with hydrogen cyanide. For some years ethylene oxide was the raw material for addition of HCN and elimination of H 2 O. Neither of these methods is used today. Around 1970 the industry switched from C 2 raw materials and classical organic chemical addition reactions to the ammoxidation of propylene. Now all acrylonitrile is made by this procedure, which involves reaction of propylene, ammonia, and oxygen at O 0 C and atm in a fluidized bed Bi 2 O 3^nMoO 3 catalyst. The yield is approximately 70%. 2CH 2 =CH-CH 3 + 2NH 3 + 3O 2 ** 2CH 2 =CH-C=N + 6H 2 O The mechanism is undoubtedly a free radical reaction that occurs very easily at the allyl site in propylene, forming the resonance-stabilized allyl radical. CH 2 =CH-(^H 2 ** ** ^H 2 -CH=CH 2 By-products of this reaction are acetonitrile, CH 3 -C=N, and hydrogen cyanide. This is now a major source of these two materials. Interestingly, the C 2 by-product acetonitrile has a bp of C, whereas acrylonitrile with three carbons has a lower bp of C, quite an unusual reversal of this physical property's dependence on molecular weight. The TWA of acrylonitrile is 2 ppm and it is on the list of "Reasonably Anticipated to Be Human Carcinogens." Table 10.2 outlines the uses of acrylonitrile. One important use of acrylonitrile is in the polymerization to polyacrylonitrile. This substance and its copolymers make good synthetic fibers for the textile industry. Acrylic is the fourth largest produced synthetic fiber behind polyester, nylon, and

4 Table 10.2 Uses of Acrylonitrile Adiponitrile 33% Acrylic fibers 25 ABS/SA resins 23 Acrylamide 9 Nitrile elastomers 3 Miscellaneous 7 Source: Chemical Profiles polyolefm. It is known primarily for its warmth, similar to the natural and more expensive fiber wool. Approximately 23% of the acrylonitrile is made into plastics, including the copolymer of styrene-acrylonitrile (SA) and the terpolymer of acrylonitrile, butadiene, and styrene (ABS). Acrylamide has a fast-growing use as a flocculent for water treatment units. Exports of acrylonitrile are over 1 billion Ib/yr. The largest use of acrylonitrile is the manufacture of adiponitrile, made by two different methods. One method is by the electrohydrodimerization of acrylonitrile. It is converted into hexamethylenediamine (HMDA), which is used to make nylon. The other adiponitrile synthesis is C 4 chemistry, which will be discussed later in this chapter, Section 8. 2CH 2 =CH-C=N -^-+ N=C-(CH 2 ) 4 -C=N ^ H 2 N-(CH 2 ) 6 -NH 2 2e' acrylonitrile adiponitrile HMDA About one third of all adiponitrile is made from acrylonitrile. In the electrodimerization of acrylonitrile a two-phase system is used containing a phase transfer catalyst tetrabutylammonium tosylate [(W-Bu) 4 N + OTs"]. The head-to-head dimerization may be visualized to occur in the following manner.

5 The by-product of acrylonitrile manufacture, HCN, has its primary use in the manufacture of methyl methacrylate by reaction with acetone. This is covered later in this chapter, Section PROPYLENE OXIDE (1,2-EPOXYPROPANE) There are two important methods for the manufacture of propylene oxide, each accounting for one half the total amount produced. The older method involves chlorohydrin formation from the reaction of propylene with chlorine water. Before 1969 this was the exclusive method. Unlike the analogous procedure for making ethylene oxide from ethylene, which now is obsolete, this method for propylene oxide is still economically competitive. Many old ethylene oxide plants have been converted to propylene oxide synthesis. The mechanism in the first step involves an attack of the electrophilic chlorine on the double bond of propylene to form a chloronium ion, which is attacked by a hydroxide ion to complete the first reaction. The dilute

6 chlorohydrin solution is mixed with a 10% slurry of lime to form the oxide, which is purified by distillation, bp 34 0 C. The yield is 90%. Propylene oxide has a TWA of 20 ppm and is on the list of "Reasonably Anticipated to Be Human Carcinogens." A new variation of the chlorohydrin process uses /-butyl hypochlorite as chlorinating agent. The waste brine solution can be converted back to chlorine and caustic by a special electrolytic cell to avoid the waste of chlorine. The second manufacturing method for propylene oxide is via peroxidation of propylene, called the Halcon process after the company that invented it. Oxygen is first used to oxidize isobutane to /-butyl hydroperoxide (BHP) over a molybdenum naphthenate catalyst at 9O 0 C and 450 psi. This oxidation occurs at the preferred tertiary carbon because a tertiary alkyl radical intermediate can be formed easily. Reaction: BHP Mechanism: (1) (2) (3) The BHP is then used to oxidize propylene to the oxide. This reaction is ionic and its mechanism follows. The yield of propylene oxide from propylene is 90%. Reaction:

7 Mechanism: The /-butyl alcohol can be used to increase the octane of unleaded gasoline or it can be made into methyl /-butyl ether (MTBE) for the same application. The alcohol can also be dehydrated to isobutylene, which in turn is used in alkylation to give highly branched dimers for addition to straight-run gasoline. Since approximately 2.2 Ib of/-butyl alcohol would be produced per 1 Ib of propylene oxide, an alternative reactant in this method is ethylbenzene hydroperoxide. This eventually forms phenylmethylcarbinol along with the propylene oxide. The alcohol is dehydrated to styrene. This chemistry was covered in Chapter 9, Section 6 as one of the syntheses of styrene. Thus the side product can be varied depending on the demand for substances such as /-butyl alcohol or styrene. Research is being done on a direct oxidation of propylene with oxygen, analogous to that used in the manufacture of ethylene oxide from ethylene and oxygen (Chapter 9, Section 7). But the proper catalyst and conditions have not yet been found. The methyl group is very sensitive to oxidation conditions. As an aside to the manufacture of propylene oxide via the chlorohydrin process let us mention use of this type of chemistry to make epichlorohydrin. allyl chloride epichlorohydrin

8 Table 10.3 Uses of Propylene Oxide Polypropylene glycol 60% Propylene glycol 25 Glycol ethers 4 Miscellaneous 11 Source: Chemical Profiles Although not in the top 50, it is an important monomer for making epoxy adhesives as well as glycerine (HO-CH 2 -CHOH-CH 2 -OH). Propylene is first chlorinated free radically at the allyl position at 50O 0 C to give ally 1 chloride, which undergoes chlorohydrin chemistry as discussed previously to give epichlorohydrin. The student should review the mechanism of allyl free radical substitution from a basic organic chemistry course and also work out the mechanism for this example of a chlorohydrin reaction. Table 10.3 summarizes the uses of propylene oxide. Propylene glycol is made by hydrolysis of propylene oxide. The student should develop the mechanism for this reaction, which is similar to the ethylene oxide to ethylene glycol conversion (Chapter 9, Section 8). Propylene glycol is a monomer in the manufacture of unsaturated polyester resins, which are used for boat and automobile bodies, bowling balls, and playground equipment. But an even larger use of the oxide is its polymerization to poly(propylene glycol), which is actually a polyether, although it has poly (propylene glycol) propylene glycol propylene oxide n = 7-35 a polyurethane

9 hydroxy end groups. These hydroxy groups are reacted with an isocyanate such as toluene diisocyanate (TDI) to form the urethane linkages in the high molecular weight polyurethanes, useful especially as foams for automobile seats, furniture, bedding, and carpets. Poly(propylene glycol) is used to make both flexible and rigid polyurethane in a 90:10 market ratio. 3. CUMENE (ISOPROPYLBENZENE) Cumene is an important intermediate in the manufacture of phenol and acetone. The feed materials are benzene and propylene. This is a Friedel- Crafts alkylation reaction catalyzed by solid phosphoric acid at C and psi. The yield is 97% based on benzene and 92% on propylene. Excess benzene stops the reaction at the monoalkylated stage and prevents the polymerization of propylene. The benzene:propylene ratio is 8-10:1. (excess) Interestingly, if benzene is left out similar conditions are used to manufacture the trimer and tetramer of propylene. The cumene is separated by distillation, bp C. The mechanism of the reaction involves electrophilic attack of the catalyst on the double bond of propylene to form the more stable secondary cation, which reacts with the TI cloud of benzene to give a delocalized ion. Deprotonation rearomatizes the ring.

10 A major industry shift to zeolite-based catalyst systems is expected to lower production costs and improve product yield. Approximately 95% of the cumene is used to make phenol and acetone. A small amount is used to make a-methylstyrene by dehydrogenation. This material is used in small amounts during the polymerization of styrene to vary the properties of the resulting copolymer. catalyst 4. ACETONE (2-PROPANONE) Presently there are two processes that make acetone in large quantities. The feedstock for these is either isopropyl alcohol or cumene. In the last few years there has been a steady trend away from isopropyl alcohol and toward cumene, but isopropyl alcohol should continue as a precursor since manufacture of acetone from only cumene would require a balancing of the market with the co-product phenol from this process. This is not always easy to do, so an alternate acetone source is required. In fact, isopropyl alcohol may become attractive again since cumene can be used to increase octane ratings in unleaded gasoline, and phenol, as a plywood adhesive, has its ups and downs with the housing industry. The percentage distribution of the two methods is given in Table In the minor route isopropyl alcohol, obtained from the hydrolysis of propylene, is converted into acetone by either dehydrogenation (preferred) or air oxidation. These are catalytic processes at 50O 0 C and psi. The acetone is purified by distillation, bp 56 0 C. The conversion per pass is 70-85% and the yield is over 90%. Table 10.4 Manufacture of Acetone Year From Isopropyl Alcohol From Cumene % 20%

11 dehydrogenation air oxidation The main route, the formation of phenol and acetone from cumene hydroperoxide, involves a fascinating rearrangement of cumene hydroperoxide where a phenyl group migrates from carbon to an electrondeficient oxygen atom. This was discovered by German chemists Hock and Lang in 1944 and commercialized in 1953 in the U.S. and U.K. The hydroperoxide is made by reaction of cumene with oxygen at C until 20-25% of the hydroperoxide is formed. The oxidation step is similar to that of cyclohexane to cyclohexane hydroperoxide and will be treated in Chapter 11, Section 4. Students should be able to work out this mechanism on their own with this help! Concentration of the hydroperoxide to 80% is followed by the acid-catalyzed rearrangement at 70-8O 0 C. The overall yield is 90-92%. cumene cumene hydroperoxide phenol acetone Side products are acetophenone, 2-phenylpropan-2-ol, and a- methylstyrene. Acetone is distilled first at bp 56 0 C. Vacuum distillation acetophenone 2-phenylpropan-2-ol oc-methylstyrene recovers the unreacted cumene and yields a-methylstyrene, which can be hydrogenated back to cumene and recycled. Further distillation separates

12 phenol, bp C, and acetophenone, bp C. The weight ratio of acetone:phenol is 0.6:1.0. The mechanism of the rearrangement is an excellent practical industrial example of a broad type of rearrangement, one occurring with an electrondeficient oxygen. The mechanism is given in Fig cumene hydroperoxide a protonated hydroperoxide simultaneous resonance stabilized a hemiketal phenol resonance stabilized acetone Figure 10.2 Mechanism of the cumene hydroperoxide rearrangement.

13 Table 10.5 Uses of Acetone Acetone cyanohydrin 45% Bisphenol A 20 Solvent 17 Aldol chemicals 8 Miscellaneous 10 Source: Chemical Profiles Table 10.5 gives the uses of acetone. A very important organic chemical that just missed the top 50 list, methyl methacrylate, is made from acetone, methanol, and hydrogen cyanide. Approximately 1.2 billion Ib of this compound is manufactured and then polymerized to poly(methyl methacrylate), an important plastic known for its clarity and used as a glass substitute. The synthesis is outlined as follows. methyl methacrylate The first reaction is a nucleophilic addition of HCN to a ketone, the second is a dehydration of an alcohol and hydrolysis of a nitrile, and the third is esterification by methanol. Aldol chemicals refer to a variety of substances desired from acetone involving an aldol condensation in a portion of their synthesis. The most important of these chemicals is methyl isobutyl ketone (MIBK), a common solvent for many coatings, pesticides, adhesives, and pharmaceuticals. Approximately 0.17 billion Ib of MIBK were made in recent years. The synthesis is outlined on the next page.

14 diacetone alcohol mesityl oxide MIBK Diacetone alcohol is a solvent used in hydraulic fluids and printing inks. Recall that the aldol condensation is an example of a variety of carbanion reactions used to make large molecules from smaller ones. An aldehyde or a ketone with at least one hydrogen on the carbon next to the carbonyl will react to give the aldol condensation. The mechanism is given as follows. 5. BISPHENOL A (BPA) Bisphenol A is manufactured by a reaction between phenol and acetone, the two products from the cumene hydroperoxide rearrangement. The temperature of the reaction is maintained at 5O 0 C for about 8-12 hr. A slurry

15 of BPA is formed, which is neutralized and distilled to remove excess phenol. Some o,p isomer is formed along with the predominance of p,p isomer. bisphenol A The student should develop the mechanism of this reaction using the following stepwise information: (1) protonation of the carbonyl; (2) electrophilic attack on the aromatic ring; (3) rearomatization by proton loss; (4) another protonation, but then loss of a water molecule; and (5) electrophilic attack and rearomatization. The major uses of BPA are in the production of polycarbonate resins (63%) and epoxy resins (27%). Polycarbonates have major outlets in automotive parts, compact discs, eyeglasses, and sheet and glazing applications, and have caused bisphenol A consumption to more than double during the past decade. Epoxy resins are two-component adhesives for very strong bonding. Miscellaneous uses include flame retardants (mostly tetrabromobisphenol A) and other polymer manufacture. Polycarbonate grade bisphenol A is >99% p,p isomer. The epoxy grade is 95% p,p. The p,p and o,p isomers can be separated by a combination of distillation and crystallization. 6. H-BUTYRALDEHYDE (BUTANAL) Butyraldehyde is made from propylene by the oxo process, also known as hydroformylation. Synthesis gas (CO + H 2 ) is catalytically reacted with propylene to the butyraldehydes. The approximate yields are 67% n- butyraldehyde and 15% isobutyraldehyde. H-butyraldehyde isobutyraldehyde

16 The classic oxo catalyst is octacarbonyldicobalt at C and 250 atm. This reacts with hydrogen to give hydridotetracarbonyl cobalt, the active catalyst in the oxo process. The mechanism of carbonylation/hydrogenation involves addition of the alkene to form a TI complex, followed by alternating additions of H, CO, and H. Newer catalysts are being studied to increase the ratio of w-butyraldehyde to isobutyraldehyde. The main use of w-butyraldehyde is the production of n-butyl alcohol by hydrogenation. w-butyl alcohol is used for ester synthesis, especially butyl acetate, aery late, and methacrylate, common solvents for coatings. butyl acetate butyl acrylate butyl methacrylate

17 7. CHEMICALS FROM THE C 4 FRACTION Chemicals obtained from petroleum having four carbons are manufactured at a considerably lower scale than ethylene or propylene derivatives. Only five C4 compounds butadiene, acetic acid, vinyl acetate, isobutylene, and methyl /-butyl ether (MTBE) appear in the top 50. The manufacture of butadiene and isobutylene, as well as the separation of other C 4 compounds from petroleum, is described in Chapter 8, Sections 3-5. Acetic acid was discussed as a derivative of ethylene in Chapter 9, Section 3 and is discussed as a derivative of methane in Chapter 12, Section 3. Vinyl acetate was discussed in Chapter 9, Section 4. A few important derivatives ofc 4 chemistry will be briefly mentioned here as well as MTBE. 8. BUTADIENE DERIVATIVES Besides butadiene, another important monomer for the synthetic elastomer industry is chloroprene, which is polymerized to the chemically resistant poly chloroprene. It is made by chlorination of butadiene follow by dehydrochlorination. As with most conjugated dienes, addition occurs either 1,2 or 1,4 because the intermediate allyl carbocation is delocalized. The 1,4- isomer can be isomerized to the 1,2-isomer by heating with cuprous chloride. Reaction: 15%NaOH -HCl 10O 0 C chloroprene Intermediate: Another derivative of butadiene, hexamethylenediamine (HMDA), is used in the synthesis of nylon. We have already met this compound earlier in this chapter since it is made from acrylonitrile through adiponitrile.

18 Approximately two thirds of all adiponitrile is made from 1,3-butadiene and 2 moles of hydrogen cyanide. This is an involved process chemically and it is summarized in Fig Butadiene first adds one mole of HCN at 6O 0 C with a nickel catalyst via both 1,2- and 1,4-addition to give respectively 2- methyl-3-butenonitrile (2M3BN) and 3-pentenonitrile (3PN) in a 1:2 ratio. The 1,2-addition is the usual Markovnikov addition with a secondary carbocation intermediate being preferred. Fig shows an ADN reactor. Next isomerization of the 2M3BN to 3PN takes place at 15O 0 C. Then more HCN, more catalyst, and a triphenylboron promoter react with 3PN to form 5% methylglutaronitrile (MGN) and mostly adiponitrile (ADN). The ADN is formed from 3PN probably through isomerization of 3PN to 4-pentenonitrile and then anti-markovnikov addition of HCN to it. The nickel catalyst must play a role in this last unusual mode of addition, and a steric effect may also be operating to make CN" attack at the primary carbon rather than a cationically preferred secondary carbon. A complicated set of extractions and distillations is necessary to obtain pure ADN. Even then the hexamethylenediamine (HMDA) made by hydrogenation of ADN must also be distilled through seven columns to purify it before polymerization to nylon. Fig pictures some HMDA distillation units. ADN HMDA Figure 10.3 Manufacture of adiponitrile and hexamethylenediamine from 1,3-butadiene.

19 Figure 10.4 Reactors used in the conversion of 1,3-butadiene and HCN to adiponitrile. (Courtesy of Du Pont) Figure 10.5 Distillation columns associated with the purification of hexamethylenediamine. (Courtesy of Du Pont)

20 9. METHYL /-BUTYL ETHER (MTBE) In 1984 methyl /-butyl ether (MTBE) broke into the top 50 for the first time with a meteoric rise in production from 0.8 billion Ib in 1983 to 1.47 billion Ib in 1984 to be ranked 47 th. In 1990 it was 24 th with production over 6 billion Ib, and in 1995 it was 12 th at 18 billion Ib. A full discussion of the current economic status of MTBE is given in Chapter 7, Section 4 as the important gasoline octane enhancer. That is its only major use. MTBE is manufactured by the acid catalyzed electrophilic addition of methanol to isobutylene. or ion exchange catalyst MTBE 10. OTHER C 4 DERIVATIVES An important antioxidant for many products is butylated hydroxytoluene (BHT), more properly named 4-methyl-2,6-di-/-butylphenol. Acid-catalyzed electrophilic aromatic substitution of a /-butyl cation at the activated positions ortho to the hydroxy group of/?-cresol yields this product, p- Cresol is obtained from coal tar or petroleum. BHT For many years maleic anhydride (MA) was made from benzene by oxidation and loss of two moles of CO2. Even as late as % of maleic

21 anhydride was made from benzene. However, the new Occupational Safety and Health Administration (OSHA) standards for benzene plants required modifications in this process, and butane is also cheaper than benzene. As a result since 1989 all maleic anhydride is now made from butane. This has been a very rapid and complete switch in manufacturing method. The mechanism is not well understood, but the intermediates in the process are butadiene and furan. The uses of maleic anhydride are furan summarized in Table Unsaturated polyester resins are its prime use area. Food acidulants include fumaric and malic acids. Malic acid competes with citric acid as an acidulant for soft drinks, and it is added to products that contain aspartame, the artificial sweetener, because it makes aspartame taste more like sugar. Agricultural chemicals made from MA include daminozide (Alar ), a growth regulator for apples which in 1989 was found to be carcinogenic because of a breakdown product, unsymmetrical dimethylhydrazine (UMDH). Alar is needed to keep the apple on the tree, to make a more perfectly shaped, redder, firmer apple, and to maintain firmness in stored apples by reducing ethylene production. Table 10.6 Uses of Maleic Anhydride Unsaturated polyester resins 62% Lube oil additive 11 Copolymers 7 Agricultural chemicals 4 Fumaric acid 3 Malic acid 3 Miscellaneous 10 Source: Chemical Profiles

22 fumaric acid malic acid daminoazide (Alar*) UDMH The oxo process is used to convert the 4 fraction to C 5 derivatives. Synthesis gas is catalytically reacted with 1-butene to give pentanal which can be hydrogenated to 1-pentanol (w-amyl alcohol), giving a route to C 5 compounds in larger amounts than what would be available from C 5 alkanes in petroleum. Suggested Readings catalyst Chemical Profiles in Chemical Marketing Reporter, , , , , , , , , and Kent, Riegel's Handbook of Industrial Chemistry, pp Szmant, Organic Building Blocks of the Chemical Industry, pp Wiseman, Petrochemicals, pp Writeoff and Reuben, Industrial Organic Chemicals, pp