Dimerization of Isobutylene

Isobutylene could be dimerized in the presence of an acid catalyst to diisobutylene. The product is a mixture of diisobutylene isomers, which are used as alkylating agents in the plasticizer industry and as a lube oil additive (dimerization of olefins is noted in Chapter 3). 

Butadiene is a diolefmic hydrocarbon with high potential in the chemical industry. In 1955, it was noticed that the assured future of butadiene (CH2=CH-CH=CH2) lies with synthetic rubber. . . the potential of butadiene is in its chemical versatility. .. its low cost, ready availability, and great activity tempt researchers. 

Butadiene is a colorless gas, insoluble in water but soluble in alcohol. It can be liquefied easily under pressure. This reactive compound polymerizes readily in the presence of free radical initiators. 

Butadiene is mainly obtained as a byproduct from the steam cracking of hydrocarbons and from catalytic cracking. These two sources account for over 90% of butadiene demand. The remainder comes from dehydrogenation of n-butane or n-butene streams (Chapter 3). The 1998 U.S. production of butadiene was approximately 4 billion pounds, and it was the 36th highest-volume chemical. Worldwide butadiene capacity was nearly 20 billion pounds. 

Butadiene is easily polymerized and copolymerized with other monomers. It reacts by addition to other reagents such as chlorine, hydrocyanic acid, and sulfur dioxide, producing chemicals of great commercial value. 

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The strong catalytic activity of anhydrous hydrogen fluoride results from the abiUty to donate a proton, as in the dimerization of isobutylene (see Butylenes) ... 

It seems certain that acid-catalyzed true oligomerization occurs via a carbocatio-nic mechanism as first suggested by Whitmore.5 Thus, the formation of diisobutylenes by dimerization of isobutylene in the presence of acidic catalysts occurs according to the following equation ... 

SP-Isoether [SnamProgetti] A process for making an Ao-octane alkylate on a plant designed to make methyl /-butyl ether. It is based on the selective dimerization of isobutylene, followed by hydrogenation to Ao-octanc. Developed by Snamprogetti in 2000. See also Nexoctane. 

Problem 6.18 What products would you expect from each of the dimers of isobutylene (Sec. 6.15) upon cleavage by (a) ozonolysis, (b) KMn04 ... 

Problem 6.29 (a) Suggest a mechanism for the dimerization of isobutylene, (CII3)2C= CII2. (b) Why does (CH3)3C+ add to the tail carbon rather than to the head carbon (c) Why are the Bronsted acids H2S04 and HF typically used as catalysts, rather than HC1, HBr, or HI ... 

In the polymerization of cyclic sulfides, cyclization becomes particularly important because of the enhanced basicity (and nucleophilicity) of the linear sulfides in comparison with their parent monomers43). The simplest cyclic oligomer, formed in the polymerization of thiiranes, is a dimer-1,4-dithiane or its derivatives. 1,4-Dithiane was first observed by Bell in the polymerization of ethylene sulfide 441 later Price isolated the styrene sulfide dimer 45) and recently Goethals obtained dimers of isobutylene and cyclohexene sulfides 46 . Structures of these dimers are ... 

It is known that the kinetics of the MTBE synthesis has a negative reaction order with respect to methanol. That means that at lower concentrations of methanol the reaction towards MTBE becomes faster. A faster reaction means that the concentration of the reactant methanol decreases inside the catalyst. On the other hand the concentration of isobutylene increases inside the catalyst. If this happens, the side reaction of dimerization of isobutylene occurs, lowering the selectivity towards MTBE. This effect can also be seen if the size of the catalyst particles is decreased. Smaller particles are more selective than large ones large particles are deactivated by plugging the catalyst with dimer, starting from the center of the particle. Only an outer shell is the active part of the catalyst particle. 

Alkylphenols are produced by reacting phenol with propylene or isobutylene oligomers. As shown in Figure 13.5, the reaction of a propylene trimer with phenol produces various isomers of nonylphenol, the most common feedstock for alkylphenol-based nonionic surfactants. Dimerization of isobutylene yields an octene which leads to octyl(diisobutylene)phenol. Similarly, propylene tetramer, on reaction with phenol, yields dodecyl(tetrapropylene)phenol. 

It is still unclear how the initiation step in alkene metathesis occurs and how the initial carbene forms. Commercial applications of metathesis include the triolefin process, in which propylene is converted to ethylene and butene, the neohexene process, in which the dimer of isobutylene, Me3CCH=CMe2, is metathesized with ethylene to give Me3CCH=CH2, an intermediate in the manufacture of synthetic musk, and a 1,5-hexadiene synthesis from 1,5-cy-clooctadiene and ethylene. Two other applications, SHOP and ROMP (Shell higher olefins process and ring-opening metathesis polymerization), are discussed in the next section. 

The dimer of isobutylene, 2,4,4-trimethylpent-l-ene, was treated with BQt and co-initiators and product analysis provided insight into the reactivity of propagating isobutylene cations toward counterions derived from BClj." Similar studies conducted with t-butyl chloride as the model for polyisobutene chain termini elucidated the mechanism of interaction between the propagating ions and aluminium alkyl-based counterions." "... 

This process is used by Nan Ya Plastics Corporation to produce, starting from an isomer mixture of octene the alcohol, INA and has a capacity >100 000 mt/a. After 2-EH, INA is the most important plasticizer alcohol, which is a complex mixture of linear and branched Cg alcohols. The most important plasticizer produced from it is diisononyl phthalate (DINP). The raw material sources can be, for example, a Cg fraction from the extraction of Fluid Catalytic Cracking (FCC) C -based polygas, the product of dimerization of butene from a raffinate-2 fraction using the octol or dimersol process, or diisobutylene from the dimerization of isobutylene from raffinate-1. 

Variations on the process make higher olefins and aromatics such as cumeme for petrochemical applications. Figure 21 shows the main chemical reaction for the dimerization of isobutylene. 

Diisobutylene is obtained from dimerization of isobutylene, which in turn is obtained from fractionation of refinery gases see Figure 11.13. 

Emig has recently studied the oxidative dimerization of isobutylene to 2,5-dimethylhexadiene (DMH). Under aerobic conditions, a conversion of 24% was obtained with a selectivity of 38% to DMH giving a single-pass yield of 9.1%. Under anaerobic oxidation, the conversion dropped to 11%, the yield remained constant at 9.9%, and the selectivity to DMH increased from 38 to 90%. This is a remarkable example of CO2 reduction using a two-step process. 

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