Isobutene is preferably isolated next in the separation of the C₄ raffinate, since it differs from the remaining C₄ components in its branching and higher reactivity.

All current industrial processes rely on the further chemical reaction of isobutene, and shape-selective isolation is not practiced.

The molecular sieve separation process exploits the methyl branching which makes isobutene too bulky to be adsorbed in the very uniform 3- 10  pores of the molecular sieve ('Olefin-Siv' process of UCC). Only n-butenes and are adsorbed, and then desorbed usinga higher boiling hydrocarbon. In this way, isobutene with 99% purity can be obtained from the C₄ raffinate.

Isobutene is the most reactive compound in the C₄ raffinate and this property can be utilized for a chemical separation. In practice, four processes have been successfully commercialized. Of these, only the addition of water or alcohols (earlier , but now also isobutanol) can be used as a reversible process.
1. Hydration of isobutene in the presence of dilute mineral acid or an acid ion exchanger to form tert-butanol. possibly followed by cleavage to regenerate isobutene and water:



2. Addition of methanol to isobutene over an acid ion exchange resin to form methyl terf-butyl ether (MTBE), and possible regeneration of isobutene and CH₃OH:



3. Oligomerization of isobutene with acidic catalysts to form 'diisobutene', a mixture of the double bond isomers of 2,4,4-trimethylpentene, preferentially:



Further isobutene can add to the 'diisobutene', resulting in 'triisobutene' and higher oligomers.
4. Polymerization of isobutene in the presence of a Lewis acid catalyst to form polyisobutene:



The C₄ raffinate is the feedstock for all four processes.

Isobutene can be removed by reaction and then regenerated as pure isobutene (Routes 1 and 2). On the other hand, it is irreversibly oligomerized in Route 3, and irreversibly Polymerized in Route 4.

To 1:

In the commercial hydration of isobutene 50-60% H₂SO₄ is generally used. Isobutene is removed from the C₄ raffinate as tert-butanol in a countercurrent extraction at 10-20°C. After dilution with H₂O the tert-butanol is vacuum distilled from the acidic solution and used as an intermediate or cleaved to regenerate isobutene.

A process using a cation exchange resin (analogous to the manufacture of MTBE) has recently been developed by Hiils, and is already in commercial use.

Nippon Oil conducts the hydration of isobutene with aqueous HCl in the presence of a metal salt. During the extraction of the C₄ raffinate, tert-butanol and tert-butyl chloride are formed. Both can be cleaved to regenerate isobutene.

One commercial process for the dehydration of tert-butanol, which is, e.g., also obtained as a cooxidant in the Oxirane rocess for the manufacture of propylene oxide was developed at Arco. In this process, the reaction is carried out in the gas phase at 260- 370°C and about 14 bar in the presence of a modified Al₂O₃ catalyst (e.g., with impregnation of SiO₂) with a high surface area. The conversion of tert-butanol is about 98%, with a high selectivity to isobutene. Other processes take place in the liquid phase at 150°C in the presence of a heterogeneous catalyst.

To 2:

The manufacture of methyl tert-butyl ether takes place in the liquid phase at 30- 100°C and slight excess pressure on an acid ion exchange resin. Either two separate reactors or a two-stage shaft reactor are used to obtain nearly complete conversion (> 99 %) of the isobutene. Because of the pressure-dependent azeotrope formed from methanol and MTBE, preparation of pure MTBE requires a multistep pressure distillation. Alternatively, pure MTBE can be obtained by adsorption of methanol on a scavenger in a process recently developed by Erdolchemie and Bayer. Methanol can be separated from organic solvents (e. g., MTBE) by pervaporation, a new separation technique. Pervaporation uses a membrane that works like a sieve, holding back particles larger than a particular size. Pervaporation is already widely used commercially for the removal of water from organic solvents.

In the reaction of isobutene with methanol described above, all other components of the C₄ fraction (Raffinate Ⅱ) remain unchanged, except a small portion of the diolefins and alkynes which polymerize and shorten the lifetime of the ion exchanger. Erdolchemie has recently developed a bifunctional catalyst containing Pd which, in the presence of small amounts of hydrogen, catalyzes the hydration of diolefins and acetylenes. The etherification of isobutene is not affected, except that the lifetime of the catalyst is increased.

(ETBE) is produced commercially from isobutene and ethanol in a similar process.

The catalytic addition of isobutanol to isobutene is used by BASE The ether is then separated from the C₄ fraction by distillation, and then catalytically cleaved back into isobutanol and isobutene. The isobutanol is recycled for formation of more ether, while isobutene is used for polymerization.

The reaction of isopentene from the C₄ fraction with methanol to produce tert-amyl methyl ether (TAME) can be performed over an acid ion exchange resin or the new bifunctional catalyst from Erdolchemie, analogous to the production of MTBE. The first plant began production in the United Kingdom in 1987. Additional TAME plants have either been put into operation or are scheduled.

After the first commercial manufacture of MTBE in Western Europe by Snamprogetti/Anic in 1973 and Hiils in 1976, global capacity had already reached 22.2x10⁶ tonnes per year in 1996 with 11.0, 3.4, and 0.30 x10⁶ tonnes per year in the USA, Western Europe, and Japan, respectively, making MTBE the ether with the highest production rate worldwide.

Although it is possible to regenerate isobutene over acidic oxides in the gas phase at 140-200°C (still practiced by Exxon and Sumitomo), most MTBE is added to gasoline to increase the octane number.

Even as the processing of the C₄ fraction was increasingly shifted to removal of isobutene as MTBE, growing demand for MTBE necessitated process development to supply larger amounts of isobutene.

The C₄ fraction remaining after MTBE production (Raffinate Ⅱ) contains n-butenes that can be isomerized to give additional isobutene. An Al₂O₃ catalyst whose surface has been modified with SiO₂ is used in a process from Snamprogetti. At 450-490°C, n-butenes isomerize with a conversion of ca. 35% and selectivity to isobutene of ca. 81 %. An analogous process was also developed by, e. g., Kellogg.

Additional isobutene can also be obtained from n-butane, wich is first isomerized to isobutane in, e.g., the Butamer process (UOP), the ABB Lummus Crest process, or the Butane Isom process (BP), and then dehydrogenated, e. g., with the Catofin process (Houdry, ABB Lummus Crest) or other processes from UOP, Phillips , Snamprogetti and others.

To 3:

The oligomerization of isobutene is acid-catalyzed and takes place at temperatures around 100°C.

The process developed by Bayer uses an acidic ion-exchanger as catalyst, at 100°C and about 20 bar, suspended in the liquid phase. Isobutene is dimerized and trimerized in a strongly exothermic reaction. Conversion is 99% with a dimer: trimer ratio of 3 : 1. The catalyst is centrifuged off and the mixture of n-butenes, C₈ and C₁₂ olefins is separated by distillation. The isobutene content of n-butene is thereby lowered to about 0.7 wt%.

The advantages of this process lie in its simple technology. However, the simultaneous isomerization of the double bond - i. e., formation of 2-butene from I-butene - can be disadvantageous. This isomerization occurs only to a limited extent in the hydration process.

In a BASF process, the isobutene recovered from the C₄ fraction by formation of an ether with isobutanol is polymerized with a special BF, catalyst system to give polyisobutene with a molecular weight between 1000 and 2500. This polyisobutene is used for the manufacture of additives to fuels and lubricants. A plant with a production capacity of 60000 tonnes per year (1995) is in operation in Belgium.

To 4:

The polymerization of isobutene, e. g., by the Cosden process, is conducted in the liquid phase with AlCl₃ as catalyst. Polyisobutene with molecular weight between 300 and 2700 is obtained. Only a small amount of n-butenes copolymerize.

After removing isobutene, the remaining fraction contains, in addition to the n-butenes, only butanes. A further separation is not generally undertaken since the saturated hydrocarbons remain unchanged during some of the further reactions of the butenes, e. g., the hydration to butanols, and can be removed as inert substances. In principle, a separation of the 2-butenes (cis and trans) and 1-butene is possible by distillation. The butanes can be separated from the n-butenes by extractive distillation.

New developments have led to another possibility for separating the C₄ fraction. Here the 1-butene in the butadiene-free Raffinate I, consisting mainly of butenes, is catalytically isomerized to 2-butene. A modified Pd catalyst in the presence of H₂ is used, either in the gas phase (UOP process) or in the liquid phase (IFP process). The boiling points of isobutene and 2-butene are sufficiently different to allow their separation by fractionation. The first firms to treat the C₄ fraction in this way were Phillips Petroleum and Petro-Tex. They use the 2-butene for production of alkylate gasoline with a higher octane number than that produced from isobutene.