Isomerization (petrochemicals)

This article deals with the chemical-technical process of isomerization ; for the isomerization of enantiomers into the racemate, see racemization ; for isomerization reactions in general, see rearrangement and isomerization .

Isomerization of n -pentane (left) in 2-methylbutane: the empirical formula does not change.

In petrochemicals, isomerization is the change in the atomic sequence or arrangement of a saturated hydrocarbon into another isomer while maintaining its molecular mass with the aim of increasing the octane number. The content of high-octane aromatic components in fuels is limited by legislation; on the other hand, modern engines require higher fuel quality. The aim of isomerization is therefore to increase the octane number of n -alkanes. The octane number of n- heptane is 0, while the isomeric2,4-Dimethylpentane has an octane rating of 80.

Many isomers have the same or nearly the same binding energy , which means that conversion occurs relatively freely. The isomerization usually takes place under acidic catalysis or under hydrogen pressure over acidic noble metal catalysts.

history

The isomerization of n -alkanes into iso- alkanes using aluminum chloride catalysts was first described by Costin Nenitzescu and Alexander Drâgan in 1933. They found that the isomerization of longer-chain alkanes took a back seat to cracking reactions . While n- butane could be isomerized into iso- butane largely without loss , this rate was very low with n- heptane.

When the tests were checked, it was found that pure aluminum chloride had no effect on isomerization. The presence of promoters such as hydrogen chloride , traces of chloroalkanes, and the presence of olefins were necessary to achieve isomerization.

mechanism

The isomerization proceeds under acid catalysis via a carbenium ion mechanism:

Isomerization of pentane (hydrogen atoms omitted for clarity)

The mechanism explains the necessary presence of hydrogen chloride and traces of olefins, without which the reaction does not proceed. Olefin concentrations of 0.01% are sufficient:

${\displaystyle \mathrm {AlCl_{3}\ +\ HCl\longrightarrow } }$ ${\displaystyle \mathrm {H^{+}\ +\ AlCl_{4}^{-}} }$

${\displaystyle \mathrm {HAlCl_{4}\ +\ R-CH{=}CH_{2}\longrightarrow } }$ ${\displaystyle \mathrm {R{-}CH^{+}{-}CH_{3}\ +\ AlCl_{4}^{-}} }$

The olefins can be formed from alkanes by cracking reactions during the reaction.

The isomerization of the low molecular weight alkanes is an equilibrium reaction. Herbert Koch succeeded in isomerizing n -hexane with aluminum chloride and hydrogen chloride at room temperature. The following equilibrium composition was found:

Equilibrium composition of the hexane isomers at 20 ° C

Chemical name	Mol%
n -hexane	4th
2-methylpentane	23
3-methylpentane	10
2,2-dimethylbutane	56
2,3-dimethylbutane	7th

process

Attempts at the purely thermal isomerization of n- butane and n- pentane at 700 ° C. only resulted in cracking, even with very short residence times. The use of a catalyst is necessary in order to achieve isomerization to give higher octane products. It is an equilibrium reaction in which, depending on the temperature, an equilibrium is established between the various isomers.

The isomerization of light gasoline with the components n -pentane and n -hexane is carried out industrially on a large scale in order to increase the octane number. For this purpose, bifunctional catalysts are used which have both acidic and hydrogenation-active, metallic platinum or palladium centers. In addition to the isomerization products, shorter-chain alkanes and olefins are also formed during the process through cracking reactions:

${\displaystyle \mathrm {C_{7}H_{16}\longrightarrow } }$ ${\displaystyle \mathrm {C_{6}H_{12}\ +\ CH_{4}} }$

To avoid cracking and coking reactions, the process is often carried out under hydrogen pressure . The resulting olefins are hydrogenated again to the alkane.

When n- pentane is isomerized , mainly isopentane (2-methylbutane) is formed. Neopentane (2,2-dimethylpropane) does not form. The isomerization of n -hexane under similar conditions, on the other hand, yields large quantities of methylpentanes as well as neohexane (2,2-dimethylbutane), which has a quaternary carbon atom.

Table of research and motor octane numbers for some hexane isomers

Chemical name	RON	MOZ
n -hexane	24.8	26.0
2-methylpentane	73.4	73.5
3-methylpentane	74	74
2,2-dimethylbutane	94	93
2,3-dimethylbutane	102	101
Cyclohexane	83	77

Penex process

The Penex process is a continuous catalytic process for the isomerization of pentane and hexane fractions. The process was commercialized by UOP LLC in 1958 . Chlorinated aluminum oxide fixed bed catalysts are used as catalysts . The octane number of light straight-run naphtha is increased from 50 to 60 to approximately 82 to 86 per process step. By separating low-octane components by distillation or molecular sieves , the octane number can be improved to about 91 to 93 units. The low octane fractions are fed back into the process.

Web links

Commons : isomerization  - collection of pictures, videos and audio files

Individual evidence

1. ↑ Costin D. Nenitzescu, Alexander Drâgan: On the action of aluminum chloride on n -hexane and n -heptane, alone and in the presence of halogen derivatives. A conversion from paraffin to cycloparaffin hydrocarbons. In: Reports of the German Chemical Society (A and B Series). 66, 1933, pp. 1892-1990, doi: 10.1002 / cber.19330661223 .
2. ↑ George Calingaert, Harold A. Beatty: The Isomerization of Normal Heptane. In: Journal of the American Chemical Society. 58, 1936, pp. 51-54, doi: 10.1021 / ja01292a014 .
3. ↑ Herman Pines, RC Wackher: Isomerization of Alkanes. I. Effect of Olefins upon the Isomerization of n-Butane in the Presence of Aluminum Halide — Hydrogen Halide Catalyst. In: Journal of the American Chemical Society. 68, 1946, pp. 595-599, doi: 10.1021 / ja01208a020 .
4. ↑ RC Wackher, Herman Pines: Isomerization of Alkanes. III. The Water — Aluminum Halide Reaction and Isomerization of n-Butane with the Reaction Product. In: Journal of the American Chemical Society. 68, 1946, pp. 1642-1646, doi: 10.1021 / ja01212a084 .
5. ↑ HS Bloch, Herman Pines, Louis Schmerling: The Mechanism of Paraffin Isomerization. In: Journal of the American Chemical Society. 68, 1946, p. 153, doi: 10.1021 / ja01205a519 .
6. ↑ Herbert Koch, Herbert Richter: The isomerization equilibrium of hexanes. In: Reports of the German Chemical Society (A and B Series). 77, 1944, pp. 127-132, doi: 10.1002 / cber.19440770211 .
7. ↑ GCA Schuit, H. Hoog, J. Verheus: Investigations into the isomerization of aliphatic and alicyclic hydrocarbons. In: Recueil des Travaux Chimiques des Pays-Bas. 59, 1940, pp. 793-810, doi: 10.1002 / recl.19400590817 .
8. ↑ ST you: Isomerization Reactions . In: G. Ertl, H. Knözinger, J. Weitkamp (Eds.): Handbook of Heterogeneous Catalysis , Vol. 4, Wiley-VCH, Weinheim, 1997, ISBN 978-3-527-29212-7 . Pp. 1998-2017.
9. ↑ BL Evering, EL d'Ouville: Experimental Equilibrium Constants for the Isomeric Hexanes. In: Journal of the American Chemical Society. 71, 1949, pp. 440-445, doi: 10.1021 / ja01170a019 .
10. ↑ Isomerization, Honeywell UOP ( Memento of the original dated May 12, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.