Transalkylation

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In organic chemistry, transalkylation is a chemical reaction involving the transfer of an alkyl group from one organic compound to another. The reaction is used for the transfer of methyl and ethyl groups between benzene rings. This is of particular value in the petrochemical industry [1] to manufacture p-xylene, styrene, [2] and other aromatic compounds. Motivation for using transalkylation reactions is based on a difference in production and demand for benzene, toluene, and xylenes. Transalkylation can convert toluene, which is overproduced, into benzene and xylene, which are under-produced. [3] Zeolites are often used as catalysts in transalkylation reactions. [4]

Contents

Disproportionation

Reaction of toluene to produce benzene and xylene Disproportionation of toluene.jpg
Reaction of toluene to produce benzene and xylene

Transalkylation, as used by the petrochemical industry, is often used to convert toluene into benzene and xylenes. This is achieved through a disproportionation reaction of toluene in which one toluene molecule transfers its methyl group to another one. The reaction is not selective, and the xylene produced can be ortho, meta, or para. There is a higher demand for para xylene, so it is often separated, and the mixture is allowed to reequilibrate to give more para product. [3]

Diethylbenzenes

Diethylbenzenes arise as side-products of the alkylation of benzene with ethylene, which is conducted on a very large scale. Since there is only a limited market for diethylbenzene, much of it is recycled by transalkylation to give ethylbenzene: [1]

M/R Ratio

Transalkylation of toluene and trimethylbenzene to produce xylene. In this example, the M/R ratio is 2. Transalkylation.jpg
Transalkylation of toluene and trimethylbenzene to produce xylene. In this example, the M/R ratio is 2.

This type of reaction can also be performed with toluene and trimethylbenzene to produce xylene. The reaction occurs via equilibrium, so the product is not pure xylene. Many products are produced with varying numbers of methyl groups. The quantities in which each product is produced depends on the M/R ratio. This is the ratio of the number of methyl groups to the number of benzene rings in all of the substrates. For example, in the disproportionation of toluene, the M/R ratio is 1. Side reactions in which alkanes are produced reduce the number of methyl groups available which decreases the M/R ratio. This can be mitigated by adding compounds with higher numbers of methyl groups, such as trimethylbenzene. The ratio of products produced depends only on the M/R ratio so different starting materials can produce the same compounds via transalkylation. [3]

Zeolite catalysts

Transalkylation reactions of six to ten carbon methylated aromatics are often performed with the cofeed of hydrogen gas, over a zeolite based solid catalyst. Industrial processes operate the transalkylation reactor at elevated temperature and pressure to achieve desired process economics. Zeolites are micro-crystalline solids composed of tetrahedral AlO
4 and SiO
4 building blocks. These crystals are porous in nature with characteristic micropore channels, cavities. Zeolite is known as one class of molecular sieve because of their channel openings are often between 0.4 and 1.5 nanometers, just enough for the molecules to pass through. Aromatics molecules enter and exit these channels at different rates, also called diffusion. In addition to their molecular sieving effect, zeolites have weakly bonded protons originated from its chemical composition. These are chemical active centers for acid-catalyzed transalkylation reaction.

Zeolites of varying sizes are used to perform transalkylation on different substrates. For example, zeolites with a pore size of 5.5Å are suitable for benzene, toluene, xylenes and trimethylbenzenes transalkylations. [5] [3] [6]

Phenols

Transalkylation is employed in the commercial production of aromatics beyond the usual BTX feedstocks. For example, 4-tert-butylphenol is produced in part via two transalkylation reactions. [7] In one example, tert-butylphenyl ether is isomerized to the phenol:

Additionally, 2,4-di-tert-butylphenol is converted to 4-tert-butylphenol by treatment with phenol by transalkylation:

Transalkylation in conjunction with the Hock rearrangement contributes to the production of 1,3-diisopropylbenzene, a precursor to resorcinol. [8]

1,3-Diisopropylbenzene is produced via transalkylation. 1,3-Diisopropylbenzene via transalkylation.svg
1,3-Diisopropylbenzene is produced via transalkylation.

See also

Related Research Articles

<span class="mw-page-title-main">Aromatic compound</span> Compound containing rings with delocalized pi electrons

Aromatic compounds usually refers to organic compounds "with a chemistry typified by benzene" and "cyclically conjugated " The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying Hückel's Rule. Aromatic compounds have the following general properties:

<span class="mw-page-title-main">Phenols</span> Chemical compounds in which hydroxyl group is attached directly to an aromatic ring

In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (−OH) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, C
6H
5OH. Phenolic compounds are classified as simple phenols or polyphenols based on the number of phenol units in the molecule.

<span class="mw-page-title-main">Toluene</span> Chemical compound

Toluene, also known as toluol, is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the odor associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) attached to a phenyl group. As such, its systematic IUPAC name is methylbenzene. Toluene is predominantly used as an industrial feedstock and a solvent.

<span class="mw-page-title-main">Petrochemical</span> Chemical product derived from petroleum

Petrochemicals are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as maize, palm fruit or sugar cane.

<span class="mw-page-title-main">Xylene</span> Organic compounds with the formula (CH3)2C6H4

In organic chemistry, xylene or xylol are any of three organic compounds with the formula (CH3)2C6H4. They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are substituted determines which of three structural isomers results. It is a colorless, flammable, slightly greasy liquid of great industrial value.

Mesitylene or 1,3,5-trimethylbenzene is a derivative of benzene with three methyl substituents positioned symmetrically around the ring. The other two isomeric trimethylbenzenes are 1,2,4-trimethylbenzene (pseudocumene) and 1,2,3-trimethylbenzene (hemimellitene). All three compounds have the formula C6H3(CH3)3, which is commonly abbreviated C6H3Me3. Mesitylene is a colorless liquid with sweet aromatic odor. It is a component of coal tar, which is its traditional source. It is a precursor to diverse fine chemicals. The mesityl group (Mes) is a substituent with the formula C6H2Me3 and is found in various other compounds.

<span class="mw-page-title-main">Steric effects</span> Geometric aspects of ions and molecules affecting their shape and reactivity

Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape (conformation) and reactivity of ions and molecules. Steric effects complement electronic effects, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlapping electron clouds result in structured groupings of molecules stabilized by the way that opposites attract and like charges repel.

<span class="mw-page-title-main">Isobutylene</span> Unsaturated hydrocarbon compound (H2C=C(CH3)2)

Isobutylene is a hydrocarbon with the chemical formula (CH3)2C=CH2. It is a four-carbon branched alkene (olefin), one of the four isomers of butylene. It is a colorless flammable gas, and is of considerable industrial value.

Aromatization is a chemical reaction in which an aromatic system is formed from a single nonaromatic precursor. Typically aromatization is achieved by dehydrogenation of existing cyclic compounds, illustrated by the conversion of cyclohexane into benzene. Aromatization includes the formation of heterocyclic systems.

Chlorotoluene is a group of three isomeric chemical compounds. They consist of a disubstituted benzene ring with one chlorine atom and one methyl group.

2,6-Di-<i>tert</i>-butylphenol Chemical compound

2,6-Di-tert-butylphenol is an organic compound with the structural formula 2,6-((CH3)3C)2C6H3OH. This colorless solid alkylated phenol and its derivatives are used industrially as UV stabilizers and antioxidants for hydrocarbon-based products ranging from petrochemicals to plastics. Illustrative of its usefulness, it prevents gumming in aviation fuels.

1,2,4-Trimethylbenzene, also known as pseudocumene, is an organic compound with the chemical formula C6H3(CH3)3. Classified as an aromatic hydrocarbon, it is a flammable colorless liquid with a strong odor. It is nearly insoluble in water but soluble in organic solvents. It occurs naturally in coal tar and petroleum (about 3%). It is one of the three isomers of trimethylbenzene.

<span class="mw-page-title-main">Durene</span> Chemical compound

Durene, or 1,2,4,5-tetramethylbenzene, is an organic compound with the formula C6H2(CH3)4. It is a colourless solid with a sweet odor. The compound is classified as an alkylbenzene. It is one of three isomers of tetramethylbenzene, the other two being prehnitene (1,2,3,4-tetramethylbenzene) and isodurene (1,2,3,5-tetramethylbenzene). Durene has an unusually high melting point (79.2 °C), reflecting its high molecular symmetry.

<span class="mw-page-title-main">Prehnitene</span> Organic compound

Prehnitene or 1,2,3,4-tetramethylbenzene is an organic compound with the formula C6H2(CH3)4, classified as an aromatic hydrocarbon. It is a flammable colorless liquid which is nearly insoluble in water but soluble in organic solvents. It occurs naturally in coal tar. Prehnitene is one of three isomers of tetramethylbenzene, the other two being isodurene (1,2,3,5-tetramethylbenzene) and durene (1,2,4,5-tetramethylbenzene). It is a relatively easily oxidized benzene derivative, with E1/2 of 2.0 V vs NHE.

<span class="mw-page-title-main">BTX (chemistry)</span> Mixtures of benzene, toluene, and the three xylene isomers

In the petroleum refining and petrochemical industries, the initialism BTX refers to mixtures of benzene, toluene, and the three xylene isomers, all of which are aromatic hydrocarbons. The xylene isomers are distinguished by the designations ortho –, meta –, and para – as indicated in the adjacent diagram. If ethylbenzene is included, the mixture is sometimes referred to as BTEX.

<span class="mw-page-title-main">Alkylbenzene</span> Family of organic compounds

An alkylbenzene is a chemical compound that contains a monocyclic aromatic ring attaching to one or more saturated hydrocarbon chains. Alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups. The simplest member, toluene, has the hydrogen atom of the benzene ring replaced by a methyl group. The chemical formula of alkylbenzenes is CnH2n-6.

<span class="mw-page-title-main">Diethylbenzenes</span> Chemical compound

Diethylbenzene (DEB) refers to any of three isomers with the formula C6H4(C2H5)2. Each consists of a benzene ring and two ethyl substituents. The meta and para have the greater commercial significance. All are colorless liquids.

2,4,6-Tri-<i>tert</i>-butylphenol Chemical compound

2,4,6-Tri-tert-butylphenol (2,4,6-TTBP) is a phenol symmetrically substituted with three tert-butyl groups and thus strongly sterically hindered. 2,4,6-TTBP is a readily oxidizable aromatic compound and a weak acid. It oxidizes to give the deep-blue 2,4,6-tri-tert-butylphenoxy radical. 2,4,6-TTBP is related to 2,6-di-tert-butylphenol, which is widely used as an antioxidant in industrial applications. These compounds are colorless solids.

Fulvio Cacace was an Italian chemist.

4-<i>tert</i>-Butylphenol Organic aromatic compound

4-tert-Butylphenol is an organic compound with the formula (CH3)3CC6H4OH. It is one of three isomeric tert-butyl phenols. It is a white solid with a distinct phenolic odor. It dissolves in basic water.

References

  1. 1 2 Griesbaum, Karl; Behr, Arno; Biedenkapp, Dieter; Voges, Heinz-Werner; Garbe, Dorothea; Paetz, Christian; Collin, Gerd; Mayer, Dieter; Höke (2000). "Hydrocarbons". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a13_227.
  2. ↑ Styrene process with recycle from dehydrogenation zone Archived 2012-03-20 at the Wayback Machine
  3. 1 2 3 4 Tsai, Tseng-Chang "Disproportionation and Transalkylation of Alkylbenzenes over Zeolite Catalysts". Elsevier Science, 1999
  4. ↑ USapplication 20100022814,Goncalvez De Almeida; Jose Luis& Berna Tejeroet al.,"Catalytic transalkylation of dialkyl benzenes",published 2010-01-28, assigned to Petroquimica Espanola, S.A. Petresa
  5. ↑ Harmer, Mark A.; Sun, Qun (2001). "Solid acid catalysis using ion-exchange resins". Applied Catalysis A: General. 221 (1–2): 45–62. doi:10.1016/S0926-860X(01)00794-3.
  6. ↑ Al-Khattaf, Sulaiman; Ali, Syed A.; Aitani, Abdullah M.; Žilková, Naděžda; Kubička, David; Čejka, Jiří (2014). "Recent Advances in Reactions of Alkylbenzenes over Novel Zeolites: The Effects of Zeolite Structure and Morphology". Catalysis Reviews. 56 (4): 333–402. doi:10.1080/01614940.2014.946846. S2CID   97415930.
  7. ↑ Fiege, Helmut; Voges, Heinz-Werner; Hamamoto, Toshikazu; Umemura, Sumio; Iwata, Tadao; Miki, Hisaya; Fujita, Yasuhiro; Buysch, Hans-Josef; Garbe (2000). "Phenol Derivatives". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_313.
  8. ↑ K. W. Schmiedel; D. Decker (2012). "Resorcinol". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a23_111.pub2.
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