Reference product: o-cresol [kg]
Location: RoW - Rest-of-World
The process “o-cresol, at plant, GLO" is modelled for the production of o-cresol from phenol. Raw materials are modelled with a stoechiometric calculation. Emissions are estimated. Energy consumptions, infrastructure and transports are calculated with standard values.
Cresol (C7H8O; methylphenol) occurs in three isomeric forms: o-cresol (CAS 95-48-7), m-cresol (CAS 103-39-4), and p-cresol (CAS 106-44-5). As pure substances, o- and p-cresol are crystalline; m-cresol is a viscous oil at room temperature. The cresols have a phenolic odour and are colourless, but become yellow to brown after a time. Because water dissolves freely in cresols, they absorb moisture from the air. Since 1965, when the recovery of cresols from coal tar and spent refinery caustics had become insufficient to meet the rising demand, these compounds have been increasingly produced by synthesis. The processes now in use are
– Alkali fusion of toluenesulfonates
– Alkaline chlorotoluene hydrolysis
– Splitting of cymene hydroperoxide
– Methylation of phenol in the vapor phase
The first three processes start from toluene and were developed from the corresponding benzene – phenol syntheses; to some extent they are even carried out in converted plants that formerly produced phenol. In the following descriptions, attention will be primarily paid to the differences between these processes. The methylation of phenol is a process specifically developed to produce cresols and xylenols. Each process gives a different isomer distribution, and all are, therefore, of individual importance.
Methylation of Phenol
Synthetic cresol, especially o-cresol, and xylenol are now produced largely by methylation of phenol with methanol in the presence of catalysts. 2,6-Xylenol is produced almost entirely by this method.
The process consists of only one reaction step, but is based on phenol, which is relatively expensive; the separation of products from the reaction mixture is also costly. This is partly because the boiling points of several components are very near one another, and partly because in some cases the purity of the product must meet very strict requirements.
The reaction can be carried out either in the vapor phase or in the liquid phase. Vapor-phase methylation is used mainly to manufacture pure o-cresol and/or pure 2,6-xylenol.
At atmospheric or slightly elevated pressure, a superheated mixture of the vapors of phenol, methanol, and water is passed at a liquid hourly space velocity (LHSV) of 1 – 2 h–1 over a metal oxide catalyst fixed in a multitubular reactor of stainless steel at a temperature of 300 – 460 °C. The reaction temperature required depends on the nature of the catalyst and on the desired composition of the product. The temperature is permitted to deviate from this level by only a few degrees.
The heat of the reaction is dissipated by boiling organic heat-transfer media, by circulating salt melt, or through the generation of high-pressure steam. The slight loss of catalytic activity that occurs in the course of time is compensated by raising the temperature accordingly. Carbon deposits are removed from time to time by burning. The presence of water in the feedstock mixture suppresses the decomposition of methanol, extends the regeneration cycle, and prolongs the lifetime of the catalyst, which may then endure for several thousand hours.
The product mixture leaving the reactor gives up some of its heat to the feedstock mixture in a heat exchanger, and is then condensed. The uncondensable reaction products formed from the decomposition of methanol (H2, CH4, CO2, and CO) are used as fuel gas in steam production. Aqueous methanol is first distilled from the liquid reaction product and then recycled. Separation into methanol, water containing phenol, and the phenols can be achieved in a single distillation column with sidestream. It is also possible to decant an aqueous methanol phase and to recycle it after adsorbing the contained phenols with an resin adsorbent (desorbing can be achieved with methanol). Water remaining in the product is then removed either by azeotropic distillation with toluene or as an azeotropic mixture with unconverted phenol. In the latter case, it is necessary to strip the phenol from the wastewater. The dehydrated mixture is then fractionated into phenol ether, phenol, o-cresol (≥ 99.5 %), and 2,6-xylenol in an additional series of highly efficient distillation columns operated continuously under vacuum. The phenol ether – phenol fraction is recycled. The distillation residue, which contains, among other compounds, 2,3-, 2,4-, and 2,5-xylenol and 2,4,6- and 2,3,6-trimethylphenol, is usually burned as a fuel oil. If the catalyst has high ortho selectivity, pure (99.5 %) 2,6-xylenol can be obtained directly. If that is not the case or if the reaction conditions are too severe (high temperature, long residence time), the 2,6-xylenol fraction will contain m- and p-cresol, which must be removed by special methods (Section Separation). The ratio of o-cresol to 2,6-xylenol can be controlled by altering the methanol – phenol ratio, and if the o-cresol is recycled, 2,6-xylenol comprises the sole product. Over an extended production time, the yield (selectivity) of o-cresol and 2,6-xylenol, calculated on the basis of reacted phenol, is 90 – 98 %.
Methylation in the Liquid Phase
Phenol and an equimolar amount of methanol are reacted in an autoclave at 350 – 400 °C in the presence of 2 – 3 wt % aluminum methylate, with the water of the reaction, together with some of the methanol, being distilled through a small column. After the catalyst residue has been removed by filtration, the product is worked up by distillation. Methanol, anisole, and phenol are recycled. The yield of cresols and xylenols is ca. 80 % , calculated on the basis of converted phenol (ca. 60 %). The main product is o-cresol; m- and p-cresol, 2,6-, 2,4-, and 2,3-xylenol, and trimethylphenols are also formed.
Most of the o-cresol manufactured in Europe is chlorinated to 4-chloro-o-cresol, the starting material for the chlorophenoxyalkanoic acids 4-chloro-2-methylphenoxyacetic acid (MCPA), 2-(4-chloro-2-methylphenoxy)-propionic acid (MCPP), and γ-(4-chloro-2-methylphenoxy)butyric acid (MCPB), which are important as selective herbicides. A considerably smaller proportion is nitrated to 4,6-dinitro-o-cresol (DNCO), which has both herbicidal and insecticidal properties, and is also used as a polymerization inhibitor for the production and distillation of styrene.
Highly pure o-cresol is increasingly processed, especially in Japan, to epoxy – o-cresol novolak resins (ECN resins), which are used as sealing materials for integrated circuits (chips). o-Cresol (of common quality) is also used to modify traditional phenol–formaldehyde resins.
o-Cresol is also important as a precursor of various dye intermediates, of which the most important in terms of quantity is o-cresotinic acid produced by the Kolbe synthesis. This acid finds further use in the manufacture of pharmaceuticals, whereas its methyl esters serve as dyeing auxiliaries.
An appreciable amount of o-cresol is used as a solvent, either directly or after hydrogenation to 2-methylcyclohexanol or 2-methylcyclohexanone. In the form of its carbonate ester, o-cresol constitutes a starting material in the synthesis of coumarin. The alkylation of o-cresol with propene gives carvacrol, which is used as an antiseptic and in fragrances. In addition, small amounts of o-cresol are, e.g., alkylated with isobutene and used as starting materials in the production of various antioxidants, for the production of components for thermal recording materials, and for pharmaceuticals.
Frischknecht R., Jungbluth N., Althaus H.-J., Doka G., Dones R., Heck T., Hellweg S., Hischier R., Nemecek T., Rebitzer G. and Spielmann M. (2007) Overview and Methodology. Final report ecoinvent v2.0 No. 1. Swiss Centre for Life Cycle Inventories, Dübendorf, CH, retrieved from: www.ecoinvent.org.
Gendorf (2000) Umwelterklärung 2000, Werk Gendorf. Werk Gendorf, Burgkirchen as pdf-File under: http://www.gendorf.de/pdf/umwelterklaerung2000.pdf
Helmut Fiege: Cresols and Xylenols. Published online: 2002. In: Ullmann's Encyclopedia of Industrial Chemistry, Seventh Edition, 2004 Electronic Release (ed. Fiedler E., Grossmann G., Kersebohm D., Weiss G. and Witte C.). 7 th Electronic Release Edition. Wiley InterScience, New York, Online-Version under: DOI: 10.1002/14356007.a08_025
[This dataset has been generated using the system model “Substitution, consequential, long-term". A system model describes how activity datasets are linked to form product systems. This consequential system model uses substitution (also known as "system expansion") to substitute by-product outputs. It includes only activities to the extent that they are expected to change in the long-term as a consequence of small-scale changes in demand, following a consequential approach, taking into account both constrained markets and technology constraints.
Version 3 of the ecoinvent database offers three system models to choose from. For more information, please visit: https://www.ecoinvent.org/database/system-models-in-ecoinvent-3/system-models-in-ecoinvent-3.html)]
methylation of phenol