Alcohol dehydrogenation reactions are necessary to produce aldehydes and ketones. Ketones are obtained from secondary alcohols, and aldehydes from primary alcohols. Copper, silver, copper chromites, zinc oxide, etc. serve as catalysts in the processes. It should be noted that, compared to copper catalysts, zinc oxide is more stable and does not lose activity during the process, however, it can provoke a dehydration reaction. In general, the reaction of dehydrogenation of alcohols can be represented as follows:
In industry, the dehydrogenation of alcohols produces compounds such as acetaldehyde, acetone, methyl ethyl ketone, and cyclohexanone. The processes proceed in a stream of water vapor. The most common processes are:
1. carried out on a copper or silver catalyst at a temperature of 200 - 400 ° C and atmospheric pressure. The catalyst is some kind of Al 2 O 3 , SnO 2 or carbon fiber supported with silver or copper components. This reaction is one of the components of the Wacker process, which is an industrial method for obtaining acetaldehyde from ethanol by dehydrogenation or oxidation with oxygen.
2. can proceed in different ways, depending on the structural formula of its starting material. 2-propanol, which is a secondary alcohol, is dehydrogenated to acetone, and 1-propanol, being a primary alcohol, is dehydrogenated to propanal at atmospheric pressure and a process temperature of 250–450 °C.
3. also depends on the structure of the starting compound, which affects the final product (aldehyde or ketone).
4. Methanol dehydrogenation. This process is not fully understood, but most researchers highlight it as a promising process for the synthesis of formaldehyde that does not contain water. Various process parameters are proposed: temperature 600 - 900 °C, active component of the catalyst zinc or copper, silicon oxide carrier, the possibility of initiating the reaction with hydrogen peroxide, etc. At the moment, most of the formaldehyde in the world is produced by the oxidation of methanol.
Divinyl and isoprene can also be obtained by dehydration of the corresponding glycols or unsaturated alcohols. The last reaction is an intermediate stage in the industrial production of divinyl by the method of S. V. Lebedev - from ethyl alcohol: 120_Chapter 8. Diene hydrocarbons_ By this method, in ...(ORGANIC CHEMISTRY)
Splitting of water from alcohols (dehydration):
Acid reagents are used as dehydration catalysts: sulfuric and phosphoric acids, alumina, etc. The order of splitting off is most often determined by Zaitsev's rule (1875): during the formation of water, hydrogen is most easily split off from the neighboring least hydrogenated carbon atom...(ORGANIC CHEMISTRY)
Alcohol oxidation
Alcohols are more easily oxidized than hydrocarbons, and the carbon at which the hydroxyl group is located is the first to be oxidized. The most suitable oxidizing agent in laboratory conditions is a chromium mixture. In industry - atmospheric oxygen in the presence of catalysts. Primary...(ORGANIC CHEMISTRY)
Oxidation of ethyl alcohol to acetic acid.
Ethyl alcohol is oxidized to acetic acid under the influence of acetic acid bacteria of the genera Gluconobacter and Acetobacter. They are Gram-negative chemoorganoheterotrophic, non-spore-forming, rod-shaped organisms, motile or immobile. Acetic acid bacteria of these genera differ from each other in ...(FUNDAMENTALS OF MICROBIOLOGY)
Catalytic dehydrogenation of paraffins
An important industrial method is also the catalytic dehydrogenation of paraffins over chromium oxide: Most laboratory methods for obtaining olefins are based on the reactions of elimination (elimination) of various reagents: water, halogens or hydrogen halides from the corresponding derivatives of saturated ...(ORGANIC CHEMISTRY)
Specialty: chemical technology
Department: inorganic chemistry and chemical technology
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COURSE WORK
By discipline: Industrial catalysis
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On the topic: Catalytic dehydrogenation
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Designation of work KR - 02068108 - 240100 - 2015
Student Fazylova L.A.
Login 435
Head _______________ Kuznetsova I.V.
Voronezh - 2015
Introduction
Production of catalysts for dehydrogenation of alkylaromatic hydrocarbons.
Catalytic dehydrogenation of alkanes
Equipment for catalytic dehydrogenation of alkanes
Regeneration of catalysts.
List of used literary sources
Introduction
Dehydrogenation - the reaction of splitting off hydrogen from a molecule of an organic compound; is reversible, the reverse reaction is hydrogenation. The equilibrium shift towards dehydrogenation is promoted by an increase in temperature and a decrease in pressure, including dilution of the reaction mixture. Hydrogenation-dehydrogenation reaction catalysts are metals 8B and 1B subgroups (nickel, platinum, palladium, copper, silver) and semiconductor oxides (Fe 2 O 3 , Cr 2 O 3 , ZnO, MoO 3).
Dehydrogenation processes are widely used in industrial organic synthesis:
1) by dehydrogenation of alcohols, formaldehyde, acetone, methyl ethyl ketone, cyclohexanone are obtained.
2) by dehydrogenation of alkylaromatic compounds, styrene, α-methylstyrene, vinyltoluene, divinylbenzene are obtained.
3) paraffin dehydrogenation produces: olefins (propylene, butylene and isobutylene, isopentene, higher olefins) and dienes (butadiene and isoprene)
Catalytic dehydrogenation of alcohols
Alcohol dehydrogenation reactions are necessary to produce aldehydes and ketones. Ketones are obtained from secondary alcohols, and aldehydes from primary alcohols. Copper, silver, copper chromites, zinc oxide, etc. serve as catalysts in the processes. It should be noted that, compared to copper catalysts, zinc oxide is more stable and does not lose activity during the process, however, it can provoke a dehydration reaction. In general, the reaction of dehydrogenation of alcohols can be represented as follows:
In industry, the dehydrogenation of alcohols produces compounds such as acetaldehyde, acetone, methyl ethyl ketone, and cyclohexanone. The processes proceed in a stream of water vapor. The most common processes are:
Ethanol dehydrogenation carried out on a copper or silver catalyst at a temperature of 200 - 400 ° C and atmospheric pressure. The catalyst is some kind of Al 2 O 3 , SnO 2 or carbon fiber supported with silver or copper components. This reaction is one of the components of the Wacker process, which is an industrial method for obtaining acetaldehyde from ethanol by dehydrogenation or oxidation with oxygen.
Methanol dehydrogenation. This process is not fully understood, but most researchers highlight it as a promising process for the synthesis of formaldehyde that does not contain water. Various process parameters are proposed: temperature 600 - 900 °C, active component of the catalyst zinc or copper, silicon oxide carrier, the possibility of initiating the reaction with hydrogen peroxide, etc. At the moment, most of the formaldehyde in the world is produced by the oxidation of methanol.
2. Production of catalysts for alcohol dehydrogenation processes
Known catalyst for the dehydrogenation of alcohols containing oxides, 5 zinc and iron. The newest is a catalyst for the dehydrogenation of alcohols, which is an oxide of yttrium or a rare earth element 10 selected from the group including neodymium, praeodymium, ytterbium ..
The disadvantage of the known catalysts is their insufficiently high activity and selectivity.
The goal of science is to increase the activity and selectivity of the catalyst for the dehydrogenation of alcohols. This goal is achieved in that the catalyst based on oxides of yttrium or a rare earth element selected from the group including neodymium, praseodymium, ytterbium, additionally contains technetium.
The introduction of technetium into the catalyst makes it possible to increase the activity of the catalyst, which is expressed in an increase in the degree of alcohol conversion by 2-5 times and a decrease in the temperature of the onset of the dehydrogenation reaction by 80-120 0 C. In this case, the catalyst acquires purely dehydrogenating properties, which makes it possible to increase selectivity. In the reaction of dehydrogenation of alcohol, for example, isopropyl alcohol to acetone up to 100%.
Such a catalyst is obtained by impregnating the preformed catalyst particles with a technetium salt solution. The volume of the solution exceeds the bulk volume of the catalyst by 1.4–1.6 times. The amount of technetium in the catalyst is determined by specific radioactivity. The wet catalyst is dried. The dry product is heated for 1 hour in a stream of hydrogen, first at 280-300 0 C (to convert pertechnetate into technetium dioxide), then at 600-700 0 C for 11 hours (to reduce technetium dioxide to metal).
Example. The catalyst is prepared by impregnating yttrium oxide with a solution of ammonium pertechnetate, the volume of which is 1.5 times that of yttrium oxide. The impregnated catalyst particles are dried at 70-80 0 C for 2 hours. Then reduction is carried out in a hydrogen flow for 1 hour at 280 0 C at a temperature of 600 C.
The study of catalytic activity is carried out on the example of the decomposition of isopropyl alcohol in a flow type installation. Catalyst weight
0.5 g at a volume of 1 cm. The size of the catalyst particles is 1.5 - 2 mm. Specific surface area 48.5 m/g. The alcohol feed rate is 0.071 ml/min.
The decomposition of isoaropyl alcohol on the proposed catalyst occurs only in the direction of dehydrogenation with the formation of acetone and hydrogen; no other products were found. On yttrium oxide without the addition of technetium, the decomposition of isopropyl alcohol proceeds in two directions: dehydrogenation and dehydration. The increase in catalyst activity is the greater, the higher the amount of technetium introduced. Catalysts containing 0.03 - 0.05% technetium are selective, leading the process in only one direction towards dehydrogenation.
3. Dehydrogenation of alkylaromatic compounds
The dehydrogenation of alkylaromatic compounds is an important industrial process for the synthesis of styrene and its homologues. In most cases, the process catalysts are iron oxides promoted by potassium, calcium, chromium, cerium, magnesium, and zinc oxides. Their distinctive feature is the ability to self-regenerate under the influence of water vapor. Phosphate, copper-chromium and even catalysts based on a mixture of iron oxide and copper are also known.
The processes of dehydrogenation of alkylaromatic compounds proceed at atmospheric pressure and at a temperature of 550 - 620 ° C in a molar ratio of raw materials to water vapor of 1:20. Steam is necessary not only to reduce the partial pressure of ethylbenzene, but also to maintain the self-regeneration of iron oxide catalysts.
The dehydrogenation of ethylbenzene is the second step in the process of obtaining styrene from benzene. At the first stage, benzene is alkylated with chloroethane (Friedel-Crafts reaction) on an aluminum-chromium catalyst, and at the second stage, the resulting ethylbenzene is dehydrogenated to styrene. The process is characterized by a high activation energy of 152 kJ/mol, due to which the reaction rate strongly depends on temperature. That is why the reaction is carried out at high temperatures.
In parallel, in the process of dehydrogenation of ethylbenzene, side reactions occur - coke formation, skeletal isomerization and cracking. Cracking and isomerization reduce the selectivity of the process, and coking affects the deactivation of the catalyst. In order for the catalyst to work longer, it is necessary to periodically carry out oxidative regeneration, which is based on the gasification reaction, which “burns out” most of the coke from the catalyst surface.
The fundamental problem that arises when alcohol oxidation to aldehydes, is that aldehydes are very easily subjected to further oxidation compared to the original alcohols. In fact, aldehydes are active organic reducing agents. Thus, during the oxidation of primary alcohols with sodium dichromate in sulfuric acid (Beckmann mixture), the aldehyde that is formed must be protected from further oxidation to carboxylic acid. It is possible, for example, to remove the aldehyde from the reaction mixture. And this is widely used, since the boiling point of the aldehyde is usually lower than the boiling point of the original alcohol. In this way, first of all, low-boiling aldehydes can be obtained, for example, acetic, propionic, isobutyric:
Picture 1.
Better results can be obtained if glacial acetic acid is used instead of sulfuric acid.
To obtain high-boiling aldehydes from the corresponding primary alcohols, chromic acid tert-butyl ester is used as an oxidizing agent:
Figure 2.
In the oxidation of unsaturated alcohols with tert-butyl chromate (in aprotic nonpolar solvents), multiple bonds are not engaged, and unsaturated aldehydes are formed in high yields.
Sufficiently selective is the oxidation method, which uses manganese dioxide in an organic solvent, pentane or methylene chloride. For example, allyl and benzyl alcohols can thus be oxidized to the corresponding aldehydes. Output alcohols are slightly soluble in non-polar solvents, and aldehydes, which are formed as a result of oxidation, are much better soluble in pentane or methylene chloride. Therefore, carbonyl compounds pass into the solvent layer and thus contact with the oxidizing agent and further oxidation can be prevented:
Figure 3
It is much easier to oxidize secondary alcohols to ketones than to oxidize primary alcohols to aldehydes. The yields here are higher, since, firstly, the reactivity of secondary alcohols is higher than that of primary ones, and, secondly, ketones, which are formed, are much more resistant to oxidizing agents than aldehydes.
Oxidizing agents for the oxidation of alcohols
For the oxidation of alcohols as oxidizing agents, reagents based on transition metals - derivatives of hexavalent chromium, four and seven valent manganese - have found the widest application.
For the selective oxidation of primary alcohols to aldehydes, the $CrO_3$ complex with pyridine - $CrO_(3^.) 2C_5H_5N$ (Sarrett-Collins reagent) is currently considered to be the best reagent. Corey's reagent - pyridinium chlorochromate $CrO_3Cl^-C_5H_5N^ +H$ in methylene chloride. The red $CrO_(3^.) 2C_5H_5N$ complex is obtained by slow interaction of $CrO_(3^.)$ with pyridine at 10-15 $^\circ$C. Orange pyridinium chlorochromate is obtained by adding pyridine to a solution of chromium (IV) oxide in 20% hydrochloric acid. Both of these reagents are soluble in $CH_2Cl_2$ or $CHCl_3$:
Figure 4
These reagents provide very high yields of aldehydes, but pyridinium chlorochromate has the important advantage that this reagent does not affect the double or triple bonds in the starting alcohols and is therefore particularly effective for the preparation of unsaturated aldehydes.
To obtain $α¸β$-unsaturated aldehydes by oxidation of substituted allyl alcohols, manganese(IV) oxide $MnO_2$
Examples of reactions of alcohols with these oxidizing agents are given below:
Catalytic dehydrogenation of alcohols
Strictly speaking, the oxidation of alcohols to carbonyl compounds is reduced to the elimination of hydrogen from the molecule of the original alcohol. Such cleavage can be carried out not only using the previously discussed oxidation methods, but also using catalytic dehydrogenation. Catalytic dehydrogenation is the process of splitting hydrogen from alcohols in the presence of a catalyst (copper, silver, zinc oxide, a mixture of chromium and copper oxides) both with and without oxygen. The dehydrogenation reaction in the presence of oxygen is called the oxidative dehydrogenation reaction.
Finely dispersed copper and silver, as well as zinc oxide, are most often used as catalysts. The catalytic dehydrogenation of alcohols is especially convenient for the synthesis of aldehydes, which are very easily oxidized to acids.
The above catalysts are applied in a highly dispersed state on inert carriers with a developed surface, for example, asbestos, pumice. The equilibrium of the catalytic dehydrogenation reaction is established at a temperature of 300-400 $^\circ$C. To prevent further transformation of the dehydrogenation products, the reaction gases must be rapidly cooled. Dehydrogenation is a very endothermic reaction ($\triangle H$ = 70-86 kJ/mol). Hydrogen formed can be burned if air is added to the reaction mixture, then the overall reaction will be highly exothermic ($\triangle H$ = -(160-180) kJ / mol). This process is called oxidative dehydrogenation or autothermal dehydrogenation. Although dehydrogenation is used mainly in industry, this method can also be used in the laboratory for preparative synthesis.
Saturation dehydrogenation of aliphatic alcohols occurs in good yields:
Figure 9
In the case of high-boiling alcohols, the reaction is carried out under reduced pressure. Unsaturated alcohols under dehydrogenation conditions are converted into the corresponding saturated carbonyl compounds. Hydrogenation of the multiple $C = C$ bond occurs with hydrogen, which is formed during the reaction. To prevent this side reaction and to be able to obtain unsaturated carbonyl compounds by catalytic dehydrogenation, the process is carried out in a vacuum at 5-20 mm Hg. Art. in the presence of water vapor. This method allows you to get a number of unsaturated carbonyl compounds:
Figure 10.
Application of alcohol dehydrogenation
The dehydrogenation of alcohols is an important industrial method for the synthesis of aldehydes and ketones, such as formaldehyde, acetaldehyde, and acetone. These products are produced in large volumes by both dehydrogenation and oxidative dehydrogenation on a copper or silver catalyst.
