The products of dehydrogenation of secondary alcohols are. Substitution of hydroxyl group

Specialty: chemical technology

Department: inorganic chemistry and chemical technology

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Head of the department

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COURSE WORK

Discipline: Industrial catalysis

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On the topic: Catalytic dehydrogenation

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Work designation KR – 02068108 – 240100 - 2015

Student Fazylova L. A.

Login 435

Head _______________ Kuznetsova I.V.

Voronezh – 2015

Introduction

Production of catalysts for dehydrogenation processes of alkyl aromatic hydrocarbons.

Catalytic dehydrogenation of alkanes

Equipment for catalytic dehydrogenation of alkanes

Regeneration of catalysts.

List of references used

Introduction

Dehydrogenation is the reaction of the removal of hydrogen from a molecule of an organic compound; is reversible, the reverse reaction is hydrogenation. A shift in equilibrium towards dehydrogenation is facilitated by an increase in temperature and a decrease in pressure, including dilution of the reaction mixture. The catalysts for the hydrogenation-dehydrogenation reaction are metals of 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 we obtain: formaldehyde, acetone, methyl ethyl ketone, cyclohexanone.

2) dehydrogenation of alkyl aromatic compounds produces: styrene, α-methylstyrene, vinyltoluene, divinylbenzene.

3) dehydrogenation of paraffins produces: olefins (propylene, butylene and isobutylene, isopentene, higher olefins) and dienes (butadiene and isoprene)

Catalytic dehydrogenation of alcohols

Dehydrogenation reactions of alcohols are necessary to produce aldehydes and ketones. Ketones are obtained from secondary alcohols, and aldehydes from primary alcohols. Catalysts in the processes are copper, silver, copper chromites, zinc oxide, etc. It is worth noting that, compared to copper catalysts, zinc oxide is more stable and does not lose activity during the process, but can provoke a dehydration reaction. In general, the dehydrogenation reactions of alcohols can be presented as follows:

In industry, the dehydrogenation of alcohols produces compounds such as acetaldehyde, acetone, methyl ethyl ketone and cyclohexanone. The processes take place 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 any carrier of Al 2 O 3, SnO 2 or carbon fiber, on which silver or copper components are deposited. This reaction is one of the components of the Wacker process, which is an industrial method for producing acetaldehyde from ethanol by dehydrogenation or oxidation with oxygen.

Methanol dehydrogenation. This process has not been fully studied, but most researchers highlight it as a promising process for the synthesis of water-free formaldehyde. Various process parameters are offered: temperature 600 - 900 °C, active catalyst component zinc or copper, silicon oxide carrier, possibility of initiating the reaction with hydrogen peroxide, etc. Currently, most of the world's formaldehyde is produced by the oxidation of methanol.

2. Production of catalysts for alcohol dehydrogenation processes

A known catalyst for the dehydrogenation of alcohols contains oxides of zinc and iron. The newest catalyst is for the dehydrogenation of alcohols, which is an oxide of yttrium or a rare earth 10 element selected from the group including neodymium, paradymium, 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 by the fact 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 allows increasing selectivity. In the reaction of dehydrogenation of alcohol, for example isopropyl, into acetone up to 100%.

Such a catalyst is prepared by impregnating pre-formed catalyst particles with a technetium salt solution. The volume of the solution exceeds 1.4 ─ 1.6 times the bulk volume of the catalyst. The amount of technetium in the catalyst is determined by the 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 the volume of yttrium oxide. The impregnated catalyst particles are dried at 70-80 0 C for 2 hours. Then reduction is carried out in a stream of hydrogen for 1 hour at 280 0 C at a temperature of 600 C.

The study of catalytic activity is carried out using the example of the decomposition of propyl alcohol in a flow-type installation. Catalyst weight

0.5 g with a volume of 1 cm. The catalyst particle size is 1.5 - 2 mm. Specific surface 48.5 m/g. The alcohol flow rate is 0.071 ml/min.

The decomposition of isoaropylic alcohol on the proposed catalyst occurs only in the direction of dehydrogenation with the formation of acetone and hydrogen; no other products were detected. On yttrium oxide without the addition of technetium, the decomposition of isopropyl alcohol proceeds in two directions: dehydrogenation and dehydration. The higher the amount of introduced technetium, the greater the increase in catalyst activity. Catalysts containing 0.03 - 0.05% technetium are selective, leading the process in only one direction towards dehydrogenation.

3. Dehydrogenation of alkylaromatic compounds

Dehydrogenation of alkyl aromatic compounds is an important industrial process for the synthesis of styrene and its homologues. The catalysts for the process in most cases 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 alkyl aromatic compounds occur 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.

Dehydrogenation of ethylbenzene is the second stage of the process of producing styrene from benzene. At the first stage, benzene is alkylated with chloroethane (Friedel-Crafts reaction) on a chromium-alumina 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 is highly dependent on temperature. That is why the reaction is carried out at high temperatures.

In parallel, during the dehydrogenation of ethylbenzene, side reactions occur - coke formation, skeletal isomerization and cracking. Cracking and isomerization reduce the selectivity of the process, and coke formation 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 a gasification reaction that “burns out” most of the coke from the surface of the catalyst.

Dehydrogenation reactions of alcohols are necessary to produce aldehydes and ketones. Ketones are obtained from secondary alcohols, and aldehydes from primary alcohols. Catalysts in the processes are copper, silver, copper chromites, zinc oxide, etc. It is worth noting that, compared to copper catalysts, zinc oxide is more stable and does not lose activity during the process, but can provoke a dehydration reaction. In general, the dehydrogenation reactions of alcohols can be presented as follows:

In industry, the dehydrogenation of alcohols produces compounds such as acetaldehyde, acetone, methyl ethyl ketone and cyclohexanone. The processes take place 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 any carrier of Al 2 O 3, SnO 2 or carbon fiber, on which silver or copper components are deposited. This reaction is one of the components of the Wacker process, which is an industrial method for producing acetaldehyde from ethanol by dehydrogenation or oxidation with oxygen.

2. can proceed in different ways, depending on the structural formula of its original substance. 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. it also depends on the structure of the starting compound, which affects the final product (aldehyde or ketone).

4. Methanol dehydrogenation. This process has not been fully studied, but most researchers highlight it as a promising process for the synthesis of water-free formaldehyde. Various process parameters are offered: temperature 600 - 900 °C, active catalyst component zinc or copper, silicon oxide carrier, possibility of initiating the reaction with hydrogen peroxide, etc. Currently, most of the world's formaldehyde is produced by the oxidation of methanol.

The fundamental problem that arises when alcohols are oxidized to aldehydes is that aldehydes are very easily subject to further oxidation compared to the starting alcohols. Essentially, 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 parent alcohol. In this way, first of all, low-boiling aldehydes can be obtained, for example, acetic, propionic, isobutyric:

Figure 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, tert-butyl ester of chromate acid is used as an oxidizing agent:

Figure 2.

During the oxidation of unsaturated alcohols with tert-butyl chromate (in aprotic non-polar solvents), multiple bonds are not occupied, and unsaturated aldehydes are formed in high yields.

The oxidation method, which uses manganese dioxide in an organic solvent, pentane or methylene chloride, is quite selective. For example, alyl and benzyl alcohols can be oxidized in this way to the corresponding aldehydes. The output alcohols are slightly soluble in non-polar solvents, and the aldehydes that are formed as a result of oxidation are much more 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.

The oxidation of secondary alcohols into ketones is much easier than the oxidation of primary alcohols into aldehydes. The yields here are higher because, firstly, the reactivity of secondary alcohols is higher than that of primary alcohols, 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, the most widely used oxidizing agents are reagents based on transition metals - derivatives of hexavalent chromium, four and seven valent manganese.

For the selective oxidation of primary alcohols to aldehydes, the best reagents are currently considered to be the complex of $CrO_3$ with pyridine - $CrO_(3^.) 2C_5H_5N$ (Sarrett-Collins reagent); the Corey reagent - pyridinium chlorochromate $CrO_3Cl^-C_5H_5N^ is also widely used +H$ in methylene chloride. The red complex $CrO_(3^.) 2C_5H_5N$ is obtained by slow interaction of $CrO_(3^.)$ with pyridine at 10-15 $^\circ$С. Orange pyridinium chlorochromate is prepared 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 allylic alcohols, manganese (IV) oxide $MnO_2$ is a universal oxidizing agent

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 comes down to the abstraction of hydrogen from the starting alcohol molecule. Such elimination can be carried out not only using the previously discussed oxidation methods, but also using catalytic dehydrogenation. Catalytic dehydrogenation is the process of removing hydrogen from alcohols in the presence of a catalyst (copper, silver, zinc oxide, a mixture of chromium and copper oxides) both with and without the participation of 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. Catalytic dehydrogenation of alcohols is especially convenient for the synthesis of aldehydes, which are very easily oxidized to acids.

The above-mentioned catalysts are applied in a highly dispersed state onto 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 $^\circ$С. To prevent further transformation of the dehydrogenation products, the reaction gases must be cooled quickly. Dehydrogenation is a very endothermic reaction ($\triangle H$ = 70-86 kJ/mol). The hydrogen produced 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 primarily in industry, the 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. Under dehydrogenation conditions, unsaturated alcohols 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 makes it possible to obtain a whole range of unsaturated carbonyl compounds:

Figure 10.

Applications of alcohol dehydrogenation

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 both by dehydrogenation and oxidative dehydrogenation over a copper or silver catalyst.

Protonation of alcohols in a non-nucleophilic environment ultimately leads to dehydration. Dehydration of alcohols occurs when heated in concentrated sulfuric acid, phosphoric acid or in a superacid medium - a mixture of antimony pentafluoride and fluorosulfonic acid. As a result of the elimination of water, the alkoxonium cation forms a carbocation as an unstable intermediate. In a non-nucleophilic environment, the carbocation loses a proton to form alkenes. The slowest stage of the entire process is the conversion of the alkoxonium cation into a carbocation:

The given sequence of transformations is typical for monomolecular elimination reactions E 1 (chapter 10). Direction E 1-elimination is determined by Zaitsev’s rule, i.e. Among the reaction products, the alkene most substituted at the double bond predominates.

Elimination by mechanism E 1 is typical for tertiary and secondary alcohols. Secondary alcohols undergo dehydration when heated with 85% phosphoric acid at 160-170 o C or with 60-70% sulfuric acid at 90-100 o C, the direction of dehydration corresponds to Zaitsev’s rule:

Dehydration of tertiary alcohols can be carried out already in 20-50% sulfuric acid at 85-100 o C:

Tertiary alcohols dehydrate so easily that selective dehydration of a diol containing a tertiary and primary hydroxyl group is possible:

The dehydration of many alcohols is accompanied by a rearrangement consisting of 1,2-migration of an alkyl group or hydride ion. Such rearrangements are typical for processes involving carbocations as intermediate particles. The observed order of decrease in the reactivity of alcohols: tertiary > secondary > primary and the presence of rearrangements are consistent with carbocationic E 1-mechanism of dehydration:

For primary alcohols, a different mechanism of dehydration in concentrated sulfuric acid is probably realized. Primary alcohols undergo dehydration under much more severe conditions compared to secondary and tertiary alcohols. Propanol-1 gives propylene when heated with 96% sulfuric acid at 170-190 °C; under the same conditions, ethylene is obtained from ethanol:

Primary alcohols, when reacting with sulfuric acid, easily form half-esters of sulfuric acid. E In this case, the half-ester apparently undergoes 2-elimination, and the role of the base is played by the hydrogen sulfate anion or water:

The given mechanism of dehydration of primary alcohols seems more likely, but the mechanism cannot be excluded E 2, in which the substrate is an alkoxonium cation and the base is a hydrogen sulfate ion:

Under milder conditions, when heating the simplest primary alcohols with 96% sulfuric acid at 130 -140 °C, ethers are predominantly obtained. The mechanism of this transformation is the alkylation of the primary alcohol either by the action of a sulfuric acid half-ester or by interaction with an alkoxonium cation, kinetically both of these mechanisms S N 2-substitutions are indistinguishable:

This method produces the simplest ethers - diethyl, dipropyl and dibutyl ethers and cyclic ethers, for example tetrahydrofuran or dioxane. Secondary and tertiary alcohols dehydrate under these conditions to form alkenes:

Another disadvantage of this method for preparing ethers is that it is not suitable for preparing unsymmetrical ethers from two alcohols, since in this case a mixture of all three possible products ROR, R"OR" and ROR" is formed.

For intra- or intermolecular dehydration of alcohols, especially in industry, instead of sulfuric acid, it is more convenient to use anhydrous aluminum oxide as a dehydrating agent. Heterogeneous catalytic dehydration of primary, secondary and tertiary alcohols over alumina at 350-450 o C leads to alkenes:

Acid-catalyzed dehydration of alcohols does not exhibit the required regioselectivity, and in most cases mixtures of different isomeric alkenes are obtained. For example, when 1-octanol is dehydrated with phosphoric acid, a mixture of 2-octene and a small amount of 3-octene is formed, which does not contain the expected 1-octene. This is due to the ease of isomerization and skeletal rearrangements in the intermediate carbocations formed. These older methods for the synthesis of alkenes and ethers are gradually being replaced by modern regioselective and stereoselective methods for creating a carbon-carbon double bond.