A fundamental problem that arises during the oxidation of alcohols to aldehydes is that aldehydes are very easily subject to further oxidation compared to the starting alcohols. In fact, aldehydes are active organic reducing agents. So, when primary alcohols are oxidized with sodium bichromate in sulfuric acid (Beckmann's mixture), the aldehyde that forms must be protected from further oxidation to carboxylic acid. You can, for example, remove the aldehyde from the reaction mixture. And this is widely used because the boiling point of the aldehyde is usually lower than the boiling point of the starting alcohol. In this way, first of all, low-boiling aldehydes, for example, acetic, propionic, isobutyric aldehydes, can be obtained:

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

Figure 2.

When unsaturated alcohols are oxidized with tert-butyl chromate (in aprotic non-polar solvents), multiple bonds are not occupied, 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.

The oxidation of secondary alcohols to ketones is much easier than of primary alcohols to aldehydes. The yields are higher here, since, firstly, the reactivity of secondary alcohols is higher than that of primary alcohols, and, secondly, ketones, which are formed, are much more resistant to the action of oxidants than aldehydes.

Oxidants for the oxidation of alcohols

For the oxidation of alcohols, reagents based on transition metals - derivatives of hexavalent chromium, tetra- and seven-valence manganese, are most widely used as oxidants.

For the selective oxidation of primary alcohols to aldehydes, the best reagents are currently considered to be the $ CrO_3 $ complex with pyridine - $ CrO_ (3 ^.) 2C_5H_5N $ (Sarrett-Collins reagent); the Corey reagent - pyridinium chlorochromate $ CrO_3Cl ^ -C_5H_5N + 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 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 an important advantage in that this reagent does not affect the double or triple bonds in the starting alcohols and is therefore especially effective for the preparation of unsaturated aldehydes.

To obtain $ α¸β $ -unsaturated aldehydes by oxidation of substituted allyl alcohols, manganese (IV) oxide $ MnO_2 $ is a universal oxidizing agent

Examples of reactions of alcohols with these oxidants 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. This elimination can be carried out not only using the previously discussed oxidation methods, but also using catalytic dehydrogenation. Catalytic dehydrogenation is the process of elimination of 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.

The most commonly used catalysts are finely dispersed copper and silver, as well as zinc oxide. 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). The hydrogen formed can be burned if air is added to the reaction mixture, then the total 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, 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 are converted under dehydrogenation conditions to the corresponding saturated carbonyl compounds. The multiple $ C = C $ bond is hydrogenated 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 number of unsaturated carbonyl compounds:

Figure 10.

Application of dehydrogenation of alcohols

The dehydrogenation of alcohols is an important industrial method for the synthesis of aldehydes and ketones, for example formaldehyde, acetaldehyde, acetone. These products are produced in large volumes by both dehydrogenation and oxidative dehydrogenation on a copper or silver catalyst.

Depending on the type of hydrocarbon radical, as well as, in some cases, the peculiarities of attachment of the -OH group to this hydrocarbon radical, compounds with a hydroxyl functional group are divided into alcohols and phenols.

Alcohols refers to compounds in which the hydroxyl group is attached to a hydrocarbon radical, but not directly attached to the aromatic nucleus, if there is one in the structure of the radical.

Examples of alcohols:

If the structure of a hydrocarbon radical contains an aromatic nucleus and a hydroxyl group, while it is connected directly to the aromatic nucleus, such compounds are called phenols .

Examples of phenols:

Why are phenols isolated in a class separate from alcohols? After all, for example, the formulas

are very similar and give the impression of substances of the same class of organic compounds.

However, the direct connection of the hydroxyl group with the aromatic nucleus significantly affects the properties of the compound, since the conjugated system of π-bonds of the aromatic nucleus is also conjugated with one of the lone electron pairs of the oxygen atom. Because of this, the O - H bond in phenols is more polar than in alcohols, which significantly increases the mobility of the hydrogen atom in the hydroxyl group. In other words, phenols have significantly more pronounced acidic properties than alcohols.

Chemical properties of alcohols

Monohydric alcohols

Substitution reactions

Substitution of a hydrogen atom in a hydroxyl group

1) Alcohols react with alkali, alkaline earth metals and aluminum (purified from the protective film Al 2 O 3), while metal alcoholates are formed and hydrogen is released:

The formation of alcoholates is possible only when using alcohols that do not contain water dissolved in them, since in the presence of water, alcoholates are easily hydrolyzed:

CH 3 OK + H 2 O = CH 3 OH + KOH

2) The esterification reaction

The esterification reaction is the interaction of alcohols with organic and oxygen-containing inorganic acids, leading to the formation of esters.

This type of reaction is reversible, therefore, in order to shift the equilibrium towards the formation of an ester, it is desirable to carry out the reaction under heating, as well as in the presence of concentrated sulfuric acid as a dehydrating agent:

Substitution of a hydroxyl group

1) Under the action of hydrohalic acids on alcohols, the hydroxyl group is replaced by a halogen atom. As a result of this reaction, haloalkanes and water are formed:

2) When a mixture of alcohol vapors with ammonia is passed through heated oxides of some metals (most often Al 2 O 3), primary, secondary or tertiary amines can be obtained:

The type of amine (primary, secondary, tertiary) will depend to some extent on the ratio of the starting alcohol to ammonia.

Elimination (cleavage) reactions

Dehydration

Dehydration, which actually means the elimination of water molecules, in the case of alcohols differs by intermolecular dehydration and intramolecular dehydration.

At intermolecular dehydration alcohols, one water molecule is formed as a result of the elimination of a hydrogen atom from one alcohol molecule and a hydroxyl group from another molecule.

As a result of this reaction, compounds belonging to the class of ethers (R-O-R) are formed:

Intramolecular dehydration alcohols proceeds in such a way that one water molecule is split off from one alcohol molecule. This type of dehydration requires somewhat more stringent conditions, consisting in the need to use a noticeably stronger heating in comparison with intermolecular dehydration. In this case, one alkene molecule and one water molecule are formed from one alcohol molecule:

Since the methanol molecule contains only one carbon atom, intramolecular dehydration is impossible for it. With the dehydration of methanol, only ether (CH 3 -O-CH 3) can be formed.

It is necessary to clearly understand the fact that in the case of dehydration of asymmetric alcohols, intramolecular elimination of water will proceed in accordance with the Zaitsev rule, i.e. hydrogen will be split off from the least hydrogenated carbon atom:

Dehydrogenation of alcohols

a) Dehydrogenation of primary alcohols upon heating in the presence of metallic copper leads to the formation aldehydes:

b) In the case of secondary alcohols, similar conditions will lead to the formation ketones:

c) Tertiary alcohols do not enter into a similar reaction, i.e. are not subjected to dehydrogenation.

Oxidation reactions

Combustion

Alcohols readily undergo combustion reactions. This produces a large amount of heat:

2СН 3 -ОН + 3O 2 = 2CO 2 + 4H 2 O + Q

Incomplete oxidation

Incomplete oxidation of primary alcohols can lead to the formation of aldehydes and carboxylic acids.

In the case of incomplete oxidation of secondary alcohols, only ketones can be formed.

Incomplete oxidation of alcohols is possible when various oxidizing agents act on them, for example, such as atmospheric oxygen in the presence of catalysts (metallic copper), potassium permanganate, potassium dichromate, etc.

In this case, aldehydes can be obtained from primary alcohols. As you can see, the oxidation of alcohols to aldehydes, in fact, leads to the same organic products as dehydrogenation:

It should be noted that when using oxidants such as potassium permanganate and potassium dichromate in an acidic medium, a deeper oxidation of alcohols, namely to carboxylic acids, is possible. In particular, this is manifested when using an excess of the oxidizing agent when heated. Secondary alcohols can be oxidized under these conditions only to ketones.

ULTIMATE MULTI-ATOMIC ALCOHOLS

Substitution of hydrogen atoms for hydroxyl groups

Polyhydric alcohols as well as monohydric react with alkali, alkaline earth metals and aluminum (removed from the filmAl 2 O 3 ); in this case, a different number of hydrogen atoms of hydroxyl groups in the alcohol molecule can be replaced:

2. Since the molecules of polyhydric alcohols contain several hydroxyl groups, they influence each other through a negative inductive effect. In particular, this leads to a weakening of the O — H bond and an increase in the acidic properties of hydroxyl groups.

B O The higher acidity of polyhydric alcohols is manifested in the fact that polyhydric alcohols, in contrast to monohydric alcohols, react with some heavy metal hydroxides. For example, you need to remember the fact that freshly precipitated copper hydroxide reacts with polyhydric alcohols to form a bright blue complex solution.

So, the interaction of glycerin with freshly precipitated copper hydroxide leads to the formation of a bright blue solution of copper glycerate:

This reaction is high quality for polyhydric alcohols. To pass the exam, it is enough to know the signs of this reaction, and it is not necessary to be able to write down the interaction equation itself.

3. Just like monohydric alcohols, polyhydric alcohols can enter into an esterification reaction, i.e. react with organic and oxygen-containing inorganic acids with the formation of esters. This reaction is catalyzed by strong inorganic acids and is reversible. In this regard, during the esterification reaction, the resulting ester is distilled off from the reaction mixture in order to shift the equilibrium to the right according to Le Chatelier's principle:

If carboxylic acids with a large number of carbon atoms in the hydrocarbon radical, resulting from such a reaction, enter into a reaction with glycerin, esters are called fats.

In the case of etherification of alcohols with nitric acid, a so-called nitrating mixture is used, which is a mixture of concentrated nitric and sulfuric acids. The reaction is carried out with constant cooling:

An ester of glycerin and nitric acid, called trinitroglycerin, is an explosive. In addition, a 1% solution of this substance in alcohol has a powerful vasodilator effect, which is used for medical reasons to prevent an attack of a stroke or heart attack.

Substitution of hydroxyl groups

Reactions of this type proceed according to the mechanism of nucleophilic substitution. Interactions of this kind include the reaction of glycols with hydrogen halides.

So, for example, the reaction of ethylene glycol with hydrogen bromide proceeds with the successive replacement of hydroxyl groups by halogen atoms:

Chemical properties of phenols

As mentioned at the very beginning of this chapter, the chemical properties of phenols differ markedly from the chemical properties of alcohols. This is due to the fact that one of the lone electron pairs of the oxygen atom in the hydroxyl group is conjugated with the π-system of conjugated bonds of the aromatic ring.

Reactions involving a hydroxyl group

Acidic properties

Phenols are stronger acids than alcohols and are very slightly dissociated in aqueous solution:

B O The higher acidity of phenols in comparison with alcohols in terms of chemical properties is expressed in the fact that phenols, unlike alcohols, are capable of reacting with alkalis:

However, the acidic properties of phenol are less pronounced than even one of the weakest inorganic acids - carbonic. So, in particular, carbon dioxide, when passed through an aqueous solution of alkali metal phenolates, displaces free phenol from the latter as an even weaker acid than carbonic acid:

It is obvious that phenol will also be displaced from phenolates by any other stronger acid:

3) Phenols are stronger acids than alcohols, and alcohols react with alkali and alkaline earth metals. In this regard, it is obvious that phenols will also react with these metals. The only thing is that, unlike alcohols, the reaction of phenols with active metals requires heating, since both phenols and metals are solids:

Substitution reactions in the aromatic nucleus

The hydroxyl group is a substituent of the first kind, which means that it facilitates the occurrence of substitution reactions in ortho- and pair- positions in relation to yourself. Reactions with phenol take place under much milder conditions than benzene.

Halogenation

The reaction with bromine does not require any special conditions. When bromine water is mixed with a phenol solution, a white precipitate of 2,4,6-tribromophenol is instantly formed:

Nitration

When phenol is exposed to a mixture of concentrated nitric and sulfuric acids (nitrating mixture), 2,4,6-trinitrophenol is formed - a crystalline yellow explosive:

Addition reactions

Since phenols are unsaturated compounds, their hydrogenation in the presence of catalysts to the corresponding alcohols is possible.

Specialty: chemical technology

Department: Inorganic Chemistry and Chemical Technology

APPROVED

Head of the Department

_____________________) (Signature, Surname, initials)

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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 _______________ I. V. Kuznetsova

Voronezh - 2015

Introduction

Production of catalysts for the dehydrogenation of alkyl aromatic hydrocarbons.

Catalytic dehydrogenation of alkanes

Equipment for the catalytic dehydrogenation of alkanes

Regeneration of catalysts.

List of used literary sources

Introduction

Dehydration is a reaction of hydrogen elimination from an organic compound molecule; is reversible, the reverse reaction is hydrogenation. The shift of 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, formaldehyde, acetone, methyl ethyl ketone, cyclohexanone are obtained.

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

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

Catalytic dehydrogenation of alcohols

Dehydrogenation reactions of alcohols are necessary to obtain aldehydes and ketones. Ketones are derived from secondary alcohols, and aldehydes from primary alcohols. Copper, silver, copper chromites, zinc oxide, etc. are used as catalysts in the processes. It should be noted that, in comparison with copper catalysts, zinc oxide is more stable and does not lose activity during the process, but 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 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 Al 2 O 3, SnO 2 support or carbon fiber on which silver or copper components are supported. This reaction is one of the components of the Wacker process, which is an industrial method for producing acetaldehyde from ethanol by its dehydrogenation or oxidation with oxygen.

Dehydrogenation of methanol... This process is not fully understood, but most researchers identify it as a promising process for the synthesis of formaldehyde that does not contain water. Various process parameters are offered: temperature 600 - 900 ° C, active catalyst component zinc or copper, support silicon oxide, 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 latest is a catalyst for the dehydrogenation of alcohols, which is an oxide of yttrium or a rare earth element selected from the group consisting of neodymium, praeodymium, ytterbium ..

The disadvantage of the known catalysts is their insufficiently high activity and selectivity.

The aim 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 prepared by impregnating the preformed 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 ° C (to convert pertechnetate into technetium dioxide), then at 600-700 ° 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 ° C for 2 hours. Then reduction is carried out in a stream of hydrogen for 1 hour at 280 ° 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-through installation. Catalyst weight

0.5 g with 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: dehydration and dehydration. The increase in the activity of the catalyst is the greater, the higher the amount of introduced technetium. 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 alkylaromatic compounds is an important industrial process for the synthesis of styrene and its homologues. The process catalysts in most cases are iron oxides promoted with potassium, calcium, chromium, cerium, magnesium, 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 steam of 1:20. Steam is required not only to lower the partial pressure of ethylbenzene, but also to maintain the self-regeneration of iron oxide catalysts.

The dehydrogenation of ethylbenzene is the second stage in the process of producing styrene from benzene. In the first stage, benzene is alkylated with chloroethane (Friedel-Crafts reaction) on a chromium alumina catalyst, and in the second, the resulting ethylbenzene is dehydrogenated to styrene. The process is characterized by a high value of the 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, in the process of ethylbenzene dehydrogenation, 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 the gasification reaction, "burning" most of the coke from the catalyst surface.