Hydration of olefins to alcohols

Hydration means, in general, addition of the elements of water to a substance. Most of these reactions are non-catalytic or homogeneously catalysed processes. In this section, only hydration of olefins to alcohols, of acetylene to acetaldehyde, and of alkene oxides to glycols will be treated, since they are typical reactions where the application of solid catalysts has become important. 

Originally, the hydration of olefins to alcohols was carried out with dilute aqueous sulphuric acid as the catalyst. Recently, the direct vapour phase hydration of olefins with solid catalysts has become the predominant method of operation. From the thermodynamic point of view, the formation of alcohols by the exothermic reaction (A) is favoured by low temperatures though even at room temperature the equilibrium is still in favour of dehydration. To induce a rapid reaction, the solid catalysts require an elevated temperatue, which shifts the equilibrium so far in favour of the olefin that the maximum attainable conversion may be very low. High pressures are therefore necessary to bring the conversion to an economic level (Fig. 11). To select an optimum combination of reaction conditions with respect to both rate limitation and equilibrium limitation, 

The catalytic hydration of olefins can also be performed in a three-phase system solid catalyst, liquid water (with the alcohol formed dissolved in it) and gaseous olefin [258,279,280]. The olefin conversion is raised, in comparison with the vapour phase processes, by the increase in solubility of the product alcohol in the excess of water [258]. For these systems with liquid and vapour phases simultaneously present, the equilibrium composition of both phases can be estimated together with vapour-liquid equilibrium data [281]. For the three-phase systems, ion exchangers, especially, have proved to be very efficient catalysts [260,280]. With higher olefins (2-methylpropene), the reaction was also performed in a two-phase liquid system with an ion exchanger as catalyst [282]. It is evident that the kinetic characteristics differ according to the arrangement (phase conditions), i.e. whether the vapour system, liquid vapour system or two-phase liquid system is used. However, most kinetic and mechanistic studies of olefin hydration were carried out in vapour phase systems. 

The best catalysts for olefin hydration are not necessarily those which have proved most satisfactory for the reverse reaction. Some of the successful hydration catalysts are not typical dehydration catalysts. The more obvious reasons are (i) different adsorption characteristics of the catalyst is desirable, e.g. stronger adsorption of olefin relative to alcohol, (ii) under the conditions used for the hydration, ether formation cannot be suppressed as readily as in the dehydration, (iii) at high pressures, the olefins tend to polymerise much more than at the low pressures used for the dehydration. 

It is apparent that all the catalysts cited are acidic in nature. The relation between the acidity and activity of catalysts was investigated and 

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Examples are given of common operations such as absorption of ammonia to make fertihzers and of carbon dioxide to make soda ash. Also of recoveiy of phosphine from offgases of phosphorous plants recoveiy of HE oxidation, halogenation, and hydrogenation of various organics hydration of olefins to alcohols oxo reaction for higher aldehydes and alcohols ozonolysis of oleic acid absorption of carbon monoxide to make sodium formate alkylation of acetic acid with isobutylene to make teti-h ty acetate, absorption of olefins to make various products HCl and HBr plus higher alcohols to make alkyl hahdes and so on. 

The models described above assume that the reaction occurs only in the liquid phase. In some cases, such as isomerization of cyclopropane to propylene on a silica-alumina catalyst,43 reduction of crotonaldehyde over a palladium catalyst,45 and hydration of olefins to alcohols over tungsten oxide,58 the reactions could occur in the gas as well as in the liquid phases. 

Hydration of olefins to alcohols is equilibrium limited and hence CD is potentially suitable for such applications. The catalysts used for the process are acidic catalysts such as cation-exchange resins or zeolites. The hydration of isobutylene to produce tert-h ity alcohol via CD results in a higher conversion and there is no need to recycle the water. The hydration process is catalyzed by acidic ion-exchanged resins at 85°C and about 1200 kPa. The CD process configuration involves feeding the isobutylene below the catalyst zone and the water is fed above the catalyst zone. Flooding of the reaction zone is introduced in the process to improve the contact between the catalyst and the liquid and to ensure that the water is in constant contact with the catalyst sites. Flooding of the catalyst zone apparently improves the catalyst lifetime and performance because catalyst deactivation is caused by mass transfer and liquid distribution problems. Some recent publications on the hydration of isobutylene include a patent and a study of the kinetics of the hydration process and discussions on the merits of the application of CD for hydration. 

Direct hydration of olefins to alcohols without the use of sulfuric acid (known for many years but never successful until the discovery of new catalysts in Europe at the end of the second world war). 

The hydration of olefins to alcohols has been carried out on a large scale by hydrolyzing the sulfuric acid esters formed by the absorption of the olefins in sulfuric acid. In the case of the higher olefins these reactions occur with comparative ease. Thus, isobutylene may be hydrated to tertiary butanol in cold, moderately concentrated sulfuric acid.81 Some of the pentenes and heptenes may be hydrated in dilute (5 to 10 per cent) solutions of formic, acetic, or oxalic acids as well as in weak solutions of the mineral acids.83 With 60 per cent concentrations of hydroiodic acid, isobutylene yields the iodide almost exclusively, but at lower concentrations increasing amounts of the alcohol are fonned84 Similar phenomena attend the absorption of the higher olefins in hydrobromic acid. Hydrochloric acid, on the other hand, does not show such marked activity toward the higher olefins ind is practically devoid of activity toward ethylene. 

Hydration of Olefins. The earliest and still the largest production of chemicals from petroleum hydrocarbons was based on the hydration of olefins to produce alcohols by the employment of sulfuric acid. The addition of olefins to sulfuric acid to form alkyl sulfates and dialkyl sulfates takes place on simple contact of the hydrocarbons with the acid. To keep down polymerization and isomerization of the hydrocarbons, the temperature is kept relatively low, usually below 40° C. and commonly considerably lower than that (18). The strength of the sulfuric acid used depends on the olefin to be absorbed. Absorption of ethylene requires an acid concentration higher than 90%, whereas propylene and butylenes are readily absorbed in 85% acid or less. The alkyl and dialkyl sulfate solutions, on dilution and heating, are hydrolyzed to the alcohols plus small amounts of by-product ethers. After distilling off the organic products, the dilute sulfuric acid is reconcentrated and re-used. 

Vapor-phase dehydration over catalysts such as alumina is also practiced. Hydration of olefins to produce alcohols, usually over an acidic catalyst, produces substantial quantities of ethers as by-products. The reverse reaction, ethers to alcohols, can be accomplished by recycling the ethers over a catalyst. 

Lyondell Chemical Co. Direct olefin hydration Olefins (ethylene or propylene, butene) Direct hydration of olefins to corresponding alcohol in vapor phase ether is prime side reaction NA NA... 

A major limitation on the production of alcohols by olefin hydration is the fact that the products consist almost solely of secondary or tertiary alcohols (excepting, of course, ethyl alcohol). The normal or primary alcohols are made by other means (but also from petroleum hydrocarbons). It appears more difficult to prepare C5 and higher alcohols by the hydration of olefins since they are produced commercially by other means. One of the problems encountered (81) is excessive polymerization of the higher olefins when contacted with aqueous sulfuric acid. 

There is also an apparent trend in manufacturing operations toward simplification by direct processing. Examples of this include the oxidation of ethylene for direct manufacture of ethylene oxide the direct hydration of ethylene to produce ethyl alcohol production of chlorinated derivatives by direct halogenation in place of round-about syntheses and the manufacture of acrolein by olefin oxidation. The evolution of alternate sources, varying process routes, and competing end products has given the United States aliphatic chemical industry much of its vitality and ability to adjust to varying market conditions. 

Fig. 1.11. Net reaction (a) for the hydration of olefins (R = CH3, R = H) or (b) for the addition of alcohol to olefins (R = CF3, R = alkyl) via the reaction sequence (1) solvomercuration of the olefin (for mechanism, see Figure 3.37 regioselectivity Figure 3.38) (2) reduction of the alkylmercury compound obtained (for mechanism, see Figure 1.12).

This indicates the two-fold function of petroleum chemicals the hydration of olefins led to alcohols and to the family of derivatives of alcohols already made from other sources and, on the other hand, the olefin oxides and their derivatives were new industrial chemicals not previously made. 

Ethyl Alcohol and Higher Alcohols. Two general processes exist for hydrating olefins to alcohols. The first is by absorption of olefins in inorganic acids, primarily sulfuric acid, followed by hydrolysis of the intermediate ester. The second process is the direct catalytic hydration of the olefin. The literature on these processes, especially as applied to ethanol and 2-propanol manufacture, is well covered in the general works listed above, and especially by Brooks (22). German work on the catalytic hydration of olefins is described by Kammermeyer and Carpenter (55). The preparation of amyl alcohols via chlorination of pentanes and hydrolysis of the halides is described by Kenyon (56). 

A number of other processes have been proposed and patented for the hydration of olefins in the presence of aqueous solutions of various salts and acids under such conditions that liquid is present at temperatures of about 200° C. and above. However, commercial yields of alcohols have not been reported from these processes. Usually, fairly dilute solutions only of the stronger mineral acids have been used or proposed in order to avoid the polymerizing effects otherwise encountered. 

The proper choice of catalysts for the vapor phase hydration of olefins under pressure to form alcohols is a very important factor. Apparently, catalysts active in promoting the hydration reaction are likewise active toward promotion of the undesirable polymerization reactions since this latter reaction often proceeds at a more rapid rate than that of alcohol formation as evidenced by the high yields of polymers and low yields of alcohols. The use of catalysts to lower the temperature for the reaction is necessitated by the fact that as the temperature is increased to obtain more favorable rates, the equilibrium conversion to alcohol becomes lower, and the tendency to polymerize is increased. Also, the catalyst must not promote dehydrogenation of the alcohol to form hydrogen and aldehyde since at the temperature of operation the equilibrium is very favorable for this reaction as has been pointed out in a previous chapter. Thus, the reaction, isobutanol = isobutyl aldehyde -f hydrogen has an equilibrium constant corresponding to about 72 per cent decomposition at 450° C even with 100 atmospheres of hydrogen pressure. 

The difficulties attending the catalytic vapor phase hydration of olefins, while not apparent from the claims made in the patents which have been obtained for such processes, are serious and numerous. Aside from those already mentioned, the difficulties of separating the alcohol from the dilute liquid condensate by distillation and of purifying the alcohols from hydrocarbon polymers by a process of chlorination or selective absorption must be overcome. In view of the success that has attended the hydration of olefins, particularly those higher than ethylene, by means of absorption in sulfuric acid followed by dilution and distillation, it is probable that direct hydration processes at the present stage of the art will be unable to compete as long as cheap sulfuric acid is available. 

In conclusion it may be said that the vapor phase hydration of olefins under pressure to form alcohols does not appear promising because of the major difficulties involved, notwithstanding the rather favorable theoretical possibility. On the other hand, considerable evidence has been amassed and a number of patents issued to show that hydration of olefins in solution is a commercial possibility. This early material has been very carefully collected and summarized by Brooks. 3... 

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