Equilibria olefin hydration

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. 

Summarizing, the in situ UV-Vis, XANES, and EXAFS studies of Bonino et al. [49] and of Prestipino et al. [50] on hydrated and anhydrous peroxo/hy-droperoxo complexes on crystalhne microporous and amorphous meso-porous titanosilicates have shown, for the first time, the equilibriiun between r] side-on and end-on complexes. The amount of water is the key factor in the equilibrium displacement. In this regard please note that, owing to the hydrophobic character of TS-1, substrates such as olefins are the dominant species in the channels. This fact assures a relatively local low concentration of water, which in turn guarantees a sufficient presence of the active end-on... 

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,... 

Also in the 1930s, detailed studies about the thermodynamic stability of adducts of silver(I) with olefins were carried out by Howard Lucas and coworkers, who determined the equilibrium constants between the hydrated Ag+ ion and the corresponding cationic olefin silver(I) complex in dilute aqueous solutions of silver nitrate [25]. In the context of this work, Saul Winstein and Lucas made an initial attempt to describe the interaction between Ag+ and an olefin by quantum mechanics [26]. Assisted by Linus Pauling, they explained the existence of olefin silver(I) compounds in terms of resonance stabilization between the mesomeric forms shown in Fig. 7.4. Following this idea, Kenneth Pitzer proposed a side-on coordination of Ag+ to the olefin in 1945 and explained the stability of the corresponding 1 1 adducts as due to an argentated double bond , in analogy to his concept of the protonated double bond [27]. He postulated that the unoccupied s-orbital of silver(l) allowed the formation of a bond with the olefin, similar to the s-orbital of the proton. 

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. 

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 acid-catalysed hydration of olefins is a reversible process (Scheme 1, p. 4) for which the rate-determining stage involves the protonation of the olefin. As a result, the kinetics of the reaction may be complicated if hydration is not substantially complete under the reaction conditions. In dilute solution, however, this is generally not the case, and some equilibrium constants for the reaction... 

The possibility of hydrating various olefins such as isobutylene, propylene, isoa-mylene, and cyclohexene has also been successfully explored in separate studies [5, 43-47]. In most cases, alcohol, being a higher-boiling component, is collected at the bottom of the column operated under total reflux. Hydration of cydohexene represents a peculiar example of vapor-liquid-liquid equilibrium in an RD column a subject that has not been studied in depth. 

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