Dehydration of alcohols

Abstract

Catalytic vapor phase dehydration of i-propanol, n-butanol, i-butanol, s-butanol and t-butanol is studied in a fixed bed flow reactor under isothermal conditions over various catalysts to understand the influence of catalyst texture on dehydration activity and the mechanism of dehydration of alcohols. The catalysts used for the above studies are different preparations of alumina (alumina from (i) aluminium nitrate, (ii) aluminium isopropoxide, and (iii) sodium aluminate), silica-alumina, 10 molecular sieve, thoria, and acid-treated thoria. These catalysts are characterised by measuring their properties, namely, acidity (at pKa = +3.3), specific surface area, pore size distribution and pore volume. Alumina from various preparations and silica-alumina, activated at 500°C, are having nearly the same specific surface area and pore size distribution. 10X molecular sieve has the highest specific surface area and thoria and acid-treated thoria the lowest. Silica-alumina has the highest acidity, followed by 10X molecular sieve and alumina catalysts. Thoria and alumina from sodium aluminate did not show any acidity on their surface. It is also observed that in the case of alumina from aluminium nitrate, heat treatment at higher temperatures (630 and 730°C) decreases the surface area as well as the acidity. However, acidity per unit surface area is found to remain constant. Acid treatment with sulfuric acid increases the acidity of thoria catalyst and the acidity is found to be proportional to the concentration of sulfuric acid. Kinetic experiments are conducted with suitable flow rates and particle size of catalysts where the internal and external diffusional effects are at a minimum and these effects are shown to be negligible by calculations also. Liquid product analysis is done by vapor phase gas chromatography and gas analysis by Orsat apparatus. Out of the ten catalysts employed for the dehydration of i-propanol, three catalysts, namely, alumina (from aluminium nitrate), silica-alumina, and 10X molecular sieve, activated at 500°C, are selected for the dehydration of the other alcohols. The effect of time factor and temperature on conversion are studied. It is found that conversion to olefin continuously increases with time factor as well as temperature, while conversion to ether depends on the particular temperature employed. No ether is observed with i-butanol, s-butanol and t-butanol. An apparent compensation effect is found to exist amongst various catalysts and different alcohols. Hence the reaction velocity constants are taken as a measure of the catalyst activity. For the dehydration of alcohols it is found that the acidity is an important factor in determining the catalyst activity. In the case of i-propanol dehydration catalytic activity could be related to the acidity of the catalysts when the catalyst properties are changed as a result of pretreatment, i.e., heat treatment or acid treatment. The acidities of acid-treated thoria catalysts could not be correlated with the activation energies of other catalysts. In general, it is observed that an increase in acidity of the catalyst decreases the activation energy for the reaction. Among the catalysts, alumina (from aluminium nitrate), 10X molecular sieve and silica-alumina, it is found that alumina is more active than the other two catalysts except in the case of t-butanol. In the case of t-butanol dehydration, silica-alumina and 10X molecular sieve are found to be more active than alumina and the catalytic activity is in accordance with the acidity of these catalysts. The behaviour of 10X molecular sieve and silica-alumina is found to be similar except in the case of s-butanol dehydration. The reactivities of various alcohols are found to be in the order: Alumina (from aluminium nitrate): t-butanol > s-butanol ~ i-propanol > i-butanol > n-butanol. 10X molecular sieve: t-butanol > s-butanol > i-propanol > i-butanol > n-butanol. Silica-alumina: t-butanol > s-butanol > i-propanol > i-butanol > n-butanol. The activation energies for different alcohols over alumina are in the same range (23 to 26 kcal/mole). Over 10X molecular sieve and silica-alumina, the activation energies of n-butanol and i-butanol are nearly the same (21.3 and 21.9 kcal/mole; 18.7 and 17.5 kcal/mole respectively), of s-butanol and i-propanol are approximately equal (25.1 and 23.2 kcal/mole; 21.7 and 19.1 kcal/mole respectively); and of t-butanol are different from other alcohols and are comparatively low (9.7 kcal/mole; 10.3 kcal/mole respectively). Kinetic data, analysed by Hougen-Watson approach for determining the rate-controlling step, did not lead to any consistent rate-controlling step at all the temperatures studied. The data are analysed for structure-reactivities of different alcohols using linear free energy relationships represented by Taft equation. It is found that ?-methyl substitution increases the reactivity of the alcohol. Over silica-alumina, the data follow Taft equation indicating that the same mechanism may be operating with different alcohols. Similarly, over 10X molecular sieve, Taft equation correlated the data with the exception of s-butanol, indicating that the same mechanism may be operating. However, over alumina though ?-methyl substitution increased the activity of the alcohol, the data could not be correlated by Taft equation indicating that different mechanisms may be operating with different alcohols. The data are also analysed with respect to the stabilities of carbonium ions represented by ionization potentials. Over silica-alumina, the reactivities of alcohols are in accordance with the ionization potentials, showing a linear relationship between ln kc and ionization potential. This indicates that the same mechanism may be operating with different alcohols over this catalyst. Over 10X molecular sieve, the values of ln kc could be correlated with ionization potentials, indicating that the same mechanism may be operating except in the case of s-butanol. Over alumina the ionization potentials do not follow a linear relationship with ln kc indicating that different mechanisms may be operating with various alcohols. For the dehydration of alcohols on silica-alumina catalysts a carbonium ion mechanism is reported in literature, while over alumina catalysts a two-center mechanism is reported to be valid for the dehydration of primary and secondary alcohols and a carbonium ion mechanism for the dehydration of tertiary alcohols. 10X molecular sieve is also reported to be similar in nature to that of silica-alumina. This may explain why the reactivities of various alcohols could not be correlated in the case of alumina catalyst, as also the activities of various catalysts. In conclusion, the catalytic activity of a catalyst could be correlated with its texture when (i) the catalyst properties are changed as a result of pretreatment, namely, heat treatment or acid treatment, and (ii) the same mechanism is operating.