Tert-Butanol dehydration

Knifton, J.F. Sanderson, J.R. Stockton, M.E. Tert-butanol dehydration to isobutylene via reactive distillation. Catal. Lett. 2001, 73 (1), 55-57. 

Esterification of linalool requires special reaction conditions since it tends to undergo dehydration and cyclization because it is an unsaturated tertiary alcohol. These reactions can be avoided as follows esterification with ketene in the presence of an acidic esterification catalyst below 30 °C results in formation of linalyl acetate without any byproducts [71]. Esterification can be achieved in good yield, with boiling acetic anhydride, whereby the acetic acid is distilled off as it is formed a large excess of acetic anhydride must be maintained by continuous addition of anhydride to the still vessel [34]. Highly pure linalyl acetate can be obtained by transesterification of tert-butyl acetate with linalool in the presence of sodium methylate and by continuous removal of the tert-butanol formed in the process [72]. 

Different approaches to the kinetics of alcohol dehydration were attempted by two groups of authors [118,119]. In one case, it has been assumed that the active surface of alumina is formed either by free hydroxyl groups or by surface alkoxyl groups. The rate equation was then derived on the basis of the steady-state assumption a good fit to the experimental data was obtained [1118]. The second model was based on the fact that water influences the adsorption of an alcohol and diminishes the available surface. The surface concentrations of tert-butanol and water were taken from independent adsorption measurements and put into the first-order rate equation a good description of integral conversion data was achieved [119]. 

Pines and Manassen [7] suggested that tertiary alcohols are dehydrated by the El mechanism involving the formation of more or less free car-bonium ions, secondary alcohols by a mechanism lying somewhere between El and E2 (i.e. synchronous with a ionic contribution) and primary alcohols by a concerted E2 mechanism. However, the large kinetic isotope effect for the dehydration of fully deuterated tert-butanol on alumina [122] indicates that, even in this case, some synchrony must exist. Alumina strongly prefers the concerted process and with other catalysts the situation may differ. 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

These relations seem to be valid for the dehydration of primary alcohols, but secondary and tertiary alcohols may need other combinations of acidic and basic sites. It has been observed that the dehydration of tert-butanol was more sensitive to the presence of strongly acidic sites than the reaction of methanol, but both processes required basic sites [8]. All this is in accordance with the dynamic model of elimination mechanisms presented in Sect. 2.1, which allows transition from El to E2 or further to ElcB according to the structure of the reactant and the nature of the catalyst. 

The sulfate anion radical is not a very strong hydrogen acceptor. It acquires an atomic hydrogen from organic substrates at significantly smaller rates a compared with the rates for one-electron oxidations. For instance, dehydration rate constants are 107, 106 and 105 I.-mole -sec 1 for methanol, tert-butanol, and acetic acid, respectively (Goldstein Mc-Nelis 1984 Zapol skikh et al. 2001). Such a peculiarity is very important for the selectivity of ion radical syntheses with the participation of SOT. 

Tert-butanol and 1-propanol were chosen because a marked difference in reactivity between the two alcohols was expected a priori. Transition metal oxides may also catalyze the formation of ketones or aldehydes by oxidative dehydrogenation providing they have a significant number of basic/acid site pairs [10], With both HY and WZ, the alcohol dehydration to ether and oxidative dehydrogenation pathways were essentially negligible. 

Isobutene, the main feedstock, is obtained in the form of raffinate from steam crackers, which make up an estimated 40% of MTBE feedstock throughout the world. Isobutene in the form of butene-butane fractions from fluid catalytic crackers represents 28% of MTBE feedstocks isobutene from dehydrogenation of isobutane represents 12% of MTBE feedstocks and isobutene by dehydration of tert-butanol represents 36% of MTBE feedstocks. The Butamer process is often used for the primary butane isomerization, while the Catofln and Olefex processes are commonly used for the isobutane dehydrogenation. 

TiOj-SiOj materials are interesting from the point of view of their use as catalysts and supports [11-13], Thus, die dispersion of titania on, or in, silica generates Lewis acid sites [14] and opens die possibility to modulate the redox properties of the Ti atoms. In this work we check the behavior of these samples for an acid catalyzed reaction the dehydration of tert-butanol [15], looking for correlations between structure and activity. 

Textural data of studied samples and catalytic properties for the dehydration of tert-butanol... 

The yield of propylene oxide is about 94% and approximately 2.2 mol of the co-product tert-butanol is produced per mol of propylene oxide. From this ratio it becomes immediately understandable that it is essential for an economic indirect propylene oxidation process to find a good market for the coupling product, here tert-butanol. For the isobutane hydroperoxidation reaction propylene is converted with pure oxygen at 120-140 °C, applying pressures of 25-35 bar. The non-catalyzed reaction takes places in the liquid-phase and acetone is formed as a minor by-product. The subsequent epoxidation is carried out in the liquid phase at 110-135 °C under 40-50 bar pressure in five consecutive reactors. The reaction is catalyzed by a homogeneous molybdenum naphthenate catalyst. The co-product tert-butanol can be dehydrated and is afterwards converted into methyl tert-butyl ether (MTBE), an important fuel additive for lead-free gasoline. 

Xu et al. reported that imder neutral conditions in water in the temperatiue range of 225-320°C, tert-butanol undergoes rapid dehydration to form isobutylene. The alkylation... 

It has already been mentioned that the K, in NCW is several orders of magnitude greater than in water at room temperature. Thus, as shown previously, acid and base catalysis can be facilitated without the use of additional acid. Certainly CO2 reacts with water to form carbonic acid and, as a consequence, the concentration of hydronium ion in NCW can be increased by enriching the medium with CO2. From an environmental point of view this procedure wiU not only facilitate specific acid-catalyzed reactions but will not require neutralization of the acid after the reaction is complete. A simple cooling and depressurization will eliminate the CO2 and phase separates the product(s) of reaction. Thus, Aleman et al. have reported that the conversion of mesitoic acid to mesitylene over a period of 120 min at 250°C increased from 50 to 80% in the presence of 10 bar (rt) of CO2. Hunter and Savage reported the dehydration of cyclohexanol in water at 250 and 275°C and the reaction of p-cresol with tert-butanol in water at 275°C in the absence and presence of CO2. Their results indicated that in the presence of CO2 the rate of dehydration of the cyclohexanol increased by more than a factor of 2 and the rate of formation of 2-tert-butyl-4-methylphenol increased 40-120%. Modest increases in rate were reported for the hydration of cyclohexene to cyclohexanol. 

The epoxide is used as a monomer for polymer production. The byproduct ethylbenzene alcohol can be dehydrated to styrene, also a monomer for the production of polymers. If isobutane is used, iso-butylhydroperoxide replaces ethylbenzene-hydroperoxide as the oxidant. The byproduct tert-butanol can be converted with methanol to an ether that is an important additive in new environmental friendly gasolines. Complexes of Mo, V, or Ti are used in homogeneous epoxidation catalysis, while heterogeneous Ti02/Si02 catalysts can be used also. The active sites consist of a titanium ion with a fourfold coordination of oxygen in a tetrahedral geometry. Titanium acts essentially as a Lewis acid to activate the 0-0 bond in the hydroperoxide. 

Major methods for isobutene production are from a C4 stream of a steam cracker, from a catalytic cracker butene-butane stream, through dehydration of tert-butanol (which is obtained from a propene oxide process) and through isomerisation of n-butane to isobutene and subsequent dehydrogenation to isobutene (Obenaus et al. 2000 van Leeuwen et al. 2012 Romanow-Garcia et al. 2007). 

With the use of these sensitivity-enhancement approaches the stable alkoxide intermediates were indeed detected by C CP/MAS NMR spectroscopy. Isopropoxide was the first alkoxy intermediate reliably identified in propylene labeled with C in the CH= group) conversion on zeolite HY [15]. Isopropoxide exhibits the signal at 87 ppm from the labeled C atom, which is characteristic of the (CH3)2CH fragment bonded to an oxygen atom of the zeolite framework (Fig. 20). Later, other alkoxide intermediates were detected and characterized. It was demonstrated that methoxides [121,122] and ethoxides [122,123] formed from methyl and ethyl iodides and also from methanol and ethanol on H-ZSM-5 and CsX zeolites. Isobutoxy [124] and tert-butoxy [90] intermediates resulted from the dehydration of isobutanol and tert-butanol on HZSM-5. Alkoxides are highly reactive species. For example, surface methoxides are effective methylating agents in their reactions with methanol, water, ammonia, alkyl halides, HCl, CO, acetonitrile, and aromatic compounds [125]. 

The hydroperoxide process involves oxidation of propjiene (qv) to propylene oxide by an organic hydroperoxide. An alcohol is produced as a coproduct. Two different hydroperoxides are used commercially that result in / fZ-butanol or 1-phenylethanol as the coproduct. The / fZ-butanol (TBA) has been used as a gasoline additive, dehydrated to isobutjiene, and used as feedstock to produce methyl tert-huty ether (MTBE), a gasoline additive. The 1-phenyl ethanol is dehydrated to styrene. ARCO Chemical has plants producing the TBA coproduct in the United States, Erance, and the Netherlands. Texaco has a TBA coproduct plant in the United States. Styrene coproduct plants are operated by ARCO Chemical in the United States and Japan, Shell in the Netherlands, Repsol in Spain, and Yukong in South Korea. 

The / f/-butanol (TBA) coproduct is purified for further use as a gasoline additive. Upon reaction with methanol, methyl tert-huty ether (MTBE) is produced. Alternatively the TBA is dehydrated to isobutylene which is further hydrogenated to isobutane for recycle ia the propylene oxide process. 

Finally, a word of caution when using [BF4] and [PF ]" ionic liquids - they are not stable and give off HF, particularly when heated in the presence of a proton source or a metal salt [21]. There are many examples of this in this chapter. An example of a HF-catalyzed reaction is ether formation from alcohols is a classic acid-catalyzed reaction. An ether formation reaction was found to occur in a range of [BF4] ionic liquids, with an example being the addition of methanol to ten-butanol to form methyl-tert-butyl ether (MTBE) [306]. The author is of the opinion that [Bp4] ionic liquids (even hydrophobic ones) can dehydrate alcohols to ether and refers to these ionic liquids as dehydrators. All that is happening here is a simple HF-catalyzed reaction. With many authors not aware of this phenomenon, they resort to all kinds of inappropriate explanations for what is occurring. 

Praill, having discovered the efficiency of acylium perchlorates as acylating agents, decided to examine the esterification of /err-butanol as opposed to its dehydration to isobutene. Using acetic anhydride and perchloric acid mixtures, both tert-h xty acetate and isobutene were rapidly produced and in accordance with the known alkyl oxygen fission of tertiary esters, the proportion of isobutene increased with time. In these reactions where acetic anhydride was in excess, crystalline material was deposited in the mixture. Later this was shown to be 2,4,6-trimethylpyrylium perchlorate, identified by its conversion to 2,4,6-trimethyl pyridine and its picrate. When isobutene itself was acylated using acetic anhydride and perchloric... 

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