Isobutene hydration reactions

Equation (1) consists of various resistance terms. l/Kj a is the gas absorption resistance, while 1/ K,a corresponds to the maleic anhydride diffusion resistance and l/i k represents the chemical reaction resistance. The reaction rate data obtained under the reaction conditions of 250°C and 70 atm were plotted according to equation (1). Although catalytic reaction data with respect to time on stream were not shown here, a linear correlation between reaction rate data and catalyst loading was observed as shown in Fig. 2. The gas absorption resistance (1/ a) was -1.26 h, while the combined reaction-diffusion resistance (lJK,a + 1 T]k) was determined to be 5.57 h. The small negative value of gas absorption resistance indicates that the gas-liquid diffusion resistance was very small and had several orders of magnitude less than the chanical reaction resistance, as similarly observed for the isobutene hydration over Amberlyst-15 in a slurry reactor [6]. This indicates that absorption of malei c anhydride in solvent was a rapid process compared to the reaction rate on the catalyst surface. 

Cation-exchange resins are used as catalysts in the produdion of MTBE (methyl tertiary-butyl ether, 2-methoxy-2-methylpropane) and various other oxygenates and, lately, also in the dimerization of isobutene [30]. Other commercial applications of the cation-exchange resins indude dehydration of alcohols, alkylation of phenols, condensation readions, alkene hydration, purification of phenol, ester hydrolysis and other reactions [31]. The major producers of ion-exchange resins are Sybron Chemicals Incorporated [32] (Lewatit resins), Dow Chemical Company [33] (DOWEX resins), Purolite [28] (Purolite resins), and Rohm and Haas Company [27] (Amberlyst resins). 

The volume of activation, A V, which is — RT 8 lnk)Jdp, has been suggested as a criterion of mechanism (Whalley, 1964). Known volumes of activation for A-SB2 reactions, measured near 25°, seem to be limited to olefin hydration (—12 cm3 mole-1 for isobutene at 35°, Baliga and Whalley, 1965) and allylmercuric iodide cleavage ( — 11 cm3 mole-1 at 25°, Halpem and Tinker, 1965). It is impossible to generalize from so few examples, but, in principle, it seems possible that AV is less dependent on structural ramification than AS, and therefore easier to interpret. Against this must be weighed the experimental difficulties in... 

Heteropolyacids are much more active than mineral acids for several types of homogeneous reactions in both organic solvents and aqueous solution [4, 8]. The enhancement is generally greater in organic solvents. For the hydration of isobutene in a concentrated aqueous HPA solution (above 1.5 mol dm-3), the reaction rate is about 10 times greater than for mineral acids [21]. This rate enhancement is attributed to the combination of stronger acidity, stabilization of protonated intermediates, and increased solubility of alkenes [21]. In this case, the selectivity is also much improved with HPA catalysts. 

The analysis of isotope effects to give an exponent cq is useful when a Bronsted exponent cannot be obtained by measuring catalytic coefficients for a series of acids. In the hydration of isobutene, general catalysis is unobservable and the reaction is dominated by hydronium ion catalysis. A value of j ca. 0.9 was obtained [44] from eqn. (135) by measuring solvent and product isotope effects. This result is of the expected size, as discussed in Sect. 4.7. 

Other industrial processes that have taken advantage of the process intensification deriving from the introduction of reactive (catalytic) distillation are (i) production of high purity isobutene, for aromatic alkylation (ii) production of isopropyl alcohol by hydration of propylene (iii) selective production of ethylene glycol, which involves a great number of competitive reactions and (iv) selective desulfurization of fluid catalytic cracker gasoline fractions as well as various selective hydrogenations. Extraction distillation is also used for the production of anhydrous ethanol. 

Addition of water follows Markovnikov s rule in principle, but mixtures are formed rather than the single homogeneous adduct expected according to the rule.57 Some olefins are hydrated in the presence of dilute sulfuric acid (isobutene), but others require concentrated acids (propene, 1- and 2-butene). Negative substituents on the doubly bonded atoms reduce the reactivity (as for cinnamic acid and dichloroethylene), whereas alkyl groups increase it. In all cases reaction should be at as low a temperature as possible when sulfuric acid is used. Too great a concentration of sulfuric acid often causes polymeri-... 

Processes employing r-butanol carry out the hydration of isobutene in acidic medium, according to the following exothermic reaction ... 

The remarkable activity of these catalysts in aqueous solution has been put to commercial use for reactions such as the hydration of propene, -butene, and isobutene (Misono, 1987 Onoue et al., 1978). 

While the acid strength of HPAs is high, they have a limitation in their use in catalysis, and this is due to their low surface area in the solid state (SlOm g- ), which corresponds to the external surface area of the crystal. When the reactants have a polar character, however, HPAs can take up polar molecules in amounts that correspond to more than 100 surface layer, and in this case their catalytic behaviour has been called bulk type catalysis [31"). Therefore, in the case of catalytic reactions involving polar molecules, they occur not only at the surface but also in the bulk solid of certain HPAs. The practical effect is that the catalytic system behaves like a highly concentrated solution, and this explains why these solids have been named pseudoliquids [31 j. Under the pseudoliquid conditions all acid sites are accessible to reactants, and the benefits of the system have been used commercially for reactions such as the hydration of propylene and n-butenc, separation of isobutene, and polymerization of tetrahydrofuran 20, 31". ... 

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