Isomerization of n-butene

Clinoptilolite Isomerization of n-butene, the dehydration of methanol to dimethyl ether, and the hydration of acetylene to acetaldehyde... 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

Tphe isomerization of olefins over acidic catalysts has been carefully A studied in the past few years. Hightower and Hall (1, 2) have studied the isomerization of n-butenes over silica-alumina. They were able to interpret their results in terms of a simple model involving the 2-butyl carbonium ion as a common intermediate. More recently Lombardo and Hall studied the isomerization of the same olefins over Na-Y-zeolite. They showed that the reaction was first order in conversion as well as time (3), that the isomers could be directly interconverted (4), and that activity sharply increased with water addition reaching a saturation value (5). There are, however, reports in the literature which are at variance with this idea. Dimitrov et al. (7, 8) explained their results for n-butene isomerization on Na-X-zeolite in terms of a free radical type mechanism. As discussed more thoroughly elsewhere (4) others have argued about the nature of catalytic activity on zeolites (9-13). 

The isomerizations of n-butenes and n-pentenes over a purified Na-Y-zeolite are first-order reactions in conversion as well as time. Arrhenius plots for the absolute values of the rate constants are linear (Figure 2). Similar plots for the ratio of rate constants (Figure 1), however, are linear at low temperatures but in all cases except one became curved at higher temperatures. This problem has been investigated before (4), and it was concluded that there were no diffusion limitations involved. The curvature could be the result of redistribution of the Ca2+ ions between the Si and Sn positions, or it could be caused by an increase in the number of de-cationated sites by hydrolysis (6). In any case the process appears to be reversible, and it is affected by the nature of the olefin involved. In view of this, the following discussion concerning the mechanism is limited to the low temperature region where the behavior is completely consistent with the Arrhenius law. 

The compensation trend present in data reported by Shannon et al. (285) for the isomerization of n-butenes over a number of different oxide catalysts is given in Table V, P (omitting from the calculation the point for MnO, which shows a marked deviation). Dehydrogenation of cyclohexane over oxides (286) exhibited similar behavior the calculated line is given in Table V, Q. Hydrocarbon exchange over alumina (287) also gave a slight compensation trend, for which e = 0.132 and ae = 0.024. 

Such a scheme for the catalytic isomerization of n-butenes over A1203 has been studied in detail previously [11]. Each reaction has a rate that is a function of both the gas composition and the surface state. In this case the assumption that the concentration of surface intermediates on the catalyst is a function of the gas composition is often used. It is a hypothesis about a quasi-steady state that is considered in detail in what follows. According to this hypothesis, for the reaction under study there exist three functions of the gas composition, w1w2, and w3, so that the kinetic equations can be written as... 

Fig. 2.2 Reaction pathway for the monomolecular isomerization of n-butenes on acid catalysts...

Skeletal Isomerization of n-Butenes Cataiyzed by Medium-Pore Zeolites and Aiuminophosphates... 

The reaction of u-butenes to give isobutylene is cataly zed by a wide variety of solid acids but requires relatively high temperature. Typical catalysts include alumina, halogenated alumina, amorphous silica-alumina, supported phosphoric acid, and supported tungsten or molybdemmi oxide. The most characteristic features of the skeletal isomerization of n-butenes... 

Recently, the use of medium-pore molecular sieves as catalysts for the isomerization of n-butenes has aUowed the development of new famihes of catalysts that are highly efficient for this reaction. Several patents claiming the use of zeolitic and nonzeohtic molecular sieves for the skeletal isomerization of C4—C5 olefins have been published the catalysts are ferrierite (5, 6) and the nonzeolitic molecular sieves MeAPO and MeAPSO (7). Since the appearance of the patents, important scientific contributions have appeared in the literature. 

The aim of this review is to describe the most interesting results characterizing the skeletal isomerization of n-butenes catalyzed by zeolitic and nonzeolitic molecular sieves and to discuss the state of the art of the isomerization mechanism, the nature and location of the active sites responsible for the selectivity for isobutylene, and the influence of the pore dimensions and pore structures of the molecular sieves. 

To better understand why and how microporous molecular sieves (aluminosilicates and aluminophosphates) are highly suitable catalysis for the skeletal isomerization of n-butenes, it is important to discuss the mechanisms of the reactions that control the formation of the desired isobutylene and to evaluate the relative importance of secondary reactions that may... 

It has been suggested that the isomerization of n-butenes to give isobutylene may proceed either through a monomolecular or through a bimolecu-lar mechanism. 

The following sections are a summary of the characteristics of the zeolites and aluminophosphates that have been investigated as catalysts for skeletal isomerization of n-butenes. 

SKELETAL ISOMERIZATION OF n-BUTENES framework viewed along [001]... 

Metal aluminophosphates (MeAPO) contain framework metal (Me), aluminum, and phosphorus. When the metal is divalent (e.g., Zn +, Co +, and Mg +) and substitutes for aluminum, a negatively charged framework results, with H+, for example, serving to compensate the charge. Many aluminophosphate molecular sieves have been synthesized. SAPO-11 and MeAPO-11 have interesting catalytic properties. Their structures have onedimensional 10-ring channels. The 10-ring pore aperture is elliptical with dimensions 0.39 x 0.63 nm. Table 1 is a summary of the characteristics of the molecular sieves which have been used for the skeletal isomerization of n-butenes. 

V. Skeletal Isomerization of n-Butenes Catalyzed by Medium-Pore Microporous Molecular Sieves... 

The overall reaction scheme for the skeletal isomerization of n-butenes (including both the mono- and bimolecular processes) is valid for aU acidic catalysts. However, the relative amounts of the carbenium ion intermediates involved either in the monomolecular or in the bimolecular reaction paths, as well as their relative rates of conversion, can be dramatically different for the various molecular sieve catalysts and can depend crucially on the sizes of the channels and their structural configurations as well as on the acidity of the catalyst. 

An idea that was tested experimentally is that if the bimolecular mechanism prevails 19), the products formed from n-butene reactants, on the one hand, and from any of the possible isomers formed by dimerization of n-butenes (such as 3,4-dimethylhex-l-ene), on the other hand, should be similar. Thus, the transformations of 2,2,4-trimethylpent-2-ene, 3,4-dimethylhex-2-ene, and methylheptenes were investigated with micro-porous catalysts such as MnAPO-11 and SAPO-11. The results are summarized in Fig. 11, in which the ratio (propene pentene) /n-butenes is plotted for different reactants. The data show that with a selective isomerization catalyst, this ratio is quite low (<0.1) for n-butene reactants in contrast, it is quite high (approximately 0.8) when 3.4-dimethylhex-2-ene or methylheptenes are the reactants, indicating that these compounds are not intermediates in the selective isomerization of n-butenes. Consequently, the isobutylene formed on selective catalysts results from a monomolecular process. Th ese results are considered to be good indirect evidence that the bimolecular reaction is not selective for isobutylene formation. 

The mechanism of skeletal isomerization of n-butenes may be rationalized in terms of the steps presented previously the key reaction intermediate is the 5-butyl cation. The predominent structure of the adsorbed intermediate was recently considered to be an alkoxy 50), which cither adds to one butene molecule and cracks into C3, C4, or C5 fragments (the bimolccular mechanism) or rearranges into isobutylene (the monomolecular mechanism) via a primary carbenium ion. 

The proposed pathway will be more favorable kinetically than that suggested for the true monomolecular process, whereby a primary carbenium ion is formed. To further test the idea that carbonaceous residues are the active and selective sites for the skeletal isomerization of n-butenes, the authors reported results showing that the rate of isobutylene formation catalyzed by ferrierite passed through a maximum as the conversion continuously decreased (Fig. 12) (51). 

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