Skeletal isomerization of n-butenes

Generally, the mechanisms of acid-catalyzed reactions involve carbenium ion intermediates, which, in the case of olelin reactants, are formed by the addition of a proton to the double bond  

Secondary or tertiary carbenium ions are formed, depending on the branching of the reactant olefin. In the case of 1-butene and 2-butene, the overall reaction scheme for the skeletal isomerization is identical. Protonation of 1-butene or of 2-butene generates x-butyl cations. 

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. 

Brouwer (8) studied the isomerization of isotopically labeled butane CH.3 — CH.2 — CH.2 — CH3. He proposed the conversion of s-butyl cation to give a substituted cyclopropyl carbenium ion intermediate  

Such an intermediate explains the - C scrambling in the butane carbon chain without the formation of isobutane ring opening at the C3 — C2 carbon bond results in the formation of x-butyl cation, whereby the labeled C is no longer a terminal carbon  

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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... 

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... 

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. 

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). 

The role of the carbonaceous residues in the skeletal isomerization of n-butenes was also questioned by Houzvicka and Ponec (49). These authors observed that in a continuous flow reactor and at high n-butene conversions, the relative concentration of isobutylene increased w ith TOS, with the results being similar to those reported by Guisnet et. al. (51). However, the ratio isobutylene/all butenes decreased with TOS. It was concluded that at the initial stage of the reaction, a fraction of the isobutylene product reacted subsequently with n-butenes to form byproducts including carbona-... 

It is clear from the previously discussed results that on a selective catalyst the skeletal isomerization of n-butenes proceeds through the monomolecu-... 

These results arc important because they clearly demonstrate the role of shape selectivity in the selective skeletal isomerization of n-butene. Similar results and interpretation were obtained independently by Kwak et al (JJ), who modified the ferrierite catalyst by replacing part of the protons with Mg +. They observed a smaU decrease in microporosity which was related to an increase in the isobutylene selectivity. Again, it was demonstrated that the micropore size is a key factor governing the selectivity for isobutylene. 

The deactivation of the molecular sieve catalysts during the skeletal isomerization of n-butenes has not received much attention with regard to how it affects the reaction mechanism. In general, medium-pore molecular sieves deactivate slowly (Fig. 16) and much less than catalysts with open surfaces. In this section, the magnitude of deactivation is represented by the ratio yield of isobutylene/WHSV of n-butenes. According to this criterion, the stability with respect to TOS of the most efficient molecular sieves is as follows ... 

The literature related to selective skeletal isomerization of n-butenes catalyzed by medium-pore zeolites and Me-aluminophosphates leads to the following conclusions ... 

In summary, acid sites on FER have different size constraints from a structural point of view. Coke (predominantly aromatic in nature) formation is limited to < 11 wt. % of the micropore volume of FER. Coke formation modifies desirable polymerization (dimerization) reactions. Such blocking produces the pore shapes and limits access to more strongly acidic sites that catalyze less significant contributions for shape selectivity for skeletal isomerization of n-butene. TPD results suggest that adsorption of NH3,1-C4H8 and i-C4Hs is shape selective.62... 

Figure 13. Yield of iso-butene from the skeletal isomerization of n-butene.

There is a continuous flow of new zeolite structures which might have a considerable potential in catalysis. Recent examples are NCL-1, NU-86, NU-87, SSZ-26 and MCM-22. However, zeolites with intrinsically chiral channels are not yet in sight, therefore stereoselective catalysis in zeolites still relies on the presence of chiral guests. Important reactions studied over acid zeolite catalysts include skeletal isomerization of n-butenes and of long-chain... 

A positive effect of the presence of carbonaceous deposits on the skeletal isomerization of n-butene over a protonic ferrierite zeolite could also be related to a possible reaction mechanism in which the tertiary carbenium ions, which are located in the zeolite pores, act as catalytically active sites. Catalysis was found initially to proceed via oligomerization... 

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