Butenes, Isomerism

The isomerization of 1-butene to cis- and trans- 2-butene onPd/C/Nafion and Pd-Ru/Nafion electrodes is one of the most remarkable and astonishing electrochemical promotion studies which has appeared in the literature.39,40 Smotkin and coworkers39,40 were investigating the electrocatalytic reduction of 1-butene to butane on high surface area Pd/C and Pd-Ru cathodes deposited on Nafion 117 when, to their great surprise, they observed at slightly negative overpotentials (Fig. 9.31) the massive production of 1-butene isomerization, rather than reduction, products, i.e. cis- and trans-2-butenes. This is extremely important as it shows that electrochemical promotion can be used also to enhance nonredox catalytic reactions such as isomerization processes. 

Butene Isomerization on Pd Fully Dispersed on C Electrodes Deposited on Nafion... 

The electrochemical promotion of 1-butene isomerization to 2-butene (cis-and trans-) using Nafion as the solid electrolyte and finely dispersed Pd deposited on carbon as the electrode has been described in section 9.2.2.14,15 Faradaic efficiency, A, values up to 28 were obtained in this remarkable study. The Pd dispersion is near complete on the high surface area C support.14,15... 

Boudart, 176, 367 Bonding orbitals, 38, 301 Butene isomerization, electrochemically promoted, 466... 

In the case of n-butene isomerization it was demonstrated (Figure 2) that the ideal micro-pore topology led to retardation of the C8 dimer intermediate and that the catalyst based on the ferrierite structure was close to optimal in this respect [1). For selective isodewaxing a one-dimensional pore structure which constrained the skeletal isomerization transition state and thereby minimized multiple branching such as the SAPO-11 structure was found to meet these criteria. Clearly, these are ideal systems in which to apply computational chemistry where the reactant and product molecules are relatively simple and the micro-porous structures are ordered and known in detail. 

Hartmann, M., A. Piippl et al. (1996). Ethylene dimerization and butene isomerization in nickel-containing MCM-41 and A1MCM-41 mesoporous molecular sieves An electron spin resonance and gas chromatography study. J. Phys. Chem. 100 9906-9910. 

Your associate, Kem Injuneer, has been testing a new catalyst for selective butene isomerization reactions. He says that the only reactions that occur to any appreciable extent over this material are ... 

There is an absence of cis-to-trans isomerization with conversion or time for the C8 (1,5-cyclooctadiene) polymer. This is shown from 52 to 58% conversion after 1 to 16 hours reaction time in Table II and III. The above review (A0, A2, A3, A5) shows that the cis structure in polymers from 1,5-cyclooctadiene using various chloride catalysts fell below 50% cis even to 20% cis units this means that the second cis double bond from the monomer underwent extensive cis-to-trans isomerization following the ring-opening of the first cis bond. Where cis-2-butene isomerizes to trans structure using other catalyst preparations, there is no evidence of this for cis-2-butene using the iodine system. However, polymer molecular... 

Fig. 24. cis-Butene isomerization over zinc oxide O, eis-butene (right-hand ordinate) A, [Pg.45]

Figure 24 shows the cfs-butene isomerization over zinc oxide as a function of time at room temperature (7/). On a per unit area basis the initial rate at room temperature is 4 X 1010 molecules/sec cm2, a rate roughly one third that reported for alumina (69). Since the activation energy for alumina is less than that found for zinc oxide, this means that zinc oxide is comparable (on a per unit area basis) to alumina as an isomerization catalyst at slightly higher temperatures. 

Results with butene are not as extensive as those with propylene. Nevertheless, on the basis of the ground work laid by the more extensive propylene studies, we are able to apply similar criteria to the more limited data for butene and conclude that a x-allyl species forms. Some preliminary studies suggest that two x-allyl species form from 1-butene (65), corresponding to the syn and anti forms. The results for propylene, the fact that x-allyl species form from butene, and the fact that zinc oxide is an effective catalyst for butene isomerization strongly suggest that these x-allyls are intermediates in the isomerization reaction. 

The conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

C4 Butesom [Butene isomerization] A process for isomerizing linear butenes to isobutene, catalyzed by a zeolite. The isobutene is intended for use as an intermediate in the production of ethers for use as fuel additives. Developed by UOP in 1992. See also C5 Pentesom. 

Patents assigned to Mobil (217) describe the use of boron trifluoride supported on several porous carriers. BF3 supported on silica was found to exhibit a slightly higher performance with added water in the alkylation of a mixed alkene feed at 273 K. It was also shown that self-alkylation activity was considerably lower than that with HF as catalyst. Another patent (218) describes the use of a pillared layered silicate, MCM-25, promoted with BF3 to give a high-quality alkylate at temperatures of about 273 K. BF3 was also supported on zeolite BEA, with adsorbed water still present (219). This composite catalyst exhibited low butene isomerization activity, which was evident from the inferior results obtained with 1-butene. At low reaction temperatures, the product quality was superior to that of HF alkylate. 

Additional evidence for the greater stability of the cis conformation of allylic anions is provided by other base catalyzed isomerization studies of 1-butene and 1-pentene. It was found that the thermodynamically less stable cis isomers of 2-butene and 2-pentene were the major products of the reaction182-186). Furthermore, m-2-butene isomerizes, under the same conditions, faster than the tram isomer to give 1-butene. 

Diadsorbed diolefins, 30 33 Diadsorbed species, 30 61, 71 Diagonalized matrix, 32 284—286, 288 ammonia synthesis, 32 294—297 n-butane dehydrogenation, 32 309-313 butenes isomerization, 32 305-308 1-butene to 1,3-butadiene dehydrogenation, 32 297-298... 

In the 1-butene isomerization example which we discuss in Section III,B we solved Eq. (65) for (x/u) by Newton s method on a high-speed computer to facilitate this otherwise laborious approximation method. The solutions to Eqs. (66) and (67) are then straightforward. From the definitions of u, x, y,... 

We are indebted to Mr. Paul Wiersma, who carried out the experimental and much of the theoretical work on 1-butene isomerization. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. 

A somewhat similar stereoselectivity has been found for butene isomerization in acid catalysis — both heterogeneous and liquid phase (16). However, this is interpreted as being due to an attraction of the electron system for positive hydrogen ions, whereas attraction of electrons for sodium ions has not been demonstrated. 

Sato et al. [195] have studied the surface borate structures and the acidic properties of alumina-boria (3-20 wt.%) catalysts prepared by impregnation method using B(MAS)-NMR measurements and TPD of pyridine, as well as their catalytic properties for 1-butene isomerization. The number of Brpnsted acid sites was found to increase with increasing boria content, and the catalytic activity was explained by the strong Brpnsted acid sites generated by BO4 species on the surface of alumina. 

This assignment is supported, indirectly, by the measured activities of these experiments. For example, the activities were measured to be (in M/g-catalyst hour) 0.98 (MS-Hz) 0.61 (MS-Dz) 180 (US-Hz) and 190 (US-Dz). Hence, the 250-fold greater activities of the ultrasound systems is consistent with the expected, more rapid, statistical C-H/D dissociation process as compared to the conventional (e.g., stirred/silent) mediated systems. Additional support for this model arises from a study of gas phase cA-2-butene isomerization to fra/rs-2-butene [15] at 291 K. Here the c O extrapolated trans deuterium number of -0.27 is supportive of C3-H/D elimination predicted by tra/jsition-state theory in this system at thermal equilibrium (e.g., vibrational temperature equal to tra/jslational temperature). 

Recent studies of the kinetics and mechanism of n-butene isomerization over lanthanum oxide by Rosynek et al. (28) indicate that for this catalyst interconversion of the two 2-butene isomers (s4 in Example 8) is very slow and in that case the system could be described by mechanism m3. Studies by Goldwasser and Hall (29) indicate that as temperature is increased, there is appreciable direct conversion via s4 so that one or both of the other two direct mechanisms may be involved. These authors suggest that further studies with all three isomers, at several temperatures and with tracers, would be desirable. 

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