Reactions of the n-butenes

Whilst the use of deuterium allows a deeper insight into the mechanism of catalytic reactions than was previously possible, it nevertheless does not allow an absolutely rigorous analysis to be made. One of the major problems in ethylene—deuterium and propene—deuterium studies is that there is no method whereby the true fraction of olefin which has undergone an olefin—alkyl—olefin cycle and reappeared in the gas phase as olefin-d0 can be determined. This is especially true for reactions on metals such as palladium, ruthenium and rhodium where the olefin exchange results sug- 

Relatively few reports of the catalysed reactions of n-butenes with hydrogen were extant up to the early 1960 s. Those studies which had been performed were mainly concerned with nickel as catalyst. The major problem was the difficulty of chemical analysis of the reaction products. However, with the advent of gas chromatography as a general analytical technique, the analysis of reaction products has become a relatively simple task and, accordingly, over the last 15 years the hydrogenation of higher olefins has received considerable attention. 

One of the earliest studies of n-butene hydrogenation was that reported by Twigg [121] who observed that, for the reaction of l butene with hydrogen over a nickel wire between 76 and 126°C, both hydrogenation and double-bond migration occurred. Hydrogenation and double-bond migration followed the same kinetic rate law, namely 

The reactions of the n-butenes with hydrogen and with deuterium catalysed by supported noble Group VIII metals have been extensively studied [103,123—127]. The variation of the butene composition with the extent 

Activation energies for butene hydrogenation and isomerisation over a nickel wire catalyst [122] 

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The reactions of the n -butenes with deuterium have been studied over alumina-supported platinum and iridium [103] and palladium [124]. In general, the results obtained are similar to those discussed above for ethylene—deuterium and propene—deuterium reactions. A comparison of the deuteroalkane distributions over platinum is shown in Fig. 17. 

Product distributions from the reaction of the n-butenes with deuterium over palladium—alumina catalysts [124] ( D o = ( c4H8)o = 60 torr... 

The Reactions of the n-Butenes with Hydrogen and with Deuterium... 

In the rhodium- and platinum-catalysed reactions [167], it is of significant interest that the 7V-profiles calculated from the distributions of deuterium in the n-butane (Table 30) appear to bear no clear relationship to the 7V-profiles of the n-butenes formed simultaneously. This observation has been interpreted as indicating that either the butene which undergoes further hydrogenation never desorbs as butene, or that the sites responsi-... 

All alkylates produced by second-step reactions in which isobutylene was the feed olefin had relatively low ROM s and contained relatively large amounts of LE s, DMH s, and HE s as compared to alkylates produced from butyl sulfate. The LE content of the alkylate generally varied between 23 to 28%, and the HE content between 30 to 40% (based on analyses with the temperature-programmed unit). The calculated RON values of the alkylate varied from about 88-91. The compositions of the TMP family were considerably different for the isobutylene runs (see Table III) as compared to those of the n-butene run (see Table II). For runs using isobutylene, 2,2,4-TMP accounted for about 60% of the TMP family. The LE s had similar compositions, however, at a given temperature for both n-butene and isobutylene runs. 

As indicated earlier (2,3) for sulfuric acid alkylations, the rates of reaction for olefins are often higher during the initial stages of alkylation than the rates of reaction for Isobutane. In the case of n-butenes, isomerization of the n-butenes occurs readily in the presence of sulfuric acid, see Reactions H-1 and H-2. Considerable information on these isomerizations were reported earlier (5) the rates of isomerization Increase as the amounts of excess acid or as the acidity of the acid phase increase. Certainly at the conditions employed in commercial alkylators, isomerization would be very rapid. 

The normal butenes were pyrolyzed in the presence of steam in a nonisothermal flow reactor at 730°-980°C and contact times between 0.04 and 0.15 sec to obtain conversion covering the range between 3% and 99%. Isomerization reactions accompanied the decomposition of these olefins however, the decomposition was the dominant reaction under these conditions. Pyrolysis of 1-butene is faster than that of either cis- or trans-2-butene. Methane, propylene, and butadiene are initial as well as major products from the pyrolysis of the n-butenes. Hydrogen is an initial product only from the 2-butenes. Ethylene appears to be an initial product only from 1-butene it becomes the most prominent product at high conversions. Over the range of conditions of potential practical interest, the experimental rate expressions for the disappearance of the respective butene isomers, have been derived. 

Over the range of conditions, 1-butene decomposes more rapidly than either of the 2-butene isomers. Double-bond shift and geometrical isomerization accompany the decomposition of the n-butenes however, skeletal isomerization does not occur, as isobutene is not found among the products of the pyrolysis. Isomerization reactions apparently are kinetically controlled, as equilibrium distributions are not generally observed. Trans cis ratios in the products do not correspond to equilibrium at either the maximum or the average reactor temperatures, and in some cases the ratio falls below equilibrium values based on American Petroleum Institute (API) data (14). However, none of these data exceed the equilibrium values based on more recent thermodynamic data (15). 

The synthesis of MTBE is carried out in the liquid phase over a fixed bed of ion exchange resin in the form. The rate of reaction of isobutene with methanol is much higher than that of the n-butenes (isobutene forms a relatively stable tertiary carbenium ion in the first step), which enables the selective conversion of isobutene in the presence of the M-butenes. In fact, streams with an isobutene content as low as 5% can be converted. 

The second stage in the process is required because the MTBE formation is an equilibrium reaction. The temperature needed ( 100°C) to achieve a sufficiently high rate of conversion means a decrease in isobutene equilibrium conversion (XiB = 0.9 at 65°C, Xjb = -0.75 at 100°C). The main side reaction in the MTBE process is the dimerization of isobutene towards di-isobutene (two isomers). Side reactions with essentially no significance are the formation of f-butyl alcohol (due to the presence of water as feed impurity), the formation of dimethyl ether from methyl alcohol, and the oligomerization of isobutene towards tri- and tetramers. A (three stage) process is also in operation which tolerates butadiene. The butadiene/ methyl alcohol reaction is faster than that of the n-butenes but consider-... 

It was also discovered that these reactions can be conducted intramolecular-ly and that they remain stereospecific. For example, imino ene reaction of the N-sulfonyl imine derived from 246 produced two cis <5-lactones 248 and 250 as a 9 1 mixture (Scheme 44). The formation of the major (E)-product 248 can be rationalized by invoking the more favorable pericydic endo ene transition state 247. The minor (Z)-product 250 would arise from endo ene transition state 249, which suffers from A1,3 strain between the allylic methyl substituent and the cis vinyl hydrogen. Rather surprisingly, the Z -olefin isomer corresponding to 246 gave the same 9 1 mixture as did the E isomer. Based on the related loss of stereoselectivity with Z-2-butene (vide supra) it was again postulated that the... 

Mechanism 4 is of considerable importance when HF is the catalyst for the reaction with all normal olefins. With HF catalysts, 10-20% of propylene is often converted to propane and 2-5% of the n-butenes to n-butane. Mechanism 4 has no importance when sulfuric acid is the catalyst and either n-butenes or propylene is used as olefins. Some n-pentane is produced, however, when n-pentenes are used in the presence of sulfuric acid (16). Isobutane consumption is several percent higher for alkylation reactions using HF than those using sulfuric acid. Olefin consumption is, however, essentially unchanged as a result of mechanism 4. 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

However, very recent studies by Fish and his co-workers (467) with butyltin compounds showed that the primary, metabolic reaction is not Sn-C bond-cleavage but carbon hydroxylation of the n-butyl group. Using [l- C]tetrabutyltin in an in vitro study, the major, primary metabolite was identified as a 2-hydroxybutyltributyltin derivative that underwent a rapid /3-elimination reaction to afford 1-butene and a tri-butyltin compound (467). 

Noguchi, H., Yoda, E., Ishizawa, N., Kondo, J. N., Wada, A., Kobayashi, H. and Domen, K. (2005) Direct observation of unstable intermediate species in the reaction of trans-2-butene on ferrierite zeolite by picosecond infrared laser spectroscopy. J. Phys. Chem B, 109, 17217-17223. 

The isomerization of light olefins is usually carried out to convert -butenes to isobutylene [12] with the most frequently studied zeolite for this operation being PER [30]. Lyondell s IsomPlus process uses a PER catalyst to convert -butenes to isobutylene or n-pentenes to isopentene [31]. Processes such as this were in larger demand to generate isobutene before the phaseout of MTBE as a gasoline additive. Since the phaseout, these processes often perform the reverse reaction to convert isobutene to n-butenes which are then used as a metathesis feed [32]. As doublebond isomerization is much easier than skeletal isomerization, most of the catalysts below are at equilibrium ratios of the n-olefins as the skeletal isomerization begins (Table 12.5). 

Dihydromuscimol (49) is a conformationally restricted analogue of the physiologically important neurotransmitter y-aminobutyric acid (GABA) and has been prepared using the cycloaddition of dibromoformaldoxime to A-Boc-allylamine followed by N-deprotection with sodium hydroxide (Scheme 6.52) (278). The individual enantiomers of dihydromuscimol were obtained by reaction of the bromonitrile oxide with (5)-( + )-l,2-0-isopropylidene-3-butene-l,2-diol, followed by separation of the diastereoisomeric mixture (erythro/threo 76 24), hydrolysis of respective isomers, and transformation of the glycol moiety into an amino group (279). 

Skov, H Th. Benter, R. N. Schindler, J. Hjorth, and G. Restelli, Epoxide Formation in the Reactions of the Nitrate Radical with 2,3-Dimethyl-2-butene, cis- and trans-2-Butene, and Isoprene, Atmos. Environ., 28, 1583-1592 (1994). 

The rhodium—charcoal-catalysed hydrogenation of the n-pentenes at room temperature exhibits similar results to those found with platinum [145], Whilst at first sight this might appear to be unusual, the apparent lack of isomerisation can be readily accounted for by assuming that, as with the n-butenes [125], isomerisation only becomes the predominant reaction at relatively high temperatures, (E — Eh ) being positive. 

In order to obtain detailed information about the reaction mechanism, the results obtained from the reaction of buta-1 2-diene with deuterium were used to calculate / /-profiles and hence theoretical deuterobutene distributions as described in Sect. 4.4. The distributions of deuterium in the n-butenes, buta-1 3-diene and buta-1 2-diene together with the butene / -profiles and calculated deuterobutene distributions are shown in Table 25. 

It is also possible that the unimolecular reaction takes place with the molecule in the electronic ground state, but it requires very intense fields to generate so-called multiphoton or direct overtone transitions that is, transitions from the vibrational ground state of the type 0 —> n, where n > 1. The opening of the cyclo-butene ring to form butadiene is an example of a unimolecular reaction induced by direct overtone excitation ... 

Recently, Angelescu et a/.[92] have studied the activity and selectivity for dimerization of ethylene of various catalysts based on Ni(4,4-bipyridine)Cl2 complex coactivated with A1C1(C2H5)2 and supported on different molecular sieves such as zeolites (Y, L, Mordenite), mesoporous MCM-41 and on amorphous silica alumina. They found that this type of catalyst is active and selective for ethylene dimerization to n-butenes under mild reaction conditions (298 K and 12 atm). The complex supported on zeolites and MCM-41 favours the formation of higher amounts of n-butenes than the complex supported on silica alumina, which is more favourable for the formation of oligomers. It was also found that the concentration in 1-butene and cw-2-butene in the n-butene fraction obtained with the complex supported on zeolites and MCM-41, is higher compared with the corresponding values at thermodynamic equilibrium. 

Fig. 14. Composition change of produced n-butenes with reaction time. Reaction conditions catalyst, RhY (0.30 g) activated by evacuation at 300°C for 1 hr temperature, 0 C ethylene initial pressure, 200 Torr. (Reproduced from Ref. 140 with permission from the authors.) O, 1-butene , trans-2-butene 3, cis-2-butene.

MTBE is produced by reacting methanol and isobutylene under mild conditions in the presence of an acid catalyst. The isobutylene feed is either mixed butylenes, a butylenes stream from catalytic cracking, or a butylenes coproduct from ethylene production. The reaction conditions are mild enough to permit the n-butenes to pass through without ether formation. Figure 10.31 shows a typical process for making MTBE. 

Another approximately 1.5 billion lb of isobutylene goes into other chemical uses. These applications include polybutenes and derivatives of high-purity isobutylene such as butyl rubber, polyisobutylenes, and substituted phenols. Isobutylene is more reactive than the n-butenes, but many of its reactions are readily reversible under relatively mild conditions. 

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