Isobutene butene isomers

There are four butene isomers three unbranched, normal butenes (n-butenes) and a branched isobutene (2-methylpropene). The three n-butenes are 1-butene and cis- and trans- 2-butene. The following shows the four butylene isomers ... 

The industrial reactions involving cis- and trans-2-butene are the same and produce the same products. There are also addition reactions where both 1-butene and 2-butene give the same product. For this reason, it is economically feasible to isomerize 1-butene to 2-butene (cis and trans) and then separate the mixture. The isomerization reaction yields two streams, one of 2-butene and the other of isobutene, which are separated by fractional distillation, each with a purity of 80-90%. Table 2-3 shows the boiling points of the different butene isomers. 

The reactions of Y with four butene isomers, namely 1-butene, cis-2-butene, trans-2-butene, and isobutene, were studied at a collision energy (/ ycoii) of 26.6kcal/mol (see Table 2). In reactions with 1-butene and cis- and trans-2-butene, four processes were observed ... 

Upon examining the data for the reactions of all four butene isomers (Fig. 37), the most striking observation is that the data for all four isomers are quite similar, except that there is no YH2 formed from isobutene. In addition, the branching ratios for each isomer are similar, except that 4>ych2 OyCiHe, is approximately a factor of two greater for isobutene than for the other isomers, and for propene, YCH2 is a much more important channel than is YH2 (Fig. 40), a situation that is exactly the opposite to that for the butene reactions (Fig. 37). 

Bismuth phosphate has been investigated as a catalyst for aromatization of the four different butene isomers at 550°C. An optimal catalyst has an atomic ratio Bi/P = 2 (Sakamoto et al. [271]). Isobutene is converted at short contact times (r 0.3 sec) to dimers and to aromatics, with a selectivity of 29% each. n-Butenes give much lower yields. 

Investigation of n-butane conversion over H-forms of the ferrierite and theta-1 zeolites demonstrated that the isobutene selectivities were similar (and low) for these catalysts. The maximum selectivities (7-8 %) were obtained at low n-butane conversions (5-10 %) and decreased with increasing conversion of n-butane due to olefin interconversion and aromatisation reactions. Isobutene was in equilibrium with the other butene isomers due to the high isomerisation activity of the parent zeolites. The maximum selectivity to butenes, which was observed at low conversions, was around 20 %. This value reflects a moderate contribution of the dehydrogenation steps in n-butane transformation over H-forms of the ferrierite and theta-1 zeolites and indicates an important role of the n-butane protolytic cracking steps over these two catalysts. 

Then, the fert-butyl cation intermediate can attack a molecule of butene to give the corresponding C8 carbocation. Depending on the particular butene isomer that is alkylated, a different C8 carbocation will be formed (2,2,4-TMP+ from isobutene, 2,2,3-TMP+ from 2-butene, and 2,2-DMH+ from 1-butene) ... 

Since alkylate compositions from the four butene isomers are basically similar, the butenes are thought to isomerize considerably, approaching equilibrium composition prior to isobutane alkylation. Such a postulation is at variance v/ith previously published alkylation mechanisms. The Isomerization step yields predominantly isobutene which then polymerizes and forms a 2,2,4-trimethylpentyl carbonium ion, a precursor of 2,2,4-trimethylpentane, the principal end product. The 2,2,4-trimethylpentyl ion is also capable of isomerization to other trimethylpentyl ions and thus yields other trimethylpentanes, principally 2,3,4-trimethyl-pentane and 2, 3, 3-trimethylpentane. 

The following facts are the basis for butene isomerization (I) There is a basic similarity in the composition of alkylates produced from all four butene isomers. (2) Alkylate molecules, once formed, are relatively stable under alkylation conditions and do not isomerize to any appreciable extent alkylate fractions having the same carbon number ore not equilibrated (see Table I). (3) Thermodynamic equilibrium between the butene olefins highly favors isobutene formation at alkylation temperatures. (4) Normal butenes p>roduce only small and variable amounts of normal butane, thus indicating only a small and variable amount of chain initiation from normal butenes. Yet the alkylate composition shows a high concentration of trimethylpentanes and a low concentration of dimethylhexanes. (5) A few of the octane isomers can be explai.ned only by isomerization of the eight-carbon skeletal structure this isomerization occurs while isobutene dimer is in ionic form. For example, 2,3,3- and 2,3,4-trimethylpentanes... 

Results from this laboratory for steam pyrolysis of isobutene were reported earlier (I), and this chapter describes the pyrolysis of the three normal butene isomers 1-butene, m-2-butene, and trans-2-butene. Reaction schemes are derived from the product distributions, and kinetic parameters are established. 

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 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-... 

The ozonolysis of simple alkenes was studied in two different apparatus a 2 litre stirred tank reactor (for ethene, propene, fran.y-2-butene, butadiene, and isoprene) coupled via molecular beam sampling to a matrix isolation FTIR set-up [20, 21], and a 570 L spherical glass vessel "big sphere" (for ethene, 2-butene isomers, isobutene, and isoprene) where products were identified by Fl lR spectroscopy, GC and a scrubber sampling unit for analysis with HPLC (for peroxides) and IC (for organic acids). In the latter system, two extreme humidity conditions, one with 0.5 ppm and the other with 2 x 10" ppm (corresponding to ca. 60 % relative humidity at 298 K) were used, which are referred to as "dry" and "wet" conditions, respectively. Results of the studies performed in the "big sphere" are summarised here. 

In addition, the neutral products formed in charge-transfer reactions usually retain the structures their ionic precursors had at the time of reaction. For example, when the different butene isomers (1-butene, 2-butene, or isobutene) are ionized with 10-eV photons and the ions are allowed to react with a compound whose ionization potential is lower than that of any C4H8 isomer... 

The three isomers constituting n-hutenes are 1-hutene, cis-2-hutene, and trans-2-hutene. This gas mixture is usually obtained from the olefinic C4 fraction of catalytic cracking and steam cracking processes after separation of isobutene (Chapter 2). The mixture of isomers may be used directly for reactions that are common for the three isomers and produce the same intermediates and hence the same products. Alternatively, the mixture may be separated into two streams, one constituted of 1-butene and the other of cis-and trans-2-butene mixture. Each stream produces specific chemicals. Approximately 70% of 1-butene is used as a comonomer with ethylene to produce linear low-density polyethylene (LLDPE). Another use of 1-butene is for the synthesis of butylene oxide. The rest is used with the 2-butenes to produce other chemicals. n-Butene could also be isomerized to isobutene. ... 

Reactions with isobutene led to channels (5), (7), and (8), but no evidence for process (6) was observed. Time-of-flight (TOF) spectra for all four isomers were similar, so only data for the Y + cis-2-butene reaction will be shown. A Newton diagram for this reaction is shown in Fig. 33. 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

There is evidence that both ionic and free radical species are involved in the degradation and depolymerization of poly (olefin sulfone) s by high energy radiation (70). Thus, the yields of olefins from poly (1-butene sulfone) at 30 °C (the sample was heated to 70 °C during removal of the gaseous products) are shown in Table II. The butene is not solely 1-butene, but comprises significant proportions of all three isomers, 1-butene, 2-butene and isobutene. 

Recently we reported that HiiC Hjj), could also be distinguished from three other isomers lNi+-butene, Ni -isobutene, nickelacyclopentane cation) on the basis of its unique photodissociation spectrum and photoproduots [15]. In particular N1(C photodissoclates to give two... 

The isomers of butene are but-l-ene (with only a plane of symmetry, symmetry point group Cs), ( )-but-2-ene (with a horizontal plane of symmetry, a twofold axis of symmetry perpendicular to it, and a centre of symmetry, symmetry point group C2h), (Z)-but-2-ene (with a twofold proper axis of symmetry and two planes of symmetry containing this axis, symmetry point group C2V) and isobutene (2-methylpropene) with two mutually perpendicular planes of symmetry and on the line of intersection of these two planes there is a twofold axis of symmetry. The symmetry point group is therefore C2v. These results can be verified using the flow chart in the appendix. 

As with the pure olefin feeds, lowering the reaction temperature Inhibited side reactions. The yield of primary alkylation products such as 23DMH and 23DMP Increased, as the formation of other DMH Isomers and 24DMP was Inhibited. TMP s are both a primary product and a byproduct. Thus, TMP s from Isobutene and 2-butene Increased as TMP s from propylene and 1-butene decreased. The total TMP yield was optimized at 27 C. 

By comparison with conventional HF and H2SO4 alkylation catalysts as shown below, Amberlyst XN-IOIO/BF3 showed relatively much less difference among these three olefin isomers. HF alkylation discriminates strongly against 1-butene while H2SO4 alkylation gives relatively poor results with isobutene (, 3 ) This suggests that the resin/BF3 system is unique and not related to conventional systems. 

Figure 5 demonstrates the sensitivity of the primary products of this lumped H-abstraction reaction by varying the probability of methyl substitution, i.e. by varying the relative amount of the different classes of isomers (mono-, di-, tri-, tetra-methyl and so on). While ethylene and 1-butene selectivities decrease with the increase in degree of methyl substitution, methyl radical, 2-butene and isobutene formation is enhanced. 

In discussing processes in olefins, it is convenient to divide the reactions into two classes simple particle transfer and carbon-addition reactions. The relative importance of these two types of reaction is also dependent on the structure of the reactants. The presence of the isobutene type structure (CH2=C(CH3)CH2—) in either the reactant ion or neutral molecule favours the simple particle transfer reaction, mainly because of the large cross-section for the formation of parent-plus-one ions. The 2-butene neutral molecule is more like isobutene than 1-butene with regard to the relative importance of simple particle transfer and carbon addition reactions [284]. However, the magnitude of the cross-section for simple particle transfer reactions in 2-butene (2-P + 2-M reaction) is much closer to that in 1-butene (1-P + 1-M reaction) than to that in isobutene (iso-P + iso-M reaction) [284, 299]. The same is true for pentene isomers i.e. the cross-sections for simple particle transfer reactions in 1-pentene and 2-pentenes are almost the same and are much lower than that in 2-methyl-l-butene. The simple particle transfer cross-sections for other branched pentenes, i.e. 2-methyl-2-butene and 3-methyl-l-butene, are even smaller than those for 1- and 2-pentenes, while the proportion of transfer reactions is higher than the corresponding proportions for 1- and 2-pentenes. The proportion is, of course, highest for 2-methyl-l-butene which has the isobutene type structure. 

Comparison with other molecular sieves (Fig. 11) shows that the yields obtained with FER are very high indeed. As elaborated upon elsewhere [6-8], we have proposed that the isomerisation involves a bi-molecular mechanism in which e.g. di-methylhexene isomers crack selectively to isobutene and n-butene (Fig. 12). The mono-molecular mechanism requires the energetically unfavourable primary carbenium ions. Molecular modelling [7] has provided support for this mechanism in that the branched octenes can be formed in the intra-crystalline voids of FER but their diffusion out of the pores is hindered. 

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