N-Butene dimerization

Butene, catalytic oxidation, 35 168-169 n-Butene dimerization, 31 36-37 composition change, 31 25-26 But-2-en-l-ol oxidation, 41 307 Butler-Volmer equation, 40 89 Butterfly cluster compounds, 38 294-295 Butyl alcohols... 

Thus propene dimers, e. g., at 50 °C, have a composition of 22 % n-hexenes, 72 % 2-methylpentenes, and 6% 2,3-dimethylbutenes. Under the same conditions, isomer structures of n-butene dimers are 6 % n-octenes, 59 % 3-methylheptenes, and 34 % 3,4-dimethylhexenes propene-butene codimerization yields the following isomer distribution 12% n-heptenes, 12% 2-methylhexenes, 40% 3-methyl-hexenes and 35 % 2,3-dimethylpentenes. The relative rate constants for codimerization are... 

C9 Isononyl alcohol, n-butenes dimer derived alcohols... 

Friedlander, R., Make plasticizer olefins via n-butene dimerization. Hydrocarbon Process., Int. Ed., 65, 31, 1986. 

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. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

The new Difasol process for manufacturing isooctenes consumes less catalyst. The process dimerizes n-butene in a continuous two-phase operation that uses the industrial Dimersol nickel catalyst dissolved in a chloroaluminate ionic liquid. The n-bu-tenes are introduced continuously into the reactor, and the products are only poorly miscible with ionic liquid, and separate in settler. The process shows 70-80% conversion with 90-95% selectivity (Freemantle, 1998). 

Isobutylene is more reactive than n-butene and has several industrial uses. It undergoes dimerization and trimerization reactions when heated in the presence of sulfuric acid. Isobutylene dimer and trimers are use for alkylation. Polymerization of isobutene produces polyisobutenes. Polyisobutenes tend to be soft and tacky, and do not set completely when used. This makes polyisobutenes ideal for caulking, sealing, adhesive, and lubricant applications. Butyl rubber is a co-polymer of isobutylene and isoprene containing 98% isobutene and 2% isoprene. 

It was shown that room-temperature molten salts derived from the combination of 1,3-dialkylimidazolium chloride and A1C13 can be used as solvents in two-phase catalytic dimerization of propene to give hexenes catalyzed by Ni(II) compounds (125). The effects of phosphane ligands coordinated to nickel and operating variables were also investigated (126). The dimerization products separate as an organic layer above the molten salt. This reaction has been carried out with n-butenes as the reactant and cationic nickel complex catalysts dissolved in organochloroaluminate liquids (127). 

Dimerization and aromatization have been reported for isobutene and n-butenes, analogous to propene, over catalysts like Bi203. Isobutene radicals dimerize more easily than n-butene radicals, which are less stable and rapidly form butadiene. 

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. 

FIGURE 1 Manufacture of butene-1 (n-butene) by dimerization of ethylene. 

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. 

A number of zeolite-based catalysts are active for the dimerization of ethylene. The major products are n-butenes (1-butene, tram-2-butene, m-2-butene), i.e.,... 

Under very mild conditions (0-20°C, 200 Torr ethylene pressure), ethylene was shown to be selectively dimerized to n-butenes over RhY (140). As shown in Fig. 14, 1-butene was formed initially but further isomerized to an equilibrium composition of -butenes with increasing reaction time. In a comparative experiment using HY as a typical solid-acid catalyst, no ethylene conversion was measurable up to 200°C, and at higher temperatures unselective polymerization and cracking reactions occurred. This provided good evidence that the selective dimerization over RhY did not proceed via a carbenium ion mechanism. 

Yields OCT process selectivityto propylene is typically greaterthan 98%. Overall conversion of n-butenes is 85-92%. Ethylene and butenes feed-streams can come from steam crackers or many refinery sources and in varying concentrations. Alternatively, butenes can come from ethylene dimerization, which is also licensed by Lummus. 

The explanation of the experimental results is that the alkylation proceeds In two steps - first ethylene dimerization takes place (7) and then n-butene (or its precursor) formed alkylated with Isobutane as follows ... 

Reactions between Isobutylene and sulfuric acid were very different than those between n-butene and sulfuric acid. At least three types of reactions occurred. These were dimerization to form Cb olefins, formation of heavy ends Including Cg to Ci4 hydrocarbons (many of which are olefins), and acid-soluble hydrocarbons. These latter hydrocarbons probably Included conjunct polymers and perhaps t-butyl sulfate. As will be reported later... 

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. 

Treatment with oxalic acid has been described as a method for selective removal of the external acid sites of medium-pore zeolites 61). PER and ZSM-23 zeolites were treated with a 1-M solution of oxalic acid at 353 K overnight 39, 62). The characterization of the acid sites showed that the treated materials had a low number of external acid sites compared with the untreated materials and, when used in n-butene isomerization, they exhibited an improved isobutylene selectivity. It was also observed that acid-treated PER does not have a high selectivity for isobutylene formation. It was inferred (62) that the cavities in ferrierite at the intersections of 8- and 10-ring channels are large enough to accommodate butene dimer intermediates, thus favoring the unselective bimolecular path. In contrast, when the external acid sites are removed from a zeolite with a unidimensional pore system (e.g., ZSM-23), the initial isobutylene selectivity is higher (nearly 80%) than that of the untreated sample. 

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 2. Dimerization/cracking mechanism in n-butene skeletal isomerization over ferrierite catalysts.

Nonregioselective dimerization is widely used on an industrial scale for propene, n-butenes and ethylene (Institut Fran9ais du Petrole s Dimersol process). The catalyst results from the interaction of a nickel organic salt, soluble in a paraffinic hydrocarbon solvent, and an ethylaluminum chloro compound the active species is formed in situ inside the dimerization reactor. 

Mixed butenes obtained by ethylene dimerization are used for paraffinic alkylation (isobutane + n-butene —> trimethylpentanes) or to make propene by a subsequent metathesis reaction (ethylene + 2-butene —> 1 propene cf. Section 2.3.3). Higher ethylene oligomers are also used as high-octane-number gasoline components. 

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 amount of coke deposited on the ferrierite at the end of the catalytic test reaches levels between 5.7 and 9.1% for different temperatures, 1-butene partial pressures, and WHSV, as can be seen in Figures 4A-C. Upon increasing the temperature, the amount of coke reaches a maximum at 380°C. Increasing the WHSV increases the amount of coke deposited. Lowering the 1-butene partial pressure, decreases the amount of coke formed. ITie hi amount of coke formed under the different operational conditions studied could be related with the capacity of the ferrierite sample to form coke on the external acid sites where dimerization followed by cracking can take place. According to these results, coke is formed by a parallel mechanism, i.e., coke is formed from the 1-butene. The relationship between the amoimt of coke and both the space velocity (WHSV) and the 1-butene partial pressure points towards such a coking mechanism. When the space velocity is increased, the n-butene... 

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