2-butene alkylation

Other olefins that are commercially alkylated are isobutene and 1- and 2-butenes. Alkylation of isobutene produces mainly 2,2,4-trimethylpen-tane (isooctane). 

The activity and decay behaviour of the different porous heteropolycompounds were compared in two reactions requiring strong acid sites the n-butane isomerization and the isobutane/2-butene alkylation. Although these two reactions are important in the petroleum refining industry, n-butane isomerization is often used as a "test reaction" since it is known that this reaction requires very strong acid sites and only a limited number of oxides are active in this reaction, under mild conditions (T = 473 K). 

Catalytic activity in liquid phase hbutane/butene alkylation... 

RON values of various alkanes and the C54. composition of isobutane/butene alkylates produced with various acids in laboratory... 

Using 1-butene as the feed alkene in most cases does not lead to dimethylhexanes as expected, but also to a mixture of TMPs. These are formed in a rapid isomerization of the linear butenes, almost to equilibrium compositions, in which the 2-butenes are strongly favored. On the other hand, some of the DMH-isomers produced in 2-butene alkylation also stem from a rapid isomerization of the feed. 

Recently, mesoporous aluminosilicates with strong acidity and high hydrothermal stability have been synthesized via self-assembly of aluminosilicate nanoclusters with templating micelles. The materials were found to contain both micro- and mesopores, and the pore walls consist of primary and secondary building units, which might be responsible for the acidity and stability (181). These materials were tested in isobutane/n-butene alkylation at 298 K, showing a similar time-on-stream behavior to that of zeolite BEA. No details of the product distribution were given. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

Cesium salts of 12-tungstophosphoric acid have been compared to the pure acid and to a sulfated zirconia sample for isobutane/1-butene alkylation at room temperature. The salt was found to be much more active than either the acid or sulfated zirconia (201). Heteropolyacids have also been supported on sulfated zirconia catalysts. The combination was found to be superior to heteropolyacid supported on pure zirconia and on zirconia and other supports that had been treated with a variety of mineral acids (202). Solutions of heteropolyacids (containing phosphorus or silicon) in acetic acid were tested as alkylation catalysts at 323 K by Zhao et al. (203). The system was sensitive to the heteropoly acid/acetic acid ratio and the amount of crystalline water. As observed in the alkylation with conventional liquid acids, a polymer was formed, which enhanced the catalytic activity. 

Nafion-H, a perfluorinated sulfonic acid resin, is another strongly acidic solid that has been explored as alkylation catalyst. Rprvik et al. (204) examined unsupported Nafion-H with a nominal surface area of 0.2 m2/g (surface area of a swellable polymer is difficult to define) in isobutane/2-butene alkylation at 353 K and compared it with a CeY zeolite. The zeolite gave a better alkylate and higher conversion than Nafion-H, which produced significant amounts of octenes and heavy-end products. The low surface area of the resin and questions about the accessibility of the sulfonic acid groups probably make the comparison inadequate. 

To increase the surface area, the resin can be supported on porous carriers, or it can be directly incorporated into silica by a sol-gel preparation technique. Both methods have been used by Botella et al. (205), who compared several composite Nafion/silica samples with varying surface areas and Nafion loadings for isobutane/2-butene alkylation at 353 K. Furthermore, supported and unsupported Nafion samples were used. As expected, the unsupported resin with its low... 

Triflic acid has also been supported on a porous silica carrier (220). The authors emphasized the importance of a strong interaction between the acid and the support to prevent leaching of the acid. In pulsed liquid-phase isobutane/ 1-butene alkylation experiments at 298 K, the catalysts produced a very high-quality alkylate, made up almost exclusively of isooctanes. With silanol groups on the silica surface or with added water, triflic acid was found to form a monohydrate that was firmly grafted to the silica surface. 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

TABLE 13.3 Isobutane/2-Butene Alkylation over Acidic Salts of 12-Tungstophosphoric Acids... 

Moreover, the efficiency of these catalysts could be modihed by tailoring the nature of the metal oxide support and/or reaction conditions (especially the reaction temperature). In this way, interesting conclusions can be obtained when comparing the isobutane/2-butene alkylation catalyzed on two of the most studied catalysts, that is, beta zeolite and sulfated zirconia, when operating at different reaction temperatures. (Table 13.2). ... 

Figure 13.6 Variation of both the conversion of 2-butene and the selectivity to trimethyl-pentanes (TMP), with the nature of the metal oxide support obtained during the isobutene/ 2-butene alkylation at 32°C over sulfated supported on different metal oxides. (After Ref. 35.)...

TABLE 13.4 Isobutane/2-Butene Alkylation over H3PW12O40/ S04 /Zr02 Catalyst... 

In the case of heteropolyoxometalates, the compositions can also be tailored in order to achieve good catalytic properties. Table 13.3 shows the variation of the catalytic performance of acidic salts of 12-tungstophosphoric acids during the iso-butene/2-butene alkylation. ... 

The use of a polyfunctional catalyst could enhance the life of the catalyst. A clear example is the use of H3PWi2O40-SO4 /ZrO2 mixtures for isobutane/ butenes alkylation (Table 13.4-). However, modifications of the t) pe of reactor could also favor extended catalyst longevity." During the last few years, other alternatives have been proposed that favor a better catalyst regeneration and/or lower catalyst deactivation the use of supercritical isobutene regeneration or dense-C02 enhanced the reaction media. ... 

M.F. Simpson, J. Wei, and S. Sudaresan. Kinetic analysis of isobutane/butene alkylation over ultrastable H-Y zeoUte. Ind. Eng. Chem. Res., 35 3861-3873,... 

Weitkamp, J. and Traa, Y. (1999) Isobutane/butene alkylation on solid catalysts. Where do we stand. Catal. Today, 49,193-199. 

Sarsani, V.R. and Subramaniam, B. (2009) Isobutane/butene alkylation on microporous and mesoporous solid acid catalysts probing the pore transport effeds with liquid and near critical reaction media. Green Chem., 11, 102-108. 

Petkovic, L.M. and Ginosar, D.M. (2004) The efiect of supercritical isobutane regeneration on the nature of hydrocarbons deposited on a USY zeolite catalyst utilized for isobutane/butene alkylation. Appl. Catal. A, 275, 235-245. 

Beta scission of a carbenium ion is an elementary step that is inihated by the weakening of the bond beta to the positive charge, leading to a smaller carbenium ion and an alkene. This elementary step is further discussed in Sections 13.8.1, 13.8.3.1 and 13.8.4 within the context of alkene skeletal isomerization, isobutane-2-butene alkylation and alkane cracking, respectively. 

Corma, A., Martinez, A., Arroyo, P.A., Monteiro, J.L.F., and Sousa-Aguiar, E.F. (1996) Isobutane/2-butene alkylation on zeolite beta influence of post-synthesis treatments. Appl. Catal. A, 142,... 

Nivarthy, G.S., Seshan, K., and Lercher, J.A. (1998) The influence of acidity on zeolite H-BEA catalyzed isobutene /N-butene alkylation. Micropor. 

Stocker, M., Mostad, H., and Rorvik, T. (1994) Isobutane/2-butene alkylation on faujasite- type zeolites. Catal. Lett., 28, 203-209. 

Rorvik, T., Mostad, H.B., Karlsson, A., and Ellestad, O.H. (1997) Isobutane/ 2-butene alkylation on fresh and regenerated La-EMT-51 compared with H-EMT. The catalysts selectivity changes at high butene conversion in a slurry reactor. Appl. Catal. A, 156, 257-283. 

Feller, A., Barth, J.-O., Guzman, A., Zuazo, I., and Lercher, J.A. (2003) Deactivation pathways in zeolite-catalyzed isobutane/butene alkylation. 

J.A. (1998) Influence of the activation temperature on the physical properties and catalytic activity of La-X zeolites for isobutane/n-butene alkylation. Micropor. Mesopor. Mater., 22, 379-388. 

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