Alkane Isomerization

Traditionally, the isomerization of light alkanes (n-C5 and n-C6, light straight run gasoline) have been carried out using bifunctional systems, that is, Pt-AlCl3/Al2O3 (UOP, BP) or Pt-Mordenite (Shell, Mobil, Norton, Su-Oil, IFP) catalyst.17,18 A clori-nated alumina-based catalyst act at a low operation temperature that is favorable in equilibrium.17 This allow as to obtain isomerates of about 83 RON. However, it has the disadvantages of being highly sensitivity to water in the feed (requiring the use of expensive dryers) and having low catalyst stability (requiring the supplement of some organic chloride chemicals to preserve its catalytic activity).

The zeolite catalysts require high operating temperatures.18 Although this reaction condition favors the production of an isomerate with a RON of about 78, these catalysts possess a tolerance to feedstock poisons such as sulfur or water.

These reactions can also be performed over a strong acid catalyst at reaction temperatures that are lower than over zeolites. Because of this, isomerization of n-butane over ZrO2-supported sulfate catalysts was initially proposed by Hino and Arata.13 They proposed these catalysts as being effective in butane isomerization at room temperature, a reaction that does not take place, even in 100% sulphuric acid. For this reason, these catalysts were considered as solid superacids, since they are active and selective in the isomerization of «-butane to isobutane at

■ Conversion

Selectivity H Yield

Beta 250°C

Beta 300°C

Figure 13.4 Isomerization of «-butane to isobutane on beta zeolite and sulfated zirconia catalysts at different reaction temperatures.

Beta 250°C

Beta 300°C

Figure 13.4 Isomerization of «-butane to isobutane on beta zeolite and sulfated zirconia catalysts at different reaction temperatures.

lower temperatures than zeolites (Figure 13.4).19 In the last decade, many research workers have studied the improvement of catalytic behavior, catalyst decay, and the resistance of poisons.13-15

Thus, if the incorporation of some metal oxides indicated a notable improvement in the catalytic activity (permitting it to operate at lower reaction temperatures),20,21 the incorporation of metals, especially Pt and working in the presence of H2, has prolonged the life of the catalysts.22,23 However, new catalyst formulations have recently increased the resistance of these catalysts to such poisons as water or sulfur during the isomerization of n-C5 and n- C6 paraffins.24-26 Nevertheless, the use of other anions, by supporting WO3 or MoO27 or heteropolya-cids,28 which have higher thermal stability, can also be interesting alternative routes to develop new catalytic systems.

On the one hand, high-throughput techniques can be used to achieve more specific catalysts,25 while the use of conditions favoring a reduction in coke formation during the reaction (i.e., the use supercritical conditions)29 could also be of crucial importance in the rapid incorporation of these catalysts into industrial processes.

In addition to this, solid acid catalysts can also be used in the hydroisomeriza-tion cracking of heavy paraffins,30 or as co-catalysts in Fischer-Tropsch processes.31 In the first case, it could also be possible to transform inexpensive refinery cuts with a low octane number (heavy paraffins, n-C8-20) to fuel-grade gasoline (C4-C7) using bifunctional metal/acid catalysts. In the last case, by combining zeolites with platinum-promoted tungstate modified zirconia, hybrid catalysts provide a promising way to obtain clean synthetic liquid fuels from coal or natural gas.

TABLE 13.1 Alkylation of Isobutane with 2-Butenes on HF or H2SO4

Catalyst

HF

H2SO4

Reaction Temperature, K

293

273

Yield, in wt %

Product:

Isopentane

0.3

2.2

Dimethylbutanes

Methylpentanes

0.9

2.6

Branched-C7

2.2

2.7

2,2,4-

Trimethylpentanes

85.6

80. 0

2,3,4-

2,3,3-

Dimethylhexanes

6.9

8.1

Methylheptanes

C^-hydrocarbons

4.1

4.4

Source: After Ref. 32.

Source: After Ref. 32.

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Responses

  • awate
    Why zeolites require high temperatures for isomerization?
    8 years ago

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