Butane isomerization catalysts

Z. Hong, K.B. Fogash, and J.A. Dumesic, Reaction kinetic behavior ofsulfated-zirconia catalysts for butane isomerization, Catalysis Today 51, 269-288 (1999). 

Butamer [Butane isomerization] A process for converting n-butane into iso-butane conducted in the presence of hydrogen over a dual-functional catalyst containing a noble metal. Developed by UOP and licensed worldwide since 1959. In 1992, more than 55 units had been licensed. 

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 M-butane over Zr02-supported sulfate catalysts was initially proposed by Hino and Arata. 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 n-butane to isobutane at... 

Butane isomerization is usually carried out to have a source of isobutane which is often reacted with C3-C5 olefins to produce alkylate, a high octane blending gasoline [13]. An additional use for isobutane was to feed dehydrogenation units to make isobutene for methyl tert-butyl ether (MTBE) production, but since the phaseout of MTBE as an oxygenate additive for gasoline, this process has decHned in importance. Zeolitic catalysts have not yet been used industriaUy for this transformation though they have been heavily studied (Table 12.1). 

Butane Isomerization. Five processes for butane isomerization were in commercial use by the end of World War II. These processes differ primarily in the method of contacting the hydrocarbon with the catalyst. Two are vapor-phase processes, which require periodic discard and replacement of the catalyst bed the other three are carried out in the liquid phase and are continuous with respect to catalyst addition and withdrawal. 

The other vapor-phase butane isomerization process, developed cooperatively by the Anglo-Iranian Oil Co. and the Standard Oil Development Co., is somewhat similar to the Isocel process. In the AIOC-Jersey process (18), the reactor is initially filled with bauxite, and aluminum chloride is sublimed into the vaporized feed as necessary to maintain the desired catalyst activity. Upflow of vapor through the reactor is the customary arrangement. Since carry-over of aluminum chloride is not excessive at the usual rates of catalyst addition, about half of the commercial plants employing this process were not equipped with guard chambers. 

The butane isomerization process developed by the Universal Oil Products Co. is shown in Figure 4. In this process (3), the feed is maintained essentially in the liquid phase under pressure. Part of the feed is by-passed through a saturator, where it dissolves aluminum chloride. The feed later picks up hydrogen chloride and passes through the reactor, which is packed with quartz chips. Some insoluble liquid complex is formed, and this adheres to the quartz chips. The aluminum chloride in the feed is preferentially taken up by the complex, which thus maintains an active catalyst bed. The complex slowly drains through the reactor, losing activity en route. It arrives at the bottom in essentially spent condition and is discarded. Aluminum chloride carried overhead in the reactor products is returned to the reactor from the bottom of the recovery tower. The rest of the process is the same as in the vapor-phase processes. 

Studies with sulfated zirconia promoted with Pt309 and industrial chlorinated Pt on AI2O3 isomerization catalysts310 led to the same conclusion, namely, the intermolecular mechanism operative for M-butane isomerization. A significant difference, however, is that on the industrial catalysts extensive hydride and methyl shifts taking place in the intermediates prior to P scission do not lead to a random distribution of the labels. Instead, a binomial distribution with one and three 13C atoms is observed.310 This is indicative of the involvement of the 31 carbocationic intermediate. 

An IR study of SbF5-Al203 after addition of pyridine to the catalyst shows an absorption at 1460 cm-1, which is assigned to pyridine coordinated with the Lewis acid site, and shows another absorption at 1540 cm-1, which is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. When the catalyst is heated up to 300°C, the IR band of the pyridinium ion disappears, whereas the absorption for the Lewis acid is still present. The fact that the catalyst is still active for ra-butane isomerization suggests that the Lewis acid sites are the active sites for the catalysis.111... 

C. Y. Hsu, C. R. Heimbuch, C. T. Armes, and B. C. Gates, A highly active solid superacid catalyst for n - butane isomerization a sulfated oxide containing iron, manganese and zirconium, J. Chem. Soc. Chem. Commun. 1645-1646 (1992). 

Over SbFs/metal oxides, it was proposed that n-butane isomerization proceeded via carbenium ion intermediates formed by hydride ion transfer from n-butane to the Lewis acid sites [55]. The details of sul-fated zirconia as a hydrocarbon conversion catalyst have been summarized by Davis et al. [56],... 

Solid superacids of the sulfated zirconia type were found active for n-butane isomerization at low reaction temperatures (50). These catalysts, however, were rapidly deactivated with time on stream. The isomerization selectivity and the stability of sulfated zirconia catalysts can be incerased by the introduction of Pt and by carrying out the reaction in the presence of H2. Higher catalytic activities were obtained when Pt was impregnated after the impregnation of zirconia gel with 0.5 M H2SO4 (51). Sulfated zirconia promoted with Fe or Mn showed an even higher activity than unpromoted SZ for the low temperature isomerization of n-butane (52). 

In this context, additional isobutane availabilities could be derived from n-butane isomerization, an operation which offers two variants, one of them induding the recyding of unconverted n-butane after separation. As a rule, isomerization takes place in the gas phase, around 150 to 200"C, under hydrogen pressure (13 to 15.10 Pa absolute), in the presence of a fixed catalyst bed of the reforming type, based on platinum (0.35 per cent weight) deposited on alumina promoted by traces of organic chlorides. Ooce-through... 

Figure 17.1 summarizes the effect of the calcination temperature on the catalytic activity for butane isomerization over sulfated zirconias prepared from different zirconia gels dried at the optimum temperatures. The figure indicates that the maximum activities are approximately the same for different catalysts even though... 

The butanes show little tendency to crack or disproportionate (7) thus butane isomerization is fairly straightforward. However, the suppression of side reactions becomes more difficult as the molecular weight increases. With pentanes, disproportionation to isobutane and hexane is pronounced, amounting to as much as 63%. A typical composition of pentane disproportionation products is shown in Table III. Besides lowering the yield of isopentane, such side reactions shorten the life of the catalyst. Adding small amounts of cyclic hydrocarbons (7,15,18)... 

Fig. 20. Process variables for butane isomerization. Shell liquid-phase process. Conditions (unless otherwise noted) temperature, 176°F. residence time, 13-15 minutes AlCU, 7.5 wt.% HCl, 4.0 wt.% catalyst-to-hydrocarbon ratio, 1/1.

Pentane isomerization was used to increase the critical supply of aviation gasoline toward the end of the war. Two processes—one developed by Shell and one by Standard Oil Company (Indiana)—were commercialized. The pentane processes differ from butane isomerization mainly in the use of somewhat milder conditions and an inhibitor to suppress side reactions. In general, the problems of the butane processes are inherent also in pentane isomerization, but the quality of the feed stock is less important. Olefins can be as high as 0.2 %, although 0.05 % is preferable. The hexane content should not exceed about 5%, and sulfur and water contents should be as low as in the butane process. Catalyst life is much shorter than in the butane processes only about 30-50 gallons of isopentane are produced per pound of aluminum chloride. 

Solid acid catalysts play an important role in hydrocarbon conversion reactions in the chemical and petroleum industries [1,2]. Many kinds of solid acids have been found their acidic properties on catalyst surfaces, their catalytic action, and the structure of acid sites have been elucidated for a long time, and those results have been reviewed by Arata [3]. The strong acidity of zirconia-supported sulfate has attracted much attention because of its ability to catalyze many reactions such as cracking, alkylation, and isomerization. Sulfated zirconia incorporating Fe and Mn has been shown to be highly active for butane isomerization, catalyzing the reaction even at room temperature [4]. 

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