Butane isomerization catalyst activity

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... 

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. 

Low temperature isomerization catalysts are of the Friedel Crafts type, such as AICI3 and AlBr3, activated with HX, and dissolved in a suitable solvent such as SbCl3. Application of these extremely acidic and corrosive systems requires special handling and disposal of the catalyst and careful pretreatment of the feed-stock to remove contaminating materials. Low temperature isomerization (< 100° C) is used mainly for isomerization of w-butane, which is generally available in sufficient purity by normal refinery processes. 

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). 

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). 

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... 

Catalysts active in the isomerization of n-butane have been synthesized by depositing sulfate ions on well-crystallized defective cubic structures based on ZrOz. This technique for introduction of sulfates does not result in any significant changes in the bulk properties of zirconium dioxide matrix. Active sulfated catalysts were prepared on the basis of cubic solid solutions of ZrOz with calcium oxide and on the basis of cubic anion-doped ZrOz. The dependence of the catalytic activity on the amount of calcium appeared to have a maximum corresponding to 10 mol.% Ca. Radical cations formed after adsorption of chlorobenzene on activated catalysts have been used as spin probes for detection of strong acceptor sites on the surface of the catalysts and estimation of their concentration. A good correlation has been observed between the presence of such sites on a catalyst surface and its activity in isomerization of n-butane. 

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]. 

Sulfated zirconias, nowadays a well established class of acid solids first reported by Holm and Bailey [2], and systematically studied by Arata [3] and Tanabe et aL [4], are considered as potential alternative catalysts for the skeletal isomerization of n-butane. These catalysts have recently found a commercial application (Par-Isom Process of UOP) for the isomerization of light naphtha (Cs-Ce), but since they are less active than Pt-chlorinated aluminas, there is a real interest for improving their catalytic performance [5]. 

As with these materials, a reaction which is not catalyzed by even the strongest liquid acids under comparable conditions proceeds rapidly the problem arises of whether or not there are principal differences between liquid and solid acids. It was already mentioned that carbenium ions are well identified by NMR in liquid acids but not on solid surfaces. The alkoxy groups identified by Kazansky might suggest that it is more difficult to form carbenium ions on solids, but this assumption obviously fails to explain the superior activity of some solid acid catalysts in butane isomerization. 

It was found that by treating either n-butane or isobutane with 10 mole % deuterium bromide-aluminum bromide catalyst for 20 hours at 25°, no isomerization of the butanes occurred and only 6 and 9.5% of the deuterium exchanged with n-butane and isobutane, respectively. When, however, 0.1 mole % butenes was added to n-butane and the isomerization reaction was carried out under the same experimental conditions, over 40% of the butane isomerized to isobutane and 92% of the deuterium underwent an exchange reaction. These results indicate clearly that olefins take an active part in isomerization. The results obtained are in agreement with the proposed mechanism of isomerization. 

A kinetic model for n-butane isomerization over sulfated zirconia catalysts is proposed involving two types of active sites on the catalyst surface and a bi-molecular reaction mechanism. Deactivation kinetics are included in which the two different active sites deactivate at different rates. The proposed model more accurately captures the activity trends observed experimentally with respect to time on stream behavior compared to a single site model with deactivation. 

Sulfated zirconia has been widely studied as a hydrocarbon conversion catalyst (1,2), especially for n-butane isomerization. The catalyst is most active when an optimal ratio of Lewis and Bronsted sites, resulting from its state of hydration, are present on the catalyst surface (3). The presence of both acid sites has shown to be critical in the low temperature activity of sulfated zirconia (4). 

Following the conjecture that two separate active sites could be responsible for the activity trends on sulfated zirconia catalysts, an elementary step kinetic model of the reaction with deactivation is proposed. The model involves the presence of two active sites on the catalyst surface, the bimolecular mechanism for n-butane isomerization for which evidence has been shown (7), and deactivation of both active sites. To our knowledge, a detailed model based on a mechanistic pathway for the bimolecular mechanism with a kinetic description of the deactivation of the two sites at different rates has not yet been proposed. 

Deactivation studies on conventional sulfated zirconia at varying partial pressures of n-butane in helium showed two regions of deactivation as reported in the literature. Reaction studies with hydrogen as the diluent gas led to distinct changes in the two regions of activity. These changes could be rationalized by the presence of two active sites on the catalyst surface. A two active site, elementary step bimolecular reaction model is proposed for n-butane isomerization over optimally hydrated sulfated zirconia catalysts. Experimental data... 

Carbon monoxide has been widely used to characterise the hydroxyl groups of solid acids, since its interaction with acidic protons modifies the strength of the C-O bond.I Analysis by IR spectroscopy is a tool of choice to obtain information on the surface of the catalyst. CO acts as a basic probe and according to previous studies, its complexation with Lewis acid sites (LAS) decreases the acidity and consequently, the activity of the catalyst in n-butane isomerization. However, it is uncertain if the interaction of CO with LAS is the main origin of catalyst deactivation, since n-butane isomerisation is also inhibited by CO in the absence of LAS. 

Since SSITKA can decouple the apparent rate of reaction into the contribution from the intrinsic activity ( the reciprocal of surface residence time of intermediates) and the nrnnber of active sites ( surface concentration of intermediates), the cause of deactivation of a catalyst during reaction can often be revealed. SSITKA has been used in a number of studies for this purpose. Catalyst deactivation during n-butane isomerization and selective CO oxidation are good examples. Deactivation studies are conducted by collecting isotopic transient data at particular times-on-stream as deactivation occurs. 

The feed to a butane (C4) isomerization unit should contain maximum amounts of n-butane and only small amounts of isobutane, pentanes, and heavier material. The feed is dried, combined with dry makeup hydrogen, and charged to the reactor section at 230 to 340° F (110 to 170°C) and 200 to 300 psig (1480 to 2170 kPa). H2 is not consumed by isomerization reactions, but it suppresses polymerization of the olefin intermediates that are formed during the reaction. A small amount of organic chloride promoter, which is added to maintain catalyst activity, converts completely to HCl in the reactors. 

It is known that sulfated zirconia, especially promoted with Fe, Mn, and other cations exhibits a high activity in n-butane isomerization [21-23]. The attempt to prepare a composite material such as copper oxide-promoted sulfated zirconia (CuSZ) supported on MCM-41 by impregnation [24] seems to be successful. -Hexane isomerization on the CuSZ/MCM-41 catalyst resulted in 2-MP, 3-MP and 2,3-DMB are the major isomerization products. 

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