Catalysts isomerization

Isomerization processes developed slowly because of the low demand for higher octane numbers and operational problems with aluminum chloride, the first catalyst to be developed. Despite high activity at 115-120°C, aluminum chloride has the major disadvantages of the formation of sludges and acid corrosion of equipment. 

Note Exhact n-hydrocarbons with a shape-selected zeolite. 

The introduction of dual-function catalysts for naphtha lefomiing and the demand for high-octane gasoline led to further interest in isomerization. The platinum/alumina (cUorided) catalysts were a success despite the resulting lower conversion to high-octane products from the need to operate at higher temperatures. The Shell Hysomer process, which used a 5A-zeolite to separate low-octane paraffins from the product allowed operators to recycle unconverted feed and achieve almost 100% conversion.  

A further important development was the use of hydrogen-exchanged mor-denite zeohte as the catalyst support The zeolite was more stable and water tolerant than the chlorided alumina and did not need chloride addition during operation. The zeohte catalyst had a lower activity than that of alumina, but the need to operate at a shghtly higher operating temperature was acceptable since the normal paraffins could be recycled. Other zeolites, such as p-zeolite, have also been used as the acid support. 

Operating conditions for the different catalysts are given in Table 6.22. Hydrogen is usuahy added to minimize coke formation but is not used in the reactions taking place. 

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Historically, the isomerization catalysts have included amorphous siUca-aluminas, zeoHtes, and metal-loaded oxides. AH of the catalysts contain acidity, which isomerizes the xylenes and if strong enough can also crack the EB and xylenes to benzene and toluene. Dual functional catalysts additionally contain a metal that is capable of converting EB to xylenes. 

Extraction of C-8 Aromatics. The Japan Gas Chemical Co. developed an extraction process for the separation of -xylene [106-42-3] from its isomers using HF—BF as an extraction solvent and isomerization catalyst (235). The highly reactive solvent imposes its own restrictions but this approach is claimed to be economically superior to mote conventional separation processes (see Xylenes and ethylbenzene). 

Bisphosphites such as (7) combine excellent reactivity, straight-chain selectivity, and high resistance to the typical phosphite degradation reactions (29). Further, the corresponding 0x0 catalysts are excellent olefin isomerization catalysts so that high normal-to-branched isomer ratios are obtained even from internal olefins, enabling, in certain instances, the use of inexpensive mixed isomer olefin feedstocks. 

Chloro-l,2-butadiene [25790-55-0] is mainly of historical iaterest (2). It is formed from vinylacetylene and HCl ia the absence of an isomerization catalyst. In the usual process for chloroprene usiag cuprous chloride, a portion of this isomer may be formed initially and then isomerize, but most of the chloroprene is apparently formed directly by the addition. 

The demand for aviation gasoline during World War II was so great that isobutanc from alkylation feedstock was insufficient. This deficiency was remedied by isomerization of abundant normal butane into isobutane using the isomerization catalyst aluminum chloride on alumina promoted by hydrogen chloride gas. 

Cobalt catalysts such as HCo(CO)4 are widely used for hydroformyla-tion of higher alkenes, despite the higher temperatures and pressures required. The main reason for this is that these catalysts are also efficient alkene isomerization catalysts, allowing a mix of internal and terminal alkenes to be used in the process. Catalyst recovery is more of a problem here, involving production of some waste and adding significantly to the complexity of the process. A common recovery method involves treating the catalyst with aqueous base to make it water soluble, followed by separation and subsequent treatment with acid to recover active catalyst (4.3). 

A non-acidic isomerization catalyst system has unexpectedly emerged from recent studies by French workers [4] in the area of Mo-oxycarbides. Although at an early stage of development, these new materials exhibit high selectivities for the isomerization of paraffins such as n-heptane. An alternative non-carbenium ion mechanistic route to achieve isomerization of higher alkanes could potentially overcome some of the limitations of conventional solid acid based catalyst systems. 

Corma, A., Serra, J.M. and Chica, A. (2003) Discovery of new paraffin isomerization catalysts based on S042-/Zr02 and WOx/Zr()2 applying combinatorial techniques. Catal. Today, 81, 495. 

Serra, J.M., Chica, A. and Corma, A. (2003) Development of a low temperature light paraffin isomerization catalysts with improved resistance to water and sulphur by combinatorial methods. Appl. Catal. A Gen., 239, 35. 

A new isomerization catalyst can be prepared by the modification of silica-supported nickel with tetrabutyltin. This catalyst is capable of the selective isomerization of 3-carene to 2-carene.269... 

Figure 24 shows the cfs-butene isomerization over zinc oxide as a function of time at room temperature (7/). On a per unit area basis the initial rate at room temperature is 4 X 1010 molecules/sec cm2, a rate roughly one third that reported for alumina (69). Since the activation energy for alumina is less than that found for zinc oxide, this means that zinc oxide is comparable (on a per unit area basis) to alumina as an isomerization catalyst at slightly higher temperatures. 

Tacticities of the polypropylenes above 60% [mmmm] highlights the tendency of the backward oriented substitution (4a) to produce plastomeric materials. During experiments in liquid propylene, no similarity is observed between the two isomeric catalysts. Catalyst 4b/borate does not follow the same trend in the catalytic performance as previously mentioned, but leads to polypropylenes with sufficient amount of isotactic sequences and relatively high molecular weight for the design of plastomeric polypropylenes. 

Not all acids are equally active isomerization catalysts. With zeolite H-BEA, nearly identical selectivities are achieved when the feed is 1-butene instead of 2-butene (48). In general, even mildly acidic zeolites are excellent catalysts for double-bond shift isomerization. Sulfuric acid also produces nearly identical... 

The major distinction between the two classes of catalysts is that the members of the former group are olefin isomerization catalysts, while the cobalt cyanide and the chromium catalysts are not23-25. 

The isomerization catalysts are hydride complexes, and they can convert the unconjugated dienes or polyenes to conjugated systems through double-bond migration. This process occurs by an M—H addition-elimination process. 

The rather low concentration of the desired p-xylene component in the Parex unit feed means a large fraction of the feed stock contains other A8 components that are competing for adsorption sites in the adsorbent zeoHte cages. Due to this typically lean feed, a significant hike in the Parex unit capacity can be obtained by even a small increase in the composition of the p-xylene. Techniques to increase the p-xylene feed concentration include greater dealkylation of the ethylbenzene in the Isomar unit by converting from an ethylbenzene isomerization catalyst to... 

Table 12.3 Zeolitic pentane and hexane isomerization catalysts. 

Introduction of Pt significantly enhances zeolite isomerization catalyst stabiUty and alters the reaction pathways. The Pt/acid ratio not only changes the isomeriza-tion/cracking ratio, but also changes the ratio of mono/di-branched isomers in Pt/Y [14]. High Pt dispersion and close proximity to acid sites correlate with high n-hexane conversion as well as high isomerization selectivity [20, 21]. 

Historically, the earliest C8 aromatic isomerization catalysts tended to use amorphous supports with a halogen such as chloride or fluoride. Due to water sensitivity and corrosion issues, these were replaced by large-pore zeolites such as mordenite. The larger pore size was more favorable toward bimolecular transalkylation, whereas the chlorided alumina support tended to promote cracking. In both... 

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