Zeolite paraffin isomerization

Par-Isom [Paraffin isomerization] A process for isomerizing light naphtha in order to improve the octane number. The proprietary catalyst was developed by Cosmo Oil Company and Mitsubishi Heavy Industries, and the process was developed by UOP. The oxide catalyst is claimed to be more efficient than zeolite catalysts currently used for this process. 

Total Isomerization Also called TIP. An integrated process which combines light paraffin isomerization, using a zeolite catalyst, with the IsoSiv process, which separates the unconverted normal paraffins so that they can be returned to the reactor. Developed by Union Carbide Corporation and now licensed by UOP. The first plant was operated in Japan in 1975 by 1992, more than 25 units had been licensed. 

Paraffin isomerization of heavy alkane feeds is often used to alter the cloud or pour point of diesel or lube fractions. Catalysts for this reaction are almost always dual-function catalysts of Pt supported on a one-dimensional zeolite. Using a onedimensional zeolite allows control of the isomerized product to contain few branches, usually methyl branches (Table 12.4). 

Metal-Zeolite Catalyzed Light Paraffin Isomerization... 

As in previous conferences, the section on catalysis contains the most papers. A general review of the different reactions which can be catalyzed by zeolites is presented by Kh. M. Minachev. H. W. Kouwenhoven discusses the isomerization of paraffins on zeolites. Cracking, isomerization, and electron transfer reactions are discussed in several papers. Correlations between particular activities and physicochemical properties are covered. Selectivities related to crystal size and molecular shapes are also studied. Most of the work is still done on modified Y zeolites, but mor-denite and erionite also receive attention. 

The application of zeolitic materials cls catalysts in paraffin isomerization is discussed. Particular attention is given to catalyst preparation variables such as sodium removal for zeolite Y and mordenite. Dual function catalysts based on these zeolites are compared with respect to activity. A reaction mechanism for paraffin isomerization over zeolitic dual function catalysts, on the basis of literature and own data, is presented. 

In the reaction mechanisms described above the acidity of the catalyst plays an important role. Zeolites can be converted into the H+ form and as such are powerful catalysts for acid-catalyzed reactions. We discuss below some aspects of isomerization catalyst preparation to demonstrate factors which influence the activity of catalysts based on zeolites. In this discussion we are concerned with zeolite Y and mordenite. Data on paraffin isomerization over dual function catalysts besed on other zeolites are scarce, and no data have been published showing that materials like zeolite X, zeolite L, offretite, zeolite omega, or gmelinite can be converted into catalyst bases having an isomerization activity comparable with that of H-zeolite Y or H-mordenite. 

It has been claimed that noble metal dual function catalysts based on H-mordenite are more active for paraffin isomerization than their counterparts based on H-zeolite Y (25). For both zeolites the isomerization activity depends strongly on the degree of sodium removal and comparison of low sodium Pd-H-mordenite and low sodium Pd-H-zeolite Y for isomerization of n-hexane at 250° C shows that both materials have about the same activity (Table IV), the Y sieve based material being slightly more active. 

Short Chain Paraffins Isomerization on Pt/heta Catalysts. Influence of Framework and Extraframework Zeolite Composition. 

It is found that Mode E behaves similarly to the zeolite free Pt-Re/Al203 Both catalysts have a relatively high proportion of isomer products which could be formed over the metal surface via a bond-shift mechanism [8]. Isomers are formed by doublebond isomerization and skeletal isomerization reactions at both the acid sites of the alumina support and the metal sites. The later provides a dehydrogenation-hydrogenation function and the acid sites an isomeiization function for the olefins to dehydrogenate from paraffins over the metal function, since it is known that olefin isomerization proceeds much quicker than the respective paraffin isomerization [8]. On the other hand, branched paraffins are less easily cracked than linear ones [10]. Therefore, once isomers are formed over conventional reforming catalysts, they are likely to be the final products. Evidently, the isomerization of paraffin requires the metal function in the bimetallic catalyst, and so does the paraffin aromatization. This can also explain the obseiwed decrease in the isomers and aromatics production with time-on-Hne since it is well- known that coke preferentially deposits on a metal surface first [14]. 

Paraffin isomerization over dual function catalysts based on zeolite Y and mordenite has been reviewedand a reaction mechanism was proposed in which olefin-paraffin equilibrium is established and carbonium ions are formed from both paraffins and olefins. The isomerization of n-hexane and hydrocrack-... 

These catalytic properties contrast sharply with those of medium pore zeolites such as LZ-105 and with silicalite. Thus, medium pore SAPO s are considerably less active than LZ-105 for olefin, paraffin and aromatic conversions when compared at the same temperature. However, they are more selective for olefin and paraffin isomerizations when evaluated at comparable conversions. 

Travers, Ch., Marcilly, Ch. Roots, F., Des Courieires, Th., Perrand, A., Fajulat, F., and Boulet, M., (1991), "New Zeolite Catalyst for Paraffin Isomerization", AIChE Spring Meeting, Houston. 

Quite a few attempts have been made to use HZSM-5 and HZSM-11 zeolites in paraffin isomerization. Eor instance, the effect of the Si/Al ratio on the properties of Ir/Pt-HZSM-5 zeolite in n-pentane isomerization was studied [17]. The increase in the Si/Al ratio decreased the number of strong Bronsted and Lewis acid sites and the catalytic activity in n-pentane isomerization. Ir/Pt-HZSM-5 with a Si/Al ratio of 23 showed the highest activity toward -pentane isomerization. The optimum conditions of -pentane isomerization over Ir/Pt-HZSM-5 gave the following values of the -pentane conversion, isopentane selectivity, and isopentane yield 63.0%, 98.2%, and 61.9%, respectively. 

MIDW [Mobil isomerization dewaxing] A petroleum refining process which improves yield and quality by isomerizing and selectively cracking paraffins in waxy oils. The catalyst is a noble metal, supported on a zeolite. Developed by Mobil Corporation from 1991 to 1996. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

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