Zeolites isomerization

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]. 

The earliest industrial zeolitic isomerization process was the Hysomer process, formerly offered for license by Shell. Currently UOP offers a zeolite- and Pt-con-taining catalyst HS-10 in the fixed-bed UOP TIP process [3]. A similar catalyst Hysopar was introduced by Sud-Chemie [22] in the CKS Isom process (Cepsa-Kellogg-Sud Chemie). Recently there were reports of IMP-02 and CI-50 commercial catalysts from China [23] and Russia [24]. 

Gusher, N.A. (1997) UOP TIP and once-through zeolitic isomerization processes, in Handbook of Petroleum Refining Processes, 2nd edn (ed. R.A. Meyers), McGraw-Hill, pp. 9.29-9.39. 

Hysomer [Hydroisomerization] A process for converting K-pentane and K-hexane into branched-chain hydrocarbons. Operated in the vapor phase, in the presence of hydrogen, in a fixed bed of a mordenite catalyst loaded with a platinum. Developed by Shell Oil Company and licensed worldwide through UOP. Used in the Total Isomerization process. Now called the UOP Once-Through (OT) Zeolitic Isomerization process. 

It follows from the above results that the presence of metal cations mostly in the zeolite intersections (MeH-ZSM-5) increases the zeolite para-selectivity, while their contribution to the zeolite alkylation activity is not significant. On the other hand, the metal cation presence in the "surface" sites (FeSiH-ZSM-5, Mn O H-ZSM-S) substantially enhances the zeolite isomerization activity. As the para-selectivity of the MeH-ZSM-5 zeolites is higher than that of the pure H-ZSM-5, the metal cation location in the channel intersections should cause steric hindrances owing to the diameter of the metal cations (Fe 0.64 S, Mn 0.80 2 and Al 0.50 2) this effect (which is also necessarily affected by the number of cations present in the zeolite) was not found with the relatively small A1 cation. Then the contribution of the metal cation to the transport limitation of the bulkier isomers and/or to the selectivity of the initial alkylation step should exceed the contribution of the cations to the isomerization activity. 

Isomerization of 1-acetyl-2-methoxynaphthalene was investigated over HFAU, HBEA and HMFI zeolites (batch reactor, T=120°C). Due to its pore size, HMFI was inactive for isomerization while HFAU is about 3 times more active than HBEA. This can be attributed to the easier desorption of the isomers from the HFAU pores. However, the selectivity of 2-acetyl-6-methoxynaphthalene (the desired isomer) is favoured over HBEA. Analysis of the compounds retained in the zeolite pores show that the reaction occurs inside the micropores of the zeolites. Indeed, the desired isomer was found to be retained in the pores of HMFI showing that even for this zeolite, isomerization occurs in its micropores and that the desorption of the reaction products appears to be the limiting step. 

Scheme 7-1 Bifunctionality of metal-doped zeolites isomerization and hydrogenation...

Zeolite and Molecular Sieve-Based Process. Mobil has commercialized several xylene isomerization processes that are based on ZSM-5. Amoco has developed a process based on a medium-pore borosiUcate molecular sieve. 

Spectroscopy at variable temperatures enables us to reveal linkage isomerism of adsorption, when certain molecule fonu with the same site two or more complexes with different geometry and chemical properties. The most studied so far is the case of CO in zeolites, when besides the usual C-bonded complexes with the cations or OH-groups, energetically unfavorable O-bonded complexes ai e formed. 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

Upon passing bromobenzene and hydrogen over zeolite Pt-H-beta dehydrobromination followed by hydrogenation and isomerization takes place. In this way undesired aromatic bromides can be recycled. 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

The scope and mechanism of the isomerization of arylamines to methyl-substituted aromatic heterocycles have been studied. Aniline, toluidines, naphthylamines and m-phenylenediamine all gave the corresponding ortho-methyl-substituted aza-aromatics when exposed to high NHj pressure and elevated temperature in the presence of acid catalysts, e.g., zeolites. The yiel of pyridines formed by this process range from low to moderate <95JC(155)268>. 

Separation of isomers is an application where zeolite membranes could be specifically interesting because of their well-defined pores that lead to molecular sieving effects. An application that is often considered is the xylene isomerization and related reactions. 

Table 10.2 Performance of several zeolite membrane reactors in the xylene isomerization reaction. 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

The carbeniiun ion so formed then reacts in the ICC 1 manner except perhaps for not abstracting a hydride ion from another alkane. Although initial views that zeolites in general were super acids have come into question, definite super acids have been found such as calcined H2S04 Zr(0H)4 which catalyze the isomerization of alkanes at low T. 

Acid-catalysed rearrangement of epoxides is another widely used reaction in the fine chemicals industry. Here again the use of solid acid catalysts such as zeolites is proving advantageous. Two examples are shown in Fig. 2.25 the isomerization of rsophorone oxide (Elings et al., 1997) and the conversion of a-pinene oxide to campholenic aldehyde (Holderich et al., 1997 Kunkeler etal., 1998). Both products are fragrance intermediates. 

Tanabe and Hdlderich (1999) have given an extensive statistical survey of industrial processes using solid acids/bases as catalysts. Over 300 solids and bases have been covered. A variety of reactions like alkylation, isomerization, amination, cracking, and etherification with catalysts like zeolites, oxides, complex oxides, phosphates and ion-exchange resins have been covered. Over 120 industrial processes are referred with 180 different catalysts. 

Isomerization of a-pinene epoxide to campholenic aldehyde, an intermediate for perfumery chemicals, has been carried out elegantly with ultra stable Y-zeolite. 

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