Zeolites alkane isomerization

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

Much progress has been made in understanding the catalytic activity of zeolites for several type of reactions. The number of reactions catalyzed by zeolites has been extended, and new multi-component polyfunctional catalysts with specific properties have been developed. In addition to cracking and hydrocracking, reactions such as n-alkane isomerization, low temperature isomerization of aromatic C8 hydrocarbons, and disproportionation of toluene are industrially performed over zeolite-containing catalysts. Moreover, introduction of various compounds (C02, HCl) into reaction mixtures allows one to control the intensity and selectivity of the reactions. There are many reviews on the catalytic behavior of zeolites and even more original papers and patents. This review emphasizes the results, achievements, and trends which we consider to be most important. 

The natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

We conclude that there is no evidence for WZ catalysts having superacidic properties or sites with the acidic character that would be necessary for initiation of catalysis by alkane protonation. In as much as WZ catalysts are some four orders of magnitude more active than zeolites for alkane isomerization,26 it is clear that there is no one-to-one correlation between acid strength of WZ and its catalytic activity. We therefore infer that although the acidity of WZ catalysts is important in alkane conversion catalysis, the reaction is most likely initiated by a reaction other than protonation of the alkane by the catalyst or a species formed from it. 

In order to reduce the reaction temperature of acid catalysed alkane conversion reactions one can reduce the temperature by replacing carbonium formation by a route via the carbenium ion by protonation of alkenes generated by metal-catalysed (group 8-10 metals, e.g. Pt, Pd) dehydrogenation of alkanes. The metals can be readily dispersed in the micropores of a zeolite. A lowering of the reaction temperature is especially useful for alkane isomerization. A low temperature favours the branched product and inhibits consecutive reactions. 

The possibility of selective reactions on the external surface of zeolites, more exactly at the pore mouth, was recently addressed by Martens et al. (56, 57) to explain the unusual selectivity of several intermediate pore size zeolites and especially of ZSM-22 (TON) in long chain n-alkanes isomerization. Over PtHTON, this isomerization is very selective towards monobranched isomers even though... 

The catalytic properties associated with the molecular shape-selectivity exhibited by ZSM-5 are now well known. Recent work by Martens et al. (1995) has revealed that the external surfaces of zeolite crystals have also to be considered as potential shape-selective environments. Thus, strong evidence has been obtained for a lock-and-key model, which involves a form of pore mouth catalysis with bulky long-chain molecules that cannot penetrate into the intracrystalline micropores. The proposed lock-and-key model for n-alkane isomerization over ZSM-22 zeolite (with tubular pore openings of 0.55 x 0.45 nm) seems likely to be valid for other catalytic reactions. 

The basic concept underlying alkylation reactions of aromatics is the formation of a stabilized carbocation able to attack nucleophilic substrates. Hydrocarbon cracking and hydrocracking, alkane isomerization, and olefin alkylation are important processes based on related alkane carbocation chemistry in the production of various types of hydrocarbons such as the branched ones for high octane gasolines. Zeolites and metal oxides are the preferred catalysts. 

Microporous and, more recently, mesoporous solids comprise a class of materials with great relevance to catalysis (cf. Chapters 2 and 4). Because of the well-defined porous systems active sites can now be built in with molecular precision. The most important catalysts derived from these materials are the acid zeolites. The acid site is defined by the crystalline structure and exhibits great chemical and steric selectivities for catalytic conversions, such as fluid catalytic cracking and alkane isomerization (cf. Chapter 2). In Section 9.5 we discuss the synthesis of zeolites and, briefly, of mesoporous solids. 

Tungstated zirconia (WZ) catalysts have been proposed as viable candidates for alkane isomerizations, especially those heavier than C4 [1]. In this work, alcohol dehydrations were used to rank the acidity of WZ with respect to that of HY zeolite, and to evaluate the ability of these catalysts to resist coking during a reaction that yields an olefin molecule as the primary product. The results from temperature-programmed reaction and infrared spectroscopy studies allowed us to gain some insight into the relative stability of WZ. 

Recent results are presented illustrating principal mechanistic differences between alkane isomerization in liquid acids and over solid acids, including bifunctional catalysts. Isotopic labeling shows that butane isomerization over solid acids proceeds preferentially as a bimolecular process, i.e. via a Cg intermediate, which subsequently decomposes, preferentially into two iso-Cn structures. Bronsted acid sites in zeolites form chemical bonds with metal clusters. The resulting metal-proton adducts function as "collapsed bifunctional sites". 

An inspection of the industrial use of zeolites as catalysts shows, however, that only a rather limited number of zeolite topologies are currently used in major industrial processes. Among the more important ones are ultrastable Y (USY) (FAU), rare-earth-exchanged faujasite-type (X, Y) (FAU) andZSM-5-type (MFI) zeolites in fluid catalytic cracking (FCC) of oil fractions [4] noble-metal-loaded U SY for hydroisomerization and hydrocracking of naphtha feedstocks [5] mordenite (MOR) and zeolite Omega (MAZ) -based catalysts for C4-C6 alkane isomerization [6] zeolites ZSM-23 (MTT), ZSM-35 (FER), ZSM-5 for selective oil dewaxing [7] ZSM-5, silicalite (MFI), MCM-22 (MWW), Beta-type (BEA) zeolites for aromatics alkylation to yield ethylbenzene, p-xylene. 

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. 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

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... 

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). 

At higher temperatures, C—H and C—C bonds may be similarly broken. Thus, zeolite catalysts may be used for (i) alkylation of aromatic hydrocarbons (cf. the Friedel-Crafts reactions with AICI3 as the Lewis acid catalyst), (ii) cracking of hydrocarbons (i.e., loss of H2), and (Hi) isomerization of alkenes, alkanes, and alkyl aromatics. 

In isomerizations, zeolites have special merit in their ability to admit straight-chain but not branched-chain molecules into the pores. Thus, normal alkanes up to n-Ci4H30 can penetrate the pores of zeolite 5A to reach the cavities where the C—C or C—H bonds may be catalytically broken the fragments, on reemerging from the pores, can recombine as isomerized molecules. The reverse process is not possible, since the isomers, having... 

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