Selectivity - alkane isomerization

S. Ruba, B. C. Gates, P. Vijayanand, R. R. Grasselli, and H. Rnozinger, An active and selective alkane isomerization catalyst iron - and platinum - promoted tungstated zirconia, J. Chem. Soc. Chem. Commun. 321-322 (2001). 

Kuba, S., Gates, B.C., Grasselli, R.K., and KnSzinger, H. An active and selective alkane isomerization catalyst Iron- and platinum-promoted tungstated zirconia. 

Previous studies have concluded that 4-, 5-, and 6-coordinate W species are present on AI2O3 and Ti02 supports [17,30] depending on surface W density and on hydration state. The present study has detected W03-like distorted octahedral domains at all surface densities and irrespective of hydration on Zr02. These species catalyze alkane isomerization reactions with much higher turnover rate and selectivity than dispersed WOx moieties on alumina or titania. 

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). 

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. 

An Estimate of Fractional Selectivities for Alkane Isomerization Reactions over Metals ... 

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. 

HETEROGENEOUSLY CATALYZED ALKANE ISOMERIZATION -TOWARDS 100% SELECTIVITY... 

Keywords alkanes-isomerization iron-promoter n-pentane-isomerization platinum-promoter selectivity tungsten oxide zirconia-tungstated-acidity zirconia-tungstated-redox properties... 

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. 

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. 

In this section we present experimental evidence for a bifunctional alkane isomerization mechanism obtained by selective poisoning of the acidic sites of Pd-NiSMM with pyridine, which was pulse-injected into the liquid hydrocarbon feed stream. The possibility of additional poisoning of the metallic sites was checked by studying the hydrogenation of benzene and the isomerization and ring opening of methylcyclopentane (MCP). 

In the work of the author and his associates on bimetallic catalysts comprising various combinations of Group VIII and Group IB metals, it was discovered that the activity of the Group VIII metal for hydrogenolysis reactions of hydrocarbons was decreased markedly by the presence of the Group IB metal (11-15). It was shown that the inhibition of hydrogenolysis leads to improved selectivity for alkane isomerization reactions (11) and for reactions in which saturated hydrocarbons are converted to aromatic hydrocarbons (12,14,15). Interest in bimetallic catalysts increased markedly with the discovery of this selectivity phenomenon. 

In all of these studies, the presence of the Group IB metal, copper or gold, with the Group VIII metal, nickel or palladium, led to inhibition of hydrogenol-ysis on the Group VIII metal. As a consequence, selectivities for isomerization of the n -alkanes to branched alkanes, and for aromatization of methylcy-dopentane to benzene, were improved. The studies of the Netherlands workers on nickel-copper alloys, like those conducted in our laboratories on the same system, eliminated the possibility of complications caused by the presence of a carrier and were therefore useful for clarifying the selectivity phenomenon. 

The reactivity pattern displayed by platinum crystal surfaces for alkane isomerization reactions is completely different from that for aromatization. Studies revealed that maximum rates and selectivity (rate of desired reaction /total rate) for butane isomerization reactions are obtained on the flat crystal face with the square unit cell. Isomerization rates for this surface are four to seven times higher than those for the hexagonal surface. Isomerization rates are increased to only a small extent by surface irregularities (steps and kinks) on the platinum surfaces (Figure 7.39). 

Further transformations of these intermediates lead to either olefins (dehydrogenation) or branched alkanes (isomerization). However, usually heterogeneous processes are not highly selective and afford simultaneously products of dehydrogenation and isomerization (as well as of other reactions, e.g., cracking and condensation). 

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. 

ZrOj and sulfatcd ZrOj have also been used as suppons for HPAs. In this way. Pt promoted Alo 3Ho.sPWi204o on Zr02 gives excellent activity and selectivity for Cy+ alkane isomerization [P1,P2], and it is a better catalyst than the pure acid supported on either Zr02 or Si02. In the case of... 

---