Martin Franciscus Maria Post

RELATION BETWEEN PROPERTIES AND PERFORMANCE OF ZEOLITES IN PARAFFIN CRACKING

The effects of zeolite structure, acid site density, and reaction temperature on the mode of n-hexane cracking have been studied. The results can be consistently explained in terms of the occurrence of two different cracking routes, viz. the classical β-scission route and the monomolecular protolytic pathway. The relative contribution of each of the two cracking routes is expressed by the “cracking mechanism ratio” (CMR): a high value of this index points to a relatively high contribution of the protolytic cracking route, whereas a low value indicates that the classical β-scission route is the main cracking pathway. With increasing temperature, decreasing aluminium content, and decreasing pore dimensions the relative contribution of the (monomolecular) protolytic cracking route increases as compared to the (bimolecular) classical route. On the basis of these results the variation of the constraint index (i.e., the ratio of the first-order rate constants for conversion of n-hexane and 3-methylpentane) with aluminium content and reaction temperature can be easily rationalized. Furthermore, the present results also provide an explanation for the change in the activation energy of n-hexane cracking with aluminium content in zeolite ZSM-5 (MFI). Finally, it is tentatively concluded that the classical cracking route is favoured by the presence of two adjacent acid sites and by low reaction temperatures. Consequently, the relation between the reaction rate constant and the lattice aluminium content changes with the reaction temperature; whereas at 811 K the cracking constant varies linearly with the aluminium content, at 623 K the order in the aluminium content increases to about 2.

Industrial applications of zeolite catalysis

For several decades zeolites have found application as catalysts in the oil refining and petrochemical industry, because of their superior activity, stability and selectivity in major conversion and upgrading processes as compared with their amorphous equivalents. The continuing drive for higher quality transportation fuels and chemical products in addition to increasing environmental pressures provides opportunities for further enhancement of their role in industrial catalysis. The development of zeolite catalysis is increasingly supported by advances in preparation, characterisation and testing of catalysts for the determination of property/performance relationships. The explosive growth in computational chemistry and step changes in the range of hydrocarbons accessible to quantitative study have firmly established the role of zeolite modelling, and attractive alternatives to experimentation may be imminent. Development cycles will consequently shorten and future applications of zeolites can be envisaged in processing a wider range of feedstocks, in higher selectivity and stability catalysts and in non-acid catalysts.

An Infrared and Catalytic Study of Isomorphous Substitution in Pentasil Zeolites

Abstract The catalytic sites associated with aluminium-, iron- and gallium-containing silicates of MFI structure have been characterized by infrared (IR) spectroscopy and in catalytic experiments in which n-hexane conversion served as a test reaction. The protonic forms of the metal silicates exhibit characteristic absorptions in the v(OH) region which are assigned to M(OH)Si bridging hydroxyls: Fe, Ga, Al in order of decreasing frequency. Rate constants for n–hexane conversion were found to increase linearly with the concentration of a particular trivalent metal ion in the silicate material, and this observation, together with the IR studies, provides direct evidence that Al, Ga and Fe can be incorporated in the MFI framework, giving rise to Bronsted acidity. The catalytic activity expressed per M(OH)Si site decreases in the order Al, Ga, Fe, which is in agreement with the direction of change in IR frequency of the absorption associated with the bridging hydroxyl group. A simple but straight-forward model is offered to account for the variation in acidity observed.

On the dynamics of organic-zeolite interactions: Tetramethylammonium in sodalite

The dynamics of tetramethylammonium (TMA) occluded in a β-cage of zeolite sodalite has been investigated in a comparative molecular dynamics (MD) and n.m.r. ( 13 C, 2 H) study. Concerning deuterium n.m.r., this is the first time that measurements on deuterated organics that are occluded during zeolite synthesis have been performed. The main objective was to understand the dynamics of organic-zeolite interactions on a molecular level, which may enhance to the insight into the role of the organic during zeolite synthesis. MD calculations as well as solid-state deuterium and crosspolarization magic angle spinning n.m.r. results indicate that TMA occluded in the sodalite β-cage has a considerable degree of rotational freedom. MD revealed this rotational freedom to be caused mainly by the dynamics of the zeolite lattice; the distance between diametrically opposed oxygen atoms in the β-cage was found to fluctuate by as much as 0.4 A.

Deposition of iron from iron-carbonyl onto a working Co-based Fischer Tropsch catalyst: The serendipitious discovery of a direct probe for diffusion limitation

During fixed-bed pilot plant runs to test silica-supported cobalt-based Fischer Tropsch catalysts deposition of iron from iron carbonyl was encountered due to some contamination of the carbon monoxide feed gas. From analysis of the used catalysts it turned out that the iron (1-2 %m/m) had been deposited preferentially at the center of the 1.4 mm-sized particles. Furthermore, from transmission electron microscopy it appeared that iron had reacted with silica to form iron hydro-silicates, thus not deteriorating the catalytic performance. The iron distribution over the individual particles is concluded to reflect the intraparticle CO profile originating from diffusion limitation during catalysis. Deposition of iron from Fe(CO) 5 during Fischer Tropsch catalysis can be considered as a probe for diffusion limitation and is a direct experimental evidence for the existence of concentration gradients in catalyst particles according to the classic Thiele-Wheeler concept.

ION-Exchange of Ni2+ IONS in Siliceous ZSM-5

Ion exchange of Ni 2+ and K + ions in siliceous ZSM-5 with widely varying aluminium contents (20 Si/Al 250) has been studied. Whereas regardless of the aluminium content near-stoichiometric exchange can be achieved with the monovalent K + ions, only limited exchange is observed with the divalent Ni 2+ ions. With decreasing aluminium content the degree of Ni 2+ exchange drops markedly. These results are interpreted using a model which relates the position of the extra framework Ni 2+ ions to a specific configuration of the aluminium ions with regard to each other in the lattice.