Active Acid Sites in Zeolite Catalyzed Iso-butane/cis-2-Butene Alkylation (2004)

Control of emissions of olefins, sulfur and aromatic compounds, especially benzene, from automotive gasoline engines becomes every day more strict due to new environmental standards. That implies that, in the refineries, the pool gasoline components have to be adjusted to meet the required specifications. Branched alkanes with high octane number are the more suitable component of environmental friendly gasoline. They are industrially produced by alkylation of iso-butane with C3-C5 alkenes. Therefore, it is expected, that the demand for alkylation catalysts will increase in the coming years. While the products from alkylation are perfect gasoline components, the catalysts used currently, sulfuric and anhydrous hydrofluoric acids, are toxic and corrosive. That has stimulated the effort of many research groups to the development of more environmental friendly and safer alternative process, the majority focused on zeolite-base catalysts. Although most of the investigations have shown that the product distribution in the alkylate obtained with zeolites as catalysts is comparable to that from industrial liquid acid technology, the rapid deactivation of zeolite has impeded their implementation on large-scale. As a consequence, and due to the relatively high cost of zeolitic catalysts (compared specially to those of sulfuric acid), an efficient and economic regeneration method of the deactivated zeolite must be designed. Moreover, in order to develop a competitive process based on zeolites, a deep knowledge of the physicochemical properties of these catalysts, especially their acidity, associated with the understanding of the effect of reaction parameters, have to be reached. In this thesis special attention was paid to understand the nature of the acid sites in FAU- and BEA-type zeolites. In Chapter 2 the acidic properties of La-X zeolite at different preparation steps were thoroughly characterized by in situ IR spectroscopy, under vacuum and flow conditions. It was found that the rehydration of the materials calcined only one time affects the distribution of different hydroxyl groups. Rehydration leads to dealumination and, as consequence, the concentration of silanol groups (IR band at 3740 cm-1) and of Lewis acid sites increases. This in turn results in an enhanced thermal stability of the rare earth zeolite for the next steps of catalyst preparation. Chapter 3 dealt with the comparison between La-X based catalysts and different forms of Y zeolite, H-EMT and H-BEA based catalysts. IR characterization of the acid sites present in these zeolites has shown that the kind of hydroxyl groups responsible for Bronsted acidity is highly dependent on the zeolite investigated. The differences in density, location and strength of the Bronsted acid sites led to catalyst lifetimes in iso-butane/cis-2-butene alkylation varying from 1 to 13 hours. Chapter 4 was focused on the influence of the activation temperature on the catalytic performance of La-X based catalysts. IR spectroscopy, 1H NMR and TPD experiments have evidenced the presence of physisorbed water, its amount being dependent on the activation temperature. IR spectroscopy of adsorbed pyridine has showed that water on the catalyst does not modify appreciably the concentration of Bronsted acid sites. Adsorption of iso-butane/cis-2-butene monitored by IR spectroscopy, along with the activity tests carried out with the sample activated at different temperatures, has shown that the presence of water influences mainly the formation of oligomers. More C5-C7 products were observed on samples activated at higher temperatures.