Abstract

Climate change is considered one of the most pressing challenges facing humankind today. The Earth’s average surface temperature has increased by almost two folds since the pre-industrial era, mainly because of the anthropogenic release of carbon dioxide into the atmosphere. Lately, circular carbon economy (CCE) has emerged as an effective framework to manage and reduce carbon dioxide emissions. One of the key enablers of CCE is electro-fuels (e-fuels), synthetic liquid fuels made from captured carbon dioxide and electrolytic hydrogen.

This thesis addresses one of the reactions families with potential to facilitate the production of e-fuels, which is alkene oligomerization. The reaction is typically catalyzed by zeolites, crystalline solids with well-defined pore openings that confer shape selectivity effects related both to the diffusion of reactants and products and to the formation of transition states. The thesis begins with a general introduction to zeolites and shape selectivity in zeolites. Subsequently, diffusion in zeolites, its theory and measurement, is thoroughly discussed. The application of zeolites in alkene oligomerization is then examined through three main perspectives.

First, the effects of reactant transport limitations on alkene oligomerization turnover rates were examined. Turnover rates varied significantly with crystallite size in medium pore zeolites ZSM-5 and ZSM-22, a strong indication of the presence of reactant transport limitations. Consistent with reaction-transport formalisms, reactant transport limitations were most intense at high oligomerization temperatures. Another examined aspect of transport limitations was their effects on branching. In ZSM-5, for example, transport limitations were found to favor the formation of linear oligomers over branched ones, an effect best interpreted in terms of product shape selectivity. Therefore, the degree of branching in oligomers was a strong function of crystallite size.

Second, the reactivity of silicalite-1, a siliceous variant of ZSM-5 which lacks traditional zeolite Brønsted and Lewis acidity, in alkene oligomerization was investigated. Silicalite-1 was found to be mildly reactive towards alkene oligomerization. Characterization by several techniques revealed that the origin of this reactivity is H-bonded silanol groups, acting as weak Lewis acid sites. Increasing the accessibility to H-bonded silanol groups by means of desilication improved significantly the alkene oligomerization reactivity.

Finally, the mechanism of formation of deactivating species in alkene oligomerization was studied. Two main deactivation pathways were identified in alkene oligomerization over ZSM-5 and zeolite beta. The first deactivation pathway proceeds by the formation of highly alkylated benzenes such as 1,3-di-tert-butylbenzene which are too bulky to leave zeolite pores and end up retained as coke species. Representing an onset of deactivation, the formation of dienes and cyclopentenyl cations as hydrocarbon pool species drives the formation of these alkylated benzenes. The second deactivation pathway proceeds by the formation of slow diffusing oligomers such as tetradecane and hexadecane, whose long backbones render them entrapped within zeolite pores. The coupling of monoenyl carbenium ions is responsible for formation of these oligomers.

Overall, this thesis advances the current understanding of how phenomena such as transport limitations and deactivation influence alkene oligomerization turnover rates and selectivity. Furthermore, the thesis leads to a reconsideration of the origin of reactivity in zeolites in the context of alkene oligomerization.