Manipulation of zeolite active site acidity and atomic structure to control hydrocarbon conversion and selectivity

Abstract

Hydrocarbon conversion processes change the molecular weight, structure, and H/C ratio of the feed stream and are vital for supplying fuels, petrochemicals, and many other valuable molecules to the world. These processes include cracking, isomerization, dehydrogenation, hydrotreating, polymerization, and many more. Zeolites – nanoporous crystalline aluminosilicates that have molecular-sized pores and channels – are among the most frequently used catalysts. They can be prepared from just silicon and oxygen – a purely siliceous sample – or with heteroatom substitution into the framework. If the heteroatom is trivalent, such as aluminum, the framework becomes negatively charged and a cation must be added for charge balance. When a proton is used for charge balance, a Brønsted acid site is generated, and the acid strength of the proton can be modulated by the trivalent heteroatom substituted into the framework, affecting the relative rates of the hydrocarbon conversion processes. In this thesis, we investigated the influence of acid strength on high-pressure catalytic cracking and dehydrogenation for aircraft endothermic cooling, as well as methanol conversion to hydrocarbons (MTH), by changing the heteroatom substituted into the zeolite framework. ☐ The first process investigated was high-pressure catalytic cracking of n-pentane on acidic H-[Al]ZSM-5 to produce light olefins. Although ethylene and propylene were the targeted products, these were rapidly consumed via secondary bimolecular reactions, such as hydride transfer and oligomerization, leading to a reduction in reaction endothermicity. The second process investigated was high-pressure dehydrogenation of C5-C7 normal paraffins using supported molybdenum carbide nanoparticles. To reduce the secondary bimolecular reactions described above, weakly acidic H-[B]ZSM-5 was used as a support for molybdenum. By using H-[B]ZSM-5 as a support instead of γ-Al2O3, we were able to maintain high selectivity to the primary dehydrogenation product (>90%), while observing over a 100 percent increase in reactant consumption rates. ☐ The final process investigated was methanol conversion to produce olefins, with the intent of producing olefins larger than ones formed by zeolites such as H-SAPO-34 and H-[Al]ZSM-5. H-[Fe]Beta was selected for this reaction as iron zeolites have an acid strength sufficiently strong to catalyze methanol conversion, but slowly catalyzes hydride transfer reactions, a necessary condition to minimize aromatics formation. By using H-[Fe]Beta for the conversion of dimethyl ether, olefins were produced with greater than 90 percent selectivity with isobutene being the major product. ☐ Our research has shown that by changing the heteroatom substituted into the zeolite framework, we can adjust the acidity of the catalyst and alter the chemistry of hydrocarbon conversion processes to enhance selectivity and yield.