Organic Structure Directing Agent Free Synthesis of Small Pore Zeolite Catalysts for the Methanol-to-Olefins Reaction

Abstract

Light olefins, ethylene and propylene, are two of the highest produced petrochemicals globally. The methanol-to-olefins (MTO) reaction is a promising route for making these chemicals from non-petroleum feedstocks such as natural gas and coal that has been successfully commercialized. The catalysts employed for this reaction are typically microporous molecular sieves with Bronsted acidity, e.g., zeolites and silicoaluminophosphates, that generally require costly organic structure directing agents (OSDAs) to synthesize. This thesis explores an alternative, low cost method for synthesizing small pore zeolite catalysts for the MTO reaction without the use of OSDAs. Small pore zeolites are first synthesized in the absence of OSDAs. The resulting high-aluminum materials are then converted into useful catalysts via high temperature (500-800°C) steam treatments that extract a portion of the framework aluminum, thereby modifying the acid site concentration, pore structure and catalytic behavior of the materials. This synthesis method is demonstrated on three small pore zeolite structures that are prepared without using OSDAs: CHA, RHO and KFI. In the as-synthesized forms, these materials deactivate rapidly when evaluated as catalysts for the MTO reaction due to their high aluminum contents. Upon steam treatment, however, improved catalyst lifetimes and olefin selectivities are observed that are attributed to a decrease in the total Bronsted acid site concentration and the creation of mesopores that facilitate transport of reactants and products. Improvements in the activity were observed for all three of the zeolites chosen for investigation with CHA-type zeolites performing best, though differences in olefin selectivities were observed. Poisoning of acid sites located in the mesopores and on external surface of the steamed zeolites did not change the observed product distribution, suggesting that these differences do not arise from secondary reactions of olefins and instead may be attributed to differences in the pore structures. Overall, the successful demonstration of this catalyst preparation method on three different zeolite structures suggests that it may be a useful route for converting any small pore zeolite that can be synthesized without using OSDAs into catalysts that may be useful for reactions like MTO.