Synthesis of Faujasite (FAU) Zeolites Applied for Acid Catalysis

Abstract

Faujasite (FAU) zeolites are extensively used in petrochemical refining with more than 50% of global gasoline supplies produced using fluid catalytic cracking, which primarily relies on FAU-based catalytic formulations. In this thesis, we accomplished control of Al siting and accessibility, manipulation/control of FAU crystal habit, and template-free synthesis of low Al content FAU materials. FAU materials circumscribe protons in two different void environments with potential for shape selectivity being greatest in sodalite cages (< 4.5 Å) that are inaccessible for reaction in pristine FAU materials, because these protons can only be accessed via six-membered ring openings of diameter < 3Å. Here, we prove that FAU zeolites (with Si/Al ratio of ca. 1.7) undergo mild dealumination at moderate ion exchange conditions (0.01 to 0.6 M of NH4NO3 solutions) resulting in protons circumscribed by sodalite cages becoming accessible for reaction without conspicuous changes to bulk crystallinity. Protons in sodalite cages show higher rate constants of propane dehydrogenation and cracking than protons in supercages plausibly due to confinement effects being more prominent in smaller voids. Low-silica FAU zeolites (namely zeolite X, Si/Al = 1~1.5) hitherto find very limited or no catalytic application as solid acids because of their limited stability under conditions required to introduce acid sites in these materials. We report synthetic protocols based on moderate ion exchange (0.01 M of NH4NO3 solution) that enable only Na+ circumscribed by large pore 12-membered ring supercages to be removed in low-silica FAU materials but allows occluded alkali cations within small-pore sodalite cages to remain unperturbed, and the latter stabilize the framework of low-silica FAU materials for H+-exchange. We report that H+-containing zeolite X samples catalyze protolytic reactions at temperatures ~800 K enabling their application as solid acid catalysts. In order to explore new recipes of FAU zeolites prepared via organic-free routes, we demonstrate the use of Machine Learning to organize and extend procedures for crystallizing zeolites to accomplish desirable structural characteristics and achieve improved catalytic activity. Through ML, synthesis conditions were identified to enhance the Si/Al ratio of high purity FAU zeolite to the hitherto highest level (i.e., Si/Al = 3.5) achieved via direct (not seeded), and organic structure-directing-agent-free synthesis from sodium aluminosilicate sols. The analysis of the ML algorithms’ results offers the insight that reduced Na2O content is key to formulating FAU materials with high Si/Al ratio. An acid catalyst prepared by partial ion exchange of the new high-Si/Al-ratio FAU (Si/Al = 3.5) exhibits improved proton reactivity in propane cracking and dehydrogenation compared to the catalyst prepared from the previously reported highest Si/Al ratio (Si/Al = 2.8).