Consequences of Mesopores in Zeolitic Frameworks on Reactivity and Selectivity in Complex Reaction Pathways
- Type: Conference Presentation
- Conference Type: AIChE Annual Meeting
- Presentation Date: November 17, 2020
- Duration: 17 minutes
- Skill Level: Intermediate
- PDHs: 0.30
Zeolite and zeotype materials are archetypal microporous crystalline materials and are attractive for acid-catalyzed conversions of hydrocarbons and oxygenates, sourced from traditional fossil-based or renewable feedstocks, to fuels and chemicals. The voids and channels surrounding active protons, in the case of Brønsted acidic zeolites, provide a solvating or confining environment that can affect the stability of transition states and reactive intermediates that mediate the desired conversions. These voids of molecular dimension can, however, restrict diffusion of molecules to and from the active sites, which impacts rates, selectivities, and deactivation. The addition of mesopores to zeolite crystals through varied synthetic protocols is one method utilized to alleviate diffusion constraints in these materials, yet, their direct effect on reaction mechanisms, especially complex ones that suffer from diffusion-enhanced secondary reactions and ones that can exploit protons in different environments, is not well understood. Here, we probe two different diffusion-limited regimes to understand mesopore connectivity and acid site location: i) the ingression of bulky reactants into zeolite pores from the liquid phase (via Friedel-Crafts alkylation of 1,3,5-trimethylbenzene with benzyl alcohol) and ii) the egression of products that are larger and bulkier than the reactant molecules (via oligomerization of light alkenes). These reactions were studied on microporous and hierarchical analogs of lab-synthesized and commercial zeolites with 3D (BEA, MFI) and 1D (MOR) pore networks. Lower catalyst efficiencies were observed for purely microporous zeolites, in agreement with hindered diffusion. However, fractional selectivities do not increase unilaterally for hierarchical zeolites and instead depend on solvation effects and mesopore connectivity. Results from these reactions yield fundamental insight into hierarchical zeolite structure-function relations that extend broadly to other zeolite-catalyzed processes for more efficient chemical and fuel production.
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