(691g) Quantifying the Role of Structure Directing Agents and Framework Al Location on Xylene Isomer Selectivity during Toluene Methylation Reactions

Zeolite-catalyzed toluene methylation by dimethyl ether (DME) is a desirable route to produce p-xylene, a key plastics precursor. Zeolites are mesoporous aluminosilicates synthesized using structure directing agents (SDA) that contain a diversity of Brønsted acid sites associated with distinct Al locations and arrangements. MFI zeolites are traditionally synthesized using tetra-n-propylammonium (TPA⁺), and kinetic measurements during toluene methylation in these zeolites (4 kPa toluene, 66 kPa DME, 403 K at <1% conversion) result in low p-xylene selectivity (22–30%) with o-xylene being the major product (64–66% selectivity). However, when using unconventional SDAs such as ethylenediamine (EDA) and 1,4-diazabycyclo[2.2.2]octane (DABCO), the measured p-xylene selectivity was consistently above 70%, suggesting that SDAs used during synthesis impact xylene isomer selectivity during toluene methylation reactions. Here, we use density functional theory to explore the role of Al location on xylene formation transition states and assess the relative stability of Al near SDAs across all MFI 12 unique T-sites to rationalize experimentally measured selectivities. At T12, toluene methylation barriers are 125, 135, and 131 kJ mol^−1 for o-, m-, and p-xylene formation respectively, suggesting a slight preference towards o-xylene. At T4, an Al location far from channel intersections, barriers suggest a stronger preference toward p-xylene, with barriers for o- and m-xylene being 24 and 5 kJ mol^−1 higher. This is consistent with the experimentally measured selectivities, suggesting that SDA of choice during synthesis biases aluminum location and thus influences xylene selectivity. To test this hypothesis, we also show that DABCO results in a more equitable T-site distribution than TPA⁺, thus increasing Al populations in more-confined locations such as T4. These experimental and theoretical data indicate that tuning Al siting in MFI leads can be used to increase p-xylene selectivity. This was further explored by extending toluene methylation reactions to TON, BEA, and MCM41 (unconfined) catalysts.