Isomers of organic structure directing agent influence the Al distribution of AEI zeolite and catalytic performance in methane oxidation

Abstract

Small pore zeolites have been reported to show salient performance in methane oxidation reactions. Among them, the synthesis of AEI zeolite is highly dependent on organic structure-directing agents (OSDA). The isomer OSDAs have been informed to influence the crystallization rate and spatial Al distribution on the crystalline particle of AEI zeolite. Still, its impact on the framework Al distribution of the AEI zeolite is not well known, which significantly impacts the subsequent loading of metal cations and thus active sites available for catalytic reactions. We herein report the impact of the isomer OSDA for synthesizing AEI-type aluminosilicate zeolite on the Al distribution. The discrepancy of Al distribution in as-synthesized AEI zeolites directed by trans and cis-rich OSDAs was identified by ²⁷Al MQMAS/MAS NMR and density functional theory (DFT). The cis-OSDA gave a higher intensity of bands at 60–63 ppm as the shoulder of tetracoordinated Al. The trans-rich OSDA guided to a slightly higher Al content and a slightly higher proportion of Al pairs than the cis-one. The DFT calculation indicated that trans- and cis-OSDAs preferentially led to the Al atoms at the T3 and T1 sites, respectively. The exchanged Cu/AEI zeolites with varied Cu content were applied in the direct and continuous oxidation of methane reaction and exhibited different activities. Given the results, we report here the methane activation properties of the Cu/AEI zeolites and how the isomer identity of the OSDA impacts the activity of Cu/AEI in methane oxidation to methanol. This work highlighted that isomer OSDA influenced the Al distribution as well as crystallization kinetics and gave an insight into the design of the OSDA for zeolite synthesis.