Incorporation and electron transfer of anthracene in pores of ZSM-5 zeolites. Effect of Brønsted acid site density

Abstract

The sorption course of anthracene (ACENE-3) into Brønsted-acidic medium pore MFI zeolites was monitored by in situEPR and diffuse reflectance UV-visible absorption over one year. Weighed amounts of solid ACENE-3 were merely exposed to H_nZSM-5 (H_n(AlO₂)_n(SiO₂)_96−n), with the following Brønsted acid site (BAS) densities, n = 0.0, 0.17, 0.57, 0.95, 2.0, 3.4, 6.6, dehydrated at 623 K under argon. The weighed amounts correspond to 1 ACENE-3 per zeolite unit cell. ACENE-3 is found to be incorporated as intact molecules in purely siliceous MFI (silicalite-1). Monte Carlo simulations indicate that ACENE-3 lies in the intersection of straight and zigzag channels. In contrast, the presence of BASs on the inner surface of channels induces spontaneous ionization of ACENE-3 (ionization potential = 7.44 eV). The charge separation as ACENE-3˙⁺@H_nZSM-5˙⁻ is caused by the strong Coulombic field gradient of Si–O⁻(H⁺)–Al BAS in the absence of any Lewis acid site. The rate and yield of ionization are found to increase dramatically with BAS density increase. The stabilization of ACENE-3˙⁺@H_nZSM-5˙⁻ is explained by the tight fit between the rod-shape ACENE-3 and the channel dimensions and especially by the compartmentalization of ejected electrons as AlO₄H˙⁻ centers away from the initial site of ionization. The final charge recombination occurs after more than one year and leads to ACENE-3 occluded in the straight channel in close proximity to BAS without any protonation of ACENE-3 (pK_a = −13.5).