Styrene oligomerization as a molecular probe reaction for Brønsted acidity at the nanoscale

The Brønsted acid-catalyzed oligomerization of 4-fluorostyrene has been studied on a series ofH-ZSM-5 zeolite powders, steamed under different conditions, with a combination of UV-Vismicro-spectroscopy and Scanning Transmission X-ray Microscopy (STXM). UV-Vis microspectroscopyand STXM have been used to monitor the relative formation of cyclic and lineardimeric carbocations as a function of the steaming post-treatment (i.e., parent vs. steaming at600, 700 and 800 1C). It was found that the UV-Vis band intensity ratios of linear to cyclicdimeric species increase from 0.79 (parent H-ZSM-5) over 1.41 (H-ZSM-5 steamed at 600 1C) and1.88 (H-ZSM-5 steamed at 700 1C) to 2.33 (H-ZSM-5 steamed at 800 1C). STXM confirms thistrend in reaction product selectivity, as the relative intensities of the transitions attributed to thepresence of the cyclic dimer in the carbon K-edge spectra decrease with increasing severity of thesteaming post-treatment. Furthermore, STXM reveals spatial heterogeneities in reaction productformation within the H-ZSM-5 zeolite powders at the nanoscale. More specifically, a shrinkingcarbon core–shell distribution was detected within the zeolite aggregates, in which the relativeamount of cyclic dimeric species is higher in the core relative to the shell of the zeolite aggregateand the relative amount of cyclic dimeric species in the zeolite core gradually decreases withincreasing severity of the steaming post-treatment. These differences are rationalized in terms ofspatial differences in Brønsted acidity within H-ZSM-5 zeolite powders as well as by changes inthe formation process of linear and dimeric carbocations within H-ZSM-5 micro- and mesopores.