Transmission ultraviolet–visible–near-infrared spectroscopic investigations of the formation of carbocations in zeolites. Re-examination of propene/CoNaY and extension to trans-butene/CoNaY, propene/H-mordenite and cyclopropane/H-mordenite

Combined Operando UV/Vis/IR Spectroscopy Reveals the Role of Methoxy and Aromatic Species during the Methanol‐to‐Olefins Reaction over H‐SAPO‐34

“…The band shift from λ =275 nm to 295 nm could owe to the formation of monoenylic carbocations,16 which can be an indication of olefin formation. Monoenylic species are allylic carbocations and can be formed by protonation of olefins on a Brønsted acid site and subsequent hydride abstraction 22. The band shift from λ =390 nm to 413 nm is ascribed to the formation of more alkylated benzene carbocations, methylated naphthalene carbocations, or PA neutral species 10d.…”

“…More precisely, the band located at λ ≈295 nm grows in intensity, most likely from light olefins produced and trapped on the catalyst owing to the low diffusion properties of the material at this stage 16. 22, 24 In addition, the band at λ ≈410 nm continues to increase in intensity during the deactivation period while the band at λ =345 nm starts to disappear, which also suggests that the λ =345 nm band most likely originates from the reaction intermediates in olefin production. Furthermore, the band at λ =600 nm assigned to PA species grows during the whole reaction period of the material until reaching a maximum when the catalyst is fully deactivated at 653 K, which implies that the formation of this highly conjugated species may play an important role in the deactivation of the catalyst, more specifically by gradually reducing the accessibility properties of the material until complete blockage.…”

Generation, Characterization, and Transformations of Unsaturated Carbenium Ions in Zeolites

“…Therefore, in the UV−vis range it should be more advantageous to replace the transmission by the reflectance method. However, from comparison of both methods, using zeolites Sendoda et al gave a preference to the transmission mode, followed by other research groups. , The advantages of the transmission technique are a demand for a lower amount of sample, an easier sample handling concerning degassing and admission of adsorbate, no need for an extra diffuse reflectance accessory, and the omission of data conversion into the Schuster−Kubelka−Munk function. The typical experimental equipment for transmission studies on zeolites is shown in Figure …”

Improved experimental setup for in situ UV-vis-NIR spectroscopy under catalytic conditions

“…hydrocarbon deposits, during catalytic reactions. 7 Only in a few cases can the samples be pressed into thin wafers of high transmittance -as successfully done with zeolites [8][9][10] -so that the spectroscopy can be performed in transmission mode. Although the transmission mode was shown advantageous in some cases, 11 many investigations must be conducted in reflection due to high absorption and scattering.…”