UV Raman studies of hydrocarbons in zeolites: (I) coke chemistry, (II) benzene in MFI, (III) quantitative Raman analysis.

摘要

UV Raman spectroscopy is a powerful tool to investigate hydrocarbons-zeolite systems, including the coke formation mechanism during MTH reactions, the sorbet-framework interactions and the quantitative analysis of catalytic reactions.;First, methanol, dimethyl ether and ethylene reactions were carried out on H-MFI with different Si/Al atomic ratios to study the coke formation process, especially in the early stages. Conjugated polyolefin, alkylated cyclopentadienyl species, alkylated benzene, and polyaromatic hydrocarbons of fluorene and pentacene have been identified among the retained species by in-situ UV Raman and ex-situ GC-MS measurements. A detailed mechanism of coke formation on MFI is proposed based on spectroscopic detection of these species.;Second, benzene and toluene absorbed in zeolite MFI and USY were used as probes to study the host-guest interactions between aromatic molecules and the catalyst framework using FT-Raman, UV-Raman and fluorescence spectroscopy. Combinations of non-totally symmetric fundamental vibrations with A2u , E1u and E2u symmetry appeared prominently in UV Raman spectra of adsorbed benzene inside the MFI framework. These bands were not observed in the FT-Raman spectra and UV Raman spectrum of liquid benzene. The appearance of these combinations is due to the resonance Raman effects of vibronic coupling between the ground and electronically excited states of benzene combined with distortion of the adsorbed benzene molecule due to interactions with the pore walls in the electronically excited state.;Third, quantitative application of UV Raman in coke chemistry has been discussed. The variations in Raman cross-sections between different species are shown to place limitations on the quantification of coke because of the changing population of polyaromatic species during reactions. In this study, we discuss the possibility of quantifying the Raman cross-sections using alkylated naphthalene species as model compounds. For visible Raman measurements, differences in Raman cross-sections can be neglected. However, for UV Raman spectra, because of the variation in resonance enhancement effects, the Raman cross-sections of the naphthalene derivatives can change by as much as a factor of 2, which places a fundamental uncertainty on the relationship between the Raman intensity and the concentration of polyaromatic species.