Search

Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by (13)C MAS NMR. Kinetics of the reaction, which was monitored by (1)H MAS NMR in situ at the temperature of 573K, provided information about the reaction mechanism. Simulation of the experimental kinetics within the frames of the possible kinetic schemes of the reaction demonstrates that a large amount of methane evolved under ethane aromatization arises from the stage of direct ethane hydrogenolysis.

To clarify the issue on Brønsted acidity enhancement by the presence of zinc species in H‐ZSM‐5 zeolite (G.D. Qi, et al. Angew. Chem.‐Int. Edit. 2016, 55, 15826), the influence of Zn2+ cationic species, introduced into the zeolite, on the strength of Brønsted acid sites (BAS) has been analyzed by solid‐state NMR and FTIR techniques using trimethylphosphine (TMP) and CO as probe molecules. Comparative analysis of the acidity for H‐ZSM‐5 and Zn2+/H‐ZSM‐5 zeolites has been performed. TMP adsorbed on both zeolite samples exhibits same signals at 6.3 and −4.5 ppm in 1H and 31P MAS NMR spectra, respectively, assigned to protonated TMP. FTIR spectroscopy has shown that both zeolite samples, H‐ZSM‐5 and Zn2+/H‐ZSM‐5, display the same red frequency shift (ΔνOH⋅⋅⋅CO=−318 cm−1) of the O−H stretching band and the same blue frequency shift (ΔνCO=+32 cm−1) of the CO stretching band for CO adsorption complex with BAS. The results obtained do not provide any evidence for the existence of some specific BAS with enhanced acid strength for Zn2+‐exchanged zeolite compared to the strength inherent to pure acid‐form zeolite. It is thus inferred the strength of BAS in H‐ZSM‐5 zeolite is not affected by the modification of the zeolite with Zn2+ cationic species. [ABSTRACT FROM AUTHOR]

Abstract: Acidic OH groups of acid-form and Zn and Ga-modified zeolite beta (BEA) have been characterized with 1H MAS NMR and by IR spectroscopy of adsorbed carbon monoxide. It is demonstrated that OH groups, which exhibit a vibration band at 3740cm−1, reveal acidity, which is similar to that of the OH groups with the band at 3610cm−1 according to the value of the low frequency shift of OH vibrations with adsorbed CO (Δν OH/CO =300cm−1). The IR band 3740cm−1 corresponds to the signal at ca. 2.1ppm in 1H MAS NMR spectrum. The OH groups with signal 2.1ppm are involved in H/D exchange with methane-d 4 similar to acidic OH groups with the signals 4.0–5.1ppm. The signals at 2.1ppm in 1H MAS NMR and at 3740cm−1 in IR are attributed to the strongly acidic silanol groups of the faulted structure of the zeolite. The silanols with the signals at 1.8ppm and 3745cm−1 are weakly acidic (ΔνOH/CO =85cm−1) and are not involved in the H/D exchange. Loading of the zeolite with Zn affords a notable decrease of the concentration of strongly acidic SiOHAl groups, whereas the quantity of these groups does not decrease upon loading the zeolite with Ga. [Copyright &y& Elsevier]