Persistent Carbenium Ions in Zeolites Characterized by NMR Spectroscopy

Persistent Carbenium Ions in Zeolites Characterized by NMR Spectroscopy 

Keto-Enol Isomerization on Acidic Zeolite HZSM-5 Evidenced by H/D Exchange 

The H/D exchange between the methyl groups of adsorbed acetone molecules and the Bronsted acid sites of zeolite HZSM-5 was also observed upon adsorption of C-2-acetone on a deuterated catalyst (D,HZSM-5, nsi/ Ai = 21.5) at room temperature (Figs 20c and d). The C MAS NMR spectrum of C-2-acetone adsorbed on zeolite D,HZSM-5 (Fig. 20e) consists of the carbonyl signal at 223 ppm with a featured sideband pattern and a methyl signal at 29 ppm. No significant 

The aldol condensation is an important reaction in organic synthesis leading to the formation of C-C bonds. This reaction is catalyzed by zeolites (121-127). To understand the mechanisms involved, various spectroscopic methods (122-125, 

128-130) and theoretical calculations (131,132) have been employed, mainly to investigate the nature of the adsorbed aldehydes and ketones on various zeolite catalysts. It was suggested that the keto-enol isomerization is the first step of this reaction (132). In situ MAS NMR spectroscopy has been applied to characterize the chemical state of acetone adsorbed on zeolite HZSM-5 (133). The observed H/D exchange between Bronsted acid sites and adsorbed acetone molecules indicates the formation of an enol via a concerted action of bridging hydroxyl protons and neighboring framework oxygen atoms in zeolite HZSM-5 (133). 

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A.4. Persistent Carbenium Ions in Zeolites Characterized by NMR Spectroscopy... 

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