Following Solid‐Acid‐Catalyzed Reactions by MAS NMR Spectroscopy in Liquid Phase—Zeolite‐Catalyzed Conversion of Cyclohexanol in Water

Abstract

A microautoclave magic angle spinning NMR rotor is developed enabling in situ monitoring of solid-liquid-gas reactions at high temperatures and pressures. It is used in a kinetic and mechanistic study of the reactions of cyclohexanol on zeolite HBEA in 130 °C water. The (13) C spectra show that dehydration of 1-(13) C-cyclohexanol occurs with significant migration of the hydroxy group in cyclohexanol and the double bond in cyclohexene with respect to the (13) C label. A simplified kinetic model shows the E1-type elimination fully accounts for the initial rates of 1-(13) C-cyclohexanol disappearance and the appearance of the differently labeled products, thus suggesting that the cyclohexyl cation undergoes a 1,2-hydride shift competitive with rehydration and deprotonation. Concurrent with the dehydration, trace amounts of dicyclohexyl ether are observed, and in approaching equilibrium, a secondary product, cyclohexyl-1-cyclohexene is formed. Compared to phosphoric acid, HBEA is shown to be a more active catalyst exhibiting a dehydration rate that is 100-fold faster per proton.

Sealed rotors for in situ high temperature high pressure MAS NMR

“…It means that the 13 C-isotope scrambling of 1- 13 C-cyclohexanol, 1- 13 C-cyclohexene, and cyclohexene hydration back reaction are more significant in H-Beta catalyst, which is probably due to cyclohexyl carbenium as intermediate species exists in zeolite catalyst. 20 To demonstrate that the experimental results obtained using the in situ NMR method are comparable with those obtained using a traditional batch autoclave, we report the turnover frequency (TOF) of cyclohexanol dehydration to cyclohexene at different temperatures and the corresponding activation energy values (Figure 3b) for both the homogeneous and heterogeneous catalyst systems in Table S1. Note that while slight deviations in the TOF are observed for the homogeneous (H 3 PO 4 ) catalyzed reaction, the determined values (approximately 148 kJ·mol −1 ) for the activation energy are nearly identical.…”

Alkylphenols to phenol and olefins by zeolite catalysis: a pathway to valorize raw and fossilized lignocellulose

“…66 On another account, effects have been attributed to confinement effects or nest effects caused by water within the zeolite pores. 51 Conversely, water could also act as a sweeping agent, forcing formed products to diffuse away from the zeolite, 67 and rendering the role of water to be mostly adsorptive, hence physical, rather than chemical. However, clear evidence of the role of water has thus far remained absent.…”

Highly efficient and robust Mg0.388Al2.408O4 catalyst for gas-phase decarbonylation of lactic acid to acetaldehyde

“…As mentioned above, for Mg 0.388 Al 2.408 O 4 catalyst predominant weak-medium acidity as well as an excellent acid-base balance plays an important role in durability and acetaldehyde selectivity. It is known that strong acidic sites have a strong catalysis for decomposition of C-C bond, resulting in deposition of carbon or formation of coke on the catalyst surface [51,60,63]. Thus the catalyst with more strong acidic sites rapidly deactivates in the process of the catalytic decarbonylation reaction of LA due to covering the active sites of the catalyst surface by the formed carbon or cokes.…”