Beckmann Rearrangement of ¹⁵N-Cyclohexanone Oxime on Zeolites Silicalite-1, H-ZSM-5, and H-[B]ZSM-5 Studied by Solid-State NMR Spectroscopy

Abstract

By means of solid-state 15N NMR spectroscopy, evidence for the formation of nitrilium ions as intermediates of the Beckmann rearrangement of 15N-cyclohexanone oxime to epsilon-caprolactam on silicalite-1, H-ZSM-5, and H-[B]ZSM-5 is reported. The zeolites under study are characterized by different acid strengths (silicalite-1 < H-[B]ZSM-5 < H-ZSM-5). Depending on the nature of catalytically active surface OH groups, reactant and product molecules exist in the nonprotonated or protonated state. In addition, formation of byproducts such as 5-cyano-1-pentene and epsilon-aminocapric acid as a result of dehydration and hydrolysis of the reactant and product molecules, respectively, were observed.

Influence of dopant substitution mechanism on catalytic properties within hierarchical architectures

“…They all contain mesopores of approximately 40 Å in diameter as well as the microporous AFI framework. Additionally, by using a combination of spectroscopic acid characterization techniques, the HP Me AlPO-5 s have all been found to contain silanol sites as well as the isolated Brønsted acid sites; both are known to be active in the Beckmann rearrangement [54]. Interestingly, the type I substituted HP Co AlPO-5 also had a significant quantity of Lewis acidity, type II substituted HP Ti AlPO-5 had minimal Lewis acidity, whereas HP Si AlPO-5 had no observable Lewis acidity.…”

Chemical diversity of zeolite catalytic sites

“…These hydroxyl nests have mild acidity, and are capable of facilitating the Beckmann rearrangement of cyclohexanone oxime (XII, see Figure 4). The mechanism of this reaction has now been elucidated, and the formation of the carbeniumion-like species XIII has been confirmed using NMR spectroscopy (Marthala et al, 2006). 18 In Figure 4, the negative charge formed on the zeolite framework is localized on a siloxy group (Si-O À ).…”

“…The mechanism of this reaction has now been elucidated, and the formation of the carbeniumion-like species XIII has been confirmed using NMR spectroscopy (Marthala et al, 2006). 18 In Figure 4, the negative charge formed on the zeolite framework is localized on a siloxy group (Si-O À ). In practice, because this siloxy group is part of a network of chemically equivalent groups, the negative charge is effectively delocalized between several oxygen atoms in the defect site (Fernandez et al, 2006).…”

“…Acid aluminosilicate and borosilicate zeolites do catalyze this reaction, but with much lower selectivity (Marthala et al, 2006). 18 They bind the e-caprolactam strongly reducing the availability of the active sites for further reaction, and promoting secondary reactions, such as oligomerization of the product. The accumulation of these byproducts fills the zeolite micropores obstructing access to the active sites.…”

Aplicación de la resonancia magnética nuclear (RMN) al estudio de procesos catalíticos sostenibles: transposición de Beckmann y acilación de sustratos aromáticos

“…118 No obstante, trabajos realizados posteriormente utilizando RMN de sólidos in situ muestran que la ciclohexanona oxima N-protonada se forma a temperatura ambiente sobre la zeolita B-ZSM-5, del mismo modo que ocurría en aluminosilicatos micro y mesoporosos, con centros ácidos Brønsted como centros activos. 147 En los últimos años, se ha investigado la naturaleza de los centros activos para la reacción de transposición empleando métodos espectroscópicos in situ. Los resultados obtenidos mediante FT-IR indican que los centros ácidos Brønsted, grupos silanoles enlazados por puentes de hidrógeno y silanoles "nests", localizados en los poros de las zeolitas y en el mesoporoso MCM-41, son activos en la reacción.…”