Kinetic Study on Gas-Phase Beckmann Rearrangement of Cyclohexanone Oxime to ε-Caprolactam over a Silica-Supported Niobia Catalyst

Abstract

Reaction kinetics of the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam was studied over a NbO x /SiO2 catalyst in a fixed-bed reactor. Kinetic measurements were carried out by variation of cyclohexanone oxime partial pressure and reactor temperature in the range of 5 to 80 hPa and 360 to 420 °C, respectively. It was found that ethanol, used as solvent in addition to toluene, is essential for high catalytic performance. Reaction rates as a function of cyclohexanone oxime partial pressure display Langmuir-type behavior, but the results could not be interpreted satisfactorily on the basis of a Langmuir–Hinshelwood mechanism. Another kinetic scheme is proposed, involving a kinetic adsorption step for cyclohexanone oxime instead of an adsorption equilibrium. Activation energies for the adsorption step and for the Beckmann rearrangement reaction were found to be 154 and 68 kJ·mol–1, respectively. Substitution of ethanol by alcohols with longer chain length (e.g., n-hexanol) resulted in significantly higher ε-caprolactam selectivities.

Identification of Vicinal Silanols and Promotion of Their Formation on MCM-41 via Ultrasonic Assisted One-Step Room-Temperature Synthesis for Beckmann Rearrangement

“…The Supporting Information is available free of charge on the ACS Publications website. 1 H MAS NMR (Fig. S1), quantitative results of silanol groups (Table S1) AUTHOR INFORMATION…”

“…Nylon-6, ε-caprolactam (CPL) is a key intermediate. [1][2][3] Globally, the annual production of CPL is around 4.5 billion kilograms. 4 Most of the CPL is produced by the Beckmann rearrangement of cyclohexanone oxime (CHO) using concentrated sulfuric acid in a large scale.…”

“…1 H MAS NMR signals of siliceous catalysts in the range of δ1H = 1.2 to 2.6 ppm are assigned to surface SiOH groups. Because of the low contents of the29 Si MAS NMR signals due to Q 2 (Si(OSi)2(OH)2) species inFigure 3, the 1 H MAS NMR signals must be mainly caused by isolated and vicinal SiOH groups, respectively, i.e.…”

The Molecular Design of Active Sites in Nanoporous Materials for Sustainable Catalysis

“…Under liquid-phase conditions, increasing the acid strength of Al-MCM-41 and Al-SBA-15 by applying a sulfonic acid (-SO 3 H) modification has proven to be beneficial to catalyst performance [ 54 ]. Nb/SiO 2 systems containing extra-framework NbO 4 -tetrahedra have also proven to be highly effective catalysts for the Beckmann rearrangement [ 55 , 56 , 57 , 58 ]. TEM imaging has revealed the presence of extra-framework niobium sites at higher loading, with Nb-clustering favoured over framework substitution ( Figure 5 ) [ 55 ].…”

“…In kinetic studies, both the adsorption energy of cyclohexanone oxime (154 kJ mol −1 ) and the activation energy for the Beckmann rearrangement (68 kJ mol −1 ) were found to be considerably lower than for the conventional Silicalite-1 catalyst [ 56 ]. Nb-doped materials have also proven active under liquid-phase conditions.…”