Catalyzed Beckmann rearrangement of cyclohexanone oxime in heterogeneous liquid/solid system: Part 1: Batch and continuous operation with supported acid catalysts

Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam has been studied in heterogeneous liquid/solid system at room temperature and pressure. In this first part preparation, characterization and reactivity of impregnated strong acids on solid support have been investigated, with particular attention to H2SO4/SiO2 catalysts. The influence of many preparative variables on catalyst's microstructure and reactivity has been studied. Surface characterization suggests a progressive filling of the silica pores as the amount of acid increases without a sensible modification in the silica porosity. Conversion and selectivity in Beckmann rearrangement are directly related to the amount of acid loaded on the solid catalyst as well as to the kind of support. Measurement of kinetic parameters together with the apparent activation energy of few kJ mol-1 and the inspection of Carberry and Weeler-Weisz number suggest that diffusion phenomena affect the kinetics. Comparing liquid/liquid and liquid/solid systems, Beckmann rearrangement occurs only with really strong acid and employing aprotic solvent. On the contrary, oxime hydrolysis occurs also with solid or liquid acid catalysts with low protonation ability which generally do not allow the rearrangement under mild conditions. Fast catalyst deactivation has always been observed, suggesting a relation with oxime hydrolysis by which neutralization of the acid phase on catalyst surface occurs.

Graphical abstractBeckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam has been studied in heterogeneous liquid/solid system at room temperature and pressure. Preparation, characterization and reactivity of impregnated strong acids on solid support have been investigated, with particular attention to H2SO4/SiO2 catalysts. Run conditions were as follows: T, 298 K; reaction volume, 20 ml; solvent, 1,2 dichloroethane; catalyst, 500 mg; cyclohexanone oxime concentration, 0.24–0.72 mol dm−3. Figure optionsDownload full-size imageDownload as PowerPoint slide