Influence of Na+ on the synthesis of silicalite-1 catalysts for use in the vapor phase Beckmann rearrangement of cyclohexanone oxime

Abstract

Silicalite-1 was hydrothermally synthesized in the presence of different concentrations of Na+ using tetrapropylammonium hydroxide (TPAOH) as a template. The synthesis was followed by a base treatment. The silicalite-1samples were characterized using X-ray diffraction, scanning electron microscopy, N2 adsorption, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and NH3 temperature-programmed desorption. The samples were used as catalysts for the vapor phase Beckmann rearrangement of cyclohexanone oxime. During the synthesis, the sodium ions were incorporated onto the silicalite-1 crystals, but were then removed by the base treatment. All the catalysts exhibited nearly complete conversion of cyclohexanone oxime to ɛ-caprolactam with selectivities grater than 95%. Addition of less than 2.5 mol-% Na+ (relative to TPAOH) did not influence the catalytic properties. However, for Na+ concentrations ⩽5 mol-%, the particle sizes of silicalite-1 increased and the catalytic activities decreased, which can be attributed to carbon deposition. The results in this work are of great importance for the polymer industry.