Active sites on SBA-15 in the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam

Modified SBA-15 materials were studied as catalysts in the vapor-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam. Their combined micropore and mesopore system allows the investigation of the influence of pore dimensions, acid site concentration and strength, and location on the catalytic activity. A treatment utilizing H2SO4 allows to study the contribution of silanol groups as potential catalytically active sites in mesopores and micropores independent of one another. Applying 29Si solid-state NMR and DRIFTS experiments using a probe molecule, surface and in-wall silanol groups can be discriminated and quantified, making analysis correlating location, acidity, and catalytic activity possible. Combining all results leads to conclusions concerning the location of the catalytic reaction, the nature of the catalytic active sites, and their properties, which provides a basis for a rational optimization of catalysts.