Calcination and deboronation of B-MFI applied to the vapour phase Beckmann rearrangement

High silica MFI are very active and selective catalysts for the vapour-phase Beckmann rearrangement, especially because they are provided with defect sites, namely silanols nests. In order to increase the amount of H-bonded silanols it is possible to deboronate B-MFI catalysts. Deboronation can be performed mainly by post-synthesis modifications. The nature of the catalytically active sites has been modified through post-synthesis treatments, i.e. calcination, and basic treatments or steaming with washing in acid solution. Basic treatment is found to reversibly modify the active sites because it partially preserves silanols from condensation during calcination. Conversely, the use of diluted acid solution is more efficient in eliminating extra-framework boron, thus enhancing boralite catalytic performances. Unfortunately, there is no clear dependence of catalytic results on the post-synthesis modification of materials. Treatment in basic or acid media provides materials with different catalytic performances. Furthermore, the boron distribution inside the lattice, unlike boron content, affects the catalytic performances because cyclohexanone oxime conversion and caprolactam selectivity do not linearly depend on the B content.