AI-containing mesoporous catalysts with MCM-41 structure and high surface area have been successfully prepared by direct hydrothermal synthesis. Various characterization techniques including XRD, N-2 adsorption, ICP-OES, NMR, FT-IR and adsorption of pyridine coupled to FT-IR spectroscopy were employed. A detailed study about the relationships existing between the different synthesis parameters, the physico-chemical properties, the acid behavior and the catalytic performance is presented in this contribution. We have suggested that the active sites for the Beckmann rearrangement of cyclohexanone oxime are nest silanols (at structural defects) of very weak Bronsted acid character, which is associated with the introduction of Al into the mesoporous framework. In all the cases, employing soft reaction conditions, we have obtained selectivity to caprolactam of 100%. Finally, a Si/Al initial molar ratio of 20, a stirring of synthesis gel for 7 h and a hydrothermal treatment of 6 days seem to be the optimum synthesis conditions to obtain the highest framework Al incorporation, more abundant weak acid sites (silanol nests) and maximum yield for xi-caprolactam. (C) 2012 Elsevier B.V. All rights reserved.