Influence of the synthesis conditions on the physicochemical properties and acidity of Al-MCM-41 as catalysts for the cyclohexanone oxime rearrangement

[Display omitted] ► Al-MCM-41 sieves have been successfully prepared by direct synthesis method. ► The incorporated Al depends on the hydrothermal synthesis time and stirring of gel. ► The presence of very weak acid nest silanols could be associated with framework Al. ► Beckmann rearrangement of CHO was carried out with selectivity to ξ-C of 100%. ► Acid silanol nests are proposed as the active sites for this reaction. Al-containing mesoporous catalysts with MCM-41 structure and high surface area have been successfully prepared by direct hydrothermal synthesis. Various characterization techniques including XRD, N2 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 Brønsted 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 7h 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 ξ-caprolactam.