Influence of the acid-base properties in Si-MCM-41 and B-MCM-41 mesoporous materials on the activity and selectivity of ε-caprolactam synthesis

B-MCM-41 mesoporous molecular sieves have been prepared with different B contents (Si/B = 40-10). Three synthesis procedures were compared: (i) direct synthesis by using tetraethyl orthosilicate as the silica source, boric acid as the boron source and the surfactant cetyl-trimethylammonium bromide, (ii) direct synthesis with a post-synthesis treatment in NH4F solution and (iii) capillary impregnation of mesoporous Si-MCM-41. The solid structures were analysed by XRD, N-2 adsorption, TG-DTA and DRIFT. The surface acidity was determined by TPD and DRIFT by using pyridine as the probe molecule. Furthermore, the presence of acid and basic sites was shown from the conversion of 2-methyl-3-butyn-2-ol. This study indicated that samples obtained by direct synthesis had both acid and basic properties, while fluorinated samples had mainly acid properties and those synthesised by capillary impregnation exhibited predominantly basic properties. Likewise, an increase in boron content produced a decrease in both the intensity of the XRD peaks and BET surface area as well as an increase in total-acidity. In the absence of deactivation processes (pulse method), at 573 K, silicates and borosilicates MCM-41 catalysed the formation of by-products of epsilon-caprolactam, although this process was favoured on solids with mainly basic proper-ties. Under continuous flow reaction conditions, at 723 K, results indicated that the incorporation of boron to the Si-MCM-41 framework did not affect the activity and product distribution. Finally, when acetonitrile was used as the solvent in a continuous flow system, although selectivity to epsilon-caprolactam improved, the by-products of e-caprolactam adsorption were also favoured resulting in the deterioration of catalyst life. (c) 2005 Elsevier B.V. All rights reserved.