(NH4)2SiF6-modified ZSM-5 as catalysts for direct hydroxylation of benzene with N2O: 1. Influence of the treatment method

Two samples of MFI-type zeolites (A, SiO2/Al2O3=30, 0.02 wt% Fe, and B, SiO2/Al2O3=20, 0.03 wt% Fe) were dealuminated with (NH4)2SiF6 by three different methods. Method (i) consisted of aqueous treatment in a buffered solution followed by thorough washing with distilled water. Method (ii) consisted of isomorphous substitution in the solid state at moderate temperatures followed by thorough washing with deionized water. Method (iii) involved the removal of aluminum fluoro complexes by thermal decomposition after the preceding solid-state isomorphous substitution procedure. The materials were characterized by ICP-AES, physisorption of N2, XRD, FT-IR, PyFT-IR, and 1H, 27Al, 29Si, and 19F MAS NMR prior to and after catalytic testing for direct hydroxylation of benzene to phenol. It was shown that the materials were loaded with considerable amounts of F− in the form of Al–F and Si–F prior to the reaction, which disappeared quantitatively after the reaction. Catalytic properties, in terms of activity and the rate of deactivation, could be correlated to acid–base and textural properties. Materials prepared according to method (iii) showed the best catalytic activity. Materials prepared according to method (ii) showed moderate activity, whereas the formation of phenol was hardly detected over materials prepared according to method (i).