On the presence of internal silanol groups in ZSM-5 and the annealing of these sites by steaming

Abstract

In a recent interesting letter Dessau et al. presented FTIR and 29Si (CP) MAS NMR results providing further evidence for the presence of internal silanol groups in ZSM5. The decrease in the silanol groups during a steaming procedure accompanied by a dramatic enhancement of the resolution of the @ signal in 29Si MAS NMR spectra was reported ( I). Recently, we described a very similar ordering of the lattice and decrease in internal silanol groups that we observed during prolonged heat treatment in laboratory air of high-silica ZSM-5 in the presence of alkaline ions. By means of XPS the Na+ or K+ ions were found to concentrate at the external surface of the zeolite particles during this procedure (2). Depending on the synthesis procedure we always find considerable amounts of Na+ or K+ ions (typically 0.75 to 1.3 ions per unit cell) in high-silica ZSM-5 even after thoroughly washing with water. They are located in terminal siloxy groups that can be converted into silanol groups upon ion exchange with diluted hydrochloric acid. Regrettably, the amounts of alkaline ions in the samples studied by Dessau et al. are not described very precisely. Only once do they mention an ion exchange with ammonium ions in order to remove Na+. But the corresponding FTIR spectrum does not exhibit the typical v(NH) bands which should indicate that the ion exchange was effective (3). Therefore, we think it to be possible that sodium ions were still present during steaming in that case. However, it is interesting to conclude from Dessau’s work that heat treatment at an increased vapor pressure of water probably accelerates the process of lattice improvement. While the evidence for internal silanol groups in ZSM-5 is rather convincing we would like to propose a different view concerning the arrangement of silanol groups and the mechanism of rearrangement in the lattice. Silylation of silanol groups in our ZSM-5 with trimethylchlorosilane (TCS) and subsequent analyses of the products by means of 29Si CP MAS NMR clearly indicate that silanol groups are not isolated but rather clustered (2, 4). Isolated silanol groups as suggested by Dessau er al. should yield exclusively trimethylsiloxysilanes. Instead, we find the secondary and tertiary products (dimethyldisiloxysilanes and methyltrisiloxysilanes) which can be formed only if further neighboring silanol groups are present. The ease of the formation of tertiary silylation products as compared with the silylation of, e.g., silica gels, indicates that in the ZSMJ samples silanol groups are in arrangements favoring the insertion of TCS. We think that high-silica ZSM-5 typically contains clusters of four terminal groups each (e.g., silanol groups) where T atoms are missing in the lattice. During prolonged heat treatment (or steam-