Isotopic Exchange of Oxygen between Proton-Exchanged Zeolites and Water

Abstract

The time courses of the title reaction were analyzed by using a closed circulation system. It was found that there are three kinds of framework oxygen, OI, OII, and OIII, that is, easily, moderately, and hardly exchanging oxygens. The respective rate constants of the exchange reaction, k1, k2, and k3, were almost constant, or independent of the zeolite structures and silica/alumina ratios. In contrast, the numbers of OI and OII (n1 and n2) decreased with increasing silica/alumina ratios, whereas that of OIII (n3) increased. n1 + n2 and n3 are approximately in agreement with the numbers of bridging oxygens of Si−O−Al and Si−O−Si, respectively. Next, IR measurement was applied to confirm the origin of the active framework oxygens. Upon the exchange reaction on H-MFI-23, one acidic OH band at 3612 cm-1 readily shifted to lower frequency while the silanol band at 3735 cm-1 and ν(Si−O−T (T = Si or Al)) bands at 1219 and 1092 cm-1 shifted slightly. The 18O contents in the bands, evaluated by the deconvolution of the spectra, decreased in the following order: acidic OH ≫ Si−O−T = silanol. A similar order was observed on H-FAU-5.6, H-MOR-16, or H-LTL-6.0. Most of oxygens corresponding to the acidic OH bands could exchange with 18O upon treatments in a large excess of H218O. It follows that OI and OII are the bridging oxygens bound to Al and exchangeable with water by the contribution of neighboring proton, and OIII is the bridging oxygen between two Si atoms. It was further found that the exchange reaction, Si−18O−Al + Si−16O−Si → Si−16O−Al + Si−18O−Si, starts to proceed above 673 K and is dominant at 873 K.