Oxygen exchange mechanisms in zeolite chabazite under steaming conditions

Under hydrothermal conditions, zeolite frameworks can readily incorporate oxygen from water, via reactive oxygen exchange. This indicates that zeolite frameworks are highly labile and reactive to water, even when stable against full hydrolytic dissolution. However, the routes by which oxygen is exchanged between water and framework have not been established. In this work, we identify the preferable oxygen exchange mechanisms in the zeolite chabazite (CHA) and compare them to hydrolysis and other framework healing mechanisms under the low water concentrations relevant for steaming conditions. We find that oxygen exchange occurs at defect sites that are created by the first hydrolysis step, both in Al-O and Si-O bonds and is competitive with subsequent hydrolysis and non-exchange framework healing processes. Furthermore, we determine the effect of increased water concentration, finding that for both Al-O and Si-O bonds, the second water can either moderately enhance or hinder both hydrolysis and O-exchange, depending on the geometry of the site. This implies that local water concentration is an important factor with varied effects on zeolite stability as a function of water loading. This work provides feasible routes of oxygen exchange in CHA, which together with hydrolytic pathways, govern the stability and mesoporosity of these important porous materials.