Physicochemical and catalytic characterization of non-hydrothermally synthesized Mg-, Ni- and Mg–Ni-saponite-like materials

Abstract

Saponite-like materials with Mg 2+ or Ni 2+ ions and their mixture in the octahedral sheet were non-hydrothermally synthesized. Their protonic forms were prepared through deammoniation or by treatment with HCl. The products were characterized by physical methods, gas-phase adsorption of amines and the catalytic transformation of 2-propanol and cumene via pulse and flow methods. The materials have a `house-of-card' texture composed of platelets with small lateral dimensions and exhibit a high surface area and pore volume. Subsequent deammoniation resulted in the formation of non-framework Ni 2+ and possibly Mg 2+ ions. Mg-saponite, prepared in this way, shows predominantly non-framework species formed by tetrahedrally coordinated Al 3+ ions whereas its acid-treated counterpart contains Al 3+ in both framework and non-framework positions. Deammoniated Ni-saponite exhibits a peculiar catalytic behaviour, which manifests itself in the fact that the activity in the 2-propanol decomposition in a pulse experiment is low while under flow conditions it becomes the most active. This is ascribed to generation during the reaction of new acidic centres due to the dissociative adsorption of produced water on site pairs formed by coordinatively unsaturated non-framework Ni 2+ and framework oxygen ions. Cumene conversion tests confirm this explanation. The high activity and stability of acid-treated Mg-saponite in cumene dealkylation underlines its potential use for high-temperature acid-catalyzed reactions. The Brønsted acidity of the saponite-like materials is related to their layer charge, cation exchange capacity and diminished with increasing Ni content, whereas the Lewis acidity increased. Both the gas-phase amine chemisorption and catalytic tests provide the same order of protonic acidity: Mg > MgNi > Ni. The origin of the acidic sites from deammoniation or HCl treatment is discussed.