Characterization and catalytic activity of aluminum- and aluminum/tetramethylammonium-exchanged bentonites

Three montmorillonites SWy-2 (Wyoming, USA), STx-1 (Texas, USA) and SAz-1 (Cheto, Arizona, USA) and a saponite Sap-Ca (California, USA) were activated with different amounts of Al3+ and tetramethylammonium (TMA(+)) cations to obtain materials with a combined Al3+ and TMA(+) content equal to their CECs. The catalytic activity was evaluated by reacting alpha-pinene at 120 degrees C for 1 h to yield camphene, limonene, and other minor products and compared to clays prepared without TMA(+). The total conversion over Sap-Ca, STx-1, SWy-2, and SAz-1 was 90%, 80%, 65%, and 25%, respectively, in samples treated with Al3+ alone. Similar yields were obtained over Al3+/TMA(+)-exchanged forms when the TMA(+) content was less than 20% CEC, but in general the presence of TMA(+) significantly reduced the total conversion. The activity of the Al3+-exchanged clays for this test reaction is attributed to their ability to remain expanded at the temperature of the activation process thus permitting easy access of the reagent. The comparatively low conversion over catalysts derived from SAz-1 was attributed to its high layer charge which restricted access to the active centers in the clay gallery.