INVESTIGATION OF THE SURFACE-STRUCTURE AND BASIC PROPERTIES OF CALCINED HYDROTALCITES

In this work, we investigated the influence of preparation method, calcination temperature, and Mg : Al atomic ratio on the basic properties of calcined hydrotalcites. The steady-state decomposition of 2-propanol at 593 K and atmospheric total pressure was studied to compare the surface reactivities of the mixed oxides with those of pure MgO and Al2O3. In general, the activity and selectivity to propanone formation followed the order MgO ≥ calcined hydrotalcities ≫ alumina, indicating that catalytically active basic sites are present on the calcined hydrotalcites. However, the selectivity to propanone is influenced significantly by the method of hydrotalcite synthesis. In addition, the observed activation energies and pre-exponential factors for propanone formation compensated each other over mixed oxides of different Mg : Al atomic ratios, resulting in similar areal reaction rates. The observed compensation effect can be explained by a shift in the rate-determining elementary step on a nonuniform surface. No significant influence of calcination temperature on the surface area and 2-propanol reactivity of hydrotalcite was observed for temperatures of 723-923 K, whereas calcination of hydrotalcite at 1173 K resulted in lower surface area, decreased selectivity to propanone, and formation of separate oxide phases. Temperature-programmed desorption (TPD) of CO2 from calcined hydrotalcites indicated that the incorporation of aluminum into MgO suppresses the formation of high-strength base sites normally associated with pure MgO; however, low-strength base sites are still present on the mixed oxide. Based on results from 27Al NMR, XPS, CO2 TPD, and 2-propanol reaction, we speculate that some Al is located at the surface but is in a local oxidic environment not favorable for acid-catalyzed reactions that typically occur over pure alumina. © 1992.