Acid-base properties of Mg-Ni-Al mixed oxides using LDH as precursors

The thermal behavior of a series of Mg/Ni/Al hydrotalcites with (Mg + Ni)/A l = 2 and different Mg/Ni ratios has been studied (by means of X-ray diffra ction (XRD), thermogravimetry and X-ray photoelectron spectroscopy (XPS), a s well as the acid-base properties of the mixed oxides resulting from their calcination at 723 K (by means of adsorption microcalorimetry). Up to 500 K, only the physisorbed and interlayer water is released, causing a small loss of crystallinity. The decomposition of carbonate anions begin s above 500 K, but this loss does not destroy the LDH structure which remai ns up to temperatures between 600 and 650 K. Above, the structure collapses and dehydroxylation occurs, leading to the formation of mixed oxides with large surface areas. At 723 K, a temperature usually chosen for the catalyt ic reactions, about 30% of the initial carbonates are still detected and po ison a part of the strongest basic sites. The. decarbonation is practically completed at 923 K. When the stability of the carbonates increases with th e nickel content, the stability of the hydroxides increases, on the contrar y, with the magnesium content. The NH3 adsorption data indicate an increase of the concentration and stren gth of acid sites with the nickel content. The compound SO2 seems to be mor e efficient than CO2 for the analysis of the basicity, because. of its high er acidity and the presence of remaining carbonates in the mixed oxides. Th e highest concentration of basic sites is observed for the Mg/Ni/Al sample with low magnesium and high nickel contents. This gives evidence of the syn ergy effect of these metals favoring the basic properties and the role of M gO as promoter. Above 723 K, the concentration of the basic sites of mixed oxides decreases, which can be related to a more complete dehydroxylation. (C) 2001 Elsevier Science BN. All rights reserved.