Acidic Character of Metal-Loaded Amorphous and Crystalline Silica−Aluminas Determined by XPS and Adsorption Calorimetry

Abstract

The XPS technique in combination with microcalorimetry was used to picture the acid character of metal ion loaded zeolite and silica−alumina samples. Co, Cu, and Ni ions were loaded on ZSM-5 and SiO2−Al2O3 (about 1 mmol/g) by three different procedures: ion exchange, impregnation, and solid-state ion-exchange. The samples prepared by ion exchange of the two matrixes presented high values of the 2 p3/2 XPS bands of the metal ions, indicating the occurrence of charge transfer from the ions to the support matrix. This permitted the stabilization of the metal phases as isolated ions rather than as oxide clusters. Microcalorimetric experiments of ammonia adsorption were performed in order to determine the number, strength (i.e., adsorption enthalpy), and strength distribution of the acid sites on the samples and on the relevant matrixes. The N 1s XPS lines of ammonia adsorbed on the surfaces were decomposed into two component peaks, assigned to Brönsted (average value of BE, 402.2 eV) and Lewis (average value of BE, 400.4 eV) acid sites. The relative intensities of the two peak components were measured for the quantitative determination of Brönsted and Lewis acid site concentrations. Coupling the information from adsorption calorimetry and XPS spectroscopy of N 1s adsorbed lines, the absolute numbers of Brönsted and Lewis acid sites on each sample were determined. The two support matrixes were protonic acids (65 and 60% of Brönsted acid sites for ZSM-5 and SiO2−Al2O3, respectively), and the acidity of ZSM-5 was greater than that of SiO2−Al2O3 considering both the number of total acid sites and the acid strength. The presence of metal ions deposited on the two matrixes deeply changed the respective proportions of Brönsted and Lewis sites. A huge increase in the Lewis acid site population of the ZSM-5-based samples (70, 85, and 90% of Lewis sites for the samples containing Co, Cu, and Ni, respectively) and of the SiO2−Al2O3-based samples prepared by ion exchange (55, 60, and 70% of Lewis sites for the samples containing Co, Cu, and Ni, respectively) was observed.