A spectroscopy and catalysis study of the nature of active sites of MgO catalysts: Thermodynamic Brønsted basicity versus reactivity of basic sites

Abstract

The relationship between thermodynamic Brønsted basicity and reactivity of basic sites of MgO samples was investigated by means of methanol deprotonation followed by IR and the conversion of 2-methylbut-3-yn-2-ol (MBOH), respectively. The relative distribution of basic oxide ions, O 2− LC, in low coordination (where LC = 3C, 4C, and 5C refer to tri-, tetra-, and penta-coordinated oxide ions, respectively) was modulated using different preparation routes. The resulting samples were classified on the basis of the relative distributions of O 2− LC ions determined by photoluminescence. The influence of the coordination of O 2− LC ions on the basic properties was studied for clean surfaces obtained after high-temperature (⩾1023 K) evacuation of CO 2 and water; the lower the coordination of O 2− LC ions, the higher the deprotonation ability and the reactivity of basic sites of the catalyst. The hydroxylation of clean MgO surfaces was studied and its influence on Brønsted basicity determined. Despite a low deprotonation ability, hydroxylated surfaces are more reactive than clean surfaces. The direct influence of OH groups on reactivity of basic sites was evidenced by correlating the latter and the amount of isolated OH groups evaluated by in situ diffuse reflectance Fourier transform infrared spectroscopy. It can be inferred that the peculiar reactivity of OH groups compared with O 2− LC ions is due to the variable stability of the alcoholate intermediate formed on both kinds of basic sites. On hydroxylated surfaces, because OH groups are poor Brønsted bases, the number of alcoholate species is lower than on clean surfaces, but these intermediates are less stabilized and so are more reactive.