Acidity Characterization of Amorphous Silica–Alumina

Abstract

Surface characterization of amorphous silica–alumina\n(ASA)\nby CO<sub>ads</sub> IR, pyridine<sub>ads</sub> IR, alkylamine temperature-programmed\ndesorption (TPD), Cs<sup>+</sup> and Cu­(EDA)<sub>2</sub><sup>2+</sup> exchange, <sup>1</sup>H NMR, and <i>m</i>-xylene isomerization\npoints to the presence of a broad range of Brønsted and Lewis\nacid sites. Careful interpretation of IR spectra of adsorbed CO or\npyridine confirms the presence of a few very strong Brønsted\nacid sites (BAS), typically at concentrations lower than 10 μmol/g.\nThe general procedure for alkylamine TPD, which probes both Brønsted\nand Lewis acidity, is modified to increase the selectivity to strong\nBrønsted acid sites. Poisoning of the <i>m</i>-xylene\nisomerization reaction by a base is presented as a novel method to\nquantify strong BAS. The surface also contains a weaker form of BAS,\nin concentrations between 50 and 150 μmol/g, which can be quantified\nby CO<sub>ads</sub> IR. Cu­(EDA)<sub>2</sub><sup>2+</sup> exchange\nalso probes these sites. The structure of these sites remains unclear,\nbut they might arise from the interaction of silanol groups with strong\nLewis acid Al<sup>3+</sup> sites. The surface also contains nonacidic\naluminol and silanol sites (200–400 μmol/g) and two forms\nof Lewis acid sites: (i) a weaker form associated with segregated\nalumina domains containing five-coordinated Al, which make up the\ninterface between these domains and the ASA phase and (ii) a stronger\nform, which are undercoordinated Al sites grafted onto the silica\nsurface. The acid catalytic activity in bifunctional <i>n</i>-heptane hydroconversion correlates with the concentration of strong\nBAS. The influence of the support electronegativity on the neopentane\nhydrogenolysis activity of supported Pt catalysts is considerably\nlarger than that of the support Brønsted acidity. It is argued\nthat strong Lewis acid sites, which are present in ASA but not in\nγ-alumina, are essential to transmit the Sanderson electronegativity\nof the oxide support to the active Pt phase.