Surface characterization of amorphous silica− alumina (ASA) by CO ads IR, pyridine ads IR, alkylamine temperature-programmed desorption (TPD), Cs + and Cu-(EDA) 2 2+ exchange, 1 H NMR, and m-xylene isomerization points to the presence of a broad range of Brønsted and Lewis acid sites. Careful interpretation of IR spectra of adsorbed CO or pyridine confirms the presence of a few very strong Brønsted acid sites (BAS), typically at concentrations lower than 10 μmol/g. The general procedure for alkylamine TPD, which probes both Brønsted and Lewis acidity, is modified to increase the selectivity to strong Brønsted acid sites. Poisoning of the m-xylene isomerization reaction by a base is presented as a novel method to quantify strong BAS. The surface also contains a weaker form of BAS, in concentrations between 50 and 150 μmol/g, which can be quantified by CO ads IR. Cu(EDA) 2 2+ exchange also probes these sites. The structure of these sites remains unclear, but they might arise from the interaction of silanol groups with strong Lewis acid Al 3+ sites. The surface also contains nonacidic aluminol and silanol sites (200−400 μmol/g) and two forms of Lewis acid sites: (i) a weaker form associated with segregated alumina domains containing five-coordinated Al, which make up the interface between these domains and the ASA phase and (ii) a stronger form, which are undercoordinated Al sites grafted onto the silica surface. The acid catalytic activity in bifunctional nheptane hydroconversion correlates with the concentration of strong BAS. The influence of the support electronegativity on the neopentane hydrogenolysis activity of supported Pt catalysts is considerably larger than that of the support Brønsted acidity. It is argued that strong Lewis acid sites, which are present in ASA but not in γ-alumina, are essential to transmit the Sanderson electronegativity of the oxide support to the active Pt phase.

By following the selective H/D exchange of acidic hydroxyl groups in aluminosilicates by IR spectroscopy, clear evidence is provided for the existence in amorphous silica-aluminas (ASAs) of Brønsted acid sites comparable in strength to the bridging hydroxyl groups in zeolites. The method distinguishes various types and strengths of strong Brønsted acid sites in aluminosilicates (zeolites, clays, ASAs) and can be used to quantify them. The concentration of strong Brønsted acid sites in ASAs is 2-3 orders of magnitude lower than that in zeolites. The acid catalytic activity in n-alkane hydroconversion correlates well with the number of strong Brønsted acid sites, as probed by our method, for steam stabilized zeolites, clays, and ASAs. The strong Brønsted acid sites in these aluminosilicates arises from hydroxyl groups located between aluminum and silicon occupied oxygen tetrahedra. Differences in Brønsted acidity are caused by differences in the concentrations of strong Brønsted acid sites.

The most acidic OH groups in silica-aluminas (zeolites, clays, amorphous silica-aluminas) can be made to react selectively with C(6)D(6) to give acidic OD groups; quantification by IR spectroscopy shows that differences in the overall Brønsted acidity of aluminosilicates are dominated by differences in the density of sites of similar acid strength.

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