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Acidity Characterization of Amorphous Silica–Alumina

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journal contribution
posted on 20.02.2016, 09:14 by Emiel J.M. Hensen, Dilip G. Poduval, Volkan Degirmenci, D.A J. Michel Ligthart, Wenbin Chen, Françoise Maugé, Marcello S. Rigutto, J.A. Rob van Veen
Surface characterization of amorphous silica–alumina (ASA) by COads IR, pyridineads IR, alkylamine temperature-programmed desorption (TPD), Cs+ and Cu­(EDA)22+ exchange, 1H 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 COads IR. Cu­(EDA)22+ 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 Al3+ 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 n-heptane 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.