Acidity Characterization of Amorphous Silica–Alumina

Abstract

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.

Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

“…2.0 ppm), hydroxyl units bound to Lewis acid sites (LAS) and extra-framework aluminum species (EFAl) through hydrogen bonding (2.4 -2.8 ppm), Brønsted acid sites (3.8 ppm) and hydrogen bonded hydroxyls (Si-OHnest) (4.3 ppm). [33][34][35] In addition, a peak at 0.8 ppm due to isolated Al-OH is particularly visible in the 30P sample. The origin and the attribution of the broad component centred at 6.6 ppm is not straight forward.…”

A real support effect on the hydrodeoxygenation of methyl oleate by sulfided NiMo catalysts

“…The acidic properties of these support materials, mostly taken from literature [46], are collected in Table 1. The number of strong Brønsted acid sites (BAS) in these samples were obtained by decomposition of adsorbed isopropylamine, the number of Lewis acid sites by infrared spectroscopy of adsorbed CO at 80 K. The number of BAS on silica and γ-alumina is negligible.…”

“…The number of strong Brønsted acid sites (BAS) in these samples were obtained by decomposition of adsorbed isopropylamine, the number of Lewis acid sites by infrared spectroscopy of adsorbed CO at 80 K. The number of BAS on silica and γ-alumina is negligible. The silica-alumina material contains about 3.6 μmol/g, a much smaller number than present in crystalline aluminosilicate zeolites [46]. The activated carbon support contains only a very small amount of strong BAS.…”

“…γ-alumina (Ketjen CK-300), silica (Shell) and activated carbon (NORIT RX-3 extra) were used as received. The acidic properties of the used ASA, SiO 2 and Al 2 O 3 have been reported elsewhere [46]. The textural properties of all support materials are collected in Table 1.…”

Sequential Condensation and Hydrodeoxygenation Reaction of Furfural-Acetone Adduct over Mix Catalysts Ni/SiO₂ and Cu/SiO₂ in Water

“…1b) showed the low degree crystallinity of SiO2 by the formation of physical interaction between silica and metal [12][13]. Changing the crystallinity of SiO2 to amorphous after deposition of the metal increased the acidic surface of the catalyst and then enhance the catalytic activity [14].…”