Presence Of Lewis Acid Sites Research Articles

The characterization of tungsten-oxide-modified zirconia supports for dual functional catalysis

The results of catalytic titration measurements indicated that WOx/ZrO2 catalysts prepared by coprecipitation have higher acid site density and strength, and are more active for pentane isomerization, than catalysts prepared by impregnation. The coprecipitated WOx/ZrO2 contains ∼0.004 meq-H+/g-catalyst. XPS of chemisorbed 2,6-dimethylpyridine and pyridine revealed the presence of Lewis acid sites as well as strong and weak Bronsted acid sites on the surface of these catalysts. The concentration of strong Bronsted sites was close to the concentration of Lewis sites, suggesting that the high acidity of these materials may originate from a conjugate Bronsted–Lewis site. Pt/WOx/ZrO2 undergoes a reversible loss of hydrogen chemisorption capacity with increasing hydrogen pretreatment temperature, accompanied by a reversible loss of pentane isomerization activity. This loss can be attributed to a strong Pt-reduced tungsten oxo species interaction (SMSI). Room temperature hydrogen spillover is observed in the Pt/WOx/ZrO2 (∼16% W) catalyst, consistent with the presence of bulk WO3 in this material as observed by TPR.

Surface characterization of alumina-supported catalysts prepared by sol–gel method. Part I. Acid–base properties

Surface characterization of Al2O3, In2O3–Al2O3 and Ga2O3–Al2O3 prepared by sol–gel method was made focusing on the acid–base properties. Cyclohexanol conversion as a model reaction revealed that acid sites rather than basic sites are present predominantly on the surface. The presence of Lewis acid sites was confirmed by pyridine adsorption followed by Fourier transform infrared (FT-IR) spectroscopy. In addition, a certain amount of Bronsted acid sites were also revealed by the adsorption of 2,6-dimethylpyridine (DMP). From the adsorption of CO, it was found that the addition of Ga2O3 into alumina leads to a decrease of Lewis acid strength but to an increase of the number of sites, while In2O3 addition caused a global decrease of Lewis acidic properties of alumina. Using CO2 as a probe to study the basicity, the number of surface basic sites on alumina was found to be decreased by the In2O3 and Ga2O3 additives. A good correlation between the catalytic activity for 3,3-dimethylbut-1-ene isomerisation and the surface acidity estimated by CO and DMP adsorption was supposed. The combination of model reactions and adsorption of specific probe molecules gives us detailed information concerning the surface acid–base properties.

Comparison of the acid strength of dealuminated H-faujasites determined by 1H NMR after water adsorption

The acidities of five steamed H-faujasites with similar framework Si/Al ratios have been compared using 1H NMR (MAS at room temperature and broad line at 4 K), water being used as base. One of the samples, S, was not treated after steaming; the other samples were washed with one of the following aqueous solutions: (i) nitric acid (sample A), (ii) the following agents (with subsequent cation exchange if necessary): Na 2H 2EDTA (once for sample E1 and twice for E2); ammonium hexafluorosilicate (sample F). The number of Brønsted acid sites (BAS) corresponds to the framework Al concentration for all samples except A, for which it corresponds to the bulk Al concentration. The ‘acidity coefficient’, χ a, which is equal to the number of hydroxonium ions per BAS when enough water is adsorbed for each BAS to interact with a water molecule, is 0.40 (±10%) for properly washed samples. It is about 0.2 for S and E1. For all samples, an increase of the number of hydroxonium ions per BAS is observed for higher concentrations of adsorbed water; for the washed samples, it can be related quantitatively to the presence of Lewis acid sites.

Microporosity and Acidity Properties of Alumina Pillared Titanates

A titanate with lepidocrocite-like structure has been intercalated by hexylammonium (HA) and tetrabuthylammonium (TBA) organic cations, which were further exchanged with aluminum polyoxocations. The pillared structure of these materials was found to be stable up to 500 °C and exhibited a basal spacing of 1.4 nm. Nitrogen adsorption isotherms were measured to determine the surface areas and pore structure of these pillared titanates, and the effect of intercalation temperature and heat treatment on these surface characteristics is discussed. The micropore volume values were related to the amount of alumina and to the heat-treatment temperature. Most of the pillared titanates prepared from HA−Ti showed a sharp distribution of micropores with the average diameter of 2 nm, whereas the comparable materials obtained from TBA−Ti showed a broad distribution of pore diameter centered 4 nm in addition to micropores. Infrared spectroscopy monitoring the pyridine adsorption showed the presence of Lewis acid sites, an...

Application of Inverse Gas Chromatography to the Study of the Surface Properties of Slates

AbstractInverse gas chromatography (IGC) at zero surface coverage was used for determining the surface characteristics of a slate sample. Previously this sample had been characterized by means of X-ray diffraction (XRD), chemical analysis, infrared (IR) spectroscopy and nitrogen adsorption. Quartz, muscovite and chlorite were the minerals foming the slate. A well-defined mesoporous structure with pores of 15 Å of interlayer distance was observed. Chemical analysis and IR spectroscopy corroborated the X-ray results. The surface characteristics of the slate were defined in relation to nonspecific and specific interactions with organic molecules. In accordance with the nonspecific interactions, the London component (γSD) of the surface free energy gave an estimation of the surface energy of the slate sample. The obtained values for γSD were 140.0, 124.8, 108.2 and 96.8 mN m-1 at 100, 110, 120 and 130 °C, respectively. These values were characteristic of an inorganic sample of high surface energy. The values of the thermodynamic variables—differential heat of adsorption, free energy and entropy of adsorption—were in accordance with these results. At the same time, specific interactions were characterized by the ηπ parameter and the acid-base (KA, KB) indices. The positive value of ηπ (0.09 kJ mol-1) indicated the presence of Lewis acidic sites on the slate surface. The value of KA (acid index) of 1.17 was higher than the value of KB (base index) of 0.37; this result confirmed the acidic nature of the studied slate's surface.

EXAFS and NMR investigation of zinc, manganese and cobalt substituted aluminophosphates with the chabazite structure

Zinc, manganese and cobalt containing aluminophosphates with the chabazite (CHA) structure type (MeAPO-34, MeZn, Mn and Co) have been synthesized hydrothermally in the presence of tetraethylammonium hydroxide. All samples showed a very high crystallinity without impurity phases, as evidenced by X-ray diffraction and scanning electron microscopy. The results of EXAFS spectroscopy on as-synthesized samples show that the metals possess 4 oxygen atoms in the first coordination shell which is an indication for a tetrahedral coordination in the lattice. The decrease of the EXAFS amplitude for some samples upon calcination is in agreement with the presence of Lewis acid sites, as previously reported. 31P NMR confirmed the incorporation of metals in the AlPO 4-34 structure. For Zn and Mn, the 31P NMR spectra show various lines attributed to different P( nAl, 4- nMe) units and the calculation of the fraction of metal in the samples after deconvolution of the spectra is in very good agreement with elemental analysis. Comparison of the relative intensities of the lines with those obtained by a binomial theorem showed that metals randomly substitute Al in the AlPO 4-34 framework. Incorporation of Co in the samples was suggested by a strong decrease in both T 1 and T 2 relaxation times with respect to SAPO-34 or ZnAPO-34, thus confirming EXAFS data.

Decomposition of n-butane on alkaline-earth mordenites

The acidity of alkaline-earth mordenites, as observed by the ir spectra of adsorbed pyridine, has been correlated with their activity as catalysts for n-butane decomposition. The results indicate that Brønsted acid sites are the origin of the catalytic activity. However, the presence of Lewis acid sites is necessary for the reaction to take place, suggesting an inductive effect of these sites on the Brønsted sites. The presence of alkaline-earth cations in decationated mordenites decreases the activity of the original sample; this influence increases with decreasing ionic radius. This result is interpreted as a weakening of the inductive effect produced by the structural Lewis sites through a fall in their number by the cation exchange process. The reactant molecules are coordinatively bound to the cations to an extent which is strongly dependent on their electrostatic field, thus influencing the accessibility of the Brønsted sites.

Infra-red study of adsorption and oxidation of ammonia on silica-supported platinum and silica

An infra-red study of the adsorption of ammonia on silica-supported platinum in the region 3600-1900 cm–1 over a temperature range 25–130°C yields four spectral features: a strong broad absorption at 2960 cm–1 due to the perturbed ν(OH ···) in a surface SiOH ··· NH3 complex, a shoulder at 3280 cm–1 and two fairly sharp medium bands at 3425 and 3345 cm–1. Only the 3345 and 3280 cm–1 bands remain at 130° and are assigned to the asymmetric and symmetric stretching modes of NH3 chemisorbed at Pt sites. The 3425 cm–1 band is attributed to a stretching mode of NH3 adsorbed at surface hydroxyl sites. The spectra of NH3+ O2 mixtures over silica-supported platinum have been recorded at 25°C intervals over the temperature range 25–200°C. Up to 150°C the same pattern of behaviour is observed as for NH3 adsorbed on silica-supported platinum. Above this temperature a doublet at 2235 and 2215 cm–1 appears due to gaseous N2O. On cooling, a new strong band is found at 3480 cm–1 which is due to the H2O formed by oxidation and subsequently adsorbed on the catalyst. The same experiments performed over silica in the temperature range 25–320°C show that silica itself is also able to catalyze ammonia oxidation at temperatures above 220°C. The presence of bands at 3350 and 3280 cm–1 renders suspect the use of such a doublet as a criterion for the presence of Lewis acid-sites.