G. Coudurier

Acidic and catalytic properties of CsxH3−xPW12O40 heteropolyacid compounds

The heteropolyacid of Keggin structure (H3PW12O40) and several of its cesium salts have been synthesized and characterized for their acidic properties. Chemical analysis, TGA, NH3 adsorption-desorption and31P MAS-NMR techniques permit the characterization of the Brønsted acidity. The catalytic properties were studied forn-butane isomerization and for methanol conversion to dimethyl ether at 200 and 180°C, respectively. Our conclusion is that, in term of conversion, the more acidic catalysts are in the range Cs2/2.7 if considering a wide range of acid strengths evidenced by methanol conversion. The range is Cs2/Cs2.1 if one considers only very strong acidity as evidenced byn-butane isomerization.

Properties of boron-substituted ZSM-5 and ZSM-11 zeolites

Using NMR and IR spectroscopy boron-substituted pentasil zeolites are found to contain four-coordinated B in the hydrated state and three-coordinated B in the dehydrated state. In addition a new tetrahedral B site assigned to (HO)2B(H2O)(OSi) was characterized by its NMR quadrupole coupling constant and asymmetry parameter. Ammonia adsorption microcalorimetry gave heats of adsorption of ~65 kJ/mol for HBZSM-11 and showed that B-substituted pentasils have only very weak acidity. Calcination at 800 °C increased the heat of NH3 adsorption to ~170 kJ/mol by creation of strong Lewis acid sites. The lack of strong Bronsted acid sites in HBZSM-11 was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation With methanol.

Basicity and basic catalytic properties of zeolites

In the present review article one describes basicity for zeolites from theoretical and experimental points of view. Zeolites appear to be rather soft bases, their basicity increasing within the series Li < Na < K < Rb < Cs. The physical methods used for characterizing basic sites either themselves (anionic) species: 02−, A104−, OH−) or through adsorption of acidic probes, as CO2, SO2, organic acids, pyrrole, etc., are described. A review of basic type reactions catalyzed by zeolites is also given. The influence of Lewis acidity of exchangeable cations and of basicity of framework oxide ions on adsorption processes is discussed.

Acid–base properties of metallic oxide catalysts studied by conversion of propan-2-ol

Conversion of propan-2-ol in the absence of air has been used to characterize the acid–base properties of metallic oxides. Two families of solids were studied: the first, niobic acid and oxides exhibited only acidic properties, i.e. resulted only in formation of propene, the second, mixed cobalt and nickel molybdates in their α or β forms, exhibited acidic and basic properties i.e. resulted in the formation of both propene and acetone. It was also observed that β-phase molybdates are more active and slightly more selective for propene than the α-phase molybdates.

Molecular design of active sites in partial oxidation reactions on metallic oxides

Abstract Though a variety of examples, it is shown that the catalytic oxidation reactions which operate via a Mars and van Krevelen mechanism imply ensembles of atoms containing variable valence cations and Lewis bases such as O2−,OH− or PO43− able to act in the different elementary steps. The structure and the size of these ensembles, defined as ‘inorganic oxide molecular clusters’, determine their reactional specificity. The examples chosen show that the ‘designs’ of such ensembles are strongly dependent on the nature of the material, on their morphology (structure sensitivity as for MoO3, vanadyl phosphates), and on their structure (vanadyl phosphates, iron phosphates). It is also possible to build such ensembles upon dispersion of an active oxide on an inert support (MoO3/SiO2, V2O5/TiO2).

ZrO2-SO42-Catalysts. Nature and Stability of Acid Sites Responsible for n-Butane Isomerization

Abstract The nature of acid sites responsible for n-butane isomerization at 150°C has been studied for ZrO 2 -SO 4 2− samples prepared and/or activated in different ways and using several techniques as infrared spectroscopy of adsorbed pyridine and catalytic tests. It is proposed that Bronsted and Lewis acid sites must be simultaneously present on the catalyst surface to ensure superacidic properties.

Niobium oxide based materials as catalysts for acidic and partial oxidation type reactions

Abstract Niobic acid, H 8 Nb 6 O 19 · x H 2 O, was synthesized and studied for its acidic features as a function of its dehydroxylation extent. It was observed to be strongly acidic, using NH 3 adsorption calorimetry and isopropanol conversion reaction as probe techniques, and to be weakly acidic on its dehydrated form, Nb 2 O 5 . The mixed oxide Al 2 o 3 :Nb 2 o 5 in 1:1 molar ratio prepared from aluminum and niobium oxalates was shown to be amorphous up to 1023 K where it crystallized in the form of AINbO 4 and to exhibit higher acidity than Nb 2 O 5 dehydrated, as expected from Tanabes model but much less acidic than the niobic acid form. The single Nb 2 o 5 and mixed A1 2 O 3 :Nb 2 o 5 oxides were used as supports for grafting MoO x and VO x species, respectively. It was observed that for MoO x /Nb 2 o 5 samples new strong acid sites were created while the redox properties were not as satisfactory as expected. For VO x /Al 2 O 3 :Nb 2 O 5 samples, at relatively low coverage (ca. 15°10), acidic features were not appreciably modified (only slightly enhanced) while new redox features were generated resulting in a rather satisfactory mild oxidation catalyst. The degree of condensation of VO x species on the mixed oxide surface, including some possible epitaxial grafting, even if not clearly characterized, appeared to play a determining role in oxidative dehydrogenation properties. This constitutes a new example of structure sensitivity in partial oxidation reactions on oxides.

Influence of preparation method on the acidity of MoO3(WO3)/ZrO2 catalysts

Zirconia-supported Mo and W oxides have been prepared by three methods: a conventional impregnation technique, a molten salt method and calcination of zirconium hydroxide impregnated with Mo(W) salt. The state of the Mo(W) and the acidity of the solids were studied, versus Mo(W) loading and calcination temperature. It has been shown that superficial acidity depends strongly on the preparation method used. Among the systems studied, solids prepared by calcination of impregnated zirconium hydroxide manifested the strongest Lewis and Bronsted acidity. The latter was, however, lower than the acidity of sulfated zirconias, as revealed by catalytic tests. A model of the structure of the acidic centres was proposed.

Comparison of the reactivities of H3PW12O40 and H4SiW12O40 and their K+, NH4+ and Cs+ salts in liquid phase isobutane/butene alkylation

Abstract Activity in n-butane isomerization reaction of various alkaline salts of H 3 PW 12 O 40 and H 4 SiW 12 O 40 was shown to be strongly dependent on the strength and number of accessible protons whereas the stability with time on stream was correlated to the presence of mesoporosity. For the liquid iC 4 /C 4 =continuous alkylation reaction, the strength and the number of acid sites appeared less important than the existence of mesoporosity indicating that the diffusion of the reactants and of the products plays an important role in this reaction.

X-ray diffraction and solid-state n.m.r. studies of AEL molecular sieves: Effect of hydration

Abstract Interpretation of X-ray powder diffraction spectra by using profile refinement techniques allowed the determination of (i) the positions of P, Al and O atoms in AEL-type materials (AlPO 4 -11, SAPO-11) and (ii) the effect of adsorbed water molecules on the geometry of the previously evacuated AEL framework. The as-synthesized sample (hereafter labeled SYN form) and the 550°C calcined form (CAL form) crystallize in the acentric Ima2 space group with 3P + 3Al sites per asymmetric unit, whereas the rehydrated material (WAT form) adopts the acentric Pna 2 1 space group with 5P + 5Al sites per asymmetric unit. In MAS -n.m.r. spectroscopy, a broad 31 P peak observed for the SYN and the dehydrated CAL forms is centered near δ = −30.7 ppm and is interpreted as the evelope of three overlapping components with 2:1:2 intensity ratios (8P1:4P2:8P3). For the hydrated WAT form two peaks centered at δ = −23 and −29 ppm having an intensity ratio of 1:4 have been attributed to the 4P1 and the 4P2 + 4P3 + 4P4 + 4P5 sites, respectively. The first peak corresponds to the P1 species, which is attached to OH and/or water molecules, as shown by proton cross-polarization experiments. The second broad peak, which corresponds to the remaining P atoms, is interpreted as four overlapping peaks of equal intensity. Water molecules were shown by X-ray diffraction experiments to form dimeric clusters and to be located close to P1 in the elliptical channels. It is suggested that this atom could be substituted by Si atoms in the SAPO-11 samples, as supported by the observed 1:4 intensity ratio and by the fact that the intensity of the component corresponding to P1 decreases with increasing Si content.