Anne Davidson

Role of silanol groups in the incorporation of V in β zeolite

Aqueous solutions of ammonium metavanadate of increasing concentrations have been contacted with a b zeolite before . . sample Alb ,S irAls 11 and after sample Sib ,S irAl) 1000 dealumination in a 13 N HNO solution. UV-visible and 3 51 V NMR characterizations of the V-loaded zeolites show that mainly octahedral V species are formed in Alb whereas, up . . to a loading of 1.75 V atoms per unit cell 2.3 V wt.% , only pseudotetrahedral V species nonhydroxylated SiO V5O and 3 . . . hydroxylated SiO HO V5O are observed in Sib. XRD and FT-IR results indicate that the V atoms present in Sib are 2 incorporated in the framework in a pseudotetrahedral environment. These V species are resistant to washing either with y1 . water or with an aqueous solution of NH OAc 1 mol l , 12 h . In contrast, the octahedral V species present in VAlb 4 . . whatever the V loading and in VSib at loadings higher than 1.75 V per unit cell are eliminated upon washing showing that these species are loosely bound to the zeolite. As demonstrated by FT-IR, the incorporation of V in the Si b framework involves silanol groups formed upon dealumination. This incorporation generates new Bronsted acidic sites. New FT-IR ¨ y1. bands 3650, 3620, 980, 950 cm are formed whose attribution is proposed. The maximum amount of V atoms . incorporated in Sib 1.75 per unit cell is lower than the potential amount of framework vacant T-sites generated by . dealuminating the b framework 5.3 per unit cell . However, this V amount is 20 times larger than that usually found in V-loaded zeolite prepared by hydrothermal synthesis. Therefore, this study confirms that dealuminating a zeolite is an attractive method to generate silanol groups which can be used to react with vanadium complexes and allow the incorporation in the zeolitic walls of high amounts of V atoms resistant to washing treatments. q 2000 Elsevier Science B.V. All rights reserved.

Modifying the walls of mesoporous silicas prepared by supramolecular-templating

The present review is not exhaustive and rather endeavors to illustrate ways to improve the wall stability of ordered mesostructured silicas in presence of water. In situ X-ray diffraction of the phase transformations of hybrid (surfactant containing) silicas show how synthesis temperature, pH and duration control wall chemistry (thickness, condensation and hydrolysis). Ex situ studies of calcined (surfactant free, empty mesopores) silicas show how wall stability (in boiling water, under steam) is affected by synthesis conditions and/or post-synthesis treatments.

Effect of calcination/rehydration treatments on the environment of V in β zeolite

A dealuminated zeolite β has been contacted with solutions of ammonium metavanadate to incorporate vanadium as tetrahedral V species which are not removed under treatment with aqueous solution of ammonium acetate. The effects of calcination and rehydration on the environment of the V species have been studied by diffuse reflectance UV-visible and 51V NMR. The reducibility of the V species has been followed by TPR and ESR. The dehydroxylation/rehydroxylation process occurring in the β framework upon dehydration/rehydration has been monitored by 29Si MAS NMR and FT-IR spectroscopy.

Fine tuning of the interaction between Pluronic surfactants and silica walls in SBA-15 nanostructured materials

Recent efforts made to understand the nature of the interactions between Pluronic surfactants and silica walls in SBA-15 materials and to modify these interactions by post-synthesis hydrothermal treatments are summarized.

Monitoring of the state of silver in porous oxides during catalyst preparation

Abstract Silver-supported porous oxides are promising heterogeneous catalysts for air treatment. Classical ways of preparation are incipient wet impregnation or ion exchange (in case of zeolites) of a support with silver nitrate. Nevertheless, this precursor is known for its photosensitivity. The latter can lead to preparations that are not well controlled yet. In this contribution, the influence of the nature of the oxide support and of the thermal activation conditions towards the state and the dispersion of silver in the catalyst is investigated. Complementary characterization techniques are used, i.e. Temperature Programmed Reduction (TPR) coupled with mass spectrometry (MS), UV-Visible spectroscopy (UV-Vis.) and Transmission Electron Microscopy (TEM).

Accessibility of Co3O4 particles patterned in SBA-15

Abstract Crystalline Co 3 O 4 nanoparticles were patterned in the pores of SBA-15 silicas using the “two solvents” technique (Co/Si atomic ratios between 0.10 and 0.35, pore sizes: 5.1, 5.6 and 7.3 nm). Normalized surface areas, calculated from N 2 sorption experiments, reveal a partial plugging of the pores which, for a given Co/Si atomic ratio, increases when the pore size decreases. The CO oxidation catalytic reactivity of the Co 3 O 4 particles either embedded or extracted from silica confirms that the Co 3 O 4 particles are partially accessible to reagents. Despite accessibility limitations, the reactivity of the patterned Co 3 O 4 particles compares favorably with the very few data available in the literature. Unexpectedly low activation energies are even measured. Finally, H 2 -TPR experiments reveal that accessibility limitations are correlated with the formation of hardly reducible Co silicate species.

Thermal stability of V-loaded AlPO4-5 materials

A progressive sintering of the porosity of both V-free and V-loaded AlPO4-5 samples is observed upon calcination in air at increasing temperatures. This sintering leads to the coexistence of porous AlPO4-5 and dense tridymite. As established by cristallinity and pore volume measurements. the presence of vanadium decreases by 100 to 300°C the threshold temperature at which this sintering begins, in spite of the low V amount in the samples (V/(V+Al+P) atomic ratio of 0.5–1%). This threshold temperature depends on the method used to introduce V (by hydrothermal synthesis, under static conditions or agitation, and/or by grinding vanadia with a V-free AlPO4-5) and on the morphology of the AlPO4-5 crystals as observed by scanning electron microscopy. These factors are related to the dispersion of the V species within the porosity. Since only very small amounts of intraporosity VIV species (VIV/VT<1%) are detected by ESR, whatever the extent of the Sintering which generates tridymite, it is concluded that the thermal stability of V-loaded AlPO4-5 is determined by the dispersion of Vv species within the porosity. This conclusion is in line with earlier propositions made to explain the thermal behaviour of V-loaded zeolites and V-contamined FCC catalysts.