Daniel Brunel

Cesium oxide encapsulation in faujasite zeolites effect of framework composition on the nature and basicity of intrazeolitic species

Abstract Post-synthetic modification of basic CsNaX and CsNaY zeolites was performed by impregnation with cesium acetate at varoous loadings followed by thermal decomposition of the cesium acetate into oxide. A comparative study of the nature and basic character of intrazeolitic species in CsNaX and CsNaY zeolites is reported. Crystallinities of modified X zeolites are largely retained after activation at 550°C for six hours. Under the same activation conditions modified Y zeoolites are less stable as evidenced by XRD, N 2 sorption, 27 Al and 29 Si MAS NMR and stepwise thermal desorption of CO 2 (TPD). The modified CsNaY zeolite crystallinities were largely maintained when the activation temperature was lowered to 400°C. The TPD of CO 2 below 500°C allows the differentiation of the structures of guest cesium species occluded in the host CsNaX or CsNaY zeolites. A shift of the desorption peak maximum from 250 to 150°C accounts for a higher basicity of the species within the pores of the CsNaX host than in the CsNaY one. Linear correlations between the amount of desorbed CO 2 and the cesium loading suggest a homogeneously dispersed loading up to 16 and 24 cesium atoms per unit cell for the modified X and Y zeolites, respectively. The formation of oxide (Cs 2 O) inside the cages of the CsNaX zeolite is proposed. In the case of the modification of the CsNaY zeolite various structures are discussed involving either the formation of local lattice cesium silicate or aluminate defects or the encapsulation of cesium oxide.

Factors affecting the efficiency of hybrid chiral mesoporous silicas used as heterogeneous inorganic–organic catalysts in the enantioselective alkylation of benzaldehyde

Abstract Chiral hybrid organic–inorganic materials were prepared by anchoring optically active β-aminoalcohols such as (−)-ephedrine on MCM-41 type silica as a member of micelle-templated silicas (MTS). Silylation of the surface was performed with halogenopropyltrimethoxysilane. Then, (−)-ephedrine was anchored through halogen substitution. These materials were used as chiral auxiliaries in the heterogeneous enantioselective catalysis of the alkylation of benzaldehyde by diethylzinc. This work deals with the study of the various factors such as accessibility to the catalytic sites and coverage of the inorganic surface, which affect their efficiency (activity, selectivity and enantioselectivity).

Chiral mesoporous templated silicas as heterogeneous inorganic–organic catalysts in the enantioselective alkylation of benzaldehyde

Abstract New mesoporous templated silicas of the MCM-41 family covalently linked to chiral ephedrine are used as heterogeneous chiral auxiliaries in the enantioselective alkylation of benzaldehyde by diethylzinc. Promising results are obtained with these inorganic–organic catalysts. Various factors which may affect the activity, selectivity and enantioselectivity are studied. Comparison with homogeneous catalysis is carried out.

Characterization of basicity in alkaline cesium-exchanged X zeolites post-synthetically modified by impregnation: A TPD study using carbon dioxide as a probe molecule

Interesting basic zeolites have been prepared by impregnating CsNaX zeolite with various loadings of cesium acetate in order to improve, after calcination, the basic character of the solid and to benefit from the cage effect in fine chemistry. In the range of 0 up to 26 cesium atoms per unit cell, X-ray powder diffraction shows that the crystallinity is largely retained after the generation of the basic species. The microporous volume values indicate that an interesting space is available for fine chemistry. For characterizing the surface chemical properties of the basic post-synthetically modified CsNaX zeolites, the stepwise thermodesorption of CO2 (TPD) has been studied. Experimental conditions allowing an accurate determination of the basic species are described. The modified zeolites with various loadings have been compared under these conditions. For loadings below 16 cesium atoms per unit cell, a linear relationship is established between the amount of desorbed CO2 (molecules per unit cell), up to 550°C, and the impregnated cesium loading (atoms per unit cell). This result supports the hypothesis that the active cesium species are homogeneously located inside the cages of the CsNaX zeofite. The slope of the straight line (0.44) indicates that one cesium oxide per zeolite cage is generated, leading upon CO2 adsorption to a carbonate species (Cs2CO3). For loadings higher than 16 cesium atoms per unit cell, a saturation is observed for the amount of desorbed CO2 up to 550°C along with an increase of the amount of desorbed CO2 above 550°C corresponding to a deposition of basic species on the external surface of the solids.

Oxidation of sulfoxides to sulfones by hydrogen peroxide over Ti-containing zeolites

The oxidation reaction of dibutylsulfoxide to the corresponding sulfone by dilute hydrogen peroxide is investigated over Ti-containing zeolites as catalysts; Ti-beta is more active than TS-1, as for the oxidation of sulfides under similar conditions. The kinetic parameters are determined over Ti-beta in t-butanol as solvent, and the kinetic law of the sulfoxide oxidation reaction is established. The influence of the reaction temperature and effect of the solvent are also reported.

Activity in the Knoevenagel condensation of encapsulated basic cesium species in faujasite CsNaX or CsNaY

Abstract The activity of basic species generated in zeolite CsNaX or CsNaY by cesium acetate impregnation and calcination under a synthetic air flow has been investigated. The Knoevenagel condensation of benzaldehyde and ethylcyanoacetate was used as a model reaction. The evaluation of the number of basic sites was carried out by CO 2 TPD. Correlations between initial rates and number of basic sites allowed the determination of the TOF for the modified zeolites. It depends mainly on the host zeolite composition. Moreover, activity varies strongly with crystallinity especially for zeolites Y at high loadings.

Design of mesoporous aluminosilicates supported (1R,2S)-(−)-ephedrine: evidence for the main factors influencing catalytic activity in the enantioselective alkylation of benzaldehyde with diethylzinc

Abstract (−)-Ephedrine, used as a model β-amino alcohol, was covalently anchored on mesoporous micelle templated aluminosilicates (Al-MTS) through nucleophilic substitution of halogenoalkyl(aryl)silane chains previously grafted on the surface. The covalent grafting was performed either by silylation (method a) or by surface sol–gel (method b). The latter method provided higher loading. However, the higher loading lowers the resulting mesoporous volume. The coupling alkyl halide moiety was then substituted with (1R,2S)-(−)-ephedrine. Used as chiral auxiliaries in the heterogeneous enantioselective catalysis of the alkylation of benzaldehyde by diethylzinc, these materials showed properties which depend mainly on the grafting method. The best results (activity, enantioselectivity) were obtained with catalysts prepared from supports featuring high initial pore diameter. The effect of the regular porosity on the efficiency and enantioselectivity was shown. Dilution of catalytic sites by alkyl groups and rigidification of the linker were also studied.

Catalytic enantioselective addition of diethylzinc to benzaldehyde induced by immobilized ephedrine: comparison of silica and MCM-41 type mesoporous silicates as supports

The synthesis and characterization of a chiral amino-alcohol ((1R-2S)-ephedrine) immobilized on MCM-41 type mesoporous silicas (MTS: Micelle Template Silicas) are described. The activity of these supported catalysts in the enantioselective addition of diethylzinc to benzaldehyde are reported, and compared with those obtained with the corresponding silica gel supported catalysts. The observed differences are discussed in terms of the nature of the grafting which depends on the support structure.

29-O-02 Towards total hydrophobisation of MCM-41 type silica surface

Publisher Summary This chapter presents the refinement of the procedure for MCM-41 surface fictionalization with various octylsilanes to obtain a maximum surface coverage. Two different types of surface modification using different grafting agents are performed on micellar templated silica (MTS) samples. MTS samples possessing three different pore sizes are functionalized by the hydrolysis and polymerization of pre-adsorbed monolayer of octylsilanes on MTS surface or by chlorine substitution from chlorooctylsilane with nucleophilic assistance. The effect of various experimental parameters on the hydrophobicity and the grafted chain loading is also analyzed.

Rearrangement of Epoxides Using Modified Zeolites

Summary The rearrangement of styrene oxide into phenylacetaldehyde was studied over various zeolites (H-ZSM-5, HY, H-offretite). It was first shown that both external and internal acidic sites are involved in that easy isomerization. Moreover, a comparative study of the rearrangement of this epoxide and of its hindered analog, l-phenyl-1,2-epoxycyclohexene, on silanated offretite, allowed a discrimination between the activities of these sites.