Monique Laspéras
École nationale supérieure de chimie de Montpellier
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Featured researches published by Monique Laspéras.
Microporous Materials | 1996
Monique Laspéras; Hélène Cambon; Daniel Brunel; Isabel Rodriguez; P. Geneste
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.
Tetrahedron-asymmetry | 1999
N. Bellocq; Sébastien Abramson; Monique Laspéras; Daniel Brunel; P. Moreau
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).
Tetrahedron-asymmetry | 1998
Monique Laspéras; N. Bellocq; Daniel Brunel; P. Moreau
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.
Microporous Materials | 1993
Monique Laspéras; Hélène Cambon; Daniel Brunel; Isabel Rodriguez; P. Geneste
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.
Journal of Molecular Catalysis A-chemical | 1998
Isabel Rodriguez; Hélène Cambon; Daniel Brunel; Monique Laspéras
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.
Tetrahedron-asymmetry | 2002
Sébastien Abramson; Monique Laspéras; Daniel Brunel
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.
Studies in Surface Science and Catalysis | 1997
N. Bellocq; Daniel Brunel; Monique Laspéras; P. Moreau
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.
Topics in Catalysis | 2000
Sébastien Abramson; Nathalie Bellocq; Monique Laspéras
Heterogeneous enantioselective alkylation of benzaldehyde with diethylzinc was performed using (-)-ephedrine grafted on mesoporous micelle templated aluminosilicates or silicates, as chiral auxiliary. Supports were characterized by a regular mesoporosity and a same initial pore diameter. Immobilization of the chiral aminoalcohol was performed through covalent anchoring of 3-halogenopropyltrimethoxysilane (XPTMS) and substitution of the halogen by (-)-ephedrine. Comparison of the efficiency of the catalysts was carried out. Results were analyzed taking into account the accessibility to the catalytic sites by changing their density (decrease of XPTMS concentration, spacing of the sites by alkyl goups) and the effect of the uncovered mineral surface on activities and enantioselectivities.
Journal of Molecular Catalysis A-chemical | 2001
Sébastien Abramson; Monique Laspéras; Bich Chiche
New solid chiral auxiliaries are used in the enantioselective alkylation of benzaldehyde with diethylzinc. These auxiliaries are obtained by direct immobilization of their homogeneous counterparts, (−)-ephedrine and (−)-O-methyl-ephedrine, on the surface of mesoporous micelle templated silicates and aluminosilicates. Hybrid materials are characterized, and the interactions between the heteroatoms of the organic compounds and the surface are studied by FTIR spectroscopy. Efficiency of such hybrid solids is discussed.
Studies in Surface Science and Catalysis | 1995
Monique Laspéras; Isabel Rodriguez; Daniel Brunel; Hélène Cambon; P. Geneste
Abstract Basic CsNaX and CsNaY zeolites have been impregnated with various loadings of cesium acetate in order to improve, after calcination, the basic character of the solids and to study their application in fine organic chemistry. The characterizations of the two kinds of solids, CsNaX n Cs and CsNaY n Cs zeolites, show that their cristallinity is largely retained after modification and that their behaviour is totally distinct. The nature and the basicity of intrazeolitic cesium oxides has been determined by stepwise thermodesorption of CO 2 (TPD). The nature arises from linear relationships between the number of CO 2 molecules desorbed per unit cell, up to 550°C, and the impregnated cesium loading (atoms per unit cell) for cesium loadings before 16 Cs and 9 Cs for modified CsNaX and CsNaY zeolites, respectively. These linear relationships suggest that active cesium species are homogeneously located inside the cages. The values of the slopes, 0.44 and 0.23 for the CsNaX n Cs and CsNaY n Cs zeolites, respectively, account for the proposed nature of intrazeolitic species: Cs 2 O inside the CsNaX zeolite and (Cs 2 O) 2 inside the CsNaY zeolite. The variation of the desorption maximum from 250°C to 150°C indicates that the basicity of the occluded oxide Cs 2 O is higher than the basicity of the intrazeolitic cluster (Cs 2 O) 2 . The volumetric measurements for the CsNaY n Cs solids by N 2 adsorption parallel the CO 2 desorption showing that, for loadings higher than 13 cesium atoms per CsNaY unit cell, the results can be explained by diffusional constraints in relation with the cluster size. A good agreement is obtained between these results and the base activity of the modified zeolites from measurements of the initial rates of the model Knoevenagel reaction, in liquid phase. Relationships activity/basicity have been established which lead to the evaluation of the activities of intrazeolitic cesium oxides.