Daniel Brunel

Functionalized micelle-templated silicas (MTS) and their use as catalysts for fine chemicals

Functionalization of micelle-templated silica (MTS) was achieved by silanation of the MTS surface by 3-amino and 3-halogeno-propyl alkoxysilane in an apolar solvent. The grafted halogeno organic moieties were further modified by halogen substitution of other functions such as piperidine, tetraalkyl guanidine, (-)-ephedrine, 3-[N,N′-bis-3-salicyldenamino propylamine] (Salpr) and 3-[N,N′-bis-3-(3,5-di-tert-butylsalicyldenamino) propylamine] (tSalpr). During the modification procedure, the regular mesoporous structure of the MTS support was preserved. MTS-bound primary and tertiary amine functions were effective immobilized catalysts for Knoevenagel condensation and selective monoglyceride synthesis. MTS-bound 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was an efficient base catalyst for performing transesterification reaction. Chiral β-aminoalcohols such as (-)-ephedrine grafted on MTS allowed the catalysed alkylation of benzaldehyde by diethylzinc with high conversion and selectivity in 1-phenyl-propanol and moderate enantioselectivity in the (1R) enantiomorph. Manganese (III) Salpr and tSalpr complexes linked to the MTS surface catalysed styrene epoxidation using iodosylbenzene. The efficiency of this latter catalysis should be improved by the use of other liganding molecules more resistant than salen-type ligands towards oxidation media.

Transition-metal ligands bound onto the micelle-templated silica surface

Abstract This paper reviews recent works on the design of immobilized transition-metal ligands on solid supports. After an overview of some results concerning ligand anchorage on polymeric supports and encapsulation of transition-metal complexes inside layered or zeolitic minerals, the grafting of ligands onto the silicic wall surface of micelle-templated silicas (MTS) is reported. MTS silicas featuring a regular mesoporous system of pore-monodispersed size and exhibiting larger pores than zeolites, provide a new opportunity to allow anchorage of organic moieties through the silanation procedure. Mn(III) Salpr and t Salpr complexes bound onto the MTS surface are active in epoxidation reaction using PhIO as oxygen donor. Anchorage of (1 R ,2 S )-ephedrine has been also investigated with the aim to obtain benefit from the MTS structure effect. These new supported chiral catalysts are active in alkylation of benzaldehyde with diethylzinc although less enantioselective than the corresponding homogeneous catalyst. The effect of dispersion of active sites and of surface passivation has been investigated and discussed in terms of the nature of the support surface.

Catalysis using multifunctional organosiliceous hybrid materials

Organic-inorganic hybrid materials with different levels of structuration and porous hierarchy and one or several types of active sites in the framework can catalyze multistep chemical processes in a one-pot reactor system following a cascade of reaction events. It will show how the different active sites can act in a synergistic or in a consecutive way following a similar functionality model to biological multisite catalysts. Research on this subject for heterogeneous catalysts is still in the beginning stage and very interesting results can be expected if we are able to successfully combine the properties of organic and inorganic catalysts.

MCM-41 type silicas as supports for immobilized catalysts

MCM-41 type silicas were covalently grafted with various functional alkoxysilanes (RO)3Si(CH2)3 X with X = Cl, NH(CH2)2NH2 and NHC(O)N-Salpr. The functions of the first two organic moieties already attached were further transformed into respectively 2-(NHCH2)Pyr, 4-(NH(CH2)2NHCO)Pyr also known as organic ligands of transition metals. The grafted mesoporous silicas were characterized by IR, MAS-NMR spectroscopies, thermogravimetry, nitrogen adsorption and elemental analysis. The coupling reactions of organic ligands do not modify the covalent grafting bonds. Pore opening shrinks at increasing organic coverage. The diameter of the channels accessible to nitrogen varies from 33 to 13 . A total surface lining by concentrically oriented organic chains is suggested by the decrease in the nitrogen adsorption enthalpy as a function of the organic coverage.

Comparison of two MCM-41 grafted TEMPO catalysts in selective alcohol oxidation

A new heterogeneous system for catalytic oxidations has been developed by immobilising TEMPO onto ordered mesoporous silica (MCM-41-type). The immobilisation was performed according to different methods: (i) direct grafting of the silica surface with 3-(trimethoxysilyl)propyloxy-TEMPO obtained by coupling 4-hydroxy-TEMPO and allyl bromide followed by hydrosilylation; (ii) coupling of 4-carboxy-TEMPO with 3-aminopropylsilane previously grafted on the mesoporous silica surface; (iii) coupling of the N-succinimide ester of 4-carboxy-TEMPO with the same previously grafted 3-aminopropylsilane. Efficiency and anchoring of the TEMPO were studied by various spectroscopic techniques. The textural characteristics of the mesoporous support were preserved during the different modifications. Catalytic tests on the use of the materials in the oxidation of primary alcohols proved their activity and stability under the reaction conditions.

Amine functions linked to MCM-41-type silicas as a new class of solid base catalysts for condensation reactions

MCM-41 type silica surfaces were grafted with primary amino groups through silanation process. Tertiary amino groups covalently bound to the surface were prepared through displacement of previously anchored halogen atom by piperidine. The catalytic activities of the immobilized amino groups were studied in the Knoevenagel condensation of benzaldehyde with ethylcyanoacetate. The mechanism of the catalyst action is discussed

Supercritical CO2 dried chitosan: an efficient intrinsic heterogeneous catalyst in fine chemistry

Chitosan microspheres are used as catalysts for the synthesis of monoglyceride by fatty acid addition to glycidol. Microspheres are obtained by drying gel beads of the natural polymer under supercritical CO2 conditions, which makes the access to the polymer functional groups easy.

Preferential grafting of alkoxysilane coupling agents on the hydrophobic portion of the surface of micelle-templated silica

Silica having a regular mesoporous structure induced by micelle templating (MTS) was functionalised with n-propyltrimethoxysilane under anhydrous conditions. Support modification was characterised by various methods (X-ray diffraction, FTIR spectroscopy, thermogravimetry, nitrogen adsorption volumetry, microcalorimetry and FTIR of water adsorption). Functionalisation takes place mainly at the hydrophobic patches of the surface, involving surface siloxane bridges, and occurs through a nucleophilic displacement at the silicon atom by the alkylsiloxane group of the silylating agent. Hydrophilic patches of the surface, consisting of hydrogen-bonded silanol groups, do not constitute, in anhydrous conditions, active sites for the covalent anchorage of the organosilane moieties on the MTS surface.

Synthesis and characterisation of ibuprofen-anchored MCM-41 silica and silica gel

A non-steroidal anti-inflammatory drug (ibuprofen) has been anchored inside the mesoporous channels of MCM-41-type silica and on a silica gel surface. The relevant anchoring procedure through an ester function has been investigated. It uses the epoxide ring opening of 3-glycidoxypropylsilane grafted on the silica surface by the carboxylic-group-containing ibuprofen. The control of the surface modification and of the anchoring efficiency was achieved by comparison of spectroscopic data with those obtained using homogeneous counterparts. The use of nanostructured silica allowed an accurate verification of the different surface modifications and also a higher drug loading.

29Si and 13C MAS-NMR characterization of surface modification of micelle-templated silicas during the grafting of organic moieties and end-capping

The surface of micelle-templated silicas (MTS) was functionalized by silylation with 3-chloropropyltriethoxysilane in anhydrous toluene. The modified sample (Cl–MTS) was then end-capped by vapor phase treatment with hexamethyldisilazane (HMDZ), which led to the AZA–Cl–MTS sample. The textural properties of the support were preserved during the different modifications. A comparison of 29Si MAS-NMR spectra of Cl–MTS with those of the parent support (MTS) showed that silylation occurred through nucleophilic attack at silicon atoms located in the hydrophobic part of the surface. This mechanism differed from the sol-gel mechanism, which involves H-bonded silanol groups during silylation in polar or aqueous solvent. Residual silanol groups of the Cl-MTS surface were not totally end-capped during HMDZ treatment. This result is explained by the steric hindrance of the trimethylsiloxane groups, which would act as umbrellas that protect the surrounding silanol groups.