Johan Alauzun

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

It is shown that surface NMR spectra can be greatly enhanced using dynamic nuclear polarization. Polarization is transferred from the protons of the solvent to the rare nuclei (here carbon-13 at natural isotopic abundance) at the surface, yielding at least a 50-fold signal enhancement for surface species covalently incorporated into a silica framework.

Evidence for Metal-Surface Interactions and Their Role in Stabilizing Well-Defined Immobilized Ru-NHC Alkene Metathesis Catalysts

Secondary interactions are demonstrated to direct the stability of well-defined Ru-NHC-based heterogeneous alkene metathesis catalysts. By providing key stabilization of the active sites, higher catalytic performance is achieved. Specifically, they can be described as interactions between the metal center (active site) and the surface functionality of the support, and they have been detected by surface-enhanced (1)H-(29)Si NMR spectroscopy of the ligand and (31)P solid-state NMR of the catalyst precursor. They are present only when the metal center is attached to the surface via a flexible linker (a propyl group), which allows the active site to either react with the substrate or relax, reversibly, to the surface, thus providing stability. In contrast, the use of a rigid linker (here mesitylphenyl) leads to a well-defined active site far away from the surface, stabilized only by a phosphine ligand which under reaction conditions leaves probably irreversibly, leading to faster decomposition and deactivation of the catalysts.

Tailored Ru‐NHC Heterogeneous Catalysts for Alkene Metathesis

The introduction of N-heterocyclic carbene ligands (NHC) has led to major breakthroughs in homogeneous catalysis. However, such homogeneous catalysts can still suffer from deactivation and problems related to catalyst cost and recovery, as well as metal separation from the organic substrates. In the case of the very challenging and promising reaction of alkene metathesis, these drawbacks have probably been delaying the development of economical industrial processes. One possible solution would be the development of an efficient heterogeneous catalysts that is highly active (TON and TOF), stable (minimum recycling and leaching) and tolerant to functional groups. Despite numerous efforts in this area (involving permanent grafting of Ru-NHC complexes on various supports or other immobilization strategies), heterogeneous catalysts has not fulfilled the aforementioned requirements. Recently, tailored made organic–inorganic materials have proved to be an alternative and advantageous route towards highly active and well-defined heterogeneous catalysts. In particular, fully characterized well-defined Ir-NHC materials displayed catalytic performances comparable to those of homogeneous homologues. This has been attributed to the careful control of the catalyst preparation: synthesis of materials containing regularly distributed NHC-moieties and subsequent selective functionalization into Ir-NHC species, leading to the “single-site” nature of these catalysts. Here, we describe the preparation of highly active and stable Ru-NHC alkene metathesis catalysts through surface organometallic chemistry on hybrid mesostructured materials (Scheme 1).

Novel monolith-type boron nitride hierarchical foams obtained through integrative chemistry

A novel class of monolith-type boron nitride hierarchical foams has been prepared through an integrative chemistry-based synthetic path. These materials contain interconnected pores in the nanometre to the micrometre range with high porosity (∼75 vol%), a specific surface area up to 300 m2 g−1 and a resistance toward mechanical stress making them suitable for innovative applications.

Hydrophilic conditions: a new way for self-assembly of hybrid silica containing long alkylene chains

For the first time, hybrid materials with long-range order (lamellar and even 2D hexagonal) were obtained during hydrolysis–polycondensation of α,ω-bis(trimethoxysilyl)alkanes thanks to hydrophobic van-der-Waals type interactions using highly hydrophilic conditions, the nanostructure depending mainly on the alkylene chain lengths.

Direct synthesis of bifunctional mesoporous organosilicas containing chelating groups in the framework and reactive functional groups in the channel pores

This paper describes the direct synthesis of bifunctional organosilica containing chelating groups in the framework and reactive functional groups in the channel pores. These materials were obtained in one step by direct synthesis using co-condensation of a bridged organosilica with chelating properties, tetraethylorthosilicate (TEOS) and an organotrialkoxysilane ZSi(OR)3 with Z = cyanopropyl, mercaptopropyl, chloropropyl in the presence of a non-ionic triblock co-polymer (P123) as structure directing agent. Bridged organosilicas used in this study include: 1,4,8,11-tetrakis(triethoxysilylpropyl)-1,4,8,11-tetraazacyclotetradecane and the corresponding copper(II) chloride complex. The influence of different parameters (concentration of the triblock copolymer P123, cosurfactant, hydrothermal treatment) on the textural and structural characteristics of the materials was investigated. The materials were characterized by 13C and 29Si NMR experiments, nitrogen gas adsorption, powder X-ray diffractrion (XRD), elemental analysis and thermogravimetric analysis (TGA). The activity of the chelating groups for complexation–decomplexation reactions was also investigated.

An original synthesis of highly ordered organosilica with a high content of thiol groups

Well ordered bridged organosilica highly functionalised with disulfide groups were obtained by self-assembly of alpha,omega-bis(trimethoxysilyl)alkyldisulfide under hydrophilic conditions; the reduction of disulfide cores to SH groups gave rise to material having a high mercury ion adsorption capacity.

Direct synthesis of ordered mesoporous silica containing iodopropyl groups. A useful function for chemical modifications

In this paper, we report a clever approach permitting, in one step, the synthesis of ordered (hexagonal phase) mesoporous silica functionalized with iodopropyl groups [I(CH2)3] located in the pore channels. This material was obtained by the hydrolysis and co-condensation of tetraethylorthosilicate (TEOS) and iodopropyltriethoxysilane (I(CH2)3Si(OEt)3) in the presence of P123 as a non-ionic structure-directing agent and hydrogen iodide as an acid catalyst. All materials were characterised by X-ray diffraction measurements, transmission electron spectroscopy, nitrogen adsorption analyses, elemental analyses, and solid-state 13C and 29Si NMR spectroscopy. The accessibility and reactivity of this functional group was investigated, showing the advantage of the iodo group for the introduction of bulky or hydrophilic groups.

Simultaneous Phase Transfer and Surface Modification of TiO2 Nanoparticles Using Alkylphosphonic Acids: Optimization and Structure of the Organosols

An original protocol of simultaneous surface modification and transfer from aqueous to organic phases of anatase TiO2 nanoparticles (NPs) using alkylphosphonic acids (PAs) is studied. The influence of the solvent, the nature and concentration of the PA, and the size, concentration, and aggregation state of the TiO2 NPs was investigated. Complete transfer was observed for linear alkyl chains (5, 8, 12, and 18 C atoms), even at very high sol concentrations. After transfer, the grafted NPs were characterized by (31)P solid-state MAS NMR. The dispersion state of NPs before and after phase transfer was monitored by dynamic light scattering (DLS). Small-angle neutron scattering (SANS) was used to characterize the structure of PA-grafted NPs in the organic solvent. Using a quantitative core-shell model cross-checked under different contrast conditions, it is found that the primary particles making up the NPs are homogeneously grafted with a solvated PA-layer. The nanometric thickness of the latter is shown to increase with the length of the linear carbon chain of the PA, independent of the size of the primary TiO2 NP. Interestingly, a reversible temperature-dependent aggregation was evidenced visually for C18PA, and confirmed by DLS and SANS: heating the sample induces the breakup of aggregates, which reassemble upon cooling. Finally, in the case of NPs agglomerated by playing with the pH or the salt concentration of the sols, the phase transfer with PA is capable of redispersing the agglomerates. This new and highly versatile method of NP surface modification with PAs and simultaneous transfer is thus well suited for obtaining well-dispersed grafted NPs.

Generic way for functionalised well-ordered cubic mesoporous silica via direct synthesis approach

Applications of functionalised mesoporous silica are dependent upon the location, the quantity and the accessibility of the functional groups attached in the inner pores, particularly in the case of cubic ordered porous structure. Facilitating such material synthesis could be of great interest. Herein, we present a controlled and generic route to functionalised and well-ordered cubic mesoporous silica by direct synthesis and an efficient method for complete template removal without structure collapse nor loss of organic content, leading to better functional accessibility and thus material capability. Materials were obtained by co-condensation of tetraethylorthosilicate (TEOS) and an organotriethoxysilane Σ-(CH2)3-Si(OR′)3 (with R′ = Me or Et and Σ = CN, SH, CH3COCHCOCH3 (acac) and PO(OEt)2) in the presence of F127 as structure-directing agent in acidic media. All materials were characterised by XRD, TEM, TGA, 13C and 29Si NMR spectroscopies, nitrogen gas adsorption–desorption measurements as well as elemental analyses. Functional accessibility and adsorption capacity of such materials were proved by complexation of lanthanide ions thanks to acac groups.