Surface molecular engineering in the confined space of templated porous silica

International audienc

Toward dual function patterning onto surface of as-made mesostructured silica

International audienceDual functionalisation on the surface of a mesostructured silica, LUS-1, is investigated here using a novel approach based on the use of so-called “molecular stencil patterning”. This is processed via sequential grafting, the principle of which is based on retention or partial displacement of the templating surfactant. The latter provides the patterning from mutual electrostatic repulsion during the grafting of the first function; here chloropropyldimethylsilyl (CPDMS) or trimethylsilyl (TMS) groups. Then, the second functionalisation step is performed with removal of the remaining surfactant leading to an overall full coverage. The order of the grafting sequence is important and, most of the first function is effectively retained during the second grafting step. Though optimisation is still needed, the concept seems to apply. To cite this article: S. Abry and al, C.R. Chimie 8 (2005)

Understanding the enantioselectivity of a heterogeneous catalyst: the influence of ligand loading and of silica passivation

International audienceOne of the major drawbacks of heterogeneous catalysts is an inferior catalytic performance relative to their homogeneous counterparts. This is often attributed to high local concentration of the catalyst, and in certain cases to various active groups of the heterogeneous support and to its steric effects. We tested the influence of these factors in the case of a silica-grafted bis(oxazoline) catalyst used in the Diels–Alder reaction, by varying the ligand loading and the degrees of passivation of the silica. We show that, in the present case, the enantioselectivity of the catalyst is linearly correlated with the passivation of the silica, and high ligand loadings can be used without damaging the performance of the catalyst. This is, to the best of our knowledge, the first example of a correlation between silica passivation and the enantioselectivity of a heterogeneous catalyst

Site isolation and coordination control of a transition metal ion by molecular surface engineering in mesoporous silica: the case of a bio-inspired copper–polyamine grafted complex

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Nanoblock Aggregation−Disaggregation of Zeolite Nanoparticles: Temperature Control on Crystallinity

International audienceDuring the induction period of silicalite-1 formation at 80 °C, primary nanoblocks of 8?11 nm self-assemble together into fragile nanoflocculates of ca. 60 nm that dislocate and reappear according to a slow pseudoperiodical process. Between 22 and 32 h, the nanoflocculates grow up to 350 nm and contain ill- and well-oriented aggregates of ca. 40 nm. After 48 h, only ill-faceted monodomains of ca. 90 nm remains, which self-assemble into larger flocculates of ca. 450 nm. For crystal growth performed at 90 °C, most of the final aggregates exhibit ill-oriented assembly. This is consistent with a trial-and-error block-by-block building mechanism that turns into an irreversible and apparently faster process at 90 °C, causing definitively ill-oriented product. The nanoblocks, aggregates, and flocculates were characterized in nondiluted, nondiluted and ultrasonicated, or diluted and ultrasonicated solutions, using mainly dynamic light scattering and cryo-high-resolution transmission electron microscopy at various tilted angles

Metal dispersion, accessibility and catalytic activity in methane oxidation of mesoporous templated aluminosilica supported palladium

International audiencePalladium catalysts using templated mesostructured porous silicas and aluminosilicas of MCM-41 type as supports were synthesized with various metal loadings and particle sizes as well as different metal accessibilities to the reactants. The metal was deposited by reacting an aqueous solution of [Pd(NH3)4]2+ complexes with the support, the template of which was partially extracted. The evolution of the support characteristics was monitored at different steps of preparation using X-ray diffraction (XRD), N2 physisorption and transmission electron microscopy (TEM). Particle size and metal accessibility were evaluated combining information from XRD line broadening, TEM observations, UV–visible spectra and H2 uptakes. The PdO reducibility was investigated using temperature-programmed reduction (TPR). In the templated mesoporous aluminosilicate, 27Al-MAS-NMR revealed that Al3+ occupied mostly tetrahedral sites, a fraction of which adopted an octahedral environment in the presence of palladium. According to the light-off curves, the catalytic methane oxidation activity is enhanced in pure silica supports where partial pore wall collapse has occurred. In contrast, for aluminosilica supports the beneficial effect of Al3+ on metal dispersion and catalytic activity was counterbalanced by partial metal encapsulation. Optimizing palladium particle size and avoiding as much as possible metal encapsulation give rise to catalysts more active than the conventional alumina supported palladium catalysts

Bio-inspired manganese mesoporous silica hybrid material as a water compatible antioxidant

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Surface engineering and palladium dispersion in MCM-41 for methane oxidation

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Electron Spin Resonance Quantitative Monitoring of Five different Radicals in γ-Irradiated Polyvinylidene Fluoride

International audienceThe complex superimposition of electron spin resonance (ESR) signals of the five different radicals generated on PVDF upon γ-rays exposure is fully simulated for the first time, including g factors as well as aα and aβ hyperfine splitting constants of proton or fluorine in α or β positions from the radical. The starting parameters were selected based on a thorough literature survey on their better-known fluorocarbons and perhydrogenated radical analogs and discussed in terms of correspondence with radicals in PVDF. In particular, the electronegativity difference between F and H atoms is taken into account since it directly affects the electron delocalization in the radical. The finally obtained ESR parameters are consistent with the spectroscopic characteristics and the chemical stability of the radicals. Since the absolute concentration of each radical is accessible, the simulation was also applied to monitor their stability upon annealing for different exposure times at 373 K. The resulting trend is in full adequacy with the expected relation between electron delocalization, mobility and stability

Design of a bio-inspired copper (II) Schiff base complex grafted in mesoporous silica for catalytic oxidation

International audienceControlled grafting of a copper (II) complex with a Schiff base ligand (LA = N-(salicylaldimine)-(N′-propyltrimethoxysilane)-diethylenetriamine) on a mesoporous MCM-41 type of silica (LUS) was accomplished using a pattering technique to allow a homogeneous distribution of isolated copper sites on the solid using trimethylsilyl groups as dispersing function. XRD patterns suggest that the final material LUS-CuLA exhibits a well-ordered 2D hexagonal structure. Elemental analysis, solid UV–Vis and EPR spectroscopies indicate that the copper (II) coordination sphere is most likely of a 3N1O type with an additional acetate ion as a ligand. Almost all the Cu(II) species grafted on the solid were EPR active (2.6 wt% active species for 2.8 wt% total copper measured from ICP-MS analysis) confirming the presence of monomeric copper species. Preliminary catalytic tests for phenol hydroxylation show that the supported system presents similar activity as the molecular analogue