New carbon nanofiber/graphite felt composite for use as a catalyst support for hydrazine catalytic decomposition

Graphite felt supporting 40 nm diameter carbon nanofibers was synthesized and successfully used as a support for a high loaded iridium catalyst (30 wt%) in the decomposition of hydrazine; a strong mechanical resistance and a high thermal conductivity led to a very efficient and stable catalyst as compared to that used industrially, iridium supported on a high surface area alumina.     

Synthesis of CoFe2O4 nanowire in carbon nanotubes. A new use of the confinement effect

Cobalt ferrite nanowires with an average diameter of 50 nm and lengths up to several micrometers were synthesized inside carbon nanotubes under mild reaction conditions using the confinement effect provided by the carbon tubular template.     

Beta zeolite supported on silicon carbide for Friedel-Crafts fixed-bed reactions

Beta zeolite supported on silicon carbide, with high thermal conductivity and high mechanical strength, was successfully used as an active and stable catalyst for Friedel-Crafts reactions in a fixed bed configuration.     

Quantitative measurement of the Brönsted acid sites in solid acids: toward a single-site design of Mo-modified ZSM-5 zeolite

On the basis of our previous H/D exchange studies devoted to the quantification of the number of Br?nsted acid sites in solid acids, we report here an innovative approach to determine both the amount and the localization of Mo atoms inside the Mo/ZSM-5 catalyst, commonly used for the methane dehydroaromatization reaction. The influence of Mo introduction in the MFI framework was studied by means of BET, X-ray diffraction, 27Al magic angle spinning NMR, NH3 temperature-programmed desorption, and H/D isotopic exchange techniques. A dependence was found between the decrease of acidic OH groups and the Mo content. Depending on the Si/Al ratio of the zeolite, i.e., the proximity of two Br?nsted acid sites, the Mo atoms substitute a different number of OH groups. Consequently, a chemical structure was proposed to describe the geometry of the Mo complex in the channels of the ZSM-5 zeolite.     

Improved synthesis of 1,4-dideoxy-1,4-imino-d-galactitol, an inhibitor of E. coli K12 UDP-Gal mutase and mycobacterial galactan biosynthesis

The E. Coli K12 UDP-Gal mutase inhibitor 1 was prepared from d-glucose in 5 steps (42% overall yield). The 4-azido galactose derivative 11, leading to 1, was formed by treatment of galactose dithioacetal 7 with mercuric oxide and mercuric chloride in acetone. To obtain 7, acetal 3 was tosylated or triflated and treated with NaN₃.     

Autoassembly of nanofibrous zeolite crystals via silicon carbide substrate self-transformation

ZSM-5 zeolite nanofibers with a size of 90 nm and lengths up to several micrometers were prepared via in-situ silicon carbide support self-transformation. The morphology and aggregation degree of these zeolite nanofibers could be modified by adjusting the pH conditions, the nature of the mineralizer (OH- or F-), or the synthesis duration. The novelty consists of the preparation of zeolite nanowires without the use of any organogelating agent, along with controlled macroscopic shapes (extrudates, foam monolith) for direct use as a structured reactor. Finally, these materials are catalytically active in the conversion of methanol to gasoline range hydrocarbons (MTG process) and hence exhibit the typical solid acidity of zeolitic materials.     

A new recyclable Pd catalyst supported on vertically aligned carbon nanotubes for microwaves-assisted Heck reactions

Palladium supported on vertically aligned multi-walled carbon nanotubes (Pd/VA-CNTs) is used as catalyst for the C-C coupling reactions of p-iodonitrobenzene with styrene and ethyl acrylate under microwaves irradiation. Pd/VA-CNTs catalyst exhibits higher activity compared to Pd supported on activated charcoal, under the same reaction conditions. Due to the microwaves irradiation, the kinetics of the reaction is strongly accelerated compared to that obtained with a traditional heating mode. The macroscopic form of aligned CNTs support allows an easy recovery of the catalyst, avoiding a costly post-reaction filtration. In addition, the interaction between the active phase and the support leads to the negligible leaching of palladium during recycling tests. The observed results indicate that Pd/CNTs is a recyclable and stable heterogeneous catalytic system.     

3D-TEM characterization of nanometric objects

The increasing level of research that is nowadays performed on the nanoscale requires specific powerful tools to characterize objects on that scale. We demonstrate in this work the usefulness of three-dimensional transmission electron microscopy (3D-TEM) used in a quantitative way to image and characterize nanomaterials with complex structures and morphologies. The tomographic recording process is a powerful tool to improve the signal-to-noise ratio when imaging nano-objects that cannot strongly extinguish electrons and to clear up the ambiguity of image interpretation due to superposition effects. The resulting ability to distinguish between the “inner” and the “outer” parts of an object as well as to determine its 3D characteristics can in turn yield quantitative information and constitutes the main focus of this paper. Complex morphologies and internal structures on the nanometer scale can thus be resolved in all spatial dimensions, and numerical densities of particles or porosities can be quantified. For porous materials, it is also possible to get the connectivity of the pores, their shapes and distribution. The 3D-TEM technique associates tomographic recording to a careful repositioning of the recorded 2D images, followed by a 3D reconstruction. It allows the recovery of a spatial resolution close to (1.5 nm)³ that can be used to perform quantitative analysis relevant to almost all types of nanometric samples encountered when 3D information down to a few nanometers is required.     

Monitoring the chemical vapor deposition growth of multiwalled carbon nanotubes by tapered element oscillating microbalance

The growth of multiwalled carbon nanotubes (MWCNTs) produced by a catalytic chemical vapor deposition (CCVD) process has been monitored using a tapered element oscillating microbalance (TEOM) probe. This technique displays a high sensitivity (<1 microg). Growths in the TEOM microreactor are investigated with catalytic particles (Fe, Ni) dispersed on different supports. First, high surface area FeAl2O3 or Fe (Ni) exchanged on zeolite powders is used. Second, growths are performed on array of nickel dots or FeSi-nc particles dispersed on large holes patterned on Si(100) substrates. An accurate monitoring of the early stages of growth permits a precise evaluation of the growth rates and shows substantial differences between these samples which greatly differ by the surface area. On catalysts dispersed on Si(100) the mass uptake is linear throughout the process. On high surface area catalysts, however, a saturation of the mass uptake is indifferently observed. This saturation is explained either by diffusion limitation by the growing MWCNTs or by internal diffusion through the pores or external diffusion through the grains of the catalyst. The kinetic dependence with partial pressure of the incoming C2H6:H2 gas mixture is then explored on the FeAl2O3 catalyst. A linear dependence of the MWCNT growth an (P(C2H6)/P(H2))(1/2) is found. A simple model is then developed that accounts for this dependence only if an associative and competitive adsorption of ethane is the rate determining step of the overall process. These results thus bring insight to improve and control the CCVD growth kinetics of MWCNTs.     

Carbon nanotubes decorated α-Al2O3 containing cobalt nanoparticles for Fischer-Tropsch reaction

A new hierarchical composite consisted of multi-walled carbon nanotubes (CNTs) layer anchored on macroscopic α-Al₂O₃ host matrix was synthesized and used as support for Fischer-Tropsch synthesis (FTS). The composite constituted by a thin shell of a homogeneous, highly entangled and structure-opened carbon nanotubes network and it exhibited a relatively high and fully accessible specific surface area of 76 m²·g^?1, compared with that of 5 m²·g^?1 of the original α-Al₂O₃ support. The metal-support interaction between carbon nanotubes surface and cobalt precursor and high effective surface area led to a relatively high dispersion of cobalt nanoparticles. This hierarchically supported cobalt catalyst exhibited a high FTS activity along with an extremely high selectivity towards liquid hydrocarbons compared with the cobalt-based catalyst supported on pristine α-Al2O₃ or on CNTs carriers. This improvement can attribute to the high accessibility of composite surface area comparing with the macroscopic host structure alone or to the bulk CNTs where the nanoscopic dimension induced a dense packing with low mass transfer which favoured the problem of reactants competitive diffusion towards the cobalt active site. In addition, intrinsic thermal conductivity of decorated CNTs could help the heat dissipating throughout the catalyst body, thus avoiding the formation of local hot spots which appeared in high CO conversion under pure syngas feed in FTS reaction. Cobalt supported on CNTs decorated α-Al₂O₃ catalyst also exhibited satisfied high stability during more than 200 h on stream under relatively severe conditions compared with other catalysts reported in the literature. Finally, the macroscopic shape of such composite easily rendered its usage as catalyst support in a fixed-bed configuration without facing problems of transport and pressure drop as encountered with the bulk CNTs.