Carbon nanotubes as a 1D template for the synthesis of air sensitive materials: about the confinement effect

Cobalt ferrite and cobalt iron nanowires with an average diameter of 50 nm and lengths up to several micrometers were synthesized inside multi-walled carbon nanotubes (MWNTs) under mild reaction conditions, i.e. 100 °C and atmospheric pressure, using an aqueous nitrate precursor salt filling the tubes. The concept of a confinement effect inside carbon nanotubes has been advanced to explain the formation of CoFe2O4 under such mild reaction conditions. The formation of caps near the tube tips at the beginning of the nitrate decomposition meant that each nanotube was considered as a closed nanoreactor, in which the reaction conditions could be very different to the macroscopic conditions outside the tube. The subsequent reduction of the CoFe2O4 allowed to obtain CoFe nanowires cast in the carbon nanotubes. These nanowires exhibit a high resistance towards oxidation, whereas bulk CoFe is known to undergo oxidation at room temperature and atmospheric pressure. This phenomenon was attributed to oxygen diffusion problems due to the confinement effect of the carbon nanotubes

Functional carbons and carbon nanohybrids for the catalytic conversion of biomass to renewable chemicals in the condensed phase

The production of chemicals from lignocellulosic biomass provides opportunities to synthesize chemicals with new functionalities and grow a more sustainable chemical industry. However, new challenges emerge as research transitions from petrochemistry to biorenewable chemistry. Compared to petrochemisty, the selective conversion of biomass-derived carbohydrates requires most catalytic reactions to take place at low temperatures (\u3c 300 °C) and in the condensed phase to prevent reactants and products from degrading. The stability of heterogeneous catalysts in liquid water above the normal boiling point represents one of the major challenges to overcome. Herein, we review some of the latest advances in the field with an emphasis on the role of carbon materials and carbon nanohybrids in addressing this challenge

N-functionalized carbon nanotubes as solid basic catalysts for biomass conversion

In the view of declining fossil fuel energy resources and rising oil prices, it is necessary to develop new ways to satisfy the energy needs and the production of chemicals. An alternative route is the use of biomass, in fact, it can serve as a sustainable source of renewable fuels and high value chemicals and materials [1-2]. Thus new catalysts need to be developed. In particular basic catalysts will play an important role for many reactions involving biomass transformation such as transesterification, dehydration, aldol condensation, or isomerization reactions, for example [3]. Nitrogen-containing carbon nanotubes (N-MWCNT) appear to be a promising basic catalyst [4]. In fact, in contrast to the existing heterogeneous basic catalysts (such as hydrotalcite, MgO, CaO) they are chemically stable and they do not suffer of problem of leaching. In this work we developed a new route to synthesize N-MWCNTs by grafting different ammines (diethyl-ethylamine, ethylamine and pyrrolidine) on the surface of the MWCNTs. Thus we investigated the potential use of N-MWCNTs as solid catalysts in the transesterification of triglycerides, model reaction for basic catalysts. In particular we study the influence of the basicity of the different amines on the catalytic activity and the stability of the catalysts after recycling reactions

Selective Glucose Isomerization to Fructose via Nitrogen-doped Solid Base Catalyst Derived from Spent Coffee Grounds

In this work, glucose isomerization to fructose was conducted via a solid base biochar catalyst derived from spent coffee grounds and melamine. The X-ray photoelectron spectroscopy (XPS) spectra identified the majority of pyridinic nitrogen on the biochar surface, which imparted the strong base character of the catalyst. Activity of the catalyst was evidenced by fast conversion of glucose (12%) and high selectivity to fructose (84%) in 20 min at a moderate temperature (120 °C) compared to recently reported immobilized tertiary amines at comparable N concentrations (10-15 mol% relative to glucose). By increasing the reaction temperature to 160 °C, fructose yield achieved 14% in 5 min. The base biochar catalyst showed superior selectivity (\u3e80%) to commonly used homogeneous base catalysts such as aqueous hydroxides and amines (50-80%) and comparable catalytic activity (~20 mol% conversion within 20 min). Moreover, co-solvent of acetone in the reaction system may increase the overall basicity by stabilizing protonated water clusters via hydrogen bonding, which led to faster conversion and higher fructose selectivity than those in water. Approximately 19% fructose was obtained at 160 °C, and the basic sites on the biochar catalyst were stable in hydrothermal environment as indicated by acid-base titration test. Therefore, nitrogen-doped engineered biochar can potentially serve as solid base catalyst for biorefinery processes

Nitrogen-functionalized carbon nanotubes as a basic catalyst for biomass conversion reactions

Biomass conversion to transportation fuels and chemicals is a growing field of research due to the depletion of fossil fuels feedstock. New catalysts, optimized for carbohydrates conversion, need to be developed. In this context, basic heterogeneous catalysts will play a major role for dehydration, hydrolysis, (trans)esterification, aldol condensation, alkylation or isomerization reactions for example. In contrast to existing basic heterogeneous catalysts, MWCNTs-based catalysts are chemically stable (no leaching) and relatively easy to tailor on a nano- and macro-level (controlled porosity). Therefore, nitrogen-functionalized multiwalled carbon nanotubes (N-MWCNTs) appear to be a promising basic catalyst and catalyst support [1,2]. Unfortunately, the nitrogen concentration, its location in/on the nanotube and the nature of the formed N-containing functional groups are difficult to control by common synthesis techniques like by catalytic chemical vapor deposition (CCVD) or by post-treatments [3]. In addition, it is still unclear which functional groups are required to reach high catalytic activities. Thus, we synthesized N-MWCNTs catalysts by grafting desired N-containing molecules on the MWCNTs’ surface. In order to avoid the drawbacks of the traditional SOCl2 route, a new procedure has been designed. The obtained catalysts have been tested in the transesterification of glyceryl tributyrate, as a model triglyceride for biodiesel synthesi

Interfacial charge distributions in carbon-supported palladium catalysts

Controlling the charge transfer between a semiconducting catalyst carrier and the supported transition metal active phase represents an elite strategy for fine turning the electronic structure of the catalytic centers, hence their activity and selectivity. These phenomena have been theoretically and experimentally elucidated for oxide supports but remain poorly understood for carbons due to their complex nanoscale structure. Here, we combine advanced spectroscopy and microscopy on model Pd/C samples to decouple the electronic and surface chemistry effects on catalytic performance. Our investigations reveal trends between the charge distribution at the palladium–carbon interface and the metal’s selectivity for hydrogenation of multifunctional chemicals. These electronic effects are strong enough to affect the performance of large (~5 nm) Pd particles. Our results also demonstrate how simple thermal treatments can be used to tune the interfacial charge distribution, hereby providing a strategy to rationally design carbon-supported catalysts

Carbon Dioxide Methanation for Human Exploration of Mars: A Look at Catalyst Longevity and Activity Using Supported Ruthenium

Overarching Purpose: To design a carbon dioxide methanation/Sabatier reaction catalyst able to withstand variable conditions including fluctuations in bed temperature and feed flow rates for 480 days of remote operation to produce seven tons of methane. Current Study Purpose: Examine supported Ruthenium as a carbon dioxide methanation catalyst to determine the effects support properties have on the active phase by studying activity and selectivity. Objective: The remote operation of the Mars ISRU (In Situ Resources Utilization) lander to produce rocket fuel prior to crew arrival on the planet to power an ascent vehicle. Constraints: Long-term operation (480 days); Variable conditions: Feed gas flow rates, Feed gas flow ratios, Reactor bed temperature

Effective Dispersion of MgO Nanostructure on Biochar Support as a Basic Catalyst for Glucose Isomerization

Glucose isomerization to fructose is one of the most important reactions in the field of biomass valorization. We demonstrate wood waste valorization with MgCl2 salt to synthesize an environment-friendly catalyst (i.e., MgO-biochar), which exhibits effective glucose-to-fructose isomerization with over 30% fructose yield and 80% selectivity at only 100 °C for 30 min in water as a green medium. This study highlights that one-step synthesis can effectively disperse and tether MgO nanostructures to the biochar matrix, which displays a significant reduction of Mg leaching compared to MgO-biochars produced by two-step synthesis and pure MgO. The MgCl2 acts as a porogen that facilitates the formation of a porous biochar structure and dispersion of nanostructured MgO. We identify key parameters of impregnation media (ethylene glycol, ethanol, and water) and pyrolysis conditions (600/750 °C in N2/CO2 atmosphere) that are responsible for adjusting the reactivity and stability of MgO, which enable the design of more effective and recyclable biochar catalysts. Weak interactions between MgCl2 and biomass in the presence of aqueous miscible organic solvents as shape-directing agents are accountable for fast leaching of Mg from the MgO-biochar surface. The FTIR spectra confirm the existence of various coordinations on the hydroxylated surfaces of MgO-biochar surfaces. The mesoporous structures of the biochar support enhance the stability of MgO moieties as revealed by BET, XRD, and Raman analyses. Given the benefits of effective MgO dispersion on the biochar support, we can reduce the amount of MgO active species involved in each reaction run, which mitigates over-reaction compared to pure MgO catalysts and achieves high fructose yield and selectivity for three consecutive cycles