|Synthesis of layered double hydroxide/MWNT nanocomposites for catalytic applications|
|Ainara Garcia-Gallastegui, Almudena Celaya Sanfiz, Maurice Mourad, Mohamed Mokhtar , Abdullah Asiri, Sulaiman N. Basahel, Shaeel A. Al-Thabaiti , Abdulrahman O. Alyoubi , Neal Skipper, Milo Shaffer|
|Layered double hydroxides (LDHs) are promising materials for many practical applications such as catalysis, adsorption, pharmaceutics, photochemistry and electrochemistry. In particular, LDHs are successfully used as catalysts for several reactions including self-condensation, cross-aldol condensation of aldehydes and ketones, Knoevenagel condensation, Claisen–Schmidt condensation, Michael addition, transesterification and alkylation. Among LDH materials, Mg–Al hydrotalcites are widely used as catalysts in several base-catalysed reactions, for which OH- acts as active Brønsted base site. The actual active sites participating in catalysis are thought to be situated at the edges of the platelets. |
Our aim is to increase the catalytic efficiency of Mg-Al LDH by supporting it on an open framework structure that stabilises a high surface area for the active catalyst and maximises accessibility. In order to achieve this objective, LDH nanoparticles were precipitated directly onto the multi-walled carbon nanotubes (MWNTs), achieving a good dispersion. Process parameters, such as pressure and the addition sequence of the metal or base solution were changed to gain an understanding of the precipitation procedure and resulting structural characteristics.
Five different LDH/MWNT mass ratios: 4%, 3%, 2%, 1% and 0.5% w/w were prepared in order to study the dependence of the structural and physical properties of the LDH materials on the MWNT supports. For each of the five LDH/MWNT samples Thermogravimetric Analysis (TGA), N2 physisorption, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Spectroscopy (EDS) and X-Ray Diffraction (XRD) measurements were performed. We observed that the surface area of the LDH/MWNT materials increases with decreasing the ratio due to the high surface area of MWNT. The crystallite size in the (003) direction of the LDH unit cell measured by XRD, decreases from 30 nm to 12 nm when the mass ratio decreases from 4% to 0.5%. The catalytic activities of the obtained Mg-Al LDH supported materials have been investigated using a standard reaction: the self-condensation of acetone.
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Figure 1. Representative SEM images of (a) LDH nanoparticles, (b) LDH/MWNT w/w: 4%, and (c) w/w: 3%, (d) w/w: 2%, (e) w/w: 1%, (f) w/w: 0.5%.
 Winter F., Koot V., Jos van Dillen A., Geus J. W., de Jong K. P. J. Catal. 2005, 236, 91