Nitrogen-doped carbon nanotubes : controlled synthesis, physicochemical characterization and application as Pd catalyst supports in a hydrogenation reaction.
Catalysis is a fundamental pillar in the development of a sustainable economy that is in line with the principles of green chemistry. This drives the need to develop novel catalytic systems whose activity and selectivity towards the desired products are controllable. Carbon nanotubes (CNTs) and nitrogen-doped CNTs (N-CNTs) are smart materials that are potentially novel metal catalyst supports or catalysts. Thus, it is vital to develop procedures for synthesizing CNTs and N-CNTs that utilize green principles and aid the control of the materials‘ physicochemical properties. CNTs and N-CNTs can be synthesized via a chemical vapour deposition (CVD) technique by use of organometallic compounds such as ferrocenyl derivatives as catalysts. These ferrocenyl derivatives can be synthesized by use of solvents such as benzene, ethanol and DMF, however, a ‗greener‘ approach is to use a solvent-free synthesis. This research focussed on utilizing a solvent-free mechanochemical approach to synthesize novel and known ferrocenyl derivatives used as new catalysts in the controlled synthesis of N-CNTs. This research also aimed at developing a synthetic procedure that yields N-CNTs with modulated physicochemical properties. Lastly, this study aimed at utilizing the N-CNTs as Pd supports (Pd/N-CNTs) and exploiting their nitrogen content or species to control the rate of nitrobenzophenone reduction. Nitrogen- or halogen-containing ferrocenyl derivatives were synthesized via a solvent-free mechanochemical approach. The ferrocenyl derivatives were used as new catalysts in the CVD synthesis of N-CNTs. Halogens in ferrocenyl derivatives, or oxygen derived from ethyl benzoate, were used to control the nitrogen content and species incorporated into N-CNTs. The synthesized N-CNTs were purified and used as supports for Pd nanoparticles. Both N-CNTs and Pd/N-CNTs were characterized by using microscopy, spectroscopy and surface characterization techniques. This included transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction, Fourier-transform infrared spectroscopy and temperature programmed reduction (TPR). For comparison CNTs were also synthesized, characterized and used as Pd supports. The catalytic performance of Pd/N-CNTs was evaluated in the hydrogenation of nitrobenzophenone. The effect of surface area, pressure, temperature and solvent polarity on the catalyst‘s performance was also evaluated. A simple mechanochemical solvent-free technique was developed and used to synthesize eight novel and one known nitrogen/halogen-containing ferrocenyl derivatives. The reactions were found to occur readily at ambient temperature and pressure and more so with relatively shorter reaction times compared to the known classical Knoevenagel condensation reactions. The electron-withdrawing strength of the para-substituent on the novel ferrocenyl derivatives influenced the yield, product selectivity and spectroscopic properties of these compounds. The X-ray structures of five new compounds were reported. Selected 1,1′-ferrocenyldiacrylonitriles having similar structures which differ only in their para substituents (para-CN, para-CF₃ and para-Cl) were used as novel catalysts in the synthesis of N-CNTs. The syntheses were conducted at 850 °C by use of either acetonitrile or pyridine as the carbon and nitrogen source. Syntheses conducted by use of pyridine as a nitrogen source yielded N-CNTs and carbon spheres (CS) while those conducted with acetonitrile (external nitrogen source) yielded N-CNTs and carbon nanofibres (CNFs). Amongst the three tested catalysts, the para-CF₃ catalyst gave the highest yield of N-CNTs in either pyridine or acetonitrile. In addition, the para-CF₃ catalyst in pyridine gave N-CNTs with the highest nitrogen-doping level, that were kinked, and less thermally stable than those obtained with the para-CN and -Cl catalysts in pyridine. This implied that fluorine heteroatoms enhanced the nitrogen-doping of N-CNTs. Thus, the expediency of using a fluorine heteroatom to control doping of N-CNTs is reported. When using acetonitrile, both the fluorinated and chlorinated ferrocenyl derivatives yielded metal-filled N-CNTs of controllable diameters. Another ferrocenyl derivative, (3-ferrocenyl-2-(4-cyanophenyl) acrylonitrile), and oxygen derived from ethyl benzoate were also used to synthesize aligned N-CNTs containing 6.4-15.7 wt. % of nitrogen in either acetonitrile or a solution of acetonitrile and ethyl benzoate. For comparison, N-CNTs were synthesized in toluene. The use of 3-ferrocenyl-2-(4-cyanophenyl)acrylonitrile in acetonitrile as a nitrogen and carbon source selectively yielded mainly N-CNTs, while use of toluene as a carbon source yielded both N-CNTs and CS. The addition of oxygen enhanced the nitrogen-content of N-CNTs. However, the higher nitrogen-containing N-CNTs were found to be less graphitic and showed a higher base constant (Kb) compared to N-CNTs synthesized without oxygen. The outer diameters of the N-CNTs decreased upon increasing the oxygen composition in the synthesis precursors from 1-4 wt. % oxygen. In addition, the alignment of N-CNTs increased upon addition of oxygen. Electrical conductivity measurements of N-CNTs showed a negative relationship between the amount of oxygen in the starting materials and the conductivity of N-CNTs. Thus, 3-ferrocenyl-2-(4-cyanophenyl)acrylonitrile and oxygen were successfully used to control the physicochemical properties of N-CNTs during synthesis. This provided a novel synthetic approach, which was further used to selectively incorporate pyrrolic-nitrogen species into N-CNTs. The physicochemical properties of pyrrolic-N-CNTs with different pyrrolic contents were evaluated. Pd was loaded on pyrrolic-N-CNTs and the influence of pyrrolic-nitrogen on Pd species evaluated by use of XPS analysis. XPS evaluation of Pd/N-CNTs revealed that the abundance of Pd²⁺ increased as the quantity of pyrrolic nitrogen increased. The increase of Pd²⁺ species was accredited to the formation of stable Pd-N coordination complexes, which increased the dispersion of Pd²⁺ nanoparticles. The catalytic performance of Pd/N-CNTs was tested in the hydrogenation of aminobenzophenone. Pd/N-CNTs showed a higher efficiency and selectivity towards nitro-reduction than Pd/CNTs and commercial Pd/AC. The increased efficiency was attributed to the pyrrolic-nitrogen present in Pd/N-CNTs. The catalytic performance of Pd/N-CNTs increased with increase in surface area and temperature but decreased with increase in pressure. An aprotic solvent enhanced the selective reduction of nitrobenzophenone to aminobenzophenone more than a protic solvent. In general, these Pd/N-CNTs are promising catalysts for use in industrial processes involving the selective reduction of substituted nitro-arenes to substituted anilines.