Porous Nitrogen-doped Carbon Materials Supported Metal Catalyst For Selective Hydrogentaion OfAromatics

The selective hydrogenation of aromatics is one of the most important reaction in chemical industry and petroleum refining industry. The hydrogenation products of aromatics and heterocyclic aromatics are used extensively for fine chemical industry and organic synthesis. On the other hand, deep hydrogenation of aromatics in diesel fuel has a positive effect on the performance and quality of fuels. Therefore, preparing a high-performance and environmentally friendly heterogeneous catalyst for the chemoselective hydrogenation of aromatic compounds is urgently needed.In this paper, we synthesized two kinds of N-doped porous carbon materials with different biomass-derived compounds as precursors. First, we synthesized a novel mesoporous N-doped carbon ?CN? by hydrothermal method using glucosamine hydrochloride as precursor and colloidal silica as hard template. Second, we designed fascinating hierarchical porous N-doped carbon materials ?NHPC? derived from biomass-derived a-cellulose via a modified "leavening" strategy, which is a green and sustainable process. And then, the obtained porous N-doped carbon materials were sent to deposit metal nanoparticles and were tested in the hydrogenation of aromatic compounds.Firstly, RuPd alloy nanoparticles ?3.6 nm? uniformly dispersed on N-doped carbon ?RuPd/CN? was prepared via a simple ultrasound-assisted co-reduction method. The RuPd/CN is highly active, selective and stable in the hydrogenation of benzoic acid to cyclohexane carboxylic acid under mild conditions with a TOF up to 2066 h^-1. It was found that the bimetallic RuPd/CN catalyst exhibited a substantially enhanced activity compared with the monometallic catalysts ?Ru/CN and Pd/CN?. The reason for higher performance of the RuPd/CN catalyst is considered to be the increased Ru⁰/Ruⁿ⁺ ratio induced by the electronic interaction between Ru and Pd as evidenced by various characterizations. Notably, the different phenomenon of activity platform on different catalysts ascribed to the effect of hydrogen pressure was newly observed and further explained by the first-principle studies. Moreover, the factors influencing the adsorption modes of BA, especially the configuration of the carboxyl group are investigated preliminarily in first-principle giving a distinct insight from the former work. The reason why the carboxyl group in benzoic acid does not undergo hydrogenation, which results in the superior selectivity ?>99%?, is also revealed by the comparison of the thermodynamics of hydrogenation and dissociation of the carboxyl group.Secondly, on the way to explore superior hydrogenation catalysts, Ir-based catalysts with a record catalytic activity (up to 40 h^-1) for the hydrogenation of benzoic acid to cyclohexane carboxylic acid under mild reaction conditions ?85 ?,0.1 MPa H₂, in water? have been successfully developed. By excluding various factors, experimental results showed that the main factor governing the activity discrepancy of the Ir-based catalysts is actually the dispersing stability of the supports ?such as N-doped carbon, active carbon, SBA-15 and various metal oxides? in the reaction solution, rather than the interaction between the Ir active component and the supports. Combining with theoretical investigation from first principle, an activity volcano curve considering the competing adsorption between reactants ?H₂? and solvent ?H₂O? for aqueous aromatic ring hydrogenation was given out for the first time. The high activity of Ir can be deduced by the proper discrepancy of dissociation energies or adsorption energies between H₂ and H₂O on the catalysts. This activity volcano curve provides guidance for further rational design of promising catalysts for benzoic acid or even aromatic ring hydrogenation in true reaction conditions for practical applications.Finally, Ruthenium nanoparticles ?2.6 nm? uniformly dispersed on a three-dimensional ?3D? interconnected hierarchical porous N-doped carbon ?Ru/NHPC? has been successfully developed, serving as a highly active and stable catalyst for the selective hydrogenation of aromatics under mild conditions. A novel "leavening'" strategy, i.e. biomass-derived a-cellulose as carbon precursor and ammonium oxalate as both nitrogen source and foaming agent, affords the NHPC material large surface area (870 m²g^-1), excellent hierarchical nanostructure act as convenient mass transfer channel and high ability in stabilizing and dispersing Ru nanoparticles. The Ru/NHPC catalyst exhibited a substantially enhanced activity in the hydrogenation of toluene (TOF up to 39000 h^-1) and quinoline (TOF up to 2858 h^-1) in comparison with the Ru/HPC ?3D-hierarchical porous carbon without nitrogen doped? and Ru/AC ?commercial activated carbon? under the same reaction conditions. Further investigations indicate that the 3D interconnected porous structure and N-doping are contributed to the improved diffusion and mass transfer, homogeneous dispersion of Ruthenium nanoparticles and high percentage of Ru⁰ ?active sites?, which result in considerable catalytic performance.In summary, we successfully prepared porous N-doped carbon materials derived from biomass, various metal nanoparticles ?Pd, PdRu and Ir? were then trapped in these porous N-doped carbon materials, which served as highly active catalyst for the chemoselective hydrogenation of aromatics ?benzoic acid, toluene and quinoline?. A series of characterizations and activity tests accompanied by theoretical calculations was carried out to study the superior catalytic performance and reaction mechanism. The results indicate that the key factors influencing the hydrogenation performance attributed to the doped nitrogen-related changes of electronic structure for support and nanoparticles, the pore structure of supports and the competing adsorption between reactants and solvent. The results obtained from experiment and theoretical calculations could provide guidance for further rational design of promising catalysts for aromatic ring hydrogenation in true reaction conditions for practical applications.