Building And Catalytic Mechanism Study Of Hydrogenation Systems Based On Using Aliphatic Alcohols As Hydrogen Source

Chemical engineering throughout the time enabled many of the fundamental breakthroughs in human society. Catalytic hydrogenation is one of the most important chemical processes, since its first observation by Paul Sabatier (1912 Nobel Prize), have already plays a tremendous role in energy generation, environmental protection, materials and fine chemical production etc. However, catalytic hydrogenation uses pure gaseous H2 as the reactant which makes it a quite risky process in industry. Catalytic transfer hydrogenation (CTH), proposed by Braude and Linstead in 1954, carries out reduction of organics by hydrogen donor (such as secondary alcohols "isopropanol", hydrazine, formic acid/formates etc.) rather than using H2-gas. But CTH is different from catalytic hydrogenation in terms of mechanism, i.e., the CTH is the exchange/transfer of "H atom" between molecules, hydrogen donor and acceptor, while the catalytic hydrogenation forms "adsorbed H" on the surface of catalyst. The CTH using homogeneous metal complex as the catalyst, and present industry application mainly limited to the asymmetry selective hydrogenation.In this thesis, based on the study of acetophenone and phenol hydrogenation, and the recent literature work on aqueous-phase reforming for H2 production, we proposed a novel liquid system of catalytic hydrogenation using "aliphatic alcohols:methanol or ethanol" as the hydrogen source. This system is different from that CTH, which also using alcohols (mainly secondary one) as hydrogen donor, because "activated H", as in the catalytic hydrogenation using H2-gas, was identified as the key intermediate for the reaction in the proposed system (desorption of "activated H" result in the production of H2). Although the proposed system involving "activated H" as the catalytic hydrogenation with H2-gas, they are also different since the properties of the "activated H" in terms of amounts, distribution, and adsorption/desorption behavior are different between these two processes. Therefore, the catalytic hydrogenation using aliphatic alcohols as hydrogen source also a potential way improving the catalytic selectivity. The detailed results for the present thesis are shown below.The carbon nanotube and activated carbon supported Pd catalyst (Pd/CNTs and Pd/C) shown dramatic selectivity difference (～95% vs～5%) in the hydrogenation of acetophenone to a-phenylethanol. The catalytic mechanism and DFT studies on the adsorption of a-phenylethanol suggested that the different selectivity can be explained by the different adsorption model of a-phenylethanol. When a-phenylethanol adsorbed on Pd/CNTs, the hydroxyl-group point-up, far away from the support, which inhibited the occurrence hydrogenolysis. While, when a-phenylethanol adsorbed on Pd/C, the hydroxyl-group point-down, close to the support, which favors the occurrence of hydrogenalysis (usually taken place at the interface between metal and support).The hydrogenation of phenol in water solvent shown quite higher activity than that in methanol. "Thermo-regulable" water-organic biphasic system was proposed for the hydrogenation of phenol in aqueous solvent, because phenol is hydrophobic below 339 K, but it is hydrophilic above 339 K. Catalytic hydrogenation of phenol usually carried out at hydrophilic temperature (>339 K), therefore, the reaction rate should not be limited by the solubility problem. While in this thesis, we also observed that the Raney Ni adsorbs more phenol in water than the methanol. After reaction (<339 K), both reactant and products are hydrophobic; the separation of solvent (water) was simplified.For the system of "catalytic hydrogenation using aliphatic alcohols as hydrogen sources", the combination of "catalytic hydrogenation of nitrobenzene and phenol" and "aqueous-phase reforming of methanol" was investigated over the commercial Raney Ni catalyst. The feasibility of the proposed system "aliphatic alcohols as hydrogen source for catalytic hydrogenation:liquid phase in-situ hydrogenation" was proved with this initial test/try. In order to understand well of the proposed system, apparent kinetics of phenol in-situ hydrogenation over Raney Ni catalyst were investigated (the reaction order with respect to phenol and hydrogen for cyclohexanolα1 andβ1 are 0.93 and 3.82, respectively, and for cyclohexanoneα2 andβ2 are 1.09 and 3.47, respectively. The activation energy of phenol in-situ hydrogenation for cyclohexanol (Ea1) and cyclohexanone (Ea2) are 67.8 and 80.2 kJ.mol-1, respectively). Additionally, a series of La-promoted Pd/Al2O3 catalyst was prepared for the liquid phase in-situ hydrogenation of phenol to cyclohexanone. The role of lanthanum on the Pd/Al2O3 catalyst was characterized by BET, CO chemisorption, XRD, and H2-TPR. The presence of lanthanum improves the Pd particle dispersion and the TOF for phenol in-situ hydrogenation over the Pd/Lax-Al2O3 catalyst was observed. Moreover, we also observed that the "aliphatic alcohols as hydrogen source for catalytic hydrogenation" is of general applicability. Based on the design of multifunctional catalyst, hydrogenation involved combination reactions, such as "in-situ hydrogenation coupled with alkylation" and "in-situ hydrogenation coupled with amination" etc. were also realized. For example, pyridines hydrogenation/alkylation for N-alkylpiperidines using aliphatic alcohols as hydrogen source was realized over the Pd/Al2O3 or Fe-Pd/Al2O3 catalyst; the use of methanol as hydrogen source for the direct synthesis of imines from nitroarenes and carbonyl compounds was realized over Au-Pd/Al2O3 catalysts. Among these studies, the superiority of using aliphatic alcohols as hydrogen source was well established in terms of the catalytic selectivity.Finally, the catalytic mechanism, in particular the hydrogen providing manner, of the "catalytic hydrogenation using aliphatic alcohols as hydrogen source" was investigated. The desorptions and reduction properties of the "activated H" from different hydrogen source were investigated by means of temperature-programmed desorption (TPD) and phenol-temperature programmed surface reaction in liquid phase (liquid phase-TPSR). The results indicated that the amounts of the hydrogen adsorbed on the catalyst could be regulable through the change of hydrogen source and hydrogen providing manner. The use of aliphatic alcohols as hydrogen source could provide limited amount/unsaturated of hydrogen (inhomogeneously distributed) on the surface of the catalyst through the aqueous-phase reforming or dehydrogenation. Isotope tracking studies using D2O suggest that hydrogen providing manner using aliphatic alcohols as hydrogen source including, aqueous phase reforming, dehydrogenation, and the coupling of the two manners, which could also depending on the reaction conditions and properties of the catalyst.