Preparation Of MWCNTs-doped Co-Cu Oxide Catalyst For Higher Alcohol Synthesis From Syngas

The higher alcohols (C2+-alcohols) have been confirmed to be a better and cleaner automobile fuel with high octane number, and lower emissions of NOx, ozone, CO, and aromatic vapors. Recently, methyl tert-butyl ether (MTBE) as additive of oil-based fuel has been prohibited to be used in some countries or regions due to the new legal requirements in environment protection, which is greatly renewed interest in hydrogenation of syngas to the C2+-oxygenates as gasoline blends. Nevertheless, the existing technology of higher alcohol synthesis (HAS) is still on a small scale. The single-pass-conversion of the feed syngas and selectivity for C2+-alcohols were both relatively low. Development of catalysts with high efficiency and selectivity for C2+-alcohols has been one of the key objectives for research and development efforts.Multiwalled carbon-nanotubes (MWCNTs, simplified CNTs thereafter) are a kind of novel nano-carbon material. This type of nano-carbon possesses highly graphitized tube-wall, nanosized channel and sp2-C-constructed surface. They display high thermal/electrical conductivity and excellent performance for adsorption and activation of H2, which render this kind of nano-carbon full of promise as a novel catalyst promoter.Recently, using home-made CNTs as the promoter, a CNTs-promoted Co-Cu catalyst for HAS from syngas has been developed in our lab, and the preliminary results have been reported. In this dissertation, condition-parameters of preparation for the catalyst were investigated and optimized, and the results provide the reliable basis for the optimized design and preparation of this type of catalysts. The following results were obtained.1. It was experimentally demonstrated that the CNTs-adding amount and Co/Cu molar ratio have marked effect on the reactivity and selectivity of the HAS. The results of optimization of the catalyst composition showed that the optimal CNTs-adding amount was 11% (mass percentage) and the optimal Co/Cu molar ratio was 3:1. Over the Co3Cu1-11%CNTs catalyst under 5.0 MPa, 573 K, V(H2)/V(CO)/V(CO2)/V(N2) = 45/45/5/5, GHSV = 10000 ml/(h.g), the observed CO-conversion reached 39.1%, and the selectivity of total (C1~8) alcohols and dimethyl ether (DME) combined reached 74.3%, and the corresponding space-time-yield (STY) of C2~8 alcohols and DME combined reached 1072 mg/(h?g); the S(C2-8-alc.+DME)/S(MeOH) = 17.8 (C-based selectivity ratio) in the obtained total oxygenated products, a good prospect for such oxygenated products to be used as gasoline additives.2. It was experimentally evidenced that the presence of a proper amount of CO2 in the feed-gas over the CNTs-promoted Co-Cu system was highly beneficial, instead of harmful, to CO conversion and selective formation of C2+-alcohols, with 5% of CO2-adding amount being optimal. It was also experimentally found that the addition of a proper amount of CO2 to the feed-gas did not cause an obvious change in the Ea for the HAS reaction, most likely implying that the addition of a minor amount of CO2 to the feed syngas did not alter the major reaction pathway of HAS.3. The XRD characterization of the oxide precursor of catalyst revealed that in the process of catalyst preparation, the formation of highly dispersed CuCo2O4 spinel crystallite phase in the oxide precursor of catalyst was utmost important for generation, through H2-reduction, of solid-solution-type bimetal CoiCuj sites catalytically active for HAS. The XRD measurement also showed that the calcination temperature has marked effect on the formation of CuCo2O4 spinel crystallite phase. When the temperature is not enough high, CuCo2O4 spinel crystallite phase is difficult to form; while too high temperature of calcination would be easy to lead to decomposition of CuCo2O4 spinel crystallite phase, with 623 K as calcination temperature being suitable.4. In H2-TPR investigation, based upon the difference in specific H2-consumed amount and referring to the temperature of main H2-reduction peak, it could be inferred that the increasing order of reducibility of the tested three catalysts was as follows: Co3Cu1-11%CNTs > Co2Cu1-11%CNTs > Co1Cu5-11%CNTs, which was in line with the observed sequence of reaction activity of HAS over these catalysts. 5. The XPS measurement showed that little difference in valence-state of the surface Cu-species, but significant difference in that of the surface Co-species, existed between the three tested catalysts, Co3Cu1-11%CNTs, Co5Cu1-11%CNTs and Co1Cu5-11%CNTs. On the catalyst with the optimal composition, Co3Cu1-11%CNTs catalyst, the concentration of the surface Co0-species decreased markedly companied with the amount of the surface CoOOH species enhancing evidently. It was probably such types of surface Con+-species (CoOOH) that was closely related to the highly selective formation of HAS.6. The results of H2-TPD tests showed that the catalyst with the optimal composition had considerably higher ability to adsorb and activate H2. The excellent performace of the CNT-promoted catalyst for adsorption and activation of H2 played important role in enhancing the reactivity and selectivity of HAS and inhibiting the side-reaction of water-gas-shift (WGS).