| The characteristics of Chinese energy structure is "lacking oil, little gas, rich coal ", which decides coal resources is a main energy in China. However, the extensive use of coal has brought a series of serious issures, such as air pollution and ecological damage. Conversion of syngas into synthetic fuels and chemicals is an effective way to realize the clean utilization of coal resources. Higher alcohols have high octane numbers, which makes the most potential application of them as an additive for gasoline or replace the methyl tert-buthyl ether (MTBE) for the reduction of exhaust emission. Furthermore, higher alcohols can be directly used as neat fuels and a feedstock for the synthesis of a variety of valuable chemicals. Among several active catalyst systems, molybdenum-based catalysts have drawn special interest due to their excellent sulfur tolerance. On the basis of our previous work, this work developed a highly active Mn-doped K-Co-Mo catalyst via a sol-gel method, which exhibited an unusual performance for the synthesis of higher alcohols.In particular, its selectivity to C2+ alcohols reached the highest level achieved to date for Mo-based catalysts. The catalyst structures and CO adsorption properties were well characterized by synchrotron radiation and other techniqies and the structure-performance relationship was established. The main contents and conclusions were shown as below:The chapter 1 is a literature review, which introduces the background knowledge and the latest research progress of the related work. First, we introduce the background and significance of conversion of syngas into alcohols, followed by a detailed review of the types of active catalyst systems for the alcohols synthesis, the progress in industrialization and the formation mechanism of alcohols etc. Then the applications of Mn promoter in the CO hydrogenation reaction, including F-T synthesis and alcohols synthesis were introduced. Finally, the reserach significance and contents of this dissertation were proposed.In chapter 2, the effect of Mn promoter on structure and performance of K-Co-Mo catalyst was studied. A series of Mn-doped catalysts with Mn/Mo molar ratios varying from 0.00 to 0.25 were prepared by a modified sol-gel method using citric acid as a complexant. The Co, Mo and Mn elements were distributed uniformly on the surface of catalysts. The addition of Mn promoter promoted the interaction between Co and Mo species, forming well crystallized Co-Mo-O active phases. The presence of Mn promoter reduced the amount of tetrahedral Mo6+ species, promoted the formation of octahedral Mo4+ species and inhibited the further reduction of Mo4+ and Co2+ species, thus decreasing the hydrogenation activity of the catalysts. Furthermore, the presence of Mn reduced the amount of strong acid sites significantly and meanwhile promoted the formation of weak acid sites, which had a positive effect on the synthesis of alcohols. It was found that the incorporation of Mn can enhance the adsorption of linear- and bridge-type CO significantly, which contributed to the formation of alcohols and growth of carbon chain and thus increased the selectivity to C2+alcohols.The activity testing results showed that the incorporation of Mn significantly increased the activity of the K-Co-Mo catalyst for the synthesis of higher alcohols. The catalyst with a Mn/Mo molar ratio of 0.15 exhibited the best performance for alcohol synthesis. On this catalyst, the STY of total alcohols was 91.4 g·kg-1·h-1, approximately 6.2 times as high as that of the Mn-firee sample under the conditions of 5.0 MPa,573 K,4800 h-1, and a H2 to CO molar ratio of 2. In particular, the selectivity to alcohol increased sharply from from 4.2% to 51.6%. The MeOH/C2+OH ratio decreased from 2.38 to 0.19 when the Mn/Mo ratio increased to 0.25. The portion of methanol in total alcohol was suppressed remarkably and the ethanol became the predominant product.In chapter 3, the influence of K/Mo molar ratio and reaction conditions, including the reaction temperature and gas hourly space velocity (GHSV), on the activity of Mn-promoted catalyst for alcohol synthesis were investigated. The CO conversion and space-time-yield (STY) of total alcohols decreased gradually with the K/Mo ratio increasing from 0.085 to 0.115. However, the selectivity to alcohols increased with an incease in the K content and reached a maximum at K/Mo ratio of 0.115. Further increasing the K content resulted in a decrease of alcohol selectivity. It is noted that with the increase of K concentration, the fraction of methanol decreased gradually while that of higher alcohols increased significantly. The result indicated that the addition of K promoter was conducive to the formation of higher alcohols.Under the conditions of 5.0 MPa,6000 h-1, when the reaction temperature increased from 290℃ to 330 ℃, the CO conversion increased, while the selectivity to alcohols decreased. The ratio of MeOH/C2+OH decreased gradually from 0.22 to 0.18, indicating that increasing reaction temperature promoted the formation of higher alcohols. The space time yield (STY) of total alcohols increased significantly with an increase in the reaction temperature and reached a maxnium at 320℃. At higher temperature, the STY of total alcohols began to decrease despite the increment of CO conversion.With an increase in the GHSV, CO conversion decreased gradually, while the alcohol selectivity and STY increased. However, the amount of methanol in total alcohol also increased and the formation of C2+ alcohols was suppressed. The result suggested that a high GHSV was unfavorable for the formation of C2+ alcohols. Under the conditions of 5.0 MPa,320℃,6000 h-1, the alcohol STY and selectivity of the catalyst (Mn/Mo=0.15) were 148.3 g·kg-1·h-1 and 58.6%, respectively. The MeOH/C2+OH ratio decreased to 0.19. The mass fraction of C2+ alcohols in the total alcohols approached 77.7%. As compared to the reported Mo-based catalyst in the literatures, the presented Mn doped K-Co-Mo catalyst showed better activity for the synthesis of higher alcohols. In particular, the selectivity to C2+ alcohols reached the highest level achieved to date for Mo-based catalysts.In chapter 4, we summarized our work, discussed the existing problems, and provided an outlook of the future work. |
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