Design, Performance And Mechanism Of The New Iron Fischer-Tropsch Synthesis For Lower Olefins Catalysts

Fischer-Tropsch synthesis (FTS) is a classical topic of great significance because of the approach of post-petroleum times. For decades, people have attempted to develop iron-based FTS catalysts with high lower olefins (C2=-C4=) selectivity. The poor selectivity to lower olefins is the challenging problem for this route. Based on the advancement of carbon-based nanotubes and the research progress in our group, this dissertation will focus on the design, performance and mechanism of FTS catalysts supported on nitrogen-doped carbon nano-materials. The main contents of this dissertation are summarized as follows.1. By means of the anchoring effect and the intrinsic basicity of nitrogen-doped carbon nanotubes (NCNTs), the iron nanoparticles are conveniently immobilized on NCNTs without pre-modification. The FTS catalytic performances for NCNTs, carbon nanotubes (CNTs) and activated carbon (AC) supported iron-based catalysts were compared. It was found that the Fe/NCNTs catalyst presents superb catalytic performance in FTS to give lower olefins with a high C2=-C4= selectivity, high activity and stability, which is much better than the corresponding Fe/CNTs and Fe/AC catalysts. In order to reveal the origin of excellent catalytic performance for Fe/NCNTs, a variety of characterization methods and theoretical calculation were used to explore the mechanism of NCNTs. The results show the high C2=-C4= selectivity is well-correlated with the intrinsic basicity of the NCNTs support, which enhances the dissociative CO adsorption and promotes the lower-olefin desorption on the derived Fe/NCNTs catalyst. The high activity is attributed to the promoted reduction of iron oxide and accelerated formation of the active χ-Fe5C2 phase for the Fe/NCNTs catalyst due to participation of the nitrogen. The high stability mainly results from the anchoring effect and the intrinsic basicity of the NCNTs support, which could prevent the loss of active species and basic sites during operation. All of these merits originate from the participation of the nitrogen, as supported by our experimental characterization and theoretical calculation. The convenient construction, excellent performance, and rational correlation of properties with structural features for the Fe/NCNTs catalyst suggest a new strategy for the development of advanced FTS catalysts to give lower olefins that is based on the intrinsic basicity of abundant N-doped carbon nanostructures.2. Taking advantage of the anchoring effect of nitrogen, three Fe/NCNTs catalysts were prepared by incipient wetness impregnation method, colloidal method and deposition-precipitation method, denoted as Fe/NCNTs-IWP, Fe/NCNTs-C and Fe/NCNTs-DP, respectively. The influence of preparation methods on the particle size and morphology, reduction and carbonization of the active species, as well as the FTS catalytic performance were systematically examined. The results indicate that the incipient impregnation method could achieve high dispersion, smaller particles and narrower size distribution (8±4 nm), leading to the easier reduction and carbonization of the iron nanoparticles compared with those prepared by the other two methods. The FTS catalytic performance of the Fe/NCNTs-IWP catalyst is much better than those of the Fe/NCNTs-C and the Fe/NCNTs-DP catalysts in terms of the high lower olefins selectivity, high catalytic activity and stability. Colloid method got the different morphologies of iron particles with the average size of 13±7 nm. The active iron species was susceptible to oxidation during the reaction, which causes poor catalytic activity and stability. The deposition-precipitation method got the largest particles of 19±11 nm which were difficult to be reduced and carbonized, leading to the sharp decline of the catalytic activity and stability after 15 h reaction. The size-dependence of the catalytic performance for the Fe/NCNTs catalysts in this study are generally in consistent with those in literatures for the iron catalysts supported on carbon nanotubes. These results should be suggestive for exploring the advanced FTS catalysts with the abundant N-doped carbon nanostructures.3. Some influence factors on FTS catalytic performance were investigated and optimized, including metal loading, reaction temperature, feed gas space velocity, K promoter. In terms of the high lower olefins selectivity, high catalytic activity and stability, the optimal reaction conditions are metal loading of 10 wt%, reaction temperature of 300℃, and space velocity of 4200 mL h-1 g-1. The catalytic performance can be further improved by adding 0.5 wt% of K promoter. Fe/NCNTs catalyst presents the superb catalytic performance in FTS to lower olefins, such as the high C2--C4-selectivity up to 46.7% as well as high activity and stability, which is comparable to the results from the mainstream literature.4. Taking advantage of the anchoring effect of nitrogen, a novel Fe-based nano-composite catalyst was conveniently constructed using nitrogen-doped carbon nanocages (NCNCs) as support. Up to 40 wt% of iron species could be well dispersed on NCNCs due to the large surface area and high nitrogen content of the support. The FTS results showed that Fe/NCNCs catalyst with 40 wt% iron loading presents the high activity and 72 hours stability. This proposes an effective way to design nano-composite catalyst with potential applications.