| Catalytic conversion of syngas to ethanol is one of the most potential routes for fuel or fuel additive. Syngas can be obtained from various sources, such as through gasification of biomass, coal and other carbonaceous waste, reforming of natural gas. Currently, in China, syngas is mainly produced by the gasification of coal. Although the high cost of rhodium-based catalyst, it is still the interest due to its higher ethanol selectivity. Rhodium-based catalyst has not yet been industrially applied, one problem is that the balance of activity and selectivity could not obtained. To achieve this goal, the catalyst is still the key technique and should be further studied. In this paper, Al2O3was chosen as carrier, the effects of Fe and/or Mn and type of Al2O3on the catalytic performance of Rh-based catalyst were investigated. In addition, the effect of rhodium on CuCo catalyst was studied.The effects of Fe on CO adsorption, dissociation, desorption and hydrogenation ability were studied over Rh/Al2O3catalyst. RhFe/Al2O3catalysts were prepared by co-impregnation. The effects of Fe loading (2～10wt%) and H2reduction temperature (200～420℃) on Rh-based catalysts performance were characterized by N2adsorption, H2-TPR, H2-TPD, TEM, XPS and DRIFTS. H2-TPR patterns show that a strong interaction occurs between Rh and Fe. More Fe species are reduced at higher Fe content, which is in favor of hydrogenation. The DRIFTS results indicate that higher Fe loading suppresses CO dissociation, and improves the hydrogenation capability and CO desorption rate of catalysts. Compared with unpromoted Rh catalyst, the addition of4wt%Fe can stabilize gem dicarbonyl and linear adsorbed CO species. Increasing reduction temperature increases Rh reduction degree and CO adsorption over2Rh4Fe/Al2O3, but more Fe0species enrichment on its surface. The experimental results indicate that the catalyst with4wt%Fe exhibites better ethanol selectivity. Higher Fe loading and reduction temperature will result in more Fe0species which favor the formation of methanol.The investigation of composition optimization, catalytic performance and Fe impregnation sequence about Mn and Fe co-promoted Rh/Al2O3catalysts were carried out. A statistical analysis was adopted to optimize catalyst compositions. The effects of Rh loading (0～3wt%), Fe loading (2～10wt%) and Mn loading (0.5-2.5wt%) of RhMnFe/γ-Al2O3were studied through response surface methodology (RSM) combined with a central composite rotatable design (CCRD). A linear and a quadratic models were proposed to correlate the three variables to the two responses:CO conversion and ethanol selectivity, respectively. The predicted values for CO conversion and ethanol selectivity were in a good agreement with the experimental values. The optimum catalyst compositions for achieving the maximum ethanol selectivity (27.8%) and CO conversion at a moderate level of20%were as follows:Rh loading of2.5wt%, Mn loading of2.5wt%and Fe loading of4wt%. CO hydrogenation performance was compared over Mn and/or Fe promoted Rh/γ-Al2O3catalysts. The results show that Mn and Fe co-promoted Rh/γ-Al2O3catalyst exhibits superior catalytic activity and better ethanol selectivity. The DRIFTS results show that Mn promoter stabilizes the adsorbed CO on Rh+and Fe promoter stabilizes adsorbed CO on Rh+and Rh0. The higher ethanol selectivity of Mn and Fe co-promoted Rh/γ-Al2O3sample is due to the existence of more (Rhx0-Rhy+)-O-Fe3+(Fe2+) active species and close contact among Mn, Fe and Rh.Mn and Fe co-promoted Rh catalysts with identical metal loading were prepared by impregnation method with different Fe impregnation sequence. The results show that a tilted adsorbed CO, which favors CO dissociation, was observed on the surface of Fe-RhMn catalyst in which Fe was impregnated firstly. Clearly, RhMn-Fe catalyst, in which Fe was inpregnated finally, is more favorable for dissociative adsorption of hydrogen, while co-impregnated RhMnFe catalyst provides more sites to adsorb CO molecularly. The best ethanol selectivity was achieved on the catalyst prepared by co-impregnation of Fe and RhMn due to the synergistic interaction between Fe, Rh and Mn, which promote the generation of more active sites for ethanol formation. The effects of temperature(235-290℃), space velocity (1800-5400mL/(g.h)) and pressure (0.4-3.6MPa) on the catalytic performance were investigated. With the rise of temperature, CO conversion increases and ethanol selectivity decreases. Increasing pressure, both CO conversion and ethanol selectivity increase. When the space velocity increases, CO conversion decreases, while ethanol selectivity increases first and then decreases.The effects of different types of Al2O3on the catalytic performance of RhFe/Al2O3catalyst were investigated. Different types of Al2O3were prepared by co-precipitation, sol-gel and hydrothermal synthesis method. Compared with the catalysts supported on commercial Al2O3, the catalysts supported on Al2O3prepared by co-precipitation and sol-gel method formed smaller particles. A strong interaction exist between small particles with Al2O3and a weak interaction occurs between Rh and Fe. There is an opposite interaction effect for the catalysts supported on nano Al2O3and nanofibres Al2O3prepared by hydrothermal synthesis method. The DRIFTS spectra in the region of3400～3800cm-1show that the hydroxyl groups bonded to catalyst surfaces are quite different. Compared with commercial Al2O3, nano and nanofiber Al2O3supported Rh-based catalysts have more isolated hydroxyl groups. Isolated hydroxyl group may involve in the transformation from Rh0to Rh+, which may be the active sites for CO insertion to form ethanol.The effects of rhodium on the structure and the activity of the supported CuCo/Al2O3catalyst for CO hydrogenation were investigated. The CuCo/Al2O3and CuCoRh/Al2O3samples were characterized by N2-adsorption, XRD, H2-TPR, H2-TPD, XPS, TEM and DRIFTS. The results indicate that ethanol selectivity and CO conversion were both improved by the addition of lwt%Rh to CuCo/Al2O3catalyst. The addition of rhodium to CuCo/Al2O3catalyst results in modification of metal dispersion, reducibility and adsorption. The DRIFTS results indicate that the addition of lwt%rhodium can improve the hydrogenation capability of CuCo/Al2O3catalyst at pressure2MPa and temperature260℃. The alcohol distribution over un-promoted and rhodium promoted CuCo/Al2O3catalyst obeys Anderson-Schulz-Flory rule and higher chain growth probability is obtained on1wt%rhodium promoted catalyst. |
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