Abstract
Fischer-Tropsch (FT) synthesis is a well-known technology that enables the conversion of syngas, a mixture of CO and H2, into liquid hydrocarbon fuels and chemicals. The FT derived fuels are of high quality, being almost free of sulphur, nitrogen, aromatics. However, owing to the sequential chain-growth mechanism, the hydrocarbon distribution of the FT synthesis product is usually very broad and it is extremely daunting to obtain hydrocarbon fractions with a specific carbon-chain range such as gasoline (C5-C9) or diesel (C10-C20). The present study was focused on the use of Zr- and Ce- modified mesoporous alumina (MA) supported Co catalysts for the production of diesel. In particular, it was intended to establish correlations between the product selectivity when varying the catalyst properties and the effect of process parameters, such as temperature, pressure, and space velocity.
The specific objectives of this research included a study into a new preparation method of Zr- and Ce- modified mesopours alumina supports, an investigation into the influence of the promoter amount on the properties of the MA supported Co catalysts, and their catalytic performance in the FT synthesis, as well as a systematic evaluation of the effect of process conditions (temperature, pressure and space velocity) on the performance of these catalysts in terms of CO conversion and diesel selectivity. To achieve these objectives, two series of Zr- and Ce-modified MA supports were firstly synthesised using an isomorphic substitution and a post-impregnation methods. Co was then deposited onto these supports to generate Zr- and Ce-modified MA supported Co catalysts. Their properties were analysed using different techniques, including N2 adsorption–desorption, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Finally, they were subjected to the evaluation of catalytic performance in the FT synthesis under different process parameters.
The study of the Zr-modified MA supported Co catalysts showed that the catalyst with 5% Zr content (10Co/5ZrMA-(s)) exhibited the highest CO conversion (38.9%) and diesel selectivity (34.6%) as well as the lowest methane selectivity (7.7%). The introduction of Zr was shown to inhibit the strong interaction of Co species with the alumina supports and improve the reducibility of Co oxides. Besides, the MA supported Co catalysts generally showed better catalytic performance than the non-mesoporous commercial alumina supported counterpart. It was also found that the Zr promoted catalysts prepared by the isomorphic substitution method were superior over the ones prepared by the post-impregnation method in terms of catalytic performance. Meanwhile, the key process parameters including temperature, pressure, space velocity and feed composition were demonstrated to have marked effects on the catalytic performance of the as-prepared catalysts.
The investigation into the Ce-modified MA supported Co catalysts revealed that the catalyst with 2% Ce content (10Co/2CeMA-(s)) showed the best performance in the FT synthesis in terms of activity and product selectivity. By modifying the catalysts with Ce, Co dispersion on the catalysts was improved whereas the reduction of Co species was decreased. It proved, once again, the superiority of the isomorphic substitution method over the post-impregnation method as the catalysts prepared by the former showed higher catalytic activity and diesel selectivity but lower methane selectivity than the ones prepared by the latter. In addition, the catalyst 10Co/2CeMA-(s) achieved the lowest methane selectivity and highest diesel selectivity under the process condition of T = 503 K, P = 3.0 MPa, GHSV = 3.0 NL g–1 h–1, and H2/CO ratio = 1.2.
The outcomes of the present research have invaluable implications in the development and deployment of these MA supported Co catalysts in the FT synthesis. The understanding of promoting mechanism was enhanced and the effects of process conditions and preparation methods on the performance of catalysts were better known. This thesis provides strong confidence in the use of this catalyst system and also a scientific base to modify the current catalysts.
The specific objectives of this research included a study into a new preparation method of Zr- and Ce- modified mesopours alumina supports, an investigation into the influence of the promoter amount on the properties of the MA supported Co catalysts, and their catalytic performance in the FT synthesis, as well as a systematic evaluation of the effect of process conditions (temperature, pressure and space velocity) on the performance of these catalysts in terms of CO conversion and diesel selectivity. To achieve these objectives, two series of Zr- and Ce-modified MA supports were firstly synthesised using an isomorphic substitution and a post-impregnation methods. Co was then deposited onto these supports to generate Zr- and Ce-modified MA supported Co catalysts. Their properties were analysed using different techniques, including N2 adsorption–desorption, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Finally, they were subjected to the evaluation of catalytic performance in the FT synthesis under different process parameters.
The study of the Zr-modified MA supported Co catalysts showed that the catalyst with 5% Zr content (10Co/5ZrMA-(s)) exhibited the highest CO conversion (38.9%) and diesel selectivity (34.6%) as well as the lowest methane selectivity (7.7%). The introduction of Zr was shown to inhibit the strong interaction of Co species with the alumina supports and improve the reducibility of Co oxides. Besides, the MA supported Co catalysts generally showed better catalytic performance than the non-mesoporous commercial alumina supported counterpart. It was also found that the Zr promoted catalysts prepared by the isomorphic substitution method were superior over the ones prepared by the post-impregnation method in terms of catalytic performance. Meanwhile, the key process parameters including temperature, pressure, space velocity and feed composition were demonstrated to have marked effects on the catalytic performance of the as-prepared catalysts.
The investigation into the Ce-modified MA supported Co catalysts revealed that the catalyst with 2% Ce content (10Co/2CeMA-(s)) showed the best performance in the FT synthesis in terms of activity and product selectivity. By modifying the catalysts with Ce, Co dispersion on the catalysts was improved whereas the reduction of Co species was decreased. It proved, once again, the superiority of the isomorphic substitution method over the post-impregnation method as the catalysts prepared by the former showed higher catalytic activity and diesel selectivity but lower methane selectivity than the ones prepared by the latter. In addition, the catalyst 10Co/2CeMA-(s) achieved the lowest methane selectivity and highest diesel selectivity under the process condition of T = 503 K, P = 3.0 MPa, GHSV = 3.0 NL g–1 h–1, and H2/CO ratio = 1.2.
The outcomes of the present research have invaluable implications in the development and deployment of these MA supported Co catalysts in the FT synthesis. The understanding of promoting mechanism was enhanced and the effects of process conditions and preparation methods on the performance of catalysts were better known. This thesis provides strong confidence in the use of this catalyst system and also a scientific base to modify the current catalysts.
| Original language | English |
|---|---|
| Qualification | Masters |
| Awarding Institution |
|
| Supervisors/Advisors |
|
| Award date | 21 Jun 2016 |
| Publication status | Unpublished - 2016 |