Catalyst formulations for use in microstructured reactors for conversion of synthesis gas to liquids
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Potential natural gas reserves in the world are more than 6000 trillion cubic feet. However, approximately 25% of these gas reserves are located offshore with no economic feasibility to be produced, transported and sold. This calls for finding solution for utilization of the resources. Exploiting offshore natural gas presents challenges that possibly could be overcome by offshore conversion of gas to liquids (e.g. methanol, synthetic gasoline/diesel (Fischer-Tropsch technology) or dimethyl ether (DME)). For offshore gas-to-liquids (GTL), the production unit would require compact, efficient, robust, lightweight, reliable and safe technologies. Microstructured reactors may present an interesting potential for offshore GTL technology. Development of catalysts for use in microstructured reactors is an important part to get a more viable technology. The purpose of the present work is preparation, characterization and performance of different catalyst formulations for use in microstructured reactors for conversion of synthesis gas to liquids. The work focuses mainly on catalyst systems for methanol synthesis from synthesis gas (paper I-III). In addition, another part of the work (paper IV) focus is on a macroporous-structured alumina material, which featured as a micro-scale structured support, then used as support for Cobased catalysts for Fischer-Tropsch (FT) synthesis. The Cu-based coatings were prepared using different techniques: slurry coating of CuO/ZnO/Al2O3 obtained via 2-stage co-precipitation, sol-gel coating of Al2O3 followed by Cu-Zn impregnation, colloid coating of Al2O3 followed by Cu-Zn impregnation, and colloid coating of Al2O3 followed by depositionprecipitation of Cu-Zn. The coated monoliths were characterized (XRD, BET, N2O titration) and studied in the methanol synthesis reaction at 80 bar. Comparison was made to similarly prepared powder catalysts subjected to characterization and laboratory scale FBR experiments. Monoliths with high activity for the methanol synthesis reaction were obtained slurry coating of CuO/ZnO/Al2O3, whereas the impregnation and deposition-precipitation methods gave insufficient Cu dispersion. Higher activity of the monolith relative to FBR experiments with the same catalyst was ascribed to the thermal properties of the steel monolith. The successful slurry coating technique obtained was employed for a microreactor configuration of stacked foils (SFMR). The stacked foil microstructured reactor (SFMR) for producing methanol from synthesis gas could be demonstrated at high pressure, proved up to 80 bar. It is furthermore shown that two different active catalyst foil coatings could be prepared and tested, Pd/CeO2 and CuO/ZnO/Al2O3. The performance of the catalyst-reactor systems could also be compared to that of a laboratory scale fixed-bed reactor (FBR) containing catalyst particles of similar composition. The differences between two reactor types – SFMR and FBR – are found using the 2-stage co-precipitated CuO/ZnO/Al2O3 catalyst. The slurry coating technique was used for coating SFMR foils. Only minor differences in performance exist between the two reactors that can be related to different temperature properties of the two systems. The Pd/CeO2 catalysts show high initial activity in both SFMR and FBR, but deactivate significantly to reach steady-state after about 60-120 hours. This may be explained by good interfacial contact between Pd and CeO2 created during preparation and reduction to form sites that are gradually lost under reaction conditions by a combination of sintering/agglomeration and enhanced coverage of the Pd by ceria layers. The activity of the Pd/CeO2 foil coating prepared via a sol-gel procedure is substantially better than the Pd/CeO2 particles prepared by depositionprecipitation and applied in the FBR (although the Pd/CeO2 powder has higher Pd dispersion), initially as well as after stabilization. This is ascribed to the Pd nanoparticles of the powder catalyst being partly covered by the ceria upon preparation and reduction. This prevents the accessibility of the Pd to the gaseous reactants. Similar phenomena seem to occur also in the foil coating, but because of the preparation resulting in larger Pd particles residing on top of a ceria layer, the Pd or Pd-CeO2 interface remains more accessible to the reactants. The maximum methanol productivity obtained in the SFMR is higher for the Pd than the Cu system on a mole of active metal basis (Pd/Cu), although at higher temperature and significantly higher methane by-product formation. MPS-Al2O3 support was successfully synthesized from Al2O3 nanoparticles and sacrificed PS beads. The ratio of the particles played a key role in controlling the morphology of MPS-Al2O3 support. The structure obtained contained uniformly spherical pores which were interconnected throughout the whole sample. The support was confirmed having α-phase alumina, but had relatively higher water adsorption capacity than that of a conventional α-Al2O3, giving a higher Co dispersion of Co/MPS-Al2O3 than that of Co/α-Al2O3. The SSITKA studies showed that this higher dispersion helped improving rate of reaction, comparable with the Co/γ-Al2O3 catalyst, but without sacrificing the selectivity as compared with α-Al2O3. The MPS-Al2O3 supported cobalt catalyst hence seems to combine the advantages of both Co/α-Al2O3 and Co/γ-Al2O3 catalysts.