Influencia del soporte catalítico en la producción de combustibles sintéticos vía Fischer-Tropsch

The present work is part of a research program that deals with the study of catalysts and their use in high-pressure reactions of industrial interest that is being developed in the Department of Chemical Engineering at the University of Castilla-la Mancha. Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed polymerization process, which converts syngas (CO and H2) into a wide variety of hydrocarbons. In this reaction, a great number of desired (paraffins, olefins and alcohols) and undesired products (aldehydes, ketones, acids, esters, carbon, etc) can be obtained, indicating the complexity of the reaction process. Over the last years, the importance of petroleum-derived fuels, the different issues of this energy source, and the desire of energetically independence of some advanced countries has caused that FTS research has gained a renewed relevance. Moreover, the study of different materials as catalyst supports stands out among the other research topics. Thus, the aim of this PhD work is to study the influence of the catalyst support in the production of synthetic fuels via FTS. Three different kind of materials: new materials synthesized by the company A, carbon nanostructures prepared in our laboratory, and silicon carbide supplied by SICAT Catalysts were considered. The Chapter 1 deals with the use of the first kind of materials mentioned above as the support. In this study, the influence of these materials on both the active phase (metal) dispersion and ageing, and the catalytic activity and selectivity towards hydrocarbon diesel fraction was studied. It could be observed that the higher the active phase amount, the higher the reactants conversion was. However, the diesel production was maximized with intermediate amounts of the active phase onto the supports. Once the amount of active phase was optimized, the effect of the three materials supplied, presenting different porosity, on the deposition and dispersion of the resulting catalyst was evaluated. Results showed that the catalyst with the highest support porosity was the most active in FTS. Moreover, the rate of secondary reactions was increased. Finally, it was checked that the most active catalyst also presented the highest thermal and hydrothermal resistances. In addition, it showed a good catalytic performance after ageing processes with N2/H2O mixtures. Chapter 2 was related to the study of the influence of the use of carbon nanofibers (CNFs) as the support of cobalt-based catalysts in the FTS. Firstly, CNFs prepared at three different temperatures (1023, 873 and 723 K, leading to samples named to as CNF-1, CNF-2 and CNF-3, respectively) were used as the support of cobalt-based catalysts in the FTS, being compared their performance. Both the supports and the cobalt catalysts were characterized by nitrogen adsorption-desorption, TPR, XRD and TGA whereas the metal content of the cobalt-based catalysts was analyzed by ICP. The activity and selectivity of the CNFs-supported catalysts were studied at 523 K, 20 bar and H2/CO = 2. Catalysts Co/CNF-1 and Co/CNF-2 were very active and showed high selectivity to CH4 and CO2, without further deactivation, whereas the less active catalyst Co/CNF-3 led to the highest selectivity to long-chain hydrocarbons (C5+) and presented a remarkable deactivation. Used catalysts were characterized by nitrogen adsorption-desorption, XRD and TGA. Results confirmed that all the catalysts underwent fouling as a consequence of C5+ hydrocarbons formation whereas catalyst Co/CNF-3 presented a notably sintering by coalescence. Then, the FTS-CO hydrogenation process was compared to that of CO2 on catalyst Co/CNF-2. The influence of the CO2 content in the feed stream (H2/CO/CO2 ratio) on the reaction performance in terms of conversion and selectivity to the different products was described. Results showed that the CO hydrogenation was controlled by a FTS regime, whereas CO2 hydrogenation was controlled by a methanation process. When feed was composed of CO and CO2 mixtures, the catalytic activity decreased with respect to that obtained with a CO2-free feed stream. Moreover, the presence of CO2 in the feed stream favored the formation of lighter hydrocarbons and could block the production of further CO2 via WGS reaction. In Chapter 3, four ß-silicon carbide (ß-SiC) samples supplied by SICAT Catalysts were used as the support of FTS catalysts. They consisted of the parent material (SiC-A), SiC-A modified by a pore agent (SiC-B) or purified by an acidic treatment (SiC-C), and SiC-B purified by the referred acidic treatment (SiC-D). The pore agent treatment resulted in a modification of the pore size distribution, whereas the acidic one led to materials with less metal impurities content. The effects of these pretreatments in the resulting materials were characterized by nitrogen adsorption-desorption, Hg intrusion porosimetry, TPR, XRD, TGA and NaOH titrations. All the samples were used as supports in cobalt-based catalyst, which were also characterized by the techniques mentioned above as well as by ICP and oxygen pulses. It was observed that the acid washing, in addition to remove some metal impurities, increased the number of acid sites over the silicon carbide, which seemed to promote the reducibility of the cobalt particles of the corresponding catalyst. On the other hand, the addition of pore agent strongly increased the macropore volume of the silicon carbides, which favored the FTS products desorption, keeping the catalytically active sites available. All these facts caused that the catalyst supported on pore- and acid-treated SiC presented the highest catalytic activity. Finally, Chapter 4 constituted a deeply study of the performance of bimetallic FTS cobalt and iron catalysts supported on carbon nanostructures. Firstly, cobalt and/or iron supported on CNF were prepared. They were characterized by ICP, nitrogen adsorption-desorption, TPR, XRD and XPS. It was observed that the performance of the bimetallic catalysts was quite different to that of the monometallic ones. Regarding the bimetallic catalysts, the higher the content in cobalt, the higher the CO conversions were observed. However, the presence of iron in these catalysts avoided an excessive production of CH4. The bimetallic sample with the highest Co loading (10Co5Fe/CNF) was the most active catalyst for the FTS reaction. This fact was attributed to the better dispersion of cobalt particles due to the presence of iron, the higher reduction of the latter to Fe0 due to the presence of the former and the larger surface concentration of the active phases. In this catalyst, the high surface concentration of both active phases (Co and Fe) respect to the rest of bimetallic catalysts would lead to both the activation of a higher proportion of reactants and an easier desorption of the products, which favored the formation of shorter hydrocarbons. Then, the physicochemical properties of CNF and CNS-supported bimetallic Co/Fe catalysts were compared. These supports were characterized by nitrogen adsorption-desorption, TPR, XRD and Raman spectroscopy. Catalysts were in turn characterized by ICP, nitrogen adsorption-desorption, TPR, XRD and TEM. Results showed that the bimetallic catalysts supported over CNF presented the biggest metal particles, which favored the catalytic conversion of CO, whereas that supported over CNS presented well-dispersed metal particles that strongly interacted with the support and promoted the conversion of CO in a lesser extent, but favored the growth of hydrocarbon chains