Investigation of Fundamental Properties of Catalysts for Direct and Indirect Synthesis of DME
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Dimethyl ether (DME) has potential as an environmentally friendly fuel. The main advantage of DME as a fuel lies in its clean combustion; the emissions of soot and particulate matter, as well as NOx and SOx are significantly reduced compared to emissions from diesel combustion. DME is produced in a two-step process today, where methanol synthesis is followed by methanol dehydration. The production of DME directly from synthesis gas represents a more thermodynamically favorable synthesis route, and should thus be more economic. In this thesis, catalysts relevant for DME production have been investigated. The goal of this project was to relate the activity and the stability of relevant catalysts in separate reactions to production of DME directly from synthesis gas. The acidity of the dehydration catalysts was however found to be very interesting, and quite challenging. The aim of the project was therefore shifted to focus more on catalyst characterization, especially the acidic catalysts, than originally intended. Some of the results included in this thesis are taken from a project on the same topic, which was conducted during the fall of 2013. These results are included in an attempt to present a more complete picture. A Cu/ZnO/Al2O3 catalyst was synthesized, characterized and tested. Both batches of catalyst were found to have sodium concentrations well above the acceptable limit. Sodium is considered a catalyst poison, which can affect the activity of the catalyst. The high concentration of sodium indicates that the catalyst precursor was insufficiently washed prior to drying and calcination. It is therefore likely that also nitrate residues were inadequately removed. Nitrate residues have been shown to promote metal agglomeration during calcination. Metal agglomeration may have affected both the activity and the stability of the catalyst. Both the BET surface area and the copper dispersion of the synthesized catalyst were found to be low compared to similar catalysts. Low activity was observed, and the catalyst deactivated rapidly, compared to the commercial catalyst. Metal particle growth may explain the low BET surface area as well as the low dispersion, but sodium may also be a factor. Both the presence of sodium and the low dispersion of copper may explain the low activity of the catalyst. Metal agglomeration during calcination may also have led to the poor stability, as interparticle distance have been shown to be an important factor for sintering. It is common for copper based catalysts to loose some activity during the first 1000 hours of operation due to sintering. The commercial catalyst was seen to continuously loose activity over a time period of 14 days. Temperature profiles of the catalyst bed measured at different points in time supported sintering as deactivation mechanism. The kinetics of the methanol synthesis was investigated by varying the temperature of the catalyst bed. The CO conversion increased as a function of temperature before it leveled off near the equilibrium conversion. The apparent activation energy was found to be about 54 kJ/mol, in reasonable agreement with literature. The acidic dehydration catalysts were investigated by temperature-programmed desorption (TPD) with ammonia and/or isopropylamine. The acid site concentration was found to be well correlated with the aluminium content for the zeolites. The zeolites were found to have two main groups of acid sites, in good agreement with literature. Both groups of acid sites were found to have contributions of both Brønsted and Lewis acid sites. Batches of ion-exchanged zeolites were prepared. Protons associated with Brønsted acid sites were attempted to be replaced with sodium ions, to reduce the amount of Brønsted acid sites. The zeolites were analyzed by ICP-MS to determine the extent of ion-exchange, but the results seemed unreliable and were therefore disregarded. The acidity of the ion-exchanged zeolites was also investigated. Two main groups of acid sites were found also for these zeolites. Even though the results are somewhat uncertain due to the unknown extent of ion-exchange, it was found that sodium had predominantly replaced protons associated with strong Brønsted acid sites. This is in good agreement with literature. Adsorption calorimetry with ammonia was attempted for one of the zeolites. Establishing the experimental protocol turned out to be challenging, but qualitatively reasonable results were achieved.