Selective and efficient synthesis of ethanol from dimethyl ether and syngas

Dominik Bjørn Rasmussen

Research output: Book/ReportPh.D. thesis

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Abstract

The modern society is heavily dependent on fossil fuels, which are a limited resource. The transport sector is responsible for a large share of the combined fuel consumption and there is a strong political and environmental incentive to make it less dependent on oil. Ethanol (EtOH) can play an important role as a gasoline additive or substitute and a catalytic process has been demonstrated, in which dimethyl ether (DME) produced from synthesis gas is converted to methyl acetate (MA), which is subsequently converted to EtOH and methanol (MeOH). MeOH can afterwards be easily converted to DME, using well-established processes. Syngas can be produced from biomass, making the entire process sustainable and environmentally friendly. The main benefit of this method is its unprecedented selectivity towards EtOH, while MeOH, the primary by-product, and the unreacted syngas are easily recycled. The reaction of MA to EtOH and MeOH is facile and can be performed efficiently using known technologies. The formation of MA is on the other hand difficult because no stable and active catalyst have been identified for the reaction yet. Mordenite has been shown to be the most active catalyst for the reaction but it is not sufficiently active or stable to be applied industrially. In this PhD project, the formation of MA over Mordenite has been studied experimentally and by density functional theory (DFT) calculations. The DFT study of the reaction path has shown that ketene is a reaction intermediate, a result with has been confirmed experimentally using mass spectroscopy. Ketene is a reactive molecule, which easily forms polymers. Consequently, its presence in the catalyst may be one of the reasons for the rapid deactivation. Additionally, it was demonstrated by DFT calculations that MA is primarily formed in the side pockets of Mordenite and that the reaction of CO with methyl groups is the rate limiting reaction step. Thus, in contrast to the previous DFT studies, the DFT model developed here is fully consistent the experimental results. An experimental study of the reaction kinetics has shown that MA inhibits the reaction. A kinetic model, taking this effect into account, was developed and it could accurately describe the dependence of the reaction rates on the amount of catalyst and the partial pressures of the reactants. The product inhibition of the reaction rate makes it difficult to scale the process up, as 2 it sets a limit on the maximum concentration of MA in the catalyst, relatively to the concentrations of the reactants. A study of the deactivation rate has revealed that for a fixed reaction rate, the deactivation rate of the catalyst increases with increasing DME concentration; for a fixed DME concentration in the feed, the deactivation rate decreases with increasing MA concentration. However, the precise connection is still unknown. The results of this PhD project contribute significantly to the understanding of the reactions taking place on Mordenite during MA synthesis and form a firm foundation for the future studies.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages143
Publication statusPublished - 2015

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