The selectivity and stability of catalysts for the single-stage production of dimethyl ether from gas mixtures containing CO2, CO and H2 is studied. A Cu/ZnO/Al2O3 catalyst was used for the hydrogenation of carbon oxides while the dehydration of methanol was catalysed by γ-alumina, phosphated γ-alumina or protonic zeolites. The influence that the proximity between the methanol synthesis and the methanol dehydration functions has on the catalytic stability is evaluated for each catalyst pair under different feed compositions. The assessment of the effect of zeolite topology on the deactivation and the by-product formation is also aimed. The intimate contact between Cu/ZnO/Al2O3 and γ-alumina is found to cause fast deactivation on both catalysts during the first hours of the experimental runs. It is shown that the detrimental interaction is more severe under CO-rich conditions than for gas mixtures with high CO2 contents. The addition of phosphorus to the γ-alumina up to the monolayer capacity mediates the loss of activity that occurs in the presence of high CO concentrations. The independent characterisation of the spent catalysts gives insights on the mechanism responsible for the observed loss of activity . Five different zeolite structures were considered to be studied as dehydration catalysts: mordenite, ZMS-5, ferrierite, theta-1 and ZSM-23. It is found that the deactivation of zeolites during the single-stage synthesis of DME is dictated by the deposition of carbon residues which is related to the topology of the solid acid. In the conversion of CO/H2, the hydrocarbon distribution depends on the nature of the entrained organics and it is demonstrated that a careful selection of crystallite size and channel structure of the zeolite allows the conversion and selectivity to be controlled. Under CO2 and H2, ZSM-5 is shown to be considerably more active than γ-alumina and very selective towards dimethyl ether. Experiments using different SiO2 to Al2O3 ratios indicate that the deactivation depends on the acid site density rather than on the acid strength.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:668217 |
| Date | January 2014 |
| Creators | Montesano Lopez, Raúl |
| Contributors | Chadwick, David |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/26597 |
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