Reaktivität und Struktur komplexer Sulfide in der oxidativen Methankupplung mit weichen Oxidationsmitteln
Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung
Zusammenfassung der Projektergebnisse
The goals of this project as stated in the project description for the DFG fellowship were as follows: Firstly, a wider range of catalysts in the oxidation of CH4 with soft oxidants should be explored. Secondly, it was proposed to obtain information on catalyst structure and composition from analytical studies. Thirdly, reactor studies for elucidating the reaction mechanisms of CH4 oxidation with S2 were suggested. First experiments were conducted on Pd/ZrO2, on which high C2H4 selectivities were observed previously in CH4 coupling with S2. Upon studying the catalytic properties of Pd/ZrO2 as a function of the Pd content, it was found that some oxides that do not contain noble metals are highly selective towards C2H4. Thus, 10 oxides were more closely investigated in CH4 coupling with S2: C2H4 selectivities of up to 33 % are found at CH4 conversions of 5 % - 12 %. C2H4 selectivities of >25 % were only observed on non-coking catalysts, this suggests that some degree of coking resistance facilitates the optimum CH4 to C2H4 conversion. Of course, these values are currently below commercial interest. Note however that the desired reaction product C2H4 is by far the most prominent C2 product. The C2H4/C2H6 ratio is 8.9 - 12.4, which to our knowledge is far higher than in any previous study using O2 as oxidant that reported on the C2H4 and C2H6 selectivity. Furthermore, oxidative coupling of CH4 with S2 is still largely unexplored, and further studies on optimizing catalysts and reaction conditions are likely to provide higher ethylene yields in future studies. Information obtained from analytical studies (XRD, Raman spectroscopy, XPS, Combustion analysis,) are summarized in the Figure below. Generally, a mixture of oxides and sulfides is present after reaction with the CH4/S2 mixture. The 10 oxides form two types of chalcogenides at reaction conditions: On type A catalysts, MgO, Sm 2O3, ZrO2, La2O3, and WO3, significant amounts of coke containing O and S are found after reaction. Beneath the coke layer, a metal sulfide forms during SOCM. On type B catalysts that form from the oxides of Ti, Fe, Zn and Cr, only minor surface C is present in addition to an oxide and a sulfide. Complete conversion of the surface oxides to sulfides might be expected after long S2 exposure. Surprisingly however, surface oxide species are observed by XPS on type B catalysts after reaction. EXAFS studies in collaboration with Dr. Jeffrey Miller found that surface ZnO completely transforms to ZnS with bond distances that are almost idendical to bulk ZnS. The reaction mechanism of CH4 conversion with S2 was studied by varying the reaction time and performing reactor experiments with the educts C2H6, C2H2 and CS2. It is interesting how very different the reaction mechanism of CH4 coupling with S2 is to the analogous reaction using the oxidant O2. With the oxidant S2, C2H6 is by far more reactive than C2H4 whereas similar reactivities for C2H4 and C2H6 have often be observed with O2 as oxidant. CS2 is formed to a large extent directly from CH4 whereas many studies using the oxidant O2 found that CO2 is formed largely via C2 intermediates.
