ausblenden:
Schlagwörter:
-
Zusammenfassung:
During the Ph.D. project described in this thesis, the Cu colloid-based catalysts were developed for studying the methanol synthesis reaction in both quasi-homogeneous and heterogeneous systems. This research should help to address unclear issues concerning the heterogeneous systems using solid catalysts, concerning the reaction mechanism, the active sites, the roles of the components, etc.
The Cu colloids were prepared using a Bönnemann route - reductive stabilization. The Cu(acac)2, as Cu precursor, was simultaneously reduced and stabilized by either alkylaluminium or alkylzinc in THF solution under Ar protection. The stabilizers applied could be extended to four different types of alkylaluminium or alkylzinc compounds, including Al(n-butyl)3, Al(n-octyl)3, Zn(ethyl)2 and Zn(n-butyl)2. The structural properties of these different Cu colloids were intensively investigated by various characterization techniques, such as TEM, UV-Vis, XRD and XAS. They all confirmed that the Cu precursor was well reduced to form Cu nanocrystals and the particle sizes were in a range of 3-6 nm with a narrow size distribution. However, it was difficult to further tune the particle size despite of variation of synthesis parameters. The in situ XAS measurements suggested a possible colloid formation mechanism: Cu(II) was reduced directly to Cu(0) without going through Cu(I). The Cu colloids were proven to be stable without agglomeration in storage under Ar protection for a long time.
In order to investigate their catalytic performance in methanol synthesis from synthesis gas feed, the Cu colloids were tested directly in a quasi-homogeneous phase. They all exhibited high activity in methanol formation and the methanol productivity reached values as high as 23.3 molMeOH/(kgCu·h). They were much more active than the benchmark catalyst -KATALCOJM 51-8 (Johnson Matthey Catalysts, Cu/ZnO/Al2O3)- in THF suspension tested under the same reaction conditions. Surprisingly, even those Cu colloids only stabilized by alkylaluminium were highly active without the presence of Zn species that are usually considered to be necessary in a solid catalyst. Moreover, the use of Cu colloids favored the methanol synthesis at lower temperature and the methanol formation already started at 130 °C. An on-line product analysis demonstrated the formation of methyl formate that is most probably considered as an intermediate rather than a by-product. The Cu colloids remained active for up to 40 hours during reaction. Different techniques were applied to reveal the reason(s) for their activity. It was found that though the Cu colloids all decomposed, the core of the Cu particles still remained metallic. In contrast, the metal alkyl shell was oxidized, which formed Cu nanoparticles supported on ZnO or Al2O3. These components might provide the activity in a THF suspension for a long time. Furthermore, a series of experiments was designed to explore the nature of both Cu core and metal alkyl stabilizing shell as well as the structure-activity relationship of Cu colloids. Among the different systems, Ag colloid, Ni colloid and non-metal alkyl-stabilized Cu colloids showed no activity in methanol synthesis, whereas Mg(n-butyl)2-stabilized colloids showed low activity. It was clearly demonstrated that the activity of the Cu colloids is provided by the synergy of both Cu core and stabilizing shell, and there were strong interactions of Cu-Al and Cu-Zn, probably associated with sites on the surface of Cu nanoparticles.
With the aim of maintaining the high activity of the Cu colloids in a heterogeneous system, supported Cu nanoparticles were prepared by a colloidal deposition method, using ordered mesoporous materials (SBA-15 and CMK-5) and metal oxides (ZrO2 and ZnO) as supports. All the supported Cu nanoparticles showed stable activity throughout the whole gas-phase reaction under similar conditions used in an industrial process. It was found that the supports did have significant influence on the activity of Cu nanoparticles, and their interactions with Cu nanoparticles were different. Cu nanoparticles stabilized on SBA-15 and CMK-5 were much less active, and there was obvious particle agglomeration. In contrast, some of those stabilized by ZrO2 and ZnO were nearly as active as the benchmark catalyst and the highest methanol productivity reached 50.8 molMeOH/(kgCu·h). The high activity might be due to the formation of Cu/Al2O3/ZrO2(ZnO) systems, similar to the active components in technical catalysts.
The Cu colloid-based catalyst system was established for studying methanol synthesis. Additional insight in some aspects, including the reaction mechanism and the active sites, could be obtained. In particular, Cu colloids were proven to be highly active in a quasi-homogeneous phase at lower temperature. However, the high activity of Cu nanoparticles could not be well maintained by solid supports in a gas-phase reaction, and their activities were only at the same level as those of technical catalysts.
