Study of Acidic and Basic Properties of Silica-Magnesia Catalysts in the Conversion of Ethanol into 1,3-Butadiene
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The Lebedev process, in which ethanol is converted into 1,3-butadiene, proceeds through multiple elementary steps, either acid- or base catalyzed. The acidic and basic properties of catalysts are thus related to the catalytic performance; in particular, a high amount of ethylene production indicates a high concentration of accessible acidic sites present in the catalyst. In this thesis, the acidic and basic properties of differently prepared silica-magnesia catalysts, used in the Lebedev process, were studied with both bulk- and spatially-resolved characterization techniques. Furthermore, the influence of doping the silica-magnesia catalysts with small dispersed CuO particles on the acidic and basic properties was studied. All catalysts contained silica and magnesia in equimolar ratios and were prepared via either a co-precipitation route (CP and CP-MOD) or via wet-kneading employing different silica sources (WK, WK-MOD, WK-AER and WK-BIG). A second series of these catalysts was promoted with 1 wt% CuO using incipient wetness impregnation. NH3-TPD showed that the CP catalyst contained the highest concentration of acidic sites and the WK catalyst the lowest. The CP-MOD, WK-MOD and WK-AER showed similar concentrations of acidic sites. Upon CuO promotion, the concentration of acidic sites decreased for all samples, except for the WK-AER catalyst, for which it increased significantly. Pyridine-FTIR showed that CP contained the highest concentration of acidic sites and WK the lowest, which is consistent with the results from NH3-TPD. Moreover, pyridine-FTIR showed that the strongest acidic sites are present in the CP catalyst and the weakest in the CP-MOD. Spectra of CuO-promoted catalysts showed that CuO particles are anchored to the surface hydroxyl groups, with a preference for the hydroxyl groups on the magnesia moiety. Upon CuO-promotion, it was shown that the concentration of acidic sites decreased significantly for the co-precipitated samples and slightly for the wet-kneaded catalysts; this is not in line with NH3-TPD, which showed an increase for WK-AER. Pyridine-FTIR showed that WK lost all acidic sites upon CuO-promotion. The acidic sites in CP and CP-MOD became weaker, shown by a lower desorption temperature, whereas the WK-MOD and the WK-AER catalysts became stronger upon CuO-doping, shown by a higher desorption temperature. A study employing Hammett indicators showed that WK contained the strongest basic sites and CP the weakest. Deuterated chloroform-FTIR showed that all catalysts contained weak and strong basic sites, with WK containing the strongest and CP the weakest. Moreover, it was shown that magnesia only contains weak basic sites, which is inconsistent with the results from the Hammett indicator study, showing the presence of strong basic sites in magnesia. Upon CuO-promotion, the CP catalyst shows a considerable decrease in concentration of basic sites. For the WK catalyst, CuO-promotion considerably influences the strength of the basic sites. Promotion with CuO does not cause a significant variation in the basic properties of the WK-MOD catalyst.