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Project 1: Acid-base & redox properties of promoted sulfated zirconia
Zusammenfassung:
This thesis investigated the influence of handling and storage on the structure of sulfated zirconia catalysts (SZ as well as FeSZ and MnSZ containing 0.5 or 2.0 wt% metal each) prepared from a commercial precursor (calcination at 823 or 923 K) as well as their activity in n-butane isomerization. Proofs for oxidative dehydrogenation (ODH) as initial step in activation of n butane leading to formation of a carbenium ion as reactive intermediate are presented.
The catalysts were characterized by BET surface area analysis, X-ray diffraction (XRD), X ray absorption spectroscopy (XAS), thermogravimetric measurements (TG) combined with differential scanning calorimetry (DSC) and on-line mass spectrometry (MS), as well as Fourier transform infrared spectroscopy in transmission (FTIR) and diffuse reflectance (DRIFTS) by adsorption of probe molecules and combined with on-line gas chromatography (GC) in situ on the operating catalyst. Catalytic tests were performed at 323–378 K (1 or 5 vol% n butane in N2, atmospheric pressure) after activation at 723–773 K in IR cells or in fixed bed reactors. To enhance the capacity and allow simultaneous tests of catalytic performance, an apparatus enabling three parallel catalytic reactor tests on laboratory scale was designed, using a fluidized sand bed for rapid heating and cooling and a Micro-GC for fast separation and quantification of reactant (n butane) and product (isobutane).
It could be shown that sulfated zirconia catalysts are sensitive materials that undergo structural changes combined with a loss in activity when subjected to mechanical stress like pressing or milling, treatments that are commonly used to prepare samples for different measurements. Furthermore, the catalysts undergo changes in structure and activity with time of storage (up to 9 months), whereby water saturation as well as the absence of oxygen in the atmosphere enhance their velocity. Keeping MnSZ in a laboratory cupboard turned out to be preferable to storage in a glovebox. The knowledge about the sensitivity of sulfated zirconia materials is a vital prerequisite for making valid structure–activity relationships.
It could be shown that during n-butane-isomerization water is formed on sulfated zirconia catalysts and that the rate during the induction period of rising conversion is linearly correlated to the amount of water. Since the carbenium ion intermediates are built up during the induction period, the simultaneous formation of water points towards ODH as the responsible reaction. Sulfate is proven to be the oxidizing agent in total oxidation of n-butane at high temperatures (573 K) under formation of CO2, water and H2S. After isomerization of n-butane at lower temperatures (373 K) with concomitant water formation, a decrease of the sulfate IR bands is observed, indicating sulfate to be the oxidizing agent also in this process. In the presence of manganese, the rate is higher for the same amount of water; the intrinsic turnover frequency per carbenium ion is increased from 10 6 s 1 in SZ to 10 4 s 1. So far it has been believed that promoters such as iron or manganese facilitate the reaction initiation; here an effect of a promoter on the isomerization itself is proven.
