14:40 - 15:00 Catalysis

## Structure-activity correlations of FexOy/SBA-15 model catalysts for selective oxidation of propene

Nina S. Genz and Thorsten Ressler

Technische Universität Berlin, Straße des 17. Juni, 10623, Berlin, GER

Motivation

In catalysis research understanding structure-activity correlations of functional materials is an important subject. Therefore, model systems with reduced structural and chemical complexity are frequently used. In heterogeneous catalysis, dispersing metal oxides on well-defined support materials, such as SBA-15, is a promising approach towards model systems. Supported iron oxide catalysts constitute a promising alternative for more expensive metal catalysts, not only for various selective oxidations but also for Friedel-Crafts alkylations and Fischer-Tropsch synthesis.1 Here, FexOy/SBA-15 catalysts are investigated as suitable model systems for selective oxidation of propene. The influence of both varying iron loading and varying precursor on structural and chemical properties of the catalysts is reported.2

Experimental

Mesoporous support material, SBA-15, was synthesized according to procedures described by Zhao et al..3 FexOy/SBA-15 samples were prepared by incipient wetness technique using aqueous solutions of two different Fe(III) precursors. Structural characterization was conducted by $\ce{N2}$ physisorption, XRD, and DR-UV-Vis measurements. In situ TPR measurements of FexOy/SBA-15 were performed in 5 % $\ce{H2}$ in 95 % Ar atmosphere. In situ XAS experiments were performed at Fe K edge in 5 % Propene, 5 % $\ce{O2}$ in He atmosphere at beamline Petra III at HASYLAB, DESY.

Results

FexOy/SBA-15 catalysts showed both precursor-dependent and precursor-independent differences in structural, chemical, and kinetic properties. Sizes of supported iron species were influenced by both varying iron loading and varying precursor. An increasing iron loading correlated with an increasing size of supported iron species. Fe(III)-citrate precursor induced FexOy sizes smaller than those obtained from Fe(III)-nitrate precursor. Independent of the used precursor, FexOy/SBA-15 catalysts possessed iron species with a local structure corresponding to that of $\alpha$-$\ce{Fe2O3}$ and $\beta$-FeOOH. The amount of $\beta$-FeOOH-like iron species increased with decreasing iron loading independent of the used precursor. Compared to FexOy/SBA-15 catalysts obtained from nitrate precursor, catalysts obtained from citrate precursor possessed significantly higher amounts of $\beta$-FeOOH-like iron species. Moreover, FexOy/SBA-15 catalysts exhibited significant variations in catalytic activity. Acrolein selectivity increased with decreasing iron loading for FexOy/SBA-15 obtained from citrate precursor. Conversely, for FexOy/SBA-15 obtained from nitrate precursor acrolein selectivity increased with increasing iron loading. Additionally, all FexOy/SBA-15 samples exhibited significantly varying reducibility. These differences were attributed to both precursor effects and effect of iron loading. In situ characterization of reduction of supported iron species obtained from nitrate precursors revealed an increasing reducibility and decreasing apparent activation energy of the rate-determining step with increasing iron loading. A lower apparent activation energy in reduction correlated with a stronger reduced Fe oxidation state during catalysis. Our results demonstrated that structure-activity correlations of FexOy/SBA-15 catalysts were significantly influenced by the used precursors.

1. Z. Gabelica et al., J Therm Anal Calorim 95 2009 445–454.

2. N. S. Genz et al., Journal of Analytical Methods in Chemistry, Article ID 6205297, 2017.

3. Zhao, D., Science 279. 1998, 5350, 548–552.