Investigations of cobalt oxide catalysts supported on SBA-15 for selective oxidation of propene
Linda Klag, Nina S. Genz and Thorsten Ressler
Technische Universität Berlin, Straße des 17. Juni, 10623, Berlin, GER
The selective oxidation of propene to acrolein is a well-established reaction in chemical industry. Usually it proceeds over multicomponent metal oxide catalysts with high structural and chemical complexity. Since catalytic processes are still not completely understood, establishing correlations between catalyst structure and activity is required. Thus, less complex model catalysts are commonly used for research purposes. In addition to previously reported metal oxide systems 1 2, CoxOy species supported on SBA-15 might be suitable model systems for selective oxidation of propene. Therefore, cobalt oxide catalysts on SBA-15 with various cobalt loadings (0.5 – 5.8 wt%) and two different cobalt(II) precursors were investigated. The influence of varying cobalt loadings and using two different precursors on cobalt oxide catalyst structure and activity during selective oxidation of propene is reported.
Mesoporous support material SBA-15 was prepared as described in literature by Zhao et al..3 Incipient wetness technique was applied for synthesis of CoxOy on SBA-15 using two different precursors and various cobalt loadings (0.5 – 5.8 wt%). The two precursors used were either Cobalt(II)-Nitrate-Hexahydrate dissolved in water (pH = 4) or Cobalt(II)-Nitrate-Hexahydrate dissolved in water where citric acid was added (pH = 1). Structural characterization of the samples was accomplished by N2 physisorption, XRD, DR-UV-Vis and TEM measurements. GC-MS measurements under catalytic conditions were performed in 5% O2, 5% propene in helium.
Structure of SBA-15 was preserved after synthesis of CoxOy on SBA-15 with incipient wetness technique. A non-uniform distribution of CoxOy species was obtained partly inside and outside the SBA-15 pores. Also, a CoxOy species size effect dependent on cobalt loading and precursor was observed. Regardless of the precursor, higher Co loadings resulted in larger CoxOy species. However, the addition of citric acid to the precursor solution led to smaller CoxOy species. An influence of the pH values of the precursor solutions could be excluded. Moreover, CoxOy species supported on SBA-15 showed an influence of cobalt loading and precursor on catalyst activity and selectivity during selective oxidation of propene. Thus, CoxOy species size was correlated with catalyst activity and selectivity. Samples based on aqueous Cobalt(II)-Nitrate-Hexahydrate precursor solution showed a higher rate of acrolein formation and a higher acrolein selectivity with smaller CoxOy species. In contrast, the addition of citric acid to the precursor solution resulted in higher rate of acrolein formation and higher acrolein selectivity with larger CoxOy species. Accordingly, structure-activity correlations of CoxOy on SBA-15 are dependent on both cobalt loading and precursor. Small and isolated cobalt species seem to be required for high acrolein selectivity and high rate of acrolein formation during selective oxidation of propene.