Iron and molybdenum mixed oxides supported on SBA-15 as green model catalysts for selective oxidation of propene
Nina S. Genz and Thorsten Ressler
Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623, Berlin, GER
Motivation In the last years, awareness of depleting fossil feedstocks rose, and green chemistry became a highly relevant topic. The concept of green chemistry aims to achieve the overall goal of sustainability for chemicals and processes. Besides, chemicals and processes conform with green chemistry if they are less toxic for human health and environment.1 This affects also the selective oxidation of propene. Propene is the second most demanded product in petrochemical industry. Therefore, interest in further improvement of already well-investigated selective oxidation of propene to acrolein reappeared. An improvement in both selectivity towards acrolein and propene conversion would lead to a considerably increased cost efficiency. Moreover, selective oxidation of propene would become greener. Supported iron-based catalysts constitute a promising greener alternative for more expensive metal catalysts. Here, a binary model catalyst system was synthesized by changing the chemical composition of the FexOy/SBA-15 catalysts by molybdenum addition. Therefore, iron and molybdenum mixed oxides supported on SBA-15 were synthesized and characterized regarding structure, reducibility, and catalytic performance in selective oxidation of propene.2 3
Experimental Mesoporous support material, SBA-15, was synthesized according to procedures described by Zhao et al.4 MoxOy_FexOy/SBA-15 samples were prepared by incipient wetness technique using an aqueous solution of ammonium iron(III) citrate ( 18% Fe, Roth) and ammonium heptamolybdate tetrahydrate ( 99%, Fluka). The pH value of the aqueous oxide precursor solution was adjusted to 7.5-8. Structural characterization was conducted by N2 physisorption, XRD, and DR-UV-Vis measurements. In situ TPR measurements of FexOy/SBA-15 were performed in 5 % H2 in 95 % Ar atmosphere. In situ GC-MS experiments were performed in 5 % propene, 5 % O2 in He atmosphere.
Results Iron and molybdenum mixed oxides supported on nanostructured silica SBA-15 were successfully synthesized with various Mo/Fe atomic ratios ranging from 0.07/1.0 to 0.57/1.0. Molybdenum addition yielded both a pronounced dispersion and electronic effect on iron oxidic species. Increasing Mo/Fe atomic ratio induced a decreasing species size due to the dispersion effect of molybdenum. Moreover, smaller and higher dispersed iron oxidic species possessed a lower reducibility compared to larger and less dispersed species. Additionally, a charge transfer from iron to oxygen, and further to molybdenum in Fe-O-Mo structure units yielded a strengthened Fe-O bond, and hence, hindered reducibility. This was ascribed to the electronic effect of molybdenum. Catalytic performance of MoxOy_FexOy/SBA-15 model catalysts was studied under selective propene oxidation conditions. Molybdenum addition resulted in an increased acrolein selectivity and a decreased selectivity towards total oxidation products. Influence of molybdenum addition on catalytic performance was correlated with both dispersion and electronic effect of molybdenum. The strengthened Fe-O bonds, and the lower reducibility, with increasing Mo/Fe atomic ratio, led to an inferior total oxidation.