Effects of catalyst crystal structure on the oxidation of propene to acrolein

Abstract

Please cite this article in press as: Z. Zhai, et al., Effects of catalyst crystal structure on the oxidation of propene to acrolein, Catal. Today (2015), http://dx.a b s t r a c t Bismuth molybdate is known to be active for the oxidation of propene to acrolein and its activity can be altered by substitution of other elements (e.g. Fe, V, W) into the scheelite phase of ␣-Bi 2 Mo 3 O 12 . This work has further revealed that the apparent activation energy for acrolein formation correlates with the band gap of the catalyst measured at reaction temperature. It is, therefore, of interest to establish to how the crystal structure of the catalyst affects the activation energy. We report here an investigation of propene oxidation conducted over Bi, Mo, V oxides having the aurivillius structure with the composition Bi 4 V 2-x Mo x O 11+x/2 (x = 0−1) and compare them with oxides having the scheelite structure with the composition Bi 2-x/3 Mo x V 1-x O 12 (x = 0−1). The aurivillius-phase catalysts again show a correlation between the apparent activation energy and the band gap of the oxide, and the only difference being that for a given band gap, the apparent activation energy for the aurivillius-phase catalysts is 1.5 kcal/mol higher than that for the scheelite-phase catalysts. This difference is attributed to the lower heat of propene adsorption on the aurivillius-phase catalysts. A further finding is that for catalysts with band gaps greater than ∼2.1 eV, the acrolein selectivity is ∼75% for the conditions used and independent of the propene conversion. When the band gap falls below ∼2.1 eV, the intrinsic selectivity to acrolein decreases rapidly and then decreases further with increasing propene conversion. This pattern shows that when the activity of oxygen atoms at the catalyst surface becomes very high, two processes become more rapid -the oxidation of the intermediate from which acrolein is formed and the sequential combustion of acrolein to CO 2 .

Synergy Effects of the Mixture of Bismuth Molybdate Catalysts with SnO2/ZrO2/MgO in Selective Propene Oxidation and the Connection between Conductivity and Catalytic Activity

“…Kinetics study of propene oxidation on bismuth molybdate catalyst has reported that the rates of formation of the main product acrolein and byproducts CO and CO 2 are all nearly first order, corresponding to propene, and therefore the selectivity toward acrolein, CO, and CO 2 remains almost constant. 15,56 Since the selectivity of acrolein (85−90%) was observed to be significantly higher than those of acetaldehyde (0−5%) and CO x (5−10%), 12,15,17,28,29,56 therefore, the reaction rates of other by products (acetaldehyde and CO x ) are not mentioned in detailed in this study. The catalytic activity, instead, is expressed by the number of moles of acrolein formed per gram of catalyst (r w , mol/g•s) or per m 2 of surface area of the catalyst per second (r s , mol/m 2 •s) using the following formula: 1.…”

“…54 Recent studies also reported linear correlations between the measured catalytic apparent activation energies for propene oxidation to acrolein and the band gap energies for several different families of multicomponent, mixed metal oxides. 55,56 Moreover, that band gap property, which correlates to the conductivity of the materials, 57 was also reported as an accurate descriptor for the energy to generate oxygen vacancy on metal oxides 58,59 and is very relevant for the incorporation of lattice oxygen into the reactant molecule. Therefore, those pieces of evidence further support our assumption that the conductivity could be one of the factors that reflect the catalytic activity in the oxidation of propene to acrolein.…”

“…r w (C 3 H 4 O) is therefore proportional to the conversion of propylene and the selectivity to acrolein. As was shown in previous studies, the usage of reaction rate is a very convenient choice in evaluating the catalytic activity 15,28,29,32,56. 3.…”

Bismuth Molybdate-Based Catalysts for Selective Oxidation of Hydrocarbons

“…When the band gap falls below 2.1 eV, the intrinsic selectivity to acrolein decreases rapidly and then decreases further with increasing propylene conversion. This pattern shows that when the activity of oxygen atoms at the catalyst surface becomes very high, two processes become more rapid-the oxidation of the intermediate from which acrolein is formed and the sequential combustion of acrolein to CO 2 [59].…”

“…Thus, the reaction occurs via a Mars and Van Krevelen mechanism in which propylene adsorbs reversibly and then reacts with an oxygen atom of the catalyst. This rate-limiting step leads to cleavage of one of the C-H bonds of the methyl group of the adsorbed propylene and results in the formation of an adsorbed OH group and a loosely adsorbed allyl radical that is rapidly stabilized as an adsorbed vinylalkoxide [59]. The sites for chemisorption and dissociation of gaseous oxygen molecule are spatially and structurally distinct from the active sites, at which adsorption and oxidation of propylene take places.…”