(244b) Microkinetic Assessment of Propane Selective Oxidation to Acrylic Acid

Microkinetic modeling is gaining more and more attention during the last years as an efficient way to bridge the gap between surface science and applied catalysis. The overall behavior of catalytic materials is assessed through a microkinetic description based on elementary steps [1]. The understanding of the reaction mechanisms and the control of selectivity in catalysis by oxides, such as the challenging 1-step propane selective oxidation [2], requires an integrated effort between theory, modeling, surface characterization and reactivity testing. In this work, a microKinetic Engine (μKE) is used for solving a set of differential algebraic equations and for optimizing kinetic parameters through regression with propane selective oxidation to acrylic acid over propene and acrolein as example reaction [3].

Experimental data were obtained at two temperature levels, i.e., 350°C and 380°C and four space times, i.e., 3.75, 7.5, 10 and 12.5 kg·s·mol^-1. The propane partial pressure was 5000 Pa and the molar oxygen to propane ratio amounted around 2. A MoV_0.3Te_0.17Nb_0.1O_x catalyst was used which is known to give among the highest acrylic acid selectivities and yields. Because of the limited number of experimental considered, data the elementary steps were classified into slow and fast elementary steps. In order to be able to tune the selectivity, a second rate coefficient for the slow steps in the formation of acrolein was used. Hence, three kinetic coefficients are to be determined. The propane conversion, acrylic acid yield and selectivity are qualitatively if not quantitatively described by the model, vide Figures 1?3. The reverse surface elementary reactions, i.e., from propene to propane (steps 6-8), acrolein to propene (steps 15-17) and acrylic acid to acrolein (steps 23-25) were found to occur only at low rates at the operating conditions used, vide Table 1. Two among the three steps considered for catalyst re-oxidation and water formation are in quasi-equilibrium. The estimates for the rate coefficients are reported in Table 2. The calculated t-values for the individual significance of the parameter estimates are around 10, which is sufficiently higher than the tabulated value of 2. Also the F-value for the global significance of the regression is sufficiently high, i.e., between 500 and 1000. The 1^st rate coefficient controls the propane conversion while the second describes the potentially fast and, hence, quasi-equilibrated elementary steps. The third one is specific for acrolein production and, as a consequence, is responsible for changes in the acrylic acid selectivity and yield, vide Table 2.

The microKinetic Engine is a valuable tool in assessing high-throughput generated data. Estimates for fundamental kinetic parameters can be compared for a series of catalysts allowing correlating catalyst composition and synthesis procedure with the behavior at relevant operating conditions. This not only leads to a fundamental understanding of the observed phenomena but also provides a means for further catalyst design and development.

References

[1] Centi G., Perathoner S., Int. J. Mol. Sci. 2 (2001) 183?196.

[2] Lin M. M., Appl. Catal. A: Gen. 207 (2001) 1?16.

[3] Wong H.-W., Cesa C. M., Golab T. J., Brazdil F. J., Green H. W., Appl. Catal. A: Gen. 303 (2006) 177?191.

Figures

Figure 1: Experimental and calculated conversion of propane versus space time at 350 and 380°C

Figure 2: Experimental and calculated selectivity of acrylic acid versus space time at 350 and 380°C

Figure 3: Experimental and calculated yield of acrylic acid versus space time at 350 and 380°C

Tables

Table 1: Elementary steps and correspondingly assigned rate coefficients and calculated reaction rates at 350, 380⁰C.

Table 2: Estimates for the three rate coefficients at 350 and 380⁰C.