Abstracts Chemistry

Bulk and Surface Characteristics of Model M1 and M2 PhaseCatalysts for Propane Ammoxidation to Acrylonitrile

by Jungwon Woo

Direct ammoxidation of propane to acrylonitrile (ACN) has received significant attention of the scientific community in recent decades because propane is cheaper, more abundant and environmentally friendlier than the current propylene feedstock. The MoVTeNbOx M1 phase catalysts display the highest propane conversion and ACN yield (~ 60 mol. %). However, the ACN yield over the M1 phase is insufficiently high in order to replace the current Sohio process, employing the propylene feedstock to produce ACN with a ~ 81 mol. % yield. Therefore, there is currently a global effort in understanding the atomic structure and catalytic behavior of these catalysts in order to design improved M1 phase catalysts for propane ammoxidation. Accordingly, this PhD thesis aimed to pursue the following objectives: 1) improve the High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image analysis and provide accurate metal site occupancies in the M1 phase; 2) develop new probability models which correlate the catalytic performance with metal distributions of the M1 phase; 3) elucidate the nature of M1/M2 phase cooperation for all chemical compositions. The HAADF-STEM image simulations were performed in order to determine accurate metal distributions in the MoVTeTaO M1 phase catalyst for propane ammoxidation. QSTEM simulation software was chosen due to the excellent agreement between experimental and simulated HAADF-STEM images. The QSTEM-based HAADF-STEM image analysis method successfully provided accurate metal distributions in the MoVTeTaO M1 phase as compared to previously reported metal occupancies determined by the Z2-based HAADF-STEM image analysis. Three probability models (Model 1-3) were advanced and investigated in this thesis based on metal distributions determined by the QSTEM-based HAADF-STEM image analysis. The correlations between the Mo/V distributions in MoVTeTaO M1 phase catalysts and their catalytic behavior in propane ammoxidation were proposed for Model 1 (based on the probability of finding 1-2 V5+ cations in the proposed S3-S4-S4-S7-S7 catalytic center), Model 2 (based on the total V content in the S2-S4-S4-S7-S7 catalytic center), and Model 3 (based on the probability of finding more than 2 V cations in the S2-S4-S4-S7-S7 catalytic center). Model 1 suggested V5+ in S3 may activate propane. Model 2 and Model 3 emphasized the importance of total V content and multiple VOx sites in the S2-S4-S4-S7-S7 catalytic center for catalytic reactivity in propane ammoxidation, respectively.The kinetic study of the MoV(Te,Sb),(Nb,Ta)Ox M1 and M2 phases in propylene ammoxidation indicated that the M2 phases are less active than the corresponding M1 phases in propylene ammoxidation. The findings of this study do not support the existence of the synergy effect for any M1/M2 compositional variant. Instead, the observed behavior of MoV(Te,Sb),(Nb,Ta)O catalysts was consistent with partial loss of some surface active species from the M1 phase surface during the H2O2 treatment and generation of fresh ab… Advisors/Committee Members: Guliants, Vadim (Committee Chair).