Reactivity Descriptors for C-H Bond Activation and C-O Bond Formation in Hydrocarbons on Metal Oxide Catalysts
- Type: Conference Presentation
- Conference Type: AIChE Annual Meeting
- Presentation Date: November 11, 2021
- Duration: 20 minutes
- Skill Level: Intermediate
- PDHs: 0.50
C-H bonds in C2H4 are stronger than in C2H6. Consequently, facile sequential C2H4 conversions involve C-O bond formation at vinylic carbons instead of direct C-H activation. Experiment and density functional theory (DFT) suggest that 0.4 nm one-dimensional pores in M1 phase MoVTeNb oxides enhance C2H4 selectivity by van der Waals stabilization of C2H6 C-H activation transition states and suppression of C-O bond formations. The difference in DFT derived transition state energies between C2H4 epoxidation (representative C-O formation reaction) and C2H6 C-H activation for M=O terminal O-atoms in V2O5 and MoVTeNbO are higher for O-atoms with less negative H-atom addition energy (HAE). This difference for M-O-M and M2-O-M bridging O-atoms is much higher than M=O O-atoms of comparable HAE on both types of oxides. Such higher C-O bond formation activation energies are consistent with greater steric hindrances determined by greater oxide framework distortion energies for more coordinated lattice O-atoms. These analyses suggest that weaker abstractors and bridging O-atoms enhance selectivity for C2H6 ODH, and the inaccessibility on terminal O-atom in M1 phase pores is essential for selectivity.
C3H6 contains, in addition to vinylic, allylic C-H bonds that are much weaker than C-H bonds in C3H8. Thus, suppressing C-O formation at vinylic carbons is insufficient for hindering facile sequential C3H6 oxidations. Here, catalyst abstractor strength and O-atom coordination exhibit analogous effects on the preference for vinylic C-O formation over allylic C-H activation in the unavoidable secondary reactions, which leads to higher selectivity to the C-H activation mediated acrolein formation inside the pores of M1-phase.
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