Toward Concurrent Engineering of the M1-Based Catalytic Systems for Oxidative Dehydrogenation (ODH) of Alkanes

Abstract

In the oxidative dehydrogenation (ODH) of alkanes, some important advances of the last decade have made it possible to accelerate the development and industrial insertion of the M1 based catalytic systems. These catalysts may be fine-tuned to account for the inevitable variability of different chemicals and impurities in the feedstocks. The latter may include, among others, different blends of shale gases, with different ratios of the C1-C3 alkanes and impurities such as sulfur, phosphorus, etc. In this article, we review the recent progress achieved in our understanding of the crystal structures and the oxidative dehydrogenation (ODH) reaction mechanisms of the multi-metal oxide (MMO) M1 catalyst. Firstly, the complex crystal structure of the M1 phases has been examined using quantum mechanics (QM), reactive force field (ReaxFF), and machine learning (ML) approaches. Secondly, we discussed the ODH mechanism on the M1 phase based on the QM simulations including the finite cluster model and the periodic slab model. Finally, we proposed a catalyst design approach to improve the selectivity of the M1 phase based upon the ODH reaction mechanism. We also briefly discuss the concept of the CE (“Concurrent Engineering”, introduced by the European Space Agency). The development of the CE concepts may be applied to the M1 catalytic systems in the future allowing businesses to be agile and react fast to the changing production conditions, thereby making them uniquely competitive in the ODH of alkanes and other areas.

Appendix 1 An Example of Molecular Dynamics Modeling of the ODH of Propane on M1

The reaction mechanism of ODH of propane has been examined using the quantum mechanics simulations [33, 88,89,90,91]. However, the detailed reaction processes are not been fully known. Here we applied the ReaxFF [39] molecular dynamics (MD) simulations to examine the ODH of propane on M1 catalyst.

We first optimized the M1 crystal structure using the ReaxFF. It is quite complex because of the partial occupation of the sites Mo/Te/V. Here we used one possible crystal structure in which five V atoms are close to the Te S12 site [33]. This configuration is predicted to be efficient for the ODH process based on the QM simulations [33]. After the crystal is optimized, we constructed a five-layer slab model of M1-phase and added 5 propane (C₃H₈) molecules into the slab model. There are two surfaces in this model because of the periodic boundary condition.

The system was first equilibrated at temperature for 25 ps. Then we heated the system to a higher temperature of 500 K within 10 ps. To observe the ODH of C₃H₈ in short MD timescale, we applied the cook-off simulations in which the C₃H₈ molecules are heated from 500 to 3000 K within 25 ps. The M1 slab remained at 500 K to avoid the thermal degradation at high temperature. A time step of 0.25 fs was applied for integrating the equations of motion in the MD simulations.

To analyze the fragments in the ReaxFF MD simulations, we applied the molecular fragment recognition analysis algorithm using the connectivity matrix and bond orders at 0.25 ps intervals. Independent molecules are identified if their bond orders are smaller than 0.3, and then were assigned with specific ID numbers to track the reaction paths. The breaking or formation of bonds because of thermal fluctuations was avoided by a time window of 1.0 ps in which every bond must exist for 1.0 ps.

To uncover the chemical reactions of the propane on M1 phase, we first examined the important fragments including C₃H₈, C₂H₄, H₂ and C₂H₆ as representatives during the ReaxFF MD cook-off simulation, as shown in Fig. 1. During the initial heating-up process, from 500 to 2500 K, no chemical reactions are observed. At ~ 2600 K, we observed the first reaction in which the C₃H₈ is decomposed to C₂H₅ and CH₃. The C₂H₅ is decomposed to one C₂H₄ and one free H radical soon. Meanwhile the CH₃ is bonded to the surface O. As the temperature continuously increase, more C₃H₈ molecules are decomposed, leading to the production of more C₂H₄ and H radicals. Some H radicals are combined in gas phase and form H₂ molecules at ~ 2800 K. It is interesting to notice that one C₃H₆ fragment (CH₃–CH = CH₂) forms because the M1 phase absorbs H from C₃H₈ (Fig. 8)

We then examined the initial C₃H₈ decomposition mechanism, as shown in Fig. 2. The C₃H₈ molecule is dissociated into C₂H₅ and CH₃; then, the CH₃ is bonded to the O–Mo. The Mo is bonded to Te, suggesting that the Te may play some roles in the C₃H₈ decomposition. The initial decomposition does not occur in V-rich region as suggested by the QM simulations. This may be due to the small area of V-rich region in our model. It is interesting to notice that one C₃H₈ molecules are constrained in the hollow region on the lower left part of our supercell. However, this molecule is not decomposed in our simulation since only Mo atoms are in this hollow region. This suggests the importance of Te atoms in the C₃H₈ decomposition reaction (Fig. 9).