(699d) Regeneration of Methane Dehydroaromatization Using Periodic and Pulse Feeding of CO2 and H2

Natural gas is a valuable feedstock because it is an abundant, cheap, and clean form of hydrocarbon feedstock. Direct conversion routes of methane to chemicals avoid the syngas production step, which simplifies the process significantly, minimizes the emission of CO_2, and reduces the process energy consumption. One of these direct routes is the methane conversion to aromatics, known as methane dehydroaromatization (MDA). The reaction starts when methane contacts a heterogeneous catalyst at high temperatures, typically 700ËšC, and atmospheric pressure. Mo/ZSM-5 is one of the most studied heterogeneous catalysts used for this process, with a Mo content mostly ranging from 2-10wt%. This catalyst is claimed to be bifunctional. The metal sites in the catalyst activate the methane C-H bond, and the acid sites oligomerize the intermediate molecule into aromatic products, mainly benzene, naphthalene, and a significant amount of hydrogen. This reaction route is challenging because of its endothermicity, high-temperature requirement, and the thermodynamic equilibrium limitation with a single-pass methane conversion of around 10% at a temperature as high as 700ËšC. The other major challenge is the very fast catalyst deactivation due to carbon formation within a couple of hours.

In the effort to address the deactivation challenge, multiple ideas were explored so far. The first is modifying the metal composition or adding promoters and additives. The second is co-feeding other molecules or modes of regeneration as continuous, periodic, or pulse as shown in Figure 1. Critical items to consider when dealing with deactivation and regeneration are long-term stability, conditions needed for regenerations, type of coke formation (soft or hard) as it requires different regeneration conditions, ways to minimize metal sintering and avoid back-formation of Mo-oxide that is an inactive catalyst.

Several research groups such as (Honda et al., 2003) [1], (Sun et al., 2015) [2], and (Song et al., 2017) [3] tried periodic feeding of H₂ as a way to improve the catalytic performance. Others like (Chen et al., 2020) [4] tried to co-feed CO₂ alongside methane in smaller quantities. However, there has not been a clear conclusion about the relation between the reaction and regeneration time for either of these molecules [5]. In this work, experimental studies were conducted to understand how H₂, CO_2, or a feed of both affect the rate of regeneration when being fed in pulse and periodic modes as reductive and oxidative regeneration molecules, respectively. The choice of the regeneration mode and molecules are made with the aim to avoiding high temperature responsible for metal sintering, minimize the formation of hard carbon, and maximize production rate by minimizing the time needed for regeneration. In all of the experiments performed in this study, the reactor was kept at a fixed temperature during both the reaction and regeneration steps. The same 6wt% Mo-ZSM5 catalyst and a methane gas cylinder of 95% CH₄/Ar were kept fixed. A reference experiment was performed with continuous methane feed. The product was analyzed using a GC connected downstream of the reactor. The ratio of reaction to regeneration time was adjusted with a ratio of 1:1 to 1:3, and 0.5-1 minute of pulses were applied.

The results shown in Figure 2 indicate that CH₄-H₂ with 15-05 min periodic switch resulted in the highest methane conversion and benzene yield throughout 5h of reaction, in comparison with the continuous methane feed without periodic switch. However, both the CH₄-CO₂ and CH4-H₂+CO₂ periodic switch led to lower methane conversion and a very low benzene yield. When combining H₂+CO_2, water formation was observed in the reactor outlet, which indicates that reveres water gas shift reaction (CO₂+H₂->CO+H₂O) occurred. Water can have a negative impact on the MDA reaction, which could justify the observed low conversion and yield in this case. [6]

Although the experimental work carried out was very valuable, relying on it completely is limiting and expensive. Using a kinetic model that captures the rate of deactivation and regeneration is needed to identify optimal conditions more quickly and guide the experimental work. Therefore, another part of this work will focus on creating such a kinetic model and comparing it to the observed experimental finding. The approach explored here can be extended to other co-feeding molecules and regeneration conditions toward overcoming the deactivation challenge and bringing the MDA reaction a step closer toward commercialization.

References

[1] K. Honda, T. Yoshida, and Z. G. Zhang, “Methane dehydroaromatization over Mo/HZSM-5 in periodic CH 4-H2 switching operation mode,” Catal. Commun., vol. 4, no. 1, pp. 21–26, 2003, doi: 10.1016/S1566-7367(02)00242-X.

[2] C. Sun et al., “Methane dehydroaromatization with periodic CH4-H2 switch: A promising process for aromatics and hydrogen,” J. Energy Chem., vol. 24, no. 3, pp. 257–263, 2015, doi: 10.1016/S2095-4956(15)60309-6.

[3] Y. Song, Q. Zhang, Y. Xu, Y. Zhang, K. Matsuoka, and Z. G. Zhang, “Coke accumulation and deactivation behavior of microzeolite-based Mo/HZSM-5 in the non-oxidative methane aromatization under cyclic CH4-H2 feed switch mode,” Appl. Catal. A Gen., vol. 530, pp. 12–20, 2017, doi: 10.1016/j.apcata.2016.11.016.

[4] X. Chen et al., “Promoting Mechanism of MCAR/MDA Coupling Reaction Under Oxygen-Rich Condition to Avoid Rapid Deactivation of MDA Reaction,” Catal. Letters, vol. 150, no. 7, pp. 2115–2131, 2020, doi: 10.1007/s10562-020-03114-1.

[5] S. J. Han et al., “Non-oxidative dehydroaromatization of methane over Mo/H-ZSM-5 catalysts: A detailed analysis of the reaction-regeneration cycle,” Appl. Catal. B Environ., vol. 241, no. April 2018, pp. 305–318, 2019, doi: 10.1016/j.apcatb.2018.09.042.

[6] K. Skutil and M. Taniewski, “Some technological aspects of methane aromatization (direct and via oxidative coupling),” Fuel Process. Technol., vol. 87, no. 6, pp. 511–521, 2006, doi: 10.1016/j.fuproc.2005.12.001.