Simultaneously, as the methanol/air ratio increased, the reaction rate first increased and then decreased. The side reaction that occurs in the synthesis of DMM (continuous bimolecular coupling of methanol and its derivative intermediates with FA) forms by-products and DMM21. As the methanol/air ratio increased, the enhancement of these secondary reactions led to an increase in the FA conversion rate and decreases in the DMM formation rate and selectivity22. At the same time, it was observed that the MF selectivity increased from 5.9% to 10.8%, which is consistent with the expected properties of the methanol bimolecular dehydration reaction.
Fig. 2 . Example of long-term reaction test using the Fe-Mo/ZSM-5 catalyst.
The long-term experimental results regarding the Fe-Mo/ZSM-5 catalyst in the circulating regenerating fluidized bed are shown in Fig. 2 (Table S2). The methanol conversion rate and DMM selectivity remained basically unchanged in the first 30 minutes. After 60 minutes, the catalyst activity decreased significantly, the methanol selectivity decreased to 87.6%, and the DMM selectivity decreased to 87.0%. It can be seen from Figure 2 that the catalytic activity was stable and high catalytic activity was still maintained with the extension of the fluidization bed reaction time, with a DMM yield of about 76%. The Fe-Mo/ZSM-5 catalyst showed better stability in the circulating regenerating fluidized bed, which was consistent with actual industrial FA production. The scanning electron microscopy (SEM) image (Fig. S1) of the catalyst after the long-term stability test showed that its apparent morphology did not change, with fresh HZSM-5 also having a hexagonal prism morphology23, nor was it found on the surface. The presence of four phases, Fe2(MoO4)3, Fe2O3, MoO3, and β-Fe2(MoO4)3, was confirmed by X-ray diffraction (XRD) analysis (Fig. 3)24. In addition, there was no significant change in the XRD peak intensity before and after recycling, and no diffraction peak of graphitic carbon was found, indicating that there was no carbon deposit on the surface of the catalyst or the carbon deposit value was very low25. Therefore, the Fe-Mo/HZSM-5 catalyst has high resistance to carbon deposition and stability. This also showed that the regeneration effect of the circulating regenerating fluidized bed was superior, effectively achieving catalyst regeneration and utilization.