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.