Fig.3. SEM image and XRD pattern of the catalyst after stability inspection .

3.2 In-situ infrared exploration of reaction mechanism

In the process of methanol oxidative polycondensation, there will be hydroxyl groups, carbonyl groups, and large lattice oxygen, which will generate formaldehyde, dimethyl ether, methyl formate and other by-products.. In this study, the oxidative polycondensation of O₂+CH₃OH on Fe-Mo/ZSM-5 was analyzed using DRIFTS. Fig.4 shows the DRIFTS spectra of methanol adsorbed on Fe-Mo/ZSM-5 catalyst for different durations. The peak at about 3400–3600 cm⁻¹ corresponded to the tensile vibration band of -OH and the peak at 2890 cm⁻¹ corresponded to the antisymmetric and symmetric tensile vibration bands of the C-H bond on methanol²⁶. The peak at 2750 cm⁻¹corresponded to the stretching vibration of HCOO-²⁷. The tensile vibration band at 1650-1725 cm^-1 belongs to the carbonyl group, the tensile vibration band at about 1430 cm^-1 belongs to -COO-, and the vibration band at 1100±50 cm^-1 belongs to the ether bond R-O-R. The peak at 2120 cm⁻¹ corresponded to the stretching vibration of -OH on the formate group²⁸. At the same time, the deformation vibration of methyl ether in methylal can be observed at 900-1000 cm⁻¹, which indicates that methanol is adsorbed on the surface of the catalyst to generate methoxy groups, and then desorbed to obtain CH₃O for further polycondensation reaction²⁹. Fig.3 shows that, at a reaction time of 0.5 min, the characteristic peak intensities corresponding to the hydroxyl, carbonyl, and formic groups were all high, and the peak intensities corresponding to the ether bonds and methoxy groups were very weak. The large and broad peaks at 3500–2750 cm−1 in the first 3 minutes were mainly due to the formation of FA during the initial oxidation of methanol and the formation of a large number of water molecules³⁰. As the reaction time increased, the vibration peak intensity of the carbonyl and formic acid groups gradually weakened, the peak intensity of -COO- increased slightly, and the methyl ether peak of DMM intensified⁵. These changes showed that methanol was first oxidized to form FA and a small amount of dimethyl ether (DME) on the Fe-Mo/ZSM-5 catalyst and then FA was further oxidized to form formic acid³¹. Then, FA and methanol underwent polycondensation to form DMM, and formic acid and methanol underwent polycondensation to form MF. After 7 minutes, there was almost no change in the peak intensities, which indicated that the reaction became stable. Therefore, DRIFTS analysis of methanol adsorption on the Fe-Mo/ZSM-5 catalyst for different reaction durations confirmed the previous theoretical speculation of all the chemical reactions and products that could occur in the process of preparing DMM from methanol by the one-step method³².

Fig.4. In situ DRIFTS spectra of the adsorption–oxidation- polycondensation of CH₃OH on Fe-Mo/HZSM-5 catalysts, as a function of time on reaction. (a) 0.5 min, (b) 2 min, (c) 3 min, (d) 5 min, (e) 7 min, (f) 10 min.

In-situ infrared spectroscopy was used to investigate the reaction of methanol on the Fe-Mo/ZSM-5 catalyst surface for different reaction times. This revealed that it took 7 minutes for the reaction to stabilize from contact. Therefore, under the same conditions, in-situ infrared spectroscopy was used to investigate the effect of changes in the Mo-Fe ratio and the Si-Al ratio of the carrier on the reaction process³³, with the results shown in Fig.5 (A) and (B), respectively. Table S3 shows the results of our previous findings on the catalytic activity of different Mo-Fe and Si-Al ratios³⁴. Fig.5 (A) shows that as the Mo-Fe ratio increased, the peak intensity of the carbonyl, ether, and methoxy groups firstly weakened and then increased³⁵. When Mo:Fe = 2, the DMM methyl ether peak reached maximum intensity, which was consistent with the results in Table S3. Our team’s previous research¹⁹ (adapted from ref. 8) found that Mo:Fe = 2 was the optimal ratio and revealed that underpinning the Mo-Fe catalytic effect was the mutual promotion of the formation of the Fe₂(MoO₄)₃ octahedral crystal structure and molybdenum oxide tetrahedron formation. With an increase in the ratio of Mo to Fe, the mutual promotion was greater, resulting in the further oxidation of part of the formaldehyde obtained by methanol oxidative dehydrogenation to obtain formic acid and MF as by-products³⁶. Fig.5 (B) shows that the Fe-Mo/ZSM-5 catalyst with Si:Al = 80 had the smallest peak intensities corresponding to hydroxyl, carbonyl, ether, and methoxy groups, and the highest intensity of the DMM methyl ether peak. Additionally, the methyl ether peak intensity of the Fe-Mo/ZSM-5 catalyst with Si:Al = 40 was greater than that of the Fe-Mo/ZSM-5 catalyst with Si:Al = 60. In previous studies³⁷ (adapted from ref. 9), our team revealed that the tetrahedral coordination of Al provided Brönsted acid (B acid) sites and Lewis acid (L acid) sites on the Si-Al framework. Differences in the catalyst Si:Al ratio directly affected the distribution of B acid and L acid sites³⁸.

Fig.5. In situ DRIFTS spectra of the adsorption–oxidation- polycondensation of CH₃OH on different catalysts.(A) Fe-Mo/HZSM-5 catalysts with different Mo-Fe ratios (a) Mo-Fe ratio=1, (b) Mo-Fe ratio=2, (c) Mo-Fe ratio=3; (B) Fe-Mo/HZSM-5 catalysts with different Si : Al ratio (d) Si : Al ratio=40, (e) Si : Al ratio=60, (f) Si : Al ratio=80.

Fig.5 (C) shows the in-situ infrared spectroscopy results of Fe-Mo/ZSM-5 with different Si:Al ratios under the same conditions. In HZSM-5, the weak peak at 3745 cm^–1 and the strong peak at 3610 cm^–1 were attributed to the terminal silanol group (Si-OH) and the acidic bridged hydroxyl group (Si-OH-Al), respectively³⁹. The Si-OH-Al group is considered to be a B acid center, and the B acidity strength affects its structure. Fig.4(C) shows that the Si-OH-Al peak intensity gradually increased with increasing Si:Al, which proved that the B acidity of the catalyst gradually increased⁴⁰. An important control step in the DMM production from methanol is the methanol acetalization stage, in which the B acid site of the catalyst plays a vital role. If the catalyst has no B acid sites, it is difficult for methanol to undergo acetalization to form DMM. Therefore, the in-situ infrared spectroscopy results in the present study further confirmed this previous theory. In-situ infrared spectroscopy also showed that the abundant oxygen vacancies on the Fe-Mo/ZSM-5 surface promoted the adsorption and oxidation of methanol to FA. These species could undergo further polycondensation at the acidic site of ZSM-5 to obtain DMM, which led to a positive shift in the reaction equilibrium. This synergistic effect of oxidation centers and acid centers may be the reason for the excellent catalytic performance obtained.

Based on these results, we summarized and proposed the reaction mechanism of methanol forming DMM under the action of Fe-Mo/ZSM-5 catalyst, as shown in Fig.6. Methanol and O₂ were chemically adsorbed on the catalyst surface for the first time, with surface hydroxyl and oxygen vacancies, respectively. The presence of surface oxygen vacancies promoted the adsorption of O₂and further converted it into active oxygen species (O₂+Fe³⁺-Mo⁶⁺→O^2-, O^-), which react with the adsorbed methanol to form FA⁴¹. The FA was further oxidized at this oxidation vacancy to form formate, which directly decomposed into CO and H₂O. CO may exist as an intermediate and react quickly with O₂ to form CO₂, and therefore be undetected by in-situ infrared spectroscopy. Methanol and FA were chemically adsorbed on the catalyst surface for the second time with surface hydroxyl and carbonyl groups respectively⁴². The presence of L acid sites on the Si-Al framework of the catalyst and B acid sites provided by the tetrahedral coordination of Al promoted the polycondensation of methanol and FA to form DMM, and at the same time generated a molecule of H₂O. H₂O reacts with active oxygen to generate -OH (O^2-+H₂O→2-OH), and with formate to generate CO₂ and H₂O⁴³. After the chemical adsorption of formic acid and methanol, MF was generated by the catalysis of L acid sites on the Si-Al framework. Methanol generated DME by the catalysis of B acid sites provided by the tetrahedral coordination of Al. Since FA was the main intermediate in the formation of the target product DMM, the conversion of methanol to FA was likely to be the rate-determining step in the entire process. With the continuous supply of oxygen, the consumption of O^2– and O^– would promote the coordination of two terminal oxygens and the Mo double bond in the Fe₂(MoO₄)₃octahedron through the appearance of Mo⁵⁺ on the catalyst surface. This would coordinate the supply, resulting in methanol hydroxyl hydrogen activation to produce methoxy species. As these are intermediates formed by FA, the next step of the reaction could proceed quickly. Therefore, combined with our previous research findings, it could be concluded that the Fe-Mo/HZSM-5 catalyst has both acidic active centers and oxidation active centers. The former are due to the synergistic effect of the L acid on the Si-Al framework and the B acid provided by the tetrahedral coordination of Al, promoting acidic active centers. The latter are due to the surface Fe₂(MoO₄)₃ octahedral crystal structure and the molybdenum oxide tetrahedron forming shared, mutually promoting oxidation active centers. To use Fe-Mo/ZSM-5 dual functional catalyst to selectively obtain more target products, it is necessary to achieve synergistic selective catalysis by these two activation centers.