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+→O2-,
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
(O2-+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 O2– 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.