Promotion effect of Fe in mordenite zeolite on carbonylation of dimethyl ether to methyl acetate

Article abstract of DOI:10.1039/c4cy01580k

A series of Fe-modified mordenite zeolite samples were synthesized by a template-free method and employed in dimethyl ether (DME) carbonylation reaction for the production of methyl acetate (MAc). XRD, UV-Vis, and UV-Raman characterization studies proved that Fe atoms have been introduced into the mordenite zeolite framework by partial substitution of Al atoms, which led to evident changes of activity and MAc selectivity. With the increase of iron content (as metal) from 0.0 to 3.6 wt%, DME conversion first increased and then decreased. MAc selectivity and catalyst stability were enhanced for all Fe-modified samples. TG and GC-MS analysis of deactivated catalysts showed that the amount of coke retained in the catalysts decreased as the iron content of the zeolites increased. The enhancement effects were expounded in terms of the decrease of the acid strength and acid density in the 12MR channels of mordenite after introduction of Fe, resulting in the reduction of carbon deposition.

Full text of DOI:10.1039/c4cy01580k

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Promotion effect of Fe in mordenite zeolite
on carbonylation of dimethyl ether to methyl
Cite this: DOI: 10.1039/c4cy01580k
acetate†
Hui Zhou,abc Wenliang Zhu,ab Lei Shi,ab Hongchao Liu,ab Shiping Liu,abc
ab
Shutao Xu,ab Youming Ni,ab Yong Liu,ab Lina Liabc and Zhongmin Liu
*
A series of Fe-modified mordenite zeolite samples were synthesized by a template-free method and
employed in dimethyl ether (DME) carbonylation reaction for the production of methyl acetate (MAc). XRD,
UV-Vis, and UV-Raman characterization studies proved that Fe atoms have been introduced into the
mordenite zeolite framework by partial substitution of Al atoms, which led to evident changes of activity
and MAc selectivity. With the increase of iron content (as metal) from 0.0 to 3.6 wt%, DME conversion first
increased and then decreased. MAc selectivity and catalyst stability were enhanced for all Fe-modified
samples. TG and GC-MS analysis of deactivated catalysts showed that the amount of coke retained in the
catalysts decreased as the iron content of the zeolites increased. The enhancement effects were
expounded in terms of the decrease of the acid strength and acid density in the 12MR channels of
mordenite after introduction of Fe, resulting in the reduction of carbon deposition.
Received 1st December 2014,
Accepted 6th January 2015
DOI: 10.1039/c4cy01580k
www.rsc.org/catalysis
studies on DME carbonylation to MAc with zeolite catalysts
(H-MOR, H-FER and H-ZSM5) and reported that the HMOR
1. Introduction
The carbonylation reaction of dimethyl ether (DME) to methyl
acetate (MAc) has triggered wide research interest1–8 because
the product of MAc hydrogenation, ethanol, can be applied
in wide fields.9–13 This novel production process of ethanol is
highly competitive compared with the traditional processes
of hydration of ethylene and fermentation of sugars and
corns because DME can be readily derived from synthesis gas
(CO + H2, syngas).14 The copper-based catalyst for the
hydrogenation reaction has been used in commercial opera-
tions. The major challenge in research on this novel process
is developing effective heterogeneous zeolite catalysts for DME
carbonylation reaction with excellent performance in terms of
activity, selectivity and stability.
zeolite could catalyze DME carbonylation reaction to MAc at
low temperatures (423–463 K) with remarkable selectivity
(>99% MAc selectivity), but the MAc productivity was below
the expected commercial target. Also, the HMOR catalyst with
8MR and 12MR channels had the highest MAc synthesis rate
and the controlling step of carbonylation reaction (3)
occurred in the 8MR channels. Quantum chemical calcula-
tion results5 showed that only the T3-O33 positions in the
8MR channels of mordenite were selective for acetyl genera-
tion. The highly selective properties of the HMOR catalyst
was not only due to the size of 8MR channels but also to the
unusual orientation of the methoxy group with respect to the
8MR channel (parallel to the cylinder axis).5 The in situ
solid-state NMR spectroscopy studies also confirmed that
the important surface acetyl intermediate species could only
be identified in the 8MR channels, not in the active sites in
the 12MR channels which favor the formation of hydrocar-
bons.6 According to the present understanding of this reac-
tion, catalyst stability, one of the important factors for
industrial application, can be enhanced through the occupa-
tion or removal of the acid sites in the 12MR channels of
mordenite, where the formation of high molecular weight
hydrocarbons like aromatics occurs, resulting in the deacti-
vation of catalysts. Shen et al. reported the related study on
the development of improved zeolite catalysts for DME
carbonylation by pre-adsorption of pyridine7 or selective
dealumination of the H-mordenite zeolite,8 obtaining
In 1984, Fujimoto et al. first reported results from studies
of vapor carbonylation of methanol with heterogeneous
zeolite catalysts. They found that HY and HMOR zeolite cata-
lysts demonstrated some carbonylation activity with low MAc
selectivity.1 Iglesia and his co-workers2–4 initiated detailed
a National Engineering Laboratory for Methanol to Olefins, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, P. O. Box 110, 116023 Dalian,
PR China. E-mail: liuzm@dicp.ac.cn; Fax: +86 0411 84379518;
Tel: +86 0411 84379518
b Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian, PR China
c Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy01580k
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considerable improvement in the performance of MOR cata-
lysts for DME carbonylation reaction.
The iron content was determined by XRF analysis
(PANalytical AXIOS).
Introduction of metal atoms in the framework or extra-
framework of zeolites is an important route for changing the
physiochemical properties of zeolites and thus improving
their performance in reactions.15 Brian Ellis et al. reported that
H-MOR and Cu-MOR were active and selective for methanol
vapor phase carbonylation reaction under moderate operating
conditions.16 For DME carbonylation reaction, introduction of
Cu and Ag by ion exchange to replace H counterions was
reported in publications and patents.17,18 FTIR and solid state
NMR studies proved that Cu–H-MOR have two neighboring
sites, bridged hydroxyl and neighboring Cu+, and contributed
to the formation of MAc. Besides the activity, the product
selectivity and stability of catalysts are major challenges in
the development of DME carbonylation reaction. Zeolites
containing Fe in their frameworks showed excellent perfor-
mance in many catalytic reactions.19,20 Here, we report the
results from studies of DME carbonylation with Fe-modified
mordenites which were synthesized by a template-free hydro-
thermal method. The physical structure of the Fe3+ framework-
substituted mordenites was characterized by XRD, UV-Vis,
UV-Raman and N2-adsorption and the acid site strength and
density were characterized by 31P MAS NMR and FTIR spectro-
scopy after the adsorption of base probe molecules.
UV-Vis reflectance spectra were collected on a Varian Cary
spectrophotometer from powdered Fe-containing samples
with BaSO4 used as a reference.
IR spectra in the framework vibration region were re-
corded with a spectral resolution of 4 cm−1 using the KBr
technique (weight ratio of the sample to KBr, 1 : 100).
UV-Raman spectra were recorded on a DL-2 Raman spec-
trometer, with a collection time of 300 s. A 244 nm line of a
LEXEL laser was used as the excitation source. The laser
power on the sample was less than 5 mW. The spectrometer
used for Raman scattering was an Acton triple monochroma-
tor. A Princeton CCD detector was used to collect the spectra.
Nitrogen sorption experiments were performed at 77 K on
a Micromeritics ASAP-2020 system in static measurement
mode. The tested samples were degassed at 623 K for 4 h
prior to each measurement. BET surface areas were calculated
by the standard multi-point method, and the pore volume was
determined by N2 adsorption at a relative pressure of 0.98.
The 31P MAS NMR spectra were recorded on a Bruker
Avance III 600 spectrometer using a 4 mm probe. The
31P MAS NMR spectra were acquired with high-power
decoupling, a repetition time of 4 s, a π/4 pulse length of 2 μs
and a spinning speed of 12 kHz. The 31P NMR chemical
shifts were referenced to 85% H3PO4. In a typical adsorption
experiment, the FeHMOR zeolite was subjected to dehydra-
tion before adsorption of trimethylphosphine oxide (TMPO)
probe molecules. The temperature of the sample increased to
673 K at a rate of 2 K min−1 and maintained for 8 h and then
cooled to room temperature. After the TMPO dissolved in
CH2Cl2 was added to the sample in a three-neck round-
bottomed flask in a N2 glovebox, the sample was stirred
overnight in the N2 atmosphere. The removal of CH2Cl2 was
conducted at about 323 K. The sealed sample vessel was fur-
ther subjected to baking treatment at 453 K for 8 h to ensure
the uniform adsorption of TMPO on the channels of
mordenite. Then the TMPO crystals that did not react with
the sample were evacuated at 453 K.
Fourier transform infrared (FT-IR) spectra after pyridine
adsorption were obtained on a Bruker Tensor 27 instrument
with a resolution of 4 cm−1. Samples were pressed into a
self-supporting wafer (R = 0.65 cm, 15 mg) and evacuated
(2 × 10−2 Pa) in an IR cell at 673 K for 1 h prior to each
measurement. Adsorption of pyridine was conducted at
573 K for 15 min to ensure saturated loading and then
evacuation was carried out for 1 h prior to acquisition of
IR spectra. The densities of Brønsted and Lewis acid sites
were calculated from the IA values of the difference spectra
at 1540 and 1450 cm−1, respectively, using the extinction
coefficients reported by Emeis.21
Thermal gravimetric (TG) analysis of deactivated catalysts
was conducted on an SDT-Q600 instrument (TA, USA). In a
typical TG experiment, about 20 mg of the sample was pre-
treated with N2 (100 mL min−1) at 473 K for 1 h to remove the
adsorbed moisture from the catalyst sample. After cooling to
2. Experimental
2.1 Catalyst preparation
Iron nitrate ijFeIJNO3)3Ĵ9H2O], sodium aluminate [NaAlO2], and
silica sol (30% Si content) were used as initial Fe, Al, and Si
sources, respectively. Si/Al in sol was 10 and the desired Fe con-
tents were 0.0 wt%, 0.9 wt%, and 1.8 wt%, 3.6 wt% in the pro-
posed product Na(x+y)FexAlySizO2(x+y+z), estimated by the formula:
Fewt
Fewt%

100%
Fewt  Alwt  Owt  Siwt  Nawt
The gel was sealed in a Teflon beaker and heated at 453 K
with agitation. After crystallization for 20 h, the solid product
was filtered and washed with deionized water. The products
were labeled as FeNaMOR-0, FeNaMOR-0.9, FeNaMOR-1.8
and FeNaMOR-3.6. HMOR samples were obtained from
NaMOR samples by ion exchange. The NaMOR sample was
dispersed in NH4NO3 aqueous solution (1 mol L−1, 1 g of the
solid in 10 mL of the solution) at 353 K for 2 h and then
filtered and washed with deionized water. After repeating
ion exchange three times, the resulting solid was dried at
393 K overnight in ambient air and then calcined in flowing
dry air (20 mL min−1 g−1) at 673 K for 8 h. The as-prepared
catalysts were labeled FeHMOR-0, FeHMOR-0.9, FeHMOR-1.8
and FeHMOR-3.6.
2.2. Catalyst characterization
X-Ray powder diffractograms were recorded at room temperature
using Cu Kα radiation (PANalytical X' Pert Pro diffractometer).
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Table 1 Relative crystallinities and iron content of the FeHMOR samples
room temperature in N2, the sample was heated to 1073 K in
an air flow (100 mL min) at a heating rate of 10 K min−1.
Organic species trapped in the channels of the mordenite
zeolites during reactions were analyzed following the proce-
dures as described in the literature.22 Spent catalysts were
dissolved in 20% hydrofluoric acid solution. The organic
phase was extracted with dichloromethane IJCH2Cl2) and then
analyzed using an Agilent 7890A/5975C GC/MSD.
Relative
crystallinity
Si/(Al + Fe)
(XRF)
Iron content, wt%
(XRF)
FeHMOR-0
100%
96%
104%
56%
7.5
7.2
7.1
6.6
0.04
0.89
1.62
3.05
FeHMOR-0.9
FeHMOR-1.8
FeHMOR-3.6
2.3. Catalytic tests
FeHMOR-0 indicated that the two samples exhibit a good crystal
state after the introduction of iron but the 56% relative crystal-
linity of FeHMOR-3.6 showed that the framework of mordenite
collapsed to some extent as the iron content increased from
0 to 3.6%. As summarized in Table 1, the iron element con-
tent measured by XRF is about 0.04, 0.89, 1.62 and 3.05 wt%
respectively and is in accordance with the designed contents.
The substitution of Fe for Al and the increased iron content
resulted in the progressive loss of the IR framework bands'
sharpness and shifting of the bands to lower wavenumbers, as
shown in Fig. S2.† The band most sensitive to the substitution
is the internal asymmetric stretching vibration of Si–O–T
observed at 1084 cm−1 for aluminosilicate.23 The differences in
terms of this band between the four samples indicated that Fe
was already in the framework of the Fe-modified HMOR sam-
ples.23 The information in Fig. S2† is consistent with the results
of XRD presented above and further confirms the existence of
Fe in the framework of the mordenites.
Fig. S3† shows the UV-Vis reflectance spectra measured for
FeNaMOR samples containing different contents of iron at
room temperature. Two common features are observed in the
spectra of the Fe-modified mordenites: the strong absorption
bands in the 200–300 nm interval due to charge transfer from
the ligand to isolated framework Fe3+ ions, and the weak
bands in the range of 370–450 nm related to the d–d transi-
tions of Fe3+ ions with a tetrahedral symmetry.23–26 Therefore,
Fe in the modified Na-mordenites was dominated by tetrahe-
dral Fe3+ ions.
DME carbonylation experiments were performed in a contin-
uous flow fixed-bed stainless steel reactor. 1.0 g of the cata-
lyst (40–60 mesh) was packed in the reactor and pretreated in
N2 at 673 K for 2 h. After cooling the catalyst sample to 473 K,
a reactant gas mixture (5% DME,35% CO, and 60% H2) was
introduced into the reactor at a gas hourly space velocity
(GHSV) of 1500 mL (g h)−1. The reaction pressure was 3.0
MPa. The outlet gas was analyzed online using a gas chro-
matograph (Agilent 7890A) equipped with TCD and FID
detectors.
3. Results and discussion
3.1. Structure analysis
The XRD patterns of Fe-modified HMOR samples with vari-
ous iron contents are shown in Fig. 1 and Table 1. The typical
diffraction peaks of mordenite-type zeolites are clearly
observed for the four samples without any other crystalline
diffraction peaks. As shown in the diffraction patterns in
Fig. S1,† it is obvious that the peaks shift to the lower fre-
quencies with the increase of iron content, indicating the
increase of d values, the basal spacing. This is the prominent
proof that the larger Fe3+ (0.63 Å) was introduced in the
mordenite framework, replacing the smaller Al3+ (0.53 Å).23
The relative crystallinities of the samples were calculated
according to the sum of the areas of the seven strong peaks
between 2θ of 5 and 30°, assuming that the relative crystallin-
ity of the FeHMOR-0 zeolite was 100%. The comparison of
the 96% and 104% relative crystallinities of FeHMOR-0.9
and FeHMOR-1.8 with the 100% relative crystallinity of
UV resonance Raman spectroscopy is a unique, practical
and powerful technique for the detection of isolated transi-
tion metal atoms in the frameworks of zeolites.27–29 The laser
line at 244 nm was used as the excitation source because it
was close to the charge transfer bands at 230 nm. In the reso-
nance Raman spectra of the transition metal substituted zeo-
lites, the observed bands at about 500 and 1100 cm−1 could
be taken as the characteristic bands for the presence of tran-
sition metal ions in tetrahedral coordination in the frame-
work sites of the zeolites.28
The UV resonance Raman spectra of the FeHMOR samples
with various iron contents excited by the 244 nm excitation
line are shown in Fig. 2. The bands centered at about 400,
450, 800 cm−1 are the characteristic bands of the MOR struc-
ture.30 The bands at 520 and 1050 cm−1 are the resonance
Raman bands associated with the isolated iron species in the
framework due to the Fe–O–Si symmetric and asymmetric
stretching modes of the isolated tetrahedral iron ions in the
silica framework, respectively.28,31 As in the report on the UV
Fig. 1 X-ray diffraction patterns of the FeHMOR samples.
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Fig.
2 UV resonance Raman spectra of the FeHMOR samples:
Fig. 3 UV-Vis spectra of the FeHMOR samples.
a) FeHMOR-0.9, b) FeHMOR-1.8, and c) FeHMOR-3.6.
summarized in Table 2. Comparing the pore volume and
micropore volume as well as the BET surface and micropore
area, FeHMOR zeolites also proved to be microporous mate-
rials. The decrease of nitrogen uptake was observed for the
FeHMOR-3.6 sample, which can possibly be attributed to the
blocking or filling of micropores by extra-framework species
or the low relative crystallinity.
Raman spectra of Fe-ZSM-5,29 the band at 1165 cm−1 is
related to the framework crystallinity of the zeolites. The
appearance of this band in Fig. 2 for FeHMOR-0.9 and
FeHMOR-1.8 indicated that the iron atoms were tightly coor-
dinated in the framework of FeHMOR. However, the remark-
able decrease in the intensity of the band for the FeHMOR-
3.6 sample illustrated that its crystal state was poor and some
iron ions were removed from the framework. Similar results
were also observed from the relative crystallinities in Table 1.
The ion exchange of H with Na induced clear modification
of the UV-Vis spectra: the appearance of a new peak at about
275 nm and the almost disappearance of the d–d region.
These results were confirmed using the UV-Vis spectra of the
FeHMOR samples in Fig. 3 and the difference spectrum
(FeHMOR − FeNaMOR) in Fig. S4.† According to the litera-
ture, the strong charge transfer absorption at about 277 nm
represents the Fe3+ species in octahedral complexes, and the
broad absorption at about 330 nm reveals the presence of
octahedral Fe3+ present in small clusters, such as that
observed for clustered Fe3+ in hydroxides.24 As shown in
Fig. 3, the decrease of the absorption at 230 nm and increase
at 275 nm suggest that the process of ion exchange of H with
Na, including ion exchange, drying and calcination, resulted
in the removal of some partial tetrahedral Fe3+ from the
framework and formation of extra-framework octahedral iron
species. The peaks in the difference spectra in Fig. S4†
indicate that some extra-framework Fe3+ species may be in
octahedral complexes and some in small clusters. The color
of the FeNaMOR zeolites is completely white, indicating the
absence of colored oxides of iron in the bulk. But the Fe-containing
HMORs were light brown in color, typical of samples
containing iron oxides and hydroxides.32,33 The diffraction
peaks ascribed to iron oxides were not found in the XRD spectra
of the four Fe-HMORs after careful comparison and matching.
The iron oxides may be highly dispersed in the samples.
3.2. Catalytic performance
Changes in DME conversion and MAc selectivity with time-
on-stream (TOS) for all four catalysts are shown in Fig. 4. The
reaction conditions, including the reaction temperature, pres-
sure, gas hourly space velocity and feed composition, were almost
the same and thus the obtained DME conversion was indica-
tive of the corresponding catalyst activities in the four tests.
The catalytic activities in all experiments initially increased
with time and reached maximum values of DME conversion,
and then began to decline with further increase in time.
Obviously, FeHMOR-0.9 had the highest activity (85%),
higher than that of the FeHMOR-0 catalyst (75%), whereas
the FeHMOR-3.6 catalyst was the least active (about 40%
DME conversion). With the increase of iron content from 0.0
to 3.6%, the maximum value of catalyst activity among the
four tests was achieved by FeHMOR-0.9, not by FeHMOR-3.6
or FeHMOR-0. This result is consistent with the changing
trend of the BET surface areas and pore volumes, suggesting
a close correlation between them in the carbonylation reac-
tion of DME. Detailed research is required to verify the valid-
ity of this hypothesis. Also, the corresponding stabilities of
all four catalysts exhibited obvious improvement, as shown
in Fig. 4b. In addition, the induction periods of all four cata-
lysts were prolonged from 2 to 5 h with the increase of iron
content. It was obvious that an appropriate amount of iron in
the framework of the mordenite catalysts favored the catalytic
performance of DME carbonylation catalysts.
The N2 adsorption curves of all four samples with or with-
out Fe exhibit type I curves (Fig. S5†), the typical N2 adsorp-
tion–desorption isotherms of microporous materials. The cor-
responding pore volumes and BET surface areas are
3.3. Analysis of coke retained in deactivated catalysts
The coke in the four deactivated catalysts after DME carbon-
ylation reactions for 12 h at 473 K and 3 MPa was
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Table 2 Textural properties of zeolites with different iron contents
FeHMOR-0
FeHMOR-0.9
FeHMOR-1.8
FeHMOR-3.6
Pore volume (cm3 g−1
Micropore volume (t-plot, cm3 g−1
BET surface area (m2 g−1
Micropore area (t-plot, cm3 g−1
)
0.201
0.171
399
0.210
0.177
417
0.200
0.176
403
0.184
0.163
320
)
)
)
367
382
377
292
characterized by TG analysis and the results are shown in
Fig. 5a and b. All four profiles show two distinct weightless-
ness peaks in Fig. 5b. The first peak at low temperature
can be attributed to the combustion of soft coke (e.g. reac-
tion intermediates such as surface bound methyl and acetyl
associated with the formation of MAc) and the second peak
at high temperature can be assigned to the removal of
heavy coke formed during the reaction.7,34,35 Fig. 5b shows
the combustion temperatures of coke in DTG. It can be
observed that the combustion temperatures of heavy coke
are significantly different from each other: 843, 803, 748,
and 742 K for FeHMOR-0, FeHMOR-0.9, FeHMOR-1.8 and
FeHMOR-3.6 respectively, decreasing with increasing iron
content of the catalysts. The same trend was obtained for the
combustion temperature of soft coke. The difference in
the combustion temperature of hard and soft coke demon-
strates that the composition of coke was different in the four
samples and the coke molecules became lighter with increas-
ing iron content.
The weight loss of the deactivated FeHMOR-0 catalyst is
10.58 wt%, 81 percent hard coke and 19 percent soft coke.
On the other hand, that of the deactivated FeHMOR-1.8 is
10.81 wt% coke with 66% hard coke and 34% soft coke. As
the iron amount increased to 3.6 wt%, however, the hard
coke and soft coke in the used FeHMOR-3.6 became 45% and
55%, respectively, with a coke amount of 7.02 wt%, much
less than that in the used FeHMOR-0 catalyst. Therefore the
reactions of coke formation and transformation of soft coke
into hard coke were suppressed effectively due to the intro-
duction of iron.
After 12 h of DME carbonylation reaction at 473 K and
3 MPa, the deactivated catalysts were dissolved in HF and
Fig.
4 DME conversion (a) and MAc selectivity (b) of DME
carbonylation over Fe-modified HMOR catalysts. Reaction conditions:
473 K, 3.0 MPa, 5%DME, 35%CO, and 60%H2.
Fig. 5 Thermal analysis of coke in the deactivated catalysts (a: TG;
b: DTG).
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their chemical compositions are described in detail in
Fig. S6.† The structures on the chromatograms are peak iden-
tifications made by comparison with the NIST database, and
the asterisk (*) represents the internal standard IJC2Cl6). The
adamantane species and methylbenzene species, mainly
before the retention time of 25 min, were detected in all
spent catalysts. And the species after the retention time of 25
min were mainly bulky polycyclic aromatics. Some higher
molecular weight chemical compounds were also detected.
The amounts of polycyclic aromatics confined in FeHMOR-0
after 25 min of retention time are obviously more than
those in FeHMOR-1.8 and FeHMOR-3.6. The relatively low
density of organic species means slight coke deposition dur-
ing DME conversion, indicating that Fe in the mordenite zeo-
lites can effectively inhibit carbon deposition, which was also
confirmed by TG analysis.
The size of TMPO (kinetic diameter ca. 0.55 nm) and pyri-
dine (kinetic diameter: 0.585 nm) is slightly smaller than that
of the 12-MR channels (0.67 × 0.70 nm) of mordenite, but
much larger than that of the 8-MR channels (0.28 × 0.57 nm).
TMPO and pyridine would selectively adsorb on the acidic
sites located in the 12-MR channels.7,40 Therefore 31P MAS
NMR of adsorbed TMPO has been used to probe surface acid-
ity, as the 31P chemical shift of TMPO is sensitive to acid
strength.41–43 The amount of acid sites in the 12MR channels
was characterized by FTIR after adsorption of pyridine.
Fig. S8† displays the 31P MAS NMR spectra of the TMPO-
adsorbed samples. The signals between 80 and 50 ppm are
ascribed to TMPO adsorbed on Brønsted acid sites or
chemisorbed species.40 The intense signal at 46 ppm is
ascribed to physisorbed or weakly adsorbed TMPO. The solid
TMPO contributed to the resonance at ~39 ppm.41 The higher
chemical shift of 31P reveals stronger acid strength.
As a competitive reaction of DME carbonylation to MAc,
DME to olefin reaction forms adamantane species and
methylbenzenes species, the important intermediates in the
formation of heavy hydrocarbons.36 Accommodation of large
hydrocarbon species in the channels of zeolite catalysts
greatly limits the mass transfer of reactants and products.
All four spectra reveal similar features and can be
deconvoluted into three characteristic resonances by Gauss-
ian simulation. The deconvolution results and relative area
ratios for the different sites are shown in Table 3.
It is well-known that the higher chemical shift of 31P value
represents stronger acid strength. Thus acid sites with chemi-
cal shifts at site I were identified as strong acid sites, those at
site II were moderate acid sites and those at site III weak acid
sites. As shown in Table 3, the proportions of strong and
moderate acid sites decreased and that of weak acid sites
increased correspondingly as the iron content increased from
0.0 to 3.6 wt%. The greater percent of acid sites with lower
acid strength would depress the tendency of coke formation.
Therefore, the decline of acid strength resulted in the
decrease of the amount of coke retained in the catalysts after
the introduction of iron atoms.
When
a significant fraction of the 12MR channels of
mordenites is occupied by heavy hydrocarbons, catalysts start
to deactivate with a decrease in DME conversion and MAc
selectivity.
3.4. Characterization of the acid sites in the 12MR channels
of mordenite
DME adsorbed on the acid sites in the 12MR channels of
mordenite could convert into olefins causing the formation
of heavy hydrocarbons through oligomerization and hydride
transfer reactions. The intersection of the 12MR channels
and 8MR channels would be blocked if the amount of heavy
hydrocarbons were enough, which makes the diffusion path-
way inefficient for reactants and products.7,37 Therefore,
removing and/or neutralizing and/or weakening the acid sites
in the 12MR channels would be an effective method to
improve the stability of MOR catalysts while keeping a high
MAc activity.
The T2 and T4 sites in the 12MR channel are the two ener-
getically preferred positions for Fe substitution in the MOR
framework, as shown in Fig. S7.†38 The acid sites, with Fe in
the framework of the zeolites, are much less active and do
not readily catalyze hydride-transfer reactions, thus a much
lower tendency to form coke during reactions of olefins is
exhibited.39 Therefore, it is supposed that the partial replace-
ment of Al by Fe in the framework would suppress the coke
formation reaction to some extent. The information about
the concentration and strength of the acid sites in the
zeolites would illustrate the current reaction results and the
coke composition retained in the catalysts.
Pyridine-IR measurements were performed to investigate
the acid density mainly in the 12MR channels of mordenite;
the results are shown in Fig. 6. In general, the two adsorption
peaks at ~1450 and 1540 cm−1 correspond to the characteris-
tic vibration peaks of pyridine molecules adsorbed on Lewis
and Brønsted acid sites, forming PyL (pyridine bonded to
Lewis sites) and Py+ (pyridinium ions), respectively.44 The
adsorption peak at around 1490 is usually classified as
Brønsted and Lewis acid sites.
The concentrations of Brønsted and Lewis acid sites in the
FeHMOR samples were calculated from the IA area of the
bands at about 1540 and 1450 cm−1 according to the extinc-
tion coefficients reported by Emeis.21 The calculation results
are shown in Fig. 7. The concentration of acid sites which
Table 3 Distributions of Brønsted acid sites on various catalysts as
detected by 31P MAS NMR using TMPO as a probe molecule
Site I ~82 ppm
area (%)
Site II ~67 ppm
area (%)
Site III ~51 ppm
area (%)
FeHMOR-0
4.81
4.65
1.93
1.98
39.99
42.39
36.37
35.13
55.20
52.96
61.71
62.89
FT-IR and solid state NMR spectroscopy techniques have
been widely used to characterize the acid sites of zeolites and
other solid catalysts by adsorbing basic probe molecules.
FeHMOR-0.9
FeHMOR-1.8
FeHMOR-3.6
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a consequence, the performance of the FeHMOR samples
was markedly improved. With the increase of iron content
from 0.0 to 3.6 wt%, DME conversion increased and then
decreased. Also, the corresponding stabilities of all four cata-
lysts continued to increase. These results are due to the inhi-
bition of coke formation in the 12MR channels of mordenite
during DME carbonylation reaction. The corresponding
amount of coke in the used catalysts was reduced with the
increase of iron content. And the deposited chemical com-
pounds on the catalysts were primarily adamantane species,
methylbenzenes and polycyclic aromatics. The results
obtained in this study with iron modified mordenite catalysts
are encouraging, and further studies on the improvement of
their performance are required.
Fig. 6 Pyridine-IR spectra of the FeHMOR samples: a) FeHMOR-0;
b) FeHMOR-0.9; c) FeHMOR-1.8; and d) FeHMOR-3.6.
Acknowledgements
We acknowledge the financial support from the National Nat-
ural Science Foundation of China (grant no. 21473182 and
21303106).
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