Insights into the Pyridine-Modified MOR Zeolite Catalysts for DME Carbonylation

Article abstract of DOI:10.1021/acscatal.9b04890

Pyridine-modified mordenite (MOR) zeolite catalysts have attracted great attention in recent years due to their unique shape selectivity within eight-membered ring (8-MR) side pockets for dimethyl ether (DME) carbonylation to methyl acetate (MA) and syngas conversion to ethylene. Herein, aimed at elucidating pyridine modification-carbonylation activity relationships and developing high-performance catalysts, we investigated the adsorption/desorption behaviors of pyridine on MOR zeolites with varying Si/Al ratios and their impact on DME carbonylation. Instead of the previously proposed selective adsorption of pyridine in 12-MR channels, pyridine is revealed to penetrate into 8-MR side pockets of MOR zeolites and interact with acidic hydroxyls therein. Upon heating, pyridine in pockets desorbs preferentially, likely arising from the lower stability of pyridine adspecies in constrained spaces. This well explains the observed increment of carbonylation activity following the increase of pretreatment temperature. Unprecedentedly, high MA yield (7.2 mmol/(h g)) has been achieved on pyridine-modified MOR (Si/Al = 13.8) under controlled pyridine desorption conditions, resulting from the joint contributions of better diffusion properties and larger amounts of active acid sites. Moreover, the catalytic activity of Br?nsted acid sites within 8-MR pockets is demonstrated to be inhomogeneous, closely associated with their locations.

Full text of DOI:10.1021/acscatal.9b04890

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Article
Insights into the Pyridine-modified MOR
Zeolite Catalysts for DME Carbonylation
Kaipeng Cao, Dong Fan, Lingyun Li, Benhan Fan, Linying
Wang, Dali Zhu, Quanyi Wang, Peng Tian, and Zhongmin Liu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b04890 • Publication Date (Web): 11 Feb 2020
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Insights into the Pyridine-modified MOR Zeolite
Catalysts for DME Carbonylation
Kaipeng Caoa, b, Δ, Dong Fana, Δ, Lingyun Lia, Benhan Fana, b, Linying Wanga, Dali Zhua, b, Quanyi
Wanga, Peng Tian*, a and Zhongmin Liu*, a
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a National Engineering Laboratory for Methanol to Olefins, Dalian National Laboratory for
Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, China.
b University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049,
China.
*Corresponding authors: tianpeng@dicp.ac.cn; liuzm@dicp.ac.cn
ABSTRACT Pyridine-modified MOR zeolite catalyst has attracted great attention in recent years due
to its unique shape selectivity within 8-MR side pockets for DME carbonylation to methyl acetate
(MA) and syngas conversion to ethylene. Herein, aiming to elucidate the pyridine modification-
carbonylation activity relationships and develop high-performance catalyst, we investigated the
adsorption/desorption behaviors of pyridine on MOR zeolites with varying Si/Al ratios and their
impact on DME carbonylation. Instead of the previously proposed selective adsorption of pyridine in
12-MR channels, pyridine is revealed to penetrate into 8-MR side pockets of MOR zeolites and
interact with acidic hydroxyls therein. Upon heating, pyridine in pockets desorbs preferentially, likely
arising from the lower stability of pyridine adspecies in constrained spaces. This well explains the
observed increment of carbonylation activity following the increase of pretreatment temperature.
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Unprecedentedly high MA yield (7.2 mmol/h/g) has been achieved on pyridine-modified MOR
(Si/Al=13.8) under controlled pyridine desorption conditions, resulting from the joint contributions of
better diffusion properties and larger amount of active acid sites. Moreover, the catalytic activity of
Brønsted acid sites within 8-MR pockets is demonstrated to be inhomogeneous, closely associated
with their locations.
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KEYWORDS MOR, DME carbonylation, FTIR, zeolite, pyridine adsorption
INTRODUCTION
Since the first report of selective carbonylation of dimethyl ether (DME) over zeolite catalysts by
Iglesia and his colleagues in 2006, this reaction has been attracting great attention in the field of
heterogeneous catalysis1-3, because it offers an effective way to enhance the value of C1 chemicals via
the formation of C-C bonds. Methyl acetate (MA), the highly selective product of DME carbonylation,
could be further hydrogenated to produce ethanol, which acts as an important chemical and additive
for transport fuel4. Thus, DME carbonylation indeed opens a novel route for ethanol production from
sources such as coal and natural gas. In 2017, a commercial DMTE (DME-to-Ethanol) process with a
production capacity of 100 000 tons of ethanol per year, developed by Dalian Institute of Chemical
Physics (DICP), has been successfully put into operation in Shaanxi, China5.
Mordenite (MOR) and ferrierite (FER) zeolites have been demonstrated to be the most efficient
catalysts for the DME carbonylation6-10. In particular, MOR exhibits the highest carbonylation activity
and selectivity to the desired product MA. MOR framework consists of parallel 12-membered ring
(12-MR, 6.5×7.0 Å) and 8-MR channels (2.6×5.7 Å) along the c axis direction, both of which are
interconnected by 8-MR side pockets (3.4×4.8 Å) along the b axis direction 11-12. As the 8-MR
channels along the c axis are too narrow for most molecules to penetrate, MOR practically behaves as
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a unidimensional zeolite in catalytic reactions. Previous works have evidenced that the mechanism of
DME carbonylation on acidic zeolites involves the adsorption of DME on Brønsted acid sites (BAS)
to form the methoxy species and their subsequent reaction with CO to yield acetyl intermediates,
which further react with DME to deliver MA13; the rate-determining step on MOR zeolite is the
formation of acetyl intermediates, which preferentially occurs in the 8-MR side pockets with spatial
confinement14-16. Several research groups have reported that DME carbonylation rates are proportional
to the number of BAS in the 8-MR side pockets17-19. In addition, Corma et al. investigated the
carbonylation activities of BAS at different crystallographic positions by quantum-chemical
methods15. They concluded that the T3-O33 position inside 8-MR pockets is the only one selective for
carbonylation; both the space-confinement effect of the 8-MR pockets and the unusual orientation of
the methoxy groups at T3-O33 positions contribute to the unique carbonylation behavior. The acid
sites in the 12-MR main channels, on the other hand, are prone to side reactions (DME to
hydrocarbons), resulting in rapid carbon deposition and quick deactivation of the MOR catalysts15, 20
Very recently, based on Monte Carlo and molecular dynamics simulation, Zheng et al. also visited the
different roles of the 8-MR and 12-MR channels in MOR zeolites, concluding that the CO aggregation
properties inside the 8-MR side pockets also contributed to the unique catalytic performance of the 8-
MR channels21.
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To improve the catalytic stability of MOR zeolites in the DME carbonylation, several strategies
have been developed via selectively removing/poisoning the acid sites inside the 12-MR channels,
including high-temperature steam treatment (dealumination)22, acid/alkaline treatment (dealumination)
23-24, pyridine modification25, etc. Among them, pyridine modification strategy represents the most
effective one, as the catalytic lifespan of pyridine pre-adsorbed MOR zeolites is evidently superior to
those prepared by other methods. Very recently, pyridine-modified MOR zeolite was also reported to
be highly selective catalyst for syngas conversion to ethylene and methanol to olefins reaction26-27
,
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verifying again the effectiveness of such strategy and the peculiarity of the 8-MR side pockets with
spatial confinement.
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Based on carefully prepared H-MOR zeolite (Si/Al≈10) without dealumination, Maache et al. firstly
reported that the BAS inside the 12-MR channels can be selectively titrated by pyridine, whereas the
acid sites located in the 8-MR pockets are inaccessible due to the large molecule diameter of pyridine28.
However, it is very difficult to avoid the occurrence of dealumination during the practical preparation
(such as calcination or pretreatment process) of MOR zeolites28-32, which would create silanol or
aluminol defects inside the zeolite framework. This is also the case for other zeolites with lower Si/Al
ratios30, 33-34.The generation of defect sites in MOR zeolite may change the diffusion properties of
reactants/products and the accessibility of BAS in 8-MR side pockets. It is thus important to learn the
adsorption/desorption behaviors of pyridine molecules on the de facto imperfect MOR zeolite and
their influence on the catalytic property, which would prompt the improvement of pyridine-modified
MOR catalyst.
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In this contribution, the sorption behaviors of pyridine on MOR zeolites with varying Si/Al ratios
(Si/Al = 7-20) were monitored by FTIR and their impact on the DME carbonylation was explored.
Spectroscopic results evidenced that pyridine could actually interact with the hydroxyl protons in the
8-MR side pockets of these MOR catalysts. However, pyridine desorbs preferentially from the pockets
when heated, which accounts for the good stability and high carbonylation activity of the pyridine-
modified catalysts. The effect of Si/Al ratios on pyridine modification and DME carbonylation activity,
and the inhomogeneity of the active centers inside the 8-MR side pockets were also studied.
EXPERIMENTAL SECTION
Catalyst preparation
MOR zeolites were prepared through a hydrothermal synthesis method. Sodium hydroxide, sodium
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aluminate, tetraethylammonium hydroxide solution (TEAOH, 35 wt%), and H2O were first mixed at
room temperature and stirred until completely dissolved. Subsequently, under intense stirring, silica
sol was added dropwise into the above solution. After aging at the ambient temperature for 1 h, seed
crystals (2 wt% relative to SiO2) were added and the mixture underwent further agitation for another
1 h. The molar compositions of the obtained reaction gels are listed in Table S1. The mixture was
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further transferred into a 100 mL autoclave and crystallized at 170 C for 48 hours under static
conditions. After crystallization, the solid products were washed with distilled water, recovered by
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centrifugation and dried at 120 C for 12 h. To remove the organic amines, the sample was heated
(1 °C/min) to 550 °C under dry air and kept at this temperature for 4 h. The resultant sample was
further transformed to its proton type by ammonium exchange within 1 mol/L NH4NO3 solution (1
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h@80 C, repeated 3 times) and calcination at 550 °C (temperature ramp rate: 1 °C/min) for 4 h
under dry air.
Catalyst characterization
The powder X-ray diffraction (XRD) patterns of the as-synthesized samples were recorded using
PANalytical X’Pert PRO X-ray diffractometer with Cu Kα radiation (λ = 0.154059 Å) at 40 kV and
40 mA from 5o to 50o. The elemental compositions of the as-synthesized samples were determined on
Philips Magix-601 X-ray fluorescence (XRF) spectrometer. The scanning electron microscope (SEM)
images of the synthesized samples were recorded on a Hitachi SU8020 SEM equipment. N2 adsorption
and desorption analysis was carried out on a Micromeritics ASAP 2020 analyzer. The total surface
area was calculated based on BET method. The micropore volume and area were derived by t-plot
method. Temperature programmed desorption of ammonia (NH3-TPD) analysis was performed using
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100 mg powder of the H-MOR zeolites at the temperature range of 100-700 C after the sample
pretreatment at 450 °C for 1 hour under a He flow using Micromeritics Autochem II 2920 equipped
with a thermal conductivity detector (TCD). Solid-state NMR spectra were recorded on a Bruker
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Avance III 600 MHz spectrometer. Before 1H MAS NMR measurements, the samples were dehydrated
at 420ꢀ°C under a vacuum below 10-3ꢀPa for 20ꢀh. 1H MAS NMR spectra were collected at 600.13ꢀMHz
with a 3.2 mm HXY probe using a Hahn-Echo pulse sequence with a π/2 pulse of 3.2 μs, a 10ꢀs recycle
delay, and a spinning speed of 20ꢀkHz. The chemical shifts were referenced to adamantane at 1.74ꢀppm.
Fourier transform infrared spectra of pyridine (Py-FTIR) analysis were recorded on Bruker Tensor
27 instrument with an MCT detector. The samples (20 mg) were pressed into a self-supporting wafer
and placed into a quartz cell. Subsequently, the samples were pretreated at 400 °C for 30 minutes
under vacuum. After pretreatment, the samples were cooled down to room temperature. The spectra
of MOR samples without pyridine perturbation were subsequently recorded from 4000 to 1000 cm-1.
Afterwards, pyridine was introduced into the cell until complete saturation, followed by evacuation of
the IR cell for 30 min to remove physisorbed pyridine. The desorption process of pyridine was
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monitored by heating the sample up to desired temperatures with a heating rate of 5 C/min under
vacuum. Each spectrum was obtained by averaging 64 scans with a resolution of 4 cm-1 and all the
spectra are recorded at room temperature. Note that the heating rate of the treatment processes was
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initially set at 1 C/min in order to avoid the dealumination of the samples. However, the resultant
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spectra show no obvious difference with those obtained at 5 C/min, and the experiments are
consequently conducted with a heating rate of 5 oC/min (Figure S1).
Catalytic testing
DME carbonylation reactions were performed in a fixed-bed reactor. 0.5 g of the catalyst (40-60
mesh) was loaded into the reactor and pretreated (heating rate 1 °C/min) in Ar at 400 °C for 2 h.
Afterwards, the sample was purged with pyridine-Ar mixture (30 mL/min) for 20 min at 200 °C and
subsequently flushed with Ar at certain temperature (200, 300, 400, 500 °C), until no obvious pyridine
can be detected by gas chromatograph. Finally, the temperature was reduced to 200 oC and a reactant
gas mixture (DME: CO: N2=5:35:60) was introduced with a GHSV of 3600 mL/g/h. The reaction
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pressure was 2.0 MPa. The products were analyzed online by gas chromatograph (Agilent 7890B)
equipped with TCD and FID detectors using TDX-1 packed column and Pora PLOT Q capillary
column, respectively.
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RESULTS AND DISCUSSION
Structural and compositional analysis
Five MOR zeolites with Si/Al ratios between 7 and 20 were synthesized. The elemental
compositions of the as-synthesized samples, derived from XRF analysis, are listed in Table 1. To
clarify, the samples are denoted MOR(7.0), MOR(9.7), MOR (13.8), MOR(16.5) and MOR(19.4),
with the values in the parentheses indicating their Si/Al ratios. The XRD patterns of the as-synthesized
samples are presented in Figure S2, all of which could be well-indexed to the MOR structure,
indicating their good purity. SEM images (Figure S3) reveal that the MOR samples with lower Si/Al
ratios have aggregated morphologies of nano-grains, which is likely owing to the use of seeds for the
synthesis. With the increase of Si/Al ratios (samples MOR(13.8), MOR(16.5) and MOR(19.4)), the
primary particles become more closely packed and even merge into bulk particles. The change in
morphology following the increase of Si/Al ratios is consistent with the increased crystallinity as
evidenced by the XRD results. The N2 physisorption isotherms of the samples are displayed in Figure
S4, which can be regarded as type I or composite of type I plus type IV isotherms. For samples MOR
(7.0) and MOR(9.7), very small hysteria loops of H4-type are observed likely resulting from the
stacking voids of nano-grains. The corresponding textural properties are listed in Table 1. The
micropore surface areas and micropore volumes of the samples are in the range of 371-394 m2 g-1 and
0.18-0.19 cm3 g-1 respectively, verifying their good crystallinity.
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Table 1 Elemental compositions and textural properties of the samples
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Surface areab (m2 g-1)
Si/Al
Pore volumec (cm3 g-1)
Sample
ratioa
SBET
Smicro
Sext
Vtotal
Vmicro
Vmeso
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8
9
MOR(7.0)
MOR(9.7)
MOR(13.8)
MOR(16.5)
MOR(19.4)
7.0
9.7
438
461
459
438
447
371
394
390
384
394
67
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69
54
53
0.25
0.26
0.25
0.24
0.24
0.18
0.19
0.19
0.19
0.19
0.07
0.07
0.06
0.06
0.05
13.8
16.5
19.4
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aDerived from XRF analysis. bSBET: BET surface area, Smicro: t-plot micropore surface area, Sext = SBET – Smicro. cVtotal
:
total volume absorbed at P/P0=0.99, Vmicro: t-plot micropore volume, Vmeso=Vtotal – Vmicro
.
Acid properties
NH3-TPD was employed to probe the acid properties of the samples, and the corresponding profiles
are presented in Figure S5. The existence of two obvious NH3 desorption peaks could be observed for
all the investigated samples. The low-temperature desorption peaks centered around 195 oC are due to
NH3 desorption from weak acid sites and physiosorbed NH3, while the peaks centered at relatively
high temperatures are attributed to NH3 desorption from strong acid sites 24, 35-39. The NH3 desorption
temperatures associated with strong acid sites rise in parallel with the increase of Si/Al ratio until 13.8,
indicating a gradual increase of acid strength. The reason could be attributed to the decreased number
of Al atoms occupying the Next-Nearest-Neighbor (NNN) sites in the MOR framework 30, 40. With the
further increase of Si/Al ratio to 16.5 and 19.4, the high-temperature desorption peak moves towards
lower temperature (from 530 to 500 oC). Given that the chance of Al atoms at the NNN sites is low in
the samples with high Si content, this shift should not be interpreted as the decrease of acid strength.
Likely, the reduced acid density on the high-Si samples lightens the probability of re-adsorption of
desorbed NH3 and thus leads to the low-temperature shift41-43. The deconvoluted results of the NH3-
TPD profiles are presented in Figure S5 and Table 2. Herein, the NH3-desorption at higher temperature
range is deconvoluted into two peaks, representing the moderate and strong acid sites, respectively.
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The total acid amounts of the samples show a decreasing trend with the increase of the Si/Al ratios.
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Table 2 Acid properties of the H-MOR samples derived from NH3-TPD and FTIR of pyridine adsorption
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NH3-TPD (mmol/g)
Py-FTIRa (mmol/g)
BASc LASc BAS/LAS
Samples
Weak
Moderate
Strong
M+Sb
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MOR(7.0)
MOR(9.7)
MOR(13.8)
MOR(16.5)
MOR(19.4)
0.83
0.60
0.51
0.44
0.38
0.32
0.22
0.18
0.15
0.12
0.67
0.66
0.68
0.59
0.52
0.98
0.88
0.86
0.75
0.64
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-
-
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-
-
0.76
0.66
0.57
0.10
0.09
0.07
7.60
7.33
8.14
aPy-FTIR was used to evaluate the ratio of Brønsted to Lewis acid sites based on the integrated areas of absorption
bands at 1540 and 1450 cm-1.44 bM+S = Moderate+Strong. All the acid sites of the three high-Si MOR samples can
be reached by pyridine (Figure 2). cThe amounts of BAS and LAS were derived from the ratio of BAS/LAS and the
(M+S) acid amount of NH3-TPD.
Figure 1 illustrates the FTIR spectra of the hydroxyl vibration region of the H-MOR samples. The
absorption at around 3740 cm-1 is due to the terminal silanol groups on external surface of the
samples45. The signal at 3660 cm-1 is ascribed to the aluminol of extra-framework (EFAl) species46-47
,
which is more prominent on the samples with low Si/Al ratios (< 10). When the Si/Al ratio increases
to 13.8 or even higher, the band at 3660 cm-1 could hardly be discerned, indicating that the low-Si
samples are more prone to dealumination. Simultaneously, two new bands at 3690 and 3490 cm-1
emerge in the spectrum of sample MOR(13.8) and their intensities rise with the further increment of
the Si/Al ratio. The broad band at around 3490 cm-1 could be unambiguously ascribed to the H-bonded
silanol groups inside the silanol nests 48-49. However, the assignment of the band at 3690 cm-1 is not
straight-forward. There are two possible origins, i.e. the silanol groups inside silanol nests with trivial
hydrogen bond perturbation50 or hydroxyl pairs featuring less hydrogen bonding49. Considering the
synchronous increase of the two bands in intensity with the Si/Al ratios, it is assumed that they are
both closely related to the silanol nests. The appearance of silanol and aluminol on the samples is also
confirmed by 1H MAS NMR spectra (Figure S6). The deconvoluted results confirm that the relative
percentage of Al-OH decreases following the increase of sample Si/Al ratios, yet that of Si-OH gives
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an opposite trend.
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Figure 1 FTIR spectra in the ν(OH) vibration region of the H-MOR samples.
The most prominent hydroxyl band shown in Figure 1 is located at 3610 cm-1, corresponding to
Brønsted hydroxyl groups, which is actually a composite band comprised of high-frequency (HF) and
low-frequency (LF) components 17, 19, 51-52. It has been demonstrated that the HF band (around 3612
cm-1) and LF band (around 3590 cm-1) are associated with bridging hydroxyls in the 12-MR and 8-
MR channels respectively, which behave distinct performances in the DME carbonylation reaction17.
A deconvolution analysis of the Brønsted hydroxyls was thus carried out to determine the acid sites
distribution17, 19 (Figure S7). To eliminate the interference of the silanol and aluminol groups on the
deconvolution, herein NH3 was employed to selectively titrate the Brønsted hydroxyls at 150 oC. As
the bands due to the silanol and aluminol groups were barely perturbed by NH3, isolated bands
corresponding to the bridging hydroxyls could be derived by subtracting the NH3-titrated spectra from
the original ones and used for the deconvolution analysis (Figure S8). Based on the assumption that
the two hydroxyl groups possess similar molar extinction coefficient, it is calculated that around 50 %
of the total BAS locates inside 8-MR side pockets, independent of the Si/Al ratios of the samples
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Figure 2 FTIR spectra of the ν(OH) vibration region of the H-MOR samples before and after pyridine
adsorption at different temperatures.
Figure 2 presents the hydroxyl vibration region of the FTIR spectra of the samples before and after
pyridine adsorption. Pyridine, as one of the most common probe molecule to assess the acidity of
zeolite catalysts, has been reported to be effective to distinguish the acid sites distribution of MOR
zeolite, via selective titration of the acid sites inside the 12-MR channels7, 23-24, 45, 53. However, for
samples MOR(13.8), MOR(16.5) and MOR(19.4), the bands associated with the Brønsted hydroxyls
are completely eroded upon pyridine adsorption at room temperature. It implies that pyridine
molecules could actually penetrate into the 8-MR side pockets and capture the protons therein. Such
contradiction with the previous findings might result from the existence of defect sites in the samples
(Figure 1 and S6), which enlarges the window size of the pockets.
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For low-Si samples H-MOR(7.0) and H-MOR(9.7), only a small amount of acid sites could be
titrated by pyridine at room temperature. Such distinct pyridine adsorption behaviors, as compared to
those of high-Si MOR samples, should be owing to their higher acid density, which causes higher
local pyridine density and thus larger diffusion resistance. In addition, the relatively high EFAl content
in H-MOR(7.0) and H-MOR(9.7) may also aggravate the difficulty in pyridine diffusion. With the
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adsorption temperature rising to 100 C, the accessibility of BAS is improved due to the enhanced
diffusion, but no preferential location is observed for pyridine molecules.
Pyridine desorption
The temperature-dependent desorption of pyridine is monitored by FTIR (Figure 3 and Figure S9).
The desorption behaviors are observed to be qualitatively similar for high-Si samples MOR(13.8),
MOR (16.5) and MOR (19.4), showing a gradual recovery of acidic hydroxyls with the temperature
ramp until 400 oC. As seen from Figure 3, although the recovery degree at the same temperature differs
among the three samples, it is clear that large proportion of BAS is not restored even after desorption
at 400 oC, in consistence with the high acid strength of the BAS as revealed by the NH3-TPD results.
More interestingly, pyridine is observed to escape merely from 8-MR side pockets, as evidenced by
the gradual grow of the symmetrical band centered below 3592 cm-1. This phenomenon seems
contradict to the higher acid strength of the BAS in the 8-MR pockets as revealed previously20, 54. The
logic is speculated to be related to the existence of certain steric effect between pyridine and 8-MR
pockets. It is acknowledged that strong coulombic field exists in the negative framework of zeolites,
and it could disturb the electron clouds of the accommodated guest molecules55. This disturbance is
especially prominent when the size of guest molecules matches well with the zeolite cages. Under
such circumstance, a contraction of the molecular orbitals of guest molecules would occur due to
strong electronic confinement effect56-57. Therefore, the enhanced lattice vibration of 8-MR windows
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at higher temperature, together with the assistance of the delocalized electron cloud in 8-MR pockets,
might help breathe out the adsorbed pyridine in their proximity, leading to preferential recovery of 8-
MR hydroxyls. The high recovery proportion on MOR(13.8) shown in Figure 3D possibly relates with
its relatively intact framework, which exerts stronger confinement effect on pyridine adspecies in 8-
MR pockets and thus reduces their adsorption stability.
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Figure 3 The ν(OH) bands of the H-MOR samples after pyridine desorption at different temperatures
(A, B, C) and the recovery percentage of BAS at different desorption temperatures (D). The
calculations in Figure D are based on the variation of the band areas at 1540 cm-1 (Figure S9), because
the absorption coefficients of the acidic ν(OH) bands (3650-3550 cm-1) may vary with the locations
of acidic hydroxyls13, 17, 58
.
Another interesting phenomenon is that the absorption bands due to the exposed hydroxyls show a
gradual shift towards higher frequencies (from 3585 to 3592 cm-1) with the increase of desorption
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temperature, implying the heterogeneity of the BAS on MOR zeolite. Lukyanov et al. once carried out
FTIR investigation on the BAS in MOR zeolite. By using Fourier Self-Deconvolution method13, they
identified the existence of six different Brønsted hydroxyl bands arising from different
crystallographic positions, among which three HF bands at 3625, 3617 and 3609 cm-1 are located in
the 12-MR channels, two LF bands at 3591, 3581 cm-1 are located in the 8-MR pockets, and one band
at 3599 cm-1 is at the intersections between the pockets and 12-MR channels. These results are in good
congruence with our findings.
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For pyridine-modified low-Si samples MOR(7.0) and MOR(9.7), although their acid sites cannot
be completely covered after pyridine adsorption at 100 oC, they behave similar desorption behaviors
to high-Si samples with preferential recovery of acid sites in 8-MR pockets upon heating (Figure S10).
In addition, sample MOR(7.0) shows a more obvious HF shift of the bridging hydroxyl band following
the increase of desorption temperature. Its hydroxyl peak moves to 3601 cm-1 after desorption at 400
oC, implying an increased pyridine desorption from the 12-MR channels. This phenomenon might be
due to the interference between the adjacent protonated pyridine in 12-MR channels with high acid
density.
Catalytic performance for DME carbonylation
The DME carbonylation performances of the pyridine-modified H-MOR zeolites were evaluated
under different pyridine desorption conditions, and the corresponding DME conversion and MA
selectivity versus time on stream (TOS) are illustrated in Figure 4. All the investigated catalysts exhibit
an induction period with lower DME conversion at the beginning of the reaction, which should be due
to the interaction of DME with the BAS and the formation of surface methoxyl species14. As the
reaction proceeds, the conversion rises rapidly and reaches a steady plateau. Similar variation trend
could be observed for MA selectivity, giving a steady value of around 100%. Moreover, the steady
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DME conversions increase in parallel with pyridine desorption temperatures (200-400 oC) for all the
samples. The rationale lies in the facts that pyridine actually enters the 8-MR pockets during its pre-
adsorption procedure; the carbonylation activity centers inside the pockets are gradually exposed
following the increase of desorption temperature. It is noted that further increasing the desorption
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temperatures to 500 C would lead to a decreased carbonylation activity and stability (Figure S11).
Obvious deactivation trend can be observed within 20 h. This should be caused by the exposure of
acid sites in 12-MR channels15, 20, 29 and the carbonization of part of pyridine at high temperature10.
These results imply that pyridine desorption temperatures essentially determine the carbonylation
activity and stability of the pyridine-modified MOR catalysts.
Figure 4 DME conversion and MA selectivity over pyridine-modified H-MOR catalysts after
pretreatment at different pyridine desorption temperatures. Reaction conditions: 200oC, 2 MPa,
DME/CO/N2 = 5/35/60, GHSV=3600 mL/g/h.
The DME conversion activity follows a volcanic trend as a function of the Si/Al ratios. MOR(13.8)
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exhibits the best DME carbonylation activity, independent of the pyridine desorption temperatures.
The reason behind this should be multifold. Firstly, MOR(13.8) possesses relatively strong acidity, as
revealed by NH3-TPD analysis. Besides, for sample MOR(7.0) with lower Si/Al ratio, the excessive
acid density causes an over-crowdedness of pyridine molecules within the 12-MR channels (see
pyridine-FTIR section), and consequently exerts negative effect on the diffusion of reactants and
products. For high-Si samples MOR(13.8), MOR(16.5) and MOR(19.4), the relatively low acid
densities are beneficial to the improvement of mass transfer. Compared to MOR(13.8), the inferior
catalytic performances of MOR(16.5) and MOR(19.4) should result from their fewer active sites. In
consequence, sample MOR(13.8) with higher acid strength and optimal acid density, exhibits the best
DME carbonylation performance. In addition, the existence of defects such as silanol nests or
aluminols is less obvious for sample MOR(13.8). The negative effect of these defects on the DME
carbonylation could not be excluded and deserves further study.
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Figure 5 The MA space-time yield (bar chart, TOS = 20 h) and the percentage of exposed BAS (black
circles) at different pyridine desorption temperatures on the H-MOR zeolites (A, B and C). Figure D
presents the variation of the TOF of the catalysts following the BAS exposure. The percentage of
exposed BAS is from Figure 3D. The exposed BAS amount for TOF calculation is evaluated based on
the total BAS in Table 2 and its exposed percentage. They are mainly from the 8-MR pockets when
desorption temperature is less than 400 oC.
Figure 5 presents the space-time yield (STY) of MA and the percentage of exposed BAS at different
desorption temperatures over the high-Si H-MOR catalysts. It can be seen that the STY of MA on
MOR(13.8) reaches as high as 7.2 mmol/h/g when the pyridine desorption temperatures are higher
than 300 oC. This value is the highest ever reported for MOR-catalyzed DME carbonylation reaction
(Table S2). However, it is noted that the increase of STY is not linearly proportional to the increased
BAS exposure, despite the fact that the majority of these BAS is from the 8-MR pockets (see Figure
3). The turnover frequency (TOF) of the samples at different desorption temperatures is further
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calculated and plotted against the percentage of exposed BAS. As shown in Figure 5D, the TOF value
drops significantly following the exposure of BAS for all the samples, which reveals that the catalytic
activity of the BAS inside the 8-MR pockets is inhomogeneous; the BAS exposed at lower desorption
temperatures possess superior catalytic activity. As the above Py-FTIR results have demonstrated that
the LF acidic hydroxyls of the 8-MR pockets are preferentially exposed upon pyridine desorption, it
is concluded that these sites with LF vibrations have the highest carbonylation activity.
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To explore the location of the LF hydroxyls in the pockets, we tried to prepare pyridine-exchanged
MOR(13.8) by ion-exchanging the calcined Na-form sample with pyridine aqueous solution and found
that the Na/Al ratio of the sample decreased from 0.76 to 0.13 after the ion exchange. Previous work
reported that there are three positions for Na+ cations in the structure of MOR59; the ratio of Na+ in
unconstrained and constrained environments is 1:228. Herein, such a low Na content implies the
incorporation of pyridine into the pockets. We suppose that the residual Na+ cations are located at the
bottom of the pockets, where the constrained environment limits the exchange with bulky hydrated
pyridine ions. As shown in Figure S12, extremely low activity is observed for the pyridine-exchanged
MOR(13.8). This suggests that the BAS at the bottom of the 8-MR pockets should be the most active
sites for DME carbonylation, which correspond to the LF acidic hydroxyls in the FTIR spectra.
CONCLUSIONS
The adsorption/desorption behaviors of pyridine on a series of MOR zeolites and their impact on
DME carbonylation have been investigated in order to elucidate the pyridine modification-catalytic
property relationships and to improve the catalyst performance. It is demonstrated that pyridine
molecules could penetrate into 8-MR pockets and interact with the Brønsted hydroxyls therein. This
is likely owing to the existence of defect sites in the MOR framework, which enlarges the window
size of the pockets. The Si/Al ratios of MOR zeolites influence the pyridine adsorption process. Low
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Si/Al ratios, corresponding to high acid density and relatively large amount of EFAL species, would
cause increased diffusion limitation and incomplete acidity coverage. However, for MOR with higher
Si/Al ratio (> 10), all the acid sites can be easily reached by pyridine molecules. With the ramp of
pyridine desorption temperature (≤ 400 oC), the BAS in 8-MR side pockets are preferentially recovered,
leading to incremental conversion. Pyridine desorption temperature essentially determines the
carbonylation activity and stability of the catalysts. The DME carbonylation activity follows a volcanic
trend with the increase of Si/Al ratios. The optimal Si/Al ratio is around 13.8, over which high MA
yield of 7.2 mmol/g/h can be achieved under controlled pyridine desorption temperature (300 ~ 400
oC). The higher acid density, stronger acidity and better diffusion properties corporately contribute to
the excellent catalytic performance. The catalytic activity of the BAS in the 8-MR pockets is rather
inhomogeneous, and the LF bridging hydroxyls siting at the bottom of the side pockets have superior
carbonylation activities.
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AUTHOR INFORMATION
Author contributions
Δ(Kaipeng Cao, Dong Fan) These authors contributed equally.
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information
The initial gel compositions, the ν(OH) bands of the H-MOR pretreated with different heating rates,
XRD, SEM, N2 physisorption isotherms, NH3-TPD, 1H MAS NMR spectra, the deconvoluted ν(OH)
bands of FTIR spectra, the derivation of the isolated Brønsted ν(OH) band for deconvolution, the FTIR
spectra of pyridine ring-related vibration regions, the ν(OH) bands of the low-Si H-MOR samples after
pyridine desorption at different temperatures, the DME carbonylation activities of the samples after
pyridine desorption at 500oC, the detailed procedure for pyridine exchange and the DME
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carbonylation activities of the pyridine-exchanged sample, the summary of the product STY in the
zeolite-catalyzed DME carbonylation reaction.
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ACKNOWLEDGMENT
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We acknowledge the National Natural Science Foundation of China (No. 91545104, 21606221,
21676262 and 20191091) and Key Research Program of Frontier Sciences, Chinese Academy of
Sciences (Grant No. QYZDB-SSW-JSC040).
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