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Synthesis of hierarchical porous H-mordenite zeolite for carbonylation of dimethyl ether
Journal Pre-proof Synthesis of hierarchical porous H-mordenite zeolite for carbonylation of dimethyl ether Haibing Sheng, Weixin Qian, Haitao Zhang, Peng Zhao, Hongfang Ma, Weiyong Ying PII:
S1387-1811(19)30809-1
DOI:
https://doi.org/10.1016/j.micromeso.2019.109950
Reference:
MICMAT 109950
To appear in:
Microporous and Mesoporous Materials
Received Date: 12 October 2019 Revised Date:
2 December 2019
Accepted Date: 9 December 2019
Please cite this article as: H. Sheng, W. Qian, H. Zhang, P. Zhao, H. Ma, W. Ying, Synthesis of hierarchical porous H-mordenite zeolite for carbonylation of dimethyl ether, Microporous and Mesoporous Materials (2020), doi: https://doi.org/10.1016/j.micromeso.2019.109950. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
In this work, a series of hierarchical porous mordenite zeolites were prepared by using n-Butylamine and Polyacrylamide as soft templates for carbonylation of dimethyl ether to methyl acetate. The introduction of a suitable soft template improved the porosity of the catalysts. An appropriate number of soft template induced to obtain more framework Al (Alf), thus increasing the number of Brønsted acid sites, which promoted conversion of DME and MA selectivity. In addition, the amount of acid site in the 12-MR was reduced after the introduction of soft template, which inhibited coke formation of mordenite during DME carbonylation reaction. The mass transfer efficiency was improved and the coke deposition was decreased with the introduction of mesopores.
1
Synthesis of Hierarchical Porous H-Mordenite Zeolite for Carbonylation
2
of Dimethyl Ether
3
Haibing Sheng, Weixin Qian, Haitao Zhang, Peng Zhao, Hongfang Ma*, Weiyong
4
Ying
5
Engineering Research Center of Large Scale Reactor Engineering and Technology,
6
Ministry of Education, State Key Laboratory of Chemical Engineering, East China
7
University of Science and Technology, Shanghai 200237, China
8
ABSTRACT: A series of hierarchical porous mordenite zeolites were prepared by
9
adding soft template during hydrothermal process for carbonylation of dimethyl ether
10
(DME) to methyl acetate (MA). The synthesized mordenite catalysts were
11
systematically characterized by XRD, BET, ICP-AES, NH3-TPD, Py-IR, FTIR,
12
HRTEM, SEM, TG, 27Al NMR,
13
confirmed that mordenite zeolites with mesoporous structure showed more framework
14
aluminum and more Brønsted acid sites of 8-membered ring (8-MR), which promoted
15
conversion of DME and MA selectivity. The hierarchical porous mordenite had
16
increased the mass transfer efficiency and showed less acidity of 12-membered ring
17
(12-MR), thus suppressing the formation of coke. Measurements of changes in the
18
chemical composition of the coke by GC-MS showed the growth mechanism of coke
19
molecules in the mordenite zeolites.
20
Keywords: hierarchical mordenite, soft template, carbonylation, dimethyl ether,
21
methyl acetate
22
29
Si NMR and GC-MS. The characterization results
23
1. Introduction
24
As an important chemical raw material, ethanol has received much attention as
25
sustainable and environmental advantages. In particular, ethanol is a good choice as a
26
promising alternative to fossil fuels [1]. Comparing with the traditional ethanol
27
synthesis route by ethylene hydration and biomass fermentation, the synthesis of
28
ethanol by dimethyl ether (DME) carbonylation to methyl acetate (MA) and MA
29
hydrogenation has drawn much attention recently because of its high atom economy,
30
MA selectivity and environmental friendliness [2, 3]. As the former studies on this
31
new route, the DME carbonylation is the critical process [4, 5]. Several zeolites such
32
as mordenite (MOR) [6], ferrierite (FER) [7] and heteropoly acids (HPAS) [8] were
33
used for DME carbonylation. Compared with the FER and solid acid catalysts, the
34
MOR catalysts showed better catalytic activity in carbonylation reaction because of
35
its unique crystal structure and acid stability.
36
Previous studies had shown that Brønsted acid sites were mainly the active site of
37
DME carbonylation reaction [9, 10]. The extent of dealumination by thermal
38
treatment changed the Si/Al ratio of the framework and increased the number of
39
Brønsted acid sites [11]. The mechanisms of zeolites dealumination had been studied
40
by density functional theory (DFT) calculations [12]. Wang group had demonstrated
41
that the amount of Brønsted acid sites in the 8-MR had increased by changing the
42
composition of Si and Al and adding different structure-direction agent [13]. However,
43
the 12-MR channels showed low selectivity to methyl acetate and highly favored the
44
formation of hydrocarbons [14]. Therefore, it was an effective way to promote the
45
stability of the catalyst by suppressing hydrocarbon formation in 12-MR channels.
46
The 12-MR pores could be poisoned by pre-adsorption of pyridine, which prolonged
47
the catalytic lifetime of HMOR [15, 16]. High temperature steam treatment could
48
remove the framework Al species in 12-MR, thereby improving the stability of
49
HMOR [17]. Besides, introduction of metal ions by ion exchange could also improve
50
the amount of Brønsted acid sites [18, 19].
51
Hierarchical Porous catalysts showed excellent properties in their higher
52
diffusivity of reactants and products. Maleki et al. [20, 21] had reported that the
53
preparation of hierarchical nanocatalyst by ultrasound irradiation is a particularly
54
novel method. Mesoporous mordenite had been obtained by acid or alkaline treatment,
55
which improved the properties of texture and exhibited higher acidity of the catalysts
56
[22, 23]. Svelle et al. [24] studied that the mesopore formation by desilication had
57
become a simple way to influence the concentration of Brønsted acid sites. Wang et al.
58
[25] reported that the hierarchical structures were introduced into the mordenite by
59
using Polyethylene glycol (PEG) as template, which increased the number of strong
60
acid sites and limited the coke depositions. Li et al. [26] had synthesized mesoporous
61
mordenite using [3-(trimethoxysilyl)propyl][hexadecyl dimethyl ammonium] chloride
62
(TPHAC) as soft template, which showed a large number of Brønsted acid sites and
63
had a good catalytic performance. However, it is ambiguous that the relationship
64
between Brønsted and Lewis acid sites by introducing mesopores into HMOR catalyst.
65
The mechanism of coke molecules growth have rarely been reported in dimethyl ether
66
carbonylation to methyl acetate.
67
In this work, the hierarchical porous mordenites were obtained by addition soft
68
templates during the process of synthesis. The purpose of this work was to increase
69
the activity and stability of mordenite in dimethyl ether carbonylation. Besides, the
70
acidity and coke deposition of mordenite were also studied. Characterization of the
71
catalysts was performed by BET, XRD, ICP-AES, Py-IR, HRTEM, SEM, NH3-TPD,
72
27
73
for the industrialization of dimethyl ether to ethanol.
74
2. Experimental section
75
2.1. Catalyst preparation
Al NMR, FTIR, TG, 29Si NMR and GC-MS. This work provided a theoretical basis
76
The hierarchical porous mordenite zeolites were synthesized by the hydrothermal
77
method using n-Butylamine (Titan, 99.5%) and Polyacrylamide (PAM) (Adamas,
78
nonionic, MW: 8 million) as soft templates. In a typical procedure, 0.98 g of sodium
79
aluminate (NaAlO2) and a certain amount of sodium hydroxide (1.58 g) were
80
dissolved in 50 ml of deionized water. Then, 30.48 g of silica sol (30 wt % SiO2) was
81
added to the above solution and stirred at room temperature for 2 h. Next, the soft
82
template of n-Butylamine or PAM was added to the mixture. The mixture was sealed
83
in a Teflon-lined autoclave and crystallized at 443 K for 4 days. The slurry was
84
collected by filtration, dried at 383 K for 12 h, and finally calcined at 823 K for 4 h.
85
The resulting sample was Na-MOR. The mole composition of the catalyst was
86
1.0SiO2: 0.04Al2O3: 0.2Na2O: 26H2O. The HMOR catalyst was derived from
87
Na-MOR upon ion-exchanged with 1 M ammonium nitrate (NH4NO3). 5.0 g calcined
88
Na-MOR powder was dispersed into 50 ml of 1 M NH4NO3. The mixture was
89
refluxed at 353 K for 3 h. Then, the solid was obtained by filtration, dried at 383 K for
90
12 h, and finally calcined at 823 K for 4 h. The mass ratio of PAM/SiO2 is 0.87,
91
denoting as HMOR-PAM. The prepared catalysts of adding n-Butylamine were
92
designated as HMOR-xNBA, where x was defined the mole weight of n-Butylamine
93
(0.1, 0.2, 0.4). For comparison, the resulting sample without addition of soft template
94
was named HMOR-parent.
95
2.2. Catalyst characterization
96
The Ar adsorption-desorption isotherms was carried out on Micrometrics ASAP
97
2020 device. The specific surface area was calculated by using BET method, and the
98
pore volume and pore size were estimated by BJH method. The Powdered X-ray
99
diffraction (XRD) profiles of the catalysts were performed on a D/Max-2550VB/PC
100 101 102
with Cu Kα (λ= 0.15416) radiation over the 2-theta range of 10-80°. The actual contents of Si and Al in catalysts were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES).
103
HRTEM images were recorded using an EOL2010 microscope operating at 300
104
kV. The sample powder was dispersed in absolute ethanol, and then dried under
105
infrared lamp. SEM was performed to observe the crystallite size and morphology
106
carrying out on a Philips Fei Quanta 200F microscope.
107
NH3-TPD experiments were performed on a Micromerities Autochem II 2920
108
chemisorption apparatus. Before the TPD measurements, 0.2 g of sample was
109
preprocessed in flowing He (35 ml/min) at 773 K for 1 h, and later cooled to 323 K.
110
Then the catalyst was exposed to 10 % NH3-He mixture for 30 min until saturation.
111
TPD analysis was recorded from 373 to 1073 K with 10 °C min-1 heating rate, and the
112
quantity of NH3 desorbed was detected by a thermal conductivity detector (TCD).
113
The
27
Al and
29
Si MAS NMR experiments were conducted on a Bruker AV-500
114
spectrometer at at 156.39 MHz and 99.33 MHz, respectively. The 27Al chemical shifts
115
were referenced to Al(NO3)3 and the 29Si chemical shift to tetramethylsilane (TMS).
116 117
Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor 27 spectrometer with a diffuse reflectance attachment and with a resolution of 4 cm-1.
118
Thermogravimetric (TG) studies of the spent samples were initially heated from
119
100 ℃ to 850 ℃ under an air flow rate of 30 ml/min. A STA 409 PC thermal analyzer
120
were used for recording the number of coke depositions on the spent mordenite.
121
GC-mass spectroscopy (GC-MS) analyses were performed on an Agilent 6890
122
series gas chromatograph with a FID detector. After dissolving the spent catalyst by
123
33 vol% hydrofluoric acid solution, the residue was extracted by CH2Cl2 at room
124
temperature. The oven temperature was increased from 60 ℃ to 220 ℃ at 5 ℃/min,
125
and held isothermally for 5 min. The carry gas was helium at flow rate of 70 ml/min.
126
2.3. DME carbonylation
127
Catalytic activities were evaluated using continuous flow fixed-bed reactor with
128
an internal diameter of 10 mm. 0.5 mg of the catalyst (40-60 mesh) was packed into
129
the reactor, employing a thermocouple to control the temperature. Before each
130
experiment, the catalyst was pretreated under a flow of N2 (20 ml/min) for 2 h at 593
131
K. After cooling to 473 K, the reactant mixture (10% DME, 50% CO, 40% N2,
132
mol/mol) was introduced into the reactor with 20 ml/min flow rate at 1 MPa. The
133
reaction products were vaporized by heating at 120 ℃. The outlet gas was analyzed
134
online using a gas chromatograph (Agilent 7890 A).
135
136
2.4. Catalyst regeneration
137
Catalyst regeneration consisted in oxidizing the coke deposited onto the catalyst in
138
air flow for 2 h at 823 K. The reaction over the regenerated catalyst exhibited similar
139
conversion to that of the fresh catalyst (Fig. S1). This result proves that the main
140
reason for the deactivation of the catalyst is carbon deposits, and the removal of
141
carbon deposits can restore the catalyst's activity without loss of active sites.
142
3. Results and discussion
143
3.1. Porosity and morphology of catalysts
144
The textural and morphological properties of catalysts were exhibited in Fig. 1.
145
The physicochemical properties of the catalysts were showed in Table 1. All the
146
samples exhibited Langmuir type I isotherms with H4-type hysteresis loop, indicating
147
the hierarchical porous texture. As can be seen from the pore diameter distribution,
148
the mesopores were introduced inside the catalysts by addition of n-Butylamine and
149
PAM soft templates during the synthesis process. From HMOR-parent to
150
HMOR-0.2NBA, the pore size increased from 2.4 to 3.3 nm, the surface area
151
increased from 269 to 289 m2/g, but the micropore volume decreased from 0.15 to 0.1
152
cm3/g and the external area increased from 38 to 66 m2/g. The HMOR-PAM catalyst
153
exhibited greater surface area, pore size and more mesopores comparing with
154
HMOR-parent. However, the HMOR-0.4NBA could not be developing more
155
mesopores, because excessive soft template blocked the formation of mordenite
156
crystal, which was in accordance with the result of XRD.
157 158
Fig. 1. Argon adsorption and desorption isotherm (a) and pore size distribution (b) of the samples.
159
Table 1
160
Textural properties of HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
161
HMOR-PAM samples. SBET
Sext
Dpore
VMa
VTb
(m2/g)
(m2/g)
(nm)
(cm3/g)
(cm3/g)
HMOR-parent
269
38
2.4
0.18
0.21
HMOR-0.1NBA
276
47
2.8
0.11
0.23
HMOR-0.2NBA
289
66
3.3
0.07
0.25
HMOR-0.4NBA
257
46
2.5
0.13
0.18
HMOR-PAM
296
58
3.2
0.10
0.24
Sample
162
a
Micropore volume.
163
b
Total pore volume.
164
The XRD patterns of the mordenite zeolite catalysts after calcination were showed
165
in Fig. 2. As can be seen, all catalysts revealed the presence of mordenite XRD
166
patterns, indicating that the original HMOR framework was not changed with the
167
addition of n-Butylamine and PAM soft templates. The diffraction peaks at 2θ = 20.5°,
168
22.3°, 25.8°, 31.0° are typical of HMOR crystalline phase [27]. Additionally, as can
169
be seen from Fig. 2, the intensities of their diffraction peaks exhibited relativity good
170
crystallinity comparing with HMOR-parent except HMOR-0.4NBA, The crystallinity
171
of the HMOR-parent was calculated from the relative intensities of four characteristic
172
HMOR-parent peaks appearing at around 2θ = 20.5°, 22.3°, 25.8°and 31.0° by
173
assuming 100% crystallinity. The crystallinity of HMOR-0.4NBA and HMOR-PAM
174
were 120 % and 112 %, respectively. However, the crystallinity of the
175
HMOR-0.4NBA was decreased from 100% on the HMOR-parent to 73%, meaning
176
that the appropriate amount of soft template increased the crystallinity of the catalyst.
177
[28].
178 179
Fig. 2. XRD patterns of the samples.
180
The microstructure of the hierarchical mordenite catalysts were examined by SEM
181
and HRTEM. The shape of HMOR-parent was like pillars-assembled structure. After
182
the addition of 0.2 mole n-Butylamine, the shape of the particle was not affected.
183
However, as can be seen from Fig. 3d and Fig. 3f, a crystalline material having
184
mesopores were obtained. Moreover, the crystal shape of HMOR-PAM differed from
185
HMOR-parent, and the particle size is smaller. Compared with the HMOR-parent
186
sample, HMOR-0.2NBA and HMOR-PAM catalyst showed obvious porosity and
187
roughness. A few bright spots appeared in the HRTEM images of HMOR-0.2NBA
188
and HMOR-PAM (Fig3. d and f), corresponding to holes in the crystals, which could
189
be mesopores. This indicated that an appropriate amount of soft template helped for
190
obtaining mesoporous mordenite.
191
192
193 194
Fig. 3. SEM and HRTEM images of HMOR-parent (a, b), HMOR-0.2NBA (c, d) and
195
HMOR-PAM (e, f) samples.
196
Fig. 4 showed the
27
Al MAS NMR spectra of samples, which was an important
197
experimental technique to insight about the coordination of aluminum sites in
198
mordenite zeolites. The area ratio of 54/0 ppm was put on the side. The signal at
199
around 54 ppm corresponded to the tetrahedrally framework Al (Alf) and the weak
200
peak at around 0 ppm belonged to the octahedrally corrdinated extra-framework (Alef)
201
[29, 30]. Reule et al. [31] had reported that the number of Lewis acidity was
202
determined by the number of Alef, while Alf was associated with Brønsted acid sites.
203
In Fig. 4, the relative intensity of the peak at 54 ppm increased apparently with the
204
addition of n-Butylamine and PAM soft templates except HMOR-0.4NBA, which
205
indicated that an appropriate number of soft template induced parent mordenite to
206
obtain more Brønsted acid sites.
207 208 209
Fig. 4. 27Al MAS NMR spectra of samples.
To further uncover the Si and Al atomic coordination environments, the
210
experiments of
211
peaks centered at -96, -105 and -112 ppm were evident in the 29Si MAS NMR spectra,
212
which corresponded to Si (2Al), Si (1Al) and Si (0Al) respectively [32]. As shown in
213
Fig S2, it can be found that the proportion of Si (1Al) increased obviously, while the
214
proportion of Si (2Al) decreased with the addition of soft template except
215
HMOR-0.4NBA. This indicated that a suitable amount of soft template induced to
216
more AlO4- tetrahedral, increasing crystallinity, which was consistent with XRD
217
studies [33].
218
3.2.Acidic properties of the catalysts
29
Si MAS NMR spectra were recorded (Fig S2, ESI). Three major
219
The NH3-TPD patterns of HMOR-parent, HMOR-xNBA (0.1, 0.2, 0.4),
220
HMOR-PAM catalysts were displayed in Fig. 5. The quantitative estimation of acid
221
strength distribution at different regions were summarized in Table 2. On the HMOR
222
samples, two typical ammonia desorption peaks were observed at 150 and 500 ℃,
223
respectively, which were weak acids and strong acids. The peak at low temperature
224
(about 150 ℃) of NH3 adsorption could be due to physically adsorbed or
225
hydrogen-bonded NH3 [13]. The high temperature (about 500 ℃) of NH3 adsorption
226
could be ascribed to the Brønsted acid sites of framework Al atoms [34, 17].
227
Comparing with the HMOR-parent sample, the strong acid sites slightly increased by
228
the addition of n-Butylamine and PAM soft templates except HMOR-0.4NBA. This
229
indicated that an appropriate soft template induced the atoms of Al to enter into the
230
position of the strong acid sites. However, excessive n-Butylamine soft template
231
partially destroyed the structure of the mordenite, prevented the formation of more
232
framework aluminum, and reduced the Brønsted acid sites, which was well correlated
233
with the characterizations of XRD and 27Al NMR.
234
It had been reported that less Si/Al showed more Brønsted acid sites [26]. From
235
Table 2 we can see that the catalysts with adding soft templates had a slightly lower
236
Si/Al comparing with HMOR-parent except HMOR-0.4NBA. Besides, an appropriate
237
soft templates would lead to more framework (Alf). As a result, according to the
238
percentage of framework Al, the number of Brønsted acid sites could be ordered as
239
follow:
240
HMOR-0.2NBA >HMOR-PAM >HOR-0.1NBA >HMOR-parent >HMOR-0.4NBA,
241
which was in line with the results of FTIR.
242 243
Fig. 5. NH3-TPD patterns of HMOR-parent, HMOR-xNBA, HMOR-PAM samples.
244
Table 2
245
The amount of acid sites of HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
246
HMOR-PAM samples. NH3-uptake (µmol/g)a
Molar
Alf (%)c
Alef (%)c
Sample Weak
Strong
(Si/Al)b
HMOR-parent
106.2
124.5
11.8
75.2
24.8
HMOR-0.1NBA
112.4
138.7
11.3
82.7
17.3
HMOR-0.2NBA
114.5
147.8
10.7
87.6
12.4
HMOR-0.4NBA
94.1
114..6
13.6
62.4
37.6
HMOR-PAM
108.6
154.4
10.6
85.3
14.7
247
a
Determined by NH3-TPD analysis.
248
b
Determined by ICP-AES analysis.
249
c
Determined by 27Al MAS NMR (Area percentage).
250
Fig. 6 showed the O-H stretching region of FTIR spectra of HMOR-parent,
251
HMOR-xNBA, HMOR-PAM. The bands at around ~3660 cm-1 were assigned to the
252
stretching vibration of extra-framework Al atoms, while the bands at ~3734 cm-1
253
corresponded to the terminal silanol group [35, 36]. The bands at around ~3606 and
254
~3545 cm-1 were ascribed to the Brønsted acid sites of O-H stretching vibration in the
255
8-MR and 12-MR channels, respectively [37]. The amount of acid sites for
256
HMOR-parent, HMOR-xNBA (0.1, 0.2, 0.4) and HMOR-PAM were shown in Table 3.
257
Compared with HMOR-parent of FTIR spectroscopy, the adsorption peak at ~3545
258
cm-1 increased remarkably by the addition of n-Butylamine and PAM soft templates,
259
because the soft template leaded to more aluminum in the framework, which was
260
consistent with the result of 27Al NMR. However, in the case of excessive addition of
261
n-Butylamine, the adsorption peak at ~3545 cm-1 of HMOR-0.4NBA showed a
262
downward trend due to the agglomeration of n-Butylamine during the synthesis
263
process, thus blocking the formation of framework Al and extra-framework Al.
264
Py-IR adsorption was recorded to probe the Brønsted and Lewis acid sites in 12-MR
265
(Fig. 7). The two bands at 1450 and 1540 cm-1 were ascribed to the C-H deformation
266
vibrations of pyridine adsorbed on Brønsted and Lewis acid sites, as reported in
267
literature [38, 39]. The band at 1490 cm-1 was usually assigned to the combinations of
268
pyridine with both Brønsted and Lewis acid sites [40]. The bands at around
269
1400-1700 cm-1 represented the adsorption of pyridine on HMOR-parent,
270
HMOR-xNBA (0.1, 0.2, 0.4) and HMOR-PAM samples. Table 3 showed the number
271
of Brønsted and Lewis acid around 1540 and 1450 cm-1, respectively. After soft
272
templates loading, the acidity of the 12-MR was reduced, and the possible reason was
273
that the molecular diameters of NBA (0.43 nm) and PAM (hydrolysate) are closer to
274
12-MR, so it was easier to enter in 12-MR during hydrothermal process, thus
275
preventing the formation of 12-MR framework Al and extra-framework Al.
276
Furthermore, with excess addition of n-Butylamine, the structure of HMOR-0.4NBA
277
was destroyed, and there was almost no Brønsted and Lewis acidity of 12-MR. The
278
turnover frequency (TOF) of DME carbonylation on the HMOR-parent sample was
279
estimated to be 1.0 h-1, while it was slightly increased to 1.2 h-1 on the
280
HMOR-0.1NBA catalyst. The TOF over HMOR-PAM was 1.5 h-1 (Table 3). These
281
results are clear evidence that the acidic sites in the 8-MR pores are the active centers
282
for DME carbonylation.
283
284 285
Fig. 6. FTIR spectra in the O-H stretching region of HMOR-parent, HMOR-0.1NBA,
286
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM samples.
287 288
Fig. 7. Py-IR spectra of the HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
289
HMOR-PAM catalysts.
290
Table 3
291
The distribution acid sites and DME carbonylation TOF of HMOR-parent, HMOR-0.1NBA,
292
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM catalysts. B acid of 12MRa
L acid of 12MRa
B acid of 8MRb
Total B acid
Carbonylation
（µmol/g）
（µmol/g）
（µmol/g）
（µmol/g）
TOF (h-1)
HMOR-parent
44.2
147.3
134.1
178.3
1.0
HMOR-0.1NBA
32.4
112.5
144.2
176.6
1.2
HMOR-0.2NBA
21.1
104.3
175.2
196.3
1.6
HMOR-0.4NBA
5.4
68.78
82.3
87.7
0.7
HMOR-PAM
18.6
94.3
169.8
188.4
1.5
Sample
293
a
Determined from the adsorption of pyridine.
294
b
Determined from the infrared spectra of the O-H stretching region.
295
3.3. Catalytic tests and carbon deposition analysis.
296
Fig. 8 shows a set of results for the carbonylation of DME over HMOR-parent,
297
HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM samples. The
298
DME conversion and MA selectivity of HMOR-xNBA (0.1, 0.2, 0.4) catalysts
299
initially increased with the increasing of the n-Butylamine mole, then began to decline
300
with further increased the mole of n-Butylamine. The reason was that too much
301
n-Butylamine soft template affected the formation of the framework Al and reduced
302
the number of Brønsted acid site. The best activity over HMOR-0.2NBA and
303
HMOR-PAM were attributed that a suitable amount of soft template induced more
304
framework aluminum, and provided more Brønsted acid sites. Guisnet et al. [41, 42]
305
had an opinion that the CH3CO* intermediate was formed and stabilized just in the
306
unique acidic of the 8-MR, which was the key to improve catalytic activity in DME
307
carbonylation.
308 309
Fig. 8. The performances of DME carbonylation over HMOR-parent, HMOR-0.1NBA,
310
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM samples. Reaction conditions: P = 1.0 MPa, T =
311
473 K, GHSV = 2400 mL/ (g · h).
312
The thermo gravimetry (TG) technique was used to analyze the coke deposition of
313
the spent catalysts. The resulting TG profiles were given in Fig. 9. The first stage of
314
the mass loss from 100 to 300 ℃ was corresponded to the loss of moisture and
315
physical adsorptions of DME and MA. The second stage from 300 to 650 ℃ was
316
attributed to the oxidation of heavy coke [43]. As shown in Fig. 9, comparing with the
317
HMOR-parent sample, the formation of heavy coke decreased curiously with the
318
addition of n-Butylamine and PAM soft templates except HMOR-0.4NBA. This
319
indicated that a suitable soft template showed fewer carbon depositions.
320 321
Fig. 9. TG profiles of HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
322
HMOR-PAM samples
323
For gaining further insights into the coke deposition, the spent catalysts after 10
324
h of dimethyl ether carbonylation reaction were dissolved in hydrofluoric acid, then
325
subjected to GC-MS analysis. Their chemical compositions were described in detail in
326
Fig. 10 and the corresponding results were shown in Table 4. The structure of
327
chromatogram was identified by the peak obtained by comparing with the NIST
328
(National Institute of Standards and Technology) database. Prior to the 20-minute
329
retention time, the species detected in all spent samples were mainly cyclenes species
330
and methylbenzene species. After the retention time of 20 min, some bulky polycyclic
331
aromatics were also detected such as naphthalene and anthracene compounds. The
332
amounts of cyclenes species and methylbenzene species confined in HMOR-parent
333
were obviously more than those in HMOR-xNBA (0.1, 0.2), which demonstrated that
334
an appropriate number of soft template could restrain coke deposition, which was also
335
consisted with TG analysis. As can be seen from Fig. 10b, the cyclenes species and
336
methylbenzene species increased obviously with prolonging the reaction time.
337 338
Fig. 10. GC-MS analysis of soluble coke retained in (a) HMOR-parent, HMOR-0.1NBA,
339
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM at TOS=10 h and (b) HMOR-0.2NBA at
340
TOS=10, 20, 30, 40 h.
341
Table 4
342
Main components in the soluble extraction from the spent catalyst Number
Name
1
1,4-dimethyl-benzene
2
1,2,3,5-Tetramethylbenzene
Structure
Formula
# CAS
C8H10
106-42-3
C10H14
527-53-7
3
3-ethylcyclopentene
C7H12
694-35-9
4
1,2,4-Trimethylbenzene
C9H12
95-63-6
5
1,5,5,6-Tetramethyl-1,3-cyclohexadiene
C10H16
514-94-3
6
1,2,3-Trimethylbenzene
C9H12
526-73-8
7
1,2,3,4-teramethyl-Benzene
C10H14
488-23-3
1,2,3,4,5-Pentamethylbenzene
C11H16
700-12-9
hexamethylbenzene
C12H18
87-85-4
1,4,5,8-Tetramethylnaphthalene
C14H16
2717-39-7
8
9 10
343
344
4. Conclusions
345
In this work, a series of hierarchical porous mordenite zeolites were prepared by
346
using n-Butylamine and Polyacrylamide as soft templates for carbonylation of
347
dimethyl ether to methyl acetate. The introduction of a suitable soft template
348
improved the porosity of the catalysts. An appropriate number of soft template
349
induced to obtain more framework Al (Alf), thus increasing the number of Brønsted
350
acid sites, which promoted conversion of DME and MA selectivity. In addition, the
351
amount of acid site in the 12-MR was reduced after the introduction of soft template,
352
which inhibited coke formation of mordenite during DME carbonylation reaction. The
353
mass transfer efficiency was improved and the coke deposition was decreased with
354
the introduction of mesopores.
355
Acknowledgements
356
We acknowledge the financial support from the Fundamental Research Funds for the
357
Central Universities (No.222201917013).
358
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of
nanocrystal℃assembled
hierarchical
mordenite
zeolites
with
A series of hierarchical porous mordenite zeolites were prepared by adding soft templates. Mordenite with mesoporous showed more framework aluminum and more Brønsted acid sites of 8-MR. The hierarchical porous mordenite increased the mass transfer efficiency and showed less acidity of 12-MR. The coke deposition was decreased with the introduction of mesopores.
Declaration of Interest Statement We believe that our work could be of interest to the readers of Microporous and Mesoporous Materials. It would be very kind, if you could consider this manuscript to be published. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal. All authors have seen the manuscript and approved to submit to your journal. Meanwhile, we believe the paper may be of particular interest to the readers of your journal as the content is original.
S1387-1811(19)30809-1
DOI:
https://doi.org/10.1016/j.micromeso.2019.109950
Reference:
MICMAT 109950
To appear in:
Microporous and Mesoporous Materials
Received Date: 12 October 2019 Revised Date:
2 December 2019
Accepted Date: 9 December 2019
Please cite this article as: H. Sheng, W. Qian, H. Zhang, P. Zhao, H. Ma, W. Ying, Synthesis of hierarchical porous H-mordenite zeolite for carbonylation of dimethyl ether, Microporous and Mesoporous Materials (2020), doi: https://doi.org/10.1016/j.micromeso.2019.109950. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
In this work, a series of hierarchical porous mordenite zeolites were prepared by using n-Butylamine and Polyacrylamide as soft templates for carbonylation of dimethyl ether to methyl acetate. The introduction of a suitable soft template improved the porosity of the catalysts. An appropriate number of soft template induced to obtain more framework Al (Alf), thus increasing the number of Brønsted acid sites, which promoted conversion of DME and MA selectivity. In addition, the amount of acid site in the 12-MR was reduced after the introduction of soft template, which inhibited coke formation of mordenite during DME carbonylation reaction. The mass transfer efficiency was improved and the coke deposition was decreased with the introduction of mesopores.
1
Synthesis of Hierarchical Porous H-Mordenite Zeolite for Carbonylation
2
of Dimethyl Ether
3
Haibing Sheng, Weixin Qian, Haitao Zhang, Peng Zhao, Hongfang Ma*, Weiyong
4
Ying
5
Engineering Research Center of Large Scale Reactor Engineering and Technology,
6
Ministry of Education, State Key Laboratory of Chemical Engineering, East China
7
University of Science and Technology, Shanghai 200237, China
8
ABSTRACT: A series of hierarchical porous mordenite zeolites were prepared by
9
adding soft template during hydrothermal process for carbonylation of dimethyl ether
10
(DME) to methyl acetate (MA). The synthesized mordenite catalysts were
11
systematically characterized by XRD, BET, ICP-AES, NH3-TPD, Py-IR, FTIR,
12
HRTEM, SEM, TG, 27Al NMR,
13
confirmed that mordenite zeolites with mesoporous structure showed more framework
14
aluminum and more Brønsted acid sites of 8-membered ring (8-MR), which promoted
15
conversion of DME and MA selectivity. The hierarchical porous mordenite had
16
increased the mass transfer efficiency and showed less acidity of 12-membered ring
17
(12-MR), thus suppressing the formation of coke. Measurements of changes in the
18
chemical composition of the coke by GC-MS showed the growth mechanism of coke
19
molecules in the mordenite zeolites.
20
Keywords: hierarchical mordenite, soft template, carbonylation, dimethyl ether,
21
methyl acetate
22
29
Si NMR and GC-MS. The characterization results
23
1. Introduction
24
As an important chemical raw material, ethanol has received much attention as
25
sustainable and environmental advantages. In particular, ethanol is a good choice as a
26
promising alternative to fossil fuels [1]. Comparing with the traditional ethanol
27
synthesis route by ethylene hydration and biomass fermentation, the synthesis of
28
ethanol by dimethyl ether (DME) carbonylation to methyl acetate (MA) and MA
29
hydrogenation has drawn much attention recently because of its high atom economy,
30
MA selectivity and environmental friendliness [2, 3]. As the former studies on this
31
new route, the DME carbonylation is the critical process [4, 5]. Several zeolites such
32
as mordenite (MOR) [6], ferrierite (FER) [7] and heteropoly acids (HPAS) [8] were
33
used for DME carbonylation. Compared with the FER and solid acid catalysts, the
34
MOR catalysts showed better catalytic activity in carbonylation reaction because of
35
its unique crystal structure and acid stability.
36
Previous studies had shown that Brønsted acid sites were mainly the active site of
37
DME carbonylation reaction [9, 10]. The extent of dealumination by thermal
38
treatment changed the Si/Al ratio of the framework and increased the number of
39
Brønsted acid sites [11]. The mechanisms of zeolites dealumination had been studied
40
by density functional theory (DFT) calculations [12]. Wang group had demonstrated
41
that the amount of Brønsted acid sites in the 8-MR had increased by changing the
42
composition of Si and Al and adding different structure-direction agent [13]. However,
43
the 12-MR channels showed low selectivity to methyl acetate and highly favored the
44
formation of hydrocarbons [14]. Therefore, it was an effective way to promote the
45
stability of the catalyst by suppressing hydrocarbon formation in 12-MR channels.
46
The 12-MR pores could be poisoned by pre-adsorption of pyridine, which prolonged
47
the catalytic lifetime of HMOR [15, 16]. High temperature steam treatment could
48
remove the framework Al species in 12-MR, thereby improving the stability of
49
HMOR [17]. Besides, introduction of metal ions by ion exchange could also improve
50
the amount of Brønsted acid sites [18, 19].
51
Hierarchical Porous catalysts showed excellent properties in their higher
52
diffusivity of reactants and products. Maleki et al. [20, 21] had reported that the
53
preparation of hierarchical nanocatalyst by ultrasound irradiation is a particularly
54
novel method. Mesoporous mordenite had been obtained by acid or alkaline treatment,
55
which improved the properties of texture and exhibited higher acidity of the catalysts
56
[22, 23]. Svelle et al. [24] studied that the mesopore formation by desilication had
57
become a simple way to influence the concentration of Brønsted acid sites. Wang et al.
58
[25] reported that the hierarchical structures were introduced into the mordenite by
59
using Polyethylene glycol (PEG) as template, which increased the number of strong
60
acid sites and limited the coke depositions. Li et al. [26] had synthesized mesoporous
61
mordenite using [3-(trimethoxysilyl)propyl][hexadecyl dimethyl ammonium] chloride
62
(TPHAC) as soft template, which showed a large number of Brønsted acid sites and
63
had a good catalytic performance. However, it is ambiguous that the relationship
64
between Brønsted and Lewis acid sites by introducing mesopores into HMOR catalyst.
65
The mechanism of coke molecules growth have rarely been reported in dimethyl ether
66
carbonylation to methyl acetate.
67
In this work, the hierarchical porous mordenites were obtained by addition soft
68
templates during the process of synthesis. The purpose of this work was to increase
69
the activity and stability of mordenite in dimethyl ether carbonylation. Besides, the
70
acidity and coke deposition of mordenite were also studied. Characterization of the
71
catalysts was performed by BET, XRD, ICP-AES, Py-IR, HRTEM, SEM, NH3-TPD,
72
27
73
for the industrialization of dimethyl ether to ethanol.
74
2. Experimental section
75
2.1. Catalyst preparation
Al NMR, FTIR, TG, 29Si NMR and GC-MS. This work provided a theoretical basis
76
The hierarchical porous mordenite zeolites were synthesized by the hydrothermal
77
method using n-Butylamine (Titan, 99.5%) and Polyacrylamide (PAM) (Adamas,
78
nonionic, MW: 8 million) as soft templates. In a typical procedure, 0.98 g of sodium
79
aluminate (NaAlO2) and a certain amount of sodium hydroxide (1.58 g) were
80
dissolved in 50 ml of deionized water. Then, 30.48 g of silica sol (30 wt % SiO2) was
81
added to the above solution and stirred at room temperature for 2 h. Next, the soft
82
template of n-Butylamine or PAM was added to the mixture. The mixture was sealed
83
in a Teflon-lined autoclave and crystallized at 443 K for 4 days. The slurry was
84
collected by filtration, dried at 383 K for 12 h, and finally calcined at 823 K for 4 h.
85
The resulting sample was Na-MOR. The mole composition of the catalyst was
86
1.0SiO2: 0.04Al2O3: 0.2Na2O: 26H2O. The HMOR catalyst was derived from
87
Na-MOR upon ion-exchanged with 1 M ammonium nitrate (NH4NO3). 5.0 g calcined
88
Na-MOR powder was dispersed into 50 ml of 1 M NH4NO3. The mixture was
89
refluxed at 353 K for 3 h. Then, the solid was obtained by filtration, dried at 383 K for
90
12 h, and finally calcined at 823 K for 4 h. The mass ratio of PAM/SiO2 is 0.87,
91
denoting as HMOR-PAM. The prepared catalysts of adding n-Butylamine were
92
designated as HMOR-xNBA, where x was defined the mole weight of n-Butylamine
93
(0.1, 0.2, 0.4). For comparison, the resulting sample without addition of soft template
94
was named HMOR-parent.
95
2.2. Catalyst characterization
96
The Ar adsorption-desorption isotherms was carried out on Micrometrics ASAP
97
2020 device. The specific surface area was calculated by using BET method, and the
98
pore volume and pore size were estimated by BJH method. The Powdered X-ray
99
diffraction (XRD) profiles of the catalysts were performed on a D/Max-2550VB/PC
100 101 102
with Cu Kα (λ= 0.15416) radiation over the 2-theta range of 10-80°. The actual contents of Si and Al in catalysts were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES).
103
HRTEM images were recorded using an EOL2010 microscope operating at 300
104
kV. The sample powder was dispersed in absolute ethanol, and then dried under
105
infrared lamp. SEM was performed to observe the crystallite size and morphology
106
carrying out on a Philips Fei Quanta 200F microscope.
107
NH3-TPD experiments were performed on a Micromerities Autochem II 2920
108
chemisorption apparatus. Before the TPD measurements, 0.2 g of sample was
109
preprocessed in flowing He (35 ml/min) at 773 K for 1 h, and later cooled to 323 K.
110
Then the catalyst was exposed to 10 % NH3-He mixture for 30 min until saturation.
111
TPD analysis was recorded from 373 to 1073 K with 10 °C min-1 heating rate, and the
112
quantity of NH3 desorbed was detected by a thermal conductivity detector (TCD).
113
The
27
Al and
29
Si MAS NMR experiments were conducted on a Bruker AV-500
114
spectrometer at at 156.39 MHz and 99.33 MHz, respectively. The 27Al chemical shifts
115
were referenced to Al(NO3)3 and the 29Si chemical shift to tetramethylsilane (TMS).
116 117
Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor 27 spectrometer with a diffuse reflectance attachment and with a resolution of 4 cm-1.
118
Thermogravimetric (TG) studies of the spent samples were initially heated from
119
100 ℃ to 850 ℃ under an air flow rate of 30 ml/min. A STA 409 PC thermal analyzer
120
were used for recording the number of coke depositions on the spent mordenite.
121
GC-mass spectroscopy (GC-MS) analyses were performed on an Agilent 6890
122
series gas chromatograph with a FID detector. After dissolving the spent catalyst by
123
33 vol% hydrofluoric acid solution, the residue was extracted by CH2Cl2 at room
124
temperature. The oven temperature was increased from 60 ℃ to 220 ℃ at 5 ℃/min,
125
and held isothermally for 5 min. The carry gas was helium at flow rate of 70 ml/min.
126
2.3. DME carbonylation
127
Catalytic activities were evaluated using continuous flow fixed-bed reactor with
128
an internal diameter of 10 mm. 0.5 mg of the catalyst (40-60 mesh) was packed into
129
the reactor, employing a thermocouple to control the temperature. Before each
130
experiment, the catalyst was pretreated under a flow of N2 (20 ml/min) for 2 h at 593
131
K. After cooling to 473 K, the reactant mixture (10% DME, 50% CO, 40% N2,
132
mol/mol) was introduced into the reactor with 20 ml/min flow rate at 1 MPa. The
133
reaction products were vaporized by heating at 120 ℃. The outlet gas was analyzed
134
online using a gas chromatograph (Agilent 7890 A).
135
136
2.4. Catalyst regeneration
137
Catalyst regeneration consisted in oxidizing the coke deposited onto the catalyst in
138
air flow for 2 h at 823 K. The reaction over the regenerated catalyst exhibited similar
139
conversion to that of the fresh catalyst (Fig. S1). This result proves that the main
140
reason for the deactivation of the catalyst is carbon deposits, and the removal of
141
carbon deposits can restore the catalyst's activity without loss of active sites.
142
3. Results and discussion
143
3.1. Porosity and morphology of catalysts
144
The textural and morphological properties of catalysts were exhibited in Fig. 1.
145
The physicochemical properties of the catalysts were showed in Table 1. All the
146
samples exhibited Langmuir type I isotherms with H4-type hysteresis loop, indicating
147
the hierarchical porous texture. As can be seen from the pore diameter distribution,
148
the mesopores were introduced inside the catalysts by addition of n-Butylamine and
149
PAM soft templates during the synthesis process. From HMOR-parent to
150
HMOR-0.2NBA, the pore size increased from 2.4 to 3.3 nm, the surface area
151
increased from 269 to 289 m2/g, but the micropore volume decreased from 0.15 to 0.1
152
cm3/g and the external area increased from 38 to 66 m2/g. The HMOR-PAM catalyst
153
exhibited greater surface area, pore size and more mesopores comparing with
154
HMOR-parent. However, the HMOR-0.4NBA could not be developing more
155
mesopores, because excessive soft template blocked the formation of mordenite
156
crystal, which was in accordance with the result of XRD.
157 158
Fig. 1. Argon adsorption and desorption isotherm (a) and pore size distribution (b) of the samples.
159
Table 1
160
Textural properties of HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
161
HMOR-PAM samples. SBET
Sext
Dpore
VMa
VTb
(m2/g)
(m2/g)
(nm)
(cm3/g)
(cm3/g)
HMOR-parent
269
38
2.4
0.18
0.21
HMOR-0.1NBA
276
47
2.8
0.11
0.23
HMOR-0.2NBA
289
66
3.3
0.07
0.25
HMOR-0.4NBA
257
46
2.5
0.13
0.18
HMOR-PAM
296
58
3.2
0.10
0.24
Sample
162
a
Micropore volume.
163
b
Total pore volume.
164
The XRD patterns of the mordenite zeolite catalysts after calcination were showed
165
in Fig. 2. As can be seen, all catalysts revealed the presence of mordenite XRD
166
patterns, indicating that the original HMOR framework was not changed with the
167
addition of n-Butylamine and PAM soft templates. The diffraction peaks at 2θ = 20.5°,
168
22.3°, 25.8°, 31.0° are typical of HMOR crystalline phase [27]. Additionally, as can
169
be seen from Fig. 2, the intensities of their diffraction peaks exhibited relativity good
170
crystallinity comparing with HMOR-parent except HMOR-0.4NBA, The crystallinity
171
of the HMOR-parent was calculated from the relative intensities of four characteristic
172
HMOR-parent peaks appearing at around 2θ = 20.5°, 22.3°, 25.8°and 31.0° by
173
assuming 100% crystallinity. The crystallinity of HMOR-0.4NBA and HMOR-PAM
174
were 120 % and 112 %, respectively. However, the crystallinity of the
175
HMOR-0.4NBA was decreased from 100% on the HMOR-parent to 73%, meaning
176
that the appropriate amount of soft template increased the crystallinity of the catalyst.
177
[28].
178 179
Fig. 2. XRD patterns of the samples.
180
The microstructure of the hierarchical mordenite catalysts were examined by SEM
181
and HRTEM. The shape of HMOR-parent was like pillars-assembled structure. After
182
the addition of 0.2 mole n-Butylamine, the shape of the particle was not affected.
183
However, as can be seen from Fig. 3d and Fig. 3f, a crystalline material having
184
mesopores were obtained. Moreover, the crystal shape of HMOR-PAM differed from
185
HMOR-parent, and the particle size is smaller. Compared with the HMOR-parent
186
sample, HMOR-0.2NBA and HMOR-PAM catalyst showed obvious porosity and
187
roughness. A few bright spots appeared in the HRTEM images of HMOR-0.2NBA
188
and HMOR-PAM (Fig3. d and f), corresponding to holes in the crystals, which could
189
be mesopores. This indicated that an appropriate amount of soft template helped for
190
obtaining mesoporous mordenite.
191
192
193 194
Fig. 3. SEM and HRTEM images of HMOR-parent (a, b), HMOR-0.2NBA (c, d) and
195
HMOR-PAM (e, f) samples.
196
Fig. 4 showed the
27
Al MAS NMR spectra of samples, which was an important
197
experimental technique to insight about the coordination of aluminum sites in
198
mordenite zeolites. The area ratio of 54/0 ppm was put on the side. The signal at
199
around 54 ppm corresponded to the tetrahedrally framework Al (Alf) and the weak
200
peak at around 0 ppm belonged to the octahedrally corrdinated extra-framework (Alef)
201
[29, 30]. Reule et al. [31] had reported that the number of Lewis acidity was
202
determined by the number of Alef, while Alf was associated with Brønsted acid sites.
203
In Fig. 4, the relative intensity of the peak at 54 ppm increased apparently with the
204
addition of n-Butylamine and PAM soft templates except HMOR-0.4NBA, which
205
indicated that an appropriate number of soft template induced parent mordenite to
206
obtain more Brønsted acid sites.
207 208 209
Fig. 4. 27Al MAS NMR spectra of samples.
To further uncover the Si and Al atomic coordination environments, the
210
experiments of
211
peaks centered at -96, -105 and -112 ppm were evident in the 29Si MAS NMR spectra,
212
which corresponded to Si (2Al), Si (1Al) and Si (0Al) respectively [32]. As shown in
213
Fig S2, it can be found that the proportion of Si (1Al) increased obviously, while the
214
proportion of Si (2Al) decreased with the addition of soft template except
215
HMOR-0.4NBA. This indicated that a suitable amount of soft template induced to
216
more AlO4- tetrahedral, increasing crystallinity, which was consistent with XRD
217
studies [33].
218
3.2.Acidic properties of the catalysts
29
Si MAS NMR spectra were recorded (Fig S2, ESI). Three major
219
The NH3-TPD patterns of HMOR-parent, HMOR-xNBA (0.1, 0.2, 0.4),
220
HMOR-PAM catalysts were displayed in Fig. 5. The quantitative estimation of acid
221
strength distribution at different regions were summarized in Table 2. On the HMOR
222
samples, two typical ammonia desorption peaks were observed at 150 and 500 ℃,
223
respectively, which were weak acids and strong acids. The peak at low temperature
224
(about 150 ℃) of NH3 adsorption could be due to physically adsorbed or
225
hydrogen-bonded NH3 [13]. The high temperature (about 500 ℃) of NH3 adsorption
226
could be ascribed to the Brønsted acid sites of framework Al atoms [34, 17].
227
Comparing with the HMOR-parent sample, the strong acid sites slightly increased by
228
the addition of n-Butylamine and PAM soft templates except HMOR-0.4NBA. This
229
indicated that an appropriate soft template induced the atoms of Al to enter into the
230
position of the strong acid sites. However, excessive n-Butylamine soft template
231
partially destroyed the structure of the mordenite, prevented the formation of more
232
framework aluminum, and reduced the Brønsted acid sites, which was well correlated
233
with the characterizations of XRD and 27Al NMR.
234
It had been reported that less Si/Al showed more Brønsted acid sites [26]. From
235
Table 2 we can see that the catalysts with adding soft templates had a slightly lower
236
Si/Al comparing with HMOR-parent except HMOR-0.4NBA. Besides, an appropriate
237
soft templates would lead to more framework (Alf). As a result, according to the
238
percentage of framework Al, the number of Brønsted acid sites could be ordered as
239
follow:
240
HMOR-0.2NBA >HMOR-PAM >HOR-0.1NBA >HMOR-parent >HMOR-0.4NBA,
241
which was in line with the results of FTIR.
242 243
Fig. 5. NH3-TPD patterns of HMOR-parent, HMOR-xNBA, HMOR-PAM samples.
244
Table 2
245
The amount of acid sites of HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
246
HMOR-PAM samples. NH3-uptake (µmol/g)a
Molar
Alf (%)c
Alef (%)c
Sample Weak
Strong
(Si/Al)b
HMOR-parent
106.2
124.5
11.8
75.2
24.8
HMOR-0.1NBA
112.4
138.7
11.3
82.7
17.3
HMOR-0.2NBA
114.5
147.8
10.7
87.6
12.4
HMOR-0.4NBA
94.1
114..6
13.6
62.4
37.6
HMOR-PAM
108.6
154.4
10.6
85.3
14.7
247
a
Determined by NH3-TPD analysis.
248
b
Determined by ICP-AES analysis.
249
c
Determined by 27Al MAS NMR (Area percentage).
250
Fig. 6 showed the O-H stretching region of FTIR spectra of HMOR-parent,
251
HMOR-xNBA, HMOR-PAM. The bands at around ~3660 cm-1 were assigned to the
252
stretching vibration of extra-framework Al atoms, while the bands at ~3734 cm-1
253
corresponded to the terminal silanol group [35, 36]. The bands at around ~3606 and
254
~3545 cm-1 were ascribed to the Brønsted acid sites of O-H stretching vibration in the
255
8-MR and 12-MR channels, respectively [37]. The amount of acid sites for
256
HMOR-parent, HMOR-xNBA (0.1, 0.2, 0.4) and HMOR-PAM were shown in Table 3.
257
Compared with HMOR-parent of FTIR spectroscopy, the adsorption peak at ~3545
258
cm-1 increased remarkably by the addition of n-Butylamine and PAM soft templates,
259
because the soft template leaded to more aluminum in the framework, which was
260
consistent with the result of 27Al NMR. However, in the case of excessive addition of
261
n-Butylamine, the adsorption peak at ~3545 cm-1 of HMOR-0.4NBA showed a
262
downward trend due to the agglomeration of n-Butylamine during the synthesis
263
process, thus blocking the formation of framework Al and extra-framework Al.
264
Py-IR adsorption was recorded to probe the Brønsted and Lewis acid sites in 12-MR
265
(Fig. 7). The two bands at 1450 and 1540 cm-1 were ascribed to the C-H deformation
266
vibrations of pyridine adsorbed on Brønsted and Lewis acid sites, as reported in
267
literature [38, 39]. The band at 1490 cm-1 was usually assigned to the combinations of
268
pyridine with both Brønsted and Lewis acid sites [40]. The bands at around
269
1400-1700 cm-1 represented the adsorption of pyridine on HMOR-parent,
270
HMOR-xNBA (0.1, 0.2, 0.4) and HMOR-PAM samples. Table 3 showed the number
271
of Brønsted and Lewis acid around 1540 and 1450 cm-1, respectively. After soft
272
templates loading, the acidity of the 12-MR was reduced, and the possible reason was
273
that the molecular diameters of NBA (0.43 nm) and PAM (hydrolysate) are closer to
274
12-MR, so it was easier to enter in 12-MR during hydrothermal process, thus
275
preventing the formation of 12-MR framework Al and extra-framework Al.
276
Furthermore, with excess addition of n-Butylamine, the structure of HMOR-0.4NBA
277
was destroyed, and there was almost no Brønsted and Lewis acidity of 12-MR. The
278
turnover frequency (TOF) of DME carbonylation on the HMOR-parent sample was
279
estimated to be 1.0 h-1, while it was slightly increased to 1.2 h-1 on the
280
HMOR-0.1NBA catalyst. The TOF over HMOR-PAM was 1.5 h-1 (Table 3). These
281
results are clear evidence that the acidic sites in the 8-MR pores are the active centers
282
for DME carbonylation.
283
284 285
Fig. 6. FTIR spectra in the O-H stretching region of HMOR-parent, HMOR-0.1NBA,
286
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM samples.
287 288
Fig. 7. Py-IR spectra of the HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
289
HMOR-PAM catalysts.
290
Table 3
291
The distribution acid sites and DME carbonylation TOF of HMOR-parent, HMOR-0.1NBA,
292
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM catalysts. B acid of 12MRa
L acid of 12MRa
B acid of 8MRb
Total B acid
Carbonylation
（µmol/g）
（µmol/g）
（µmol/g）
（µmol/g）
TOF (h-1)
HMOR-parent
44.2
147.3
134.1
178.3
1.0
HMOR-0.1NBA
32.4
112.5
144.2
176.6
1.2
HMOR-0.2NBA
21.1
104.3
175.2
196.3
1.6
HMOR-0.4NBA
5.4
68.78
82.3
87.7
0.7
HMOR-PAM
18.6
94.3
169.8
188.4
1.5
Sample
293
a
Determined from the adsorption of pyridine.
294
b
Determined from the infrared spectra of the O-H stretching region.
295
3.3. Catalytic tests and carbon deposition analysis.
296
Fig. 8 shows a set of results for the carbonylation of DME over HMOR-parent,
297
HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM samples. The
298
DME conversion and MA selectivity of HMOR-xNBA (0.1, 0.2, 0.4) catalysts
299
initially increased with the increasing of the n-Butylamine mole, then began to decline
300
with further increased the mole of n-Butylamine. The reason was that too much
301
n-Butylamine soft template affected the formation of the framework Al and reduced
302
the number of Brønsted acid site. The best activity over HMOR-0.2NBA and
303
HMOR-PAM were attributed that a suitable amount of soft template induced more
304
framework aluminum, and provided more Brønsted acid sites. Guisnet et al. [41, 42]
305
had an opinion that the CH3CO* intermediate was formed and stabilized just in the
306
unique acidic of the 8-MR, which was the key to improve catalytic activity in DME
307
carbonylation.
308 309
Fig. 8. The performances of DME carbonylation over HMOR-parent, HMOR-0.1NBA,
310
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM samples. Reaction conditions: P = 1.0 MPa, T =
311
473 K, GHSV = 2400 mL/ (g · h).
312
The thermo gravimetry (TG) technique was used to analyze the coke deposition of
313
the spent catalysts. The resulting TG profiles were given in Fig. 9. The first stage of
314
the mass loss from 100 to 300 ℃ was corresponded to the loss of moisture and
315
physical adsorptions of DME and MA. The second stage from 300 to 650 ℃ was
316
attributed to the oxidation of heavy coke [43]. As shown in Fig. 9, comparing with the
317
HMOR-parent sample, the formation of heavy coke decreased curiously with the
318
addition of n-Butylamine and PAM soft templates except HMOR-0.4NBA. This
319
indicated that a suitable soft template showed fewer carbon depositions.
320 321
Fig. 9. TG profiles of HMOR-parent, HMOR-0.1NBA, HMOR-0.2NBA, HMOR-0.4NBA,
322
HMOR-PAM samples
323
For gaining further insights into the coke deposition, the spent catalysts after 10
324
h of dimethyl ether carbonylation reaction were dissolved in hydrofluoric acid, then
325
subjected to GC-MS analysis. Their chemical compositions were described in detail in
326
Fig. 10 and the corresponding results were shown in Table 4. The structure of
327
chromatogram was identified by the peak obtained by comparing with the NIST
328
(National Institute of Standards and Technology) database. Prior to the 20-minute
329
retention time, the species detected in all spent samples were mainly cyclenes species
330
and methylbenzene species. After the retention time of 20 min, some bulky polycyclic
331
aromatics were also detected such as naphthalene and anthracene compounds. The
332
amounts of cyclenes species and methylbenzene species confined in HMOR-parent
333
were obviously more than those in HMOR-xNBA (0.1, 0.2), which demonstrated that
334
an appropriate number of soft template could restrain coke deposition, which was also
335
consisted with TG analysis. As can be seen from Fig. 10b, the cyclenes species and
336
methylbenzene species increased obviously with prolonging the reaction time.
337 338
Fig. 10. GC-MS analysis of soluble coke retained in (a) HMOR-parent, HMOR-0.1NBA,
339
HMOR-0.2NBA, HMOR-0.4NBA, HMOR-PAM at TOS=10 h and (b) HMOR-0.2NBA at
340
TOS=10, 20, 30, 40 h.
341
Table 4
342
Main components in the soluble extraction from the spent catalyst Number
Name
1
1,4-dimethyl-benzene
2
1,2,3,5-Tetramethylbenzene
Structure
Formula
# CAS
C8H10
106-42-3
C10H14
527-53-7
3
3-ethylcyclopentene
C7H12
694-35-9
4
1,2,4-Trimethylbenzene
C9H12
95-63-6
5
1,5,5,6-Tetramethyl-1,3-cyclohexadiene
C10H16
514-94-3
6
1,2,3-Trimethylbenzene
C9H12
526-73-8
7
1,2,3,4-teramethyl-Benzene
C10H14
488-23-3
1,2,3,4,5-Pentamethylbenzene
C11H16
700-12-9
hexamethylbenzene
C12H18
87-85-4
1,4,5,8-Tetramethylnaphthalene
C14H16
2717-39-7
8
9 10
343
344
4. Conclusions
345
In this work, a series of hierarchical porous mordenite zeolites were prepared by
346
using n-Butylamine and Polyacrylamide as soft templates for carbonylation of
347
dimethyl ether to methyl acetate. The introduction of a suitable soft template
348
improved the porosity of the catalysts. An appropriate number of soft template
349
induced to obtain more framework Al (Alf), thus increasing the number of Brønsted
350
acid sites, which promoted conversion of DME and MA selectivity. In addition, the
351
amount of acid site in the 12-MR was reduced after the introduction of soft template,
352
which inhibited coke formation of mordenite during DME carbonylation reaction. The
353
mass transfer efficiency was improved and the coke deposition was decreased with
354
the introduction of mesopores.
355
Acknowledgements
356
We acknowledge the financial support from the Fundamental Research Funds for the
357
Central Universities (No.222201917013).
358
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359
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A series of hierarchical porous mordenite zeolites were prepared by adding soft templates. Mordenite with mesoporous showed more framework aluminum and more Brønsted acid sites of 8-MR. The hierarchical porous mordenite increased the mass transfer efficiency and showed less acidity of 12-MR. The coke deposition was decreased with the introduction of mesopores.
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