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Fabrication of highly (110)-Oriented ZIF-8 membrane at low temperature usingnanosheet seed layer
Chenhan Zhang, Jiahui Yan, Taotao Ji, Dongying Du, Yanwei Sun, Liangliang Liu,Xiongfu Zhang, Yi Liu
PII: S0376-7388(21)00858-9
DOI: https://doi.org/10.1016/j.memsci.2021.119915
Reference: MEMSCI 119915
To appear in: Journal of Membrane Science
Received Date: 10 August 2021
Revised Date: 17 September 2021
Accepted Date: 22 September 2021
Please cite this article as: C. Zhang, J. Yan, T. Ji, D. Du, Y. Sun, L. Liu, X. Zhang, Y. Liu, Fabricationof highly (110)-Oriented ZIF-8 membrane at low temperature using nanosheet seed layer, Journal ofMembrane Science (2021), doi: https://doi.org/10.1016/j.memsci.2021.119915.
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© 2021 Published by Elsevier B.V.
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Author Statement
Chenhan Zhang: completed the main experiments and the analysis of relevant
experimental results. Jiahui Yan: assisted in drawing the schematic illustration and the
test of gas separation performance. Taotao Ji and Dongying Du: assisted in drawing
the crystal structure of ZIF-8. Yanwei Sun: assisted in the test and analysis of gas
separation performance. Liangliang Liu: assisted in morphological characterization
and the analysis of relevant experimental results. Xiongfu Zhang: helped with the
analysis of gas permeation results. Yi Liu: proposed the major idea, projected relevant
experiments, and jointly wrote the manuscript.
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Fabrication of Highly (110)-Oriented ZIF-8 Membrane at Low 1
Temperature Using Nanosheet Seed Layer 2
Chenhan Zhang a, Jiahui Yan a, Taotao Ji a, Dongying Du b, Yanwei Sun a, Liangliang Liu a, Xiongfu 3 Zhang a, Yi Liu a,* 4
a State Key Laboratory of Fine Chemicals, PSU-DUT Joint Centre for Energy Research, School of Chemical Engineering, 5 Dalian University of Technology, Ganjingzi District, Dalian, 116024, China 6 b Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, 130024, 7 China 8
Abstract 9
Preferred orientation represents a main concern for microstructure control and performance 10
optimization of MOF membranes. To date, it remains a challenging task to precisely control 11
preferred orientation of ZIF-8 membrane. Herein, we prepared highly (110)-oriented ZIF-8 12
membrane by oriented secondary growth. Among various elements, preparation of uniform ZIF-8 13
nanosheet seeds through thermal conversion of identically shaped ZIF-L precursors, spin-coating-14
assisted oriented deposition of (110)-oriented ZIF-8 seed layer, and controlled in-plane secondary 15
growth at low temperature were found crucial for obtaining well-intergrown (110)-oriented ZIF-8 16
membrane whose H2/CO2 ideal selectivity exceeded H2/N2 and H2/CH4 under ambient conditions, 17
owing to preferential CO2 adsorption capacity of not only PEI but also ZIF-8 nanosheets compared 18
with their bulk counterparts. Our research highlighted the significance of preferred orientation 19
regulation in tailoring the gas permeation behavior of MOF membranes. 20
Keywords: metal-organic framework, ZIF-8, nanosheets, gas separation, membranes 21
1. Introduction 22
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Membrane-based technology has offered unexampled opportunities for efficient separation [1, 2] 23
considering its advantages over other separation technologies like low energy consumption, nearly-24
zero pollutant discharge, low capital cost, small footprint, and easy operation. Metal-organic 25
framework (MOF) [3-6], as a burgeoning class of hybrid material comprising metal ions/clusters 26
bridged by organic linkers, has been considered as ideal membrane candidates for separation because 27
of its adjustable composition and structure, tailorable pore size, and diverse functionality. Among 28
them, ZIF-8 membrane [7, 8] has aroused widespread attention for the potential applications in 29
hydrogen purification and olefin/paraffin separation owing to its appropriate pore size, exceptional 30
chemical stability, thermal stability, and relative ease of fabrication. 31
In recent decades, noticeable development has been made in the preparation of high quality ZIF-8 32
membranes enabling efficient gas separation. For instance, Xing and Pan et al. prepared PDMS 33
layer-coated ZIF-8 membranes on commercial ceramic tubes to realize efficient C3H6/C3H8 34
separation under high pressure [9]. The PDMS layer effectively blocked the inter-crystalline defects 35
and suppressed lattice flexibility of the ZIF-8 membrane, resulting in significantly improved 36
separation factor for C3H6/C3H8 gas mixture at the expense of a slight loss in C3H6 permeance. Yao et 37
al. prepared ZIF-8@CNF composite membranes with excellent CO2/N2 and CO2/CH4 selectivity 38
through combining in-situ growth with vacuum filtration [10]. He et al. combined ZIF-8@GO flake-39
like composite fillers with the ultrasound-assisted pre-Zn(II)-doping strategy to prepare Pebax-based 40
MMMs showing concurrently enhanced CO2 permeability and CO2/N2 selectivity [11]. Wang et al. 41
obtained alumina hollow fiber-supported ZIF-8 membrane exhibiting superior H2/N2 selectivity by 42
secondary growth. Maintaining appropriate sodium formate content and using thick-walled Teflon-43
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liner were found crucial in warranting the well-intergrowth of relatively thin ZIF-8 membranes [12]. 44
Wang et al. proposed an in-situ current-driven synthesis approach [13, 14] to prepare ZIF-8 45
membrane with suppressed lattice flexibility and enhanced C3H6/C3H8 selectivity. Agrawal et al. 46
developed a rapid heat treatment (RHT) strategy [15] to increase the lattice stiffness of ZIF-8 47
membrane which exhibited remarkably enhanced selectivity towards CO2/CH4 and CO2/N2 gas pairs. 48
Jeong et al. prepared ZIF-7-8 membrane via corresponding mixed ligands [16], which led to effective 49
pore aperture reduction and separation performance improvement. Tsapatsis et al. [17] proposed a 50
vapor-phase-ligand-treatment (VPLT) strategy to partially replace 2-methylimidazole (Hmim) 51
ligands with 2-amino-benzimidazole (2abIm), which concurrently reduced the effective pore aperture 52
and facilitated the preferential adsorption of CO2, resulting in improved CO2/N2 and 53
CO2/CH4 selectivity. 54
It is worth noting, however, majority of previous studies mainly focused on tailoring structural 55
properties (e.g., framework flexibility) of ZIF-8 membranes on a microscopic scale. In contrast, 56
precise microstructural control (like preferred orientation) of ZIF-8 membranes remains largely 57
unexplored to date except Caro’s early work on the preparation of preferentially (100)-oriented ZIF-8 58
membrane [18]. More recently, Eddaoudi et al. reported the preparation of preferentially (110)-59
oriented, ultrathin ZIF-8 membrane showing unprecedented and unique CH4–n–C4H10 mixture 60
permeation behavior via the liquid-phase epitaxy (LPE) method [19]. In the same manner, Valadez 61
Sánchez et al. prepared monolithic (110)-oriented ZIF-8 SURMOF membrane support on Au thin 62
layer-modified porous α-Al2O3 substrate via LPE layer-by-layer deposition. Prepared ZIF-8 63
membrane showed certain selectivity towards ethene/ethane gas mixture [20]. The preferred 64
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orientation of molecular sieve membranes has been proven to exert profound effects on their 65
separation performances due to ordered alignment of pore channels, decrease of intercrystalline 66
defects and therefore, considerable reduction in diffusion resistance [4, 21-24]. Therefore, it is 67
meaningful to prepare highly oriented ZIF-8 membranes to achieve unique permeation properties 68
(Fig. S1). Compared with in situ growth protocol [25, 26], secondary growth enabled more accurate 69
regulation over preferred orientation [22, 27, 28] due to the effective separation of nucleation and 70
epitaxial growth processes. 71
Among various factors affecting microstructure and functionality of MOF membranes, MOF seeds 72
have proven to be especially important. So far as preferred orientation control is concerned, uniform 73
MOF nanosheets (NSs) are considered as ideal seed candidates because of the ease of deposition, 74
better thickness control, and versatile functionality. Taking into account of potential benefits derived 75
from MOF NS seeds, in this study, we prepared highly (110)-oriented ZIF-8 membrane using 76
uniform ZIF-8 NS seeds derived from identically shaped ZIF-L precursors [29-31], which not only 77
facilitated the deposition of oriented seed layers, but also led to enhanced CO2 capacity compared 78
with their bulk counterparts. Moreover, carrying out secondary growth at low temperature was 79
confirmed crucial for the elimination of grain boundary space while maintaining the optimized 80
orientation inherited from ZIF-8 seed layer (Fig. 1). As far as we know, this work represented the 81
first report on the fabrication of highly (110)-oriented ZIF-8 membrane via secondary growth method. 82
Gas separation results showed that H2/CO2 ideal selectivity of obtained ZIF-8 membrane exceeded 83
H2/N2 and H2/CH4 gas pairs under ambient conditions, which has been rarely reported in previous 84
studies. 85
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86
Fig. 1. Schematic illustration of preparing highly (110)-oriented ZIF-8 membrane. 87
2. Experimental Section 88
2.1. Reagents and materials 89
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%, Xiya Reagent), Zinc acetate (Zn(CH3COO)2, AR, 90
Macklin), 2-methylimidazole (Hmim, 98%, Macklin), N,N-dimethylformamide (DMF, 99.8%, 91
Kemiou), methanol (CH4O, 99.5%, Macklin), ethanol (C2H6O, 99.7%, Tianjin Kemiou), and 92
ethylene imine polymer (PEI, Mw = 1800, 99%, Aladdin) were used as received without further 93
purification. α-Al2O3 substrates were obtained from Fraunhofer IKTS, Germany. 94
2.2. Preparation of leaf-shaped ZIF-L NSs 95
Uniform leaf-shaped ZIF-L NSs were synthesized as reported in the literature [32]. Initially, 1.30 g 96
Hmim and 0.59 g Zn(NO3)2·6H2O were dissolved in 40 mL deionized water via continuous stirring. 97
Subsequently, above homogeneous aqueous solutions were blended under stirring and then stirred 98
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under ambient conditions for 4 h, which led to the formation of white precipitates. The above 99
precipitates (i.e., ZIF-L NSs) were collected via repeated centrifugation (6000 rpm, 5 min), washed 100
with DI water for several times, and dried overnight before further use. 101
2.3. Synthesis of ZIF-8 NSs 102
ZIF-8 NSs with leaf-like morphology were prepared as reported in the literature with slight 103
modification [30, 31]. Initially, 150.0 mg of leaf-shaped ZIF-L crystals were uniformly dispersed in 104
144 ml DMF and 48 ml ethanol and then sonicated for 5 min. Subsequently, the above solution was 105
subjected to solvothermal treatment at 70 °C for 30 h. The above precipitates (i.e., ZIF-8 NSs) were 106
isolated via repeated centrifugation (6000 rpm, 5 min), washed with methanol, and finally dried 107
overnight before further use. 108
2.4. Synthesis of ZIF-8 microcrystals and nanocrystals 109
ZIF-8 microcrystals and nanocrystals were prepared as reported in the literature with slight 110
modification [33, 34]. For the synthesis of ZIF-8 microcrystals, initially, 1.58 g Hmim and 1.05 g 111
Zn(CH3COO)2 were dissolved in 120 mL methanol, respectively. Subsequently, above homogeneous 112
solutions were blended under stirring for 5-10 min and then kept at room temperature for 24 h to 113
form white precipitates. The above precipitates were collected via repeated centrifugation (8000 rpm, 114
10 min), washed with methanol for several times, and dried overnight before further use. For the 115
synthesis of ZIF-8 nanocrystals, initially, 1.62 g Hmim and 1.47 g Zn(NO3)2·6H2O were dissolved in 116
100 mL methanol, respectively. Subsequently, above homogeneous solutions were blended under 117
stirring for 5-10 min and then kept at room temperature for 24 h to form white precipitates. The 118
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above precipitates were collected via repeated centrifugation (12000 rpm, 20 min), washed with 119
methanol for several times, and dried overnight before further use. 120
2.5. Oriented deposition of the ZIF-8 seed layer 121
Oriented seed layer was obtained via spin-coating. Initially, we prepared ZIF-8 seed suspension by 122
adding 0.025 g PEI (Mw = 1800) and 0.010 g of ZIF-8 NSs into 5 g methanol. Subsequently, the 123
above suspension was stirred at room temperature overnight after sonication for 10 min. In the next 124
step, 0.1 ml of prepared ZIF-8 seed suspension was dropped on the surface of dry α-Al2O3 disk, and 125
then spin-coating was conducted at 3000 rpm for 60 s. In the end, ZIF-8 seed layer-coated substrate 126
was dried overnight before further use. 127
2.6. Secondary growth of highly (110)-oriented ZIF-8 seed layer 128
Initially, 2.27 g Hmim and 0.11 g Zn(NO3)2·6H2O were dissolved in 20 mL deionized (DI) water 129
under ice-water bath, respectively [35]. Subsequently, metal source aqueous solution was added in 130
Hmim aqueous solution under continuous stirring. After further stirring under ice-water bath for 20 131
min, obtained precursor solution was poured in a 50 mL autoclave containing vertically placed ZIF-8 132
seed layer-coated α-Al2O3 substrate. The syntheses were conducted at ~5 °C for 12 h by keeping the 133
autoclave in a refrigerator. After reaction, the substrate was taken out, gently rinsed with deionized 134
(DI) water and dried under ambient conditions overnight. For the secondary growth at 30, 60, 90 and 135
120 °C, the same synthesis strategy was used except for the reaction temperature. 136
2.7. Preparation of ZIF-8 membrane with nano-sized seeds 137
Procedure for the preparation of ZIF-8 membrane with nano-sized seeds was follow: First, the 138
ZIF-8 seed suspension was prepared by adding 0.025 g PEI (Mw = 1800) and 0.015 g ZIF-8 139
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nanocrystals in 5 g methanol. After sonication for 10 min, the above suspension was further stirred at 140
room temperature overnight. Subsequently, 0.2 ml of the above ZIF-8 seed suspension was dropped 141
on the α-Al2O3 disk, followed by spin-coating at 3000 rpm for 60 s. In the next step. ZIF-8 seed 142
layer-coated substrate was dried overnight before further use. Finally, secondary growth was 143
conducted following the same procedure as depicted in section 2.6. 144
2.8. Gas permeation test 145
During the implementation of permeation test, volumetric flow rate on both feed and permeate 146
sides was maintained at 50 mL/min; meanwhile, the pressure difference of feed side and permeate 147
side was maintained at 1 bar. Gas phase composition was analyzed by 7890B GC, Agilent. The 148
separation factor of binary mixture permeation (αA/B) could be described as follows: 149
150
x represented volume fractions of different components (A, B) in the feed side, and y corresponded 151
to volume fractions in permeate side. 152
2.9. Characterization 153
X-ray diffraction (XRD) patterns were collected on Rigaku SmartLab diffractometer. 154
Thermogravimetric analysis (TGA) results were obtained on NETZSCH (TG 209) thermal analyzer 155
under air or N2 atmosphere. Scanning electron microscopy (SEM) images were evaluated by 156
FlexSEM 1000 instrument (Hitachi Co.). Physical adsorption analysis was obtained on Mike ASAP 157
2020 Plus. 158
3. Results and Discussion 159
3.1. Synthesis of ZIF-8 NSs 160
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The first step was preparation of ZIF-8 NS seeds. In a recent study, Oh et al. [30] reported that leaf-161
shaped ZIF-L NSs could be transformed to ZIF-8 NSs preserving identical morphology through 162
simple thermal treatment. Motivated and inspired by this, herein we attempted to prepare ZIF-8 NSs 163
following the same route. A solution-based route for the preparation of uniform ZIF-L NSs has been 164
well established in a previous study [32]. As shown in Fig. 2a and b, prepared ZIF-L NSs with the 165
pore size of 0.34 nm in the (110) direction exhibited special leaf-like appearance with a general 166
crystal size of 5×2 μm and a thickness of ca. 150 nm. Subsequently, as affirmed by SEM images and 167
XRD patterns (Fig. 2c-e), prepared ZIF-L NSs were transformed into more thermodynamically stable 168
ZIF-8 NSs maintaining the same morphology through solvothermal treatment in DMF-ethanol binary 169
solvent. 170
171
Fig. 2. SEM images of (a, b) leaf-shaped ZIF-L NSs and (c, d) ZIF-8 NSs; (e) PXRD patterns of prepared ZIF-L NSs and 172
ZIF-8 NSs; and (f) CO2 and N2 adsorption isotherms of ZIF-8 NSs, ZIF-8N and ZIF-8M at 298 K. The scale bar is 1 μm. 173
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In view of potential impacts of ZIF-8 seeds on gas permeation properties of prepared membrane, 174
herein physical adsorption analysis was performed on prepared ZIF-8 NSs. In parallel, 70 nm-sized 175
ZIF-8 nanocrystals (defined as ZIF-8N) and 4 μm-sized ZIF-8 microcrystals (defined as ZIF-8M) were 176
prepared further for comparison [33, 34] (Fig. S2). CO2 and N2 adsorption isotherms of ZIF-8 NSs, 177
ZIF-8N and ZIF-8M measured at 298 K were shown in Fig. 2f and Fig. S3. It was found that on the 178
one hand, all these samples showed similar N2 adsorption capacity; on the other hand, ZIF-8 NSs 179
exhibited considerably higher CO2 adsorption capacity (32.91 cm3·g-1) than both ZIF-8N and ZIF-8M, 180
which was potentially advantageous for improving the H2/CO2 separation selectivity of 181
corresponding membranes. Besides, the pore volume and BET surface area of prepared ZIF-8 NSs 182
reached 0.66 cm³/g and 1304 m²/g , respectively, which was comparable with other reported ZIF-8 183
particles [8, 36, 37] (shown in Table S1), thereby indicating that prepared ZIF-8 NSs were of high 184
quality. In addition, their thermal stability was further evaluated by TGA. As shown in Fig. S4, 185
prepared ZIF-8 NSs exhibited excellent thermal stability under both N2 and air atmospheres, which 186
was ideally suited for operation under harsh conditions. 187
3.2. Oriented deposition of ZIF-8 seed layer 188
The second step involved oriented deposition of ZIF-8 seed layer via spin-coating. It was observed 189
that after spin-coating under optimized experimental conditions (Fig. S5), the porous α-Al2O3 190
substrate was fully covered with uniform ZIF-8 NSs (Fig. 3a). Cross-sectional SEM image further 191
showed that the thickness of seed layer reached ~800 nm (Fig. 3b). Furthermore, the XRD results 192
(Fig. 4) indicated that obtained ZIF-8 seed layer was dominantly (110)-oriented. 193
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Fig. 3. (a, b) Top and cross sectional SEM images of oriented seed layer; and (c, d) top and cross sectional SEM images 195
of oriented membrane. The scale bar is 1 μm. 196
3.3. Secondary growth of highly (110)-oriented seed layer 197
Subsequently, secondary growth was carried out to seal intercrystalline gaps between neighboring 198
seeds on the precondition of preserving preferred orientation inherited from the seed layer. Aqueous 199
solution approach has been confirmed to be an excellent strategy for synthesizing high performance 200
ZIF-8 membranes since Hmim could be easily deprotonated to achieve better intergrowth [35]. 201
Previous researches indicated that high performance ZIF-8 membranes could be obtained in a wide 202
range of reaction temperatures [35, 38, 39]. By referring to the strategy proposed by Lai et al. [35], 203
initially secondary growth was conducted at reaction temperatures of 30, 60, 90 and 120 °C, 204
respectively. Nevertheless, XRD results indicated that all ZIF-8 membranes prepared at these 205
temperatures were randomly oriented (Fig. S6). SEM images further showed that not only the 206
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membrane surface became rough but also massive twin crystals were aggregated on the surface of 207
membrane (Fig. S7). 208
209
Fig. 4. XRD patterns of simulated ZIF-8, (110)-oriented seed layer and (110)-oriented membrane, respectively. Note: * 210
represented diffraction peaks of porous α-Al2O3 substrate. 211
Fortunately, our results showed that undesired twin growth could be effectively suppressed by 212
carrying out secondary growth at low reaction temperature (~5 °C). SEM images (Fig. 3c, d) 213
indicated that well-intergrown, twin-free membrane displayed a thickness of 1.1 μm had been 214
emerged on porous α-Al2O3 substrate under optimized reaction conditions. Corresponding XRD 215
pattern exhibited remarkable diffraction peaks corresponding to (110) and (220) planes of ZIF-8 216
crystallites, which convincingly demonstrated the dominance of (110)-preferred orientation inherited 217
from the oriented seed layer (Fig. 4). This can be ascribed to reduced nucleation and growth rate of 218
ZIF-8 crystalline grains in the synthetic solution and therefore, effective suppression of undesired 219
twin growth accompanying with a substantial reduction in reaction temperature. As far as we know, 220
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this work represented the first report on the fabrication of highly (110)-oriented ZIF-8 membrane via 221
secondary growth method. Moreover, considering higher CO2 adsorption capacity of ZIF-8 NS seeds 222
compared with their bulk counterparts, it is expected that prepared (110)-oriented ZIF-8 membrane 223
may exhibit superior H2/CO2 selectivity. 224
3.4. Evaluation of gas separation performance 225
Finally, single gas permeation tests and mixed binary gas separation tests were operated under 226
ambient conditions to evaluate permeation behavior of pure gase (such as H2, CO2, N2 and CH4) and 227
equimolar gas mixtures (H2/CO2, H2/N2 and H2/CH4) through prepared (110)-oriented ZIF-8 228
membrane. The relationship between kinetic diameters and permeance of gas molecules was shown 229
in Fig. 5a. It was observed that the permeance of H2 remarkably exceeded CH4, N2 and CO2; 230
moreover, the ideal selectivity of H2/CO2, H2/N2 and H2/CH4 gas pairs reached 12.9, 7.7 and 11.7, 231
respectively (Fig. 5a), which was higher than their Knudsen diffusion coefficients (i.e., 4.7, 3.7 and 232
2.8), thereby illustrating the dominance of molecular sieving mechanism and the existence of few 233
grain-boundary defects. 234 Journ
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Fig. 5. (a) Single-gas and equimolar binary gas mixture permeation results under ambient conditions; (b) single-gas and 236
equimolar binary gas mixture permeation results at 150 °C; (c) H2 permeance, CH4 permeance and ideal selectivity of 237
equimolar H2/CH4 gas pair on prepared ZIF-8 membrane as a function of operation temperature; (d) evaluation of 238
equimolar H2/CH4 gas mixture separation performance at different operating temperatures; (e) H2 permeance, CO2 239
permeance and ideal selectivity of equimolar H2/CO2 gas pair on prepared ZIF-8 membrane as a function of operation 240
temperature; (f) evaluation of equimolar H2/CO2 gas mixture separation performance at different operating temperatures; 241
(g) long-term stability of (110)-oriented ZIF-8 membrane towards equimolar H2/CO2 gas mixture separation under 242
ambient conditions; and (h) long-term stability of (110)-oriented ZIF-8 membrane towards equimolar H2/CH4 gas 243
mixture separation at 150 °C. 244
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It should be noted that the permeance of gas molecules was not inversely proportional to their 245
kinetic diameters, which was contradictory to common experimental observations. Particularly, the 246
ideal selectivity of H2/CO2 equimolar gas pair on highly (110)-oriented ZIF-8 membrane exceeded 247
H2/N2 and H2/CH4 gas pairs under ambient conditions. To verify whether the ZIF-8 seeds exerted 248
influence on the H2/CO2 selectivity, herein ZIF-8 membrane was further prepared from nano-sized 249
ZIF-8 seeds while keeping other synthetic conditions unchanged (shown in Fig. S8 and S9). The 250
ideal H2/CO2, H2/N2 and H2/CH4 selectivity of prepared ZIF-8 membrane reached 10.0, 5.5 and 10.5 251
at room temperature. Nevertheless, we noticed that the ideal H2/CO2 selectivity was lower than not 252
only the ideal H2/CH4 selectivity of the same membrane, but also the ideal H2/CO2 selectivity of 253
prepared (110)-oriented ZIF-8 membrane. We therefore concluded that the increased H2/CO2 254
selectivity could be due to preferential CO2 adsorption capacity of not only PEI but also ZIF-8 255
nanosheets, which led to certain reduction in CO2 diffusion coefficients and therefore, enhanced ideal 256
selectivity of H2/CO2 within the framework of ZIF-8 membrane [40]. 257
Subsequently, the evaluation of the separation performance of prepared (110)-oriented membrane at 258
different operating temperatures was conducted. It was found that permeance and SF of binary gas 259
mixture strongly relied on the operating temperature. By raising the operation temperature gradually 260
from 30 to 150 °C, H2 permeance and SF of equimolar H2/CO2, H2/N2 and H2/CH4 steadily increased; 261
simultaneously, CO2, N2 and CH4 were slightly decreased (as shown in Fig. 5b-f and Fig. S10), 262
which could be explicated by adsorption–diffusion model: Higher operating temperature led to lower 263
adsorption capacity of CO2, N2 and CH4 in the pores of ZIF-8, which inevitably blocked the diffusion 264
of highly mobile and rarely adsorbed H2. Since a higher free microporous volume for the diffusion of 265
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H2 was established at higher operating temperature, both H2 permeance and SF of equimolar H2/CO2, 266
H2/N2 and H2/CH4 mixtures were concurrently enhanced [41, 42]. To the best of our knowledge, the 267
H2/CO2 selectivity of our ZIF-8 membrane was one of the highest in comparison with previously 268
published results (Table S2). 269
Furthermore, long-term stability was investigated under ambient conditions. Gas permeantion test 270
results revealed that both SF of equimolar H2/CO2 gas mixture and permeance of H2 showed no 271
discernible degradation within 20 h (shown in Fig. 5g), which vividly confirmed reliable long-term 272
operation stability. Furthermore, long-term thermal stability was further evaluated at 150 °C and 1 273
bar. Gas permeantion test results revealed that both SF of equimolar H2/CH4 and permeance of H2 274
remained unchanged within 50 hours (shown in Fig. 5h), which was indicative of its excellent 275
thermal stability. 276
4. Conclusions 277
To sum up, in this study we prepared highly (110)-oriented ZIF-8 membrane through combining 278
oriented deposition of ZIF-8 NSs with controlled in-plane secondary growth. It was found that the 279
following elements were vital for guaranteeing the formation of ZIF-8 membrane with desired 280
microstructure: 1) Preparation of uniform ZIF-8 NS seeds exposing (110)-dominant facets through 281
simple solvothermal treatment of identically shaped ZIF-L precursors. 2) Carrying out secondary 282
growth at low temperature for the suppression of undesired twin growth. Gas permeation results 283
indicated that H2/CO2 ideal selectivity of prepared (110)-oriented ZIF-8 membrane exceeded 284
H2/N2 and H2/CH4 gas pairs under ambient conditions, owing to preferential CO2 adsorption capacity 285
of not only PEI but also ZIF-8 nanosheets compared with their bulk counterparts. Our research 286
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highlighted the significance of preferred orientation regulation in tailoring the gas permeation 287
behavior of MOF membranes. 288
Acknowledgements 289
This work was supported by National Natural Science Foundation of China (22078039, 21176231), 290
Fok Ying-Tong Education Foundation of China (171063), Science and Technology Innovation Fund 291
of Dalian (2020JJ26GX026), Liaoning Revitalization Talents Program (XLYC1807084), the 292
Technology Innovation Team of Dalian University of Technology (DUT2017TB01), Science Fund 293
for Creative Research Groups of the National Natural Science Foundation of China (22021005), and 294
National Key Research and Development Program of China (2019YFE0119200). 295
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Highly (110)-oriented ZIF-8 membrane was prepared by oriented secondary
growth showing unique gas permeation behavior
Uniform ZIF-8 nanosheet seeds were prepared through simple thermal conversion
of leaf-shaped ZIF-L precursors
Carrying out secondary growth at low temperature was essential for suppressing
undesired twin growth
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Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Yi Liu reports financial support was provided by National Natural Science Foundation of China. Yi Liu reports financial support was provided by Fok Ying-Tong Education Foundation of China. Yi Liu reports financial support was provided by Science and Technology Innovation Fund of Dalian. Yi Liu reports financial support was provided by Liaoning Revitalization Talents Program. Yi Liu reports financial support was provided by Science Fund for Creative Research Groups of the National Natural Science Foundation of China. Yi Liu reports financial support was provided by National Key Research and Development Program of China.
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