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MDI (P84) copolyimide mixed-matrix membranes by incorporating submicrometer-sized [Ni3(HCOO)6] framework crystals
Journal Pre-proof Enhanced CO2/CH4 separation performance of BTDA-TDI/MDI (P84) copolyimide mixed-matrix membranes by incorporating submicrometer-sized [Ni3(HCOO)6] framework crystals Lujie Sheng, Ya Guo, Dan Zhao, Jizhong Ren, Shudong Wang, Maicun Deng PII:
S1875-5100(19)30375-0
DOI:
https://doi.org/10.1016/j.jngse.2019.103123
Reference:
JNGSE 103123
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 23 October 2019 Revised Date:
4 December 2019
Accepted Date: 19 December 2019
Please cite this article as: Sheng, L., Guo, Y., Zhao, D., Ren, J., Wang, S., Deng, M., Enhanced CO2/CH4 separation performance of BTDA-TDI/MDI (P84) copolyimide mixed-matrix membranes by incorporating submicrometer-sized [Ni3(HCOO)6] framework crystals, Journal of Natural Gas Science & Engineering, https://doi.org/10.1016/j.jngse.2019.103123. 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 B.V.
Effect of the [Ni3(HCOO)6] loading on CO2/CH4 separation performance of P84/[Ni3(HCOO)6] MMMs.
Enhanced CO2/CH4 separation performance of BTDA-TDI/MDI (P84) copolyimide mixed-matrix membranes by incorporating submicrometer-sized [Ni3(HCOO)6] framework crystals
Lujie Shenga,b,#, Ya Guoa,b,#, Dan Zhaoa, Jizhong Rena,*, Shudong Wanga, Maicun Denga
a
National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b
University of Chinese Academy of Sciences, Beijing 100049, China
#
These authors contributed to this work equally
*
Corresponding Author
Fax: (86)-0411-84379613; Phone: (86)-0411-84379961 E-mail address: [email protected]
Dedicated to the 70th anniversary of Dalian Institute of Chemical Physics, CAS
1
ABSTRACT The incorporation of metal-organic frameworks (MOFs) into mixed-matrix membranes (MMMs) is gaining widespread attention because of the combined advantages of easy processability and superior separation performance. In this paper, submicrometer-sized
[Ni3(HCOO)6]
frameworks
were
incorporated
into
BTDA-TDI/MDI (P84) matrix as filler material to separate CO2 from CH4. When the loadings of [Ni3(HCOO)6] frameworks in MMMs were 7 and 15 wt%, obvious enhancements in gas separation properties were obtained. Especially at the loading of 15 wt%, CO2 permeability and the CO2/CH4 selectivity increased 76% (from 0.72 Barrer to 1.26 Barrer) and 52% (from 44 to 67), respectively, compared with the pristine P84 membrane. The partial pore blockage was found by the high-pressure gravimetric adsorption measurements and it was deemed to be profitable to separate CO2/CH4. In order to better understand the gas separation behavior, the characterizations
of
adsorption-desorption,
[Ni3(HCOO)6] CO2
and
frameworks
CH4
adsorption,
(XRD,
SEM,
Ar
ATR-FTIR,
TGA)
and
P84/[Ni3(HCOO)6] MMMs (SEM, ATR-FTIR, XRD, TGA, stress–strain tests) were performed. Keywords: CO2 separation; mixed-matrix membranes; metal-organic framework; partial pore blockage; P84
2
1. Introduction CO2 is an undesired component in natural- and bio-gas because it will decrease the calorific value and corrode the pipeline (Saqib et al., 2019; Wu and Mosleh, 2019). Conventional methods for CO2 removal rely on cryogenic distillation, absorption and adsorption, which are costly processes. Membrane-based gas separation processes are affordable approaches for CO2 removal because of the operational simplicity, energy efficiency and environmentally friendly advantages. Recently, polymeric membranes have been demonstrated effectively in natural gas sweetening successfully. However, the membranes are limited by the trade-off relationship of “Robeson upper bond” (Robeson, 1991; Robeson, 2008). Some solutions have been proposed to break through the “Robeson upper bound”. Thereinto, mixed-matrix membranes (MMMs) (Amooghin et al., 2019; Chuah et al., 2018; Farashi et al., 2019; Rezakazemi et al., 2014; Sanaeepur et al., 2019; Seoane et al., 2015) are promising alternatives which are often composed of polymer and porous fillers. They compromise the easy fabrication of the polymer matrix and the particular separation capacities of the porous materials. However, conventional porous materials (Dunn et al., 2019; Zhao et al., 2014a; Zhao et al., 2014b; Zhao et al., 2019), such as zeolites and carbon molecular sieves have poor interfacial adhesion with the polymer matrix. Metal-organic frameworks (MOFs) are new promising alternatives for the fabrication of MMMs (Amedi and Aghajani, 2016; Jomekian et al., 2016). The strong bonds between metal ions and organic ligands allow building up one-, two-, and three-dimensional structures. There are relatively stronger interfacial adhesions between the polymer matrix and MOFs because of the organic linkers of MOFs. In general, the surface property and pore structures of MOFs can be easily tuned if necessary and the MOFs have lower density and higher pore volumes. Therefore, MMMs composed of polymer and MOFs have been recognized as new generation MMMs for CO2/CH4 separation. For gas separation, the incorporation of MOFs into polymeric matrix was 3
investigated for the first time by Won et al. (Won et al., 2005). Recently, many different MOFs have been applied to fabricate MMMs (Adams et al., 2010; Bae et al., 2010; Basu et al., 2010; Liu et al., 2011; Ordonez et al., 2010). Bae et al. (Bae et al., 2010) prepared MMMs with excellent gas-separation properties by adding submicrometer-sized ZIF-90 into the polyimide 6FDA-DAM. Su et al. (Su et al., 2016) combined UIO-66-NH2 with polysulfone matrix to prepare hybrid membranes. The MOFs NH2-MIL-101(Al) or NH2-MIL-53(Al) were used as dispersed phase in MMMs with two polymer matrix polyimides (Seoane et al., 2013). Chen et al. (Chen et al., 2018) introduced the CO2-philic KAUST-7 (NbOFFIVE-1-Ni) into 6FDA-Durene polyimide to make MMMs for separating CO2/CH4. Most researches have focused on the MOFs which have three-dimensional pore structures, such as MOF-5 (Perez et al., 2009), KAUST-7 (NbOFFIVE-1-Ni) (Chen et al., 2018), ZIFs (Guo et al., 2018), UiO-66 (Hossain et al., 2019) and so forth. The MOFs which have one-dimensional pore structures were seldom incorporated to polymer matrix to prepare MMMs. Additionally, relatively few MOF-based MMMs exhibit both enhanced gas permeability and selectivity compared with the pristine membranes. [Ni3(HCOO)6] is a member of the [M3(HCOO)6] family (M = Mn, Fe, Co, Ni, etc.) (Wang et al., 2007) and the frameworks have one-dimensional pore structure. Previous researches mainly focused on the gas adsorption separation properties of these frameworks. For example, Li et al. (Li et al., 2008) synthesized the frameworks [M3(HCOO)6]·DMF (M = Mn, Co, Ni) by solvothermal method and they exhibited good gas/hydrocarbon adsorption selectivity. Guo et al. (Guo et al., 2017) prepared [Ni3(HCOO)6] frameworks successfully to separate CH4 from N2. Wang et al. (Wang et al., 2018) added [Ni3(HCOO)6] into poly(styrene-b-butadiene-b-styrene) (SBS) polymer to fabricate MMMs for CH4/N2 separation, the result showed the increased permeability but unchanged selectivity. However, as far as we know, there is no study about MMMs with the [Ni3(HCOO)6] filler to separate CO2/CH4. P84 copolyimide was a potential membrane material (Favvas et al., 2016; Sazali et al., 2018) because its high thermally stability, chemical stability and high gas selectivity. And some particles such as ZIF-8 (Guo et al., 2018) and Cu3BTC2 (Ploegmakers et al., 2013) 4
have been incorporated into P84 matrix to fabricate MMMs previously. In this paper, the ultra-microporous [Ni3(HCOO)6] frameworks were used as filler material in P84 membrane matrix to separate CO2/CH4 for the first time due to its good separation selectivity for CO2/CH4. Submicrometer-sized (about 400 nm) crystals of [Ni3(HCOO)6] were prepared by a solvothermal method and the P84/[Ni3(HCOO)6] MMMs were prepared by solution-blending method. The physical properties of P84/[Ni3(HCOO)6] MMMs were characterized. The gas permeation properties and high-pressure gravimetric adsorption measurements of CO2 and CH4 were investigated. 2. Experimental 2.1. Materials
Fig. 1. The structure of P84.
Commercial co-polyimide P84 (Fig. 1) powders were dried in a vacuum oven over night. N-Methyl pyrrolidone (NMP) was provided by Sinopharm Chemical Reagent Co. Ltd. Pure CH4 and CO2 were supported by Dalian gases company. Nickel (II) nitrate hexahydrate [Ni(NO3)2·6H2O, 98%], nickel (II) acetate tetrahydrate [Ni(CH3COO)2]·4H2O, 98%], formic acid (HCOOH, 98%), methanol (CH3OH, 99%), and ethanol (CH3CH2OH, 99.7%) were supplied by Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All chemicals of analytical grade were used without further purification from commercial vendors. 2.2. Synthesis of [Ni3(HCOO)6] crystals Nickel (II) acetate tetrahydrate (24.28 g, 0.097 mol) and formic acid (31.40 g, 0.68 mol) were dissolved in methanol (150 ml) in a three-necked flask. The reactants were magnetically stirred at room temperature to form a homogenous mixture, followed by heating at 70 oC for 12 h under moderate mechanical stirring. After cooling down to room temperature, the resultant precipitates were centrifuged and 5
washed with methanol twice. Subsequently, the product was dried in a vacuum oven at 150 oC overnight. 2.3. Fabrication of P84/[Ni3(HCOO)6] MMMs The P84/[Ni3(HCOO)6] MMMs were fabricated by solution-blending method. The [Ni3(HCOO)6] crystals and P84 powders were dissolved in NMP, respectively. The solution of [Ni3(HCOO)6]/NMP was treated by sonicator at 50 °C for 2 hours and then poured slowly into P84/NMP solution under vigorously stirring to form a homogenous mixture. The mixture was casted by an automatic membrane coater at 60 °C. After evaporation in the glass oven for 24 h, the membranes were dried at 150 °C in a vacuum oven for 48 h. The [Ni3(HCOO)6] loading was calculated as following: [ () ] () =
[() ]
Where [() ] and are the weight of [Ni3(HCOO)6] frameworks and P84, respectively. 2.4. Characterization of [Ni3(HCOO)6] and P84/[Ni3(HCOO)6] MMMs The crystal size and morphology of [Ni3(HCOO)6] frameworks and the cross-section morphology of MMMs samples were characterized by the scanning electron microscope (SEM, JSM-7800F, operated at 1.0 kV). The wide angle X-ray diffraction (XRD) patterns (Cu Kα radiation with a wave length of 0.154 nm) of [Ni3(HCOO)6] and MMMs were obtained to clarify the crystalline structures. Fourier transform infrared (FTIR) with spectroscopy attenuated total reflectance (ATR) spectra of all membranes were obtained at 400–4000 cm-1 by FTIR spectrometer (Nicolet iS 50). Thermogravimetric analysis (TGA) of [Ni3(HCOO)6] frameworks and MMMs were performed using NETZSCH STA 449 F3 analyzer in argon atmosphere (20 ml/min, 30-800 °C) with a heating rate of 10 °C/min. Low pressure adsorption with argon at 87.3 K was conducted to analyze the pore structure of [Ni3(HCOO)6] frameworks using a Quantachrome Autosorb-iQ2 automatic volumetric instrument. The micropore volume, BET surface area, and pore size distribution were calculated 6
according to the Ar adsorption isotherms. The micropore volume and pore size distributions were calculated using Dubinin-Raduskevitch (DR) method and nonlinear density-functional theory (DFT), respectively. Adsorption isotherms of CH4 and CO2 on [Ni3(HCOO)6] frameworks were obtained at 303 K in the range of 0–100 kPa using Quantachrome Autosorb-iQ2 automatic volumetric instrument. High-pressure adsorption isotherms of CO2 and CH4 of [Ni3(HCOO)6] frameworks and MMMs were performed via the Hiden Intelligent Gravimetric Analyzer (30 °C, 0-10 bar). 2.5. Stress–strain tests The mechanical properties were characterized by stress–strain tests. They were measured with a speed of 5 mm/min via Instron universal material testing system at room temperature. The gauge length and width of all samples were 30 and 5 mm, and the thickness tester was used to determine the thickness of samples. 2.6. Gas permeation measurements The pure gas permeation properties of CH4 and CO2 for pristine P84 and P84/[Ni3(HCOO)6] MMMs were determined by a constant volume/variable pressure technique. Gas permeability and ideal selectivity of CO2/CH4 were determined by the following equation: =
1 273.15 ' * 76 & ( ∆ +
α-.//-1 =
/
Where P is the gas permeability (Barrer, 1 Barrer=10-10 cm3(STP) cm cm-2 s-1 cmHg-1); T is the operating temperature (K), V is the downstream volume (cm3), ( is the membrane area (cm2); is the thickness of membrane (cm); ∆ is the transmembrane pressure difference (cmHg) and
23 24
is the rate of pressure increase at
the permeate side at steady state. 3. Results and discussion 3.1. Characterization of the as-synthesized [Ni3(HCOO)6] frameworks 7
Fig. 2a shows the XRD pattern of the as-synthesized [Ni3(HCOO)6] sample. The XRD peak positions agree well with reported ones (Wang et al., 2007), confirming the successful synthesis of the specific MOF structure. Fig. 2b indicates the SEM image of the [Ni3(HCOO)6] sample. The crystals are approximately sphere-like particles with an average diameter of about 400 nm. Argon adsorption-desorption isotherms confirm the microporous characteristic of the [Ni3(HCOO)6] sample (Fig. 2c). The BET surface area of [Ni3(HCOO)6] sample is 293 m2/g and the micropore volume of the sample is 0.097 cm3/g. The pore size distribution of [Ni3(HCOO)6] frameworks was calculated based on the Ar adsorption isotherm (insert in Fig. 2c). The analysis shows that the pore size distribution is relatively narrow, mainly concentrate nearby 0.46 nm. Obviously, the pore size is within the range of ultra-micropore (below 0.7 nm), which is beneficial to the separation of small gas molecules (Guo et al., 2017; Wenzel et al., 2009). (a)
(c)
(b)
(d)
Fig. 2. (a) PXRD pattern, (b) SEM micrographs, (c) Ar adsorption isotherms at 87.3 K and pore size distribution and (d) CO2 and CH4 adsorption isotherms at 303 K for [Ni3(HCOO)6] frameworks. 8
The excellent textural properties of [Ni3(HCOO)6] frameworks promise distinct separation ability for CO2/CH4. Adsorption equilibrium isotherms of CO2 and CH4 at 303 K on [Ni3(HCOO)6] frameworks are plotted in Fig. 2d. The CO2 uptake at 303 K and 1 bar is 1.62 mmol/g. Comparatively, the CH4 uptake is merely 0.64 mmol/g. The significant difference in adsorption capacity indicates the excellent adsorptive selectivity for CO2 over CH4. It is attributed to the fact that CO2 has larger polarizability and quadruple moment than that of CH4 (Li et al., 2009), thus the interaction between CO2 and the [Ni3(HCOO)6] frameworks is stronger. Additionally, the smaller kinetic diameter of CO2 (0.33 nm) than that of CH4 (0.38 nm) also contributes to the larger adsorption capacity of CO2. 3.2. Characterization of P84/[Ni3(HCOO)6] MMMs Fig. 3 shows the cross-section morphologies of the pristine P84 and P84/[Ni3(HCOO)6] MMMs with different [Ni3(HCOO)6] loadings. It can be seen that the pristine P84 membrane is highly smooth and defect-free. The submicrometer-sized [Ni3(HCOO)6] particles show good adhesion with P84 polymer chains and they are dispersed homogeneously in P84 polymer matrix at the [Ni3(HCOO)6] loadings of 7 and 15 wt%. Furthermore, the appearance of polymer network around the [Ni3(HCOO)6] frameworks also indicates strong interfacial interactions (Ordonez et al., 2010). When the loading of [Ni3(HCOO)6] frameworks increased to 25 wt%, the agglomeration of the submicrometer-sized particles was observed, which would decrease CO2/CH4 separation performance. P84
7 wt%
15 wt%
25 wt%
Fig. 3. Cross-sectional SEM images of the pristine P84 and P84/[Ni3(HCOO)6] MMMs.
9
Fig. 4. ATR-FTIR spectra of the pristine P84 and P84/[Ni3(HCOO)6] MMMs.
Fig. 5. XRD of the pristine P84 and P84/[Ni3(HCOO)6] MMMs. Fig. 4 shows the ATR-FTIR spectra of pristine P84 membrane, the [Ni3(HCOO)6] frameworks and the MMMs over the wavenumber of 400-2100 cm-1. The functional groups of pristine P84 membrane such as OC-N-CO at 1716 (symmetric stretching mode) and 1779 cm-1 (asymmetric stretching mode), O=C-N at 1509, 1093 (symmetric stretching mode) and C-N at 1360 cm-1 (asymmetric stretching mode) can be seen. For [Ni3(HCOO)6], the band at 780 cm-1 is attributed to the symmetric deformation of O-C-O, the strong bands at 1326, 1582 and 1608 cm-1 are due to the symmetric and asymmetric stretching vibration of O-C-O, and the bands at 1392 and 10
1408 cm-1 are due to the asymmetric deformation of O-C-O. These typical bands of P84 and [Ni3(HCOO)6] are expected in P84/[Ni3(HCOO)6] MMMs. When the loading of [Ni3(HCOO)6] increases, the intensity of vibration peaks of [Ni3(HCOO)6] frameworks in P84/[Ni3(HCOO)6] MMMs increased. XRD characterization of pristine P84 and P84/[Ni3(HCOO)6] MMMs are displayed in Fig. 5. The pristine P84 membrane has a broad peak which suggests the amorphous structure. The typical sharp diffraction peaks of [Ni3(HCOO)6] frameworks are present in P84/[Ni3(HCOO)6] MMMs and the intensity increase with the increased [Ni3(HCOO)6] loading. These results indicate that the structure of [Ni3(HCOO)6] remain unchanged when preparing the MMMs.
Fig. 6. TGA curves of the pristine P84 and P84/[Ni3(HCOO)6] MMMs. Fig. 6 shows the TGA curves of pristine P84 and the P84/[Ni3(HCOO)6] MMMs. For both the pristine P84 and MMMs, there are almost no weight loss before 200 °C. And for the pristine P84 membrane, the weight loss at 350 °C is probably caused by the evaporation of trapped solvent and crosslinking of P84. The temperature of 350 °C is related to the Tg of P84, considering that the increased mobility of polymer chains above Tg are favorable to solvent desorption (Joly et al., 1999). When the temperature is about 570 °C, the pristine P84 membrane has a weight drop of 20%, indicating the thermal degradation process. Finally, the pristine P84 membrane lost 42% of its original weight. The first weight drop of [Ni3(HCOO)6] at about 120 °C suggests that 11
the evaporation of solvent and the second weight drop at about 220 °C indicates the thermal decomposition of [Ni3(HCOO)6]. The MMMs containing [Ni3(HCOO)6] frameworks with 7, 15 and 25 wt% loadings show obvious weight loss stages between 220 and 350 °C, indicating the decomposition of [Ni3(HCOO)6] frameworks. As we can see, higher loadings of [Ni3(HCOO)6] weaken the thermal stability of the MMMs, compared with the pristine P84 membrane. 3.3. Gas transport properties of P84/[Ni3(HCOO)6] MMMs 3.3.1. The pure gas permeation
A
B
Fig. 7. Effect of [Ni3(HCOO)6] loading on gas separation performance of P84/[Ni3(HCOO)6] MMMs. 12
The pure gas permeability (CO2 and CH4) and the ideal selectivity of CO2/CH4 at 30 °C and 5 bar are shown in Fig. 7. The permeabilities of CO2 and CH4 of the pristine P84 membrane are 0.716 and 0.016 Barrer. For the [Ni3(HCOO)6] loadings of 7 and 15 wt%, the CO2 permeability increases 46% and 76%, and simultaneously the CO2/CH4 selectivity increases 42% and 52%, respectively, compared with the pristine P84 membrane. This also indicates that there are no defects in the interface of [Ni3(HCOO)6] frameworks and P84 matrix. At the [Ni3(HCOO)6] loading of 25 wt%, the CO2 permeability is 1.43 Barrer, but the CO2/CH4 selectivity decreases to 43.4 which is much lower than that of 15 wt%. And when the [Ni3(HCOO)6] loading is 35 wt%, the CO2/CH4 selectivity decreases obviously to 12.4. CO2/CH4 selectivity decreases at higher loadings (25 wt% and 35 wt%) mainly due to the agglomeration of the [Ni3(HCOO)6] frameworks which can be seen in Fig. 3. And when the agglomeration of [Ni3(HCOO)6] frameworks forms, the defects form too, so CO2/CH4 selectivity decreases at higher loading of [Ni3(HCOO)6]. In Wang’s work (Wang et al., 2018), they focused on CH4/N2 separation by incorporating [Ni3(HCOO)6] into poly(styrene-b-butadiene-b-styrene) (SBS) polymer, resulting that the enhanced permeability and unchanged selectivity, but the CO2/CH4 separation properties were not investigated. In this work, we combine P84 and [Ni3(HCOO)6] which have attractive CO2/CH4 adsorption selectivity for the first time to separate CO2/CH4. The enhancement of both CO2 permeability and CO2/CH4 selectivity may be profitable to further investigation in other applications. The CO2/CH4 separation performance of the prepared membranes in this work is presented on Robeson graph (Comesana-Gandara et al., 2019; Robeson, 1991; Robeson, 2008), which is shown in Fig. 8 and the related values are displayed in Table 1. To get close to the upper bound lines, many researchers have made much efforts. As can be seen in Fig 8, the CO2/CH4 performance of some membranes such as 6FDA-durene polyimide based MMMs is closer to the new Robeson’s upper bound (2019). Compared with other MMMs (6FDA-durene polyimide, Matrimid and Pebax), P84 based MMMs has higher CO2/CH4 selectivity. The CO2 permeability of pristine P84 membrane is 0.72 Barrer in this work, when the [Ni3(HCOO)6] frameworks are 13
incorporated into P84 matrix (7 wt% and 15 wt%), the gas performance is much closer to the Robeson’s upper bound (1991). Additionally, P84 material has higher thermal stability and chemical stability (Sheng et al., 2019), so the P84/[Ni3(HCOO)6] MMMs are potential to be applied in industry.
Fig. 8. Comparison of CO2/CH4 separation performance of MMMs reported in previous literature and this work.
14
Table 1 Comparison of results with MMMs reported in literature and this work Number in Fig. 8
Polymer
Filler
αCO2/CH4
1
6FDA-durene
KAUST-7
33
1030
(Chen et al., 2018)
2
6FDA-durene
ZIF-71
13
4006
(Japip et al., 2014)
3
Matrimid
UIO-67
45
20
(Perez et al., 2009)
4
Matrimid
MIL-53
52
12
(Dorosti et al., 2014)
5
Matrimid
ZIF-8
40
20
(Kertik et al., 2017)
6
Pebax
NH2-MIL-53
23
149
(Meshkat et al., 2018)
7
Pebax
SAPO-34
17
338
(Zhao et al., 2014a)
8
Pebax
MWNTs-NH2
15
361
(Zhao et al., 2014b)
9
P84
ZIF-8
93
11
(Guo et al., 2018)
10
P84
Nanodiamond
75
1.6
(Pulyalina et al., 2018)
11
P84
/
44
0.72
This study
12
P84
[Ni3(HCOO)6]
62
1.0
This study
13
P84
[Ni3(HCOO)6]
67
1.3
This study
PCO2 (Barrer)
References
3.3.2. Investigation of the effect of [Ni3(HCOO)6] incorporation on gas permeation properties The permeation of gases through the P84/[Ni3(HCOO)6] MMMs are affected by the properties of the gas adsorption and diffusivity. The pore size of [Ni3(HCOO)6] frameworks is about 0.46 nm, which is larger than the gas molecules of CO2 (0.33 nm) and CH4 (0.38 nm). However, when the guest molecules such as polymer chains block 15
the nanospace of porous materials, the nanospace can favor excluding the larger molecules (Ban et al., 2015; Guo et al., 2018).
Fig. 9. Adsorption isotherms of CO2 and CH4 on [Ni3(HCOO)6] and P84/[Ni3(HCOO)6] MMMs. The
adsorption
properties
for
the
[Ni3(HCOO)6]
frameworks
and
P84/[Ni3(HCOO)6] MMMs are shown in Fig. 9, MMMs exhibit obvious difference between the adsorption amount of CO2 and CH4. The adsorption amount of CO2 improves with the increasing loading of [Ni3(HCOO)6]. Taking 7 wt% and 15 wt% P84/[Ni3(HCOO)6] MMMs as examples, the actual and theoretical values of adsorption amount are shown in Fig. 10, where the theoretical values are calculated with additive model (Uptaketheoretical=
w
1+w
Uptake[Ni3(HCOO)6]+61-
w 1+w
8 UptakeP84). By
comparison, it can be found that the actual adsorption amount of MMMs is lower than the theoretical one, which may be due to the partial pore blockage of the [Ni3(HCOO)6].
16
A
B
Fig. 10. Comparison of theoretical and actual adsorption amount of CO2 (A) and CH4 (B) for P84/[Ni3(HCOO)6] MMMs. At 5 bar, the MMMs with the [Ni3(HCOO)6] loadings of 7 wt% and 15 wt% exhibit CO2 adsorption almost equivalent to the theory values of the MMMs with 2 wt% and 7 wt% [Ni3(HCOO)6], respectively (Fig. 10A). However, the MMMs with 7 wt% [Ni3(HCOO)6] exhibit almost the same CH4 adsorption in comparation with pristine P84 membrane, indicating that CH4 can not enter the nanopores of [Ni3(HCOO)6]. And at the loading of 15 wt% [Ni3(HCOO)6], the MMMs exhibit the CH4 adsorption almost equivalent to the theory values of the MMMs with 2.4 wt% [Ni3(HCOO)6] (Fig. 10B). From the above analysis, it can be inferred that the partial pore blockage 17
favor excluding the larger molecules (CH4), suggesting that the pore size of [Ni3(HCOO)6] frameworks after partial pore blockage may be between 0.33 and 0.38 nm, which CO2 can get into while CH4 can not. So, the adsorption property of larger gas molecule (CH4) is more sensitive to the partial pore blockage. Compared with the pristine P84 membrane, the CO2 permeability (Fig. 7A) and its adsorption amount (Fig. 10A) for the MMMs with 15 wt% [Ni3(HCOO)6] loading increases 76 % and 16 % at 5 bar, respectively, so the CO2 diffusivity in MMMs increases 52 %. However, the CH4 permeability (Fig. 7A) of the MMMs with 15 wt% [Ni3(HCOO)6] loading is almost the same as the pristine P84 membrane, but its adsorption amount (Fig. 10B) increases, which resulting in the low CH4 diffusivity. It is obvious that the incorporation of [Ni3(HCOO)6] into P84 matrix can improve the CO2 diffusivity in spite of the partial pore blockage, but the CH4 diffusion in MMMs is prevented because of its larger size. The simplified diagram of the defect-free MMMs (7 and 15 wt% loadings) is shown in Fig. 11. It can be found that the P84 polymer chains has strong interaction with [Ni3(HCOO)6] so that there are no non-selective defects, and the pores are blocked by the polymer chains. Some pores are severely blocked by polymer chains so that both CO2 and CH4 can not get into. Other pores are partially blocked that CO2 can pass though preferentially because of its smaller size and plasticizing ability. The profitable role of the partial pore blockage on the gas transport properties of MMMs was also demonstrated in other literatures (Chung et al., 2007; Li et al., 2005).
Fig. 11. The pore blockage of [Ni3(HCOO)6] in P84/[Ni3(HCOO)6] MMMs. (3D molecular structure: Dark grey-C, light grey-H, red-O, blue-N, green-Ni.) 18
3.3.3. The mechanical properties of P84/[Ni3(HCOO)6] MMMs
A
B
Fig. 12. Effect of [Ni3(HCOO)6] loading on mechanical properties of P84/[Ni3(HCOO)6] MMMs. The mechanical properties of the pristine P84 and P84/[Ni3(HCOO)6] MMMs are shown in Fig. 12. The loading of [Ni3(HCOO)6] in MMMs has a little influence on Young’s modulus of MMMs. The tensile strength and the elongation at break decrease slightly when the loadings of [Ni3(HCOO)6] are 7 and 15 wt% in comparation with the pristine P84 membrane. This indicates that [Ni3(HCOO)6] dispersed in P84 matrix homogeneously. When the [Ni3(HCOO)6] loadings are 25 and 35 wt%, the tensile strength decreases 46% and 76%, and the elongation at break decreases 61% and 82%, 19
respectively, compared with the pristine P84 membrane. This is probably due to the agglomeration of [Ni3(HCOO)6] particles and the reduction of polymer chain entanglement. 4. Conclusions Submicrometer-sized [Ni3(HCOO)6] framework crystals, which have uniform ultra-micropore with micropore size of 0.46 nm, were synthesized by solvothermal method. They were incorporated into P84 matrix successfully for the first time, resulting in P84/[Ni3(HCOO)6] MMMs with good potential to separate CO2 from CH4. Detect-free MMMs were obtained when the [Ni3(HCOO)6] loadings were not more than 15 wt% and their gas separation performance was obviously enhanced. When the loading increased to 25 wt%, the agglomeration of [Ni3(HCOO)6] particles occurs, which resulting in the increase of CO2 permeability and decrease of CO2/CH4 selectivity. The partial pore blockage of the nanospace of [Ni3(HCOO)6] frameworks was confirmed by the high-pressure gravimetric adsorption measurements, which was profitable to separate CO2/CH4. Considering the cheapness and ease of preparation of [Ni3(HCOO)6] frameworks, the aforementioned MMMs promise good candidates for further research in CO2/CH4 separation. Acknowledgments This work was supported by the National Natural Science Foundation of China (21908215). References Adams, R., Carson, C., Ward, J., Tannenbaum, R., Koros, W., 2010. Metal organic framework mixed matrix membranes for gas separations. Microporous Mesoporous Mater. 131, 13-20. Amedi, H.R., Aghajani, M., 2016. Gas separation in mixed matrix membranes based on polyurethane containing SiO2, ZSM-5, and ZIF-8 nanoparticles. J. Nat. Gas Sci. Eng. 35, 695-702. Amooghin, A.E., Mashhadikhan, S., Sanaeepur, H., Moghadassi, A., Matsuura, T., Ramakrishna, S., 2019. Substantial breakthroughs on functionled design of 20
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Wang, S., Guo, Q., Liang, S., Li, P., Li, X., Luo, J., 2018. Ni3(HCOO)6 /Poly(styrene-b-butadiene-b-styrene) Mixed-matrix membranes for CH4/N2 gas separation. Chem. Eng. Technol. 41, 353-366. Wang, Z., Zhang, B., Zhang, Y., Kurmoo, M., Liu, T., Gao, S., Kobayashi, H., 2007. A family of porous magnets, M3(HCOO)6 (M = Mn, Fe, Co and Ni). Polyhedron 26, 2207-2215. Wenzel, S.E., Fischer, M., Hoffmann, F., Froeba, M., 2009. Highly porous metal-organic framework containing a novel organosilicon linker - A promising material for hydrogen storage. Inorg. Chem. 48, 6559-6565. Won, J.G., Seo, J.S., Kim, J.H., Kim, H.S., Kang, Y.S., Kim, S.J., Kim, Y.M., Jegal, J.G., 2005. Coordination compound molecular sieve membranes. Adv. Mater. 17, 80-+. Wu, K.-Y., Mosleh, A., 2019. Effect of temporal variability of operating parameters in corrosion modelling for natural gas pipelines subject to uniform corrosion. J. Nat. Gas Sci. Eng. 69, 102930. Zhao, D., Ren, J., Li, H., Hua, K., Deng, M., 2014a. Poly(amide-6-b-ethylene oxide)/SAPO-34 mixed matrix membrane for CO2 separation. J. Energy Chem. 23, 227-234. Zhao, D., Ren, J., Li, H., Li, X., Deng, M., 2014b. Gas separation properties of poly(amide-6-b-ethylene oxide)/amino modified multi-walled carbon nanotubes mixed matrix membranes. J. Membr. Sci. 467, 41-47. Zhao, J., Xie, K., Liu, L., Liu, M., Qiu, W., Webley, P.A., 2019. Enhancing plasticization-resistance of mixed-matrix membranes with exceptionally high CO2/CH4 selectivity through incorporating ZSM-25 zeolite. J. Membr. Sci. 583, 23-30.
25
1. [Ni3(HCOO)6] frameworks were incorporated into P84 matrix to separate CO2/CH4. 2. Defect-free MMMs were prepared and characterized at 7 wt% and 15 wt% loadings. 3. CO2/CH4 separation performance was obviously enhanced for the defect-free MMMs. 4. The pore blockage of [Ni3(HCOO)6] frameworks was profitable for gas separation.
Lujie Sheng: Conceptualization, Methodology, Formal analysis, Investigation, Writing-Original draft preparation Ya Guo: Methodology, Investigation, Formal analysis Dan Zhao: Writing – Review & Editing, Founding acquisition Jizhong Ren: Supervision, Writing – Review & Editing Shudong Wang: Supervision Maicun Deng: Supervision
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:
S1875-5100(19)30375-0
DOI:
https://doi.org/10.1016/j.jngse.2019.103123
Reference:
JNGSE 103123
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 23 October 2019 Revised Date:
4 December 2019
Accepted Date: 19 December 2019
Please cite this article as: Sheng, L., Guo, Y., Zhao, D., Ren, J., Wang, S., Deng, M., Enhanced CO2/CH4 separation performance of BTDA-TDI/MDI (P84) copolyimide mixed-matrix membranes by incorporating submicrometer-sized [Ni3(HCOO)6] framework crystals, Journal of Natural Gas Science & Engineering, https://doi.org/10.1016/j.jngse.2019.103123. 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 B.V.
Effect of the [Ni3(HCOO)6] loading on CO2/CH4 separation performance of P84/[Ni3(HCOO)6] MMMs.
Enhanced CO2/CH4 separation performance of BTDA-TDI/MDI (P84) copolyimide mixed-matrix membranes by incorporating submicrometer-sized [Ni3(HCOO)6] framework crystals
Lujie Shenga,b,#, Ya Guoa,b,#, Dan Zhaoa, Jizhong Rena,*, Shudong Wanga, Maicun Denga
a
National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b
University of Chinese Academy of Sciences, Beijing 100049, China
#
These authors contributed to this work equally
*
Corresponding Author
Fax: (86)-0411-84379613; Phone: (86)-0411-84379961 E-mail address: [email protected]
Dedicated to the 70th anniversary of Dalian Institute of Chemical Physics, CAS
1
ABSTRACT The incorporation of metal-organic frameworks (MOFs) into mixed-matrix membranes (MMMs) is gaining widespread attention because of the combined advantages of easy processability and superior separation performance. In this paper, submicrometer-sized
[Ni3(HCOO)6]
frameworks
were
incorporated
into
BTDA-TDI/MDI (P84) matrix as filler material to separate CO2 from CH4. When the loadings of [Ni3(HCOO)6] frameworks in MMMs were 7 and 15 wt%, obvious enhancements in gas separation properties were obtained. Especially at the loading of 15 wt%, CO2 permeability and the CO2/CH4 selectivity increased 76% (from 0.72 Barrer to 1.26 Barrer) and 52% (from 44 to 67), respectively, compared with the pristine P84 membrane. The partial pore blockage was found by the high-pressure gravimetric adsorption measurements and it was deemed to be profitable to separate CO2/CH4. In order to better understand the gas separation behavior, the characterizations
of
adsorption-desorption,
[Ni3(HCOO)6] CO2
and
frameworks
CH4
adsorption,
(XRD,
SEM,
Ar
ATR-FTIR,
TGA)
and
P84/[Ni3(HCOO)6] MMMs (SEM, ATR-FTIR, XRD, TGA, stress–strain tests) were performed. Keywords: CO2 separation; mixed-matrix membranes; metal-organic framework; partial pore blockage; P84
2
1. Introduction CO2 is an undesired component in natural- and bio-gas because it will decrease the calorific value and corrode the pipeline (Saqib et al., 2019; Wu and Mosleh, 2019). Conventional methods for CO2 removal rely on cryogenic distillation, absorption and adsorption, which are costly processes. Membrane-based gas separation processes are affordable approaches for CO2 removal because of the operational simplicity, energy efficiency and environmentally friendly advantages. Recently, polymeric membranes have been demonstrated effectively in natural gas sweetening successfully. However, the membranes are limited by the trade-off relationship of “Robeson upper bond” (Robeson, 1991; Robeson, 2008). Some solutions have been proposed to break through the “Robeson upper bound”. Thereinto, mixed-matrix membranes (MMMs) (Amooghin et al., 2019; Chuah et al., 2018; Farashi et al., 2019; Rezakazemi et al., 2014; Sanaeepur et al., 2019; Seoane et al., 2015) are promising alternatives which are often composed of polymer and porous fillers. They compromise the easy fabrication of the polymer matrix and the particular separation capacities of the porous materials. However, conventional porous materials (Dunn et al., 2019; Zhao et al., 2014a; Zhao et al., 2014b; Zhao et al., 2019), such as zeolites and carbon molecular sieves have poor interfacial adhesion with the polymer matrix. Metal-organic frameworks (MOFs) are new promising alternatives for the fabrication of MMMs (Amedi and Aghajani, 2016; Jomekian et al., 2016). The strong bonds between metal ions and organic ligands allow building up one-, two-, and three-dimensional structures. There are relatively stronger interfacial adhesions between the polymer matrix and MOFs because of the organic linkers of MOFs. In general, the surface property and pore structures of MOFs can be easily tuned if necessary and the MOFs have lower density and higher pore volumes. Therefore, MMMs composed of polymer and MOFs have been recognized as new generation MMMs for CO2/CH4 separation. For gas separation, the incorporation of MOFs into polymeric matrix was 3
investigated for the first time by Won et al. (Won et al., 2005). Recently, many different MOFs have been applied to fabricate MMMs (Adams et al., 2010; Bae et al., 2010; Basu et al., 2010; Liu et al., 2011; Ordonez et al., 2010). Bae et al. (Bae et al., 2010) prepared MMMs with excellent gas-separation properties by adding submicrometer-sized ZIF-90 into the polyimide 6FDA-DAM. Su et al. (Su et al., 2016) combined UIO-66-NH2 with polysulfone matrix to prepare hybrid membranes. The MOFs NH2-MIL-101(Al) or NH2-MIL-53(Al) were used as dispersed phase in MMMs with two polymer matrix polyimides (Seoane et al., 2013). Chen et al. (Chen et al., 2018) introduced the CO2-philic KAUST-7 (NbOFFIVE-1-Ni) into 6FDA-Durene polyimide to make MMMs for separating CO2/CH4. Most researches have focused on the MOFs which have three-dimensional pore structures, such as MOF-5 (Perez et al., 2009), KAUST-7 (NbOFFIVE-1-Ni) (Chen et al., 2018), ZIFs (Guo et al., 2018), UiO-66 (Hossain et al., 2019) and so forth. The MOFs which have one-dimensional pore structures were seldom incorporated to polymer matrix to prepare MMMs. Additionally, relatively few MOF-based MMMs exhibit both enhanced gas permeability and selectivity compared with the pristine membranes. [Ni3(HCOO)6] is a member of the [M3(HCOO)6] family (M = Mn, Fe, Co, Ni, etc.) (Wang et al., 2007) and the frameworks have one-dimensional pore structure. Previous researches mainly focused on the gas adsorption separation properties of these frameworks. For example, Li et al. (Li et al., 2008) synthesized the frameworks [M3(HCOO)6]·DMF (M = Mn, Co, Ni) by solvothermal method and they exhibited good gas/hydrocarbon adsorption selectivity. Guo et al. (Guo et al., 2017) prepared [Ni3(HCOO)6] frameworks successfully to separate CH4 from N2. Wang et al. (Wang et al., 2018) added [Ni3(HCOO)6] into poly(styrene-b-butadiene-b-styrene) (SBS) polymer to fabricate MMMs for CH4/N2 separation, the result showed the increased permeability but unchanged selectivity. However, as far as we know, there is no study about MMMs with the [Ni3(HCOO)6] filler to separate CO2/CH4. P84 copolyimide was a potential membrane material (Favvas et al., 2016; Sazali et al., 2018) because its high thermally stability, chemical stability and high gas selectivity. And some particles such as ZIF-8 (Guo et al., 2018) and Cu3BTC2 (Ploegmakers et al., 2013) 4
have been incorporated into P84 matrix to fabricate MMMs previously. In this paper, the ultra-microporous [Ni3(HCOO)6] frameworks were used as filler material in P84 membrane matrix to separate CO2/CH4 for the first time due to its good separation selectivity for CO2/CH4. Submicrometer-sized (about 400 nm) crystals of [Ni3(HCOO)6] were prepared by a solvothermal method and the P84/[Ni3(HCOO)6] MMMs were prepared by solution-blending method. The physical properties of P84/[Ni3(HCOO)6] MMMs were characterized. The gas permeation properties and high-pressure gravimetric adsorption measurements of CO2 and CH4 were investigated. 2. Experimental 2.1. Materials
Fig. 1. The structure of P84.
Commercial co-polyimide P84 (Fig. 1) powders were dried in a vacuum oven over night. N-Methyl pyrrolidone (NMP) was provided by Sinopharm Chemical Reagent Co. Ltd. Pure CH4 and CO2 were supported by Dalian gases company. Nickel (II) nitrate hexahydrate [Ni(NO3)2·6H2O, 98%], nickel (II) acetate tetrahydrate [Ni(CH3COO)2]·4H2O, 98%], formic acid (HCOOH, 98%), methanol (CH3OH, 99%), and ethanol (CH3CH2OH, 99.7%) were supplied by Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All chemicals of analytical grade were used without further purification from commercial vendors. 2.2. Synthesis of [Ni3(HCOO)6] crystals Nickel (II) acetate tetrahydrate (24.28 g, 0.097 mol) and formic acid (31.40 g, 0.68 mol) were dissolved in methanol (150 ml) in a three-necked flask. The reactants were magnetically stirred at room temperature to form a homogenous mixture, followed by heating at 70 oC for 12 h under moderate mechanical stirring. After cooling down to room temperature, the resultant precipitates were centrifuged and 5
washed with methanol twice. Subsequently, the product was dried in a vacuum oven at 150 oC overnight. 2.3. Fabrication of P84/[Ni3(HCOO)6] MMMs The P84/[Ni3(HCOO)6] MMMs were fabricated by solution-blending method. The [Ni3(HCOO)6] crystals and P84 powders were dissolved in NMP, respectively. The solution of [Ni3(HCOO)6]/NMP was treated by sonicator at 50 °C for 2 hours and then poured slowly into P84/NMP solution under vigorously stirring to form a homogenous mixture. The mixture was casted by an automatic membrane coater at 60 °C. After evaporation in the glass oven for 24 h, the membranes were dried at 150 °C in a vacuum oven for 48 h. The [Ni3(HCOO)6] loading was calculated as following: [ () ] () =
[() ]
Where [() ] and are the weight of [Ni3(HCOO)6] frameworks and P84, respectively. 2.4. Characterization of [Ni3(HCOO)6] and P84/[Ni3(HCOO)6] MMMs The crystal size and morphology of [Ni3(HCOO)6] frameworks and the cross-section morphology of MMMs samples were characterized by the scanning electron microscope (SEM, JSM-7800F, operated at 1.0 kV). The wide angle X-ray diffraction (XRD) patterns (Cu Kα radiation with a wave length of 0.154 nm) of [Ni3(HCOO)6] and MMMs were obtained to clarify the crystalline structures. Fourier transform infrared (FTIR) with spectroscopy attenuated total reflectance (ATR) spectra of all membranes were obtained at 400–4000 cm-1 by FTIR spectrometer (Nicolet iS 50). Thermogravimetric analysis (TGA) of [Ni3(HCOO)6] frameworks and MMMs were performed using NETZSCH STA 449 F3 analyzer in argon atmosphere (20 ml/min, 30-800 °C) with a heating rate of 10 °C/min. Low pressure adsorption with argon at 87.3 K was conducted to analyze the pore structure of [Ni3(HCOO)6] frameworks using a Quantachrome Autosorb-iQ2 automatic volumetric instrument. The micropore volume, BET surface area, and pore size distribution were calculated 6
according to the Ar adsorption isotherms. The micropore volume and pore size distributions were calculated using Dubinin-Raduskevitch (DR) method and nonlinear density-functional theory (DFT), respectively. Adsorption isotherms of CH4 and CO2 on [Ni3(HCOO)6] frameworks were obtained at 303 K in the range of 0–100 kPa using Quantachrome Autosorb-iQ2 automatic volumetric instrument. High-pressure adsorption isotherms of CO2 and CH4 of [Ni3(HCOO)6] frameworks and MMMs were performed via the Hiden Intelligent Gravimetric Analyzer (30 °C, 0-10 bar). 2.5. Stress–strain tests The mechanical properties were characterized by stress–strain tests. They were measured with a speed of 5 mm/min via Instron universal material testing system at room temperature. The gauge length and width of all samples were 30 and 5 mm, and the thickness tester was used to determine the thickness of samples. 2.6. Gas permeation measurements The pure gas permeation properties of CH4 and CO2 for pristine P84 and P84/[Ni3(HCOO)6] MMMs were determined by a constant volume/variable pressure technique. Gas permeability and ideal selectivity of CO2/CH4 were determined by the following equation: =
1 273.15 ' * 76 & ( ∆ +
α-.//-1 =
/
Where P is the gas permeability (Barrer, 1 Barrer=10-10 cm3(STP) cm cm-2 s-1 cmHg-1); T is the operating temperature (K), V is the downstream volume (cm3), ( is the membrane area (cm2); is the thickness of membrane (cm); ∆ is the transmembrane pressure difference (cmHg) and
23 24
is the rate of pressure increase at
the permeate side at steady state. 3. Results and discussion 3.1. Characterization of the as-synthesized [Ni3(HCOO)6] frameworks 7
Fig. 2a shows the XRD pattern of the as-synthesized [Ni3(HCOO)6] sample. The XRD peak positions agree well with reported ones (Wang et al., 2007), confirming the successful synthesis of the specific MOF structure. Fig. 2b indicates the SEM image of the [Ni3(HCOO)6] sample. The crystals are approximately sphere-like particles with an average diameter of about 400 nm. Argon adsorption-desorption isotherms confirm the microporous characteristic of the [Ni3(HCOO)6] sample (Fig. 2c). The BET surface area of [Ni3(HCOO)6] sample is 293 m2/g and the micropore volume of the sample is 0.097 cm3/g. The pore size distribution of [Ni3(HCOO)6] frameworks was calculated based on the Ar adsorption isotherm (insert in Fig. 2c). The analysis shows that the pore size distribution is relatively narrow, mainly concentrate nearby 0.46 nm. Obviously, the pore size is within the range of ultra-micropore (below 0.7 nm), which is beneficial to the separation of small gas molecules (Guo et al., 2017; Wenzel et al., 2009). (a)
(c)
(b)
(d)
Fig. 2. (a) PXRD pattern, (b) SEM micrographs, (c) Ar adsorption isotherms at 87.3 K and pore size distribution and (d) CO2 and CH4 adsorption isotherms at 303 K for [Ni3(HCOO)6] frameworks. 8
The excellent textural properties of [Ni3(HCOO)6] frameworks promise distinct separation ability for CO2/CH4. Adsorption equilibrium isotherms of CO2 and CH4 at 303 K on [Ni3(HCOO)6] frameworks are plotted in Fig. 2d. The CO2 uptake at 303 K and 1 bar is 1.62 mmol/g. Comparatively, the CH4 uptake is merely 0.64 mmol/g. The significant difference in adsorption capacity indicates the excellent adsorptive selectivity for CO2 over CH4. It is attributed to the fact that CO2 has larger polarizability and quadruple moment than that of CH4 (Li et al., 2009), thus the interaction between CO2 and the [Ni3(HCOO)6] frameworks is stronger. Additionally, the smaller kinetic diameter of CO2 (0.33 nm) than that of CH4 (0.38 nm) also contributes to the larger adsorption capacity of CO2. 3.2. Characterization of P84/[Ni3(HCOO)6] MMMs Fig. 3 shows the cross-section morphologies of the pristine P84 and P84/[Ni3(HCOO)6] MMMs with different [Ni3(HCOO)6] loadings. It can be seen that the pristine P84 membrane is highly smooth and defect-free. The submicrometer-sized [Ni3(HCOO)6] particles show good adhesion with P84 polymer chains and they are dispersed homogeneously in P84 polymer matrix at the [Ni3(HCOO)6] loadings of 7 and 15 wt%. Furthermore, the appearance of polymer network around the [Ni3(HCOO)6] frameworks also indicates strong interfacial interactions (Ordonez et al., 2010). When the loading of [Ni3(HCOO)6] frameworks increased to 25 wt%, the agglomeration of the submicrometer-sized particles was observed, which would decrease CO2/CH4 separation performance. P84
7 wt%
15 wt%
25 wt%
Fig. 3. Cross-sectional SEM images of the pristine P84 and P84/[Ni3(HCOO)6] MMMs.
9
Fig. 4. ATR-FTIR spectra of the pristine P84 and P84/[Ni3(HCOO)6] MMMs.
Fig. 5. XRD of the pristine P84 and P84/[Ni3(HCOO)6] MMMs. Fig. 4 shows the ATR-FTIR spectra of pristine P84 membrane, the [Ni3(HCOO)6] frameworks and the MMMs over the wavenumber of 400-2100 cm-1. The functional groups of pristine P84 membrane such as OC-N-CO at 1716 (symmetric stretching mode) and 1779 cm-1 (asymmetric stretching mode), O=C-N at 1509, 1093 (symmetric stretching mode) and C-N at 1360 cm-1 (asymmetric stretching mode) can be seen. For [Ni3(HCOO)6], the band at 780 cm-1 is attributed to the symmetric deformation of O-C-O, the strong bands at 1326, 1582 and 1608 cm-1 are due to the symmetric and asymmetric stretching vibration of O-C-O, and the bands at 1392 and 10
1408 cm-1 are due to the asymmetric deformation of O-C-O. These typical bands of P84 and [Ni3(HCOO)6] are expected in P84/[Ni3(HCOO)6] MMMs. When the loading of [Ni3(HCOO)6] increases, the intensity of vibration peaks of [Ni3(HCOO)6] frameworks in P84/[Ni3(HCOO)6] MMMs increased. XRD characterization of pristine P84 and P84/[Ni3(HCOO)6] MMMs are displayed in Fig. 5. The pristine P84 membrane has a broad peak which suggests the amorphous structure. The typical sharp diffraction peaks of [Ni3(HCOO)6] frameworks are present in P84/[Ni3(HCOO)6] MMMs and the intensity increase with the increased [Ni3(HCOO)6] loading. These results indicate that the structure of [Ni3(HCOO)6] remain unchanged when preparing the MMMs.
Fig. 6. TGA curves of the pristine P84 and P84/[Ni3(HCOO)6] MMMs. Fig. 6 shows the TGA curves of pristine P84 and the P84/[Ni3(HCOO)6] MMMs. For both the pristine P84 and MMMs, there are almost no weight loss before 200 °C. And for the pristine P84 membrane, the weight loss at 350 °C is probably caused by the evaporation of trapped solvent and crosslinking of P84. The temperature of 350 °C is related to the Tg of P84, considering that the increased mobility of polymer chains above Tg are favorable to solvent desorption (Joly et al., 1999). When the temperature is about 570 °C, the pristine P84 membrane has a weight drop of 20%, indicating the thermal degradation process. Finally, the pristine P84 membrane lost 42% of its original weight. The first weight drop of [Ni3(HCOO)6] at about 120 °C suggests that 11
the evaporation of solvent and the second weight drop at about 220 °C indicates the thermal decomposition of [Ni3(HCOO)6]. The MMMs containing [Ni3(HCOO)6] frameworks with 7, 15 and 25 wt% loadings show obvious weight loss stages between 220 and 350 °C, indicating the decomposition of [Ni3(HCOO)6] frameworks. As we can see, higher loadings of [Ni3(HCOO)6] weaken the thermal stability of the MMMs, compared with the pristine P84 membrane. 3.3. Gas transport properties of P84/[Ni3(HCOO)6] MMMs 3.3.1. The pure gas permeation
A
B
Fig. 7. Effect of [Ni3(HCOO)6] loading on gas separation performance of P84/[Ni3(HCOO)6] MMMs. 12
The pure gas permeability (CO2 and CH4) and the ideal selectivity of CO2/CH4 at 30 °C and 5 bar are shown in Fig. 7. The permeabilities of CO2 and CH4 of the pristine P84 membrane are 0.716 and 0.016 Barrer. For the [Ni3(HCOO)6] loadings of 7 and 15 wt%, the CO2 permeability increases 46% and 76%, and simultaneously the CO2/CH4 selectivity increases 42% and 52%, respectively, compared with the pristine P84 membrane. This also indicates that there are no defects in the interface of [Ni3(HCOO)6] frameworks and P84 matrix. At the [Ni3(HCOO)6] loading of 25 wt%, the CO2 permeability is 1.43 Barrer, but the CO2/CH4 selectivity decreases to 43.4 which is much lower than that of 15 wt%. And when the [Ni3(HCOO)6] loading is 35 wt%, the CO2/CH4 selectivity decreases obviously to 12.4. CO2/CH4 selectivity decreases at higher loadings (25 wt% and 35 wt%) mainly due to the agglomeration of the [Ni3(HCOO)6] frameworks which can be seen in Fig. 3. And when the agglomeration of [Ni3(HCOO)6] frameworks forms, the defects form too, so CO2/CH4 selectivity decreases at higher loading of [Ni3(HCOO)6]. In Wang’s work (Wang et al., 2018), they focused on CH4/N2 separation by incorporating [Ni3(HCOO)6] into poly(styrene-b-butadiene-b-styrene) (SBS) polymer, resulting that the enhanced permeability and unchanged selectivity, but the CO2/CH4 separation properties were not investigated. In this work, we combine P84 and [Ni3(HCOO)6] which have attractive CO2/CH4 adsorption selectivity for the first time to separate CO2/CH4. The enhancement of both CO2 permeability and CO2/CH4 selectivity may be profitable to further investigation in other applications. The CO2/CH4 separation performance of the prepared membranes in this work is presented on Robeson graph (Comesana-Gandara et al., 2019; Robeson, 1991; Robeson, 2008), which is shown in Fig. 8 and the related values are displayed in Table 1. To get close to the upper bound lines, many researchers have made much efforts. As can be seen in Fig 8, the CO2/CH4 performance of some membranes such as 6FDA-durene polyimide based MMMs is closer to the new Robeson’s upper bound (2019). Compared with other MMMs (6FDA-durene polyimide, Matrimid and Pebax), P84 based MMMs has higher CO2/CH4 selectivity. The CO2 permeability of pristine P84 membrane is 0.72 Barrer in this work, when the [Ni3(HCOO)6] frameworks are 13
incorporated into P84 matrix (7 wt% and 15 wt%), the gas performance is much closer to the Robeson’s upper bound (1991). Additionally, P84 material has higher thermal stability and chemical stability (Sheng et al., 2019), so the P84/[Ni3(HCOO)6] MMMs are potential to be applied in industry.
Fig. 8. Comparison of CO2/CH4 separation performance of MMMs reported in previous literature and this work.
14
Table 1 Comparison of results with MMMs reported in literature and this work Number in Fig. 8
Polymer
Filler
αCO2/CH4
1
6FDA-durene
KAUST-7
33
1030
(Chen et al., 2018)
2
6FDA-durene
ZIF-71
13
4006
(Japip et al., 2014)
3
Matrimid
UIO-67
45
20
(Perez et al., 2009)
4
Matrimid
MIL-53
52
12
(Dorosti et al., 2014)
5
Matrimid
ZIF-8
40
20
(Kertik et al., 2017)
6
Pebax
NH2-MIL-53
23
149
(Meshkat et al., 2018)
7
Pebax
SAPO-34
17
338
(Zhao et al., 2014a)
8
Pebax
MWNTs-NH2
15
361
(Zhao et al., 2014b)
9
P84
ZIF-8
93
11
(Guo et al., 2018)
10
P84
Nanodiamond
75
1.6
(Pulyalina et al., 2018)
11
P84
/
44
0.72
This study
12
P84
[Ni3(HCOO)6]
62
1.0
This study
13
P84
[Ni3(HCOO)6]
67
1.3
This study
PCO2 (Barrer)
References
3.3.2. Investigation of the effect of [Ni3(HCOO)6] incorporation on gas permeation properties The permeation of gases through the P84/[Ni3(HCOO)6] MMMs are affected by the properties of the gas adsorption and diffusivity. The pore size of [Ni3(HCOO)6] frameworks is about 0.46 nm, which is larger than the gas molecules of CO2 (0.33 nm) and CH4 (0.38 nm). However, when the guest molecules such as polymer chains block 15
the nanospace of porous materials, the nanospace can favor excluding the larger molecules (Ban et al., 2015; Guo et al., 2018).
Fig. 9. Adsorption isotherms of CO2 and CH4 on [Ni3(HCOO)6] and P84/[Ni3(HCOO)6] MMMs. The
adsorption
properties
for
the
[Ni3(HCOO)6]
frameworks
and
P84/[Ni3(HCOO)6] MMMs are shown in Fig. 9, MMMs exhibit obvious difference between the adsorption amount of CO2 and CH4. The adsorption amount of CO2 improves with the increasing loading of [Ni3(HCOO)6]. Taking 7 wt% and 15 wt% P84/[Ni3(HCOO)6] MMMs as examples, the actual and theoretical values of adsorption amount are shown in Fig. 10, where the theoretical values are calculated with additive model (Uptaketheoretical=
w
1+w
Uptake[Ni3(HCOO)6]+61-
w 1+w
8 UptakeP84). By
comparison, it can be found that the actual adsorption amount of MMMs is lower than the theoretical one, which may be due to the partial pore blockage of the [Ni3(HCOO)6].
16
A
B
Fig. 10. Comparison of theoretical and actual adsorption amount of CO2 (A) and CH4 (B) for P84/[Ni3(HCOO)6] MMMs. At 5 bar, the MMMs with the [Ni3(HCOO)6] loadings of 7 wt% and 15 wt% exhibit CO2 adsorption almost equivalent to the theory values of the MMMs with 2 wt% and 7 wt% [Ni3(HCOO)6], respectively (Fig. 10A). However, the MMMs with 7 wt% [Ni3(HCOO)6] exhibit almost the same CH4 adsorption in comparation with pristine P84 membrane, indicating that CH4 can not enter the nanopores of [Ni3(HCOO)6]. And at the loading of 15 wt% [Ni3(HCOO)6], the MMMs exhibit the CH4 adsorption almost equivalent to the theory values of the MMMs with 2.4 wt% [Ni3(HCOO)6] (Fig. 10B). From the above analysis, it can be inferred that the partial pore blockage 17
favor excluding the larger molecules (CH4), suggesting that the pore size of [Ni3(HCOO)6] frameworks after partial pore blockage may be between 0.33 and 0.38 nm, which CO2 can get into while CH4 can not. So, the adsorption property of larger gas molecule (CH4) is more sensitive to the partial pore blockage. Compared with the pristine P84 membrane, the CO2 permeability (Fig. 7A) and its adsorption amount (Fig. 10A) for the MMMs with 15 wt% [Ni3(HCOO)6] loading increases 76 % and 16 % at 5 bar, respectively, so the CO2 diffusivity in MMMs increases 52 %. However, the CH4 permeability (Fig. 7A) of the MMMs with 15 wt% [Ni3(HCOO)6] loading is almost the same as the pristine P84 membrane, but its adsorption amount (Fig. 10B) increases, which resulting in the low CH4 diffusivity. It is obvious that the incorporation of [Ni3(HCOO)6] into P84 matrix can improve the CO2 diffusivity in spite of the partial pore blockage, but the CH4 diffusion in MMMs is prevented because of its larger size. The simplified diagram of the defect-free MMMs (7 and 15 wt% loadings) is shown in Fig. 11. It can be found that the P84 polymer chains has strong interaction with [Ni3(HCOO)6] so that there are no non-selective defects, and the pores are blocked by the polymer chains. Some pores are severely blocked by polymer chains so that both CO2 and CH4 can not get into. Other pores are partially blocked that CO2 can pass though preferentially because of its smaller size and plasticizing ability. The profitable role of the partial pore blockage on the gas transport properties of MMMs was also demonstrated in other literatures (Chung et al., 2007; Li et al., 2005).
Fig. 11. The pore blockage of [Ni3(HCOO)6] in P84/[Ni3(HCOO)6] MMMs. (3D molecular structure: Dark grey-C, light grey-H, red-O, blue-N, green-Ni.) 18
3.3.3. The mechanical properties of P84/[Ni3(HCOO)6] MMMs
A
B
Fig. 12. Effect of [Ni3(HCOO)6] loading on mechanical properties of P84/[Ni3(HCOO)6] MMMs. The mechanical properties of the pristine P84 and P84/[Ni3(HCOO)6] MMMs are shown in Fig. 12. The loading of [Ni3(HCOO)6] in MMMs has a little influence on Young’s modulus of MMMs. The tensile strength and the elongation at break decrease slightly when the loadings of [Ni3(HCOO)6] are 7 and 15 wt% in comparation with the pristine P84 membrane. This indicates that [Ni3(HCOO)6] dispersed in P84 matrix homogeneously. When the [Ni3(HCOO)6] loadings are 25 and 35 wt%, the tensile strength decreases 46% and 76%, and the elongation at break decreases 61% and 82%, 19
respectively, compared with the pristine P84 membrane. This is probably due to the agglomeration of [Ni3(HCOO)6] particles and the reduction of polymer chain entanglement. 4. Conclusions Submicrometer-sized [Ni3(HCOO)6] framework crystals, which have uniform ultra-micropore with micropore size of 0.46 nm, were synthesized by solvothermal method. They were incorporated into P84 matrix successfully for the first time, resulting in P84/[Ni3(HCOO)6] MMMs with good potential to separate CO2 from CH4. Detect-free MMMs were obtained when the [Ni3(HCOO)6] loadings were not more than 15 wt% and their gas separation performance was obviously enhanced. When the loading increased to 25 wt%, the agglomeration of [Ni3(HCOO)6] particles occurs, which resulting in the increase of CO2 permeability and decrease of CO2/CH4 selectivity. The partial pore blockage of the nanospace of [Ni3(HCOO)6] frameworks was confirmed by the high-pressure gravimetric adsorption measurements, which was profitable to separate CO2/CH4. Considering the cheapness and ease of preparation of [Ni3(HCOO)6] frameworks, the aforementioned MMMs promise good candidates for further research in CO2/CH4 separation. Acknowledgments This work was supported by the National Natural Science Foundation of China (21908215). References Adams, R., Carson, C., Ward, J., Tannenbaum, R., Koros, W., 2010. Metal organic framework mixed matrix membranes for gas separations. Microporous Mesoporous Mater. 131, 13-20. Amedi, H.R., Aghajani, M., 2016. Gas separation in mixed matrix membranes based on polyurethane containing SiO2, ZSM-5, and ZIF-8 nanoparticles. J. Nat. Gas Sci. Eng. 35, 695-702. Amooghin, A.E., Mashhadikhan, S., Sanaeepur, H., Moghadassi, A., Matsuura, T., Ramakrishna, S., 2019. Substantial breakthroughs on functionled design of 20
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25
1. [Ni3(HCOO)6] frameworks were incorporated into P84 matrix to separate CO2/CH4. 2. Defect-free MMMs were prepared and characterized at 7 wt% and 15 wt% loadings. 3. CO2/CH4 separation performance was obviously enhanced for the defect-free MMMs. 4. The pore blockage of [Ni3(HCOO)6] frameworks was profitable for gas separation.
Lujie Sheng: Conceptualization, Methodology, Formal analysis, Investigation, Writing-Original draft preparation Ya Guo: Methodology, Investigation, Formal analysis Dan Zhao: Writing – Review & Editing, Founding acquisition Jizhong Ren: Supervision, Writing – Review & Editing Shudong Wang: Supervision Maicun Deng: Supervision
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: