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ethane separation
Journal Pre-proof Synthesis and modification of moisture-stable coordination pillared-layer metalorganic framework (CPL-MOF) CPL-2 for ethylene/ethane separation Huan Xiang, Ahmed Ameen, Jin Shang, Yilai Jiao, Patricia Gorgojo, Flor R. Siperstein, Xiaolei Fan PII:
S1387-1811(19)30641-9
DOI:
https://doi.org/10.1016/j.micromeso.2019.109784
Reference:
MICMAT 109784
To appear in:
Microporous and Mesoporous Materials
Received Date: 23 August 2019 Revised Date:
26 September 2019
Accepted Date: 30 September 2019
Please cite this article as: H. Xiang, A. Ameen, J. Shang, Y. Jiao, P. Gorgojo, F.R. Siperstein, X. Fan, Synthesis and modification of moisture-stable coordination pillared-layer metal-organic framework (CPLMOF) CPL-2 for ethylene/ethane separation, Microporous and Mesoporous Materials (2019), doi: https:// doi.org/10.1016/j.micromeso.2019.109784. 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.
Synthesis and modification of moisture-stable CPL-2 MOF for ethylene/ethane separation GRAPHICAL ABSTRACT
Synthesis and modification of moisture-stable coordination pillared-layer metal-organic framework (CPL-MOF) CPL-2 for ethylene/ethane separation Huan Xianga, Ahmed Ameena, Jin Shangb,c, Yilai Jiaod, Patricia Gorgojoa, Flor R. Sipersteina,∗, Xiaolei Fana,∗ a
School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, United
Kingdom b
City University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park,
Nanshan District, Shenzhen, China c
School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
d
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72
Wenhua Road, Shenyang 110016, China
Abstract Adsorptive separation of ethylene/ethane (C2H4/C2H6) binary mixture has growing interest in petrochemical industries compared to the conventional energy-intensive cryogenic distillation. Development of moisture-stable materials with high selectivity is of great importance to accomplish C2H4/C2H6 separation. Coordination pillared-layer metal-organic framework (CPL-MOF) CPL-2 was synthesised at room temperature, and then modified by silver ions impregnation to enhance the selectivity towards ethylene over ethane. The synthesised CPL-2 and Ag/CPL-2 MOFs have excellent moisture stability which was confirmed by the dynamic water vapour adsorption analysis under 90% relative humidity, showing no significant framework decomposition, even at 50 °C. The calculated selectivity based on gravimetric single-component gas adsorption experiments shows the significantly improved C2H4/C2H6 selectivity from 1.4 to 26.1 after loading 10 wt.% (theoretical) of silver ions on CPL-2. Breakthrough experiments for C2H4/C2H6 (1:1, v/v) mixture suggest that both CPL-2 and 10 wt.% Ag/CPL-2 can achieve the binary mixture separation, and 10 wt.% Ag/CPL-2 shows relatively better dynamic separation performance compared to parent CPL-2. The good adsorption selectivity and moisture stability allow CPL-MOF to be a class of promising porous materials for further exploitation in the separation of C2H4/C2H6 mixtures. Additionally, the method presented here can potentially be extended to other CPLs with different pore sizes for alkene/alkane separations. Keywords: Ethylene; Ethane; Adsorption; Coordination pillared-layer metal-organic framework (CPL-MOF); Selectivity; Moisture stability
∗
Corresponding authors.
Email address: [email protected] (FS). Email address: [email protected] (XF). 1
1. Introduction Light hydrocarbons are important energy resources and feedstocks for the production of many petrochemical products [1, 2]. Alkene/alkane separation by alternative processes to expensive cryogenic distillation is now considered as one of the seven chemical separations to change the world [3]. To date, the industrial practice of separating alkene/alkane mixtures is mainly based on the cryogenic distillation [4]. Cryogenic distillation must be performed at low temperature and high pressure, which makes it an energy-intensive process [5]. For ethylene/ethane mixture separation, it needs to be carried out at about −25 °C and 23 bar in a column with over 100 trays due to the similar boiling point of ethylene (−104 °C) and ethane (−89 °C) [6-8]. Therefore, highly energy efficient separation methods are demanded to replace the current industrial standards for ethylene/ethane separation. Among all alternative technologies, adsorptive separation is regarded as the promising option, especially with the emergence of various functional porous adsorbents [9, 10]. Developing an adsorbent with high selectivity (towards the target guest molecule), as well as excellent stability, is the key step to enable the adsorptive separation in practice. Metal-organic frameworks (MOFs), as a new class of porous materials, have attracted considerable attention due to their combined properties of large specific surface area, pore volume, functionalised and adjustable pore structure and surface [11-13]. During the past decades, an increasing number of MOFs, e.g. Fe2(O2)(dobdc) [14], ZIFs [15-17], IRMOF-8 [18, 19], MAF-49 [20], PCN-250 [8], MIL-142A [21] and Ni(bdc)(ted)0.5 [22], were proposed to be ethane-selective materials and investigated for ethane/ethylene (C2H6/C2H4) separation using the typical volumetric ethane/ethylene composition of 1:15 simulating the industrially cracked gas mixture [20, 23]. However, only Fe2(O2)(dobdc) with the Fe-peroxo sites shows the strong interaction with ethane, other MOFs do not have specific adsorption sites, and hence the adsorption affinity of ethane molecule onto them is mainly dominated by the van der Waals interactions. Fe2(O2)(dobdc) exhibits a ethane selectivity of 4.4 at 25 °C, and MAF-49 shows an excellent selectivity (ca. 9 at 43 °C) at 1 bar, while lower selectivities have also been reported for the same material (ca. 2.7 at 25 °C) [14]. Other adsorbents show a poor selectivity of about 2 at atmospheric pressure, even though the good selectivity could be obtained at the very low pressure, e.g. 4.8 on In-soc-MOF-1 at 25 °C and 1 kPa [23]. For ethylene-selective adsorbents, MOFs with open metal sites (OMSs), such as HKUST-1 [24-27] and MOF-74 [28-31] have been widely studied due to the specific interaction between OMSs and the carbon-carbon double bond in the ethylene molecule. For example, Fe-MOF-74 has been reported to have a selectivity of up to 13–18 for an equimolar mixture of ethylene and ethane at 45 °C [28]. However, OMSs interact strongly with water molecules as well, resulting in material decomposition and poor moisture or water stability (i.e. stability in water vapour or liquid water) [32-36], especially at elevated temperatures, limiting their potential in practical settings [37, 38]. Therefore, the development of MOFs with high selectivity and hydrothermal stability is always of great importance for separation applications, including the ethylene/ethane separation. Over the recent years, pillared-layer MOFs consisting of two types of organic linkers have been proposed and studied in the separation of alkene/alkane mixtures, which are exemplified by the coordination pillared-layer (CPL) MOFs. CPL-MOFs are structured by scaffolding 2D Cu(II) and pyrazine-2,3-dicarboxylate (pzdc) layers using dipyridyl ligands (such as pyrazine, 4,4′-bipyridine 2
and 1,2-di(4-pyridyl)ethylene, as the bridging pillars) [39]. CPL-MOFs can be facilely synthesised under mild conditions (e.g. at room temperature) and their pore sizes can be easily modified by changing the pillar ligands (L). They also show good flexibility and unique guest-responsive nature. CPL-1 (L = pyrazine) with a pore size of 4×6 Å2 has already been investigated in the separation of propylene/propane [4] and ethylene/ethane [40] mixtures. Both adsorption isotherm tests and breakthrough experiments revealed that CPL-1 exhibited the selective adsorption of the ethylene molecules with a selectivity of 3.8 from breakthrough experiments. In general, selectivity is expected to decrease in larger pore materials, but they may have lower mass transfer resistances leading to sharper breakthrough curves, and more importantly, it may be possible to functionalise or modify them without causing much pore blocking. It has been previously reported that an increase in ethylene selectivity can be achieved using the Ag(I) ions impregnated materials via the formation of π-complexes with ethylene molecules [41, 42]. Aguado et al. have shown for the first time that absolute ethylene/ethane separation could be achieved by ethane exclusion on Ag-exchanged zeolite A (AgA) [43]. As a result, AgA showed 100% ethylene selectivity. Ag(I) was also introduced into (Cr)-MIL-101-SO3H MOF, leading to a much higher selectivity (ca. 16) than the original MOF (ca. 1.15) at 30 °C and 1 bar [44, 45]. Ag-decorated NUS-6(Hf) MOF showed an ethylene/ethane selectivity of 6 at 25 °C and 1 bar, which was 5 times higher than that of NUS-6(Hf) [46]. Moreover, Ag(I) ions have also been successfully dispersed on other materials, such as faujasite-type zeolites (X and Y) [6, 43], SBA-15 [42], MCM-41 [47, 48], ETS-10 [49], SiO2 [47] and resin [6, 50], to improve alkene/alkane separation performance for practical applications. In this work, CPL-2 (L = 4,4′-bipyridine) with a pore size of 9×6 Å2 was selected as the model MOF and synthesised at room temperature for separating binary ethylene/ethane mixture. In addition, the moisture stability of CPL-2 was also evaluated under dynamic humid conditions (from 0% to 90% relative humidity). Ag(I) ions modification of CPL-2 was also conducted to explore the possibility of improving the selectivity of CPL-MOFs for ethylene/ethane separation. 2. Experimental 2.1. Materials Copper(II) perchlorate hexahydrate (Cu(ClO4)2·6H2O), 98%), 2,3-pyrazinedicarboxylic acid (H2pzdc, 97%), 4,4′-bipyridine (bpy, 98%), methanol (≥ 99.9%), nitric acid (HNO3, ≥ 65%) and hydrochloric acid (HCl, 37%) were purchased from Sigma Aldrich. Sodium hydroxide (NaOH, 97%) was purchased from Honeywell. Silver nitrate (AgNO3) was purchased from Fisher and ethanol (absolute) was purchased from VWR Chemicals. All chemicals were used as received without further purification. C2H4 (99.9%), C2H6 (99.0%) and N2 (99.99%) gases were purchased from BOC Ltd Company. 2.2. Synthesis of materials Standard hydrothermal synthesis was used in this work to prepare CPL-2 [51]. H2pzdc (1 mmol, 168.1 mg) and bpy (0.5 mmol, 78 mg) were dissolved in a 100 mL mixture containing 50 mL NaOH aqueous solution (0.04 M) and 50 mL ethanol. The mixture was stirred for 10 min until a 3
clear solution was obtained. The resulting solution was then added slowly to an aqueous solution (100 mL) containing Cu(ClO4)2·6H2O (1 mmol, 370 mg). The final mixture was stirred for 3−24 h at room temperature. After synthesis, the blue precipitate was filtered and then washed with water and methanol to remove the unreacted chemicals, and then activated by drying at 100 °C and atmospheric pressure overnight in a convention oven for further use (i.e. the as-synthesised CPL-2). Silver ions modified CPL-2 MOFs (denoted as Ag/CPL-2) were prepared using the incipient wetness impregnation method. The as-synthesised CPL-2 (0.4 g) was immersed in a 1:1 mixture of ethanol/water solution (8 mL) containing different amounts of silver nitrate (0.19 mmol, 0.41 mmol and 0.65 mmol). The mixture was stirred continuously for 4 h at room temperature, and then dried at 60 °C overnight to give the Ag/CPL-2 samples with different Ag(I) loadings, denoted as x wt.% Ag/CPL-2 (x wt.% represents the theoretical concentration of silver ions on CPL-2 adsorbent, i.e. 5 wt.%, 10 wt.% and 15 wt.%, respectively). 2.3. Characterisation of materials Powder X-ray diffraction (PXRD) was performed on a Philips X’Pert X-ray diffractometer with CuKα1 radiation (λ = 1.5406 Å, 40 kV, 40 mA) using a step scan mode (0.02° per step) at 2θ ranging from 5° to 50°. Scanning electron microscopy (SEM) was carried out using FEI Quanta 250 ESEM instrument with a high voltage mode of 20 kV. All samples were coated with platinum using Cressington Pt sputter coater under vacuum prior to test. Qualitative composition analysis of materials was acquired by an Oxford energy-dispersive X-ray spectroscopy (EDS) system. Nitrogen (N2) adsorption/desorption isotherms at −196.15 °C were obtained using Micrometrics ASAP 2020 analyser. The degassing was carried out at 100 °C and under vacuum for 12 h. Surface area was determined using the Brunauer-Emmett-Teller (BET) method. Thermal gravimetric analysis (TGA) was performed using a TA-Q5000 analyser. Samples were heated from ambient temperature to 700 °C at a heating rate of 10 °C/min under N2 at 60 mL/min. Dynamic water vapour adsorption analyses at 25 °C and 50 °C with relative humidity values ranging from 0% to 90% (10% per step) were measured using a dynamic vapour sorption (Surface Measurements Systems, DVS 1) equipment. Prior to the water vapour adsorption, all samples were dried at 0% relative humidity for 3 h. The actual concentration of silver ions on Ag/CPL-2 samples was analysed by inductively coupled plasma optical emission spectrometer (ICP-OES, PlasmaQuant PQ9000). All samples were dissolved by HCl and HNO3 for the ICP-OES analysis and the 328.07 nm line was used to detect the Ag(I) ion in the samples [52]. In ICP-OES analysis, silver standard with a concentration of 1000 mg/L was diluted to 0–40 mg/L to obtain a standard curve of silver ions. 2.4. Single-component gas adsorption of C2H4, C2H6 and N2 on materials Ethylene, ethane and nitrogen adsorption isotherms on developed CPL-2 and Ag/CPL-2 materials were measured using an intelligent gravimetric analyser (Hiden Isochema, IGA-001). The IGA analysis was based on the static gravimetric technique, i.e. using a microbalance to measure the weight changes as a function of pressure at a constant temperature of 25 °C, 35 °C and 50 °C, respectively. Langmuir model based on monolayer adsorption theory was employed to study the experimental
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adsorption isotherms, which is commonly expressed as Eq. 1: N KP N = sat (1) 1 + KP where K (bar−1) is the Langmuir constant, N (mmol/g) is the equilibrium adsorption uptake at the corresponding adsorption pressure P (bar) and Nsat (mmol/g) is the saturation capacity for monolayer adsorption. The ideal adsorbed solution theory (IAST) [53, 54] was combined with the Langmuir model to calculate the C2H4/C2H6 selectivity. The selectivity (S) of C2H4 over C2H6 can be calculated as Eq. 2:
S=
xethane xethylene
(2)
yethane yethylene
where xethane and xethylene are the mole fraction of C2H6 and C2H4 in the adsorbed phase (on CPL-2 MOF adsorbent), whereas yethane and yethylene are their corresponding mole fraction in the gas bulk phase. The isosteric heat of adsorption (Qst, kJ/mol) was calculated by the Clausius-Clapeyron equation based on the adsorption isotherms measured at 25–50 °C, and the equation can be expressed as: ∂ ln P Qst = RT 2 ( )n (3) ∂T where R (J/(mol·K)) is the gas constant, T (K) is the temperature, P (bar) is the pressure, and n (mmol/g) is the gas adsorption uptake.
2.5. Breakthrough experiments on materials The breakthrough experiments of an equimolar mixture (C2H4/C2H6 = 1:1, v/v) were carried out in a dynamic breakthrough set-up at 25 °C (Fig. S1). Activated sample (about 1 g) with a particle size of 250–425 µm was packed into a stainless-steel column (φ 4×120 mm), and the void space of the two ends were filled with glass wool. The column packed with sample was firstly purged with helium (He, 50 mL/min) for 30 min at 25 °C before the breakthrough test. The total flow rate of the binary mixture was controlled to be 2 mL/min, regulated by the mass flow controller (MFC). A gas chromatography (Aglient 490 Micro GC) with a PoraPLOT U (PPU) column and a thermal conductivity detector (TCD) was used for on-line analysis of the outlet gases from the packed column. 3. Results and discussion 3.1. Characterisation of CPL-2 CPL-2 can be easily synthesised at room temperature, which was confirmed by XRD (Fig. 1a) and SEM (Fig. S2) analysis of samples from the systematic study of varying synthesis time from 3−24 h. Characteristic peaks at 2θ = 6.4°, 8.8°, 10.3° and 12.6°, matching the simulated results [55, 56], are identified in all the as-synthesised CPL-2 MOFs, confirming the presence of the pillared-layer structure in the crystalline domain. N2 physisorption of the synthesised CPL-2 (Fig. 1b and Table 5
S1) shows the reversible typical type I isotherm for microporous CPL-2 and gave the BET value of about 546 m2/g, which is in good agreement with the reported ones [51, 57] in the literature (e.g. 530 m2/g measured by Meza-Morales et al. [57].
Fig. 1. (a) PXRD patterns of CPL-2 synthesised at different times and (b) N2 adsorption/desorption isotherms of CPL-2 (synthesised for 24 h) at −196.15 °C with reported results from García-Ricard et al. [51] and Meza-Morales et al. [57].
The intrinsic thermal stability of the as-synthesised CPL-2 was assessed by TGA under N2. Fig. S3 depicts the according weight loss profile of CPL-2, showing the three-stage weight loss of CPL-2 under the condition used. The initial weight loss of about 12.6 wt.% between room temperature to 100 °C was due to the desorption of water molecules, which are weakly present within the pores of CPL-2 due to physisorption. The second stage of abrupt weight loss started at around 260 °C, corresponding to the collapse of pillar ligands. The final stage was gradual, representing the decomposition of the pzdc linker in the remaining 2D structure. It led to the remaining weight registered as ca. 20.9% which are copper species. According to the unit cell composition, CPL-2 is composed of 54 wt.% pzdc, 25 wt.% bpy and 21 wt.% Cu. Hydrophobicity is a desirable feature of the adsorbent for light hydrocarbons separation applications. For MOF-based adsorbents, poor water and moisture stability, specifically MOFs with OMSs [33, 34], is one of the major issues for the further exploration towards the practical applications. In this work, the hydrothermal stability of CPL-2 was studied at two different temperatures of 25 °C and 50 °C using dynamic water vapour adsorption technique [33] at the relative humidity of 0–90% (Fig. 2). The water uptake profiles in Fig. 2a and 2b are significantly different from those observed in MOFs with OMSs, such as HKUST-1 [33, 58, 59]. Here, the system is reversible whereas in HKUST-1 [33] apparently 6.7 mmol/g (ca. 20%) of the adsorbed water remains in the material after the desorption cycle at 25 °C. The adsorbed water molecules are easily to desorb from CPL-2, suggesting that the interaction between water molecules and CPL-2 is weak. This weak interaction is due to no OMSs existed in CPL-2 MOF. The metal Cu(II) in CPL-2 is fully coordinated with two linkers (pzdc and bpy), resulting in no specific adsorption site accessible for water molecules. The adsorption/desorption isotherms of water vapour on CPL-2, i.e. the uptake of water over the 6
dry mass of CPL-2 as a function of the relative pressure (ratio of water vapour pressure to the water vapour pressure at 90% relative humidity) are shown in Fig. 2c and 2d. It was found that the adsorption of water on CPL-2 was reversible in all cycles under the condition used, suggesting the physisorption of water on CPL-2. PXRD analysis (Fig. S4) confirms that the crystalline structure of CPL-2 MOF was retained after the cyclic water vapour adsorption tests (4 cycles). Beside, CPL-2 also exhibits good stability under dynamic humid conditions at 50 °C, whereas in MOFs with OMSs like HKUST-1, rapid degradation is observed at high temperatures [58], indicating the potential of CPL-2 as adsorbent for practical applications in the presence of water vapour.
Fig. 2. Cyclic water vapour adsorption and desorption at different partial vapour pressures on CPL-2 at (a) 25 °C and (c) 50 °C and water vapour adsorption/desorption isotherms on CPL-2 at (b) 25 °C and (d) 50 °C.
3.2. Gas adsorption isotherms on CPL-2 The single-component adsorption/desorption isotherms of ethylene, ethane and nitrogen on CPL-2 were measured over a pressure range of 0–10 bar at 25, 35 and 50 °C, respectively, as shown in Figs. 3Fig. 3a–3c. All adsorption isotherms are typical type I isotherms (i.e. monolayer adsorption within microporous adsorbents), which is characterised by a plateau after an equilibrium saturation point. The adsorption of the three probing gases on CPL-2 is reversible, suggesting that CPL-2 can be regenerated easily in the adsorption process. CPL-2 prefers to adsorb C2H4 over C2H6 and N2, as shown in Fig. 3. Table 1 shows the
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polarisability (α) and quadruple moment (Θ) of the gases studied to help rationalise the observed adsorption behaviour. The polarisabilities of C2H4 and C2H6 are significantly larger than the nitrogen polarisability, but if the adsorption was driven exclusively by dispersion interactions, then the adsorption of C2H6 would be more favourable than the adsorption of C2H4. Given that C2H4 is preferentially adsorbed over C2H6, then we can infer that electrostatic interactions play a significant role in these systems because the quadruple moment of C2H4 is higher than C2H6. Despite N2 having a similar quadruple moment to C2H4, its adsorption is weak due to its low polarisability compared to the other gases studied. A summary of the gas adsorption uptake at 10 bar and at different temperatures is presented in Table S2, showing that the gas uptake of CPL-2 decreased with the increase in adsorption temperature for all three gases, confirming the gas adsorption is exothermic. At zero coverage, the isosteric heats of adsorption were 23.5, 16.5 and 10.6 kJ/mol for C2H4, C2H6 and N2, respectively. Langmuir model was applied to study C2H4 and C2H6 adsorption isotherms, and the fitting parameters are listed in Table S3. Fig. 3d shows the IAST predicted C2H4/C2H6 selectivity for their binary mixtures based on the obtained fitting parameters. The adsorption selectivity was in the range of 1.1–1.6, and slightly increased with an increasing pressure. The calculated C2H4/C2H6 selectivity of CPL-2 is relatively lower than those of the previously reported C2H4-selective MOFs with OMSs, e.g. CuBTC (ca. 2) [26] and Fe-MOF-74 (ca. 13–18) [28]. Besides, the selectivities of different mixture compositions (C2H4/C2H6 = 1:1 and 15:1, v/v)) were comparable, indicating the mixture composition would not affect the adsorption selectivity of CPL-2 under the conditions used.
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Fig. 3. C2H4, C2H6 and N2 adsorption/desorption isotherms on CPL-2 at: (a) 25 °C, (b) 35 °C and (c) 50 °C, and (d) IAST selectivity for C2H4/C2H6 (1:1 and 15:1, v/v) mixtures on CPL-2. Solid symbols represent adsorption isotherms, whereas open symbols represent desorption isotherms.
Table 1 Relevant physical parameters of ethylene, ethane and nitrogen [10, 31].
Adsorbate C2H4 C2H6 N2
Θ (×10−26 esu cm2)
α (×10−25 cm3) 42.52 44.3–44.7 17.40
1.50 0.65 1.52
The density in the adsorbed phase, calculated as the amount adsorbed at a given pressure divided by the pore volume, is expected to increase slightly with pore size at high pressures due to the ability of molecules to pack better in larger pores. This coincides with observations from Kaneko et al. [60] who studied the adsorption of three different probing gases (Ar, N2 and CO2) on CPL-1, 2, 5 with a pore size of 4×6, 9×6 and 11×6 Å2, respectively, and found that space-filling ratio (i.e. the ratio of the void volume obtained from XRD analysis to the micropore volume measured by gas adsorption) increased with an increase in the rectangular pore size, i.e. CPL-5 > CPL-2 > CPL-1. Similar observations have been found in this work for C2H4 and C2H6 when comparing the density of the fluid in the adsorbed phase, calculated as the amount adsorbed divided by the pore volume. At 25 °C and 1 bar (see Table S4), the densities of C2H4 and C2H6 on CPL-2 are approximately 8.3 and 7.0 mmol/cm3, which are significantly larger than those of CPL-1, where the densities are less than 1.4 mmol/cm3. 3.3. Modification of CPL-2 using Ag(I) ions The impregnation method was used to incorporate silver ions in CPL-2 for modifying its selectivity in C2H4/C2H6 separation. PXRD characterisation of the resulting Ag/CPL-2 samples shows all characteristic diffraction peaks of CPL-2 (Fig. S6), confirming the intactness of the pillared-layer structure of CPL-2 after the modification. However, the intensity of diffraction peaks of Ag/CPL-2 decreased by increasing the Ag(I) loading possibly due to inclusion of guest molecules in the porous framework of CPL-2, accordingly, 15 wt.% Ag/CPL-2 shows the lowest diffraction peak intensity.
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SEM analysis of Ag/CPL-2 MOFs (Fig. 4 and S4) shows the typical squared slab morphology of CPL-2. Additionally, Ag/CPL-2 MOFs also exhibit the comparable thermal stability to that of the parent CPL-2 (Fig. S8). Thus, the introduction of Ag(I) ions did not alter the crystal structure, morphology and thermal stability of parent CPL-2 significantly. According to comparative EDS mapping characterisation of CPL-2 and Ag/CPL-2 (using 5 wt.% Ag/CPL-2 as the representative), as shown in Fig. 4, silver species were successfully loaded on CPL-2 MOF. The calculated concentrations of silver ions in Ag/CPL-2 MOFs using ICP-OES were lower than the theoretical values: 3.96, 5.11 and 8.38 wt.%, measured versus 5, 10 and 15 wt.% theoretical values. The loss of silver species might happen during the preparation and workup of Ag/CPL-2 MOFs.
Fig. 4. EDS images of CPL-2 and 5 wt.% Ag/CPL-2.
Ag/CPL-2 MOFs showed similar adsorption behaviour to the parent CPL-2 in N2 physisorption, as shown in Fig. 5a, but with significantly lower capacity under the condition used in BET measurements, indicating the micropore blockage due to Ag(I) impregnation. The adsorption capacity and the calculated BET surface area and pore volume are inversely related to the Ag(I) loading (Fig. 5a and Table S5). Chang and co-workers also observed a reduction in surface area and pore volume upon impregnation of (Cr)-MIL-101-SO3H MOF with Ag(I) [44], but the reduction they observed was on the order of 30% whereas in this work a reduction of 80% in the pore volume is observed. It should be noted that the cages in MIL-101 materials have an approximate diameter of 30−34 Å [61], whereas the pore diameter in CPL-2 is around 5−6 Å [57], therefore the presence of Ag(I) ions can have a more significant effect in blocking the access to the micropore volume. Pore size distribution calculated by Horvath-Kawazoe (HK) method (Fig. 5b) shows that the porous structure of Ag/CPL-2 consists of micropores mainly with the pore width of 7−11 Å.
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Fig. 5. (a) N2 adsorption/desorption isotherms at −196.15 °C and (b) pore width distributions of Ag/CPL-2 MOFs.
The moisture stability of Ag/CPL-2 MOFs was evaluated at 25 °C using DVS (Fig. S10), showing weak physisorption and no degradation of the materials (Fig. S11), suggesting a similar stability to the parent material. The water uptake decreases with Ag(I) loading (Fig. S10), but the water uptake close to condensation pressure is practically insensitive to the Ag(I) loading, suggesting that some ions may be on the external surface and hydrate at P/P90 = 1.0. 3.4. Improved selectivity to C2H4 of Ag/CPL-2 The adsorption selectivity of Ag/CPL-2 MOFs was determined from pure component gas adsorption isotherms as shown in Fig. 6. Considering the adsorption processes are generally carried out at ambient temperature and pressure, all gas adsorption isotherms on Ag/CPL-2 were measured at 25 °C and over a pressure range of 0–1 bar. Ag/CPL-2 MOFs showed the reduced adsorption capacity of both gases compared to the parent CPL-2 MOF, especially for the sample with the theoretical 15 wt.% Ag(I) loading. As seen from Fig. 6a–6c, the C2H4 adsorption uptakes on Ag/CPL-2 MOFs are significantly larger than those of C2H6 after modification. It reveals that Ag(I) ions interact strongly with C2H4 molecules by forming π-complexes with the carbon-carbon double bonds in C2H4, and then enhance the selectivity towards C2H4 over C2H6. The π bond formed between silver ions and C2H4 is a weak chemical bond, which means chemisorption occurs on Ag/CPL-2 MOFs, in addition to physisorption due to the confinement of the MOFs. It could be confirmed by the calculated isosteric heats of adsorption on Ag/CPL-2 (using 5 wt.% Ag/CPL-2 as the representative), as shown in Fig. 7. The isosteric heat of C2H4 adsorption on Ag/CPL-2 was obviously higher than that on the parent CPL-2, i.e. 42.9 kJ/mol versus 23.5 kJ/mol at zero coverage. Conversely, for C2H6 adsorption, the isosteric heats on both adsorbents were comparable (15.2 kJ/mol on Ag/CPL-2 and 16.5 kJ/mol on Ag/CPL-2). Moreover, the interaction with Ag(I) results in a larger adsorption per unit area for both gases, but the increase is significantly more important for C2H4 than for C2H6.
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Fig. 6. Adsorption isotherms of C2H4 and C2H6 on CPL-2 and Ag/CPL-2 at 25 °C: (a) C2H4, (b) C2H6, and (c) comparison of equilibrium adsorption capacity of C2H4 and C2H6 at 1 bar, and (d) IAST selectivity for C2H4/C2H6 (1:1, v/v) mixture on CPL-2. Solid symbols represent the experimental adsorption isotherms, whereas the dashed lines represent the isotherms fitted by the theoretical model.
Fig. 7. Isosteric heats of C2H4 and C2H6 adsorption on (a) CPL-2 and (b) 5 wt.% Ag/CPL-2.
The experimental adsorption isotherms on Ag/CPL-2 were also studied by Langmuir model and the fitting parameters are listed in Table S6. The IAST selectivity obtained using Eq. 2 is shown in Fig. 6d. Similarly, the selectivity of Ag/CPL-2 also increased as pressure increased, but more rapidly (ca. 12
32%, 74% and 90% for 5 wt.%, 10 wt.% and 15 wt.% Ag/CPL-2, respectively) than the parent CPL-2 MOF (ca. 16%). Compared to the selectivity of 1.4 of CPL-2 at 1 bar and 25 °C, the selectivities of 5 wt.%, 10 wt.% and 15 wt.% Ag/CPL-2 were much higher, i.e. 4.2, 26.1 and 63.7, respectively. The C2H4/C2H6 selectivity obviously increased with increasing Ag(I) loading. It also confirmed that C2H4 molecules interact strongly with the Ag(I) ions on CPL-2. 3.5. Breakthrough experiments Breakthrough experiments were performed at 25 °C and 1 bar to evaluate the dynamic separation performance of CPL-2 and 10 wt.% Ag/CPL-2, which exhibited the best adsorption performance considering both adsorption capacity and selectivity. Fig. 8 shows the experimental breakthrough curves for C2H4/C2H6 (1:1, v/v) mixture on parent CPL-2 and 10 wt.% Ag/CPL-2. The breakthrough time of C2H4 was longer than that of C2H6 on both adsorbents, indicating that the solid adsorbents prefer to adsorb C2H4 over C2H6 molecules, which is consistent with the isosteric heats of adsorption (Fig. 7). The breakthrough curves demonstrated that the binary mixture could be separated on both CPL-2 and 10 wt.% Ag/CPL-2 in the dynamic adsorption process, and 10 wt.% Ag/CPL-2 displayed slightly better separation performance than CPL-2, as evidenced by the larger difference in C2H4 and C2H6 concentrations of the effluent. It also confirmed that the introduction of Ag(I) ions enhanced the interactions between adsorbents and C2H4 molecules.
Fig. 8. Breakthrough curves for C2H4/C2H6 (1:1, v/v) mixture on (a) CPL-2 and (b) 10 wt.% Ag/CPL-2 at 25 °C and 1 bar.
3.6. Effect of water vapour on adsorption capacity of materials CPL-2 and 10 wt.% Ag/CPL-2 MOFs exhibited good moisture stability at 25 °C, having possibility of being further explored as adsorbents under humid conditions. Indeed, the influence of water vapour on the adsorption equilibrium of ethylene, ethane and their mixtures is important, and an understanding of the nature of such effects. However, the equilibrium adsorption experiments were based on the gravimetric method, which is not capable to distinguish the relevant selectivity, i.e. the weight change of the sample cannot reflect the adsorbed proportion of guest molecules when a mixture is used. Additionally, the presence of water vapour in the continuous stream for the dynamic adsorption of gas mixtures of C2H4/C2H6, i.e. the breakthrough experiment, is also challenging due to the in-line GC analysis (water vapour can damage the GC column, affecting its
13
separation performance of other gases for analysis). Accordingly, additional experiments (i.e. the equilibrium and breakthrough adsorption) were performed using the water vapour saturated samples to gain more information regarding this aspect. Prior to the tests, CPL-2 and 10 wt.% Ag/CPL-2 were exposed at 100% relative humidity (i.e. the water vapour partial pressure equals the saturated water vapour pressure) at 25 °C for 0.5 h, which was achieved by flowing wet N2 at 50 mL/min through the materials (the wet N2 flow was achieved by saturating dry N2 via a bubbler containing deionised water). The gas adsorption isotherms and IAST selectivity on two pre-treated adsorbents with water vapour are presented in Fig. S13 and Fig. S14 (and Table S6), respectively. In comparison with the results obtained on the pristine adsorbents, the relevant adsorption capacity and selectivity are comparable, suggesting that the water vapour pre-treatment did not affect the intrinsic adsorption property of the two adsorbents. Moreover, as shown in Fig. S15, both adsorbents could also achieve C2H4/C2H6 (1:1, v/v) mixture separation after being saturated by water vapour, and the pre-treated 10 wt.% Ag/CPL-2 also exhibited better dynamic separation performance than the pre-treated CPL-2 adsorbent, confirming the presence of water vapour has a negligible effect on the separation performance of the materials under study.
4. Conclusions Herein, we presented the synthesis and modification of moisture-stable CPL-2 MOFs (via Ag(I) impregnation to give Ag/CPL-2 MOFs) and the application of CPL-2 and Ag/CPL-2 MOFs in ethylene/ethane separation. The synthesis of CPL-2 is simple with fast crystallisation, i.e. 3 h is sufficient to prepare CPL-2 MOF at room temperature. CPL-2 demonstrated the outstanding moisture stability under dynamic humid conditions at 25 °C and 50 °C, showing the reversible water vapour adsorption under the conditions used. This feature allows CPL-2 to be exploited as the promising adsorbent in humid environment. The obtained results also confirmed that CPL-2 has larger adsorption capacity compared to CPL-1 with smaller pore networks. It provides a good understanding about the pore size effect on the selection of materials for ethylene/ethane separation, indicating that different CPLs can be also developed and exploited for hydrocarbon separations. Micropore blockage of CPL-2 framework was measured by N2 physisorption analysis after the introduction of Ag(I) ions on CPL-2, showing a significant decrease in adsorption capacity for Ag/CPL-2 MOFs comparing to the parent CPL-2. However, the Ag species enhanced the interaction between Ag/CPL-2 adsorbents and ethylene molecules, resulting in the improved selectivity, e.g. 26.1 at 1 bar and 25 °C for 10 wt.% Ag/CPL-2 MOF compared to 1.4 of the parent material. Single-component gas adsorption experiments using C2H4, C2H6 and N2 were studied to assess the effect of polarisability and electrostatic interactions (quadrupole moment) upon the adsorbent modification. The results showed that despite the reduction in the surface area in the modified materials, the amount adsorbed per unit area increases. The IAST selectivity for C2H4/C2H6 mixture increased in the presence of Ag(I). This suggests that electrostatic (or in this case quadrupole-ion) interactions dominate. The trade-off between adsorption capacity and selectivity of porous adsorbents should be 14
considered for practical gas separation. In the case of CPL-2 MOF, this study showed that 10 wt.% Ag/CPL-2 by Ag(I) impregnation gave the best adsorption performance considering both capacity and selectivity towards C2H4 over C2H6. Breakthrough experiments for C2H4/C2H6 (1:1, v/v) mixture confirmed that 10 wt.% Ag/CPL-2 showed comparatively better dynamic separation performance than parent CPL-2. The good selectivity combined with the moisture stability makes CPL-2 MOFs worthy of being further developed and explored for the separation of ethylene/ethane mixtures, especially under conditions where water vapour exists. Although the presence of water vapour had a negligible effect on the adsorption properties of CPL-2 materials under the conditions used, a systematic study of the effect of water vapour on the adsorption capacity and selectivity should be considered in the future for a comprehensive assessment of the adsorbent’s potential in practical applications. Acknowledgements HX thanks The University of Manchester President’s Doctoral Scholar Award and the China Scholarship Council (file no. 201606150068) for supporting her PhD research. We are also grateful to Dr. S. Holmes for the N2 adsorption/desorption measurements.
References [1] S.Y. Pu, J.W. Wang, L.Y. Li, Z.G. Zhang, Z.B. Bao, Q.W. Yang, Y.W. Yang, H.B. Xing, Q.L. Ren, Performance comparison of metal organic framework extrudates and commercial zeolite for ethylene/ethane separation, Ind. Eng. Chem. Res. 57 (2018) 1645–1654 https://doi.org/10.1021/acs.iecr.7b04391. [2] H. Xiang, X.L. Fan, F.R. Siperstein, Design of 2D materials for selective adsorption: a comparison between Monte Carlo
simulations
and
direct
numerical
integration,
Mol.
Syst.
Des.
Eng.
3
(2018)
636–644
https://doi.org/10.1039/c8me00024g. [3] D.S. Sholl, R.P. Lively, Seven chemical separations to change the world, Nature 532 (2016) 435–437
https://doi.org/10.1038/532435a. [4] Y. Chen, Z. Qiao, D. Lv, C. Duan, X. Sun, H. Wu, R. Shi, Q. Xia, Z. Li, Efficient adsorptive separation of C3H6 over C3H8
on
flexible
and
thermoresponsive
CPL-1,
Chem.
Eng.
J.
328
(2017)
360–367
https://doi.org/10.1016/j.cej.2017.07.044. [5] L. Mei, Y.F. Wu, X. Zhou, J. Yan, F. Xu, Z. Li, Adsorption performance of MIL-100(Fe) for separation of olefin-paraffin
mixtures,
J.
Taiwan
Inst.
Chem.
Eng.
70
(2017)
74–78
https://doi.org/10.1016/j.jtice.2016.10.047. [6] R.T. Yang, E.S. Kikkinides, New sorbents for olefin/paraffin separations by adsorption via π-complexation, AIChE J. 41 (1995) 509–517 https://doi.org/10.1002/aic.690410309.
15
[7] B. Li, Y. Zhang, R. Krishna, K. Yao, Y. Han, Z. Wu, D. Ma, Z. Shi, T. Pham, B. Space, J. Liu, P.K. Thallapally, J. Liu, M. Chrzanowski, S. Ma, Introduction of π-complexation into porous aromatic framework for highly selective adsorption of ethylene over ethane, J. Am. Chem. Soc. 136 (2014) 8654–8660 https://doi.org/10.1021/ja502119z. [8] Y.W. Chen, Z.W. Qiao, H.X. Wu, D.F. Lv, R.F. Shi, Q.B. Xia, J. Zhou, Z. Li, An ethane-trapping MOF PCN-250 for highly
selective
adsorption
of
ethane
over
ethylene,
Chem.
Eng.
Sci.
175
(2018)
110–117
https://doi.org/10.1016/j.ces.2017.09.032. [9] Z.B. Bao, G.G. Chang, H.B. Xing, R. Krishna, Q.L. Ren, B.L. Chen, Potential of microporous metal-organic frameworks
for
separation
of
hydrocarbon
mixtures,
Energy
Environ.
Sci.
9
(2016)
3612–3641
https://doi.org/10.1039/c6ee01886f. [10] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal-organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504 https://doi.org/10.1039/b802426j. [11] V.V. Butova, M.A. Soldatov, A.A. Guda, K.A. Lomachenko, C. Lamberti, Metal-organic frameworks: structure, properties,
methods
of
synthesis
and
characterization,
Russ.
Chem.
Rev.
85
(2016)
280–307
https://doi.org/10.1070/rcr4554. [12] Y. He, W. Zhou, R. Krishna, B. Chen, Microporous metal-organic frameworks for storage and separation of small hydrocarbons, Chem. Commun. 48 (2012) 11813–11831 https://doi.org/10.1039/c2cc35418g. [13] H.C. Zhou, S. Kitagawa, Metal-organic frameworks (MOFs), Chem. Soc. Rev. 43 (2014) 5415–5418
https://doi.org/10.1039/c4cs90059f. [14] L. Li, R.B. Lin, R. Krishna, H. Li, S. Xiang, H. Wu, J. Li, W. Zhou, B. Chen, Ethane/ethylene separation in a metal-organic
framework
with
iron-peroxo
sites,
Science
362
(2018)
443–446
https://doi.org/10.1126/science.aat0586. [15] Y. Wu, H. Chen, D. Liu, Y. Qian, H. Xi, Adsorption and separation of ethane/ethylene on ZIFs with various topologies: Combining GCMC simulation with the ideal adsorbed solution theory (IAST), Chem. Eng. Sci. 124 (2015) 144–153 https://doi.org/10.1016/j.ces.2014.07.019. [16] D.L. Chen, N.W. Wang, C.H. Xu, G.M. Tu, W.D. Zhu, R. Krishna, A combined theoretical and experimental analysis on transient breakthroughs of C2H6/C2H4 in fixed beds packed with ZIF-7, Microporous Mesoporous Mater. 208 (2015) 55–65 https://doi.org/10.1016/j.micromeso.2015.01.019. [17] M. Hartmann, U. Bohme, M. Hovestadt, C. Paula, Adsorptive separation of olefin/paraffin mixtures with ZIF-4, Langmuir 31 (2015) 12382–12389 https://doi.org/10.1021/acs.langmuir.5b02907. [18] J. Pires, M.L. Pinto, V.K. Saini, Ethane selective IRMOF-8 and its significance in ethane-ethylene separation by 16
adsorption, ACS Appl. Mater. Interfaces 6 (2014) 12093–12099 https://doi.org/10.1021/am502686g. [19] R.S. Pillai, M.L. Pinto, J. Pires, M. Jorge, J.R. Gomes, Understanding gas adsorption selectivity in IRMOF-8 using molecular simulation, ACS Appl. Mater. Interfaces 7 (2015) 624–637 https://doi.org/10.1021/am506793b. [20] P.Q. Liao, W.X. Zhang, J.P. Zhang, X.M. Chen, Efficient purification of ethene by an ethane-trapping metal-organic framework, Nat. Commun. 6 (2015) 8697 https://doi.org/10.1038/ncomms9697. [21] Y.W. Chen, H.X. Wu, D.F. Lv, R.F. Shi, Y. Chen, Q.B. Xia, Z. Li, Highly adsorptive separation of ethane/ethylene by
an
ethane-selective
MOF
MIL-142A,
Ind.
Eng.
Chem.
Res.
57
(2018)
4063–4069
https://doi.org/10.1021/acs.iecr.7b05260. [22] W. Liang, F. Xu, X. Zhou, J. Xiao, Q. Xia, Y. Li, Z. Li, Ethane selective adsorbent Ni(bdc)(ted)0.5 with high uptake and its significance in adsorption separation of ethane and ethylene, Chem. Eng. Sci. 148 (2016) 275–281
https://doi.org/10.1016/j.ces.2016.04.016. [23] H. Wu, Y. Chen, D. Lv, R. Shi, Y. Chen, Z. Li, Q. Xia, An indium-based ethane-trapping MOF for efficient selective
separation
of
C2H6/C2H4
mixture,
Sep.
Purif.
Technol.
212
(2019)
51–56
https://doi.org/10.1016/j.seppur.2018.11.005. [24] Q.M. Wang, D.M. Shen, M. Bulow, M.L. Lau, S.G. Deng, F.R. Fitch, N.O. Lemcoff, J. Semanscin, Metallo-organic molecular sieve for gas separation and purification, Microporous Mesoporous Mater. 55 (2002) 217–230
https://doi.org/10.1016/S1387-1811(02)00405-5. [25] J. Ploegmakers, S. Japip, K. Nijmeijer, Mixed matrix membranes containing MOFs for ethylene/ethane separation-Part B: Effect of Cu3BTC2 on membrane transport properties, J. Membr. Sci. 428 (2013) 331–340
https://doi.org/10.1016/j.memsci.2012.11.013. [26] T.M. Nicholson, S.K. Bhatia, Electrostatically mediated specific adsorption of small molecules in metallo-organic frameworks, J. Phys. Chem. B 110 (2006) 24834–24836 https://doi.org/10.1021/jp065102b. [27] V.F.D. Martins, A.M. Ribeiro, A. Ferreira, U.H. Lee, Y.K. Hwang, J.S. Chang, J.M. Loureiro, A.E. Rodrigues, Ethane/ethylene separation on a copper benzene-1,3,5-tricarboxylate MOF, Sep. Purif. Technol. 149 (2015) 445–456
https://doi.org/10.1016/j.seppur.2015.06.012. [28] E.D. Bloch, W.L. Queen, R. Krishna, J.M. Zadrozny, C.M. Brown, J.R. Long, Hydrocarbon separations in a metal-organic
framework
with
open
iron(II)
coordination
sites,
Science
335
(2012)
1606–1610
https://doi.org/10.1126/science.1217544. [29] Z. Bao, S. Alnemrat, L. Yu, I. Vasiliev, Q. Ren, X. Lu, S. Deng, Adsorption of ethane, ethylene, propane, and propylene
on
a
magnesium-based
metal-organic
framework, 17
Langmuir
27
(2011)
13554–13562
https://doi.org/10.1021/la2030473. [30] U. Bohme, B. Barth, C. Paula, A. Kuhnt, W. Schwieger, A. Mundstock, J. Caro, M. Hartmann, Ethene/ethane and propene/propane separation via the olefin and paraffin selective metal-organic framework adsorbents CPO-27 and ZIF-8, Langmuir 29 (2013) 8592–8600 https://doi.org/10.1021/la401471g. [31] S.J. Geier, J.A. Mason, E.D. Bloch, W.L. Queen, M.R. Hudson, C.M. Brown, J.R. Long, Selective adsorption of ethylene over ethane and propylene over propane in the metal-organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn), Chem. Sci. 4 (2013) 2054–2061 https://doi.org/10.1039/c3sc00032j. [32] J.B. DeCoste, G.W. Peterson, B.J. Schindler, K.L. Killops, M.A. Browe, J.J. Mahle, The effect of water adsorption on the structure of the carboxylate containing metal-organic frameworks Cu-BTC, Mg-MOF-74, and UiO-66, J. Mater. Chem. A 1 (2013) 11922–11932 https://doi.org/10.1039/c3ta12497e. [33] N. Al-Janabi, P. Hill, L. Torrente-Murciano, A. Garforth, P. Gorgojo, F. Siperstein, X. Fan, Mapping the Cu-BTC metal-organic framework (HKUST-1) stability envelope in the presence of water vapour for CO2 adsorption from flue gases, Chem. Eng. J. 281 (2015) 669–677 https://doi.org/10.1016/j.cej.2015.07.020. [34] N. Al-Janabi, A. Alfutimie, F.R. Siperstein, X. Fan, Underlying mechanism of the hydrothermal instability of Cu3(BTC)2
metal-organic
framework,
Front.
Chem.
Sci.
Eng.
10
(2016)
103–107
https://doi.org/10.1007/s11705-015-1552-0. [35] J. Canivet, A. Fateeva, Y. Guo, B. Coasne, D. Farrusseng, Water adsorption in MOFs: fundamentals and applications, Chem. Soc. Rev. 43 (2014) 5594–5617 https://doi.org/10.1039/c4cs00078a. [36] N.C. Burtch, H. Jasuja, K.S. Walton, Water stability and adsorption in metal-organic frameworks, Chem. Rev. 114 (2014) 10575–10612 https://doi.org/10.1021/cr5002589. [37] Y. Li, R.T. Yang, Gas adsorption and storage in metal-organic framework MOF-177, Langmuir 23 (2007) 12937– 12944 https://doi.org/10.1021/la702466d. [38] Z. Liang, M. Marshall, A.L. Chaffee, CO2 adsorption, selectivity and water tolerance of pillared-layer metal organic
frameworks,
Microporous
Mesoporous
Mater.
132
(2010)
305–310
https://doi.org/10.1016/j.micromeso.2009.11.026. [39] H. Sakamoto, R. Kitaura, R. Matsuda, S. Kitagawa, Y. Kubota, M. Takata, Systematic construction of porous coordination
pillared-layer
structures
and
their
sorption
properties,
Chem.
Lett.
39
(2010)
218–219
https://doi.org/10.1246/cl.2010.218. [40] K. Kishida, Y. Watanabe, S. Horike, Y. Watanabe, Y. Okumura, Y. Hijikata, S. Sakaki, S. Kitagawa, DRIFT and theoretical
studies
of
ethylene/ethane
separation 18
on
flexible
and
microporous
[Cu2(2,3-pyrazinedicarboxylate)2(pyrazine)]n,
Eur.
J.
Inorg.
Chem.
2014
(2014)
2747–2752
https://doi.org/10.1002/ejic.201402085. [41] R.T. Yang, Adsorbents: fundamentals and applications, John Wiley & Sons 2003. [42] C. Wu, J.J. Wang, Y. Fang, Z. Wang, D.J. Fei, X. Han, Y.G. Dang, Comparative Study of Ag+-Based Adsorbents Performance
in
Ethylene/Ethane
Separation,
J.
Chem.
Eng.
Data
64
(2019)
611–618
https://doi.org/10.1021/acs.jced.8b00806. [43] S. Aguado, G. Bergeret, C. Daniel, D. Farrusseng, Absolute molecular sieve separation of ethylene/ethane mixtures with silver zeolite A, J. Am. Chem. Soc. 134 (2012) 14635–14637 https://doi.org/10.1021/ja305663k. [44] G. Chang, M. Huang, Y. Su, H. Xing, B. Su, Z. Zhang, Q. Yang, Y. Yang, Q. Ren, Z. Bao, B. Chen, Immobilization of Ag(I) into a metal-organic framework with -SO3H sites for highly selective olefin-paraffin separation at room temperature, Chem. Commun. 51 (2015) 2859–2862 https://doi.org/10.1039/c4cc09679g. [45] Y. Zhang, B. Li, R. Krishna, Z. Wu, D. Ma, Z. Shi, T. Pham, K. Forrest, B. Space, S. Ma, Highly selective adsorption of ethylene over ethane in a MOF featuring the combination of open metal site and π-complexation, Chem. Commun. 51 (2015) 2714–2717 https://doi.org/10.1039/c4cc09774b. [46] Y.X. Wang, Z.G. Hu, Y.D. Cheng, D. Zhao, Silver-decorated hafnium metal-organic framework for ethylene/ethane separation, Ind. Eng. Chem. Res. 56 (2017) 4508–4516 https://doi.org/10.1021/acs.iecr.7600517. [47] J. Padin, R.T. Yang, New sorbents for olefin/paraffin separations by adsorption via π-complexation: synthesis and effects of substrates, Chem. Eng. Sci. 55 (2000) 2607–2616 https://doi.org/10.1016/s0009-2509(99)00537-0. [48] F. Iucolano, P. Aprea, D. Caputo, C. Colella, M. Eic, Q. Huang, Adsorption and diffusion of propane and propylene in Ag+-impregnated MCM-41, Adsorption 14 (2008) 241–246 https://doi.org/10.1007/s10450-008-9106-0. [49] A. Anson, Y. Wang, C.C.H. Lin, T.M. Kuznicki, S.M. Kuznicki, Adsorption of ethane and ethylene on modified ETS-10, Chem. Eng. Sci. 63 (2008) 4171–4175 https://doi.org/10.1016/j.ces.2008.05.038. [50] S.U. Rege, J. Padin, R.T. Yang, Olefin/paraffin separations by adsorption: π-complexation vs. kinetic separation, AIChE J. 44 (1998) 799–809 https://doi.org/10.1002/aic.690440405. [51] O.J. García-Ricard, A.J. Hernández-Maldonado, Cu2(pyrazine-2,3-dicarboxylate)2(4,4'-bipyridine) porous coordination sorbents: Activation temperature, textural properties, and CO2 adsorption at low pressure range, J. Phys. Chem. C 114 (2010) 1827–1834 https://doi.org/10.1021/jp9103068. [52] M. Galiova, J. Kaiser, K. Novotny, J. Novotny, T. Vaculovic, M. Liska, R. Malina, K. Stejskal, V. Adam, R. Kizek, Investigation of heavy-metal accumulation in selected plant samples using laser induced breakdown spectroscopy and laser
ablation
inductively
coupled
plasma
mass
spectrometry, 19
Appl.
Phys.
A
93
(2008)
917–922
https://doi.org/10.1007/s00339-008-4747-0. [53] W.Y. Huang, X. Zhou, Q.B. Xia, J.J. Peng, H.H. Wang, Z. Li, Preparation and adsorption performance of GrO@Cu-BTC
for
separation
of
CO2/CH4,
Ind.
Eng.
Chem.
Res.
53
(2014)
11176–11184
https://doi.org/10.1021/ie501040s. [54] Y. Jiao, Z. Li, Y. Ma, G. Zhou, S. Wang, G. Lu, The studies on gas adsorption properties of MIL-53 series MOFs materials, AIP Adv. 7 (2017) 085009 https://doi.org/10.1063/1.4999914. [55] R. Matsuda, T. Tsujino, H. Sato, Y. Kubota, K. Morishige, M. Takata, S. Kitagawa, Temperature responsive channel uniformity impacts on highly guest-selective adsorption in a porous coordination polymer, Chem. Sci. 1 (2010) 315–321 https://doi.org/10.1039/c0sc00272k. [56] H. Chen, K. Riascos-Rodríguez, M.E. Marcano-González, A.J. Hernández-Maldonado, Cu2(pzdc)2L [L=dipyridyl-based ligands] porous coordination polymers: Hysteretic adsorption and diffusion kinetics of CO2 and CH4, Chem. Eng. J. 283 (2016) 806–815 https://doi.org/10.1016/j.cej.2015.08.017. [57] P.J. Meza-Morales, D.A. Gomez-Gualdron, R.R. Arrieta-Perez, A.J. Hernandez-Maldonado, R.Q. Snurr, M.C. Curet-Arana, CO2 adsorption-induced structural changes in coordination polymer ligands elucidated via molecular simulations and experiments, Dalton Trans. 45 (2016) 17168–17178 https://doi.org/10.1039/c6dt02994a. [58] N. Al-Janabi, H. Deng, J. Borges, X. Liu, A. Garforth, F.R. Siperstein, X. Fan, A facile post-synthetic modification method to improve hydrothermal stability and CO2 selectivity of CuBTC metal-organic framework, Ind. Eng. Chem. Res. 55 (2016) 7941–7949 https://doi.org/10.1021/acs.iecr.5b04217. [59] N. Al-Janabi, V. Martis, N. Servi, F.R. Siperstein, X.L. Fan, Cyclic adsorption of water vapour on CuBTC MOF: Sustaining the hydrothermal stability under non-equilibrium conditions, Chem. Eng. J. 333 (2018) 594–602
https://doi.org/10.1016/j.cej.2017.09.197. [60] D. Li, K. Kaneko, Molecular geometry-sensitive filling in semi-rectangular micropores of organic-inorganic hybrid crystals, J. Phys. Chem. B 104 (2000) 8940–8945 https://doi.org/10.1021/jp000660q. [61] G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I. Margiolaki, A chromium terephthalate-based solid with unusually large pore volumes and surface area, Science 309 (2005) 2040–2042
https://doi.org/10.1126/science.1116275.
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Synthesis and modification of moisture-stable CPL-2 MOF for ethylene/ethane separation HIGHLIGHTS
− CPL-2 MOF exhibits preferential adsorption of ethylene over ethane. − CPL-2 MOF shows excellent moisture stability under humid conditions. − Ethylene/ethane selectivity was enhanced from 1.4 to 26.1 by silver ions modification.
S1387-1811(19)30641-9
DOI:
https://doi.org/10.1016/j.micromeso.2019.109784
Reference:
MICMAT 109784
To appear in:
Microporous and Mesoporous Materials
Received Date: 23 August 2019 Revised Date:
26 September 2019
Accepted Date: 30 September 2019
Please cite this article as: H. Xiang, A. Ameen, J. Shang, Y. Jiao, P. Gorgojo, F.R. Siperstein, X. Fan, Synthesis and modification of moisture-stable coordination pillared-layer metal-organic framework (CPLMOF) CPL-2 for ethylene/ethane separation, Microporous and Mesoporous Materials (2019), doi: https:// doi.org/10.1016/j.micromeso.2019.109784. 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.
Synthesis and modification of moisture-stable CPL-2 MOF for ethylene/ethane separation GRAPHICAL ABSTRACT
Synthesis and modification of moisture-stable coordination pillared-layer metal-organic framework (CPL-MOF) CPL-2 for ethylene/ethane separation Huan Xianga, Ahmed Ameena, Jin Shangb,c, Yilai Jiaod, Patricia Gorgojoa, Flor R. Sipersteina,∗, Xiaolei Fana,∗ a
School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, United
Kingdom b
City University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park,
Nanshan District, Shenzhen, China c
School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
d
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72
Wenhua Road, Shenyang 110016, China
Abstract Adsorptive separation of ethylene/ethane (C2H4/C2H6) binary mixture has growing interest in petrochemical industries compared to the conventional energy-intensive cryogenic distillation. Development of moisture-stable materials with high selectivity is of great importance to accomplish C2H4/C2H6 separation. Coordination pillared-layer metal-organic framework (CPL-MOF) CPL-2 was synthesised at room temperature, and then modified by silver ions impregnation to enhance the selectivity towards ethylene over ethane. The synthesised CPL-2 and Ag/CPL-2 MOFs have excellent moisture stability which was confirmed by the dynamic water vapour adsorption analysis under 90% relative humidity, showing no significant framework decomposition, even at 50 °C. The calculated selectivity based on gravimetric single-component gas adsorption experiments shows the significantly improved C2H4/C2H6 selectivity from 1.4 to 26.1 after loading 10 wt.% (theoretical) of silver ions on CPL-2. Breakthrough experiments for C2H4/C2H6 (1:1, v/v) mixture suggest that both CPL-2 and 10 wt.% Ag/CPL-2 can achieve the binary mixture separation, and 10 wt.% Ag/CPL-2 shows relatively better dynamic separation performance compared to parent CPL-2. The good adsorption selectivity and moisture stability allow CPL-MOF to be a class of promising porous materials for further exploitation in the separation of C2H4/C2H6 mixtures. Additionally, the method presented here can potentially be extended to other CPLs with different pore sizes for alkene/alkane separations. Keywords: Ethylene; Ethane; Adsorption; Coordination pillared-layer metal-organic framework (CPL-MOF); Selectivity; Moisture stability
∗
Corresponding authors.
Email address: [email protected] (FS). Email address: [email protected] (XF). 1
1. Introduction Light hydrocarbons are important energy resources and feedstocks for the production of many petrochemical products [1, 2]. Alkene/alkane separation by alternative processes to expensive cryogenic distillation is now considered as one of the seven chemical separations to change the world [3]. To date, the industrial practice of separating alkene/alkane mixtures is mainly based on the cryogenic distillation [4]. Cryogenic distillation must be performed at low temperature and high pressure, which makes it an energy-intensive process [5]. For ethylene/ethane mixture separation, it needs to be carried out at about −25 °C and 23 bar in a column with over 100 trays due to the similar boiling point of ethylene (−104 °C) and ethane (−89 °C) [6-8]. Therefore, highly energy efficient separation methods are demanded to replace the current industrial standards for ethylene/ethane separation. Among all alternative technologies, adsorptive separation is regarded as the promising option, especially with the emergence of various functional porous adsorbents [9, 10]. Developing an adsorbent with high selectivity (towards the target guest molecule), as well as excellent stability, is the key step to enable the adsorptive separation in practice. Metal-organic frameworks (MOFs), as a new class of porous materials, have attracted considerable attention due to their combined properties of large specific surface area, pore volume, functionalised and adjustable pore structure and surface [11-13]. During the past decades, an increasing number of MOFs, e.g. Fe2(O2)(dobdc) [14], ZIFs [15-17], IRMOF-8 [18, 19], MAF-49 [20], PCN-250 [8], MIL-142A [21] and Ni(bdc)(ted)0.5 [22], were proposed to be ethane-selective materials and investigated for ethane/ethylene (C2H6/C2H4) separation using the typical volumetric ethane/ethylene composition of 1:15 simulating the industrially cracked gas mixture [20, 23]. However, only Fe2(O2)(dobdc) with the Fe-peroxo sites shows the strong interaction with ethane, other MOFs do not have specific adsorption sites, and hence the adsorption affinity of ethane molecule onto them is mainly dominated by the van der Waals interactions. Fe2(O2)(dobdc) exhibits a ethane selectivity of 4.4 at 25 °C, and MAF-49 shows an excellent selectivity (ca. 9 at 43 °C) at 1 bar, while lower selectivities have also been reported for the same material (ca. 2.7 at 25 °C) [14]. Other adsorbents show a poor selectivity of about 2 at atmospheric pressure, even though the good selectivity could be obtained at the very low pressure, e.g. 4.8 on In-soc-MOF-1 at 25 °C and 1 kPa [23]. For ethylene-selective adsorbents, MOFs with open metal sites (OMSs), such as HKUST-1 [24-27] and MOF-74 [28-31] have been widely studied due to the specific interaction between OMSs and the carbon-carbon double bond in the ethylene molecule. For example, Fe-MOF-74 has been reported to have a selectivity of up to 13–18 for an equimolar mixture of ethylene and ethane at 45 °C [28]. However, OMSs interact strongly with water molecules as well, resulting in material decomposition and poor moisture or water stability (i.e. stability in water vapour or liquid water) [32-36], especially at elevated temperatures, limiting their potential in practical settings [37, 38]. Therefore, the development of MOFs with high selectivity and hydrothermal stability is always of great importance for separation applications, including the ethylene/ethane separation. Over the recent years, pillared-layer MOFs consisting of two types of organic linkers have been proposed and studied in the separation of alkene/alkane mixtures, which are exemplified by the coordination pillared-layer (CPL) MOFs. CPL-MOFs are structured by scaffolding 2D Cu(II) and pyrazine-2,3-dicarboxylate (pzdc) layers using dipyridyl ligands (such as pyrazine, 4,4′-bipyridine 2
and 1,2-di(4-pyridyl)ethylene, as the bridging pillars) [39]. CPL-MOFs can be facilely synthesised under mild conditions (e.g. at room temperature) and their pore sizes can be easily modified by changing the pillar ligands (L). They also show good flexibility and unique guest-responsive nature. CPL-1 (L = pyrazine) with a pore size of 4×6 Å2 has already been investigated in the separation of propylene/propane [4] and ethylene/ethane [40] mixtures. Both adsorption isotherm tests and breakthrough experiments revealed that CPL-1 exhibited the selective adsorption of the ethylene molecules with a selectivity of 3.8 from breakthrough experiments. In general, selectivity is expected to decrease in larger pore materials, but they may have lower mass transfer resistances leading to sharper breakthrough curves, and more importantly, it may be possible to functionalise or modify them without causing much pore blocking. It has been previously reported that an increase in ethylene selectivity can be achieved using the Ag(I) ions impregnated materials via the formation of π-complexes with ethylene molecules [41, 42]. Aguado et al. have shown for the first time that absolute ethylene/ethane separation could be achieved by ethane exclusion on Ag-exchanged zeolite A (AgA) [43]. As a result, AgA showed 100% ethylene selectivity. Ag(I) was also introduced into (Cr)-MIL-101-SO3H MOF, leading to a much higher selectivity (ca. 16) than the original MOF (ca. 1.15) at 30 °C and 1 bar [44, 45]. Ag-decorated NUS-6(Hf) MOF showed an ethylene/ethane selectivity of 6 at 25 °C and 1 bar, which was 5 times higher than that of NUS-6(Hf) [46]. Moreover, Ag(I) ions have also been successfully dispersed on other materials, such as faujasite-type zeolites (X and Y) [6, 43], SBA-15 [42], MCM-41 [47, 48], ETS-10 [49], SiO2 [47] and resin [6, 50], to improve alkene/alkane separation performance for practical applications. In this work, CPL-2 (L = 4,4′-bipyridine) with a pore size of 9×6 Å2 was selected as the model MOF and synthesised at room temperature for separating binary ethylene/ethane mixture. In addition, the moisture stability of CPL-2 was also evaluated under dynamic humid conditions (from 0% to 90% relative humidity). Ag(I) ions modification of CPL-2 was also conducted to explore the possibility of improving the selectivity of CPL-MOFs for ethylene/ethane separation. 2. Experimental 2.1. Materials Copper(II) perchlorate hexahydrate (Cu(ClO4)2·6H2O), 98%), 2,3-pyrazinedicarboxylic acid (H2pzdc, 97%), 4,4′-bipyridine (bpy, 98%), methanol (≥ 99.9%), nitric acid (HNO3, ≥ 65%) and hydrochloric acid (HCl, 37%) were purchased from Sigma Aldrich. Sodium hydroxide (NaOH, 97%) was purchased from Honeywell. Silver nitrate (AgNO3) was purchased from Fisher and ethanol (absolute) was purchased from VWR Chemicals. All chemicals were used as received without further purification. C2H4 (99.9%), C2H6 (99.0%) and N2 (99.99%) gases were purchased from BOC Ltd Company. 2.2. Synthesis of materials Standard hydrothermal synthesis was used in this work to prepare CPL-2 [51]. H2pzdc (1 mmol, 168.1 mg) and bpy (0.5 mmol, 78 mg) were dissolved in a 100 mL mixture containing 50 mL NaOH aqueous solution (0.04 M) and 50 mL ethanol. The mixture was stirred for 10 min until a 3
clear solution was obtained. The resulting solution was then added slowly to an aqueous solution (100 mL) containing Cu(ClO4)2·6H2O (1 mmol, 370 mg). The final mixture was stirred for 3−24 h at room temperature. After synthesis, the blue precipitate was filtered and then washed with water and methanol to remove the unreacted chemicals, and then activated by drying at 100 °C and atmospheric pressure overnight in a convention oven for further use (i.e. the as-synthesised CPL-2). Silver ions modified CPL-2 MOFs (denoted as Ag/CPL-2) were prepared using the incipient wetness impregnation method. The as-synthesised CPL-2 (0.4 g) was immersed in a 1:1 mixture of ethanol/water solution (8 mL) containing different amounts of silver nitrate (0.19 mmol, 0.41 mmol and 0.65 mmol). The mixture was stirred continuously for 4 h at room temperature, and then dried at 60 °C overnight to give the Ag/CPL-2 samples with different Ag(I) loadings, denoted as x wt.% Ag/CPL-2 (x wt.% represents the theoretical concentration of silver ions on CPL-2 adsorbent, i.e. 5 wt.%, 10 wt.% and 15 wt.%, respectively). 2.3. Characterisation of materials Powder X-ray diffraction (PXRD) was performed on a Philips X’Pert X-ray diffractometer with CuKα1 radiation (λ = 1.5406 Å, 40 kV, 40 mA) using a step scan mode (0.02° per step) at 2θ ranging from 5° to 50°. Scanning electron microscopy (SEM) was carried out using FEI Quanta 250 ESEM instrument with a high voltage mode of 20 kV. All samples were coated with platinum using Cressington Pt sputter coater under vacuum prior to test. Qualitative composition analysis of materials was acquired by an Oxford energy-dispersive X-ray spectroscopy (EDS) system. Nitrogen (N2) adsorption/desorption isotherms at −196.15 °C were obtained using Micrometrics ASAP 2020 analyser. The degassing was carried out at 100 °C and under vacuum for 12 h. Surface area was determined using the Brunauer-Emmett-Teller (BET) method. Thermal gravimetric analysis (TGA) was performed using a TA-Q5000 analyser. Samples were heated from ambient temperature to 700 °C at a heating rate of 10 °C/min under N2 at 60 mL/min. Dynamic water vapour adsorption analyses at 25 °C and 50 °C with relative humidity values ranging from 0% to 90% (10% per step) were measured using a dynamic vapour sorption (Surface Measurements Systems, DVS 1) equipment. Prior to the water vapour adsorption, all samples were dried at 0% relative humidity for 3 h. The actual concentration of silver ions on Ag/CPL-2 samples was analysed by inductively coupled plasma optical emission spectrometer (ICP-OES, PlasmaQuant PQ9000). All samples were dissolved by HCl and HNO3 for the ICP-OES analysis and the 328.07 nm line was used to detect the Ag(I) ion in the samples [52]. In ICP-OES analysis, silver standard with a concentration of 1000 mg/L was diluted to 0–40 mg/L to obtain a standard curve of silver ions. 2.4. Single-component gas adsorption of C2H4, C2H6 and N2 on materials Ethylene, ethane and nitrogen adsorption isotherms on developed CPL-2 and Ag/CPL-2 materials were measured using an intelligent gravimetric analyser (Hiden Isochema, IGA-001). The IGA analysis was based on the static gravimetric technique, i.e. using a microbalance to measure the weight changes as a function of pressure at a constant temperature of 25 °C, 35 °C and 50 °C, respectively. Langmuir model based on monolayer adsorption theory was employed to study the experimental
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adsorption isotherms, which is commonly expressed as Eq. 1: N KP N = sat (1) 1 + KP where K (bar−1) is the Langmuir constant, N (mmol/g) is the equilibrium adsorption uptake at the corresponding adsorption pressure P (bar) and Nsat (mmol/g) is the saturation capacity for monolayer adsorption. The ideal adsorbed solution theory (IAST) [53, 54] was combined with the Langmuir model to calculate the C2H4/C2H6 selectivity. The selectivity (S) of C2H4 over C2H6 can be calculated as Eq. 2:
S=
xethane xethylene
(2)
yethane yethylene
where xethane and xethylene are the mole fraction of C2H6 and C2H4 in the adsorbed phase (on CPL-2 MOF adsorbent), whereas yethane and yethylene are their corresponding mole fraction in the gas bulk phase. The isosteric heat of adsorption (Qst, kJ/mol) was calculated by the Clausius-Clapeyron equation based on the adsorption isotherms measured at 25–50 °C, and the equation can be expressed as: ∂ ln P Qst = RT 2 ( )n (3) ∂T where R (J/(mol·K)) is the gas constant, T (K) is the temperature, P (bar) is the pressure, and n (mmol/g) is the gas adsorption uptake.
2.5. Breakthrough experiments on materials The breakthrough experiments of an equimolar mixture (C2H4/C2H6 = 1:1, v/v) were carried out in a dynamic breakthrough set-up at 25 °C (Fig. S1). Activated sample (about 1 g) with a particle size of 250–425 µm was packed into a stainless-steel column (φ 4×120 mm), and the void space of the two ends were filled with glass wool. The column packed with sample was firstly purged with helium (He, 50 mL/min) for 30 min at 25 °C before the breakthrough test. The total flow rate of the binary mixture was controlled to be 2 mL/min, regulated by the mass flow controller (MFC). A gas chromatography (Aglient 490 Micro GC) with a PoraPLOT U (PPU) column and a thermal conductivity detector (TCD) was used for on-line analysis of the outlet gases from the packed column. 3. Results and discussion 3.1. Characterisation of CPL-2 CPL-2 can be easily synthesised at room temperature, which was confirmed by XRD (Fig. 1a) and SEM (Fig. S2) analysis of samples from the systematic study of varying synthesis time from 3−24 h. Characteristic peaks at 2θ = 6.4°, 8.8°, 10.3° and 12.6°, matching the simulated results [55, 56], are identified in all the as-synthesised CPL-2 MOFs, confirming the presence of the pillared-layer structure in the crystalline domain. N2 physisorption of the synthesised CPL-2 (Fig. 1b and Table 5
S1) shows the reversible typical type I isotherm for microporous CPL-2 and gave the BET value of about 546 m2/g, which is in good agreement with the reported ones [51, 57] in the literature (e.g. 530 m2/g measured by Meza-Morales et al. [57].
Fig. 1. (a) PXRD patterns of CPL-2 synthesised at different times and (b) N2 adsorption/desorption isotherms of CPL-2 (synthesised for 24 h) at −196.15 °C with reported results from García-Ricard et al. [51] and Meza-Morales et al. [57].
The intrinsic thermal stability of the as-synthesised CPL-2 was assessed by TGA under N2. Fig. S3 depicts the according weight loss profile of CPL-2, showing the three-stage weight loss of CPL-2 under the condition used. The initial weight loss of about 12.6 wt.% between room temperature to 100 °C was due to the desorption of water molecules, which are weakly present within the pores of CPL-2 due to physisorption. The second stage of abrupt weight loss started at around 260 °C, corresponding to the collapse of pillar ligands. The final stage was gradual, representing the decomposition of the pzdc linker in the remaining 2D structure. It led to the remaining weight registered as ca. 20.9% which are copper species. According to the unit cell composition, CPL-2 is composed of 54 wt.% pzdc, 25 wt.% bpy and 21 wt.% Cu. Hydrophobicity is a desirable feature of the adsorbent for light hydrocarbons separation applications. For MOF-based adsorbents, poor water and moisture stability, specifically MOFs with OMSs [33, 34], is one of the major issues for the further exploration towards the practical applications. In this work, the hydrothermal stability of CPL-2 was studied at two different temperatures of 25 °C and 50 °C using dynamic water vapour adsorption technique [33] at the relative humidity of 0–90% (Fig. 2). The water uptake profiles in Fig. 2a and 2b are significantly different from those observed in MOFs with OMSs, such as HKUST-1 [33, 58, 59]. Here, the system is reversible whereas in HKUST-1 [33] apparently 6.7 mmol/g (ca. 20%) of the adsorbed water remains in the material after the desorption cycle at 25 °C. The adsorbed water molecules are easily to desorb from CPL-2, suggesting that the interaction between water molecules and CPL-2 is weak. This weak interaction is due to no OMSs existed in CPL-2 MOF. The metal Cu(II) in CPL-2 is fully coordinated with two linkers (pzdc and bpy), resulting in no specific adsorption site accessible for water molecules. The adsorption/desorption isotherms of water vapour on CPL-2, i.e. the uptake of water over the 6
dry mass of CPL-2 as a function of the relative pressure (ratio of water vapour pressure to the water vapour pressure at 90% relative humidity) are shown in Fig. 2c and 2d. It was found that the adsorption of water on CPL-2 was reversible in all cycles under the condition used, suggesting the physisorption of water on CPL-2. PXRD analysis (Fig. S4) confirms that the crystalline structure of CPL-2 MOF was retained after the cyclic water vapour adsorption tests (4 cycles). Beside, CPL-2 also exhibits good stability under dynamic humid conditions at 50 °C, whereas in MOFs with OMSs like HKUST-1, rapid degradation is observed at high temperatures [58], indicating the potential of CPL-2 as adsorbent for practical applications in the presence of water vapour.
Fig. 2. Cyclic water vapour adsorption and desorption at different partial vapour pressures on CPL-2 at (a) 25 °C and (c) 50 °C and water vapour adsorption/desorption isotherms on CPL-2 at (b) 25 °C and (d) 50 °C.
3.2. Gas adsorption isotherms on CPL-2 The single-component adsorption/desorption isotherms of ethylene, ethane and nitrogen on CPL-2 were measured over a pressure range of 0–10 bar at 25, 35 and 50 °C, respectively, as shown in Figs. 3Fig. 3a–3c. All adsorption isotherms are typical type I isotherms (i.e. monolayer adsorption within microporous adsorbents), which is characterised by a plateau after an equilibrium saturation point. The adsorption of the three probing gases on CPL-2 is reversible, suggesting that CPL-2 can be regenerated easily in the adsorption process. CPL-2 prefers to adsorb C2H4 over C2H6 and N2, as shown in Fig. 3. Table 1 shows the
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polarisability (α) and quadruple moment (Θ) of the gases studied to help rationalise the observed adsorption behaviour. The polarisabilities of C2H4 and C2H6 are significantly larger than the nitrogen polarisability, but if the adsorption was driven exclusively by dispersion interactions, then the adsorption of C2H6 would be more favourable than the adsorption of C2H4. Given that C2H4 is preferentially adsorbed over C2H6, then we can infer that electrostatic interactions play a significant role in these systems because the quadruple moment of C2H4 is higher than C2H6. Despite N2 having a similar quadruple moment to C2H4, its adsorption is weak due to its low polarisability compared to the other gases studied. A summary of the gas adsorption uptake at 10 bar and at different temperatures is presented in Table S2, showing that the gas uptake of CPL-2 decreased with the increase in adsorption temperature for all three gases, confirming the gas adsorption is exothermic. At zero coverage, the isosteric heats of adsorption were 23.5, 16.5 and 10.6 kJ/mol for C2H4, C2H6 and N2, respectively. Langmuir model was applied to study C2H4 and C2H6 adsorption isotherms, and the fitting parameters are listed in Table S3. Fig. 3d shows the IAST predicted C2H4/C2H6 selectivity for their binary mixtures based on the obtained fitting parameters. The adsorption selectivity was in the range of 1.1–1.6, and slightly increased with an increasing pressure. The calculated C2H4/C2H6 selectivity of CPL-2 is relatively lower than those of the previously reported C2H4-selective MOFs with OMSs, e.g. CuBTC (ca. 2) [26] and Fe-MOF-74 (ca. 13–18) [28]. Besides, the selectivities of different mixture compositions (C2H4/C2H6 = 1:1 and 15:1, v/v)) were comparable, indicating the mixture composition would not affect the adsorption selectivity of CPL-2 under the conditions used.
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Fig. 3. C2H4, C2H6 and N2 adsorption/desorption isotherms on CPL-2 at: (a) 25 °C, (b) 35 °C and (c) 50 °C, and (d) IAST selectivity for C2H4/C2H6 (1:1 and 15:1, v/v) mixtures on CPL-2. Solid symbols represent adsorption isotherms, whereas open symbols represent desorption isotherms.
Table 1 Relevant physical parameters of ethylene, ethane and nitrogen [10, 31].
Adsorbate C2H4 C2H6 N2
Θ (×10−26 esu cm2)
α (×10−25 cm3) 42.52 44.3–44.7 17.40
1.50 0.65 1.52
The density in the adsorbed phase, calculated as the amount adsorbed at a given pressure divided by the pore volume, is expected to increase slightly with pore size at high pressures due to the ability of molecules to pack better in larger pores. This coincides with observations from Kaneko et al. [60] who studied the adsorption of three different probing gases (Ar, N2 and CO2) on CPL-1, 2, 5 with a pore size of 4×6, 9×6 and 11×6 Å2, respectively, and found that space-filling ratio (i.e. the ratio of the void volume obtained from XRD analysis to the micropore volume measured by gas adsorption) increased with an increase in the rectangular pore size, i.e. CPL-5 > CPL-2 > CPL-1. Similar observations have been found in this work for C2H4 and C2H6 when comparing the density of the fluid in the adsorbed phase, calculated as the amount adsorbed divided by the pore volume. At 25 °C and 1 bar (see Table S4), the densities of C2H4 and C2H6 on CPL-2 are approximately 8.3 and 7.0 mmol/cm3, which are significantly larger than those of CPL-1, where the densities are less than 1.4 mmol/cm3. 3.3. Modification of CPL-2 using Ag(I) ions The impregnation method was used to incorporate silver ions in CPL-2 for modifying its selectivity in C2H4/C2H6 separation. PXRD characterisation of the resulting Ag/CPL-2 samples shows all characteristic diffraction peaks of CPL-2 (Fig. S6), confirming the intactness of the pillared-layer structure of CPL-2 after the modification. However, the intensity of diffraction peaks of Ag/CPL-2 decreased by increasing the Ag(I) loading possibly due to inclusion of guest molecules in the porous framework of CPL-2, accordingly, 15 wt.% Ag/CPL-2 shows the lowest diffraction peak intensity.
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SEM analysis of Ag/CPL-2 MOFs (Fig. 4 and S4) shows the typical squared slab morphology of CPL-2. Additionally, Ag/CPL-2 MOFs also exhibit the comparable thermal stability to that of the parent CPL-2 (Fig. S8). Thus, the introduction of Ag(I) ions did not alter the crystal structure, morphology and thermal stability of parent CPL-2 significantly. According to comparative EDS mapping characterisation of CPL-2 and Ag/CPL-2 (using 5 wt.% Ag/CPL-2 as the representative), as shown in Fig. 4, silver species were successfully loaded on CPL-2 MOF. The calculated concentrations of silver ions in Ag/CPL-2 MOFs using ICP-OES were lower than the theoretical values: 3.96, 5.11 and 8.38 wt.%, measured versus 5, 10 and 15 wt.% theoretical values. The loss of silver species might happen during the preparation and workup of Ag/CPL-2 MOFs.
Fig. 4. EDS images of CPL-2 and 5 wt.% Ag/CPL-2.
Ag/CPL-2 MOFs showed similar adsorption behaviour to the parent CPL-2 in N2 physisorption, as shown in Fig. 5a, but with significantly lower capacity under the condition used in BET measurements, indicating the micropore blockage due to Ag(I) impregnation. The adsorption capacity and the calculated BET surface area and pore volume are inversely related to the Ag(I) loading (Fig. 5a and Table S5). Chang and co-workers also observed a reduction in surface area and pore volume upon impregnation of (Cr)-MIL-101-SO3H MOF with Ag(I) [44], but the reduction they observed was on the order of 30% whereas in this work a reduction of 80% in the pore volume is observed. It should be noted that the cages in MIL-101 materials have an approximate diameter of 30−34 Å [61], whereas the pore diameter in CPL-2 is around 5−6 Å [57], therefore the presence of Ag(I) ions can have a more significant effect in blocking the access to the micropore volume. Pore size distribution calculated by Horvath-Kawazoe (HK) method (Fig. 5b) shows that the porous structure of Ag/CPL-2 consists of micropores mainly with the pore width of 7−11 Å.
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Fig. 5. (a) N2 adsorption/desorption isotherms at −196.15 °C and (b) pore width distributions of Ag/CPL-2 MOFs.
The moisture stability of Ag/CPL-2 MOFs was evaluated at 25 °C using DVS (Fig. S10), showing weak physisorption and no degradation of the materials (Fig. S11), suggesting a similar stability to the parent material. The water uptake decreases with Ag(I) loading (Fig. S10), but the water uptake close to condensation pressure is practically insensitive to the Ag(I) loading, suggesting that some ions may be on the external surface and hydrate at P/P90 = 1.0. 3.4. Improved selectivity to C2H4 of Ag/CPL-2 The adsorption selectivity of Ag/CPL-2 MOFs was determined from pure component gas adsorption isotherms as shown in Fig. 6. Considering the adsorption processes are generally carried out at ambient temperature and pressure, all gas adsorption isotherms on Ag/CPL-2 were measured at 25 °C and over a pressure range of 0–1 bar. Ag/CPL-2 MOFs showed the reduced adsorption capacity of both gases compared to the parent CPL-2 MOF, especially for the sample with the theoretical 15 wt.% Ag(I) loading. As seen from Fig. 6a–6c, the C2H4 adsorption uptakes on Ag/CPL-2 MOFs are significantly larger than those of C2H6 after modification. It reveals that Ag(I) ions interact strongly with C2H4 molecules by forming π-complexes with the carbon-carbon double bonds in C2H4, and then enhance the selectivity towards C2H4 over C2H6. The π bond formed between silver ions and C2H4 is a weak chemical bond, which means chemisorption occurs on Ag/CPL-2 MOFs, in addition to physisorption due to the confinement of the MOFs. It could be confirmed by the calculated isosteric heats of adsorption on Ag/CPL-2 (using 5 wt.% Ag/CPL-2 as the representative), as shown in Fig. 7. The isosteric heat of C2H4 adsorption on Ag/CPL-2 was obviously higher than that on the parent CPL-2, i.e. 42.9 kJ/mol versus 23.5 kJ/mol at zero coverage. Conversely, for C2H6 adsorption, the isosteric heats on both adsorbents were comparable (15.2 kJ/mol on Ag/CPL-2 and 16.5 kJ/mol on Ag/CPL-2). Moreover, the interaction with Ag(I) results in a larger adsorption per unit area for both gases, but the increase is significantly more important for C2H4 than for C2H6.
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Fig. 6. Adsorption isotherms of C2H4 and C2H6 on CPL-2 and Ag/CPL-2 at 25 °C: (a) C2H4, (b) C2H6, and (c) comparison of equilibrium adsorption capacity of C2H4 and C2H6 at 1 bar, and (d) IAST selectivity for C2H4/C2H6 (1:1, v/v) mixture on CPL-2. Solid symbols represent the experimental adsorption isotherms, whereas the dashed lines represent the isotherms fitted by the theoretical model.
Fig. 7. Isosteric heats of C2H4 and C2H6 adsorption on (a) CPL-2 and (b) 5 wt.% Ag/CPL-2.
The experimental adsorption isotherms on Ag/CPL-2 were also studied by Langmuir model and the fitting parameters are listed in Table S6. The IAST selectivity obtained using Eq. 2 is shown in Fig. 6d. Similarly, the selectivity of Ag/CPL-2 also increased as pressure increased, but more rapidly (ca. 12
32%, 74% and 90% for 5 wt.%, 10 wt.% and 15 wt.% Ag/CPL-2, respectively) than the parent CPL-2 MOF (ca. 16%). Compared to the selectivity of 1.4 of CPL-2 at 1 bar and 25 °C, the selectivities of 5 wt.%, 10 wt.% and 15 wt.% Ag/CPL-2 were much higher, i.e. 4.2, 26.1 and 63.7, respectively. The C2H4/C2H6 selectivity obviously increased with increasing Ag(I) loading. It also confirmed that C2H4 molecules interact strongly with the Ag(I) ions on CPL-2. 3.5. Breakthrough experiments Breakthrough experiments were performed at 25 °C and 1 bar to evaluate the dynamic separation performance of CPL-2 and 10 wt.% Ag/CPL-2, which exhibited the best adsorption performance considering both adsorption capacity and selectivity. Fig. 8 shows the experimental breakthrough curves for C2H4/C2H6 (1:1, v/v) mixture on parent CPL-2 and 10 wt.% Ag/CPL-2. The breakthrough time of C2H4 was longer than that of C2H6 on both adsorbents, indicating that the solid adsorbents prefer to adsorb C2H4 over C2H6 molecules, which is consistent with the isosteric heats of adsorption (Fig. 7). The breakthrough curves demonstrated that the binary mixture could be separated on both CPL-2 and 10 wt.% Ag/CPL-2 in the dynamic adsorption process, and 10 wt.% Ag/CPL-2 displayed slightly better separation performance than CPL-2, as evidenced by the larger difference in C2H4 and C2H6 concentrations of the effluent. It also confirmed that the introduction of Ag(I) ions enhanced the interactions between adsorbents and C2H4 molecules.
Fig. 8. Breakthrough curves for C2H4/C2H6 (1:1, v/v) mixture on (a) CPL-2 and (b) 10 wt.% Ag/CPL-2 at 25 °C and 1 bar.
3.6. Effect of water vapour on adsorption capacity of materials CPL-2 and 10 wt.% Ag/CPL-2 MOFs exhibited good moisture stability at 25 °C, having possibility of being further explored as adsorbents under humid conditions. Indeed, the influence of water vapour on the adsorption equilibrium of ethylene, ethane and their mixtures is important, and an understanding of the nature of such effects. However, the equilibrium adsorption experiments were based on the gravimetric method, which is not capable to distinguish the relevant selectivity, i.e. the weight change of the sample cannot reflect the adsorbed proportion of guest molecules when a mixture is used. Additionally, the presence of water vapour in the continuous stream for the dynamic adsorption of gas mixtures of C2H4/C2H6, i.e. the breakthrough experiment, is also challenging due to the in-line GC analysis (water vapour can damage the GC column, affecting its
13
separation performance of other gases for analysis). Accordingly, additional experiments (i.e. the equilibrium and breakthrough adsorption) were performed using the water vapour saturated samples to gain more information regarding this aspect. Prior to the tests, CPL-2 and 10 wt.% Ag/CPL-2 were exposed at 100% relative humidity (i.e. the water vapour partial pressure equals the saturated water vapour pressure) at 25 °C for 0.5 h, which was achieved by flowing wet N2 at 50 mL/min through the materials (the wet N2 flow was achieved by saturating dry N2 via a bubbler containing deionised water). The gas adsorption isotherms and IAST selectivity on two pre-treated adsorbents with water vapour are presented in Fig. S13 and Fig. S14 (and Table S6), respectively. In comparison with the results obtained on the pristine adsorbents, the relevant adsorption capacity and selectivity are comparable, suggesting that the water vapour pre-treatment did not affect the intrinsic adsorption property of the two adsorbents. Moreover, as shown in Fig. S15, both adsorbents could also achieve C2H4/C2H6 (1:1, v/v) mixture separation after being saturated by water vapour, and the pre-treated 10 wt.% Ag/CPL-2 also exhibited better dynamic separation performance than the pre-treated CPL-2 adsorbent, confirming the presence of water vapour has a negligible effect on the separation performance of the materials under study.
4. Conclusions Herein, we presented the synthesis and modification of moisture-stable CPL-2 MOFs (via Ag(I) impregnation to give Ag/CPL-2 MOFs) and the application of CPL-2 and Ag/CPL-2 MOFs in ethylene/ethane separation. The synthesis of CPL-2 is simple with fast crystallisation, i.e. 3 h is sufficient to prepare CPL-2 MOF at room temperature. CPL-2 demonstrated the outstanding moisture stability under dynamic humid conditions at 25 °C and 50 °C, showing the reversible water vapour adsorption under the conditions used. This feature allows CPL-2 to be exploited as the promising adsorbent in humid environment. The obtained results also confirmed that CPL-2 has larger adsorption capacity compared to CPL-1 with smaller pore networks. It provides a good understanding about the pore size effect on the selection of materials for ethylene/ethane separation, indicating that different CPLs can be also developed and exploited for hydrocarbon separations. Micropore blockage of CPL-2 framework was measured by N2 physisorption analysis after the introduction of Ag(I) ions on CPL-2, showing a significant decrease in adsorption capacity for Ag/CPL-2 MOFs comparing to the parent CPL-2. However, the Ag species enhanced the interaction between Ag/CPL-2 adsorbents and ethylene molecules, resulting in the improved selectivity, e.g. 26.1 at 1 bar and 25 °C for 10 wt.% Ag/CPL-2 MOF compared to 1.4 of the parent material. Single-component gas adsorption experiments using C2H4, C2H6 and N2 were studied to assess the effect of polarisability and electrostatic interactions (quadrupole moment) upon the adsorbent modification. The results showed that despite the reduction in the surface area in the modified materials, the amount adsorbed per unit area increases. The IAST selectivity for C2H4/C2H6 mixture increased in the presence of Ag(I). This suggests that electrostatic (or in this case quadrupole-ion) interactions dominate. The trade-off between adsorption capacity and selectivity of porous adsorbents should be 14
considered for practical gas separation. In the case of CPL-2 MOF, this study showed that 10 wt.% Ag/CPL-2 by Ag(I) impregnation gave the best adsorption performance considering both capacity and selectivity towards C2H4 over C2H6. Breakthrough experiments for C2H4/C2H6 (1:1, v/v) mixture confirmed that 10 wt.% Ag/CPL-2 showed comparatively better dynamic separation performance than parent CPL-2. The good selectivity combined with the moisture stability makes CPL-2 MOFs worthy of being further developed and explored for the separation of ethylene/ethane mixtures, especially under conditions where water vapour exists. Although the presence of water vapour had a negligible effect on the adsorption properties of CPL-2 materials under the conditions used, a systematic study of the effect of water vapour on the adsorption capacity and selectivity should be considered in the future for a comprehensive assessment of the adsorbent’s potential in practical applications. Acknowledgements HX thanks The University of Manchester President’s Doctoral Scholar Award and the China Scholarship Council (file no. 201606150068) for supporting her PhD research. We are also grateful to Dr. S. Holmes for the N2 adsorption/desorption measurements.
References [1] S.Y. Pu, J.W. Wang, L.Y. Li, Z.G. Zhang, Z.B. Bao, Q.W. Yang, Y.W. Yang, H.B. Xing, Q.L. Ren, Performance comparison of metal organic framework extrudates and commercial zeolite for ethylene/ethane separation, Ind. Eng. Chem. Res. 57 (2018) 1645–1654 https://doi.org/10.1021/acs.iecr.7b04391. [2] H. Xiang, X.L. Fan, F.R. Siperstein, Design of 2D materials for selective adsorption: a comparison between Monte Carlo
simulations
and
direct
numerical
integration,
Mol.
Syst.
Des.
Eng.
3
(2018)
636–644
https://doi.org/10.1039/c8me00024g. [3] D.S. Sholl, R.P. Lively, Seven chemical separations to change the world, Nature 532 (2016) 435–437
https://doi.org/10.1038/532435a. [4] Y. Chen, Z. Qiao, D. Lv, C. Duan, X. Sun, H. Wu, R. Shi, Q. Xia, Z. Li, Efficient adsorptive separation of C3H6 over C3H8
on
flexible
and
thermoresponsive
CPL-1,
Chem.
Eng.
J.
328
(2017)
360–367
https://doi.org/10.1016/j.cej.2017.07.044. [5] L. Mei, Y.F. Wu, X. Zhou, J. Yan, F. Xu, Z. Li, Adsorption performance of MIL-100(Fe) for separation of olefin-paraffin
mixtures,
J.
Taiwan
Inst.
Chem.
Eng.
70
(2017)
74–78
https://doi.org/10.1016/j.jtice.2016.10.047. [6] R.T. Yang, E.S. Kikkinides, New sorbents for olefin/paraffin separations by adsorption via π-complexation, AIChE J. 41 (1995) 509–517 https://doi.org/10.1002/aic.690410309.
15
[7] B. Li, Y. Zhang, R. Krishna, K. Yao, Y. Han, Z. Wu, D. Ma, Z. Shi, T. Pham, B. Space, J. Liu, P.K. Thallapally, J. Liu, M. Chrzanowski, S. Ma, Introduction of π-complexation into porous aromatic framework for highly selective adsorption of ethylene over ethane, J. Am. Chem. Soc. 136 (2014) 8654–8660 https://doi.org/10.1021/ja502119z. [8] Y.W. Chen, Z.W. Qiao, H.X. Wu, D.F. Lv, R.F. Shi, Q.B. Xia, J. Zhou, Z. Li, An ethane-trapping MOF PCN-250 for highly
selective
adsorption
of
ethane
over
ethylene,
Chem.
Eng.
Sci.
175
(2018)
110–117
https://doi.org/10.1016/j.ces.2017.09.032. [9] Z.B. Bao, G.G. Chang, H.B. Xing, R. Krishna, Q.L. Ren, B.L. Chen, Potential of microporous metal-organic frameworks
for
separation
of
hydrocarbon
mixtures,
Energy
Environ.
Sci.
9
(2016)
3612–3641
https://doi.org/10.1039/c6ee01886f. [10] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal-organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504 https://doi.org/10.1039/b802426j. [11] V.V. Butova, M.A. Soldatov, A.A. Guda, K.A. Lomachenko, C. Lamberti, Metal-organic frameworks: structure, properties,
methods
of
synthesis
and
characterization,
Russ.
Chem.
Rev.
85
(2016)
280–307
https://doi.org/10.1070/rcr4554. [12] Y. He, W. Zhou, R. Krishna, B. Chen, Microporous metal-organic frameworks for storage and separation of small hydrocarbons, Chem. Commun. 48 (2012) 11813–11831 https://doi.org/10.1039/c2cc35418g. [13] H.C. Zhou, S. Kitagawa, Metal-organic frameworks (MOFs), Chem. Soc. Rev. 43 (2014) 5415–5418
https://doi.org/10.1039/c4cs90059f. [14] L. Li, R.B. Lin, R. Krishna, H. Li, S. Xiang, H. Wu, J. Li, W. Zhou, B. Chen, Ethane/ethylene separation in a metal-organic
framework
with
iron-peroxo
sites,
Science
362
(2018)
443–446
https://doi.org/10.1126/science.aat0586. [15] Y. Wu, H. Chen, D. Liu, Y. Qian, H. Xi, Adsorption and separation of ethane/ethylene on ZIFs with various topologies: Combining GCMC simulation with the ideal adsorbed solution theory (IAST), Chem. Eng. Sci. 124 (2015) 144–153 https://doi.org/10.1016/j.ces.2014.07.019. [16] D.L. Chen, N.W. Wang, C.H. Xu, G.M. Tu, W.D. Zhu, R. Krishna, A combined theoretical and experimental analysis on transient breakthroughs of C2H6/C2H4 in fixed beds packed with ZIF-7, Microporous Mesoporous Mater. 208 (2015) 55–65 https://doi.org/10.1016/j.micromeso.2015.01.019. [17] M. Hartmann, U. Bohme, M. Hovestadt, C. Paula, Adsorptive separation of olefin/paraffin mixtures with ZIF-4, Langmuir 31 (2015) 12382–12389 https://doi.org/10.1021/acs.langmuir.5b02907. [18] J. Pires, M.L. Pinto, V.K. Saini, Ethane selective IRMOF-8 and its significance in ethane-ethylene separation by 16
adsorption, ACS Appl. Mater. Interfaces 6 (2014) 12093–12099 https://doi.org/10.1021/am502686g. [19] R.S. Pillai, M.L. Pinto, J. Pires, M. Jorge, J.R. Gomes, Understanding gas adsorption selectivity in IRMOF-8 using molecular simulation, ACS Appl. Mater. Interfaces 7 (2015) 624–637 https://doi.org/10.1021/am506793b. [20] P.Q. Liao, W.X. Zhang, J.P. Zhang, X.M. Chen, Efficient purification of ethene by an ethane-trapping metal-organic framework, Nat. Commun. 6 (2015) 8697 https://doi.org/10.1038/ncomms9697. [21] Y.W. Chen, H.X. Wu, D.F. Lv, R.F. Shi, Y. Chen, Q.B. Xia, Z. Li, Highly adsorptive separation of ethane/ethylene by
an
ethane-selective
MOF
MIL-142A,
Ind.
Eng.
Chem.
Res.
57
(2018)
4063–4069
https://doi.org/10.1021/acs.iecr.7b05260. [22] W. Liang, F. Xu, X. Zhou, J. Xiao, Q. Xia, Y. Li, Z. Li, Ethane selective adsorbent Ni(bdc)(ted)0.5 with high uptake and its significance in adsorption separation of ethane and ethylene, Chem. Eng. Sci. 148 (2016) 275–281
https://doi.org/10.1016/j.ces.2016.04.016. [23] H. Wu, Y. Chen, D. Lv, R. Shi, Y. Chen, Z. Li, Q. Xia, An indium-based ethane-trapping MOF for efficient selective
separation
of
C2H6/C2H4
mixture,
Sep.
Purif.
Technol.
212
(2019)
51–56
https://doi.org/10.1016/j.seppur.2018.11.005. [24] Q.M. Wang, D.M. Shen, M. Bulow, M.L. Lau, S.G. Deng, F.R. Fitch, N.O. Lemcoff, J. Semanscin, Metallo-organic molecular sieve for gas separation and purification, Microporous Mesoporous Mater. 55 (2002) 217–230
https://doi.org/10.1016/S1387-1811(02)00405-5. [25] J. Ploegmakers, S. Japip, K. Nijmeijer, Mixed matrix membranes containing MOFs for ethylene/ethane separation-Part B: Effect of Cu3BTC2 on membrane transport properties, J. Membr. Sci. 428 (2013) 331–340
https://doi.org/10.1016/j.memsci.2012.11.013. [26] T.M. Nicholson, S.K. Bhatia, Electrostatically mediated specific adsorption of small molecules in metallo-organic frameworks, J. Phys. Chem. B 110 (2006) 24834–24836 https://doi.org/10.1021/jp065102b. [27] V.F.D. Martins, A.M. Ribeiro, A. Ferreira, U.H. Lee, Y.K. Hwang, J.S. Chang, J.M. Loureiro, A.E. Rodrigues, Ethane/ethylene separation on a copper benzene-1,3,5-tricarboxylate MOF, Sep. Purif. Technol. 149 (2015) 445–456
https://doi.org/10.1016/j.seppur.2015.06.012. [28] E.D. Bloch, W.L. Queen, R. Krishna, J.M. Zadrozny, C.M. Brown, J.R. Long, Hydrocarbon separations in a metal-organic
framework
with
open
iron(II)
coordination
sites,
Science
335
(2012)
1606–1610
https://doi.org/10.1126/science.1217544. [29] Z. Bao, S. Alnemrat, L. Yu, I. Vasiliev, Q. Ren, X. Lu, S. Deng, Adsorption of ethane, ethylene, propane, and propylene
on
a
magnesium-based
metal-organic
framework, 17
Langmuir
27
(2011)
13554–13562
https://doi.org/10.1021/la2030473. [30] U. Bohme, B. Barth, C. Paula, A. Kuhnt, W. Schwieger, A. Mundstock, J. Caro, M. Hartmann, Ethene/ethane and propene/propane separation via the olefin and paraffin selective metal-organic framework adsorbents CPO-27 and ZIF-8, Langmuir 29 (2013) 8592–8600 https://doi.org/10.1021/la401471g. [31] S.J. Geier, J.A. Mason, E.D. Bloch, W.L. Queen, M.R. Hudson, C.M. Brown, J.R. Long, Selective adsorption of ethylene over ethane and propylene over propane in the metal-organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn), Chem. Sci. 4 (2013) 2054–2061 https://doi.org/10.1039/c3sc00032j. [32] J.B. DeCoste, G.W. Peterson, B.J. Schindler, K.L. Killops, M.A. Browe, J.J. Mahle, The effect of water adsorption on the structure of the carboxylate containing metal-organic frameworks Cu-BTC, Mg-MOF-74, and UiO-66, J. Mater. Chem. A 1 (2013) 11922–11932 https://doi.org/10.1039/c3ta12497e. [33] N. Al-Janabi, P. Hill, L. Torrente-Murciano, A. Garforth, P. Gorgojo, F. Siperstein, X. Fan, Mapping the Cu-BTC metal-organic framework (HKUST-1) stability envelope in the presence of water vapour for CO2 adsorption from flue gases, Chem. Eng. J. 281 (2015) 669–677 https://doi.org/10.1016/j.cej.2015.07.020. [34] N. Al-Janabi, A. Alfutimie, F.R. Siperstein, X. Fan, Underlying mechanism of the hydrothermal instability of Cu3(BTC)2
metal-organic
framework,
Front.
Chem.
Sci.
Eng.
10
(2016)
103–107
https://doi.org/10.1007/s11705-015-1552-0. [35] J. Canivet, A. Fateeva, Y. Guo, B. Coasne, D. Farrusseng, Water adsorption in MOFs: fundamentals and applications, Chem. Soc. Rev. 43 (2014) 5594–5617 https://doi.org/10.1039/c4cs00078a. [36] N.C. Burtch, H. Jasuja, K.S. Walton, Water stability and adsorption in metal-organic frameworks, Chem. Rev. 114 (2014) 10575–10612 https://doi.org/10.1021/cr5002589. [37] Y. Li, R.T. Yang, Gas adsorption and storage in metal-organic framework MOF-177, Langmuir 23 (2007) 12937– 12944 https://doi.org/10.1021/la702466d. [38] Z. Liang, M. Marshall, A.L. Chaffee, CO2 adsorption, selectivity and water tolerance of pillared-layer metal organic
frameworks,
Microporous
Mesoporous
Mater.
132
(2010)
305–310
https://doi.org/10.1016/j.micromeso.2009.11.026. [39] H. Sakamoto, R. Kitaura, R. Matsuda, S. Kitagawa, Y. Kubota, M. Takata, Systematic construction of porous coordination
pillared-layer
structures
and
their
sorption
properties,
Chem.
Lett.
39
(2010)
218–219
https://doi.org/10.1246/cl.2010.218. [40] K. Kishida, Y. Watanabe, S. Horike, Y. Watanabe, Y. Okumura, Y. Hijikata, S. Sakaki, S. Kitagawa, DRIFT and theoretical
studies
of
ethylene/ethane
separation 18
on
flexible
and
microporous
[Cu2(2,3-pyrazinedicarboxylate)2(pyrazine)]n,
Eur.
J.
Inorg.
Chem.
2014
(2014)
2747–2752
https://doi.org/10.1002/ejic.201402085. [41] R.T. Yang, Adsorbents: fundamentals and applications, John Wiley & Sons 2003. [42] C. Wu, J.J. Wang, Y. Fang, Z. Wang, D.J. Fei, X. Han, Y.G. Dang, Comparative Study of Ag+-Based Adsorbents Performance
in
Ethylene/Ethane
Separation,
J.
Chem.
Eng.
Data
64
(2019)
611–618
https://doi.org/10.1021/acs.jced.8b00806. [43] S. Aguado, G. Bergeret, C. Daniel, D. Farrusseng, Absolute molecular sieve separation of ethylene/ethane mixtures with silver zeolite A, J. Am. Chem. Soc. 134 (2012) 14635–14637 https://doi.org/10.1021/ja305663k. [44] G. Chang, M. Huang, Y. Su, H. Xing, B. Su, Z. Zhang, Q. Yang, Y. Yang, Q. Ren, Z. Bao, B. Chen, Immobilization of Ag(I) into a metal-organic framework with -SO3H sites for highly selective olefin-paraffin separation at room temperature, Chem. Commun. 51 (2015) 2859–2862 https://doi.org/10.1039/c4cc09679g. [45] Y. Zhang, B. Li, R. Krishna, Z. Wu, D. Ma, Z. Shi, T. Pham, K. Forrest, B. Space, S. Ma, Highly selective adsorption of ethylene over ethane in a MOF featuring the combination of open metal site and π-complexation, Chem. Commun. 51 (2015) 2714–2717 https://doi.org/10.1039/c4cc09774b. [46] Y.X. Wang, Z.G. Hu, Y.D. Cheng, D. Zhao, Silver-decorated hafnium metal-organic framework for ethylene/ethane separation, Ind. Eng. Chem. Res. 56 (2017) 4508–4516 https://doi.org/10.1021/acs.iecr.7600517. [47] J. Padin, R.T. Yang, New sorbents for olefin/paraffin separations by adsorption via π-complexation: synthesis and effects of substrates, Chem. Eng. Sci. 55 (2000) 2607–2616 https://doi.org/10.1016/s0009-2509(99)00537-0. [48] F. Iucolano, P. Aprea, D. Caputo, C. Colella, M. Eic, Q. Huang, Adsorption and diffusion of propane and propylene in Ag+-impregnated MCM-41, Adsorption 14 (2008) 241–246 https://doi.org/10.1007/s10450-008-9106-0. [49] A. Anson, Y. Wang, C.C.H. Lin, T.M. Kuznicki, S.M. Kuznicki, Adsorption of ethane and ethylene on modified ETS-10, Chem. Eng. Sci. 63 (2008) 4171–4175 https://doi.org/10.1016/j.ces.2008.05.038. [50] S.U. Rege, J. Padin, R.T. Yang, Olefin/paraffin separations by adsorption: π-complexation vs. kinetic separation, AIChE J. 44 (1998) 799–809 https://doi.org/10.1002/aic.690440405. [51] O.J. García-Ricard, A.J. Hernández-Maldonado, Cu2(pyrazine-2,3-dicarboxylate)2(4,4'-bipyridine) porous coordination sorbents: Activation temperature, textural properties, and CO2 adsorption at low pressure range, J. Phys. Chem. C 114 (2010) 1827–1834 https://doi.org/10.1021/jp9103068. [52] M. Galiova, J. Kaiser, K. Novotny, J. Novotny, T. Vaculovic, M. Liska, R. Malina, K. Stejskal, V. Adam, R. Kizek, Investigation of heavy-metal accumulation in selected plant samples using laser induced breakdown spectroscopy and laser
ablation
inductively
coupled
plasma
mass
spectrometry, 19
Appl.
Phys.
A
93
(2008)
917–922
https://doi.org/10.1007/s00339-008-4747-0. [53] W.Y. Huang, X. Zhou, Q.B. Xia, J.J. Peng, H.H. Wang, Z. Li, Preparation and adsorption performance of GrO@Cu-BTC
for
separation
of
CO2/CH4,
Ind.
Eng.
Chem.
Res.
53
(2014)
11176–11184
https://doi.org/10.1021/ie501040s. [54] Y. Jiao, Z. Li, Y. Ma, G. Zhou, S. Wang, G. Lu, The studies on gas adsorption properties of MIL-53 series MOFs materials, AIP Adv. 7 (2017) 085009 https://doi.org/10.1063/1.4999914. [55] R. Matsuda, T. Tsujino, H. Sato, Y. Kubota, K. Morishige, M. Takata, S. Kitagawa, Temperature responsive channel uniformity impacts on highly guest-selective adsorption in a porous coordination polymer, Chem. Sci. 1 (2010) 315–321 https://doi.org/10.1039/c0sc00272k. [56] H. Chen, K. Riascos-Rodríguez, M.E. Marcano-González, A.J. Hernández-Maldonado, Cu2(pzdc)2L [L=dipyridyl-based ligands] porous coordination polymers: Hysteretic adsorption and diffusion kinetics of CO2 and CH4, Chem. Eng. J. 283 (2016) 806–815 https://doi.org/10.1016/j.cej.2015.08.017. [57] P.J. Meza-Morales, D.A. Gomez-Gualdron, R.R. Arrieta-Perez, A.J. Hernandez-Maldonado, R.Q. Snurr, M.C. Curet-Arana, CO2 adsorption-induced structural changes in coordination polymer ligands elucidated via molecular simulations and experiments, Dalton Trans. 45 (2016) 17168–17178 https://doi.org/10.1039/c6dt02994a. [58] N. Al-Janabi, H. Deng, J. Borges, X. Liu, A. Garforth, F.R. Siperstein, X. Fan, A facile post-synthetic modification method to improve hydrothermal stability and CO2 selectivity of CuBTC metal-organic framework, Ind. Eng. Chem. Res. 55 (2016) 7941–7949 https://doi.org/10.1021/acs.iecr.5b04217. [59] N. Al-Janabi, V. Martis, N. Servi, F.R. Siperstein, X.L. Fan, Cyclic adsorption of water vapour on CuBTC MOF: Sustaining the hydrothermal stability under non-equilibrium conditions, Chem. Eng. J. 333 (2018) 594–602
https://doi.org/10.1016/j.cej.2017.09.197. [60] D. Li, K. Kaneko, Molecular geometry-sensitive filling in semi-rectangular micropores of organic-inorganic hybrid crystals, J. Phys. Chem. B 104 (2000) 8940–8945 https://doi.org/10.1021/jp000660q. [61] G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I. Margiolaki, A chromium terephthalate-based solid with unusually large pore volumes and surface area, Science 309 (2005) 2040–2042
https://doi.org/10.1126/science.1116275.
20
Synthesis and modification of moisture-stable CPL-2 MOF for ethylene/ethane separation HIGHLIGHTS
− CPL-2 MOF exhibits preferential adsorption of ethylene over ethane. − CPL-2 MOF shows excellent moisture stability under humid conditions. − Ethylene/ethane selectivity was enhanced from 1.4 to 26.1 by silver ions modification.