Adsorption separation of acetylene and ethylene in a highly thermostable microporous metal-organic framework

Accepted Manuscript Adsorption separation of acetylene and ethylene in a highly thermostable microporous metal-organic framework Yingcai Zhao, Jiawei Wang, Zongbi Bao, Huabin Xing, Zhiguo Zhang, Baogen Su, Qiwei Yang, Yiwen Yang, Qilong Ren PII: DOI: Reference:

S1383-5866(17)31678-7 https://doi.org/10.1016/j.seppur.2017.11.044 SEPPUR 14200

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

27 May 2017 3 November 2017 19 November 2017

Please cite this article as: Y. Zhao, J. Wang, Z. Bao, H. Xing, Z. Zhang, B. Su, Q. Yang, Y. Yang, Q. Ren, Adsorption separation of acetylene and ethylene in a highly thermostable microporous metal-organic framework, Separation and Purification Technology (2017), doi: https://doi.org/10.1016/j.seppur.2017.11.044

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Adsorption separation of acetylene and ethylene in a highly thermostable microporous metal-organic framework Yingcai Zhao,1 Jiawei Wang,1 Zongbi Bao,* Huabin Xing, Zhiguo Zhang, Baogen Su, Qiwei Yang, Yiwen Yang, and Qilong Ren*

Key Laboratory of Biomass Chemical Engineering of the Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China

* Corresponding author: Tel.: +86-571-87952773; Fax: +86-571-87952773; Email: [email protected]; [email protected] 1

These two authors contribute equally to this work.

Abstract A magnesium-based metal-organic framework with high hydrothermal stability, Mg-CUK-1, was investigated for the adsorption separation of acetylene from ethylene. The synthesized MOF remained excellent thermostability even near 500 oC according to TGA results. Characterization techniques including FTIR, SEM, and PXRD were utilized to confirm the detailed structure and properties of the Mg-based MOF. It exhibited moderate adsorption capacities of C2H2 (3.01 mmol/g at 100 kPa and 273 K). Moreover, it showed preferential adsorption of C2H2 over C2H4 with C2H2/C2H4 adsorption selectivity over 3, outperforming some popular MOFs. Furthermore, the isosteric heat of C2H2 (< 21 kJ/mol) and C2H4 (< 18 kJ/mol) were extremely lower than those MOFs with open metal sites. From an industrial point of view, this MOF could be synthesized in absence of organic solvent, drastically reducing the risk of environmental pollution in the production process. Besides, such advantages as excellent hydrothermal stability, cheap and non-poisonous metal ion resource, high C2H2 adsorption capacity, and low isosteric heat make the Mg-based MOF promising in practical application. Keywords: gas adsorption, metal-organic framework, acetylene, ethylene, separation

1. Introduction Ethylene (C2H4), as one of the most significant basic chemicals in the petrochemical industry, is widely used in the production of polyethylene and other chemical products. High quality of C2H4 is required especially in the production of polyethylene. Generally, C2H4 is prepared via the cryogenic distillation of the cracking gas while a small amount of acetylene (C2H2) as an impurity of about 1% is unavoidably generated. The presence of minor acetylene as low as 40 ppm will pose a severe impact in the process of producing polyethylene [1]. Not only can it lead a Ziegler-Natta catalyst to be poisoned during the polymerization of C2H4 affecting the quality of polyethylene product [2], but also excess C2H2 can generate solid metal acetylides that block the gas flow and cause explosion [3]. Therefore, it’s an urgent and crucial challenge to efficiently remove C2H2 from C2H4 for the preparation of polymer-grade ethylene. Currently, adsorption may be the most potential alternative in that it is economical and energy-efficient compared with cost and energy-intensive partial hydrogenation or solvent extraction [4-6]. As a new family of porous adsorbent materials, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have shown amazing potential in the fields of gas storage and separation [7-16] due to its exceptionally high surface area, tunable pores and multiple functionalities. Nonetheless, there are few MOFs having been applied in industrial application nowadays. Some primary limitations have hindered the transition to the industrial practice. Firstly, many MOFs are not stable in the existence of environment moisture. For example, Fe-MOF-74 reported by Bloch

et al. [17] showed a high C2H2 adsorption capacity of 6.8 mmol/g but poor stability in air and humidity. Secondly, most ligands for preparing MOFs are costly or commercially unavailable. For instance, Chen et al. [18] synthesized a microporous enantiopure mixed-metal-organic frameworks (M’MOF) with a remarkably high adsorption selectivity of C2H2/C2H4; however, its metalloligand is fairly expensive. Thirdly, the processes of preparing MOF materials are not environmentally friendly. Organic solvents like N,N-dimethylformamide and N,N-dimethylacetamide were commonly adopted for preparation of MOF materials. Removal of these solvents was typically performed by exchanged with methanol or acetone. Large amount of organic solvents are consumed in the subsequent purification [19]. As a consequence, it’s still challenging and urgent to develop a stable, easily-prepared MOF adsorbent with economical and accessible raw materials. Moreover, low regeneration energy cost is required in industrial practice as well. For these purposes, we found a magnesium-based MOF, Mg-CUK-1 with the formula of [Mg3(2,4-pdc)2(μ3-OH)2]·9H2O (2,4-pdc=2,4-pyridinedicarboxylic acid), with satisfactory C2H2 uptake and moderate adsorption selectivity for C2H2/C2H4 as well as lower regeneration energy compared to the MOFs with high density of open metal sites. Mg-CUK-1, crystallizing in the monoclinic space group P21/c, was firstly reported by Humphrey et al. for the efficient separation of p-divinylbenzene [20]. Mg(II)

centers

are

octahedrally

coordinated

with

O

and

N

atoms

of

2,4-pyridinedicarboxylic acid, forming rhombic apertures and 1D zigzag channels (Figure 1). Guest water molecules in the channels can be removed by gentle heating

under vacuum. Apart from its thermal and chemical stability due to the higher hardness of Mg(II) than transition metals, Mg(II) is also lighter, cheaper and relatively non-poisonous compared with most transition metals, making it a commendable alternative for scale-up industrial practice. Moreover, the absence of organic solvent during the whole preparation process drastically reduces the risk of environmental pollution, accordant with the concept of “Green Chemical Industry”. The resulting Mg-CUK-1 was characterized by powder X-ray diffraction (PXRD), attenuated total reflection

Fourier

adsorption-desorption

transform

infrared

isotherms

and

spectroscopy

thermal

gravimetric

(ATR-FTIR), analysis

CO2 (TGA).

Single-component adsorption isotherms of C2H2 and C2H4 were measured at 273, 298 and 313 K, and ideal adsorbed solution theory (IAST) [21] was applied to calculate the C2H2/C2H4 adsorption selectivity. The isosteric heats of C2H2 and C2H4 on Mg-CUK-1 were estimated by virial method.

Figure 1. (a) Structure of Mg-CUK-1 after degassing viewed along the z-axis. (b) The pillared zigzag channel for Mg-CUK-1 viewed along the x-axis. (Mg, green; O, red; C, grey; N, blue; H, white)

2. Experimental 2.1. Materials 2,4-pyridinedicarboxylic acid was purchased from Aladdin Co. (China). Magnesium chloride (MgCl2) and potassium hydroxide (KOH) were got from Sinopharm Chemical Reagent Co., Ltd. (China). C2H2 (99%) and C2H4 (99.99%) were purchased from Jingong Co., Ltd. (China). All chemicals were used as received without any purification. 2.2. Synthesis of Mg-CUK-1 Mg-CUK-1 was synthesized and purified following the slightly modified method reported before [20]. 2,4-Pyridinedicarboxylic acid (1 mmol, 167 mg), KOH aqueous solution (2.0 mol/L, 2.0 mL), MgCl2 (1.5 mmol, 142.5 mg), and H2O (3.0 mL) were added into a 25 mL Teflon-lined stainless steel autoclave and stirred for 30 min for the sufficient mixing. Then the mixture was heated at 200 oC for 15 h and cooled down to room temperature during 6 h subsequently. To obtain the pure Mg-CUK-1 crystals, as-synthesized product was sonicated in H2O for 3 times until the supernatant became colorless. Finally the colorless prismatic crystals were dried at 80 oC under vacuum for the further study. 2.3. Characterization The attenuated total reflection Fourier transform infrared spectra (ATR-FTIR) were performed in a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific Inc., USA) with diamond as an internal reflection element at an incident angle of 45o, using

a resolution of 4 cm-1 and 32 scans and between 400 and 4000 cm-1. Scanning electron microscope (SEM) was carried out on a SIRION-100 instrument (FEI Corp., Netherlands). It was performed on the samples previously dried and sputter-coated with a thin Au layer. CO2 adsorption-desorption isotherms were obtained at 195 K on a Micromeritics ASAP 2460 adsorption apparatus (Micromeritics Instrument Corp., USA), and the surface area and pore volume were calculated by the method reported [22]. Each sample was degassed at 100 oC for 6 h under ultrahigh vacuum before measurement. Powder X-ray diffraction (PXRD) patterns of the samples were recorded with an XPert diffractometer (Panalytical Corp., Netherlands) using Cu Kα (λ=0.1543 nm) radiation at 40 kV from 3o to 30o (2θ angle range) with a step size of 0.02o. The thermal gravimetric analysis (TGA) was performed in a Pyris 1 TGA instrument (Perkin-Elmer Corp., USA) from 30 to 800 oC in N2 atmosphere at a constant rate of 10 oC/min. 2.4. Adsorption measurements The C2H2 and C2H4 single-component adsorption isotherms were carried out at 313, 298 and 273 K on a Micromeritics ASAP 2050 adsorption apparatus (Micromeritics Instrument Corp., USA) after degassed at 100 oC for 6 h under ultrahigh vacuum. Breakthrough experiments were carried out in a stainless steel column (50 mm x 4.6 mm ID) manually packed with 0.62 g dehydrated Mg-CUK-1 under atmospheric pressure and 298 K. Before packed in a glove-box, the sample was degassed under ultrahigh vacuum at 100 oC for 6 h. The mixed gas (C2H2:C2H4=1:99, v/v) was introduced with a constant flow rate of 0.4 mL/min at 298 K, and outlet gas

passing through the column was analysed using a GC-8A gas chromatography (Shimadzu, Japan) with a flame ionization detector (FID). After breakthrough experiments, a helium gas flow-rate (20 mL/min) was utilized to regenerate the column at 313 K until no signal of C2H2 and C2H4 was detected on GC. 2.5. Calculation for adsorption selectivity and isosteric heat of adsorption Single-component adsorption isotherms were analysed by utilizing the well-defined Dual Site Langmuir-Freundlich (DSLF) method

bA p vA bB p vB q  q A, sat  qB , sat 1  bA p vA 1  bB p vB

(1)

where p (kPa) is the pressure of the bulk gas when at equilibrium with the adsorbed phase (kPa), q (mmol/g) is the gas uptake of an adsorbent, qA,sat and qB,sat (mmol/g) are the saturation capacities of site A and site B, bA and bB (1/kPa) are the affinity coefficients to the sites A and B, and vA and vB represent the deviations from an ideal homogeneous surface. Adsorption selectivity is established by IAST for C2H2/C2H4 binary mixtures (50:50 or 1:99, v/v) in Mg-CUK-1. The adsorption selectivity, Sij, is defined by the following equation Sij 

xi / x j yi / y j

(2)

where xi and xj are the equilibrated adsorption capacity of component i and j, respectively, and yi and yj are the molar fractions of component i and j in gas phase, respectively. The isosteric heats of adsorption (Qst), revealing the average interactions of

adsorbent- adsorbate at a specific surface coverage, are calculated by fitting isotherms at two temperatures with the following virial method [23, 24]. ln P  ln N 

n 1 m ai N i   bi N i  T i 0 i 0

(3)

where P is the pressure (kPa), N is the amount adsorbed (mmol/g), T is the temperature (K), ai and bi are virial coefficients, and m and n determine the number of coefficients required to adequately describe the isotherms. The values of the virial coefficients a0 through am are then used to calculate the isosteric heat of adsorption by using the following expression m

Qst   R ai N i

(4)

i 0

3. Results and discussion 3.1. Characterization of Mg-CUK-1 To verify the binding sites of the metal cations and ligands, the FTIR method was employed. As shown in Figure 2, strong bands in the range of 1650-1300 cm-1 are correspongding to vas(COO), vs(COO) and v(C-C) vibrations, suggesting the existence of the carboxylate group. Especially, the characteristic band of 1633 cm-1 is attributed to the vibration of C=O [25]. Mg-O stretching vibration appears at 427 cm-1, revealing the coordination mode of the ligands and metal cations [26].

Transmittance

1633

427 2000 1800 1600 1400 1200 1000 800 600 400

Wavenumber (cm-1) Figure 2. FTIR spectrum of Mg-CUK-1.

As presented in Figure 3, Mg-CUK-1 had a rod-like morphology and average crystallite sizes of 500 x 100 μm.

Figure 3. The SEM image of the Mg-CUK-1.

PXRD patterns were used to ensure the exact crystal structure synthesized in the study. From Figure 4, spiculate and refined peaks were exhibited, illustrating the intact crystlline structure of Mg-CUK-1. Surprisingly, different from the most of

MOFs it demonstrated extraordinarily high stability. The PXRD pattern was changeless and retained all of the peaks after exposed in air for 7 days. The stable structure of Mg-CUK-1 provides great potential for industrial practice.

Intensity (a.u.)

a week later as synthesized

b a 5

10

15

20

2 (degree)

25

30

Figure 4. PXRD patterns of (a) as-synthesized Mg-CUK-1, and (b) Mg-CUK-1 exposed in air for 7 days.

As an important factor for the performence of the adsorbent, especailly for gas adsorption

seperation,

porosity

of

Mg-CUK-1

was

studied

via

CO2

adsorption-desorption isotherms at 195 K. As presented in Figure 5, Mg-CUK-1 possessed BET surface area of 546 m2/g. Importantly, its porosity was retained completely (561 m2/g) even after exposed in the air for 7 days, indicating its excellent stability further.

150 120 90

1.8 1.5

dV/dw (cm /gnm)

60

3

CO2 uptake (cm3/g STP)

180

30

1.2 0.9 0.6 0.3 0.0 0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Pore width (nm)

0 0.0

0.2

0.4

0.6 P /P

0.8

1.0

0

Figure 5. CO2 adsorption isotherms at 195 K for fresh Mg-CUK-1 (red), and Mg-CUK-1 exposed in air for 7 days (blue). The inset was the pore size distribution of fresh Mg-CUK-1.

Figure 6 showed the TGA curve of Mg-CUK-1. The weight loss was observed to occur in three steps. A sharp mass loss of around 27 wt% lined with the loss of solvent outside and inside the cavities of the adsorbent was discoveried between 30 and 85 oC. The curve remained level until 480 oC, meaning that Mg-CUK-1 could perfectly reserve its crystalline structure at rather high temperature. This property is quite important for thermal regernation of adsorbents in the thermal swing adsorption (TSA) application. Another rapid decline occured after 480 oC due to the decomposition of the ligands and collapse of the frameworks.

100 90

Weight loss (%)

80 70 60 50 40 30 20 10

100

200

300

400 T

500

600

700

800

o

( C)

Figure 6. TAG profile for Mg-CUK-1 at N2 atmosphere.

3.2. Single-component adsorption isotherms of C2H2 and C2H4 As its outstanding properties, such as excellent hydrothermal stability, low cost and relative nontoxicity, Mg-CUK-1 has caught our attention to evaluate its potential for important industrial separation of C2H2 and C2H4. As reported, Mg-CUK-1 is extremely easily dehydrated by heated under vacuum at mild temperature (50 oC) for 1 h [20]. For the complete dehydration, the samples were degassed at 100 oC for 6 h under ultrahigh vacuum before measurement. The single-component adsorption isotherms of C2H2 and C2H4 were collected at 273, 298, and 313 K. As shown in Figure 7, the adsorbent exhibited moderately high C2H2 uptake and relatively low C2H4 uptake. The adsorption capacities of C2H2 and C2H4 for Mg-CUK-1 were 3.01 mmol/g and 1.87 mmol/g at 100 kPa and 273 K. Furthermore, it showed remarkably different adsorption behaviors with respect to C2H2 and C2H4. The isotherm curves of C2H2 increased rapidly below 15 kPa; however, the uptake of C2H4 increased more

slowly than C2H2. These differences between adsorption behaviors for C2H2 and C2H4 indicated that Mg-CUK-1 might have good selectivity of C2H2 over C2H4, which encouraged us to deeply explore its ability for selective separation of C2H2/C2H4 binary mixture.

3.5

Gas uptake (mmol/g)

3.0 2.5 2.0 1.5 1.0 0.5 0.0

0 10 20 30 40 50 60 70 80 90 100 110 P

(kPa)

Figure 7. Single-component adsorption isotherms of C2H2 (solid) and C2H4 (hollow) in Mg-CUK-1 at 273 (green), 298 (blue), and 313 K (red).

3.3. IAST adsorption selectivity for C2H2/C2H4 binary mixture For this purpose, we utilized ideal adsorbed solution theory (IAST) to calculate the adsorption selectivity of Mg-CUK-1 for C2H2/C2H4 binary mixture (both 50/50 and 1/99, v/v). The single-component adsorption isotherms of both gases were firstly correlated with the dual site Langmuir-Freundlich model. The fitting parameters of the dual site Langmuir-Freundlich were summarized in Table 1. These values were used for predicting the selectivity of competitive adsorption between C2H2 and C2H4

by IAST method. In Figure 8, the adsorption selectivities of Mg-CUK-1 for C2H2 and C2H4 (50/50, 1/99, v/v) were 3.36 and 3.13 at 273 K and 100 kPa, respectively. As shown in Figure 9, we have compared the IAST adsorption selectivities of C2H2/C2H4 binary mixture (1/99, v/v) between Mg-CUK-1 and other popular MOFs at similar temperature. It can be seen that the adsorption selectivity of Mg-CUK-1 at 100 kPa was higher than MOF-74 series [17, 27] and NOTT-300 [28].

Table 1. Dual Site Langmuir-Freundlich parameters fit for Mg-CUK-1 at 273 K Site A

Site B -1

qA,sat/mmol g

bA/kPa

1.4239 37.3131

0.0909 0.0003

IAST selectivity (C2H2/C2H4)

C2H2 C2H4

-1

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

vA

qB,sat/mmol g-1

bB/ kPa-1

vB

1.6162 0.8878

7.7711 1.2335

0.0108 0.0774

0.6901 1.3378

R2 0.9999 0.9999

10 20 30 40 50 60 70 80 90 100 P

(kPa)

Figure 8. IAST calculation for C2H2/C2H4 (50:50 (red), and 1:99 (blue)) adsorption selectivity for Mg-CUK-1 at 273 K.

IAST selectivity (C2H2/C2H4)

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

10 20 30 40 50 60 70 80 90 100 P

(kPa)

Figure 9. IAST calculation for C2H2/C2H4 (1:99, v/v) adsorption selectivity for Mg-CUK-1 (298 K, red), FeMOF-74 (318 K, blue), MgMOF-74 (296 K, green), CoMOF-74 (296 K, orange), and NOTT-300 (293 K, purple).

3.4. Isosteric heats of adsorption It’s also very important to pay attention to the regeneration energy of the adsorbent in industrial application. As a result, the isosteric heat of adsorption (Qst) was calculated by the virial method (Table 2). As shown in Figure 10, Mg-CUK-1 demonstrated the Qst of less than 21 kJ/mol for C2H2 and less than 18 kJ/mol for C2H4, which were significantly lower than that of MOF-74 series (FeMOF-74: 46.5 and 47 kJ/mol; MgMOF-74: 41 and 42 kJ/mol for C2H2 and C2H4) [27] and NOTT-300 (32 and 16 kJ/mol for C2H2 and C2H4) [28] which have abundant coordinately unsaturated metal sites. It’s notable that the Qst of Mg-CUK-1 were also appreciably lower than the MOFs possessing narrow pore windows, such as M’MOF-3a, UTSA-100a. It implied that Mg-CUK-1 was energy-efficient and required less regeneration energy

than other reported MOFs.

Table 2. Isosteric heats of adsorption fitting parameters for Mg-CUK-1 Adsorbent C2H2 C2H4

a1

a2

a3

a4

a5

b0

b1

b2

R2

-2155.22 -1732.54

-965.14 -1889.50

700.04 2873.76

-173.19 -2247.31

62.89 1204.62

-9.75 -248.38

9.68 8.98

2.36 3.51

-1.19 -2.41

0.9986 0.9992

30 25

Qst (kJ/mol)

Mg-CUK-1

a0

20 15 10 5 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Gas uptake (mmol/g)

Figure 10. Isosteric heats of adsorption for C2H2 (red) and C2H4 (blue) of Mg-CUK-1.

3.5．C2H2/C2H4 breakthrough experiments The breakthrough experiments with C2H2/C2H4 binary mixtures (1:99, v/v) were studied so as to confirm the industrial potential of Mg-CUK-1 for the removal of C2H2 from C2H4. The C2H2/C2H4 mixture passed through a packed bed of Mg-CUK-1 powder at 298 K with a total flow of 0.4 mL/min. From the result of Figure 11, C 2H4 reached adsorption saturation within 10 min after a fast breakthrough; however, C 2H2 didn’t approach saturation until 150 min. It’s worth noting that C2H2 and C2H4 nearly broke through simultaneously though the C2H2/C2H4 binary mixtures were efficiently separated. It’s probably attributed that light Mg(II) without empty d orbits had weaker

interaction with unsaturated bonds compared with transition metals, as well the large pore windows more than 9 Å couldn’t effectively sieve C2H2/C2H4 by molecular sieving effect. Therefore, the separation effect will be further enhanced by judicious tuning the pore size. The separation selectivity based on the saturation adsorption capacities were 1.81. The decreased separation selectivity compared with IAST results might be related with the packing performance.

1.0 0.8 C2H2

0.6

1.0 0.8

0.4

C/C0

C/C0

C2H4

0.6 0.4 0.2

0.2

0.0 30 32 34 36 38 40 42 44

t (min)

0.0 40

60

80

100

t (min)

120

140

160

Figure 11. Experimental breakthrough curves for C2H2 (red, solid) and C2H4 (blue, hollow) binary mixture (1:99, v/v) separation. The inset is the partial enlarged view from 30 to 45 min.

4. Conclusions To realize the efficient separation of C2H2/C2H4 binary mixture in industry, we employed a Mg-based MOF, Mg-CUK-1, with moderate C2H2 uptake and adsorption selectivity for C2H2 and C2H4. The low cost and hypotoxicity of Mg metal salt, and simple synthetic procedures are beneficial to the large-scale industrial application.

The absence of organic solvent during the preparation process drastically reduces the risk of environmental pollution, which is a significant advantage in industry compared with other MOFs. The structure of Mg-CUK-1 was characterized through FTIR, SEM, PXRD, and BET results. The excellent stability was confirmed by TGA results. Single-component adsorption isotherms of C2H2 and C2H4 were obtained at 273, 298, 313 K, respectively. High adsorption capacity of C2H2 reaching up to 3.01 mmol/g was realized at 273 K and 100 kPa; however, C2H4 uptake was relatively low, which caused a satisfactory adsorption selectivity of more than 3 superior to some previously reported MOFs. Additionally, the extremely prominent Qst, which was lower than most of the MOFs, implied less energy required during the regeneration process and led to a significant energy saving.

Acknowledgement This work was supported by Zhejiang Provincial Natural Science Foundation of China (LR17B060001), National Natural Science Foundation of China (No. 21436010 and 21376205) and National Key Research and Development Plan (No. 2016YFB0301500)..

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Graphical Abstract

Highlights 

The studied MOF material is highly thermostable until nearly 500 oC.

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High acetylene uptake and moderate adsorption selectivity were achieved.

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Very low isosteric heat of C2H2 adsorption (< 21 kJ/mol) was observed.