Preferential Adsorption Performance of Ethane in a Robust Nickel-Based Metal-Organic Framework for Separating Ethane from Ethylene.

Development of an ethane-selective adsorbent to separate ethane from ethylene is a challenging issue with great significance for ethylene purification. The adsorptive separation technique based on physical adsorption holds a great promise to address this issue. Herein, we report a robust ethane-selective metal-organic framework, Ni(BODC)(TED), and investigate its separation performance on C2H6/C2H4. The as-synthesized Ni(BODC)(TED) exhibits excellent water vapor stability and high capacity of C2H6 molecules with an uptake of 3.36 mmol/g at 298 K and 100 kPa, higher than those of many adsorbents reported in recent years. Its C2H6/C2H4 selectivity predicted by the ideal adsorbed solution theory (IAST) model reaches 1.79. A molecular simulation is applied to unveil the preferential adsorption mechanism of ethane. Calculation shows that five strong C-H···H interactions are formed between C2H6 and the framework of Ni(BODC)(TED), and the isosteric heat of ethane on Ni(BODC)(TED) is 27.02 kJ/mol, higher than that of ethylene, resulting in preferential adsorption of ethane. Ni(BODC)(TED) would become a promising member of the family of ethane-selective materials for the industrial separation of ethane from ethylene.

Introduction

Ethylene is one of the most important raw materials in the petrochemical industry and is also the largest output chemical product in the world, which is widely used in polymer manufacturing and organic chemical synthesis.[1−4] Ethylene and its derivatives account for more than 75% of the petrochemical products and thus play an important role in industrial economy. To date, ethylene has been produced through steam cracking and then separated from the cracked gas consisting of an ethylene/ethane mixture. Currently, separation of ethylene and ethane is based on cryogenic distillation, which is an energy-intensive process.[5−7] It was reported that the energy used for ethylene separation accounts for 0.3% of the total global energy consumption every year, equivalent to the energy consumption of Singapore.[8−10] Adsorption separation is considered to be the most potential candidate to replace cryogenic distillation for the separation of ethylene and ethane. In the cracked gas consisting of a mixture of ethylene and ethane, ethylene is the major composition, while ethane is the low-content composition. Therefore, the use of an ethane-selective adsorbent can be more efficient than an ethylene-selective adsorbent for purification of ethylene with polymer-grade purity.[11] Thus, development of an ethane-selective adsorbent is critical to obtain polymer-grade ethylene.[12]

In recent years, many efforts have been made to develop solid porous materials, such as zeolites,[13,14] carbon materials,[15−19] and metal–organic frameworks (MOFs),[20−24] for separating ethane and ethylene. Among them, MOFs exhibit outstanding adsorption capacity and separation performance for C2H4/C2H6 separation, owing to their extraordinary porosity and tunable structure. Basically, MOFs for ethylene/ethane separations can be summarized into two categories according to their nature of selectivity: ethylene-selective MOFs and ethane-selective MOFs.[25]

Ethylene-selective MOFs, such as M2(dobdc),[26] M2(m-dobdc),[27] [Ca(C4O4)(H2O)],[28] Cu+@MIL-101,[29] NOTT-300,[30] and M-gallate,[31] can preferentially adsorb ethylene through π-complexation or a sieving effect. Although these ethylene-selective materials possess high C2H4/C2H6 selectivity, they are unsuitable to efficiently separate ethylene from ethane for production of polymer-grade ethylene because the majority component of the cracked gas is ethylene. The use of ethylene-selective MOFs to separate polymer-grade ethylene would lead to more capital investment and higher energy consumption compared to the use of ethane-selective MOFs, which was stated and discussed previously.[2]

Hence, development of ethane-selective MOF adsorbents would be interesting for the separation of low-content ethane from ethylene since in this case less adsorbents are required and the use of only one adsorption process can efficiently remove low-content ethane so that polymer-grade ethylene could be obtained, which simplifies the separation process and lowers the energy consumption of the purification process greatly. Gücüyener et al. reported the first example of an ethane-selective MOF material, ZIF-7, with a C2H6 uptake of 1.90 mmol/g at 298 K and 100 kPa.[32] Liao et al. synthesized MAF-49 and reported its remarkable selectivity of 9 at 316 K.[33] Besides, IRMOF-8,[34] PCN-250,[35] and Ni(BDC)(ted)0.5[36] were reported to take up C2H6 preferentially, as well. An ultra-microporous MOF with excellent water vapor stability, Cu(Qc)2, was revealed to achieve high ideal adsorbed solution theory (IAST) selectivity of 3.4 but low C2H6 adsorption capacity of 1.85 mmol/g at 298 K and 1 bar.[37] Chen reported that Fe2(O2)(dobdc) bound ethane strongly and its C2H6/C2H4 selectivity reached as high as 4.4 at 298 K and 1 bar, yet its water vapor stability seemed to be poor.[38] Therefore, it is still a great challenge to develop a MOF material with high stability, C2H6 capacity, and C2H6/C2H4 selectivity.

Here, we reported a robust MOF, Ni(BODC)(TED), and its preferential adsorption performance toward ethane. The robust Ni(BODC)(TED) is synthesized and then characterized. Single-component adsorption isotherms of C2H6 and C2H4 on Ni(BODC)(TED) were measured separately at different temperatures, and the isosteric heats of adsorption were calculated. The IAST-predicted C2H6/C2H4 selectivity was given to evaluate the separation performance of Ni(BODC)(TED) for C2H6 and C2H4 binary mixtures. A comparison of Ni(BODC)(TED) and some other adsorbents in terms of C2H6/C2H4 selectivity and C2H6 uptake was made. Moreover, the preferential adsorption mechanism of ethane over ethylene in Ni(BODC)(TED) was studied using a molecular simulation and then reported.

Results and Discussion

Characterization of the Materials

Figure shows the powder X-ray diffraction (PXRD) patterns of Ni(BODC)(TED). As shown in Figure , the PXRD patterns of the as-synthesized sample show the characteristic peaks at 8.1, 9.4, 12.5, and 16.4° for Ni(BODC)(TED), which match well with the simulated pattern as well as the reported data,[39] which confirms that Ni(BODC)(TED) was synthesized successfully with high purity and crystallinity.

Figure 1

PXRD patterns of Ni(BODC)(TED).

Moist stability is one of the most important parameters to determine whether MOFs can be put into realistic applications or not. The moist stability of Ni(BODC)(TED) was also examined. As shown in Figure , there is no significant loss of crystallinity after the sample was exposed to air for over 1 week, suggesting its excellent stability. This is also proved by the N2 adsorption–desorption isotherms (Figure S5). Owing to the low-polarity methylene group on the bicyclo[2.2.2]octane-1,4-dicarboxylic acid (BODC) ligand and the TED ligand, the pore environment of Ni(BODC)(TED) is hydrophobic, leading to the excellent water vapor stability.

Figure S1 presents the scanning electron microscopy (SEM) images of Ni(BODC)(TED). The crystals of Ni(BODC)(TED) exhibit a relatively rodlike morphology, with an average length of approximately 100 μm. The sharp and clean edges of the crystals prove that Ni(BODC)(TED) has a high crystallinity.

The N2 adsorption–desorption isotherms of Ni(BODC)(TED) were measured at 77 K. As shown in Figure , the N2 isotherms of Ni(BODC)(TED) exhibit a type-I isotherm, showing that Ni(BODC)(TED) is a microporous material, and its Brunauer–Emmett–Teller (BET) surface area reaches 961 m2/g. The pore size distribution curve of Ni(BODC)(TED) shows two distinct peaks at 0.804 and 1.179 nm, as shown in Figure (inset), which is suitable for C2H6 and C2H4 loading.

Figure 2

N2 adsorption/desorption isotherms at 77 K and the pore size distribution (inset) of Ni(BODC)(TED).

The Thermogravimetric analysis (TGA) curve was collected to investigate the thermostability of Ni(BODC)(TED) under an argon atmosphere. As shown in Figure , two main steps of weight loss can be observed: (i) the first negligible weight loss of 5% before 603 K is related to the removal of guest molecules such as H2O or N,N-dimethylformamide (DMF); (ii) the second sharp weight loss of 70%, which occurred at 603 K, can be attributed to the decomposition of the frame structure, implying that Ni(BODC)(TED) is thermally stable up to 603 K.

Figure 3

TGA curve of Ni(BODC)(TED).

Adsorption Isotherms of Ethylene and Ethane on Ni(BODC)(TED)

Figure shows the single-component adsorption isotherms of ethane and ethylene on Ni(BODC)(TED) at different temperatures. Ni(BODC)(TED) exhibited preferential adsorption of C2H6 over C2H4 at the tested temperatures. The equilibrium adsorption capacities of ethane and ethylene on Ni(BODC)(TED) reached 3.36 and 2.61 mmol/g at 298 K and 100 kPa, respectively. The cyclic adsorption isotherms of C2H6 and C2H4 further confirmed the excellent stability of Ni(BODC)(TED), as shown in Figure S6. The behavior of preferential adsorption of C2H6 over C2H4 on Ni(BODC)(TED) demonstrated the stronger interaction between ethane and Ni(BODC)(TED) than that of ethylene, which could be ascribed to the methylene group from the BODC ligand and the TED ligand generating nonpolar pore surfaces in Ni(BODC)(TED).

Figure 4

(a) C2H6 and C2H4 adsorption isotherms on Ni(BODC)(TED) at 298 K and adsorption isotherms of (b) C2H4 and (c) C2H6 on Ni(BODC)(TED) at different temperatures.

Table S3 summarizes the C2H6/C2H4 separation performances of ethane-selective adsorbents (including carbon materials, zeolites, and MOFs). The data indicated that the C2H6 uptake capacity (3.36 mmol/g) of Ni(BODC)(TED) at 100 kPa is notably higher than that of some benchmark adsorbents, such as MAF-49 (1.73 mmol/g),[33] Cu(Qc)2 (1.85 mmol/g),[37] and Fe2(O2)(dobdc) (3.31 mmol/g).[38]

Ethylene/Ethane Adsorption Selectivity of Ni(BODC)(TED)

Selectivity is an important indicator to evaluate the separation performance of materials. The ideal adsorbed solution theory (IAST) model was applied to calculate the adsorption selectivity from the experimental pure component isotherms of C2H6 and C2H4.[35,40]

Figure depicts the IAST-predicted selectivity at 298 K and 100 kPa of Ni(BODC)(TED) for binary mixtures (C2H4/C2H6 = 15:1, v/v). The C2H6/C2H4 selectivity ranges from 1.76 to 1.84 at 0–100 kPa and reaches 1.79 at 100 kPa, which is higher than 1.5 for ZIF-4[41] and 1.4 for UTSA-33.[42] Besides, the dynamic separation performance of Ni(BODC)(TED) was also examined by a breakthrough experiment in which separation of the C2H6/C2H4 binary mixture was carried out at ambient conditions, as shown in Figure S4. In addition, the breakthrough curve of ethylene showed a roll up, which can be attributed to the competitive adsorption of ethane. Figure b and Table S3 present the C2H6/C2H4 selectivity of some adsorbents reported in recent years. Ni(BODC)(TED) shows a moderate IAST-predicted selectivity; however, it features excellent moist stability, which would make it highly promising for realistic applications.

Figure 5

(a) IAST-predicted selectivity of C2H6/C2H4 mixtures (1:15, v/v) on Ni (BODC)(TED) at 298 K and (b) comparison of C2H6/C2H4 adsorption performance on adsorbents reported in the literature.

Isosteric Heats of Adsorption

Isosteric heats of adsorption is one of the key thermodynamic parameters for evaluating the interaction between an adsorbate and an adsorbent.[43,44]

Figure shows the isosteric heats of ethane and ethylene adsorption on Ni(BODC)(TED). As illustrated in Figure , the isosteric heat of ethane is estimated to be 27.08 kJ/mol at 100 kPa, which was higher than that of ethylene, further confirming the strong adsorption affinity between ethane molecules and frameworks of Ni(BODC)(TED) compared to ethylene. On the other hand, in comparison with an ethylene-selective adsorbent based on π-complexation, the isosteric heats of ethane on Ni(BODC)(TED) is significantly low, implying that less energy would be consumed for regeneration of Ni(BODC)(TED).

Figure 6

Isosteric heats of adsorption (Qst) of C2H6 and C2H4 for Ni(BODC)(TED).

Adsorption Mechanism of Ethylene and Ethane in Ni(BODC)(TED)

To gain a further insight into the adsorption mechanism, the Sorption module in Materials Studio 7.0 was used in this section to calculate the adsorption energy and the optimal adsorption site of C2H4/C2H6 in the material. Figure illustrates the energy distribution of C2H4 and C2H6 on Ni(BODC)(TED). Obviously, the optimal interaction energy of ethane (∼−33.26 kJ/mol) is higher than that of ethylene (∼−28.24 kJ/mol), suggesting the stronger interaction of ethane toward the framework than ethylene. We also calculated the preferential adsorption sites of C2H6 and C2H4 in Ni(BODC)(TED). As shown in Figure , ethane was tightly trapped by five H atoms from the BODC ligand and the adjacent TED ligand through strong C–H···H interactions with C–H···H distances in the range of 2.50–2.82 Å, while ethylene was bound with five weaker C–H···H interactions with longer C–H···H distances of 2.71–3.73 Å, which was the reason for the preferential adsorption of ethane over ethylene. Therefore, the stronger interaction between C2H6 and the framework in Ni(BODC)(TED) enables the material to become an ethane-selective adsorbent.

Figure 7

Interaction energy distribution of ethane and ethylene during adsorption in Ni(BODC)(TED).

Figure 8

Preferential binding sites for (a) ethane and (b) ethylene around the adsorption sites in Ni(BODC)(TED) (C, O, H, N, and Ni are represented by gray, red, white, blue, and green, respectively).

Conclusions

In summary, we reported a robust metal–organic framework Ni(BODC)(TED) for separation of C2H6/C2H4. The adsorption capacities of ethane and ethylene on Ni(BODC)(TED) reached 3.36 and 2.61 mmol/g at 298 K and 100 kPa, respectively. The IAST selectivity was up to 1.8, showing the relatively good separation for the C2H6/C2H4 binary mixtures. In addition, Ni(BODC)(TED) also showed excellent water vapor stability. The molecular simulation revealed the stronger affinity of the Ni(BODC)(TED) framework with C2H6 compared to C2H4, which is formed by five strong C–H···H interactions between C2H6 and the Ni(BODC)(TED) framework, resulting in the preferential adsorption of ethane over ethylene. Ni(BODC)(TED) would be an outstanding material and might become another promising member in the family of ethane-selective materials for industrial ethylene purification.

Experimental Section

Materials

Nickel nitrate [Ni(NO3)2·6H2O, ≥99%] was provided by Tianjin Damao company. 1,4-Diazabicyclo[2.2.2]octane (TED, ≥99%) was obtained from Shanghai Aladdin Bio-Chem Technology Co., LTD. N,N-Dimethylformamide (DMF, ≥99.5%) was obtained from Guangzhou Chemical Reagent Factory. Bicyclo[2.2.2]octane-1,4-dicarboxylic acid (BODC, ≥97%) was purchased from Jilin Chinese Academy of Sciences, Yanshen Technology Co., Ltd. Ethylene (99.999%), ethane (99.999%), helium (99.999%), nitrogen (99.999%), and binary gas mixtures of C2H4/C2H6 (15/1, v/v) were bought from KODI Company. All chemicals were commercially available and used as received without further purification.

Synthesis of Ni(BODC)(TED)

The Ni(BODC)(TED) sample was synthesized by the hydrothermal synthesis method according to the literature with some modifications.[39] Typically, Ni(NO3)2·6H2O (93 mg, 0.32 mmol), TED (18 mg, 0.16 mmol), and BODC (63 mg, 0.32 mmol) were mixed in 8 mL DMF in a 20 mL vial, followed by sonication until a homogeneous solution was obtained. The vial was then heated to 393 K and kept for 48 h. After that, the mother liquor was decanted, and the obtained green powder was washed with DMF and EtOH. After being activated in a vacuum oven at 120 °C, the fresh Ni(BODC)(TED) was obtained.

Characterization of Samples

The N2 adsorption–desorption isotherms at 77 K of the sample was measured on a Micromeritics ASAP 2460. All samples were degassed at 120 °C overnight. The Brunauer–Emmett–Teller (BET) and Langmuir surface areas were calculated by the BET and Langmuir equations. The total pore volume was calculated according to the amount of nitrogen adsorbed at 100 kPa. The density functional theory (DFT) model was applied to estimate the porosity distribution. Powder X-ray diffraction (PXRD) was performed on a Bruker AXS D8 Advance using Cu Kα (λ = 1.5406 Å) radiation.

Scanning electron microscopy (SEM) was carried out on a Hitachi SU8010 instrument. The powder sample was dried and sputter-coated with a thin layer of gold prior to tests.

The TGA curve was collected on a Discovery TGA 55 simultaneous thermal analyzer (TA, America) in the temperature range of 30–700 °C with a 10 °C/min heating rate under an Ar atmosphere.

Adsorption Isotherm Measurement

The C2H4 and C2H6 single-component adsorption isotherms were collected on a 3Flex surface characterization analyzer (Micromeritics) at 288, 298, and 308 K. Before transferring into a testing tube, the ethanol-soaked sample was filtered and degassed at 393 K for 8 h for each measurement.

Moist Stability Test

To investigate the stability of Ni(BODC)(TED), the ethanol-soaked sample was filtered, followed by exposure to dry air. Then, the sample was exposed to humid air with a relative humidity of 80–90% for 1 week. After that, the sample was collected, and its crystallinity was characterized by the PXRD measurement.

Simulation Details

To get a deep understanding of the adsorption mechanism, in this section, the Sorption module in Materials Studio 7.0 was used to calculate the adsorption energy and the optimal adsorption site of C2H4 and C2H6 molecules in the material. Before modeling the adsorption mechanism, the Forcite section was applied to optimize the model of the crystal from the Cambridge Crystal Data Center. The optimization algorithm was SMART, and the convergence criteria were less than 5.0 × 10–4 kcal/mol for force, 1.0 × 10–5 kcal/mol for energy, and 1.0 × 10–6 Å for displacement. The Metropolis Monte Carlo method was used to simulate the adsorbate–adsorbent interactions using the universal force field.[45] Four kinds of movements, including exchange, rotation, regrowth, and translation, were used to describe the adsorption behavior of the adsorbate in the adsorbent structure. Ni(BODC)(TED) was kept rigid. The cutoff radius was 1.4 nm, and the number of initialization and equilibration cycles was 2.0 × 106.