Microporous Metal-Organic Frameworks with Hydrophilic and Hydrophobic Pores for Efficient Separation of CH4/N2 Mixture.

Highly selective removal of N2 from unconventional natural gas is considered as a viable way to increase the heat value of CH4 and reduce the greenhouse effect caused by the direct emission of CH4/N2 mixture. In this work, a three-dimensional Cu-MOF with two different types of micropores was synthesized, exhibiting a high selectivity for CH4/N2 (10.00-12.67) and the highest sorbent selection parameter value (65.73) among the reported materials. The CH4 molecule interacts with the framework to form multiple van der Waals interactions both in hydrophilic and hydrophobic pores, indicated by density functional theory calculations to gain a deep insight into the adsorption binding sites. In contrast, the weak polarity feature of the hydrophobic pore and the occupied open-metal sites in the hydrophilic pore result in a very low adsorption uptake of N2. The excellent separation performance combining the good stability and regenerability guarantees this Cu-MOF to be a promising adsorbent for an efficient separation of the CH4/N2 mixture.

Introduction

Nowadays, the continuous consumption of fossil fuels has raised many issues-related to energy and environment, creating an increasing demand for the development of clean energy. Natural gas (the main component is methane, CH4) is considered as an alternative energy source due to its clean and economical features and has become one of the fastest-growing energy source in the world.[1−6] A large amount of CH4 is emitted from conventional natural gas and unconventional natural gas (such as landfill gas, shale gas, and coal bed methane). The latter resource with a substantial amount of CH4 is very difficult to utilize directly because the high concentration of nitrogen (N2) lowers the heat enthalpy. Current treatment, like direct emission, will lead to a serious greenhouse effect.[2] Therefore, it is of significance to efficiently separate the CH4/N2 mixture, which is however a great challenge because of the very similar physical properties CH4 and N2.[7,8] To date, several technologies have been used, such as cryogenic distillation,[9] membrane separation,[10−12] and adsorption separation,[13] where pressure swing adsorption (PSA) is regarded as an promising industrial and commercial technology,[14,15] due to the advantages of low investment cost, simple operation, flexibility, and energy conservation.[16,17] The key is suitable adsorbent. Different types of adsorbents have been investigated, while the separation performance is still very low so far,[1−9] especially for the integrated consideration of selectivity and productivity simultaneously.

As a new kind of porous material, metal–organic frameworks (MOFs) have exhibited potential applications in gas separation owing to the good designability as well as structural and chemical adjustability.[15,18−21] Due to their unique structural and chemical features, several MOFs have been studied to separate CH4/N2 with better separation performances than other porous materials.[1,7,9] However, it still needs to be further enhanced. In this work, a three-dimensional (3D) Cu–MOF was selected to separate the CH4/N2 mixture. The hydrophilic and hydrophobic pores endow this material with a large difference in the interaction between CH4 and N2 with the framework. As a result, a high selectivity for CH4/N2 is obtained, especially for the sorbent selection parameter (SSP)[22] that can evaluate the selectivity and productivity in the mostly industrialized pressure swing adsorption (PSA) process. In addition, this MOF has good stability and regenerability, providing a great potential to separate the CH4/N2 mixture in practical application.

Results and Discussion

Preparation and Characterization for Cu–MOF

Cu–MOF was synthesized using copper salt and H3BTC, and the structure is given in Figure .[23] Copper clusters with a paddle wheel structure are connected by monomethyl BTC ester. The unique structure of Cu–MOF is due to the esterification of BTC linker during the synthesis process. Such a structure is different from the well-known Cu–BTC because the paddle wheel copper clusters are occupied by μ2-coordinated carboxylate O atoms of the paddle wheel unit in the adjacent sheet after the material is dehydrated, effecting that the Cu open-metal sites are not exposed. This change in coordination environment causes the dimensionality of the frameworks to vary from 2D to 3D with the formation of two different types of micropores.[23,24] The hydrophilic pore is formed by the arrangement of a copper paddle wheel, and the hydrophobic pore is formed by ester groups on organic ligands. Powder X-ray diffraction (PXRD) patterns in Figure agree well with the simulated patterns, indicating the successful synthesis with good crystallinity. The PXRD patterns at 2θ of 10–13° is due to the trace amount of unreacted Cu–BTC, which has also been observed in literature.[23] As indicated in literature,[23] the best yield for the esterified process reaches 100% and yields in excess of 95% could be obtained easily. With long heating times, the amount of Cu–BTC can be lower than 1% as an impurity. As shown in Figure S10a, the BET specific surface areas and the pore sizes are 110 m2/g and 7 and 5 Å, respectively.

Figure 1

Illustration of the crystal structure of Cu–MOF: (a) view of hydrophobic and hydrophilic pores along the c-axis; (b) view along the b-axis; and (c) coordination environment of Cu- and μ2-coordinated carboxylate O atoms of paddle wheel unit in the adjacent sheet (Cu, orange; O, red; C, gray; H, white).

Figure 2

Powder X-ray diffraction patterns for Cu–MOF.

Gas Separation Performance of Cu–MOF

Single-component gas adsorption experiments of CH4 and N2 were first performed at 298 K, and the results are given in Figure . The adsorption capacity of CH4 is about 14.17 cm3/g at 298 K and 1.0 bar, while that of N2 is relatively very low (2.40 cm3/g), indicating the interaction between CH4 and the framework is stronger than that for N2. To investigate the separation performance, ideal adsorbed solution theory (IAST)[25,26] was used to calculated the selectivities using different fitting models. As shown in Figures b and S6 in the Supporting Information (SI), the IAST-predicted selectivities are about 10.8–11.5 (10.8 at 1.0 bar) for a dual-site Langmuir model, 10.67–12.66 (11.18 at 1.0 bar) for a single-site Langmuir–Freundlich model, 10.00–12.67 (11.14 at 1.0 bar) for the dual-site Langmuir–Freundlich model, and 10.94 for the Toth model at the range of tested pressures. Details can be found in the SI. These values are higher than most of the reported values for various porous materials, except for Co–MOF.[1] While the adsorption capacity of CH4 in Co–MOF is about 9 cm3/g, it is only 64% of that in Cu–MOF in this work. In addition, the ratio of working capacity is valuable for estimating the separation performance of adsorbent in practical conditions.[27] For Cu–MOF, this value is about 5.90, much higher than that of all the reported materials owing to the very low adsorption capacity of N2.

Figure 3

(a) Single-component adsorption isotherms of CH4 and N2 in Cu–MOF at 298 K. (b) IAST predicted selectivity of equimolar CH4/N2 mixture at 298 K. (c) Adsorption capacity of CH4 after the treatment of the material using water (inset: the comparison of PXRD). (d) Results of regeneration experiment.

From the practical point of view, the adsorbent should have good water stability, since the humidity usually exists in many industrial separation processes.[28] In fact, it is observed that the water in gas mixtures may lower the performance to separate the CH4/N2 mixture.[29] Therefore, stability test was performed by treating the MOF sample using water for three days. Experimental results demonstrate that Cu–MOF exhibits good water stability, and the adsorption capacity of CH4 can be well-maintained as shown in Figure c. This may be attributed to the different coordination environments in this Cu–MOF compared to other typical MOFs with Cu open-metal sites, such as Cu–BTC. These structures can be easily destroyed by water molecules. As shown in Figure , the open-metal sites on copper paddle wheels are occupied by μ2-coordinated carboxylate O atoms of the paddle wheel unit in adjacent sheets, resulting in the framework remaining intact in the presence of water.[23,24] Furthermore, regeneration experiment of the material was carried out at 298 K and 1.0 bar. Figure d shows that the adsorption capacity of CH4 remains unchanged even after 10 cycles, indicating the excellent regenerability.

In practical separation processes, such as the typical PSA process, an excellent adsorbent should possess high selectivity and high adsorption capacity simultaneously. The former can reduce the number of cycles for the treatment of an input stream to reach a desired concentration, and the latter may lower the overall cost. In this aspect, the SSP parameter is a comprehensive indicator of the separation performance that can reflect the cyclic nature of the adsorption processes.[22] One bar and 0.1 bar are used for adsorption and desorption conditions, respectively, considering the absence of the adsorption data at high pressures in literature. From Figure b, it is obvious that the SSP value of Cu–MOF is about 65.73 and much higher than that of all the reported materials (Figure b), demonstrating that this Cu–MOF is an ideal candidate for separating the CH4/N2 mixture.

Figure 4

Comparison of the separation performance of CH4/N2: (a) selectivities as a function of the adsorption capacity of CH4 at 298 K and 1.0 bar and (b) SSP at 298 K. The data of MIL-101-Cr were collected at 293 K.

To confirm the application of materials in practical applications, a breakthrough experiment was performed using a mixture gas mixture of CH4/N2 (50/50%) with a constant flow rate of 5 mL/min at 298 K. As shown in Figure , N2 was first detected (about 2.5 min). The breakthrough of CH4 (about 7.5 min) is obviously later than that of N2, demonstrating that CH4 molecules competitively adsorb in the framework over N2. The effluent purity of N2 is about 96%, indicating that Cu–MOF could separate the CH4/N2 mixture at ambient conditions.

Figure 5

Breakthrough experiment curves for the binary mixture component of CH4 and N2 (50/50, v/v) at a constant flow rate of 5.0 mL/min under 298 K and 1.0 bar.

To study the separation mechanism of Cu–MOF, computations were performed for CH4 and N2 in the framework. Geometric optimization results in Figure a indicate that the distances between the hydrogen atom of CH4 and the C–H of ester groups in organic ligands in the hydrophobic pore and negative oxygen sites in the hydrophilic pore are about 2.383–2.956 and 2.900–3.205 Å on average, respectively, which are shorter than those for N2 (about 3.378–3.648 and 3.428–3.841 Å, respectively), indicating the interaction between CH4 and the framework is stronger than that for N2. The calculated binding energies of 23.0 and 19.0 kJ/mol for CH4 in different types of pores are higher than those for N2 (14.2 and 14.0 kJ/mol). As shown in Figure a, a CH4 molecule can interact with three C–H atoms in the ester groups in the hydrophobic pore or three negative O atoms in the hydrophilic pore to form multiple van der Waals interactions.[1,30] In contrast, Cu atoms in the paddle wheels are coordinated by μ2-coordinated carboxylate O atoms, resulting in a relatively weak interaction of the N2 molecule in the hydrophilic pores. In the hydrophobic pore, the weak polarity group C–H also exhibits a weak interaction with N2.[31] In fact, Qst of N2 in Cu–MOF is lower than those in other porous materials.[1,2,7,32] Therefore, the adsorption capacity of N2 is very low, leading to the higher SSP than those in the reported materials. It should be noted that Qst of CH4 is moderate compared to the values in other materials, which can reduce the regeneration cost since the cost of energy in the regeneration process is a major factor to be considered in the practical industrial application.[1]

Figure 6

CH4 and N2 adsorption binding sites in the hydrophobic and hydrophilic pores of Cu–MOF by DFT calculation: (a) CH4 and (b) N2.

Conclusions

In this work, a kind of Cu–MOF with hydrophilic and hydrophobic pores was synthesized, exhibiting a high selectivity and the highest SSP value among the reported values for the CH4/N2 mixture. The DFT calculations confirm that the CH4 molecule interacts with the framework to form relatively stronger multiple van der Waals interactions through the H atoms of CH4 with three C–H in the ester groups on the organic ligands in the hydrophobic pore and three negative O atoms in the hydrophilic pore. In contrast, the interaction with N2 molecule is weak due to the unique coordination environment of Cu in the paddle-wheel structure. Furthermore, this MOF has good water and thermal stabilities, as well as regenerability. These results indicate that Cu–MOF might be a potential candidate for the separation of CH4/N2 in practical industries.

Experimental Section

Materials

All general chemicals reagents and solvents (AR grade) are commercially available and used directly as received without further purification. Cu(NO3)2·3H2O and 1,3,5-benzenetricarboxylic acid (H3BTC) were derived from Alfa Aesar. Methanol (CH3OH) was obtained from Sinopharm Chemical Reagent Co. Ltd.

Characterizations

Powder X-ray diffraction (PXRD) data were recorded on a D8 Advance X diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å). The measurement of a single gas sorption isotherm and the pore structure were performed by a Quantachrome Autosorb-IQ instrument. Thermogravimetric analysis (TGA) was carried out by GA Q50 in an air atmosphere (20 mL/min, from 30 to 800 K with the rate of 5 K/min). The FT-IR spectroscopy data were obtained by the Nicolet 6700 FTIR instrument. Morphology of the material was characterized by Hitachi S-4700. For the breakthrough experiment, the concentration of the outlet gases from the column was analyzed by a gas chromatograph (Shimadzu, GC-2014C) at 298 K and 1.0 bar.

Synthesis of MOFs

The material was synthesized using the method as described in the literature.[23] Cu(NO3)2·3H2O (0.991 g, 4.1 mmol) and H3BTC (0.862 g, 4.1 mmol) were mixed with CH3OH (10 mL) and distilled water (10 mL) in a 25 mL Teflon-lined steel autoclave. The reactor was placed in an oven and heated to 383 K for 7 days. After that, the reactor was cooled to room temperature. The obtained blue crystals were filtered and washed several times with CH3OH solvent.

Gas Adsorption Measurement

The adsorption isotherms of CH4 and N2 at 273, 298, and 323 K were measured using a Quantachrome Autosorb-IQ instrument. The sample was degassed at 393 K for 12 h prior to measurement. For each gas adsorption measurement, the weight of the sample is about 600 mg.

Breakthrough Measurement

Five grams of material was packed into the column (10 × 150 mm2) with the void space filled by silica wool and purged with helium (25 mL/min) at 393 K for 6 h. After that, heating and helium purge were stopped until the device was cooled down to room temperature. Then, the mixture of CH4 and N2 (CH4/N2 = 50:50%) at the total flow rate of 5 mL/min was passed through the column. The concentration of the outlet gases from column was analyzed by a gas chromatograph (Shimadzu, GC-2014C).

Calculation of Adsorption Selectivity

The selectivity of the material was calculated by using the ideal adsorbed solution theory (IAST) based on the isotherm data of the single-component experiment.[25,26]S represents the ideal selectivity of the material, which can be defined aswhere q and q are the adsorbed quantity of the components i and j, respectively, and y and y are the gas molar fractions of components i and j, respectively.

Working capacity[22] was can be defined asSorbent selection parameter (SSP)[22] can be calculated aswhere Sads and Sdes are the selectivities of i and j components, respectively, and Nads and Ndes are the adsorption capacities of i and j components, respectively. The superscripts of “ads” and “des” represent the adsorption and desorption process, respectively.

Isosteric Heat of Adsorption

The coverage-dependent isosteric heat of adsorption is calculated using the Clausius–Clapeyron equation[33]where R is the molar gas constant (8.314 J/K/mol) and P and T are the pressure and the temperature of isotherm i, respectively.

Theoretical Calculations

Binding energy of guest molecules with MOF was calculated using the density functional theory (DFT)[34] method in Dmol3[3] of Materials Studio. Generalized gradient approximation with the Perdew–Burke–Ernzerh functional was applied for the calculations, combining the double numerical plus d-functions basis set. Self-consistent field (SCF) calculations were performed, and the convergence criterion is 10–5 Ha in energy. To accelerate the SCF convergence, thermal smearing was used with a value of 0.005 Ha to orbital occupation.