Article pubs.acs.org/IECR
Selective Adsorption of Light Alkanes on a Highly Robust Indium Based Metal−Organic Framework Yongwei Chen,‡ Zhiwei Qiao,‡ Daofei Lv, Houxiao Wu, Renfeng Shi, Qibin Xia,* Haihui Wang, Jian Zhou,* and Zhong Li School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China S Supporting Information *
ABSTRACT: Highly robust indium based InOF-1 was synthesized by the solvothermal method for the separation of C3H8/CH4 and C2H6/CH4. Its adsorption and separation performance was investigated by isotherms (CH4, C2H6, and C3H8) and breakthrough experiments. Results showed that C3H8, C2H6, and CH4 adsorption capacities of InOF-1 were 4.25, 4.14, and 0.64 mmol/g at 298 K and 100 kPa. The adsorption selectivities of C3H8/CH4 and C2H6/CH4 equimolar mixtures were up to 91 and 17 based on ideal adsorbed solution theory (IAST). Importantly, the potential of industrial separation was also confirmed by breakthrough experiments. Furthermore, we employed the configurational-biased grand canonical Monte Carlo (CB-GCMC) method to simulate their adsorption behaviors in InOF-1. It was found that the relatively higher percentages of adsorbate−adsorbate interactions in total energy and the electrostatic interactions of C2H6 and C3H8 made more efficient use of the large free space in the center pores of InOF-1 than that of CH4. Additionally, InOF-1 exhibited excellent chemical stability with retaining its framework in both acidic and alkaline solutions ranging from pH 3 to 11 for 12 h. Therefore, the research for robust InOF-1 is of great importance for industrial light alkane separation. on.15−21 In terms of selective separation of light hydrocarbons, Long et al.7,22 intensively investigated the effect of different metals within MOF-74 for the separation of C2 and C3 hydrocarbons. MOF-74 showed that the adsorption selectivities for C2H4/C2H6 and C3H6/C3H8 separations were both up to 15 owing to the high density of open metal sites that can selectively interact with unsaturated hydrocarbons. The UTSA series, such as UTSA-30 and UTSA-33−UTSA-36, were also systematically reported about regarding their separation of C1− C3 light hydrocarbons by Chen and co-workers.23−27 Particularly, the C3H8, C2H6, and CH4 adsorption uptakes of UTSA-35 were 130.8, 73.0, and 6.9 mg/g; the separation selectivities of C3H8 and C2H6 over CH4 are even in excess of 80 and 20, respectively, at 296 K and 1 atm. Li and coworkers28 unveiled effective separation of C1−C4 paraffins by various gate-opening pressures for different adsorbate molecules on RPM3-Zn since the H-bond strength affected the gateopening pressure. However, one of the main challenges is that most MOFs are sensitive to water or moisture, much less stable under acidic or alkaline harsh conditions from a practical perspective.29,30 The poorly inherent water or chemical stability of MOFs will largely
1. INTRODUCTION C1−C3 light alkanes, including CH4, C2H6, and C3H8, are very important raw chemical feedstocks and energy resources in the petrochemical industry.1,2 For example, CH4 has been viewed as one of the most promising clean petroleum replacements for fueling future vehicle transportation.3 Furthermore, CH4, C2H6, and C3H8 are widely used as important raw chemicals and primary building blocks of essential commodities in our daily life.4,5 Thus, to fully utilize these light alkanes, effective separation of those hydrocarbons is important for the petrochemical industry to get high quality and purity of such basic chemicals.6 To date, cryogenic distillation still remains the most commonly applied method for industrial separation of those small hydrocarbons.7 However, this process requires extremely intensive capital input and high energy cost.8,9 As an alternative, adsorption separation is proposed as a more economical and cost-effective technology.10 Thus, it is crucial for designing and synthesizing new adsorbents with high adsorption capacity and selectivity. Compared with other types of porous materials, a novel type of adsorbents called metal−organic frameworks (MOFs) has attracted considerable attention for gas storage and separation.11−13 They are constructed by metal cations or clusters and organic ligands via coordination bonds.14 MOFs show great potential for gas adsorptive separation, because of their unique designable framework structures, extremely high porosity, adjustable chemical functionality, structural flexibility, and so © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
December 27, 2016 February 28, 2017 March 24, 2017 March 24, 2017 DOI: 10.1021/acs.iecr.6b05010 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
radiation with a scan speed of 2 deg/min and a step size of 0.02° in 2θ. Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 instrument through a sample powder previously dried and sputter-coated with a thin layer of gold. Thermogravimetric analysis (TGA) of samples was carried out on a TA Q500 instrument heating from 30 to 700 °C in nitrogen atmosphere at a rate of 10 °C/min. N2 isotherms of the samples were measured at 77 K on an accelerated surface area and porosimetry system ASAP 2020 (Micromeritics, USA). The pore textural properties were estimated by Brunauer−Emmett−Teller (BET) equation, and the pore size distributions were calculated by the density functional theory (DFT) method. 2.4. Adsorption Experiments. The volumetric adsorption experiments were obtained under low pressure, using a 3Flex Surface Characterization Analyzer (Micromeritics, USA). The temperature was controlled by circulated water in which the sample tube was immersed. Prior to measurement, 60−80 mg samples were degassed at 150 °C for 8 h. The gas adsorption isotherm was obtained under pressure ranging from 0 to 100 kPa. Ultrahigh purity grade CH4 (99.99%), C2H6 (99.99%), and C3H8 (99.99%) were used. 2.5. Chemical Stability. To test the chemical stability, activated InOF-1 was studied; approximately 100 mg samples were placed in vials containing 20 mL of HCl or NaOH solutions with different pH values. Samples were allowed to sit statically at room temperature for 12 h. Afterward, the samples were washed with deionized water and then collected by centrifugation without any extra treatment. The high stability was verified by PXRD and N2 adsorption measurements. 2.6. Breakthrough Experiments. The breakthrough experiments were performed by using a self-assembly experimental setup as shown in Figure S1 of the Supporting Information. The gas flow rates of CH4, C2H6, and C3H8 were controlled by mass flow meters. The gas mixtures of pure gas were delivered to the gas mixer and then to the adsorption column, where the temperature could be controlled at an accuracy of 0.1 K. In this work, the flow rate of gas mixtures through the adsorption column was controlled to be 5 NmL/ min. Then 160 mg of InOF-1 powder was packed into a stainless tube with an inner diameter of 0.4 cm and a length of 10 cm; the void space was filled by glass wool. The effluent from the column was monitored by gas chromatography (GC9560, Shanghai Wuhao, China) with a flame ionization detector at the outlet. The breakthrough curves of C3H8/CH4 and C2H6/CH4 equimolar mixtures on InOF-1 were obtained at 298 K and 100 kPa. 2.7. Simulation Models and Methods. Originally from the experimental data, the atomic structures of InOF-1 were refined after removing solvent molecules. The largest cage diameter and surface area of InOF-1 were estimated by Zeo+ +,34 and the void fraction and pore size distribution were calculated by RASPA.35 The void fraction and surface area were estimated by He and N2 as probes, respectively. The framework atoms were described here by Lennard-Jones (LJ) and electrostatic potentials:
impede their practical applications, where the adverse impact on their overall performance will emerge due to their relatively inferior tolerance to water, acid, or base.31 Consequently, this challenge has motivated numerous researchers to make efforts to target robust MOFs, which is a prerequisite property for real applications. Herein, ideal MOFs as adsorbent materials for gas separation should not only have high adsorption capacity and selectivity, but also more importantly have good water stability. Indium based InOF-1 was reported by Qian et al.32 and our previous work,33 and showed excellent water stability and superior CO2 adsorption property. To the best of our knowledge, no research has been done to investigate the light alkane adsorptive separation by InOF-1. In this work, indium based InOF-1 was chosen as a candidate for the adsorptive separation of C1−C3 light alkanes owing to its superior adsorption property and robust structure. By systematic investigation on the adsorption behaviors of C1−C3 alkanes in InOF-1, results showed that this material had superior C3H8/CH4 and C2H6/CH4 separation performance. More importantly, InOF-1 showed excellent chemical stability that was not susceptible to acidic and alkaline solutions in the pH range from 3 to 11. To disclose the mechanism for gas adsorption and framework behavior on gas loading, we executed a theoretical simulation to predict the gas molecular distributions inside InOF-1.5 Configurational-biased grand canonical Monte Carlo (CB-GCMC) simulation was employed to provide further insight into the factors that can affect binding interactions between adsorbed alkane molecules and porous host. Hence, we can understand the key interactions responsible for selective separation of C1−C3 alkane hydrocarbons. The separation performance of C3H8/ CH4 and C2H6/CH4 mixtures is further discussed and reported in section 3.
2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were commercially available and used without further purification. Indium nitrate hydrate [In(NO3)3·5H2O, 99.99%] was purchased from Aladdin. The organic ligand 3,3′,5,5′-biphenyltetracarboxylic acid (H4bptc, C16H10O8, 98%) was obtained from Beijing HWRK Chem Co. Ltd. N,N-Dimethylformamide (DMF, 99%), HNO3 (65−68%), and acetone (99%) were obtained from Guangdong Guanghua Sci-Tech Co. Ltd. Acetonitrile (CH3CN, 99%) was obtained from Shanghai Kunling chemical reagent Co. Ltd. 2.2. Preparation of InOF-1. All the synthetic procedures were performed in 25 mL sealed glass vials under autogenous pressure. The preparation of InOF-1 was according to Qian et al.’s work32 with modifications. Briefly, InOF-1 precursor components were simultaneously dispersed in a mixture solvent in the following steps: H4bptc (33 mg, 0.10 mmol) and In(NO3)3·5H2O (156 mg, 0.40 mmol) were stirred in the mixture solvent of 5 mL of DMF and 5 mL of CH3CN with an additional 0.2 mL of HNO3 for 5 min to be completely dissolved. Then, the sealed glass vial was heated at 85 °C for 3 days to obtain colorless crystals [In2(OH)2(BPTC)]·6H2O (InOF-1). The colorless crystals were centrifuged and then soaked in 50 mL of acetone for 72 h at room temperature, during which acetone was decanted and freshly replenished daily. The sample was collected by centrifugation and evacuated at 150 °C for 8 h. Finally, the activated InOF-1 was obtained. 2.3. Characterizations. Powder X-ray diffraction (PXRD) experiments were conducted on a Bruker D8 Advance X-ray diffractometer operating at 40 kV and 40 mA, by using Cu Kα
⎡⎛ ⎞12 ⎛ ⎞6 ⎤ σij σij μij (r ) = ∑ 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠
∑
qiqj 4πε0rij
(1)
where εij and σij are the well depth and collision diameter; rij is the distance between atoms i and j; qi is the atomic charge of B
DOI: 10.1021/acs.iecr.6b05010 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research atom I; and ε0 = 8.8542 × 10−12 C2 N−1 m−2 is the permittivity of vacuum. As listed in Table S1, the LJ potential parameters were adopted from the universal force field (UFF).36 A number of simulation studies showed that UFF could accurately predict gas adsorption and diffusion in various MOFs.37,38 The atomic charges were estimated by the MEPO-Qeq method, which has been demonstrated to be fast and accurate to evaluate electrostatic interactions.39 The three gas molecules CH4, C2H6, and C3H8 were represented by the all-atom OPLS force field, and the parameters are listed in Table S2.40 The Lorentz− Berthelot combining rules were employed to calculate the cross interactions. The adsorption behaviors of pure CH4, C2H6, and C3H8 in InOF-1 were simulated by the CB-GCMC method. The frameworks were assumed to be rigid with framework atoms frozen during simulations. A spherical cutoff of 12.8 Å with long-range correction was used to calculate the LJ interactions, whereas the electrostatic interactions were calculated by the Ewald sum. All GCMC simulations were carried out using the RASPA package.35 The simulation was run for 20 000 cycles with the first 10 000 cycles for equilibration and the last 10 000 cycles for ensemble averages. Each cycle consisted N trial moves (N, number of adsorbate molecules), including translation, rotation, regrowth, and swap. It was found that further increasing the number of cycles had an insignificant effect on simulation results.
synthesized with high crystallinity. The good correspondence of the simulated PXRD pattern with that of the as-synthesized sample suggested the high purity of sample. It was noticeable that the pattern of activated sample remained analogous to that of the as-synthesized sample, demonstrating that both the structural integrity and rigidity were maintained after the acetone activation treatment. The SEM images of InOF-1 to determine the morphology of activated sample are presented in Figure 2. These images obviously show that the morphology of InOF-1 was welldefined uniform cubic crystals with an individual crystallite size of about 15 μm. The relatively large crystallite size was possibly attributed to the large ligand H4bptc. In addition, these images indicate that InOF-1 was well crystallized by the solvent-based preparation method. We performed N2 adsorption and desorption isotherms at 77 K to investigate the permanent porosity of the framework. As shown in Figure S2, the N2 isotherm was typical type I sorption behavior with a saturated uptake of 250 cm3/g(STP), which was an indication of the microporous nature of InOF-1. Accordingly, the BET and Langmuir surface areas of the measured InOF-1 were 982 and 1082 m2/g, respectively, giving a total pore volume of 0.385 cm3/g. Our measured surface areas were close to those reported ones (1065 and 1093 m2/g),32 suggesting that the structure remained intact and suggesting complete activation. A pore distribution analysis based on the DFT calculation demonstrated that the pore distribution was predominantly around 7 Å. It also meant that the pores in the frameworks were comparatively uniform. 3.2. Thermal and Chemical Stability. Figure S3 shows the TGA of InOF-1. It was distinctly shown that the activated InOF-1 was thermally stable up to 400 °C under nitrogen atmosphere. For this measured material, two obvious mass loss and two main differential thermogravimetric (DTG) peaks were clearly noticed. We assigned that the first weight loss step of 11.7% was in the range 30−100 °C, corresponding to the evaporation of guest molecules adsorbed on the surfaces. With the continuous increase of temperature up to 400 °C, there was only 5.9% gradual mass loss. This mass loss was probably attributed to the removal of occluded molecules in the pores. Upon temperatures above 400 °C, the collapse of the whole structural framework took place due to the decomposition of the organic ligand. Thus, the decomposition temperature was slightly above 400 °C. The thermal stability of InOF-1 was somewhat comparable with those of the formerly reported Zrbased compounds UiO-66, UiO-67, and PCN-128.41,42 Furthermore, in order to examine the chemical stability of InOF-1, we carried out stability tests at different pH values from 3 to 11. The pH values were adjusted by changing the
3. RESULTS AND DISCUSSION 3.1. Characterization of InOF-1. Figure 1 shows the PXRD patterns of InOF-1 in different states. It was clearly
Figure 1. PXRD patterns of InOF-1.
shown that the characteristic peaks at 8.0, 9.2, 14.7, 16.3, and 18.7°, which were consistent with those simulated in the literature,32 indicated that the crystal InOF-1 was successfully
Figure 2. SEM images of InOF-1. C
DOI: 10.1021/acs.iecr.6b05010 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
InOF-1 increased with the hydrocarbon chain length elongation. Specifically, the C3H8 isotherm showed a steep adsorption isotherm in the region of low pressure (
