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Selective Adsorption of Ethane over Ethylene in PCN-245: Impacts of Interpenetrated Adsorbent Daofei Lv, Renfeng Shi, Yongwei Chen, Ying Wu, Houxiao Wu, Hongxia Xi, Qibin Xia, and Zhong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19414 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018
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ACS Applied Materials & Interfaces
Selective Adsorption of Ethane over Ethylene in PCN-245: Impacts of Interpenetrated Adsorbent Daofei Lv1, Renfeng Shi1, Yongwei Chen1, Ying Wu1,*, Houxiao Wu1, Hongxia Xi1, Qibin Xia1,*, and Zhong Li1 1
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China
ABSTRACT: The separation of ethane from ethylene using cryogenic distillation is an energy-intensive process in industry. With lower energetic consumption, adsorption technology provides the opportunities for developing industry with economic sustainability. We report an iron based metal-organic framework PCN-245 with interpenetrated structures as an ethane-selective adsorbent for ethylene/ethane separation. The material maintains stability up to 625 K, and even after exposure to 80% humid atmosphere for 20 days. Adsorptive separation experiments on PCN-245 at 100 kPa and 298 K indicated that ethane and ethylene uptakes of PCN-245 were 3.27 mmol and 2.39 mmol, respectively, and the selectivity of ethane over ethylene was up to 1.9. Metropolis Monte Carlo calculations suggested that the interpenetrated structure of PCN-245 created greater interaction affinity for ethane than ethylene through the crossing organic linkers, consistent with the experimental results. This work highlights the potential application of adsorbent with interpenetrated structure for ethane separation from ethylene. KEYWORDS: ethane-selective adsorbent, ethylene/ethane separation, stability, adsorption capacity, adsorption selectivity, interpenetrated structure 1. INTRODUCTION Ethylene is one of the most important chemical feedstocks in the petrochemical *
Corresponding authors.
E-mail addresses:
[email protected] (Qibin Xia),
[email protected] (Ying Wu).
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industry for manufacturing polymers such as polystyrene, polyethene, polyester, and other chemical organics.1 In industry, ethylene is usually generated by refining crude oil and thermal decomposition of ethane, and at the same time, some ethane byproducts are inevitably produced.2 To obtain high-purity ethylene, the separation of ethane and ethylene is very necessary. Owing to the similar molecular sizes and volatilities between ethylene and ethane, the industrial separation of their mixtures at large scale is especially challenging.3,4 Traditional ethylene/ethane separations, which uses cryogenic distillation performed at low temperatures (~248 K) and high pressures (~2.306 MPa), resulting in an extremely energy-intensive process in industry.5,6 The energy consumption for purifying ethylene and propylene is about 0.3% of the global total energy consumption, which is almost equivalent to the energy consumption of Singapore in one year.7 Thus, to save energy, it is highly demanded for developing more energetically effective technologies for separating ethylene from ethane.8 Adsorptive separation performed at ambient temperature and pressure is believed to an alternative possibility for separating ethylene from ethane.9 Adsorbents are the key to superior performance of separating ethane and ethylene.10 Numerous conventional adsorbents have been widely investigated for ethylene/ethane separations. For example, it was reported that ethylene and ethane uptakes for natural mordenites were about 0.91 and 0.38 mmol/g at 293 K and 50 kPa, respectively.11 Yang and co-workers5 reported that the adsorption capacities of ethylene and ethane on zeolite 4A separately were 2.8 and 2.4 mmol/g at 298 K and 100 kPa. Furthermore, Yang’s group5 also found that Bergbau-Forschung carbon molecular sieve (CMS) with pore sizes (0.3~0.5 nm) could completely exclude ethane, but its ethylene uptake was only 1.1 mmol/g at 298 K and 100 kPa. Obviously, these conventional adsorbents have the drawbacks of low adsorption capacities or poor selectivity for the adsorptive separation of ethane and ethylene. Hence, developing new adsorbents with high ethylene and ethane adsorption capacities and superior ethylene/ethane selectivity is desirable. Compared with conventional adsorbents such as porous carbon-based materials and
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molecular sieves, metal-organic frameworks (MOFs) demonstrate outstanding potentials for ethylene/ethane separation owing to their tunable pore properties, diverse fascinating topologies and ultra-high surface areas and pore volumes.12-15 Whereas, most reported adsorbents (including MOFs) exhibit preferential adsorption of ethylene over ethane through π-complexation interactions, which is a kind of weak chemical bonds, that may consume more energy in desorption compared to traditional physisorption.16 Furthermore, the composition of ethylene in the cracked gas mixture is much higher than that of ethane, indicating that ethylene-selective adsorbents require larger adsorbent consumption and space occupation than ethane-selective adsorbents. More importantly, if using ethylene-selective adsorbents in a fix-bed separation process, the purity of ethylene can just reach 99%+ after a complete adsorption-desorption cycle, and at least four times such cycles will be need to achieve ethylene with polymerization grade purity (99.95%+).9 These drawbacks can be overcome by using ethane-selective adsorbents, which not only obviously reduce energy consumption, but also improve the purity of ethylene. Moreover, using ethane-selective adsorbents implies that high-grade ethylene can be directly obtained merely after the adsorption process in a breakthrough operation, which can greatly simplify the separation operation and device. However, few materials have been reported to preferentially adsorb ethane over ethylene.9,17 ZIF-7 and ZIF-8 selectively adsorbed ethane from ethylene/ethane mixture, but the adsorbed amounts of ethane were only 1.8 mmol/g and 2.5 mmol/g at 100 kPa and 298 K, respectively.18,19 ZJU-30, UTSA-33a and ZIF-69 were reported as ethane-selective adsorbents, whereas their ethane/ethylene selectivities calculated with IAST method were comparatively low (98.5%, A.R.] was purchased from Tianjin Chemical Plant (Tianjin,
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China). Biphenyl-4,4'-dicarboxylic (H2bpdc, 98%, A.R.) was bought from J&K Scientific Co., Ltd (Beijing, China). Sodium acetate anhydrous (CH3COONa, ≥99.0%, A.R.), acetic acid glacial (CH3COOH, ≥99.5%, A.R.), ethanol (EtOH, ≥99.0%, A.R.) and N,N-dimethylformamide (DMF, ≥99.5%, A.R.) were obtained from Guangdong Guanghua Chemical Factory Co., Ltd (Guangzhou, China). The gases used in the adsorption experiments were ethane (99.99%), ethylene (99.99%) and nitrogen (99.99%). Helium (99.999%) was used as a backfill gas in the adsorption experiments. All the used gases were purchased from Zhuozheng Gas Co., Ltd (Guangzhou, China). 2.2.
Preparation
of
Preformed
[Fe3(μ3-O)(CH3COO)6]
Cluster.
[Fe3(μ3-O)(CH3COO)6] cluster synthesis was carried out according to reported procedure with some modifications.25 First, Fe(NO3)3·9H2O (8.08 g, 0.02 mmol) and sodium acetate anhydrous (25.4262 g, 0.31 mol) were separately dissolved in 50 mL of distilled water. Second, solutions were mixed and stirred for 12 h at room temperature. Then, the brown precipitate was filtered off, washed with water and ethanol. Finally, the product was dried at 343 K for 8 h under vacuum. 2.3. Preparation of PCN-245. PCN-245 was synthesized according to previously study with modifications.25 Briefly, [Fe3(μ3-O)(CH3COO)6] cluster (0.2 g), H2bpdc (0.2 g), acetic acid glacial (4 mL) and DMF (40 mL) were transformed into a 100 mL teflon holders and synthesis was carried out at 423 K for 12 h. After slowly cooling down to ambient temperature, orange square bulk crystals were collected by filtration and washed with DMF. Then, these products were soaked in 80 mL of DMF for 36 h (refreshing DMF every 12 h) to remove the excess of unreacted reactants, including organic linkers, inorganic metal clusters and acetic acid glacial. Finally, soaked PCN-245 crystals were dried under vacuum at 423 K for 6 h. 2.4. Characterization. The phase purity and crystallinity of the obtained PCN-245 sample were characterized by powder X‐ray diffraction (PXRD) on a Bruker D8 Advance X-ray diffractometer with Cu‐Kα radiation operated at 40 kV and 40 mA in a scanning range of 5-50° at 5°/min. The morphologies of the synthesized PCN-245 sample were observed in a Hitachi SU-70 scanning electron microscope (SEM).
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PCN-245 sample powder was previously sputter coated with a thin layer of gold before scanning. Thermogravimetric analysis (TGA) was carried out using a HTG-1/2 instrument at a heating rate of 10 K/min under flowing nitrogen atmosphere in the temperature range from 300 K to 973 K. N2 adsorption/desorption isotherms at 77 K were recorded with a Micromeritics ASAP 2460 analyzer. The specific surface area was calculated using Brunauer-Emmett-Teller (BET) equation in the range P/P0 = 0.01-0.06 and density functional theory (DFT) method was used to calculate the pore size distribution. 2.5. Measurement of Ethane and Ethylene Adsorption Isotherms. Ethane and ethylene adsorption isotherms at 288 K, 298 K and 308 K were measured up to 100 kPa employing a 3Flex Surface Characterization Analyzer. Data points on ethane and ethylene isotherms were collected while the change rates of quantity adsorbed kept lower than 0.01% for 10 seconds. The adsorption temperature was precisely controlled by putting sample cell into a thermostatic bath. Before each measurement, the sample (~100 mg) was degassed under vacuum (
