Article pubs.acs.org/IECR
Silver-Decorated Hafnium Metal−Organic Framework for Ethylene/ Ethane Separation Yuxiang Wang, Zhigang Hu, Youdong Cheng, and Dan Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore S Supporting Information *
ABSTRACT: At present, the dominant technology for olefin/ paraffin separations is cryogenic distillation, which is extremely energy-intensive. Developing advanced adsorbent materials would be a first step toward cost-effective alternatives such as pressure swing adsorption or temperature swing adsorption. In this work, we report a silver-decorated hafnium metal−organic framework, NUS-6(Hf)-Ag, as a C2H4-selective adsorbent for C2H4/C2H6 separation. NUS-6(Hf)-Ag was synthesized by the modulated hydrothermal approach followed by ion-exchange reactions. The successful introduction of Ag(I), confirmed by X-ray photoelectron spectroscopy and energy-dispersive spectroscopy, increased the ideal C2H4/C2H6 selectivity of the resultant NUS-6(Hf)-Ag material to 6, which was 5 times that of NUS-6(Hf). Breakthrough experiments further confirmed the C2H4/ C2H6 separation performance of NUS-6(Hf)-Ag, indicating coadsorption C2H4/C2H6 selectivities greater than 3. In addition, NUS-6(Hf)-Ag was found to maintain its crystallinity and the oxidation state of Ag(I) after recyclability tests. This study clearly demonstrates the promising potential of Ag-decorated metal−organic frameworks as adsorbent materials in adsorption-based C2H4/C2H6 separations.
■
INTRODUCTION Short-chain olefins are among the most important feedstocks for the chemical industry.1 Currently, light olefins are mainly obtained from the steam cracking of naphtha or ethane, involving extensive operations for olefin/paraffin separation.2,3 Owing to the similar physical properties between olefins and paraffins, the industrial separation of their mixtures is performed by cryogenic distillation using distillation towers with more than 150 trays operating at high pressures (7−28 bar) and low temperatures (183−258 K).4 Such energyintensive processes constitute about 20% of the energy consumption of the overall cracking process, whose energy cost is 26−31 GJ/t (in terms of ethylene) for naphtha cracking and 17−21 GJ/t for ethane cracking.1,5 Considering the huge market for short-chain olefins, even a small improvement in the olefin/paraffin separation process to reduce energy consumption can make a significant difference in terms of reducing the operating costs and carbon footprint.6 Several alternative processes for olefin/paraffin separation with lower energy consumption and operating costs have been studied,7−9 one of which is membrane-based separation using facilitated-transportation membranes.10−14 The essential working principle of this type of membrane is the double bonding between metal ions (e.g., silver ion) and olefin molecules. The σ components of these bonds are formed by the overlap of the full π molecular orbitals of the olefins with the vacant outermost s orbitals of the metal, whereas the π components result from the back-donation of electrons from the full outer d orbitals of Ag(I) to the vacant π* orbitals of the olefins.5,11 © 2017 American Chemical Society
Another approach is adsorption-based separation such as pressure swing adsorption (PSA), vacuum swing adsorption (VSA), and temperature swing adsorption (TSA) using adsorbent materials including silicate,15 zeolites,15−18 porous organic frameworks,19−21 and more recently metal−organic frameworks (MOFs).9,22−30 MOFs are porous crystalline materials constructed from metal-containing secondary building units (SBUs) and organic linkers bearing various functionalities.31−34 Compared with other adsorbent materials, MOFs are promising for sorption-based olefin/paraffin separations owing to their ultrahigh specific surface areas, diversified pore sizes and geometries, and various modification approaches for tuning their gas sorption behaviors.35−39 Basically, MOFs for olefin/paraffin separations can be divided into two categories according to their selectivities: alkane-selective MOFs 24,25,40−43 and alkene-selective MOFs.26−29,44−46 Alkane-selective MOFs can be used to separate alkanes from gas mixtures to afford polymer-grade olefins (e.g., 99.95% for C2H4) after a single adsorption step in breakthrough operation,24 although studies of these MOFs are still very limited. A successful strategy toward synthesizing alkane-selective MOFs is to judiciously construct their pores so that paraffin molecules can be preferentially adsorbed in the frameworks through cooperative intermolecular interactions Received: Revised: Accepted: Published: 4508
February 6, 2017 March 15, 2017 March 27, 2017 April 4, 2017 DOI: 10.1021/acs.iecr.7b00517 Ind. Eng. Chem. Res. 2017, 56, 4508−4516
Article
Industrial & Engineering Chemistry Research such as hydrogen-bonding or π−π stacking interactions.24,25 For alkene-selective MOFs, although olefin productivities based on these MOFs are not as efficient as those based on their alkane-selective counterparts,17,40 the design strategies for the MOFs are relatively facile and straightforward.17 Apart from constructing MOFs with small aperture sizes to perform molecular-sieving and kinetic separation, as reported recently,26,47−49 one general guideline is to fix unsaturated metal sites (either as parts of SBUs or as functional groups attached to the organic ligands) into the MOFs.27,44,50−53 For instance, Bloch et al. reported the olefin/paraffin separation performance of Fe-MOF-74.27 Olefin molecules introduced into this MOF can interact strongly with the Fe(II) open sites lining its one-dimensional channels through the donation of easily polarizable π-bond electrons from the olefin molecules to the Fe open sites. Similarly, C2H4 can form π-complexes with Ag(I), whereas C2H6 cannot. Exploiting the different behaviors of C2H4 and C2H6, Li et al. reported a silver-functionalized porous aromatic framework, denoted as PAF-1-SO3Ag,19 that demonstrated a C2H4 uptake of 4.1 mmol g−1 and a C2H4/ C2H6 selectivity of 27 at 100 kPa and 296 K. Chang et al.28 and Zhang et al.29 extended this strategy to the postsynthetic modification of MOFs and reported independently the silverdecorated Cr MOF MIL-101-SO3Ag and its application in ethylene/ethane separation. This silver-decorated MOF was able to separate C2H4 from C2H6 through selective adsorption, with a C2H4 capacity over 2.5 mmol g−1 and a selectivity of over 10 at ambient conditions. However, the separation performance was not examined by experimental breakthrough tests. Because of the high toxicity of Cr and stringent synthetic requirements, Cr MOFs are not easily mass-produced with low cost. Recently, Zr or Hf MOFs such as the UiO-66 series have been extensively studied because of their excellent stabilities and mild synthetic conditions.54−56 However, Zr or Hf MOFs have rarely been reported for olefin/paraffin separations, mainly because of the lack of strong interaction sites in these frameworks.9 Herein, we report the synthesis, characterization, and gas separation performance of the Ag-decorated Hf MOF NUS-6(Hf)-Ag, which exhibits a decent C2H4/C2H6 separation selectivity of 6, as well as good recyclability. In addition, the synthesis of NUS-6(Hf)-Ag can be easily scaled up using environmentally benign modulated hydrothermal (MHT) synthesis57 and postsynthetic ion exchange (PSIE), making this MOF a promising adsorbent material for industrial olefin/ paraffin separations.
ments were carried out using monochromatic Al Kα radiation (1486.6 eV) at 15 kV as the excitation source. Elemental analyses for carbon, hydrogen, nitrogen and sulfur were performed using an Elementar vario MICRO cube, whereas metal content analyses were conducted by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin-Elmer Optima 5300DV). The porosity of the packed column for breakthrough studies was determined by mercury intrusion porosimetry (Micromeritics Autopore III 9420). Modulated Hydrothermal Synthesis of NUS-6(Hf). Monosodium 2-sulfoterephthalic acid (2.6 g, ∼9.6 mmol) and HfCl4 (3.2 g, ∼10 mmol) were dissolved in 100 mL of water/ acetic acid (30/20, v/v) solution and heated at 90 °C for 24 h to yield a powder product. The product was soaked in anhydrous methanol for 3 days at room temperature, during which time the extract was decanted and fresh methanol was added three times. After removal of the methanol by decanting, the sample was briefly dried in the open air to afford a white powder as the final product (yield: 80%). Solvothermal Synthesis of UiO-66(Hf). The solvothermal synthesis of UiO-66(Hf) was conducted based on the modification of a reported procedure.58 Briefly, benzene-1,4dicarboxylic acid (83 mg, ∼0.5 mmol) and HfCl4 (160 mg, ∼0.5 mmol) dissolved in 20 mL of dimethylformamide (DMF)/formic acid (18/2, v/v) mixed solvent were loaded into a Teflon-lined autoclave and heated at 123 °C for 40 h. The product was soaked in anhydrous methanol for 3 days at room temperature, during which time the extract was decanted and fresh methanol was added every day. Then the sample was treated with anhydrous dichloromethane similarly for another 3 days. This process was carried out to wash out residual reagents trapped inside the pores. After removal of the dichloromethane by decanting, the sample was dried under a dynamic vacuum at 120 °C for 24 h to afford the final product (yield: 52%). Postsynthetic Ion Exchange. MOF precursor [NUS6(Hf) or UiO-66(Hf), 500 mg, before activation] and AgBF4 (2.5 g, 12.8 mmol) were suspended in 100 mL of acetonitrile/ water (1/1, v/v) solution and stirred at room temperature overnight. The remaining solid was collected by centrifugation and washed with acetonitrile three times. The whole process was conducted under darkness and repeated three times to maximize the metalation of the sulfonic acid sites. Finally, the product was dried in the open air under darkness for further characterization (yield: 70%). Gas Sorption Measurements. Gas sorption isotherms were measured up to 1 bar using a Quantachrome Autosorb iQ surface area and pore size analyzer. Before the measurements, the sample (∼50 mg) was degassed under reduced pressure (
