Article pubs.acs.org/jced
Adsorption of Phenol and p‑Nitrophenol from Aqueous Solutions on Metal−Organic Frameworks: Effect of Hydrogen Bonding Baojian Liu,* Fan Yang, Yuanxing Zou, and Yong Peng Zhejiang Provincial Key Lab for Biological and Chemical Processing Technologies of Farm Products, School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, Hangzhou, Zhejiang 310023, China S Supporting Information *
ABSTRACT: Three metal−organic frameworks (MOFs), MIL-100(Fe, Cr) and NH2-MIL-101(Al), were prepared, and their adsorption equilibria for phenol and p-nitrophenol (PNP) from water were investigated. All three MOFs show similar and limited adsorption capacities for phenol, but NH2-MIL101(Al) reveals exceptional adsorption capacity for PNP, greatly exceeding those of MIL-100(Fe, Cr). MIL-100(Fe, Cr) possess similar adsorption affinity for phenol and PNP, which suggests that the effect of metal ions and the coordinatively unsaturated sites in MOFs show negligible effect for phenol and PNP adsorption from water. NH2-MIL-101(Al) exhibits superior adsorption capacity for PNP and uniquely higher adsorption selectivity for PNP over phenol than a benchmark activated carbon. The remarkable adsorption affinity is attributed to the hydrogen bonding between PNP and the amino groups in NH2-MIL-101(Al). Phenol and PNP displayed a fast adsorption kinetics on NH2-MIL-101(Al) and followed a pseudo-second-order kinetic model. This work highlights that introducing functional groups into MOFs through an organic linker is a promising way to tailor MOFs for aqueous adsorption and separation.
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INTRODUCTION Metal−organic frameworks (MOFs), which are built up from clusters or chains of metal ions and organic linkers bearing multiple complexing functions, are a new class of organic− inorganic hybrid porous materials. MOFs have extra-high specific surface areas, highly ordered pore structures, and easy tunability of pore size and shapes.1 MOFs are more versatile for application-oriented tailing than conventional adsorbents, and thus have attracted considerable research for gas storage,2,3 gas/ liquid separation,4 CO2 capture,5 catalysis,6 and removal of hazardous materials from gas and liquid streams.7 To date, publications concerning aqueous-phase adsorption of organic contaminants using MOFs are limited to benzene,8 organic dyes,9−14 pharmaceuticals, and personal care products,15,16 phenol and p-cresol,17 nitrobenzene,18 bisphenol A (an endocrine disrupting chemical),19 and 2,4-dichlorophenoxyacetic acid (a herbicide).20 Effective capture of organic pollutants from water relies on the specific interactions between the adsorbates and MOFs. The pore structure,12 the coordinatively unsaturated sites,13 the metal ions of MOFs,14 and the breathing effect of the MIL-53 framework17,19 have been reported to play a dominant role for adsorption. The charge interactions with guest molecules were illustrated to give an improved adsorption affinity of MOFs in the aqueous phase,10,11,16,20 and introducing functional groups into MOFs through postsynthetic modification is another way to enhance the adsorption affinity of MOFs toward solutes in the aqueous phase due to the electrostatic interaction.9 However, the effort © 2014 American Chemical Society
devoted to understand the interaction or mechanism of adsorption on MOFs in aqueous media is still not sufficient, thus making this underexplored avenue of research deserve greater scrutiny. Phenolic compounds are toxic and carcinogenic to human beings, animals, and wildlife even at low concentrations and are thus being considered as the priority pollutants by the U.S. Environmental Protection Agency (U.S. EPA). Phenolic compounds commonly exist in many industrial effluents including but not limited to those of the petrochemical, pharmaceutical, plastics, coal, and steel industries. The maximum concentration of phenol and its derivatives in wastewater is 1 ppm (mg·kg−1);21 therefore, the removal of phenolic compounds from contaminated water has attracted much attention, but the removal process is still challenging because of the high stability and solubility of these compouds in water. Adsorption is a competitive method for their removal especially at low concentrations. Activated carbons22,23 and polymeric resins24−26 are the most widely used adsorbents. In this work, three water-stable isotypic MOFs (MIL-100(Fe, Cr) and NH2-MIL-101(Al)) were prepared to comprehensively investigate their adsorption behaviors for phenol and pnitrophenol (PNP) from aqueous solutions. The adsorption capacities and selectivity of both phenol and PNP on three Received: November 25, 2013 Accepted: March 12, 2014 Published: March 20, 2014 1476
dx.doi.org/10.1021/je4010239 | J. Chem. Eng. Data 2014, 59, 1476−1482
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detected at 270 nm. The amount adsorbed was calculated by the material balance:
MOFs and a benchmark activated carbon were discussed to illustrate the effect of central metal ions and functional groups in MOFs on the adsorption behavior of phenolic compounds from water. The adsorption mechanism and kinetics on the best-performing MOF were also considered. The aim of this work is to illustrate that incorporating functional groups into MOFs and the hydrogen bonding between MOFs and adsorbates would greatly enhance the adsorption capacities and selectivity in aqueous adsorption.
qe =
ρV (C0 − Ce) m
(1)
ρV (C0 − Ct ) (2) m −1 where qe and qt are the amount adsorbed (mmol·g ) at equilibrium and at time t, V is the volume of the liquid solution (cm−3), C 0 and C are the initial and residue solute concentrations at time t or at equilibrium (mmol·kg−1), m is the mass of adsorbent (g), and ρ is the density of water (1·10−3 kg·cm−3). qt =
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EXPERIMENTAL SECTION Materials. All chemicals used in this work were purchased from commercial suppliers and used without further purification. Chromium(VI) oxide (CrO3, 0.995 mass fraction) and 1,3,5-benzenetricarboxylic acid (H3BTC, 0.98 mass fraction) were supplied by J&K Scientific Ltd. (Shanghai, China). AlCl3·6H2O (0.99 mass fraction) and 2-aminoterephthalic acid (0.99 mass fraction) were supplied by Sigma−Aldrich (Beijing, China). Metallic iron powder (0.98 mass fraction), N,N-dimethylformamide (DMF, HPLC grade), hydrofluoric acid (HF, 0.40 mass fraction), nitric acid (HNO3, 0.65−0.68 mass fraction), absolute ethanol (AR grade), phenol (0.995 mass fraction), PNP (0.995 mass fraction), and salicylic acid (0.995 mass fraction) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). A commercial activated carbon was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd., China. MIL-100(Fe), MIL-100(Cr), and NH2-MIL-101(Al) were synthesized and activated according to the published procedures.27−29 The powder X-ray diffraction (PXRD) patterns of the synthesized MOFs were recorded on a Rigaku R-Axis Spider diffractometer equipped with a Cu Kα X-ray source and the activated MOFs were determined by the N2 adsorption−desorption isotherms at 77 K by Micromeritics ASAP 2020. The detailed synthesis and characterization of MIL-100(Fe, Cr) and NH2-MIL-101(Al) were reported in the Supporting Information. Adsorption Experiments. The batch adsorption experiments were conducted at 303 K using a temperature-controlled shaker. Prior to adsorption, the adsorbents were evacuated under vacuum at 423 K overnight to ensure total activation, and (100 to 1000) mg·kg−1 phenol and PNP solutions were prepared using deionized water. About 0.100 g of activated MOFs were mixed with 15 cm3 solutions containing different concentrations of phenol or PNP ((100 to 1000) mg·kg−1) in a set of vials (20 cm3 volume). To ensure sufficient mixing, the vials were put horizontally in the shaker and shaken at 200 rpm for 24 h at 303 K, which was confirmed to be sufficient to reach adsorption equilibrium. After adsorption equilibrium, the solutions were filtered using a syringe filter (PTFE, hydrophilic, 0.25 μm). For the kinetic studies, exactly 0.090 g of NH2-MIL-101(Al) were added into a series of vials (20 cm3 volume) containing 15 cm3 of (400 and 1000) mg·kg−1 phenol or PNP. The vials were shaken at 200 rpm at 303 K, and one vial was taken out and filtered at preset time intervals to analyze the solute concentration in the filtrate. The solute concentrations were analyzed by a Waters e2695 high-performance liquid chromatography (HPLC) instrument equipped with a Waters 2489 UV−vis detector and an ODSC18 column. The mobile phase was (60 + 40) cm3 (methanol + water), and the flow rate was 1.0 cm3·min−1. The ODS-C18 column was maintained at 323 K, and phenol and PNP were
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RESULTS AND DISCUSSION Characterization of MOFs. The crystal structures of the synthesized MOFs were confirmed to be MIL-100(Fe, Cr) and NH2-MIL-101(Al) since the collected PXRD patterns are in good accordance with the simulated ones as shown in Figure 1.
Figure 1. Experimental and simulated PXRD patterns of (a) MIL100(Fe, Cr) and (b) NH2-MIL-101(Al).
The activated MOFs were characterized by N2 adsorption/ desorption isotherms at 77 K (Figure S1 in the Supporting Information), and the specific surface areas were evaluated by the Brunauer−Emmett−Teller (BET) method. The BET surface areas are 1675, 1695, and 1942 m2·g−1 for MIL100(Fe), MIL-100(Cr), and NH2-MIL-101(Al), respectively. Adsorption Equilibrium Data. The adsorption isotherms of phenol on three MOFs and an activated carbon (as a benchmark adsorbent) at 303 K are presented in Figure 2. The adsorption capacities follow the order of activated carbon ≫ MIL-100(Fe) ≈ MIL-100(Cr) ≈ NH2-MIL-101(Al). MIL1477
dx.doi.org/10.1021/je4010239 | J. Chem. Eng. Data 2014, 59, 1476−1482
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adsorbents for removing phenolic compounds from water. NH2-MIL-101(Al) exhibits tremendous capacity for PNP and is superior or equal to some of the best-performing hypercrosslinked polymeric adsorbents. For example, at 303 K and 0.3 mmol·kg−1 equilibrium concentration, NH2-MIL-101(Al) adsorbed 1.0 mmol·g−1, whereas the highly cross-linked cyanomethyl styrene/divinylbenzene copolymers adsorbed only 0.25 mmol·g−1.33 At the equilibrium concentration of 0.719 mmol·kg−1, the amounts adsorbed on NH2-MIL-101(Al), Amberlite XAD-4, HJ-1, NJ-8, and NDA-7 were 1.2, 0.4,24 1.0,25 1.25,34 and 1.26 mmol·g−1,24 respectively. Conventional adsorbents such as activated carbons,23 polymeric resins,26,33,34 and natural organic/inorganic adsorbents35,36 usually adsorbed more PNP than phenol because of the lower water solubility of PNP than phenol (11.6 and 83 g· L−1 H2O at 293 K, respectively) and the possible interaction of the nitro group of PNP with the adsorbent surfaces. It is expected that the adsorption capacity of PNP will be higher than that of phenol on MOFs. However, MIL-100(Fe) and its Cr analogue show very similar adsorption affinities for both phenol and PNP. The lower solubility of PNP does not lead to any higher adsorption capacity on MIL-100(Fe, Cr). The adsorption capacities of phenol and PNP on NH2-MIL-101(Al) and the activated carbon are compared as shown in Figure 4.
Figure 2. Adsorption isotherms of phenol on three MOFs and a benchmark activated carbon at 303 K: ■, activated carbon; red ●, NH2-MIL-101(Al); blue ▲, MIL-100(Fe); pink ▼, MIL-100(Cr).
100(Fe, Cr) almost have identical adsorption capacities and low affinities for phenol, which implies that different metal ions in MIL-100(Fe, Cr) do not give any evident adsorption interaction. The interaction between water molecules and MIL-100(Fe, Cr) is very strong and the binding energy calculated by density functional theory (DFT) is very high.14 The coordinatively unsaturated metal sites in MIL-100(Fe, Cr) would be bonded with H2O30,31 thus preventing their interaction with the phenol molecules. Phenol is weakly acidic with a pKa value of 9.89 at 298 K. The amino groups in NH2MIL-101(Al) provide weak Brønsted basic sites32 and presumably it would have acid−base interaction with phenol, but there is no any improvement of adsorption capacity on NH2-MIL-101(Al). For all three MOFs, the adsorption capacities for phenol are very limited and remain almost at the same level as MIL-53(Cr) within the same range of equilibrium concentration (
