Selective Adsorption of n-Alkanes from n-Octane on Metal-Organic

To understand the selective adsorption of C12 on MAF-6 more, the adsorption of C12 from C8 over MAF-6 was investigated in detail and compared with tha...
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Selective Adsorption of n‑Alkanes from n‑Octane on Metal-Organic Frameworks: Length Selectivity Biswa Nath Bhadra† and Sung Hwa Jhung*,† †

Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 702-701, Republic of Korea ABSTRACT: The liquid-phase adsorption of n-alkanes (from n-octane (C8) solvent) with different chain lengths was carried out over three metal-organic frameworks (MOFs), viz., metal-azolate framework-6 (MAF-6), copper-benzenetricarboxylate (Cu-BTC), and iron-benzenetricarboxylate (MIL-100(Fe)), and a conventional adsorbent activated carbon (AC). MAF-6 and Cu-BTC were found to have significant selectivity for the adsorption of n-dodecane (C12) and n-heptane (C7), respectively, from C8. Selectivity for C12 on MAF-6 was also observed in competitive adsorption from binary adsorbate systems. To understand the selective adsorption of C12 on MAF-6 more, the adsorption of C12 from C8 over MAF-6 was investigated in detail and compared with that over AC. The obtained selectivities over MAF-6 and Cu-BTC for C12 and C7, respectively, might be explained by the similarity between cavity size of adsorbents and molecular length of n-alkanes. In the case of AC and MIL-100(Fe), no specific adsorption selectivity was observed because the cavity sizes of the two adsorbents are larger than the size of the n-alkanes used in this study. The adsorption capacities (qt) of n-alkanes over AC and MIL-100(Fe) decreased and increased, respectively, as the polarity (or length) of the adsorbates increased, probably because of nonpolar and polar interactions between the adsorbents and n-alkanes. On the basis of the results obtained, it can be concluded that matching the cavity size (of adsorbents) with the molecular length (of n-alknaes) is more important parameter than the MOF’s hydrophilicity/hydrophobicity for the selective adsorption/separation of alkanes. KEYWORDS: adsorption, n-alkanes, cavity size, chain length, metal-organic frameworks

1. INTRODUCTION Separating n-alkanes/olefins from mixtures has been achieved through various processes, including low-temperature distillation, extractive distillation, membrane techniques, and adsorption (chemical or physical).1 It is important to separate n-alkanes from their branched isomers or aromatics in petroleum refineries to improve the octane number of gasoline2,3 and the low-temperature properties of aviation fuels.4 However, separating saturated hydrocarbons from mixtures is very challenging because of their similar structures, properties, and reactivities.5 Liquid-phase adsorption is one of the most attractive methods for the adsorptive separation of nalkanes/olefins because it has several advantages, such as ease of operation and low energy consumption.6,7 Recently, nanoporous materials, such as metal-organic frameworks (MOFs)8−14 and mesoporous materials,15−17 have attracted substantial attention because of their fascinating structures, facile synthesis, and wide range of applications.8−23 In particular, MOFs are ideal adsorbents for gas-/liquid-phase adsorption because of their large surface areas, surface functionalities and high adsorption affinities. Among the wide variety of MOF applications, adsorptive removal/separation, such as for the separation of chemicals,19,20 water purification,23−27 and fuel upgrading,28−33 has been explored. Several influencing factors, such as acid−base interactions, H-bonding, π-complexation, electrostatic interactions, coordination, hydrophobic interactions, and solvent polarity have been discussed in © XXXX American Chemical Society

terms of their effects on the selective/efficient adsorption of various adsorbates by MOFs.23,34 Variations in existing pore topologies and the internal and external surface properties of MOFs have expanded the applications of MOFs, facilitating their use for the adsorptive separation of various chemicals. Owing to their distinct metal atoms, pore structures, and cavity sizes, MOFs have also been applied as potential adsorbents for the separation of hydrocarbons, including aromatic and aliphatic species.35−39 For example, MIL-47,40 Zn-terephthalate (monoclinic-MOF),41 MIL-53,42 and MgCUK-143 can separate xylene isomers and ethylbenzene from mixtures. The characteristics of vapor phase adsorption of nalkanes (up to C9) over flexible MOFs, e.g., MIL-53(Al, Cr)44 or MIL-88 (Fe)45 and rigid MOF MIL-4746 have also been explored by both experimental44−46 and molecular simulation46 studies. In contrast, M2(dobdc) MOFs (MOF-74 or CPO-27; M = Mg, Mn, Fe, Co, Ni, or Zn) have shown interesting performance in the separation of saturated/unsaturated hydrocarbon mixtures because of the difference in charge density at the uncovered M2+ ion and the permeation selectivities in the one-dimensional channels of M2(dobdc).47,48 Supramolecular interactions have also been discussed regarding the separation of unsaturated hydrocarbons using the MOF NOTT-300.49 Received: January 16, 2016 Accepted: February 23, 2016

A

DOI: 10.1021/acsami.6b00608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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washed with deionized water, and dried at 100 °C in a vacuum oven. Cu-BTC was prepared from a solution of H3BTC (2.0 mmol) and Cu(NO3)2·3H2O (3.65 mmol) in 24 mL of water/ethanol (1/1, w/ w).30 The homogeneous reactant mixture was transferred to a Teflon autoclave and then placed in a preheated oven (140 °C) for 12 h. The resulting Cu-BTC crystals were separated by filtration and dried at 100 °C using a vacuum oven. 2.3. Characterization. The crystal phases of the synthesized MOFs were characterized using an X-ray diffractometer (D2 Phaser, Bruker, Germany) with Cu−Kα radiation. The porosities (in terms of the surface areas and pore volumes) of studied adsorbents were measured at −196 °C using an analyzer (Tristar II 3020, Micromeritics). All of the adsorbents were evacuated at 150 °C up to 12 h before nitrogen adsorption. The Brunauer−Emmett−Teller (BET) equation and t-plots were applied to evaluate the surface areas and micropore volumes, respectively. The adsorbed amounts of nitrogen (at P/P0 = 0.99) were used to determine the total pore volumes of the studied samples. 2.4. Adsorption Experiments. All of the solutions were prepared from their stock solutions (5000 ppm) in C8. Solutions consisting of one and two n-alkanes in C8 were termed single and binary adsorbate systems, respectively, and were prepared from stock solutions. The necessary dilution was performed to prepare solutions with the desired concentrations (100−2000 ppm) for the adsorption experiments. The adsorbents were evacuated overnight using a vacuum oven at 100 °C and stored in a desiccator. The amounts of adsorbent (5 mg) and solution (5 mL) were kept constant for each adsorption experiment, and slurries were mixed with a shaker (Lab Companion, Incubator Shaker) at 250 rpm and room temperature for a predetermined time. After the preset adsorption time, the solutions were removed from the solid adsorbents with a syringe filter (polytetrafluoroethylene, hydrophobic, 0.5 μm), and the remaining concentrations of n-alkanes were measured using a GC (DS Science, IGC 7200) equipped with a flame ionization detector. The mathematical eqs 1−4 commonly used in adsorption studies were applied to obtain the adsorbed quantities qt (mg/g), adsorption kinetic constants k2 (g/mg/h), maximum adsorption capacities Qo (mg/g), and separation factors RL for adsorption of the studied nalkanes onto porous adsorbents.63−65

MOF-based stationary phases for the high-performance liquid chromatographic (HPLC)50 or gas chromatographic (GC) 51,52 separation of hydrocarbons have also been developed. For example, MIL-101,50,51 MOF-5,53 and CuBTC-based54 chromatographic capillaries are capable of separating aromatic50,51/aliphatic54 hydrocarbons selectively, even though some MOFs are moisture sensitive. Well-known hydrophobic MOFs, such as zeolitic imidazolate framework-8 (ZIF-8)52 and metal-azolate framework-6 (MAF-6),55 have been applied for the GC separation of n-alkanes (up to C10). Small hydrocarbons that exhibit different gate opening pressures (based on their molecular lengths) can be separated on Zn2(bpdc) (RPM3-Zn).56 In addition, good matching between micropore size/shape of MOF-50857 and Cu-MOF ([Cu(hfipbb)(H2hfipbb)0.5])58 with a kinetic diameter of nalkanes can be applied to separate n-alkanes (up to C6) from their branched isomers.57,58 However, to the best of our knowledge, there is currently no study on the liquid-phase adsorption of n-alkanes (especially ones as large as nhexadecane) over MOFs via simple batch adsorption experiments. Therefore, it is interesting to investigate the adsorption selectivity of n-alkanes from n-octane (C8) over MOFs using a simple liquid-phase batch adsorption. MAF-6 and conventional AC were considered as adsorbents for the adsorption of nonpolar hydrocarbons (n-alkanes) because of the large cavity and high hydrophobicity.34,55,59 On the other hand, two widely studied MOFs (Cu-BTC and MIL-100(Fe)) were selected based on their cavity size60,61 (smaller and larger, respectively, than MAF-6) and hydrophilicity.62 The selected adsorbents were used to adsorb n-alkanes (namely, n-hexane (C6), nheptane (C7), n-decane (C10), n-dodecane (C12), npentadecane (C15), and n-hexadecane (C16)) from C8. In this study, C8 was used as a solvent because C8 is a very common solvent and is a main component of gasoline.

2. EXPERIMENTAL SECTION

qt =

2.1. Materials. All chemicals were purchased from commercially available sources and used without further purification. Benzenetricarboxylic acid (C 6H3(CO2H)3, 98%) and 2-ethylimidazole (C5H8N2, 98%) were purchased from Sigma-Aldrich. C6 (C6H14, 99%), C7 (C7H16, 99%), C8 (C8H18, 99%), C10 (C10H22, 99%), C12 (C12H26, 99%), C15 (C15H32, 99%), and C16 (C16H34, 99%) were obtained from Alfa Aesar. Cyclohexane (C6H6, 99%) and granular activated carbon (AC; size, 2−3 mm) were procured from Duksan Pure Chemical Co. Ferric nitrate nonahydrate (Fe(NO3)3·9H2O, 99%) and copper nitrate trihydrate (Cu(NO3)2·3H2O, 99%) were obtained from Samchun Chemicals. Aqueous ammonia solution (NH3, 25%) and ethanol (C2H6O, 98%) were acquired from OCI chemicals. Zinc hydroxide (Zn(OH)2, 98%) was purchased from DC Chemicals. 2.2. Synthesis of Adsorbents. MAF-6,55 MIL-100(Fe),30 and Cu-BTC30 were synthesized following previously reported methods. The synthesis of MAF-6 was conducted at room temperature following a specific mixing order. First, 2 mmol of Zn(OH)2 was dissolved in concentrated aqueous ammonia solution (25%, 40 mL). Then, 2ethylimidazole (4 mmol) was dissolved in a premixed solvent (30 mL) of ethanol and cyclohexane (6.66% (v), and the first solution was added dropwise to the second solution with gentle stirring.55 The resultant slurry was stirred for an additional 15 min, and then, the white precipitate was filtered and washed with methanol. The obtained microcrystalline powder was dried at 100 °C in a vacuum oven. MIL100(Fe) was synthesized hydrothermally from a reactant mixture (composition was 1Fe(NO3)3·9H2O:0.67H3BTC:270H2O) using a Teflon-lined reactor.30 The reactor containing the precursors was placed in a preheated electric oven (160 °C) and allowed to stand for 12 h. After the completion of the reaction, the solid was filtered,

(C i − Cf )V m

(1)

where qt (mg/g) is the adsorbed amount at time t, Ci (mg/mL) is the initial concentration of the adsorbate, C f (mg/mL) is the concentration after adsorption, V (mL) is the volume of solution for each adsorption, and m (g) is the mass of the adsorbent used for the adsorption experiment.

qt =

qe 2k 2t 1 + qek 2t

(2)

where qe (mg/g) is the amount adsorbed at equilibrium, qt (mg/g) is the amount adsorbed at time t, t (h) is the adsorption time, and k2 (g/ mg/h) is the pseudo-second-order rate constant. Ce C 1 = e + qe Qo Q ob

(3)

where Ce (mg/L) is the equilibrium concentration of the adsorbate, qe (mg/g) is the amount adsorbed at equilibrium, Qo (maximum capacity, mg/g) is the Langmuir parameter, and b (L/mg) is the Langmuir constant.

RL =

1 1 + bCo

(4)

where RL is the separation factor, b (L/mg) is the Langmuir constant, and Co (mg/L) is the initial adsorbate concentration. B

DOI: 10.1021/acsami.6b00608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. Properties of Adsorbents and Adsorbates. The powder X-ray diffraction (XRD) patterns of the synthesized materials were obtained and compared with their simulated patterns, as shown in Figure 1. The exact match between the

Table 2. Dielectric Constants and Calculated Lengths of Selected n-Alkanes n-alkanes hexane (C6) heptane (C7) octane (C8) decane (C10) dodecane (C12) pentadecane (C15) hexadecane (C16)

dielectric constant (εr)

length (Å) (Tanford’s)a

length (Å) (modified Tanford’s)b

1.886 1.921 1.948 1.985 2.012

9.09 10.36 11.62 14.15 16.68

10.93 12.19 13.46 15.98 18.51

2.039

20.48

22.31

2.046

21.74

23.58

a

Tanford’s formula: lmax = 1.5 + 1.265n. bModified Tanford’s formula: lmax = 1.265(n − 1) + 4.6; where “n” is the number of carbon atoms in a chain.

hydrocarbons were collected from the database of Christian Wohlfarth.66 The polarity of n-alkanes ranges from 1.886 to 2.046 and increases with the carbon number (from C6 to C16). The lengths of the studied n-alkanes were estimated using both Tanford’s formula, {lmax = 1.5 + 1.265n}67 and the modified Tanford’s formula, {lmax = 1.265(n − 1) + 4.6},68 where “n” is the number of carbon atoms in the n-alkane chain. 3.2. Adsorption Results. The liquid-phase adsorption of C6, C7, C10, C12, C15, and C16 was carried out over AC, MAF-6, Cu-BTC, and MIL-100(Fe) from C8 solutions, and the obtained results are shown in Figure 3. The adsorbed amounts

Figure 1. XRD patterns of (a) MAF-6, (b) Cu-BTC, and (c) MIL100(Fe). Crystal structures of each MOFs are also shown as insets.

patterns of the synthesized MOFs and the simulated ones confirmed the successful synthesis of MAF-6 (Figure 1a), CuBTC (Figure 1b), and MIL-100(Fe) (Figure 1c). The textural properties of adsorbents (as shown in Table 1) were obtained from the nitrogen adsorption isotherms (Figure 2). The MOFs’ high porosities also confirmed the successful syntheses. Table 1. Textural Properties of Adsorbents, AC, MAF-6, CuBTC, and MIL-100(Fe) adsorbents

SABET (m2/g)

PVtotal (cm3/g)

PVmicro (cm3/g)

AC MAF-6 Cu-BTC MIL-100(Fe)

1016 1317 1219 1998

0.56 0.71 0.69 0.89

0.29 0.47 0.49 0.41

Figure 3. Effect of carbon numbers of n-alkanes on adsorbed quantities over (a) MAF-6 and AC and (b) Cu-BTC and MIL100(Fe).

(qt values) of n-alkanes over AC (enlarged by three times) and MAF-6 are presented in Figure 3a. The Figure 3a indicates that MAF-6 efficiently captured C12 (from C8) compared with other alkanes and that this material absorbs shorter n-alkanes less preferentially than longer ones. In contrast, the qt values of n-alkanes over AC decreased monotonously as the length of the sorbate molecules increased (Figure 3a). The qt values of nalkanes over Cu-BTC and MIL-100(Fe) were also compared

Figure 2. Nitrogen adsorption and desorption isotherms of AC, MAF6, Cu-BTC, and MIL-100(Fe). Filled and open symbols represent the adsorption and desorption, respectively.

The properties, such as the polarity (dielectric constant) and length, of the studied n-alkanes (C6, C7, C10, C12, C15, and C16) are shown in Table 2. The dielectric constants of selected C

DOI: 10.1021/acsami.6b00608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces (Figure 3b) as a function of the selected n-alkanes’ lengths. Curiously, the highest qt on Cu-BTC was found for C7, whereas, n-alkane adsorption onto MIL-100(Fe) increased monotonously as the size of the n-alkanes increased. The obtained results suggest that selective interactions between adsorbents (such as MAF-6 and Cu-BTC) and adsorbates (such as C12 and C7, respectively) occur. To investigate the adsorption selectivity of C12 over MAF-6 more, competitive adsorptions were performed using mixtures (called binary adsorbate systems) of C6 and C12 or C12 and C16 in C8; the obtained results are shown in Figure 4. The

Figure 5. Effect of contact time on adsorption of C12 from n-octane onto AC and MAF-6. Condition: 0.005 g of adsorbents in 5.0 mL of solution (1000 ppm).

Table 3. Second-Order Kinetic Constants (k2) and Langmuir Parameters (Q0 and b-Values) for Adsorptions of nDodecane from n-Octane over AC and MAF-6a kinetics adsorbents AC MAF-6 a

k2 (g mg−1 h−1) −2

6.9 × 10 1.3 × 10−2

Langmuir parameters R2

Qo (m2/g)

0.998 0.995

1.1 × 10 5.5 × 101 1

b (mg/L) −3

5.7 × 10 1.1 × 10−2

R2 0.999 0.998

The correlation parameters (R2) are also shown.

Table 3. The k2 value for C12 adsorption on AC (6.9 × 10−2 g mg−1 h−1) is 5 times higher than that of MAF-6 (1.3 × 10−2 g mg−1 h−1) (Table 3), which might be attributable to AC’s larger pore size (∼39 Å)59 compared to MAF-6 (7.6 Å).55 The isotherms shown in Figure 6a were obtained after 24 h of C12 (100−2000 ppm) adsorption from C8 solution over MAF-6 and AC. On the basis of the adsorption isotherms, MAF-6 was again confirmed to be a more efficient sorbent than AC, which is in agreement with the adsorption results obtained after various times (Figure 5). Langmuir parameters, such as maximum adsorption capacities (Qo, mg/g) and b-values, and correlation factors (R2) were determined from the Langmuir plots (Figure 6b) and are summarized in Table 3. The Qo for C12 on MAF-6 (5.5 × 101 mg/g) is 5 times higher than that on AC (1.1 × 101 mg/g), revealing the preferential adsorption of C12 over MAF-6. MAF-6’s b-value (1.1 × 10−2 mg/L) was higher than that of AC (5.7 × 10−3 mg/L), further verifying that MAF-6 is an efficient sorbent for C12. To better understand the adsorption results, the equilibrium parameters (RL values) for the adsorption of C12 over MAF-6 and AC were evaluated over the studied adsorption ranges and plotted against the initial C12 concentrations (Figure 6c). The relatively low RL values obtained for MAF-6 also supports that this material as being the very favorable adsorbent for C12 because lower RL values indicate more favorable adsorption. The results described above indicate that there is a special interaction between C12 (one of six n-alkanes used in this study) and MAF-6 that does not occur with AC, MIL-100(Fe), or Cu-BTC. Polar and nonpolar interactions cannot be the reason for this selectivity because the polarity increases as the size of the n-alkanes increases (Table 2). Moreover, the kinetic diameters of the n-alkanes studied here are very similar to each other (∼3.9 Å).58 To identify the reason for this interesting selectivity, the adsorbent’s cavity size and the adsorbate’s chain length were considered. MAF-6 is known to be highly hydrophobic (either its outer or inner surface) and has a fairly

Figure 4. Adsorbed quantities of C6, C12, and C16 over (a) MAF-6 and (b) AC in single or binary sorbate system.

overall qt values in the binary adsorbate systems were lower than those in single adsorbate systems because of the competition between adsorbates in the binary system. Although the qt values over MAF-6 were low in binary systems, this material exhibited clear selectivity for C12 (Figure 4a), as observed in the single adsorbate system (Figure 3a). No remarkable selectivity was observed (smaller alkanes were adsorbed a bit more than larger ones) over AC in the binary systems (Figure 4b), similar to the result obtained in the single adsorbate system, although the adsorbed amounts decreased slightly. To facilitate better understanding the adsorption of C12 over MAF-6, adsorption experiments of C12 (initial concentration of 1000 ppm) were performed over various times in C8 solvent. The effects of contact time on the adsorption of C12 over MAF-6 and AC are shown in Figure 5. C12 was almost completely absorbed on AC and MAF-6 after 6 and 12 h, respectively. A commonly used kinetic model, the “pseudosecond-order nonlinear kinetic model”,64 was applied to estimate the adsorption kinetics. The solid lines in Figure 5 represent the calculated qt values obtained using the nonlinear model of the pseudo-second-order kinetic equations. The calculated kinetic constants (k2) of the adsorption of C12 are listed in Table 3. The high accuracy of the model was confirmed by the high correlation parameters (R2) as shown in D

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Figure 6. (a) Adsorption isotherms, (b) Langmuir plots, and (c) effects of initial concentrations on RL values for the adsorption of C12 over AC and MAF-6 from n-octane. Conditions: 0.005 g of adsorbents in 5.0 mL of solution (100−2000 ppm).

large cavity (18.4 Å).55 According to Tanford’s formula67 or the modified Tanford’s formula,68 C12 has a molecular length of 16.7 or 18.5 Å (Table 2), which is very similar to the cavity size of MAF-6 (18.4 Å).55 Therefore, the observed selective adsorption of C12 over MAF-6 might be attributable to the similarity in the adsorbent’s cavity size and the n-alkane’s length. In other words, the results obtained for the selective adsorption of C12 over MAF-6 (as shown in Figures 3 and 4) indicate that the origin of the selectivity might be the good match between the cavity size of MAF-6 and the length of C12. To confirm the effect of the match between the alkane chain length and the adsorbent cavity size on the selective adsorption of n-alkanes, the adsorbed amounts of selected n-alkanes over a hydrophilic MOF, Cu-BTC,62 was also investigated. Cu-BTC adsorbs C7 more efficiently than any other tested linear alkane (Figure 3b). The cavity size of Cu-BTC (11.1 or 13.2 Å)60 is very close to the length of C7 (10.36 or 12.19 Å) (Table 2), again suggesting the importance of the similarity of the adsorbent’s cavity size and the n-alkane’s length for selective adsorption. Shape- and size-selective adsorptive separation of C4 or C5 from its branched isomers and from longer homologues has been conducted over microporous Cu-MOF58 or rare-earth fcuMOF.38 The selective adsorptive separation of aromatics based on suitable cavity size has also been achieved over MIL-4740 and monoclinic Zn-MOF.41 However, the present results are meaningful because they constitute the unusual observation of selectivity (based on length similarity) in the liquid-phase adsorption of n-alkanes over MOFs. Because of the very similar sizes of the C12 chain (Table 2) and the MAF-6 cavity, the C12 molecule can enter the cavity of MAF-6 easily. The two terminal methyl groups of C12 may interact with the inner walls of MAF-6, which is highly hydrophobic and can thus adsorb C12 effectively. In contrast, MAF-6’s adsorption capacities for shorter n-alkanes (C6:10.93

Å or C7:12.19 Å) are substantially lower than that for C12. C6 and C7 are much shorter than the MAF-6’s cavity (18.4 Å); therefore, they can rapidly move through the MAF-6 cavity and result in low qt values. Although cavity-based selectivity may not be relevant for the adsorption of C6 and C7 from C8, these compounds are adsorbed (in low quantities) from the C8 solvent (which has very similar polarity) because of nonpolar interactions (see below) between the less-polar hydrocarbons and the highly hydrophobic surface of MAF-6. Conversely, C15 (22.31 Å) and C16 (23.58 Å) are longer (Table 2) than MAF6’s cavity (18.4 Å), which could inhibit their ability to access it. As a result, their qt values (for C15 and C16) are low; however, because of the flexible nature of these n-alkane chains, the qt values are higher than those of C6 and C7. Unlike MAF-6 and Cu-BTC, AC and MIL-100(Fe) did not exhibit appreciable selectivity for one particular alkane. As shown in Figure 3, the qt values over AC and MIL-100(Fe) monotonously decreased and increased, respectively, as the polarity (or size) of the adsorbates increased. These poor selectivities might be because of their larger cavity sizes (∼39 Å for AC;59 25 or 29 Å for MIL-100(Fe)61) relative to the lengths of the studied alkanes. The qt values over AC and MIL-100(Fe) decreased and increased, respectively, with the polarity of the adsorbates increased (Figure 3). As shown in Table 2, the polarity (or dielectric constant) and length of alkanes increase as the number of carbons increases. AC and MIL-100(Fe) are typical hydrophobic and hydrophilic materials,34,62 respectively. Therefore, nonpolar interactions between n-alkane molecules and AC’s hydrophobic surface might be the reason underlying the monotonously increasing adsorption observed as the alkane size (or polarity) decreased. On the contrary, the preferential adsorption of long-chain alkanes with high polarity (such as C16 and C15) over MIL-100(Fe) might depend upon polar E

DOI: 10.1021/acsami.6b00608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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interactions. Adsorption mechanisms based on polar/nonpolar interactions have been reported previously.69 The interesting adsorption properties of hydrophobic MAF-6 for C12 (and hydrophilic Cu-BTC for C7) lead us to conclude that matching the adsorbent’s cavity size with the n-alkane’s molecular length is more important than the MOF’s hydrophilicity/-phobicity (or the occurrence of polar/nonpolar interactions) for the adsorption/separation of linear alkanes.

4. CONCLUSION Linear alkanes were adsorbed from C8 over three MOFs and conventional AC to elucidate important parameters affecting liquid-phase adsorption. MAF-6 and Cu-BTC showed selectivity for the adsorption of C12 and C7, respectively, from C8 because of the good matches between the adsorbent’s cavity size and the adsorbate’s length, suggesting the importance of adsorbent cavity size and adsorbate length for the adsorption/separation of n-alkanes. In contrast, the adsorption of n-alkanes over AC and MIL-100(Fe) decreased and increased monotonously, respectively, as the polarity (and size) of the n-alkanes increased, probably because of nonpolar and polar interactions, respectively, given that AC is hydrophobic and MIL-100(Fe) is hydrophilic. These results lead us to conclude that matching the adsorbent’s cavity size with the n-alkane’s molecular length is a more important parameter than the MOF’s hydrophilicity/-phobicity for the selective adsorption/separation of alkanes.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 82-53-950-5341. Fax: 82-53-950-6330. E-mail: sung@ knu.ac.kr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Grant Number 2013R1A2A2A01007176).

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ABBREVIATIONS AC = activated carbon Cu-BTC = copper-benzenetricarboxylate MAF-6 = metal-azolate framework-6 MIL-100(Fe) = iron-benzenetricarboxylate MOFs = metal-organic frameworks C6 = n-hexane C7 = n-heptanes C8 = n-octane C10 = n-decane C12 = n-dodecane C15 = n-pentadecane C16 = n-hexadecane

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DOI: 10.1021/acsami.6b00608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX