Dichotomous adsorption behaviour of dyes on an amino-functionalised metal–organic framework, amino-MIL-101(Al)

Abstract

An amino-functionalised metal–organic framework (MOF), aluminium aminoterephthalate (amino-MIL-101(Al)), has been applied to the adsorptive removal of dyes (cationic methylene blue, MB, and anionic methyl orange, MO) from aqueous solutions in order to examine the effect of the amino group on sorption behaviour. Adsorption isotherms and thermodynamic studies indicated the spontaneous adsorption of MB with a maximum adsorption capacity at 30 °C (762 ± 12 mg g_MOF⁻¹) higher than those observed for MB on other MOFs and most other materials. In contrast, lower adsorption capacities were observed in the adsorption of the same dye on the analogous non-amino-functionalised framework (MIL-101(Al), 195 mg g⁻¹) and in the adsorption of MO by amino-MIL-101(Al) (188 ± 9 mg g⁻¹), suggesting that an electrostatic interaction between the amino groups of the MOF and the cationic dye MB may have contributed to the high adsorption capacity. The adsorptions of both dyes on amino-MIL-101(Al) were spontaneous, endothermic, and entropy-driven, as is common for dye adsorptions. However, the ΔS value obtained for the adsorption of MB (346 J mol⁻¹ K⁻¹) was extreme. Further analysis demonstrated that after exposure to MB, the ordered amino-MIL-101(Al) structure was absent, ∼30% of the Al³⁺ was lost to solution, and significant changes occurred in the X-ray photoelectron spectrum of the MOF. On the other hand, the MOF structure was intact following the adsorption of MO. Several groups have exploited electrostatic interactions to improve dye adsorption; however, these proved excessive in the case of MB (but not MO) adsorption on amino-MIL-101(Al).

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1. Introduction

Metal–organic frameworks (MOFs)¹ are crystalline porous materials constructed from multifunctional ligands and metal ions. Recently, MOFs have become popular for their diverse, porous structures, high surface areas and potential applications in the fields of catalysis,² gas adsorption/storage,³ biomedicine and drug delivery,⁴ separation,^2c,5 adsorption of organic molecules,⁶ luminescence,⁷ electrode materials,⁸ and magnetism,⁹ and as carriers for nanomaterials.¹⁰ In the particular field of adsorption, the removal of dyes,¹¹ alkylaromatics and phenols,¹² pharmaceuticals (furosemide and sulfasalazine),¹³ and sulfur compounds^6c,14 from the liquid phase have been reported.

We became interested in the use of MOFs to remove dyes from contaminated water because considerable amounts of coloured wastewater are generated from industries such as the textile, leather, paper, printing, dyestuff, and plastic industries.¹⁵ Dyes are generally difficult to degrade because they are very stable to light and oxidation;¹⁶ however, water quality is highly influenced by colour,¹⁵ and even a small amount of dye is highly visible and undesirable. Moreover, many dyes are considered to be toxic and even carcinogenic.^15–17 Physical, chemical and biological methods have been investigated for the removal of dyes from contaminated water,^15–17 and adsorption is considered competitive among these because it is highly efficient, economically feasible, and requires only simple design.^16,18 MOFs have some advantages as adsorbents in that their structures and pore sizes are tunable.¹ Further, they contain polar and polarisable bonds, and can even bear open (or, in aqueous solution, water-coordinated) coordination sites on metal atoms. Thus electrostatic or coordinative interactions can increase^11c,19 (or decrease^19a) the dye-sorption capacity of a structure. Electronic interactions have also been invoked in the sorption of dyes on MOF-235, a cationic MOF with intercalated anions.^11b

In addition to as-synthesised MOFs,¹¹ some tailored materials have been tested in dye adsorption. Inspired by Ma and co-workers, who reported the adsorption of anionic dyes on ammonium-functionalised MCM-41,²⁰ Jhung and co-workers grafted ethylenediamine onto MIL-101 (MIL = Materials of Institute Lavoisier) to produce sorbents that adsorbed the anionic dye methyl orange (MO, Fig. S1†).^11c The best adsorption was obtained when MIL-101 was functionalised with the diamine and then protonated, presumably forming dangling ammonium groups; however, even the diamine-functionalised framework was a more effective sorbent than bare MIL-101. Studies on the impact of pH suggested that electrostatic interactions assisted the adsorption of the anionic dye even on the diamine-functionalised framework. More recently, Li et al. formed a MOF–graphite oxide composite that adsorbed more methylene blue (MB, Fig. S1†) than the MOF alone.^19b

Though an amino-functionalised MOF has been tested in the adsorption of anionic MO, we reasoned that a functional group bearing a lone pair of electrons should also improve the sorption of cationic dyes such as MB. Rather than use the post-synthesis grafting method that was demonstrated by Jhung and co-workers,^11c we chose to study the adsorption of both MB and MO from aqueous solution onto amino-functionalised MIL-101(Al) (amino-MIL-101(Al)). Amino-MIL-101(Al) is built from supertetrahedral building units formed by aminoterephthalate ligands and trimeric Al(III) octahedral clusters,²¹ and thus bears an amino group on every linker. Whereas these supertetrahedral enclose tetrahedral micropores, the extended structure of amino-MIL-101(Al) is composed of two types of quasi-spherical mesoporous cages formed by 12 pentagonal and 16 hexagonal faces (Fig. 1) with windows 1.2 and 1.6 nm, respectively, in diameter.²² It is an active catalyst for the condensation of benzaldehyde and ethyl cyanoacetate,²² and its iron analogue, amino-MIL-101(Fe), has been used in drug delivery²³ and medical imaging.²⁴ Unfunctionalised MIL-101(Cr) has been used as a sorbent for the cationic dye malachite green; however, its positive surface charge was detrimental to adsorption capacity.^19a

Fig. 1 (a) Octahedral cluster of three AlO₅(OH) units with the organic ligand aminoterephthalate. These clusters and ligands form the framework of amino-MIL-101(Al), and each cluster is shown in the bottom diagram as a green octahedron. (b) Supertetrahedral building units of amino-MIL-101(Al) formed by aminoterephthalate ligands and trimeric AlO₅(OH) octahedral clusters. Green = trimeric cluster; red = O; black = C; blue = N; yellow = H. Only the N-bound H atoms are shown.

2. Experimental

Amino-MIL-101(Al) was synthesised using a reported solvothermal method.²¹ 2-Aminoterephthalic acid (0.56 g, HO₂C–C₆H₃NH₂–CO₂H, Sigma–Aldrich, 99%) and aluminum chloride hexahydrate (0.51 g, AlCl₃·6H₂O, Sigma–Aldrich, 99%) were mixed with 30 mL of N,N-dimethylformamide ((CH₃)₂NCHO, Sigma–Aldrich, 99.8%) and stirred until a clear solution was formed. The resulting mixture was placed in a Teflon-lined reactor and heated for 6 h at 130 °C in a microwave (MARS-5, CEM, 300 W). The resulting yellow powder was filtered under vacuum, washed with acetone, and dried at 100 °C under vacuum. To remove organic species trapped within the pores, the product was activated in methanol at 80 °C overnight and then dried at 100 °C under vacuum.

The products were analysed by X-ray diffraction (Siemens D5000, CuKα radiation) over 2θ = 2–20°. The crystallite morphologies were examined using a field emission scanning electron microscope (FESEM, Zeiss ultra Plus) operated at 5 kV. FTIR spectra were obtained with a Bruker IFS66V spectrometer. Nitrogen physisorption was carried out using an Autosorb-iQ (Quantachrome Instruments, USA) adsorption unit at liquid nitrogen temperature (−196 °C). Samples were evacuated at 150 °C for 12 h prior to analysis. Surface areas were calculated using the Brunauer–Emmett–Teller (BET)²⁵ model from the nitrogen adsorption isotherms over P/P₀ = 0.05–0.15. The Barrett–Joyner–Halenda (BJH)²⁶ method was used to calculate the pore size distribution from the adsorption branch. Thermal stability was studied by heating the sample in a thermogravimetric analyser (TGA, TA Instruments, Q500) at 5 °C min⁻¹ under an air flow of 60 mL min⁻¹. X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo scientific, UK; X-ray source: monochromated Al Kα, Power: 164 W (10.8 mA and 15.2 kV); binding energy reference: C1s = 285.0 eV for adventitious hydrocarbon) was used to examine the chemical states of different elements in amino-MIL-101(Al) before and after MB adsorption. ²⁷Al spectra were recorded at UNSW on a Bruker Avance III 700 operating at 182.49 MHz, using a 2.5 mm triple resonance CPMAS probe. Samples (∼10 mg) were packed into 2.5-mm o.d. rotors and spun at the magic angle at 20 kHz and at ambient temperature. Data was acquired using a single pulse (∼14°, π/12 pulse) with a 1-s recycle delay. Finally, ICP-OES measurements were performed on a Varian 720-ES spectrometer. The wavelength monitored was 396.152 nm, and the calibration curve was prepared from dilutions of a 1000-ppm Al³⁺ solution in 2% HNO₃(aq.) (Choice Analytical).

Stock solutions of MB and MO (1000 ppm each, assuming that ρ_solution = 1.0 mg L⁻¹) were prepared by dissolving solid MB (C₁₆H₁₈ClN₃S·3H₂O, MW: 373.9, Sigma–Aldrich, ≥82% dye content) and MO (C₁₄H₁₄N₃NaO₃S, MW: 327.33, Sigma–Aldrich, ≥85% dye content) separately in ultrapure water (18.2 MΩ cm, Sartorius Arium 611D). The dye contents were considered when calculating concentrations; thus, for 1.0 g MB, a mass of (1.0/0.82) g was used. Working solutions of MB and MO were prepared by sequential dilution of the stock solution with ultrapure water. Stock solutions of MB and MO ([dye] = 1–30 ppm]) were used to construct calibration curves (Fig. S2†) for absorbance at 665 and 464 nm respectively, using a Varian Cary 50UV UV-visible spectrophotometer.

Before adsorption, the adsorbent was dried overnight under vacuum at 100 °C and kept in a desiccator. In a typical adsorption experiment, the adsorbent (∼5 mg) was weighed precisely and added to an aqueous dye solution (100 mL) with a known dye concentration between 20 and 200 ppm. The dye solutions containing the adsorbents were mixed well with magnetic stirring and maintained for a fixed time (5 min to 12 h) at 30 °C in a thermal control chamber (Thermoline Scientific). After a pre-determined time, the solution was separated from the adsorbent with a syringe filter (0.45 μm, Millex, Millipore). The final dye concentration was calculated by comparing the UV-vis absorbance to the appropriate calibration curve; samples with [dye]₀ > 30 ppm were diluted before analysis. The adsorption quantity was measured from the calibration curve using eqn (1):

where q_t is the amount of MB or MO, in mg, adsorbed onto 1 g adsorbent; [dye]₀ and [dye]_t are the MB or MO concentration before and after adsorption, respectively; V is the volume, in mL, of MB or MO solution used in the adsorption experiment, and m_adsorbent is the mass of the adsorbent, in g.

Adsorption rates were fit to both pseudo-second- and pseudo-first-order models.²⁷ Maximum adsorption capacities were calculated using Langmuir adsorption isotherms obtained after adsorption for 12 h. In order to determine ΔG_ads, ΔH_ads and ΔS_ads, the adsorptions were repeated at 40 and 50 °C, and van't Hoff plots were prepared.

3. Results and discussion

Amino-MIL-101(Al) was synthesised in the microwave at 130 °C. Its XRD pattern is shown in Fig. 2a, and is similar to the calculated pattern for MIL-101(Cr)²⁸ and the experimental patterns for amino-MIL-101(Al),²¹ amino-MIL-101(Fe),²⁹ and MIL-101(Cr).^28,30 This evinced the formation of the MIL-101 phase (Fig. S3†). A scanning electron microscopy (SEM) image of our amino-MIL-101(Al) (Fig. 2b) showed a homogeneous surface morphology with an average particle diameter of approximately 800 nm. The FT-IR spectrum of amino-MIL-101(Al) (Fig. 2c) displayed the two characteristic bands of the amino group: the δ_(N–H) (scissoring) vibration at 1624 cm⁻¹ and the ν_(C–N) absorption distinctive of aromatic amines at 1336 cm⁻¹.^22,31 The nitrogen adsorption–desorption isotherm (Fig. 2d) measured at −196 °C showed the characteristic steps and type VI isotherm observed for MIL-101 structures.²⁸ The Brunauer–Emmett–Teller (BET) surface area,²⁵ calculated from the adsorption isotherm over P/P₀ = 0.05–0.15, was 1980 m² g⁻¹, similar to the reported BET surface area of amino-MIL-101(Al) (2100 m² g⁻¹).²² The pore size distribution (Fig. S4†) of amino-MIL-101(Al), calculated from the adsorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) equation,²⁶ was bimodal, with narrow peaks at pore sizes of 1.10 and 1.52 nm. The purified amino-MIL-101(Al) was stable in air up to 492 °C (Fig. S5†). The adsorption ability of amino-MIL-101(Al) was first tested by exposing 10 mg of the solid to 50 mL of a 50-ppm aqueous MB solution for 6 h, after which the supernatant appeared colourless (Fig. S6†). Thus we examined the kinetics and thermodynamics of the adsorption. The saturation adsorption was investigated by combining 5 mg of amino-MIL-101(Al) with 100 mL of aqueous MB ([MB]₀ = 20–40 ppm) for 12 h. The quantity of MB adsorbed over this time is displayed in Fig. 3a, and the source UV-vis spectra are shown in Fig. S7.† Saturation occurred after approximately 2 h, and the total amount of MB adsorbed increased with [MB]₀, demonstrating favourable adsorption characteristics at high MB concentrations. The same has been observed for other dye adsorptions on MOFs.^11b,c,19a The reaction with [MB]₀ = 40 ppm was performed three times, and the maximum adsorption was reproducible within 2% (see Table 1 and Fig. 4). The rate of MB adsorption onto amino-MIL-101(Al) was measured over a 12-h period. Like many dye-adsorption reactions, this adsorption could not be described rigorously by first- or second-order kinetics, so true kinetic constants were not extracted. However, in order to compare the rate of MB adsorption on amino-MIL-101(Al) to those on reported sorbents, the change in MB concentration with time was treated with two common kinetic models. Fig. S8a† shows the adsorption of MB from solutions with initial MB concentrations of 20, 30 and 40 ppm, fit to a pseudo-second-order kinetic model (see ESI† for details).²⁷ The calculated values of k₂, along with the relevant correlation coefficients (R²), are shown in Tables 1 and S2.† The value of k₂ for the adsorption of MB on amino-MIL-101(Al) increased slightly with increased initial dye concentration, similar to previous observations,^11b,c,19a,32 but contrary to the case of malachite green on MIL-100(Fe).^19a The adsorption data were also fit to a pseudo-first-order kinetic model (see ESI and Fig. S8b†), and the extracted values of k₁ and R² are displayed in Tables 1 and S3.† The rate of adsorption was clearly better described by pseudo-second-order than pseudo-first-order kinetics. Nevertheless, the values of both k₂ and k₁ determined for the adsorption of MB on amino-MIL-101(Al) were much higher than those reported for MOF-235,^11b activated carbon,³³ chitosan-g-poly(acrylic acid)–vermiculite hydrogel composite,³⁴ magnetic modified beer yeast,³⁵ alginate–hydrolysed oak sawdust composite,³⁶ papaya seed,³⁷ grass waste,³⁸ and graphene,³⁹ as shown in Table 2. Not only did amino-MIL-101(Al) adsorb MB more quickly than many adsorbents, but it also took up large amounts of the dye. Adsorption isotherms were measured after 12 h, and the maximum adsorption capacity was calculated from the plot in Fig. 4b according to the Langmuir equation (eqn (2)).⁴⁰Here, C_e is the equilibrium concentration of dye in solution (mg g⁻¹), q_e is the amount of dye adsorbed (mg g⁻¹), Q_o is maximum adsorption capacity (mg g⁻¹) and b is the Langmuir constant (L mg⁻¹). The maximum adsorption capacity of MB by amino-MIL-101 (Q_o), i.e. the reciprocal of the slope of a plot of C_e/q_e against C_e (Fig. 4), was 762 ± 12 mg g⁻¹ (Table 1), or approximately 0.54 equivalents of MB per repeat unit in the MOF. An uptake of 762 mg g⁻¹ MB is larger than those reported over adsorbents like MOF-235,^11b activated carbon,^33a–c,41 jute fiber,⁴² poly(amic acid)-modified biomass of baker's yeast,⁴³ filtrasorb,⁴⁴ coal, hair, cotton waste,⁴⁵ papaya seed,³⁷ grass waste,³⁸ diatomite,⁴⁶ Al-MCM-41⁴⁷and graphene³⁹ (Table S1†). MB adsorption over amino-MIL-101(Al) was carried out at three temperatures to observe its temperature dependence. Fig. 5 shows the change of MB concentration in solution after 12 h with 5 mg amino-MIL-101(Al) at 30, 40 and 50 °C. UV-visible spectra (Fig. S9†) showed the final concentration of MB in solution decreasing sharply with increasing temperature, and the adsorption isotherms and corresponding Langmuir plots derived from this data (Fig. 6a and b, respectively) confirmed that the adsorption capacity increased with increasing temperature, suggesting an endothermic adsorption.^11b,c,19,48 Langmuir constants were calculated from the data in Fig. 6b, and their natural logarithms were plotted against T⁻¹ in a van't Hoff plot (Fig. 6b, inset) to produce values for the enthalpy and entropy of adsorption according to eqn (3).

Fig. 2 Characterisation data for amino-functionalised MIL-101(Al): (a) XRD pattern; (b) SEM image; (c) FT-IR spectrum; (d) nitrogen adsorption–desorption isotherm recorded at −196 °C.

Fig. 3 (a) Effect of contact time on the adsorption of MB or MO by 5 mg amino-MIL-101(Al) at 30 °C with [dye]₀ = 20, 30 or 40 ppm; (b) isotherms for MB and MO adsorption after 12 h of adsorption.

Table 1 Textural and reactivity properties of amino-MIL-101(Al)

Adsorbent	Pore size (nm)	BET Surface area^a (m^2 g^−1)	Total pore volume^b (cm^3 g^−1)	Dye	Kinetic data for [dye]0 = 40 ppm^c				Adsorption capacity, Qo^d (mg g^−1)
					Pseudo-second-order kinetic constant (g mg^−1 min^−1)		Pseudo-first-order kinetic constant (min^−1)
					k 2	R ^2	k 1	R ^2
a Calculated over P/P0 = 0.05–0.15. b Calculated for P/P0 = 0.99. c The data were not completely described by either first- or second-order models, so the rate ‘constants’ depend on [dye]0. Constants and R^2 values were determined by fitting data to standard kinetic models; see Fig. S8 and S14 for the relevant plots and Tables S2 and S3 for data collected at [dye]0 = 20 or 30 ppm. d Measured after a 12 h adsorption at 30 °C using Langmuir equation; see Fig. 4 and S13.
Amino-MIL-101	1.10	1980	1.02	MB	(2.6 ± 1.3) × 10^−3	0.999	(7 ± 3) × 10^−2	0.9	762 ± 12
	1.52			MO	(1.3 ± 0.3) × 10^−4	0.993	(8.7 ± 0.9) × 10^−3	0.98	188 ± 9

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Fig. 4 (a) Isotherm for MB adsorption over 5 mg of amino-MIL-101(Al) after 12 h at 30 °C; and (b) Langmuir plot of the isotherm of (a).

Table 2 Comparison of pseudo-first-order and pseudo-second-order kinetic constants (k₁ and k₂) for MB adsorption on amino-MIL-101(Al) with those for other adsorbents

Adsorbent	[dye]0 (ppm)	k 1 (min^−1)	k 2 (g mg^−1 min^−1)	Ref.
a Not determined.
Amino-MIL-101(Al)	40	0.07 ± 0.03	0.0026 ± 0.0013	This work
MOF-235	40	—^a	2.18 × 10^−4	11b
Bamboo-based activated carbon	200	0.00129	1.27 × 10^−4	33c
Activated carbon from periwinkle	200	0.0152	1.88 × 10^−4	33b
Magnetic cellulose beads entrapping activated carbon	374	—^a	1.41 × 10^−4	33d
Chitosan-g-poly(acrylic acid)–vermiculite hydrogel composite	1000	0.0370	5.75 × 10^−4	34
Magnetic modified beer yeast	300	0.0300	9.00 × 10^−5	35
Alginate–hydrolysed oak sawdust composite	200	0.0134	1.61 × 10^−3	36
Papaya seed	300	0.0304	1.70 × 10^−3	37
Grass waste	380	0.0283	1.70 × 10^−3	38
Graphene	40	0.0096	1.00 × 10^−4	39

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Fig. 5 Digital images of MB solutions before and after 12 h over 5 mg of amino-MIL-101(Al) at 30, 40 and 50 °C.

Fig. 6 (a) Isotherms for MB adsorption over 5 mg of amino-MIL-101(Al) for 12 h at 30, 40 and 50 °C; (b) Langmuir plots of the isotherms in (a) and (inset) van't Hoff plot of the Langmuir constants b as a function of temperature, used to calculate the ΔH and ΔS of the MB adsorption over amino-MIL-101(Al).

The enthalpy change ΔH for MB adsorption over amino-MIL-101(Al) was positive, +77.9 kJ mol⁻¹, confirming that the adsorption was endothermic. The entropy change ΔS was +346 J mol⁻¹ K⁻¹. According to eqn (4), the free energies of adsorption at 30, 40 and 50 °C were −26.19 ± 0.04, −30.1 and −33.0 kJ mol⁻¹, respectively (Table 3). These negative free energies confirmed that adsorption was spontaneous under the experimental conditions used.

ΔG = −RT ln b (4)

Table 3 Maximum adsorption capacities and thermodynamic parameters of MB and MO adsorption over amino-MIL-101(Al) and MB adsorption over MIL-101(Al)

Dye	MOF	T (°C)	Q o (mg g^−1)^a	ΔG^a (kJ mol^−1)	ΔH^a (kJ mol^−1)	ΔS^a (J mol^−1 K^−1)
a Adsorption isotherms and van't Hoff plots for MB and MO adsorption on amino-MIL-101 are given in Fig. 6 and S15, respectively, and the adsorption isotherm and van't Hoff plot for MB on MIL-101(Al) is given in Fig. S23.
MB	Amino-MIL-101(Al)	30	762 ± 12	−26.19 ± 0.04	77.9	346
		40	1032	−30.1
		50	1409	−33.0
MO	Amino-MIL-101(Al)	30	188 ± 9	−21.0 ± 0.3	0.489	74.3
		40	192.1	−22.8
		50	198.6	−23.5
MB	MIL-101(Al)	30	195	−23.3	0.603	78.8
		40	216	−24.1
		30	195	−23.3

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The high positive value of ΔS for the adsorption of MB by amino-MIL-101(Al) indicated a significant loss of order. Moderate increases in entropy (ΔS = 50–200 J mol⁻¹ K⁻¹) are common during in dye adsorption;^{11c,17,19,48b,49} when observed for dye adsorption on MOFs, these values have been attributed to the displacement of multiple surface-bound water molecules by a single (larger) dye molecule.^11c,19a However, the magnitude of ΔS for MB adsorption on amino-MIL-101(Al) was much greater than the values measured for MO adsorption on MIL-101(Cr)^11c or malachite green adsorption on MIL-100(Fe);^19a only the adsorption of MO on MOF-235 has shown a larger value of ΔS.^11b The value of ΔH measured here was also high compared to other dye adsorptions on MIL materials.^11c,19a In an effort to understand the dramatic thermodynamic parameters measured here, we examined the MB–MOF composite. Thus, following exposure to MB in solution ([MB]₀ = 40 ppm, 50 °C, 12 h), the MOF was filtered from solution and dried at 100 °C under vacuum for 12 h. X-ray photoelectron spectroscopy (XPS; Fig. S12 and S13, Tables S4 and S5†) of the as-synthesised and recovered dyes showed that the electron binding energies associated with some of the carbon and nitrogen atoms in the MOF structure changed upon dye exposure. In particular, a new C1s peak appeared at 287.8 eV, and several other C1s peaks decreased in intensity (Fig. S13†), suggesting significant structural alteration of the MOF after the absorption of the MB dye. It was clear from the XRD patterns of the amino-MIL-101(Al) before and after adsorption (Fig. 7, cf. a and b) that the adsorption of MB interrupted its long-range order, which likely contributed to the high positive entropy change observed. This structural change could not have been the result of reaction with water, as amino-MOF-101(Al) was stable in pure water under the reaction conditions (up to 50 °C for 12 h; see Fig. 7d).

Fig. 7 Powder X-ray diffraction patterns of amino-MIL-101(Al): (a) before adsorption; (b) after adsorption of MB; (c) after adsorption of MO; (d) after 12 h in H₂O at 50 °C. (e) Powder X-ray diffraction pattern of MIL-101 after adsorption of MB after 12 h at 50 °C.

MOFs can, in some instances, change their structures upon strong interaction with an adsorbate,⁵⁰ and this appears to occur in the present case, though at the expense of an ordered structure. We therefore considered the possibility that the interaction between MB and amino-MIL-101(Al) should be described as a reaction rather than an adsorption, that is, whether metal–carboxylate bonds were broken upon dye adsorption. The as-synthesised and recovered dyes were thus examined using solid-state magic-angle spinning (MAS) ²⁷Al spectroscopy (Fig. S22†). The spectrum showed a peak at δ = −10.0 ppm, which was similar to the ²⁷Al NMR chemical shift measured for the as-synthesised material (δ = −7.3 ppm, Fig. S22†). Both were indicative of octahedral coordination at Al; however, the sample that was exposed to MB showed a signal that was less symmetric, and could have represented more than one type of Al nucleus. Further, as would be expected for both pore-filling by an adsorbate or structural collapse, the MB–MOF composite had a much lower surface area (154 m² g⁻¹) than the as-synthesised amino-MIL-101(Al) (1980 m² g⁻¹, see above). However, the MB–MOF composite showed a clear hysteresis loop over P/P₀ = 0.45–0.80, suggesting the formation of mesopores. Most telling, though, was the aluminium content of the solution after MB adsorption. An aluminium concentration of 1.8 ppm was measured, indicating that ∼30% of the metal was lost from the amino-MIL-101(Al). This, combined with the change in the XRD pattern of the solid, indicated that MB reacted with some, but not necessarily all, of the amino-MIL-101(Al).

The disruption of the amino-MIL-101(Al) structure upon MB exposure explains the significantly unfavourable enthalpy upon adsorption, as at least some of the metal–carboxylate bonds that held the structure together were broken and weakened. This loss was apparently greater than the combined favourable enthalpy change from the electrostatic interactions and any van der Waals forces between the dye and the MOF. Moreover, based upon the values of ΔH and ΔS for the reaction, the destruction of the ordered MOF structure, and the dissolution of Al³⁺ ions, appears to be the driving force for the interaction. This is in contrast to the case of MB adsorption on a MOF–graphite oxide material, which occurred with more moderate thermodynamic parameters (ΔH = 39.26 kJ mol⁻¹, ΔS = 167.5 J mol⁻¹ K⁻¹), as the XRD pattern of that material upon recycling still reflected an ordered crystalline phase.^19b It is not clear whether a loss of crystallinity was responsible for the high ΔS values observed in the adsorptions of MB and MO on MOF-235, as that material was not characterised after adsorption.^11b

In order to determine whether the amino groups in the amino-MIL-101(Al) material were involved in its interaction with MB, we tested an analogous adsorption. Thus, the amino-free parent framework, MIL-101(Al), was exposed to aqueous solutions of MB at 30, 40, and 50 °C. The adsorption curves, Langmuir isotherms and van't Hoff plots are shown in Fig. S23† and summarised in Table 3. MIL-101(Al) took up significantly less MB than its amino-functionalised analogue, adsorbing a maximum of 195 mg g⁻¹ at 30 °C. This was approximately one-fourth of the capacity of amino-MIL-101(Al) for MB, and only slightly higher than the capacity of MOF-235 for the same dye.^11b Contrary to the adsorption of MB on amino-MIL-101(Al), that on the unfunctionalised MIL-101(Al) was only weakly temperature-dependent. This was borne out in the thermodynamic parameters of the reaction (Table 3); the reaction was nearly thermoneutral, and incurred a modest entropy gain that was well within the range common for dye adsorptions.^{11c,17,19,48b,49} The lower adsorption capacity of MIL-101(Al) for MB, as well as the unexceptional ΔH and ΔS values observed for the adsorption reaction, suggested that this adsorption did not induce a significant change in the crystal structure of the MOF, and this was confirmed by the XRD pattern of MIL-101(Al) after MB adsorption (Fig. 7e), which still showed the MIL-101 phase.

Given the severe impact of dye adsorption on the MOF structure, we also investigated the structure of the adsorbed dye. Thus, samples of the dye–MOF composites that were produced by adsorption at 30, 40, and 50 °C were sonicated in deionised H₂O for 12 h, and the UV-vis spectrum of the resulting solutions were measured (Fig. S10†). In all cases, these displayed the UV spectrum of MB (λ_max = 666 nm), indicating that at least some of the MB was adsorbed without reacting further with the MOF. However, complete recovery of the dye was not obtained, so we cannot exclude the possibility that some of the dye was altered upon reaction with the MOF.

The regeneration of an adsorbent is important for its commercial feasibility and, although amino-MIL-101(Al) adsorbed more MB than any other MOF tested to date, the changes to its structure upon adsorption were inauspicious for its reuse. We attempted to regenerate the used adsorbent (amino-MOF–MB) by filtering it from the solution, washing several times with water, and drying at 100 °C overnight. It was then activated with deionised water under ultrasound for 60 min^11b,c and tested as an adsorbent over 12 h at 30 °C (Fig. S11†). Though the maximum adsorption capacity of fresh amino-MIL-101(Al) for MB was 762 ± 12 mg g⁻¹, the value for the ‘regenerated’ material was only 14.8 mg g⁻¹. Thus the degradation of the MOF structure that occurred upon MB adsorption had a strong negative impact on its reusability.

We had originally chosen an amino-functionalised MOF as a sorbent with the expectation that the Lewis basic amine group would interact with the cationic dye. To further study the importance of these electrostatic interactions, we compared the adsorption of MB by amino-MIL-101(Al) to that of an anionic dye, methyl orange (MO). The quantity of MO adsorbed increased, though not dramatically, at higher initial dye concentrations (Fig. 3b). Fig. S14† shows the adsorption isotherms and Langmuir plots for MO adsorption over amino-MIL-101(Al). The maximum adsorption capacity (Q_o) of amino-MIL-101(Al) for MO adsorption, calculated from eqn (2) using data from Fig. S14b,† was 188 ± 9 mg g⁻¹ (average of three experiments), which is four times lower than that for MB. The dyes have similar molecular weights, so the comparison is similar on a molar basis. Significantly though, the adsorption capacity of amino-MIL-101(Al) for MO is higher than those of other reported adsorbents, including diatomite,⁴⁶Spirodela polyrrhiza biomass,⁵¹ MCM-22,^49a magnetic maghemite–chitosan nanocomposite,⁵² MOF-235^11b and fly ash,⁵³ as shown in Table S6.†

The adsorption of MO on amino-MIL-101(Al) was also evaluated as a function of time, and fit to pseudo-first-order and pseudo-second-order models to calculate values for k₁ and k₂ (Fig. S15† and Table 1). Both increased slightly upon increasing the initial concentration of MO. Whereas the adsorption of MB on amino-MIL-101(Al) had been much better represented mathematically by pseudo-second-order kinetics, the case for MO adsorption was more ambiguous, with the models giving similar quality descriptions of the data (see Table 1). This indicated that the rates of MB and MO adsorption were not controlled by the same factors. Fig. 3c and S18† compare the adsorptions of MO and MB on amino-MIL-101(Al); the latter was adsorbed approximately an order of magnitude faster (Fig. S19 and S20†).

The thermodynamics of the interaction between MO and amino-MIL-101(Al) were also examined (Fig. S16†). The reaction was favourable, but both ΔG_ads and the maximum adsorption capacity were relatively insensitive to temperature (Table 3). Unlike the adsorption of MB, which was highly endothermic, that of MO had ΔH ∼ 0. This implied that, unlike MB adsorption, MO adsorption had little impact on the structure of amino-MIL-101(Al); any disruption of the MOF structure upon MO adsorption was almost completely compensated by the van der Waals forces between the dye and the high-surface-area MOF structure. The entropy change upon MO adsorption was also modest (ΔS = 74.3 J mol⁻¹ K⁻¹, falling in the range generally observed for dye adsorption,^{11c,17,19,48b,49}), consistent with the suggestion that it had little impact on the MOF structure. This was also supported by the XRD pattern of amino-MIL-101(Al) after MO adsorption (Fig. 7c), which was similar to that of the MOF alone (Fig. 7a). Nevertheless, the surface area of amino-MIL-101(Al) dropped dramatically, to 123 m² g⁻¹, upon exposure to MO ([MO]₀ = 40 ppm, T = 50 °C, t = 12 h), indicating that significant dye adsorption occurred within the pores of the material. On the other hand, no significant hysteresis loop was noted in the N₂ physisorption isotherm of the MO–MOF composite (Fig. S21†). Like the ²⁷Al NMR spectrum of the MB–MOF composite, that of the MO–MOF composite showed a peak near 0 ppm (at δ = −9.5 ppm; Fig. S22†) that was less symmetrical than the peak seen for amino-MIL-101(Al). Given the complex NMR peak shapes observed, and the significant differences between the thermodynamic and X-ray diffraction data obtained for the interaction of amino-MIL-101(Al) and each of the dyes, we do not interpret this as evidence that the same reactions that occur between amino-MIL-101(Al) and MB occur between the MOF and MO. Rather, we can say that the Al atoms in amino-MOF-101(Al) remain octahedrally coordinated after exposure to either dye; further investigations will be necessary before conclusions can be drawn from the line shapes observed in the ²⁷Al NMR studies. Further evidence that the interaction between MO and amino-MIL-101(Al) was an adsorption could be gleaned from analysis of the solution following adsorption; only 0.07 ppm Al was detected (cf. 1.8 ppm following exposure to aqueous MB, see above).

The lack of drastic structural change in amino-MIL-101(Al) upon MO adsorption suggested that it might be a reusable adsorbent for this substrate. Thus, the used adsorbent was regenerated using the procedure described above and applied to a fresh solution of MO. The procedure was then repeated. In these second and third uses of amino-MIL-101(Al), its maximum adsorption capacities for MO were 99.3 and 19.5 mg g⁻¹ (Fig. S17†). Thus, the MOF lost approximately half of its adsorption capacity upon reuse, and 80% of its remaining capacity in an additional use. Though significant, this change was small compared to the case for MB, where Q₀ dropped to <2% of its initial value upon a single reuse.

The kinetic, thermodynamic, and structural data all indicated that amino-MIL-101(Al) adsorbed cationic MB and anionic MO via different mechanisms. Although this was, to some degree, by design, in that an amino-bearing MOF was chosen in order to favour the adsorption of cationic dyes, the effect proved much larger than anticipated, and the adsorption of MB occurred at the expense of the MOF structure. To our knowledge, this effect has not been reported previously; however, it should clearly be considered in the ongoing design of tailored adsorbents for dyes.

4. Conclusions

Harmful dyes (cationic MB and anionic MO) can be efficiently removed from contaminated water by the amino-functionalised MOF, amino-MIL-101(Al). This MOF showed a superior adsorption capacity (up to 762 mg g⁻¹) and high adsorption rate for MB adsorption, but only one-fourth as much of the anionic dye MO. Moreover, MIL-101(Al) absorbed significantly less MB than its amine-functionalised counterpart. Thus the electrostatic interaction between the cationic MB and the electron lone pairs on the amino groups in the MOF led to a remarkable material for MB adsorption. However, the large entropy increase that drove the reaction between amino-MIL-101(Al) and the cationic dye reflected a concomitant disruption of the MOF structure and dissolution of aluminium ions; this was not observed upon adsorption of the anionic dye. Attempts to reuse the sorbent demonstrated that, although fresh amino-MIL-101(Al) quickly adsorbed significant amounts of MB, it was a very poor sorbent upon reuse. On the other hand, though as-synthesised amino-MIL-101(Al) adsorbed only one fourth as much MO, its structure was better conserved, and it could be reused more effectively, following that interaction. Thus, although the flexible synthesis of MOFs allowed us to construct a material tailored to the removal of a cationic dye from solution, its affinity for the dye proved greater than its stability. Nevertheless, the high surface area of the MOF meant that it was still a good sorbent for an anionic dye, and it was more stable in that reaction.

Acknowledgements

E. Haque is grateful to the University of Sydney for an International Scholarship. A. T. Harris acknowledges ongoing support from the Australian Research Council (ARC) through a Future Fellowship. The authors are grateful to Dr Bill Gong of the University of New South Wales for performing the X-ray photoelectron spectroscopy, and to Drs Aditya Rawal & James Hook of the NMR Facility, Mark Wainwright Analytical Centre, University of NSW, Australia, for performing the ²⁷Al NMR spectroscopy, as well as to Dr Shuranjan Sarkar, Kyungpook National University, South Korea, for assistance with manuscript graphics.

Notes and references

1. (a) G. Férey, Chem. Soc. Rev., 2008, 37, 191; (b) S. Kitagawa, R. Kitaura and S.-i. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334; (c) O. M. Yaghi, M. O'Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705.
2. (a) L. Alaerts, E. Séguin, H. Poelman, F. Thibault-Starzyk, P. A. Jacobs and D. E. De Vos, Chem.–Eur. J., 2006, 12, 7353; (b) P. Forster and A. Cheetham, Top. Catal., 2003, 24, 79; (c) P. Horcajada, S. Surblé, C. Serre, D.-Y. Hong, Y.-K. Seo, J.-S. Chang, J.-M. Grenèche, I. Margiolaki and G. Férey, Chem. Commun., 2007, 2820; (d) Y. K. Hwang, D.-Y. Hong, J.-S. Chang, S. H. Jhung, Y.-K. Seo, J. Kim, A. Vimont, M. Daturi, C. Serre and G. Férey, Angew. Chem., Int. Ed., 2008, 47, 4144; (e) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450; (f) L. Ma, C. Abney and W. Lin, Chem. Soc. Rev., 2009, 38, 1248; (g) L. G. Qiu, A. J. Xie and L. D. Zhang, Adv. Mater., 2005, 17, 689; (h) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kim, Nature, 2000, 404, 982; (i) C.-D. Wu, A. Hu, L. Zhang and W. Lin, J. Am. Chem. Soc., 2005, 127, 8940; (j) M. Yoon, R. Srirambalaji and K. Kim, Chem. Rev., 2012, 112, 1196; (k) R.-Q. Zou, H. Sakurai and Q. Xu, Angew. Chem., Int. Ed., 2006, 45, 2542.
3. (a) B. Chen, N. W. Ockwig, A. R. Millward, D. S. Contreras and O. M. Yaghi, Angew. Chem., Int. Ed., 2005, 44, 4745; (b) H. Chun, D. N. Dybtsev, H. Kim and K. Kim, Chem.–Eur. J., 2005, 11, 3521; (c) M. Dincǎ and J. R. Long, J. Am. Chem. Soc., 2005, 127, 9376; (d) D. N. Dybtsev, H. Chun and K. Kim, Angew. Chem., Int. Ed., 2004, 43, 5033; (e) L. J. Murray, M. Dinč and J. R. Long, Chem. Soc. Rev., 2009, 38, 1294; (f) F. Nouar, J. F. Eubank, T. Bousquet, L. Wojtas, M. J. Zaworotko and M. Eddaoudi, J. Am. Chem. Soc., 2008, 130, 1833; (g) J. L. C. Rowsell, E. C. Spencer, J. Eckert, J. A. K. Howard and O. M. Yaghi, Science, 2005, 309, 1350; (h) J. L. C. Rowsell and O. M. Yaghi, Angew. Chem., Int. Ed., 2005, 44, 4670; (i) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724; (j) S. Couck, J. F. M. Denayer, G. V. Baron, T. Rémy, J. Gascon and F. Kapteijn, J. Am. Chem. Soc., 2009, 131, 6326; (k) M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O'Keeffe and O. M. Yaghi, Science, 2002, 295, 469; (l) B. Kesanli, Y. Cui, M. R. Smith, E. W. Bittner, B. C. Bockrath and W. Lin, Angew. Chem., Int. Ed., 2005, 44, 72; (m) Y. Li and R. T. Yang, J. Am. Chem. Soc., 2005, 128, 726; (n) J. Liu, P. K. Thallapally, B. P. McGrail, D. R. Brown and J. Liu, Chem. Soc. Rev., 2012, 41, 2308; (o) Y. Liu, J. F. Eubank, A. J. Cairns, J. Eckert, V. C. Kravtsov, R. Luebke and M. Eddaoudi, Angew. Chem., Int. Ed., 2007, 46, 3278; (p) A. R. Millward and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 17998; (q) R. E. Morris and P. S. Wheatley, Angew. Chem., Int. Ed., 2008, 47, 4966; (r) L. Pan, M. B. Sander, X. Huang, J. Li, M. Smith, E. Bittner, B. Bockrath and J. K. Johnson, J. Am. Chem. Soc., 2004, 126, 1308; (s) X. Zhao, B. Xiao, A. J. Fletcher, K. M. Thomas, D. Bradshaw and M. J. Rosseinsky, Science, 2004, 306, 1012.
4. (a) P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, G. Férey, R. E. Morris and C. Serre, Chem. Rev., 2012, 112, 1232; (b) P. Horcajada, C. Serre, G. Maurin, N. A. Ramsahye, F. Balas, M. Vallet-Regí, M. Sebban, F. Taulelle and G. Férey, J. Am. Chem. Soc., 2008, 130, 6774; (c) P. Horcajada, C. Serre, M. Vallet-Regí, M. Sebban, F. Taulelle and G. Férey, Angew. Chem., Int. Ed., 2006, 45, 5974.
5. (a) R. Kitaura, K. Seki, G. Akiyama and S. Kitagawa, Angew. Chem., Int. Ed., 2003, 42, 428; (b) J.-R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009, 38, 1477; (c) J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112, 869; (d) S. Ma, D. Sun, X.-S. Wang and H.-C. Zhou, Angew. Chem., Int. Ed., 2007, 46, 2458.
6. (a) B. Chen, C. Liang, J. Yang, D. S. Contreras, Y. L. Clancy, E. B. Lobkovsky, O. M. Yaghi and S. Dai, Angew. Chem., Int. Ed., 2006, 45, 1390; (b) C. Chen, M. Zhang, Q. Guan and W. Li, Chem. Eng. J., 2012, 183, 60; (c) K. A. Cychosz, A. G. Wong-Foy and A. J. Matzger, J. Am. Chem. Soc., 2008, 130, 6938; (d) X.-X. Huang, L.-G. Qiu, W. Zhang, Y.-P. Yuan, X. Jiang, A.-J. Xie, Y.-H. Shen and J.-F. Zhu, CrystEngComm, 2012, 14, 1613; (e) L. Pan, D. H. Olson, L. R. Ciemnolonski, R. Heddy and J. Li, Angew. Chem., Int. Ed., 2006, 45, 616; (f) T. K. Trung, P. Trens, N. Tanchoux, S. Bourrelly, P. L. Llewellyn, S. Loera-Serna, C. Serre, T. Loiseau, F. Fajula and G. Férey, J. Am. Chem. Soc., 2008, 130, 16926; (g) X. Wang, L. Liu and A. J. Jacobson, Angew. Chem., Int. Ed., 2006, 45, 6499.
7. (a) M. D. Allendorf, C. A. Bauer, R. K. Bhakta and R. J. T. Houk, Chem. Soc. Rev., 2009, 38, 1330; (b) Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2012, 112, 1126; (c) D. T. de Lill, N. S. Gunning and C. L. Cahill, Inorg. Chem., 2004, 44, 258; (d) B. V. Harbuzaru, A. Corma, F. Rey, P. Atienzar, J. L. Jordá, H. García, D. Ananias, L. D. Carlos and J. Rocha, Angew. Chem., Int. Ed., 2008, 47, 1080; (e) Z. Li, G. Zhu, X. Guo, X. Zhao, Z. Jin and S. Qiu, Inorg. Chem., 2007, 46, 5174.
8. G. Férey, F. Millange, M. Morcrette, C. Serre, M.-L. Doublet, J.-M. Grenèche and J.-M. Tarascon, Angew. Chem., Int. Ed., 2007, 46, 3259.
9. (a) N. Guillou, C. Livage, M. Drillon and G. Férey, Angew. Chem., Int. Ed., 2003, 42, 5314; (b) S. M. Humphrey, T. J. P. Angliss, M. Aransay, D. Cave, L. A. Gerrard, G. F. Weldon and P. T. Wood, Z. Anorg. Allg. Chem., 2007, 633, 2342; (c) D. Maspoch, D. Ruiz-Molina and J. Veciana, J. Mater. Chem., 2004, 14, 2713.
10. (a) S. Hermes, F. Schröder, R. Chelmowski, C. Wöll and R. A. Fischer, J. Am. Chem. Soc., 2005, 127, 13744; (b) H. R. Moon, J. H. Kim and M. P. Suh, Angew. Chem., Int. Ed., 2005, 44, 1261.
11. (a) A. A. Adeyemo, I. O. Adeoye and O. S. Bello, Toxicol. Environ. Chem., 2012, 94, 1846; (b) E. Haque, J. W. Jun and S. H. Jhung, J. Hazard. Mater., 2011, 185, 507; (c) E. Haque, J. E. Lee, I. T. Jang, Y. K. Hwang, J.-S. Chang, J. Jegal and S. H. Jhung, J. Hazard. Mater., 2010, 181, 535.
12. (a) E. Haque, J. W. Jun, S. N. Talapaneni, A. Vinu and S. H. Jhung, J. Mater. Chem., 2010, 20, 10801; (b) M. Maes, S. Schouteden, L. Alaerts, D. Depla and D. E. De Vos, Phys. Chem. Chem. Phys., 2011, 13, 5587; (c) M. Maes, F. Vermoortele, L. Alaerts, J. F. M. Denayer and D. E. De Vos, J. Phys. Chem. C, 2010, 115, 1051.
13. K. A. Cychosz and A. J. Matzger, Langmuir, 2010, 26, 17198.
14. (a) S. Achmann, G. Hagen, M. Hämmerle, I. M. Malkowsky, C. Kiener and R. Moos, Chem. Eng. Technol., 2010, 33, 275; (b) G. Blanco-Brieva, J. M. Campos-Martin, S. M. Al-Zahrani and J. L. G. Fierro, Fuel, 2011, 90, 190; (c) K. A. Cychosz, A. G. Wong-Foy and A. J. Matzger, J. Am. Chem. Soc., 2009, 131, 14538.
15. G. Crini, Bioresour. Technol., 2006, 97, 1061.
16. S. Chen, J. Zhang, C. Zhang, Q. Yue, Y. Li and C. Li, Desalination, 2010, 252, 149.
17. A. Mittal, A. Malviya, D. Kaur, J. Mittal and L. Kurup, J. Hazard. Mater., 2007, 148, 229.
18. M. Rafatullah, O. Sulaiman, R. Hashim and A. Ahmad, J. Hazard. Mater., 2010, 177, 70.
19. (a) S.-H. Huo and X.-P. Yan, J. Mater. Chem., 2012, 22, 7449; (b) L. Li, X. L. Liu, H. Y. Geng, B. Hu, G. W. Song and Z. S. Xu, J. Mater. Chem. A, 2013, 1, 10292.
20. Q. Qin, J. Ma and K. Liu, J. Hazard. Mater., 2009, 162, 133.
21. E. Stavitski, M. Goesten, J. Juan-Alcañiz, A. Martinez-Joaristi, P. Serra-Crespo, A. V. Petukhov, J. Gascon and F. Kapteijn, Angew. Chem., Int. Ed., 2011, 50, 9624.
22. P. Serra-Crespo, E. V. Ramos-Fernandez, J. Gascon and F. Kapteijn, Chem. Mater., 2011, 23, 2565.
23. P. Horcajada, T. Chalati, C. Serre, B. Gillet, C. Sebrie, T. Baati, J. F. Eubank, D. Heurtaux, P. Clayette, C. Kreuz, J.-S. Chang, Y. K. Hwang, V. Marsaud, P.-N. Bories, L. Cynober, S. Gil, G. Férey, P. Couvreur and R. Gref, Nat. Mater., 2010, 9, 172.
24. K. M. L. Taylor-Pashow, J. D. Rocca, Z. Xie, S. Tran and W. Lin, J. Am. Chem. Soc., 2009, 131, 14261.
25. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309.
26. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 1951, 73, 373.
27. Y. S. Ho and G. McKay, Process Biochem., 1999, 34, 451.
28. (a) G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé and I. Margiolaki, Science, 2005, 309, 2040; (b) D.-Y. Hong, Y. K. Hwang, C. Serre, G. Férey and J.-S. Chang, Adv. Funct. Mater., 2009, 19, 1537.
29. S. Bauer, C. Serre, T. Devic, P. Horcajada, J. Marrot, G. Férey and N. Stock, Inorg. Chem., 2008, 47, 7568.
30. S. H. Jhung, J.-H. Lee, J. W. Yoon, C. Serre, G. Férey and J.-S. Chang, Adv. Mater., 2007, 19, 121.
31. M. Kandiah, M. H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M. Tilset, C. Larabi, E. A. Quadrelli, F. Bonino and K. P. Lillerud, Chem. Mater., 2010, 22, 6632.
32. B. H. Hameed and A. A. Rahman, J. Hazard. Mater., 2008, 160, 576.
33. (a) S. Altenor, B. Carene, E. Emmanuel, J. Lambert, J.-J. Ehrhardt and S. Gaspard, J. Hazard. Mater., 2009, 165, 1029; (b) O. S. Bello, I. A. Adeogun, J. C. Ajaelu and E. O. Fehintola, Chem. Ecol., 2008, 24, 285; (c) B. H. Hameed, A. T. M. Din and A. L. Ahmad, J. Hazard. Mater., 2007, 141, 819; (d) X. Luo and L. Zhang, J. Hazard. Mater., 2009, 171, 340.
34. Y. Liu, Y. Zheng and A. Wang, J. Environ. Sci., 2010, 22, 486.
35. J.-X. Yu, L.-Y. Wang, R.-A. Chi, Y.-F. Zhang, Z.-G. Xu and J. Guo, Environ. Sci. Pollut. Res., 2013, 20, 543.
36. M. M. A. El-Latif, A. M. Ibrahim and M. F. El-Kady, J. Am. Sci., 2010, 6, 267.
37. B. H. Hameed, J. Hazard. Mater., 2009, 162, 939.
38. B. H. Hameed, J. Hazard. Mater., 2009, 166, 233.
39. T. Liu, Y. Li, Q. Du, J. Sun, Y. Jiao, G. Yang, Z. Wang, Y. Xia, W. Zhang, K. Wang, H. Zhu and D. Wu, Colloids Surf., B, 2012, 90, 197.
40. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361.
41. (a) E. N. El Qada, S. J. Allen and G. M. Walker, Chem. Eng. J., 2008, 135, 174; (b) G. Duman, Y. Onal, C. Okutucu, S. Onenc and J. Yanik, Energy Fuels, 2009, 23, 2197; (c) S. Timur, E. Ikizoglu and J. Yanik, Energy Fuels, 2006, 20, 2636; (d) J. Yamashita, M. Shioya, T. Kikutani and T. Hashimoto, Carbon, 2001, 39, 207.
42. S. Senthilkumaar, P. R. Varadarajan, K. Porkodi and C. V. Subbhuraam, J. Colloid Interface Sci., 2005, 284, 78.
43. J.-x. Yu, B.-h. Li, X.-m. Sun, J. Yuan and R.-a. Chi, Appl. Biochem. Biotechnol., 2010, 160, 1394.
44. F. Raposo, M. A. De La Rubia and R. Borja, J. Hazard. Mater., 2009, 165, 291.
45. G. McKay, J. F. Porter and G. R. Prasad, Water, Air, Soil Pollut., 1999, 114, 423.
46. M. A. Al-Ghouti, M. A. M. Khraisheh, S. J. Allen and M. N. Ahmad, J. Environ. Manage., 2003, 69, 229.
47. S. Eftekhari, A. Habibi-Yangjeh and S. Sohrabnezhad, J. Hazard. Mater., 2010, 178, 349.
48. (a) S. Wang, L. Li, H. Wu and Z. H. Zhu, J. Colloid Interface Sci., 2005, 292, 336; (b) K. P. Singh, D. Mohan, S. Sinha, G. S. Tondon and D. Gosh, Ind. Eng. Chem. Res., 2003, 42, 1965.
49. (a) S. Wang, H. Li and L. Xu, J. Colloid Interface Sci., 2006, 295, 71; (b) Z.-M. Ni, S.-J. Xia, L.-G. Wang, F.-F. Xing and G.-X. Pan, J. Colloid Interface Sci., 2007, 316, 284; (c) A. Bhatnagar, E. Kumar, A. K. Minocha, B.-H. Jeon, H. Song and Y.-C. Seo, Sep. Sci. Technol., 2009, 44, 316.
50. (a) C. Serre, C. Mellot-Draznieks, S. Surblé, N. Audebrand, Y. Filinchuk and G. Férey, Science, 2007, 315, 1828; (b) D. Tanaka, K. Nakagawa, M. Higuchi, S. Horike, Y. Kubota, T. C. Kobayashi, M. Takata and S. Kitagawa, Angew. Chem., Int. Ed., 2008, 47, 3914.
51. P. Waranusantigul, P. Pokethitiyook, M. Kruatrachue and E. S. Upatham, Environ. Pollut., 2003, 125, 385.
52. R. Jiang, Y.-Q. Fu, H.-Y. Zhu, J. Yao and L. Xiao, J. Appl. Polym. Sci., 2012, 125, E540.
53. P. Janoš, H. Buchtová and M. Rýznarová, Water Res., 2003, 37, 4938.

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Footnote

† Electronic supplementary information (ESI) available: Structures and calibration curves for dyes, characterisation of amino-MIL-101(Al) before and after dye exposure, UV-vis spectra and adsorption data, rate data and treatment, adsorption of MB on MIL-101(Al). See DOI: 10.1039/c3ta13589f

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