Synthesis, Characterization and Adsorption Properties for Al-based Metal-organic Framework

1. INTRODUCTION

Over the last several years, the need for an effective and efficient removal of dyes from sewage has become increasingly important because organic dyes in sewage block sunlight in water, thereby reducing the rate of photosynthesis. In addition, some synthetic dyes are toxic and have strongly negative effects on humans and can even cause cancer^[1]. Congo red (CR) is a typical benzidine direct azo dye and its molecular structure is illustrated in Scheme 1. As shown, CR has two unsaturated structured chromophores (azo groups) and four auxochromos (amino and sulfonate groups), and the polysubstituted structure of the azo groups impedes degradation and decolorization. Many methods have been developed to treat sewage, including photocatalytic, electrochemical and microbial oxidation^[2]. Compared with other methods, the absorption method has been applied in many cities as an economical and effective method for treating sewage because it is inexpensive, requires low energy, does not generate secondary pollution, has a long life cycle, and supports regeneration^[3]. The objective of this study is to identify compounds that are able to adsorb CR in practice.

Scheme1

Scheme1.  Molecular structure of Congo red

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Metal-organic frameworks (MOFs), which consist of a three-dimensional network structure of crystal materials, have attracted much attention due to their many potential applications, including organic pollution absorption^[4], chem-sensors^{[5, 6]}, and photocatalysis^[7].

At present, the use of MOFs to remove organic dyes from the environment is an important and timely research topic. For example, MOFs–Cr–BDCs can efficiently absorb methyl orange^[8], MOF-235 has excellent absorption efficiencies for organic dyes MO and MB (477 and 187 mg/L, respectively), higher than that of activated carbon^[9], MIL-100(Fe) and functionalized MOF-100(Fe) can absorb malachite green^[10], and nitrobenzene can be removed from sewage with MIL-53(Al)^[11]. The effective removal of organic dyes in solution via MOF materials is a promising area of research in the field of materials and environmental science. In other applications of MOFs, Al-BTC MOFs were first used as an absorbent for the removal of organic dyes in solution.

In the present study, the MOF material, Al–BTC, was facilely prepared via a solvothermal reaction, and was then characterized by several means, such as Fourier-transform infrared (FTIR) spectroscopy, X-ray diffractometry (XRD), scanning electronic microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The adsorption behavior of Al–BTC MOFs for CR in terms of its adsorption kinetics was found that the adsorption of CR onto the MOF was well described by the pseudo-second-order equation. Al–BTC appears to be a promising material for organic dye adsorption from aqueous solutions. This paper reports the development of a MOF-based adsorbent that can selectively remove CR in aqueous solution.

2. CALCULATION METHODS

2.1 Materials

All reagents in this study, such as aluminum nitrate nonahydrate, 1, 3, 5-benzenetricarboxylic acid (BTC), and CR, were purchased from the Shantou City West Long Chemical Co. All other chemicals were used as received without further treatment. Distilled water was used for all sample preparations and CR adsorption processes.

2.2 Preparation of Al-BTC MOFs

Al-BTC MOFs was prepared through a solvothermal reaction between 3 mmol Al(NO₃)₃·9H₂O and 5 mmol H₃BTC, which was introduced into a solution of 50 mL of N, N-dimethylformamide (DMF). The mixture was stirred gently for 10 min at 25 ℃, placed into a 100 mL Teflon liner, and then into a metallic Parr digestion bomb at 150 ℃ for 20 h. When the mixture returned to 25 ℃, the obtained jelly-like solid was recovered by filtration, washed twice with acetone, and then dried in air at room temperature. Finally, the resulting product was dried overnight at 120 ℃ under vacuum conditions.

2.3 Characterization of the Al-BTC MOFs

FT-IR spectra were recorded in transmission mode from 2000 to 400 cm^-1 on a Nicolet Magna-IR 170 using KBr discs. XRD pattern of the sample was recorded on a Bruker D8 diffractometer with a CuKα ray target (λ = 0.1542 nm) at a target voltage of 40 kV, target current of 40 mA, scanning range 2θ of 10~30°, and a scan rate of 6 °·min^-1. Scanning electron microscopy (SEM) images were captured on a scanning electron microscope (S-4800) at an accelerating voltage of 5 kV.

2.4 Adsorption kinetic experiments

Adsorption experiments were carried out at room temperature. The concentration of CR in the supernatant solution before and after adsorption was determined using a UV spectrophotometer at 502 nm. Exactly, 300 mL of CR solution with a known initial concentration (40, 60, or 80 mg/L) was shaken at a constant agitation speed (200 rpm) with a specific dose of Al-BTC (0.2 g/L). At appropriate time intervals, 5 mL of the solution was removed and centrifuged for 5 min at 4000 rpm to separate the solid particles. The concentration of the CR solution was then obtained using the standard calibration curve.

The amount of dye uptake Q_e (mg/g) was calculated using the following equation:

where C₀ is the initial concentration of the CR solution (mg/L), C_e is the CR concentration after adsorption (mg/L), V is the volume of the solution (mL), and m is the mass of adsorbent (g).

3. RESULTS AND DISCUSSION

3.1 Characterization of the Al-BTC MOFs

The FT-IR spectrum of the Al-BTC MOFs sample is as shown in Fig. 1. The broad absorption bands at 1666 and 1585 cm^-1 were assigned to the asymmetric stretching of the C=O group in BTC^3-, the peaks at 1461 and 1394 cm^-1 were attributed to the symmetric stretching of the C–O group in BTC^3-, and the peaks at 538 and 420 cm^-1 in the lowfrequency region were consistent with the Al–O generated by Al complexation to the carboxyl O atoms.

Figure 1

Figure 1.  IR spectra of the Al-BTC MOFs sample

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The XRD spectrum of the as-prepared Al-BTC MOFs sample is shown in Fig. 2. The phase peak was similar to that of the compound reported^[12]. However, the structure was not conclusively determined and may have contained mixed (MIL-100(Al)^[13] and MIL-96(Al)^[14]) or unknown phases. SEM images of the as-prepared Al-BTC MOFs are shown in Fig. 3. As shown in Fig. 3a, the Al-BTC MOFs exhibited massive, particle sizes, while it can be seen in Fig. 3b that the surface of Al-BTC MOFs consists of small particles. The morphology of Al-BTC MOFs is composed of a non-uniform particle size, as shown in the figures, and it can be seen that the inner block was porous and could therefore be used for dye adsorption.

Figure 2

Figure 2.  Pattern of Al-BTC MOFs (a), simulation of MIL-100(Al) (b) and MIL-96(Al) (c)

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Figure 3

Figure 3.  SEM images of Al-BTC MOFs

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3.2 Effect of the initial dye concentration

As shown in Fig. 4, the amount of adsorption changed over time, so did the Al-BTC MOFs sorbent adsorption of different initial concentrations of CR solution. (1) The amount of CR adsorbed by the Al-BTC MOFs increased as the concentration of the Congo red solution increased. The initial concentrations of the Congo red solution were 40, 60, and 80 mg/L, and the maximum amount adsorbed by Al–BTC MOFs was 138.82, 217.99, and 301.05 mg/g, respectively. (2) Thirty minutes prior to the reaction, the rate of adsorption of the Congo red solution decreased within 30~60 min, the time of the reaction was 180 min, and the adsorption equilibrium eventually reached the original concentration of Congo red. The number of adsorption sites on Al–BTC MOFs increased, and due to adsorption, the concentration of the Congo red solution was gradually reduced and the adsorption sites on Al–BTC MOFs were gradually occupied by the Congo red, which hindered the reaction. This is why the adsorption rate began to decrease until it eventually reached the adsorption equilibrium. This is consistent with the results reported by Wang et al., which indicated MFe₂O₄ (M = Mn, Fe, Co, or Ni) had similar absorption results for Congo red^[15].

Figure 4

Figure 4.  Effect of the contact time and initial concentration on the adsorption of CR on Al-BTC MOFs

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3.3 Adsorption kinetics

To describe the adsorption mechanism of the sorption of CR onto Al-BTC MOFs from a liquid solution, two common kinetic models, namely, the pseudo-first-order and pseudosecond-order models, were used to investigate the adsorption rate of the reaction system.

The pseudo-first-order model can be expressed by the following linear form^[16]:

Where Q_e and Q_t (mg/g) are respectively the amounts of adsorbed CR at equilibrium at time t, k₁ (1/min) is the pseudo-first-order rate constant, and t (min) is the time. The k₁ value in linear form can be calculated from the slope of the plot of ln (Q_e – Q_t) versus t.

The rate of a pseudo-second-order reaction is mainly dependent on the amount of solute adsorbed on the surface of adsorbent and the amount adsorbed at equilibrium.

The pseudo-second-order model^[17] is as follows:

Where k₂ (g/(mg·min)) represents the pseudo-second-order rate constant. The values of k₂ and Q_e can be experimentally determined from the intercept and slope of the plot of t/Q_t versus t.

Linear plots of the pseudo-first-order and pseudo-seconorder kinetic models at three initial concentrations of 40, 60, and 80 mg·L^-1 are shown in Fig. 5. The corresponding kinetic parameters of k₁, k₂, Q_{e1, cal}, Q_{e2, cal}, and R² obtained from the two models are listed in Table 1. From Fig. 5, it can be seen that the pseudo-first-order kinetic curves do not fit well with the experimental kinetic data as the corresponding correlation coefficients for the three different concentrations were low. In contrast, the linear plots of t/Q_t versus t resulted in higher correlation coefficients that were close to unity, which indicates that the adsorption kinetics of CR onto the Al–BTC MOFs was consistent with the pseudo-second-order kinetic model. Based on the deviation between the calculated and experimental Q_e values listed in Table 1, it can be seen that there were only minor deviations between these values for the former model while significant differences for the latter one. Thus, the pseudo-second-order kinetic model was found to be more accurate when describing the CR adsorption process over Al-BTC MOFs in solution.

Figure 5

Figure 5.  Linear plots of pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model

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Table 1

Table 1.  Pseudo-first (second)-order Rate Constants at Different Initial CR Concentrations

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[CR](mg/L)	Qe, exp(mg/g)	Pseudo-first-order model				Pseudo-second-order model
		k1 (1/min)	Qe1, cal (mg/g)	R2		k2 (g/(mg·min))	Qe2, cal (mg/g)	R2
40	137.64	0.0171	25.174	0.6329		2.749 × 10-3	139.470	0.9999
60	216.36	0.0239	84.800	0.9563		8.723 × 10-4	221.239	0.9999
80	296.35	0.0253	214.935	0.9519		2.498 × 10-4	312.500	0.9997

4. CONCLUSION

Al-BTC MOFs material was successfully prepared via a facile solvothermal reaction from Al(NO₃)₃ and H₃BTC in a DMF solution, and characterized by FTIR, XRD, SEM and XPS. The MOFs showed high adsorption capacity for CR from an aqueous solution. The adsorption kinetics was evaluated using the pseudo-first-order and pseudo-secondorder equations, in which the adsorption of CR onto the as-prepared MOFs could be best described by the pseudosecond-order equation. The as-prepared MOFs adsorbent seems to be a promising material in practice for dyes removal from aqueous solution.

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