MIL-53(Al): An Efficient Adsorbent for the Removal of Nitrobenzene

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MIL-53(Al): An Efficient Adsorbent for the Removal of Nitrobenzene from Aqueous Solutions Dinesh V. Patil, Phani B. Somayajulu Rallapalli, Ganga P. Dangi, Rajesh J. Tayade, Rajesh S. Somani,* and Hari C. Bajaj* Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Gijubhai Badheka Marg, Bhavnagar 364002, Gujarat, India ABSTRACT: MIL-53(Al), hydrothermally synthesized and purified by solvent extraction, was used as adsorbent for the removal of nitrobenzene from aqueous solution. Pristine MIL-53(Al) and MIL-53(Al) loaded with various amounts of nitrobenzene were characterized by X-ray diffraction analysis with cell indexation study, thermogravimetric analysis, Fourier transform infrared spectroscopy, and BET surface area. A simulation study of nitrobenzene adsorption on MIL-53(Al) was performed. The adsorption study of nitrobenzene on MIL-53(Al) was carried out at 30 ( 1 °C using batch experiments. The amount of nitrobenzene adsorbed decreases with an increase in the temperature from 30° to 60 °C and pH from 8 to 11, whereas no significant difference was observed in acidic pH. The adsorption data were fitted to Sips and RedlichPeterson isotherm models. The adsorption capacity of nitrobenzene on MIL-53(Al) obtained was 610 mg/g, higher than that of zeolites (267.2 mg/g) and organoclays (100 mg/g), but, lower than that of modified commercial activated carbons (1443.53 mg/g).

1. INTRODUCTION Metalorganic framework (MOFs) made up of metal ions connected by organic linkers, formed a three-dimensional (3D) porous framework with a 1-D, 2-D, and 3-D channel system.13 Because of their diverse properties such as higher surface area,4,5 uniform but tunable pore size,6,7 and functionalizable pore walls,1,810 the MOFs are considered as potential candidates for gas separation, gas storage, catalysis, ion exchange, microelectronics, and health care applications.11 The rational and pragmatic approach to the selection of organic linkers and metals of suitable coordination in order to get the desire pore size make these MOFs a versatile material. The disadvantages associated with the MOFs are their low thermal and chemical stability and sensitivity to moisture. The metal organic frameworks designated as MIL-n (Materials of Institute Lavoisier) used for different gas adsorption studies are promising candidates for H2, CH4, and CO2 adsorption.1214 A noticeable feature of MIL-53(Al) is its remarkable thermal stability up to 500 °C compared to other MOFs, which are stable below 400 °C. Owing to their ability of “breathing” upon adsorption of water and CO2, these materials have attracted much attention3 as MIL-53(Al or Cr) can adjust its cell volume in a reversible manner to optimize the interactions between guest molecules and framework, with no evidence of bond breaking. A mechanism of nitrobenzene adsorption on MOF material has been reported in literature and is based on the quenching effect of nitrobenzene on luminescence intensities.15,16 Such a quenching effect has been attributed to the charge transfer from the benzene ring of benzenedicarboxylate ligands to the nitrobenzene as the electron withdrawing NO2 group makes nitrobenzene an electron deficient molecule. In addition the flat structure of nitrobenzene strongly favors the ππ interaction between nitrobenzene and the MOF framework. r 2011 American Chemical Society

Activated carbons have been widely used for the removal of organic and inorganic pollutants such as benzoic acid, nitrobenzene, pyridine, and copper ions for the purification of water;1722 however, regeneration of activated carbon is difficult and expensive.23 Alternative adsorbents such as zeolites and organoclays have also been investigated.2428 As compared to zeolites, organoclays, and activated carbons, MOFs have higher surface area, porosity, crystallinity and pore volume and their use for the removal of organic pollutants for the purification of water has not been much explored. Pollution due to nitrobenzene has become a global toxicological concern, as nitrobenzene has been nominated by the National Institute of Environmental Health Sciences for listing in the Report on Carcinogens.29 The U.S. EPA has surveyed nitrobenzene levels in effluents from 4000 publicly owned treatment works and industrial sites. The highest concentrations of nitrobenzene in effluent are associated with wastewaters from the organics and plastics industries, with some reported levels exceeding 100 ppm.30 The main source of nitrobenzene in the environment is from the industries producing nitrobenzene or using it to produce other products like pesticides, drugs, dyes, synthetic rubber, and lubricating oil.29 Direct contact of a small amount of nitrobenzene with eyes and skin may cause mild irritation, whereas, repeated exposures to high concentration can result in the condition methemoglobinemia which affects the oxygen carrying ability of blood.29 Nitrobenzene was reported to be highly resistant to degradation or to inhibit the biodegradation of other pollutants; however, these effects were observed at concentration (g50 mg/L) of nitrobenzene, much higher than those detected in ambient waters.31 Received: March 3, 2011 Accepted: August 10, 2011 Revised: August 10, 2011 Published: August 10, 2011 10516

dx.doi.org/10.1021/ie200429f | Ind. Eng. Chem. Res. 2011, 50, 10516–10524

Industrial & Engineering Chemistry Research For the treatment of aqueous nitrobenzene, biodegradation, ozone/UV advance oxidation process, UV/Fe (III)-enhanced ozonation process, and degradation in presence of TiO2 have been reported.31 The present study discusses the adsorption of nitrobenzene on hydrothermally synthesized MIL-53(Al). The Sips and Redlich Peterson equilibrium isotherm equations were used to test their validity for the experimental equilibrium sorption data. The kinetics of adsorption and the effect of pH as well as temperature on adsorption process were examined. Finally, desorption of nitrobenzene by MIL-53(Al) was studied to determine reversibility of adsorption.

2. MATERIALS AND METHODS 2.1. Materials. 1,4-Benzene dicarboxylic acid (purity 98%), aluminum nitrate, Al(NO3)3 3 9H2O (purity 98.5%), nitrobenzene (purity 99%), and N,N-dimethylformamide (purity 99%) were purchased from S.D. Fine Chemicals, India, and were used without any further purification. Methanol (purity 99.8%) was purchased from Nice Chemicals Pvt. Ltd., India, and used as received. The aqueous solution of nitrobenzene was prepared by dissolving nitrobenzene in deionized water without pH adjustment, over the range of concentration studied. 2.2. Synthesis of MIL-53(Al). MIL-53(Al) was synthesized by the hydrothermal method as reported by Loiseau et al.3 A 13 g aliquot of aluminum nitrate (Al(NO3)3 3 9H2O) and 2 3 88 g of 1,4-benzenedicarboxylic acid (BDC) in 50 mL of water was autoclaved at 220 °C for 72 h. The product was filtered and washed with water to ensure the removal of nitric acid formed during the synthesis. It was then purified by a solvent extraction method,32,33 using N,N-dimethylformamide (DMF) to remove the unreacted BDC, and dried in vacuum for 2 h. Further it was treated with methanol in order to replace the DMF molecules trapped inside the cavities of the product. Finally, it was filtered, washed with methanol, and dried in an air oven at 80 °C for 2 h. 2.3. Characterization of MIL-53(Al) Adsorbent. Powder X-ray diffraction analysis was carried out using Philips X0 pert MPD system in the 2θ range of 550° using Cu KR1 (λ = 1.54056 Å). The cell parameters of MIL-53(Al) and nitrobenzene loaded MIL-53(Al) were deduced using a Treor (trial and error) method using X0 pert Highscore Plus software (version 2.2.3) with scaning rate of 0.02 degree per 4 s. The Fourier transform infrared (FTIR) spectroscopy of the MIL-53(Al) before and after adsorption of nitrobenzene was performed on PerkinElmer spectrum GX FT-IR instrument. The samples were pelletized using KBr, and the spectrum was recorded in the range of 4004000 cm1 with a resolution of 4 cm1. Thermogravimetric analysis was carried out from 30 to 600 °C (TGA/DTA analyzer (Mettler Toledo) under argon atmosphere at the heating rate of 10 °C/min. The surface area of the purified MIL53(Al) sample was measured on a static volumetric adsorption system (Micromeritics Instrument corporation, USA, modelASAP 2010), obtaining N2 adsorption/desorption isotherms at 77.4 K up to 1 bar pressure. Prior to the adsorption measurement the sample was degassed overnight under vacuum (5 103 mmHg) at 200 °C. The C, H, N analysis study was carried out using a CHNS/O analyzer, Perkin-Elmer, series 2400. 2.4. Kinetic Studies of Nitrobenzene Adsorption. The effect of contact time on the uptake of nitrobenzene on MIL53(Al) was investigated at 30 ( 1 °C. The kinetic experiments were studied using two different initial concentration of nitrobenzene

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(50 and 250 mg/L) and 25 mg of MIL-53(Al) by taking 250 mL of nitrobenzene solution of known concentration in stoppered conical flasks. The mixture was stirred at 400 rpm. At predecided time intervals, the sample from each flask was withdrawn, centrifuged, and analyzed for the nitrobenzene concentration using Shimadzu-2550 (UVvis spectrophotometer at λmax = 267 nm). The amount of nitrobenzene adsorbed on the MIL53(Al) was calculated using eq 1. Q e ¼ ðC0 Ce ÞV =m

ð1Þ

Where Qe = quantity of nitrobenzene adsorbed on the MIL53(Al) (mg/g); C0 = initial concentration of nitrobenzene in aqueous solution (mg/L); Ce = equilibrium concentration of nitrobenzene in aqueous solution (mg/L); V = volume of the solution (L); m = mass of the adsorbent (g). 2.5. Nitrobenzene Adsorption Studies. For the adsorption of nitrobenzene on the MIL-53(Al), batch type experiments were conducted in 500 mL stoppered round-bottom flasks at 30 ( 1 °C by taking 250 mL of nitrobenzene solution of varying concentrations (10 to 250 mg/L) and 25 mg of MIL-53(Al) under stirring (400 rpm). The solution from the mixture was withdrawn after 30 min interval (kinetic study confirmed that the saturation reaches maximum within 20 min, vide infra). The solutions collected after each experiment were centrifuged and analyzed for nitrobenzene using a UV spectrophotometer. The maximum adsorption capacity observed was 610 ( 10 mg/g at 30 °C. 2.6. Molecular Simulation Methods and Models. Conventional grand canonical Monte Carlo (GCMC) simulations were performed for nitrobenzene adsorption in MIL-53ht (Al)3 (high temperature form) to obtain adsorption isotherms, which relate the loading (i.e., the weight fraction of adsorbate in the adsorbate/adsorbent system) to the bulk pressure of the adsorbate gas in equilibrium with the adsorbent. Using GCMC simulation we also calculated the isosteric heat of adsorption and the adsorption site of nitrobenzene in MIL-53(Al) pores. The initial atomic coordinates of the hybrid porous framework were taken directly from the refined structure obtained by X-ray diffraction.3 H atoms were added to the organic groups and to the μ2 position, using the H-adding facility in the Accelrys Materials Studio Visualizer software.34 The framework structure was energy minimized by using the energy minimization option in the Cerius2 software.35 For GCMC simulation we need force field and atomic charges as the input parameters. The atomic partial charges (Mullikan charges) of the hybrid porous framework were taken from the work of Ramsahye et al.36 The nitrobenzene molecule geometry was optimized by DFT method. The Accelrys DMol3 code was used for these calculations, performed using the PW91 GGA density functional and the double numerical plus polarization (DNP) basis set. The partial charges for the atoms in the nitrobenzene model were extracted using the Mullikan charge partitioning method. The adsorbateadsorbate and adsorbateadsorbent interactions were modeled by using a repulsion-dispersion 12-6 Lennard-Jones (LJ) potential and a Coulombic contribution. For GCMC simulation we used universal forcefield (UFF) as implemented in the Cerius2 software. The simulations were performed using the grand canonical Monte Carlo implementation available in the Accelrys Cerius2 software package. These calculations were performed at 303 K using 16 unit cells (4 2 2) of MIL-53ht (Al) with typically 1 107 Monte Carlo steps, the framework structure being considered rigid. The Ewald summation method was used for 10517

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calculating electrostatic interactions and the short-range interactions were calculated with a cutoff distance of 12 Å.

3. RESULT AND DISCUSSIONS 3.1. X-ray Powder Diffraction Analysis. The X-ray powder diffraction pattern of MIL-53(Al) and MIL-53(Al) loaded with different nitrobenzene (NB) amount is shown in Figure 1. The XRD pattern confirms that the synthesized material is MIL53(Al). The diffraction pattern clearly indicated that the material is well crystalline, and the peaks obtained are in good agreement with the literature pattern3 of MIL-53(Al) framework which exists in room temperature (MIL-53lt (Al), monoclinic Cc) and high temperature (MIL-53ht (Al) Imma orthorhombic) phases. There was no change in the crystallinity level and peak position of the PXRD in the case of nitrobenzene loaded MIL-53(Al) or MIL-53(Al) recovered after the experiment was performed at pH 11, with all samples having a triclinic phase. The cell parameter data after the loading of different amounts of nitrobenzene and for a bare sample are shown in Table 1. The MIL-53lt (Al) in the monoclinic phase is well matched with the reported phase having a cell volume of 1107.21 Å3. The NB adsorbed MIL-53(Al) has a triclinic phase with a cell volume of 839.87 (for MIL-53-pH-11, 14.5 mg/75 mg adsorbed NB), 1256.76 (for MIL-53, 24 mg/ 75 mg adsorbed NB), and 1097.54 Å3 (for MIL-53, 36 mg/75 mg adsorbed NB). The cell volume of the MIL-53(Al) decreased from 1107.21 to 839.87 Å3 after loading 14.5 mg/75 mg. As the NB loading increased from 14.5 to 24 mg/75 mg, the cell volume increased to 1256.76 Å3, higher than that of the bare material. Further, an increase in NB loading to 36 mg/75 mg led to a decrease in the cell volume to 1097.54. These values suggested the breathing nature of MIL-53(Al) upon nitrobenzene adsorption; however literature has been cited for such a breathing phenomenon due to the hostguest interaction.3,37,38 When a lesser amount of NB adsorbed due to the interactions of organic linker, a shrinkage of cells of the MIL-53(Al) framework occurred, resulting in the decrease of its cell volume. As the amount of NB adsorption increased the cell again reopened and expanded which resulted in the increment of its cell volume. Further adsorption of NB occurred due to pore filling which resulted in a decrease of its cell volume. The cell volume of MIL-5324 mg-NB is higher than that of bare MIL-53lt (Al) because of the flexible nature of the MIL-53(Al) framework. While the amount of NB adsorption increased, the cell volume decrease for samples MIL-5336 mgNB may be due to the excess loading of NB. 3.2. Thermogravimetric Analysis. The thermal stability of the purified MIL-53(Al) analyzed by thermogravimetric analysis from 30 to 600 °C (Figure 2) depicts the initial weight loss at 100 °C due to the dehydration process and corresponds to the removal of water molecules. The second weight loss at 500 °C corresponds to the collapse of MIL-53(Al) framework, indicating that the MIL-53(Al) framework is thermally stable up to 500 °C. 3.3. Fourier Transforms Infrared (FTIR) Spectroscopy. The FTIR spectra of MIL-53(Al) depicted the bands at 1608 and 1512 cm1 corresponding to the asymmetric stretching of the COO group, whereas bands at 1435 and 1417 cm1 correspond to the symmetric stretching of the COO group (Figure 3). The spectra of MIL-53(Al) with adsorbed nitrobenzene showed two additional vibrational bands at 1524 cm1 (asymmetric stretching of the aromatic NO2 group) and at 1345 cm1 due to the symmetric stretching of aromatic NO2 group, whereas the band observed at 1345 cm1 corresponds to the symmetric

Figure 1. XRD pattern of the purified MIL-53(Al) and MIL-53(Al) loaded with different amounts of nitrobenzene and the MIL-53(Al) recovered from the experiment carried out at pH-11.

stretching of the aromatic NO2 group. These values clearly indicate the inclusion of nitrobenzene within the MIL-53(Al) framework. 3.4. BET Surface Area Measurement. The surface area measurement on MIL-53(Al) using N2 adsorptiondesorption method exhibited Type-I isotherm (Figure 4), characteristic of microporous materials. The surface area calculated from N2 adsorptiondesorption data at 77.4 K using BET equations was found to be 1235 m2/g, which is little higher compared to literature values3 1140 m2/g. The increment in surface area may be due to the change in activation protocol of MIL-53(Al).32,33 3.5. Effect of pH on Adsorption. The adsorption of nitrobenzene was studied over a wide pH range of 211, as pH affects the adsorption process and may also affect the structural stability of MIL-53(Al). The pH of the solution was adjusted either by 0.1 N HCl or NaOH solution. The adsorption of nitrobenzene was 610 ( 10 mg/g at 30 °C, in the pH range of 26 (at pH 2, 551.65 mg/g; pH 4, 550.69 mg/g; pH 6, 545 mg/g) (Figure 5). No significant effect of pH was observed on the adsorption capacity of nitrobenzene as MIL-53(Al) is stable in acidic conditions. However, as the pH was increased from 8 to 11 the adsorption capacity decreased: at pH 8, 525.14 mg/g; pH 10, 498.92 mg/g. At pH 11, 192.79 mg/g capacity was observed. The adsorption of nitrobenzene in acidic medium (pH = 26) was not affected as compared to that in neutral conditions, but its adsorption drastically decreased in basic medium (pH = 811). In the framework of MIL-53(Al) the metal center acts as hydrophilic center and the benzene ring in the organic linker acts as hydrophobic center. According to Ronen Zangi et al,39 the hydroxide ions are physically adsorbed at the water/hydrophobic interface. The driving force for the adsorption is the preferential orientation of the water molecules in the first two layers away from the hydrophobic surface. There exists a preferential orientation that generates an alternating net charge distribution along the surface normal. The interaction energy of this electrical potential gradient with the permanent dipole moment of the hydroxide ion renders the adsorption process possible. The simulation studies also revealed that the adsorption takes place due to the ππ stacking interactions between the nitrobenzene molecules and the organic linker, that is, on the hydrophobic surface of the 10518



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Table 1. Cell Indexation Parameters of MIL-53lt (Al), MIL-53(Al) Loaded with Different Amounts of Nitrobenzene, and MIL53(Al) Recovered from Experiment Performed at pH = 11 samples MIL-53lt (Al)

MIL-5324 mg NB

MIL-5336 mg NB

MIL-53-pH-11

unit cell volume (Å3)

unit cell parameters a = 10.53 Å

b = 16.89 Å

c = 6.398 Å

R = 90°

β =103.39°

γ = 90°

a = 8.915 Å

b = 9.888 Å

c = 17.012 Å

R = 65.198°

β = 70.626°

γ = 71.589°

a = 6.37 Å

b = 11.19 Å

c = 18.628 Å

R = 62.83°

β = 94.25°

γ = 111.03°

a = 6.153 Å R = 60.45°

b = 9.171 Å β = 82.27°

c = 17.267 Å γ = 86.91°

Figure 2. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) plot of MIL-53(Al).

material. In basic pH conditions active sites for nitrobenzene adsorption (the benzene rings of linker) were occupied by hydroxide ions which prevent the further adsorption of nitrobenzene. The maximum adsorption capacity was observed at neutral pH, hence further studies were performed at pH 7.2. 3.6. Effect of Temperature on Adsorption. The effect of temperature on nitrobenzene adsorption was studied at 30, 40, 50, and 60 °C with 100 mL aqueous solution containing 250 mg/L nitrobenzene and 25 mg of MIL-53(Al) in the batch experiments. No significant difference in the equilibrium adsorption of nitrobenzene from 30 to 40 °C was observed. However above 50 °C there was a gradual decrease in adsorption capacity (Figure 6). The increase in temperature weakens the interaction between the adsorbate and adsorbent molecules. Moreover, adsorption being exothermic, the adsorbate has a tendency to desorb from solid phase to solution. Thus, increase in temperature results in the decrease of adsorption of nitrobenzene. 3.7. Nitrobenzene Adsorption Kinetic Study. The kinetic studies revealed that the adsorption process takes place very fast and 60% of the nitrobenzene gets adsorbed within 1 min of contact time, whereas the saturation time for nitrobenzene was 20 min. There was no significant change in concentration of nitrobenzene after 20 min of contact time.

1107.21

1256.76

1097.54

839.87

Figure 3. FTIR spectra of MIL-53(Al) before and after adsorption of nitrobenzene.

The kinetics of the nitrobenzene adsorption on the MIL53(Al) has been analyzed by pseudo-first-order and pseudosecond-order equations.40 The pseudo-first-order equation relates the adsorption rates to the amount of nitrobenzene adsorbed at time “t” (eq 2 and 3): dQ t =dt ¼ k1 ðQ e Q t Þ

ð2Þ

lnðQ e Q t Þ ¼ ln Q e k1 t

ð3Þ

where Qe and Qt are adsorbed amount of nitrobenzene at equilibrium and time t, respectively, expressed in mg per gram; k1 is a pseudo-first-order kinetic constant expressed in min1. The pseudo-second-order equation may be written as eq 4 and 5: dQ t =dt ¼ k2 ðQ e Q t Þ2

ð4Þ

t=Q t ¼ 1=k2 Q e 2 þ t=Q e

ð5Þ

where k2 (g mg constant. 10519

1

1

min ) is the pseudo-second-order rate



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Figure 4. N2 adsorption/desorption isotherms of MIL-53(Al) at 77.4 K.

The pseudo-first-order equation was not found suitable to describe the kinetic study of the nitrobenzene adsorption over MIL-53(Al); however, the adsorption data was better fitted to pseudo-second-order equation (Figure 7) indicating that the sorption process depends on adsorbent and adsorbate. The data for second-order rate constants are given in Table 2. 3.8. Equilibrium Adsorption studies. The isotherm models of Sips (eq 6) and RedlichPeterson (eq 7) were used to describe the equilibrium adsorption.40 Sips Equation: Qe ¼

Q m K s Ce 1=n 1 þ K s Ce 1=n

Figure 5. Effect of pH on equilibrium adsorption of nitrobenzene on MIL-53(Al) (1% error bar; adsorption conditions: T = 30 ( 1 °C, Co = 250 mg/L, V = 0.100 L, m = 0.025 g).

ð6Þ

where Ks (g/L)1/n is the Sips constant related with affinity and Qm (mg/g) is the Sips maximum adsorption capacity. RedlichPeterson Equation: Q e ¼ K RP Ce =1 þ RRP Ce β

ð7Þ β

where KRP (L/g) and RRP (mg/L) are RedlichPeterson constants and β is the RedlichPeterson exponent (dimensionless). The results obtained showed the applicability of the models over a wide range of concentration (Figure 8). Both Sips and RedlichPeterson models were found suitable to estimate the model parameters; however, the Sips model fit slightly better (as evident from correlation coefficient). The values obtained for the Sips and RedlichPeterson constants are shown in Table 3. 3.9. Molecular Simulation Study. The adsorption isotherm of nitrobenzene in MIL-53ht (Al) at 303 K and varied pressure range were computed from GCMC simulation and nitrobenzene adsorption on MIL-53(Al) from aqueous solution at 303 K (Figure 9A,B). The isotherm obtained from GCMC simulation depicted that the loading of nitrobenzene remains constant with increasing pressure. Even at very low pressure (1 103 kPa) nitrobenzene is adsorbed on MIL-53ht (Al), demonstrating its use for the sensing of nitrobenzene. The molecular graphics snapshot of nitrobenzene adsorbed in MIL-53(Al) is shown in Figure 10. From the snapshots it is clear that nitrobenzene molecules are adsorbed in the pores of MIL-53(Al) framework

Figure 6. Effect of temperature on nitrobenzene equilibrium adsorption on MIL-53(Al) (0.5% error bar; adsorption conditions: Co = 250 mg/L, V = 0.100 L, m = 0.025 g, pH = 7.2).

in such a fashion that there is a stacking between the nitrobenzene molecules and 1,4-benzenedicarboxylate linker of the framework. The observed high isosteric heat of adsorption (110.8 kJ mol1) may be due to the ππ stacking interactions between the nitrobenzene molecules and the organic linker. The high adsorption energy of nitrobenzene suggests that this material may be suitable for the detection of low levels of nitrobenzene as well as for the removal of nitrobenzene from the polluted water. 3.10. Regeneration of Adsorbent. Recovery of the adsorbate as well as regeneration of the adsorbent is an important process in the wastewater treatment; hence, regeneration of the MIL53(Al) was attempted using methanol for desorption of nitrobenzene. The regeneration of the MIL-53(Al) was studied using 25 mg of adsorbent with 50 mL (250 mg/L) solution of nitrobenzene. The desorption of nitrobenzene was carried out by mixing 25 mg of MIL-53(Al) loaded with 11.94 mg of nitrobenzene and 50 mL of methanol in a conical flask. The mixture was stirred at 10520



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Figure 7. Kinetic study of nitrobenzene adsorption on MIL-53(Al): (a) effect of contact time and (b) pseudo-second-order plot (adsorption conditions: T = 30 ( 1 °C, Co = 50 and 250 mg/L, V = 0.250 L, m = 0.025 g, pH = 7.2).

Table 2. Parameters of Kinetics of Nitrobenzene Adsorption on MIL-53(Al) K2 (g mg1 min1)

C0 (mg/dm3)

R2

50

0.0020

0.9999

250

0.4375

0.9999

Figure 8. Isotherms of nitrobenzene adsorption from aqueous solution on MIL-53(Al) obtained at 30 ( 1 °C (adsorption conditions: t = 30 min, V = 0.250 L, m = 0.025 g, pH = 7.2).

Table 3. Model Parameters for the Adsorption of Nitrobenzene on MIL-53(Al) isotherm models Sips

RedlichPeterson

Qmax (mg/g)

625

KRP (L/g)

Ks (g/L)1/n

14.87

β

1.35

R2

1.0

R2

0.9999

1/n

0.5

Figure 9. (A) Simulated isotherm of nitrobenzene (vapor) on MIL53ht (Al) up to 102 kPa at 303 K. (B) Adsorption isotherm of nitrobenzene (from aqueous phase) on MIL-53(Al) at 303 K.

11.625

400 rpm for 1 h. After 1 h, 5 mL of solution was taken out, centrifuged, and analyzed for nitrobenzene concentration. About 99%

of nitrobenzene was desorbed. Methanol was chosen as it has been used for replacement of guest molecules trapped inside the pores of MIL-53(Al).32 Moreover, because of the large difference in boiling point of nitrobenzene (210.9 °C) and methanol (64.7 °C) nitrobenzene can be easily recovered. The regenerated adsorbent was separated from the solution by centrifugation and dried in an 10521



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oven at 70 °C to remove the trapped methanol inside the cavities of MIL-53(Al) and was again used for the adsorption of nitrobenzene up to three cycles. In the second and third cycles desorption was found to be 98.15 and 98.68%, respectively (Table 4). The nitrobenzene adsorption capacity of MIL-53(Al) is six times higher than that of the organoclays, whereas two times compared to that of faujasite zeolite (Table 5). Compared to that of modified activated carbon it is lower, whereas it is comparable to that of activated carbon(S-23).

3.11. Elemental Analysis. The C, H, N, analysis of MIL53(Al) and MIL-53(Al) loaded with various amounts of nitrobenzene was carried out using CHNS/O, Perkin Elmer, series II, 2400. The MIL-53(Al) (75 mg) sample was loaded with various amounts of nitrobenzene such as 24, 36, and 41 mg using 250 mg/L concentration of solution. The percentages of C, H, and N were found to increase gradually with the loading of nitrobenzene (Table 6), whereas in the case of the experiment performed at pH 11, the percentages of C, H, and N were found to be less compared to that of all the nitrobenzene loaded samples due to the lesser adsorption of nitrobenzene in basic medium. The percentage of hydrogen was slightly higher in the bare MIL53(Al) compare to that of the NB loaded sample, which may be due to the presence of adsorbed water molecules as there is remote change of adsorption for water molecules due to the presence of nitrobenzene in a MIL-53(Al) cavity. 3.12. Surface Area and Nitrobenzene Adsorption Uptake. There was no correlation between the nitrobenzene uptake and the surface area of the materials (Table 5), that is, the surface area of the material is not the main criteria for its uptake of nitrobenzene. The surface area and nitrobenzene uptake of MIL-53(Al), activated carbon (S-23),41 and modified activated carbon18 are 1235, 1250, 926 m2/g and 610, 550.8, 1443.53 mg/g, respectively. The high uptake in modified activated carbon is mainly due to the pH point of zero charge (pHpzc), number of surface oxygen groups, and good development of mesopores along with presence of micropores.18 It has been established that for the adsorption of organics molecules in activated carbon the presence of mesopores along with micropores enhances their adsorption capacity especially for large molecules. The increased number of mesopores can reduce the length of the diffusion path to the micropores, accelerate the pore diffusion of nitrobenzene molecules, and decrease the resistance to diffusion.18 The NB adsorption capacity of MIL-53(Al) is lower than that of activated carbon although its surface area is higher; this may be due to nonexistence of mesopores in MIL-53(Al).

Figure 10. Snapshots of nitrobenzene configuration in MIL-53ht (Al).

Table 4. Reuse of MIL-53(Al) up to Three Cycles MIL-53(Al) Reuse

nitrobenzene adsorbed (mg/25 mg)

% desorption

I-cycle

11.94

99.04

II-cycle

11.31

98.15

III-cycle

10.43

98.68

4. CONCLUSION The present study revealed the suitability of MIL-53(Al) as an adsorbent for the removal of nitrobenzene from the aqueous solution. The kinetics of nitrobenzene adsorption over MIL-53(Al) can be described by a pseudo-second-order equation. The kinetic

Table 5. Comparison of Maximum Adsorption Capacities of Nitrobenzene with Other Porous Materials material

maximum adsorption capacity (mg/g)

surface area (m2/g)

modified activated carbon

1443.53

926.4

Liu et al.18

activated carbon(S-23) faujasite

550.8 267.2

1250 750

Reungoat et al.41 Reungoat et al.41

organoclays

100

5.12

Patel et al.28

MIL-53(Al)

610

1235

this study

references

Table 6. Elemental Analysis of MIL-53lt (Al), MIL-53(Al) Loaded with Different Nitrobenzene Amounts, and MIL-53(Al) Recovered from the Experiment Performed at pH 11. sample

% C, calculated (observed)

% H, calculated (observed)

MIL-53

46.15 (43.49)

2.40 (2.81)

00, (0.25)

MIL-5324 mg NB

49.17 (49.91)

2.81 (2.47)

2.76, (3.07)

MIL-5336 mg NB MIL-5341 mg NB

50.12 (50.77) 50.53 (51.35)

2.94 (2.56) 2.99 (2.59)

3.69, (3.48) 4.02 (4.16)

MIL-53-pH-11 (14.5 mg NB)

48.15 (46.95)

2.67. (2.20)

1.84, (2.94)

10522

% N, calculated (observed)


Industrial & Engineering Chemistry Research study showed that, equilibrium is reached within 20 min, whereas 60% of the nitrobenzene was adsorbed within a minute. No significant effect of acidic pH was observed on equilibrium adsorption, but adsorption capacity decreased above pH = 8, whereas equilibrium adsorption capacity decreases above 50 °C. The cell indexation study of MIL-53(Al) loaded with various amounts of nitrobenzene described the breathing nature of MIL-53(Al). The Sips and RedlichPeterson models were used to estimate the model parameter. Regeneration of the adsorbent was successfully carried out up to three cycles using methanol for desorption of nitrobenzene.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +91-0278-2567760, 2471793. Fax: +91-0278-2567562. E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT We are thankful to Council of Scientific and Industrial Research (CSIR) for a senior research fellowship (to Phani B. S. Rallapalli) and funding under Network Project: NWP 0010. The authors wish to thank the analytical science discipline, especially Pragnya Bhatt, CSMCRI, for their technical assistance in instrumental analysis. The authors also wish to thank Thillai Siva Kumar and Manu V. for their help in carrying out experimental work. ’ ABBREVIATION Qe = quantity of nitrobenzene adsorbed at equilibrium (mg/g) C0 = initial concentration of nitrobenzene (mg/L) Ce = equilibrium concentration of nitrobenzene (mg/L) V = volume of the solution (L) m = mass of the adsorbent (g) Qt = quantity of nitrobenzene adsorbed at time t (mg/g) k1 = pseudo-first-order kinetic constant (min1) k2 = pseudo-second-order kinetic constant (g mg1 min1) T = temperature (°C) t = contact time (min) Qm = Sips maximum adsorption capacity (mg/g) Ks = Sips constant (g/L)1/n KRP = RedlichPeterson constant (L/g) RRP = RedlichPeterson constant (mg/L)β β = RedlichPeterson exponent (dimensionless) λmax = wavelength (nm) ’ REFERENCES (1) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Addaoudi, M.; Kim, J. Reticular Synthesis and the Design of New Materials. Nature 2003, 423, 705. (2) Krungleviciute, V.; Lask, K.; Heroux, L.; Migone, A. D.; Lee, J.-Y.; Li, J.; Skoulidas, A. Argon Adsorption on Cu3(benzene-1,3,5tricarboxylate)2(H2O)3 MetalOrganic Framework. Langmuir 2007, 23, 3106. (3) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.; Henry, M.; Bataille, T.; Ferey, G. A Rationale for the Large Breathing of the Porous Aluminum Terephthalate (MIL-53) upon Hydration. Chem.—Eur. J. 2004, 10, 1373. (4) Ferey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surble, S.; Margiolaki, I. A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area. Science 2005, 309, 2040. (5) Li, Y.; Yang, R. T. Gas Adsorption and Storage in Metal Organic Framework MOF-177. Langmuir 2007, 23, 12937.

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MIL-53(Al): An Efficient Adsorbent for the Removal of Nitrobenzene

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