Activated MIL-53(Al) for Efficient Adsorption of Dichloromethane and Trichlormethane

Activation process designed to remove the solvent and unreacted ligands is the key step for the synthesis of metalorganic frameworks (MOFs). MIL-53(Al), a subclass of MOFs, was prepared using various synthesis and activation methods. Especially, a combined activation method was proposed. The performance of activation methods was evaluated by the specific surface area of the samples. The mechanism of activation methods was confirmed by the loading induced structural transition of MIL-53(Al) using X-ray diffraction. MIL-53(Al) activated by the combined method has the best performance and the adsorption capacities of vaporous dichloromethane (CH2Cl2) and trichlormethane (CHCl3) on this sample was investigated at 298 K and 101 kPa. The results of adsorption experiment show that MIL-53(Al) has larger adsorption capacity in CH2Cl2 than CHCl3, which is consistent with the conclusion of molecular simulation (Grand Canonical Monte Carlo).


INTRODUCTION
Dichloromethane (CH 2 Cl 2 ) and trichlormethane (CHCl 3 ) as two kinds of volatile organic compounds (VOCs) are the pollutants in the air, and how to deal with them has become a focus of the society (Narita et al., 1986;Schlosser et al., 2014;Kujawa et al., 2015).A number of add-oncontrol techniques, including filtration, adsorption, membrane separation, ionization, ozonation, and photocatalysis, have been developed to remove them from contaminated air (Koh et al., 2009;Ma et al., 2009).Among these techniques, adsorption is one of the most effective technique for VOCs removal.Porous materials including zeolites, resins, and activated carbons are the most frequently utilized adsorbents (Ganesh et al., 2014;Dai et al., 2015;de Fonseca et al., 2015;Palau et al., 2015).Metal-organic frameworks (MOFs), which are constructed of metal ions and organic linkers, have attracted considerable interests due to their high porosity and large adsorption capacity (Ren et al., 2015;Xu et al., 2015).Moreover, a specific subclass of MOFs, MIL-53 (MIL = Material of Institut Lavoisier), exhibits a reversible structural transformation named breathing effect upon adsorption and desorption.MIL-53 is made of parallel onedimensional M(OH) chains (M = Al 3+ , Cr 3+ , Fe 3+ ), linked together by 1,4-benzenedicarboxylate (BDC) ligands to form linear diamond-shaped pores that are wide enough to accommodate small guest molecules (Férey et al., 2003;Loiseau et al., 2004).MIL-53 may "breathe" between two distinct conformations called large-pore phase (lp) and narrow-pore phase (np), which has a remarkable difference in cell volume of up to 40% (Bourrelly et al., 2005;Serra-Crespo et al., 2012).
Solvothermal reaction based on N,N-dimethylformamide (DMF) is a most commonly used method to synthesize MIL-53(Al) (Ortiz et al., 2011;Pera-Titus et al., 2012;Serra-Crespo et al., 2012).Hereafter, this method was named as single solvothermal method (SSM).To enhance the specific surface area, a two-step double solvothermal method (DSM) which takes water and DMF as the solvents was developed.As the pores of as-synthesized (AS) samples are full of solvent and unreacted ligands which will dramatically decrease its activities, the AS samples should be activated before its applications (Alaerts et al., 2008).A major problem for developing efficient synthesis techniques is to optimize the activation method.Calcination in high temperature (HT) and solvent exchange (SE) are frequently-used activation methods for MOFs (Serre et al., 2007;Boutin et al., 2010).In HT method, the AS samples are put in shallow beds and heated in a muffle furnace at 573-623 K for about three days to remove residues from the pores.However, the high temperature also may cause destroy of framework structures for MOFs with low thermal stability.Fortunately it is appropriate for MIL-53.On the other hand, MOFs can also be activated by SE method.In SE activation method, the residues of synthesis reaction will be replaced by other solvent which has lower boiling point such as methanol.SE method has the advantage of mild treatment conditions and is especially suitable for the low thermo-stable structures (Hong et al., 2009).However, these two methods are not quiet efficient to activate MIL-53.In HT activation method, BDC molecules are still trapped in the pores because they sublime only at high temperature higher than 675 K. On the other hand, in SE activation method, the solvent can't escape from the pores completely and the trapped solvent molecules decrease the specific surface area.Besides the two activation methods, a more effective combined activation method to activate MIL-53(Al) sample completely had been presented (Rallapalli et al., 2010;Ghoufi et al., 2012).The advantages of the combined activation method and the drawbacks of the HT and SE methods will be illustrated comprehensively.It should be noted that the mechanism of the activation methods has not been explained clearly.According to the paper proposed the combined method, the high temperature during calcination may affect the textural properties of the material and packing which may result in lower specific surface area and pore volume.The formation of metal oxide by-product by calcination also reduces the surface area and pore volume (Tayade et al., 2006).However, this consideration is ambiguous and vague and is not a conclusion based on the exact experimental results.Clear understanding the mechanism of the activation is the prerequisite for developing new activation methods and the application of MIL-53.So far, the optimized activation method based on different synthesize methods has not been reported.
In this work, we designed a parallel experiment to optimize activation methods of MIL-53(Al) based on the synthesis method comprehensively and analyzed the mechanism of the activation process based on the both synthesis.MIL-53(Al) activated by the most efficient method was applied to adsorption for VOCs.Both molecular simulation (Grand Canonical Monte Carlo) and experimental techniques (Nitrogen adsorption-desorption, X-ray diffraction and a specific adsorption experiment) allowed a complete elucidation of the influence of activation on adsorption of CH 2 Cl 2 and CHCl 3 by MIL-53(Al).
In the SSM, 3.15 mmol of the metal precursor (Al(NO 3 ) 3 • 9H 2 O) and 3.15 mmol BDC are dissolved in 45 mL DMF, mixed in a Teflon insert and placed in an autoclave.The autoclave was kept at 423 K for 3 days.Then the white powder product was filtered off.After dried in the air, these products labelled as MIL-53(Al) AS by SSM.
The alternate way, DSM, the same dose of Al(NO 3 ) 3 • 9H 2 O and BDC were dissolved in 45 mL deionized water.The solution was then placed in a Teflon-lined autoclave and subjected to hydrothermal synthesis at 423 K for 5 h in an oven.The product was separated from solution and placed in the autoclave again with 45 mL DMF and kept at 423 K for 20 h.Then, the solvent was replaced by fresh DMF and the solvothermal treatment was repeated twice.This white powder product labelled as MIL-53(Al) AS by DSM.

Activation
AS sample needs to be activated because there are lots of residues trapped in the pores.Three activation methods were used to remove the residues within the pores, which were named as SE, HT and SE + HT, respectively.I. SE The SE method was carried out by immersing samples in methanol for 10 hours.The sample was dried at room temperature for 10 hours under air atmosphere and the obtained fine powder product labelled as SE sample.II.HT In HT method, the AS sample was heated with a rate of 60 K h -1 from room temperature to 603 K.At this temperature, the sample was calcined for 60 h.After cooling, the powder product was obtained and labelled as HT sample.III.SE + HT The combined method is the SE and HT in order.And the product was labelled as SE + HT sample.

Characterization
Powder X-ray diffraction patterns (XRD) were recorded at room temperature with Bruker D8 Advance X-ray diffractometer at 40 kV, 100 mA with a scan speed of 10° min -1 and a step size of 0.01° in 2θ range 4 to 20°, using Cu Kα radiation (wavelength λ = 0.15406 nm).Nitrogen adsorption/desorption measurement data were recorded with Quadrasorb SI at liquid nitrogen temperature.The specific surface areas were calculated from the adsorption branch in the relative pressure interval from 0.005 to 0.1 using both the Brunauer-Emmett-Teller (BET) equation and Langmuir equation.

Adsorption Experiment
CH 2 Cl 2 and CHCl 3 were also supplied by Beijing Chemical Works and without further purification.VOCs adsorption was performed gravimetrically at room temperature (298 K) and 101 kPa using a fixed fluidized bed unit.The synthetic samples (m 0 , 20 mg), SE + HT by DSM and SE + HT by SSM, were distributed over the ceramic wool and loaded in a quartz tube with 1.5 mm inner diameter at the beginning of the experiments, CH 2 Cl 2 and CHCl 3 were used as the VOCs source.The gas concentration of outlet of quartz tube was detected by a gas chromatography (GC).When the gas concentration remained unchanged, the adsorption of VOCs saturated.The difference in weight (∆m) of the tube before and after the adsorption experiment were detected and the adsorbed amounts (n ads ) was calculated by the equation: n ads = ∆m/(m 0 × M), M as the molar mass of the adsorbates.

THEORETICAL MODELING
VOCs adsorption in MIL-53(Al) was studied using Grand Canonical Monte Carlo (GCMC) simulations implemented in the multipurpose simulation code MUSIC (Gupta et al., 2003) with typically 5 × 10 7 Monte Carlo steps including 2.5 × 10 7 steps for equilibration.The simulations were conducted at 298 K and 101 kPa using the structure of the lp phase as rigid, considering simulation boxes as 8 (2 × 2 × 2) unit cells.The periodic models of the lp phase were built using the atomic coordinates previously reported (Ortiz et al., 2011).The intermolecular interactions were considered as follows: the set of Lennard -Jones (LJ) parameters for describing the lp phase frameworks were created by combining the generic Universal Force Field for the inorganic part of the crystalline lattice and the Dreiding interatomic potential for the organic part (Rallapalli et al., 2010).

Conformation State Analysis
MIL-53, a kind of flexible MOFs, has two conformation states.The equilibrium conformation state with a given temperature and pressure depends on the loading of guest molecules.The breathing transitions between lp and np phases are spontaneous process upon the adsorbed molecules, such as CO 2 (Rallapalli et al., 2010;Ganesh et al., 2014) and H 2 O (Chen et al., 2013).Loaded molecules induce a significant stress inside the elastic framework that is in the order of several to tens of MPa.This adsorption induced stress may be either positive or negative depending on the loading amount, which leads the framework's expansion or contraction.If the pores are empty, the equilibrium conformation is lp phase.When molecules are adsorbed, the crystal transits to np phase in the low loading adsorption.As more molecules as adsorbed, np phase transits to lp phase.Eventually lp phase becomes equilibrium again when the pores are full of molecules.
These two conformation states, lp and np phases, can be distinguished by XRD according to their different lattice parameters.The structural transition of the MIL-53(Al) solid upon activation was shown in Fig. 1.
The XRD patterns synthesized by SSM (a) and DSM (b) are shown in Fig. 2. Each figure contains the XRD patterns of AS samples, samples activated by different methods and computed XRD patterns.The periodic lattice models of MIL-53(Al) were built as a rigid lattice using the atomic coordinates available in the literature (Serre et al., 2002;Boutin et al., 2010).The corresponding diffraction angle of lp phase and np phase were labled θ lp and θ lp .As shown in Fig. 2a and 2b, the calculated MIL-53(Al) lp (magenta line) and np (black line) possesses typical peaks at 2θ lp = 8.72°, 15.04°, 17.48° and 2θ np = 12.3°, 17.5°, respectively.
By comparing with the simulated XRD pattern of lp phase, it is clear that AS (red line) samples are in a lp structure with pores full of residues, such as BDC, DMF, which influence the scattering peaks.The SE activation method has no significant effect on the XRD patterns and the SE (blue line) sample is still an lp structure.However, the typical XRD pattern of MIL-53(Al) HT (green line) is the combination of signals of lp and np phases, which agrees with the simulated pattern of lp and np.This indicated that most of DMF were removed.Furthermore, the typical XRD pattern of lp was observed after treating AS with the SE + HT activation (cyan line).The structure is only in the lp phase with a unit cell volume of 1436.35Å 3 , which is coincident with the reported lp phase (Loiseau et al., 2004).
For DSM (see Fig. 2(b)), the pattern of MIL-53(Al) AS (red line) suggests that it is a combination of lp phase with pores full of residues and np phase.Different from the AS sample synthesized by SSM, DSM has H 2 O molecules as additional residues.There is little water located in the pores, which generates the np phase.The XRD pattern of MIL-53(Al) SE (blue line) becomes more complex than that of AS samples.Moreover, the two phases coexist in the samples activated by HT method (green line) since both of their characteristic XRD peaks are observed simultaneously.With SE + HT method, only the lp phase pattern (cyan line) was observed, which confirms that pores had been emptied without any residues, i.e., fully activated.

Optimal Activation
N 2 adsorption is used to measure the free volume in the pores of MIL-53, which is roughly equal to the difference between the volume of unit cell and that occupied by the solvents and BDC molecules.The adsorption/desorption isotherms are shown in Fig. 3.They all display a type I isotherm without hysteresis upon desorption.Whether SSM (see Fig. 3(a)) or DSM (see Fig. 3(b)), the N 2 adsorption capacity of MIL-53 was increased after the SE (blue line) activation.DMF was replaced by methanol which is more easily to escape from pores.This leads to higher N 2 adsorption volume of SE samples than that of AS samples.Due to the XRD pattern's peak strength of SE samples is similar with that of AS samples, the sample is still in lp phase, but with the pores full of methanol molecules mostly.Compared to MIL-53(Al) SE, the N 2 adsorption capacity of the HT samples (green line) further increased for both SSM and DSM than AS samples.This drastic increase can be attributed to the evaporation of solvent at the high temperature, 603 K.By this means, most of the pores in lp phases were emptied, which was indicated by the increased free volume.A further increase in pore volume of MIL-53(Al) was observed with SE + HT (black line) method.This indicates that the residues in SE samples are much easier to remove than that in the AS samples.And it is easier to transform the lp phase with the full pores to another lp phase with empty pores.The N 2 adsorption capacity of MIL-53(Al) synthesized by DSM was much higher than that synthesized by SSM even for the AS sample, which indicates DSM is a more efficient synthesized method.
Removal of residues from the samples has been confirmed by both BET and Langmuir specific surface areas calculated from the nitrogen adsorption isotherms.They are the most important physical parameters for the application of porous materials.The specific surface areas of MIL-53(Al) samples obtained by different synthesis and activated methods is illustrated in Fig. 4. The BET specific surface areas have a 294% increase based on the DSM and a 133% increase based on the SSM for SE vs. AS.And for HT vs. AS, the BET specific surface areas have a 781% increase based on SSM and a 271% increase based on DSM.For SE vs. HT, the BET specific surface areas have a 265% increase based on SSM and a 204% increase based on DSM.That is why HT is a frequently-used activation method rather than SE.For the combined method (SE + HT) vs. HT, the BET specific surface areas have a further 120% increase based on SSM and 108% based on DSM.This increase can be attributed to that BDCs are still trapped in the pores in HT method.Compare the AS sample synthesized by SSM and DSM, the two-step method, DSM, are more effective in getting the raw material.As the result, the surface areas also confirm that DSM synthesis method combined with SE + HT activation method is the best way to obtain the activated MIL-53(Al).

Activation Mechanism Analysis
The guest molecules of the pores in the activating process are illustrated in Fig. 5.For SSM, the specific surface area of AS sample is very small because the pores are full of the BDC and DMF.In SE activation process, BDC and most of DMF were replaced by methanol, hence only the methanol and DMF exist in the pores (see Fig. 5(a)).Methanol is more volatile than DMF, so the free volume of SE sample is larger than that of AS sample.In HT method, the DMF and BDC were removed from the pores.However, some BDC were still trapped in the pores for the sublime only at high temperatures of 698 K.So the BET specific surface area of HT sample is still lower than the theoretical prediction result.In the combined SE + HT method, only the solvent molecules (methanol and DMF) filled in the pores after SE activation and the residual solvent can be removed by HT activation efficiently.Compared with the HT method, the combined method empties the pores occupied by these solvent molecules and improves the specific surface areas dramatically.
Different from SSM, H 2 O molecules also exist in AS sample (shown in Fig. 5(b)).BDC is insoluble in deionized water and sublimes only at high temperatures of 675 K, so the sample which had been synthesized in the water should react again with the rest of metal salt and remove the redundant BDC in the DMF solution.As H 2 O is easier to escape from pores than DMF, the AS sample's BET specific surface areas of SSM are higher than that of DSM.Similar to SSM, only solvents molecules existed in the pores after  SE activation and BDC existed in the pores after HT activation.So the BET specific surface areas of SE samples and HT samples are all higher than that of AS samples and HT activation is a more effective method than SE activation.SE + HT activation, the most efficient activation, can make the residues escape from the pores and get the large phase structure.While SSM and DSM are all lp phase after SE + HT, the BET specific surface area of the samples synthesized by DSM is higher than SSM because some BDC molecules still trapped within the pores after SE.DSM can remove the BDC more efficiently than SSM.This indicates the DSM is a more effective method than SSM.Above all, DSM provides a better raw material with a small amount of BDC, SE activation removes the BDC and HT activation removes the solvents completely.

Adsorption Analysis
Using MIL-53(Al) activated by SE + HT based on SSM and DSM as the adsorbent, we carried out the adsorption experiments at 298 K and 101 kPa.The results of adsorption studies are presented in Fig. 6.In the adsorption of CH 2 Cl 2 and CHCl 3 , the adsorption mounts for DSM is higher than that for SSM.The results are consistent with the N 2 adsorption results, which is further proved that DSM is a more effective method to remove BDC than SSM.For both methods, the adsorbed amounts of CH 2 Cl 2 in all samples are higher than that of CHCl 3 because the kinetic diameter of CH 2 Cl 2 (0.32 nm) is smaller than CHCl 3 (0.65 nm).For the samples synthesized by DSM, the extra empty pores contain more small CH 2 Cl 2 than CHCl 3 .Therefore, the differences of adsorption amounts between DSM and SSM for CH 2 Cl 2 larger than that for CHCl 3 .The adsorption mounts of CH 2 Cl 2 and CHCl 3 in MIL-53 is much higher than that of activated carbon (3.18 mmol g -1 for CH 2 Cl 2 ) and Na-Y 5.5 zeolite (3 mmol g -1 for CHCl 3 ), respectively (Kawai et al., 1994;Shim et al., 2003).
At 298 K and 101 kPa, the adsorption of CH 2 Cl 2 and CHCl 3 on MIL-53(Al) were simulated by the GCMC simulation.In the simulation box, the average molecules of CH 2 Cl 2 and CHCl 3 are 33.45 and 31.36,respectively.It is nearly equivalent to 4.18 and 3.92 molec uc -1 (mollec uc -1 = molecules (unit cell) -1 ), respectively.This indicates that the adsorption capacity on MIL-53(Al) is, CH 2 Cl 2 > CHCl 3 .In addition, the adsorbed amounts of CH 2 Cl 2 and CHCl 3 on SE + HT samples prepared by DSM were 15.71 mmol g -1 and 8.14 mmol g -1 , respectively.These amounts were higher than that by SSM (9.90 mmol g -1 and 7.59 mmol g -1 ).This shows that the SE + HT by DSM method can remove the residues more thoroughly than that of SSM.Besides, the order of the adsorbed amounts is, CH 2 Cl 2 > CHCl 3 .At 298 K and 101 kPa, the snapshots of the GCMC simulation box with CH 2 Cl 2 and CHCl 3 were simulated by GCMC (shown in Fig. 7).The result of GCMC is consistent with the experiment in the adsorption capacity of CH 2 Cl 2 and CHCl 3 .Both the simulation and experiment results indicate that the adsorption capacity of VOCs on MIL-53(Al) is, CH 2 Cl 2 > CHCl 3 .

CONCLUSIONS
Based on different synthesis processes, an in-depth comprehensive study has been provided to reveal an effective method for MIL-53(Al) activation.Compared with two general activation methods based on two synthesis processes of MIL-53(Al), the method combined DSM synthesis and SE + HT activation can remove the unreacted ligand and solvents molecules trapped in the pores thoroughly and has been demonstrated to be a more efficient way to obtain pure .Using this method, the lp phase MIL-53(Al) with emptied pores has been confirmed by the XRD pattern and the high BET specific surface area (1301.86m 2 g -1 ).The mechanism of these activations has been elucidated via the adsorption induced structural transition between the large and narrow pores.VOCs adsorption experiment showed that MIL-53(Al) prepared by the most efficient way in this work had higher CH 2 Cl 2 vapor adsorption capacity (15.71 mmol g -1 ) than CHCl 3 (8.14mmol g -1 ) and it also exhibited higher adsorption capacity than that of traditional activated carbon.The results of the adsorption experiment are consistent with that of the GCMC simulation.

Fig. 2 .
Fig. 2. XRD patterns of treated MIL-53(Al)s.(a) Prepared with Single Solvent.(b) Prepared with Double Solvent.AS: without treatment; SE: solvent exchange with methanol; HT: calcined for 72 h; lp: and np: calculated by lp and np modal.

Fig. 3 .
Fig. 3. Nitrogen adsorption/desorption isotherms of MIL-53(Al)s: (a) Prepared with Single Solvent.(b) Prepared with Double Solvent.Empty and filled symbols refer, respectively, to adsorption and desorption data.The curves are a guide to the eye.

Fig. 4 .
Fig. 4. Change of the specific surface are of MIL-53(Al)s according to the different activated and calculated method: ■: BET specific surface areas •: Langmuir specific surface areas.

Fig. 5 .
Fig. 5.The guest molecules trapped within pores during the process of activation with AS made by (a) SSM and (b) DSM.

Fig. 6 .
Fig. 6.The adsorption amounts figures of SE + HT by DSM and SSM.