Impact of the functionalization onto structure transformation and gas adsorption of MIL-68(In)

A series of functionalization –NH2, –Br and –NO2 has been performed on MIL-68(In) material in order to improve the porosity features of the pristine material. The functional groups grafted onto the ligand and the molar ratios of the ingredient indicate a profound influence on product formation. With the incremental amount of metal source, product structures undergo the transformation from MIL-68 to MIL-53 or QMOF-2. The situation is different depending on the variation of the ligands. Gas (N2, Ar, H2 and CO2) adsorption–desorption isotherms were systematically investigated to explore the impact of the functionalization on the porous prototypical framework. Comparison of adsorption behaviour of N2 and Ar indicates that the polar molecule exhibits striking interaction to N2 molecule, which has a considerable quadrupole moment. Therefore, as a probe molecule, Ar with no quadrupole moment is more suitable to characterize the surface area with the polar groups. Meanwhile, Ar adsorption result confirms that the negative influence on the surface area stems from the size of the substituting groups. The uptake of H2 and CO2 indicates that the introduction of appropriate polar organic groups can effectively enhance the adsorption enthalpy of relative gases and improve the gas adsorption capacity apparently at low pressure. The introduction of –NO2 is in favour of improving the H2 adsorption capacity, while the grafted –NH2 groups can most effectively enhance the CO2 adsorption capacity.

A series of functionalization -NH 2 , -Br and -NO 2 has been performed on MIL-68(In) material in order to improve the porosity features of the pristine material. The functional groups grafted onto the ligand and the molar ratios of the ingredient indicate a profound influence on product formation. With the incremental amount of metal source, product structures undergo the transformation from MIL-68 to MIL-53 or QMOF-2. The situation is different depending on the variation of the ligands. Gas (N 2 , Ar, H 2 and CO 2 ) adsorptiondesorption isotherms were systematically investigated to explore the impact of the functionalization on the porous prototypical framework. Comparison of adsorption behaviour of N 2 and Ar indicates that the polar molecule exhibits striking interaction to N 2 molecule, which has a considerable quadrupole moment. Therefore, as a probe molecule, Ar with no quadrupole moment is more suitable to characterize the surface area with the polar groups. Meanwhile, Ar adsorption result confirms that the negative influence on the surface area stems from the size of the substituting groups. The uptake of H 2 and CO 2 indicates that the introduction of appropriate polar organic groups can effectively enhance the adsorption enthalpy of relative gases and improve the gas adsorption capacity apparently at low pressure. The introduction of -NO 2 is in favour of improving the H 2 adsorption capacity, while the grafted -NH 2 groups can most effectively enhance the CO 2 adsorption capacity.

Introduction
In the last two decades, metal-organic frameworks (MOFs) have developed as promising candidates for adsorption of gas, such as H 2 , CO 2 , CH 4 , owing to their ultra-high porosity, large surface area, welldefined open channels and structural diversity [1 -9]. However, practical and industrial applications of those materials in realistic and non-ideal environments need to meet more rigorous requirements such as higher thermal stability and higher adsorption capacity. So, it is necessary to enhance the performance of the MOFs. It is experimentally or computationally demonstrated that functionalization of the porous MOFs with the appropriate substituents is an effective approach for performance optimization, such as introducing the metal unsaturated site (Li þ , Cu 2þ ), post-synthetic approaches or graft functional substance onto a coordinative unsaturated framework or the use of ligands with functional group to generate functionalized MOFs [10][11][12][13][14]. Zhang et al. made 24 MOF-177 structures with different functional groups on the triangular BTB linker, having one or more functionalities [15]. It has been found that the position of the functional groups on the BTB unit shows the selection for the framework net. Meanwhile, the mixing of functionalities (-H, -NH 2 , -C 4 H 4 and -NO 2 ) into MOF-177 leads to the enhancement of hydrogen uptake by 25%. Xia et al. study the effect of functional groups on the hydrogen storage properties of MOF-808-X (X ¼ -OH, -NO 2 , -CH 3 , -CN, -I) with the GCMC method [16]. That the simulated H 2 uptakes isosteric heat at 77 K indicates that all these substituents have a favourable impact on the hydrogen storage capacity, and -CN is the most promising substituent to improve H 2 uptake. Frysali et al. reported that the OSO 3 H functional group possesses the highest binding energy with CO 2 among the 14 selected functional groups with RI-MP2/aug-cc-pVTZ method [17]. Depending on the functional group, CO 2 acts as Lewis acid and Lewis base simultaneously. This cooperative effect stabilizes the formed complex and affects the location of CO 2 in the optimized geometries. Mu et al. study the effect of the chemical properties of nine organic linkers on CO 2 /CH 4 mixture separation in MOFs with a hierarchical multiscale approach combining GCMC simulation and DFT calculation [18]. The results show that the organic linkers decorated with the electron-donating groups can strengthen the distribution of the electrostatic field in the pores of MOFs and greatly enhance the adsorption selectivity of the CO 2 /CH 4 mixture. In addition, this work also demonstrates that the steric hindrance indicates the negative effects on the separation behaviour. Despite extensive research work, there is plenty of room to explore the impact of the functionalization onto the properties of MOFs due to the rapidly growing species of MOFs.
Motivated by it, this work is devoted to improving the porosity of the prototypical material by implementing different functional groups. To realize functionalization, high stability of the prototype framework and the channels with appropriate dimension should be required. Therefore, MIL-68(In) with high thermal stability and high BET surface area, reported by Volkringer and co-workers, is the priority as the prototype structure. There exist two kinds of the channels with sufficient diameter openings (approx. 6.0 and approx. 16 Å ) in the skeleton which makes it possible to further introduce the specific substituent groups into the pore to implement channel functionalization [19]. Meanwhile, -NH 2 , -Br and -NO 2 with different dimension and polarity were selected as functional groups to probe the influence onto the gas adsorption behaviour of the prototype MIL-68(In).
But, related to MIL-type materials, some interesting structural transformations have been reported due to different important factors, such as pH, time, temperature and guest molecules. Perea-Cachero et al. described a method for the reversible crystalline transformation between MIL-53(Al) and MIL-68(Al), which had higher thermal stability and crystallinity than the as-synthesized MIL-68(Al) [20]. Carson et al. reported topological isomers MIL-101(V) and MIL-88B(V) converted into MIL-47(V) by a thermal treatment (2008C) [21]. Liédana et al. demonstrated that caffeine played the role of a structure directing agent or template, which led NH 2 -MIL-88B(Fe) to undergo a reversible transformation into NH 2 -MIL-53(Fe) [22]. During our synthesis process, a series of regular crystalline structural transformations among MIL-68(In)_X, MIL-53(In)_X and QMOF-2 were observed along with the adjustment of the reactants. MIL-53(In) (sra topology) is polymorph with MIL-68(In), constructed from infinite chains of inorganic InO 4 (OH) 2 octahedra linked by the apical trans hydroxyl functions with the flexible framework and unique rhombic cavities (ca 8.5 Å ) [23]. QMOF-2 constructed from distorted pseudo-tetrahedral coordinated indium centres linked by carboxylate ligands with the high symmetry of the b-quartz network [24]. To the best of our knowledge, no attention, however, has ever been paid to study the impact of the functionalization onto the structural transformation of these MOFs to date, which is one main aspect of our study.
In this work, powder X-ray diffraction analysis, IR spectroscopy, 1 H NMR spectroscopy, TG analysis and the gas (N 2 , Ar, H 2 , CO 2 ) sorption behaviours were fully investigated to determine the influence of the functionalization on the prototype MIL-68(In). The results highlight the effectiveness of the functionalization of the porous MOFs to improve the porous characteristics. Notably, our current work provides an insight into the dramatical influence of the functional groups grafted on the ligand as well as the molar ratios of the ingredient onto the structural formations of MIL-type materials.

Synthesis
Experiments on the investigation of structure transformation rule were carried out in accordance with the following strategy. A mixture of In(NO 3 ) 3 . xH 2 O (x mmol), H 2 BDC-NH 2 ( y mmol) and DMF (1.24 ml, 17.30 mmol) was placed in a Teflon-lined home-made stainless steel autoclave (2.40 ml) and heated for 5 or 24 h at 1258C in an oven. The resulting powder was filtered and then washed with DMF. Metal/ligand molar ratios were modulated in three different synthesis systems, which were respectively M : L ¼ 3 : 1, 1 : 1, 1 : 3 or 1 : 6. The MIL-68(In) sample was prepared according to the reported recipe [19].

Activation process
To remove the guest molecules, an activation process was performed on each crude sample of MIL-68(In)_X before gas adsorption measurements. The as-synthesized samples are soaked in the organic solvents with low boiling point (MIL-68(In)_Br with anhydrous ethanol, MIL-68(In)_NH 2 and MIL-68(In)_NO 2 with anhydrous methanol) for 3 days and the solvents were changed twice a day. After the filtration, each sample was heated (MIL-68(In)_Br at 1508C, MIL-68(In)_NH 2 and MIL-68(In)_NO 2 at 2008C) under the flow of N 2 for 5 h. Before the measurement, the samples were degassed again by using the 'degas' function of the surface area analyser at a corresponding temperature for 12 h in vacuum.

Techniques of characterization
The IR spectra were recorded (400-4000 cm -1 region) on a SHIMADZU IRAffinity-1 Fourier-transform infrared spectrometer by using the KBr pellets method. The PXRD patterns were carried out on an STOE STADI-P diffractometer equipped with a curved germanium (111) primary monochromator and a linear position-sensitive detector using Cu Ka1 radiation, l ¼ 1.5406 Å . The patterns were registered in the 3-508 2u range with a scanning step of 0.128 s -1 . The size and the morphology of the crystals were determined by scanning electron microscopy (SEM) using a Philips XL 30 FEG microscope. TGA was performed under nitrogen with a heating rate of 58C min 21 up to 8008C using a Perkin -Elmer TGA 7 thermogravimetric analyser. 1 HNMR spectra were carried out on a Bruker 400 UltraShield TM by using tetramethylsilane as standard.
Argon adsorption isotherms were performed on a Quantachrome Antosorb-IQ-C apparatus. Ar isotherms at 87 K were measured in a liquid argon bath using an 87 K sensor. Nitrogen, hydrogen and carbon dioxide adsorption isotherms were performed on a Micromeritics Tristar II 3020 apparatus. H 2 isotherms at 77 K were measured in a liquid nitrogen bath using a 77 K sensor, while H 2 isotherms at 87 K in a liquid argon bath using an 87 K sensor. CO 2 isotherms at 273 K were measured in an ice-water bath using a 273 K sensor, while CO 2 isotherms at 298 K were measured at room temperature controlled by central air-conditioning.

Structure transformation analysis
Based on the pre-experimental studies, solvent, temperature and time were fixed in three different reaction systems, while the metal/ligand molar ratio was variable. Because the crystal size was not appropriate for performing SCXRD, PXRD was used to characterize the crystalline structure of the samples. The PXRD pattern is depicted in figure 1. Corresponding morphological changes were tracked by SEM, as shown in figure 2.
The PXRD results show that the molar ratios of the ingredients have a profound influence on product formation. The similar variation trend can be observed in the presence of H 2 BDC_Br and H 2 BDC_NO 2 . Exclusive MIL-68(In)_X is formed with an excess amount of In 3þ (M : L ¼ 3 : 1). While with M : L ¼ 1 : 1, the Bragg peak assigned to MIL-53(In)_X is discovered in the corresponding PXRD spectra, which indicates that structural transformation is discovered as the amount of the ligands increases. With M : L ¼ 1 : 3, absolute MIL-53(In)_X is observed in the presence of excess ligands. The morphological transformation, from hexagonal needle shape to rhombic block shape, can be clearly observed in the corresponding SEM images ( figure 2a,b). The analogous crystalline-state transformation between MIL-53(Al) and MIL-68(Al) was reported by Perea-Cachero and co-workers [20]. The transformation mechanism is regarded as the exchange or removal of guest molecules from the pores.
However, the PXRD results show that dissimilar structural transformation occurred with the incremental amount of H 2 BDC_NH 2 . As the amount of the ligand switched from 3 : 1 to 1 : 1, the structural transformation is similar to another two synthesis systems from MIL-68(In)_X to the mixed phase (MIL-68(In)_X and MIL-53(In)_X). But, the Bragg peak assigned to QMOF-2 can be obviously observed in the PXRD spectra of M : L ¼ 1 : 3, which indicates that disparate structural transformation process is taking place with an excess amount of H 2 BDC_NH 2 . Choi and co-workers reported the  analogous structural transformation from MIL-68(In) to QMOF-2, and verified that MIL-68(In) is an initial kinetic product and QMOF-2 is a final thermodynamic product of In 3þ with an excess amount of H 2 BDC [25]. To verify the tenability of this conclusion in In 3þ and H 2 BDC_NH 2 system, the amount of the ligand was continuously increased to M : L ¼ 1 : 6, and the reaction time was prolonged to 24 h. The PXRD result validates that exclusive QMOF-2 is formed. Seen from the corresponding SEM images, the morphological transformation goes from hexagonal needle shape to hexagonal needle shape mixed with rhombic block shape, and then to rhombic block shape mixed with two-based pointed hexagonal rod shape, finally to two-based pointed hexagonal rod shape (figure 2c). It is interesting to note that distinguishing crystal structure formations occurred in the three systems, with the same M : L ¼ 1 : 3. Referring to the three pristine crystallographic frameworks, in MIL-68 and MIL-53 In 3þ adopted in hexa-coordination with six O atoms, four of which come from four monodentate carboxyl ligands, two from m-connecting OH groups (electronic supplementary material, figure S1a). Whereas, in QMOF-2 In 3þ adopted in octa-coordination with eight O atoms come from four chelating bidentate carboxyl ligands (electronic supplementary material, figure S1b). Therefore, the carboxylic ligands are coordinated to metal centres more steadily and closely in the crystallographic configuration of QMOF-2 relatively. Compared to the NH 2 group, Br and NO 2 groups are with larger dimensions leading to the larger space resistance. The higher energy barrier needs to be overcome, which makes it more difficult to the formation of final thermodynamic product QMOF-2 with H 2 BDC_Br and H 2 BDC_NO 2 .

Structure description of MIL-68(In)_X
The unit cell parameters of MIL-68(In)_X were determined from PXRD using DICVOL4 algorithm [26]. The comparison of the lattice parameters of MIL-68(In)_X and MIL-68(In) is shown in table 1, which exhibits the minimum variation among the four structures. Meanwhile, the perfect fit between PXRD pattern of as-synthesized samples and simulated spectrum of MIL-68(In) shows the insertion of the substituting groups does not lead to obvious structure variation (electronic supplementary material, figure S2). The Kagomé-like MIL-68(In) prototype framework has been sustained after the decoration of different substituting groups. PXRD analysis demonstrates that the skeleton is sustained without the obvious variation after modification; however, the emplacement of the substituting groups leads to the different surface area for MIL-68(In)_X. We analyse that the improved N 2 adsorption capacity generating from the interactions of polar groups-N 2 molecule, which has a considerable quadrupole moment (table 2). Thus, the surface area calculated from the N 2 adsorption quantity results in the higher value instead of the actual one.

Ar adsorption measurement
To verify our conjecture and to obtain the actual surface area and pore volume, argon with zero quadrupole moment was chosen to be the probe molecule. And the adsorption result is shown in figure 3b. In contrast  . The result confirms that the influence of the size of the substituting groups on the surface area cannot be ignored [28][29][30]. Larger groups lead to more loss in the surface area or pore volume. The results of N 2 and Ar adsorption are summarized in table 3 for comparison. When the gas molecule is used to probe the surface area and pore volume of porous materials, the feature of the probe molecules and the pore wall of the porous materials cannot be ignored. The interaction between gas molecules and the pore wall can lead to the deformation of the probe molecules and then affect the gas adsorption capacity, which generates the deviation of the calculated surface area from the actual value. Such influence was proved to be evident in the case that N 2 molecules with a considerable quadrupole moment were used to probe the surface area of the porous material with polar pore wall.

Calculation method of adsorption enthalpy
To further understand H 2 and CO 2 adsorption behaviour of MIL-68(In)_X, the coverage-dependent isosteric heat of adsorption (hereinafter Q st for short) was determined. At first, the combined isotherm data were modelled with a virial-type expression [31,32], where a i and b i are independent parameters, P is the pressure (mmol g -1 ), N is the uptake amount (mmol g -1 ), T is the temperature (K ), and m and n determine the number of terms required to adequately describe the isotherm, in all cases, m 6 and n 3. Then from the results from the upper fitting, the independent parameter a i was used to calculate the isosteric heat of adsorption with the following expression: where R is the universal gas constant of 8.3147 J K -1 mol -1 .

H 2 adsorption measurement
The  S BET (m 2 g -1 ) S Langmiur (m 2 g -1 ) uptake (cm 3 g -1 ) S BET (m 2 g -1 ) S Langmiur (m 2 g -1 ) MIL  4d). The comparison result shows the subequal molecules per unit cell. However, considering the reductive surface area of the functionalized materials, the modification of organic groups has a positive influence on the capacity of H 2 uptake, which is obviously reflected at low pressure. As shown in the inset of figure 4, the functionalized materials indicate higher or more rapid H 2 absorption capacity. To further understand the H 2 adsorption behaviour, the H 2 Q st of each sample was determined from fits of 77 and 87 K isotherms, as shown in figure 5a. The Q st of the functionalized materials shows the similar variation tendency. At the onset of the adsorption, the Q st reaches around 7.00 kJ mol -1 , and then decreases monotonically to approximate 5.20 kJ mol -1 at higher H 2 pressures and remains steady at this value throughout the adsorption process. However, the maximum Q st of MIL-68(In) is 5.86 kJ mol -1 at low coverage, and then decreases to 4.07 kJ mol -1 as the H 2 loading increases. The comparison of the low-coverage Q st between the functionalized materials and MIL-68(In) demonstrates that appropriate functionalization can contribute to the improved H 2 Q st , which implies the existence of the excess interaction between adsorbent and adsorbate generated from the organic groups along the pore wall. It is proved that functionalization of benzene with an electron-donating group such as -NH 2 could enhance the interaction between H 2 and the phenyl rings regardless of the centroid or perpendicular direction. Although the affinity between H 2 and the phenyl rings may be weakened by the electron-withdrawing groups (-Br and -NO 2 ), it can be greatly enhanced due to the constrictions of the strong polarity of the two substituting groups, which can well make up for the former loss [33,34]. With the pore surface being occupied, the Q st gradually decreases to a constant.
To facilitate the contrast, the H 2 adsorption results of MIL-68(In) and MIL-68(In)_X are listed in table 4. H 2 uptake of the isomorphic MIL-68(In)_X is dominated by the surface area or pore volume at high pressure. However, the introduction of polar organic groups can effectively enhance the H 2 Q st , especially -NO 2 and -NH 2 , thus effectively improving the H 2 adsorption capacity at low pressure.   6d). Considering the reductive surface area of the functionalized materials, the comparison result shows the -NH 2 group has the obviously positive influence on the capacity of CO 2 uptake. Besides -NH 2 , the insertion of the -NO 2 group apparently enhances the CO 2 adsorption capacity at low pressure, as shown in the inset of figure 6. The Q st of CO 2 was studied to further understand the adsorption properties, which is determined by fits of the 273 K and 298 K sorption data. As shown in figure 5b, at the lowest coverage MIL-68(In)_NH 2 indicates the highest value up to 24.3 kJ mol 21 , and then followed by MIL-68(In)_NO 2 (23.6 kJ mol -1 ) and MIL-68(In)_Br (20.0 kJ mol -1 ). All the values decrease gradually to 16.7 kJ mol 21 as the CO 2 loading increases. However, the Q st of MIL-68(In) is around 14.5 kJ mol 21 during the overall process, which effectively certifies that the polar organic groups are the favoured adsorption sites for CO 2 adsorption and thus the insertion of these groups is an effectual approach to improve CO 2 adsorption enthalpy. The higher value of MIL-68(In)_NO 2 compared to MIL-68(In)_Br is in agreement with the greater attraction expected between the stronger polar functional group -NO 2 and CO 2 , which has a significant quadrupole moment (table 2). With reference to the highest value of CO 2 adsorption enthalpy of MIL-68(In)_NH 2 , it is believed to be generated from a combination of CO 2 -amine interactions, which is evidenced by other amino group modified MOF materials or computational studies [35 -37]. During the CO 2 coverage, the overlap of the favoured sites leads to gradual decrease in CO 2 adsorption enthalpy.   To facilitate the contrast, the CO 2 adsorption results of MIL-68(In) and MIL-68(In)_X are listed in table 5. CO 2 uptake of the isomorphic MIL-68(In)_X is also affected by the surface area or pore volume at high pressure. However, the introduction of appropriate polar organic groups, outstandingly the -NH 2 group, can effectively enhance the CO 2 Q st , and apparently improve the CO 2 adsorption capacity at low pressure.

Conclusion
We have demonstrated a series of crystalline structural transformations, among MIL-68(In)_X, MIL-53(In)_X or QMOF-2. It is notable to find the profound influence of the formula and the functional groups of the ligands on the product formation. It is proved that Ar with zero quadrupole moment is more suitable to probe the surface area of MIL-68(In)_X, in order to avoid the  influence of the interactions of polar groups-N 2 molecule. Ar adsorption result confirms that the influence of the size of the substituting groups on the surface area cannot be ignored. H 2 and CO 2 uptake of the MOF materials indicates that although the relative gas adsorption at high pressure is dominated by the surface, the appropriate grafted functions can effectively enhance the adsorption enthalpy, and apparently improve the gas adsorption capacity at low pressure. In contrast, the modification of -NO 2 is best for H 2 adsorption, while the grafted -NH 2 is most beneficial to CO 2 adsorption. Based on the good porosity, remarkable stability and the existence of the polar sites in the framework, tentative further in-depth work on the functionalized MIL-68(In) materials will be undertaken in our laboratories to explore the other functional features.
Data accessibility. The datasets supporting this article have been uploaded as part of the electronic supplementary material.