Post-synthetic modulation of the charge distribution in a metal–organic framework for optimal binding of carbon dioxide and sulfur dioxide

Modulation of pore environment is an effective strategy to optimize guest binding in porous materials.


Introduction
The release of CO 2 and SO 2 into the atmosphere from combustion of fossil fuels causes signicant environmental problems and health risks. 1,2 The wholesale replacement of the carbon-based energy supply is highly challenging, not least because of the existing infrastructure. 3 It is therefore vital to mitigate the emissions of these acidic gases post combustion. At present, several technologies are used to capture CO 2 and SO 2 , such as amine scrubbing, absorption in organic solvents, ionic liquids and limestone slurry. [4][5][6][7] However, the considerable costs and the substantial energy input required for system regeneration signicantly limit their long-term application. Powerful drivers therefore exist to develop new systems showing high adsorption capacity, selectivity, storage density and excellent reversibility to sequester these gases.
Metal-organic frameworks (MOFs) constructed from metal ions and clusters bridged by organic ligands are an emerging class of porous materials showing highly crystalline structures and great promise for gas adsorption and storage. 8 MOFs can exhibit very high surface areas and, more importantly, have tunable pore environments with predictable pore size 9 and can incorporate specic functional groups. [10][11][12][13][14] MOFs have been studied extensively as sorbents for CO 2 under post-combustion conditions. 11,[13][14][15][16][17][18][19][20][21][22] In contrast, adsorption of SO 2 in MOFs has been rarely reported due to the limited stability of coordination compounds to highly reactive and corrosive SO 2 . [21][22][23][24][25][26][27] Development of new stable porous materials with optimal SO 2 and CO 2 adsorption property thus remains signicant challenge. Optimising the interactions between hosts and substrate molecules to enhance the storage capacity, density and selectivity is the key to overcoming these barriers. For this reason, visualisation of the host-guest interactions involved in the MOF-gas binding interactions is crucial for the design of new materials. Interrogation of adsorption mechanisms by in situ experiments as a function of gas loading can afford key insights into the preferred binding sites within pores and the interaction to the pore interior. 13,14,21,22 Open metal sites 11,28 and pendent functional groups (e.g., amine, hydroxyl group and nitrogencontaining aromatic rings) 13,14,21,22,29 have been found to act as specic sites for CO 2 and SO 2 binding.
Ionic liquids are composed of cations such as ammonium, pyridinium, phosphonium and imidazolium groups, and the solubility of CO 2 and SO 2 in these systems is oen high owing to strong acid-base interaction. 6,[30][31][32] The incorporation of functionalized organic ligands within MOFs, [23][24][25] and post synthetic modication can be used to control the number and types of functional groups within the pores. [15][16][17] Cleavage of C-N bonds and hydrolysis of methyl viologen in alkaline solution have been observed. 18,19 However, the dealkylation of pre-formed MOFs has not been reported previously, although adsorption of CO 2 in two imidazolium-pyridinium cation-containing MOFs has been observed. 10,33 We report the synthesis of a highly unusual charged material MFM-305-CH 3 , [Al(OH)(L)]Cl, [(H 2 L)Cl ¼ 3,5dicarboxy-1-methylpyridinium chloride] incorporating cationic (methylpyridinium) and anionic (chloride) components giving the material zwitterionic features. By heating MFM-305-CH 3 at 180 C, the 1-methylpyridiniumdicarboxylate ligand undergoes in situ demethylation to give the pyridine-based neutral complex MFM-305 showing the same overall framework topology. Demethylation coupled to loss of chloride anion exposes the bridging hydroxyl group in the MOF pore for hydrogen bonding to substrates, with MFM-305-CH 3 and MFM-305 showing distinct charge distributions and pore environments decorated with different functional groups. This provides a unique platform to investigate the precise roles of Lewis acid, Lewis base, chloride, methyl, pyridine and hydroxyl groups in the binding of guest molecules. Through in situ synchrotron X-ray diffraction, neutron diffraction, IR, 2 H NMR and neutron spectroscopic experiments, the binding of CO 2 and SO 2 has been comprehensively investigated in these two porous materials. All experiments show that the binding domains for CO 2 and SO 2 molecules are directly affected by the tuning of surface charge distribution and functional groups. Signicantly, simultaneously enhanced adsorption capacity and selectivity have been observed on going to MFM-305. We also report a unique study of structural dynamics of restricted guest molecules in MOFs as a function of temperature, showing the unprecedented mobility of CO 2 molecule within the pores.  Views of (c) MFM-305-CH 3 and of (d) MFM-305. The pore size is $4.6 Â 4.6Å for MFM-305-CH 3 and $5.6 Â 5.6Å for MFM-305 taking into consideration van der Waals radii. The methyl groups (olive) and chloride ions (green) in MFM-305-CH 3 ; N atoms (blue) and hydroxyl group (olive) in MFM-305. 10. 34 The total free solvent volume in MFM-305-CH 3 -solv was estimated by PLATON/SOLV to be 28.5%. 35 Guest solvent molecules in the pores can be removed by heating at 110 C under vacuum for 10 h to give the desolvated material MFM-305-CH 3 , which retains the structure of the solvated material as determined by SPXRD. As expected, the framework of desolvated MFM-305-CH 3 is cationic since it incorporates pyridinium moieties, and these are balanced by chloride ions that hydrogen bond to the hydroxyl groups and aromatic -CH groups on the pyridinium ring [Cl /H O ¼ 2.01(1)Å; Cl /H C ¼ 2.47(1)Å]. 36,37 As a result, the m 2 -OH groups in MFM-305-CH 3 are hindered by the Cl À ions and are thus not accessible to guest molecules, leaving 1-methylpyridinium and chloride ion as potential sites for guest interaction. The stability and rigidity of the framework in MFM-305-CH 3 has been conrmed by variable temperature PXRD (50-550 C) (Fig. S2 †), which conrms framework decomposition above 450 C.

Structural analysis of MFM-305
TGA-MS measurements of desolvated MFM-305-CH 3 shows that the methyl and chloride groups can be removed from the pore at 150-300 C (Fig. S3 †) giving the iso-structural neutral material MFM-305. To prepare a bulk sample of MFM-305, assynthesized MFM-305-CH 3 -solv was heated at 180 C under vacuum for 16 h to completely remove the guest solvents and CH 3 + /Cl À (Fig. S4 †). A change in color from white to pale yellow is observed on going from MFM-305-CH 3 to MFM-305. SPXRD analysis of MFM-305 conrms that it retains the same space group I4 1 /amd, but shows a slight contraction along the a/b axis (D ¼ 0.06%) and c axis (D ¼ 4%) with an extended channel size of 5.6 Â 5.6Å. The total free solvent volume in MFM-305 was estimated by PLATON/SOLV to be 39.9%. 35 The most signicant change is conversion of the methylpyridinium species in MFM-305-CH 3 to a free pyridyl moiety in MFM-305 and formal loss of CH 3 Cl. The bridging hydroxyl groups in MFM-305 are now exposed because of the removal of chloride (see below). Moreover, the pyridyl groups are now accessible within the pores of MFM-305, and IR spectroscopy conrms the loss of the stretching vibration of the -CH 3 group at 2988 cm À1 on going from MFM-305-CH 3 to MFM-305 ( Fig. S7 †). The complete removal of Cl À in MFM-305 has been conrmed by XPS spectroscopy ( Fig. S5 and S6 †). Interestingly, the post-synthetic modication leads to distinct pore environments for isostructural MFM-305-CH 3 and MFM-305, and thus provides an excellent platform to examine their capabilities of guest binding and selectivity. To the best of our knowledge, this is the rst example of studying the mechanism of guest adsorption within an isostructural pair of MOFs with charged and neutral pore environments of this kind (Table 1).
The excellent stability of these two MOFs motivated us to measure the adsorption isotherm of SO 2 . At 298 K and 1.0 bar, MFM-305-CH 3 and MFM-305 show SO 2 adsorption capacities of 5.16 and 6.99 mmol g À1 , respectively ( Fig. 2c and d). Importantly, the SO 2 uptake is fully reversible in both materials, and no loss of crystallinity or porosity was observed for the regenerated samples. At 298 K and 1.0 bar, the SO 2 uptake of MFM-305 is notably higher than that of most solid sorbents in the literature (Fig. S29, † Table  S2 †), and only lower than that of MFM-300(In) (8.28 mmol g À1 ), Mg-MOF-74 (8.6 mmol g À1 ), Ni(bdc)(ted) 0.5 (9.97 mmol g À1 ) (H 2 bdc ¼ terephthalic acid; ted ¼ triethylenediamine), MFM-202a (10.2 mmol g À1 ) and SIFSIX-1-Cu (11.01 mmol g À1 ), which have higher surface areas of 1071, 1525, 1783, 2220 and 3140 m 2 g À1 , respectively. [21][22][23][24][25][26][27] At 303 K, MFM-305 shows an adsorption capacity  3 and MFM-305 exhibit very steep adsorption proles for SO 2 between 0 and 20 mbar, leading to an uptake of 3.59 and 3.94 mmol g À1 , respectively, accounting for 70% and 56% of the total uptake at 1.0 bar. In contrast, under the same conditions, MFM-305-CH 3 and MFM-305 show a much lower uptake of CO 2 (i.e., 0.15 and 0.40 mmol g À1 , respectively). Moreover, the uptakes of N 2 under the same conditions are negligible (<0.01 mmol g À1 ) in these two materials. The isosteric heat of adsorption (Q st ) for CO 2 in MFM-305-CH 3 and MFM-305 both lie in the range of 29-34 kJ mol À1 ; on average the Q st of MFM-305 is ca. 3 kJ mol À1 higher than MFM-305-CH 3 (Fig. 2e). The Q st for adsorption of SO 2 in MFM-305 and MFM-305-CH 3 is estimated to be 39-43 kJ mol À1 and 30-32 kJ mol À1 , respectively (Fig. 2g). MFM-305 displays a higher Q st value for SO 2 uptake than MFM-305-CH 3 , indicating the presence of enhanced host-guest binding affinity upon the pore modication. To further evaluate their potential for gas separation, the selectivities for CO 2 /N 2 (S CN ), SO 2 /CO 2 (S SC ) and SO 2 /N 2 (S SN ) at 298 K have been calculated using ideal adsorbed solution theory (IAST) 40 over a wide range of molar compositions (i.e., 1 : 99 to 50 : 50) (Fig. 2f, h, S31 and S32 †). Signicantly, for a 5 : 95 mixture of SO 2 /CO 2 and a 15 : 85 mixture of CO 2 /N 2 , MFM-305-CH 3 and MFM-305 both show exceptionally high IAST selectivities, and these are enhanced by the pore modication. The calculations for S SN are subject to large uncertainties due to the extremely low uptake of N 2 . The adsorptive removal of low concentration SO 2 by MFM-305 has been conrmed by breakthrough experiments in which a stream of SO 2 (2500 ppm diluted in He/N 2 ) was passed through a packed bed of MFM-305 under ambient conditions (Fig. 3a). As expected, He and N 2 were the rst to elute through the bed, whereas SO 2 was retained selectively under dry condition (Fig. 3a). On saturation (dimensionless time > 500), SO 2 breaks through from the bed and reaches saturation gradually. The ability of MFM-305 to capture SO 2 in the presence of moisture has also been demonstrated by breakthrough experiments using a wet stream of SO 2 (Fig. 3b). In the presence of water vapor, the breakthrough of SO 2 from MFM-305 slightly reduces to 420 (dimensionless time) as a result of competitive adsorption of water. Thus, the marked differences in adsorption proles, uptake capacities, Q st between SO 2 , CO 2 and N 2 , the corresponding IAST selectivity and the dynamic adsorption experiments indicate the potential of MFM-305-CH 3 and MFM-305 have the potential to act as selective adsorbents for CO 2 and SO 2 .

Determination of binding domains for adsorbed CO 2 and SO 2
We sought to determine the binding domains for CO 2 and SO 2 in the pores of MFM-305-CH 3 and MFM-305 since comparison between the binding sites within these two materials affords a direct observation of the effect of pore environment on guest binding. High resolution SPXRD data were collected at 198 K for CO 2 -loaded samples and at 298 K for SO 2 -loaded samples at 1 bar. SPXRD data enabled full structural analysis via Rietveld renement (Fig. S8-S22 †) to yield the positions, orientations and occupancies of adsorbed CO 2 and SO 2 molecules (Fig. 4 and S33 †). Overall, all gas-loaded samples retain the I4 1 /amd space group and two crystallographically independent binding sites (I and II) are observed in each case.
In  3 are comparable to that observed in the crystal structure of SO 2 (3.10Å), 43 conrming the very efficient packing of adsorbed SO 2 molecules leading to its high observed storage density. In MFM-305, SO I 0 2 (occupancy ¼ 0.39) primarily binds to the pyridyl N-atom via a dipole interaction [OSO/N ¼ 2.78(1)Å] and also forms hydrogen bonds with the exposed hydroxyl group [-OH/OSO ¼ 3.42(4)Å] as well as the surrounding -CH groups [-CH/OSO ¼ 2.63(3) and 3.14(3)Å]. Further dipole interactions were observed between SO 2 molecules on sites I 0 and II 0 with intramolecular distance of 4.29(2)Å. Overall, these observations conrm that the methyl group and -CH groups are the primary binding sites in MFM-305-CH 3 for both CO 2 and SO 2 . In contrast, modulation of the pore environment in MFM-305 induces notable shis of binding sites to the free pyridyl nitrogen center and hydroxyl group. This study offers a comprehensive understanding of the synergistic effect of functional groups on the binding of CO 2 and SO 2 in these materials.

Analysis of host-guest binding via inelastic neutron scattering (INS)
To directly visualise the multiple supramolecular host-guest binding modes in these systems, INS spectra of bare and CO 2 -loaded MFM-305-CH 3 and MFM-305 (1 gas/Al) were collected at 5 K (Fig. 5). In addition, the structural models obtained from X-ray crystallographic studies were optimized by Addition of CO 2 in MFM-305-CH 3 is accompanied by signicant change to peaks at 13 and 27 meV, indicating stiffening of the lattice modes as a result of CO 2 inclusion. Large intensity changes were observed for the peaks at 20 meV, indicating the hindrance of rotation motion of -CH 3 groups upon CO 2 binding, consistent with the formation of hydrogen bonds as observed in the structural models. This is further accompanied by small red shis for the peaks between 85 and 200 meV (bending modes of -CH groups). Thus, the INS result conrms that the methyl and -CH groups are the effective binding sites for CO 2 in MFM-305-CH 3 . Addition of CO 2 in MFM-305 leads to similar changes of peaks at 19, 28 and 37 meV. Signicant changes to peak at 53, 83, 115 and 119 meV indicate that the bridging hydroxyl is directly involved in binding to CO 2 . The peaks between 120 and 200 meV also show notable changes in the C-H modes on CO 2 binding. This result conrms unambiguously that the -OH group and pyridine ring are the primary binding site for CO 2 in MFM-305, in excellent agreement with the crystallographic results. Thus, the INS results have conrmed the shis of primary binding sites upon the modi-cation of pore charge distribution.

Analysis of CO 2 binding via in situ synchrotron infrared micro-spectroscopy
In order to study the interaction between adsorbed CO 2 molecules and the MOF hosts at 298 K, an in situ synchrotron IR micro-spectroscopic study 22 was carried out as a function of CO 2 loading. Upon desolvation of MFM-305-CH 3 under a dry He ow, an absorption band at 3690 cm À1 corresponding to the n(m 2 -OH) stretching mode is observed. Upon dosing CO 2 up to 1.0 bar, this peak remained at the same position, but the peak intensity increased slightly indicating a through-space effect due to the weak interaction between Cl À and CO 2 molecules (Fig. 5b). In contrast, upon dosing desolvated MFM-305 with 1.0 bar CO 2 , the peak at 3690 cm À1 shis very slightly but the peak intensity decreases notably (Fig. 5d) indicating the presence of an strong interaction between CO 2 and hydroxyl groups, entirely consistent with the crystallographic study. The combination bands of the -C]Cvibrations 44 within MFM-305-CH 3 centered at 1555 cm À1 shi to 1560 cm À1 , and that at 1552 cm À1 in MFM-305 shis to 1566 cm À1 upon CO 2 loading, consistent with the formation of supplementary interactions between CO 2 molecules and pyridinium or pyridyl rings (Fig. S40 †). Thus, the observed changes in IR experiments gives further evidence and supports the distinct interactions between guest CO 2 molecules and these two porous MOFs.

Analysis of host-CO 2 binding via in situ 2 H NMR spectroscopy
The -CH 3 group in the 1-methylpyridinium dicarboxylate linker represents a fast stochastic rotor or natural isolated gyroscope. As such, its rotational parameters can be used to track the possible binding interaction with the guest molecules. 45 We were interested to probe further the dynamic changes of the methyl groups on CO 2 binding by synthesising MFM-305-CD 3 and studying it by solid state 2 H NMR spectroscopy over a wide temperature range (90-300 K). MFM-305-CD 3 was obtained using the same synthetic route described above but using the deuterated ligand, 3,5-dicarboxy-1-methyl-d 3 -pyridinium chloride. Two samples were used in this 2 H NMR study: guest-free MFM-305-CD 3 and CO 2 -loaded MFM-305-CD 3 . Typically the -CD 3 group is expected to exhibit very fast uniaxial rotation and hence its dynamics can be probed by 2 H NMR spin-lattice (T 1 ) relaxation, which is sensitive to rapid (rate > 10 6 s À1 ) motions. 45,46 The T 1 relaxation curves (Fig. 6) show for both materials that the temperature dependence is characterised by two regions of monotonous decrease separated by a local minimum (marked as a and c in Fig. 6). Such behavior shows that in addition to the usual uniaxial rotations, the methyl groups perform another type of motiona faster one, as it governs the relaxation curve notably at lower temperatures (marked b and d in Fig. 6). In CO 2 -loaded MFM-305-CD 3 , both modes are notably slower which is reected in the behavior of the T 1 curves behavior at $180 K and $100 K. A quantitative analysis requires a model of rotation (Fig. 6d and e): considering the close contact of the -CD 3 group with the neighboring linker (d $ 3Å), it is reasonable to assume that the slower (a, c) motion (k 1 , k 1 0 ) reects the uniaxial rotation of the -CD 3 group about its C 3 symmetry axis aligned along the N-CD 3 bond, while the faster (b, d) motion (k 2 , k 2 0 ) represents small angular librations of the rotating axis restricted within borders AEg lib .
Parameters derived within such a model conrm that the rotation along N-CD 3 axis (E 1 ¼ 4.2 kJ mol À1 , k 10 ¼ 2.1 Â 10 10 s À1 ) reaches a rate of k 1 $ 10 8 s À1 at 100 K (a), while in the presence of the CO 2 it is $10 times slower with k 1 0 $ 10 7 s À1 (E 1 0 ¼ 4.2 kJ mol À1 , k 10 0 ¼ 1.8 Â 10 9 s À1 ), and reaches 10 8 s À1 at 180 K (c). Hence, by interacting with CO 2 , the pre-exponential factor of the -CD 3 rotation is affected. This indicates that although the interaction with CO 2 is not sufficient to increase the activation barrier of the rotation, the dynamic density of guests around the methyl groups is tight enough and sufficient to slow down the rotation rate by 10-fold by random collisions. Similarly, the libration mode is affected as well and the preexponential is faster and reaches its characteristic minimum 45 below 90 K (b, d). For MFM-305-CD 3 its rate at 100 K is k 2 $ 10 11 s À1 , while for CO 2 -loaded MFM-305-CD 3 it is notably slower and can be resolved unambiguously (E 2 0 ¼ 5.1 kJ mol À1 , k 20 0 ¼ 0.65 Â 10 13 s À1 ) with k 2 0 $ 10 10 s À1 (100 K).
Notably, the amplitude of the restricted librations also sense the presence of CO 2 with g lib $ 5 decreasing to g lib 0 $ 2 . This decrease in libration angle coupled with the strong deceleration of both motional modes by $10 times in the presence of CO 2 evidences the interaction of CO 2 guests with the methyl groups within the MOF pores.
Structural dynamics of restricted CO 2 molecules in the pore To investigate the dynamics of host-guest interaction, we sought to study the structural exibility of restricted CO 2 molecules (e.g., positions, orientations and occupancies) within the pores of MFM-305-CH 3 and MFM-305 via variable temperature SPXRD. Le Bail analysis reveals changes in lattice parameters of CO 2 -loaded samples as a function of temperature (Fig. 7, Table S3 †). As the temperature decreases from 273 K to 117 K, the lattice parameters of CO 2 -loaded MFM-305-CH 3 contract along all directions (DV/V ¼ 0.7%), whereas MFM-305 expands along c axis (Dc/c ¼ 1.3%) and contracts along the a/ b axes (Da/a ¼ 0.02%) as the temperature decreases from 270 K to 100 K (Fig. S41 †). Throughout the temperature range studied, two independent binding sites for CO 2 molecules were observed in both samples. In general, the intermolecular distances of MOF-CO 2 and CO 2 -CO 2 decrease continuously as the temperature decreases (Tables S3 and S4 †). The CO 2 supply was maintained at 1.0 bar on going from room temperature to 198 K. In MFM-305-CH 3 the occupancy of CO 2 (I + II) increased from 0.60 to 0.70 from 273 K to 230 K, with CO I 2 hydrogen bonding with the methyl group and the -CH groups on the pyridyl ring. The hydrogen bond distances [OCO I -H 3 C] decrease steadily from 3.10(2) to 2.38(2)Å from 273 K to 117 K, indicating that the strength of these hydrogen bonds is highly sensitive to temperature (Fig. 7, Table S4 †). Interestingly, as the temperature decreases, we observe signicant rearrangement of restricted CO 2 molecules at site I and II. CO I 2 molecules move closer to the pore surface and rotate to drive the oxygen atoms closer to the ligands to form stronger hydrogen bonds. CO I 2 and CO II 2 retain their T-shape arrangement, but move closer to each other. From 273 K to 117 K, the ratio [i.e. CO I 2 /(CO I 2 + CO II 2 ) Â 100%] of the occupancy of site I decreases from 50(2)% to 40(2)%, while the occupancy of site II increases from 50(2)% to 60(1)%. The activation energy (E a ) of the site conguration and occupancy was calculated using the Arrhenius equation as a function of temperature. From 273 K to 117 K, E a for site I is 0.233 kJ mol À1 , and for site II it is À0.128 kJ mol À1 . The changes of site conguration and occupancy reveal that the host-guest binding is highly sensitive to temperature via intra-pore re-arrangement.
Variable temperature PXRD data were collected for MFM-305 using the same method. CO I 0 2 forms hydrogen bonds with hydroxyl group and the -CH groups of the pyridyl ring, and the free N-center of the pyridyl ring interacts with CO 2 via dipole interactions. The hydrogen bond distance (OCO I 0 -HO) and the distance of CO 2 to the N-center (N-OCO I 0 ) both reduce from 4.20(3) to 3.34(4) and from 3.16(1) to 2.96(1)Å, respectively, as the temperature is lowered from 270 K to 100 K. The intermolecular distances between CO I 0 2 and CO I 00 2 (OCO I 0 -OCO II 0 and OCO I 0 -OCO II 0 ) reduce from 5.03(2) to 3.07(1)Å and 5.04(3) to 2.94(1)Å, respectively, from 270 to 100 K (Fig. 7  parallel to the pyridine ring at 270 K, and rotate to be almost parallel to CO I 0 2 with reducing temperature. From 270 K to 198 K, the E a of CO I 0 2 and CO II 0 2 are 2.068 and À2.842 kJ mol À1 , respectively, and from 198 K to 100 K, the E a values are À0.125 kJ mol À1 and 0.154 kJ mol À1 for CO I 0 2 and CO II 0 2 , respectively. The E a for adsorbed CO 2 molecules in MFM-305 is signicantly higher than that of MFM-305-CH 3 at 270-198 K, indicating the formation of stronger host-guest interactions in MFM-305. This study conrms that the host-guest interaction between functional groups and CO 2 molecules in these two MOFs follows the trend of hydroxyl/pyridyl groups-CO 2 > CO 2 -CO 2 > methyl group-CO 2 . Thus, the simultaneous enhancements of adsorption capacity and host-guest binding affinity upon pore modication on going from MFM-305-CH 3 to MFM-305 can be fully rationalised. The crystal structure of CO 2 -loaded MFM-305-CH 3 was also determined at 7 K by neutron powder diffraction which conrms retention of the space group I4 1 /amd. Interestingly, a completely new structure was resolved where only one binding domain for adsorbed CO 2 was located near the methyl group (Fig. S34 †). The adsorbed CO 2 molecules are disordered about a 2-fold rotation axis and in a cross-tunnel mode interacting with the methyl group with a CH 3 /OCO distance of 3.58(2)Å. CO 2 also binds to the pyridiniumhydrogen atoms with a CH/OCO distance at 3.08(2)Å. This result conrms the signicant impact of temperature on the binding sites and orientation of adsorbed CO 2 molecules in MFM-305-CH 3 .

Conclusions
We have reported here the synthesis and characterisation of porous MFM-305-CH 3 , and its transformation via post-synthetic demethylation to give the isostructural, neutral MFM-305. The post-synthetic modication has enabled direct modulation of the pore environment, including changes in charge distribution and accessible functional groups. Signicantly, MFM-305 shows simultaneously enhanced CO 2 and SO 2 uptake and CO 2 /N 2 and SO 2 /CO 2 selectivities over MFM-305-CH 3 ; these two factors are widely known to display a trade-off in porous materials. A comprehensive investigation of the host-guest binding using a combinations of synchrotron X-ray and neutron powder diffraction, INS, 2 H NMR and IR spectroscopy and modelling has unambiguously revealed the role of Lewis acid, Lewis base, chloride ions, methyl and hydroxyl groups in the supramolecular binding of guest molecules within the pores of both MOFs. Considering that these two MOFs have similar pore shape and size, the distinct binding mechanisms to guest molecules between these two samples is a direct result of the charge modulation. We have also conrmed that post-synthetic modication via dealkylation of the as-synthesised metalorganic framework is a powerful route to the synthesis of materials incorporating active polar groups, in this case a free pyridyl N-donor. Thus, deprotection of the as-synthesized MOF allows the synthesis of materials that cannot as yet, in our hands, be synthesized directly.

Conflicts of interest
The authors declare no competing nancial interests.