Binding of carbon dioxide and acetylene to free carboxylic acid sites in a metal–organic framework

The functionalisation of organic linkers in metal–organic frameworks (MOFs) to improve gas uptake is well-documented. Although the positive role of free carboxylic acid sites in MOFs for binding gas molecules has been proposed in computational studies, relatively little experimental evidence has been reported in support of this. Primarily this is because of the inherent synthetic difficulty to prepare MOF materials bearing free, accessible –COOH moieties which would normally bind to metal ions within the framework structure. Here, we describe the direct binding of CO2 and C2H2 molecules to the free –COOH sites within the pores of MFM-303(Al). MFM-303(Al) exhibits highly selective adsorption of CO2 and C2H2 with a high selectivity for C2H2 over C2H4. In situ synchrotron X-ray diffraction and inelastic neutron scattering, coupled with modelling, highlight the cooperative interactions of adsorbed CO2 and C2H2 molecules with free –COOH and –OH sites within MFM-303(Al), thus rationalising the observed high selectivity for gas separation.

groups [O/O = 2.637( 14) Å], which helps to stabilise the overall structure.This conrms the approach to overcome the inherent preference of carboxylic acid groups to bind to metal centres via the use of strongly acidic conditions combined with multitopic ligands that can support hydrogen bonding.Along the crystallographic c axis the widest aperture is ∼8 Å, and the -COOH groups taking van der Waals radii into account are separated by ca 2.3 Å (Fig. 1).The phase purity of bulk samples of MFM-303(Al) was conrmed by PXRD (Fig. S1 †).Desolvated MFM-303(Al) shows a surface area of 724 m 2 g −1 (Fig. S4 †).
CO 2 isotherms were recorded over a wide temperature range (273-333 K) and showed adsorption-desorption hysteresis, which increased with increasing temperature (Fig. 2a).This is in contrast to many previously reported MOFs, in which hysteresis loops are enhanced at low temperature, 22,23 and implies that MFM-303(Al) has a small degree of temperature/guestdependant framework exibility.The isosteric heat (Q st ) and entropy (DS) of adsorption of CO 2 were calculated using the van't Hoff equation (Fig. 2b).The value of Q st decreases from 37 to 33 kJ mol −1 with CO 2 loading up to 0.5 mmol g −1 and then increases to 36 kJ mol −1 between 0.5 and 2.5 mmol g −1 .This indicates that the strong binding sites are occupied early on initial uptake of CO 2 and then notable CO 2 /CO 2 interactions are generated with increasing surface coverage.The value of DS decreases continuously with increasing loading, indicating an increase in local order of the system, again consistent with the formation of CO 2 /CO 2 interactions at higher loadings.
Pores with restricted geometry or with narrow apertures have been shown to have high potential in the eld of molecular separation, particularly where the components of mixtures have similar molecular properties, such as sizes and volatilities. 24sotherms for C 2 hydrocarbons were recorded on MFM-303(Al) at 273-333 K. Hysteresis loops are present for all the C 2 hydrocarbons and become more prominent at higher temperature (Fig. 2c  and d).For C 2 H 2 , an uptake of 2.65 mmol g −1 is observed at 293 K and 1 bar, which decreases to 2.24 mmol g −1 at 333 K.The hysteresis loop becomes prominent at temperatures above 313 K.For C 2 H 4 , a lower uptake of 1.75 mmol g −1 was measured at 293 K and 1 bar, decreasing to 1.19 mmol g −1 at 333 K.The hysteresis loop is much more pronounced than for C 2 H 2 , being apparent for measurements at all temperatures.Meanwhile, an uptake for C 2 H 6 of 1.45 mmol g −1 at 293 K and 1 bar was observed, reducing to 0.30 mmol g −1 at 333 K.The values for Q st for C 2 H 2 , C 2 H 4 and C 2 H 6 were measured to be 38-42, 34-32 and 30-24 kJ mol −1 , respectively, the order of Q st values being consistent with the observed uptake capacities and inversely related to the magnitude of the observed hysteresis.This suggests that the adsorbates with the largest molecular size do not enter the framework as readily and once inside become trapped in the pore, which results in a hysteresis loop and a lower value of Q st but higher DS (Fig. S8 †).Adsorption of CH 4 shows broad hysteretic loops and much lower uptakes (Fig. S5 †).
The presence of hysteresis loops in the isotherms suggests that there is a possible structural change from the activated to the guest-loaded material.The magnitude of the hysteresis loop is inversely proportional to the observed values of Q st with more strongly interacting adsorbates having a narrower loop.This suggests that the structural change can be modulated by the choice of guest, and molecules with stronger interactions can more readily overcome the energy barrier to re-organisation. 25lso, for the C 2 hydrocarbons, the uptake capacities and values of Q st are inversely proportional to the kinetic diameter of the adsorbate (Table S3 †).The hysteresis loops are dependent on temperature, becoming wider at higher temperature, consistent with other MOFs that feature guest-dependent exibility; 26 this is in contrast to capillary condensation in rigid porous structures. 27Within MFM-303(Al), the intramolecular hydrogen bonding can reorientate from that in the activated empty structure in order to optimise interaction of the -COOH moieties with guest molecules.The presence of exibility in sorbent materials is oen benecial to gas separations, and materials can breathe and/or undergo a gate-opening response dependent on the size and shape of the molecule.The pressure under which structural transformation occurs is unique for each adsorbent, and thus excellent separation performance and selective uptake of target molecules can be achieved. 28,29he adsorption selectivity of MFM-303(Al) for equimolar binary mixtures was estimated using ideal adsorbed solution theory (IAST) based upon single-component isotherms. 30,31nterestingly, MFM-303(Al) exhibits an excellent IAST selectivity of 15.0 for C 2 H 2 over C 2 H 4 at 1 bar and 293 K (Fig. S11 †).This compares well with other MOFs in the literature.For example, at 298 K, MOF-74(Fe) possesses a selectivity of 2.08 (ref.32) whilst UTSA-100 achieves 10.72 (ref.33) and NbU-1 reaches 5.9. 34The   In contrast, three binding sites (I-III) were determined for adsorbed C 2 H 2 molecules in MFM-303(Al) (Fig. 3d-g In situ INS measurements were conducted for MFM-303(Al) as a function of loading of CO 2 and of C 2 H 2 (Fig. 4).The  MFM-303(Al), bearing free, accessible -COOH groups and incorporating narrow pores, shows selective uptake of CO 2 and C 2 H 2 .This selectivity is a result of multiple factors: rstly, the larger value of Q st for C 2 H 2 than C 2 H 4 shows that MFM-303(Al) has a stronger binding preference, with a lower DS indicating more ordered pores when loaded with C 2 H 2 .The strong interaction of MFM-303(Al) with CO 2 and C 2 H 2 was revealed through in situ SXPD and INS/DFT studies with adsorbed C 2 H 2 and CO 2 being well-ordered within the structure via interaction with the free carboxylic acid and hydroxyl groups, whilst also being conned within the pores.In comparison, C 2 H 4 shows an overall lower uptake and a weaker interaction with the MOF, related also to its larger kinetic diameter.Interestingly, adsorption of small gas molecules in MFM-303(Al) shows hysteresis, which increases with increasing temperature and molecular size of the gas molecules.
MFM-303(Al) illustrates the importance of the free -COOH group in binding with CO 2 and C 2 H 2 molecules, and demonstrates that there is much potential in developing MOFs containing free carboxylic acid sites for selective gas separation.More broadly, the incorporation of functional groups on the interior of MOF materials provides favourable binding interactions with target guest molecules, and is a key strategy for improving selective uptake.This can be combined with optimisation of the pore geometry for the target gas molecules, enabling them to pack efficiently within the pores, thus increasing the overall uptake and selectivity.There is a careful balance to be struck, as efficient packing due to restricted pore geometry and volume may lead to an overall lower uptake when compared with materials with larger pore volumes.Thus, it is important to consider if the intended application is molecular separation (with greater emphasis on selectivity) or gas storage (with greater emphasis on total uptake/packing density).This work also emphasises the power of in situ SXPD and INS studies to reveal and understand the nature of host-guest interaction.This deserves further attention in studies of other porous solids showing exceptional adsorption selectivity.
excellent selectivity is corroborated by comparison to other reported MOFs containing acidic -OH and -COOH groups, such as Ni-gallate 35 and NTU-72 (ref.36) which show exceptional selectivities of 43.7 and 56 for 1 : 99 mixtures, though in these examples their potential for other mixtures of hydrocarbons was not fully explored.A high selectivity for CO 2 /CH 4 is also observed for MFM-303(Al) (56.1 at 293 K and 1 bar), which again compares well with other MOFs (Table S2 †).Thus, the restricted pores of MFM-303(Al) lead to enhanced selectivity for C 2 H 2 and CO 2 .Dynamic breakthrough separation of an equimolar mixture of C 2 H 2 /C 2 H 4 and C 2 H 4 /C 2 H 6 was undertaken at 333 K (Fig. 2e and f).Clear separation of components was observed in both cases with selective retention of C 2 H 2 and C 2 H 4 demonstrating the potential of MFM-303(Al) as a sorbent material for molecular separation.Two unique binding domains (I and II) for CO 2 have been determined using Rietveld renement of SXPD data for CO 2loaded MFM-303(Al) (Fig.3a-c).Notable interactions are observed between CO 2 molecules and the free -COOH and bridging -OH sites in the pore.CO 2 I (occupancy of 1.0) is stabilised by moderate hydrogen bonding interactions to the bridging hydroxyl group [m 2 -OH/O]C=O = 3.21(1) Å] and the free -COOH [O]C]O/O = 2.67(1) Å].CO II 2 [occupancy of

Fig. 2
Fig. 2 Adsorption and breakthrough data for MFM-303(Al).(a) CO 2 sorption isotherms recorded from 273-333 K, with inset showing the low pressure region, highlighting the presence of a hysteresis loop with increasing temperature; (b) isosteric heat of adsorption (Q st ) calculated from isotherms using the van't Hoff equation; adsorption isotherms for C 2 hydrocarbons in MFM-303(Al) at (c) 293 K and (d) 333 K; breakthrough plots for 50 : 50 mixtures of hydrocarbons (e) C 2 H 2 /C 2 H 4 (f) C 2 H 4 /C 2 H 6 over a fixed-bed packed with MFM-303(Al) at 333 K and 1 bar (sample mass = 2.33 g; total flow rate = 4 mL min −1 ).
difference spectra were obtained by subtracting the INS spectrum of bare MFM-303(Al) from the spectra of gas-loaded samples.DFT calculations of the INS spectra of both the bare and gas-loaded systems show excellent agreement with experimental data, allowing the full assignment of the vibrational modes.The difference spectrum of the CO 2 -loaded sample showed several changes.Difference peaks I and II are both combinations of vibrational modes, most signicantly the outof-plane wagging of -OH in the -COOH group, along with aromatic C-H asymmetric bending and C-H out-of-plane bending modes.Peak III is identied as a blueshi of the mode at 959-979 cm −1 , assigned to the bridging -OH group rocking parallel and in the plane of the [AlO 4 (OH) 2 ] chain.Peak IV is assigned to a combination of vibrational modes, most signicantly the O-H of COOH in-plane wagging and C-O-H deformation, along with a contribution from the aromatic C-H in-plane wagging.For C 2 H 2 , ve notable changes are observed in the difference spectrum.Peak I centred at 667 cm −1 arises from a combination of modes: the bridging -OH along-chain wagging, C 2 H 2 bending and aromatic C-H out-of-plane wagging.Peak II is also assigned to a combination of modes, the antisymmetric C-H binding in C 2 H 2 , and the in-plane wagging of the -OH group of the free carboxylic acid group.Peak III at 850 cm −1 arises from the O-H of the free COOH inplane wagging mode, whilst peaks IV and V arise from a combination of the aromatic C-H out-of-plane wagging and the free carboxylic acid group O-H in-plane wagging, respectively.These observations on the host-guest binding dynamics

Fig. 3
Fig. 3 The crystal structure of gas-loaded MFM-303(Al) determined via Rietveld refinement of high-resolution SXPD data showing two unique sites for CO 2 (a-c) and three for C 2 H 2 (d-f) with partial occupancies (aluminium, blue; carbon, black; oxygen, red; hydrogen, white; CO I 2 , dark blue; CO II 2 , dark green; C 2 H 2 I , orange; C 2 H 2 II , pink; C 2 H 2 III , light green).Views down the crystallographic c-axis: (a) and (d) packing of adsorbed molecules; (b) the interaction of CO 2 I with the free -COOH and m-OH groups of the framework; (c) medium-strong interactions of CO II 2 with free -COOH groups; (e and f) interactions of C 2 H I 2 and C 2 H 2 II with free -COOH groups; (g) interaction of C 2 H 2 III with free -COOH groups and the phenyl ring of the framework.

Fig. 4
Fig. 4 (a) (top) INS spectra of the activated, CO 2 -loaded and C 2 H 2 -loaded MFM-303(Al); (bottom) comparison between INS spectra of MOF using DFT and experimental INS data.(b) Difference spectra between the bare and gas-loaded MFM-303(Al) with key vibrational modes indicated which change in intensity upon gas loading (dashed horizontal line indicates y = 0 for each spectrum).(c and d) Visualisation of the vibrational modes that correspond to the indicated peaks in the difference spectrum using DFT-calculated INS spectra.The presence of framework flexibility contributes to the observed discrepancies between the simulated and experimental spectra.