Computational and Experimental Assessment of CO2 Uptake in

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2017, 29, 10469−10477

Computational and Experimental Assessment of CO2 Uptake in Phosphonate Monoester Metal−Organic Frameworks Benjamin S. Gelfand,† Racheal P. S. Huynh,† Sean P. Collins,‡ Tom K. Woo,*,‡ and George K. H. Shimizu*,† †

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

‡

S Supporting Information *

ABSTRACT: Phosphonate monoesters (PMEs) as ligands for metal−organic frameworks can potentially direct topology, enhance water stability, and modify pore chemistry. Here, we show, experimentally and computationally, not only that is the ratio of phosphonate to phosphonate monoester significant, but also that gas sorption depends on the distribution of the monoesters in the structure. A phosphonate monoester ligand, 1,3,5-tri(4-phosphonato)benzene-tris(monoethylester), was coordinated to copper(II) to form two different frameworks based on the same copper−phosphonate chain building units, one dense (1) and the other with an experimental surface area over 1000 m2 g−1 (CALF-33-Et3). One of the three phosphonate monoesters in CALF-33-Et3 can be hydrolyzed to make an isostructural material, CALF-33-Et2H, with approximately the same surface areas but vastly superior CO2 sorption. Controlling the hydrolysis at this site allowed the partially hydrolyzed variants, CALF-33-Et3−xHx (where 0 < x < 1), to be prepared and their gas sorption studied by experiment and simulation to determine CO2 binding sites and binding energies. These results show that each PME group can impact multiple gas sorption sites meaning that clustering versus random distributions of ester groups gives very different gas uptake. Finally, an algorithm is put forward that allows the CO2 uptake of the hydrolyzed MOF to be simulated by algebraically combining the isotherms of the nonhydrolyzed and fully hydrolyzed forms. This method can be used to assess both the degrees of ester hydrolysis and the distribution of ester groups in the solid.

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INTRODUCTION Metal−organic frameworks (MOFs), are a class of porous materials formed from metal atoms or clusters linked by organic ligands.1 MOFs have the potential to be used for a variety of applications,2−7 with commercial production and application for some already beginning. An advantage of most MOFs is that they are crystalline, allowing their exact structure to be determined and structure−property relationships to be studied and modeled. However, crystallography only represents an averaged picture and cannot give information on local structure, such as defects or distribution of nonstoichiometric groups. There have been reports of systems containing multiple functional groups on the same ligand core, which show that the functional groups are randomly incorporated into the structures rather than forming large domains of a single functional group.8,9 Knowledge of local structure will only increase in importance as MOF materials find broader application. Many MOFs use carboxylate or azolate ligation to balance strong bonding with order, though there are a variety of other coordinating groups that have been successfully employed. Phosphonates are an alternative coordinating group that have been used to make materials with excellent thermal, chemical, and hydrolytic stability.10−13 Unfortunately, in the absence of structure directing components, monophosphonates or linear © 2017 American Chemical Society

diphosphonates default to dense materials, or solids without accessible voids because of the phosphonate group’s various protonation states and subsequent numerous accessible coordination modes.14−17 One way of decreasing the number of coordinative variables and directing formation of porous structures is by including a single ester on the phosphonates, a phosphonate monoester (PME), making them more akin to carboxylates in their coordination.18−21 In terms of gas sorption, PMEs have relatively weak interactions with guests, such as CO2, as the esters are typically alkyl groups that have nonspecific interactions with guests. One intriguing methodology is to use the PME to direct the structure while removing the esters in situ, leaving a hydrogen phosphonate.22 Previously, we have reported a new framework composed of copper(II) and benzene-1,3,5-triphosphonate monoisopropyl ester, CALF-30 (CALF = Calgary Framework). This material had Cu−PME chains linked by the trigonal core to form an ultra-microporous honeycomb structure that adsorbed CO2 but not gases with larger kinetic diameters, such as N2 or CH4.20 Here we report two materials inspired by CALF-30, where Received: September 28, 2017 Revised: November 21, 2017 Published: November 22, 2017 10469

DOI: 10.1021/acs.chemmater.7b04108 Chem. Mater. 2017, 29, 10469−10477

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Chemistry of Materials

Scheme 1. Procedure for Synthesizing H3L-Et3, Beginning with an Acid-Catalyzed Trimerization of 4′-Bromoacetaphenone to the Tribromo Precursor, Then a Nickel-Catalyzed Arbuzov Reaction to the Tris(phosphonate diethyl ester), and Finally Hydrolysis to the Tris(phosphonate monoethyl ester)

additional phenyl spacers have been added to the central benzene ring, H3L-Et3 (Scheme 1). Different synthetic conditions are employed, and despite the inorganic building units being nearly identical, 1, [Cu3(L-Et3)2]n, forms a nonporous material (and so is not given a CALF designation), and CALF-33-Et3 [Cu3(L-Et3)2]n, a material with surface areas exceeding 1000 m2 g−1. By more subtly altering the synthetic conditions in CALF-33-Et3, it is possible to selectively hydrolyze one of the PMEs to give CALF-33-Et2H, an isostructural material but with vastly improved CO2 binding. CALF-33-Et3 and its selective hydrolysis were the subject of a preliminary communication.23 Here, a combination of molecular simulations, experimental adsorption isotherms, and isotherm modeling are used to ascertain the precise binding sites of carbon dioxide in the ester and de-esterified forms. The present study also examines the partially hydrolyzed analogues, CALF-33-Et3−xHx (where 0 < x < 1). The results from this component of the study are unexpected in that, for example, the 50% hydrolyzed system behaves almost like the fully esterified material for carbon dioxide capture. These results are interpreted in terms of the individual binding sites and the effect on gas sorption of the distribution of functional groups in the MOF.

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RESULTS AND DISCUSSION The preparations of the two polymorphs, compounds 1 and CALF-33-Et3, are similar. Under solvothermal conditions (48 h at 120 °C in ethanol/water, identical to that of the triphosphonato benzene analogue, CALF-30), the dense structure 1 can be synthesized as large blue crystals. Compound 1 crystallizes in the P21/n space group with one ligand and one and a half copper atoms per asymmetric unit (Figure S1). The overall structure can be described as two different types of inorganic columns bridged by the 1,3,5-triphenylbenzene cores of the phosphonate monoester ligands. The first inorganic column in 1 has a ladderlike structure where Cu1 adopts a Jahn−Teller distorted square pyramidal geometry (Cu−Oeq = 1.915(4)−1.962(4) Å, ∑Oeq−Cu−Oeq = 364.49°, Cu−Oax = 2.338(4) Å, ∠aveOeq−Cu−Oax = 95.48°). The second inorganic column has Cu2, which is half the occupancy of Cu1, with a square planar geometry (Cu−O = 1.917(5)−1.933(5) Å, ∑∠O−Cu−O = 360.00°) (Figure S3). Each ligand has three inequivalent phosphonate esters with different bridging modes. According to Harris’ notation,24 P1 and P2 each adopt a 2.110 coordination mode, and P3 adopts a 3.210 coordination mode, where each uncoordinated oxygen atom is the ester (Figures S4 and S5). Along the a axis (Figure 1a,b), the square pyramidal Cu1−PME chains bridge four ligands per inorganic unit

Figure 1. Net structures of 1 in (a) the ball-and-stick model and (b) space-filling model along with (c) the distorted square pyramidal copper−phosphonate chains and (d) square planar copper− phosphonate chains, which act as secondary building units. H, C, O, P, and Cu atoms are beige, gray, red, pink, and blue, respectively. For clarity, H atoms have been omitted in parts a, c, and d, as well as the esters in parts c and d.

(Figure 1c) while the square planar Cu2−PME chains (Figure 1d) bridge two ligands. The average aryl plane is approximately perpendicular to the square planar copper−PME chains at 80.295° (Figure S9). 10470


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(Cu−O = 1.910(5)−1.912(5) Å, ∑∠O−Cu−O = 360.00°) (Figure S7). Each ligand has three symmetrically inequivalent phosphonates; according to Harris’ notation, P1 and P2 adopt a 2.110 coordination mode, and P3 adopts a 3.210 coordination mode (Figures S4 and S8),23 where each uncoordinated oxygen atom has the ethyl ester attached or is protonated. The net structure (Figure 2a,b) consists of square pyramidal Cu1−PME chains (Figure 2c) and square planar Cu2−PME chains (Figure 2d) running parallel to the a axis while the average aryl plane is approximately parallel to the square planar copper−PME chains at 0.973° (Figure S9). The overall structure is an open framework with one-dimensional channels measuring approximately 16.1 Å × 7.2 Å, including van der Waals radii. In analyzing the single crystal of CALF-33, when examining the residual electron density around the phosphonate oxygen atoms, it was only possible to locate two of the three ethyl ester groups. Initially, this was assumed to be because of a highly fluxional and disordered ester but was later determined, via 31P NMR, to be a result of a selective in situ hydrolysis of one ester position.24 Despite many attempts, it has not yet been possible to grow single crystals of the full ethyl ester form of CALF-33. Upon comparison of the structures of 1 and CALF-33, it is evident that the two materials are similar in their composition and building units but adopt very different topologies. A summary of the structural similarities and differences can be found in Table 1. Both materials have one ligand and one and a half copper atoms per asymmetric unit. Each ligand has three unique phosphonates; two adopt a 2.110 coordination mode, and the third adopts a 3.210 coordination mode (Figure S4). Each structure also contains two different copper atoms, one in a square planar geometry, and the other in a distorted square pyramidal geometry (Figures S3 and S7). This combination of phosphonate modes and copper coordination geometries results in two distinct building units, a square pyramidal copper−phosphonate chain (Figures 1c and 2c) and a square planar copper−phosphonate chain (Figures 1d and 2d). Both 1 and CALF-33 can be described as the linking of dimeric Cu1 columns and monomeric Cu2 columns. The Cu1 columns are bridged by two PO3 groups of L in one direction, and then these assemblies are further linked to the monomeric Cu2 column by the remaining PO3 group on each ligand. In Figures 2 and 4, the general shape of each inorganic column with respect to the protruding ligands can be seen. For both 1 and CALF-33, the dimeric Cu1 columns adopt a pinwheel-like shape, and the monomeric Cu2 columns have ligands extending in a linear fashion. However, whereas, in CALF-33, the Cu1 columns are doubly bridged by L molecules to form pores, in 1, the Cu1 columns are only singly bridged by L molecules (Figure 3). The remaining coordination is occupied by L molecules bridging to the Cu2 column, effectively filling the pore that exists in CALF-33. The ester group plays a vital role in the formation of both 1 and CALF-33. In the absence of the esters (i.e., the triphosphonic acid as ligand), a poorly crystalline material is formed in synthetic conditions analogous to both 1 and CALF33 (Figure S10). The ester restricts the protonation states that are possible and likely also acts to kinetically slow the selfassembly. As previously reported with simple diphosphonate coordination, using PMEs rather than phosphonic acids results in chains rather than layers as building units,18,20,25 which allows, with appropriate guest molecules, for the highly porous CALF-33 to be formed. The preparations of dense 1 and porous CALF-33 differ only by the addition of 1,3-

There is no apparent porosity in 1 as the crystal structure shows no solvent accessible voids. Upon probing of alternative synthetic conditions, another polymorphic crystalline phase was found. The addition of 1,3-diisopropylbenzene to an otherwise identical preparation results in the formation of CALF-33 (Figure 2). CALF-33 crystallizes in the P1̅ space group also

Figure 2. Net structures of CALF-33: (a) ball-and-stick model; (b) space-filling model; (c) the distorted square pyramidal copper− phosphonate chains; (d) square planar copper−phosphonate chains, which act as secondary building units. H, C, O, P, and Cu atoms are beige, gray, red, pink, and blue, respectively. For clarity, H atoms have been omitted in parts a, c, and d, as well as the esters in parts c and d. The absent ester has been highlighted by an orange circle in part a.

with one ligand and one and a half copper atoms per asymmetric unit (Figure S6). Again, two types of inorganic columns are observed analogous to 1, and they show isomorphous structural features. Cu1 adopts a distorted Jahn−Teller square pyramidal coordination geometry (Cu− Oeq = 1.914(4)−1.965(5) Å, ∑Oeq−Cu−Oeq = 362.38°, Cu− Oax = 2.319(4) Å, ∠aveOeq−Cu−Oax = 95.44°), and the half occupied Cu2 adopts a square planar coordination geometry 10471


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Chemistry of Materials Table 1. Structural Comparison of 1 and CALF-33 1 unit cell formula crystal system space group a, b, c (Å) α, β, γ (deg) V (Å3) fractional porosityb evacuated density (g cm−3) Ar−PO3R coordination modes geometry Cu−Oeq (Å) ∑∠Oeq−Cu−Oeq (deg) Cu−Oax (Å) ∠aveOeq−Cu−Oax (deg) geometry Cu−O (Å) ∑∠O−Cu−O (deg) angle between the inorganic ribbon and organic layers (deg) C−P−O−Cusquare‑planar torsion (deg)

[Cu3(L-Et3)2] Cu3C56H52O18P6 monoclinic P21/n 5.039(1), 27.642(6), 23.662(5) 90, 94.95(3), 90 3283.54(11) 0 1.462 2.110, 2.110, 3.210 Cu1 Jahn−Teller square pyramidal 1.915(4)−1.962(4) 364.49 2.338(4) 95.48 Cu2 square planar 1.917(5)−1.933(5) 360.00 Additional Parameters 80.295 8.151, 83.971, −83.971, −8.151

CALF-33-Et3a [Cu3(L-Et3)2] Cu3C56H52O18P6 triclinic P1̅ 5.0813(1), 15.1585(3), 29.3246(7) 83.882(1), 85.728(1), 82.705(2) 2223.42(8) 0.118 1.080 2.110, 2.110, 3.210 Jahn−Teller square pyramidal 1.914(4)−1.965(4) 362.38 2.319(4) 95.44 square planar 1.910(5)−1.912(5) 360.00 0.973 109.367, 179.835, −179.835, −109.367

a This is for the structure of CALF-33 with the ethyl ester added in silico30 for a more consistent comparison. bCalculated from the solvent accessible surface area with a probe radius of 1.8 Å (approximating N2).30

Figure 3. Simplified nets found in 1 (left) and CALF-33 (right). Aryl rings, dimeric Cu1 columns, and monomeric Cu2 columns are represented by gray, green, and orange, respectively.

diisopropylbenzene. In this system, 1,3-diisopropylbenzene can act as a structure directing agent in a few different ways:26 (1) It can act as a filler, having a stabilizing energetic contribution to make the less dense structure, CALF-33, favorable. (2) It can preorganize the constituent species, such as allowing L-Et3 dimers or oligomers to nucleate in the more open arrangement by providing more hydrophobic groups with which the ligand could interact rather than the polar solvents present. (3) It can act as a template where L-Et33− is better solubilized by additional C−H···π interactions, which results in the formation of the 1D channels occupied by solvent molecules.27 Though the exact nature of this structure directing agent has not been confirmed, it is possible that these different effects are happening in conjunction. By subtle variation of the synthetic conditions, CALF-33 can be made with different and controllable levels of ester hydrolysis at the susceptible site (on Cu2, circled in Figure 2a). Although the reason why the hydrolysis occurs at just this one site is not fully understood, the hydrolysis occurs in solution followed by incorporation into the framework. Hydrolysis of the ester with retention of the network does

not occur in any condition tested (see Figures S11 and S12 and the Supporting Information). Given the voids evident in CALF-33, both the unhydrolyzed (CALF-33-Et3) and monohydrolyzed (CALF-33-Et2H) frameworks were synthesized to determine the impact of hydrolysis on the structure. Powder X-ray diffraction (PXRD; Figure S13) confirms that the unhydrolyzed and monohydrolyzed frameworks form isostructural materials by comparing with the simulated pattern from crystallography. As the two bookend structures have been previously communicated, their sorption behavior will only be summarized here prior to presenting more detailed analysis of the system.24 From N2 sorption at 77 K (Figure S14), CALF-33-Et3 has a Langmuir surface area of 1017 m2 g−1 [BET (Brunauer−Emmett−Teller) = 842 m2 g−1], which compares well to the simulated surface area range 916− 1021 m2 g−1 (1.8 Å probe radius); the range arises as uptake depends on the orientation of the esters. Despite the high surface area, CALF-33-Et3 has a low CO2 uptake of 0.92 mmol g−1 at 278 K and 1200 mbar (Figure S17). From N2 sorption at 77 K (Figure S18), CALF-33-Et2H has a Langmuir surface area of 950 m2 g−1 (BET = 810 m2 g−1), corresponding well to the simulated surface area of 969 m2 g−1 (1.8 Å probe radius). 10472


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Chemistry of Materials Hydrolysis of a single ester results in nearly double the CO2 uptake to 1.83 mmol g−1 at 278 K and 1200 mbar (Figure S21). This outcome is congruent with the removal of a hydrophobic group and a new surface with an acidic site as well as possibly more exposed metal sites. To probe water stability, a freshly activated sample of CALF-33-Et3 was exposed to water, yielding a dramatic structural change by PXRD (Figure S22). Even exposure to ambient air caused a structural change and decrease in Langmuir surface area of 287 m2 g−1 (Figures S22 and S23), giving CALF-33-Et3, a hydrolytic stability classification of 1C.28 After the changes that full ester hydrolysis at the Cu2 site has on gas sorption properties were observed, experiments were performed to determine the effect of partial hydrolysis at the Cu2 site. By controlling the water content in the synthesis of CALF-33, intermediate amounts of ester hydrolysis at the susceptible site are possible giving CALF-33-Et3−xHx (0 < x < 1). Initially, CALF-33-Et2.53H0.47 was synthesized and structural retention confirmed (Figures S25 and S26). N2 sorption at 77 K (Figure S40) indicates that CALF-33-Et2.53H0.47 has a Langmuir surface area of 1076 m2 g−1 (BET = 895 m2 g−1), larger than the surface areas observed for either the full ester or monohydrolyzed materials. This increased surface area could be a result of two factors: (1) additional pore texturing, whereby the presence of some ethyl esters causes additional surfaces for adsorption as they protrude into the pore, and (2) the lower molecular weight caused by losing some ethyl ester, when compared to CALF-33-Et3. While a 50% hydrolyzed material may have been expected to give CO2 uptake halfway between CALF-33-Et3 and CALF-33-Et2H, surprisingly, the CO2 sorption for CALF-33-Et2.53H0.47 was only slightly higher than the unhydrolyzed analogue. Further study was needed to understand this phenomenon. Previously, it had been assumed that the increased CO2 uptake in CALF-33-Et2H over CALF-33-Et3 was a result of Lewis acid/base interactions, involving the hydrogen phosphonate and/or the Cu center in the hydrolyzed materials. In a situation where half the ethyl esters are hydrolyzed, there may be a potential for the remaining ethyl esters to still block the hydrogen phosphonate group from interacting with CO2. A simple geometrical rotation of the ethyl esters about the C−O single bond (Figure 4a) shows that they have a maximum lateral radius of approximately 4.8 Å, including van der Waals radii (Figure 4b).29 However, on the basis of this simple model, the ethyl esters are not sufficiently large to block Lewis acid/ base interactions between the hydrogen phosphonate on the framework and CO2 even if the hydrogen phosphonate has ethyl esters on both sides (Figure 4c). At this point, molecular simulations were performed to elucidate the interactions that allow CALF-33-Et2H to perform much better than its unhydrolyzed analogue and give insight toward developing a model to explain the results from the partially hydrolyzed materials.29 GCMC (Grand Canonical Monte Carlo) simulations of CO2 adsorption in each material were performed at 0.15−1.2 bar, at 298 K, and with the binding sites localized from analyzing the resulting probability distributions.29 Full details about the GCMC simulations are given in the Supporting Information. The CO2 uptakes of CALF-33-Et3 and CALF-33-Et2H at 1 bar and 278 K were found to be 1.04 and 1.70 mmol g−1, respectively, in reasonable agreement with experiment. Importantly, simulations reproduce the large increase in CO2 uptake upon hydrolysis of a single ester.

Figure 4. (a) Considering the ethyl ester as freely rotating, it is confined to a cone in CALF-33-Et3. (b) The cone radius is ∼2.2 Å (dark gray circle) and ∼4.2 Å (lighter gray circle), excluding and including van der Waals radii, respectively. (c) Despite the ester cones’ volume, they are not sufficiently large to block a hydrogen (darker beige and lighter beige are without and with van der Waals radii, respectively). C, O, P, and Cu atoms are gray, red, pink, and blue, respectively. H atoms are omitted for clarity.

In the hydrolyzed material, CALF-33-Et2H, a single dominant CO2 binding site was identified that runs nearly parallel to the coordination plane of the Cu atoms as shown in Figure 5a. In this binding site, one of the CO2 O atoms was within 2.71 Å of the Cu atom, while the carbon atom of the CO2 was found to be within 2.90 Å of the oxygen atom of the hydrolyzed ester group. The binding site was found to have a CO2 binding energy of 30.5 kJ mol−1, where 45% of the net attractive interaction can be attributed to electrostatics with the remainder being due to dispersion. The large electrostatic component of the interaction is consistent with the OCO2−Cu and CCO2−OMOF distances identified. In the material with no hydrolysis, CALF-33-Et3, two unique binding sites were identified as shown in Figure 5b. In both binding sites, the CO2 lies in the pockets formed by hydrophobic aryl groups and the ethyl groups of the esters. There are no significant interactions between the Cu and the CO2 guest molecules with the closest Cu−OCO2 distance being 6.82 Å. The binding sites were found to have binding energies of 24.8 and 20.1 kJ mol−1, which are notably lower than the binding energies found in CALF-33-Et2H. Interestingly, these two binding sites are dominated by dispersion interactions with only ∼8% or less of the net binding coming from electrostatics. These results suggest that the ethyl ester in CALF-33-Et3 provides enough steric hindrance to prevent CO2 from strongly interacting with the Cu center or the bridging oxygen atoms, as compared to that in the hydrolyzed structure CALF-33-Et2H, which in turn leads to weaker CO2 binding in CALF-33-Et3. To examine the partially hydrolyzed structure CALF-33Et2.53H0.47, we created a structural model in which the PMEs and hydrogen phosphonates alternate along the square planar copper−phosphonate building units. Most interestingly, this model for CALF-33-Et2.53H0.47 also showed a CO2 uptake comparable to that of CALF-33-Et3 as was observed experimentally. Analyses of the binding sites (Figure S28) identified from the GCMC simulations also show no direct 10473


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Figure 5. CO2 binding sites identified in (a) CALF-33-Et2H and (b) CALF-33-Et3 from simulation. H, C, O, P, and Cu atoms are beige, gray, red, pink, and blue, respectively. In part a, CO2 and atoms interacting with it are shown as ball-and-stick representation. Aryl H atoms are omitted for clarity.

CO2−Cu interactions (O···Cu > 3 Å). Instead, CO2 primarily maximizes nonspecific, dispersion dominated interactions in a manner similar to that observed for CALF-33-Et3. A simple predictive model to gauge the distribution of the esters based on CO2 sorption was sought. As the clustered versus isolated phosphonate ester groups impact CO2 sorption differently, it was hypothesized that, by making a linear combination of the CO2, isotherms of CALF-33-Et3 and CALF-33-Et2H could give insight into the extent of clustering (eq 1), where N(p, T) is the adsorption of the framework at a given temperature and pressure, and a is the contribution of CALF-33-Et3. Nmixed = (a)NCALF‐33‐Et 2H + (1 − a)NCALF‐33‐Et3

Figure 6. Statistical distribution of different environments around a Cu center in partially hydrolyzed CALF-33 structures: (a) PME + PME, (b) PO3H + PME, (c) PME + PO3H, and (d) PO3H + PO3H. H, C, O, P, and Cu atoms are beige, gray, red, pink, and blue, respectively. The large spheres indicate the space that ethyl or protons can occupy, including van der Waals radii.

(1)

There are four possible environments with respect to the adjacent potentially open copper sites and the ester groups (Figure 6): (a) PME + PME, (b) PO3H + PME, (c) PME + PO3H, or (d) PO3H + PO3H. On the basis of these four environments, it is possible to propose the distribution of PMEs and hydrogen phosphonates. If a clustered distribution is assumed (i.e., large sections of PMEs and large sections of hydrogen phosphonates), then the probability for environments a−d would be 1 − x, ∼0, ∼0, and x, respectively, where x is the portion of hydrolyzed sites. Conversely, if a random distribution of PMEs and hydrogen phosphonates is assumed, then the probability for environments a−d would be (1 − x)2, x − x2 [from x(1 − x)], x − x2, and x2, respectively, again where x is the portion of hydrolyzed sites. In the case of a random

distribution, it was shown previously that environments b and c, those copper sites surrounded by a single PME and a single hydrogen phosphonate, behave similarly to environment a, PME + PME, so the probabilities can be simplified to 1 − x2 and x2 for a and d. Thus, having an experimental isotherm for a partially hydrolyzed sample, one can gain insights to the extent of hydrolysis and/or the distribution of the esters by combining the equations of the isotherms with appropriate weightings to match the experimental data. That is, it is possible to simulate the isotherm by a f ractional combination of gas sorption isotherms (FCI). Using eq 1 and the assumptions outlined above, it becomes possible to differentiate between clustered 10474


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studies on the degraded solid sample gave x = 0.29 ± 0.02 (Figure S26), a result in good agreement. As a separate check, Figure 7 also shows the curve estimated by the random model, eq 3, which clearly does not fit. A similar clustered control test was performed with CALF-33-Et2.90H0.10, which gives the hydrolysis amount to be 0.10 ± 0.01 for both FCI and NMR (see Figures S38−S41). Thus, this validated that the clustered FCI model gave predictions that fit clustered experimental data and that the fittings also matched the extent of hydrolysis. The FCI method was then applied to a sample hydrolyzed in situ, a procedure that would presumably generate randomly distributed ester groups. In this experiment, the solid was degraded and the degree of hydrolysis measured by NMR. NMR gave a degree of hydrolysis of 0.47 ± 0.03. FCI was performed on CALF-33-Et2.53H0.47 at four temperatures (Figure 8 and Figures S49−S51). Curves of best fit with the random hydrolysis model estimated the degree of hydrolysis as 0.43 ± 0.02. As further confirmation, a second sample with a different amount of hydrolysis, CALF-33-Et2.09H0.91, was also synthesized and characterized. For CALF-33-Et2.09H0.91, FCI calculations give the random distribution of hydrolysis to be 0.88 ± 0.01 (Figures S55−S58), which matches very well to the 0.91 ± 0.05 from 31P NMR. There have been notable examples of mixed functional group MOFs in the past.8,9 These frameworks have been characterized both crystallographically and by NMR, similar to the methods here. In those reports, varying not just the types of functional groups but also their distributions was shown to drastically affect the materials’ selectivity over the single component parent frameworks in a synergistic manner. The inorganic building units in CALF-33 are similar to those in many phosphonate ester MOF materials in that they are 1D chains. The present study presents a method to assess both the extent of hydrolysis of phosphonate ester groups and also their distribution in the MOFs if pure isotherms of the nonhydrolyzed and fully hydrolyzed forms are accessible. The present results show that the FCI method will be most valuable in systems that are insoluble (excluding NMR determination of hydrolysis) and/or amorphous. Moreover, the potential for FCI extends beyond phosphonate ester MOFs to larger classes of mixed porous solids.

(where a = x) and random (where a = x2) distribution using eqs 2 and 3, respectively. Nmixed = (x)NCALF‐33‐Et 2H + (1 − x)NCALF‐33‐Et3

(2)

Nmixed = (x 2)NCALF‐33‐Et 2H + (1 − x 2)NCALF‐33‐Et3

(3)

On the basis of these equations and assumptions, if the degree of hydrolysis is known, the differences between clustered and random distribution should be easily identified (see Figures 7 and 8). Conversely, if assumptions can be made

Figure 7. Experimental 278 K CO2 sorption isotherms for CALF-33Et 2H (purple), CALF-33-Et 3 (orange), and mech-CALF-33Et2.71H0.29 (black ×), the standard employed for clustered ester distribution. FCI generated isotherms are shown for random distributions (green) and clustered distributions (blue), confirming the match with the clustered standard. FCI calculations used x = 0.29 as determined from NMR analysis, for the random distribution.

Figure 8. Experimental 278 K CO2 sorption isotherms for CALF-33Et2H (purple), CALF-33-Et3 (orange), and CALF-33-Et2.53H0.47 (black ×), the standard employed for clustered ester distribution. FCI generated isotherms are shown for random distributions (green) and clustered distributions (blue), confirming the match with the random distribution. FCI calculations used x = 0.47, as determined from NMR analysis, for the clustered distribution.

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CONCLUSIONS

Here, two new frameworks have been synthesized from a trigonal planar phosphonate monoester and Cu(II). Compound 1 possesses no porosity while CALF-33-Et3 has a Langmuir surface area of over 1000 m2 g−1, despite being composed of nearly identical building units. In CALF-33-Et3, one ester per ligand can be hydrolyzed, resulting in an isostructural material (CALF-33-Et2H) with approximately double the uptake of CO2 at ambient conditions. Simulations are employed to determine the binding sites of CO2 and to understand the effect of the hydrolysis. Finally, a methodology involving the fractional combination of sorption isotherms was developed to determine the distribution of hydrolysis in CALF33-Et3−xHx (where x = the amount of hydrolysis). These results confirmed that random and clustered distributions give very different gas uptake and determined the in situ hydrolysis to be random in nature.

about the degree of clustering, then this method could be used to determine the extent of hydrolysis. This may be much more useful in systems which are more robust and where NMR study of degraded solids is not an option. As a proof of concept, a mechanical mixture of CALF-33-Et3 and CALF-33-Et2H was made by selecting arbitrary amounts of the two solids. CO2 isotherms were collected for this sample at four different temperatures. As, in this experiment, the ester groups are known to be in the clustered form, eq 2 was employed and x varied until a curve of best fit was obtained for isotherms at four different temperatures. Figure 7 shows the outcome of this fitting for one temperature. Considering the data for all four temperatures (Figure 7 and Figures S31−S33) a degree of hydrolysis of 0.26 ± 0.02 is obtained from FCI corresponding to a composition of CALF-33-Et2.74H0.26. NMR 10475


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04108. Synthetic details for 1 and CALF-33; NMR of hydrolysis experiments; and PXRD data and gas sorption analyses for all studies on partially hydrolyzed samples (PDF) CIF file for C56H52Cu3O18P6 (CIF) CIF file for C60H60Cu3O18P6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sean P. Collins: 0000-0001-6153-8515 Tom K. Woo: 0000-0003-0073-3901 George K. H. Shimizu: 0000-0003-3697-9890 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

We thank Alberta Innovates Technology Futures for a Strategic Grant and the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding the CREATE Carbon Capture Initiative and an Alexander Graham Bell Canada Graduate Scholarship to B.S.G. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS BET, Brunauer−Emmett−Teller; CALF, Calgary Framework; MOF, metal−organic framework; PME, phosphonate monoester; FCI, Fractional Combination of Isotherms; GCMC, Grand Canonical Monte Carlo; PXRD, powder X-ray diffraction; NMR, nuclear magnetic resonance; CIF, crystallographic information file

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REFERENCES

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Chemistry of Materials (27) Jorgensen, W. L.; Severance, D. L. Aromatic-Aromatic Interactions: Free Energy Profiles for the Benzene Dimer in Water, Chloroform, and Liquid Benzene. J. Am. Chem. Soc. 1990, 112 (13), 4768−4774. (28) Gelfand, B. S.; Shimizu, G. K. H. Parameterizing and grading hydrolytic stability in metal−organic frameworks. Dalt. Trans. 2016, 45 (9), 3668−3678. (29) See the Supporting Information for full details. (30) Materials Studio, version 6.0; Accelrys Software Inc.

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