Connecting Defects and Amorphization in UiO-66 and MIL-140 Metal-organic Frameworks: A Combined Experimental and Computational Study

The mechanism and products of the structural collapse of the metal-organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal-ligand bonding in each case. The amorphous products contain inorganic-organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.


Introduction
Crystalline metal-organic frameworks (MOFs) continue to be of interest to the scientific community due to their high surface areas and related potential for gas sorption, separations, drug delivery and catalysis. 1,2 4][5] The use of pressure, temperature or shear stress to induce transitions between crystalline and amorphous states is of particular intrigue, [6][7][8] due to possible uses in reversible sorption, conductive and multiferroic applications. 9Whilst computational or experimental characterization of purely inorganic 10 or organic 11 amorphous frameworks is known, structural insight into amorphous MOFs (aMOFs) is largely limited to those which adopt similar network topologies to the zeolite family. 12anochemistry, or ball-milling is increasingly utilized to synthesize crystalline MOFs in relatively large quantities with minimal solvent use. 13Curiously, MOFs are highly susceptible to collapse using the same treatment. 14Whilst this can be mitigated by pore-filling prior to treatment, it remains problematic given the use of ball-milling as a common post-processing method to increase external surface area. 15,16 his propensity for collapse has previously been linked to the low shear moduli of the family, 17 although recent reports have emerged that appear to contradict this argument. 18,19  zirconium-based MOF UiO-66 [Zr 6 O 4 (OH) 4 (O 2 C-C 6 H 4 -CO 2 ) 6 ] 20 crystallizes in the space group Fm3m, and consists of Zr 6 O 4 (OH) 4 octahedra connected together by benzene-1,4-dicarboxylate (bdc) linkers in three dimensions. 21The inorganic cluster has a twelve-fold coordination and each Zr 4+ ion is connected to four intra-cluster oxygen atoms, and four distinct carboxylate linkers (Figs.1a, 1b). 22The presence of such high coordinate inorganic centers is thought to confer a large shear modulus upon the framework, though rapid amorphization upon ball-milling is still observed. 18e two related MOFs, MIL-140B [ZrO(O 2 C-C 10 H 6 -CO 2 )] and MIL-140C [ZrO(O 2 C-C 12 H 8 -CO 2 )], also collapse relatively quickly upon ball-milling.Respectively crystallizing in the space groups Cc and C2/c, each contains seven-coordinate Zr 4+ ions, which are connected to four 2,6-napthalene dicarboxylate (ndc) and 4,4'-biphenyldicarboxylate (bpdc) ligands respectively.Purely inorganic ZrO chains line the c-axis of the cells and delimit triangular channels (Fig. 1c), which are larger in MIL-140C due to the larger bpdc organic linker used (Fig. 1e).Whilst π-stacking along the c-axis between all bpdc ligands (i.e.those bisecting the lozenge-shaped channels) is observed in MIL-140C, only 50% of the corresponding ndc ligands in MIL-140B participate in this stabilization (Fig. 1d, 1f). 23 2 The MIL-140B crystal structure proposed in ref 23 , viewed along the c-axis, along with (d) π -stacking distances along the c-axis between the ndc ligands.Red -4.46 Å, green -3.63 Å. (e) The MIL-140C crystal structure proposed in ref 23 , with (f) corresponding π -stacking distances (green -3.94 Å) along the c-axis between the bpdc linkers.Zr atoms are depicted as light blue polyhedra, O is red, C is gray and H is white.
Connections between the mechanical properties of MOFs and their defect content are of interest.UiO-66 has previously been demonstrated to be prone to linker vacancies, 22,24 the extent of which can be tuned to yield differential adsorption and thermo-mechanical properties. 25,26 pecifically, recent work has pointed to the direct coordination of H 2 O and solvent molecules to Zr 4+ nodes in defective UiO-type structures. 27,28 ifferences between simulated and experimental cell parameters of solvothermally synthesized MIL-140B point towards the presence of defects in this framework, though microwave based syntheses are recorded as producing a defect free material. 29,30 otential links between defects and amorphization mechanisms are therefore of much interest, alongside the structure and properties of the resultant amorphous products.
We use infrared spectroscopy and pair distribution function (PDF) measurements to structurally characterize the products of the ball-milling induced collapse of the rigid UiO-66, MIL-140B and MIL-140C frameworks.Density functional theory (DFT) calculations reveal the presence of defects in MIL-140B.We build defective models, and alongside 13 C solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR), use these to suggest the likely nature of the defects in the framework.
Overall, we find a quantifiable partial breakage of metal-ligand bonding in each case, though, perhaps surprisingly, the backbone ZrO chains of the MIL-140 frameworks are also damaged in the process.
The [Zr 6 O 4 (OH) 4 ] clusters of UiO-66 however remain intact.Infrared spectroscopy revealed the emergence of a band centered at ca.1700 cm-1 upon amorphization of the MIL-140 samples, which is assigned to the uncoordinated carbonyl stretching frequency (Fig. 2).

Powder X-ray Diffraction and Infrared Spectroscopy
An increase in intensity of this band was also noted upon collapse of UiO-66, where uncoordinated bdc ligands within the pores result in a small feature at 1700 cm-1 in the crystalline sample. 31

Nuclear Magnetic Resonance Spectroscopy
The qualitative observation of partial destruction of metal-ligand bonding motivated us to perform solidstate 13 C MAS NMR on the compounds.The experimental spectrum of UiO-66 (Fig. 3a) contains three peaks.Based on prior work, 31 resonances at 128 ppm and 137 ppm are assigned to the two types of carbon on the bdc aromatic ring, whilst the carboxylate-binding group gives rise to the signal at 170 ppm.The 13 C NMR spectrum of MIL-140B (Fig. 3b) is significantly more complicated, containing multiple signals in the 120 ppm -140 ppm region and two distinct resonances at 173.5 ppm and 175 ppm.A further increase in complexity is noted in the 13 C MAS NMR experimental spectrum of MIL-140C (Fig. 3c), which now contains three resonances in the region 170-175 ppm.
Upon amorphization, significant peak broadening of all signals is observed in the experimental spectra, though an indication of retention of the bdc ligand in a m UiO-66 is given by the two identifiable signals which make up the broad peak at low chemical shift (Fig. 3a).Coalescence of the signals belonging to non-carboxylate carbon atoms in a m MIL-140B* is observed (Fig. 3b), while three distinct features in the 120-145 ppm 13 C spectrum of a m MIL-140C remain apparent (Fig. 3c).Loss of chemically distinct ligand environments in the MIL-140 frameworks was confirmed by coalescence of carboxylate peaks at ca. 170 ppm in each case.

PAGE 2
A small additional feature at ca. 182.5 ppm appears upon amorphization for all three samples (Fig. 3d).
The higher chemical shift of this peak is consistent with loss of carboxylate coordination to the Zr 4+ ions upon structural collapse.Integration traces of the carboxylate signals in the amorphous samples yielded the approximate ratio of this emergent peak to the main peak.In a m UiO-66, the new feature was found to account for ca.6.8 % of the total intensity from the carboxylate carbon, suggesting a low degree of Zr-OOC bond breaking.This is raised to ca. 13.2 % and 11.0 % respectively in MIL-140B and MIL-140C, implying a higher degree of coordinate bond breaking in these samples upon amorphization.

Non-defective modeling of MIL-140B
In order to understand and better assign individual features in the experimental 13 C NMR spectra of MIL-140B and MIL-140C, DFT calculations were performed using their previously reported crystal structures, 23 which were derived using a computationally assisted strategy from the crystal structure of MIL-140A (the bdc-based isoreticular structure); the only one solved by Rietveld refinement of X-ray powder diffraction data. 23Simulated cell parameters and volumes for MIL-140B and MIL-140C were obtained in the present work upon fully relaxing coordinates and cell parameters, using four different DFT approaches; one with no dispersion correction (PBE), two with dispersion correction using the semi-empirical vdW method by means of D2 and D3 corrections (PBE-D232 and PBE-D333), and a fourth one employing one of the non-local van der Waals functionals (optB88-vdW). 34We first discuss attempts at structural optimization of alternative, non-defective MIL-140B structures, before considering defective MIL-140B models and their relative energetics, and then their respective Upon comparison of DFT results with the experimental values for MIL-140B (Table 1), all employed DFT methods converge systematically to a larger cell volume than the experimental one (7.5 % error).This mainly emanates from the recurrent overestimation of the lattice constant a found at ~28 Å (4.8 % error), while b and c are predicted with excellent accuracy (within 1.5 % error).This cell expansion occurs when dispersion corrections are omitted (PBE entry), and is surprisingly maintained when they are taken into account, a result which is not sensitive to the methodology by which the dispersion is calculated (PBE-D2, -D3 and optB88-vdW entries).A further geometry optimization of MIL-140B, constraining its cell parameters to the experimentally determined values (PBE-D3//exp entry), reveals it is much less stable by 1.57 eV than its relaxed MIL-140B counterpart (PBE-D3 entry), pointing towards an inconsistency between the structural model used and the experimentally determined cell parameters.Expectedly, the simulated 13 C spectrum of MIL-140B is in very poor agreement with the experimental one, in both regions of carboxylate and aromatic carbons (Fig. S1a), supporting a hypothesis of a different, or defective MIL-140B structure not captured in the initial model of ref 22 .
The situation is markedly different with MIL-140C, with excellent predictions from DFT structure optimizations (Table 1 and Fig. S2).While dispersion correction-free DFT calculations slightly overestimate the a cell parameter (1.6% error) and cell volume, V (4.3% error), all dispersion-corrected DFT calculations yield excellent agreement when compared to experiment, particularly the optB88-vdW entry with errors reduced to 0.4 % and 0.6% for a and V, respectively.As in all MIL-140B calculations, the c parameter is systematically very well predicted.This reflects the structural rigidity associated with the inorganic subunit of the zirconium oxide chains along that direction.Taking the dispersion effects into account improves the simulation of the MIL-140C crystal structure and highlights the importance of the π -stacking stabilizing interactions along the c-axis, which take place with the involvement of all bpdc ligands at regular center-to-center distances of 3.9 Å (Fig. 1f).The simulated NMR spectra provide significantly better agreement with observed NMR shifts, supporting a consistent initial structural model (Fig. S2).
The failure of DFT calculations to predict the correct cell volume and lattice parameters for MIL-140B reveals that its stabilization at the smaller observed cell parameters emanates from additional structural features.This together with NMR discrepancies prompted us to an in-depth investigation of the structure and defects in MIL-140B, as a case study.

Enhanced π Stacking in MIL-140B
A first variant of the reported MIL-140B structure, hereby referred to as MIL-140B(r), where (r) indicates the partial rotation of linkers, was therefore constructed by imposing a 180° rotation around the b-axis to the ndc linkers not participating in the π-stacking (Fig. 4a).The resultant MIL-140B(r) model is hence one in which a full π-stacking of ndc linkers along the c-axis occurs.As a result, the fully relaxed MIL-140B(r) model is 0.38 eV lower in energy than the parent MIL-140B, compared at the same level of theory (Table 1, PBE-D3 entry).This stabilization is assigned mainly to the now enhanced π-stacking along c.In addition, the sole rotation of these ndc linkers induces a large contraction along the a-axis, of 0.4-0.7 Å depending on the level of theory, with an error reduced to 2-3%.

Addition of uncoordinated H 2 O
While being closer to experimental findings, MIL-140B(r) still exhibits overestimated cell parameters.
Given recent reports of the defects in UiO-materials being caused by H 2 O or solvent molecules, 27,35 we thus further constructed several non-defective structures, where low concentrations of water are introduced as adsorbed molecules.Table 2 summarizes the optimized cell parameters and their relative energies obtained at the PBE-D3 level of theory.
Two hydrated models were derived from MIL-140B and MIL-140B(r), respectively (labeled "H 2 O"), by adding one adsorbed water molecule per cell (Fig. 4b, 4d).Upon full relaxation, the water molecule preferentially forms hydrogen bonds with two carboxyl oxygen atoms (1.99 and 1.95 Å) of two neighboring ndc linkers stacked along c, with minor change in cell volumes.The rotated hydrated model was found to be of lower energy (by 0.35 eV) than the non-rotated one, which is ascribed entirely due to the enhanced π-stacking in MIL-140B(r) (see Table 1).Addition of a second uncoordinated water to both models (labeled 2H 2 O), to a site at the opposite end of the organic linker to the first molecule, did not result in any cell modification nor further stabilization.

Defect Modelling H 2 O coordination to Zr combined with linker displacement
In further models, H 2 O was allowed to coordinate to Zr 4+ ions, forming defective MIL-140B structures.
Four possible defect incorporation pathways, (a)-(d) were considered (Figure 5), respectively leading to models A-D (Fig. S3).Models arising from the original MIL-140B structure are given the subscript '0', and those arising from MIL-140B(r), are given the subscript 'r', e.g.A 0 and A r respectively for pathway 'a'.PAGE 2 given in eV with respect to the structures with adsorbed (not coordinated) water molecules.'r' refers to models constructed with rotated linkers for π-stacking enhancement while "0" refers to models constructed from the parent MIL-140B.A, B, C and D refer to the various defect pathways (a, b, c, d) in Figure 5. 'e' refers to models with extended coordination of water molecules along the a-axis.
In pathway 'a', one end of an ndc linker was displaced into a monodentate state, allowing coordination  Turning to defect B, we considered translation of the entire ndc linker induced by the chemisorption of two water molecules on the opposite Zr-centers (pathway 'b' in Figure 5).As a result, the linker becomes monodentate on one side and bidentate on the other.The resultant models, B r and B 0 , are +0.26 and +0.54 eV higher in energy than the reference model, respectively.
A third type of defect yielded model C 0 (pathway 'c', in Figure 5).Whilst the defect resembles that in pathway 'b', the linker binds to two neighboring Zr-centers along the c direction.This structure was deemed unfavourable by +0.82 eV with respect to the reference defect-free model MIL-140B2H2O.
A fourth defect structure, modeled by following pathway 'd' in Figure 5 Comparison of predicted 13C NMR shifts from defective models 13 C NMR chemical shift calculations were performed on our defective MIL-140B variants in order to evaluate the pertinence of these models with respect to available experimental data.Calculation of the 13 C NMR chemical shifts for the MIL-140B(r) model yielded significantly better agreement with the experimentally observed shifts than those attained directly from MIL-140B (Fig. S1b).This is most notable in the increased complexity of the splitting pattern in the aromatic carbon region that covers more consistently the whole range of observed peaks.The concordance of such splitting between the experimental data and this MIL-140B(r) model suggests that rotation of ndc linkers might indeed occur so as to favor π-stacking locally.
This improved agreement in the 120-140 ppm aromatic region is retained in hydrated models, i.e.A re , B r and D r , in which H 2 O is introduced directly to the Zr coordination sphere.The region of the NMR spectra attributed to carboxylate shifts (170-190 ppm) of A re , B r and D r , however, shows a lower level of agreement with the experimental spectra of MIL-140B with a large dispersion of the calculated values compared to the experimental ones.
Whilst possible, defect-incorporating models (A re , B r and D r ), for the reported MIL-140B structure have been suggested, it is equally interesting that some of the features in the predicted NMR shifts agree well with those of a m MIL-140B.The range of chemical shifts across the spectra observed shows excellent agreement with those for the amorphized MIL-140B sample.The predicted 13 C NMR spectrum of model B r however, is the only one to exhibit a peak in the region 185 ppm -190 ppm, which is identified to be a fingerprint of one carboxylate end being bound in a bidentate fashion to Zr 4+ .Since this is closely reminiscent of the feature which appears upon amorphization, it provides some evidence as to the introduction of defects upon ball-milling of the structure.

Pair Distribution Function Studies
Analysis of the pair distribution function, or the weighted histogram of atom-atom distances, 36 of amorphous inorganic zeolites 37 and MOFs, 12 has in the past yielded useful information on the chemical structure of complex amorphous systems.Room temperature total scattering data were therefore collected on crystalline and amorphous samples using synchotron radiation (λ = 0.1722 Å, Q max = 22 Å - 1 ).The resultant structure factors S(Q) (Figs.S4-S6) of a m UiO-66 and a m MIL-140C are devoid of Bragg peaks, confirming the amorphous sample nature.Expectedly, that of a m MIL-140B* contains some features due to Bragg scattering.After suitable corrections using the GudrunX software, 38 the data were converted to the corresponding PDFs by Fourier Transform. 39To facilitate assignment of features, partial PDFs were calculated using the PDFgui software (Figs.S7-S9). 40   The near-identical nature of the MIL-140C framework, aside from the slightly longer organic linker, led to the labeling of corresponding distances in the PDFs with the same notation as that used in the MIL-140B PDF.Similar differences between amorphous and crystalline frameworks are observed in the PDFs of MIL-140C and a m MIL-140C (Fig. 7c, d).In particular, the same lengthening of nearest Zr-Zr distances (N) upon amorphization is witnessed.In addition, very similar reductions in the intensity of the peak belonging to nearest neighbor Zr-O correlations at 4.5 Å (O) are also observed whilst there is very little intensity in further Zr-O correlations (P-R).
This observation provides confidence that the ZrO backbone shared by both MIL-140B and MIL-140C, undergoes significant distortion during ball-milling, though some order persists in the PDF of a m MIL-140C, to 17.6 Å (S, T and U).These distances, as in a m MIL-140B*, correspond to two Zr atoms separated by the larger bpdc organic linker (Fig. 7d).

Conclusions
The results further contribute to the area of non-crystalline metal-organic frameworks and provide insight into the relationship between defects and amorphization.We have identified possible modes of defect incorporation into the structure of MIL-140B; Defects investigated include coordinated water to Zr centers combined with linker displacement.Such defective models provide significantly better agreement with the experimentally observed structural cell parameters than the defect free ones.DFT calculations of the chemical shifts of some models also show similar features to those witnessed experimentally upon amorphization of MIL-140B.We anticipate that the occurrence of such defects might be applicable to other MOFs, given the prevalence of carboxylate linkers in this family.
In addition to solid state NMR, we have used pair distribution function analysis to show that the amorphization of the prototypical UiO-66 framework proceeds via partial breakage of Zr-carboxylate bonds, though the rigid Zr 6 O 4 (OH) 4 inorganic building unit appears to remain intact.This is in stark contrast to MIL-140B and MIL-140C, where in addition to metal-carboxylate bond breaking, the inorganic backbone is also heavily damaged during the ball-milling process.This mechanism bears a large similarity to the changes seen in the ball-milling of zeolites, where M-O (M = Si, Al) bonds are observed to be broken, 41 whilst the inorganic distortions are to be expected given that pure Zirconia undergoes phase transitions and eventual amorphization when subjected to ball-milling. 42e different behavior of the two inorganic units is ascribed to the larger interconnected nature of the inorganic clusters in UiO-66, where 24 Zr-O bonds hold the unit together.This is in contrast to the ZrO chains of the MIL-140 frameworks, where in a unit containing the same number of Zr ions, only 16 Zr-O bonds hold the chain together.Whilst structural defects will also play a role in determining mechanical stability, the research here may be combined with increasing metal-ligand bond strengths, 20 to yield 'strong' MOFs capable of resisting external stresses.

calculated 13 C 2 Table 1 .
schemes.PBE-D3//exp entry refers to a geometry optimization of MIL-140B at the experimental lattice

Figure 4 .
Figure 4. Crystal structure of MIL-140B(r), formed by rotating 50% of the ndc linkers which lie along

of one H 2 O
molecule to the newly created defect site.Upon relaxation, the chemisorbed water remained bound to Zr and established hydrogen bonds with its two surrounding carboxyl oxygen atoms (1.80 Å).Interestingly, model A r is isoenergetic to the relative ground state MIL-140BH 2 O structure (0.02 eV): the energy penalty for breaking Zr-O(linker) bond (~0.3 eV) and altering the π-stacking is compensated by the coordination of the water molecule to Zr.In addition, unlike the (non-coordinating) adsorption of water, the chemisorption of water on the Zr-center in A r leads to a significant cell contraction along a

2 Figure 5 .
Figure 5. Formation of different types of defective structures upon direct binding of water on a Zr- All PDFs of crystalline species PAGE 2 are, as expected, dominated by correlations involving Zr, due to the larger X-ray scattering cross section of Zr relative to C, O and H. Due to the difficulties in obtaining reliable data below ~ 1 Å from X-ray total scattering instruments, the first peak in the PDF of crystalline UiO-66 belonging to a physical atom-atom correlation appears at ~ 1.3 Å, which corresponds to C-C or C-O direct linkages (A, Fig. 6a).Other features below 6 Å (B-F) are assigned to various Zr-Zr and Zr-O inter-cluster separations.At longer distances, overlapping contributions from similarly spaced Zr-C atom pairs results in peak broadening and renders precise assignment of the features at ~ 6.5 Å and ~ 8.6 Å challenging.The large relative scattering cross-section of Zr results in sharp features above 10 Å (G-K) in the PDF of UiO-66, which can be ascribed to Zr atom pairs joined through a bdc linker (Fig. 6b).

Figure 7 .
Figure 7. (a) PDF data for MIL-140B and a m MIL-140B*.Labels of peaks below 8 Å correspond to the

Table 2 .
Optimized cell parameters (PBE-D3 level of theory) of MIL-140B defective variants containing different amount of adsorbed or coordinated water molecules per unit cell.Energies are

A 0 27.79 13.46 7.87 90.34 92.13 90.52 2944.2 +0.21 A 0e 27.56 13.49 7.85 90.79 90.12 91.10 2918.8 +0.37 B r 27.40 13.41 7.85 90.67 89.54 87.65 2882.5 +0.26 B 0 27.69 13.47 7.85 89.43 92.26 89.51 2925.6 +0.54 C 0 27.77 13.46 7.85 90.18 93.30 89.41 2930.2 +0.82 D r 27.10 13.45 7.86 89.45 87.19 89.77 2861.3 +0.37 (
up to~0.3Å) when compared to the water-free model, making A r a promising defective model candidate.Extension of this defect with a second coordinated H 2 O molecule along the a-axis (model A re ) induces a relatively low energetic penalty (+0.40 eV).More importantly, this results in a further cell contraction of this defective MIL-140B model of the a lattice constant (a= 26.63 Å), in excellent agreement with the experimentally determined value for the MIL-140B structure of 26.71 Å.Identical treatment of MIL-140BH 2 O, the non-rotated system, gave rise to models A 0 and A 0e , which are only 0.21 eV and 0.37 eV higher in energy than the reference ground state of MIL-140B with uncoordinated water molecule, respectively, with however again poor agreement in lattice parameters.