When Polymorphism in Metal–Organic Frameworks Enables Water Sorption Profile Tunability for Enhancing Heat Allocation and Water Harvesting Performance

The development of thermally driven water‐sorption‐based technologies relies on high‐performing water vapor adsorbents. Here, polymorphism in Al–metal–organic frameworks is disclosed as a new strategy to tune the hydrophilicity of MOFs. This involves the formation of MOFs built from chains of either trans‐ or cis‐ µ‐OH‐connected corner‐sharing AlO4(OH)2 octahedra. Specifically, [Al(OH)(muc)] or MIP‐211, is made of trans, trans‐muconate linkers, and cis‐µ‐OH‐connected corner‐sharing AlO4(OH)2 octahedra giving a 3D network with sinusoidal channels. The polymorph MIL‐53‐muc has a tiny change in the chain structure that results in a shift of the step position of the water isotherm from P/P0 ≈ 0.5 in MIL‐53‐muc, to P/P0 ≈ 0.3 in MIP‐211. Solid‐state NMR and Grand Canonical Monte Carlo reveal that the adsorption occurs initially between two hydroxyl groups of the chains, favored by the cis‐positioning in MIP‐211, resulting in a more hydrophilic behavior. Finally, theoretical evaluations show that MIP‐211 would allow achieving a coefficient of performance for cooling (COPc) of 0.63 with an ultralow driving temperature of 60 °C, outperforming benchmark sorbents for small temperature lifts. Combined with its high stability, easy regeneration, huge water uptake capacity, green synthesis, MIP‐211 is among the best adsorbents for adsorption‐driven air conditioning and water harvesting from the air.


Introduction
The development of many sustainable technologies involving the adsorption of water vapor strongly relies on the discovery of high-performing porous materials.[3][4][5][6] In water-based adsorptiondriven heat exchangers, that is adsorptiondriven chillers (ADCs) and adsorption heat pumps (AHPs), heating or cooling, for example, for air-conditioning, are achieved by reversible multicycling adsorptiondesorption of water vapor into/from a porous adsorbent.The regeneration of the material at each cycle could be realized by applying a (low temperature) renewable energy, such as solar or waste heat (see the working principle in Figure S1, Supporting Information). [7,8]ADCs and AHPs are regarded as a clean and sustainable alternative to compressor-based The development of thermally driven water-sorption-based technologies relies on high-performing water vapor adsorbents.Here, polymorphism in Almetal-organic frameworks is disclosed as a new strategy to tune the hydrophilicity of MOFs.This involves the formation of MOFs built from chains of either trans-or cis-µ-OH-connected corner-sharing AlO 4 (OH) 2 octahedra.Specifically, [Al(OH)(muc)] or MIP-211, is made of trans, trans-muconate linkers, and cis-µ-OH-connected corner-sharing AlO 4 (OH) 2 octahedra giving a 3D network with sinusoidal channels.The polymorph MIL-53-muc has a tiny change in the chain structure that results in a shift of the step position of the water isotherm from P/P 0 ≈ 0.5 in MIL-53-muc, to P/P 0 ≈ 0.3 in MIP-211.Solid-state NMR and Grand Canonical Monte Carlo reveal that the adsorption occurs initially between two hydroxyl groups of the chains, favored by the cis-positioning in MIP-211, resulting in a more hydrophilic behavior.Finally, theoretical evaluations show that MIP-211 would allow achieving a coefficient of performance for cooling (COPc) of 0.63 with an ultralow driving temperature of 60 °C, outperforming benchmark sorbents for small temperature lifts.Combined with its high stability, easy regeneration, huge water uptake capacity, green synthesis, MIP-211 is among the best adsorbents for adsorption-driven air conditioning and water harvesting from the air.[14] AWH represents, therefore, an attractive means for the delivery of drinking/fresh water in remote and arid areas. [15,16]For both technologies, the applied porous material should, among other criteria, adsorb water vapor at low relative humidity (5% ≤ RH ≤ 40%) and release it with a minimal energy penalty.[40][41][42] Regarding tuning the pore size and/or shape of MOFs, polymorphism could be a new approach to modulate the water sorption properties of a MOF while maintaining its building blocks.Polymorphism means the occurrence of a compound in diverse crystalline structures possessing the same chemical composition (stoichiometry), but differ in the spatial arrangement of the atoms which leads to variation in physicochemical properties.50] For instance, two polymorphs of MOF Tl I (TCNQ) (TCNQ = 7,7,8,8-tetracyanoquinodimethane), which crystallize in the space groups P2 1 /c and P2/c, respectively, displayed dramatically different conductivity properties. [51]The nbo topology of Cu 2 (1,4-benzenedicarboxylate) was reported to feature a remarkably high affinity for linear alkanes due to its small pores, unlike its polymorphs with rhr and lvt topologies, respectively. [52]However, this approach has not yet been addressed, to the best of our knowledge, for improving MOFs for water adsorption and related applications.
In the case of the zigzag-spaced, pseudo-linear trans,transmuconate linker (from trans,trans-1,3-butadiene-1,4-dicarboxylate = muc 2− ), some of us recently obtained, using this linker, a new Al-MOF, MIL-53-muc, [76] showing MIL-53's topology similarly to the aluminum fumarate MOF (MIL-53-Fum), [77] where its framework is made up of rod-like chains of trans-µ-OH-corner-sharing AlO 6 octahedra.This was expected since muconate can be regarded as a twofold extension of the fumarate linker.The water sorption profile of MIL-53-muc displays an S-shaped isotherm like MIL-53-Fum, but with a higher uptake capacity and the step position shifted to higher relative pressures (P/P 0 = 0.5-0.6),compared with that of MIL-53-Fum whose step is found at a relative pressure of P/P 0 = 0.2-0.3. [72]his is in line with an increase in hydrophobic behavior, rationally attributed to the extension of the hydrophobic organic chain in the muconate linker and lower confinement effect due to larger channels.Hence, the resultant water sorption profile of MIL-53-muc renders this MOF unsuitable for AHT applications when paired with water as cooling fluid, since the step position of the isotherm is located beyond P/P 0 = 0.4.
Herein, we disclose polymorphism in aluminum muconate MOFs which consists of a simple twisting of the linear chain of trans-µ-OH-connected corner-sharing AlO 6 octahedra into a helical chain of cis-µ-OH-connected corner-sharing AlO 6 octahedra (Figure 2).Remarkably, without any significant difference in pore size/shape and volume, such a subtle structural modification results, however, in a drastic change in the water sorption profile, as it switches the inflection point (α)-that is, the relative pressure at which half of the total adsorption capacity is reached-of the less hydrophilic aluminum muconate phase MIL-53-muc from α = 0.54 to α = 0.29 in the (much) more hydrophilic new phase MIP-211.In-depth investigations combining Density Functional Theory (DFT) and in situ solidstate NMR (ssNMR) along with Grand Canonical Monte Carlo (GCMC) simulations were carried out to study the water sorption mechanism in both aluminum muconate MOFs, and thus, to understand the origin of this remarkable difference.MIP-211 was then assessed as a potential adsorbent material applicable for adsorption cooling due to its S-shaped water vapor isotherm with a steep uptake at a relative pressure of P/P 0 = 0.26-0.3, a high uptake capacity of 0.50 g H2O g MOF −1 at this relative pressure and excellent stability upon cycling.

MOF Synthesis
The newly obtained aluminum muconate, MOF MIP-211, was initially synthesized from the reaction of t,t-muconic acid (H 2 muc) with aluminum sulfate octadecahydrate [Al 2 (SO 4 ) 3 •18H 2 O] in a water/DMF (3/1) mixture (DMF = N,N′-dimethylformamide) under reflux conditions.The solvothermal treatment (T = 120 °C) of similar reaction mixture lead to the formation of a crust-like unidentified crystalline material (see powder X-ray diffraction, PXRD, pattern in Figure S5, Supporting Information).The reported aluminum muconate phase MIL-53-muc was instead obtained from the reaction of H 2 muc with aluminum nitrate hexahydrate [Al(NO 3 ) 3 •6H 2 O] in a water/DMF mixture under solvothermal conditions (see synthesis details in the Experimental Section). [72]The reflux treatment of a similar mixture leads to the formation of the phase of MIL-53-muc still, albeit with poor crystallinity (see PXRD pattern in Figure S5 1b) confirmed this hypothesis.Furthermore, analyses of the NMR parameters calculated by DFT calculations and obtained from solid-state NMR (ssNMR) measurements were found to be in agreement with this hypothesis, as MIP-211 and MIL-53-muc featured very similar 1 H MAS spectra and 13 C CP MAS spectra (Figure 1d,e; Figures S37,S38, Supporting Information) attesting that the linker has very similar configurations in both MOFs.One could nevertheless note a narrower 13 C line width for MIP-211 (see spectra simulation in Figure S6, Supporting Information, and resulting parameters in Tables S1,S11, Supporting Information), pointing to a slightly more ordered structure.MIL-53-muc 1 H lines were, however, narrower than for the MIP-211 ones (Figure S7 and Table S2, Supporting Information) but only for the most shielded 1 H peak at 2.1 ppm, ascribed to the bridging µ-OH of the chain based both on its position, [78] and on the 27 Al{ 1 H} correlation experiments (Figure S8, Supporting Information).This broadening observed for this µ-OH peak in ssNMR in MIP-211 may reveal a dynamic behavior in interaction (possibly exchanging) with a small amount of residual water in the pores.MIP-211 and MIL-53-muc displayed also very similar infrared and Raman spectra (Figures S15,S16, Supporting Information), with a band at about 3600 cm −1 ascribed to µ-OH , which also corroborates closely related polymorphs.Besides, scanning electron microscopy (SEM) showed rectangular prismatic microcrystals of ≈0.3 µm wide and ≈0.5-2 µm long for MIL-53-muc, against elongated rectangular bipyramid microcrystals of ≈2.5 µm wide and ≈8.5 µm long for MIP-211 (Figure S17, Supporting Information).
MIP-211 exhibits a type I nitrogen-adsorption isotherm at 77 K (Figure 1c) associated with a BET surface area of 1450 m 2 g −1 and pore volume of 0.60 cm 3 g −1 .This is close to the values of MIL-53-muc whose BET area and pore volume are 1500 m 2 g −1 and 0.63 cm 3 g −1 , respectively.Theoretical surface areas (2140 m 2 g −1 and 2200 m 2 g −1 ) and free pore volumes (0.86 cm 3 g −1 and 0.85 cm 3 g −1 ) calculated from the DFT optimized crystal structures of MIP-211 and MIL-53-muc solids, respectively, also confirm that both solid possess nearly identical porosities although values are slightly higher than the experimental ones most likely assigned to the presence of remaining solvent in the pores.c) nitrogen sorption isotherms at 77 K (adsorption and desorption branches are represented by full and empty symbols, respectively).d) 1 H, e) 13 C, and f) 27 Al MAS ssNMR spectra of MIP-211 and MIL-53-muc.

Structural Aspects
The structure of MIP-211 was solved using the direct space method in FOX, [79] and then refined by the Rietveld method from high-resolution powder X-ray diffraction (HR-PXRD) data using Fullprof (see more details in the Supporting Information). [80]The dried and hydrated MIP-211 crystallize in the tetragonal centrosymmetric space group  S4-S6, Supporting Information), in agreement with a rigid character of this framework.DFT optimization performed on the dried form of this solid shows that the most energetically favorable crystal structure is associated with a slightly expanded unit cell volume (<4.0%) as compared to the experimental one (see Table S10, Supporting Information).A close inspection of the crystal structure of MIP-211 reveals that it is made up of helical chains of cis-connected corner-sharing AlO 6 octahedra, as observed for CAU-10H and MIL-160. [81]However, neighboring chains in MIP-211 are translation images of each other, unlike MIL-160 where neighboring chains are mirror images of each other (Figure S21, Supporting Information).The chains in MIP-211 are bridged by t,t-muconate linkers (muc), whereby each carboxylate group of the muc linker connects two Al centers from the same chain in a bidentate bridging (µ-k 2 O:O' mode) fashion.This leads to a 3D network forming 1D sinusoidal channels with a square-shaped cross-section of ≈8.5 × 8.5 Å 2 (Figure 2; Figures S22,S36, Supporting Information).MIP-211 can therefore be regarded as a polymorph of MIL-53-muc resulting from screw-twisting the straight AlO chains of trans-corner-sharing AlO 6 octahedra in the structure of MIL-53-muc, all other elements being equal (Figure 2; Figures S22,S23, Supporting Information).So far, to our knowledge, such a polymorphism has not been reported for MOFs with the same ligand.This can be accounted to the degree of flexibility that muc ligand allows.Indeed, a close comparison between the ligands in the two MOF structures reveals, as shown in Figure S24, Supporting Information, some conformational differences needed for their accommodation within each of the frameworks.
The 27 Al spectra (Figure 1f) displayed a single well-defined quadrupolar-like component for MIP-211, while a more complex line shape is found for MIL-53-muc associated with a distribution of environments. 27Al MQMAS experiments (Figure S9, Supporting Information) confirmed this analysis with the presence of at least a second component for MIL-53-muc.DFT-calculated 27 Al NMR parameters and corresponding NMR spectra reported in supporting information Table S12, and Figures S37c,S38c S3, Supporting Information) and the DFT calculations (Table S12 and Figures S37c,S38c, Supporting Information) showed marked differences between the two MOFs corroborating that the structural dissimilarities between MIP-211 and MIL-53-muc stem primarily from the connection linking the AlO 6 octahedra (cis-connected vs trans-connected) and therefore the shape of the resulting AlO chains.
Interestingly, MIP-211 is to the best of our knowledge, the first reported Al-MOF made up of helical Al-O chains which incorporate a linear linker.This suggests that analogous polymorphs of MIL-53-type Al-MOFs with linear linkers like terephthalate and fumarate could be conceivable.

Water Sorption Profile
The investigation of their water vapor sorption behavior showed that MIL-53-muc displays a single-step sigmoidal (S-shaped) water sorption isotherm with the large step located at the relative pressure P/P 0 = 0.5-0.6 and a moderate hysteresis loop between the adsorption and desorption branches (Figure 3a).A step located above P/P 0 = 0.4 indicates that MIL-53-muc can be regarded as a more hydrophobic material, [8,32,52] and could have a slight water-induced network flexibility as often observed for MOFs with the MIL-53 topology. [82]At 25 °C, The water uptake capacity of 0.50 g H2O g −1 MOF at P/P 0 = 0.6 slightly increases to 0.60 g H2O g −1 MOF at P/P 0 = 0.9.Unlike MIL-53-muc, MIP-211 features an S-shaped water vapor isotherm with no hysteresis loop between the adsorption and desorption branches, which confirms the rigidity of MIP-211's framework.At 25 °C, the adsorbed amount of water increases gradually with increasing relative pressure up to 0.10 g H2O g MOF −1 , right below P/P 0 = 0.3, which is followed by a steep uptake to 0.50 g H2O g MOF −1 at about P/P 0 = 0.3, and eventually reaches 0.60 g H2O g MOF −1 at P/P 0 = 0.9 (Figure 3a).Interestingly, the step position in the water isotherm of MIP-211 is drastically shifted to a lower relative pressure of about P/P 0 = 0.3, compared with that of MIL-53-muc, meanwhile the water uptake capacity both at the step position and the total water capacity remains almost unchanged in agreement with their similar pore volumes.The step position of the water isotherm at P/P 0 = 0.3 makes MIP-211 a comparatively far more hydrophilic solid.This shift was unexpected since the two MOFs are both made up of the same muconate linker and AlO-chains with bridging µ-OH-groups having only a different configuration.Moreover, both frameworks feature similar pore geometries (lozenge-vs square-shaped 1D channels, Figure 2c,d), and essentially identical surface areas and pore volumes (vide supra).Therefore, at first sight, one would expect both MOFs to also feature similar water affinity.Remarkably, the polymorphism between MIL-53-muc and MIP-211, which consists of twisting the straight linear AlO chains into helical AlO chains, appears to be a strategy to tune the hydrophilicity or in general the guest affinity to MOFs.
It is worth mentioning that, among the benchmark Al-MOFs already investigated for water-adsorption-based applications, the water sorption isotherm of MIP-211 is particularly close to that of CAU-23 in terms of the inflection points position (α = 0.27 and 0.29 at 25 °C; Figure 3e), the steepness of the uptake step (almost vertical for both MOFs), the insignificant hysteresis loop, and the enthalpy of water adsorption (ΔH ads = −48.2,and −48.5 kJ mol −1 for CAU-23, [58] and MIP-211, respectively; vide infra).However, the IBU (Inorganic Building-Unit) in CAU-23 consists of chains of mixed cis-and trans-µ-OH-connected AlO 6 octahedra, unlike chains of exclusively cisµ-OH-connected AlO 6 octahedra in MIP-211, the latter having also a far higher water uptake and working capacity (around the step) compared with CAU-23.Meanwhile, one should also highlight the polymorph of CAU-23, namely MIL-53-TDC, consisting of chains of exclusively trans-µ-OH-connected AlO 6 octahedra, that features slightly lower hydrophilicity, manifested by the large water uptake step located at a relative pressure slightly over P/P 0 = 0.3 (Figure S25, Supporting Information).The minor shift of the inflection point at 25 °C from α = 0.35 in MIL-53-TDC to α = 0.29 in CAU-23, which was not previously emphasized, is likely due to the partial presence of cisconnected AlO 6 octahedra in the framework of CAU-23.Consequently, unlike MIP-211 the sole presence of only cis-µ-OHconnected AlO 6 octahedra can be seen as a way to induce a drastic shift of the water isotherm step toward lower relative pressures, with respect to MIL-53-muc (only trans-µ-OH).This comparison establishes the polymorphism of trans-connected versus cis-connected AlO 6 octahedra as a new approach to significantly tune (increase) the hydrophilicity (or to adjust the position of the water pressure step) of Al-MOFs.

Probing the Crystal Structure-Water Sorption Relationship by ssNMR
To get more insights on the interactions between water and the MOF frameworks, we have deployed advanced ssNMR studies.Upon hydration, the 13 C signals appeared not affected whereas 1 H peaks shift and 27 Al lines broaden (see Figure S11, Supporting Information), which is related to the increase of pore volume when filled with water.For both MOFs, the peak related to water dominates the 1 H spectra at around 4 ppm, the peak related to µ-OH environments at 2.1 ppm disappears and that related to the muconate slightly shifts (≈0.2 ppm) without changing line widths.This is better seen upon hydration with D 2 O which reduces considerably the water peak (Figure S12, Supporting Information) evidencing at least two water components and a small peak near 5 ppm.The former are assigned to the H 2 O molecules in contact with the pore walls and inside the pores, [33] while the later peak is undoubtedly assigned to µ-OH environments based, again, on 27 Al{ 1 H} double resonance D-HMQC (dipolar heteronuclear multiple-quantum coherence) experiments which dramatically increases its intensity (Figures S8,S12, Supporting Information).In the case of MIL-53-muc, this µ-OH peak is found at 4.6 ppm with a width of 0.84 ppm whereas for MIP-211 its maximum is at 5.1 ppm with a width of 0.37 ppm (see Figure S13 and Table S2, Supporting Information).The strong positive shift seen for both compounds indicates a strong µ-OH/H 2 O chemical interaction, and the bigger line width for MIL-53-muc than MIP-211 suggests a stronger interaction for the former.This is the opposite behavior with respect to the dried sample (see Figure S12, Supporting Information), suggesting that the degree of hydration may play a significant role in the µ-OH/water interaction.In all cases, the hydrophilic character seems to be controlled by the µ-OH sites.
Furthermore, the resolution provided by the hydration with D 2 O was exploited to measure the water self-diffusion coefficient using 1 H pulse-field gradient (PFG) NMR under magic angle spinning (Figure S14, Supporting Information).For both compounds, the main component at 4.0 ppm diffuses faster (D S = 1.82 × 10 −9 m 2 s −1 for MIL-53-muc and D S = 1.41 × 10 −9 m 2 s −1 for MIP-211) and is related to bulk water not interacting with the pore walls, but, as expected, diffusing slower than in bulk water at room temperature (D = 2.01 × 10 −9 m 2 s −1 ). [83]Additional components at 4.3 ppm and 3.7 ppm for MIL-53-muc exhibit D S = 1.34 × 10 −9 m 2 s −1 and 1.47 × 10 −9 m 2 s −1 , respectively, whereas for MIP-211 a single additional component is found at 4.3 ppm with D S = 1.12 × 10 −9 m 2 s −1 .The reduced self-diffusion coefficient confirms their attribution to water molecules in interaction with the pore walls.All types of water molecules are diffusing slower in MIP-211 than MIL-53-muc as a consequence of stronger interactions with the walls and in line with a lower hydrophobicity of this compound.The absolute values of D S retrieved here are in line with those obtained in MIL-100(Fe), [84] but much higher than those of MIL-53(Cr) or MIL-100(Al). [85,86]This could be due to the fact that, to our best knowledge, for the first time, our PFG measurements are performed under MAS conditions, with deuterated water and at high magnetic field (17.6 T).

Molecular Understanding of the Water Sorption Mechanism
GCMC simulations were performed to shed light on the microscopic origin of the water adsorption performance of MIL-53-muc and MIP-211.As shown in Figure S39, Supporting Information, the overall shape of the experimental water adsorption isotherms, particularly the positions of the step on the P/P 0 axis are well reproduced for both MOFs confirming the lower hydrophobicity of MIP-211 compared to MIL-53-muc.The slight overestimation of the total uptakes at saturation pressure is attributed to larger free pore volumes of the DFT optimized geometries compared to the experimental porosity determined from N 2 adsorption measurements (Table S10, Supporting Information).Both MOFs possess very similar hydrophobic pore confinement (≈8.5 Å in diameter, Figure S36, Supporting Information).However, the density of the hydrophilic µ-OH sites is much higher (r O(μ-OH)-O(μ-OH) = 2.8 Å vs 3.75 Å) in the AlO helical chain of MIP-211 solid compared to that in the AlO rod of MIL-53-muc framework.As such, due to the strong dominance of the hydrophobic character of the confined pore geometry, the hydrophilic µ-OH sites of MIL-53-muc are able to adsorb water molecules only at a relative pressure, P/P 0 = 0.50 (Figure 4a).Upon increment of water vapor pressure, those water molecules anchored to the µ-OH (primary nucleation sites) enable the adsorption of additional water molecules and form hydrogen bonds with other neighboring µ-OH sites of the pore walls (Figure 4b).The associated adsorption enthalpy calculated at the beginning of this water clustering process is quite low (≈−33 kJ mol −1 ) in line with the hydrophobicity of MIL-53-muc.Notably, the radial distribution functions (RDFs) for H 2 O/H 2 O and H 2 O/µ-OH pairs calculated at this intermediate pore filling pressure (viz., P/P 0 = 0.60) indicate that although water molecules strongly interact with the µ-OH sites with a separating distance of 2.7 Å. Intermolecular water-water interactions supersede the former as evidenced by the relatively higher intensity of the corresponding RDF peak centered around 2.8 Å (Figure 4g).Further, an increase of the water vapor pressure to P/P 0 = 0.75 leads to a complete filling of MIL-53-muc porosity as depicted in Figure 4c.
In the case of MIP-211, at the beginning of the adsorption process at P/P 0 = 0.30, we note that a single water molecule effectively interacts with two µ-OH sites (Figure 4d) resulting in a simulated adsorption enthalpy of −45 kJ mol −1 , which is higher than the value obtained for MIL-53-muc and in line with its overall reduced dominance of the hydrophobic character.The RDFs presented in Figures 4h for the intermolecular O(H 2 O)…O(µ-OH) interaction show a first peak at 2.8 Å accompanied by a wider shoulder spanning up to 3.2 Å, corroborating that these µ-OH sites are likely to attract water molecules much strongly as compared to MIL-53-muc.As shown in Figure 4e, a subtle increase of water vapor pressure from 0.30 to 0.35, over half of the MIP-211 porosity is filled with water molecules.These adsorbed water molecules form additional hydrogen bonds with other µ-OH sites in vertical (along the channel) and lateral directions facilitating a quick filling of the entire porosity at P/P 0 = 0.40 (see Figure 4f).

Application of MIP-211 for Adsorption-Driven Heat Transformation
The stepwise water vapor sorption isotherm of MIP-211, with the step pressure located at P/P 0 = 0.3, indicates this material to be a promising adsorbent candidate for water-adsorption-driven chillers.In this regard, MIP-211 holds a record water uptake capacity (0.6 g H2O g MOF −1 , corresponding to a volumetric uptake capacity of 0.48 mL H2O mL MOF −1 ; the calculated cell density is ρ = 0.811 g. mL −1 ) among all microporous Al-MOF materials that have been investigated for heat reallocation applications (see Figure 3; Table S7, Supporting Information).The water uptake of MIP-211 at P/P 0 = 0.3 surpasses by far that of the benchmark CAU-23 (0.37 g g −1 ), MIL-160 (0.35 g g −1 ), KMF-1 (0.39 g g −1 ), CAU-10H (0.30 g g −1 ), MOF-303 (0.40 g g −1 ), MIL-53-Fum (0.35 g g −1 ), MOF-333 (0.38 g g −1 ), and MIL-53-TDC (0.30 g g −1 ) (see Figure 3f).The isosteric heat of adsorption was calculated by applying the Clausius-Clapeyron equation to linearly interpolated high-resolution water adsorption isotherms collected at 15, 25, and 35 °C exhibiting step positions at relative pressures of 0.27, 0.29, and 0.31, respectively (Figure 3b).The resultant value (Figure 3c; Figure S31, Supporting Information) of ≈−48 kJ mol −1 is in excellent agreement with the GCMC calculated adsorption enthalpy and the value expected by a simple Dubinin approximation at 25 °C (47.0 kJ mol −1 ), and is only slightly higher than the evaporation enthalpy of water (40.8 kJ mol −1 ) due to the high relative pressure at the uptake step.The moderate value of the enthalpy of adsorption suggests that the water adsorbed by MIP-211 could be desorbed with moderate amounts of regeneration heat resulting in higher cycle efficiencies.
The robustness of MIP-211 was finally tested in order to assess its practical usage for thermally-driven adsorption cooling applications.Thermogravimetric analysis (TGA) and variable temperature-depended powder X-ray diffraction (VT-PXRD) revealed that the framework of MIP-211 is thermally stable up to 250 °C, meanwhile, it decomposes at 300 °C.MIL-53-muc is slightly more thermally stable up to 350 °C (see Figures S25,S26, Supporting Information).Noteworthy, MIP-211 maintains its crystalline integrity and porosity over stirring the material for 24 h in acidic (pH 2) and basic (pH = 12) aqueous solutions, as well as in boiling water.Nevertheless, about 14% decrease in the BET surface area and pore volume was observed for the material treated in boiled water (see Figures S27,S28, Supporting Information).Furthermore, multiple water adsorption/desorption cycles were performed with MIP-211, which showed only a slight (<2%) decrease in the water uptake capacity during the first seven cycles, which remained constant for the rest of the first round of 20 cycles (Figure 3d).Noteworthy, the second round of 20 cycles with the same material did not show any change in the crystallinity, porosity, or water uptake capacity of the material (Figure 3d; Figure S30, Supporting Information).All aforementioned hydrothermal, chemical, and multicycle water ad-/adsorption stabilities make therefore MIP-211 a highly suitable adsorbent to be applied for adsorption-driven heat transfer.
For AHT, the possible application temperatures and the expected efficiencies may directly be estimated from equilibrium material properties.For expected power densities which are crucial for economic viability, additional investigations on sorption dynamics need to be carried out in further studies.Possible application temperature levels are determined by the characteristic temperature difference ΔT ch (Figure S33, Supporting Information), the difference between the adsorbent temperature and the pure liquid refrigerant at the same equilibrium pressure, which is around 19 K.The temperature difference between the heat source for desorption and heat sink for condensation, the temperature thrust, needs to be larger than ΔT ch , proving a driving temperature difference to overcome all transport resistances during regeneration (refer to Figure S32, Supporting Information).The temperature difference between the heat sink for adsorption and the heat source for evaporation, the temperature lift ΔT lft , must be smaller than ΔT ch , providing driving forces for adsorption. [87]Typically, applications require driving forces of about 10 K. Thus, MIP-211 is suitable for an adsorption-based cooling application with temperature conditions of, for example, 20, 30, and 60 °C (or 25, 35, and 65 °C, etc.) for cooling, heat rejection to the ambient and driving heat, respectively.That is, the material is perfectly suitable for low-lift applications like moderate cooling of buildings or data centers without dehumidification, but less for high-lift applications like refrigeration or cooling under high ambient temperatures.Obviously, the low-lift characteristic allows exploiting low-grade driving heat sources like waste or solar heat.Application to large-lift applications, like air conditioning in hot climates or refrigeration will not be possible.
The expected coefficient of performance (COP) is estimated for an adsorption-based cooling application.We assume a coated fin and tube heat exchanger, as detailed by Wittstadt, [88] coated with 0.1 kg m −2 of adsorbent per heat exchanger area resulting in a total heat capacity of the heat exchanger per adsorbent mass of 7.7 kJ kg −1 K −1 .A configuration with a separated evaporator and condenser is assumed with a driving force of 8 K in total, allowing for ΔT drv = 4 K for the adsorber heat exchanger and ΔT drv = 4 K for the evaporator and the condenser respectively.The cooling COP is calculated based on the energy balance of the adsorption heat transformation cycle: [89] COP cooling L H 15 In technically realized adsorption cycles, power output becomes small when close to equilibrium which is avoided by early cycle switching.Therefore, we assume the technically exploitable loading to be between 15% and 85% of the equilibrium loading spread: More details are given in the Supporting Information.The resulting cooling COP (Figure 5 top) has been investigated to answer the questions: a) what driving temperature T H is required to run a cycle with a small lift of 10 K (T L = 20 °C and T M = 30 °C fixed)?b) what heat rejection temperature must not be exceeded to run a cycle with a driving temperature level that is high enough for all materials, that is, a large temperature thrust (T L = 20 °C and T H = 90 °C fixed).This checks at which temperature the cycle runs into a thrust limitation (Figure 5a) or a lift limitation (Figure 5b).Results show a superior performance of MIP-211 for small lifts, where a driving temperature of 60 °C is sufficient to allow for a maximum cooling COP of 0.63.This outperforms all other materials investigated here, especially MIL-53-Fum and CAU-23, [90,58] which both have a similar temperature characteristic.No other material is known with a higher performance under these conditions.The step in temperature dependency of the COP is directly related to the step-like isotherm as it depends on the temperature and whether this step can be exploited for the cycle or not.The results also confirm the aforementioned low-lift characteristic, which manifests as a strong sensitivity to the temperature level of the heat rejection T M (Figure 5b).As soon as T M exceeds 31 °C (for T L = 20 °C), the COP decreased dramatically as it does for MIL-53-Fum or CAU-23.For these harsher conditions, more hydrophilic materials like CAU-10H, and later MIL-160 are required. [91,92]The state-of-the-art silica gel (Siogel, Oker Chemie) is outperformed by MIP-211 under most conditions, [93] with a considerable increase compared to aluminum fumarate (MIL-53-Fum), which is applicable to similar temperature conditions.
This "technically-expectable" approach differs from the "ideal material COP" which is popular in fundamental material literature but where the heat capacity of any heat exchanger structure, driving temperature differences, and reduced loading spread are all omitted: Adv. Mater.2024, 36, 2211302 However, all three aspects are inherently necessary for any technical realization of an adsorption chiller.Thus, results suggest unrealistically high efficiencies for given temperature conditions.If not well put in context, this can lead to disappointment in view of experimental results of built adsorption chillers.Moreover, even as a theoretical figure of merits this quantity has strong limitations to represent the performance difference of different adsorbents: as soon as the loading spread achieved in the cycle exceeds a threshold of 0.1-0.2g g −1 , the denominator becomes strongly dominated by the first summand.Thus, ΔX may be cancelled out in the fraction, and the material COP merely depends on adsorption and evaporation enthalpies disregarding any further improvement of the loading spread.
Nevertheless, for the sake of comparison, we have calculated this COP for different benchmarks.The results (Figure 5 bottom) show an ideal COP of about 0.8 for most of the adsorbents investigated as long as the temperature thrust is large enough (Figure 5c) and the temperature lift is small enough (Figure 5d).However, when compared to the results for the cooling COP, these temperature limits are strongly shifted, suggesting lower driving temperatures and higher heat rejection temperatures because of neglected driving temperature differences.Moreover, the big uptake step, characteristic for most MOFs, is blurred and the little linear uptake at lower relative pressures dominates.As all working pairs start to show some kind of minimal uptake as soon as ΔT thr > ΔT lft , the ideal COP suddenly increases at T H > 40 °C in Figure 5c.This also leads to the misleading finding that Siogel outperforms all MOFs for low driving temperature levels.All in all, the material COP is an interesting concept to investigate the fundamental thermodynamics of adsorption chiller cycles but tells nothing about the technical performance of different working pairs.Note that the conditions investigated here differ from those used, for example, by Lenzen et al. (T L = 10 °C, T M = 30 °C), [62] as both MIP-211 and CAU-23 have negative cooling COPs under these conditions and only the material COP suggest a high performance.A detailed comparison is given in Figure S35 (Supporting Information).

MIP-211 for Water Harvesting from Air
Besides its applicability for ultra-low-temperature-driven cooling, and as a preliminary prospect, we have investigated MIP-211 as a potential candidate adsorbent for sorbent-assisted water harvesting from air.The combined high uptake capacity, position of the water isotherm at a relative humidity RH = 30% (P/P 0 = 0.3), multicyclic stability, and low regeneration temperature of MIP-211 (only 60-65 °C) make indeed this material particularly interesting for this application.As a representative showcase, in an arid desert area, the temperature can fluctuate between 38 °C (during the day) and 15 °C (at night), while the relative humidity fluctuates between RH = 10% (during the day) and RH = 40% (overnight).The low regeneration temperature of only 65 °C would make it possible to entirely desorb the water from MIP-211 using only solar radiation.In a passive situation, whereby the MIP-211-assisted water harvester operates without any auxiliary energy supply, the working capacity under the aforementioned desert conditions reaches 0.5 L H2O kg MOF day −1 (corresponding to a single water adsorption-desorption cycle per day).4 kg of MIP-211 would still supply 2 L of water, which satisfies the drinkable water demand of an adult.This value surpasses significantly the water deliverable capacity of benchmark MOFs: MOF-801 (0.25 L H2O kg MOF day −1 ), [94] MOF-160 (0.31-0.33 L H2O kg MOF day −1 ), [95] and MOF-303 (0.38 L H2O kg MOF day −1 ) [59] for a single daily cycle.For the evaluation of MIP-211 in a context of active regeneration, whereby the water harvester achieves multiple cycles per day, using active auxiliary systems for heating, transport, and condensation (e.g., driven by solar electricity), the water sorption dynamics of MIP-211 shall still to be determined in further studies.

Exploratory Synthesis Optimization
In view of their intended use, the next generation MOFs should also, among other criteria, be synthesized under nontoxic, energy saving, and large scalable synthetic routes. [96]For this reason, the sustainable production of MIP-211 in a DMF-free synthesis was tentatively explored using several aluminum sources in water.Interestingly, in a preliminary attempt, MIP-211 was alternatively obtained from the reaction of H 2 muc with hydrated aluminum hydroxide (Al(OH) 3 •xH 2 O) in water under reflux conditions (see details in the Experimental Section; Figures S18-S20, Supporting Information).Although, this promising protocol still needs to be further optimized to reach pure MIP-211, the MOF could be obtained at the g-scale in a relative short time reaction (6 h).

Conclusion
This work demonstrates that not only polymorphism is possible in Al-MOFs with chain-like IBUs, whereby a given aluminumdicarboxylate MOF could be structural formed with either rodlike chains of trans-connected or helical chains of cis-connected corner-sharing AlO 6 octahedra, depending on the synthesis conditions employed but is a powerful tool to tune the water sorption properties of MOFs.The cis-µ-OH-connectivity results indeed in drastic increase of the hydrophilicity compared to that of the MOF with the trans-µ-OH-connectivity, due to the ease of water molecule to preferable bridge between two neighboring µ-OH groups of the IBU, which is more improbable for trans-connected µ-OH groups.The new polymorphism, therefore, induces a strong change in the hydrophilicity of Al-MOFs and the relative pressure position of the step of water sorption isotherm.In the case of the newly discovered hydrothermally stable MIP-211, its water sorption profile and uptake capacity surpass most of the benchmark materials investigated for water sorption applications while exhibiting a very low temperature of regeneration.This concept which is here well exemplified by using t,t-muconate is expected to be generalized to other dicarboxylate linkers.

Experimental Section
Materials and Methods: All chemicals were purchased from Alfa Aesar and used without further purification.Al 2 (SO 4 ) 3 •18H 2 O,

Scheme 1 .
Scheme 1. Structures and opening angles of some dicarboxylate linkers that have been used to construct Al-MOFs made up of AlO chains.Linkers that yield Al-MOFs made up of helical AlO chains (left), linkers that yield Al-MOFs made up of mixed helical and/or straight AlO chains (middle), and linkers that yield Al-MOFs made up of straight AlO chains (right).Shared corners of AlO 6 octahedra are bridging µ-OH groups, which are transpositioned or cis-positioned in straight or helical chains, respectively.Opening angles of V-shaped dicarboxylates were measured from their structure from cif files (CSD-Refcodes: MIL-160, PICBAH; CAU-10, CELZUQ; KMF-1, KUZPUT; CAU-23, ZOVHUQ; MOF-303, CAMTET; MOF-333, CAMYEY).
, Supporting Information).The comparison of PXRD patterns of MIP-211 and MIL-53-muc (Figure 1a; Figure S4, Supporting Information) gave the first indication of the structural difference between the two materials.The formula of dehydrated MIP-211 suggested from CHN elemental analysis (see supporting information) is shown to be [Al(OH)(muc)], as in the case of MIL-53-muc.This suggested that MIP-211 and MIL-53-muc are aluminum muconate polymorphs.The modeled structure and the Rietveld refinement of data from high-resolution PXRD data for MIP-211 (Figure
I4 1 /acd, with similar unit cell parameters of a = b = 23.4208(3)Å and c = 10.9733(3)Å (dried form), and a = b = 23.4148(3)Å and c = 10.9510(3)(hydrated form) (see Tables , Supporting Information, are in very good agreement for MIP-211 with a predicted single Al site with quadrupolar coupling constant (C Q ) of 4.93 MHz and asymmetry parameter η Q of 0.53 in excellent agreement with the corresponding experimental values of 5.1 MHz and 0.48, respectively.Regarding MIL-53-muc, the DFT calculations reveal two almost equivalent Al associated with very similar C Q (8.04 MHz and 7.93 MHz) and η Q (0.18 and 0.19) that match with the values experimentally obtained for one Al site (8.7 MHz and 0.18), however they do not support the existence Adv.Mater.2024, 36, 2211302

Figure 2 .
Figure 2. Comparative structures of MIL-53-muc (left) and MIP-211 (right), showing t,t-muconate linkers bridging a) two straight AlO chains in MIL-53-muc and b) two helical AlO chains in MIP-211.The projection along the [001] direction disclosing the 1D channels with lozenge-shaped crosssection in c) MIL-53-muc and d) square-shaped cross-section in MIP-211.Al, pink, and blue polyhedral in MIL-53-muc and MIP-211, respectively; C, gray; O, red.Water molecules and hydrogen atoms are omitted for clarity.

Figure 4 .
Figure 4. GCMC simulated adsorption mechanism of water for MIL-53-muc and MIP-211 MOFs at T = 298 K. Top panel: water adsorption snapshots for MIL-53-muc at a) P/P 0 = 0.50, b) P/P 0 = 0.60, and c) P/P 0 = 0.75.Middle panel: water adsorption snapshots for MIP-211 at d) P/P 0 = 0.30, e) P/P 0 = 0.35, and f) P/P 0 = 0.40.Subfigures (a) and (d) also show a close-up view of typical water interaction with μ-OH sites in MIL-53-muc and MIP-211, respectively.Atom color codes: O red; H white, C grey, Al pink, adsorbed water molecules are presented in green for easy distinction.Bottom panel shows the radial distribution functions calculated for the water/water and water/µ-OH pairs in g) MIL-53-muc at P/P 0 = 0.60 and h) MIP-211 at P/P 0 = 0.35.

Figure 5 .
Figure 5.Estimated cooling COP under typical application conditions (top) and ideal material COP (bottom) for variations of the high temperature level (left) and the mid temperature level (right) for MIP-211 compared to selected MOFs and a state-of-the-art silica gel (Siogel), showing a superior performance of the novel MIP-211 as long as a low-enough temperature for heat rejection can be provided.Note that for cooling COPs the temperatures of the exploited heat sink/source differ from the temperatures at material level (driving temperature difference) whereas for material COPs this difference is neglected resulting in drastic overestimations of possible mid-temperature levels and underestimation of required high-temperature levels.Negative COPs are set to zero.