Solvent Impact on the Properties of Benchmark Metal–Organic Frameworks: Acetonitrile‐Based Synthesis of CAU‐10, Ce‐UiO‐66, and Al‐MIL‐53

Abstract Herein is reported the utilization of acetonitrile as a new solvent for the synthesis of the three significantly different benchmark metal–organic frameworks (MOFs) CAU‐10, Ce‐UiO‐66, and Al‐MIL‐53 of idealized composition [Al(OH)(ISO)], [Ce6O4(OH)4(BDC)6], and [Al(OH)(BDC)], respectively (ISO2−: isophthalate, BDC2−: terephthalate). Its use allowed the synthesis of Ce‐UiO‐66 on a gram scale. While CAU‐10 and Ce‐UiO‐66 exhibit properties similar to those reported elsewhere for these two materials, the obtained Al‐MIL‐53 shows no structural flexibility upon adsorption of hydrophilic or hydrophobic guest molecules such as water and xenon and is stabilized in its large‐pore form over a broad temperature range (130–450 K). The stabilization of the large‐pore form of Al‐MIL‐53 was attributed to a high percentage of noncoordinating −COOH groups as determined by solid‐state NMR spectroscopy. The defective material shows an unusually high water uptake of 310 mg g−1 within the range of 0.45 to 0.65 p/p°. In spite of showing no breathing effect upon water adsorption it exhibits distinct mechanical properties. Thus, mercury intrusion porosimetry studies revealed that the solid can be reversibly forced to breathe by applying moderate pressures (≈60 MPa).


Introduction
Over the past years metal-organic frameworks( MOFs) have gained much attention because of their potentiala pplications in, for example, gas storage [1] and separation, [2] sensing, [3] catalysis, [4] heat transformation, [5] and the medical sector. [6] This led to intense research activities on the development of new and synthetically challenging MOFs with in some cases highly complex structures. [7] Nowadays,a pplicability, sustainability,a nd simplicity of the syntheses are comingm ore and more into the focus of interest, because the transition between the exploration of new MOFs anda pplication-oriented research needs to be realized.
One approacht oc reate more sustainable synthesis routes is to replaceh azardous and environmentally unfriendly solvents by less harmful ones,b asically followingt he twelve principles of green chemistry. [8] In common MOF syntheses often aprotic and highly polar solvents are required to dissolve organic molecules( linkers) in sufficient amounts.U nfortunately,t hese solvents (a prominente xample is N,N-dimethylformamide (DMF)) pose many risks regarding safety,o ccupational health, ande nvironmental impact. [9] In some selected cases, water has already been used as an alternative solvent, considering for example the synthesis of ÀCOOH-functionalized Zr-UiO-66, [10] Al-MIL-53, [11] or CAU-10. [12] Nonetheless, water-based routes are often either limited to very polar organic linker molecules or require high temperatures to realizes ufficient dissolution of less polar reactants. For solvents in general, harmfulnessi so ften linked to their beneficial properties. [9a] The use of acetonitrile has been only very little explored in solvothermalM OF syntheses. [13] Acetonitrile is commercially mainly produced in the Sohio process through catalytic ammoxidation of propene. [14] Hydrogen cyanide and acetonitrile occur as byproducts. Acetonitrile has similar solvent properties as those of DMF,which are desired in MOF syntheses,w hile being considered as less hazardous. Am ajor concern regarding DMF is its reproductive toxicity.I ti sc ommon for amides and not observed for acetonitrile. [9a] Both solvents are covered and rated by multiple solvent selection guidelines comprising al arge number of different criteria. In 2014, Prat et al. published as urveyo fs olvent selectiong uides, which combined all previousg uides in one comprehensive guide with improved consensus. [9b] In Table 5ofr eference[9b],acetonitrile is still listed as ap roblematic solvent and cannot be considered as fully green, but it is one of the least hazardous options for aproticp olar solvents to date and capable of dissolving smaller organicm olecules with low polarity in sufficient amounts.
As at ypical illustration, an acetonitrile-baseds ynthetic strategy was applied by using different metal ions and linker molecules (Table S2). On the basis of our previousw ork on Al-and Ce-MOFs, the following three compounds were chosen for a more detailed study:1 )CAU-10 [Al(OH)(ISO)] is an aluminum MOF,w ell-known for its applicability in adsorption-driven chilling (ISO 2À :i sophthalate). [12] It exhibitsi nfinite helical chains of cis corner-sharing AlO 6 polyhedra, whicha re interconnected by V-shaped isophthalate ions to form at hree-dimensional network with square-shaped, sinusoidal pore channels ( Figure 1, top left). [15] 2) Ce-UiO-66 [Ce 6 O 4 (OH) 4 (BDC) 6 ]r epresents the redox-active [16] cerium-based analogue of Zr-UiO-66 [17] andcontains hexanuclear cerium-oxygen clusters ([Ce 6 O 4 (OH) 4 ] 12 + ) (BDC 2À :t erephthalate). Each cluster is coordinated to 12 other clusters via terephthalate ions, which leads to af cu topology ( Figure 1, bottoml eft). [16a] 3) Al-MIL-53 [Al(OH)(BDC)],aMOF composed of infinite chains of trans corner-sharingA lO 6 octa-hedra that are interconnected by terephthalate ions, represents as pecial type of framework. Its wine-rack structureh as the ability to undergo reversible phase transitions upon adsorptiono fg uest molecules or temperature change (Figure 1, right). [11,19] This behavior is called "breathing effect".W hen, for example, water molecules are adsorbed onto the 1D pore channels of the activated/empty structure (denoted as ht form, ht:h igh temperature;a lso known as open or large-pore form), they create strong hydrogen bonds with bridging OH groups of the inorganic buildingu nit (IBU). These interactions force the framework to contract into its narrow-pore (np)/low-temperature( lt) form (also known as closed-pore (cp) form if no guest molecules are present). The process is reversible upon water desorption. [11] This breathing behavior has also been observedu nder mechanical pressure, which makes this material and its isoreticular analogues promising shock absorbers. [20] Here, we presentt he syntheses of CAU-10, [12,15] Ce-UiO-66, [16a] and Al-MIL-53 [11] from acetonitrile as well as the characterization of the products with as pecial focus on changes of the framework flexibility of Al-MIL-53.

Results and Discussion
The screening of different metal ions and linker molecules using acetonitrile as the solvent resulted in the formation of well-known MOFs (Table S2, Figures S7 and S8). For ad etailed study the compounds CAU-10, Ce-UiO-66, and Al-MIL-53w ere selected. The solvothermalr eaction of aluminum nitrate nonahydrate, Al(NO 3 ) 3 ·9H 2 O, with nearly insoluble isophthalic or terephthalic acid in acetonitrile at 130 8Cf or 23 hy ieldedC AU-10 or Al-MIL-53, respectively.F or Ce-UiO-66 am ilder synthesis route is feasible. Thus, terephthalica cid was reacted with aqueous cerium ammonium nitrate, (NH 4 ) 2 [Ce(NO 3 ) 6 ], solution in acetonitrile at 100 8Cf or 2h under reflux.I ti sr emarkable that the reaction time for Ce-UiO-66 could be increased to 2h, because DMF-based syntheses for this material are commonly limitedt ov ery short reaction times of about 15 to 30 min. At longerr eactiont imes, DMF decomposition to dimethylamine and formic acid becomes dominant and the thermodynamically favored product, cerium(III) formate, Ce(O 2 CH) 3 ,i sf ormed. [16a, 21] This also limits the scalability of the synthesis, and usually only small quantities of Ce-MOFs have been obtaineds o far.T hus, by utilizing acetonitrile as the solvent the reaction could be carried out at the 240 mL scale, which resulted in a yield of more than 7gof this compound.
All as-synthesized (as) compounds are not yet pure, which is due to only partially reacted metal species or small amounts of linker residues.A dditionally,a cetamide occurs as am inor impurity.I ti st he intermediate of the acetonitrile hydrolysis, which ultimately leads to the formationo fa cetic acid and ammonia.T he latter manifestsi tself through pressure build-upi n the reactionv essel. The hydrolysis is enabledb yw ater,w hich originates from hydration water or the solvent, as well as catalyticallya ctive Lewis acidic metal ions, Al 3 + or Ce 4 + . [22] One should keep in mind that the final hydrolysis product, acetic acid, possibly acts as amodulator during nucleation and crystal growth of the title compounds. [23] However,s oluble metal spe- Figure 1. Structures of CAU-10 [18] (top left), Ce-UiO-66 [16a] (bottom left),a s well as narrow-(top right) and open-poreform (bottom right)o fs tandard Al-MIL-53 [11] with unitc ell edges. AlO 6 octahedra are showninl ight blue and Ce 6 O 32 polyhedra in light orange. Hydrogen atoms are not displayed. cies represent the largestp art of impurities and can therefore be easily removed through solventt reatment by using DMF, acetone, ethanol, or water.
Ce-UiO-66 (as) was treated with DMF in order to remove unreactedt erephthalica cid. Subsequents olvente xchange with acetonef inally yields as lightly defective product (section Thermogravimetric and Elemental Analysis in the Supporting Information). Because of smaller amounts of impurities,w ater treatment is sufficient for CAU-10 (as), whereas Al-MIL-53 (as) requires preliminaryw ashing with ethanol. Otherwise X-ray amorphousa luminum hydroxide is formed, whichc omplicates the purificationp rocess significantly.D etailed descriptions of the synthesis and purification procedures of the three title compounds are given in the Supporting Information (section Synthetic Procedures).
After purification,a ll products were characterizedb yp owder X-ray diffraction (PXRD)( Figure 2) and their cell parameters were determined by LeBailfits (Table S3).
The PXRD patterns of CAU-10 andC e-UiO-66a re well in line with their theoretical patterns. The comparison of the PXRD pattern of Al-MIL-53 with the theoreticalp atterns of standard Al-MIL-53 and V-MIL-47 [VO(BDC)] in their large-pore forms reveals that the synthesized product exhibits as tructure that is rather reminiscent of the latter.T he structural similarity with V-MIL-47 is underlined by the corresponding LeBail fit, carried out starting from the cell parameters of V-MIL-47 ( Figure S4), which shows ag ood match of the calculated andr eportedc ell parameters (Table S3).
Compositional analysis by thermogravimetric and elemental analyses as well as infrared( IR) spectroscopy and nitrogen adsorptione xperiments confirmt he successful synthesis of CAU-10, Ce-UiO-66,a nd Al-MIL-53. Details are given in the Supporting Information. The micropore volumes and specific surface areas as determinedb yt he Brunauer-Emmett-Teller (BET) methodf rom nitrogen adsorption isotherms by utilizing the approachr eported by Rouquerol et al. [25] are summarized in Ta ble 1a nd compared to valuesp reviously reported in the literature.
The Al-MIL-53 of this work is compared to standard Al-MIL-53-ht and to an onbreathing MIL-53 analogue (MIL-53-is, is: imidazolium salt). [26] The BET areaso fA l-MIL-53 and MIL-53-is are similara nd significantly lower than those reportedf or conventional Al-MIL-53-ht. The lower BET area for MIL-53-is was attributed to the presence of g-AlO(OH), whicha lso stabilizes its large-pore form and thus inhibits its breathing. [26] For the present Al-MIL-53, the PXRD and spectroscopic analysis did not yield experimental evidencefor such an impurity.
Chem. Eur.J.2020, 26,3877 -3883 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim ure S21). In contrast, the Al-MIL-53 of this work is more hydrophobic than standard Al-MIL-53 [27] ands hows as ignificantly increasedo ne-step water uptake of 310 mg g À1 in the range p/ p8 = 0.45-0.65. Standard Al-MIL-53 is limited to am aximum water adsorption capacity of 90 mg g À1 because of its pore contraction (breathing) upon water adsorption, and its major uptake lies within the range p/p8 = 0.2-0.4. [27] Therefore, the unexpected behavior of the synthesized Al-MIL-53 can be related to the absence of such ab reathing effect. This is also in line with the hydrophobic character of the material, because the hydrophobic phenyl rings of the linkersa re much better accessible for guest molecules when the large-pore form is retained. [11,28] It is noteworthy that the adsorption anddesorption branch lie within the range (45 to 65 %) recommended by ASHRAE (American Societyo fH eating, Refrigerating, and Air Conditioning Engineers) for relative indoor humidity of occupied spaces. [29] To getherw ith the materials' comparably high capacityi nt he given range it makes it ap otentialc andidate for indoor moisture control applications. [29] It could be most preferably used to effectively stabilize moisture levels between 55 and 65 %r elative humidity. To furtheru nderstand and evaluate the materialp roperties, detailed solid-state NMRs pectroscopic measurements and Hg intrusion porosimetry studies were carriedo ut. 1 H, 13 C, 27 Al high-resolution solid-state NMR spectroscopic experiments suggesta nu nusually high defect concentration within the framework, which mostp robably inhibits the breathing behaviori nt he present case.
In the 1 Hs ingle-pulse (SP) NMR spectrum ( Figure 4a)t wo dominant features at 8.0 and 2.6 ppm with an intensity ratio of 4:1a re discernable, which were attributed to the four aromatic protons of the terephthalate linkersa nd the single proton of the bridging hydroxide groups (Al-OH-Al) between two AlO 6 octahedra. [11,30] The small low-field resonance at 13 ppm is characteristicf or unconnected carboxylic acid groups of the linkers. Because the presenceo ffree terephthalic acid is unlikely duetot he synthesis and purification conditions, this signal implies that roughly 15 %o ft he linkers( Ta ble S9) are in ad efect state with only one of the two carboxylic acidg roups being deprotonated and coordinated to the Al 3 + ions. This is also in line with a weak intensity IR band at 1702 cm À1 that is ascribed to aromatic carboxylic acid groups ( Figure S23, Ta ble S7). The resonances at 6.3 and 2.6 ppm are attributedt or esidual acetamide arising from the decomposition of acetonitrile with approximatelyo ne acetamide molecule per 10 linker molecules. [31] Af urtherr esonance at 3.9 ppm is caused by adsorbed water molecules. [32] This assignment is in line with the spectralf ingerprint of the 13 CCPM AS (cross-polarizationm agic-angle-spinning) spectrum ( Figure 4b). The resonances at 130 and 137 ppm are associated with the aromatic CH and quaternary carbon atoms of the linker.T he resonance at 171 ppm is assigned to carboxylate groups of connecting terephthalate linkers. Again,i ts downfield shoulder corroborates the existence of small residuals of protonated groups,a sp roposed above. [11] Furthermore, as mallr esonance in the aliphatic regiona t 21 ppm ( Figure S28) matches residual acetamide.
The 27 Al single-pulse magic-angle-spinning (SP MAS) NMR spectrum (Figure 4c)r eveals two distinctively different aluminum coordinatione nvironments. Both resonances exhibit the typical shapes for as econd-order quadrupolar broadening.T he 2D satellite-transition magic-angle-spinning (STMAS)s pectrum ( Figure S29) furthermore reveals ad istribution of about 1MHz for the quadrupolarc oupling constants, probably arising from the disorder caused by the ÀCOOH defects within the framework. The main component (blue line, Figure 4c), with an isotropic chemical shift d iso = 2ppm, aq uadrupolar coupling constant of C q = 8.8 MHz, and an anisotropy of h q = 0i st ypicalf or  27 Al SP MASN MR spectrao fAl-MIL-53. The 1 HSPN MR spectrum (black lines)isp resentedt ogether with ad econvolution by using pseudo-Voigt profiles (red and blue lines). The experimental 27 Al SP NMRs pectrum( black) was deconvoluted with two resonances (blue and green)w ith line shapes typical for as econd-order quadrupolarbroadening. The resulting simulated line shape is depicted as ar ed line. Chem. Eur.J.2020, 26,3877-3883 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim aluminumi nad efect-free Al-MIL-53 environment. [11] The significant high-field shift of d iso of about 9.6 ppm together with the reduction of C q to 6.0 MHz for the minority contribution (green line, Figure 4c)a re suggestive of aluminum in ac oordination with less than six equally strong contacts. [33] The integrated intensity amountst o1 6% of the overall signal intensity, which matches the defect concentration as determined from the 1 HMAS NMR spectra.
To additionally probe the stability of the large-pore form as af unctiono ft emperature under the influence of hydrophobic guest molecules, like xenon, [34] variable-temperature (vt) 129 Xe NMRs pectra( Figure 5) were recorded, using hyperpolarized xenon gas. [35] At RT two resonances at 0ppm typical for gas-phase xenon anda t7 5ppm characteristic for xenon adsorbed within the large-pore form of Al-MIL-53 are visible. [34,36] To wards lower temperature the chemical shift for 129 Xe adsorbed in Al-MIL-53 slowly and continuously increases towards 330 ppm at 140 K. The absence of ab istable state around 220 Ks ignaled by ad iscontinuous increaseo ft he shift by roughly4 0ppm [34, 36a] demonstrates that the presentA l-MIL-53 sample does not change from al arge-to an arrow-pore form upon cooling. Between 260 to 190 Kt he signali sc omposed of several, overlapping shapes that, in accordance to the literature, can be assigned to clusters of adsorbed xenon atoms. [37] This significant number of defects in the present Al-MIL-53 materiali sv ery likely to interferew ith its lattice dynamics and might thus inhibitt he typical breathing behavior of the framework as demonstrated by the water adsorption ( Figure 3) and vt 129 Xe NMRe xperiments ( Figure 5).
Mercury intrusion porosimetry of Al-MIL-53 over two cycles revealed that the solid, in spite of showing no breathing effect upon water adsorption or cooling, can be forced to breathe reversibly by applying moderate pressures ( % 60 MPa). The applied pressure is significantly higher than the pressure required to contract standard Al-MIL-53 (18 MPa), whichs hows an irre-versible structural switching. [20c] Furthermore, the resulting compression-decompression curves( Figure 6) exhibit ad ifferent evolution than those in previous studies dedicated to similar materials. [20b-e, 38] The first step of the first cycle represents the filling of the intrusionc ell and the de-agglomerationo ft he powder particles (secondary agglomerates) ( Figure 7B). Because the powder is   . Schematic illustration of the mercury intrusionprocess for Al-MIL-53. A) Agglomerated powder particles (represented as blue spheres for simplification) before mercury intrusion. B) De-agglomerationofs econdary powder agglomerates. C) Mercury penetration of inter-particle voids (primary agglomerates). D) Phase transition from large-to closed-pore form. For the closed-pore stateblue filled diamonds representthe Al-MIL-53c haintype IBU viewed along the c-axis and dark blue sticks represent the terephthalate linkers. In the same way dashed lines and nonfilled objects indicate the large-pore state before pore closure. composed of very smalli ntergrownc rystallites ( Figure S25-S27), the subsequents tep can be assigned to the mercury penetration into the inter-particlevoids (primary agglomerates) ( Figure 7C). This is also in line with the average particle size of 140 to 270 nm as determined from mercury intrusion itself by the Mayer-Stowe method. [39] The first two steps are indeed typical for aggregated powders that are not porous towards mercury. [39,40] The last step of the first cycle shows av olume variation (DV)o f0 .41 mL g À1 whichi ss imilar to the material's micropore volume( 0.46 mL g À1 )a sd etermined from nitrogen adsorption data (Table 1). It corresponds to the phase transition of Al-MIL-53 from its large-pore form into its closed-pore (cp) form because of the externalc ompressionb ym ercury (mercuryc annot penetrate the small micropores of the framework) ( Figure 7D).
The calculatedc ell volumer eduction of the large-pore form based on the volume of mercury intruded is 566 3 .T herefore, the obtained Al-MIL-53-cp has ac ell volume of 940 3 ,w hich is well in line with the value of 947 3 reported by Loiseau et al. for standard narrow-pore Al-MIL-53. [11] Upon retraction the structureo pens again and the large-pore form is again obtained (first retraction step). Afterwards, am ajor part but not all of the mercury is released from the inter-particlev oids (second retraction step). It is aw ell-known phenomenont hat mercury is not quantitatively expelled by such as ample because some mercury will be trapped in the larger inter-particle pores. [40b,c] The second intrusion/retraction cycle furthers hows that the contractiono fA l-MIL-53 is reversible.
Compared to the flexible version of the solid the contraction of the large to the closed form occurs at higher pressure ( % 60 MPa vs. 18 MPa)w ith as imilarv ariation of volume DV % 37 %. [20c] The associatedw ork for one cycle of compression and decompression is close to 25 Jg À1 .
These findings are well in line with the presence of defects within the material as observed by NMR spectroscopy.B ecause of the defects, which are stabilizing the large-pore form, more force is required to contractt he framework. Nevertheless,t he small size/degree of intergrowth of the Al-MIL-53 particles (Figure S25-S27)c ould be another factor that inhibits the breathing effect through mechanical hindrance.

Conclusions
In conclusion, we were able to synthesize an umber of different MOFs using acetonitrile as the solvent, demonstrating its potentiali nM OF syntheses. It is capable of sufficiently dissolving smaller organic linker molecules with low polarity at moderate temperatures while being less hazardoust han many other aprotic polar solvents such as DMF.T he three title compounds CAU-10, Ce-UiO-66,a nd Al-MIL-53 were characterized in depth. The Ce-MOFc ould be obtained in gram-scale quantities for the first time because of increased redox stability in this solvent. Surprisingly,t he choice of acetonitrile affected the breathing properties of the synthesized Al-MIL-53 drastically. While the typical structuralf lexibility upon water adsorption was absent, mercury intrusion measurements revealed that the materialc an be reversibly forced to breath when ap ressure of about 60 MPa is applied, which is significantly highert han the pressure required to irreversibly contract standard Al-MIL-53 (18 MPa). This behavior can most likelyb ea scribed to defects within the structure of the material. These consist of linker units which only coordinate througho ne carboxylate group, whereas the second group remains protonated. 15 %o ft he linker molecules exhibit this defective state. Another factor could be the small size/degree of intergrowth of the particles that might lead to am echanical inhibition of the breathing effect. The missing flexibility of Al-MIL-53 results in distinct water adsorption properties with ah igh uptake of 310 mg g À1 in the range between 0.45 and 0.65 p/p8 making the material ap otential candidate for indoor humidity control applications.