Design of a robust and strong-acid MOF platform for selective ammonium recovery and proton conductivity

Metal–organic frameworks (MOFs) are potential candidates for the platform of the solid acid; however, no MOF has been reported that has both aqueous ammonium stability and a strong acid site. This manuscript reports a highly stable MOF with a cation exchange site synthesized by the reaction between zirconium and mellitic acid under a high concentration of ammonium cations (NH4+). Single-crystal XRD analysis of the MOF revealed the presence of four free carboxyl groups of the mellitic acid ligand, and the high first association constant (pKa1) of one of the carboxyl groups acts as a monovalent ion-exchanging site. NH4+ in the MOF can be reversibly exchanged with proton (H+), sodium (Na+), and potassium (K+) cations in an aqueous solution. Moreover, the uniform nanospace of the MOF provides the acid site for selective NH4+ recovery from the aqueous mixture of NH4+ and Na+, which could solve the global nitrogen cycle problem. The solid acid nature of the MOF also results in the proton conductivity reaching 1.34 × 10−3 S cm−1 at 55 °C by ion exchange from NH4+ to H+.


Introduction
9][20][21] Historically, the acidic points of solid materials have played important roles in ion-conductive materials by providing dissociable ions at the acidic point.7][28] However, the number of studies on MOFs as ionexchange materials is still limited.Some MOFs are reported to adsorb heavy metal ions such as lead, arsenic, selenium, mercury, silver, and palladium, [29][30][31][32][33][34] while no MOFs have been reported that can execute reversible ion exchange for small monovalent cations like ammonium (NH 4 + ), sodium (Na + ), and potassium (K + ) in water.The signicant barrier for the solidacid MOF is the poor stability in water. 35Early MOFs were very susceptible to decomposition in water or water vapor.Later, many MOFs with improved chemical stability were reported.For example, UiO-66 (ref. 36) shows outstanding strength thanks to the strong metal-ligand interaction between the zirconium cation and the carboxy group of terephthalic acid.The combination of the uniform nanopore and ion-exchanging sites of MOFs could expand the scope for the new solid materials endowed with high ionic selectivity and high chemical stability.8][39][40] In addition, the selective capture of NH 4 + could solve the global nitrogen cycle problem.Today, 1.8 billion tons of ammonia are produced annually by the Haber-Bosch process, 41 and the accumulation of ammonia in the soil and sea has become a serious issue. 42,43The recovery and reuse of NH

Results and discussion
Hydrothermal synthesis of ZrOCl 2 $8H 2 O (8.3 mM), mellitic acid (0.13 M) with a high concentration of ammonium chloride (2.3 M) and acetic acid (8.7 M) in an aqueous solution produced white crystalline powders.Optical microscope and SEM images show that the crystals have an octahedral shape with a size of 5 to 15 mm (Fig. 1a and b and S1 †).The ratio of carbon to nitrogen was found to be 12 : 1 (mol : mol) by elemental analysis (Table S1  We conrmed the structure of Zr-mel-NH 4 by single-crystal X-ray diffraction analysis (Fig. 1c and Table S2 †).The MOF contains Zr 6 O x (OH) 8−x clusters bridged by two carboxy groups of the mellitic acid, and the other four carboxy groups remain uncoordinated.The calculated formula agrees with the result of the elemental analysis.Zr-mel-NH 4 has an Im 3m space group with a = 41.547(2)Å, which is about twice the cell constant of UiO-66 (a = 20.7004(2)Å). 36 The large cell constant originates from the long-range superlattice structure.The Zr 6 O 4 (OH) 4 clusters in UiO-66 are connected to the neighboring 12 clusters by the linker.In contrast, Zr-mel-NH 4 is composed of two types of Zr clusters, type I and type II (Fig. 1d and S2 †).The type I cluster connects to six linkers, and the type II cluster is coordinated by eight linkers (Fig. 1e and f).The absence of the linkers caused the periodic lack of Zr clusters and resulted in the superlattice structure.
We examined the thermal stability by thermogravimetric analysis of Zr-mel-NH 4 and powder XRD patterns of the MOF aer heating.The TG curve showed a two-step weight loss at 25 °C and 300 °C corresponding to water elimination and the decomposition of the organic linker in MOFs, respectively (Fig. S3 †).PXRD patterns of the Zr-mel-NH 4 aer thermal treatment showed the structural change starting at 90 °C, and the structure became amorphous at 100 °C (Fig. 2a).These results suggest that the structure of Zr-mel-NH 4 is collapsed by dehydration.The chemical stability of the Zr-mel-NH 4 was evaluated from the powder XRD patterns aer soaking in various aqueous solutions.The PXRD patterns of Zr-mel-NH 4 aer immersion in hydrochloric acid (HCl, pH 0) and sodium hydroxide (NaOH, pH 10) solutions are unchanged from that of the pristine Zr-mel-NH 4 (Fig. 2b), showing that the structures are highly stable under both acidic and basic conditions.Zr-mel-NH 4 also has durability with 60 mM NH 4 Cl, NaCl, and KCl aqueous solutions, and the octahedral shape of the MOF crystals was maintained aer immersion in these solutions (Fig. S1 †).The BET surface area of the Zr-mel-NH 4 was evaluated by nitrogen (N 2 ) gas adsorption analysis (Fig. 2c).Pristine Zr-mel-NH 4 showed N 2 uptake up to 0.2P × P 0 −1 , and the BET surface area was determined to be 876 m 2 g −1 , which is comparable to that of UiO-66 and its derivatives (Table S3 †). 45Saito-Foley pore size analysis 46,47 shows that a uniform pore with a cavity size of 7 Å is present in the Zr-mel-NH 4 (Fig. S4 †), which agrees with the cavity size expected from the single-crystal XRD analysis.The MOFs maintained their permanent porosities aer soaking in NH 4 Cl, NaCl, and KCl salt solutions (Fig. 2c).It is to be noted that the degradation of porosity is observed for the MOF soaked in HCl, and the capacity was recovered by the additional ion exchange with NH 4 Cl (Fig. S5 †).The degradation of the porosity was derived from the collapse of the MOF by the elimination of H 2 O, and the collapse of the MOF was accelerated by the Zr-mel-H.The PXRD patterns are unchanged aer the N 2 gas adsorption experiment (Fig. S6 †).As discussed below, the NH 4 cations in the MOFs soaked in HCl, NaCl, and KCl are thought to be fully replaced by H, Na, and K, respectively.
UiO-66 is widely known for its high stability in water due to the strong coordination bonds with the high-valence zirconium.Therefore, we have attempted to use UiO-66-SO 3 Na, UiO-66-(COOH) 2 , and zirconium-sulfoterephthalate MOF, 48 and all of them have acid groups capable of trapping NH 4 + .However, these MOFs were unstable in an aqueous NH 4 + solution, probably due to the reaction with trace NH 3 (Fig. S7-S9 †).The high stability of Zr-mel-NH 4 with NH 4 + arises from the synthesis procedure where NH 4 Cl is added to the reaction mixture so that only the MOFs stable with NH 4 + can survive and maintain their framework in the reaction.We investigated the ion-exchanging properties of Zr-mel-NH 4 with H + , Na + and K + (Fig. 3a).The amount of desorbed NH 4 + and the amount of adsorbed exchanging ions were evaluated by ion chromatography, and the result shows that nearly 100% of NH 4 + in Zr-mel-NH 4 was replaced by H + , Na + , and K + , which is hereaer called Zr-mel-X (X = H, Na, and K) (Table S1 †).The existence of NH 4 + , Na + , and K + in Zr-mel-X was conrmed by SEM-EDX analysis (Fig. S10 †).Then, the ionexchange capability of the Zr-mel-H with NH 4 + , Na + , and K + was studied.We conrmed that an equimolar amount of the H + on the linker of Zr-mel-H was replaced by NH 4 + , Na + , and K + , respectively (Fig. S11 †).The ion exchange between NH 4 + and H + was repeated three times, and the reversibility was conrmed by PXRD patterns and SEM image (Fig. S12-S14 †).
To investigate the affinity between Zr-mel-H and each of the cations, the ion-exchange experiments were performed at the initial concentrations of 1, 2, 3, 5, 7, 10, 20, and 30 mmol L −1 .The Langmuir adsorption isotherms of Zr-mel-H with NH 4 + , Na + , and K + show greater adsorption amounts with NH 4 + and K + than with Na + below 10 mmol L −1 (Fig. 3b).The Langmuir adsorption equation is as follows: where q e is the number of adsorbed cations, C e is the adsorbate concentration, respectively.The Langmuir plots show good linearity (Fig. S15 †), and the ion-exchange capacity and affinity were evaluated by the tting of the plots.The maximum capacity (q max ) of Zr-mel-H was xed at 1.30 mol kg −1 , which is calculated from the result of elemental analysis.The equilibrium constant (K) of Zr-mel-H for NH 4 + and K + is higher than that for Na + , showing higher affinity with NH 4 + and K + (Table S4 †).Fig. 3c shows the ion-exchanging rate of Zr-mel-H in the mixed solution of NH 4 Cl and NaCl (NH 4 + : Na + = 30 : 30/mM).
Notably, 76% of H + on the linkers was exchanged with NH 4 + , whereas only 4% of the H + was exchanged with Na + .Moreover, the exchanging rate of Zr-mel-H with NH 4 + remains higher than that with Na + , even at a higher concentration of Na + than NH 4 + (NH 4 + : Na + = 30 : 120/mM).This high selectivity for NH 4 + is an advantage for the ammonia recovery from wastewater containing a large amount of Na + .Note that the concentration of K + in wastewater is generally lower than both NH 4 + and Na + , and the little selectivity between NH 4 + and K + should pose less of a problem compared to the competitive adsorption of Na + (Fig. S16 †). 49The adsorption affinity in water between the guest ions and MOFs is related to the hydration state of the ions.The cations are strongly hydrated in water and become dehydrated when adsorbed by the MOF.The lower dehydration energy required for so cations like NH 4 + compared to hard cations like Na +50-54 results in a greater tendency of NH 4 + to be dehydrated and adsorbed inside the MOF.This selectivity is unique to inorganic ion-exchange materials with regulated nanopore structure, 55 in contrast to the conventional ion-exchanging polymer resins that accommodate the cations under hydrated states.
The number of acid points, spatially interconnected pores, and ligand defects in solid acids is associated with high proton conductivity. 48,56For investigating the relation between proton conductivity and water uptake, the water vapor adsorption analysis of Zr-mel-NH 4 and Zr-mel-H was executed.From Fig. 4a, Zr-mel-NH 4 and Zr-mel-H adsorb 487 and 405 cm 3 $(STP) per g at 90% RH, corresponding to 74 and 59 equivalents of water, respectively.Hysteresis between the adsorption and desorption steps was observed in the RH range of 0-50%.Zr-mel-H showed a rise in water adsorption at a higher humidity than that of Zr-mel-NH 4 .
The powders of Zr-mel-NH 4 and Zr-mel-H were pelletized to perform an AC-impedance analysis (Fig. S17 †), and the ionic conductivity (s) was evaluated from the tting curves of the Nyquist plots (Fig. S18 †).The s of Zr-mel-NH 4 and Zr-mel-H was measured from 15 to 55 °C at 95% RH (Fig. S19 †).8][59] The activation energy of Zr-mel-NH 4 and Zr- mel-H shows the same value of 0.12 eV, which indicates that proton transport takes place in a Grotthuss mechanism.The ion exchange from NH 4 + to H + increases the acidity of the acid point and conductivity.Humidity dependency of the resistance was examined by variable humidity impedance measurement from 30 to 95% RH at 25 °C (Fig. S20 and S21 †).Two cycles of humidity change were performed to conrm the repeatability.The s of Zr-mel-NH 4 and Zr-mel-H were 2.83 × 10 −4 S cm −1 and 3.58 × 10 −4 S cm −1 at 95% RH, respectively (Fig. 4c and S22 †).The hystereses were observed to be similar to water adsorption isotherms, and their reversibility was also conrmed.Protonated samples have larger hysteresis of conductivity than Zr-mel-NH 4 .This implies that higher water pressure is required to adsorb the water in Zr-mel-H, which is in good agreement with the results of the water vapor adsorption analysis.Zr-mel-NH 4 and Zr-mel-H maintained their structure aer being placed under proton conductivity measurement conditions (Fig. S23 and S24 †).

Conclusions
In summary, we report the rst MOF consisting of zirconium and mellitic acid that can reversibly exchange monovalent cations such as H + , NH 4 + , Na + , and K + in water.Single-crystal XRD analysis revealed that Zr-mel-NH 4 has the structure of UiO-66 with periodic defects and four uncoordinated carboxy groups on the linkers.Elemental analysis indicates that one of them is exchanged with H + and acts as an ion-exchange site.The MOF maintains its structure and permanent porosity stably in acid (pH 0), alkaline (pH 10), and aqueous NH 4 Cl, NaCl, and KCl solutions from PXRD patterns, nitrogen adsorption isotherms, and SEM observations.Zr-mel-NH 4 exhibits reversible ion-exchange between NH 4 + and H + , Na + , and K + .The ion-exchange site within the uniform nanoporous structure provides selective NH 4 + recovery from a mixture of aqueous NH 4 + and Na + solution, in accordance with the difference in the hydration state of NH 4 + and Na + .Furthermore, by ionexchanging from NH 4 + to H + , the proton conductivity reached ca. 10 −3 S cm −1 due to the increasing proton donor property.These achievements suggest that Zr-mel-NH 4 could be applied as a solid acid platform in various applications.

Fig. 1
Fig. 1 (a) Optical microscope image and (b) SEM image of Zr-mel-NH 4 .(c) Crystal structure of Zr-mel-NH 4 .Blue, red and gray balls show Zr, O and C atoms, respectively.H atoms and water molecules are omitted for clarity.(d) The connection between two types of zirconium clusters within the unit cell.(e) Type I Zr cluster connecting with six type II clusters, viewed from the (111) and (110) directions.(f) Type II Zr cluster connecting with four of both type I and type II clusters, viewed from the (100) and (001) directions.