pKa-Directed Incorporation of Phosphonates into MOF-808 via Ligand

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pKa-Directed Incorporation of Phosphonates into MOF-808 via Ligand Exchange: Stability and Adsorption Properties for Uranium Wen Zhang, An Bu, Qingyuan Ji, Luofu Min, Song Zhao, Yuxin Wang, and Jing Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10920 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

pKa-Directed Incorporation of Phosphonates into MOF-808 via Ligand Exchange: Stability and Adsorption Properties for Uranium Wen Zhang,*, † An Bu,† Qingyuan Ji,† Luofu Min,† Song Zhao,† Yuxin Wang,† Jing Chen*, ‡ †State

Key Laboratory of Chemical Engineering, Tianjin Key Laboratory of Membrane Science

& Desalination Tech-nology, and School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China ‡Collaborative

Innovation Center of Advanced Nuclear Energy Technology, Beijing Key Lab of

Radioactive Waste Treatment, and Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China *To

whom correspondence should be addressed. E-mail: [email protected].

[email protected]. KEYWORDS pKa; Phosphonates; MOF-808; Ligand Exchange; Stability; Uranium adsorption.

ACS Paragon Plus Environment

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ABSTRACT

We report a class of pKa-directed, precise incorporation of phosphonate ligands into a zirconiumbased metal-organic framework (Zr-MOF), MOF-808, via ligand exchange. By replacing of formate ligands with methylphosphonic acid (MPA), ethanephosphonic acid (EPA), and vinylphosphonic acid (VPA), whose pKa values are slightly larger than that of the benzenetricarboxylic acid (BTC) linker in MOF-808, daughter MOFs can be synthesized without controlling the stoichiometric amounts of added MPA. The methylphosphonate MOFs (808MPAs) demonstrate high porosities, with only small changes in pore diameter and specific surface area when compared with parental MOF-808. PXRD patterns and structure refinements indicate the expansion of the lattice for all MOFs after decorating of methylphosphonate ligands. The XPS spectra reveal a charge redistribution of the Zr6 node after ligand exchange. FTIR and 31P MAS NMR spectra, combined with DFT calculation, suggest that the methylphosphonate ligand is connected to the Zr6 node as CH3P(O)(OZr)(OH) species with an accessible acidic P-OH group. Besides, 808-MPAs demonstrate excellent chemical stability in concentrated HCl, concentrated HNO3, hot water and 0.2 mol/L trifluoroacetic acid solutions. Impressively, 808-MPAs show ultrafast adsorption performance for uranyl ions using the ion exchange property of P-OH sites in their cavity environment, with an equilibrium time of 10 min, much quicker than the previous adsorbents. The present study demonstrates a series of important proof-of-concept examples of the pKa-directed Zr-MOFs with tunable phosphonate-terminated ligands, which can extend to other phosphonate-functionalized Zr-based framework platforms in the near future.


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INTRODUCTION

Zirconium-based metal-organic frameworks (Zr-MOFs) are attracting growing interest and foreseen as one of the most promising MOFs for actual applications, due to their unique outstanding robustness and stability.1, 2 Compared with other metal nodes, Zr6 nodes in Zr-MOFs exhibit much more resistant to aggressive environments, such as heterocatalysis in oxidizing atmosphere,3 CO2 capture from hot and moist industrial flue gases,4, 5 and removal of radionuclides in strong acidic liquid wastes.6, 7 Coordinated to Zr6 nodes, the organic ligands play a significant role for the structures and properties of Zr-MOFs materials.8, 9 At present, the reported Zr-MOFs are based mostly on carboxylates.10 Phosphonate is a kind of organic ligands with versatile application values, such as catalyst supporters,11 radioactive ion sorbents12-15 and proton conductors.16, 17 Integration of Zr-MOFs and phosphonate ligands can take full advantage of both.

However, it is quite difficult to obtain conventional phosphonate Zr-MOFs via combining Zr oxo clusters and phosphonate linkers via the one-pot method. Because of the strong affinity between Zr(IV) and phosphonates, their products tend to form 2D densely packed layered structures with little porous structures.18 To solve this problem, sophisticated spacer structures, such as rigid trigonal structure, are introduced into the phosphonate linkers.4, 6 With the help of these spacers, the formation of 3D porous structures could be achievable. This synthetic strategy is effective and encouraging, but the design and synthesis of the complicated phosphonate linkers with 3D steric hindrance are difficult and high cost. Therefore, extensive efforts are still required to seek out simple and facile strategies to incorporate phosphonates into Zr-MOFs Post-synthetic ligand exchange is an easy and gentle strategy to modify the original MOFs using various metal-ligand combinations.19 Till now, several benchmark Zr carboxylate MOFs have been


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used as the parent's plat-forms to produce various MOFs successfully.20 In fact, phenylphosphonate ligand was also introduced into Zr-MOFs by ligand exchange, and the stoichiometric ratio of the phenylphosphonic acid (PPA)/Zr6 node should be limited strictly to preserve the MOF structure.21 To avoid the limitation of stoichiometry, diphenylphosphinic acid (DPPA) with large steric hindrance was employed to make bonds to the Zr6 nodes without destroying the MOF structure.22, 23 However, to the best of our knowledge, there is no previous report about integrating of phosphonate ligands into any Zr-MOFs without controlling the stoichiometric amounts of used phosphonic acids. As we know, MOFs are governed by Lewis acid-base coordination theory, and the ligand pKa can be used as a primary metric of the metalligand bond strength (here we use pKa to represent pKa1 for the multidentate acidic ligands).18, 24 Ligand pKa can also predict whether targeted MOF structures would be water and thermally stable24, 25 and their degradation kinetics.26 Besides, we also learn that the carboxylic acid ligand possessing of lower pKa is more competitive for node ligation than the carboxylic acid with higher pKa.5, 27, 28 Inspired by these literature works, the pKa values of new ligands associated with the postsynthetic ligand exchange is firstly correlated to phosphonate ligands in this work. We report a pKa-directed strategy to covalently integrate phosphonate ligands into Zr-based MOFs without the restriction on the amount of added phosphonic acids. Particularly, MOF-808 is selected as the parent's platform because of the high chemical stability of benzenetricarboxylic acid (BTC) linkers, due to its three anchor sites connected to Zr6 nodes.29-33 Besides, its larger spherical internal pore, with diameters of 18 Å, could accommodate abundant and large molecules, such as ethylenediaminetetraacetic (EDTA) acids,34 spiropy-rans35 and dipeptides.36 Firstly, we employed several common organic phosphonic acids with different pKa, as well as phosphoric acid (HPA),


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to coordinate with the Zr6 nodes in MOF-808. Remarkably, the conjugate base of the phosphonic/phosphoric acids (PA) bearing lower pKa is more aggressive for Zr6 node ligation, while the methylphosphonic acid (MPA), ethanephosphonic acid (EPA), and vinylphosphonic acid (VPA), with slightly higher pKa values than that of BTC can preserve the framework of MOF808. Then, we examined the stability of methylphosphonate MOF-808s (808-MPAs) in different acidic environments. Furthermore, 808-MPAs were used as adsorption materials for U(VI) uptake. Because of the regular porosities and easy accessible ion exchange P-OH sites, 808-MPAs exhibit ultrafast uptake rates to U(VI). This study is expected to provide a guidance for post-synthetic phosphonate Zr-MOF materials before any synthesis attempt is made. And the established strategy of integration of phosphonic acid with Zr-MOFs would provide a great application prospect in the removal of actinides from radioactive waste liquids. MATERIALS AND METHODS Materials. MPA, EPA, VPA, HPA, BTC, PPA, 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), 2-hydroxyphosphonocarboxylic acid (HPAA), and trifluoroacetic acid (TFA) were purchased from Aladdin. Microcrystalline MOF-808 was synthesized according to the literature procedures.29, 37 Synthesis of 808-MPAs. MOF-808 powder (0.50 g) was immersed in 50 mL MPA solution with a certain concentration for 24 h under stirring at room temperature. The mixture was filtered and the solid product was then solvent exchanged sequentially with water (3 × 50 mL), acetone (3 × 50 mL) and CH2Cl2 (3 × 50 mL) dried at 150 °C in vacuum to yield a white powder. The powders treated via MPA solutions with the concentrations of 25 mmol/L, 50 mmol/L, 75 mmol/L and 100 mmol/L, were denoted as 808-MPA25, 808-MPA50, 808-MPA75 and 808-MPA100, respectively.


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Synthesis of 808-EPAs/VPAs/HPAAs/HEDPs/HPAs/PPAs. The synthetic procedure was similar to that for 808-MPAs. 0.50 g MOF-808 powder was immersed in 50 mL PA solution for 24 h under stirring at room temperature. Following the same post-processing protocol, white powder was obtained. For clarity, we use 808-Xy to delegate the products from different PA with various concentrations. X is the abbreviation of the PA solute, and y is the certain concentration of PA with the unit of mmol/L. For example, we use 10 mmol/L PPA solution to treat MOF-808, and the resulted powder after post-processing is named 808-PPA10. And 808-Xs means all the products treated by X solutions. Analytical techniques. Powder X-ray diffraction (PXRD) patterns were recorded using a Bruker D8-Focus diffractometer. The surface morphology was examined by scanning electron microscopy (SEM, Nova Nanosem 430) and transmission electron microscopy (TEM, JEOL JEM2100). N2 adsorption isotherm was recorded on a Quantachrome Autosorb-1 volumetric gas adsorption analyzer. FTIR spectra were performed in KBr pellets on Nicolet Nexus 470. Solution 1H

NMR and 31P spectra were acquired on a Bruker Avance-400 MHz NMR spectrometer after

digesting the samples (10 mg) in HF (0.5mL, 40%)/DMSO-d6 (20 uL). The 1H NMR spectra of the PA solutions after ligand exchange were recorded using acrylic acid as the internal standard to monitor the loss of ligands. The functional groups of the 808-MPAs and 808-HPAs were analyzed via an X-ray photoelectron spectroscopy (XPS, Thermo Electron PHI-5000 II, C 1s at 285 eV) and 31P

solid-state magic angle spinning nuclear magnetic resonance (MAS NMR, Varian Infinity plus

300 MHz). Stability Tests. 50 mg 808-MPA100 was soaked in 50 mL hot water (100 °C), or 5 mL acidic solutions, including concentrated HNO3, concentrated HCl, 0.2 mol/L TFA and 1 mol/L TFA solutions, for one day. After filtration and drying under vacuum at 60 °C, the samples were


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characterized by PXRD and FTIR, and the filter liquors were tested by 1H NMR to detect the loss of ligands. DFT Calculations. Gaussian 09 was used to calculate the pKa values at the B3LYP/(6-311++G(d, p) level, using the literature method.24, 26 We also use Gaussian 09 to optimize the geometries of MOF-808 and its methylphosphonate products, as well as obtain their energies. The basis sets of Lanl2dz were employed for Zr atom, and 6-31G** for the rest atoms. The stationary point energies were also refined via the addition of diffuse functions in 6-31++G** set. All the energies obtained at the RM06L/(6-31++G**+Lanl2dz) level, corrected by zero-point energies at the same level. A cluster model, [Zr6O8(HCOO)12]4- was adopted to simulate MOF-808, in which the BTC linkers are substituted via formate ligands.36, 38 The energy for substitution is assessed by the binding energy

: =

FA

+

complex

MPA

MOF

808

where the Ecomplex represents the total energy of the complex products, EFA, EMPA and EMOF-808 represent the total energies of the HCOOH, MPA and MOF-808, respectively. Uranium adsorption. Solutions containing 100 ppm U(VI) were prepared via dissolution of uranium nitrate in HNO3 solutions and further dilution. 10 mg sample was added into 10 mL U(VI) solution in a flask stirring for 24 h at 25 °C. The solution was separated by filtration with 0.22 um filter membrane and then used for analysis. The concentration of the U(VI) in solutions was determined by Arsenazo III spectrophotometric method.39 Each adsorption was performed in triplicate and the average value was recorded.


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RESULTS AND DISCUSSION pKa-directed incorporation Table 1. pKa (pKa1) value of different phosphonic/phosphoric acids PA EPA

Structure HO

MPA

HO

VPA

HO

BTC

OH P O

OH P

O OH

P

O

O OH O O OH OH

HPA

HO

PPA

OH

HO P

O OH

HO P

O

Exp.

DFT

2.43,40 2.3941 2.41,42 2.3840

5.21

2.74,43 2.6044 2.12,45 2.1346

3.34

1.97,47 2.1448 2.03,49 1.7550

1.27

4.89

4.88

2.96

HEDP

HO OH O OH P P OH O OH

1.87,51 1.4352

1.11

HPAA

OH O O P OH HO OH OH

1.2053

0.74

Directed by the pKa controlled synthetic strategy, we sought phosphonic acid ligands that could not destroy the parents’ structure with 12-connected Zr6 nodes in MOF-808. To meet such a challenge, we screened seven phosphonic or phosphoric acid (PA) compounds with different pKa values, including MPA, EPA, VPA, HPA, PPA, HEDP and HPAA, to study the relationship between the stoichiometric ratios of the added PA/Zr6 node and the structure of their products. Their experimental pKa values from literatures are listed in Table 1. We also used DFT calculation to obtain their theoretical values. Both the experimental and calculated pKa values reveal that MPA EPA and VPA have pKa values close to that of BTC, but much higher than that of other four phosphonic acids and phosphoric acid. The scheme of incorporation of phosphonates/phosphate into MOF-808 is shown in Figure 1. The DFT calculated value of VPA is lower than that of BTC,


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Figure 2. PXRD patterns of experimental and simulated MOF-808 (a) and the resulted products, 808-Xy, where solute X is (b) MPA, (c) EPA, (d)VPA, (e)HPA, (f) PPA, (g) HPAA or (h) HEDP solution, and y is the initial concentration of X, mmol/L.

1H

NMR spectra (Figure 3(a) and Figure S4-S8) for digested MOF-808 and 808-MPAs showed

that the extent of phosphonate per Zr6 node varied from 3 to 6 by varying the concentration of MPA between 25 mmol/L and 100 mmol/L. The peak integral areas ratios of FA/TBC for digested 808-MPAs are all less than 5%, indicating activation of MPA-808s after heat treatment at 150 °C


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under vacuum. The formulas of 808-MPAs are listed in Table S1 according to their 1H NMR spectra. We also monitored the change of MPA solutions via 1H NMR (Figure 3(b)), and no BTC signals but displaced formic acid (FA, HCOOH) signals are measured, indicating the successful exchange of MPA and FA. That is, even when a large excess of MPA was used, no release of BTC was detected, suggesting that MPA does not replace the carboxyl groups of BTC in the MOF-808 framework. We calculated the amounts of FA and MPA in solutions before and after ligand exchange (Table S1). Interestingly, the decreased amounts of MPA from solutions are larger than the amounts MPA from digested MPA-808 samples, and the solution with initial MPA concentration MPA of 25 mmol/L has the largest increased amounts of FA after ligand exchange (Table S1). These phenomena indicate that there are strong interactions between 808-MPA and FA/MPA molecules, leading to adsorption of PA and MPA into the MOFs’ cavity after ligand exchange. There is also only formic acid but no BTC signals in the VPA solutions after ligand exchange (Figure S9), indicating the successful exchange of MPA and FA.

(a)Digested MPA-808y (b)MPA solutions

(c) HPA solutions after exchange

Displaced FA

Displaced FA

MPA

BTC Leakage

Residual MPA

after exchange

FA

BTC MOF-808

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


y=100

y=100

y=75

y=75 y=50 y=25

9.0

8.5

8.0 1.5 1.0 9.0

y=50 y=25

8.5 8.0 1.5 1.0 9.0 NMR (ppm)

8.5

8.0

1H


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Figure 3. 1H NMR spectra of (a) digested 808-MPAs, (b) MPA solutions after the ligand exchange, and (c) HPA solutions after the ligand exchange. y is the initial concentrations of MPA or HPA, mmol/L. When the concentrations of HPA/PPA/HPAA are less than or equal to 25 mmol/L (added PA/Zr6 node ratio, 3.4), the crystal structure of MOF-808 can be preserved in their daughter products according to PXRD patterns (Figure 2(e)-(g) and Figure S10-S12). And the replacement of formate ligands by HPA/PPA/HPAA is also achieved, according to the 1H NMR spectra analysis of their immersing solutions (Figure 3(c) and Figure S13-S14). However, when further increasing the concentration of HPA/PPA/HPAA solutions to 50 and 100 mmol/L, the samples have lost their original crystal structures. These results suggest the structural breakdown of MOF-808 when exposed to high concentrations of HPA/PPA/HPAA solutions. Different from the reported linker loss of UN-1000 reacted in PPA solutions,21 there is almost no BTC linker leakage of MOF-808 in PPA solutions (Figure S13), indicating that not all the three –COO- sites of one BTC linker are displaced by PPA. When treating with HEDP solutions, the framework collapse of MOF-808 is more obvious, even in an extremely low concentration (2-5 mmol/L) of HEDP solutions (Figure 2(h) and Figure S15). And the release of BTC linkers was detected even in 10 mmol/L HEDP solution (Figure S16). Altough the pKa of HEDP is larger than that of HPAA, it has a more destructive ability. That is due to the double phosphonic acid groups in one HEDP molecule, which can attack the adjacent two Zr6 nodes at the same time. Besides, after immersing in PA solutions, lower 2, shifts were observed for the samples without framework collapse, shown in the insets of Figure 2, indicating the expansion of the lattices for these MOFs after ligand exchange. The data from Le Bail fitting for MOF-808 and 808-MPA100


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reveal that the unit cell volume expanded from 42959 Å3 to 43892 Å3 (Figure S17-S18 and Table S2). We demonstrate the morphology of MOF-808 before and after ligand exchange via TEM (Figure 4 and Figure S19) and SEM (Figure S20) images. The TEM images of original MOF-808 (Figure 4(a)) have a distinct light and shade contrast for crystals. The dark outline of MOF-808 crystal exhibits its regular octahedral morphology. After ligand exchange, the octahedral morphology is preserved in all the products and there are still sharp outlines for the samples (Figure 4(b)-(h)). The phosphonate/phosphate ligand incorporation does not detectably change the size or shape of the MOF crystallites. From EDS Elemental mapping images of 808-MPA-100 (Figure 4(i)-(l)), we can find that the distributions of Zr, O and P elements were homogenous, implying a uniform distribution of methylphosphonate ligands. (a) MOF-808

(b) 808-MPA25

200 nm

(e) 808-HPAA100

(d) 808-PPA10

200 nm

(f) 808-MPA100

(j) P

200 nm

200 nm

(g) 808-HPA100

200 nm

200 nm

(i) 808MPA-100 Mapping

(c) 808-HPA10

(h) 808-PPA100

200 nm

200 nm

(k) O

(l) Zr

Figure 4. (a)-(h) Typical TEM images of MOF-808 and its products after ligand exchange in PA solutions, and (i)-(l) Elemental mapping images of 808-MPA100.

MOF porosities were evaluated via N2 adsorption isotherms (Figure 5(a)). The DFT surface areas of MOF-808, 808-MPA25, 808-MPA50, 808-MPA75, 808-MPA100 and 808-HPA25, are 2205 m2/g, 1762 m2/g, 1628 m2/g, 1557 m2/g, 1436 m2/g and 1125 m2/g, respectively. The


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incorporation of phosphonate/phosphate into the framework leads to a decrease in the specific surface areas and pore sizes (Figure 5(b)). The high concentrations of HPA can destroy the pore structure, resulting in a small surface area for 808-HPA100, about 27 m2/g, and no uniform pore channel was obtained. 650

(b)

(a)

MOF-808

MOF-808 808-MPA25 808-MPA50

510

808-MPA25

3

N2 adsorption (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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370

808-MPA50

808-MPA75 808-MPA100

808-MPA75

230

808-HPA25 808-MPA100

90

808-HPA100

-50

0.0

0.2

0.4

0.6

p/po

0.8

808-HPA25

1.0 0

2

4

Pore size (nm)

Figure 5. (a) N2 adsorption-desorption isotherms of MOF-808 and 808-MPAs/HPAs and (b) their pore size distributions.

We also employed FTIR to further illustrate the framework collapse process (Figure 6 and Figure S21-24). According to literatures, the P=O and P-OH stretch peaks of HPA,48 PPA,54 HEDP55 and HPAA56 were labeled in Figure 6. After ligand exchange, all the P-OH stretching bands of the samples are broadened and shifted to the longer wavelength sides, indicating coordination reactions to form the bonds of P-O-Zr. And the P=O stretching bands of these samples are shifted to the shorter wavelength sides, because of the strong interaction between P=O and Zr atom. The absence of the P=O and P-OH bands, in the final amorphous products, 808-HPA100, 808-PPA100, 808-HEDP100 and 808-HPAA100, can be interpreted as evidence for a multidentate bonding


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mode in these samples.57 Hence, it can be inferred that the HPA/PPA/HPAA/HEDP replace the formate ligand attached to the Zr6 nodes of MOF-808 firstly, and then replace the BTC ligands to form an amorphous zirconium phosphonate/phosphate salts. (b)

(c)

(d) HEDP

P=OH

P=O

808-HEDP2 808-HEDP50 808-HEDP10

808-HPAA100

808-HEDP100

808-PPA100

808-HPA100

808-HPAA50 808-PPA50

808-HPA75

808-PPA25

808-HPAA25

808-PPA10

808-HPA50

HPAA 808-HPAA10

P—OH

P=O

PPA

Ionic P—O

P—OH Ionic P—O Associated P=O Free P=O

HPA 808-HPA25

P=OH

(a) P=O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1200 1000 8001200 1000 8001200 1000 800 1200 1000 800

Wavenumber (cm-1)

Figure 6. FTIR spectra for (a)HPA/808-HPAs, (b) PPA/808-PPAs, (c) HPPA/808-HPPAs, and (d) HEDP/808-HEDPs.

We also notice that the steric structure of the ligands has a significant effect on the ligand exchange.22 We used a diphenylphosphinic acid (DPPA) with low pKa of 1.8058 but large steric hindrance, to exchange the ligands of MOF-808. The results show that the structure of MOF-808 cannot be preserved after treated by 100 mmol/L DPPA solution (Figure S25). However, the peaks of at 4.3°, 8.3° and 8.7° can still be observed for 808-DPPA100. This is in contrast with the other 808-X100 samples in this work. The seven phosphoric/phosphonic acid used here has only one alkyl group, with no such prominent steric effect as the two alkyl groups in DPPA. As a result, we


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could evolve that the pKa of the ligands is the main determining factor for the reactivity of phosphoric/phosphonic acid in the MOF-808 platform. To our best knowledge, the 808-MPA100/EPA100/VPA100 are the first reported Zr-MOFs treated using excess phosphonic acid without framework collapse. Meanwhile, in these cases, we achieve a maximum loading of phosphonic acid for each Zr6 node. Particularly, the pendant vinyl group in VPA100 can be used for further introduction of various ligands with different applications via addition reactions. Overall, we demonstrate that ligand pKa is a particularly useful and simple indicator to predict the stability of Zr-MOFs in the ligand exchange procedures, providing a proofof-concept sample for post-synthetic phosphonate Zr-MOF materials. Ligand Binding Mode of 808-MPAs We used XPS to reveal electronic characteristics of MOF-808 and 808-MPAs. The Zr 3d peaks of 808-MPAs shift to higher binding energy, indicating that the Zr atoms are more positively charged after ligand exchange, suggesting the charge redistribution within the Zr6 nodes (Figure 7(a)). The Zr 3d electrons of 808-HPA samples also have the same shift with 808-MPAs (Figure S26). The XPS spectra for the P atoms in 808-MPAs samples have the same binding energy at 133.6 eV and similar feature of peak shape, indicating the binding modes of MPA in these samples are uniform (Figure 7(b)). From P atom ratios in XPS analysis, we get the MPA contents in 808MPAs is 2.10 mmol/g, 2.45 mmol/g, 2.77 mmol/g and 3.58 mmol/g for 808-MPA25, 808-MPA50, 808-MPA75 and 808-MPA100, respectively. These contents are very close to the data obtained from 1H NMR analysis (Table S1). The P 2p peaks for 808-HPAs shift to the high binding energy when increasing the concentration of HPA, indicating their different phosphate binding modes (Figure S27).


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(a) Zr 3d

808-MPA75 808-MPA100

Intensity

MOF-808 808-MPA25 808-MPA50

(b)1P 2p

186

184

182

1 (c) C 1s

-C6H4 ring

-COO69%

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31%

P-CH3

13% 75%

12% 136

134

132

292

290

288

286

284

282

Binding Energy (eV)

Figure 7. XPS spectra of the MOF-808 and 808-MPAs: (a) Zr 3d, (b) P 2p and (c) C 1s.

From Figure 7(c), the C 1s peak of MOF-808 can be deconvoluted into two peaks at binding energies of 285.0 eV and 288.9 eV, which correspond to C=C and –COO- groups in C6H4 rings, respectively. For 808-MPA100, a new peak at 285.9 eV is attributed to P–C bond in methylphosphonate ligands. The sharp drop in the peak area of –COO- groups is due to the dissociation of the formate ligands from Zr6 nodes. The areas of –COO- peak and P–CH3 peak in 808-MPA100 are approximately equal, implying their almost equal amount. Therefore, we could infer that 6 formate ligands and 6 phosphonate ligands are bonded to one Zr6 node, which is in accordance with the 1H NMR Analysis. FTIR of 808-MPAs could provide insight into the vibrational modes of the phosphonate species bonded to Zr6 nodes (Figure 8 (a)). The position and assignment of peaks were labeled according to literature59. The P–OH stretching vibrations of MPA are shifted to higher wavenumbers after ligand exchange to form P-O-Zr linkers. Different from the 808-HPAs/PPAs/HPAAs/HDEPs, the


17


P=O groups are presented in all the 808-MPAs/EPAs/VPAs and their intensity increases with the exchanged MPA (Figure 8 (a) and Figure S28-S29). Meanwhile, after ligand exchange, P=O peaks are broadened and shifted slightly to the shorter wavenumber, indicating the redistribution of charge density for P=O groups ((Figure 8 (a)). In consideration of charge-balancing, we infer that a phosphonate ligand CH3P(O)(OZr)(OH) binds to a Zr6 node by chelating a terminal Zr(IV) to form one P-O-Zr bond.

P—OH 953 P—CH3 1010

O—H 1113 Bend 1153

P=O 1308 1319 MPA

808-MPA25 76%

23%

2% 808-MPA50

75%

MOF-808

P—O—Zr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

808-MPA25

2%

24%

808-MPA75

808-MPA50

78% 20%

2% 808-MPA100

808-MPA75

CH3P(O)(OZr)(OH) CH3P(O) (OZrO) 29% CH3P(O) 68% (OZr)2

808-MPA100

3%

1400

1200

1000

800

-1

Wavenumber (cm )

30 15 0 31P MAS NMR (ppm)

Figure 8. (a) FTIR and (b) 31P MAS NMR spectra for 808-MPAs.

The

31P

MAS-NMR spectra of 808-MPAs are given in Figure 8(b). Peak fitting of the

31P

spectra manifests three peaks with the same chemical shifts for 808-MPAs, of 7, 15 and 23 ppm. As we know, the coordination of P-OH to Zr will lead to an upfield shift of 31P MAS-NMR about 8 ppm.13, 60 Because the

31P

chemical shift of MPA was 28 ppm,61 the signal at 7 ppm can be

allocated to doubly coordinated phosphonate CH3P(O)(OZr)2, and the peaks at 15 ppm and 23 ppm can be both assigned as phosphonate coordinated to a single Zr atom, with different binding modes.


18

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In detail, the peak at 23 ppm can be allocated to CH3(O)(OZr)(OH), and the peak at 15 ppm can be allocated to CH3P(O)(OZrO) with two phosphonate oxygen atoms linked to one Zr atom, due to a higher degree of condensation will cause a downfield shift.60, 62 From 31P MAS NMR, most phosphonate ligands in the MPA-808 are CH3(O)(OZr)(OH) species, which is basically consistent with the FTIR evidence. Because the formation of ZrF62- species, MPA was released into the HF solution (Figure S30). For comparison, the

31P

MAS NMR spectra of 808-HPAs are shown in Figure S31, with

parameters listed in Table S3. The phosphates have four different binding modes to the Zr6 nodes. The predominant peak of amorphous 808-HPA100 at YF5 ppm is likely being (ZrO)2(O)POH.13 However, for the 808-HPA10 and 808-HPA25 crystals, the predominate peaks are at about -11 ppm, assigned to (ZrO)(O)P(OH)2. After structural collapse, all the spectrum is shifted upfield, suggesting more P-OH groups was attached with Zr6 nodes. Besides, we should notice that we could not get the binding modes via digested samples. The FTIR and 31P MAS NMR data strongly suggest that the CH3P(O)(OZr)(OH) species is the main ligand anchored to the 808-MPA samples. To further explore the ligand binding mode configurations of this species, we optimized the geometric structures of MOF-808 and its three mono-substituted products, 808-MPAsingle1/2/3 by replacing the single formate ligand with single deprotonated MPA ligand. All three configurations have one P-O3-Zr1 bond formed by POH groups and Zr6 nodes (Figure 9). The difference is: (a) 808-MPAsingle1 features P=O groups closing to Zr2 atom; (b) 808-MPAsingle2 features unanchored P-OH groups closing to Zr1 atom; (b) 808-MPAsingle3 features unanchored P-OH group closing to Zr2 atom. The optimized geometries show that the acidic proton on unanchored Y E groups has a strong tendency to form hydrogen bond ? Y YEZ @ A with a -3-oxo ligand (Figure 9, H1…O1). From the energy


19


Page 20 of 37

calculation, we learn that the three substitution reactions are all exothermic ([E < 0) by more than 80 kJ/mol. Relative to 808-MPAsingle2, 808-MPAsingle1 and 808-MPAsingle3 lay 2.11 kJ/mol and 5.94 kJ/mol lower in potential energy, respectively. The small difference in relative potential energy implies that these configurations have almost the same stability and hence they are in close competition with each other.

O3

Zr1

O1

O2

O4

H1 1.845 Zr2

Zr2

Zr1

O1

[E=-84.68 kJ/mol

(a) MOF-808

(b) 808-MPAsingle1 O2

O2

O3

O4

Zr1

O4

O3

H1 1.745

H1 Zr2

1.845 Zr2

Zr1 O1

O1

[E=-82.56 kJ/mol

(c) 808-MPAsingle2

[E=-88.50 kJ/mol

(d) 808-MPAsingle3

Figure 9. Optimized geometries of the MOF-808 ([Zr6O8(HCOO)12]6Y cluster) and the three binding modes of [Zr6O8(HCOO)11(CH3P(O)(OZr)(OH))]6Y systems. Color code: Zr = deep green, O = red, C = gray, P = purple, and H = white.

DFT calculation also provided a good approximation for the change of relevant interatomic distances in the Zr6 nodes (Table S4). The calculated Zr1–O1 and Zr1···Zr2 distances for MOF808 are 2.113 Å and 3.374 Å, respectively, which are close to experimental data.29 After ligand exchange, both the calculated Zr1–O1 and Zr1···Zr2 distances have an increase, indicating the lattice expansion of Zr6 nodes. The energy and the character of the highest occupied molecular


20

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


orbitals (HOMOs) of 808-MPAsingle1/2/3 are different from that of MOF-808 (Figure S32). The character and the energy of the lowest unoccupied molecular orbitals (LUMOs) are also altered. In the case of 808-MPA, the LUMO is delocalized over the six Zr atoms equally. However, for 808-MPAsingle1, the LUMO is delocalized over the phosphonate ligands. And in 808MPAsingle2/3, the LUMOs are mainly delocalized over the carboxylate ligands in the opposite positions of phosphonate ligands. These changes indicate that the methylphosphonate ligands alter the electron distribution of the Zr6 nodes, and could promote the further replacement of formate ligands with phosphonate ligands.36 Stability of 808-MPAs We investigated the thermostability of 808-MPA via thermogravimetric analysis (Figure S33). The result reveals MOF-808 lost 35 wt% weight at 350 °C. The initial weight losses were mainly due to the loss of the solvent molecules trapped in the pores, followed by formate ligands, and then decomposition of the BTC ligands, with ZrO2 as the remaining material at 700 °C. 808-MPAs have much better thermostability than MOF-808. Particularly, the weight loss of 808-MPA100 is below than 5% before 400 °C, followed by decomposition of the BTC and phosphonate ligands, with 78% weight remaining at 700 °C. As we know, the zirconium phosphonates, with good heatresisting properties, are usually used as the flame retardants.63,

64

Hence, the formation of

zirconium phosphonates could be responsible for the enhancement of their thermostability.


21


1

Internal standard (b) Concentrated HCl MPA

(a)

Leakage

BTC Leakage

1.8%

2.0%

Concentrated HNO 808-MPA100

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

3

2%

2.2%

0.2 mol/L HTFA

Concentrated HCl

pKa-Directed Incorporation of Phosphonates into MOF-808 via Ligand

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