Hf Metal

Article pubs.acs.org/crystal

Modulator Effects on the Water-Based Synthesis of Zr/Hf Metal− Organic Frameworks: Quantitative Relationship Studies between Modulator, Synthetic Condition, and Performance Zhigang Hu,∥ Ioannina Castano,∥ Songnan Wang, Yuxiang Wang, Yongwu Peng, Yuhong Qian, Chenglong Chi, Xuerui Wang, and Dan Zhao* Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore S Supporting Information *

ABSTRACT: The modulated synthesis of metal−organic frameworks (MOFs) remains empirical and challenging. Modulators are often applied and assumed capable of facilitating crystal growth by adjusting the reaction kinetics. However, most of the current studies are based on qualitative analysis and performance-leading synthesis, while no quantitative insights between modulator feature and MOF performance have been offered. In this work, we carried out a comprehensive study of the effects of three modulators (acetic acid, formic acid, trifluoroacetic acid) on the water-based modulated synthesis of UiO-66-type MOFs by using Zr or Hf as the building block and fumarate as the ligand. The modulator effects on crystallinity, yield, morphology, pore size, defects, porosity, stability, and gas separation performance of resultant MOFs have been discussed. A relationship between optimal molar ratio y and pKa value of modulator x is modeled as y = 12.72 + 0.193 × exp(1.276x). For MOF synthesis using ligands of different acidity, it tends to follow the equations of y = −40.78 + 39.1x and y = −21.7 + 25.58x for acetic acid and formic acid, respectively. Our results have thus provided pioneering quantitative analysis and synthetic guidelines on the further synthesis of water-stable MOFs that require modulators.

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morphology of the MOF products.34 Despite the abundant use and frequent success of acid modulators in MOF synthesis, a quantitative understanding of the role of acidity of both the modulator and ligand is limited and rarely reported. In order to precisely predict the modulation behaviors for the rational and optimal synthesis of MOFs, quantitative investigations on the modulators and their effect in crystal growth are highly aspired and conducive to further rationally guide the MOF synthesis. In this work, we carried out comprehensive and quantitative studies of modulator effects by probing modulator parameters (acidity, amount), synthetic conditions, and the properties of resulting MOF products to establish a relationship between the modulator, synthetic condition, and product performance. Specifically, we adopted the modulated hydrothermal (MHT) synthesis32,42 of Zr and Hf MOFs with fumarate (FMA) as the ligand using three common carboxylic acid modulators with varying pKa values: trifluoroacetic acid (TFA), formic acid (FA), and acetic acids (AA) (Scheme 1). By comparing the crystallinity, yield, morphology, defect concentration, surface area, pore size, and gas uptake of the resultant MOFs, we have obtained insights into the modulation mechanism and models (optimal molar ratio of modulator to ligand versus pKa of

INTRODUCTION Metal−organic frameworks (MOFs) are porous, crystalline, and hybrid materials formed by pervasive coordination bonds between metal ions and organic ligands.1−3 Their applications in fields such as gas storage and separation, catalysis, sensing, and drug delivery have been widely explored due to their excellent tunability in pore size, shape, and functionality.4−7 The majority of current research on MOFs focuses on the synthesis of water-stable MOFs, such as MILs,8−10 ZIFs,11−13 Zr/Hf-MOFs,14−26 and hydrophobic moieties modified MOFs,27−30 etc. During the synthesis of these MOFs, especially MILs and Zr/Hf-MOFs, acid modulators have been repeatedly reported to be capable of facilitating crystal growth through adjusting reaction kinetics. Modulators have been often assumed to play two important roles in the modulated synthesis of Zr MOFs: (1) to facilitate the formation of Zr6O4(OH)4 clusters and thus the growth of crystals; (2) to slow down the crystal growth rate avoiding fast precipitation of amorphous products.31,32 Since Behrens’s pioneering studies in the role of acetic acid (AA) and benzoic acid (BA) as modulators in the synthesis of UiO-66 and UiO-67 MOFs,31 a variety of modulators such as trifluoroacetic acid (TFA), hydrochloride acid (HCl), formic acid (FA), or even water have been investigated.33−41 The use of HCl that yields UiO-66 with BET surface area as high as 1580 m2 g−1 by Farha and his coworkers once again confirms the importance of these modulators in affecting the crystallinity, porosity, and © XXXX American Chemical Society

Received: January 16, 2016 Revised: March 4, 2016

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DOI: 10.1021/acs.cgd.6b00076 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

acidity of the modulators (Figures 1c and S1−4 and Table S1). Taking both the acidity (TFA > FA > AA) and the amount (TFA < FA < AA) of the modulators into consideration, it is suggested that increasing modulator acidity and decreasing its amount have an equivalent effect in affecting the crystallinity of resultant MOFs. Lastly, the acidity and optimal ratio correlate in a similar fashion for different metal groups (Zr versus Hf) (Figure 1c). Besides crystallinity, yield is another important factor to consider especially in terms of the large scale MOF production in industry.45 The modulator effects on the yield of MOFs are summarized in Figure 1b,e−f. The expected proportional correlation between crystallinity and yield is reflected in the case of ZrFMA (Figure 1b). However, HfFMA (Figure 1e) synthesized using a molar ratio of 35 has the highest yield of 72%. This is likely due to the precipitation of amorphous products (poorer crystallinity) that AA has the least ability in inhibiting crystal growth.32 A proximate negative correlation between the yield and crystallinity of the MOFs can be obtained when comparing the yield of MOFs at the optimal molar ratio (Figure 1f), which could be attributed to the fact that increased acidity may slow down the coordination reactions and thus facilitate the crystal growth.46 So far, formic acid as the modulator gives the best synthetic results in terms of MOF crystallinity and yield. This conclusion is also consistent with the current literature.37,39,47 However, in practical applications, material properties such as morphology and stability are also needed to be considered for scenarios such as product packaging and durability. We observed completely different MOF morphologies (each examined at their optimal molar ratio) using the three modulators (Figures 2a and S5). The AA modulator yields Zr- and Hf- FMA MOFs with an octahedral crystal shape similar to UiO-66-type MOFs.14 TFA modulated reactions, on the other hand, form MOFs as small (∼30 nm in diameter) sphere particles. This deviation in morphology from the parental UiO-66 MOFs can be explained

Scheme 1. (a) Modulators with Different pKa Values and (b) Crystal Structure of Zr/Hf-FMA MOFs

modulator and ligand) to predict optimal water-based synthetic conditions for stable Zr/Hf MOFs.

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RESULTS AND DISCUSSION The carboxylic groups of modulators are often believed to form clusters with metal cations that modulate the crystallization processes during the crystal formation of MOFs.43 We chose these three modulators with decreasing acidity (thus increasing pKa) in the order of TFA (0.3) > FA (3.74) > AA (4.76) to examine the hypothesis that acidity can greatly influence the modulation effect.44 We first examined the degree of crystallinity indicated by the relative powder X-ray diffraction (PXRD) intensity of Zr/Hf-FMA MOFs using different modulators (Figure 1a,d). For the same modulator, the relative intensity of PXRD peaks can be used to indicate the optimal modulator to ligand molar ratio (FA and AA in this case). Across different modulators, a decreasing optimal molar ratio (indicating the relative amount of modulator used) from AA (70), FA (53) to TFA (13) is consistent with the increasing

Figure 1. Modulator effects on the crystallinity (a, d), yield (b, e, f), and relationship of optimal molar ratio of modulator to ligand versus modulator acidity (c). B



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Figure 2. Modulator effects on the morphology (a), stability (reflected by decomposition temperature) (b), defect concentration (c), BET surface area (d), and pore size (e). (f) Relationship of MOF stability versus MOF defect concentration.

Figure 3. Relationship of CO2 uptake, Qst values, and IAST CO2/N2 adsorption selectivity versus modulator and defect concentration: (a, e) 0.15 bar; (b, f) 1 bar; (c, g) low-coverage heat values of CO2; (d, h) IAST CO2/N2 adsorption selectivity (assuming a mixture of CO2:N2 = 15:85 at 298 K and 1 bar) versus modulator and defect concentration.

missing ligands in the secondary building units (SBUs) of MOFs, which creates imperfections inside crystals leading to reduced stability.51 Among the MOFs with the same modulator concentration, HfFMA tends to have more defects (Figure 2c), which can be attributed to the slow reaction kinetics of Hf salts leading to incomplete crystallization.43 Assuming the same defect concentration, HfFMA is supposed to have higher thermal stability than ZrFMA (Figure 2f) due to the higher dissociation energy of Hf−O bonds (802 kJ mol−1) than Zr−O bonds (776 kJ mol−1).24 In short, a more acidic modulator (with lower pKa value, e.g., FA) is thus recommended if higher stability of MOFs is required. We also collected N2 sorption isotherms at 77 K to evaluate the porous texture of resultant MOFs including their BET surface area, porosity, and pore size distribution (Figures 2d,e and S7−8 and Table S2). Roughly

by the impeded crystal formation due to the abundant coordination interactions between the acid modulator and the substrates/MOFs. Interesting structures of nanoparticle (10− 20 nm) aggregates are obtained with FA modulator for both Zrand Hf-FMA MOFs, which is likely due to the existence of molecular-level self-assembly during the crystallization processes.48 Thermogravimetric analysis (TGA) is commonly used to probe the thermal stability and defect concentration of MOFs.49,50 TGA curves (Figure S6) reveal a trend between the increased modulator acidity (from AA to FA and TFA) versus the enhanced stability and decreased defect concentration (Figure 2b,c). The inversed correlation between stability and defect concentration is reiterated in Figure 2f. This is consistent with the fact that defect formation is due to the C



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Figure 4. Relationships between optimal molar ratio (modulator to ligand) and pKa (acidity) value of modulator (a) and ligand (b).

the highest CO2 uptake of 0.76 and 2.50 mmol g−1 at 0.15 and 1 bar among all the synthesized Zr-MOFs in this study, respectively. As for HfFMA, among all the synthesized HfMOFs in this study, using 13 equiv of TFA (13TFA for short) with least defects (1.4/12), it has the highest CO2 uptake of 0.55 and 1.63 mmol g−1 at 0.15 and 1 bar, respectively. Both of them are higher than UiO-66(Zr) (0.37, 1.79 mol g−1) though with much lower surface area,52 which could be possibly due to the reduced pore sizes (∼5 Å) favorable for gas adsorption.28 A similar trend is also observed for zero-coverage Qst of CO2 (Figures 3c,g and S13 and Table S2). The highest Qst is obtained for HfFMA using 13TFA (29.1 kJ mol−1), followed by ZrFMA using 53FA (29 kJ mol−1), which is much higher than that of UiO-66(Zr) (∼24.5 kJ mol−1)32 and UiO-66(Hf) (22.8 kJ mol−1),42 indicating the stronger interactions between CO2 and Zr/Hf-FMA of smaller pore size. Besides CO2 uptake capacity, CO2/N2 selectivity is also important in evaluating an adsorbent’s applicability for CO2 separation.57 The adsorption-based binary gas separation performance was evaluated based on ideal adsorption solution theory (IAST), and the results are shown in Figures 3d,h and S14 and Table S2.58 The IAST CO2/N2 selectivity was calculated by assuming a CO2/N2 mixture (molar ratio, 15:85) at 298 K identical to flue gas composition. Similar to the trend of Qst, ZrFMA using 13TFA shows the highest IAST CO2/N2 selectivity of 78.8 at 1 bar (Figure 3d), which is 315 and 275% higher than that of UiO-66(Zr) (∼19)52 and UiO66(Hf) (∼21),42 respectively, followed by decreasing order from 53FA (52.6) to 70AA (44). For HfFMA, using 13TFA also gave the highest IAST CO2/N2 selectivity of 48.3 (Figure 3h), followed by 53FA (26.8) and 70AA (20.8). They are all higher than that of pristine UiO-66 (∼20),52 which can be explained by the reduced pore size highly favorable for gas adsorption.28,59 These results suggest that Zr/Hf-FMA with smaller pores might be more promising in CO2 capture compared to their parent UiO-66 with relatively larger pores. In a short summary, the modulator effects on CO2 uptake, Qst of CO2 and CO2/N2 selectivity tend to be the same since these properties are mainly controlled by the pore size and Lewis acid sites.28 As discussed above, we have addressed the modulator effects on crystallinity, yield, morphology, pore size, defects, porosity, stability, and gas separation performance of resultant Zr/HfFMA MOFs. We would also like to establish a quantitative relationship between the optimal synthetic condition versus the acidity of both modulators and ligands, which can be conducive for the further synthesis of other MOFs.43 We cited the results of propionic acid (PA) and pKa values of ligands in the

speaking, more acidic modulator leads to higher porosity and surface area of MOFs (Figure 2d). The ZrFMA synthesized in this study exhibits a high BET surface area (960 m2 g−1), which is higher than the solvothermally synthesized one (856 m2 g−1)46 and is comparable to the one synthesized in water at 120 °C (970 m2 g−1).41 Similar correlation between higher surface area and more acidic modulator was observed in HfFMA, with the highest BET surface area in this study (761 m2 g−1) obtained using TFA as the modulator. The slight drop in surface area from Zr to HfFMA is consistent with the fact that larger and heavier Hf atoms increase molecular weight, reduce crystal volume, and thus increase the crystal density.52 We summarize several observations so far: (1) for all modulators, MOFs with high crystallinity tend to have high surface area; (2) modulator effects on the pore size of MOFs vary across different reaction parameters such as modulator acidity, with the exception that smaller pore size is commonly observed in TFA-modulated samples (Figure 2e); (3) different metal building units (Zr vs Hf) are nonetheless influenced by the modulators in a similar fashion (Table S2). The structural defects of MOFs may enhance substrate− material interactions and can be used for gas adsorption and catalysis applications.53 Previous studies have demonstrated the feasibility to fine-tune the type and concentration of defects by using different modulators during MOF synthesis.54 However, quantitative examination of the defect effects on stability and gas uptake performance (e.g., gas uptake capacity and gas affinity) of resultant MOFs has yet to be extensively explored. In this study, we focus the gas separation performance on CO2 capture, especially postcombustion CO2 capture where 15% of CO2 needs to be separated from the flue gas containing N2 (75%) and other minor components under ambient conditions.52 The relationships between CO2 uptake capacity and isosteric heat of adsorption (Qst) versus modulator and defect concentration are shown in Figures 3 and S9−13 and Table S2. The CO2 uptake capacity of HfFMA decreases at a higher structure defect concentration (Figure 3e−g), while there is no defined relationship between the two parameters for ZrFMA (Figure 3a−c). This discrepancy can be explained by combinatorial effects55,56 of different pore size (which would affect the surface area of MOFs) and Lewis acidity (which contributes to different adsorption affinity) of the two metal ions: (1) smaller Zr yields stronger Lewis acid sites than Hf; (2) CO2 adsorption of ZrFMA is governed by both the porosity and Lewis acid sites while HfFMA is only dominated by porosity.44 Being synthesized using 53 equiv (molar ratio of modulator to ligand) of FA with moderate defects (2.4/12), ZrFMA has D



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literature,16,41,60 and also summarized our previous and present work.32,39,42 We thus plot the optimal molar ratio (y) of modulator to ligand versus the acidity (x) of modulator or ligand. The results are shown in Figure 4. The pKa values for TFA, FA, AA, and PA are 0.3, 3.74, 4.76, and 4.88, respectively, following the order of decreasing acidity. As expected, the stronger the acidity, the less amount of modulator (refers to optimal molar ratio of modulator to ligand) required (Figure 4a). For different metal salts (Zr vs Hf), they shows a similar effect. Considering the modulating effect caused by acidity, we can expect an almost constant optimal ratio when using modulators of strong acidity (pKa 0−1) and an optimal ratio of infinity (∞) when the pKa of the modulator reaches above 5 (insoluble in water), meaning unable to modulate the reaction at all. The fitted equation between optimal molar ratio (y) and pKa value (x) of modulators can be written as y = 12.72 + 0.193 × exp(1.276x). Another important factor influencing the modulated synthesis of MOFs is the acidity of ligand itself. The relationship between optimal molar ratio (y) and pKa value of ligands (x) is shown in Figure 4b. A roughly linear relationship can be seen from the data for both AA and FA as the modulator. For the same modulator (AA or FA) using different ligands, the less acidic the ligand (higher pKa values), the more amount of modulator is required. However, a plateau can be imagined because the acidity of modulators will ultimately dominate the reaction. If the amount of modulator is increased further (less water), the ligand becomes harder to dissolve in aqueous media and that will not be covered in this plot. As for the same ligand using different modulators, a modulator with lower acidity (AA) tends to require a larger amount of modulator to assist the crystallization processes probably by slowing down the coordination reactions and prohibiting fast precipitation of amorphous products.44 The representative equations for AA and FA as the modulator are found to be y = −40.78 + 39.1x and y = −21.7 + 25.58x, respectively. The empirical fittings of these equations reported herein represent the relationship between the amount of modulator (ratio of modulator to ligand) and acidity of modulator/ligand for the best condition of crystal growth, which is controlled by the ligand-exchange reaction and crystal growth rate. It needs to be pointed out that the obtained conclusions in this study may not be applicable to other solvents as solvent will play a determinant role in the synthesis of MOFs and the effect is quite complicated. In this study, we focus on water as the solvent due to its environmentally friendly features suitable for the green process of scaled-up synthesis. Although semiempirical, these equations can be useful to guide the future trials of water-based modulated synthesis of MOFs.

were able to correlate the modulator feature and MOF performance. The relationship between optimal molar ratio y and pKa value (acidity) of modulators x can be obtained as y = 12.72 + 0.193 × exp(1.276x). For MOF synthesis using ligands of different acidity, it tends to follow the equations of y = −40.78 + 39.1x and y = −21.7 + 25.58x for AA and FA, respectively. Our results have for the first time provided quantitative analysis and synthetic guidelines on the hydrothermal synthesis of stable Zr/Hf MOFs that require modulators.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00076. Experimental details, calculations, gas sorption data, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

Z.H. and I.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National University of Singapore (CENGas R-261-508-001-646) and Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410-112, AcRF Tier 2 R-279-000-429-112).

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CONCLUSIONS In summary, we have carried out a comprehensive study of modulator effects on the water-based modulated synthesis of MOFs exemplified by using Zr or Hf salts as the inorganic component and fumarate (FMA) as the ligand. The modulator effects on crystallinity, yield, morphology, pore size, defects, porosity, stability, and CO2 separation performance of resultant MOFs have been discussed. We found that different modulators may have an equivalent effect on the crystallinity and yield of MOFs due to the underlying factors governed by the acidity rather than the type of modulators. By studying the performance indicators, such as surface area, pore size, defects, stability, CO2 working capacity, and CO2/N2 selectivity, we E



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