Abstract Water-stable Hf/Zr-Fumarate (FMA) metal-organic frameworks (MOFs) are highly promising for gas separation because of their postulated small pore size compared to other UiO-66-type MOFs and the low cost of fumaric acid. However, the efficient synthesis of these MOFs remains a big challenge. A mild, green, scalable modulated hydrothermal (MHT) method was applied for the synthesis of these MOFs. Specifically, acetic acid (AA), formic acid (FA), and trifluoroacetic acid (TFA) were used as the modulators. Various water to modulator solvent ratios were studied to investigate the effects of modulators on surface area and gas uptake properties. The MHT synthesized Hf/Zr-FMA MOFs displayed excellent hydrothermal stability, high surface area, high working capacity, and CO 2 selectivity. Modulated Hydrothermal Synthesis and Optimization of Hf/Zr-Fumarate Metal-Organic Frameworks Ioannina Castano 1, Zhigang Hu 2, Dan Zhao 2 1. Department of Chemistry, University of San Francisco, San Francisco, CA 94117, United States 2. Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, , Singapore MOFs are crystalline inorganic hybrid network structures made of coordination bonds between metal ions and polydentate organic linking groups. 1 Applications include gas storage, molecular separation, catalysis, sensing, and drug delivery. 1 They are known to have high porosities, tunable pore size, structures, and functionality. 2,3 The synthetic approach used in this study differs from the traditional solvothermal approach which requires high temperature and pressure, expensive toxic solvents, and intricate separation processes. 3-5 A MHT approach was used to optimize the synthesis of Hf/Zr- FMA MOFs. This approach is green (aqueous solutions), mild (100 °C, 1 atm), and scalable (up to kilograms). 3 Background Methods Modulated Hydrothermal Synthesis of MOFs Fumaric acid (FMA) organic ligand (0.6 g, ~5 mmol) and ZrCl 4 (1.2 g, ~5 mmol) or HfCl 4 (1.6 g, ~5 mmol) were suspended in 50 mL of water/modulator mixed solvent (Table 1). Heated under reflux at 105 °C for 24 h to yield a white powder. Residual reagents removed by water and ethanol washes. Dried under a dynamic vacuum at 120 °C for 24 h. Characterization Degassed under reduced pressure (<10 -2 Pa) at 150 °C for 10 h. N 2 sorption isotherms at 77 K were used to determine the pore size distribution, pore volume, and BET surface area data. Data gathered from powder X-ray diffraction (PXRD), Brunauer- Emmett-Teller (BET) surface area, field emission scanning electron microscopy (FE-SEM), and thermogravimetric analysis (TGA). Results Figure 1. Summary of PXRD patterns of Zr/Hf-FMA MOFs using best ratios of different modulators As the crystallinity of the MOF increased, the yield was reduced. For ZrFMA MOFS, the ratio with the greatest crystallinity displayed the best yields, but that was not the case for HfFMA (ratio of 35 showed the highest yield of 72%). Figure 2. N 2 sorption isotherms at 77 K of MHT synthesized of (a) ZrFMA and (b) HfFMA N 2 isotherms at 77 K displayed that an increase in modulator acidity increases the porosity and surface area. There is no absolute linear relationship between porosity and crystallinity as MOFs with the best crystallinity did not always show the highest BET surface area (FA) while some poorly crystallized MOFs displayed a high surface area (TFA) (Figures 1, 2). Figure 3. Pore size distribution of MHT synthesized (a) ZrFMA and (b) HfFMA The modulator effect on the pore size varies but TFA tends to give a smaller pore size (Figure 3). All Hf/Zr-FMA MOFs tested for pore width displayed major pores at approximately 5, 8, and 11.8 Å (Table 1). The more acidic the modulator, the larger the BET surface area (Table 1). Table 1. Summary of BET Surface Area Data For ZrFMA, there exists a balance between CO 2 uptake and defect concentration while an inversely linear relationship between CO 2 uptake and defect concentration was observed for HfFMA (Figure 4, Table 2). Figure 4. CO 2 sorption isotherms (a) ZrFMA and (b) HfFMA (filled, adsorption; open, desorption) at 273 and 298 K Figure 6. FESEM patterns of MHT synthesized ZrFMA and HfFMA MOFs: AA: (a) & (b); FA: (c) & (d); TFA: (e) & (f). References 1. Bradshaw, D.; Hankari, S.E.; Spagnolo, L.L. Chem. Soc. Rev. 2014, 43, Zhou, H. C.; Kitagawa, S. Chem. Soc. Rev. 2014, 43, Hu, Z.; Peng, Y.; Kang, Z.; Qian, Y.; Zhao, D. Inorg. Chem. 2015, 54, WiBmann, G.; Schaate, A.; Lilienthal, S.; Bremer, I.; Schneider, A. M. Elsevier. 2011, 152, Stock, N.; Biswas, S. Chem. Rev. 2012, 112, Acknowledgments This research was conducted in the Chemical & Biomolecular Engineering Department at the National University of Singapore. This work was supported by the National Science Foundation under award number DMR# and the American Chemical Society IREU program. Both Zr- and Hf- FMA using AA as the modulator gave an octahedral shape, which is also the origin of UiO-66-type crystal structure. FA exhibited complex aggregate morphology composed of nanoparticles (~100 nm) (Figure 6). TFA showed small spherical shapes (~30 nm). TGA curves displayed the general trend that with an increase in acidity from AA to FA and TFA, the relative stability increases and the defect concentration decreases (Figure 5, Table 2). Figure 5. TGA curves of MHT synthesized Hf/Zr-FMA MOFs Table 2. Summary of TGA and gas uptake properties Hf/Zr-FMA MOFs were successfully synthesized using the MHT approach. Of the modulator to ligand ratios tested, 70 AA, 53 FA, and 13 TFA proved to have the best crystallinity for Hf/Zr-FMA MOFs. By studying performance indicators, such as surface area, pore size, defects, stability, and CO 2 working capacity, we were able to optimize Hf/Zr-FMA MOFs. PXRD data displayed that the optimal modulator to ligand ratios for TFA, FA and AA are 13, 53, and 70, respectively (Figure 1). The more acidic the modulator, the less required. Although Zr and Hf have different reaction kinetics, they showed a similar effect in terms of acidity on optimal ratios. Conclusions Aggregation could be due to inner crystal interaction between the modulator and polar groups. Irregular, aggregated crystals correspond to unstable or poorly synthesized samples.