Modulated Hydrothermal Synthesis of UiO-66(Hf)-Type Metal–Organic

Article pubs.acs.org/IC

Modulated Hydrothermal Synthesis of UiO-66(Hf)-Type Metal− Organic Frameworks for Optimal Carbon Dioxide Separation Zhigang Hu, Anjaiah Nalaparaju, Yongwu Peng, Jianwen Jiang, and Dan Zhao* Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore S Supporting Information *

ABSTRACT: Recently, there has been growing interest in hafnium (Hf) metal−organic frameworks (MOFs). These MOFs may perform better as gas adsorbents than zirconium (Zr) MOFs due to the presence of Brønsted acid sites with high affinity toward adsorbates, together with the outstanding chemical and hydrothermal stabilities similar to their Zr analogues. However, Hf-MOFs have been rarely reported due to the lack of effective synthetic methods. We herein report a modulated hydrothermal synthesis of UiO-66(Hf)-type MOFs. Among these MOFs, UiO-66(Hf)-(OH)2 possesses a very high CO2 gravimetric uptake of 1.81 mmol g−1 at 0.15 bar and 298 K, which is 400% higher than that of UiO-66(Hf) (0.36 mmol g−1). It also exhibits a record-high volumetric CO2 uptake of 167 v/v at 1 bar and 298 K. Ideal adsorbed solution theory calculations showed a CO2/N2 (molar ratio 15:85) selectivity of 93 and CO2/H2 (molar ratio 30:70) selectivity above 1700. Breakthrough simulations also confirmed its optimal CO2 separation attribute. Our results have demonstrated for the first time the strong potential of Hf-MOFs for advanced adsorbents for high-performance CO2-related separations.

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INTRODUCTION Metal−organic frameworks (MOFs), built from coordination bonds between inorganic metal nodes and organic ligands, have become the frontier of porous materials after exponential development during the past two decades.1−6 Recent study has focused on water-stable MOFs toward practical applications, such as UiO-66(Zr)-type MOFs.7−13 As a new member of water-stable MOFs, Hf-MOFs have only been recently developed.14 UiO-66(Zr) and UiO-66(Hf) should have similar physical properties because both Zr and Hf belong to group IV elements. Moreover, in view of the fact that the dissociation energy of the Hf−O bond (802 kJ mol−1) is higher than that of Zr−O bond (776 kJ mol−1), Hf should be more oxophilic, and the μ3-OH groups in the Hf-containing secondary building units (SBUs) of UiO-66(Hf) could possibly function as Brønsted acid sites that are stronger than those in the Zrcontaining SBUs of UiO-66(Zr).15 This unique feature may grant Hf-MOFs more intriguing properties in the applications of catalysis and gas separation.16 Since the initial report of UiO-66(Hf),14 several attempts have been devoted to the synthesis of novel Hf-MOFs.15,17−22 However, traditional solvothermal synthesis of Hf-MOFs © XXXX American Chemical Society

usually ends up with either amorphous solids or viscous gels, possibly because of the slower reactivity of Hf salts.23 Therefore, a new synthetic approach for Hf-MOFs remains highly desired and a key issue to fully reveal the properties and potential applications of these promising materials. In our previous study, we have developed a modulated hydrothermal (MHT) approach to synthesize a series of UiO66(Zr)-type MOFs in a green, mild, and scalable way.13,24 In this work, we demonstrate a similar synthesis of a series of UiO66(Hf)-type MOFs with high porosity, excellent stability, and superior CO2 separation performance. In addition, the synthesis of these Hf-MOFs can be easily scaled up to tons of throughput with repeatable quality, proving their practical applications as promising advanced adsorbents for high-performance CO2related separations.

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EXPERIMENTAL SECTION

Materials and Methods. All of the reagents were obtained from commercial suppliers and used without further purification. Fieldemission scanning electron microscope (FE-SEM) analyses were Received: October 7, 2015

A

DOI: 10.1021/acs.inorgchem.5b02312 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry conducted on an FEI Quanta 600 SEM (20 kV) equipped with an energy-dispersive spectrometer (EDS, Oxford Instruments, 80 mm2 detector). Samples were treated via Pt sputtering before observation. Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8 Advance X-ray powder diffractometer equipped with a Cu-sealed tube (λ = 1.54178 Å) at a scan rate of 0.02 degree s−1. Thermogravimetric analysis (TGA) was performed using a Shimadzu DTG-60AH thermal analyzer under a flowing N2 gas (100 mL min−1) with a heating rate of 10 °C min−1. MHT Synthesis of UiO-66-Type MOFs. In general, organic ligands (∼5 mmol) and HfCl4 (1.7 g, ∼5.2 mmol) were suspended in 50 mL of water/acetic acid (30/20, v/v) solution and heated under reflux for at least 20 h to yield a powder product. The product was soaked in anhydrous methanol for 3 days at room temperature, during which time the extract was decanted and fresh methanol was added three times. After removal of methanol by decanting, the sample was dried under vacuum at 120 °C for 24 h to yield the final product with a yield of 75−92% based on the overall weight of ligand and metal salt. Solvothermal Synthesis of UiO-66(Hf). The solvothermal synthesis of UiO-66(Hf) was based on the modification of a reported procedure.22 Briefly, benzene-1,4-dicarboxylic acid (BDC) (83 mg) and HfCl4 (160 mg) dissolved in 20 mL of dimethylformamide (DMF)/formic acid (18/2, v/v) mixed solvent were loaded into a Teflon-lined autoclave and heated at 123 °C for 40 h. The product was soaked in anhydrous methanol for 3 days at room temperature, during which time the extract was decanted and fresh methanol was added every day. Then the sample was treated with anhydrous dichloromethane similarly for another 3 days. This process was carried out to wash out residual reagents trapped inside the pores. After removal of dichloromethane by decanting, the sample was dried under a dynamic vacuum at 120 °C for 24 h to afford the final product with a yield of 52% based on the overall weight of ligand and metal salt. Gas and Water Vapor Sorption Measurements. Gas and water vapor sorption isotherms of UiO-66(Hf)-type MOFs were measured up to 1 bar using a Micromeritics ASAP 2020 surface area and pore size analyzer. Before the measurements, the sample (∼80 mg) was degassed under reduced pressure (1700 30 38.8 3.6

28.2 14.4 8.2 27 61 13 5.8 2.7

23.4 16.2 8.1 24 59 16 6.4 2.4

a CO2/N2 = 15:85, 298 K 1 bar. bCO2/H2 = 30:70, 298 K 1 bar. cCO2/CH4 = 50:50, 298 K 1 bar. dCH4/H2 = 50:50, 298 K 1 bar. eCH4/N2 = 50:50, 298 K 1 bar. fNot determined.

bar and 298 K, UiO-66(Hf)-(OH)2 also has the highest CO2 uptake capacity (4.06 mmol g−1), which is comparable to some of the best adsorbent materials such as PPN-SO3Li (3.7 mmol g−1),32 SIFSIX-2-Cu-i (5.41 mmol g−1),28 biomass-derived carbons (5.8 mmol g−1),33 and industrial benchmark material zeolite 13X (5.0 mmol g−1)34 but is still much lower than that of Mg-MOF-74 (8.0 mmol g−1)35 (Figure 6 and Table S1). The CO2 uptake capacities under the same conditions (1 bar and

Figure 5. Gas sorption isotherms of UiO-66(Hf)-(OH)2 (filled, adsorption; open, desorption).

for postcombustion CO2 capture is defined.30 This value is 400% higher than that of UiO-66(Hf) (0.36 mmol g−1) and is even 61% higher than that of the best UiO-66(Zr)-type MOFs, UiO-66(Zr)-NH2 (1.12 mmol g−1).31 To the best of our knowledge, this is so far the highest CO2 working capacity obtained among all of the water-stable pristine MOFs without chemical decorations (e.g., amine grafting). It is possibly a synergistic effect of Brønsted acid sites and small pore size of UiO-66(Hf)-(OH)2 that boosts its CO2 uptake capacity.15,28 The second highest CO2 working capacity belongs to UiO66(Hf)-NH2 (0.93 mmol g−1), followed by UiO-66(Hf)(COOH)2 (0.40 mmol g−1) and UiO-66(Hf)-(F)4 (0.28 mmol g−1). In terms of the total gravimetric CO2 uptake at 1

Figure 6. Comparison of CO2 uptake capacities of UiO-66(Hf)-type MOFs with zeolite 13X and other prominent MOFs (dotted line represents the level of volumetric CO2 uptake of 167 v/v). D

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interested in is precombustion CO2 capture, where CO2 needs to be separated from syngas containing H2.44,45 The IAST CO2/H2 selectivity was calculated by assuming a CO2/H2 binary mixture with a molar ratio of 30/70 at 298 K. The highest CO2/H2 selectivity belongs to UiO-66(Hf)-(OH)2 (>1700), which is among the top of MOFs reported so far including Mg-MOF-74 (∼859)46 and SIFSIX-3-Zn (>1800).28 The last but not least interesting gas separation is natural gas upgrading, in which CO2/CH4 separation is a major process.47 The IAST CO2/CH4 selectivity was calculated by assuming an equal molar CO2/CH4 binary mixture at 1 bar and 298 K. UiO66(Hf)-(OH)2 has a moderate CO2/CH4 selectivity (∼30), which is higher than that of UiO-66(Hf) (12), UiO-66(Zr) (4.9),10 MOF-5 (∼15),48 and MOF-177 (∼4.5)48 but still lower than that of Mg-MOF-74 (∼137)46 and SIFSIX-3-Zn (231).28 So far, UiO-66(Hf)-(OH)2 has been identified to possess the highest gravimetric CO2 uptake capacity (1.81 and 4.06 mmol g−1 at 0.15 and 1 bar, 298 K, respectively), highest volumetric CO2 uptake capacity (167 v/v at 1 bar, 298 K), highest IAST CO2/N2 selectivity (∼93), CO2/CH4 selectivity (∼30), and CO2/H2 selectivity (>1700) among all the reported waterstable pristine MOFs. In addition, its high water and chemical stability (stable from pH 1 to 10, Figure 8) and easy scale-up have indicated its strong potential for industrial CO2 separation.

298 K) of UiO-66(Hf)-NH2, UiO-66(Hf)-(COOH)2, and UiO-66(Hf)-(F)4 are 2.80, 1.20, and 0.82 mmol g −1, respectively. It is worth noting that UiO-66(Hf)-(F)4 has a CO2 uptake lower than that of UiO-66(Hf)-(COOH)2, although they have comparable surface areas. This is probably because UiO-66(Hf)-(F)4 has a much weaker interaction with CO2 due to its nonpolar fluorine functional groups.36,37 We need to point out that for CO2 capture from large stationary sources such as postcombustion CO2 capture from power plants, the volumetric CO2 uptake capacity is more relevant as it is the volume instead of weight of the CO2 capture units that determines the process design.38 Due to its high density, UiO66(Hf)-(OH)2 possesses a superior volumetric CO2 uptake capacity of 167 v/v at 1 bar and 298 K, which is higher than that of the benchmark zeolite 13X and some of the prominent MOFs reported so far (Figure 6 and Table S1). Isosteric heat of adsorption (Qst) was calculated to fully unveil the CO2 adsorption strength of Hf-MOFs. As expected, UiO-66(Hf)-(OH)2 has the highest CO2 low-coverage Qst of 28.4 kJ mol−1 (Figure 7 and Table 2), which is substantially

Figure 7. Qst of CO2 in UiO-66(Hf)-type MOFs.

higher than that of UiO-66(Zr) (∼24 kJ mol−1)10 and UiO66(Hf) (∼22.8 kJ mol−1) but is lower than other highly efficient CO2 adsorbents such as zeolite 13X (38 kJ mol−1),34 PPN-6-SO3NH4 (40 kJ mol−1),39 Mg-MOF-74 (∼50 kJ mol−1),40 SIFSIX-3-Zn (45 kJ mol−1),28 and SIFSIX-3-Cu (54 kJ mol−1).41 However, a lower Qst actually indicates an easy reversibility of adsorption−desorption cycles and is highly beneficial for the adsorbent regeneration process because a balance between separation performance and energy cost is highly sought in real industrial operations. As expected, UiO66(Hf)-(F)4 has the lowest CO2 low-coverage Qst of 23.4 kJ mol−1, and Qst values of 28.2 and 25.4 kJ mol−1 were obtained for UiO-66(Hf)-(COOH)2 and UiO-66(Hf)-NH2, respectively (Table 2). The sorption-based binary gas separation performance of these Hf-MOFs was further evaluated based on IAST,42 and the results are shown in Table 2. The first interesting gas separation is postcombustion CO2 capture, in which 15% of CO2 needs to be removed from a flue gas mixture containing ∼75% of N2 at 1 bar and 298 K.30 The IAST CO2/N2 selectivity at 298 K was calculated by assuming a CO2/N2 binary mixture with a molar ratio of 15/85 to mimic the flue gas composition. Similar to the trend of Qst, UiO-66(Hf)-(OH)2 has the highest CO2/N2 selectivity of 93 at 1 bar, which is 343% higher than that of UiO-66(Hf) (∼21), 145% higher than that of the optimal UiO66(Zr)-(COOH)2 (∼38),8 close to that of Mg-MOF-74 (∼148),43 but still much lower than that of the top adsorbents such as PPN-6-SO 3 NH 4 (∼796) 39 and SIFSIX-3-Zn (∼1818).28 The second gas separation process we are

Figure 8. Stability tests of UiO-66(Hf)-(OH)2 under various conditions: (a) PXRD patterns; (b) N2 sorption isotherms at 77 K.

Column Breakthrough Simulation. In order to further evaluate the gas separation performance of UiO-66(Hf)-(OH)2 under practical working conditions, we carried out column breakthrough simulations for this MOF as packed adsorbents in a fixed-bed under an energy-efficient pressure swing adsorption (PSA) process.49 Simulation methodology has been wellestablished50,51 and successfully used to predict binary CO2 breakthrough profiles in our previous studies.52−54 Herein, we E

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breakthrough time between CO2 and other gases (N2, H2, and CH4) strongly indicates that UiO-66-(OH)2 is able to effectively separate CO2 in the operations such as postcombustion CO2 capture (CO2/N2 separation), precombustion CO2 capture (CO2/H2 separation), and natural gas upgrading (CO2/CH4 separation). The operations of precombustion CO2 capture and natural gas upgrading are typically carried out at high pressures. Therefore, the breakthrough profiles for CO2/H2 and CO2/ CH4 mixtures were also predicted at a higher pressure of 25 bar (Figure 9d). Compared to the results obtained at 1 bar, the high-pressure breakthrough time of CO2 is reduced to 40.5 and 12 in CO2/H2 and CO2/CH4 mixtures, respectively. This is because of higher CO2 concentrations under high pressures that lead to quicker adsorption saturation in the fixed-bed. On the other hand, this also suggests that a frequent regeneration is required for the adsorbent in the fixed-bed PSA process. Therefore, we carried out repeated CO2 adsorption/desorption tests to check the regeneration feasibility of UiO-66(Hf)(OH)2, which should be of equal importance for industrial applications. The result showed that there was no significant loss of CO2 uptake capacity in UiO-66(Hf)-(OH)2 after five cycles (Figure 9e), indicating excellent regeneration and cycle performance of this MOF. These features suggest a low-energy, mild, and easy regeneration process of using UiO-66(Hf)(OH)2 for CO2-related gas separations. The next question we want to answer is the effect of moisture, which exists everywhere and may affect the CO2 separation performance of adsorbent materials.56,57 Water vapor sorption behaviors of Hf-MOFs were thus examined (Figure 10 and Table 1). Both UiO-66(Hf)-(OH)2 and UiO-

predicted the breakthrough profiles of UiO-66(Hf)-(OH)2 for the mixed gases including CO2/N2 (15/85), CO2/H2 (30/70), and CO2/CH4 (50/50) at 298 K and 1 bar based on pure gas adsorption isotherms (Figure 9 and Table S2). The break-

Figure 9. (a−d) Fixed-bed breakthrough simulations for UiO-66(Hf)(OH)2: (a) CO2/N2 (15/85) at 1 bar and 298 K, (b) CO2/H2 (30/ 70) at 1 bar and 298 K, (c) CO2/CH4 (50/50) at 1 bar and 298 K, and (d) CO2/H2 (30/70) and CO2/CH4 (50/50) at 25 bar and 298 K. (e) CO2 adsorption−desorption cycles of UiO-66(Hf)-(OH)2.

through profiles for each mixture can be characterized by different regimes. Taking CO2/N2 as an example, first, nearly pure N2 emits from the bed, whereas CO2 remains trapped within the bed due to strong adsorption into the packed UiO66(Hf)-(OH)2. After a certain period of time, CO2 breakthrough occurs and gradually reaches its feed composition. Meanwhile, N2 concentration at the outlet drops to its feed composition. The x-axis is normalized by the characteristic time (bed length/interstitial velocity) and presented as dimensionless. To quantify, a breakthrough time τ is defined as the time when gas concentration at the outlet is 0.01%. Typically, the longer the breakthrough time of CO2, the higher the retention time of CO2, and the more efficient the gas separation is. For CO2/N2 separation, the simulated breakthrough time is 681 for CO2, which is much higher than that of simulated CuTDPAT (84)54 and [HO2C]100%-H2P-COF (∼50).55 Similar breakthrough behaviors were observed for CO2/H2 and CO2/ CH4 mixtures, in which the breakthrough times of CO2 are 532.5 and 371.3, respectively, which are also higher than that of Cu-TDPAT (126 and 90.3, respectively).54 Because of their weaker interactions with UiO-66(Hf)-(OH)2, H2 and CH4 have much shorter breakthrough time. Such a large difference of

Figure 10. Water vapor sorption isotherms of UiO-66(Hf)-type MOFs at 273 K (filled, adsorption; open, desorption).

66(Hf)-NH2 showed type I water sorption isotherms at 273 K (Figure 10), which are identical to that of zeolites but quite different from the sigmoidal shape of UiO-66.57 Large amounts of water uptake were observed for this two MOFs at P/P0 < 0.3 [290 and 330 cm3 g−1 for UiO-66(Hf)-(OH)2 and UiO66(Hf)-NH2, respectively], indicating their highly hydroscopic nature, which might be detrimental to the CO2 separation performance. However, the adsorbed water vapor can be almost completely removed by reducing pressure without changing temperature (pressure swing desorption). This is quite different from zeolites in which removing moisture requires high vacuum under high temperature (normally above 200 °C) because of their strong affinity toward water. The almost reversible water vapor adsorption−desorption behavior of Hf-MOFs suggests their mild water affinity and easy regeneration processes even in moisturized conditions. Therefore, these Hf-MOFs should be F

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suitable for a two-step (dehydration towers followed by separation towers) PSA operation for postcombustion CO2 capture. It is interesting to note that, although with a BET surface area comparable to that of UiO-66(Hf)-(COOH)2, UiO-66(Hf)-(F)4 has a nearly flat water isotherm with a low water uptake at low pressure ranges, indicating its hydrophobic nature. This indicates a large design freedom of MOFs through ligand functionalization, which might be helpful in the tailored synthesis of hydrophobic MOFs for CO2 separation with minimized moisture interference. The real CO2 separation performance of Hf-MOFs in the presence of moisture remains to be comprehensively investigated, and the relevant research is ongoing in our laboratory.

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CONCLUSIONS In conclusion, we have successfully synthesized a series of UiO66(Hf)-type MOFs via a modulated hydrothermal approach. Among these MOFs, UiO-66(Hf)-(OH)2 possesses the highest gravimetric CO2 uptake of 1.81 mmol g−1 at 0.15 bar and 298 K among all the reported water-stable pristine MOFs and recordhigh volumetric CO2 uptake of 167 v/v at 1 bar and 298 K due to its high crystal density. IAST calculations indicated excellent CO2/N2 (15/85) selectivity of 93 and CO2/H2 (30/70) selectivity above 1700. Breakthrough simulations and cyclicity tests also confirmed that UiO-66(Hf)-(OH)2 could act as an efficient adsorbent for optimal CO2-related gas separations. Our results have for the first time demonstrated a strong potential of Hf-MOFs as promising advanced adsorbents for high-performance CO2/N2, CO2/H2, and CO2/CH4 separations. Moreover, the synthetic approach and results will surely open a new realm of synthesis and applications of Hf-MOFs.

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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.inorgchem.5b02312. Calculation methods, gas sorption data, and breakthrough simulation details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s): A US provisional patent (No. 62/132,608) has been filed on 13 March 2015 based on the presented result.

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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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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02312 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02312 Inorg. Chem. XXXX, XXX, XXX−XXX