Boron Trifluoride Gas Adsorption in Metal–Organic Frameworks

Communication pubs.acs.org/IC

Boron Trifluoride Gas Adsorption in Metal−Organic Frameworks Paul W. Siu,† John P. Siegfried,† Mitchell H. Weston,† Patrick E. Fuller,† William Morris,† Christopher R. Murdock,† William J. Hoover,† Rachelle K. Richardson,† Stephanie Rodriguez,† and Omar K. Farha*,†,‡,§ †

NuMat Technologies, 8025 Lamon Avenue, Skokie, Illinois 60077, United States Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‡

S Supporting Information *

Recently, we reported the first MOF-based sorption study of PH3,17 another highly toxic gas used widely in semiconductor fabrication. Results showed that the MOF-74 series exhibited significantly higher PH3 uptake than conventionally used activated carbon. We envisioned that similar PH3 capacity enhancement could also be achieved with BF3 using MOFs. To this end, we report the first BF3 sorption study of MOFs, specifically MOF-74-M18 (where M = Co, Mn, Mg, and Cu), HKUST-1,19,20 and MOF-505.21 Their sorption capacities were compared to that of the conventionally used activated carbon to highlight the potential of MOFs in storage, delivery, and abatement of BF3. Caution! Boron trif luoride is a highly toxic gas. Extreme caution is required in its handling and usage. Samples of MOF-74-M (where M = Co, Mn, Mg, and Cu) were synthesized following modified procedures [see the Supporting Information (SI), section S-2, for details]. Their Brunauer−Emmett−Teller surface areas were calculated from measured N2 isotherms and were consistent with those of previously reported samples (see Figures S5−S11 and Table S1 in the SI). Their bulk purity was validated by comparing their powder X-ray diffraction (PXRD) patterns with their simulated patterns from single-crystal data (see Figures S1−S4 in the SI). The activated samples of MOF-74-M (where M = Co, Mn, Mg, and Cu) and activated carbon were exposed to gaseous BF3, and their BF3 isotherms were measured at 21 °C (Figure 1 for MOF-74-M, where M = Co, Mn, and Mg, Figure 2 for MOF-74Cu and activated carbon, and section S-5 in the SI for BF3 isotherm details). The cobalt, manganese, and magnesium analogues of MOF-74 exhibited a steep initial rise in BF3 uptake, reaching near-saturation at 6.7 × 10−3 bar (6.7 × 10−3 bar is also incidentally the pressure used to measure deliverable capacity upon desorption vide infra). The relative BF3 uptake at 6.7 × 10−3 bar is in the following order: MOF-74-Co (6.4 mmol/g) > MOF-74-Mn (3.2 mmol/g) > MOF-74-Mg (1.5 mmol/g) > activated carbon (1.0 mmol/g) ≈ MOF-74-Cu (0.9 mmol/g). The BF3 capacities for MOF-74-Co, MOF-75-Mn, and MOF74-Mg measured at 0.9 bar are 7.3, 3.5, and 2.1 mmol/g, respectively. The enhanced BF3 uptake of MOF-74-Co and MOF-74-Mn over activated carbon (2.5 mmol/g) can be attributed to the positive interaction between MOF-74-M

ABSTRACT: Coordinatively unsaturated metal−organic frameworks (MOFs) were studied for boron trifluoride (BF3) sorption. MOF-74-Mg, MOF-74-Mn, and MOF-74Co show high initial uptake (below 6.7 × 10−3 bar) with negligible deliverable capacity. The BF3 isotherm of MOF74-Cu exhibits gradual uptake up to 0.9 bar and has a deliverable gravimetric capacity that is more than 100% higher than activated carbon. Two other Cu2+ MOFs, MOF-505 and HKUST-1, have slightly lower deliverable capacities compared to MOF-74-Cu.

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xtensive investigation into metal−organic frameworks (MOFs) in recent years has propelled this class of crystalline porous materials into the frontier of gas storage and separation applications.1,2 This structurally diverse class of materials can be rationally designed by modulating their constituent metal nodes and organic linkers to afford MOFs with a variety of physical, chemical, mechanical, and sorptive properties. Given this tailorability, coupled with their higher porosity than that of conventional adsorbents, such as carbons and zeolites, MOFs are promising candidates as next-generation materials for the safe storage, delivery, and abatement of industrially significant toxic gases.3−8 One of these high value industrially significant toxic gases is boron trifluoride (BF3), a gas that is widely utilized as a reagent or catalyst in organic reactions,9,10 as a source for fluoride,11 and as a dopant in semiconductor fabrication.12,13 Its high toxicity is exhibited by a permissible exposure limit (PEL) of 1 ppm and an immediately dangerous to life or health (IDLH) concentration of 25 ppm.14 In the presence of water or humid air, BF3 gas will hydrolyze and releases HF, which is also a highly corrosive and poisonous gas and is readily absorbed through skin. While commercially available liquids and solutions of BF3 adducts (e.g., boron trifluoride methyl sulfide, boron trifluoride−methanol, and boron trifluoride diethyl etherate) offer ease of handling of BF3 for many organic reactions, the stringent purity requirements in semiconductor fabrication necessitate the need for pure gaseous BF3.13 Conventional sorbents, based on activated carbons,15,16 have been investigated for their BF3 sorption properties; however, studies with MOFs have thus far been lacking. © XXXX American Chemical Society

Received: September 19, 2016

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DOI: 10.1021/acs.inorgchem.6b02273 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. BF3 isotherms measured at 21 °C for MOF-74-M (MOF-74Mg, yellow circles; MOF-74-Mn, blue squares; MOF-74-Co, purple diamonds). P0 = 1 atm. Closed symbols: adsorption. Open symbols: desorption.

Figure 3. BF3 isotherms measured at 21 °C for HKUST-1 (purple squares) and MOF-505 (green triangles). P0 = 1 atm. Closed symbols: adsorption. Open symbols: desorption.

Although the ultrahigh overall BF3 uptake for HKUST-1 and MOF-505 makes them suitable for abatement applications, the resulting deliverable capacity for these Cu2+ MOFs containing open-metal sites was lower than MOF-74-Cu because of the significant hysteresis. Thus, the presence of Cu2+ open-metal sites in MOF-74-Cu cannot alone account for its enhanced BF3 deliverable capacity. The significant hysteresis observed in HKUST-1 and MOF-5 can be attributed to the coordination geometry around the copper metal center. While MOF-74-M, HKUST-1, and MOF505 all contain coordinatively unsaturated Cu2+ metal sites that can interact with BF3, they differ in the coordination geometry around the Cu2+ centers. All activated MOF-74-M structures are comprised of square-pyramidal metal MO5 clusters (Figure 4); Figure 2. BF3 isotherms measured at 21 °C for MOF-74-Cu (red squares) and activated carbon (black triangles). P0 = 1 atm. Closed symbols: adsorption. Open symbols: desorption.

open-metal sites with BF3 and illustrates the potential of these materials as superior sorbents in BF3 abatement applications. Unlike abatement applications, delivery applications rely on reversible capacities (i.e., deliverable capacities) rather than nonreversible total capacities. In the case of ion-implantation devices, the working delivery pressures range from 0.92 to 6.7 × 10−3 bar. To that end, the BF3 uptake of MOF-74-Cu does not reach saturation at 0.9 bar. The BF3 isotherm of MOF-74-Cu (Figure 2) exhibits a gradual, near-linear uptake and reaches 4.7 mmol/g at 0.9 bar. The deliverable capacity of MOF-74-Cu, based on the desorption component of the BF3 isotherm from 0.9 to 6.7 × 10−3 bar, is 3.8 mmol/g and is significantly higher than that observed for activated carbon (1.5 mmol/g). In contrast, the isostructural MOF-74-Co, MOF-74-Mn, and MOF74-Mg showed a negligible deliverable capacity under the same conditions, indicating that Cu2+ open-metal sites in MOF-74-Cu play an active role in modulating the reversible deliverable capacity. We were interested to see whether other MOFs containing Cu2+ open-metal sites would also exhibit high deliverable BF3 capacities. To this end, HKUST-120 and MOF-50521 were synthesized in accordance to reported procedures and their BF3 isotherms were measured at 21 °C (Figure 3). Interestingly, unlike the high initial uptake of BF3 at 6.7 × 10−3 bar observed in MOF-74-M (M = Co, Mn, and Mg), HKUST-1 and MOF-505 exhibited step uptake at higher pressures (0.1 and 0.05 bar, respectively), producing type VI like isotherms.

Figure 4. Left: MOF-74-Cu crystal structure and CuO5 squarepyramidal cluster (inset). Right: HKUST-1 crystal structure and Cu2O8 square-planar paddlewheel cluster. Inset: ball-and-stick model. Color code: copper, brown; oxygen, red; carbon, gray; hydrogen, white.

however, of the MOF-74 analogues examined here, MOF-74-Cu is the only MOF that contains a transition metal with a d9 electronic configuration. The interaction of a BF3 molecule with the open coordination site within MOF-74-M likely results in a pseudooctahedral geometry around the metal center. For MOF74-Cu, the Jahn−Teller effect likely plays a role in distorting the local environment around the Cu2+ center, thus elongating and weakening the Cu-BF3 interaction. A similar effect has been noted for MOF-74-Cu with CO222 and H2.23 This distortion, in turn, benefits the desorption of BF3 from MOF-74-Cu and B

DOI: 10.1021/acs.inorgchem.6b02273 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry ORCID

enhances its deliverable capacity over the other MOF-74 analogues mentioned vide supra. In contrast to MOF-74-Cu, the coordination geometry around the Cu2+ centers in activated HKUST-1 and MOF-505 is squareplanar and is further arranged into paddlewheels (Figure 4). Substrates can only interact with one of the two axial open coordination sites; therefore, the subsequent interaction with a BF3 molecule would only result in a square-pyramidal local environment. The Jahn−Teller effect is unlikely to play a role for HKUST-1 and MOF-505, which led to stronger interaction and in return much lower deliverable BF3 capacity than that for MOF-74-Cu. The favorable deliverable BF3 capacity of MOF-74-Cu prompted us to examine its potential in BF3 storage and delivery applications. As shown in Figure 5, the deliverable capacity is

Mitchell H. Weston: 0000-0002-4574-5888 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare the following competing financial interest(s): The authors have a financial interest in NuMat Technologies, a startup company that is commercializing metalorganic frameworks.

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ACKNOWLEDGMENTS NuMat Technologies thanks Northwestern University for PXRD access. NuMat Technologies also graciously acknowledges funding from the NSF SBIR program (Award 1430682).

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Figure 5. BF3 deliverable capacity (measured between 650 and 5 Torr) for MOF-74-Cu after multiple cycles.

retained after 10 adsorption and desorption cycles. Moreover, after a BF3 isotherm, MOF-74-Cu was evacuated at room temperature to remove the leftover BF3 heel. Subsequent N2 isotherms (see Figure S12 and Table S1 in the SI) revealed retention of the surface area: 1315 m2/g (before BF3 isotherm) and 1295 m2/g (after BF3 isotherm). In summary, we have shown the first study for the storage, delivery, and abatement of boron trifluoride using MOFs. In the case of abatement, HKUST-1 exhibited significant capture and retention of BF3. In the case of the storage and delivery of BF3, MOF-74-Cu well outperforms other MOF-74 analogues and other MOFs with Cu2+ coordination sites and outperforms activated carbon (3.8 mmol/g for MOF-74-Cu and 1.5 mmol/g for activated carbon). Additionally, MOF-74-Cu was shown to be highly stable to the Lewis acidic BF3 gas by retaining its performance after multiple filling and dispensing cycles.

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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.6b02273. Synthesis methods, PXRD patterns, and nitrogen and boron trifluoride isotherms (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (O.K.F.). C

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DOI: 10.1021/acs.inorgchem.6b02273 Inorg. Chem. XXXX, XXX, XXX−XXX