Selective decontamination of the reactive air pollutant nitrous acid via node-linker cooperativity in a metal–organic framework

The environmental pollutant nitrous acid is rapidly and selectively sorbed and converted to benign products in the metal–organic framework UiO-66-NH2.


S1 Synthesis
Unless otherwise noted, all reagents were purchased from commercial sources and used without further purification.

S1.2 Synthesis of UiO-66 and UiO-66-NH2
UiO-66 and UiO-66-NH2 were synthesized using the same procedure by Katz et al. 2 Each MOF was synthesized in 25 mL Duran® glass bottles. Using the preparation of Zwoliński et al. 3 the isolated UiO-66-NH2 product was refluxed in 25 mL of MeOH for 24 h to remove any formylated UiO-66-NH2. The purified UiO-66-NH2 was filtered, dried, and stored wrapped in tinfoil in the dark.

S1.3 Synthesis of Zn2(BDC-NH2)2(DABCO)
Synthesis of Zn2(BDC-NH2)2(DABCO) was carried out using the literature preparation from McGrath et al. 4 In a 25 mL Duran® glass bottle 0.0947 g (0.844 mmol) DABCO was sonicated to dissolution in 7 mL DMF. To this, 0.5010 g (1.684 mmol) of Zn(NO3)2•6H2O was added along with another 7 mL aliquot of DMF and the resultant solution was sonicated to dissolution. Finally, 0.2675 g (1.526 mmol) of H2BDC-NH2 was added with an additional 7 mL aliquot of DMF and the final solution was sonicated to dissolution. Subsequently, the sample was placed into a 120 °C oven for 48 h. After 48 h, the sample was removed from the oven and the cooled crystals were transferred to a 50 mL centrifuge tube and centrifuged for 5 min at 5000 rpm. The mother liquor was then decanted off and the sample was washed with three 10 mL aliquots of DMF followed by three 10 mL aliquots of MeOH under the same centrifugation conditions mentioned above. The crystals were characterized by NMR.

S2.1 Nuclear Magnetic Resonance (NMR)
1 H-NMR analyses were measured using a Bruker AVANCE III 300 MHz NMR. The solution phase NMR of all MOF samples were prepared using ca. 5 mg of MOF first dissolved in 2-3 drops of D2SO4. Once dissolved, approximately 1 mL of DMSO-d6 was added as the NMR lock solvent. It should be noted that small chemical shift differences can occur between samples due to the change in dielectric constant caused by the slight variance in pH between samples.

S2.2 Infrared (IR) Spectroscopy
A Bruker Alpha FTIR spectrometer equipped with an ATR crystal was used for IR characterizations. The scanning range was 500 cm -1 -4000 cm -1 .

S2.3 Surface Area (SA) Analysis
All samples were thermally activated using a Micromeritics Smart VacPrep gas adsorption sample preparation instrument. The samples were initially heated to 90 °C while a vacuum level below 1.00 mmHg was reached at 5.00 mmHg s -1 . Subsequently, the samples were heated to 150 °C for 500 min at a ramp rate of 5 °C min -1 . The nitrogen-accessible gas adsorption isotherms (77 K) were measured using a 3Flex Surface Characterization instrument, with the accompanying MicroActive software suites. The accessible surface area was calculated based on Brunauer-Emmett-Teller (BET) theory using the 4-point criterion. 5

S2.4 Powder X-Ray Diffraction (PXRD)
Powder X-ray diffractograms were measured on a Rigaku Ultima IV X-ray diffractometer with a Cu X-ray source operating at 40 kV x 44 mA (1.76 kW) and a scintillation counter detector.
A continuous scan mode was applied between 2θ = 5° -30° with a sampling width of 0.020° and a scan rate of 1.000 °/min. Fig. S2. Schematic of the nitrous acid gas-phase source connected to the MOF sample holder and the downstream air generator and detector.

S3.1 Custom Instrumentation for Nitrous Acid Generation
The generation of gas-phase nitrous acid was based on an acid-displacement reaction (Fig. S2). [6][7] To accomplish this, an HCl permeation tube is necessary. To make a stable/tunable nitrous acid source, a 120 sccm stream of nitrogen gas is divided.
Each 60 sccm stream flows through PFA tubing (ID = 1 mm, OD = 3 mm), with one stream S8 bubbled through water to produce a 100% relative humidity (RH) nitrogen stream. The other stream flows through a heated channel in an aluminum block containing the porous HCl permeation tube. The two streams are recombined to yield 50% RH gaseous HCl. The humid HCl flows over solid NaNO2 residing in the second channel of the heating block to produce gas-phase nitrous acid. This nitrous acid gas is subsequently used for MOF breakthrough experiments. Higher Al block temperatures result in higher mass-emission rates of HCl and increased nitrous acid production. The output concentration of the nitrous acid source is measured using an Ecotech Serinus 40 Oxides of Nitrogen Analyzer (American Ecotech, Warren, RI).

S3.3 Oxides of Nitrogen Analyzer
To The general detection principle of the Serinus 40 involves reacting nitric oxide with ozone (O3) to generate excited state NO2 * . During luminescent relaxation, a photon is released and detected by a photomultiplier. 8 Using this technique, nitric oxide can be measured easily in the O3reaction channel. Other nitrogen oxides must first be converted to nitric oxide before they can be measured. The mechanisms for nitrogen dioxide and nitrous acid analysis are synonymous and involve converting either species into nitric oxide using the Mo catalyst heated to 325 °C in the NOx channel. Thus, nitrogen dioxide is determined by difference between the NOx and nitric oxide channels. If only nitrous acid is introduced to the NOx channel, then the instrument can also be exploited to measure nitrous acid, which is well documented. 9 To ensure that we are producing nitrous acid and not nitrogen dioxide, a control experiment is run periodically in which a sodium carbonate (Na2CO3)-coated annular denuder (URG Corp., Chapel Hill, NC) is used to remove nitrous acid from our gas stream. [10][11][12] Under these conditions, the analyzer is only measuring nitrogen S10 dioxide and nitric oxide. With the denuder in line, no downstream nitrogen dioxide, or nitric oxide were observed. Thus, our nitrous acid source was producing nitrous acid and no NOx.
The mass-emission rate of nitrous acid was determined by bubbling the output flow through 1 mM KOH solution. Under this condition, nitrous acid is converted back to the nitrite anion. The nitrite concentration was subsequently measured by anion ion chromatography with conductivity detection (IC). The separation was performed using a ThermoScientific ICS-2100 fitted with AS11-HC guard and analytical columns, a KOH eluent generator system (EGC-III), running a gradient elution program at 30 °C and a mobile phase flow of 1.5 mL/min, followed by suppressed conductivity detection. 13 Table S2: Breakthrough times for the first sorption cycle featured in the main text represented in different units (column 2, 3, and 4 correspond to the first sorption curve in Fig. 2a, 2b, and 2c respectively, while column 5 corresponds to the first sorption curve in Fig. 4b).

S4 MOF Characterization
UiO   S4. Nitrous acid breakthrough curves for one sample of UiO-66. The sample was run through three sorption and desorption cycles. Each desorption cycle flowed a dry (0% RH) N2 gas stream through the sample. Blue lines separate sorption and desorption curves, while black lines separate different cycles on the same MOF sample. Note the minimal desorption using 0% RH, and the spike in concentration when the MOF is exposed in subsequent (cycles 1 and 2) sorption cycles due to the presence of 50% RH in the subsequent HONO gas stream sorption cycle. The present work illustrates that, unlike the 50% RH desorption in Fig. 2   Note that for the post-exposed Zn2(BDC-NH2)2DABCO samples, there is no evidence of BDC-OH suggesting that BDC-NH2 did not react with HONO in Zn2(BDC-NH2)2DABCO. Furthermore, the small amount of BDC-N2 + that may be present, is likely due to homogeneous reaction between BDC-NH2 and adsorbed nitrous acid during D2SO4 dissolution; this also confirms that the chemistry observed in the main text is heterogeneous in origin.