Functional Monolithic Polymeric Organic Framework Aerogel as

May 9, 2012 - Synopsis. A polymeric organic framework in the form of monolith, Mon-POF, was prepared from terephthalaldehyde and 1,5-dihydroxynaphthal...
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Functional Monolithic Polymeric Organic Framework Aerogel as Reducing and Hosting Media for Ag nanoparticles and Application in Capturing of Iodine Vapors Alexandros P. Katsoulidis,† Jiaqing He,‡ and Mercouri G. Kanatzidis*,† †

Department of Chemistry and ‡Department of Materials Science and Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois, 60208-3113, United States S Supporting Information *

ABSTRACT: Monolithic aerogels of polymeric organic framework (Mon-POF) with a high density of OH functional groups were synthesized through solvothermal polymerization of terephthalaldehyde and 1,5-dihydroxynaphthalene. This POF material presents high surface area of 1230 m2 g−1 having micro-, meso-, macropores, and low bulk density of 0.15 g cm−3. The evolution of the porous properties is controlled with the polymerization rate. Mon-POF is stable under acidic and basic conditions. The presence of high number of OH functional groups provides the monolith with ion-exchange properties as well as reducing properties. The Mon-POF adsorbs Ag+ from aqueous solution to deposit Ag nanoparticles into the pores at a high loading content ∼25 wt % of the composite material. The Ag loaded monolith captures significant amount of I2 vapor and fixes it effectively in the form of β-AgI. KEYWORDS: functional porous polymer, monolith, Ag nanoparticles, iodine capture

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aerogels. Resorcinol-formaldehyde (RF)5 aerogel is the first member of this family, and subsequently melamine-formaldehyde,6 phenol-furfural,7 phloroglucinol-formaldehyde8 aerogels were reported. Other organic aerogels that are based on polyurethane9,10 and polyurea11 have interesting thermal insulation properties. Also polydicyclopentadiene12 and polyimide13 aerogels prepared with ring-opening metathesis polymerization exhibit good insulation and mechanical properties. Crystalline aerogels of polystyrene14 and poly(4-methylpentene-1)15 are obtained after cooling hot solutions of these polymers. Cellulose aerogels were derived from aqueous cellulose/NaOH solutions.16 Organic aerogels are easily transformed to carbons by pyrolysis at temperature above 500 °C. Porous polymer monoliths are a separate class of materials having similar porous properties to organic aerogels. They were mainly developed by Frechet et al.17−19 from direct

INTRODUCTION Porous organic materials are attracting considerable scientific attention for their potential opportunities for new science and a multitude of technological applications.1 Particularly interesting is the development of hybrid, inorganic-organic, and purely organic micro- and mesoporous materials with functional groups for targeted applications.2 Aerogels are a special class of porous materials derived from highly cross-linked inorganic or organic gels that are dried using supercritical fluid processing.3 Aerogels are obtained in two different forms: monoliths and fluffy powders. Monoliths are single piece objects that retain the shape of the wet gel, and they are sufficiently mechanically stable for handling.4 Aerogels exhibit a great variety of physical properties like low refractive index, low dielectric constant, low bulk density, and high surface area and are being investigated for use for applications such as Cerenkov detectors, low k dielectrics, thermal insulation, sorbents, and catalysts.3,4 Organic aerogels received great attention after the fundamental investigation of Pekala on phenolic resins based © 2012 American Chemical Society

Received: March 3, 2012 Revised: April 23, 2012 Published: May 9, 2012 1937

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Scheme 1. Polymerization Reaction between Terephthaladehyde and 1,5-Dihydroxynaphthalenea

a

The molecular structure of the polymer having (i) terephthalaldehyde and (ii) 1,5-dihydroxynaphthalene in the middle. polymerization corresponded to 94%. Similarly two more aerogels were synthesized under the same conditions using 1 mmol of each monomer and 0.5 mL of aq. HCl 1 M (Mon-POFcc) and 1 mmol of each monomer and 1 mL of aq. HCl 1 M (Mon-POFc). Preparation of Ag Nanoparticles in Mon-POF. A piece of Mon-POF, 150 mg, placed in 50 mL of aqueous solution 1 M NaOH to exchange the protons of −OH groups with Na+. After 3 h the Na+ exchanged Mon-POF was collected through filtration and washed with H2O. The wet Na+Mon-POF was placed in 50 mL of H2O resulting in pH = 10. In that system 300 mg of AgNO3 were added and allowed to react overnight. The collected monolithic piece was washed extensively with H2O, soaked in ethanol to exchange the H2O, and dried again with supercritical CO2. The final product was called Ag@Mon-POF. Capture of Iodine. A 500 mg portion of I2 was transferred in a two neck round-bottom flask. On the top of the flask a fritted glassware was connected where a piece (≈50 mg) of Ag@Mon-POF was placed. Iodine vapors were produced after heating the flask at 70 °C, and they were driven upward with the nitrogen flow connected from the side neck of the flask. Characterization Methods. N2 adsorption-desorption isotherms were measured at 77 K. The measurements were carried out in an ASAP 2020 and in a Tristar 3020 porosimeter of Micromeritics. The specific surface area was calculated according to the Brunauer− Emmett−Teller (BET) method (0.05 < P/P0 < 0.25). Total pore volume was estimated from the adsorbed amount at P/P0 = 0.97. Micropore volume was determined from t-plots.25 NLDFT (cylindrical model) was applied to obtain the pore size distribution. The skeletal density of the aerogel was determined with helium pycnometry using the Accupyc II 1340 of Micromeritics. The bulk (geometrical) density was calculated from the physical dimensions of the aerogel. Solid state NMR spectra were recorded in a Varian 400 ATX spectrometer operating 100 MHz for 13C and 400 for 1H. The 13C CPMAS measurements carried out at spinning rate 10 kHz. Two pulse phase modulation (TPPM) 1H decoupling was applied during acquisition. The 13C are given relative to tetramethylsilane as 0 ppm and calibrated by using adamantane as a secondary reference. X-ray diffraction (XRD) powder patterns were collected on a CPS 120 Inel difractometer equipped with CuKa radiation. UV−vis−NIR diffuse reflectance spectra (DRS) were recorded with a Shimadzu UV3101PC spectrophotometer. BaSO4 powder was used as the 100% reflectance standard. The reflectance data were converted to absorption according to the Kubelka−Munk equation a/S = (1 − R)2/2R, where R is the reflectance and a and S are the absorption and scattering coefficient, respectively.26 Thermogravimetric analysis was performed in a Shimadzu TGA-50 thermal analyzer by heating each sample (≈10 mg) from room temperature to 600 °C with a ramping rate of 5 °C min−1 under nitrogen or air flow. Scanning electron microscopy (SEM) images were collected in a Hitachi S-3400N instrument with an accelerating voltage of 20 kV. High magnification SEM images were collected on a Leo 1525 (Carl Zeiss Microimaging

polymerization within a mold. Other porous polymer monoliths have been prepared based on polyacrylonitrile20 and methylacrylate.21 Recently, we have reported the synthesis of microporous and mesoporous polymeric organic frameworks.22,23 Both families are based on the polymerization of phloroglucinol (1,3,5trihydroxybenzene) with several benzaldehydes under solvothermal conditions. Those frameworks are very stable because of C−C bonds that are produced from the nucleophilic attack of phloroglucinol to the carbonyl groups. Herein we present a new monolithic polymeric organic framework, Mon-POF, that is derived from the polymerization of 1,5-dihydroxynaphthalene with terephthalaldehyde. It exhibits high surface area of 1230 m2 g−1, low bulk density, 0.15 cm3 g−1, and its pores are decorated with −OH functional groups. This monolith is very stable after soaking in strong acidic and basic solutions of pH = 0 and pH = 14, respectively, for 1 d. We demonstrate that Na+ exchanged Mon-POF has reductive properties producing Ag nanoparticles (5−10 nm), without any additional reagent. The Ag loaded monolith, Ag@Mon-POF contains Ag at 25% of its mass and is able to capture iodine forming mainly AgI but also in the molecular form. Capture of iodine is of special interest because of the long-lived radioactive 129I (half-time life 1.57 × 107 years) that is produced from the fission process in nuclear reactors.24

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EXPERIMENTAL SECTION

Materials. All reagents and solvents were used as received unless noted otherwise. Terephthalaldehyde, 1,5-dihydroxynaphthalene, silver nitrate, iodine, and 1,4-dioxane were purchased from Aldrich Chemical Co. Tetrahydrofuran (THF), ethanol, HCl, and NaOH were purchased from VWR. Synthesis of Monolith. In a round-bottom flask an amount of 0.320 g (2 mmol) of 1,5-dihydroxynaphthalene and 0.268 g (2 mmol) of terephthalaldehyde were added in 5 mL of dioxane. The mixture was kept under stirring at 70 °C and 30 min later 1 mL of aq. HCl 1 M was added. The mixture was allowed to react for 3 h and then transferred to a Teflon lined autoclave which was purged with N2 to remove the air and placed in an oven at 220 °C for 4 d. After cooling at room temperature, a brown piece having the internal shape of the autoclave was obtained. The monolith was placed in a beaker with THF and left undisturbed for 3 days to wash out any unreacted and oligomeric species. The solvent was decanted and refilled twice each day. After three days THF was replaced with ethanol where the monolith stayed for 2 days. Finally the monolith was supercritically dried with CO2 using the Autosamdri 815B instrument of Tousimis. The mass of the dried product was 0.52 g, and the yield of the 1938

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Inc.). Before measurement, the samples were sputter coated with gold. Transmission electron microscopy (TEM) investigations were carried out in a JEOL 2100F transmission electron microscope operating at a 200 kV accelerating voltage. The sample was dispersed in ethanol and mounted on a carbon coated copper grid.

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RESULTS AND DISCUSION The polymerization between terephthalaldehyde and 1,5dihydroxynaphthalene is depicted in Scheme 1. Each carbonyl group reacts with two dihydroxynaphthalene molecules eliminating a water molecule, thus terephthalaldehyde is linked with four molecules of 1,5-dihydroxynaphthalene (i). On the other hand 1,5-dihydroxynaphthalene has four reaction sites, ortho- and para- positions of each hydroxyl group, and it reacts with four terephthalaldehydes as well (ii). In this way an extended and highly cross-linked polymeric framework is created. In the past we have shown that no catalyst is needed for the solvothermal polymerization between phloroglucinol and terephthaladehyde.22 However, 1,5-dihydroxynaphthalene is less nucleophilic and less reactive than phloroglucinol having only one hydroxyl group per aromatic ring instead of three of phloroglucinol. To polymerize 1,5-dihydroxynaphthalene with terephalaldehyde, HCl was used to activate the carbonyl groups. The formation of the polymer according to the abovementioned reaction is proved with solid state 13C CPMAS NMR. The typical spectrum of Mon-POF is presented in Figure 1 where the side bands of the big peak are denoted with

Figure 2. Thermogravimetric analysis of Mon-POF under N2 (red line) and air (blue line).

The untreated sample after the solvothermal synthesis has the internal shape of the autoclave (Figure 3a) and exhibits bulk

Figure 3. Images of (a) as prepared and (b) dried Mon-POF, and (c) the cross-section of the dried Mon-POF.

(geometrical) density of 1.27 g cm−3. Even though the sample looks dry it contains significant amount of dioxane, and its mass, 4.8 g, is much higher than the monomers’ mass, 0.588 g. After solvent exchange with THF and ethanol and CO2 supercritical drying, the shape and the dimensions of the monolith are preserved (Figure 3b), and the bulk density is decreased to 0.15 g cm−3. It can be also seen from the crosssection image that the texture of the sample is very homogeneous (Figure 3c). Despite the low bulk density, Mon-POF is strong enough to support a zirconia ball on the top of it, which is 250 times heavier than the monolithic piece (Supporting Information, Figure S1). Mon-POF is handled easily, and it can be cut to smaller pieces with a blade. The SEM image (Figure 4) shows that Mon-POF consists of agglomerat-

Figure 1. Solid State 13C CPMAS NMR of Mon-POF. The bullets correspond to the sidebands of the 128 ppm peak.

bullets. The resonance of the aldehyde carbonyl carbons, 195 ppm, does not appear, and a new one emerges at 44 ppm which is attributed to the methyne bridge carbons. Reacted ortho and para carbons of 1,5-dihydoxynaphthalene exhibit a signal at 116 ppm. The big peak at 128 ppm peak is assigned to the aromatic carbons, and the resonance at 150 ppm corresponds to phenoxy carbons. A shoulder at 145 ppm corresponds to carbons 1 and 4 of the aldehyde originating ring. We observed no peaks in the spectra in the range of 55−75 ppm indicating the absence of any adsorbed solvent molecule, dioxane, THF, or ethanol used in the synthesis and washing procedure. The Mon-POF is completely dry after supercritical drying, as proved from the thermogravimetric analysis (TGA) curves (Figure 2) where no mass loss is observed up to 330 °C either under N2 or under air. The aerogel is gradually decomposed under N2 at elevated temperatures, but it is considered thermally stable retaining the 70% of its initial mass at 600 °C. Under air the aerogel is oxidized rapidly and the combustion is completed at 530 °C.

Figure 4. SEM image of Mon-POF.

ing nanoparticles 20−40 nm in diameter. The skeletal density of dried Mon-POF was measured to be 1.37 g cm−3 according to helium pycnometry. The porosity, ε, of Mon-POF was calculated from eq 1 where pb and ps are the bulk and skeletal density, respectively, and found to equal 89%. 1939

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⎛ p ⎞ ε = ⎜⎜1 − b ⎟⎟ × 100% ps ⎠ ⎝

aerogels were obtained in lower yield, 84 and 90% for MonPOFcc and Mon-POFc, respectively, (Table 1) in comparison to 94% of Mon-POF. They exhibit the similar molecular structure as Mon-POF since the 13C CPMAS NMR spectra are very similar (Supporting Information, Figures S3 and S4) and the N2 adsorption-desorption isotherms are also of type II (Supporting Information, Figures S5 and S6). The porous properties of the monoliths are listed in Table 1. The specific surface areas and the total pore volumes of MonPOFs follow the same trend as the yield of the polymerization what is increased with the reaction rate. On the other hand the values of micropore volume show the opposite trend decreasing in moving from Mon-POFcc to Mon-POF. The evolution of the porous properties can be explained by considering the framework’s extension and relaxation. At low reaction rate the fragments of the framework have time to relax and achieve better packing before growing larger and resulting in materials with higher microporosity. On the other hand at higher reaction rates the framework grows rapidly with less time to pack efficiently forming larger and more tortuous polymeric units, and this produces materials with higher total pore volume. The effect of the reaction rate on the porous properties of monoliths is represented in Figure 6.

(1)

A typical N2 adsorption-desorption isotherm of Mon-POF is presented in Figure 5a. It corresponds to isotherm Type II

Figure 5. (a) N2 adsorption-desorption isotherm and (b) NLDFT pore size distribution of Mon-POF.

according to the IUPAC classification27 suggesting the predominant macroporous character of the monolithic aerogel but at the same time the high uptake at very low relative pressure gives evidence of significant microporosity as well. The specific surface area (BET) is calculated to be 1230 m2 g−1 and the total pore volume 1.46 cm3 g−1 (P/P0 = 0.97). The existence of micropores on Mon-POF is proved using the tplot25 method (Supporting Information, Figure S2.) that shows a micropore volume of 0.2 cm3 g−1 (Table 1). The pore size distribution is calculated applying NLDFT and exhibits a maximum at 6 Å psd (Figure 5b) and some lower peaks in the whole range of diameters showing that beyond micropores the distribution of pores is very broad. The distribution in the range of macropores (>500 Å) cannot be obtained from N 2 porosimetry. The formation of a gel is strongly influenced from the parameters that affect the polymerization reaction rate as temperature, catalyst, and concentration of reactants.28 To investigate the effect of reaction rate on the formation of MonPOFs various samples were synthesized using half concentration of starting materials and/or half amount of catalyst, aqueous solution HCl 1M, under the same temperature profile. Mon-POFcc has been synthesized using 1 mmol of monomers per 5 mL of dioxane, instead of 2 mmol for Mon-POF, and 0.5 mL of HCl 1M, instead of 1 mL for MonPOF. Mon-POFc has been synthesized using 1 mmol of monomers per 5 mL of dioxane and 1 mL of HCl. From both syntheses, monolithic

Figure 6. Evolution of Mon-POFs porosity with concentration of monomers in dioxane and amount of catalyst.

As the previous POFs compounds22,23 MonPOFs show also semiconductor like optical absorption properties. The solid state absorption spectra of Mon-POFs are given in Figure 7.

Figure 7. Solid state electronic absorption spectra of Mon-POFs.

Table 1. Synthesis Parameters, Porous Properties, and Yield of Mon-POFs sample

concentration - catalyst

specific surface area (m2 g−1)

total pore volume (cm3 g−1)

micropore volume (cm3 g−1)

yield (%)

Mon-POFcc Mon-POFc Mon-POF

1 mmol/5 mL - 0.5 mL HCl 1 mmol/5 mL - 1 mL HCl 2 mmol/5 mL - 1 mL HCl

946 1117 1230

0.84 1.17 1.46

0.24 0.22 0.20

84 90 94

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M (pH = 14) respectively. Neither of the monoliths dissolved but remained as a single piece. Their 13C CPMAS NMR spectra (Supporting Information, Figures S7 and S8) after the acid and base treatment were the same in comparison to raw Mon-POF. After the base treatment, there was one additional peak for Mon-POF(NaOH), at 165 ppm, which is assigned to phenoxy carbons with exchanged protons with Na+ cations. A similar phenomenon of partial exchange of protons with Na+ was observed in phloroglucinol POFs as well.22 N2 adsorptiondesorption isotherms (Supporting Information, Figures S9 and S10) revealed that both monoliths retained more than 75% of their surface area. Mon-POF(HCl) exhibit a surface area of 1059 m2 g−1 and Mon-POF (NaOH) show a surface area of 929 m2 g−1. The porosity of Mon-POF decreased very slowly with time, and no precautions are needed for the monolith’s storage. The N2 adsorption-desorption isotherms were measured again in 3 and 6 month intervals after synthesis (Supporting Information, Figures S11 and S12) and the specific surface area decreased to 1049 and 914 m2 g−1 respectively. Silver Deposition. The functionalization of Mon-POF with Na+ ion-exchange prompted us to investigate some basic ionexchange properties with metals such as Ag+. Our original goal was to see if Mon-POF was capable of exchanging its Na+ for Ag+ ions. The materials did indeed lose its Na and picked up Ag (Supporting Information, Figure S13) but to our surprise the Ag was reduced to nanoparticles. Clearly, the Mon-POF has reductive properties and is capable of reducing AgNO3 probably after ion-exchange as the silver ions enter the material. TEM images of Ag@Mon-POF (Figure 8) show spherical Ag nanoparticles of 5−10 nm in diameter well dispersed in the

Mon-POF exhibits a band gap of 2.3 eV and a broad absorption feature at >4 eV. The other two samples, Mon-POFc and MonPOFcc, have band gaps at 2.5 eV, and the intensity of absorption from 2 to 4 eV is remarkable weaker than MonPOF. We ascribe the differences in optical properties to the size of the polymeric units in each monolith. As we mentioned above the higher the reaction rate the greater the size of the polymeric unit and greater the delocalization of electrons within the framework. Thus, Mon-POF prepared at higher reaction rate exhibits the lowest band gap and the highest optical absorption. The molecular structure of MonPOF as depicted in Scheme 1 is not highly conjugated; however, protons from the methyne linkages may be released at 220 °C, as they are fairly acidic, forming highly conjugative segments in the frameworks (Scheme 2). Scheme 2. Possible Conjugated Structure of Mon-POF

The stability of Mon-POF in water was tested under acidic and basic conditions. Two pieces of Mon-POF were soaked for 24 h in aqueous solutions of HCl 1 M (pH = 0) and NaOH 1

Figure 8. (a, b) TEM images of Ag nanoparticles (dark spots) dispersed in Mon-POF. (c, d) STEM images of Ag nanoparticles (bright spots) in Mon-POF. 1941

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polymeric framework. The XRD pattern of Ag@Mon-POF (Figure 9a) shows broad peaks that correspond to nanocrystal-

Figure 10. (a) XRD pattern and (b) thermogravimetric analysis of Ag@Mon-POF after iodine capture. Figure 9. (a) XRD pattern of Ag@Mon-POF. The bullets correspond to Ag diffraction peaks. (b) N2 adsorption-desorption isotherm of Ag@Mon-POF.

dispersive spectroscopy (EDS, Supporting Information, Figure S16) was 2:3 showing excess of I compared to Ag indicating that in addition to reaction with Ag, iodine vapors are also physically adsorbed in the pores of the monolith. This is verified from the TGA graph (Figure 10b) of the sample after iodine capture where a broad mass loss step is observed from 90 to 240 °C, (I2 bp = 184 °C). In addition to the high Ag loading, 25 wt %, the effective capture and storage of I2 on Ag@ Mon-POF is facilitated from the high dispersion and the small size of the Ag nanoparticles as well as the macroporous structure of the monolith that allows iodine vapors to diffuse readily and access the metallic phase. The continued interest for alternative waste forms for 129I using materials that can provide higher waste loadings makes the Mon-POF presented here of significant interest. To capture iodine, aerogels, silver-loaded zeolites, and more recently chalcogels and MOFs have been studied for confinement of iodine radioactive wastes in recent years and are under investigation as waste forms for 129I.24,39 MonPOFs appear to be an attractive alternative to these systems bringing special advantages such as high loadings, extreme pH stability, and mechanical robustness.

line cubic Ag (pdf 04-0783, ICDD) with an average crystallite size of ∼25 Å according to the Scherrer equation.29 The monolith after the reaction with AgNO3 was soaked in ethanol and dried with supercritical CO2. The loading of Ag to MonPOF is significant and equals to ∼25 wt % as estimated from thermogravimetric analysis. Interestingly the TGA also shows that in air the combustion of Ag@Mon-POF is catalyzed by the Ag nanoparticles and occurs in a very narrow temperature range, from 230 to 250 °C (Supporting Information, Figure S14), in comparison to pristine Mon-POF that burned out in the range 300 to 520 °C (Figure 2). The residue of Ag@MonPOF’s TGA was analyzed with XRD presenting sharp diffraction peaks of elemental Ag (Supporting Information, Figure S15). Ag@Mon-POF exhibits an N2 adsorption desorption isotherm (Figure 9b) of type II as the pristine monolith and specific surface area of 690 m2 g−1 revealing that the deposited Ag nanoparticles did not block the pores of the monolith. Ag@Mon-POF composite material combines the properties of low density porous substrate with metallic nanoparticles. Silver nanoparticles have been widely exploited in several fields like catalysis,30 optics,31 antimicrobials,32 biosensing,33 and SERS,34 and they have been stabilized on macroporous polymeric substrates through the reduction of AgNO3 from NaBH4 or hydrazine at maximum content ∼7% wt.35 In our case AgNO3 is reduced without any additional reductant, and the reductive properties of Na+Mon-POF is attributed to the phenolic OH groups that can be correlated to the reducing activity of natural polyphenols which are wellknown for their antioxidant properties.36 Ag-nanoparticles have been synthesized from the reduction of Ag+ using natural polyphenols and plant extracts as reductants.37 Natural tannin grafted collagen fiber acts as reductant and stabilizer of Ag nanoparticles, but the Ag content is as low as 2% wt.38 Iodine Capture. With the Ag@Mon-POF at hand we explored whether the silver laden material could capture iodine vapor and stabilize it in the form of AgI, eq 2. This reaction is relevant for the capture and storage of radioisotopes released during reprocessing of spent nuclear fuel and particularly 129iodine.24 1 Ag@Mon‐POF + I 2 → AgI@Mon‐POF (2) 2 The XRD pattern of Ag@Mon-POF after exposure in iodine vapors for 2 h is shown in Figure 10a where the Bragg peaks correspond to hexagonal β-AgI (pdf 09-0374, ICDD), and those of the metallic Ag are absent. The atomic ratio of Ag:I in the material after iodine treatment according to energy

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CONCLUSIONS We have demonstrated a new functional polymeric organic framework in the form of a monolith which is prepared from the polymerization of terephthalaldehyde and 1,5-dihydroxynaphthalene. The Mon-POF exhibits high surface area, low bulk density, and it is remarkably stable under strong acidic and basic media. The monolith has OH functional groups and shows reductive properties after the exchange of protons with Na+. Ag spherical nanoparticles, 5−10 nm, are deposited inside the pores of Mon-POF from the reduction of AgNO3 without any additional reductant. The high Ag content and the macroporous character make Ag@Mon-POF an excellent adsorbent for iodine vapors stabilizing them in the form of βAgI as well as molecular I2. This makes Mon-POF materials promising in nuclear waste remediation concerning the capture of 129I and possibly the recovery of precious metals. The synthesis of this polymeric organic framework is expected to be highly scalable because of its high yield of preparation, the low cost of starting materials, and its long-term stability.

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ASSOCIATED CONTENT

S Supporting Information *

N2 adsorption desorption isotherms, t-plots, solid state NMR, EDS, TGA, and XRD. This material is available free of charge via the Internet at http://pubs.acs.org. 1942

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by DOE-EERE is gratefully acknowledged (Grant DEFG36-08GO18137/A001). REFERENCES

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dx.doi.org/10.1021/cm300696g | Chem. Mater. 2012, 24, 1937−1943