Porous Silsesquioxane Imine Frameworks as Highly Eï¬cient

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Porous Silsesquioxane Imine Frameworks as Highly E#cient Adsorbents for Cooperative Iodine Capture Mateusz Janeta, Wojciech Bury, and Slawomir Szafert ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03023 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Materials & Interfaces

Porous Silsesquioxane Imine Frameworks as Highly Efficient Adsorbents for Cooperative Iodine Capture Mateusz Janeta,‡ Wojciech Bury,‡* Sławomir Szafert Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland Supporting Information ABSTRACT: The efficient capture and storage of radioactive iodine (129I or 131I), which can be formed during nuclear energy generation or storage of nuclear waste, is of paramount importance. Herein we present highly efficient aerogels for reversible iodine capture, namely Porous Silsesquioxane-Imine Frameworks (PSIF), constructed by a condensation of octa(3-aminopropyl)silsesquioxane cage compound (OAS-POSS) and selected multitopic aldehydes. The resulting PSIFs are permanently porous (Brunauer-Emmet-Teller (BET) surface areas up to 574 m2/g), thermally stable and present a combination of micro-, meso- and macropores in their structures. The presence of a high number of imine functional groups in combination with silsesquioxane cores results in extremely high I2 affinity with uptake capacities up to 485%wt, which is the highest reported to date. Porous properties can be controlled by the strut length and rigidity of linkers. In addition, PSIF-1a could be recycled at least 4 times while maintaining 94% I2 uptake capacity. Kinetic studies of I2 desorption show two strong binding sites with apparent activation energies of 77.0 kJ/mol and 89.0 kJ/mol. These energies are considerably higher than the enthalpy of sublimation of bulk I2. KEYWORDS: microporous polymer, iodine capture, gas storage, porosity, aerogel, imine, silsesquioxane Chart 1. Structural motifs used for improved interaction with I2.

INTRODUCTION Electrical energy production and consumption is in the focal point of worldwide discussion. Increasing global emission of greenhouse gases forces the utilization of energy sources other than fossil fuels. In this view, nuclear power is still one of the major alternatives, however, volatile radioactive waste (e.g. 129I, 14 CO2, 85Kr, 3H) generated from nuclear fuel raises many concerns and constitutes a major challenge for present technologies. One of the biggest issues is the generation of highly volatile radioactive iodine-containing species. The influence of I2 on the human body is related to the proper function of thyroid gland, which is responsible for fundamental biological functions. In this regard, effective capture and storage of radioactive iodine-129, because of its long radioactive half-life (1.57×107 years), high volatility, and harmful effects on humans and the environment, still requires finding of more efficient solutions. From a practical perspective, adsorption of I2 vapor onto a solid adsorbent has many advantages over traditional liquid scrubbing methods.1,2 To date, various groups of porous solids have been tested for I2 capture including zeolites,3 metal-organic frameworks (MOFs),4–6 covalent-organic frameworks (COFs)7, aerogels,8 porous organic cages,9 and lately, porous organic polymers.10–21 Recent reports emphasize the fact that I2 sorption capacity does not simply correlate with the classical sorption parameters like surface area or pore volume of adsorbents. Moreover, determination of binding modes of molecular I2 within porous organic polymers, especially these containing various functional groups, is a difficult task, due to irregular pore distributions, random adsorption sites, lack of structural order, and high activation barrier for diffusion of I2 in the host structure, due to high molar mass of I2. Therefore, in the design of new materials for this application, other factors, that influence the I2 loading capacity and efficiency, should be considered, especially the host-guest and guest-guest specific or synergistic interactions.5 Recently, a number of very promising candidates for I2 capture have been reported, including conjugated polymer frameworks (Chart 1,

I),11–14 nitrogen-rich porous frameworks (II),15–18 and charged polymeric frameworks (III).19,20 To attain desired 3D materials with imprinted geometries and functions, it is of great importance to properly select the building units and chemical reactions that allow their successful incorporation. In this view, covalently bonded porous organic materials are usually built of lightweight elements and are characterized by strong covalent linkages, which translates into their 3D structures with high stability and porosity.22–29 Due to a constant development of organic synthetic methodology this group of materials is growing very fast, thus, multifunctional, symmetric and highly connected molecular building blocks are of great importance to further expand this group of materials. Polyhedral oligomeric silsesquioxanes (POSS) constitute an interesting, yet not very explored in this regard, group of multifunctional building blocks. Molecular silsesquioxanes are organosilicon compounds described by the chemical formula (RSiO3/2)n (where n = 6, 8, 10, 12; R = H, alkyl, aryl, Chart 1, IV), and are considered as the smallest existing silica nanoparticles, due to their diameters in the range of 1–3 nm. Moreover, POSS cages offer unusual connectivity such as T6, T8, T10, and T12, however, the most widely studied are

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a)

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N N

8Cl

N +

+

N

N Si O Si O Si O Si O O O O O Si Si O O Si O Si

N N N

L1 or L2

N

Et3N, DMSO N

N

N +

O Si

N N

Si O O

N

Si O Si O Si O Si O O O O O Si Si O O Si O Si

N

L3, L4, or L5 Et3N, DMSO

N N

+ NH3

+

N

+ NH3

O

c)

H

O

O

O

O

H

O

H H

O

H

N

H

L1

O

H

H H

L2

O

H

L3

O

H

H

OH

O

O

H

HO O

N

H

O

O

N

N

N

H

N

N

N

N

O

N

N

NH3

OAS-POSS

H

N

O

N

b)

N

N

O Si O O Si

NH3

N

N

NH3

+

NH3 O Si O Si O O O Si Si O

O

N

+

NH3

NH3

N

N

N

-

OH O

H

O

H

L4

L5

d)

Figure 1. (a) Schematic representation of the synthesis of PSIF-1–5; (b) bifunctional and (c) trifunctional prolinkers used in the synthesis; (d) photographs showing the activated PSIF-1–5 materials.

cubic T8 cages.30 Another advantage of POSS building blocks lies in their high thermal stability and chemical versatility, which means that various R substituents can be covalently attached to the silicon atom. A handful of polymeric networks based on POSS cages exist in the literature, e.g. cross-linked polymers,31–35 monoliths with hierarchical structure,36,37 copolymers,38 membranes,39 however none of them has been utilized for I2 capture. Herein, we report on the synthesis and characterization of a new family of POSS-based hybrid porous aerogels, which we termed Porous Silsesquioxane-Imine Frameworks (PSIF) and their use as highly effective I2 adsorbents. In our approach we considered three design criteria. Firstly, we envisioned that the presence of POSS cages in the porous structure linked by imine moieties could cooperatively increase the I2 sorption capacity (Chart 1, IV). This hypothesis was based on previous reports of Laine and co-workers, who demonstrated preferential interaction of Br2 with POSS cage [SiO1.5Ph]8, leading to unusual reactivity and regioselectivity.40,41 Secondly, the desired porous materials should be thermally stable what is of crucial importance since, typically, industrial gas streams containing I2 vapors are operated at 75 °C. Finally, mesoporous channels would be beneficial for the diffusion of I2 vapor within the porous structure.

EXPERIMENTAL SECTION Preparation of synthons. Octa(3-aminopropyl) silsesquioxane hydrochloride (OAS-POSS),42 compound 1,43 2',5'-dimethyl-[1,1':4',1''-terphenyl]-4,4''-dicarbaldehyde (L2),44 1,3,5-benzenetricarbaldehyde (L3),45 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (L4),46,47 and 1,3,5-tris(4-

formylphenyl)benzene (L5),48 were prepared according to the previously reported procedures. 1,4-phthalaldehyde (99%) (L1) was purchased from Sigma-Aldrich. Synthesis of PSIFs. A 20 mL pyrex vial was charged with OAS-POSS (0.200 g, 0.170 mmol), dimethyl sulfoxide (10 mL), triethylamine (0.190 mL, 1.364 mmol) and 1,4-phthalaldehyde / 2',5'-dimethyl-[1,1':4',1''-terphenyl]-4,4''-dicarbaldehyde (0.682 mmol) / 1,3,5-benzenetricarbaldehyde / 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde / 1,3,5tris(4formylphenyl)benzene (0.454 mmol). The mixture was sonicated for 2 minutes. The vial was placed in an oven at 60 °C for 1–12 h. After the reaction a white sol was formed. Activation of PSIFs by supercritical carbon dioxide drying. Supercritical CO2 activation was carried out in a Tousimis Samdri-PVT-3D instrument. Prior to the activation process, the as-prepared samples were washed exhaustively with ethanol in a Soxhlet extraction apparatus over 24 h. Then samples were placed inside the pressure vessel and the ethanol was exchanged with liquid CO2. After 6 h of repetitive venting and soaking with liquid CO2, the chamber was heated above the critical point of CO2 and slowly vented overnight (12 h). Activation of PSIFs by freeze-drying. Prior to the activation process, the as-synthesized samples were washed exhaustively with ethanol in a Soxhlet extraction apparatus for 24 h, next for 24 h with cyclohexane. After washing the samples were slowly frozen at −20 °C. Then, the samples were freeze-dried without heating under 0.2 mbar vacuum for 10 h. Experimental procedure for the uptake of I2 vapor by PSIF aerogels. A 6 dram vial of activated PSIF (20.0–50.0 mg)


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and an excess of crystalline iodine were placed in a sealed polypropylene container and heated at 75 °C under ambient pressure. After specific period of time, the container was cooled down to room temperature and the PSIF sample was quickly weighed. The I2 uptake of PSIF was calculated by weight gain: α = (m2 − m1)/m1, where α is the I2 uptake, m1 and m2 are the mass of PSIF sample before and after being exposed to I2 vapor, respectively. The I2 capture experiments were conducted three times and a good repeatability was observed.

RESULTS AND DISCUSSION Synthesis and activation of PSIFs. Previous studies demonstrated that OAS-POSS is a versatile precursor for building functionalized molecular assemblies based on amide or imine linkages.42,43,49,50 Moreover, our investigations have shown that desired imine derivatives can be formed, starting from OASPOSS, under very mild conditions, in the presence of triethylamine as a base. 3D imine-linked PSIFs were constructed from OAS-POSS and selected multitopic aldehydes. The utilization of this methodology for the preparation of 3D PSIFs is shown in Figure 1. The OAS-POSS precursor was obtained in a onestep hydrolytic condensation, previously reported by us, 42 using commercially available (3-aminopropyl)triethoxysilane and appropriate amount of HCl. As prolinkers we selected 2- and 3topic aromatic aldehydes L1–L5, which have been previously employed in the construction of classical covalent organic frameworks (COFs).47,51–54 It is worthy to note that L4 prolinker contains -OH groups, which allow for the formation of chemically stable enamine linkages in the final PSIF, which are not prone, in contrast to imines, to hydrolytic cleavage. 55 The prolinkers L1–L5 and OAS-POSS were mixed in equimolar amounts in the presence of triethylamine as a deprotonating agent to attain full conversion of substrates into desired PSIFs. The reactions were carried out in dimethyl sulfoxide (DMSO) at 60 °C in tightly sealed reaction vials over 1–12 h. The products PSIF-1–5 were formed as gels occupying full volume of the starting liquid phase (see Figure S1 in the Supporting Information). To remove byproducts and residual solvents the obtained gels were firstly separated by filtration and then exhaustively washed with ethanol in a Soxhlet extractor for 24 hours. The obtained PSIF alcogels were further desolvated using different methods. We attempted three methods commonly employed for activation of porous polymers or MOFs.56 The thermal activation appeared to be unsuitable for our materials leading to dramatic shrinkage of the framework into xerogel phase and loss of its porosity (see Table 1 and Figure S2c in the Supporting Information). Therefore, we resorted to milder desolvation methods, i.e. freeze-drying with cyclohexane and drying with supercritical CO2. Both methods resulted in successful activation of PSIF-1 to PSIF-5, without collapsing their 3D frameworks (see PSIF-1a and PSIF-1b in Table 1 and Figures S2a and S2b in the Supporting Information). As a result PSIF1a–5a were obtained as spongy solids as shown in Figure 1d. Structural characterization of PSIFs. The activated aerogels PSIF-1a–5a were insoluble in typical organic solvents (for more details see the solubility tests in the Supporting Information) and in water. To elucidate the chemical composition of PSIF-1a–5a we resorted to spectral analysis in solution and solid state. Small samples of respective PSIFs were digested in a mixture of DMSO-d6 and D2SO4 to obtain homogenous solutions. This solvent mixture allowed the hydrolysis of iminebased frameworks into their starting components. Only in the

case of PSIF-4a it was impossible to dissolve the sample due to the high chemical stability of PSIF-4a. For PSIF-1a the resulting 1H NMR spectrum consists of characteristic signals of 1,4phthalaldehyde and the starting OAS-POSS (see Figure 2). Integration of the relative intensities of aldehyde and OAS-POSS signals showed that the ratio of di- or trialdehyde to OAS-POSS in the final PSIF was preserved. Importantly, the POSS cage is stable under acidic conditions and does not decompose during sample preparation. To further confirm this observation we acquired 29Si NMR spectra in a solution and in the solid state, which are shown in Figure 2. As a reference we used the previously characterized molecular POSS compound of the formula [SiO1.5(CH2)3N=CHPh]8 (1).43 The 29Si NMR spectrum of 1 in solution contains only one resonance at -66.5 ppm, and similar result was obtained for a digested sample of PSIF-1a, which means that the cage rearrangement did not occur. Likewise, 29 Si CP-MAS spectrum recorded for a solid sample of PSIF-1a also showed only one symmetric resonance at -66.5 ppm. Thus, these results unambiguously confirmed that POSS-based cages were present in the resulting structure of the PSIF material.

Figure 2. (a) Solution 1H NMR spectrum of digested sample of PSIF-1a in DMSO-d6/D2SO4 mixture; (b) 29Si NMR spectra of: 1 in solution (bottom), digested PSIF-1a in DMSO-d6/D2SO4 mixture (middle), and solid sample of PSIF-1a (top, CP-MAS).

Additional structural features of PSIF-1a–5a were analyzed by means of 13C cross-polarized magic angle spinning (CPMAS) solid-state NMR spectroscopy as shown in Figure 3. The 13 C CP-MAS spectrum of PSIF-1a contains two resonances with chemical shifts at 137.6 and 127.8 ppm that are associated with aromatic carbon atoms of the phenylene rings and three signals at 63.9, 24.8 and 9.8 ppm indicating the presence of POSS cage. The comparison of spectra of PSIF-1a with spectra of both substrates clearly shows that there is no unreacted substrates remaining in the product (Figure S3, Supporting Information). Moreover, the presence of characteristic peak at 159.2 ppm confirms the quantitative formation of imine linkages and a lack of unreacted aldehyde or amine groups. In contrast, PSIF-4a might exist in two tautomeric forms: a nonaromatic keto-enamine tautomer (heteroradialene, C=O form) and an aromatic enol-imine (OH form) tautomer. In this case, based on 13 C and 15N CP-MAS spectral data, we confirmed the exclusive formation of enamine form (Figures S16 and S17, Supporting Information).57,58 To gain further insights into the chemical structure of the studied PSIFs we recorded Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra (Figure 3). Notably, in PSIF-1a–5a materials new peaks around 1644 cm−1 appear, corresponding to the imine νC=N stretching vibration. Importantly, these spectra do not exhibit the



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Figure 3. DRIFT spectra (left) and 13C CP-MAS NMR (right) of PSIF-1a–5a; asterisks denote the spinning sidebands.

aldehyde νC=O stretching vibrations of the monomers near 1690 cm−1, which confirms complete transformation of aldehyde groups into imines and completeness of crosslinking reactions. Additionally, DRIFT analysis for PSIF-4a proves a formation of enamine linkages. Characteristic vibrations at 1103 cm−1 of the Si-O-Si moieties can be observed in spectra of all PSIFs. The lack of absorptions of SiOH groups supports the preservation of the Oh symmetry of the POSS cage,43 which further supports our observations from solution and solid state NMR analysis (vide supra). Thermogravimetric studies of PSIF aerogels. The thermal and chemical stabilities of desolvated (activated) PSIF-1a–5a were also investigated by means of thermogravimetric analysis (TGA) under an oxidative environment (O2:N2 = 40:60). The TGA profiles of the studied networks are presented in Figure 4a and indicate their high thermal stability up to 300 °C, which can be attributed to the presence of rigid silsesquioxane cages. The thermogravimetric analyses up to 1000 °C under oxidative conditions allowed to determine a composition of these materials based on their ceramic yields. These calculations were performed assuming SiO2 as the final product of the thermal decomposition.50 For example, for PSIF-1a the ceramic yield of 38.1%wt was recorded, which is in a very good agreement with the theoretical value of 37.7%wt. The largest difference between theoretical and calculated values of weight loss were observed for PSIF-4a. In this case initial change of 6.0% can be observed, below 80 °C, due to the removal of residual EtOH, which possibly was not completely eliminated during supercritical CO2 drying procedure. The presence of residual EtOH was also confirmed by 13C CP-MAS spectrum. The stronger interaction of PSIF-4a with EtOH, as compared to the other studied PSIFs, can be attributed to the formation of strong hydrogen bonds between alcohol molecules and heteroradialene moieties. Porosity studies of PSIF aerogels. N2 sorption studies. The porosity of PSIFs was investigated by nitrogen adsorption-desorption measurements at 77 K (Figure 4b). The sorption isotherms represent a mixed I and IV type indicating the presence of micropores, large mesopores and macropores in the PSIFs. The density functional theory (DFT) analysis derived pore size

distribution (PSD), using a carbon slit pore model for fitting, exhibited a contribution from micropores of 14–17 Å (Figure 4b). The high content of large mesopores and macropores is manifested by a steep increase of adsorption branches in the range of high relative pressures (p/p0=0.8–1.0) for PSIF-1a–5a, which is also shown in the DFT incremental pore volume distribution plot (Figure S30, Supporting Information). The Brunauer–Emmett–Teller (BET) specific surface areas for PSIF-1a–5a were determined over the relative pressure (p/p0) range of 0.02–0.20, applying the consistency criteria by Rouquerol et al.,59 and these values are summarized in Table 1. The highest BET area was recorded for PSIF-5a (574 m2/g), whereas the lowest value was obtained for PSIF-4a (210 m2/g). It is noticeable that the extension of organic linkers does not lead to increase of N2 sorption properties, most probably due to catenation of polymeric frameworks containing extended linkers (cf. PSIF-1a vs. PSIF-2a and PSIF-3a vs. PSIF-5a). The total pore volumes of PSIFs were estimated from single-point nitrogen uptakes at p/p0 = 0.98 and are listed in Table 1. Skeletal density of PSIF aerogels. To further investigate porous structure of PSIFs we measured the skeletal density of obtained aerogels by employing helium pycnometry. This technique is a very convenient measure of textural properties of porous materials, especially for aerogels. The values of skeletal densities for PSIF-1a–5a are presented in Table 1. For example, for purely silica-based aerogels, skeletal densities in the range of 1.7–2.2 g/cm3 are typically obtained.60,61 Thus, the values obtained for PSIFs clearly indicate that these materials represent structure characteristic for aerogels, which were maintained after the supercritical drying process. Comparing the values of skeletal density and total pore volumes for studied systems other correlations can be observed. For PSIF-1a and PSIF-2a the pore volumes are similar (1.13 vs 0.89 cm3/g), whereas the difference in skeletal density values is much more pronounced (1.12 vs. 1.66 g/cm3 respectively). Similar observation can be made for PSIF3a/PSIF-5a pair, where pore volumes reach 1.50 and 1.41 cm3/g and skeletal densities are 1.22 and 1.77 g/cm3, respectively. We believe that the observed differences suggest high degree of catenation in PSIF-2a and PSIF-5a structures.


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ACS Applied Materials & Interfaces a)

b)

38.1% (37.7%)

24.9% (27.0%)

32.9% (39.0%)

36.4% (41.1%)

25.1% (25.4%)

c)

d)

Figure 4. Stability and sorption studies of PSIF-1a-5a: (a) TGA profiles under aerobic conditions (O2:N2 = 40:60), arrows show ceramic yield, theoretical values in parentheses; (b) N2 adsorption–desorption isotherms at 77 K (filled symbols: adsorption; open symbols: desorption), inset shows the DFT pore size distribution; (c) CO2 isotherms at 273 K; (d) CO2 isosteric heat of adsorption plots.

Interaction of PSIFs with CO2. To further explore the nature of pore surfaces of the measured PSIFs we investigated the interaction of these materials with CO2. The CO2 isotherms measured in the 263 K–293 K temperature range were used for calculation of isosteric heats of adsorption (Qst). For PSIF-1a we used single-site Langmuir (SSL) fit, whereas for PSIF-2a, -3a, -4a and -5a the best isotherms fits were obtained when the dualsite Langmuir (DSL) model was employed, and the Clausius−Clapeyron method was used for Qst calculations (see Table 1 and Figure 4d). The comparison of CO2 isotherms collected at 273 K is shown in Figure 4c. In this plot one can observe that PSIF-1a and PSIF-2a exhibit very similar interactions with CO2, which is manifested by very similar isotherm traces and total CO2 uptakes. For PSIF-1a Qst value is around 25 kJ/mol, whereas for PSIF-2a initial Qst reaches 27 kJ/mol and then this value lowers to 22 kJ/mol. Much stronger interactions with CO2 were observed for PSIF-3a, PSIF-4a and PSIF-5a, which exhibit roughly twice as much CO2 uptake as the former materials. The calculation of initial Qst for PSIF-5a and PSIF-3a yielded very high values of 37.4 kJ/mol and 33.0 kJ/mol respectively, which are amongst the highest values reported to date for various porous materials, including microporous polymer networks (15.6−33 kJ/mol),62–65 COFs (24.1–43.5),66,67 MOFs (15−35 kJ/mol),68 ions impregnated MOFs (37.4–34.5 kJ/mol)69 and

carbons (20–37.1 kJ/mol),70,71 but are still below the energy of chemisorption (40 kJ/mol).72 The interaction of PSIF-4a with CO2 gives Qst values in the range of 27–31 kJ/mol and suggests a slightly different mechanism of adsorbate-adsorbent interaction. These specific interactions can be attributed to the presence of enamine –NH groups of the floroglucinol moiety (cf. 15 N, 13C NMR and DRIFT data for PSIF-4a). Table 1. Porosity parameters of PSIFs. SBET [m2/g]

Vpore [cm3/g]

[g/cm3]d

s

Qst(CO2) [kJ/mol]

PSIF-1aa

320±2

1.13

1.118±0.003

24.7e

PSIF-1bb

256±1

0.82

1.581±0.008

–

PSIF-1cc

4.9±0.05

0.01

–

–

PSIF-2aa

262±1

0.89

1.663±0.005

27.2–22.4f

PSIF-3aa

473±2

1.50

1.221±0.003

33.0–22.0f

PSIF-4aa

209.6±0.5

0.42

1.800±0.007

31.1–26.7f

PSIF-5aa

574±2

1.41

1.772±0.006

37.4–23.6f

Activation procedure: a supercritical CO2 drying, b freeze-drying with cyclohexane, c conventional heating; d skeletal density; e SSL model; f DSL model.



a)

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b)

Figure 5. (a) Gravimetric I2 uptake of PSIFs as a function of time at 75 °C. The solid line represents the fit with the pseudo-first or -second order kinetic model; (b) column graph of maximum adsorption capacities of I 2 vapors in PSIFs and other porous materials.

Capture and release of molecular iodine by PSIFs. To date various types of porous materials have been tested as sorbents for I2 vapors. As mentioned before, recent studies demonstrated high efficiency of porous organic polymers for this application. Thus, in the next step, we tested the PSIF materials in sorption of I2 vapor. Small samples of activated materials were placed in pre-weighed vials and exposed to I2 vapor in a sealed polypropylene container (for details see the Experimental Section). The container was kept at 75 °C in an oven during the experiment. The mass of the sample was monitored over selected time periods until no further mass change could be observed, reaching a plateau (typically after 15 h). During the exposure to I2 vapors the color of PSIFs gradually changed from off-white to dark brown, as shown in Figure 6a. The obtained kinetic curves are presented in Figure 5a and the calculated kinetic parameters are listed in Table S11. The I2 uptake can be expressed in terms of a gravimetric or volumetric capacity (Table 2). To our pleasant surprise the PSIF materials demonstrated very good gravimetric I2 capture capacities, reaching a record-breaking value of 4.85 g/g for PSIF-1a and placing other three of PSIFs in the top ten list of all of the studied materials to date (see Figure 5b and Table S12). For each material, the adsorbed I2 quantity can be expressed as I2/T8 ratio, which shows the number of moles of I2 adsorbing per one POSS cage (see Table 2). For PSIF-1a, -2a, -3a and -5a, which contain 12 oxygen atoms and 8 C=N linkages per POSS cage (R8Si8O12), these numbers reach roughly 20 mol I2/mol T8, suggesting that both structural units contribute to I2 sorption. For PSIF-4a, however, this value is around 12 mol I2/mol T8, which suggest that only POSS cages interact with I2, whereas enamine linkages do not provide significant contribution to I2 sorption. The experimental evidence of these interactions comes from the DRIFT spectra obtained for PSIF1a sample exposed to I2 vapor over various periods of time (Figure S37, Supporting Information). In the presence of I2 vapor the Si-O-Si vibration mode of the POSS cage is shifting towards higher wavenumbers νSi-O-Si (from 1103 cm-1 to 1120 cm-1 and from 1188 cm-1 to 1204 cm-1). A more pronounced shift can be observed for vibrations of imine linkages νC=N (from 1644 cm-1 to 1666 cm-1) and the bending mode of Si-C bonds δSi-C (from 829 cm-1 to 794 cm-1). To correlate the experimental data with theory we tested pseudo-first and pseudo-second order kinetic models. As one

can see in Table S11, the correlation coefficient values, obtained using the pseudo-second-order kinetic model, fit experimental data satisfactorily for PSIF-1a, -2a, -4a and -5a (R2 > 0.9999), whereas for PSIF-3a the experimental data show favorable compliance with pseudo-first order kinetic model (R2 = 0.9997). When analogous sorption experiment was performed at 60 °C we noticed a slight change in sorption kinetics, in this case pseudo-first order kinetic model had to be utilized for PSIF-2a and -3a. It is noticeable that the maximum I2 loading for studied PSIFs could be reached at both temperatures. Table 2. I2 sorption capacity of PSIF materials. I2 gravimetric capacity [g/g]a

I2 volumetric capacity [g/cm3]b

I2/T8 [mol/ mol]

released I2 (adsorbed) [%]c

PSIF-1a

4.85±0.12

5.42

24.3

80(83)

PSIF-2a

3.46±0.09

5.75

25.8

58(78)

PSIF-3a

4.11±0.12

5.01

18.9

77(80)

PSIF-4a

2.44±0.06

4.39

11.9

69(71)

PSIF-5a

3.01±0.09

5.34

21.6

70(75)

I2 gravimetric capacity = 𝑚𝐼2 /𝑚𝑃𝑆𝐼𝐹 I2 volumetric capacity = I2 gravimetric capacity × s; c based on TGA (N2:O2 = 60:40); released I2 = [𝑚𝐼2 /(𝑚𝐼2 + 𝑚𝑃𝑆𝐼𝐹 )] × 100%. a

;b

The microstructure and morphology of PSIFs were also examined by scanning electron microscopy (SEM) supported by energy-dispersive X-ray spectroscopy (EDS). Figures 6b, and S22 (Supporting Information) show SEM images of the prepared samples of PSIF-1a–5a. Based on SEM images it can be seen that all PSIFs have uniform morphology and contain spherical macropores with the pore openings in the range of 0.1–2.0 µm. The porous skeleton is formed by highly interlocked polymeric fibers. It is reasonable that the macroporous structure has a beneficial role in the vapor phase transport of I2 towards sorption sites inside the bulk phase. The SEM-EDS analysis of I2@PSIF-1a (Figures 6d–f, Figures S23 and S24 in the Supporting Information) allowed to have a closer look into distribution of I2 in I2@PSIF-1a and confirmed that iodine was evenly distributed in the PSIF.


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To test the binding strength of adsorbed iodine within the framework TGA desorption experiments were performed under oxidative conditions (N2:O2 = 60:40). As it was shown above the aerobic TGA run for PSIF-1a resulted in quantitative formation of SiO2. Thus, if the end weight of the TGA profile is normalized to 100%, which corresponds to SiO2, then the initial sample mass should reach 265% for ideal PSIF-1ideal.73 For PSIF-1a we obtained 260%, which is in a very good agreement with the ideal value. Similar processing of the TGA curve for I2@PSIF-1a, which involved normalization to match the SiO2 level of PSIF-1a, allowed the determination of the quantity of I2 entrapped inside the porous structure during sorption. Figure 6g shows the comparison of the TGA profiles for both samples, in this plot it can be clearly seen that the major part of the I2 desorption precedes the decomposition of the parent framework. It is also possible to estimate that, in the temperature range from 25 to 310 °C, the desorbed I2 amounts to 80% of the initial sample mass of I2@PSIF-1a, which is in good agreement with the content of I2 (83%) in a freshly loaded sample of I2@PSIF-1a.

isothermal tests conducted at constant heating rates (β) (e.g. collecting several TGA curves at various β). Thus, for PSIF-1a we performed TG measurements at heating rates of 5, 10 and 20 K/min. As shown in Figure S42 (Supporting Information), the increase of β leads to a shift of the maximum of I2 desorption rate (denoted as Tm) towards higher temperatures. The analysis of DTG traces shows that the desorption proceeds in two steps. To extract Tm values from DTG data we performed deconvolution analysis of the observed peaks, which yielded two values of Eaa of 77.0 kJ/mol and 89.0 kJ/mol, which are higher than the enthalpy of sublimation of bulk I2 (62.4 kJ/mol). These values suggest that there are two major environments for I2 sorption. Likewise, when the I2-loaded PSIF was heated under vacuum at 100 °C for 2 h, the iodine release efficiency as high as 95% was observed. Alternatively, adsorbed I2 can also be efficiently removed by soaking in ethanol. To check if I2 adsorption is a reversible process recycling test were performed by taking the I2-loaded sample I2@PSIF-1a and heating it at 100 °C for 2 h under dynamic vacuum. After that the sample was reused for I2 capture under previously described conditions. In 4 consecutive runs the I2 uptakes were 94%, 92%, 91% and 90% (Figure S36, Supporting Information), indicating that PSIF-1a can be efficiently recycled and reused without significant loss of I2 capture capacity.

SUMMARY AND CONCLUSION:

Figure 6. (a) Photographs of PSIF-1a before and after I2 sorption; (b)–(d) SEM images of: (b) PSIF-1a before I2 capture; (c) PSIF1a after I2 removal; (d) I2@PSIF-1a; (e)–(f) EDS mapping in I2@PSIF-1a of silicon and iodine respectively; (g) TGA traces of PSIF-1a and I2@PSIF-1a, collected with the heating rate β = 10 K/min, inset shows the corresponding DTG curves.

Apart from studying thermal stability, TGA analysis is a valuable tool in studying kinetic processes in solids, including estimation of apparent activation energies (Eaa). In general, for a simple one-stage process, an activation energy is the smallest energy barrier which has to be overcome to result in a physical or chemical process. The desorption of I2 from PSIF framework can be envisioned as a physical process accompanied with an energy barrier, which is a function of different factors including I2 diffusion, framework deformation, mass and heat transfer. For estimation of Eaa of physical or chemical processes Kissinger method has often been applied.74 This method relies on the analysis of the process rate from data obtained at several non-

In summary, a new family of Porous Silsesquioxane-Imine Frameworks (PSIF) was designed and synthesized by imine condensation approach starting from octa(3-aminopropyl)silsesquioxane cage compound (OAS-POSS) and selected multitopic aldehydes. The resulting PSIFs possess 3D micromeso-macroporous structures with permanent porosity and high thermal stability. These aerogels were tested in sorption of I2 vapor at 75 °C. For PSIF-1a I2 uptake of 485%wt was obtained, which is the highest value reported to date. Preferential interaction of I2 with PSIFs can be attributed to the cooperative interactions of POSS cages and imine moieties in the porous framework. Moreover, low value of skeletal density for PSIF-1a results in the highest gravimetric uptake of I2, whereas micromeso-macroporous structure of PSIF aerogels has a beneficial impact on I2 sorption kinetics. The analysis of apparent activation energies suggests that desorption from PSIF-1a is a multistep process with two binding sites for I2 molecules. Recyclability experiments show that PSIF-1a can be reused several times without significant framework decomposition nor drop in capture efficiency.

ASSOCIATED CONTENT SUPPORTING INFORMATION Materials, characterization methods, synthesis and characterizations of prolinkers. 1H, 13C and 29Si NMR spectra, solid-state 13C, 29Si and 15N CP-MAS NMR spectra, TGA data, SEM/EDS images and CO2 isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions ‡These authors contributed equally.

Notes



The authors declare no competing financial interest.

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ACKNOWLEDGMENT The authors would like to thank the National Centre for Research and Development (Grant TANGO1/266660/NCBR/2015) (S.S.) and the National Science Centre, Poland (UMO2014/14/E/ST5/00652 (W.B.) and UMO-2016/21/N/ST5/03293 (M.J.)) for support of this research.

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The molecular formula of PSIF-1ideal can be represented as (Si8O12N8C56H72)n which gives a molar mass 2.65 times larger than the molar mass of 8 molecules of SiO2. Blaine, R. L.; Kissinger, H. E. Homer Kissinger and the Kissinger Equation. Thermochim. Acta 2012, 540, 1–6.


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ACS Applied Materials & Interfaces Table of Contents Figure


Porous Silsesquioxane Imine Frameworks as Highly Eï¬cient

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