Cobalt-Chelated Polyamine Brushes on Solid Microspheres for Rapid

Apr 30, 2019 - Cobalt-Chelated Polyamine Brushes on Solid Microspheres for Rapid Binding and Chemical Storage of Molecular Oxygen. Ahmet Ince , Ece ...
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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Cobalt-Chelated Polyamine Brushes on Solid Microspheres for Rapid Binding and Chemical Storage of Molecular Oxygen Ahmet Ince, Ece Tukenmez, Niyazi Bicak, and Bunyamin Karagoz* Department of Chemistry, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SOUTHERN QUEENSLAND on 05/08/19. For personal use only.

S Supporting Information *

ABSTRACT: Methylated poly(vinyl amine) brushes generated on PS-DVB microspheres were demonstrated as excellent macroligands, forming hydrophilic Co(II) chelates capable of reversible molecular oxygen binding. The oxygenation is extremely fast when counteranions of cobalt are replaced with hydroxyl ions. The experiments carried out using oxygen sensor, FT-IR, TGA techniques, and gas volumetric measurements showed that the polymer complexes with either 4/1 or 2/1 [amine]/[Co] ratios form 1:1 molecular oxygen adducts which are stable up to 110−120 °C. In order to impart the chain extension effect, poly(vinyl amine-stat-DADMAC) brushes were also generated on the microspheres by using DADMAC as additional comonomer. Fast oxygenation, high oxygen binding capacity (ca. 80 mL/g), and long-term stability of the oxygen adducts make the polymeric complex superior to current systems. The cobalt complex system presented can be considered as a high capacity solid matrix for dissociation and chemical storage of the molecular oxygen from air. tractive reversible oxygen-binding materials.26 Perhaps solidstate lithium pentacyanocobaltate with 2 mol of DMF has the highest capacity (∼7%) of a solid material ever reported as an oxygen carrier.27 However, moist air quickly reduces its capacity. Recently, Reye and co-workers described a one-pot process for incorporation of CoSalene to mesoporous silica by covalently bound imidazole and pyridine axial ligands. The resulting material has been employed for reversible binding of molecular oxygen.28 The hydrophobic nature of the supporting polymers and low thermal stability of their metal complexes are general problems of those systems.29 In a previous study, the synthesis, oxygen affinity, and oxygen-releasing properties of nonhydrolyzable highly hydrophilic linear polyvinyl amine−cobalt complexes have been demonstrated.30 This material is a promising candidate for chemical storage of oxygen due to its high and rapid oxygenbinding capacity and smooth oxygen release in the temperature range of 70−120 °C. The only drawback of this material in the solution form might be difficult handling in the process. In view of the synthetic approach, our group also demonstrated a new method to get poly(NVF-co-DADMAC) brushes with high yields on the PS-DVB microspheres with nonhydrolyzable linkage.31 It was thought that the random coil chains in the brush could be converted into a stretched state by the

1. INTRODUCTION Since the discovery of reversible oxygen binding of salicylaldimine−cobalt complex by Tsumaki,1 the subject has found considerable interest both in academia and in industry.2−6 Various Co(II) complexes of salicylaldimine derivatives,7 ethylenediamine-bridged bis-β-diketones,8 porphyrins,9 and cyclams10 have been demonstrated to bind molecular oxygen by forming either 1:1 or 2:1 oxygen adducts depending on the donating behavior of the ligating groups11 and steric effects12 of the substituting groups. Among the cobalt complexes either with small ligating molecules13−16 or carbon nanotubes,17−20 a couple of studies on polymer-based cobalt complexes were also demonstrated in the the literature for storage or separation of air oxygen.21,22 Tsuchida et al. described the design of oxygen-selective membranes based on polymer-supported CoSalenes with poly(vinyl imidazole) as an axial ligand.23 The Shinohara group studied oxygenation of polymer-bound tetraaza porphyrin−cobalt complexes.24 They found that oxygen binding occurs at subzero temperatures, but oxygenation practically stops at temperatures higher than 50 °C as in the case of low molecular weight porphyrin complexes. Hsiue et al. reported oxygen-permeable membranes based on CoSalene-anchored epoxidized styrene−butadiene− styrene terpolymers.25 A literature survey revealed that salicyladimine is the most common ligating group for generation of oxygen-trapping centers on the polymer supports. Few examples appear on the use of ligands other than the salicyladimine. Lei and co-workers reported polystyrene-tethered 2,2′-bipyridyl−cobalt complexes as at© XXXX American Chemical Society

Received: Revised: Accepted: Published: A

February April 24, April 30, April 30,

15, 2019 2019 2019 2019 DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Method. P(NVF-stat-DADMAC) brushes were created on PS-DVB microbeads in two steps by surface-initiated photoiniferter polymerization (SI-PIP). In the first one, NVF monomer was used in SI-PIP to obtain hydrophilic PNVF thin layer on the surface of microbeads. After getting a monomer-compatible surface on the microbeads, NVFDADMAC monomer mixtures with 3 different molar ratios (0%, 10%, and 20% (mol/mol)) were used for grafting as described elsewhere31 and resulted in thick brush layers. In a typical procedure, 1.0 g of morpholine dithiocarbamatetethered microbeads was mixed with 2.5 mL of NVF and 5 mL of distilled water in a Schlenk tube under continuous nitrogen flow. Then the sealed tube was put in a photoreactor (8 × 12 W), and SI-PIP was carried out under light at 360 nm for 24 h. After filtration the resulting crude microspheres were purified by washing 100 mL of distilled water and acetone to remove homopolymer residues. The final product was dried at 40 °C under vacuum overnight and then weighed as 2.1 g. Grafting yield was found to be 110% gravimetrically. In the second step, the NVF-grafted microspheres were separated into three parts, and chain extension photopolymerization was carried out for each one with the same approach in the presence of NVF/0%, 10%, and 20% DADMAC (mol/mol) monomer mixture for 48 h. The resulting densely grafted bead samples were isolated in a similar fashion. 2.4. Hydrolysis of Poly(vinyl formamide) Surface Brushes. To hydrolyze formamido groups of poly(vinyl formamide) units in the brushes, the grafted resin samples were soaked into 4 M HCl aqueous solution and the suspension was stirred for 24 h at room temperature. The filtered microbeads were purified by washing with distilled water, resuspended in 25 mL of freshly prepared 4 M HCl solution, and heated at 70 °C for 8 h. Resulting microbeads were filtered and washed with water (3 × 100 mL). To obtain the resin in free amine form, the wet microbeads were transferred into 50 mL of cooled NaOH solution (2 M) and interacted for 24 h at room temperature. The amine functional microbeads were filtered and washed with water (5 × 100 mL), methanol (30 mL), and diethyl ether (20 mL). The purified material was dried under vacuum at 40 °C for 16 h. 2.5. Amine Contents of the Hydrolyzed Microbeads and Methylated Derivatives. The amine content of the hydrolyzed microbeads and methylated derivatives was determined using the same acid titration process. In a typical procedure, a sample (ca. 0.2−0.3 g) was mixed with 20 mL of 1 M HCl aqueous solution and interacted for 24 h at 25 °C. Then 10 mL of the filtrate from the reaction mixture was titrated with 0.05 M NaOH solution in the presence of phenolphthalein to estimate the residual acid concentration. The amine contents of the microspheres were then calculated from the gap of the initial and final acid concentrations. 2.6. Methylation of Primary Amino Groups of Poly(vinyl amine) Brushes. A 10 g amount of the resin with hydrolyzed surface brushes was swelled in 100 mL of distilled water. To this mixture was added 27.7 g (0.22 mol) of dimethyl sulfate while stirring at 0 °C, and stirring was continued for 1 h at room temperature. The reaction flask was placed in an ice bath, and 16.6 g (0.44 mol) of NaOH in 100 mL of water was added dropwise to the flask. The mixture was stirred for 8 h at room temperature and filtered. Methylation of the crude product with dimethyl sulfate was repeated once again using the same procedure. The resulting material was

electrostatic repulsion of the quaternary groups (polyelectrolyte effect) by inserting DADMAC units into the brushes. In this way the hairy brushes gained extra hydrophilicity, easy reachability, and pseudohomogenous behavior. Herein, the oxygen storage30 and chain extension behavior on the solidsupported resin31 features were combined on the brushes to accomplish oxygen storage on the solid-supported material with an easy handling and packaging ability. Resulting material could be separated easily from the medium by a simple filtration process and used repeatedly. Also, the solidsupported polymer complexes could be stored in the oxygencharged state for a long time at room temperature and used as a solid oxygen storage. In the study, first poly(N-vinyl formamide-co-diallyldimethylammonium chloride) P(NVF-co-DADMAC) brushes (up to 500% w/w) were generated on morpholine dithiocarbamate functional PS-DVB (10% cross-linked) microspheres by the surface-initiated photoiniferter polymerization (SI-PIP) technique. After hydrolysis of the PNVF segments on hairy brushes in acidic medium, the methylation reaction was carried out with dimethyl sulfate and yielded methylated poly(vinyl amine) brushes capable of efficient chelating with Co(II). As known from the literature, cobalt complexes of low molecular weight multiamine ligands (cyclams) give more stable oxygen adducts.10 Thus, methylated poly(vinyl amine) brushes were prepared as ligand for creating thermally stable oxygen adducts. In such structures the mobility of the graft chains is expected to provide a nearly homogeneous reaction medium for reversible binding with molecular oxygen. The insolubility of the resulting polymer complex, on the other hand, was considered to allow easy separation from the reaction medium by simple filtration and possible storage of the oxygenated product in the long term. Reversible oxygenation characteristics of their cobalt complexes of dense polyamine surface brushes were investigated by TGA, conventional spectroscopic techniques, and gas volumetric measurements.

2. EXPERIMENTAL SECTION 2.1. Materials. N-Vinyl formamide, divinylbenzene, and styrene were purchased from E. Merck and redistilled under reduced pressure. Diallyldimethylammonium chloride (DADMAC) (65% aqueous solution), morpholine, bromoacetyl bromide, and dibenzoyl peroxide were obtained from Aldrich and used without further purification. Mercuric acetate (E. Merck), carbon disulfide (E. Merck), and all other chemicals were used as purchased. Polystyrene-divinylbenzene (PS-DVB) cross-linked microbeads were synthesized by using aqueous suspension polymerization of styrene and divinylbenzene with a molar ratio of 9 to 1 (Gum Arabic was used as stabilizer) as described before.32 The obtained microbeads were sieved, and the fraction in the size range of 210−420 μm was used for further surface-initiated photopolymerization step. 2.2. Functionalization of PS-DVB Microbeads with Morpholine Dithiocarbamate Iniferter Groups. The following steps were performed to get the morpholine dithiocarbamate functional PS-DVB microbeads: (i) acetoxy mercuration of PS-DVB microspheres, (ii) chlorine group exchange with saturated aqueous solution of NaCl, (iii) interaction with bromoacetyl bromide, and (iv) reaction with an excess amount of sodium salt of morpholine dithiocarbamate in DMF as described before.31 2.3. Surface-Initiated Polymerization NVF and DADMAC on PS-DVB Microspheres by Photoiniferter B

DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Scheme 1. Preparation and Stepwise Functionalization of PS-DVB Microspheres with Morpholine Dithiocarbamate Groups

Scheme 2. Formation of P(NVF-co-DADMAC) Hairy Grafts on PS-DVB Resin by Photoiniferter Technique

washed with distilled water (5 × 200 mL) and acetone (2 × 50 mL) and dried under vacuum at 60 °C overnight. Also, each modification on the resin was characterized by FT-IR (Figure S1). 2.7. Complexation and Oxygenation of the Methylated Poly(vinyl amine) Brushes on the Resin. Complexation of methylated poly(vinyl amine) brushes on the PS-DVB microbeads with cobalt was accomplished by applying the previously described method.30 Chelation was performed with excess amine−cobalt(II) molar ratios (2/1 mol/mol) under continuous nitrogen flow due to oxygen sensitiveness of the microbeads. In this procedure, 1.5 mmol of amine-containing microbeads (0.2 g) was wetted by mixing 15 mL of distilled water in a flask. Then 5 mL of 0.75 mmol of Co(NO3)2·6H2O and 7.5 mL of 2 M KOH aqueous solutions were added into this flask sequentially and kept under stirring for 0.5 h. The resulting microbeads were purified by washing with water (100 mL) and acetone (20 mL) and dried at 40 °C for 24 h under vacuum. Afterward, the typical oxygenation procedure was done as given in the literature.30 Simply, the complex microbeads were interacted with 20 mL of water, and air oxygen was bubbled (30 mL s−1) for 5.0 min through the solution. The color of the resin turned dark green from beige during air exposure. Then the same purification and drying process was followed as indicated above. 2.8. Tests for Oxygenation of the Cobalt Complexes on the Brushes. A homemade gas volumetric system (see Figure S2 in the Supporting Information) combined with an oxygen sensor (ALMEMO MA2390-3 oxygen meter with Ahlborn Fy 9600-O2 oxygen sensor) was tethered to the gas volumetric setup for in situ tracking of the released oxygen. For

the deoxygenation a sample of oxygen-loaded polymer complex (ca. 0.2−0.5 g) was added to 25 mL of diethylene glycol in a 100 mL two-necked flask equipped with a digital thermometer temperature and a nitrogen inlet. The flask was placed in an oil bath equipped with a digital thermometer where the temperature was increased gradually by 10 °C per 10 min. One of the necks of this flask was attached to another flask involving the oxygen sensor via a silicone tube as depicted in Figure S2. An inverted buret dipped into a tap water bath was tethered to the second flask. The releasing oxygen volume was determined by the water level in the buret.

3. RESULTS AND DISCUSSION 3.1. Generation of Methylated Poly(vinyl amine) Brushes on PS-DVB Microspheres. Morpholine dithiocarbamate functional PS-DVB microbeads were prepared with the procedure as given in the literature.31 The general scheme of the modification is depicted in Scheme 1. The gravimetrically found bromide content (0.81 mmol/g) of the microspheres implied 0.75 mmol/g of morpholine dithiocarbamate density by assuming full conversion in the last step. Then the photoiniferter technique was employed for graft copolymerization of NVF-DADMAC monomer mixture from the morpholine dithiocarbamate functional PS-DVB microspheres. High grafting degrees (426−568%) were attained from a mixture of NVF with 0%, 10%, and 20% (mol/mol) of DADMAC content (Scheme 2). The NVF segments were hydrolyzed by acid treatment as suggested by the Witek group to give vinyl amine-DADMAC copolymer brushes.31,33 C

DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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good stability against oxidation, PVAm graft chains were methylated. Smoothness and spherical form of the PS-DVB microbeads are proved with SEM images (Figure 1). After surface grafting, the spherical form of the microbeads is not destroyed. Moreover, the thick cracked layers demonstrate the occurrence of polymerization on the surface of microparticles. Also, enlargement of the particle size on PAGR0 and PAGR20 indirectly proves the occurrence of a high grafting yield. In addition to SEM images, the surface area of PAGR0 is obtained with BET analysis. According to thee analysis, the single-point surface area at p/p0 of PAGR0 microbeads was measured as 0.9389 m2/g. As seen, the low surface area might be explained by the occurrence of the polymerization on the surface and inner part of the microbeads as well. Even if the reachable surface area is blocked, this does not affect the complexation properties of the hairy brushes. The BJH adsorption and desorption cumulative volume of pores are measured as 0.005296 and 0.005112 cm3/g, respectively. Furthermore, the BJH adsorption average pore width (4 V/A) is measured as 49.792 Å, and desorption is found as 50.415 Å. 3.2. Reversible Oxygen Binding via Cobalt Complexes of Methylated PVAm Brushes on PS-DVB Microspheres. Full names of PS-DVB microspheres with PVAm brushes were abbreviated as PAGR0, PAGR10, and PAGR20, where PAGR stands for polyamine-grafted resin and the numerals denote percentage of DADMAC contents of the brushes. Cobalt complexes of these chelating resins formed by action with aqueous Co(II) solution are light brown in color but turn dark green upon standing for 5−10 min in open atmosphere, implying their oxygen sensitivity. Figure 2 shows optical images of tertiary amine functional PAGR10 microspheres and their oxygenated cobalt complex as a representa-

Although PAGR20 has the lowest grafting degree value, the oxygen adsorption capacity demonstrates the highest value due to supplying the reachable chelating ligands on hairy brushes (Table 1). This might be the chain expansion effect of the DADMAC which prevents the random coil formation of hairy brushes. Table 1. Amine Contents of PS-DVB Microspheres with PVAm and Methylated PVAm Copolymer Brushes with Varying DADMAC Contents [DADMAC] ratio of the monomer mixture (mol/mol)

grafting degreea (%)

amine content after hydrolysisb found (theor.) (mmol/g)

mass increase in methylation (%)

amine content after methylationb (mmol/g)

0% 10% 20%

568 455 426

11.0 (11.96) 9.0 (9.42) 7.6 (7.65)

34.0 29.1 25.6

8.3 7.2 6.2

a

Grafting degrees were assigned by mass increases of the bead samples. bCalculated by acid titration.

The amine content of the resin beads obtained by hydrolysis of the PNVF units was found to be 11.3 mmol/g, which is slightly lower than the theoretical value, 11.96 mmol/g (Table 1). The ratio of the practical to theoretical amine content (7.6/ 7.65) is highest for the case of the hydrolysis product of the microspheres with the surface brushes having 20% DADMAC. This can be ascribed to an almost quantitative hydrolysis yield of this sample due to the chain extension effect (polyelectrolyte effect) of the quaternary groups of the surface brushes. Amino groups of the hydrolysis products were methylated by action of dimethyl sulfate. Since tertiary amine−cobalt complexes show

Figure 1. SEM images of bromoacetyl functional PS-DVB, PAGR10, and PAGR20. D

DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Photos of tertiary amine functional PAGR10 microspheres (A) and their dark green cobalt complex after 10 min of air exposure (B).

Scheme 3. Proposed Reversible Oxygenation Mechanism of the Cobalt Complexes onto the Microspheres

tive example. Figure 2A demonstrates the PAGR10 adsorbent having a beige color just before aeration. This beige color turns into a dark green color by oxygenation of polymer−cobalt complexes (shown in Figure 2B). Herein, we assigned the oxygenation time as 1 h, which indicates actually the complete oxygenation period. Normally, the color change is observed between 5 and 10 min. To inspect the effect of amine/cobalt ratio on the oxygen binding, PAGR0 was prepared using 0.25 and 0.50 mol of Co(II) per mole of the amino group of this resin. After equilibration of the resulting complexes with air stream in diethylene glycol, the samples were heated stepwise to 130 °C and the oxygen evolved was measured by the gas volumetric method. The volume of oxygen evolved from the complex with a 4/1 [Amine]/[Cobalt] ratio was about one-half of that of polymer complex with a 2/1 [amine]/[Cobalt] ratio. Interestingly, the measured molar ratio of oxygen to cobalt was almost equivalent (1.01 and 0.92 mol of O2 per mole of cobalt) in both cases (see Table 1), implying formation of stable 1:1 oxygen adducts for complexes with 4/1 and 2/1 [Amine]/[Cobalt] ratios. From this point of view, 1 g of bead sample (PAGR0) for complexes with 4/1 and 2/1 [Amine]/ [Co] ratios, O2 binding capacity were calculated to be 2.095 and 3.818 mmol, respectively. These results are also consistent with the literature; for instance, in Yang’s paper, the oxygenbinding capacity of CoSalene-tethered nanoporous materials was 1.06 mmol/g, based on a 1:2 (O2:Co2+) adduct.15

The only difference was the slightly high deoxygenation temperature of the former (120 °C), compared with those of the 2/1 [L]/[M] resin complexes (110 °C). The difference can be attributed to the slightly high thermal stability of the oxygen adducts of the tetraaza complex possessing three chelate rings. This means that the oxygenated resin complexes with a 2/1 [L]/[M] ratio also show reasonable stability, most probably due to the hydroxyl counterions of the cobalt involved. Similar high oxygen binding capacities were noted for the resins with polyDADMAC segments when the [Amine]/ [Co] ratio was kept at 2/1. It is noteworthy that oxygenation by air exposure is very slow without exchanging the counteranions of cobalt (chloride or nitrate) by KOH addition. It was determined that a special axial ligand is not essential when counteranions of Co(II) is exchanged with OH− ions. Considering special attention paid for the axial ligand to attain stable oxygen adducts, this result seems to be controversial to the earlier reports. However, Marchaj et al. published about the pH dependency and kinetics of oxygenation of some cobalt(II) complexes.34 In this interesting article the authors stated that high pH values had a minor effect on oxygenation of the axial ligand. Moreover, the probability of dioxygen binding by cobalt−cyclam complexes in basic medium in the absence of an axial ligand was reported in the same paper as well. After a while such an unconventional oxygenation behavior was also observed by Ceccanti et al.35 In the study the role of the axial E

DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. Oxygen evolutions of the oxygenated PAGR0, PAGR10, and PAGR20 complexes with 2/1 [amine]/[Co] ratios measured by the gas volumetric method as a function of temperature in diethylene glycol.

Table 2. Oxygen Storage and Releasing Characteristics of the Resins with Cobalt-Chelated PVAm Surface Brushes chelating resin b

PAGR0 PAGR0 PAGR10 PAGR20

tert-amine content(mmol/g)

[amine]/[Co] (mol/mol)

oxygenation capacity (mL)a

[O2]/[Co] (mol/mol)

oxygen releasing temperature

8.3 8.3 7.2 6.2

4/1 2/1 2/1 2/1

51 80 79 79

1.01 0.92 0. 98 1.05

∼120 °C 110 ± 2 °C 110 ± 2 °C

a

Volume of released oxygen per gram of the resin at room temperature. bMeasured in monoethylene glycol.

methylated PVAm and P(VAm-co-DADMAC) oxygen evolving values were found to be 20 ± 2 and 18.4 ± 2 mL per 0.8 mmol of the cobalt complexes, respectively. FT-IR spectra of PAGR10 (a) and its oxygenated cobalt complexes (b) are given in Figure 4 in order to demonstrate

ligand was suggested as enhancing the pH of the solution instead of coordinating. Even if those outcomes are coherent with the present results, it is not sufficient to state the exact mechanism of the “hydroxyl ion effect”. Here, in the cobalt− oxygen coordination system, the presence of the hydroxyl groups as counterion induces formation of stable oxygen adducts even for the case with 2/1 [Amine]/[ Co] ratio and the residual coordination cites of cobalt are filled with a water molecule. The proposed reversible oxygenation mechanism of the tertiary amine−cobalt complexes on the hairy brushes of the microspheres with a [L]/[M]: 2/1 ratio is demonstrated in Scheme 3. Presumably the quick molecular oxygen binding of this system depends on the ease of replacement with hydroxyl anions. As a consequence, the oxidation state of cobalt is +3 after oxygen loading; one of the hydroxyl groups must stay as a counterion around cobalt due to the electroneutrality, as mentioned by Nikon and Martell.36 Figure 3 exhibits s-shaped deoxygenation profiles starting around 70 °C and stabilized at 110 °C in diethylene glycol for the PAGR series (0, 10, and 20) of the complexes. Meanwhile, the chemical structure of the microbeads is quite stable and does not have an affect (or disintegrate) at these temperatures. The oxygen evolutions were measured by the gas volumetric method (see Figure S3 in the Supporting Information) and found at around 80 mL (O2 per gram of the bead sample) per 4.15 mmol of the cobalt content, implying formation of nearly 1:1 oxygen adducts on the resin complexes. Experiments show that PAGR0 can bind 2.959 mmol/g oxygen, but the theoretical oxygen capacity of PAGR0 is 3.33 mmol/g. This result shows that PAGR0’s oxygen binding yield is 89% (Table 2). Slight deviations of [O2]/[Co] for the cases with PAGR0 and PAGR20 are in the experimental error limits. Slightly higher oxygen binding capacities for the complexes of PAGR10 and PAGR20 must be due to the chain extension effect of the quaternary ammonium groups’ repulsion that supply the complex sites nearby the PS-DVB core accessible for the dissolved oxygen. These results were quite coherent with the previous study30 in which cobalt complexes of

Figure 4. FT-IR spectra of PAGR10 (a) and its cobalt complex after oxygenation (b).

the molecular oxygen binding onto resin complexes. In Figure 4b the peaks at around 1300−1600 cm−1 belonging PAGR10 undergo overlapping with a broad band due to the restricting mobility of the graft chains upon complexing with cobalt. The broad O−H vibration band of PAGR10 centered at 3400 cm−1 must be associated with the hydrophilic nature of the resin. Disappearance of O−H vibration band in Figure 4b might be due to the low hydrophilicity of its cobalt complex in the dry state. A new sharp peak emerging at 3600 cm−1 might be associated with the hydroxyl anion of the oxygenated cobalt complex. F

DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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minimum at 70 °C in the DTG curve can be assigned as the optimum starting temperature of the oxygen release. The maximum around 110 °C can be assigned as the upper limit of the deoxygenation temperature. The DTG curve also shows an initial decomposition temperature of the polymer at 200 °C. Obviously, this is a considerably low temperature that must be associated with decomposition of the graft chains promoted by the catalytic effect of the oxygenated cobalt complex. One interesting aspect of the oxygenated CoSalenes is their oxygen release at room temperature in the presence of benzene and chloroform.40 This phenomenon has been ascribed to exchange of the axial ligands with benzene and chloroform to facilitate breaking of the Co−O bond. Benzene can be considered as a π-donor ligand, although why toluene is not effective is still in question. By not having π or unpaired electrons it is difficult to understand ligand exchange with chloroform. To inspect any oxygen-releasing effect, we studied various solvents such as benzene, toluene, xylene, dichloromethane, ethanol, diethylene glycol, butanol, isopropanol, and monoethylene glycol, none of which resulted in oxygen release at room temperature. It is verified by the oxygen sensor that an instantaneous oxygen evolution was affected by acid (1.0 M H2SO4) treatment at room temperature. In this experiment the acid solution was slowly added by a syringe to the oxygenated cobalt complex of PAGR20 in nitrogen atmosphere (Figure S3), and the oxygen gas was monitored by an oxygen sensor in a vessel connected to the reaction flask. Figure 6 (right) shows the increment on oxygen percentage as a function of acid volume. Correspondingly, the sudden rise occurs on the oxygen level after addition of 9 mL of H2SO4. This means that 5.6 mol of hydronium ion is adequate to release the oxygen complexed per mole of cobalt. Considering 5 protons for the decomposition of each Co(III) complex moiety, this result implies that deoxygenation of the resin complex at room temperature can be achieved only after decomposition of the complex by acid treatment to give protonated tertiary amines and Co(II)−sulfate. Indeed, PAGR20 particles become nearly colorless at the end, and the aqueous solution turns to a faint pink, which is the original color of free Co(II) salt. To inspect the long-term stability of the oxygenated resin complexes, a sample of aerated PAGR20 complex was filtered and washed with distilled water and then left to stand at room temperature for 2 weeks. The oxygen content of the sample measured by the gas volumetric method was found to be the same (80 mL) as that of PAGR20, implying long-term stability of the molecular oxygen adduct. Regarding the high oxygen binding capabilities and longterm stabilities, the resin complexes seem to be promising as a solid matrix for oxygen storage. However, Co(II) complexes are also known as good catalysts for oxidation of alcohols and benzylic carbons41,42 at relatively high temperatures. These complexes have also been demonstrated to act as catalyst also for epoxidation of alkenes.43 Therefore, cobalt complexes of the resins presented in this work are expected to behave as oxidation catalyst. This may bring limitation in the recycling and long-term use of such polymer complexes by destructing the ligating brushes. To investigate decomposition of the resin by self-oxidation, oxygenation and deoxygenation of PAGR20 complex was repeated for 10 cycles without measuring the evolved oxygen in the intermediate steps.

Generally the O−O vibration of the superoxo group is known to be IR inactive and invisible in most cases owing to the symmetry of this group.37 Therefore, the small peak observed at 894 cm−1 cannot be assigned as the O−O vibration of the superoxo group. This peak must be deformation vibration of the hydroxyl ion.38 In this work XPS was used to confirm the presence of the molecular oxygen in the aerated resin complexes. Thus, Figure 5a shows XPS of metal-free PAGR10 containing base element

Figure 5. X-ray photoelectron spectrum of PAGR10 (a) and its oxygenated cobalt complex (b) scanned in the 0−1400 eV range.

peaks. The peaks at 285 and 400 eV represent C 1s and N 1s core levels of the carbon and nitrogen atoms of the resin, respectively. The weak O 1s peak at 532 eV can be ascribed to the existence of residual hydroxide contaminants in this sample. Complexation with Co(II) and the following oxygenation results in suppressing the C 1s and N 1s core level peaks due to the shielding effect of the metal ion, so the peaks become almost invisible. The peak associated with the binding energy for Co 2p3 appears at 781.7 eV, which is slightly higher than that of Co(II). This can be attributed to Co(III) rather than Co(II) as reported in the literature.39 The most significant difference in Figure 5b is the appearance of a strong O 1s core level peak at 532 eV. A high percentage of this peak (51.5%) compared to that of PAGR10 (3.1%) can be considered as more direct evidence for the binding of molecular oxygen. Moreover, the presence of an observable O2 Auger electron band (O2−KLL) at 974 eV is additional evidence for the binding of molecular oxygen during aeration. In order to verify the oxygen evolution temperatures estimated from the deoxygenation curves, TGA of the oxygenated PAGR0 complex with a 2/1 [L]/[M] ratio was carried out in a nitrogen atmosphere. The TGA curve of the sample (Figure 6) represents about 3.5% mass loss up to 120 °C, which can be ascribed to the oxygen evolution. The

Figure 6. TGA of the oxygenated PAGR0−cobalt complex taken in nitrogen atmosphere (left) and oxygen releasing (monitored with oxygen sensor) from PAGR20-cobalt complex (1.0 g) by action of H2SO4 solution (1.0 M). G

DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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Only 0.0076% capacity loss was observed in the 10th experiment. The volume of the evolved oxygen measured at last by the gas volumetric method was determined to be the same, implying stability of this resin complex at least within 10 cycles.

4. CONCLUSION Methylated poly(vinyl amine-stat-DADMAC) brushes generated on PS-DVB microspheres were demonstrated to be efficient macroligands to form donor−acceptor complexes with Co(II) ions. These complexes with either 2/1 or 4/1 [amine]/ [cobalt] ratios undergo rapid oxygenation by aeration in water or in diethylene glycol to give 1:1 oxygen adducts when counteranions of the cobalt are replaced by hydroxyl ions. This result is also quite consistent with the former study, in which the linear polyvinylamine-based complexes bound 0.8 mol of oxygen per mole of cobalt.30 Having high capacities (80 L/kg resin), fast oxygenation capabilities, and reusability, complexes of PAGR series of resin materials are superior to the current cobalt systems. These resin complexes can be considered as a useful high-capacity solid matrix for harvesting and chemical storage of molecular oxygen. Moreover, the resin complexes might also be useful for supplying nitrogen by adsorbing the oxygen component of air.

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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.iecr.9b00892. Discussion on monomer selection for the brushes, characterization of the microspheres with FT-IR, discussion on methylation process of the brushes, and experimental setup for testing oxygenation and deoxygenation of the polymer complexes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bunyamin Karagoz: 0000-0003-1191-218X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Turkish Scientific and Technological Research Council (TUBITAK) is greatly acknowledged for financial support (Project No. 109T803). This work was also supported by Istanbul Technical University, BAP 39921.

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DEDICATION This paper is dedicated to the memory of our wonderful colleague, Prof. Niyazi Bicak, who recently passed away. REFERENCES

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DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.9b00892 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX