Effect of Different Acid Initiators on Branched Poly(propylenimine

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Article

Effect of Different Acid Initiators on Branched Poly(propyleneimine) Synthesis and CO2 Sorption Performance Michele L Sarazen, Miles A Sakwa-Novak, Eric W. Ping, and Christopher W Jones ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00512 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Effect of Different Acid Initiators on Branched Poly(propyleneimine) Synthesis and CO2 Sorption Performance Michele L. Sarazen,1 Miles A. Sakwa-Novak,2 Eric W. Ping,2 and Christopher W. Jones1* 1School

of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA 2Global Thermostat LLC, 660 Madison Ave., New York, NY 10065 *to whom correspondence should be addressed: [email protected]

Abstract Branched poly(propylenimine) (PPI) provides an alternative to the prototypical amine polymer, commercially available branched poly(ethylenimine) (PEI), in composite adsorbents for CO2 capture. Herein, we investigate the synthesis of PPI via cationic ring opening polymerization of azetidine using various acid initiators (HBr, HClO4, HCl, CH3SO3H) and polymerization times, impacting the molecular weight and CO2 sorption behavior. The polymerization kinetics and the amine distribution (i.e. primary:secondary:tertiary ratios) are monitored with 1H-NMR during polymerization and a basic ion exchange resin is used to neutralize charged amine centers and to remove unreacted acid. The polymers are impregnated into the model porous oxide support, mesoporous silica SBA-15, and the CO2 capacities under both simulated ambient air and flue gas conditions are elucidated. In parallel, the oxidative stability of the PPI based sorbents is assessed and compared with the prototypical PEI sorbents. Sorbents with 30 wt.% polymers synthesized using HBr and HClO4 exhibit higher CO2 capacities than those made with HCl or CH3SO3H. Sorbents from HBr polymers only lost 24 % of their CO2 capacity after 12 h of oxidation in air at 383 K. Even trace amounts of residual ClO4- anions in HClO4 initiated polymers, though, accelerated oxidation (decreased CO2 capacity by 64 %). Extended resin treatments were needed to leave undetectable Cl content in these polymers, which resulted in sorbents that are much more oxidatively stable. Keywords Adsorption, polyamines, carbon capture, mesoporous materials, polymer synthesis

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Reduction of atmospheric CO2 via emissions mitigation, e.g. capture from flue-gas, and by negative emissions technologies, e.g. capture from ambient air (direct-air-capture; DAC) is imperative in order to limit continued effects of global climate change.1 Solid sorbents offer a set of sustainable and regenerable carbon capture materials with some potential advantages compared to benchmark sorbents based on liquid amines.2–4 While various aminopolymers are active for CO2 capture, poly(ethylenimine) (PEI) is by far the most well-studied in CO2 separations when supported in mesoporous solids.5–18 Under dry conditions, primary and secondary amines act as the adsorptive/reactive centers, while tertiary amines also become active in humid environments.19 It has also been found that the secondary amines are more susceptible to degradation at elevated temperatures in the presence of oxygen, which results in loss of active centers and thus, CO2 capture capacity.20–24 The resistance to oxidative degradation is critical for DAC adsorbents due to the presence of oxygen in the dilute CO2 feed streams. While this suggests that polymers with only primary amines are desired, with studies utilizing primary amine rich materials such as poly(allyl amine) (PAA) demonstrating enhanced oxidative resistance, such primary amines are susceptible to degradation via other routes, such as urea formation.15,22,25 Recent efforts to increase the oxidation stability of aminopolymers investigated the effects of carbon spacing within the polymer backbone. Small oligomers and dendrimers comparing ethyl versus propyl backbones indicated amines separated by propyl linkers were more stable under oxidative conditions.26 Further, the higher molecular weight linear analog of poly(propylenimine) also demonstrated high CO2 capacity and oxidative stability.27 This motivated an effort to synthesize and utilize branched poly(propylenimine) (PPI),26,28,29 which is expected to have higher


Page 2 of 23

Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sorbent-site accessibility than the linear analog. Further, the complexity of linear PPI synthesis27 compared to branched PPI would impact the scalability/cost of using these polymers. The synthesis of higher molecular weight PPI polymers was initially reported via cationic ring opening polymerization,30,31 while the polymerization of functionalized azetidine derivatives has been more well-studied.32–34 Previously, we investigated the effects of synthesis conditions such as reaction time, temperature, and monomer to perchloric acid initiator ratio on molecular weight, polydispersity, and degree of branching of the obtained PPI; the efficacy of various neutralization techniques was also examined.29 The PPI-based adsorbents were demonstrated to be promising materials for further development and elaboration for CO2 capture; however, their stability after oxygen exposure and the effects of varying the acid initiator on polymer structure and performance has not been explored. Here, we investigate the synthesis of branched PPI via cationic ring opening polymerization of azetidine using various acid initiators (HBr, HClO4, HCl, CH3SO3H); further, PPI polymers with different molecular weights were prepared by changing the polymerization time. The polymerization kinetics and primary:secondary:tertiary amine distribution (i.e. degree of branching) were monitored with solution-phase 1H-NMR during the reaction. A basic resin was used to neutralize charged amine centers after reaction, as well as to remove unreacted acid, with the presence of residual heteroatoms determined by elemental analysis. The CO2 capacities under both simulated ambient air and flue gas conditions of silica-based PPI sorbents made with these polymers were explored. The results suggest that at a given polymer weight loading, sorbents made with polymers synthesized using HBr and HClO4 exhibit higher CO2 capacities than those made with HCl or CH3SO3H. However, sorbents from HClO4 polymers with even trace residual HClO4 lost much more CO2 capture capacity after harsh oxidative treatments than HBr-derived polymers.



Page 4 of 23

Extended treatments were needed for Cl to become undetectable by elemental analysis; sorbents made with these polymers demonstrated improved oxidative stability. Thus, branched PPI represents a promising alternative to branched PEI as it is shown to be more resistant to degradation in the presence of oxygen. Experimental Procedures Preparation and Polymerization of Azetidine Azetidine was obtained after distillation of azetidine hydrochloride over potassium hydroxide.35 Potassium hydroxide was dissolved in deionized water in a 2:1 molar ratio using a 100 mL round-bottom flask. Azetidine hydrochloride (97 %, Alfa Aesar, 1:3 molar ratio with respect to KOH) was added to the solution. Azetidine was isolated by distillation and confirmed using NMR: 1H NMR (400.0 MHz, CDCl3, TMS): δ (ppm) 1.99, 2.25, 3.55;

13C

NMR (100.6

MHz, CDCl3, TMS): δ (ppm) 22.1, 48.2 (Supplemental Information; SI, Fig. S1). The resulting liquid was stored in the freezer. Polymerizations were run according to previous methods using azetidine30 and aziridine.36 In a 25 mL pressure tube, acid was added azetidine in 2 mL of methanol, yielding a H+:monomer ratio of 3.2. The acids were perchloric acid (70 % in H2O), hydrobromic acid (48 % in H2O), hydrochloric acid (37 % in H2O), and methanesulfonic acid (>99%). The pressure tube was sealed, placed in an oil bath at 353 K. Aliquots were taken using glass pipettes at various time increments and monitored via 1H-NMR using D2O as the solvent. The resulting polymer was concentrated by rotary evaporation. The polymer (100 mg) in 10 mL water was neutralized using AMBERSEP 900 (OH) basic resin (Sigma-Aldrich; 25 mL) by stirring for 6 h or 12 h. PPI was collected after removal of the resin via filtration followed by rotary evaporation. Relative amounts of primary, secondary, and tertiary amines as well as residual ring species were determined from 1H-NMR


Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectra, where peak assignments in CDCl330 and NMR predictive software were used for identification. Aqueous GPC was performed on a Shimadzu HPLC system with a refractive index detector (RID-10A) and cationic columns (TSKgel G3000-G6000-PWxl-CP Guard, G3000-PWxlCP, and G5000-PWxl-CP) with an eluent phase of 0.4 M acetic acid with 0.3 M NaNO3 flowed at 1 mL/min. Qualitative molecular weight distributions were also determined using MALDI (matrixassisted laser desorption/ionization; Bruker AutoFlex III). 2.2 Synthesis of SBA-15 Mesoporous silica SBA-15 was synthesized according to a previously reported procedure.35 Pluronic P123 block copolymer ((EO)20(PO)70(EO)20) (24 g) was dissolved in deionized water (636 mL) and 12.1 M HCl (120 mL) and stirred for 3 h. Tetraethyl orthosilicate (TEOS; 46.6 g) was added dropwise and the mixture stirred for an additional 20 h at 313 K. The solution was then heated to 373 K, held with no stirring for 24 h, and then quenched with deionized water (400 mL). The precipitate was filtered, washed with copious amounts of deionized water, then aged at 348 K for 12 h. Finally, the material was heated to 473 K at 1.2 K/min, held at 473 K for 1 h, heated to 823 K at 1.2 K/min, held at 823 K for 12 h, and cooled to room temperature. 2.3 Synthesis and Characterization of Aminopolymer/SBA-15 Composites SBA-15 (200 mg) was dried at 383 K under vacuum (< 20 mTorr) overnight and then methanol (10 mL) was added. A given amount of amine polymer was dissolved in 5 mL methanol, added to the SBA-15 solution and stirred for at least 6 h at room temperature. Methanol was removed via rotory evaporation at room temperature and the material was further dried overnight at room temperature under vacuum. Combustion TGA (Netzsch STA409PG) under a flow of nitrogen-diluted air was used to determine the organic content via mass loss from 398 K to 1173



K. Elemental compositions (C, N, Cl, Br, S) of the adsorbents were measured at Atlantic Microlabs (Norcross, GA). Nitrogen physisorption isotherms at 77 K were collected on a Micromeritics Tristar 3020 instrument after being degassed for 12 h at 383 K. The BET method was used to determine surface areas; pore volumes and pore size distributions were calculated using the NLDFT equilibrium model in the Quantachrome VersaWin software package. 2.4 CO2 Adsorption Capacities and Oxidative Resistances CO2 adsorption capacities were measured gravimetrically on a TA Instruments Q500 TGA. The pretreatment for all samples involved heating to 383 K at 5 K/min under flowing N2 and held for 3 h. The samples were cooled to 308 K and equilibrated for 1 h. The gas flow was switched to either 10% CO2/N2 or 0.04% CO2/N2. Oxidative resistances were determined by completing one CO2 adsorption cycle in the TGA, followed by ramping to 383 K at 5 K/min under N2, and holding under a flow of ultra-zero grade air for 12 or 24 h (time denoted in the main text). The sample was then cooled to 308 K in flowing N2, equilibrated for 1 h and then another CO2 adsorption cycle was performed. The oxidative resistance was calculated as the ratio of CO2 capacity before and after the oxidation and given as a percentage. Results and Discussion Polymerization of azetidine The reaction progress for azetidine polymerization at 353 K in a closed vessel is shown in Figure 1 for HBr initiated polymerization; similar monitoring was completed with the other acid initiators as well. It was previously reported that the monomer is first converted to a dimeric species (N-(3-aminopropyl)azetidine) before additional polymerization steps.29,30 The peaks downfield from 3.0 ppm in D2O are characteristic of protons on the α-carbon relative to the


Page 6 of 23

Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nitrogen in either the monomer or in the dimer ring species. The appearance of broader peaks reflects the formation of polymeric species as opposed to small oligomers. Specifically, the peak around 1.6-1.7 ppm is indicative of the β-protons to any linear amine group, while the three peaks at 2.7 ppm, 2.6 ppm and 2.4 ppm are characteristic of α-protons to a primary, secondary, tertiary amine group, respectively. In order to calculate the degree of branching, the areas of the peaks between 2.4 and 2.7 ppm are normalized by their representative number of α-protons (i.e. two for primary amines, four for secondary amines and six for tertiary amines).

Figure 1. 1H-NMR in D2O at different time points during polymerization with initial azetidine:hydrobromic acid concentration ratio=3.2 and temperature=353 K. 1H-NMR

is also used to calculate the degree of polymerization, which is the number of

moles of hydrogen atoms in the polymer relative to those in the ring species, either in the monomer



Page 8 of 23

and dimer (Eq. 1). Further, the degree of polymerization can be used to determine the molecular weight, where Mn is the number averaged molecular weight and the Mo is the molecular weight of the monomer. Because previous NMR and GC investigations suggested the dimer becomes the major reactive species after several hours of reaction after all the monomer is converted to dimer,29,30 any rings were attributed to this dimer species. The degree of polymerization increases as a function of time for all four of the acid initiators used (Fig. 2a). The initial rates of polymerization (< 30 hours) are fairly similar for the acids despite very different acid strengths. However, the continued polymerization (i.e. the stage beyond initial formation of dimers and small oligomers) is significantly different and trends according to the pKa values of the acids, which are -10, -9, -6.3, and -1.9 for HClO4, HBr, HCl, and CH3SO3H, respectively. 𝑚𝑜𝑙 𝐻 𝑝𝑜𝑙𝑦𝑚𝑒𝑟

𝑀𝑛

DP = 𝑚𝑜𝑙 𝐻 𝑚𝑜𝑛𝑜𝑚𝑒𝑟/𝑑𝑖𝑚𝑒𝑟 = 𝑀 𝑜

(1)

Figure 2. (a) Degree of polymerization (Eq. 1) calculated from 1H-NMR as a function of polymerization time at azetidine:acid concentration ratio of 3.2 and 353 K for different acid initiators: () HClO4, () HBr, () HCl, and (▲) CH3SO3H. (b) Distribution of amines calculated from 1H-NMR in D2O for polymers synthesized with various acid initiators after 120 h of polymerization.


Page 9 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


With increasing the polymerization time, the polymer becomes more brached as primary amines are converted to tertiary amines (Fig. S2 for CH3SO3H initiated polymerization). The number of secondary amines initially increases but then plateaus after the first 24 h, which is consistent with rings remaining as the reaction centers. After 120 h of polymerization and resin treatment, the final degree of branching is shown for the polymer for each acid in Figure 2b. In previous work,29 it was observed that the number of primary amines decreased after neutralization with the basic resin, suggesting the loss of low molecular weight, primary amine rich dimers or oligomers. This was further supported by MALDI spectra of the polymer before and after resin treatment and thermogravimetric analysis at higher temperatures in He to observe polymer vaporization.29 It was also observed that the polymeric peaks in the neutralized samples shifted upfield, which is consistent with the neutralization of protonated amines. Polymers synthesized using CH3SO3H have similar numbers of primary and secondary amines, which is consistent with the lower values of degree of polymerization, and thus lower molecular weight, as shown in Figure 2a. Polymers synthesized using HClO4, HBr or HCl have similar secondary amine contents, but use of stronger the acids correlates with more tertiary amines relative to primary amines. This trend is held both before and after the resin treatment; however, the loss of primary amines for HClO4 polymers is much more significant (maximum 25% versus 10% for HBr), indicating that the interaction of small oligomers with the resin is different depending on the different anions used in the synthesis. The ramification of the interaction with polymers, residual acid, and resin will be discussed in the next section. All polymers, however, have similar polydispersities, as indicated from aqueous phase GPC and MALDI (Figs. S3-S4).

CO2 adsorption properties and oxidative resistances of aminopolymer/SBA-15 hybrid materials



The various polymers were impregnated into SBA-15 and the composites were investigated using thermogravimetric analysis (TGA; Fig. S5a), differential scanning calorimetry (DSC; Fig. S5b), and N2 physisorption (Table S1). All of the polymer sorbents showed similar degradation temperatures around 250 °C, as opposed to untreated polymers that have decomposition temperatures near 300 °C, consistent with the salt form of the polymer being more stable than the neutralized form.29 The CO2 capacity (mmol CO2/g SiO2) and amine efficiency (mmol CO2/mmol N) of these PPI/silica hybrids were obtained under dry conditions using 0.04% (400 ppm) and 10% CO2 balanced with helium. All four PPI/silica composite materials synthesized using different acid initiators shown in Figure 3 have similar polymer weight loadings (~30 wt.%). At both CO2 concentrations, polymers made with HClO4 and HBr perform better than those made with HCl or CH3SO3H. Although the HClO4 polymer composite has a higher CO2 capacity than the HBr polymer composite (0.31 versus 0.25 mmol CO2/g SiO2, respectively, at 0.04% CO2), their amine efficiencies are similar (0.039 and 0.040 mmol CO2/mmol N, respectively). Initial oxidation treatments (12 h in air at 383 K) showed that sorbent made using HClO4initiated polymer retained less than 40 % of its initial capacity. Conversely, the HBr, HCl, and CH3SO3H polymer composites retained 69-76 % of their initial capacity after the harsh oxidation treatments (Table 1, entries 1-4). These losses were due primarily to oxidation rather than loss of polymer, as the fraction of polymer in the composite, as measured via combustion TGA analysis, was similar before and after the treatment. Thus, the HClO4-initiated polymer proved to be substantially more susceptible to oxidation than the other materials.

Table 1. Summary of amine loadings, oxidative resistances, residual acid counter ions for various polymer/SBA-15 composite sorbents.


Page 10 of 23

Page 11 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


acalculated bcalculated

Acid

Amine Loading

Oxidative Resistancea

mol X/mol Nb

Initiator

(mmol N/g SiO2)

at 0.04 % CO2

where X is Cl, Br, S

HClO4

8.17

37 %

0.029

HBr

6.05

76 %

Effect of Different Acid Initiators on Branched Poly(propylenimine

Recommend Documents