Phosphorus Dendrimer Derived Solid Sorbents for CO2 Capture from

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Environmental and Carbon Dioxide Issues

Phosphorous Dendrimer Derived Solid Sorbents for CO2 Capture from Post-Combustion Gas Streams Samuel J. Thompson, Mustapha Soukri, and Marty Lail Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01764 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Phosphorous Dendrimer Derived Solid Sorbents for CO2 Capture from Post-Combustion Gas Streams Samuel John Thompson, Mustapha Soukri,* and Marty Lail RTI International, Post Office Box 12194, Research Triangle Park, NC 27709-2194, USA

KEYWORDS: Carbon Capture, Solid Sorbent, Simulated Flue Gas, Phosphorous Dendrimer

ABSTRACT: Solid amine sorbents were prepared via cross-linking of polyamine compounds and polyaldehyde phosphorous dendrimers through a reductive amination approach. The CO2 adsorption capacities from inert gas streams and simulated flue gas were determined using a thermogravimetric analyzer (TGA) and a packed-bed reactor (PBR) respectively. The sorbents adsorbed CO2 from inert CO2 gas streams up to 7.3 wt.% at 65 °C and 10.7 wt.% at 25 °C, while adsorbing CO2 from simulated flue gas up to 7.4 wt.% at 65 °C. The amines are covalently bound in the sorbent, providing a robust material that is stable at elevated temperatures and does not leach upon exposure to moisture. The sorbents underwent extensive cycle testing, with no degradation observed up to 45 cycles on the TGA and up to 25 cycles on the PBR, while displaying rapid kinetics for both adsorption and desorption of CO2.

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1. Introduction The increasing industrial emissions of greenhouse gases is having a dramatic effect on global warming and climate change. Carbon dioxide, the main contributor to the greenhouse gas effect, is mainly produced from electricity generation and large manufacturing facilities, equating to 40% and 20% of global emissions respectively.1 Research and the implementation therein of cost-effective carbon capture and sequestration (CSS) technologies in large industrial settings has become an attractive solution to this problem. The selective separation and adsorption of CO2 from post-combustion gas streams has been accomplished at large scales using aqueous amine systems (e.g. monoethanolamine [MEA]).1,2 However, these liquid solvent systems to-date suffer from significant cost and high-energy usage requirements, averting their employment in industrial power plant settings. Solid sorbents have been proposed as a potential economically-feasible technology for the selective removal of CO2 from flue gas streams on the industrial scale.1,3,4 Solid sorbents may adsorb CO2 via a physisorption mode, common among metal-organic frameworks, activated carbon, and zeolite-based materials, and/or a chemisorption mode, which typically requires amine functionality to form carbamates. Solid sorbents generally benefit from low heats of regeneration, lower heat capacities, and favorable reaction kinetics with CO2.3 Specifically, amine-based solid sorbents provide no liquid waste5 and afford greater resistance to decomposition from common flue gas contaminants,4 while demonstrating high capacities at flue gas temperatures (50-80 °C) under conditions with moisture.1 Amine-based sorbents have been intensely investigated over the past decade. Amine solid sorbents are majorly prepared by one of three techniques: 1) physical impregnation of amine compounds into porous supports; 2) grafting of amines covalently to the support; or 3)


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condensation of amine and silica small molecules to form hyperbranched aminosilicas.1 The first reported solid amine sorbent for CO2 capture was disclosed by Tsuda and Fujiwara in 1992, in which polyethyleneimines (PEIs) and macrocyclic polyamines were covalently attached to silica gels.6 In 1995, Leal and coworkers attached 3-aminopropyl groups to silica gel for reversible capture of CO2 at room temperature.7 An important development in this field was made in 2002 when Xu and coworkers reported an impregnation of mesoporous silica supports (e.g. MCM-41) with PEI to achieve sorbents with high CO2 capacities.8 After these initial report, numerous silica and carbon amine-based sorbents have been prepared through the impregnation method to produce a wide array of cheaper and effective sorbents.9–13 The impregnation of amines in solid supports is the simplest preparative technique, however, these materials suffer from significant amine leaching once they are exposed to steam due to unstable physical interactions of the sorbent.14,15 To improve amine sorbent stability, the amines can be covalently tethered to a solid support by grafting condensation methods. Covalently linked amine materials are thermally stable and adsorb moderate to high capacities of CO2. This practice was first established by Huang and coworkers in 2003, grafting amines to MCM-48.16 Amine monomers can be polymerized with porous materials for covalent linking to the supports, as shown by Liang and coworkers in 2008, or by the pring opening polymerization of aziridine grafted to a SBA-15 support by Jones and coworkers in the same year.16–18 Recently, Barron and coworkers reduced the regeneration temperature of PEI-functionalized sorbents with carbon nanomaterial supports, obtaining capacities up to 8.0 wt.% at 25 °C with a low regeneration temperature of 75 °C.19 Although many amine-incorporated sorbents have been prepared to date, most do not provide high adsorption capacities, long-term stability, and practical preparative methods.4


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Our group has been investigating solid sorbents for the direct capture of CO2 from postcombustion gas streams to help tackle the environmental impacts of the industrial release of CO2.20,21 We recently disclosed the synthesis and properties of novel sorbents that utilize polyaldehyde

phosphorus

dendrimers

(P-dendrimers),

to

crosslink

polyethyleneimine

compounds to form solid materials for CO2 capture up to 13.6 wt.% (Scheme 1).22 Under simulated flue gas conditions, these solid sorbents can achieve high CO2 loading capacities, display fast adsorption and regeneration kinetics and can operate at a wide range of temperatures. Importantly, the amines are covalently bonded – thus no leaching occurs over long-term usage. In this report we examined the usage of less-expensive, short-chain polyamines to react with polyaldehyde P-dendrimers for CO2 capture. The most promising sorbent is composed of up to 14.9 wt.% nitrogen and achieved CO2 capacities up to 7.3 wt.% from inert CO2 streams and 7.4 wt.% under simulated flue gas streams when adsorbing at 65 °C. This sorbent has been tested through 45 cycles of adsorption and regeneration with no loss of activity observed. Notably, the sorbent could achieve capacities up to 10.7 wt.% CO2 at ambient temperature.

Scheme 1. General approach to forming cross-linked solid sorbent.

2. Methods and Materials Materials and Instruments. All solvents and reagents were reagent grade and used as received. Thin layer chromatography (TLC) analysis was run on silica gel plates purchased from EMD


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Chemical (silica gel 60, F254). 1H NMR and Avance (300 MHz for 1H, 121 MHz for

31

31

P NMR spectra were recorded on a Bruker

P). Chemical shifts are reported as parts per million

(ppm) using residual solvent signals as internal standard (CHCl3, δ = 7.26 ppm for 1H NMR). Data for 1H NMR were presented as follows: chemical shifts (δ, ppm), multiplicity (s = singlet, d = doublet, t = triplet, dd = doublet of doublets, m = multiplet), coupling constant (Hz), and integration. The chemical shifts of peaks found were reported for

31

P NMR spectra. Fourier

transformed infrared spectra were obtained on a PerkinElmer Spectrum 100FT-IR spectrometer on neat samples (ATR FT-IR). Scanning electronic microscopy (SEM) images were obtained using an FEI Quanta 200 variable pressure scanning electron microscope. Thermal stability measurements were conducted on a Mettler Toledo thermogravimetric analyzer (TGA) using a 5 °C/min step to 1000 °C under an air atmosphere. Nitrogen sorption isotherms at 77 K were obtained with a Micromeritics ASAP 2020 apparatus. Prior to measurement, the samples were degassed for 6 h at 120 °C. The surface area was determined assuming a surface coverage of the nitrogen molecule estimated at 13.5 Å. Carbon dioxide sorption isotherms were obtained at 0 °C. Elemental Analysis was conducted on an Elemental Analyzer Flash 2000 C/H/N/S instrument.

Preparation of Cross-linked sorbents. The phosphorous dendrimers were prepared using literature procedures,23–25 with more detailed experimental conditions and characterization found in the Supporting Information. The sorbent preparation was carried out in round-bottom flasks under air atmosphere with commercially available polyamines. The sorbent preparation for each P-dendrimer and polyamine was optimized (See Supporting Information for optimization). For example, the synthesis of 1-G0-TEPA proceeds as follows: To a 50 mL round bottom flask was added hexa(4-formylphenoxy)cyclotriphosphazene 1-G0 (0.5 g, 0.58 mmol, 1 equiv.) and


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tetrahydrofuran (10 mL, 0.06 M). The flask was heated until all solids were dissolved and left to stir open to air. While warm, a solution of TEPA (0.55 g, 2.9 mmol, 5 equiv.) in 3 mL tetrahydrofuran was added rapidly (under 5 s) to the above stirring mixture. In under 10 s, a white solid formed and was allowed to stir (or sit if the stir bar was frozen) for 1 hour. Then, the solids were isolated via vacuum filtration, washed with tetrahydrofuran (50 mL), crushed with a mortar and pestle, and placed in a new 100 mL round bottomed flask. The solids were suspended in 40 mL of tetrahydrofuran and 20 mL of methanol while stirring open to air. To this mixture was added anhydrous sodium borohydride (430 mg, 12 mmol, 20 equiv.) at room temperature and the reaction was left to stir for 14 hours under nitrogen. The mixture was then filtered under vacuum and the solid obtained was washed with 50 mL of distilled water, 50 mL of methanol, and 25 mL of diethyl ether. The washings produced a white powder that was further dried under reduced pressure, resulting in 470 mg of 1-G0-TEPA as a white powder.

Adsorption from Inert CO2 Gas. All CO2 capture analyses from inert CO2 gas streams were conducted with a thermogravimetric analyzer (TGA) from Mettler Toledo. The capacity of the sorbent was determined by the weight change of the sorbent after switching the carrier gas from a pure helium stream to a CO2 stream balanced with the carrier gas. In a typical experiment: a solid sorbent (5-30 mg) is placed on a pre-weighed platinum pan which is lowered into the TGA reactor. Under a helium atmosphere with a flow rate of 60 mL/min, the sample was heated to 120 °C for 10 minutes, at which point no more weight loss was observed. The reactor temperature was adjusted to 65 °C and then pure CO2 was introduced at a flow rate of 60 mL/min for 50 minutes (with 30 ml/min of helium balance gas). For multiple cycles, the sample would be


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reheated to 120 °C for 10 minutes to desorb CO2 under a helium atmosphere (60 mL/min) and the remaining procedure would be followed as described for as many cycles as was desired.

Adsorption from 15 vol% CO2 from Simulated Flue Gas. The CO2 capture analyses were performed on a packed-bed reactor (PBR) system designed and constructed by RTI International (RTI, North Carolina, USA) (Figure 1). The use of this packed-bed reactor system has previously been reported for simulating flue gas conditions, specifically of flue gas derived from a coal fired power plant (SCF).20 The feed gas composition was adjusted by changing the flow rate of individual gases controlled by mass flow controllers. Water vapor was introduced to the gas stream by flowing the mixture of air and N2 to the temperature controlled humidifier (A) to the preset moisture content. The CO2 stream combined with the N2/Air/H2O mixed gas at the outlet of the humidifier to avoid the CO2 from dissolving in the water in the humidifier, which would result in an increased acidity of the water and inaccurate CO2 content in the simulated flue gas stream. The effluent of the PBR (B) entered a condenser (C) and water collector (D) to remove water vapor prior to entering the CO2 analyzer. The CO2 concentration in the PBR effluent was analyzed using a Horiba CO2 analyzer (VA-3000). The PBR was operated autonomously by National Instruments Lookout program coupled with customized PLC control developed by RTI's engineers. In a typical experiment, approximately 1.5 g of adsorbent was mixed with silicon carbide beads, a filler and heat transfer material, then loaded into the PBR, which was made of a stainless steel column (1.27 cm inner diameter and 20 cm length). The test undergoes 5 stages: 1) feed test, 2) purge, 3) adsorption, 4) regeneration, and 5) cooling. The feed test stage allows the system to adjust the temperature of the process lines, humidifier, and reactor to defined values


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for the adsorption stage and to analyze and adjust the feed gas composition at the same flow rate being used in the adsorption stage. The feed gas was tailored to simulated flue gas conditions of flue gas from a coal fired power plant (SCF). The SCF flue gas has following composition: CO2 = 15 vol%, O2 = 4.5 vol%, and water vapor = 5.65 vol% in balance with N2. When the CO2 concentration in the feed gas is stable, the system will enter the purge stage by flowing N2 through the system until no CO2 is detected in the outlet gas of the reactor. The adsorption stage is initiated by switching a valve to allow CO2-laden feed gas to enter the reactor with a flow rate of 150 mL/min. The end of the adsorption process was determined by the breakthrough point for CO2 which occurs when CO2 concentration in outlet gas reached 99.95% of that in the feed gas. The end of the adsorption stage may also be determined by the minimum and maximum adsorption times, which was defined as 35 and 40 min. The regeneration stage follows the adsorption and is carried out by flowing humidified N2 through the reactor at the flow rate of 150 mL/min at a desired temperature. The regeneration end point was determined by the minimum or maximum desorption times of 25 and 30 min, respectively. The cooling of the system is performed by flowing N2 at the flow rate of 350 mL/min to reduce reactor temperature to either the temperature of the adsorption stage or to room temperature (if the tests are complete). The CO2 loading (wt.%) was calculated from regeneration data by dividing the total amount of CO2 desorbed by the mass of the adsorbent and multiplying with 100. The CO2 loading is defined as the amount of CO2, which desorbs at a specific regeneration condition and does not include CO2 lean loading. Each test consisted of 25 adsorption-regeneration cycles and the CO2 loading that is reported was taken as the average between all cycles.


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Figure 1. Packed-bed reactor (PBR) illustration. A, B, C, and D are humidifier, packed bed reactor, vapor condenser, and water collector, respectively

3. Results and Discussion Sorbent Design and Synthesis A priority in our sorbent design was to covalently tether amines to a solid network or support to prevent leaching when exposed to moisture present in flue gas. We believed that a compound with multiple electrophilic sites could react with a polyamine compound to covalently bind the amines as a cross-linked product. With this idea in mind, we became interested in electrophilic dendridic monomers because they can be designed to have an assortment of terminal reactive sites. Dendridic molecules have been successfully grafted onto solid supports with free amine functionalities to provide CO2-capturing solid sorbents.26–31 In 2013, two unsupported solid dendrimers, one with deposited Mg/Al ions on tethered hydroxyl groups and one with rigid triazene repeating units, were successful for capturing CO2 but suffered from poor capacities and regeneration properties.32,33 P-dendrimers are comprised of phosphorous atoms in the structural building blocks of the molecule and tend to make thermally robust materials (Figure 2).34 In 2016, Muhammed and


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coworkers reported a P-dendrimer with a melamine hemiaminal repeating unit that could achieve 18.9 wt.% CO2 at 25 °C under inert conditions.35 In the same year, Majoral, Caminade, and coworkers reported silica grafted P-dendrimers which attained low CO2 capacities when adsorbing at 30 °C.36 Neither report disclosed desorption kinetics with CO2, extended stability analysis, or the capture of CO2 under simulated flue gas conditions. Recently, we disclosed the preparation of CO2 solid sorbents from polyaldehyde P-dendrimers with PEIs which maintained an average CO2 capacity of 13.1 wt.% over 350 cycles of simulated flue gas testing with adsorption at 65 °C. In continuing our effort develop P-dendrimer sorbents for CO2 capture, we envisioned reacting terminal aldehyde containing P-dendrimers as cross-linking agents with less expensive, short-chain polyamine compounds (e.g. tetraethylenepentamine) to directly form a solid sorbent. The P-dendrimer repeating unit would allow for multi-reactive sites to promote cross-linking, as well as helping to improve the thermal stability of the sorbent. Due to the nonordered cross-linking reaction between the polyamines and polyaldehyde P-dendrimer, the use of short-chain polyamines could form sorbents with different characteristics and capacities than sorbents formed from PEI.


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O Ar =

Ar Ar N Ar P P Ar N N P Ar Ar

S Ar

P

O

Ar G0

Ar 2-Gx

Me N N P S O O

1-Gx Ar =

O

O

G1 O

Ar =

Me N N P S O O

O

G2

O S O P N N O Me O

N S N P O Me O

O

O

Figure 2. P-dendrimers used to produce solid polyamine sorbents.

Our evaluation began with P-dendrimer hexakis(4-formylphenoxy)cyclo(triphosphazene) (1G0) for its ease of synthesis, scalability, and 6 reactive sites.37 Tetraethylenepentamine (TEPA) was used as the polyamine component. The installation and solidification of TEPA was accomplished through a 1-pot, 2-step reductive amination procedure (Scheme 2). Addition of TEPA to 1-G0 instantaneously resulted in the formation of a white solid as a result of imine bond formation between the aldehydes and primary amines. The intermediate was separated, suspended in a mixture of THF/MeOH, and the imine bonds were reduced with sodium borohydride to yield sorbent 1-G0-TEPA as a white powder in 91% yield (see Supporting Information for optimization details). Infrared spectroscopic evidence showed the disappearance of the aldehyde C=O stretch at 1695 cm-1 with a gain at 1645 cm-1 for the imine C=N stretch, which upon reduction disappeared, supporting that the imine intermediates were successfully reduced in the solid state (Figure 3).


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Scheme 2. Synthesis of 1-G0-TEPA solid sorbent.

1-G0-Dendrimer

Aldehyde TEPA

Transmittance (%)

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

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Intermediate

Imine

1-G0-TEPA

Amine

New C-N

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Figure 3. Infrared spectra comparison of solid sorbent with the starting materials and reaction intermediate.

Temperature Effect on Adsorption Capacity The CO2 capacity of 1-G0-TEPA was analyzed via thermogravimetric analysis (TGA) with an adsorption temperature of 65 °C at 1 atm of CO2. Adsorption at 65 °C was selected for CO2 capture within common flue gas temperatures from the pulverized coal power plants.38 1-G0TEPA was found to adsorb 6.9 wt.% CO2 over 50 minutes at 65 °C (Figure 4). After 300 minutes of exposure to an inert CO2 stream, saturation of the sorbent was attained with 7.3 wt.%


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CO2 adsorbed. The adsorption kinetics with CO2 were fast, with 90% capacity achieved within the first 5 minutes of CO2 exposure (Figure 5a). Regeneration of sorbent proceed smoothly, with complete removal of CO2 realized within the first 5 minutes of heating at 120 °C under helium atmosphere. Impressively, this sorbent maintained full activity over 45 repeated cycles of adsorption and regeneration, with no degradation observed (Figure 5b). The complete removal of adsorbed CO2 in the regeneration cycle suggests that the amines react with CO2 via carbamate intermediates, not through irreversible urea formation. The temperature effect on the adsorption was then examined. The sorbent was found to adsorb 8.9 wt.%, 8.05 wt.%, and 6.07 wt.% CO2 at 25 °C, 45 °C, and 85 °C respectively (Figure 4). Breakthrough at 25 °C from 300 minutes of exposure attained a maximum capacity of 10.7 wt.% CO2. The high capacity of 1-G0-TEPA can be attributed to the 14.9 wt.% nitrogen composition of the material, determined via Elemental Analysis. The kinetics for CO2 capture were increased as the temperature was increased, as observed in Figure 4. However, the adsorption capacity decreases with elevated temperatures due to the exothermic adsorption of CO2 to amines.39 25 oC 45 oC 65 oC 85 oC

10

8 CO2 Capacity (wt%)

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6

4

2

0 0

10

20

30

40

50

Time (min)

Figure 4. TGA curves displaying the temperature effect on 1-G0-TEPA sorbent capacity.


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8

a)

o

Adsorption, 65 C

o

Desorption, 120 C

10

b)

1-G0

7 8 CO2 Capacity (wt%)

6 CO2 Capacity (wt%)

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

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5 4 3 2

6

4

2

1 0

0

0

100 200 300 400 500 600 700 800 900 Time (min)

0

5

10

15

20 25 Cycles

30

35

40

45

Figure 5. a) TGA CO2 adsorption/desorption over 10 cycles for 1-G0-TEPA from an inert CO2 stream. b) Average capacity over 45 adsorption/desorption cycles of 1-G0-TEPA from an inert CO2 stream.

Effect of P-Dendrimer on Sorbent Capacity A parametric study was performed to determine the best P-dendrimer and polyamine partner to produce a high adsorbing CO2 polymer. The number of reactive aldehyde functionalities on the P-dendrimer was increased through iterative transformations to grow the P-dendrimer into a larger, polymeric type compound with additional aldehyde functional groups; with each step termed a generation.40–42 Two expanded P-dendrimers of the first (1-G1) and second (1-G2) generations were prepared, providing compounds with 12 and 24 terminal aldehydes respectively (Figure 1). Polymers prepared from those generations attained capacities at 65 °C of 4.0 wt.% for 1-G1-TEPA and 2.1 wt.% for 1-G2-TEPA (Figure 6a). The sorbents made from 1-G1 and 1G2 were stable over repeated cycle testing, with no decrease of CO2 capacity observed over 10 cycles (Figure 6b). Notably, as the P-dendrimer generation was increased, the overall CO2 capacity was observed to decrease. The decrease in capacity for sorbents of generation 1 and 2


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P-dendrimers may be due to the formation of more densely cross-linked sorbents (folding and coalescing), formed from the increase in number of aldehyde reactive sites, preventing amine accessibility to CO2. Additionally, as the P-dendrimer generation increases, the overall mass of the P-dendrimer molecule increases which can result in a lower capacity of CO2 by weight.

b)

7 1-G0

6

1-G1 5

1-G2 2-G0

4

2-G1

3 2

7 6

CO2 Capacity (wt%)

a)

CO2 Capacity (wt%)

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

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1-G0 1-G1

5

1-G2 2-G0

4

2-G1

3 2 1

1

0

0 0

10

20 30 Time (min)

40

50

0

1

2

3

4

5 6 Cycles

7

8

9

10

Figure. 6. P-dendrimer generation and geometric effect on sorbent CO2 capacity over a) 50 minutes and b) 10 repeated cycles.

P-dendrimer 2-G0 and its first generation (2-G1) were synthesized to examine the effect of the P-dendrimer precursor. These molecules have terminal aldehydes that are more localized in space than 1-G0, which should affect the cross-linking reaction. The optimized CO2 capacities for these sorbents were 1.2 wt.% and 1.7 wt.% CO2 at 65 °C, dramatically lower than sorbents prepared from 1-G0 and 1-G1 (Figure 5). Both 1-G0 and 2-G1 have 6 terminal reactive sites, with 1-G0 shaped like an hourglass and 2-G1 pyramidal shaped.43 The geometry of 2-G0 potentially inhibits polymer growth by acting as an end-cap to oligomers, which is supported by lower 11.4 wt.% nitrogen content of 2-G1-TEPA.36


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The structural morphology of these sorbents was examined with N2 adsorption-desorption isotherms (Table 1). Due to a non-ordered cross-linking polymerization, the sorbents have low surface areas of 12.7 – 19.5 m²/g (measured by BET analysis). These materials appear to be nonporous, based on the small pore volumes measured on average of ~0.005 mL/g and pore sizes between 11.3 – 17.1 Å. Notably, the nitrogen content of the material is similar between the best performing sorbents from each P-dendrimer generation. As the P-dendrimer generation was increased from 1-G0 to 1-G2, the surface areas and pore sizes decrease in the bulk material, highlighting that the amines are less to the CO2 in the later for 1-G1 and 1-G2 in comparison with

1-G0.

Table 1. Textural and physical analysis of 1-Gx-TEPA sorbents.

Pore diameter (Å)a

Pore volume, Vp [mL/g]a

Surface area, SBET [m2/g]a

Nitrogen content [wt.%]b

1-G0-TEPA

14.1

0.0069

19.5

14.9

1-G1-TEPA

17.1

0.0054

12.7

15.1

1-G2-TEPA

11.3

0.0050

17.7

14.7

Name

[a] Values determined from nitrogen adsorption-desorption isotherms. [b] Values determined from CHN Analysis.

Scanning electron microscopy (SEM) was used to understand the textural composition of the polymer network and bulk molecule organization. SEM revealed that the molecules pack in both small and large aggregates and look amorphous (Figure 7). The aggregates appear to stack as flakes and no major textural difference was observed between the other prepared sorbents. The random and rapid polymerization with polyamines unsurprisingly forms a nonporous and unordered material.


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Figure 7. Scanning electron microscopy images of a) 1-G0-TEPA (20 µm), b) 1-G0-TEPA (50 µm), c) 1-G1-TEPA (50 µm), and d) 1-G2-TEPA (100 µm).

Effect of Amine Component on Sorbent Capacity Sorbents could be prepared from other polyamine compounds as well, with diethylenetriamine (DETA) and ethylenediamine (EDA) each reacting with 1-G0 to form solid sorbents. The capacities obtained were 1.4 wt.% and 2.6 wt.% CO2 for 1-G0-DETA and 1-G0-EDA respectively over the course of 10 cycles (Figure 8). A branched-amine derivative, tris(2aminoethyl)amine (termed Triamine), underwent the polymerization to afford a sorbent with 1.4 wt.% CO2 capacity. In contrast to the cross-linked sorbents produced herein, previously reported silica-supported amine functionalized P-dendrimers had capacities of 1.2 wt.% CO2 from inert gas streams (Figure 8).36 The crosslinking reaction between P-dendrimers and polyamines was designed to create a solid sorbent with a high amine loading for maximum CO2 capture. Evaluation of the amine efficiencies of the polyamine P-dendrimer sorbents revealed that a significant portion of the amine content of the sorbent was inactive for CO2 capture, with the best


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performing sorbent 1-G0-TEPA having an amine efficiency of 0.22 (Table 2). The nonporous Pdendrimer polyamine sorbents likely offer active amines only on the surface of the sorbent. A majority of the amine content is beneath the surface of the sorbent and is incapable of capturing CO2. The amine efficiencies of the crosslinked polyamine P-dendrimer sorbents have similar results with the hyperbranched aminosilicas reported by Jones and coworkers which were also prepared by an unselective polymerization reaction.45 Sorbents prepared from polyamine compounds with less amine content (e.g. EDA) unsurprisingly resulted in sorbents with decreased CO2 capacities (Table 2). The active amine content was generally predictive of the CO2 capacity of the sorbent. Sorbents formed from DETA were expected to exceed EDA sorbents in capacity, however, as seen in Table 2, the DETA sorbent had a low active amine content of 1.56 mmol/g versus the EDA sorbent with 4.96 mmol/g. This result highlights the non-ordered crosslinking reaction that takes place between the polyamines and polyaldehyde dendrimer.

10 9 8 CO2 Capacity (wt%)

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

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7

6.9

6 5 4

4 3

2.6

2.2

2

1.4

1.4

1.2

1 0 1-G0 -TEPA

1-G1 -TEPA

2-G0 -TEPA

1-G0 -DETA

1-G0 -EDA

1-G0 Silica Grafted -Triamine Dendrimers

Solid Sorbent

Figure 8. P-dendrimer solid sorbents for CO2 capture.


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Table 2. CO2 adsorption capacities, active amine content, and amine efficiencies P-dendrimer derived solid amine sorbents from inert CO2 gas streams. CO2 Adsorption Capacity [mmol/g]

CO2 Active Amine Content [mmol/g]a

Amine Efficiency [mol CO2/mol amine]

1-G0-TEPA

1.55

7.17

0.22

1-G1-TEPA

0.91

5.26

0.17

1-G2-TEPA

0.48

4.16

0.11

2-G0-TEPA

0.27

9.39

0.03

2-G1-TEPA

0.39

1.69

0.23

1-G0-DETA

0.32

1.56

0.20

1-G0-EDA

0.59

4.96

0.12

1-G0-Triamine

0.32

4.37

0.07

Name

[a] Amine content calculated from sample CHNS Analysis and subtracting the inactive nitrogen components of the P-dendrimer subunit.

Sorbent Physical Properties In general, the solid sorbents prepared from P-dendrimers were stable, maintaining full CO2 capacity over 10 cycles of the temperature swing measurements. The stability is largely attributed to the covalent linking of amine compounds via C–N bonds to the P-dendrimer from the reductive amination procedure. These sorbents were examined for thermal stability and the TGA curves show two weight loss events at 240 °C and 510 °C (Figure 9). From 240 – 400 °C, the carbon-carbon bonds begin fragmenting alongside the oxidation of the amines, and after 510 °C the organics begin burning off leaving behind the P-dendrimer core. The 1-G0-TEPA sorbent has a higher mass composition of the thermally stable P=N heteroatoms, which leads to an increased char yield compared to the 1-G1-TEPA sorbent.34 The resistance of these materials to


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Energy & Fuels

thermal decomposition is notable. Thermal degradation of amine impregnated sorbents occurs commonly above 150 °C.8,46–49 Only grafted amine sorbents, which have lower CO2 capacities, have comparative thermal stabilities above 200 °C.50,51 The P-dendrimer sorbents are also insoluble in both organic and aqueous media, a trait common among cross-linked polymers.52 Moreover, the sorbents are stable to both strong acids and strong bases. Exposure of 1-G0-TEPA to either a 12 N HCl or 50 wt.% NaOH solution over the course of 3 months did not dissolve or degrade the sorbent.

100

1-G0-TEPA

90

1-G1-TEPA

80 70 Weight (%)

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

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60 50 40 30 20 10 0 0

100 200 300 400 500 600 700 800 900 1000 o

Temperature ( C)

Figure 9. TGA curves under air of a) 1-G0-TEPA and b) 1-G1-TEPA heated at a rate of 5 °C per min from 25 to 1000 °C.

Cyclic Adsorption/Desorption Testing Under Simulated Flue Gas After the conclusion of P-dendrimer sorbent screening, the highest performing sorbents, 1-G0-

TEPA and 1-G1-TEPA, were selected for CO2 capture from simulated flue gas. Carbon capture from flue gas is particularly challenging due to: a) the low concentration of CO2 in the gas stream (~15 vol%); b) the necessity for the selective adsorption of CO2 over other molecules; c)


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stability requirement with oxygen and water; and d) an operating temperature range of 50-75 °C for adsorption and 100 – 120 °C for regeneration.53 Using a packed-bed reactor (Figure 1), 1-G0-

TEPA was exposed to a gas stream consisting of 14.77% CO2, 5.65% H2O, and 4.5% O2 balanced by nitrogen at 65 °C for capture. Regeneration of the sorbent proceeded at 120 °C using 5.65% H2O balanced by nitrogen. The adsorption/desorption process was carried out over 25 cycles, with 1-G0-TEPA maintaining an average capacity of 7.4 wt.% CO2 (Figure 10). The capacity was preserved over 25 cycles with negligible change in its chemical composition (Table

3), highlighting the stability of this sorbent under cyclic conditions. However, 1-G1-TEPA captured only 4.2 wt.% CO2 over 25 cycles.

10 1-G0-TEPA 1-G1-TEPA 8 CO2 Capacity (wt%)

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Energy & Fuels

6

4

2

0 0

5

10

15

20

25

Cycles

Figure 10. Simulated flue gas capture by 1-G0-TEPA and 1-G1-TEPA P-dendrimer solid sorbents.


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Table 3. Elemental analysis of 1-G0-TEPA before and after 25 cycles of CO2 capture under simulated flue gas conditions.[a] 0 Cycles

25 Cycles

N: 14.9%

N: 14.8%

C: 53.4%

C: 53.8%

H: 6.6%

H: 6.8%

[a] Amine content calculated from sample CHNS Analysis and subtracting the inactive nitrogen components of the P-dendrimer subunit.

Notably, 1-G0-TEPA and 1-G1-TEPA reached higher capacities under the flue gas conditions than from the inert CO2 gas stream. Both 1-G0-TEPA and 1-G1-TEPA sorbents are composed mainly of secondary amines, which have been shown to oxidize faster than primary amine-based sorbents.27 Over 25 cycles no degradation was observed with simulated flue gas containing 4.5 vol% O2, however, more extensive cyclic testing is required to evaluate the potential oxidative degradation. The reaction between the sorbents’ amines with CO2 and water may result in both bicarbonate and carbamate formation as well as preventing urea formation, providing an increase in adsorption capacity. The trend of increasing capacity under humid conditions with aminedense sorbents has been previously observed.18,54–56 The 5.65 vol% water in the simulated flue gas stream is believed to be adsorbed by the solid sorbent and fully removed during the regeneration cycle. The water in the gas stream improves the CO2 capacity of the sorbent, does not damage the integrity of the sorbent simply because the amine is covalently bonded to phosphorus dendrimer core, and does not compete for reactive sites with the amines. The adsorption of CO2 in simulated flue gas by 1-G0-TEPA in the packed-bed was rapid. Fast kinetics were observed throughout the entirety of the cycle testing, as shown in Figure 11 and


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Figure 12. Exposure of the sorbent to the simulated flue gas at 65 °C promoted adsorption at a rate of 1.6 wt.% per min for the first 5 minutes, with 98% of the sorbent’s maximum capacity attained within the first 10 minutes. These results show that the adsorption of CO2 is not kinetically limited. Similarly, the regeneration of the sorbent proceeded quickly. As the packedbed reactor’s temperature was ramped upward CO2 began desorption at 70 °C and after holding the regeneration temperature at 120 °C, 76% of the CO2 was desorbed within the first 10 minutes of thermal treatment. Importantly, the kinetics of the adsorption and regeneration of the sorbent were not changed over 25 cycles. The first cycle of adsorption and regeneration with 1-G0-

TEPA was slightly slower in both instances compared to cycles 10 and 25. It is likely that during the first cycle both adsorbed water and CO2 promote a rearrangement of the sorbent’s amines which provides even faster kinetics in subsequent cycles. 10

130

120

8

6

4

100

90

80

o

Cycle 1 Cycle 10 Cycle 25 Temperature

Temperature C

110

CO2 Capacity (wt%)

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

Energy & Fuels

2 70

0 60

1

6 11 16 21 26 31 36 41 46 51 56 61 66 71 Time (min)

Figure 11. Adsorption and regeneration CO2 traces for cycles 1, 10, and 25 using 1-G0-TEPA on a packed-bed reactor.


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Energy & Fuels

8 7 6 CO2 Capacity (wt%)

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

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5 4 3

Regeneration Adsorption Cycle 1 Cycle 1 Cycle 10 Cycle 10 Cycle 25 Cycle 25

2 1 0 0

2

4

6

8

10

Time (min)

Figure 12. Kinetics of adsorption and regeneration of 1-G0-TEPA capturing simulated flue gas.

4. Conclusions Numerous amine-based materials have been developed for the capture of CO2 from flue gas streams. Generally, these sorbents are derived from the impregnation of amines into the pores of a support or grafting of the amines to silica or polymer supports. Much focus in the field has been placed on impregnated sorbents, which can be easily prepared and achieve high capacities. Amines impregnated into supports (e.g. MCM-41, SBA-15, carbon black, alumina, etc) have achieved capacities up to 17.2 wt.% CO2 from pure CO2 gas streams at 75 °C.1 Application of amine solid sorbents for simulated flue gas capture is more rare.4 Two of the highest performing materials, MCM-41 impregnated with PEI (13.6 wt.% CO2 at 75 °C)10 and commercially available Zeolite Y60 impregnated with TEPA (18.9 wt.% CO2 at 60 °C),57 can adsorb high capacities, but they suffer from decreased thermal stabilities and leaching of the amines from water solvation. To prevent leaching, amines have been grafted to supports (e.g. 3aminopropyltriethoxysilane), however, these sorbents are often limited by lower degrees of


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Energy & Fuels

amine functionality causing decreased capacities (

Phosphorus Dendrimer Derived Solid Sorbents for CO2 Capture from

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