Chemical Engineering Journal 221 (2013) 166–175

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Chemical Engineering Journal

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Post-spinning infusion of poly(ethyleneimine) into polymer/silicahollow fiber sorbents for carbon dioxide capture

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.01.086

⇑ Corresponding authors. Tel.: +1 404 385 1683; fax: +1 404 894 2866.E-mail addresses: cjones@chbe.gatech.edu (C.W. Jones), william.koros@chbe.

gatech.edu (W.J. Koros).

Ying Labreche a, Ryan P. Lively b, Fateme Rezaei a, Grace Chen a, Christopher W. Jones a,⇑,William J. Koros a,⇑a School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332-0100, USAb Algenol Biofuels, 28100 Bonita Grande Drive, Bonita Springs, FL 34315, USA

h i g h l i g h t s

" Polymer/silica/PEI hollow fibers are proposed as sorbents for CO2 capture." CA/silica/PEI fibers are made via a post-spinning amine infusion technique." Effects of synthesis conditions on CO2 capacity are determined." Multi-fiber module used to monitor CO2 and water adsorption from simulated flue gas." Amine-containing fibers may be useful in RTSA CO2 capture process.

a r t i c l e i n f o

Article history:Received 3 December 2012Received in revised form 25 January 2013Accepted 28 January 2013Available online 6 February 2013

Keywords:PEIHollow fiber sorbentFlue gasCO2 captureRTSA

a b s t r a c t

Amine-loaded hollow fiber sorbents for CO2 capture from dilute gas streams are created using a novelpost-spinning amine-infusion technique. This technique infuses poly(ethyleneimine) (PEI) into celluloseacetate/mesoporous silica hollow fiber sorbents during the solvent exchange steps after dry-jet,wet-quench, non-solvent induced phase separation spinning. A suitable post-spinning infusing solutionwas found to be 10% PEI in methanol with an infusion time of 4 h. After amine infusion, the 51 wt% silicahollow fiber sorbents are demonstrated to have a nitrogen loading of 0.52 mmol/g-fiber and a CO2 uptakeof 1.2 mmol/g-fiber, at equilibrium. Amine-loaded fibers are packaged into a shell-and-tube module andexposed on the shell side to simulated flue gas with an inert tracer (10 mol% CO2, 80 mol% N2 and 10 mol%He at 100% relative humidity; 1 atm, 35 �C). The fibers are shown to have a breakthrough CO2 capacity of0.58 mmol/g-fiber and CO2 uptake after 20 min of 0.92 mmol/g-fiber (1 atm and 35 �C). Under the sameconditions, the water uptake was found to be 3.2 mmol/g-fiber. The preparation of amine-containingpolymeric hollow fibers and demonstration of their CO2 adsorption properties is an important steptowards realizing new, scalable process configurations for supported amine sorbents relevant topost-combustion CO2 capture.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The combustion of fossil fuels for electricity generation will playa key role in our global energy supply for the foreseeable future;however, fossil fuel combustion results in the emission of largequantities of CO2. Rising atmospheric CO2 concentration may havenegative impacts on the environment, contributing to climatechange and acidification of oceans [1–3]. Post-combustion carboncapture and storage (CCS) has been proposed as a method forreducing CO2 emissions [4,5]. Currently, absorption processes

based on alkanolamine/water solutions can be used for CO2 cap-ture [6]; however, these techniques are encumbered with thedeleterious effects of equipment corrosion, alkanolamine toxicity,and high energy consumption [7,8]. Furthermore, water is a non-contributing thermal ballast, amines are volatile, and waterevaporates during stripping, resulting in latent heat of vaporizationlosses. Consequently, processes based on solid adsorbents thathave (i) high CO2 capacities, (ii) high CO2 selectivities, (iii) goodadsorption/desorption kinetics, and (iv) good sorbent regenerabil-ities have been proposed as potentially energy-efficient alterna-tives to liquid absorption processes [6,9–13].

Adsorption processes utilizing temperature or pressure swingsorbent regeneration strategies have been demonstrated as poten-tially lower cost alternatives to absorption with amine solutions

Table 1Silica sources and their properties.a

Silica A (ES757) B (PD9024) C (C803)

Bulky density (g/cm3) 0.22–0.26 0.16–0.18 0.07–0.60Particle size (lm) 25 6.5 3.8Pore diameter (nm) 19.5–21.5 11.0 18.5–20Pore volume (cm3/g) 1.13 1.04 0.85BET surface area (m2/g) 195–295 294–400 209Supplier PQ corporation PQ corporation W.R. Grace

a Code names in parentheses are manufacturers’ code names.

Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175 167

[6,14–16]. Sorbents that selectively adsorb CO2 – including amine-modified silica materials [11,12,17–20], amines supported on por-ous carbon [21,22], amines supported on other metal oxides, suchas alumina [23,24], zeolites [25–30], metal oxides [9] and metal–organic frameworks [31–34] – have been investigated extensively.Each of these sorbents has some limitations, as described below.

MOFs and zeolites are typically hydrophilic, and their applica-tions toward CO2 separation from flue gases often necessitates adrying stage. To circumvent such drying, significant efforts havebeen devoted to identifying materials that do not contain hydro-philic adsorption sites. Hydrophobic microporous solids are morerobust towards the presence of water vapor [35], but tend to havelower CO2 uptakes. Metal oxides, which chemically absorb CO2, of-ten require high temperatures for regeneration [9]. For theseadsorbents, flue gas reheating may be required, thus decreasingthe energy efficiency of the process.

Amine adsorbents, although promising under controlled labora-tory testing conditions typical of academia, can be degraded undermore practical processing conditions. For example, amine materi-als can be degraded under high temperature conditions in concen-trated CO2 to form urea [36–38], although Sayari has shown thiscan be mitigated either by choice of amine type or the presenceof humidity [38,39]. In addition, amines can be degraded underoxidizing conditions and at elevated temperatures, again as a func-tion of amine type [40,41], although typical oxygen concentrationsin flue gas are in a modest range that does not make oxidationhighly problematic. Amine-modified silica materials can also de-grade in concentrated steam [42].

As chemisorbents, amine adsorbents also have relatively highheats of adsorption, meaning that effective large scale processeswill require efficient heat-integration to capture the significantadsorption exotherms. Amine adsorbents are most often studiedin fixed bed processes at the laboratory scale [43], where previouswork has shown that heat effects do not impact the adsorptiondynamics [44,45]. However, on a process scale, fixed beds are un-likely to allow for the heat integration needed and thus alternateprocess designs are needed that allow for heat integration and topotentially mitigate the stability shortcomings of amine adsor-bents described above.

Recently, Lively and Koros introduced the use of polymeric/inorganic hybrid hollow fiber contactors loaded with CO2 adsor-bents as a new, scalable process configuration for post-combustionCO2 capture [46]. In this approach, polymeric hollow fibers deriva-tive of those already prepared on commercial scales for gas separa-tion membranes are prepared and loaded with large volumes ofsolid sorbent materials with high CO2 affinities. Unlike traditionalpolymer fibers for membrane applications, these hollow fibershave several unique aspects to their design. First, very high vol-umes of adsorbent materials are included, typically 60–75% by vol-ume. Second, the polymeric phase is designed to have an open andbicontinuous pore network, allowing for rapid mass transferthrough the fiber wall to the supported sorbent particles. Third, adense lumen layer is installed in the fiber bore to restrict transportfrom the shell side of the fibers to the bore and vice versa. This de-sign yields fibrous structures that are ideally suited for applicationas combined sorption and heat transfer devices in a rapid temper-ature swing adsorption (RTSA) process [47,48]. Cycle times are ex-pected to be on the order of 3–4 min. This RTSA approach usingcomposite hollow fibers for post-combustion CO2 capture wasoriginally demonstrated by Lively and Koros using cellulose acetatefibers and zeolite 13X as the adsorbent in the fibers [46,48].

This hollow-fiber platform appears well-suited for use withamine adsorbents. Amine adsorbents would benefit from a pro-cess design that allows for efficient recovery of the adsorptionexotherms, which is facilitated by the hollow-fiber structure. Inaddition, by separating the heating and cooling fluids from the

sorbents behind the bore-side lumen layer, direct contacting ofaminosilica adsorbents with steam can be avoided [42]. In thiswork, solid supported amine adsorbents are reported as thefiller phase in hollow fiber sorbents for the first time. It isdemonstrated that the direct-spinning of polymeric hollowfibers containing pre-synthesized aminosilica materials can beproblematic. However, hollow fibers spun using commercial CAand commercial (amine-free) mesoporous silica components aresuccessfully spun and these fibers are subsequently infused withan amine solution during the hollow fiber formation solvent ex-change step. These amine-loaded hollow fiber sorbents are evalu-ated for CO2 adsorption in gravimetric systems and are loadedinto shell-and-tube modules for testing under simulated fluegas conditions, as the first step in developing a RTSA amine/hol-low fiber process concept.

2. Experimental

2.1. Materials

Cellulose acetate (CA) (MW 50 000, Sigma–Aldrich) andpoly(vinylpyrrolidone) (PVP) (MW 55 000, Sigma–Aldrich) wereused for formation of the hollow fiber sorbents. All polymers weredried in vacuum at 110 �C for 1 day to remove moisture before use.N-methyl-2-pyrrolidone (NMP) (Reagent Plus 99%, Sigma–Aldrich)was used as the solvent for the polymer-spinning dope. Methanol(MeOH) (99.8%, ACS Reagent, Sigma–Aldrich) and hexanes (ACS Re-agent, >98.5%, Baker) were used for the solvent exchange portion ofthe fiber formation process, which occurs after spinning. All sol-vents and non-solvents were used as-received with no purificationor modification. Poly(ethyleneimine) (PEI) (MW 800, Sigma–Aldrich) was used as the amine source in the post-spinning amineinfusion step, as discussed below. Three different commercial silicamaterials were used in this work. The name, source and propertiesof the silica materials are listed in Table 1.

2.2. Solid supported amine adsorbents

PEI was dispersed in methanol by stirring at room temperaturefor 1 h (ratio of 20 mL methanol/g silica) and then dried silica pow-der was added to the solution followed by stirring overnight.MeOH was removed by rotovap at a bath temperature of 40 �C.This produced ‘‘class 1’’ aminosilica materials [12,17], which weredried in a vacuum oven. The class 1 sorbent powders were soni-cated in NMP and the material characteristics before and afterthe sonication were evaluated. The sorbents were sonicated in a(polymer-free) dope solution (NMP/water weight ratio ca. 8) for35 s using a 100 W sonication horn and then the solids were recov-ered and analyzed via N2 (physisorption at 77 K) and CO2 adsorp-tion experiments.

2.3. Fiber formation

A typical spinning dope contains polymer CA, sorbent particles,NMP (solvent), water (non-solvent), and additives (PVP). The

Table 2Dope composition, wt% (all the class 1 aminosilica loadings are 50 wt%).a

Code F-Ab F-A/PEIc F-B F-B/PEI F-C F-C/PEI

Silica type A (ES757) A class1 B (PD9024) B class1 C (C803) C class1CA 9.33 12.50 10.60 12.50 10.09 11.11PVP 3.73 5.00 4.24 5.00 4.05 4.44Silica 13.98 12.50 15.54 12.50 13.38 22.22NMP 64.23 61.63 61.50 61.63 64.19 54.79Water 8.73 8.37 8.12 8.37 8.29 7.44

a All fibers in the table were prepared with the traditional solvent exchangemethod (three times water, three times MeOH and three times hexane), asdescribed above.

b F-X – denotes bare silica fiber. X is A, B or C.c F-X/PEI – denotes a class 1 aminosilica fiber. X is A, B or C.

Table 3Spinning parameters.

Air gap height 3 cmTake-up rate 5–20 m/minQuench bath medium and temperature H2O, 25–50 �CSpinneret temperature 25–50 �CBore fluid composition 20/80 NMP/H2OExtrusion rate Core: 200–1000 mL/h, bore:

50–500 mL/h

168 Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175

polymers and fillers (silica or class 1) were dried at 110 �C in a vac-uum oven overnight prior to use. Bare silica or class 1 aminosilicafillers were added to 80% of the required NMP/water and sonicatedusing a 100 W sonication horn. The mixture was stirred and soni-cated alternately for 1 h to obtain a well dispersed suspension. A‘‘prime’’ dope was made from 20% polymers and 20% of the re-quired NMP/water and was stirred for 48 h on a roller. The dis-persed silica mixture and prime dope were mixed together. Thismixture was stirred and sonicated alternately for 1 h and thenthe rest of polymers were added and mixed with mechanical stirrerfor 4 h at 50 �C to completely dissolve the polymer to form the finalspin-ready dope. The hollow fibers were formed using a non-sol-vent phase inversion technique commonly referred to as ‘‘dry-jet,wet-quench spinning’’ [49]. Spinning dopes were co-extruded witha bore fluid through a spinneret into a non-solvent water quenchbath to induce phase separation and form a porous fiber. The finalpolymer dope solution conditions chosen are listed in Table 2. Fi-bers were spun with bare silica particles, as well as class 1 amino-silica materials, pre-loaded with PEI. The fibers were removed fromthe take-up drum by clean cuts using a sharp blade and then they

Fig. 1. Solvent exchange-based PEI infu

were placed in a bath of deionized water for 3 days – wherein thewater was changed every day – to remove residual solvent. Afterthe third day, the fibers (approximately 75 g) were solvent ex-changed by immersing in three successive aliquots (400 mL) ofmethanol for 20 min each followed by three successive aliquots(400 mL) of hexanes for 20 min each. Fibers were removed fromthe bath and allowed to dry in a fume hood in air for 1 h, thenplaced in a vacuum oven and dried for 2 h at 100 �C. The completespinning conditions are given in Table 3.

2.4. Post-spinning infusion of PEI in fiber sorbents

The post-spinning infusion of PEI into CA/bare silica fibers isschematically shown in Fig. 1. After spinning the fibers, the fiberswere subjected to a methanol solvent exchange process, followedby treatment in methanol/PEI. Subsequently, the fibers were ex-changed with hexanes, and then the fibers were dried. As an exam-ple, a 57 wt% C803 silica/CA hollow fiber sorbent was made usingthe aforementioned spinning dopes and procedures. The fiberswere removed from the take-up drum by cutting cleanly using asharp blade and then they were placed in a bath of deionized waterfor 3 days, wherein the water was changed every day, to removeresidual solvent. After the third day, the fibers (approximately75 g) were solvent exchanged by immersing in two successive ali-quots (400 mL) of methanol for 20 min each, followed by treat-ment (with varying immersion times, 1–20 h) in either (i)400 mL of methanol or (ii) 20 wt% PEI/methanol. Next, the fiberswere subjected to 1 h immersion in 400 mL of hexanes before dry-ing. Fibers were removed from the bath and allowed to dry in afume hood in air for 1 h, then placed in a vacuum oven and driedfor 2 h at 100 �C.

2.5. Materials characterization

The final silica content of the hollow fiber sorbents was deter-mined by thermogravimetic analysis (TGA) (NETZSCH InstrumentsSDT 409 PG) via combustion of the organic component in air. Aseparate TGA (TA Instruments SDT Q500) was also used for CO2

adsorption characterization using a gas composed of 0.1 atm ofCO2 and 0.9 atm of He. The samples were heated first to 120 �C un-der He flow to remove moisture, and then cooled to 35 �C undernitrogen flow. The dry CO2 containing gas (30 mL/min) was flowedover the samples during the sorption tests. A weight gain was ob-served due to CO2 absorption in the sample, and the uptake pergram dry silica or dry fiber (mol/g-fiber) was calculated as theCO2 capacity.

sion CA/silica hollow fiber process.

2

2

2

bed

upstream

downstream2

2

2

2 2

bed

Fig. 2. Multicomponent competitive adsorption system.

Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175 169

Scanning electron microscopy (SEM) (Leo 1530, Leo ElectronMicroscopy, Cambridge, UK) was used to evaluate the fiber sorbentpore structure and the polymer–silica interfaces. The fibers weresevered and put on the SEM sample holder for cross-sectionalimages. The samples were sputter-coated with a 10–20 nm thickgold coating (Model P-S1, ISI, Mountain View, CA) before beingtransferred to the SEM for imaging.

Elemental analysis was performed by Columbia Analytical Ser-vices, Inc. (Kelso, WA).

2.6. Fiber module characterization in a simulated flue gas flow system

Fibers were cut and mounted in modules composed of four fi-bers per module. Characterization of the fiber modules was per-formed in a simulated flue gas flow system similar as reported ina previous work [50]. The fiber module was degassed under flow-ing N2 at 110 �C prior to CO2 adsorption tests. Adsorption experi-ments were carried out by flowing a simulated flue gas (10 mol%CO2, 80 mol% N2 and 10 mol% He at 100% RH) through the shell sideof the temperature-controlled hollow fiber adsorbent module at60 mL/min at 37 �C, 1 atm (gas space hourly velocity: 4500 h�1).Helium was used as an inert tracer, N2 acted as the carrier gas,and CO2 was the adsorbate of interest. The concentration of CO2

at the module outlet was transiently measured by mass spectrom-etry (ultrahigh vacuum Pfeiffer Vacuum QMS 200 Prisma Quadru-pole Mass Spectrometer) until equilibrium was reached. The totalCO2 uptake was found by integrating the area bounded by theCO2 and He elution molar flow rate fronts (the He tracer accountsfor the mean residence time within the system) (see Fig. 2).

3. Results and discussion

Silica-supported amine adsorbents have been classified intothree general types [12,51]. Class 1 materials composed of aminepolymers (PEI) physically loaded into porous silica supports wereused here. Two approaches to preparation of class 1 aminosilica/CAhollow fiber sorbents are described. In the first ‘‘pre-synthesized’’approach, class 1 aminosilica materials were spun with CA to

create hollow fiber sorbents. In the second approach, which wasmore successful, hollow fibers were spun from CA and bare silicasupports, and PEI was introduced after the fibers were spun by anovel post-spinning PEI-infusion process during the solventexchange steps, which is described here for the first time.

Three different commercial porous silicas were used. Effectiveclass 1 sorbents require supports with pore openings large enoughto allow entry of the amine polymers and large pore volumes toprovide sufficient amine capacity. For fiber spinning applications,it was further expected that the support particle size would havean effect on the ability to spin homogeneous fibers. To this end, arange of commercial silica materials with appropriate porositycharacteristics but varying particles sizes, labeled silicas A, B andC in Table 1, were used.

The hollow fiber sorbents were formed using a non-solventphase inversion technique commonly referred to as ‘‘dry-jet, wet-quench spinning’’ [49]. Spinning ‘‘dopes,’’ containing CA, sorbentparticles, NMP (solvent), water (non-solvent), and additives (PVP)were co-extruded with a bore fluid through a spinneret into anon-solvent water quench bath to induce phase separation andform a porous fiber. PVP was chosen as a pore former due to itsmacrovoid suppression properties, as well as its ability to promotepore network formation [52]. The function of the bore fluid was tokeep the fiber open and also to induce a phase transition and con-trol fiber morphology near the inner surface through phase inver-sion. To maximize mass transfer rates to the sorbents, the so-called‘‘sieve in a cage’’ morphology is desirable, whereby the dispersedparticles are not attached at all points on the particle surface tothe polymer matrix [46].

The polymer dope solutions used to spin various fibers arelisted in Table 2. The complete fiber spinning conditions are givenin Table 3 and these parameters are the primary factors that deter-mine final fiber size. Typically, the desired outer diameter is con-trolled by adjusting the draw ratio (which is the ratio of theextrusion rate to the take-up rate). The ultimate outer-to-inner fi-ber radii ratio is largely determined by the polymer-to-bore volu-metric flow rate ratio. The membrane structure, pore size andpore size distribution are determined by many factors including

0.0

0.5

1.0

1.5

2.0

2.5

CO

2C

apic

ity (m

mol

/g

sorb

ent o

r fib

er)

Powder or fiber sample

Fig. 4. Dry weight normalized CO2 capacities for class 1 powders (A/PEI, B/PEI andC/PEI) and ‘‘directly spun’’ class 1 hollow fiber sorbents (F-A/PEI, F-B/PEI, and F-C/

170 Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175

the spinneret temperature, air gap length, quench air temperatureand velocity, and type of solvent. In this work, the small air gap –approximately 3 cm – allows the process to approach wet-spinningwithout actually submerging the spinneret, which might causephase separation and plugging in the spinneret annulus. The lowair gap and lack of volatile solvent was desirable to avoid formationof a dense outer resistive skin layer via evaporative solvent loss,which can be minimized by using NMP as the only solvent [46].After spinning, the wet fibers were subjected to a solvent exchangeprocess, whereby lower surface tension fluids replace high surfacetension fluids to prevent capillary forces from collapsing the porestructure during fiber drying [53].

Fibers spun with bare silica materials as the dispersed phase arelabeled F, and fibers spun with pre-synthesized class 1 aminosilicafillers are denoted F-X/PEI (Table 2), X is silica A, B or C.

PEI).

3.1. Fiber sorbents by direct-spinning of class 1 aminosilica materials

Electron micrographs of fiber sorbents produced using the var-ious silica materials in the form of bare silica materials (F-A, F-B,and F-C) as well as class 1 aminosilica materials (F-A/PEI, F-B/PEI,and F-C/PEI) are shown in Fig. 3. From the SEM images, the fibersqualitatively possess the desired highly porous state. Furthermore,the bores are well-centered (Fig. 3iv and vi). Fig. 3v shows a mag-nified image of F-B/PEI, which shows the dispersed nature of thesilica and the porosity of the CA fiber. The fiber wall thicknesswas approximately 300–400 lm, which has previously beenshown to provide sufficient sorption capacity while still ensuringrapid mass transfer [50]. The fibers composed of A-silica particleswhere more difficult to spin. From the SEM images we can seethe pores between particles are larger than the internal particlepores, which are not visible. Since the diffusion equilibration timescales as the particle dimension squared, larger particles have muchlarger internal mass diffusion resistances during adsorption anddesorption, so smaller particles are clearly preferred. Therefore, sil-ica A particles were deemed poorly suited for this application. Incontrast, the fibers constructed from silicas B and C appeared to

Fig. 3. Electron micrographs of cross-sections of the hollow fiber sorbents containing pu

have homogeneous, well-dispersed silica particles and were there-fore further used throughout the rest of this work.

The CO2 capacities of the as-synthesized, class 1 aminosilicapowders and the hollow fiber sorbents prepared by spinning class1 aminosilica powders (F-A/PEI, F-B/PEI, and F-C/PEI) were deter-mined via TGA using dry, 10% CO2 in helium at atmospheric pres-sure. The adsorption capacities are shown in Fig. 4. The class 1powder samples all displayed CO2 capacities around 1–2 mmol/g-sorbent, while the directly spun class 1 fiber samples all had verylow CO2 capacities. These results clearly demonstrate that the di-rect spinning of pre-synthesized class 1 aminosilica sorbents didnot result in composite fibers with good CO2 sorption characteris-tics. Elemental analysis of the F-C fibers spun using class 1 amino-silica powders shows that the majority of the PEI was lost from thesamples during fiber synthesis. For example, the class 1 powders B/PEI and C/PEI had amine loadings of 11–12 mmol/g-sorbent in theas-synthesized form, but after spinning, the fibers made with thesesorbents – F-B/PEI and F-C/PEI – had amine loadings of about0.8 mmol/g-fiber. PEI, which is not covalently bound to the silicain class 1 materials, likely leaches out of the silica during either

re silica particles (i–iii); and fibers containing aminosilica class 1 adsorbents (iv–vi).

Table 4Structural properties of supported PEI silica sorbents before and after sonication tests.

Material Surfaceareaa

(m2/g)

Porevolumea

(cm3/g)

Porediametera

(nm)

PEI loadingb

(mmol/g-sorbent)

CO2 capacityc

(mmol/g-sorbent)

A/PEI 24 0.22 18.2 11.9 2.05A/PEI Sd 110 0.75 17.7 7.8 1.54C/PEI 37 0.33 26.1 11.9 2.15C/PEI Sd 105 0.81 20.8 7.3 1.43

a From N2 physisorption analysis at 77 K.b From combustion of organics via TGA.c From CO2 adsorption using dry, 10% CO2 via TGA.d S: sonicated.

Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175 171

the dope formation process or the hollow fiber sorbent spinningprocess. In the former, the aminosilica material is sonicated inNMP with the polymer (CA), whereas in the latter, the fiber exitsthe spinneret and plunges into a water bath; PEI has good solubil-ity in both NMP and water.

To probe whether PEI leaches from the silica matrix during dopepreparation, the class 1 sorbent powders were sonicated in NMPand the material characteristics before and after the sonicationwere evaluated. The sorbents were sonicated in a (polymer-free)dope solution (NMP/water weight ratio ca. 8) for 35 s using a100 W sonication horn and then the solids were recovered andanalyzed via N2 (physisorption at 77 K) and CO2 adsorption exper-iments. The amine loadings of the sorbents post-sonication werealso accessed via TGA. The data in Table 4 show that for two silicasupports, a significant fraction of PEI was leached from the sor-bents due to sonication in NMP, and these results show a concom-itant reduction in the CO2 capacity. It can be seen from these datathat the amine loading and CO2 capacity were decreased for bothtypes of silica impregnated with PEI. The porosity generally in-creased after exposing the class 1 materials to sonication, whichwas likely a result of the amine leaching from the silica into solu-tion. These results indicate that approximately 30–40% of theloaded PEI leached from the porous silicas during the dope prepa-ration process, which suggests that the remaining PEI likely lea-ched out during the spinning process in the water bath.

3.2. Post-spinning amine infusion into hollow fiber sorbents with baresilica fillers

Because the direct spinning of class 1 aminosilica sorbents re-sulted in composite fibers that contained very little PEI, an alter-nate approach to creating silica/PEI-containing hollow fibers was

Fig. 5. SEM images of fibers prepared from CA and bare silica C followed by PEI infusionprepared without hexanes solvent exchange (b).

needed. A conceptually simpler and highly scalable process wasdeveloped, whereby the PEI was infused into composite fibersconstructed of CA and bare, porous silica particles during thepost-spinning solvent exchange steps that are used to stabilizethe delicate pore structure of the fibers before drying. In thisapproach, a PEI/methanol soaking step was added to the normalsolvent exchange process after fiber spinning. This approach isreferred to here as the post-spinning infusion (PSI) method.

Typically, fibers are removed from the water bath after spin-ning and then subjected to solvent exchange with water threetimes (once a day), then methanol (three times, each time for20 min) and then hexanes (1–3 times, each time for 20 min), fol-lowed by drying in air, and then a vacuum oven. Before alteringthese solvent exchange steps and adding an amine-infusion step,the ability to streamline the process by reduction of solvent ex-change steps was investigated. Specifically, it was of interest todetermine if the number of hexanes exchange steps could bereduced or eliminated. Fig. 5 shows SEM images of the post-spinning infused hollow fiber sorbent made from silica C (F-C-PSI)with and without a 1 h hexanes solvent exchange step. In partic-ular, Fig. 5b shows the fiber pore morphology without thehexanes solvent exchange step. These micrographs qualitativelyshow that the fiber pore morphology did not collapse and thehighly porous state was maintained. CO2 adsorption capacity testsdemonstrated that both fibers had the same capacity, 1.10 mmol/g-fiber.

To further investigate possible fiber sorbent pore morphologydifferences as a function of processing conditions, helium perme-ation measurements were performed on fibers containing bare sil-ica (no PEI) prepared using hexanes solvent exchange one andthree times after three rounds of MeOH exchange along with thepost-spinning infused fiber prepared with and without hexanes ex-change (one time) after the PEI infusion. The permeation pressurewas 25 psig and the flux through the fiber was determined at 35 �C.All the fiber modules gave helium permeances of 70,000–80,000 GPUs, as shown in Table 5, regardless of the solvent ex-change method used. PSI fibers had slightly lower He permeanceafter PEI filling into the fiber, but were within the experimentaluncertainty. These permeances are in a similar range to the previ-ously spun CA/13X hollow fiber sorbents [50], suggesting that thefiber pore network is quite open in all cases, which implies that theinternal mass transport resistances through the fiber wall are min-imal. These experiments suggest that the fiber porosity will be sta-ble enough to infuse PEI after only methanol exchange, and thepores will not collapse during PEI-infusion. Furthermore, two or

via PSI, including F-C-PSI prepared with hexanes solvent exchange (a) and F-C-PSI

Table 5He permeances at 35 �C and 25 psig of CA/silica fibers and a CA/silica/PEI fiberprepared by post-spinning infusion.

Fiber code F-C 3-timehex

F-C 1-timehex

F-C-PSI withouthex

F-C-PSI 1 timehex

He permeance,GPUa

80,900 ± 530b 80,700 ± 620 75,400 ± 450 76,600 ± 520

a GPU = 10�6 (cm3(STP))/(cm2 s cmHg).b Error bars are from three fiber modules, with each module was tested three

times. 0.60.70.80.91.01.11.21.31.4

0 10 20 30

CO

2C

apac

ity (m

mol

/g-fi

ber)

Concentration of PEI in MeOH, wt%

1 h

4 h

20 h

Fig. 6. Effect of post-spinning infusion parameters on CO2 capacities of CA/silica/PEIhollow fiber sorbents prepared via post-spinning infusion of PEI (F-C-PSI materials).

172 Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175

three hexanes solvent exchange steps can possibly be eliminated,thus removing extra processing steps and extraneous chemical use.

The CO2 capacity, nitrogen loading and silicon loading of thepost-infused fiber sorbents (F-C-PSI) and the ‘‘directly spun’’ class1 aminosilica-loaded hollow fiber sorbents (F-C/PEI and F-B/PEI)are shown below in Table 6. The PSI fibers appear to provide sim-ilar performance both with and without the secondary hexanessolvent exchange, indicating that for these fibers, the extra pro-cessing step may not be needed. In addition, based on the dataand discussion provided above regarding loss of amines from fibersprepared via direct-spinning, which is verified by elemental analy-sis here, it is not surprising that post-spinning infused fiber sorbentsample, F-C-PSI, performed significantly better than the directspinning samples (F-C/PEI and F-B/PEI), which lacked significantamounts of PEI due to leaching during the dope formation andspinning process.

3.3. Optimization of post-infusion conditions

Having established PSI as a viable method to create compositeCA/silica/PEI hollow fibers sorbents, the post-spinning infusionparameters were varied to determine the controlling factors thatlead to high CO2 uptake. In particular, the PEI concentration inMeOH and the infusion time were varied. The CO2 capacities ofthe hollow fiber sorbents were tested under various conditions(Fig. 6). For 10 wt% PEI solutions and an infusion time of 4 h, the fi-bers were observed to have an equilibrium CO2 capacity of1.25 mmol/g-fiber. When the PEI concentrations were higher than20%, the CO2 capacities started to decrease. When the PEI concen-tration in MeOH was higher than 30%, the fiber was found to bedamaged (the fiber became transparent and collapsed) and alsoexhibited a much lower CO2 capacity – 0.56 mmol/g-fiber. Asshown in Fig. 6, an infusion time of 4 h appears appropriate,whereas an infusion time of 20 h appears unnecessarily long. Thesorption half time to reach the quasi-equilibrium CO2 capacitywas approximately 10 s for all of the samples studied.

Table 6CO2 capacities of fibers prepared via post-spinning infusion of PEI (F-C-PSI) and viadirect-spinning of class 1 aminosilica samples (F-C/PEI and F-B/PEI).

Fiber CO2 capacitya

(mmol/g-fiber)N loadingb

(mmol/g-fiber)Si loadingb

(mmol/g-fiber)

F-C-PSI 1.10 ± 0.07c,d 5.2e 5.1F-C/PEI 0.06 ± 0.01 0.84 6.9F-B/PEI 0.04 ± 0.01 0.76 3.3

a From CO2 adsorption using dry, 10% CO2 via TGA.b From elemental analysis; total fiber materials includes CA, silica and PEI but

little PVP as the PVP is largely lost to the water bath during the spinning process.c Error bars represent testing of two different batches of fibers.d Pure CA polymer fibers have a CO2 capacity of 0.12 mmol/g-fiber.e Bare silica fiber have 0.06 N mmol/g-fiber. This may be caused by remaining

solvent (NMP) in the pores of the silica.

3.4. ‘‘Rechargeable’’ post-spinning infusion method

An advantage of the PSI approach to sorbent synthesis is that itoffers a simple method for recharging fibers that have lost theiramine species via leaching or degradation after use. To investigatethe possibility of adding PEI to fiber sorbents that have alreadybeen through the solvent exchange procedure (three times MeOH),fibers with bare silica fillers (dried using the standard solvent ex-change procedure) were post-spinning infused in either a 10 wt%and 20 wt% PEI/MeOH solution for 1 h or 4 h, respectively (sampleslabeled F-C-RPSI). The hexane solvent exchanges were skipped (asshown above), and the fibers were dried in air for 1 h and thendried in a vacuum oven at 100 �C for 2 h. These samples were com-pared with a similar fiber prepared by the solvent exchange post-spinning infusion of PEI (F-C-PSI in Table 7) method describedabove. The fibers prepared via both approaches were found to havesimilar CO2 capacities and He permeance. These results demon-strate that it may be feasible to ‘‘recharge’’ fiber sorbents that havelost some of their sorption capacity over extended use by re-infus-ing the amines by simply rewetting and drying the fiber using asolvent exchange drying process, assuming any damaged aminescan be removed. In these first pass experiments, a 10% PEI concen-tration and 4 h infusing time are sufficient for ‘‘re-activating’’ thefibers.

3.5. Sorption performance of amine-loaded hollow fiber sorbentmodules under flowing simulated flue gas

Post-spinning infused fibers (F-C-PSI) with a 4 h PEI infusiontime in 10 wt% PEI/MEOH were assembled into a 23 cm long 4-fi-ber module in a 1=4

00 tube with two embedded thermocouples. Forthis initial study, no barrier layer was used in the bore [54]; thus,the fibers operated in a ‘‘non-isothermal’’ mode, without bore-sidecooling fluid. The module packing fraction was 53%. The layout ofthe module is shown in Fig. 7, and a shell-side feed at 100% RHwith 10 mol% CO2, 80 mol% N2 and 10 mol% He was used.

Table 7CO2 capacity and He permeance of fibers prepared via PEI ‘‘recharging’’ (F-C-RPSI) andthe fresh fiber post-spinning amine infusion (F-C-PSI).

Fiber samples CO2 capacity, soaktime = 1 h (mmol/g-fiber)

CO2 capacity, soaktime = 4 h (mmol/g-fiber)

He permeance,1 h (GPU)

F-C-PSI, 20%PEI

1.10 ± 0.07 0.93 ± 0.14 75,400 ± 450

F-C-RPSI, 20%PEI

1.03 ± 0.05 1.10 ± 0.14 73,600 ± 470

F-C-PSI, 10%PEI

1.12 ± 0.06 1.25 ± 0.08 74,500 ± 370

F-C-RPSI, 10%PEI

0.94 ± 0.12 1.07 ± 0.03 74,000 ± 430

Fig. 7. Schematic representation of hollow fiber sorbent prototype module.

0 200 400

02468

1012

Helium CO2

H2O

Con

cent

ratio

n (m

ol%

)

Time (s)

0 200 400

0.0

2.5

Z/Z0=0.38 Z/Z0=0.61

Tem

pera

ture

diff

eren

tial (

o C)

Time (s)

(a)

(b)

Fig. 8. (a) CO2, He and H2O elution profiles for amine loaded hollow fiber sorbentmodules (F-C-PSI) and (b) corresponding thermal profiles.

Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175 173

Fig. 8a shows the flue gas elution profile from the fiber sorbentcolumn, while Fig. 8b shows the corresponding temperature re-lease profiles. Due to the high CO2 adsorption capacities of the fibersorbents under these conditions as well as the He/CO2 streambeing exposed to a clean bed, a helium overshoot is observed,wherein helium fills the void space of the fiber sorbent and is sub-sequently displaced by the moving CO2 front, which results in theslight increase in He concentration observed (a phenomenonknown as ‘‘roll-up’’ [15]). This qualitatively indicates rapid masstransfer through both the silica sorbents and the fiber wall (fibersorbents with zeolite 13X have similar roll-up profiles [50]). TheCO2 breakthrough capacity at 10% product leakage (essentiallywhen the eluent CO2 concentration reaches 1 mol%) is approxi-mately 0.58 mmol/g-fiber with a propagation front velocity of0.38 cm/s, which is similar to experiments using zeolite 13X asthe filler phase in the fiber sorbent [50]. The CO2 uptake after20 min of exposure was 0.9 mmol/g-fiber (the true equilibrium isnot reached in these experiments). A thermal wave was observed,as expected. The sorption enthalpy release was monitored and theresulting thermal front propagated at a velocity of 0.38 cm/s. Thesmall temperature excursions observed (�3 �C), the axial reductionof the intensity of the thermal front and the similar mass and ther-mal propagation front velocities indicate that the sensible heat ofthe module itself is dominating the heat transfer in these small

systems [50]. Lively and Koros’ previous analysis indicates thatlarge scale modules (in which the module does not play such animportant role in heat transfer) will have temperature excursionsin excess of 40 �C, confirming the need for bore-side cooling [50].Finally, the ‘‘equilibrium’’ water uptake was determined to be3.2 mmol/g-fiber. The propagation velocity of the water front wasdetermined to be approximately 0.035 cm/s. Due to the slowerpropagation of water and the persistent thermal cycles (which willdesorb the sorbed water at the front of the column), it is likely thatthe majority of the bed (�90%) will adsorb CO2 in nearly dry con-ditions, which has important implications for CO2 capacity, ther-mal regeneration, cooling loads and amine performance. Morecomplete tests are needed to further elucidate the dynamic behav-ior of these sorbents, and a barrier layer will need to be installed tocomplete the RTSA sorbent module. However, these studies are be-yond the scope of this initial proof of concept study, which demon-strates that amine-based fiber sorbents can be created using anovel post-spinning infusion methodology, developed here, andthat they can capture dilute CO2 under 100% RH conditions andhave meaningful breakthrough capacities.

4. Conclusions

‘‘Direct spinning’’ of class 1 aminosilica materials to create CA/silica/PEI hollow fiber sorbents gave materials with very low CO2

capacities and low PEI content, due to amine leaching during dopepreparation and the fiber spinning process. To circumvent thisissue, hollow fiber sorbents were spun using CA as the polymerbinder and bare porous silica as the filler material, and a newpost-spinning infusion (PSI) process whereby poly(ethyleneimine)was infused into the fibers, was demonstrated to load the fiberswith amines. This PSI technique was determined to be robust tovariations in operating conditions and was straight-forward toconduct, and resulted in CA/silica/PEI fiber sorbents with CO2

uptakes of 1.2 mmol CO2/g-fiber. The highly scalable PSI-approachalso facilitates possible recharging of fiber sorbents that have lostsome of their sorption capacity in use by re-infusing amines afterfiber rewetting, followed by solvent exchange and drying.

Fibers were assembled into hollow fiber modules and CO2

breakthrough experiments were performed. The humid CO2 break-through uptake at 10% product leakage was 0.58 mmol/g fiber,while the CO2 uptake after 20 min was 0.92 mmol/g-fiber. Wateruptake was determined to be 3.2 mmol/g-fiber. Furthermore, thewater front propagated at a velocity of approximately 0.035 cm/s,while CO2 propagated at a velocity of approximately 0.38 cm/s.These results indicate that water in flue gas may potentially pene-trate only 10% of the length of the column before the sorption stepis completed, which has important implications for amine perfor-mance, as well as heating and cooling loads.

It is anticipated that hollow fiber sorbent based RTSA processesincorporating amine-modified materials may also be suitable forcapture from ultra-dilute gases, such as air [13,55–63].

174 Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175

Acknowledgement

The authors acknowledge the DOE-NETL under contract DE-FE0007804 and GE for financial support. However, any opinions,findings, conclusions, or recommendations expressed herein arethose of the author(s) and do not necessarily reflect the views ofthe DOE or GE.

References

[1] C.E. Powell, G.G. Qiao, Review: polymeric CO2/N2 gas separation membranesfor the capture of carbon dioxide from power plant flue gases, J. Membr. Sci.279 (2006) 1–49.

[2] J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K.Maskell, C.A. Johnson, Climate Change 2001: The Scientific Basis, CambridgeUniversity Press, Berlin, 2001. p. 787.

[3] Z. Zhang, K. Wang, J.D. Atkinson, X. Yan, X. Li, M.J. Rood, Z. Yan, Sustainable andhierarchical porous Enteromorpha prolifera based carbon for CO2 capture, J.Hazard. Mater. 229–230 (2012) 183–191.

[4] C. Azar, K. Lindgren, E. Larson, K. Mollersten, Carbon capture and storage fromfossil fuels and biomass – costs and potential role in stabilizing theatmosphere, Clim. Change 74 (2006) 47–79.

[5] C.S. Song, Global challenges and strategies for control, conversion andutilization of CO2 for sustainable development involving energy, catalysis,adsorption and chemical processing, Catal. Today 115 (2006) 2–32.

[6] D. Aaron, C. Tsouris, Separation of CO2 from flue gas: a review, Sep. Sci.Technol. 40 (2005) 321–348.

[7] R.S. Haszeldine, Carbon capture and storage: how green can black be? Science325 (2009) 1647–1652.

[8] J.D. Seader, E.J. Henley, Separation Process Principles, John Wiley & Sons, NewYork, 1998.

[9] S. Choi, J.H. Drese, C.W. Jones, Adsorbent materials for carbon dioxide capturefrom large anthropogenic point sources, ChemSusChem 2 (2009) 796–854.

[10] N. Hedin, L. Chen, A. Laaksonen, Sorbents for CO2 capture from flue gas –aspects from materials and theoretical chemistry, Nanoscale 2 (2010) 1819.

[11] D.M. D’Alessandro, B. Smit, J.R. Long, Review: carbon dioxide capture:prospects for new materials, Angew. Chem. Int. Ed. 49 (2010) 6058–6082.

[12] P. Bollini, S.A. Didas, C.W. Jones, Amine-oxide hybrid materials for acid gasseparations, J. Mater. Chem. 21 (2011) 15100–15120.

[13] C.W. Jones, CO2 capture from dilute gases as a component of modern globalcarbon management, Annu. Rev. Chem. Biomol. Eng. 2 (2011) 31–52.

[14] R.T. Yang, Gas Separation by Adsorption Processes, Imperial College Press,London, 1997.

[15] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley &Sons, Inc., New York, 1984.

[16] D.M. Ruthven, F. Shamsuzzaman, K.S. Knaebel, Pressure Swing Adsorption,John Wiley and Sons, Inc., New York, 1994.

[17] X.C. Xu, C.S. Song, J.M. Andresen, B.G. Miller, A.W. Scaroni, Novelpolyethyleneimine modified mesoporous molecular sieve of MCM-41 type asadsorbent for CO2 capture, Energy Fuels 16 (2002) 1463–1469.

[18] T. Tsuda, T. Fujiwara, Y. Taketani, T. Saegusa, Amino silica gels acting as acarbon dioxide absorbent, Chem. Lett. 21 (1992) 2161–2164.

[19] R.S. Franchi, P.J.E. Harlick, A. Sayari, Applications of pore-expandedmesoporous silica. 2. Development of a high-capacity, water-tolerantadsorbent for CO2, Ind. Eng. Chem. Res. 44 (2005) 8007–8013.

[20] J.C. Hicks, J.H. Drese, D.J. Fauth, M.L. Gray, G. Qi, C.W. Jones, Designingadsorbents for CO2 capture from flue gas-hyperbranched aminosilicas capableof capturing CO2 reversibly, J. Am. Chem. Soc. 130 (2008) 2902–2903.

[21] D. Wang, C. Sentorun-Shalaby, X. Ma, C.S. Song, High-capacity and low-costcarbon-based ‘‘molecular basket’’ sorbent for CO2 capture from flue gas,Energy Fuels 25 (2011) 456–458.

[22] M.L. Gray, Y. Soong, K.J. Champagne, J. Baltrus, R.W. Stevens Jr., P. Toochinda,S.S.C. Chuang, CO2 capture by amine-enriched fly ash carbon sorbents, Sep.Purif. Technol. 35 (2004) 31–36.

[23] W. Chaikittisilp, H.-J. Kim, C.W. Jones, Mesoporous alumina-supported aminesas potential steam-stable adsorbents for capturing CO2 from simulated flue gasand ambient air, Energy Fuels 25 (2011) 5528–5537.

[24] M.G. Plaza, C. Pevida, B. Arias, J. Fermoso, A. Arenillas, F. Rubiera, J.J. Pis,Application of thermogravimetric analysis to the evaluation of aminated solidsorbents for CO2 capture, J. Therm. Anal. Calorim. 92 (2008) 601–606.

[25] J.A. Dunne, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Calorimetric heats ofadsorption and adsorption isotherms. 2. O2, N2, Ar, CO2, CH4, C2H6, and SF6 onNaX, H-ZSM-5, and Na-ZSM-5 zeolites, Langmuir 12 (1996) 5896–5904.

[26] P.J.E. Harlick, F.H. Tezel, An experimental adsorbent screening study for CO2

removal from N2, Micropor. Mesopor. Mater. 76 (2004) 71–79.[27] J.A. Dunne, R. Mariwals, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Calorimetric

heats of adsorption and adsorption isotherms. 1. O2, N2, Ar, CO2, CH4, C2H6, andSF6 on silicalite, Langmuir 12 (1996) 5888–5895.

[28] P.J.E. Harlick, F.H. Tezel, Adsorption of carbon dioxide, methane, and nitrogen:pure and binary mixture adsorption by ZSM-5 with SiO2/Al2O3 ratio of 30, Sep.Sci. Technol. 37 (2002) 33–60.

[29] P.J.E. Harlick, F.H. Tezel, Adsorption of carbon dioxide, methane and nitrogen:pure and binary mixture adsorption for ZSM-5 with SiO2/Al2O3 ratio of 280,Sep. Purif. Technol. 33 (2003) 199–210.

[30] N.R. Stuckert, R.T. Yang, CO2 capture from the atmosphere and simultaneousconcentration using zeolites and amine-grafted SBA-15, Environ. Sci. Technol.45 (2011) 10257–10264.

[31] A.R. Millward, O.M. Yaghi, Metal–organic frameworks with exceptionally highcapacity for storage of carbon dioxide at room temperature, J. Am. Chem. Soc.127 (2005) 17998–17999.

[32] R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe, O.M. Yaghi,High-throughput synthesis of zeolitic imidazolate frameworks and applicationto CO2 capture, Science 319 (2008) 939–943.

[33] S. Keskin, T.M. van Heest, D.S. Sholl, Can metal–organic framework materialsplay a useful role in large scale carbon dioxide separations? ChemSusChem 3(2010) 879–891.

[34] J.R. Li, Y. Ma, M.C. McCarthy, J. Sculley, J. Yu, H. Jeong, P.B. Balbuena, H. Zhou,Carbon dioxide capture-related gas adsorption and separation in metal–organic frameworks, Coord. Chem. Rev. 255 (2011) 1791–1823.

[35] A. Samanta, A. Zhao, G.K.H. Shimizu, P. Sarkar, R. Gupta, Post-combustion CO2

capture using solid sorbents: a review, Ind. Eng. Chem. Res. 51 (2011) 1438–1463.

[36] T.C. Drage, A. Arenillas, K.M. Smith, C.E. Snape, Thermal stability ofpolyethylenimine based carbon dioxide adsorbents and its influenceon selec-tion of regeneration strategies, Micropor. Mesopor. Mater. 116 (2008) 504–512.

[37] A. Sayari, Y. Belmabkhout, E. Da’na, CO2 deactivation of supported amines:does the nature of amine matter? Langmuir 28 (2012) 4241–4247.

[38] A. Sayari, A. Heydari-Gorji, Y. Yang, CO2-induced degradation of amine-containing adsorbents: reaction products and pathways, J. Am. Chem. Soc. 134(2012) 13834–13842.

[39] A. Sayari, Y. Belmabkhout, Stabilization of amine-containing CO2 adsorbents:dramatic effect of water vapor, J. Am. Chem. Soc. 132 (2010) 6312–6314.

[40] P. Bollini, S. Choi, J.H. Drese, C.W. Jones, Oxidative degradation of aminosilicaadsorbents relevant to postcombustion CO2 capture, Energy Fuels 25 (2011)2416–2425.

[41] G. Calleja, R. Sanz, A. Arencibia, E.S. Sanz-Perez, Influence of drying conditionson amine-functionalized SBA-15 as adsorbent of CO2, Top. Catal. 54 (2011)135–145.

[42] W. Li, S. Didas, P. Bollini, S. Choi, J.H. Drese, P.M. Eisenberger, C.W. Jones,Structural changes of silica mesocellular foam supported amine-functionalizedCO2 adsorbents upon exposure to steam and oxygen, ACS Appl. Mater.Interfaces 2 (2010) 3363–3372.

[43] R. Serna-Guerrero, A. Sayari, Modeling adsorption of CO2 on amine-functionalized mesoporous silica. 2: kinetics and breakthrough curves,Chem. Eng. J. 161 (2010) 182–190.

[44] P. Bollini, N.A. Brunelli, S.A. Didas, C.W. Jones, Dynamics of CO2 adsorption onamine adsorbents. 2. Insights into adsorbent design, Ind. Eng. Chem. Res.(2012), http://dx.doi.org/10.1021/ie3017913.

[45] P. Bollini, N.A. Brunelli, S.A. Didas, C.W. Jones, Dynamics of CO2 adsorption onamine adsorbents. 1. Impact of heat effects, Ind. Eng. Chem. Res. (2012), http://dx.doi.org/10.1021/ie301790a.

[46] R.P. Lively, R.R. Chance, B.T. Kelley, H.W. Deckman, J.H. Drese, C.W. Jones, W.J.Koros, Hollow fiber adsorbents for CO2 removal from flue gas, Ind. Eng. Chem.Res. 48 (2009) 7314.

[47] R.P. Lively, R.R. Chance, W.K. Koros, Enabling low-cost CO2 capture via heatintegration, Ind. Eng. Chem. Res. 49 (2010) 7550–7562.

[48] R.P. Lively, R.R. Chance, J.A. Mysona, V.P. Babu, H.W. Deckman, D.P. Leta, H.Thomann, W.K. Koros, CO2 sorption and desorption performance of thermallycycled hollow fiber sorbents, Int. J. Greenhouse Gas Control 10 (2012) 285–294.

[49] S. Husain, Mixed Matrix Dual Layer Hollow Fiber Membranes for Natural GasSeparation, Ph.D. Dissertation, Georgia Institute of Technology, Atlanta, GA,2006.

[50] R.P. Lively, Hollow Fiber Sorbents for Post-combustion CO2 Capture, Ph.D.Dissertation, Georgia Institute of Technology, Atlanta, GA, 2011.

[51] W. Li, S. Choi, J.H. Drese, M. Hornbostel, G. Krishnan, P.M. Eisenberger, C.W.Jones, Steam-stripping for regeneration of supported amine-based CO2

adsorbents, ChemSusChem 3 (2010) 899–903.[52] J.J. Qin, Y. Li, L.S. Lee, H. Lee, Cellulose acetate hollow fiber ultrafiltration

membranes made from CA/PVP 360 K/NMP/water, J. Membr. Sci. 218 (2003)173–183.

[53] B.S. Minhas, T. Matsuura, S. Sourirajan, Solvent-exchange Drying of CelluloseAcetate Membranes for Separation of Hydrogen–Methane Gas Mixtures,Reverse Osmosis and Ultrafiltration, vol. 281, ACS Symposium Series, 1985(Chapter 33).

[54] R.P. Lively, J.A. Mysona, R.R. Chance, W.J. Koros, Formation of defect-free latexfilms on porous fiber supports, ACS Appl. Mater. Interfaces 3 (2011) 3568–3582.

[55] C. Gebald, J.A. Wurzbacher, P. Tingaut, T. Zimmermann, A. Steinfeld, Amine-based nanofibrillated cellulose as adsorbent for CO2 capture from air, Environ.Sci. Technol. 45 (2011) 9101–9208.

[56] N.R. Stuckert, R.T. Yang, CO2 capture from the atmosphere and simultaneousconcentration using zeolites and amine-grafted SBA-15, Environ. Sci. Technol.45 (2011) 10257–10264.

[57] S. Choi, J.H. Drese, P.M. Eisenberger, C.W. Jones, Application of amine-tetheredsolid sorbents for direct CO2 capture from the ambient air, Environ. Sci.Technol. 45 (2011) 2420–2427.

Y. Labreche et al. / Chemical Engineering Journal 221 (2013) 166–175 175

[58] S. Choi, M.L. Gray, C.W. Jones, Amine-tethered solid adsorbents coupling highadsorption capacity and regenerability for CO2 capture from ambient air,ChemSusChem 4 (2011) 628–635.

[59] Y. Belmabkhout, R. Serna-Guerrero, A. Sayari, Amine-bearing mesoporous silicafor CO2 removal from dry and humid air, Chem. Eng. Sci. 65 (2010) 3695–3698.

[60] S. Choi, J.H. Drese, P.M. Eisenberger, C.W. Jones, AIChE Annual Meeting,Nashville, 2009.

[61] T.M. McDonald, W.R. Lee, J.A. Mason, B.M. Wiers, C.S. Hong, J.R. Long, Captureof carbondioxide from air and flue gas in the alkylamine-appended

metal�organic framework mmen-Mg2(dobpdc), J. Am. Chem. Soc. 134(2012) 7056–7065.

[62] S. Choi, T. Watanabe, T.-H. Bae, D.S. Sholl, C.W. Jones, Modification of the Mg/DOBDC MOF with amines to enhance CO2 adsorption from ultradilute gases, J.Phys. Chem. Lett. 3 (2012) 1136–1141.

[63] A.R. Kulkarni, D.S. Sholl, Analysis of equilibrium-based TSA processes for directcapture of CO2 from air, Ind. Eng. Chem. Res. 51 (2012) 8631–8645.