Monolithic carbon aerogels from bioresources and their application for CO2 adsorption

Monolithic binder-free CO2 adsorbents with high adsorption capacity, selectivity, adsorption-desorption kinetics, and regenerability are highly desired to both reduce the environmental impact of anthropogenic CO2 emissions and purify valuable gases from CO2.

Available online 11 June 2021

1387-1811/© 2021 The Authors Published by Elsevier Inc This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Monolithic carbon aerogels from bioresources and their application for

aDivision of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-971 87, Luleå, Sweden

bChemical Technology, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-97 187, Luleå, Sweden

cMechanical & Industrial Engineering (MIE), University of Toronto, Toronto, ON, M5S 3G8, Canada

A R T I C L E I N F O

Keywords:

Monolithic carbon aerogels

CO 2 adsorbents

Bioresources

CO 2 /N 2 selectivity

Lignin

A B S T R A C T Monolithic binder-free CO2 adsorbents with high adsorption capacity, selectivity, adsorption-desorption kinetics, and regenerability are highly desired to both reduce the environmental impact of anthropogenic CO2 emissions and purify valuable gases from CO2 Herein, we report a strategy to prepare monolithic carbonaceous CO2 ad-sorbents from low-cost and underutilized bioresources, which enabled the formation of a delicate anisotropic, hierarchical porous structure With optimized material composition and processing conditions, the biobased carbon adsorbent demonstrated a CO2 adsorption capacity of 4.49 mmol g-1 at 298 K and 100 kPa, relatively weak adsorbent-adsorbate affinity, good CO2/N2 selectivity, and advantageous hydrophobicity against water vapor Moreover, the unique anisotropic porous structure provided high stiffness and good flexibility to the adsorbent in the axial and radial directions, respectively We confirmed that this type of carbon adsorbent could

be packed in a column for dynamic CO2 capture independent of any binders, indicating its promising future for further development toward widespread utilization

1 Introduction

The capture and recovery of CO2 are vital tasks as the rate of

anthropogenic CO2 emissions has overtaken the natural carbon cycle,

posing risks to the climate [1] The separation of CO2 from other

valu-able gases has also attracted attention owing to the requirements for

their purification For example, biogas contains 20%–50% of CO2 which

needs to be removed to improve the calorific value and make biogas a

competitive biofuel [2] Traditional CO2 capture methods are normally

based on chemical absorption, such as amine scrubbing, which has high

energy consumption owing to the need for solvent regeneration, suffers

from amine degradation problems, and causes equipment corrosion [3

4] By contrast, physical capture methods using porous adsorbents have

been proposed and investigated since the 1980s [5–8], because they rely

on molecular sieving effects and weak adsorbent-adsorbate interactions

that lead to easier regeneration and a longer lifespan of the adsorbents

Various materials such as zeolites, activated carbons, mesoporous

sil-icas, and metal-organic frameworks have been developed for CO2

adsorption and separation [9–15] Among these materials, activated

carbons have several advantages, including low cost, large specific

surface area, and high thermal and chemical stability [16,17] However,

the use of conventional activated carbons for CO2 capture is limited by their relatively low adsorption capacity and large structural variation, depending on the base material [9]

Other types of porous carbon materials have been investigated to improve the CO2 adsorption capacity of carbonaceous adsorbents, such

as carbon molecular sieves [18,19], nitrogen-doped porous carbons [20–23], and carbon nanotubes [24–27] Although superb capacity and selectivity have been reported for many of these adsorbents, the use of non-renewable resources and complicated manufacturing processes still pose as significant obstacles for widespread applications Considering the sustainability requirements, high-performance carbonaceous ad-sorbents produced from renewable bioresources via simple and straightforward processes are highly desired Recently, we have suc-cessfully developed carbon aerogels from bio-based raw materials, lignin and cellulose nanofibers (CNFs), which showed great potential for use as CO2 adsorbents [28] Lignin as one of the main components of wood possesses a high aromatic content, and it is the byproduct of the pulp and paper industry most in need of utilization [29], and CNFs, which can be isolated from various plants, are promising green nano-materials with large aspect ratios and strong mechanical properties [30] The derived monolithic carbon aerogels possessed a hierarchical porous

* Corresponding author

E-mail address: shiyu.geng@ltu.se (S Geng)

Contents lists available at ScienceDirect Microporous and Mesoporous Materials

journal homepage: www.elsevier.com/locate/micromeso

https://doi.org/10.1016/j.micromeso.2021.111236

Received 17 March 2021; Received in revised form 6 June 2021; Accepted 8 June 2021

structure with homogeneous, tracheid-like macropores oriented in the

same direction, which was attributed to the ice-templating and

carbonization process that we used This interesting structure is

ex-pected to be favorable for CO2 capture because it contributes to the fast

adsorption and desorption kinetics and also helps reach high adsorption

capacity [9] Therefore, this type of carbonaceous adsorbent should be

further modified and explored for higher CO2 capture efficiency

The main aim of this study was to improve the CO2 adsorption

per-formance of lignin/CNF-based carbon aerogels to the next level, and to

evaluate their properties as monolithic adsorbents The effects of

different material compositions and processing factors, including the

type of lignin, lignin/CNF ratio, solid content of the starting suspension,

and carbonization time, on the final porous structure and properties of

the carbon aerogels were systematically investigated The optimized

adsorbent prepared without any activation process exhibited an

excel-lent CO2 adsorption capacity, which was competitive with many

acti-vated carbonaceous adsorbents synthesized using more complex

procedures with lower yields reported in the literature [21,31–38] The

CO2 selectivity against N2 and water vapor adsorption of the adsorbent

were also measured, as they are important in gas separation and

puri-fication applications Moreover, as most of the conventional and

previ-ously investigated adsorbent materials are in powder form and their

performance can be reduced by the binder blocking effect when applied

with a binder, adsorbents in monolithic form are preferable [18]

Consequently, the bulk behaviors of the lignin/CNF-based carbon

aer-ogels obtained from forward-step change breakthrough tests and

me-chanical tests are presented, which indicated that these monolithic

carbon adsorbents derived from bioresources had the capability to be

used without any additional materials or support in various CO2 capture

applications

2 Results and discussion

2.1 Synthesis of monolithic carbon aerogels

The manufacturing process, from the lignin/CNF suspensions to the

final carbon aerogels, is schematically shown in Fig 1a, and

represen-tative images of the lignin/CNF precursor and carbon aerogel are

demonstrated in Fig 1b and c, respectively A commercial kraft lignin

from traditional kraft pulping process was first used to determine the

optimized lignin/CNF ratio, suspension solid content, and carbonization

time, as shown in Table 1 Another type of lignin generated from the

newly developed LignoBoost process [39], named lignoboost lignin, was then chosen to investigate the effects of different types of lignin on the structure and properties of the derived carbon aerogels This is because the lignoboost lignin possesses higher purity and higher content of phenolic hydroxyl, enol ether and stilbene, and lower content of ash,

methoxy group and β-O-4 structure compared to the traditional kraft

lignin [40] The CNFs used in this study had an average diameter of 1.7

nm and a carboxylation degree of 0.72 mmol g− 1, which were well characterized in our previous work [28] The sample codes, material compositions, and processing parameters of all prepared carbon aero-gels are presented in Table 1 The whole carbonization process included three isothermal steps at 100, 500, and 1200 ◦C in sequence, while the carbonization time mentioned in the following discussion is only related

to the isothermal time at 1200 ◦C

2.2 Physical and thermal properties of carbon aerogels and their raw materials

The viscosity of the lignin/CNF suspensions, as shown in Fig 2a, varied significantly with the lignin/CNF ratio, solid content, and lignin type The K75/25-3w and K85/15-6w suspensions had viscosities of 96 and 91 mPa s, respectively, which were much higher than those of K85/ 15-3w (26 mPa s), due to the higher CNF content (25 wt% vs 15 wt% of dry weight) and solid content (6 wt% vs 3 wt%) Compared to the K85/ 15-3w, the L85/15-3w suspension interestingly showed a lower viscos-ity of 11 mPa s likely due to both the difference in the charge content between the lignoboost lignin and the kraft lignin and the different number of hydrophilic groups in them [41] It is obvious from Fig 1b and c that the lignin/CNF precursors contracted drastically during the carbonization, and the material yield and volume shrinkage were in the range of 33%–37% and 61%–68%, respectively, as illustrated in Fig 2b and Table S1 Among the carbon aerogels derived from the kraft lignin, K85/15-6w-1h had the highest yield (37%) and the lowest volume shrinkage (61%), caused by the higher solid content, while there was no considerable difference between the K85/15-3w carbon aerogels with various carbonization times at 1200 ◦C (1, 2, and 3 h) The carbon aerogel derived from lignoboost lignin (L85/15-3w-1h) also showed quite high yield and low volume shrinkage, especially when compared

to K85/15-3w-1h, which had the same material composition and pro-cessing condition except the lignin type Fig 2c reveals that the porosity

of all carbon aerogels (calculated using Eq (1)) reached approximately 98%, indicating the super lightweight nature of the carbon aerogels, which is attributed to the ice-templating induced tracheid-like macropores

Fig 1 (a) Schematic preparation processing from lignin/CNF aqueous

sus-pensions to carbon aerogels, and representative illustrations of (b) a lignin/CNF

precursor and (c) a carbon aerogel

Table 1

Coding, composition, and carbonization time of samples prepared in this study Sample

code Lignin type Composition (w/ w) Solid content a (wt

%)

Carbonization time b (h) Lignin TOCNF

K75/25-

K85/15-

K85/15-

K85/15-

K85/15-

L85/15-

a Solid content of the related lignin/CNF suspensions without considering the weight of the added NaOH

b Isothermal holding time at 1200 ◦C

To individually understand the behaviors of the kraft lignin, the

lignoboost lignin, and the CNFs during the carbonization process, their

thermal degradation features were characterized by thermogravimetric

analysis (TGA), as shown in Fig 3a and b Both types of lignin

experi-enced a small weight loss in the starting stage up to 100 ◦C due to

moisture evaporation, and their primary degradation occurred at

250–450 ◦C caused by the release of volatiles including aromatics,

al-kyls, carbonyls, CO, and CO2 [42,43] It is obvious from the derivative

thermogravimetry (DTG) curves that the kraft lignin reached the highest

rate of weight loss at a lower temperature (310 ◦C) than the lignoboost

lignin (380 ◦C), while the weight loss of the lignoboost lignin was much

higher than that of the kraft lignin at the primary degradation stage The

lignoboost lignin also showed better thermal stability at higher

tem-perature, as its weight loss rate gradually declined with increasing

temperature and was close to 0 after 730 ◦C, while the weight of the kraft

lignin continued to decrease This can be related to the higher yield of

the lignoboost lignin-derived carbon aerogel after carbonization at

1200 ◦C (Fig 2b) The possible reason is that the larger quantity of

phenolic hydroxyl, enol ether, and stilbene structures present in the

lignoboost lignin could trigger more crosslinking reactions at high

temperatures compared to the kraft lignin [44,45], which resulted in a

more stable carbon structure As a comparison, the CNFs started to

degrade from 200 ◦C, attributed to their thermally unstable

anhydro-glucuronic acid units [46], which led to a lower residue content

of 23% at 900 ◦C than that of the kraft lignin (44%) and lignoboost lignin

(35%) This indicates that the CNFs not only contributed to the shape

stability of the lignin/CNF precursors, due to their fiber geometry with a

large aspect ratio, but also acted as sacrificial templates for generating

cavities during carbonization, which resulted in a hierarchical porous

structure in the materials The thermal degradation behaviors of

K85/15-3w and L85/15-3w precursors were also characterized to

further compare the effects of the different types of lignin, and their TGA

curves are exhibited in Fig S1 Due to the presence of the TOCNFs, both

precursors had the primary degradation stage at a lower temperature region (200–400 ◦C) with two main DTG peaks compared to that of the neat kraft lignin and lignoboost lignin (250–450 ◦C) K85/15-3w pre-cursor showed the highest rate of weight loss at a lower temperature than that of L85/15-3w precursor, which corresponds to the DTG curves

of the neat lignin (Fig 3a) In addition, it can be noticed that the TGA residue content of L85/15-3w precursor at 900 ◦C (32.2%) was even lower than the yield of the L85/15-3w-1h carbon aerogel carbonized at

1200 ◦C (37.4%, Fig 2b) The possible reasons include that the isothermal step at 500 ◦C during the carbonization process increased the thermal stability of the precursor owing to the lignin crosslinking effect, and some volatiles and tars generated during carbonization could not be removed as efficient as in the TGA test caused by the much bulkier sample size The carbon structure of the carbon aerogels was analyzed

by Raman spectroscopy, and the results are shown in Fig 3c and Fig S2 All samples exhibited typical D (~1338 cm− 1) and G (~1584 cm− 1) bands of carbon materials, which are usually assigned to the breathing mode of disordered carbon atoms and the in-plane bond-stretching of

carbon sp2 sites, respectively Farrari et al interpreted that in the case of amorphous carbon with nanocrystalline graphite, the intensity of D band is proportional to the graphitic cluster area, which is instead related to ordering [47] The inset of Fig 3c clearly demonstrates that both K85/15-3w-2h and K85/15-3w-3h exhibited higher D-band in-tensity compared to K85/15-3w-1h, indicating that the size of graphite crystallites in the carbon aerogels was increased by extending the carbonization time to 2 h, while it remained constant when the time was extended to 3 h

2.3 Morphology and pore structure of carbon aerogels

The morphology of both the cross section and the longitudinal sec-tion (Fig S3) of the carbon aerogels was investigated using scanning electron microscopy (SEM), as illustrated in Fig 4 All carbon aerogels

Fig 2 (a) Viscosity of the lignin/CNF suspensions (b) Yield and volume shrinkage of the carbon aerogels from their lignin/CNF precursors and (c) porosity of the

carbon aerogels (solid columns: K85/15-3w samples with different carbonization time, columns with patterns: other samples)

Fig 3 TGA curves of (a) kraft lignin and lignoboost lignin as well as (b) 2,2,6,6-tetramethylpiperidinyl-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers

(TOCNFs), and (c) Raman spectra of K85/15-3w carbon aerogels with different carbonization time (the intensity was normalized according to that of G bands)

exhibited an anisotropic structure with longitudinal macropores because

of the ice-templating technique, and Figs S4 and S5 confirm that this

unique structure remained homogenous at a relatively large scale (more

than 1 mm2 in cross section) Compared to K85/15-3w-1h, K75/25-3w-

1h, and K85/15-6w-1h showed macropores with a more regular shape

and less wrinkled cell walls (Fig 4a), especially in the case of the former

This can be associated with the higher CNF concentration and solid

content in the starting lignin/CNF suspensions, which induced the

for-mation of a more rigid cell structure after ice-templating and

carbon-ization There was no distinct difference between the morphology of the

carbon aerogels with different carbonization times, as shown in Fig 4b

For the carbon aerogels with different types of lignin (Fig 4c), it is

interesting that the aerogel with the lignoboost lignin (L85/15-3w-1h)

also demonstrated less wrinkled and smoother cell walls, which could be

attributed to the better thermal stability of the lignoboost lignin in the

high-temperature range, corresponding to the above-mentioned yield/

volume shrinkage and TGA results

Inspired by our earlier work [28], all carbon aerogels obtained in this

study were washed with distilled water after carbonization to remove

possible residual impurities and subsequently increase their porosity

and surface area, which acted as key factors in CO2 adsorption To

confirm the effects of washing, Brunauer− Emmett− Teller (BET) surface

area analysis and energy-dispersive X-ray spectroscopy (SEM-EDX) were

carried out for both unwashed and washed samples, and the results are

shown in Fig 5, Table 2, and Table S2 The N2 adsorption isotherms for

the BET analysis are shown in Figs S6 and S7 All washed samples

demonstrated a much higher surface area compared to the unwashed

ones (Fig 5), and one possible reason is the removal of salt compounds

that expose the previously blocked pores This can be verified by the

EDX data shown in Table 2, where the oxygen, sodium, and potassium

contents in the kraft lignin-derived samples decreased considerably after

washing, while the relative carbon contents increased However, the

lignoboost lignin-derived carbon aerogel (L85/15-3w-1h) did not show

a similar tendency, and the carbon content remained same, while the

oxygen content was slightly increased by washing This indicates that

removing salts was not the only reason of the increased surface area, but

the removal of some tars generated during carbonization could also have

contributed K85/15-3w-1h showed a higher surface area than that of

K75/25-3w-1h and K85/15-6w-1h, owing to its more accessible

micro-pores (563 m2 g− 1 of micropore-surface area after washing compared to

290 and 185 m2 g− 1, respectively; Table S2), which is probably caused

by the different cell wall structure (Fig 4a), inducing different heat

transfer and carbonization effects [48] Moreover, it was obvious that increasing carbonization time from 1 h to 2 h significantly reduced the surface area of the carbon aerogels and slightly increased the average pore size (from 1.6 to 1.9 nm, after washing) This was likely because more severe migration of carbon atoms resulted in pore merging in the samples with a longer carbonization time at 1200 ◦C [49]

2.4 Adsorption properties of carbon aerogels

The CO2 adsorption isotherms of all the washed carbon aerogels were measured in the 0–120 kPa pressure range and at three different tem-peratures, which were 273, 298, and 323 K Fig 6a and b reveal that L85/15-3w-1h surprisingly exhibited a superior CO2 adsorption capacity compared to K85/15-3w-1h at all temperatures, which at 273 K reached 1.94 and 6.28 mmol g− 1 at 10 and 100 kPa, respectively This was not in agreement with the BET surface area results (Fig 5), in which L85/15- 3w-1h showed a drastically lower surface area (380 m2 g− 1 compared

to 643 m2 g− 1) This phenomenon implied that a large number of ultra- micropores could be present in L85/15-3w-1h, which were accessible for

CO2 while not detectable in BET measurements using N2 at 77 K because

Fig 4 SEM images of (a) K75/15-3w-1h, K85/15-3w-1h and K85/15-6w-1h (up: cross section, bottom: longitudinal section; scale bar: 50 μm), (b) K85/15-3w carbon aerogels with different carbonization time (cross section; scale bar: 50 μm), and (c) carbon aerogels derived from different types of lignin (up: cross sec-tion, bottom: longitudinal section; scale bar: 20 μm)

Fig 5 BET surface area and pore size of all carbon aerogels before and after

washing (solid columns: K85/15-3w samples with different carbonization time, columns with patterns: other samples)

of the smaller quadrupole moment and more diffusional restrictions of

N2 when compared to CO2 [17,50,51] The data of CO2 adsorption

ca-pacity of all samples as a function of BET surface area are summarized in

Fig 6c and d and Table S3 Among the kraft lignin-derived carbon

aerogels, K85/15-3w-1h possessed the largest surface area and showed

the highest CO2 adsorption capacity at 10 kPa, because of its large

quantity of accessible micropores (Table S2) However, at 100 kPa, a

similar CO2 adsorption capacity was observed for all kraft lignin-based

samples, except for K85/15-6w-1h, which had a much smaller surface

area To further investigate this, Fig S8 compares the CO2 adsorption

isotherms recorded at 273 K of the K85/15-3w carbon aerogels with

different carbonization times With increasing carbonization time, the

isotherm slope in the relatively high-pressure range increased positively,

and the adsorption quantity of K85/15-3w-3h exceeded that of both

K85/15-3w-1h and K85/15-3w-2h at pressures higher than 85 kPa This

was likely due to the larger pore size of the samples with longer

carbonization times (Table S2), resulting in the condensation of CO2

molecules in the pores at high pressure A similar behavior, that of a

steep isotherm slope at high pressure, was also observed for

K75/25-3w-1h (Fig S9), which had a pore size of 2.1 nm In addition, by calculating the CO2 adsorption enthalpy (ΔH) and entropy (ΔS) of the

samples from their isotherms at various temperatures (Eq (2) and Eq

(3)), it can be seen in Fig 7 that the absolute values of ΔH (|ΔH|) of

K75/15-3w-1h, K85/15-3w-2h, and K85/15-3w-3h are greater than the other two kraft lignin-based samples, which illustrates that they can provide stronger affinity for CO2 L85/15-3w-1h exhibited a relatively low affinity for CO2 (|ΔH| is 19.98 kJ mol− 1), while still showed promising CO2 adsorption capacity, even when compared to many literature-reported carbonaceous adsorbents prepared with various activation processes (Table 3), revealing its excellence not only in CO2

capture but also in adsorbent regeneration

As L85/15-3w-1h outperformed all the other samples, showing the highest CO2 adsorption capacity, it was selected to evaluate the CO2/N2

selectivity and water vapor adsorption, as well as the monolithic prop-erties, including the column breakthrough adsorption capacity and mechanical properties Fig 8a shows the comparison between the CO2

and N2 adsorption isotherms of L85/15-3w-1h at 298 K The CO2/N2

selectivity was estimated to be 21 from the ratio of the slope of the

Table 2

Elemental composition (in at %) of the carbon aerogels before and after washing according to the SEM-EDX

Fig 6 (a,b) CO2 adsorption isotherms with Langmuir model-fitting curves of K85/15-3w-1h and L85/15-3w-1h at 273, 298 and 323 K; and (c,d) the quantity of adsorbed CO2 at the pressures of 10 and 100 kPa of all prepared carbon aerogels at the various temperatures used (i: K85/15-6w-1h, ii: L85/15-3w-1h, ii: K85/15-3w- 2h, iv: K85/15-3w-3h, v: K75/25-3w-1h, vi: K85/15-3w-1h)

isotherm linear regions in the low pressure range (≤10 kPa), which is

higher than or comparable to that of many modified carbon materials

[22,52,53] The water vapor adsorption of the sample was evaluated at

298 K in the 0–3.169 kPa pressure range (up to the saturated vapor

pressure) [54] The isotherm is shown in Fig 8b and can be described as

a Type-V isotherm according to the IUPAC classification [55] This

in-dicates that the amount of adsorbed water was relatively low at

pres-sures below 1.5 kPa, while significant capillary condensation of water

molecules occurred at higher pressures Finally, the sample reached a

high-water adsorption capacity due to pore filling This corresponds to

the typical water vapor adsorption behavior of hydrophobic carbon materials with a hierarchical porous structure [56,57] The adsorption capacity of L85/15-3w-1h was recorded as 0.46 mmol g− 1 at a pressure

of 1 kPa, which is considerably lower than that of bituminous coal-based activated carbons at the same temperature (approximately 1–2 mmol

g− 1 at 298 K) [58,59] This is probably because the high carbonization temperature (1200 ◦C) used for the carbon aerogels led to a more hy-drophobic surface with fewer sorption sites for water molecules

Fig 8c illustrates the results from the forward-step change break-through experiment for the L85/15-3w-1h with a 10 kPa CO2/90 kPa N2

mixture, which was used to simulate the realistic conditions of post- combustion capture The test setup is depicted in Fig S11, where several carbon aerogels were loaded in the column to achieve a suffi-cient ratio of adsorbent length to diameter By integrating the area be-tween the sample curve and the curve of the empty column, a dynamic

CO2 adsorption capacity of 0.67 mmol g− 1 was obtained The value was lower than that obtained from the equilibrium measurement at the same pressure and temperature (1.11 mmol g− 1 at 10 kPa CO2 and 298 K) because of the competitive adsorption from N2 According to the above- mentioned CO2/N2 selectivity of L85/15-3w-1h (a value of 21), its theoretical CO2 adsorption capacity under a 10 kPa CO2/90 kPa N2 at-mosphere can be calculated as 0.63 mmol g− 1, which corresponds very well with the measured result

2.5 Mechanical properties of monolithic carbon aerogels

The mechanical properties of L85/15-3w-1h in both the axial and radial directions were characterized by compression testing, and the acquired stress-strain curves are shown in Fig 9 The sample exhibited distinct anisotropic mechanical behaviors, which was attributed to the ice-templating induced tracheid-like porous structure The elastic modulus of the sample in the axial direction was as high as 2.64 MPa (Table S4) with a specific elastic modulus of 61.2 kNm kg− 1 (calculated using Eq (4)), while in radial direction the sample showed good flexi-bility, reaching 50% of the strain before collapse The excellent me-chanical properties of the carbon aerogels with a porosity as high as 98% make them a viable choice for CO2 capture applications without any supporting materials or binders, demonstrating their potential as inde-pendent CO2 adsorbents in the future

3 Conclusions

In summary, we have demonstrated that carbon aerogels based on lignin and CNFs with anisotropic, hierarchical porous structures were successfully prepared via a straightforward procedure combining ice- templating and carbonization By tailoring the CNF concentration, solid content, and carbonization time, the structure of the carbon aer-ogels, including the carbon structure, morphology, porous structure, and surface area, were varied, which consequently led to different CO2

adsorption capabilities The different lignin structure generated from different types of lignin affected their thermal degradation behaviors during carbonization, also resulting in significant variations in the structure and properties of the obtained carbon aerogels The carbon aerogel containing lignoboost lignin reached a CO2 adsorption capacity

of 4.49 mmol g− 1 at 298 K and 100 kPa, which is competitive among previously reported carbon-based adsorbents, while possessing a good

CO2/N2 selectivity of 21 and a low water vapor adsorption capacity up

to 1.5 kPa of vapor pressure Furthermore, we have also discovered that carbon aerogels had excellent mechanical properties and could be used alone in a column for CO2 capture without the need for any additional binders, which is expected to initiate the further development of monolithic carbonaceous CO2 adsorbents with large adsorption capac-ity, high selectivcapac-ity, fast adsorption-desorption kinetics, and easy regeneration

Fig 7 Enthalpy (ΔH) and entropy (ΔS) of CO2 adsorption of all carbon aerogels

(solid columns: K85/15-3w samples with different carbonization time, columns

with patterns: other samples)

Table 3

Comparison of CO2 adsorption capacity at 100 kPa and 298 K between L85/15-

3w-1h and other carbonaceous adsorbents made from various precursors

pre-pared by different processes, as reported in literature

Material

code Carbon resource Processing COadsorption 2

capacity (mmol g − 1 )

Reference

CP-2-600 Polypyrrole Nitrogen-doping

and chemical activation

3.84 [ 21 ]

MFB-600 Melamine and

formaldehyde Pyrolysis 2.25 [31]

GKOSA50 Olive stones Pyrolysis and

physical activation

2.43 [ 32 ]

AA750 Almond shell Pyrolysis and

physical activation

2.66 [ 33 ]

HCM-

DAH-1-

900-1

Resorcinol and

formaldehyde Nitrogen-doping, pyrolysis, and

physical activation

3.30 [ 34 ]

AS-2-600 Sawdust Hydrothermal

carbonization and chemical activation

4.82 [ 35 ]

NCP-800 Coal tar pitch Nitration and

pyrolysis 2.20 [36] Y–K-600 Yeast Pyrolysis and

chemical activation

4.77 [ 37 ]

L85/15-

3w-1h Lignoboost lignin and

nanocellulose

Pyrolysis 4.49 This work

4 Material and methods

4.1 Materials

Kraft lignin with a low sulfonate content (M w of ~10,000) was

purchased from Sigma-Aldrich, Sweden AB Lignoboost lignin was

supplied by Domtar Plymouth pulp mill (NC, USA) with a M w of 6772

and a purity of 96.5% [40] Cellulose nanofibers (CNFs) were prepared

through 2,2,6,6-tetramethylpiperidinyl-1-oxyl radical

(TEMPO)-me-diated oxidation treatment followed by a homogenization process, as

described in our previous study [28] Sodium hydroxide (NaOH, pure

pellets) was purchased from Merck KGaA, Germany All chemicals were

used as received

4.2 Preparation of lignin/CNF precursors

Kraft lignin/CNF suspensions with various lignin to CNF ratios and

different solid contents were obtained by directly mixing kraft lignin,

the CNF suspension (1 wt%), and distilled water at room temperature for

2 h To prepare the lignoboost lignin/CNF suspension, the lignin was

first dissolved in a NaOH solution, where the amount of NaOH was 14 wt

% of the lignin dry weight, and then the lignin solution was stirred

together with the CNF suspension Afterwards, both types of lignin/CNF

suspensions were ice-templated unidirectionally at a freezing rate of 10

K/min to form an anisotropic structure The ice-templating setup is

described in detail in our earlier work [28] The frozen samples were

then freeze-dried using an Alpha 2–4 LD plus freeze-dryer (Martin Christ

GmbH, Germany) to generate porous lignin/CNF precursors

4.3 Preparation of carbon aerogels

The carbon aerogels were prepared by the carbonization of the lignin/CNF precursors under a nitrogen atmosphere using a tube furnace (RHTC-230/15, Nabertherm GmbH, Lilienthal, Germany) The heating procedure included temperature ramp from room temperature to 100 ◦C with an isothermal holding time of 2 h, ramp from 100 to 500 ◦C and holding for 100 min, and a final ramp up to 1200 ◦C with different holding times of 1, 2, and 3 h The heating rate of all ramp steps was

5 ◦C/min Then, the carbonized samples were washed with distilled water five times at 30-min intervals and dried in an oven at 80 ◦C overnight to obtain the final carbon aerogels

4.4 Characterizations

The viscosity of the lignin/CNF suspensions was measured using an SV-10 Vibro viscometer (A&D Company, Tokyo, Japan) at 22 ◦C TGA was performed using a TA Q500 thermogravimetric analyzer (TA In-struments, New Castle, DE, USA) under a nitrogen atmosphere, with a temperature range from room temperature to 900 ◦C and a heating rate

of 10 ◦C/min The porosity of the carbon aerogels was calculated as follows:

P =

(

1 − ρ∗

ρ

)

where ρ and ρ * denote the density of the solid carbon (2.1 g cm− 3) [54] and the bulk density of the carbon aerogels, respectively The carbon structure was characterized by Raman spectroscopy using a Bruker Senterra dispersive Raman spectroscope (Bruker Corp., Billerica, MA, USA) with a 533 nm laser beam SEM was conducted using a JEOL JSM

6460 L V scanning electron microscope (JEOL Ltd., Tokyo, Japan) The elemental composition of the samples was examined by SEM-EDX equipped with a silicon drift detector (Oxford X-MaxN 50 mm2, Ox-ford Instruments, OxOx-fordshire, UK) BET surface area analysis was car-ried out according to N2 adsorption tests at 77 K using a Gemini VII 2390a analyzer (Micromeritics Instrument Corp., Norcross, GA, USA) The samples were degassed at 300 ◦C for 4 h prior to the tests CO2

adsorption measurements were performed with an ASAP 2020 Plus BET analyzer (Micromeritics Instrument Corp., Norcross, GA, USA) The pressure ramp was from 0 to 120 kPa at 273, 298, and 323 K, respec-tively, after the degas step Based on the adsorption isotherm data at low

pressure (<20 kPa), the enthalpy (ΔH) and entropy (ΔS) of the CO2

adsorption of each sample were calculated using the Langmuir adsorp-tion model and van’t Hoff equaadsorp-tion:

1

q e

= 1 (

K eq Q m

)

C e

+ 1

ln K eq= − ΔH

RT+

ΔS

Fig 8 (a) CO2 and N2 adsorption isotherms of L85/15-3w-1h at 298 K, and (b) water vapor adsorption isotherm and (c) forward-step change breakthrough curves with CO2/N2 mixture (volume ratio of 10/90) of L85/15-3w-1h and the empty column at 298 K

Fig 9 Representative stress-strain curves of L85/15-3w-1h from compression

testing in both axial and radial directions

where C e and q e represent the pressure and the amount of adsorbed CO2

at equilibrium, respectively, K eq denotes the equilibrium constant, Q m is

related to the maximum capacity of the adsorbent, T is the temperature,

and R is the Avogadro constant N2 and water vapor adsorption

iso-therms at 298 K were also collected from the ASAP 2020 Plus BET

analyzer Breakthrough experiment was performed at room temperature

in the setup shown in Fig S11 The diameter of the adsorption column

was 20 mm To eliminate the gap between the carbon aerogels and

column, the carbon aerogels were surrounded by Teflon tape and packed

in the column The bulk volume of the carbon aerogels was 44 mL

Before the experiment, the carbon aerogels were dried at 100 ◦C

over-night under a flow of N2 A digital mass flow controller (Bronkhorst, HI-

TEC, Netherlands) was used to feed the CO2/N2 gas mixture (10%/90%

by volume) to the column at a flow rate of 65 ml/min The CO2

con-centration at the outlet of the column was measured using an online

carbon dioxide analyzer (CA-10, Sable Systems International, North Las

Vegas, NV, USA) A blank experiment was conducted with an empty

column Compression testing was conducted using a TA Q800 dynamic

mechanical analyzer (TA Instruments, New Castle, DE, USA) At least

four specimens were cut into cubes with an edge length of

approxi-mately 1 cm and tested at 30 ◦C with a strain rate of − 5% min− 1 in both

the axial and radial directions (parallel and perpendicular to the freezing

direction of ice templating, respectively) Average values of elastic

modulus (E) and yield strength were calculated, and the specific elastic

modulus (E s) of the aerogel was determined as follows:

E s=E

CRediT authorship contribution statement

Shiyu Geng: Conceptualization, Methodology, Investigation,

Su-pervision, Data curation, Formal analysis, Visualization, Writing –

original draft, Writing – review & editing Alexis Maennlein:

Investi-gation, Validation Liang Yu: InvestiInvesti-gation, Validation, Formal analysis,

Writing – review & editing Jonas Hedlund: Funding acquisition,

Re-sources, Supervision, Writing – review & editing Kristiina Oksman:

Funding acquisition, Project administration, Resources, Supervision,

Writing – review & editing

Declaration of competing interest

The authors declare that they have no known competing financial

interests or personal relationships that could have appeared to influence

the work reported in this paper

Acknowledgment

This work was financially supported by Swedish Strategic Research

Program Bio4Energy Maria Harila is acknowledged for sample

prepa-ration for the breakthrough test Prof Mohini Sain is thanked for kindly

providing the lignoboost lignin

Appendix A Supplementary data

Supplementary data to this article can be found online at https://doi

org/10.1016/j.micromeso.2021.111236

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