1

Optimization of the pore structure of biomass-based carbons in

relation to their use for CO2 capture at low and high pressure regimes

Marta Sevilla,a*

Abdul Salam M. Al-Jumialy,b Antonio B. Fuertes,

a Robert

Mokayab*

a Instituto Nacional del Carbón (CSIC), Francisco Pintado Fe, 26, Oviedo 33011, Spain

b School of Chemistry, University of Nottingham, University Park, NG7 2RD

Nottingham, UK.

* Corresponding author: martasev@incar.csic.es (M. Sevilla);

r.mokaya@nottingham.ac.uk (R. Mokaya)

Keywords: carbon capture, porosity, adsorption, melamine, pressure, activated carbon

2

Abstract

A versatile chemical activation approach for the fabrication of sustainable porous

carbons with a pore network tunable from micro- to hierarchical micro-/mesoporous is

hereby presented. It is based on the use of a less corrosive and less toxic chemical, i.e.

potassium oxalate, than the widely used KOH. The fabrication procedure is exemplified

for glucose as precursor, although it can be extended to other biomass derivatives

(saccharides) with similar results. When potassium oxalate alone is used as activating

agent, highly microporous carbons are obtained (SBET ~ 1300 - 1700 m2 g

-1). When a

melamine-mediated activation process is used, hierarchical micro-/mesoporous carbons

with surface areas as large as 3500 m2 g

-1 are obtained. The microporous carbons are

excellent adsorbents for CO2 capture at low pressure and room temperature, being able

to adsorb 4.2 - 4.5 mmol CO2 g-1

at 1 bar and 1.1 - 1.4 mmol CO2 g-1

at 0.15 bar. On the

other hand, the micro-/mesoporous carbons provide record-high room temperature CO2

uptakes at 30 bar of 32 - 33 mmol g-1

CO2 and 44 - 49 mmol g-1

CO2 at 50 bar. The

findings demonstrate the key relevance of pore size in CO2 capture, with narrow

micropores having the leading role at pressures < 1 bar and supermicropores/small

mesopores at high pressures. In this regard, the fabrication strategy presented here

allows fine-tuning of the pore network to maximize both the overall CO2 uptake and the

working capacity at any target pressure.

3

1. Introduction

Porous carbon materials, as exemplified by activated carbons (ACs), have been

commonly used in catalysis1 and adsorption processes for water decontamination

2 and

are currently in high demand for applications in fields such as energy storage

(supercapacitors and Li-ion batteries),3-5

energy production (electrocatalysts or

electrocatalyst supports for fuel cells),6-8

and CO2 capture or gas storage (H2, CH4),9-12

besides environmental remediation (heavy metals, dyes, Hg, H2S, SO2, etc.).13-15

In

particular, CO2 capture by physical adsorption using porous carbons as adsorbents for

pressure/temperature swing adsorption processes is regarded as a promising alternative

technology to the traditional amine-based liquid-phase absorption process owing to

lower energy requirements for adsorbent regeneration, lower cost and being

environmentally friendly.9, 16-19

Depending on whether CO2 capture is targeted at low

(post-combustion capture) or high pressures (pre-combustion capture), a suitably

targeted design of the porous structure is necessary. Thus, for low CO2 partial pressure

(≤ 1 bar), several studies have demonstrated the key role of narrow micropores (< 0.8 -

1 nm) owing to the enhanced adsorption potential in such pores,20-24

whereas for high

pressure CO2 capture, pore filling of larger pores takes place and therefore well-

developed supermicroporosity and/or mesoporosity is important.25-28

One of the main

attractions of porous carbons is the ease with which their porosity may be designed. In

this regard, carbons generated via nanocasting techniques,29-31

and the so-called carbide-

derived-carbons (CDCs)24, 32, 33

have shown great potential for precise control of their

pore size distribution (PSD). However, these techniques are complex, time-consuming

and often involve toxic reagents or products, which hampers their commercial

exploitation. Therefore, on the basis of economic considerations, simplicity and

environmental friendliness, activation approaches continue to be the main choice for

4

porous carbon synthesis.3, 5, 34

Similarly, in recent years, the choice of carbon precursor

for activated carbons has shifted from fossil sources to renewable sources such as

biomass.3, 34-36

Within this context, many improvements have been made in relation to

the ability to control the porous structure of carbons, especially those generated by

chemical activation approaches. Amongst chemical activating agents, KOH is the

leading choice for the production of highly porous carbons, routinely achieving BET

surface areas in excess of 2500 - 3000 m2 g

-1, along with a relatively well-controlled

PSD.5 This control of porosity, normally within the micro-/supermicropore regime, is

possible by varying the activation conditions, mainly temperature and the amount of

KOH used.5 Moreover, we have recently shown that by adding melamine to the mixture

of KOH and biomass, the PSD can be extended into the mesopore region thus creating

hierarchical carbons, which opens up the door for KOH-derived ACs to other

applications such as high pressure gas storage, adsorption of biomolecules or ionic

liquid-based supercapacitors where larger pores are required.26, 37

However, despite

having favorable characteristics for the production of advanced porous carbons, the

large-scale industrial implementation of KOH as activating agent is hindered by its

toxicity and high corrosiveness.38

Thereby, the quest for more environmentally friendly

and less corrosive activating chemicals or processes is a priority in porous carbon

synthesis. Furthermore, the developed synthesis procedures should be versatile and

allow control of the PSD over a wide range of pore size in order to broaden the portfolio

of applications for the developed materials.

Herein a versatile and less corrosive activating chemical agent, i.e. potassium

oxalate, is studied for the development of porous carbon materials with controlled

textural properties. As carbon precursor, glucose has been used based on the fact that it

is derived from biomass, although the synthesis procedure described herein can be

5

successfully applied and with similar results for other biomass-derived precursors. The

versatility of the synthesis procedure is demonstrated by the synthesis of two types of

carbon materials: (i) microporous carbons (by using potassium oxalate alone) that have

excellent performance for CO2 capture at low pressures and (ii) hierarchical micro-

/mesoporous carbons (by using potassium oxalate+melamine) with excellent capacity

for CO2 capture at high pressures.

2. Experimental section

2.1 Synthesis of microporous carbon materials

-D-Glucose (96%, Aldrich) was chemically activated using potassium oxalate

monohydrate (Alfa Aesar). Briefly, glucose was thoroughly mixed with potassium

oxalate monohydrate (potassium oxalate/glucose weight ratio = 3.6) in a mortar.

Afterwards, the mixture was introduced in a high-alumina crucible and subjected to a

high temperature treatment (800 or 850 ºC) in a vertical tube furnace under N2 flow

(heating rate = 5 ºC min-1

), and held at the desired temperature for 1 or 5 h before

cooling. Purification of the samples was done with dilute hydrochloric acid and then

distilled water until neutral pH. The ACs thus produced were dried in an oven at 120 ºC

for 3 h. The resulting glucose-derived ACs were designated as G-T, where T is

activation temperature (the sample activated for 5 h was labeled by adding “-5” to the

aforementioned designation). For comparison purposes, glucose-derived hydrochar (0.5

M, 180 ºC, 5 h), sucrose and starch were activated at 850 ºC, and eucalyptus sawdust at

800 ºC, using the same ratio of activating agent to precursor. These samples were

denoted, respectively, as HC-850, S-850, A-850 and SW-800. The product yield was

calculated using the formula: product yield = mass of AC/mass of precursor 100.

6

2.2 Synthesis of micro-/mesoporous carbon materials

-D-Glucose (96%, Aldrich) was chemically activated using a mixture of potassium

oxalate monohydrate (Alfa Aesar) and powdered melamine (Aldrich). Glucose was

thoroughly mixed with potassium oxalate monohydrate and melamine powder

(potassium oxalate/glucose weight ratio = 1.8 to 3.6 and melamine/glucose weight ratio

= 1 to 3) in a mortar. Afterwards, the mixture was introduced in a high-alumina crucible

and subjected to a high temperature treatment at 800 ºC (3 ºC min-1

) in a vertical tube

furnace under N2 flow and held at the desired temperature for 1 h (Caution: precautions

should be taken when extracting the crucible from the furnace as KCN is contained in

the solid residue). Purification of the samples was also accomplished with dilute

hydrochloric acid (Caution: precautions should be taken when adding hydrochloric

acid as HCN is produced), then washed with distilled water until neutral pH and finally

dried in an oven at 120 ºC for 3 h. The ACs thus synthesized were labeled G-y-z, where

y is potassium oxalate/glucose weight ratio and z is the melamine/glucose weight ratio.

For comparison purposes, cellulose and eucalyptus sawdust were activated under

similar conditions, and the resulting ACs were denoted, respectively, as C-3.6-1 and

SW-3.6-2. The product yield was calculated as the mass of AC/mass of precursor 100.

2.3 Physical and chemical characterization

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)

micrographs were obtained respectively on a Quanta FEG650 (FEI) instrument and a

JEOL (JEM 2100-F) apparatus. The determination of the textural properties of the

carbon materials was performed through nitrogen physisorption at −196 °C on a

Micromeritics ASAP 2020 sorptometer. The apparent surface area (SBET) was calculated

from the N2 isotherms using the Brunauer-Emmett-Teller (BET) method and an

appropriate relative pressure range.39, 40

The total pore volume (Vp) was determined

7

from the amount of nitrogen adsorbed at P/Po ~ 0.95. The micropore volume (Vm) was

obtained by applying the Dubinin-Radushkevich equation (DR)41

or the Quenched-Solid

Density Functional Theory (QSDFT). The PSDs were determined by applying the

QSDFT method to the nitrogen adsorption data (slit pore model). Bulk elemental

composition (C, H, N and O) was determined using a LECO CHN-932 microanalyzer.

Thermogravimetric analysis (TGA) was performed on a TA Instruments Q6000 TGA

system. Raman spectra were obtained on a Horiba (LabRamHR-800) spectrometer

(laser wavelength = 514 nm and power = 25 mW). The curve fitting was done with a

combination of five Gaussian–Lorentzian line shapes that gave the minimum fitting

error. X-ray diffraction (XRD) patterns were acquired on a Siemens D5000 instrument

(40 kV and 20 mA, CuK radiation). Surface elemental composition was determined

through X-ray photoelectron spectroscopy (XPS) on a Specs spectrometer, using Mg

K (1253.6 eV) radiation from a double anode (150 W). Binding energies for the high-

resolution spectra were calibrated by setting C 1s to 284.6 eV.

2.4 CO2 adsorption measurements at atmospheric pressure

CO2 adsorption isotherms at 25 °C and up to 1 bar were acquired on a Nova 4200e

(Quantachrome) static volumetric analyzer. The adsorption kinetics of CO2, and CO2

adsorption–desorption cycling were measured at a temperature of 25 ºC using a

thermogravimetric analyser (C.I. Electronics) and following the procedure previously

reported by us. 42

2.5 CO2 adsorption measurements at high pressure

The CO2 uptake measurements (25 ºC, 0-50 bar) were performed with a Hiden XEMIS

intelligent gravimetric analyser. Prior to CO2 uptake determination, the carbons were

outgassed under vacuum at 250 °C typically overnight. Buoyancy corrections were

applied, and the measurements determined the excess CO2 uptake from which the total

8

storage capacity could be determined using the equation; θT = θExc + dCO2 × VT, where;

θT is total CO2 uptake, θExc is excess CO2 uptake, dCO2 is density (mmol g-1

) of CO2 gas

at the relevant temperature and pressure (from NIST website (http://www.nist.gov/)),

and VT is pore volume of the carbon.

2.6 H2 adsorption measurements at high pressure

Hydrogen uptake (25 ºC, 0-50 bar) was measured by gravimetric analysis with a Hiden

XEMIS intelligent gravimetric analyser using 99.9999% purity H2, additionally purified

by a molecular sieve filter. Previously to analysis, the carbon sample was outgassed

under vacuum at 250 °C overnight.

3. Results and Discussion

3.1 Structural properties of the microporous carbons and performance as CO2

sorbents at low pressure

The morphology and porous structure of glucose-derived carbons chemically activated

with potassium oxalate was analyzed by means of SEM and TEM/HRTEM. As revealed

by the SEM micrographs in Figure 1a-b and Figure S1a (Supporting information), the

morphology is dominated by sheet-like and foam-like particles. This morphology

contrasts with that achieved by potassium hydroxide, which is characterized by irregular

particles,43

but holds some similarity to that achieved via activation with potassium

bicarbonate, with respect to having a 3D framework with large macropores.44

We have

previously found that carbonization of some organic salts of potassium or sodium such

as potassium citrate and sodium gluconate45

gives rise to carbons with a sheet-like

morphology. However, it should be noted that potassium oxalate yields no carbon

(carbon yield is less than 0.02 %) when thermally treated under inert atmosphere.

9

Therefore, in the present scenario, potassium oxalate acts not only as an activator but

also as structure directing agent by inducing sheet-like morphology on the carbon

derived from the carbonization of glucose. It is likely that this effect is made possible by

the melting of both potassium oxalate (390 ºC) and glucose. Indeed, similar results have

been reported for the production of carbon materials using the molten salt

approximation.46, 47

In this regard, we have recently used potassium oxalate as activating

agent for sawdust-derived hydrochar, wherein the morphology of the precursor is

retained.48

To further ascertain the necessity for a melting process towards sheet-like

morphology, a precursor with a well-defined particle morphology such as glucose-

derived hydrochar, which is composed of microspheres, was activated with potassium

oxalate. As evidenced by Figure S1b, the spherical morphology is well preserved. In

order to further confirm the melting hypothesis, a saccharide that melts, i.e. sucrose,

was used as precursor. As evidenced by Figure S1c-d, similar to glucose, a sheet-like

morphology is obtained. A closer inspection of the glucose-derived particles by TEM

shows many transparent sheets with crumpled edges and foam-like particles with thin

walls (Figure 1c and Figure S2). Further magnification by HRTEM (Figure 1d) shows a

disordered porous structure, typical of ACs.

The pore development of these materials was further analyzed by means of N2

physisorption. The corresponding isotherms and PSDs are shown in Figure 2 and the

textural characteristics are compiled in Table 1. As can be seen in Figure 2a,

independently of the carbonization temperature or duration, all the isotherms are Type I

with pronounced adsorption at relative pressures < 0.05, indicating highly microporous

materials. This is corroborated by the PSDs in Figure 2b, which show no pores > 2 nm,

and the textural data listed in Table 1. Thus, all the materials have a relatively large

BET surface area of 1300 to 1700 m2 g

-1 and a pore volume in the 0.5-0.7 cm

3 g

-1 range,

10

with the micropore volume representing ≥ 94 % of the total pore volume. Independently

of the kind of biomass employed in the process, the textural characteristics are similar,

as revealed by the data in Table S1. As shown by previous studies, these textural

properties are beneficial for CO2 adsorption at ambient conditions.20, 21, 49

A rise in the

activation temperature slightly increases the pore volume, whereas rising the activation

duration at 850 ºC from 1 h (i.e., G-850) to 5 h (i.e., G-850-5) substantially enhances the

pore volume and also causes an enlargement of pore size owing to the progressive

gasification of carbon (reaction 4 below) by the CO2 evolved from the decomposition of

K2CO3 (reaction 3 below).

The whole activation process can be described by the following reactions:

(1) Decomposition of potassium oxalate in the range of ~ 500-580 ºC (see Figure S3a-

b): C2O2K2 → K2CO3 + CO

(2) Redox reaction between the carbon produced from glucose and the potassium

carbonate produced from potassium oxalate at T > 700 ºC (Figure S3b), which causes

the partial etching of carbon atoms under controlled activation conditions: K2CO3 + 2 C

→ 2 K + 3 CO

(3) Slow partial decomposition of potassium carbonate at 850 ºC (Figure S3a): K2CO3

→ K2O + CO2

(4) Partial carbon gasification at 850 ºC: C + CO2 → 2 CO

The progressive gasification of carbon at longer activation duration is further

supported by a decrease of the activation yield (see Table 1). It is worth mentioning that

the yield of the activation process (25-30 %) is higher than that of the pyrolysis of

glucose, which is ca. 20 % (Figure S3b). Analogous results are obtained in the case of

sucrose (carbonization yield = 23 % vs. activation yield = 29 %). As previously

mentioned, attempts at direct carbonization of potassium oxalate yielded no carbon,

11

which suggests that potassium oxalate catalyzes the dehydration and polymerization

reactions that take place during the initial stages of the carbonization process. This is

supported by the TGA curves in Figure S3b, which show that the weight loss ascribed

to dehydration/polymerization of glucose in a glucose/potassium oxalate mixture occurs

at a substantially lower temperature (i.e., ca. 150 ºC vs. 190 ºC for pure glucose). This

result agrees with the visual observation that the potassium oxalate+glucose mixture

turns brown at a considerably lower temperature compared to glucose alone.

The microstructure of the glucose-derived microporous carbons was studied by

XRD and Raman spectroscopy. Before acid washing, the XRD pattern shows the

presence of un-reacted potassium carbonate (Figure S4a). The potassium carbonate is

completely removed by acid washing, as supported by the XRD pattern in Figure S4a,

which additionally shows the typical broad (002) band at 2~24.7º and (10) band at

2~43º characteristic of amorphous carbon. The disordered nature of these materials is

confirmed by the first order Raman spectrum in Figure S4b. The D band at ca. 1350 cm-

1 associated with a double-resonance Raman process in disordered carbon is quite

intense and broad, overlapping with a similarly broad G band (at 1580 cm-1

), which is

ascribed to bond stretching of all pairs of sp2 atoms in both rings and chains.

50, 51 The

ratio of integrated intensities, ID/IG, that measures the degree of ordering in carbon

materials, possesses a value of 1.42.

The CO2 capture ability of these highly microporous materials was investigated

at 25 ºC and pressure of up to 1 bar, which are the conditions relevant to post-

combustion CO2 capture. The CO2 uptake isotherms are depicted in Figure 3a and the

adsorption capacity at various pressures is compiled in Table 2. These highly

microporous materials with a PSD mainly centered below 1 nm adsorb large amounts of

CO2 at 1 bar and 25 ºC, i.e. 4.2-4.5 mmol CO2 g-1

, values which are very competitive

12

compared to the highest reported uptakes to date (see Table S2). Furthermore, enhanced

CO2 adsorption (1.1-1.4 mmol CO2 g-1

) is obtained at typical flue gas conditions (CO2

partial pressure of 0.15 bar),16, 17

which highlights the potential of these materials as

sorbents for post-combustion CO2 capture. The superior performance of sample G-800

is explained by the presence of the narrowest micropores (see Figure 2b) which gives

rise to an enhanced interaction with the CO2 molecules, as can be deduced from Figure

S5, where the fraction of CO2 adsorbed is represented as a function of the increase of

pressure. Thereby, the fraction of CO2 adsorbed by sample G-800 at any given pressure

is higher than for the other two materials, especially compared to G-850-5, which

possesses a broader PSD in the micro-supermicropore range (see Figure 2b).

For post-combustion CO2 capture using pressure swing adsorption (PSA) or

vacuum swing adsorption (VSA) systems, a more relevant parameter than the uptake

capacity is the working capacity which takes into account the adsorption and desorption

(i.e., regeneration) pressures. In this regard, a typical PSA system may adsorb at a

pressure of 6 bar and desorb at 1 bar, whereas for VSA systems, adsorption may occur

at 1.5 bar and evacuation at ca. 0.05 bar.52

Hence, considering the typical flue gas

conditions, i.e. CO2 partial pressure of 0.15 bar, a PSA system would work between 0.9

and 0.15 bar, and a VSA system between 0.225 and 0.0075 bar. The corresponding

working capacities of these materials, included in Table 2, are still very attractive, in the

1.4-1.7 mmol CO2 g-1

for a VSA system and 2.8-3.1 mmol CO2 g-1

for a PSA system.9,

17, 53 As deduced from the data in Table 2, G-800 is especially suited for a VSA system,

which works in the lowest range of pressures and hence only the narrowest micropores

are filled with CO2. Meanwhile, G-850-5 is best suited for a PSA system, as this

material, on one hand, contains larger micropores ensuring a lower adsorption at the

regeneration pressure and, on the other hand, has a high micropore development

13

providing a large CO2 uptake at 0.9 bar. These results show the importance of a fine-

tuning of the porosity depending on the range of targeted pressures rather than only

considering the final pressure.

Besides large adsorption and working capacities, a sorbent to be used in swing

adsorption systems should be easily regenerated and should keep its performance with

cycling. Here the regeneration and cycling performance was tested by successive cycles

of CO2 adsorption at 1 bar followed by desorption under He, all at room temperature.

As exemplified by sample G-800 in Figure 3b, both adsorption and desorption processes

are rapid, with 95% of CO2 being adsorbed in 4 min and desorbed in ~ 3 min, in spite of

the narrow microporosity present in this material. The rapid sorption may be attributed

to the sheet-like particle morphology that entails short diffusion paths. Furthermore, no

discernible changes take place in the CO2 uptake or sorption kinetics after five

adsorption-desorption cycles.

3.2 Structural properties of the micro-/mesoporous carbons and performance as

CO2 sorbents at high pressure

In order to generate mesopores in the ACs targeted at high pressure CO2

adsorption, melamine was added to the potassium oxalate/glucose mixture employed in

the activation process. We have recently shown that the melamine-mediated KOH- and

KHCO3-activation of biomass-derived hydrochar is an effective strategy for producing

highly porous carbons with SBET > 3000 m2 g

-1 and a well-balanced micro-mesoporosity

(Vmicrop/Vmesop ~ 0.8 - 1).37, 54

The nitrogen sorption isotherms in Figure 4a-b and the

PSDs in Figure 4c-d show that the potassium oxalate/melamine mixture can also

generate highly micro-/mesoporous carbons from glucose. Carbon materials with

apparent surface areas as large as ~ 3300-3500 m2 g

-1 and Vmicrop/Vmesop ~ 0.6 - 0.9 are

14

obtained (Table 1). The volume of mesopores can be tuned through the modification of

the potassium oxalate/glucose and melamine/glucose weight ratios. Thus, for a given

potassium oxalate/glucose ratio, increase of the melamine/glucose ratio from 0 to 2

raises the percentage of pore volume corresponding to mesopores from nil to 63%,

along with a three-fold rise in the BET surface area (Table 1). The change in the PSD

from a microporous material (all pores < 1.2 nm) to a micro-/mesoporous material

(bimodal PSD centered at 0.9-1 nm and 2.1-2.6 nm) is evident in Figure 4c. Similarly,

increase of the potassium oxalate/glucose ratio (at a melamine/glucose ratio = 2), also

enhances the proportion of mesoporosity (Table 1). Figure 4d shows that all the

materials have a bimodal PSD in the micro-mesopore range, with pore maxima at 0.9-1

nm and 1.5-2.6 nm. Similar results are obtained for other saccharides such as cellulose

(see Table S1). However, in the case of lignocellulosic biomass such as eucalyptus

sawdust, an intermediate situation between that of saccharides and of hydrochar48

is

observed. Thus, as shown in Figure S6 and deduced from the data in Tables 1 and S1,

the amount of mesopores decreases following the trend: glucose (63%) > eucalyptus

sawdust (34%) > glucose-derived hydrochar (19%). This can be attributed to a higher

resistance (lower reactivity) of lignin (and also hydrochar) to the activating agent

potassium oxalate+melamine compared to saccharides owing to its higher degree of

aromatization.

The creation of porosity with the rise in synthesis temperature for a

glucose/potassium oxalate/melamine ratio = 1 / 3.6 / 2 was analyzed by N2 sorption.

The isotherms and PSDs are depicted in Figure 5, while the textural properties are

compiled in Table S2. Both the isotherms and PSDs indicate gradual enhancement of

the pore volume and an enlargement of the pore size as the synthesis temperature raises.

Thus, the total pore volume increases from 0.39 cm3 g

-1 at 500 ºC up to 2.72 cm

3 g

-1 at

15

800 ºC and, in parallel, the percentage of pore volume corresponding to micropores

decreases from 85% (500 ºC) to 37% (800 ºC), as shown in Table S2. For activation at

650 ºC, the material is still highly microporous with an apparent surface area and pore

volume surpassing those of glucose activated simply with potassium oxalate (see Table

1). The largest mesopore development takes place at 750 ºC, the temperature at which

the redox reaction (2) occurs. This is accompanied by a large decrease in the yield of

product, from 28% at 650 ºC to 9% at 750 ºC, and the largest removal of nitrogen

(Table S2). Further raise of the synthesis temperature from 750 to 800 ºC slightly

enhances the proportion of pores of size ~ 2.3 nm at the expense of a 1% drop in

product yield.

Although the incorporation of melamine into the activation mixture greatly

modifies the porous structure of the carbons, it does not seem to affect the particle

morphology, as revealed by the SEM image in Figure 6a. Closer inspection of the

particles via TEM evidences the micro-mesoporous structure of the material (see Figure

6b), in agreement with the N2 sorption data.

In addition to inducing modifications in the porous network of the carbon

materials, the presence of melamine also effects N-doping of the final carbon material,

in a manner similar to that of melamine-mediated KOH- and KHCO3-chemical

activation processes.37, 54

Materials activated using a high melamine/glucose ratio or low

potassium oxalate/glucose ratios are heavily N-doped, with N content > 15 %. The

nitrogen is homogeneously distributed within the particles, as inferred by EDX mapping

(Figure S7). It is noteworthy that the highly porous carbons with BET surface areas >

3000 m2 g

-1 retain 1-4 wt% N. Materials combining ultra-large surface areas, a

hierarchical porosity and moderate nitrogen contents are rarely found, yet they are

potentially very useful for energy-related or environmental applications.26, 42, 54-57

XPS

16

analysis further shows that the existing N-species are the typical ones in activated

carbons, i.e. N-pyridinic, N-pyrrolic/N-pyridonic, N-quaternary and pyridine-N-oxide,

in varying proportions depending on the synthesis conditions (Figure S8a). Both N-

doping and the large amount of pores in these materials lead to an increase of the D

band in the first order Raman spectra (see Figure S8b). Thus, the ratio of integrated

intensities, ID/IG, increases from 1.42 for G-800 to 2.15 and 2.36 for G-3.6-2 and G-1.8-

2, respectively.

The high pressure CO2 uptake of these materials was investigated at 25 ºC and

up to 50 bar. The total CO2 adsorption isotherms are depicted in Figure 7a, whereas the

excess adsorption isotherms are given in Figure S9 and the CO2 uptake at various

pressures is listed in Table 2. The highly micro-/mesoporous carbons with apparent

surface areas above 3300 m2 g

-1 provide record-high CO2 uptakes of 32-33 mmol g

-1

CO2 at 30 bar and 44-49 mmol g-1

CO2 at 50 bar (see Table S4 for comparison with

literature data). Furthermore, these ultra-highly porous materials are very far from

saturation even at 50 bar. On the contrary, the microporous carbon G-850-5 is already

saturated for pressures > 15-20 bar, but demonstrates the highest CO2 uptake for

pressures below 4 bar, ca. 8 mmol g-1

CO2. This different pressure-dependence

performance of the diverse materials can be directly correlated with their textural

properties and, more specifically, with their cumulative pore volume of pores below

certain size, which further supports the key relevance of pore size in CO2 capture (and

also suggests the negligible influence of surface chemistry). Thus, three performance

regions can be identified in the CO2 isotherms in Figure 7a: i) P < 4 bar (region I), ii) 4

bar < P < 23 bar (region II), and iii) P > 23 bar (region III). The three regions are

identified in the graph of cumulative pore volume versus pore size in Figure 7b. In this

way, for P < 4 bar, the material which provides the highest CO2 uptake is the one with

17

the largest volume of pores below 1.3 nm (see correlation in Figure S10a). In this case,

owing to the low relative pressure (P/Po ~ 0.06), only small micropores can be filled

following a volume-filling mechanism as a result of the enhanced adsorption potential

caused by the overlapping of the potential fields of neighboring pore walls. For P < 23

bar (relative pressure ~ 0.36), the highest CO2 uptake is provided by the material with

the largest volume of pores below 2.4 nm (see correlation in Figure S10b), which

explains why the highly microporous material still behaves better than a micro-

/mesoporous material such as G-2.3-2 at such high pressures, as the former has a larger

volume of pores below 2-2.4 nm. At the highest evaluated pressure, i.e. 50 bar (relative

pressure ~ 0.78), the material with the largest pore volume provides the highest CO2

uptake (see correlation in Figure S10d). In the case of an intermediate pressure such as

30 bar, the best correlation is achieved with the volume of pores below 3 nm (see Figure

S10c). These results agree with previous studies at high pressures - albeit with slight

differences in the key pore sizes,25, 58, 59

and clearly show that CO2 can fill

supermicropores/small mesopores (i.e., pores with lower adsorption potential) at higher

relative pressures by means of a surface coverage mechanism, similar to N2

physisorption. Indeed, it has previously been shown that CO2 behaves in a similar

manner to N2 if a comparable range of relative pressures is considered for both

adsorbates.60

The stronger adsorption potential in micropores is again revealed by

representing the fraction of CO2 adsorbed as a function of pressure, as in Figure S11. In

fact, it can be clearly seen that the larger the percentage of microporosity in the

material, the higher the interaction of the material with the CO2 molecules.

Nevertheless, for high pressures (P > 20 bar), micropores are already saturated and

mesopores are essential for achieving a high CO2 uptake.

18

The working capacity of these materials in a pre-combustion CO2 capture PSA

system was evaluated based on typical conditions of a pre-combustion gas stream, i.e.

~40 bar and 40% CO2.61

Setting the regeneration pressure at 2 bar, these conditions

imply that the PSA system would work between a pressure of 16 and 0.8 bar. As can be

deduced from the values given in Table 2, the PSA working capacity of these micro-

mesoporous materials is still impressive, at ca. 19 mmol CO2 g-1

. By increasing the

adsorption pressure of the PSA system up to 75 bar (PCO2 = 30 bar), values higher than

29 mmol CO2 g-1

are obtained (Table 2). As can be deduced from the comparison in

Table S4, the working capacity of the materials here developed is also much higher than

that of the best carbon adsorbents reported up to date. A closer inspection of the data in

Table 2 reveals the importance of mesopores for high pressure CO2 capture using a PSA

system, confirming our previous results.26, 62

Thus, the difference in CO2 uptake

between G-850-5 and G-2.3-2 is only 10 % at 30 bar and G-850-5 outperforms G-2.3-2

by 14% at 16 bar. However, G-2.3-2 outperforms G-850-5 on the basis of working

capacity by more than 40% in the 1-30 bar range and matches G-850-5 in the 0.8-16 bar

range, which is due to the much lower uptake of the micro-/mesoporous G-2.3-2

material at the regeneration pressure (1 bar), i.e. 2.1 mmol CO2 g

-1 vs. 4.5 mmol CO2

g

-1

for the microporous material G-850-5.

For pre-combustion CO2 capture, another important characteristic is the

selectivity against H2, since CO2 has to be separated from H2 in shifted-syngas. Typical

conditions of the pre-combustion gas might be 40% CO2 and 55% H2 (40 bar).61

We

therefore analyzed the H2 adsorption isotherm of the best adsorbent, i.e. G-3.6-2, which

is compared with the corresponding CO2 uptake isotherm in Figure 8. As can be seen, at

the highest evaluated pressure, i.e. 50 bar, the excess CO2 uptake (41 mmol g-1

) is much

higher than the excess H2 uptake (2.8 mmol g-1

), which provides an equilibrium CO2/H2

19

selectivity for the pure gases of ca. 15, rising to 16 at 40 bar. Furthermore, considering

the typical conditions of shifted-syngas indicated above, the selectivity for the CO2/H2

mixture was also determined, using the ideal adsorbed solution theory (IAST) model.

According to the IAST model, the selectivity (SIAST) for the CO2/H2 mixture is

calculated as: SIAST = (𝑛𝐶𝑂2 𝑛𝐻2⁄ )/(𝑃𝐶𝑂2 𝑃𝐻2⁄ ), where 𝑛𝐶𝑂2 and 𝑛𝐻2 are the CO2 and H2

uptakes (in mmol g-1

) and 𝑃𝐶𝑂2 and 𝑃𝐻2 are the partial pressure of CO2 and H2

respectively. In this way, the CO2/H2 selectivity is determined to be 23. Therefore, G-

3.6-2 combines an ultra-large CO2 uptake, a high working capacity and a good

selectivity against H2 at the conditions relevant to pre-combustion capture. This

combination of features makes it a promising candidate for pre-combustion CO2

capture.

4. Conclusions

In summary, biomass-based highly porous carbons with a pore structure tunable from a

microporous to a hierarchical micro-/mesoporous network have been produced by a

chemical activation approach using potassium oxalate, a less corrosive and less toxic

substance compared to KOH, as activating agent. Direct activation of glucose with

potassium oxalate yields highly microporous carbons (SBET ~ 1300-1700 m2 g

-1),

which

show excellent CO2 uptake at low pressures and room temperature (1.1-1.4 mmol CO2

g-1

at 0.15 bar and 4.2-4.5 mmol CO2 g-1

at 1 bar). On the other hand, a melamine-

mediated activation approach produces hierarchical micro-/mesoporous carbons with

surface area as large as 3500 m2 g

-1 and pore volume of up to 2.7 cm

3 g

-1. The

percentage of pore volume corresponding to mesopores can be tuned through

modification of the potassium oxalate/glucose and melamine/glucose weight ratios.

These micro-/mesoporous carbons provide record-high CO2 uptakes compared to

20

previously reported carbon adsorbents at room temperature and high pressure. Thus,

they are able to adsorbed 32-33 mmol g-1

CO2 30 bar and 44-49 mmol g-1

CO2 at 50 bar.

The CO2 uptake at various pressures correlates well with the volume of pores of a

critical size, which clearly supports the key relevance of pore size in CO2 capture and

provides a guideline for the development of pressure-targeted high-performance CO2

sorbents. Furthermore, precise tuning of the porosity is essential to achieve not only a

high CO2 uptake, but also a high working capacity in pressure or vacuum swing

adsorption systems and a good CO2/H2 selectivity.

Acknowledgments

This research study was funded by the FICYT Regional Project (GRUPIN14-102), and

the Spanish MINECO-FEDER (CTQ2015-63552-R). We thank the government of Iraq

for funding a PhD studentship for Abdul Salam Al-Jumialy.

Supporting Information

SEM, SEM-EDX and TEM images of porous carbons, thermogravimetric analysis

curves, XRD patterns, fraction of CO2 adsorbed as a function of pressure at low and

high pressure, pore size distributions, N1s XPS and Raman spectra, high-pressure

excess CO2 uptake isotherms, correlation between the CO2 uptake and various pressures

and the cumulative pore volume of pores below certain key size.

References

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24

Table 1. Physico-chemical properties of the porous carbons obtained by chemical

activation of glucose with potassium oxalate or potassium oxalate+melamine.

a Total pore volume was determined at a P/Po of ~ 0.95.

b Micropore volume was determined by

using the Dubinin-Radushkevich equation or the QSDFT PSD. The percentage of pore volume

that corresponds to micropores is indicated in parenthesis.

Table 2. CO2 uptake of the porous carbons obtained by chemical activation of glucose

with potassium oxalate or potassium oxalate+melamine.

a Range of pressure: 0.225-0.0075 bar.

b Range of pressure: 0.9-0.15 bar.

c Range of pressure:

0.8-16 bar. d Range of pressure: 1-30 bar.

Activating

agent Code

Yield

(%)

Textural properties Chemical composition

[wt %]

SBET

[m2 g

-1]

Vp

[cm3 g

-1]

a

Vmicro

[cm3 g

-1]

b

C O N

Potassium

oxalate

G-800 30 1270 0.50 0.49 (98) - - -

G-850 28 1330 0.53 0.51 (96) - - -

G-850-5 25 1690 0.72 0.67 (93) - - -

Potassium

oxalate

+

Melamine

G-1.8-2 37 1240 0.69 0.47 (68) 74.3 6.8 17.3

G-2.3-2 22 1520 0.96 0.52 (54) 73.5 9.6 15.4

G-2.7-2 10 3310 2.36 1.00 (42) 90.1 5.4 3.8

G-3.6-2 8 3460 2.72 1.00 (37) 92.5 4.4 2.7

G-3.6-1 14 3470 2.37 1.10 (46) 94.4 4.2 1.0

G-3.6-3 30 870 0.59 0.30 (51) 65.4 11.9 20.6

Code

CO2 uptake at 25 ºC

(mmol g-1

)

Working capacity at 25 ºC

(mmol g-1

)

0.15 bar 1 bar 30 bar 50 bar Low pressure High pressure

VSAa PSA

b PSA

c PSA

d

G800 1.4 4.5 - - 1.7 2.9 -

G850 1.2 4.2 - - 1.5 2.8 -

G850-5 1.1 4.5 14.3 15.4 1.4 3.1 8.9 9.8

G-2.3-2 - 2.1 15.8 22.3 - - 9.1 13.7

G-2.7-2 - 2.1 31.7 45.9 - - 18.6 29.6

G-3.6-2 - 1.5 32.6 49.1 - - 18.8 31.1

G-3.6-1 - 2.5 31.9 44.4 - - 19.4 29.4

25

Figure 1. a and b) SEM, c) TEM and d) HRTEM images of glucose-derived carbon

chemically activated with potassium oxalate (G-850).

400 nm

a b

c d

30 mm

26

Figure 2. a) N2 sorption isotherms and b) pore size distributions of the porous carbons

obtained by chemical activation of glucose with potassium oxalate.

Figure 3. a) CO2 sorption isotherms at 25 ºC over the pressure range of 0 to 1 bar for

the microporous carbons and b) CO2 adsorption-desorption cycles at 25 ºC

corresponding to sample G-800 (CO2 concentration: 100%).

1 2 3

0

0.5

1.0

1.5

2.0

GOxK800

GOxK850

GOxK850-5

dV

(d)

(cm

3 n

m-1 g

-1)

Pore size (nm)

0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

Adsorb

ed v

olu

me (

cm

3 S

TP

g-1)

Relative pressure (p/po)

G-800

G-850

G-850-5

a) b)

a) b)

0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

CO

2 u

pta

ke (

mm

ol g

-1)

Pressure (bar)

G-800

G-850

G-850-5

0 20 40 60 80 1000

1

2

3

4

5

C

O2 u

pta

ke (

mm

ol g

-1)

Time (min)

27

Figure 4. a and b) N2 sorption isotherms, and c and d) pore size distributions of porous

carbons obtained by chemical activation of glucose with a mixture of potassium oxalate

and melamine.

1

0

0.5

1.0

1.5

G-1.8-2

G-2.3-2

G-2.7-2

G-3.6-2

dV

(d)

(cm

3 n

m-1 g

-1)

Pore size (nm)

0 0.2 0.4 0.6 0.8 1.00

500

1000

1500

2000

Adsorb

ed v

olu

me (

cm

3 S

TP

g-1)

Relative pressure (p/po)

G-3.6-0

G-3.6-1

G-3.6-2

G-3.6-3

0 0.2 0.4 0.6 0.8 1.00

500

1000

1500

2000

Adsorb

ed v

olu

me (

cm

3 S

TP

g-1)

Relative pressure (p/po)

G-1.8-2

G-2.3-2

G-2.7-2

G-3.6-2

a) b)

c) d)

1

0

0.5

1.0

1.5

G-3.6-0

G-3.6-1

G-3.6-2

G-3.6-3

dV

(d)

(cm

3 n

m-1 g

-1)

Pore size (nm)

28

Figure 5. a) N2 sorption isotherms and b) pore size distributions of the porous carbons

obtained by carbonization at different temperatures (500-800 ºC) of a mixture of

glucose/potassium oxalate/melamine = 1 / 3.6 / 2.

0 0.2 0.4 0.6 0.8 1.00

500

1000

1500

2000

A

dsorb

ed v

olu

me (

cm

3 S

TP

g-1)

Relative pressure (p/po)

500 ºC

650 ºC

750 ºC

800 ºC

Increase of TIncrease

of T

a) b)

1

0

0.5

1.0

1.5

w/o melamine, 800 ºC

500 ºC

650 ºC

750 ºC

800 ºC

dV

(d)

(cm

3 n

m-1 g

-1)

Pore size (nm)

29

Figure 6. a) SEM and b) HRTEM pictures of the micro-/mesoporous carbon G-2.7-2.

3 mm

a b

30

Figure 7. a) High-pressure CO2 total uptake isotherms at 25 ºC over the pressure range

of 0-50 bar and b) cumulative pore volume versus pore size for the microporous carbon

G-850-5 and several micro-/mesoporous carbons.

Figure 8. H2 and CO2 excess uptake isotherms at 25 ºC for the micro-/mesoporous

carbon G-3.6-2.

0 5 25 300

0.5

1.0

1.5

2.0

2.5

G-850-5

G-2.3-2

G-2.7-2

G-3.6-2

G-3.6-1

Cum

ula

tive p

ore

volu

me (

cm

3g

-1)

Pore size (nm)

a) b)

I II III I IIIII

0 10 20 30 40 500

10

20

30

40

50

CO

2 u

pta

ke (

mm

ol g

-1)

Pressure (bar)

G-850-5

G-2.3-2

G-2.7-2

G-3.6-2

G-3.6-1

0 10 20 30 40 500

10

20

30

40

Gas u

pta

ke (

mm

ol g

-1)

Pressure (bar)

Excess CO2

Excess H2

31

TOC