Sustainable porous carbons with a superior performance for CO
2
capture
Marta Sevilla and Antonio B. Fuertes
*
Instituto Nacional del Carbón (CSIC), P.O. Box 73, 33080 Oviedo, Spain
*
Corresponding author (E-mail: abefu@incar.csic.es)
Abstract
Sustainable porous carbons have been prepared by chemical activation of hydrothermally
carbonized polysaccharides (starch and cellulose) and biomass (sawdust). These materials
were investigated as sorbents for CO
2
capture. The activation process was carried out under
severe (KOH/precursor=4) or mild (KOH/precursor=2) activation conditions at different
temperatures in the 600-800ºC range. Textural characterization of the porous carbons showed
that the samples obtained under mild activating conditions exhibit smaller surface areas and
pore sizes than those prepared by employing a greater amount of KOH. However, the mildly
activated carbons exhibit a good capacity to store CO
2
, which is mainly due to the presence
of a large number of narrow micropores (< 1 nm). A very high CO
2
uptake of 4.8 mmol⋅g
-1
(212 mg
CO
2
⋅g
-1
) was registered at room temperature (25ºC) for a carbon activated at 600ºC
using KOH/precursor=2. To the best of our knowledge, this result constitutes the largest ever
recorded CO
2
uptake at room temperature for any activated carbon. Furthermore, we
observed that these porous carbons have fast CO
2
adsorption rates, a good selectivity for
CO
2
-N
2
separation and they can be easily regenerated.
2
Broader context
The mitigation of carbon dioxide emissions is attracting widespread attention due to the fact
that this gas is the main anthropogenic contributor to climate change. Among the possible
strategies for CO
2
abatement, that of capture and storage has attracted keen interest. In this
regard, the use of solid sorbents to capture CO
2
by means of pressure, temperature or vacuum
swing adsorption systems constitutes a promising alternative. To accomplish this objective
the sorbents need to satisfy important conditions: i) low-cost and high availability, ii) a large
CO
2
uptake, iii) a high sorption rate, iv) a good selectivity between CO
2
and other competing
gases (i. e. N
2
) and v) an easy regenerability. However, the development of a solid sorbent
that satisfies all these conditions has so far proved to be complex. Here we present a novel
route for the preparation of carbon-based porous sorbents of CO
2
from a low-cost sustainable
biomass product (sawdust). The results obtained show that these carbon sorbents exhibit very
large CO
2
adsorption uptakes of up to 4.8 mmol⋅g
-1
(212 mg
CO
2
⋅g
-1
) at room temperature
(25ºC), a value that far exceeds those reported in the literature for activated carbons.
Furthermore, these carbon sorbents exhibit high sorption rates, a good CO
2
-N
2
selectivity and
they can be easily regenerated.
3
Introduction
The control of anthropogenic CO
2
emissions is a crucial matter in view of the
significant role that this gas plays in global climate change. In recent years, great efforts have
been directed towards the development of new technologies for CO
2
capture and its storage,
the improvement of energy efficiency and the generation of energy from non-fuel sources.
For the capture of CO
2
, the most popular technology is the absorption process using
alkanolamine solvents.
1
However, this process presents several disadvantages, such as a high
energy consumption, solvent regeneration, the corrosion of the equipment and toxicity.
2
A
promising alternative technology to the liquid-phase absorption process is to use porous
solids as sorbents for capturing CO
2
by means of pressure, temperature or vacuum swing
adsorption systems.
3-8
To this end, numerous porous solids including zeolites, metal-organic
frameworks (MOFs), porous carbons or organic-inorganic hybrid sorbents have been
investigated.
9-11
Hybrid sorbents require costly and multi-step fabrication procedures that
involve the impregnation or grafting of porous solids (i. e. silica or carbons) with different
types of amines.
12-16
Furthermore, these materials require high regeneration temperatures and
undergo a substantial loss of adsorption capacity after several cycles. Of the sorbents
mentioned so far, porous carbons have several important advantages in terms of cost,
availability, large surface area, an easy-to-design pore structure, hydrophobicity and low
energy requirements for regeneration. However, most activated carbons exhibit CO
2
uptakes
below ~ 3-4 mmol CO
2
⋅g
-1
sorbent (25ºC, 1 atm), the value reported as representing the
minimum working capacity necessary to compete with the liquid-phase amine systems.
17
In
order to improve the CO
2
-adsorption capacity of porous carbons, a large amount of research
effort has been directed towards the creation of superficial basic sites via the incorporation of
nitrogen groups into the carbon framework.
18-20
However, until now this strategy has only
produced moderate enhancements of CO
2
uptakes. Thus, Hao et al. recently reported a
maximum CO
2
adsorption capacity of 3.13 mmol⋅g
-1
(25ºC, 1 atm) for N-enriched carbon
4
samples (N content up to 1.9 wt %).
20
Wahby et al. showed that carbons with high surface
areas produced by the chemical activation of petroleum pitch exhibit an excellent
performance for CO
2
adsorption (4.7 mmol⋅g
-1
at 25ºC and 1 atm).
21
This result clearly
demonstrates that porous carbons with a suitable pore structure are excellent sorbents for CO
2
capture.
Taking into account the potential scale involved in the production of porous carbons
for CO
2
capture, the use of renewable sources for fabricating these materials would seem
highly desirable. This could be achieved by employing biomass or biomass-derived products
as precursors for the production of carbon sorbents for CO
2
capture. Unfortunately, to date
this alternative has hardly been explored to this end. In this paper we present a novel route for
the preparation of carbon-based CO
2
sorbents from sustainable biomass products. The results
obtained show that these carbon materials exhibit large CO
2
adsorption uptakes, which are far
superior to those reported in the literature for activated carbons. The present work focuses
above all on the relationship between the porous characteristics of the carbon sorbents and
their capacity for CO
2
adsorption. Interestingly, the porosity of these carbons can be easily
designed by simply modifying the operational conditions (i. e. activation temperature and the
amount of activating agent). It is worth mentioning that, if the pore characteristics of these
carbons are properly designed, their capacity for CO
2
adsorption can be optimized and fully
exploited.
Experimental
Synthesis of porous carbons
Carbonaceous materials were prepared by hydrothermal carbonization of the following
substances: potato starch (Sigma-Aldrich), cellulose (Aldrich) and eucalyptus sawdust.
Briefly, an aqueous dispersion of the raw material (concentration: 320 g L
-1
) was placed in a
stainless steel autoclave, heated up to 250ºC and maintained at this temperature for 2 h. The
5
resulting carbonaceous solid, denoted as hydrochar (HC), was recovered by filtration, washed
with distilled water and dried.
The hydrochar materials were chemically activated using potassium hydroxide
(Sigma-Aldrich). Briefly, a HC sample was thoroughly mixed with KOH at the desired
weight ratio (KOH/HC=2 or 4), after which the mixture was heat treated up to the target
temperature (heating rate: 3ºC min
-1
) under a nitrogen gas flow and held at this temperature
for 1 h. The samples were then thoroughly washed several times with 10 wt % HCl to remove
any inorganic salts, washed with distilled water until neutral pH and finally dried in an oven
at 120ºC for 3 h. The activated carbons thus synthesized were denoted as AX-x-z, where X
refers to the raw material (A: starch, C: cellulose and S: sawdust), y the KOH/HC weight
ratio and z the activation temperature (in ºC).
Characterization of materials
The morphology of the samples was examined by Scanning Electron Microscopy (SEM)
using a Zeiss DSM 942 microscope. Transmission electron micrographs (TEM) were taken
on a JEOL (JEM-2000 FX) apparatus operating at 200 kV. The nitrogen sorption isotherms
and textural properties of the carbons were determined at -196
o
C using a conventional
volumetric technique (Micromeritics ASAP 2020). The surface area was calculated by the
BET method from the adsorption data obtained in the relative pressure (p/p
o
) range of 0.04 to
0.2. The total pore volume was determined from the amount of nitrogen adsorbed at
p/p
o
=0.99. The pore size distribution (PSD) was calculated via a Non Local Density
Functional Theory (NLDFT) method using nitrogen adsorption data and assuming a slit pore
model. The micropore surface area and total micropore volume (pore size < 2 nm) were
obtained via a t-plot analysis. The volume of the narrow micropores (< 0.7 mm) was
determined by the applying the Dubinin-Radushkevitch (D-R) equation to the CO
2
adsorption
data at 0ºC.
22
