CO₂ Adsorption on Nanocomposite Pollen-Derived Carbons: Synergic Effect of KOH Activation and MgO Incorporation

Abstract

Novel nanocomposites synthesized with pollen grain, as a precursor, were tested for CO₂ adsorption performance. The pollen-derived carbons were obtained by direct calcination (PC), calcination followed by KOH-chemical activation (PCKOH), and MgO incorporation using high-energy ball-milling (PC-MgO and PCKOH-MgO). All the as-prepared materials were texturally, morphologically and structurally characterized by N₂ physisorption, scanning electron microscopy (SEM-EDS) and X-ray diffraction (XRD); CO₂ adsorption/desorption was measured by thermogravimetry. The four materials were able to adsorb CO₂ at 30 ºC and 1 bar, but PCKOH-MgO, with the high surface area (12.4 m² g^-1), total pore volume of (0.13 cm³ g^-1) and microporosity, was the best (2.06 mmol g^-1 and percentage of adsorption (Q_ad) = 9.06%, at 1 bar). This result is due to the synergic effect of both synthesis methods. Neither temperature (from 30 to 100 ºC), nor pressure (from 1 to 15 bar), improved the adsorption process of PCKOH-MgO. CO₂ desorption times at 300 ºC were 47 min for PCKOH-MgO and 30 min for the other adsorbents, showing the possibility of cycling. Based on the obtained results, the PCKOH-MgO activated carbon nanocomposite stands as a promising adsorbent to be potentially used for industrial CO₂ capture applications and to contribute to long-term solutions to avoid dangerous climate-change and reduce CO₂ emissions.

Keywords:
pollen-derived carbons; KOH-chemical activation; MgO ball milling incorporation; textural properties; CO₂ adsorption/desorption

Introduction

CO₂ is mostly emitted, at great scale, by thermal power plants and other large energy facilities that use fossil-based fuels (natural gas, oil and coal);¹,² the drastic increase in atmospheric CO₂ concentrations has contributed to the phenomenon known as “global warming”.³ To mitigate increasing CO₂ levels and/or reduce the negative environmental impacts, several methods and technologies for CO₂ capture have been developed and are recognized as potential solutions.⁴ Among these methods, CO₂-adsorption in solid adsorbents has been extensively studied due to low operating cost, low energy requirements, low secondary waste generation and applicability over a wide range of temperatures and pressures.⁵,⁶,⁷,⁸ Carbon-based materials are considered as some of the most dominant low-cost adsorbents due to their textural characteristics, which confer them high capacity and selectivity towards CO₂ capture under ambient conditions. The adsorption of physical gases is mainly governed by pore diffusion, which allows an easy transport of gaseous molecules into the porous structure. Large surface area and large pore volume of the structure offer more active sites for the entrapment of CO₂ molecules.⁹ Thus, the search for novel carbon-based adsorbents with improved textural, morphological, and structural properties is essential to develop suitable strategies.

Carbonaceous materials can be easily prepared from a direct carbonization of abundantly-available and inexpensive carbon-containing precursors (mainly biomasses), such as seaweed, beer yeast and microorganisms that are rich in lignocellulose, cellulose, hemicellulose, lignin, proteins, water and sugars.¹⁰ Many biomass precursors have been studied for CO₂ capture purposes;¹¹,¹²,¹³ however, the research concerning the biomass of pollen grains is scarce.¹⁴ Pollen grains are produced by a great variety of flowers and are collected by bees during the pollination season; these grains are mainly constituted by the rigid structure of a sporopollenin biopolymer.¹⁵ Pollen grains were chosen for the present research as a favorable precursor of porous carbons for CO₂ capture.

Chemical activation of carbon-based materials using activating agents, such as KOH, is a process that improves the textural characteristics of these materials. This hydroxide acts as a dehydrating and oxidizing agent. The process involves wet impregnation of carbon precursors with KOH, which confers the activated carbons a better-developed micro-pore structure with large surface area and porosity, characteristics that optimize its performance and use in CO₂ capture applications.¹⁶,¹⁷,¹⁸ The incorporation of alkaline earth metallic oxides to an adsorbent, by means of high-energy ball milling, generates surfaces even more reactive towards CO₂ molecules and thereby enhances the separation efficiency. The method is based on mechanochemistry and is a low-cost effective alternative of producing nanocomposites with excellent surface characteristics.¹⁹,²⁰ The mechano-synthesis diminishes the particle size and increases the activity of reactants.²¹

Magnesium oxide (MgO) has been explored as one of the best adsorbents for CO₂ due to its unique features, such as a direct physicochemical relationship with the adsorbate.²²,²³ MgO adsorbs CO₂ at > 300 ºC by a predominant chemisorption process producing stable carbonates. Afterwards, MgO can be regenerated by direct calcination of these carbonates, releasing CO₂.²⁴,²⁵ Other studies have shown that CO₂ can be adsorbed by MgO at relatively-low temperatures; this is attributable to the basic oxide surface that is able to capture the weakly-acidic CO₂.²⁶,²⁷ In fact, MgO adsorbs CO₂ below 200 ºC and can be regenerated at relatively-low temperatures.²⁵

For this purpose, biochar, chemical activation and incorporation of MgO by ball milling, were implemented for a series of nanocomposites, which were systematically prepared, characterized, and tested for CO₂ adsorption. The objective of the present work was to explore the synergistic effect of two relevant processes: KOH activation and MgO incorporation by high-energy ball-milling, on the CO₂ adsorption properties of the pollen-derived carbons. Thus, the adsorbents were: pollen-derived carbons obtained by direct calcination (PC), by calcination followed by KOH-chemical activation (PCKOH), and by MgO incorporation by high-energy ball milling (PC-MgO and PCKOH-MgO). To the best of our knowledge, this is the first report on the preparation of porous carbons from pollen grains using the mentioned methods of synthesis for CO₂ adsorption. The application of these novel as-prepared absorbents represents a new contribution to the gas adsorption area.

Experimental

Materials and reagents

Bee-collected pollen grains, purchased in the city of Toluca, State of Mexico, were the carbon precursors. Magnesium nitrate hexahydrate Mg(NO₃)₂.6H₂O (99 wt.% purity), urea NH₂CONH₂ (99-100 wt.% purity), potassium hydroxide KOH (86% purity) and hydrochloric acid HCl (37% purity) were obtained from Sigma-Aldrich, Merck and Baker ACS; all chemicals were of analytical grade and used without further purification. Extra-dry carbon dioxide (CO₂, 99.9% purity), helium (He, 99.9% purity) and nitrogen (N₂, 99.9% purity) were supplied by Infra Mexico Co. Figure 1 summarizes the material synthesis processes, which will be described below.

Figure 1
Scheme of the synthesis processes of the CO₂ adsorbents: pollen carbon (PC), KOH-activated pollen carbon (PCKOH), MgO-incorporated pollen carbon (PC-MgO) and KOH-activated, MgO-incorporated pollen carbon (PCKOH-MgO).

Pollen-derived carbons and KOH activation

The pollen grain-derived carbons were prepared by a direct calcination, followed by a wet KOH chemical activation. For this purpose, the bee-collected pollen grains were thoroughly washed with distilled water to remove soluble impurities and dried at 110 °C for 2 h. Then, the pollen grains were placed into a porcelain crucible and carbonized at 800 °C for 1 h, at ambient atmosphere. The obtained carbonized material was grounded in an agate mortar and sieved in a 60 mesh to a uniform size of 0.25 mm. This as-prepared carbonized material was labeled as “PC”.

An aliquot of PC was subjected to a KOH chemical activation by wet impregnation, as per our previous work²⁸ and using a KOH/PC mass ratio of 3:1 at a constant activation temperature of 600 °C and under the pressure of 1 bar, for 2 h. The resulting material was first washed with 10% HCl (v/v) to remove the excess of KOH, and afterwards, with abundant distilled water for several times until a pH of 6.0-7.0 was reached. It should be noted that KOH is not incorporated into the carbon; the purpose of the process is to improve the textural properties. The resulting activated carbonaceous material was dried at 110 °C for 18 h and its corresponding label was “PCKOH”.

MgO powder synthesis

The magnesium oxide (MgO) powder was prepared via the chemical combustion method, as described by Granados-Correa et al.²⁹ Molar amounts of Mg(NO₃)₂.6H₂O and NH₂CONH₂ were mixed and suspended in 1 mL of distilled water until a homogeneous solution was obtained. The mixture was gently heated to obtain a humid integrated solid, which was calcined for 5 min at 800 °C.

Preparation of pollen-derived carbon-MgO nanocomposites by ball milling

A mechanochemical method, by means of ball milling, was carried out employing a three-dimensional Spex-type ball mill. The calcined pollen grains (PC), and the KOH-chemically-activated carbon (PCKOH) were mixed separately with magnesium oxide powder (MgO) at a composition percentage of 95:5. The high-energy ball milling process was executed for 1.5 h, using a ball/sample weight ratio of 6:1, a speed of 780 rev min^-1, and 0.6 mL of hexane as a control agent. Approximately 6 g of each sample were prepared under these conditions. The obtained adsorbent samples were identified as “PC-MgO” and “PCKOH-MgO”, respectively.

Pollen grains derived carbon characterization

The adsorbents (PC, PCKOH, PC-MgO and PCKOH-MgO) were characterized texturally, morphologically and structurally by N₂ physisorption, scanning electron microscopy (SEM-EDXS, Tokyo, Japan) and X-ray diffraction (XRD) Siemens Analytical, Madison WI. N₂ physisorption measurements were done at liquid nitrogen temperature (77 K) with a Micromeritics Belsorp Max Inc. Japan equipment (BEL Japan, Osaka, Japan) and the samples were degassed prior to measurement, using a nitrogen stream at 300 °C for 2 h, for removal of any moisture or possible contaminant. The N₂-physisorption measurement data allowed the calculation of the Brunauer-Emmet-Teller surface area (S_BET), total pore volume (V_T), mean pore diameter (d_p) and pore size distribution via the Barrett-Joyner-Halenda (BJH) method. The porous morphologies of as-prepared samples were visualized and analyzed by a JEOL-JMS-5900LV electron microscope fitted with an energy dispersive X-ray (EDXS) detector for elemental analysis. The samples were mounted onto aluminum brackets using aluminum tapes and were previously sputter-coated with gold by using a Denton Vacuum DESK II for 90 s. The structures of as-prepared samples were visualized via X-ray diffraction (XRD), by using a Bruker D8 Discover X-ray diffractometer, equipped with Cu Kα radiation (λ = 1.54060 Å). The diffraction data was collected between 10° and 75° 2θ with at 0.04 s of step size, and the lines were identified according to their referring diffraction lines of corresponding standard samples by using the Joint Committee of the Powder Diffraction Standard (JPCDS) cards.

CO₂ adsorption experiments

Adsorption experiments were conducted in a steel Parr-type high-pressure reactor (50-mL capacity). Approximately, 5 mg of samples were exposed to a constant ultra-dry CO₂ gas flow during 1 h inside said device. Before CO₂ adsorption experiments, the samples were heated at 400 °C for 3 h to release humidity and possible ambient impurities. First, CO₂ adsorption performance was evaluated at 30 °C under ambient pressure (1 bar). These experiments were also carried out at different adsorption conditions of temperature and pressure.

Thereafter, the samples were subjected to thermogravimetric (TGA/DTA) analysis. Measurements were done using a calorimeter (TA Instruments SDT Q600, New Castle DE, USA) coupled to a mass spectrometer (TA Instruments-Waters), heating the samples from 20 °C up to 300 °C and under a helium atmosphere at a flow rate of 100 mL min^-1. Under these conditions, TGA desorption curves were obtained, in which the weight loss was equivalent to the uptake of CO₂ by the pollen-derived carbon into the high-pressure reactor. The adsorption capacity of CO₂ per gram of adsorbent (mmol g^-1) and the percentage of adsorption (Q_ad, %) were calculated considering the weight loss of the TGA curves and the molar mass of CO₂ (0.044 in g mmol^-1).

Results and Discussion

Material characterization

The selection of an adsorbent must be based on the textural, morphological and structural characteristics of the material, because these physicochemical properties considerably influence the CO₂ capture ability. Table 1 shows the main textural characteristics of as-prepared pollen-derived carbons. The PC exhibited a complete carbonization attributable to the loss of volatile matter and disintegration of lignocellulosic organic material of the natural pollen grains. The production yield of the carbon from the pollen grains after direct carbonization in a muffle furnace at 800 ºC during 1 h was 34.5%. In fact, this raw material showed high volatile matter and low ash content, so, it is a good precursor for carbon production. PC carbon presents low textural characteristics for CO₂ capture purposes (BET surface area of 1.49 m² g^-1, total pore volume of 0.0077 m³ g^-1 and high average mesopore diameter of 20.61 nm), compared to the other as-prepared pollen grain-derived carbons. According to Sentorun-Shalaby et al.,³⁰ the low BET surface area of the directly-carbonized materials can be attributed to an extensive degassing of the process at such high temperatures, which resulted in a widening of the pores and a partial collapse of the porous structure.

Table 1
Main textural properties of pollen-derived carbons and composites

Sample	SBET / (m2 g-1)	dp / nm	Vpa / (cm3 g-1)	Type of N2 ads-des isotherm	Pore size distribution
PC	1.49	20.61	0.008	type IV (mesoporous)	rp, peak (area) = 1.64 nm Vµp = 0.007 cm3 g-1 Aµp = 1.77 m2 g-1
PC-MgO	10.21	8.61	0.22	type IV (mesoporous)	rp, peak (area) = 1.21 nm Vµp = 0.19 cm3 g-1 Aµp = 31.3 m2 g-1
PCKOH	4.20	3.08	0.024	type IV (mesoporous)	rp, peak (area) = 1.64 nm Vµp = 0.023 cm3 g-1 Aµp =3.64 m2 g-1
PCKOH-MgO	12.39	41.81	0.130	type IV (mesoporous)	rp, peak (area) = 10.76 nm Vµp = 0.13 cm3 g-1 Aµp =10.40 m2 g-1

• ^aTotal pore volume with relative pressure p/po = 0.99, calculated via the Barrett-Joiner-Halenda method; S_BET: surface area; d_p: mean pore diameter; r_p,peak(area): radii pore, peak (area); A_µp: micropore surface area; V_µp: total micropore volume.

The pore structure of PCKOH was improved due to the KOH chemical activation process of the carbonized material, which induced the development of pores and increase in surface area.²⁸,³¹ The pore distribution of PCKOH, determined via the Barret-Joyner-Halenda (BJH) method, revealed better textural factors than for PC. The pores contribute to rapid CO₂ molecule transport through channels, which is beneficial for gas diffusion and CO₂ adsorption capacity.³² The results of pore size distribution for PC and PCKOH showed that mesopores were developed in both, as normally occurs in calcined materials. It would be expected that, due to the chemical activation, microporous structures be present in PCKOH, and effectively, this material showed a mixture of micropores and mesopores, with predominance of the latter; it is probable that the impregnation of KOH prevented the formation of abundant micropores.

When PC and PCKOH were combined with the MgO powder and subjected to high-energy ball milling, the obtained products, PC-MgO and PCKOH-MgO, resulted in nanocomposites and their surface areas were notably improved (Table 1). The high-energy ball milling process can break the grains into ultrafine particles, thereby considerably increase the surface area.³³ The total pore volume (V_p) of PC was so small that it would seem that such carbon did not have pores; a similar behavior was also observed with pollen elsewhere.¹⁴ This value increased slightly with KOH activation, but more notably with MgO incorporation; the V_p of PC-MgO and PCKOH-MgO were 0.22 and 0.13 cm³ g^-1, respectively. These results indicate that the processes played a critical role in the determination of physicochemical characteristics. Thus, it is possible that the ball milling process did not only increase the external surface area by reducing the grain size up to a nanometric scale, but also increased the internal surface area by opening the internal pore networks.³⁴

Regarding the pore size distribution, abundant mesopores were generated in the structures; the radii sizes, r_p, peak (area), are between 1.21 and 10.76 nm, and these values are classified as “mesoporous”, according to the International Union of Pure and Applied Chemistry (IUPAC).³⁵ Ball milling may lead to the opening and enlargement of the pores due to the strong impacts during ball-milling process; this was more notorious for PCKOH-MgO, with the largest radius size. Micropores favor the fast pore diffusion of molecules due to their narrow wall. On the contrary, the CO₂ physisorption process may usually be slower in mesoporous materials, considering the kinetic diameter of the CO₂ molecule (0.33 nm).³⁶ However, wider pores increased the total pore volume (V_p) and the surface area (A_µp); thus, the mesopores were able to allow CO₂ molecule diffusion into the pore structure.³⁷

The N₂ adsorption-desorption isotherms obtained from these composites at a relative pressure of p/pº = 0.99 (figure not shown) exhibited type IV isotherms, according to the IUPAC classification; moreover, these curves included hysteresis, attributable to capillary condensation in mesoporous materials. Therefore, remarkable homogeneous pore structures were developed by the direct carbonization treatment of pollen grains, followed by KOH chemical activation and high-energy ball milling treatment with MgO powders, thus generating nanocomposites. That integral process revealed the suitability of the use of a natural biomass of low cost, based on pollen grains, which is an excellent precursor to produce carbons designed for efficient and selective CO₂ capture.

Analysis of the surface morphology images of the as-prepared pollen-derived carbons, evaluated by SEM at 2500× magnification (Figure 2), revealed morphological uniformity in particle size and porosity. The MgO powder was fully incorporated on the surface-active biochar. The micrographs also confirmed that the high-energy ball milling process altered the material particle sizes, changing them from the millimetric to nanometric scale (< 100 nm), resulting in the acquisition of unique nanocomposites. When the particle size of the adsorbent surface is reduced, the reactivity of the adsorbent increases, due to the accessibility of the nanoparticles.³⁸ These results mean that it is possible to obtain CO₂ pollen-derived carbons through the high-energy ball milling method with high-quality morphological characteristics, which may be decisive for efficient CO₂ capture. Pollen-derived carbons and MgO nanocomposites are constituted mainly by C, O, Si, P, K and Ca (in addition to Mg in PC-MgO and PCKOH-MgO), coming from the precursors; the presence of other elements, as contaminants, was discarded.

Figure 2
SEM micrographs (2500×) of the pollen-derived carbons and composites: (a) PC (b) PC-MgO (c) PCKOH and (d) PCKOH-MgO.

The elemental mapping images of pollen-derived carbons and nanocomposites prepared under different synthesis processes allowed for the confirmation that the elements were homogeneously dispersed and C, O and Mg were the main elements contained in these samples (Figures 3, 4, 5 and 6).

Figure 3
Elemental mapping images of PC sample.

Figure 4
Elemental mapping images of PC-MgO sample.

Figure 5
Elemental mapping images of PCKOH sample.

Figure 6
Elemental mapping images of PCKOH-MgO sample.

Regarding the structural study, the XRD patterns of as-prepared pollen-derived carbons are displayed in Figure 7. Specifically, the as-prepared pollen-derived carbon, obtained from direct calcination (PC), was amorphous, with a structure similar to coal and some disarray in the molecular chain of carbon, because of the calcination treatment. This biochar presented some traces of SiO₂ at 23 and 50 in 2θ degrees. XRD patterns of the composites clearly showed the presence of their components, in addition to the biochar itself; KHCO₃ in PCKOH, MgO in PC-MgO and both in PCKOH-MgO. It is noticeable that XRD patterns of PC-MgO and PCKOH-MgO, which were prepared by ball milling, both showed widened peaks, which may reveal nanocrystalline structures, attributable to the defects and/or dislocations induced by the treatment. The ball milling treatment provoked significant effects in the structural characteristics of the materials, since this method entails the achieving of a nanocrystalline structure. A similar trend was seen with the preparation of other biochar-based nanocomposites.¹⁹

Figure 7
XRD patterns of pollen-derived carbons and composites: (a) PC, (b) PC-MgO, (c) PCKOH and (d) PCKOH-MgO.

CO₂ adsorption

The adsorption capacity of PC, PCKOH, PC-MgO and PCKOH-MgO was tested via exposure to a CO₂ gas flow at 30 °C and 1 bar. The thermal decomposition process after CO₂ exposure was examined via TGA, between 25 and 300 °C, considering CO₂ adsorbed per gram of material (mmol g^-1). The obtained TGA curve profiles for CO₂ uptake as a function of temperature (°C) are shown in Figure 8. The appearance of the curves of PC and PC-MgO is virtually identical and the same occurs with those of PCKOH and PCKOH-MgO. This may be attributable to the influence of the KOH activation process on the textural, morphological and structural characteristics of the pollen-derived carbons. Regarding the amounts of CO₂ adsorbed by PC, PCKOH, and PC-MgO, they are all ca. 0.8 mmol g^-1 (Q_ad ca. 3.05%). On the contrary, the nanocomposite PCKOH-MgO, reached a CO₂ adsorption capacity of 2.06 mmol g^-1 (Q_ad ca. 9%), under the same adsorption conditions. This result highlights the improved characteristics of this carbon, which are probably due to a synergic effect of both the KOH activation and the surface reactivity of the MgO. So, the abundance of mesopores, with high pore volume, the high surface area, and the nanocrystalline structure lead up a more intense CO₂ molecule diffusion.

Figure 8
TGA curve profiles for CO₂ uptake at 30 ºC and 1 bar of pollen grain-derived carbons: (a) PC, (b) PC-MgO, (c) PCKOH and (d) PCKOH-MgO.

For illustrative purposes, Table 2 includes data about the CO₂ uptake reported in literature, considering carbons obtained from diverse biomass precursors and activated by KOH, MgO or both. Comparison of data, as that presented in Table 2, is sometimes an arduous task because of the diversity of conditions; certainly, each playing an important role. The plant biomass/biochar precursors are comprised of cell walls that contains some polymers like cellulose, hemicelluloses (polysaccharides) and the polyphenol lignin.³⁹,⁴⁰ The structure is diverse from one plant to another; thus, the procedures required to prepare an activated carbon are not always the same. The calcination temperature (450-850 ºC),¹² the activation process with chemical agents and the subsequent calcination (< 600 ºC) are responsible for the pore formation in the activated/composite biochar. For instance, calcination at 500 ºC was better that at 700 ºC for the generation of greater pore volume into Yellow mombin fruit stone carbon;⁴¹ whereas bee-collected pollens were well-degraded at 800 ºC¹⁴ and present work. The chemical agents to choose are numerous; perhaps, one of the most popular is KOH, either impregnated¹⁴,⁴¹ or incorporated.⁴² MgO has also garnered interest for chemical activation, either impregnated⁴³,⁴⁴,⁴⁵,⁴⁶,⁴⁷,⁴⁸ or incorporated (present work). Therefore, CO₂ uptake values (mmol g^-1) are diverse, depending on the described parameters and the specific adsorption conditions of temperature and pressure; usually, 25-30 ºC and 1 bar.

Table 2
Data of CO₂ uptake reported in literature for biomass-biochars activated with KOH, MgO or both

Adsorbent precursor	Activation	CO2 adsorption conditions	CO2 uptake / (mmol g-1)	Reference
Pollen	KOH	1 bar, 25 °C	3.42	14
Yellow mombin fruit stones	KOH	1 bar, 25 °C	7.3	41
Bamboo sawdust	KOH	1 bar, 25 °C	3.49	42
Coal activated carbon	-	1 bar, 52.5 °C	13.3	43
	MgO	1 bar, 52.5 °C	28.9
Palm kernel shell	KOH-MgO	1 bar, 25 °C	0.44	44
Bamboo	MgO	1 bar, 25 °C	6.5	45
Whitewood	MgO	1 bar, 25 °C	1.12	46
Activated carbon	KOH	1 bar, 25 °C	3.51	47
Coconut	MgO	1 bar, 100 °C	0.19	48
Pollen	-	1 bar, 30 °C	0.75	present study
	KOH		0.88
	MgO		0.69
	KOH-MgO		2.06

The CO₂ adsorption process certainly depends on the accessibility of the gas to the porous structures of the material, but the roles of temperature and pressure may be equally crucial. So, both factors were evaluated with regard to PCKOH-MgO, which resulted in an improved CO₂ adsorption capacity.

The CO₂ adsorption experiments with the nanocomposite PCKOH-MgO were carried out at 30, 50, 75 and 100 ºC and at a pressure of 1 bar. Figure 9a shows the TGA curves obtained, in which CO₂ adsorption diminishes in an inversely-proportional manner, with regard to temperature (Figure 9b), from 2.06 to 1.49 meq g^-1, when temperature was increased from 30 to 100 ºC. These results indicate that physi-sorption is the dominant mechanism for CO₂ adsorption in the studied solid-gas system, in which the nature of the adsorption process is determined by the forces exerted between the adsorbate and the adsorbent into a system.⁴⁹ Temperature can exhibit different effects on the CO₂ adsorption behavior, increasing or decreasing the adsorption kinetics or even allowing release of the molecules trapped in its porous structure. At the experimental temperature range, the attractive forces between CO₂ molecules and the active sites were not sufficiently strong, as it would have happened in chemisorption; consequently, the external diffusion resistance of trapped CO₂ molecules decreased and the adsorbed CO₂ molecules were released.⁵⁰ A similar effect of temperature on CO₂ adsorption was observed with KOH-activated biomass carbon; the phenomenon may be attributable to the exothermicity of the process and to the high speed of the molecules that prevent adsorption.¹⁴,⁴¹,⁴³

Figure 9
TGA curve profiles for CO₂ desorption from the nanocomposite PCKOH-MgO: (a) Under different adsorption temperatures (30, 50, 75 and 100 ºC) and 1 bar, and (b) decrease of CO₂ uptake as a function of adsorption temperature.

The following CO₂ adsorption experiments were carried out with PCKOH-MgO at a constant temperature of 30 ºC and under pressures of 1, 5, 10 and 15 bar. The obtained TGA curves, as can be seen in Figure 10, show that the amount of CO₂ adsorbed by PCKOH-MgO decreased, from 2.05 to 1.6 mmol g^-1, with an increase of the adsorption pressure from 1 to 15 bar, respectively. The CO₂ partial pressure did not show any effect on the mass transfer rate, contrary to what should occur in a characteristic physisorption process on a pure and microporous activated carbon. This behavior may be explained by the effect exerted by MgO into the adsorbent, modifying the surface reactivity, even if it improved the CO₂ adsorption capacity regarding PC in PC-MgO. It is worth mentioning that for industrial applications, it is important to operate the CO₂ capture at ambient pressure, because the atmospheric pressure of a gas flow from fossil fuel power plants is around ambient pressure. An opposite effect of pressure on CO₂ adsorption was reported with an MgO/ activated carbon.⁴³

Figure 10
TGA curve profiles for CO₂ of the nanocomposite PCKOH-MgO: (a) under different adsorption pressures (1, 5, 10 and 15 bar) and 30 ºC, and (b) decrease of CO₂ uptake as a function of adsorption pressure.

The studies on the CO₂ adsorption-desorption behavior may reveal the cyclic stability of the adsorbent and that great stability allows energy expenses to decrease CO₂ capture technologies are implemented. Some cycling results have shown that the MgO-based materials present a significant stability when reused, because they do not suffer sintering or significant wear.⁵¹,⁵² The interest in knowing the behavior of the materials of this study, with regard to desorption-adsorption processes give way to further experimentation. Fast or low kinetics could be achieved for the CO₂ desorption steps using temperature-swing operations, even while maintaining the CO₂ gas flow.

The experiments focused to predict the CO₂ desorption behavior were carried out at 300 ºC, because it was expected that, at this temperature, the resistance exerted by the diffusion of the gas in the porous network of the material would be overcome, that the textural properties of the materials be unaffected, and moreover, that the adsorptive properties would not be degraded.

Samples of PC, PCKOH, PC-MgO, and PCKOH-MgO were first exposed to 30 ºC and 1 bar for CO₂ adsorption, and the values of the adsorbed CO₂ were 0.75, 0.88, 0.69 and 2.10 mmol g^-1, respectively. These batches were evaluated by thermogravimetry, at a programmed temperature of 300 °C for 1 h. In this case, TGA curves were plotted as CO₂ uptake as a function of time. Ideal desorption times were defined at the point when the values stopped decreasing at the TGA curve and the CO₂ uptake value became stable. The results are displayed in Figure 11 and Table 3. The complete CO₂ desorption from PC, PCKOH and PC-MgO under 300 ºC was achieved in ca. 30 min, whereas complete CO₂ desorption from PCKOH-MgO was slower (46.8 min). This difference in CO₂ desorption behavior of PCKOH-MgO may be due to the textural and morphological characteristics, to the abundance of micropores in the network that were not saturated nor blocked in a short time, and to the absence of active sites on the particle surface.

Figure 11
TGA curve profiles of CO₂ desorption from pollen-derived carbons, as a function of time, at 300 ºC and 1 bar.

Table 3
Desorption times of the CO₂ adsorbents: PC, PCKOH, PC-MgO and PCKOH-MgO

Adsorbent	Initial CO2 / (mmol g-1)	Desorption time / min	CO2 desorbed / (mmol g-1)
PC	0.75	29.4	0.75
PCKOH	0.88	28.9	0.88
PC-MgO	0.69	30.2	0.69
PCKOH-MgO	2.10	46.8	2.14

The CO₂ desorption performance of the original biochar, PC, was not improved by KOH chemical activation (PCKOH), nor by MgO incorporation via ball milling (PC-MgO), but by the synergic effect of both methods, as seen in PCKOH-MgO. The adsorption mechanism in carbonous materials depends mainly on their properties, such as surface area and porosity. These parameters may be large enough to allow a good diffusion of the gaseous adsorbate and achieve its physisorption by means of the van der Waals interactions with the surface atoms of the pollen carbons. The chemical activation with KOH and the incorporation of MgO by the high-energy ball milling process enhanced the adsorption characteristics of the pollen carbons. Furthermore, MgO, upon contact with a weakly-acidic molecule such as CO₂, reacts to form magnesium carbonate. Studies about the adsorption behavior of CO₂ in this solid-gas system using kinetic, equilibrium and thermodynamic models that allow us to fully understand and predict this entire adsorption process and to control the CO₂ pollution resulting from the burning of fossil fuels, are still required.

Conclusions

Novel adsorbents were successfully prepared by using natural pollen grain carbons, KOH activation and incorporation of MgO via ball milling. These adsorbents were: a pollen biochar (PC), a KOH-activated pollen biochar (PCKOH), a composite pollen biochar/MgO (PC-MgO) and a composite KOH-activated/MgO (PCKOH-MgO). All these materials were able to adsorb CO₂, at 30 ºC and 1 bar. with PCKOH-MgO being the best adsorbent, as it was homogeneous in size of its particles, with larger surface area and total pore volume, and with a better CO₂ adsorption capacity (2.06 mmol g^-1, Q_ad = 9.06%), at 30 ºC and 1 bar, than PC, PCKOH and PCMgO, as the CO₂ adsorption capacities of other adsorbents were ca. 0.8 mmol g^-1 (Q_ad ca. 3.05%). Therefore, the improved adsorption capacity of PCKOH-MgO, compared with the other adsorbents, may be due to the combined advantages of both processes (KOH activation and MgO incorporation via ball milling). Both processes increased the presence and dispersion of basic active sites and favored the interaction of the weakly-acidic CO₂ molecules with MgO. Regarding the CO₂ desorption, 300 ºC were enough to attain the complete desorption from the four adsorbents in 47 min, for PCKOH-MgO, and in 30 min, for the others. The data suggests a possibility of recycling for these materials; especially, for PCKOH-MgO. Ensuring the success of this process requires studies of the material before being exposed to CO₂ again, which was not the purpose of this work. This research showed that the PCKOH-MgO can be used as a potential adsorbent for industrial CO₂ capture applications and contribute to long-term solutions for the abatement of global warming.

Acknowledgments

The authors acknowledge financial support from the Instituto Nacional de Investigaciones Nucleares, Mexico, through Project (QU-001). The authors also thank the technical support of Iris-Zoet Malpica and Elvia Moreno-Morales.

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Edited by

Editor handled this article: Izaura C. N. Diógenes

Publication Dates

• Publication in this collection
  20 Sept 2024
• Date of issue
  2025

History

• Received
  30 May 2024
• Accepted
  30 Aug 2024