Engineering Porous Carbons from Furfural-Derived Humins toward CO₂ Adsorption

Abstract

Furfural-derived humins were produced through acid-catalyzed hydrothermal synthesis, and a series of humin-based carbon materials with different pore structures were prepared via high-temperature carbonization and KOH activation. It was found that the carbon material activated at 800 °C with a KOH-to-humin ratio of 2/3 exhibited excellent CO₂ adsorption performance, whereby the activation temperature and KOH ratio synergistically modulated the porous structure. At 0 °C and 1 bar, its adsorption capacity reached 6.06 mmol/g, and the CO₂ uptake was highly linearly correlated with the volume of micropores smaller than 0.79 nm. Moreover, the 800 °C-2/3KOH material has exhibited an excellent CO₂/N₂ adsorption selectivity of 25.16, a moderate heat of adsorption (14–24 kJ/mol), and good recyclability. This study provides a novel approach to valorize humin-based porous carbons as efficient, sustainable, and economical CO₂ adsorbents.

1. Introduction

Carbon dioxide (CO₂) is the main greenhouse gas, and its increased emissions significantly impact global climate change and environmental pollution. To address this challenge, the development of carbon capture and sequestration (CCS) technology is receiving increasing attention. Although a variety of methods for CO₂ capture have been proposed, such as chemical absorption, physical adsorption, and membrane separation, achieving a balance between economic feasibility, efficiency, and environmental friendliness remains a critical issue in current research. −

In recent years, biomass-derived carbon materials have emerged as a novel class of adsorbents, gaining attention in CO₂ adsorption research. Compared to other carbon materials, biomass-derived carbon materials offer a variety of remarkable advantages, such as renewability, environmental friendliness, low cost, porosity, and tunable surface chemistry. , To fully utilize biomass resources and enhance their adsorption performance, various preparation methods for biomass-derived carbon materials have been developed, including carbonization, activation, and heteroatom doping. Carbonization is the foundational step in converting biomass into carbon materials, involving thermal decomposition under an inert atmosphere to remove volatile components and retain a carbon skeleton. Activation expands the pore network and increases specific surface area, making it the most effective method to boost adsorption capacity. Besides, biomass-derived carbon materials often lack specific functional groups for selective adsorption. Therefore, doping with heteroatoms (N, O, S, P) or grafting organic molecules tailors surface chemistry, improving interactions with the adsorbate and increasing the adsorption capacity. ,

In addition to directly converting biomass into biomass-derived carbon materials, the conversion of biomass into platform chemicals would inevitably generate dark-brown, insoluble byproducts known as humins. − The formation of humins greatly diminishes the yield of valuable platform chemicals and reduces the economy of biomass conversion. , Though much effort has been made to inhibit the formation of humins, it cannot be totally suppressed yet. Thus, the valorization of humins seems quite crucial for making the whole biomass conversion process more economical and environmentally friendly. In the past, humins are mostly considered a combustible fuel to supply heat and power back into the biorefinery process, which is rather wasteful. Recently, the valorization of humins as a CO₂ adsorbent has attracted research interests. In 2018, Hedin and co-workers synthesized HMF-derived humins by rapid addition of HMF into conc. H₂SO₄, whereas the solids generated were washed with diethyl ether and heated at 400 °C. Comparably high CO₂ uptake of 5.27 mmol/g at 0 °C and 1 bar was obtained. They further synthesized humins based on monosaccharides, disaccharides, and polysaccharides, leading to a maximum of 4.25 mmol/g CO₂ uptake starting from glucose.

So far, how to modulate the humins structure to fulfill better applications in CO₂ adsorption remains challenging. Previously, we have investigated the structure and formation mechanism of humins derived from versatile biomass, − which provided an important basis to disclose the correlation between humin structure and CO₂ adsorption performance. In 2024, Yang et al. captured a key intermediate, 2-(4-furfur-2-al)-4-hydroxy-2-cyclopenten-1-one, and demonstrated that Piancatelli rearrangement, electrophilic substitution, etherification, and aldol condensation were involved in the formation of furfural-derived humins. Herein, this work highlights the feature of carbon materials from furfural-derived humins, exhibiting excellent CO₂ adsorption capacity.

2. Materials and Methods

2.1. Materials and Equipment

Furfural (98%) was supplied by Energy Chemical Co., Ltd. (Shanghai, China) and was freshly distilled under vacuum prior to use. CO₂ and N₂ adsorption and desorption were measured using Quantachrome NOVA2000E and Quantachrome Autosorb-IQ instruments, respectively. The SEM images were acquired using a Hitachi S-4800 scanning electron microscope.

2.2. Synthesis of Furfural-Derived Humins

A mixture of furfural (0.684 g) and H₂SO₄ (0.504 g, 50%) was heated in a Teflon-lined autoclave preheated to 170 °C for 10 min under magnetic stirring. After the reaction, the humin solid was filtered, washed to neutrality, and dried under high vacuum at 80 °C, with a yield of 58.8%.

2.3. General Procedure for Carbonization or Activation of Humins

In order to improve the adsorption capacity of humins, carbonization and activation were carried out, and the carbonization temperature was 500–900 °C. For the carbonization process, the conventional steps are to prepare humins by calcination for 2 h under a 40 mL/min N₂ flow in a high-temperature tube furnace.

In addition, KOH was used as the chemical activator for activation, and the activated product was called x–y KOH (x is its activation temperature and y is the mass ratio of KOH to humins). For example, 800 °C-2/3KOH means that chemical activation is performed under the condition that the mass ratio of KOH to humins is 2/3 at 800 °C. For the activation process, a certain mass of KOH solution was mixed with humins, impregnated for 12 h, and dried in a vacuum oven at 80 °C for 8h. Afterward, it was calcinated for 2 h under 40 mL/min N₂ flow in a high-temperature tube furnace. Then, the solid was washed to neutrality with deionized water and dried under high vacuum at 80 °C for 8 h.

3. Results and Discussion

3.1. CO₂ Adsorption Performance under Different Conditions

First, CO₂ adsorption experiments were conducted for furfural-derived humins, and their CO₂ adsorption capacity was 0.65 mmol/g at 0 °C and 1 bar. In order to enhance the CO₂ adsorption capacity, the humins were carbonized at high temperatures ranging from 500 to 900 °C, and the yields of carbon materials are shown in Figure S1. With the carbonization temperature rising from 500 to 700 °C, the adsorption capacity of CO₂ increased from 1.45 to 3.22 mmol/g (Figure a). When the carbonization temperature further increased to 900 °C, high-molecular-weight volatiles may soften and sinter, causing the total pore volume of the carbon material to shrink, which thereby resulted in a decrease of CO₂ adsorption capacity to 2.74 mmol/g.

1.

CO₂ adsorption capacity of the humin-based carbon materials obtained under different carbonization temperatures at 0 °C and 1 bar (a); under different activation conditions at 0 °C and 1 bar (b); and at 25 °C and 1 bar (c).

Among various activators, KOH is widely used due to its advantages such as high surface area, narrow pore size distribution, low environmental pollution, and low cost. Therefore, KOH was selected to activate the humin material to further improve the CO₂ adsorption capacity. Three optimal carbonization temperatures (700, 800, and 900 °C) were chosen for activation. The KOH-to-humin mass ratios were set at 0.5, 2/3, and 1. As chemical activation by KOH generates microporosity in carbon materials, yet excessive KOH/humin ratios (>1:1) induce structural collapse and reduce micropore volume. The yields of carbon materials obtained under different activation temperatures and KOH ratios are shown in Figure S2. After activation, the CO₂ adsorption capacity of humin-based carbon materials increased significantly, with the 800 °C-2/3KOH material exhibiting the most pronounced improvement. At 0 °C and 1 bar, their CO₂ uptake reached 6.06 mmol/g, approximately twice that of the carbonized material (Figure b). The elemental analysis results show that after activation with 2/3 KOH at 800 °C, the carbon content of humin-based carbon material increased significantly from 66.83 to 83.35%, while hydrogen and oxygen contents decreased from 4.16 to 1.88% and 29.01 to 14.77%, respectively. This indicates the effective removal of oxygen-containing functional groups and the enrichment of carbon structure during activation.

3.2. Relationship between CO₂ Adsorption and Pore Structure

It is vital to correlate the structure of humin-based carbon materials with the CO₂ adsorption performance. Table provides an overview of the pore characteristics of humin-based materials, including specific surface area, total pore volume, and micropore volume. There is no clear dependency between the total pore volume/micropore volume and the amount of CO₂ adsorbed, suggesting that the total pore volume or micropore volume does not limit the CO₂ uptake. Hence, more in-depth investigations were conducted as follows.

1. Pore Structure and CO₂ Uptake of Humin-Based Carbon Materials.

sample	S BET (m2/g)	V t‑plot (cm3/g)	V total (cm3/g)	V 0.79 (cm3/g)	CO2 uptake at 0 °C (mmol/g)
humins	34.14	0.003	0.196	0.032	0.65
500 °C	349.07	0.065	0.400	0.070	1.45
600 °C	600.84	0.166	0.475	0.137	2.72
700 °C	651.88	0.178	0.521	0.161	3.22
800 °C	664.40	0.181	0.575	0.154	3.15
900 °C	471.78	0.122	0.442	0.134	2.74
700 °C-0.5KOH	750.73	0.269	0.549	0.219	4.48
700 °C-2/3KOH	544.93	0.194	0.419	0.212	4.40
700 °C-1KOH	949.43	0.269	0.539	0.222	4.56
800 °C-0.5KOH	752.11	0.272	0.484	0.209	4.15
800 °C-2/3KOH	1231.24	0.445	0.729	0.302	6.06
800 °C-1KOH	1191.48	0.329	0.576	0.197	4.04
900 °C-0.5KOH	1186.78	0.435	0.731	0.230	4.66
900 °C-2/3KOH	1455.70	0.544	0.870	0.279	5.71
900 °C-1KOH	1498.31	0.235	0.803	0.183	3.90

^aSurface area was calculated using the BET method.

^bEvaluated by the t-plot method.

^cTotal pore volume at P/P ₀ = 0.99.

^dVolume of micropores (<0.79 nm) obtained from the CO₂ adsorption data at 0 °C.

As shown in Figure , the micropores of the humin-based carbon materials are distributed across three primary ranges: 0.3–0.4, 0.4–0.7, and 0.7–1 nm. The activation temperature mainly dictates the dominant distribution ranges of micropores, while the mass ratio of KOH/humins provides more precise control over the pore volume within these specific ranges.

2.

Pore size distribution of humins carbonized and activated at (a) 700, (b) 800, and (c) 900 °C, obtained from the CO₂ adsorption data.

Under activation at 700 °C (Figure a), the micropores of the humin-based carbon materials are mainly distributed in two ranges: 0.3–0.4 and 0.4–0.7 nm. Notably, the micropore volume within 0.4–0.7 nm shows negligible differences between carbonized and activated samples. The key distinction lies in the 0.3–0.4 nm range. However, the CO₂ adsorption capacities of 700 °C-0.5KOH (4.48 mmol/g), 700 °C-2/3KOH (4.40 mmol/g), and 700 °C-1KOH (4.56 mmol/g) samples are nearly identical, indicating that pores of 0.3–0.4 nm contribute minimally to CO₂ uptake.

When the activation temperature increased from 700 to 800 °C (Figure b), the humin-based carbon materials developed a distinct micropore distribution in the 0.7–1 nm range, while the micropore volume in the 0.3–0.4 nm range decreased. For the 800 °C-2/3KOH sample exhibiting optimal CO₂ adsorption, the micropore volume within 0.4–0.7 nm significantly increased. Two conclusions can be drawn from this phenomenon: (1) pore widening occurs with elevated temperatures, where narrower micropores expand; and (2) pores of 0.4–0.7 nm are more dominant in CO₂ adsorption.

Upon further increasing the activation temperature to 900 °C (Figure c), continued micropore widening was observed. With the increase of KOH ratios, the micropore volume in the 0.3–0.4 nm range decreased, while that in the 0.7–1.0 nm range increased, and even the mesopore volume showed a slight increase (Figures S3 and S4). Consequently, the 900 °C-1KOH sample exhibited reduced CO₂ adsorption capacity (3.90 mmol/g) compared to 900 °C-2/3KOH (5.71 mmol/g), suggesting that pores of 0.7–1.0 nm and larger also contribute minimally to CO₂ uptake.

Accordingly, the correlation between the volume of micropores (limited by the pore size within the 0.35–1.48 nm range) and CO₂ adsorption was investigated in detail. Then, the R ² values to quantify the linear dependence of CO₂ adsorption capacity on micropore volume below a discrete pore size were calculated (Figures a and S5). As shown in Figure b, the linear fitting factor R ² initially increased with the increase of pore size, reached its maximum critical value at 0.79 nm (R ² = 0.99768), and then gradually decreased. Thereby, the relationship between the CO₂ adsorption and pore structure of humin-based carbon materials has been elucidated, revealing that the volume of micropores smaller than 0.79 nm showed the best linear correlation with CO₂ adsorption.

3.

(a) Optimal linear relationship between CO₂ uptake and the volume of micropores less than 0.79 nm. (b) Correlation between linear fitting factor R ² and pore size.

3.3. Morphology Transformation

Scanning electron microscopy (SEM) was employed to examine the morphology of humin-based carbon materials. As shown in Figure , the noncarbonized/activated humins exhibit smooth spherical structures with low microporosity. After carbonization at 800 °C, more volatiles are released, the microspheres are broken, and the material changes from spherical to fragmented. After KOH activation, the carbon material with the optimal CO₂ adsorption performance (800 °C-2/3KOH) displays distinctly different morphological features: it maintains a spherical framework but develops a reticulated surface texture with unique pore architecture. This structural evolution arises from K⁺ intercalation during KOH activation, which simultaneously generates new micropores. , With the ratio of KOH increased from 2/3 to 1, the reticulated surface texture disappears due to excessive ablation by KOH, with micropores collapsed, resulting in reduced CO₂ adsorption capacity.

4.

SEM images of humin-based carbon materials.

3.4. Selectivity, Heat of Adsorption, and Recyclability

To comprehensively evaluate the performance of an adsorbent material, in addition to its adsorption capacity, selectivity, and heat of adsorption are also important, which reflect the practical effectiveness and economic viability of the material. In industrial settings, CO₂ emissions are typically mixed with N₂. High selectivity means that the carbon material can preferentially adsorb CO₂, reducing the capture of other gases. This enhances separation efficiency. It also implies that large volumes of inert gases like N₂ remain unabsorbed, which reduces the burden during the material’s regeneration process.

To investigate the selectivity of furfural-derived humins, CO₂ and N₂ adsorption isotherms were measured on the material exhibiting the highest CO₂ adsorption capacity (800 °C-2/3KOH), as shown in Figure . At 0 °C, the adsorption capacities of CO₂ and N₂ on the 800 °C-2/3KOH material were 6.06 and 0.69 mmol/g, respectively, which decreased to 4.10 and 0.4 mmol/g, respectively, at 25 °C. Then, the selectivity of the 800 °C-2/3KOH material was determined using adsorption isotherms combined with ideal adsorption solution theory (IAST) calculations. The selectivity coefficient (S) is calculated using the following formula:

$$S=\frac{V_1/V_2}{P_1/P_2}$$

where V ₁ and V ₂ represent the adsorbed amounts of CO₂ and N₂ at respective pressures, while P ₁ and P ₂ denote the equilibrium partial pressures of CO₂ and N₂ in the gas phase. For selectivity calculations, 1 bar total pressure was regarded as the standard experimental protocol, with a gas composition of CO₂/N₂ = 10:90 (mimicking flue gas from coal-fired power plants, typically containing 10–15% CO₂). Thus, V ₁ is the CO₂ uptake at 0.1 bar, V ₂ is the N₂ uptake at 0.9 bar, P ₁ is 0.1 bar, and P ₂ is 0.9 bar. Therefore, the 800 °C-2/3KOH sample exhibits S values of 25.16 at 0 °C and 28.43 at 25 °C, respectively, which are among the highest reported for biomass-derived carbon materials. These exceptional selectivities enable the precise capture of substantial CO₂ from CO₂/N₂ mixtures.

5.

Adsorption isotherms of CO₂ and N₂ of the 800 °C-2/3KOH material at (a) 0 °C and 1 bar and (b) 25 °C and 1 bar.

The isosteric heat of adsorption (Q _st) provides insight into the strength of the interaction between CO₂ and the adsorbent during the adsorption process. It can be used to distinguish between physical adsorption and chemical adsorption, which reflects the adsorption capacity and regeneration performance of the material. If Q _st is too high (>50 kJ/mol), it may involve chemical adsorption, whereby desorption requires a significant amount of energy, increasing the operational costs in industrial applications. Conversely, a lower Q _st indicates that the physical adsorption is the adsorption mechanism, and the material can be more easily desorbed after adsorption, reducing the energy consumption for regeneration. Thereby, carbon materials with moderate Q _st (20–40 kJ/mol) are most favorable for industrial CO₂ adsorption.

Q _st is usually calculated using the Clausius–Clapeyron equation. By measuring adsorption isotherms at different temperatures, the variation of adsorption capacity with temperature is obtained. The adsorption isotherms are fitted using the Langmuir–Freundlich model, and Q _st is then calculated using the following formula:

$$\ln P=\frac{-Q_{\mathrm{st}}}{RT}+\ln C$$

As shown in Figure , Q _st ranges from 14 to 24 kJ/mol, indicating that the adsorption of CO₂ by humin-based carbon material under the 800 °C-2/3 KOH condition is a physical adsorption process (Tables S1, S2, and Figure S6). In the literature, many sorbents exhibiting heterogeneous adsorption behavior show a decrease in Q _st with increasing CO₂ loading. ,− In contrast, Q _st in this work increases with the increase of CO₂ adsorption capacity, which may be attributed to the enhanced intermolecular interactions between CO₂ and the adsorbent. This trend also indicates the occurrence of multilayer CO₂ adsorption on the adsorbent, a behavior that has been similarly reported in CO₂ adsorption studies. ,,,

6.

Heat of adsorption of CO₂ on the 800 °C-2/3KOH material.

Furthermore, after five consecutive adsorption–desorption cycles (Figure ), the adsorption capacity of the 800 °C-2/3KOH material exhibited substantial stability with no significant decline in adsorption capacity throughout the cycles. This observation highlights the excellent cycling performance of the humin-based adsorbent studied in this research. As shown in Table , compared with other biomass-derived carbon materials, furfural-derived humins as a precursor for carbon materials have exhibited a high CO₂ adsorption capacity at 0 °C (6.06 mmol/g), high selectivity (25.16), and a moderate heat of adsorption (14–24 kJ/mol), demonstrating the great potential of humin-based carbon materials as a CO₂ adsorbent.

7.

Adsorption–desorption for 5 cycles of the 800 °C-2/3KOH material at 25 °C and 1 bar.

2. Comparison of CO₂ Adsorption Performance Obtained from Biomass.

feedstock	pyrolysis temp. (°C)	activator	CO2 uptake at 0 °C (mmol/g)	CO2/N2 Selectivity	Q st (kJ/mol)	ref
pomegranate peels	700	KOH	6.03	15.1
carrot peels	700	KOH	5.64	8.1
fern leaves	700	KOH	4.52	5.6
bamboo shoot shells	700	K2CO3	5.74	14.9	13–36
lotus petiole	700	KOH	6.05		26–41
pomelo peel	800	ZnCl2	4.83	38
olive pomace	600	K2CO3	2.63	29
water chestnut shell	750	NaBO2·4H2O	3.30	18	22–39
coconut shell	700	KOH, thiourea	6.46	17	22–37
HMF	400		5.27	19	42–44
glucose	400		4.25		29–34
cellulose	600	CO2	3.47	14.5	25–54
furfural	800	KOH	6.06	25.16	14–24	this study

4. Conclusions

Biomass-based carbon materials have shown great potential in the field of CO₂ adsorption due to their wide availability, low cost, and environmental friendliness. In this study, furfural-derived humins were produced through acid-catalyzed hydrothermal synthesis, followed by high-temperature carbonization and KOH activation. It was found that the carbon material activated at 800 °C with a KOH/humins ratio of 2/3 exhibited exceptional CO₂ adsorption performance up to 6.06 mmol/g at 0 °C and 1 bar. The synergistic regulation of activation temperature and KOH ratio effectively modulated the micropore volume, whereby a strong linear correlation was revealed between the CO₂ uptake and micropore volume in pores smaller than 0.79 nm. Furthermore, the material showed remarkable CO₂/N₂ selectivity up to 25.16, moderate adsorption heat (14–24 kJ/mol), and good recyclability. This research not only provides a new value-added utilization method for humin-based carbon materials as a CO₂ adsorbent but also makes the biomass conversion process more economical and environmentally friendly.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c13677.

• Yields of carbon materials; N₂ adsorption isotherms; pore size distribution; CO₂ uptake; CO₂ adsorption isotherms; and calculation of adsorption heat (PDF)

The authors declare no competing financial interest.