Study of CO 2 adsorption and separation using modified porous carbon

Abstract

Porous carbon and La₂O₃/porous carbon materials are synthesized for the study of CO₂ adsorption and separation by the volumetric method. The synthesized adsorbents are characterized by X-ray diffraction, N₂ adsorption–desorption isotherms, Raman spectra and scanning electron microscopy with energy-dispersive X-ray analysis. Characterization results confirm the existence of porosity in the synthesized carbon materials and uniform distribution of lanthanum(III) oxide on porous carbon. The CO₂ adsorption capacity for porous carbon and La₂O₃/porous carbon is 21 and 33 cm³ g⁻¹, respectively, at 298 K and 1 bar. High adsorption of CO₂ is obtained for La₂O₃/porous carbon because of the electrostatic interaction between La₂O₃ and CO₂. Moreover, the N₂ adsorption capacity is 2.8 cm³ g⁻¹ for porous carbon and 2.2 cm³ g⁻¹ for La₂O₃/porous carbon at 298 K and 1 bar. The change in N₂ adsorption is due to the decrease in surface area. For La₂O₃/porous carbon, the selectivity of CO₂/N₂ is 33.5 and the heat of CO₂ adsorption is 36.5 kJ mol⁻¹ at low adsorption of CO₂. It also shows constant CO₂ adsorption capacity in each adsorption cycle.

Introduction

Global warming is a worldwide environmental issue which is contributed to carbon dioxide. It is liberated by the combustion of fossil fuels coinciding with the high growth of the automobile and petrochemical industries. ¹ Day by day, the concentration of CO₂ is increasing and has reached 400 ppm, which is more than pre-industrial levels. Hence, greater effort has to be made to control the CO₂ concentration levels in the atmosphere. Carbon capture and separation (CCS) techniques on solid materials is an important method to lower CO₂ concentration levels, as is the liquid amine solution absorption technique. In power plants, liquid amine solutions are mostly used to capture CO₂. However, high energy is required for regeneration and occurs corrosion of the pipeline system. To overcome these drawbacks, solid materials are used for CCS. So far, materials such as activated carbon, ² zeolites ³ and mesoporous silica ⁴ have been used for CO₂ capture and separation.

Activated carbon is a cost-effective adsorbent and has a high surface area. However, the production of activated carbon on a large scale is hindered because of less renewable sources. Porous carbon synthesis from renewable sources is an important aspect. Renewable sources such as sawdust, ⁵ rice husk, ⁶ waste tea, ⁷ biodiesel industry solid residues, ⁸ cotton stalks, ⁹ olive and peach stones,^10,11 palm oil ash ¹² and coffee grounds ¹³ have been used to synthesize porous carbon materials. Pongamia pinnata fruit seeds are used for the production of biodiesel. During the production of biodiesel, a large quantity of pongamia pinnata fruit hulls is disposed of without commercial use. From these, porous carbon can also be synthesized by pyrolysis.

The CO₂ adsorption of porous carbon can be enhanced by the incorporation of basic metal oxide and a heteroatom, which generates basic sites on the porous carbon. The Nitrogen-doped porous carbon-derived from Hazelnut-shell has shown CO₂ adsorption capacity of 97 cm³ g⁻¹ at 298 K and 1 bar. ¹⁴ Similarly, d-glucose used as a source for N-doped porous carbon synthesis and reported CO₂ adsorption capacity of 88 cm³ g⁻¹ at 298 K and 1 bar, respectively. ¹⁵ Porous carbon derived from sustainable biomass stalk showed CO₂ adsorption capacity of 82.5 cm³ g⁻¹ at 298 K and 1 bar. ¹⁶ MgO-incorporated mesoporous carbon has shown a CO₂ adsorption capacity of 46.8 cm³ g⁻¹ at 298 K and 1 bar ¹⁷ and CeO₂-doped mesoporous carbon demonstrated 39.7 cm³ g⁻¹ of CO₂ adsorption at 303 K and 1 bar. ¹⁸ The CO₂ adsorption was higher on Cu₂O and NiO-incorporated porous carbon compared to bare porous carbon.^19,20 Recently, our group reported mesoporous carbon supported MgO for CO₂ capture. This material showed 37.6 cm³ g⁻¹ of CO₂ adsorption on 10 wt% MgO-incorporated mesoporous carbon which was higher than bare mesoporous carbon CO₂ adsorption 20.2 cm³ g⁻¹ at 298 K and 1 bar. ²¹ For the reported adsorbents, high CO₂ adsorption was obtained for metal oxide incorporated porous carbon due to the electrostatic interaction between the metal oxide and CO₂. In this paper, we have studied the influence of a low content of lanthanum(III) oxide on a porous carbon towards CO₂ adsorption and separation. Moreover, CO₂/N₂ selectivity, heat of CO₂ adsorption and CO₂ adsorption cycles were studied.

Results and discussion

Structural characterization

Figure 1(a) shows the X-ray diffraction (XRD) of porous carbon and lanthanum oxide incorporated porous carbon. Two broad diffraction peaks were observed at 2θ = 24° and 43.5° for the (002) and (100) planes, respectively. These were the main characteristic peaks of porous carbon. ²² Moreover, lanthanum oxide–incorporated porous carbon showed diffraction peaks similar to porous carbon and no peaks related to La₂O₃ appeared. This indicated that La₂O₃ was highly dispersed on porous carbon and/or not detectable by XRD analysis. Chanapattharapol et al. ²³ have also reported similar results with iron oxide doped MCM-41 for CO₂ adsorption. The Raman spectra of porous carbon and La₂O₃/porous carbon are shown in Figure 1(b). Porous carbon shows Raman bands at 1334 and 1582 cm⁻¹, which are related to the D-band and the G-band, respectively. ²⁴ The D-band corresponds to disordered carbon, whereas the G-band corresponds to graphitic carbon. The ratio of the intensity of D and G-bands (I_D/I_G) was 0.87 for porous carbon and 0.95 for La₂O₃/porous carbon. The high I_D/I_G ratio of La₂O₃/porous carbon indicates a decrease in the graphitic nature of porous carbon by the incorporation of lanthanum oxide. ²⁵

Figure 1.

(a) XRD patterns, and (b) Raman spectra of porous carbon and La₂O₃/porous carbon.

The porosity of the synthesized adsorbents was determined by measuring the N₂ adsorption–desorption isotherms at 77 K. Figure 2 shows the N₂ isotherms of porous carbon and La₂O₃/porous carbon, and the textural properties are presented in Table 1. Pristine porous carbon showed high adsorption of N₂ below the relative pressure P/P₀ = 0.1 and a hysteresis loop between the relative pressure of 0.4 and 1.0. As per the classification of porous materials by IUPAC, the isotherm of the porous carbon material is similar to type I and type IV isotherms. ²⁶ This indicated that the synthesized porous carbon has micropores and mesopores. The specific surface area, pore volume and pore size of porous carbon were 826 m² g⁻¹, 0.89 cm³ g⁻¹ and 4.3 nm, respectively. Similarly, La₂O₃/porous carbon showed an isotherm curve similar to that of pristine porous carbon. However, the amount of N₂ adsorption was less, which indicated that the pores of pristine porous carbon were occupied with lanthanum oxide. Hence, a change in the textural properties was observed. The specific surface area, pore volume and pore size of La₂O₃/porous carbon were 715 m² g⁻¹, 0.76 cm³ g⁻¹ and 4.2 nm, respectively. Lou et al. ²⁷ have also observed a change in textural properties by the occupation of ruthenium nanoparticles within the pores of porous carbon material.

Figure 2.

(a) N₂ adsorption–desorption isotherms (closed symbol: adsorption, open symbol: desorption) and (b) pore size distribution of porous carbon and La₂O₃/porous carbon.

Table 1.

Textural properties and metal composition of the synthesized samples.

Sample	SBET (m2 g−1) a	Vtotal (cm3 g−1) b	Vmicro (cm3 g−1) c	Vmeso (cm3 g−1) d	Pore size (nm) e	La content (wt%) f
Porous carbon	826	0.89	0.12	0.77	4.3	NF
La2O3/porous carbon	715	0.76	0.10	0.66	4.2	2.97

a

BET surface area.

b

Total pore volume at P/P₀ = 0.99.

c

Micropore volume obtained from the t-plot.

d

Mesopore volume obtained by subtracting V_micro from V_total.

e

Average pore size by BET.

f

From EDX, NF: not found.

Morphological images with chemical composition for porous carbon and La₂O₃/porous carbon are shown in Figure 3. Disordered carbon particles were observed for porous carbon (Figure 3(a)). In La₂O₃/porous carbon, the loaded La₂O₃ covers the surface of the disordered porous carbon (Figure 3(b)). From energy-dispersive X-ray (EDX) results, the amount of lanthanum was 2.97 wt%.

Figure 3.

SEM with EDX images of (a) porous carbon and (b) La₂O₃/porous carbon.

Study of CO₂ and N₂ adsorption

The adsorption of CO₂ and N₂ has been studied by the volumetric method using porous carbon and La₂O₃/porous carbon. The CO₂ and N₂ adsorption isotherms are shown in Figure 4. CO₂ adsorption increased on increasing the pressure, but no equilibrium was attained for both adsorbents. The amount of CO₂ adsorption was 21 cm³ g⁻¹ for porous carbon and 33 cm³ g⁻¹ for La₂O₃/porous carbon at 298 K and 1 bar. The high adsorption of CO₂ with La₂O₃/porous carbon was due to the electrostatic interaction between lanthanum oxide and CO₂. Along with the CO₂ adsorption study, N₂ adsorption was also studied under similar conditions to those used for the CO₂ adsorption study. The amount of N₂ adsorption was 2.8 cm³ g⁻¹ for porous carbon and 2.2 cm³ g⁻¹ for La₂O₃/porous carbon at 298 K and 1 bar. The decrease in N₂ adsorption was due to a change in the surface area.

Figure 4.

CO₂ and N₂ adsorption of porous carbon and La₂O₃/porous carbon (closed symbol: CO₂ adsorption, open symbol: N₂ adsorption).

In flue gas, carbon dioxide is a major component. So, it is essential to study CO₂/N₂ selectivity. The initial slope method was used to calculate CO₂/N₂ selectivity. ²⁸ Figure 5 shows the CO₂/N₂ selectivity with porous carbon and La₂O₃/porous carbon. For porous carbon, the CO₂/N₂ selectivity was 14.5, whereas for La₂O₃/porous carbon, the selectivity was 33.5. High selectivity was obtained for La₂O₃/porous carbon because of the high adsorption of CO₂ and the selectivity value was higher than those of some reported adsorbents such as HKUST-1 and Mg-MOF-74.^29,30

Figure 5.

CO₂/N₂ selectivity on porous carbon and La₂O₃/porous carbon.

The adsorption behaviour of an adsorbent can be calculated by fitting of the experimental CO₂ adsorption with the Freundlich and Langmuir-Freundlich isotherm models. ³¹ The isotherm models can be written as follows

$$\begin{array}{l}\mathrm{Freundlich}\mathrm{isotherm}\mathrm{model}:\mathrm{Q}={\mathrm{k}}_{\mathrm{F}}{\mathrm{P}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}\\ \mathrm{L}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{m}\mathrm{u}\mathrm{i}\mathrm{r}-\mathrm{F}\mathrm{r}\mathrm{e}\mathrm{u}\mathrm{n}\mathrm{d}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{l}:\mathrm{Q}=\frac{{\mathrm{Q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{K}{\mathrm{P}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}}{1+\mathrm{K}{\mathrm{P}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}}\end{array}$$

where Q is the adsorption capacity at equilibrium (cm³ g⁻¹); Q_max is the maximum adsorption capacity (cm³ g⁻¹); P is the pressure (kPa); k_F and K are the Freundlich and Langmuir–Freundlich constants, respectively; and n is the heterogeneity factor. The experimental CO₂ adsorption of porous carbon and La₂O₃/porous carbon are fitted with the Freundlich and Langmuir–Freundlich isotherm models, as shown in Figure 6. The fitting parameters are presented in Supplemental Table S1. The Langmuir–Freundlich isotherm model was well-fitted with the experimental CO₂ adsorption, regression co-efficient R² > 0.999. The Q_max was higher for La₂O₃/porous carbon because of the strong interaction of CO₂ with lanthanum oxide.

Figure 6.

Fitting of isotherm models for CO₂ adsorption on porous carbon and La₂O₃/porous carbon.

In gas adsorption studies, the heat of adsorption (Q_st) is an important parameter. It describes the interaction between the adsorbate and adsorbent. The Q_st can be calculated using the virial equation ³²

$$\begin{array}{l}\mathrm{V}\mathrm{i}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\mathrm{l}\mathrm{n}\mathrm{P}=\mathrm{l}\mathrm{n}\mathrm{N}+\frac{1}{\mathrm{T}}\sum _{\mathrm{i}=0}^{\mathrm{m}}{\mathrm{a}}_{\mathrm{i}}{\mathrm{N}}^{\mathrm{i}}+\sum _{\mathrm{j}=0}^{\mathrm{n}}{\mathrm{b}}_{\mathrm{j}}{\mathrm{N}}^{\mathrm{j}}\\ \mathrm{H}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:{\mathrm{Q}}_{\mathrm{s}\mathrm{t}}=-\mathrm{R}\sum _{\mathrm{i}=0}^{\mathrm{m}}{\mathrm{a}}_{\mathrm{i}}{\mathrm{N}}^{\mathrm{i}}\end{array}$$

where P is the pressure in Torr, N is the gas uptake in cm³ g⁻¹, T is the temperature in K, R is the universal gas constant (8.314 JK⁻¹ mol⁻¹), Q_st is the heat of adsorption in kJ mol⁻¹, ai and bj are the virial coefficients, and m and n are the number of coefficients. To calculate the heat of CO₂ adsorption for porous carbon and La₂O₃/porous carbon, CO₂ adsorption at 303 K was measured for both samples (see Supplemental Figure S1). The amount of CO₂ adsorption was less at 303 K compared with CO₂ adsorption at 298 K because of the increase in the kinetic energy of CO₂. The measured CO₂ adsorption isotherms were fitted with the virial equation (Supplemental Figure S2). From the virial fitting parameters, the heat of CO₂ was calculated. The heat of CO₂ adsorption of porous carbon and La₂O₃/porous carbon is shown in Figure 7. For porous carbon, Q_st was 18.9–14.5 kJ mol⁻¹, whereas for La₂O₃/porous carbon was 36.5–33.4 kJ mol⁻¹. A high Q_st was obtained for La₂O₃/porous carbon because of the strong interaction between CO₂ and lanthanum oxide. At low adsorption of CO₂, a high Q_st was obtained. With an increase in CO₂ adsorption, the Q_st decreased due to a decrease in the number of active adsorption sites.

Figure 7.

The heat of CO₂ adsorption of porous carbon and La₂O₃/porous carbon.

Multiple CO₂ adsorption cycles have been studied to determine the adsorption stability of La₂O₃/porous carbon at 298 K (Figure 8). Before studying the CO₂ adsorption cycle, the adsorbent was degasified at 473 K for 2 h under vacuum to remove the adsorbed CO₂. The amount of CO₂ adsorption was constant in each adsorption cycle. The CO₂ adsorption of La₂O₃/porous carbon was compared with the CO₂ adsorption capacity of previously reported adsorbents (Table 2). The synthesized La₂O₃/porous carbon showed 33 cm³ g⁻¹ of CO₂ adsorption at 298 K and 1 bar, which was in-between the CO₂ adsorption capacity of mesoporous N-doped CeO₂ and S-doped microporous carbon.^34,36

Figure 8.

Multiple CO₂ adsorption cycles of La₂O₃/porous carbon.

Table 2.

Comparison of the CO₂ adsorption capacity of La₂O₃/porous carbon with reported adsorbents.

Sample	Adsorption method	CO2 adsorbed (cm3 g−1) at 298 K, 1 bar	Reference
ZIF-98	Volumetric	34	Wang et al. 33
S-doped microporous carbon	Volumetric	54	Xia et al. 34
N-doped microporous carbon	Volumetric	53.7	An et al. 35
Mesoporous N-doped CeO2	Gravimetric	24 (303 K)	Wang et al. 36
PEHA-MIL-101	Volumetric	29	Anbia et al. 37
Zeolite-13X	Volumetric	38	McEwen et al. 38
La2O3/porous carbon	Volumetric	33	Present work

Conclusion

In this work, we have studied CO₂ adsorption and separation by the volumetric method using porous carbon and La₂O₃/porous carbon. The high CO₂ adsorption capacity was obtained on La₂O₃/porous carbon compared to bulk porous carbon at 298 K and 1 bar because of electrostatic interaction of La₂O₃ with CO₂ and CO₂/N₂ selectivity was also higher on La₂O₃/porous carbon. The heat of CO₂ adsorption was 36.5 kJ mol⁻¹ at low coverage of CO₂ for La₂O₃/porous carbon and CO₂ adsorption capacity was constant in each adsorption cycle. Therefore, a basic metal oxide can be incorporated on porous carbon to increase the CO₂ adsorption and separation.

Experimental

Lanthanum nitrate hexahydrate [La(NO₃)₃.6H₂O, 99.9%] and orthophosphoric acid (H₃PO₄, 85%) were purchased from Sigma-Aldrich, India, and used without purification. Distilled water was used to synthesize the adsorbents. High purity gases (carbon dioxide, nitrogen and helium) were purchased from BOC, India.

Porous carbon was synthesized by using pongamia pinnata fruit hulls which were collected from the forest region of Telangana, India. First, the fruit hulls were washed, dried then made into a powder. The powder was activated using phosphoric acid at room temperature for 24 h followed by drying at 373 K for 12 h. The dried sample was calcined under nitrogen gas with a flow rate of 50 mL/min at 723 K for 4 h, with a heating rate of 5 K/min, and then cooled to room temperature. The sample was washed with distilled water until the pH reached 7, then it was dried at 373 K overnight to afford porous carbon. ³⁹ Porous carbon supported lanthanum oxide was synthesized by the impregnation method. ⁴⁰ About 0.1 g of lanthanum nitrate hexahydrate was dissolved in 10 mL of distilled water, stirred for 10 min then 1 g of porous carbon was added. The mixture was stirred at room temperature for 1 h and then dried at 373 K overnight. The dried compound was calcined under nitrogen gas at 873 K for 3 h.

XRD patterns were recorded on a Rigaku Ultima-IV X-ray diffractometer using Ni-filtered Cu-K_α radiation operated at a voltage of 40 kV and a current of 30 mA in the scan range of 2θ = 10°–80° with a step size of 0.02°/s. A Micrometric ASAP 2020 porosity analyzer was used to measure N₂ adsorption–desorption isotherms at 77 K. Prior to the adsorption study, about 0.1 g of the sample was degassed at 473 K for 2 h under vacuum. The multipoint Brunauer–Emmett–Teller (BET) surface area was calculated in the relative pressure range of 0.05–0.3, the total pore volume at a relative pressure of 0.99, the micropore volume by the t-plot method and the mesopore volume was calculated by subtracting the micropore volume from the total pore volume. The pore size distribution was plotted using non-local density functional theory. A LabRAM HR800 spectrometer was used to record the Raman spectra. The morphology and elemental composition of each sample were determined from scanning electron microscopy with energy-dispersive X-ray (SEM with EDX) analysis using a ZEISS Sigma 300 scanning electron microscopy analyzer.

The adsorption of CO₂ and N₂ was carried out using a Micromeritics ASAP 2020 gas adsorption analyzer at low pressure and at 298 K. A thermostatic bath connected with water circulating jacket was used to control the sample temperature, and helium gas was used to determine the free space of the sample tube. Before the gas adsorption measurement, 0.1 g of the sample was activated at 473 K for 2 h under vacuum to remove moisture or adsorbed gases and then cooled to the gas adsorption temperature. Ultra-high pure gases were used to measure the adsorption isotherms. The initial slope method was used to calculate the selectivity of CO₂/N₂. The virial method was used to calculate the heat of CO₂ adsorption using adsorption isotherms obtained at 298 and 303 K. The CO₂ adsorption cycles were also studied at 298 K to calculate the adsorption stability of an adsorbent.

Supplemental Material

Supplementary_material_7 – Supplemental material for Study of CO2 adsorption and separation using modified porous carbon

Supplemental material, Supplementary_material_7 for Study of CO2 adsorption and separation using modified porous carbon by Madhavi Jonnalagadda, Rumana Anjum, Harshitha Burri and Suresh Mutyala in Journal of Chemical Research