Introduction

The increase in greenhouse gas (GHG) emissions, especially carbon dioxide (CO2), in the atmosphere leads to global climate change, which has become a serious issue worldwide today1. Excessive GHGs in the atmosphere are responsible for various environmental problems, such as a rise in the sea water level, an increase in the earth’s temperature, and an increasing number of ocean storms, floods, etc2. The 26th UN Climate Change Conference of the Parties (COP26) resulted in an agreement that requires countries to decrease their GHG emissions by reducing the use of fossil fuels and replacing it with clean energy by 20303,4. Currently, there are many useful methods to reduce or capture GHG emissions, such as renewable technologies5,6, energy saving technologies7, and carbon capture, utilization, and storage (CCUS)8,9,10. Of these, CCUS is one of the interesting technologies and is based on CO2 adsorption by various solid adsorbents, such as activated carbon11,12,13, zeolites14,15,16, sepiolite17, heteroatom-doped porous carbons18,19, porous organic polymers20, metal-organic frameworks (MOFs)21, reduced graphene oxide22, and recycled waste materials23,24,25,26. These strategies are also compatible with the Bio-Circular-Green Economic Model (BCG)27 to create economic and environmental sustainability and to reduce the waste from many industries28. However, after CO2 adsorption in the adsorbent material, it is economically and environmentally important to make value-added products from the CO2. The possible ways used to convert CO2 into valuable hydrocarbon (HC) compounds include thermochemical, photocatalytic, and electrochemical conversions29,30,31. These concepts are mainly focused on this study, as they are the part of the challenge of how to utilize waste materials as adsorbents and how to convert CO2 to value-added chemicals.

In this study, the recycling of waste tires (WTs) was used to develop an adsorbent for CO2 capture, since WTs are a non-biodegradable material, due to their complex structure after vulcanization, and the annual global production of WTs is currently around 1.4 billion pieces every year32. At present, WTs are mainly disposed of by the traditional methods of landfill, which requires a large space, or burning, which accumulates toxic air pollution33. Therefore, the use of recyclable WTs as an adsorbent is beneficial for reducing the huge amount of their waste.

Several researchers have focused on the transformation of WTs for use in various applications. For instance, WTs have been pyrolyzed and chemically modified to produce a magnetic pyrochar adsorbent for wastewater treatment34,35,36,37. Pyrolytic WT products have been prepared via impregnation as a catalyst for producing hydrogen (H2) and carbon nanotubes38,39,40,41,42,43. Pyrochar from WT pyrolysis was treated by chemical activation to prepare an electrode for energy storage44,45,46,47. Finally, activated carbon from WT pyrolysis was formed by physical and chemical activation as a solid adsorbent for CO2 capture48,49,50.

However, these techniques prepared the obtained materials through a thermal process that consumes a high amount of energy, and the process is complicated. To solve such problems, modifying or functionalizing a WT adsorbent with amines on its surface benefits the easy preparation of a solid adsorbent. Natural rubber (NR) latex foam with amine-modified silica51,52 or modified NR with amine-modified silica particles was found to enhance the CO2 adsorption53. Therefore, this study focused on amine-modified WTs. It is interesting to note that the WTs used in this study consisted of some additives, such as carbon black or silica, which could enhance the efficiency of CO2 adsorption54,55. In addition, the presence of vulcanization in WTs could improve the thermal properties of the material56. Although, amine-modified WTs have been studied previously as an absorbent in wastewater treatment57, their application for CO2 capture has not been demonstrated yet.

To reduce the emission level of GHGs, CO2 can also be converted into valuable HC compounds or fuel, such as methane (CH4), ethane (C2H6), carbon monoxide (CO), methanol, and ethanol. There are three possible routes for CO2 conversion to higher value products as follows: photocatalytic conversion, electrochemical reduction, and thermochemical conversion58,59,60,61,62,63,64. In this study, the electrochemical reduction of CO2 was examined in the conversion of CO2 to more reduced chemical species using electrical energy. The electrochemical reduction can support renewable energy exploitation to create an artificial carbon-neutral cycle. Also, it can be easily carried out in aqueous media at room temperature and atmospheric pressure.

Copper (II) oxide (CuO) and copper (I) oxide (Cu2O) are both used as catalysts in the conversion of CO2 due to their unique properties that facilitate certain chemical reactions. Their application in the CO2 conversion processes is motivated by their redox properties, high surface area, ability to chemisorb CO2 molecules, presence of active sites, involvement in catalytic cycles, and capability to control the selectivity65,66,67,68. These catalysts play a crucial role in developing sustainable methods for utilizing CO2 as a feedstock to produce valuable chemicals and fuels.

Accordingly, the aim of this study was to modify WT samples as an adsorbent material for CO2 capture. The WT samples were directly impregnated with tetraethylenepentamine (TEPA) or polyethyleneimine (PEI) to enhance the CO2 adsorption capacity. A series of adsorbents prepared with different amine types (PEI or TEPA) and amine loading levels [0–15% (w/w)] was investigated for their CO2 capture under ambient temperature and atmospheric pressure. The adsorption kinetics were explored by fitting three adsorption kinetic models (pseudo 1 st order, pseudo 2nd order, and Avramis’ models). The reusability of the selected TEPA-modified WT was evaluated over 10 successive adsorption-desorption cycles, while for practical application, the CO2 captured in the TEPA-modified WT was converted to value-added HCs via the electrochemical CO2 reduction reaction (CO2RR) using a CuO: Cu₂O catalyst with a 1:2 molar ratio.

Materials and methods

Materials

The WTs, mainly unusable tires from automobiles, were provided as a powder with an average particle size of 250 μm from the Union Commercial Development Co., Ltd, Thailand. The TEPA (189.3 g/mol), PEI (800 g/mol), Cu2O (98% purity analytical grade), and CuO (AR grade) were purchased from Sigma-Aldrich Co., Ltd. Absolute ethanol (99.99%), potassium bicarbonate (KHCO3; AR grade), and isopropyl alcohol (AR grade) were from Qrec Chemical Ltd., New Zealand. Super C65 nano carbon black conductive, used as a conductive additive in electrodes, was from MSE supplies™, USA. Ethylene glycol (AR grade) was purchased from KEMAUS Chemical, Australia. All chemicals were used as received without further purification.

Preparation of the amine-modified WT samples

The WT powders were modified by amines (TEPA or PEI) using wet impregnation. The selected amine was dissolved in 50 mL ethanol with stirring at ambient temperature (about 30 °C) for 30 min. Then, 30 g of WT powder was mixed with the amine solution by stirring at ambient temperature for 3 h. Finally, the slurry was then dried in an oven at 60 °C for 48 h. To optimize the amine loading level, amine loadings were varied at 0, 2.5, 5, 10, and 15% (w/w, based on 100 g WT). The amine-modified WT samples were coded WTxP or WTxT for PEI and TEPA modification, respectively, where x represents the amine % (w/w) loading level. The amine efficiency of the amine-doped composite that showed the maximum CO2 adsorption capacity was calculated based on previous work by Sakwa-Novak et al.69 In this study, the amine efficiency of WT10T and WT10P was approximately 0.1 and 0.08, respectively.

Characterization of the amine-modified WT samples

The WT samples before and after amine modification were characterized by nitrogen (N2) adsorption-desorption Micromeritics, Asap 2020, USA) to determine the Brunauer-Emmett-Teller (BET) surface area (SBET) and total pore volume (Vp). The samples were degassed under a N2 flux for 2 h at 120 °C. Scanning electron microscopy (SEM; JEOL, JSM-6480LV, USA) at an accelerating voltage of 15 kV was used to investigate the WT morphology before and after amine modification. The samples were placed on a stub and then coated with gold before testing. The elemental analysis was examined using a CHN/S analyzer (Perkin Elmer, PE-2400, USA), while the composition of the samples was evaluated using Fourier-transformed infrared spectroscopy (FTIR, Thermo Ficher, Nicolet iS5, USA). To confirm the CO2 adsorption-desorption in the adsorbent, Raman spectroscopy (DXR Raman microscope, Thermo Scientific, USA) was examined and recorded from 100 to 4000 cm−1.

The powder rubber samples (0.4 g) contained in tea bag were immersed in 100 mL of toluene for 72 h. After that the surface liquid was rapidly removed by blotting with filter paper. The samples were dried in an oven at 80 °C until at constant weight. The swelling ratio (Q), sol-gel fraction, and crosslink density were calculated according to Eqs. (1)–(4), respectively70,71.

$$\:Q=\frac{{{W}_{2}-W}_{1}}{{W}_{1}}$$
(1)
$$\:Sol\:fraction\:\left(\%\right)=\frac{{{W}_{1}-W}_{3}}{{W}_{1}}$$
(2)
$$\:Gel\:fraction\:\left(\%\right)=100-\:Sol\:fraction\:\left(\%\right)$$
(3)
$$\:Crosslink\:density\:\left(\frac{mol}{{cm}^{3}}\right)=\frac{-[\text{ln}\left(1-{v}_{r}\right)+{v}_{r}+\chi{v}_{r}^{2}]}{{v}_{0}[{v}_{r}^{1/3}-\frac{{v}_{r}}{2}]}$$
(4)

where W1 is the initial sample weight, W2 is the sample weight after swelling, W3 is the dried sample weight after swelling, vr is the volume fraction of the rubber network in the swollen gel, vo is the molar volume of the toluene (106.2 cm3/mol)70, and χ is a rubber-solvent interaction parameter (0.393)71. The three parameters—swelling ratio, sol-gel fraction, and crosslink density—are studied to evaluate the properties of WT, specifically before and after modification. These parameters help in understanding the effects of amine incorporation on the tires’ properties, providing insight into how the amine modification influences the material’s structural and functional characteristics.

CO2 adsorption-desorption

The CO2 adsorption-desorption of the unmodified and amine-modified WT samples was performed in a stainless-steel reactor as shown in Fig. 1. The dried sample (3 g) was loaded into the reactor and then heated under vacuum pressure at 60 °C under a N2 flow rate of 300 mL/min for 20 min to release existing bound gases. After that, the reactor was cooled down to room temperature, and then the gas was switched to CO2 [12% and 99% (v/v)] at a flow rate of 70 mL/min at ambient temperature and atmospheric pressure. The CO2 amount in the effluent was detected using a CO2 sensor (CM-40401, United States of America). The CO2 adsorption capacity was then calculated based on the difference between the CO2 adsorption breakthrough curve areas. The CO2 desorption of each sample was performed by the combination of vacuum and temperature swing adsorption (VTSA) process. The adsorbent after CO2 adsorption was heated at 60 °C under reduced pressure with a N2 flow rate of 100 mL/min and then cooled to ambience temperature. Finally, the sample was subjected to 10 sequential repeated CO2 adsorption-desorption cycles to evaluate its reusability.

Fig. 1
figure 1

Illustration of the CO2 adsorption-desorption and CO2 conversion equipment.

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Kinetic study

The pseudo 1 st order, pseudo 2nd order, and Avrami’s kinetic models were used to fit the experimental adsorption kinetic data to investigate the adsorption mechanism of the adsorbent using Eqs. (5)–(7), respectively72,

$$Pseudo \, 1^{st} \, order: \:{q}_{t}=\:{q}_{e}\left[1-\text{exp}\left(-{k}_{1}t\right)\right]$$
(5)
$$Pseudo \, 2^{nd} order: \:{q}_{t}=\:\frac{{k}_{2}{q}_{e}^{2}}{1+{k}_{2}{q}_{e}t}t$$
(6)
$$Avrami{'}s \, model: \:{q}_{t}=\:{q}_{e}\left[1-{\text{exp}\left(-{k}_{A}t\right)}^{n}\right]$$
(7)

where \(\:t\) is the adsorption time (min), \(\:{q}_{e}\) is the adsorption capacity at equilibrium (mg\(\cdot\)g−1), \(\:{q}_{t}\) is the adsorption capacity at time \(\:t\) (min), \(\:{k}_{1}\) is the pseudo 1 st order rate constant (min− 1), \(\:{k}_{2}\) is the pseudo 2nd order rate constant (g\(\cdot\)mg−1\(\cdot\)min− 1), \(\:{k}_{A}\) (min− 1) is the Avrami’s rate constant, and \(\:n\) is the order of the kinetic equation.

CO2 conversion

The catalyst for the CO2 electrochemical conversion by the CO2RR was prepared as a 1:2 mol ratio of CuO: Cu2O on carbon paper. The electrochemical test was performed using a three-electrode cell on a H-cell station. A platinum plate of 1 × 2 cm in size was employed as the counter electrode and Ag/AgCl (in 3 M sodium chloride solution) soaked in a luggin capillary was used as the reference electrode. The cathode and anode compartments were separated by a piece of proton exchange membrane (Nafion 117, Dupont) to prevent re-oxidation of the products. The 0.1 M KHCO3 electrolyte solution was purged with CO2 [12% and 99% (v/v)] for 10 min before applying a reduction potential of −1.7 V (vs. Ag/AgCl). The gaseous products after the chronoamperometric electrolysis were detected by gas chromatography equipped with a thermal conductivity detector (GC-TCD) using helium as the carrier gas. The overall summary of this research is shown in Fig. 2.

Fig. 2
figure 2

The overall summary of this research.

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Results and discussion

Characterization of the amine-modified WT

The FTIR spectra of the WT before and after amine modification are shown in Fig. 3. The peak located at 2914 cm−1 corresponded to the aliphatic C–H unsymmetrical stretching vibration of CH3– groups and the peak at 2847 cm−1 was attributed to the aliphatic C–H symmetrical stretching vibration of –CH2– groups71,73,74. These results indicated that the WT used in this study may contain both synthetic and natural rubbers. The peak at 1536 cm−1 was assigned to the C-S bond from the sulfur crosslinks formed during the WT production75. After the WT was modified by TEPA or PEI, a broad peak at 3210 cm−1 was evident, which represented the N-H symmetric and asymmetric stretching vibration of amine groups76,77. The N-H bending of the secondary amine [-N(R)H] was represented at 1650 cm−1, while the asymmetric and symmetric bending of primary amine (-NH2) were observed at 1560 and 1430 cm−1, respectively76,78,79. It is interesting to note that the peak intensity of N-H stretching and bending for WT10P was clearly higher than that for WT10T due to the high numbers of amine active sites in the PEI structure. In addition, based on the elemental analysis (Table 1), the nitrogen contents of WT10P were higher than those of WT10T. According to these results, the WT was successfully modified by TEPA or PEI.

Fig. 3
figure 3

Representative FTIR spectra of the (a) WT, (b) WT10T, and (c) WT10P.

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Table 1 Elemental analysis and textual properties of the WT without and with different amine modification.
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However, the peak at 1536 cm−1 representing the C–S bond in both the WT10T and WT10P was markedly reduced or disappeared, while the sulfur content in WT10T (2.72%) and WT10P (2.44%) was decreased compared to that in the WT (3.09%) due to the devulcanization by amines80,81. Table 2 displays the swelling ratio, sol-gel fraction, and crosslink density of WT without and with various amine modifications. The swelling ratio of WT10T (4.11) and WT10P (3.91) was higher than that of unmodified WT (3.56). The sol fraction of WT10T and WT10P tended to be increased and the crosslink density of WT10T (23.30 × 10−3 mol/cm3) and WT10P (23.55 × 10−3 mol/cm3) was lower than that of WT (27.32 × 10−3 mol/cm3). These results confirmed that the amines (TEPA and PEI) could break up the sulfur bonds in rubber vulcanizates, resulting in a decreased level of C–S bonds. Accordingly, TEPA and PEI serve not only as modifiers of WT but also as devulcanizing agents.

Table 2 Swelling ratio, sol-gel traction, and crosslink density of the WT without and with different amine modification.
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The morphology of the WT before and after amine modification is shown as SEM images in Fig. 4. The WT surface contained some small pores and had a SBET and total Vp of 2.02 m2∙g−1 and.

5.80 × 10−4 cm3∙g⁻¹, respectively (Table 1). After amine modification, the modified WT surface became rougher and lost its elasticity, resulting in an increased SBET and total Vp. Although, the total Vp of WT10T and WT10P was increased after amine modification, their values seemed to be low when compared to those previously reported for other solid adsorbent materials, i.e., MOFs, carbon materials, silica, and zeolite82,83,84,85. Accordingly, the adsorbent materials prepared in this study exhibited the characteristics of a non-porous material.

Fig. 4
figure 4

Representative SEM images (1,500 x magnification) of (a) WT, (b) WT10T, and (c) WT10P.

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Effect of the amine type, amine loading level, and CO2 concentration on the CO2 adsorption

The WT powder was modified by two amine types (TEPA and PEI) at varying amine loading levels [2.5–15% (w/w)] to form the CO2 adsorbents, which were then compared under the same operating condition of an ambient temperature and atmospheric pressure. The breakthrough curve of the adsorbents (Fig. 5) reveals distinct adsorption behaviors. When comparing amine types, the TEPA-modified WTs showed a low sensitivity of breakthrough behavior with the amine loading level, while the PEI-modified WTs exhibited a high sensitivity in their breakthrough capacity with the amine loading level. This can be explained by the structural effects of amine polymers on adsorbent performance. Branched amine polymers may obstruct CO2 adsorption compared to that of linear amine polymers. The breakthrough curves of WTs with different amine loadings (Fig. 5a and b) corresponded with the CO2 adsorption capacity of the amine-modified WT with various amine loadings (Fig. 5c). The CO2 adsorption capacity of the unmodified WT was about 7.10 mg/g of adsorbent, while the CO2 adsorption capacity of the WT modified with TEPA or PEI increased with increasing amine loading levels up to 5% or 10% (w/w) for PEI and TEPA, respectively. With an amine loading in the range of 2.5–5% (w/w), the CO2 adsorption capacity of the PEI-modified WT increased more than that for the TEPA-modified WT. This is presumably due to the difference in chemical structure of both amines, where PEI is an organic polyamine polymer with a network structure, while TEPA is a linear structure. Therefore, there are more basic sites in PEI than in TEPA to bind the CO2 molecules86.

Increasing the amine loading level to 10% (w/w) decreased the CO2 adsorption capacity of WT10P slightly, while that for WT10T was still increased compared to that with a 5% (w/w) loading level. This suggested that a high concentration of the high molecular weight PEI made the PEI-modified WT sample stickier, resulting in a decreased surface area and high CO2 diffusion resistance, whilst this behavior was not observed in the case of WT10T. At a 15% (w/w) amine loading level, the CO2 adsorption capacities of the WT15T and, especially, WT15P decreased markedly compared to those at 10% (w/w) due to the adsorbent surface becoming sticky. The morphology of the amine-modified adsorbents with various amine loading levels are represented in supplementary information (Figs. S1 and S2).

The FTIR spectral analysis was used to ascertain the nature of WT10T and WT10P before and after CO2 adsorption as well as CO2 desorption (Fig. 6). Before adsorption, the peaks of each adsorbent were located at 3210 cm−1, assigned to the N-H symmetric and asymmetric stretching vibration of amine groups. The peaks located at 1650 and 1560 cm−1 were attributed to the N-H bending of the secondary and primary amines, respectively. After CO2 adsorption, the peak located at 1535 cm−1, which represented the asymmetric stretching vibration of the carboxylate anion (COO−) of ammonium carbamate, was observed in both WT10T and WT10P87,88,89,90,91. This indicated that the CO2 molecules could be chemisorbed on the amine-impregnated WT. However, the interaction between CO2 molecules and amino groups was destroyed after the CO2 desorption under regeneration using the VTSA process, resulting in the presence of N-H bending peaks at 1650 and 1560 cm−1, similar to the peaks of the sorbent materials before CO2 adsorption.

Fig. 5
figure 5

Breakthrough curves of WT with different (a) TEPA and (b) PEI loading levels; and (c) the CO2 adsorption capacity of the WT and TEPA- or PEI-modified WT at different amine loading levels [2.5–15% (w/w)] at ambient temperature and atmospheric pressure.

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Fig. 6
figure 6

Representative FTIR spectra of (a) WT10T and (b) WT10P before and after CO2 adsorption as well as their CO2 desorption.

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Additionally, the CO2 capture in the adsorbent materials could be confirmed by Raman spectroscopy (Fig. 7). The two distinct bands at 1360 and 1570 cm−1 represented the carbon black in WT92,93 and corresponded to the disordered carbon (D band) and the graphite (G band) structures, respectively93. Before CO2 adsorption (Fig. 7a), the intensity peak ratio (ID/IG = I1360/I1570) presented the same value (0.85) because the carbon black in all samples was the same content and did not adsorb or react with CO2 molecules. After CO2 adsorption (Fig. 7b), the peak intensity of the band at 1360 cm−1 seemed to be increased due to a band overlap of CO2 molecules at 1370 and 1245 cm−1 and D band of carbon black as previous research52,94. The I1360/I1570 ratio of WT10P and WT10T was then increased to 0.89 and 0.91, respectively. Whist, the I1360/I1570 of WT did not change because the small amount of CO2 molecules was absorbed. Accordingly, the CO2 molecules could be adsorbed more favorably on the amine-impregnated adsorbent materials compared to the neat WT.

Fig. 7
figure 7

Representative Raman spectra of the WT, WT10T, and WT10P (a) before and (b) after CO2 adsorption.

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The CO2 adsorption capacity of WT10T at ambient (30 °C), 45 °C, and 60 °C is summarized in Fig. 8d. When the temperature increased during the CO2 adsorption, the CO2 adsorption of WT10T tended to be reduced. This was due to the exothermic nature of the adsorption reaction95,96, where increasing the temperature decreased the adsorption rate. As a result, the time for CO2 to flow through the adsorbent material, and the absorption level was reduced. This behavior is different from the previous study that used NR filled with N-[(3-trimethoxysilyl)propyl]ethylenediamine and N-[(3-trimethoxysilyl)propyl]diethylenetriamine)52. This may be due to the different amine chemical structure, rubber nature, and modified silica particles.

Fig. 8
figure 8

Experimental CO2 adsorption capacity of WT10T with the corresponding fits to the different kinetic models at (a) 30 °C, (b) 45 °C, and (c) 60 °C; and (d) the CO2 adsorption capacity of WT10T at various temperatures.

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Overall, WT10T had the highest CO2 adsorption capacity (10.41 ± 0.24 mg∙g⁻¹ of adsorbent) at an ambient temperature and atmospheric pressure. Thus, this sample was then chosen for further study of kinetics, regeneration, and CO2RR. It is interesting to note that the effect of CO₂ concentration on Faradaic efficiency (FE) is significant. As the CO₂ concentration increased from 12% (v/v) to 99% (v/v), the CO₂ adsorption capacity of the WT10T adsorbent notably increased, from 10.41 ± 0.243 to 59.98 ± 1.577 mg∙g⁻¹ under ambient temperature and atmospheric pressure (Table 3). This enhancement in CO₂ adsorption was accompanied by an increase in FE, with both CH₄ and C₂H₄ detected as gas products. The increase in FE with higher CO₂ concentration can be attributed to more efficient interactions between the adsorbent and CO₂ molecules at higher concentrations, leading to better electrochemical conversion. Therefore, factors such as amine type, amine loading level, and CO₂ concentration play important roles in influencing both the CO₂ adsorption capacity and the Faradaic efficiency of the amine-modified WT adsorbent.

Table 3 The CO2 adsorption capacity of WT10T at different CO2 concentrations and the Faradaic efficiency (FE) of gas products from the CO2RRa using a 1:2 mol ratio cuo: Cu2O catalyst.
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Table 4 Comparison of CO2 adsorption capacities of various adsorbent materials.
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Here in this study, the comparison of CO2 adsorption capacity for different adsorbent materials is summarized in Table 4. The observed variation in the CO2 adsorption capacity was due to the difference in specific properties and testing conditions of each sorbent material under a similar condition, WT10T could be considered as a potential alternative solid sorbent material for CO2 capture. It has a high CO2 adsorption capacity, is easy to prepare, low-cost, low energy requirement, and is environmentally friendly. In addition, the advantage of using WTs to make an absorbent material is that it reduces the current large-scale amount of rubber waste.

Kinetic adsorption. Kinetic modeling is an important step in evaluating the CO2 adsorption mechanism of an adsorbent material. The experimental CO2 adsorption capacities of the WT10T sample were fitted to each kinetic model (Fig. 8a and c), with the results summarized in Table 5. The pseudo 1 st order and pseudo 2nd order models were clearly not suitable for describing the adsorption kinetics of WT10T due to their low R2 value, whereas Avrami’s model fitted well with the adsorption experimental data (R2 > 0.97 with the lowest root mean square error and mean square error values). Thus, CO2 could interact with WT10T by both physisorption with the rubbery matrix and by chemisorption with the amine groups, in agreement with the Avrami’s model order (na) being in the range of 0.82–1.27100.

Table 5 Kinetic model and the corresponding kinetic parameters of CO2 adsorption for WT10T at ambient temperature and atmosphere pressure.
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Regeneration of adsorbent materials

To evaluate the stability of the amine-modified adsorbent materials, the adsorption-desorption cycle of WT and WT10T was investigated over 10 sequential cycles using the VTSA process. The relationship between the CO2 adsorption capacity and the cycle number for both WT and WT10T are shown in Fig. 9. The CO2 adsorption capacity of the unmodified WT decreased by 5.72% in the 2nd cycle compared to the 1 st cycle. This is because the heterogeneity in sorbent’s active site was treated during the first regeneration cycle. Thereafter, the CO2 adsorption capacity then remained almost constant until the 6th cycle, which can be explained by the stability of the active component in the sorbent with the employed regeneration method. Finally, the CO2 adsorption capacity gradually declined again by 13.55% and 16.47% at the 7th and 10th cycle (compared to the 1 st cycle), respectively. This decline was due to the loss of active components in the sorbent material, as evidenced by CHNS results. Thus, the WT adsorbent material showed a poor regeneration stability. In contrast, the CO2 adsorption capacity of WT10T seemed to be stable over all 10 cycles, decreasing by less than 3%. Accordingly, the regeneration stability of WT10T was better than that of WT, which reflected that the amine sites on WT10T are not volatile and do not show oxidative degradation during the regeneration process101,102. This result was also supported by the CHNS analysis, where the nitrogen contents of WT10T before and after regeneration seemed to be constant (Table 1).

Fig. 9
figure 9

Successive CO2 adsorption-desorption cycles of WT and WT10T.

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Electrochemical CO2RR

To study the catalytic activity and the effect of the CO2 concentration on the catalytic performance of the CO2RR using a 1:2 mol ratio CuO: Cu2O on carbon paper catalyst, the CO2 was supplied to the CO2RR electrochemical cell at 12% and 99% (v/v). The resulting GC-TCD chromatogram of the CO2RR is shown in Fig. 10. With 12% (v/v) CO2, the CO2RR gave CO and H2 at a 0.15% and 5.99% Faradaic efficiency (FE), respectively. The relatively low FE values might come from the low CO2 concentration in the electrolyte solution. When the CO2 concentration was increased to 99% (v/v), the CO2RR produced CO, H2, CH4, and ethylene (C2H4) at a 25.12%, 26.84%, 0.78%, and 5.14% FE, respectively. These observations indicated that to obtain HC products from the electrochemical CO2RR, a high concentration of CO2 needs to be released from the adsorption process. It is important to know that the experiment in this study was conducted as a batch scale, which is a limitation for CO2 conversion. Adjusting the methodology to a continuous system and increasing the CO2 concentration will enhance the efficiency of CO2 conversion into valuable products using CuO and Cu2O catalysts.

Fig. 10
figure 10

Representative GC-TCD chromatogram after CO2RR of WT10T with a CO2 concentration of (a) 12% (v/v) and (b) 99% (v/v).

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Conclusion

In this study, different amine-modified WT samples were successfully prepared using TEPA and PEI, and then developed as an adsorbent for CO2 capture. The prepared adsorbents exhibited a non-porous material nature. Based on the FTIR and CHNS analyses, the specific amine spectrum was found on the WT surface and the nitrogen content was increased after modification. The optimum condition to obtain the highest CO2 adsorption capacity of the modified WT under a CO2 flow rate of 70 mL min−1, ambient temperature, and atmospheric pressure was modification of the WT using 10% (w/w) TEPA to give a CO2 adsorption capacity of 10.41 mg g−1 of adsorbent. The kinetic adsorption of WT10T fitted well with Avrami’s model, supporting both physical and chemical adsorptions. Finally, WT10T had a high stability in regeneration over at least 10 cycles. Accordingly, WT10T is an alternative CO₂ adsorbent that could potentially be used as a sorbent material for CO₂ capture. The CO2 present in the adsorbent material could undergo direct conversion to CO, H2, CH4, and C2H4 through CO2RR when CuO/Cu2O on carbon paper served as the catalyst. Moreover, elevated CO2 levels corresponded to a higher FE in HC product generation. This research can be related to the concept of BCG and CCUS and shows one way of how to potentially utilize waste materials as adsorbents and reduce the waste from rubber industries.