Synthesis of metal-organic framework derived porous carbon for CO₂ capture

The drive to capture CO₂ at point sources has intensified demand for porous adsorbents that combine high capacity with easy regeneration. This research converts ZIF-8 (zeolitic imidazolate framework-8) into nitrogen-doped porous carbons by direct carbonisation at 700, 800, 900, and 1000 °C under argon, and evaluates their CO₂ adsorption performance at 25 °C and pressures up to 1 bar. BET surface areas increased from 480 m²/g (700 °C) to a maximum of 1520 m²/g (1000 °C), while nitrogen content decreased from 11.3 to 3.8 wt% over the same temperature range. The sample carbonised at 900 °C (ZC-900) offered the best trade-off, with a BET area of 1340 m²/g, nitrogen content of 6.1 wt%, and a CO₂ uptake of 4.81 mmol/g at 25 °C and 1 bar. The CO₂/N₂ selectivity calculated by ideal adsorbed solution theory was 28.4 for ZC-900. Recyclability over five adsorption-desorption cycles showed less than 7% capacity loss. A preliminary cost comparison suggests that ZIF-8-derived carbons can be produced at roughly 18–24 USD/kg, positioning them competitively against commercial activated carbons for post-combustion CO₂ capture.

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Trade-off between Surface Area and Nitrogen ContentClose ×

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CO₂ (mmol/g) and CO₂/N₂: 700 vs 800 vs 900 vs 1000Close ×

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CO₂ (mmol/g) and CO₂/N₂: ZC-900 (this work) vs ZIF-67-C (900 °C) vs MIL-53-C (800 °C) vs Norit RB4 vs Zeolite 13XClose ×

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Introduction

The concentration of atmospheric CO₂ surpassed 424 ppm in 2024, and the Intergovernmental Panel on Climate Change estimates that limiting global warming to 1.5 °C will require annual capture of 5-10 gigatonnes of CO₂ by mid-century [1,2] . Post-combustion capture from power-plant flue gas is the nearest-term option, but the incumbent aminescrubbing technology carries a steep energy penalty for solvent regeneration-roughly 30% of the plant's output [3] . Solid porous adsorbents offer a lower-energy alternative. Activated carbons, zeolites, and metal-organic frameworks (MOFs) have all been tested, each with trade-offs between capacity, selectivity, moisture tolerance, and cost [4,5] . MOFs possess the highest surface areas on record but are often too expensive and moisture-sensitive for largescale flue-gas service [6] . A practical middle ground is to use MOFs as sacrificial templates: carbonising them under inert atmosphere pyrolyses the organic linkers into a carbon scaffold while preserving the porous architecture. If the linker contains nitrogen-as in ZIF-8's 2methylimidazolate-the resulting carbon inherits nitrogen dopants that strengthen CO₂ binding through Lewis acid-base interactions [7,8] . Several groups have reported ZIF-8derived carbons with BET areas exceeding 1000 m²/g, but systematic optimisation of carbonisation temperature alongside a cost assessment is uncommon [9,10] . This research fills that gap by carbonising ZIF-8 at four temperatures, characterising the products by N₂ physisorption, XRD, XPS, and TGA, and measuring CO₂ uptake, CO₂/N₂ selectivity, and cyclic stability. A simple cost model is included to judge industrial viability. Work was carried out at the Alexandria Mediterranean University, Alexandria, between May 2023 and January 2024 [11] .

Materials and Methods

Materials and Methods

Materials

Zinc nitrate hexahydrate (≥ 98%), 2-methylimidazole (99%), and methanol (HPLC grade) were from Sigma-Aldrich Egypt. ZIF-8 was synthesised by mixing zinc nitrate (2.93 g) and 2-methylimidazole (6.49 g) in methanol (200 mL), stirring at 25 °C for 24 h, collecting the white precipitate by centrifugation, washing with methanol (3×), and drying at 80 °C for 12 h [6,9] .

Methods

Dried ZIF-8 powder was placed in an alumina boat and carbonised in a tube furnace under flowing argon (100 mL/min) at 700, 800, 900, or 1000 °C for 3 h (heating rate 5 °C/min). The resulting black powders (ZC-700 to ZC-1000) were washed with 1 M HCl to remove residual zinc, rinsed with water, and dried at 100 °C. N₂ physisorption isotherms (77 K) were recorded on a Micromeritics ASAP 2020 to determine BET surface area and pore-size distribution. XRD (Bruker D8, Cu-Kα), XPS (Thermo K-Alpha), and TGA (TA Q500, air, 10 °C/min) provided structural, compositional, and thermal data. CO₂ and N₂ adsorption at 25 °C and 0-1 bar was measured volumetrically on the same ASAP 2020. CO₂/N₂ selectivity was estimated by ideal adsorbed solution theory at 0.15 bar CO₂ / 0.85 bar N₂ (simulated flue gas). Cyclic stability was tested over five adsorption-desorption cycles at 25/150 °C. All work was at Alexandria Mediterranean University, May 2023 to January 2024 [8,11] .

Performance Evaluation

The CO₂ uptake of ZC-900 (4.81 mmol/g at 25 °C, 1 bar) was benchmarked against literature values for other MOFderived carbons: ZIF-67-derived carbon (3.6 mmol/g), MIL-53-derived carbon (2.9 mmol/g), and commercial Norit activated carbon (3.2 mmol/g) [7,10] . ZC-900 outperformed all three comparators under identical conditions. Its CO₂/N₂ selectivity of 28.4 also exceeded those of undoped porous carbons (typically 10-18), confirming the benefit of nitrogen doping for preferential CO₂ binding [4,5] . The isosteric heat of CO₂ adsorption at low coverage was 32.7 kJ/mol-high enough for effective capture yet low enough to permit thermal regeneration at 150 °C without excessive energy input.

Cost Analysis

A preliminary cost estimate considered raw materials (zinc nitrate, 2-methylimidazole, methanol), energy (carbonisation at 900 °C for 3 h, HCl washing), and labour at Egyptian rates. At laboratory scale (100 g batches), the estimated production cost was 24 USD/kg. Scaling to a 10kg pilot batch and recovering methanol by distillation reduced the estimate to approximately 18 USD/kg, competitive with high-grade commercial activated carbons (15-30 USD/kg) but above commodity-grade carbons (5-8 USD/kg) [3,9] . The nitrogen-doping advantage-higher capacity and selectivity per kilogram-could offset the modest cost premium in applications where adsorbent volume or column size is a constraint. area increased monotonically from 480 to 1520 m²/g, while nitrogen content fell from 11.3 to 3.8 wt%. ZC-900 struck the optimum balance (1340 m²/g, 6.1 wt% N, 4.81 mmol/g CO₂) [7,8] . Figure 1 plots CO₂ uptake against BET surface area. The data follow a concave-upward trend, but ZC-1000, despite having the highest area, underperforms ZC-900 in uptakea consequence of its lower nitrogen content, which reduces the density of basic adsorption sites [5,10] . Table 2 benchmarks ZC-900 against literature MOF-derived and commercial carbons. ZC-900's capacity and selectivity rank at the top of reported values, while its isosteric heat stays within the easy-regeneration window [4,9] . .03 to 4.68 mmol/g-indicating good cyclic stability with less than 7% loss after five cycles [8,11] .

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CO₂ uptake (25 °C, 1 bar) versus BET surface area for ZIF-

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Adsorbent	CO₂ (mmol/g)	CO₂/N₂	Qst (kJ/mol)	Cost (USD/kg)
ZC-900 (this work)	4.81	28.4	32.7	18–24
ZIF-67-C (900 °C)	3.6	21.2	29.4	22–28
MIL-53-C (800 °C)	2.9	16.8	27.1	30–40
Norit RB4	3.2	12.4	24.3	15–20
Zeolite 13X	4.3	25.1	36.8	8–12

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Benchmarking ZC-900 against literature MOF-derived

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Results

Results

Discussion

The non-monotonic relationship between surface area and CO₂ uptake observed at 1000 °C highlights a well-known trade-off in MOF-derived carbon design: higher temperatures drive more complete graphitisation and pore development but simultaneously expel nitrogen dopants that serve as Lewis-basic CO₂ binding sites [7,8] . At 900 °C the material retains 6.1 wt% nitrogen-predominantly pyridinic and pyrrolic species, as confirmed by XPS-while developing a hierarchical pore structure with both micropores (< 2 nm, responsible for high CO₂ capacity) and mesopores (2-10 nm, ensuring fast diffusion kinetics) [5] .

The CO₂/N₂ selectivity of 28.4 compares favourably with zeolite 13X (about 25 under similar conditions) but falls short of functionalised amine-grafted silicas (40-60), which pay for their higher selectivity with slower kinetics and larger regeneration energy [3,4] . For post-combustion capture from coal-fired flue gas at roughly 15% CO₂, a selectivity above 20 is generally considered workable, placing ZC-900 within the practical range [10] . The cost estimate of 18-24 USD/kg positions ZIF-8-derived carbons as a mid-range adsorbent: more expensive than commodity activated carbons but cheaper than most MOFs and amine-grafted materials. The higher per-kilogram capacity (4.81 vs 3.2 mmol/g for Norit) means that fewer kilograms are needed per tonne of CO₂ captured, partially offsetting the price premium [9] . A limitation is the absence of moisture in the adsorption tests; flue gas typically contains 5-10% water vapour, which can compete for adsorption sites and reduce CO₂ capacity by 10-30% [6] .

Conclusion

ZIF-8 was carbonised at four temperatures to produce nitrogen-doped porous carbons for CO₂ capture. The sample prepared at 900 °C (ZC-900) delivered the best combination of BET surface area (1340 m²/g), nitrogen content (6.1 wt%), CO₂ uptake (4.81 mmol/g at 25 °C, 1 bar), and CO₂/N₂ selectivity (28.4). Cyclic stability over five adsorption-desorption cycles was satisfactory, with less than 7% capacity loss. Preliminary cost analysis estimated a production cost of 18-24 USD/kg, competitive with highgrade commercial activated carbons. Future work should test ZC-900 under humid conditions simulating real flue gas and evaluate breakthrough performance in a packed-bed column. Exploring other MOF templates with higher nitrogen content-such as ZIF-67 or ZIF-62-may further improve CO₂ selectivity without sacrificing surface area.

Acknowledgement

Acknowledgements Funding SourcesThis research was supported by the Science, Technology and Innovation Funding Authority of Egypt. Institutional SupportThe Department of Materials Chemistry at Alexandria Mediterranean University provided all instrumentation including the ASAP 2020 and tube furnace. Contributions Not Qualifying for AuthorshipMs. Mariam El-Sayed assisted with XPS measurements.

Figures and tables from the paper

Sample	Temp (°C)	BET (m²/g)	N (wt%)	CO₂ (mmol/g)	CO₂/N₂	Vmicro (cm³/g)
ZC-700	700	480	11.3	1.82	14.6	0.18
ZC-800	800	920	8.4	3.56	22.1	0.34
ZC-900	900	1340	6.1	4.81	28.4	0.51
ZC- 1000	1000	1520	3.8	4.48	19.3	0.58

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Textural properties, nitrogen content, CO₂ uptake, and

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CO₂/N₂ selectivity and CO₂ capacity of ZC-900 over five

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