Decoding the N-doping mechanism for enhanced CO2 adsorption on biochar

Abstract. A primary environmental concern making global warming worse is the continuous release of CO2 into the atmosphere. One feasible approach to this issue is the geo-sequestration of CO2 involving CO2 adsorption and sequestration in saline aquifer formations. This study explores the possibility of improving CO2 adsorption in sandstone formations by using fly ash nanoparticles, a by-product. Pre- and post-nano-treated Berea sandstone samples are tested for CO2 adsorption at pressures ranging from 0.1 to 10 MPa, temperature of 40°C, and salinity of 10 wt% NaCl. High-pressure-temperature volumetric CO2 adsorption equipment was used to quantify CO2 adsorption under reservoir conditions. The findings reveal that Xanthan Gum (XG) polymer concentration of 0.04 wt% stabilized and well-suspended the fly ash nanofluid, exhibiting a zeta potential below -32.2 mV. The findings revealed that the total CO2 adsorption was 45.07 mmol/g for the nano-treated sample and 41.56 mmol/g for the untreated sample. Additionally, the fly ash nano-treated sample demonstrated superior CO2 adsorption across all pressure points. Specifically, when CO2 reached a supercritical state over 7.38 MPa, the nano-treated sample showed a maximum CO2 adsorption of 39.17 mmol/g, while untreated samples reached 36.40 mmol/g. Adsorption isotherm analysis revealed that the Toth model provided the best fit for the experimental data, with an R2 value of 0.9708, indicating heterogeneous and multilayer adsorption behavior. The isotherm analysis suggests that multilayer adsorption, driven by van der Waals forces, was predominant due to the energy favorability of forming multiple adsorption layers rather than a monolayer. The study outcome suggests the feasibility of fly ash nanoparticles as a promising nanofluid to improve CO2 adsorption in CO2 geo-sequestration projects.

ZnO/Cu/Al2O3 catalysts are used in industrial applications to produce methanol from CO2 gas. Because of the high barriers and also of the poor interaction between CO2 and the catalysts, current methods require extreme conditions, thereby significantly increasing production costs. In the search for more efficient catalysts based on the ZnO/Cu/Al2O3 system, this work investigates CO2 adsorption, activation, and hydrogenation on N-doped ZnO clusters supported on Cu(111). Our results show that N-doping significantly enhances CO2 adsorption compared to pure ZnO/Cu. Even in a hydrogen-rich environment, N-doped ZnO/Cu systems show a dramatically higher CO2 adsorption energy than the undoped ZnO/Cu configuration, sufficient to counterbalance the gas-phase entropic penalty in a wide range of conditions. In addition to enhancing CO2 adsorption, the overall reaction barrier to CO2 reduction is significantly reduced, enabling methanol production through a reverse water–gas shift (RWGS) pathway. These findings demonstrate that N-doping of ZnO/Cu catalysts can enhance CO2 adsorption and activation, thereby facilitating methanol synthesis under less demanding reaction conditions.

Enhancement of CO2 adsorption on natural zeolite, modified clinoptilolite with cations, amines and ionic liquids

Enhancement of CO2 adsorption on phenolic resin-based mesoporous carbons by KOH activation

To enhance the CO2 adsorption of almond shell-derived activated carbon (AC) samples treated with cold oxygen plasma, the samples were impregnated with cholinium-amino acid ionic liquids ([Cho][AA] ILs) using the vacuum-assisted impregnation method. The physicochemical and textural properties of the resulting composites (ILs@AC) were characterized using various techniques, including Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) spectroscopy, and Brunauer-Emmett-Teller (BET) surface area measurement. The CO2 adsorption performance of the samples was evaluated using a quartz crystal microbalance (QCM) over a temperature range of 288.15–308.15 K and gas pressures up to 1 bar. The IL@AC composite materials exhibited notably improved CO2 adsorption capacities compared to pristine AC. The CO2 adsorption isotherms onto the IL@AC composite samples closely conformed to the Langmuir isotherm model, indicating the dominant involvement of strong intermolecular interactions, particularly driven by amine functionalities. Meanwhile, the results revealed that [Cho][His]@AC showed lowered CO2 adsorption capacity compared to [Cho][Pro]@AC and [Cho][Gln]@AC. Among the studied ionic liquids, [Cho][Pro]@AC showed the highest absorption capacity (2.332 mmol·g−1 at 288 K and 1 bar). This was due to the obstruction of internal pores within the AC structure caused by excessive amine incorporation into its porous framework. In the meantime, for a deeper insight into the impregnation process of ILs onto the AC surfaces and their potential interactions with CO2 molecules, we conducted density-functional theory (DFT) calculations using the ωB97XD/6-31 + G(d,p) method. The calculated interaction energies, ranging from − 1.19 to − 1.44 eV, along with calculated quantum chemical descriptors, indicated a notable stabilization of IL species on the AC surfaces, with high affinity toward CO2 molecules.

Postsynthetic functionalization of magnesium 2,5-dihydroxyterephthalate (Mg-MOF-74) with tetraethylenepentamine (TEPA) resulted in improved CO2 adsorption performance under dry and humid conditions. XPS, elemental analysis, and neutron powder diffraction studies indicated that TEPA was incorporated throughout the MOF particle, although it coordinated preferentially with the unsaturated metal sites located in the immediate proximity to the surface. Neutron and X-ray powder diffraction analyses showed that the MOF structure was preserved after amine incorporation, with slight changes in the lattice parameters. The adsorption capacity of the functionalized amino-Mg-MOF-74 (TEPA-MOF) for CO2 was as high as 26.9 wt % versus 23.4 wt % for the original MOF due to the extra binding sites provided by the multiunit amines. The degree of functionalization with the amines was found to be important in enhancing CO2 adsorption, as the optimal surface coverage improved performance and stability under both pure CO2 and CO2/H2O coadsorption, and with partially saturated surface coverage, optimal CO2 capacity could be achieved under both wet and dry conditions by a synergistic binding of CO2 to the amines as well as metal centers.

Global warming is primarily driven by the rapid accumulation of carbon dioxide (CO₂) in the atmosphere. Metal–organic frameworks (MOFs) have emerged as promising materials for mitigating CO₂ emissions due to their tunable porosity, large surface area, and structural flexibility. Among them, MIL-53(Al) has attracted widespread interest owing to its excellent thermal and chemical stability. Recent studies show that modifying MOFs with secondary metals can significantly enhance their CO₂ adsorption performance. In this work, a one-pot synthesis method was employed to incorporate calcium (Ca), functioning as a Lewis basic metal, into the MIL-53(Al) framework to produce a series of bimetallic materials: MIL-53(Al, Ca)-1, -2, and − 3. These materials were further treated with NaOH to yield MIL-53(Al, Ca, Na)-1, -2, and − 3. Introducing Ca into the framework increased the affinity toward CO₂—a Lewis acidic gas—thereby enhancing adsorption. All materials were characterised and compared with pristine MIL-53(Al). CO₂ adsorption was evaluated at 273 K under both low pressure (up to 100 kPa) and high pressure (up to 980 kPa), while N₂ adsorption was measured at 273 K and 100 kPa. N₂ uptake decreased significantly with increasing Ca content, from 0.7177 mmol·g⁻¹ for MIL-53(Al) to 0.173 mmol·g⁻¹ for MIL-53(Al, Ca)-3 and MIL-53(Al, Ca, Na)-3. Moderate Ca incorporation improved CO₂ adsorption: at 100 kPa, MIL-53(Al, Ca)-1 and MIL-53(Al, Ca, Na)-1 achieved 4.09 and 3.56 mmol·g⁻¹, respectively, compared with 2.19 mmol·g⁻¹ for MIL-53(Al). However, excessive Ca loading (samples − 3) reduced CO₂ uptake due to structural distortion and reduced surface area. Despite this, the highly Ca-modified samples (-3) exhibited superior CO₂/N₂ selectivity. These findings demonstrate that controlled Ca incorporation can tune the adsorption behaviour and separation efficiency of MIL-53-type MOFs, offering a promising pathway for designing high-performance adsorbents for CO₂ capture and CO₂/N₂ separation. Graphical Abstract

Synthesis of hydrophobic CuCl/LaA modified by butyltrichlorosilane towards enhanced CO adsorption under humid environment

Pebax mixed matrix membrane with bimetallic CeZr-MOFs to enhance CO2 separation

We synthesized a series of azo-linked porphyrin-based porous organic polymers (APPs) with linear, bent, and trigonal linkers (APP-1 to APP-6) and with directly connected tetraphenylporphyrin units (APP-7a, APP-7b and APP-8). The synthesized APPs are amorphous solids demonstrating good thermal stability and diverse BET surface areas. APPs with linkers showed significantly higher surface areas (469 to 608 m2 g-1) compared to those with directly connected tetraphenylporphyrin units (0.3 to 23 m2 g-1). Higher surface areas correlated with enhanced CO2 adsorption, particularly for APP-1, APP-2, and APP-5 with experimental CO2 uptake values of 41 mg g-1, 38 mg g-1, and 38 mg g-1, respectively, at 306 K. The computational study supported the experimental findings and provided insights on how surface area and the local landscape affect the CO2 adsorption. Although the computational models were based on ideal structures, while the experiments revealed the materials were amorphous, the calculated CO2 adsorption capacities were roughly comparable to the experimental results, particularly for the 3D systems (APP-5 and APP-6) and the 2D systems with directly connected building units (APP-7 and APP-8). Porphyrin units in the framework serve as additional binding sites for CO2, especially when unhindered and available on either side of the porphyrin plane. This work highlights the potential of 2D layered APPs and 3D topologies for efficient CO2 capture.

Enhanced CO2 adsorption in nano-ZIF-8 modified by solvent assisted ligand exchange

Metal-organic frameworks (MOFs) are promising adsorbents for CO2 capture due to readily tunable porosity and diverse functionality; however, their performance is deteriorated by the presence of H2O in a flue gas. Fluorinated MOFs (FMOFs) may impede H2O interaction with frameworks and enhance CO2 adsorption under humid conditions. In this study, a multiscale computational screening study is reported to identify the top FMOFs for CO2 capture from a wet flue gas. Initially, geometric properties as well as heats of H2O adsorption are used to shortlist FMOFs with a suitable pore size and weak H2O affinity. Then, grand-canonical Monte Carlo simulations are conducted for adsorption of a CO2/N2/H2O mixture with 60% relative humidity in 5061 FMOFs. Based on the adsorption performance, 19 FMOFs are identified as top candidates. It is revealed that the position of F atom, rather than the amount, affects CO2 adsorption; moreover, N-decorated FMOFs are preferential for selective CO2 adsorption. Finally, the hydrostability of the top FMOFs is confirmed by first-principles molecular dynamics simulations. From a microscopic level, this study provides quantitative structure-performance relationships, discovers hydrostable FMOFs with high CO2 capture performance from a wet flue gas, and would facilitate the development of new MOFs toward efficient CO2 capture under humid conditions.

Accelerated prediction of Cu-based single-atom alloy catalysts for CO2 reduction by machine learning

Dynamic fluxionality and enhanced CO adsorption in the presence of coadsorbed H2O on free gold cluster cations

The incorporation of multiple metal ions in metal-organic frameworks (MOFs) through one-pot synthesis can induce unique properties originating from specific atomic-scale spatial apportionment, but the extraction of this crucial information poses challenges. Herein, nondestructive solid-state NMR spectroscopy was used to discern the atomic-scale metal apportionment in a series of bulk Mg1-xCox-MOF-74 samples via identification and quantification of eight distinct arrangements of Mg/Co ions labeled with a 13C-carboxylate, relative to Co content. Due to the structural characteristics of metal-oxygen chains, the number of metal permutations is infinite for Mg1-xCox-MOF-74, making the resolution of atomic-scale metal apportionment particularly challenging. The results were then employed in density functional theory calculations to unravel the molecular mechanism underlying the macroscopic adsorption properties of several industrially significant gases. It is found that the incorporation of weak adsorption sites (Mg2+ for CO and Co2+ for CO2 adsorption) into the MOF structure counterintuitively boosts the gas adsorption energy on strong sites (Co2+ for CO and Mg2+ for CO2 adsorption). Such effect is significant even for Co2+ remote from Mg2+ in the metal-oxygen chain, resulting in a greater enhancement of CO adsorption across a broad composition range, while the enhancement of CO2 adsorption is restricted to Mg2+ with adjacent Co2+. Dynamic breakthrough measurements unambiguously verified the trend in gas adsorption as a function of metal composition. This research thus illuminates the interplay between atomic-scale structures and macroscopic gas adsorption properties in mixed-metal MOFs and derived materials, paving the way for developing superior functional materials.