Critical review of existing nanomaterial adsorbents to capture carbon dioxide and methane

Separation of CO2/CH4 gas mixtures using nanoporous graphdiyne and boron-graphdiyne membranes: influence of the pore size

Nanoporous carbon-based membranes have garnered significant interest in gas separation processes owing to their distinct structure and properties. We have investigated the permeation and separation of the mixture of CO2 and CH4 gases through membranes formed by thin layers of porous graphdiyne (GDY) and boron graphdiyne (BGDY) using Density Functional Theory. The main goal is to investigate the effect of the pore size. The interaction of CO2 and CH4 with GDY and BGDY is weak, and this guarantees that those molecules will not be chemically trapped on the surface of the porous membranes. The permeation and separation of CO2 and CH4 through the membranes are significantly influenced by the size of the pores in the layers. The size of the hexagonal pores in BGDY is large in comparison to the size of the two molecules, and the passing of these molecules through the pores is easy because there is no barrier. Then, BGDY is not able to separate CO2 and CH4. In sharp contrast, the size of the triangular pores in GDY is smaller, comparable to the diameter of the two molecules, and this raises an activation barrier for the crossing of the molecules. The height of the barrier for CO2 is one half of that for CH4, the reason being that CO2 is a linear molecule which adopts an orientation perpendicular to the GDY layer to cross the pores, while CH4 has a spherical-like shape, and cannot profit from a favorable orientation. The calculated permeances favor the passing of CO2 through the GDY membrane, and the calculated selectivity for CO2/CH4 mixtures is large. This makes GDY a very promising membrane material for the purification of commercial gases and for the capture of the CO2 component in those gases.

In silico capture and activation of methane with light atom molecules

Methane (CH₄) can be captured in silico with a light atom molecule containing only C, H, Si, O, and B atoms, respectively. A tripodal peri-substituted ligand system was employed, namely, [(5-Ph₂B-xan-4-)₃Si]H (1, xan = xanthene), which after hydride abstraction (1+) carries four Lewis acidic sites within the cationic cage structure. In a previous study, this system was shown to be able to capture noble gas atoms He-Kr (Mebs & Beckmann 2022). In the corresponding methane complex, 1+CH4, a polarized Si⁺⋯CH₄ contact of 2.289 Å as well as series of (H₃)CH⋯O/C_Ph hydrogen bonds enforce spatial CH₄ fixation (the molecule obeys C₃-symmetry) and slight activation. A trigonal-pyramidal Si-CH^eq₃-H^ax local geometry is thereby approached with H^ax-C-H^eq angles decreased to 103.7°. All attempts to replace the Lews acidic -BPh₂ fragments in 1 with basic -PR₂ (R = Ph, tBu) fragments indeed increased intra-molecular hydrogen bonding between host molecule and CH₄, and thus caused stronger activation of the latter, however ultimately resulted in the formation of energetically favorable quenched structures with short P-Si contacts, making CH₄ binding hard to achieve. The electronic situation of two hypothetic methane complexes, 1+CH4 and [(5-tBu₂P-xan-4-)₃SiCH₄]⁺ (2+CH4), was determined by a set of calculated real-space bonding indicators (RSBIs) including the Atoms-In-Molecules (AIM), non-covalent interactions index (NCI), and electron localizability indicator (ELI-D) methods, highlighting crucial differences in the level of activation. The proposed ligand systems serve as blueprints for a more general structural design with adjustable trigonal ligand systems in which central atom, spacer fragment, and functional peri-partner can be varied to facilitate different chemical tasks.

Use of Multiscale Data-Driven Surrogate Models for Flowsheet Simulation of an Industrial Zeolite Production Process

The production of catalysts such as zeolites is a complex multiscale and multi-step process. Various material properties, such as particle size or moisture content, as well as operating parameters—e.g., temperature or amount and composition of input material flows—significantly affect the outcome of each process step, and hence determine the properties of the final product. Therefore, the design and optimization of such processes is a complex task, which can be greatly facilitated with the help of numerical simulations. This contribution presents a modeling framework for the dynamic flowsheet simulation of a zeolite production sequence consisting of four stages: precipitation in a batch reactor; concentration and washing in a block of centrifuges; formation of droplets and drying in a spray dryer; and burning organic residues in a chain of rotary kilns. Various techniques and methods were used to develop the applied models. For the synthesis in the reactor, a multistage strategy was used, comprising discrete element method simulations, data-driven surrogate modeling, and population balance modeling. The concentration and washing stage consisted of several multicompartment decanter centrifuges alternating with water mixers. The drying is described by a co–current spray dryer model developed by applying a two-dimensional population balance approach. For the rotary kilns, a multi-compartment model was used, which describes the gas–solid reaction in the counter–current solids and gas flows.

Biogas improvement as renewable energy through conversion into methanol: A perspective of new catalysts based on nanomaterials and metal organic frameworks

In recent years, the high cost and availability of energy sources have boosted the implementation of strategies to obtain different types of renewable energy. Among them, methane contained in biogas from anaerobic digestion has gained special relevance, since it also permits the management of a big amount of organic waste and the capture and long-term storage of carbon. However, methane from biogas presents some problems as energy source: 1) it is a gas, so its storage is costly and complex, 2) it is not pure, being carbon dioxide the main by-product of anaerobic digestion (30%–50%), 3) it is explosive with oxygen under some conditions and 4) it has a high global warming potential (27–30 times that of carbon dioxide). Consequently, the conversion of biogas to methanol is as an attractive way to overcome these problems. This process implies the conversion of both methane and carbon dioxide into methanol in one oxidation and one reduction reaction, respectively. In this dual system, the use of effective and selective catalysts for both reactions is a critical issue. In this regard, nanomaterials embedded in metal organic frameworks have been recently tested for both reactions, with very satisfactory results when compared to traditional materials. In this review paper, the recent configurations of catalysts including nanoparticles as active catalysts and metal organic frameworks as support materials are reviewed and discussed. The main challenges for the future development of this technology are also highlighted, that is, its cost in environmental and economic terms for its development at commercial scale.

Development of a High-Accuracy Statistical Model to Identify the Key Parameter for Methane Adsorption in Metal-Organic Frameworks

The geometrical and topological features of metal-organic frameworks (MOFs) play an important role in determining their ability to capture and store methane (CH4). Methane is a greenhouse gas that has been shown to be more dangerous in terms of contributing to global warming than carbon dioxide (CO2), especially in the first 20 years of its release into the atmosphere. Its accelerated emission increases the rate of global temperature increase and needs to be addressed immediately. Adsorption processes have been shown to be effective and efficient in mitigating methane emissions from the atmosphere by providing an enormous surface area for methane storage. Among all the adsorbents, MOFs were shown to be the best adsorbents for methane adsorption due to their higher favorable steric interactions, the presence of binding sites such as open metal sites, and hydrophobic pockets. These features may not necessarily be present in carbonaceous materials and zeolites. Although many studies have suggested that the main reason for the increased storage efficiencies in terms of methane in the MOFs is the high surface area, there was some evidence in certain research works that methane storage performance, as measured by uptakes and deliveries in gravimetric and volumetric units, was higher for certain MOFs with a lower surface area. This prompted us to find out the most significant property of the MOF, whether it be material-based or pore-based, that has the maximum influence on methane uptake and delivery, using a comprehensive statistical approach that has not previously been employed in the methane storage literature. The approach in our study employed various chemometric techniques, including simple and multiple linear regression (SLR and MLR), combined with different types of multicollinearity diagnostics, partial correlations, standardized coefficients, and changes in regression coefficient estimates and their standard errors, applied to both the SLR and MLR models. The main advantages of this statistical approach are that it is quicker, provides a deeper insight into experimental data, and highlights a single, most important, parameter for MOF design and tuning that can predict and maximize the output storage and capture performance. The significance of our approach is that it was modeled purely based on experimental data, which will capture the real system, as opposed to the molecular simulations employed previously in the literature. Our model included data from ~80 MOFs and eight properties related to the material, pore, and thermodynamics (isosteric adsorption energy). Successful attempts to model the methane sorption process have previously been conducted using thermodynamic approaches and by developing adsorption performance indicators, but these are either too complex or time-consuming and their data covers fewer than 10 MOFs and a maximum of three MOF properties. By comparing the statistical metrics between the models, the most important and statistically significant property of the MOF was determined, which will be crucial when designing MOFs for use in storing and delivering methane.

Biogas and Biomethane Production and Usage: Technology Development, Advantages and Challenges in Europe

In line with the low-carbon strategy, the EU is expected to be climate-neutral by 2050, which would require a significant increase in renewable energy production. Produced biogas is directly used to produce electricity and heat, or it can be upgraded to reach the “renewable natural gas”, i.e., biomethane. This paper reviews the applied production technology and current state of biogas and biomethane production in Europe. Germany, UK, Italy and France are the leaders in biogas production in Europe. Biogas from AD processes is most represented in total biogas production (84%). Germany is deserving for the majority (52%) of AD biogas in the EU, while landfill gas production is well represented in the UK (43%). Biogas from sewage sludge is poorly presented by less than 5% in total biogas quantities produced in the EU. Biomethane facilities will reach a production of 32 TWh in 2020 in Europe. There are currently 18 countries producing biomethane (Germany and France with highest share). Most of the European plants use agricultural substrate (28%), while the second position refers to energy crop feedstock (25%). Sewage sludge facilities participate with 14% in the EU, mostly applied in Sweden. Membrane separation is the most used upgrading technology, applied at around 35% of biomethane plants. High energy prices today, and even higher in the future, give space for the wider acceptance of biomethane use.

RSM Modeling and Optimization of CO2 Separation from High CO2 Feed Concentration over Functionalized Membrane

The challenges in developing high CO₂ gas fields are governed by several factors such as reservoir condition, feed gas composition, operational pressure and temperature, and selection of appropriate technologies for bulk CO₂ separation. Thus, in this work, we report an optimization study on the separation of CO₂ from CH₄ at high CO₂ feed concentration over a functionalized mixed matrix membrane using a statistical tool, response surface methodology (RSM) statistical coupled with central composite design (CCD). The functionalized mixed matrix membrane containing NH₂-MIL-125 (Ti) and 6FDA-durene, fabricated in our previous study, was used to perform the separation performance under three operational parameters, namely, feed pressure, temperature, and CO₂ feed concentration, ranging from 3.5–12.5 bar, 30.0–50.0 °C and 15–70 mol%, respectively. The CO₂ permeability and CO₂/CH₄ separation factor obtained from the experimental work were varied from 293.2–794.4 Barrer and 5.3–13.0, respectively. In addition, the optimum operational parameters were found at a feed pressure of 12.5 bar, a temperature of 34.7 °C, and a CO₂ feed concentration of 70 mol%, which yielded the highest CO₂ permeability of 609.3 Barrer and a CO₂/CH₄ separation factor of 11.6. The average errors between the experimental data and data predicted by the model for CO₂ permeability and CO₂/CH₄ separation factor were 5.1% and 3.3%, respectively, confirming the validity of the proposed model. Overall, the findings of this work provide insights into the future utilization of NH₂-MIL-125 (Ti)/6FDA-based mixed matrix membranes in real natural gas purification applications.

Conversion of Carbon Dioxide into Methanol Using Cu–Zn Nanostructured Materials as Catalysts

Nowadays, there is a growing awareness of the great environmental impact caused by the enormous amounts of carbon dioxide emitted. Several alternatives exist to solve this problem, and one of them is the hydrogenation of carbon dioxide into methanol by using nanomaterials as catalysts. The aim of this alternative is to produce a value-added chemical, such as methanol, which is a cheaply available feedstock. The development of improved materials for this conversion reaction and a deeper study of the existing ones are important for obtaining higher efficiencies in terms of yield, conversion, and methanol selectivity, in addition to allowing milder reaction conditions in terms of pressure and temperature. In this work, the performance of copper, zinc, and zinc oxide nanoparticles in supported and unsupported bimetallic systems is evaluated in order to establish a comparison among the different materials according to their efficiency. For that, a packed bed reactor operating with a continuous gas flow is used. The obtained results indicate that the use of bimetallic systems combined with porous supports, such as zeolite and activated carbon, is beneficial, thus improving the performance of unsupported materials by four times.

Photocatalytic degradation of cefixime in aqueous solutions using functionalized SWCNT/ZnO/Fe3O4 under UV-A irradiation

In this study, SWCNT/ZnO/Fe₃O₄ heterojunction composite was prepared for enhancing the degradation of β-lactam drugs such as cefixime (CFX) from an aqueous solution. The effects of several factors such as pH, initial concentration of CFX, and photocatalyst dose were investigated. Among them, pH was the most effective parameter for the degradation of CFX. Pareto graph revealed that the degradation process was accelerated at acidic conditions. The surface morphology test such as scanning electron microscopy (SEM) was applied to enlighten the surface of the functionalized SWCNT/ZnO/Fe₃O₄ photocatalyst. Highly advanced analyzes such as X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive Spectrometry (EDX), Fourier-Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and point of zero charge were included to explain the structure of the photocatalyst. The response surface methodology's results show that the optimum CFX efficiency was fully achieved at 94.19%. The optimal conditions with lower standard error (2.08) were given as pH of 5.93, 22.76 ppm of CFX, and 0.46 g L^-1 of the amount of photocatalyst. Besides, the obtained photocatalyst can be easily used many times owing to its high reusability. SWCNT/ZnO/Fe₃O₄ photocatalyst might be recommended to be used for the mineralizing of drug compounds such as antibiotics in water. Moreover, thiazol-2-ol, N-(dihydroxymethyl)-2-(2-hydroxythiazol-4-yl)acetamide,(S)-N-(2-amino-1-hydroxy-2-oxoethyl)-2-(2 hydroxythiazol-4-yl), and 2-(2-hydroxythiazol-4-yl)-N-((2R,3R)-2-mercapto-4-oxoazetidin-3-yl)acetamide were among the detected intermediates products from the cefixime degradation in the process.

Research Progress and Reaction Mechanism of CO2 Methanation over Ni-Based Catalysts at Low Temperature: A Review

The combustion of fossil fuels has led to a large amount of carbon dioxide emissions and increased greenhouse effect. Methanation of carbon dioxide can not only mitigate the greenhouse effect, but also utilize the hydrogen generated by renewable electricity such as wind, solar, tidal energy, and others, which could ameliorate the energy crisis to some extent. Highly efficient catalysts and processes are important to make CO2 methanation practical. Although noble metal catalysts exhibit higher catalytic activity and CH4 selectivity at low temperature, their large-scale industrial applications are limited by the high costs. Ni-based catalysts have attracted extensive attention due to their high activity, low cost, and abundance. At the same time, it is of great importance to study the mechanism of CO2 methanation on Ni-based catalysts in designing high-activity and stability catalysts. Herein, the present review focused on the recent progress of CO2 methanation and the key parameters of catalysts including the essential nature of nickel active sites, supports, promoters, and preparation methods, and elucidated the reaction mechanism on Ni-based catalysts. The design and preparation of catalysts with high activity and stability at low temperature as well as the investigation of the reaction mechanism are important areas that deserve further study.

The Synthesis of Magnetic Nitrogen-Doped Graphene Oxide Nanocomposite for the Removal of Reactive Orange 12 Dye

Herein, we report the nanofabrication of magnetic calcium ferrite (CaFe2O4) with nitrogen-doped graphene oxide (N-GO) via facile ultrasonication method to produce CaFe2O4/N-GO nanocomposite for the potential removal of reactive orange 12 (RO12) dye from aqueous solution. The successful construction of the nanocomposite was confirmed using different characterization techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform-infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The magnetic properties were studied using vibrating sample magnetometer (VSM) indicating ferromagnetic behavior of the synthesized materials that facilitate their separation using an external magnetic field after adsorption treatment. The addition of N-GO to CaFe2O4 nanoparticles enhanced the BET surface area from 24 to 52.93 m2/g as resulted from the N2 adsorption-desorption isotherm. The adsorption of the synthesized nanomaterials is controlled by several parameters (initial concentration of dye, contact time, adsorbent dosage, and pH), and the RO12 dye removal on the surface of CaFe2O4 nanoparticles and CaFe2O4/N-GO nanocomposite was reached through the chemisorption process as indicated from the kinetic study. The adsorption isotherm study indicated that the adsorption process of RO12 dye was best described through the Langmuir isotherm approving the monolayer adsorption. According to the Langmuir model, the maximum adsorption capacity for RO12 was 250 and 333.33 mg/g for CaFe2O4 nanoparticles and CaFe2O4/N-GO nanocomposite, respectively. The adsorption capacity offered by CaFe2O4/N-GO nanocomposite was higher than reported in the literature for adsorbent materials. Additionally, the regeneration study indicated that CaFe2O4/N-GO nanocomposite is reusable and cost-effective adsorbent. Therefore, the nanofabricated CaFe2O4/N-GO hybrid material is a promising adsorbent for water treatment.

Metal-Organic Frameworks (MOFs) as methane adsorbents: From storage to diluted coal mining streams concentration

Ventilation Air Methane emissions (VAM) from coal mines lead to environmental concern because their high global warming potential and the loss of methane resources. VAM upgrading requires pre-concentration processes dealing with high flow rates of very diluted streams (<1% methane). Therefore, methane separation and concentration is technically challenging and has important environmental and safety concerns. Among the alternatives, adsorption on Metal-Organic Frameworks (MOFs) could be an interesting option to methane selective separation, due to its tuneable character and outstanding physical properties. Most of the works devoted to the methane adsorption on MOFs deal with methane storage. Therefore, these works were reviewed to determine the properties governing methane-MOF interactions. In addition, the metallic ions and organic linkers roles have been identified. With these premises, decisive effects in the methane adsorption selectivity in nitrogen/methane lean mixtures have been discussed, since nitrogen is the most concentrated gas in the VAM stream, and it is very similar to methane molecule. In order to fulfill this overview, the effect of other aspects, such as the presence of polar compounds (moisture and carbon dioxide), was also considered. In addition, engineering considerations in the operation of fixed bed adsorption units and the main challenges associated to MOFs as adsorbents were also discussed.

Combined GCMC, MD, and DFT Approach for Unlocking the Performances of COFs for Methane Purification

Covalent organic frameworks (COFs) are promising materials for gas storage and separation; however, the potential of COFs for separation of CH₄ from industrially relevant gases such as H₂, N₂, and C₂H₆ is yet to be investigated. In this work, we followed a multiscale computational approach to unlock both the adsorption- and membrane-based CH₄/H₂, CH₄/N₂, and C₂H₆/CH₄ separation potentials of 572 COFs by combining grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Adsorbent performance evaluation metrics of COFs, adsorption selectivity, working capacity, regenerability, and adsorbent performance score were calculated for separation of equimolar CH₄/H₂, CH₄/N₂, and C₂H₆/CH₄ mixtures at vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions to identify the best-performing COFs for each mixture. Results showed that COFs could achieve selectivities of 2-85, 1-7, and 2-23 for PSA-based CH₄/H₂, CH₄/N₂, and C₂H₆/CH₄ separations, respectively, outperforming conventional adsorbents such as zeolites and activated carbons for each mixture. Structure-performance relations revealed that COFs with pore sizes <10 Å are promising adsorbents for all mixtures. We identified the gas adsorption sites in the three top-performing COFs commonly identified for each mixture by DFT calculations and computed the binding strength of gases, which were found to be on the order of C₂H₆ > CH₄ > N₂ > H₂, supporting the GCMC results. Nucleus-independent chemical shift (NICS) indexes of aromaticity for adsorption sites were calculated, and the results revealed that the degree of linker aromaticity could be a measure for the selection or design of highly alkane-selective COF adsorbents over N₂ and H₂. Finally, COF membranes were shown to achieve high H₂ permeabilities, 4.57 × 10³ -1.25 × 10⁶ Barrer, and decent membrane selectivities, as high as 4.3, outperforming polymeric and MOF-based membranes for separation of H₂ from CH₄.

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO₂ emissions. Solid materials capable of reversibly absorbing CO₂ have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO₂ sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO₂ absorption both at surfaces and within solid materials.

Zr-MOFs for CF4/CH4, CH4/H2, and CH4/N2 separation: towards the goal of discovering stable and effective adsorbents

Zirconium metal-organic frameworks (MOFs) can be promising adsorbents for various applications as they are highly stable in different chemical environments. In this work, a collection of Zr-MOFs comprised of more than 100 materials is screened for CF₄/CH₄, CH₄/H₂, and CH₄/N₂ separations using atomistic-level simulations. The top three MOFs for the CF₄/CH₄ separation are identified as PCN-700-BPDC-TPDC, LIFM-90, and BUT-67 exhibiting CF₄/CH₄ adsorption selectivities of 4.8, 4.6, and 4.7, CF₄ working capacities of 2.0, 2.0, and 2.1 mol kg^-1, and regenerabilities of 85.1, 84.2, and 75.7%, respectively. For the CH₄/H₂ separation, MOF-812, BUT-67, and BUT-66 are determined to be the top performing MOFs demonstrating CH₄/H₂ selectivities of 61.6, 36.7, and 46.2, CH₄ working capacities of 3.0, 4.1, and 3.4 mol kg^-1, and CH₄ regenerabilities of 70.7, 82.7, and 74.7%, respectively. Regarding the CH₄/N₂ separation, BUT-67, Zr-AbBA, and PCN-702 achieving CH₄/N₂ selectivities of 4.5, 3.4, and 3.8, CH₄ working capacities of 3.6, 3.9, and 3.5 mol kg^-1, and CH₄ regenerabilities of 81.1, 84.0, and 84.5%, in successive order, show the best overall separation performances. To further elucidate the adsorption in top performing adsorbents, the adsorption sites in these materials are analyzed using radial distribution functions and adsorbate density profiles. Finally, the water affinities of Zr-MOFs are explored to comment on their practical use in real gas separation applications. Our findings may inspire future studies probing the adsorption/separation mechanisms and performances of Zr-MOFs for different gases.

CO2 Capture by Hydroxylated Azine-Based Covalent Organic Frameworks

Covalent organic frameworks (COFs) RIO-13, RIO-12, RIO-11, and RIO-11m were investigated towards their CO₂ capture properties by thermogravimetric analysis at 1 atm and 40 °C. These microporous COFs bear in common the azine backbone composed of hydroxy-benzene moieties but differ in the relative number of hydroxyl groups present in each material. Thus, their sorption capacities were studied as a function of their textural and chemical properties. Their maximum CO₂ uptake values showed a strong correlation with an increasing specific surface area, but that property alone could not fully explain the CO₂ uptake data. Hence, the specific CO₂ uptake, combined with DFT calculations, indicated that the relative number of hydroxyl groups in the COF backbone acts as an adsorption threshold, as the hydroxyl groups were indeed identified as relevant adsorption sites in all the studied COFs. Additionally, the best performing COF was thoroughly investigated, experimentally and theoretically, for its CO₂ capture properties in a variety of CO₂ concentrations and temperatures, and showed excellent isothermal recyclability up to 3 cycles.

Efficient CH4/CO2 Gas Mixture Separation through Nanoporous Graphene Membrane Designs

Nanoporous graphene membranes have drawn special attention in the gas-separation processes due to their unique structure and properties. The complexity of the physical understanding of such membrane designs restricts their widespread use for gas-separation applications. In the present study, we strive to propose promising designs to face this technical challenge. In this regard, we investigated the permeation and separation of the mixture of adsorptive gases CO2 and CH4 through a two-stage bilayer sub-nanometer porous graphene membrane design using molecular dynamics simulation. A CH4/CO2 gashouse mixture with 80 mol% CH4 composition was generated using the benchmarked force-fields and was forced to cross through the porous graphene membrane design by a constant piston velocity. Three chambers are considered to be feeding, transferring, and capturing to examine the permeation and separation of molecules under the effect of the two-stage membrane. The main objective is to investigate the multistage membrane and bilayer effect simultaneously. The permeation and separation of the CO2 and CH4 molecules while crossing through the membrane are significantly influenced by the pore offset distance (W) and the interlayer spacing (H) of the bilayer nanoporous graphene membrane. Linear configurations (W = 0 Å) and those with the offset distance of 10 Å and 20 Å were examined by varying the interlayer spacing between 8 Å, 12 Å, and 16 Å. The inline configuration with an interlayer spacing of 12 Å is the most effective design among the examined configurations in terms of optimum separation performance and high CO2 and CH4 permeability. Furthermore, increasing the interlayer distance to 16 Å results in bulk-like behavior rather than membrane-like behavior, indicating the optimum parameters for high selectivity and permeation. Our findings present an appropriate design for the effective separation of CH4/CO2 gas mixtures by testing novel nanoporous graphene configurations.

Great Location: About Effects of Surface Bound Neighboring Groups for Passive and Active Fine-Tuning of CO2 Adsorption Properties in Model Carbon Capture Materials

Improved carbon capture materials are crucial for managing the CO₂ level in the atmosphere. The past focus was on increasing adsorption capacities. It is widely known that controlling the heat of adsorption (ΔH_ads ) is equally important. If it is too low, CO₂ uptake takes place at unfavorable conditions and with insufficient selectivity. If it is too high, chemisorption occurs, and the materials can hardly be regenerated. The conventional approach for influencing ΔH_ads is the modification of the adsorbing center. This paper proposes an alternative strategy. The hypothesis is that fine-tuning of the molecular environment around the adsorbing center is a powerful tool for the adjustment of CO₂ -binding properties. Via click chemistry, any desired neighboring group (NG) can be incorporated on the surfaces of the nanoporous organosilica model materials. Passive NGs induce a change in the polarity of the surface, whereas active NGs are capable of direct interaction with the active center/CO₂ pair. The effects on ΔH_ads and on the selectivity are studied. A situation can be realized which resembles frustrated Lewis acid-base pairs, and the investigation of the binding-species by solid-state NMR indicates that the push-pull effects could play an essential role not only in CO₂ adsorption but also in its activation.

CO2 adsorption mechanisms on MOFs: a case study of open metal sites, ultra-microporosity and flexible framework

In this review the CO₂ adsorption mechanisms of MOF-74-Mg, HKUST-1, SIFSIX-3-M, and ZIF-8 are explored, highlighting their preferential adsorption sites, CO₂–MOF complex configuration, adsorption dynamics, bonding angle, and water stability.

Dynamics of phase transitions in Na2TiO3 and its possible utilization as a CO2 sorbent: a critical analysis

Thermally-driven structural changes of Na2TiO3 and their effect on its carbonation behaviour.

Adsorption of methane on single metal atoms supported on graphene: Role of electron back-donation in binding and activation

We consider single metal atoms supported on graphene as possible candidate systems for on-board vehicular storage of methane or for methane activation. We use density functional theory to study the adsorption of one and two molecules of methane on such graphene-supported single atoms, where the metal atom M is a 3d-transition metal (Sc to Zn). Our results suggest that M = Sc, Ti, and V are the best candidates for gas storage applications, while Ni and Co seem particularly promising with respect to activation of the C-H bond in methane. We find a strong and linear correlation between the adsorption energy of methane and the degree of back-donation of electrons from occupied metal d-states to antibonding methane states. A similar correlation is found between the elongation of C-H bonds and electron back-donation. An important role is played by the graphene substrate in enhancing the binding of methane on metal atoms, compared to the negligible binding observed on isolated metal atoms.

Biomass-Derived Carbon Molecular Sieves Applied to an Enhanced Carbon Capture and Storage Process (e-CCS) for Flue Gas Streams in Shallow Reservoirs

It is possible to take advantage of shallow reservoirs (<300 m) for CO₂ capture and storage in the post-combustion process. This process is called enhanced carbon capture and storage (e-CCS). In this process, it is necessary to use a nano-modifying agent to improve the chemical-physical properties of geological media, which allows the performance of CO₂ selective adsorption to be enhanced. Therefore, this study presents the development and evaluation of carbon sphere molecular nano-sieves (CSMNS) from cane molasses for e-CSS. This is the first report in the scientific literature on CSMNS, due to their size and structure. In this study, sandstone was used as geological media, and was functionalized using a nanofluid, which was composed of CNMNS dispersed in deionized water. Finally, CO₂ or N₂ streams were used for evaluating the adsorption process at different conditions of pressure and temperature. As the main result, the nanomaterial allowed a natural selectivity towards CO₂, and the sandstone enhanced the adsorption capacity by an incremental factor of 730 at reservoir conditions (50 °C and 2.5 MPa) using a nanoparticle mass fraction of 20%. These nanofluids applied to a new concept of carbon capture and storage for shallow reservoirs present a novel landscape for the control of industrial CO₂ emissions.

Shaping the Future of Fuel: Monolithic Metal-Organic Frameworks for High-Density Gas Storage

The environmental benefits of cleaner, gaseous fuels such as natural gas and hydrogen are widely reported. Yet, practical usage of these fuels is inhibited by current gas storage technology. Here, we discuss the wide-ranging potential of gas-fuels to revolutionize the energy sector and introduce the limitations of current storage technology that prevent this transition from taking place. The practical capabilities of adsorptive gas storage using porous, crystalline metal-organic frameworks (MOFs) are examined with regard to recent benchmark results and ultimate storage targets in this field. In particular, the industrial limitations of typically powdered MOFs are discussed while recent breakthroughs in MOF processing are highlighted. We offer our perspective on the future of practical, rather than purely academic, MOF developments in the increasingly critical field of environmental fuel storage.

Influence of water-film-forming-unit on the enhanced removal of carbon dioxide from mixed gas using water absorption apparatus

This research uses tap water to absorb carbon dioxide from mixed gas (N₂ and CO₂) in an absorption apparatus coupled with a water-film-forming-unit (WFFU). The objective is to assess the benefits of using a WFFU to enhance CO₂ removal efficiency at low pressure conditions. Based on our results, the WFFU significantly improves CO₂ capture at 0.30 MPa in a water absorption system with two WFFUs. The CO₂ removal efficiency was 20% greater than for systems without WFFUs. Moreover, statistical data attained by the Taguchi analysis method showed that the number of WFFUs used in the absorption system has the greatest influence on CO₂ removal efficiency (contribution percentage = 50.65%) compared to gas pressure, initial CO₂ concentration, gas-to-liquid ratio, and liquid temperature. We also thoroughly investigated the effects of these factors on CO₂ removal performance. The optimum conditions for CO₂ removal efficiency in a system equipped with two WFFUs are low temperature, low gas-to-liquid ratio, low gas pressure (0.25-0.30 MPa), and high inlet CO₂ concentration. These findings could provide an effective method for capturing CO₂ from exhaust gases, and thus help mitigate global warming.

Potential applications of graphene-based nanomaterials as adsorbent for removal of volatile organic compounds

In recent years, graphene-based materials (GBMs) have been regarded as the core technology in diverse research fields. Consequently, the demand for large-scale synthesis of GBMs has been increasing continuously for various fields of industry. These materials have become a competitive adsorbent for the removal of environmental pollutants with improved adsorption capacity and cost effectiveness through hybridization or fabrication of various functionalities on their large surface. In particular, their applicability opens up new avenues for the adsorptive removal of volatile organic compounds (VOCs) (e.g., through the build-up of efficient air purification systems). This review explored the basic knowledge and synthesis approaches for GBMs and their performances as adsorbent for VOC removal. Moreover, the mechanisms associated with the VOC removal were explained in detail. The performance of GBMs has also been evaluated along with their present limitations and future perspectives.

An overview on trace CO2 removal by advanced physisorbent materials

This review paper focuses on various gas processing technologies and materials that efficiently capture trace levels of carbon dioxide (CO₂). Fundamental separation mechanisms such as absorption, adsorption, and distillation technology are presented. Liquid amine-based carbon capture (C-capture) technologies have been in existence for over half a century, however, liquid amine capture relies upon chemical reactions and is energy-intensive. Liquid amines are thus not economically viable for broad deployment and offer little room for innovation. Innovative C-capture technologies must improve both the environmental footprint and cost-effectiveness. As a promising alternative, physisorbents have many advantages including considerably lower regeneration energy. Generally, existing classes of physisorbent materials, such as metal-organic frameworks (MOFs) and zeolites are selective toward C-capture. However, their selectivity is currently not high enough to remove trace levels (e.g., ~1%) of CO₂ from various natural gas process streams. This review summarizes the current advancements in physisorbent materials for CO₂ capture. Here, key performance parameters needed to select the most suitable candidate are highlighted. Furthermore, this review discusses the scope for the development of better performing CO₂ selective physisorbents from both environmental and economic perspectives. In addition, hybrid ultra microporous materials (HUMs), characterized mainly by ultra-micro pores (<0.7 nm), are discussed in reference to C-capture. Various characteristics of HUMs result in high selectivity and applicability in difficult separations such as the gas sweetening and C-capture from complex humid mixed gas streams.

Highly permeable tubular silicalite-1 membranes for CO2 capture

Membranes represent one of the most promising alternatives for CO₂ separation and capture. Zeolites membranes, in particular, that can withstand high temperatures and pressures, offer energy efficient way to capture CO₂ compared to conventional separation techniques such as amine absorption. In this work, silicalite-1/ceramic composite membranes were prepared on the inner surface of zirconium oxide and/or titanium oxide tubular supports by a pore plugging hydrothermal synthesis. Five types of supports with different pore sizes ranging from 0.14 to 1.4 μm, were studied. The synthesized membranes were characterized by scanning electron microscope (SEM), electron diffraction spectrometer (EDS), x-ray diffraction (XRD), and gas permeation with pure and mixed gas feeds. All membranes showed high concentrations of Si within the active layer of the support, suggesting successful pore-plugging of the membranes. The greater the pore size of the active layer of the support, the greater was the concentration of Si observed. In addition, large coffin-shape crystals, which are characteristics of silicalite-1, were also observed on top of each membrane. The analysis of XRD micrographs revealed that the crystals were mostly oriented with either the a- or b-axes perpendicular to the membrane surface, which is desirable from the point of view of minimizing the resistance to gas transport through the zeolite membrane. Except for the membranes synthesized using the supports with 0.14 μm pores, all membranes were very selective with CO₂/N₂ permselectivities up to 30 at low-pressure differentials. At the same time, the membranes were very permeable with CO₂ permeance in the order of 10^-6 mol m^-2 Pa^-1 s^-1. Assuming the thickness of the selective layer to be equivalent to the thickness of the active layer of the support, all membranes fell above the revised Robeson upper-bound line for CO₂/N₂ separation.

An Enhanced Carbon Capture and Storage Process (e-CCS) Applied to Shallow Reservoirs Using Nanofluids Based on Nitrogen-Rich Carbon Nanospheres

The implementation of carbon capture and storage process (CCS) has been unsuccessful to date, mainly due to the technical issues and high costs associated with two main stages: (1) CO₂ separation from flue gas and (2) CO₂ injection in deep geological deposits, more than 300 m, where CO₂ is in supercritical conditions. This study proposes, for the first time, an enhanced CCS process (e-CCS), in which the stage of CO₂ separation is removed and the flue gas is injected directly in shallow reservoirs located at less than 300 m, where the adsorptive phenomena control CO₂ storage. Nitrogen-rich carbon nanospheres were used as modifying agents of the reservoir porous texture to improve both the CO₂ adsorption capacity and selectivity. For this purpose, sandstone was impregnated with a nanofluid and CO₂ adsorption was evaluated at different pressures (atmospheric pressure and from 3 × 10^-3 MPa to 3.0 MPa) and temperatures (0, 25, and 50 °C). As a main result, a mass fraction of only 20% of nanomaterials increased both the surface area and the molecular interactions, so that the increase of adsorption capacity at shallow reservoir conditions (50 °C and 3.0 MPa) was more than 677 times (from 0.00125 to 0.9 mmol g^-1).

Tuning porosity in macroscopic monolithic metal-organic frameworks for exceptional natural gas storage

Widespread access to greener energy is required in order to mitigate the effects of climate change. A significant barrier to cleaner natural gas usage lies in the safety/efficiency limitations of storage technology. Despite highly porous metal-organic frameworks (MOFs) demonstrating record-breaking gas-storage capacities, their conventionally powdered morphology renders them non-viable. Traditional powder shaping utilising high pressure or chemical binders collapses porosity or creates low-density structures with reduced volumetric adsorption capacity. Here, we report the engineering of one of the most stable MOFs, Zr-UiO-66, without applying pressure or binders. The process yields centimetre-sized monoliths, displaying high microporosity and bulk density. We report the inclusion of variable, narrow mesopore volumes to the monoliths’ macrostructure and use this to optimise the pore-size distribution for gas uptake. The optimised mixed meso/microporous monoliths demonstrate Type II adsorption isotherms to achieve benchmark volumetric working capacities for methane and carbon dioxide. This represents a critical advance in the design of air-stable, conformed MOFs for commercial gas storage.

While metal–organic frameworks exhibit record-breaking gas storage capacities, their typically powdered form hinders their industrial applicability. Here, the authors engineer UiO-66 into centimetre-sized monoliths with optimal pore-size distributions, achieving benchmark volumetric working capacities for both CH₄ and CO₂.

A Porous Carbon with Excellent Gas Storage Properties from Waste Polystyrene

In this paper, we describe the synthesis and gas adsorption properties of a porous carbonaceous material, obtained from commercial expanded polystyrene. The first step consists of the Friedel-Craft reaction of the dissolved polystyrene chains with a bridging agent to form a highly-crosslinked polymer, with permanent porosity of 0.7 cm 3 /g; then, this polymer is treated with potassium hydroxide at a high temperature to produce a carbon material with a porous volume larger than 1.4 cm 3 / g and a distribution of ultramicro-, micro-, and mesopores. After characterization of the porous carbon and determination of the bulk density, the methane uptake was measured using a volumetric apparatus to pressures up to 30 bar. The equilibrium adsorption isotherm obtained is among the highest ever reported for this kind of material. The interest of this product lies both in its excellent performance and in the virtually costless starting material.

Sustainability criteria for assessing nanotechnology applicability in industrial wastewater treatment: Current status and future outlook

Application of engineered nanomaterials for the treatment of industrial effluents and to deal with recalcitrant pollutants has been noticeably promoted in recent years. Laboratory, pilot and full-scale studies emphasize the potential of this technology to offer promising treatment options to meet the future needs for clean water resources and to comply with stringent environmental regulations. The technology is now in the stage of being transferred to the real applications. Therefore, the assessment of its performance according to sustainability criteria and their incorporation into the decision-making process is a key task to ensure that long term benefits are achieved from the nano-treatment technologies. In this study, the importance of sustainability criteria for the conventional and novel technologies for the treatment of industrial effluents was determined in a general approach assisted by a fuzzy-Delphi method. The criteria were categorized in technical, economic, environmental and social branches and the current situation of the nanotechnology regarding the criteria was critically discussed. The results indicate that the efficiency and safety are the most important parameters to make sustainable choices for the treatment of industrial effluents. Also, in addition to the need for scaling-up the nanotechnology in various stages, the study on their environmental footprint must continue in deeper scales under expected environmental conditions, in particular the synthesis of engineered nanomaterials and the development of reactors with the ability of recovery and reuse the nanomaterials. This paper will aid to select the most sustainable types of nanomaterials for the real applications and to guide the future studies in this field.

Polydopamine Nanoparticle-Coated Polysulfone Porous Granules as Adsorbents for Water Remediation

Water purification technologies possibly based on eco-sustainable, low cost, and multifunctional materials are being intensively pursued to resolve the current water scarcity and pollution. In this scenario, polysulfone hollow porous granules (PS-HPGs) prepared from scraps of the industrial production of polysulfone hollow fiber membranes were recently introduced as adsorbents and filtration materials for water and air treatment. Here, we report the functionalization of PS-HPGs with polydopamine (PD) nanoparticles for the preparation of a new versatile and efficient adsorbent material, namely, PSPD-HPGs. The in situ growth of PD under mild alkaline oxidative polymerization allowed us to stably graft PD on polysulfone granules. Enhanced removal efficiency of ofloxacin, an antibiotic drug, with an improvement up to 70% with respect to the pristine PS-HPGs, and removal of Zn(II) and Ni(II) were also observed after PD modification. Remarkably, removal of Cu(II) ions with an efficiency up to 80% was observed for PSPD-HPGs, whereas no adsorption was found for the PD-free precursor. Collectively, these data show that modification with a biocompatible polymer such as PD provides a simple and valuable tool to enlarge the field of application of polysulfone hollow granules for water remediation from both organic and metal cation contaminants.

Outlook of carbon capture technology and challenges

The greenhouse gases emissions produced by industry and power plants are the cause of climate change. An effective approach for limiting the impact of such emissions is adopting modern Carbon Capture and Storage (CCS) technology that can capture more than 90% of carbon dioxide (CO₂) generated from power plants. This paper presents an evaluation of state-of-the-art technologies used in the capturing CO₂. The main capturing strategies including post-combustion, pre-combustion, and oxy - combustion are reviewed and compared. Various challenges associated with storing and transporting the CO₂ from one location to the other are also presented. Furthermore, recent advancements of CCS technology are discussed to highlight the latest progress made by the research community in developing affordable carbon capture and storage systems. Finally, the future prospects and sustainability aspects of CCS technology as well as policies developed by different countries concerning such technology are presented.

Landfill leachate treatment by sorption in magnetic particles: preliminary study

Leachates are still an open issue in environmental protection. Many of the applied methods for their treatment present low efficiency and thus need to be used collectively. In practice reverse osmosis is mostly used, as it is the most effective option, regardless of its cost. Magnetic methods to treat effluents have been used for water and wastewater treatment by the use of magnetic particles together with magnetic separation for the removal of contaminants. However, large-scale applications are few or even non-existent when we deal with complex contaminated media such as landfill leachates, for which not even research studies at laboratorial scale with real samples have been done yet. In this work, we apply for the first time magnetic sorption for the treatment of leachates, and close the full cycle by studying the regeneration and re-use of the magnetic particles; we also study the influence of the concentration of magnetic particles, the use of several pre-treatment methodologies and the type of particle used in the process, in real landfill samples from the waste treatment plant of Salamanca (Spain), for the removal of COD, NO₃^-, NO₂^-, NH₄⁺, Total-N, PO₄^3-, SO₄^2- and Cl^-. Regeneration of the magnetic particles after being used in the sorption stage is also studied, as well as their efficiency regarding their re-use. It is also determined the optimum number of batches for complete desorption and for regeneration of the particles, the effect of successive regeneration and re-use cycles, the use of two different regeneration methods, the efficiency of the desorption, the effect of the quantity of solvent and the influence of the time of sorption. Due to its innovative character and the complexity of the media, this work represents a first preliminary approach and, although some promising results have been obtained, further studies are required to completely understand and evaluate the proposed treatment process.

Photo-Fenton abatement of aqueous organics using metal-organic frameworks: An advancement from benchmark zeolite

A new and environmentally benign photocatalyst is introduced in this study, which was synthesized via incipient wetness impregnation onto MIL-47(V) using an ethanolic Fe(III) chloride solution. The resultant materials were characterized by XRD, FE-SEM, and HR-TEM analyses. The photocatalytic capability of Fe/MIL-47 towards removal of methylene blue (MB) was evaluated in comparison to MIL-53(Al), Cu/MIL-47, and Fe/zeolite-Y. The unmodified MIL-47 achieved 55% MB removal after 20-min exposure to UV/H₂O₂, through photodegradation as the dominant mechanism. Incorporation of Fe species into MIL-47 significantly increased the MB removal rate by 2.4-fold and accomplished nearly complete removal (98.2%) in 60 min, outcompeting the performance of Cu/MIL-47 and Fe/zeolite-Y. Based on the results of XRD, the impregnation of Fe retained the crystalline characteristics of MIL-47. The significance of temperature, catalyst dose, pH, and molar ratio of H₂O₂:MB was also evaluated in governing the photocatalytic activity of Fe/MIL-47. The reusability of Fe/MIL-47 was evidenced through its repetitive uses in MB photodegradation. The current work highlighted the potential of Fe impregnation for modification of MOFs in order to fabricate highly efficient and water-stable heterogeneous photocatalyst for degradation of organic pollutants. With the use of an economical and environmentally safe reagent (i.e., Fe), robust photocatalyst can exhibit high sustainability to warrant clean environmental remediation.

CO2 capture and storage: A way forward for sustainable environment

The quest for a sustainable environment and combating global warming, carbon capture, and storage (CCS) has become the primary resort. A complete shift from non-renewable resources to renewable resources is currently impossible due to its major share in energy generation; making CCS an imperative need of the time. This study, therefore, aims to examine the reckoning of carbon dioxide (CO₂), measurement methods, and its efficient capture and storage technologies with an ambition to combat global warming and achieve environmental sustainability. Conventionally, physical, geological and biological proxies are used to measure CO₂. The recent methods for CO₂ analyses are spectrometry, electrochemical gas sensors, and gas chromatography. Various procedures such as pre, post, and oxyfuel combustion, and use of algae, biochar, and charcoal are the promising ways for CO₂ sequestration. However, the efficient implementation of CCS lies in the application of nanotechnology that, in the future, could provide a better condition for the environment and economic outlooks. The captured carbon can be stored in the earth crust for trillions of years, but its leakage during storage can raise many issues including its emissions in the atmosphere and soil acidification. Therefore, global and collective efforts are required to explore, optimize and implement new techniques for CCS to achieve high environmental sustainability and combat the issues of global warming.

Technology Evolution in Membrane-Based CCS

In recent years, many CO2 capture technologies have been developed due to growing awareness about the importance of reducing greenhouse gas emissions. In this paper, publications from the last decade addressing this topic were analyzed, paying special attention to patent status to provide useful information for policymakers, industry, and businesses and to help determine the direction of future research. To show the most current patent activity related to carbon capture using membrane technology, we collected 2749 patent documents and 572 scientific papers. The results demonstrated that membranes are a developing field, with the number of applications growing at a steady pace, exceeding 100 applications per year in 2013 and 2014. North American assignees were the main contributors, with the greatest number of patents owned by companies such as UOP LLC, Kilimanjaro Energy Inc., and Membrane Technology and Research Inc., making up 26% of the total number of published patents. Asian countries (China, Japan, and Korea) and international offices were also important knowledge sources, providing 29% and 24% of the documents, respectively. Furthermore, this paper highlights 10 more valuable patents regarding their degree of innovation and citations, classified as Y02C 10/10 according to the Cooperative Patent Classification (CPC) criteria.

High-silica zeolites for adsorption of organic micro-pollutants in water treatment: A review

High-silica zeolites have been found to be effective adsorbents for the removal of organic micro-pollutants (OMPs) from impaired water, including various pharmaceuticals, personal care products, industrial chemicals, etc. In this review, the properties and fundamentals of high-silica zeolites are summarised. Recent research on mechanisms and efficiencies of OMP adsorption by high-silica zeolites are reviewed to assess the potential opportunities and challenges for the application of high-silica zeolites for OMP adsorption in water treatment. It is concluded that the adsorption capacities are well-related to surface hydrophobicity/hydrophilicity and structural features, e.g. micropore volume and pore size of high-silica zeolites, as well as the properties of OMPs. By using high-silica zeolites, the undesired competitive adsorption of background organic matter (BOM) in natural water could potentially be prevented. In addition, oxidative regeneration could be applied on-site to restore the adsorption capacity of zeolites for OMPs and prevent the toxic residues from re-entering the environment.