Low-pressure CO2 Research Articles

The effects of long-term supercritical CO2 exposure on Zululand Basin core samples

Carbon dioxide (CO2) sequestration in geological formations is a viable solution for ensuring coal-based energy supply whilst reducing CO2 emissions. The variation in the microstructure and composition of geological formations before and after CO2 storage plays a significant role in CO2 sequestration. Three core samples obtained from the Zululand Basin in South Africa were exposed to supercritical CO2 (ScCO2) in the presence of water under typical hydrothermal conditions (175 bar, 70 °C) for up to 2 months. The samples were characterized pre- and post-treatment using X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), low pressure nitrogen (N2) adsorption and low pressure CO2 adsorption. Physical and chemical structural changes were observed in all three samples after treatment, to varying degrees. Mineral alterations were observed in the three samples, including plagioclase and calcite dissolution/precipitation and quartz composition changes. Dissolution of organic components and surface chemistry alterations were found in two of the core samples. Increases in pore volume, surface area and CO2 adsorption capacity were observed in all studied samples after CO2 treatment.

Controls of Distinct Mineral Compositions on Pore Structure in Over-Mature Shales: A Case Study of Lower Cambrian Niutitang Shales in South China

Investigating the impacts of rock composition on pore structure is of great significance to understand shale gas occurrence and gas accumulation mechanism. Shale samples from over-mature Niutitang formation of Lower Cambrian in south China were measured by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption to elucidate the controls of distinct mineral composition on pore development. Two distinct lithofacies, namely siliceous shale and argillaceous shale, were ascertained based on their mineral composition. Due to the variability of mineral composition in different lithofacies, pore structure characteristics are not uniform. Pores in siliceous shales are dominated by interparticle pores and organic matter (OM) pores, among which the interparticle pores are mainly developed between authigenic quartz. Furthermore, most of these interparticle pores and cleavage-sheet intraparticle pores within clay minerals are usually filled by amorphous organic matter that is host to OM pores. Due to the lack of rigid minerals, argillaceous shale was cemented densely, resulting in few interparticle pores, while cleavage-sheet intraparticle pores within clay minerals are common. Comparing siliceous shales with argillaceous shales, specific surface areas and pore volumes are higher on the former than on the latter. The content of total organic carbon (TOC) and authigenic quartz have a great influence on micropore structures, but less on mesopore structure for siliceous shales. The rigid framework structure formed by authigenic quartz is believed to be able to prevent primary interparticle pores from mechanical compaction and facilitate the formation of organic matter-associated pores. In terms of argillaceous shales, due to the lack of authigenic quartz, interparticle pores were rarely developed and its pore structure is mainly controlled by illite content.

An efficient method to prepare aryl acetates by the carbonylation of aryl methyl ethers or phenols

A low pressure of CO was used to prepare aryl acetates directly.

Applicability Analysis of Determination Models for Nanopores in Coal Using Low-Pressure CO₂ and N₂ Adsorption Methods.

The development characteristics of nanopores (with pore sizes <200 nm) in coal are a key factor affecting the accumulation and migration of coalbed methane (CBM). Thus, an appropriate determination method and calculation model are essential for accurate nanopore representation. Based on the experiments of low-pressure CO₂ adsorption (LP-CO₂GA) at 273 K and low-pressure N₂ adsorption (LP-N₂GA) at 77 K on four coals with different ranks, the abilities of different models (e.g., Langmuir, Dubinin-Radushkevich (D-R), Dubinin-Astakhov (D-A), Brunauer-Emmett-Teller (BET) and nonlocal density functional theory (NLDFT)) to accurately predict the pore parameters were analyzed. The results showed that (1) for LP-N₂GA, the Langmuir model is only suitable for gas adsorptions at low relative pressure conditions (P/P0 < 0.01), and its error value increased with the relative adsorption pressure. The fitting results of the D-R model showed good agreement with the D-A model under low relative pressure of LP-CO₂GA (P/P0 < 0.01), and the D-A model had more accurate fitting results. The BET model is more accurate than the other models (φ = -1.2733%) only in the interval of LP-N₂GA with 0.05 < P/P0 < 0.35. The data also showed that the NLDFT model can maintain a higher fitting accuracy for LPCO₂/N₂GA processes at relative adsorption pressures from 0.001-0.9996. (2) Using LP-CO₂GA with the Langmuir, D-R, D-A, and NLDFT models, the micropore specific surface area (SSA; 66.9570-248.6736 m²/g) and pore volume (0.0201- 0.0997 cm³/g) were obtained, while the values of meso-/macropore SSA (0.0007-2.3398 m²/g) and pore volume (0.0036-0.04 cm³/g) were calculated by LP-N₂GA with the BET and NLDFT models. The results showed that the fitting accuracy in descending order was the D-R, D-A, Langmuir and NLDFT models. (3) In combination with the applicable model range, LP-CO₂GA with the NLDFT model was recommended for micropore analysis of the coal pore sizes from 0.36-1.1 nm, while LP-N₂GA combined with the NLDFT model was recommended for nanopore analysis of pore sizes from 1.1-200 nm. (4) The characteristics of pore development in the Beiloutian coal were analyzed using LP-CO₂/N₂GA combined with the NLDFT model. It was found that a pore volume and SSA less than 1.0 nm accounted for 88.82% of the total pore volume and 98.05% of the total SSA, indicating that micropores in coal are the main space for CBM storage and are key physical factors for the occurrence and migration of coalbed methane. The conclusions of this article will provide a basis for the accurate calculation of nanopores in coal.

Co(III)-Salen immobilized cellulose nanocrystals for efficient catalytic CO2 fixation into cyclic carbonates under mild conditions

Searching for green, recyclable and highly efficient catalyst for the synthesis of cyclic carbonates from CO2 is of great importance because it is profitable for reducing the greenhouse effects and meets the principles of green chemistry. Herein, a series of cellulose nanocrystals, either the pristine or modified ones (TEMPO oxidized and Co(III)salen immobilized), were explored as catalysts for cycloaddition of epoxides and carbon dioxide. The impact of surface properties on the performance of the as-made catalysts was investigated. Co(III)-salen grafted cellulose nanocrystals was proven to be the most effective catalyst in this study, which could afford excellent yield up to 99 % after 24 h even under low CO2 pressures of 0.1 MPa. They can be easily recovered and reused for at least 4 times, demonstrating their excellent stability. We found that the surface functional groups such as enriched sulfate or carboxylic groups could also account for the enhanced catalytic activity. This work highlights the applications of green and sustainable nanoparticles in a cycloaddition reaction and offers a sustainable solution in industrial catalysis related to CO2 conversions.

Applicability of Low-Pressure CO2 and N2 Adsorption in Determining Pore Attributes of Organic-Rich Shales and Coals

Low-pressure gas adsorption (LPGA) using N2 and CO2 has been widely used by researchers to evaluate the porous structures present within shales and coals. For a suite of shale and coal samples from...

Preparation of low-cost activated carbons from amazonian nutshells for CO2 storage

Low-cost carbonaceous adsorbents with microporous structures and a moderate specific surface area (up to 1630 m2 g−1) were prepared from different amazonian nutshells by one-step chemical activation with KOH for CO2 storage. In-depth micropore structure analysis and surface characterization were performed using adsorption isotherms at 298 K and carbon dioxide isotherms at 273 K. The Brazilian nutshell and Cascara Capuassau exhibit a remarkable CO2 uptake capacity of 5.14 mmol g−1 and 5.13 mmol g−1 at 273 K, respectively up to 1 atm. The highest CO2 adsorption at 298 K equal to 3.67 mmol g−1 was observed on activated carbon produced from Cascara Capuassau. The low-pressure CO2 capacities are well correlated with the volume of small micropores (<0.7 nm) rather than the total micropore volume and surface area. The great mesoporosity of activated carbons is beneficial to ensure higher CO2 capture at high uptake pressure.

Advanced pore characterization and adsorption of light gases over aerogel-derived activated carbon

Abstract Activated carbon produced by etching with CO2 is an attractive adsorbent material because of its ultrahigh surface area above 1000 m2/g and substantial pore volume. This being stated, because etched carbon was only reported recently, many of its properties, such as its textural properties, high-pressure adsorption capacities towards CO2, N2, H2, CO, and CH4, and dynamic performance, have never been thoroughly investigated. Accordingly, this study explored these characteristics for the first time. The textural and adsorptive properties were also assessed for zeolite 5A, a metal-organic framework (Ni MOF-74), and activated carbon provided by Ingevity corporation to allow for comparison to established benchmarks. The textural properties were assessed by a combination of N2 physisorption at 77 K and CO2 adsorption at 273 K using a combination of t-plot, Dubinin–Radushkevich, 2D-NLDFT, and Horvath–Kawazoe models. Therein, it was determined that the etched carbon had a hierarchical porosity, with pores appearing in both the ultramicro- and meso-porous ranges. From the adsorption isotherms at 298 K and 0–20 bar, the etched carbon exhibited adsorption capacities of 20, 9, 5, 4.5, and 1 mmol g−1 for CO2, CH4, CO, N2, and H2 respectively, indicating that the samples high (i.e., 95%) pore volume made it selective towards multiple species at high pressure. Having said this, when the material was examined for dynamic CO2 adsorption from N2, H2, and CH4 mixtures at 25 °C and 1 bar, it achieved CO2/CH4, CO2/H2, and CO2/N2 selectivities of 2.4, 15.4, and 6.6 mol/mol, respectively, meaning that it can be used for kinetic separations. The high dynamic selectivity was retained for CO2/H2 at 20 bar, however, the CO2/CH4 breakthrough fronts were much broader compared to the low-pressure experiments, meaning that the two species' diffusivities were competitive. As such, EC-RF was concluded to be effective for low-pressure CO2 separations as well as for high pressure CO2/H2 separations. More importantly, this study provides an in-depth characterization and better understanding of EC-RF's properties, which is imperative to its use as an adsorbent material.

High-Performing Deep Learning Regression Models for Predicting Low-Pressure CO2 Adsorption Properties of Metal–Organic Frameworks

Metal–organic frameworks (MOFs) have garnered interest as potential solid sorbent materials for postcombustion CO2 capture. With a seemingly infinite design space, high-throughput computational scr...

Calcination under low CO2 pressure enhances the calcium Looping performance of limestone for thermochemical energy storage

The Calcium Looping performance of limestone for thermochemical energy storage has been investigated under novel favorable conditions, which involve calcination at moderate temperatures under CO2 at low pressure (0.01 and 0.1 bar) and carbonation at high temperature under CO2 at atmospheric pressure. Calcining at low CO2 pressures allows to substantially reduce the temperature to achieve full calcination in short residence times. Moreover, it notably enhances CaO multicycle conversion. The highest values of conversion are obtained for limestone samples calcined under 0.01 bar CO2 at 765 °C. Under these conditions, the residual conversion is increased by a factor of 10 as compared to conditions involving calcination under CO2 at atmospheric pressure. The enhancement of CaO conversion is correlated to the microstructure of the CaO samples obtained after calcination. As seen from SEM, BET surface and XRD analysis, calcination under low CO2 pressure leads to a remarkable decrease of pore volume and CaO crystallite size. Consequently, CaO surface area available for carbonation in the fast reaction-controlled regime and therefore reactivity in short residence times is promoted.

A reaction mechanism for vibrationally-cold low-pressure CO2 plasmas

The use of plasmas for CO2 utilization has been under investigation in recent years following a wave of environmental awareness. In this work, previously published experimental results on vibrationally cold CO2 plasmas are modelled to define a reaction mechanism, i.e. a set of reactions and rate coefficients validated against benchmark experiments. The model couples self-consistently the electron and heavy particle kinetics. In turn, the simulated results are validated against measurements taken in CO2 DC glow discharges in a relatively large range of experimental conditions: at pressures from 0.4 to 5 Torr, reduced electric fields ranging from 50 to 100 Td and gas flowing from 2 to 8 sccm. The model predicts the measured values of product formation (CO and O) as well as discharge power and electric field. After validation, a thorough analysis of the model’s results is presented, including: electron properties, species densities, power distribution into different excitation channels and main creation and destruction mechanisms of the main species. It is shown that, although vibrational populations are low, they have a significant effect on the electron properties and thus on the electric field and conversion. Moreover, the shape of the EEDF is significantly dependent on the dissociation degree. The role of electronically excited states on CO2 dissociation is also analyzed, showing that the first electronic excited state of CO can have a beneficial or detrimental effect in further producing CO and O in the discharge.

Variations in pore structure of marine shale from the same horizon of the Longmaxi Formation with changing position in a small-scale anticline: Implications for the influence of structural deformation

Nanopores are well developed in shale, and are influenced by many rock properties, such as mineral composition, TOC (Total Organic Carbon) and thermal maturity. However, the influence of structural deformation on pore properties of shale is still not well understood. In this study, 19 samples from the same horizon of the lower Silurian Longmaxi shale, but from different positions in a small-scale anticline of Wuping Village, Chongqing (China), were collected. Low pressure N2 and CO2 adsorption experiments were performed to study the influence of structural deformation on the pore structure. The results show that the mineral composition, TOC and thermal maturity of all samples are similar, but the specific surface areas and pore volumes exhibit great differences in different positions of this anticline. The mean N2-BJH pore volumes, the mean N2-BET specific surface areas, the mean CO2 D-R pore volumes, as well as the mean CO2 D-R specific surface areas exhibit decreasing trends for the samples from the core part to the sub-core part, and to the limb part of the studied anticline, respectively. Structural deformation has shown a considerable influence on the micropores and mesopores, especially for the pores lower than 10 nm because that tectonic stress increases from the core part to the sub-core part and the limb part of the anticline. Moreover, the structural curving degree characterized using curvature shows good correlation with the pore volume and specific surface area. Compared to the limb part, the core part of the anticline with high curvature is located in the tension environment, and it is beneficial for pore development and gas accumulation. Our study implies that moderate structural deformation can provide more specific surface area for absorbed gas and pore volume for free gas at the core part of the structure on a larger scale.

Studies on the influence of moisture on the sorption and structural properties of hard coals

In the conducted research, the effect of moisture on CO2 sorption and transport processes and changes occurring in the coal pore structure were investigated in detail. The measurements were carried out under isothermal (313 K) and isobaric conditions (0.1−0.8 MPa) on the original device - a manometric sorptomat. Coal sorption capacities, Langmuir sorption isotherms, effective diffusion coefficients and structural parameters were analysed. The obtained sorption capacities of the tested samples relative to CO2 for pressures 0.1, 0.3 and 0.8 MPa confirmed that moisture significantly (even up to 6 times) reduces the sorption capacity of coal relative to CO2, and the largest reduction in sorption capacities was noted on coal with the lowest vitrinite reflectance. It was observed that the increase of the sorption equilibrium pressure decreases the effect of the reduction of the sorption capacity of coal under the influence of moisture, and above 1.0 MPa this effect disappears completely. The critical moisture content was estimated, above which there is no further reduction of the coal sorption capacity due to the increase in moisture content. It was observed that the influence of moisture on CO2 diffusion in its pore space is not clear. Structural analysis were carried out by low-pressure CO2 adsorption at 273 K and 0−0.1 MPa. After increasing the coal moisture content, the surface area decreased by 50 % and a decrease in the volume of available pores with diameters in the range of 0.5−0.6 nm was noted.

Electrochemical Approaches toward CO2 Capture and Concentration

Carbon capture and concentration of low partial pressure CO2 in air and flue gas is a key step in carbon abatement strategies. Traditional CO2 capture methods employ temperature or pressure swings;...

Meso- and microporosity characteristics of Miocene lignite and subbituminous coals in the Kınık coalfield (Soma Basin, W. Turkey)

Three Miocene-age coal seams - a lower seam (kM2, subbituminous), a middle seam (kM3, subbituminous), and an upper seam (kP1, lignite) - in the Kınık coalfield, located in the southwesternmost part of the Soma Basin in western Turkey, and factors controlling mesoporosity and microporosity characteristics, were investigated using low-pressure N2 and CO2 adsorption techniques, respectively. In addition, this study is also aimed to compare controlling factors on meso- and microporosity characteristics of Soma Basin and Late Carboniferous Zonguldak, and Late Miocene-Pliocene Dombayova Basins. It is found that distinct differences with regard to meso- and microporosity exist among the three seams (kM2, kM3 and kP1) investigated in the Kınık coalfield. Specifically, samples from kP1 have greater mesopore surface areas and volumes than those from kM3. In addition, samples from the upper part of kM2 have significantly lower mesopore surface area and volume than from the lower part of this seam and from kM3 and kP1. Samples from the lower part of kM2 have higher meso- and micropore surface area and volume values than kM3 and kP1 samples. The increase in mesopore surface area and volume, along with the increase in ash yield toward the lower part of kM2, suggests that mesoporosity characteristics in this seam appear to be controlled mainly by mineral matter content. The results of this study show that meso- and micropore surface area and volume of Miocene subbituminous coal (kM2) in Kınık and Eynez coalfields of the Soma Basin have higher values than lignite from the Dombayova Basin and bituminous coal from the Zonguldak Basin. The differences in mesopore characteristics between the Soma and the Zonguldak Basins could be related to differences in rank. In addition, the mesoporosity of coal seams in the Dombayova Basin is more similar to seams in the Zonguldak Basin. These conflicting data suggest that rank might not be a controlling factor on the meso- and micropore characteristics of the basins investigated in this paper. Nevertheless, the common presence of structured huminite macerals in the Soma Basin seems to have caused relatively higher microporosity surface area and volume values, whereas the common presence of illite, particularly in the lower parts of kM2, resulted in higher values of mesopore surface area and volume in the Soma Basin samples. Consequently, maceral and mineralogical compositions of the studied Turkish coals seem to exert major controls on meso- and micropore characteristics.

Transport and Interfacial Properties of Mixed Molten Carbonate/Hydroxide Electrolytes by Molecular Dynamics Simulations

We use molecular dynamics simulations based on a recently developed force field to obtain the viscosity, ionic conductivity, and liquid–vapor surface tension of molten alkali-metal carbonate–hydroxide mixtures over a range of cation and hydroxide compositions. Recent experimental and simulation studies have suggested that molten carbonates contain non-negligible amounts of hydroxide ions in the presence of water at low partial pressures of CO2. However, due to the high temperatures (≃600 °C or higher) required to melt pure alkali carbonates and their mixtures, there is a lack of experimental thermodynamic and transport data for these molten phases. Here, we deploy a recently parametrized force field for molten alkali carbonates and hydroxides [ J. Chem. Theory Comput. 2020, 16, 5736−5746, DOI: 10.1021/acs.jctc.0c00285] to simulate some physical properties using atomistic molecular dynamics. Our predictions show a consistent decrease in viscosity and an increase in ionic conductivity with increasing hydroxide fractions, whereas a higher Li+ mole fraction leads to an increase in both viscosity and ionic conductivity. The computed surface tension values exhibit an upward trend with higher asymmetry in cation and anion sizes. Structural analysis suggests that in the carbonate–hydroxide melts the smaller/harder ions, OH– and Li+, are more favored at the interface than the larger/softer ions, CO32– and K+/Na+. The current approach provides a systematic route to obtaining physicochemical properties of molten alkali carbonate and hydroxide mixtures over a large domain of chemical compositions and thus can steer future experimental research for molten carbonates and their applications.

Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2.

Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal–NHC bond distances and bond strengths are governed by ligand-to-metal σ- and π-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC π-acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC.

Gain dynamics in a CO2 active medium optically pumped at 4.3 μm

A pulsed ∼2 mJ Fe:ZnSe laser tunable around ∼4.3 μm is used to optically pump mixtures of CO2 and He to create gain at 10 μm. A conventional low-pressure CO2 laser operating on both regular (001-100) and sequence (002-101) bands is used to study the gain dynamics of the optically pumped CO2 amplifier. Time-resolved measurements of the CO2 asymmetric stretching mode vibrational temperature, T3, as well as the translational temperature, T, are made. The measured T3 value of ∼2500 K is much higher than that typically measured in discharge pumped CO2 lasers. High gain coefficients ∼30%/cm in the optically pumped active medium are attributed to the efficient storage of energy in the asymmetric stretching mode and the selective population of the upper laser level. The measured optical-to-optical energy conversion efficiency of ∼30% for 10 μm lasing at sub-atmospheric pressures is close to the theoretical quantum limit of 40% and, thus, supports our claim of gain dynamics optimization. It is concluded that a joule-class 4.3 μm pump laser will be required for the amplification of sub-picosecond 10 μm pulses in a multi-atmosphere optically pumped CO2 active medium.

Single‐Atom Electrocatalysts from Multivariate Metal–Organic Frameworks for Highly Selective Reduction of CO2 at Low Pressures

AbstractSingle‐atom catalysts (SACs) are of great interest because of their ultrahigh activity and selectivity. However, it is difficult to construct model SACs according to a general synthetic method, and therefore, discerning differences in activity of diverse single‐atom catalysts is not straightforward. Herein, a general strategy for synthesis of single‐atom metals implanted in N‐doped carbon (M1‐N‐C; M=Fe, Co, Ni and Cu) has been developed starting from multivariate metal–organic frameworks (MOFs). The M1‐N‐C catalysts, featuring identical chemical environments and supports, provided an ideal platform for differentiating the activity of single‐atom metal species. When employed in electrocatalytic CO2 reduction, Ni1‐N‐C exhibited a very high CO Faradaic efficiency (FE) up to 96.8 % that far surpassed Fe1‐, Co1‐ and Cu1‐N‐C. Remarkably, the best‐performer, Ni1‐N‐C, even demonstrated excellent CO FE at low CO2 pressures, thereby representing a promising opportunity for the direct use of dilute CO2 feedstock.

Single-Atom Electrocatalysts from Multivariate Metal-Organic Frameworks for Highly Selective Reduction of CO2 at Low Pressures.

Single-atom catalysts (SACs) are of great interest because of their ultrahigh activity and selectivity. However, it is difficult to construct model SACs according to a general synthetic method, and therefore, discerning differences in activity of diverse single-atom catalysts is not straightforward. Herein, a general strategy for synthesis of single-atom metals implanted in N-doped carbon (M1 -N-C; M=Fe, Co, Ni and Cu) has been developed starting from multivariate metal-organic frameworks (MOFs). The M1 -N-C catalysts, featuring identical chemical environments and supports, provided an ideal platform for differentiating the activity of single-atom metal species. When employed in electrocatalytic CO2 reduction, Ni1 -N-C exhibited a very high CO Faradaic efficiency (FE) up to 96.8 % that far surpassed Fe1 -, Co1 - and Cu1 -N-C. Remarkably, the best-performer, Ni1 -N-C, even demonstrated excellent CO FE at low CO2 pressures, thereby representing a promising opportunity for the direct use of dilute CO2 feedstock.