CO2 Adsorption Isotherms Research Articles

Preparation of molded biomass carbon from coffee grounds and its CH4/N2 separation performance

Coffee grounds are promising precursors for excellent porous carbon adsorbents. During the preparation of the porous carbons, sodium silicate was used as a binder and pore-forming agent, and extrusion molding technology was used to prepare them in a columnar form. After carbonization, steam activation and silica removal by alkaline washing, high-strength columnar porous carbon adsorbents (CGCs) were obtained. Their CH4/N2 separation performance was studied by multicomponent breakthrough experiments. The Brunauer-Emmett-Teller (BET) surface area of CGC-1.5 (where 1.5 is the mass ratio of a 9 wt% sodium silicate aqueous solution to the coffee grounds) is 527 m2·g-1. Both the N2 and CO2 adsorption isotherms show that the CGCs are rich in micropores and mesopores, with the micropores size mainly centered at about 0.48 nm. FT-IR results show that CGC-1.5 has abundant oxygen-containing functional groups. At 298 K and 0.1 MPa, its equilibrium adsorption capacity for CH4 is 0.87 mmol·g-1, and the separation selectivity for a CH4/N2 mixture (3/7, vol/vol) is 10.3, which is better than most biomass-based porous carbon adsorbents and crystalline materials. Dynamic breakthrough tests show that CGC-1.5 has an excellent CH4/N2 separation performance at both high and atmospheric pressures. The dynamic selectivities at 298 K, 0.11 and 0.5 MPa are 10.4 and 17.9, respectively. The adsorption capacity is unchanged after 10 adsorption-desorption cycles. The mechanical strength of CGC-1.5 is as high as 123 N·cm-1, which meets the criteria of industrial applications.

High pressure CO2 adsorption onto Malaysian Mukah-Balingian coals: Adsorption isotherms, thermodynamic and kinetic investigations

CO2 sequestration into coalbed seams is one of the practical routes for mitigating CO2 emissions. The adsorption mechanisms of CO2 onto Malaysian coals, however, are not yet investigated. In this research CO2 adsorption isotherms were first performed on dry and wet Mukah-Balingian coal samples at temperatures ranging from 300 to 348 K and pressures up to 6 MPa using volumetric technique. The dry S1 coal showed the highest CO2 adsorption capacity of 1.3 mmol g−1, at 300 K and 6 MPa among the other coal samples. The experimental results of CO2 adsorption were investigated using adsorption isotherms, thermodynamics, and kinetic models. Nonlinear analysis has been employed to investigate the data of CO2 adsorption onto coal samples via three parameter isotherm equilibrium models, namely Redlich Peterson, Koble Corrigan, Toth, Sips, and Hill, and four parameter equilibrium model, namely Jensen Seaton. The results of adsorption isotherm suggested that the Jensen Seaton model described the experimental data well. Gibb's free energy change values are negative, suggesting that CO2 adsorption onto the coal occurred randomly. Enthalpy change values in the negative range established that CO2 adsorption onto coal is an exothermic mechanism. Webber's pore-diffusion model, in particular, demonstrated that pore-diffusion was the main controlling stage in CO2 adsorption onto coal matrix. The activation energy of the coals was calculated to be below −13 kJ mol−1, indicating that adsorption of CO2 onto coals occurred through physisorption. The results demonstrate that CO2 adsorption onto coal matrix is favorable, spontaneous, and the adsorbed CO2 molecules accumulate more onto coal matrix. The observations of this investigation have significant implications for a more accurate measurement of CO2 injection into Malaysian coalbed seams.

Efficient separation of CO2 from methane at high pressures by modified microporous titanosilicates; design and breakthrough studies

In this work, type-4 titanosilicate (TS-4) having a micro-pore size of 3–4 Å, was synthesized from inexpensive source materials, such as commercial silicon dioxide, with the template-free hydrothermal method. The XRD analysis confirmed formation of the TS-4 crystalline phase. Then, strontium cation modification was performed using SrCl2 solution to adjust the pore structure and increase the basicity of the surface to improve CO2 selectivity versus methane. In this study, the adsorption isotherms of methane and CO2 were measured in the range of 0 to 50 bars, at two different temperatures of 298 K and 308 K, then the ideal selectivities of CO2/CH4 were calculated at different pressures. The ideal CO2/CH4 selectivity was determined at 19.04 and 38.49 for TS-4 and Sr-TS-4, respectively, at the pressure of 50 bar and the temperature of 298 K. The isosteric heat of adsorption of CO2 was calculated by Van’t Hoff equation as −29.63 kJ/mol and −26.22 kJ/mol, respectively. Low CO2 heat of adsorption of Sr-TS-4 and high selectivity of CO2/CH4 at high pressures, against zeolites, offer Sr-TS-4 as an economic and efficient adsorbent for cyclic adsorption processes. To study the dynamic selectivity of CO2/CH4, dynamic adsorption experiments were conducted at the temperature of 298 K, pressures of 7 and 15 bars, flow rates of 10 and 20 Nml/min in a bed of 2.7 gr adsorbent, with the inlet molar fraction of 5–95 % CO2-CH4 mixture. Comparison of the breakthrough curves indicated that Sr-TS-4 had a longer breakpoint and a larger used ratio of the length at any feed flow rate. The maximum relative dynamic selectivity of CO2/CH4 at 15 bar, by the Sr-TS-4, was obtained 95.29 at CO2 breakpoint of 40 min, whereas maximum relative dynamic selectivity, by the TS-4, was obtained 63.54 at CO2 breakpoint of 30 min. It is concluded that Sr-TS-4 would be an efficient adsorbent for separation of CO2 from natural gas in commercial scales at high pressures with high methane recovery.

Insights into CO2 capture in porous carbons from machine learning, experiments and molecular simulation

Porous carbon is one of the important CO2 adsorbents being developed at present. However, interpreting the potential mechanism of CO2 adsorption by porous carbon is still challenging due to their various functional groups, different structural characteristics and different adsorption conditions (temperature and pressure) during CO2 adsorption. Here, this study firstly applied machine learning to study the effects of pore structure, chemical properties, and adsorption conditions on CO2 adsorption performance based on 1594 CO2 adsorption datasets, and to predict CO2 adsorption capacity. The results show that the R2 of the random forest (RF) model is above 0.97 on the training and test data, which has good prediction performance. According to RF analysis results, the nitrogen groups of porous carbon have the greatest impact on CO2 capture at 0–0.15 bar, while ultra-micropores have the greatest impact on CO2 capture at 0.15–1 bar. Subsequently, we prepared three kinds of porous carbons with different pore structures and functional groups, and carried out CO2 adsorption isotherm tests. The results were consistent with the results of machine learning. However, the above results hardly reveal the effect of functional group type and pore size on CO2 capture. Finally, the relative importance of pore size and functional group on CO2 adsorption under different pressures was calculated by molecular simulation, and the mechanism of CO2 adsorption by a single pore size and functional group species was revealed. The results based on the aforementioned machine learning, experimental data and molecular simulation are of great significance for predicting gas adsorption and guiding the development of the carbon-based adsorbents.

A Better Understanding of the SBA-15 Pores Filling through Textural Changes in CMK-3 Carbon Synthesis and Its CO2:CH4 Adsorption Selectivity

This text reports the morphological and textural behavior of the synthesis stages of a CMK-3 carbon type using a silicon matrix of the SBA-15 type calcined at 823 K as a template. During the synthesis, three intermediate materials were obtained because of (i) the addition of sucrose to the SBA-15 template (CCMK3-1st), (ii) the addition of sucrose to the CCMK3-1st material (CCMK3-2nd), and (iii) the carbonization by pyrolysis of the by-product CCMK3-2nd (CCMK3-F). The texture of the above materials was found by analyzing the N2 adsorption isotherms, applying the classical adsorption theories to obtain the BET-specific surface and the meso- and micropore distributions by the BJH and Dubinin–Astakhov (DA) methods, respectively, in addition to the non-localized density functional theory (NLDFT). Similarly, with high resolution, the samples were analyzed morphologically by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Finally, the adsorption isotherms of CO2 and CH4 of the CMK-3 sample were obtained at six different temperatures in the interval of 243 to 303 K to evaluate the behavior of the isosteric enthalpy of adsorption (qst) and its CO2:CH4 ideal selectivity. The final CMK-3 carbon presented two families of micro- and mesopores of 1.5 and 3.2 nm, nanopipe diameters of 3.5 nm, and a specific surface area of 1350 m2/g. It also presented values of 6.0 and 2.4 mmol/g adsorbed CO2 and CH4 at 243 K, respectively, and strong intermolecular interactions, with qst values higher than 22 kJ/mol reflected in high selectivity values for an ideal mixture of CO2:CH4 (30:70%).

Influence of regeneration conditions on cyclic CO2 adsorption on NaA zeolite at high pressures

This work aimed to determine the influence of different regeneration conditions on CO2 adsorption on NaA zeolite at high pressures through cyclic processes. To reach this goal, NaA characterization, high-pressure adsorption isotherms, heat of adsorption and working capacities for CO2/NaA system were performed. NaA zeolite characterization showed a high crystalline and stable microporous structure with 0.43 nm pore diameter and 247 m² g-1 specific area. CO2 adsorption isotherms are strongly favorable, showing adsorption uptakes equal to 4.73, 4.54 and 4.44 mol kg-1 (c.a. 20 wt%) at 40 bar and 20, 30 and 40 °C, respectively. Jensen-Seaton model fits experimental data and was used to determine CO2/NaA heats of adsorption from 10 to 40 kJ mol-1, which are characteristic of physisorption process. This was corroborated by constant working capacity for all regeneration processes, reaching 1.44, 3.27, and 4.27 mol kg-1 for REG-1 (1 bar, 30 °C), REG-2 (vacuum, 30 °C) and REG-3 (vacuum, 150 °C) regeneration processes, respectively. The working capacity simulation showed a strong influence of pressure and practically insensitive to adsorption temperature from 20° to 40°C. Therefore, NaA zeolite is a regenerative material showing a high potential for utilization in high pressure cyclic adsorption processes, such as fixed and fluidized beds, for CO2 separation when pressure and temperature are changed in blowdown/regeneration step.

Tunable Interlayer Shifting in Two-Dimensional Covalent Organic Frameworks Triggered by CO2 Sorption.

Two-dimensional covalent organic frameworks (2D COFs) have been widely viewed as rigid porous materials with smooth and reversible gas sorption isotherms. In the present study, we report an unusual hysteresis step in the CO2 adsorption isotherm of a 2D COF, TAPB-OMeTA. In situ powder X-ray diffraction (PXRD) measurements, computational modeling, and Pawley refinement indicate that TAPB-OMeTA experiences slight interlayer shifting during the CO2 adsorption process, resulting in a new structure that is similar but not identical to the AA stacking structure, namely, a quasi-AA stacking structure. This interlayer shifting is responsible for the step in its CO2 adsorption isotherm. We attribute the interlayer shifting to the interactions between COF and CO2, which weaken the attraction strength between adjacent COF layers. Notably, the repulsion force between the methoxy groups on the backbone of TAPB-OMeTA is essential in facilitating the interlayer shifting process. After further increasing the size of side groups by grafting poly(N-isopropylacrylamide) oligomers to the TAPB-OMeTA backbone via surface-initiated atom transfer radical polymerization (SI-ATRP), we observed a second interlayer shifting and two adsorption steps in the CO2 adsorption isotherm, suggesting tunability of the interlayer shifting process. Density functional theory (DFT) calculations confirm that the quasi-AA stacking structure is energetically preferred over AA stacking under a CO2 atmosphere. These findings demonstrate that 2D COFs can be "soft" porous materials when interacting with gases, providing new opportunities for 2D COFs in gas storage and separation.

CO2 capture using in-situ polymerized amines into pore-expanded-SBA-15: Performance evaluation, kinetics, and adsorption isotherms

Over the last decades, carbon dioxide capture has attracted lots of attention and different solution have been proposed, yet the global CO2 concentration is still increasing along with its environmental impacts. Covalently tethered amines were in-situ polymerized into mesoporous silica support, namely PE-SBA-15, for the reversible temperature swing CO2 adsorption. The in-situ polymerization approach shows high loadings of amines in comparison to the grafting technique which was confirmed via thermal decomposition and CHN elemental analysis. The covalent tethering of amines, evidenced by the characteristic absorption peaks via FTIR, resulted in good thermal stability of the sorbent and constant CO2 uptake during multicycle operation. Porosimetry analysis, pore volume and BET surface area, showed a noticeable decrease upon the functionalization of support indicating that most of the pores of the support are occupied by the amines. CO2 capture of the sorbent at different concentrations and temperatures, from 400 ppm for the direct air CO2 capture to 15 % CO2 for coal-powered plants showed good performance. A reduction in CO2 uptake was noticed upon the decrease in adsorption temperature with an optimum temperature of 25 °C. Adsorption kinetics were found to be best represented by a pseudo-second order model with significant adsorption rates. Dry adsorption isotherms of CO2 and N2 were studied at different temperatures and the CO2 isotherms were found to follow Langmuir dual site (LDS) model with a very high selectivity towards CO2. This study shows that in-situ polymerization is a key route for mitigating the continuous increase in atmospheric CO2 concentration.

Discovering the impact of targeted defects in SP-MOF for CO2 capture from flue gas in presence of humidity through computational modelling

The imperfections in crystalline porous solid materials through defects and further accelerating the separation performances of MOFs are growing interest in recent times. Herein, a novel class of [Cu(SiF6)(4,4′-bipyridine)2] square pillared (SP) material is used to analyse the impact of targeted defect amendment for CO2 capture from flue gas via specific force field-based molecular simulation. Additionally, the performances of CO2 separation from flue gas composition in the presence of humidity are more realistically predicted with the aid of a theoretically modelled breakthrough curve from their co-adsorption isotherms.

Influence of chain length on structural properties of carbon molecular sieving membranes and their effects on CO[formula omitted], CH[formula omitted] and N[formula omitted] adsorption: A molecular simulation study

Carbon molecular sieving membranes (CMSM) derived from 6FDA/BPDA-DAM polymer precursors (6F-CMSM) show excellent adsorption and selectivity towards CO2. Using molecular dynamics (MD) simulations, we investigate how the length of individual carbon chains determines the morphologies emerging from the same monomers. Smaller chains produce densely packed structures, whereas longer chains arrange in layers, creating larger voids comprising both micropores and ultra-micropores. The pure gas adsorption isotherms of CO2, CH4, and N2 inside various 6F-CMSM morphologies, obtained from MD, provide theoretical insights into most feasible structures formed in experiments, thereby providing an estimate of the corresponding 6F-CMSM density. Using grand canonical Monte Carlo simulations, we observe that high isosteric heats of adsorption for CO2 leads to enhanced adsorption capacity and high sorption selectivity from binary mixtures of these gases. Studying the asphericity of carbon chains, we report the existence of an optimum chain length of highest curvature, leading to low density, high porosity, and high CO2 adsorption capacities that compare well with available experimental data, without compromising sorption selectivity of 6F-CMSM. This work provides a theoretical basis of how slight modifications of the physical arrangements of chemical components lead to significant changes in porosity and gas adsorption capacities of molecular sieving membranes.

In-situ construction of BiOBr/Bi2WO6 S-scheme heterojunction nanoflowers for highly efficient CO2 photoreduction: Regulation of morphology and surface oxygen vacancy

The improvements of charge transfer efficiency and CO2 capture ability are of particular importance to the photocatalytic CO2 reduction activity of semiconductor photocatalysts. Herein, a BiOBr/Bi2WO6 S-scheme heterojunction with intimate interfacial contact is in-situ synthesized by a facile one-step hydrothermal method. The nanoflower morphology of the heterojunction is elaborately regulated based on the “allometric growth” mechanism, while the concentration of surface oxygen vacancies (SOVs) is readily tuned by low-temperature calcination duration. Due to the unique nanoflower morphology and rich SOVs, the CO2 capture ability is significantly enhanced, confirmed by the CO2 adsorption isotherms and density functional theory (DFT) calculations. The construction of S-scheme heterojunction and the introduction of SOVs also lead to the remarkably improved efficiency of charge separation and transfer. The BiOBr/Bi2WO6 heterojunction exhibits excellent photocatalytic CO2 reduction activity with a CO production rate of 55.17 μmol·g−1·h−1 without using any sacrificial agent and cocatalyst, surpassing most reported photocatalysts. In addition, the formation of key intermediate *COOH during CO2 photoreduction on the photocatalyst surface is determined by in-situ FT-IR spectra. This work not only provides a new strategy for the construction of highly efficient S-scheme heterojunctions, but also sheds light on the optimization of photocatalytic performance through defect and morphology engineering.

Design of highly microporous activated carbons based on walnut shell biomass for H2 and CO2 storage

Low-cost walnut shell-based carbons with high microporosity were prepared by simple one-step carbonization with chemical activation using KOH, exhibiting the promising potential to be a very good CO2 and H2 adsorbent. The physicochemical properties of the obtained carbons were characterized by N2 and CO2 adsorption isotherms, X-ray powder diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and elemental analysis. The activated carbon AC-800 was characterized by a highly developed specific surface area of 1868 cm2/g and a high micropore content of 0.94 cm3/g. It highly exhibited CO2 uptake in 1 bar was up to 9.54, 5.17, 4.33 mmol/g for 0, 25 and 40 °C, respectively. In addition, the H2 storage capacity was 3.15 mmol/g at 40 bars. Significantly, confirmed an exceptionally high dependency of CO2 and H2 uptake vs micropores structure of activated carbon. AC-800 also shows good selectivity for CO2/N2 and fast adsorption kinetics that be easily regenerated with superior cyclic stability after multiple cycles The experimental isotherm data of activated carbon produced from walnut shells were analyzed using Langmuir, Freundlich, Temkin, Sips, and Toth isotherm equations. The fitting details showed that the multitemperature Toth equation is a powerful tool to mathematically represent CO2 and H2 isotherms on activated carbon. The easy way of preparation and high capture abilities endow this kind of activated carbon attractive as a promising adsorbent for CO2 and H2 storage.

Thermodynamic characteristics of CO2 adsorption on β-cyclodextrin based porous materials: Equilibrium capacity function with four variables

This work focuses on establishing a kind of extended adsorption model equation including four variables relevant to preparation conditions (activation temperature and heating rate) except adsorption situations (adsorption temperature and adsorption pressure). And the equation is used for thermodynamic analysis of CO2 adsorption on a promising cyclodextrin based porous material. Novelty, those isotherms of CO2 adsorption on the porous materials prepared at different activation temperatures and heating rates are experimentally obtained to investigate the effects of activation temperature and heating rate on CO2 adsorption. Then the parameters of the model equation are quantified based on 589 data points from the isotherms by the least squares regression method combined with the steepest descent mode. Research results indicate that high performance for CO2 uptake appears on the porous material prepared at high activation temperature and low heating rate. Differently with the previous studies, the adsorption site energy distribution is found to exhibit an asymmetrical peak and the peak site energy increases with an increase of activation temperature but the decrease of heating rate. Moreover, a series of interesting phenomena of those thermodynamic state parameters via adsorption temperature and pressure are verified based on both extended Slip model and D-R model, which have not been reported before.

Low-cost preferential different amine grafted silica spheres adsorbents for DAC CO2 removal

DAC CO2 capture is gaining wide attention as one of the most difficult carbon approaches to tackle climate change. In this work, different pore-size silica spheres were grafted using different amine groups such as APTES, APTMS, and Diamine. Herein, all samples based on the wet and dry grafting method were used for CO2 adsorption isotherm at room temperature and pressure (298 K and 1 bar). The sample based on the wet grafting (Silica-APTES-W) sample shows the highest CO2 uptake 1.67 mmol/g. Also, the adsorption isotherm of the Silica-APTES-W sample was showed a high capacity of CO2 1.2 mmol/g at 25 °C, which describes the strong physical interaction between CO2 and amine. The isosteric adsorption of Silica-APTES-W also confirmed that the physical adsorption was dominant because of low adsorption heat ranging from 23 to 37 kJ/mol. Also, the fixed bed experiment was conducted with 2000 ppm CO2 that obtains the optimal working capacity 4.5 mL/g with the lowest regeneration temperature 90 °C. It was shown that Silica-APTES-W sample was superior performance for DAC CO2 capture in practical applications.

Fabrication and application of novel core–shell MIL-101(Cr)@UiO-66(Zr) nanocrystals for highly selective separation of H2S and CO2

Removal of H2S and CO2 is essential in diverse industrial applications such as natural gas treatment for avoiding their corrosion, toxicity, and environmental damages. To improve selective separation of H2S and CO2 at ambient temperature, we designed novel core–shell-structure MOFs based on a porous MIL-101(Cr) as seeds and a less porous UiO-66(Zr) shell. MU-4 and MU-8 nanocrystals, with different shell growth times, were synthesized and characterized by different techniques. The adsorption isotherms of H2S, CO2, CH4, and N2 on individual and hybrid samples were measured, using a volumetric method in a wide pressure region and at different temperatures. MU nanocrystals exhibited high adsorption capacity for both H2S (20.62 mmol g−1 at 15 bar) and CO2 (17.21 mmol g−1 at 35 bar), elevated up to 84.4 % and 121.8 % compared to pristine UiO-66(Zr). Most importantly, ideal adsorbed solution theory (IAST) showed that adsorption selectivity of H2S/CH4, H2S/N2, CO2/CH4, and CO2/N2 was significantly improved up to 7.8 %, 47.0 %, 32.1 % and 59.9 % at highest pressure compared to pristine MIL-101(Cr). Besides additional open metal sites, this high separation efficiency was ascribed to the size exclusion separation due to optimized pores diameter of MU nanocrystals. According to impressive preferential gas adsorption along with excellent cyclic adsorption–desorption performance and thermal stability, MU nanocrystals can broaden the horizon of H2S and CO2 capture and separation.

Increased CO2 Affinity and Adsorption Selectivity in MOF-801 Fluorinated Analogues.

The novel ZrIV-based perfluorinated metal–organic framework (PF-MOF) [Zr6O4(OH)4(TFS)6] (ZrTFS) was prepared under solvent-free conditions using the commercially available tetrafluorosuccinic acid (H2TFS) as a bridging ditopic linker. Since H2TFS can be seen as the fully aliphatic and perfluorinated C4 analogue of fumaric acid, ZrTFS was found to be isoreticular to zirconium fumarate (MOF-801). The structure of ZrTFS was solved and refined from X-ray powder diffraction data. Despite this analogy, the gas adsorption capacity of ZrTFS is much lower than that of MOF-801; in the former, the presence of bulky fluorine atoms causes a considerable window size reduction. To have PF-MOFs with more accessible porosity, postsynthetic exchange (PSE) reactions on (defective) MOF-801 suspended in H2TFS aqueous solutions were carried out. Despite the different H2TFS concentrations used in the PSE process, the exchanges yielded two mixed-linker materials of similar minimal formulae [Zr6O4(μ3-OH)4(μ1-OH)2.08(H2O)2.08(FUM)4.04(HTFS)1.84] (PF-MOF1) and [Zr6O4(μ3-OH)4(μ1-OH)1.83(H2O)1.83(FUM)4.04(HTFS)2.09] (PF-MOF2) (FUM2– = fumarate), where the perfluorinated linker was found to fully replace the capping acetate in the defective sites of pristine MOF-801. CO2 and N2 adsorption isotherms collected on all samples reveal that both CO2 thermodynamic affinity (isosteric heat of adsorption at zero coverage, Qst) and CO2/N2 adsorption selectivity increase with the amount of incorporated TFS2–, reaching the maximum values of 30 kJ mol–1 and 41 (IAST), respectively, in PF-MOF2. This confirms the beneficial effect coming from the introduction of fluorinated linkers in MOFs on their CO2 adsorption ability. Finally, solid-state density functional theory calculations were carried out to cast light on the structural features and on the thermodynamics of CO2 adsorption in MOF-801 and ZrTFS. Due to the difficulties in modeling a defective MOF, an intermediate structure containing both linkers in the framework was also designed. In this structure, the preferential CO2 adsorption site is the tetrahedral pore in the “UiO-66-like” structure. The extra energy stabilization stems from a hydrogen bond interaction between CO2 and a hydroxyl group on the inorganic cluster.

Single-layer cluster adsorption model and its application for the estimation of the CO2 structure on metal-organic frameworks

The concept of single-layer cluster adsorption and a new scientific understanding of single-layer adsorption by porous and non-porous sorbents are proposed. The model expands the understanding of the physical adsorption, where sorbate-sorbate bonds make a significant contribution to the adsorption energy. The fundamental difference between proposed adsorption picture and the traditional one is the representation of one sorption center as the center of fixation of several molecules for porous systems. An analytical expression for adsorption isotherm have been obtained from equilibrium coefficients expressions of the adsorption processes, expressed via surface coverage fraction by adsorbed molecules. The developed model supplements the adsorption theory. A geometric method is described for the parameters determining of the simplified model. The model is applied to the interpretation of CO2 adsorption isotherms by metal-organic frameworks MOF-177, IRMOF-1, IRMOF-6 and IRMOF-11.

Temperature Effects in Flexible Adsorption Processes for Amorphous Microporous Polymers.

A collection of atomistic molecular simulations is reported that illustrate the impact of adsorption temperature on species uptake and adsorbate-induced structural rearrangement for amorphous polymers of intrinsic microporosity. Temperature-sensitive structural rearrangement is evaluated by contrasting two methods: standard grand canonical Monte Carlo simulations using a rigid framework approximation and a combined Monte Carlo/molecular dynamics approach that fully incorporates framework flexibility. We report single-component gas phase adsorption isotherms for CH4, C2H4, C2H6, C3H6, C3H8, and CO2 across a temperature range of 250-400 K for models of an archetypal polymer of intrinsic microporosity, PIM-1. A quadratic model is presented that captures two main mechanisms of temperature-dependent adsorption-induced deformation of PIM-1 up to a relative swelling of 1.15: thermal expansion and an increased propensity to swell as a function of species uptake. Two case studies are reported that highlight the critical role of operating temperature in industrial storage and separation applications. The first study focuses on methane storage and delivery applications using a pressure-temperature swing adsorption application (PTSA). We demonstrate that larger working capacities are accompanied by increased volumetric strain between adsorption-desorption steps. The second case study considers PIM-1 as an adsorbent to separate an exemplar ternary syngas mixture at operating temperatures ranging 300-550 K. A temperature threshold of ∼400 K is identified, beyond which adsorption-induced PIM-1 swelling is negligible and the solubility selectivity-loading curve transitions to exhibiting a nearly linear relationship.

Insights into the performance of hybrid graphene oxide/MOFs for CO2 capture at process conditions by molecular simulations

Hybridization of metal organic frameworks (MOFs) with graphene oxide (GO) is used to improve the CO2 adsorption performance of MOFs, but the underlying mechanism of this process is still unclear. This study provides a general framework to understanding the mechanism of CO2 adsorption and separation on GO/CuBTC and GO/UTSA-16 in order to optimize the synthesis of the desired material. For this purpose, molecular models mimicking the experimentally available hybrid materials were developed and studied by molecular simulations. Once the models were validated with available experimental data, a systematic study on the effect of different structure variables was performed, searching for the best hybridization procedure for this application, in a predictive manner. It has been confirmed that the interface between GO and MOFs produces strong interactions with CO2, which, together with the smaller pore sizes, significantly enhances the adsorption performance at low pressures. Moreover, the performance of the most promising hybrid GO/MOFs structures from pure CO2 adsorption isotherms for separating CO2 from nitrogen were predicted by GCMC based on binary mixtures (15CO2:85 N2) and a temperature swing adsorption (TSA) process. Among the different materials/compositions explored, GO/CuBTC with the highest GO content (i.e., 65% wt.) and under the premise of no stacking of GO, shows the best results in terms of key performance indicators: CO2/N2 adsorption selectivity (120 at 313 K), working capacity (1.794 mmol/g at a desorption temperature of 443 K), and a specific energy consumption (0.534 GJ/tonne-CO2) comparable to amine scrubbing.

Development of in situ polymerized amines into mesoporous silica for direct air CO2 capture

Direct air CO2 capture (DAC) is inevitable to achieve negative emissions and lower CO2 concentration in the atmosphere. The use of class III supported amine materials has shown great potential as effective materials for CO2 capture from both flue gas and low concentrations of CO2. In this work, a linear polyethylene amine tethered to mesoporous silica foam was synthesized by controlled in situ cationic ring opening polymerization of 2-methyl-2-oxazoline. The sorbent was characterized based on porosity, FTIR, elemental analysis, and surface morphology. The CO2 capture performance at different temperatures was measured using a volumetric method, whereas the dynamic breakthrough analysis (DBA) technique was evaluated using simulated air of 400 ppm CO2 at different temperatures and relative humidities. The CO2 adsorption isotherms were studied in the temperature range from 5 to 80 °C and were fitted using different isotherm models. To evaluate the effects of moisture on the CO2 capture performance, breakthrough experiments were performed using 400 ppm CO2 at various relative humidities (0–65%). It was found that the CO2 uptake calculated from the breakthrough experiment is enhanced at higher moisture contents and decreases at higher temperatures. The maximum CO2 uptake was 1.50 mmol/g at 25 °C and 65% RH. The CO2 adsorption kinetics of the sorbent was found to follow the Avrami model, whereas the dual site Langmuir model was found to be the best fit for the adsorption isotherms. The developed sorbent exhibited a stable cyclic performance and retained its initial CO2 uptake during 10 consecutive cycles. This study demonstrates that in situ polymerization of amines into porous supports is a viable route for designing sorbents with high CO2 adsorption performance for DAC applications.