CO2 Adsorption Measurements Research Articles

Study of micropores accessibility in coals and activated carbon using immersion heats with C1–C4 alkanols

Two natural coals and a sample of activated carbon were studied with respect to micropores accessibility for C1–C4 alkanols using immersion calorimetry. From the immersion enthalpies, micropores volumes were calculated according to the Stoeckli–Kraehenbuehl (SK) equation, and obtained values were compared with the surface characteristics of gas adsorption measurements. Validity of the SK equation was found to be problematic for natural coals giving micropores volumes up to three times exceeding those of the CO2 adsorption measurements. Reason for such discrepancy is discussed, with specific interactions between coals and alkanols likewise marked swelling of coals in the presence of alkanols being recognised as possible explanation. A drop in immersion heats with branched butanols was generally observed, enabling at least semi-quantitative evaluation of the abundance of micropores with diameters below ca. 0.7 nm that are inaccessible for tert-butanol.

The Role of Multiwall Carbon Nanotubes in Cu‐BTC Metal‐Organic Frameworks for CO2 Adsorption

The discovery of natural gas fields with a high content of CO2 in world gas reservoirs poses new challenges for CO2 capture. This work investigates the use of the metal‐organic framework (MOF) Cu‐BTC and hybrid MWCNTs@Cu‐BTC for CO2 adsorption. Cu‐BTC and hybrid MWCNTs@Cu‐BTC were synthesized by the solvothermal method. The results of imaging of intact MOF pores in Cu‐BTC and hybrid MWCNTs@Cu‐BTC nanocrystals by high‐resolution transmission electron microscopy (HRTEM) under liquid nitrogen conditions are presented. Physical characterizations of the solid adsorbents were made by using a selection of different techniques, including field‐emission scanning electron microscopy (FESEM), X‐ray powder diffraction (XRD), Fourier transform infrared (FT‐IR) spectroscopy, thermogravimetric analysis (TGA), Brunauer–Emmet–Teller (BET) surface area, and CO2 adsorption and physisorption measurements. HRTEM and FESEM confirmed that Cu‐BTC has an octahedral shape and that the surface morphology of Cu‐BTC changes by the intercalation of MWCTNs. The results show that the modified Cu‐BTC improved the CO2 adsorption compared to pure Cu‐BTC. The increase in the CO2 uptake capabilities of hybrid MWCNTs@Cu‐BTC was ascribed to the intercalation of MWCNTs with Cu‐BTC crystals. The CO2 sorption capacities of Cu‐BTC and hybrid MWCNTs@Cu‐BTC were found to increase from 1.91701 to 3.25642 mmol/g at ambient conditions.

IAST predictions vs co-adsorption measurements for CO2 capture and separation on MIL-100 (Fe)

Metal–organic frameworks (MOFs) are considered since last decade as promising materials notably for the development of gas separation and purification by adsorption processes. Selectivity and working capacity are important parameters for adsorbent selection. Unfortunately, these parameters are mostly estimated from pure component isotherms without comparison with mixture experimental data because these measurements are time consuming and complicated. The aim of this study is to compare IAST mixture simulations from pure component isotherms with co-adsorption measurements considering promising industrial adsorption processes for a mesoporous MOF, MIL-100 (Fe). This adsorbent has a good selectivity for CO2 over permanent gases. Three adsorption processes have been studied (i) CO2 capture: a CO2/N2 separation by PSA or mixed membrane process, (ii) biogas and (iii) natural gas purification: a CO2/CH4 separation by PSA process. Pure component isotherms (CO2, CH4 and N2 at 303.15 K) and binary mixture equilibrium measurements (CO2/N2 at 1 and 4 bar and 303.15 K and CO2/CH4 at 303.15 K and 1 and 4 bar for biogas and 30 bar for natural gas) have been realized. A particular attention has been paid for the pure component representation in order to obtain IAST predictions as accurate as possible. The pressure ranges and the theoretical models are selected to get the best fitted curves of pure component adsorption measurements. In these conditions, the comparison between experimental and simulated values in terms of adsorbed quantities and selectivities allows us to conclude that in the case of mixtures studied in this work on MIL-100 (Fe), IAST is a suitable model for such applications.

Applying Alkyl-Chain Surface Functionalizations in Mesoporous Inorganic Structures: Their Impact on Gas Flow and Selectivity Depending on Temperature.

Porous inorganic capillary membranes are prepared to serve as model structures for the experimental investigation of the gas transport in functionalized mesopores. The porous structures possess a mean pore diameter of 23 nm which is slightly reduced to 20 nm after immobilizing C16-alkyl chains on the surface. Gas permeation measurements are performed at temperatures ranging from 0 to 80 °C using Ar, N2, and CO2. Nonfunctionalized structures feature a gas transport according to Knudsen diffusion with regard to gas flow and selectivity. After C16-functionalization, the gas flow is reduced by a factor of 10, and the ideal selectivities deviate from the Knudsen theory. CO2 adsorption measurements show a decrease in total amount of adsorbed gas and isosteric heat of adsorption. It is hypothesized that the immobilized C16-chains sterically influence the gas transport behavior without a contribution from adsorption effects. The reduced gas flow derives from an additional surface resistance caused by the C16-chains spacially limiting the adsorption and desorption directions for gas molecules propagating through the structure, resulting in longer diffusion paths. In agreement, the gas flow is found to correlate with the molecular diameter of the gas species (CO2 < Ar < N2) increasing the resistance for larger molecules. This affects the ideal selectivities with the relation [Formula: see text]. The influence on selectivity increases with increasing temperature which leads to the conclusion that the temperature induced movement of the C16-chains is responsible for the stronger interaction between gas molecules and surface functional groups.

Investigation of methane diffusion in low-rank coals by a multiporous diffusion model

To study the methane diffusion behaviors of low-rank coals (Ro,m of 0.45% and 0.58%), a combination of CO2 adsorption, N2 adsorption/desorption, and mercury intrusion porosimetry (MIP) measurements were used to characterize the pore structure of coals, and a new multiporous diffusion model was established to model methane diffusion under dry and moist conditions during the methane adsorption process. The results indicate that the pore structure of low-rank coal samples exhibits a multimodal pore size/volume distribution and has a greater percentage of microporosity and mesoporosity. The multiporous model provides a better fit than the bidisperse model, which deviates significantly from the data, especially during the initial diffusion stage. Based on the multiporous diffusion model, the macropore diffusivity (10−4∼10−3 s−1) is generally one to three orders of magnitude greater than the mesopore diffusivity (10−5∼10−4 s−1) and micropore diffusivity (10−6∼10−5 s−1). Moreover, both the macropore diffusivity and micropore diffusivity show a decreasing trend with increasing pressure, whereas a strong positive correlation exists between the mesopore diffusivities and pressure, indicating that the effect of pressure on methane diffusion in pores with different sizes is different during the adsorption process. This difference may be due to the competition between the different mechanisms of gaseous methane diffusion and the swelling of the coal matrix caused by gas adsorption. Furthermore, the moisture-reduced methane diffusivities is mainly due to the moisture in the coal matrix that adsorbs to the pore surface and occupies the pore space, as well as changes the pore structure according to the effect of mineral swelling from the adsorbing moisture. Therefore, these results may have a significant implication for understanding the transport mechanism of methane in coals and the design of enhanced CBM recovery.

Exploring aramid as emerging contender for CO2 capture

To prevent CO2 accumulation in the atmosphere generated from scorching of fossil fuels, carbon capture and sequestration (CCS) technology is considered as a potential route to mitigate the emissions of CO2 from reaching the atmosphere. Power generation from sources such as gas, coal and biomass can fulfill the energy demand more readily than many other sources of electricity production. Thus these sources may be retained as important alternative option in the global energy cycle. In order to curtail CO2, porous aramid network was fabricated by the condensation of 1,3,5-benzenetricarbonyl trichloride and 1,3-phenylenediamine in 1,4-dioxane solvent. Aramid was characterized for various analyses including FTIR, XRD, TGA, BET surface area and pore size analysis, FESEM and CO2 adsorption measurements. Excellent thermal stability was provided by strong amide linkages in the polymer backbone. Optimum CO2 uptake of aramid was achieved to be 23.14mg·g−1 at 273K at 0.1 MPa. The basic amide groups of network structure showed greater affinity for CO2. Excellent thermal stability of aramid makes it a promising sorbent for CO2 capture in adverse conditions.

Influence of Co-adsorbates on CO2 induced phase transition in functionalized pillared-layered metal–organic frameworks

Co-adsorption measurements reveal the flexible and gas separation behavior of functionalized pillared-layered MOFs when exposed to gas mixtures.

Nanoporous chalcogenides for adsorption and gas separation.

The adsorption and gas separation properties of amorphous porous chalcogenides such as GeS2 are investigated using statistical mechanics molecular simulation. Using a realistic molecular model of such amorphous adsorbents, we show that they can be used efficiently to separate different gases relevant to environmental and energy applications (H2, CO2, CH4, N2). In addition to shedding light on the microscopic adsorption mechanisms, we show that coadsorption in this novel class of porous materials can be described using the ideal adsorbed solution theory (IAST). Such a simple thermodynamic model, which allows avoiding complex coadsorption measurements, describes the adsorption of mixture from pure component adsorption isotherms. Our results, which are found to be in good agreement with available experimental data, paves the way for the design of gas separation membranes using the large family of porous chalcogenides.

Aramid as potential solid Sorbent for CO2 capture

The escalating level of green house gases and in particular CO2 emissions is an issue of major concern because of its link to potential climate change. CO2 capture and sequestration (CCS) is a promising strategy to control CO2 emission in the atmosphere. In sorption-based CO2 separation technologies, sorbent selection is very crucial. In order to mitigate the CO2 emissions, solid aramid sorbent was synthesized by a facile one pot polyamidation reaction between melamine and terephthaloyl chloride in 1,4-dioxane as the reaction medium. Aramid was probed using Fourier transform Infrared spectroscopy (FTIR), X-ray diffraction, thermogravimetric analysis, Brunauer-Emmett-Teller surface area and pore size analysis, field emission scanning electron microscopy and CO2 adsorption measurements. CO2 adsorption capacity of aramid was determined at temperatures of 273 and 298 K at a pressure of 1 bar. The polymeric material displayed the highest CO2 uptake equal to 2.01 cm3/g at 273 K relative to 1.46 cm3/g at 298 K and at 1 bar respectively. The results revealed that aramid is capable of capturing CO2, thus acting as a promising CO2 scavenger in severe environment.

New High- and Low-Temperature Phase Changes of ZIF-7: Elucidation and Prediction of the Thermodynamics of Transitions.

We have found that the 3D zeolitic imidazolate framework ZIF-7 exhibits far more complex behavior in response to the adsorption of guest molecules and changes in temperature than previously thought. We believe that this arises from the existence of different polymorphs and different types of adsorption sites. We report that ZIF-7 undergoes a displacive, nondestructive phase change upon heating to above ∼700 °C in vacuum, or to ∼500 °C in CO2 or N2. This is the first example of a temperature-driven phase change in 3D ZIF frameworks. We predicted the occurrence of the high-temperature transition on the basis of thermodynamic arguments and analyses of the solid free-energy differences obtained from CO2 and n-butane adsorption isotherms. In addition, we found that ZIF-7 exhibits complex behavior in response to the adsorption of CO2 manifesting in double transitions on adsorption isotherms and a doubling of the adsorption capacity. We report adsorption microcalorimetry, molecular simulations, and detailed XRD investigations of the changes in the crystal structure of ZIF-7. Our results highlight mechanistic details of the phase transitions in ZIF-7 that are driven by adsorption of guest molecules at low temperature and by entropic effects at high temperature. We derived a phase diagram of CO2 in ZIF-7, which exhibits surprisingly complex re-entrant behavior and agrees with our CO2 adsorption measurements over a wide range of temperatures and pressures. We predicted phase diagrams of CH4, C3H6, and C4H10. Finally, we modeled the temperature-induced transition in ZIF-7 using molecular dynamics simulations in the isobaric-isothermal ensemble, confirming our thermodynamic arguments.

Enhancement in CO2 adsorption capacity and selectivity in the chalcogenide aerogel CuSb2S4 by post-synthetic modification with LiCl

The new chalcogel CuSb2S4 was obtained by reacting Cu(OAc)2·H2O with KSbS2 in a water/formamide mixture at room temperature. In order to modify the gas adsorption capacity the synthesized CuSb2S4 aerogel was loaded with different amounts of LiCl. CO2 adsorption measurements on the CuSb2S4 aerogel before and after treatment with LiCl showed more than three times increased uptake of the LiCl-modified chalcogel. The selectivities of the gas pairs CO2/H2 and CO2/CH4 in the LiCl-treated chalcogel are 235 and 105 respectively and amongst the highest reported for chalcogenide-based aerogels. In comparison with other porous materials like zeolites, activated carbon and most of the Metal Organic Frameworks (MOFs) or Porous Organic Frameworks (POFs), our synthesized aerogels show good air and moisture stability. Although, the CO2 storage capacity of our aerogels is relatively low, however the selectivity of CO2 over H2 or CH4 in LiCl-loaded aerogels are higher than in zeolites, activated carbon as well as some MOFs like Cu-BTC and MOF-5 etc.

Adsorption equilibrium of carbon dioxide on ammonia-modified activated carbon

The equilibrium adsorption isotherms of carbon dioxide on a commercial granular activated carbon (GAC) and an ammonia-modified GAC (OXA-GAC) were measured using a static volumetric method. CO2 adsorption measurements were performed at three different temperatures (303, 318, and 333K), and pressures up to 1atm. The obtained equilibrium data were fitted to the Freundlich, Sips, and Toth isotherms using a semi-empirical approach to differentiate the contributions of physical and chemical adsorption to the total CO2 uptake. The isotherm parameters were determined independently for each mechanism by non-linear regression. To evaluate the adequacy of the fit of the isotherm models, two different error functions (i.e., the average relative error and the nonlinear regression coefficient) were calculated. The Toth semi-empirical equilibrium model provided the best fit to the experimental data, with average relative errors of less than 3% observed at all temperatures. The isosteric heats of CO2 adsorption onto the ammonia-modified adsorbent and onto the untreated adsorbent were determined using the Clausius–Clapeyron equation. The loading dependence of the isosteric enthalpy of CO2 adsorption over the OXA-GAC reflected an energetic heterogeneity of the adsorbent surface. The initial isosteric heats of adsorption of 70.5kJmol−1 and 25.5kJmol−1 correspond to the adsorption of CO2 on the modified and untreated adsorbents, respectively, and these values were in excellent agreement with the zero-coverage heats of adsorption obtained using the temperature-dependent parameters of the proposed model.

Selective adsorption of CO2 on a regenerable amine-bentonite hybrid adsorbent

The ability of magnesium form of bentonite modified by monoethanolammonium cations (MEA+-Mg-bentonite) to reversibly and selectively adsorb CO2 over other light gases, such as CH4 and N2 has gravimetrically been investigated using a Magnetic Suspension Balance (MSB) equipment. Before CO2 adsorption measurements, untreated bentonite and MEA+-Mg-bentonite were characterized by XRD, FTIR, TGA and BET techniques to establish their structural characteristics, thermal stability and specific surface area and porosity characteristics. The results showed that MEA+-Mg-bentonite adsorbed higher CO2 amount compared to untreated bentonite under similar conditions. The equilibrium isotherms for CO2 adsorption on untreated bentonite and MEA+-Mg-bentonite were measured at 25, 50 and 75°C and a pressure range of 01.0bar. It was found that MEA+-Mg-bentonite has static CO2 adsorption capacities of 0.79, 0.69 and 0.56mmol/g, compared to 0.45, 0.36 and 0.30mmol/g for untreated bentonite at temperatures of 25, 50 and 75°C and a pressure of 1bar, respectively. The calculated values of the thermodynamic parameters revealed that the process involving CO2 adsorption on MEA+-Mg-bentonite adsorbent was a physical, spontaneous and exothermic. Moreover, the results showed moderate and slightly high equilibrium selectivities for CO2 over CH4 on MEA+-Mg-bentonite with the values of 1.36, 4.85 and 3.43 at 25, 50 and 75°C, respectively. However, MEA+-Mg-bentonite showed equilibrium selectivity for CO2 over N2 as high as 13.79, 9.91 and 6.32 at 25, 50 and 75°C, respectively. Regenerative ability in short runs of isothermal CO2 adsorption–desorption cycles revealed that MEA+-Mg-bentonite adsorbent was able to undergo several CO2 adsorption–desorption cycles without noticeable decrease in its adsorption capacity. Clearly, amine-bentonite hybrid adsorbents are promising inexpensive materials for CO2 capture.

Production of α,β-unsaturated esters via Knoevenagel condensation of buthyraldehyde and ethyl cyanoacetate over amine-containing carbon catalyst

Amine-containing carbon catalysts were synthesized to study the production of α,β-unsaturated esters by Knoevenagel condensation of buthyraldehyde and ethyl cyanoacetate. The solids were characterized by XRD, XPS, FTIR, elemental analyses, CO2 adsorption measurements and N2 adsorption-desorption isotherms. Monoethanolamine-functionalization on the carbon led to structural changes while keeping relatively high surface areas. Although a great number of basic sites provided a good performance in the reaction, leaching of these sites at elevated temperatures accounted for deactivation of the catalyst. The best performance towards ethyl 2-cyano-3-propylacrylate ester was obtained for ACN10 (conversion=10.7%; selectivity=100%) at 348K, being reused for five times.

Synthesis and analysis of CO2 adsorbents based on cerium oxide

In order to find materials suited for CO2 capture from exhaust gas, CO2 adsorbents based on single-metal oxide consisting of SiO2, Al2O3, ZrO2 and CeO2 were prepared and their CO2 chemical adsorption amounts were measured using CO2 pulse injection. In the CO2 adsorption measurement, the CeO2-based adsorbents exhibited the largest CO2 adsorption capacity. CO2 temperature programmed desorption (TPD) analysis revealed CO2 adsorbed on CeO2 desorbed mainly from 323K to 473K with its peak at 393K, and relatively small amount of desorption continued to 723K. To study CO2 adsorption and desorption mechanism, CO2 adsorbed on CeO2 and their temperature dependences were analyzed using in situ Fourier transform-infrared (FT-IR) spectroscopy. FT-IR peaks from four CO2 adsorbed species (monodentate carbonate, bidentate carbonate, polydentate carbonate, and hydrogen carbonate species) were identified. Bidentate carbonate and hydrogen carbonate were main species, and the latter desorbed at around 473K, which was the lowest temperature among species observed. CO2 desorption temperature from other species were approximately 573K (bidentate carbonate and monodentate carbonate) and 723K (polydentate), and it was consistent with CO2 TPD analysis. This indicates that the formation of hydrogen carbonate species on CeO2 is a key process to decrease desorption temperature for CO2 capture.

Selective hydrogenation of benzoic acid over Au supported on CeO2 and Ce0.62Zr0.38O2: Formation of benzyl alcohol

The gas-phase hydrogenation of benzoic acid was studied over Au supported on CeO2 and Ce0.62Zr0.38O2 (CZ). HAADF-STEM has established formation of nanoscale (mean=1.5–2nm) Au particles, which is consistent with CO adsorption measurements. Incorporation of Au facilitated partial support reduction during TPR to 573K where the presence of Zr increased oxygen mobility, resulting in a greater degree of Ce4+ reduction (to Ce3+) in Au/CZ, as demonstrated by oxygen storage capacity and XPS measurements. Hydrogenation of an aqueous benzoic acid feed generated benzaldehyde and benzyl alcohol with a higher rate over Au/CZ that is attributed to the action of oxygen vacancies, which activate the carboxyl function for hydrogen attack. A parallel/consecutive kinetic model has been applied to quantify catalytic selectivity. A concerted (single step) conversion is proposed for Au/CeO2 that involves bridging interaction of the benzoate with Ce cations and Au nanoparticles with hydrogen addition. A stepwise conversion on Au/CZ is achieved via a Mars and van Krevelen mechanism with benzoic acid activation at an oxygen vacancy and reaction with surface hydrogen to generate benzaldehyde as a reactive intermediate that is converted to benzyl alcohol via nucleophilic CO attack. Switching from an aqueous to ethanolic feed increased rate due to greater oxygen vacancy availability with higher selectivity to benzaldehyde and appreciable toluene formation over Au/CZ.

Probing the potential of polyester for CO2 capture

Global warming, the major environmental issue confronted by humanity today, is caused by rising level of green house gases. Carbon capture and storage technologies offer potential for tapering CO2 emission in the atmosphere. Adsorption is believed to be a promising technology for CO2 capture. For this purpose, a polyester was synthesized by polycondensation of 1,3,5-benzenetricarbonyl trichloride and cyanuric acid in pyridine and dichloromethane mixture. The polymer was then characterized using FT-IR, TGA, BET surface area and pore size analysis, FESEM and CO2 adsorption measurements. The CO2 adsorption capacities of the polyester were evaluated at a pressure of 1bar and two different temperatures (273 and 298K). The performance of these materials to adsorb CO2 at atmospheric pressure was measured by optimum CO2 uptake of 0.244mmol/g at 273K. The synthesized polyester, therefore, has the potential to be exploited as CO2 adsorbent in pre-combustion capture process.

Effect of solvent on the CO2 capture ability of polyester: A comparative study

A polyester was prepared by condensing 1,3,5-benzenetricarbonyl trichloride with phloroglucinol using two different solvents. Techniques used for characterization include FTIR, XRD, TGA, BET surface area and pore size analysis, FESEM and CO2 adsorption measurements. The highest CO2 uptake of 5.16cm3/g (0.23mmol/g) at 273K and 1bar was achieved by polyester synthesized in DMAc, indicating its superior performance than the polyester prepared using 1,4-dioxane due to the more porous nature and better surface area. Such types of thermally stable polyesters are expected to be the good candidates for CO2 adsorption in harsh environmental conditions.

Oxygen-containing functional group-facilitated CO2 capture by carbide-derived carbons.

A series of carbide-derived carbons (CDCs) with different surface oxygen contents were prepared from TiC powder by chlorination and followed by HNO3 oxidation. The CDCs were characterized systematically by a variety of means such as Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ultimate analysis, energy dispersive spectroscopy, N2 adsorption, and transmission electron microscopy. CO2 adsorption measurements showed that the oxidation process led to an increase in CO2 adsorption capacity of the porous carbons. Structural characterizations indicated that the adsorbability of the CDCs is not directly associated with its microporosity and specific surface area. As evidenced by elemental analysis, X-ray photoelectron spectroscopy, and energy dispersive spectroscopy, the adsorbability of the CDCs has a linear correlation with their surface oxygen content. The adsorption mechanism was studied using quantum chemical calculation. It is found that the introduction of O atoms into the carbon surface facilitates the hydrogen bonding interactions between the carbon surface and CO2 molecules. This new finding demonstrated that not only the basic N-containing groups but also the acidic O-containing groups can enhance the CO2 adsorbability of porous carbon, thus providing a new approach to design porous materials with superior CO2 adsorption capacity.

Fibrous Architectures of Porous Coordination Polymers–Alumina Composites Fabricated by Coordination Replication

Abstract Polycrystalline fibers of porous coordination polymers (PCPs) were synthesized from free-standing alumina nanofiber materials by a coordination replication process. The diameter and morphology of fibrous PCP architectures were controlled by the concentration of aluminum salts and the pH of the reaction solution of replication, respectively. CO2 adsorption measurement proved the presence of hierarchical porosity composed of both micropores and mesopores in PCP fibers.