Isosteric Heat Of CO2 Adsorption Research Articles

Template-directed fabrication of MIL-101(Cr)/mesoporous silica composite: Layer-packed structure and enhanced performance for CO2 capture

A novel hybrid material constituted of MIL-101(Cr) and mesoporous silica was successfully assembled through an in-situ hydrothermal method. The MCM−41 with well-ordered mesopores acted as the structure-directing agent, which regulated the growth of MIL-101(Cr) crystals along a certain direction and restricted the expansion of framework. Meanwhile, the hydroxyl groups existed in MCM−41 preferentially coordinated with the Cr3+ centers in MOF, followed by the layer-packed arrangement of MIL-101(Cr) nanocrystals on the surface of matrix. The structural characterizations further revealed that the introduction of MCM−41 could increase the micropore volume and specific surface area. The MIL-101(Cr)@MCM−41 exhibited higher CO2 uptake capacity and adsorption rate compared with the original MIL-101(Cr) at 298 K and 1 bar. Via ideal adsorbed solution theory (IAST), it could be further predicted that the composite was more inclined to adsorb CO2 than N2. The calculated isosteric heats of CO2 adsorption and desorption activation energy demonstrated that the interaction between CO2 molecules and the composite was also enhanced. The as-prepared MIL-101(Cr)@MCM−41 showed good reusability and could be easily regenerated without any reduction in its CO2 adsorption capacity. Hence, this study opened up a new pathway for designing hierarchical porous structured MOF-based materials with advanced gas separation performance.

Synthesis and CO2 adsorption properties of hydrotalcite-like compounds prepared from aluminum saline slag wastes

This study describes the synthesis of hydrotalcite-like materials by the co-precipitation method using aluminum extracted from saline slags wastes. Saline slags were chemically treated with 2 mol/dm3 aqueous solutions of NaOH for 2 h using a reflux system. The aluminum aqueous solutions were used as precursors with cobalt, magnesium and nickel nitrates, and Na2CO3, to obtain hydrotalcite-like materials with two mole M2+/Al3+ ratios, 2:1 and 4:1. The resulting solids were characterized by X-ray diffraction, termogravimetric analyses, nitrogen adsorption at −196 °C and scanning electron microscopy. After thermal treatment at 200 °C, the CO2 adsorption at 50, 100 and 200 °C was evaluated under dry conditions. The results showed a remarkable sorption capacity of 5.26 mmol/g at a pressure of 80 kPa and at a temperature of 50 °C for the MgAl-2:1 sample, with sorption capacities considerably higher than those reported in the literature for hydrotalcites under similar conditions. The Henry’s law constants were obtained directly from the adsorption isotherms at low pressures and the values found are between 0.01 and 4.20 mmol/kPa·g. The isosteric heats of CO2 adsorption found, using the Clausius-Clapeyron equation, were in the range of 5.2 to 16.8 kJ/mol.

Controlled synthesis of conjugated polycarbazole polymers via structure tuning for gas storage and separation applications

A series of conjugated microporous polymers (CMPs) based on 1,3,6,8-tetrabromocarbazole (N4CMP-1–5) is synthesized via Suzuki cross-coupling or Sonogashira polycondensation. The porosity properties and surface area of these polymer networks can be finely tuned by using a linker with different geometries or strut length. These polymers show the Brunauer-Emmett-Tellerthe (BET) surface areas ranging from 592 to 1426 m2 g−1. The dominant pore sizes of the polymers on the basis of the different linker are located between 0.36 and 0.61 nm. Gas uptake increases with BET surface area and micropore volume, N4CMP-3 polymer can capture CO2 with a capacity of 3.62 mmol g−1 (1.05 bar and 273 K) among the obtained polymers. All of the polymers show high isosteric heats of CO2 adsorption (25.5–35.1 kJ mol−1), and from single component adsorption isotherms, IAST-derived ideal CO2/N2 (28.7–53.8), CO2/CH4 (4.6–5.2) and CH4/N2 (5.7–10.5) selectivity. Furthermore, N4CMPs exhibit the high CO2 adsorption capacity of 542–800 mg g−1 at 318 K and 50 bar pressure. These data indicate that these materials are a promising potential for clean energy application and environmental field.

Populus wood biomass-derived graphene for high CO2 capture at atmospheric pressure and estimated cost of production

Abstract In the present study, populus wood biomass (PWB), as a precursor, was first carbonized under a nitrogen atmosphere and the obtained carbon was used to prepare graphene via chemical activation using KOH. The graphene samples were used to study as adsorbents for CO2 capture. The effect of different parameters, such as various KOH/C weight ratios (2:1–4:1 g/g), different heating temperatures (750–950 °C) and heating time (30–90 min), on the surface characteristics and CO2 uptake capacity of the as-synthesized graphene was investigated. Various characterization techniques including elemental, TGA, BET, TEM, FT-IR and Raman were used. Under the experimental conditions of KOH/C impregnation ratio of 3:1, heating temperature of 850 °C and heating time of 60 min, the graphene sample showed a BET surface area of 1317.1 m2/g, total pore volume of 0.604 m3/g, up to 94% microporocity, and an average micropore diameter size of 1.84 nm. The measured uptake capacity of 7.2 mmol/g at a pressure of 1 bar and 293 K by sample G-3-850-60 was among the highest reported values in the literature data for carbon base materials. The isosteric heat of CO2 adsorption onto the graphene samples were computed from the Clausius–Clapeyron equation, which was consistent with the physical nature of adsorption. Furthermore, these nano-adsorbents exhibited a high CO2 adsorption capacity, thus proving to be good candidates for cost-effective CO2 capture and storage for the downstream application on an industrial scale. In addition, the results of our laboratory investigations explore a process for scaling-up a complete flow diagram of graphene production from populous wood biomass and an economic evaluation of graphene production is estimated at a cost of about $25 per kg.

Equilibrium and Kinetic Behaviour of CO2 Adsorption onto Zeolites, Carbon Molecular Sieve and Activated Carbons

This study was carried out to characterize CO2 adsorption equilibrium and kinetics of commercial adsorbents that have potential for use in the pressure swing adsorption (PSA) process and also to provide a better understanding of CO2 adsorption behaviour under wide ranges of operating conditions. A comprehensive set of data and the analysis for CO2 adsorption equilibrium and kinetics are presented for six commercial adsorbents, i.e. zeolite 13x, zeolite 5A, zeolite 4A, carbon molecular sieve (MSC-3R), and activated carbons (GCA-830 and GCA-1240). The adsorption equilibrium and the kinetic data were taken at a temperature range of 293-333 K and pressure up to 35 atm. The CO2 adsorption isotherm was found to follow typical a type-I isotherm classification according to IUPAC, representing a monolayer adsorption mechanism. Among the six commercial adsorbents tested, activated carbon GCA-1240 offered the highest adsorption capacity, and zeolite 4A provided the lowest capacity. The obtained isotherm data were correlated as a function of temperature and pressure to fit with different model equations (i.e. Langmuir, Toth, Sips, and Prausnitz). The Sips model showed the best fit with the equilibrium data for zeolite 13X, zeolte 5A, zeolite 4A, and carbon molecular sieve (MSC-3R) while the Prausnitz model provided an excellent fit with the data for activated carbons (GCA-830 and GCA-1240). The isosteric heat of CO2 adsorption was also estimated for individual adsorbents according to the Clausius-Clapeyron equation. The CO2 adsorption kinetic, presented in terms of mass transfer coefficients (k), were analyzed from the plots of CO2 uptake rate using the well-recognized linear driving force (LDF) model. The k values were correlated by non-linear regression to reveal effects of adsorption temperature and pressure. Activation energies of CO2 adsorption on the individual adsorbents were also calculated and correlated according to the Arrhenius equation.

Synthesis of Hierarchically Structured Hybrid Materials by Controlled Self-Assembly of Metal-Organic Framework with Mesoporous Silica for CO2 Adsorption.

The HKUST-1@SBA-15 composites with hierarchical pore structure were constructed by in situ self-assembly of metal-organic framework (MOF) with mesoporous silica. The structure directing role of SBA-15 had an obvious impact on the growth of MOF crystals, which in turn affected the morphologies and structural properties of the composites. The pristine HKUST-1 and the composites with different content of SBA-15 were characterized by XRD, N2 adsorption-desorption, SEM, TEM, FT-IR, TG, XPS, and CO2-TPD techniques. It was found that the composites were assembled by oriented growth of MOF nanocrystals on the surfaces of SBA-15 matrix. The interactions between surface silanol groups and metal centers induced structural changes and resulted in the increases in surface areas as well as micropore volumes of hybrid materials. Besides, the additional constraints from SBA-15 also restrained the expansion of HKUST-1, contributing to their smaller crystal sizes in the composites. The adsorption isotherms of CO2 on the materials were measured and applied to calculate the isosteric heats of adsorption. The HS-1 composite exhibited an increase of 15.9% in CO2 uptake capacity compared with that of HKUST-1. Moreover, its higher isosteric heats of CO2 adsorption indicated the stronger interactions between the surfaces and CO2 molecules. The adsorption rate of the composite was also improved due to the introduction of mesopores. Ten cycles of CO2 adsorption-desorption experiments implied that the HS-1 had excellent reversibility of CO2 adsorption. This study was intended to provide the possibility of assembling new composites with tailored properties based on MOF and mesoporous silica to satisfy the requirements of various applications.

Structure–Thermodynamic‐Property Relationships in Cyanovinyl‐Based Microporous Polymer Networks for the Future Design of Advanced Carbon Capture Materials

Nitrogen‐rich solid absorbents, which have been immensely tested for carbon dioxide capture, seem until this date to be without decisive molecular engineering or design rules. Here, a family of cyanovinylene‐based microporous polymers synthesized under metal‐catalyzed conditions is reported as a promising candidate for advanced carbon capture materials. These networks reveal that isosteric heats of CO2 adsorption are directly proportional to the amount of their functional group. Motivated by this finding, polymers produced under base‐catalyzed conditions with tailored quantities of cyanovinyl content confirm the systematical tuning of their sorption enthalpies to reach 40 kJ mol−1. This value is among the highest reported to date in carbonaceous networks undergoing physisorption. A six‐point‐plot reveals that the structure–thermodynamic‐property relationship is linearly proportional and can thus be perfectly fitted to tailor‐made values prior to experimental measurements. Dynamic simulations show a bowl‐shaped region within which CO2 is able to sit and interact with its conjugated surrounding, while theoretical calculations confirm the increase of binding sites with the increase of PhCC(CN)Ph functionality in a network. This concept presents a distinct method for the future design of carbon dioxide capturing materials.

Thermodynamics and regeneration studies of CO2 adsorption on activated carbon

AbstractThe kinetic and thermodynamic studies of CO2 adsorption on activated carbon were carried out in a fixed bed. Six different CO2 concentrations (from 5% up to 18 vol.%) and four temperatures (303 K, 313 K, 323 K, and 333 K) were applied to investigate dynamic adsorption behaviors. It showed that the increase in temperature led to faster adsorption kinetics, but also reduced the adsorption capacity. An isotherms modeling was made by the Langmuir isotherm model. The experimental isotherms data were well fitted. Then, based on the adsorption isotherms, the isosteric heats of CO2 adsorption were determined by Clausius‐Clapeyron equation. The calculated isosteric heat declined slightly with the adsorbed amount on activated carbon, and had an average value of 16.09 kJ/mol, which is much lower than the actual heat of adsorption measured by calorimeter. Moreover, regeneration experiments by vacuum evacuation (<10 kPa) were conducted to assess the regeneration efficiency (RE) of activated carbon. Regeneration efficiency increased with the reduction of vacuum pressure and reached over 90% at the vacuum of 4 kPa. A temperature swing integrated into the vacuum evacuation process resulted in a remarkable increase of RE with an increase in regeneration temperature, which confirmed the favorable effect of temperature on regeneration of activated carbon and providing almost full regeneration. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

Ultrafast room temperature synthesis of GrO@HKUST-1 composites with high CO2 adsorption capacity and CO2/N2 adsorption selectivity

An ultrafast synthesis method is developed for the preparation of composites of graphite oxide and HKUST-1, GrO@HKUST-1. Fast synthesis of GrO@HKUST-1 composites can be quickly achieved at room temperature within 1min. The synthesized composites were characterized by XRD, SEM, N2 adsorption, FTIR and TGA. The isotherms of CO2 and N2 on the as-synthesized materials were measured and the isosteric heats of CO2 adsorption were estimated. The CO2/N2 adsorption selectivities were predicted by means of ideal adsorbed solution theory (IAST). Results show that the GrO@HKUST-1 composites have higher BET surface area and pore volume than the parent HKUST-1. The CO2 adsorption capacity of 2GrO@HKUST-1 (2% of GrO) is up to 9.02mmol/g at 1bar and 273K, giving an increase of 32% in comparison of the parent HKUST-1. The isosteric heat of CO2 adsorption on 2GrO@HKUST-1 is slightly higher than that of HKUST-1, suggesting the stronger interaction between CO2 molecules and 2GrO@HKUST-1. The CO2/N2 adsorption selectivity of 2GrO@HKUST-1 is significantly enhanced over pristine HKUST-1. At 1bar, the CO2/N2 adsorption selectivity of 2GrO@HKUST-1 is up to 186, while that of parent HKUST-1 is only 103. This rapid room temperature synthesis route is promising for new MOF-based composites.

Adsorption behavior of carbon dioxide and methane in bituminous coal: A molecular simulation study

The adsorption behavior of CO2, CH4 and their mixtures in bituminous coal was investigated in this study. First, a bituminous coal model was built through molecular dynamic (MD) simulations, and it was confirmed to be reasonable by comparing the simulated results with the experimental data. Grand Canonical Monte Carlo (GCMC) simulations were then carried out to investigate the single and binary component adsorption of CO2 and CH4 with the built bituminous coal model. For the single component adsorption, the isosteric heat of CO2 adsorption is greater than that of CH4 adsorption. CO2 also exhibits stronger electrostatic interactions with the heteroatom groups in the bituminous coal model compared with CH4, which can account for the larger adsorption capacity of CO2 in the bituminous coal model. In the case of binary adsorption of CO2 and CH4 mixtures, CO2 exhibits the preferential adsorption compared with CH4 under the studied conditions. The adsorption selectivity of CO2 exhibited obvious change with increasing pressure. At lower pressure, the adsorption selectivity of CO2 shows a rapid decrease with increasing the temperature, whereas it becomes insensitive to temperature at higher pressure. Additionally, the adsorption selectivity of CO2 decreases gradually with the increase of the bulk CO2 mole fraction and the depth of CO2 injection site.

Indolo[3,2-b]carbazole-containing hypercrosslinked microporous polymer networks for gas storage and separation

Two indolo[3,2-b]carbazole-containing hypercrosslinked microporous organic polymers have been successfully synthesized via Friedel-Crafts alkylation reaction in the presence of anhydrous FeCl3. It was found that the polymer InCz-HCP2 produced from the monomer with more reaction active sites shows a higher Brunauer-Emmet-Teller specific surface area up to 1421 m2 g−1 than InCz-HCP1. Both polymers show high isosteric heats of CO2 adsorption over 27 kJ mol−1 because the incorporation of indolo[3,2-b]carbazole unit into the skeleton of the polymers enhanced the binding affinity between the pore wall of the polymer and CO2 molecules. InCz-HCP2 shows a hydrogen uptake ability of 1.68 wt% (77.3 K/1.13 bar) and a carbon dioxide capture capacity of 3.58 mmol g−1 at 273 K/1.13 bar with a good CO2/N2 selectivity of 29:1 at 273 K. With those adsorption properties, these materials could be considered as potential candidates for gas storage and separation.

Enhanced CO2 adsorption on Al-MIL-53 by introducing hydroxyl groups into the framework

Substitution of 25% of terephthalic acid with 2,5-dihydroxyl terephthalic acid in Al-MIL-53 enhances the isosteric heat of CO2 adsorption remarkably, obtaining a 5.5 wt% CO2 uptake at 298 K and 0.2 bar.

Experimental and numerical study of CO2 adsorption on copper benzene-1,3,5-tricarboxylate (Cu-BTC) metal organic framework

The amount and isosteric heat of CO2 adsorption in copper benzene-1,3,5-tricarboxylate (Cu-BTC) metal organic framework are synchronously experimentally studied with the combined PCTProE&E and Calvet Calorimeter under the ambient temperature of 35°C and pressure range 0–1200kPa. A grand canonical Monte Carlo numerical model is proposed to predict the CO2 adsorption amount combined with isosteric heat. The numerical model is validated with experimental data, and the obtained adsorption snapshots can provide a deep insight for the adsorption structure at molecular level. The CO2 adsorption amount and heat have three contributions: the Lennard-Jones potential, electrostatic interactions between CO2 and Cu-BTC, and electrostatic interactions between CO2 and CO2. The contribution rate ranges for the adsorption amount of the three mechanisms are 48.45–69.66%, 28.43–48.41%, and 1.91–3.14%, respectively. The third contribution can be ignored. The L-J potential controls the adsorbed CO2 molecules in the tetrahedron-shaped pockets of Cu-BTC, whereas the electrostatic interactions between CO2 and Cu-BTC control the adsorbed CO2 molecules in the larger square-shaped channels of Cu-BTC.

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.

Modeling the performances of a CO2 adsorbent based on polyethylenimine-functionalized macro-/mesoporous silica monoliths

An adsorbent based on polyethylene oxide-templated silica monoliths with hierarchical uniform porosity was functionalized with polyethylenimine and was used as a substrate for CO2 adsorption. Such material, denoted PEI-MonoSil, was characterized by means of thermogravimetric analysis and N2 adsorption/desorption at 77 K, in order to prove that polymer chains efficiently filled the pores of functionalized samples. CO2 adsorption isotherms on PEI-MonoSil were evaluated at T = 298, 313, 333, and 348 K for pressures up to 100 kPa by means of a volumetric technique. CO2 adsorption data revealed a meaningful dependence of the CO2 adsorption capacity on temperature, with the highest capacity registered at 348 K. CO2 adsorption on PEI-MonoSil was convincingly modeled by means of the Toth isotherm. The comparison between the results obtained in the present work and those relative to CO2 adsorption on other nanoporous substrates allowed to highlight a notable affinity of PEI-MonoSil towards CO2 and a high level of heterogeneity in the adsorbent/adsorbate interaction. Eventually, the modeling effort also allowed to evaluate the isosteric heat of CO2 adsorption as a function of the fractional coverage of PEI-MonoSil, showing values that are comparable with experimentally estimated values of the enthalpy of reaction between CO2 and monoethanolamine solutions.

Strong binding site molarity of MOFs and its effect on CO2 adsorption

Metal organic frameworks (MOFs) with strong binding sites usually have remarkable CO2 adsorption capacity at ambient conditions. A new material parameter, the strong binding site molarity, defined as the number of the strong binding sites divided by the molecular weight of the unit cell of MOF, is introduced to understand CO2 adsorption on these MOFs. The CO2 adsorption capacity can be correlated to the strong binding site molarity in 1:1 ratio. The strong binding site coverage by CO2 molecules can also be correlated to the isosteric heat of CO2 adsorption at a given pressure. The strong binding site molarity is used to predict CO2 adsorption amount of several new types of MOFs to show its uses in selection or design of MOFs for CO2 capture.

Bifunctionalized conjugated microporous polymers for carbon dioxide capture

A novel kind of bifunctionalized conjugated microporous polymers containing sultone and hydroxyl groups has been synthesized via palladium-catalyzed Sonogashira–Hagihara cross-coupling reaction of bromophenol blue with 1,4-diethynylbenzene or 1,3,5-triethynylbenzene. The resulting polymers show high specific surface area up to 1470 m2 g−1 and good thermal stability. BFCMP-2 exhibits a hydrogen uptake ability of 156 cm3 g−1 (∼1.39 wt%) at 77 K/1.13 bar and a carbon dioxide uptake capacity of 2.77 mmol g−1 at 273 K/1.13 bar. Compared with most other reported non-functionalized conjugated microporous polymers, both of the bifunctionalized polymer networks show relatively high isosteric heat of CO2 adsorption (25 kJ mol−1) due to the introduction of the polar functional groups of sultone and hydroxy enhanced the binding affinity between the polymer networks and CO2 molecules. The results demonstrate that the introduction of strong polar groups into a polymer skeleton is an efficient strategy to produce CO2-philic microporous organic polymers with enhanced binding affinity with CO2 molecules.

Synthetic Control of Pore Properties in Conjugated Microporous Polymers Based on Carbazole Building Blocks

A series of conjugated microporous polymers (CMPs) from 1,3,6,8‐tetrabromocarbazole and its alkylated derivative is synthesized via Suzuki cross‐coupling polycondensation. These polymer networks are stable in common organic solvents and thermally stable. The pore properties (pore size and surface area) of this kind of CMPs can be tuned by using either a linker with different geometries or carbazole substituted with alkyl groups. All of the polymers show high isosteric heats of CO2 adsorption (27.1–30.8 kJ mol−1) because the incorporation of nitrogen atoms into the skeleton of the CMP enhances the interaction between the pore wall and CO2 molecules. The polymer PPTBC shows a high Brunauer–Emmett–Teller specific surface area up to 917 m2 g−1 with a high CO2 uptake ability of 2.93 mmol g−1 at 1.13 bar/273 K. These data show that these materials are potential candidates for applications in post‐combustion CO2 capture and sequestration technology. image

Molecular basis for the high CO2 adsorption capacity of chabazite zeolites.

CO2 adsorption in Li-, Na-, K-CHA (Si/Al=6,=12), and silica chabazite zeolites was investigated by powder diffraction. Two CO2 adsorption sites were found in all chabazites with CO2 locating in the 8-membered ring (8MR) pore opening being the dominant site. Electric quadrupole-electric field gradient and dispersion interactions drive CO2 adsorption at the middle of the 8 MRs, while CO2 polarization due to interaction with cation sites controls the secondary CO2 site. In Si-CHA, adsorption is dominated by dispersion interactions with CO2 observed on the pore walls and in 8 MRs. CO2 adsorption complexes on dual cation sites were observed on K-CHA, important for K-CHA-6 samples due to a higher probability of two K(+) cations bridging CO2. Trends in isosteric heats of CO2 adsorption based on cation type and concentration can be correlated with adsorption sites and CO2 quantity. A decrease in the hardness of metal cations results in a decrease in the direct interaction of these cations with CO2.

Comprehensive study of carbon dioxide adsorption in the metal–organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Analysis of the CO2 adsorption properties of a well-known series of metal–organic frameworks M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts.