CO2 Sorption Research Articles

Ligand effects on gas adsorption in nanoporous phthalocyanine crystals.

X-ray diffraction is used to study the sorption of CO and NO in two phthalocyanine nanoporous crystals (PNCs) with 4,4' bipyridine or 4,4' bipyrimidine trans coordinated to open Co2+ sites, demonstrating how the trans coordinated ligands influence the gas sorption properties and structures of the PNCs.

Modulated synthesis of hcp MOFs for preferential CO2 capture.

We herein show that the μ2-OH bridging groups in the double M12O8(OH)14 clusters of hcp UiO-66 could act as a preferential CO2 sorption site, compared to fcu UiO-66. As such, hcp-UiO-66-0.015 shows a high binary CO2 sorption capacity of 0.31 mmol g-1 and CO2/N2 (15/85) mixture selectivity of 87 at 298 K and 1 bar, which holds great potential for post-combustion CO2 capture.

Mg2-xCaxAl layered double hydroxide-derived mixed metal oxide porous hexagonal nanoplatelets for CO2 sorption.

Porous hexagonal nanoplatelets of mixed metal oxide (MMO) derived from the calcination of MgAl layered double hydroxide exhibits a CO2 sorption capacity of 1.99 mmol g-1 at 30 °C, with a retention of 87% sorption capacity over 10 carbonation-decarbonation cycles and a CO2 sorption capacity of 1 mmol g-1 at 200 °C with a 40% increase in capacity over 10 cycles. The high sorption capacity is attributed to the porous nanoplatelet structure of the MMO with a BET surface area of 115 m2 g-1, which enables increased CO2 diffusion. Upon partially replacing magnesium with calcium (33, 50 and 66 mol%), the CO2 sorption capacity of the MMO increases with an increase in temperature. MMO derived from LDH, in which 66% of magnesium is replaced by calcium (MgCaAl-66), delivers CO2 sorption capacities of 1.38, 1.31, 2.50, 4.85 and 7.75 mmol g-1 at 200, 300, 350, 400 and 600 °C, respectively, which is significant for application in the sorption-enhanced water gas shift (SEWGS) process. MgCaAl-66 MMO exhibits a sorption capacity of 1 mmol g-1, which is stable over 10 cycles at 200 °C, and a sorption capacity of 3.68 mmol g-1 at 400 °C with 85% capture efficiency retention over 10 cycles. While the incorporation of Ca2+ serves multiple purposes such as increasing basic defect sorption sites and improving stability to repress the sintering-induced limitation of MMO over sorption cycles, the porous nanoplatelets act as individual sorbent units resisting volume changes through carbonation-decarbonation cycles.

Novel, facile, and scalable synthesis of magnesium based adsorbents via the freeze-drying technique for CO2 capture

Physico-chemical sorption of CO2 over leaf-flake structures obtained via the freeze drying technique.

Structural and sorption characteristics of an aerogel composite material loaded with flufenamic acid: insights from MAS NMR and high-pressure NOESY studies.

The structural and sorption characteristics of a composite material consisting of a silica aerogel loaded with flufenamic acid were investigated using a variety of nuclear magnetic resonance techniques. The composite structure was analyzed using magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, which revealed significant interactions between the aerogel matrix and the FFA molecules. Solid-state 29Si NMR provided insights into the aerogel's stability, while 1H and 13C NMR confirmed the presence of FFA in the matrix, with signals from FFA molecules observed alongside tetraethoxysilane (TEOS) groups. Ethanol-induced desorption of FFA led to narrowed spectral lines, suggesting the breaking of intermolecular hydrogen bonds. 19F MAS NMR spectra indicated changes in FFA local environments upon loading into AG pores. Evaluation of CO2 sorption characteristics using 13C NMR demonstrated a slower sorption rate for AG + FFA than that for pure AG, attributed to decreased pore volume. Furthermore, nuclear Overhauser effect spectroscopy (NOESY) was employed to explore the conformational behavior of FFA within the aerogel matrix. The results indicated a shift in conformer populations, particularly those related to the rotation of one cyclic fragment relative to the other. These findings provide insights into the structural and sorption characteristics of the AG + FFA composite, which are valuable for developing novel drug solid forms.

CO2-actuated spin transition tuning in an interdigitated Hofmann-type coordination polymer.

The increased anthropogenic emission level of CO2 urges the development of CO2-responsive materials, but is it possible to regulate the inherent electronic properties through weak physisorption of a ubiquitous gas such as CO2? Herein, we intended to answer this imperative question by the first case of CO2-actuated variable spin-state stabilisation in an interdigitated Hofmann-type coordination polymer [FeIIPd(CN)4L2] (1, L = methyl isonicotinate), showing a wide shift in transition temperature (T eq) from 178 K at P CO2 = 0 kPa to 229 K at P CO2 = 100 kPa. Interestingly, the emergence of a stepped behaviour in the heating process below P CO2 = 10 kPa and overlapping magnetic susceptibility values above P CO2 = 10 kPa elucidate the selective LS state stabilisation solely correlated with the extent of CO2 accommodation. Based on the magnetic response and phase transition diagrams obtained under respective P CO2 , a plausible scenario of the spin-state switching can be interpreted as (1ls + ) → (1hs + ) → 1hs at P CO2 ≤ 10 kPa, → 1hs at 100 kPa < P CO2 ≥ 32 kPa and → → 1hs at 100 kPa, where 1 and 1' represent CO2-free and CO2-encapsulated states, respectively. The cooperative CO2 sorption with SCO based on the varied CO2 pressure corroborates a novel case for developing CO2-responsive magnetic materials henceforth.

Chemical shaping of CPO-27-M (M = Co, Ni) through an in situ crystallization within chitosan hydrogels.

The preparation of MOF composites is considered as an effective method to address the challenges of shaping MOFs and to create porous solids with enhanced properties and broader applications. In this study, CPO-27-Co was crystallized via a simple strategy within porous chitosan beads. The resulting CS@CPO-27-Co composites were tested for CO2 sorption and they demonstrated promising performances by exceeding 3 mmol(CO2) g-1. The versatility of this strategy was further demonstrated by replacing cobalt(II) ions with nickel(II), also leading to the isostructural CPO-27 framework.

Thermal fluctuation-induced selective CO2 uptake of seemingly nonporous N,C-protected dipeptide crystals as elucidated by in situ X-ray crystallographic analysis

A novel N,C-protected dipeptide deposited nonporous but yet CO2-sorbing molecular crystals, of which the CO2 sorption state was disclosed.

Evaluation of Nitrogen-Doped Adsorbents Based on Reduced Graphene Oxide as Platforms for CO2 Capture

CO2 emissions into the atmosphere have been rapidly rising due to human activities, resulting in the escalation of global warming. To mitigate climate change, it is imperative to develop materials for CO2 capture with high CO2 capacity and low production costs. Herein, we developed a facile method to obtain adsorbents based on reduced graphene oxide (rGO) sheets, NrGO(1 - X)700, where X represents the mass of diethylenetriamine (DETA) (X = 1, 2 and 4 g) used in the preparation. The materials NrGO(1 - 1)700, NrGO (1 - 2)700, and NrGO(1 - 4)700 were obtained from graphene oxide dispersions, followed by DETA impregnation and chemical activation with K2CO3. N2 isotherms demonstrated that the materials simultaneously presented micro and mesopores with similar values of specific surface area (280.16 to 310.32 m2 g-1), pore volume (0.26 to 0.28 cm3 g-1) and pore size (3.78 to 3.80 nm). CO2 sorption experiments revealed that the material NrGO(1 - 4)700, containing the highest amount of pyridinic, graphitic, and amino nitrogen functionalities, showed the best CO2 adsorption capacity. Diffuse reflectance Fourier transformed infrared spectroscopy experiments indicated stronger solid-gas interactions for NrGO(1 - 4)700 than for the other materials.

Fluidized bed chemical looping for CO2 capture and catalytic methanation using dual function materials

CO2 capture from combustion flue gas combined to its catalytic hydrogenation to synthetic methane is considered as a promising technology in the field of Carbon Capture and Utilization (CCU). In this work, the integrated CO2 capture and methanation process was investigated in an innovative chemical looping configuration using dual function materials (DFMs) recirculated alternately between two interconnected bubbling fluidized bed reactors. By physically separating the CO2 capture step and the catalytic hydrogenation reaction in two coupled fluidized bed reactors it is possible to effectively control and independently optimize the operating temperature of each half cycle while running the process continuously. A high-performing Lithium-Ruthenium/Al2O3 was selected to investigate the effect of the specific temperature level for the CO2 capture and the methanation phases in the range 200 - 400 °C, checking the stability and repeatability of the CO2 sorption and catalytic performance over 5 repeated cycles for each operating condition. Subsequently, under the best conditions in terms of methanation performance, a similar Na-promoted dual function material was also tested. The DFMs performance appeared to be quite reproducible over the cycles, but it was subject to kinetic limitations, especially in the case of Na-Ru/Al2O3. Interestingly, the methane yield approached 100 % under the highest tested temperatures for the Li-based DFM. Despite some limitations due to the experimental purge phases of the lab-scale system, the study provides the proof-of-concept of the process which enables the possibility of decoupling the two steps with the aim of a large potential intensification.

Insight into the effect of pressure on the CO2 capture capacity and kinetics by a biochar-ionic liquid composite

Agave bagasse fibers-derived biochar (BCw) impregnated with 1-butyl-3-methylimidazolium acetate ionic liquid (IL) exhibited a considerably CO2 capture capacity at atmospheric (1.47 mmol/g at 25 °C and 1 bar) and moderately high pressure (1.32 mmol/g at 25 °C and 8.5 bar) under dynamic and static systems, respectively. Biochar-IL composites improved the CO2 capture capacity, kinetics, and selectivity (CO2/N2) by 4–90% compared with than of the bulk adsorbents. By increasing the working manometric pressure from 1 to 8.5 bar in a static high-pressure system, the CO2 capture capacity and kinetics of impregnated-biochar improved 2 times. Additionally, the biochar-IL composites were regenerated under a pressure swing adsorption (PSA) arrangement, where more than 70% of the desorption was attributed to the depressurization of the system. The proposed CO2 sorption mechanism suggests gas-solid physisorption and gas-liquid chemisorption at low pressure. At moderately high pressure (≥ 1 bar), the textural properties of the adsorbent play an important role. Moreover, this study provided evidence that the CO2 capture is not only related to the textural properties of the biochar, but also to its surface chemistry. The biochar-IL composite materials reported herein could be an option for CO2 capture at low and moderate pressures, which could reduce operating costs if implemented at large scale for CO2 separation from flue gases under a PSA design.

Performance of Li exchange hierarchical X zeolite for CO2 adsorption and H2 separation

Li exchange hierarchical X zeolite (Li-H-X) was prepared by decationization of X zeolite by NH4Cl solution followed by Li exchange and subsequent calcination. NH4-X and Li-X were prepared to compare the results of Li-H-X. XRD, FE-SEM, EDS, N2 adsorption–desorption, and micro-/mesoporous volume were performed. The higher mesoporosity was confirmed in Li-H-X due to the framework dealumination during decationization. Static and dynamic sorption capacity of sorbents was evaluated to identify the performance of sorbent. The CO2 and H2 equilibrium adsorption capacity of Li-H-X was found to be 9.6 mmol g−1 and 0.78 mmol g−1, respectively, at 298 K and 20 bar, which was 25 % (CO2) and 30 % (H2) higher than Li-X. Static experimental data were validated using the Langmuir, Freundlich, and Sips models. The CO2 & H2 dynamic sorption capacity of Li-H-X sorbent for binary gas (CO2/H2, 25/75 %) was 4.145 mmol g−1 and 0.258 mmol g−1 at 303 K and 10 bar. The higher sorption capacity of Li-H-X was obtained due to large micro-/mesoporous volume of sorbent, which may allow to access unoccupied sites at higher pressure. This result reveals that micro-/mesoporous structure of zeolite adsorbs significantly higher CO2 from binary gas stream, which can use to separate pure H2 from gas stream.

Cold Temperature Direct Air CO2 Capture with Amine-Loaded Metal-Organic Framework Monoliths.

Zeolites, silica-supported amines, and metal-organic frameworks (MOFs) have been demonstrated as promising adsorbents for direct air CO2 capture (DAC), but the shaping and structuring of these materials into sorbent modules for practical processes have been inadequately investigated compared to the extensive research on powder materials. Furthermore, there have been relatively few studies reporting the DAC performance of sorbent contactors under cold, subambient conditions (temperatures below 20 °C). In this work, we demonstrate the successful fabrication of adsorbent monoliths composed of cellulose acetate (CA) and adsorbent particles such as zeolite 13X and MOF MIL-101(Cr) by a 3D printing technique: solution-based additive manufacturing (SBAM). These monoliths feature interpenetrated macroporous polymeric frameworks in which microcrystals of zeolite 13X or MIL-101(Cr) are evenly distributed, highlighting the versatility of SBAM in fabricating monoliths containing sorbents with different particle sizes and density. Branched poly(ethylenimine) (PEI) is successfully loaded into the CA/MIL-101(Cr) monoliths to impart CO2 uptakes of 1.05 mmol gmonolith-1 at -20 °C and 400 ppm of CO2. Kinetic analysis shows that the CO2 sorption kinetics of PEI-loaded MIL-101(Cr) sorbents are not compromised in the monoliths compared to the powder sorbents. Importantly, these monoliths exhibit promising working capacities (0.95 mmol gmonolith-1) over 14 temperature swing cycles with a moderate regeneration temperature of 60 °C. Dynamic breakthrough experiments at 25 °C under dry conditions reveal a CO2 uptake capacity of 0.60 mmol gmonolith-1, which further increases to 1.05 and 1.43 mmol gmonolith-1 at -20 °C under dry and humid (70% relative humidity) conditions, respectively. Our work showcases the successful implementation of SBAM in making DAC sorbent monoliths with notable CO2 capture performance over a wide range of sorption temperatures, suggesting that SBAM can enable the preparation of efficient sorbent contactors in various form factors for other important chemical separations.

Low-Temperature Composite CO2 Sorbents Based on Amine-Containing Compounds

The use of technologies based on combustion of carbon-containing fossil fuel leads to emission of large amounts of CO2, one of the main greenhouse gases, into the atmosphere. To reduce the CO2 level in the atmosphere, systems for CO2 sorption from various gas sources are being developed. The systems allowing the CO2 sorption and desorption at low temperatures (25–200°С) are of most interest. Most frequently, such systems are composite materials consisting of a porous support and a CO2 chemisorbent dispersed on it. Low-volatile amine-containing compounds are the most promising among organic chemisorbents. Classification of the amine-containing sorbents with respect to the preparation procedure is discussed. The procedures include impregnation, covalent grafting, and in situ polymerization on the support surface. The impregnation procedure is simple and cheap in implementation. The sorption characteristics of materials prepared by impregnation depend on the efficiency of the dispersion of the active component, which is determined by the characteristics of the support pore structure, in particular, by the ability of the pore surface for chemical or electrostatic interaction with the supported amine-containing compound. The covalent grafting is based on immobilization of alkoxyaminosilanes on the surface of porous silica materials. The supports for implementing this approach should contain a large amount of silanol groups on the surface and should have the pore size sufficient for the efficient transport of CO2 molecules to amino groups. The main drawback of the grafting method is low thickness of the amine-containing component layers obtained. In situ polymerization is used for preparing materials with high content of grafted functional groups. Provided that the blocking of support pores is excluded in the course of in situ polymerization, materials of this type exhibit the highest sorption capacity for CO2.

Carbon capture from low CO2 concentration stream through ternary alkali metal nitrates promotion of MgO sorbents at intermediate temperature and low space velocity

Many MgO sorbents exhibit high CO2 uptake in the presence of high concentration CO2 and aided by high space velocity, which unfortunately leads to low CO2 capture efficiency from the gas stream. This study investigates the effect of Li/Na/K ratio on sorption performance in dilute CO2 streams (15% CO2) using alkali metal nitrates-promoted MgO sorbents at a low space velocity of 20 ml/g·min in fixed bed reactor. Study shows that the ratio of Li/Na/K in ternary alkali nitrate salt mixture significantly affects the induction periods for the second phase CO2 sorption, which is a key contributor to high CO2 uptake. High LiNO3 content in ternary mixture leads to long induction period, and therefore lower CO2 capacity during the 4 h period. Conversely, high ratios of NaNO3 and KNO3 leads to shorter induction periods. Among the samples, MgO with 9 mol% (Li0.18Na0.52K0.3) exhibits the highest CO2 sorption capacity of 5.56 mmol/g at 240°C in 4 h (20.6% CO2 capture efficiency) and 3.18 mmol/g after 5 cycles (10.8% CO2 capture efficiency). Comparative studies show that optimal temperatures for CO2 sorption are higher when using pure CO2 (280 to 320°C) than when using 15% CO2 (240°C) due to the thermodynamic equilibrium. CO2-TPD results indicate that CO2 desorption proceeds in two phases similar to CO2 sorption, and the presence of alkali salt in MgO also promotes the decomposition of bulk carbonate to CO2. It is shown that molten salt migration and agglomeration of MgO particles are likely the factors that lead to decline in CO2 uptake over the sorption-desorption cycles.

Potassium nickel hexacyanoferrate–polyacrylonitrile composite for removing cobalt, strontium, and cesium from radioactive laundry wastewater

AbstractThe applicability of potassium nickel hexacyanoferrate–polyacrylonitrile (KNiFC–PAN) for the sorption of Co2+, Sr2+, and Cs+ from radioactive laundry wastewater generated in nuclear power plants was investigated. Competitive sorption of Co2+, Sr2+, and Cs+ onto KNiFC–PAN was studied for single, binary, and ternary solutions. The Langmuir, Freundlich, Kargi–Ozmıhci, Koble–Corrigan, and Langmuir–Freundlich models predicted the single‐sorption data (R2 ≥ 0.942, sum of squared error ≤ 0.105). The sorption isotherms were nonlinearly favorable (Freundlich coefficient, NF = 0.288–0.842). According to the Langmuir, Freundlich, Kargi–Ozmıhci, Koble–Corrigan, and Langmuir–Freundlich models, at pH 5 (C0 = 20 mM), KNiFC−PAN exhibited the highest maximum sorption capacity (qmL) for Cs+ among the investigated cations, wherein the primary mechanism was physical sorption. The competition between the metal ions in the binary and ternary systems reduced the respective sorption capacities. Binary and ternary sorption models, such as the ideal adsorbed solution theory (IAST) model coupled with Freundlich (IAST–Freundlich), IAST–Langmuir, and IAST–Langmuir–Freundlich models, were fitted to the experimental data; among these, the IAST–Freundlich model was the most accurate for the binary and ternary systems. The presence of sodium 4‐n‐octylbenzenesulfonate and dodecylbenzene–sulfonic acid sodium salt as anionic surfactants strongly affected the sorption capacity on KNiFC–PAN owing to increased distribution coefficients (Kd) of Cs+, Sr2+, and Co2+. Thus, KNiFC–PAN is promising for removing Cs+, Sr2+, and Co2+ from radioactive laundry wastewater.

Modeling of methane and carbon dioxide sorption capacity in tight reservoirs using Machine learning techniques

The viability of enhanced coalbed methane recovery (ECBM) and enhanced shale gas recovery (ESGR) are abundantly explored in various studies since they present a solution for ongoing energy demand and environmental crisis. As a matter of challenge, the prediction of the gas sorption profile poses a significant obstacle in the development of such resources. In this regard, mathematical models, numerical simulations, empirical correlations, and experimental measurements suffer from over-simplification, computationally extensive calculations, time-consuming, and costly processes. This work examines different machine learning methods, from shallow to deep learning, to investigate their capability to model 3804 data points of methane (CH4) and/or carbon dioxide (CO2) sorption capacity in shale and coal at different thermodynamic conditions. Percentage of CO2 in injection gas, rock type, total organic carbon (TOC), moisture, temperature, and pressure were taken as input. Hyperparameter tuning was executed by Optuna optimizer. According to the results, the random forest (RF) was the most proficient predictor for both test subsets of CH4 (MAE = 0.0864, MSE = 0.0231, RMSE = 0.1520) and CO2 (MAE = 0.0529, MSE = 0.0533, RMSE = 0.2308) sorption capacity. Based on the trend prediction simulation, RF predictors were able to properly capture the single and binary sorption capacities. More importantly, they successfully predict the abnormal descending behavior of CO2 sorption in high pressures. In addition, a sensitivity analysis was carried out to acquire the importance of input features and model hyperparameters in the training process. Input feature impacts were consistent with technical expectations of geology and reservoir engineering aspects. Both models are capable of being applied to lab scale and further developed for their application in reservoir scale simulators.

Functional Activated Biocarbons Based on Biomass Waste for CO2 Capture and Heavy Metal Sorption.

Inexpensive porous activated biocarbons were prepared from biomass and agriculture waste following the method of thermal and hydrothermal carbonization and activation with superheated water vapor. The activated biocarbons were characterized by nitrogen adsorption-desorption at 77 K, SEM, XRD, Raman spectrometry, FTIR spectroscopy, determination of particle size, and elemental composition by XRF. The specific surface area was in the range of 240-709 m2/g, and the total pore volume was from 0.12 to 0.43 cm3/g. The percentage of microporosity in activated biocarbons was 89-92%. These activated biocarbons have been used for CO2 and heavy metal sorption. Activated biocarbons based on pine cones and birch prepared by thermal carbonization and activation with superheated water vapor had the highest ability to capture CO2 and amounted to 6.43 and 6.00 mmol/g at 273 K, as well as 4.57 and 4.22 mmol/g at 298 K, respectively. The best activated biocarbon was characterized by unchanged stability after 30 adsorption and desorption cycles. It was proved that the adsorption of CO2 depends on narrow micropores (<1 nm). Activated biocarbons have also been analyzed as effective adsorbents for removing Cu2+, Zn2+, Fe2+, Ni2+, Co2+, and Pb2+ ions from aqueous solutions. Activated biocarbons are effective adsorbents for the removal of lead and zinc ions from aqueous solutions.

Amino acid-based ionic liquid as a promising CO2 sorption increasing agent by aqueous MDEA solution

The present study aims to enhance the efficiency of CO2 absorption by aqueous MDEA solutions and reduce the energy consumption of the process by designing combined absorbent systems. For this, a novel ionic liquid based on the amino acid glycine, known as bis(2-hydroxyethyl)dimethylammonium glycinate ([M2E2A][Gly]), was synthesized and investigated as a carbon dioxide sorbent. The composition was confirmed using 1H NMR. The sorption capacity of neat [M2E2A][Gly], its aqueous solutions and amine + IL aqueous mixtures was evaluated gravimetrically at different temperatures and concentrations of comprising components. The study observed an increase in both the sorption capacity and the rate of chemical reaction with carbon dioxide when using this compound as an additive. The H2O/MDEA/[M2E2A][Gly] absorbent (50/30/20 wt%) exhibited the highest sorption capacity of 2.98 mol CO2·kg-1solution. Furthermore, a mechanism of CO2 interaction with the H2O/MDEA/[M2E2A][Gly] absorbent was suggested and confirmed using FTIR spectroscopy. To assess the effect of the IL concentration on the absorbent’s viscosity behavior, studying rheological properties of absorbent with different contents of [M2E2A][Gly] were studied.

Discovery and In Situ Crystallization Studies of Cerium-Based Metal-Organic Frameworks with V-Shaped Linker Molecules.

We report the discovery and characterization of two porous Ce(III)-based metal-organic frameworks (MOFs) with the V-shaped linker molecules 4,4'-sulfonyldibenzoate (SDB2-) and 4,4'-(hexafluoroisopropylidene)bis(benzoate) (hfipbb2-). The compounds of framework composition [Ce2(H2O)(SDB)3] (1) and [Ce2(hfipbb)3] (2) were obtained by using a synthetic approach in acetonitrile that we recently established. Structure determination of 1 was accomplished from 3D electron diffraction (3D ED) data, while 2 could be refined against powder X-ray diffraction (PXRD) data using the crystal structure of an isostructural La-MOF as the starting model. Their framework structures consist of chain-like inorganic building units (IBUs) or hybrid-BUs that are interconnected by the V-shaped linker molecules to form framework structures with channel-type pores. The composition of both compounds was confirmed by PXRD, elemental analysis, as well as NMR and IR spectroscopy. Interestingly, despite the use of (NH4)2[CeIV(NO3)6] in the synthesis, cerium ions in both MOFs occur exclusively in the + III oxidation state as determined by X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). Thermal analyses reveal remarkably high thermal stabilities of ≥400 °C for the MOFs. Initial N2 sorption measurements revealed the peculiar sorption behavior of 2 which prompted a deeper investigation by Ar and CO2 sorption experiments. The combination with nonlocal density functional theory (NL-DFT) calculations adds to the understanding of the nature of the different pore diameters in 2. An extensive quasi-simultaneous in situ XANES/XRD investigation was carried out to unveil the formation of Ce-MOFs during the solvothermal syntheses in acetonitrile. The crystallization of the two Ce(III)-MOFs presented herein as well as two previously reported Ce(IV)-MOFs, all obtained by a similar synthetic approach, were studied. While the XRD patterns show time-dependent MOF crystallization, the XANES data reveal the presence of Ce(III) intermediates and their subsequent conversion to the MOFs. The addition of acetic acid in combination with the V-shaped linker molecule was identified as the crucial factor for the formation of the crystalline Ce(III/IV)-MOFs.