Molecular basis for the high CO2 adsorption capacity of chabazite zeolites

Effect of temperature and extraframework cation type on CHA framework flexibility

The sorption properties of zeolites are controlled by several factors, i.e. Si/Al ratio of the aluminosilicate framework [AlSiO4]-, the type and position of the extraframework (EF) cations, and the applied temperature. Here we investigate the flexibility of CHA framework as a function of EF cation-content and temperature (20-350 °C). Two CHA forms (Na- and Cu-CHA) with Si/Al = 2 were analysed. The main objectives were: (i) to shed light on the HT behaviour of Na-CHA, for which contrasting results exist in literature; (ii) define the role of temperature and EF cation-type in the response to the heating stimuli. We show that at 75 °C, Na-CHA undergoes a severe contraction of the unit-cell volume (-12%) accompanied by a symmetry lowering (R-3m to I2/m). The transformation is reversible, if the dehydrated Na-CHA is exposed to ambient conditions. In contrast, Cu-CHA experiences a significant different dehydration path, which involves minor changes of the CHA framework, and a net positive thermal-expansion after dehydration. The implications of the observed transformations for gas separation processes are finally discussed.

Sorbent selection for the recovery of gallium and indium from aqueous solutions: a sustainable approach to the recovery of strategic metals from LED lamps

Gallium and indium, metals present in light-emitting diode (LED) lighting technology, can be effectively recovered from aqueous solutions by sorption. For this purpose, carbonaceous materials, such as activated carbon, or low-cost biosorbents as beer bagasse, spent coffee grounds or peanut shells, and a low-cost zeolite as chabazite, were characterized by BET, FTIR, XRD, and SEM analysis prior use. Protonated chabazite, with high surface area (505 m2/g) and a Si/Al molar ratio of 3.4, showed high sorption capacities for gallium (56 mg/g) and indium (92 mg/g), which is 10 to 30 times higher than those of our carbonaceous materials (T = 298 K, pH < 3, dosage = 1 g/L). Sorption experiments with both metals in solution showed a competitive effect between gallium and indium for the sorption sites of the chabazite, showing more affinity toward gallium than indium. Ga3+sorbed/In3+sorbed molar ratio above 2 was achieved for the same initial concentration of both metals, increasing to almost 3 when the initial gallium concentration increased, which was appropriate since gallium concentration tends to be higher in LED chips. However, the sorption capacity for both metals was always around 0.35 mmol Ga + In/g. The selectivity of the chabazite was conditioned by different behavior of both metals in aqueous solution at the sorption pH (below 3.5) being the predominant species in solution Ga(OH)2+ for gallium and In3+ for indium. Sorption with protonated chabazite can be used in the treatment of spent LEDs leachate for the dual purpose of water purification and selective metal separation.

Privileged zeolitic sites for humid CO2 adsorption: K+ in double eight-membered rings

Humid CO2 adsorption in K+-exchanged zeolites featuring double-eight membered ring (D8R) structures results in CO2 outcompeting and desorbing dimeric water under equilibrated conditions, which is not observed for either the H+-form of the same zeolites or larger-pore zeolites.

Research on carbon dioxide capture materials used for carbon dioxide capture, utilization, and storage technology: a review

In recent years, climate change has increasingly become one of the major challenges facing mankind today, seriously threatening the survival and sustainable development of mankind. Dramatically increasing carbon dioxide concentrations are thought to cause a severe greenhouse effect, leading to severe and sustained global warming, associated climate instability and unwelcome natural disasters, melting glaciers and extreme weather patterns. The treatment of flue gas from thermal power plants uses carbon capture, utilization, and storage (CCUS) technology, one of the most promising current methods to accomplish significant CO2 emission reduction. In order to implement the technological and financial system of CO2 capture, which is the key technology of CCUS technology and accounts for 70-80% of the overall cost of CCUS technology, it is crucial to create more effective adsorbents. Nowadays, with the development and application of various carbon dioxide capture materials, it is necessary to review and summarize carbon dioxide capture materials in time. In this paper, the main technologies of CO2 capture are reviewed, with emphasis on the latest research status of CO2 capture materials, such as amines, zeolites, alkali metals, as well as emerging MOFs and carbon nanomaterials. More and more research on CO2 capture materials has used a variety of improved methods, which have achieved high CO2 capture performance. For example, doping of layered double hydroxides (LDH) with metal atoms significantly increases the active site on the surface of the material, which has a significant impact on improving the CO2 capture capacity and performance stability of LDH. Although many carbon capture materials have been developed, high cost and low technology scale remain major obstacles to CO2 capture. Future research should focus on designing low-cost, high-availability carbon capture materials.

Self-Assembled Core-Shell Structure MgO@TiO2 as a K2CO3 Support with Superior Performance for Direct Air Capture CO2

Traditional carbon capture and storage technologies for large point sources can at best slow the rate of increase in atmospheric CO2 concentrations. In contrast, direct capture of CO2 from ambient air, or "direct air capture" (DAC), offers the potential to become a truly carbon-negative technology. Composite solid adsorbents fabricated by impregnating a porous matrix with K2CO3 are promising adsorbents for the adsorption capture of CO2 from ambient air. Nevertheless, the adsorbent can be rapidly deactivated during continuous adsorption/desorption cycles. In this study, MgO-supported, TiO2-stabilized MgO@TiO2 core-shell structures were prepared as supports using a novel self-assembled (SA) method and then impregnated with 50 wt % K2CO3 (K2CO3/MgO@TiO2, denoted as SA-KM@T). The adsorbent exhibits a high CO2 capture capacity of ∼126.6 mg CO2/g sorbent in direct air adsorption and maintained a performance of 20 adsorption/desorption cycles at 300 °C mid-temperature, which was much better than that of K2CO3/MgO. Analysis proved that the core-shell structure of the support effectively inhibited the reaction between the active component (K2CO3) and the main support (MgO) by the addition of TiO2, resulting in higher reactivity, thermal stability, and antiagglomeration properties. This work provides an alternative strategy for DAC applications using adsorbents.

Unary Adsorption Equilibria of Hydrogen, Nitrogen, and Carbon Dioxide on Y-Type Zeolites at Temperatures from 298 to 393 K and at Pressures up to 3 MPa

The equilibrium adsorption of CO2, N2, and H2 on commercially available Zeolite H-Y, Na-Y, and cation-exchanged NaTMA-Y was measured up to 3 MPa at 298.15, 313.15, 333.15, 353.15, and 393.15 K gravimetrically using a magnetic suspension balance. The chemical and textural characterization of the materials was carried out by thermogravimetric analysis, helium gravimetry, and N2 (77 K) physisorption. We report the excess and net isotherms as measured and estimates of the absolute adsorption isotherms. The latter are modeled using the simplified statistical isotherm (SSI) model to evaluate adsorbate-adsorbent interactions and parametrize the data for process modeling. When reported per unit volume of zeolite supercage, the SSI model indicates that the saturation capacity for a given gas takes the same value for the three adsorbents. The Henry's constants predicted by the model show a strong effect of the cation on the affinity of each adsorbate.

Investigating the effect of textural properties on CO2 adsorption in porous carbons via deep neural networks using various training algorithms

The adsorption of carbon dioxide (CO2) on porous carbon materials offers a promising avenue for cost-effective CO2 emissions mitigation. This study investigates the impact of textural properties, particularly micropores, on CO2 adsorption capacity. Multilayer perceptron (MLP) neural networks were employed and trained with various algorithms to simulate CO2 adsorption. Study findings reveal that the Levenberg-Marquardt (LM) algorithm excels with a remarkable mean squared error (MSE) of 2.6293E-5, indicating its superior accuracy. Efficiency analysis demonstrates that the scaled conjugate gradient (SCG) algorithm boasts the shortest runtime, while the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm requires the longest. The LM algorithm also converges with the fewest epochs, highlighting its efficiency. Furthermore, optimization identifies an optimal radial basis function (RBF) network configuration with nine neurons in the hidden layer and an MSE of 9.840E-5. Evaluation with new data points shows that the MLP network using the LM and bayesian regularization (BR) algorithms achieves the highest accuracy. This research underscores the potential of MLP deep neural networks with the LM and BR training algorithms for process simulation and provides insights into the pressure-dependent behavior of CO2 adsorption. These findings contribute to our understanding of CO2 adsorption processes and offer valuable insights for predicting gas adsorption behavior, especially in scenarios where micropores dominate at lower pressures and mesopores at higher pressures.

CO2 Physisorption over an Industrial Molecular Sieve Zeolite: An Experimental and Theoretical Approach

The present work studies the adsorption of CO2 using a zeolitic industrial molecular sieve (IMS) with a high surface area. The effect of the CO2 feed concentration and the adsorption temperature in conjunction with multiple adsorption-desorption cycles was experimentally investigated. To assess the validity of the experimental results, theoretical calculations based on well-established equations were employed and the values of equilibrium, kinetic, and thermodynamic parameters are presented. Three additional column kinetic models were applied to the data obtained experimentally, in order to predict the breakthrough curves and thus facilitate process design. Results showed a negative correlation between temperature and adsorption capacity, indicating that physical adsorption takes place. Theoretical calculations revealed that the Langmuir isotherm, the Bangham kinetic model (i.e., pore diffusion is the rate-determining step), and the Thomas and Yoon-Nelson models were suitable to describe the CO2 adsorption process by the IMS. The IMS adsorbent material maintained its high CO2 adsorption capacity (>200 mg g-1) after multiple adsorption-desorption cycles, showing excellent regenerability and requiring only a mild desorption treatment (200 °C for 15 min) for regeneration.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.

Truly combining the advantages of polymeric and zeolite membranes for gas separations

Mixed-matrix membranes (MMMs) have been investigated to render energy-intensive separations more efficiently by combining the selectivity and permeability performance, robustness, and nonaging properties of the filler with the easy processing, handling, and scaling up of the polymer. However, truly combining all in one single material has proven very challenging. In this work, we filled a commercial polyimide with ultrahigh loadings of a high-aspect ratio, CO₂-philic Na-SSZ-39 zeolite with a three-dimensional channel system that precisely separates gas molecules. By carefully designing both zeolite and MMM synthesis, we created a gas-percolation highway across a flexible and aging-resistant (more than 1 year) membrane. The combination of a CO₂-CH₄ mixed-gas selectivity of ~423 and a CO₂ permeability of ~8300 Barrer outperformed all existing polymer-based membranes and even most zeolite-only membranes.

Importance of the polarity on nanostructured silica materials to optimize the hydrolytic condensation with molecules related to CO2 adsorption

The present work reports the changes for the mesoporous materials SBA-15 and KIT-6 associated with the structural, textural, and chemical properties when they are subjected to thermo-alkaline treatment. Despite the fact that the silica supports have not a strong affinity for CO₂ adsorption, the adsorption enthalpy profiles (ΔH_ads) reported that the substrates subjected to the thermo-alkaline treatment (S15H and K6H) have a greater energetic affinity towards CO₂ capture if compared to the precursory solids (S15 and K6). The ΔH_ads is - 26.7 kJ mol^-1 at 0.15 mmol g^-1 by supported S15H and K6H while the ΔH_ads is - 20. 7 kJ mol^-1 and - 18.7 kJ mol^-1 by K6 and S15, respectively, at the same CO₂ coverage. Furthermore, the CO₂ adsorption performances by the hydrolytic condensation between silica supports and the N´- (3-trimethoxysilylpropyl)diethylenetriamine (NAEPTES) or 3-aminopropiltriethoxysilane (APTES) are presented and it can be seen that the best performer for CO₂ adsorption is reported for the S15HN since it is able to absorb 0.93 mmol at 0.15 atm at 318 K. Thereby, the outcomes show that the effects of porous curvature and the magnitude of the amine species are parameters to be considered, as well as the thermo-alkaline treatment, in order to improve the subsequent surface reactions on silica supports. The materials were characterized by XRD, TEM, and N₂ adsorption at 77 K, NIR, and pyridine thermodesorption using Fourier Transform Infrared Spectroscopy (FTIR-Py), NMR for ²⁹Si and ¹³C, DSC, and CO₂ adsorption.

Carbon Dioxide Reforming of Methane over Nickel-Supported Zeolites: A Screening Study

As the utilization of zeolites has become more frequent in the dry reforming of methane (DRM) reaction, more systematic studies are required to evaluate properly the influence of zeolites’ composition and framework type on the performance. Therefore, in this work, a step-by-step study was performed with the aim of analyzing the effects of Ni loading (5, 10 or 15 wt.% over USY(3) zeolite), Si/Al ratio (3, 15 or 38 on USY zeolites with 15 wt.% Ni) and framework type (USY, BEA, ZSM-5 or MOR for 15 wt.% Ni and Si/Al ratios of ≈40) on catalysts’ properties and performances. Increasing Ni loadings enhanced CH4 and CO2 conversions even though the catalysts’ stability was decreasing over the time. The variation of the Si/Al ratio on USY and the use of different zeolites had also a remarkable impact on the catalytic performance. For instance, at 500–600 °C reaction temperatures, the catalysts with higher basicity and reducibility exhibited the best results. However, when the temperature was further increased, catalysts presenting stronger metal–support interactions (nickel nanoparticles located in mesoporous cavities) displayed the highest conversions and stability over time. In brief, the use of 15 wt.% Ni and a USY zeolite, with both micro- and mesopores and high surface area, led to the best performances, mainly attributed to a favorable number of Ni0 active sites and the establishment of stronger metal–support interactions (due to nanoparticles confinement inside the mesopores).

Ga+-Chabazite Zeolite: A Highly Selective Catalyst for Nonoxidative Propane Dehydrogenation

Ga-chabazite zeolites (Ga-CHA) have been found to efficiently catalyze propane dehydrogenation with high propylene selectivity (96%). In situ Fourier transform infrared spectroscopy and pulse titrations are employed to determine that upon reduction, surface Ga₂O₃ is reduced and diffuses into the zeolite pores, displacing the Brønsted acid sites and forming extra-framework Ga⁺ sites. This isolated Ga⁺ site reacts reversibly with H₂ to form GaH_x (2034 cm^-1) with an enthalpy of formation of ∼-51.2 kJ·mol^-1, a result supported by density functional theory calculations. The initial C₃H₈ dehydrogenation rates decrease rapidly (40%) during the first 100 min and then decline slowly afterward, while the C₃H₆ selectivity is stable at ∼96%. The reduction in the reaction rate is correlated with the formation of polycyclic aromatics inside the zeolite (using UV-vis spectroscopy) indicating that the accumulation of polycyclic aromatics is the main cause of the deactivation. The carbon species formed can be easily oxidized at 600 °C with complete recovery of the PDH catalytic properties. The correlations between GaH_x vs Ga/Al ratio and PDH rates vs Ga/Al ratio show that extra-framework Ga⁺ is the active center catalyzing propane dehydrogenation. The higher reaction rate on Ga⁺ than In⁺ in CHA zeolites, by a factor of 43, is the result of differences in the stabilization of the transition state due to the higher stability of Ga³⁺ vs In³⁺. The uniformity of the Ga⁺ sites in this material makes it an excellent model for the molecular understanding of metal cation-exchanged hydrocarbon interactions in zeolites.

Calcium Poly(Heptazine Imide): A Covalent Heptazine Framework for Selective CO2 Adsorption

Potassium poly(heptazine imide) (KPHI) has recently garnered attention as a crystalline carbon nitride framework with considerable photoelectrochemical activity. Here, we report a Ca²⁺-complexed analogue of PHI: calcium poly(heptazine imide) (CaPHI). Despite similar polymer backbone, CaPHI and KPHI exhibit markedly different crystal structures. Spectroscopic, crystallographic, and physisorptive characterization reveal that Ca²⁺ acts as a structure-directing agent to transform melon-based carbon nitride to crystalline CaPHI with ordered pore channels, extended visible light absorption, and altered band structure as compared to KPHI. Upon acid washing, protons replace Ca²⁺ atoms in CaPHI to yield H⁺/CaPHI and enhance porosity without disrupting crystal structure. Further, these proton-exchanged PHI frameworks exhibit large adsorption affinity for CO₂ and exceptional performance for selective carbon capture from dilute streams. Compared to a state-of-the-art metal organic framework, UTSA-16, H⁺/CaPHI exhibits more than twice the selectivity (∼300 vs ∼120) and working capacity (∼1.2 mmol g^-1 vs ∼0.5 mmol g^-1) for a feed of 4% CO₂ (1 bar, 30 °C).

Propane dehydrogenation over extra-framework In(i) in chabazite zeolites

Indium on silica, alumina and zeolite chabazite (CHA), with a range of In/Al ratios and Si/Al ratios, have been investigated to understand the effect of the support on indium speciation and its corresponding influence on propane dehydrogenation (PDH). It is found that In2O3 is formed on the external surface of the zeolite crystal after the addition of In(NO3)3 to H-CHA by incipient wetness impregnation and calcination. Upon reduction in H2 gas (550 °C), indium displaces the proton in Brønsted acid sites (BASs), forming extra-framework In+ species (In-CHA). A stoichiometric ratio of 1.5 of formed H2O to consumed H2 during H2 pulsed reduction experiments confirms the indium oxidation state of +1. The reduced indium is different from the indium species observed on samples of 10In/SiO2, 10In/Al2O3 (i.e., 10 wt% indium) and bulk In2O3, in which In2O3 was reduced to In(0), as determined from the X-ray diffraction patterns of the product, H2 temperature-programmed reduction (H2-TPR) profiles, pulse reactor investigations and in situ transmission FTIR spectroscopy. The BASs in H-CHA facilitate the formation and stabilization of In+ cations in extra-framework positions, and prevent the deep reduction of In2O3 to In(0). In+ cations in the CHA zeolite can be oxidized with O2 to form indium oxide species and can be reduced again with H2 quantitatively. At comparable conversion, In-CHA shows better stability and C3H6 selectivity (∼85%) than In2O3, 10In/SiO2 and 10In/Al2O3, consistent with a low C3H8 dehydrogenation activation energy (94.3 kJ mol-1) and high C3H8 cracking activation energy (206 kJ mol-1) in the In-CHA catalyst. A high Si/Al ratio in CHA seems beneficial for PDH by decreasing the fraction of CHA cages containing multiple In+ cations. Other small-pore zeolite-stabilized metal cation sites could form highly stable and selective catalysts for this and facilitate other alkane dehydrogenation reactions.

Gating effect for gas adsorption in microporous materials-mechanisms and applications

In the past two decades, various microporous materials have been developed as useful adsorbents for gas adsorption for a wide range of industries. Considerable efforts have been made to regulate the pore accessibility in microporous materials for the manipulation of guest molecules' admission and release. It has long been known that some microporous adsorbents suddenly become highly accessible to guest molecules at specific conditions, e.g., above a threshold pressure or temperature. This anomalous adsorption behavior results from a gating effect, where a structural variation of the adsorbent leads to an abrupt change in the gas admission. This review summarizes the mechanisms of the gating effect, which can be a result of the deformation of the framework (e.g., expansion, contraction, reorientation, and sliding of the unit cells), the vibration of the pore-keeping groups (e.g., rotation, swing, and collapse of organic linkers), and the oscillation of the pore-keeping ions (e.g. cesium, potassium, etc.). These structural variations are induced either by the host-guest interaction or by an external stimulus, such as temperature or light, and account for the gating effect at a threshold value of the stimulus. Emphasis is given to the temperature-regulated gating effect, where the critical admission temperature is dictated by the combined effect of the gate opening and thermodynamic factors and plays a key role in regulating guest admission. Molecular simulations can improve our understanding of the gate opening/closing transitions at the atomic scale and enable the construction of quantitative models to describe the gated adsorption behaviour at the macroscale level. The gating effect in porous materials has been widely applied in highly selective gas separation and offers great potential for gas storage and sensing.

Cs-RHO Goes from Worst to Best as Water Enhances Equilibrium CO2 Adsorption via Phase Change

The strong affinity of water to zeolite adsorbents has made adsorption of CO₂ from humid gas mixtures such as flue gas nearly impossible under equilibrated conditions. Here, in this manuscript, we describe a unique cooperative adsorption mechanism between H₂O and Cs⁺ cations on Cs-RHO zeolite, which actually facilitates the equilibrium adsorption of CO₂ under humid conditions. Our data demonstrate that, at a relative humidity of 5%, Cs-RHO adsorbs 3-fold higher amounts of CO₂ relative to dry conditions, at a temperature of 30 °C and CO₂ pressure of 1 bar. A comparative investigation of univalent cation-exchanged RHO zeolites with H⁺, Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ shows an increase of equilibrium CO₂ adsorption under humid versus dry conditions to be unique to Cs-RHO. In situ powder X-ray diffraction indicates the appearance of a new phase with Im3̅m symmetry after H₂O saturation of Cs-RHO. A mixed-cation exchanged NaCs-RHO exhibits similar phase transitions after humid CO₂ adsorption; however, we found no evidence of cooperativity between Cs⁺ and Na⁺ cations in adsorption, in single-component H₂O and CO₂ adsorption. We hypothesize based on previous Rietveld refinements of CO₂ adsorption in Cs-RHO zeolite that the observed phase change is related to solvation of extra-framework Cs⁺ cations by H₂O. In the case of Cs-RHO, molecular modeling results suggest that hydration of these cations favors their migration from an original D8R position to S8R sites. We posit that this movement enables a trapdoor mechanism by which CO₂ can interact with Cs⁺ at S8R sites to access the α-cage.

Preparation and characterization of templated porous carbons from sucrose by one-pot method and application as a CO2 adsorbent

The templated porous carbons were prepared from sucrose by one-pot method. In this method in which the pre-synthesis of the hard template is eliminated, the porous carbons were produced by organic-inorganic self-assembly of sucrose, tetraethyl ortosilicate (TEOS), Pluronic P123 and n-butanol in an acidic medium, and subsequent carbonization. The synthesis parameters such as sucrose amount, TEOS molar ratio and carbonization temperature were evaluated for describing their effects on the pore structures of the synthesized carbons. The prepared porous carbons were characterized by N₂ adsorption, thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) techniques. The carbon dioxide adsorption uptakes of the obtained porous carbons were determined at 1 bar and 273 K. The templated carbon obtained with the lowest TEOS molar ratio exhibited the highest BET surface area of 1289 m²/g and micropore volume of 0.467 cm³/g, and showed the highest CO₂ uptake of 2.28 mmol/g.

Recent Advances in Catalytic Confinement Effect within Micro/Meso-Porous Crystalline Materials

Micro/meso-porous crystalline materials with a well-defined pore structure, such as zeolites, carbon nanotubes, and metal-organic frameworks, are of great significance in the development of catalytic systems for scientific and industrial demands. The confinement effect aroused by pore features of porous crystalline materials has triggered great interest in heterogeneous catalysis. Catalytic reactions in confined spaces exhibit unique behaviors compared to those observed on bulk materials. More interestingly, chemical reactivity can be modulated in different ways by the confinement effect, despite the fact that the mechanism on how the confinement effect changes the reaction remains unclear. In this review, a systematic discussion and fundamental understanding is provided concerning the concept of confinement effect, highlighting the impact of confinement effects on diffusion, adsorption/desorption, and catalytic reaction in typical micro/meso-porous crystalline materials, including zeolites, carbon nanotubes, and metal-organic frameworks. Relevant studies demonstrate that confinement effect affords not only shape selectivity against reactants/products, but also modulates surface electron distribution of active species confined within porous environments, thereby successively affecting the catalytic reactivity, selectivity, and stability. This review provides a useful guide for researchers attempting to design excellent porous crystalline catalysts based on the concept of confinement effect in heterogeneous catalysis.

Study on the Synthesis of Chabazite Zeolites via Interzeolite Conversion of Faujasites

The interzeolite conversion of faujasite (FAU-type) zeolites to chabazite (CHA-type) zeolite in the presence of N,N,N-trimethyladamantammonium and N,N,N-dimethylethylcyclohexylammonium cations was investigated over a large compositional range by carefully controlling the reaction mixture compositions. Highly crystalline CHA zeolites were also obtained by the transformation of several zeolite types including EMT, LTL, LEV, RTH, and MFI frameworks. The formation of CHA zeolite from FAU zeolite precursors was substantially faster than that from zeolite L with a similar composition. High-silica CHA zeolites were also produced successfully using a mixture of TMAda with a number of less expensive organic structure-directing agents. The CHA zeolite materials have been synthesized with high crystallinity and with a Si/Al ratio ranging from 5 to 140. Our data support the importance of structural similarity between the zeolite precursors, nucleation/crystallization processes, and the zeolite product in the interzeolite conversion compared to conventional amorphous aluminosilicate gels. Our synthetic methods could be used to prepare other 8-membered ring zeolites such as AEI and AFX frameworks, potential candidates for selective catalytic reduction of NOx, light olefin production, and CO2 abatement.

Advances in Post-Combustion CO2 Capture by Physical Adsorption: From Materials Innovation to Separation Practice

The atmospheric CO₂ concentration continues a rapid increase to its current record high value of 416 ppm for the time being. It calls for advanced CO₂ capture technologies. One of the attractive technologies is physical adsorption-based separation, which shows easy regeneration and high cycle stability, and thus reduced energy penalties and cost. The extensive research on this topic is evidenced by the growing body of scientific and technical literature. The progress spans from the innovation of novel porous adsorbents to practical separation practices. Major CO₂ capture materials include the most widely used industrially relevant porous carbons, zeolites, activated alumina, mesoporous silica, and the newly emerging metal-organic frameworks (MOFs) and covalent-organic framework (COFs). The key intrinsic properties such as pore structure, surface chemistry, preferable adsorption sites, and other structural features that would affect CO₂ capture capacity, selectivity, and recyclability are first discussed. The industrial relevant variables such as particle size of adsorbents, the mechanical strength, adsorption heat management, and other technological advances are equally important, even more crucial when scaling up from bench and pilot-scale to demonstration and commercial scale. Therefore, we aim to bring a full picture of the adsorption-based CO₂ separation technologies, from adsorbent design, intrinsic property evaluation to performance assessment not only under ideal equilibrium conditions but also in realistic pressure swing adsorption processes.

Unusually Low Heat of Adsorption of CO2 on AlPO and SAPO Molecular Sieves

The capture of CO₂ from post-combustion streams or from other mixtures, such as natural gas, is an effective way of reducing CO₂ emissions, which contribute to the greenhouse effect in the atmosphere. One of the developing technologies for this purpose is physisorption on selective solid adsorbents. The ideal adsorbents are selective toward CO₂, have a large adsorption capacity at atmospheric pressure and are easily regenerated, resulting in high working capacity. Therefore, adsorbents combining molecular sieving properties and low heats of adsorption of CO₂ are of clear interest as they will provide high selectivities and regenerabilities in CO₂ separation process. Here we report that some aluminophosphate (AlPO) and silicoaluminophosphate (SAPO) materials with LTA, CHA and AFI structures present lower heats of adsorption of CO₂ (13-25 kJ/mol) than their structurally analogous zeolites at comparable framework charges. In some cases, their heats of adsorption are even lower than those of pure silica composition (20-25 kJ/mol). This could mean a great improvement in the regeneration process compared to the most frequently used zeolitic adsorbents for this application while maintaining most of their adsorption capacity, if materials with the right stability and pore size and topology are found.

Selective Adsorption of CO2 on Zeolites NaK-ZK-4 with Si/Al of 1.8-2.8

Zeolites with appropriately narrow pore apertures can kinetically enhance the selective adsorption of CO₂ over N₂. Here, we showed that the exchangeable cations (e.g., Na⁺ or K⁺) on zeolite ZK-4 play an important role in the CO₂ selectivity. Zeolites NaK ZK-4 with Si/Al = 1.8-2.8 had very high CO₂ selectivity when an intermediate number of the exchangeable cations were K⁺ (the rest being Na⁺). Zeolites NaK ZK-4 with Si/Al = 1.8 had high CO₂ uptake capacity and very high CO₂-over-N₂ selectivity (1190). Zeolite NaK ZK-4 with Si/Al = 2.3 and 2.8 also had enhanced CO₂ selectivity with an intermediate number of K⁺ cations. The high CO₂ selectivity was related to the K⁺ cation in the 8-rings of the α-cage, together with Na⁺ cations in the 6-ring, obstructing the diffusion of N₂ throughout the zeolite. The positions of the K⁺ cation in the 8-ring moved slightly (max 0.2 Å) toward the center of the α-cage upon the adsorption of CO₂, as revealed by in situ X-ray diffraction. The CO₂-over-N₂ selectivity was somewhat reduced when the number of K⁺ cations approached 100%. This was possibly due to the shift in the K⁺ cation positions in the 8-ring when the number of Na⁺ was going toward 0%, allowing N₂ diffusion through the 8-ring. According to in situ infrared spectroscopy, the amount of chemisorbed CO₂ was reduced on zeolite ZK-4s with increasing Si/Al ratio. In the context of potential applications, a kinetically enhanced selection of CO₂ could be relevant for applications in carbon capture and bio- and natural gas upgrading.

CO2 Adsorption Capacities in Zeolites and Layered Double Hydroxide Materials

The development of technologies that allow us to reduce CO₂ emissions is mandatory in today's society. In this regard, we present herein a comparative study of CO₂ adsorption over three types of materials: zeolites, layered double hydroxides (LDH), and zeolites coated LDH composites. The influence of the zeolite Si/Al ratio on zeolites sorption capacity along with the presence of mesopores was investigated. By comparing these results with the well-known performance of LDHs, we aim to provide insights on the factors that may influence the CO₂ capture capacity over zeolites, thus providing useful tools for tuning their properties upon post-treatments.

Carbonate-Promoted Drift of Alkali Cations in Small Pore Zeolites: Ab Initio Molecular Dynamics Study of CO2 in NaKA Zeolite

An effect of deblocking of small size (8R, D8R) pores in zeolites due to cation drift is analyzed by using ab initio molecular dynamics (AIMD) at the PBE-D2/PAW level. The effect of carbonate and hydrocarbonate species on the carbon dioxide uptake in NaKA zeolite is demonstrated. It is shown that a hydrocarbonate or carbonate anion can form strong complexes with K⁺ cation and withdraw it from the 8R window, so that the probability of CO₂ diffusion through 8R increases. For the first time, correlations between cationic and HCO₃^-/CO₃^2- positions are demonstrated in favor of their significant interaction leading to the cationic drift from 8R windows. This phenomenon explains a nonzero CO₂ adsorption in narrow pore zeolites upon high Na/K exchange. In a gas mixture, such deblocking effect reduces the separation factor because of the possible passage of both components through the plane of partly open 8R windows.

CO2 adsorption in Y zeolite: a structural and dynamic view by a novel principal-component-analysis-assisted in situ single-crystal X-ray diffraction experiment

The increasing efficiency of detectors and brightness of X-rays in both laboratory and large-scale facilities allow the collection of full single-crystal X-ray data sets within minutes. The analysis of these `crystallographic big data' requires new tools and approaches. To answer these needs, the use of principal component analysis (PCA) is proposed to improve the efficiency and speed of the analysis. Potentialities and limitations of PCA were investigated using single-crystal X-ray diffraction (XRD) data collected in situ on Y zeolite, in which CO2, acting as an active species, is thermally adsorbed while cooling from 300 to 200 K. For the first time, thanks to the high sensitivity of single-crystal XRD, it was possible to determine the sites where CO2 is adsorbed, the increase in their occupancy while the temperature is decreased, and the correlated motion of active species, i.e. CO2, H2O and Na+. PCA allowed identification and elimination of problematic data sets, and better understanding of the trends of the occupancies of CO2, Na+ and water. The quality of the data allowed for the first time calculation of the enthalpy (ΔH) and entropy (ΔS) of the CO2 adsorption by applying the van 't Hoff equation to in situ single-crystal data. The calculation of thermodynamic values was carried out by both traditional and PCA-based approaches, producing comparable results. The obtained ΔH value is significant and involves systems (CO2 and Y zeolite) with no toxicity, superb stability and chemical inertness. Such features, coupled with the absence of carbonate formation and framework inertness upon adsorption, were demonstrated for the bulk crystal by the single-crystal experiment, and suggest that the phenomenon can be easily reversed for a large number of cycles, with CO2 released on demand. The main advantages of PCA-assisted analysis reside in its speed and in the possibility of it being applied directly to raw data, possibly as an `online' data-quality test during data collection, without any a priori knowledge of the crystal structure.

Rational Synthesis of Chabazite (CHA) Zeolites with Controlled Si/Al Ratio and Their CO2 /CH4 /N2 Adsorptive Separation Performances

Separation of CO₂ from CH₄ and N₂ is of great significance from the perspectives of energy production and environment protection. In this work, we report the rational synthesis of chabazite (CHA) zeolites with controlled Si/Al ratio by using N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as an organic structure-directing agent, wherein the dependence of TMAdaOH consumption on the initial Si/Al ratio was investigated systematically. More TMAdaOH is required to direct the crystallization of CHA with higher Si/Al ratio. Once the product Si/Al ratio is larger than 24, the amount of TMAdaOH consumption remains nearly constant. CHA zeolites with different Si/Al ratios and charge-compensating cations were then applied for the separation of CO₂ /CH₄ /N₂ mixtures. The equilibrium selectivities predicted by ideal adsorbed solution theory (IAST) and ideal selectivities calculated from the ratio of Henry's constants for both CO₂ /CH₄ and CO₂ /N₂ decrease with the zeolite Si/Al ratio increasing, whereas the percentage regenerability of CO₂ presents the opposite trend. Therefore, there is a trade-off between adsorption selectivity and regenerability for the adsorbents. There is a weaker interaction between CO₂ molecules and the H-type zeolites than that on the Na-type ones, thus a higher regenerability can be achieved. This study indicates that it is possible to design CHA zeolites with different physicochemical properties to meet various adsorptive separation requirements.

Dynamic Adsorption of CO2/N2 on Cation-Exchanged Chabazite SSZ-13: A Breakthrough Analysis

Alkali-exchanged SSZ-13 adsorbents were investigated for their applicability in separating N₂ from CO₂ in flue gas streams using a dynamic breakthrough method. In contrast to IAST calculations based on equilibrium isotherms, K⁺ exchanged SSZ-13 was found to yield the best N₂ productivity, comparable to Ni-MOF-74, under dynamic conditions where diffusion properties play a significant role. This was attributed to the selective, partial blockage of access to the chabazite cavities, enhancing the separation potential in a 15/85 CO₂/N₂ binary gas mixture.

Enhanced Trace Carbon Dioxide Capture on Heteroatom-Substituted RHO Zeolites under Humid Conditions

Boron and copper heteroatoms were successfully incorporated into the frameworks of high-silica RHO zeolite by adopting a bulky alkali-metal-crown ether (AMCE) complex as the template. These heteroatom-doped zeolites show both larger micropore surface areas and volumes than those of their aluminosilicate analogue. Proton-type RHO zeolites were then applied for the separation of CO₂ /CH₄ /N₂ mixtures, as these zeolites have weaker electric fields and, thus, lower heats of adsorption. The adsorption results showed that a balance between working capacity and adsorption heat could be achieved for these heteroatom-doped zeolites. Ideal adsorbed solution theory predictions indicate that these zeolites should have high selectivities even for remarkably dilute sources of CO₂ . Finally, the heteroatom-substituted zeolites, especially the boron-substituted one, could be thermally regenerated rapidly at 150 °C after full hydration and maintained high regenerability for up to 30 cycles; therefore, they are potential candidates for trace CO₂ removal under humid conditions.

Computational evaluation of aluminophosphate zeotypes for CO2/N2 separation

Zeolites and structurally related materials (zeotypes) have received considerable attention as potential adsorbents for selective carbon dioxide adsorption. Within this group, zeotypes with aluminophosphate composition (AlPOs) could be an interesting alternative to the more frequently studied aluminosilicate zeolites. So far, however, only a few AlPOs have been characterised experimentally in terms of their CO₂ adsorption properties. In this study, force-field based grand-canonical Monte Carlo (GCMC) simulations were used to evaluate the potential of AlPOs for CO₂/N₂ separation, a binary mixture that constitutes a suitable model system for the removal of carbon dioxide from flue gases. A total of 51 frameworks were considered, all of which have been reported either as pure AlPOs or as heteroatom-containing AlPO derivatives. Prior to the GCMC simulations, all structures were optimised using dispersion-corrected density-functional theory calculations. The potential of these 51 systems for CO₂/N₂ separation was assessed in preliminary calculations (Henry constants and CO₂ uptake at selected pressures). On the basis of these calculations, 21 AlPOs of particular interest were selected, for which 15 : 85 CO₂/N₂ mixture adsorption isotherms were calculated up to 10 bar. For adsorption-based separations using an adsorption pressure of 1 bar (vacuum-swing adsorption), AlPOs with GIS, ATN, ATT, and SIV topologies were predicted to be most attractive, as they combine high CO₂/N₂ selectivities (75 to 140) and reasonable CO₂ working capacities (1 to 1.7 mmol g^-1). Under pressure-swing adsorption conditions, there is a tradeoff between selectivity and working capacity: while highly selective AlPOs like GIS reach only moderate working capacities, the frameworks with the highest working capacities above 2 mmol g^-1, AFY, KFI, and SAV, have lower selectivities between 25 and 35. To gain atomic-level insights into the host-guest interactions, interaction energy maps were computed for selected AlPOs. The computational assessment presented here can guide future experimental efforts in the development and optimisation of AlPO-based adsorbents for selective CO₂ adsorption.

High-Throughput Synthesis and Structure of Zeolite ZSM-43 with Two-Directional 8-Ring Channels

The aluminosilicate zeolite ZSM-43 (where ZSM = Zeolite Socony Mobil) was first synthesized more than 3 decades ago, but its chemical structure remained unsolved because of its poor crystallinity and small crystal size. Here we present optimization of the ZSM-43 synthesis using a high-throughput approach and subsequent structure determination by the combination of electron crystallographic methods and powder X-ray diffraction. The synthesis required the use of a combination of both inorganic (Cs⁺ and K⁺) and organic (choline) structure-directing agents. High-throughput synthesis enabled a screening of the synthesis conditions, which made it possible to optimize the synthesis, despite its complexity, in order to obtain a material with significantly improved crystallinity. When both rotation electron diffraction and high-resolution transmission electron microscopy imaging techniques are applied, the structure of ZSM-43 could be determined. The structure of ZSM-43 is a new zeolite framework type and possesses a unique two-dimensional channel system limited by 8-ring channels. ZSM-43 is stable upon calcination, and sorption measurements show that the material is suitable for adsorption of carbon dioxide as well as methane.

On the Structure-Property Relationships of Cation-Exchanged ZK-5 Zeolites for CO2 Adsorption

The CO₂ adsorption properties of cation-exchanged Li-, Na-, K-, and Mg-ZK-5 zeolites were correlated to the molecular structures determined by Rietveld refinements of synchrotron powder X-ray diffraction patterns. Li-, K-, and Na-ZK-5 all exhibited high isosteric heats of adsorption (Q_st ) at low CO₂ coverage, with Na-ZK-5 having the highest Q_st (ca. 49 kJ mol^-1 ). Mg²⁺ was located at the center of the zeolite hexagonal prism with the cation inaccessible to CO₂ , leading to a much lower Q_st (ca. 30 kJ mol^-1 ) and lower overall uptake capacity. Multiple CO₂ adsorption sites were identified at a given CO₂ loading amount for all four cation-exchanged ZK-5 adsorbents. Site A at the flat eight-membered ring windows and site B/B* in the γ-cages were the primary adsorption sites in Li- and Na-ZK-5 zeolites. Relatively strong dual-cation adsorption sites contributed significantly to an enhanced electrostatic interaction for CO₂ in all ZK-5 samples. This interaction gives rise to a migration of Li⁺ and Mg²⁺ cations from their original locations at the center of the hexagonal prisms toward the α-cages, in which they interact more strongly with the adsorbed CO₂ .

Impact of chabazite SSZ-13 textural properties and chemical composition on CO2 adsorption applications

A narrow pore zeolite was synthesized with different Si/Al ratios and micro- to nanoparticle size where both played an important role in CO₂ adsorption.

An Experimental and Kinetic Study of the Sorption of Carbon Dioxide onto Amine-Treated Oil Fly Ash

A new CO2adsorbent is produced from waste oil fly ash (OFA). Ammonium hydroxide solution is used to convert OFA to activated carbon. Then, the product is used for the adsorption of CO2from a nitrogen/carbon dioxide (N2/CO2) gas mixture. The OFA samples are characterized by several techniques. Chemical treatment of OFA considerably changed its surface morphology. In particular, its surface area, as determined by BET measurements, increased from 59 to 318 m2/g. The amine-functionalized ash had a monolayer adsorption capacity of 74.51 mg/g and was obtained at relative pressure,0.05<p/ps<0.35. A kinetics study showed that the CO2adsorption capacity of OFA increased with increasing CO2flow rates and concentrations and decreasing the relative humidity. Unlike physical adsorption, the chemisorption process resulted in increased adsorption capacity with increasing temperatures over the range 0–40°C. We also found that the adsorption process was endothermic (80–173 kJ/mol). The isotherm data for the adsorption process were fitted using different models. The saturation capacity determined from the Sips model, which corresponds to the sum of the saturation capacities of all of the adsorbed layers, was 540.3 mg/g of ash.

Framework stabilization of Si-rich LTA zeolite prepared in organic-free media

Zeolite HOU-2 (LTA type) is prepared with the highest silica content (Si/Al = 2.1) reported for Na-LTA zeolites without the use of an organic structure-directing agent. The rational design of Si-rich zeolites has the potential to improve their thermal stability for applications in catalysis, gas storage, and selective separations.