The Role of Water during CO₂ Adsorption by Ca-Based Sorbents at High Temperature

Reactions of CaO, MgO, and decarbonated dolomite (CaOMgO) with CO₂ and added water have been studied with the goal of understanding fundamental issues related to these materials’ performance as CO₂ sorbents. We used a fixed bed reactor, in situ XRD, and DRIFTS to monitor the extent and kinetics of carbonation, surface reactions, and performance loss during repetitive adsorption–desorption cycles at industrial relevant conditions. From reactor and in situ XRD experiments, we found that water is essential to reach high carbonation levels (solid conversion >40%) of CaO and CaOMgO, which is in contrast to a situation where only a small fraction (<10%) of the capacity is used. Water has a more pronounced effect when applying CaOMgO as sorbent as compared to CaO, both when considering solid conversions and carbonation rates. DRIFTS shows that water together with CO₂ do in fact react at the MgO surface into carbonates species. Furthermore, H₂CO₃ may be important for exploiting CaO and CaOMgO materials because hydrogen carbonate is observed as a surface species only during reactions with water

NMR-Based Carbamate Decomposition Constants of Linear Primary Alkanolamines for CO₂ Capture

The amine carbamate related equilibrium (RNHCOO^– + H₂O ⇆ RNH₂ + HCO₃^–) has been investigated with ¹³C NMR (Nuclear Magnetic Resonance) spectroscopy for a series of linear primary alkanolamines, and the apparent carbamate decomposition equilibrium constants have been estimated. A quantitative NMR method for the calculation of the concentration of the species formed in solution has been provided, including the assessment of each of the fast exchanging proton species (whose nuclei resonate at the same chemical shifts in the NMR spectra). For this purpose, NMR-based calibration curves were utilized and an alternative method was applied for validation. The overall results showed that the amount of carbamate found at the equilibrium decreased as the length of the carbon chain increased, while the corresponding apparent carbamate decomposition equilibrium constants featured the same order of magnitude (10^–2)

SAPO-37 microporous catalysts: revealing the structural transformations during template removal

<p>We have studied the structural behavior of SAPO-37 during calcination using simultaneous <i>in situ</i> powder X-ray diffraction (PXRD) and mass spectroscopy (MS) in addition to <i>ex situ</i> thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A spike in the unit cell volume corresponding to template removal (tracked using the occupancy of the crystallographic sites in the SAPO-37 cages) is revealed from the XRD data and is strongly correlated with the DSC curve. The occupancy of the different template molecules in the faujasite (FAU) and sodalite (SOD) cages is strongly related to the two mass loss steps observed in the TGA data. The templates act as a physical stabilizing agent, not allowing any substantial unit cell response to temperature changes until they are removed. The FAU cages and SOD cages have different thermal response to the combustion of each template. The FAU cages are mainly responsible for the unit cell volume expansion observed after the template combustion. This expansion seems to be related with residual coke from template combustion. We could differentiate between the thermal response of oxygen and T-atoms. The T–O–T angle between two double 6-rings and a neighboring T–O–T linkage shared by SOD and FAU had different response to the thermal events. We were able to monitor the changes in the positions of oxygen and T-atoms during the removal of TPA⁺ and TMA⁺. Large changes to the framework structure at the point of template removal may have a significant effect on the long-term stability of the material in its activated form.</p

Nanoporous Intergrowths: How Crystal Growth Dictates Phase Composition and Hierarchical Structure in the CHA/AEI System

Some of the most important nanoporous materials that are used for industrial applications are formed as intergrowths between structurally related phases. Further, the specific properties and functions are often strongly related to the nature of these intergrowths. By their nature such structures are notoriously difficult to characterize in detail and thereby formulate a structure/property relationship. We approach the problem of the industrially relevant CHA/AEI intergrowth system by getting insight into not only the structure of the materials but also the crystal-growth mechanism and show that the former is crucially dependent upon the latter. Through a detailed X-ray diffraction analysis with optimization of the CHA/AEI layer stacking sequence, it is shown that up to three distinct components are present. These consist of the two end member structures intimately cocrystallizing with an intergrowth structure. The intergrowth composition is further corroborated by nuclear magnetic resonance and unit cell measurements. The mechanism by which these complex intergrowth structures form is revealed by atomic force microscopy that shows there are at least two competing mechanisms of growth at the surface: layer-by-layer and spiral. This has profound consequences on the resulting intergrowth materials, as intergrowth formation is not permitted in spiral growth. The competition from the lower energy spiral growth at screw dislocations does not allow intergrowth formation and consequently results in blocks of pure-phase AEI or CHA. Owing to this competitive growth nature, the different possibilities furnish the material with its higher level hierarchical structure

Nanoporous Intergrowths: How Crystal Growth Dictates Phase Composition and Hierarchical Structure in the CHA/AEI System

Some of the most important nanoporous materials that are used for industrial applications are formed as intergrowths between structurally related phases. Further, the specific properties and functions are often strongly related to the nature of these intergrowths. By their nature such structures are notoriously difficult to characterize in detail and thereby formulate a structure/property relationship. We approach the problem of the industrially relevant CHA/AEI intergrowth system by getting insight into not only the structure of the materials but also the crystal-growth mechanism and show that the former is crucially dependent upon the latter. Through a detailed X-ray diffraction analysis with optimization of the CHA/AEI layer stacking sequence, it is shown that up to three distinct components are present. These consist of the two end member structures intimately cocrystallizing with an intergrowth structure. The intergrowth composition is further corroborated by nuclear magnetic resonance and unit cell measurements. The mechanism by which these complex intergrowth structures form is revealed by atomic force microscopy that shows there are at least two competing mechanisms of growth at the surface: layer-by-layer and spiral. This has profound consequences on the resulting intergrowth materials, as intergrowth formation is not permitted in spiral growth. The competition from the lower energy spiral growth at screw dislocations does not allow intergrowth formation and consequently results in blocks of pure-phase AEI or CHA. Owing to this competitive growth nature, the different possibilities furnish the material with its higher level hierarchical structure

Selective Charging Behavior in an Ionic Mixture Electrolyte-Supercapacitor System for Higher Energy and Power

Ion–ion interactions in supercapacitor (SC) electrolytes are considered to have significant influence over the charging process and therefore the overall performance of the SC system. Current strategies used to weaken ionic interactions can enhance the power of SCs, but consequently, the energy density will decrease due to the increased distance between adjacent electrolyte ions at the electrode surface. Herein, we report on the simultaneous enhancement of the power and energy densities of a SC using an ionic mixture electrolyte with different types of ionic interactions. Two types of cations with stronger ionic interactions can be packed in a denser arrangement in mesopores to increase the capacitance, whereas only cations with weaker ionic interactions are allowed to enter micropores without sacrificing the power density. This unique selective charging behavior in different confined porous structure was investigated by solid-state nuclear magnetic resonance experiments and further confirmed theoretically by both density functional theory and molecular dynamics simulations. Our results offer a distinct insight into pairing ionic mixture electrolytes with materials with confined porous characteristics and further propose that it is possible to control the charging process resulting in comprehensive enhancements in SC performance