Adsorption equilibria and kinetics of CH4 and N2 on commercial zeolites and carbons

Ca-Based Layered Double Hydroxides for Environmentally Sustainable Carbon Capture

The process of carbon dioxide capture typically requires a large amount of energy for the separation of carbon dioxide from other gases, which has been a major barrier to the widespread deployment of carbon capture technologies. Innovation of carbon dioxide adsorbents is herein vital for the attainment of a sustainable carbon capture process. In this study, we investigated the electrified synthesis and rejuvenation of calcium-based layered double hydroxides (Ca-based LDHs) as solid adsorbents for CO2. We discovered that the particle morphology and phase purity of the LDHs, along with the presence of secondary phases, can be controlled by tuning the current density during electrodeposition on a porous carbon substrate. The change in phase composition during carbonation and calcination was investigated to unveil the effect of different intercalated anions on the surface basicity and thermal stability of Ca-based LDHs. By decoupling the adsorption of water and CO2, we showed that the adsorbed water largely promoted CO2 adsorption, most likely through a sequential dissolution and reaction pathway. A carbon capture capacity of 4.3 ± 0.5 mmol/g was measured at 30 °C and relative humidity of 40% using 10 vol % CO2 in nitrogen as the feed stream. After CO2 capture occurred, the thermal regeneration step was carried out by directly passing an electric current through the conductive carbon substrate, known as the Joule-heating effect. CO2 was found to start desorbing from the Ca-based LDHs at a temperature as low as 220 °C as opposed to the temperature above 700 °C required for calcium carbonate that forms as part of the Ca-looping capture process. Finally, we evaluated the cumulative energy demand and environmental impact of the LDH-based capture process using a life cycle assessment. We identified the most environmentally concerning step in the process and concluded that the postcombustion CO2 capture using LDH could be advantageous compared with existing technologies.

Highly Selective Mixed Potential Methanol Gas Sensor Based on a Ce0.8Gd0.2O1.95 Solid Electrolyte and Au Sensing Electrode

A Ce_0.8Gd_0.2O_1.95-based mixed potential type sensor attached with a commercially available Au paste sensing electrode material was fabricated to detect methanol. The optimum working temperature of the sensor was 545 °C, and the response value to 100 ppm methanol was -53 mV. The selectivity of the sensor was poor. The addition of a 4A molecular sieve filter layer and the method of pattern recognition were combined to improve it. Only gas molecules smaller than the pore diameter of the 4A molecular sieve were able to pass through the zeolite channel, and the selectivity coefficient of the sensor to methanol was improved by adding the filter layer. Meanwhile, there was an obvious distinction between the response and recovery times of the sensor toward methanol, ethanol, acetone, n-butanol, and n-pentanol. Next, the pattern recognition method was adopted. The relationship between the response value and the logarithm of gas concentration and the relationship between the maximum rate of the response process and the gas concentration were plotted separately. By comprehensively considering the two characteristic parameters of the response value and the maximum value of the differential response signal, the purpose of qualitative identification of gas types and quantitative analysis of gas concentrations was hopefully achieved.

High-Performance Carbon Molecular Sieves for the Separation of Propylene and Propane

High-performance carbon molecular sieves (CMSs) for the separation of propylene (C₃H₆) and propane (C₃H₈) were synthesized in this study by chemical vapor deposition (CVD) of benzene on the pore entrances of activated carbon. The C₃H₆ and C₃H₈ separation characteristics of the CMSs were controlled by altering the amount of carbon deposited during CVD, and the resulting characteristic curve featuring the kinetic selectivity of C₃H₆ over C₃H₈ as a function of the adsorption rate constant of C₃H₆ is considered to be the upper bound of the C₃H₆-C₃H₈ separation factor for current CMSs because of the presence of previously reported CMS data under this curve. Additionally, CMS models were constructed using grand canonical molecular dynamics (GCMD) simulations mimicking the process of CVD, which revealed that the kinetic selectivity of C₃H₆ over C₃H₈ strongly depended on the size of the pore entrances at the level of 0.01 nm, and that strict control of the pore-entrance size was crucial for obtaining high-performance CMSs for C₃H₆-C₃H₈ separation. This was essentially achieved by controlling the duration of CVD, which led to the experimental realization of CMSs with a C₃H₆ selectivity over C₃H₈ of >2000 and a high uptake rate of C₃H₆. A design guideline for the development of high-performance CMSs for C₃H₆-C₃H₈ separation was proposed based on theoretical calculations performed using idealized carbon structures, which extracted the characteristics of the CMS models obtained from the GCMD simulations.

Adsorption kinetics and equilibria of two methanol samples with different water content on activated carbon

To investigate the influence of fluid purity on the adsorption properties, adsorption kinetics and adsorption equilibria of two methanol samples with different water content on an activated carbon were studied. The purity of the methanol samples was 98.5% and 99.9%. Measurements were conducted at 298 K and 318 K using a magnetic suspension balance and cover a wide p/p0 range. To determine effective diffusion time constants and mass transfer coefficients, adsorption kinetics were evaluated using an isothermal and a nonisothermal Fickian diffusion model, and the linear driving force model. The pressure dependence of the kinetic parameters was studied and discussed. A small influence of sample purity on the adsorption equilibria was observed, as the purer methanol sample showed slightly higher equilibrium loadings than the less pure sample. However, significantly faster adsorption kinetics were observed for the purer sample at all temperature and pressure conditions. Compared to the less pure sample, the determined effective diffusion time constants and the mass transfer coefficients were up to 98% and 35% higher, respectively.

Temperature dependence of adsorption hysteresis in flexible metal organic frameworks

"Breathing" and "gating" are striking phenomena exhibited by flexible metal-organic frameworks (MOFs) in which their pore structures transform upon external stimuli. These effects are often associated with eminent steps and hysteresis in sorption isotherms. Despite significant mechanistic studies, the accurate description of stepped isotherms and hysteresis remains a barrier to the promised applications of flexible MOFs in molecular sieving, storage and sensing. Here, we investigate the temperature dependence of structural transformations in three flexible MOFs and present a new isotherm model to consistently analyse the transition pressures and step widths. The transition pressure reduces exponentially with decreasing temperature as does the degree of hysteresis (c.f. capillary condensation). The MOF structural transition enthalpies range from +6 to +31 kJ·mol^-1 revealing that the adsorption-triggered transition is entropically driven. Pressure swing adsorption process simulations based on flexible MOFs that utilise the model reveal how isotherm hysteresis can affect separation performance.

A review of common practices in gravimetric and volumetric adsorption kinetic experiments

The availability of commercial gravimetric and volumetric systems for the measurement of adsorption equilibrium has seen also a growth of the use of these instruments to measure adsorption kinetics. A review of publications from the past 20 years has been used to assess common practice in 180 cases. There are worrying trends observed, such as lack of information on the actual conditions used in the experiment and the fact that the analysis of the data is often based on models that do not apply to the experimental systems used. To provide guidance to users of these techniques this contribution is divided into two parts: a discussion of the appropriate models to describe diffusion in porous materials is presented for different gravimetric and volumetric systems, followed by a structured discussion of the main trends in common practice uncovered reviewing a large number of recent publications. We conclude with recommendations for best practice to avoid incorrect interpretation of these experiments.

Influence of Mineral Composition of Chars Derived by Hydrothermal Carbonization on Sorption Behavior of CO2, CH4, and O2

The doping of SiO₂ and Fe₂O₃ into hydrochars that were produced by the hydrothermal carbonization of cellulose was studied with respect to its impact on the resulting surface characteristics and sorption behavior of CO₂, CH₄, and O₂. During pyrolysis, the structural order of the Fe-doped char changed, as the fraction of highly ordered domains increased, which was not observed for the undoped and Si-doped chars. The Si doping had no apparent influence on the oxidation temperature of the hydrochar in contrast to the Fe-doped char where the oxidation temperature was reduced because of the catalytic effect of Fe. Both dopants reduced the micro-, meso- and macroporous surface areas of the chars, although the Fe-doped chars had larger meso- and macroporosity than the Si-doped char. However, the increased degree in the structural order of the carbon matrix of the Fe-doped char reduced its microporosity relative to the Si-doped char. The adsorption of CO₂ and CH₄ on the chars at temperatures between 273.15 and 423.15 K and at pressures up to 115 kPa was slightly inhibited by the Si doping but strongly suppressed by the Fe doping. For O₂, however, the Si doping promoted the observed adsorption capacity, while Fe doping also showed an inhibiting effect.

Temperature-regulated guest admission and release in microporous materials

While it has long been known that some highly adsorbing microporous materials suddenly become inaccessible to guest molecules below certain temperatures, previous attempts to explain this phenomenon have failed. Here we show that this anomalous sorption behaviour is a temperature-regulated guest admission process, where the pore-keeping group's thermal fluctuations are influenced by interactions with guest molecules. A physical model is presented to explain the atomic-level chemistry and structure of these thermally regulated micropores, which is crucial to systematic engineering of new functional materials such as tunable molecular sieves, gated membranes and controlled-release nanocontainers. The model was validated experimentally with H₂, N₂, Ar and CH₄ on three classes of microporous materials: trapdoor zeolites, supramolecular host calixarenes and metal-organic frameworks. We demonstrate how temperature can be exploited to achieve appreciable hydrogen and methane storage in such materials without sustained pressure. These findings also open new avenues for gas sensing and isotope separation.