CO₂ Capture with Mesoporous Silicas Modified with Amines by Double Functionalization: Assessment of Adsorption/Desorption Cycles

Experimental Study on Renewable Porous Carbon Dioxide Adsorbent Materials for Space Shuttles

Porous adsorbent material is promising to be used to regeneratively remove CO2 from space shuttles. In this work, the amount and isosteric heat of CO2 adsorption in solid amine are experimentally studied at pressures ranging from 0 to 6 bar and temperatures ranging from 20 °C to 60 °C. The amount and isosteric heat of water adsorption in the solid amine is tested at different humidities (relative humidity 30–80%). The effective thermal conductivity of the solid amine at different atmospheres (air, N2, CO2 and water), pressures and temperatures is also investigated. The results show that the best temperature for CO2 adsorption in the solid amine is 45 °C under dry conditions. The amount of water adsorption increases with enhanced humidity, while the isosteric heat of water adsorption remains a constant value. The effective thermal conductivity of the solid amine increases with an increase in pressure. The adsorbed phase (CO2 and water) in the solid amine makes a contribution to improving the effective thermal conductivity of solid amine particles. The above findings can help design a better adsorption system in space.

Theoretical and experimental study on the fluidity performance of hard-to-fluidize carbon nanotubes-based CO2 capture sorbents

Carbon nanotubes-based materials have been identified as promising sorbents for efficient CO2 capture in fluidized beds, suffering from insufficient contact with CO2 for the high-level CO2 capture capacity. This study focuses on promoting the fluidizability of hard-to-fluidize pure and synthesized silica-coated amine-functionalized carbon nanotubes. The novel synthesized sorbent presents a superior sorption capacity of about 25 times higher than pure carbon nanotubes during 5 consecutive adsorption/regeneration cycles. The low-cost fluidizable-SiO2 nanoparticles are used as assistant material to improve the fluidity of carbon nanotubes-based sorbents. Results reveal that a minimum amount of 7.5 and 5 wt% SiO2 nanoparticles are required to achieve an agglomerate particulate fluidization behavior for pure and synthesized carbon nanotubes, respectively. Pure carbon nanotubes + 7.5 wt% SiO2 and synthesized carbon nanotubes + 5 wt% SiO2 indicates an agglomerate particulate fluidization characteristic, including the high-level bed expansion ratio, low minimum fluidization velocity (1.5 and 1.6 cms−1), high Richardson—Zaki n index (5.2 and 5.3 > 5), and low Π value (83.2 and 84.8 < 100, respectively). Chemical modification of carbon nanotubes causes not only enhanced CO2 uptake capacity but also decreases the required amount of silica additive to reach a homogeneous fluidization behavior for synthesized carbon nanotubes sorbent.

A Brief Overview of Recent Progress in Porous Silica as Catalyst Supports

Porous silica particles have shown applications in various technological fields including their use as catalyst supports in heterogeneous catalysis. The mesoporous silica particles have ordered porosity, high surface area, and good chemical stability. These interesting structural or textural properties make porous silica an attractive material for use as catalyst supports in various heterogeneous catalysis reactions. The colloidal nature of the porous silica particles is highly useful in catalytic applications as it guarantees better mass transfer properties and uniform distribution of the various metal or metal oxide nanocatalysts in solution. The catalysts show high activity, low degree of metal leaching, and ease in recycling when supported or immobilized on porous silica-based materials. In this overview, we have pointed out the importance of porous silica as catalyst supports. A variety of chemical reactions catalyzed by different catalysts loaded or embedded in porous silica supports are studied. The latest reports from the literature about the use of porous silica-based materials as catalyst supports are listed and analyzed. The new and continued trends are discussed with examples.

K2CO3-Impregnated Al/Si Aerogel Prepared by Ambient Pressure Drying for CO2 Capture: Synthesis, Characterization and Adsorption Characteristics

A new potassium-based adsorbent for CO₂ capture with Al aerogel used as support is proposed in this work. The adsorbents with different surface modifiers (tetraethyl orthosilicate (TEOS) and trimethyl chlorosilane (TMCS)) and different K₂CO₃ loadings (10%, 20%, 30% and 40%) were prepared by sol-gel and iso-volume impregnation processes with ambient pressure drying. The CO₂ adsorption performance of the adsorbents were tested by a fixed-bed reactor, and their adsorption mechanisms were studied by scanning electron microscopy (SEM), Brunauer Emmett Teller (BET), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and X-ray fluorescence spectrometry (XRF). Furthermore, the adsorption kinetics and the cycling performance were investigated. The results show that using TEOS to modify the wet gel can introduce SiO₂ to increase the strength of the skeleton. On the basis of TEOS modification, TMCS can further modify –OH, thus effectively avoiding the destruction of aerogel structure during ambient drying and K₂CO₃ impregnation. In this work, the specific surface area and specific pore volume of Al aerogel modified by TEOS + TMCS are up to 635.32 cm²/g and 2.43 cm³/g, respectively. The aerogels without modification (Al-B), TEOS modification (Al/Si) and TEOS + TMCS modification (Al/Si-TMCS) showed the best CO₂ adsorption performance at 20%, 30% and 30% K₂CO₃ loading, respectively. In particular, the CO₂ adsorption capacity and K₂CO₃ utilization rate of Al/Si-TMCS-30K are as high as 2.36 mmol/g and 93.2% at 70 degrees Celsius (°C). Avrami’s fractional order kinetic model can well fit the CO₂ adsorption process of potassium-based adsorbents. Al-B-20K has a higher apparent activation energy and a lower adsorption rate during the adsorption process. After 15 adsorption-regeneration cycles, Al/Si-TMCS-30K maintain a stable CO₂ adsorption capacity and framework structure, while the microstructure of Al/Si-30K is destroyed, resulting in a decrease in its adsorption capacity by nearly 30%. This work provides key data for the application of Al aerogel in the field of potassium-based adsorbent for CO₂ capture.

Enhanced CO2 Adsorption on Nitrogen-Doped Carbon Materials by Salt and Base Co-Activation Method

Nitrogen-doped carbon materials with enhanced CO₂ adsorption were prepared by the salt and base co-activation method. First, resorcinol-formaldehyde resin was synthesized with a certain salt as an additive and used as a precursor. Next, the resulting precursor was mixed with KOH and subsequently carbonized under ammonia flow to finally obtain the nitrogen-doped carbon materials. A series of samples, with and without the addition of different salts, were prepared, characterized by XRD (X-ray powder diffraction), elemental analysis, BET (N₂-adsorption-desorption analysis), XPS (X-ray photoelectron spectroscopy) and SEM (Scanning electron microscopy) and tested for CO₂ adsorption. The results showed that the salt and base co-activation method has a remarkable enhancing effect on the CO₂ capture capacity. The combination of KCl and KOH was proved to be the best combination, and 167.15 mg CO₂ could be adsorbed with 1 g nitrogen-doped carbon at 30 °C under 1 atm pressure. The materials characterizations revealed that the introduction of the base and salt could greatly increase the content of doped nitrogen, the surface area and the amount of formed micropore, which led to enhanced CO₂ absorption of the carbon materials.

Effects of Alkali Metal (Li, Na, and K) Incorporation in NH₂⁻MIL125(Ti) on the Performance of CO₂ Adsorption

A series of titanium-based, metal–organic framework (MOF) materials, xM@NH₂-MIL125(Ti) (x is the alkali metal loading percentage during the synthesis; M = Li, Na, K), have been synthesized solvothermally. Alkali metal doping in the NH₂–MIL125(Ti) in situ solvothermal process demonstrated a vital modification of the material structure and surface morphology for the CO₂ adsorption capacity at ambient conditions. By changing the reactants’ precursor, including different kinds of alkali metal, the morphology of xM@NH₂–MIL125(Ti) can be adjusted from a tetragonal plate through a circular plate to a truncated octahedron. The variation of the alkali metal loading results in substantial differences in the CO₂ adsorption. The properties of xM@NH₂–MIL125(Ti) were evaluated via functional group coordination using FT-IR, phase identification based on X-ray diffraction (XRD), surface morphology through scanning electron microscopy (SEM), as well as N₂ and CO₂ adsorption by physical gas adsorption analysis. This work reveals a new pathway to the modification of MOF materials for high-efficiency CO₂ adsorption.

Study on Adsorption Mechanism and Failure Characteristics of CO₂ Adsorption by Potassium-Based Adsorbents with Different Supports

In order to obtain the adsorption mechanism and failure characteristics of CO₂ adsorption by potassium-based adsorbents with different supports, five types of supports (circulating fluidized bed boiler fly ash, pulverized coal boiler fly ash, activated carbon, molecular sieve, and alumina) and three kinds of adsorbents under the modified conditions of K₂CO₃ theoretical loading (10%, 30%, and 50%) were studied. The effect of the reaction temperature (50 °C, 60 °C, 70 °C, 80 °C, and 90 °C) and CO₂ concentration (5%, 7.5%, 10%, 12.5%, and 15%) on the adsorption of CO₂ by the adsorbent after loading and the effect of flue gas composition on the failure characteristics of adsorbents were obtained. At the same time, the microscopic characteristics of the adsorbents before and after loading and the reaction were studied by using a specific surface area and porosity analyzer as well as a scanning electron microscope and X-ray diffractometer. Combining its reaction and adsorption kinetics process, the mechanism of influence was explored. The results show that the optimal theoretical loading of the five adsorbents is 30% and the reaction temperature of 70 °C and the concentration of 12.5% CO₂ are the best reaction conditions. The actual loading and CO₂ adsorption performance of the K₂CO₃/AC adsorbent are the best while the K₂CO₃/Al₂O₃ adsorbent is the worst. During the carbonation reaction of the adsorbent, the cumulative pore volume plays a more important role in the adsorption process than the specific surface area. As the reaction temperature increases, the internal diffusion resistance increases remarkably. K₂CO₃/AC has the lowest activation energy and the carbonation reaction is the easiest to carry out. SO₂ and HCl react with K₂CO₃ to produce new substances, which leads to the gradual failure of the adsorbents and K₂CO₃/AC has the best cycle failure performance.

Environmentally Friendly Gelatin/β-Cyclodextrin Composite Fiber Adsorbents for the Efficient Removal of Dyes from Wastewater

In this paper, environmentally friendly gelatin/β-cyclodextrin (β-CD) composite fiber adsorbents prepared by electrospinning were used for the removal of dyes from wastewater. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and a universal materials tester were employed to characterize the internal structures, surface morphologies and mechanical strength of the composite fiber adsorbents. Additionally, the fiber was evaluated as an adsorbent for the removal of methylene blue (MB) from aqueous solution. The effects of the raw material ratio, pH, temperature, concentration and adsorption time were studied. The results show that the gelatin/β-CD composite fiber adsorbents possess excellent mechanical strength and high adsorption efficiency for MB. The adsorption equilibrium and adsorption kinetics are well-described by the Langmuir isotherm model and the pseudo-second-order kinetic model, respectively. The theoretical maximum adsorption capacity is 47.4 mg·g-1. Additionally, after nine successive desorption-adsorption cycles, the removal rate is still over 70%. Moreover, the gelatin/β-CD composite fiber adsorbents exhibit excellent adsorption capability for basic fuchsin, gentian violet, brilliant blue R and malachite green dyes. Therefore, owing to the characteristics of degradability, low cost and high-efficiency, the gelatin/β-CD composite fiber can be used as an efficient adsorbent for the removal of dyes from wastewater.