Adsorption of CO<Subscript>2</Subscript>-containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for CO<Subscript>2</Subscript> separation

Adsorption of CO₂, N₂, CH₄ and H₂ on triamine-grafted pore-expanded MCM-41 mesoporous silica (TRI-PE-MCM-41) was investigated at room temperature in a wide range of pressure (up to 25 bar) using gravimetric measurements. The material was found to exhibit high affinity toward CO₂ in comparison to the other species over the whole range of pressure. Column-breakthrough dynamic measurements of CO₂-containing mixtures showed very high selectivity toward CO₂ over N₂, CH₄ and H₂ at CO₂ concentrations within the range of 5 to 50%. These conditions are suitable for effective removal of CO₂ at room temperature from syngas, flue gas and biogas using temperature swing (TS) or temperature-pressure swing (TPS) regeneration mode. Moreover, TRI-PE-MCM-41 was found to be highly stable over hundreds of adsorption-desorption cycles using TPS as regeneration mode.     

Preparation and performance of potassium-based sorbent using SnO<Subscript>2</Subscript> for post-combustion CO<Subscript>2</Subscript> capture

Potassium-based sorbents using γ-Al₂O₃ or TiO₂ as a support or an additive material have disadvantages in terms of their thermal stability and cyclic CO₂ capture. To overcome the shortcomings of these sorbents, a novel potassium-based sorbent (KSnI30) using SnO₂ was developed in this study. The KSnI30 sorbent formed only K₂CO₃ and SnO₂ phases without any inactive alloy species even after calcination at high temperatures (500–700 °C), indicating the good thermal stability of the KSnI30 sorbent regardless of the calcination temperature. Furthermore, the KSnI30 sorbent has an excellent regeneration property (above 98 %), as well as high CO₂ capture capacities (89–94 mg CO₂/g sorbent). Its excellent regeneration property is due to the formation of a KHCO₃ phase without by-products during CO₂ sorption. These results of the present study demonstrate that the SnO₂ shows promise as a new support or an additive material to replace TiO₂ and γ-Al₂O₃ in the preparation of a regenerable potassium-based sorbent for post-combustion CO₂ capture with good thermal stability and excellent regeneration property.     

FT-IR study on CO<Subscript>2</Subscript> adsorbed species of CO<Subscript>2</Subscript> sorbents

The stability of amine-functionalized silica sorbents prepared through the incipient wetness technique with primary, secondary, and tertiary amino organosilanes was investigated. The prepared sorbents were exposed to different gaseous streams including CO₂/N₂, dry CO₂/air with varying concentration, and humid CO₂/air mixtures to demonstrate the effect of the gas conditions on the CO₂ adsorption capacity and the stability of the different amine structures. The primary and secondary amine-functionalized adsorbents exhibited CO₂ sorption capacity, while tertiary amine adsorbent hardly adsorbed any CO₂. The secondary amine adsorbent showed better stability than the primary amine sorbent in all the gas conditions, especially dry conditions. Deactivation species were evaluated using FT-IR spectra, and the presence of urea was confirmed to be the main deactivation product of the primary amine adsorbent under dry condition. Furthermore, it was found that the CO₂ concentration can affect the CO₂ sorption capacity as well as the extent of degradation of sorbents.     

Effect of various aminosilanes functionalized inside nanoporous silica on CO<Subscript>2</Subscript> adsorption performance

Three different aminosilanes ((3-aminopropyl)trimethoxysilane (1NS), N-[3-(trimethoxysilyl) propyl]ethylenediamine (2NS), N¹-(3-trimethoxysilylpropyl)diethylenetriamine (3NS)) were grafted covalently inside nanoporous silica (NPS-1) with a large surface area to prepare CO₂ adsorbents. The prepared CO₂ sorbents were evaluated for their CO₂ sorption capacity, kinetic behavior, temperature programmed desorption (TPD) and textural properties. Grafting efficiency of 1NS was better due to the smaller molecular size compared to 2NS and 3NS, which are difficult to react with the hydroxyl group of the silica surface due to steric hindrance. The highest adsorption capacity of 7.0 wt% was observed for the 2NS/NPS-1 adsorbent, followed by 5.2 wt% for 1NS/NPS-1, then 5.0 wt% for 3NS/NPS-1. The adsorption capacity of 2NS/NPS-1 was highest at 30 °C, and it gradually decreased as the adsorption temperature increased. TPD analysis showed that the reaction of primary amine of 2NS with CO₂ inside the nanoporous silica could form less thermally stable carbamic acid and carbamate compared to 1NS and 3NS.     

Preparation and characterization of waste ion-exchange resin-based activated carbon for CO<Subscript>2</Subscript> capture

Waste ion-exchange resin was utilized as precursor to produce activated carbon by KOH chemical activation, on which the effects of different activation temperatures, activation times and impregnation ratios were studied in this paper. The CO₂ adsorption of the produced activated carbon was tested by TGA at 30 °C and environment pressure. Furthermore, the effects of preparation parameters on CO₂ adsorption were investigated. Experimental results show that the produced activated carbons are microporous carbons, which are suitable for CO₂ adsorption. The CO₂ adsorption capacity increases firstly and then decreases with the increase of activation temperature, activation time and impregnation rate. The maximum adsorption capacity is 81.24 mg/g under the condition of 30 °C and pure CO₂. The results also suggest that waste ion-exchange resin-based activated carbons possess great potential as adsorbents for post-combustion CO₂ capture.     

Effect of amine double-functionalization on CO<Subscript>2</Subscript> adsorption behaviors of silica gel-supported adsorbents

Amine double-functionalized adsorbents were fabricated using silica gel as supports and their capabilities for CO₂ capture were examined. Aminopropyltrimethoxysilane (1N-APS), and N¹-(3-trimethoxysilylpropyl)diethylenetriamine (3N-APS) were used as grafted amine compounds, and tetraethylenepentamine and polyethyleneimine were used as impregnated species. The influence of double-functionalization method on the CO₂ adsorption performance and textural properties of adsorbents was investigated. The adsorption capacity, the amine efficiency, and the thermal stability of double-functionalized sorbents depend strongly upon molecular variables associated with two different functional states (i.e., chemically grafted and physically impregnated amines). The temperature dependence of adsorption isotherms reveals that the CO₂ adsorption behavior in the double-functionalized adsorbents follow the diffusion limitation model proposed by Xu et al. (Energy Fuels 16:1463–1469, 2002) where the CO₂ adsorption is helped by the diffusion of impregnated amines. It is also found that the adsorption isotherm in the double-functionalized sorbent system with a proper choice for grafted and impregnated amines is nearly independent of temperature, which may offer a novel means to fabricate practically useful sorbents that can be used in a wide range of temperature without loss of CO₂ adsorption capacity.     

High-performance CO<Subscript>2</Subscript> capture on amine-functionalized hierarchically porous silica nanoparticles prepared by a simple template-free method

The adsorption of CO₂ on polyethyleneimine (PEI)-functionalized hierarchically porous silica nanoparticles (PSNs), prepared by using rice husk as a silica source via a simple template-free method, was reported in this study. Compared with traditional alkaline fusion and surfactant-templating methods for preparing waste-derived porous silica materials as CO₂ adsorbents, this method holds specific important advantages in being an inexpensive, and energy-saving process with faster production rate. The results revealed that the (NH₄)₂SiF₆ salt formed during the synthetic process served as an effective porogen, which can be readily removed by washing with water. Additionally, the total pore volumes of PSNs materials were strongly correlated to the amount of (NH₄)₂SiF₆. When evaluated as a support of PEI for CO₂ adsorption, 55PEI/PSNs(12/14) could reach 159 mg/g at 75 °C under 15 % CO₂, which was remarkably superior to those using waste silicate precursors reported in the previous literature. It was demonstrated that both PEI loading, and total pore volume of the PEI/silica composite sorbents, played key roles on CO₂ adsorption. Besides, 55PEI/PSNs(12/14) also showed high stability during 20 cycles of adsorption–desorption operation, implying its high potential in post-combustion CO₂ capture.     

Pressure swing adsorption for CO<Subscript>2</Subscript> capture in Fischer-Tropsch fuels production from biomass

Environmental concerns and oil price rises and dependency promoted strong research in alternative fuel sources and vectors. Fischer-Tropsch products are considered a valid alternative to oil derivatives having the advantage of being able to share current infrastructures. As a renewable source of energy, synthesis gas obtained from biomass gasification presents itself as a sustainable alternative. However, prior to hydrocarbon conversion, the bio-syngas must be conditioned, which includes the removal of carbon dioxide for subsequent sequestration and capture. A pressure swing adsorption cycle was developed for the removal and concentration of CO₂ from the bio-syngas stream. Activated carbon was chosen as adsorbent. The simulation results showed that it was possible to produce a (H₂ + CO) product with a H₂/CO stoichiometric ratio of 2.14 (suitable as feed stream for the Fischer-Tropsch reactor) and a CO₂ product with a purity of 95.18%. A CO₂ recovery of 90.3% was obtained. A power consumption of 3.36 MW was achieved, which represents a reduction of about 28% when compared to a Rectisol process with the same recovery.     

Preparation and characterization of (Pebax 1657 + silica nanoparticle)/PVC mixed matrix composite membrane for CO<Subscript>2</Subscript>/N<Subscript>2</Subscript> separation

In this work, the films of poly(ether-block-amide) (Pebax 1657) and hydrophilic/hydrophobic silica nanoparticles (0–10 wt%) were coated on a poly(vinyl chloride) (PVC) ultrafiltration membrane to form new mixed matrix composite membranes (MMCMs) for CO₂/N₂ separation. The membranes were characterized by SEM, FTIR, DSC and XRD. Successful formation of a non-porous defect-free dense top layer with ~4 μm of thickness and also uniform dispersion of silica nanoparticles up to 8 wt% loading in Pebax matrix were confirmed by SEM images. The gas permeation results showed an increase in the permeance of all gases and an increase in ideal CO₂/N₂ selectivity with the increase in silica nanoparticle contents. Comparison between the incorporation of hydrophilic and hydrophobic silica nanoparticle into Pebax matrix revealed that the great enhancement of CO₂ solubility is the key factor for the performance improvement of Pebax + silica nanoparticle membranes. The best separation performance of the hydrophilic silica nanoparticle-incorporated Pebax/PVC membrane for pure gases (at 1 bar and 25 °C) was obtained with a CO₂ permeability of 124 barrer and an ideal CO₂/N₂ selectivity of 76, i.e., 63 and 35% higher than those of neat Pebax membrane, respectively. The corresponding values for hydrophobic silica nanoparticle-incorporated Pebax/PVC membrane were 107 barrer for CO₂ permeability and 61 for ideal CO₂/N₂ selectivity. Also the performances of MMCMs improved upon pressure increase (1–10 bar) owing to the shift in plasticizing effect of CO₂ towards the higher pressures. In addition, an increase in permeabilities with a decrease in ideal selectivity was observed upon temperature increase (25–50 °C) due to the intensification of chain mobility.     

Insights into amine-based CO<Subscript>2</Subscript> capture: an ab initio self-consistent reaction field investigation

Ab initio many-body perturbation theory (MP2/6-311++G(,dp)), density functional theory (B3LYP/6-31++G(d,p)) and self-consistent reaction field (IEF-PCM UA HF/6-31G(d)) calculations have been used to study the CO₂ capture reagents NH₃, 2-hydroxyethylamine (MEA), diaminoethane (EN), 2-amino-1-propanol (2A1P), diethanolamine (DEA), N-methyl-2-hydroxyethylamine (N-methylMEA), 2-amino-2-methyl-1-propanol (AMP), trishydroxymethylaminomethane (tris), piperazine (PZ) and piperidine (PD). This study involved full conformational searches of the capture amines in their native and protonated forms, and their carbamic acid and carbamate derivatives. Using this data, we were able to compute Boltzmann-averaged thermodynamic values for the amines, carbamates and carbamic acid derivatives, as well as equilibrium constants for a series of ‘universal’ aqueous capture reactions. Important findings include (i) relative pK _a values for the carbamic acid derivatives are a useful measure of carbamate stability, due to a particular chemical resonance which is also manifest in short computed N–CO₂H bonds at both levels of theory, (ii) the computational results for sterically hindered amines such as AMP and tris are consistent with these species forming carbamates which readily hydrolyse and (iii) the amine-catalysed reaction between OH⁻ and CO₂ to generate bicarbonate correlates with amine pK _a. Thermodynamic data from the ab initio computations predicts that the heterocycles PD and PZ and the acyclic sorbent EN are good choices for a capture solvent. AMP and tris perform poorly in comparison.     

Dry Potassium-Based Sorbents for CO<Subscript>2</Subscript> Capture

Dry potassium-based sorbents were prepared by impregnation with potassium carbonate on supports such as activated carbon (AC), TiO₂, Al₂O₃, MgO, CaO, SiO₂ and various zeolites. The CO₂ capture capacity and regeneration property of various sorbents were measured in the presence of H₂O in a fixed bed reactor, during multiple cycles at various temperature conditions (CO₂ absorption at 50–100 °C and regeneration at 130–400 °C). The KAlI30, KCaI30, and KMgI30 sorbents formed new structures such as KAl(CO₃)₂(OH)₂, K₂Ca(CO₃)₂, K₂Mg(CO₃)₂, and K₂Mg(CO₃)₂·4(H₂O), which did not completely convert to the original K₂CO₃ phase at temperatures below 200 °C, during the CO₂ absorption process in the presence of 9 vol.% H₂O. In the case of KACI30, KTiI30, and KZrI30, only a KHCO₃ crystal structure was formed during CO₂ absorption. The formation of active species, K₂CO₃·1.5H₂O, by the pretreatment with water vapor and the formation of the KHCO₃ crystal structure after CO₂ absorption are important factors for absorption and regeneration, respectively, even at low temperatures (130–150 °C). In particular, the KTiI30 sorbent showed excellent characteristics with respect to CO₂ absorption and regeneration in that it satisfies the requirements of a large amount of CO₂ absorption (87 mg CO₂/g sorbent) without the pretreatment with water vapor, unlike KACI30, and a fast and complete regeneration at a low temperature condition (1 atm, 150 °C). In addition, the higher total CO₂ capture capacity of KMgI30 (178.6 mg CO₂/g sorbent) than that of the theoretical value (95 mg CO₂/g sorbent) was explained through the contribution of the absorption ability of MgO support. In this review, we introduce the CO₂ capture capacities and regeneration properties of several potassium-based sorbents, the changes in the physical properties of the sorbents before/after CO₂ absorption, and the role of water vapor and its effects on CO₂ absorption.     

Organometallic coordination polymers based on silver carboxylates: [AgCF<Subscript>3</Subscript>CO<Subscript>2</Subscript>(2,3-Et<Subscript>2</Subscript>Pyz)] and [AgCF<Subscript>3</Subscript>CO<Subscript>2</Subscript>(Bpeta)]

Coordination polymers [AgCF₃CO₂(2,3-Et₂Pyz)](I)(2,3-Et₂Pyz-C₈H₁₂N₂) and [AgCF₃CO₂(Bpeta)] (II) (Bpeta is 4′4-bipyridylethane, C₁₂H₁₂N₂) are synthesized. Their structures are determined. The crystals of compound I are monoclinic, space group P2(1)/n, a = 7.185(1), b = 14.754(1), c = 12.317(1)Å, β = 97.09(1)°, V = 1295.7(2) ų, ρ_calcd = 1.831 g/cm³, Z = 4. Structure I consists of infinite chains of doubled polymeric chains joined by silver carboxylate dimers [[Ag₂(CF₃CO₂)₂(Et₂Pyz)₂]_∞. The coordination polyhedron of Ag⁺ is a distorted tetrahedron. The crystals of compound II are orthorhombic, space group Pbca, a = 13.555(3), b = 13.991(3), c = 16.449(3) Å, V = 3119.5(11) ų, ρ_calcd = 1.725 g/cm³, Z = 8. Doubled polymeric chains with the Ag…Ag bond (3.16 Å) are also formed in structure II. Supramolecular layers are formed in the structure due to the weak π-π-stacking interaction between the aromatic groups of chains. The CF₃CO ₂ ^? anion is weakly bound to Ag⁺ (Ag-O_avg 2.790 Å).     

CO<Subscript>2</Subscript> and SO<Subscript>2</Subscript> uptake by oil shale ashes

In the present research, CO₂ and SO₂ binding ability of different oil shale ashes and the effect of pre-treatment (grinding, preceding calcination) of these ashes on their binding properties and kinetics was studied using thermogravimetric, SEM, X-ray, and energy dispersive X-ray analysis methods. It was shown that at 700 °C, 0.03–0.28 mmol of CO₂ or 0.16–0.47 mmol of SO₂ was bound by 100 mg of ash in 30 min. Pre-treatment conditions influenced remarkably binding parameters. Grinding decreased CO₂ binding capacities, but enhanced SO₂ binding in the case of fluidized bed ashes. Grinding of pulverized firing ashes increased binding parameters with both gases. Calcination at higher temperatures decreased binding parameters of both types of ashes with both gases studied. Clarification of this phenomenon was given. Kinetic analysis of the binding process was carried out, mechanism of the reactions and respective kinetic constants were determined. It was shown that the binding process with both gases was controlled by diffusion. Activation energies in the temperature interval of 500–700 °C for CO₂ binding with circulating fluidized bed combustion ashes were in the range of 48–82 kJ mol⁻¹, for SO₂ binding 43–107 kJ mol⁻¹. The effect of pre-treatment on the kinetic parameters was estimated.     

Thermal behavior of LDH 2CuAl.CO<Subscript>3</Subscript> and 2CuAl.CO<Subscript>3</Subscript>/Pd

Cu/Al layered double hydroxide (LDH) can be used as a catalyst for important processes such as cross-coupling reactions. This property may be improved by adding palladium by either impregnation or intercalation. Therefore, the LDH matrix and its composites with Pd⁰ or [PdCl₄]^2? have been prepared. By powder X-ray diffraction, FT-infrared spectroscopy, thermogravimetric and elemental analysis it was determined the LDH formula Cu₄Al₂(OH)₁₂CO₃.4H₂O, with malachite as the second phase. The LDH thermal decomposition occurs between 120 and 600 °C, having as intermediates the double oxi-hydroxide and the mixed oxide phases. At 800 °C the residue is composed of CuO and CuAl₂O₄. The composites were obtained employing [PdCl₄]^2? and Pd₂(dba)₃ as precursors, and the solvent choice for this process was shown to be of significant importance: the materials obtained using DMF had Pd impregnated in the surface, while the usage of water promoted the intercalation of [PdCl₄]^2? in the LDH matrix. The thermogravimetric analysis was able to distinguish the mode of supporting palladium between the composites being a reliable characterization for such task.     

Thermal behavior of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> mesoporous gels

Summary Titania-based photocatalytic materials were prepared by sol-gel method using Fe³⁺ and polyethyleneglycol (PEG₆₀₀) as additives. Thermogravimetry (TG), differential thermal analysis (DTA) and evolved gas analysis (EGA) with MS detection were used to elucidate processes that take place during heating of Fe³⁺ containing titania gels. The microstructure development of the Fe₂O₃/TiO₂ gel samples with and without PEG₆₀₀ admixtures was characterized by emanation thermal analysis (ETA) under in situ heating in air. A mathematical model was used for the evaluation of ETA results. Surface area and porosity measurements of the samples dried at 120&deg;C and the samples preheated for 1 h to 300 and 500&deg;C were compared. From the XRD measurements it was confirmed that the crystallization of anatase took place after thermal heating up to 600&deg;C.     

Application of thermogravimetric analysis to the evaluation of aminated solid sorbents for CO<Subscript>2</Subscript> capture

In this work a series of solid sorbents were synthesized by immobilizing liquid amines on the surface of a mesoporous alumina. The samples were chemically characterized and BET surface areas calculated from the N₂ adsorption isotherms at 77 K. The CO₂ capture performance of the sorbents and their thermal stability was studied by thermogravimetric methods. The effect of amine loading on the CO₂ capture performance of the prepared sorbents was also evaluated. Analysis of TG-DTG curves showed that thermal stabilization of the amines is significantly improved by immobilizing them on an inorganic support. Temperature-programmed CO₂ adsorption tests from 298 K up to 373 K at atmospheric pressure, proved to be a useful technique for assessing the capacity of sorbents for CO₂ capture. Alumina impregnated with diethylenetriamine presented the highest CO₂ adsorption capacities throughout the tested temperature range.     

Photocatalytic reduction of CO<Subscript>2</Subscript> over TiO<Subscript>2</Subscript> based catalysts

At present, carbon dioxide is considered the largest contributor among greenhouse gases. This review covers the current state of problem of carbon dioxide emissions from industrial and combustion processes, the principle of photocatalysis, existing literature related to photocatalytic CO₂ reduction over TiO₂ based catalysts and the effects of important parameters on the process performance including light wavelength and intensity, type of reductant, metal-modified surface, temperature and pressure. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

NaBO<Subscript>2</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-Na<Subscript>2</Subscript>WO<Subscript>4</Subscript> quaternary system

This is the first study of the NaBO₂-Na₂CO₃-Na₂MoO₄-Na₂WO₄ quaternary system by differential thermal analysis. Na₂[MoO_4(x)WO_{4(1 − x)}] solid solutions in the quaternary system are found to not decompose.