Adsorption of CO2 on Hydrotalcite‐like Compounds in a Fixed Bed

Abstract

The reduction of carbon dioxide emission from flue gases can be achieved using post‐combustion technologies, such as adsorption employing efficient solid sorbents. In this work, the adsorption of CO₂ on hydrotalcite‐like Al‐Mg compounds partially carbonated was studied using dynamic and static methods. The breakthrough curves were obtained at different flow gas rates in the range 60 to 100 mL/min and total pressure 1.0 atm. Different mixtures of CO₂ diluted in helium were used (3–20% v/v) at temperatures in the range 29 to 350°C. The experimental equilibrium data were described according to a Langmuir‐like equation. The capacity of adsorption presented a weak dependence on the temperature due to opposite effects of increasing of entropy and increasing of MgO (non‐carbonated) content in the adsorbent at high temperatures. The linear driving force model was suitable to describe the breakthrough curves. The dispersion and mass transfer coefficients were calculated by theoretical correlations and the model described quite very well the adsorption of CO₂ on hydrotalcite‐like compounds in a fixed bed in any temperature.     

Production of Monoacylglycerols from Fully Hydrogenated Palm Oil Catalyzed by Hydrotalcite Loaded with K2CO3

This study reports a new method of producing high-purity monoacylglycerols (MAGs) by glycerolysis of fully hydrogenated palm oil (FHPO) catalyzed by hydrotalcite loaded with K₂CO₃ (K₂CO₃/HT). The effects of reaction temperature, reaction time, catalyst (K₂CO₃/HT) loading, and mass ratio of FHPO to glycerol on glycerolysis were investigated. The selected conditions included a reaction temperature of 200°C, K₂CO₃/HT loading at 0.8 wt.% (FHPO mass), a 5:2 mass ratio of FHPO to glycerol, and a reaction time of 2 h. Under these selected conditions, the yield of MAGs in the acylglycerol phase reached 46.8 wt.%. A two-stage molecular distillation was introduced to purify MAGs, and the final MAG product was obtained with a purity of 96.6 wt.% and a recovery of 96.8%. Furthermore, the recycled K₂CO₃/HT was reactivated with restored catalytic efficiency through impregnation, carbonation, and recalcination.     

CO2 Adsorption by Modified Palm Shell Activated Carbon (PSAC) Via Chemical and Physical Activation and Metal Impregnation

The aim of this study was to verify the ability of nickel-impregnated palm shell activated carbon (PSAC) for CO₂ adsorption and compare its performance with the chemically and physically activated PSAC. Sodium hydroxide and CO₂ were used as activating agents for chemical and physical activation, respectively. Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) was used as a precursor for metal impregnation. The effect of different chemical loadings (NaOH: 20–50 wt%), metal impregnation (Ni(NO₃)₂·6H₂O: 16–28 wt%), and heat treatment time (1–4 h) was studied as parameters. Adsorption capacity was calculated using breakthrough graphs. The effect of humidity on CO₂ adsorption and desorption of CO₂ was also investigated in this study. The results revealed that chemically modified PSAC yields the highest adsorption capacity (48.2 mg/g) compared to other methods of activation. Interestingly, it was found that the adsorption capacity of nickel-impregnated PSAC was similar to other types of metal-impregnated activated carbon. Humidity gave a negative effect on CO₂ adsorption. In summary, results showed that chemical activation is an efficient technique to modify PSAC for CO₂ adsorption.     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.     

Simplified synthesis of K<Subscript>2</Subscript>CO<Subscript>3</Subscript>-promoted hydrotalcite based on hydroxide-form precursors: Effect of Mg/Al/K<Subscript>2</Subscript>CO<Subscript>3</Subscript> ratio on high-temperature CO<Subscript>2</Subscript> sorption capacity

Hydrotalcite was synthesized from hydroxide-form precursors to prepare a novel high-temperature CO₂ sorbent, and the effect of Mg/Al ratio on CO₂ sorption was studied. To enhance the CO₂ sorption capacity of the sorbent, K₂CO₃ was coprecipitated during the synthetic reaction. X-ray diffraction analysis indicated that the prepared samples had a well-defined crystalline hydrotalcite structure, and confirmed that K₂CO₃ was successfully coprecipitated in the samples. The morphology of the hydrotalcite was confirmed by scanning electron microscopy, and N₂ adsorption analysis was used to estimate its surface area and pore volume. In addition, thermogravimetric analysis was used to measure its CO₂ sorption capacity, and the results revealed that the Mg: Al: K₂CO₃ ratio used in the preparation has an optimum value for maximum CO₂ sorption capacity.     

CO2 reforming of CH4 over Ni/Mg–Al oxide catalysts prepared by solid phase crystallization method from Mg–Al hydrotalcite-like precursors

Ni supported catalysts were prepared by the solid phase crystallization (spc) method starting from hydrotalcite (HT) anionic clay based on [Mg₆Al₂(OH)₁₆CO₃ ^2–]H₂O as the precursor. The precursors were prepared by the co-precipitation method from nitrates of the metal components, and then thermally decomposed, in situ reduced to form Ni supported catalysts (spc-Ni/Mg–Al) and used for the CO₂ reforming of CH₄ to synthesis gas. Ni²⁺ can well replace the Mg²⁺ site in the hydrotalcite, resulting in the formation of highly dispersed Ni metal particles on spc-Ni/Mg–Al. The spc-catalyst thus prepared showed higher activity than those prepared by the conventional impregnation (imp) method such as Ni/-Al₂O₃ and Ni/MgO. When Ni was supported by impregnation of Mg–Al mixed oxide prepared from Mg–Al HT, the activity of imp-Ni/Mg–Al thus prepared was not so low as those of Ni/-Al₂O₃ and Ni/MgO but close to that of spc-Ni/Mg–Al. The relatively high activity of imp-Ni/Mg–Al may be due to the regeneration of the Mg–Al HT phase from the mixed oxide during the preparation, resulting in an occurring of the incorporation of Ni²⁺ in the Mg²⁺ site in the HT as seen in the spc-method. Such an effect may give rise to the formation of highly dispersed Ni metal species and afford high activity on the imp-Ni/Mg–Al.     

CO2 capture using some fly ash-derived carbon materials

Adsorption is considered to be one of the more promising technologies for capturing CO₂ from flue gases. For post-combustion capture, the success of such an approach is however dependent on the development of an adsorbent that can operate competitively at relatively high temperatures. In this work, low cost carbon materials derived from fly ash, are presented as effective CO₂ sorbents through impregnation these with organic bases, for example, polyethylenimine aided by polyethylene glycol. The results show that for samples derived from a fly ash carbon concentrate, the CO₂ adsorption capacities were relatively high (up to 4.5 wt%) especially at high temperatures (75 °C), where commercial active carbons relying on physi-sorption have low capacities. The addition of PEG improves the adsorption capacity and reduces the time taken for the sample to reach the equilibrium. No CO₂ seems to remain after desorption, suggesting that the process is fully reversible.     

CO2 capture by adsorption with nitrogen enriched carbons

The success of CO₂ capture with solid sorbents is dependent on the development of a low cost sorbent with high CO₂ selectivity and adsorption capacity. Immobilised amines are expected to offer the benefits of liquid amines in the typical absorption process, with the added advantages that solids are easy to handle and that they do not give rise to corrosion problems. In this work, different alkylamines were evaluated as a potential source of basic sites for CO₂ capture, and a commercial activated carbon was used as a preliminary support in order to study the effect of the impregnation. The amine coating increased the basicity and nitrogen content of the carbon. However, it drastically reduced the microporous volume of the activated carbon, which is chiefly responsible for CO₂ physisorption, thus decreasing the capacity of raw carbon at room temperature.     

CO2 adsorption performance of polyethyleneimine-modified ion-exchange resin

A series of solid amine adsorbents were prepared by the template method with ion-exchange resin (D001) as the carrier and polyethyleneimine (PEI) as the modifier. The absorbents were characterized by energy disperse spectroscopy (EDS), scanning electron microscope (SEM), N₂ adsorption–desorption, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) techniques. The effects of PEI loading, adsorption temperature and influent velocities on CO₂ adsorption capacity in a fixed-bed reactor were investigated. The results show that the solid amine adsorbent prepared by the template method had a better PEI dispersion, stability and CO₂ adsorption capacity. The maximum CO₂ adsorption capacity was 3.98 mmol·g^?1 when PEI loading was 30%, the adsorption temperature was 65°C and the influent velocity was 40 mL·min^?1. The CO₂ adsorption capacity decreased only by 9.50% after 10 cycles of adsorption–desorption tests. The study of kinetics indicates that both chemical adsorption and physical adsorption occurred in the CO₂ adsorption process. The CO₂ adsorption process included fast breakthrough adsorption and gradually approaching equilibrium stage. The particle internal diffusion process was the control step for CO₂ adsorption.     

Mesoporous Silica Sorbents Impregnated with Blends of Tetraethylenepentamine and Alkanolamine for CO2 Separation

Mesocellular silica foam (MSU-F) supports were functionalized via wet impregnation with various amine and alcohol compounds for use as high-capacity adsorbents for CO₂ separation. The effect of the amino, hydroxyl, and ether functional groups in the impregnating mixture on the CO₂ adsorption capacity was investigated. Chemical adsorption was controlled by the composition of the compounds, and the blending effect on the adsorption performance was dependent on the temperature. MSU-F (30 wt.%) impregnated with a mixture of tetraethylenepentamine (40 wt.%) and aminoethylethanolamine (30 wt.%) showed a high adsorption capacity of 5.4 mmol/g at 333 K for 15 kPa CO₂.     

NOx storage/reduction catalysts based in cobalt/copper hydrotalcites

The use of materials based on hydrotalcites as NO_x storage/reduction (NSR) catalysts has been investigated, examining their activity at low temperature and their resistance to poisons such as H₂O and SO₂. The results obtained show that catalysts derived from Mg/Al hydrotalcites containing copper or cobalt is active at low temperatures, specially the samples containing 10 or 15% of Co. The addition of 1 wt% of transition metals with redox properties such as Pt, Pd, V and Ru to the hydrotalcite increases its activity because the combination of the redox properties of these metals and the acid-base properties of the hydrotalcite. The best results were obtained with the catalyst derived from a hydrotalcite with a molar ratio Co/Mg/Al = 15/60/25 and containing 1 wt% V. This material shows a higher activity, at low temperatures and in the presence of H₂O and SO₂, than a Pt–Ba/Al₂O₃ reference catalyst.     

Adsorptive removal of trace NO from a CO2 stream over transition-metal-oxide-modified HZSM-5

This study investigates the adsorption performance of a number of absorbents prepared by incipient wetness impregnation of 3.85 wt% Fe, Co, Ni, Mn, Co and Ce oxides on HZSM-5 for the removal of trace NO (150–200 ppm) from a CO₂ stream to produce food-grade CO₂. The adsorbents were characterized using X-ray diffraction, X-ray fluorescence, and N₂ adsorption with their performances evaluated in a fixed-bed flow system. The investigations revealed NO removal of better than 0.1 ppm from the CO₂ stream. The breakthrough capacities of the adsorbents for NO removal in the presence of O₂ were found to significantly increase. The converse was observed in the case of NO adsorption selectivity. Co/HZSM-5(25) was found to be the best adsorbent under all the conditions tested producing CO₂ purities of better than 99.9999%. This is significantly higher than for Fe–Mn mixed oxides, claimed to be the best NO adsorbent reported in the literature for CO₂ purification.     

Triptycene-based microporous polyimides: Synthesis and their high selectivity for CO2 capture

A series of triptycene-based porous polyimides (STPIs) were synthesized by condensation polymerizations. The structure and properties of these STPIs were characterized by IR, solid ¹³C NMR, powder XRD, SEM, TEM and gas absorption. As STPI-2 with the high thermal stability, they display excellent adsorption ability to CO₂ uptake capacity of 14.6 wt% (273 K), and exhibit the high selectivity of CO₂/N₂ of 107.     

Separation of CO2/CH4 mixtures with the MIL-53(Al) metal–organic framework

Adsorptive separation of CH₄/CO₂ mixtures was studied using a fixed-bed packed with MIL-53(Al) MOF pellets. Such pellets of MIL-53(Al) were produced using a polyvinyl alcohol binder. As revealed by N₂ adsorption isotherms, the use of polyvinyl alcohol as binder results in a loss in overall capacity of 32%. Separations of binary mixtures in breakthrough experiments were successfully performed at pressures varying between 1 and 8 bar and different mixture compositions. The binary adsorption isotherms reveal a preferential adsorption of CO₂ compared to CH₄ over the whole pressure and concentration range. The separation selectivity was affected by total pressure; below 5 bar, a constant selectivity, with an average separation factor of about 7 was observed. Above 5 bar, the average separation factor decreases to about 4. The adsorption selectivity is affected by breathing of the framework and specific interaction of CO₂ with framework hydroxyl groups. CO₂ desorption can be realised by mild thermal treatment.     

混合胺改性SBA-15的二氧化碳吸附特性

为实现廉价高效的二氧化碳捕集，新型燃烧后CO₂捕集固体吸附材料的设计和开发具有重要的研究意义。为提高CO₂吸附量，胺功能化改性吸附剂的方法主要有湿浸渍和表面嫁接。基于此，提出了“混合胺”修饰的概念，把湿浸渍和表面嫁接两种改性技术结合起来。把3-氨丙基三甲氧基硅烷（APTS）嫁接到分子筛SBA-15孔道表面，再把聚乙烯亚胺（PEI）浸渍到载体孔道的间隙，制备出高密度胺功能化的CO₂吸附剂。主要考察了不同含量的PEI和APTS功能化SBA-15的结构性能、CO₂吸附量及胺吸附效率。CO₂吸附结果表明，混合胺功能化SBA-15吸附主要依赖于动力学扩散。其中，SBA-15-（APTS-0.5-PEI-50），SBA-15-（APTS-1.0-PEI-50）和SBA-15-（APTS-2.0-PEI-30）在75℃时具有很好的吸附潜力。混合胺功能化SBA-15的胺吸附效率介于单纯嫁接和单纯浸渍的胺功能化SBA-15之间。     

负载型钾基CO2吸收剂的结构表征和碳酸化反应特性

The structure identification and carbonation characteristics of several potassium-based supported sorbents for CO₂ capture were investigated with TGA,XRD,XRF,SEM and N₂ adsorption method.Potassium-based sorbents were prepared by impregnation with potassium carbonate on supports,such as cocoanut activated charcoal（AC1）,coal active carbon（AC2）,silica gel（SG）and diatomite（DT）using the iso-volume impregnation method.The results showed that the K₂CO₃ loading amounts of AC1,AC2,SG and DT were 24.1%,22.5%,21.7% and 19.1% respectively.The surface area and pore volume of K₂CO₃/AC1 and K₂CO₃/AC2 were larger than others.In contrast,those of K₂CO₃/DT were small.For K₂CO₃/SG,pore volume was large but surface area was small.K₂CO₃/AC1 and K₂CO₃/AC2 showed excellent carbonation capacity.However,the carbonation capacities of K₂CO₃/SG and K₂CO₃/DT were low.The difference in carbonation capacity of those sorbents were attributed to the difference in pore structure,leading to different supported sorbents.     

Effect of Si/Al Ratio on CO2 – CH4 Adsorption and Selectivity in Synthesized SAPO-34

Impact of Si/Al ratio on the adsorption capacity and separation selectivity of CO₂/CH₄ in SAPO-34 has been investigated. SAPO-34 samples were synthesized with two Si/Al ratios (0.2 and 0.3). A batch adsorption volumetric apparatus was used to measure the adsorption equilibrium capacity and derive the equilibrium isotherms. The tests were performed in a wide range of pressure from normal to 3000 kPa and three levels of temperatures from 277 to 298 K. Results proved decreasing Si/Al ratio, from 0.3 to 0.2, improved CO₂ separation from CH₄.     

NOx storage behavior and sulfur-resisting performance of the third-generation NSR catalysts Pt/K/TiO2–ZrO2

The NO_x storage and reduction (NSR) catalysts Pt/K/TiO₂–ZrO₂ were prepared by an impregnation method. The techniques of XRD, NH₃-TPD, CO₂-TPD, H₂-TPR and in situDRIFTS were employed to investigate their NO_x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO_x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO₂-TPD results indicate that the decomposition performance of K₂CO₃ is largely determined by the surface property of the TiO₂–ZrO₂ support. The interaction between the surface hydroxyl of the support and K₂CO₃ promotes the decomposition of K₂CO₃ to form –OK groups bound to the support, leading to low NO_x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K₂CO₃ species and Lewis acid sites gives rise to high NO_x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO₂–ZrO₂ support is 650 °C.     

Comparison of NO adsorbing ability and process on Pt/Mg/Al oxide catalysts prepared by different methods

Pt/Mg/Al metal oxide catalysts were prepared by impregnation and co-precipitation methods, respectively. These samples were characterized by BET, XRD and NO-TPD; their NO _X storage property and adsorbing intermediate species were investigated with NSC and FTIR. The results showed that the prepared methods exert significant influence on the physical structure properties and the adsorption abilities of NO. (Pt)/Mg/Al samples prepared by impregnation (IM) have larger specific areas and higher NO _X storage capacity than (Pt)/Mg/Al catalysts prepared by co-precipitation (CP). The intermediate species of NO adsorbing process indicated that NO was firstly adsorbed as bridged nitrites both on Pt/Mg/Al (IM) and on Pt/Mg/Al (CP), then on Pt/Mg/Al (IM) the nitrites transferred into monodentate and bidentate nitrate species while on Pt/Mg/Al (CP) the nitrites only transferred into monodentate nitrate species.     

Exploring Potential Methods for Anchoring Amine Groups on the Surface of Activated Carbon for CO2 Adsorption

Activated carbon can be effectively modified for CO₂ adsorption with amine groups due to their high affinity for CO₂. Using approaches such as impregnation, some modifiers containing amine groups are physically adsorbed on the surface of carbon, whereas other amine groups can be directly or indirectly chemically bound to the activated carbon matrix. In the context of exploring potential techniques for grafting amine groups onto activated carbon surfaces, we herein review the literature on modifications applied to different materials and supports for a variety of applications, limited to neither activated carbon nor CO₂-adsorption applications. We focus on the processes of grafting amine groups and the parameters influencing these processes. Moreover, the mechanism of CO₂ adsorption involving amine groups is discussed.