Aluminophosphates for CO₂ separation

Zeolites in Adsorption Processes: State of the Art and Future Prospects

Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.

Structural Transformation-Involved Synthesis of Nanosized ERI-Type Zeolite and Its Catalytic Property in the MTO Reaction

Nanosized ERI-type aluminophosphate was prepared by the calcination of a precursor material (denoted as ECNU-38P) synthesized using 1,1,6,6-tetramethyl-1,6-diazacyclododecane-1,6-diium hydroxide (TDDH) as a structure-directing agent. The structure of ECNU-38P is related to ERI topology but exhibits a highly disordered manner and contains both four- and six-coordinated Al atoms. In situ XRD patterns revealed a rarely reported temperature-induced three-dimensional (3D)-to-3D structural transformation from ECNU-38P to the ordered ERI-type ECNU-38 zeolite at 573-623 K. Nanosized ERI-type silicoaluminophosphate Si-ECNU-38 was also obtained by introducing Si atoms into the synthetic system of ECNU-38P. The catalytic performance of ERI-type silicoaluminophosphates in the methanol-to-olefin (MTO) reaction was revealed to be highly related to the crystal sizes.

Carbon dioxide capture with zeotype materials

This review describes the application of zeotype materials for the capture of CO2 in different scenarios, the critical parameters defining the adsorption performances, and the challenges of zeolitic adsorbents for CO2 capture.

Advances in Post-Combustion CO2 Capture by Physical Adsorption: From Materials Innovation to Separation Practice

The atmospheric CO₂ concentration continues a rapid increase to its current record high value of 416 ppm for the time being. It calls for advanced CO₂ capture technologies. One of the attractive technologies is physical adsorption-based separation, which shows easy regeneration and high cycle stability, and thus reduced energy penalties and cost. The extensive research on this topic is evidenced by the growing body of scientific and technical literature. The progress spans from the innovation of novel porous adsorbents to practical separation practices. Major CO₂ capture materials include the most widely used industrially relevant porous carbons, zeolites, activated alumina, mesoporous silica, and the newly emerging metal-organic frameworks (MOFs) and covalent-organic framework (COFs). The key intrinsic properties such as pore structure, surface chemistry, preferable adsorption sites, and other structural features that would affect CO₂ capture capacity, selectivity, and recyclability are first discussed. The industrial relevant variables such as particle size of adsorbents, the mechanical strength, adsorption heat management, and other technological advances are equally important, even more crucial when scaling up from bench and pilot-scale to demonstration and commercial scale. Therefore, we aim to bring a full picture of the adsorption-based CO₂ separation technologies, from adsorbent design, intrinsic property evaluation to performance assessment not only under ideal equilibrium conditions but also in realistic pressure swing adsorption processes.

Morphology, Activation, and Metal Substitution Effects of AlPO4-5 for CO2 Pressure Swing Adsorption

Aluminophosphate, AlPO₄-5, an AFI zeotype framework consisting of one-dimensional parallel micropores, and metal-substituted AlPO₄-5 were prepared and studied for CO₂ adsorption. Preparation of AlPO₄-5 by using different activation methods (calcination and pyrolysis), incorporation of different metals/ions (Fe, Mg, Co, and Si) into the framework using various concentrations, and manipulation of the reaction mixture dilution rate and resulting crystal morphology were examined in relation to the CO₂ adsorption performance. Among the various metal-substituted analogs, FeAPO-5 was found to exhibit the highest CO₂ capacity at all pressures tested (up to 4 bar). Among the Fe-substituted samples, xFeAPO-5, with x being the Fe/Al₂O₃ molar ratio in the synthesis mixture (range of 2.5:100–10:100), 5FeAPO-5 exhibited the highest capacity (1.8 mmol/g at 4 bar, 25°C) with an isosteric heat of adsorption of 23 kJ/mol for 0.08–0.36 mmol/g of CO₂ loading. This sample also contained the minimum portion of extra-framework or clustered iron and the highest mesoporosity. Low water content in the synthesis gel led to the formation of spherical agglomerates of small 2D-like crystallites that exhibited higher adsorption capacity compared to columnar-like crystals produced by employing more dilute mixtures. CO₂ adsorption kinetics was found to follow a pseudo–first-order model. The robust nature of AlPO₄-5–based adsorbents, their unique one-dimensional pore configuration, fast kinetics, and low heat of adsorption make them promising for pressure swing adsorption of CO₂ at industrial scale.

Selective Adsorption of CO2 on Zeolites NaK-ZK-4 with Si/Al of 1.8-2.8

Zeolites with appropriately narrow pore apertures can kinetically enhance the selective adsorption of CO₂ over N₂. Here, we showed that the exchangeable cations (e.g., Na⁺ or K⁺) on zeolite ZK-4 play an important role in the CO₂ selectivity. Zeolites NaK ZK-4 with Si/Al = 1.8-2.8 had very high CO₂ selectivity when an intermediate number of the exchangeable cations were K⁺ (the rest being Na⁺). Zeolites NaK ZK-4 with Si/Al = 1.8 had high CO₂ uptake capacity and very high CO₂-over-N₂ selectivity (1190). Zeolite NaK ZK-4 with Si/Al = 2.3 and 2.8 also had enhanced CO₂ selectivity with an intermediate number of K⁺ cations. The high CO₂ selectivity was related to the K⁺ cation in the 8-rings of the α-cage, together with Na⁺ cations in the 6-ring, obstructing the diffusion of N₂ throughout the zeolite. The positions of the K⁺ cation in the 8-ring moved slightly (max 0.2 Å) toward the center of the α-cage upon the adsorption of CO₂, as revealed by in situ X-ray diffraction. The CO₂-over-N₂ selectivity was somewhat reduced when the number of K⁺ cations approached 100%. This was possibly due to the shift in the K⁺ cation positions in the 8-ring when the number of Na⁺ was going toward 0%, allowing N₂ diffusion through the 8-ring. According to in situ infrared spectroscopy, the amount of chemisorbed CO₂ was reduced on zeolite ZK-4s with increasing Si/Al ratio. In the context of potential applications, a kinetically enhanced selection of CO₂ could be relevant for applications in carbon capture and bio- and natural gas upgrading.

pH Responsive Carboxymethyl Chitosan/Poly(amidoamine) Molecular Gate Membrane for CO2/N2 Separation

Efficient carbon dioxide separation is an emerging field of interest in the era of energy scarcity and environmental calamity. The present study focuses on the versatile aspects of carboxymethyl chitosan and dendrimer in terms of CO₂ separation. A comprehensive study has been accomplished to inspect the physicochemical properties of the prepared membrane. The mixed gas (CO₂/N₂) separation performances have been measured varying the temperature (60-110 °C) and sweep/feedwater flow ratio (0.33-3). The blend membrane containing 10 weight (wt.)% dendrimer presented highest CO₂ permeance of ∼100 GPU and CO₂/N₂ selectivity ∼149 on maintenance of sweep/feedwater flow ratio 2.33 and 1.67, respectively, at an operating temperature of 90 °C. The remarkable performance displayed by the membrane has been explained with reference to the dendrimer molecular gate mechanism and the salting out effect offered by carboxymethyl chitosan matrix.

Proton Acidity and Proton Mobility in ECR-40, a Silicoaluminophosphate that Violates Löwenstein's Rule

The silicoaluminophosphate zeotype ECR-40 contains linkages of AlO4 tetrahedra via a common oxygen atom, thereby violating the famous "Löwenstein's rule". In this work, a combination of static density functional theory (DFT) calculations and DFT-based ab-initio molecular dynamics (AIMD) simulations were employed to study the acidity and mobility of protons associated with such unusual linkages. It was found that the Al-O-Al linkages are preferentially protonated, as deprotonation causes a local accumulation of negative charge. The protons at these linkages possess a somewhat lower Brønsted acidity than those at Si-O-Al links. AIMD simulations for fully hydrated ECR-40 predicted a partial deprotonation of the Al-O-Al linkages, whereas Si-O-Al linkages were fully deprotonated. Frequently, a coordination of water molecules to framework Al atoms was observed in the vicinity of the Al-O-Al links. Hence, these linkages appear prone to break upon dehydration, potentially explaining why Löwenstein's rule is mostly obeyed in materials formed in aqueous media.

Enhancing CO2 Adsorption and Separation Properties of Aluminophosphate Zeolites by Isomorphous Heteroatom Substitutions

Mg, Co-substituted aluminophosphate zeolites with ERI framework topology (denoted as MgAPO-ERI and CoAPO-ERI) have been synthesized under hydrothermal conditions by using N, N, N', N'-tetramethyl-1,6-hexanediamine as organic template. Their CO₂ adsorption properties are investigated in comparison to those of the pure aluminophosphate counterpart AlPO-ERI. CoAPO-ERI shows the highest CO₂ uptake of 57.3 cm³ g^-1 (273 K and 1 bar) and the highest isosteric heat of 39.0 kJ mol^-1 among the three samples. Importantly, the incorporation of Mg²⁺ and Co²⁺ ions in the framework of AlPO-ERI can greatly improve the adsorption selectivities of CO₂ over CH₄ and N₂. Whereafter, transient breakthrough simulations were investigated and further proved the advantages of heteroatoms for separations. These results demonstrate that isomorphous heteroatom substitutions in aluminophosphate zeolites play a key role in enhancing CO₂ adsorption and separation abilities.

Molecular Layer Deposition-Modified 5A Zeolite for Highly Efficient CO2 Capture

Effective pore mouth size of 5A zeolite was engineered by depositing an ultrathin layer of microporous TiO₂ on its external surface and appropriate pore misalignment at the interface. As a result, a slightly bigger N₂ molecule (kinetic diameter: 0.364 nm) was effectively excluded, whereas CO₂ (kinetic diameter: 0.33 nm) adsorption was only influenced slightly. The prepared composite zeolite sorbents showed an ideal CO₂/N₂ adsorption selectivity as high as ∼70, a 4-fold increase over uncoated zeolite sorbents, while maintaining a high CO₂ adsorption capacity (1.62 mmol/g at 0.5 bar and 25 °C) and a fast CO₂ adsorption rate.

Computational evaluation of aluminophosphate zeotypes for CO2/N2 separation

Zeolites and structurally related materials (zeotypes) have received considerable attention as potential adsorbents for selective carbon dioxide adsorption. Within this group, zeotypes with aluminophosphate composition (AlPOs) could be an interesting alternative to the more frequently studied aluminosilicate zeolites. So far, however, only a few AlPOs have been characterised experimentally in terms of their CO₂ adsorption properties. In this study, force-field based grand-canonical Monte Carlo (GCMC) simulations were used to evaluate the potential of AlPOs for CO₂/N₂ separation, a binary mixture that constitutes a suitable model system for the removal of carbon dioxide from flue gases. A total of 51 frameworks were considered, all of which have been reported either as pure AlPOs or as heteroatom-containing AlPO derivatives. Prior to the GCMC simulations, all structures were optimised using dispersion-corrected density-functional theory calculations. The potential of these 51 systems for CO₂/N₂ separation was assessed in preliminary calculations (Henry constants and CO₂ uptake at selected pressures). On the basis of these calculations, 21 AlPOs of particular interest were selected, for which 15 : 85 CO₂/N₂ mixture adsorption isotherms were calculated up to 10 bar. For adsorption-based separations using an adsorption pressure of 1 bar (vacuum-swing adsorption), AlPOs with GIS, ATN, ATT, and SIV topologies were predicted to be most attractive, as they combine high CO₂/N₂ selectivities (75 to 140) and reasonable CO₂ working capacities (1 to 1.7 mmol g^-1). Under pressure-swing adsorption conditions, there is a tradeoff between selectivity and working capacity: while highly selective AlPOs like GIS reach only moderate working capacities, the frameworks with the highest working capacities above 2 mmol g^-1, AFY, KFI, and SAV, have lower selectivities between 25 and 35. To gain atomic-level insights into the host-guest interactions, interaction energy maps were computed for selected AlPOs. The computational assessment presented here can guide future experimental efforts in the development and optimisation of AlPO-based adsorbents for selective CO₂ adsorption.

Food wastes derived adsorbents for carbon dioxide and benzene gas sorption

Food wastes are produced worldwide in large quantities that could have potential to produce higher value products, including industrial adsorbents. The present work attempts valorization of food waste by CO₂ activation and functionalization through nitric acid and melamine treatment. The prepared porous materials were subjected to gas phase adsorption of CO₂ and benzene gases. The resultant highly porous carbon materials with surface area range from 797 to 1025 m²/g were synthesized showing uptake capacities of 4.41, 4.07, 4.18 and 4.36 mmol/g of CO₂ and 345, 305, 242.5 and 380.7 mg/g of C₆H₆ respectively for PyF515, PyF520, PyF715 and PyF720 in the absence of doped carbon matrix. Differential thermogravimetric (DTG) analysis showed the thermostability of the precursors to validate selected initial pyrolysis temperatures (500 and 700 °C). C₆H₆ sorption lies mainly in the physisorption region for all adsorbents ensuring re-generation potential. PyF720 and PyF520 recorded the highest isosteric enthalpy of 64.4 kJ/mol and 48.7 kJ/mol respectively, despite the low degree of coverage of the latter. Thus, PyF515 and PyF720 demonstrated the potential for use as sustainable and cost effective adsorbents for benzene gas containment suitable for swing adsorption system.

Composite 5A zeolite with ultrathin porous TiO2 coating for selective gas adsorption

A composite zeolite adsorbent was prepared by conformally depositing an ultrathin porous TiO₂ coating on the external surface of the 5A zeolite by molecular layer deposition (MLD) and subsequent calcination.

Carbon dioxide capture utilizing zeolites synthesized with paper sludge and scrap-glass

The present work introduces the study of the CO2 capture process by zeolites synthesized from paper sludge and scrap glass. Zeolites ZSM-5, analcime and wairakite were produced by means of two types of Structure Directing Agents (SDA): tetrapropilamonium (TPA) and ethanol. On the one hand, zeolite ZSM-5 was synthesized using TPA; on the other hand, analcime and wairakite were produced with ethanol. The temperature programmed desorption (TPD) technique was performed for determining the CO2 sorption capacity of these zeolites at two sorption temperatures: 50 and 100 °C. CO2 sorption capacity of zeolite ZSM-5 synthesized at 50 °C was 0.683 mmol/g representing 38.2% of the value measured for a zeolite ZSM-5 commercial. Zeolite analcime showed a higher CO2 sorption capacity (1.698 mmol/g) at 50 °C and its regeneration temperature was relatively low. Zeolites synthesized in this study can be used in the purification of biogas and this will produce energy without increasing the atmospheric CO2 concentrations.

Oxidative degradation of silica-supported polyethylenimine for CO2 adsorption: insights into the nature of deactivated species

The oxidative degradation of polyethylenimine-impregnated mesoporous SBA-15 silica for CO2 capture was investigated at the molecular level. The adsorbents were exposed to flowing air at different temperatures, and their degree of deactivation was evaluated through the measurement of CO2 adsorption capacity prior and subsequent to air exposure. A solvent-extraction method was employed to isolate the deactivated species from the silica support. The extracted species were investigated by a variety of 1D and 2D NMR techniques such as (13)C, (1)H, (1)H-(15)N HMBC, (1)H-(13)C HMQC, and (1)H-(13)C HMBC. This in-depth investigation showed that they contain predominantly fragments involving imine and carbonyl groups. Several structural units were conclusively established.

Aluminophosphate monoliths with high CO2-over-N2 selectivity and CO2 capture capacity

Monoliths of microporous aluminophosphates (AlPO₄-17 and AlPO₄-53) were structured by binder-free pulsed current processing.

Cation-exchanged SAPO-34 for adsorption-based hydrocarbon separations: predictions from dispersion-corrected DFT calculations

The interaction of C₂ and C₃ hydrocarbons with cation-exchanged SAPO-34 materials is studied using DFT-D calculations, permitting predictions regarding their suitability for alkene–alkane separations.

Zeolites and related sorbents with narrow pores for CO2 separation from flue gas

Adsorbents with small pores are especially relevant for capturing carbon dioxide at large emission sources.

Synthesis of DNL-6 with a high concentration of Si (4 Al) environments and its application in CO(2) separation

The synthesis of DNL-6 with a high concentration of Si (4 Al) environments [Si/(Si+Al+P)=0.182 mol, denoted as M-DNL-6] is demonstrated. This represents the highest reported concentration of such environments in silicoaluminophosphate (SAPO) molecular sieves. Adsorption studies show that the high Si (4 Al) content in M-DNL-6, with an increased number of Brønsted acid sites in the framework, greatly promotes the adsorption of CO(2). M-DNL-6 exhibits a large CO(2) uptake capacity of up to 6.18 mmol g(-1) at 273 K and 101 kPa, and demonstrates high ratios of CO(2)/CH(4) and CO(2)/N(2) separation. From breakthrough and cycling experiments, M-DNL-6 demonstrates the ability to completely separate CO(2) from CH(4) or N(2) with a dynamic capacity of approximately 8.0 wt % before breakthrough. Importantly, the adsorbed CO(2) is easily released from the adsorbent through a simple gas purging operation at room temperature to regain 95 % of the original adsorption capacity. These results suggest that M-DNL-6 can be used as a potential adsorbent for CO(2) capture in pressure swing adsorption processes.

ZK-5: a CO₂-selective zeolite with high working capacity at ambient temperature and pressure

The increased carbon dioxide concentration in the atmosphere caused by combustion of fossil fuels has been a leading contributor to global climate change. The adsorption-driven pressure or vacuum swing (PSA/VSA) processes are promising as affordable means for the capture and separation of CO₂. Herein, an 8-membered-ring zeolite ZK-5 (Framework Type Code: KFI) exchanged with different cations (H⁺, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺) was synthesized as novel CO₂ adsorbent. The samples were characterized by SEM, energy-dispersive X-ray spectroscopy (EDAX), XRD, and gas adsorption (CO₂ and N₂). The Toth adsorption model was used to describe the CO₂ adsorption isotherms, and the isosteric heats of adsorption were calculated. CO₂ capture adsorbent evaluation criteria such as working capacity, regenerability and CO₂/N₂ selectivity were applied to evaluate the zeolite adsorbents for PSA/VSA applications. The in situ FTIR CO₂ adsorption spectra show that physisorption accounts for the largest fraction of the total CO₂ adsorbed. The CO₂ adsorption analysis shows that Mg-ZK-5 is the most promising adsorbent for PSA applications with the highest working capacity (ΔN(CO₂)=2.05 mmol g⁻¹), excellent selectivity (α(CO₂/N₂)=121), and low isosteric heat. Li-, Na- and K-ZK-5 with good working capacity (ΔN(CO₂)=1.55-2.16 mmol g⁻¹) and excellent selectivity (α(CO₂/N₂)=103-128) are promising CO₂ adsorbents for the VSA working region.