Single-gas Adsorption Isotherms Research Articles

Desolvation-Degree-Induced Structural Dynamics in a Rigid Cerium-Organic Framework Exhibiting Tandem Purification of Ethylene from Acetylene and Ethane.

Due to the industrial requirements for high production and high quality of ethylene, efficient purification of ethylene from acetylene and ethane is of prime importance but challenging. Dynamic metal-organic frameworks (MOFs) have demonstrated intriguing structural dynamics and diverse applications recently. Among them, although a few flexible ones have exhibited interesting ethylene purification capability, rigid ones were yet barely investigated for such purpose. In this regard, a cerium(III)-based MOF was solvothermally synthesized, which is rigid and assembled from rod molecular building blocks associated with coordinative N,N-dimethylformamide (DMF) molecules. After liberating different degrees of DMF ligands via heating under vacuum or acetone exchange, both partially desolvated compounds of Ce-MOF-1 and Ce-MOF-2 were concertedly isolated in a fashion of single-crystal to single-crystal transformation. Although both newly generated materials crystallize in the same space group, they exhibit dissimilar unit cell parameters and slightly distinct ultramicropore sizes and pore microenvironments, thanks to the discrepancy in the desolvation degree. Consequently, Ce-MOF-1 and Ce-MOF-2 individually demonstrate C2H2- and C2H6-selective adsorption behavior, resulting in the potential tandem separation of C2H4 from C2H2 and C2H6 mixtures. The above results were successfully supported by not only single gas adsorption isotherms but also grand canonical Monte Carlo (GCMC) calculation studies and dynamic breakthrough experiments. The present work may pave the way for rigid MOFs aiming at advancing applications via solid-state structural dynamics.

Zirconium and Aluminum MOFs for Low-Pressure SO2 Adsorption and Potential Separation: Elucidating the Effect of Small Pores and NH2 Groups.

Finding new adsorbents for the desulfurization of flue gases is a challenging task but is of current interest, as even low SO2 emissions impair the environment and health. Four Zr- and eight Al-MOFs (Zr-Fum, DUT-67(Zr), NU-1000, MOF-808, Al-Fum, MIL-53(Al), NH2-MIL-53(Al), MIL-53(tdc)(Al), CAU-10-H, MIL-96(Al), MIL-100(Al), NH2-MIL-101(Al)) were examined toward their SO2 sorption capability. Pore sizes in the range of about 4-8 Å are optimal for SO2 uptake in the low-pressure range (up to 0.1 bar). Pore widths that are only slightly larger than the kinetic diameter of 4.1 Å of the SO2 molecules allow for multi-side-dispersive interactions, which translate into high affinity at low pressure. Frameworks NH2-MIL-53(Al) and NH2-MIL-101(Al) with an NH2-group at the linker tend to show enhanced SO2 affinity. Moreover, from single-gas adsorption isotherms, ideal adsorbed solution theory (IAST) selectivities toward binary SO2/CO2 gas mixtures were determined with selectivity values between 35 and 53 at a molar fraction of 0.01 SO2 (10.000 ppm) and 1 bar for the frameworks Zr-Fum, MOF-808, NH2-MIL-53(Al), and Al-Fum. Stability tests with exposure to dry SO2 during ≤10 h and humid SO2 during 5 h showed full retention of crystallinity and porosity for Zr-Fum and DUT-67(Zr). However, NU-1000, MOF-808, Al-Fum, MIL-53(tdc), CAU-10-H, and MIL-100(Al) exhibited ≥50-90% retained Brunauer-Emmett-Teller (BET)-surface area and pore volume; while NH2-MIL-100(Al) and MIL-96(Al) demonstrated a major loss of porosity under dry SO2 and MIL-53(Al) and NH2-MIL-53(Al) under humid SO2. SO2 binding sites were revealed by density functional theory (DFT) simulation calculations with adsorption energies of -40 to -50 kJ·mol-1 for Zr-Fum and Al-Fum and even above -50 kJ·mol-1 for NH2-MIL-53(Al), in agreement with the isosteric heat of adsorption near zero coverage (ΔHads0). The predominant, highest binding energy noncovalent binding modes in both Zr-Fum and Al-Fum feature μ-OHδ+···δ-OSO hydrogen bonding interactions. The small pores of Al-Fum allow the interaction of two μ-OH bridges from opposite pore walls with the same SO2 molecule via OHδ+···δ-OSOδ-···δ+HO hydrogen bonds. For NH2-MIL-53(Al), the DFT high-energy binding sites involve NHδ+···δ-OS together with the also present Al-μ-OHδ+···δ-OS hydrogen bonding interactions and C6-πδ-···δ+SO2, Nδ-···δ+SO2 interactions.

Selective CO2 Sorption Using Compartmentalized Coordination Polymers with Discrete Voids*.

Carbon capture and storage with porous materials is one of the most promising technologies to minimize CO2 release into the atmosphere. Here, we report a family of compartmentalized coordination polymers (CCPs) capable of capturing gas molecules in a selective manner based on two novel tetrazole-based ligands. Crystal structures have been modelled theoretically under the Density Functional Theory (DFT) revealing the presence of discrete voids of 380 Å3 . Single gas adsorption isotherms of N2 , CH4 and CO2 have been measured, obtaining a loading capacity of 0.6, 1.7 and 2.2 molecules/void at 10 bar and at 298 K for the best performing material. Moreover, they present excellent selectivity and regenerability for CO2 in mixtures with CH4 and N2 in comparison with other reported materials, as evidenced by dynamic breakthrough gas experiments. These frameworks are therefore great candidates for separation of gas mixtures in the chemical engineering industry.

Interplay of Tri- and Bidentate Linkers to Evolve Micropore Environment in a Family of Quasi-3D and 3D Porous Coordination Polymers for Highly Selective CO2 Capture.

In the design and construction of porous materials, these with exceptional structure and composition are often highly expected, as they may offer unique nanopore space for desired applications. Here, a new family of quasi-3D and a 3D porous coordination polymers (PCPs) (termed NTU-43 to NTU-50) were constructed via an evolution strategy from a layered structure (termed NTU-42). Single gas adsorption isotherms of CO2, N2, and CH4 display the dependency of gas capacity on optimized effects of pore size, functionality, and charged framework of these quasi-3D PCPs, where NTU-45 and NTU-46, the two with NH2-BDC and OH-BDC bidentate linkers (NH2-BDC = 2-aminoterephthalic acid and OH-BDC = 2-hydroxyterephthalic acid) have demonstrated outstanding ability for selective CO2 uptake. To the best of our knowledge, this is the first time to well explore the synergistic effects toward gas adsorption on a platform of quasi-3D frameworks. More importantly, the efficient CO2 capture from CO2/CH4 and CO2/N2 mixtures has been also validated by breakthrough experiments under continuous and dynamic conditions at 298 K.

Pure and mixture adsorption equilibria of methane and nitrogen onto clinoptilolite: effects of Cs+ and Fe3+ exchanged cations on separation performance

Cation exchanged clinoptilolite has emerged as an adsorbent that may be used for the treatment of natural gas type mixtures containing excess amounts of nitrogen. In this study, natural clinoptilolite was modified through Fe3+ and Cs+ cation exchange. The resultant mineral phase and elemental composition were verified and the extra framework cation type, charge, and distribution is shown to have a profound effect on the observed adsorptive properties. Single gas adsorption isotherms were conducted at 15 °C and/or 30 °C for CH4 and N2 using a microgravimetric adsorption analyser, and the ideal equimolar CH4/N2 selectivity values were determined for the natural and modified clinoptilolites. Natural clinoptilolite presents selectivity values close to 1. However, we demonstrate that clinoptilolite cation-exchanged with Cs+ and Fe3+ cations is rendered preferentially adsorptive for CH4 over N2 with a higher CH4/N2 selectivity than many adsorbents reported in the literature. The experimental CH4–N2 binary adsorption behavior was evaluated for the first time on raw clinoptilolite and was compared to the modified clinoptilolites with Cs+ and Fe3+ exchanged cations using the concentration pulse chromatographic technique. The experimental binary isotherms showed non-ideal behavior with competitive adsorption between CH4 and N2. In most cases, the theoretical models could not adequately describe the experimental adsorption behavior. Within the temperature range studied of 15 °C and 30 °C, the binary CH4/N2 separation factors range from 2.7 to 5.3, and 3.9 to 7.3 for Cs+ and Fe3+ exchanged clinoptilolite, respectively. These results are comparable or better than those found in literature for activated carbon which typically ranges from 2.1 to 5.5 and may serve as an alternative adsorbent for this type of separation which warrants further investigation.

D2/H2 adsorption selectivity on FAU zeolites at 77.4 K: Influence of Si/Al ratio and cationic composition

Equilibrium D2/H2 adsorption selectivity was determined at 77.4 K below 1000 hPa for a series of FAU type zeolites X exchanged with different cations (Li+, Na+, K+, Mg2+, Ca2+, Ba2+ and Mn2+). In addition NaY, DAY (dealuminated Y) and pure silica CHA and MFI zeolites were studied. Two experimental approaches were used to determine the D2/H2 adsorption selectivity: direct determination at the thermodynamic equilibrium from manometric coadsorption experiments and calculations by Ideal Adsorbed Solution Theory (IAST) from single gas adsorption isotherms. While these two approaches are not in quantitative agreement, they reveal similar trends. At low loading (<20 molec/uc) exchanged zeolites X in the selectivity diminishes with increasing cation size from 5.7 for MgX to 2.4 for KX: MgX > MnX > LiX > CaX ≈ NaX > KX ≈ BaX. In contrast, at high loading the selectivity is not influenced by the material composition being around 1.5 ± 0.2 for all studied materials. For cation-free zeolites (CHA and MFI) the latter value is observed for all loadings. The key role of cations in D2/H2 selectivity is explained by the fact that at low loading the adsorption proceeds through strong guest-cation interactions while at high loading weaker interactions are involved. Surprisingly, the selectivity in the low loading range is found to be correlated with the cation size and not with the interaction strength (estimated from the value of the Langmuir constant). This behavior of FAU type zeolites is thus different from that of MOFs or that predicted by simple theoretical models based on a one dimensional gas in a harmonic potential.

CH4/CO2 and CH4/C2H6 gas separation using a flexible one-dimensional copper(II) porous coordination polymer

Porous coordination polymers (PCPs) that show a gate-type adsorption originating from structural flexibility represent a new concept for developing gas separation materials. Here, the gas adsorption and separation properties of a flexible one-dimensional (1D) copper(II) PCP, [Cu(BF4)2(1,3-bis(4-pyridyl)propane)2]n, were evaluated. The single-gas adsorption isotherms revealed different gate-opening pressures for CO2 and C1∼C4 linear alkanes, suggesting great possibility for the separation of mixtures of these gases. The gas separation properties were characterized by performing breakthrough experiments at ambient temperature using CH4/CO2 and CH4/C2H6 mixtures. The results demonstrated the practical utility of 1D PCPs in gas separation applications. Single-crystal X-ray diffraction analysis demonstrated high structural adjustability for the incorporation of a variety of guest molecules.

Nanoporous Porphyrin Polymers for Gas Storage and Separation

This article describes the synthesis of four porous polymers containing Ni–porphyrin units with Brunauer–Emmet–Teller (BET) specific surface areas up to 1711 m2/g achieved. The isotherm gas adsorptions of hydrogen, methane and carbon dioxide over these polymers were measured. The adsorption selectivity for methane and carbon dioxide over nitrogen were also investigated. While the initial isosteric heat of adsorption (ΔHads) was around 8–9 kJ/mol for hydrogen, it reached 23 kJ/mol for methane and 29 kJ/mol for carbon dioxide. CO2/N2 selectivity as high as 19 (calculated from single gas adsorption isotherms) was also achieved with one of these four polymers.

Adsorption of Binary Gas Mixtures on Strongly Heterogeneous Surfaces

The integral equation (IE) approach coupled with a rectangular adsorption energy distribution and Mean Field Approximation (MFA) is used to describe the adsorption of single gases and their binary mixture on a heterogeneous solid surface. In particular, it is important to stress that all theoretical expressions shown are the exact solution of IE approach. During the analysis of single-gas adsorption isotherms also accompanying calorimetric effects are investigated. The problem of correlations between the best-fit parameters describing energetic heterogeneity and energy of intermolecular interactions is discussed. The predictions of the theory are compared with the experimental literature data. It is shown that the new theoretical adsorption isotherms give the results that are very close to the experimental data.

CO2–CH4 phase equilibria on modified multi‐walled carbon nanotubes using Gibbs excess energy models based on vacancy solution theory

AbstractCO2 and CH4 equilibrium adsorption are predicted by Excess Gibbs energy models based on vacancy solution theory, for single and binary mixture on Multi‐Walled Carbon Nanotubes (MWCNTs) functioned by –NH2 group. The experimental data of single gas adsorption isotherms were obtained at moderate pressures and temperatures using the volumetric method in a static gaseous set up. Firstly, the equilibrium pressures related to the adsorbed amounts, for single gases, were correlated on Wilson and Flory–Huggins activity coefficient equations based on vacancy solution theory and the model parameters were determined by fitting the model on the experimental data. Secondly, the pure component parameters were implemented in extended Wilson and Flory–Huggins equations for CO2 and CH4 mixture to predict the gas–solid phase equilibria. The results showed fairly good agreement between the experiments and both Gibbs models. Finally, the studied models were compared with the popular model of Extended Langmuir. The results revealed more accurately and precisely prediction of Wilson and Flory–Huggins against Langmuir model for mixed gas of CO2 and CH4 on MWCNT–NH2. © 2011 Canadian Society for Chemical Engineering

Application of the Vacancy Solution Theory to Describe the Enthalpic Effects Accompanying Mixed-Gas Adsorption

The possibility of utilizing vacancy solution theory (VST) to study the enthalpic effects accompanying mixed-gas adsorption equilibria is presented. Besides heterogeneity, the interaction effects by using the regular adsorbed solution, Flory-Huggins, and Wilson models of nonideality in the adsorbed phase are taken into account. To predict adsorption phase diagrams and calorimetric effects in the mixed-gas adsorption system, only a knowledge of the single-gas adsorption isotherms and accompanying calorimetric effects is required. The possibility of simplification of the obtained theoretical expressions is shown. The obtained agreement between theory and experiment is very satisfactory.

The theoretical study of the heat effects accompanying mixed-gas adsorption by using the non-ideal adsorbed solution theory and the Flory–Huggins activity coefficient equations

The possibilities of the non-ideal adsorbed solution approach to study enthalpic effects accompanying mixed-gas adsorption equilibria is presented. The model of calculations take into account the surface energetic heterogeneity and the effects of the lateral interactions between adsorbed molecules by using Flory–Huggins activity coefficient equations. The theoretical expressions for predicting isosteric heats of adsorption in the gas mixture are relatively simple and accurate. To predict phase diagrams and calorimetric effects in the mixed-gas adsorption only the knowledge of single-gas adsorption isotherms and accompanying calorimetric effects is required.

Theoretical Description of Adsorption from Binary Liquid Non-Electrolyte Solutions with Unlimited Component Miscibility on to Microporous Solids

The 'solution' analogues of four single-gas adsorption isotherms [i.e. the Langmuir–Freundlich (LF), the generalized Langmuir (GL), the Toth (T) and the LF–GL equations] were examined. The derived expressions were applied to the description of the experimental adsorption data relating to the sorption of a binary liquid solution containing components of unlimited miscibility on to microporous solids. Optimization of the equation parameters, as well as a comparison of the results obtained with those determined earlier for sorption in systems where the components were of limited miscibility, was undertaken.

The application of NIAS approach to describe enthalpic effects accompanying mixed-gas adsorption

The possibilities of the non-ideal adsorbed solution (NIAS) approach to study enthalpic effects accompanying mixed-gas adsorption equilibria is presented. Besides heterogeneity the interaction effects are taken into account. The obtained theoretical expressions for predicting isosteric heats of adsorption in the gas mixture are relatively simple and accurate. To predict phase diagrams and calorimetric effects in the mixed-gas adsorption only the knowledge of single-gas adsorption isotherms and accompanying calorimetric effects is required.

Theoretical Description of the Calorimetric Effects Accompanying the Mixed-Gas Adsorption Equilibria by Using the Ideal Adsorbed Solution Theory

The theoretical prediction of multicomponent adsorption equilibria from the single-component data is one of the most important problems in adsorption. The possibilities of the ideal adsorbed solution approach to study enthalpic effects accompanying the mixed-gas adsorption equilibria are presented. The obtained theoretical expressions for predicting isosteric heats of adsorption in a gas mixture are relatively simple and accurate. To predict phase diagrams and calorimetric effects in the mixed-gas adsorption, only the knowledge of single-gas adsorption isotherms and accompanying calorimetric effects is required.

Single and Mixed Gas Adsorption Equilibria of Carbon Dioxide/Methane on Activated Carbon

Single gas adsorption isotherms of methane and carbon dioxide on micro-porous Norit RB1 activated carbon were determined in a gravimetric analyser in the temperature range of 292 to 349 K and pressures to 0.8 Mpa. Furthermore binary isotherms of carbon dioxide and methane mixtures were determined at 292 K and pressures up to 0.65 MPa. Adsorbed phase compositions were determined from the gravimetric data by the rigorous thermodynamic method of Van Ness. These experimental binary equilibrium data were compared with equilibrium data calculated by the Ideal Adsorbed Solution (IAS) model. Only moderate agreement could be obtained. Finally, activity coefficients, accounting for the non-ideality of the adsorbate mixture, were calculated from the experimental data. The Wilson equation, derived for bulk solutions, was fitted on these activity data and the Wilson interaction parameters were determined. The Wilson equation proved to correlate the experimental data reasonably. However, the Wilson interaction parameters are not only completely different from those found for bulk solutions, but also the physical interpretation of these parameter values is completely lacking. It is concluded that new solution models should be developed encompassing both non-ideal solution behaviour and surface heterogeneity.

On the theoretical origin and applicability of the potential theory approach to predict mixed-gas adsorption on solid surfaces from single-gas adsorption isotherms

A rigorous theoretical development of the Potential Theory Approach for predicting mixed-gas adsorption on solids from single-gas adsorption isotherms is given. That approach requires very simple computer calculations making it very attractive for process design and control in industrial applications of mixed-gas adsorption on solids. It is shown that the “coalescence” operation can be treated as a purely numerical operation not involving the knowledge or assumptions about some physical quantities the meaning of which is not clear. However, it is also shown that the traditional coalescence operation is not the best way to elucidate the values of the parameters which are necessary to predict mixed-gas adsorption equilibria. A more effective way to determine the values of these parameters better, based on a theoretical analysis of single-gas adsorption isotherms, is proposed.

Equilibrium adsorption of gases and liquids on heterogeneous adsorbents — A practical viewpoint

The equilibrium characteristics of industrial adsorbents are determined primarily by the energetic heterogeneity of fluid-solid interactions. This heterogeneity is caused by thedetailed structural and chemical properties of the solid; it is modeled as a distribution of sites of different energy levels so that an overall extensive property of a heterogeneous adsorbent is the sum of individual contributions from each type of site. The energy distribution of an adsorbent is estimated from its experimental adsorption isotherms using a model of surface heterogeneity. Alternatively, theories of energetic heterogeneity provide a basis for generating analytical adsorption isotherm equations that cover wide ranges of pressure and temperature and are therefore useful for process design calculations. Although the energetic heterogeneity of solid adsorbents is usually very complex, models containing two or three constants for the energy distribution are adequate for practical purposes. The heterogeneous model is particularly successful in predicting multicomponent adsorption equilibria from single-gas adsorption isotherms.