Temperature-dependent molecular sieving of fluorinated propane/propylene mixtures by a flexible-robust metal-organic framework

The electronics industry necessitates highly selective adsorption separation of hexafluoropropylene (C3F6) from perfluoropropane (C3F8), which poses a challenge due to their similar physiochemical properties. In this work, we present a microporous flexible-robust metal-organic framework (Ca-tcpb) with thermoregulatory gate opening, a rare phenomenon that allows tunable sieving of C3F8/C3F6. Remarkably, the temperature-dependent adsorption behavior enhances the discrimination between the larger C3F8 and the smaller C3F6, resulting in unprecedented C3F6/C3F8 selectivity (over 10,000) compared to other well-known porous materials at an optimal temperature (298 K). Dynamic breakthrough experiments demonstrate that high-purity C3F8 (over 99.999%) could be obtained from a C3F6/C3F8 (10:90) mixture under ambient conditions. The unique attributes of this material encompass exceptional adsorption selectivity, remarkable structural stability, and outstanding separation performance, positioning it as a highly promising candidate for C3F6/C3F8 separation. Single-crystal structural analysis of C3F6-loaded Ca-tcpb and theoretical calculations elucidate the host-guest interaction via multiple intermolecular interactions.

Amino-Functionalized Microporous MOFs for Capturing Greenhouse Gases CF4 and NF3 with Record Selectivity

The capture and separation of fluorinated gases (F-gases) from N₂ has the potential to not only reduce greenhouse gas emissions but also provide economic benefits for the semiconductor industry. In this work, two Ni-based metal-organic frameworks (MOFs), Ni-MOF (Ni(ina)₂, ina = isonicotinic acid) and amine-functionalized NH₂-Ni-MOF (Ni(3-ain)₂, 3-ain = 3-aminoisonicotinic acid), were constructed for capturing F-gases (CF₄ and NF₃). At ambient conditions, both materials exhibit very high CF₄ sorption capacities (2.92 mmol g^-1 for Ni-MOF and 2.69 mmol g^-1 for NH₂-Ni-MOF). In addition, NH₂-Ni-MOF exhibited a record selectivity of 46.3 for the CF₄/N₂ mixture at 298 K and 100 kPa, surpassing all benchmark adsorbents, including Ni-MOF (34.7). The kinetic adsorption tests demonstrated that Ni-MOF and NH₂-Ni-MOF performed well for CF₄/N₂ and NF₃/N₂ mixtures. According to grand canonical Monte Carlo (GCMC) simulations, CF₄ or NF₃ interacts with NH₂-Ni-MOF by multiple van der Waals interactions, resulting in stronger interaction than N₂. More importantly, dynamic breakthrough experiments verified the practical separation potential of the two materials for CF₄/N₂ and NF₃/N₂ mixtures.

Pore-Structure Control in Metal-Organic Frameworks (MOFs) for Capture of the Greenhouse Gas SF6 with Record Separation

In the electronics industry, the efficient recovery and capture of sulfur hexafluoride (SF₆ ) from SF₆ /N₂ mixtures is of great importance. Herein, three metal-organic frameworks with fine-tuning pore structures, Cu(peba)₂ , Ni(pba)₂ , and Ni(ina)₂ , were designed for SF₆ capture. Among them, Ni(ina)₂ has perfect pore sizes (6 Å) that are comparable to the kinetic diameter of sulfur hexafluoride (5.2 Å), affording the benchmark binding affinity for SF₆ gas. Ni(ina)₂ exhibits the highest SF₆ /N₂ selectivity (375.1 at 298 K and 1 bar) and ultra-high SF₆ uptake capacity (53.5 cm³  g^-1 at 298 K and 0.1 bar) at ambient conditions. The remarkable separation performance of Ni(ina)₂ was verified by dynamic breakthrough experiments. Theoretical calculations and the SF₆ -loaded single-crystal structure provided critical insight into the adsorption/separation mechanism. This porous coordination network has the potential to be used in industrial applications.

HF Resistant Porous Aromatic Frameworks for Electronic Special Gases Separation

Here we report two HF acid resistant porous aromatic frameworks as adsorbents for high value-added electronic special gases (e.g., SF₆, NF₃, CF₄, Xe, Kr) separation. The New-PAF-1 and N-SO₃H exhibit exceptional adsorption selectivity for Xe and F-gases from semiconductor exhaust gas along with high physicochemical stability and excellent reusability, which have been collaboratively confirmed by single-component gas adsorption experiments, time-dependent adsorption rate tests, dynamic breakthrough experiments and regeneration tests. The theoretical calculations based on DFT and Mulliken atomic charge analyses elucidated the adsorption mechanism of New-PAF-1 and N-SO₃H toward F-gases, Xe, Kr, and N₂ at molecular level, including adsorption site, binding energy and electrostatic potentials distribution. The systematic investigation sufficiently manifests that PAFs can act as highly stable porous adsorbents in harsh operating conditions.

Separation of CF4/N2, C2F6/N2, and SF6/N2 Mixtures in Amorphous Activated Carbons Using Molecular Simulations

The capture and separation of CF₄, C₂F₆, and SF₆ and their mixtures containing nitrogen is a challenging process. To solve this, we propose the use of saccharose coke-based carbons as membranes for the adsorption and separation of these gases. By means of advanced techniques of Monte Carlo and molecular dynamics simulations, we have studied the adsorption and diffusion of CF₄, C₂F₆, and SF₆ as well as their mixtures with nitrogen in three HRMC carbon models, namely, CS400, CS1000, and CS1000a. We have computed the adsorption isotherms of the single components and the heat of adsorption as a function of the adsorbed concentration. We have also calculated the competitive adsorption of fluoride molecules and nitrogen at two different molar fractions, 0.1 and 0.9. We have computed the transport properties of the adsorbed gases in terms of the self-diffusivities and corrected diffusivities. The performance of the membranes for the targeted separations has been characterized by the calculation of the permselectivity. Our results indicate that the activated amorphous carbon CS1000a is an efficient adsorbent for the capture of the fluoride adsorbates as well as their purification from nitrogen-based mixtures.

NMR relaxation and modelling study of the dynamics of SF6 and Xe in porous organic cages

The porous solid formed from organic CC3 cage molecules has exceptional performance for rare gas separation. NMR spectroscopy provides a way to reveal the dynamical details by using experimental relaxation and diffusion measurements. Here, we investigated T₁ and T₂ relaxation as well as diffusion of ¹²⁹Xe and SF₆ gases in the CC3-R molecular crystal at various temperatures and magnetic field strengths. Advanced relaxation modelling made it possible to extract various important dynamical parameters for gases in CC3-R, such as exchange rates, activation energies and mobility rates of xenon, occupancies of the cavities, rotational correlational times, effective relaxation rates, and diffusion coefficients of SF₆.

One-step conversion of agro-wastes to nanoporous carbons: Role in separation of greenhouse gases

Highly microporous carbons have been synthesized from four types of agro-wastes of lignin, walnut shells, orange peels and apricot seeds by one-step carbonization/activation with potassium hydroxide (KOH) in varying ratios. The resultant carbons demonstrated BET specific surface areas of 727-2254 m²/g, and total pore volumes 0.34-1.14 cm³/g. These are higher than the majority of agro-waste derived carbons reported in the literature. For all the carbons, CO₂ adsorption at 298 K was higher than SF₆ followed by N₂ suggesting a possible separation of CO₂ and SF₆ from N₂. The adsorbed amounts of CO₂ at 298 K and 273 K and at pressures up to 760 Torr were 7.24 and 9.4 mmol/g, respectively which, to the best of our knowledge, are the highest CO₂ uptakes in these temperatures by any carbon material reported so far. For all the gases, selectivity, mixed adsorption isotherms and adsorption breakthrough have been simulated from experimental data.

Porous Organic Cages for Sulfur Hexafluoride Separation

A series of porous organic cages is examined for the selective adsorption of sulfur hexafluoride (SF6) over nitrogen. Despite lacking any metal sites, a porous cage, CC3, shows the highest SF6/N2 selectivity reported for any material at ambient temperature and pressure, which translates to real separations in a gas breakthrough column. The SF6 uptake of these materials is considerably higher than would be expected from the static pore structures. The location of SF6 within these materials is elucidated by X-ray crystallography, and it is shown that cooperative diffusion and structural rearrangements in these molecular crystals can rationalize their superior SF6/N2 selectivity.

Wide Carbon Nanopores as Efficient Sites for the Separation of SF6 from N2

SF₆ and SF₆-N₂ mixed gases are used widely as insulators, but such gases have high greenhouse gas potential. The separation of SF₆ from SF₆-N₂ mixed gases is an inevitable result of their use. Single-walled carbon nanohorns (CNHs) were used here for a fundamental study of the separation of SF₆ and N₂. The diameters of the interstitial and internal nanopores of the CNHs were 0.7 and 2.9 nm, respectively. The high selectivity of SF₆ over N₂ was observed only in the low-pressure regime in the interstitial 0.7 nm nanopores; the selectively was significantly decreased at higher pressures. In contrast, the high selectivity was maintained over the entire pressure range in the internal 2.9-nm nanopores. These results showed that the wide carbon nanopores were efficient for the separation of SF₆ from the mixed gas.

Zeolites for the selective adsorption of sulfur hexafluoride

Molecular simulations have been used to investigate at the molecular level the suitability of zeolites with different topology on the adsorption, diffusion and separation of a nitrogen–sulfur hexafluoride mixture containing the latter at low concentration.

Evaluation of the potential of volatile organic compound (di-methyl benzene) removal using adsorption on natural minerals compared to commercial oxides

This study is dedicated to the investigation of the potential of volatile organic compounds (VOC) adsorption over low cost natural minerals (bentonite and diatomite). The performances of these solids, in terms of adsorption/desorption properties, were compared to commercial adsorbents, such as silica, alumina and titanium dioxide. The solids were first characterized by different physico-chemical methods and di-methyl benzene (dMB) was selected as model VOC pollutant for the investigation of adsorptive characteristics. The experiments were carried out with a fixed bed reactor under dynamic conditions using Fourier Transform InfraRed spectrometer to measure the evolution of dMB concentrations in the gaseous stream at the outlet of the reactor. The measured breakthrough curves yields to adsorbed amounts at saturation that has been used to obtain adsorption isotherms. The latters were used for determination of the heat involved in the adsorption process and estimation of its values using the isosteric method. Furthermore, the performances of the studied materials were compared considering the adsorption efficiency/cost ratio.

Adsorption equilibrium of sulfur hexafluoride on multi-walled carbon nanotubes

The adsorption of sulfur hexafluoride (SF(6)) on multi-walled carbon nanotubes (MWNTs) was investigated. The properties of MWNTs were characterized and the adsorption capacities of SF(6) on MWNTs at different concentrations and temperatures were collected. H(2)SO(4)/H(2)O(2) oxidation or KOH activation of MWNTs has effectively introduced the surface oxides and modified the microstructure without destruction of their graphitic crystalline structure. The MWNTs, especially the modified samples, are expected to be promising adsorbents for SF(6) removals from air. The saturated capacities of SF(6) with a concentration of 518 ppmv on the MWNTs ranged from 278 to 497 mg/g at 25 degrees C. The Toth equation has been reported to fit the adsorption data better than the Freundlich or Langmuir equation. The pi-pi dispersion interaction followed by the multi-layer adsorption and the electron donor-acceptor interaction were proposed to be the major adsorption mechanisms, depending on the adsorption temperature. The isosteric heat of adsorption, ranging from 51 to 124 kJ/mol with a loading of 30-300 mg/g, decreased with increasing SF(6) loading, reflecting the energetic heterogeneity of the MWNTs. These results suggest that the adsorption of SF(6) on MWNTs could be associated with binding to defect sites.