Adsorption Separation of R-22, R-32 and R-125 Fluorocarbons using 4A Molecular Sieve Zeolite

Capture Fluorocarbon and Chlorofluorocarbon from Air Using DUT-67 for Safety and Semi-Quantitative Analysis

Fluoro- and chlorofluorocabons (FC/CFCs) are important refrigerants, solvents, and fluoropolymers in industry while being toxic and carrying high global warming potential. Detection and reclamation of FC/CFCs based on adsorption technology with highly selective adsorbents is important to labor safety and environmental protection. Herein, the study reports an integrated method to combine capture, separation, enrichment, and analysis of representative FC/CFCs (chlorodifluoromethane(R22) and 1,1,1,2-tetrafluoroethane (R134a)) by using the highly stable and porous Zr-MOF, DUT-67. Gas adsorption and breakthrough experiments demonstrate that DUT-67 has high R22/R134a uptake (124/116 cm3 g-1) and excellent R22/R134a/CO2 separation performance (IAST selectivities of R22/CO2 and R134a/CO2 ranging from 51.4 to 33.3, and 31.1 to 25.8), even in rather low concentration and humid conditions. A semi-quantitative analysis protocol is set up to analyze the low concentrations of R22/R134a based on the high selective R22/R134a adsorption ability, fast adsorption kinetics, water-resistant utility, facile regeneration, and excellent recyclability of DUT-67. In situ single-crystal X-ray diffraction, theoretical calculations, and in situ diffuse reflectance infrared Fourier transform spectra have been employed to understand the adsorption mechanism. This work may provide a potential adsorbent for purge and trap technique under room temperature, thus promoting the application of MOFs for VOCs sampling and quantitative analysis.

Exploring the Potential of Metal-Organic Frameworks for the Separation of Blends of Fluorinated Gases with High Global Warming Potential

The research on porous materials for the selective capture of fluorinated gases (F-gases) is key to reduce their emissions. Here, the adsorption of difluoromethane (R-32), pentafluoroethane (R-125), and 1,1,1,2-tetrafluoroethane (R-134a) is studied in four metal-organic frameworks (MOFs: Cu-benzene-1,3,5-tricarboxylate, zeolitic imidazolate framework-8, MOF-177, and MIL-53(Al)) and in one zeolite (ZSM-5) with the aim to develop technologies for the efficient capture and separation of high global warming potential blends containing these gases. Single-component sorption equilibria of the pure gases are measured at three temperatures (283.15, 303.15, and 323.15 K) by gravimetry and correlated using the Tóth and Virial adsorption models, and selectivities toward R-410A and R-407F are determined by ideal adsorption solution theory. While at lower pressures, R-125 and R-134a are preferentially adsorbed in all materials, at higher pressures there is no selectivity, or it is shifted toward the adsorption R-32. Furthermore, at high pressures, MOF-177 shows the highest adsorption capacity for the three F-gases. The results presented here show that the utilization of MOFs, as tailored made materials, is promising for the development of new approaches for the selective capture of F-gases and for the separation of blends of these gases, which are used in commercial refrigeration.

Separation of Azeotropic Hydrofluorocarbon Refrigerant Mixtures: Thermodynamic and Kinetic Modeling for Binary Adsorption of HFC-32 and HFC-125 on Zeolite 5A

Hydrofluorocarbons (HFCs) have been used extensively as refrigerants over the past four decades; however, HFCs are currently being phased out due to large global warming potentials. As the next generation of hydrofluoroolefin refrigerants are phased in, action must be taken to responsibly and sustainably deal with the HFCs currently in circulation. Ideally, unused HFCs can be reclaimed and recycled; however, many HFCs in circulation are azeotropic or near-azeotropic mixtures and must be separated before recycling. Previously, pure gas isotherm data were presented for both HFC-125 (pentafluoroethane) and HFC-32 (difluoromethane) with zeolite 5A, and it was concluded that this zeolite could separate refrigerant R-410A (50/50 wt % HFC-125/HFC-32). To further investigate the separation capabilities of zeolite 5A, binary adsorption was measured for the same system using the Integral Mass Balance method. Zeolite 5A showed a selectivity of 9.6-10.9 for HFC-32 over the composition range of 25-75 mol % HFC-125. Adsorbed phase activity coefficients were calculated from binary adsorption data. The Spreading Pressure Dependent, modified nonrandom two-liquid, and modified Wilson activity coefficient models were fit to experimental data, and the resulting activity coefficient models were used in Real Adsorbed Solution Theory (RAST). RAST binary adsorption model predictions were compared with Ideal Adsorbed Solution Theory (IAST) predictions made using the Dual-Site Langmuir, Tóth, and Jensen and Seaton pure gas isotherm models. Both IAST and RAST yielded qualitatively accurate predictions; however, quantitative accuracy was greatly improved using RAST models. Diffusion behavior of HFC-125 and HFC-32 was also investigated by fitting the isothermal Fickian diffusion model to kinetic data. Molecular-level phenomena were investigated to understand both thermodynamic and kinetic behaviors.

Design of Ionic Liquids for Fluorinated Gas Absorption: COSMO-RS Selection and Solubility Experiments

In recent years, the fight against climate change and the mitigation of the impact of fluorinated gases (F-gases) on the atmosphere is a global concern. Development of technologies that help to efficiently separate and recycle hydrofluorocarbons (HFCs) at the end of the refrigeration and air conditioning equipment life is a priority. The technological development is important to stimulate the F-gas capture, specifically difluoromethane (R-32) and 1,1,1,2-tetrafluoroethane (R-134a), due to their high global warming potential. In this work, the COSMO-RS method is used to analyze the solute-solvent interactions and to determine Henry's constants of R-32 and R-134a in more than 600 ionic liquids. The three most performant ionic liquids were selected on the basis of COSMO-RS calculations, and F-gas absorption equilibrium isotherms were measured using gravimetric and volumetric methods. Experimental results are in good agreement with COSMO-RS predictions, with the ionic liquid tributyl(ethyl)phosphonium diethyl phosphate, [P₂₄₄₄][C₂C₂PO₄], being the salt presenting the highest absorption capacities in molar and mass units compared to salts previously tested. The other two ionic liquids selected, trihexyltetradecylphosphonium glycinate, [P₆₆₆₁₄][C₂NO₂], and trihexyl(tetradecyl)phosphonium 2-cyano-pyrrole, [P₆₆₆₁₄][CNPyr], may be competitive as far as their absorption capacities are concerned. Future works will be guided on evaluating the performance of these ionic liquids at an industrial scale by means of process simulations, in order to elucidate the role in process efficiency of other relevant absorbent properties such as viscosity, molar weight, or specific heat.

Difluoromethane (HFC-32) and Pentafluoroethane (HFC-125) Sorption on Linde Type A (LTA) Zeolites for the Separation of Azeotropic Hydrofluorocarbon Refrigerant Mixtures

Worldwide use of hydrofluorocarbons (HFCs) is currently being regulated and phased out because of high global warming potentials (GWPs). Separation techniques for recycling refrigerants are needed so that HFCs can be dealt with responsibly. Many HFCs currently in use are azeotropic or near-azeotropic refrigerant blends and must be separated so that the components can be recycled and repurposed effectively. One such refrigerant is R-410A, which is a near-azeotropic 50/50 wt % mixture of pentafluoroethane (HFC-125) and difluoromethane (HFC-32). This study examined the use of the LTA zeolites for separating HFC-32 from HFC-125. Pure gas isotherms were measured using a XEMIS gravimetric microbalance with zeolites 3A, 4A, and 5A. Reversible sorption was observed for HFC-32 with zeolites 4A and 5A, whereas irreversible sorption was observed for HFC-125 with zeolite 5A. Negligible sorption was observed for HFC-125 with zeolites 3A and 4A, and although sorption of HFC-32 with zeolite 3A was observed, the process was slow, making the sorbent not commercially viable. The enthalpy of adsorption was predicted using the vapor adsorption equilibrium (VAE) analogue of the Clausius-Clapeyron equation and measured using a calorimeter for HFC-125 and HFC-32 with zeolite 5A and for HFC-32 with zeolite 4A. Molecular-level interactions between the LTA zeolites and HFCs were discussed and used to interpret pure gas isotherms and enthalpy of adsorption results. Overall, zeolites 4A and 5A were found to be good candidates for kinetically and thermodynamically separating R-410A, respectively.

Integration of Stable Ionic Liquid-Based Nanofluids into Polymer Membranes. Part II: Gas Separation Properties toward Fluorinated Greenhouse Gases

Membrane technology can play a very influential role in the separation of the constituents of HFC refrigerant gas mixtures, which usually exhibit azeotropic or near-azeotropic behavior, with the goal of promoting the reuse of value-added compounds in the manufacture of new low-global warming potential (GWP) refrigerant mixtures that abide by the current F-gases regulations. In this context, the selective recovery of difluorometane (R32, GWP = 677) from the commercial blend R410A (GWP = 1924), an equimass mixture of R32 and pentafluoroethane (R125, GWP = 3170), is sought. To that end, this work explores for the first time the separation performance of novel mixed-matrix membranes (MMMs) functionalized with ioNanofluids (IoNFs) consisting in a stable suspension of exfoliated graphene nanoplatelets (xGnP) into a fluorinated ionic liquid (FIL), 1-ethyl-3-methylpyridinium perfluorobutanesulfonate ([C2C1py][C4F9SO3]). The results show that the presence of IoNF in the MMMs significantly enhances gas permeation, yet at the expense of slightly decreasing the selectivity of the base polymer. The best results were obtained with the MMM containing 40 wt% IoNF, which led to an improved permeability of the gas of interest (PR32 = 496 barrer) with respect to that of the neat polymer (PR32= 279 barrer) with a mixed-gas separation factor of 3.0 at the highest feed R410A pressure tested. Overall, the newly fabricated IoNF-MMMs allowed the separation of the near-azeotropic R410A mixture to recover the low-GWP R32 gas, which is of great interest for the circular economy of the refrigeration sector.