Formation Behaviors and Selective Separation of the Binary (SF6+N2) Gas Mixtures Using Hydroquinone Clathrate

doi:10.21203/rs.3.rs-3310238/v1

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Formation Behaviors and Selective Separation of the Binary (SF6+N2) Gas Mixtures Using Hydroquinone Clathrate

https://doi.org/10.21203/rs.3.rs-3310238/v1

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After reacting hydroquinone (HQ) powder with pure sulfur hexafluoride (SF₆) and nitrogen (N₂), and binary gas mixtures of (SF₆ + N₂) with various compositions at experimental pressures of 20 bar and 40 bar, the HQ samples are analyzed using a series of analytic methods to obtain both qualitative and quantitative information. As a result, it was found that N₂ can be enclathrated into cavities of the formed HQ clathrate if the simplified partial pressure of N₂ is sufficiently large (occupation of 10% per one cavity when the simplified partial pressure is 32 bar, and occupation of 40% per one cavity when the pressure of pure N₂ is 40 bar). In addition, additional experiments were performed to evaluate the N₂ storage in the clathrate depending on the experimental pressure. When the pressure of pure N₂ was increased up to 80 bar, the occupation per one clathrate cage was calculated to be 55%, which corresponded to 18.7 L of N₂ at STP conditions per 1 kg of HQ. Because higher pressure is required to form the HQ clathrate and the stored amount of gas molecules is smaller compared with other technologies such as adsorption or hydrate-based process, a lot of additional investigations will be necessary, focusing on reducing energy consumption and increasing the gas storage.

Organic Clathrate

Greenshouse Gas

Air Pollution

Sulfur Hexafluoride

Separation

Today, global warming due to the greenhouse effect has become a critical environmental issue. The greenhouse effect means that heat from the earth remains in the atmosphere instead of radiating into the universe, causing the average temperature on earth to steadily increase. Carbon dioxide (CO₂) has been targeted as a main contributor to global warming because it is released into the air in huge volumes by the combustion of fossil fuels. However, in addition to CO₂, sulfur hexafluoride (SF₆), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), methane (CH₄), and nitrous oxide (N₂O) have also been targeted as greenhouse gases for emission reduction by the Kyoto Protocol [1, 2]. Among the six greenhouse gases, SF₆ is widely used as an insulating gas, a plasma etching contrast agent, and thermoacoustic insulator due to its excellent chemical stability, non-toxic, non-flammable, and good arc extinguishing and dielectric properties [3]. The use of gases as an electrical insulator has many advantages compared with liquid and solid insulation, including a relatively low weight, low cost, simple equipment manufacturing process, and the ability to insulate moving parts [4].

SF₆ is usually used in the form of a binary SF₆ + N₂ gas mixture to reduce the environmental impact. However, SF₆ is the most potent greenhouse gas: its global warming potential is 23,900 times higher than that of CO₂ [3, 5], and it has a very long lifetime of 3,200 years in the atmosphere [1, 3, 6]. Accordingly, SF₆ should be separated and recycled from the binary insulating gas mixtures to avoid it being emitted into the atmosphere, and to minimize its global warming impact.

Conventional gas separation can be performed with membranes (especially polymer membranes) or cryogenic distillation [3]. The latter has the disadvantages of enormous energy consumption and complex operation [4]. Recently, adsorbent materials such as zeolites, activated carbons, silica alumina, and metal organic frameworks (MOF) have been studied for separating SF₆ by the adsorption method. MOFs are considered to be a next-generation adsorbent, since they have high surface area, high porosities, and well-defined pore sizes [4, 7, 8]. P.-J. Kim et al. [7] synthesized MOF granules and reported qualitative and quantitative results for the separation of SF₆ from binary SF₆ + N₂ gas mixtures.

As a noble gas separation technology, many researchers have reported experimental results by forming clathrate compounds. A clathrate compound is a crystal compound formed by interactions between a host and guest species under specific conditions (usually, under high pressures and low temperatures). Hydrogen-bonded host molecules form a 3-dimensional lattice structure with cavities where guest molecules (usually, gaseous species with low molecular weight) can be accommodated by van der Waals forces, so that the crystal structure is stabilized.

In particular, gas hydrates whose host material is water can be formed with a variety of gases. They can be employed for hydrate-based separation or a concentration process for a specific component, because each guest species has inherent formation conditions (pressures and temperatures). SF₆ can also act as a guest to form a gas hydrate. G. Ko and Y. Seo [1] reported the formation behaviors of SF₆ hydrates from macroscopic and kinetic experiments. In addition, E. K. Lee et al. [9] also reported the formation of SF₆ hydrates, and suggested a hydrate-based SF₆ separation from binary SF₆ + N₂ gas mixtures by means of spectroscopic measurements.

However, when water is used as a separation material, vapor can contaminate the separated gas component. Moreover, because the hydrate-based process basically requires low temperatures near the freezing point it requires significant energy consumption for cooling. If a host material that is solid state at room temperature can be used to form the clathrate compound, contamination due to its vapor and energy consumption for cooling can be mitigated. In this regard, Lee et al. proposed a dry synthesis of a clathrate compound from hydroquinone (HQ, with a solid state at room temperature) without using any solvent [10]. In addition, Lee et al. [11] also demonstrated that many gaseous materials can be guest species and captured into the clathrate compound, by providing spectroscopic measurements. Moreover, the clathrate compound prepared from HQ was found to be applicable to gas separation. Unlike a gas hydrate, because the HQ clathrate only has one type of cavity structure, it can easily achieve the selective separation of a specific component from various gas mixtures [12, 13].

This study investigates a potential application of the HQ clathrate for the separation of SF₆ from binary SF₆ + N₂ gas mixtures. Since N₂ is already known to be captured in the cavities of the HQ clathrate [11], this investigation is mainly focused on the enclathration of SF₆ and separation from the binary gas mixtures.

HQ samples were reacted with pure SF₆ and N₂, and the binary (SF₆ + N₂) gas mixtures with various compositions were prepared at two experimental pressures of 20 and 40 bar. Then, the samples were analyzed by spectroscopic and elemental analysis instruments to obtain both qualitative and quantitative information. Experimental and calculated results obtained in this study can provide useful information regarding the crystal structure and cavity size of the HQ clathrate compound.

Pure HQ with a minimum purity of 99 mol% was supplied by Sigma–Aldrich Chemicals Co. The binary (SF₆ + N₂) gas mixtures had gas compositions of 20 mol%, 40 mol%, 60 mol%, and 80 mol% SF₆ (analyzed compositions of 19.99 mol%, 39.96 mol%, 59.96 mol%, and 80.00 mol% SF₆) balanced with N₂. In addition to the gas mixtures, pure SF₆ (purity of 99.9 mol%) and N₂ gases (purity of 99.999 mol%) were manufactured and purchased from Daemyoung Special Gas Co (Republic of Korea). These materials were used without further purification or treatment. To prepare the HQ clathrate, the HQ particles were ground into fine powders (particle sizes < 100 µm) to promote the gas-solid reaction. Then the HQ powders were charged in a high-pressure reactor (made from 316 stainless steel, having an internal volume of approximately 200 cm³) and allowed to react with 20 and 40 bar pressurized binary gases. An external reservoir cell (~ 500 cm³) to compensate pressure drops due to the clathrate formation was also attached to the reactor.

After reaction for 14 d at room temperature, the samples were collected for spectroscopic measurements, after releasing the pressurized gas from the reactor and the reservoir. A multi-purpose X-ray diffractometer (SMD 3000 model, Advanced Scientific Instrumentation Co.) was used to identify the crystal structures of the HQ samples. The X-rays were monochromatized to a wavelength of 1.5406 Å using a parabolic mirror and channel-cut crystal. This instrument had a maximum output power of 3 kW. Approximately 0.5 g of the powdered HQ sample was placed on a flat plate; reflections from the HQ samples were collected from 8.0° to 50.0° with a step size of 0.05° and a step time of 3 s. All of the powder X-ray diffraction (XRD) measurements were performed at ambient temperature. For solid-state ¹³C NMR, an Agilent DD2 400 MHz spectrometer was used. The ¹³C NMR spectra were collected at ambient temperature by placing samples within a 1.6 mm HFXY fast magic angle spinning (MAS) probe and recorded with a spinning rate of 20 kHz. The pulse length of the proton was 2 µs, and a phase repetition delay of 10 s under proton decoupling was employed.

The downfield carbon resonance peak of adamantane, which was assigned a chemical shift of 38.3 ppm at 300 K, was used as an external chemical shift reference. A dispersive Raman spectrometer (Horiba Jobin Yvon, Lab-RAM ARAMIS model) was used with a 460 mm focal length monochromator and air cooled charge-coupled device (CCD) detector. The excitation source was an Ar-ion laser emitting a 514 nm line with a power of 40 mW. The Raman spectroscopic measurements were conducted at ambient temperature. The components (C, H, N, and S) in the HQ samples were determined using an elemental analyzer (Thermo Scientific, Flash 2000 Series). Approximately 3.0 mg of each sample was analyzed using the dynamic flash combustion method with a thermal conductivity detector (TCD). All the analytic instruments were located in the Analysis Center for Research Advancement of Korea Advanced Institute for Science and Technology (KAIST, Daejeon, Republic of Korea). To minimize dissociation of the clathrate out of the formation conditions, pressure release and sample collection were performed right before all the measurements. Basically, the same experimental procedures as our previous reports were used in this study [10–13].

When pure HQ is subjected to sufficiently high pressure, the HQ molecules align in a more regular way and are converted into a clathrate compound with a cavity structure to accommodate guest molecules. During this conversion, the crystal structure of pure HQ, α-form in the space group of R3 (a = b = 38.46 Å, c = 5.650 Å) is transformed into β-form in the same space group (a = b = 16.650 Å, c = 5.453 Å) [10, 14].

Figure 1 shows the Raman spectra of HQ samples reacted with pure SF₆ and N₂, and binary (SF₆ + N₂) gas mixtures with various compositions at the experimental temperature of 298.2 K and pressures of 20 and 40 bar. Many Raman signals have been obtained to distinguish the crystal structures of HQ (that is, the α- and β-forms) [15], but the three stretching mode signals in the range of 1600 ~ 1620 cm^− 1 can be the most useful.

For the α-form, that is, pure or unreacted HQ, among the three signals the middle one is strongest, while in the β-form clathrate compound the middle one of the three signals is the weakest. For the HQ samples prepared at 20 bar (not shown in the figure), all of the HQ samples with pure or mixed gases were found to exist in the form of unreacted HQ, α-form. Most of the HQ samples prepared at 40 bar were also found to be α-form, except for two samples of β-form reacted with pure N₂ and 80% N₂ + 20% SF₆ mixed gases. Both samples showed an additional signal at about 2320 cm^− 1. Because the signal comes from the N-N stretching mode, it can be a direct evidence of the formation of the clathrate compound and the enclathration of N₂ in the cavity [11].

To determine the crystal structure and lattice parameters, the same HQ samples were then analyzed using the powder X-ray diffraction (PXRD) method, which supported the results obtained from the Raman measurements. Most of the HQ samples prepared at two experimental pressures showed the R3 structure of α-form, except for two HQ samples prepared with pure N₂ and (80% N₂ + 20% SF₆) mixed gases at 40 bar, which revealed the β-form structure. Lattice parameters calculated from the obtained PXRD patterns were found to be a = b = 38.344 ± 0.031 Å and c = 5.650 ± 0.013 Å for the α-form, and to be a = b = 16.435 ± 0.023 Å and c = 5.612 ± 0.011 Å for the β-form, which shows good agreement with the literature [11, 14].

In order to support the previous results and obtain quantitative information, the same HQ samples were also analyzed using the ¹³C solid-state nuclear magnetic resonance (NMR) spectroscopic method. Because most of the HQ samples show the α-form NMR spectrum, three NMR spectra for pure HQ powder, and two HQ samples reacted with pure SF₆ and pure N₂ at 298.2 K and 40 bar are provided in Fig. 2 to avoid the repetition of the same spectra.

HQ (1,4-dihydroxybenzene) is a molecule with a substitution of two hydroxy groups at the first and the fourth carbon atoms (para position) in a benzene ring. Therefore, the two chemically equivalent atoms of OH-substituted and OH-unsubstituted carbons provide two carbon signals. However, due to the molecular arrangement in the crystal structure, OH-substituted carbon signal is detected as a triple signal with an area ratio of 2:3:1 at around 150 ppm, while the OH-unsubstituted carbon signal is obtained as a multiple signal in the range of 116 ~ 120 ppm [14, 16].

The ¹³C NMR spectra for the pure HQ powder showed good agreement with this reported pattern. In addition, the HQ sample reacted with pure SF₆ at 40 bar also showed the same NMR spectrum as that of pure HQ, which indicates HQ was not converted into the clathrate compound as identified from the Raman measurements. When HQ in converted into the clathrate compound, the arrangement of HQ molecules become more aligned in the crystal structure. In this regard, the carbon signal from the OH-substituted atom becomes a distinct single peak, while the signal from the OH-unsubstituted atom is changed into two peaks with an area ratio of 1:1 [14, 16].

The HQ sample prepared at an N₂ pressure of 40 bar showed a ¹³C NMR spectrum similar to that of the clathrate compound, β-form, which indicates that N₂ molecules were captured into the clathrate cavity. It should be noted that the OH-substituted atomic signal at the chemical shift of about 150 ppm is not a single peak. Instead, weak shoulder peaks were detected at both sides of the intense peak, and multiple signals with weak intensities in the range of 116 ~ 120 ppm demonstrated that unreacted (pure) HQ co-exists with the HQ clathrate.

As mentioned earlier, the three atomic signals from the OH-substituted carbons of HQ had an area ratio of 2:3:1, while the signal was changed into a single peak when HQ was converted into the clathrate form. Therefore, based on the numerical integration of the three peaks, the conversion, or the ratio of unreacted (pure) HQ to HQ clathrate, can be calculated. In other words, the conversion can be calculated as the percentage of (the area of the middle signal – the area of the right signal × 3) in the total area of all the three peaks. Using this calculation, the conversion of HQ into the clathrate form in the N₂-loaded HQ sample at 40 bar was found to be 66.2%.

Elemental analysis of the HQ samples was also performed to identify another important quantitative information, that is, the amount of N₂ trapped in the HQ clathrate. The chemical formula of the HQ clathrate compound is expressed as 3 HQ·x Gas, which means that three HQ molecules form a cavity, and that x gas molecules occupy a cavity. Because the HQ clathrate is a non-stoichiometric compound, x has a value from 0 to 1 depending on the formation conditions (pressure, temperature, composition and so on).

Figure 3 shows a plot of the number of N₂ molecules (the value of x) trapped in a cavity of the HQ clathrate compound, which comes from the elemental analysis. As shown in the plot, and as mentioned previously, most of the HQ samples do not show the enclathration of N₂ molecules, while two HQ clathrate samples (prepared with pure N₂ and mixed (80% N₂ + 20% SF₆) gases) obviously exhibited N₂ storage in the HQ clathrate. However, the amounts of stored N₂ molecules was found to be 10% and 40% for the mixed and pure N₂ gases, respectively.

Even though the remaining HQ samples were not converted into the clathrate form, the amount of N₂ storage was found to increase in accordance with increasing N₂ concentration in the gas mixtures, and with increasing experimental pressure. Because such amount is very small, the calculated amount is thought to be attributed to contamination, due to N₂ in the air during the measurement preparation. In addition, it is also attributed to a small amount of N₂ molecules attached (or adsorbed) to α-form HQ molecules in the initial stage of the clathrate formation mechanism. Because the experimental pressure are not sufficiently high, such α-form HQ cannot progress the clathrate formation mechanism further.

A series of analytic measurements revealed that SF₆ did not react with HQ to form a clathrate compound, while N₂ can be trapped in the cavity of the converted HQ clathrate. The main factor affecting the enclathration behaviors of two species is thought to be the molecular size. The larger SF₆ (kinetic diameter of 5.49 Å) is too large to stably occupy the HQ clathrate cavity, while smaller N₂ (kinetic diameter of 3.64 Å) can be enclathrated into the HQ clathrate cavity [4]. Because conversion into an HQ clathrate is only about 66% even at a pressure of 40 bar, the effect of experimental pressure on conversions was investigated by increasing the experimental pressure up to 80 bar.

Figure 4 shows a graph of conversion into an HQ clathrate depending on pure N₂ pressures from 20 bar to 80 bar. As plotted in the graph, the conversion exceeded 90% when the N₂ pressure was increased to 60 bar. Considering the HQ clathrate (β-form) partially formed at high N₂ concentration in the binary gas mixtures at the experimental pressure of 40 bar, it can be said that the HQ clathrate can be formed with a simplified N₂ partial pressure of 32 bar.

In other words, selective N₂ separation due to the HQ clathrate formation from the binary SF₆ + N₂ gas mixtures began slightly at the simplified N₂ partial pressure of 30 bar, and gradually increased to a conversion of 90% when the N₂ pressure rose to 60 bar or higher.

Although conversion into an HQ clathrate clearly increased with increased experimental pressure, and N₂ molecules were enclathrated into the clathrate cavity in a selective way, the maximum occupation of N₂ molecules was found to be 55% of the total cavity even at the highest experimental pressure, 80 bar. This corresponds to 18.7 L of N₂ at the STP condition per 1 kg of HQ (Fig. 5).

Such high pressure or limited storage amount is inefficient compared with other technologies such as adsorption or hydrate-based process, which suggests that many disadvantages need to be resolved, and that more investigations should be performed. In addition, a lot of HQ is required to realize a clathrate-based separation process because N₂ rather than SF₆ is enclathrated, which increases the total cost of the process. Of course, the cost may be somewhat reduced by recycling HQ after desorption of the N₂ molecules from the N₂-loaded HQ clathrate.

Considering the overall results in this study, the clathrate-based separation process is applied to the SF₆ separation from the binary gas mixtures within the limits of low SF₆ concentrations and high N₂ partial pressures. To address the identified disadvantages and expand application areas, additional studies to find another host species including supramolecules to capture SF₆ selectively and to reduce the clathrate formation pressure are thought to be necessary.

In order to verify the potential application of the HQ clathrate compound to a noble separation/recovery technology for the most potent greenhouse gas, SF₆, the HQ samples prepared with pure and mixed (SF₆ + N₂) gases at two experimental pressures of 20 and 40 bar were analyzed so as to obtain qualitative and quantitative information. The experimental results showed the selective enclathration of N₂ occurred when the experimental pressure including partial pressure was 30 bar or larger.

In addition, the amount of stored N₂ molecules and the conversion into the HQ clathrate were also investigated by increasing pure N₂ pressures up to 80 bar. Overall, the HQ clathrate-based separation can selectively separate N₂ from the binary SF₆ + N₂ gas mixtures with various compositions when the partial N₂ pressure is sufficiently high. The requirements of high operating pressure and large amounts of HQ to capture N₂ rather than SF₆ are disadvantages, and need to be overcome compared with other technologies such as adsorption or hydrate-based separation. Therefore, additional investigations are necessary that focus on the search for an organic host material with a larger cavity size in the clathrate, or using a solvent to reduce operating pressures. Moreover, the energy costs for desorption and recycling of the host material should be taken into account in the future works.

ACKNOWLEDGEMENTS

This work was supported by the research grant of the Kongju National University in 2022. This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2022R1F1A1076249).

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Formation Behaviors and Selective Separation of the Binary (SF6+N2) Gas Mixtures Using Hydroquinone Clathrate

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Abstract

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INTRODUCTION

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

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ACKNOWLEDGEMENTS

References

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