Adsorption Properties of ZSM-5 Molecular Sieve for Perﬂuoroisobutyronitrile Mixtures and Its Fluorocarbon Decomposition Products

: Perﬂuoroisobutyronitrile (C 4 F 7 N), an environment-friendly insulating gas, has excellent insulating properties and has the potential to be used in gas-insulated equipment when mixed with CO 2 . Selecting suitable adsorption materials to adsorb the decomposition products of the C 4 F 7 N mixture can ensure the safe and stable operation of the gas-insulated equipment and the personal safety of the operators in the electric power industry. The adsorption characteristics of the ZSM-5 molecular sieve on C 4 F 7 N and its ﬁve ﬂuorocarbon decomposition products were investigated by adsorption experiments. The results show that the ZSM-5 molecular sieve has a certain adsorption effect on six ﬂuorocarbon gases; the adsorption performance of C 3 F 6 and C 3 F 8 are the best, with an adsorption efﬁciency over 85%, while the concentration of CO 2 and C 4 F 7 N is affected by the ZSM-5 molecular sieve. At the same time, the paper based on the Metropolis Monte Carlo simulation of Materials Studio software found that the ZSM-5 molecular sieve has the strongest adsorption effect on C 4 F 7 N molecules and the weakest adsorption effect on CO 2 molecules. The stronger the polarity of the gas molecule, the more obvious the adsorption effect of molecular sieve structure on it. As a result, the ZSM-5 molecular sieve could be used in tail gas puriﬁcation of insulated equipment, as well as to provide solutions for the development and production of protective equipment.


Introduction
SF 6 is a stable gas with excellent insulation and arc extinguishing performance, which is widely used in various gas insulation equipment However, the global warming potential (GWP) value of SF 6 is high, about 23,500 times that of CO 2 . In the "Kyoto Protocol" promulgated in 1997, it was clearly stated that SF 6 is a greenhouse gas [1][2][3][4]. With the increasing global greenhouse effect on the environment, the control of greenhouse gas emissions has become the primary task of protecting the global environment. Some countries and regions have introduced policies and regulations to limit the usage and emissions of SF 6 . Particularly, China has pledged to reduce SF 6 emissions to 55~60% of 2005 levels by 2019 [5]. It is one of the research hotspots to find a new type of environment-friendly insulating medium that can replace SF 6 in the electric power industry. 3M, in conjunction with General Electric, introduced the "g3", an environmentally friendly insulating gas, which is mainly 4~10% perfluoroisobutyronitrile (C 4 F 7 N) and CO 2 . The insulation performance of C 4 F 7 N is about twice that of SF 6 , and the GWP of C 4 F 7 N is only 2090. Although the liquefaction temperature is high (−4.7 • C), it has potential application in engineering when mixed with a buffer gas (CO 2 , N 2 ) [6,7].

Adsorption Experiment Method
Considering the actual concentration of C 4 F 7 N mixtures and common fluorocarbon gases, the relevant parameters of the adsorbate gas concentration used in this paper are shown in Table 1. The dilution background of each low-concentration standard gas was He, and the concentration error was ±3 ppm. The three adsorbate gases with high concentrations were made up of 99.2% C 4 F 7 N, 99.999% CO 2 , and 99.999% He. The ZSM-5 molecular sieve material was provided by Pioneer Nano Company, and the adsorption experimental device is shown in Figure 1. The detailed experimental procedure is shown in Table 2.

Adsorption Experiment Method
Considering the actual concentration of C4F7N mixtures and common fluoro gases, the relevant parameters of the adsorbate gas concentration used in this pa shown in Table 1. The dilution background of each low-concentration standard He, and the concentration error was ±3 ppm. The three adsorbate gases with high trations were made up of 99.2% C4F7N, 99.999% CO2, and 99.999% He. The ZSM-5 ular sieve material was provided by Pioneer Nano Company, and the adsorption mental device is shown in Figure 1. The detailed experimental procedure is show ble 2.

Concentration Detection
For the concentration detection of low-concentration gases (CF3H, CF4, C2F6, C C3F8), the instrument used was a Shimadzu QP-2020NX gas chromatography-ma trometer, equipped with a special capillary column (CP-Sil5CB) for gas qualitativ mination detection, using 99.999% He as carrier gas. The column temperature adop programmed heating method: 0-9 min maintained at 32 °C unchanged, then the ature was raised to 120 °C at a constant rate for 3 min after 9 min, and finally the ature was kept unchanged to maintain the temperature at 120 °C. The inlet temp was 200 °C, the split ratio was 50:1, the 200 °C EI ion source was used as the MS ion and the sample inlet temperature was 200 °C. After gas sampling, ion scanning w to quantitatively analyze the characteristic ions of each fluorocarbon gas. The corre  Step Operation Content 1 Checking the tightness of the device 2 Loading with 3 g of adsorption material 3 Pumping of the chamber to vacuum 4 Heating activates the adsorbent 5 Vacuuming for five minutes to remove the residual impurity gas in the chamber 6 Filling the chamber with gas to an absolute pressure of 0.15 MPa, and then letting it stand for 48 h. 7 Detecting the concentration of the gas in the chamber after 48 h

Concentration Detection
For the concentration detection of low-concentration gases (CF 3 H, CF 4 , C 2 F 6 , C 3 F 6 , and C 3 F 8 ), the instrument used was a Shimadzu QP-2020NX gas chromatography-mass spectrometer, equipped with a special capillary column (CP-Sil5CB) for gas qualitative determination detection, using 99.999% He as carrier gas. The column temperature adopted the programmed heating method: 0-9 min maintained at 32 • C unchanged, then the temperature was raised to 120 • C at a constant rate for 3 min after 9 min, and finally the temperature was kept unchanged to maintain the temperature at 120 • C. The inlet temperature was 200 • C, the split ratio was 50:1, the 200 • C EI ion source was used as the MS ion source, and the sample inlet temperature was 200 • C. After gas sampling, ion scanning was used to quantitatively analyze the characteristic ions of each fluorocarbon gas. The corresponding characteristic ion mass-to-charge ratios (m/z) of the six gases were 69, 69, 119, 131, 69, respectively.
For the detection of high-concentration gases (C 4 F 7 N and CO 2 ), the instrument used was a Thermo Scientific Nicolet iS50 infrared spectrometer. The optical path length of the gas cell in the infrared spectrometer was 2.4 m, the volume was 200 mL, and the material of the windows at both ends of the gas cell was potassium bromide. The number of scans was set to 1 and the resolution to 4 before collecting the absorption spectrum signal. A specific absorption peak was selected and the intensity of each gas absorption peak was compared before and after the experiment to quantitatively analyze the change in concentration. For the analysis of C 4 F 7 N concentration, the characteristic peaks in the range of 2220~2290 cm −1 were selected. For CO 2 concentration analysis, characteristic peaks with wavenumbers ranging from 3650 to 3755 cm −1 were selected.  For the detection of high-concentration gases (C4F7N and CO2), the ins was a Thermo Scientific Nicolet iS50 infrared spectrometer. The optical pat gas cell in the infrared spectrometer was 2.4 m, the volume was 200 mL, an of the windows at both ends of the gas cell was potassium bromide. The nu was set to 1 and the resolution to 4 before collecting the absorption spect specific absorption peak was selected and the intensity of each gas absorp compared before and after the experiment to quantitatively analyze the cha tration. For the analysis of C4F7N concentration, the characteristic peaks i 2220~2290 cm −1 were selected. For CO2 concentration analysis, characteris wavenumbers ranging from 3650 to 3755 cm −1 were selected.   It can be seen from the figure that the concentration of each gas decre experiment. The adsorption properties of the ZSM-5 molecular sieve for fiv orocarbons were different, and the adsorption properties for C3F6 and C3F8 The concentrations of the three gases decreased from 200 ppm to 25 ppm, spectively, and the adsorption efficiency was over 85%. The concentratio creased from 200 ppm before the experiment to 61 ppm after the experimen sorption efficiency was over 65%, but it was weaker than that of C3F6 and centration of CF3H and CF4 decreased from 200 ppm before the experime and 156 ppm after the experiment, and the adsorption efficiency did not e It can be seen from the figure that the concentration of each gas decreased after the experiment. The adsorption properties of the ZSM-5 molecular sieve for five kinds of fluorocarbons were different, and the adsorption properties for C 3 F 6 and C 3 F 8 were the best. The concentrations of the three gases decreased from 200 ppm to 25 ppm, and 6 ppm respectively, and the adsorption efficiency was over 85%. The concentration of C 2 F 6 decreased from 200 ppm before the experiment to 61 ppm after the experiment, and the adsorption efficiency was over 65%, but it was weaker than that of C 3 F 6 and C 3 F 8 . The concentration of CF 3 H and CF 4 decreased from 200 ppm before the experiment to 161 ppm and 156 ppm after the experiment, and the adsorption efficiency did not exceed 25%. In general, the ZSM-5 molecular sieve adsorbed most of the fluorocarbon gases, although the adsorption performance of some fluorocarbon gases was not outstanding, and the concentration of fluorocarbon decomposition products produced by the equipment due to various failures generally did not exceed the concentration parameters set in this paper. Thus, ZSM-5 molecular sieve could adsorb fluorocarbon decomposition products in electrical insulation equipment. Figure 3a shows the characteristic peaks corresponding to the 15% C 4 F 7 N standard gas before and after the experiment. The figure shows that the characteristic peak of C 4 F 7 N had no obvious change after the experiment, and the two characteristic peak curves coincide. Comparing the absorbance in the 2260~2280 cm −1 band, the absorbance of the characteristic peak before the experiment was slightly higher than the absorbance after the experiment, which shows that the ZSM-5 molecular sieve exhibited a partial adsorption capacity for C 4 F 7 N. Figure 3b shows the characteristic peaks corresponding to the 15% CO 2 standard gas before and after the experiment. The curve of absorbance changes with the number before and after the experiment was roughly the same in this band. When the wave number was constant, the absorbance before the experiment was slightly higher than that after the experiment, so the ZSM-5 molecular sieve also shows partial adsorption capacity to CO 2 .

Analysis of Results
Chemosensors 2022, 10, x FOR PEER REVIEW 5 of 13 characteristic peak before the experiment was slightly higher than the absorbance after the experiment, which shows that the ZSM-5 molecular sieve exhibited a partial adsorption capacity for C4F7N. Figure 3b shows the characteristic peaks corresponding to the 15% CO2 standard gas before and after the experiment. The curve of absorbance changes with the number before and after the experiment was roughly the same in this band. When the wave number was constant, the absorbance before the experiment was slightly higher than that after the experiment, so the ZSM-5 molecular sieve also shows partial adsorption capacity to CO2. To calculate the change of main gas concentration before and after the experiment, this paper used Beer-Lambert law to calculate the two concentrations after the experiment through the relevant parameters of the standard gas concentration and the light absorption degree before the experiment [23]. To ensure the accuracy of the calculation results, this paper used two parameters, the peak area of the characteristic peak and the intensity of the absorbance, to estimate the concentration reduction after the experiment. Figure 4 shows the changes of the two parameters of the two gases before and after the experiment.
In the infrared absorption spectrum of C4F7N, the curve in the 2260~2290 cm −1 band was integrated to calculate the peak area of the characteristic peaks before and after the experiment. The intensity of the absorbance corresponding to the wave velocity of 2272 cm −1 was selected to quantitatively analyze the concentration of C4F7N before and after the experiment. In the infrared absorption spectrum of CO2, the curve in the band of 3650~3755 cm −1 was integrated, the peak area of the characteristic peak was calculated, and the intensity of the absorbance corresponding to the wave velocity of 3725 cm −1 was selected to quantitatively analyze the CO2 concentration before and after the experiment. Figure 4a shows the corresponding characteristic peak areas of the two gases before and To calculate the change of main gas concentration before and after the experiment, this paper used Beer-Lambert law to calculate the two concentrations after the experiment through the relevant parameters of the standard gas concentration and the light absorption degree before the experiment [23]. To ensure the accuracy of the calculation results, this paper used two parameters, the peak area of the characteristic peak and the intensity of the absorbance, to estimate the concentration reduction after the experiment. Figure 4 shows the changes of the two parameters of the two gases before and after the experiment.   Table 3 shows the absorption rate of CO2 and C4F7N in the ZSM-5 molecular sieve calculated with two parameters. Comparing the calculation results of the adsorption rate of the two gases, the adsorption rate calculated by the characteristic peak area was slightly higher than the result calculated by the intensity of the absorbance. From the perspective of the adsorption effect, the error calculated by the two methods can be ignored. According to the calculated adsorption rate of the two gases, the adsorption effect of ZSM-5 molecular sieve on CO2 and C4F7N was not obvious, the average absorption rate of CO2 was 2.1%, and the average adsorption rate of C4F7N was 4.5% [23]. Table 3. Adsorption rates of CO2 and C4F7N standard gases by ZSM-5 molecular sieve.

Peak Area
Absorbance Intensity adsorption rate of CO2 2.3% 1.9% adsorption rate of C4F7N 5.6% 3.4% Figure 5 shows the infrared spectra of C4F7N and CO2 in the gas mixtures before and after the adsorption experiment. The concentration of C4F7N and CO2 in the gas mixtures before and after the experiment is calculated by the characteristic peak area and peak intensity. Table 4 shows the adsorption rates of CO2 and C4F7N in ZSM-5 molecular sieve for different parameters in the gas mixtures. It can be seen from Table 3 that the concentration of CO2 after the adsorption experiment is slightly higher than that before the experiment, and the average adsorption rate is −2.05%. The concentration of C4F7N is lower than that before the experiment, and the average adsorption rate is 5.9%. It shows that the ZSM-5 molecular sieve in the mixed gas mainly adsorbs C4F7N gas, and C4F7N inhibits the adsorption of CO2. In the infrared absorption spectrum of C 4 F 7 N, the curve in the 2260~2290 cm −1 band was integrated to calculate the peak area of the characteristic peaks before and after the experiment. The intensity of the absorbance corresponding to the wave velocity of 2272 cm −1 was selected to quantitatively analyze the concentration of C 4 F 7 N before and after the experiment. In the infrared absorption spectrum of CO 2 , the curve in the band of 3650~3755 cm −1 was integrated, the peak area of the characteristic peak was calculated, and the intensity of the absorbance corresponding to the wave velocity of 3725 cm −1 was selected to quantitatively analyze the CO 2 concentration before and after the experiment. Figure 4a shows the corresponding characteristic peak areas of the two gases before and after the experiment, and Figure 3b shows the corresponding absorbance intensities of the two gases before and after the experiment. It can be seen from Figure 4 that the parameters of the two gases after the experiment were lower than before the experiment. To reflect the adsorption efficiency of the ZSM-5 molecular sieve material for the two gases, the adsorption rate was calculated according to the characteristic peak area before and after the experiment and the decrease in absorbance intensity. Table 3 shows the absorption rate of CO 2 and C 4 F 7 N in the ZSM-5 molecular sieve calculated with two parameters. Comparing the calculation results of the adsorption rate of the two gases, the adsorption rate calculated by the characteristic peak area was slightly higher than the result calculated by the intensity of the absorbance. From the perspective of the adsorption effect, the error calculated by the two methods can be ignored. According to the calculated adsorption rate of the two gases, the adsorption effect of ZSM-5 molecular sieve on CO 2 and C 4 F 7 N was not obvious, the average absorption rate of CO 2 was 2.1%, and the average adsorption rate of C 4 F 7 N was 4.5% [23]. Table 3. Adsorption rates of CO 2 and C 4 F 7 N standard gases by ZSM-5 molecular sieve.

Peak Area
Absorbance Intensity adsorption rate of CO 2 2.3% 1.9% adsorption rate of C 4 F 7 N 5.6% 3.4% Figure 5 shows the infrared spectra of C 4 F 7 N and CO 2 in the gas mixtures before and after the adsorption experiment. The concentration of C 4 F 7 N and CO 2 in the gas mixtures before and after the experiment is calculated by the characteristic peak area and peak intensity. Table 4 shows the adsorption rates of CO 2 and C 4 F 7 N in ZSM-5 molecular sieve for different parameters in the gas mixtures. It can be seen from Table 3 that the Chemosensors 2022, 10, 121 7 of 13 concentration of CO 2 after the adsorption experiment is slightly higher than that before the experiment, and the average adsorption rate is −2.05%. The concentration of C 4 F 7 N is lower than that before the experiment, and the average adsorption rate is 5.9%. It shows that the ZSM-5 molecular sieve in the mixed gas mainly adsorbs C 4 F 7 N gas, and C 4 F 7 N inhibits the adsorption of CO 2 . ZSM-5 molecular sieve has good adsorption capacity for most of the fluorocarbon decomposition products of the environmentally friendly insulating medium C4F7N/CO2 mixtures and can achieve complete adsorption for a few fluorocarbon gases. It has the potential to be used in the treatment and purification of exhaust gas from gas-insulated equipment, and it can also provide solutions for the development and production of related protective equipment. However, the ZSM-5 molecular sieve also has a certain adsorption effect on the main insulating medium; specifically, it would affect the concentration of C4F7N in the mixtures. If the ZSM-5 molecular sieve can not only adsorb the fluorocarbon decomposition products but also ensure that the insulation performance of the gas is not affected, the proportion of C4F7N in the mixture can be increased to 1.04~1.08 times. Table 4. Adsorption rates of CO2 and C4F7N in gas mixtures by ZSM-5 molecular sieve.

Model Building and Parameter Setting
To clarify the adsorption mechanism and real adsorption capacity of ZSM-5 zeolite porous materials for several fluorocarbon molecules and main insulating gases from the microstructure, the adsorption process was simulated by using the adsorption module in Materials studio simulation software. The initial model was built according to the ZSM-5 molecular sieve in the International Molecular Sieve Association (IZA) database. The simulation was performed using 2 × 2 × 2 cells with cell parameters: a = 4.00 nm, b = 3.98 nm, c = 5.34 nm, α = β = γ = 90°. To be close to the actual experimental molecular sieve structure composition, the Si atoms in the initial model were randomly replaced with Al atoms, and then the Na + equilibrium charge was introduced. The optimized structure of NaZSM-5 with a Si/Al ratio of 47 is shown in Figure 6, which consists of vertical straight channels (5.3 × 5.6 Å) and transverse S-shaped channels (5.5 × 5.1 Å).
According to reference [24], the atomic distribution charges in adsorption are as follows: O(−0.4 e), Si(+0.766 e), Al(+0.575 e), Na (+1 e). The model structures of the five carbon-fluorine decomposition products and two main insulating gas molecules were opti-  Table 4. Adsorption rates of CO 2 and C 4 F 7 N in gas mixtures by ZSM-5 molecular sieve.

Peak area
Absorbance intensity adsorption rate of CO 2 −1.8% −2.3% adsorption rate of C 4 F 7 N 3.9% 8.0% ZSM-5 molecular sieve has good adsorption capacity for most of the fluorocarbon decomposition products of the environmentally friendly insulating medium C 4 F 7 N/CO 2 mixtures and can achieve complete adsorption for a few fluorocarbon gases. It has the potential to be used in the treatment and purification of exhaust gas from gas-insulated equipment, and it can also provide solutions for the development and production of related protective equipment. However, the ZSM-5 molecular sieve also has a certain adsorption effect on the main insulating medium; specifically, it would affect the concentration of C 4 F 7 N in the mixtures. If the ZSM-5 molecular sieve can not only adsorb the fluorocarbon decomposition products but also ensure that the insulation performance of the gas is not affected, the proportion of C 4 F 7 N in the mixture can be increased to 1.04~1.08 times.

Model Building and Parameter Setting
To clarify the adsorption mechanism and real adsorption capacity of ZSM-5 zeolite porous materials for several fluorocarbon molecules and main insulating gases from the microstructure, the adsorption process was simulated by using the adsorption module in Materials studio simulation software. The initial model was built according to the ZSM-5 molecular sieve in the International Molecular Sieve Association (IZA) database. The simulation was performed using 2 × 2 × 2 cells with cell parameters: a = 4.00 nm, b = 3.98 nm, c = 5.34 nm, α = β = γ = 90 • . To be close to the actual experimental molecular sieve structure composition, the Si atoms in the initial model were randomly replaced with Al atoms, and then the Na + equilibrium charge was introduced. The optimized structure of NaZSM-5 with a Si/Al ratio of 47 is shown in Figure 6, which consists of vertical straight channels (5.3 × 5.6 Å) and transverse S-shaped channels (5.5 × 5.1 Å). distribution and dynamic diameter are shown in Table 5. The seven gas m introduced into NaZSM-5 separately for kinetic simulations.

H Atom
Al Atom    According to reference [24], the atomic distribution charges in adsorption are as follows: O(−0.4 e), Si(+0.766 e), Al(+0.575 e), Na (+1 e). The model structures of the five carbon-fluorine decomposition products and two main insulating gas molecules were optimized using the GGA and PBE basis groups in the Dmol3 module. The lowest energy configuration obtained is shown in Figure 7, and the corresponding atomic charge distribution and dynamic diameter are shown in Table 5. The seven gas molecules were introduced into NaZSM-5 separately for kinetic simulations.
Chemosensors 2022, 10, x FOR PEER REVIEW distribution and dynamic diameter are shown in Table 5. The seven gas molecule introduced into NaZSM-5 separately for kinetic simulations.

H Atom
Al Atom

Molecule
Charge (e) Dynam (g) C 4 F 7 N. The simulation is based on the Metropolis sampling method, and the non-bond interaction is described by the Lennard-Jones potential energy and Coulomb interaction: In Equation (1), U is the interaction potential energy between two molecules, i and j are different atoms, σ is the position diameter, is the depth of the potential well, q is the charge parameter set for each atom, and 0 is the vacuum dielectric constant. Combined with the actual operating environment of the equipment, the pressure was set to 0.15 MPa and the temperature was set to 300 K in the simulation process. The Universal Force Field was selected for simulation. Both the molecular sieve model and the adsorbate gas molecules were regarded as rigid structures. Electrostatic and van der Waals interactions were calculated by Ewald and atom-based methods respectively [24,25]. The 1 × 10 6 step of the Monte Carlo method process was used to keep the balance, and the 1 × 10 5 step was used to calculate the heat of adsorption of the gas molecules in the porous structure. The Lennard-Jones parameters of molecular sieve and adsorbent atom based on universal force field are shown in Table 6.  Figure 8 shows the distribution of seven gas molecules in the structure of the NaZSM-5 molecular sieve after the adsorption simulation, and the red dots in the molecular sieve structure indicate the possible locations of gas molecules being adsorbed. From the Figure, the adsorption sites of CF 3 H and CF 4 molecules in the carbon-fluorine decomposition product molecules are widely distributed in the molecular sieve structure, with adsorption sites in both S-type pore channels and straight pore channels. This indicates that CF 3 H and CF 4 molecules are small in size and can diffuse freely in the molecular sieve structure, and the skeleton of the molecular sieve does not have a significant inhibitory effect on the two gas molecules. The adsorption sites of C 2 F 6 , C 3 F 6, and C 3 F 8 molecules are mainly distributed at the intersection of S-type pore channels and straight pore channels, and a few adsorption sites are in the longitudinal straight pore channels. This shows that the volume of these three gas molecules is larger than the diameter of the S-shaped pore channel, and it is difficult to enter the pore channel and interact with the internal atoms, and the molecular sieve skeleton has a binding effect on the movement of the three gas molecules. From Figure 8f, the adsorption sites of CO 2 molecules are distributed in two different pore channels in the molecular sieve structure. On the one hand, the CO 2 molecules are small in size, and the molecules can move across the pore channels from one pore cage to another when they diffuse; on the other hand, the atoms on the molecular sieve skeleton have an average interaction potential for CO 2 molecules, and there is no significant difference in the adsorption produced by the pore cages and pore channels. The adsorption sites of C 4 F 7 N molecules in the molecular sieve structure are mainly concentrated at the pore crossings. The range of C 4 F 7 N adsorption sites is smaller than the range of the three bulkier fluorocarbon gases. This can indicate that C 4 F 7 N molecules are larger than the molecular sieve pore size, and it is difficult to cross the pores to enter the inside of the molecular sieve structure, so intermolecular forces are generated mainly with the atoms on the pore cage to bind the C 4 F 7 N molecules.

Analysis of Simulation Results
Chemosensors 2022, 10, x FOR PEER REVIEW 10 of 13 From Figure 8f, the adsorption sites of CO2 molecules are distributed in two different pore channels in the molecular sieve structure. On the one hand, the CO2 molecules are small in size, and the molecules can move across the pore channels from one pore cage to another when they diffuse; on the other hand, the atoms on the molecular sieve skeleton have an average interaction potential for CO2 molecules, and there is no significant difference in the adsorption produced by the pore cages and pore channels. The adsorption sites of C4F7N molecules in the molecular sieve structure are mainly concentrated at the pore crossings. The range of C4F7N adsorption sites is smaller than the range of the three bulkier fluorocarbon gases. This can indicate that C4F7N molecules are larger than the molecular sieve pore size, and it is difficult to cross the pores to enter the inside of the molecular sieve structure, so intermolecular forces are generated mainly with the atoms on the pore cage to bind the C4F7N molecules. The adsorption heat is an important parameter reflecting the strength of adsorption when gas is adsorbed in the structure of the molecular sieve. The adsorption heat parameters of seven gas molecules in the molecular sieve structure were calculated. The adsorption heat probability curves of seven gas molecules in the NaZSM-5 molecular sieve struc- The adsorption heat is an important parameter reflecting the strength of adsorption when gas is adsorbed in the structure of the molecular sieve. The adsorption heat parameters of seven gas molecules in the molecular sieve structure were calculated. The adsorption heat probability curves of seven gas molecules in the NaZSM-5 molecular sieve structure are shown in Figure 9. The results show that the adsorption heat of the seven gas molecules in the Nazsm-5 molecular sieve is −124.0~−49.3 kJ/mol. Table 7 shows the average adsorption heat of seven gas molecules in the Nazsm-5 molecular sieve.  Figure 9. Probability distribution of adsorption heat of seven gas molecules on Nazsm-5 molecular sieve. Among the five-carbon and fluorine gas molecules, CF4 is a non-polar molecule with a small and stable structure, and the molecular sieve structure exhibits a low heat of adsorption of −60.5 kJ/mol for it. The average adsorption heat of CF3H is −89.2 kJ/mol and is slightly higher than that of CF4. Although CF3H molecule volume is smaller than CF4, it is not easy to be bound by the molecular sieve skeleton, but its polar molecules are more likely to interact with the atoms on the skeleton to be bound, and when the molecular sieve skeleton and atomic force field act simultaneously, the two gases are subject to the same adsorption when they do molecular motion, so the adsorption rate of molecular sieve for the two gases in the adsorption experiment is approximately equal. The adsorption heat of C2F6 in the molecular sieve structure is between non-polar molecules (CF4) and polar molecules (C3F6 and C3F8). Due to the symmetric structure of the C2F6 molecule, the molecular polarity is weak, but its larger size is more likely to interact with the atoms on the skeleton to produce strong induced attraction and then be bound in the molecular sieve skeleton, so in practice the molecular sieve material shows a certain adsorption capacity for C2F6. The two polar molecules C3F6 and C3F8 have larger average adsorption heats in the molecular sieve, −113.1 kJ/mol and −96.5 kJ/mol, respectively, and produce stronger interactions with the atoms in the molecular sieve. At the same time, the molecular volume of the two is large and also bound by the molecular sieve skeleton, so the NaZSM-5 molecular sieve shows a better adsorption capacity for it, and the adsorption rate of the molecular sieve for the two in the adsorption experiment is up to 80% or more.
For the two main insulating gas molecules, CO2 and C4F7N, CO2 has the smallest average adsorption heat in the molecular sieve structure at −49.3 kJ/mol, for which the NaZSM-5 molecular sieve shows a weak interaction. C4F7N has the largest adsorption heat in the molecular sieve structure at −124.0 kJ/mol, for which the molecular sieve is between strong physical adsorption and weak chemisorption. The NaZSM-5 molecular sieve has a strong adsorption effect on C4F7N. This could explain the phenomenon that the molecular sieve has a more pronounced effect on the concentration of C4F7N in the adsorption ex-  Among the five-carbon and fluorine gas molecules, CF 4 is a non-polar molecule with a small and stable structure, and the molecular sieve structure exhibits a low heat of adsorption of −60.5 kJ/mol for it. The average adsorption heat of CF 3 H is −89.2 kJ/mol and is slightly higher than that of CF 4 . Although CF 3 H molecule volume is smaller than CF 4 , it is not easy to be bound by the molecular sieve skeleton, but its polar molecules are more likely to interact with the atoms on the skeleton to be bound, and when the molecular sieve skeleton and atomic force field act simultaneously, the two gases are subject to the same adsorption when they do molecular motion, so the adsorption rate of molecular sieve for the two gases in the adsorption experiment is approximately equal. The adsorption heat of C 2 F 6 in the molecular sieve structure is between non-polar molecules (CF 4 ) and polar molecules (C 3 F 6 and C 3 F 8 ). Due to the symmetric structure of the C 2 F 6 molecule, the molecular polarity is weak, but its larger size is more likely to interact with the atoms on the skeleton to produce strong induced attraction and then be bound in the molecular sieve skeleton, so in practice the molecular sieve material shows a certain adsorption capacity for C 2 F 6 . The two polar molecules C 3 F 6 and C 3 F 8 have larger average adsorption heats in the molecular sieve, −113.1 kJ/mol and −96.5 kJ/mol, respectively, and produce stronger interactions with the atoms in the molecular sieve. At the same time, the molecular volume of the two is large and also bound by the molecular sieve skeleton, so the NaZSM-5 molecular sieve shows a better adsorption capacity for it, and the adsorption rate of the molecular sieve for the two in the adsorption experiment is up to 80% or more.
For the two main insulating gas molecules, CO 2 and C 4 F 7 N, CO 2 has the smallest average adsorption heat in the molecular sieve structure at −49.3 kJ/mol, for which the NaZSM-5 molecular sieve shows a weak interaction. C 4 F 7 N has the largest adsorption heat in the molecular sieve structure at −124.0 kJ/mol, for which the molecular sieve is between strong physical adsorption and weak chemisorption. The NaZSM-5 molecular sieve has a strong adsorption effect on C 4 F 7 N. This could explain the phenomenon that the molecular sieve has a more pronounced effect on the concentration of C 4 F 7 N in the adsorption experiment.
The results of theoretical calculation correspond to the results of adsorption experiments. The results show that the adsorption of small non-polar gas molecules on the NaZSM-5 molecular sieve is weak, followed by small positive gas molecules. The larger chemical polarity molecules can interact with the atoms inside the molecular sieve to create an induced attraction that allows the gas to be partially absorbed. The stronger the molecular polarity, the more obvious the adsorption effect of molecular sieve structure on gas molecules, and the better the actual adsorption effect on gas.

Conclusions
In this paper, the adsorption properties of the ZSM-5 molecular sieve on the C 4 F 7 N/CO 2 mixtures and its five common fluorocarbons were investigated by gas adsorption experiments and molecular dynamics simulation. The following conclusions were obtained: (1) ZSM-5 molecular sieve has a certain adsorption effect on six kinds of fluorocarbon gases, of which the adsorption performance of C 3 F 6 , and C 3 F 8 is the best, with an adsorption efficiency of over 85%. (2) The concentration of the two main insulating gases CO 2 and C 4 F 7 N will be affected by the ZSM-5 molecular sieve; if the insulation performance of the gas is not affected in practical application, the proportion of C 4 F 7 N in the mixture can be increased 1.04~1.08 times. (3) NaZSM-5 molecular sieve has the strongest adsorption to C 4 F 7 N and the weakest adsorption to CO 2 . The stronger the polarity of the gas molecule, the more obvious the adsorption effect of molecular sieve structure.