In a high vacuum space environment, the molecular contaminants released by some organic space materials^[1⇓-3], such as plastics, adhesives, lubricants, silicones, epoxies, potting compounds, and other similar materials, deposit on the surface of sensitive optical components, thermal control components, and solar panels, which seriously affects the performance and service life of the spacecraft^[4⇓-6]. At the same time, the molecular contaminants mentioned above also pollute the manned cabins to affect the equipment in the cabin and the health of astronauts^[7-8]. Conventional methods such as selecting low outgassing materials and increasing the number of thermal vacuum bake on the ground can no longer meet the design requirements of long-life spacecraft. The problem of on-orbit molecular pollution has become an important unfavorable factor to restrict the development of long- life and high-performance spacecraft. It is urgent to develop new technologies to control space contaminations^[9⇓⇓-12].

Thanks to the rich pore structure, porous adsorbents can collect space contaminations in real time^[13]. As a space adsorbent material, in addition to the molecular adsorption function, it is also required to have good space environmental stability and low outgassing performance. Zeolite molecular sieve is an inorganic adsorbent material with good space environmental stability which can withstand harsh environments such as space particle irradiation and atomic oxygen erosion. The zeolite molecular adsorber coating (MAC) is one kind of new and innovative materials, which is developed by National Aeronautics and Space Administration (NASA) to reduce the risk of space contaminations^[2,14]. The advantage of zeolite molecular adsorber coating is that it can effectively collect molecular contaminants in real time, and when the contaminant molecules move to the surface and inside the coating, they will be effectively adsorbed by the zeolite adsorbent. As a porous inorganic material coating, MAC mainly includes inorganic zeolite filler and inorganic silica sol binder. It offers impressive absorptive capabilities for different contaminants^[15]. In the extreme ultra-high vacuum and variable temperature space environment, the capturing and adsorbing process of zeolite on contaminants are more complicated. However, the current research on zeolite MAC is still mainly based on the experimental test of the adsorption capacity, and the in-depth analysis of the adsorption and diffusion processes of contaminant molecules in zeolites is still lacking in the on-orbit environment. Therefore, the MAC with ultra-high adsorption capacity cannot be individually designed for different space missions. Computational simulation is a good substitute for experimental testing to efficiently explore the adsorption process of zeolite on different contaminants in ultra-high vacuum conditions^{[16⇓⇓-19]}.

The main volatiles of aerospace cables, silicone rubber, rubber gaskets, etc. are toluene, phthalates, and siloxane molecules, and the models of some typical organic molecules, including toluene, dimethyl phthalate and octamethyl cyclotetrasiloxane were established by the Material Studio in this work, and the X-ray diffraction patterns and adsorption process of NaY zeolite were calculated and compared with the experiment data. In order to study the adsorption process of the contaminants under the on-orbit environment, the adsorption isotherm, adsorption heat and density distribution of different contaminants in NaY zeolite also were calculated and predicted. This work could provide some theoretical data for the development and individual design of zeolite molecular adsorber coating.

In this work, toluene(C₇H₈), dimethyl phthalate (C₁₀H₁₀O₄), octamethyl cyclotetrasiloxane (C₈H₂₄O₄Si₄) were used as target contaminants. As shown in Fig. 1, these contaminant models were established by using Material Studio software package.

An original FAU zeolite with the chemical composition of Si₉₆Al₉₆O₃₈₄ was established by the Material Studio software package, and its Si/Al ratio was 1:1. The space group of original FAU zeolite was Fd-3z, a=b=c=25.028 Å, α=β=γ=90°^[20-21]。Then 42 aluminum atoms were changed randomly to Si atoms in accordance with the Si/Al ratio (2.6) of the experiment NaY zeolite and the substituting process must satisfy the Löwenstein rule. 54 sodium atoms need to be introduced for charge balance by three different ways. The first method refers to the distribution rule of cations reported in the literatures^[22-23]. The monovalent alkali metal ions of NaY zeolite occupied the sites of Ⅰ, Ⅰ′ and Ⅱ, and SⅡ (site of Ⅱ) was close to being fully occupied, when SⅠ (site of Ⅰ) and SⅠ′ (site of Ⅰ′) were occupied partly. Therefore, for 54 sodium ions in this work, 32 of them firstly occupied the SⅡ. As for the distribution of SⅠ and SⅠ′, some different attempts were tried. Then the geometric structures of different NaY zeolites were optimized by using the Forcite module in the Material Studio software. With respect to the second way, sodium atoms were randomly placed one by one and the zeolites with the lowest energy were optimized in 54 times. In the third way, sodium atoms were placed randomly and then the zeolite was optimized in one time. NaY zeolites with the composition of Na₅₄Si₁₃₈Al₅₄O₃₈₄ and the Si/Al ratio of 2.6 were established and different optimized zeolites above were named NaY2.6 opt1, NaY2.6 opt2, NaY2.6 opt3, respectively. The final sites of 54 sodium ions of NaY opt1 were 32SⅡ, 13SⅠ, 8SⅠ′ and 1SⅢ′, and the distributions of sodium ions in NaY opt2 and NaY opt3 zeolite structures were 2SⅠ, 19SⅠ′, 30SⅡ, 2SⅢ, 1SⅢ' and 5SⅠ, 12SⅠ′, 15SⅡ, 9SⅢ, 13SⅢ′, respectively. The NaY zeolite was semi-ionic with atoms carrying the following partial charges: Si(+2.4e), Al(+1.4e), O(-1.2e), Na(+1e)^[24-25]. The structures of three NaY zeolites were shown in Fig. 2.

The balls have four colors including purple, yellow, pink, and red, representing atoms of sodium, silicon, aluminum, and oxygen, respectively

Grand Canonical Monte Carlo (GCMC) technique was applied to simulate the adsorption process between NaY zeolite and different adsorbates^[19,26]. This technique is in the constant μVT ensemble, which means chemical potential, volume and temperature of the whole system are fixed. The force fields used as calculating potential energy of the system are the force between atoms or molecules of the studied system, including bonded and non-bonded interactions. The interactions between zeolites and other three absorbates as well as between adsorbates and adsorbates took place through the non-bonded energy which is the sum of van der Waals interaction and the electrostatic energy by the Formula (1) when the former is expressed by Lennard-Jones (LJ) potential energy and the latter is expressed by Ewald method calculation^[26-27]:

where U is total potential energy, r_ij is the distance between atoms i and j, ε_ij and σ_ij are the LJ potential parameters, ε₀ is the vacuum permittivity (ε₀=8.85×10^-12 C²·N^-1·m^-2), q_i and q_j are the partial charges for atoms i and j. The box length of NaY zeolite along each direction was longer than 24 Å. So a cut off distance of 12 Å was applied to all LJ interactions^[28-29]. The long-range electrostatic interactions were accounted for using the Ewald sum technique.

The number of molecules or atoms in the simulation process directly determines the running speed and ability of the computer, and the amount of calculation could be reduced by setting valid boundary conditions in all three directions. Therefore, only one zeolite crystal cell with 1×1×1 was used as a simulated unit. Meanwhile, the skeleton atoms including silicon atoms and aluminum atoms were fixed at the coordinate position. The contaminant molecules remained rigid, and these molecules could only move and rotate without deformation in the simulation process.

In this work, the Sorption module and COMPASS force-field in Material Studio simulation package were used to calculate the adsorption process of NaY zeolites on different contaminants^[30-31]. The Monte Carlo interactions were set to 1×10⁷ when the first 5×10⁶ steps were calculated only for equilibration and the second 5×10⁶ steps were calculated for predicting the adsorption capacity.

NaY zeolite was purchased from Nankai University Catalyst Co., Ltd. (China). The phase of NaY zeolite samples was studied by Bruker D2 PHASER powder diffractometer with Cu Kα radiation operated at 40 kV and 200 mA. Toluene adsorption isotherm of NaY zeolite was measured by the Intelligent Gravimetric Analyzer (3H2000-PW, Beishide Instrument Technology (Beijing) Co., Ltd) at 298 K, and the sample in the vessel was vacuumed up to 105 mbar and outgassed at 573 K for 24 h before measurements.

The established NaY zeolite models are verified by comparing the simulated X-ray diffraction patterns with the actual measurement. As shown in Fig. 3, the characteristic peaks of three NaY zeolite models calculated by the Reflex Module of Material Studio are very similar^[26], which are 10°, 11.75°, 15.45°, 18.4°, 20.05°, 23.3°, 26.65°, 30.3°, and 30.95°. The experimental characteristic peaks of NaY zeolite are 10.13°, 11.88°, 15.63°, 18.65°, 20.33°, 23.61°, 27.01°, 30.7°, and 31.34°. Therefore, the diffraction peak positions of simulated XRD patterns are highly consistent with the experimental results, indicating that the NaY zeolite models meet the simulation requirements.

The experimental result of toluene adsorption isotherms and the simulated adsorption isotherms of three NaY zeolites at 298 K are demonstrated in Fig. 4. It can also be seen that the three simulated NaY zeolites have a similar trend, rising rapidly at low pressure and remaining stable with the pressure increasing. The saturated toluene adsorption capacity simulated by different NaY zeolite models is about 36 per cell, which is close to the experimental data (36.2 per cell) in Fig. 4(b) and other simulated data in the literatures^{[20,36 -37]} in Table 1. It is important to note that the saturated adsorption isotherm of NaY opt3 is sufficiently accurate in respect of the experimental data.

Considering the convenience of establishing NaY zeolites models and the validation of simulation method, the NaY2.6 opt3 model is the most suitable one, which is selected as the targeted simulation model in the following calculation of adsorption process.

Temperature and pressure are two significant factors in the space environment, and different temperatures (173, 223, 273, 298, and 373 K) in the range from ultra-high vacuum to standard atmospheric pressure are applied to simulate the adsorption capacity of NaY zeolite on toluene. As shown in Fig. 5, due to the limitation of the total pore volume, the adsorption capacity of toluene cannot be increased by adjusting the temperature and pressure at relatively high pressures (1 Pa<P<101 kPa). Temperature has little influence on the toluene adsorption capacity of NaY zeolite that is kept at about 36 per cell. However, in the range of low pressure (P<1 Pa), as the temperature increasing, the adsorption capacity of NaY zeolite shows a great difference in Fig. 5(b), and the toluene adsorption capacity at high temperature is smaller than that at low temperature. When pressure is lower than 10^-7 kPa, the adsorption capacity at 373 K is close to zero while that is about 25 per cell at 298 K. The adsorption capacity is basically maintained at 36 per cell at lower temperatures.

The adsorption heat of NaY zeolite on toluene were simulated and shown in Fig. 5(c). It can be observed that the adsorption heat is about 105 kJ/mol, which is close to the experimental data (95 kJ/mol) measured by temperature-programmed desorption method^[34] and the simulated data (105.5 kJ/mol)^[35], representing the strong interactions between toluene and NaY zeolite. In addition, the adsorption heat decreases from 111 kJ/mol (173 K, 10^-7 kPa) to 94 kJ/mol (373 K, 10^-7 kPa) at low pressure. With the increase of pressure, the adsorption heat and the adsorption capacity are kept at about 110 kJ/mol and 36 per cell, respectively. Temperature influences the interaction between toluene and NaY zeolite, which can be expressed by the wave form of the adsorption heat. At low pressure, the adsorption capacity is relatively stronger, while the desorption is more difficult, resulting in a higher adsorption capacity. Overall, the NaY zeolite has a bright application prospect in high vacuum condition.

In order to further understand the adsorption behaviors of NaY zeolites on toluene, the density distributions were investigated. As shown in Fig. 6, different density distributions are represented at the pressure of 10^-6 Pa, 10^-4 Pa, and 1 Pa, respectively. The yellow ball and stick model are the NaY zeolite framework and the red scatter plots represent the presence possibility of toluene in Fig. 6. It can be seen that adsorption locates of toluene in NaY zeolites are distributed along with the interior of super- cage, especially close to twelve-membered ring. There are no toluene molecules existing in the sodalite cage (Fig. 7) due to the toluene size (6.3 Å) is close to the diameter of six-membered ring of sodalite cage and smaller than that of super-cage (12 Å). With the increase of pressure, more and more toluene molecules are adsorbed inside the super-cage, including the position near twelve-membered ring and the center of super-cage in Fig. 6.

As shown in Fig. 8, the adsorption capacity of NaY zeolite on C₁₀H₁₀O₄ and C₈H₂₄O₄Si₄ molecules were calculated at different temperatures and pressures. With the increase of temperature, the adsorption capacity of NaY zeolite for C₁₀H₁₀O₄ and C₈H₂₄O₄Si₄ demonstrate obvious differences in the range of different pressures. In the range of low pressure (P<1 Pa), the adsorption capacity of two molecules is really influenced by temperature. The adsorption capacity of NaY zeolite on C₁₀H₁₀O₄ is only about 17 per cell when temperature up to 473 K. Similarly, the C₈H₂₄O₄Si₄ adsorption capacity reduces when temperature up to 348 K. However, the adsorption capacity of NaY zeolite on two contaminants still maintains a high value when the pressure is 10^-7 kPa. In the higher pressure range (1 Pa<P<101 kPa), the adsorption capacity of two molecules is basically unchanged with the increase of temperature, and the saturated adsorption capacity is about 17 per cell (C₁₀H₁₀O₄) and 8 per cell (C₈H₂₄O₄Si₄), respectively.

The adsorption heat of NaY zeolite on C₁₀H₁₀O₄ and C₈H₂₄O₄Si₄ was studied at different temperatures in Fig. 9. The adsorption heat of C₈H₂₄O₄Si₄ decreases with the increase of temperature, while that of C₁₀H₁₀O₄ changes slightly with different temperatures. The difference of adsorption heat is related to the structure of C₁₀H₁₀O₄ and C₈H₂₄O₄Si₄. Compared with toluene, the molecular structures of C₁₀H₁₀O₄ and C₈H₂₄O₄Si₄ are more complex and larger, resulting in higher adsorption heat and smaller adsorption capacity.

In Fig. 10, density distributions of C₁₀H₁₀O₄ and C₈H₂₄O₄Si₄ at pressure of 1 Pa were demonstrated. It can be seen apparently that the adsorption sites of both molecules are distributed within super-cage rather than sodalite cage. Compared with the adsorption behavior on toluene (Fig. 7), the red scatter plots in Fig. 10 are less and distributions of two molecules are more concentrated. Considering the larger cell diameter and more complex structures of C₁₀H₁₀O₄ and C₈H₂₄O₄Si₄, this result is reasonable.

In this work, the models of NaY zeolite and different contaminants were established successfully. The effects of temperature and pressure on the adsorption capacity and adsorption heat on different molecules were carefully studied. Three molecules including toluene, dimethyl phthalate and octamethyl cyclotetrasiloxane exhibit different adsorption capacities on NaY zeolite and the values were about 36 per cell, 17 per cell and 8 per cell, respectively, which is related to their molecular structures. These contaminants can be effectively adsorbed by NaY zeolite under ultra-high vacuum situation, so the molecular adsorber coating based on NaY zeolite has a significant application prospect.