Molecular mechanism of selective adsorption and separation of n-butane/i-butane on MFI zeolite

doi:10.21203/rs.3.rs-2604157/v1

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Molecular mechanism of selective adsorption and separation of n-butane/i-butane on MFI zeolite

https://doi.org/10.21203/rs.3.rs-2604157/v1

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published 17 Nov, 2023

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MFI zeolite is one of the most attractive molecular sieve materials for selective adsorption and separation of C4 hydrocarbon isomers. The correlation of channel structure and adsorption thermodynamics and kinetic behavior of butane isomers has been explored using Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The results of single com-ponent and binary competitive adsorption isotherms of n-butane/i-butane at 273 K, 303 K, 343K, and 373 K substantiated the excellent low pressure loading and adsorption selectivity for n-butane on MFI zeolite. The density distribution maps and the RDFs data revealed that there are different preferential adsorption sites for n-butane and i-butane molecules in MFI zeolite. Due to the van der Waals repulsion occurring, the adsorption energy of i-butane is less than that of n-butane, which makes that i-butane cannot be apt to be adsorbed in the two channels, but preferentially in the cross channel of MFI zeolite. Moreover, the mean square displacement (MSD) and calculated diffusion coefficient proved that the diffusion rate of n-butane is faster than that of i-butane by more than two orders of magnitude. These findings clarify the molecular mechanism of selective adsorption and separation of n-butane/i-butane on the rational designed MFI zeolite-based materials.

Adsorption

MFI zeolite

butane

separate

C4 hydrocarbons, occurring in oilfield gas, wet natural gas, and side-products from petroleum processing, should not be used mainly as fuel, but as the chemical raw materials for high value-added comprehensive utilization[1]. However, due to the high-energy-consumption and high cost in traditionally separation process by distillation, only about 12% of butane is further processed in the chemical industry. Therefore, the development of less energy intensive adsorption and membrane separation approach are particularly promising for efficient separation of mixed C4 hydrocarbons.

Zeolites, also known as molecular sieve, are the most highly reported porous materials used in the adsorption and membrane separation processes due to their excellent adsorption selectivity[2–4]. The most common types of zeolite-based adsorbents and membranes for n/i-butane isomers separation are the MFI aluminosilicate zeolites[5–10].

MFI zeolites, belonging to the pentasil family, which consist of two kinds of crossed 10-membered oxygen ring (10-MR) holes, as the straight channels (5.3 Å×5.6 Å), and the zigzag channels (5.1 Å×5.5 Å)[11]. The kinetic dimensions of n-butane (4.7 Å) and i-butane (5.3 Å) are very close to the channel sizes of MFI zeolite. It is widely recognized that the mechanism of size exclusion and the molecular sieving effect for the excellent separation ability of butane isomers by MFI zeolite membranes. However, it is puzzling that MFI zeolite with different morphological features exhibit significantly different adsorption and separation performance in C4 hydrocarbon separations[1, 3, 12]. Specifically, MFI zeolite membranes presents a satisfactory selectivity and permeance, with the variation range for the selectivity is around 6 and 140, and the variation range for the permeance is around 1 × 10 ^− 9 and 9.8 × 10 ^− 7 mol/m² s Pa. While, the orientations of MFI membranes also affect its separation factor that the c-orientation (or h0h-) has the highest value followed by a-orientation and b-orientation[13]. The thicknesses of MFI membrane are also reported by Hedlund et al. that the highest permeances (9.8 × 10 ^− 7 mol/m² s Pa) at 298 K is obtained by only 0.5 µm thickness material. However, its selectivity is only 9 for the mixtures due to the defects in the membrane. To solve this problem, Liu et al. demonstrated that the introduction of hierarchy easily enabled concurrent thickness reduction (ca. 390 nm) and preferentially (h0h) orientation control of MFI zeolite membranes with a satisfactory performance for the separation of butane isomers. By synthesized the MFI membranes via solution-based protocols, trade-off limits between the n-/i-butane selectivity and n-butane permeance is surpassed[5].

In addition to morphological features of the MFI membranes, the separation performance of MFI zeolite membranes is also affected critically by both the temperature and pressure of the system. The maximum separation factor of n-butane/i-butane mixture are observed by Sun et al. at 363 K[14]. Generally, the n-butane permeance increases with increase in temperature. The facts appear to tell a different story that, because of the permeance of n-butane is decreased as the temperatures drop, the separation factor is decreased for the mixture [12, 15–17]. It is clarified that, in the cross channel of MFI framework, the bulky i-butane molecules can be easily gathered as the i-butane flux increased[18, 19]. Sun et al.[14] found that i-butane molecules can preferential adsorbed in the pores of the MFI framework because of the increase of its partial pressure, which will decrease the separation factor. However, Wang et al.[20] found that the stronger adsorption ability of n-butane in MFI zeolite than that of i-butane lead to the decrease of the separation factor [21].

Previous studies demonstrated that the separation performance of C4 hydrocarbon isomers in MFI zeolite are sensitive to the morphological features and the thermodynamic conditions, including temperature, pressure, and component concentration in the separation system, but the molecular mechanisms of selective adsorption and separation of n-butane/i-butane on MFI zeolite has not been unambiguously clarified due to the complexity of MFI crystal structure and the complex interaction between the host channel structure and the guest C4 hydrocarbon molecules.

In this study, the intrinsic impact of the MFI channel structure on the adsorption thermodynamics and kinetic behaviour of butane isomers was explored by using Giant Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Based on the calculation results of this work, the molecular mechanism of selective adsorption and separation of n-butane/i-butane on MFI zeolite was revealed. The results of the current simulation work provide a theoretical foundation for the design of the improved zeolite-based adsorbents and membranes with tailored morphological features and channel structures to improve experiments involving the efficient separation of n/i-butane isomers.

2.1 Zeolite framework

The structure of siliceous MFI zeolite model was available in the Database of Zeolite Structures (http://www.iza-structure.org/databases/). The periodic supercells containing 8 (2 × 2 × 2) lattices were constructed from one-unit cell. MFI zeolite, which contains 3-D intersecting channels, have two kinds of channels, which are the straight channels (5.3 × 5.6 Å) along [010] direction and tortuous channels (5.1 × 5.5 Å) along [100] direction, and they are intersected together to form the cross section. All the structure atoms of MFI were assumed to be rigid during the GCMC simulation, which has little affection on the calculation of adsorption properties[22]. The Metropolis Monte Carlo (MMC) was chosen. For the framework, the charge of silica and oxygen atoms were obtained by the charge equilibration (QEq_charged1.1) approach method, which is based on the atomic configuration and the electronegativities of the various atoms[23]. Thus, the partial charge of silica atom and oxygen atom in the framework was 1.25 e and − 0.65 e in average, respectively. The same method was also used for the n-butane and i-butane molecules during all the calculations.

2.2 Simulation details

GCMC simulations were carried out with the Sorption model in Materials Studio 5.5 (Accelary. US) to simulate the single n-butane and i-butane adsorption in the zeolites, as well as their mixtures. For all the simulations, the Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS 2.8) forcefield, which is established by the first principles ab-initio that enables accurate and simultaneous prediction of both gas-phase and zeolite properties[24], was used in this study to measure the valence and nonbonding interactions of zeolite frameworks with adsorbates (host–guest) and adsorbates with adsorbates (guest and guest). And the simulation result meet well with the work of Zhu et al.[25], as shown in Figure S1. The interactions were modelled by Lenard–Jones (LJ) potential[26] plus Coulomb potential, with the equation of:

$${E}_{pot}=\sum _{i>j}\frac{{q}_{i}{q}_{j}}{{r}_{ij}}+\sum _{i>j}{E}_{ij}\left[2{\left(\frac{{r}_{ij}^{0}}{{r}_{ij}}\right)}^{9}-3{\left(\frac{{r}_{ij}^{0}}{{r}_{ij}}\right)}^{6}\right]$$

where r_ij is the distance between i and j sites, q_i and q_j are the charges on the interaction centre. All atoms are treated as nonbonded, and interact with each other through a pairwise LJ-9-6 function as Eq. 1 displayed. The parameters represent the effective total contribution of both metallic bonds and long-range van der Waals interactions. Ewald summation method[27] is used to deal with the electrostatic interactions with a calculation accuracy of 4.184×10^− 5 kJ/mol; the Atom-based method, in which a simple approach to the calculation of long-range non-bond interactions is the direct method, is selected for van der Waals force interactions, where non-bond interactions are simply calculated to a cutoff distance (12 Å) and interactions beyond this distance are ignored.

During the GCMC calculation, the production step was set to 5 × 10⁶ cycles after 5 × 10⁶ cycles equilibration. Each of the steps attempted to move every adsorbate once. The distribution of movement for Metropolis Monte Carlo method was chosen randomly with a fixed probability, which was 40% exchange, 20% translation, 20% rotation and 20% conformation.

After the simulation of isotherm, the fix loading of the adsorbed molecules was also calculated to analyse the adsorption distribution density and the energy distribution. During the fix loading calculation, a trajectory containing snapshots of the adsorption configuration was collected during the simulation, and then the Radial Distribution Function (RDF) was used for confirming the adsorption sites of adsorbed molecules on MFI models. Before the calculation of RDF, the minimum distance between the H atoms of molecules and O atoms of MFI framework was collected for the RDF calculation, which can count on the positions between molecules and MFI skeletons.

For the MD calculation, in order to ensure satisfactory conservation, 1.0 fs was employed as the calculation step of the system. The system was performed using 2000 ps NVT-MD simulations after 500 ps equilibration steps at 303 K, and a Nose–Hoover thermostat[28] was used for temperature control. The mean square displacement (MSD) of alkanes was used in analyzing the diffusion coefficients and is defined by the following equation

$${D}_{s}=\underset{t\to \infty }{lim}\frac{1}{6t}⟨{\left|\overrightarrow{r}\left(t\right)-\overrightarrow{r}\left(0\right)\right|}^{2}⟩$$

where D_s is the self-diffusion coefficient, r(0) and r(t) are the initial position vector of the molecule and the position vector of the molecule at time t separately.

The adsorption of n-butane and i-butane molecules adsorption in the three kinds of channels of MFI zeolite was calculated by DFT method. The calculations were carried out by using the Cambridge Serial Total Energy Package (CASTEP) plane-wave code[29] in the Materials Studio 5.5 software package, Accelyrs. The unit cell of MFI framework is utilized for the n-butane and i-butane adsorption. The Perdew-Burke-Ernzerh generalized gradient approximation (GGA-PBE) function[30] with TS dispersion correction method[31], which can descript the weakly bonded systems including the van der Waals (vdW) interactions, were selected to optimize the structures of adsorption state at unit cell of MFI zeolite. All calculations were performed with a Gamma k-point with cutoff energy of 500 eV. The force threshold and self-consistent tolerance were set to 0.03 eV Å⁻¹ and 10^− 6 eV per atom, respectively.

The optimized adsorption structure is then prepared to calculate the weak interaction by Independent Gradient Model (IGM) method[32]. The IGM calculation is calculated by Multiwfn maintained by Tian Lu[33].

2.3 Adsorption models

The studies of the adsorption are always started with pure gas adsorption. The Dual-Site Langmuir-Freundlich (DSLF) model and ideal adsorbed solution theory (IAST) method could be combined to simulate the adsorption properties for mixture gas. The DSLF model was employed to fit the adsorption isotherms with absolute adsorption amount:

$$q={q}_{m1}\frac{{b}_{1}{p}^{{n}_{1}}}{1+{b}_{1}{p}^{{n}_{1}}}+{q}_{m2}\frac{{b}_{2}{p}^{{n}_{2}}}{1+{b}_{2}{p}^{{n}_{2}}}$$

where q_m1 and q_m2 represent the saturated adsorption capacities of the two sites (mmol/g), n₁ and n₂ represent the deviation of the two sites from the ideal one (dimensionless), b₁ and b₂ represent the affinity coefficients of two sites (1/Pa), and p represents the total pressure (Pa). Other models were also employed to fit the adsorption isotherms, e.g., Langmuir, Toth, Langmuir-Freundlich, dual site Langmuir, etc., the DSLF model was used herein that it yielded satisfying fitting results with R² all above 0.999 (see Table S1 for the fitting results). These fitting parameters in Table 1 were used in the IAST modelling. IAST is the most widespread multicomponent adsorption theory to predict multicomponent adsorption equilibrium from single component adsorption isotherms[34].In the IAST, the adsorbed phase is considered an ideal solution, which is an acceptable assumption or can offer a useful initial result for a more refined interpretation.

Selectivity is an important factor for describing the competition adsorption performances, which can be defined by the following formula:

$${S}_{ij}=\left(\frac{{x}_{i}}{{x}_{j}}\right)/\left(\frac{{y}_{i}}{{y}_{j}}\right)$$

where x_i, x_j are the molar fraction of species i and j in the adsorbed phase, while y_i, y_j are the molar fractions of species i and j in the gas phase.

3.1. Adsorption and Competitive Adsorption Properties of n-butane/i-butane in/on MFI zeolite

The adsorption isotherms of single component n-butane and i-butane in/on MFI zeolite at 273 K, 303 K, 343K, and 373 K were simulated and fitted with DSLF model. The simulated data points and the fitting data lines are presented in Fig. 1, and the fitted parameters of DSLF model are listed in Table S1. As shown in the Fig. 1, the adsorption isotherms of n-butane and i-butane are apparently different. For the n-butane, the adsorption amounts increased sharply in a narrow pressure region (the isotherms at four temperatures are almost parallel), then the adsorption amounts increased slowly and approached saturation. The initial adsorption pressure (from 10^− 6 kPa to 10^− 3 kPa) and saturated adsorption pressure (from 10^− 3 kPa to 10⁰ kPa) of n-butane almost increased by three orders of magnitude with the increase of temperature (from 273 K to 373 K). By contrast, the adsorption isotherms of i-butane show higher initial adsorption pressure (from 10^− 5 kPa to 10^− 2 kPa), and higher saturated adsorption pressure (from 10⁰ kPa to 10³ kPa), whose slopes are obviously smaller (spanning five orders of pressure magnitude) than that of n-butane (spanning three orders of pressure magnitude) for all the temperatures. These results indicate a relatively lower adsorption rate of i-butane than that of n-butane on MFI zeolite, agreeing well with the conclusions as reported in the literature[8, 35, 36]. Also of note is that there are inflection points and platforms on the isotherms of i-butane when the adsorption amounts increased about 0.8 mmol/g, which concludes that the adsorption mode of i-butane on MFI zeolite changes with the increase of pressure. It can be speculated that the adsorption site of isobutane has changed. This is also different from the adsorption behaviour of n-butane. This kind of “kink” is also observed by other experimental and simulated results[25, 37, 38], and it is said that the branched i-butane molecules have a strong preference for the intersections, which lead to a two-step adsorption behaviour.

Figure 2a presents the IAST estimations of component loadings, q_i, of n-butane and i-butane for adsorption of n-/i-butane mixtures in MFI zeolite at four different temperatures (273 K, 303 K, 343K, and 373 K) and total gas-phase pressure p_t = p₁ + p₂ = 100 kPa, plotted as a function of the mole fraction of n-butane in the bulk gas phase, y_n−butane. Figure 2b plots the n-/i-butane adsorption selectivity as a function of the mole fraction of n-butane in the bulk gas phase. The adsorption amount of n-butane and i-butane vary dramatically when the mole fraction of n-butane in the bulk gas phase is larger than 0.2. The adsorption of i-butane is lower than 0.1 mmol/g when the component of n-butane is larger than 0.2. This means that the tiny addition of n-butane, which is strongly adsorbed, can influence the adsorption of i-butane significantly. The selectivity is well satisfactory that is similar to most of the selectivity of MFI mem-brane (between 10 and 100)[1]. The selectivity is decreased as the temperature increases, which means that the low temperature is benefit the separation of butane isomers.

After the unary adsorption isotherm analysis, the n-/i-butane mixtures were considered on MFI zeolite simulated with multi-component GCMC to study the competitive adsorption behaviour. Figure 3a plots the binary competitive adsorption isotherms of n-butane/i-butane mixtures with the mole ratio of n-butane: i-butane = 1:1 of GCMC calculation (point data) and the simulated results by IAST method (line data) at four different temperatures. The adsorption selectivity for GCMC calculation is also presented in Fig. 3b. Firstly, it can be observed that, when the mixture gas adsorbed on the MFI zeolite, the adsorption amount of n-butane is much more than i-butane, which is agree with the experimental result[38]. It also can be seen that there is a certain deviation in the competitive adsorption isotherms obtained by the two algorithms (cf. Figure 3a). It is mainly reflected that the saturated adsorption capacity of n-butane is smaller, while that of isobutane is larger for the GCMC results than that for IAST results in the high adsorption pressure range. The adsorption selectivity calculated from multi-component GCMC isotherms is significantly affected by the adsorption temperature and pressure. Specifically, at low temperature (273 K and 303 K), the adsorption selectivity decreases with the increase of pressure, while at high temperature (343 K and 373 K), it first increases with the increase of pressure and then decreases until a maximum in separation factor is reached at about 10 kPa. It can be observed a maximum in n-butane/i-butane adsorption selectivity of 15 at 343 K. The above simulation results are very consistent with the experimental results, confirming that the separation factor is increased when the temperature increase to a maximum. Then, as the temperature continues to grow up, the separation selective decrease due to the increase of i-butane adsorption amount and the decrease of the n-butane permeance compared with its permeances obtained at lower temperatures[18, 19].

3.2 Correlation between Adsorption Sites of N-butane and Isobutane and Pore Structure of MFI Zeolite

Figure 4 displays the density distribution of n-butane on MFI zeolite at different loading from 5 to 65 molecules/cell (Correspondingly from 0.11 to 1.41 mmol/g) at 303 K. The domains with densely distributed red dots represent the lowest energy and preferred adsorption positions for n-butane molecules. It can be seen that n-butane molecules are preferentially adsorbed in sinusoidal and straight channels when the loading is low (5 and 15 molecules/cell), and scarcely distributed in cross channels. With the increase of n-butane loading, the red dot area in the straight channel gradually becomes continuous. When the loading is higher than 35 molecules/cell (0.76 mmol/g), almost all straight channels are filled with n-butane molecules. While, the red dot regions in the sinusoidal channels are not continuous, and even if saturated adsorption is reached, they are always disconnected near the cross channel. These results suggest that the n-butane molecules are preferentially adsorbed and distributed in the two channels of MFI zeolite, especially in the straight channel, but resist to stay near the cross channel. This may be applicable to explain the excellent n-butane permeability of MFI membrane materials with (h0h) orientation control growth.

Figure 5 depicts the density distribution of i-butane on MFI zeolite at different loading of 5 to 65 molecules/cell (Correspondingly from 0.11 to 1.41 mmol/g) at 303 K. The domains with densely distributed red dots represent the lowest energy and preferred adsorption positions for i-butane molecules. It is obvious different from n-butane that the i-butane molecule is preferentially located in the intersection channel. With the increase of loading (less than 25 molecules/cell), the adsorption of n-butane is still mainly distributed in the cross channel, and only when the adsorption amount reaches a certain value (approx. 35 molecules/cell) will occur the adsorption process in the two channels. These results can well explain why the inflection points and platforms appear on the isotherms of i-butane when the adsorption amounts increased about 0.8 mmol/g. In addition, it can be seen that even if saturated adsorption is achieved, the adsorption distribution characteristics of i-butane in MFI zeolite channels are also in a local regional distribution state. This is obviously different from the results of n-butane. This may be due to the fact that the dynamic size (5.3 Å) of isobutane molecule is too close to the pore diameter of MFI zeolite, resulting in the spatial steric restriction.

Figure 6 depicts the density distribution of n-butane (red dot regions) and i-butane (green dot regions) competitive adsorption on MFI zeolite under different adsorption pressure at 303 K. It is obvious that under the pressure conditions, the green dot areas representing i-butane only appears the intersection channels. However, the red dots representing n-butane are mainly distributed nicely in both straight and zigzag channels. The results show that the competitive adsorption of n-butane and i-butane in MFI zeolite mainly occurs at the intersection channels[37]. The experimental results have been confirmed that n-butane and i-butane permeances as well as the n-butane/i-butane separation factor decrease with increase in pressure[36–38]. However, the essential cause of this result is still unknown. Sun et al.[14] found that i-butane molecules can preferential adsorbed in the pores of the MFI framework because of the increase of its partial pressure, which will decrease the separation factor. However, Wang et al.[20] found that the stronger adsorption ability of n-butane in MFI zeolite than that of i-butane lead to the decrease of the separation factor [21]. On the basis of theoretical calculation results, it can be stated that the intrusion of i-butane in the intersection channels hinders the effective permeation of n-butane in MFI membrane.

By calculating the distance between the particular molecules or atoms, which is usually called Radial Distribution Function (RDF), it can clearly identify the relative locations of particles. Figure 7 gives the RDFs between H atoms of n-butane (a) / i-butane (b) and the nearest framework O atom of the MFI zeolite and their changing rules with the increase of loading at 303 K, while Figure S2 presents the RDF results for all the temperatures. It can be seen that a sharp peak was observed at 2.978 Å in the RDFs between H atoms of n-butane and the nearest framework O atom of the MFI zeolite, which hardly change with the increase of loading. This indicates that the adsorption mode of n-butane in MFI channel is relatively identical, and is not affected by loading change. This circumstance is the same for all the other temperatures thar the most profitable distance is around 2.8 Å. By contrast, the RDF results of i-butane are completely different from n-butane that the peak at 3.011 Å becomes wider, and splits into two peaks at 2.663 Å and 3.674 Å with the increase of loading. Combined with the results of density distribution map in Fig. 5, it can be concluded that this is caused by the adsorption of isobutane in the two channels, indicating that the most probable distance between the H atoms of i-butane molecule adsorbed in the channels and the nearest framework O atom of the MFI zeolite is smaller than that of n-butane. Figure S3 presents i-butane molecule adsorbed in three kinds of different channels, the structures of which were optimized by the Density Functional Theory (DFT) calculation. The result also shows that the distance distribution between O and H atoms for its adsorption in the sinusoidal and straight channels becomes more wider than cross channel, and the shortest distance of the O-H pair is also diminished compared with the cross channel. Once this distance is less than the van der Waals radius, repulsive force will be generated between isobutane molecules and zeolite framework, which may weaken the adsorption capacity.

3.3 Comparative Analysis of Energy in the Adsorption Process N-butane and Isobutane Molecules on the MFI Zeolite

The adsorption isosteric heat change curves related to adsorption capacity calculated by Clausius–Clapeyron relation with DSLF fitting data which are illustrated in Fig. 8, and the parameters of DSLF are listed in Table S1.

The adsorption isosteric heat of n-butane is apparently higher than that of i-butane, which means that the interaction between n-butane and MFI zeolite is stronger than that of i-butane. Moreover, the adsorption isosteric heat of n-butane increases linearly with the increase of the loading explained as the contribution of intermolecular force. The adsorption isosteric heat of i-butane fluctuated with the increase of the loading, which is caused by the change of the adsorption sites of isobutene in MFI zeolite. This is consistent with the above results of adsorption isotherms and the density distribution.

As shown in Fig. 9, the distribution of adsorption energies for the n-butane and i-butane in MFI zeolite are different because of the different interaction between adsorbed molecule and zeolite framework. Though the main adsorption energies in-creased (negative sign means exothermic) as the adsorption amount increased for both n-butane and i-butane molecule, i.e., the main adsorption energy for n-butane is in-creased from 61.2 to 66.7 kJ/mol, the change of the distribution of these two molecules is also different that the adsorption of n-butane become more localized than i-butane as the adsorption amount increased. For n-butane molecules, all the channels can dis-tribute the molecules, which means that, when the adsorption amount increased, there are not changed much in the adsorption location. The increase of n-butane adsorption energy would mainly due to the strong interaction between molecules and zeolite. But for i-butane molecule, when the adsorption amount increases, the adsorption energy distribution area become broad that the left starting point is shifted from 57 kJ/mol to nearly 70 kJ/mol. Also, the height of the distribution peak is diminished to half of its value from 0.8 to 0.4. These changes indicate that the adsorption energy has two different types when the adsorbed i-butane increase, which is consistent of the isotherm results that it has a platform as the adsorption amount increased to 1.0 mmol/g (about 46 molecules per super-cell). From Fig. 4b, it can be clearly seen that the energy distribution is shifted significantly when the loading of i-butane molecule is larger than 45 molecules per cell.

The interaction mode of n-butane and i-butane molecules and MFI zeolite are mainly van der Waals (vdW) interactions. In order to evaluate the strength of the vdW interactions, Independent Gradient Model (IGM) method[32] is used to calculated inter fragment interactions separately for the adsorption of n-/i-butane on the three kinds of channels (cross channel, straight channel, and sinusoidal channel) using on promolecular approximation. Figure 10 shows the δ_g^inter 2D plots and colored maps of δ_g inter isosurface versus sign(λ₂)ρ (insert) of adsorbed n-butane and i-butane molecules interactions with the framework of MFI zeolite at different positions, in which the default color transition is blue-green-red. The same δ_g^inter scale over the range of 0.04 a.u. has been deliberately chosen for the sake of comparison. Two pairs of spikes appear in the 2D plot of δ_g^inter for the six adsorption systems, associated with vdW contacts, both in the attractive (blue dots) and repulsive (red dots) parts. [33, 39].

In that series of 3 models of n-butane, we observe that the van der Waals attraction between n-butane molecule and zeolite framework at three adsorption sites is slightly greater than the van der Waals repulsion. By contrast, there are significant differences among the three i-butane models corresponding to different adsorption positions. In the cross channel, the van der Waals attraction and repulsion of isobutane are relatively weak due to the longer distance between H atoms of i-butane and the nearest framework O atom of the MFI zeolite (cf. Figure 7). It is clear that the van der Waals repulsion in straight channel and sinusoidal channel occur judged from the increasing δ_g^inter value of the red dot area becomes significantly larger than the other models. This is strong evidence to explain why i-butane does not tend to be adsorbed in the straight channel and sinusoidal channel of MFI zeolite.

3.4 Isotropic Diffusion Properties of N-butane and I-butane Molecules on the MFI Zeolite

The theoretical diffusion ability of n-butane and i-butane molecules in the MFI zeolite, respectively, is investigated via molecular dynamics (MD) simulations. Figure 11 display the mean square displacement (MSD) and diffusion coefficient of n-butane and i-butane on MFI zeolite at 303 K. The calculated total self-diffusion coefficient of n-butane molecules (D_T=3.06×10^–9 m²/s) is more than 2 orders of magnitude than that for the i-butane (D_T=2.18×10^–11 m²/s). The same conclusion is also applicable to the self-diffusion coefficient in each channel direction for n-butane and i-butane molecules diffusion in the straight channels (D_y) and the sinusoidal channels (D_x) of MFI zeolite, respectively. All above results clearly demonstrate that n-butane exhibits superior diffusion ability and may foster better permeability than that of i-butane. The results of diffusion simulation are in good agreement with the n-butane/i-butane separation selectivity reported in literatures[5–7]. Hence, the separation mechanism based on molecular diffusion dynamics have been clarified for n-butane/i-butane separation on the MFI membranes.

Both the GCMC and IAST calculations were showed that the separation property for n-butane/i-butane mixtures is excellent on MFI zeolite. Though the unary adsorption amount of n-butane and i-butane is nearly the same, the adsorption of i-butane is influenced significantly by n-butane when binary adsorption. Because of the relatively strong van der Waals repulsion in straight channel and sinusoidal channel, i-butane is preferred to adsorb on cross channel first, and limit its diffusion; however, n-butane can adsorb on any channel of MFI zeolite. The methodology of combining the molecular simulation with the IGM method in this work can be used for the future research about adsorption and separation.

Ethical Approval

This is not applicable in this article.

Competing interests

This is not applicable in this article.

Authors' contributions

Conceptualization, Y.Q., and L.S.; methodology, H.W., and Q.L.; software, Q.L.; validation, H.W., J.X. and Q.L.; formal analysis, H.W.; investigation, L.Z., and Y. Q.; resources, Z.S., and H. L.; data curation, L.S., and Z. S.; writing—original draft preparation, H.W. and Q.L.; writing—review and editing, Y.C. and L.S.; visualization, Q.L.; supervision, Z.S.; project administration, Y.Q.; funding acquisition, L.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China, grant numbers 21902068, and U20A20120; and by PetroChina Innovation Foundation (2020D-5007-0401)

Availability of data and materials

This is not applicable in this article.

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No competing interests reported.

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Molecular mechanism of selective adsorption and separation of n-butane/i-butane on MFI zeolite

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Journal Publication

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Abstract

Figures

1 Introduction

2 Materials And Methods

2.1 Zeolite framework

2.2 Simulation details

2.3 Adsorption models

3 Results And Discussion

3.1. Adsorption and Competitive Adsorption Properties of n-butane/i-butane in/on MFI zeolite

3.2 Correlation between Adsorption Sites of N-butane and Isobutane and Pore Structure of MFI Zeolite

3.4 Isotropic Diffusion Properties of N-butane and I-butane Molecules on the MFI Zeolite

4 Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

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