Intermolecular interactions governing adsorption of bulky molecules on ZIF-8: Insight from adsorption kinetics of benzene and 6-membered ring alicyclic hydrocarbons

doi:10.21203/rs.3.rs-7276006/v1

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Intermolecular interactions governing adsorption of bulky molecules on ZIF-8: Insight from adsorption kinetics of benzene and 6-membered ring alicyclic hydrocarbons

https://doi.org/10.21203/rs.3.rs-7276006/v1

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This study examined the adsorption kinetics of benzene and six-membered ring alicyclic hydrocarbons on Zeolitic imidazolate framework-8 (ZIF-8) to elucidate the interactions between the adsorbed molecules and linkers on the adsorption behavior of bulky molecules. The temperature dependence of the Fickian diffusion coefficient was analyzed to determine the activation entropy and energy of diffusion. The significant negative values observed for the activation entropy, from − 172 to − 217 J K^− 1 mol^− 1, indicate a restriction on the degree of freedom of the adsorbate molecules within the diffusion transition state, thereby reducing the diffusion coefficients. The ZIF-8 differentiates between 1,3- and 1,4-cyclohexadiene based on kinetic mechanisms, indicating its potential as an isomeric molecular sieve. The molecular orientations of the adsorbate molecules traversing the ZIF-8 aperture were investigated using Monte Carlo simulations. Order parameter analysis revealed that the molecular orientation was constrained to minimize the energy barrier with passage through the aperture. Fourier-transform infrared (FT-IR) spectroscopy and ¹³C cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) measurements supported the CH-π interactions between the adsorbate molecules and 2-methylimidazolate linkers. This interaction maintained the orientation of the adsorbate molecules, facilitating their passage through the ZIF-8 apertures. CH-π interactions influence molecular orientation control in the adsorption and diffusion of large molecules containing π electrons in porous materials with organic ligands.

Adsorption kinetics

Molecular sieving

Diffusion

Activation entropy

CH-π interaction

The growing interest in hydrogen as a prospective energy source has led to increased demand for hydrogen as a renewable resource in recent years. To establish a dependable hydrogen supply, there is an urgent need to construct large-scale storage facilities, and economically viable transportation systems [1–3]. The organic hydride process is a promising strategy for hydrogen supply [4, 5] involving the storage of hydrogen in saturated hydrocarbons such as methylcyclohexane, which can be dehydrogenated to produce unsaturated hydrocarbons like toluene for hydrogen extraction [6, 7]. The efficiency of the hydrogen extraction process can be improved by continuously removing toluene from the reaction system. Therefore, the development of molecular sieve technologies capable of effectively separating toluene from methylcyclohexane–toluene mixtures while minimizing energy consumption is essential. Beyond the methylcyclohexane/toluene system, the advancement of molecular sieve materials for the selective separation and adsorption of saturated and unsaturated compounds in bulky hydrocarbons with high hydrogen densities is anticipated to further enhance the organic hydride method [8–12].

Zeolitic Imidazolate Framework 8 (ZIF-8), a type of metal-organic framework, is one of the leading candidates for such molecular sieve materials. The ZIF-8 is represented by the formula [Zn(C₄H₅N₂)₂]_n, consisting of Zn²⁺ ions and the 2-methylimidazolate anion (C₄H₅N₂⁻, MeIm⁻) [13]. It possesses a sodalite structure characterized by micropores with a diameter of 11.4 Å, interconnected by eight 6-membered ring (6MR) openings measuring 3.4 Å in diameter [14]. This material exhibits significant potential for gas adsorption and separation applications [15, 16], which can be attributed to its exceptional molecular sieving properties stemming from its flexible framework. The rotational motion of the bridging ligand modifies the apparent size of the 6MR opening, allowing the capture of molecules larger than 3.4 Å [17, 18]. This phenomenon, referred to as "gate adsorption," results from the framework's flexibility [19–21]. The unique molecular sieving behavior of ZIF-8 is characterized by a nearly linear relationship between the activation energy for gas molecular diffusion and the size of the adsorbed molecules [18], although the effective diffusion coefficient derived from the adsorption rate decreases with the kinetic diameter of the adsorbed molecules. This diffusion behavior is linked to the swing effect, which is induced by the torsional motion of the imidazole ligand around the Zn–Im–Zn axis [22–24]. Significant fluctuations in the bridging ligands have been confirmed using various spectroscopic techniques, including far-infrared absorption spectroscopy [25], inelastic neutron scattering [26], terahertz spectroscopy [27], and solid-state NMR [28, 29] methods.

However, bulky molecules that exhibit significant molecular shape anisotropy may adopt an energetically favorable orientation for navigating through the 6MR aperture in an orientation that minimizes the molecular cross-sectional area. In previous studies, it was observed that for benzene and its derivatives (toluene and xylene), the minimum molecular cross-section rather than the average molecular diameter predominantly influenced the molecular orientation of the adsorbed molecules as they traversed the 6MR aperture [30, 31]. Recently, Poryvaev et al. reported that benzene was adsorbed onto ZIF-8 with an adsorption selectivity of approximately 270% from a benzene/cyclohexane solution containing > 90% cyclohexane [32]. Their findings indicated that the interaction between benzene and cyclohexane with the 6MR aperture of ZIF-8 was governed by the molecular thickness of the adsorbed molecules, suggesting that the molecular orientation was critical for the passage of benzene and cyclohexane through the 6MR aperture. Thus, for adsorbed molecules with bulky and highly anisotropic geometries, both the intermolecular interactions with the 6MR aperture (enthalpic contributions) and the molecular orientation during passage through the aperture (entropic contributions) were expected to significantly influence their diffusion behavior.

To sustain a favorable molecular orientation for passage through the 6MR aperture, highly directional interactions likely govern the orientation of the adsorbed molecules, as opposed to isotropic intermolecular interactions such as dispersion forces. Identifying the intermolecular interactions that contribute to the maintenance of molecular orientation is anticipated to enhance the theoretical understanding of the adsorption behavior of bulky molecules on ZIF-8, particularly regarding diffusion phenomena. Potential candidates for such interactions include hydrogen bonds, tetrel bonds, CH-π interactions, and halogen-π interactions [33, 34]. We hypothesized that the double bond of 2-methylimidazolate, the bridging ligand of ZIF-8, would facilitate CH-π interactions.

This study aimed to elucidate the molecular-level adsorption mechanism of bulky hydrocarbons in ZIF-8. Initially, we focused on the adsorption kinetics of benzene and several 6-membered ring alicyclic hydrocarbons on ZIF-8, with an emphasis on evaluating the influence of intermolecular interactions on diffusion behavior. The selected alicyclic compounds (1,3-cyclohexadiene, 1,4-cyclohexadiene, cyclohexene, and cyclohexane, summarized in Scheme S1 in supporting information) had a similar projected molecular diameter to benzene but differed in molecular thickness and π-electron content (see Table S1, in SI). These variations facilitate the analysis of the diffusion coefficients and activation parameters in relation to the molecular dimensions and the presence of π-electrons. This study employed Monte Carlo simulations to investigate the critical process of molecules traversing the 6MR aperture of ZIF-8, with particular emphasis on the orientation of bulky molecules during this process. Additionally, Fourier-transform infrared absorption spectroscopy and ¹³C cross-polarization magic-angle sample spinning (CPMAS) NMR techniques were utilized to assess adsorbate-adsorbent interactions, aiming to identify specific intermolecular interactions that influence molecular orientation as the adsorbate molecules navigate through the 6MR aperture. These research strategies collectively offer molecular-level understanding of the adsorption mechanism of bulky hydrocarbon molecules on ZIF-8.

2.1 Chemicals

The ZIF-8 (Basolite Z1200, BASF) and 1,3-cyclohexadiene (97% purity) were obtained from Sigma-Aldrich. Benzene (purity > 99.5%), cyclohexane (purity > 99.5%), 1,4-cyclohexadiene (purity > 95.0%), and cyclohexene (purity > 97.0%) were purchased from Fujifilm Wako Pure Chemicals Co., Ltd. The reagents utilized were Wako Special Grade for benzene and cyclohexane and Wako 1st Grade for 1,4-cyclohexadiene and cyclohexene. All the reagents were used without further purification. For this study, benzene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, cyclohexene, and cyclohexane are referred to as BZ, 1,3-CHD, 1,4-CHD, CHEN, and CHEX, respectively.

2.2 Characterization

2.2.1 Nitrogen adsorption isotherm

Prior to use, ZIF-8 was activated by heating under vacuum (0.1 Torr) at 150°C for 12 h. Nitrogen adsorption isotherms were recorded using a Gemini 2375 apparatus (Shimadzu Corporation) to determine the pore volume and specific surface area of ZIF-8. The isotherms were recorded over a relative pressure (p/p₀) range of 0.0001–0.98, where p₀ represents the saturated vapor pressure of nitrogen at its boiling point of liquid nitrogen (77 K). The porosity of ZIF-8 was evaluated from the nitrogen adsorption isotherm (Figure S1, Supplementary Information), which exhibited a characteristic Type I (a) pattern according to the IUPAC classification. The BET surface area was calculated to be 1074 m² g^–1, with a corresponding pore volume of 0.61 cm³ g^–1.

2.2.2 Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) was performed using a field-emission scanning electron microscope SU6600 (Hitachi High-Technologies Co.) to analyze the size distribution and morphology of the crystallites. The SEM images are shown in Figure S2 (Supplementary Information). The average particle diameter, based on the crystalline size distribution (Figure S3), was determined to be 450 nm, corresponding to a radius of 225 nm.

2.2.3 Fourier transferred infrared (FT-IR) absorption spectroscopy

Fourier-transform infrared (FT-IR) measurements were performed using an FT/IR-4600AC spectrometer (JASCO Co.) over the wavenumber range of 400–4000 cm^–1 at room temperature with a resolution of 1 cm^–1. To prevent the amorphization of the ZIF-8 sample due to uneven pressure during mortar grinding, KBr pellet samples were prepared by compressing the powder sample between two KBr plates. Spectra were recorded in transmittance units and subsequently converted to absorbance values.

2.2.4 Solid-state ¹³C NMR spectroscopy

High-resolution solid-state ¹³C NMR spectra were obtained using a Bruker Avance III 400WB spectrometer equipped with a superconducting magnet (9.4 T), which provided resonance frequencies of 400.25 and 100.64 MHz for the ¹H and ¹³C nuclei, respectively. Free induction decay (FID) signals for ¹³C nuclei were acquired using a single-pulse sequence with ¹H decoupling under magic-angle sample spinning (MAS) conditions and a ¹H decoupling MAS cross-polarization (CP) pulse sequence. The powdered samples were packed in a 4 mm-diameter cylindrical zirconia rotor and spun at 10 kHz for MAS. Adamantane was used as an external standard for the chemical shift, with a CH₂ peak referenced at 38.5 ppm (relative to tetramethylsilane).

2.3 Measurement of time dependence of adsorption uptake

The ZIF-8 powder sample underwent pretreatment (heating at 150°C in a vacuum of 0.1 Torr for 12 h). The samples with an average weight of 7–15 mg were placed in an aluminum pan (5 mm × 3 mm depth). The aluminum pan was placed in a hermetically sealed vessel containing 3 mL of the appropriate solvent. The presence of air in the container had a minimal effect on solvent vapor adsorption, as nitrogen (N₂) and oxygen (O₂) behave as supercritical gases at ambient temperatures. The adsorbates used in this study were BZ, 1,3-CHD, 1,4-CHD, CHEN, and CHEX. The vessel was affixed to a base constructed from thermally insulating styrofoam and placed within a temperature-controlled enclosure with an accuracy of ± 0.5°C. Vapor adsorption was conducted at 30, 40, and 50°C under saturated vapor pressures of the solvent. The temperature dependence of the saturated vapor pressure is shown in Figure S4 (Supplementary Information). At a constant temperature, BZ, 1,3-CHD, and CHEX exhibited similar vapor pressures within ± 1 kPa, whereas CHEN and 1,4-CHD displayed slightly lower vapor pressures, with differences of 1.5 kPa for CHEN and 5 kPa for 1,4-CHD. Consequently, the influence of the vapor pressure of each solvent on the adsorption rate was expected to be negligible. The samples were weighed at intervals until a stable value was reached, with weight measurements exhibiting an accuracy of ± 5%.

2.4 Monte Carlo simulations

A Monte Carlo simulation was performed using proprietary software at a constant temperature of 298 K under NVT conditions, with a minimum of 100,000 trials. The force field parameters —including Lennard-Jones parameters and partial charges—assigned to the atoms forming the 6MR aperture were consistent with those used in the molecular dynamics (MD) simulations of ZIF-8 conducted by Parkes et al. [35, 36]. The configuration of the adsorbed molecule was determined by optimizing its structure using density functional theory (DFT) calculations. The Gaussian 16 software package [37] was employed, utilizing the B3LYP functional and 6-31G(d) basis set for the calculations. In the Monte Carlo simulations, the OPLS-AA force field parameters [38, 39] were used to define the Lennard-Jones parameters and partial charges of the adsorbed molecules. The 6MR structure employed in the simulation was modeled by extracting one-eighth of the unit cells dimension (a/2, a/2, a/2) from the ZIF-8 crystal (lattice constant a). The vector from the lattice point (0, 0, 0) to (1/2, 1/2, 1/2) was designated as the Z-axis in the reference coordinate system. The origin of this reference coordinate system was set at the center of the 6MR plane (lattice point (1/4, 1/4, 1/4)). The direction toward lattice point (0, 0, 0) was defined as negative (–), and that toward (1/2, 1/2, 1/2) as positive (+). The 6MR structure is shown in Figure S5. At a specified Z value, the mass center of the adsorbed molecule was displaced over a 4 Å × 4 Å plane centered on the reference coordinate axis. The most energetically favorable configuration was then determined using Monte Carlo simulation. The opening of the 6MR aperture was adjusted by systematically rotating the tilt angle of the 2-methylimidazole bridging ligand along its N-N axis. The tilt angle was defined as the angle between the molecular plane of the bridging ligand and the 6MR aperture plane. Variations in tilt were expressed as positive (+Δθ) or negative (–Δθ) deviations from the ligand’s original orientation of the crystal lattice. The adsorbed molecule was moved along the Z-axis in the reference coordinate system from + 4 Å to − 4 Å in 0.2 Å increments, and the potential energy between the adsorbed molecule and the 6MR for each Z value was evaluated by varying Δθ. For the order parameter analysis, the simulations were carried out in the Z value from + 5 Å to − 5 Å.

3.1 Time dependence of adsorption uptake

Figure 1 depicts the time-dependence of the adsorption uptake of individual hydrocarbons on ZIF-8 at 303 K, illustrating the relationship between the amount of adsorption and the square root of the adsorption time. Initially, all the vapor species exhibited a linear increase in the adsorption amount, which was proportional to the square root of time, suggesting that micropore diffusion was the primary mechanism governing the adsorption of individual solvent vapors on ZIF-8. These observations can be characterized using the Fickian diffusion model, which is applicable to spherical particles [40]. Assuming that the concentration of adsorbed molecules at the center of the particle is negligible (i.e., in the short diffusion time region), the model can be approximated as

$$\:m\left(t\right)={m}_{\text{e}\text{x}\text{t}}+{m}_{\infty\:}\frac{6}{\sqrt{\pi\:}}\sqrt{\frac{{D}_{\text{f},\text{i}\text{n}\text{i}}t}{{r}_{\text{a}\text{v}\text{e}}^{2}}}$$

where D_f,ini denotes the Fickian diffusion coefficient, which is derived from the initial slope of the time-dependent adsorption curve; r_ave represents the average radius of the spherical particles; m_∞ indicates the saturation adsorption amount; and m_ext refers to the adsorption amount present on the surface of the particle [41]. The diffusion coefficient D_f,ini can be calculated from the slope of the linear portion depicted in Fig. 1, assuming that the saturation adsorption amount is known. The data indicated that BZ and 1,4-CHD achieved saturation during adsorption, whereas 1,3-CHD approached saturation. In contrast, the CHEN and CHEX did not reach saturation during this period. For convenience, the adsorption quantity recorded at the longest adsorption time was designated as the saturation adsorption quantity, m_∞, at equilibrium. The D_f,ini values for each adsorbed molecule were extracted from the linear segments, as shown in Fig. 1. Figure S6 (SI) presents the time-dependent adsorption amounts at 313 and 323 K. Table 1 summarizes the D_f,ini values obtained at 303, 313, and 323 K. Notably, despite being structural isomers with identical molecular weights, 1,3-CHD and 1,4-CHD exhibited distinct diffusion coefficients. This finding suggests that ZIF-8 can differentiate between structural isomers based on their respective adsorption behavior through kinetic mechanisms. This observation is particularly significant, as it implies that ZIF-8 can potentially serve as a molecular sieve for structural isomers—an ability with important implications for separation applications.

Table 1
Diffusion coefficients of adsorbate molecule determined from the initial slope of adsorption uptake curve
Adsorbate	D_f,ini / m² s^− 1
	303 K	313 K	323 K
BZ	1.3 (1) × 10^‒19	2.2 (1) × 10^‒19	2.8 (1) × 10^‒19
1,4-CHD	5.6 (1) × 10^‒20	7.9 (1) × 10^‒20	9.5 (1) × 10^‒20
1,3-CHD	3.2 (1) × 10^‒20	3.5 (1) × 10^‒20	5.4 (1) × 10^‒20
CHEN	1.2 (1) × 10^‒20	1.8 (1) × 10^‒20	2.2 (1) × 10^‒20
CHEX (chair)	8.2 (1) × 10^‒21	1.1 (1) × 10^‒20	1.7 (1) × 10^‒20

The member in parentheses presents the experimental error.

When larger molecules are adsorbed onto ZIF-8, they must navigate through the 6MR aperture, which consists of six zinc (Zn) atoms and 2-methylimidazole bridging ligands. A previous study established that, for benzene and its derivatives adsorbed on ZIF-8 [30], traversal through the 6MR aperture represents the rate-limiting step in the diffusion process. Furthermore, the movement of bulky molecules through the 6MR aperture is primarily governed by configurational diffusion, which is characterized by a restricted molecular orientation and rotation. Consequently, the temperature dependence of D_f,ini should be assessed using the following equation derived from Eyring's transition state theory [42] rather than the traditional Arrhenius equation:

$$\:{D}_{\text{f},\text{i}\text{n}\text{i}}=\text{e}{\lambda\:}^{2}\frac{kT}{h}{\text{e}}^{\varDelta\:{S}^{\ddagger}/R}{\text{e}}^{{-E}_{a}/RT}$$

where E_a signifies the diffusion activation energy, ΔS^‡ represents the diffusion activation entropy, h denotes Planck's constant, k is Boltzmann's constant, and λ indicates the distance between the molecular jumps. During the diffusion process, molecules jump between adjacent lattices via 6MR apertures with λ = 3^1/2a/2. The lattice constant a is 1.701 nm at 298 K [22], resulting in a λ of 1.473 nm.

Figure 2 illustrates a logarithmic representation of D_f,iniꞏT ^‒1 versus the reciprocal temperature (Eyring plot). The slope and intercept of the graph correspond to the activation energy and entropy, respectively. The activation parameters for each adsorbed molecule are listed in Table 2.

Table 2
Diffusion coefficients at 303 K and activation parameters determined from the temperature dependence of the diffusion coefficients.
Adsorbate	σ_average /nm	E_a / kJ mol^− 1	ΔS^‡/ J K^− 1 mol^− 1
BZ	0.525	28 (2)	‒172 (5)
1,4-CHD	0.537	19 (2)	‒208 (10)
1,3-CHD	0.538	18 (2)	‒217 (10)
CHEN	0.548	24 (2)	‒205 (6)
CHEX (chair)	0.558	27 (1)	‒200 (3)
‒

Figure 3 presents the molecular size dependence of D_f,ini at 303 K and the activation parameters E_a and ‒ΔS^‡ for individual hydrocarbons. The semi-logarithmic plot of D_f,ini shows a linear decrease with increasing molecular size, whereas the standard plots of E_a and ‒ΔS^‡ exhibited a V-shaped dependence on the molecular size. Specifically, E_a reached its minimum for 1,4-CHD and 1,3-CHD, which possessed intermediate molecular sizes, and attained its maximum values for BZ and CHEX, representing the smallest and largest molecular sizes, respectively. This observation suggests that the passage of adsorbed molecules through 6MR aperture is governed by the combination of molecular size and specific host-guest interactions at the 6MR aperture site. The activation energy E_a can be lowered by stabilizing the transition state or raised by stabilizing the adsorbed molecules within the pores. A lower E_a enhances the likelihood of adsorbed molecules overcoming the diffusion energy barrier at a specific temperature. Conversely, ΔS^‡ exerts an opposing influence on D_f,ini. The substantial negative activation entropy indicates a pronounced restriction on the degrees of freedom of the adsorbed molecule within the diffusion transition state, which leads to a reduced diffusion frequency. In BZ, the effect of ΔS^‡—despite having the smallest absolute value—surpasses that of the largest E_a, resulting in the highest D_f,ini value among the compounds. For 1,4-CHD and 1,3-CHD, the E_a values are less than 20 kJ mol^‒1; however, their ΔS^‡ values reach up to ca. 210 J K^‒1 mol^‒1. The competing effects of these opposing factors may result in the contribution of activation entropy surpassing that of activation energy, thereby causing a decline in D_f,ini as the molecular size increases. In BZ, 1,4-CHD, and 1,3-CHD, E_a and ΔS^‡ exhibited a trade-off relationship. In contrast, for CHEN and CHEX, both the high E_a values and the relatively large absolute values of ΔS^‡ decrease the D_f,ini values. In the following sections, we examine the pronounced effects of molecular orientation on the diffusion of adsorbed molecules through the 6MR and identify the specific intermolecular interactions that significantly influence the E_a.

3.2 Monte Carlo simulation for 6MR passage of adsorbate molecules

Monte Carlo simulations were employed to elucidate the potential energy profile and molecular orientation of the adsorbed molecules as they traverse the 6MR aperture. In this context, the 6MR aperture, characterized by a tilted angle (Δθ = 0) as derived from the crystal structure, presents an activation energy required for molecular passage with activation energies ranging from several hundred to several thousand kJ mol^‒1. Consequently, it is nearly impossible for molecules to pass through the aperture using only the thermal energy available at room temperature. This observation implies that the 6MR aperture must undergo enlargement to facilitate the passage of target molecules. To quantify this, we evaluated the activation energy required for molecular passage as a function of the tilted angle of the bridging ligand by systematically varying this angle. By employing polynomial interpolation on the acquired data, we identified a tilted angle that aligned with the experimentally measured values. Figure 4 illustrates the energy profile associated with passage through the 6MR aperture, corresponding to the experimental activation energy for each adsorbate, as determined by the time dependence of the adsorption uptake. Two minima were observed on either side of the 6MR aperture. The energy minimum at the CH₃ neck (Z/Å ~ − 2) was deeper than that at the CH neck (Z/Å ~ +3). This finding suggests that the passage of molecules trapped in the CH₃ neck through the 6MR aperture is energetically unfavorable. Consequently, the passage of molecules through the 6MR aperture is only considered when they approach from the CH neck side [30]. In this case, the energy barrier encountered by an adsorbed molecule navigating through the 6MR aperture is defined as the difference (ΔE = E_CH,max − E_CH,min) between the maximum energy (E_CH,max) located near the CH neck (+ 0.5 ≤ Z/Å ≤ +1) and the minimum energy (E_CH,min) at the CH neck (Z/Å ~ +3). Notably, in the case of CHEX, an additional energy maximum was detected on the outside of the CH neck (Z/Å ~ +2). The presence of the dual maxima in the energy profile aligns with the findings from previous studies. This behavior supports the use of a model that incorporates pre-equilibrium adsorption at the CH neck, represented as: A(g) + M(s) ⇌ AM*(s) ⇌ AM(s) [30]. In this model, the apparent activation energy can be expressed as ΔE = E_a,a – E_a,d + E_a,in, where E_a,a denotes the energy barrier for pre-adsorption at the 6MR aperture, E_a,d is the energy barrier for the desorption of the pre-adsorbed molecule, and E_a,in is the energy barrier for passage through the CH neck. The activation energy evaluated by MC simulation, the resulting tilted angle of the bridging ligand, and the aperture opening diameter are listed in Table 3. The correlation between the tilted angle and the opening diameter of the 6MR aperture, which reproduces the experimental values, appears to align more closely with MIN-1 than with MIN-2, as proposed by Webster et al. [43] (see Scheme S1 and Table S1 in the SI). This suggests that the energy barrier for the passage of the adsorbed molecule through the 6MR aperture is significantly influenced by the cross-sectional area of the adsorbed molecule, indicating that the molecular orientation is highly constrained during this process. This conclusion was consistent with the findings of our previous study.

Table 3
Experimental and simulated activation energies of adsorbed molecules passing through the 6MR aperture, tilted angles of the bridging ligand, and average diameters of the 6MR aperture.
Adsorbate	ΔE / kJ mol^− 1	Δθ_c / deg.	d_p,ave / nm	MIN-1^b)/nm	MIN-2 ^b)/nm
BZ	27.6	17.0	0.47	0.3277	0.6628
1,4-CHD	19.3	21.1	0.49	0.3845	0.6612
1,3-CHD	17.5	19.9	0.49	0.3845	0.6612
CHEN	24.2	20.55	0.49	0.4414	0.6595
CHEX (chair)	27.0 ^a)	24.05	0.52	0.4982	0.6580

a. This value corresponds to the apparent activation energy when adsorption pre-equilibration occurs: ΔE = E_a,a – E_a,d + E_a,in.

b. Critical dimensions of adsorbate molecule for entry into zeolite pores. In the slit-shaped pores, the size of the adsorption in the minimum dimension, MIN-1, was determined. In the cylindrical pores, the size of the molecule in two directions is considered: the minimum dimension, MIN-1, and the next smallest dimension, MIN-2.

To further analyze the orientation of the adsorbed molecules, we defined two order parameters:

$\:{S}_{\perp\:}=\frac{1}{2}\left(3⟨{\text{cos}}^{2}\alpha\:⟩-1\right)$	(3a)
$\:{S}_{\parallel\:}=\frac{1}{2}\left(3⟨{\text{cos}}^{2}\beta\:⟩-1\right)$	(3b)

where α and β defined the angles formed by the vectors perpendicular and parallel to the molecular plane, respectively, with respect to the perpendiculars passing through the center of the 6MR aperture. The characterization of these vectors and their orientation within the reference coordinate system are shown in Figure S7 (see SI). Figure 5 presents the order parameter for each molecule as it traverses the 6MR aperture. An animation depicting the molecular passage through the 6MR is provided in the SI. When the molecular orientation is isotropically distributed, the order parameter is zero. However, as each molecule passed through the aperture, both S_⊥ and S_|| deviated from zero, indicating that the orientation of the molecules was constrained by the aperture. Notably, the negative value of S_⊥ suggests that the molecules navigate the aperture in an orientation in which the molecular plane is nearly perpendicular to the aperture plane. This behavior was exhibited by all the adsorbate molecules. This indicated that the passage of adsorbed molecules through the 6MR aperture was accompanied by significant orientational constraints. In other words, the translational and rotational degrees of freedom of the adsorbed molecules were considerably restricted during their traversal through the 6MR aperture. Consequently, the observed significant negative activation entropy can be attributed to the transition of molecules in the vapor to a diffusion transition state, in which the molecular orientation is markedly constrained at the 6MR aperture.

Conversely, S_|| represents the variation in the rotational orientation of the molecules around an axis perpendicular to the molecular plane and exhibited different behaviors for each adsorbate molecule in the range − 3 ≤ Z/Å ≤ +3. For BZ, 1,4-CHD, CHEN, and CHEX, the values ranged from − 0.5 < S_|| < +0.5, whereas for 1,3-CHD, 0 < S_|| < +1. In BZ and 1,4-CHD, S_|| exhibited a bipolar change centered at Z = 0, indicating that the molecule turned its orientation around an axis perpendicular to the molecular plane and then turned it back. In the CHEN and CHEX, the variation in S_|| showed a similar trend, although the range of the Z-axis was narrow. This feature implies that these molecules undergo twisting and wiggling around the axis perpendicular to the molecular plane when traversing the 6MR aperture (see the animation provided in the SI). In contrast, for 1,3-CHD, S_|| varied continuously from 0 to + 1 during the passage through 6MR, suggesting that the molecular orientation around the perpendicular axis changes progressively throughout the diffusion process. That is, the 1,3-CHD molecule rotates around the axis as it moves through the aperture (see the animation in the SI). This rotation is believed to minimize steric hindrance within the 6MR aperture by reducing friction between the molecule and the framework. This behavior differs from that of other molecules and may partly explain why 1,3-CHD exhibits a lower activation energy for diffusion than the other molecules. Distinguishing between 1,3-CHD and 1,4-CHD is crucial for understanding the factors influencing the diffusion coefficient of ZIF-8. The differences in the E_a and S_|| behaviors observed in the MC simulations between 1,3-CHD and 1,4-CHD may reflect the slight differences in the molecular cross-sections and arrangement of π bonds within the molecules.

3.3 Analysis of adsorbate-adsorbent interactions by spectroscopies

3.3.1 FT-IR spectroscopy

Figure 6 displays the symmetric C–H stretching bands associated with the 2-methylimidazole group. The full wavenumber regions of the infrared spectra are shown in Figure S8 (SI). Notably, no shift was observed in the band upon cyclohexane adsorption, while a pronounced redshift was observed over the wavenumber range of 1–2 cm^‒1 upon benzene and cyclic alkene adsorption. This shift was more pronounced for 1,3-CHD, 1,4-CHD, and BZ than for CHEN. This observation suggests the elongation of C–H bond, which is attributed to the attractive interaction between the π-electrons of the adsorbed molecules and the C–H bond of 2-methylimidazole [31]. One plausible attractive interaction is the CH-π interaction, which has been reported for various organic compounds, metal complexes, and molecular assemblies [33]. It has been established that CH-π bonding induces stretching of the C–H bonds, resulting in a redshift of the CH stretching band [44, 45]. Additionally, the protons of the methyl group may function as proton donors in CH-π interactions, thereby leading to an anticipated redshift of the CH symmetric stretching band in the methyl group of the 2-methylimidazolate moiety [31]. However, the symmetric CH stretching band of the methyl group overlaps with the CH stretching bands of the adsorbate molecules. Deconvolution of the infrared bands potentially confirmed the redshift of the CH symmetric stretching band of the methyl group. However, the accuracy of the resultant redshift from this deconvolution was insufficient for a meaningful discussion of the minor wavenumber shifts observed in this study. The elongation of the C–H bonds and the corresponding redshift of the CH symmetric stretching band were also qualitatively corroborated by the molecular assembly of 2-methylimidazole with benzene, 1,3-CHD, and 1,4-CHD, optimized using DFT calculations (Figure S9, Figure S10, and Table S3). Generally, intermolecular CH-π interactions occur within 3 Å [33] and exhibit a high degree of directionality, with the orientation of the C–H bond aligned with that of the π-orbital. Consequently, the C–H and π bonds tend to be oriented nearly perpendicular to each other. Following the formation of the CH-π bond, the adsorbed molecules were arranged such that their molecular planes were positioned at an angle that was nearly perpendicular to the 6MR aperture plane. This orientation facilitates the ingress of adsorbed molecules into the 6MR apertures. Thus, the CH-π interaction between the 2-methylimidazole ring and the adsorbed molecules is crucial for governing the molecular orientation necessary for the effective passage through the 6MR aperture for benzene and cyclic alkenes.

3.3.2 ¹³C CP/MAS NMR spectroscopy

The ¹³C CP/MAS NMR spectra of ZIF-8, both in its pure form and with various adsorbates are shown in Fig. 7. Cross-polarization (CP) measurements are particularly effective for highlighting immobile molecular segments within solid matrices while attenuating signals from more mobile components. Consequently, the highly mobile adsorbed molecules appeared as minor signals in the spectra. The predominant peaks are attributed to the 2-methylimidazole moiety, suggesting that the ZIF-8 framework exhibited greater rigidity and reduced mobility than the adsorbate molecules. Specifically, the resonance lines at approximately 14, 124, and 151 ppm correspond to the methyl group, CH carbon of the imidazole ring, and quaternary carbon of the 2-methylimidazolate moiety, respectively. Figures 8(a), 8(b), and 8(c) provide enlarged views of these spectral components. Notably, the adsorption of benzene and cyclic alkenes resulted in downfield shifts of 0.5–1.3 ppm in the resonance lines associated with the methyl group and CH carbon of the imidazole ring, whereas the adsorption of cyclohexane did not elicit such shifts. The resonance lines corresponding to quaternary carbons remained largely unchanged after adsorption. These observations suggest a significant interaction between the π-electrons of the adsorbed molecules and both the methyl group and the CH carbon of the 2-methylimidazolate moiety. Our previous studies indicated that both the methyl and CH protons of the imidazole ring can act as proton donors in CH-π bond. According to Pople's point magnetic dipole model [46, 47], the shielding effect of carbon along a perpendicular line through the center of the molecular plane of the benzene ring is described by the equation Δσ_C = 51.96/R³, where R represents the average distance between the carbon nucleus of interest and the molecular plane in angstroms (Å). For the optimized structure of the 2-methylimidazole-benzene complex derived from DFT calculations (see Figure S9), R was determined to be 3.27 Å, which yielded a shielding effect of approximately 1.5 ppm. In the case of CH-π bond, the C-H bond is anticipated to elongate owing to the attraction of the proton by the π electrons, resulting in an anti-shielding effect on the ¹³C nucleus. Scheiner reported an anti-shielding effect of 200 ppm/Å for the tetrel bond of a methyl group [48]. To account for the observed downfield shift, an average elongation of approximately 0.01–0.015 Å in the C-H bond is necessary, which is consistent with the expected bond-length increase associated with a strong CH-π bond. In summary, the downfield shifts in the resonance lines resulting from the adsorption of benzene and cyclic alkenes can be attributed to the CH-π bond between the CH protons of the methyl group and imidazole ring and π electrons of the adsorbed molecules.

3.4 Local structure of molecular passage through 6MR

This study elucidates the intermolecular interactions that dictate molecular orientation during the adsorption of bulky molecules onto ZIF-8, particularly as these molecules navigate through 6MR apertures. The results suggest a specific configuration for benzene and six-membered-ring alicyclic hydrocarbons as they traverse the 6MR aperture (Fig. 9). The CH-π bond are pivotal in influencing the molecular orientation of benzene and cyclic alkenes during their passage through the aperture. As these molecules approach the CH bottleneck of the 6MR, they form CH-π bonds with the CH groups of the 2-methylimidazole ring at this bottleneck. This highly directional CH-π bonding results in an nearly perpendicular (T-shaped) alignment of the imidazole ring and adsorbed molecule. This arrangement ensured that the adsorbed molecules maintained a configuration that minimized their molecular cross-sectional areas (molecular planes) upon entering the 6MR aperture. After successfully passing through the aperture, the adsorbed molecule is stabilized by forming an additional CH-π bond with the methyl group on the methyl bottleneck side (Figure S10).

In contrast, cyclic alkanes such as cyclohexane primarily engage in dispersion-force interactions. The optimal molecular orientation for enhancing these interactions occurs when the plane of the adsorbed molecule is parallel to the 2-methylimidazole ring, as this alignment maximizes the contact area between them. Consequently, a molecule approaching the CH bottleneck side of 6MR is captured by dispersion force interactions with the methyl group and quaternary carbon in the 2-methylimidazolate moieties. This results in the adsorbed molecule orienting its minimum molecular cross-section (molecular face) towards the 6MR aperture, thereby facilitating its entry. After passing through the 6MR aperture, the molecule stabilizes through dispersive force interactions with the three methyl groups on the methyl bottleneck side. Cyclohexene, being a cyclic alkene, also exhibits significant contributions from dispersion force interactions due to its substantial proportion of saturated hydrocarbon components, in addition to the CH-π interactions (see Figure S10).

Finally, ZIF-8 exhibited remarkably different diffusion coefficients for 1.3-CHD and 1,4-CHD. This result is of particular interest because it highlights the potential of ZIF-8 as a molecular sieve for isomer separation. Further investigation is necessary to elucidate these detailed mechanisms. However, the isomer distinction may involve the symmetry of the phonon modes associated with the swing effect of the bridging ligands that constitute the 6MR, as well as the symmetry of the adsorbate molecule. Additionally, considering the aspect of configuration diffusion, this isomer distinction might be linked to the internal arrangement of π electrons through the CH-π bond, which controls the molecular orientation with respect to the 6MR aperture.

In this study, we investigated the adsorption kinetics of benzene and six-membered-ring alicyclic hydrocarbons on ZIF-8, to clarify the interactions between the adsorbed molecules and linkers in relation to the adsorption behavior of larger molecules. Monte Carlo simulations were employed to elucidate the local structural dynamics of the molecules as they navigated through the six-membered ring aperture of ZIF-8. Additionally, the FT-IR and ¹³C cross-polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) were used to identify the adsorbate-adsorbate interactions that influence molecular orientation during transit through the aperture.

The findings indicate that the diffusion of benzene and other six-membered ring hydrocarbons through the six-membered ring aperture is predominantly governed by a significant negative activation entropy ranging from − 172 to − 217 J K^− 1 mol^− 1. This observation suggests a pronounced effect of the molecular orientation on the transit of adsorbate molecules through the apertures. Monte Carlo simulations further elucidated this phenomenon, revealing that the aperture diameter significantly affected the activation energy required for the molecular passage. Order parameter analysis revealed that the molecular orientation was tightly constrained and aligned in a manner that minimizes the energy barrier for traversing the aperture. Spectroscopic analyses employing FT-IR and ¹³C CP/MAS NMR techniques identified a specific CH-π interaction between the π electrons of the adsorbed molecules and the methyl group and imidazole ring CH of the 2-methylimidazolate linker. For unsaturated cyclic hydrocarbons possessing π electrons, the CH-π interaction predominantly governs the molecular orientation during aperture transit. Conversely, saturated cyclic hydrocarbons are primarily influenced by dispersion force interactions with the methyl groups of the bridging ligand. These orientation-controlling interactions are anticipated to play a critical role in governing molecular adsorption and diffusion behaviors of other metal-organic frameworks (MOFs), porous coordination polymers (PCPs), and porous organic materials (POMs) composed of organic materials.

Finally, the distinct diffusion coefficients of 1.3-CHD and 1,4-CHD are of particular interest. This indicated the potential of ZIF-8 as a molecular sieve for structural isomers.

Acknowledgements

The authors thank Dr. Naoya Inazumi and Dr. Yasuto Todokoro of the Analytical Instrument Facility, Graduate School of Science, Osaka University, for their helpful advice, guidance, and instructions regarding the solid-state NMR measurements. We would like to thank Editage (www.editage.jp) for English language editing.

Author Contributions

Takahiro Ueda: Conceptualization, methodology, validation, resources, software, formal analysis, visualization, writing–original draft, review, editing, supervision, funding acquisition, and project administration. Yuta Yamada: Methodology, investigation, data curation, and visualization. Kota Fujii: Investigation, DFT calculations, data curation, and visualization. Ryota Mihara: Investigation, Monte Carlo simulation, data curation, and visualization. Taku Iiyama: Methodology, investigation, software, data curation, validation, visualization, review, and editing. Yasutaka Hamada: Investigation, DFT calculations, data curation, and visualization. Mitsutaka Okumura: Methodology, software, and supervision.

Funding

This research was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) (Grant Numbers JP21K04979, JP24K8360). This work was the result of using research equipment shared in the MEXT Project to promote the public utilization of advanced research infrastructure (Program for supporting the construction of core facilities), Grant Numbers JPMXS0441200021, JPMXS0441200023, and JPMXS0441200024.

Availability of data and materials

The data presented in this article will be available from the corresponding author on reasonable request.

Ethical Approval

Not applicable.

Conflicts of interest

The authors declare no conflicts of interest.

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Intermolecular interactions governing adsorption of bulky molecules on ZIF-8: Insight from adsorption kinetics of benzene and 6-membered ring alicyclic hydrocarbons

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental

2.1 Chemicals

2.2 Characterization

2.2.1 Nitrogen adsorption isotherm

2.2.2 Scanning electron microscopy (SEM)

2.2.3 Fourier transferred infrared (FT-IR) absorption spectroscopy

2.2.4 Solid-state ¹³C NMR spectroscopy

2.3 Measurement of time dependence of adsorption uptake

2.4 Monte Carlo simulations

3 Results and Discussion

3.1 Time dependence of adsorption uptake

3.2 Monte Carlo simulation for 6MR passage of adsorbate molecules

3.3 Analysis of adsorbate-adsorbent interactions by spectroscopies

3.3.1 FT-IR spectroscopy

3.3.2 ¹³C CP/MAS NMR spectroscopy

3.4 Local structure of molecular passage through 6MR

4 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Intermolecular interactions governing adsorption of bulky molecules on ZIF-8: Insight from adsorption kinetics of benzene and 6-membered ring alicyclic hydrocarbons

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental

2.1 Chemicals

2.2 Characterization

2.2.1 Nitrogen adsorption isotherm

2.2.2 Scanning electron microscopy (SEM)

2.2.3 Fourier transferred infrared (FT-IR) absorption spectroscopy

2.2.4 Solid-state 13C NMR spectroscopy

2.3 Measurement of time dependence of adsorption uptake

2.4 Monte Carlo simulations

3 Results and Discussion

3.1 Time dependence of adsorption uptake

3.2 Monte Carlo simulation for 6MR passage of adsorbate molecules

3.3 Analysis of adsorbate-adsorbent interactions by spectroscopies

3.3.1 FT-IR spectroscopy

3.3.2 13C CP/MAS NMR spectroscopy

3.4 Local structure of molecular passage through 6MR

4 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

2.2.4 Solid-state ¹³C NMR spectroscopy

3.3.2 ¹³C CP/MAS NMR spectroscopy