Confined Water Vapor in ZIF-8 Nanopores.

Metal-organic frameworks (MOFs) possess an ordered and size-controllable porous structure, making them an interesting heterogeneous confining environment for water. Herein, molecular dynamics simulations are employed to investigate the structure of confined water vapor in zeolitic imidazolate framework-8 (ZIF-8) nanopores. Water dimers, which are rarely observed in liquid or water vapor, can form in ZIF-8 at room temperature. The six-ring-member gate is the main location of a water dimer in ZIF-8. The terminal methyl and CH groups of the imidazole linker interact with the water dimer by relatively weak hydrogen bonding. The above-presented findings provide a foundation for the elucidation of water confined in ZIF-8 and demonstrate the potential of obtaining low-order clusters of water by MOFs.

Introduction

Metal–organic frameworks (MOFs) consist of metal ions or oxide nodes connected via organic linkers based on the coordination chemistry principle.[1] Due to their flexible and tunable porous structures, MOFs have attracted considerable attention, wherein a wide range of applications have been reported, for example, gas storage, separation, sensing, and catalysis.[2−5] Although the relation-property tunability is commonly used in materials studies,[6−8] MOF–guest interactions dictate their properties and, in-turn, applications. It is known that water from ubiquitous atmospheric moisture can combine with an MOF–guest system and influence its structure, properties, and applications.[9−12]

MOFs can utilize their nanosized pore structures to frustrate hydrogen-bond (H-bond) networks of water, forming heterogeneous confining environments.[13,14] On the one hand, nanoconfined water exhibits properties that differ substantially from those of bulk water and can lead to anomalously low dielectric constant and significantly reduced diffusion coefficient.[15,16] Additionally, interactions between water and the surrounding confining environment can promote unique properties.[17] On the other hand, MOF properties can change via inclusion of water. For example, exceptional conductivity of MOFs has been achieved under high relative humidity.[18] Considering the above aspect of water, the applications of MOFs in real world must consider their moisture or water stability. Potentially, water molecules can gradually replace metal-coordinated linkers of MOFs via hydrolysis.[19] Even though MOFs possess a strong metal-linker coordination bond and hydrophobic nature, they remain unstable under liquid water conditions.[20,21] Therefore, the MOF/water ratio and contact time can affect the amount of dissolved MOF crystallites. A small amount of water, such as water vapor in atmosphere, actually will not rapidly destruct most of the MOFs. Still, the water structure inside the confining MOF environment is interesting. The actual properties of MOFs must consider the contribution from ubiquitous water in air.

In this paper, zeolitic imidazolate framework-8 (ZIF-8), a type of MOF consisting of large cavities (1.16 nm) connected by small apertures (0.34 nm), was chosen as a template to confine water molecules. ZIF-8 is unstable in high amounts of water but exceptionally stable in the atmosphere.[20−23] The interactions and configurations between the water–water and water–ZIF-8 framework were represented through the structural reconstruction method at an atomic level. The results could be used to review the ZIF-8 pore structure confining water vapor.

Results and Discussion

Using the above-described interaction models, ZIF-8 can accommodate up to six water molecules per unit cell (Figure a), with a water density of 0.0366 kg/m3. This density corresponds to a water vapor density of 0.0361 kg/m3 (107 °C).[24]

Figure 1

Water molecules confined by ZIF-8 framework nanopores. Purple and green denote the O and H atoms of the water molecule, respectively.

Radial distribution functions (RDFs) resulting from the types of empirical models were used to analyze the water structure inside the ZIF-8 nanopore (Figure ). Different water models (SPC/E, TIP3P, and TIP4P) were employed to compute the water RDF (Figures and S2). It can be found that the TIP4P models can provide more information. An intense peak at 2.65–2.85 Å related to the O–O RDF is observed when using all water models employed here (Figures and S2). Increasing the water–water distance can generate another O–O RDF peak at ∼4.35 Å for both TIP4P/2005 and TIP4P/ice water models but 4.65 Å for the TIP4P-Ew model. Since the possible water dimers and trimers in this study, the optimized forms (TIP4P/2005 and TIP4P/ICE) are the most suitable model. According to Figures and S2, no more peaks related to O–O RDF at >5.0 Å region were observed. However, the O–O RDF of low density and unconfined water shows an additional peak at approx. 7.0 Å position.[25] Furthermore, in this case, the intensity ratio of 1st to 2nd O–O RDF peak is approx. 4.38 (TIP4P/Ice model), which is much larger than that of free and low density water (1.49) computed by empirical potential structure refinement simulations.[25] A larger intensity ratio implies broken hydrogen bonds between the first and second coordination shells. Thus, the confining environment from ZIF-8 frustrates the H-bond networks of water.

Figure 2

RDF of the water structure inside ZIF-8 nanopores. (a) TIP4P-Ew, (b) TIP4P/2005, and (c) TIP4P/ICE water model.

Furthermore, the orientation of water molecules inside ZIF-8 nanopores can be analyzed by H–H and O–H RDF curves (Figure ). Excluding the 1st peaks corresponding to the distance of the intrawater molecule, two peaks (2.35 and 3.75 Å for TIP4P/2005 and TIP4P/Ice model) exist for the H–H RDF. In the case of the O–H RDF, two peaks at a relatively short distance (1.85 and 3.25 Å) are observed. Considering the nonlinear configuration of the water molecule, H atoms between adjacent waters cannot align face-on. However, complete back-to-back orientation of two adjacent water molecules cannot form the two peaks related to the O–H RDF.

For clarity, all peaks of RDF curves are shown as Donut charts (Figure ). The circle lines and colors correspond to the peaks highlighted in Figure . The O atom of one water molecule is located at the center position of the Donut chart. Correspondingly, both H atoms of the water molecule are located at the most inner circle line. The position of the current water molecule is fixed at the center and another water molecule is adjusted to fulfill all the RDF peaks. Only one possible position and orientation for the latter water molecule can be seen in Figure .

Figure 3

Analysis of the peaks in the RDF for water: (a) O–H and O–O; (b) H–H. Pink and green denote O and H atoms of the water molecule, respectively. The line colors are the same as in Figure . The TIP4P/Ice water model was employed here.

The planar configuration shown in Figure can be converted to the space configuration shown in Figure a by rotating one water molecule and forming a H–H–O–H dihedral angle of 90°. The symmetrical dimer configuration ensures that the last RDF peak for H–H is located at the position of 3.75 Å (Figure ). Furthermore, a possible trimer interwater configuration can be constructed in Figure b. The longest O–O distance of ∼4.30 Å in the trimer corresponds to the O–O RDF peak at ∼4.35 Å (Figure b). The distance between two H atoms at both ends of the trimer is larger than 4.30 Å, where no H–H RDF peak is observed (Figure ). Therefore, the trimer’s interwater configuration is actually rare in the ZIF-8 nanopore. Considering the pore geometry of the ZIF-8 framework (∼11 and ∼3.4 Å diameter of cage and gate, respectively), it is difficult to find other complicated structures consisting of more water molecules due to the frustration from the narrow and ordered gate.

Figure 4

Preferential (a) dimer and (b) trimer interwater configuration inside ZIF-8 nanopores.

The dipole–dipole orientations for water dimers as a function of their distance was employed to provide a deep insight into the local orientation (Figure ). Compared to the crystal phases of water, the weak fluctuations in the radial dependence of the cosine in this work can be observed once the distance is larger than 4 Å.[26] This indicates that the frustrate networks of water were obtained in ZIF-8. The hydrogen bonds around water molecules in this study were analyzed by computing the fractions of water molecules forming from 0 to 6 hydrogen bonds with other water molecules (Figure ). Figure shows that the water molecules forming 3 (∼40%) and 2 hydrogen bonds (∼33%) are dominant. However, the proportion of water molecules forming 1 and 4 hydrogen bonds is ∼8 and ∼17%, respectively. An average number of water–water hydrogen bonds per water molecule is 2.7, which agrees well with the reported results for water confined in other MOFs.[27]

Figure 5

Angle (cos θ) between the dipole moment vectors of two water molecules as a function of their distance.

Figure 6

Percent fraction of water molecules forming from 0 to 6 hydrogen bonds with other water molecules.

Bulk water has a tetrahedral or “ring-and-chain”-like structure constructed by the hydrogen bonding network.[28] However, water vapor behaves much like any other gas, where molecule occasional interactions, instead of the hydrogen bonding network, governed this phase. As an intermediate structure between free bulk water and water vapor, the water dimer is rare and short-lived at low temperatures and moderate pressures. In early 1957, the water dimer received considerable attention as a necessary starting point of studying myriad water forms.[29] Supersonic molecular beams or the solid N2 matrix at an extremely low temperature have been used to generate trapped water dimers.[29,30] In this study, ZIF-8 nanopores provided the confining environment to frustrate the H-bond networks of water and water dimers formed at room temperature. The near linear structure (Figure a) of the Cs symmetry with a O–O distance of 2.75 Å is in good agreement with the reported experimental and theoretical results.[31−33]

The water-ZIF configuration must be revealed to understand the interactions between water and the surrounding confining environment. 100,000 frames of structure from 50 ns molecular dynamics (MD) running are used to compute the distance of all atomic pairs between water and ZIF-8. The nearest distance between each water molecule and ZIF-8 in every frame has been recorded and distributed. It should be noted that this computation considers all possible atomic pairs based on the search for the nearest distance, which can be directly used to analyze the preferential adsorption sites.

Figure shows that the nearest distance distribution curves can be divided into two parts. The first part relates to the distribution peaks located at a short distance, including the shaded area (C2 and N) of imidazole linker pairing with water. The second part corresponds to the terminal part (C1 and C3) of the imidazole linker and zinc node, which are far away from water. Both H and O (Figure S3) atoms of the water molecule show the same trend of approaching to the CH- terminal part of the linker.

Figure 7

Distribution of the nearest distance between the water guest and ZIF-8 host atoms. ZIF-8 atomic labels are shown in the inset picture. The TIP4P/Ice water model was employed here.

The ZIF-8 framework consists of the six-ring-member gate (Figure ) and four-ring-member gate (Figures S4 and S5). The water dimer, which is commonly found in this study (Figure ), is placed at the six-ring-member gate to satisfy the water-ZIF distance distribution peaks shown in Figure . The resulting configuration shows that the water dimer bridges the two opposite linkers of the six-ring-member gate (Figure ). One water molecule approaches the methyl group, and another is close to CH- terminals of the imidazole ring. The water-ZIF configuration (Figure ) is in good agreement with the distance distribution peaks shown in Figure . However, it is difficult to find a proper position for the water dimer located at the four-ring-member (Figures S4 and S5) gate to meet with the RDF peaks shown in Figure . Thus, the preferential site for locating the water dimer is the six-ring-member gate of ZIF-8.

Figure 8

Preferential configuration of water along the six-ring-member gate of ZIF-8: (a) side view and (b) top view. The measurement colors are the same as in Figure .

Conclusions

In conclusion, this work is based on molecular dynamic simulations devoted to systematically studying the structure of nanoconfined water vapor in ZIF-8. ZIF-8 nanopores can frustrate H bond networks of water, allowing the water dimer configuration to preferentially form. It is found that the water dimer bridges the opposite linkers of the six-ring-member gate, which are far from the zinc node and N atoms of ZIF-8. From the point of view of the development of MOF materials for practical applications, the nanoconfined water structure provides evidence for re-examination of the remarkable adsorption, separation, and other bulk properties of MOFs. Furthermore, MOFs are promising template materials to obtain a low order cluster of water under moderate conditions.

Computational Method

The ZIF-8 unit cell with a lattice parameter of 16.985 Å was employed in this study. 2 × 2 × 2 unit cells (2208 atoms) with periodic boundary conditions were applied along the three main coordinate directions that constitute the simulation box. The Verlet-velocity integration algorithm with a time step of 1.0 fs was used for all types of interactions (i.e., bonding, van der Waals, and electrostatic interactions). The bonding and nonbonding force parameters of ZIF-8 were determined from our previous publication,[34] and a 16.0 Å cutoff value was set for all nonbonding interactions. Three common types of water models (SPC/E,[35] TIP3P,[36] TIP4P-Ew,[37] TIP4P/Ice,[37] and TIP4P/2005[37]) were selected in this study. SPC/E and TIP3P consisted of a three-site model, whereas TIP4P consisted of a four-site water model (Figure S1). The TIP4P-Ew model was used for simulations of periodic systems including Ewald summation to evaluate electrostatic energy; then, in 2005, the TIP4P/2005 model was developed to improve the reproduction of the phase diagram of water, and finally, the TIP4P/ICE model was optimized for the simulation of solid phases of water, although it performs well also for liquid water. Lorentz–Berthelot (LB) combination rules were used to calculate the cross-potential parameters.

MD simulations were performed using LAMMPS software.[38,39] Initially, geometric optimization of the atomic positions at zero temperature employed the conjugate gradient (CG) algorithm. Then, the simulated system was equilibrated for 10 ns at 300 K temperature (Langevin thermostat[40]) in the canonical ensemble (NVT). After the thermal equilibration step, the production step with 50 ns microcanonical ensemble (NVE) run was performed for the structure and dynamic analysis.