3.1. Structure of the ZL-FL Composites at Different Pressure Conditions
shows selected powder patterns of the three ZL-FL composites compressed in s.o. as a function of pressure. With increasing pressure the peak intensities decrease and the peak profiles become broader. Notwithstanding this, complete X-ray amorphization is not achieved up to the highest investigated pressure (about 6 GPa). All the observed peaks are consistent with the P/6mmm s.g.
, thus ruling out any P
-induced phase transition. The patterns collected upon decompression demonstrate that the P
-induced effects are almost completely reversible. In fact, the features of the ambient-pressure pattern (Pamb
) and the unit cell parameters are rather well recovered upon P release. The decrease of the cell parameters for ZL/0.5FL, ZL/1FL, ZL/1.5FL samples up to 6 GPa are: ∆a
= −3%, −2.7%, −2.3%; ∆c
= −4.6%, −4.4%, −3.7% accounting for a ∆V = −10.1%, −9.5% and −8.0%, respectively (See Figure 5
and Table S1
). These values indicate that the compressibility changes with the loading and that the FL molecules hosted in the channels stiffen the structure.
The hexagonal lattice undergoes a slight anisotropic compression (Figure 5
), with c
as the most compressible axis. This is probably due to the presence of bonds among the FL carbonyl groups and KD potassium cations, lying along the a
direction, that stiffen the structure.
The framework P
-induced deformations can be summarized as follows (see Table 2
and Figure 6
In the ZL/0.5FL at 2 GPa, both the diameters (O1–O1 and O2–O2) of the 12 MR shorten. The shortening of the 12 MR O1–O1 diameter is reflected in the lengthening of the O1–O1 diameter of 8 MR channel (ǁ ), which becomes more elliptical. Upon pressure release, the original values of the 12 MR diameters are almost regained (remaining slightly smaller than those observed at Pamb
), while the 8 MR ones are strongly lengthened with respect to the original values, in accordance with the increase of a
parameter (see Table 1
and Table S1
In the ZL/1FL sample, O2–O2 (12 MR) diameter decreases with pressure while O1–O1 increases—probably due to the presence of a larger numbers of FL molecules in the channels with respect to the ZL/0.5FL system. The O1–O1 (12 MR) increase is balanced by the decrease of O1–O1 diameter of the 8 MR channel running alongside (ǁ ): as a consequence, 8 MR becomes less elliptical. Upon pressure release, the 12 MR opening remains slightly smaller than that observed at Pamb. The significant enlargement of the 8 MR leads to an overall value for a parameter comparable with that at Pamb.
The 8 MR window (8 MR ‖ ) (O1–O1, O6–O6), parallel to the c axis, becomes more circular at 2 GPa in both the samples. Once the pressure is released the starting values are regained.
The O3–O5–O3 and O5–O3–O5 angle variations indicate that the D6R slightly increases its ditrigonal distortion in both samples. Upon pressure release the Pamb features are almost recovered.
Extra Framework Species
The distribution of the K cations in both composites at Pamb
can be described as follows (see Table S2 and S3
site KB—in the centre of the cancrinite cage—is fully occupied and coordinated to six framework oxygen atoms O3;
site KC—in the centre of the 8 MR channel—is fully occupied and coordinated to four oxygen atoms O5;
site KD—near the wall of the main 12 MR channel—is partially occupied and coordinated to six oxygen atoms (O4, O6), two water molecules (WH and WI) and to the oxygen atom of FL molecule (OFL).
, in ZL/0.5FL and ZL/1.0FL composites the water molecules (14.7 and 9.7 per unit cell, respectively) are distributed over three extra framework sites (WH, WI, WJ). All of them are located in the main channel. WH site is present only in ZL/0.5FL composite, the other two sites have the same positions of the oxygen atom and of the C3 carbon atom of FL molecule (labelled OFL/WI and C3/WJ, respectively, in Ref. [56
Upon compression at 2 GPa the following structural features are observed:
the distances between KB, KC and the coordinating framework oxygen atoms (Table S3
and Ref. [56
]) decrease as a consequence of the shape modifications of both 8 MR channel aperture and D6R. The distances between the cation in the main channel (KD) and O4 and O6 decrease as well. All these effects are more marked in the ZL/1FL system.
OFL–KD distance decreases in ZL/0.5FL and remains almost constant in ZL/1.0FL.
Compression induces the splitting of OFL/WI and C3/WJ sites, which in the Pamb
structures of ZL/0.5FL and ZL/1FL samples [56
], occupy single sites. After the splitting, WJ increases its distance from the framework O2 atom, approaching WI site (Table S3
). After pressure release, the original positions are recovered in the ZL/0.5FL composite, while this does not happen in the ZL/1FL sample.
At 2 GPa, the shape, orientation and arrangement of the fluorenone molecules in the main channel do not change with respect to ambient pressure.
All these P
-induced deformations are reversible and, once the pressure is released, the original features of the zeolite and the distances among the FL molecules are almost recovered (Table 2
, Table S2 and Table S3
3.2. Structure of the ZL/1.5FL Composite from First-Principles Molecular Dynamics
As evidenced by the above-discussed data, XRPD refinements provided a satisfactory description of the P-induced structural modifications of the composites characterized by low and moderate dye content. On the other hand, the great number of low-occupancy sites found for water and fluorenone molecules and the high symmetry hindered the structural refinement of the sample containing the maximum amount of dye. Such a difficulty was previously encountered in the room pressure refinement of the ZL/1.5FL composite and it was overcome by integrating the experimental data on cell parameters with theoretical modelling for the atomic coordinates [56
Hence, encouraged by this result, we exploited again theoretical modelling for achieving an atomistic structural description of the ZL/1.5FL composite at high-pressure conditions. Practically, we used the room-pressure coordinates as an initial guess to determine the composite structure at cell parameters corresponding to 1.95 GPa and we run first-principles molecular dynamics simulations for both P = Pamb and P = 1.95 GPa in order to study the pressure-induced changes of the supramolecular organization inside the zeolite nanochannels at molecular-level detail.
The first remarkable observation is that the unique dye-architecture found at room pressure remains stable at high pressure conditions (Figure 7
), with minimal alterations of its intermolecular distances and essentially without significant perturbation of the FL molecular geometry, apart from slight instantaneous distortions from the ideal gas-phase structure, mainly ascribable to thermal motion.
Significantly, the leading interaction stabilizing the confined fluorenone superstructure, i.e., the coordination to potassium cations, not only is maintained but it appears also to be, on average, strengthened upon compression. Indeed, by comparing the pair distribution functions relative to the K-OFL interaction, we deduce a significant shortening of the minimum coordination distance, which passes from 3.1 to 2.9 Å in going from room pressure to 1.95 GPa (Figure 8
). Moreover, the splitting of the first maximum into two peaks becomes much more evident upon compression, indicating that the interaction of the carbonyl group and potassium may have different degrees of strength among the hyperconfined fluorenone molecules. Taken as a whole, these data indicate that the supramolecular architecture of dyes responds to pressure essentially by approaching with the carbonyl groups the potassium cations in the 12 MR channel, without undergoing appreciable modifications of both intra- and inter-molecular distances.
By considering now water molecules, which share the channel space with fluorenone, a striking similarity emerges by comparing their pair distribution functions at room pressure and 1.95 GPa (Figure 9
). Actually, for both water protons (Figure 9
a) and water oxygens (Figure 9
b) the two curves are almost superposable, showing also very close positions for first maximum peaks of water-water hydrogen bonding (corresponding, at room pressure, to Hw
distances of 1.85 and 2.80 Å, respectively—see upper panels of Figure 9
a,b). In particular, only the shoulder of the Hw
peak—found at 1.78 Å at Pamb
—appears to be very slightly displaced towards greater distances. Also the interaction of water with framework oxygens—which, as normally found for zeolitic water, is weaker than water–water hydrogen bonding [143
]—appears to be nearly unperturbed upon compression and characterized by Hw
distances of 1.95 and 2.90 Å, respectively (see centre panels of Figure 9
a,b). Furthermore, the interaction of water with potassium cations is nearly unaffected by pressure at short range distances (with first maximum position at 2.90 Å, see Figure 9
b, bottom panel), indicating that the coordination environment of the zeolite L extra framework cations is, on average, spectacularly insensitive to compression: no variation of the K coordination number occurs and distances from water oxygens remain basically unaltered. Finally, very minor changes are detected for the water-carbonyl interaction: the first peak position passes from 1.80 to 1.75 Å upon compression, indicating a slightly stronger interaction of water molecules with the carbonyl group of the dye (Figure 9
a, bottom panel).
Interestingly, whereas in the ZL/0.5FL and ZL/1FL composites the O6–O6 distance (i.e., the maximum diameter of the 12 MR channel) either remains constant (ZL/1FL) or slightly shortens (ZL/0.5FL) with pressure (Table 2
), such a distance increases in the ZL/1.5FL composite. Accordingly, the calculations also predict a decrease of the 12 MR window (i.e., the channel opening) of ZL/1.5FL with compression (see O1–O1 and O2–O2 distances in Table 3
). This behaviour is due to the peculiar close-packing arrangement of the extra framework content at maximum dye loading (ZL/1.5FL). Specifically, as compression occurs mainly along the channel axis, the guest species—water molecules and dye nanoladder—can only respond by further clustering in the maximum-diameter region of the channel (Figure 7
), thus explaining the O6–O6 lengthening and the 12 MR window narrowing. The different pressure response of the composites according to the dye loading is a consequence of the stiffening/template effects of the extra framework content, already observed in several high-pressure studies on zeolites and zeolite-based materials (see, e.g., Refs. [62
Taken together, besides the impressive stability of the fluorenone nanoladder, these results underline a quite surprising and important feature of the confined supramolecular system: even though water is smaller than the dye and hence in principle more easily displaceable upon compression, the arrangement of the water molecules in ZL/0.5FL and ZL/1FL is only slightly modified by the application of hydrostatic pressure—in particular, WJ increases its distance from the framework O2 atoms—and remains essentially unperturbed in ZL/1.5FL. This is ascribable to the fact that the Pamb
structure of the ZL/1.5FL composite has already a close-packing arrangement of the extra framework species. In particular, all water molecules are fully stabilized by coordination to potassium cations and by the network of hydrogen-bond interactions with fluorenone carbonyl groups, water and framework oxygens [56