We use powder x-ray diffraction under variable temperature to study the thermal expansion of the metal–organic framework (MOF) PCN-222. The thermal expansion increases drastically in magnitude following more aggressive heating, which is rationalized by enhanced flexibility upon guest removal. Moreover, the thermal response strongly depends on the temperature: the volumetric expansivity nearly quadruples and the expansion along c changes sign upon cooling. Our results highlight the large flexibility of MOFs and the role of guest species.

Metal–organic frameworks (MOFs) are a fascinating class of materials with a wealth of potential applications.1 They consist of inorganic nodes joined by organic linkers to form extended porous networks. Due to their open structure, they are considerably more flexible than ceramics,2,3 which enhances the sensitivity to external stimuli such as temperature or pressure. As a result, MOFs often display counterintuitive mechanical behavior, including negative thermal expansion (NTE),4,5 negative Poisson’s ratios,6,7 or negative linear compressibility.8,9 Such anomalous elastic properties may ultimately lead to applications, such as zero-strain composites, sensors, or actuators.4,10

The mechanical properties of porous materials are often influenced by interstitial guest species, which can have drastic effects. By way of example, inclusion of CO2 in Co[Fe(CN)5NO] continuously tunes the thermal expansion from −6 to +8 MK−1, as the gas inhibits the transverse vibrations responsible for NTE.11 This type of guest-induced NTE suppression is frequently observed in framework materials with phonon-driven NTE,12,13,15,44 although guest-enhanced expansion is also possible.15 In either case, guest inclusion in porous solids is an important tool toward the control of thermal expansion, which is relevant for diverse applications ranging from, e.g., high-precision optics to cookware.16 

Most studies on the guest-dependent thermal behavior of MOFs have focused on microporous MOFs, e.g., the archetypical HKUST-117,18 and MOF-5.5,13 Less experimental attention has been devoted to mesoporous MOFs, although molecular simulations predict a strong correlation between the pore diameter and the NTE magnitude.19 Since larger pore volumes allow for greater uptake of guests, stronger guest effects can be expected. Furthermore, the study of mesoporous MOFs at non-ambient conditions helps evaluate their structural stability and propensity for phase transitions during thermal fluctuations, which is relevant for future applications. Thermal expansion studies of mesoporous MOFs are, therefore, not only fundamentally interesting but may also be useful for applied research.

In this manuscript, we use variable-temperature powder x-ray diffraction (XRD) to study the thermal expansion of PCN-22220 (also known as MOF-54521 and MMPF-622) as a function of varying amounts of guest content. PCN-222 is composed of Zr6(OH)4O4 clusters and tetrakis(4-carboxyphenyl)porphyrin linkers (Fig. 1).20 

FIG. 1.

PCN-22220 viewed along the [1̄00] and [110] directions. Zr is shown in blue, the linker in black, and the pores in yellow (mesopore), cyan (micropore), and pink (intercluster pore).

FIG. 1.

PCN-22220 viewed along the [1̄00] and [110] directions. Zr is shown in blue, the linker in black, and the pores in yellow (mesopore), cyan (micropore), and pink (intercluster pore).

Close modal

It adopts a Kagome-like structure (csq topology) with the hexagonal space group P6/mmm. Three types of pores are present, mesoporous hexagonal channels, triangular microporous channels, and small intercluster pockets, along the c direction connecting adjacent mesopores (Fig. 1). The high porosity, stability, and the affinity of porphyrin to metal ions have sparked interest in the use of PCN-222 for catalytic and sensing applications.23–25 It is also a good candidate for the exploration of thermomechanical responses due to its hierarchical porosity and high thermal stability. We will come to show that the thermal expansion varies strongly depending on the temperature range and the thermal history, which we attribute to guest effects.

PCN-222 was synthesized and activated according to literature procedures (supplementary material).20,26 Rietveld refinements using the published hexagonal crystal structure20 give a good fit to the data (Figs. S1 and S2). The Zr concentration by mass is 22.1%, which is in excellent agreement with the theoretical value of ∼23%. Nevertheless, Zr-MOFs are known to feature many types of vacancies (defects),27 and to eliminate any sample-specific behavior,28 all measurements were recorded on the same sample batch. Thermogravimetric analysis shows an initial mass loss of ∼20%, followed by a large mass loss around 450 °C, corresponding to decomposition (Fig. S10). After activation, the initial mass loss is smaller and the decomposition temperature reduced to 400 °C. Assuming that the initial mass loss is entirely due to loss of guest water, this leads to an approximate water content of seven molecules per cluster.

Powder XRD patterns of PCN-222 were collected in the temperature range 273–123–473–293 K in 10 K intervals [Figs. 2(a) and S1]. This is referred to as the hydrated sample, as water is inevitably present in the pores. Upon cooling from 273 K, the a lattice parameter initially decreases while c expands, leading to a small negative volumetric thermal expansion, αV. As the temperature is reduced further, αa and αc continuously decrease in magnitude and change sign around 173 K (Fig. 3), denoted as the switching temperature, Ts. Below Ts, PCN-222 exhibits NTE along a and positive thermal expansion (PTE) along c, while αV remains negative and increases in magnitude.

FIG. 2.

The thermal evolution of a and c of PCN-222 in its (a) hydrated, (b) in situ dehydrated, and (c) heat-treated state. Data points are colored according to the order of data collection, where the first (final) collected data point is yellow (blue). The data were measured as follows: (a) upon cooling followed by heating, whereas the converse was true for (b) and (c). Temperatures where αc changes sign are given by the dashed lines. The arrows indicate the initial direction of heating/cooling.

FIG. 2.

The thermal evolution of a and c of PCN-222 in its (a) hydrated, (b) in situ dehydrated, and (c) heat-treated state. Data points are colored according to the order of data collection, where the first (final) collected data point is yellow (blue). The data were measured as follows: (a) upon cooling followed by heating, whereas the converse was true for (b) and (c). Temperatures where αc changes sign are given by the dashed lines. The arrows indicate the initial direction of heating/cooling.

Close modal
FIG. 3.

The coefficients of thermal expansion as a function of temperature calculated over seven data points for the as-made, hydrated (black); as-made, dehydrated (blue); and heat-treated, dehydrated (cyan) PCN-222. The pore content thus decreases from black to blue to cyan. Approximate temperature regions referred to in the text are highlighted by gray bars.

FIG. 3.

The coefficients of thermal expansion as a function of temperature calculated over seven data points for the as-made, hydrated (black); as-made, dehydrated (blue); and heat-treated, dehydrated (cyan) PCN-222. The pore content thus decreases from black to blue to cyan. Approximate temperature regions referred to in the text are highlighted by gray bars.

Close modal

Above 300 K, PCN-222 rapidly contracts along a and expands along c, which is attributed to the dehydration of the atmospheric water present in the pores [Fig. 2(a)]. To investigate the thermal response of the water-free MOF, data were collected using the temperature protocol 303–473–123–303 K, such that the framework dehydrates in situ [Fig. 2(b) and S3]. The BET surface area increases slightly following heating to 473 K (Fig. S12), consistent with loss of guest species. Henceforth referred to as dehydrated PCN-222, this system shows NTE along both a and c from 473 to 193 K. Similar to hydrated PCN-222, the thermal expansion along c gradually decreases in magnitude and below Ts = 193 K, αc becomes positive (Fig. 3). The expansivity of a remains negative, but its absolute value increases [Figs. 2(b) and 3].

Since the thermal response of PCN-222 correlates with the thermal history, diffraction patterns were also measured following heat treatment. The sample was heated to 573 K under flowing Ar and kept in the gas stream at ambient temperature overnight, prior to the collection of XRD data. 1H nuclear magnetic resonance (NMR) spectroscopy of digested samples indicates that as-made (i.e., not heat-treated) PCN-222 contains dimethylformamide (DMF) and its decomposition products dimethylamine and formic acid. A comparison of the integrated intensity of the peaks corresponding to the methyl groups of the guests to the intense resonances from the phenyl-group protons on the linker indicates ∼0.1 DMF and 0.1 dimethylamine molecules per linker. After heat treatment, these signals are noticeably reduced, indicating removal of guests (Fig. S13). Yet, a weak signal from the modulator benzoic acid remains.

The heat-treated MOF dehydrates like the as-made ones upon heating from ambient temperature. This stems from the inevitable rehydration of the sample from air exposure during transfer to the diffractometer. Following the in situ dehydration, the lattice parameters differ by ∼0.2–0.4 Å between the as-made and heat-treated samples [Fig. 2(c)]. Two additional very-weak reflections appear in the XRD pattern of the heat-treated PCN-222 after dehydration (Fig. S7), indicating a phase transition or the onset of decomposition. No structural solution was found for the additional reflections, but, since the vast majority of reflections are satisfactorily modeled by P6/mmm, refinements to obtain the thermal expansion were carried out using this space group. The potential phase transition is left for future studies. Sorption measurements following evacuation to 673 K show a reduced pore volume relative to evacuation at lower temperatures (Fig. S12). This could arise from the onset of decomposition, as this is close to the decomposition temperature found by TGA.

When cooling the dehydrated, heat-treated PCN-222 from 473 K, NTE along a and very small PTE along c occur until ∼373 K (Ts1) [Figs. 2(c) and 3]. At this temperature, αc turns negative, and the magnitude of αa decreases to near-zero thermal expansion. The expansivities change sign abruptly, unlike the continuous changes observed for the as-made MOFs. A second step change in the thermal expansion occurs at 153 K (Ts2), where αc switches from negative to positive and αa increases in magnitude but remains negative. As a result, the absolute volumetric thermal expansion quadruples. The expansivities in the different temperature regimes are tabulated in Table S4.

In addition to the changes in the thermal expansion, the diffraction patterns exhibit changes in the intensity distribution upon cooling (Figs. S1, S3, and S5). The 100 reflection increases in intensity below ∼200 K, which is not accounted for by the published model of PCN-222 (Figs. S2, S4, and S6).20 The intensity enhancement occurs at similar temperatures as the sign switching of αc (and αc for the hydrated MOF) and may provide a clue to the origin of this behavior. No significant features were observed in the differential scanning analysis (Fig. S11), suggesting that the intensity changes do not result from a simple phase transition.

Fourier difference maps provide some insight into the intensity changes upon cooling. At low T, the Fourier maps indicate additional electron density in the triangular micropores and intercluster pores, which is not visible at higher T. This could arise from guests in the pores, such as benzoic acid and DMF from the synthesis—in line with the NMR spectra. To quantify the interstitial electron density, a C atom with a large thermal parameter (Beq = 20) was placed at (0, 0.5, 0.5) and its occupancy refined. The occupancy increases below Ts for all three samples (Fig. S8). Subsequently, C occupancies of the mesopore and intercluster pore were refined, with the total occupancy constrained to the maximum value found in the refinement outlined above. This shows an apparent relocation of guests from the mesopore to the intercluster pore upon cooling (Fig. S8). Thus, the pore content may migrate toward the intercluster pores upon cooling or cluster at specific sites within the same pore. In any case, the refinements suggest a possible temperature dependence of the guest arrangements.

NU-100029—the pyrene-based analog of PCN-222—undergoes a thermally induced local distortion of the Zr6(OH)4O4 cluster above 373 K, which has a lifetime of weeks.30 To investigate whether PCN-222 exhibits similar behavior, pair distribution function (PDF) analysis of PCN-222 at various temperatures was carried out (Fig. S9). While a small distortion occurs upon heating, this is reversible and local cluster distortions can be ruled out as a cause for the thermal behavior upon cooling.

The thermal response of PCN-222 is clearly strongly dependent on both the thermal history and temperature range, as summarized in Fig. 3. At low T, heat-treated PCN-222 (cyan) exhibits the largest absolute expansivities and hydrated PCN-222 (black) is the most rigid. The volumetric expansivities reach ∼80 MK−1 at most, which is large but not unprecedented for MOFs.4,31,43 The values agree with the correlation between mesopore diameter and volumetric expansivity predicted by Evans et al.19 At intermediate T, hydrated PCN-222 shows a small PTE along a and NTE along c, whereas the dehydrated MOFs exhibit near-zero expansion along both axes. Thus, the heat treatment has a smaller effect in this intermediate temperature range, but the hydration clearly matters. Finally, at high T, αa of heat-treated PCN-222 becomes more negative, whereas αc increases to more positive values. These changes are not mirrored by the as-made, dehydrated sample, indicating that heat treatment is important for the high-temperature thermal response. The general trend points toward enhanced flexibility upon more aggressive heating, which affects both a and c, although their expansivities show opposite signs.

The question now arises: What causes the large difference in thermal response between the samples? The large changes in lattice parameter following heating suggest dehydration, which rationalizes the changes between the hydrated and dehydrated samples. As the NMR spectra show loss of DMF and its decomposition products following heat treatment, it can be assumed that the heat-treated sample has lower pore content. In other words, more aggressive heat treatment reduces the pore content, which in turn changes the flexibility and thermal expansion behavior.

The thermal expansion of PCN-222 is also dictated by the temperature range—expansivities change below ∼200 K, irrespective of the thermal history. Based on the Fourier maps analysis, we hypothesize that this may stem from a reorientation of the guests from the mesopore to the micropore and intercluster pores. However, other mechanisms may also be at play and further studies are required. Similar temperature dependence of the thermal expansion sign was noted for solvated DUT-49, and rationalized in terms of the solidification of interstitial solvent upon cooling.32 The changes in thermal expansion for PCN-222 occur just below the freezing point of DMF, and the solidification of guest solvent could also be relevant here.

NTE in frameworks generally arises from low-energy transverse phonon modes and a possible mode in PCN-222 is depicted in Fig. 4. It involves a concerted rotation of the triangular units of the Kagome network, and resembles the NTE-driving vibration in the structurally related Zn(CN)2.33,34 This mode can be used to speculatively explain the thermal dependence of the expansivity. Below Ts, the refinements suggest low pore occupancy of the mesopore (Fig. S8), which would enable large vibrational amplitudes and drive a sizable NTE. Above Ts, the guests appear to predominantly reside in the mesopores, reducing the free space. Consequently, the transverse vibrations are obstructed and the NTE behavior is inhibited—or even completely suppressed—as for hydrated PCN-222, which has the largest guest content. Guest-induced NTE suppression is commonly observed in porous frameworks, where the thermal expansion is caused by phonon-based mechanisms.11,13,14,15 Hence, the suggested vibrational mode and thermal guest redistribution speculatively rationalize the temperature dependence of αa.

FIG. 4.

Rigid-body rotations of the PCN-222 Kagome structure proposed to be responsible for the NTE behavior along a and b.

FIG. 4.

Rigid-body rotations of the PCN-222 Kagome structure proposed to be responsible for the NTE behavior along a and b.

Close modal

To conclude, both the temperature range and thermal history influence the thermal expansion of PCN-222. More aggressive heat treatment enhances the flexibility and leads to larger absolute expansion. Furthermore, the sign of the thermal expansion along c—and for hydrated PCN-222, also along a—changes upon cooling. While sign switching of the thermal expansion is known in some oxides,35,36 intermetallics,37 and organics,38,39 it is still relatively rare. At low temperatures, the absolute linear expansivities of PCN-222 increase steeply to ∼30–60 MK−1 (|αa|) and ∼10–50 MK−1 (|αc|), with the larger changes seen for samples with lower pore content. Consequently, |αV| varies drastically from almost 0 up to 80 MK−1.

The dependence on the thermal history is believed to result from different amounts of pore content, affecting the flexibility. We also speculate that the arrangement of interstitial guests interplays with the expansivity, thus causing the thermally induced changes between NTE and PTE. By way of context, the thermal expansion of the isostructural NU-1000 depends on the gas atmosphere, and the preferred guest location varies between different gases.40 This also points toward a coupling between guest arrangements and elastic properties. While further mechanistic understanding is needed, the pore content is clearly a crucial consideration when investigating the thermal response of mesoporous MOFs.

Our manuscript highlights many avenues for further research. Studies using local techniques would help uncover the exact location of the guests and their migration upon heating/cooling, which is not only key to the flexibility, but also to applications such as gas storage or catalysis. Likewise, the study of defects in porphyrin MOFs is nascent,41,42 yet may provide a further means to tuning the elastic properties.28 Finally, further studies on the interplay between guests and flexibility in mesoporous MOFs would be interesting and in comparison with microporous systems reveal any dependence on the pore size.

The supplementary material includes full experimental details, XRD patterns, lattice parameters, PDF data, TGA traces, BET data, and NMR spectra.

We acknowledge funding through a Humboldt research fellowship to HLBB. C. Koschnick (MPI-FKF), M. J. Cliffe (Nottingham), and C. S. Coates (Cambridge) are acknowledged for useful discussions, and J. Nuss (MPI-FKF), M.-L. Schreiber (MPI-FKF), and L. Yao (MPI-FKF) for assistance with DSC, ICP, and NMR, respectively. This work was supported by the Max Planck Society, the Center for NanoScience and the DFG Cluster of Excellence e-conversion (Grant No. EXC2089).

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material