Tuning Pt and Cu sites population inside functionalized UiO-67 MOF by controlling activation conditions

Abstract

The exceptional thermal and chemical stability of the UiO-66, -67 and -68 classes of isostructural MOFs [J. Am. Chem. Soc., 2008, 130, 13850] makes them ideal materials for functionalization purposes aimed at introducing active centres for potential application in heterogeneous catalysis. We previously demonstrated that a small fraction (up to 10%) of the linkers in the UiO-67 MOF can be replaced by bipyridine-dicarboxylate (bpydc) moieties exhibiting metal-chelating ability and enabling the grafting of Pt(II) and Pt(IV) ions in the MOF framework [Chem. Mater., 2015, 27, 1042] upon interaction with PtCl₂ or PtCl₄ precursors. Herein we extend this functionalization approach in two directions. First, we show that by controlling the activation of the UiO-67-Pt we can move from a material hosting isolated Pt(II) sites anchored to the MOF framework with Pt(II) exhibiting two coordination vacancies (potentially interesting for C–H bond activation) to the formation of very small Pt nanoparticles hosted inside the MOF cavities (potentially interesting for hydrogenation reactions). The second direction consists of the extension of the approach to the insertion of Cu(II), obtained via interaction with CuCl₂, and exhibiting interesting redox properties. All materials have been characterized by in situ X-ray absorption spectroscopy at the Pt L₃- and Cu K-edges.

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1. Introduction

Metal–organic frameworks (MOFs) are crystalline, porous solids consisting of metal ions or clusters, coordinated with organic molecules. The large number of combinations for inorganic and organic building units offers an almost infinite variety of structural solutions with a wide range of properties.^1–7 An additional degree of freedom is the functionalization of the MOFs that further extends the range of possibly obtainable structures.^8–14 For about one decade, MOFs have been considered as potential candidates for shape-selective heterogeneous catalysis for reactions running at mild conditions.^15–24

As the metal sites in the cornerstones of most of the MOF structures show a maximum of one coordination vacancy (see e.g. the HKUST-1 (ref. 25–27) and the CPO-27, or MOF-74,^28–31 cases), non-functionalized MOFs have limited application in catalysis, where at least two coordination vacancies are required in the active site. Consequently, functionalization represents an attractive way to introduce active sites in MOF structures.

The recently discovered UiO-66, -67 and -68 classes of iso-structural MOFs are obtained by connecting Zr₆O₄(OH)₄ inorganic cornerstones with 1,4-benzene-dicarboxylate (bdc), 4,4′-biphenyl-dicarboxylate (bpdc) or 4,4′-terphenyl-dicarboxylate (tpdc) linkers, for the UiO-66, UiO-67 and UiO-68 MOFs, respectively.^32–34 Due to their outstanding stability at high temperatures, high pressures and in the presence of different solvents, these materials are among the few MOFs already commercialized for potential applications in the fields of catalysis, gas storage, and gas purification. For the same reasons the UiO-66, -67 and -68 family has already been subjected to several functionalization procedures, involving both the Zr₆O₄(OH)₄ inorganic cornerstone and the organic linkers, as well as intentionally toned defect insertion.^35–38 As far as the inorganic cornerstone functionalization is concerned, Zr atoms have been partially or totally substituted with Hf^39,40 or Ce^41–44 atoms. Substitution with cerium allows alteration of the Ce(IV) ↔ Ce(III) redox chemistry, providing some reactivity to the cornerstone. Coming to the linker functionalization, several routes have been undertaken, including: (i) grafting of –NH₂, –NO₂, –Br groups σ-bonded to the bdc ring;^45–49 grafting of Cr(CO)₃ complexes π-bonded to the bdc ring;⁵⁰ (iii) encapsulation of Pt,^51,52 Pd,^53,54 Au,⁵⁵ AuPd,⁵⁶ Ru,⁵⁷ Ag,⁵⁸ or metal nanoparticles (NPs) inside the tetrahedral and octahedral MOF cavities. Moreover, for UiO-67, the substitution of a small fraction of bpdc linkers by bipyridine-dicarboxylate (bpydc) moieties, exhibiting metal-chelating ability, enables the path for post-synthetic grafting of metal ions in the MOF framework upon interaction with metal precursors.^51,59,60

Herein we extend this functionalization approach into two directions. First, by tuning the H₂ flow during the activation procedure of the UiO-67-Pt we can move from a material hosting isolated framework Pt(II) exhibiting two coordination vacancies (potentially interesting for C–H bond activation) to the formation of very small Pt NPs hosted inside the MOF cavities (potentially interesting for hydrogenation reactions). The second direction consists of the extension of the approach to the insertion of Cu(II), obtained via interaction with CuCl₂, leading to interesting redox properties already evidenced in other Cu-containing MOFs.^61–63

The fact that MOFs are mainly constituted by low Z elements (C, O, N, H) implies that they are almost transparent to hard X-rays;^64,65 this allows the collection of high quality transmission X-ray absorption spectra at the metal K- or L-edges, characterized by an optimized edge jump Δμx as high as 1.0–1.5, resulting in accurate data, analyzable up to 15–20 Å⁻¹ (vide infraFig. 3a). This makes X-ray absorption techniques both in the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) regions a technique of choice in characterizing the electronic and structural configuration of metal centers hosted in MOF structures, as testified by many previous studies.^{28,29,32–34,40,48,59,64,66–90} In this study we have characterized Pt- and Cu-functionalized UiO-67 MOFs via in situ X-ray absorption spectroscopy at the Pt L₃- and Cu K-edges, respectively.

2. Experimental and methods

2.1. Materials

All chemicals were used as received. ZrCl₄ (Sigma) must be dry and of high purity for successful synthesis.

2.1.1. Synthesis of 2,2′-bipyridine-5,5′-dicarboxylic acid. In a 25 mL beaker glass, 6.0 g HNO_3(aq) (65%) was slowly added to 9.0 mL deionized water to obtain 26 wt% HNO₃. 1.50 g 5,5′-dimethyl-2,2′-bipyridine was added and quickly dissolved in the acid. The solution was transferred to a 25 mL Teflon-lined autoclave which was heated to 160 °C and kept at that temperature for 16 hours. The autoclave was allowed to slowly cool to room temperature before opening. A crystalline, weakly yellow powder was isolated by filtration and then washed 5 times with approximately 50 mL portions of water. The filtrate was cooled to 273 K and carefully brought to pH 7 by addition of saturated NaHCO₃ solution, at which a weakly yellow precipitate was formed. This solid was isolated by filtration and washed in the same manner as above. ¹H-NMR confirmed that the two solids were both pure H₂bpydc. The yield was 1.74 g (92%).

Crystals of H₂bpydc suitable for single crystal XRD were obtained directly from the autoclave. A complete data set was acquired on a Bruker D8 Venture equipped with a Photon 100 detector, using Mo Kα radiation (λ = 0.71073 Å). Data reduction was performed with the Bruker Apex3 Suite, the structure was solved with ShelxT⁹¹ and refined with ShelxL.⁹² Olex2 was used as the user interface.⁹³ The H₂bpydc molecules form 1D H-bonded chains analogous to the previously reported structure,⁹⁴ but the packing mode is slightly different.

2.1.2. Synthesis of UiO-67-bpy(PtCl₂) by postsynthetic modification. 116 μL (6.5 mmol) H₂O, 502 mg (2.15 mmol) ZrCl₄ and 2.63 g (21.5 mmol) benzoic acid was added to 50 mL of DMF in a beaker glass, and the mixture was stirred until a clear solution was obtained. It was then heated to 393 K while stirring gently. 469 mg (1.94 mmol) H₂bpdc and 110 mg (0.22 mmol) H₂bpydc was then added, and a clear solution was quickly obtained. The solution was transferred to a round-bottom flask and kept at 393 K for 72 hours while stirring in a reflux apparatus. A colorless powder was isolated by filtration, and then immediately suspended in 20 mL fresh DMF while still wet. The suspension was kept at 373 K while stirring to extract unreacted precursor and modulator from the pores. The powder was isolated by filtration, washed twice with 15 mL portions of hot DMF (373 K), 5 times with 30 mL portions of dry acetone, and then dried in air at 423 K.

The material, 1000 mg, was submerged in a solution of 112 mg K₂PtCl₄ in 20 mL DMF and kept at 373 K for 24 hours while stirring. A bright yellow powder was isolated by filtration, washed twice with 50 mL portions of hot DMF (373 K), 2 times with 50 mL portions of dry acetone, and then dried in air at 423 K.

2.1.3. Synthesis of UiO-67-bpy(CuOHCl) by postsynthetic modification. The starting UiO-67-bpy MOFs were synthesized by a standard solvothermal method, as described in more detail in previous studies,⁵⁹ by reacting ZrCl₄ with different mixtures of H₂bpdc and H₂bpydc linkers (10 wt% of bpydc) in a solution of dimethylformamide (DMF). 5 molar equivalents of benzoic acid were added to obtain a modulator effect, which resulted in a porous MOF with monodispersed particles and well-defined crystallinity.⁹⁵ The Cu-functionalized Cu-UiO-67-bpy derivatives were prepared employing a post synthesis modification approach, by submerging the MOF powder in solutions of CuCl₂ dihydrate in 2-propanol, and heating the solution at reflux conditions for one hour. The resulting bright green powders were washed in three cycles with 2-propanol.

2.2. Pt L₃- and Cu K-edge XAS data collections, EXAFS data analysis

Pt L₃-edge (11 560 eV) and Cu K-edge (8980 eV) XAS data were collected at the I811 beamline of the Max Lab II source (Lund, Sweden).⁹⁶ Max Lab II was operated at 1.5 GeV with a uniform current between 250 and 100 mA. The white beam produced by a liquid He-cooled superconducting wiggler was monochromatized by a horizontally sagittally focused double-crystal Si(111) monochromator. At the Cu K-edge the monochromator was detuned to 20% to minimize the third harmonic, while this procedure is not needed at the Pt L₃-edge. Spectra were collected in transmission mode using 30 cm ionization chambers for I₀ and I₁; the intensity I₂ transmitted by a Pt (or Cu) reference foil located after the sample was measured using a photodiode, and the resulting spectrum was employed for energy alignment purposes.

All XAS spectra were measured employing a home-made cell allowing sample activation in temperature and gas dosage under in situ or operando conditions,⁹⁷ monitoring the evolution of the XAS features while controlling the temperature and gas feed. The XAS data reduction and EXAFS extraction procedure was performed using the Athena codes.⁹⁸ The parametric EXAFS data analysis was performed with IFEFFIT⁹⁹ code that employs phases and amplitudes computed by FEFF6 code,^100,101 as detailed elsewhere.⁵⁹

2.3. Pt L₃-edge XANES simulations

For all clusters simulating the local environments of Pt atoms hosted inside the UiO-67-Pt MOF and successive modification by chemical reactivity with H₂, Br₂ and toluene-3,4-dithiol (H₂tdt), the ground-state electronic structures and atomic geometries were calculated using the hybrid DFT B3LYP level of theory¹⁰² using the ADF-2015 program package.^103,104 The QZ4P basis set was used in all calculations. Thus, each atomic orbital was represented as a combination of four Slater type orbitals with different exponential powers and four polarization functions. We first obtain eigenvalues and corresponding wave functions. The matrix elements are then evaluated for transitions between the 2p core level and unoccupied MOs using dipole transition operators. In order to compare with experimental XANES spectra, a convolution of calculated matrix elements was performed with a Lorentzian profile using the energy-dependent line width. In the pre-edge region, the width of the Lorentzian profile corresponds to a core hole lifetime broadening for Pt. This value is then increased in higher energy intervals with a smooth arctangent function. The parameters of the matrix element calculations (grid step, size number of unoccupied MOs) and energy convolution are fixed once for all complexes. The calculated spectra were subsequently aligned according to the energy value of the Pt 2p orbital, thus reproducing the chemical shift for different species. A rigid shift with the identical value was applied for all spectra in order to align the energy scale between experimental data and theoretical calculations.

3. Results and discussion

3.1. Pt-functionalization of UiO-67 MOF: state of the art

Introducing a chemically active Pt site as part of the UiO-67 framework is of great interest as platinum has rich redox chemistry, showing 0, II and IV stable oxidation states. Moreover, certain square planar Pt(II) coordination complexes are known to be active in C–H bond activation,¹⁰⁵ for example see the scheme reported in Fig. 1b and c. In particular, dichlorobipyrimidyl platinum(II), PtCl₂(BPYM), performs the catalytic oxidation in fuming or concentrated sulfuric acid, achieving high yields of methanol with selectivity higher than 90%.^106,107 It is consequently of potential interest to investigate the possibility to heterogenize such a process anchoring the active Pt(II) complex on some high surface area material such as that recently shown by the group of Schüth, for polymers first^108,109 and for N-doped carbons¹¹⁰ successively, or as done more recently by Øien et al.⁵⁹ who have succeeded in functionalizing UiO-67 with (H₂bpydc)PtCl₂ or (H₂bpydc)PtCl₄ units, substituting 10% of the standard bpdc linkers. The authors used EXAFS, XANES and valence-to-core resonant inelastic X-ray scattering techniques to prove the insertion of Pt atoms in the expected framework position of UiO-67 (see Fig. 1c).

Fig. 1 Part (a): structure of the standard bpydc linker used for the synthesis of the non-functionalised UiO-67 MOF. Part (b): structure of the (H₂bpydc)PtCl₂ linker inserted in the UiO-67 structure showing 2 N and 2 Cl in the first coordination shell of Pt(II) and showing the distortion induced on the two rings by N/C substitution and Pt insertion. Part (c): three-dimensional representation of the Pt(II)-functionalized UiO-67 MOF. Colour code: H white, C grey, N blue, O red, Cl green, Pt cyan. Previously unpublished figure.

Using EXAFS and XANES, the local coordination environment and the oxidation state of Pt can be monitored under in situ conditions. The elimination of chloride ligands from Pt in a continuous gas flow of diluted H₂ (3% H₂ in He) have been monitored by EXAFS during temperature ramping (Fig. 2a). The spectrum collected at room temperature (black curve) exhibits both the first shell Pt–N and the Pt–Cl contributions centred around 1.5 and 1.9 Å in the phase uncorrected Fourier transform (FT), highlighted by vertical blue and green dashed lines, respectively. Upon increasing the temperature, both contributions decrease in intensity because of the increased Debye–Waller factors (σ_N² and σ_Cl²). Starting from about 600 K the Pt–Cl contribution shows a much more relevant decrease in temperature than the Pt–N contribution, suggesting that the system starts losing chlorine ligands. A standard EXAFS analysis failed because of the high correlation between the coordination numbers (N_Pt–N; N_Pt–Cl) and the thermal parameters (σ_N²; σ_Cl²).⁵⁹ The problem was solved by applying an advanced data analysis approach briefly summarized hereafter.

Fig. 2 Part (a): k³-weighted, phase uncorrected FT of Pt L₃-edge EXAFS spectra collected during the in situ H₂-TPR experiments on the PtCl₂(H₂bpydc) functionalized UiO-67-Pt(II) MOF in the 300 K (black spectrum)–750 K (blue spectrum) range performed with diluted H₂ flow (3% H₂ in He). Part (b): quantitative data analysis of the set of spectra shown in part (a) by modelling the temperature dependence of the Debye–Waller factors (σ_N² and σ_Cl²) based on the Einstein model. Previously unpublished figure replotting data published in ref. 59.

First, we worked only on the sub-set of data in the interval between RT and 473 K. In this temperature range no bond breaking occurs, so it was possible to fix N_N = N_Cl = 2. On that sub-set of in situ EXAFS data a parametric refinement was performed, commonly employed in XRPD Rietveld refinements,^111,112 adopting the Einstein model to describe the temperature dependence of both σ_N² and σ_Cl² factors. The Einstein model approximates the vibrational density of states as a Dirac delta function spiked at a single frequency named the Einstein frequency (ω_E). The model assumes that the Pt–N pairs (or Pt–Cl) behave as a quantum harmonic oscillator of mass equal to the reduced mass of the atomic pair (M = 13.070 and 30.004 amu for the Pt–N and Pt–Cl pairs, respectively). Under such assumptions, the σ²(T) behaviour is straightforwardly determined by the only ω_E parameter according to eqn (1):^59,113

Θ _E is the Einstein temperature of the Pt–N (or Pt–Cl) bond, related to the Einstein frequency by the relationship: ħω_E = k_BΘ_E, where ħ = 1.055 × 10⁻³⁴ J s is the reduced Planck constant and k_B = 1.38 × 10⁻²³ J K⁻¹ is the Boltzmann constant.

This approach allowed us to reduce the number of parameters used to optimize the thermal factors of the series from ∼40 to only two, Θ_E(Pt–N) and Θ_E(Pt–Cl), with a consequent reduction of the correlation among the optimized parameters and thus a reduction of the relative error bars. Once the Einstein temperatures Θ_E(Pt–N) = (709 ± 63) K and Θ_E(Pt–Cl) = (333 ± 9) K were obtained, the dependence of both σ_N² and σ_Cl²vs. T was straightforwardly obtained viaeqn (1) and extrapolated on the whole set of data (i.e. also above 473 K). This strategy allowed stable fits in the whole temperature range while optimizing both N_N and N_Cl, as shown in Fig. 2b. From this data analysis, it is evident that both N_N and N_Cl are stable to the stoichiometric values of 2.0 up to 575 K, when they start to decrease together. However, while N_Cl decreases almost linearly to 0.4 at 750 K, N_N undergoes a fast decrease to 1.6 at 610 K and then remains almost stable, with a value at 750 K of 1.4. This means that a prolonged activation in diluted H₂ of the UiO-67-Pt(II) MOF in the 610–640 K interval will result in a minimal loss of Pt-sites, which will lose the Pt–N connection with the framework, but break of an important fraction of the Pt–Cl bonds. The experiment reported in Fig. 2 proved that this activation temperature interval is ideal to obtain a material where most of the Pt species are still linked to the MOF framework, exhibiting the coordination vacancies needed to make the UiO-67-Pt(II) material a potential heterogeneous catalyst.⁵⁹ The presence of coordination vacancies at platinum sites was also directly testified by IR spectroscopy of adsorbed CO. No evidence of a Pt–Pt signal of an aggregated platinum phase was observed in the experiment reported in Fig. 2.

EXAFS was also used to prove the high reactivity of the Pt(II) species hosted in the UiO-67 MOF. We followed the liquid-phase ligand exchange with toluene-3,4-dithiol (H₂tdt) and the liquid-phase oxidative addition of Br₂ to Pt (see Fig. 3). All the observed reactions take place without any degradation of the framework, as testified by parallel XRPD experiments.

Fig. 3 Part (a): experimental Pt L₃-edge k³χ(k) spectra of UiO-67-Pt(II) before (black line) and after interaction with H₂tdt (red line) and Br₂ (blue line). Part (b): modulus of the k³-weighted, phase uncorrected FT of the experimental EXAFS spectra reported in part (a), solid lines, same color code as in part (a). The corresponding best fits are also reported, as dashed lines of similar colour. Part (c): schematic representation of the reactivity of Pt(II) species in functionalized UiO-67-Pt MOFs that has been highlighted in the EXAFS study reported in Fig. 2 and in parts (a and b) of this figure. The sketched square bi-pyramid represents the octahedral large cavity of UiO-67, measuring about 16 Å in diagonal.^32,34 Previously unpublished figure replotting data published in ref. 59.

Also the XANES part of the XAS spectrum is sensitive to the changes undergone by the local environment of Pt(II) along the chemical reactions reported in Fig. 3. Upon a change in the Pt oxidation state, the Pt L₃-edge XANES will show a very small edge shift while it will exhibit a noticeable variation of the intensity of the “white-line” peak.^59,114–116 Indeed, the XANES part of the Pt L₃-edge mainly derives from the promotion of core 2p_3/2 electrons into empty 5d_3/2, 5d_5/2 and 6s valence states, so mainly probing the unoccupied density of 5d-states and partially 6s-states. The XANES spectra reported in Fig. 4a for the UiO-67-Pt(II) MOF before (red line) and after interaction with H₂tdt (blue line) and Br₂ (green line) clearly follow this phenomenological trend. In particular, interaction with H₂tdt (blue spectrum in Fig. 4a) does not affect the white line intensity, affecting only the post edge and EXAFS region of the spectrum; on this basis it was concluded that a ligand exchange reaction occurs where two Cl ligands are exchanged with the two S atoms of the bulky H₂tdt unit. Conversely, interaction with Br₂ (green spectrum in Fig. 4a) results in a significant increase in the white line intensity, testifying an oxidation process from Pt(II) to Pt(IV).^59,116

Fig. 4 Part (a): experimental Pt L₃-edge XANES spectra of UiO-67-Pt(VI) (green) and of UiO-67-Pt(II) before (red line) and after interaction with H₂tdt (blue line) and Br₂ (green line). Part (b): as for part (a), theoretical spectra computed with ADF code.

On a more quantitative ground, simulation of the XANES spectra, on the H₂bpydcPtCl₄, H₂bpydcPtCl₂, H₂bpydcPt-tdt and H₂bpydcPtBr₄ molecular fragments (Fig. 4b), was able to reproduce correctly the variation of the white line intensity, and post edge features.¹¹⁶ The data reported in Fig. 4 testifies the potentialities of the XANES simulations.¹¹⁷

3.2. Tuning the Pt site population inside UiO-67-Pt MOF by controlled H₂-TPR

The interesting results obtained by us in the Pt-functionalisation of UiO-67 (here summarized in Section 3.1), together with the relevance of hosting reactive metal NPs inside the MOF cavities,^51–58 encouraged us to investigate different activation conditions in order to be able to tune the population of Pt phases inside functionalised UiO-67, from isolated Pt sites anchored to the MOF framework with Pt exhibiting two coordination vacancies (potentially interesting for C–H bond activation) to small Pt NPs (potentially interesting for hydrogenation reactions).

In the following two subsections, we report the XANES and EXAFS results, and the corresponding data analysis, obtained by thermal activation of the functionalised UiO-67-Pt MOF under an inert flow (He, 3.2.1) and under a concentrated H₂ flow (10% H₂ in He, 3.2.2). In both cases the EXAFS data have been analysed using a parametric refinement allowing us to obtain, along the activation procedure, the first shell Pt coordination numbers (N_Pt–N, N_Pt–Cl and, when present, N_Pt–Pt) with the minimal correlation possible with the corresponding Debye–Waller parameters. From such data, the fractions of the different platinum phases present in the sample have been obtained, as outlined hereafter. The following nomenclature has been adopted for the different relative fractions of Pt species formed in the experiments: f_bpyCl2 corresponds to Pt atoms coordinated to the bpydc linker in the MOF framework, still containing the Cl ligands; f_bpy stands for Pt atoms coordinated to the bpydc linker in the MOF framework after the loss of two Cl ligands; f_extra represents non-aggregated extra-framework Pt atoms; and f_NPs stands for extra-framework Pt atoms aggregated in NPs. While the f_bpyCl2, f_bpy and f_NPs fractions correspond to well defined Pt species, f_extra does not. Although it is fully reasonable to infer that, once the Pt–N bonds within the framework are broken, Pt atoms will behave as isolated species for a while before being incorporated into a pre-existing NP (or before interacting with other isolated Pt atoms to produce a new NP), the local environment of such Pt species is very difficult to predict. Most probably such Pt atoms will experience a large variety of different local environments resulting in an average contribution to the measured EXAFS spectrum that is very low because of heterogeneity. This fact has been very well documented by EXAFS studies on Fe-substituted zeolites showing that the amplitude of the experimental EXAFS signal is strongly dimmed once Fe atoms migrate from the framework into extra-framework sites,^118–120 contributing only in the very short k-range (typically up to 6 Å⁻¹).

3.2.1. Thermal activation of UiO-67-Pt under inert atmosphere. Fig. 5a and b report the evolution of the Pt L₃-edge XANES and EXAFS spectra during in situ thermal activation of the functionalized UiO-67-Pt MOF under an inert He flow in the 300–750 K range. The evolution of both XANES and EXAFS spectra observed under an inert flow is very similar to what is observed working under a diluted H₂ flow,⁵⁹ see Fig. 2a for the EXAFS part. Also, in this case we did not observe any evidence of Pt–Pt bonds in the FT of the EXAFS data (Fig. 5b), evidencing the absence of formation of Pt metal NPs. According to this evidence, the fractions of the remaining three Pt phases are obtained as follows:

f_NPs = 0; f_bpyCl2 = N_Cl/2; f_extra = (2 − N_N)/2; f_bpy = 1 − f_bpyCl2 − f_extra (2)

Fig. 5 Part (a): evolution of the Pt L₃-edge XANES spectra during in situ thermal activation of the functionalized UiO-67-Pt MOF under an inert He flow in the 300–750 K range. The inset shows a cartoon description of the dominant final phase (framework-coordinated Pt atom that has lost the two Cl ligands). Part (b): as part (a) for the k³-weighted, phase uncorrected FT of the corresponding EXAFS data. Also highlighted with blue and green dashed lines are the typical positions of the Pt–N and Pt–Cl contributions, respectively. Part (c): fraction of the different Pt phases present in the sample, estimated from the N_Pt–N and N_Pt–Cl coordination numbers obtained from the parametric refinement of the whole set of EXAFS data, according to the assumptions reported in eqn (2).

The analysis of the evolution during the activation experiment of the relative fractions of the different Pt phases performed according to eqn (2) is reported in Fig. 5c. Up to 550 K, within the experimental errors, f_bpyCl2 = 1 and f_extra = f_bpy = 0. Then, f_bpyCl2 starts to rapidly decrease down to 0.1 in an almost linear fashion with the increasing temperature; an opposite trend is observed for f_bpy, reaching 0.6 at 750 K, while f_extra exhibits an abrupt increase at ca. 570 K, and subsequently stabilizes at around 0.3.

3.2.2. Thermal activation of UiO-67-Pt under concentrated H₂ flow. Evolution of the Pt L₃-edge XANES and EXAFS spectra during the in situ H₂-TPR experiment of the PtCl₂(H₂bpydc) functionalized UiO-67-Pt(II) MOF performed in a concentrated H₂ flow (10% H₂ in He) is reported in parts (a) and (b) of Fig. 6, respectively. In this case, the experiment consists of a linear temperature increase up to 625 K with a heating ramp of 5 K min⁻¹, followed by almost 3 h in isotherm (see the open grey triangles in Fig. 6c).

Fig. 6 Part (a): evolution of the Pt L₃-edge XANES spectra during the in situ H₂-TPR experiment of the PtCl₂(H₂bpydc) functionalized UiO-67-Pt(II) MOF performed in a concentrated H₂ flow (10% H₂ in He). The inset shows a cartoon description of the dominant final phase (metal NPs). Part (b): as part (a) for the k³-weighted, phase uncorrected FT of the corresponding EXAFS data. Also highlighted with blue, green and orange dashed lines are the typical positions of the Pt–N, Pt–Cl and Pt–Pt contributions, respectively. Part (c), full black, red and orange symbols, left ordinate axis: fraction of the different Pt phases present in the sample, estimated from the N_Pt–N, N_Pt–Cl and N_Pt–Pt coordination numbers obtained from the parametric refinement in the whole set of EXAFS data, according to the assumptions reported in eqn (5). Part (c), open grey triangles, right ordinate axis: evolution of the sample temperature with time, highlighting the linear increase up to 625 K in 65 min followed by 2 h 55 min in isotherm.

Different to what was observed in the previous cases, where N_Pt–Cl started to decrease significantly before N_Pt–N, in the present case both N_Pt–N and N_Pt–Cl remain, within the associated error bars, equal to 2 during the whole heating ramp up to 625 K (indicating f_bpyCl2 = 1) and start to decrease in the same way during the isotherm (data not reported for brevity). This indicates that, under a concentrated H₂ flow, when framework Pt(II) species lose the Pt–Cl bond they almost simultaneously also lose the Pt–N bond and thus the connectivity with the MOF framework, thus becoming extra-framework species. Based on this evidence, we can assume f_bpy = 0, while f_bpyCl2 can be determined by averaging the contribution of both Pt–Cl and Pt–N bonds: f_bpyCl2 = (N_Cl/2 + N_N/2)/2.

A second important difference, very evident in Fig. 6b, is the structured signal in the 2.5–3.5 Å range, typical of the single and multiple scattering Pt–Pt contributions of the fcc metal phase, testifying the formation of Pt NPs. Hence, the EXAFS data analysis also provides a first shell N_Pt–Pt coordination number. Unfortunately the fraction of Pt atoms in the NPs phase cannot be straightforwardly deduced from the N_Pt–Pt number obtained from the EXAFS data analysis, because the average coordination number of a fcc NP increases with the particle size, asymptotically reaching the value of 12 of the bulk in the first shell.^121–125 Consequently, the N_Pt–Pt ↔ f_NPs relationship can be established only if the particle size distribution has been determined by an independent SAXS^126–133 or TEM^{74,124,133–135} study.

Indeed, at the end of the H₂-TPR experiment reported in Fig. 6, the sample was cooled down to 300 K under He flow, recovered and subjected to a TEM study (see Fig. 7b and c for two selected micrographs taken at different magnifications), giving the particle size distribution w_P(d_i) reported in Fig. 7a (orange bars) and obtained by selecting the NPs in 12 classes of amplitude 1 nm. w_P(d_i) reports the fraction of NPs that have a diameter d falling in the class centred in d_i and results in an average NP diameter of 〈d〉_P = 3.5 nm with a standard deviation of 1.2 nm. The orange NP distribution w_P(d_i) is the standard output from a TEM analysis, but is however not the correct one to be used to extrapolate the average coordination number measured in the EXAFS study. Indeed, the measured EXAFS signal is the average signal coming from all the Pt atoms in the sample. This means that a large particle contributes more to the EXAFS signal than a small one, because it contains more atoms. The orange w_P(d_i) distribution, weighted by particles, has to be corrected by weighting for the corresponding volume fraction into w_V(d_i), defined as follows:

Fig. 7 Part (a): Pt NP distributions (weighted by number of particles (orange) or by volume (blue): w_P(d_i) and w_V(d_i), respectively, see eqn (3)) obtained from the TEM analysis of the PtCl₂(H₂bpydc) functionalized UiO-67-Pt(II) MOF measured after the H₂-TPR XAS experiment reported in Fig. 6. Parts (b) and (c) report two micrographs at different magnifications.

The volume-weighted NP distribution, obtained from w_P(d_i) viaeqn (3), is reported in Fig. 7a (orange bars) and results in a (volume-weighted) average NP diameter of 〈d〉_V = 5.5 nm with a standard deviation of 1.5 nm. Comparing w_P(d_i) and w_V(d_i), the more important role played by the high-d region is evident. Knowing the average coordination number of an fcc NP with a given diameter, N(d_i), the volume-weighted NP distribution allows us to obtain the average coordination number of the whole particle distribution estimated from the TEM analysis (N_TEM) as:

Once N_TEM has been determined, the fraction of Pt atoms occurring as NPs is obtained as f_NPs = N_Pt/N_TEM, while the fraction of Pt atoms in non-aggregated extra-framework positions is obtained by the difference between the unit and the sum of the other two fractions. Summarizing:

f_bpy = 0; f_bpyCl2 = (N_Cl/2 + N_N/2)/2; f_NPs = N_Pt/N_TEM; f_extra = 1 − f_bpyCl2 − f_NPs. (5)

The evolution of the relative fractions of the different Pt phases during the in situ H₂-TPR experiment in a concentrated H₂ flow is summarized in Fig. 6c. The weakness of this approach is the fact that we assume the same NP size distribution along the whole H₂-TPR experiment. Nonetheless, this approach provides a correct evaluation of f_NPs for the last points of the treatment, which is the required information in the view of potential application in catalysis.

Within the experimental error, along the whole heating ramp up to 625 K, Pt atoms maintain the same square planar coordination with two nitrogen and two chlorine ligands in their first coordination shell. During the isotherm, they break simultaneously the bonds with both the Cl ligands and the MOF framework, becoming extra-framework species, that progressively aggregate into NPs. After 3 h of isotherm at 625 K the distribution among the different Pt phases reaches an asymptotic equilibrium.

3.3. Structure and redox properties of isolated Cu sites in UiO-67-Cu MOF

The interesting results obtained on the Pt-functionalised UiO-67 MOF encouraged us to extend the study using other metals. Because of its interesting redox chemistry, our first choice was on copper. We consequently prepared a Cu-functionalized UiO-67 MOF by contacting the bpydc-containing UiO-67-bpy MOF with a CuCl₂ dihydrate precursor.⁶⁰ The synthesis method succeeded in grafting isolated Cu(II) ions on the functionalised bpydc linkers of the UiO-67 framework but, contrary to what was expected, the local environment of copper was not composed of two nitrogen and two chlorine atoms as it was the case for the Pt-functionalised MOF (see Fig. 1b and c).

Indeed, just from a first simple comparison of the XANES (Fig. 8a) and EXAFS (Fig. 8b) data collected on as-prepared UiO-67-Cu (black) and on the tBbpyCuCl₂ model compound (green, where Cu(II) is in a square planar geometry coordinating 2 N and 2 Cl atoms, see Fig. 8c) clearly testifies that the local environment of Cu atoms in the two materials is significantly different. The EXAFS data clearly shows a lack of an EXAFS signal in the (1.5–2.1) Å region in the MOF, suggesting the presence of only one Cl atom in the first coordination shell of the Cu(II).

Fig. 8 Part (a): Cu K-edge XANES spectra of the tBbpyCuCl₂ model compound (green) and of the UiO-67-Cu MOF before and after the in situ thermal activation in an N₂ flow (black and blue spectra, respectively). Part (b): as part (a) for the k³-weighted, phase uncorrected FT of the corresponding EXAFS data. Also highlighted with blue and green dashed lines are the typical positions of the (Cu–N or Cu–OH) and Cu–Cl contributions, respectively. Part (c): structure of the tBbpyCuCl₂ model compound as obtained from the single-crystal XRD refinement (data collected at 100 K). Parts (d) and (e): DFT-optimized structures of the Cu(II)ClOH(H₂bpydc) and Cu(I)Cl(H₂bpydc) complexes used as a starting point for the EXAFS fits of the UiO-67-Cu MOF before and after the in situ thermal activation. Atom colour code: Cu: orange; O: red; N: blue; Cl: green, C: gray; H: white. Previously unpublished figure reporting spectra and structures from ref. 60.

Fig. 9 reports the XANES spectra (part a) and the k³-weighted, phase uncorrected (part b) FT EXAFS spectra for the exemplificative case of UiO-67-Cu during the thermal treatment in an inert gas flow, from RT (as-prepared state, black) to 523 K (blue). Spectra collected at intermediate temperatures are also reported, as thin gray lines.

Fig. 9 Part (a): evolution of the Cu K-edge XANES spectra during the in situ thermal activation in an N₂ flow of the CuClOH(H₂bpydc) functionalized UiO-67-Cu(II) MOF in the 300–523 K range. The inset shows a cartoon description of the dominant final phase (framework-coordinated Cu(I) species with a Cl ligand). Part (b): as part (a) for the k³-weighted, phase uncorrected FT of the corresponding EXAFS data. Also highlighted with blue and green dashed lines are the typical positions of the (Cu–N or Cu–OH) and Cu–Cl contributions, respectively. Part (c), full black and blue symbols, left ordinate axis: fraction of the different Cu phases present in the sample estimated according to a linear combination of the XANES spectra of the starting and final phases. Part (c), open grey triangles, right ordinate axis: evolution of the sample temperature with time, highlighting the linear increase up to 523 K in 23 min followed by 13 min in isotherm. Previously unpublished figure reporting spectra from ref. 60.

The XANES spectrum for the as-prepared material is characterized by the absence of any defined pre-edge/edge peaks, and by a rather intense white line feature at ∼8996.6 eV, typical of Cu(II) centres coordinated to water molecules and, eventually, OH groups.^136–141 As also supported by the comparison with the tBbpyCuCl₂ model compound, the overall shape and the edge energy position in the XANES spectrum of the as-prepared material are consistent with a 2+ oxidation state of copper. The spectral changes observed during thermal activation involve a clear decrease of the white line feature, accompanied by a significant shift of the absorption edge towards lower energies (ca. 3.6 eV), as evaluated by monitoring the energy position of the first main maximum of the first derivative spectra.^142,143 At the end of the heating ramp, a shoulder starts to be distinguishable in the XANES edge-rising region at ca. 8983 eV, falling in an energy range typical of two- or three-coordinated Cu(I) sites.^144,145 These evidences suggest a ligand loss process associated with the reduction of the starting Cu(II) sites into Cu(I), as thermal treatment proceeds. Such spectral modifications are also accompanied by the sample color change from green to light-green/white as the temperature increases, supporting the formation of coordinatively unsaturated Cu(I) sites after mild thermal treatment in an inert atmosphere.

We succeeded in reproducing the EXAFS and XANES spectra of the as prepared UiO-67-Cu MOF using as a starting model that obtained from the optimization of the [Cu(II)(bpydc)(OH)Cl] complex (see Fig. 8d) at the DFT level of theory with VASP 5.3 code,^146,147 using a kinetic energy cut-off for the plane-wave basis set of 400 eV. The EXAFS-optimized distances were: R_Cu–OH = 1.91 ± 0.01 Å, R_Cu–N = 2.026 ± 0.006 Å and R_Cu–Cl = 2.265 ± 0.006 Å, in reasonable agreement with the DFT-optimized distances.⁶⁰ The same holds for the DFT-optimized [Cu(I)(bpydc)Cl] complex (see Fig. 8e), that was able to correctly reproduce the EXAFS spectrum of the N₂-activated UiO-67-Cu MOF, resulting in R_Cu–N = 1.989 ± 0.008 Å and R_Cu–Cl = 2.249 ± 0.007 Å. The progressive evolution from the [Cu(II)(bpydc)(OH)Cl] complex into the [Cu(I)(bpydc)Cl] complex upon thermal activation in an inert atmosphere has been quantified by linear combination of the XANES spectra and is reported in Fig. 9c.

Further insights into the nature and reactivity of these [Cu(I)(bpydc)Cl] complexes, representing an interesting platform for future experiments on Cu(I)-catalyzed reactions, were achieved by monitoring with XAS and FTIR the interaction of the thermally-treated material with the CO probe molecule. Both techniques pointed out the efficient and reversible formation of Cu(I)-mono-carbonyl adducts with CO, compatible with [Cu(I)(bpydc)(CO)Cl] species as highlighted by EXAFS fitting.

The overall picture is confirmed by XANES simulations performed with the FDMNES code¹¹⁷ for the key experimental conditions investigated, including as-prepared MOFs in air, after thermal treatment in N₂-flux and upon interaction with CO.⁶⁰

4. Conclusions

A post-synthesis functionalisation approach has been successfully employed to graft Pt(II) and Cu(II) atoms on the UiO-67 MOF functionalized with 10% of bpydc ligands. The in situ EXAFS analysis, for all the series of samples at RT, demonstrates that the Pt atoms are grafted to the bpydc-linkers in a square planar coordination with two Cl and two N atomic neighbors. XAS spectroscopy was used in operando conditions to study the activation of UiO-67-Pt functionalized MOFs, demonstrating that, by tuning the activation conditions, two types of catalytically active sites can be formed in the cavities of the MOF: (i) isolated, highly uncoordinated framework [double bond splayed left]Pt species bound to the two N atoms of the functionalising bpydc linkers of the MOF and (ii) Pt NPs. XAS was used for monitoring the temperature-dependent behavior of UiO-67-Pt during the thermal activation process, in different gas feeds (pure He, 3% H₂/He and 10% H₂/He). EXAFS analysis of the data collected during operando H₂-TPR was performed using the Einstein model to predict the temperature dependence of the Debye–Waller factors and to reduce the correlation among the optimized parameters. In a 10% H₂/He flow, EXAFS clearly shows the loss of both Cl and N scattering contributions, while the rising of an intense signal at around 2.7 Å, associated with the Pt–Pt contributions, unambiguously indicates Pt NPs formation. Conversely, in a pure He flow, the only significant change observed during TPR is the progressive decrease of the Pt–Cl single scattering contribution, leading to the conclusion that the Pt grafted to the bpydc-linkers remains naked. In general, Pt sites in UiO-67-Pt MOFs tend to form Pt NPs in the presence of H₂ and remain naked, but still grafted to the bpydc-linkers, in an inert gas flow. This study demonstrates that two families of different catalytically active Pt sites can be generated in UiO-67-Pt MOFs by properly tuning the activation conditions. In situ IR, ex situ TEM and catalytic tests support the operando XAS study.

By combining in situ and operando XAS and FTIR spectroscopy, we also assessed the successful incorporation of well-defined Cu complexes in the UiO-67 framework in the form of the [Cu(II)ClOH(H₂bpydc)] complex, that evolves into [Cu(I)Cl(H₂bpydc)] upon thermal activation in an inert atmosphere. We explored the local coordination geometry, redox properties and reactivity of the dominant Cu species formed in different conditions relevant to potential future applications in catalysis. EXAFS fits and XANES simulations, based on DFT-optimized geometries, yielded detailed structural and electronic information on the major Cu-species formed.

The methodology of the post-synthesis functionalisation of the UiO-67 MOF, here described for PtCl₂ and CuCl₂, has been recently successfully extended using RhCl₃ and IrCl₃ precursors, while a first attempt to insert gold via HAuCl₄ as the precursor failed.¹⁴⁸ The same strategy has been followed by the group of Long, who succeeded in preparing UiO-67-M grafting NiBr₂,¹⁴⁹ CuCl₂,¹⁵⁰ CoCl₂,¹⁵⁰ FeBr₂,¹⁵⁰ and Cr(CO)₄ ¹⁵⁰ on functionalized H₂bpydc linkers in UiO-67. In this regard, very interesting are also the works of the Lin group who synthesized UiO-66-like class of MOFs using long [Ir(ppy)₂(bpy)]⁺-derived dicarboxylate ligands; successive photoreduction of K₂PtCl₄ resulted in Pt nanoparticles hosted inside the large MOFs cavities.⁵¹ Successively, the same group used Re(CO)₃Cl grafted to 4,4′-(2,2′-bipyridine-5,5′-diyl)dibenzoate¹⁵¹ and FeBr₂ grafted to 4,4′-(1,10-phenanthroline-3,8-diyl)dibenzoic acid¹⁵² linkers to obtain metal functionalized MOFs with UiO-66-like structure with interesting catalytic activities. Of interest is also the post-synthesis functionalization strategy used to insert the β-diketiminate functionality in a MOF of UiO-topology. Metalation of the β-diketiminate-MOF with iron, copper, and cobalt metal salts afforded the desired MOF-supported β-diketiminate-M complexes (M = Fe, Cu, and Co).⁸⁸

Acknowledgements

LB, ALB, AAG, AVS and CL acknowledge the Megagrant of the Russian Federation Government to support scientific research at the Southern Federal University, no. 14.Y26.31.0001. KAL acknowledges the support from the Russian Foundation for Basic Research, Project no. 16-32-00572 mol_a. The authors are grateful to MAX-lab for the allocation of the beam time (proposal 20140449) and to Dr Stefan Carlson for the technical support during the experiments at the I811 beamline.

Notes and references

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