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 PtCl or PtCl 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 CuCl, and exhibiting interesting redox properties. All materials have been characterized by in situ X-ray absorption spectroscopy at the Pt L- and Cu K-edges.

The duality of UiO-67-Pt MOFs: connecting treatment conditions and encapsulated Pt species byoperandoXAS

“…The two maxima at 2.2 Å and 2.6 Å correspond both to the SS of a single Pt-Pt contribution of Pt metal (a double-shaped modulus of the FT is typical of a single distance in EXAFS when the scattering atom is a high Z one such as e.g. Ag, 155 In 156 or Pt 92 ).…”

“…Fig. 8 presents the distribution of particle size (parts (a), (d) and (g)) weighted by the number (blue bars) and by the volume (green bars) of the particles 92,157 and the TEM microscopy images at low (parts (b), (e) and (h)) and at high (parts (c), (f) and (i)) magnification. In all the samples, Pt-NPs are well dispersed in the MOF matrix and no evidence of NP aggregation has been observed.…”

“…For this reason, when EXAFS and TEM studies are compared, the relevant size distribution is the volume-weighted one (w V (d i ), green histograms in Fig. 8a, d and g), 92,157 that is obtained from the particle weighted one according to eqn (2):…”

“…As for the first shell Pt-Pt coordination number, N Pt , it was fixed based on TEM results that allows us to obtain the average N Pt based on the volume-weighted particle size distribution reported in Fig. 8a, d and g, 92,[157][158][159]161,162 assuming a spherical shape for the Pt-NPs, see eqn (3). The results of the EXAFS analysis of the three samples after the TPR are reported in Table 6, while the quality of the fit can be appreciated in Fig.…”

“…65 In this respect, there are multiple choices of relatively easy ways to functionalize the linker of UiO-67 with metal sites by substituting a fraction of the standard bpdc linkers with a bipyridine-dicarboxylate (bpydc) linker, as described in the Experimental section. In such a way, Fe(II), 86 Ni(II), 87 Cu(II), [88][89][90][91][92] Ru(II), 93 Rh(III), 94,95 Pd(II), 96 Re(I), 93 Ir(III) 93,95 Pt(II), 92,[97][98][99][100] and Pt(IV) 92,98,99 have already been successfully grafted to the two N atoms of the functionalized bpydc linker of different MOFs. In this paper, we present a study done on UiO-67 functionalized with Pt, a metal chosen for its extensive and interesting chemistry in catalysis.…”