Local Platinum Environments in a Solid Analogue of the Molecular Periana Catalyst

Abstract

Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl₂, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl₂. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl₂. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl₂, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl₂, along with differences that account for their respective properties

Covalent Triazine‐based Frameworks—Tailor‐made Catalysts and Catalyst Supports for Molecular and Nanoparticulate Species

“… Electron microscopy images of the atomic dispersion of Pt within the Pt‐CTF (15 wt %) catalyst by Soorholtz et al. ; a) EDX line scan of a catalyst cross section and AC‐HAADF‐STEM images of the catalyst (b–c) as‐made and (d–e) after 4 methane oxidation reaction cycles for 2.5 h at 215 °C and 90 bar CH 4 in oleum (20 wt % SO 3 ). Reprinted with permission.…”

“…However,s ince CTFs are thermally and chemically stable, the immobilized catalystc an be used within the same harsh reaction environmenta nd thuse nables simple workup through filtration.Infact, comparableturnover numbers(TON) to the molecular systems at 75 %s electivity towards methanolh ave been reported for at least five recycling steps, with Pt II being the predominant species( > 95 %) confirmed via XPS. In addition, the platinum sites were analyzed prior to and after catalysis with ac ombination of several sophisticated analytical methods, [84] including solid-state 195 Pt nuclear magnetic resonance (NMR) spectroscopy,a berration-corrected scanning transmission electron microscopy (AC-STEM, Figure 1), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), illustrating nicely the great analytical challenge that has to be overcome to unravel the exact nature and chemical surrounding of such solid molecular catalyst systems. Even thoughc ostly synthesis procedures for CTF materials might hamper scale-up and thus industrial application,t he resultsr egardings tructure-activity correlations of such systems could be transferred and even enhanced further on other N-rich supports for targeted catalystdesign.…”

Multifunctional Single‐Site Catalysts for Alkoxycarbonylation of Terminal Alkynes

“…The O 1s peak at 532.5 eV and S 2p peak at 168.2 eV were associated with the −SO 3 H (Figure 3 C and D). Two peaks at approximately 399.0 and 400.1 eV were ascribed to N atoms in C−N and C−N−Pd, respectively, indicating that N coordinated with the Pd species (Figure E) . The P 2p gives two peaks at approximately 131.4 and 133.8 eV, which correspond to P atoms in C−P and C−P−Pd, respectively (Figure F) .…”

Metal–organic and covalent organic frameworks as single-site catalysts

“…7(a) ). 167 – 169 K 2 [PtCl 4 ] was chosen as the platinum precursor and its successful coordination to the bipyridine moieties within the CTF was confirmed by XPS. Catalysed methane to methanol oxidation was conducted in an oleum medium at high temperature and pressure (215 °C and 40 bar).…”