Synthesis of 17O-Labeled Cs2WO4 and Its Ambient- and Low-Temperature Solid-State 17O MAS NMR Spectra

Sulphur retention and in-situ preparation of metal sulphide catalysts during activation of petroleum coke

Petroleum coke (petcoke) containing sulphur has limited direct applications, but stockpiling the material creates an environmental issue. Although chemical activation can be used to valorise the petcoke to activated carbon, sulphur is released creating alternative environmental problems. In this study, a new activation method for high sulphur content (∼6.5 wt%) petcoke was developed to retain sulphur and prepare transition metal sulphide catalysts simultaneously. Petcoke was mixed with tungsten and nickel precursors and then activated by KOH at 600 °C in the presence of steam. After washing, the activated petcoke had a sulphur content of 5.1 wt%, which was much higher than that in the absence of steam during activation (0.4 wt%). Sulphur was also retained (>4 wt% of sulphur) when other transition metals including molybdenum and cobalt were used. Characterization by XRD, XPS, and SEM-EDS suggested that sulphur was retained on the activated petcoke in the form of metal sulphides. Further thermodynamic analysis of the system revealed that in the presence of steam an H₂S/H₂ mixture was generated, and this mixture promoted the formation of the metal sulphide species when metal precursors were introduced. The prepared metal sulphide catalysts were active for several reactions including the photoreduction of CO₂. Overall, this study provided an effective method to prepare metal sulphide catalysts from sulphur containing carbonaceous waste.

Unleashing the Potential of 17 O NMR Spectroscopy Using Mechanochemistry

¹⁷ O NMR spectroscopy has been the subject of vivid interest in recent years, because there is increasing evidence that it can provide unique insight into the structure and reactivity of many molecules and materials. However, due to the very poor natural abundance of oxygen-17, ¹⁷ O labeling is generally a prerequisite. This is a real obstacle for most research groups, because of the high costs and/or strong experimental constraints of the most frequently used ¹⁷ O-labeling schemes. Here, we show for the first time that mechanosynthesis offers unique opportunities for enriching in ¹⁷ O a variety of organic and inorganic precursors of synthetic interest. The protocols are fast, user-friendly, and low-cost, which makes them highly attractive for a broad research community, and their suitability for ¹⁷ O solid-state NMR applications is demonstrated.

Direct observation of ¹⁷O-¹⁸⁵/¹⁸⁷Re ¹J-coupling in perrhenates by solid-state ¹⁷O VT MAS NMR: temperature and self-decoupling effects

(17)O MAS NMR spectra recorded at 14.1T and room temperature (RT) for (17)O-enriched samples of the two perrhenates, KReO4 and NH4ReO4, exhibit very similar overall appearances of the manifold of spinning sidebands (ssbs) for the satellite transitions (STs) and the central transition (CT). These overall appearances of the spectra are easily simulated in terms of the usual quadrupole coupling and chemical shift interaction parameters. However, a detailed inspection of the line shapes for the individual ssbs of the STs and, in particular, for the CT in the spectrum of KReO4 reveals line-shape features, which to our knowledge have not before been observed experimentally in 1D MAS NMR spectra for any quadrupolar nucleus, nor emerged from simulations for any combination of second-order quadrupolar interaction and chemical shift anisotropy. In contrast, such line-shape features are not observed for the corresponding ssbs (STs and CT) in the 14.1T RT (17)O MAS NMR spectrum of NH4ReO4. Considering the additional interaction of a combination of residual heteronuclear (17)O-(185/)(187)Re dipolar and scalar J coupling between this spin pair of two quadrupolar nuclei, spectral simulations for KReO4 show that these interactions are able to account for the observed line shapes, although the expected (1)J((17)O-(185/)(187)Re) six-line spin-spin splittings are not resolved. Low-temperature, high-field (21.1T) (17)O VT MAS NMR spectra of both KReO4 and NH4ReO4 show that full resolution into six-line multiplets for the centerbands are achieved at -90°C and -138°C, respectively. This allows determination of (1)J((17)O-(187)Re)=-268Hz and -278Hz for KReO4 and NH4ReO4, respectively, i.e., an isotropic (1)J coupling and its sign between two quadrupolar nuclei, observed for the first time directly from solid-state one-pulse 1D MAS NMR spectra, without resort to additional 1D or 2D experiments. Determination of T1((187)Re) spin-lattice relaxation times, observed indirectly through a 2D (17)O EXSY experiment for NH4ReO4 at several low temperatures, show that the dynamics observed for the ReO4(-) anion in the (17)O VT MAS NMR spectra at low temperatures are caused by self-decoupling of (1)J((17)O-(187)Re). The (1)J((17)O-(187)Re) values determined here for ReO4(-) from solid-state (17)O MAS NMR, along with literature (1)J((17)O-M) values for oxoanions (M being a quadrupolar nucleus) obtained from liquid-state NMR, have allowed correlations to be established between the reduced coupling constant (1)K((17)O-M)=2π(1)J((17)O-M)/(γ17OγMℏ) and the atomic number of M.