Michael Brorson

Activities of unsupported second transition series metal sulfides for hydrodesulfurization of sterically hindered 4,6-dimethyldibenzothiophene and of unsubstituted dibenzothiophene

The applicability of transition metal sulfides (TMS) from the second transition series in deep hydrodesulfurization (HDS) was examined and compared to that of a traditional, supported CoMo/Al2O3 catalyst. Sulfides of Nb, Mo, Ru, Rh and Pd were studied for HDS of dibenzothiophene (DBT) and 4,6‐dimethyldibenzothiophene (4,6‐Me2DBT). Measurements were carried out with unsupported TMS samples at different temperatures and H2S partial pressures. The trend in DBT HDS activities agreed quite well with those found by previous authors. It was furthermore found that the activities of the metal sulfides towards the sterically hindered molecule 4,6‐Me2DBT closely followed those for DBT. This is somewhat surprising since the direct sulfur abstraction route was of major importance for DBT while the prehydrogenation route, in which ring‐hydrogenation in the DBT skeleton precedes desulfurization, was prevalent for 4,6‐Me2DBT. This suggests that common steps are involved in the two routes. For the unsupported metal sulfides, ring‐hydrogenated but not desulfurized DBT and 4,6‐Me2DBT products were found in much larger amounts than for supported and promoted MoS2‐based catalysts. This can be rationalized as being due to a relatively higher hydrogenation/desulfurization selectivity ratio for the different transition metal sulfides. Inhibition by H2S was found to be most pronounced near the center of the transition series.

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.

Site preferences of NH4+ in its solid solutions with Cs2WS4 and Rb2WS4 from multinuclear solid-state MAS NMR.

Solid solutions of NH(4)(+) in Cs(2)WS(4) and Rb(2)WS(4) are obtained by precipitation/crystallization from aqueous solutions. By means of (14)N, (87)Rb, and (133)Cs magic angle spinning NMR, compositions and extraordinarily accurate NH(4)(+)-site preferences are established for these materials.

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

Following several seemingly straightforward but unsuccessful attempts to prepare a sample of (17)O-enriched Cs(2)WO(4), we here report a simple, aqueous procedure for synthesis of pure Cs(2)WO(4), if so desired, enriched in (17)O. The purpose for the preparation of (17)O-enriched Cs(2)WO(4) is to record its solid-state (17)O MAS NMR spectrum, which would allow for a determination of its quadrupole coupling and chemical shift anisotropy (CSA) parameters and thereby for a comparison with the corresponding (33)S and (77)Se parameters in the related compounds M(2)WS(4) and M(2)WSe(4). These compounds are isomorphous and crystallize in the orthorhombic space group Pnma, and Cs(2)WO(4) turns out to be the only alkali metal tungstate with the Pnma crystal structure. Therefore, it has been mandatory to use Cs(2)WO(4) and not K(2)WO(4) (space group C2/m) for which CSA data have previously been published, to achieve a reliable comparison with the (33)S and (77)Se data and thus allow assignment of the three different sets of (17)O NMR parameters to the three distinct oxygen sites (O(1,1), O(2), and O(3)) in the Pnma crystal structure of Cs(2)WO(4). Because the ambient temperature (17)O MAS NMR spectrum of Cs(2)WO(4) exhibits a dynamically broadened singlet, resorting to low-temperature (-83 °C) conditions at 21.15 T was necessary and resulted in a high-resolution (17)O MAS spectrum that allowed both (17)O quadrupole coupling and CSA parameters to be determined. As no quadrupole coupling data were obtained from the earlier investigation on K(2)WO(4), the present results for Cs(2)WO(4) prompted a reinvestigation of the (17)O MAS spectrum for K(2)WO(4), which actually also shows the presence of (17)O quadrupole couplings for all three oxygen sites. These data for Cs(2)WO(4) and K(2)WO(4) are consistent and result in unambiguous assignments of the parameters to the three distinct oxygen sites in their crystal structures.