Catalysis Science & Technology最新文献

Even though α-arylation of ketones is attractive for direct C-H functionalization of organic substrates, the method largely relies on phosphine-ligated palladium complexes. Only recently, efforts have focused on developing nitrogen-based ligands as a more sustainable alternative to phosphines, with pyridine-functionalized pyridinium amidate (pyr-PYA) N,N'-bidentate ligands displaying good selectivity and activity. Here, we report on a second generation set of catalyst precursors that feature a 5-membered N-heterocycle instead of a pyridine as chelating unit of the PYA ligand to provide less steric congestion for the rate-limiting transmetalation of the enolate. To this end, new heterocycle-functionalized PYA palladium(ii) complexes containing an oxazole (5b), N-phenyl-triazole (5c), N-methyl pyrazole (5d), N-phenyl-pyrazole, (5e), N-xylyl-pyrazole (5f), and N-isopropyl-pyrazole (5g) were synthesized compared to the parent pyr-PYA complex 5a. Less packing of the palladium coordination sphere was evidenced from solid state X-ray diffraction analysis. While the catalytic activity of the oxazole system was lower, all other complexes showed higher activity. In particular, complex 5g comprised of an electron-donating and sterically demanding iPr-pyrazole chelating PYA ligand is remarkably stable towards air and moisture and shows outstanding catalytic activity with complete selectivity (>99% yield) and turnover frequencies up to 1200 h^-1, surpassing that of parent 5a by one order of magnitude and rivalling the most active phosphine-based palladium systems. Kinetic studies demonstrate a first order rate-dependence on palladium and the substrate. Some deviation of linearity together with poisoning experiments suggest a mixed homogeneous/heterogeneous pathway, though the reproducible kinetics of in situ catalyst recycling experiments strongly point to a molecularly defined active species, demonstrating the high potential of PYA-based ligands.

In this work, we study the reducibility of a PdO precursor placed by strong electrostatic adsorption on either NiO or SiO₂ of NiO/SiO₂ obtained by incipient wetness impregnation. The catalysts were characterized by HAADF-STEM, quasi-in situ XPS, CO IR spectroscopy and H₂ chemisorption as a function of the reduction temperature and evaluated for their performance in cinnamaldehyde hydrogenation. PdO on SiO₂ requires reduction at higher temperatures to achieve appreciable rates of cinnamaldehyde hydrogenation. Pd placed on NiO particles dispersed on the SiO₂ support can already be reduced at room temperature and show a higher activity in cinnamaldehyde hydrogenation, which is argued to be due to the higher Pd dispersion obtained at low reduction temperatures.

With the increasing interest in developing catalytic materials based on atomically dispersed transition metals on heterogeneous supports, it is necessary to have an atomic-level understanding of the factors that impact their structural and electronic properties and, ultimately, their reactivity. In this contribution, we address and elucidate with electronic structure calculations open questions related to the ethane dehydrogenation mechanism on silica-supported mononuclear Fe(II) and Fe(III) sites. Contrary to prior hypotheses, we determine that the σ-metathesis on Fe(II) sites is an unlikely dehydrogenation mechanism. On tricoordinate and tetracoordinate Fe(II)@SiO₂, the reaction proceeds via heterolytic C–H bond activation and β-hydride elimination facilitated by spin-crossing. Atomically dispersed Fe(III) grafted on SiO₂ exhibits a more complex behavior as it seems to be undergoing autoreduction and we propose a new redox ethane dehydrogenation mechanism which, remarkably, is energetically competitive with the heterolytic C–H activation mechanism previously identified for other transition metals.

Correction for ‘Integrated adsorption and photocatalytic degradation of VOCs using a TiO₂/diatomite composite: effects of relative humidity and reaction atmosphere’ by Guangxin Zhang et al., Catal. Sci. Technol., 2020, 10, 2378–2388, https://doi.org/10.1039/D0CY00168F.

The metal-free visible-light-driven CO₂ reduction reaction was achieved by using a fluorine-atom-modified COF, i.e. N3F4-COF, which exhibited over a 5-fold enhancement compared to the pristine COF for syngas production. The activity can be further improved by incorporating a Ru-based photosensitizer, and the syngas ratio could be regulated from 2.57 to 0.14 (CO/H₂) by altering the hydrophilicity of the photosensitizers.

Ru/Nb₂O₅ is effective for furfural aqueous reductive conversion. Systematic characterization, kinetic studies and in situ DRIFT tests demonstrated that the Ru^δ+ species abundance is highly correlated with the adsorption behavior of the substrate, key intermediate and product (furfural, 2-cyclopentenone, and cyclopentanone), which governs the FFR conversion rate and cyclopentanoid product selectivity.

Designing highly efficient catalysts for driving the hydrogen production from formic acid (FA) dehydrogenation is of considerable practical importance for future hydrogen economies. Herein, ultrafine Pd nanoparticles (NPs) with lattice strain induced by the incorporation of tripeptide (TPT) anchored on commercial Vulcan XC-72R carbon (Pd/C-TPT) are fabricated by a facile wet-chemical process. Remarkably, the obtained Pd/C-TPT catalyst presents an extraordinary catalytic activity towards dehydrogenation of FA without any additives, giving an initial turnover frequency (TOF) value as high as 2102 mol H₂ per mol Pd per h at 323 K, which is 8.4 times than that of the Pd/C catalyst, and even much higher than most of other superior Pd catalysts reported so far. The characterization results reveal that the strain effect and size effect caused by the strong interaction between the Pd NPs and TPT tune the electronic structure of Pd for optimized formate adsorption. This study provides more flexibility for a facile and controllable synthesis strategy of efficient catalysts toward FA dehydrogenation for hydrogen production.

A series of four molecular ruthenium hydrido complexes supported by previously reported rigid PC_carbeneP pincer ligand frameworks were evaluated as formic acid dehydrogenation (FAD) catalysts. The ligands in the complexes L_RRu(H)X (R = H, NMe₂; X = Cl, κ²-O₂CH) differ in the electron richness by substitution on the aryl groups linking the di-iso-propylphosphine arms to the central carbene donor. We find that only the unsubstituted (R = H) chloro and formato complexes are effective catalyst precursors; the NMe₂ substituted derivatives decompose under catalytic conditions. However, the two compounds L_HRu(H)X are highly active (TOF = 1300–4200 h⁻¹), long lived (TON up to 122 000) and selective (dihydrogen and carbon dioxide are the sole products) at 21 °C with no base additives necessary in 13 M formic acid in water/dioxane. These performance metrics compare well with state of the art catalysts operating under ambient conditions. Mechanistic experiments support a simple two-step mechanism involving rate limiting protonolysis of the Ru–H by formic acid to release H₂ and rapid loss of CO₂via β-elimination from the resulting formato complex.

The regulation of the electronic state of catalytic sites is essential to improve the intrinsic activity of catalysts. Herein, we modulated the metal state of Au species by varying their particle size on nitrogen-rich triazine-based porous organic polymer supports. Due to the different interface percentages between Au and nitrogen species in selected supports, the electronic state of the metal can be modulated. The catalyst with the smallest Au particle size presented the most negative metallic state and the highest surface energy, thus exposing more sites for H₂ activation and providing sufficient reactive H species to CO₂ hydrogenation absorbed on the adjacent N. The designed Au/Trz-TETA (1.23 nm) exhibited high catalytic activity for the CO₂ hydrogenation to formic acid and a turnover number (TON) up to 1687 over 10 h, which is higher than that of Au/Trz-DETA (2.24 nm) and Au/Trz-TEPA (1.96 nm) with a bigger metal particle size. This work shows a size-dependent CO₂ hydrogenation for various sizes of Au metal catalysts and provides a new way for regulating the metal electronic state.

Direct construction of 1,3-disubstituted planar chiral ferrocenes (PCFs) is a challenging task. Herein, we have computationally investigated an enantioselective synthetic strategy for 1,3-disubstituted PCFs using density functional theory (DFT) methods to explore its principal characteristics and find plausible solutions to mechanistic issues. A cooperative palladium/norbornene (NBE)-catalyzed enantioselective relay remote C–H bond activation mechanism is established. The obtained results indicate that the total free energy barrier of the conversion process is 31.8 kcal mol⁻¹, which is reasonable under the studied reaction conditions. The rate-determining step consists of a combination of β-carbon elimination and protodepalladation. Calculations of multiple ortho-C–H activation pathways indicate that the acetate-assisted direct C–H activation is the most kinetically favorable route. Meanwhile, computations of a competitive side reaction pathway confirm that the faster the extrusion of the NBE group, the lower the formation probability of the ortho-substituted by-product. Furthermore, in the protodepalladation step, the acidic ligand (s)-Boc-L-Val-OH (L) most likely acts as a proton donor. Enantioselectivity calculations reveal that the high NBE olefin insertion barrier prevents the formation of the R_p isomer and is most likely responsible for the enantioselectivity of this transformation. The findings of the present work can deepen the understanding of PCF construction strategies and pave the way for the synthesis of 1,3-disubstituted PCFs.