Novel and selective strategies for platform chemical production from renewable biomass are highly attractive in respect to value-added utilization of sustainable resources. In this study, a series of low-cost, commercially available, transition metal carbonates (Zr, Ni, Mg, Zn, and Pb) were investigated for catalytic transfer hydrogenation of levulinate esters to γ-valerolactone (GVL) via the cascade process of Meerwein−Ponndorf−-Verley (MPV) reduction and lactonization reaction. Among the selected catalysts, basic zirconium carbonate is the most active, with the highest turnover frequency (TOF) of 3.1 h −1 and a surface reaction rate of 0.21 mol m −2 h −1 . At 453 K, 3.0 h, and 1.0 MPa N 2 , 100% ethyl levulinate conversion, 96.3% GVL yield, and 91.9% hydrogen donor utilization are observed due to the cooperative effect between acid (M n+ ) and base (−OH) sites. Furthermore, this catalyst shows high recyclability under the optimized conditions, where a satisfactory catalytic activity is shown even after six consecutive runs.
Heterogeneous single-metal-site catalysts have been drawing increasing interests in the field of academy and industry because of the comparable catalytic activity with homogeneous catalyst and easy separation.Here, an efficiently heterogeneous single-site Rh catalyst on activated carbon (Rh 1 /AC) was constructed, which performs three times activity than the corresponding homogeneous catalyst for methanol carbonylation. Experimental data reveals that the apparent activation energy on the Rh 1 /AC catalyst is 0.91 eV, far less than 1.54 eV of its homogeneous counterpart. Ex situ EXAFS confirms the molecular configuration of a single Rh site. DFT calculation demonstrates that the electron-donating carbonyl group on the surface of the support possesses the precedence to accommodate the singlesite Rh ions. Furthermore, difference charge density verifies that the coordinative bond between a single metal ion and a carbonyl group enhances the electronic density of the central Rh atom, consequently lowering the energy barrier of the rate-determining step of CH 3 I oxidative addition. Together with the atomic dispersion, as well as the electronic interaction between a single Rh ion and carbonyl groups, the Rh 1 /AC catalyst performs superior activity than homogeneous systems.
Heterogeneous
single-metal-site catalysts (HSMSCs) have attracted
considerable interest, but most studies have focused on the metal
atoms in the active site while ignoring the key role of ligands. The
unique coordination environment of a single-site catalyst is crucial
for realizing its potential. Constructing this kind of catalyst via a feasible and practical fabrication method is challenging.
Herein, a single-site Pd catalyst with iodide ligands supported on
activated carbon (Pd1/AC) was successfully fabricated by
atomic dispersion of large Pd nanoparticles (NPs). Intermediate I•
radicals were detected during the atomic dispersion process of Pd
NPs by in situ imaging photoelectron photoion coincidence
spectroscopy (in situ iPEPICO) with vacuum ultraviolet
synchrotron radiation. The molecular structure of single-site Pd was
established as [Pd(CO)I4(OAC)]2– through combined characterization. Alkyne dialkoxycarbonylation
with high selectivity toward 1,4-dicarboxylic acid esters (>94%)
and
high acetylene conversion (>99%) was achieved. A sulfonic promoter
on the Pd1/AC catalyst for alkyne dialkoxycarbonylation
was avoided because of the iodide ligand. Good durability and a broad
substrate scope were successfully achieved.
Direct dehydrogenation of isobutane to isobutene has drawn extensive attention for synthesizing various chemicals. The Mo-based catalysts hold promise as an alternative to the toxic CrO - and scarce Pt-based catalysts. However, the low activity and rapid deactivation of the Mo-based catalysts greatly hinder their practical applications. Herein, we demonstrate a feasible approach toward the development of efficient and non-noble metal dehydrogenation catalysts based on Mo-C hybrid nanowires calcined at different temperatures. In particular, the optimal Mo-C catalyst exhibits isobutane consumption rate of 3.9 mmol g h and isobutene selectivity of 73% with production rate of 2.8 mmol g h. The catalyst maintained 90% of its initial activity after 50 h of reaction. Extensive characterizations reveal that such prominent performance is well correlated with the adsorption abilities of isobutane and isobutene and the formation of η-MoC species. In contrast, the generation of β-MoC crystalline phase during long-term reaction causes minor decline in activity. Compared to MoO and β-MoC, η-MoC plays a role more likely in suppressing the cracking reaction. This work demonstrates a feasible approach toward the development of efficient and non-noble metal dehydrogenation catalysts.
Sulfur
poisoning is a severe problem in industrial applications,
attracting broad interest in fundamental research studies. Although
a number of studies about sulfur resistance have been implemented
in many reactions on nanoparticle catalysts, few investigations focus
on carbonylation reactions using heterogeneous single-metal-site catalysts
(HSMSCs). Herein, we present an unanticipated sulfur-promoted performance
in olefin hydrocarboxylation reactions on a single-Rh-site catalyst
supported on porous ionic polymers (Rh1/PIPs) with 1000
ppm H2S in CO feed. Ex situ EXAFS and in situ DRIFTS revealed a ternary cycle mechanism of olefin
hydrocarboxylation reactions with Rh–H complexes as predominant
active species in both pure and H2S-containing feedstock.
Moreover, the transformation of the Rh mononuclear complex with the
addition of H2S was also demonstrated. Density functional
theory studies were performed to verify the feasibility of the proposed
pathway and confirm that the energy barriers of transition states
with the sulfur ligand were much lower than those in normal feed,
for example, a decline of 3.4 kcal/mol for the rate-determining step
of migration and insertion of CO. This work provides a distinctive
example for the insight of sulfur effect on carbonylation, which could
be potentially beneficial for further applications of HSMSCs.
Heterogeneous single‐metal‐site catalysts usually suffer from poor stability, thereby limiting industrial applications. Dual Pd1−Ru1 single‐atom‐sites supported on porous ionic polymers (Pd1−Ru1/PIPs) were constructed using a wetness impregnation method. The two isolated metal species in the form of a binuclear complex were immobilized on the cationic framework of PIPs through ionic bonds. Compared to the single Pd‐ or Ru‐site catalyst, the dual single‐atom system exhibits higher activity with 98 % acetylene conversion and near 100 % selectivity to dialkoxycarbonylation products, as well as better cycling stability for ten cycles without obvious decay. Based on DFT calculations, it was found that the single‐Ru site exhibited a strong CO adsorption energy of −1.6 eV, leading to an increase in the local CO concentration of the catalyst. Notably, the Pd1−Ru1/PIPs catalyst had a much lower energy barrier of 2.49 eV compared to 3.87 eV of Pd1/PIPs for the rate‐determining step. The synergetic effect between neighboring single sites Pd1 and Ru1 not only enhanced the overall activity, but also stabilized PdII active sites. The discovery of synergetic effects between single sites can deepen our understanding of single‐site catalysts at the molecular level.
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