Bifunctional metalloporphyrins with quaternary ammonium bromides (nucleophiles) at the meta, para, or ortho positions of meso-phenyl groups were synthesized as catalysts for the formation of cyclic carbonates from epoxides and carbon dioxide under solvent-free conditions. The meta-substituted catalysts exhibited high catalytic performance, whereas the para- and ortho-substituted catalysts showed moderate and low activity, respectively. DFT calculations revealed the origin of the advantage of the meta-substituted catalyst, which could use the flexible quaternary ammonium cation at the meta position to stabilize various anionic species generated during catalysis. A zinc(II) porphyrin with eight nucleophiles at the meta positions showed very high catalytic activity (turnover number (TON)=240 000 at 120 °C, turnover frequency (TOF)=31 500 h(-1) at 170 °C) at an initial CO2 pressure of 1.7 MPa; catalyzed the reaction even at atmospheric CO2 pressure (balloon) at ambient temperature (20 °C); and was applicable to a broad range of substrates, including terminal and internal epoxides.
Serpentinizing hydrothermal systems generate H 2 as a reductant and harbor catalysts conducive to geochemical CO 2 conversion into reduced carbon compounds that form the core of microbial autotrophic metabolism. This study characterizes mineral catalysts at hydrothermal vents by investigating the interactions between catalytically active cobalt sites and silica-based support materials on H 2 -dependent CO 2 reduction. Heteroatom incorporated (Mg, Al, Ca, Ti, and Zr), ordered mesoporous silicas are applied as model support systems for the cobalt-based catalysts. It is demonstrated that all catalysts surveyed convert CO 2 to methane, methanol, carbon monoxide, and low-molecular-weight hydrocarbons at 180 °C and 20 bar, but with different activity and selectivity depending on the support modification. The additional analysis of the condensed product phase reveals the formation of oxygenates such as formate and acetate, which are key intermediates in the ancient acetyl-coenzyme A pathway of carbon metabolism. The Ti-incorporated catalyst yielded the highest concentrations of formate (3.6 mM) and acetate (1.2 mM) in the liquid phase. Chemisorption experiments including H 2 temperature-programmed reduction (TPR) and CO 2 temperatureprogrammed desorption (TPD) in agreement with density functional theory (DFT) calculations of the adsorption energy of CO 2 suggest metallic cobalt as the preferential adsorption site for CO 2 compared to hardly reducible cobalt−metal oxide interface species. The ratios of the respective cobalt species vary depending on the interaction strength with the support materials. The findings reveal robust and biologically relevant catalytic activities of silica-based transition metal minerals in H 2 -rich CO 2 fixation, in line with the idea that autotrophic metabolism emerged at hydrothermal vents.
Low-temperature
complete oxidation of ethylene by the mesoporous
silica-supported Pt catalyst is a forefront technology for food preservation.
Public implementations of the Pt catalyst have already begun, and
spectroscopy analyses on the catalytic mechanism have been reported.
In this study, density functional theory calculations were conducted
to clarify the potential energy profile and electronic mechanism of
the catalytic reaction. Based on the experimental findings, a reaction
pathway was proposed for ethylene oxidation up to CO2 formation
via the HCHO intermediate. Among several possibilities, a reaction
pathway via ethylene dioxide species is energetically plausible for
the C–C bond cleavage to generate HCHO. Particular focus was
given to the electronic effect of the silica support in the ethylene
dioxide route. The reservoir effect, in which the siloxide groups
take electrons from the Pt moiety, reduces the activation energy of
the C–C bond cleavage step by taking electrons from the σ(C–C)
orbital.
Dehydrogenative aromatization is one of the attractive alternative methods for directly synthesizing primary anilines from NH3 and cyclohexanones. However, the selective synthesis of primary anilines is quite difficult because the desired primary aniline products and the cyclohexanone substrates readily undergo condensation affording the corresponding imines (i.e., N‐cyclohexylidene‐anilines), followed by hydrogenation to produce N‐cyclohexylanilines as the major products. In this study, primary anilines were selectively synthesized in the presence of supported Pd nanoparticle catalysts (e.g., Pd/HAP, HAP=hydroxyapatite, Ca10(PO4)6(OH)2) by utilizing competitive adsorption unique to heterogeneous catalysis; in other words, when styrene was used as a hydrogen acceptor, which preferentially adsorbs on the Pd nanoparticle surface in the presence of N‐cyclohexylidene‐anilines, various structurally diverse primary anilines were selectively synthesized from readily accessible NH3 and cyclohexanones. The Pd/HAP catalyst was reused several times though its catalytic performance gradually declined.
Reactions
based on transition-metal-catalyzed C–O bond cleavage
have attracted much attention as a new synthetic method. Until now,
several intermolecular reactions via C–O bond cleavage of aryl
ethers, alkenyl ethers, esters, and others have been reported. Here
we report an unprecedented C–O bond cleavage of 3-phenoxy acrylic
acid derivatives, followed by intramolecular C–O bond formation
with alkynes. This reaction gave 2,3-disubstituted benzofurans having
useful functional groupssilyl substituents and acrylic acid
derivativesat the 2- and 3-positions, respectively. This report
also described theoretical (DFT) insights into the mechanism.
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