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•Many Rh catalysts were tested for N2O decomposition under diesel conditions.•Rh/CeO2 is the only catalyst with good activity in a wet feed after aging.•Aging does not deactivate ...Rh/CeO2 but significantly increases its activity.•Removal O2 from the catalyst has been demonstrated to be the rate limiting step.
Numerous Rh catalysts were evaluated for N2O decomposition for automotive applications. Some Rh-containing spinel materials exhibit excellent fresh activities in the absence of H2O but become inactive after hydrothermal aging or when tested in a wet feed. Rh catalysts supported on zeolites can be very active in a dry feed even after aging but are extremely sensitive to H2O. Rh/CeO2 is an exceptional catalyst for this reaction in the presence of both H2O and O2. Hydrothermal aging (750 °C/20 h) significantly increases its activity. A similar activity enhancement was found by calcining the support before Rh impregnation. XPS results show a surface enrichment of Rh species on the aged Rh/CeO2 catalyst relative to the fresh catalyst. Aberration corrected STEM images reveal that Rh is buried in the bulk on the fresh catalyst and pulled out onto the surface of the support after thermal treatments. All catalysts are inhibited by H2O with the zeolite-based Rh catalysts being the worst. The aged Rh/CeO2 catalyst is less sensitive to H2O relative to others. DRIFTS data show that H2O sensitivity is related to catalyst hydrophilicity; a high coverage of OH groups on a catalyst reduces its N2O decomposition activity. H2-TPR results show that a Rh/CeO2 catalyst can be readily reduced at < 100 °C. On a reduced Rh/CeO2 catalyst, near complete N2O conversion can be obtained with a lean feed at 250 °C for a duration equivalent to its oxygen storage capacity. The N2O-DRIFTS experiments over a pre-reduced Rh/CeO2 catalyst show that Ce3+ sites are quickly oxidized to Ce4+ upon contacting N2O at room temperature, resulting N2 and adsorbed O, with the latter being an efficient oxidizer. Based on these results, a N2O decomposition mechanism is proposed for the Rh/CeO2 catalyst.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
An efficient intramolecular palladium-catalyzed, asymmetric reductive-Heck reaction has been developed, which allowed for the synthesis of either enantiomerically enriched 3-substituted indanones or ...α-exo-methylene indanones depending on the base used.
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In the presence of the third generation Grubbs catalyst, the ring-expanding olefin metathesis of a monocyclooct-4-en-1-yl functionalized salen ligand and the corresponding Co(II)(salen) complex at ...low monomer concentrations results in the exclusive formation of macrocyclic oligomeric structures with the salen moieties being attached in an unsymmetrical, flexible, pendent manner. The TOF-MALDI mass spectrometry reveals that the resulting macrocyclic oligomers consist predominantly of dimeric to tetrameric species, with detectable traces of higher homologues up to a decamer. Upon activation under aerobic and acidic conditions, these Co(salen) macrocycles exhibit extremely high reactivities and selectivities in the hydrolytic kinetic resolution (HKR) of a variety of racemic terminal epoxides under neat conditions with very low catalyst loadings. The excellent catalytic properties can be explained in terms of the new catalyst's appealing structural features, namely, the flexible oligomer backbone, the unsymmetrical pendent immobilization motif of the catalytic sites, and the high local concentration of Co(salen) species resulting from the macrocyclic framework. This ring-expanding olefin metathesis is suggested to be a simple way to prepare tethered metal complexes that are endowed with key features(i) a high local concentration of metal complexes and (ii) a flexible, single point of attachment to the supportthat facilitate rapid and efficient catalysis when a bimetallic transition state is required.
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We describe the preparation of a compound whose molecules consist of two metal sandwich stands carrying tentacles with affinity to metal surfaces and holding an axle that carries a dipolar or a ...nonpolar rotator. The dipolar rotor exists as three pairs of enantiomers, rapidly interconverting at room temperature. When mounted on a gold surface, each molecule represents a chiral altitudinal rotor, with the rotator axle parallel to the surface. The surface-mounted rotor molecules are characterized by several spectroscopic and imaging techniques. At any one time, in about one-third of the dipolar rotors the rotator is free to turn and the direction of its dipole can be flipped by the electric field applied by an STM tip, as revealed by differential barrier height imaging. Molecular dynamics calculations suggest that electric field normal to the surface causes members of one pair of enantiomers to rotate unidirectionally.
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•Ceria can stabilize Rh surface upon aging treatment.•The synergism between Rh and ceria improved the reducibility of both Rh species and ceria support.•Ce3+ at the interface of Rh ...and ceria played important role in N2O dissociation.•New generated Rh+ and less OH group on aged Rh/CeO2 contributed to improved N2O decomposition activity with H2O and O2.
Ceria supported Rh catalysts exhibit superior activities for the decomposition of N2O in the presence of water and excess O2 comparing with other oxides supported Rh catalysts. In contrast to Rh/γ-Al2O, hydrothermal aging (750 °C for 20 h with 10% H2O in air) significantly enhances the catalytic activity of Rh/CeO2 for the decomposition of N2O. The presence of H2O, on the other hand, inhibits the reaction rate on all Rh catalysts, but this inhibition effect is less on the aged Rh/CeO2 than on its fresh state. The focus of this work is to explore the nature of this activity enhancement on the aged Rh/CeO2 catalyst and the effect of water. Ceria and alumina supported Rh catalysts were characterized by in-situ DRIFTS with CO and N2O as probe molecules to investigate the synergism between Rh species and ceria support and the interaction between water molecules and the Rh/CeO2 catalysts. The results suggest that Rh supported on ceria has more CO adsorption sites and better hydrothermal stability than that on alumina. On Rh/CeO2 catalysts, the surface Rh species was found to facilitate the reduction of Ce4+ ions to Ce3+ at the interfacial area and as a result increased the concentration of Ce3+ species relative to the CeO2 support after a reduction treatment. Ce3+ ions at the interface between Rh and CeO2 support can be easily oxidized to Ce4+ by N2O at room temperature and played an important role in the disassociation of N2O. After a hydrothermal aging treatment on Rh/CeO2, more surface Rh sites were found that can adsorb CO and N2O. In addition, on the aged Rh/CeO2 catalyst, a different type of Rh species was found to be situated in a more electron-rich environment. These combined factors may be responsible for its enhanced N2O decomposition activity on the aged Rh/CeO2 catalyst. DRITFS were also collected on fresh and aged Rh/CeO2 catalysts with different degrees of sample hydration. The intensity of the OH group at 3658 cm−1 over an aged catalyst was found to decrease with increasing temperature at a faster rate than that on the fresh catalyst. This finding may explain their difference in H2O sensitivity for the N2O decomposition reaction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A practical, one-pot synthesis of enantiopure unsymmetrical salen ligands is described, using a 1:1:1 molar ratio of a chiral diamine and two different salicylaldehydes. The new synthetic protocol ...can be readily performed in good yields (60−85%) on a multigram scale with good tolerance toward various functional groups.
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Possible modes of deactivation of Jacobsen's Co−salen catalyst during the hydrolytic kinetic resolution (HKR) of epichlorohydrin were explored by UV−vis spectroscopy, X-ray absorption spectroscopy, ...and electrospray ionization mass spectrometry, combined with recycling studies. Although an active Co(III)−salen catalyst deactivated substantially after multiple cycles without regeneration, the catalyst maintained its +3 oxidation state throughout the runs. Thus, deactivation of Co−salen during HKR was not the result of Co reduction. The mass spectrum of a deactivated material showed that catalyst dimerization does not account for the loss of activity. Results from various catalyst pretreatment tests, as well as from catalysts containing various counterions (acetate, tosylate, chloride, iodide) indicated that the rate of addition of the Co−salen counterions to epoxide forming Co−OH during the reaction correlated with deactivation. The extent of counterion addition to epoxide was influenced by the exposure time and the nucleophilicity of the counterion. An oligo(cyclooctene)-supported Co−OAc salen catalyst, which was 25 times more active than the standard Co−salen catalyst, was recycled multiple times with negligible deactivation.
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Ceria supported Rh catalysts exhibit superior activities for the decomposition of N2O in the presence of water and excess O2 comparing with other oxides supported Rh catalysts. In contrast to ...Rh/γ-Al2O, hydrothermal aging (750 °C for 20 h with 10% H2O in air) significantly enhances the catalytic activity of Rh/CeO2 for the decomposition of N2O. The presence of H2O, on the other hand, inhibits the reaction rate on all Rh catalysts, but this inhibition effect is less on the aged Rh/CeO2 than on its fresh state. The focus of this work is to explore the nature of this activity enhancement on the aged Rh/CeO2 catalyst and the effect of water. Ceria and alumina supported Rh catalysts were characterized by in-situ DRIFTS with CO and N2O as probe molecules to investigate the synergism between Rh species and ceria support and the interaction between water molecules and the Rh/CeO2 catalysts. The results suggest that Rh supported on ceria has more CO adsorption sites and better hydrothermal stability than that on alumina. On Rh/CeO2 catalysts, the surface Rh species was found to facilitate the reduction of Ce4+ ions to Ce3+ at the interfacial area and as a result increased the concentration of Ce3+ species relative to the CeO2 support after a reduction treatment. Ce3+ ions at the interface between Rh and CeO2 support can be easily oxidized to Ce4+ by N2O at room temperature and played an important role in the disassociation of N2O. After a hydrothermal aging treatment on Rh/CeO2, more surface Rh sites were found that can adsorb CO and N2O. In addition, on the aged Rh/CeO2 catalyst, a different type of Rh species was found to be situated in a more electron-rich environment. These combined factors may be responsible for its enhanced N2O decomposition activity on the aged Rh/CeO2 catalyst. DRITFS were also collected on fresh and aged Rh/CeO2 catalysts with different degrees of sample hydration. The intensity of the OH group at 3658 cm−1 over an aged catalyst was found to decrease with increasing temperature at a faster rate than that on the fresh catalyst. This finding may explain their difference in H2O sensitivity for the N2O decomposition reaction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Excellent enantioselectivities and isolated yields have been achieved for the hydrolytic kinetic resolution of epoxides using a resin‐supported dendronized R,R‐(salen)Co catalyst with catalyst ...loadings as low as 0.04 mol%, the lowest metal loadings of any heterogeneous resin‐supported (salen)Co catalyst reported to date. In addition, the supported catalysts can be recycled and reused with comparable enantioselectivities. It is hypothesized that the high catalytic activity can be attributed to the flexible linker and the dendronized framework supporting the (salen)Co moieties on the resin thereby promoting cooperativity between two metal centers. This work opens up new opportunities for the design of highly active resin‐supported catalysts that catalyze transformations through a bimetallic pathway.
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A series of poly(ethylene oxide)-substituted triphenylphosphines, Ph
3−
m
PC
6H
4-
p-(OCH
2CH
2)
n
OH
m
(PEO-TPPs;
1a
m=1,
1b
m=2,
1c
m=3;
N=
m×
n=8–25), have been prepared by the ethoxylation of ...mono-, di-, and tri-
p-hydroxytriphenylphosphines. PEO-TPPs demonstrate an inverse temperature-dependent solubility in water, and possess distinct cloud points range from 26°C to 90°C.
Based on the clouding property of PEO-TPPs, a new line of aqueous/organic two-phase catalysis termed the thermoregulated phase-transfer catalysis (TRPTC) has been described. That is, the catalyst transfers into the organic phase to catalyze a reaction at a higher temperature, and returns to the aqueous phase to be separated from the products at a lower temperature. Application of this novel strategy to the rhodium-catalyzed two-phase hydroformylation of higher olefins gave desirable results with an average turnover frequency of 180
h
−1 for 1-dodecene. The TRPTC is suitable for carrying out a reaction with extremely water-immiscible substrate in the aqueous/organic two-phase system. Thus, the application scope of the classical two-phase catalysis has been widened.
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