Phenol hydrogenation is widely studied for selective production of the chemical intermediate cyclohexanone. A plethora of studies in the literature have reported catalysts aiming to achieve high ...selectivity compared to Pd/C. However, we demonstrate that selective and high-yielding reactions are inherent features of liquid-phase phenol hydrogenation using conventional Pd/C catalysts. We also show there is a very strong dependance of selectivity upon conversion, with high selectivity being maintained until near complete consumption of the phenol, after which subsequent reaction to the unwanted, fully hydrogenated cyclohexanol occurs rapidly. Furthermore, through competitive reactions with other aromatic molecules it is demonstrated that the phenol molecule effectively self-poisons the onwards reaction of weakly bound cyclohexanone, likely by virtue of its relative adsorption strength, and this is the source of the intrinsic selectivity. The implications of this to the reaction mechanism, and in turn to the rational design of catalysts, especially for obtaining chemicals from phenolic bio-oils, are discussed.
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•Phenol self-promotes selective hydrogenation over standard Pd/C type catalysts.•Achieves ∼90 selectivity at > 96 % conversion.•Competitive adsorption experiments point to an adsorption strength effect.•Stepwise reactions occur with no evidence for direct complete hydrogenation.•Qualitatively similar effects are seen in water (relevant to biomass processing).
•Endophytic bacterial communities of plants from Pb-Zn contaminated sites are presented.•High throughput sequencing technology is used to study unculturable bacterial communities.•Culturable bacteria ...are isolated and their plant-growth promotion capabilities are investigated.•Endophyte BXGe71 can significantly enhance plant’s growth under multi-heavy metal stress.
The endophytic bacterial community of two hyperaccumulators (Arabis alpine, Dysphania ambrosioides) and Veronica ciliate was investigated by Illumina sequencing technology. In addition, the culturable endophytic bacteria (EB) were isolated and their plant-growth promotion capabilities were studied. A dataset consisting of 221,075 filtered high-quality and classifiable unique 16S rDNA gene tags, and an average of 36,846 tags with a mean length of 464-bp for each sample was generated. In total, 10801 different operational taxonomic units (OTUs) were detected, belonging to 18 bacterial phyla, 41 classes, 91 orders, 135 families, and 215 genera. Pseudomonas was the most dominant genus in both shoots and roots of the two hyperaccumulators, making up 81.56% and 81.13%, 41.60% and 77.06% of the total number of OTUs, respectively. However, both Chao 1 and Shannon indices of EB of the two hyperaccumulators were significantly lower than those of V. ciliate (P <. 05), except the Shannon index of D. ambrosioides shoots. The endophytic bacterial community of roots and shoots of A. alpine showed greater similarity with that of D. ambrosioides roots (12 km away), and clustered to one group in dendrogram, in clear contrast to that of V. ciliate, which grew closer to A. alpine (60 m away). Combining results of soil and plant analyses, we suggest that the soil properties, especially heavy metal concentration, may influence the plants endophytic bacterial community composition. Pot experiments showed that the strain BXGe71 (Microbacterium sp.) from A. alpine significantly enhanced host plants’ growth under multi-heavy metal (HM) stress (P < .05, t-test).
The unimolecular nucleophilic substitution (SN1) mechanism features prominently in every introductory organic chemistry course. In principle, stepwise displacement of a leaving group by a nucleophile ...via a carbocationic intermediate enables the construction of highly congested carbon centres. However, the intrinsic instability and high reactivity of the carbocationic intermediates make it very difficult to control product distributions and stereoselectivity in reactions that proceed via SN1 pathways. Here we report asymmetric catalysis of an SN1-type reaction mechanism that results in the enantioselective construction of quaternary stereocentres from racemic precursors. The transformation relies on the synergistic action of a chiral hydrogen-bond-donor catalyst with a strong Lewis-acid promoter to mediate the formation of tertiary carbocationic intermediates at low temperature and to achieve high levels of control over reaction enantioselectivity and product distribution. This work provides a foundation for the enantioconvergent synthesis of other fully substituted carbon stereocentres.
Since the discovery that ceria is an active catalyst for selective hydrogenation of alkynes, there has been much debate on the catalytic mechanism. In this work, we propose, based on density ...functional theory (DFT) investigations, a mechanism that involves the heterolytic dissociation of H2 at oxygen vacancies of CeO2(111), facilitated by frustrated Lewis pairs consisting of spatially separated O and Ce sites. The resulting O–H and Ce–H species effectively catalyze the hydrogenation of acetylene, avoiding the overstabilization of the C2H3* intermediate in a previously proposed mechanism. On the basis of our mechanism, we propose the doping of ceria by Ni as a means to create oxygen vacancies. Interestingly, the Ni dopant is not directly involved in the catalytic reaction, but serves as a single-atom promoter. Experimental studies confirm the design principles and demonstrate much higher activity for Ni-doped ceria in selective hydrogenation of acetylene. The combined results from DFT calculations and experiment provide a basis to further develop selective hydrogenation catalysts based on earth-abundant materials.
The performance of catalysts used for the dry reforming of methane can strongly depend on the selection of active metals, supports and promoters. This work studies their effects on the activity and ...stability of selected catalysts. Designing an economically viable catalyst that maintains high catalytic activity and stability can be achieved by exploiting the synergic effects of combining noble and/or non-noble metals to form highly active and stable bi- and tri-metallic catalysts. Perovskite type catalysts can also constitute a potent and cost effective substituent. Metal oxide supports with surface Lewis base sites are able to reduce carbon formation and yield a greater stability to the catalyst, while noble metal promoters have proven to increase both catalyst activity and stability. Moreover, a successful metal-support-promoter combination should lead to higher metal-support interacrtion, lower reduction temperature and enhancement of the anti-coking and anti-amalgamation properties of the catalyst. However, the effect of each parameter on the overall performance of the catalyst is usually complex, and the catalyst designer is often faced with a tradeoff between activity, stability and ease of activation. Based on the review carried out on various studies, it is concluded that a catalyst design must take into consideration not only the separate effects of the active metal, support and promoter, but should also include the combined and mutual interactions of these components.
The electrochemical reduction of CO2 to CO is a reaction of central importance for sustainable energy conversion and storage. Herein, structure–activity relationships of a series of imidazolium-based ...cocatalysts for this reaction are described, which demonstrate that the C4- and C5-protons on the imidazolium ring are vital for efficient catalysis. Further investigation of these findings led to the discovery of new imidazolium salts, which show superior activity as cocatalysts for the reaction, i.e., CO is selectively produced at significantly lower overpotentials with nearly quantitative faradaic yields for CO.
Spontaneous association above a threshold concentration is a hallmark of supramolecular polymerization, in which monomeric units self-assemble into polymeric aggregates through noncovalent ...interactions. This self-initiated supramolecular process differs from the conventional covalent chain-growth polymerization in that the latter often involves the use of a different chemical entity as an initiator to trigger/control the polymerization process. We report here the use of a small molecule hydrophobe, paclitaxel (PTX), as an effective promoter to induce the supramolecular polymerization of a peptide–paclitaxel conjugate, Spheropax (Spax). We found that Spax monomers alone in water self-assemble into spherical micelles of approximately 6.5 nm in diameter but, in the presence of free PTX, undergo a supramolecular polymerization process to form filamentous assemblies of several micrometers in length. Increasing the ratio of promoter to monomer (PTX/Spax) induces Spax’s directional polymerization and expedites its kinetic process. We believe these findings provide important insight into the initiator-controlled supramolecular polymerization process.
Herein, we reported a successful Suzuki–Miyaura coupling of dimethyl aryl amines to forge biaryl skeleton via Ni catalysis in the absence of directing groups and preactivation. This transformation ...proceeded with high efficiency in the presence of magnesium. Preliminary mechanism studies demonstrated dual roles of magnesium: (i) a reductant that reduced Ni(II) species to active Ni(I) catalyst; (ii) a unique promoter that facilitated the Ni(I)/Ni(III) catalytic cycle.
Promoter elements enhance the activity and selectivity of heterogeneous catalysts. Here, we show how methanol synthesis from synthesis gas over copper (Cu) nanoparticles is boosted by zinc oxide ...(ZnO) nanoparticles. By combining surface area titration, electron microscopy, activity measurement, density functional theory calculations, and modeling, we show that the promotion is related to Zn atoms migrating in the Cu surface. The Zn coverage is quantitatively described as a function of the methanol synthesis conditions and of the size-dependent thermodynamic activities of the Cu and ZnO nanoparticles. Moreover, experimental data reveal a strong interdependency of the methanol synthesis activity and the Zn coverage. These results demonstrate the size-dependent activities of nanoparticles as a general means to design synergetic functionality in binary nanoparticle systems.
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