Supported Platinum Nanoparticles Catalyzed Carbon-Carbon Bond Cleavage of Polyolefins: Role of the Oxide Support Acidity.

Supported platinum nanoparticle catalysts are known to convert polyolefins to high-quality liquid hydrocarbons using hydrogen under relatively mild conditions. To date, few studies using platinum grafted onto various metal oxide (MxOy) supports have been undertaken to understand the role of the acidity of the oxide support in the carbon-carbon bond cleavage of polyethylene under consistent catalytic conditions. Specifically, two Pt/MxOy catalysts (MxOy = SrTiO3 and SiO2-Al2O3; Al = 3.0 wt %, target Pt loading 2 wt % Pt ∼1.5 nm), under identical catalytic polyethylene hydrogenolysis conditions (T = 300 °C, P(H2) = 170 psi, t = 24 h; Mw = ∼3,800 g/mol, Mn = ∼1,100 g/mol, D = 3.45, Nbranch/100C = 1.0), yielded a narrow distribution of hydrocarbons with molecular weights in the range of lubricants (Mw = < 600 g/mol; Mn < 400 g/mol; D = 1.5). While Pt/SrTiO3 formed saturated hydrocarbons with negligible branching, Pt/SiO2-Al2O3 formed partially unsaturated hydrocarbons (<1 mol % alkenes and ∼4 mol % alkyl aromatics) with increased branch density (Nbranch/100C = 5.5). Further investigations suggest evidence for a competitive hydrocracking mechanism occurring alongside hydrogenolysis, stemming from the increased acidity of Pt/SiO2-Al2O3 compared to Pt/SrTiO3. Additionally, the products of these polymer deconstruction reactions were found to be independent of the polyethylene feedstock, allowing the potential to upcycle polyethylenes with various properties into a value-added product.

Breaking New Ground: MBene Route toward Selective Vinyl Double Bond Hydrogenation in Nitroarenes.

Doping, or incremental substitution of one element for another, is an effective way to tailor a compound's structure as well as its physical and chemical properties. Herein, we replaced up to 30% of Ni with Co in members of the family of layered LiNiB compounds, stabilizing the high-temperature polymorph of LiNiB while the room-temperature polymorph does not form. By studying this layered boride with in situ high-temperature powder diffraction, we obtained a distorted variant of LiNi0.7Co0.3B featuring a perfect interlayer placement of [Ni0.7Co0.3B] layers on top of each other─a structural motif not seen before in other borides. Because of the Co doping, LiNi0.7Co0.3B can undergo a nearly complete topochemical Li deintercalation under ambient conditions, resulting in a metastable boride with the formula Li0.04Ni0.7Co0.3B. Heating of Li0.04Ni0.7Co0.3B in anaerobic conditions led to yet another metastable boride, Li0.01Ni0.7Co0.3B, with a CoB-type crystal structure that cannot be obtained by simple annealing of Ni, Co, and B. No significant alterations of magnetic properties were detected upon Co-doping in the temperature-independent paramagnet LiNi0.7Co0.3B or its Li-deintercalated counterparts. Finally, Li0.01Ni0.7Co0.3B stands out as an exceptional catalyst for the selective hydrogenation of the vinyl C═C bond in 3-nitrostyrene, even in the presence of other competing functional groups. This research showcases an innovative approach to heterogeneous catalyst design by meticulously synthesizing metastable compounds.

Capturing Rare-Earth Elements by Synthetic Aluminosilicate MCM-22: Mechanistic Understanding of Yb(III) Capture.

We studied the mechanism underlying the solid-phase adsorption of a heavy rare-earth element (HREE, Yb) from acidic solutions employing MCM-22 zeolite, serving as both a layered synthetic clay mimic and a new platform for the mechanistic study of HREE adsorption on aluminosilicate materials. Mechanistic studies revealed that the adsorption of Yb(III) at the surface adsorption site occurs primarily through the electrostatic interaction between the site and Yb(III) species. The dependence of Yb adsorption on the pH of the solution indicated the role of surface charge, and the content of framework Al suggested that the Brønsted acid sites (BAS) are involved in the adsorption of Yb(III) ions, which was further scrutinized by spectroscopic analysis and theoretical calculations. Our findings have illuminated the roles of surface sites in the solid-phase adsorption of HREEs from acidic solutions.

Mesoporous Silica Encapsulated Platinum-Tin Intermetallic Nanoparticles Catalyze Hydrogenation with an Unprecedented 20% Pairwise Selectivity for Parahydrogen Enhanced Nuclear Magnetic Resonance.

Supported noble metals offer key advantages over homogeneous catalysts for in vivo applications of parahydrogen-based hyperpolarization. However, their performance is compromised by randomization of parahydrogen spin order resulting from rapid hydrogen adatom diffusion. The diffusion on Pt surfaces can be suppressed by introduction of Sn to form Pt-Sn intermetallic phases. Herein, an unprecedented pairwise selectivity of 19.7 ± 1.1% in the heterogeneous hydrogenation of propyne using silica encapsulated Pt-Sn intermetallic nanoparticles is reported. This high level of selectivity exceeds that of all supported metal catalysts by at least a factor of 3. Moreover, the pairwise selectivity for alkyne hydrogenation is about 2 times higher than for alkene hydrogenation, an observation attributed to the higher coverage of the former and its effect on diffusion. Lastly, PtSn@mSiO2 nanoparticles exhibited improved coking resistance, and any loss of activity is shown to be fully reversible through high-temperature oxidation-reduction cycling.

General Synthetic Strategy to Ordered Mesoporous Carbon Catalysts with Single-Atom Metal Sites for Electrochemical CO2 Reduction.

The electrochemical carbon dioxide reduction reaction (CO2 RR) is a transformative technology to reduce the carbon footprint of modern society. Single-site catalysts have been demonstrated as promising catalysts for CO2 RR, but general synthetic methods for catalysts with high surface area and tunable single-site metal composition still need to be developed to unambiguously investigate the structure-activity relationship crossing various metal sites. Here, a generalized coordination-condensation strategy is reported to prepare single-atom metal sites on ordered mesoporous carbon (OMC) with high surface areas (average 800 m2  g-1 ). This method is applicable to a broad range of metal sites (Fe, Co, Ni, Cu, Pt, Pd, Ru, and Rh) with loadings up to 4 wt.%. In particular, the CO2 RR to carbon monoxide (CO) Faradaic efficiency (FE) with Ni single-site OMC catalyst reaches 95%. This high FE is maintained even under large current density (>140 mA cm-2 ) and in a long-term study (14 h), which suits the urgently needed large-scale applications. Theoretical calculations suggest that the enhanced activity on single-atom Ni sites results from balanced binding energies between key intermediates, COOH and CO, for CO2 RR, as mediated by the coordination sphere.

Transition metal-like carbocatalyst.

Catalytic cleavage of strong bonds including hydrogen-hydrogen, carbon-oxygen, and carbon-hydrogen bonds is a highly desired yet challenging fundamental transformation for the production of chemicals and fuels. Transition metal-containing catalysts are employed, although accompanied with poor selectivity in hydrotreatment. Here we report metal-free nitrogen-assembly carbons (NACs) with closely-placed graphitic nitrogen as active sites, achieving dihydrogen dissociation and subsequent transformation of oxygenates. NACs exhibit high selectivity towards alkylarenes for hydrogenolysis of aryl ethers as model bio-oxygenates without over-hydrogeneration of arenes. Activities originate from cooperating graphitic nitrogen dopants induced by the diamine precursors, as demonstrated in mechanistic and computational studies. We further show that the NAC catalyst is versatile for dehydrogenation of ethylbenzene and tetrahydroquinoline as well as for hydrogenation of common unsaturated functionalities, including ketone, alkene, alkyne, and nitro groups. The discovery of nitrogen assembly as active sites can open up broad opportunities for rational design of new metal-free catalysts for challenging chemical reactions.

The Accessory Gene saeP of the SaeR/S Two-Component Gene Regulatory System Impacts Staphylococcus aureus Virulence During Neutrophil Interaction.

Staphylococcus aureus (S. aureus) causes a range of diseases ranging from superficial skin and soft-tissue infections to invasive and life-threatening conditions (Klevens et al., 2007; Kobayashi et al., 2015). S. aureus utilizes the Sae sensory system to adapt to neutrophil challenge. Although the roles of the SaeR response regulator and its cognate sensor kinase SaeS have been demonstrated to be critical for surviving neutrophil interaction and for causing infection, the roles for the accessory proteins SaeP and SaeQ remain incompletely defined. To characterize the functional role of these proteins during innate immune interaction, we generated isogenic deletion mutants lacking these accessory genes in USA300 (USA300ΔsaeP and USA300ΔsaeQ). S. aureus survival was increased following phagocytosis of USA300ΔsaeP compared to USA300 by neutrophils. Additionally, secreted extracellular proteins produced by USA300ΔsaeP cells caused significantly more plasma membrane damage to human neutrophils than extracellular proteins produced by USA300 cells. Deletion of saeQ resulted in a similar phenotype, but effects did not reach significance during neutrophil interaction. The enhanced cytotoxicity of USA300ΔsaeP cells toward human neutrophils correlated with an increased expression of bi-component leukocidins known to target these immune cells. A saeP and saeQ double mutant (USA300ΔsaePQ) showed a significant increase in survival following neutrophil phagocytosis that was comparable to the USA300ΔsaeP single mutant and increased the virulence of USA300 during murine bacteremia. These data provide evidence that SaeP modulates the Sae-mediated response of S. aureus against human neutrophils and suggest that saeP and saeQ together impact pathogenesis in vivo.

A Fluorescence-based Method to Study Bacterial Gene Regulation in Infected Tissues.

Bacterial virulence genes are often regulated at the transcriptional level by multiple factors that respond to different environmental signals. Some factors act directly on virulence genes; others control pathogenesis by adjusting the expression of downstream regulators or the accumulation of signals that affect regulator activity. While regulation has been studied extensively during in vitro growth, relatively little is known about how gene expression is adjusted during infection. Such information is important when a particular gene product is a candidate for therapeutic intervention. Transcriptional approaches like quantitative, real-time RT-PCR and RNA-Seq are powerful ways to examine gene expression on a global level but suffer from many technical challenges including low abundance of bacterial RNA compared to host RNA, and sample degradation by RNases. Evaluating regulation using fluorescent reporters is relatively easy and can be multiplexed with fluorescent proteins with unique spectral properties. The method allows for single-cell, spatiotemporal analysis of gene expression in tissues that exhibit complex three-dimensional architecture and physiochemical gradients that affect bacterial regulatory networks. Such information is lost when data are averaged over the bulk population. Herein, we describe a method for quantifying gene expression in bacterial pathogens in situ. The method is based on simple tissue processing and direct observation of fluorescence from reporter proteins. We demonstrate the utility of this system by examining the expression of Staphylococcus aureus thermonuclease (nuc), whose gene product is required for immune evasion and full virulence ex vivo and in vivo. We show that nuc-gfp is strongly expressed in renal abscesses and reveal heterogeneous gene expression due in part to apparent spatial regulation of nuc promoter activity in abscesses fully engaged with the immune response. The method can be applied to any bacterium with a manipulatable genetic system and any infection model, providing valuable information for preclinical studies and drug development.

A spectrum of CodY activities drives metabolic reorganization and virulence gene expression in Staphylococcus aureus.

The global regulator CodY controls the expression of dozens of metabolism and virulence genes in the opportunistic pathogen Staphylococcus aureus in response to the availability of isoleucine, leucine and valine (ILV), and GTP. Using RNA-Seq transcriptional profiling and partial activity variants, we reveal that S. aureus CodY activity grades metabolic and virulence gene expression as a function of ILV availability, mediating metabolic reorganization and controlling virulence factor production in vitro. Strains lacking CodY regulatory activity produce a PIA-dependent biofilm, but development is restricted under conditions that confer partial CodY activity. CodY regulates the expression of thermonuclease (nuc) via the Sae two-component system, revealing cascading virulence regulation and factor production as CodY activity is reduced. Proteins that mediate the host-pathogen interaction and subvert the immune response are shut off at intermediate levels of CodY activity, while genes coding for enzymes and proteins that extract nutrients from tissue, that kill host cells, and that synthesize amino acids are among the last genes to be derepressed. We conclude that S. aureus uses CodY to limit host damage to only the most severe starvation conditions, providing insight into one potential mechanism by which S. aureus transitions from a commensal bacterium to an invasive pathogen.