1,721,042 research outputs found
Methylphosphonium methylcarbonate, ylide precursor for halyde- and base-free Wittig reactions
No full textThe phosphonium salt triphenylmethylphosphonium methylcarbonate [PΦ,Φ,Φ,1][OCOOCH3] was obtained by methylation of triphenylphosphine (Ph3P) with dimethylcarbonate, adopting a green and sustainable procedure1.
The [PΦ,Φ,Φ,1][OCOOCH3] phosphonium salt was observed to possess significant P-CH3 proton acidity, and deuterium exchange experiments showed the formation of the analogous PhP3-CD3 phosphonium salt. Spontaneous deprotonation of the methyl group lead therefore to formation of the corresponding phosphorus ylide, Ph3P=CH2.
This Ph3P=CH2 ylide was tested for the Wittig reaction with benzaldehyde PhCHO, generating the desired PhC=CH2 olefination product. It was noteworthy that this Wittig reaction protocol did not require an alkyl halide or a strong base for the formation of the ylide, and could be conducted in air, making it a greener procedure.
The scope of the olefination reaction was extended to a number of carbonyl substrates, both aldehydes and ketones, with high conversions and selectivity. It was performed under mild conditions (34 – 80 °C), using a ratio ylide:carbonyl between 1.0 -3.0, in 2-methyl tetrahydrofuran (2-Me-THF) as solvent.
The study was also extended to other alkylphosphonium methylcarbonate ionic liquids ([P8,8,8,1][OCOOCH3] and [P4,4,4,1][OCOOCH3]). It was demonstrated that, depending on the reaction conditions, it was possible to achieve not only the transfer of a =CH2 fragment, but also the selective transfer of the bulkier alkyl group e.g. =CH(CH2)nCH3, giving access to a variety of olefins. Cis-trans selectivity was in the range 20-80
Halide- and base-free Wittig reaction: phosphonium methylcarbonate salts as ylide precursors
No full textElectronic structure tunability of diamonds by surface functionalization
No full textWide-bandgap semiconductors are exploited in several technological fields such as electron emission devices, energy conversion, high-power high-temperature electronics, and electrocatalysis. Their electronic properties vary significantly depending on the functionalization of the surface. Here, we investigated as a proof-of-concept the modulation of the electronic properties of one of the most common wide-bandgap semiconductors, that is, diamonds, to show the tunability of their properties by modifying the surface termination. Photoelectron spectroscopy was used to demonstrate the availability of a wide window of band bending, work function, and electron affinity. The band bending and work function were found to change by up to 360 meV and 2 eV, respectively, by varying the surface from hydrogen- to oxygen-terminated. Because of the negative electron affinity of diamonds, we were able to experimentally show the rigid shift of the whole band structure
Corrosion in Pb-acid batteries-recent developments
No full textBattery-based energy storage systems with high power/energy densities and excellent cycle efficiency are expected to play a key role in our everyday lives. Even though Lead-Acid Batteries (LABs) are the oldest electrochemical energy storage technology, they still attract a lot of interest thanks to their properties: stability, reliability, recyclability, and low cost of the raw materials. Precisely for these reasons, LAB technology will retain its strong position at least until 2030 and remain very competitive, but ongoing investments are needed to improve production and performance. The main weak points of LABs are the limited charge efficiency and cyclability, mostly due to the degradation of electrodes during the charging/discharging process. Corrosion, in particular, represents a severe problem for LABs and has been the subject of many studies. Although LABs inevitably corrode to a certain extent throughout life, runaway corrosion of the positive grid will ultimately lead to failure. This phenomenon results in capacity degradation, often termed “Premature Capacity Loss” (PCL) or in electrical shorts. This chapter, after having given a general overview of LABs, describing their different types, their chemistry, and their failure modes, focuses specifically on the problem of corrosion of the grids stressing the causes of this phenomenon and the different strategies to evaluate and reduce it
Improved mass transport imparts stability and durability in nickel catalysts for urea oxidation
No full textCalibrating the numerical simulation of the injection moulding process for liquid silicone rubber
No full textIn this work, a new Computer Aided Engineering method was developed to predict optimal design and process conditions for stretch blow moulded bottles, minimizing the cost associated with the design of a new container. The proposed approach evaluates final product characteristics by process numerical simulations in a user friendly environment. The implemented algorithm seeks for the optimal conditions in terms of preform geometry and operating parameters, subjected to process and design constraints. Special attention was devoted to material characterization, obtained by simple and inexpensive instruments, and to the material model implementation, in order to improve results accuracy. For the process simulations commercial general purpose numerical codes were used. With reference to an industrial case study, the predicted optimization results were compared to the experimental outcomes to assess the validity and the robustness of the proposed approach
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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