Catalytic cleavage of benzyl ethers

Catalytic cleavage of benzyl ethers

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Catalytic Cleavage of Benzyl Ethers: Introduction

Reaction principle:

Benzyl alcohols and benzyl ethers can be catalytically hydrogenated. The bond between the benzyl carbon atom and the oxygen atom is broken, and a new bond to the hydrogen is formed at the same time. The reaction is made possible by the particular reactivity of the benzyl position. Benzyl ethers are therefore used in preparative synthesis as a protective group for alcohols; especially if you want to remove the protective group again under neutral reaction conditions. (Most of the other protecting groups for alcohols are split off under acidic conditions.)


Structure in serine proteases

In serine proteases, the catalytic triad is formed from aspartate, histidine and serine, the amino acid residues of which are linked by hydrogen bonds. The aspartate residue is in a pocket inaccessible to the solvent and forms a hydrogen bond to the N-H group of the histidine residue. The histidine polarized in this way, in turn, forms a hydrogen bond with the second ring-bound nitrogen to the OH group of the serine residue. The hydrogen-oxygen bond is thereby strongly polarized and the nucleophilicity of the oxygen is further increased.

Structure in thiol and cysteine ​​proteases

Both catalytic di- and triads occur in thiol and cysteine ​​proteases. The catalytic diad consists of cysteine ​​and histidine, the triad of cysteine-histidine-asparagine / aspartate / glutamine or glutamic acid.

Voluminous molecules protect efficient cobalt catalyst for light-induced water splitting from disintegration.

With increasing long-term stability and falling production costs, fuel cells are slowly but surely spreading over ever larger fields of application. The hydrogen alone, which is converted into electricity using oxygen in the energy converters, is largely obtained from natural gas. In order to use the future-oriented fuel without CO2- To gain carbon-neutral emissions, it can be produced electrolytically with electricity generated from renewable sources or directly via the catalytic splitting of water. For the latter, arguably the most elegant way of water splitting, French and American scientists developed a new cobalt-based catalyst.

Fig .: Rising oxygen bubbles prove that the new cobalt-based catalyst is suitable for splitting water when exposed to light. (Image: (c) Benjamin Yin, Emory University)

Following the example of natural photosynthesis, the energy of sunlight is sufficient for the catalytic splitting of water molecules. This requires an efficient light absorber, a catalyst for the oxidation of the water to oxygen and another for the reduction of water to hydrogen. Cobalt and ruthenium catalysts have already proven to be highly efficient for water oxidation. However, up to now they have not been particularly long-lived, as they gradually decomposed due to the chemical aggressiveness of the oxygen.

"We focused our work on a catalyst without organic components, since organic components combine with oxygen and thus self-destruct," says Craig Hill, chemist at Emory University in Atlanta. Together with colleagues from the University of Paris 06, they therefore stabilized the catalyst material cobalt oxide (Co4O4) with carbon-free ligands, so-called polyoxometalates.

The complete, homogeneously structured cobalt complex could be synthesized in a reaction of cobalt oxide, phosphate salts and tungsten by simply heating it in water. The chemists tested the catalytic behavior under exposure to ultraviolet light. Buffered to a pH value of 8, the cobalt complex was able to oxidize water to oxygen most efficiently with a reaction and regeneration rate of over five Hertz (catalytic turnover) without being successively destroyed itself.

With this catalyst made of relatively cheap substances, another very promising material is available for the direct splitting of water into oxygen and hydrogen. Before a technical application, however, a suitable catalyst for the catalytic reduction to hydrogen must be found and the yield of both gases must be precisely determined and possibly increased. British researchers who only recently presented a heterogeneously structured catalyst for splitting water are apparently one step further. Their system - a gold electrode coated with numerous layers of indium phosphide (InP) nanoparticles - showed an efficiency of 60 percent when irradiated with light and a relatively low electrical voltage.

Molybdenum and sulfur thin films belong to a class of materials that can be used as photo-catalysts. Such inexpensive catalysts are needed to produce hydrogen fuel using solar energy. However, they are still not very efficient. A new instrument at BESSY II at the Helmholtz Center Berlin now shows how a light pulse changes the surface properties of the thin film and catalytically activates the material.

MoS2-Thin layers are made up of alternating layers of molybdenum atoms and sulfur atoms, which are superimposed to form two-dimensional layers. The material is a semiconductor. But even a blue light pulse with surprisingly low intensity is enough to change the properties of the surface and make it metallic. The exciting thing about it: The MoS are in this metallic phase2-Layers are also particularly catalytically active. They can then be used, for example, as catalysts for splitting water into hydrogen and oxygen. As an inexpensive catalyst, they could thus enable the production of hydrogen.

Nomi Sorgenfrei and her team have set up a new instrument at BESSY II to precisely measure the changes in the samples due to irradiation with ultrashort, weak light pulses using time-resolved electron spectroscopy. These light pulses are generated at BESSY II with femtoslicing and are therefore of low intensity. The new “SurfaceDynamics @ FemtoSpeX” instrument can also obtain meaningful measurement data for electron energies, surface chemistry and changes over time from these weak light pulses in a short time.

Analysis of the experimental data showed that the light pulse leads to a temporary accumulation of charge on the surface of the sample, which triggers the phase transition on the surface from a semiconducting state to a metallic state. “This phenomenon should also occur in other representatives of this material class of p-doped semiconducting dichalcogenides, so that there are opportunities to influence the functionality and catalytic activity in a targeted manner,” explains Sorgenfrei.

Nuclear fission and nuclear fusion

For a nuclear fission z. B. uranium-235 can be bombarded with slow neutrons (see Fig. 1). Such a neutron can excite the uranium-235 isotope so that it breaks down into several parts within a very short period of time. The core was split.

In certain cases, when uranium-235 is bombarded with slow neutrons, a krypton-89 isotope, a barium-144 isotope and 3 free neutrons are produced.

Mass ratios in nuclear fission

Fig. 2 shows the "mass ratios" during the nuclear fission of uranium ‑ 235 by neutron bombardment into a krypton ‑ 89 isotope, a barium ‑ 144 isotope and 3 free neutrons.

The splitting process of (<> _ <92> ^ <235> < rm> ) when bombarded with a neutron is only one of many possible.

The * sign means that this nucleus is excited and is still decaying with the emission of further radiation.

The mass differences in a fission reaction are of course not so great that they could be determined with a beam balance, no matter how sensitive it is. Modern mass spectrometers, however, allow a very precise determination of the mass of atoms and atomic nuclei.

In the nuclear fission of uranium-235, the sum of the masses of the reaction products is smaller than the sum of the masses of the starting products. This so-called mass defect is responsible for the fact that energy is released during nuclear fission.

Binding energy per nucleon

In Fig. 3 the binding energy per nucleon of an atomic nucleus is shown as a function of the number of its nucleons, i.e. its mass number (A ).

From this representation you can see directly that the nuclear fission shown is exothermic and thus energy is released, since the end products of the reaction in the (A ) - ( frac) Diagram are higher than the starting product uranium & # 8209235.

Estimation of the energy released

For a rough estimate of the energy that is released during this reaction, one thinks that the uranium nucleus is first divided into its components. For this, the energy is (235 cdot 7 <,> 5 , rm) necessary because the mean energy of a nucleon in uranium is about (7 <,> 5 , rm) amounts to. Now, from the (235 ) free nucleons, we build two nuclei of medium weight, in which the mean binding energy is (8 <,> 5 , rm) amounts to. During this process an energy of (235 cdot 8 <,> 5 , rm) free. All in all, one gains about the energy [235 cdot 8 <,> 5 , rm in a fission reaction - 235 cdot 7 <,> 5 , rm = 235 , rm].

Note: Since the given energies of (7 <,> 5 , < rm> ) and (8 <,> 5 , < rm> ) are only approximate values, you only get an approximate value for the released energy.

German researchers on the way to artificial photosynthesis

Mülheim an der Ruhr - The efficiency of natural photosynthesis is still unmatched when it comes to the use of solar energy. Researchers approach this energy conversion step by step. In the journal "Angewandte Chemie", German researchers are now proposing a new class of catalysts for using sunlight to split water directly into oxygen and hydrogen. In addition, the titanium disilicide used can even store the hydrogen generated in this way.

"Our catalyst splits water with a higher degree of efficiency than most other semiconductor systems that also work with visible light," says Martin Demuth from the Max Planck Institute for Bioinorganic Chemistry in Mülheim an der Ruhr. This is because the semiconductor material titanium disilicide captures a broad spectrum of sunlight and can use it directly to split water molecules. At the beginning of the catalysis reaction, a slight oxide formation on the titanium disilicide ensures the formation of the necessary catalytically active centers.

Demuth emphasizes another advantage of these catalysts. Because titanium disilicide can also store the hydrogen obtained. Other, porous storage materials are more effective, but titanium disilicide releases the hydrogen at significantly lower temperatures. The researchers are convinced of the marketability of this relatively inexpensive catalyst for hydrogen production using sunlight. That is why they patented the processes and, together with American and Norwegian partners, founded a company for further development and marketing in Lörrach.

Pentlandite as an efficient electrocatalyst

The efficient reduction of water to hydrogen under mild conditions and without the use of expensive precious metals is a desirable goal for future-oriented and sustainable energy storage. In addition, the hydrogen produced has the highest energy density of all commercial fuels and only produces water as a waste product. While only a few effective non-precious metal-containing materials are known for this purpose, enzymes - the [FeNi] -hydrogenases - operate this conversion very efficiently. These enzymes are equipped with an iron-nickel-sulfur center. Surprisingly, the mineral pentlandite has a molecular structure comparable to the enzyme and can be used as an inexpensive, robust and highly efficient electrocatalyst for proton reduction.

Although hydrogen is a clean energy carrier and its use as a future energy carrier and storage device is increasingly being discussed, around 95% of the hydrogen produced worldwide is currently still produced by splitting fossil hydrocarbon sources at high pressures and temperatures [1, 2]. The electrochemical splitting of water is hardly considered, probably due to the electrode materials usually used. In particular, precious metals such as platinum and its precious metal alloys have proven to be extremely efficient in proton reduction (HER, Hydrogen Evolution Reaction), as they very effectively generate hydrogen at low voltages. Despite the excellent properties of platinum and its alloys for the HER, the commercial use is to be classified as uneconomical due to the low natural occurrence and the associated high price. Sulfur compounds of the transition metals, especially those from the manganese, cobalt, iron and nickel groups of the periodic table, which are known for their good electrocatalytic properties in the HER [3–5], should be mentioned as alternative materials. A major disadvantage of these systems, however, is the low conductivity of the materials and the associated need to use them as nanoparticles. These nanoparticles have to be manufactured in a large number of complex process steps and laboriously applied to a conductive carrier material.

The right catalyst makes it all
But what makes a good electrocatalyst? There will probably not be a simple and generally applicable answer to this question, but key points can be defined that make a desirable catalyst. First of all, there is a high catalytic activity for the reduction of water. In particular, a low overvoltage, i.e. the additional voltage that is required to allow the electrolysis to take place, is desirable here because less energy is "lost" when storing energy in hydrogen. A rapid reduction of protons on the catalyst is also desirable and manifests itself in high current densities with low overvoltages. However, these are only two criteria for a good catalyst. A high long-term stability, preferably over years, and a high current yield, which describes the actual efficiency of an electrode, are essential for a technically feasible application. In this respect, nature is far superior to technology and has formed effective catalysts based on nickel, iron and sulfur over the course of several million years.

An electrode material that has high activity over a long period of time with low overvoltage is the mineral pentlandite. This iron-nickel sulfide, very similar to [FeNi] -hydrogenase, with the composition Fe4.5Ni4.5S.8 can be produced artificially in high-temperature synthesis [6].

Stone with potential
Based on the elements iron, nickel and sulfur, the conversion to pentlandite takes place in the absence of air at 1,100 ° C. In contrast to nature, the synthetic pentlandite does not contain any contamination from silicates or other materials and therefore does not affect the catalysis results. Another advantage of this process: the electrical conductivity of the synthetic pentlandite is significantly increased and enables the “stone” (crude pentlandite) to be used directly as an electrocatalyst (Fig. 1).

For better reproducibility of the electrocatalytic measurements, finely powdered pentlandite can also be pressed into pellets, similar to commercially available glassy carbon electrodes, and used as electrode material (Fig. 2). The electrodes obtained in this way are stable in a sulfuric acid medium, in concentrated sodium hydroxide solution and also in non-aqueous solvents without any signs of corrosion. The testing of the catalytic properties was carried out in a three-electrode setup using electrochemical measurements. This allows electrocatalytic boundary conditions, such as overvoltages, conductivity of the material and long-term stability to be measured quickly and precisely. The overvoltage of the water reduction on the produced pentlandite electrodes was in 0.5 M H2SO4 measured and is just 280 mV at a current density of 10 mA cm-². This means that the overvoltage is significantly lower compared to known nanoscale electrocatalysts such as NiS2 (315 mV), FeS2 (400 mV) and MoS2 (374 mV) with the same current densities. It is also noteworthy that, in contrast to the nanostructured systems, no loss of activity whatsoever was observed at an applied voltage of -0.8 V. On the contrary, such a treatment led to a further activation of the material, in the course of which current densities of up to 650 mA cm -1 at -0.8 V (vs RHE) could be achieved. Furthermore, this activation led to a reduction in the overvoltage to just 190 mV, which is a record for non-precious metal-containing and macroscopic electrodes (Fig. 3).

Measurements over 170 hours at this high potential did not show any reduction in activity. However, which processes on the electrode surface are important for this activation and the observed catalytic activity? X-ray spectroscopic methods have shown that various sulfur components are slowly removed from the surface over the period of activation. The surface “cleaned” in this way has a higher metallic character and “freely accessible” metal centers. This surface modification also improves the catalytic activity for generating hydrogen. The amount of hydrogen released was measured over a period of 4 hours. With an electrode surface the size of a 5-cent piece, 340 mL of hydrogen are formed within an hour (Fig. 3). Almost all of the electricity was converted into hydrogen without any significant losses.

Pentlandite as a hydrogenase model
A plausible theoretical mechanism for the catalytic reduction of protons by the mineral provided information about the high activity of the material for hydrogen formation. The proton reduction can be formulated in three steps. First a proton binds between a nickel and iron atom and is converted to a hydride by electron transfer. This hydride reacts with another proton bound to the sulfur to form molecular hydrogen. The calculated mechanism for the formation of hydrogen on the pentlandite surface is astonishingly well comparable with the enzymatic model, [FeNi] -hydrogenase.

With its good electrocatalytic properties in proton reduction, pentlandite defies precious metals and artificial nanoparticles. Not only that the composition of iron, nickel and sulfur is significantly cheaper than platinum, but also its high robustness, high efficiency and the fact that no artificial nano-structuring is required make it a potential industrially usable catalyst for hydrogen generation .

Stefan Piontek and Ulf-Peter Apfel
Inorganic Chemistry, Bioinorganic Chemistry, Ruhr University Bochum, Bochum

Dr. Ulf-Peter Apple
Inorganic Chemistry 1, Bioinorganic Chemistry
Ruhr-University Bochum
[email & # 160protected], de

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[2] C. Agrafiotis, H. von Storch, M. Roeb, C. Sattler, Renew. Sustain. Energy Rev. 2014, 29, 656-682 - DOI: 10.1016 / j.rser.2013.08.050

[3] H. Li, C. Tsai, A. L. Koh, L. Cai, A. W. Contryman, A. H. Fragapane, J. Zhao, H. S. Han, H. C. Manoharan, F. Abild-Pedersen, et al., Nat. Mater. 2015, 15, 48-53 - DOI: 10.1038 / nmat4465

[4] M.-R. Gao, Z.-Y. Lin, T.-T. Zhuang, J. Jiang, Y.-F. Xu, Y.-R. Zheng, S.-H. Yu, J. Mater. Chem. 2012, 22, 13662 - DOI: 10.1039 / C2JM31916K

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Video: Δολοφονία στα Γλυκά Νερά: Καταπέλτης η εισαγγελική πρόταση (July 2022).


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