Putting Carbon Dioxide to Good – Scientists Use Electrochemistry To Convert Carbon to Useful Molecules

 

A joint effort in chemistry has resulted in an innovative method for utilizing carbon dioxide in a positive – even beneficial – manner: through electrosynthesis, it is integrated into a series of organic molecules that play a crucial role in the development of pharmaceuticals.

During the process, the team made an innovative discovery. By altering the type of electrochemical reactor used, they were able to generate two distinct products, both of which are useful in medicinal chemistry.

The team’s paper was recently published in the journal Nature. The paper’s co-lead authors are postdoctoral researchers Peng Yu and Wen Zhang, and Guo-Quan Sun of Sichuan University in China.

The Cornell team, led by Song Lin, professor of chemistry and chemical biology in the College of Arts and Sciences, has previously used the process of electrochemistry to stitch together simple carbon molecules and form complex compounds, eliminating the need for precious metals or other catalysts to promote the chemical reaction.

For the new project, they set their sights on a more specific target: pyridine, the second-most prevalent heterocycle in FDA-approved drugs. Heterocycles are organic compounds in which the molecules’ atoms are linked into ring structures, at least one of which is not carbon. These structural units are considered to be “pharmacophores” for their frequent presence in medicinally active compounds. They are also commonly found in agrochemicals.

The researchers’ goal was to make carboxylated pyridines, i.e., pyridines with carbon dioxide appended to them. The advantage of introducing carbon dioxide to a pyridine ring is that it can change a molecule’s functionality and potentially help it bind to certain targets, such as proteins. However, the two molecules are not natural partners. Pyridine is a reactive molecule, while carbon dioxide is generally inert.

“There are very few ways of directly introducing carbon dioxide to a pyridine,” said Lin, the paper’s co-senior author, along with Da-Gang Yu of Sichuan University. “The current methods have very severe limitations.”

Lin’s lab combined its expertise in electrochemistry with Yu’s group’s specialization in utilizing carbon dioxide in organic synthesis, and they were able to successfully create carboxylated pyridines.

“Electrochemistry gives you that leverage to dial in the potential that is sufficient to activate even some of the most inert molecules,” Lin said. “That’s how we were able to achieve this reaction.”

The team’s serendipitous discovery emerged while they were conducting the electrosynthesis. Chemists typically run an electrochemical reaction in one of two ways: in an undivided electrochemical cell (in which the anode and cathode that supply the electric current are in the same solution) or in a divided electrochemical cell (whereby the anode and cathode are separated by a porous divider that blocks large organic molecules but allows ions to pass through). One approach may be more efficient than the other, but they both produce the same product.

Lin’s group found that by switching from a divided to an undivided cell they could selectively attach the carbon dioxide molecule on different positions of the pyridine ring, creating two different products: C4-carboxylation in the undivided cell and C5-carboxylation in the divided cell.

“This is the first time we discovered that by just simply changing the cell, what we call the electrochemical reactor, you completely change the product,” Lin said. “I think that mechanistic understanding of why it happened will allow us to continue to apply the same strategy to other molecules, not just pyridines, and maybe make other molecules in this selective but controlled fashion. I think that’s a general principle that can be generalized to other systems.”

While the project’s form of carbon dioxide utilization is not going to solve the global challenge of climate change, Lin said, “it’s a small step towards using excessive carbon dioxide in a useful way.”

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All you need to knowabout organic acids

 Organic acids are broadly dispersed in nature (animal, plant and microbial sources) and are produced by several fungi, yeasts and bacteria. Organic acids are categorised in the “weak” acid group that do not totally dissolve in water, and they comprise one or more carboxylic acid groups covalently linked in groups such as amides, esters and peptides.

Organic acids characteristics

The acid and base properties of organic compounds are very similar to the acid and base properties of inorganic compounds. Properties of acids include a pH less than 7, a sour taste, producing hydrogen ions when dissolved in water and being corrosive to human tissue and reactive with bases to form a salt and water. Common properties of bases include a pH more than 7, a “soapy” feel, a bitter taste and being corrosive to human tissue and reactive with acids to form a salt and water. Pigs have sensitivity for sour taste that is about tenfold than for sweet taste.

How many types of organic acids are there?

There are two types of organic acids. One has the carboxyl group (COOH group), for example acetic acid (CH3COOH) which is made by oxidising grain alcohol or by the fermentation of fruit sugar in cider. The second type has a phenol group (C6H5OH). Salicylic acid (OHC6H4COOH) is an example of an organic acid with both carboxyl and phenol groups.

Why are organic acids important?

Organic acids play a role in the regulation of basic cellular processes such as pH modification, signalling messengers and modulating transport across biological membranes, and they extensively modify the cellular, subcellular or extracellular compartments in which they are found due to their chemical properties. Therefore, organic acids can be involved in various biochemical and physiological processes in vivo. In addition, organic acids are involved in chemical modification of proteins, with high impact on the in vivo protein activity. The different roles of these compounds still remain to be explored.

Organic acids production and extraction

Chemical synthesis or fermentation are among the most used methods for organic acid production. In recent years, new techniques have been developed for fast and efficient extraction of organic compounds from different plant materials. Citric, lactic, gluconic and itaconic acids are produced industrially by microbial processes, which is a promising approach to obtain building block chemicals based on renewable carbon sources. In addition, large quantities of acetic acid are produced by bioprocesses and chemical synthesis. Microwave-assisted extraction is another technique to isolate various compounds from plants or vegetal materials for both analytical and industrial purposes.

Positive effects of organic acids

The positive effects of organic acids include:

• Antimicrobial activity;

• Decreasing the emptying rate of the stomach;

• Stimulating enzyme excretion and activity in the gut;

• Supplying nutrients to gut tissue;

• Improving mucosal integrity and function;

• Enhancing pepsin and microbial phytase activity;

• Inducing pancreatic excretion;

• Increasing protein digestion;

• Improving minerals utilisation;

• Reducing competition between the microflora and the host; and

• Improving pig health and productivity.

Organic acids can directly decrease the pH of the gut environment through the release of hydrogen ions, thus preventing or inhibiting the proliferation of acid-sensitive bacteria. The antimicrobial effect of organic acids is greater under acidic conditions and lesser at neutral pH. It is important to know that each organic acid has a microbial activity spectrum involved to a specific pH range, membrane structure and physiology in the cell of the microbiota species. In addition, organic acids are promising alternatives to antibiotics to promote nutrient digestibility by decreasing the pH of the upper region of the digestive tract.

The most used organic acids in swine nutrition

Some of the most used organic acids in swine nutrition include citric, lactic and formic acids.

Citric acid

Citric acid is an odourless, colourless compound with an acidic, sour-tasting nature. It is used predominantly as a flavouring and preserving agent in soft drinks and sweets, as a disinfectant and to stabilise or preserve medicines. Citric acid releases minerals bound to the phytate molecules, which in turn increases the utilisation of calcium, phosphorus and zinc. Dietary supplementation of citric acid improves the digestibility of dry matter, nitrogen and energy in finishing pigs and lactating sows, and the digestibility of crude protein, dry matter, fat and energy in growing pigs. In addition, citric acid supplementation to the diet of growing and finishing pigs decreases Escherichia coli counts and increases Lactobacillus counts, which improves gut health. Furthermore, in growing pigs diets supplemented with citric acid reduce the emission of acetic gas and the ammonia concentration.

Lactic acid

Lactic acid is an odourless, colourless liquid, corrosive to metals and tissue, which is produced during fermentation. Feeding post-weaning pigs with a diet supplemented with lactic acid improves average daily gain, average daily feed intake, feed efficiency and performance. In post-weaning pigs, supplementing the diet with lactic acid improves the digestibility of dry matter, increases bacilli and lactobacilli concentrations and reduces Salmonella and E. coli counts.

Formic acid

Formic acid is a colourless liquid with a pungent odour. It is corrosive to metals and tissue. Formic acid is the simplest carboxylic acid and is present in various sources such as bee venom and ant stings. It is also used as a preservative and antibacterial agent in livestock feed. Formic acid reduces bacterial nitrogen in the pig gut and improves apparent ileal digestibility of crude protein, essential amino acids, lipids, calcium and phosphorus. In addition, formic acid supplementation enhances the digestibility of dry matter in post-weaning pigs and improves gut health.

Concluding remarks

Organic acids reduce gastric pH, prevent the growth of pathogens, act as an energy source, increase apparent total tract digestibility, improve gut health and enhance growth performance and productivity. However, the effect of organic acids in practice is not always consistent due to the wide variety of available products and the various recommended effective dosages with the different combinations. Type composition, dosage, formula, feeding regimen, environment, nutrient composition of feed and the age and health status of animals all affect the efficacy of organic acids. Therefore, furthermore research is required to establish effective dosage and combination of organic acids to achieve the best possible results.

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New Technology Bonds Aluminum With Polyamides

 Celanese Engineered Materials has developed a new chemical bonding technology for joining its Zytel polyamides and aluminum—a common design challenge in numerous automotive and industrial applications. Zytel Bonding Technology works via application of a liquid solution to an aluminum surface that is then activated via hot plate welding or overmolding. The resulting structural bond leads to higher burst, tensile and shear strength than conventional adhesives, mechanical interlocking and mechanical assembly methods. In burst pressure testing, the joint held while the plastic showed a cohesive failure.

With the ability to be added at room temperature, the coating can be sprayed, brushed, dipped or applied to a substrate in an injection mold, and it features a shelf life of up to 3 years. Compatible with multiple polyamide chemistries, this new bonding technology can join PA6, PA610, PA612, PA66, and PPA with aluminum. Celanese notes that all of these materials are available within the Zytel and Zytel HTN portfolios.

The disparate materials are joined in a three-step process, starting with the removal of aluminum oxide from the aluminum part surface via water sanding, wet blasting, or plasma/laser treatments. Next, Zytel Bonding Technology is applied via spray, brush, dip or other methods at room temperature. The coating is activated in the third step by overmolding plastic onto an aluminum insert or by hot plate welding a previously molded Zytel part to the aluminum component. The chosen method depends on the geometry of the final hybrid aluminum-plastic component.

At K 2022, Giacomo Parisi, marketing director automotive electrification at Celanese (formerly Dupont), discussed development of the technology and an early potential application—a hybrid cooling plate for an electric vehicle battery module.

“Hybrid” in this case because the two-part component features a plastic bottom and aluminum top. In an electric vehicle, these plates sit below the battery modules with water glycol cycling through them to ensure the batteries stay within the optimal operating range between 20 and 40°C.

At present, most cooling plates are monomaterial metal-on-metal constructions that experience the dissipation of heat through the bottom of the part. By utilizing a Zytel polyamide, the hybrid plate features better insulative properties. The company undertook testing to ensure the plastic/metal component could withstand the pressures and water-glycol exposure to be experienced in the field by the part. In the end, a 30% glass-filled Zytel HTN family of polyamides based on PPA was chosen. In addition to its robust mechanical properties and strong chemical resistance, this material was picked on the basis of its high dimensional stability—requisite in what has to be a flat part.

From a stress perspective, the water-glycol within the cooling plate reaches pressures of approximately 10 bar (145 psi). In terms of application temperatures, on the high side, the cooling plates work to keep the battery modules from overheating and cascading into a thermal runaway situation. In colder environments, the plates work to keep the battery modules from dropping below freezing.

In addition to the cooling plate, Celanese sees potential application of the technology in EV battery plugs; structural components, like beams, brackets, pillars, and mounts; noise and vibration reduction components; filters; modules; housings; clips; quick connectors; and flow leads, among others.

The cutaway in the center shows the combined polyamide/aluminum cooling plate that sits beneath battery modules in electric vehicles.
Photo Credit: Celanese Engineered Materials

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The electron–proton bottleneck of photosynthetic oxygen evolution

Photosynthesis fuels life on Earth by storing solar energy in chemical form. Today’s oxygen-rich atmosphere has resulted from the splitting of water at the protein-bound manganese cluster of photosystem II during photosynthesis. Formation of molecular oxygen starts from a state with four accumulated electron holes, the S4 state—which was postulated half a century ago1 and remains largely uncharacterized. Here we resolve this key stage of photosynthetic O2 formation and its crucial mechanistic role. We tracked 230,000 excitation cycles of dark-adapted photosystems with microsecond infrared spectroscopy. Combining these results with computational chemistry reveals that a crucial proton vacancy is initally created through gated sidechain deprotonation. Subsequently, a reactive oxygen radical is formed in a single-electron, multi-proton transfer event. This is the slowest step in photosynthetic O2 formation, with a moderate energetic barrier and marked entropic slowdown. We identify the S4 state as the oxygen-radical state; its formation is followed by fast O–O bonding and O2 release. In conjunction with previous breakthroughs in experimental and computational investigations, a compelling atomistic picture of photosynthetic O2 formation emerges. Our results provide insights into a biological process that is likely to have occurred unchanged for the past three billion years, which we expect to support the knowledge-based design of artificial water-splitting systems. Main In all plants, algae and cyanobacteria, sunlight drives the splitting of water molecules into energized electrons and protons, both of which are needed for the reduction of CO2 and eventually carbohydrate formation2. Molecular oxygen (O2) is formed during this process, which transformed the Earth’s atmosphere during the ‘great oxygenation event’3, which began about 2.4 billion years ago. Light-driven water oxidation occurs at the oxygen-evolving complex, a Mn4CaO5 cluster bound to the proteins of photosystem II2,4 (PSII). The relationship between electron and proton transfer in the bottleneck steps of O2 formation has remained incompletely understood. We address this key step here using time-resolved Fourier transform infrared (FTIR) experiments 

Reaction cycle of photosynthetic oxygen evolution. figure 1 a, Model of the S-state cycle with sequential electron and proton removal from the oxygen-evolving site10,11,50. Starting in the dark-stable S1 state, each laser flash initiates oxidation of the primary chlorophyll donor (P680+ formation) followed by electron transfer from a tyrosine sidechain (TyrZ oxidation) and—in three of the four S-state transitions—manganese oxidation, until four electron holes (oxidizing equivalents) are accumulated by the Mn4Ca-oxo cluster in its S4 state. b, Example of tracing S-state transitions using IR absorption changes after excitation with visible-wavelength laser flashes (at zero on the time axis). The absorption changes (ΔA) are provided in optical density (OD) units. The IR transients at 1,384 cm−1 reflect symmetric stretching vibrations of carboxylate protein sidechains that sense changes in the oxidation state of manganese in the microsecond and millisecond time domain (coloured lines are simulations with time constants provided in Supplementary Table 2). Note that the scale on the x axis is linear below t = 0 and logarithmic above t = 0. c, The Mn4Ca cluster (Mn, violet; Ca, pink) in the S3 state with six bridging oxygens, the redox-active tyrosine (TyrZ), and further selected protein sidechains as well as water molecules (red spheres), based on crystal structures25. Assignment to polypeptide chains, numbering of the atoms of Mn4Ca-oxo and water molecules and hydrogen-bond distances are indicated in Supplementary Fig. 1. The two oxygens atoms that form the O–O bond in the oxygen-evolving S3 → S0 transition are indicated by red arrows.

Time-resolved tracking of O2 transition

To perform time-resolved infrared spectroscopy on PSII, we developed an FTIR step-scan experiment with automated exchange of dark-adapted PSII particles (Methods), thereby expanding previous experiments at individual wavenumbers towards detection of complete fingerprint spectra. The sample exchange system was refilled about every 60 h using PSII membrane particles with about 1.5 g of chlorophyll prepared from 40 kg of fresh spinach leaves for day and night data collection over a period of 7 months. We initiated the transitions between semi-stable S states by 10 visible light (532 nm) nanosecond laser flashes applied to the dark-adapted photosystems   Using a specific deconvolution approach based on Kok’s standard model1 (Fig. 1a), we obtained time-dependent S-state difference spectra for each of the individual transitions between the four semi-stable reaction-cycle intermediates S1, S2, S3 and S0 (for selected time courses see Extended Data Fig. 2).

We focus on the oxygen-evolution transition, S3 → S4 → S0 + O2, predominantly induced by the third laser flash, for which time courses at selected wavenumbers are shown in F (time-resolved spectra are shown in Extended Data Fig. 3). Multiexponential simulations of the time courses provided 5 time constants describing acceptor- and donor-side PSII processes, including the expected time constants of 340 µs and 2.5 ms. The 2.5-ms time constant (tO2${�}_{{\mathrm{O}}_2}$) corresponds to the reciprocal rate constant of the rate-determining step in O–O bond formation and O2 release8,9. The 340 µs time constant (tH+${�}_{{\mathrm{H}}^{+}}$) corresponds to an obligatory step of proton removal from the oxygen-evolving complex of PSII, as shown recently by time-resolved detection of X-ray absorption, UV-visible spectroscopy, recombination fluorescence and photothermal signals  resulting in a specific Mn(IV)4 Tyrox∙Z${\mathrm{T}\mathrm{y}\mathrm{r}}_{\mathrm{Z}}^{\mathrm{o}\mathrm{x}\bullet }$ metalloradical intermediate that was also trapped in low-temperature magnetic resonance experiments  ‘Obligatory’ here signifies that the O–O bond formation chemistry can proceed only after proton removal is complete, as verified by the delayed onset of signals that trace manganese oxidation states or, generally, the O2 formation chemistry , which is also visible in the top time course of Fig.  For systematic analysis of the 2D time–wavenumber data array obtained by the FTIR step-scan experiment, we exploited that the requirement for wavenumber independence of the time constants of proton removal (tH+${�}_{{\mathrm{H}}^{+}}$ = 340 µs) and the electron transfer associated with O2 formation (tO2${�}_{{\mathrm{O}}_2}$ = 2.5 ms), because they always reflect the same reaction (the same rate constant). The time constants can thus serve as a kinetic tag of the reaction in the time-resolved spectroscopic data. By simultaneous simulation of the time courses at 2,582 wavenumbers (1,800 cm−1 to 1,200 cm1) using the same set of time constants at each wavenumber, we obtained the amplitude spectra shown in Fig.  , which are denoted as decay-associated spectra (DAS).

a, IR time traces at selected wavenumbers, demonstrating the delayed onset of O–O bond formation (1,381 cm−1) and reversible changes assignable to transient sidechain deprotonation (1,571 cm−1 and 1,707 cm−1). The corresponding wavenumbers in the spectra in b–d are marked with coloured asterisks. b–d, DAS corresponding to the proton release phase (tH+${�}_{{\mathrm{H}}^{+}}$ = 340 µs, blue line) and the oxygen-evolution phase (tO2${�}_{{\mathrm{O}}_2}$ = 2.5 ms, green) as well as the steady-state difference spectrum of the S3 → S0 + O2 transition (dashed black line). Red areas b,d mark inverted 340 µs DAS and 2.5 ms DAS, indicating reversible behaviour; purple shaded areas in c highlight the similarity of the 2.5 ms DAS and the steady-state spectrum, in line with the assignment to non-transient changes in Mn oxidation state. 

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