Category: Chemistry

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A new method to dehydrogenate alkanes at ambient conditions

The chemical term alkanes, or paraffins, refers to organic compounds that consist of single-bonded carbon and hydrogen atoms, such as methane, ethane, and propane, and several other hydrocarbons. Over the years, alkanes have become widely used in organic chemistry, due to their unique chemical properties and their role in producing chemical reactions.

In recent decades, chemists have been exploring the possibility of breaking alkanes down through a process known as “non-oxidative dehydrogenation,” to attain valuable carbon feedstocks and hydrogen fuel. This could ultimately have valuable implications for the energy sector, as it would provide a low-cost method of creating  for new hydrogen-based energy solutions.

Unfortunately, the C-H bonds in alkanes can generally only be broken down at high temperatures, under ultraviolet light, or using stoichiometric oxidants. This hinders the large-scale adoption of this approach to create hydrogen fuel.

Researchers at Jilin University in China have recently introduced a possible strategy to achieve the non-oxidative dehydrogenation of alkanes in visible to near-infrared light and at room temperature. Their proposed method, introduced in a paper published in Nature Energy, entails the use of platinum/ (Pt/black TiO2) photocatalysts, in which platinum atoms are close but not fully bonded.

“The initial goal of our study was to replace the commercial TiO2 used in my previous work with black TiO2 , to enhance the light-absorption capacity of the catalyst,” Lu Li said. “In our experiments, we utilized solar light energy to drive the non-oxidative dehydrogenation of alkanes at room temperature. The introduction of clean photon energy can overcome the thermodynamic limitation, making alkanes conversion occur at ambient conditions with higher selectivity and durability.”

In initial experiments, the approach by Li and his colleagues achieved highly promising results, as it could efficiently dehydrogenate different alkanes at room temperature and in visible to near-infrared light. For cyclohexane, their photocatalysts enabled the production of H2 with a turnover number of 100,000, with the reaction effectively ongoing for 80 reaction cycles. This is a significantly better result than those achieved by thermal reactions.

For methane, on the other hand, the researchers achieved a conversion rate of 8.2%, with a 65% selectivity to propane. Finally, for C2+ alkanes, Li and his colleagues achieved a fast dehydrogenation (up to 1,440 µmol g−1 h−1) to the corresponding olefins.

“The brand-new ‘single-atom collection’ catalysts can combine the advantages of single-atom catalysts and nano-catalysts,” Li said. “This new class of catalysts may have potential applications in many important heterogeneous catalytic processes.”

In the future, the photocatalysts used by this team of researchers could prove to be highly valuable for enabling the production of hydrogen fuel from alkanes, without the need for high temperatures, UV-light, and stoichiometric oxidants. This could help to lower the cost of  production, thus potentially facilitating its use for energy applications.

“To extend this work, we are now looking for efficient photocatalysts based on cheap metals,” Li added. “Furthermore, we will extend the substrates to various non-toxic saturated hydrocarbons.”

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Developing self-complementary macrocycles with ingenious molecules

Credit: Tokyo Tech

Some biological molecules with efficient noncovalent bonding sites can use their bonding properties to create well-defined assemblies from a single class of molecules—i.e., they assemble with each other. These molecules, which are frequently seen in nature, are referred to as “self-complementary assemblies.” For instance, the p24 protein hexamer, which is part of the capsid of the HIV (human immunodeficiency virus), is composed of six protein subunits that complementarily self-assemble using many hydrogen bonds.


This phenomenon allows well-designed molecules to form higher-ordered assemblies without the metal ions that are commonly used as “joints” between monomer molecules. Indeed, many self-complementary assemblies have been reported on the basis of intrinsic hydrogen bonds, π-interactions, and coordination bonds.

However, self-complementary assembly based on host-guest systems is rare and notoriously difficult to control. In order to further our understanding of self-complementary assembly with higher-ordered structures, many new strategies have come to light in recent years.

Now, a team of researchers from the School of Science at Tokyo Institute of Technology (Tokyo Tech) might have just cracked the code to developing these innovative systems. The team, led by Assistant Professor Masahiro Yamashina and Professor Shinji Toyota, has constructed a novel self-complementary macrocycle assembly using an anthracene-based tweezer-like molecule with a pyridinedicarboxamide (PDA) linker as the monomeric species. Their work is described in Nature Communications.

“The molecule we use has an interesting property: It can bond with itself in two ways and form self-complementary structures. Not only does it show head-to-tail π-π interactions between the electron-rich tweezer tail (the anthracene groups) and the electron-deficient head, but also presents hydrogen bonding through the amide (-NH) functional group. By incorporating these two interactions, a preferential direction of self-assembly is achieved, and this guides the formation of the macrocycle,” explains Prof. Yamashina.

This type of dual interaction leads to much more control over the formation of synthetic macrocycles, and in this case, gives rise to a stable self-complementary hexameric structure upon crystallization. These hexamers can further assemble into even bigger self-complementary structures in the right conditions.

“When we added trifluoroacetic acid (TFA), we found that the cyclic hexamers further assemble into two predominant, stable supramolecular structures: rhombohedral grid assemblies and giant spherical cuboctahedra, a so-called hierarchical assembly,” Prof. Toyota says. “The latter structure is particularly impressive as it is formed from 108 monomeric tweezer units.”

Current methods to form supramolecular assemblies require metals that could harm the environment and ecosystems. The metal-free alternative method described here could produce novel supramolecular structures using a simple anthracene-based tweezer molecule. It opens the door to a new range of supramolecular assemblies with optical and electronic functions. This work adds a new important tool in the chemistry toolbox, one that is sure to play a big role in the metal-free supramolecular structures of the future.

 

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‘Smart plastic’ material is step forward toward soft, flexible robotics and electronics

Inspired by living things from trees to shellfish, researchers at The University of Texas at Austin set out to create a plastic much like many life forms that are hard and rigid in some places and soft and stretchy in others. Their success—a first, using only light and a catalyst to change properties such as hardness and elasticity in molecules of the same type—has brought about a new material that is 10 times as tough as natural rubber and could lead to more flexible electronics and robotics.


The findings are published today in the journal Science.

“This is the first material of its type,” said Zachariah Page, assistant professor of chemistry and corresponding author on the paper. “The ability to control crystallization, and therefore the physical properties of the material, with the application of light is potentially transformative for wearable electronics or actuators in soft robotics.”

Patterned sample is being stretched under uniaxial tension. Video was recorded with the sample between cross-polarizers, allowing for visualization of polymer chain alignment. The dark, opaque sections have been hardened. The transparent sections have been left soft and stretchy. Credit: The University of Texas at Austin

Scientists have long sought to mimic the properties of living structures, like skin and muscle, with synthetic materials. In living organisms, structures often combine attributes such as strength and flexibility with ease. When using a mix of different synthetic materials to mimic these attributes, materials often fail, coming apart and ripping at the junctures between different materials.

Oftentimes, when bringing materials together, particularly if they have very different mechanical properties, they want to come apart,” Page said. Page and his team were able to control and change the structure of a plastic-like material, using light to alter how firm or stretchy the material would be.

Patterned island sample is being stretched and relaxed under uniaxial tension. Video was recorded with the sample as seen (left) and between cross-polarizers (right), allowing for visualization of polymer chain alignment. The dark, opaque spots are areas that have been hardened. Credit: The University of Texas at Austin

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Chemists started with a monomer, a small molecule that binds with others like it to form the building blocks for larger structures called polymers that were similar to the polymer found in the most commonly used plastic. After testing a dozen catalysts, they found one that, when added to their monomer and shown visible light, resulted in a semicrystalline polymer similar to those found in existing synthetic rubber. A harder and more rigid material was formed in the areas the light touched, while the unlit areas retained their soft, stretchy properties.

Because the substance is made of one material with different properties, it was stronger and could be stretched farther than most mixed materials.

The reaction takes place at room temperature, the monomer and catalyst are commercially available, and researchers used inexpensive blue LEDs as the light source in the experiment. The reaction also takes less than an hour and minimizes use of any hazardous waste, which makes the process rapid, inexpensive, energy efficient and environmentally benign.

Patterned suture sample is being stretched under uniaxial tension. Video was recorded with the sample between cross-polarizers, allowing for visualization of polymer chain alignment. Credit: The University of Texas at Austin

The researchers will next seek to develop more objects with the material to continue to test its usability.

“We are looking forward to exploring methods of applying this chemistry towards making 3D objects containing both hard and soft components,” said first author Adrian Rylski, a doctoral student at UT Austin.

The team envisions the material could be used as a flexible foundation to anchor electronic components in medical devices or wearable tech. In robotics, strong and flexible materials are desirable to improve movement and durability.

Patterned sample is being melted to show complete transparency and later the opacity returning as the sample cools and returns to a semicrystalline state. Credit: The University of Texas at Austin

Henry L. Cater, Keldy S. Mason, Marshall J. Allen, Anthony J. Arrowood, Benny D. Freeman and Gabriel E. Sanoja of The University of Texas at Austin also contributed to the research.