Archive for the ‘Catalysis’ Category

Coordinating nature and photochemistry to create hydrogen

When we look to our future energy resources, the need to realise new means of renewable energy is immediately obvious. Much research is being carried out around the world into the development of systems that can generate energy – from H2 to biofuels to solar fuels – all of which place great importance on high efficiency and sustainability.

Looking at the world around us for inspiration, the obvious candidate is the photosynthetic process, where visible light is employed to convert CO2 and H2O into chemical energy. This process involves the transport of electrons through a complex series of intricately aligned porphyrin-related and protein biomolecules. We can explore the development of a system that mimics the behaviour of natural systems, with respect to the relay of electrons along a series of molecules, or, alternatively, we can take the components in these systems and exploit their properties in combination with other electronically-active but non-natural molecules.

Upon photoexcitation of [Ru(bpy)3]2+, electron transfer through a ferredoxin scaffold to a cobaloxime catalyst facilitates the production of hydrogen.It is the latter approach which Lisa Utschig and her team from Argonne National Laboratory, near Chicago in the US, employed to generate a molecular system capable of photocatalysing the production of hydrogen. In their biohybrid system, the photosensitiser ruthenium(II) tris(bipyridine), ferredoxin (a water-soluble electron transfer protein), and cobaloxime (a cobalt(II)-based catalyst), were combined to generate a miniature reaction center that mimics those which occur in biological systems. However, the Utschig group’s system has a smaller molecular weight, which allows for characterisation of the electronic processes that occur in the system.

Lisa and her colleagues found that the presence of ferredoxin in the catalytic system acted as a scaffold to stabilise the charge-separated state necessary for electron transfer and the desired production of H2. They also observed that the catalytic behaviour of the Ru(II)–Co(II) pair was only possible in the presence of ferredoxin, which acted to extend the lifetime of the otherwise transient Co(I), allowing the desired reaction to occur.

In order to fully understand and enhance the properties of the molecular systems developed to fulfil the increasing need for energy alternatives, we need to be able to probe the structure and processes that occur in the molecule; the use of smaller analogs to those that exist in nature offers a means by which to achieve this goal. The photoactivated catalyst discussed in this work is an important step forward in the development of an optimized system for use in solar fuel production.

Read this hot ChemComm article in full:
Aqueous light driven hydrogen production by a Ru–ferredoxin–Co biohybrid
S. R. Soltau, J. Niklas, P. D. Dahlberg, O. G. Poluektov, D. M. Tiede, K. L. Lulfort and L. M. Utschig
Chem. Commun., 2015, 51, 10628–10631
DOI: 10.1039/C5CC03006D

Biography

Anthea Blackburn is a guest web writer for Chemical Science. She hails originally from New Zealand, and is a recent graduate student of Northwestern University in the US, where she studied under the tutelage of Prof. Fraser Stoddart (a Scot. There, she exploited supramolecular chemistry to develop multidimensional systems and study the emergent properties that arise in these superstructures. When time and money allow, she is ambitiously attempting to visit all 50 US states.

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Improvements to a selective hydrogenation process using ionic liquids

In this ChemComm communication, Peter Claus and co-workers describe an interesting application of room temperature ionic liquids to the selective hydrogenation of 2-hexyne. Unlike many reports in the literature, where an ionic liquid acting as a solvent may enhance a particular reaction, this report outlines a solid supported catalyst system modified with an ionic liquid layer.

Such materials, known as SCILLS, (solid catalyst with an ionic liquid layer) have been investigated in a variety of hydrogenation reactions. In this work the desired reaction is the reduction of 2-hexyne to cis-2-hexene. The catalyst is 1 wt% palladium on silica, modified with various loadings of 3 common ionic liquids: BMIM hexafluorophosphate, BMIM bis(triflouoromethanesulfonyl)imide and N-butyl-N-methylpyrrolidinium dicyanamide ([BMPL][DCA]). The performance of the unmodified catalyst was compared with the yield and selectivity afforded by the SCILL systems. The best results were reported with the dicyanamide ionic liquid SCILL, ([BMPL][DCA]) at 30 wt% ionic liquid loading.

In such a process, there are several reactions that must be suppressed. As the product is an olefin, isomerisation to the trans product must be controlled, as must further hydrogenation to the fully reduced material, hexane. For a number of reasons, based on the nature and amount of chemisorbed hydrogen, and favourable dicyanamide anion interactions with palladium, the dicyanamide SCILL system is particularly effective.

Notably, this system gives improved performance in terms of selectivity and yield over the two best performing commercial catalysts for this task. For example, Lindlar´s catalyst, palladium on calcium carbonate, deactivated with lead, cannot match its performance. In this work, the authors give an example of how ionic liquids can add value to a commercial process, while also offering considerable process improvements, in terms of toxicity and arguably, simplicity. The group’s focus now turns to SCILL activity and stability in a continuous hydrogenation process.

Read this RSC Chemical Communication today!

ionic-liquid layer
Frederick Schwab, Natascha Weidler, Martin Lucas and Peter Claus
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Printable Nanoscale Catalysts with Controlled Nano-morphologies

Nanoscale metal rings and dots could find potential use in a wide range of applications including catalysis. However, the impact the morphology differences have must be unambiguously ascertained before they can be used in practical applications. For this to be achieved there needs to be a simple and efficient fabrication process that can create arrays of nanoscale metal rings or dots for study.

Won Bae Kim and team, from the School of Materials Science and Engineering at the Gwangju Institute of Science and Technology, report such a method in their new ChemComm paper. They make use of the powerful transfer printing technique, but importantly have created suitable stamps that can generate ring or dot arrays. These stamps use one dimensional carbon nanostructures that are supported within the hexagonal pores of anodic aluminium oxide, the tip shapes being controlled by ion milling conditions. After loading with a suitable catalytic metal they are then used in transfer printing onto indium tin oxide substrates.

SEM images of nanoring and nanodot stamps showing the supported one dimensional carbon structures within the AAO pores.


The team demonstrate the catalytic ability of the printed metal ring and dot arrays by studying methanol oxidation in acidic solution with platinum structures and carbon monoxide electrooxidation in alkaline solution with gold structures. With this approach they were able to study the effect of morphology on the catalytic activity – to find out which was better, rings or dots, you will have to read the ChemComm article today!

To read the details for free* check out the Chem Comm article in full:

Transfer printing of metal nanoring and nanodot arrays for use in catalytic reactions

Sang Ho Lee, Sung Mook Choi, Seungha Yoon, Huisu Jeong, Gun Young Jung, Beong Ki Cho and Won Bae Kim

DOI: 10.1039/C4CC02939A

*Access is free untill Friday 4th July through a registered RSC account – click here to register

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The chemists’ enzyme

The title of this post was taken from the website of Barry Trost, one of the world’s leading scientists and author of an astonishing 924 papers. Describing his work, he states:

One major activity in designing new reactions and reagents involves the development of “chemists’ enzymes” – non-peptidic transition metal based catalysts that can perform chemo-, regio-, diastereo-, and especially enantioselective reactions.

Chemists have, for a long time, sought to reproduce the incredible feats of nature. Natural biology has evolved over many years to achieve the efficiency and reactivity that most lab-based chemists could only dream of. Nature achieves this by employing incredibly sophisticated enzymes which are, sadly, almost impossible to replicate by a synthetic chemist due to their complexity. An alternative idea is to use these enzymes as inspiration for new catalysts and try to focus on the general reasons why they work rather than trying to create direct copies.

Supramolecular catalysts for decarboxylative hydroformylation and aldehyde reduction.

Dr Bernhard Breit, Lisa Diab and Urs Gellrich at Albert-Ludwigs-Univertat in Germany have shown in a HOT ChemComm article that a highly selective catalyst can be created when combining a metal catalyst with a directing ligand to control the reaction. In this Communication, they report excellent results using  rhodium, the classic metal of choice for hydroformylation, and a functional group for recognition of the substrate. The net effect of these features combined is that the substrate is held in a specific way at the catalytic site. As a result, the reaction which follows can only occur in a specific way. This is similar to how enzymes control the chirality.

The concept behind this catalyst is one which could be applied to a great number of different reactions – no doubt we can look forward to reading about these in the near future.

Read this HOT ChemComm article today!

Tandem decarboxylative hydroformylation–hydrogenation reaction of α,β-unsaturated carboxylic acids toward aliphatic alcohols under mild conditions employing a supramolecular catalyst system
Lisa Diab, Urs Gellrich and Bernhard Breit
Chem. Commun., 2013, Advance Article
DOI: 10.1039/C3CC45547E, Communication

Ruaraidh McIntosh is a guest web-writer for ChemComm.  His research interests include supramolecular chemistry and catalysis.  When not working as a Research Fellow at Heriot-Watt University, Ruaraidh can usually be found in the kitchen where he has found a secondary application for his redoubtable skills in burning and profanity.

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A cloak of many carbons

Catalysts can be exceedingly useful in the real world, from treating our car’s exhaust fumes to creating fertilisers.  There are many ways to make catalysts and even multiple ways to make the same catalyst.  The path that you choose to a catalyst can have a significant impact on the quality of the end product.

Eloy del Rio and team from the Structure and Chemistry of Nanomaterials group at the University of Cadiz in Spain have investigated ceria-based oxide-supported gold catalysts for carbon monoxide oxidation.  The routine for depositing the metal phase onto the oxide support and the subsequent catalyst activation step can ultimately affect the activity of the catalyst.  Catalysts prepared by deposition-precipitation with urea followed by activation under oxidising conditions result in significantly more activity than those prepared under reducing conditions.

Variation in catalyst activity under oxidising and reducing activation protocols.

This had previously been observed by others, but the reason for the difference was never discussed.  The authors set out to find out why the activity differed.  They used a suite of nano-analytical and nano-structural techniques to probe the catalysts, finding that the catalyst prepared under reducing conditions had a coat of amorphous carbon which severely hampered the catalyst activity.  This could be removed by a re-oxidation treatment that burnt away the carbon layer and produced an active catalyst similar to the one produced under oxidising conditions.

The precipitating agent used in the synthesis can also influence the resulting activities of catalysts prepared via the deposition-precipitation method.  No difference between oxidising and reducing activations is observed when sodium carbonate is used in place of urea.

To read the details, check out the ChemComm article in full:

Dramatic effect of redox pre-treatments on the CO oxidation activity of Au/Ce0.50Tb0.12Zr0.38O2-x catalysts prepared by deposition-precipitation with urea: a nano-analytical and nano-structural study
E. del Rio, M. López-Haro, J.M. Cies, J.J. Delgado, J.J. Calvino, S. Trasobares, G. Blanco, M.A. Cauqui and S. Bernal
Chem. Commun., 2013, 49, Accepted Manuscript
DOI: 10.1039/C3CC42051e

Iain Larmour is a guest web writer for ChemComm.  He has researched a wide variety of topics during his years in the lab including nanostructured surfaces for water repellency and developing nanoparticle systems for bioanalysis by surface enhanced optical spectroscopies.  He currently works in science management with a focus on responses to climate change.  In his spare time he enjoys reading, photography and art.

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