The Student Spot is a regular feature on the Radleys blog in which we invite those who are studying, researching, or practising chemistry to share their thoughts on the latest developments in the world of science.
In this edition, student Jimmy Brancho, a Chemistry Graduate Student at the University of Michigan, talks though using electrodes that respond to solar light to carry out oxidation reactions on organic substrates.
Picture an organic chemistry lab: stirring flasks crowded onto hot plates, a rotovap on every desk, a veritable forest of NMR tubes, and at least one 3-liter round-bottom that everyone secretly dreams of using someday.
But what about alligator clips, coils of copper wire, and bright blue LEDs? Is “voltage” on your list of reagents?
Green chemists from several fields are turning to some unconventional methods to reduce the amount of waste generated in some kinds of organic reactions. Borrowing from the solar energy storage playbook, researchers have begun to use solid-state semiconductor materials as photocatalysts to oxidize alcohols.
Hypothetically, semiconductor photocatalysts could cut down on the amount of reagent that needs to be added to make these reactions work, leading to big savings on energy intensity and a boost to efficiency.
In a conventional oxidation reaction, alcohol is mixed in with a chemical oxidant in some supporting solvent. The reaction usually succeeds in creating a high yield of oxidized aldehyde product, but the leftovers from the chemical oxidant not incorporated into the product mean that more waste products get generated as well.
Also, the chemical oxidant took energy to prepare, and preparing it generated still another batch of waste. Since it is only possible to use the oxidant one time, all of the energy used to make the oxidant is spent in one place.
With semiconductors, researchers are showing that we can do better than that. When light of the right energy hits a semiconductor, it creates an electron-hole pair. The hole, an energized spot on the semiconductor where electrons used to be, is hungry to get those electrons back, so it takes them from the alcohol by oxidizing it.
The semiconductor’s excited electron gets used up when another chemical in solution oxidizes the semiconductor to complete the cycle.
A typical photoelectrochemistry apparatus for organic oxidations. The front rod with the green clip is the working electrode, of which TiO2 is an example. The white clip holds the reference electrode, which controls the applied voltage. In the back with the red clip is a platinum electrode which uses TiO2’s photogenerated electrons to reduce the most willing solution species and complete the circuit. These reactions are usually run overnight.
The key point: if the chemical that oxidizes the semiconductor is something cheap, benign, and renewable, like water or oxygen, then the overall process is energetically less expensive and less wasteful than using a chemical oxidant.
Additionally, the energy used to drive the reaction can come from sunlight rather than the latent energy, probably derived from fossil fuels, stored in a chemical oxidant.
Add that in with an abundant catalyst that can be reused again and again, and things are starting to look pretty green.
The main material under study for semiconductor oxidations is titanium dioxide. TiO2 is cheap and notoriously stable in water. It is also one of the first materials ever studied for photochemical water splitting, a field of solar energy research that has provided the chemistry community with much of what it knows about how reactions happen on semiconductor surfaces.
Since TiO2 is so well-studied, there is a wealth of knowledge available for green chemists hoping to use it as a catalyst and understand what they see going on.
What are researchers seeing? One of the cornerstone reports comes from Dr. Jincai Zhao’s group at the Chinese Academy of Sciences.
Zhao’s group showed in 2008 in Angewandte Chemie that TiO2 powder and light could be used to activate the common organic oxidant TEMPO, which went on to react with a range of different alcohols to give the corresponding aldehydes. The paper showed that many different functional groups, from alkenes to halides, could survive the reaction. In fact, the selectivity for aldehydes was excellent. No values below 93% were reported.
Energy schematic of a photochemical semiconductor oxidation. After the semiconductor is exposed to light, photogenerated holes (h+) oxidize benzylamine, which goes through a net condensation reaction to produce benzylidenebenzylamine and ammonia. At the same time, photogenerated electrons (e‑) reduce oxygen to water. The environmentally benign side products are circled in green, and besides substrate, only oxygen is added as a reagent.
The main drawbacks of Zhao’s initial system are the use of expensive TEMPO and slow reaction times. Some alcohols were less than half reacted after almost a day, and the reactions became much slower upon scaling past milligram amounts.
Later on, Zhao’s group showed that in the specialized solvent benzotrifluoride, TiO2 could oxidize alcohols without TEMPO. TiO2 oxidizes the alcohol directly, then combines the oxidized intermediate with molecular oxygen to create an aldehyde where the carbonyl oxygen atom originally came from O2. Zhao’s group teased out that observation by isotope labeling.
They used “heavy” oxygen labeled with two extra neutrons to carry out the reactions, then observed whether the heavy oxygen ended up in their product. Taking the TEMPO out of the reaction greatly reduces the cost and increases the atom economy – both exciting strides for the research. However, the reactions are still hindered by slow conversion rates and small scale; the question of why the semiconductor oxidations slow down at higher scales hasn’t yet been answered.
Since its publication eight years ago, Zhao’s Angewandte report has been cited almost 200 times and has given rise to a new niche field in the semiconductor photocatalysis community.
Semiconductors with smaller band gaps capable of absorbing more light have been tested, along with non-oxide semiconductors such as zinc indium sulfide. With each paper, the range of substrates tested increases. Besides alcohols, researchers have begun to investigate converting amines to imines and other oxidized products as well.
;Alongside the photochemical work, reports of using light and electricity to affect some of the same reactions have begun to crop up. TiO2 can easily be stuck to a conductive material to create an electrode. With TiO2 electrodes in hand, chemists can employ photoelectrochemical techniques, which combine photochemistry with an additional electrical voltage. In photoelectrochemistry, TiO2’s excited electron no longer has to do a chemical reaction; the voltage provided whisks the electron away to another part of the cell, where an entirely different material uses it.
A close-up of the electrodes in action.
A photoelectrochemical reaction has more moving parts at first glance. However, the fact that each electrode can be optimized independently for the function it’s supposed to perform is a major advantage to photoelectrochemistry – TiO2 no longer has to do two things at once.
A 2014 report by Xiaofeng Huang and coworkers at Tongji University in Shanghai shows that alcohols can be selectively oxidized photoelectrochemically using a TiO2 electrode. Huang’s team examined benzyl alcohol, a substrate that Zhao had also tested, and measured how quickly they could completely oxidize it. The TiO2 electrode completed the reaction in less than half the time that it took Zhao’s TiO2 powder by itself, retaining all of the selectivity.
Huang’s study highlights another interesting facet of photoelectrochemical systems. Because the voltage at the electrode can be precisely controlled, the rate of the reaction is easily tunable. Changes in the applied potential could also affect the reaction mechanism, which means the distribution of products that come out of the reaction might change with applied potential as well. It remains to be seen to what extent chemists will take advantage of photoelectrochemistry in organic synthesis.
Based on these initial results, it’s not hard to imagine huge photochemical reactors churning out aldehyde product with great selectivity. But there are quite a few hurdles left before the research can become commercially viable. First of all, in many systems, full conversion is only obtained after very long reaction times, if at all.
Secondly, the reusability of the photocatalyst or photoelectrode has yet to be established. Even though TiO2 and other materials used in these reactions are very stable, there are other ways for them to become deactivated besides their decomposition, such as side products adsorbing to the particle surfaces.
A photochemical reactor, where a powder is simply stirred into a solution in front of a light source.
Organic oxidations are an exciting frontier for photochemists and green chemists alike. If you’re an organic chemist, it’s probably too early to throw out the Dess-Martin reagents, but keep an eye on this field for an interesting new twist on some old reactions.
Oh, and try not to get hypnotized by that blue LED glow.
The author would like to thank Aaron Proctor for helping to assemble sources for this piece, and for setting up the experiments in the photographs.
Graduate Student – University of Michigan Department of Chemistry
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