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New materials: when chemistry combines metals and organic molecules

Since the 1990s, chemistry has been able to successfully synthesize hybrid polymers by combining metals with organic molecules. Laboratories are racing to identify the most interesting new nanoporous materials and patent their synthesis processes while new industrial uses are being explored.

12
June 2017
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Since the 1990s, chemistry has been able to successfully synthesize hybrid polymers by combining metals with organic molecules. Laboratories are racing to identify the most interesting new nanoporous materials and patent their synthesis processes while new industrial uses are being explored.

Paris Innovation Review – Among the “new materials” developed by chemical sector over the past two decades, porous organic-inorganic hybrids have aroused keen interest. What is their novelty?

François-Xavier Coudert – Today, the industrial use of porous materials is mainly focused on inorganic materials. More precisely, on alumino-silicates of the zeolite family, which offer a remarkable mechanical, thermal and chemical stability. They are used in catalysis, fine chemistry, hydrocarbon cracking, gas separation, gas capture, storage, etc. These are the main applications of porous materials – think of a sponge, or a pumice stone. In simple words, alumino-silicates offer the advantage of being “microporous,” with pores (“holes”) of nanometric size which provide a larger surface inside the material to fix molecules.

Zeolites, whether natural or synthetic, have been known for a long time. But the bonding properties of atoms set a limit: we don’t have complete control over their chemistry – and very few possibilities to change it. As a result, we know how to produce several very stable materials and since the others are much more difficult to synthesize, we always fall back on the same products.

The breakthrough occurred in the 1990s. It came from organometallic chemistry, which synthesizes the so-called “coordination” polymers: one metal brick, one organic brick, one metal, one organic... and so on.

It was then discovered that some of these polymers form a three-dimensional architecture which can be porous. At the time, it was rather a curiosity. The 2000s saw the rise of the concept of metal-organic frameworks (MOF), which is the name by which it is still known today. An American researcher, Omar Yaghi, realized that this porosity could be exploited: the performance of MOFs is not much different from the usual inorganic materials, but they present much more variability. Simply because instead of Si–O or Al–O bonds, each organic molecule within the material can be functionalized. It can be replaced or modified by one or several chemical groups that have a specific function.

If, for example, a material with larger pores is required, the space between metals is increased by choosing organic molecules which have the same chemistry but are longer. If we need materials containing a specific chemical group which will bind to CO2 – for example, an amine function – it is grafted onto the organic molecule, and every organic molecule of the material will receive an amine function. Hence, the material will have a great potential for the capture of CO2.

Hence, all the fineness of organic chemistry can be applied to these materials and this opens the possibility of creating new personalized materials.

Do organic and inorganic chemistry involve very different processes?

No, not radically different. But the know-hows are not the same. It is part of a logic of specialization: when chemistry developed during the 20th century, it fragmented in different currents because of a large number of procedures for synthesis, of possible reactions. Specialization went even further: in inorganic chemistry, you can find specialists of manganese or iron... But there is no conceptual breakthrough, as such.

The meeting of organic and inorganic chemistry is therefore possible, and it happened. In practice, researchers interested in hybrids are rather specialists of the chemistry of materials – more specifically, of inorganic chemistry – for a very simple reason: adding an amine to a molecule is not very complicated for organic chemistry, a field whose frontier is much closer to biochemistry, asymmetric catalysis and biosynthesis.

On the other hand, hybrids are far more innovative if considered from the point of view of inorganic chemistry. For example, these structures share a certain level of disorder and irregularities which can lead to interesting mechanical or chemical properties. But above all, hybrids allow for much more varied combinations, whereas a strictly inorganic chemistry quickly finds its limits.

Could you please clarify this matter?

Most of the time, structures are crystalline. In any case, they are regular enough to be exploitable. Synthesis is carried out at room temperature or by heating slightly, during a few hours. After crystallization, the product is recovered under the form of a powder and characterized through an exposure to X-rays. The techniques used for synthesis and characterization aren’t revolutionary, as you will have noticed. What is truly revolutionary is the Lego game with molecular structures. These structures are potentially infinite in number. Approximately 10,000 have been fully characterized, but we also need to count all of those that have been synthesized and still haven’t been completely characterized.

Several labs have specialized in large-scale, automated synthesis: they change concentrations, synthesize many materials that are characterized in a very incomplete way. By recording the corresponding X-ray composition and diffraction pattern (a sort of ID card of the material), they obtain a database of potentially usable materials — and a head start on their discovery and synthesis, even if they don’t fully characterize the properties of the material. For now, the issue isn’t financial yet. Rather, of occupying space and defining ourselves as important players in this field.

What are the main labs working on the development of these materials?

There are three major labs: that of Omar Yaghi, in Berkeley; Susumu Kitagawa, in Kyoto; and the Institut Lavoisier of Gérard Férey, in Versailles. These are the three pioneers. Yaghi really created the concept of this molecular Lego game and popularized the term. His team specializes in identifying, for example, materials with the largest storage capacity of gas such as methane, hydrogen and CO2 backed by initiatives and funding from the US Department of Energy.

There is also a lot of research in Germany and in the United Kingdom, where an extensive network of startups and integrated labs has developed.

It has become an important area for research, in which 3,000 to 4,000 articles are published every year. I manage a Twitter feed powered by a robot that reads all chemistry journals to select only the articles in which these materials are explicitly quoted in the title or in the abstract, before retweeting them: this represents a dozen articles a day. As researchers, we focus mainly on major changes, new impressive structures, new phenomena... Conferences and word-of-mouth both help us keep a good overview of the field.

This rapidly expanding field evokes, in many respects, that of graphene, which is also very dynamic. We find ourselves again in a “nano” field, a scale that has aroused some concerns in the past.  What about MOFs?

They aren’t affected, for a very simple reason: we are not creating a new range of uses, but more modestly, a material that can replace others in established industrial applications. Nanomaterials are already implemented in industrial environment, labs or even hospitals: most hospitals have a system for the production or purification of medical oxygen, such as those manufactured by Air Liquide, with a filtration system that uses nano-porous materials. These are established uses, even if the materials could change.

Precisely, let’s talk about applications. What are they?

Many medical applications are being developed, for example, the encapsulation of drugs, active principles, targeting of cancer cells. Nano-porous materials offer interesting properties. Take the targeting of cancer cells. With conventional methods, one can certainly approach very close to the cell, but the dissemination of active ingredients isn’t linear: there’s a peak at the beginning, before it decreases. It is a first-order kinetics. For an optimization of the treatment, the administration must be constant, with a zero-order kinetics: the same dose is given over time, until nothing is left. Continuity makes the treatment effective. Nano-porous materials offer a possible route: after injection or absorption of the drug, the delivered dose is held constantly during a full week. For clinical applications, this makes a significant difference, both in terms of effectiveness of the drug but also for the mitigation of its side effects – a crucial issue, for example, in oncology.

Alongside medical applications that use very small doses, there is also a growing industrial field, that of capture of gas. Are nano-porous materials in the race?

Absolutely. Three areas can be identified here.

The first is the capture and storage of CO2. The challenge is to trap the gas and then, to bury it, with a possible, yet distant, prospect of re-use.

The second is that of separation: for example, sulfur compounds in natural gas, carbon monoxide from plant stacks.

The third is the capture and degradation, the best known example being catalytic converters: the temperature inside the pot causes targeted molecules to attach to the metal of the pot, but also to degrade into less harmful, or even harmless, compounds. MOFs have this capability and quite a few promising papers have been published on the capture and subsequent degradation of neurotoxic agents. This opens on filter cartridge applications in war zones or contaminated areas.

Most of these applications involve gases, but some also concern liquids. From an industrial point of view, the latter case is a niche market in terms of fluid dynamics: the smaller the holes, the lesser the flow. But there are many applications: I co-direct a thesis, financed by Saint-Gobain, on the filtration of heavy and radioactive metals in drinking water. Part of the solutions to the problem of fluid dynamics that I have just evoked consists in enlarging the pores of the material. But then, the contact surface decreases and so does the performance: an optimal balance needs to be found.

Catalysis must also be mentioned alongside the capture of gases. As practiced today in the industrial environment, it is very energy-intensive and therefore costly process. One of the mains reasons for which the chemical industry is considered as such an electro-intensive sector. In other words, industrialists pay a hefty bill! New-generation nanoporous materials also offer interesting methods to improve performance.

Besides industrial applications on large volumes, are there applications in fine chemistry?

Yes indeed. Here again, organic-inorganic hybrids offer solutions, with the enzyme model as general direction. Enzymatic synthesis makes it possible to fix, in the right place of an organic pocket, molecules which help obtain the exact desired reaction. Downstream of the control of these processes, there are a number of applications – sensors, for instance.

I’d like to take this opportunity to mention a detail that may be of interest to your readers: I am sometimes asked how chemical sensors are not “saturated” after several reactions. In fact, the material is constantly regenerating, for example, when heated. The issue of saturation isn’t the most relevant. Let’s get back to sensors. Today, they are everywhere. In many electronic instruments, but also in functions that, unfortunately, pervade our everyday life – the detection of explosives for example. In this particular case, the issue is no longer saturation, but rather misdetection. A surface gain, in an instrument that remains handy, offers superior performance and nano-porous materials can prove their relevance in this situation.

There are also applications in precision mechanics, for example, to detect piezoelectric variations: metal ions of nanoporous materials can feel small deformations or stress before amplifying or translating them into signals (into light, for example) so that they can be detected by electronics.

Let’s return to the bigger picture. You have mentioned many possible applications and 10,000 known structures. In terms of development, rather than research, where do we stand exactly?

At the very beginning of the journey. Among these 10,000 structures, scarcely six or seven are now marketed and, to my knowledge, today, there is only one, very recent, commercial application: a system to control the ripening of fruits during transport. It may seem minor, but the economic stakes of this device are considerable. In practice, we are talking of cylinders placed inside a container with fruits, which capture the molecules that assess the fruit maturity. Information is simply produced by a control of the content in molecules. It is then transmitted to a hygrometric regulator. The capturing power of nano-porous material is much greater than that of the material previously used in this type of application. It makes a huge difference: since the molecules that are meant to be detected don’t saturate the atmosphere, the air inside the container shows relatively low levels. The lower the detection levels, the more effective the triggered action to regulate hygrometry.

Today, one of the major industrial players, BASF, is selling units of approximately one hundred kilos. Several start-ups are also involved in mass production. In Belfast, one of these is already capable of producing approximately 50 kg per hour; another is based in Chicago, backed by a department of Northwestern University (hosting the 2016 Nobel Prize in chemistry, recognized for his work on molecular machines).

We are approaching the industrial scale and the landscape is evolving quickly. I have been working on these materials since 2007. For a long time, my colleagues and I said that these structures were really interesting from an intellectual and academic point of view, but we had no certainty about their industrial future. Researchers have a certain tendency to believe that their research topic will change the world: similar statements have been made concerning nanotubes – graphene, for example – and reality was often below expectations. For nanotubes, with the exception of a few niche applications, very little has happened on a large scale. Graphene is holding its promises, with many derived applications. As for MOFs, I am more optimistic today than I was ten years ago: the landscape is better organized, things are falling into place and significant investments have been incurred.

You mentioned BASF, but mostly startups. Are major industrial players investing in this new field?

They are interested in our work, but they are more cautious, for easily understandable reasons. MOFs aren’t a new material that opens up on new applications. For the time being, they have displaced established practices in the industrial community since the 1960s. As a result, the bar is much higher. It is much easier in the case of niche applications. But for gas capture where there is an existing industrial park, investments, a know-how... industrialists don’t see yet the interest of investing massively, if only because technologies used today are inexpensive. Activated carbon, for example, is obtained by burning agricultural waste (sugar cane, coconut, banana). When you buy it on site, it can have a negative cost! When you go to see an industrialist and offer them a material based on zinc – before even listening to you, they are already calculating the price of zinc... Competition is therefore represented by materials with average performance which prove very advantageous in terms of costs.

I’m not even talking about risks in terms of supply, especially for elements whose exploitation depends on that of another element: there has been a lot of talk about rare earths in recent years, but there is also helium (a by-product of the extraction of radon, from which it originates). To realize this, one must look at a periodic table colored according to the risks in terms of supply! This can be reflected in our research: if I try to develop a new material, I would definitely avoid using cobalt or cadmium and focus on iron or zinc. Today, this type of problem is central for researchers who work on batteries intended to be produced in large quantities. We have known for a long time, for example, that lithium is associated with many geopolitical problems, and it is also a by-product of the upgrading of potassium. It is therefore a matter of urgency to develop batteries using other elements, such as sodium. This type of constraint creates new branches of research and guides researchers working on this subject, such as Jean-Marie Tarascon at the Collège de France. Returning to the area we are interested in, these issues, though not alien to our concerns, are far from being central.

Francois-XavierCoudert
Researcher at Institut de Recherche de Chimie Paris (CNRS / Chimie ParisTech PSL University)