Damien Debecker and his team have developed a hybrid catalyst that produces chemical cascade reactions of unprecedented efficiency. It’s a very useful invention for improving chemical processes. Ultimately, some drugs could be produced in a greener way. Also, biomass molecules, such as the ‘waste’ molecule glycerol, could be transformed more rapidly for the production of biopolymers.
If you ever want to play the apprentice chemist, one of the first things you'll have to deal with is catalysis. Sound scary? Fear not! There’s nothing cataclysmic about catalysis, so don’t let it intimidate you. Granted, provocation is what catalysis is about. It’s an integral part of our daily life, the subtle basis of thousands of chemical reactions which occur naturally in our environment and industries.
Catalysis is ‘the acceleration of a chemical reaction’, explains Damien Debecker, a professor and researcher at the Institute of Condensed Matter and Nanosciences. ‘And 90% of industrial plastics, textiles, household goods, pharmaceuticals, fuels, etc., are made by using catalysis and thus catalysts.’ From making bread with baker's yeast to our vehicles’ exhaust-reducing catalytic converters, catalysis is everywhere. It’s therefore important to understand what a catalyst is and above all to find the best possible catalysts to optimise chemical processes. They make it possible for two substances (or reagents) to react effectively to each other for the purpose of forming a specific new product.
Hybrid catalysts for cascade reactions
This is the focus of Damien Debecker and his research team: finding new catalysts for developing sustainable chemical processes such as recovering molecules from biomass, converting CO2 into ‘e-fuels’, achieving greener synthesis of molecules with high added value, etc. ‘There are three subcategories of catalysts’, he explains. ‘The first subcategory is homogeneous catalysts, which are substances that dissolve in a liquid reaction medium to form a single phase. The second is heterogeneous catalysts which are solid and insoluble in the reaction medium; they’re interesting because we can recover and reuse them after the reaction has taken place. The last subcategory is enzymes, proteins whose role is promoting chemical reactions within living cells, namely biochemical reactions. These three subcategories of catalysts are generally used in different fields of application. It’s a relatively compartmentalised science.’ Prof. Debecker's team is focused mainly on developing new heterogeneous catalysts. But he’s also interested in enzymes. ‘Enzymes are soluble, they were developed by nature during Darwinian evolution and are specialised in carrying out a very precise chemical reaction with great efficiency. They’re “perfect” catalysts but we can’t reproduce them in chemistry and they require gentle handling.’ For some applications, it seems worthwhile to combine the two types of catalysis, for example by carrying out a cascade reaction, where the enzyme performs a first transformation, the product of which then serves as a substrate for the heterogeneous synthetic catalyst.
Prof. Debecker's idea was to immobilise enzymes directly on a solid catalyst to prevent them from dissolving and to be able to recover them after the reaction. ‘It’s to bridge the gap between heterogeneous and enzymatic catalysis,’ he says. It’s a challenge his team recently met successfully and the results of which have been published in the prestigious journal Chemical Science. They did so by imagining a hybrid catalyst that combines the effect of a solid catalyst and an enzyme. The approach they followed to create such a catalyst was using zeolites, solid catalysts created by chemists and whose structure is microporous. ‘But zeolite pores are too small for enzymes to enter them and zeolites have few anchor points for fixing enzymes onto their surface’, Prof. Debecker says. Here the researchers got creative: they used aerosol processing to create microspheres of zeolite nanocrystals, using them as very small building blocks to form a hollow sphere. ‘This trick allowed us to load the interior of the microspheres with enzymes and thus obtain a true heterogeneous hybrid catalyst!’ Prof. Debecker explains. ‘We have two catalytic entities “for the price of one” and the microspheres can contain a large amount of enzymes, which further strengthens overall catalytic capacity. This bifunctional solid makes it possible to carry out cascade reactions that have never been produced before with such efficiency.’
Testing the concept for biomass recovery
The targeted reaction with this new catalyst was a reaction model to test the principle of this hybrid catalyst based on the encapsulation of enzymes in zeolite microspheres. ‘The new hybrid catalyst design has proven itself in our cascade reaction model,’ Prof. Debecker says, ‘but it’s promising for other types of reactions.’ It’s now a question of identifying cascade reactions for which there’s great interest in working with this zeolite-enzyme combination for synthesising new molecules, reducing complexity or intensifying current chemical processes. ‘In a project that just started, we’re studying whether our concept can be applied to other reactions, particularly those focusing on biomass recovery. For example, glycerol, a “waste” molecule, must undergo two catalytic stages to be transformed into lactic acid, which we use to make biopolymers.’