The peculiar catalytic properties of water in nanoconfinement

By Massimo Bocus and Veronique Van Speybroeck

We are all familiar with the concept of an acid aqueous solution: a lemonade rich in citric acid or the muriatic acid used as a drain cleaner both fall under this category, and are both examples of Brønsted acids. Chemically, this means that they release positively-charged hydrogen atoms, or protons, to the surrounding water molecules (H2O). A water molecule captures the proton and forms a hydronium ion (H3O+). Many chemical reactions are catalyzed by Brønsted acids, which means that the rate at which they occur dramatically increases if water is acidified.

Not all Brønsted acids are tiny molecules. Zeolites, in particular, are materials containing sparse sites that can donate a proton and, for this reason, are called Brønsted acid sites, or BAS for short. Zeolites are primary solid catalysts and adsorbents with a variety of applications in the chemical industry. Most of the chemicals around you (like plastic, gasoline, and so on) have likely seen a zeolite at some point during their production process. Zeolites are nanoporous materials. This means that they have pores, made of channels and/or cages depending on the zeolite type, just barely larger than a single molecule (10-9 m). Interestingly, when water molecules enter a nanopore, they stop behaving like regular liquid water as, instead of being fully surrounded by other water molecules, every molecule is squeezed by the pore walls. This has a dramatic effect on its properties and on the catalytic activity of hydronium ions.

Our experimental partners in the group of Bert Sels (KU Leuven) and Bert Maes (University of Antwerp) are leading experts in the conversion of biomass. Instead of relying on oil to obtain commodity chemicals, such as plastic and pharmaceuticals, they want to use the scraps of the paper industry. These contain lignin, a complex polymeric molecule which can nowadays be deconstructed into simpler building blocks ready to be reassembled into useful compounds (Figure 1).

Figure 1. A schematic overview of the lignin-to-chemicals process, where lignin is efficiently depolymerized into simpler molecules. These are converted into basic building blocks for the chemical industry, exploiting, among others, hot pressurized water in zeolites. The resulting products are then used to synthesize commodity chemicals, such as polymers.

The reaction we were interested in is the O-demethylation of guaiacol to catechol, a process in which guaiacol (a lignin-derived molecule) loses a carbon atom, forming the simpler and more useful catechol. In the lab, our collaborators observed that this acid-catalyzed reaction proceeds much faster in zeolites than in liquid water. Fascinated by this finding, we investigated the reaction using advanced molecular simulations able to reproduce the complex experimental conditions (high temperature and pressure).

We discovered that the undercoordinated (i.e., not fully surrounded by water) hydronium ions in the zeolite are much more active than in bulk liquid water, resulting in a better catalyst for the guaiacol O-demethylation reaction. But that’s not all. Indeed, by exploiting the simulations to construct hypothetical catalysts with different pore geometry and water content, we found that the 3-dimensional spatial organization of the water, the BAS, and the reacting guaiacol also plays a major role in modulating the reaction kinetics. That is, undercoordination is per se insufficient, and the rational design of new and better catalysts for the chemical industry of tomorrow should take into account such spatial organization as well. The final goal of molecular simulations is, after all, not only to explain but also to guide experimental discoveries.

Our simulations, based on ab initio molecular dynamics, are computationally very demanding and require weeks to complete even when using tens of computing cores on state-of-the-art hardware. This would not have been possible without the large amount of computing time granted by VSC over multiple projects.

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Read the full article in Nature Catalysis here