Catalytic processes are key to provide higher-value molecules, such as hydrocarbons, from abundant ones, such as H2 or CO2. For more than a century, heterogeneous catalysis has been using metal-containing nanoparticles as catalysts to perform the conversion of chemicals under fairly harsh reaction conditions (typically, several bars of pressure and a temperature higher than 150°C). In this paradigm, the catalytic process can hardly produce fragile molecules with a high degree of functionality, because these would be destroyed (or burnt) right away.
The NanoFLP project is shifting the paradigm by targeting much softer catalytic conditions, typically, with pressures below 3 bars and temperatures below 150°C. The key is to boost the reactivity of known metal or metal oxide nanoparticles.
This is explored using the “Frustrated Lewis Pair” concept. Associating bulky and strong Lewis acid and base creates a Frustrated Lewis Pair (FLP). Traditionally, both FLP partners are molecules. Molecular FLPs have shown excellent abilities to catch and dissociate small molecules such as H2 in a heterolytic way, under mild conditions. The driving force is the destabilization of the initial acid-base adduct, sterically frustrated: it liberates a reactive pocket that catches the small molecule guest, and strongly lowers the activation energy for bond dissociation.
The pristine and challenging concept of NanoFLP consists in replacing one of the molecular FLP partner, either the acid or the base, by an inorganic nanoparticle: the other molecular partner will adsorb on the surface and boosts the reactivity of the nanoparticle by creating a frustrated active site.
Three families of inorganic nanoparticles (metals, acidic oxides, basic oxides) were investigated, illustrating the two schemes: nanoparticle is either the Lewis acid or the Lewis base. We used probe molecules (such as CO2, H2) to investigate the nature and reactivity of the active sites. All the catalytic reactions were performed under much milder conditions (rt.-150 °C, 1-3 bars) than those required using similar nanoparticles in the absence of the molecular partner. We also worked to describe the nanoparticle surface and the dynamics of the molecular partner using benchtop and synchrotron spectroscopies such as near-ambient-pressure x-ray photoelectron spectroscopy.
Conclusions:
In NanoFLP, we were able to identify a number of promising nanoparticles-ligand pairs that behaved as catalysts under mild reaction conditions. While these based on a basic nanoparticle (such as MgO) and an acidic molecule (such as a borane) showed disappointing performances, these based on the opposite scheme (an acidic nanoparticle and a basic molecule) opened a new route for the hydrogenation of alkynes. For example, we demonstrated that well-chosen nickel and cobalt-based nanoparticles, associated with phosphines of adequate steric hindrance and donating character, were prone to act as catalysts for the low-temperature hydrogenation of phenylacetylene and its derivative.
Moreover, we strongly developed in the framework of this research the use of stereo-electronic maps to rationalize the performances of nanoparticle-phosphine pairs. This approach allowed us to propose that a heterolytic cleavage of the H-H bond happened at low temperature between the nanoparticle surface and the phosphorus atom of the phosphine.
