Abstract

Sustainable and Scalable Flow Synthesis of Catalytic Colloidal Nanocrystals

Noah Malmstadt, Professor of Chemical Engineering and Materials Science, University of Southern California

New catalytic materials are central to addressing the current climate crisis. In particular, nanoscale crystals of metals as well as metal phosphides, carbides, and oxides are promising materials for upgrading biofuels and converting captured carbon dioxide. Industrial adoption of these materials has been limited by the energy intensive and difficult-to-scale solid-phase routes by which they’re typically synthesized. We are developing solution-phase routes to synthesize these materials as colloidal nanocrystals in millifluidic flow reactors. These routes minimize energy usage, allow for superior phase and size control of products, and facilitate scaling via millifluidic parallelization.

A general drawback to solution-phase synthesis is the cost of the solvent—in terms of economics, environmental risks, and heath hazards. To address this, we have been developing chemical routes to nanocrystal fabrication in sustainable ionic liquid solvents. Ionic liquids—organic salts that are molten at room temperature—have safety advantages due to their low volatility and non-flammability. They are also uniquely good solvents for nanocrystal fabrication due to their ability to stabilize early stage nuclei and to serve as weak surface ligands to growing nanocrystals. However, they are expensive—typically prohibitively so for industrial solvent applications. We have therefore developed actively controlled in-flow methods for recycling ionic liquid solvents used in nanocrystal synthesis. A techno-economic analysis of this recycling approach shows the recycled ionic liquid solvent cost is less than that of a typical volatile organic compound solvent.