Pd Nanoparticle-Decorated Melamine Sponges for Recyclable and Scalable Solar-Thermal Catalysis in Suzuki Coupling Reactions

Sunlight is a renewable energy source that can be converted into various forms of energy, with recent advances in solar-thermal materials enabling near 100% efficiency in converting sunlight to heat. This eco-friendly heating method has applications such as solar-thermal desalination, where heat enhances seawater evaporation for collecting potable water. Solar-thermal materials can selectively raise the surface temperature of seawater, with porous films being particularly effective. In chemical reactions, solar-thermal heating can replace traditional energy-intensive methods. Park et al. demonstrated solar-thermal acceleration of Suzuki coupling reactions using Pd-decorated sponges, but their system had limitations, such as unscalable sponge fabrication, limited reaction scalability, and high Pd content (8.1 mol%). To address this, we developed a scalable solution process for creating Pd nanoparticle monolayers on melamine sponges, achieving high solar-thermal and catalytic activity. Additionally, we introduced multiphasic Suzuki coupling reactions, where oxidative addition occurs in the ethanol phase and transmetalation in the aqueous phase, increasing product mass per illuminated area. Despite alternating contact between the phases, the high catalytic activity of the Pd monolayer allowed us to reduce Pd content to 0.41 mol%, 20 times lower than the previous system. This approach demonstrates the scalability of solar-thermal catalysis, achieving gram-scale reactions with reusability across 10 cycles.
To enhance the scalability of solar-thermal Suzuki coupling reactions, we developed a multiphasic reaction system, which effectively addressed the solubility challenges commonly faced in these reactions. This innovative setup consisted of an aqueous base layer and an organic layer containing dissolved reactants, allowing Pd nanoparticles to alternately contact both phases during the reaction, thereby promoting effective reactant interaction. Under simulated 1 sun illumination, the reaction mixture with Pd-sponge-A maintained a stable temperature of 62.1°C after 70 minutes, achieving an impressive conversion rate of 98.5% for synthesizing biphenyl. By stacking multiple Pd-sponge units, we successfully increased the number of moles of iodobenzene per illumination area from 0.88 to 2.63 mmol/cm^2 while consistently achieving conversion rates above 98%. These optimized conditions allowed gram-scale production, yielding 1.193 g of product with a 93.9% isolated yield, significantly improving on previous solar-thermal reactions that produced only 0.0567 g under similar conditions.
The versatility of the Pd-sponge-A system was demonstrated through various Suzuki coupling reactions, achieving conversion rates between 96.3% and 99.4% across different substrates. Additionally, the Pd-sponge-A exhibited excellent reusability, maintaining high solar-thermal and catalytic activity over ten consecutive reaction cycles without significant performance loss, indicating its potential for long-term practical applications. This study presents a scalable, eco-friendly method for solar-thermal Suzuki coupling catalysis, combining a densely packed Pd nanoparticle monolayer on melamine sponges with a multiphasic reaction system for efficient gram-scale synthesis without additional energy input for heating. This approach significantly increases the product mass per illuminated area while reducing the required Pd catalyst content. The ease of use and low energy consumption associated with the multi-cycle application of Pd-sponge-A further highlights its potential for sustainable chemical synthesis. Ultimately, this research aims to contribute to the advancement of green chemistry technologies that efficiently utilize solar energy for chemical transformations, fostering a more sustainable chemical industry.