Eco-Friendly and Scalable Multiphasic Suzuki Coupling Reactions Using a Polypyrrole@Pd Solar-Thermal Catalyst

Sunlight, the most abundant renewable energy source, can be converted into various useful forms of energy. Solar-thermal materials, which convert sunlight directly into heat, have gained significant attention as eco-friendly alternatives to fossil fuels. One application is solar-thermal desalination, where these materials enhance seawater evaporation for fresh water collection. They can also be used for organic synthesis by heating reaction solutions under sunlight, replacing traditional methods. Park et al. demonstrated solar-thermal acceleration of Suzuki coupling reactions using Pd-decorated melamine sponges, but to scale up for industrial use, more efficient catalytic systems are needed. Scalability in solar-thermal reactions is determined by product output per illumination area and increasing the reaction solution's concentration is key to overcoming heat limitations caused by larger volumes. This study introduces multi-phase reactions with immiscible, highly concentrated liquid phases, using nanocrystalline Pd-decorated polypyrrole (PPy@Pd) nanoparticles that serve as solar-thermal catalysts and Pickering emulsifiers. These nanoparticles accelerate Suzuki coupling reactions through solar-thermal heating and emulsion formation in concentrated mixtures. A gram-scale synthesis under 1 sun illumination was achieved, and the catalyst was reusable for 20 cycles. This work demonstrates the potential for scalable, eco-friendly solar-thermal reactions, offering a sustainable approach for large-scale organic synthesis.
PPy@Pd was synthesized through the chemical oxidative polymerization of pyrrole, producing PPy nanoparticles, followed by the growth of nanocrystalline Pd on their surface via chemical reduction. The resulting composite consisted of spherical PPy nanoparticles (220-400 nm) decorated with Pd nanocrystals (5-10 nm). Characterization through SEM, TEM, XRD, XPS, and DLS confirmed the structure, revealing a Pd content of 21.6 wt% and excellent solar light absorption across 280-2500 nm. A key innovation in this research was the liquid-solid-liquid triphasic reaction system, which used two immiscible liquid phases—an aqueous base and an organic phase with dissolved reactants. This system significantly increased reactant concentrations compared to conventional single-phase systems. The PPy@Pd nanoparticles acted as Pickering emulsifiers, forming water-in-ethanol emulsions with droplet sizes of 20-300 μm, which enhanced the interfacial area for improved mass transfer and reaction efficiency.
Solar-thermal Suzuki coupling reactions were performed under simulated AM 1.5G solar illumination (100 mW/cm^2) over an area of 3.14 cm^2. A 2.5 mm-thick PDMS layer reduced heat loss while maintaining light transmission. With PPy@Pd, the reaction mixture reached 60.6°C in 60 minutes, compared to 42.1°C without it. This solar-thermal heating, along with Pd's catalytic activity and the increased interfacial area, resulted in high conversion rates in various Suzuki coupling reactions. Scalability was assessed by increasing the moles of iodobenzene per illumination area from 0.63 to 3.78 mmol/cm^2, achieving conversion rates above 95% up to 3.15 mmol/cm^2. Gram-scale synthesis was also achieved, yielding 1.38 g of product with a 90.1% isolated yield, representing a 24-fold improvement in product mass per illumination area. The catalyst showed versatility across various aryl iodides and boronic acids, with conversion rates ranging from 92.3% to 99.4%, and maintained excellent reusability over 20 consecutive reaction cycles. Mechanistic studies suggested that the accelerated reaction rates were due to elevated temperatures through solar-thermal conversion, while Pickering emulsions enhanced mass transfer between the liquid phases. This research demonstrates the potential for scalable, eco-friendly organic synthesis by integrating solar energy with innovative triphasic systems.