(169e) Hydrogenation IN Microreactors with Immobilized Noble Nanocatalysts

Multi-phase hydrogenation is one of the most important reactions in converting biorenewables to fuels and/or chemicals. The conversion reactions should ideally be run under moderate temperatures and pressures to protect fragile feedstock. However, due to the low solubility of hydrogen in the aqueous phase, they have typically been run at high temperatures and pressures, which make it difficult to determine the intrinsic kinetics. In addition, the large number of catalyst candidates makes rapid and accurate evaluation of catalyst performance very desirable. To address these issues, we have combined nanocatalysis and microfluidics to create a microreactor that can be used to study the kinetics and reaction mechanisms of hydrogenation and other renewables conversions. We synthesized noble nanoparticles (Pd, Pt and Ru) stabilized by n-dodecyl sulfide by thermal decomposition. The nanoparticles ranged from 2 to 5 nm in size with a narrow size distribution, as shown by high resolution transmission electron microscopy (HRTEM) images. The resulting high surface-area-to-volume ratios enhanced hydrogenation at much lower temperatures and pressures than typically used. We fabricated microreactors by soft lithography from polydimethylsiloxane (PDMS) to greatly reduce mass transfer limitations and shorten reaction times. We then developed schemes to immobilize the catalytic nanoparticles in-situ in the microreactors to promote interaction of reactants and catalysts, and enable easy catalyst recycling^1,2. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) were used to measure the atomic concentrations of nanocatalysts on the surfaces, with the results confirming successful immobilization of the catalysts. The system was assessed by hydrogenation of 6-bromo-1-hexene at room temperature and 1 atm. of hydrogen pressure. The estimated residence time in the microreactor was 4 s, with a selectivity of 100% and complete conversions at lower substrate concentrations^3,4. Control experiments showed no hydrogenation occurred in blank microreactors. Catalyst leaching was negligible according to ICP-OES analysis, indicating robustness of the immobilization protocol. The catalyst turnover frequency was three orders of magnitude higher than in conventional batch systems⁴. The nanocatalysts were recycled at least 3 times before decreases in conversions were observed. This system has the potential to provide an important tool for efficient and rapid evaluation of catalytic nanoparticles, measurement of intrinsic kinetics, and assessment of reaction mechanisms.