(34f) Hydrogenation IN Microreactors with Immobilized CATALYTIC NANOPARTICLES

The conversion of biorenewables to fuels is receiving a great deal of attention in research laboratories across the country. An equally important but less active area is the need to produce specialty and commodity chemicals from biomass products, which requires the development of such processes as the catalytic hydrogenation of lactic acid and other platform products to commodity chemicals1. Traditional catalytic processes are subject to mass transfer limitations which make it difficult to determine the intrinsic kinetics. Biomass conversion processes also require reactions under moderate temperatures and pressures, along with fast catalyst screening to enable rapid and accurate evaluation of catalyst performance. To address these issues, we have combined nanocatalysis and microfluidics to create a microreactor that can be used to study kinetics and reaction mechanisms. We synthesized palladium and platinum nanoparticles using D-biotin, n-dodecylsulfid and 3-aminopropyltrimethoxysilane as stabilizing ligands. 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 lower temperatures and pressures. 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 recycling2. X-ray photoelectron spectroscopy (XPS) was used to measure the atomic concentrations of nanocatalysts on the surfaces, and the results confirmed successful immobilization of the catalysts. Finally we assessed the system by hydrogenation of 6-bromo-1-hexene, with an estimated residence time in the microreactor of 4 s at steady state. The reactant was converted to 1-bromohexane in the microreactors at room temperature and atmospheric pressure, with a selectivity of 100% and complete conversions at lower substrate concentrations. Control experiments showed no hydrogenation occurred in blank microreactors. The highest catalyst turnover frequencies were 23,963 h-1 for APTMS-stabilized palladium, and 9,370 h-1 for biotin-stabilized palladium. The nanocatalysts were recycled at least 5 times before an obvious decrease in conversions was observed. This system could provide a tool for efficient and rapid evaluation of catalytic nanoparticles, measurement of intrinsic kinetics, and assessment of reaction mechanisms.