Multiphase Millireactors for High Volume Nanoparticle Catalyst Synthesis
Wai Kuan Wong, Swee Kun Yap, Yen Bing Su, Yi Chen Lim, Saif A. Khan*
Department of Chemical and Biomolecular Engineering
National University of Singapore, SINGAPORE
Metallic nanoparticles dispersed in liquid media or on solid supports for heterogeneous catalysis have been vital for the synthesis of a plethora of materials, ranging from pharmaceutical drugs to polymers and fertilizers. Synthesis of nanoparticles using bottom-up approaches such as the reduction of metallic salts with strong reducing agents in bulk batch syntheses often face severe heat and mass transfer limitations. Micro-/milli-fluidic wet chemical synthesis of nanoparticles, which has been extensively studied in the recent years, is not only able to circumvent the drawbacks of conventional batch-based synthesis methods, but also allow for continuous manufacturing of nanoparticles with consistent properties.[1,2] The use of reducing agents such as sodium borohydride often results in the nucleation and growth of gas bubbles within the reactor, which disrupts the flow and ultimately leads to reactor failure. We have previously demonstrated the use of inert gas bubble reservoirs within a segmented flow to capture the evolved gas in the synthesis of ultra-small metallic nanocrystals. Here, we present scaled-up millifluidic syntheses methods that incorporate the use of strong reducing agents (hydrazine and sodium borohydride) for rapid metallic salt reduction with and without the generation of gaseous side products with liters per day throughput. The segmented flow is a simple biphasic flow consisting of an aqueous solution of metallic salt, reducing agent and stabilizer, as well as silicone oil for cases where gas evolution is absent, while a triphasic flow with an additional nitrogen gas bubble as an inert gas reservoir is used when gas evolution is expected. In both cases, the feed solutions are drawn from their respective reservoirs by means of peristaltic pumps, with the latter being preferred over syringe pumps due to limited syringe capacities. However, flow rate fluctuations are typical of peristaltic pumps, and these fluctuations dramatically influence the stability of the multiphase flow in the reactor. We also demonstrate the design of inline hydraulic damper systems for fluid delivery, which are able to damp out pressure fluctuations from the pumps, thereby allowing for the formation of a smooth multiphase flow in the millireactors. The controlled and sequential addition of reagents in our segmented flow millireactors, coupled with rapid mixing allows for the synthesis of well-defined nanoparticles with tunable size and catalytic properties. Using the synthesis of palladium nanoparticles stabilized by polyvinylpyrrolidone (PVP) as a model catalyst, we are able to achieve turnover frequencies of ~350 min-1, comparable to that achieved in small-scale batch synthesis of PdNPs.
 S. Biswas, J. T. Miller, Y. Li, K. Nandakumar, and C. S. S. R. Kumar; Small, 2012, 8, pp. 688-698
 V. S. Cabeza , S. Kuhn , A. A. Kulkarni and K. F. Jensen, Langmuir, 2012, 28(17), pp. 7007-7013
 S. A. Khan and S. Duraiswamy, Lab Chip, 2012, 12, pp. 1807-1812