(435d) Continuous Production of Magnetic and Metal-Organic Nanoparticles With Netmix Reactor

Continuous Production of Magnetic and Metal-Organic Nanoparticles with NETmix Reactor

 M. Enis Leblebici^a, Carlos M. Fonte^a, Marcelo F. Costa^a, Filipe Ataíde^b, M. Paz Garcia^c, Viviana Silva^a, Thomas Devic^d, Patricia Horcajada^d, Pedro Tavares^e, João Pedro Araújo^c, Rui Oliveira^b, Joaquim L. Faria^a, Madalena M. Dias^a, José Carlos B. Lopes*^a

^aLaboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Porto, Porto, Portugal

 ^bREQUIMTE/CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal  

^cIFIMUP/Departamento de Física, Universidade do Porto, Porto, Portugal

^dInstitut Lavoisier (CNRS 8180) Institut universitaire de France, Université de Versailles, Versailles, France

^eCQVR, Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, Vila Real, Portugal

*e-mail: lopes@fe.up.pt

More than half a century passed over the legendary lecture of Professor Richard Feynman, “There’s Plenty of Room at the Bottom” which is usually referred as the muse of nanotechnology. Today, nano-scale products find there way into a variety of promising services from more humane cancer treatments to increasing computer power/capacity, while their production scales are getting from laboratory towards industrial [1].

 Chemical coprecipitation in batch stirred vessels has been a suitable, low-energy method to achieve nanometer scale crystals. However the focus now is not only to produce nanoparticles, but to control the particle size with a narrow particle size distribution (PSD). [2, 3]. Since coprecipitation in general results in wide PSD, high energy methods such as thermal decomposition, hydrothermal, and microwave synthesis were proposed and have been shown effective for size control of nanoparticles [2]. However, the productivity and scalability of these methods are lower than coprecipitation. Coprecipitation is done by vigorously mixing two or more reactant solutions, resulting in a product, which has low solubility in the medium. This supersaturated medium results in immediate nucleation [4] and particle growth. To produce uniform particles, it is crucial to keep a homogeneous concentration of the precursor molecules/clusters throughout the medium from the moment the reactant solutions contact until the end of the particle growth stage. Maintaining the homogeneity is the reason for the vigorous mixing in stirred vessels where batch coprecipitation takes place and it is the key factor on product quality, namely the PSD and on the energy input of the process.

 This work revisits coprecipitation method focusing on continuous production of magnetic nanoparticles (MNP) and metal – organic frameworks (MOFs) with high productivity without compromising PSD. The aim is to produce MNP and MOFs nanoparticles on a continuous reactor, namely the NETmix Reactor. The NETmix Reactor is a new technology consisting of a network of mixing chambers interconnected by transport channels [5]. Networks are generated by the repetition of unit cells where each unit cell consists of one chamber and two inlet and two outlet channels oriented at a 45º angle from the main flow direction. Above a critical channel Reynolds number, the system evolves to a self-sustained oscillatory laminar flow regime inside the mixing chambers inducing local strong laminar mixing. This network system of identical units, results in a flexible static mixer, which can be scaled-up easily without changing the design of mixing units.

Magnetite synthesis with NETmix reactor was performed by mixing an aqueous solution of Fe²⁺ and Fe³⁺ salts with a base solution continuously to result in a pH value of 9.5 under inert atmosphere and 45°C. Trisodium citrate was used to stabilize particles. At this temperature and pressure, the solubility of magnetite is of order of 10^-8 mol/kg the solubility of water [6]. This degree of supersaturation favors nucleation rather than particle growth therefore resulting in small crystals. Maturation time of 10 minutes was proven sufficient to ensure total conversion of iron salts to magnetite crystals. The washed crystals were characterized using dynamic light scattering, X-Ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy and supercooled quantum interference device.  The pure magnetite crystals were found to be 4.8±0.25 nm, monocrystalline and superparamagnetic. The space-time yield of the process was 4.3 tons/m^3/day.

Two different iron-based metal-organic frameworks were also synthesized using the NETmix reactor namely iron fumarate MIL-88A [7] and iron trimesate MIL-100 [8]. An equilibrium model for iron (III) and the polycarboxylic acids was developed, in order to avoid formation of amourphous phases thus eliminating kinetic limitations. In this way, the nucleation of the MOFs is fast, which is further favored by the immediate homogenization provided by NETmix reactor which resulted in smaller and more uniform particles. The materials were characterized using dynamic light scattering, X-Ray diffraction, high-resolution transmission electron microscopy, Fourier transform infra-red spectroscopy, thermogravimetry and nitrogen adsorption (BET specific surface area). The product was shown to be of nanoscale with a narrow PSD. The space-time yield increased up to 2 tons/m^3/day since the amount of reaction time was drastically reduced.

In conclusion, continuous production of MNP and MOFs were performed with NETmix reactor. MNP with precise size control and high productivity was reached. Production of two iron-based crystalline nanoMOFs, using different organic ligands was successfully achieved with high productivity.

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