Pickering-Emulsion Strategy for Separating & Recycling Nanoparticle Catalysts

Nanoparticle Catalysts

Nanoparticle catalysts have extensive surface area/unit volume when compared to traditional catalysts.  This property makes them ideal for use in process intensified reactors such as the Holl-Reactor^®. The catalyst mass rate can be dramatically lowered and still achieve high production rates.

A Classical Emulsion

First we will talk about a classical emulsion that consists of plant based oil droplets suspended in an aqueous phase.  The hydrophobic end of the oily fatty acids a line in such a way that  they are pointed inward towards the center of the oil droplet. The hydrophilic end of the oily fatty acids point outward towards the aqueous media.

A Pickering Emulsion

In a Pickering emulsion, the oil droplets are stabilized by nanoparticles that adhere to the outside of the droplets.  The nanoparticles can be catalysts.

In summarizing, the classical emulsion uses molecular surfactants to stabilize the emulsion and the Pickering emulsion uses solid particles to stabilize an emulsion.

An Oil Water Classical Emulsion Versus an Oil Water Pickering Emulsion

Nanoparticles are particles between 1 and 100 nanometers in size.  Thus nanoparticle catalysts can not be filtered out of a reaction medium and also although membranes were considered as a possible way to separate nanoparticle catalysts at the discharge of a reactor; erosion on the membrane does occur ruling them out also. So how can we recycle catalyst particles in a process intensified reactor?

For our process intensified theoretical example we will be reacting benzene with hydrogen peroxide to produce phenol.  The nanoparticle catalyst will be Modified Titanium Silicalite.^{1, 2}

In the first tank, the Pickering W/O emulsion is created with some benzene and H₂O/H₂O₂ blend and the nanoparticle catalyst. The second tank has the benzene feed.  A Holl-Reactor^® is used for the conversion of Benzene to Phenol.  The RPM of the Holl-Reactor^® must be adjusted to maintain the Pickering Emulsion.  Note that the hydrogen peroxide and water, the aqueous phase, is inside the emulsion sphere and the catalyst is on the surface.

Separation of the Catalyst

An increase the oil fraction by recycling the organic phase can produce a Pickering emulsion/oil biphasic system (PEOBS) that makes use of the gravity after the Holl-Reactor^® to separate and recycle the catalyst. Most of product remains in the oil phase owing to smaller fraction of oil in the Pickering emulsion.³

The aqueous phase goes to the centrifuge where nanoparticles flow back to the emulsification tank. The Aqueous phase goes to flash evaporation and any remaining oily phase goes back to the phase separation.

References:

1. Direct Oxidation of Benzene to Phenol with Hydrogen Peroxide over a Modified Titanium Silicalite -Luigi Balducci Dr.,Daniele Bianchi Dr.,Rossella Bortolo Dr.,Rino D’Aloisio, Marco Ricci Dr., Roberto Tassinari and Raffaele Ungarelli

2. Dynamics of Pickering Emulsions in the Presence of an Interfacial Reaction: A Simulation Study Shuangliang Zhao, Bicai Zhan, Yaofeng Hu, Zhaoyu Fan, Marc Pera-Titus, and Honglai Liu

3. An Overview of Pickering Emulsions: Solid-Particle Materials, Classification, Morphology, and Applications   Yunqi Yang, Zhiwei Fang, Xuan Chen, Weiwang Zhang², Yangmei Xie ,Yinghui Chen, Zhenguo Liu and, Weien Yuan