Porous Photocatalytic Membrane Microreactor (P2M2): A new reactor concept for photochemistry

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Abstract

In this study, a new membrane microreactor concept for multiphase photocatalytic reactions is demonstrated. Microfabrication, photocatalyst immobilization and surface modification steps were performed to develop a Porous Photocatalytic Membrane Microreactor (P2M2). This concept benefits from a stable gas–liquid–solid (G–L–S) interface allowing a continuous supply of gaseous reactants and a reduced light path. A surface modification technique was devised to alter the wetting conditions of the reactor wall. Through a complete hydrophobization and a selective hydrophilization step by use of UV-light, we obtained a hydrophobic porous membrane support with hydrophilic photocatalytic microchannels. The photocatalytic degradations of methylene blue and phenol were used as model reactions to test the device, demonstrating promising degradation performance. We further demonstrated the effect of additional oxygen supply to the performance of the reactor for both reaction systems.

Highlights

► New microreactor design for photochemistry. ► Utilization of membrane technology in photocatalytic reactions. ► Selective surface modification and interface stabilization multiphase processes. ► Membrane assisted oxygen supply for photocatalytic degradation.

Introduction

Microreactors provide high surface to volume ratio, which leads to enhanced heat and mass transfer in these devices. Additionally, due to their miniaturized dimensions they facilitate safer operations, occupy small space and create less waste. These properties make them superior for many applications (e.g. in heterogeneous catalysis) and open new routes for chemical technology [1], [2], [3], [4], [5], [6].

In recent years, microreactors also became attractive for photochemistry and photocatalytic processes [7], [8], [9], [10]. Their high surface to volume ratio reduces mass transfer limitations, such that they overcome the setback of the low surface area of the immobilized macroscale wall reactors for photocatalysis [9], [11]. Moreover, the miniaturized dimensions in the microreactors provide a reduced path for the light decreasing photon transfer limitations and lead to high illumination homogeneity along the reactor [7], [9].

Many of the already existing concepts from microreaction technology have been adapted for photochemical and photocatalytic gas–liquid (G–L) and gas–liquid–solid (G–L–S) reactions. A remarkable reactor design for these purposes is the commercially available falling film microreactors utilized in G–L reactions [7]. In these reactors, a thin falling liquid film flows by gravitational force along a microstructured surface while it is illuminated by UV light and exposed to the co-flowing gas. Jähnisch et al. [12], [13] applied falling film microreactors for photooxidation and photochlorination reactions, in which the liquid reactant was saturated with the gaseous reactant in this continuous manner. These reactors are well-suited for G–L photochemical reactions. However, for photocatalytic G–L–S reactions these reactors can suffer from mass transfer limitations, as the gas has to diffuse through the liquid film to reach an immobilized solid catalyst on the microstructured surface.

Another possible microreactor type for the application of photocatalysis is the dispersed phase microreactors. In these reactors the gaseous reactant is dispersed in the liquid phase (e.g. slug flow, annular flow) flowing in the microchannel. The dispersed phase microreactors benefit from enhanced mixing (mass transfer) in the liquid slugs inside the microchannel [14], [15]. Lindstrom et al. [15] integrated slug flow and Matsushita et al. [16], [17] investigated annular flow in microreactors for photocatalytic model reactions and they showed significant improvement in the performance of their microreactors, compared to single phase operation. For applications in which long microchannels are required, dispersed phase operation can result in depletion of gaseous reactants due to its consumption by the reaction [18]. In addition, the presence of gas bubbles in microchannels decreases the residence time of the liquid reactant in the reactor.

The aim of this study is to demonstrate a new concept for multiphase (G–L–S) photocatalytic reactions inside microreactors: Porous Photocatalytic Membrane Microreactor (P2M2). In this concept, the contacting of the G–L–S phases is established using membrane technology. The liquid flows inside microchannels (fabricated in porous aluminum oxide: α-Al2O3), where the photocatalyst (titanium dioxide: TiO2) is immobilized on the channel wall (Fig. 1). The gas permeates through the porous wall of the membrane reaching the liquid that is flowing inside the microchannel. The microchannels are illuminated from the top by UV-light to stimulate the photocatalytic reaction.

This reactor design ensures that the liquid (L)–gas (G) interface is located at the solid (S) photocatalyst surface. We obtain this stable G–L–S interface by selective surface modification steps (hydrophobization and selective hydrophilization) of the intrinsically hydrophilic porous reactor materials. Photocatalytic degradations of methylene blue and phenol on TiO2 catalyst were selected as model reactions in this work, in order to study the performance of the microreactor and the influence of oxygen (O2) in these processes. The presence of O2 is known to improve photocatalytic degradation of organic compounds [15], [17], [16], [19]. The reactor concept presented in this study offers continuous supply of O2 to the entire photocatalytic microreactor, avoiding its depletion due to consumption.

Section snippets

Materials

Polyvinyl alcohol (PVA, Aldrich, Mowiol 8-88), MilliQ water, aluminum oxide (α-Al2O3; AKP-30, Sumitomo Chemical), nitric acid (HNO3, Sigma–Aldrich, puriss, 65%) were used for the fabrication of the porous α-Al2O3 substrates. For the preparation of the photocatalyst coating, titanium dioxide (TiO2, Evonik, Aeroxide P25, 99.5%), PVA (Aldrich, MW = 13,000–23,000 g/mol, 87–89%), acetic acid (Merck) and 2-propanol (Merck) were used. Perfluorinated octyltrichlorosilane (FOTS, Aldrich, 97%) and n-hexane

Reactor characterization

Fig. 4 shows a fabricated microchannel in porous α-Al2O3 substrate with the TiO2 layer inside after the surface modification steps. It was observed that the inner walls of the microchannel were entirely covered with TiO2 (Fig. 4b and c). The BET surface area of the TiO2 layer was found to be approximately 45 m2/g.

With the first surface modification using an FOTS coating (complete hydrophobization – Fig. 2a), the whole sample (α-Al2O3 and TiO2) was successfully hydrophobized. The hydrophobicity

Conclusions and outlook

A new microreactor concept for photocatalytic gas–liquid–solid (G–L–S) systems was introduced in this work. In contrast to the existing concepts, Porous Photocatalytic Membrane Microreactor (P2M2) exploits tuned interfaces to ensure a well-defined and stable G–L–S interface for the reaction and allows supplying the gaseous reactant continuously along the length of the microreactor. Microreactors with controlled surface properties and photocatalytic activity were successfully prepared by

Acknowledgements

This work was financially supported by Stichting voor de Technische Wetenschappen (STW, Project 07569) in Netherlands. We are grateful to J.G.F. Heeks, and J.A.M. Vrielink for technical support and analysis. The authors also greatly acknowledge J.M. Jani for the fruitful discussions and J. Bennink (Tingle.nl) for the illustrations.

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