Functional catalytic membrane development: A review of catalyst coating techniques

Highlights

• Catalyst coating techniques for catalytic membrane preparation were reviewed.

• Catalyst coating techniques were categorized into physical and chemical routes.

• Principles, advantages and drawbacks of the various techniques were emphasized.

• Perspectives on future development of catalyst coating techniques were highlighted.

Abstract

Catalytic membranes combine catalytic activity with conventional filtration membranes, thus enabling diverse attractive benefits into the conventional membrane filtration processes, such as easy catalyst reuse, antifouling, anti-microbial, and enhancing process efficiency. Up to date, tremendous progresses have been made on functional catalytic membrane preparation and applications, which significantly advances the competitiveness of membrane technologies in process industries. The present article provides a critical and holistic overview of the current state of knowledge on existing catalyst coating techniques for functional catalytic membrane development. Based on coating mechanisms, the techniques are generally categorized into physical and chemical surface coating routes. For each technique, we first introduce fundamental principle, followed by a critical discussion of their applications with representative case studies. Advantages and drawbacks are also emphasized for different surface coating technologies. Finally, future perspectives are highlighted to provide deep insights into their future developments.

Introduction

Membrane technologies offer attractive advantages in mass separation processes or chemical conversion processes owing to facile operation, small footprint, scalable capacities, and high separation efficiency [1]. Based on the membranes' functions, membrane structures, and their operation fashions, a number of different membrane processes have been developed, including micro/ultra/nano-filtration, reverse/forward osmosis, membrane reactor, membrane extractor (e.g., gas separation and pervaporation), and membrane contactor (e.g., membrane distillation and membrane condenser). However, the conventional membranes often suffer from some inherent drawbacks, such as membrane fouling, inadequate separation of low molecular sized substances and limited pollutant degradation [2], which limit their practical applications.

Integration of catalysis and chemical reactivity into membrane filtration leads to innovative and smart membrane separation systems. These chemically reactive membranes radically transform physical filtration into chemically or catalytically reactive processes that progressively eliminates the inherent limitations of traditional membrane filtration processes such as membrane fouling, pollutant rejection or degradation. Typical catalysis that have been integrated into membrane filtration include photocatalysis, catalytic ozonation, electrochemical oxidation, Fenton or Fenton-like processes. For example, the most widely studied photocatalytic membranes are enabled by membrane surface coating with photocatalysts [3]. Mélisa Hatat-Fraile et al. prepared three types of TiO₂ composite membranes (including undoped, boron-doped, and nitrogen-doped TiO₂) via sol-gel dip coating, and assessed their photocatalytic degradation of acid orange 7 (dye) through a dead-end filtration process [4]. Semiconductor catalysts (e.g., SnO₂, ZrO₂ and WO₃) and carbonaceous materials (e.g., g-C₃N₄) have been utilized to modify the inorganic or polymeric membranes, and even metallic substrate such as stainless steel. Qi Zhang et al. prepared a g-C₃N₄/ TiO₂ nanotube array membrane by imbedding g-C₃N₄ quantum dots into TiO₂ supporting layer. More than 60% of rhodamine B was removed by the g-C₃N₄/TiO₂ membrane on account of the synergistic effect of photocatalysis and membrane filtration. Additionally, both the performance of bacterial cell removal and anti-fouling capability were enhanced by the immobilized membrane. Similarly, catalytic ozonation can also be achieved on membranes coated with titania and other catalysis such as Al₂O₃, MnO₂, FeOOH, bimetallic oxides and carbonaceous materials (e.g., graphene or carbon nanotubes). Catalytic membrane filtration may also be achieved by exposing membranes to mixed oxidants generated from homogeneous catalysis such as UV/O₃, O₃/UV/H₂O₂, H₂O₂/UV, Fe²⁺/H₂O₂, FeS/H₂O₂, UV/persulfate, and UV/chlorine [5]. This type of reactive membrane systems is not within the scope and relevant discussions of this review, which primarily focuses on surface catalyst coating.

Besides catalytic properties, surface coating can also enhance other material functionalities such as durability [6], anti-abrasion [7], antifouling [8] and antimicrobial properties [9]. For instance, silver nanoparticles are reported to coat on polysulfone membranes to achieve antimicrobial properties on membrane surface [10]. Carbonaceous nanomaterials such as graphene and carbon nanotubes were coated on cellulose triacetate membranes to enhance surface electrical conductivity and electrochemical reactions for pollutant degradation [11]. The properties of catalytic membranes such as rejection, selectivity, pollutant degradation and antifouling ability could be adjusted by catalysts coatings. Successful catalyst coating not only enables reactive membrane filtration but also helps retain catalyst and prevent aggregation.

Catalytic membranes are also important for realization of process intensification to further reduce equipment/energy cost and enhance process efficiency. For example, product separation in catalytic membrane reactors can overcome thermodynamic equilibrium limitations of enormous synthesis reactions (e.g., esterification, acetalization, hydrogenation/dehydrogenation, and water-gas shift reaction) to achieve superior conversions. Alternatively, in situ catalytic degradation on the membrane surface can significantly mitigate membrane fouling and thus remarkably improve the rejection or degradation of pollutants in membrane processes.

Two main strategies have been used to prepare catalytic membranes by either incorporating the catalysts into the membrane matrix or coating on the membrane surface [12]. The former strategy is simple and straightforward, but apparently the as-prepared membranes suffer from limited overall catalytic activity due to reduced accessibility of the embedded catalysts [13]. Furthermore, integrating catalysts into the membrane matrix may compromise the properties and functionality of the pristine membrane such as physical structures, porosity, mechanical strength or elasticity and chemical selectivity [14]. Therefore, surface coating for catalysts is often a desirable means to prepare catalytic membranes, which not only enables direct catalyst exposure to the reactants, but potentially improve the mass transfer and reaction rates via a forced convection [12]. Up to now, a variety of surface coating techniques have been developed for innovative and smart catalytic membranes. However, comprehensive reviews of these surface coating techniques are still lacking, and more importantly, catalyst coating on membrane filtration materials still seem premature for industrialization due to the sophisticated procedures and some negative impacts on membrane permeability or catalyst activity.

The present paper aims to provide a critical and holistic discussion of the current state of knowledge on existing surface coating techniques for catalyst immobilization on membranes. These techniques are classified into two major sections: physical surface coating and chemical surface coating techniques. The principles of each technique are first introduced, followed by a critical discussion of their successful applications with representative case studies. Advantages and shortcomings of the surface coating technologies are also highlighted. Finally, we analyze the major challenges and give personal perspectives on these techniques to provide insights into their future developments.