Hybrid organosilica membranes and processes: Status and outlook

https://doi.org/10.1016/j.seppur.2013.08.003Get rights and content

Highlights

  • Organically bridged hybrid materials applied in hydrothermally stable membranes.

  • Fabrication possible through sol–gel technology and chemical vapor deposition.

  • Functional membranes can be deposited on ceramic and polymeric supports.

  • Bridging and terminating organic moieties allow tailoring of the separation properties.

  • The use of the organically modified silica membranes leads to important process improvements.

Abstract

In the past, the research in molecular separation membranes prepared through sol–gel technologies has been dominated by ceramic membranes. Especially, silica membranes have been studied in great depth. Steps towards hybrid organosilica membranes were taken by using pendant organic groups. However, only with the appearance of organically bridged silica, stable and reliable membranes that are suitable for large scale industrial utilization have become available. In this paper, we provide an overview of recent development of hybrid silica membranes that contain organic bridges. The freedom of choice in precursor allows for a flexible approach towards tailoring of the membrane properties. New support materials can be used by applying alternative deposition methods, such as expanding thermal plasma chemical vapor deposition. The robustness of the membrane concept allows for the design of novel separation process concepts in which the demonstrated stability is required.

Introduction

Molecular separation membranes are expected to play an important role in reaching the environmental goal to reduce CO2 emissions by approximately 50 Gt per year by 2050 [1]. Membranes can be applied as effective and energy-efficient separation systems to dehydrate wet bio-based fuels [2], [3]. Also application in the (petro)chemical industry is widely considered to significantly enhance the energy efficiency of key processes. Although exact numbers are missing, separation processes are known to be one of the more energy-intensive steps in a chemical production plant. Molecular separation membranes can play a crucial role here [4], [5], [6].

In this paper, the status and potential of hybrid organosilica membrane materials, and their use in different molecular separations are presented. An outlook is given towards further developments. Pure inorganic silica membranes have a low hydrothermal stability [7], [8], reducing the application window for dehydration of organic liquids by pervaporation to temperatures not exceeding ∼75 °C [9]. The incorporation of methyl groups [10] in the silica structure allows an increase of the application temperature to 95 °C [9]. This results in an application window with a temperature range that is similar to that of polymeric membranes like PVA [11] and polyimides [12]. Ceramic membranes will only have a value in the market when clear advantages over the commercially produced polymeric ones can be obtained. These advantages may include a higher performance, i.e. a higher selectivity and permeance, and an increased stability. As a result the application window can be broader than the more commonly used polymeric membranes. Envisaged application extensions are a higher application temperature, higher chemical stability towards strong organic solvents, acids and water, and a smaller sensitivity to swelling. The increase of operation periods under a wider range of applications will result in a positive business case for all industrial parties involved.

The focus of this review will be the preparation of membranes bridged polysilsesquioxanes precursors and their application in processes. Other types of hybrid membranes are reviewed elsewhere [13]. These current class of materials is characterized by covalent bonds between both oxygen and hydrocarbons to silicon. These materials are prepared by sol–gel processing [14] of monomers that contain an organic group that bridges typically two functional silyl groups [15]. A great variety of organic bridges can be used, including aromatic, alkyne, alkene, alkane, and functionalized moieties [16]. A general trend in mesoporous materials is that the pore diameter increases with increasing length of the bridge [16], [17], [18]. Alternatively, organic templates can be used to direct the pore formation process [19]. The positive impact of the organic bridge on the hydrothermal stability for mesoporous materials has been recognized [20]. The application of similar microporous materials in molecular separation membranes was reported as recently as 2008 [21].

This review paper is structured in a number of chapters. We start with material and preparation considerations, which include structure–property relations. With respect to preparation, our focus is on sol–gel processes, in which bridging and pendant organic groups with various structures are incorporated. This is followed by a generic overview of possible applications of these membranes. While the main focus is on pervaporation, developments in gas separation and nanofiltration are discussed as well. A pervaporation process case study is presented for the production of an acetal, which can be used as a renewable diesel additive. The topics of the last chapters are recent developments in alternative preparation methods, new support options, new processes, and an outlook towards the requirements in fundamental knowledge needed to allow further applicability.

Section snippets

Preparation of oxide membranes

Oxide microporous materials and membranes are typically prepared by conventional sol–gel approaches [14]. The mild synthesis conditions of sol–gel preparation allow the synthesis and control of nanoporous inorganic materials. Thin layers, <1 μm, can be efficiently deposited onto a support by coating with the sol before the transition to the gel phase occurs. The first membranes prepared in this way were made from tetraethylorthosilicate (TEOS) [22], [23], [24]. A microporous, pore diameter dp < 2 

Pervaporation applications

The HybSi® membrane was originally developed for pervaporation applications, and not surprisingly most application-oriented papers focus on the separation of water from organic solvents. In this section, we present the technical application window with special attention to the long-term behavior.

In the publication of 2008 [21], a long term continuous pervaporation experiment at 150 °C was reported for the first time. Data for a period of over 500 days were reported, without any indication of

The production of acetal: a case study

One of the possible applications that has been suggested for pervaporation membranes is related to the dehydration of reaction mixtures [74]. Till date, membranes have found limited utilization in this field. The breakthrough in stability of the HybSi® membrane may renew the interest in this topic. The interest in acetals is increasing in recent years [75], [76], [77]. Acetals can be considered as promising bio-based diesel additives. The production of these compounds, from an alcohol and an

Origins of the high stability

The operational stability of organically bridged silica under demanding hydrothermal conditions is unique. For periodic mesoporous materials it has been established that the inclusion of organic bridges is essential for their hydrothermal stability [20]. The solubility of organosilica in water appears to be lower than that of inorganic silica [82].

The increased stability of methylated silica as compared to pure silica has been ascribed to a shielding effect rather than to stabilization of the

New manufacturing technologies

Although less extensively applied than wet chemistry processes in the field of membrane science and technology, vapor-based deposition techniques have been applied successfully. Chemical vapor deposition is a thin-film deposition process following the thermal decomposition of one or more volatile precursors at the surface of a substrate to produce the desired film. One specific example is the counter-diffusion chemical vapor deposition method [88]. The high temperature, 600 °C, and the presence

New supports

The membranes described above are often deposited in first instance on a flat ceramic discs [51]. This geometry is very suitable for fundamental research, but is unlikely to be suitable for scaling up. A tubular geometry is more appropriate in this case. A layer deposited on the outside of a ceramic tube offers mechanical resistance and easy observation [21]. One possibility to decrease costs is to increase the surface area that can be coated in one single step. This can be achieved through the

Conclusions

In this review an overview of the current status of organic–inorganic silica-based molecular membranes has been presented. It was shown that the incorporation of especially alkyl hydrocarbon bridges greatly enhances the applicability of the sol–gel membranes. Furthermore that gas phase deposition methods can be used to make functional hybrid membranes. The main advantage of this membrane system is an exceptionally high stability under especially hydrothermal conditions. Long-term data sets for

Acknowledgements

This research was supported by the Netherlands Technology Foundation STW and the EOS technology program of the Dutch Ministry of Economic Affairs, administered by Agentschap NL, and by the Institute of Sustainable Process Technology, ISPT.

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