Microreactors and membrane microreactors: fabrication and applications

5 Microreactors and membrane microreactors: fabrication and applications P. K. SEELAM , M. HUUHTANEN and R. L. KEISKI , University of Oulu, Finland

DOI: 10.1533/9780857097347.1.188 Abstract: Process intensification (PI) is the future direction for the chemical and process industries and in this chapter, two key technologies to achieve this are discussed: microreactors and so-called membrane microreactors (MMRs). There is great potential to enhance the overall efficiency of microreactors by integrating them with membrane technologies to make MMRs and there are tremendous opportunities for the application of MMRs in many fields. This chapter reviews microreactor design, fabrication and applications as well as materials for micromembranes (MM). The integration of MMs with microreactors and the applications of the resulting MMRs are then discussed. Key words: membrane microreactor, microfluidic device, palladium micromembrane, zeolite, microfabrication.

5.1

Introduction

Process intensification (PI), which represents a new direction in modern chemical engineering, can be defined as a technological toolbox of process improvement tools (Gerven et al., 2009). It is a key area of study in green chemistry and engineering and an important part of strategies for more sustainable development. Engineers and scientists are currently working on green technologies to produce products with less or no environmental impact, and chemical reactor engineering has made great progress in this respect as a result of PI. Miniaturization is a fundamental PI concept and reducing the size of equipment has facilitated the conversion of raw materials to more useful products in a more energy efficient, safe and cost effective manner. (Anastas and Warner, 1998; Ramshaw, 1999). More detailed information on PI can be found in reports such as that by Moulijn et al. (2006) which describes the four main PI domains, namely the thermodynamic, spatial, temporal and functional domains (Moulijn et al. 2006; Van Gerven and Stankiewicz, 2009). Process-intensifying approaches have led to 188 © Woodhead Publishing Limited, 2013

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the development of several methods and equipment set-ups, in which novel reactors as well as intensive mixing, heat transfer and mass transfer devices are employed (e.g., static mixers, monolithic catalysts, microreactors, rotating devices). Moreover, other methods such as the integration of a reaction with one or more unit operations (separation, heat exchange or phase transition) into so-called multifunctional reactors (reverse-flow reactor, reactive distillation, reactive adsorption, membrane reactors, catalytic membranes, reactive extrusion and fuel cells (FCs)) are considered (Moulijn et al., 2006; Rong et al., 2008; Sanders et al., 2011). This chapter considers microreactors and MMRs in particular, being two key technologies for PI. By definition, microreactor technology is the process miniaturization of chemical reactors in sub-micron or sub-millimetre (roughly 50 μm–2 mm) range dimensions, leading to an improvement in both the physical and chemical parameters of reaction engineering. Compared to macroreactor systems, microreactors have many benefits. As well as a high surface-tovolume ratio and high heat and mass transfer rates, microreactors are safe to operate, have low operation, maintenance and construction costs, short residence times and high energy and materials efficiency (Delsman et al., 2005; Gavriilidis et al., 2002; Hessel et al., 2005a). The linear growth in scientific studies on microreactor applications, particularly in the fields of FCs for stationary and portable small-scale power systems, hydrogen production, catalytic studies, fine chemical and organic synthesis, integrated energy systems, functional chemicals and highly exothermic reactions is shown in Fig. 5.1 (Gavriilidis et al., 2002; Gokhale et al., 2005; Holladay et al., 2004; Jensen et al., 2001; Klemm et al., 2007; Pattekar and Kothare, 2004; Watts and Haswell, 2005; Zhang et al., 2004). The introduction of membranes into microstructured reactors has not yet been studied extensively in some fields. However the potential benefits of such integration are wide-ranging. The synergistic effects of multifunctional reactors are the key concept in MMR technology. In a catalytic MMR, for example, both the reaction (catalyst effect) and separation (membrane effect) are combined in a single unit. This reduction in the number of unit processes and unit operations leads to yield enhancement, with more efficient heat and mass transfer achieved (Dittmeyer et al., 2001). Integrating a membrane into a microchannel or microstructured reactor system to create an MMR is a challenging task due to the dimensions of a microreactor, where the height, width and depth are in the range of a few micrometres to submillimetres. This involves multidisciplinary fields in order to effectively design, fabricate and test MMRs. The fact that MMR involves multidisciplinary fields and covers such a wide study area makes it impossible to cover all aspects of it in a single chapter. Most of the research on micromembranes for MMRs places emphasis on Pd-based membranes for hydrogen separation, purification and

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700 Scopus Science Direct

650 600 550

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5.1 Number of published articles with the key word ‘microreactor’ in Scopus and Science Direct databases categorized by the year of publication (from 1993 to 2011) (accessed in September 2011).

production by dehydrogenation, steam reforming (SR) and water-gas-shift (WGS) reactions and zeolite MMs for Knoevenagel condensation reaction (KCR) and fine chemicals syntheses. Therefore this chapter is restricted to Pd-based and zeolite MM devices.

5.1.1

Membrane reactors

The term membrane means a permeable phase acting as a selective barrier and controlled by mass transport. A membrane can be porous or dense material, and separation takes place due to a difference in chemical potential gradients (Dittmeyer et al., 2001). There are two materials involved in an MR: a membrane and a catalyst. A membrane can have catalytic and separation functions by itself, or each material can function independently depending upon how the catalyst and membrane are incorporated in an MR. In a tubular MR, the catalyst bed is packed in the annulus or inside the tube, in which case the MR is termed a packed bed membrane reactor.

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Microreactors

Microreactors are microfluidic devices with dimensions in the sub-millimetre range that is, the width−height−diameter−length are in the range of 10–1000 μm. Generally, microreactors or microstructured reactors are different from conventional reactors, and are manufactured via different fabrication methods using silicon or glass materials. Miniaturization of process and reactors has started in the late 1980s, and various emerging applications (e.g., micro-fuel cell processors) and technologies have been driving the trend of miniaturization over the last two decades (Mills et al., 2007). According to Holladay et al. (2004), on-site and on-demand production of hazardous chemicals is the main motivating factor. Due to environmental regulations and energy security issues, alternative techniques to produce products in a more sustainable way, by reducing, for example, the formation of waste by-products and energy consumption must be found. One such alternative is microreactor technology (MRT). The first attempt to translate this concept into practice was done by DuPont scientists at the beginning of 1987. They demonstrated a prototype microreactor which was fabricated using microelectromechanical systems (MEMS) techniques. This research continued until the 1990s, with studies containing experimental chemical reactions on a miniaturized scale (Mills et al., 2007). Microreactors can play a significant role in limiting the transportation and production of hazardous chemicals, reducing by-product formation, increasing atom efficiency and safety (e.g., phosgene synthesis), and other various factors reported by Mills et al. (2007), for example. The number of patents and scientific articles in the field of MRT has grown exponentially during the last decades (Ehrfeld et al., 2000; Hessel et al., 2008). In comparison to conventional reactor systems, microreactors are easier to scale up by numbering-up (external or internal numbering). Most microreactors are made from silicon wafer or Si bulk using traditional semiconductor microfabrication methods, whilst other materials such as ceramic, glass and stainless steel have also been used in their design. The design of microreactors made from these materials is based on the application type, thermal conductivity, mechanical, electrical and electronic properties of each material.

5.2.1

Advantages and disadvantages of microreactors

To build a miniaturized process unit with an integrated microreactor system can only be considered beneficial if there is no technical or economic advantage in using a conventional system instead. It is therefore necessary to identify those reactions and processes which can be run successfully and beneficially in a microreactor compared to a conventional reactor. There

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Table 5.1 Advantages and disadvantages of microreactors (Ehrfeld 2000a; Mills et al., 2007) Advantages

Disadvantages

High surface-to-volume ratio Improved heat and mass transfer rates

Clogging or fouling Challenges in numbering-up (scale-up) Catalyst deactivation Malfunctioning of distributors Reliability for long time on stream Leaks between the channels Mixing efficiency Cost issues

Compactness Short and narrow RTD Enhanced safety Mitigation of runaway reactions Faster system response Faster research results and process development Light weight

Short residence times require fast reactions

Better process control High yield and selectivity Increased conversion Quick start and shutdown Easier scale-up Distributed production (on-site)

are several advantages, as well some disadvantages, in using microreactors as summarized in Table 5.1. Regulations concerning the safety, health and environmental issues in the process industry, as seen in the chemical process industry for example, have recently become increasingly rigorous. Thus, the potential application of MRTs has begun to receive greater attention, as the reactions in microstructured or microchannel reactors can be run in safe conditions. A number of examples can be found of MRT applications reported as process improvements in industries such as fine chemicals, organic synthesis, pharmaceuticals and hydrogen production (Hessel et al., 2005a; Jensen 1999; Jensen et al., 2001). Furthermore, the potential for portable automotive fuel processing or on-site hydrogen production (facilitated by compact, lightweight technology) are exciting applications, and good examples of efficient utilization of the key advantages of MRT (Holladay et al., 2004).

5.2.2

Flow phenomena in microreactors

The flow phenomena in microfluidic devices such as microchannels have been studied extensively during the last few decades (Ehrfeld et al., 2000; Hessel et al. 2005b; Papautsky et al., 2001). Fluid flow in microstructured or microchannel reactors is quite different from that in conventional macro reactor systems, due to the smaller hydraulic diameter of the

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channels. Generally, flow phenomena are analysed using the Navier-Stokes equations (convection−diffusion equations). However, these equations are not always capable of describing the flow phenomena in microreactors. In macro-scale conventional reactors, some parameters are neglected (e.g., molecular, interfacial tension, wall friction, roughness of surface, viscosity of the fluid and rarefactions effects) due to the large dimensions. These parameters have a significant role in microreactor design for gaseous flows. The molecular effect on the momentum transfer in directions other than the streamwise direction can increase significantly when the length of the flow channels is reduced and the continuum assumption becomes invalid (Alfadhel and Kothare, 2005; Ratchananusorn, 2007). When the length scale of the flow domain is reduced, the surface phenomena become more important, and surface effects such as wetting and spreading, for example, become dominant, whereas on the macro-scale these phenomena can be ignored. Pfahler et al. (1991) have reported that the variation in fluid properties such as viscosity can occur due to temperature variation in microfluidic transport on a micro-scale, which invalidates assumptions of constant properties (Coleman and Colin, 1999; Pfahler et al., 1991). For gaseous flows, the local statistical distribution of Maxwell–Boltzmann is assumed for the velocity of the particles in the co-moving fluid, but this assumption may breakdown when the gases flow in microchannels at high temperature or low pressures. The flow in a microreactor is predominantly laminar due to the small hydraulic diameter of the channels, making the Reynolds number (Re) very small. The diffusion paths for heat and mass transfer are also very small, thus making microreactors ideal candidates for heat and mass transfer limited reactions. In conventional macro-scale systems with large diameter flow channels, the flow is mainly dominated by the influence of gravitational force, whereas in micro-scale systems with small diameter flow channels, the flow is dominated by the wall friction force, molecular effects and viscosity (Kolb et al., 2004). For gas, the standard continuum flow regime and its deviation is described by the Knudsen number (Kn) (Equation [5.1]): Kn =

λ L

λ=

K BT

[5.1]

[5.2]

2 2π pd dm

where Kn is the ratio of two length scales, λ is the mean free path of gas molecules (Equation [5.2]), L is the characteristic length scale of the flow domain, KB is the Boltzmann constant, T is temperature, p is pressure and

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dm is the molecular diameter. When Kn > 1, it is likely that a gas molecule collides with the channel wall, rather than with another molecule. The flow behaviour may change dramatically if Kn exceeds 1. This may happen when gas flows through narrow channel are considered, and also when the temperature is high and/or pressure is low. Four different flow regimes can be distinguished based on Kn: continuum flow with no-slip boundary conditions (Kn ≤ 10−2); continuum flow with slip boundary conditions (10−2 < Kn ≤ 10−1); transitions flow (10−1 < Kn ≤ 10) and free molecular flow (Kn > 10) (Hessel et al., 2005b). In microreactors, the friction factor is not independent of wall surface roughness. Moreover, molecular interaction with the walls increases relative to intermolecular interactions when compared to macro-scale flows. In macro-scale systems, two boundary conditions will be applied, that is, a no-slip-flow in which the fluid next to the wall exhibits the velocity of the fluid normally being zero in the most common conditions, and a slip flow in which the velocity of the fluid next to the wall is not zero, and is affected by the wall friction effects and shear stress at the wall. In the case of the slip-flow conditions, a significant reduction in the friction pressure drop and thus reducing the power consumption required to feed the fluid into the microchannel reactor. For most cases in microreactors, the Kn = 0.1 continuum flow with slip boundary conditions is applied. In addition, the pressure drop inside the microreactor is minimal in comparison to that of macro-scale systems (Hessel et al., 2005b). The two main non-dimensional parameters used to characterize the fluid flow are the Re and the Darcy friction factor (f). The Re depends on four quantities: the diameter of the flow, viscosity, density and average liner velocity of the fluid (Equation [5.3]). Re =

ρ f U m Dh μ

[5.3]

where ρf is the density of the fluid (kg/m3), Um is the mean fluid velocity (m/s), Dh is the hydraulic diameter (m) and μ is the fluid viscosity (Pa·s). Dh can be calculated by Equation [5.4]: Dh =

2WH W H

[5.4]

where W is the width of the channel (m) and H is the height of the channel (m). When the hydraulic diameter decreases, the pressure difference increases by an order of two. Simultaneously, the Re decreases and the friction factor

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increases due to the pressure drop. The Darcy friction factor (f) is calculated using Equation [5.5] and the pressure drop, which is the function of the friction factor, is described by Equation [5.6]. The roughness of the microchannel walls plays a role in the friction factor, as well as in the transition to the turbulent flow regime. If the wall roughness in a microchannel structure increases, the friction factor increases in line with the pressure drop. The friction coefficient (Cf) which determines the relation between the Darcy friction factor and the Re is presented in Equation [5.7]. f =

2 Δp pD p Dh

[5.5]

2 ρL LU m

Δ = Δp

2 fLρU m 2 Dh

[5.6]

Cf = fRe

[5.7]

For a single-phase flow, in a rectangular channel for example, the flow phenomenon is assumed to be laminar layered and fully developed due to small hydraulic diameter (Dh), resulting in low Re values. A transition or even a turbulent flow regime may occur in the corrugated flow channel, resulting in larger Re values. Moreover, the rarefaction effects can also occur at normal pressures in a microchannel, resulting in a deviation from the continuum flow behaviour. With regards to slip boundary conditions, a three-dimensional Navier-Stokes equation is relevant and commonly applied for the flow in microchannels. A single-phase flow in a micro-scale reactor is similar to that in a macro-scale reactor, but the interactions between the fluid and the surface properties of the wall on the rarefaction effect for gas and liquid flow should be taken into consideration whilst determining the boundary conditions (Hessel, 2005b). The no-slip boundary condition is not valid for all cases in micro-scale flows, as these boundary conditions can result in a slip flow which can cause a reduction in the pressure drop. A two-phase flow in a microreactor is much more difficult to analyse than a single-phase flow. A two-phase flow, gas−liquid, for example, exhibits many flow regimes and parameters which will affect the flow and patterns in a microchannel (Waelchli and Rohr, 2006). The flow velocities of the dominant fluids, flow regimes (bubbly, slug, stratified, wavy and annular flow regimes) and contacting principles of the two phases are the two most significant factors (Doku et al., 2005; Serizawa et al., 2002). The ratio of flow velocities of each phase is critical and determines the flow domains. Generally, two-phase flows in a microchannel reactor are based on a gas−liquid flow, with the two phases fed separately with different types of mixing or contacting principles

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in a continuous contact phase. The regimes and transitions of flow depend on many factors, such as the geometry of the microchannels, flow velocities and the mixing or contacting principle. In the case of round tubes, slug, annular, bubbly and churn flow regimes can be observed, whereas for squared tubes irregular flow behaviour is found (Gokhale et al., 2005). As observed by Coleman and Colin (1999) and Serizawa et al. (2002), the most common flow regimes, including stratified, intermittent, annular, dispersed and bubbly, occur when air (dispersed phase) and water (continuous phase) are fed through a rectangular microchannel reactor. In both gas−liquid and liquid−liquid flows, the interfacial mass transfer between the two phases is high when the flow pattern consists of alternating plugs or bubbles (Kashid et al., 2007; Serizawa et al., 2002). Understanding flow regimes and the transitions plays an important role in designing microreactors. The manifold structures for flow distribution of compounds/streams inside microchannels should be considered thoroughly in order to ensure uniform flow, and efficient heat and mass transfer are achieved. For a liquid phase flow in a microreactor, mixing is the critical design issue due to the small Re values, and it has been found that mixing occurs due to diffusion. A concise and detailed review of characterization in the single or multiphase flow, and mixing phenomena in microchannels is reported by Aubin et al. (2010) and Doku et al. (2005).

5.3 5.3.1

Microreactor design and fabrication methods Microreactor design

The design of microreactors requires extensive knowledge with regard to material choice, fabrication methods, kinetics, transport rates, catalyst coating and loading, location of sensors and intrinsic conductivity (Gokhale et al., 2005). Computational fluid dynamics (CFD) is one possible and good method for the design and optimization of microreactor parameters (Kashid et al., 2007). Recent advances in CFD modelling and simulations provide relatively precise knowledge on flow, temperature and pressure distribution, without the need to perform any experiments. Various types and shapes of microreactors exist for single and multiphase flow systems in the field of MRT. The selection of an appropriate mixer and/ or reactor type and shape for a certain process depends on the characteristics of the reaction. For heterogeneous gas phase reactions, a microchannel device (a thin wall coated catalyst) with an integrated heat exchanger, sensors and temperature controllers can be selected. In the case of endothermic reactions, a combustion channel capable of supplying energy integrated with the reaction channels is the most optimal design, for a micro-fuel cell processor, for example. Catalytic wall and packed bed microreactors are

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appropriate choices for single-phase reactions, whereas capillary and falling film microreactors, as well as microcontactors and the aforementioned reactor types, are used for multiphase reactions (Kiwi-Minsker and Renken, 2005). From a design point of view, it is important to understand how to introduce two separate flows into one microchannel. In addition, the relative velocities of the flows have a significant influence on the resulting pattern of the multiphase flow. Another important aspect is how to introduce the catalysts’ active phase for a heterogeneous reaction where the solid catalyst is coated on the wall and/or placed as a packed bed inside a reactor. Even though the packed bed reactors are easier to fabricate than catalytic wall microreactors (CWM), CWMs are still favoured in most cases due to lower pressure drop and as they exhibit higher heat transfer rates (Kin et al., 2006). Besides choosing the proper type of a reactor, the geometry and appropriate microreactor structure are also important decisions. Multi-channel CWMs are most commonly used and have numerous advantages over conventional reactors, overcoming potential limitations related to volumetric flow rates and numbering-up for microchannel units. The dimensions of a microreactor and its channels have to be determined based on the throughput, and optimal dimensions should maximize the most important characteristics and parameters. The residence time distribution in a microchannel with a laminar flow profile is strongly dependent on the diffusion coefficients of the species, and also on the channel dimensions (Kolb et al., 2004). According to the study conducted by Tonomura et al. (2004a) an optimal design for a plate-fin microreactor typically contains parallel microchannels with inlet and outlet manifolds. The two main design parameters are thermal and fluid design. In thermal design, the main objective function is unformalization of fluid temperature, with the optimization of variables such as the microchannel shape, flow rate, coolant temperature and constraints (maximum pressure drop, reaction temperature and yield/selectivity). In fluid design, minimization of the total residence time distribution (RTD), the manifold shape and the number of microchannels are the optimization variables. Moreover, throughput, RTD and maximum pressure drop are constraints which must be considered. In the thermo−fluid design approach, by changing the channel width of the longitudinal position, a uniform temperature distribution with no hot spots has been achieved during exothermic reactions. Thus, optimal thermo−fluid design is the main goal for the microreactor system (Tonomura et al., 2004a, 2004b). McMullen and Jensen (2010) have made a review of the automation and construction materials used in integrated microreactors, highlighting various materials and fabrication methods. Generally, five types of materials were used: ceramic, glass, plastic, silicon and stainless steel (SS). The most

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common fabrication techniques are stereolithography (for ceramic), photolithography (for glass and silicon), soft lithography (for plastic), powder and injection moulding (for ceramic and plastic), wet and dry etching (for glass and silicon), laser and electric discharge machining (for ceramic), ultra machining (for glass), electro plating (for SS), micromachining (for SS) and embossing (for plastic). Each material offers specific advantages and disadvantages, and selecting appropriate materials for a microreactor depends on the reaction characteristics, along with the chemical properties and compatibility of the reactants or reagents. For high temperature reactions and separations, for example, ceramic is a good candidate with a low heat loss and chemical resistance. However, the costs are high. For high pressures and temperatures with superior heat conductivity and high aspect-ratio design, silicon and SS microreactors are the most effective choices. Plastic is preferable for fast and inexpensive development, but it is incompatible with organic solvents and not suitable for high temperatures and pressures. Most microreactors formed by silicon wafers are patterned to form microstructures or channels with heaters, sensors and catalytic reaction zones.

5.3.2

Microfabrication methods for microreactor devices

Microfabrication (MF) methods are widely used in microchemical systems design, especially in the case of MMRs where the reaction and separation are performed in a single unit. The use of MF methods for micro-devices has increased very rapidly in the design of MEMS, MSs, electronic circuits, microelectronics, semiconductors and energy systems (Jensen, 1999, 2001; Qin et al., 1998). MF techniques create new opportunities for chemical reaction engineering, and are used to build compact, efficient microreactor systems. The use of MF methods offers many advantages, including a reduction in the consumption of expensive reagents, fluidic components with small dead volumes, improved separation resulting from higher surface-to-volume ratios, integration of sensors, actuators and parallel screening, replication for multiple unit production and compactness. MF techniques used in microreactor components design and manufacturing have the potential to replace conventional energy production devices and macro-scale reactors. One such potential application is on-site hydrogen production using a micro-scale reformer unit, integrated with a micro-fuel processor (a PC-card sized microchemical system with integrated microfluidic, sensor, controller and reaction components) (Jensen, 1999). Moreover, miniaturization offers improved heat and mass transfer rates, and enables the design of more compact and efficient reaction and separation units.

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By applying MF techniques, the reaction heat can be controlled by varying the thickness and thermal conductivity of the wall. Geometrical parameters and construction materials are the key selection criteria in designing the microreactor systems, whilst for microsystems which perform chemical reactions, separations, analyses, and sensing devices, the channels, cavities, valves and electrodes all need to be designed and selected properly. In order to create MSs or micro units capable of performing the various required operations, it is important to understand which materials and methods are the most economic, reliable, accurate, stable and efficient (Ehrfeld et al., 2000). Generally, rigid materials such as Si are preferred, and many microfluidic devices are thus built on silicon substrates, along with other substrates such as crystalline and amorphous Si, glass, metal, plastic and polymers (Qin et al., 1998). Single-crystal Si substrates are used in many microsystems due to the shapes and patterns which can be reproduced with high precision by bulk and surface micromachining techniques. Moreover, Si/SiO2 is chemically and thermally stable and also extensively used in the electronics industry (e.g.in integrated circuits). Microreactors can be fabricated using high-volume and low-cost techniques, but the final price of a microreactor depends on many factors, including design parameters, materials and fabrication costs. Some of the MF techniques most widely applied in the creation of microstructured reactors are as follows (Ehrfeld et al., 2000; Gavriilidis et al., 2002; Hessel et al., 2005c; Qin et al., 1998): 1. Microlithographic techniques (photolithography, soft lithography, stereolithography, microcontact printing, moulding organic polymers, etc.); 2. LIGA – combination of deep lithography, electroforming, moulding, micromachining with laser radiation; 3. micromilling; 4. laser ablation; 5. micro moulding; 6. wet and dry chemical etching (on Silicon, glass materials); 7. electro-discharge micromachining (EDM); 8. other advanced techniques like turning, sawing, punching, embossing, drilling, laser micromachining, etc.; and 9. bonding techniques – gaskets, welding, sintering, electron-beam welding, diffusion bonding, soldering and laser welding. Fabrication of MMs has been studied by many authors, who have investigated Pd and zeolite based membranes on Si substrates. Generally, MMRs are fabricated using silicon (Si) wafer substrates for micro-fuel

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Si wafer (b)

(c)

(d)

(e)

5.2 Microfabrication procedure of a thin Pd membrane by Zhang et al. (a) Preparation of a Pd thin micromembrane on a Si wafer by dc sputtering. (b) Preparation of negative resist (65 μm thick). (c) Deposition of about 50 μm Ni layer by electroplating. (d) Removal of the negative resist. (e) Removal of Si wafer by wet etching in KOH. (f, g) SEM images of the prepared Pd micromembrane (thickness 2.5 μm) (Zhang et al., 2006) (Copyright permission 2006 Elsevier). (Continued)

processors, and function as integral components of miniature devices. Ye et al. (2005) have studied oxidized porous silicon (PS) supported thin Pd membranes. They used the MF technique to produce better adhesion between the support and the Pd with a smooth surface of PS. The microfabrication procedure for thin Pd membranes on Si substrates is presented in Fig. 5.2.

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(f)

Pd Ni metal

⫻500

5kV

50 μm

(g)

Ni metal Pd membrane

2.5 μm

5kV

⫻2, 200

10 μm

5.2 Continued

5.4

Micromembranes

In the following section, methods for the fabrication and deposition of Pd-based and zeolite MMs are discussed, as well as applications in (de) hydrogenation, SR, WGS, partial oxidation (POx) reactions and fine chemical synthesis. The research on Pd-based MMRs for hydrogen separation, purification and production (by dehydrogenation, SR and WGS reactions) has been selected as a case study, as significant research and, therefore, much information can be found in the literature on this field.

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Pd-based micromembranes

Conventional Pd-based membranes are categorized into self-supported Pd-based membranes, and supported composite membrane with a thin Pd layer deposited on porous supports. Self-supported Pd-based membranes (SSP) are available commercially, are capable of providing adequate mechanical strength and can easily be integrated into a reactor set-up. SSPs are normally relatively thick (50 μm or thicker) and exhibit low hydrogen fluxes with high hydrogen perm-selectivity (almost infinite with respect to other gases). However, the cost of Pd has increased exponentially during recent years, and as the flux is inversely proportional to the thickness, it makes SSPs an expensive choice. In order to reduce the thickness (i.e., higher fluxes) and to have better mechanical and thermal stability, thin Pd films are deposited on various supports such as porous stainless steel (PSS), Al2O3, ceramic and ZrO2. In Seelam et al. (2012), a 20 μm thick Pd thin layer was deposited by the ELP method on a PSS supported membrane module. The prepared membrane was investigated in SR reactions, and was concluded to exhibit good selectivity and a high hydrogen flux in comparison to a dense 50 μm thick Pd–Ag layer. By using a sub-micron thickness Pd-based membrane, the cost is not only reduced but the hydrogen flux is also enhanced. However, it is difficult to prepare a defect-free hydrogen separating membrane including both a Pd film and porous support. The commercially available porous supports are not defect or pinhole free materials, and also have surface imperfections such as non-uniform pore sizes, which make the metal films unable to completely cover up the support, leading to membrane defects and cracks. Furthermore, the supports may have a thick form which has a considerable higher mass transfer resistance, thus negatively affecting the separation flux (Tong, 2004). Thin film coverage of the pores of the support may also be insufficient, and thus the walls of the pore may not be covered completely by the metal film. In the studies by Bryden and Ying, (1995) a thin Pd-based film with a nanostructured membrane was deposited on porous Vycor glass disks. Leakage was found to occur due to small pinholes related to surface defects in the Vycor glass support. When exposed to hydrogen, the membrane can also cause cracks due to the expansion and contraction of Pd. In order to suppress a phase transition from α to β in the Pd metal lattice and grain-growth, alloying of Pd with other metals such as Ag, Ru, Cu, Rh, Ni is used (Bryden and Ying, 1995). The support structure is very important in avoiding pinholes or defects in the membranes that could potentially lead to gas leakages. Thus, the pore size of the top layer support must be reduced. In order to deposit an appropriate thin Pd film, avoiding surface imperfections of the support and film coverage, more advanced methods should be employed to produce durable, cost effective membranes for hydrogen separation and

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purification. Thin SSP membranes cannot withstand high pressures, and there is a clear need for strong Pd-supported membranes with good adhesion and gas-tightness, which can be improved by a reduction in pore size and surface roughness of the support (Ye et al., 2005). Most Pd-based membranes operate at high temperatures (HT) ranging from 300°C to 500°C. In addition, the hydrogen embrittlement factor, lattice strains and thermal stability are major problems. Furthermore, high temperature Pd-based MMs offer potential for application in miniaturized micro-FCs, WGS, SR and hydrogenation reactions. These membranes can be permselective to hydrogen, and in addition, occasionally to CO2 when operated at high pressures, providing even greater advantages for further processes. As these developments reveal, in the 21st century advanced MMs are developed with high permeation efficiency using micro- and nano-engineering techniques. In the study conducted by Kim (2008), HT MMs using Si wafers and SS supports were prepared, and the permeation properties for hydrogen and CO2 were reported. The permeation property of the MMs depends strongly on the geometry, morphology, preparation methods of the support and uniformity of film deposition. Chen and Gobina (2010) reported that a support with high hydrogen affinity is much more efficient, due to the resistance to hydrogen-induced failures or cracks. Generally, thin membranes exhibit higher hydrogen flux (2–10 mol/ m2s) but a selectivity decline. Thus, perm-selectivity and permeability are trade-off parameters. Tong et al. (2003, 2004, 2005a) have used microfabrication techniques to produce thin Pd–Ag MMs on substrates such as silicon wafer and MPSS. The performance of Pd-based MMs manufactured with a thickness less than 20 μm Pd and deposited using different fabrication techniques, are summarized in Table 5.2. Furthermore, palladium MMs with very thin features can be applied in microfluidic devices, and some applications are already suggested by the referred authors. Zhang et al. (2006) have also reported on thin Pd MMs with thicknesses of 2.5 μm and 0.7 mm, produced on an Si wafer for hydrogen separation applications, via MF technology and using a sputtering method to deposit the Pd film. Permeation tests were conducted using pure hydrogen and hydrogen/ CO2 mixtures at a temperature range of 200–400°C. The hydrogen permeability for this thin 2.5 μm Pd membrane was 50–60% of that achieved with a 0.7 mm membrane. Figure 5.2a–e presents the fabrication procedure for the Pd thin membranes with 50 μm Ni layer as a support, and Fig. 5.2f, g presents scanning electron micrograph images of thin Pd 2.5 μm and 50 μm Ni supports before the permeation test was done. During the SR and WGS reactions and separation processes, the inhibiting effects of CO2 and steam on the hydrogen flux for thin Pd–Ag membranes (1 μm and 0.5 μm thickness) are stronger than for thicker membranes (>10 μm) (Gielens et al., 2006). Moreover, CO2 and CO are reported to

© Woodhead Publishing Limited, 2013

Table 5.2 Performance of microfabricated Pd-based micromembranes deposited by various techniques

© Woodhead Publishing Limited, 2013

Membrane

Thickness Hydrogen (μm) flux (mol/m2·s)

Separation factor

Operating temperature (oC)

Deposition Remarks method

Pd–Ag/ceramic

0.35

0.015

250

SD

Δp = 1 atm; n = 1

Pd/MPSS Pd/SS

6 10

0.3 0.089

H2/N2 = 5.7, H2/He = 2.2 H2/He, Ar = ∞ H2/N2 > 1000

500 480

MD/ELP ELP/O

Pd–Ag Pd/ceramic

1.5–2 11.4

0.07 0.71

H2/N2 = 24 H2/N2 = 650

500 550

SP ELP

Δp = 1 atm Δp = 0.987 atm; n = 0.5 n=1 n = 0.6

Pd75%Ag25%γ-Al2O3

0.16–0.52

0.01

300

SD

n > 0.5

Pd/porous α-alumina Pd76%Ag24%/ polymeric Pd and Pd–Ag/γ-αAl2O3 Pd–Ag/ceramic Pd77%–Ag23%/Si wafer Pd–Fe5%

3–5

>0.1

H2/N2 = up to 116, H2/He = 3845 H2/N2 = 1000

300–500

MOCVD

pH2

0.25–1

0.002

H2/CO2 > 100

25

SD

n=1

0.1–1.5

0.01–0.02

H2/N2 = 30–200

25–300

0.7–1.1 1

0.05 0.5

H2/N2 = 4–80 H2/N2 > 550

350 450

MOCVD Δp = 1 atm and MS MS Δp = 1–4.93 atm CS Δp = 0.297 atm

18

10*

H2/He = 30

200

PE

Δp = 1 atm

Pd/cordierite Pd

8–16 0.2

0.001–0.005 H2/He = 40–360 350 5.2 100

ELP SD

Δp = 0.1–0.5 atm; n = 0.5 Δp = 0.56 atm; n = 0.5

1 atm n

References

Jayaraman and Lin, 1995 Goltsov, 1977 Li et al., 1998 Li et al., 1993 Collins and Way, 1993 McCool et al., 1999

05

Yan et al., 1994 Athayde et al., 1994 Xomeitakis and Lin, 1997 O’Brien et al., 2001 Tong et al., 2003 Bryden and Ying, 2002 Kim et al., 2009 Karnik et al., 2003

© Woodhead Publishing Limited, 2013

Pd–Ag/SiN wafer

0.5

4

H2/N2 > 1500

450

SD

pH2 = 0 82 atm

Tong et al., 2005a

Pd–Ag/Si wafer Pd–Ni/Si wafer

0.2 2.5

3–4 0.8

H2/Ar = 1000 –

350 200–400

EB CS

Δp = 0.96 atm pH2 0 2 atm n = 0.73

Wilhite et al., 2004 Zhang et al., 2006

Pd–Ag/Si microsieve

1

1

H2 > 1500

400–450

Δp = 0.8 atm; n =1

Keurentjes et al., 2004

Pd–Ag/Si3N4

0.75

0.02–0.95

H2/He > 1500

350–450

Single cannon SD CS

pH2 = 0.2 atm

Gielens et al., 2002

Pd

0.34

0.112

H2/N2 = 46, H2/ He = 10

250

SD

Δp = 1.1 atm; pH2 = 2 atm

Ye et al., 2005

Pd–Ag/Si3N4

1

3.6

450

CS

pH2 = 0.82 atm

Gielens et al., 2002

Pd77%–Ag23%/Si

2.2

8.4

H2/He = 1500–2000 H2/N2 = 1400

400

MS

pH2 = 25.66 atm; n = 0.5

Peters et al., 2008

Pd–Ag

5.5

0.35

400

ELP

Δp = 3.95 atm

Pd/ZrO2 Pd/PSS Pd/ceramic/PSS Pd–Ag/PSS Pd/Al2O3 HF† Pd/Al2O3 HF‡ Pd/Al2O3 HF§ Pd/Si wafer Pd–Ag23% Pd/porous alumina

10 6 5 16 5 2.5 1.5 6 0.2 1

0.2 – 0.78 0.19 0.135 0.198 0.2387 0.302 2.21

350–500 500 450 450 450 450 450 500 400 300

ELP ELP ELP ELP ELP ELP ELP ELP SD CVD

Δp = 1 atm; n = 0.5 Δp = 0.5 atm; n = 1 Δp = 3.35 atm; n = 0.5 n = 0.6 Δp = 1 atm; n = 1 Δp = 1 atm; n = 1 Δp = 1 atm; n = 1 Δp = 1 atm; n = 1 n = 0.97

Hou and Hughes, 2003 Wang et al., 2004 Su et al., 2005 Li et al., 2007 Yepes et al., 2006 Sun et al., 2006 Sun et al., 2006 Sun et al., 2006 Tong et al., 2005b Wilhite et al., 2006 Yamamoto et al., 2006 (Continued)

H2/N2 = upto 4500 He/Ar = 3.1 H2/N2 = 450 – H2/N2 = 380 H2/N2 = 340 H2/N2 = 1400 H2/N2 = 3115 H2/N2, Ar = ∞ H2/He > 1000 H2/He > 104

Table 5.2 Continued

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Operating temperature (oC)

Deposition Remarks method

References

450 500 430

SD/PECVD Δp = 0.18 atm; n = 0.73 CMS/ELP Δp = 0.2 atm ELP Δp = 1.1 atm

400 500 400

ELP IM/ELP ELP

Δp = 0.199 atm Δp = 1.1 atm Δp = 1.1 atm; n = 0.6–0.7

400

ELP

Δp = 1.1 atm; n = 0.7

3.5

H2/N2 = ∞ H2/N2 = ∞ H2/N2 = 100–200 0.086–0.134 H2/ N2=100–1000 0.056 H2/N2 ≤ 7000

350

ELP/O

McLeod et al., 2009 Tong et al., 2005c Tong and Matsumura, 2006a Uemiya et al., 1988 Tong et al., 2006b Dittmeyer et al., 2001 Dittmeyer et al., 2001 Roa and Way, 2003

Pd60%–Cu40%/Al2O3 Pd/α–Al2O3 HF†

1.5 1.1

0.499 0.4

Pd/α–Al2O3 HF‡ Pd/α–Al2O3 HF§ Pd/silicon wafer/ PDMS Pd–Ag23% Pd/PSS

2.6 0.6 4 1 4.4

Membrane

Thickness Hydrogen (μm) flux (mol/ m2·s)

Pd75%–Ag25%/SiO2 Pd–CeO2 /MPSS Pd/HF

1 13 11

1.1 0.275 0.136

Pd–Ag/PG Pd–Ag/MPSS Pd/PSS

21.6 4 5

0.067 0.28 0.155

Pd/Al2O3

7–15

Pd90%–Cu10%/Al2O3

Separation factor

H2/N2 = ∞ H2/N2 = 1000

350 370

ELP/O ELP

0.16 0.25 –

H2/N2 = 93 H2/N2 = 3000–8000 H2/N2 = 500 H2/N2 = 50 –

Δp = 1.7–2.4 atm; n = 0.7–1 Δp =1.7 atm; n = 0.5 Δp = 3.95 atm

370 370 200

ELP ELP SD

Δp = 3.95 atm Δp = 3.95 atm –

Roa and Way, 2003 Nair and Harold, 2007 Nair et al., 2007 Nair et al., 2007 Cui et al., 2000

0.05¶ 0.92

– H2/He = 1124

298 500

SD ELP

Δp = 1.1 atm n = 0.5

Naddaf et al., 2009 Chi et al., 2010

Pd/α–Al2O3

4

0.55

H2/N2 = 6600

500

ELP

pH2

4 93 atm n = 0.5

Pd77% Ag23%/SOI

0.2

46

H2/N2 > 1000

350

EB

pH2

Pd–Ag23%

1.9–3.8

18.3

H2/N2 = 2900

400

MS

pH2

Deshpande et al., 2010 25 66 atm n = 0.631 Peters et al., 2011

Pd–Ag/Si wafer

1

3.6

H2/He = 1500–2000

450

Single cannon SD

pH2

0 82 atm n = 1

0 1 2 atm n

Israni et al., 2009

05

Gielens et al., 2004

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* Standard cm3 min−1. Pd encapsulated membrane. ‡ Pd nanopore. § Aged Pd nanopore. ¶ Pa m3 s−1. †

n = Pressure exponential factor; Δp = pressure differential; pH2 = hydrogen partial pressure (retentate); SD = sputter deposition; EP = electroless plating; CVD = chemical vapour deposition; SP = spray pyrolysis; CS = co-sputtering; MS = magnetron sputtering; PSS = porous stainless steel; MOCVD = metal-organic chemical vapour deposition; PE = pulsed electrodeposition; EB = electron-beam deposition; CMS/ ELP = combined method of physical sputtering and electroless plating; O = osmotic pressure method; MPSS = macroporous stainless steel; PG = porous glass; HF = hollow fibre; MD/ELP = multidimensional plating mechanism; IM/ELP = improved method of electroless plating; CMS/ELP = combined method of physical sputtering and electroless plating; RT = room temperature; DSD = dual sputtering deposition; SOI = silicon-on-insulator. In the first column % refers to wt%.

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Handbook of membrane reactors

chemisorb on the surface of the Pd MM, which may have a negative impact on the hydrogen flux. Su et al. (2005) have reported a very high hydrogen permeability value in comparison to various other studies, using an approximate 2.5 μm thickness of Pd-supported on PSS graded with a SiO2 layer, deposited by a sol–gel technique. A new type of ultra-thin, hydrothermally stable, molecular-sieve supported hydrogen-selective membrane was fabricated as part of a project by the National Research Council, reported under DOE, and operated at high temperatures and pressures. The main goal of the project was to scale up economically, providing an easy way to manufacturing and fabricate hydrogen-selective membranes with high flux and selectivity. Not only was this achieved, but a hydrogen permeance higher than 10−7mol/m2·s·Pa with H2/CO2 selectivity greater than 100 at temperatures in the range of 500– 700°C, and pressure of 20 bar was also obtained. (Plasynski et al., 2008) Franz et al. (2000) have reported a novel Pd-based MMR with controlled selective permeation and/or increased hydrogen flux, using MF techniques and sequential steps to fabricate Pd MM as presented in Fig. 5.3a. Moreover, in-situ palladium and its alloy based MM were prepared as part of microfluidic devices, as shown in Fig. 5.3b (Gielens et al., 2004).

5.4.2

Zeolite micromembranes

Zeolites are crystalline aluminosilicates characterized by a structure comprising a three-dimensional pore system and regular framework formed by linked TO4 tetrahedral (T = Si, Al) with different morphological and physico-chemical properties. Due to their impressive selectivity and uniform pore structure, they have very efficient molecular sieving properties, and are able to separate molecules based on size and shape. Zeolite powders, films and membranes are widely used in catalysis, adsorption and separation applications (McLeary et al., 2006; Pina et al., 2011). Zeolites are cheap and widely available due to their abundance in both natural and synthetic forms. The application of zeolites in the membrane field is growing very fast, and has been the subject of increased research focus during the last few decades (McLeary et al., 2006). In this section, zeolite micromembranes for MMRs will be discussed, and synthesis methods and applications in gas separation and fine chemical synthesis will be introduced. Incorporating zeolite into microreactors presents a range of challenges, including difficulties in the production of homogeneous layers, sufficient coatings, and adhesion and reproducibility. A zeolite itself may act as either a catalyst or a membrane separating layer in microreactors. A zeolite film can be directly grown on microchannels, and during the synthesis the thickness and crystal orientation can be regulated. Many

© Woodhead Publishing Limited, 2013

Microreactors and membrane microreactors (a) Starting material: Si wafer with 0.25 μm of oxide and 0.3 μm low pressure chemical vapour deposition (LPCVD) nitride

Backside KOH etch, to form channel/membrane structure

Pattern backside (dry nitride etch followed by BOE)

Blanket deposition of Pd (200 nm) with a thin Ti (10 nm) adhesion layer.

Pattern perforations on frontside (dry nitride etch)

Opening of Pd membrane using BOE

Heater patterning and metallization (Pt/Ti)

Pt

209

Packaging

Front view

SiNx SiO2 Si Pd Al Side view

5.3 (a) Microfabrication steps for a palladium membrane microreactor (Jensen et al., 2001; Franz et al., 2000) (Copyright permission 2001 Elsevier). (LPCVD in the figure refers to low pressure chemical vapor deposition.) (b) Procedure to manufacture a microsieve-supported Pd–Ag membrane micro system using MF techniques (Gielens et al. 2004) (Copyright permission 2004 Elsevier). (Continued) © Woodhead Publishing Limited, 2013

210 (b)

Handbook of membrane reactors <110> Si wafer double-side polished Deposition of 0.3 μm SiO2 and 0.7 μm SiN

Selective removal of SiO2 and SiN with dry etching

KOH etching to create apertures

Open windows on back-side, dry etching SiN

5 μm

SixNy SiO2 Si

Remove Si completely by KOH, etch stop at SiO2

Co-sputtering of Pd/Ag membrane layer

Release of membrane by removal of SiO2 with BHF

5 μm

SixNy SiO2 Pd/Ag

5.3 Continued

factors may influence the zeolite coatings inside a microreactor, including gel composition, time, support, crystal orientation, temperature, synthesis procedure and Si/Al ratio. These factors can be manipulated to produce the desired characteristics of the selected reaction and separation. Since 2004, many articles on preparation of zeolite MMs have been published, on such areas as MFI or Sil-1 zeolite etched on the Si substrate for gas separation applications, and MMRs for KCR and fine chemical synthesis (Coronas and Santamaria, 2004; Kwan et al., 2010; Wan et al., 2001; Yeung et al., 2005). Coronas and Santamaria (2004) have reported on the use of zeolite films and interfaces in micro-scale and portable applications, including the removal of volatile organic compounds from indoor air, recovery of catalysts in homogeneous reactions, zeolitic microreactors and microseparators, for example. Moreover, zeolite coated microreactors and microseparators exhibit high surface-to-volume ratio, and are capable of high productivity as a result of the good contact between reactants and catalyst wall.

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211

In the work by Chau et al. (2003) a zeolite layer was grown uniformly on the Si wafer using a variety of fabrication steps. Firstly, a zeolite−silicon composite was fabricated via hydrothermal synthesis. This was then micro-patterned with a resistant coating, using a photolithography etching process (by UV radiation), after which it was exposed to buffered oxide etch (BOE) and photoresist stripping. In an alternative approach, zeolite nanocrystals were grown directly on silicon, with Teflon tape used prior to the growth. Leung and Yeung (2004) have investigated three different types of freestanding microfabricated zeolite MMs (Sil-1 and ZSM-5 with two different Si/Al ratios, namely 40 and 60). In their investigations, the micromembranes were prepared on silicon substrate, and SEM images of this are presented in Fig. 5.4. These zeolite MMs were subjected to gas permeation tests for hydrogen, He, CH4, CO2 and mixtures of these gases. Gas permeation characteristics are dependent on the morphology, Si/Al ratio and synthesis conditions of the zeolite. A similar study was conducted by Kwan et al. (2010) in which ZSM-5 and Sil-1 MMs (Si/Al ratio 14 to ∞) were fabricated on silicon substrate, and gas permeation tests were conducted for pure gas and gas mixtures. Synthesis parameters such as composition, thickness, orientation, crystal grain size, intergrowth and morphology were adjusted to obtain a homogeneous MS. Surface diffusion is the dominant mechanism at low temperature, and the effect of Al of ZSM-5 MM on gas permeation for single gas components (i.e., H2, He, N2, Ar and methane) is greater than the Knudsen diffusion. When the Si/Al ratio in ZSM-5 increases, the deviation from the Knudsen diffusion simultaneously increases. For propaneN2 separation, the ZSM-5 MM is able to separate the gases based on the kinetic diameters of the molecules, producing the molecular sieving effect. The ZSM-5 based MMs exhibit excellent permeance and perm-selectivity values for single, binary and ternary gas permeation and separation. The zeolite MMs are less expensive than the Pd-based membranes and offer strong competition in both hydrogen production and separation capabilities (especially in miniature devices such as microreformer or micro-fuel cell processors). Wan et al. (2001) prepared zeolites (ZSM-5, Sil-1 and TS-1) as catalysts, MMs and structural materials for micro-devices using 3–16 μm layer thickness and film orientation. Micromachining techniques were employed for the device architecture, and four different fabrication methods were compared: firstly, zeolite powder coatings on Si; secondly, with or without seeding via hydrothermal synthesis for uniform film growth on the Si; thirdly, with confined seeding and; fourthly, zeolite-silicon composite fabrication using a photolithographic etching process (by UV radiation, masking, patterning by photo resist).

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Handbook of membrane reactors

(a)

(b)

100 μm

250 μm

260 μm (c)

(d)

(e)

(f)

5.4 SEM images of (a) zeolite micromembrane units, (b) higher magnification image of a single unit, (c) cross-sectional view of support structure after zeolite growth before etching, (d) zeolite support layer deposited inside the micro-cavity which was etched on silicon, (e) a freestanding zeolite micromembrane layer after etching and (f) layer formed as a result of excessive etching (Adapted from Leung and Yeung, 2004) (Copyright permission 2004 Elsevier).

These kinds of MMs are useful for MMRs and/or incorporation into microfluidic devices with thin-films. The potential applications of such zeolite MMs include separations in pharmaceutical and fine chemical synthesis, as well as in lab-on-chip devices, sensors, zeolite micro-FCs and adsorption screening tools (Pina et al., 2011).

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Microreactors and membrane microreactors

5.5 5.5.1

213

Catalyst coating techniques and hydrogen production in microreactors Catalyst coating techniques inside the microreactors

Introducing a catalyst into a microchannel system is an important step in catalyst preparation for heterogeneously catalysed reactions. There are many methods for the incorporation of catalytic materials inside a microstructured reactor. Wash coating, packed bed catalyst filaments or structured monoliths, coatings of commercial catalysts, or catalysts as a part of microreactor fabrication (as in the direct formation of zeolite crystal on a metallic structure) may all be used (Cybulski and Moulijn, 2006). Kolb et al. (2004) reported various coating methods including spin coating, dip coating and drop coating for the incorporation of porous support materials into microchannels, and later introduced Pt by sputtering and wet impregnation. As previously suggested, wall coated catalysts are more efficient than packed bed microstructured catalysts due to issues related to pressure drop. There are two methods generally used: the material-independent method (coating technologies such as wash coating, spray coating and dip coating) or the material-dependent method (e.g., anodic oxidation of aluminium) (Hessel et al., 2005c). Microchannel plates washcoated with porous Al2O3, for example, are available commercially and metals or metal oxides can be introduced to these by impregnation. Alternatively, the commercial catalyst can be coated using a MS. Before coating the ready-made catalyst, the MSs are pre-treated to ensure strong adhesion between the wall and the catalyst. Conventional impregnation methods are employed to prepare a supported catalyst, and the procedure for washcoating by impregnation is the common procedure for catalytic MS preparation. The process for MS wash coating begins with cleaning of the MS in an ultrasonic bath, before thermal pre-treatment, positioning and masking, channel filling with suspension or slurry (e.g., γ-alumina, water and binder), removal of excess suspension, drying, calcination, pre-treatment of porous wash-coats, impregnation (the incorporation of metal and metal oxides for example), and finally drying and calcination. The washcoating of commercial catalyst powders can be carried out using a similar procedure, excluding the pre-treatment of porous wash-coats. Similarly, catalyst coatings can be used to produce MSs via thin film gas phase deposition, with chemical (up to 10 μm thickness) and physical vapour deposition methods (< 1 μm) typically used to deposit metals and metal oxide thin-films. Other methods such as electrophoretic deposition of alumina, ZnO, CeO2 and ZrO2 coating layers inside the microreactors are also used (Cybulski and Moulijn, 2006; Hessel et al., 2005c).

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Klemm et al. (2007) have reported on microstructured reactors for heterogeneously catalysed reactions (gas or liquid phase reactions) using wall catalysts. In wall coated microreactors, the mechanical stability and the adhesion between the catalyst and the wall are the most important considerations. It is important to avoid blistering of the wall catalyst during operations, and prevent wall degradation through shear stresses, bending and impact loads. Furthermore, whilst some reactions require a catalyst layer of greater thickness (> 100 μm) to achieve a desired level of yield and selectivity, the thickness of the wall catalyst must be kept uniform to prevent negative effects on flow distribution. Thus, optimum porosity can also greatly influence the catalyst mass per unit area; otherwise it can lead to cracks and thus limit the mass transfer. Based on all of this knowledge, new catalyst materials are being developed and tested in microstructured reactors to produce high levels of activity and selectivity with better stability.

5.5.2

Hydrogen production in microreactors

Hydrogen production in microreactors is studied extensively using a range of different fuels and processing technologies. Most of the studies focus on catalytic SR, as this technique offers high efficiency, commercial experience and a high hydrogen/CO ratio (Holladay et al., 2009). Generally, SR reactions are endothermic, fast and equilibrium-limited. They are based on a fuel (a hydrocarbon or alcohol) reacting with steam to give hydrogen and CO2 as major products, whilst forming CO, CH4 and coke in addition. Two key points to note are that fast heat and mass transfer rates are needed to drive the endothermic SR reaction, and that SR reactions are fast with short residence times. These two points, in combination with other motivating factors, make microreactor utilization in SR reactions beneficial. Around 70–80% of hydrogen is produced by SR from natural gas, using Ni as the catalyst in conventional macro-scale reformer systems (Pattekar and Kothare, 2004). This process is highly energy intensive and is not environmentally friendly, due to the high air emissions produced. Moreover, hydrogen is now not only used in the production of ammonia, fertilizers and in oil refineries, but also as fuel for fuel cell devices. Hydrogen can be used as a fuel in all types of FCs, especially proton exchange membrane fuel cell (PEMFC) and alkaline fuel cell (AFC) energy systems which can be used in stationary and portable electronic applications. For the production of hydrogen on-site and on-demand, it can be a challenging task to build a compact, highly efficient system, but, as previously discussed, the compactness and light weight of microreactors makes them very promising for use as on-board fuel processing micro-devices. Exploring the use of biofuels like bio-ethanol, bio-methanol, bio-butanol and bio-

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215

glycerol as raw materials for hydrogen production could be advantageous when operating at low temperatures with closed carbon cycles (Seelam et al., 2012). Studies of hydrogen production via SR of alcohols and hydrocarbon based fuels in microreactors are summarized in Table 5.2. These reactions can also be beneficial in MMRs. In microreactors, hydrogen is produced via SR, POx, WGS reaction, autothermal reforming (ATR) and other processing technologies. Delsman et al. (2005) studied the comparison between the conventional fixed-bed system and MRT for portable hydrogen production, with a complete methanol steam reformer system designed with a reformerburner (RB), which coupled endo- and exo-thermic reactions and a POx reactor with a heat exchanger for power output of 100 W and 5 kW. It was revealed that, for both levels of power, structured microreactors perform better than conventional reactors in the same conditions.

5.6

An overview of membrane microreactors

Integration of sub-micron thick membranes into devices with dimensions in the sub-millimetre range has the potential to produce highly efficient results, for example in high-purity hydrogen production. The MM can act as a permselective barrier, or be used to facilitate the addition of specific molecules to enhance microreactor performance. Moreover, the high pressures that are critical for conventional membrane systems can be operated safely in MMRs. The concept of lab-on-chip has created a range of new opportunities and has already been successfully applied to many fields, including catalysis, analytical chemistry and integration of sensors and micro-separation units. Whilst the integration of membrane functionality into microchemical systems (micro-devices) offers great benefits, designing this kind of micro unit can be challenging. In MMRs, three main units are integrated: the membrane separation, reaction zone and reactor channel units. In the previous section, the advantages, disadvantages and role in hydrogen production of microreactors have already been discussed. Table 5.3 summarizes microreactors for hydrogen production, which consist of a reformate stream of gases with other by-products, such as CO, CO2 and CH4. This stream cannot, however, be fed directly to a PEMFC as even very small amounts of CO (i.e., > 10 ppm) can damage the anode electrode of PEMFC, and the inhibiting effects of by-products lower the overall performance of the FC system. Microstructured membrane systems can be elegant, efficient alternatives to conventional catalytic CO clean up or WGS reaction systems. In order to produce on-site streams of pure or COx free hydrogen for a micro-fuel cell processor, a thin Pd-based membrane is introduced after the micro-reforming section, or the membrane is attached to the reforming catalyst in order to selectively separate hydrogen. Similar catalytic MM devices for high-purity hydrogen generation were designed by Wilhite et al.

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Table 5.3 Hydrogen production in microreactors with various dimensions and operating temperatures

© Woodhead Publishing Limited, 2013

Microreactor (its material and shape)

Dimensions

Reaction

Catalyst-coated

T (oC)

Reference

Autocatalytic Microchannel, micro-combustion PCFSF supported Stainless steel Tree shaped serpentine flow Si wafer Rectangular microchannel

D = 3 mm w = 700 μm, D = 1.5 mm, tw = 0.3 mm, tc = 10–100 μm, L = 20 mm L = 10–20 mm w = 1 mm, h = 20 mm, L = 50 mm w = 2 mm, h = 1 mm, L = 110 mm

MeSR MSR

15% NiO/Al2O3 Cu/ZnO/Al2O3 and Pt

600–840 230

Levent et al., 2003 Tadbir and Akbari, 2011

MSR ESR MSR

Cu/Zn/Al/Zr Cobalt talc CuO/ZnO/Al2O3

240–360 350 230

Zhou et al., 2009 Domínguez et al., 2011 Chen et al., 2011

ESR

Rh/CeO2

350–650

Gorke et al., 2009

ESR

Co/ZnO

200–500

Casanovas et al., 2008

ESR

CO3O4

400

Casanovas et al., 2009

ESR

Rh/CeO2/Al2O3

400–600

Peela et al., 2011

MeATR

Ni/Al2O3

600

Akbari et al., 2011

MSR

Cu/ZnO

100–300

Kim and Kwon, 2006

ESR

Ni/Al2O3

500–700

Wang et al., 2008

w = 200 μm, h = 200 μm, L = 80 mm Stainless steel w = 700 μm, d = 350 μm, tw = 750 μm, L = 78 mm Silicon micromonoliths w = 0.9 mm, D = 3–4 μm, d = 350 μm, L = 210 μm Micromachined and w = 500 μm, d = 400 μm, L = 60 welded mm Rectangular microchannel w = 340 μm, h =340 μm, L = 8.5 mm Glass rectangular w = 0.5 mm, h = 1 mm, L = 2 cm microchannel Ceramic rectangle w = 0.3 mm, h = 0.4 mm, tc = 0.3 μm, L = 20 mm microchannel

w = width; d = depth of the channel; D = diameter of the microchannel; L = length of the microchannel; h = height; tc = catalyst thickness; tw = wall thickness; MeSR = methane steam reforming; MSR = methanol steam reforming; ESR = ethanol steam reforming; MeATR = methane autothermal reforming.

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(2006) for portable power applications. The introduction of membrane phenomena in the sub-millimetre ranges, using mass transport controlled by a pressure-driven process in channels, flow and reaction domain, is also of interest. For example, hydrogen production in a tubular membrane reactor module was studied extensively using thick Pd-based membranes (Basile et al., 2011; Seelam et al., 2012) and reported in many publications. However, there have not yet been many studies focussed on membranes in microreactors for hydrogen production.

5.6.1

Methods and approaches

Jong et al. (2006) have reported different approaches and methods for integration of the membrane into microfluidic devices. These approaches are summarized as follows: (a) Direct incorporation of commercial membranes: achieved by clamping or gluing flat commercial membranes, followed by functionalisation. The membrane is introduced during micro-stereo lithography (ML) and HF membranes, rather than flat, and are used between capillaries. Sealing of the membrane with the micro-device is the main problem, particularly in the use of inorganic substrates such as glass or silicon with polymeric membranes. Furthermore glue can fill the pores of the membrane − a problem that can be solved by use of the ML technique. This approach is simple to apply to processes, and a wide choice of membrane materials and morphologies exist Jong et al. (2006). (b) Membrane preparation as part of the chip fabrication process: for many applications, this is the most appropriate method. Moreover, many authors have worked on this approach, developing methods to introduce membrane into microfluidic devices for hydrogen separation and purification using SR, WGS and KCR reactions (Chau et al., 2003; Deshpande et al., 2010; Karnik et al., 2003; Tong et al., 2003, 2005a; Wilhite et al., 2006; Zhang et al., 2006). Microfabrication techniques such as etching for well-defined pores, growing zeolite crystals on Si substrate, SiO2, porous Si, alumina or molecular-sieve materials, thin metallic film deposition on a SiO2 wafer, preparing polymeric membranes by casting and creating pores by ion track technology are just some of the approaches developed for this kind of method Wan et al. (2001). The adhesive and mechanical strength between the support structure (e.g., Silicon) and the membrane (e.g., thin Pd metal) during preparation and operation are the most important elements of this method. (c) In-situ preparation of membranes: this method is based on the fabrication of micro membranes in-situ with microfluidic channels or devices.

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One example of this method is the production of liquid membranes inside microfluidic channels, where membranes are formed between the pillars by laser induced phase separation of the acrylate monomer. This method is very complex and can only be applied to limited materials. (d) The final method is using the membrane itself as chip material: this simple method involves fabrication of a zeolite membrane into microchannels for the MMR devices, and is very promising as it offers many advantages over the other methods. No additional materials are required in this method, for example. Jong et al. (2006) studied the PDMS materials due to their high gas permeability, and exploited this characteristic in microfluidic devices. This kind of method is applied to polymeric chips, hydrogel based chips and also fabrication of completely porous chips capable of utilizing membrane as chip materials. Some key features must be considered in selecting the most desirable membrane for microfluidic devices. The chemical, mechanical, thermal and surface properties of membrane materials, as well as their compatibility with the chip, and reaction to fouling are important, as are the selectivity, porosity, morphology and geometry (flat, HF, tubular etc.) of the membrane type. Feasibility of the fabrication method is another key feature and selection of the method and the approach, from the four previously discussed approaches, is the main application-dependent choice. Finally, the availability and functionality of operating conditions such as pressure, temperature, chemicals and reagents must be considered. In combination, these features are very important for the successful design of MMR devices (Jong et al., 2006; McMullen et al., 2010; Jensen et al., 2001).

5.6.2

MMR applications

Currently listed reactions performed in MMRs are reported in Table 5.4. Most of the studies are devoted to hydrogen separation, using hydrogenextractor and hydrogen-distributor MMR devices. A microfabricated Pd–Ag MMR integrated device, using reforming catalysts to produce highpurity hydrogen for portable micro-fuel cell applications, has been studied by Wilhite et al. (2006) (Fig. 5.5). In this study, hydrogen generation on the catalyst-coated wall layer and purification by Pd–Ag MM were integrated into one compact unit, improving the overall thermal and reactor performance. This was achieved by MF of a microfluidic device, followed by the deposition of a hydrogen permselective and 0.2 μm thick Pd–Ag MM on the microchannels, with a final coating by a catalyst suspension (prepared by co-precipitation method, milling, suspension in MeOH and aluminium oxide binder addition). The coating solution was added uniformly drop wise on the membrane surface using a syringe. In this case, the membrane and

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Table 5.4 Reactions performed in membrane microreactors Membrane microreactor

Fabrication methods

Pd/PS

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Etching, BOE, DRIE, SD Pd–Ag23%/Si oxide Evaporation, sputtering, spin-casting, or electrodeposition Pd–MMR – ZSM-5/NaA/PSS

NaY/NaX/NaA/ PSS Pd–Ag multichannelled

Pd/silicon

Fabricated microchannel on porous stainless steel plate Electro-discharge micromachining Structured catalytic filament bed membrane microreactor Sputtering, wet etching, patterning, plasma etch

Thickness Reaction (μm)

Temperature (°C)

Catalyst

Membrane function

References

0.34

250

–

H2 distributor

Ye et al., 2005

0.2

1-butene hydrogenation POM

400

LaNi0.95Co0.05O3

H2 separation

Wilhite et al., 2006

0.2

MeSR

887

Ni/MgAl2O4

H2 separation

30

KCR

100

ZSM-5

Water removal

Goto et al., 2003 Yeung et al., 2005

6

KCR

100

Water removal

–

PDH

550

Cs-exchanged NaA 0.5 wt%Pt/1%Sn/ Si–Al

0.2

WGS

200

Cu/Zn/Al2O3

H2 permeation

H2 permeation

Yeung et al., 2005 Wolfrath et al., 2001

Karnik et al., 2003 (Continued)

Table 5.4 Continued

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Membrane microreactor

Fabrication methods

Thickness Reaction (μm)

Temperature (°C)

Catalyst

Membrane function

References

Pd/silicon wafer/ PDMS

4

Dehydrogenation of cyclohexane

200

Pt

H2 permeation

Cui et al., 2000

Pd/alumina

Wet and dry micromachining, PDMS moulding, bonding, etching, sputtered –

1

300

–

H2 permeation

Dense polymeric

–

–

–

–

EO separation

ZSM-5/PSS

ZSM-5 membrane microchannel on PSS plate Fabrication of porous metallic HFs, CNF on PSS, PDMS coating, ELP

25

Dehydrogenation of cyclohexane Ethylene epoxidation Oxidation of aniline

70

Titanium silicalite-1

Water removal

Yamamoto et al., 2006 Schiewe et al., 2001 Wan et al., 2005

20

Catalytic reduction of nitrite (G-L)

RT

Pd

H2 distributor

Aran et al., 2011

6

WGS

200–500

30%CuO/CeO2

H2 separation

70

PDH

550

Pt/Sn/alumina

H2 extractor

Rahman et al., 2011 Kiwi-Minsker et al., 2002

Dense polymer PDMS/CNFs/ PSS Pd/Al2O3 HF Pd–Ag/ filamentous catalyst

Filaments structured catalytic packing

Microreactors and membrane microreactors (a)

Ar

CH3OH + O2

221

Ar, H2

H2 + CO + CO2 + CH3OH

Pt resistive heaters, 200 nm

Pd–Ag permselective film, 200 nm

Catalyst washcoat

Silicon nitride, 300 nm

Silicon substrate, 0.65 mm

Silicon oxide, 250 nm

(b)

(c)

5.5 Catalytic micromembrane device for high-purity hydrogen generation: (a) micro-device unit with side-view (left) and cross section (right), (b) completed micro-device prior to assembly and (c) catalyst washcoat of microchannel with LaNi0.95Co0.05O3/Al2O3 (Wilhite et al., 2006) (Copyright permission 2006 John Wiley and Sons).

the catalyst are the active phases in which reaction and separation happen simultaneously and instantly. Goto et al. (2003) have simulated the MMR for fuel cell applications using a methane feed, and have gone on to compare the efficiencies of the three FC system configurations; one with Pd–MMR followed by PEMFC, solid-oxide fuel cell and the proton conducting solidoxide fuel cell (SOFC). The simulation results show that Pd–MMR is the most effective system for power generation in comparison to the other two systems. In Takahashi et al. (2005) a suspended MEMS based micro-fuel reformer was designed and manufactured, and the performance of the reformer evaluated. In this study, in-situ chemical vapour deposition (CVD) of the alumina catalyst bed on a membrane was used as the preparation method for better mechanical and thermal isolation of the reaction zone on the membrane. Most of the microfabricated Pd-based MMs are much more efficient than the conventional thicker or large scale devices, as reported by many authors.

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Water

Set-points from external supervisory predictive controller

Adaptive PI parameters

V1 (sp) I1

PI control V1

Adaptive PI parameters

V2 (sp)

Si substrate

PI control I2

V2

Non-permeate from lower microchannel T2

T1

Permeate hydrogen from upper microchannel

Heaters and temp. sensors Methanol Mixer/vaporizer

Catalytic reformer Palladium membrane microreactor Heaters and sensors on an insulator Pd

Resistor 1 μm 500 μm

Cu Cu

Pyrex 1000 μm

5.6 A microreactor system with integrated Pd membrane with heaters and sensors (Karnik et al., 2003) (Copyright permission 2003 IEEE). (In the figure, V1 (sp) and V2 (sp) refer to voltage regulation signal set-point 1 and 2, from and to the PI controllers. T1 and T2 refer to temperature sensors to the heaters and I1 and I2 refer to current to the resistive heaters from the PI controllers.)

In the first experiment of its kind, a Pd-based MMR for hydrogen production via SR of methanol was designed, fabricated and tested in a study conducted by Karnik et al. (2003). The MMR consists of four main components: a mixer/vaporizer for methanol and water, a catalytic steam micro-reformer, a WGS reactor with Cu and integrated Pd MM, and integrated resistors and sensors (shown in Fig. 5.6). This Pd–MMR is utilized to produce pure or CO-free hydrogen for a micro-fuel cell processor. The complete MMR was built on a silicon substrate using MEMS MF techniques. The Pd–MMR device consists of an integrated WGS reactor and a Pd MM (hydrogen gas separator) in one compact unit (as shown in Fig. 5.6) which is constructed from a Cu−SOG−Al−Pd layer (spin-of-glass (SOG), Cu as a catalyst). Cu, SOG and aluminium provide structural support to the MM Pd. In another study, a MEMS based thin Pd–MMR with 0.34 μm Pd thickness was fabricated on an oxidized PS support. The MMR system was characterized by permeation experiments with hydrogen, N2 and He and also carried out the 1-butene hydrogenation in the temperature range of around 200–250oC. The

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Microreactors and membrane microreactors Liquid

223

Liquid

Gas

Porous wall

Dense polymer layer Gas Liquid

Gas

5.7 Scheme for membrane microreactors for multiphase reactions, that is, gas−liquid reactions (left), a PSS supported dense PDMS gaspermeable membrane with CNFs as a catalyst support (right). (Aran et al., 2011) (Copyright permission 2011 Elsevier).

hydrogen flux was reported to be 0.112 mol m−2 s−1, with a partial pressure difference of 110 kPa (Ye et al., 2006). A new hybrid catalytic MMR has been designed and fabricated for gas−liquid−solid (G−L−S) reactions (Aran et al., 2011). In this MMR, carbon nanofibres (CNF) are grown as catalyst supports on the PSS substrate, and Pd catalyst are immobilized onto the support. The outer part is coated with an encapsulated hydrogen gas-permeable PDMS membrane, as shown in Fig. 5.7. This complete catalytic MMR system is used to study the G−L−S reaction, such as nitrite (NO−2 ions) reduction in water using a PSS/Pd–CNF/ PDMS reactor. Due to intrinsic reductive properties of CNFs on PSS and catalytic activity of Pd–CNF on PSS, the porous catalytic MMR is highly active and thus very promising in relation to multiphase reactions in microreactors. A new integrated ceramic membrane micro-network constructed from 8 μm thick defect-free Pd films was introduced into the alumina coated channels (Kim et al., 2009). This micromembrane system consists of three channels: SR, WSG and permeate sides. As proposed, this thermally integrated MMR system can be utilized as a portable reformer to produce highpurity hydrogen for PEMFC. The use of miniature zeolite membranes inside a microchannel reactor allows the removal of the water by-product from the aniline oxidation reaction mixture, thus prolonging the life of the TS-1 catalyst. Zeolite MMRs were studied extensively by the Yeung’s research group on KCR (Lau et al., 2003, 2007; Wan et al., 2001; Yeung et al., 2005) and, as KCR is an equilibrium or thermodynamically limited reaction, the results were particularly interesting. In this case free-standing ZSM-5 MMs fabricated on the Si chip were used in the KCR reaction, along with a zeolite (NaX) catalyst. In KCR, the water by-product is selectively removed from the condensation reaction by the MM, thus a supra-equilibrium conversion was achieved with high product purity, shifting the chemical equilibrium towards the desired product side.

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100%

Product yield

80% Equilibrium conversion

60%

40% 20% 0% 0

1

2

3

4

5

Residence time (h)

5.8 Product yield as a function of residence time for a fixed-bed reactor (triangles), a multi-channel microreactor (circles) and a multi-channel membrane microreactor (squares) (Lai et al., 2003) (Copyright permission 2006 Royal Society of Chemistry).

As shown in Fig. 5.8 a higher product yield was achieved using a multichannel MMR in comparison to FBR and a microreactor (Lai et al., 2003). Lau et al. (2007) have reported on a zeolite MMR for KCR, offering two different design approaches. In the first approach, a thin Cs-exchanged NaX catalyst powder was coated on the wall of the EDM-fabricated microchannel (thirty-five straight channels with 300 μm wide, 600 μm deep and 25 mm long). The NaA membrane was grown on the back side on the microchannel plate by pre-seeding with 150 nm NaA zeolite nanocrystals and seeds were attached using mercapto-3-propyltrimethoxysilane, before being assembled on the PSS microchannel plates. In the second design approach, a hybrid NaA membrane with a Cs-NaX catalyst film was deposited on the microchannel plate, and NaA nanocrystal membrane was grown by seeding the microchannels with a thin layer. The procedure was repeated three times until a membrane thickness of approximately 6 μm was achieved, and the faujasite X zeolite layer was then deposited on top of the membrane, before the wall was finally coated with a Cs-NaX catalyst. The KCR between benzaldehyde and ethyl cyanoacetate, ethyl acetoacetate and diethyl malonate were conducted in the zeolite MMR using NaX as the membrane and Cs−NaA as the catalyst. Zeolite MMR was found to function more efficiently than a microreactor without a membrane, due to the fact that water formed during the condensation reaction was continuously removed by the membrane, thus shifting the equilibrium towards the desired products (Lau et al., 2003, 2007).

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5.7

225

Conclusions and future trends

By selective removal of a product or addition of a reactant through a membrane, chemical equilibrium can be controlled. Integration of the reaction and separation in one unit offers great advantages, including improved efficiency over conventional systems, and MMR devices are multifunctional, novel approach to achieve such integration in reaction engineering. In order to enhance the high surface-to-volume ratios and high heat and mass transfer rates in a microreactor, it is beneficial to include functional membranes capable of helping the reactor to achieve maximum efficiency. The high degree of parallelization in micro-devices makes them ideal candidates for high throughput screening and testing devices for membrane based processes and, as such, membrane technology is foreseen to have a bright future in micro and/or chemical process technology. Enhancing this view, membranes are already being successfully exploited and applied in many areas, including analytical chemistry and gas separation, and MF techniques already exist in the semiconductor and MEMS based industries. It therefore seems greatly advantageous to build membrane based micro-devices, with decreasing costs and existing commercial experience. MF technology will be critical to the reproduction of membrane micro-devices, as well as being crucial in enabling precise control of pore sizes, depositing thin membrane separating layers and manufacturing integrated systems for application in, for example, portable electronic applications. However, much research is still needed into the application of functioning membranes inside microfluidic devices. Much research has been conducted on microstructured devices for reactions, analytical devices, catalyst screening and organic synthesis. However, methods for depositing a very thin membrane layer into the MSs have not yet been widely discussed in the scientific literature and only a few areas, such as palladium-based and zeolite MMs for microfluidic devices, have been explored. In the future, new membrane and catalyst materials will be developed and tested in microreactors, and the effect of miniaturization, use of membranes and optimization will be thoroughly investigated. It can be forecast that the application of membrane technology in microfluidic devices will become increasingly important in gas separations, volatile organic compound removal, pervaporation, emulsification, gas−liquid contactors and energy devices, especially for niche markets. Another important aspect for investigation is the catalyst, which plays an important role in catalytic MMRs. In this case, the membrane functions only to separate, and does not participate in the chemical reaction. A thorough understanding of the process for introducing an appropriate quantity of a catalyst into the MSs will be crucial in ensuring sufficient mechanical strength, adhesion of the catalyst to the walls and prevention of ageing

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phenomena. Therefore, the synergic effects of a catalyst and a membrane should be studied carefully in order to assess their robustness, selectivity and activity before they are introduced into micro-devices. Moreover, understanding the manner in which catalysts affect the membrane function will be very important. Dense Pd-based membrane reactors have been widely studied in the conventional tubular and fixed-bed reactors with thick membranes. However, only the permeation characteristics have been studied using microfabricated micro-devices, meaning research is needed into the reaction and separation characteristics in MMRs. Zeolites are very promising materials, with vast potential for use as membrane, catalyst or substrate materials for microreactors in MMR applications.

5.8

References

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5.9 5.9.1 Cf d dm D Dh H KB Kn L n p Re T tc tw Um w

Appendix: nomenclature Notation friction coefficient depth of the microchannel molecular diameter diameter of the microchannel hydraulic diameter height of the channel Boltzmann constant Knudsen number length scale pressure exponential factor pressure Reynolds number temperature catalyst thickness wall thickness mean fluid velocity width of the channel

Greek symbols Λ μ ρf

5.9.2 AFC ATR BHF BOE

mean free path fluid viscosity density of the fluid

Abbreviations alkaline fuel cell autothermal reforming buffered hydrofluoric etch buffered oxide etch

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CMS/ELP CNF CS CVD CWM DCH DOE DRIE DSD EB EDM ELP ESR FBR FC G–L–S HF HT IM/ELP KCR LIGA MD/ELP MeATR MeOH MEMS MeSR MF MOCVD MM MMR MPSS MS MSP MSR MR MRT OPM PCFSF PDH PDMS PE PECVD

combined method of physical sputtering and electroless plating carbon nanofibre co-sputtering chemical vapour deposition catalytic wall microreactor dehydrogenation of cyclohexane US Department of Energy deep reactive ion etching process dual sputtering deposition electron-beam deposition electro-discharge micro machining electroless plating ethanol steam reforming fixed-bed reactor fuel cell gas–liquid–solid hollow fibre high temperature improved method of electroless plating Knoevenagel condensation reaction lithography-electroforming-moulding-micromachining multidimensional plating mechanism methane autothermal reforming methanol microelectromechanical systems methane steam reforming microfabrication metal-organic chemical vapour deposition micromembrane membrane microreactor macroporous stainless steel microstructure magnetron sputtering methanol steam reforming membrane reactor microreactor technology osmotic pressure method porous copper fibre sintered felt propane dehydrogenation polydimethylsiloxane pulsed electrodeposition plasma-enhanced chemical vapour deposition

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Microreactors and membrane microreactors PEMFC PG PI POx PS PSS PVD RT RTD SD SEM Si SOFC SOG SP SS SSP SR UV WGS

proton exchange membrane fuel cell porous glass process intensification partial oxidation porous silicon porous stainless steel physical vapour deposition method room temperature residence time distribution sputter deposition scanning electron microscope silicon solid-oxide fuel cell spin-of-glass spray pyrolysis stainless steel self-supported Pd-based membranes steam reforming ultra violet water-gas-shift

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