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"Nanoporous materials" comprise all kinds of porous solids that possess pores in the range from about 0.2 nm up to 50 nm, irrespective of their chemical composition, their origin (natural or synthetic), and their amorphous or crystalline nature. Typical examples are zeolites and zeolite-like materials (e.g., crystalline microporous aluminophosphates and their derivatives), mesoporous oxides such as silica, metal organic frameworks, pillared clays, porous carbons, and related materials.
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Juergen Caro, and Manfred Noack
1. Introduction: Setting the Scene 2 2. Preparation of Zeolite Membranes 5 2.1. Peculiarities of zeolite membrane crystallization 5 2.2. Direct in situ crystallization on supports 8 2.3. Secondary crystallization using seeded supports 9 2.4. Use of silica nanoblocks as precursor 13 3. Separation Behavior of Molecular Sieve Membranes 14 3.1. Apparatus and definitions 14 3.2. Characterization of zeolite membranes by permporosimetry 18 3.3. Permeation of single components 22 3.4. Separation of binary mixtures 29 3.5. Case study: Hydrogen separation 33 3.6. Case study: Carbon dioxide separation 37 3.7. Membrane reactors on the laboratory scale 44 3.8. Micromembrane reactor 46 4. Industrial Applications of Zeolite Membranes 49 4.1. De-watering of ethanol and propanol by hydrophilic zeolite membranes 49 4.2. Ethanol removal from fermentation batches by hydrophobic zeolite membranes 54 4.3. Further R&D on zeolite membrane-based separation processes 57 4.4. Cost analysis: Need for cheaper supports 58 5. Novel Synthesis Concepts 62 5.1. Crystallization by microwave heating 62 5.2. Use of intergrowth supporting substances 65 5.3. Growth of oriented zeolite layers on supports 69 5.4. Bi-layer membranes 73 5.5. Metal organic frameworks as molecular sieve membranes 75 5.6. Functional zeolite films 79 5.7. Mixed matrix membranes 81 6. Outlook 82 Acknowledgment 84 References 84
The introduction of industrial membrane-based separation technologies can dramatically reduce the separation costs in comparison with thermally based separation technologies. In addition, membrane technologies allow the energy effective use and recovery of both valuable raw materials and the separation of wastes. Organic polymer membranes are increasingly used, but they suffer from stability at elevated temperatures and toward attack of organic solvents. Therefore, much effort is put into the development of temperature stable and solvent resistant inorganic membranes. Pd-based metal membranes for hydrogen separation, perovskite-type membranes for oxygen separation and zeolite-type molecular sieve membranes are on the jump into the industrial practice. This increasing application of inorganic membranes in gas separation – and on a later timescale in chemical membrane reactors – is a slow process. Because of the high investment costs, many companies prefer to play the role of an "observer." In this contribution, we reflect the state of the art of zeolite membranes. We report the first industrial application of zeolite membranes in bio-ethanol de-watering and parallel ongoing fundamental research on improving the thin zeolite layer crystallization on porous asymmetric supports following new synthesis concepts and the development of novel diagnostics. In this chapter, we also treat the molecular understanding of zeolite membrane separations since this knowledge is crucial for the proper use of zeolite membranes and for the exploration of new application fields.
1. INTRODUCTION: SETTING THE SCENE
Intelligent membrane engineering can help to realize the process intensification strategy. Integrated membrane separations and new membrane operations such as catalytic membrane reactors and membrane contactors will play a crucial role in future technologies. However, so far no inorganic membrane is used in large-scale industrial gas separation. The increase of the 235U isotope concentration in a 238U/235U mixture from 0.7% in natural uranium to approximately 3.5% for nuclear fuel applications by separation of 235UF6 and 238UF6 on porous ceramic membranes according to the Knudsen mechanism with an ideal separation factor of 1.0043 is an exception. However, nowadays exclusively gas centrifuges are used for uranium isotope separation. Membrane reactor technology has a huge potential in the development of processes that are more compact, less capital intensive, giving higher conversions and selectivities in both thermodynamically and kinetically controlled reactions, respectively. Membrane reactors are expected to save energy and costs of feed/product separation. So far, no high-temperature membrane reactor for chemical reactions is in industrial operation. The use of porous ceramic filter membranes in biotechnology is an exception.
Inorganic membranes such as ceramics, metals, and glass show promising properties different from the organic ones. They can be backwashed frequently without damaging the separation layer. Inorganic membranes are highly resistant to cleaning chemicals, they can be sterilized and autoclaved repeatedly at 130–180 °C and can withstand temperatures up to at least 500 °C. These properties recommend them for biotechnological processes as well. Inorganic membranes should have longer life spans than organic ones. The life span of a typical hydrophilic organic membrane is approximately 1 year, of a hydrophobic membrane 2 years, and of fluoropolymers up to 4 years. Inorganic membranes are, however, much more expensive than polymeric ones, and they are brittle.
Three types of inorganic membranes are near to a commercialization: Pd-based membranes in H2 separation, perovskites in O2 separation, and zeolite membranes in shape-selective separations. The regular pore structure of a zeolite molecular sieve suggests that a thin supported zeolite membrane layer can discriminate between molecules of different size and shape. The pore diameter of the separating zeolite layer is in the range of the kinetic diameter of the molecules to be separated to force molecular sieving as the determining diffusional regime. Furthermore, beside the molecular exclusion effect, due to the interplay of mixture adsorption and mixture diffusion, reasonable separation effects on zeolite membranes can be expected according to specific adsorptive interactions and/or differences in the molecular mobilities. The rapidly growing progress in the field of zeolite membranes is reflected by some recent reviews. It is, therefore, not the aim of this contribution to give a comprehensive picture of zeolite membranes and to present all the fundamentals in detail, but to highlight and evaluate recent developments.
By the end of the 1980s, the idea was born to develop zeolite membranes and the first attempts to prepare them were reported, the first patents were claimed. With some pioneering papers, R.M. Barrer triggered the experimental work on zeolite membranes. In parallel, he contributed to the theoretical understanding of mixture permeation through porous membranes. The first one and the last one of Barrers altogether 407 publications were dealing with membranes. The unit Barrer of gas permeability (flux in moles per time and area through a membrane of a given thickness and pressure difference) honours R.M. Barrer (Section 3.1).
Today, LTA (Linde Type A) membranes in the de-watering of alcohol by steam permeation or pervaporation have reached the commercial state. For shape-selective separations, other zeolite membranes with structure types such as MFI and DDR (deca-dodecasil 3R) are already in the technicum scale. Further molecular sieve structures are tested as membranes (Table 1). Most progress in the development of molecular sieve membranes was achieved for MFI-type membranes (silicalite-1 and ZSM-5) since their preparation is relatively easy. They can be synthesized highly siliceous, which provides chemical stability and allows for oxidative regeneration. Therefore, this contribution will mainly deal with MFI-type membranes. New ways of synthesis, improved permeation tests, and proper applications shall improve the zeolite membranes for their technical use. Increasing R&D activities are reflected by increasing publication activities (Fig. 1). It is the aim of this contribution to summarize the state of R&D on zeolite membranes as a relative young branch of the inorganic membrane family, 250 years after the discovery of zeolites by Cronsted. It will be shown that the application of a crystalline molecular sieve as a zeolite membrane layer offers huge promises but it is still a challenge in itself.
2. PREPARATION OF ZEOLITE MEMBRANES
2.1 Peculiarities of zeolite membrane crystallization
As it will be described in more detail in Section 3.1, for high fluxes and a proper handling of zeolite membranes, a thin zeolite layer with a thickness of 1–20 µm is crystallized on a mechanically stable support. However, the chemical compositions of the crystallization solutions and their handling for zeolite membrane preparation as a thin supported layer differ from the standard recipes for a zeolite powder crystallization [21–23].
The following points are characteristic of the zeolite layer crystallization on supports :
At sufficient supersaturation, heterogeneous nucleation takes place on both the geometric outer surface of the support and inside the pores of the support. If externally prepared seed crystals are attached to the surface of the support, primarily the crystal growth of the seeds takes place but the simultaneous secondary nucleation at the surface of the seed crystallites and in/on the support cannot be suppressed completely. Therefore, diluted crystallization solutions are used to prevent the formation of new seeds and to have only growth of the attached seeds to a continuous layer.
In the beginning of the growth of the seeds, the surface-to-volume ratio increases like in the case of the crystallization in the free solution. This is based on the effect that in the beginning of crystal growth, usually a parallel nucleation takes place, which results in a surface enlargement. In the subsequent process of crystal intergrowth, the individual crystals grow together to a continuous layer and the surface-to-volume ratio decreases drastically.
The diffusion of the precursor species in the solution is not rate limiting. Since crystal growth is controlled by a first-order surface process, the growth rate decreases with the reduction of accessible surface.
For the crystal intergrowth that is important for the sealing of voids between crystals, the viscosity of the synthesis solution should be low to enable mass transport in narrow slits. The driving force of the diffusion process is the concentration gradient. Therefore, the low viscosity should be realized rather by higher temperatures than by dilution. Another way to decrease the viscosity consists in an increase of the pH, which results in a higher concentration of low-connected silica species.
During the crystal intergrowth of isolated crystals to a continuous layer, a large slit surface is in contact with a small volume of synthesis solution. Therefore, besides crystal growth, a strong heterogeneous secondary nucleation inside the slit can occur, which can lead to a closure of the macroscopic slit pore by many small crystals with intercrystalline transport pores between them. A post-synthesis thermal or hydrothermal treatment can result in a reorganization of these domains with improved membrane properties.
The starting chemicals for the preparation of the synthesis batch should be selected with the aim to have low salt concentrations in the solution. Whereas these salts are not disturbing in the formation of the free crystals, the incorporation of neutral salt species – especially in multicrystal layer formation – can be disturbing since defect pores are formed by their thermal decomposition (e.g., NH4NO3 and carbonate decomposition).
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