Carbon-based membranes for membrane reactors

BRICENO, Universitat Rovira i Virgili,Spain, A. BASILE, ITM-CNR, Italy, 

HARAYA, National Institute of Advanced Industrial Science and Technology (AIST), Japan 

Abstract Membrane reactor research has been focused on new membrane materials to be integrated in a compact configuration. Carbon membranes have scarcely been explored in the past because of mechanical drawbacks. For this reason, it is recommended that carbon membranes are supported. However, this can cause the formation of defects which are disadvantageous in membrane reactor (MR) applications. This chapter explores the main variables to be considered in the development of carbon membranes, mainly focusing on when the carbon material has to be supported. Some applications are revised for macro and micro reactors. 

Key words membrane reactors, microfabrication, carbon membranes, gas separation, microreactors, carbon molecnlar sieve membranes (CMSM). 

Membrane reaction processes are systems where separation and reaction are carried out simultaneously, and the continuous extraction of one of the products can shift the equilibrium, enhancing yield and selectivity as compared with a traditional system. The development of membrane reactors has gone hand-in-hand with innovations in membrane materials and catalysts. Specifically, in the case of membranes, the same type of materials used to obtain them can also be adapted to support different catalysts. In terms of the separation and catalysis functions, porous membranes with permeance superior to dense membranes are the preferred candidates for use in membrane reactors these include porous oxide, zeolite, glass, metal, and, more recently, carbon membranes. Although carbon membranes are still in their infancy and have some serious challenges, such as weak mechanical strength as unsupported membranes and bad controllability and reproducibility of manufacture as supported membranes, they are believed to be promising 

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In this chapter, we first give an overview of carbon membrane materials (Section 10.2) and the classification of carbon membranes (Section 10.3). Then, unsupported carbon membranes, based on planar membranes and asymmetric hollow fiber membranes are discussed (Section 10.4). In Section 10.5, the supported CMSMs are reviewed in detail in terms of precursors, supports, fabrications and problems. In Section 10.6, carbon-based membrane reactors are discussed in detail, based on the topics of dehydrogenation reactions, hydration reactions, hydrogen production reactions, H2O2 synthesis, bio-diesel synthesis, and new carbon membranes for carbon membrane reactors (CMRs). In the end, the new concept of using carbon membranes in microscale devices (microcarbon-based membrane reactor) is outlined (Section 10.7). 

Although carbon membranes are still in their infant stage, and have some serious challenges such as weak mechanical strength for unsupported membranes and bad controllability and reproducibihty of the fabrication for supported membranes, they are believed to be promising candidates for porous membrane based membrane reactors because of their ease of fabrication, low cost of both fabrication and raw materials, molecular-sieve separation effect and high permeance. 

The overall goal of this study is to develop superior catalysts suitable for use in modem advanced slurry phase or membrane reactors. Successful investigations have produced an iron-based unsupported catalyst with high activity and extended catalyst life (improved attrition resistance). This catalyst was used by the authors to understand the phase transformations from iron oxide precursors after activation to iron carbide crystallites and to characterize the presence of characteristic amorphous carbon species that have been reported to envelop the spent Fe-FTS catalyst grains in form of surface layers. Previous studies suggest two different phases to be considered as the active phase including iron oxide and a mixture of x snd e -carbides (Table 1) and in some instances minor amounts of metallic iron, however the spatial distribution of the different... 

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