Organic Preforms of Biological Origin: Natural Plant Tissues as Templates for Inorganic and Zeolitic Macrostructures

Nature displays a huge number of biological materials with complex hierarchically built anatomies and a large variety of cellular/porous structures and functional architectures, as the result of evolutionary processes. Most of these materials exhibit micro-/macro-structural features which are difficult, or impossible, to reproduce via technical processes. Nonetheless, reproducing such biostructures into engineering materials might be the key for the development of inorganic materials with novel structures and properties. In particular, inexpensive, abundant, and renewable biological structures such as plant tissues represent, from the technical point of view, interesting preforms/templates for the manufacturing of biomorphous ceramic substrates. The open cellular anatomy provides access for infiltration with liquid or gaseous reactants to convert the biological preform or the carbon template into carbide-or oxide-based ceramics. Such biomorphous cellular ceramics can be used as novel monolithic supports for zeolite layers, producing hierarchically porous composites with unique structures and properties. On the other hand, cellular biological preforms could be used directly as sacrificial templates, in order to obtain bio-inspired self-supporting zeolite macrostructures with complex morphologies and open-porous architectures. Owing to their unique structures and functionalities, zeolite-based biomorphous materials might open new scenarios in the development of catalysts, molecular sieves, adsorbents and membranes with hierarchical porosity and organization. In this chapter we present two approaches towards the realization of biomorphous cellular inorganic zeolite-based materials/replica with hierarchical porosity. On the one hand, cellular biological supports have been subjected to a three-step biotemplating process: (i) pyrolysis of the biotemplate; (ii) liquid silicon infiltration (LSI) for the development of biomorphous SiSiC cellular ceramic supports; and (iii) support self-transformation (SST) for the growth of zeolite crystals in the inner cavities of the above-mentioned porous substrate. On the other hand, biological structures such as Luffa sponges and cellulose fibers have been used as sacrificial templates for the synthesis of self-supporting zeolite replicas, which mimic the morphology and microstructure of the organic materials.