Principles of Nanocasting

1.1 Nanocasting Concept

Casting is a 6000-year-old manufacturing process. The oldest surviving casting is a copper frog from 3200 bc. In the casting process, a liquid or a fluid material is poured into a mold, which contains a hollow cavity of the desired shape, and is then allowed to solidify (Figure 1.1). The solid casting is then ejected or broken out to complete the process. Casting is most frequently used for making complex shapes that would be otherwise difficult or uneconomical to produce by other methods. The casting process is divided into two distinct subgroups: expendable and non-expendable mold casting. Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster and investment (lost-wax technique) moldings. Non-expendable mold casting differs from expendable processes in that the mold does not need to be reformed after each production cycle.

However, the above-mentioned old casting techniques are constrained to large size with a limitation in the centimeter range. If the casting procedure is scaled down to the nanometer scale, 'nanocasting' is a suitable word to describe this process. In recent years, nanocasting started emerging and developing with the demand for creating nanomaterial arrays and nanopores. Nanocasting can also be used for making complex shapes in nanoscale that cannot be prepared or fabricated by other methods. The earliest 'nanocasting' process was utilized to produce nanowire arrays using porous anodic aluminum oxide (AAO) as a template (Figure 1.2). However, the templating process is actually not a 'nanocasting' due to the fact that the diameter of AAO is normally in the sub-micrometer range. From this viewpoint, the first 'nanocasting' technique was adopted by Kyotani and coworkers. The 'nanocasting' technique was first adopted by Kyotani and other researchers to fabricate microporous carbons using ultra-stable zeolite Y (USY) as a hard template. They fabricated microporous carbons with large surface areas (>2000m2 g-1 and large microporosity via a nanocasting process. Microporous carbon replicas exhibited very high adsorption capacities.

In 1998, Göltner and coworkers first proposed the concept termed as 'nanocasting' in mesoporous materials. They used a mesoporous silica monolith with the interconnective pore system as a confined hard template via the 'two-step' nanocasting to prepare mesoporous organic polymer networks with well-defined nanostructure. From then on, the hard templating and nanocasting processes have attracted more and more attention and become one of the most important approaches for the synthesis of porous materials, especially mesoporous materials.

Microporous carbons cast by zeolite Y normally should have the negative structure of the template. Unfortunately, the produced carbons cannot fully duplicate their parent zeolites' structure due to the fact that the pore size of the zeolite template is too small for carbon precursors to infiltrate. Kyotani and coworkers had made tremendous efforts to control nanocasting; however, partial 'zeolite' framework structure with a low periodic regularity can only be observed in a small domain of the obtained carbon products. Another reason for unsuccessful duplication of zeolitic structures is the large amount of defects caused by crystalline aluminosilicates in amorphous carbons. Since this work, nanocasting has been successfully employed in the synthesis of ordered mesoporous materials. The earliest example could be traced back to 1999 when mesoporous carbons were synthesized by using mesoporous silica as a hard template via the nanocasting strategy (Figure 1.3). Two Korean research groups independently reported the nanocasting synthesis of mesoporous carbons. Thereafter, the synthesis of ordered mesoporous materials from nanocasting strategy became a hot topic in the research field of mesostructured materials, especially for non-silicious mesostructures.

1.2 Hard Templates: Ordered Mesoporous Materials

Porous materials have both continuous skeletons and voids that can be randomly arranged (disordered pore system) or highly regular (ordered pore system) with large surface areas. According to the definition of IUPAC (International Union of Pure and Applied Chemistry), porous materials can be divided into microporous (with pore diameter < 2nm), mesoporous (pore size in the range of 2–50 nm) and macroporous materials (pore size >50nm). Among them, microporous mesoporous materials are the most widely studied for applications of sensors, shape-selective catalysis, chemical separations and electronic applications. The presence of micro- or mesopore channels allows for molecules that are accessible to the large internal surface areas and the internal active sites, enhancing shape selectivity and sorption properties.

Generally, inorganic microporous materials like zeolites or molecular sieves are often in crystalline form with narrow pore-size distributions, large surface areas and high ion exchangeable properties, which make them suitable for application in adsorption and catalysis. However, the small pore size of zeolites limits their further application in heavy oil products and the synthesis/ separation of large molecules.

1.2.1 Synthesis of Mesoporous Materials

In spite of the considerable efforts that have been made toward making large and regular pore systems, ordered mesoporous materials still remained elusive until the discovery of MCM-41 in 1992. The researchers of Mobil Company first reported a family of mesoporous silicate molecular sieves (M41S) with large surface areas (up to 1400m2 g-1) and narrow pore-size distribution (in the range of 1.5–10nm). The MCM-41 materials process 2-D hexagonal mesostructures with uniform pore size, but their silicate pore walls are amorphous. MCM-41 can be easily synthesized under the 'hydrothermal condition' in the presence of alkyltrimethylammonium surfactant cations with an alkyl chain length ranging from 8 to 22 carbon atoms. The synthesis mechanism of MCM-41 was proposed, for the first time, as the true 'template' concept, which brought out a novel concept for the scientific domain. As a consequence, a large number of ordered mesoporous silicate materials like SBA-n, KIT-n, FDU-n and HMS-n with various structures have been obtained one after another following the 'surfactant templating' method.

1.2.2 Ordered Mesoporous Materials Prepared from Soft-templating

Soft-templating is defined as a process in which organic molecules serve as a 'mold' and around which a framework is built up. The removal of these organic molecules results in a cavity which retains the same morphology and structure of the organic molecules (Figure...