Center for Nanomaterials and Chemical Reactions
New methodology for synthesizing mesoporousstructure
New methodology for synthesizing mesoporousstructure
Large-scale synthesis of mesoporousstructure using random copolymer
Angewandte Chemie Int. Ed. 2014, 53, 1 – 6
Despite the same size, the larger surfacearea is advantageous in many ways. In the field of environmental study, thereis an adsorption technique for satisfying both economic feasibility andefficiency at the same time. This technique uses the porous materials with lessweight and large surface area as efficient materials. Under a constant mass,the area of surface is maximized to produce porous materials. One of the typicalporous materials is the mesoporous structure. The term, “meso” means “mid-size”.The size of mesoporous structure ranges from millimeters (mm) to micrometers(μm). And this corresponds to the case when the diameter of a hole is 2-30nm (1nm=10-9m).In 1992, the silica mesoporous structure was developed for the first time. Forthe production, the silica molecules reacted with the surfactant molecules inorder to produce uniform crystals. After the reaction, the surfactant wasrinsed. Then, we could get porous crystals having only the remaining skeletonsmade with silica molecules. The crystal has an infinite number of fine holes witha diameter of 5nm corresponding to one-hundred-thousandth the diameter of ahuman hair. And the shape of the crystal is like a honeycomb or sponge.
These mesoporous structures are made by theuniform arrangement of holes with a constant size. During the production of themesoporous structures, the size of the holes can be adjusted to 1-30nm precisely.And their application is vast, not to mention most of materials in thenano-field belong to the mesoporous structure. The mesoporous structure is notonly the core material for various fields such as catalyst and the separationprocess, energy storage and conversion process and bio-medicine but it is usedfor synthesizing other mesoporous structures.
Structure-directing agent refers to thecompound for inducing a wide variety of materials to the mesoporous structures.Usually, amphiphilic molecules that are organic surfactants or block copolymersare used as structure-directing agents. However, if we use the new synthesizingmethod, self-assembled amphiphilic molecules will restrict the space where variousforms of mesoporous structures that are synthesized from inorganic materials cangrow.
The “self-assembly” means that materialsare assembled themselves. Molecules sometimes gather among themselves with thehelp of hydrogen bonding, van der Waals forces, electrostatic force and others,consequently becoming a bigger molecule. This is also called “self assembly”. However,when using a self-assembled structure that is made by a weak van der Waals forcebetween molecules as a structure-directing agent, it is hard to meet this requirement.Therefore, it is very difficult to transform crystalline material with a veryhigh tendency to grow itself into the mesoporous structure. For these reasons, theproduction of a structure-directing agent that is used for synthesizing mesoporousstructure can be very difficult and costly.
This indicates that we need to develop anew type of structure-directing agent for synthesizing crystalline mesoporousstructure easily or a new method for producing the structure. In other words,for synthesizing the mesoporous structure consisting of crystalline skeletons,the structure-directing agent needs to be designed for the strong inhibition ofthe crystal’s tendency to grow.
In this situation, the research group ledby director Ryong Ryoo at the Center for Nanomaterials and Chemical Reactionsfound a new method for synthesizing crystalline mesoporous structure using aspecially designed amorphous random copolymer. The core of this method is a randomcopolymer. The random copolymer that was designed by the research group has astructure that is made by a random combination with organic functional groupsand is able to combine with inorganic precursors strongly. They performed a researchon how organic functional groups attached to organic materials interact withcrystalline microporous silica that is zeolite skeleton, using tools such as X-raydiffraction analysis (XRD), High-resolution transmission electron microscopy (TEM)and 2-dimensional solid-state nuclear magnetic resonance (Solid state NMR). Here,they could reveal the principles of the induction of the mesoporous structure. Ifthe silica precursor is put in a solvent with dissolved polymer material, amorphoussilica-polymer complex is formed initially. At that time, if the process ofmaking crystalline is sustained, the skeleton that is made with silica bindsthe polymer. As a result, as crystallization progresses, polymers surround thecrystal densely. According to this feature, the 3-dimensional restraint in thepolymer skeleton is maximized. Consequently, the zeolite crystal grows only uptothe nanometer-size. And additionalgrowth is not possible. Using this synthesis mechanism, the research groupdesigned a random copolymer with functional group having a strong interactionwith a specific inorganic precursor. And using it as a structure-directingagent, they synthesized various types of crystalline mesoporous structures suchas zeolite, aluminum phosphate (AlPO4), titanium oxide (TiO2),tin oxide (SnO2) and zirconium oxide (ZrO2). The mesoporousstructures showed an irregular arrangement of very uniform size of mesopores. Especially,when the density of organic functional groups that were attached to the polymermaterials was regulated, the size of the holes of the mesoporous structurescould be adjusted. The research team led by director Ryong Ryoo presented a newmethod for synthesizing mesoporous structure using random copolymer, beyond theconventional concept that only amphiphilic substances can produce mesoporousstructure. Through the analysis of the synthesis mechanism, the research teamalso revealed that the core of the synthesis method was the sustained strongbonding between the random copolymer and the inorganic substance in the processof the crystallization.
By applying the new synthesis methoddeveloped by the research team, which is the way of using the random copolymeras a structure-directing agent, we expect that we can synthesize a new type of mesoporousstructure unlike the previous ones. Especially because a large-scale synthesisof random copolymer can be done at a time compared to conventional blockcopolymers, we expect that a large quantity of mesoporous structure can beproduced in a relative short period of time.