Research
Now we have four major research themes in our group:
- Silica membranes for gas separation and membrane reactors for hydrogen production
- Low-fouling membranes for wastewater treatment and MBR process
- Functional particles prepared by membrane- and/or microfluidic-emulsification technique
- Developing novel applications of membranes
1. Silica membranes for gas separation and hydrogen production
Tailor-made preparation of silica membranes for gas separation
We study microporous silica membranes for gas separation. The separation of gases is achieved by the “molecular-sieving mechanism”: only gases that are smaller than the pore sizes can permeate through the membranes. Thus, the control of membrane structure, in particular pore size, is one of the most important techniques to be developed. Molecular sizes of gases are not more than 1 nm, so we need to establish the methodology of controlling the pore size of the silica membranes in the subnanometer range for applying various gas separation system.
The technique to fabricate the silica membranes we employ is CVD (chemical vapor deposition) method. Our detailed investigation successfully revealed that the pore size of the silica membranes is finely tuned by the chemical structure of the silica precursors. For example, the silica membrane prepared with dimethoxydiphenylsilane (DMDPS) as silica precursor showed extremely excellent hydrogen-selective performance at broad temperature range. This breakthrough now enables us to broaden the technological application fields of the silica membranes.
Fig.1 Pore size control of the CVD silica membranes by utilizing the differences of the chemical structures of the silica precursors
Hydrogen production using membrane reactors with hydrogen-selective membranes
A membrane reactor is a system that combines “reaction” with catalysts and “separation” with membranes. As reaction and separation occur simultaneously in a membrane reactor, an endothermic equilibrium reaction can be shifted forward beyond the thermodynamic equilibrium by extracting one of the generated components from the reaction system using a component-selective membrane.
We apply such membrane reactors with hydrogen-selective membranes to various hydrogen-production reaction systems. For example, we have successfully demonstrated the production of hydrogen with purity higher than 99.9% from dehydrogenating cyclohexane using the membrane reactor with DMDPS-derived hydrogen-selective silica membrane. We also demonstrated that the direct supply of the produced gases to a commercially-available fuel cell can generate stable electric power. In addition, we can successfully develop membrane reactors for steam reforming reaction of methane, and decomposition reaction of hydrogen sulfide.
Fig.2 Dehydrogenation of cyclohexane using a membrane reactor with a DMDPS-derived silica membrane
2. Low-fouling membranes for wastewater treatment and MBR process
Development of low-fouling membranes
Fouling is one of the severest problems in membrane technologies. In particular, when aqueous solutions containing proteins or polysaccharides are filtered, the membranes are inevitably fouled severely by them. Once the membranes are fouled, it is difficult to recover the membrane performance completely. Therefore low-fouling membranes should be developed.
This low-fouling property has also been studied in the field of biomaterials because they require biocompatible, antithrombogenic surfaces. Inspired by the methodology of the biomaterial researches, we develop low-fouling membranes for wastewater treatment. Now we use zwitterionic polymers and nonionic polymers for modifying the surfaces of commercially-available membranes to achieve this low-fouling property, and excellent performances have been demonstrated. Furthermore we take particular notes of water structure at the surface of the polymers for discussing this low-fouling property. Now this approach enables to us understand little by little how the fouling occurs and how the fouling can be suppressed.
Fig.3 Fouled membrane and non-fouled membrane
Novel type of MBR that use electric field for fouling suppression
Membrane bioreactors (MBRs) are systems that combine conventional activated sludge processes and membrane filtration. Nevertheless, fouling is also an inevitable problem and remains the biggest obstacle for full-scale practical operation.
Recently we develop a novel type of MBR with an electric field applied directly to the membrane surface that will detach the foulant from the membrane surface. This is based on the concept that almost all potential foulant substances such as activated sludge and secreted polymers are charged negatively and that their accumulation on a membrane surface can be suppressed and they can even be removed from a membrane surface by applying an electric field. We successfully demonstrate this novel MBR, and are now carrying out a scale-up study.
Fig.4 Low-fouling membranes prepared using the plasma graft polymerization method : The modified membranes successfully suppressed fouling by BSA
3. Functional particles prepared by membrane- and/or microfluidic-emulsification technique
Membrane emulsification is a technique used to prepare emulsions utilizing membrane pores, where the pore size and pore size distribution are crucial factors affecting the average diameter of the emulsions and their monodispersity. The membrane emulsification technique using Shirasu Porous Glass (SPG) membranes is one of the best-known methods for obtaining monodisperse emulsions because of the sharpness of their pore size distribution. Each emulsion can be a unique space for synthesizing various kinds of functional particles with tuning their particle diameters and monodispersities. This method is favorable because it is simply scalable.
We have successfully prepared chitosan microspheres and chitosan microcapsules with hollow structures by this method. The diameter of the microspheres/microcapsules was controlled in the submicron to 10 μm range by the pore sizes of the SPG membranes, and the obtained microspheres/microcapsules were monodisperse. Furthermore, we demonstrated that the diameter of the microspheres/microcapsules could be successfully predicted by considering the mass balance of the chitosan in one emulsion droplet, based on the assumption that one chitosan microsphere is formed in one water-in-oil (W/O) emulsion droplet. Using the same technique, silver nanoparticles with diameters as small as 10 nm were successfully prepared, and the average diameter was well predicted using the same assumption. Now we try preparing other kinds of functional particles, and demonstrating a scale-up study.
Fig.5 Preparation of various functional microparticles by utilizing membrane technology
Besides this technique, we are studying microfluidic emulsification technique. These particles prepared with such emulsification techniques can be used for pharmaceutical, cosmetic, agricultural applications.
Fig.6 Multiple emulsions by utilizing the microfluidic emulsification technique
4. Developing novel applications of membranes
There are so many kinds of possibilities in membrane science and technology. We study to develop novel applications of membranes, such as energy-saving membrane separation system that can replace distillation, and particle classification method using membranes. These novel applications will become major breakthroughs in every industrial field.
Fig.7 Particle classification method using membranes