Preparation of POSS-derived robust RO membranes for water desalination

Significance Statement

Silica membranes synthesized through hydrolysis/condensation, normally referred to as the sol-gel process of tetraethoxysilane has been implemented for separation of gases. The dense structures of these silica membranes are fundamental in the separation of small gas molecules such as nitrogen, helium, and hydrogen. Unfortunately, these membranes are normally too dense for application in separation of water. Above all, silica materials with Silicon-oxygen linkages are unstable towards hydrolysis and therefore cannot be applied for separation of water for a long design life.

However, introducing ethylene bridges and implementing 1,2-bis(triethoxysilyl)ethane as the precursor is helpful in expanding the silicon-oxygen network in a bid to improve porosity, therefore, improving water permeability. Therefore, 1,2-bis(triethoxysilyl)ethane-based membranes with about 1 × 10−13 m/s·Pa water permeance and over 95% sodium chloride rejection can be implemented as reverse osmosis membranes.

A team of researchers from Hiroshima University in Japan introduced a (triethoxysilyl)ethyl group to every corner of silicon polyhedral oligomeric silsesquioxane, T8 and exposed it to hydrolysis to form bridged silica membranes for separation of water. They then analyzed the functioning of the membranes using reverse osmosis experiments applying 2000 ppm sodium chloride solution. Also, they investigated the robustness of these membranes to heat and chlorine. Their work is published in peer-reviewed journal, Desalination.

The team prepared polyhedral oligomeric silsesquioxane containing silica sols through hydrolysis of octakis(triethoxysilylethyl)-substituted polyhedral oligomeric silsesquioxane. They coated these sols on SiO2/ZrO2/TiO2 porous supports. They were then calcinated at 350 °C to come up with the membranes.

The membranes were then applied for reverse osmosis using 2000 ppm sodium chloride solution. The experiment was done at 25 °C and a pressure of about 1.0MPa. The team then determined the sodium chloride rejection and liquid permeance.

The team also immersed the synthesized membrane in sodium hypochlorite solution in the dark and pH adjusted to 7 using a buffer solution. For every 20h interval, the researchers rinsed the membrane and subjected it to reverse osmosis.

calcination temperature may lead to better separation selectivity since the network densification is improved. However, this could have led to thermal degradation of the framework. Therefore, it was paramount to establish the thermal stability of the gels. Homo-polymer gels were observed to lose weight, which proceeded in two steps. Weight loss at a temperature range of 250-350 °C was due to dehydration of the remaining silicon-hydroxyl groups as well as decomposition of Tetraethyl orthosilicate to form Si-OH which undergoes dehydration. Weight loss between 420-600 °C was due to the decomposition of the ethylene units. Copolymers, on the other hand, experienced less weight loss.

adsorption isotherms synthesized by drying the gels showed that the gels had porous characteristics. The authors analyzed water desalination performance of the membranes using reverse osmosis experiments. They recorded liquid permeance of about 1 × 10−13 m/s·Pa and sodium chloride rejection of 90% at 25 °C. Liquid permeance (at 90 °C) observed to increase while sodium chloride rejection was relatively the same. The authors observed that the membranes were robust to chlorine and heat. Their performance was unchanged after exposure to sodium hypochlorite solution.

The results of this paper indicate high application potential of the membranes for high-temperature water separation.

Preparation of POSS-derived robust RO membranes for water desalination - advances in engineering

About the author

Dr. Kazuki Yamamoto is an assistant professor at Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science. He received Dr. Eng. in 2017 from Hiroshima University. After working at Hiroshima University as a Project Researcher, he became an assistant professor at Tokyo University of Science in 2017. His research interests are polysilsesquioxane-based materials including separation membrane and porous films.

About the author

Prof. Dr. Takahiro Gunji is a professor of the Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science (TUS), Noda, Chiba, Japan. Prior to joining TUS in 1993, he worked as a post-doctoral research fellow on the synthesis of novel alkoxylated fullerene in the Department of Chemistry, University of Wisconsin at Madison, WI. He earned his PhD, MSc, and bachelor’s degrees in Engineering from TUS, Japan, in 1992, 1989, and 1987, respectively.

Prof. Dr. Takahiro Gunji is the current chairperson of the Research Group on Inorganic Polymers, the Society of Polymer Science, Japan. His current research interests are element-block polymers, sol-gel chemistry, inorganic polymer chemistry, and organic-inorganic hybrid materials.

About the author

Dr. Masakoto Kanezashi is an Associate Professor at Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University. He received his Ph.D. degree from Hiroshima University in 2005. He was a postdoctoral research fellow at Department of Chemical Engineering, Arizona State University supervised by Prof. Jerry Y. S. Lin (2005-2007). He then moved to Japan and served in Department of Chemical Engineering, Graduate School of Engineering, at Hiroshima University as an Assistant Professor supervised by Prof. Toshinori Tsuru.

His current research focuses on the development of microporous inorganic membranes for gas/liquid separation, and the evaluation of permeation mechanism of sub-nano-sized porous membranes. He received several awards, including Young Researcher Award, The Society of Chemical Engineers, Japan (2011), and Young Researcher Award, The Membrane Society of Japan (2016).

About the author

Toshinori Tsuru received his Dr. Eng. in 1991 from The University of Tokyo. After working at The University of Tokyo and Hiroshima University as a research associate and an associate professor, respectively, he was promoted to full professor in 2006. He was appointed distinguished professor in 2015 at Hiroshima University. His research interests include the preparation of subnano-nanoporous membranes and their applications to gas and liquid phase separation.

About the author

Joji Ohshita received his Dr. Eng. in 1991 from Hiroshima University. After working at Hiroshima University and Kyushu University as a research associate and an associate professor, he was appointed professor in 2007 and distinguished professor in 2014 at Hiroshima University. His research interest encompasses the development of functional materials grounded in element-based chemistry.  

Reference

Kazuki Yamamoto1, Sayako Koge1, Takahiro Gunji2, Masakoto Kanezashi3, Toshinori Tsuru3, and Joji Ohshita1. Preparation of POSS-derived robust RO membranes for water desalination. Desalination, volume 404 (2017), pages 322–327.

Show Affiliations
  1. Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
  2. Department of Pure and Applied Chemistry, Tokyo University of Science, Noda 278-8510, Japan
  3. Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

 

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