Carbon capture and storage, a key way to reduce CO2 concentrations in emissions, consists in separating the CO2 from a gas mixture using a membrane. Thus, scientists have directed their efforts towards improving these membranes. In a new study, a Japanese team presents a new method of synthesizing a promising membrane on a new silica substrate, called Si-CHA, which can make the membrane twice as permeable to CO2 than those grown according to existing methods.
As part of efforts to reach the goal of net zero emissions, researchers around the world are exploring ways to remove atmospheric carbon dioxide (CO2) and CO2 in industrial emissions. One effective way is carbon capture and storage (CCS).
In CCS, membranes separate CO2 other gases in a mixture. The physical properties of the membrane, such as its porosity, are key to the efficiency of the process. Thus, efforts to improve CCS technology have focused on improving separation membranes. Recently, a particular membrane, Si-CHA, has gained attention as a promising option for separating CO2 methane (CH4), another greenhouse gas which is also useful as an energy source in various industries. Si-CHA is a crystal composed largely of eight-membered silicon (Si) rings that create pores 0.38 nm in diameter. This is ideal for letting the CO2 molecules (0.33 nm) pass through, while retaining CH4 molecules (0.38 nm) and other larger molecules.
The most common methods for synthesizing Si-CHA membranes with uniform porosity are to grow a secondary layer of the crystal on a primary mother layer coated with a substrate. The compositions of the mother layer and the substrate determine the effectiveness of the resulting membrane. Unfortunately, there is currently no method to synthesize a membrane efficient enough for industrial use (in terms of porosity, scalability and high temperature stability).
Now, in a new study published in Membranes, Japanese scientists have developed a new method to synthesize a pure Si-CHA membrane with much higher CO2 separation performance than Si-CHA membranes developed using existing methods. This could be a first step in overcoming the obstacles to its widespread use in industry.
The key to this achievement, as lead scientist Dr Mikihiro Nomura of the Shibaura Institute of Technology (SIT) explains, is to use a porous silica substrate instead of the conventional alumina substrate to grow crystal. “Usually, Si-CHA membranes are synthesized on porous alumina substrates with a layer of Si-CHA, because this method increases the mechanical strength of the resulting membrane,” he says. “However, the aluminum in the alumina substrates dissolves in the CHA layer, reducing the pore size, and therefore, preventing so much CO.2 to cross the resulting membrane as would have been possible otherwise. Using a silica substrate eliminates this problem and improves CO2 separation efficiency. ”
Through experiments under various synthetic conditions, scientists optimized the compositions of the parent gel and the substrate to obtain the membrane with the highest CO2 separation performance for a CO2/ CH4 gas mixture. They also compared the performance of their membrane with that of Si-CHA membranes produced on alumina substrates.
They found that for a water / silica ratio of 4.2 in the parent gel, the viscosity (flowability) of the parent gel was right and the smoothest membrane was created. But the softer does not mean the best in terms of CO2 permeability. A parent gel with a water / silica ratio of 4.6 applied to a porous silica substrate and allowed to stand at 150 ° C for 8 hours gave a membrane with twice the permeability of a membrane of similar thickness. synthesized on a porous alumina substrate.
In addition, the conditions required to grow this membrane of similar thickness on an alumina substrate were 170 ° C and 70 hours, indicating that the silica substrates potentially allowed much better results with much milder synthesis conditions.
This is the first study to obtain effective Si-CHA membranes on a silica substrate and, therefore, although it provides key information on which to build, much research remains to be done before this method. synthesis itself can be used in industry. But Dr Nomura is hopeful and believes that “one day this technology will be developed enough to capture CO2 enough to maintain a carbon neutral society and reduce weather fluctuations caused by global warming. “
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