Membrane separation processes are in great demand in the industry, since separating molecules or small particles is fundamental for the quality of many products, for example, microfiltration over membranes for water desalination is very important to obtain excellent quality water, especially given the amounts of contaminants and substances that have been incorporated into water these days, and although ways have been sought to employ graphene and other single-layer structures to fabricate high-flux separation membranes, trying to provide porosity to such structures puts their crystalline integrity at risk. But a team of researchers has managed to fabricate an alternative material that could solve this problem, as it naturally contains pores.
Separation membranes are the main element of filtration systems. Source: wikimedia.org.
Membrane molecular sieving is a physical separation process that allows the removal of molecules of different sizes and characteristics from a gas or liquid stream passing through a membrane of solid material. The membranes used in the process constitute a physical barrier made of materials with very specific characteristics that retain or prevent the passage of certain molecules and allow the flow of other molecules that have been selected.
The permeability through the membranes is affected by several factors, such as the concentration of molecules on the surface, the thickness of the membrane and the porosity of the membrane, which is why the manufacture of materials that serve this purpose must have very specific characteristics.
Membranes constitute barriers of very specific materials to achieve the desired selectivity. Source: 
In this sense, many researchers are looking for single-layer materials whose integrity supports the operational requirements of the process; and in this sense, scientists from Bielefeld, Bochum and Yale have managed to fabricate a two-dimensional (2D) silicon dioxide layer that contains natural pores suitable for the separation of molecules and ions, as they announced in a publication of their results and method of fabrication of this bilayer material in the journal Nano Letters. This type of material is of great interest to industry as it could bring great benefits in many processes, such as desalination and water purification or be implemented in new types of fuel cells.
This team has achieved something very important in the development of molecular membranes, since according to results, two-dimensional silicon dioxide has a very high pore density that is very difficult to obtain in artificial membranes. For example, many scientists have tried to perforate graphene materials for the fabrication of membranes, since two-dimensional materials can be perforated very precisely to obtain pores of a desired size, but this is extremely difficult to do without damaging the material, since it loses its mechanical properties in the process.
This is why another way of making molecular sieves had to be found, and in this study, the team of scientists managed to take advantage of the crystalline network of two-dimensional silicon dioxide, demonstrating that the pores it contains by nature can be used to separate some gases from others.
To fabricate this membrane, the authors used a method called atomic layer deposition (ALD), which allowed them to grow large-area films on Au/mica substrates; the layers were then given their two-dimensional shape by means of a high-pressure process. Subsequently, the material was tested by permeation experiments with gaseous and vaporous substances.
**_The material was tested with different substances. Source: Source:
The results indicate that condensable species such as vaporized water and alcohol penetrated the 2D silica oxide layer, while nitrogen and oxygen gases could not pass through.
Although the results are promising, and this type of material is in high demand, it is still necessary to delve deeper into how many and which molecules can adhere to the membrane surface or which ones can penetrate it, however the results pave the way to explore other bilayer oxides as separation membranes, putting them at the forefront of water purification or energy storage development.
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