New Hybrid Membrane With High Hydrothermal Stability; Potential Energy-Efficient Replacement for Distillation Techniques in Biofuel Production

A new hybrid organic–inorganic nanosieve membrane with high hydrothermal stability that enables energy-efficient molecular separations, including dehydration up to at least 150° C, even after long periods of continuous exposure to water.

The hybrid membranes are suitable for dehydrating solvents and biofuels, an application for which there is a large potential market worldwide. The main advantage of membrane technology is that it consumes far less energy than common distillation techniques. The scientists also foresee opportunities in separating hydrogen gas from gas mixtures and also in water desalinization applications.

Hydrothermal stability over long periods of time has been one of the barriers to more widespread applications of molecular sieving membrane technologies in areas such as in energy-efficient separation of biomass fuel and hydrogen, dehydration of condensation reactions, and breaking of azeotropic mixtures during distillation (e.g., hydrous ethanol).

Of the materials considered for membranes—including polymers, zeolites and metal oxides—a supported 30–100 nm thin film of nanoporous silica (SiO₂) is one of the most promising systems for molecular separation of gases and liquids, according to the researchers. Amorphous silica combines high permeability for molecules <3>2O, H₂) with very low permeability for larger ones. It has high mechanical, thermal and solvent stability and can be applied at 600 °C in dry atmospheres.

However, prolonged exposure to water at temperatures as low as 60° C leads to hydrolysis, resulting in large non-selective pores and cracks.

Replacement of silica by other candidate materials, the researchers note in their paper, has not resulted in stable membranes while retaining both selectivity and a high permeation rate. Instead, the Dutch team replaced part of the ceramic links with organic links.

The cylinder is the carrier of a hybrid membrane: a layer of about 100 nanometer thickness. The insert is a close-up of the layer showing the organic links and pores. From the left of the tube, only water molecules leave the sieve.

The strategy behind the present work was to replace as many siloxane bonds as possible by hydrolytically stable Si–C links, while raising the CH_x : Si ratio to 1... We aimed at a microporous material with pore diameters of 2–4 Å, similar to the kinetic diameter of small molecules. Our prime aim was, however, to prepare a material with a high hydrothermal stability, i.e., a persisting high selectivity towards molecular separation at high temperatures in the presence of water.

After 18 months of testing of the hybrid membrane for the dehydration of n-butanol (5 wt% water) by pervaporation at 150 °C, the membrane remained stable and still highly selective.

Manufacturing the new hybrid membrane is simpler than manufacturing ceramic membranes, because the material is flexible and will not show cracks. What the new membranes have in common with ceramic membranes is the rapid flow—one advantage of this is that the membrane surface can be kept small.