Scientists from Eindhoven University of Technology in the Netherlands and University of Bologna in Italy proposed an environmentally sustainable method of carbon membranes production. Such membranes are key for hydrogen economy, they allow for fast, efficient and low-cost separation of hydrogen from methane, nitrogen and other gases in fuel recycling.
The carbon membranes per se have been viewed as a promising technology since long ago, however, their commercial applications were impeded by the “backside” of their production. Hazardous organic solvents were traditionally used at the key stage of membrane formation. One of the most common solvent, N-Methyl-2-pyrrolidone, effectively dissolves phenol resins, but is reprotoxic, poorly degradable in the environment, hence, it is gradually cycled-out under the pressure of the regulators. Eventually, it becomes more complicated and expensive to scale-up membrane technologies, especially against the background of environmental crackdown.
In their study, the researchers set a task before themselves: to replace this solvent with safer and more sustainable analogues. Two bio-solvents were selected as an alternative: γ-valerolactone and di-hydro-levo-gluco-zenon known under its commercial name Cyrene™. Both compounds are received from renewable feedstock including cellulose, and their physical and chemical properties are close to traditional aprotic solvents. This allows for efficient work with phenol resins like novolac, used to form carbon membranes after thermal processing. Such new solvents are less toxic, they have higher flash point and better conform with the industrial safety requirements.
For comparison purposes, the scientists prepared three options of solvents to apply a polymeric layer on ceramic tubes: using N-Methyl-2-pyrrolidone, Cyrene™ and γ- valerolactone. After application of the layer and drying, the intermediate products were exposed to pyrolysis in nitrogen under up to 700 °C temperature, resulting in converting the polymer into carbon membrane. The finished products undergone a series of trials, when permeability of certain gases (helium, hydrogen, nitrogen and methane) was measured under different temperatures and pressure differentials.
The membrane produced using γ-valerolactone demonstrated the most noticeable results. After optimizing the pyrolysis mode including additional isothermal curing under 400 °C, some parameters of this membrane even outstripped the sample received using the traditional toxic solvent. At room temperature, the selectivity to hydrogen-methane reached 340, and to hydrogen-nitrogen – 270, maintaining high permeability of hydrogen. Membranes based on Cyrene™ were also working, but demonstrated more modest separation performance.
To understand the causes of such difference, the researchers analyzed the structure of the received materials in detail. Using permeability and porosity measurements they studied pores distribution in the membranes and showed that the optimized γ-valerolactone-based membrane was close to the so-called “gold standard”. About 85.6 % of its pores are ultra-micropores of less than 0.6 nanometers. Such pores are the most efficient ones operating as a molecular sieve, screening small hydrogen molecules and blocking bigger molecules of methane and nitrogen. Spectroscopy confirmed that the selection of solvent affects not only the size of pores, but also the chemical composition and the degree of order of the carbon matrix after the pyrolysis.
Future plans of the scientists stipulate for switching from lab experiments to more realistic operation conditions. They intend to test the membranes on mixed gas flows simultaneously comprising hydrogen, methane, nitrogen and CO₂, to expand the range of temperatures and to perform long stability tests. On top of that, they explore the possibility of further selectivity increase by way of introducing metallic nano-particles into the carbon matrix, which will be selectively interacting with hydrogen.
