This micrograph image shows the novel adsorbent material produced using a poplar leaf as a template; the channel architecture is clearly noticeable. The graphics show the tubular structure (green) and the molecular separation process. Image: HIMS.
This micrograph image shows the novel adsorbent material produced using a poplar leaf as a template; the channel architecture is clearly noticeable. The graphics show the tubular structure (green) and the molecular separation process. Image: HIMS.

Researchers at the University of Amsterdam (UvA) in the Netherlands have devised a way to enhance the practical performance of metal-organic frameworks (MOFs). By using leaves from the black poplar tree as a template, they have produced hierarchical porous structures of mixed-metal oxide materials that can act as supports for MOF crystals. In a paper in ACS Applied Materials & Interfaces, the researchers report the unique adsorption and separation properties of this bio-inspired design.

Separation of water-alcohol mixtures is one of the most challenging problems associated with the practical adoption of bioethanol as a sustainable fuel. Produced by the microbial fermentation of plant-derived sugars, bioethanol contains both water and methanol as impurities. Obtaining fuel-grade bioethanol from these water-alcohol mixtures using traditional distillation is not practical because water and ethanol form a so-called azeotropic mixture.

The cost-effective and green alternative to distillation is adsorptive separation. In biofuels production, this method relies on the development of adsorbent materials that are highly selective towards ethanol or the impurities in the mixture. At UvA’s Research Priority Area Sustainable Chemistry, the group of Stefania Grecea develops porous molecular-based materials with just these kind of selective adsorption properties.

Suitable adsorbent materials for separation applications should have an appropriate porous structure and a high specific surface area to facilitate both the adsorption and diffusion of specific molecules. MOFs meet these requirements; not only do they have a high specific surface area, but by tuning the size and functionality of their pores at the molecular level, specific adsorption selectivities can be achieved.

However, practical applications also depend on their macroscopic properties. Often MOFs are synthesized as powders of tiny crystals, which cannot be used directly in industrial applications because they have limited packing density as well as high diffusion barriers. One solution is to shape MOFs as granules, pellets or monoliths, or to disperse them within thin films, creating membranes. But the pressure applied in such shaping methods leads to a loss of crystallinity and therefore to reduced activity of the MOF materials.

In searching for ways to improve MOF performance, the UvA researchers turned to nature; in particular, to green plant leaves. Scientists have already used natural leaves as templates for designing heterogeneous photocatalysts, as leaves are structured to provide efficient light harvesting. Such artificial leaf structures have proven to be very effective for hydrogen production, for example.

The UvA researchers took their inspiration from the natural leaf vein system that has evolved for transporting aqueous liquids. This is a hierarchical porous system consisting of many fibers and vessels of different sizes. In separation technology, hierarchically porous materials with multi-level pores often display enhanced adsorption performance compared to uniformly sized porous materials.

Using leaves of the black poplar (Populous nigra) as a template, the researchers synthesized a mixed-metal oxide material with a hierarchical porous structure via a sol-gel method. They then used this mixed-oxide artificial leaf as a support for creating a homogeneously dispersed layer of MOF crystals.

Detailed morphological studies showed that the resulting composite material possessed the desired hierarchical porous structure and that MOF crystals with a narrow size distribution are homogenously dispersed at the inner surface of the hierarchical pores.

Next, PhD student Yiwen Tang studied the water, methanol and ethanol adsorption properties of this new material, finding that it was most selective for methanol followed by ethanol followed by water. Subsequent molecular simulations of equimolar ethanol-methanol mixtures, performed by David Dubbeldam of the UvA Computational Chemistry group, showed that methanol adsorption by the material is highly selective in the low-pressure range. Moreover, the material is also effective at separating water-ethanol mixtures, with ethanol being adsorbed selectively in the low-pressure range and water adsorbed selectively at high pressures.

The researchers conclude that their bio-inspired synthetic approach is highly relevant not only for molecular separations but also as a general strategy for designing MOF composite materials for various applications, including catalysis and molecular sensing.

This story is adapted from material from the University of Amsterdam, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.