Chemists recycle shrimp waste as hydrogen-generating catalyst -

Chemists recycle shrimp waste as hydrogen-generating catalyst

The flexible balls of the biomolecule chitosan, made from shrimp waste, can be used in catalysts that generate hydrogen gas from borohydride salts. In a paper in Green Chemistry, a research team at the University of Amsterdam (UvA) demonstrates how pellets can “blow out” hydrogen bubbles without breaking. This is an important step towards practical and safe hydrogen storage and release units.

Since 2020, the Heterogeneous Catalysis and Sustainability Chemistry group at the UvA’s Van ‘t Hoff Institute for Molecular Sciences has been working on the use of alkali metal borohydride salts as hydrogen carriers in the future. These solid salts can be safely stored in air under ambient conditions and release hydrogen gas only upon reaction with water. However, controlling the release of hydrogen, and thus preventing runaway reactions, is challenging. One solution is to stabilize the solution with a base, and control the hydrogen release with a catalyst. The UvA team, led by Professor Gadi Rotenberg, is developing such stimuli in collaboration with the Austrian Competence Center for Tribology (AC2T) and Electriq Global.

Hydrogen destroys the catalyst molecules

Potential catalysts are easy to find, but getting them to work long enough to be commercially viable is not. The combination of the high reaction pH and the constant release of hydrogen bubbles destroys conventional catalysts within a few days. For example, the team has successfully engineered highly active and selective cobalt-containing catalyst molecules. However, the high activity produces large amounts of hydrogen that quickly destroy the particles.

The breakthrough came during a so-called Friday afternoon experiment when MS student Jeffrey Junk and PhD student Fran Pope decided to try encapsulating cobalt molecules in chitosan globules. Chitosan is a natural polymer that can be produced from chitin, which is the main component of the exoskeletons of insects and crustacean shells. It is biodegradable, biocompatible and widely available in a multi-ton scale, and is produced mostly from waste shrimp and crab shells.

Repeated amine groups on the backbone of chitosan make it highly soluble in aqueous acid solutions but slightly soluble in basic solutions. Thus chitosan pellets can be produced relatively easily by dropping liquid chitosan into a base solution. A critical property of chitosan domains is their flexibility, which enables them to expand during hydrogen generation. Thus they can “exhale” hydrogen bubbles without it breaking. And since it’s made at a high pH, ​​the base borohydride solution poses no problem.

Real-life potential of chitosan-based catalysts

The team tested the new catalysts in batches and continuous patterns, monitoring the reactions by measuring the flow of hydrogen generated. The few mm sized spheres loaded with 7% cobalt were sufficient to generate 40 ml hydrogen per minute in a continuous reactor for two days, showing the real life potential of this new catalyst.

According to Rothenberg, the work highlights the importance of catalyst stability as a research focus. “Many papers focus on activity and selectivity, because journals have become focused on publishing amazing results,” he says. “However, if you look at the chemical industry, none of these ‘amazing’ catalysts are used in practice. The reason is that running a successful reaction for a few hours, or even a few days, means nothing for large-scale operations. A real catalyst has to run for months and years to be economically viable. We’re not there yet.”

Hydrogen may be the energy carrier of the future, but it comes with its own set of challenges. When stored as a compressed gas or liquid, molecular hydrogen, H2, consume a lot of energy. This is a feature in some applications, but a safety concern in others. For medium-sized storage and release of mobile facilities, such as cranes, ships, and generators, other hydrogen storage methods are preferred. There are many forms of hydrogen carriers. Examples with a high hydrogen storage capacity include ammonia, methanol, formic acid, and others. However, each has its advantages and disadvantages. Methanol has a high capacity (12.5% ​​by weight) but dehydrogenation requires high temperatures and may also emit carbon dioxide2. Ammonia may contaminate H.2 Generated, a toxic gas itself under ambient conditions. Alternatively, alkaline borohydrides can provide a safe source of hydrogen, chemically bonding it as a solid salt. Reaction with water releases hydrogen, and the resulting salt by-product can be reprocessed and reused for hydrogen storage.