Optimising Aqueous Phase Reforming for Sustainable Hydrogen Production from Aqueous Side Streams

The report, "Valorisation of Light Oxygenates Present in Aqueous Side Streams via Aqueous Phase Reforming", details Johnson Matthey’s research on converting oxygenates in aqueous side streams to hydrogen (H₂) through aqueous phase reforming (APR). This work involves testing various model aqueous feeds and catalysts to optimise APR efficiency.

Catalyst Development and Initial Testing on Simple Oxygenate Compounds

The research began by preparing and characterising catalysts supported on inorganic oxides and carbon. These catalysts were initially tested on simple model compounds like methanol, acetic acid, and pyruvic acid to evaluate their effectiveness in hydrogen production by aqueous phase reforming. Early results revealed certain catalysts yielded high conversions of methanol and acetic acid with significant hydrogen selectivity. However, relatively low carbon-to-gas conversion was observed, which was tentatively attributed to the batch reactor set-up.

Advancing APR with Complex Feedstocks and Optimised Conditions

As the project progressed, researchers focused on more complex model feeds containing compounds like furanics, uronic acids, and various organic acids. Findings showed that some catalysts achieved better feed and carbon-to-gas conversions, though side reactions, such as decarboxylation and CO/CO₂ hydrogenation, often competed with APR, reducing hydrogen selectivity. To refine the process, a Design of Experiments (DOE) approach optimised reaction conditions and catalyst properties, revealing that specific catalyst compositions and reaction conditions improved hydrogen production.

To further enhance hydrogen production, a pre-hydrogenation step was introduced for feedstocks that were identified to be challenging to reform to hydrogen by aqueous phase reforming. This step transformed the furanic compounds and uronic acids into more reformable alcohol intermediates, such as sorbitol. This approach significantly improved hydrogen yields and minimised CO₂ production via feedstock thermal decomposition, underscoring its potential in APR processes.

Application to Real Aqueous Side Streams from Biorefineries

The study also examined real aqueous side streams from C5 biorefinery feed dehydration. Initial tests with model feeds based on these side streams confirmed the necessity of pre-hydrogenation to prevent feed thermal decomposition under APR conditions. This suggests that a dual-catalyst approach may be ideal: one for pre-hydrogenation and another for APR.

Key Findings and Future Applications

In conclusion, this research provides valuable insights into the feasibility of APR as a route for hydrogen production from organic materials in waste aqueous side streams. It emphasises the importance of careful catalyst selection and optimisation, as well as the critical role of pre-hydrogenation in enhancing hydrogen yield and selectivity when complex feedstocks are present. Key findings highlight the activity and hydrogen selectivity of specific catalysts on inorganic oxides or carbon, tested across a variety of complex model and real feedstocks. This work contributes significantly to the development of sustainable bio jet fuel and bio-chemical production, paving the way for further study and potential industrial applications.