From powder to 3D architecture: Ni/graphene sponges as noble-metal-free catalysts for the selective hydrodeoxygenation of 5-HMF to dimethylfuran

Abstract

The catalytic upgrading of biomass-derived platform molecules into renewable fuels is a key challenge in the transition toward sustainable energy systems. In this context, the hydrodeoxygenation (HDO) of 5-hydroxymethylfurfural (5-HMF) into 2,5-dimethylfuran (DMF) has attracted significant attention due to the excellent fuel properties of DMF and the need for efficient and scalable catalytic systems. To bridge the gap between fundamental catalyst design and practical application, conventional Ni@rGO powder catalysts were first used as a model system to optimize metal loading and understand structure–performance relationships. These insights were then translated into a three-dimensional (3D) graphene sponge architecture to overcome limitations related to mass transport, handling, and scalability. Nickel-based catalysts supported on reduced graphene oxide (Ni@rGO) were synthesized and evaluated for the hydrodeoxygenation (HDO) of 5-hydroxymethylfurfural (5-HMF) to produce 2,5-dimethylfuran (DMF), a biomass-derived compound with promising fuel properties. Catalysts with varying nickel loadings (5, 10, 20, and 30 wt%) were tested under batch reactor conditions (180 ºC, 2 MPa H2), revealing that the 20%Ni–rGO catalyst achieved the highest performance, with up to 90% conversion of 5-HMF and 71% selectivity toward DMF after 22 h. Higher Ni loadings led to reduced activity, likely due to particle agglomeration and increased Ni oxidation. Comparative studies with a noble metal catalyst (0.5%Pd-rGO) and a technical form aerogel Ni-rGO (sponge) showed that while Pd enabled rapid initial conversion, it suffered from poor long-term selectivity due to over-hydrogenation and ring-opening reactions (100% conversion but 3% DMF selectivity after 22 h). In contrast, the sponge-based Ni-rGO catalyst demonstrated 100% conversion and superior DMF selectivity (∼ 85%) with only 5 h of reaction, attributed to enhanced metal dispersion and increased active site accessibility. Product distribution analysis confirmed that Ni@rGO catalysts, especially in sponge form, follow a selective transformation pathway with minimal formation of secondary by-products such as butanol and hexanedione. These results underscore the importance of optimizing metal loading and support morphology to achieve efficient and selective HDO processes, positioning Ni@rGO as a promising candidate for upgrading biomass-derived oxygenates into high-value fuels. The 3D graphene architecture obtained through a supercritical CO2 route plays a key role in enabling efficient and scalable catalytic systems.