Title : HF-free hydrothermal synthesis of 2D-on-2D MBene@g-C3N4/g-C3N5 photo (electro) catalysts for furfural upgrading to sustainable aviation fuel
Abstract:
The electrochemical and photo-electrochemical upgrading of bio-oil derivatives represents a critical pathway toward sustainable aviation fuels and value-added chemicals [1,2]. Herein, we present the synthesis of hydrofluoric acid-free two-dimensional MBene (MB = WB, MoB,) decorated graphitic carbon nitride heterojunctions (g-C3N4/g-C3N5) for the efficient conversion of furfural to fuel precursors. MBenes were synthesized via a hydrothermal approach using acidic and alkaline media, eliminating the environmental and safety hazards associated with conventional HF-based etching methods. Systematic compositional optimization revealed that the M10G67 catalyst, comprising 10 wt% tungsten boride and a g-C3N4/g-C3N5 heterojunction with 67 mol% C3N5, exhibited the lowest optical bandgap (1.58 eV) and highest electrochemically active surface area. Under photo-electrochemical conditions, M10G67 demonstrated a ca. 38% enhancement in reduction current density compared to dark conditions, achieving an electrical energy saving efficiency of ca. 11%. The pH-dependent product distribution analysis revealed that basic conditions (pH 8.5) predominantly yielded deoxygenated ring-opened products (57% of total products), representing drop-in sustainable aviation fuels and optimal precursors for sustainable aviation fuel synthesis. Density functional theory calculations elucidated the synergistic role of MBene active sites and the carbon nitride heterojunction in facilitating ring-opening and hydrodeoxygenation pathways, with Co-doped MoB/C3N4- C3N5 (representing M–(Mo/W)Bx@g-C3N4/g-C3N5) demonstrating exceptionally favourable thermodynamics for selective 1,3-pentadiene formation. This work establishes a sustainable synthetic route to high-performance 2D-on-2D photo(electro)catalysts for biomass valorisation, offering significant implications for renewable fuel production.
HF-free synthesis: WB and MoB were obtained from WAlB and MoAlB MAB phases via hydrothermal etching in HCl (1–12 M, 393–433 K, 16–24 h) followed by NH4OH sonication. ICP analysis confirmed nearcomplete de aluminization of WAlB (Al reduced from ca. 67 to ca. 2 wt%) with boron retention (B increased from ca. 19 to ca. 60 wt%), whereas the alkaline route stripped boron (down to ca. 1 wt%) and is therefore unsuitable. The acidic route was thus selected. MBene was then co-pyrolysed in situ with urea/melamine/guanidine-carbonate/3-amino-1,2,4-triazole mixtures to yield MYGX composites (Y = MBene wt%; X = mol% g-C3N5 in the heterojunction). XRD confirmed the tetragonal WB phase (JCPDS 35-0738) and graphitic (002) reflections at 2θ ≈ 27.5°, while FTIR revealed broadening of C=N/C–N modes (1150–1700 cm?¹) and attenuation of the 810 cm?¹ s-triazine breathing mode, indicating strong WB–CN interfacial coupling.
Optical and (photo)electrochemical optimization: The composite design pairs metallic, conductive WB with a g-C3N4/g-C3N5 heterojunction in which the g-C3N5 framework provides a narrower bandgap than g-C3N4 for enhanced visible-light harvesting [3]. The optimal composition, M10G67 (10 wt% WB + g-C3N4:g-C3N5 = 1:2), exhibited the lowest Tauc bandgap (1.58 eV) versus g-C3N4 (2.63 eV), g-C3N5 (1.71 eV), and M10G100 (1.68 eV) (Figure 1). In 100 mM furfural / 200 mM H2SO4 / 20 vol% acetonitrile, M10G67 delivered the highest reduction current density (j ≈ −10.14 mA cm-2) and the lowest onset potential (~ −0.37 VAg/AgCl) among all MYGX compositions. Under simulated solar irradiation, j increased to −13.83 mA cm-2, a ca. 38% PEC enhancement over dark conditions, and the potential required to reach −10 mA cm-2 dropped from −1.12 to −1.01 VAg/AgCl, corresponding to an electrical energy saving efficiency (ηEES) of ca. 11%.
PH-controlled product selectivity: GC-MS analysis of bulk H-cell electrolysis revealed a strong pH-steering effect on the product pool. Acidic media (pH 0.5) channelled ca. 534% of carbon into polymerized humins; near-neutral conditions (pH 6) maximized ring-opening (ca. 86%) but produced mainly oxygenates (ca. 90% of the ring-opened fraction). Basic conditions (pH 8.5) inverted this split, yielding ca. 64% deoxygenated species within the ring-opened pool, i.e. ~57% of all products are fully deoxygenated ring-opened hydrocarbons, with carbon numbers C3–C24 dominated by C5–C18, directly aligned with the SAF carbon range and requiring only minimal downstream hydrotreatment.
Conclusions: A scalable, HF-free hydrothermal route to 2D-on-2D MBene@g-C3N4/g-C3N5 photo(electro)catalysts is established. Composition-tuned M10G67 combines a 1.58 eV bandgap, a ca. 38% photo-enhancement of the reduction current, and pH 8.5-controlled selectivity yielding ~57% C3–C24 deoxygenated ring-opened products. The platform is generalizable to other bio-oil furans (e.g. 5-HMF) for renewable transportation-fuel production.
Keywords: MBene, g-C3N4/g-C3N5 heterojunction, furfural electroreduction, photo-electrocatalysis, sustainable aviation fuels.
