Triphenylphosphine-functionalized hyper-crosslinked polymers for efficient hydrogen generation via sodium borohydride methanolysis

Four novel hyper-crosslinked polymers (HCP-1 to HCP-4) were synthesized via Friedel–Crafts alkylation using 4,4′-bis(chloromethyl)-1,1′-biphenyl as the crosslinker and various aromatic monomers including triphenylphosphine (TPP). These polymers were designed as metal-free catalysts for hydrogen generation via sodium borohydride methanolysis. Comprehensive characterization using FT-IR, XPS, BET, TGA, SEM, and zeta potential analyses confirmed that the materials possess thermally stable, porous networks with irregular morphologies and distinct surface charges. Under practical conditions, HCP-3 exhibited the highest catalytic activity with a hydrogen generation rate of 9857 mL H₂ min⁻¹ g⁻¹ at 303.15 K and the lowest activation energy (Ea = 32.0 kJ mol⁻¹). At elevated temperature (333.15 K), HCP-2 achieved the highest activity (37,200 mL H₂ min⁻¹ g⁻¹), reflecting the strong influence of temperature on performance trends. Despite not having the highest surface area or pore volume, the superior activity of HCP-3 at 303.15 K highlights the decisive roles of microporous architecture, electrostatic surface characteristics, and heteroatom functionality. Zeta potential analysis revealed significant reductions in surface charge after reaction, particularly for HCP-3, suggesting strong electrostatic interactions with BH₄⁻ ions. XPS data further confirmed the successful incorporation of TPP and heteroaromatic units, correlating with enhanced catalytic efficiency. Overall, the findings underscore a surface-mediated mechanism where both charge-assisted hydride attraction and structural topology govern hydrogen evolution. The metal-free and reusable nature of these catalysts supports their potential in sustainable hydrogen technologies.