Highly Selective Conversion of Diphenylmethane/o-Hydroxydiphenylmethane to Dicyclohexylmethane/Perhydrofluorene Catalyzed by an Aqueous Ru Nanoparticles System Reduced and Stabilized by Sodium Lignosulfonate

A catalytic system of Ru nanoparticles (NPs), Ru@SLSaq, dispersed in water was developed by employing sodium lignosulfonate (SLS) to reduce and stabilize ruthenium(III) cation heptaacetate (Ru3O(OAc)7) in an aqueous solution. This Ru@SLSaq catalytic system enabled the efficient hydrogenation of diphenylmethane (DPM) and o-hydroxydiphenylmethane (o-HDPM), yielding dicyclohexylmethane (DCHM) and perhydrofluorene (PHF), respectively. Characterization using UV–vis, FT-IR, Raman spectroscopy, HR-TEM, CO chemisorption, DLS, XRD, and XPS demonstrated that SLS effectively replaces the axial acetate ligands of Ru3O(OAc)7, reduces Ru in situ through the phenolic hydroxyls in its macromolecular structure, and maintains the stability of its core structure, thereby preventing the aggregation of generated Ru NPs. The obtained Ru NPs, predominantly in the Ru0 state with a Ru (100) lattice plane, were uniformly dispersed in the aqueous phase, exhibiting an average particle size of 2–3 nm. Ru@SLSaq displayed excellent catalytic activity for the hydrogenation of DPM and o-HDPM, achieving yields of 96.6% and 97.0% for DCHM and PHF, respectively, under mild conditions (n(substrate)/n(Ru) = 750, hydrogen pressure of 1.5 MPa, at 80 °C for 4 h). The calculated apparent activation energies (Ea) for the two reactions were found to be 46.5 kJ·mol–1 and 75.3 kJ·mol–1, respectively, and the catalytic system demonstrated good cycling stability. Experimental results, supported by DFT calculations, indicate that the complete hydrogenation processes of DPM and o-HDPM occur on the surface of Ru NPs via monophenyl ring adsorption, exhibiting small energy barrier differences across various pathways.