Hydrogen production via the aqueous-phase reforming of methanol catalyzed by Ru(ii) complexes of PNNP ligands

Abstract

Hydrogen (H₂) production by the aqueous-phase reforming of liquid organic hydrogen carriers such as methanol, ethanol, and formic acid has attracted much interest recently as it offers a safe and convenient means of producing H₂ on demand by mitigating the costs and safety challenges associated with the storage and transportation of H₂. Using efficient catalysts operative under easily achievable conditions is the key to the success of such reactions. Herein four Ru(II) complexes with PNNP ligands, namely [Ru(Ph₂PPhCHN(1,2-cyclohexylene)NCHPhPPh₂)Cl₂] (1), [Ru(Ph₂PPhCHNCH₂CH₂NCHPhPPh₂)Cl₂] (2), [Ru(Ph₂PPhCH₂NHCH₂CH₂NCHPhPPh₂)Cl₂] (3), and [Ru(Ph₂PPhCH₂NHCH₂CH₂NHCH₂PhPPh₂)Cl₂] (4) were synthesized and evaluated for catalyzing the aqueous-phase reforming of methanol (APRM). Comparative studies between better performing 2 (than 1) and its closely related analogs 3 and 4 revealed the profound influence of the ligands on their catalytic activities with the best-performing 2 producing a maximum TON of 12 639 after 8 days. Mechanistic studies using in situ¹H NMR and high-resolution electrospray ionization mass spectrometry (HRESI-MS) suggested the presence of two key intermediates, formulated as [Ru(Ph₂PPhCHN-CH₂CH₂-N-CH₂PhPPh₂)(H)] (I1) and [Ru(Ph₂PPhCH₂-NH-CH₂CH₂-N-CH₂PhPPh₂)(H)] (I2) bearing respectively the half and fully reduced PNNP ligands. Comparative studies revealed that I1 is more active than I2. A mechanism involving the association and activation of the substrate (i.e. CH₃OH, HOCH₂OH, and HCOOH) mediated by the cooperative Ru–N⁻ set-up, followed by the stage-wise release of H₂, is proposed.