Microwave-assisted pincer-ruthenium catalyzed Guerbet reaction for the upgradation of bio-ethanol to bio-butanol

Abstract

The upgradation of feed agnostic bio-ethanol to fuel grade n-butanol was accomplished via the pincer-ruthenium catalyzed Guerbet reaction under microwave irradiation (75 W, 110 °C) with high efficiency after only 2 h in the presence of 10 mol% sodium ethoxide (NaOEt). Among the considered carbonyl and phosphine pincer-ruthenium complexes, ruthenium carbonyl complexes based on 2,6-bis(benzimidazole-2-yl)pyridine ligands showed higher activity towards the Guerbet reaction. The corresponding pincer-ruthenium complexes based on bis(imino) pyridine ligands demonstrated poor activity. The highest rate (8534 TO h⁻¹, ca. 18% yield of n-butanol at 90% selectivity) was obtained for the ethanol upgradation catalyzed by 0.00225 mol% [(^Bim2NNN)RuCl(PPh₃)₂]Cl in the presence of 10 mol% NaOEt at 110 °C under microwave irradiation at 75 W. The best productivity (72% ethanol conversion and 42% yield of n-butanol at 57% selectivity) and turnover (ca. 13 022 TON with 22.8% yield of n-butanol at 73% selectivity) were obtained in the (^Bim2NNN)RuCl₂(CO) (0.025 mol% and 0.00225 mol% respectively) catalyzed reactions. Although DFT calculations indicate the generation of hydrogen to be the rate-determining step (RDS), the corresponding hydride species involved in the RDS was detected as its phosphine adduct by NMR studies. The hydroxy group on (p-OH^Bim2NNN)RuCl₂(CO) could successfully be anchored on various solid supports and used as an efficient heterogeneous catalyst albeit with no recyclability.