Solid base assisted n-pentanol coupling over VIII group metals: elucidation of the Guerbet reaction mechanism by DRIFTS

Liquid phase catalytic coupling of n-pentanol over VIII group metals (Pt, Pd, Ir, Ru, Rh) on a range of supports (C, Al2O3, TiO2, CeO2, ZrO2) doped with different solid bases was done varying the reaction temperature and gas atmosphere. It was shown that n-pentanol coupling in the selected reaction conditions (180 °C, 10 bar of N2) mainly resulted in the formation of the target product 2-propyl-1-heptanol (Guerbet alcohol), whereas pentanal, 2-propyl-2-heptenal, 2-propyl-1-heptanal, and 2-propyl-1-heptenol were observed in minor quantities. Temperature increase accelerates the reaction and improves selectivity to 2-propyl-1-heptanol. Acid–base properties of the supports also influenced conversion and selectivity correlating with the support base strength TiO2 (860 kJ/mol) < ZrO2 (900 kJ/mol) < C (850–910 kJ/mol) < Al2O3 (935 kJ/mol). For carbon supported catalysts conversion increases in the raw Ir < Ru < Pt < Pd. Analysis of the reaction mechanism for NaOH/Pt/Al2O3 and the relative contributions of various catalytically active phases was done by in situ Fourier transform infrared spectroscopy.