Synthesis of isoprene from 1-butanol and from diethyl ether in the presence of 3d-metal oxides nano-clusters acting as catalysts

Abstract

The conformational equilibria of 1-butanol (1-BuOH) and diethyl ether (DEE) molecules were investigated in gas and liquid phases. The geometry of the molecules of all stable isomers of 1-BuOH and DEE was fully optimised and the thermochemical quantities’ values (ΔH, ΔG and ΔS) were computed in the temperature range from close to 0 K to 1000 K at the Density Functional Theory (DFT) level. DEE shows two rotamers (namely, Tt and Tg), whereas five different rotamers were identified for 1-BuOH (namely, Tt, Tg, Gt, Gg and Gg’) with different stability, due to the presence of intramolecular interactions. Electrostatic properties of 1-BuOH, ethanol (EtOH), and DEE were also evaluated. Calculated vibration spectra of 1-BuOH and DEE were compared with the experimental FT-IR and Raman spectra recorded in neat phase and diluted in cyclohexane (CHX). The experimental vibrational spectra found the best match with the sum of calculated spectra using the Boltzmann coefficients based on the thermal stability of each conformer. The frequency shift observed for some peaks as a function of the concentration of 1-BuOH in CHX reveals the impact of hydrogen bonds between 1-BuOH molecules, in particular in the case of conformer Gg’. In contrast, very weak interactions among molecules of DEE were spectroscopically detected. The transformation reactions of {1-BuOH+EtOH} and {DEE+EtOH} mixtures at different temperatures were studied using the method of quantum mechanical calculations. The mechanism of isoprene formation from these mixtures was determined taking into account the chemisorption of 1-BuOH and DEE onto metal oxide nano-clusters M4O4 (M = Sc to Zn) and its effect on the vibrations of BuOH and DEE at the interface for M = Zn, Cu and Ti using Raman microscopic spectroscopy.