Mechanistic origins for the enhanced ethanol dehydration kinetics in H-ZSM-5 by cofeeding n-butanol

Abstract

Periodic density functional theory (DFT) with dispersion corrections is used to construct a detailed reaction network for dehydration of n-butanol/ethanol mixtures in H-ZSM-5. Apart from the direct conversion of the alcohols to olefins or di-alkyl-ethers, novel mechanisms have been explored for the formation and decomposition of a cross-ether, butyl ethyl ether. Furthermore, a novel mechanism that affects the intrinsic activity of ethanol dehydration to ethene is found, the n-butanol-assisted ethanol dehydration. Thermodynamic and kinetic parameters for all elementary reaction steps were calculated and implemented in a microkinetic model capable of simulating the dehydration of (i) pure ethanol, (ii) pure n-butanol and (iii) n-butanol/ethanol mixtures over a H-ZSM-5 catalyst. The microkinetic model was able to reasonably predict the observed experimental results. A reaction path analysis shows that the mixed ether is primarily formed through an S_N2 mechanism, where the water is split off from ethanol, except at low alcohol pressure. The mixed ether decomposes predominantly to butenes and ethanol. Contrary to pure ethanol dehydration, if sufficient n-butanol is available, ethylene is primarily formed through a novel butanol-assisted mechanism for n-butanol/ethanol mixtures, indicating the intrinsic activity for ethanol dehydration is – here beneficially – altered by cofeeding of butanol. These results hint towards the possibility of cofeeding strategies to accelerate the conversion of a less reactive reagent.