Catalytic consequences of hydroxyl group location on the rate and mechanism of parallel dehydration reactions of ethanol over acidic zeolites

The effects of zeolite topology on the dehydration of oxygen-containing molecules were probed in steady-state and isotopic chemical reactions of ethanol over proton-form zeolite materials (FER, MFI and MOR) at low temperatures (368–409 K). The measured rate of diethyl ether (DEE) synthesis was largely independent of ethanol partial pressure on all proton-form zeolites (FER, MFI, and MOR), indicating that DEE formation involves the activation of ethanol dimers. The measured rate of DEE synthesis over H-FER increased with increasing ethylene pressure in experiments done with ethanol–ethylene mixtures, reflecting the weaker adsorption of ethanol dimers on the FER framework compared to that on MFI and MOR materials, thereby resulting in the co-adsorption and reaction of ethylene with ethanol on FER materials. Ethylene production was only observed on H-MOR because the small eight-membered ring side pockets protect ethanol monomers from forming bulky ethanol dimers. Secondary kinetic isotopic effects measured for ethylene synthesis rates using C2D5OH reactants imply that the kinetically relevant step involves the cleavage of C–O bonds via a carbenium-ion transition state.

In zeolite pores large enough to accommodate ethanol dimers, ethanol preferentially dehydrates via a bimolecular pathway to generate diethyl ether since the formation of ethanol dimeric species is energetically more favorable than the formation of ethanol monomers. In zeolite channels too small to accommodate ethanol dimers, ethanol is selectively dehydrated via a unimolecular reaction pathway to generate ethylene.Figure optionsDownload high-quality image (88 K)Download as PowerPoint slide