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.