Catalytic Transformation of Ethanol with Silicalite-1: Influence of Pretreatments and Conditions on Activity and Selectivity

Silicalite-1 undergoes a reversible cycle of acid activation and base deactivation when employed as a catalyst for the vapor-phase dehydration of ethanol at 230-300°C. In particular, a pretreatment with aqueous HCl strongly promotes the production of diethyl ether at lower temperatures (230-270°C) or ethene at higher temperature (300°C). The activated catalyst becomes completely inactive upon addition of aqueous potassium acetate, whereas the catalytic effect is restored by repeating the acid treatment. Moreover, the reversible exchange of H+ with K+, introduced as KCl salt, in HCl-activated silicalite causes the suppression of the catalytic activity. Several analytical techniques, as the acidic sites characterization, surface area-porosity determination and electron microscopy images were unable to display surface modifications responsible for the important catalytic effect induced by HCl treatment. It is proposed that HCl treatment allows the formation of protonated and neutral chemical groups, such as Si-O-Si surface bridges, behaving as active sites for ethanol dehydration. On the surface of the catalyst, K+ competes with H+ and strongly inhibits alcohol dehydration.