Dehydration of butanol to butene over solid acid catalysts in high water environments

The effects were studied of high water concentrations on the kinetics of alcohol dehydration, as encountered in aqueous-phase processing of biomass-derived oxygenated hydrocarbons. These studies were carried out for dehydration of an aqueous solution of 10 wt% 2-butanol at 513 K, and at a total pressure of 52 bar to maintain the water in the liquid state. Under these high pressures of water, silica-alumina, niobium phosphate and niobic acid are found to be stable and active for the dehydration of butanol. These three catalysts showed an increase in rate after contact with liquid water, caused by an increase in the concentration of Bronsted acid sites. Zeolite catalysts (Beta, USY, H-ZSM-5) and zirconia based catalysts (WOx/ZrO2, MoOx/ZrO2, and MgO/ZrO2) were ineffective due to deactivation or low catalytic activity. The flow rate of inert gas at constant aqueous flow rate had a significant effect on the rate of butene production, due to vaporization of butanol and water. At low flow rates of gas, increasing the gas flow rate causes the preferential vaporization of butanol, leading to a decrease in the butanol pressure in the reactor and a corresponding decrease in the rate of dehydration. Above a critical gas flow rate, the liquid feed becomes completely vaporized in the reactor, and increasing the gas flow rate further leads to a decrease in the pressure of water and a corresponding increase in the rate of dehydration. In the vapor-liquid equilibrium regime, kinetic models predict that most of the catalyst is covered with multiple layers of water, and dehydration takes place by reaction of hydrated-adsorbed butanol with a hydrated surface site. In the vapor-only regime, kinetic models suggest that the fraction of vacant active sites increases with increasing gas flow rate, and dehydration takes place by reaction of adsorbed butanol with a vacant surface site. (C) 2008 Elsevier Inc. All rights reserved.