| Molecular beam scattering, gas-liquid uptake, and residence time measurements are used to explore the interfacial and bulk-phase chemistry of n-propanol molecules colliding with molten 51/49 mol % NaOH/KOH at 463 and 513 K. This basic and ionic liquid is a powerful deprotonating agent for organic molecules such as alcohols. Uptake measurements of deuterated n-propanol, CH3CH 2CH2OD (PrOD), reveal that over half of all incident molecules enter the liquid at an approach angle of 55° even at collision energies exceeding 40RTliq. At thermal collision energies, the trapping probability approaches one and 85% or more of the molecules dissolve in solution, while the remaining 15% desorb back into the gas phase after thermalizing on the surface. This extensive PrOD entry is likely driven by immediate interfacial deprotonation or by strong solvation and hydrogen bonding that pulls the molecule into the molten salt. Time-resolved measurements indicate that PrOD reacts to form PrO- within one microsecond of dissolving within the liquid.; Dissolved PrO- ions can be converted to desorbing PrOH by exposing the solution to water vapor in the range of 1 x 10 -8 to 1 x 10-6 Torr. Water is the strongest acid that can exist in molten alkali hydroxides and reacts with PrO- according to PrO- + H 2O &rlhar2; PrOH + OH-. We did not find any evidence for direct, interfacial PrOD→PrOH exchange and desorption. On the contrary, the data indicate that PrO- spends long times in solution before desorbing as PrOH, with solvation times ranging from many seconds to hours under the conditions of our experiments. Despite these long residence times, no evidence was found for the decomposition of the alcohol into CH 4 and CO2. The acidity of the molten hydroxide was also controlled indirectly by exposing the liquid to carbon dioxide gas, which readily dissolves in the molten salt to form H2O and CO32- . The cumulative data suggest that the molten NaOH/KOH eutectic is indeed a powerful deprotonating agent, one that readily captures and dissolves n-propanol but does not destroy it. Instead, the alcohol may be stored as PrO- and released over a wide range of times that can be controlled by varying the acidity of the solution. |
