Microemulsion guided synthesis of microporous vanadium pentoxide.

摘要

We have developed a process for making microporous vanadium pentoxide that proceeds via the polymerization of vanadium isopropoxide oxide in a water/sodium dioctylsulfosuccinate (Aerosol OT)/toluene reverse microemulsion. The kinetics of this method can be separated into four steps that occur at very different speeds. The first, the reaction of alkoxide and water in the microemulsion droplets to produce polyions, takes about 100 milliseconds. The second step, the conversion of these polyions into polymer ribbons, takes about ten minutes. The third step, which involves making large clusters from the ribbons, is the slowest step, taking an hour or more. In the fourth and final step, the vanadia clusters are assembled into the final microporous structure by a sequence of washing and drying steps.; In the first step, we discovered the first order rate constant {dollar}({lcub}sim{rcub}{dollar}40 1/s) for the alkoxide-microemulsion reaction is directly related to the water concentration and inversely related to the surfactant concentration. This result is consistent with diffusion of alkoxide across a new surfactant structure formed in microemulsions containing excess surfactant.; In the second step, the sol produced above forms ribbons that are characterized by a mass fractal dimension of 2-2.4. In the third step, these ribbons aggregate into clusters, characterized by a mass fractal dimension of 2.0, over a period of an hour. The growth rate is linearly dependent on the initial alkoxide and water concentrations. This is explained by a two step process involving diffusion of the droplets and reaction of the fibers within droplets.; In the fourth and final step, three types of microporous structures from vanadium pentoxide clusters are prepared: Dense films, with little surface area; low porosity films (.27) with high surface area (100 m{dollar}sp2{dollar}/g); and high porosity films (.61) with high surface area (100 m{dollar}sp2{dollar}/g). The final film microstructure depends on the solvent from which the colloids are dried as well as the amount of surfactant present during the drying. The final porosity and surface area is created through a mechanism that involves the formation of chemical bonds during the drying step.