Microemulsion-Directed Synthesis of Molecular Sieve Fibers

The aluminum phosphate molecular sieve AlPO 4 -5 with the AFI crystal structure was synthesized in a novel fibrous morphology through confined synthesis within water-in-toluene microemulsion droplets. The surfactant cetylpyridinium chloride with the cosurfactant butanol forms a thermodynamically stable single-phase microemulsion with the AlPO 4 -5 synthesis gel at room temperature. Several pseudo-ternary phase diagrams were constructed to map the single-phase microemulsion region. The phase diagrams were used as a guide for hydrothermal synthesis at various microemulsion compositions. Crystallization of AlPO 4 -5 was achieved at 180 °C either by conventional heating of the microemulsion for 6 h or microwave heating for 17 min. The crystals are much larger than the microemulsion droplets, so that the microemulsion does not simply act as a template, but rather directs crystal growth into fibers presumably by interacting with the surface of the growing crystal. The AlPO 4 -5 crystal morphology changes from individual fibers to “fan-like” fiber aggregates as the ratio of AlPO 4 -5 gel to surfactant increases. As the ratio of toluene to surfactant increases, nonporous Berlinite becomes the favored product due to partitioning of the structure-directing agent into toluene. Microwave heating produces smaller fibers and less nonporous AlPO 4 than conventional heating. The microemulsion approach presented is applicable to hydrothermal synthesis of a variety of zeolites and molecular sieves to potentially control crystal morphology.