Temperature-Dependent Halogen-Exchange Activity Studies of Zeolite-Derived Aluminum Trifluoride

Abstract

A high-surface-area (190 m 2 /g) amorphous aluminum trifluoride material ("plasma-AlF 3 ") was synthesized by plasma decomposition of zeolite, and its structural and reactivity properties were investigated. High-resolution transmission electron microscopy of plasma-AlF 3 indicates morphological features on the nanometer-scale, whereas temperature-programmed X-ray diffraction is used to determine the phase-transition temperatures of plasma-AlF 3 to β-and R-AlF 3 . Halogen-exchange reactivity is studied by temperature-programmed reaction (TPR) techniques using the dismutation of CCl 2 F 2 as a model reaction. Plasma-AlF 3 is found to possess an unexpected low-temperature (>315 °C) activity not observed with the well-known halogen-exchange catalyst β-AlF 3 . Supporting TPR studies on aluminum trifluoride hydrates are performed to correlate this new activity with an amorphous AlF 3 structure, and a simple Lewis acid model is presented to explain the reactivity data.

Recent developments in the preparation of high surface area metal fluorides

“…Formation of nano-rods in the later process was tentatively attributed to either anisotropic etching effects found in plasma or to the formation of crystalline hydrate. Namely, plasma-derived HS-AlF 3 was found to be very sensitive to moisture, after exposure to air for some days it was converted to a mixture of a-AlF 3 and b-AlF 3 Á3H 2 O [20]. During our work, formation of crystalline hydrate after longer exposures of HSAlF 3 to ambient air was also observed, but in our case only bAlF 3 Á3H 2 O was formed.…”

“…sol-gel derived HS fluorides may contain considerable amounts of organic residues while NH 4 + species may be present in the products from oxidative decomposition. As pointed out earlier, different residual by-products can considerably influence the phase transformation processes in HS-AlF 3 [20]. Investigation of HS fluorides from different preparations, containing therefore different residual species, can help to clarify the main processes that influence the stability of HS fluorides what may have important implications for their application.…”

“…Combination of both effects was crucial in preventing the thermally induced ordering and crystallisation of the fluoride phase that remained nearly amorphous with a highly distorted arrangement of AlF 6 octahedra. Thermal stability of the amorphous plasma-derived AlF 3 was investigated recently by temperature-programmed synchrotron X-ray powder diffraction [20]. Amorphous AlF 3 was found to be stable up to 330 8C, at higher temperatures it was converted first to b-AlF 3 and above 440 8C to cubic-AlF 3 , the high-temperature modification of a-AlF 3 .…”

“…Materials are described as amorphous or nearly amorphous [10,20], although the diffuse diffraction lines indicate that a low level of ordering may already be present. The nature, the degree of crystallisation, and morphology of the HS metal fluorides obtained by oxidative decomposition were studied by transmission electron microscopy (TEM, JEM 2100 microscope).…”

“…Based on the TEM results we can conclude that the HS-AlF 3 sample consists of the matrix phase composed from nanocrystalline aAlF 3 and b-AlF 3 and nano-rods consisting of b-AlF 3 . HS-AlF 3 materials from other preparations show slightly different nanostructures: sol-gel derived products usually consist of rounded 10 nm particles [10] or a mixture of 10 and 50 nm particles [14], in the plasma-derived products two types of particles were found, relatively uniform nano-rods 20-50 nm in length and 3-5 nm agglomerated particles [20]. Formation of nano-rods in the later process was tentatively attributed to either anisotropic etching effects found in plasma or to the formation of crystalline hydrate.…”