Formation of the Glycol Derivative of Boehmite by the Thermal Treatment of Gibbsite in Glycol -Reaction Mechanisms-

Abstract

Reaction of fine-particle gibbsite in glycol under the spontaneous vapor pressure of the glycol (glycothermal treatment) yields a glycol derivative of boehmite (Ref.5). In this paper, the mechanisms of this reaction were explored. With an increase in the particle size of gibbsite, the yield of the boehmite derivative decreased and the yield of well-crystallized boehmite and the amount of unreacted gibbsite increased (Table 2). Autocatalytic nature was observed during the course of the reaction (Figs.2 and 3). Addition of a small amount of water facilitated the formation of the glycol derivative of boehmite. A honeycomb like texture composed of plate-like crystals of the boehmite derivative was developed on the exterior of the particle when coarse gibbsite was used, whereas the glycothermal treatment of fine-particle gibbsite yielded bulky aggregates composed of honeycomb-like texture, combining the original particles (Figs.4-6). These results suggest that aluminum hydroxide was dissolved into a mixture of glycol and water formed by partial dehydration of gibbsite followed by ligand exchange between the water molecule in the coordination site of aluminum and the glycol molecule and then crystallization of the glycol derivative of boehmite. Formation of boehmite was interpreted by the intra-particle hydrothermal reaction originally proposed by de Boer et al. for the formation of boehmite during the thermal dehydration of gibbsite. A chimney-like structure composed of honeycomb-like texture of the glycol derivative of boehmite was found in the product from coarse gibbsite (Fig.7). This structure may have been formed when closed pore filled with water, where the intraparticle hydrothermal reaction took place, reached to the surface of the particle and a large amount of water together with aluminum species was released from the pore into the bulk solvent.