Of ¦Ã-al (oh) <sub> 3 </ Sub> Theoretical Research, The Top Site Adsorption Behavior Of Organic Small Molecules

Gibbsite is important intermediate product in alumina industry. But the long production cycle is the always main problem during the manufacture of gibbsite. It is reported that a lot of small organic molecules can inhibit the crystallizion of gibbsite from the sodium aluminate solution. But nine tenths of the existing researches limit their field in the description of the experiment phenomena, and there is still seldom research about the mechanism of this phenomenon. In this paper, the top-site adsorptions of small organic molecules on the gibbsite surface were studied by the density functional theory (DFT). This theoretical result can offer some basic data in designing new method to avoid the effects of organic compounds on the crystallization of gibbsite.1,The gibbsite crystal model was optimized by plane wave pseudopotential method, and the gibbsite crystal model in ideal condition was obtained. Using this ideal model, the slab models of several surfaces were obtained and optimized. According to the energy of every surface, it can be concluded that the stability of the gibbsite's surfaces follow this arrangement: {010}≈{110} < {112} < {111} < {{100}} < {101} < {001}.2,As a probe molecule, top-site adsorptions of H₂O on gibbsite's surfaces were investigated. This works can be a standard in doing following research about the adsorptions of small organic molecules.3,Top-site adsorptions of polyols, polycarboxylic acids and carboxylic acid ions on the gibbsite surfaces were studied using the same method as H₂O's. It can be concluded that:The interactions between small molecular organic compounds and gibbsite surfaces are mainly related to the energy of organic compounds' frontier orbital and the surfaces' Fermi level. Only if the energy of organic compounds' frontier orbital and the surfaces' Fermi level matching, can small molecular organic compounds adsorb on the gibbsite surfaces.The gibbsite's {001}, {100} and {101} surface which possess a relatively low Fermi level, prefer to interact with the polyols and polycarboxylic acids with a very low frontier orbital energy. So polyols and polycarboxylic acids can seriously affect the morphology of the gibbsite. But the three surfaces can not adsorb the carboxylic acid ions, because the frontier orbital energy of carboxylic acid ions is too high.Although the {111} surface can interact with the organic compounds with a low frontier orbital energy, the adsorption energy is not high, because that the energy differences between organic compounds' frontier orbital and the Fermi level of {111} surface do not so match. But active surfaces, such as {111} face, can obviously interact with the organic compounds with a high frontier orbital energy, for example, carboxylic acid ions. The adsorptions of carboxylic acid ions can inhibit the crystallization of gibbsite from the sodium aluminate solution.