Effect of Organic Ligands Used in Sol-Gel Process on the Formation of Mullite

The preparation of mullite by the sol-gel method using organic polydentate ligands and the effect of the raw materials and organic polydentate ligands on the formation of mullite were investigated. Two series of samples were prepared using tetraethoxyorthosilicate (TEOS) and aluminum nitrate nonahydrate, or dibutoxyethylacetoacetatoaluminum (Al(OBu)₂(AcAcEt)) as the silica and alumina sources, respectively, and using ethylene glycol (EG), 1,3-propanediol (PD), 1,3-butanediol (BD), 2-methyl-2, 4-pentanediol (MPD), diethlene glycol monoethyl ether (DEME) and ethoxyethanol as the ligands. When the alumina source was aluminum nitrate nonahydrate, mullite was apt to appear in the order of EG>PD>MPD. When Al(OBu)₂(AcAcEt) was the alumina source, the tendency toward the appearance of mullite crystalline phase was EG>BD>DEME>MPD. Between the two alumina sources, aluminum nitrate nonahydrate gave mullite much easier than Al(OBu)₂(AcAcEt). These relationships were discussed from the viewpoints of the coordination ability of the ligands and the miscibility between the silica and alumina.     

Effect of Solvent Diols and Ligands on the Properties of Sol-Gel Alumina-Silicas

Amorphous alumina-silicas were prepared from a tetra-alkoxysilane and anhydrous aluminum trichloride or an aluminum alkoxide by a sol-gel process using 2-methyl-2,4-pentanediol, pinacol, 1,2-propanediol, 2,3-butanediol or ethylene glycol as the solvent or complexing agent, and the effect of diols and alkoxy groups on the physical and chemical properties of the alumina-silicas was examined. When the diol or the alkoxy group was bulky, the alumina-silicas had relatively larger micropores, a larger pore volume and higher surface areas. In the conversion of methanol catalyzed by the alumina-silicas, the bulkier diols and alkoxides gave catalysts that produced dimethyl ether in higher yield and hydrocarbons in lower yield. Thus, when ethylene glycol was used as the diol, the best catalyst for the production of hydrocarbons, especially the production of olefins such as ethylene, propylene and butene, was obtained. Furthermore, in comparison with alumina-silica prepared by a traditional kneading process, it was found that the sol-gel alumina-silica could efficiently convert methanol to dimethyl ether and hydrocarbons, but the material prepared by kneading had a very low conversion of methanol to other compounds.