Date of Award
Doctor of Philosophy
Craig E. Barnes
Craig E. Barnes, Zi-ling Xue, Michael Best, Joseph J. Bozell
A building block approach and sequential addition methodology were utilized to prepare heterogeneous silicate catalysts containing atomically dispersed group 13 metal (B, Al, Ga) centers. The octa(trimethyltin) silsequioxane, Si8[sub]O12[sub](OSnMe3[sub])8[sub], was used as the building block for the synthesis of these materials. Reaction of the building block with a variety of group 13 metal chlorides led to the formation of cross-linked matrices. All prepared materials were characterized by gravimetric analysis, gas absorption, IR, and NMR. In addition, aluminum and boron samples where characterized by 27[sup]Al and 11[sup]B solid state NMR, and gallium samples were studied using x-ray absorption techniques.
Studies found the nature of the reaction for the aluminum and gallium species to be more complex than expected. This was manifested most prominently in the formation of tetramethyltin, Me4[sub]Sn, an unexpected byproduct that led to unpredictably high connectivity of the metal centers to the silicate matrix. This in turn gave rise to questions regarding the true structural nature of the metal sites.
Characterization of the aluminum systems indicated that multiple types of aluminum sites (4, 5 and 6 coordinate) were present in the matrix. Increased coordination was found to result in part from the in situ formation and reaction of the [Me3[sub]Sn][AlCl4[sub]] species. It was determined that the trimethyltin cation in this ionic species was responsible for formation of MeSn through abstraction of a methyl group from unreacted –OSnMe3[sub]groups remaining on the corners of the silicate building block.
While the gallium analogues showed similar behavior, XANES and EXAFS analyses showed that in nearly every material, gallium had achieved 4-coordinate tetrahedral geometry.
The boron systems behave quite differently that Al and Ga, producing no secondary byproduct, and forming stable 3-coordinate trigonal geometries. Pyridine adsorption studies showed that these trigonal species could at least in part be converted back and forth to pseudo tetrahedral structures.
Abbott, Joshua G., "The Synthesis and Characterization of Novel Group 13 Nanostructured Building Block Heterogeneous Silicate Catalysts. " PhD diss., University of Tennessee, 2012.