Polyoxotitanates and their role in titania sol-gel polymerization.

摘要

Hydrolysis of Ti(OBu{dollar}sp{lcub}rm t{rcub})sb4{dollar} with 1 equiv of water in t-BuOH at 100{dollar}spcirc{dollar}C yielded a new polyoxotitanate complex {dollar}rmlbrack Tisb{lcub}18{rcub}Osb{lcub}28{rcub}Hrbrack (OBusp{lcub}t{rcub})sb{lcub}17{rcub}{dollar}. A single crystal X-ray diffraction study of its t-BuOH solvate revealed a {dollar}rm Tisb{lcub}18{rcub}Osb{lcub}45{rcub}{dollar} metal-oxygen framework having a pentacapped Keggin structure Reaction of the complex with t-amyl alcohol led to the product {dollar}rmlbrack Tisb{lcub}18{rcub}Osb{lcub}28{rcub}Hrbrack (OBusp{lcub}t{rcub})sb{lcub}12{rcub}(OAmsp{lcub}t{rcub})sb5{dollar}, identified using NMR spectroscopy.; Partial hydrolysis and condensation of titanium alkoxides were studied in situ by using solution 170 NMR spectroscopy. The speciation and rates of formation were found to be greatly affected by reaction conditions such as the choice of alkoxide ligand, degree of hydrolysis, solvent and pH. The low molecular weight species formed did not appear to be intermediates formed under sol-gel polymerization conditions.; Solution {dollar}sp{lcub}17{rcub}{dollar}O NMR and static light scattering techniques were used to examine molecular size distributions in hydrolyzed Ti(OEt){dollar}sb4{dollar} and {dollar}rmlbrack Tisb{lcub}16{rcub}Osb{lcub}16{rcub}rbrack (OEt)sb{lcub}32{rcub}{dollar} solutions under acidic and basic (or neutral) conditions as a function of the degree of hydrolysis. Sol-gel polymerization of Ti(OEt){dollar}sb4{dollar} was observed to involve the initial formation of stable polyoxotitanates which then serve as monomers for sol-gel polymerization.; A building block approach in titania sol-gel polymerization was explored using two polyoxotitanates, {dollar}rmlbrack Tisb7Osb4rbrack (OEt)sb{lcub}20{rcub}{dollar} and {dollar}rmlbrack Tisb{lcub}16{rcub}Osb{lcub}16{rcub}rbrack (OEt)sb{lcub}32{rcub}{dollar}, as potential precursors. First, the stability of the {dollar}rmlbrack Tisb7Osb4rbrack (OEt)sb{lcub}20{rcub}{dollar} and {dollar}rmlbrack Tisb{lcub}16{rcub}Osb{lcub}16{rcub}rbrack (OEt)sb{lcub}32{rcub}{dollar} core structures toward ethanol was studied. Ethanolic solution of {dollar}rmlbrack Tisb7Osb4rbrack (OEt)sb{lcub}20{rcub}{dollar} showed significant decomposition of the {dollar}rmlbrack Tisb7Osb4{dollar}) core structure, while solution of {dollar}rmlbrack Tisb{lcub}16{rcub}Osb{lcub}16{rcub}rbrack (OEt)sb{lcub}32{rcub}{dollar} showed no signs of decomposition under the same conditions. Solid state {dollar}sp{lcub}17{rcub}{dollar}O MAS NMR experiments using selective {dollar}sp{lcub}17{rcub}{dollar}O labeling techniques indicated that the {dollar}rmlbrack Tisb{lcub}16{rcub}Osb{lcub}16{rcub}{dollar}) core was largely preserved during sol-gel polymerization. Dynamic light scattering studies comparing kinetics of {dollar}rmlbrack Tisb7Osb4rbrack (OEt)sb{lcub}20{rcub}{dollar}, {dollar}rmlbrack Tisb{lcub}16{rcub}Osb{lcub}16{rcub}rbrack (OEt)sb{lcub}32{rcub}{dollar} and Ti(OEt){dollar}sb4{dollar} sol-gel polymerization revealed that two different molecular growth pathways were followed. Nitrogen adsorption studies showed different porosities for {dollar}rmlbrack Tisb7Osb4rbrack (OEt)sb{lcub}20{rcub}{dollar}-, {dollar}rmlbrack Tisb{lcub}16{rcub}Osb{lcub}16{rcub}rbrack (OEt)sb{lcub}32{rcub}{dollar}- and Ti(OEt){dollar}sb4{dollar}-derived xerogels arising from the different precursors employed.