On the Existence and Hydrolytic Stability of Titanosiloxane Bonds in the System: Glycidoxypropyltrimethoxysilane-Water-Titaniumtetraethoxide

Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for optical applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes, their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMR spectroscopy has been used to investigate sols with molar ratios Si : Ti = 1 and H2O : OR (H) = 0.5 – 2.0 and their gels after heat treatment at 130°C. The presence of Si—O—Ti bonds in sols with a low water content (H < 0.2) and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of up to 2–3 ppm compared to corresponding signals of homo-condensed Si—O—Si bonds. The existence of Si—O—Ti bonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavage of the Si—O—Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bonds and the building up of homo-condensed species leads to a separation into areas with predominantly Ti—O—Ti and Si—O—Si bonds resulting in a decreased molecular homogeneity of the materials.     

Untersuchungen zum System Platin/Lithium/Wasserstoff

Investigations on the System Platinum/ Lithium/Hydrogen A ternary hydride with the limiting phase composition LiPtH_0.66 is synthesized by the reaction of lithium hydride with platinum at 600°C under a H₂ atmosphere. The structure, determined by X-ray and neutron diffraction experiments, is shown to be a sheetwise ordered, hexagonal close packing of the platinum and lithium atoms in which the hydrogen atoms occupy tetrahedral interstices. Pressure-composition-temperature data were obtained for the reaction 2/(x ? z) LiPtH_x? 2/(x ? z) LiPtH_z + H₂, with LiPtH_z as β-phase and LiPtH_x as ß-phase between 450–590°C, yielding a ΔH value of 134 kJ (mol H₂)^?1 and a value of 149 J (mol H₂)^?1 K^?1.     

Schmelzpunktsbestimmung

Ohne Zusammenfassung     

The Effect of Nanoscaled Metal Oxide Sols on the Structure and Properties of Glycidoxypropyltrimethoxysilane Derived Sols and Gels

Glycidoxypropyltrimethoxysilane (GPTS) is frequently used as precursor for the preparation of sol-gel derived nanoscaled hybrid polymers. The influence of nanoscaled metal oxide sols of silica, boehmite, zirconia and ceria on reactions of GPTS in ethanolic hydrolysates and in corresponding gels (epoxide ring-opening, condensation degree) was examined by liquid- and solid-state ¹³C and ²⁹Si NMR with regard to a better correlation between structure and material properties. Generally, a higher condensation degree of RSi(O_0.5)₃ units of GPTS is found after addition of metal oxide sols compared to GPTS without additives. The metal oxide sols (10 mole% series) cause an epoxide ring-opening up to 20% in GPTS hydrolysates after 24 h. A nearly complete ring opening was found in the boehmite and silica containing hybrid gels whereas gels containing ceria and other types of silica only show a low degree of ring-opening. The results show an accelerated ring-opening with increasing content of AlO/OH-species in silica sols. ¹³C NMR studies reveal that the epoxide ring-opening does not completely lead to polyether structures but to considerable amounts (up to 40%) of ethylether groups which can influence the material properties (hardness).     

A NMR Study on the Hydrolysis,Condensation and Epoxide Ring-Opening Reaction in Sols and Gels of the System Glycidoxypropyltrimethoxysilane-Water-Titaniumtetraethoxide

The examination of hydrolysis, homo- and hetero-condensation reactions, of the condensation degree and the extent of epoxide ring opening in the course of sol-gel process was carried out by means of liquid- and solid-state 29Si and 13C NMR in the system 3-glycidoxypropyltrimethoxysilane (GPTS)-titaniumtetraethoxide-water (molar ratio 1 : 1 : 1.5–14) which is frequently used for the synthesis of heterometal hybrid polymers. The monomeric silanol groups in the GPTS-prehydrolysate immediately co-condense with the Ti-tetraethoxide to Si–O–Ti bonds to an extent of about 50–60% which remain stable in sols and also in the corresponding gels at low amounts of free water (0.02 H2O/OR) in the sol. An increasing amount of free water in the sol (0.12 H2O/OR) leads to an increased hydrolytic cleavage of the heterometal bonds and to the formation of homo-condensed polysiloxanes. The condensation degree of RSiO1.5 units in the Ti-containing sols is with 30–60% relatively high in comparison to Ti-free GPTS sols (ca. 5%) whereas the condensation degree of GPTS derived gels (81%) was found to be similar to that of the Ti-containing gels (60–80%). Ti-tetraethoxide accelerates the ring opening reaction of the epoxide group in the sols in dependence on the water content. Up to 78% of the epoxide rings are opened after 24 h in the sol with the highest water content (2 H2O/OR). No epoxide rings can be detected in Ti-containing gels which derive from sols with an amount of free water of 0.12 H2O/OR. The results give a first insight into the different parallel reactions in this system and can contribute to more structure controlled syntheses of heterometal hybrid polymers.     

On the Influence of Metal Alkoxides on the Epoxide Ring-Opening and Condensation Reactions of 3-Glycidoxypropyltrimethoxysilane

The extent of the epoxide ring-opening, the formation of ethyl ether groups as one reaction product of the ring-opening and the condensation degree of RSi(O_0.5)₃ units in sols and gels of the system 3-glycidoxypropyltrimethoxysilane (GPTS)-1.5H₂O-0.01/0.1/1.0 metal alkoxide [Si(OEt)₄, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃, Ta(OEt)₅, Ti(OEt)₄, Zr(OBuⁿ)₄] in ethanol has been examined by means of liquid- and solid-state ₁₃C and ₂₉Si NMR spectroscopies. The results reveal a strong epoxide ring-opening effect of Al-alkoxides in hybrid sols after 24 h reaction time and of Zr-, Ta-, Al- and Sn-alkoxides in corresponding hybrid gels already at low concentration (1 mole%). The ring-opening rate increases in sols with higher metal alkoxide concentration (10 mole%) but decreases at 50 mole% concentration of Al-, Ti- and Zr-alkoxides. The ring-opening activity of metal alkoxides in 10 mole% hybrid sols increases after 24 h reaction time in the order Si(OEt)₄ < Ti(OEt)₄ < Zr(OBuⁿ)₄ < Ta(OEt)₅, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃. The 24 h hybrid sols and gels contain considerable amounts (up to 90%) of ethyl ether groups as reaction product of the ring-opening reaction which lowers the formation of polyether bonds. The condensation degree (c.d.) of RSi(O_0.5)₃ units of GPTS-1.5H₂O sols with 10 mole% of metal alkoxides increases up to 80% after 7 h reaction time in the order: Si(OEt)₄ Sn(OBu^t)₄ < Zr(OBuⁿ)₄ < Al(OBu^s)₃ < Al(OEtOBu)₃ < Ta(OEt)₅ < Ti(OEt)₄. An additional increase in c.d. up to 90% follows after the thermal sol-gel transformation. Generally, maximum activity of metal alkoxides in ring-opening and condensation reactions was found in sols and gels with 10 mole% additives. The effect of water on the epoxide ring-opening and on c.d. is discussed. Furthermore, the activity of metal alkoxides is compared with corresponding nanoscaled metal oxides.