Dagobert Hoebbel

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.

Inorganic-organic polymers derived from functional silicic acid derivatives by additive reaction

Abstract Inorganic-organic polymers were synthesized by additive reaction of vinyl-, allyl-, and H-silylated double four-ring (D4R) silicic acids and polymeric silicic acids. The structure and properties of the hybrid polymers were investigated by means of 29Si NMR spectroscopy, thermoanalysis and BET nitrogen adsorption measurements. Using the defined vinylsilylated D4R silicicacid [(CH2CH)(CH3)2Si]8Si8O20 and the corresponding H-silylated compound [(CH3)2HSi]8Si8O20 as precursors, the additive reaction results in a microporous polymer with a ordered Si8O20 substructure. The structural units are connected by six-membered bridges. Shorter (four-membered) or longer (seven-membered) bridges between D4R cages lead to non-porous polymeric materials. The connection of water-glass-derived silicic acid units by six-membered bridges similarly leads to porous polymers with specific surface areas of 500 m2/g. For the preparation of the porous hybrid polymers a new, simple two-step reaction route is described.

On the Hydrolytic Stability of Organic Ligands in Al-, Ti- and Zr-Alkoxide Complexes

The complexation degrees of Al-, Ti- and Zr-butoxide (M) with unsaturated and saturated β-diketones (3-allylpentane-2.4-dione-APD, acetylacetone-ACAC) and β-ketoesters (methacryloxyethyl-acetoacetate-MEAA, allylacetoacetate-AAA, ethylacetoacetate-EAA) as organic ligands (L) were examined by IR and 13 C NMR spectroscopy and were found to be L:M ≥ 1.5. The hydrolytic stability of the ligands of the metal alkoxide complexes (L:M = 1) during hydrolysis/condensation reactions at the molar ratio h (H2O : OR) = 0.5–2.0 decreases with increasing H2O:complex ratio. Furthermore, the ligand stability depends on the type of metal in the complexes and decreases in the order Al- > Zr- > Ti-butoxide complexes at h=1. The ACAC ligand likewise shows in the Al-, Ti- and Zr-butoxide complexes a high hydrolytic stability (95–100%) at h=1 within 7 days. The Ti- and Zr-butoxide complexes with β-ketoesters as ligand show at h=1> a release to a different extent e.g., up to 60% in the case of the MEAA-ligand in the Ti-butoxide complex after 2 days. In general, the hydrolytic stability of the ligands in the Ti-butoxide complexes (L:M = 1, h=1) decreases in the order ACAC > APD > AAA > EAA ≥ MEAA. The hydrolysis/condensation reaction of complexes having a weak ligand stability leads to larger particle sizes in the sols than those with stable ACAC ligands. The results contribute to a more controlled synthesis of sols and of new inorganic-organic hybrid polymers via the sol-gel process.

Gaschromatographische und 29Si‐NMR‐spektroskopische Untersuchungen an Kieselsäuretrimethylsilylestern

Kieselsauretrimethylsilylester unterschiedlicher Konstitution werden mit Hilfe der Gaschromatographie hinsichtlich ihres Retentionsindex und der Retentionstemperatur charakterisiert. Es wird gezeigt, das die Retentionsdaten der Ester im wesentlichen von der Anzahl der Trimethylsilylgruppen im Molekul abhangen und nur wenig von der Konstitution des Kieselsauregerustes beeinflust werden. Die 29Si-NMR-Spektren von neun Kieselsaure-TMS-Estern unterschiedlicher Konstitution wurden aufgenommen und die Zuordnung der Signale zu den einzelnen Baugruppen der Ester diskutiert. Auf der Grundlage dieser Ergebnisse wurden Konstitutionsuntersuchungen an hexameren Kieselsaure-TMS-Estern durchgefuhrt, die zur Identifizierung von zwei Isomeren des Bicyclohexakieselsaureesters [(CH3)3Si]10[Si6O17] fuhrten.

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(O0.5)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(OBut)4, Al(OBus)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(OBut)4, Al(OBus)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(O0.5)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(OBut)4 < Zr(OBun)4 < Al(OBus)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.

Gas chromatography of trimethylsilylated silicate anions : separation with glass capillary columns and new aspects in derivatization

Abstract The advantages of capillary-column gas chromatography (GC) in the analysis of trimethylsilylated silicate systems are demonstrated and separation parameters and measurement and calculation methods for the qualitative and quantitative analysis of different silicic acid solutions are outlined. The problems of the trimethylsilylation procedures are discussed. Results obtained by GC after derivatization with bistrimethylsilylacetamide (BSA) as the trimethylsilylating agent indicate that both the unwanted condensation and hydrolysis during silylation are suppressed if the proton-trimethylsilyl is accelerated e.g. , by a powerful trimethylsilyl donor such as BSA, and if the pH of the silylation reaction mixture is controlled. Chromatograms of some silicic acid samples are compared with those of identical samples silylated according to Lentz and Gotz-Masson, respectively. Comparison of both the qualitative and quantitative results indicates that the compositions of the original silicic acid solutions are best reflected in the chromatograms after silylation with BSA. The effect of the after-treatment of the silylated silicic acid solution with Amberlyst 15 cation-exchange resin is disscussed separately.

Silicate groupings in glassy and crystalline 2PbO·SiO2

The type and the amount of silicate groupings existing in glassy and crystalline 2PbO·SiO2 have been determined by direct chemical methods: paper chromatography, trimethylsilylation combined with gas-liquid partition chromatography and by the molybdate method. The results obtained by these three different methods are in good agreement and demonstrate, that glassy 2PbO·SiO2 and each of the three main crystalline polymorphs are characterized by its own specific silicate anion distribution: the distribution in vitreous 2PbO·SiO2 is of a polyanionic nature; in TPb2SiO4 dimetic groups [Si2O7]6− prevail; M1Pb2SiO4 contains predominantly [Si4O12]8− rings and HPb2SiO4 is a typical polysilicate with chain anions [SiO32−]n. The results fit a structural model according to which glass is a random array of discrete polyatomic groupings; the gradual transition from the glassy state to the stablest crystalline structure is connected with degradation and polymerization of silicate anions.

29Si NMR investigation of condensation reactions of diphenylsilanediol in presence of Ti-, Zr-, Al-, Sn- and Si-alkoxides

The investigation of the effects of small amounts of Ti-, Zr-, Al-, Sn- and Si-alkoxides and those of triethylamine and hydrochloric acid on the condensation reaction of diphenylsiloxanediol (DPSD) in molar ratio Si : additive = 1:0.01 and 1:0.04 by 29Si NMR shows a decrease in their activity in the order Ti≈HCl>Al>Sn≈N(Et)3≈Zr>Si after 6 h reaction time. The influence of the type of alkoxide ligands attached to the metal on the condensation rate of DPSD was found relatively low compared to the different metals. In presence of Ti-alkoxide the condensation reaction of DPSD leads to di-, tri- and tetrasiloxanediols, the latter of which dominates and crystallizes from the solution as octaphenyltetrasiloxanediol. The catalysis of DPSD solutions by Al-alkoxide and HCl results preferably in tetraphenyldisiloxanediol species even after a few days. The complexity of metal alkoxides and the hydrolytic stability of heterometallic Si−O−M bonds have been discussed as possible reasons for the differences in the catalytic activity of the metal alkoxide.

X-Ray and NMR spectroscopic characterisation of cyclic titanodiphenylsiloxanes and examination of the hydrolytic stability of their Si–O–Ti bonds

Six crystalline titanodiphenylsiloxanes have been synthesised by reaction of diphenylsilanediol (DPSD) with titanium tetraisopropoxide or its complexes with acetylacetonate (acac) as ligand. Two of them show a spirocyclic structure with the formula TiO2 [O2Si2(C6H5)4 ]2A and TiO2 [O4Si4(C6H5)8 ]2B which have already been described in the literature. Two compounds C and D were identified by X-ray analysis to have the same bicyclic structure but different coordinating solvent molecules. Tetrahydrofuran acts as a non-bridging ligand at the Ti atoms in [Ti(acac)O1.5 ]2 [OSi(C6H5)2 ]3 ·2C4H8O C while dioxane acts as a bridging ligand between the Ti atoms of neighbouring molecules of [Ti(acac)O1.5 ]2 [OSi(C6H5)2 ]3 ·3C4H8O2D. The titanodiphenylsiloxanes E and F were identified by a cyclotetrameric structure and the formulas [Ti(acac)2O]2 [OSi(C6H5)2 ]2 and [Ti(acac)2O][OSi(C6H5)2 ]3 , respectively. The titanodiphenylsiloxanes A–E were characterised by 29Si and 17O NMR spectroscopy, IR and time-of-flight mass spectrometry measurements. The hydrolytic stabilities of the Si–O–Ti bonds in the titanodiphenylsiloxanes A–E have been examined mainly by means of 29Si NMR spectroscopy. The results reveal a strong influence of the structure type of the titanodiphenylsiloxanes on the hydrolytic stability of their Si–O–Ti bonds apart from the hydrolytic conditions (amount of water, Si, Ti and H+ concentration). The hydrolytic stability of the titanodiphenylsiloxanes A–E decreases in the order cyclotetramer (E)>spirocyclo (A, B)>bicyclo (C, D). Reasons for the different hydrolytic stability are discussed. The results on the different hydrolytic stabilities of Si–O–Ti bonds can contribute to a better understanding of the synthesis of homogeneous heterometal materials on a molecular scale via the sol–gel process.

29 Si n.m.r spectroscopy reveals dynamic SiO44–group exchange between silicate anions in aqueous alkaline silicate solutions

Temperature dependent line broadening is observed in the 29Si n.m.r spectra of sodium silicate solutions which indicates dynamic exchange of SiO44– groups between different silicate anion species with the free SiO44– anion life time in the order of milliseconds.