Non-hydrolytic Sol Gel Route Research Articles

Non-hydrolytic sol–gel routes to silica

Silica was prepared by solvent-free non-hydrolytic condensation reactions from silicon tetrachloride with different oxygen donors: benzyl alcohol and tetrabenzyloxysilane (no catalysis), tetraethoxysilane, tetraisopropoxysilane, diethylether and diisopropylether (catalyzed by FeCl 3, TiCl 4, AlCl 3 or ZrCl 4). Highly condensed gels with a low hydroxyl content were obtained in high yields. A solution 29Si NMR study showed that in the presence of FeCl 3 the condensation of SiCl 4 with Si(O i Pr) 4 was accompanied by extensive redistribution of Si–Cl and Si–O i Pr bonds, leading to increasingly complex mixtures. Non-hydrolytic silica xerogels were characterized by chemical analysis, IR and 29Si NMR spectroscopy and thermogravimetric analysis. Porosity and specific surface area were determined using nitrogen adsorption–desorption. Xerogels displayed a wide variety of textures depending on the nature of the oxygen donor and of the catalyst.

Solvent-free synthesis of binary inorganic oxides

The non-hydrolytic sol–gel route has been used in the solvent-free synthesis of binary inorganic oxides based on silicon, aluminium and titanium. Where necessary, iron(iii) chloride was used as a catalyst. Clear evidence for the formation of true, amorphous binary systems was obtained only in the case of aluminosilicates. A crystalline aluminosilicate was obtained after calcination at 1000 °C. A titanium–silicon binary system gave only crystalline rutile and anatase following calcination, with no evidence for either crystalline silica or a binary oxide.

Preparation of Inorganic Oxides via a Non-Hydrolytic Sol-Gel Route

Inorganic oxides have been synthesized successfully under mild reaction conditions using a solvent-free non-hydrolytic sol-gel process based on the condensation reaction of “metal” chlorides with oxygen donors such as alkoxides, aldehydes and ethers. Iron(III) chloride was found to be an effective catalyst for the reactions. The order of reactivity of the halides was generally titanium > aluminium > silicon, but in some cases reaction was halted by premature gelation of intermediate species. Silica, alumina and titania were all prepared and characterized by various methods. Calcination studies on the silicas showed these materials to be amorphous up to at least 700°C, but devitrification occurred at 1000°C. Crystallization was studied by X-ray powder diffraction.

Incorporation of Siloxane And Cyclophosphazene Units Into Metal Oxides by a Nonhydrolytic Sol-Gel Route

AbstractA nonhydrolytic sol-gel method was used to incorporate siloxane and cyclotriphosphazene units within metal oxides. The degree of mixing of the components was investigated by EDX, XPS, and 29Si and 31P solid-state NMR. The NMR data confirmed the presence of Si-O-M and PO- M bonds.

Nonhydrolytic sol-gel routes to layered metal(IV) and silicon phosphonates

Three nonhydrolytic routes to methyl and phenylphosphonates have been developed involving the systems: (i) MCl4-RPO(OR)2 or M(OR)4-RPOCl2 (M=Si, Ti); (ii) MCl4-RPOCl2-ether (M=Sn, Ti, Zr); (iii) MCl4-RPOCl2-alcohol (M=Sn, Ti, Zr). It is noteworthy that use of nonhydrolytic conditions opens a route to water-sensitive silicon phosphonates. In most cases the phosphonates were obtained as gels. The poorly crystalline materials obtained after drying were characterised by solid-state NMR spectroscopy and their layered structure could be shown by powder X-ray diffraction. However a higher degree of crystallinity could be obtained under solvothermal conditions, either by the direct alcoholysis of chloride precursors with tert-butanol (2-methylpropan-2-ol), or by a simple postsynthesis treatment in tert-butanol.

Effect of Aging on Nonhydrolytic Alumina Xerogels

Nonhydrolytic sol-gel route is a relatively recent process which enables production of complex, multicomponent oxide materials. This process has some advantages over the conventional hydrolytic sol-gel route due to the ability to produce low-shrinkage, homogeneous, multicomponent gels. The objective of this work was to determine the effects of aging of nonhydrolytic gels on the composition, yield, phase transformations and morphology. Xerogels were prepared from aluminum chloride and isopropyl ether. Properties were studied using AgNO3 titrations, TGA/DTA, XRD, and BET analysis. We have found that the gels contain significant amount of chlorine where the Cl/Al atomic ratio ranges from 1.1–0.6 depending on the aging time. The crystallization temperature and enthalpy of crystallization decreased with aging time. The decrease of the surface area near the crystallization temperature correlates well with the decrease of the enthalpy of crystallization as a function of aging time. A closed pore phenomenon has been observed in the nonhydrolytic alumina system. Finally, analysis of the condensation degree (CD) yielding Al–O–Al bonds suggests that the rate determining step before the gel point is the alkoxy groups formation. However, during aging of the gels, the CD remains constant since the condensation of chloride with isopropoxy groups is stericly inhibited. Surface areas in the 300–650 m2/g range were obtained depending on the aging time.

Mixed oxides SiO2–ZrO2and SiO2–TiO2by a non-hydrolytic sol–gel route

SiO2–ZrO2 and SiO2–TiO2 mixed oxides with various metal contents have been prepared by a non-hydrolytic sol–gel route involving the condensation between chloride and isopropoxide functions at 110 °C. Well condensed, monolithic gels were obtained in one step, without the use of additives. The Si/M ratio of the oxide may be controlled easily by the composition of the starting mixture. The Si/Zr oxides remained amorphous after calcination for 5 h at 600 °C; IR and 29Si NMR spectroscopy showed a large amount of Si–O–Zr bonds, indicating a homogeneous distribution of the components on the atomic scale. The crystallization of tetragonal zirconia took place at higher temperature; the transformation of tetragonal to monoclinic zirconia was strongly retarded and did not take place after 2 h at 1300 °C. The crystallization of zircon (for a sample containing 50 mol% Zr) started at 1500°C and was completed after 20 h at 1500°C. IR spectroscopy indicated the presence of a limited number of Si–O–Ti bonds in all the Si/Ti oxides after calcination for 5 h at 500 °C. The sample within the stable glass region (5 mol% Ti) appeared perfectly homogeneous: it crystallized at 900 °C as single-phase cristobalite oxide, with Ti4+ ions substituting Si4+ ions at random. On the other hand, the precipitation of anatase was observed for the Si/Ti oxides with a high Ti content (20–50 mol% Ti), which are outside the stable glass region. The transformation of anatase to rutile was not observed even after 2 h at 1300 °C.

Preparation of alumina gels by a non-hydrolytic sol-gel processing method

Monolithic alumina gels were prepared by a novel non-hydrolytic sol-gel route based on the condensation reaction between aluminum alkoxides Al(OR)3 and aluminum halides AlX3, through the formation of alkyl halide RX(X = Br, Cl; R = iso-propyl, sec- or ter-butyl), around 100°C. Samples were then calcined and were found to be amorphous up to about 750°C; they kept a high specific surface area to about 900°C. This delayed crystallization is correlated with the large number of five-coordinate aluminum sites measured in dried gels by 27Al NMR spectroscopy.

Novel non-hydrolytic sol-gel route to metal oxides

Monolithic alumina and aluminosilicate gels have been prepared using a novel sol-gel process based on the non-hydrolytic condensation reaction between a metal halide and a metal alkoxide. XRD indicated that the alumina gel remained amorphous at 750°C; solid state 27Al NMR indicated the presence in the dried gel and in the amorphous calcined sample of a large amount of pentacoordinated aluminum atoms. A study of the sol formation using liquid state 27Al NMR suggested that the gel structure was reminiscent of the oligomeric structure of the chloroisopropoxide precursors. Differential thermal analysis and XRD indicated that the aluminosilicate gels were converted to mullite below 1000°C, suggesting a high degree of homogeneity in these precursors.

Monophasic Pre-Mullite Gels Prepared by a Nonhydrolytic Process

ABSTRACTA nonhydrolytic sol-gel route to aluminosilicates is proposed which avoids the problems associated with different hydrolysis rates for the precursors. Two pre-mullite gels (Al2O3/SiO2=3/2) were prepared in two different ways. The conversion to mullite was studied as a probe for chemical homogeneity. The results indicate that a monophasic precursor (i.e. homogeneous at the atomic level) may be easily obtained by etherolysis of a mixture of SiCl4 and AlCl3.