Non-hydrolytic Sol-gel Chemistry Research Articles

Suppression of phase-transition temperature in aluminium indium tungstate and aluminium indium molybdate

Aluminium indium tungstate (AlInW3O12) and aluminium indium molybdate (Al0.84In1.16Mo3O12) were synthesized by non-hydrolytic sol–gel chemistry, and their crystal structures, phase transition and thermal expansion behavior were studied using variable-temperature synchrotron powder diffraction. AlInW3O12 adopts an orthorhombic phase above 260 K and gradually transitions to a monoclinic polymorph below this temperature. Al0.84In1.16Mo3O12 also shows a gradual transition between the monoclinic and orthorhombic structures between 330 and 445 K. Both materials display much lower phase-transition temperatures than predicted on the basis of the parent compounds and Vegard's law. This suppression is attributed to the large size difference between Al3+ and In3+. Interestingly, both samples display positive thermal expansion along all unit-cell axes instead of the typically observed negative expansion of orthorhombic A 2 M 3O12 compositions.

Chemical route for synthesis of β-SiAlON:Eu2+ phosphors combining polymer-derived ceramics route with non-hydrolytic sol-gel chemistry

We report the synthesis of β-SiAlON:Eu2+ phosphors from novel single source precursors in which strictly controlled chemical composition is established at molecular scale. The two-step synthesis occurs by the chemical modification of perhydropolysilazane (PHPS) with Al(OCH(CH3)2)3 and AlCl3 in xylene at room temperature to 140 °C to introduce Al in the PHPS network while controlling the oxygen content followed by the reaction of EuCl2 with the Al-modified PHPS upon heat-treatment. Gas chromatography-mass spectrometry and thermogravimetry-mass spectrometry analyses revealed that PHPS reacted with Al(OCH(CH3)2)3 and AlCl3 via the formation of Al-N bonds. Moreover, AlCl3 reacted with nitrogen-bonded Al alkoxide residue to release 2-chloropropane in an analogy to the non-hydrolytic sol-gel reaction between metal alkoxide and metal chloride. Subsequently, AlCl3 acted as a Lewis acid catalyst to promote the Friedel-Craft alkylation between xylene solvent and the 2-chloropropane formed in-situ to afford dimethylcumene. On the other hand, EuCl2 reacted with silylamino moiety to form Eu-N bonds at around 850 °C. β-SiAlON:Eu2+ phosphors were successfully synthesized by pyrolysis of the precursors under flowing N2 or NH3 at 1000 °C, followed by heat treatment at 1800 °C for 1 h under a N2 gas pressure at 980 kPa. The polymer-derived β-SiAlON:Eu2+ (z = 0.55, Eu2+ 0.37 at%) exhibited green emission under excitation at 410 or 460 nm, and the green emission intensity under the excitation at 410 nm was found to be increased by reducing the carbon and chlorine impurities through the polymer-derived ceramics route investigated in this study.

Non-hydrolytic sol–gel synthesis of polypropylene/TiO2 composites by reactive extrusion

An original way to synthesise polypropylene/titanium dioxide (PP/TiO2) composites combining non-hydrolytic sol–gel chemistry and reactive extrusion was reported. The non-hydrolytic sol–gel reaction between titanium alkoxide as a titanium dioxide precursor and an acid anhydride as an oxygen donor at 240 °C was first evaluated and optimised in an alkane liquid medium. TiO2 was present essentially in crystalline anatase form when synthesised in squalane with a domain size around 10 nm. Based on these encouraging results, the synthesis was adapted to reactive extrusion that means at high temperature in molten viscous polypropylene and for short reaction times (few minutes maximum). The proof of concept was evidenced through the formation of amorphous TiO2-based inorganic domains of size around 1 µm within the polymer matrix from the reaction between titanium isopropoxide and acetic anhydride. A condensation degree of 79% was obtained. To optimise the polymer/filler interface, titanium dioxide precursors and oxygen donors with different alkyl chain lengths were evaluated. Hexanoic anhydride as substituent of acetic anhydride and titanium tetrakis 2-ethylhexyloxide as substituent of titanium isopropoxide were used. The oxygen donor contribution appeared to be dominant for the particle dispersion state. The polypropylene (PP)/TiO2 composite synthesised from the reaction between titanium isopropoxide and hexanoic anhydride showed the narrowest TiO2 particle diameter distribution with a mean particle size around 700 nm.

Slowing the Kinetics of Alumina Sol-Gel Chemistry for Controlled Catalyst Overcoating and Improved Catalyst Stability and Selectivity.

Catalyst overcoating is an emerging approach to engineer surface functionalities on supported metal catalyst and improve catalyst selectivity and durability. Alumina deposition on high surface area material by sol-gel chemistry is traditionally difficult to control due to the fast hydrolysis kinetics of aluminum-alkoxide precursors. Here, sol-gel chemistry methods are adapted to slow down these kinetics and deposit nanometer-scale alumina overcoats. The alumina overcoats are comparable in conformality and thickness control to overcoats prepared by atomic layer deposition even on high surface area substrates. The strategy relies on regulating the hydrolysis/condensation kinetics of Al(s BuO)3 by either adding a chelating agent or using nonhydrolytic sol-gel chemistry. These two approaches produce overcoats with similar chemical properties but distinct physical textures. With chelation chemistry, a mild method compatible with supported base metal catalysts, a conformal yet porous overcoat leads to a highly sintering-resistant Cu catalyst for liquid-phase furfural hydrogenation. With the nonhydrolytic sol-gel route, a denser Al2 O3 overcoat can be deposited to create a high density of Lewis acid-metal interface sites over Pt on mesoporous silica. The resulting material has a substantially increased hydrodeoxygenation activity for the conversion of lignin-derived 4-propylguaiacol into propylcyclohexane with up to 87% selectivity.

Controlled deposition of titanium oxide overcoats by non-hydrolytic sol gel for improved catalyst selectivity and stability

Advances in the synthetic control of surface nanostructures could improve the activity, selectivity and stability of heterogeneous catalysts. Here, we present a technique for the controlled deposition of TiO2 overcoats based on non-hydrolytic sol-gel chemistry. Continuous injection of Ti(iPrO)4 and TiCl4 mixtures led to the formation of conformal TiO2 overcoats with a growth rate of 0.4 nm/injected monolayer on several materials including high surface area SBA-15. Deposition of TiO2 on SBA-15 generated medium-strength Lewis acid sites, which catalyzed 1-phenylethanol dehydration at high selectivities and decreased deactivation rates compared to typically used HZSM-5. When supported metal nanoparticles were similarly overcoated, the intimate contact between the metal and acid sites at the support-overcoat interface significantly increased propylcyclohexane selectivity during the deoxygenation of lignin-derived propyl guaiacol (89% at 90% conversion compared to 30% for the uncoated catalyst). For both materials, the surface reactivity could be tuned with the overcoat thickness.

The Power of Non-Hydrolytic Sol-Gel Chemistry: A Review

This review is devoted to non-hydrolytic sol-gel chemistry. During the last 25 years, non-hydrolytic sol-gel (NHSG) techniques were found to be attractive and versatile methods for the preparation of oxide materials. Compared to conventional hydrolytic approaches, the NHSG route allows reaction control at the atomic scale resulting in homogeneous and well defined products. Due to these features and the ability to design specific materials, the products of NHSG reactions have been used in many fields of application. The aim of this review is to present an overview of NHSG research in recent years with an emphasis on the syntheses of mixed oxides, silicates and phosphates. The first part of the review highlights well known condensation reactions with some deeper insights into their mechanism and also presents novel condensation reactions established in NHSG chemistry in recent years. In the second section we discuss porosity control and novel compositions of selected materials. In the last part, the applications of NHSG derived materials as heterogeneous catalysts and supports, luminescent materials and electrode materials in Li-ion batteries are described.

Interfacial catalysis in and initial reaction mechanism of Al2O3 films fabricated by atomic layer deposition using non-hydrolytic sol-gel chemistry.

Atomic layer deposition (ALD) is a powerful nanofabrication technique that can precisely control the composition, structure, and thickness of thin films at the atomic scale, and is widely used in the fields of electronic displays, microelectronics, catalysis, coatings, and energy storage and conversion. ALD of metal oxide thin films can be completed using metal alkoxides as the oxygen source, which is similar to the non-hydrolytic sol-gel (NHSG) technique. Density functional theory calculations show that metal alkoxides, such as Al(OiPr)3 and Al(OEt)3, can directly form M-O bonds through strong chemisorption on the surface. Meanwhile, alkyl groups can be eliminated through the formation of alkyl halides and alkenes, which can be catalyzed by interfacial interactions between alkyl groups and the surface. Such noncovalent catalysis resulting from interfacial interaction can be termed as interfacial catalysis. This can be characterized by the difference between the interfacial interaction energies of the transition state and the corresponding intermediate based on natural bond analysis. We expect that such interfacial catalysis can be used in precursor designs, improvement of ALD of oxides and as a new characterization method for other interfacial catalysis and noncovalent catalysis processes.

Sol–gel encapsulated lithium polysulfide catholyte and its application in lithium–sulfur batteries

Careful control of sol–gel chemistry allows for the encapsulation of water-reactive lithium polysulfides

Low Temperature Synthesis and Characterization of AlScMo₃O12.

Recent interest in low and negative thermal expansion materials has led to significant research on compounds that exhibit this property, much of which has targeted the A2M3O12 family (A = trivalent cation, M = Mo, W). The expansion and phase transition behavior in this family can be tuned through the choice of the metals incorporated into the structure. An undesired phase transition to a monoclinic structure with large positive expansion can be suppressed in some solid solutions by substituting the A-site by a mixture of two cations. One such material, AlScMo3O12, was successfully synthesized using non-hydrolytic sol-gel chemistry. Depending on the reaction conditions, phase separation into Al2Mo3O12 and Sc2Mo3O12 or single-phase AlScMo3O12 could be obtained. Optimized conditions for the reproducible synthesis of stoichiometric, homogeneous AlScMo3O12 were established. High resolution synchrotron diffraction experiments were carried out to confirm whether samples were homogeneous and to estimate the Al:Sc ratio through Rietveld refinement and Vegard’s law. Single-phase samples were found to adopt the orthorhombic Sc2W3O12 structure at 100 to 460 K. In contrast to all previously-reported A2M3O12 compositions, AlScMo3O12 exhibited positive thermal expansion along all unit cell axes instead of contraction along one or two axes, with expansion coefficients (200–460 K) of αa = 1.7 × 10−6 K−1, αb = 6.2 × 10−6 K−1, αc = 2.9 × 10−6 K−1 and αV = 10.8 × 10−6 K−1, respectively.

Phase selective synthesis of copper sulfides by non-hydrolytic sol–gel methods

Copper sulfide nanoparticles have many potential applications, however, difficulties with controlled synthesis of specific polymorphs, and long term stability of phases with desirable properties, have hampered their implementation. In this work, non-hydrolytic sol–gel chemistry was used for precisely controlled synthesis of five different CuxS polymorphs, namely β-chalcocite, α-chalcocite, djurleite, low digenite and covellite. All polymorphs could be obtained as nanoparticles by adjusting reaction conditions. Notably, α-chalcocite nanoparticles were considerably more stable than any previously reported. In addition, continuous phase evolutions between several sets of copper sulfide polymorphs, and intermediate structures between certain structures, were observed. A route to synthesize digenite with an unusual stoichiometry by grinding β-chalcocite, α-chalcocite or djurleite was also discovered.

Low temperature crystallisation of mesoporous TiO2

Conducting mesoporous TiO2 is rapidly gaining importance for green energy applications. To optimise performance, its porosity and crystallinity must be carefully fine-tuned. To this end, we have performed a detailed study on the temperature dependence of TiO2 crystallisation in mesoporous films. Crystal nucleation and growth of initially amorphous TiO2 derived by hydrolytic sol-gel chemistry is compared to the evolution of crystallinity from nanocrystalline building blocks obtained from non-hydrolytic sol-gel chemistry, and mixtures thereof. Our study addresses the question whether the critical temperature for crystal growth can be lowered by the addition of crystalline nucleation seeds.

Facile Synthesis of Troilite

Low-temperature synthetic pathways can result in crystallization of metastable materials. These methods have been widely explored for the preparation of metal oxides. Adaptation of nonhydrolytic sol-gel chemistry to non-oxide systems offers an elegant route to transition-metal sulfides. The method can be exploited for the facile and reproducible synthesis of iron sulfide crystallizing in the troilite structure. This phase is only found in meteorites and planets and has previously been obtained by high-temperature or high-energy ball-milling methods. "Nonhydrolytic" sol-gel processing results in direct crystallization of troilite with no need for further calcination.

Polymorphism in yttrium molybdate Y 2Mo 3O 12

Yttrium molybdate (Y 2Mo 3O 12) has been prepared by non-hydrolytic sol–gel chemistry. The phase evolution upon heating was investigated using in situ and ex situ heat treatments combined with powder X-ray diffraction. This method has led to the isolation of two orthorhombic phases with different atomic connectivity. Yttrium adopts 6- and 7-coordinate sites in the Pbcn and Pba2 structures, respectively. Cocrystallization of both phases was observed in a narrow temperature range, suggesting that crystallization kinetics play a major role in phase formation. It was found that the Pba2 phase is the stable polymorph below 550 °C, and converts to Pbcn at higher temperatures.

Non-hydrolytic sol–gel synthesis, properties, and high-pressure behavior of gallium molybdate

A new material, Ga2Mo3O12, was prepared for the first time using non-hydrolytic sol–gel chemistry. Rietveld analysis of variable temperature X-ray diffraction data showed that this compound crystallizes in the monoclinic Sc2W3O12 structure, and that it displays positive thermal expansion up to its decomposition temperature of 650 °C. No phase transitions were observed in this temperature range. The high-pressure behavior was investigated with an in situ synchrotron diffraction study. Reversible phase transitions to new monoclinic structures occurred at approximately 3.2 and 4.1 GPa. In addition, irreversible amorphization was observed above 8 GPa.

ChemInform Abstract: A New Polymorph of ZrW2O8 Prepared Using Nonhydrolytic Sol—Gel Chemistry.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A New Polymorph of ZrW2O8 Prepared Using Nonhydrolytic Sol−Gel Chemistry

A new polymorph of ZrW{sub 2}O{sub 8}, structurally related to trigonal ZrMo{sub 2}O{sub 8}, has ben prepared at low temperatures using nonhydrolytic sol-gel chemistry. Its identity was established by powder C-ray diffraction, energy dispersive X-ray analysis, and extended X-ray absorption fine structure (EXAFS). This material is denser than the previously reported cubic and orthorhombic forms of ZrW{sub 2}O{sub 8} and decomposes into the binary oxides on prolonged heating at 700 C. An EXAFS investigation of the crystallization of ZrW{sub 2}O{sub 8} gels, prepared by nonhydrolytic methods, suggests that the coordination environments of the cations in the heat-treated gels evolve toward those found in the new form of ZrW{sub 2}O{sub 8} prior to the crystallization of this product.