Environmentally Benign Oxidation of Alkylphenols to p-Benzoquinones: A Comparative Study of Various Ti-Containing Catalysts

Catalytic properties of Ti-containing porous solids were compared in the oxidation of 2,3,6-trimethylphenol (TMP) with H₂O₂ to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate). Mesoporous titanium–silicates with di(oligo)nuclear Ti centers, metal–organic framework MIL-125 and amorphous TiO₂ demonstrated 100 % selectivity toward TMBQ. Titanium–silicates prepared by evaporation-induced self-assembly revealed superior performance in terms of product yield and catalyst reusability.     

How to reach 100% selectivity in H2O2-based oxidation of 2,3,6-trimethylphenol to trimethyl-p-benzoquinone over Ti,Si-catalysts

A comprehensive study of structure/activity/selectivity relationships and mechanistic aspects of the oxidation of 2,3,6-trimethylphenol (TMP) with aqueous H₂O₂ over a wide variety of titanium-silicate catalysts allowed us to infer requirements to an optimal catalyst and optimal reaction conditions for this reaction and to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, Vitamin E precursor) with nearly 100% selectivity at 100% substrate conversion. The main by-products in the TMP oxidation are C–C and C–O dimers, formed by coupling of intermediate phenoxyl radicals. The formation of TMBQ is favoured by (1) a poor coordinating solvent (MeCN), (2) elevated temperature (80 °C), (3) low TMP concentration (not higher than 0.1 M), (4) high H₂O₂/TMP molar ratio (ca. 3.5), and (5) low TMP/Ti ratio (<10–20). The crucial factors which determine the selectivity of Ti,Si-catalysts in TMP oxidation to TMBQ are mesoporosity and an optimal surface concentration (ca. 0.7–1.0 Ti atoms/nm²) of accessible highly dispersed, probably, dimeric Ti(IV) species. The catalysts prepared by a simple, affordable and cheap synthesis methodology via grafting titanium(IV) precursors onto the surface of commercial mesoporous silica completely fulfil these requirements and thus can be viewed as promising catalysts for environmentally benign TMBQ production.     

C3S and C2S hydration in the presence of Na2CO3 and Na2SO4

This study analyses the behavior of calcium silicates C₃S and C₂S hydrated in two alkaline media, Na₂CO₃ and Na₂SO₄. The silicates were synthesized with laboratory reagents and hydrated in water, to which solid‐state alkaline activators with 4 wt% Na₂CO₃ or 4 wt% Na₂SO₄ were added. Two‐ and 28‐day mechanical strength values were determined and the reaction products were characterized with XRD, SEM/EDX, and ²⁹Si and ²³Na MAS NMR. The findings showed that the presence of Na₂CO₃ hastened hydration kinetics and stimulated early‐age mechanical strength development in both silicates. The most significant effect of sodium sulfate, however, was observed in the 28‐day material in both silicates, in which it raised strength by stimulating the precipitation of C–S–H gels with a high percentage of Q² units.     

Photocatalytic activity of titania modified mesoporous silica for pollution control

A series of titania modified mesoporous silicates with variable Si/Ti ratios were prepared using titanium tetrabutoxide by impregnation method. The samples were characterized by different analytical techniques such as XRD, TEM, FT-IR, low temperature N₂ sorption, and UV–Vis diffused reflectance spectroscopy. The TiO₂ supported on mesoporous silica was found to be in the anatase form. Crystallite size calculated using Scherrer’s formula was found to be in the range 18–20 nm. However, the TiO₂ particle diameter estimated by TEM was 5 nm. UV–Vis spectra showed a blue-shift of the absorption edge for all the samples. The BET surface area decreased with TiO₂ loading. Photodegradation of basic dye-like methylene blue (MB) was studied on these titania modified mesoporous silica using UV irradiation. Volatile organic pollutants like phenol and toluene can also be photocatalytically degraded using these titania modified mesoporous silicates.     

Activation of lipase by sol–gel coating with hydrophobic alkyl‐substituted silicates in supercritical carbon dioxide

BACKGROUND: Sol–gel entrapment of lipases is usually performed in an aqueous solution. A novel method of sol–gel coating of lipase in supercritical carbon dioxide (SC‐CO₂) is proposed. RESULTS: Crude lipase powder (Rhizopus oryzae) coated with hydrophobic silicates, derived from dimethyldimethoxysilane and tetramethoxysilane in SC‐CO₂ at 35 °C and 15 MPa, exhibited 5–7 times higher esterification activity than that prepared via an aqueous sol–gel route. Lipase immobilized in a methyl‐substituted silica monolith was also highly activated by sol–gel coating using the same silica precursors in SC‐CO₂. CONCLUSION: Sol– gel coating in SC‐CO₂, of lipases in powder and immobilized forms with hydrophobic alkyl‐substituted silicates provides an efficient tool for the enhancement of enzymatic activities in non‐aqueous media. Copyright © 2009 Society of Chemical Industry     

Influence of Yb:Hf Ratio on Ytterbium Hafnate/Molten Silicate (CMAS) Reactivity

This study investigated the influence of YbO_1.5 concentration on the reactivity between YbO_1.5–HfO₂ thermal barrier coating (TBC) materials and silicate melts [calcium–magnesium aluminosilicate (CMAS), specifically 33CaO–9MgO–13AlO_1.5–45SiO₂, all in mol%]. Three thermal barrier oxides (TBO) with YbO_1.5 concentration between 30 and 80 mol% were tested at 1300°C and 1500°C. Porous pellets were used to study reaction layer growth and the behavior within infiltrated porosity. Complementary experiments examined the phase equilibria in melts containing varying quantities of the candidate TBOs. The effectiveness of reactive crystallization to limit interaction depth increased with the YbO_1.5 concentration in the TBO. The results indicate that the TBO must contain sufficient YbO_1.5 to satisfy the equilibrium between the melt and HfO₂‐based fluorite phase before YbO_1.5‐bearing silicates can precipitate. Increasing the YbO_1.5 availability leads to the crystallization of apatite, garnet, silicocarnotite, and/or cuspidine (alumino)silicates. Apatite was observed at 1300°C and 1500°C, garnet and silicocarnotite only appeared at 1300°C, and cuspidine was only evident at 1500°C. The implications for the design of CMAS‐resistant coatings are discussed in the context of the experimental results.     

The deactivation of Co/SiO2 catalyst for Fischer–Tropsch synthesis at different ratios of H2 to CO

The deactivation of Co/SiO₂ catalyst for Fischer–Tropsch synthesis (FTS) at different H₂ / CO ratios was investigated by XRD, FTIR, BET, XPS, TPR and H₂ chemisorption. It was found that the deactivation rate of the catalyst increased with the rise of the H₂ / CO ratio. The generation of silicates and/or hydrosilicates species was evidenced by TPR and XPS, and their amounts were monotonously enhanced with increasing H₂ / CO ratio, which suggested that the deactivation was caused by the transformation of metallic cobalt into inactive silicates and the high partial pressure of H₂ facilitated the formation of the silicates. Moreover, the percentage loss of the surface cobalt was larger than that of bulk cobalt, suggesting that the cobalt silicates and/or hydrosilicates species were formed mainly on the surface of the catalyst or in the small crystallites. For the catalyst run at H₂ / CO ratio of 1, it was observed that the sintering also contributed to the catalyst deactivation, but it was a less important factor for the deactivation.     

Experimental phase equilibria studies of the PbO–SiO2 system

Phase equilibria of the PbO–SiO₂ system have been established for a wide range of compositions: (i) liquid in equilibrium with silica polymorphs (quartz, tridymite, and cristobalite) between 740°C and 1580°C, at 60‐90 mol% SiO₂; (ii) with lead silicates (PbSiO₃, Pb₂SiO₄, and Pb₁₁Si₃O₁₇) and lead oxide (PbO) between 700°C and 810°C. A high‐temperature equilibration/quenching/electron probe X‐ray microanalysis (EPMA) technique has been used to accurately determine the compositions of the phases in equilibrium in the system. Significantly, no liquid immiscibility has been found in the high‐silica range, and the liquidus in this high‐silica region has been accurately measured. The phase equilibria information in the PbO–SiO₂ system is of practical importance for the improvement of the existing thermodynamic database of lead‐containing slag systems (Pb–Zn–Fe–Cu–Si–Ca–Al–Mg–O).     

HYDROTHERMAL SYNTHESIS AND ION EXCHANGE PROPERTIES OF THE NOVEL FRAMEWORK SODIUM AND POTASSIUM NIOBIUM SILICATES

ABSTRACT

Two novel metastable sodium niobium silicates of the empirical formula: Na_l+x?yH_y(Nb_1?xSi_x)O₃ nH₂O, where x=0.33?0.38, y<l+x, n=0,7-l.l (NbSi-Na, 6.0 Å phase), and Na_3-x H_xNb₃Si₂O₁₃ nH₂O, where x<1.5, n=2.5?3.5 (NbSi-Na, 12.6 Å phase), and two novel potassium niobium silicates: K_4?xH_xNb₄SijO₂₂nH₂O, where x<l, n=3.5-4.0 (NbSi-K., 10.0 Å phase), and K_1?xH_xNbSi₄O₁₁nH₂O, where x<0.2, n=0.4-0.5 (NbSi-K, 6.05 Å phase), were synthesized in the homogeneous alkaline reaction system NbCl₅ - SiO₂ - NaOH (KOH) -H₂O₂ - H₂O under mild hydrothermal conditions. The compounds were characterized by elemental analysis, FTIR, TGA, MAS ²⁹Si NMR and X-ray diffraction. It was found that alkali metal niobium silicates have open framework structures. Their ion exchange affinity towards alkali, alkaline earth and some transition metal ions was studied. All alkali metal niobium silicates are moderately acidic ion exchangers. Both sodium niobium silicates show a distinct affinity for Cs⁺ ion among alkali metal ions, whereas potassium niobium silicate, the NbSi-K, 10.0 Å phase, exhibits affinity for Rb⁺ ion. The affinity of the sodium niobium silicate, NbSi-Na, 6.0 Å, toward strontium ion in neutral solutions is equal or superior to the best Sr-selective inorganic ion exchangers. The sodium niobium silicate (NbSi-Na, 12.6 Å phase) exhibits extremely high affinity for Pb²⁺ ion in acidic and neutral media, and both sodium niobium silicates also show a moderate affinity for Hg²⁺ ion in neutral and highly alkaline media. These exchangers could be promising for the treatment of some specific nuclear waste and contaminated environmental and biological liquors containing lead, mercury and radioactive strontium.     

Activity of samarocene catalysts adsorbed on mesoporous silicates for the polymerization of methyl methacrylate

A samarocene complex, (C₅Me₅)₂SmMe(thf), was adsorbed on a series of mesoporous silicates of various pore sizes. Pre‐treatment of the latter with AlMe₃ before adding the complex was effective in deactivating the surface silanol functionalities. The silicates having relatively larger pore size tended to adsorb a larger amount of the complex. The polymerization of methyl methacrylate (MMA) by the complex adsorbed on the silicates with large pore sizes (>29 Å) quantitatively afforded highly syndiotactic poly(MMA)s with higher molecular weights compared with those obtained by the corresponding homogeneous system. Similar catalyst systems of smaller pore size were much less active. Copyright © 2004 Society of Chemical Industry     

Degradation mechanism of high alumina refractory bricks by reaction with deposits in a rotary kiln for fluxed iron ore pellets production

The formed deposits wear out of refractory wall linings in the rotary kiln and may cause production disturbances. This study describes the chemical composition and mineralogical phase components at the deposit/refractory interface in the rotary kiln for fluxed iron ore pellets production. The main phases of refractory bricks are corundum and mullite, while the deposits mainly contain hematite and silicates. The main phases in the deposit/refractory brick contact zone are hematite, anorthite (CaAl₂Si₂O₈), mullite, corundum, and silicates. Moreover, the hematite phases in the deposit/brick interface averagely contain 6.98 wt% Al and 1.38 wt% Ti. The silicates in the contact zone contain higher aluminium content and lower iron content than the silicates in the deposits. Finally, the thermodynamic analysis indicates that the main phases in the deposits can react with the refractory to form Al₂Fe₂O₆, CaAl₂Si₂O₈, feldspar, and liquid phases lead to the degradation of bricks in the kiln during the iron ore pellets production.     

Synthesis and characterization of ytterbium oxide: A novel CMAS-resistant environmental barrier coating material

Demand for more powerful aircraft promotes development of ceramic matrix composites and environmental barrier coating (EBC). A promising EBC material, ytterbium oxide (Yb₂O₃), was fabricated by hot pressing, and its properties were systemically investigated. The evaluation of thermal properties provides a baseline for the application of Yb₂O₃ on SiC_f/SiC or Al₂O_3f/Al₂O₃ composites. The performance in water vapor and molten calcium–magnesium–aluminosilicate (CMAS) environments indicates its excellent durability in harsh environment. Compared with rare-earth silicates, the thermochemical interactions between ytterbium oxide and CMAS changed greatly with the absence of silicon oxide. Reactions of ytterbium oxide with CMAS form several reaction products, including apatite, garnet, and silicocarnotite. The crystallization of garnet and silicocarnotite could effectively consume and solidify the CMAS melt, which prevents the melt infiltration and mitigates the further corrosion.     

The Peroxysilicate Question. 29Si-NMR Evidence for the Role of Silicates in Alkaline Peroxide Brightening of Mechanical Pulp

The influence of hydrogen peroxide on the chemistry of aqueous silicates was investigated by high-resolution ²⁹Si-NMR and electrochemical methods. The observations indicate that dissolved silicates form labile complexes with radical H₂O₂-decomposition products, possibly O₂·^?. An important role of soluble silicates in peroxide bleaching liquors thus might be to affect the activity of free radical species which mediate H₂O₂ bleaching and decomposition reactions. Attempts to prepare the widely reported peroxysilicates from silicate/H₂O₂/NaOH mixtures resulted in the isolation of Na₂O₂·8H₂O.     

Use of waste water glass as silica supplier in synthesis of pure and Mg-doped lanthanum silicate powders for IT-SOFC application

Water glass in alkali solution (Na₂SiO₃/NaOH) an abundant effluent, generated in the alkaline fusion of zircon sand, represents a potential silica source to be converted in useful silica technological application. Actually, the generation of energy by environmental-friendly method is one of the major challenges for researchers. Solid Oxide Fuel Cells (SOFC) is efficient and environmentally clean technique to energy production, since it converts chemical energy into electrical power, directly. Apatite-type lanthanum silicates are promising materials for application as an electrolyte in intermediate temperature SOFC (IT-SOFC) because of their higher ionic conductivity, in temperatures of range 600–700 °C, than conventional zirconia electrolytes. In this work, pure (La_9,56(SiO₄)₆O_2,34) and Mg-doped (La_9,8Si_5,7Mg_0,3O_26,4) lanthanum silicate were synthesized, from that rich effluent. Using the sol-gel followed by precipitation method, the single crystalline apatite phase of both silicates was obtained by thermal treatment at 900 °C of their precursors. Sintered ceramic samples reached density of higher than 90%.     

From C–S–H to C–A–S–H: Experimental study and thermodynamic modelling

It has long been known that the stoichiometry of C–S–H varies with the calcium hydroxide concentration in solution. However, this issue is still far from understood. We revisit it in both experimental and modelling aspects. A careful analysis of the solubility confirms the existence of three different C–S–H phases, defined as Ca₄H₄Si₅O₁₆, Ca₂H₂Si₂O₇ and Ca₆(HSi₂O₇)₂(OH)₂, respectively. The variation of the Ca/Si ratio of the three phases has been described by surface reactions: the increase of the Si content is accounted for by silicate bridging, the increase of calcium content and the surface charge are accounted for by reactions involving silanol groups via deprotonation and complexation with calcium. In the presence of Al in solution, the uptake of Al by C–S–H is experimentally observed. The Al content increases with Al concentration. C–A–S–H formation is modelled by the competition between silicate and aluminate tetrahedra for the bridging of the dimeric silicates in C–S–H.     

Novel synthesis and applications of yttrium silicates from a silicone resin containing oxide nano-particle fillers

In this paper, a novel method for the synthesis of yttrium silicates is presented. Such silicates are well known to be promising materials for protecting various substrates against high temperature oxidation, but they can typically be produced only after quite complicated processing. The use of preceramic polymers, in which a silicate article is obtained by direct thermal treatment in air of nano-composites consisting of silicone resins containing suitable oxide nano-particles, is a valid alternative, since the desired phases, i.e. Y-monosilicate (Y₂O₃·SiO₂) and Y-disilicate (Y₂O₃·2SiO₂) can be obtained by treatments at low temperature (1000–1400 °C). Y-disilicate could be employed for the manufacturing of dense and thick coatings on SiC foamed substrate, by simply dipping the substrates into silicone suspensions, before ceramic conversion. Y-monosilicate, that could be also useful for coatings, was found to exhibit promising characteristics, when doped with Eu₂O₃, making it of interest for application as red phosphor for LEDs.     

Phase, microstructure, and oxidation resistance of yttrium silicates coatings prepared by a hydrothermal electrophoretic deposition process for C/C composites

Oxidation-resistant yttrium silicates coatings for SiC precoated carbon/carbon composites were prepared by a novel hydrothermal electrophoretic deposition process. Sonochemical-synthesized yttrium silicates nanocrystallites, isopropanol, and iodine were respectively used as source materials, solvent, and charging agent during the deposition. Phase compositions, surface and cross-section microstructures of the as-prepared multilayer coatings were characterized by an X-ray diffractometer (XRD) and a scanning electron microscopy (SEM). The influence of deposition temperatures on the phase, microstructure, and oxidation resistance of the multilayer coated C/C composites was particularly investigated. Results show that the as-prepared outer coatings are composed of yttrium silicates crystallites with a main phase of Y₂Si₂O₇ and Y₂SiO₅. The thickness and density of the yttrium silicates coatings are improved with the increase of deposition temperature. Compared with SiC coating prepared by pack cementation, the multilayer coatings prepared by pack cementation with a later hydrothermal electrophoretic deposition process exhibit better antioxidation properties. The as-prepared multilayer coatings can effectively protect C/C composites from oxidation at 1773 K in air for 35 h with a weight loss of 0.32 × 10⁻³ g/cm².     

Substitutional reaction in Si–O network of molecular dynamics-modeled liquid Na2SiO3: Microscopic and statistical study

The study of the bond breaking and formation processes, that is, the chemical reaction, in the Si–O network structure in liquid alkali silicates at temperatures around or higher than the glass-transition temperature is important for understanding kinetic processes such as the structural relaxation of the network, viscous flow, and diffusion of the network former ions. Herein, novel methods for analyzing the reactions in a molecular-dynamics-modeled liquid Na₂SiO₃ were used to confirm the following results: (a) the substitutional reactions (in which a nonbridging O ion of a Si–O chain or a SiO₄ tetrahedron attacks the Si ion of another chain from backside of a bridging O ion, which acts as the leaving group, and the bridging O leaves the Si ion) primarily occur in the Si–O network of liquid Na₂SiO₃; and (b) The abundance ratio of Q_n species can be quantitatively reproduced by the reaction rate.     

Rare-earth silicates

Binary systems of the type Ln₂O₃SiO₂ were studied. Three types of compounds were established for them: Ln₂O, SiO₄ (1:1) diorthosilicate, Ln₂Si₂O₇ (1:2), and the apatite-like silicate, Ln_4.67O [SiO₄]₃ (7:9), the latter being stable for the large cations. Systematization of the physical-chemical properties of rare-earth silicates was made. Based on the model of ideal solutions the liquidus curves were calculated for several Ln₂O₃  Ln₂O₃SiO₂ systems. The solid solution limits were determined for complex systems formed by rare-earth and alkaline-earth silicates. A series of rare-earth single crystals of simple and complex composition were grown by crystallization from molten solutions. Classification of the same type compounds into structural groups (morphotropic series) was made and partecipation of the 4f-orbitals in bondings was considered. The spectral-luminiscence characteristics of the rare-earth silicates are given.     

2ZrO2·Y2O3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS: Part II,Interactions with Sand and Fly Ash

Based on the application of OB considerations (Part I) to various major thermal barrier coating (TBC) compositions and two types of important calcium–magnesium–alumino–silicates (CMAS)—desert sand and fly ash—the 2ZrO₂·Y₂O₃ solid solution (ss) TBC composition, with high CMAS‐resistance potential, is chosen for studying molten‐CMAS/TBC interactions. It is demonstrated that 2ZrO₂·Y₂O₃(ss) air plasma sprayed (APS) TBCs are highly resistant to high‐temperature attack by both sand‐CMAS and fly‐ash‐CMAS. Despite the differences in the compositions of the two CMASs, the overall CMAS‐attack mitigation mechanisms in both cases appear to be similar, viz reaction between 2ZrO₂·Y₂O₃(ss) APS TBC and the CMAS, and the formation of main reaction products of Y‐depleted c‐ZrO₂ and nonstoichiometric Ca–Y apatite. Large differences in the OBs (ΔΛ) between the 2ZrO₂·Y₂O₃(ss) and the CMASs are good predictors of ready reaction between this TBC and these CMASs. While the details of the CMAS‐mitigation mechanisms can depend critically on various other aspects, the OB difference (ΔΛ) calculations could be used for the initial screening of CMAS‐resistant TBC compositions.