Silicate Precursor Research Articles

Tuning Zeolite Precursor Interactions by Switching the Valence of Polyamine Modifiers.

Nonclassical mechanisms of crystal growth often involve the formation of amorphous precursors that play a direct role in what is generally referred to as crystallization by particle attachment (or CPA). One of the most studied CPA systems in the literature is zeolite MFI, which is a microporous crystal with siliceous (silicalite-1) and aluminosilicate (ZSM-5) isostructures. The self-assembly, microstructural evolution, and mechanistic role of nanoparticle precursors (1-6 nm) during silicalite-1 crystallization have been the subjects of prior investigation by combined experimental and modeling techniques. Here we investigate for the first time the effects of zeolite growth modifiers (ZGMs) on MFI precursors. ZGMs are organic molecules that alter the anisotropic rate(s) of crystal growth as a means of tailoring crystal size and/or habit. We show that most ZGMs have little effect on precursor assembly and evolution during the prenucleation stages of silicalite-1 and ZSM-5 synthesis; however, studies at varying alkalinity reveal that pH can be used as a "switch" to tune ZGM speciation and concurrently the colloidal stability of precursors. This has been proven effective for various polyamine compounds, such as spermine, that exhibit divalent (positive) charge near negatively charged nanoparticle surfaces. Our finding is consistent with colloidal models that predict a higher concentration of divalent modifiers within the diffuse double layer surrounding the surfaces of (alumino)silicate precursors. Multivalent polyamines seemingly promote precursor-precursor aggregation at elevated temperature, which is consistent with a proposed hypothesis that modifiers with two or more sufficiently spaced cationic functional moieties are capable of bridging neighboring precursor surfaces, thus overcoming an electrostatic repulsive force that contributes to their colloidal stability. Given the importance of precursor-precursor and precursor-crystal interactions in zeolite nucleation and growth, respectively, our observations provide additional insight into the role of organics in zeolite crystallization.

Eu(2+)-Activated Phase-Pure Oxonitridosilicate Phosphor in a Ba-Si-O-N System via Facile Silicate-Assisted Routes Designed by First-Principles Thermodynamic Simulation.

Eu(2+)-activated single phase Ba(2+)-oxonitridosilicate phosphors were prepared under a mild synthetic condition via silicate precursors, and their luminescent properties were investigated. Both the preferred oxonitridosilicate formation as for the available host compounds and thermodynamic stability within the Ba-Si-O-N system were elucidated in detail by the theoretical simulation based on the first-principles density functional theory. Those results can visualize the optimum synthetic conditions for Eu(2+)-activated highly luminescent Ba(2+)-oxonitridosilicates, especially Ba3Si6O12N2, as promising conversion phosphors for white LEDs, including Ba3Si6O9N4 and BaSi2O2N2 phases. To prove the simulated design rule, we synthesized the Ba3Si6O12N2:Eu(2+) phosphor using various silicate precursors, Ba2Si4O10, Ba2Si3O8, and BaSiO3, in a carbothermal reduction ambient and finally succeeded in obtaining a phase of pure highly luminescent oxonitridosilicate phosphor without using any solid-state nitride addition and/or high pressure synthetic procedures. Our study provides useful guidelines for robust synthetic procedures for developing thermally stable rare-earth-ion activated oxonitridosilicate phosphors and an established simulation method that can be effectively applied to other multigas systems.

Synthesis of Novel Titanosilicate Catalysts by Simultaneous Isomorphous Substitution and Interlayer Expansion of Zeolitic Layered Silicates

A facile and cheap method for the postsynthesis of large-pore titanosilicates from layered zeolitic silicate precursors is presented, in which isomorphous substitution of Ti for Si and interlayer expansion with mobile Si debris are realized simultaneously using aqueous H2TiF6 solution at room temperature. This versatile and convenient one-pot postsynthesis was used to construct several different 3D interlayer-expanded zeolite (IEZ) structures with tetrahedrally coordinated Ti incorporated into the framework without the use of external silane. The structural transformation and the incorporation of Ti in the preparation of IEZ-Ti-PLS-3 were monitored using Rietveld refinement and NMR studies. Furthermore, IEZ-Ti-PLS-3 exhibited unique catalytic properties in the epoxidation of alkenes with hydrogen peroxide, and was active not only for linear alkenes but also for bulky cyclic alkenes.

Nucleation of FAU and LTA Zeolites from Heterogeneous Aluminosilicate Precursors

The nucleation of many natural, biogenic, and synthetic crystals involves the initial formation of metastable precursors that provides a kinetic pathway for an amorphous-to-crystalline transformation. This nonclassical mechanism is believed to be the dominant crystallization pathway for microporous zeolites. Despite significant research on zeolite growth mechanisms, molecular level details regarding the assembly, physicochemical properties, and structural evolution of amorphous (alumino)silicate precursors remain elusive. Here we use a combination of diffraction, scattering, and microscopy techniques to characterize the amorphous precursors that assemble and evolve during the synthesis of zeolites FAU and LTA – two materials that are widely used in commercial applications such as catalysis, adsorption, separations, and ion-exchange. Nucleation occurs by a two-step mechanism involving the initial formation of aggregates that serve as heterogeneous sites for nucleation. Using colloidal silica as a reagent, ...

The formation conditions of the early Ediacaran cherts, South China

During the early Ediacaran, abundant cherts (nodules and bands) were discontinuously hosted by dolostones and mudstones of the Doushantuo Formation (635–551Ma) in the Yangtze Three Gorges area, South China. The appearance of these cherts correlates with the distribution of acanthomorphic acritarch fossils and is potentially important for the reconstruction of the biogeochemical conditions and oceanic chemistry during the early Ediacaran. However, the formation mechanisms of these cherts are still poorly understood. In this study, silicon isotopes, Ge/Si molar ratios and rare earth element (REE)+Y features are used to constrain the origin mechanism of these cherts, silica geochemical cycling and seawater redox conditions during the Early Ediacaran. Our results suggest that the studied cherts were formed by the replacement of precursor carbonate minerals, which included the dissolution of a siliceous precursor and reprecipitation of microquartz. The positive silicon isotopic composition (typically between 0.4‰ and 1.4‰) and Ge/Si molar ratios (0.04–0.95×10−6) indicate a major seawater source of Ge and Si in these cherts with minor detrital and negligible hydrothermal contamination. In the early Ediacaran Ocean, the dissolved silica may be derived from the river input with high δ30Si values. On the other hand, the output of dissolved silica was controlled by inorganic precipitation, which was subsequently fixed as microquartz in cherts during early diagenesis. During the formation of cherts, the primary REE+Y signals of seawater were preserved with little detrital influence. Meanwhile, the REE+Y patterns, obvious negative Ce anomalies and near modern seawater Y/Ho ratios indicate an oxic marine chemistry during chert formation. This oxic seawater environment may have promoted the observed diversity of Ediacaran biological systems. Finally, temporal variations related to silica sources and silicon isotopic fractionation controls in the cherts and/or ocean were also evaluated.

Design of Microporous Material HUS-10 with Tunable Hydrophilicity, Molecular Sieving, and CO2 Adsorption Ability Derived from Interlayer Silylation of Layered Silicate HUS-2.

The attractive properties of zeolites, which make them suitable for numerous applications for the energy and chemical industries and for life sciences, are derived from their crystalline framework structures. Herein, we describe the rational synthesis of a microporous material, HUS-10, utilizing a layered silicate precursor, HUS-2, as a structural building unit. For the ordered micropores to be formed, interlayer pillars that supported the original silicate layer of HUS-2 were immobilized through the interlayer silylation of silanol groups with trichloromethylsilane and a subsequent dehydration-condensation reaction of the hydroxyl groups on the preintroduced tetrahedral units. An actual molecular sieving ability, enabling the adsorption of molecules smaller than ethane, was confirmed in the ordered micropores of HUS-10. The hydrophilic adsorption could also be controlled by changing the number of methyl and hydroxyl groups in the immobilized interlayer pillars. In addition, when the adsorption behaviors of CO2, CH4, and N2 on HUS-10 were compared to those on siliceous MFI and CDO zeolites with approximately the same pore diameter, the CO2 adsorption capacity of HUS-10 was comparable. Conversely, because of the adsorption inhibition of CH4 and N2, HUS-10 exhibited larger CO2/CH4 and CO2/N2 adsorption ratios relative to those of MFI and CDO zeolites. These results reveal that the unique microporous framework structure presented by the rational structural design using the layered silicate precursor HUS-2 has the potential to separate CO2 from gas mixtures.

Interlayer expansion using metal-linker units: Crystalline microporous silicate zeolites with metal centers on specific framework sites

Interlayer expansion using silylating agents to connect layer silicates to 3D framework structures has shown to be a versatile synthesis route to new crystalline, microporous frameworks. We demonstrate here that also Me cations can be introduced on the linker sites applying the same synthesis procedure. An acidic aqueous Fe-chloride solution was used in a hydrothermal reaction to convert the layered hydrous silicate precursor RUB-36 into an interlayer expanded zeolite, containing Fe at the linker sites, Fe-IEZ-RUB-36, Si19.14Fe0.86O38(OH)4. Structure analysis from powder X-ray data using the Rietveld technique confirmed that the porous framework is stable upon calcination and contains Fe on T-sites at the linker position. SEM–EDX analysis is in agreement with the analysis of the electron density maps showing that almost every other linker T-position is occupied by Fe-ions. The material crystallizes in the monoclinic space group Pm with a = 12.200(9) Å, b = 13.981(8) Å, c = 7.369(2) Å, and β = 106.9(1)°. Applying a similar synthesis procedure, the Sn-analog, Sn-IEZ-RUB-36, Si38.6Sn1.4O76(OH)8, has been obtained and structurally characterized. Despite its limited crystallinity, Rietveld analysis of the PXRD data set confirmed the materials framework topology and chemical composition (a = 23.856(14) Å, b = 14.103(7) Å, c = 7.412(7) Å, in SG Pnm21). We conclude, that the synthesis procedure is flexible and, meanwhile, has been extended to other metal cations such as Ti, Zn, Eu and Al leading to microporous materials with potentially active metal cations on well defined sites of the silicate framework.

High-performance CO2 capture on amine-functionalized hierarchically porous silica nanoparticles prepared by a simple template-free method

The adsorption of CO2 on polyethyleneimine (PEI)-functionalized hierarchically porous silica nanoparticles (PSNs), prepared by using rice husk as a silica source via a simple template-free method, was reported in this study. Compared with traditional alkaline fusion and surfactant-templating methods for preparing waste-derived porous silica materials as CO2 adsorbents, this method holds specific important advantages in being an inexpensive, and energy-saving process with faster production rate. The results revealed that the (NH4)2SiF6 salt formed during the synthetic process served as an effective porogen, which can be readily removed by washing with water. Additionally, the total pore volumes of PSNs materials were strongly correlated to the amount of (NH4)2SiF6. When evaluated as a support of PEI for CO2 adsorption, 55PEI/PSNs(12/14) could reach 159 mg/g at 75 °C under 15 % CO2, which was remarkably superior to those using waste silicate precursors reported in the previous literature. It was demonstrated that both PEI loading, and total pore volume of the PEI/silica composite sorbents, played key roles on CO2 adsorption. Besides, 55PEI/PSNs(12/14) also showed high stability during 20 cycles of adsorption–desorption operation, implying its high potential in post-combustion CO2 capture.

Fabrication of paraffin@SiO2 shape-stabilized composite phase change material via chemical precipitation method for building energy conservation

A novel paraffin@SiO2 shape-stabilized composite phase change material (ss-CPCM) was fabricated by chemical precipitation process using sodium silicate precursor. Various techniques were used to characterize the as-prepared paraffin@SiO2 ss-CPCM so as to investigate its structure and thermal properties, including Fourier transforms infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and thermal conductivity measurement. The SEM results show that the microstructures of the prepared ss-CPCM are some uniform submicron particles. The FTIR and XRD results showed that there was no chemical interaction between paraffin and SiO2 matrix. The DSC analysis results indicated that the paraffin@SiO2 ss-CPCM achieved high encapsulation efficiency and desirable latent heat storage capability. The thermal conductivity of the composite was also significantly improved compared with pure paraffin. According to the TGA results and the thermal cycling tests, the prepared ss-CPCMs had a good thermal stability. The prepared paraffin@SiO2 ss-CPCM with enhanced heat transfer and phase change properties holds great promise for building energy conservation owing to the low cost of raw materials and the simple synthetic technique.

A systematic for oxygen isotopic variation in meteoritic chondrules

Primitive meteorites are characteristically formed from an aggregation of sub-millimeter silicate spherules called chondrules. Chondrules are known to present large three-isotope oxygen variations, much larger than shown by any planetary body. We show here that the systematic of these oxygen isotopic variations results from open-system gas–melt exchanges during the formation of chondrules, a conclusion that has not been fully assessed up to now. We have considered Mg-rich porphyritic chondrules and have modeled the oxygen isotopic effects that would result from high-temperature interactions in the disk between precursor silicate dust and a gas enriched in SiO during the partial melting and evaporation of this dust. This formation process predicts: (i) a range of oxygen isotopic composition for bulk chondrules in agreement with that observed in Mg-rich porphyritic chondrules, and (ii) variable oxygen isotopic disequilibrium between chondrule pyroxene and olivine, which can be used as a proxy of the dust enrichment in the chondrule-forming region(s). Such enrichments are expected during shock waves that produce transient evaporation of dust concentrated in the mid-plane of the accretion disk or in the impact plumes generated during collisions between planetesimals. According to the present model, gas–melt interactions under high PSiO(gas) left strong imprints on the major petrographic, chemical and isotopic characteristics of Mg-rich porphyritic chondrules.

PEG-templated mesoporous silicas using silicate precursor and their applications in desiccant dehumidification cooling systems

Polyethylene glycol (PEG) was employed as a templating agent for the preparation of mesoporous silica materials starting from sodium silicate solutions using a sol–gel method. We show that PEG can serve as a structure-directing agent resulting in silica structures with pores spanning a range of 3.3–3.9 nm depending on the PEG concentration and the PEG removal method. The use of a PEG template increased significantly the surface area from 252 m2 g−1 without PEG to >340 m2 g−1 and >634 m2 g−1 when the PEG was removed by, respectively, calcination and solvothermal extraction. It appears that calcination at high temperature caused a densification of the pore structure resulting in a smaller surface area and pore diameter. The impregnation of mesoporous silica with CaCl2 significantly increased the adsorption capacity of water. At high humidity, the composite adsorbents containing approximately 42 wt.% CaCl2 yield a maximum adsorption capacity of 1.6 times their own weight and 4.4 times that of bare silica. The adsorption is physical in nature as viewed from the adsorption energy calculated by the Dubinin–Radushkevich (D–R) equation.

Boria Fluxing of SiC in Ceramic Matrix Composite Aeropropulsion Applications

Ceramic matrix composites (CMC’s) are being sought for high temperature applications in extreme environments. The composites are comprised of silicon carbide (SiC) fibers coated with a boron nitride (BN) layer embedded in a SiC matrix. In high temperature oxidizing conditions silicon carbide and boron nitride can react with oxygen, forming silica and boria oxidation products, respectively. The boria and silica can react to form a borosilicate glass over the SiC fiber. In the presence of water vapor the boria and silica can also react with water to form HxByOz and SixOy(OH)z gaseous derivatives, respectively. The composites can eventually degrade due to excessive borosilicate glass formation. These reactions can result in the eventual depletion of the BN interphase and in rapid recession of the SiC fibers, eventually resulting in CMC degradation due to excessive borosilicate glass formation. The effect of the boria concentration in the borosilicate glass on SiC oxidation kinetics is currently unknown since the thermally grown borosilicate glass composition is not currently understood. In this effort, model borosilicate glass coatings of defined boria concentration were synthesized and applied onto SiC substrates. These glass coatings of known boria composition were synthesized by sol-gel method using silicate and borate sol-gel precursors, which were then converted to glass by heat treatment. Glass compositions ranged from 0 – 30 weight percent boria with the remainder silica. The coatings were applied onto CVD-SiC and SiC fibers by dip-coat method prior to heat treatment. The synthesis parameters were optimized to achieve adherent glass coatings on the substrates with thicknesses less than 1 µm. Free-standing glasses were made by casting molds to investigate boria volatility in isolation from SiC oxidation. Prior to high temperature oxidation and volatilization experiments, several techniques were used to characterize the sol-derived glasses. The sol-gel synthesis parameters for maximum boria retention and reproducibility were optimized by quantifying glass composition, up to 28 weight percent, by ICP-OES. Thermogravimetric weight change measurements as a function of time, temperature, and gas environment was essential in the optimization of the gel-to-glass conversion in different gas environments, corroborated by FTIR analysis, which confirmed that Si-O-B bridges exist in the glass structure. The carbon retention from trapped organics common to sol-gel derived glasses was quantified by combustion technique. Raman analysis confirmed its presence as free carbon. Further confirmation of the amorphous glass structure lacking crystalline by-products from the synthesis was done by XRD. Imaging of the glass coating surface coverage was accomplished using SEM, during which 2-3 measurements of the coating thickness on 20-30 individual coated fiber cross-sections were recorded. Following glass coating application, high temperature oxidation experiments were conducted on bare and coated CVD-SiC and also on three bare and coated fiber grades: Sylramic SiC with in-situ grown BN, desized Sylramic SiC without BN, and desized Hi-Nicalon fiber tows. The SiC substrates were exposed to three high temperatures: 1200, 1300, and 1400°C in two different gas environments: dry and wet oxygen. Comparison of bare and glass coated CVD-SiC oxidation demonstrated that the sol-gel derived glass coatings are good models for thermally grown oxides. Initial results showed that the oxidation rate on silica coated CVD-SiC decreased slightly, but repeat experiments are being conducted to determine the uncertainty in the results. The initial fiber diameters were measured by SEM on 20-25 individual bare fibers along with 2-3 measurements of glass coatings on 20-25 coated fibers prior to oxidation experiments. Quantification of fiber diameter recession and oxide growth post oxidation experiments of bare and glass coated fibers by SEM measurement analysis demonstrated in both cases that recession was significant and measurable. Additionally, free-standing sol-gel derived borosilicate glasses of defined boria concentration were exposed to these oxidation environments to measure boria volatility from the glass. Bulk boria concentration quantified by ICP-OES showed that the boria concentration remained unchanged. On the other hand, preliminary results from XPS depth profiling of the exposed glasses showed a boron concentration gradient up to a sputter depth of ~ 50 nm, whereas oxygen and silicon concentrations remained unchanged for the entire sputter depth of ~ 300 nm. Moreover, the boron surface depletion was greater in wet oxygen than in dry oxygen. These results indicate that boria volatility is a surface reaction. Preliminary results demonstrate that the sol-gel derived glass coatings are good models for thermally grown oxides of silica and borosilicate compositions. They also demonstrate that boria is volatile from the glass surface in these oxidation environments. Ensuing results from SiC oxidation of varying boria glass coating compositions will be presented.

Synthesis of mesoporous silica with controlled pore structure from bagasse ash as a silica source

Abstract Mesoporous silica with controlled pore structure was prepared from bagasse ash as a silica source. The silica in the bagasse ash was dissolved by NaOH solution to produce sodium silicate precursor for the mesoporous silica, with or without polyethylene glycol (PEG) template. Without the PEG template, the pore structure was greatly influenced by the pH and the presence of Na+ ions. The porous silica produced in the presence of Na+ possessed higher surface area and pore diameter than those produced in the absence of Na+. The surface area and pore diameter reached approximately 525 m2 g−1 and 18 nm, respectively, when the gelation pH was 3. When the gelation pH was increased, the surface area decreased while the pore diameter increased. The use of PEG template increased significantly the surface area, which reached approximately 656 m2 g−1 when the template was removed by solvothermal extraction. Calcination at high temperature caused a densification of pore structure resulting a smaller surface area, pore diameter and pore volume. The pore diameter for all cases was greater than 4 nm, indicating that the silicas were mesoporous.

Windshield-waste-driven synthesis of hydroxy sodalite

We demonstrated the synthesis of hydroxy sodalite (HS) via a simple hydrothermal process using windshield waste as the source material. Acid-treated waste glass was directly used as the silicate precursor. The syntheses were conducted under various NaOH-precursor contents for 1, 12, and 24 h. The crystal structure and morphology of the HS were identified by X-ray diffraction and field-emission scanning electron microscopy. As per the results of the analyses, the HS and zeolite-A phases formed with low NaOH content, but HS was successfully synthesized in a single phase with a Na2O:SiO2:Al2O3 molar ratio of 2.2:1:1 over the course of 24 h. For uniform and high-purity HS crystals, the sizes of the particles of windshield waste were reduced via a high-energy milling process, and then we conducted the hydrothermal procedure under same conditions. Finally, uniformly sized HS crystals were produced with an average diameter of 3 µm. These optimized conditions were not only scientifically valuable to understand the formation process, but this process should also accelerate windshield-waste recycling.

FT-IR and 29Si-NMR for evaluating aluminium–silicate precursors for geopolymers

Geopolymers are systems of inorganic binders that can be used for sustainable, cementless concrete and are formed by alkali activation of an aluminium–silicate precursor (often secondary resources like fly ash or slag). The type of aluminium– silicate precursor and its potential variations within one batch may influence geopolymer performance. Therefore, if geopolymers are to be applied for sustainable concrete with reproducible quality, a characterization tool for quality control of precursors and optimizing mix design is needed, which links the (variable) secondary resource characteristics to the performance of geopolymers. This paper shows the potential of using FT-IR and solid state 29Si-NMR for a (semi-)quantitative evaluation of secondary resources suitable for geopolymers. More specifically, the aim is to investigate whether the presence of particular aluminium–silicate phases may have a dominant influence on the strength of a simple geopolymer system. Based on deconvolution of FT-IR and NMR spectra of 9 source materials (fly ash and slag) and their geopolymers, relative amounts of different (aluminium– silicate) phases were estimated and trends with strength were evaluated. A clear trend is observed for bothNMRand FT-IR results: the precursors containing a higher amount of ‘active bonds’, predominantly from silicates with a medium amount of aluminium and alkalis incorporated in their framework, result in higher paste strengths. As such, due to its relatively quick and easy measurements, FT-IR for estimating relative amounts of these aluminium–silicate phases, can be a useful tool for evaluating geopolymer precursors

Surfactant-directed synthesis of mesoporous films made single-step by a tandem photosol-gel/photocalcination route

In view of their technological impact in materials chemistry, a simplified and more efficient synthetic route to mesoporous films is highly sought. We report, herein, a smart UV-mediated approach coupling in a one-stage process sol-gel photopolymerization and photoinduced template decomposition/ablation to making mesoporous silica films. Performed at room temperature with a solvent-free solution of silicate precursor and amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer, the synthesis relies on photoacid generation to induce the fast formation (≈10 min) of mesostructured silica/surfactant domains. Continuation of UV exposure for three additional hours enables subsequent and complete photodegradation of the polyether copolymer, resulting in ordered or disordered mesoporous silica film. One of the most attractive features is that the one-step procedure relies on a continuous illumination provided by the same conventional medium-pressure Hg-Xe arc lamp equipped with a 254 nm reflector to enhance the emission of energetic photons <300 nm. In addition to X-ray diffraction and transmission electron microscopy, time-resolved Fourier transform infrared spectroscopy has proved to be a powerful in situ technique to probe the different chemical transformations accompanying irradiation. Photocalcination strengthens the inorganic network, while allowing to preserve a higher fraction of residual silanol groups compared with thermal calcination. A polyether chain degradation mechanism based on oxygen reactive species-mediated photo-oxidation is proposed.

Phase-change characteristics and thermal performance of form-stable n-alkanes/silica composite phase change materials fabricated by sodium silicate precursor

A series of n-alkanes/silica composites as form-stable phase change materials (PCMs) were synthesized in a sol–gel process using sodium silicate precursor. The chemical compositions and structures of the synthesized composites were characterized by Fourier transform infrared spectroscopy. Scanning electric micrographs show an irregularly spherical morphology of the n-alkanes/silica composites, and transmission electric micrographs confirm that the n-alkanes have been well encapsulated by silica. These n-alkanes/silica composites keep a good sharp stability due to the support of silica wall even if the n-alkanes are in molten state. The differential scanning calorimetric analysis indicates that the phase change behaviors and characteristics of the n-alkanes/silica composites strongly depend on the carbon atom number in n-alkanes, and meanwhile, the encapsulated n-alkanes have a high thermal storage capability. The investigation on thermal performance demonstrated that the n-alkanes/silica composites achieved a high thermal conductivity, low supercooling, and good work reliability as a result of the encapsulation of n-alkanes with highly thermal conductive inorganic silica. Moreover, the thermal stability of the composites was also improved due to the protection of silica wall toward the encapsulated n-alkanes. It is anticipative that, owing to the easy availability and low cost of sodium silicate, the synthetic technology developed by this work has a high feasibility in the industrial manufacture of the form-stable PCMs.

Role of subcolloidal (nanosized) precursor species in the early stage of the crystallization of zeolites in heterogeneous systems.

A critical analysis was carried out for the purpose of understanding the role of subcolloidal (nanosized) (alumino)silicate precursor species in the early stage of crystallization of zeolites in heterogeneous systems (hydrogels). The formation and evolution of these subcolloidal species in both the solid and the liquid phases were investigated by various experimental methods such a scanning electron microscopy (SEM, FE-SEM), transmission electron microscopy, atomic force microscopy, particle size analysis, pH measurement, atomic absorption spectroscopy, and dynamic light scattering, after careful separation of intermediates from reaction mixture by two-step centrifugation treatment. The results revealed that a chain of processes (i) the formation of low-molecular-weight (LMW) silicate species, by dissolution of Al-enriched amorphous silica, and their aggregation into about 3 nm sized primary precursor species (PPSs), (ii) the formation of larger (∼3 to ∼15 nm sized) silicate precursor species (LSPSs) by a rapid aggregation/coalescence of PPSs, (iii) the formation of "gel" (primary amorphous precursor) by a random aggregation of LSPSs at room temperature, and (iv) the formation of the worm-like particles (secondary amorphous precursor) occurred in the solid phase during heating of the reaction mixture (hydrogel) from room temperature to 170 °C. It is interesting that almost the same processes occur in the liquid phase but with decreased rate according to the relative low concentration of LMW silicate species. With the above described findings, it is highly expected that the manipulation of crystallization pathway through controlling the formation/evolution of precursor species in the initial stage of the process can be achieved.

Flexible aerogels with interpenetrating network structure of bacterial cellulose–silica composite from sodium silicate precursor via freeze drying process

Bacterial cellulose (BC)–silica composite aerogels (CAs) with interpenetrating network (IPN) microstructure are prepared through a permeation sol–gel process followed by freeze drying. The IPN structure is constructed by diffusing the precursor into a three-dimensional (3D) BC matrix followed by permeating the catalyst into the BC network gradually to promote the in situ condensation of precursor to form a SiO2 gel skeleton from outside to inside. The precursor used here is Na2SiO3 instead of traditional tetraethoxysilane. This IPN structure could offer excellent mechanical properties to aerogels, and is essential to prepare flexible aerogels by freeze drying. The compression modulus of CAs could be adjusted in the range of 0.38 MPa to 16.17 MPa. The BC–silica CAs exhibit low density (as low as 0.011 g cm−3), high specific surface area (as high as 534.5 m2 g−1) and low thermal conductivity (less than 0.0369 W m−1 K−1). Furthermore, the contact angle of the hydrophobization modified CAs is as high as 145°. The outstanding hydrophobicity and the large specific surface area endow the hydrophobic CAs with excellent oil absorption capability on the water surface. Moreover, the hydrophobic CAs that had absorbed oil could be washed and recycled.

Using bagasse ash as a silica source when preparing silica aerogels via ambient pressure drying

Silica aerogels were prepared using bagasse ash as a silica source through a route involving the surface modification of wet gels and ambient pressure drying (APD). The silica in the bagasse ash was extracted using a NaOH solution to form the sodium silicate precursor for the silica aerogels. Trimethylchlorosilane (TMCS) and hexamethyldisalazane (HMDS) were used to replace the surface silanol groups with alkyl groups to prevent the condensation and shrinkage of the gel structure during APD. The prepared silica aerogels exhibited strong hydrophobicity. The silica aerogels aged in water possess a higher surface area than those aged in hexane. The mixing time for the silylating agents with the silica precursor influences the surface area of the silica aerogels. The highest surface area and pore volume of the silica aerogels were 1114m2/g and 2.16cm3/g, respectively.