Polysiloxane-Modified Mesoporous Materials

We report the preparation of polysiloxane-modified mesoporous silica gels derived from the acid catalysed hydrolysis of tetraethoxysilane (TEOS) and oligomeric silanol terminated polydimethylsiloxane (PDMS) in the presence of the non-ionic surfactant, octaethylene glycol monohexadecyl ether. The gels were characterised using thermal gravimetric analysis (TGA), infra-red (IR) spectroscopy, X-ray diffraction (XRD) and ²⁹Si solid state cross-polarisation (CP) magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. TGA and IR spectroscopy showed the loss of surfactant after calcination and a decrease in the Si–OH band at 950 cm^–1 indicated further condensation had occurred. This was confirmed by the increase in Q⁴ at –110 ppm, in ²⁹Si MAS NMR spectroscopy, which also showed that calcination had led to the redistribution of PDMS forming a T species. XRD data showed ordering within the structure, with an initial d-spacing of 45 Å, decreasing to 35 Å after calcination.     

TEOS–colloidal silica–PDMS‐OH hybrid formulation used for stone consolidation

The consolidation of materials concept, which consists of introducing a chemical substance (consolidant) into degraded stone, has been applied to architectural conservation. Silicon compounds such as tetraethoxysilane (TEOS) are frequently used as a base for commercial consolidant formulations due to their ability to form a siloxane polymer such as SiO₂. However, the silica xerogels deposited into the stone show poor performance and the gels obtained are non‐porous and tend to crack during the drying stage. In order to avoid the fractures and to improve gel properties, we propose the synthesis of a hybrid consolidant based on TEOS and fillers such as colloidal silica (200 nm in diameter) and hydroxy‐terminated polydimethylsiloxane (PDMS‐OH). Both additives enhance gel properties such as porosity and elasticity, leading to the formation of non‐fractured and permeable gels. Characterization of the hybrid xerogel was carried out by nitrogen adsorption and ²⁹Si MAS‐NMR. The properties of the hybrid xerogels were compared with those prepared from a formulation based on TEOS (T‐ME) with a composition similar to a commercial product. In order to evaluate the effectiveness of the hybrid consolidant, it was applied to tuff‐stone of historical monuments in the city of Guanajuato, Mexico. The tuff‐stone was also treated with the formulation T‐ME. Both treatments were studied by determining the percentage of consolidant deposited, evaluating changes in porosity and hardness of the treated stone. The applicability of the hybrid consolidant for the decayed tuff‐stone is under study. Copyright © 2010 John Wiley & Sons, Ltd.     

Formation of small pore SAPO-44 type molecular sieve

The hydrothermal transformation of silico–aluminophosphate gel with cyclohexylamine to SAPO-44 has been examined. The hydrothermal crystallisation products of the SAPO have been investigated by X-ray diffraction, FTIR, nitrogen and water adsorption, thermogravimetric analysis, surface analysis and ²⁷Al, ³¹P, and ²⁹Si MAS NMR. Structural changes were observed in the silico–aluminophosphate gel with and without organic template and during the hydrothermal crystallisation. The silico–aluminophosphate gel converted to pure SAPO-CHA phase in 168 h at 473 K. The surface of SAPO-44 was silicon rich as compared with that of SAPO-34 and SAPO-18. The ²⁷Al MAS NMR signal of tetrahedrally coordinated Al observed in the silico–aluminophosphate gel without the organic template was changed to octa-, penta- and tetrahedrally coordinated aluminium upon the addition of the cyclohexylamine template to the SAPO gel. After 3 h of hydrothermal treatment at 473 K however, the ²⁷Al MAS NMR signals of the octahedral and pentacoordinated aluminium were removed. This was also confirmed by ³¹P and ²⁹Si MAS NMR. The tetrahedrally coordinated P and Si were detected within 3 h at 473 K. The sorption capacity and adsorption–desorption trends of the SAPO gels and the crystallisation products were found to be different. ²⁹Si MAS NMR results indicated that the percentage of Si (4Al) and its distribution were significantly affected by the crystallization period. SAPO-44 was thermally stable up to 973 K with phase change observed over the calcination temperature of 1193 K. The SAPO gels and the crystallisation products have also been investigated for their catalytic behaviour in n-hexane and ethanol conversion reactions.     

29Si NMR研究TEOS/DDS双前驱体体系的水解动力学

原位引入有机组分对氧化硅体系改性是合成有机-无机杂化硅材料的重要方法. 利用原位的²⁹Si液体核磁, 研究了甲醇为溶剂、氨水催化条件下的四乙氧基硅烷(TEOS)和二甲基二乙氧基硅烷(DDS)原位共水解的动力学过程. 通过改变反应体系中氨和水的浓度, 拟合出单体及中间产物浓度随时间的变化曲线, 得到了TEOS和DDS各自的水解速率常数以及相应各反应物的反应级数. 与单前驱体水解一致的是, 在双前驱体系中TEOS和DDS自身的反应级数仍保持一级, 但是氨和水的反应级数都有不同程度的增大. 与单前驱体水解速率方程相比, 混合体系中TEOS的水解速率常数增大. 同时, DDS在双前驱体中比单前驱体中的水解速率常数有很大程度的减少. 水解动力学表明, TEOS和DDS在双前驱体体系中显示出更平行的水解速率. 利用固体²⁹Si MAS NMR, XPS及小角X射线散射(SAXS)手段对双前驱体体系研究得到的信息显示, 碱催化条件下原位的TEOS水解中间物与DDS中间产物的原位共缩聚程度很弱.     

光固化聚氨酯丙烯酸酯杂化材料的核磁共振谱

光固化聚氨酯丙烯酸酯杂化材料的核磁共振谱;SiO2;溶胶-凝胶法     

Effect of non-ionic surface active agents on TEOS-derived sols,gels and materials

The sol-gel process, starting from tetraethylorthosilicate precursor, is a suitable technique for the preparation of silica thin films. The use of specific organic additives, like non ionic surface-active agents, drastically modifies the gelation process and allows the preparation of microporous materials with a high microporous volume. The effects of additives on the sol, gel and material characteristics have been investigated by several methods such as ²⁹Si NMR, QELS, SAXS (for sols and gels), and N₂ adsorption, FESEM (for fired materials). It appears that the interactions of surface active agents with TEOS derived species limit condensation reactions and particle growing. A brittle gel structure is generated which leads to highly porous microporous silica after the elimination of organic chains by thermal treatment at 450°C. The material porous texture (specific surface area, pore size distribution and porous volume) can be varied especially by varying the surface active agent chain length and quantity. This kind of sol-gel system is suitable to prepare microporous silica membranes candidate for gas separation or catalytic reactor applications.     

Formation of borosilicate glasses from silicon alkoxides and metaborate esters in dry non-aqueous solvents

The reaction of metaborate esters (RO)₃B₃O₃ [R = Me, Et, ClCH₂CH₂–, Cl₃CCH₂–, ClCH₂CH₂CH₂–, (ClCH₂)₂CH–] with Si(OR)₄ (R = Me, Et), either neat or in dry propan-2-one or dry THF at room temperature, led to gels which when dried and heated in air for 20 mins at 600°C afforded borosilicate glasses in high ceramic yields. The dried gels and glasses were characterized by elemental analysis, TGA, IR, and powder XRD, and solid-state MAS ²⁹Si and ¹¹B NMR. The gelling reaction was investigated by solution ¹¹B and ²⁹Si NMR. These NMR studies indicated B–O–Si reaction intermediates and a mechanism involving alkoxy exchange and various condensation/elimination reactions of the borosilicate esters have been proposed.     

疏水介孔二氧化硅膜的制备与表征

用甲基三乙氧基硅烷(MTES)代替部分正硅酸乙酯(TEOS)作为前驱体，以聚乙烯醚-聚丙烯醚-聚乙烯醚三嵌段共聚物(P123)作有机模板剂，通过共水解缩聚反应制备了甲基修饰的介孔SiO₂膜。利用N₂吸附、FTIR、²⁹Si MAS NMR以及接触角测量仪对膜的孔结构和疏水性进行了表征。结果表明，修饰后的膜材料具有良好的介孔结构，最可几孔径为4.65 nm，孔体积为0.69 cm³·g^-1，比表面积为938.4 m²·g^-1；同时疏水性明显提高，当n_MTES/n_TEOS达到1.0时，其对水的接触角达到109°± 1.1°。气体渗透实验表明气体通过膜孔的扩散由努森机制所控制。     

Porous Siloxane-Silica Hybrid Materials by Sol-Gel Processing

Porous hybrid materials have been fabricated by sol-gel processing of tetraethoxysilane (TEOS) and 1,3,5,7-tetramethyl-tetrakis(ethyltriethoxysilane)-cyclotetrasiloxane (1) in the presence of the cationic surfactant, cetyltrimethylammonium bromide (CTAB). The chemical and physical properties of these materials have been analysed by FT-IR spectroscopy, solid state ²⁹Si NMR spectroscopy, powder X-ray diffraction and nitrogen adsorption-desorption studies. FT-IR spectroscopy established that the CTAB surfactant can be extracted from a crushed gel using ethanol as a solvent. Solid state ²⁹Si NMR spectroscopy showed the presence of D, T and Q species as expected from the structure of the precursors. Broad bands observed for the D units at –18 ppm and the T units at –63 ppm suggested that the cyclotetrasiloxane was held in a rigid environment and bound to the Q species of the silica matrix derived from the TEOS. NMR spectroscopy confirmed that solvent extraction resulted in further condensation of the silica matrix. Powder X-ray diffraction indicated that the materials possess short-range order and small domain sizes, as shown by broad diffraction peaks. The condensation induced by solvent extraction led to a decrease in the lattice and domain size of the samples, generally resulting in a less ordered material. Nitrogen adsorption-desorption isotherms were typical of microporous materials with pore diameters of 18 Å and a narrow size distribution.     

Sol–gel preparation of mesoporous Al2O3–SiO2 glasses: structural evolution monitored by solid state NMR

Glasses along the composition line 0.5Al₂O₃–xSiO₂ (1 ≤ x ≤ 6) were prepared via a novel sol–gel route using tetraethylorthosilicate and aluminum lactate as precursors. The structural evolution from solution to gel to glass is monitored by standard ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopies, revealing important insights about molecular level mechanisms occurring at the various stages of glass formation. Under the experimental conditions reported, silica and alumina precursors undergo homoatomic condensation processes when the gel is heat treated at about 100 and 300 °C, respectively, and only little heteroatomic co-condensation occurs in this temperature range. The latter is promoted only upon elimination of the residual lactate and water ligands upon annealing the gels above 300 °C. Following calcination at 650 °C, mesoporous glasses are obtained, having average pore diameter of about 3 nm and a surface areas near 500 m²/g. Si–O–Al connectivities are detected by ²⁹Si magic angle spinning (MAS)-NMR. ²⁷Al MAS-NMR spectra reveal aluminum in four-, five- and six-coordination. The spectra differ significantly from those of other sol–gel derived Al₂O₃–SiO₂ materials prepared from different precursor routes, suggesting that the lactate route results in a higher degree of compositional homogeneity.     

Development of mullite and mullite/Al2O3 precursor sols for electrophoretic deposition of oxidation protection coatings

Mullite and mullite/Al₂O₃ precursor sols have been developed for the deposition of oxidation barrier coatings on carbon fibre reinforced composites using a combination of sol–gel synthesis and electrophoresis.The sols were synthesised by controlled hydrolysis and condensation of TEOS (tetraethoxysilane) and Al(OBu^s)₃ (aluminium tri-sec-butylate). The main objective was the definition of synthesis conditions which yield sols suitable for the electrophoretic deposition (EPD). Measurements of the Electrokinetic Sonic Amplitude were used to investigate the electrokinetic properties of the sols in the as-prepared state and depending on the later addition of H₂O. ²⁹Si CP/MAS NMR spectra of dried precursor samples were recorded to study the homogeneity of Al/Si mixture. The progress of crystallisation with increasing temperature of heat treatment was examined by XRD. Oxidation protection coatings on C/C–SiC composites were prepared by EPD.Whereas a low H₂O to TEOS ratio during the sol synthesis was advantageous for a low mullite formation temperature, a high H₂O to TEOS ratio was necessary to enable the EPD. The synthesis of a sol with a low H₂O to TEOS ratio in the first step and the later modification of this sol by the addition of water was a successful method to combine the required electrokinetic properties and mullitisation temperatures below 1200 °C.     

A new study on the kinetics of Stöber synthesis by in-situ liquid 29Si NMR

Liquid-state ²⁹Si NMR was used to investigate the hydrolysis and condensation kinetics of ammonia-catalyzed tetraethoxysilane (TEOS) in methanol system. The reactive rate constants were calculated by applying first-order reaction approximation and the steady state approximation theory. The reaction orders with respect to TEOS, ammonia and water were derived, as well as the activation energies and the Arrhenius constants. It was found that the formation of intermediate species Si(OH)(OEt)₃ was the rate-limiting step and its reaction rate equation was r _TEOS=7.41×10⁻³[TEOS][NH₃]^0.333[H₂O]^0.227. Higher reactive temperature benefited the hydrolysis of TEOS. The results presented here indicated quantificationally that the formation of colloidal SiO₂ particles was controlled by the initial hydrolysis of TEOS.     

New silica hybrids elaborated by sol-gel process from bifunctional thiadiazole and 1,2,4-triazole precursors

New hybrid materials were synthesized from an inorganic precursor, (tetraethoxysilane: TEOS) and bifunctional organic precursors through the sol-gel process. Indeed, monolithic and transparent hybrid gels were obtained by the reaction of tetraethoxysilicate (TEOS) with 1,2,4-thiadiazole-3,5-diamine (1), 5-amino-1,3,4-thiadiazole-2-thiol (2), and 1H-1,2,4-triazole-3-thiol (3) in ethanol as solvent, using chloric acid (HCl 10^?1M) as catalyst. It is shown that TEOS has been cross-linked with bi-functional heterocyclic compounds: (1), (2) and (3) leading to the formation of transparent and colorous gels. The characterization was realized by infrared, ²⁹Si and ¹³C CP MAS NMR. The xerogels morphology and texture were studied by scanning electron microscopy (SEM) and Brunauer – Emmet – Teller method (BET). The obtained results show that organic adducts (1), (2) and (3) react with TEOS leading to gels in which Si–N and Si–S bridges were formed. According to N₂-physisorption results, xerogels are mesoporous with specific surface area varied from 105 to 312 m²g^?1 and their isotherms are classified as type IV. The optic and magnetic properties of hybrids were studied respectively by UV-Vis spectroscopy and RPE. This study showed that all materials are paramagnetic semiconductors.     

Preparation and characterization of novel polyimide‐silica hybrids

Polyimide‐silica (PI‐SiO₂) hybrids were prepared from a novel polyimide (PI), derived from pyromellitic dianhydride (PMDA), 1,6‐bis(4‐aminophenoxy)hexane (synthesized) and 4,4′‐oxydianiline. SiO₂ networks (5–30 wt%) were generated through sol–gel process using either tetraethylorthosilicate (TEOS) or a mixture of 3‐aminopropyltriethoxysilane‐PMDA‐based coupling oligomers (APA) and TEOS. Thin, free standing hybrid films were obtained from the respective mixtures by casting and curing processes. The hybrid films were characterized using Fourier transform infrared, ²⁹Si nuclear magnetic resonance (NMR), field emission scanning electron microscopy (FE‐SEM), energy dispersive X‐ray spectrometry and atomic force microscopy (AFM) techniques. ²⁹Si NMR results provide information about formation of organically modified silicate structures that were further substantiated by FE‐SEM and AFM micrographs. Contact angle measurements and thermogravimetric thermograms reveal that the addition of APA profoundly influences surface energy, interfacial tension, thermal stability and the residual char yield of modified hybrids in comparison to those obtained by mixing only TEOS. It was found that reduced particle size, efficient dispersion and improved interphase interactions were responsible for the eventual property enhancement. Copyright © 2012 John Wiley & Sons, Ltd.     

The Structure of Hybrid Gels by Drift and NMR Spectroscopies

Hybrid inorganic-organic gels have been prepared by the sol-gel process using tetraethoxysilane (TEOS) as precursor, mixed with a low concentration of polytetrahydrofuran (PTHF), under acid catalysis. The hybrid xerogels were characterized by DRIFTS and Solid State ¹H, ¹³C and ²⁹Si NMR. The DRIFT spectra indicate that the polymer is responsible for decreasing the number of free silanol groups in comparison to pure silica. Solid-state NMR spectra reveal the types of silicate structures formed and the conditions for establishing chemical bonds between the two phases, which are responsible for the silica network flexibility. We have concluded that it is possible to design a hybrid gel with tailored properties, even at very low polymer concentration, by selecting the appropriate preparation route.     

Effect of vinyltriethoxysilane addition on the pyrolytic conversion of tetraethoxysilane based silica gel

The effect of vinyltriethoxysilane (VTES) addition on the pyrolytic conversion of tetraethoxysilane (TEOS) based silica gel had been studied. Thermogravimetric analysis coupled with mass spectroscopy was carried out to study the thermal decomposition behavior of precursor gels. The ceramic yield of precursor gels was decreased with the increase of the VTES content. ²⁹Si magic angle spinning nuclear magnetic resonance indicated that the incorporation of VTES into TEOS not only changed the composition and structure of precursor gels, but also increased carbon-enriched SiO _x C_4?x units of silicon oxycarbide ceramics during the pyrolysis conversion. The carbon content of SiOC ceramic was almost unchanged between 1,000 and 1,500 °C. However, the O/Si ratio of the silicon oxycarbide ceramic was reduced and the free carbon content was increased with the increasing molar ratios of VTES/TEOS. Moreover, the carbothermal reduction reaction led to the free carbon content decreased with the increase of the sintering temperature.     

One-step synthesis of hydrothermally stable mesoporous aluminosilicates with strong acidity

Using tetraethylorthosilicate (TEOS), polymethylhydrosiloxane (PMHS) and aluminium isopropoxide (AIP) as the reactants, through a one-step nonsurfactant route based on PMHS-TEOS-AIP co-polycondensation, hydrothermally stable mesoporous aluminosilicates with different Si/Al molar ratios were successfully prepared. All samples exclusively showed narrow pore size distribution centered at 3.6 nm. To assess the hydrothermal stability, samples were subjected to 100 °C distilled water for 300 h. The boiled mesoporous aluminosilicates have nearly the same N₂ adsorption-desorption isotherms and the same pore size distributions as those newly synthesized ones, indicating excellent hydrothermal stability. The ²⁹Si MAS NMR spectra confirmed that PMHS and TEOS have jointly condensed and CH₃ groups have been introduced into the materials. The ²⁷Al MAS NMR spectra indicated that Al atoms have been incorporated in the mesopore frameworks. The NH₃ temperature-programmed desorption showed strong acidity. Due to the existence of large amount of CH₃ groups, the mesoporous aluminosilicates obtained good hydrophobicity. Owing to the relatively large pore and the strong acidity provided by the uniform four-coordinated Al atoms, the excellent catalytic performance for 1,3,5-triisopropylbenzene cracking was acquired easily. The materials may be a profitable complement for the synthesis of solid acid catalysts.     

Interpenetration of wood with NH2R-functionalized silica xerogels anchoring copper(II) for preservation purposes

Interpenetration of wood samples of pine sapwood (Pinus sylvestris L.) with hybrid inorganic–organic silica xerogels bearing amine functions able to coordinate copper(II) cations has been successfully carried out. These materials have been prepared by sol–gel processing TEOS/APTES mixtures inside the wood. Solid state ²⁹Si NMR data provide evidence that the interpenetrated xerogel material has the same degree of condensation of the corresponding xerogel formed outside the wood. Copper(II) is effectively vehiculated inside the wood by coordination linkages with two ammine functions well evidenced by ESR measurements. SEM/EDX investigations show that the chlorine/copper atomic ratio inside the wood is lower than that of the starting salt CuCl₂, suggesting an exchange reaction with silanol groups with the formation of Si–O–Cu linkages and HCl. This reaction could be promoted by the excess of amine functions with formation of ammonium chloride species which remain onto the surface of the wood and in the mother solution owing to a higher degree of condensation. Sodium silicate was tested in place of TEOS in order to have a cheaper and ethanol-free formulation. However, gel penetration results significantly lower than that of the corresponding formulation containing TEOS and preservation performance are lower than that of TEOS formulation against brown-rot fungal decay.     

Synthesis of Mesoporous Silica Immobilized with 3-[(Mercapto or amino)propyl]trialkoxysilane by a Simple One-pot Reaction

Sodium silicate from rice husk ash (RHA) was transformed to functionalized silica with 3‐(mercaptopropyl)‐trimethoxysilane (MPTMS) or 3‐(aminopropyl)triethoxysilane (APTES) via a simple sol‐gel technique in a one‐pot synthesis to give RHAPrSH and RHAPrNH₂. The ²⁹Si MAS NMR of RHAPrSH and RHAPrNH₂ showed the presence of T¹, T², T³, Q², Q³ and Q⁴ silicon centers. The ¹³C MAS NMR showed that RHAPrSH had chemical shifts at δ 16.59, 32.73, consistent with two of the carbon atoms of the MPTMS moiety, while the ¹³C MAS NMR of RHAPrNH₂ had chemical shifts at δ 14.58, 26.13, 47.87, consistent with the three carbon atoms of the APTES moiety. The presence of carbon, silicon, sulfur and nitrogen in RHAPrSH and RHAPrNH₂ was determined by a combination of elemental analysis and EDX study.     

The interaction of dibutyltin dilaureate with tetraethyl orthosilicate in sol-gel systems

Sols composed of dibutyltin dilaureate (DTL) and tetraethyl orthosilicate (TEOS) were prepared using a mixture of methyl ethyl ketone (MEK) and acetone as the solvent in order to study the interaction between the oligomeric Sn and Si species. The hydrolysis molar ratio r (r=nH₂O/nM (M: Si, Sn or Si+Sn) was 2. The use of an acid or basic catalyst was avoided, as the sols are intended to be used in the formulation of potential stone consolidants. The sols were studied by several spectroscopic techniques including Small Angle X-ray Scattering (SAXS), ²⁹Si and ¹¹⁹Sn NMR, Fourier Transform Infrared (FTIR) spectroscopy and X-ray diffraction (XRD). According to the spectroscopic results the lauric acid produced by the hydrolysis of DTL modifies the condensation path of the Si species, leading to the formation of two types of oligomeric chains: linear swollen and multiparticle diffusion-limited aggregates, depending on the molar ratio Sn/Si. The ²⁹Si NMR results indicated that the hydrolysis of DTL catalizes the condensation of the Si species, giving as a result higher condensation extents of the Si species in the Sn-Si sols compared to a pure Si sol. Based on the Radial Distribution functions (RDF) and the FTIR results, heterocondensation occurred.