Zirconia‐based Aerogels via Hydrolysis of Salts and Alkoxides: The Influence of the Synthesis Procedures on the Properties of the Aerogels

This contribution aims at evaluating different synthesis procedures leading to zirconia‐based aerogels. A series of undoped and yttrium‐doped zirconia aerogels have been prepared via hydrolysis and condensation reaction of different alkoxy‐ and different inorganic salt‐based precursors followed by supercritical drying. Well‐established but deleterious zirconium n‐propoxide (TPOZ) or zirconium n‐butoxide (TBOZ) were used as metal precursors in combination with acids like nitric acid and acetic acid as auxiliary agent for the generation of non‐yttrium stabilized zirconia aerogels. Yttrium‐stabilized zirconia aerogels as well as pure zirconia aerogels were obtained by the salt route starting from ZrCl₄ and crosslinking agents like propylene oxide or acetylacetone. The characteristics of the products were analyzed by nitrogen adsorption measurements, electron microscopy, and X‐ray scattering. It turned out that with respect to all relevant properties of the aerogels as well as the practicability of the synthesis procedures, approaches based on inexpensive non‐toxic salt precursors are the methods of choice. The salt‐based approaches allow not only for low‐cost, easy‐to‐handle synthesis procedures with realizable gelation times of less than 60 seconds, but also delivered the products with the highest surface area (449 m² g^?1 for ZrCl₄) within this series of syntheses.     

Stabilization of Titanium Dioxide Nanoparticles at the Surface of Carbon Nanomaterials Promoted by Microwave Heating

TiO₂ is frequently combined with carbon materials, such as reduced graphene oxide (RGO), to produce composites with improved properties, for example for photocatalytic applications. It is shown that heating conditions significantly affect the interface and photocatalytic properties of TiO₂@C, and that microwave irradiation can be advantageous for the synthesis of carbon‐based materials. Composites of TiO₂ with RGO or amorphous carbon were prepared from reaction of titanium isopropoxide with benzyl alcohol. During the synthesis of the TiO₂ nanoparticles, the carbon is involved in reactions that lead to the covalent attachment of the oxide, the extent of which depends on the carbon characteristics, heating rate, and mechanism. TiO₂ is more efficiently stabilized at the surface of RGO than amorphous carbon. Rapid heating of the reaction mixture results in a stronger coupling between the nanoparticles and carbon, more uniform coatings, and smaller particles with narrower size distributions. The more efficient attachment of the oxide leads to better photocatalytic performance.     

Function‐Led Design of Aerogels: Self‐Assembly of Alloyed PdNi Hollow Nanospheres for Efficient Electrocatalysis

One plausible approach to endow aerogels with specific properties while preserving their other attributes is to fine‐tune the building blocks. However, the preparation of metallic aerogels with designated properties, for example catalytically beneficial morphologies and transition‐metal doping, still remains a challenge. Here, we report on the first aerogel electrocatalyst composed entirely of alloyed PdNi hollow nanospheres (HNSs) with controllable chemical composition and shell thickness. The combination of transition‐metal doping, hollow building blocks, and the three‐dimensional network structure make the PdNi HNS aerogels promising electrocatalysts for ethanol oxidation. The mass activity of the Pd₈₃Ni₁₇ HNS aerogel is 5.6‐fold higher than that of the commercial Pd/C catalyst. This work expands the exploitation of the electrocatalysis properties of aerogels through the morphology and composition control of its building blocks.     

常压干燥制备酚醛树脂基炭气凝胶研究进展

以间苯二酚-甲醛为代表的酚醛树脂基炭气凝胶是一种轻质、多孔、非晶态的纳米炭材料,在催化、吸附、电化学和隔热等领域中有广阔的应用前景.但是复杂且高成本的超临界干燥工艺极大地限制了炭气凝胶的工业化制备及其应用,因此常压干燥工艺成为目前研究最为广泛的炭气凝胶制备技术之一.本文综述了酚醛树脂基炭气凝胶常压干燥制备过程的四种结构...     

常压干燥制备二氧化硅气凝胶*

二氧化硅气凝胶是典型的纳米多孔轻质材料,由于具有独特的性能并在许多领域存在潜在的应用价值而受到广泛关注。二氧化硅气凝胶的制备传统上采用超临界干燥工艺,但此工艺成本高、工艺复杂而且具有一定的危险性。为了实现二氧化硅气凝胶的大批量生产和商品化应用,研究低成本常压干燥制备技术非常必要。目前常压干燥制备工艺已取得了较大进展,本文主要介绍了二氧化硅气凝胶的常压干燥制备方法及其特点,并概述了二氧化硅气凝胶复合材料制备的最新研究进展。以纤维和聚和物为增强体的二氧化硅气凝胶复合材料改善了气凝胶的力学性能,进一步扩宽了其应用范围。     

Photocatalytic Degradation of Paracetamol in Aqueous Medium Using TiO2 Prepared by the Sol–Gel Method

Two titania photocatalysts have been prepared using the sol–gel method using TiCl₄ as a precursor, and two different alcohols, namely, ethanol or propanol (Et or Pr). The main aim of this work was to study the effect of the nature of the alcohol on the chemical, structural and photocatalytic properties for paracetamol photodegradation of the final solids. The TiCl₄/alcohol molar ratio to obtain the corresponding alkoxides (TiEt and TiPr) was 1/10. These alkoxides were calcined at 400 °C to prepare the oxide catalysts (named as TiEt400 and TiPr400). Powder X-ray diffraction (PXRD) of the original samples showed the presence of anatase diffraction peaks in sample TiPr, while TiEt is a completely amorphous material. Contrary to commercial TiO₂-P25, the PXRD diagrams of the calcined samples showed anatase as the exclusive crystalline phase in both solids. The specific surface area (S_BET) of sample TiPr400 was larger than that of sample TiEt400, and both larger than that of TiO₂-P25. The three solids have been tested in the photodegradation of paracetamol in aqueous solution. It has been established that the alcohol used influences the properties and catalytic activity of the final oxides. The synthesized solids exhibit a higher activity than commercial TiO₂-P25, because of their structural characteristics and larger S_BET.     

Preparation of SiO2-TiO2 Aerogels Using Supercritical Impregnation

The preparation of SiO₂-TiO₂ aerogels by supercritical impregnation of titanium alkoxides into silica alcogels was investigated. A mixture of CO₂ and 2-propanol with dissolved titanium tetraisopropoxide modified with acetylacetone was used as the impregnation medium. Prior to the experiments, the supercritical behaviour of the impregnation solution was investigated. The microstructure and properties of aerogels prepared by the supercritical impregnation method were almost identical to those generated by the liquid impregnation. However, the time for impregnation was substantially decreased and the homogeneity of the impregnated titanium distribution on the aerogel increased.     

Antibacterial and Chemical Characterization of Silica-Quercetin-PEG Hybrid Materials Synthesized by Sol–Gel Route

This paper aims to synthesize, via the sol–gel method, a biomaterial usable in the medical field. Here, the silica-PEG-quercetin system was evaluated in relation to the different concentrations of PEG (0, 6, 12, 24, 50 wt%) and quercetin (0, 5, 10, 15 wt%), respectively. In addition, Fourier Transform-Infrared spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), and Kirby–Bauer analyses were performed. FT-IR was used to evaluate the hybrid formation and the influence of both PEG and Quercetin in the hybrid synthesized materials, SEM was used to evaluate the morphological properties, while the Kirby–Bauer test was used to understand the ability of the materials to inhibit the growth of the assayed bacterial strains (Escherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis, and Staphylococcus aureus).     

Chemical Preparation of Ferroelectric Mesoporous Barium Titanate Thin Films: Drastic Enhancement of Curie Temperature Induced by Mesopore‐Derived Strain

Mesoporous barium titanate (BT) thin films are synthesized by a surfactant‐assisted sol–gel method. The obtained mesoporous BT thin films show enhanced ferroelectricity due to the effective strains induced by mesopores. The Curie temperature (T_c) of the mesoporous BT reaches approximately 470 °C.     

微波加热法快速合成T型分子筛

由于具有高的水热稳定性和优良的孔道结构．T型分子筛已成为一种高选择性的催化剂．在低碳化合物的催化和重整等方面有较多的应用。近年来．报道采用晶种法在无机多孔陶瓷支撑体上制备的T型分子筛膜．在脱除有机物／水混合物中的水时．表现出优异的渗透汽化分离性能。然而，提高T型分子筛膜的致密性和生长速率仍是亟待解决的问题。T型分子筛的合成研究较少．制备过程均采用普通加热（Conventional Heating．CH）法。在无模板剂的条件下．T型分子筛的结晶区间较窄，结晶速率慢．合成时间通常需要6d以上。     

块状TiO2/SiO2气凝胶的非超临界干燥法制备及其表征

分别通过TiO2和SiO2的单独溶胶和TiO2/SiO2复合凝胶,并添加干燥控制化学添加剂甲酰胺,形成比较完善的凝胶网络结构,同时通过正硅酸乙酯的乙醇溶液浸泡,低表面张力溶剂替换和分级陈化以及干燥等步骤,实现了块状TiO2/SiO2复合气凝胶的非超临界干燥制备.所得TiO2/SiO2气凝胶为无色或乳白色轻质块状多孔固体,表观密度约0.4～0.9g/cm3,孔隙率约80%～95%.它由直径约10nm的TiO2和SiO2微粒相互分散复合而成,孔洞直径约几十纳米.其相态SiO2为无定形,TiO2为锐钛矿晶型.随着焙烧温度的升高,直到800℃不发生相变化.     

Sol–Gel Entrapped Lewis Acids as Catalysts for Biodiesel Production

Replacing fossil fuels with biodiesel enables the emission of greenhouse gases to be decreased and reduces dependence on fossil fuels in countries with poor natural resources. Biodiesel can be produced by an esterification reaction between free fatty acids (FFAs) and methanol or by transesterification of triglycerides from oils. Both reactions require homogeneous or heterogeneous catalysis. Production of biodiesel catalyzed by heterogeneous catalysts seems to be the preferred route, enabling easy product separation. As we have previously shown, the Lewis acids AlCl₃ and BF₃ can serve as highly efficient catalysts under ultrasonic activation. The present study focused on the development of oleic acid (OA) esterification with methanol by the same catalysts immobilized in silica matrices using the sol–gel synthesis route. During the course of immobilization, AlCl₃ converts to AlCl₃ × 6H₂O (aluminite) and BF₃ is hydrolyzed with the production of B₂O₃. The immobilized catalysts can be reused or involved in a continuous process. The possibility of biodiesel production using immobilized catalysts under ultrasonic activation is shown for the conversion of FFAs into biodiesel in batch and continuous mode.     

Preparation of Alumina Aerogel Films by Low Temperature CO2 Supercritical Drying Process

Alumina aerogel thin films were formed by a new synthesis route. Sols were prepared by the Yoldas process. Gels were formed by sol evaporation in a few hours. Films were prepared by dip coating glass or alumina substrates into both the sols and the gels. Aerogel films with special morphology were produced for the first time by exchanging the film solvent with acetone after the dip coating, followed by supercritical drying. The morphology of the films, studied by SEM and TEM, consists of fiber-like network of round chains (≈0.1 μm thick), and pores (0.1–0.5 μm in diameter). It is shown that the fibers contain a homogeneous arrangement of sol particles, 2–4 nm in size. Formation of this microstructure can be attributed to phase separation in the alumina-water-acetone system in a 2D film geometry. A conceptual model for the film development is proposed.     

块状TiO2气凝胶的制备及其表征

随着以溶胶-凝胶法和超临界干燥技术为基础的气凝胶制备方法的逐步完善,已不断制备出多种气凝胶[1～3].由于TiO2具有半导体特性,它常被作为光催化剂而受到重视,但是TiO2气凝胶的结构强度远比SiO2气凝胶小,在制备过程中极易碎裂粉化,所以至今未见制备块状TiO2气凝胶的报道.Dagan等[4]曾用异钛酸丁酯为母体制得TiO2气凝胶,并发现水杨酸在TiO2气凝胶存在下的光解速率是一般TiO2粉末的10倍,但获得的仅为TiO2气凝胶粉末.张敬畅等[5]以无机盐为原料,采用溶胶-凝胶法结合超临界干燥技术制备了纳米级TiO2气凝胶,也未能得到块状TiO2气凝胶材料. 本文报道以正钛酸丁酯为原料制备块状TiO2气凝胶的方法,并用TEM,SEM,XRD和IR等手段对所获得的气凝胶进行了结构表征.     

Development of Amino Acids Functionalized SBA-15 for the Improvement of Protein Adsorption

Ordered mesoporous materials and their modification with multiple functional groups are of wide scientific interest for many applications involving interaction with biological systems and biomolecules (e.g., catalysis, separation, sensor design, nano-science or drug delivery). In particular, the immobilization of enzymes onto solid supports is highly attractive for industry and synthetic chemistry, as it allows the development of stable and cheap biocatalysts. In this context, we developed novel silylated amino acid derivatives (Si-AA-NH₂) that have been immobilized onto SBA-15 materials in biocompatible conditions avoiding the use of toxic catalyst, solvents or reagents. The resulting amino acid-functionalized materials (SBA-15@AA) were characterized by XRD, TGA, EA, Zeta potential, nitrogen sorption and FT-IR. Differences of the physical properties (e.g., charges) were observed while the structural ones remained unchanged. The adsorption of the enzyme lysozyme (Lyz) onto the resulting functionalized SBA-15@AA materials was evaluated at different pHs. The presence of different functional groups compared with bare SBA-15 showed better adsorption results, for example, 79.6 nmol of Lyz adsorbed per m² of SBA-15@Tyr compared with the 44.9 nmol/m² of the bare SBA-15.     

Green and Functional Aerogels by Macromolecular and Textural Engineering of Chitosan Microspheres

Tremendous interest was recently devoted to the preparation of porous and functional materials through sustainable route, including primarily the use of renewable biopolymers instead of petroleum‐sourced synthetic chemicals. Among the biopolymers available in enormous quantity, chitosan – obtained by deacetylation of chitin – stands as the sole nitrogen‐containing cationic amino‐sugar carbohydrate. This distinctively provides chitosan derivatives with plenty of opportunities in materials science. Particularly, its pH switchable solubility allowed the preparation of three‐dimensional entangled nanofibrillated self‐standing microspheres. These porous hydrogels behave as nano‐reactors to confine exogenous nanoobjects within the polysaccharide network, including sol‐gel metal alkoxide species, organometallic derivatives and isotropic and oriented nanoparticles. Besides, the interfacial interplay of chitosan with lamellar clay and graphene oxide allowed the penetration of the biopolymer inside of the galleries, which result in a complete delamination of the layered nanomaterials. The preserved gelation memory of chitosan in these formulations provides a way to access porous microspheres entangling exfoliated nanometric sheets. CO₂ supercritical drying of functional hydrogel beads enabled efficient removal of water and other liquid solvents without wall collapsing, allowing large‐scale preparation of millimetric hydrocolloidal microspheres with an open macroporous network. These functionalized lightweight biopolymer aerogels find applications in heterogeneous catalysis, sensing, adsorption, insulation and for the design of other sophisticated porous nanostructures. Beyond their tailorable molecular and textural‐engineering, the possibility for macroscopic shaping of these intriguing nanostructures opens many new opportunities, especially in additive‐manufacturing for soft and hybrid robotics.     

Silica Meets Tannic Acid: Designing Green Nanoplatforms for Environment Preservation

Hybrid tannic acid-silica-based porous nanoparticles, TA-SiO₂ NPs, have been synthesized under mild conditions in the presence of green and renewable tannic acid biopolymer, a glycoside polymer of gallic acid present in a large part of plants. Tannic acid (TA) was exploited as both a structuring directing agent and green chelating site for heavy metal ions recovery from aqueous solutions. Particles morphologies and porosity were easily tuned by varying the TA initial amount. The sample produced with the largest TA amount showed a specific surface area an order of magnitude larger than silica nanoparticles. The adsorption performance was investigated by using TA-SiO₂ NPs as adsorbents for copper (II) ions from an aqueous solution. The effects of the initial Cu²⁺ ions concentration and the pH values on the adsorption capability were also investigated. The resulting TA-SiO₂ NPs exhibited a different adsorption behaviour towards Cu²⁺, which was demonstrated through different tests. The largest adsorption (i.e., ~50 wt% of the initial Cu²⁺ amount) was obtained with the more porous nanoplatforms bearing a higher final TA content. The TA-nanoplatforms, stable in pH value around neutral conditions, can be easily produced and their use would well comply with a green strategy to reduce wastewater pollution.