Catalyst-Free Epoxidation of Limonene to Limonene Dioxide

Limonene dioxide is a platform molecule for the production of new biopolymers. First attempts at limonene epoxidation were made by using low-coordination titanium supported on SBA-16 as the catalyst using <i>tert</i>-butyl hydroperoxide as the oxidizing agent, but no limonene dioxide was obtained. When limonene was substituted by 1,2-limonene oxide, the yield of limonene dioxide was only 13% in the same conditions. Two other techniques, both using in situ generated dimethyl dioxirane by the reaction of acetone with Oxone, have been studied and compared. These reactions are carried out in semibatch conditions and at room temperature. The first double epoxidation of limonene was performed in a conventional biphasic organic–water system and the other in excess acetone. The former epoxidation of limonene using ethyl acetate as the organic phase allowed reaching 95% conversion and yielding 33% of limonene dioxide. In comparison, when the reaction was performed in acetone, a limonene dioxide yield of 97% was observed under optimized conditions. The double epoxidation of limonene should be carried out at room temperature with a flowrate of 4 mL min^–1 of aqueous Oxone for a period of 45 min with a stoichiometric excess of 30% of Oxone

Engineering Homogeneous Doping in Single Nanoparticle To Enhance Upconversion Efficiency

Upconversion nanoparticles (UCNPs) have shown considerable promises in many fields; however, their low upconversion efficiency is still the most serious limitation of their applications. Herein, we report for first time that the homogeneous doping approach based on the successive layer-by-layer method can greatly improve the efficiency of the UCNPs. The quantum yield as high as 0.89 ± 0.05% is realized for the homogeneous doping NaGdF₄:Yb,Er/NaYF₄ UCNPs, which is nearly 2 times higher than that of the heterogeneous doping NaGdF₄:Yb,Er/NaYF₄ UCNPs (0.47 ± 0.05%). The influences of spatial distributions and local relative concentrations of the dopants on the optical properties of UCNPs were investigated in the single particle level. It was found that heterogeneous doping indeed existed during the spontaneous growth process of the nanoparticles. The heterogeneous doping property can further induce many negative effects on the optical properties of UCNPs, especially the luminescent efficiency. The spatial distributions and local relative concentrations of the dopants can be well controlled by the successive layer-by-layer homogeneous doping method on the monolayer level and homogeneously distributed in the single particle level. Furthermore, by using homogeneous doping NaGdF₄:Yb,Tm as initial core, the multicolor emission intensity of NaGdF₄:Yb,Tm/NaGdF₄:A (A = Tb³⁺, Eu³⁺) core/shell nanoparticles can also exhibit 20%–30% improvement. We believe that such a homogeneous doping model can open the door to improve the upconversion optical properties by engineering the local distribution of the sensitizer, activator, host, etc., in a microcosmic and provide a track for engineering the high quality UCNPs with advanced nanostructure and optical properties

Development of Sinter-Resistant Core–Shell LaMn_<i>x</i>Fe_1–<i>x</i>O₃@mSiO₂ Oxygen Carriers for Chemical Looping Combustion

This work investigates the possibility of using LaMn_0.7Fe_0.3O_3.15@mSiO₂ as oxygen carriers for chemical looping combustion (CLC). CLC is a new combustion technique with inherent separation of CO₂ from atmospheric N₂. LaMn_0.7Fe_0.3O_3.15@mSiO₂ core–shell materials were prepared by coating a layer of mesostructured silica around the agglomerated perovskite particles. The oxygen carriers were characterized using different methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ sorption, hydrogen temperature-programmed reduction (H₂-TPR), and temperature-programmed desorption of oxygen (TPD-O₂). The reactivity and stability of the carrier materials were tested in a special reactor, allowing for short contact time between the fluidized carrier and the reactive gas [Chemical Reactor Engineering Centre (CREC) fluidized riser simulator]. Multiple reduction–oxidation cycles were performed. TEM images of the carriers showed that a perfect mesoporous silica layer was formed around samples with 4, 32, and 55 nm in thickness. The oxygen carriers having a core–shell structure showed higher reactivity and stability during 10 repeated redox cycles compared to the LaMn_0.7Fe_0.3O_3.15 core. This could be due to a protective role of the silica shell against sintering of the particles during repeated cycles under CLC conditions. The agglomeration of the particles, which occurred at high temperatures during CLC cycles, is more controllable in the core–shell-structured carriers, as confirmed by SEM images. XRD patterns confirmed that the crystal structure of all perovskites remained unchanged after multiple redox cycles. Methane conversion and partial conversion to CO₂ were observed to increase with the contact time between methane and the carrier. Indeed, more oxygen from the carrier surface, grain boundaries, and even from the bulk lattice was released to react with methane. Upon rising the contact time, less CO was formed, which is desirable for CLC application. Increasing the reaction temperature and methane partial pressure lead to enhanced conversions of CH₄ under CLC conditions

Formation Mechanism of Cubic Mesoporous Carbon Monolith Synthesized by Evaporation-Induced Self-assembly

The formation mechanism of the cubic mesoporous carbon, FDU-16, synthesized by evaporation-induced self-assembly (EISA) was investigated at the molecular level by electron paramagnetic resonance (EPR) spectroscopic techniques. This material is synthesized using F127 pluronic block copolymer [poly­(ethylene oxide)–poly­(propylene oxide)–poly­(ethylene oxide) (PEO₁₀₆-PPO₇₀-PEO₁₀₆)] as a structure-directing agent (template) and phenolic resol as a carbon precursor. Using two spin probes derived from pluronics with PEO and PPO chains of different lengths that are designed to sense different regions of the system, we followed the evaporation and thermopolymerization stages of the synthesis in situ. To make such studies possible, we have used a polyurethane foam support, placed in the EPR tube, which allows for the efficient solvent evaporation as required for EISA. We focused on the evolution of the dynamics of the template and its interactions with the resol during the reaction. We observed that during the evaporation stage the resol is distributed throughout the entire PEO blocks, all the way to the PPO–PEO interface, interacting with them via H-bonds, thus hindering the local motion of the PEO chains. At the end of this stage there is no polarity gradient along the PEO blocks, as found for traditional F127 micelles in water or during the synthesis of silica materials, and the mesostructure is not well-defined. A polarity and a resol gradient developed during the thermopolymerization stage where the polymerizing resol is driven out to the outer region of the PEO corona. This produces a corona of resin-pluronic composite and a resol-free PPO core with high mobility of the PEO segments close to the PPO–PEO interface and restricted mobility in the composite corona. During this stage the final structure sets in

Graphitic Carbon Conformal Coating of Mesoporous TiO₂ Hollow Spheres for High-Performance Lithium Ion Battery Anodes

Rational design and controllable synthesis of TiO₂ based materials with unique microstructure, high reactivity, and excellent electrochemical performance for lithium ion batteries are crucially desired. In this paper, we developed a versatile route to synthesize hollow TiO₂/graphitic carbon (H-TiO₂/GC) spheres with superior electrochemical performance. The as-prepared mesoporous H-TiO₂/GC hollow spheres present a high specific surface area (298 m² g^–1), a high pore volume (0.31 cm³ g^–1), a large pore size (∼5 nm), well-defined hollow structure (monodispersed size of 600 nm and inner diameter of ∼400 nm, shell thickness of 100 nm), and small nanocrystals of anatase TiO₂ (∼8 nm) conformably encapsulated in ultrathin graphitic carbon layers. As a result, the H-TiO₂/GC hollow spheres achieve excellent electrochemical reactivity and stability as an anode material for lithium ion batteries. A high specific capacity of 137 mAh g^–1 can be achieved up to 1000 cycles at a current density of 1 A g^–1 (5 C). We believe that the mesoporous H-TiO₂/GC hollow spheres are expected to be applied as a high-performance electrode material for next generation lithium ion batteries

Controlled Sn-Doping in TiO₂ Nanowire Photoanodes with Enhanced Photoelectrochemical Conversion

We demonstrate for the first time the controlled Sn-doping in TiO₂ nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO₂ and TiO₂, Sn dopants are incorporated into TiO₂ NWs by a one-pot hydrothermal synthesis with different ratios of SnCl₄ and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl₄ hydrolysis rate. The obtained Sn-doped TiO₂ (Sn/TiO₂) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO₂ NWs is well controlled at a low level, that is, 1–2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO₂ NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm² at 0 V vs Ag/AgCl under 100 mW/cm² simulated sunlight illumination up to ∼100% enhancement compared to our best pristine TiO₂ NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO₂ NWs in H₂ further improves their photoactivity with an optimized photoconversion efficiency of ∼1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO₂ NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO₂ materials in a variety of solar energy driven applications

Distinct Packings of Supramolecular Building Blocks in Metal–Organic Frameworks Based on Imidazoledicarboxylic Acid

When the supramolecular building block packings (face-centered, body-centered, and primitive cubic) with different interactions (hydrogen and coordination bonding) were controlled, four new structures based on octahedral M^II (M = Zn, Ni, Mn) and imidazoledicarboxylate were constructed. The interaction modes between the supramolecular building blocks affect the water stability of the structures. Furthermore, with uncoordinated carboxylate O atoms in the structures, these compounds demonstrate a strong capability of capturing metal ions in the solution

Ordered Mesoporous Platinum@Graphitic Carbon Embedded Nanophase as a Highly Active, Stable, and Methanol-Tolerant Oxygen Reduction Electrocatalyst

Highly ordered mesoporous platinum@graphitic carbon (Pt@GC) composites with well-graphitized carbon frameworks and uniformly dispersed Pt nanoparticles embedded within the carbon pore walls have been rationally designed and synthesized. In this facile method, ordered mesoporous silica impregnated with a variable amount of Pt precursor is adopted as the hard template, followed by carbon deposition through a chemical vapor deposition (CVD) process with methane as a carbon precursor. During the CVD process, in situ reduction of Pt precursor, deposition of carbon, and graphitization can be integrated into a single step. The mesostructure, porosity and Pt content in the final mesoporous Pt@GC composites can be conveniently adjusted over a wide range by controlling the initial loading amount of Pt precursor and the CVD temperature and duration. The integration of high surface area, regular mesopores, graphitic nature of the carbon walls as well as highly dispersed and spatially embedded Pt nanoparticles in the mesoporous Pt@GC composites make them excellent as highly active, extremely stable, and methanol-tolerant electrocatalysts toward the oxygen reduction reaction (ORR). A systematic study by comparing the ORR performance among several carbon supported Pt electrocatalysts suggests the overwhelmingly better performance of the mesoporous Pt@GC composites. The structural, textural, and framework properties of the mesoporous Pt@GC composites are extensively studied and strongly related to their excellent ORR performance. These materials are highly promising for fuel cell applications and the synthesis method is quite applicable for constructing mesoporous graphitized carbon materials with various embedded nanophases

High-Performance Ionic Diode Membrane for Salinity Gradient Power Generation

Salinity difference between seawater and river water is a sustainable energy resource that catches eyes of the public and the investors in the background of energy crisis. To capture this energy, interdisciplinary efforts from chemistry, materials science, environmental science, and nanotechnology have been made to create efficient and economically viable energy conversion methods and materials. Beyond conventional membrane-based processes, technological breakthroughs in harvesting salinity gradient power from natural waters are expected to emerge from the novel fluidic transport phenomena on the nanoscale. A major challenge toward real-world applications is to extrapolate existing single-channel devices to macroscopic materials. Here, we report a membrane-scale nanofluidic device with asymmetric structure, chemical composition, and surface charge polarity, termed ionic diode membrane (IDM), for harvesting electric power from salinity gradient. The IDM comprises heterojunctions between mesoporous carbon (pore size ∼7 nm, negatively charged) and macroporous alumina (pore size ∼80 nm, positively charged). The meso-/macroporous membrane rectifies the ionic current with distinctly high ratio of ca. 450 and keeps on rectifying in high-concentration electrolytes, even in saturated solution. The selective and rectified ion transport furthermore sheds light on salinity-gradient power generation. By mixing artificial seawater and river water through the IDM, substantially high power density of up to 3.46 W/m² is discovered, which largely outperforms some commercial ion-exchange membranes. A theoretical model based on coupled Poisson and Nernst–Planck equations is established to quantitatively explain the experimental observations and get insights into the underlying mechanism. The macroscopic and asymmetric nanofluidic structure anticipates wide potentials for sustainable power generation, water purification, and desalination

Protein Biomineralized Nanoporous Inorganic Mesocrystals with Tunable Hierarchical Nanostructures

Mesocrystals with the symmetry defying morphologies and highly ordered superstructures composed of primary units are of particular interest, but the fabrication has proved extremely challenging. A novel strategy based on biomineralization approach for the synthesis of hematite mesocrystals is developed by using silk fibroin as a biotemplate. The resultant hematite mesocrystals are uniform, highly crystalline, and porous nanostructures with tunable size and morphologies by simply varying the concentration of the silk fibroin and iron­(III) chloride in this biomineralization system. In particular, we demonstrate a complex mesoscale biomineralization process induced by the silk fibroin for the formation of hematite mesocrystals. This biomimetic strategy features precisely tunable, high efficiency, and low-cost and opens up an avenue to access new novel functional mesocrystals with hierarchical structures in various practical applications