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

Unraveling the Structure–Reactivity Relationship of CuFe₂O₄ Oxygen Carriers for Chemical Looping Combustion: A DFT Study

CuFe2O4 is an emerging high-performance oxygen carrier for chemical looping combustion (CLC), which is hailed as the most promising technology to reduce combustion-derived CO2 emission. CuFe2O4 oxygen carriers with minute structural differences could be largely divergent in the reactivity for the CLC process, which seems not to raise much concern by either experimental or computational studies. Herein, based on density functional theory (DFT) calculations, we compare the performance of three well-documented CuFe2O4 configurations as oxygen carriers in the CLC process and relate the reactivity difference to their structural nuances. The reaction mechanisms of representative CLC reactants (i.e., CH4, H2, and CO) over different CuFe2O4 configurations are explored in-depth. DFT calculations indicate that among different CuFe2O4 configurations, the distribution, orientation, and activity of the O/Cu/Fe sites vary largely over the respective CuFe2O4(100) surfaces, thus affecting the adsorption and oxidation of CLC reactants. Fe atoms, especially in configuration 3, are observed to exhibit a higher degree of exposure and afford lower steric hindrance to interact with CH4 and H2, thereby facilitating higher adsorption energies and lower dissociation energy barriers correspondingly. The Fe–Cu synergistic effect is revealed to promote the dissociation reaction of both CH4 and H2. CO exhibits direct oxidation to CO2 over the O sites, which generally exhibit higher CO binding energies than Cu/Fe sites. Particularly, O sites in configuration 3 are observed with generally lower oxygen vacancy formation energy as well as steric hindrance, thus affording the oxidation of CO in a more facile way. The structure–performance relationship revealed in this work is of positive significance for the design of high-performance spinel CuFe2O4 oxygen carriers

Designed Fabrication and Characterization of Three-Dimensionally Ordered Arrays of Core–Shell Magnetic Mesoporous Carbon Microspheres

A confined interface coassembly coating strategy based on three-dimensional (3-D) ordered macroporous silica as the nanoreactor was demonstrated for the designed fabrication of novel 3-D ordered arrays of core–shell microspheres consisting of Fe₃O₄ cores and ordered mesoporous carbon shells. The obtained 3-D ordered arrays of Fe₃O₄@mesoporous carbon materials possess two sets of periodic structures at both mesoscale and submicrometer scale, high surface area of 326 m²/g, and large mesopore size of 19 nm. Microwave absorption test reveals that the obtained materials have excellent microwave absorption performances with maximum reflection loss of up to −57 dB at 8 GHz, and large absorption bandwidth (7.3–13.7 GHz, < −10 dB), due to the combination of the large magnetic loss from iron oxides, the strong dielectric loss from carbonaceous shell, and the strong reflection and scattering of electromagnetic waves of the ordered structures of the mesopores and 3-D arrays of core–shell microspheres

Hierarchically Ordered Macro-/Mesoporous Silica Monolith: Tuning Macropore Entrance Size for Size-Selective Adsorption of Proteins

In this paper, hierarchically ordered macro-/meso porous silica monoliths with 3D fcc packed macropores and 2D hexagonally arranged mesopores are synthesized by using polymer colloidal crystals as the hard template and block copolymer Pluronic P123 as a soft template. Through the impregnation of the colloidal crystal hard template with an acidic ethanol solution containing silica source and P123, the entrance size of macropores can be tailored by controlling the synthesis conditions. As the acid concentration increases, the resulting mesopore sizes tend to decrease slightly in the range of 4.7−3.6 nm, meanwhile, the macropore entrance size increase gradually from 0 to ca. 200 nm. These highly ordered macro-/mesoporous silica monoliths have a macropore of about 1.0 μm with tunable window size (0−200 nm), high surface area (ca. 330 m2/g) and large pore volume (∼ 0.36 cm3/g). Biomacromolecule adsorption results show that the porous silica monolith with the macropore entrance of about 50 nm has absorption capacity of ∼16.6 mg/g for bovine serum albumin (BSA, ∼10 nm in size), much higher than that (∼3.4 mg/g) of the porous silica materials without macropore entrance, and the porous materials with or without macropore entrances exhibit similar a adsorption capacity for cytochrome c (Cyt.c, dimension: ∼ 3 nm) (∼36.8 mg/g), suggesting that the large guest molecules can be excluded by the porous silica monoliths without the macopore entrances. These results indicate that, through engineering the pore connection and multimodal pore system, porous materials with hierarchically pores and tailorable window sizes can be created for size-selective applications, such as enrichment, nanofiltration, and drug delivery

MgO-Based Granular Sorbent Pelletized by Using Ordered Mesoporous Silica as Binder for Low-Temperature CO₂ Capture

Cyclic CO2 adsorption by using MgO as a sorbent at low temperatures is considered a promising route for postcombustion CO2 capture. However, most MgO-based sorbents are in the form of fine powder and cannot be used in a fluidized bed reactor, and at the same time, suffer from a rapid loss in CO2 uptake capacity due to the decrease of surface area aroused by pore shrinking and grain sintering. In this study, mesoporous silicas with highly ordered pore structures have been used as binders, for the first time, to fabricate MgO-based sorbent pellets via a simple and scalable extrusion–spheronization approach. The obtained MgO-based pellets exhibit high porosity attributed to the nature of the mesoporous binder, leading to a significantly increased stability and CO2 uptake capacity. Especially for the low-concentration CO2 that is comparable to the flue gas from a coal-fired power plant, the results show that the ordered mesoporous silica binder provides a remarkable promotion effect and excellent stability in the capture performance. The CO2 uptake capacity of the best-performing sorbent, 20-KIT-6–100, displays a small decline of 6.86% (from 1.02 mmol of CO2/g in the first cycle to 0.95 mmol of CO2/g in the 10th cycle). It is envisaged that mesoporous materials hold great potential to be used as binders in reinforcing the metal oxide-based sorbents for flue-gas CO2 capture in practical applications

A General Chelate-Assisted Co-Assembly to Metallic Nanoparticles-Incorporated Ordered Mesoporous Carbon Catalysts for Fischer–Tropsch Synthesis

The organization of different nano objects with tunable sizes, morphologies, and functions into integrated nanostructures is critical to the development of novel nanosystems that display high performances in sensing, catalysis, and so on. Herein, using acetylacetone as a chelating agent, phenolic resol as a carbon source, metal nitrates as metal sources, and amphiphilic copolymers as a template, we demonstrate a chelate-assisted multicomponent coassembly method to synthesize ordered mesoporous carbon with uniform metal-containing nanoparticles. The obtained nanocomposites have a 2-D hexagonally arranged pore structure, uniform pore size (∼4.0 nm), high surface area (∼500 m²/g), moderate pore volume (∼0.30 cm³/g), uniform and highly dispersed Fe₂O₃ nanoparticles, and constant Fe₂O₃ contents around 10 wt %. By adjusting acetylacetone amount, the size of Fe₂O₃ nanoparticles is readily tunable from 8.3 to 22.1 nm. More importantly, it is found that the metal-containing nanoparticles are partially embedded in the carbon framework with the remaining part exposed in the mesopore channels. This unique semiexposure structure not only provides an excellent confinement effect and exposed surface for catalysis but also helps to tightly trap the nanoparticles and prevent aggregating during catalysis. Fischer–Tropsch synthesis results show that as the size of iron nanoparticles decreases, the mesoporous Fe–carbon nanocomposites exhibit significantly improved catalytic performances with C₅₊ selectivity up to 68%, much better than any reported promoter-free Fe-based catalysts due to the unique semiexposure morphology of metal-containing nanoparticles confined in the mesoporous carbon matrix

Selectivity Enhancement in Dynamic Kinetic Resolution of Secondary Alcohols through Adjusting the Micro-Environment of Metal Complex Confined in Nanochannels: A Promising Strategy for Tandem Reactions

Dichloro­(η⁶-<i>p</i>-cymene) (1-butyl-3-cyclohexyl-imidazolin-2-ylidene) ruthenium­(II) (<b>RuL</b>) was synthesized and confirmed. Five heterogeneous catalysts with similar ruthenium cores were prepared by chemical immobilization method using various silica-based supports, including mesoporous silica SBA-15 of different pore sizes (<b>Ru/Si-9</b>, <b>Ru/Si-8</b>, and <b>Ru/Si-7</b>), nonporous silica particles (<b>Ru/SiO</b>_<b>2</b>), and surface trimethylsilylated SBA-15 (<b>Ru/SiMe</b>). The dynamic kinetic resolution (DKR) of 1-phenylethanol, which includes metal–enzyme bicatalytic racemization in tandem with stereoselective acylation, gave product in 99% yield and 0% ee with homogeneous catalyst <b>RuL</b>, whereas the heterogeneous <b>Ru/Si-8</b> exhibited high catalytic activity and enantioselectivity (up to 96% yield and 99% ee). The racemization and acylation abilities of different catalysts were analyzed. The influences of pore size and surface properties for heterogeneous catalysts were investigated, and the nanocage effect was found to be the key factor in stereoselectivity. The catalyst <b>Ru/Si-8</b> performed well in reactions with various substrates and can be reused for at least seven times

In-Situ Crystallization Route to Nanorod-Aggregated Functional ZSM‑5 Microspheres

Herein, we develop a reproducible in situ crystallization route to synthesize uniform functional ZSM-5 microspheres composed of aggregated ZSM-5 nanorods and well-dispersed uniform Fe3O4 nanoparticles (NPs). The growth of such unique microspheres undergoes a NP-assisted recrystallization process from surface to core. The obtained magnetic ZSM-5 microspheres possess a uniform size (6–9 μm), ultrafine uniform Fe3O4 NPs (∼10 nm), good structural stability, high surface area (340 m2/g), and large magnetization (∼8.6 emu/g) and exhibit a potential application in Fischer–Tropsch synthesis

Solvent Evaporation Induced Aggregating Assembly Approach to Three-Dimensional Ordered Mesoporous Silica with Ultralarge Accessible Mesopores

A solvent evaporation induced aggregating assembly (EIAA) method has been demonstrated for synthesis of highly ordered mesoporous silicas (OMS) in the acidic tetrahydrofuran (THF)/H2O mixture by using poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA) as the template and tetraethylorthosilicate (TEOS) as the silica precursor. During the continuous evaporation of THF (a good solvent for PEO-b-PMMA) from the reaction solution, the template molecules, together with silicate oligomers, were driven to form composite micelles in the homogeneous solution and further assemble into large particles with ordered mesostructure. The obtained ordered mesoporous silicas possess a unique crystal-like morphology with a face centered cubic (fcc) mesostructure, large pore size up to 37.0 nm, large window size (8.7 nm), high BET surface area (508 m2/g), and large pore volume (1.46 cm3/g). Because of the large accessible mesopores, uniform gold nanoparticles (ca. 4.0 nm) can be introduced into mesopores of the OMS materials using the in situ reduction method. The obtained Au/OMS materials were successfully applied to fast catalytic reduction of 4-nitrophenol in the presence of NaHB4 as the reductant. The supported catalysts can be reused for catalytic reactions without significant decrease in catalysis performance even after 10 cycles

Free-Standing Mesoporous Carbon Thin Films with Highly Ordered Pore Architectures for Nanodevices

We report for the first time the synthesis of free-standing mesoporous carbon films with highly ordered pore architecture by a simple coating–etching approach, which have an intact morphology with variable sizes as large as several square centimeters and a controllable thickness of 90 nm to ∼3 μm. The mesoporous carbon films were first synthesized by coating a resol precursors/Pluronic copolymer solution on a preoxidized silicon wafer and forming highly ordered polymeric mesostructures based on organic–organic self-assembly, followed by carbonizing at 600 °C and finally etching of the native oxide layer between the carbon film and the silicon substrate. The mesostructure of this free-standing carbon film is confirmed to be an ordered face-centered orthorhombic Fmmm structure, distorted from the (110) oriented body-centered cubic Im3̅m symmetry. The mesoporosity of the carbon films has been evaluated by nitrogen sorption, which shows a high specific BET surface area of 700 m2/g and large uniform mesopores of ∼4.3 nm. Both mesostructures and pore sizes can be tuned by changing the block copolymer templates or the ratio of resol to template. These free-standing mesoporous carbon films with cracking-free uniform morphology can be transferred or bent on different surfaces, especially with the aid of the soft polymer layer transfer technique, thus allowing for a variety of potential applications in electrochemistry and biomolecule separation. As a proof of concept, an electrochemical supercapacitor device directly made by the mesoporous carbon thin films shows a capacitance of 136 F/g at 0.5 A/g. Moreover, a nanofilter based on the carbon films has shown an excellent size-selective filtration of cytochrome c and bovine serum albumin