Tunable Hierarchical Porosity Research Articles

3D printing of hierarchically porous monolithic TS‐1 catalyst for one-pot synthesis of ethylene glycol

Structured monolithic catalysts have attracted increasing attention in the petrochemical industry due to their advantages of high heat and mass transfer rate, easy scale-up, and reusability. Herein, we put forward a strategy for one-step preparation of SiO2-based monolithic TS-1 catalysts with optimal hierarchical scaffolds by combining the flow field simulation of the reactor and 3D printing technology. Tunable hierarchical porosity from nano to micro scales, suitable pore characteristics, and numerous active sites have been obtained by innovatively combining the spherical porous structure of TS-1/SiO2 material, direct ink writing (DIW) and post-sintering processes. Combined with computational fluid dynamics (CFD) analysis, the effects of different channel offset angles, axial and radial channel sizes on the 3D printed monolithic TS-1 in the catalytic reaction of one-step oxidation of ethylene to ethylene glycol (EG) were verified. 3D-printed monolithic TS-1 catalysts exhibited superior crush strengths (135.11 N/cm) and extraordinary catalytic efficiency, in which the yield of EG reached 82.7%, and the utilization of hydrogen peroxide reached 85.3%. Moreover, the 3D printed TS-1 monolith maintained high catalytic performance over 5 reusable cycles, which solved the problem that traditional catalysts are difficult to be manufactured, separated, and recycled, and presented considerable prospects in reactions requiring fluid agitation.

Defect Dominated Hierarchical Ti‐Metal‐Organic Frameworks via a Linker Competitive Coordination Strategy for Toluene Removal

AbstractConstructing a hierarchically porous structure in metal–organic frameworks (MOFs) can improve the accessibility of active sites and facilitate mass diffusion to enhance their adsorption and catalytic efficiency. Here, a novel linker competitive coordination strategy based on the electronegativity difference of two kinds of organic linkers is proposed to tailor the porous structure of Ti‐MOF (MIL‐125). A series of Ti‐MOFs with continuously tunable hierarchical porosity is obtained by simply adjusting the molar ratios of two organic linkers. The demonstration of toluene removal shows that the competitive coordination strategy not only contributes to wide pore size distribution to enhance the adsorption performance of toluene, but also endows good charge separation ability to facilitate photocatalytic performance. Finally, the toluene removal efficiency of optimal Ti‐MOF with mixed organic linkers of 1:1 molar ratio is 2.14 times and 1.88 times of the pristine MIL‐125 and MIL‐125(NH2), respectively. This strategy opens a new prospect to optimize Ti‐MOF properties for various heterogeneous catalysis applications.

Room-Temperature Rapid Synthesis of Two-Dimensional Metal–Organic Framework Nanosheets with Tunable Hierarchical Porosity for Enhanced Adsorption Desulfurization Performance

Two-dimensional (2D) metal–organic framework (MOF) nanosheets with tunable hierarchical porosity have promising applications owing to their fascinating physicochemical characteristics. However, one...

Rational Design of Mimic Multienzyme Systems in Hierarchically Porous Biomimetic Metal-Organic Frameworks.

A facile approach was reported to establish mimic multienzyme systems with hierarchically porous (HP) biomimetic metal-organic frameworks (MOFs) and natural enzymes for tandem catalysis. The hierarchically porous MOF HP-PCN-224(Fe) with peroxidase-like activity and tunable hierarchical porosity was synthesized via a modulator-induced strategy. HP-PCN-224(Fe) not only acts as the enzyme-immobilization matrix but also as an effective enzyme mimic, which could cooperate with the immobilized natural enzyme to catalyze the cascade reactions. The mimic multienzyme systems were used for the efficient colorimetric detection of a series of biomolecules, including glucose and uric acid. This work displays the great potential to construct highly functional biocatalysts by integrating the merits of both natural enzymes and MOF mimics, which are promising for applications in biosensing and biomimetic catalysis.

Mineral-Templated 3D Graphene Architectures for Energy-Efficient Electrodes.

3D graphene networks have shown extraordinary promise for high-performance electrochemical devices. Herein, the chemical vapor deposition synthesis of a highly porous 3D graphene foam (3D-GF) using naturally abundant calcined Iceland crystal as the template is reported. Intriguingly, the Iceland crystal transforms to CaO monolith with evenly distributed micro/meso/macropores through the releasing of CO2 at high temperature. Meanwhile, the hierarchical structure of the calcined template could be easily tuned under different calcination conditions. By precisely inheriting fine structure from the templates, the as-prepared 3D-GF possesses a tunable hierarchical porosity and low density. Thus, the hierarchical pores offer space for guest hybridization and provide an efficient pathway for ion/charge transport in typical energy conversion/storage systems. The 3D-GF skeleton electrode hybridized with Ni(OH)2 /Co(OH)2 through an optimal electrodeposition condition exhibits a high specific capacitance of 2922.2 F g-1 at a scan rate of 10 mV s-1 , and 2138.4 F g-1 at a discharge current density of 3.1 A g-1 . The hybrid 3D-GF symmetry supercapacitor shows a high energy density of 83.0 Wh kg-1 at a power density of 1011.3 W kg-1 and 31.4 Wh kg-1 at a high power density of 18 845.2 W kg-1 . The facile fabrication process enables the mass production of hierarchical porous 3D-GF for high-performance supercapacitors.

3D Printing of Artificial Leaf with Tunable Hierarchical Porosity for CO2 Photoreduction

The development of new pathways for 3D artificial photosynthetic systems (APS) with controllable architectures and tunable hierarchical porosity on a large scale is significant. Herein, we demonstrate a 3D printing approach for fabricating artificial microleaves with 3D architectures spanning orders of magnitude from nanometers to centimeters in a rapid, programmable, and scalable manner. TiO2-based inks served as a preliminary prototype, with surfactants and silica nanospheres incorporated for porosity modification. Thus, a TiO2-based ink is developed to allow for the fabrication of porosity-tunable hierarchical 3D architectures with high surface area (up to ∼259 m2g–1) and structural integrity with well-designed patterns. The artificial microleaves have macropore architectures comparable to those of natural leaves, indicating their efficient mass transfer ability. Artificial photosynthesis via CO2 reduction enhances CO and CH4 evolution on the 3D printed APS by up to 2-fold and 6-fold, respectively, com...

Bottom-Up Assembly of Silica and Bioactive Glass Supraparticles with Tunable Hierarchical Porosity.

We investigate the formation of spherical supraparticles with controlled and tunable porosity on the nanometer and micrometer scales using the self-organization of a binary mixture of small (nanometer scale) oxidic particles with large (micrometer scale) polystyrene particles in the confinement of an emulsion droplet. The external confinement determines the final, spherical structure of the hybrid assembly, while the small particles form the matrix material. The large particles act as templating porogens to create micropores after combustion at elevated temperatures. We control the pore sizes on the micrometer scale by varying the size of the coassembled polystyrene microspheres and produce supraparticles from both silica- and calcium-containing CaO/SiO2 particles. Although porous supraparticles are obtained in both cases, we found that the presence of calcium ions substantially complicated the fabrication process since the increased ionic strength of the dispersion compromises the colloidal stability during the assembly process. We minimized these stability issues via the addition of a steric stabilizing agent and by mixing bioactive and silica colloidal particles. We investigated the interaction of the porous particles with bone marrow stromal cells and found an increase in cell attachment with increasing pore size of the self-assembled supraparticles.

3D printing of concentrated emulsions into multiphase biocompatible soft materials.

3D printing via direct ink writing (DIW) is a versatile additive manufacturing approach applicable to a variety of materials ranging from ceramics over composites to hydrogels. Due to the mild processing conditions compared to other additive manufacturing methods, DIW enables the incorporation of sensitive compounds such as proteins or drugs into the printed structure. Although emulsified oil-in-water systems are commonly used vehicles for such compounds in biomedical, pharmaceutical, and cosmetic applications, printing of such emulsions into architectured soft materials has not been fully exploited and would open new possibilities for the controlled delivery of sensitive compounds. Here, we 3D print concentrated emulsions into soft materials, whose multiphase architecture allows for site-specific incorporation of both hydrophobic and hydrophilic compounds into the same structure. As a model ink, concentrated emulsions stabilized by chitosan-modified silica nanoparticles are studied, because they are sufficiently stable against coalescence during the centrifugation step needed to create a bridging network of droplets. The resulting ink is ideal for 3D printing as it displays high yield stress, storage modulus and elastic recovery, through the formation of networks of droplets as well as of gelled silica nanoparticles in the presence of chitosan. To demonstrate possible architectures, we print biocompatible soft materials with tunable hierarchical porosity containing an encapsulated hydrophobic compound positioned in specific locations of the structure. The proposed emulsion-based ink system offers great flexibility in terms of 3D shaping and local compositional control, and can potentially help address current challenges involving the delivery of incompatible compounds in biomedical applications.

Nanostructured ZnO as a structural template for the growth of ZIF-8 with tunable hierarchical porosity for CO2 conversion

We report a new approach to introduce hierarchical porosity into ZIF-8 by using three-dimensional nanostructured porous ZnO as a structural template.

Advanced nanoarchitectures of carbon aerogels for multifunctional environmental applications

AbstractIn recent decades, carbon aerogels (CAs) have attracted increasing attention. Compared with other carbon materials, CAs are available in macroscopic diverse morphologies with tunable hierarchical porosity and large specific surface area. The interconnected carbonaceous framework also endows CAs with excellent electrical properties. CA can be a promising material for various purposes through proper design and functionalization. In this article, we place an emphasis on CAs based on resorcinol-formaldehyde resin and graphene sheets. We review the state-of-the-art progress on the preparation of functionalized CAs, and the relationships between their advanced nanoarchitectures and performances in environmental remediation, including adsorption, capacitive deionization and catalysis, are discussed. The challenges in design and application of CAs are also proposed.

Graphene/ZIF-8 composites with tunable hierarchical porosity and electrical conductivity

We present a new approach to introduce hierarchical porosity along with electrical conductivity into three-dimensional metal–organic frameworks.

One-pot, template-free synthesis of a robust superhydrophobic polymer monolith with an adjustable hierarchical porous structure

Multifunctional polymer monoliths with both tunable hierarchical porosity and damage-tolerant superhydrophobic properties are fabricated via a one-pot, template-free process.

Methanol Tolerant Oxygen Reduction Activity of Nitrogen-Iron Doped Carbon in a Hollow Core Mesoporous Shell Structure

A well-defined porous carbon structure with tunable parameters in different length scales can provide a platform for systematic study of transport and confinement effects on electrochemical conversion. For example, the carbon structure with tunable hierarchical porosity composed of a hollow core within a mesoporous shell (HCMS) is used to investigate transport limitations in electrochemical capacitance [1, 2]. Recently, we synthesized the same HCMS carbon structure with doping of nitrogen and iron and investigated the electrocatalytic activity for oxygen reduction in acid medium [3]. As shown in Fig. 1, the electrocatalyst has a mesoporous shell of 50 nm thick around a hollow core of 350 nm. The pores have uniform diameter of 8 nm. The synthesis follows the procedure described in [1] and the introduction of iron nitrate and ethylene diamine as nitrogen and iron precursors [3, 4]. Despite having no precious metals, the Fe-N-HCMS electrocatalyst displays excellent performance for oxygen reduction reaction. The better mass transport of the HCMS structure leads to higher diffusion limitation currents compared to other carbon structures. The Fe-N doped HCMS catalyst also performs better at high current than a similar HCMS carbon with Pt nanoparticles but no Fe-N doping. This is attributed to uniformly distributed activity of Fe-N doped carbon as opposed to localized activity Pt nanoparticles which is vulnerable to diffusion limitation. Furthermore, Fe-N/HCMS carbon electrocatalyst is inactive towards methanol oxidation, thus rendering it ideal for methanol tolerant oxygen reduction. As shown in Fig. 2, platinum containing electrocatalysts has an overall anodic current between above 0.6 V/RHE due to methanol oxidation, whereas Fe-N-HCMS carbon is insensitive to presence of methanol. The Fe-N-HCMS electrocatalyst also display good durability in accelerated degradation tests (ADTs) of over 1000 cycles of voltage scan between 0.6 and 1.0 V/RHE. Under the same ADTs, Pt containing electrocatalysts all show large increase in overpotential at the end of 1000 cycles. Acknowledgement The work was supported by a grant from Hong Kong Research Grants Council GRF HKU 700209P) and funding from ICEE via SRT Clean energy and UDF of University of Hong Kong.

Hierarchical macro-meso-microporous ZSM-5 zeolite hollow fibers with highly efficient catalytic cracking capability.

Zeolite fibers have attracted growing interest for a range of new applications because of their structural particularity while maintaining the intrinsic performances of the building blocks of zeolites. The fabrication of uniform zeolite fibers with tunable hierarchical porosity and further exploration of their catalytic potential are of great importance. Here, we present a versatile and facile method for the fabrication of hierarchical ZSM-5 zeolite fibers with macro-meso-microporosity by coaxial electrospinning. Due to the synergistic integration of the suitable acidity and the hierarchical porosity, high yield of propylene and excellent anti-coking stability were demonstrated on the as-prepared ZSM-5 hollow fibers in the catalytic cracking reaction of iso-butane. This work may also provide good model catalysts with uniform wall thickness and tunable porosity for studying a series of important catalytic reactions.

Tunable hierarchical porosity from self-assembled chitin–silica nano-composites

We studied new mesoporous materials with original properties and obtained from self-assembled chitin–silica nano-composites. Our novel synthesis allows the controlled colloidal assembly of α-chitin nanorods (bundles of elongated chitin monocrystals) and siloxane oligomers. Calcination of nano-composites results in mesoporous silica materials. Their pore volume fraction ϕPOR (0–0.52) is strongly correlated to the initial chitin content. Using N2 sorption and TEM data, we identify and characterize primary and secondary textural units related to the imprints of chitin monocrystals (2.5 nm wide) and nanorods (20–30 nm wide) respectively. Primary textural units are preserved over a wide ϕPOR range (linear relationship between pore volume and specific surface area). The coating of monocrystals by siloxane oligomers leads to a siloxane network of fractal nature as deduced from complementary SAXS data. Beyond a critical value ϕPOR′ estimated near 0.2, the coating is partial, and the porosity becomes more open and connected. At larger scales, the arrangements of secondary textural units result in complex textures and long-range ordering, showing similarities with textural features found in natural materials. We discuss the competition between entropy-driven transitions typical of anisotropic particles and kinetic arrest due to colloidal gelation and inorganic condensation. Finally, a schematic model for texture formation is given.