Hierarchical Pore System Research Articles

Postsynthesis of FAU-type stannosilicate as efficient heterogeneous catalyst for Baeyer-Villiger oxidation

Sn-Y zeolite with hierarchical pore system was prepared using the atom-planting method from properly dealuminated USY zeolite and SnCl4 vapor. Sn ions were tetrahedrally incorporated into the zeolite framework by the reaction between the SnCl4 molecules and the “silanol nests” generated by steaming and acid treatment. The amount of incorporated Sn depended closely on the dealumination degree, in which the maximum Sn content of 2.6wt.% was achieved when the parent USY zeolite was treated by precisely controlled acid treatment. Compared with Sn-MFI, Sn-MWW, Sn-MCM-41, hydrothermally synthesized or postsynthesized Sn-Beta, thus prepared Sn-Y zeolite showed excellent catalytic properties in the Baeyer-Villiger oxidation of 2-adamantanone using hydrogen peroxide or tert-butyl hydroperoxide as oxidant, which was mainly attributed to the open pore system contributed by 3-dimensional 12-membered ring channels and dealumination-derived intracrystal mesoporosity.

New heterogeneous catalysts based on zeolites with hierarchical pore system

The results of synthesis and packing of crystallized iron-containing nanosized zeolites of MFI structure in dense and loose structures having different textural properties are reported. Data on the formation of additional meso- and macropores immediately during zeolite crystallization using polystyrene microspheres in the form of an aqueous suspension (latex) and as a dry template of closely packed particles are also presented. Advantages and disadvantages of these procedures for preparing hierarchical iron-containing MFI structure zeolites have been compared. Values of the external surface area and the total pore volume of hierarchical Fe-silicalites are significantly higher than those for a reference sample. Using the polymer template to synthesize the hierarchical material consisting of zeolite nanocrystals makes it possible can significantly reduce the synthesis time and simplify the preparation procedure.

Engineering the Morphology of Carbon Materials: 2D Porous Carbon Nanosheets for High‐Performance Supercapacitors

AbstractCompared with traditional porous carbon materials, two‐dimensional (2D) porous carbon materials with a high aspect ratio and a continuous hierarchical porous structure are promising candidates for the construction of supercapacitor electrode materials. Herein, well‐defined, graphene‐based conjugated microporous polymer sheets (G‐CMPs) were prepared in a graphene‐inspired synthetic strategy. Thereby, 4‐iodophenyl‐substituted graphene (RGO‐I) templates not only effectively hinder the aggregation between the adjacent graphene layers but also provide an efficient template for the growth of porous CMPs onto the graphene surface. Moreover, our G‐CMPs could be converted into 2D porous carbon nanosheets by pyrolysis. The synergism of high aspect ratio and high specific surface area, the presence of graphene‐based conduction paths and hierarchical pore systems ensure a high ion‐accessible surface area of the corresponding supercapacitor electrodes thus allowing accelerated electron and ion diffusion/transport and leading to high specific capacitance as well as good rate capability and cycling stability.

Catalytic Properties of Hierarchical Mordenite Nanosheets Synthesized by Self-Assembly Between Subnanocrystals and Organic Templates

An effective and rapid procedure to synthesize hierarchical MOR (Hi-MOR) nanosheets by self-assembly between subnanocrystals and organic templates was reported. The hierarchical pore systems provided a large quantity of Bronsted acid sites. Compared with conventional mordenite, Hi-MOR showed higher activity, xylene selectivity and longer catalytic life in the reaction of toluene disproportionation, thanks to its excellent diffusion from hierarchical structure. Hierarchical morednite (Hi-MOR) nanosheets were synthesized by self-assembly between subnanocrystals and organic templates.

Biological Chitin-MOF Composites with Hierarchical Pore Systems for Air-Filtration Applications.

Metal-organic frameworks (MOFs) are promising materials for gas-separation and air-filtration applications. However, for these applications, MOF crystallites need to be incorporated in robust and manageable support materials. We used chitin-based networks from a marine sponge as a non-toxic, biodegradable, and low-weight support material for MOF deposition. The structural properties of the material favor predominant nucleation of the MOF crystallites at the inside of the hollow fibers. This composite has a hierarchical pore system with surface areas up to 800 m(2) g(-1) and pore volumes of 3.6 cm(3) g(-1) , allowing good transport kinetics and a very high loading of the active material. Ammonia break-through experiments highlight the accessibility of the MOF crystallites and the adsorption potential of the composite indicating their high potential for filtration applications for toxic industrial gases.

Synthesis of Ordered Mesoporous Carbon Materials by Dry Etching.

A novel synthesis method for ordered mesoporous carbons is presented. The inverse replication of a silica template was achieved using the carbonization of sucrose within mesoporous KIT-6. Instead of liquid acid etching, as in classical nanocasting, a novel dry chlorine etching procedure for template removal is presented for the first time. The resultant ordered mesostructured carbon material outperforms carbons obtained by conventional hard templating with respect to high specific micro- and mesopore volumes (0.6 and 1.6 cm(3) g(-1) , respectively), due to the presence of a hierarchical pore system. A high specific surface area of 1671 m(2) g(-1) was achieved, rendering this synthesis route a highly convenient method to produce ordered mesoporous carbons.

Thin‐Sheet Carbon Nanomesh with an Excellent Electrocapacitive Performance

A facile yet effective chemical vapor deposition (CVD) method to prepare carbon nanomesh (CNM) with MgAl‐layered double oxides (LDO) as sacrificial template and ferrocene as carbon precursor is reported. Due to the combined effect of the LDO template and organometallic precursor, the as‐made hexagonal thin‐sheet CNM features a hierarchical pore system consisting of micropores and small mesopores with a size range of 1–6 nm, and a great number of random large mesopores with a pore size of 10–50 nm. The density, geometry, and size of the pores are strongly dependent on the CVD time and the annealing conditions. As supercapacitor electrode, the CNM exhibits an enhanced capacitance, high rate capability, and outstanding cycling performance with a much‐shortened time constant. The excellent capacitive performance is due to the presence of the large mesopores in the 2D CNM, which not only offer additional ion channels to accelerate the diffusion rate across the thin sheets but also help to make efficient use of the oxygen functional groups at the edges of large mesopores to increase the pseudocapacitance contribution.

Efficient Synthesis of Hierarchical Porous Carbons for Application in Li-S Cells

The Li-S battery is a promising candidate to meet the demands on future generation mobile power supplies due to the high theoretical gravimetrical energy density and low price of sulfur. Unfortunately, the active material sulfur exhibits a very low conductivity, which limits the performance. A nanostructured carbon matrix is required. Up to date, large numbers of novel, elaborate host structures have been explored. However, in particular, suitable and effective production processes need to be established. To overcome the existing drawbacks we focus on the development of a scalable and efficient approach for the synthesis of nanostructured porous carbon materials by using of inexpensive, commercially available ZnO nanoparticles as hard template.[1] One major benefit of the process is the in-situ removal of all pore building components through carbothermal reduction. Thus, no toxic/reactive gases are required and purification of the as-prepared porous carbon is not necessary. Adjusting the ZnO-NP template size, porous carbons with internal pore volumes as high as 3.9 cm3 g-1 and tailored pore sizes reaching from 20 nm to 200 nm with ideal template replication are obtained. The carbothermal reduction process, furthermore, facilitates the access of a hierarchical pore system with distinctive microporosity leading to very high specific surface areas even exceeding 3000 m-2 g-1. The synergetic advantage of that bimodal porosity renders them particularly suitable for the application in high energy density lithium-sulfur batteries. A high capacity of > 1200 mAh g-1-sulfur (> 750 mAh g-1-electrode) at a high sulfur loading of ≥ 3 mg cm-2 and good cycle stability was achieved even using a low amount of electrolyte. This allows for further weight reductions and maintenance of high energy density on cell level. [1] Strubel et al., Adv. Funct. Mater. 2015, 25, 287–297. doi: 10.1002/adfm.201402768.

Multi Length-Scale Quantification of Hierarchical Microstructure in Designed Microtubular SOFC Electrodes

Extrusion techniques can be used to produce microtubular SOFCs with a wide range of hierarchical microstructures. It is believed that by controlling the pore structure of the thicker supporting electrode, the transport properties can be optimised. X-ray micro-tomography has been used to image the finger-like voids in a microtubular anode. These structures are on the order of 10 microns in diameter; however, smaller characteristic lengthscales can be imaged using FIB/SEM, which reveals the packing of the sintered particles, as well as the pore structure within the particles. Quantifying the tortuosity factor at each of these lengthscales offers insight into the effect of tailoring the pores. The tortuosity factor of porous SOFC electrodes is used to account for the apparent reduction in species diffusion caused by the geometry of the network. It affects both the electrical transport in the solid phases as well as gas diffusion in the pores. The tortuosity factor is ideally calculated by solving the steady-state diffusion equation; however in systems spanning several orders of magnitude it is very computationally expensive. The figure shows the distribution of a scalar field in an interdigitated void structure, calculated using a finite element method. This work combines the calculated tortuosity factors of each lengthscale to try and understand the significance of the hierarchical pore structure. The straight, radial pores of the microtubes imaged were found to augment the system significantly compared to an isotropic network of equivalent porosity. The importance of recasting the tortuosity factor as a function of distance from the electrolyte for hierarchical pore systems is also discussed. Figure 1

The Fluid Interface Reactions, Structures and Transport (FIRST) Energy Frontier Research Center

The overarching goal of the FIRST Center, which is in its sixth year of operation, is to develop fundamental understanding and validated, predictive models of the unique nanoscale environment at fluid-solid interfaces, that will enable transformative advances in electrical energy storage and electrocatalysis. The Center is primarily based at Oak Ridge National Laboratory, and includes research at Argonne National Lab, Drexel, Penn State, Vanderbilt, and the Universities of California at Davis and Riverside, Delaware and Minnesota. In order to achieve our goal, we integrate novel substrate and electrolyte synthesis and characterization, advanced electron (TEM) and scanning probe microscopies (SPM), neutron and X-ray scattering, and multiscale computational modeling ranging from quantum Monte Carlo to classical density functional theory approaches. Electrolytes investigated include aqueous, polar organic and room temperature ionic liquids (RTILs), representing increasing cost and electrochemical stability, and decreasing viscosity, competing factors in device performance, where charge/discharge rates are affected by viscosity, while stored energy in electrochemical capacitors increases with the square of the applied potential. Organic solvents have the added problem of high volatility and flammability. We investigate the interactions of these fluids with a wide range of carbon materials and recently an entirely new class of 2D transition metal carbonitrides, known as MXenes. Like graphenic materials, MXenes can be conductive, but they can also be strongly hydrophyllic and well-suited for multivalent ion adsorption and intercalation. Our recent efforts have focused on predicting the functionality of interfacial systems for capacitive and pseudocapacitive electrical energy storage in microdevice to grid scale applications. The structures, dynamics and transport properties of RTILS at planar, curved and enclosed (nanopore) surfaces is intensely investigated, and we have begun investigating the role of imidazolium RTIL cations in co-catalyzing the highly efficient and selective electrocatalytic conversion of CO2 to CO at negatively charged, post-transition-metal electrode surfaces as a first step in Fischer-Tropsch-type processes for converting CO2 to liquid fuels and chemical feedstocks. Unique contributions of the FIRST Center include the development of 1.) Electrochemical Strain Microscopy, an SPM method that probes the expansion and contraction of both battery and nanoporous electrochemical capacitor electrodes during charging/discharging with nanometer spatial resolution; 2.) in situ, operando electrochemical flow cells S/TEM studies of ion transport and interfacial reactivity with subnanometer resolution; 3.) unique applications of neutron scattering to probe the structures and transport properties of fluids within hierarchical pore systems; 4.) onion-like carbon nanoparticles with sphereical external surfaces and graphite-like conductivity and inertness; 5.) defect-driven and selective transport of protons across large, pristine single-layer graphene sheets via a Grotthus mechanism; 6.) Electrochemical Flow Capacitor systems for grid scale applications; 7) coarse-grained models like classical density functional theory and reactive classical MD force fields to achieve fundamental and predictive understanding of interfacial system functionality at larger time and length scales; and 8.) efficient lower temperature and/or less toxic synthesis routes for a wide array of ultrahigh surface area carbon and MXene materials.

Hierarchical core–shell zeolite composite ZSM-5@SAPO-34 fabricated by using ZSM-5 as the nutrients for the growth of SAPO-34

A series of zeolite composites with a hierarchical pore system were prepared by using ZSM-5 zeolite powder as the nutrients for the growth of SAPO-34. Factors controlling the formation of the aimed zeolite composite were investigated in details. The structural, crystalline, and textural properties of the zeolite composites, as well as the references ZSM-5, SAPO-34 and physical mixture composed of ZSM-5 and SAPO-34, were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) analyses, intrusive mercury experiments, Fourier transform infrared spectroscopy (FTIR), temperature-programmed desorption of ammonia (NH3-TPD), and nitrogen adsorption/desorption techniques. The results display that the incompatibility of the core and shell layer with the different zeolite structure types, chemical compositions and crystallization conditions can be circumvented by using ZSM-5 zeolite cores as the nutrients for the growth of SAPO-34 zeolite in the shell; acidity properties and the ratios of SAPO-34/ZSM-5 in the composites can be adjusted; a hierarchical system with a mesopore centering at about 15nm and a macropore centering at about 150nm was introduced into the zeolite composite samples.

Hierarchical porous carbons as a metal-free electrocatalyst of triiodide reduction for dye-sensitized solar cells

Hierarchical porous carbons (HPCs) are prepared by a one-step hydrothermal route using soluble resols as a carbon source, polystyrene spheres and Pluronic F127 as macroporous and mesoporous templates, respectively. The resulting HPCs comprise continuous macro–meso–micropore networks on the carbon blocks, simultaneously exhibiting large surface areas (501–709 m2 g−1), which are particularly evaluated as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). Electrochemical impedance spectra (EIS), Tafel polarization curves, and cyclic voltammograms (CVs) demonstrate that the hierarchical pore system plays a crucial role in promoting the catalytic activities of the HPC CEs. In comparison with other porous carbon CEs, the HPC CEs with various macropores and appropriate micro/mesopores present better catalytic behaviors due to the easier accessibility for electrolyte ions and better utilization of the catalytically active sites, resulting in the higher photovoltaic conversion efficiencies of DSSCs. The optimum photovoltaic conversion efficiency of DSSCs based on HPC CEs is 7.22%, which can even match up to that of the cell with Pt CE (7.25%).

Attempts to design porous carbon monoliths using porous concrete as a template

The preparation of carbon monoliths with defined shape and hierarchical pore system has been investigated. A template assisted route was applied using porous concrete as the template. The synthesis route was combined with a one pot chemical activation using phosphoric acid and zinc chloride, respectively, as the activating agents. As the template caused the formation of primary macropores the activation created a secondary system of micropores which resulted in a significant increase of the specific surface area up to about 1400m2g−1. The material was characterized by means of mercury intrusion, helium pycnometry, and nitrogen adsorption at 77K.

Synthesis of mesoporous NiO nanoflake array and its enhanced electrochemical performance for supercapacitor application

Highly porous metal oxide nanoflake arrays are important for developing high-performance electrochemical devices. Herein, mesoporous NiO nanoflake arrays have been successfully prepared by a facile hydrothermal synthesis method. Except for the mesoporous walls, the NiO nanoflakes are interconnected with each other to form macroporous structure. This porous structured NiO nanoflake arrays possess a high specific surface area of ∼185m2g−1. The supercapacitor performance of the mesoporous NiO nanoflake arrays is fully characterized. The mesoporous NiO nanoflake arrays exhibit weaker polarization, higher specific capacitance and better rate capability than the common NiO nanoflake arrays. The mesoporous NiO nanoflake arrays deliver a specific capacitance of 400Fg−1 at 2Ag−1 and 339Fg−1 at 40Ag−1, respectively, higher than those of the common NiO nanoflake arrays (287Fg−1 at 2Ag−1 and 179Fg−1 at 40Ag−1). The enhanced performance is mainly due to the hierarchical pore system in the mesoporous NiO nanoflake arrays.

CO2-activated, hierarchical trimodal porous graphene frameworks for ultrahigh and ultrafast capacitive behavior

Herein, we demonstrate CO2-activated macroscopic graphene architectures with trimodal pore systems that consist of 3D inter-networked macroporosity arising from self-assembly, mesoporosity arising from the intervoids of nanosheets, and microporosity via CO2 activation. The existence of micropores residing in hierarchical structures of trimodal porous graphene frameworks (tGFs) contributes to greatly improve the surface area and pore volume, which are ∼3.8 times greater than typical values of existing 3D macroporous graphene monoliths. As confirmed by the specific capacity, the kinetic parameters, and the regeneration capability for chemical adsorption as well as the specific capacitance, the rate capability, and the cycle stability for electrochemical energy storage, the tGFs have an ideal texture for high performance capacitive materials. Furthermore, the tGFs obtain the structurally and energetically homogeneous surface active sites, which dominantly operate through the π-π interactions for adsorption. Consequently, the ultrahigh capacitance and ultrafast capacitive performance of the tGFs for both chemical and electrochemical adsorptions are attributed to hierarchical trimodal porosity and surface chemistry. These results offer a chemical approach combining self-assembly with conventional activation for the construction of 3D hierarchical structures with multimodal porosity.

SiO2-Al2O3 composite oxides with hierarchical pores as solid acid catalysts for tetrahydrofuran polymerization

A series of SiO2-Al2O3 composite oxides with different parameters of porous structure was synthesized via sol-gel process at a systematically varied pH (pH 2, 5, 7, 9, 11, 13), and characterized by transmission electron microscope, N2 adsorption-desorption measurements, X-ray powder diffraction, Fourier transform infrared spectroscopy, temperature-programmed desorption of ammonia and IR spectra of pyridine adsorption. The catalytic performance of SiO2-Al2O3 was investigated in the catalytic polymerization of tetrahydrofuran. All the SiO2-Al2O3 oxides are characterized by similar acidity but different porous properties. In the pH range of 7 to 9, the hierarchical pore system composed of mesopores and macropores is formed. Due to an enhanced accessibility of acid sites and easier diffusion of reacting molecules, the samples containing a hierarchical pore system of the catalysts show the highest yield of polytetrahydrofuran (about 48%) and an improved number-average molecular weight (Mn).

Transformation of porous glasses into MCM-41 containing geometric bodies

This contribution presents the transformation of Controlled Pore Glass (CPG) granules into MCM-41 material. Porous glasses with pore diameters between 34 and 390nm and varying pore volumes were used as starting materials. In dependence on the textural properties of the starting porous glasses and the synthesis conditions a pseudomorphic transformation of porous glass granules into hierarchically structured MCM-41 granules was accomplished. Such MCM-41 granules exhibit not only the original macroscopic shape but also the pore morphology of the starting porous glass. Based on systematic variations of alkalinity, glass pore volume and glass pore diameter the requirements for a pseudomorphic transformation are defined.Materials with a hierarchical pore system (meso- or macropores of the porous glass, ordered mesopores inside the former glass walls) could be generated via the variation of the molar ratio between cetyltrimethylammonium hydroxide and total cetyltrimethylammonium template amount (CTAOH/CTAOH+CTAB). The transformation products were characterized by nitrogen adsorption, mercury intrusion, X-ray diffraction, scanning electron and light microscopy.During the transformation the primary pore walls were found to swell into the initial pore space, which is caused by bulk density differences between the non-porous glass walls and the template containing MCM-41 material. A pore volume above 1.0cm3/g and pore diameters above 60nm were found to be prerequirements for a pseudomorphic transformation of the porous glasses into MCM-41. Furthermore, a complete transformation was only possible for glasses exhibiting sufficiently high surface area, which was not the case for very large pore diameters of around 390nm.

Hierarchical Carbide‐Derived Carbon Foams with Advanced Mesostructure as a Versatile Electrochemical Energy‐Storage Material

Highly porous carbide‐derived carbon (CDC) mesofoams (DUT‐70) are prepared by nanocasting of mesocellular silica foams with a polycarbosilane precursor. Ceramic conversion followed by silica removal and high‐temperature chlorine treatment yields CDCs with a hierarchical micro‐mesopore arrangement. This new type of polymer‐based CDC is characterized by specific surface areas as high as 2700 m2 g−1, coupled with ultrahigh micro‐ and mesopore volumes up to 2.6 cm3 g−1. The relationship between synthesis conditions and the properties of the resulting carbon materials is described in detail, allowing precise control of the properties of DUT‐70. Since the hierarchical pore system ensures both efficient mass transfer and high capacities, the novel CDC shows outstanding performance as an electrode material in electrochemical double‐layer capacitors (EDLCs) with specific capacities above 240 F g−1 when measured in a symmetrical two‐electrode configuration. Remarkable capacities of 175 F g−1 can be retained even at high current densities of 20 A g−1 as a result of the enhanced ion‐transport pathways provided by the cellular mesostructure. Moreover, DUT‐70 can be infiltrated with sulfur and host the active material in lithium–sulfur battery cathodes. Reversible capacities of 790 mAh g−1 are achieved at a current rate of C/10 after 100 cycles, which renders DUT‐70 an ideal support material for electrochemical energy‐storage applications.

Biogenic metal–organic frameworks: 2,5-Furandicarboxylic acid as versatile building block

2,5-Furandicarboxylic acid (FDA) is a promising renewable building block. It can be derived from cellulosic biomass and therefore poses no competition to the food chain. FDA is considered a valuable replacement for terephthalic acid in the industrial production of poly(ethylene terephthalate). Thus, the biogenic linker was investigated in the formation of metal–organic frameworks (MOFs). Novel crystalline compounds and gels have been synthesized and extensively characterized. The bi-functional linker forms Cu-, Al- and Fe-containing framework materials with hierarchical pore systems and high surface areas. Depending on the synthesis conditions crystalline Cu MOFs have been obtained exhibiting dense packings as well as open microporosity. In contrast, Al and Fe resulted in three-dimensional amorphous metal–organic framework gels with hierarchical pore systems ranging from micro- and meso-up to small macropores. By varying the metal connector as well as synthesis conditions the adsorption properties regarding the pore size and specific surface area can thus be tailored utilizing the versatile biogenic linker FDA that can be produced from renewable resources.

Micron-Sized Single-Crystal-like CoAPO-5/Carbon Composites Leading to Hierarchical CoAPO-5 with Both Inter- and Intracrystalline Mesoporosity

Commercial mesopores-containing carbon (carbon black BP2000) has been used as crystallization host as well as hard template of intracrystalline mesopores in CoAPO-5 materials prepared with N-methyldicyclohexylamine as specific structure-directing agent. As a result, micrometer-sized AFI-structured terminated-tip (pencil-like) hexagonal prims have been obtained. Despite the appearance of single crystals, the crystals are actually composites formed by a large number of elongated-shape CoAPO-5 and carbon nanodomains. Indeed, the removal of carbon by calcination leaves an inorganic Co-doped AlPO4-based material having a complex hierarchical pore system, formed by the expected microporosity for an AFI-structured material and both intra- and intercrystalline mesoporosity. In good agreement with the nanocomposite nature of these micrometer-sized crystals, the size of their fibers generated by burning the carbon matrix can be relatively controlled by the conditions of the calcination processes. Under optimized synthesis and calcination conditions, AFI-structured materials with high mesoporous surface area and pore volume can be afforded.