Hierarchically porous materials： Synthesis strategies and emerging applications

Great interests have arisen over the last decade in the development of hierarchically porous materials. The hierarchical structure enables materials to have maximum structural functions owing to enhanced accessibility and mass transport properties, leading to improved perfor- mances in various applications. Hierarchical porous materials are in high demand for applications in catalysis, adsorption, separation, energy and biochemistry. In the present review, recent advances in synthesis routes to hierarchically porous materials are reviewed together with their catalytic contributions.

Nitrogen-doped hierarchically porous carbon spheres for low concentration CO_(2) capture

Synthesis of spherical carbon beads with effective CO_2 capture capability is highly desirable for large scale application of CO2 sorption, but remains challenging. Herein, a facile and efficient strategy to prepare nitrogen-doped hierarchically porous carbon spheres was developed via co-pyrolyzation of poly(vinylidene chloride) and melamine in alginate gel beads. In this approach, melamine not only serves as the nitrogen precursor, but also acts as a template for the macropores structures. The nitrogen contents in the hierarchically porous carbon spheres reach a high level, ranging from 11.8 wt% to 14.7 wt%, as the melamine amount increases. Owing to the enriched nitrogen functionalities and the special hierarchical porous structure, the carbon spheres exhibit an outstanding CO_2 capture performance, with the dynamic capacity of as much as about 7 wt% and a separation factor about 49 at 25 °C in a gas mixture of CO_2/N_2(0.5:99.5, v/v).

Polysaccharide from Loquat Leaves to Prepare Hierarchical Porous Carbon as a Matrix for Active Sulfur

Fossil fuel exhaustion and overdevelopment usually lead to a recession,which is worsened by the environmental pollution.So it is of high priority to develop high-efficiency energy storage device.Here,agreen and environment-friendly strategy is devised to fabricate carbon materials from biomass.By water extraction and alcohol precipitation,polysaccharide is extracted from loquat leaves.After calcining under high temperature,hierarchical porous carbon materials(HPCM)are obtained,possessing a variety of macropores,mesopores and micropores.Such ample and hierarchical pores enable the electrolyte infiltration and the buffering of the volume expansion of sulfur in repeated electrochemical reactions.The structure stability of the entire electrode can thus be well maintained.When evaluated as the scaffold for sulfur,the electrochemical performance of carbon/sulfur composite was tested.Even after 500 cycles,the reversible capacity is retained as high as 485.4 mA·h/g at the current density of 1.6 A/g.It also offers a notable rate capability,attaining the discharge capacity of 700.7 mA·h/g at 2 C.All the electrochemical performance results prove the feasibility of the proposed strategy.

Hierarchically porous composite fabrics with ultrahigh metal–organic framework loading for zero-energy-consumption heat dissipation

The long-term safe operation of high-power equipment and integrated electronic devices requires efficient thermal management,which in turn increases the energy consumption further.Hence,the sustainable development of our society needs advanced thermal management with low,even zero,energy consumption.Harvesting water from the atmosphere,followed by moisture desorption to dissipate heat,is an efficient and feasible approach for zero-energy-consumption thermal management.However,current methods are limited by the low absorbance of water,low water vapor transmission rate(WVTR)and low stability,thus resulting in low thermal management capability.In this study,we report an innovative electrospinning method to process hierarchically porous metal–organic framework(MOF)composite fabrics with high-efficiency and zero-energy-consumption thermal management.The composite fabrics are highly loaded with MOF(75 wt%)and their WVTR value can be up to 3138 g m^(-2) d^(-1).The composite fabrics also exhibit stable microstructure and performance.Under a conventional environment(30℃,60%relative humidity),the composite fabrics adsorb water vapor for regeneration within 1.5 h to a saturated value Wsat of 0.614 g g^(-1),and a corresponding equivalent enthalpy of 1705.6 J g^(-1).In the thermal management tests,the composite fabrics show a strong cooling capability and significantly improve the performance of thermoelectric devices,portable storage devices and wireless chargers.These results suggest that hierarchically porous MOF composite fabrics are highly promising for thermal management of intermittent-operation electronic devices.

Tailored boron phosphate monolithic foam with multimodal hierarchically porous structure and its applications

The synthesis of multimodal hierarchically porous materials is of great challenge by facile approach.Herein,we assemble BPO_(4) hollow spheres into macroscopic foam materials with multimodal hierarchically porous structure by combining down-to-up process and Ostwald ripening effect.Tailored monolithic B_(2)O_(3)@BPO_(4) foams were obtained from a sticky hydrogel precursor by a one-step annealing process.The foam has the self-supporting frame of BPO_(4) hollow spheres with covering B_(2)O_(3) nanowires and shows excellent permeability and relatively high surface area due to hierarchical structure.The formation mechanism of monolithic B_(2)O_(3)@BPO_(4) foams mainly undergoes inflation,particle aggregation,and Ostwald ripening process.Monolithic foams exhibit superior catalytic activity in oxidation dehydrogenation of alkanes due to the sufficient exposure of active sites over the special frame structure.Furthermore,various monolithic functionalized BPO_(4) foam composites can be easily synthesized and exhibit superior performance in different applications including the oxidation of carbon monoxide,and the self-driven removal of organic pollutants.More interestingly,we also found the sticky hydrogel precursor possesses good heat shielding effect.This work provides a new insight for constructing multimodal hierarchically porous materials with the remaining superior property of nanoscale to cope with various challenges.

The influence of pore size distribution on thermal conductivity,permeability, and phase change behavior of hierarchical porous materials

Porous foams have been widely employed to improve heat storage rate and prevent leakage of phase change materials(PCMs).Actual porous foams have non-uniform or hierarchical pore size distribution, which is usually neglected in most researches.Here, we establish hierarchical porous models considering different pore size distributions by using Voronoi tessellations. Effects of pore size distribution on thermal conductivity, permeability, and phase change behavior of hierarchical porous foams/PCMs composites are investigated. Uneven pore size distributions are found to decrease the thermal conductivity of porous foams to some extent. On the other hand, the permeability can be reduced by 27.6%, and the heat storage rate can be improved by 7.7% by introducing moderate hierarchy without changing the total porosity. This work opens a new route to enhance heat storage performance of PCMs via leveraging hierarchy of pore size distribution of porous foams.

Heteropolyacids supported on hierarchically macro/mesoporous TiO_(2):efficient catalyst for deep oxidative desulfurization of fuel

The fabrication of polyoxometalates(POMs)-modified TiO_(2)catalysts with connected pore structure has attracted great interests due to its prominent effect on mass transfer and catalytic oxidation activity.Here,we report a series of hierarchically macro/mesoporous(M/m)phosphotungstic acid(HPW)/TiO_(2)composites,which are fabricated by colloidal crystal template method and applied as deep desulfurization catalyst in fuel oil.As-synthesized hierarchical HPW/TiO_(2)composite is interconnected by ordered macroporous channel with disordered mesoporous embedded on pore walls.Moreover,Keggintype HPW is homogeneously dispersed on the TiO_(2)matrix.Hierarchical macro/mesoporous HPW/TiO_(2)shows an excellent catalytic performance,the removal rate for dibenzothiophene in model fuel reaches 99%under the optimum conditions.This high performance of the three-dimensional ordered macropores(3DOM)HPW/TiO_(2)can be attributed to its hierarchically porous which is highly beneficial for the mass transfer during the catalytic process.Moreover,the used catalyst could be regenerated by centrifugation and only slight decreasing of the catalytic activity after five cycles.

Trimodel hierarchical yolk-shell porous materials TS-1@mesocarbon:Synthesis and catalytic application

Trimodal hierarchical yolk-shell materials consisting of TS-1 core and mesoporous carbon shell （YS-TS- I@MC） was successfully synthesized by using TS-l@mesosilica as hard template, sucrose as carbon source and organic base tetrapropylammonium hydroxide （TPAOH） as silica etching agent. The resultant YS-TS-I@MC contains the micropores （0.51 nm） in TS-1 core, the mesopores （2.9 rim） in carbon shell as well as a void or a stack pore between TS-1 fragements （TS-1 intercrystal mesopores, -18.4 nm）. Under the rigorous etching conditions, the crystalline structure of TS-1 core was well retained. The YS-TS- I@MC served as a good support for palladium nano-particles （Pd NPs） or Rh（OH）x species, giving rise to efficient bifunctional catalysts for the tandem reactions including one-pot synthesis of propylene oxide or amides.

Sulfonylcalix[4]arene-based metal-organic polyhedra with hierarchical porous structures for efficient Xe/Kr separation

Multiple space from the interior of metal-organic polyhedra(MOPs),the exterior among MOPs,and the inherent nature of big organic molecules makes MOPs as promising platform with hierarchical porous structures,especially when well-elucidated reticular chemistry principles were used.Herein we describe the preparation of a series of isoreticular octahedral MOPs featuring Zn4-p-tert-butylsulfonylcalix[4]arene clusters by the metal-directed assembly of three rigid organic ligands with different lengths.Intercage hydrogen-bonds and hydrophobic interactions between sulfonylcalix[4]arene groups direct the stacking of discrete MOPs into a novel permanent hierarchical porous material.More importantly,the optimal MOP 1-Zn exhibits high adsorption capacity of Xe and excellent Xe/Kr(20/80,v/v)separation performance,as demonstrated by adsorption isotherms,breakthrough experiments,and density functional theory calculations.Additionally,grand canonical Monte Carlo(GCMC)and dispersion-corrected density functional theory(DFT-D)theoretical calculations provide molecular-level insight over the adsorption/separation mechanism.

Generation of Hierarchical Pores in Metal-Organic Frameworks by Introducing Rigid Modulator

The fabrication of hierarchically porous metal-organic frameworks(HP-MOFs)that combine tunable mesopore sizes with high stability is highly desired but remains a technical challenge.Herein,a facile and versatile“rigid modular-assisted defect formation”strategy has been developed to transform microporous MOFs into their corresponding HP-MOFs.By controlling the modulator dose and acid amount,the pore size and mesopore percentage can be finely regulated.Based on the hierarchical pores,the mass transfer of molecules with large sizes is significantly promoted.As a result,the representative HP-UiO-66-NH_(2)-OTf,decorated with a trifluoromethanesulfonate(OTf)group to enhance Lewis acidity,exhibits excellent activity and selectivity in tandem catalysis,far superior to pristine UiO-66-NH_(2).This work provides a novel strategy to the general synthesis of stable HP-MOFs.