Self-formation phenomenon to hierarchically structured porous materials: design, synthesis, formation mechanism and applications

In this paper, we will thoroughly review a novel and versatile self-formation phenomenon that can be exploited to target porous hierarchies of materials without need of any external templates only on the basis of the chemistry of metal alkoxides and alkylmetals. These hierarchically porous materials have unique structures, which are made of either parallel funnel-like/straight macrochannels or 3D continuous interconnected macroporous foams with micro/mesoporous walls. The self-generated porogen mechanism has been proposed, leading to a series of techniques to tailor porous hierarchy, i.e. the use of different chemical precursors (single metal alkoxides, mixed metal alkoxides, single molecular precursors with two different alkoxide functionalities, alkylmetals, etc., …), the control of their hydrolysis and condensation rates (pH, chelating agents,…) and the addition of alkoxysilanes as co-reactant. Various chemical compositions from single or binary metal oxides, to aluminosilicates, aluminophosphates, silicoaluminophosphates, metallophosphates,… can be prepared, offering a panel of potential applications. Some perspectives have been proposed to transform the synthesized materials with a hierarchy of pore sizes to micro-meso-macroporous crystalline materials with zeolite architectures. The advantages of this self-formation preparation method have been discussed compared to traditional templating methods. The possibility to combine with other strategies, for example soft or hard templating, to target even more sophisticated hierarchically meso-macroporous materials with specific structure and function for various applications has been presented. The "hierarchical catalysis" concept has been re-visited.     

A review on the green synthesis of hierarchically porous zeolite

Conventional bottom-up and top-down methods for synthesizing hierarchical zeolite have led to complicated economic and environmental issues due to the requirement of expensive and hazardous organic molecules, the large amount of acid/base solution, high energy, and expensive starting materials. Besides, the bottom-up method through the hydrothermal crystallization evokes safety issues due to the high autogenous pressure. Accordingly, considerable efforts have been made to develop green route synthesis of hierarchical zeolite by eliminating the use of a solvent (solvent-free), utilizing sustainable starting materials and green secondary template (mesoporogen), as well as eliminating the use of mesoporogen (mesoporogen-free). Other routes, including recycling of mother liquor, steam-assisted conversion, gel-like-solid phase method, and silanization, are also elaborated, as they are reported to promote a green and facile approach for the synthesis of hierarchical zeolites. In this review, we provide recent progress on the development of the green synthesis of hierarchically porous zeolite.     

Solvothermal synthesis of hierarchically porous CeO2 nanopalm leaves and their photocatalytic properties

Hierarchically porous CeCO₃OH palm leaves were synthesized by using a facile solvothermal method at 180 °C utilizing cerium nitrate and ethylene glycol. Polycrystalline CeO₂ palm leaves were obtained from the thermal decomposition-oxidation process of the CeCO₃OH nanostructures. CeCO₃OH and CeO₂ palm leaves were characterized by X-ray diffraction pattern, scanning electron microscopy image, energy dispersive X-ray analyses, and ultraviolet (UV)–visible spectroscopy images. The CeO₂ palm leaves exhibited a significant photocatalytic performance in degrading azo dye acid orange II in an aqueous solution under an UV radiation. The formation mechanisms of the CeCO₃OH/CeO₂ nanostructures were described on the basis of the experimental results.     

多级孔催化剂的合成及应用(英文)

系统总结了多级孔分子筛、多级孔金属氧化物和多级孔碳基材料等多级孔催化剂材料的研究进展, 主要阐述了多级孔催化剂材料已有的制备方法及其在催化领域中的应用.     

Synthesis of hierarchically porous inorganic-metal site-isolated nanocomposites

Hierarchically porous inorganic nanocomposites have been synthesized combining interconnected macropores and mesopores with a high loading of site-isolated gold nanoparticles.     

Template-assisted synthesis of hierarchically porous Co_3O_4 with enhanced oxygen evolution activity

Oxygen evolution reaction(OER) is one of the most important reactions in the energy storage devices such as metal–air batteries and unitized regenerative fuel cells(URFCs). However, the kinetically sluggishness of OER and the high prices as well as the scarcity of the most active precious metal electrocatalysts are the major bottleneck in these devices. Developing low-cost non-precious metal catalysts with high activity and stability for OER is highly desirable. A facile, in situ template method combining the dodecyl benzene sulfuric acid sodium(SDBS) assisted hydrothermal process with subsequent high-temperature treatment was developed to prepare porous Co_3O_4 with improved surface area and hierarchical porous structure as precious catalysts alternative for oxygen evolution reaction(OER). Due to the unique structure, the as-prepared catalyst shows higher electrocatalytic activity than Co_3O_4 prepared by traditional thermal-decomposition method(noted as Co_3O_4-T) and commercial IrO_2 catalyst for OER in 0.1M KOH aqueous solution. Moreover, it displays improved stability than Co_3O_4-T. The results demonstrate a highly efficient, scalable, and low cost method for developing highly active and stable OER electrocatalysts in alkaline solutions.     

A facile synthesis of hierarchically porous Cu-BTC for efficient removal of uranium(VI)

Journal of Radioanalytical and Nuclear Chemistry - A new, effective, and economic adsorbent (PAA-Do), consist of Polyacrylamide and Dolomite was synthesized chemically and used for removal of...     

Preparation of hierarchically porous nickel from macroporous silicon

A novel approach for preparing hierarchical porous nickel structures has been developed. It is an aqueous processing of the template, macroporous silicon, which was etched in HF solution to have straight channels. By immersion in a prepared nickel solution, the silicon sidewalls were replaced by nickel deposits to form a total metallic structure. Since silicon framework was gradually consumed during the displacement reactions, the deposited nickel displayed additional feature of being highly porous on the sidewalls. With the initial straight microchannels, the metal structure was hierarchically porous.     

Facile fabrication and characterization of hierarchically porous calcium carbonate microspheres

Higher-order porous calcite microspheres exhibiting high specific surface areas, unusual morphologies and textures were fabricated by a simple precipitation reaction of CaCO3 in the presence of PSMA as a crystal modifier.     

Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths

Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol–gel reaction. Starting from AlCl₃·6H₂O and poly(ethylene oxide) (PEO) dissolved in a H₂O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H₂O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H₂O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO.     

New hierarchically porous titania monoliths for chromatographic separation media

Separation media based on hierarchically porous titania (TiO₂) monoliths for high‐performance liquid chromatography (HPLC) have been successfully fabricated by the sol–gel process of titanium alkoxide in a mild condition utilizing a chelating agent and mineral salt. The as‐gelled TiO₂ monoliths were subjected to a simple solvent exchange process from ethanol (EtOH) to H₂O followed by drying and calcination. The resultant monolithic TiO₂ columns consist of anatase crystallites with the typical specific surface area of more than 200 m²/g. The resultant monolithic TiO₂ column calcined at 200 and 400°C exhibited a good separation performance for organophosphates as well as for polar benzene derivatives in the normal‐phase mode.     

Effects of inherent/enhanced solid acidity and morphology of diatomite templates on the synthesis and porosity of hierarchically porous carbon

The inherent or enhanced solid acidity of raw or activated diatomite is found to have significant effects on the synthesis of hierarchically porous diatomite-templated carbon with high surface area and special porous structure. The solid acidity makes raw/activated diatomite a catalyst for the generation of porous carbon, and the porous parameters of the carbon products are strongly dependent on the solid acidity of diatomite templates. The morphology of diatomite also dramatically affects the textural structure of porous carbon. Two types of macroporous structures in the carbon product, the partially solid pillars and the ordered hollow tubes, derive from the replication of the central and the edge pores of diatom shell, respectively. The hierarchically porous carbon shows good capability for the adsorption of solvent naphtha and H(2), enabling potential applications in adsorption and gas storage.     

具有三维贯通多级孔道结构大孔氧化铝的制备与表征

采用铝溶胶晶种引入、结合相分离的方法制备了具有三维贯通多级孔道结构的大孔氧化铝材料。采用扫描电镜（SEM）、X射线衍射（XRD）、N₂吸附-脱附、压汞、核磁共振波谱（NMR）等测试方法对所得材料进行了表征。结果表明,该氧化铝材料具有200-600 nm的均匀分布且贯通的连续大孔孔道,经550℃焙烧即可得到结晶态γ-氧化铝。大孔氧化铝比表面积达到366 m²/g,具有以5 nm及400 nm为中心的较为集中的介孔-大孔多级孔道分布。焙烧后的样品中,铝具有四、六两种配位状态。制备过程中,聚环氧乙烷（PEO）作为诱导剂引发固-液两相分离,形成具有三维贯通多级孔道结构大孔氧化铝,而凝胶中引入铝溶胶时,AlOOH晶粒与铝交联水合物均相伴生,在凝胶过程诱导铝交联水合物转变为AlOOH,最终使大孔氧化铝在较低的焙烧温度即可转化为γ-氧化铝。     

Monitoring bisphosphonate surface functionalization and acid stability of hierarchically porous titanium zirconium oxides

To take advantage of the full potential of functionalized transition metal oxides, a well-understood nonsilane based grafting technique is required. The functionalization of mixed titanium zirconium oxides was studied in detail using a bisphosphonic acid, featuring two phosphonic acid groups with high surface affinity. The bisphosphonic acid employed was coupled to a UV active benzamide moiety in order to track the progress of the surface functionalization in situ. Using different material compositions, altering the pH environment, and looking at various annealing conditions, key features of the functionalization process were identified that consequently will allow for intelligent material design. Loading with bisphosphonic acid was highest on supports calcined at 650 °C compared to lower calcination temperatures: A maximum capacity of 0.13 mmol g(-1) was obtained and the adsorption process could be modeled with a pseudo-second-order rate relationship. Heating at 650 °C resulted in a phase transition of the mixed binary oxide to a ternary oxide, titanium zirconium oxide in the srilankite phase. This phase transition was crucial in order to achieve high loading of the bisphosphonic acid and enhanced chemical stability in highly acidic solutions. Due to the inert nature of phosphorus-oxygen-metal bonds, materials functionalized by bisphosphonic acids showed increased chemical stability compared to their nonfunctionalized counterparts in harshly acidic solutions. Leaching studies showed that the acid stability of the functionalized material was improved with a partially crystalline srilankite phase. The materials were characterized using nitrogen sorption, X-ray powder diffraction, and UV-vis spectroscopy; X-ray photoelectron spectroscopy was used to study surface coverage with the bisphosphonic acid molecules.     

一步处理法制备高水热稳定多级孔ZSM-5分子筛

ZSM-5具有较高的催化活性和独特的择形选择性,因而被广泛用于精细化工和石油炼制等工业过程.但其较小的孔道尺寸导致其在反应中尤其在催化过程中的传质受到影响,从而严重影响催化剂寿命.为了解决反应过程中分子筛中底物及产物的扩散限制问题,近年来关于介微孔复合多级孔道分子筛的研究在分子筛合成领域引起了广泛兴趣,并取得一定进展.但直接合成法存在成本及复杂性问题,因此在量产的分子筛上进行后改性引入介孔表现出明显优势.在这一大类处理过程中,碱处理造介孔因成本低以及可操作性较高而备受青睐.但由于脱硅所形成的介孔往往无序,稳定性较差,因此提高其水热稳定性具有重要意义.文献已有较多报道通过磷元素修饰抑制分子筛在水热环境中脱铝,从而提高分子筛骨架稳定性.但传统的磷元素修饰一般采用后续浸渍法,过程繁琐.本课题组开发了一步法后处理制备高水热稳定多级孔ZSM-5分子筛,将脱硅过程与磷引入过程相结合,以四乙基氢氧化磷(TEPOH)为磷源,直接处理微孔分子筛得到含磷的多级孔分子筛.相比于传统的分子筛磷修饰过程,该磷物种在处理中优先交换分子筛骨架上铝原子附近用来平衡电荷的钠离子,从而增加了磷物种与铝物种相互作用的可能性,提高了稳化骨架的效果.基于此,本文利用氮吸附-脱附、元素分析、X射线粉末衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、氨程序升温脱附(NH3-TPD)以及27Al和31P固体核磁(NMR)等一系列表征技术,证明该一步后处理法对分子筛催化性能的作用.SEM和XRD结果表明,分子筛在进行各种处理前后,相对结晶度和分子筛形貌变化不大,说明大部分微孔得到保留.TEM测试表明,经过该一步后处理法得到的分子筛实现了介孔的引入,介孔孔径在10–20 nm.元素分析结果表明,几乎所有的磷源物质在该过程中被引入到分子筛上,实现了介孔和磷物种的同时引入.NH3-TPD测试表明,含磷分子筛在老化前后较无磷分子筛具有更高的酸性位密度保留度,说明磷元素对分子筛上的酸性位起到了稳化作用.孔结构特性数据不仅说明了脱硅过程中成功引入介孔,而且含磷分子筛老化前后的介孔特性数据保留度明显高于无磷样品,实现了磷元素对介孔结构的稳化作用.27Al和31P NMR结果从理论上证明了该样品上磷元素对抑制骨架脱铝的稳化效应,证实了分子筛在处理后水热稳定性的提高.基于前期的研究工作,本文完善了磷元素对分子筛稳化作用的机理过程.TEPBr在水热活化过程中转变成磷酸盐,并修饰分子筛的骨架铝以及部分难以避免的非骨架铝.该过程中形成的磷铝物种在后续的水热老化过程中进一步修饰分子筛骨架铝,使骨架铝得到稳化.而在脱硅过程中存在的"反插铝"过程往往使大部分骨架铝位于介孔孔道中,磷元素与铝元素的相互作用同时也对介孔进行了稳化.通过正辛烯和1,3,5-三异丙苯的裂化测试发现,处理后的分子筛由于其优化后的孔道性能和酸性性质,大大提高了底物分子的转化率以及其自身的容碳能力,从而延长了催化寿命.     

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).     

Strategies for design and synthesis of porous liquids toward carbon capture and separation

Porous liquids (PLs) represent a promising category of sorbents in carbon capture and separation capable of integrating the advantages of flowing liquid and porous solid systems. Well-defined pores were engineered into liquid sorbents via liquifying molecules with stiff interior voids, dissolving rigid porous hosts in flowing liquids, or dispersing porous frameworks in high steric hindrance solvents, producing type I, II, or III PLs, respectively. Unique features of PLs have triggered broad interest in exploring their applications in carbon capture and separation, in which diverse design strategies, synthesis approaches, and enhanced performance have been reported. In this minireview, recent progress in the design, synthesis, and structural engineering of PLs and efforts towards the optimization of their carbon capture and separation behavior will be summarized, including the comparison between PLs with varied types. Porosity engineering into liquid sorbents provides opportunities to resolve challenging issues in conventional sorption and separation systems.     

A superacid-catalyzed synthesis of porous membranes based on triazine frameworks for CO2 separation

A general strategy for the synthesis of porous, fluorescent, triazine-framework-based membranes with intrinsic porosity through an aromatic nitrile trimerization reaction is presented. The essence of this strategy lies in the use of a superacid to catalyze the cross-linking reaction efficiently at a low temperature, allowing porous polymer membrane architectures to be facilely derived. With functionalized triazine units, the membrane exhibits an increased selectivity for membrane separation of CO(2) over N(2). The good ideal CO(2)/N(2) selectivity of 29 ± 2 was achieved with a CO(2) permeability of 518 ± 25 barrer. Through this general synthesis protocol, a new class of porous polymer membranes with tunable functionalities and porosities can be derived, significantly expanding the currently limited library of polymers with intrinsic microporosity for synthesizing functional membranes in separation, catalysis, and energy storage/conversion.