Functional nanostructured materials: Aerosol, aerogel, and de novo synthesis to emerging energy and environmental applications

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Original Research Paper

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Functional nanostructured materials: Aerosol, aerogel, and de novo synthesis to emerging energy and environmental applications

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Hung-Li Wang a,1, Chang-Yen Hsu b,1, Kevin C.W. Wu b,⇑, Yi-Feng Lin c,⇑, De-Hao Tsai a,⇑

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a

Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC c Department of Chemical Engineering and Research Center for Circular Economy, Chung Yuan Christian University, Taoyuan, Taiwan, ROC b

a r t i c l e

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Article history: Received 11 July 2019 Received in revised form 20 September 2019 Accepted 28 September 2019 Available online xxxx Keywords: Aerosol Aerogel De novo MOF Nanostructure

a b s t r a c t In this review, we introduce advanced synthetic methods for functional nanostructured materials (in powder form) bridging to the development in emerging energy and environmental applications. Three types of synthetic methods (aerosol-based, aerogel-based, and de novo methods) are introduced, all of which have shown to be extensively investigated as novel routes to create nanostructured materials with designed material properties (i.e., controlled size, shape, porosity, and chemical composition are to be achievable). The typical experimental setup and the general experimental procedure for material preparation via the above three synthesis routes are discussed. Complementary characterization approaches are employed to study material properties of the synthesized nanostructured materials via the three synthesis routes. Here we investigate: (1) CuxO-CeO2, Ni-CeO2, and CuxO nanoparticle-encapsulating metal– organic framework (MOF) hybrid nanoparticles synthesized via the aerosol-based method; (2) Crencapsulating MOF (Cr-MOF-199), Au-encapsulating MOF (Au@ZIF-8), and MOF-derived nanocomposites (CuO/CuCr2O4) produced via the de novo route; (3) a variety of aerogels (carbon, metal oxide, polymer) with high porosity created by the aerogel-based approach. Finally, several examples of emerging energy and environmental applications are introduced using these functional nanostructured materials, including (1) catalytic transformation to chemicals by using precious metal nanoparticles-embedded MOFs and the MOF-derived nanocomposites as the catalysts; (2) methane combustion using CuxO-CeO2 hybrid nanoparticles as catalyst, (3) methane dry reforming with CO2 using Ni-CeO2 hybrid nanoparticles as catalyst; (4) CO2 capture by fluoroalkyl silane-modified mesoporous silica and polymethylsilsesquioxane (PMSQ) aerogel membranes; (4) adsorption of organic solvent, dye, and oil by cetyltrimethylammonium bromide-modified PMSQ aerogel. Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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1. Introduction

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Nanostructured material powder has shown to be substantially attractive for a variety of energy and environmental applications. The superior properties of nanostructured materials (e.g., high surface-to-volume ratio, unique surface properties) will create new functionalities at the interface of the nanostructured materials and the media. For example, metal–organic framework (MOF) features exceptionally high surface area, large pore volume, reticular structure, and tunable properties [1–3], showing promise for providing strong metal-support interaction in gas-solid and liquidsolid heterogeneous catalysis [4]. Mesoporous aerogel materials

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⇑ Corresponding authors. E-mail addresses: [email protected] (K.C.W. Wu), [email protected] (Y.-F. Lin), [email protected] (D.-H. Tsai). 1 Equal contribution.

(i.e., pore diameter between 2 and 50 nm) with high porosity (90%), low material density, and high specific surface area possess a lot of unique functionalities for the capture of the gaseous and liquid-phase contaminants via absorption and adsorption [5,6]. Transition metal oxide-based nanoparticles are attractive as nanocatalysts due to the large surface-to-volume ratio with the ability to create a designed interface for the strong metal-support interaction (SMSI) [7–9]. Three important aspects are to be considered to successfully implement the functional nanostructured material powder into the energy and environmental applications, as they are often interplaying during the technological development (Fig. 1). Firstly, developing new concepts of energy and environmental applications via the incorporation of these new functional nanostructured materials is very important to drive the technological development. For example, the nanostructure materials have shown pro-

https://doi.org/10.1016/j.apt.2019.09.039 0921-8831/Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: H.-L. Wang, C. Y. Hsu, K. C. W. Wu et al., Functional nanostructured materials: Aerosol, aerogel, and de novo synthesis to emerging energy and environmental applications, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.039

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Fig. 1. Three important aspects of implementing the functional nanostructured material for energy and environmental applications.

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mise as catalysts for a variety of power generation (e.g., fuel cell, methane combustion) due to the large surface area per unit mass [7,8,10–12]. Secondly, establishing an advanced approach for the synthesis of the functional nanostructured material with the material properties by design is required, and the synthetic method should have excellent controllability of size, shape, porosity, composition, and surface state [7,9,13–17]. The third important aspect is about the metrology and methodology of nanomaterial characterization, which has shown to be critically important to connect the designed material properties of the synthesized nanostructured materials to the performance in their application [18–20]. As a result, the performance of these nanomaterial-manufactured products can further enhance through the rational design of these functional nanostructured materials via the use of advanced synthetic and material characterization methods. Firstly, to provide a comprehensive scope to the latest development in preparation of new functional nanostructured materials, three types of synthetic routes are discussed in this review, all of which have shown to be extensively investigated as novel routes over the decade: (1) aerosol-based synthetic method, (2) aerogelbased synthetic method and (3) de novo method. Transition metal/metal oxide hybrid nanoparticles, metal–organic framework (MOF) encapsulated with fine nanoparticles, MOF-derived composites, and oxide, polymeric and carbon aerogels are chosen as the representative nanostructured materials in this review, all of which are shown be highly attractive for emerging energy and environmental applications. Complementary characterization methods, including physical-type, microscopic, and spectroscopic approaches, are introduced for the study of material properties of these functional nanostructured materials. Finally, several recently developed functional nanostructured materials for the emerging energy and environmental applications are discussed, including catalytic transformation to chemicals, methane combustion, dry reforming, CO2 capture, and adsorption of organic solvent, dye, and oil.

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2. Controlled synthesis of functional nanostructure powder with material characterization

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2.1. Aerosol-based synthetic approach

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The commonly-used aerosol-based synthetic routes are (1) spray pyrolysis, (2) flame synthesis, (3) laser ablation, and (3) metal evaporation/condensation [7–11,21–42]. In general, the

aerosol-based synthetic methods are advantageous for continuous operation of materials without encountering the issues related to solution-based chemistry (e.g., restriction in the physical and chemical properties of the solvent) [9,11]. Aerosol spray pyrolysis method has shown a promise for the scale-up mass production of the powders of functional nanoparticles (NPs) based on its convenience in use and high throughput, where particle size and chemical composition of the NPs are controllable [41,42]. Besides, the dispersions of the multiple components within the nanostructure are maintained (i.e., at their molecular level homogeneity in solutions) during the spray drying process [9,10,21,38,43]. A schematic diagram for the typical synthesis of functional NP powder (i.e., using CuOx-CeO2 hybrid NP as example) using an aerosol spray method is shown in Fig. 2a [8]. The system consists of a nebulizer (for spraying the precursor), an aerosol diffusion drying unit composed of a preheater and a diffusion dryer, two tube furnaces connected in series (for two stages of thermal treatments on the aerosols), and an aerosol filter for collection of aerosol NP (i.e., in the powder form). In principle, initial homogeneous dispersions are maintained via gas-phase evaporation-induced selfassembly (EISA; from Step 1 to Step 3 of Fig. 2a). The dry precursor aerosols were thermally decomposed to metal oxide NPs after the 1st-stage thermal treatment (Step 4) and subsequently reduced to metallic NPs by the 2nd-stage thermal reduction using H2 (Step 5). The material properties of the functional NPs synthesized by the aerosol spray method are determined by composition of precursor, droplet size, and temperatures of the 1st-stage (e.g., calcination) and the 2nd stage (e.g., reduction, oxidation) thermal treatments [8,24,44]. Droplet size is mainly governed by the choice of the spray device. For example, electrospray generally provides small, fine droplets with a narrow size distribution in comparison to the traditional nebulizer or atomizer [45,46]. Physical properties of the precursor solution (e.g., viscosity, surface tension, density of the precursor solution) and operating conditions of the spray device (e.g., flow rate, operating temperature) will also affect the droplet size. Particle size is shown to be the most critical properties of NPs. Differential mobility analysis (DMA), a gas-phase electrophoretic approach, is often employed to in-situ characterize the physical size of the synthesized NP in aerosol state (i.e., prior to being collected as powder). Fig. 2b shows an example of using DMA to analyze the physical size of copper oxide nanoparticle (CuOx-NP) versus the concentration of Cu precursor (CCu) [9]. By increasing the CCu from 0.1 wt% to 1 wt% and 5 wt%, the averaged mobility

Please cite this article as: H.-L. Wang, C. Y. Hsu, K. C. W. Wu et al., Functional nanostructured materials: Aerosol, aerogel, and de novo synthesis to emerging energy and environmental applications, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.039

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Fig. 2. Aerosol-based synthesis of functional nanoparticle powder using an aerosol spray method. (a) Schematic diagram of the aerosol spray method. (b) Differential mobility analysis of bare CuO NPs with different precursor concentrations of Cu (CCu). (c) Representative SEM images of bare CuO NP. 1: CCu = 0.1 wt%. 2: CCu = 5 wt %. (d) HRTEM images with elemental mapping of the CuOx-based hybrid NPs. 1: CuO-CeO2 hybrid NP; 2: CuO-Al2O3 hybrid NP. (a) Reprinted with permission from Ref. [8]. (b–d) Reprinted with permission from Ref. [9].

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diameter (dp,m) increases from 59.6 nm to 64.2 nm and 66.3 nm, respectively. The results of DMA indicate that the physical size of the NPs synthesized via aerosol spray method is controllable by the tuning of the concentration of metal precursor prior to aerosol synthesis. Microscopic analyses are effective for the ex-situ characterization of the morphology and the primary diameter of the synthesized NP. From the images of scanning electron microscopy

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(SEM; Fig. 2c) [9], the CuOx-NP were spherical due to the aerosol spray drying process [9,21,22,43]. Based on a histogram-based analysis of the SEM images [9], the primary diameter of CuOx-NP increased with CCu [38.8 nm at CCu = 0.1 wt% (2c-1), and 62.8 nm at CCu = 5 wt% (2c-2)]. Besides, the homogeneity of the synthesized hybrid nanoparticle can also be analyzed using the microscopic analysis in combination with an elemental mapping by energy dispersive spectroscopy (EDS). Fig. 2d shows the HRTEM images with the elemental mapping of CuOx-CeO2 and CuOx-Al2O3 hybrid NPs synthesized via the aerosol spray method [9]. Clearly, the Cu atoms were homogeneously distributed along with the Ce or Al, confirming a homogeneous dispersion of CuO and CeO2 or Al2O3 crystallites in the synthesized NP (i.e., via the gas-phase EISA) using the aerosol spray method. The crystallinity and oxidation state of the synthesized NP is often characterized using X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). For the CuO-CeO2 hybrid NPs reduced by H2 at different temperatures (Td2), the crystalline of Ce was unchanged over different Td2 (i.e., CeO2; see Fig. 3a). The crystalline of Cu is mainly CuO for the samples with Td2 < 400 °C (i.e., CuCeOx-NP-RT, CuCeOx-NP-300, and CuCeOx-NP-400), and it transforms to a mixed crystalline of Cu and Cu2O by increasing Td2 to 600 °C (CuCeOx-NP-600). Fig. 3b shows XPS spectra of Cu 2p and Ce 3d regions. The relative intensity of shake-up peaks in the Cu 2p regions typically identified as CuO (i.e., for CuCeOx-NPRT at 944–946 eV) decreased dramatically by increasing the Td2 to 600 °C (CuCeOx-NP-600). In contrast, the XPS spectra of the Ce 3d regions were unchanged (i.e., remain as CeO2). The results of XRD and XPS show a successful selective reduction of CuO in the CuOx-CeO2 hybrid NP using the aerosol spray method (i.e., without inducing the reduction of CeO2 via a two-stage aerosol-based synthetic route). The aerosol-based synthetic route is useful for the fabrication of MOF-based hybrid nanostructures; for example, the CuO NPs encapsulated in a MOF [47]. As depicted in Fig. 4a, firstly a massive amount of liquid-air interface is created through fast evaporation of a dilute, heterogeneous aqueous solution composed of copper nitrate with stabilized MOF colloids (Step 1), after which the Cu precursor NPs are formed and homogeneously distributed in the MOF with a controlled size in the confine porous structure (Step 2). Through a gas-phase thermal decomposition of Cu precursor at a sufficient high T (i.e., without inducing the MOF decomposition), Cu precursor NPs are converted to CuO NPs and encapsulated in the MOF (Step 3). Fig. 4b-1 shows the representative HRTEM image of colloidal UiO-66 after EISA from the nebulized droplets followed by thermal treatment at T = 400 °C. Clearly, UiO-66 crystals become a spherical aggregate with a mesoporous structure, indicating a change of the secondary structure of UiO-66 by the gas-phase EISA. The CuO NPs (0.8 nm in diameter) are homogeneously distributed in the UiO-66 crystal aggregates (denoted as CuxO@UiO-66), where the presence of CuO(1 1 1) and CuO(0 0 2) confirm a complete transformation of Cu precursor to CuO. As presented in Fig. 4b-2 (using SEM plus EDS elemental mapping), Cu, Zr and O atoms are homogeneously distributed in CuxO@UiO-66, confirming that CuO NPs were uniformly dispersed in the porous structure of UiO-66 using the proposed aerosol-based synthetic route.

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2.2. De novo method

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Encapsulation of nanoparticle (NP) into a metal–organic framework (MOF) powder is an attractive route to enhance the functionality of NP [48]. In the past few years, researchers have reported extensive applications of MOFs, such as gas storage, controllable chemical delivery, selective chemical catalysis, and chemical separation [49–52]. The material properties of MOFs depend on both

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Fig. 3. Analyses of crystallinity and surface state of the synthesized CuOx-CeO2 hybrid NPs (via aerosol spray method) with different temperatures of H2-reduction (Td2) using XRD and XPS. (a) XRD patterns. (b) XPS spectra of Cu 2p and Ce 3d regions. 1: CuCeOx -NP-RT. 2: CuCeOx-NP-600. Reprinted with permission from Ref. [8].

metal joints and organic linkers. In addition to the coordination with organic linkers to form a reticular framework, metal joints also serve as active sites in catalytic reactions. The composition of the organic linkers has a significant effect on their properties. For instance, the existence of nitrogen in the organic linkers can affect the affinity of MOFs to water [53]. Except for being catalysts, MOFs are also utilized as support of functionalized catalysts [54,55], which are capable of introducing metallic nanoparticles (MNPs) [56–58]. The merits of the MOFs mentioned above, especially extraordinary high specific surface area, make them favorable for catalyst supports compared to conventional supports such as mesoporous silica, zeolite, metal oxide, and carbon-based materials. Continued on the discussion of Section 2.1, metal nanoparticle (MNP) can be loaded on the MOFs in the powder form to prepare bimetallic or functionalized catalysts by various strategies. Apart from the aerosol-based route discussed in Fig. 4, solution impregnation is shown to be the most widely utilized method, where precursors are introduced to MOFs by concentration gradient in the solution and then reduced into MNPs by adding reducing agents [59]. Although this method features facile and economical in preparation, the presence of a reducing agent stands a chance to destroy part of the MOF and leads to uncontrollable support structures. Besides, most of the guests (MNPs) are likely to load only outside the MOF rather than being encapsulated due to the fact that the aggregation of the MNPs limit their ability in penetration and subsequent deposition inside the porous structure of MOFs. Chemical vapor infiltration is another strategy to load MNPs on MOFs. In principle, organometallic compounds firstly decompose to become vapor under vacuum and high-temperature environment and then deposit onto the supports [60]. However, the method of chemical vapor infiltration is restricted by the boiling point of the chosen chemicals and is also unfavorable in largescale synthesis. Wang et al. [61] reported a synthesis method of

Fig. 4. (a) Synthesis of CuxO@MOF hybrid nanostructures using the gas-phase EISA. (b) Microscopic analysis of CuxO@UiO-66 after EISA. T = 400 °C. 1: HRTEM image analysis of CuxO@UiO-66. 2: SEM image with the elemental mapping of CuxO@ UiO-66. Reprinted with permission from Ref. [47].

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NPs-embedded MOFs without the presence of the reducing agent and stabilizer. However, the use of nitrogen- and oxygencontaining ligands limits its flexibility from further technological development. To resolve the problems mentioned above, this review focus on the synthesis strategy called ‘‘de novo”. De novo synthesis of NPs-embedded MOFs demonstrates a solution-based synthesis method that can evenly distribute MNPs inside the porous structure of MOFs. A schematic diagram for the typical synthesis of MNP-embedded MOFs by de novo synthesis is shown in Scheme 1. Firstly, two types of metal precursors (one for the metal joints and the other is used as active compound embedded inside the MOFs) were mixed in solvent followed by the addition of organic ligands (linker), which entirely dissolved in the solvent; in some cases, reducing agent was introduced into the solution and stirred under a specific temperature. After the fabrication of the MNPs-embedded MOFs, the supernatant was removed, and the precipitate was cleaned with water or organic solvent for several cycles via centrifugation cleaning and dried in a lyophilizer overnight. The morphology of as-synthesized MOF and MNPs-embedded MOF is affected by various factors. In some cases, the encapsulated MNPs dramatically alter the structure of the frame and porous size of mother MOF, while in other cases the MNPs distribute uniformly in the unscathed MOF. To determine the morphology of MNPsembedded MOFs, SEM was utilized to display the microscopic world of catalysts. Fig. 5a–d shows the SEM images of AuCl
4 @ZIF-8 over various Au/Zn ratios. Prior to the addition of AuCl
4 (pure ZIF-8; Fig. 5a) the particle size of ZIF-8 was approximately 70 nm with sharp hexagonal facets. By adding a small amount of AuCl
4 (the molar ratio of Au/Zn = 0.01; Fig. 5b), the particle size of AuCl-4
@ZIF-8 was similar to that of ZIF-8, but the hexagonal facet morphology disappeared. With a further increase of Au/Zn to 0.1 (Fig. 5c) and 0.2 (Fig. 5d), the samples exhibited an irregular shape with particle aggregation. Seetharaj et al. [62] reported that pH value strongly affects the growth of the MOFs (the deprotonation and nucleation reaction between ligands and joints). According to Le Chatelier’s principle, the presence of protons inhibits the formation of ligands. Hence, the low pH value results in the slow nucleation between ligands and joints, which gives rise to form particles with a larger size. Since the most commonly-used Au precursor, chloroauric acid, is a robust monoprotic acid, the ratio of added zinc precursor to the gold precursor has profoundly influenced the size of AuCl
4 @ZIF8. As the Au/Zn ratio raised from 0:1 to 0.2:1, the mode particle size increased from 70 nm to 150 nm. The amount of embedded MNPs can also influence the shape of particles. From Fig. 6, the controlled amount of Cr in MOF-199

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transformed the shape of the particles. In the absence of chromium (Fig. 6a), MOF-199 formed an octahedral structure. Interestingly, the morphology of MOF-199 gradually converted into regular hexahedron by increasing Cr/Cu molar ratio to 0.05:1 (Fig. 6b), 0.1: 1 (Fig. 6c), and 0.2:1 (Fig. 6d). The size of synthesized MNPsembedded MOFs is also affected by the solution temperature when de novo synthesis is in progress. Fig. 7 shows the temperature study of the particle size of Cr-embedded MOF-199. The morphology of the particle was cubic (Fig. 7a-c), and the average particle size increased from 6.3 lm (45 °C; Fig. 7d), 10.1 lm (65 °C; Fig. 7e), to 12.9 lm (85 °C; Fig. 7f) due to the increased rate of frameworks formation of MOF-199. In conclusion, the morphology of MNPs-embedded MOFs is greatly affected by the various experimental parameters, including pH, the molar ratio of reactant, and solution temperature. The crystalline of the synthesized MNPs-embedded MOFs can be evaluated by XRD patterns. As shown in Fig. 8, the characteristic diffraction peaks of the catalysts prepared by de novo method (CrMOF-199 and Au@ZIF-8) are the same with that of its mother MOFs, indicating that the loading of MNPs does not alter the crystalline of MOF. A diffraction peak located at 2h = 20° (as shown in the magnified image of Fig. 8a) indicate the connections between chromium and 1,3,5-benzenetricarboxylate (the ligands of MOF199) [65]. Also, the broad peak of Au@ZIF-8 at 2h = 38° (as shown in the magnified image of Fig. 8b) indicated that the metallic Au was bonded with ZIF-8 as nanoscale particles. Moreover, further treatment of MNPs-embedded MOFs can be employed to strengthen their structure, to adjust the porous sizes, and to prepare the functionalized catalyst. Liao et al. [63] synthesized Au-embedded, nitrogen-doped nanoporous carbon from pyrolysis of Au-embedded ZIF-8 to enhance the affinity to water and to enlarge the pore size for enabling the reactants to penetrate into the reticular structure and react with Au NPs. Liao et al. [66] also reported that charge separation and photoactivity of the titanium-based metal–organic framework (MIL-125) enhanced after thermal treatment to form CuO-embedded mesoporous TiO2. Huang et al. [64] converted Cr-embedded MOF-199 into CuO/CuCr2O4 porous composites for enhancing the mechanical strength by calcination under an oxygen-rich environment. The homogeneity of MNPs-embedded MOFs is studied by HRTEM. Fig. 9a shows the SEM image of cubic CuO/CuCr2O4 composites with a porous structure, which inherited from its mother MOF, MOF-199. The thermal treatment of the carbon in the oxidized ligands and the vacancies in the structure resulting from the absence of carbon were occupied by metal oxide. From Figs. 9b-3 and 9b-4, it is evident that chromium and copper dispersed well in MOF structures, respectively, which indicated that

Scheme 1. Synthetic scheme of nanoparticles-embedded MOFs by de novo synthesis.

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Fig. 5. SEM images of AuCl
4 @ZIF-8 with Au/Zn molar ratios of (a) 0:1, (b) 0.01:1, (c) 0.1: 1, and (d) 0.2:1. Reprinted with permission from Ref. [63].

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de novo synthesis successfully loaded MNPs on the MOFs substrates. The nitrogen adsorption/desorption isotherms and pore size distributions of Au@ZIF-8, ZIF-8, Au@NC, and ZIF-8 derived carbon are shown in Fig. 10. Au@ZIF-8 was fabricated by de novo synthesis. Au@NC and ZIF-8 derived carbon were calcined from Au@ZIF-8 and ZIF-8, respectively. From Fig. 10a, the nitrogen adsorption/desorption isotherms of Au@ZIF-8 and ZIF-8 are similar (i.e., Type I isotherm), indicating that the synthesized catalyst is a microporous solid. Moreover, these two materials are also in the ascendant of its total pore volume (1.662 and 1.107 cm3 g
1, respectively). Thus, the physical properties of synthesized catalyst inherited its mother MOFs, indicating that de novo strategy retain the merits from the chosen MOF substrates. The nitrogen adsorption/desorption isotherms confirmed that the structure retains its outstanding specific inner surface area and total pore volume for heterogeneous catalysis. The consequences remain from the comparison between Au@NC, and ZIF-8 derived carbon (Fig. 10c). They also share a similar type of nitrogen adsorption/desorption isotherm (Type 1) and close values of the specific surface area (477.7 and 558.1 m2 g
1 respectively) and the total pore volume (0.566 and 0.970 cm3 g
1 respectively). As shown in Fig. 10b, the mode pore diameter of ZIF-8 and Au@ZIF-8 are around 1.2 nm and 0.9–1.2 nm, respectively. The reducing pore diameter resulted from the loading of Au MNPs in the pores of ZIF-8. This evidence reveals that Au particles were successfully embedded inside ZIF-8 in the form of MNP. From Fig. 10d, the distributions of pore diameter of the two materials are identical when the pore diameters are smaller than 2 nm. Besides, a broad peak of pore diameter for the Au@NC ranging from 2 to 6 nm is identified in Fig. 10d. Since ZIF-8 has been reported for

its flexible frames [67], Au precursor (3.4 ± 1.4 nm) was able to penetrate through the pores (1.1 nm for ZIF-8) and loaded inside the ZIF-8 and ZIF-90 using a chemical or a thermal treatment by the solvent-free, gas‐phase method [63,67]. Hence, calcination of the ZIF-8 only enlarged pore size and distorted the frames, rather than destroyed the whole structures. Liao et al. also compared the material properties of the Auencapsulated MOF-derived carbon synthesized via the de novo method with the sample using commercially available activated carbon loaded with Au nanoparticles [63]. The results show that the sample using MOF-derived carbon had a high specific surface area and exhibited a more uniform distribution of the encapsulated Au nanoparticles than the sample using the conventional activated carbon. The results show a promise in enhancing catalytic performance (e.g., toward the oxidation of nitrophenol to aminophenol) using the MOF-derived carbon as the support for a variety of nanocatalysts.

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2.3. Aerogel-based synthesis

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Various aerogel materials were successfully developed, such as SiO2, NiO, ZnO, carbon, Fe2O3, FeOOH, lanthanide oxide, metal chalcogenide, cellulose, polyimide (PI) and polyurethane (PU), indicating that aerogels are well-developed porous materials around the world. Consequently, the aerogel materials have been applied to various fields, including thermal insulators [68], supercapacitors [69], catalysis [70], adsorption [5], chemical sensors, energy storage, pharmaceutical drug carriers and acoustic transducers [71].

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Fig. 6. SEM images of Cr-embedded MOF-199 with Cr/Cu molar ratios of (a) 0:1, (b) 0.05:1, (c) 0.1: 1, and (d) 0.2:1. Reprinted with permission from Ref. [64].

Fig. 7. SEM images of Cr-embedded MOF-199 with de novo synthesis temperature of (a) 45 °C, (b) 65 °C, (c) 85 °C. Particle size distribution was calculated from the SEM images and shows in (d), (e), and (f). Reprinted with permission from Ref. [64].

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The approach of sol-gel reaction is commonly used for the aerogel-based synthesis. Sol-gel reactions are classified into two different gel routes: one is the colloid gel route, and the other is a polymeric gel route. For the colloid gel route, the aqueous solutions of a metal salt or alkoxide precursors are firstly hydrolyzed to form the small sol particles, which is the so-called hydrolysis reaction. In the second condensation stage, these small sol particles will crosslink with each other in the solvent to construct a threedimensional (3-D) open network gel structure. After that, the as-

prepared gel structures are further dried using a supercritical fluid, such as carbon dioxide or alcohol solvents, resulting in the synthesis of aerogel materials. Taking metal alkoxide of tetraethylorthosilicate (TEOS, Si(OEt)4) for example, the hydrolysis reaction (Eq. (1)) involves a nucleophilic attack of oxygen lone pairs of H2O molecules on the Si atoms of the TEOS precursors, leading to the formation of Si(OH)4 sol. The gelation process is constituted by the continuous transformation of sol to a gel through the condensation process, as shown in Eq. (2). The 3-D network

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Fig. 8. XRD images of (a) Cr-MOF-199 compared to MOF-199, and (b) Au@ZIF-8 compared to ZIF-8. (a) Reprinted with permission from Ref. [64]. (b) Reprinted with permission from Ref. [63].

Fig. 9. SEM images of (a) CuO/CuCr2O4 composites and their corresponding elemental distribution of (b-1) carbon, (b-2) oxygen, (b-3) chromium, (b-4) copper. Reprinted with permission from Ref. [64].

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structures of „SiAOASi„ are obtained by the hydration of Si(OH)4 sols, resulting in the formation of gel structures. The as-prepared gels are further dried by a supercritical fluid, such as carbon dioxide, leading to the formation of silica aerogels [71].

SiðOEtÞ4 þ 4H2 O ! SiðOHÞ4 þ 4EtOH

ð1Þ

 Si
OH þ HO
Si ! Si
O
Si  þH2 O

ð2Þ

In our previous work [72], we successfully prepared silica aerogels with 3-D network structures using TEOS precursors. For the measurement of the pore size distribution and the specific surface area of the as-synthesized silica aerogels, the Brunauer-EmmettTeller (BET) and Barrett-Joyner-Halenda (BJH) methods are commonly utilized, respectively, based on the results of N2 adsorption/desorption isotherms. The typical Type IV isotherm and an H1 hysteresis loop are clearly observed for the as-prepared silica aerogel samples, indicating the existence of mesopores (2– 50 nm) in the silica aerogels, which is in good agreement with the average pore diameter of ca. 3 nm from the BJH desorption data, as shown in Fig. 11. The specific surface area of the silica aerogels was found to be 656 m2/g. The surface characteristic of the silica aerogels can be controlled using a suitable hydrophobic precursor or a surface modification

process. Polymethylsilsesquioxane (PMSQ) aerogels, a porous hydrophobic network structure, can be synthesized by a sol-gel process using methyltrimethoxysilane (MTMS, H3C-Si(OCH3)3) as the reactant [73]. The reaction mechanism for the preparation of waterproof PMSQ aerogels is illustrated in Fig. 12, which is similar to the sol-gel reaction using the TEOS precursor. Briefly speaking, the MTMS precursors with a hydrophobic ACH3 functional group are firstly hydrolyzed with water to form H3C-Si(OH)3. The asprepared H3C-Si(OH)3 sols are condensed with each other to form hydrophobic PMSQ aerogels with a 3-D porous network structure. The FESEM images and the corresponding water contact angle images of the PMSQ aerogels with different molar ratios of ethanol to MTMS (E/M) are shown in Fig. 13. The FESEM images indicate the 3-D porous structures of the as-prepared PMSQ aerogels. The water contact angles of the as-prepared PMSQ aerogels with E/M of 0, 1 and 2 are 154.65°, 149.71°, and 139.06°, respectively, implying the hydrophobic characteristics of the PMSQ aerogels. In our previous work [73], solvent-resistant cetyltrimethylammonium bromide (CTAB)-modified PMSQ aerogel powders were successfully synthesized using a facile sol–gel reaction. CTAB is a structure-directing agent to adsorb on the pore surface of the PMSQ aerogels to stabilize their structures, leading to achieving greater structural strength and solvent resistance. Fig. 14 shows

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Fig. 10. (a) Nitrogen adsorption/desorption of Au@ZIF-8 and ZIF-8. (b) Pore size of Au@ZIF-8 and ZIF-8. (c) Nitrogen adsorption/desorption of Au@NC and ZIF-8 derived carbon. (d) The pore size of Au@NC and ZIF-8 derived carbon. Reprinted with permission from Ref. [63].

Fig. 11. Pore size distribution and nitrogen adsorption/desorption isotherm (inset figure) of the silica aerogels. Reprinted with permission from Ref. [72].

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the FESEM, the corresponding water contact angle images, and the pore size distribution of the CTAB-modified PMSQ aerogels with molar ratios of CTAB to MTMS (C/M) of 0.001, 0.002 and 0.004. The interconnected nanoparticles are observed from the FESEM images, indicating the highly porous structures of the CTABmodified PMSQ aerogels. The water contact angles of the CTABmodified PMSQ aerogels are 166.38° at C/M = 0.001, 165.62° at C/ M = 0.002, and 166.43° at C/M = 0.004, demonstrating the superhydrophobic characteristics of the CTAB-modified PMSQ aerogels. The specific surface areas of all CTAB-modified PMSQ aerogels

are (298–342) m2/g with pore sizes peaked at (75–175) nm, which show that the CTAB-modified PMSQ aerogels are macroporous. The organic aerogels, such as cellulose, PI, PU, phenolformaldehyde, melamine-formaldehyde, and cresolformaldehyde, are commonly synthesized using the polymeric gel routes. The resorcinol-formaldehyde (RF) resin [74–76], a phenol-type formaldehyde, was first described by Pekala and coworkers. The first step is the addition reaction, which is the reaction of resorcinol (R) and formaldehyde (F) to form hydroxymethyl derivatives (ACH2OH) in the presence of the alkaline catalyst of sodium carbonate (Na2CO3), as illustrated in Fig. 15. The second step is the condensation reaction. The hydroxymethyl derivatives condense with each other to form a cross-linked wet gel consisting of methylene (ACH2A) and methylene ether (ACH2OCH2A)bridged nano-sized clusters. The as-synthesized wet gels are dried using supercritical fluid with CO2 to form the RF resins. The carbon aerogels can be further fabricated by the carbonization of RF resins. In our previous work, carbon aerogels were successfully prepared by the carbonization of RF resins under nitrogen atmosphere at 1273 K for 6 h [75]. Type IV N2 adsorption/desorption isotherm (Fig. 16a) was identified for the as-prepared carbon aerogels. The result revealed the existence of mesopores, which was in good agreement with the measured average pore diameter (10 nm) from the BJH desorption data shown in Fig. 16b. Furthermore, the specific surface area and pore volume of the as-synthesized carbon aerogels are 702 m2/g and 0.83 cm3/g, respectively.

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3. Applications of functional nanostructure powder materials

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In this section, recent emerging applications are introduced using the above-mentioned functional nanostructured materials.

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Fig. 12. The reaction mechanism of the preparation of PMSQ aerogels. Reprinted with permission from Ref. [73].

Fig. 13. FESEM and the corresponding contact angle images of the PMSQ aerogels with E/M molar ratios of (a) 0, (b) 1 and (c) 2. Reprinted with permission from Ref. [73].

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The synthetic approaches, synthesized materials, their material properties, and their applications are summarized in Table 1.

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3.1. Catalysis of methane combustion and methane dry reforming

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Methane combustion (CH4 + 2O2 ? CO2 + 2H2O) is advantageous due to its ultralow emission of pollutants and CO2 per unit energy in power generation, and it has shown to be attractive for various applications in traditional fossil fuel power generation (power plants) and an internal combustion engine of new energy vehicles [80–82]. Heterogeneous catalysis using nanoparticle powder is a promising strategy for promoting methane combustion. In principle, the activation energy and required temperature of the reaction can be reduced dramatically through the development of an active catalyst [7,8,24,81,83,84].

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Fig. 17a and b shows the conversion ratios of methane (XCH4) versus the surrounding temperature of reaction (Tsur) for the methane combustion catalyzed by Cu-only NPs and CuOx-CeO2 hybrid NPs, respectively [8]. The ratio of the initial concentration of oxygen to the initial concentration of CH4 was 0.59 (i.e., an oxygen-lean condition). Prior to the reduction of CuO (CuOx-NPRT and CuOx-NP-300), a light-off effect was identified starting at Tsur = 380 °C followed by a transient decrease in XCH4 at Tsur = 400 °C. After reduction (CuOx-NP-500), the light-off temperature reduced to 360 °C, and the transient decrease in XCH4 occurred at Tsur = 380 °C. The results imply that the reduction of CuO to Cu decreases the required temperature to ignite methane combustion [85]. Using CuOx-CeO2 hybrid NPs as the catalyst for methane combustion, XCH4 shows a similar trend of stable light-off effect at

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Fig. 14. FESEM, the corresponding contact angle images and pore size distributions of the CTAB-modified PMSQ aerogels with C/M molar ratios of (a, d) 0.001, (b, e) 0.002 and (c, f) 0.004. Reprinted with permission from Ref. [73].

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(340–360) °C. The results clearly indicate that hybridization of Cubased catalysts with CeO2 NPs improves their catalytic performance by increasing the light-off stability and activity and also further lowering the light-off temperature [7,85]. The synergistic catalysis indicates that the SMSI at the Cu-Ce-O interface plays an essential role of determining the redox feature of the catalysts and the subsequent catalytic performance [43,81,85–87]. Methane dry reforming with CO2 (DRM; CH4 + CO2 ? 2CO + 2H2) arises a great interest as an environmental-friendly route for the synthesis of value feedstock with a simultaneous reduction of two greenhouse gases, CH4 and CO2 [83,84,88–91]. Ni-based catalyst, especially with nanostructure, has shown the potential for industrial application in the catalytic DRM. The key advantage of the Ni-based nanostructured material is that it is more economical and highly available than the noble metal-based catalyst. Besides, it has been reported that a comparably high catalytic activity and selectivity achieved by choosing Ni-based nanostructured material in comparison to the noble metal-based catalyst [24,44]. The potential in the catalytic DRM can further enhance by reducing the required reaction temperature (i.e., less heat required) and improving operation stability. Using Ni-CeO2 hybrid NPs as catalysts for DRM, the XCH4 and XCO2 versus Tsur are shown in Fig. 18 [24]. For all NiCeOx-NP samples, both XCH4 and XCO2 increased with Tsur and reached 100% at Tsur = 700 °C, indicating that CeO2 significantly enhanced the cat-

alytic activity and the reaction stability of Ni-based NP toward DRM. For the NiCeOx-NP-600 and the NiCeOx-NP-800 (i.e., reduced Ni), a lower starting Tsur (400–450 °C) and a higher TOF were observed at a low temperature than the NiCeOx-NP-RT and the NiCeOx-NP-300 (i.e., remained NiO). The results indicate that the 2nd stage H2-reduction during the gas-phase synthesis of NiCeOxNP samples will effectively reduce the required Tsur for starting their catalysis of DRM [24]. Typically, DRM is accompanied with a variety of side (unwanted) reactions, including direct methane decomposition, reverse water-gas shift reaction (RWGS), and CO disproportionation (Boudouard reaction) [92]. The basicity/acidity of the catalyst plays an important role in the catalytic performance of DRM, especially to the conversion of CO2 [93]. The adsorption of CO2 (i.e., acidic gas) is favored on the basic sites of catalysts surface. For the side reactions involving coke formation, the moss-like coke formation is to be inhibited by increasing the basicity of the catalyst, where the oxidation of carbon deposits will be enhanced and then to reduce the coke formation [92,93]. The hydrogen-to-carbon monoxide (H2/CO) ratio of the products provides an indication of the catalyst selectivity toward DRM [92]. Noble metal-based catalyst (e.g., Rh-based support catalyst) is generally considered as the most potent catalysts with high activity and selectivity (conversion ratio of methane  90% and H2/ CO  0.9 at 700 °C) [94]. In the study reported by Liang et al.

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Fig. 15. The polymerization reaction of resorcinol with formaldehyde to form RF resin. Reprinted with permission from Ref. [76].

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[24], high activity (see Fig. 18) and selectivity were both achieved by using Ni-CeO2 hybrid NP as the catalyst for DRM [24]. It also reported that coke formation and RWGS are the two dominant sides reactions by operating DRM at 700 °C [24]. The oxygen vacancy at the Ni-Ce-O interface favors the dissociated adsorption of CO2 from the competitive adsorption with methane at the Ni surface [24,94]. In comparison to the results using the Ni catalysts prepared by the conventional methods (impregnation, coprecipitation) [95,96], the aerosol-based synthesized Ni-CeO2 hybrid NP demonstrates a high activity especially under low operating temperature. The results imply that a massive amount of Ni-Ce-O interface formed via gas-phase EISA is beneficial for the catalysis.

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3.2. Catalysis of chemicals transformation reactions

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In industry, catalysts are indispensable to most of the chemicals transformation reactions. Since catalyst is able to provide industrially favorable mechanism, 85 to 90 percent of reactions rely on them. In recent years, sustainable development has gradually been highlighted due to environmental issues (e.g., biomass transformation). Therefore, developing more environmentally friendly catalysts to improve the catalytic processes is a promise for future technological development of chemicals transformation reactions. The improvement of the catalytic system is able to reduce the by-products, to achieve better conversion and selectivity, to reduce the required energy for reactions, and to develop more sustainable reaction routes. Catalysts synthesized using de novo method feature the special bimetallic and reticular structure, which is much more conducive to catalytic reactions. Moreover, they also have the characteristics

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of high specific surface area, good recyclability, and porous structure. MOFs and derived porous composites synthesized by the de novo method have merit the required properties shown above, which are highly attractive for various applications in chemicals transformation reactions. In the following, three catalyst materials are introduced for environmentally friendly reactions and energy applications. Firstly, Au@NC derived from Au@ZIF-8 is employed to improve the conversion of 4-nitrophenol to 4-aminophenol compare to other Au nanoparticles enhanced catalysts [63]. Secondly, CuO@MTs obtained via Cu-doped MIL-125 is used to promote hydrogen revolution from methanol. The yield (gas) can be used as the hydrogen sources in reduction reaction or can be collected and stored for renewable energy source [66]. The other catalyst is CuO/CuCr2O4, which was derived from Cr-embedded MOF199. The composites have outstanding ability to enhanced phenol hydroxylation. The reaction route is an alternative way to produce fine chemicals instead of deriving them from fossil fuels [64] (see Fig. 19).

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3.3. CO2 capture

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Global warming caused global climate change is a severe problem due to the CO2 emission. As a result, CO2 capture has drawn substantial attention in recent years. Solid adsorbent using nanostructured powders with high specific surface areas (e.g., porous aerogel networks) for CO2 adsorption is a promising strategy for the applications of CO2 capture. The surface characteristics of metal oxide aerogels, including those prepared from SiO2, ZnO, TiO2 and Fe2O3 [97], can be easily modified by grafting a variety of silane having different functional groups (ACH3, ACF3 and

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Fig. 16. Carbon aerogel. (a) N2 adsorption/desorption isotherms. (b) Pore size distribution. Reprinted with permission from Ref. [75].

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ANH2). For example, the amine-modified SiO2 aerogels using 3aminopropyltriethoxysilane (APTES) [5], polyethyleneimine (PEI) and tetraethylenepentamine [98] have been reported extensively and widely utilized as CO2 adsorbents due to the strong binding affinity between CO2 and the amino groups. On the other hand, a novel CO2 capture approach, which is the so-called membrane contactor process with the combination of membrane and CO2 absorption process, using the hydrophobic SiO2 aerogel membranes were successfully developed in our previous work [72,79,99–102]. The pure SiO2 aerogel membranes were modified with hydrophobic fluorocarbon functional groups (ACF3)

to form hydrophobic SiO2 aerogel membranes. The waterproof SiO2 aerogel membrane serves as a gas-liquid interface between aqueous amine solution and CO2/N2 gas mixtures, where CO2 is able to pass through and to be absorbed by the aqueous amine solution, as shown in Fig. 20. The hydrophobic SiO2 aerogel membrane will prevent wetting of the membrane arouse by the absorbent (amine), thereby showing promise of attaining long periods of continuous operation of CO2 absorption under a high CO2 flux. As a result, the hydrophobic (ACF3) SiO2 aerogel membrane will become a potential membrane core for membrane contactors used in CO2 capture. The hydrophobic SiO2 aerogel membrane contactors are also promising to be used for a large-scale CO2 absorption technology (e.g., CO2 capture from the flue gas in the post-combustion process of the power plants). Since the hydrophobic membrane is the potential membrane core for membrane contactor used in CO2 capture, a waterproof PMSQ aerogel membranes was successfully developed using hydrophobic MTMS reactants without further hydrophobic fluorocarbon functional groups (-CF3) modifications, as shown in Fig. 21 [79]. The water contact angle of the as-prepared PMSQ aerogel membrane is 143°, indicating the hydrophobic characteristic of the PMSQ aerogel membranes. The CO2 absorption using the PMSQ aerogel membrane can be continuously operated for at least four days and reach a stable absorption flux of approximately 1.2 mmol/m2 s. Furthermore, the CO2 absorption of the asprepared PMSQ aerogel membrane is also demonstrated in three one-day cycles, indicating that the PMSQ aerogel membrane is not only durable but also reusable in membrane contactor for CO2 capture. Thus, the as-fabricated PMSQ aerogel membrane has excellent potential as a membrane contactor for future CO2 capture.

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3.4. Organic solvent, dye, and oil adsorption

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The adsorption process of organic solvent, dye, and oil in water is significant for water remediation. Nanostructured powders with high specific surface areas are good candidates for the adsorption process. In this regard, highly porous aerogel powders possessing high specific surface areas and 3-D network structures are very suitable to serve as an adsorbent for the organic solvent, dye, and oil adsorption process. Fig. 22 demonstrates the capacity of CTAB-modified PMSQ aerogels for the adsorption of diesel fuel, gasoline, ethanol, and n-hexane. The CTAB-modified PMSQ aerogels with C/M molar ratio of 0.002 show the highest adsorption performance due to their highest specific surface area of 342 m2/

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Table 1 A summary of synthetic approach, materials, their properties, and their applications. Synthetic approach

Materials

Material properties

Applications

Reference

Aerosol-based

CuOx-CeO2 hybrid NP

Methane combustion

[7,8]

Aerosol-based

Methane dry reforming

[24]

Aerosol-based

Ni-CeOx hybrid nanoparticle CuxO@MOF

CO oxidation

[47]

De novo

Au@NC

Reduction of 4-nitrophenol

[63,77]

De novo

CuO/CuCr2O4

Phenol hydroxylation

[64,78]

De novo Aerogel-based

CuO@MTs FAS-modified mesoporous silica aerogel membranes Hydrophobic mesoporous PMSQ aerogel membranes CTAB-modified PMSQ aerogels

Spherical hybrid cluster (100 nm) composed of CuOx and CeO2 crystallites (10–30 nm) Spherical hybrid cluster (100 nm) composed of Ni and CeO2 crystallites (10–30 nm) Spherical MOF cage (100 nm) composed of CuO crystallites (1 nm) Au nanoparticles (7.5 ± 0.8 nm) embedded in spherical MOF cage (50 nm) Cubic structure (10 nm) composed of metal oxide particles (several hundred nanometers) MOF tablets (1 µm) NP-formed 3-D porous network structures with specific surface area of 656 m2/g and average pore size of 3 nm NP-formed 3-D porous network structures with specific surface area of 357 m2/g and average pore size of 8.7 nm NP-formed 3-D porous network structures with specific surface area of 342 m2/g and average pore size of 7.9 nm

Hydrogen revolution CO2 capture

[66] [72]

CO2 capture

[79]

Adsorption for the solvents of gasoline, diesel fuel, EtOH and nhexane

[73]

Aerogel-based Aerogel-based

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Fig. 17. Activity tests of methane combustion catalyzed by Cu-only NP and CuOx-CeO2 hybrid NP over various oxidation states of Cu. (a) Cu-only NP (b) CuOx-CeO2 hybrid NP. Reprinted with permission from Ref. [8].

Fig. 18. DRM catalyzed by the NiCeOx-NP samples with different Td2. 1: XCH4 versus Tsur; 2: XCO2 versus Tsur. Reprinted with permission from Ref. [24].

Fig. 19. The conversion rate of 4-nitrophenol reduction catalyzed by Au@NC, naked Au nanoparticles derived from Au@NC, Au-loaded activated carbon (Au/AC), and ZIF-8 derived carbon. Reprinted with permission from Ref. [63].

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g. The PMSQ aerogels with a low dose of CTAB modification have the industrial potential for organic solvent and oil adsorption [73]. The impurity of phospholipids in plant oil, which inhibited the application of plant oil in diesel engines, is also targeted for the removal using the adsorption process. The powders with Lewis acid characteristic are commonly used for the phospholipid adsorption in plant oil. As a result, the Lewis acid ZrO2 nanoparticles [103] and Y2O3 cubes and flowers [104] were successfully prepared using a hydrothermal process. These works also demonstrate that the as-prepared Lewis acid ZrO2 nanoparticles and Y2O3 cubes and flowers are with outstanding performance on phospholipid adsorption from plant oils for biofuel applications. In comparison, the hydrothermal-synthesized ZrO2 NPs possess larger specific sur-

Fig. 20. Scheme for CO2 gas absorption in a hydrophobic membrane contactor using fluoroalkyl silane (FAS)-modified mesoporous silica aerogel membranes. Reprinted with permission from Ref. [72].

face areas (102 m2/g) than the commercial ZrO2 powders (58 m2/ g), resulting in a better performance on phospholipid adsorption from plant oils [103]. Dye removal is one of the crucial processes in wastewater treatment applications. As a result, ZrO2/carbon aerogel (CA) powders with monoclinic (m-ZrO2) and tetragonal (t-ZrO2) ZrO2 crystalline structures were successfully synthesized using a hydrothermal process for the adsorption of cationic RhB dyes [105]. The ZrO2/ CA powders with 100% t-ZrO2 phase present a high negative sur-

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Fig. 21. Scheme for CO2 gas absorption in a hydrophobic membrane contactor using hydrophobic mesoporous PMSQ aerogel membranes. Reprinted with permission from Ref. [79].

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4. Summary

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Functional nanostructured materials in powder form are highly attractive for a variety of emerging energy and environmental applications: (1) heterogeneous catalysts for chemical transformation, methane combustion and methane dry reforming with CO2; (2) mesoporous aerogel membrane contactor for CO2 capture; (3) adsorbent for the removal of organic solvent, dye, and oil in water. Incorporation of advanced synthetic methods with support of metrology and methodology of material characterization bridges the development of functional nanostructured materials to their emerging energy and environmental applications. Using the aerosol-based synthetic approach, CuxO-CeO2, Ni-CeO2, and CuxO nanoparticle-encapsulating metal–organic framework (MOF) hybrid nanoparticles are successfully created with homogeneous elemental distribution. De novo method demonstrates a facile, convenient route to form Cr-encapsulating MOF (Cr-MOF-199) and Au-encapsulating MOF (Au@ZIF-8) MOFs with high loading capacity. The MOF-derived nanocomposites (CuO/CuCr2O4) are to form via a further thermal treatment of the MOFs. Confined mesoporous oxides, polymers and carbon nanostructures can be expertly produced using the aerogel-based method. The review demonstrates a prototype methodology of the development of a variety of functional nanostructured materials using the advanced material synthetic methods to improve their performance for heterogeneous catalysis, gas absorption and adsorbent of contaminants. The synthetic methods also provide a facile route for mechanistic understanding and further optimization of their material properties, which expand potential window for reducing environmental pollutions and for providing alternative energy applications in the future.

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Declaration of Competing Interest

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The authors declared that there is no conflict of interest. References

Fig. 22. The adsorption capacity for the solvents of gasoline, diesel fuel, EtOH and nhexane using the CTAB-modified PMSQ aerogels with C/M molar ratios of 0.001, 0.002 and 0.004. Reprinted with permission from Ref. [73].

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face charge at pH 7, resulting in active interactions with cationic RhB dyes and excellent performance in RhB dye adsorption. The ZrO2/CA powders with 100% t-ZrO2 phase also demonstrate reusability for RhB dye adsorption. However, further separation process such as centrifugation is needed for the reuse of ZrO2/CA powders in RhB dye adsorption. As a result, magnetic adsorbents, such as c-Fe2O3/a-Fe2O3/CA powders were successfully developed for dye adsorption [106,107]. The as-prepared magnetic adsorbents can be separated by an external magnetic field to avoid further separation steps. In addition to dye adsorption, other magnetic adsorbents, such as mesoporous c-Fe2O3 nanostructures [108], Fe/CA powders [109] and Fe3O4 nanoparticles [110] were also utilized for the arsenic (As) ion adsorption. The as-prepared magnetic powders were also demonstrated an excellent performance in As ion adsorption, and these magnetic powders can be easily separated by an external magnetic field. These results demonstrate that magnetic adsorbents exhibit strong potential for use in wastewater treatment applications, such as organic dye and metal ion removal.

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Please cite this article as: H.-L. Wang, C. Y. Hsu, K. C. W. Wu et al., Functional nanostructured materials: Aerosol, aerogel, and de novo synthesis to emerging energy and environmental applications, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.039