Layered double hydroxide polymer nanocomposites for catalysis

Layered double hydroxide polymer nanocomposites for catalysis

20

Shadpour Mallakpour1,2 and Hashem Tabebordbar1 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

20.1 Introduction In recent years, the fabrication and applications of polymer nanocomposites (PNCs) have been placed in the spotlight of researchers and industrialists due to impressive improvements in their mechanical, electrical, thermal, gas permeability, flammability, and UV stability properties (Mallakpour and Javadpour, 2016c; Kotal and Bhowmick, 2015; Liu et al., 2017; de Leon et al., 2016; Mallakpour and Behranvand, 2016a; Mallakpour and Khadem, 2016d). This success can be achieved in the presence of a low content of fillers, which is a special feature in the industry. Hence, researchers are enthusiastic to create, identify, and develop various types of PNCs and have published numerous and useful reviews in this context. Various fillers are employed in the preparation of PNCs based on the types of requests and applications (Mallakpour and Khadem, 2015; Tan and Thomas, 2016; Mallakpour and Behranvand, 2016b; Mallakpour et al., 2016). Layered double hydroxide (LDH) is one of the fillers that with appropriate structure could find a special position in the field of PNC manufacturing (Maheskumar et al., 2014; Velasco et al., 2012; Mallakpour et al., 2015a; Mallakpour and Behranvand, 2017a). LDH is a class of anionic layered mineral with a brucite-like sheet structure with natural and sources. common chemical formula of LDH is defined as synthetic x1 The 21 n
31 M21 M : ð OH Þ ð A Þ signifies a divalent metal ion, 2 x=n : yH2 O, in which M 1
x x 31 n
M signifies a trivalent metal ion, and A signifies an anion. Since the interlayer anions (Cl2, F2, CO322, NO32, OH2, SO422, etc.) and employed metal ions (Cu, Zn, Mg, Ni, Co, Fe, Mn, Cr, Al, Ga, etc.) are variable, diversified chemical compositions of this anionic layered inorganic filler can be produced. Different ranges of x value have been reported for organized LDH, which generally changes in range from 0.2 to 0.33 (Han et al., 2014). To gain a better understanding, a schematic drawing of the LDH structure is shown in Fig. 20.1. The high water capacity, crystallinity, high reactivity, tunable assembly consistency, nontoxicity, and especially catalytic activities of LDHs make them appropriate for energy storage, Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00020-3 © 2020 Elsevier Ltd. All rights reserved.

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Figure 20.1 Schematic drawing of the structure of LDH compounds: (A) side view; (B) top view. (C) Octahedral units of LDH compounds assembled through hydrogen bonding of water molecules, electrostatic force of anions between the interlayer and hydroxyl group of 21 21 21 21 21 21 the sheets. M21 or Ga21. M231: Al31, Cr31, Mn31, 1 : Mn , Fe , Co , Ni , Cu , Zn 2 31 31 31 31 n2 2 2 22 22 Fe , Co , Ni or La . An : OH , NO3 , Cl , ClO2 4 , CO3 or SO4 . Source: Adapted from Mao, N., Zhou, C.H., Tong, D.S., Yu, W.H., Lin, C.X.C., 2017. Exfoliation of layered double hydroxide solids into functional nanosheet. Appl. Clay Sci. 144, 60
78. With kind permission of Elsevier.

heterogeneous and homogeneous catalysts, environmental safekeeping, additives for fabrication of nanocomposites (NCs), medicinal, makeup materials, and UV preservative applications. The identification and development of catalysts are essential because they play an effective role in the progress of chemical technology (Wang and O’Hare, 2012; Li and Duan, 2006; Costantino et al., 2013; Mallakpour et al., 2015b). The majority of chemical reactions take place in the presence of catalysts, including the production of epoxides, acids, ketones, aldehydes, and alcohols through oxidation, hydrogenation of saturated compounds, and polymerization. Today, industries rely heavily on catalysts and extensive researches are carried out to produce stable and active catalysts that act selectively (Sudha and Sivakumar, 2015; Robinson et al., 2016). The layered structure has provided appropriate conditions for the design of practical catalysts with high performance in the synthesis of organic molecules, water splitting, pollutant degradation, air conditioners, and energy storage (Barrado, 2015). Catalytic applications of LDH have various aspects that have been developed over time. The layered structure, cation-exchangeability, uniform dispersion of metal cations, adjustable basicity, high surface area, and ability to combine with metal oxides (MOs) and polymers are advantages of LDH for catalytic activity (Fig. 20.2) (Feng et al., 2015; Li et al., 2015).

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Figure 20.2 Properties and applications of supported catalysts fabricated using LDHs as supports/precursors for catalytic oxidation and hydrogenation. Source: Adapted from Feng, J., He, Y., Liu, Y., Du, Y., Li, D., 2015. Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects. Chem. Soc. Rev. 44 (15), 5291
5319. With kind permission of RSC.

The objective of this chapter is to investige the catalytic behavior of LDH and polymer/LDH NCs. Accordingly, some important catalytic applications of polymersupported LDHs are discussed separately. In this regard, the synthesis methods of polymer/LDH NCs have also been debated. It is hoped that this chapter will be useful and create a pathway for progress in the field of catalysts.

20.2 Applications of layered double hydroxides in catalysis The high potential of LDH for surface adsorption of diverse metal ions makes it conducive for utilization in redox catalysis. On the other hand, incorporation of functionalized anions between LDH layers creates an active basic surface that facilitates the redox reactions. LDH with mixed MO has been employed as ecological heterogeneous base catalysts for the synthesis of organic molecules through oxidation processes. Supported Au-LDH was used as a heterogeneous catalyst for synthesis of lactones from diols with a high turnover number (1400) (Mitsudome et al., 2009b). The procurement process of Au-LDH catalyst with the corroborant analysis is displayed in Fig. 20.3. With small Au-LDH loading without any other additive, the lactonization was performed at relatively low temperatures, which is a good advantage compared to previously reported catalysts. The catalytic activity of Au-LDH was compared with other Au-MO catalysts for aerobic oxidation of 1-phenylethanol. The Au-LDH showed excellent turnover number, as large as 200,000, with three optimum recyclings without any change in selectivity and activity (Mitsudome et al., 2009a).

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Layered Double Hydroxide Polymer Nanocomposites

Figure 20.3 A schematic of the AuNCs/LDH catalyst and (A) UV
vis spectra of the original solution of GS-AuNCs (a) and the supernatant after impregnation over LDH (using Mg3Al-LDH as the example) (b); (B) HRTEM image of GS-AuNCs (inset shows the crystalline structure of an individual NC and the histogram of the size distribution). Source: Adapted from Feng, J., He, Y., Liu, Y., Du, Y., Li, D., 2015. Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects. Chem. Soc. Rev. 44 (15), 5291
5319. With kind permission of RSC.

This distinction is due to the stability of adsorbed Au negative charge in the presence of LDH positive charges that promote the oxidation process. Studies showed that Mg/Al atomic ratio, calcination temperature, and using different M21 and M31 cations for preparation of Au-LDH catalysts affected the catalytic performance in oxidation reactions. Generally, the catalytic efficiency was enhanced with increasing calcination temperature and Mg/Al ratio. Also, replacement of cations in AuLDH structure with transition metal cations improved the efficiency of the catalyst by increasing the synergetic effect (Liu et al., 2012; Takagaki et al., 2011; Li et al., 2014). High yields of aldehydes and ketones were synthesized without any isomerization through oxidation of alcohols, with the help of heterogeneous Pd-LDH catalysts (Kakiuchi et al., 2001). The results exhibited that the presence of Bronsted basic sites on the LDH structure increases the activity of catalysts. The functionalized Pd-LDH as a recyclable catalyst was used for oxidation of benzyl alcohols. Diamine as nitrogen donor ligand stabilized the Pd sites. The benzaldehyde was produced selectively, with 94% conversion by this catalyst. Functionalized Pd-LDH

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can be simply recycled for several periods with the previous selectivity and activity (Sahoo and Parida, 2013). The Ru-LDH catalyst was applied for the tandem synthesis of quinolone. Ru species facilitate the aerobic oxidation and, in the following, basic sites of LDH manage the aldol reaction (Motokura et al., 2004). The hydrogenation reaction is another approach for the synthesis of organic species. In these reactions, catalysts play an important role in yield and selectivity. The LDH catalyst with excellent properties is also a useful candidate for hydrogenation of unsaturated bonds. The Pd-LDH catalysts with diverse mixed MO showed outstanding catalytic activity for hydrogenation of the disposed compounds. The CH2F2 was obtained selectively by hydrodechlorination of CCl2F2 with the help of Pd-LDH catalyst (Padmasri et al., 2004). The Pd-LDH was modified with arginine and was used as an active catalyst for the formation of alcohols from hydrogenation of ketones (Tao et al., 2010). The modified catalyst showed higher activity than unmodified catalyst by converting more than 97%. This advantage is originated from arginine which, with its amino group, creates a durable synergistic effect between LDH and Pd NPs. The decline in the activity of the catalyst was not observed after being reused five times, representing the stability of the prepared catalyst. Besides the electrostatic interactions between the arginine and Pd NPs, the guanidyl group contributes to the stability of the catalyst by establishing coordination with Pd21 ions. Formation of α-alkylated nitriles as vital structures for different biologically active substances was catalyzed with Pd-LDH catalyst (Motokura et al., 2005). This reaction was carried out in two phases, the basic site on LDH contributed in aldol condensation, and Pd NPs supported the hydrogenation process. This prepared catalyst was more active than Pd/C as a commercially existing catalyst in the hydrogenation reaction. Hydrogenation of colophony was performed over Ni-LDH catalyst under an optimized operating state (Huang et al., 2016). The results indicated a correlation between the reaction temperature and the rate of conversion. The greatest amount of conversion (99.73%) was obtained under applied pressure 5.5 MPa of H2 at 178 C for a duration of 90 min. The catalytic activity of the heterogeneous Co-LDH catalyst was investigated for hydrogenation of CO under adjusted conditions (Tsai et al., 2011). The Co-LDH catalyst revealed a remarkably progressive activity compared to the other prepared catalysts in the absence of a reduction supporter. Comparing the activity of prepared catalysts suggests that the support has a significant influence on the performance of the catalyst. According to the performed analysis, it was found that high surface area and thermal stability of LDH, good interaction between Co particles and LDH, and uniform dispersion of Co particles on the surface of LDH were all significant factors for the superiority of the Co-LDH catalyst. The reforming of CO and CO2 to CH4, which is called methanation, is very important due to the worthwhile environmental effects and production of a useful and applicable reactant. Also, methanation is applied to remove traces of CO and CO2 during the ammonia synthesis process. Several metal catalysts, such as Ru, Co, Fe, Rh, and Ni, with different performances have been introduced for the formation of CH4 from CO and CO2. Ni-based catalyst has practical advantages such as low cost and high activity (Li et al., 2016; Zhang et al., 2014). However, the

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Layered Double Hydroxide Polymer Nanocomposites

exothermic phenomenon in high temperature makes it deactivate quickly. The LDH with high adsorption capacity and anchoring effect has a good potential for adsorption of CO2. Hence, the flower-like Ni-LDH-Al2O3 with a high degree of stability and dispersion was used for methanation of CO/CO2 (Fig. 20.4). Due to the strong interaction between the Ni particles and LDH as a support, the new catalyst showed an extraordinary activity and after 252 h, only 7% reduction was observed in CO2 conversion (He et al., 2013). Nitrogen and sulfur oxides are known as dangerous pollutants with most environmental problems, including some diseases, acid rain, demolition of the ozone layer, and changing climate, originating from them. These threats and damages have forced researchers to develop ways for controlling these

Figure 20.4 Illustration of the formation of Ni NPs with high dispersion and high density on a hierarchical flower-like Al2O3 matrix via an in situ reduction process of a NiIIAlIII-LDH precursor; (A, B) SEM images of the flower-like NiAl-LDH; (C, D) HRTEM images of the flower-like NiAl-LDH; (E) high-angle annular dark field (HAADF) STEM images of Ni nanoparticles in the sample of flower-like NiAl-LDH; (F) profiles of CO2 conversion vs. temperature for CO2 methanation in the presence of flower-like NiAl-LDH. Source: Adapted from Feng, J., He, Y., Liu, Y., Du, Y., Li, D., 2015. Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects. Chem. Soc. Rev. 44 (15), 5291
5319. With kind permission of RSC.

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pollutants. Among different approaches, the NOx storage and reduction (NSR) strategy is the best technique for NOx removal due to user convenience, cheapness, and high efficiency. The NSR catalyst oxidized firstly the NO to NO2, formerly the produced NO2 is stored on the surface of the catalyst and finally, the adsorbed NOx is reduced to inert N2 gas. Furthermore, several NSR catalysts with various transition metals were reported for the exclusion of NOx (Dai et al., 2012; Obalova et al., 2009; Wang et al., 2012). Palomares and coworkers produced several types of LDHs with diverse approaches for the exclusion of NOx at high temperature (Palomares et al., 2012). Among them, CoAl-LDH showed the best potential for elimination of NOx in the presence of water and oxygen at 750 C. The results proposed that the catalytic activity is independent of the fabrication method but is totally dependent on the constituent metals. LDH with middle basicity is a promising catalyst for SO2 removal. For example, The MgAl-LDH showed an outstanding contribution to the eradication of SO2 in the air flow (Kameda et al., 2011; Kameda et al., 2012). The resulting SO3 from the oxidation of SO2 is dissolved in aqueous solution and then removed by instauration of LDH intercalated with detached sulfate. The removal percentage of SO2 increased well by enhancing the LDH dosage and temperature. Epoxides due to high chemical reactivity have special trade status in the chemical industry in which it is extensively employed as crude and versatile materials for surfactants, paints, epoxy resins, and synthesis of commonly used chemicals. This imperative intermediate is generally prepared by the chlorohydrin process and straight oxidation of olefins by means of molecular oxygen or organic peroxides and peracids. Straight oxidation with O2 is preferred for epoxide production on an industrial scale. Intercalation of polyoxometalates (POMs) into the LDHs produces highly stable heterogeneous catalysts for epoxidation of olefins. The selectivity of LDHPOM catalysts was compared with homogeneous Na-POM catalyst for epoxidation of several allylic alcohols in the presence of dilute H2O2 as an oxidant. The results showed that the LDH-POM catalysts without pH being regulatory are the absolute winner; this advantage can be attributed to the fine hydrothermal permanency of LDH and sustained interaction between the LDH and POM. Moreover, catalyst activity remained unchanged after continuous recycling (Liu et al., 2008, 2009).

20.3 Polymer/layered double hydroxide nanocomposites Incorporation of LDHs into the polymers produces multifunctional NCs with a wide range of applications. LDHs, due to their layered structure, high thermal stability, adjustable composition, and catalytic activity promote the optical, flame retardancy, mechanical, rheological, and thermal properties of the host polymeric medium. To date, various types of polymers as a matrix have been reported for the preparation of LDH NCs. In general, the dispersion index of LDH in the polymer environment directly affects the structure and properties of resultant NCs. Hence, the development of methods for the preparation of homogeneous polymer/LDH NCs is important.

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Layered Double Hydroxide Polymer Nanocomposites

20.3.1 Preparation of polymer/layered double hydroxide nanocomposites Various procedures have been reported for the fabrication of polymer/LDH NCs which are reviewed briefly in four principal routes: in situ LDH synthesis in the polymer solution, in situ polymerization, melt mixing, and solution blending.

20.3.1.1 In situ layered double hydroxide synthesis in polymer solution This method is a good strategy for enhancing the interfacial connections between LDHs and host polymers. The target is the gradual formation of LDHs within the predetermined polymer solution. The NCs are obtained with the implantation of the polymer chains into layers during the assembling LDH construction. The LDH formation process is accomplished by coprecipitation of two fundamental metal salts in an appropriate polymer solution. The polyester and poly(vinyl pyrrolidone) (PVP) were interposed to ZnAl-LDH by quantized precipitating respective metal salts in the presence of fixed polymer solution under fixed pH 9 at room temperature (Stimpfling et al., 2016). The achieved PVP-LDH NCs were more homogeneous due to their ability to create a gel-like suspension. The photostable poly(vinyl alcohol) (PVA)/Zn2Al-LDH NCs were fabricated based on steady coprecipitation of LDH plaques in aqueous PVA solution under constant pH 9 (Gaume et al., 2013). The Zn(NO3)2 and Al(NO3)2 were applied as constructive metal salts of LDH with a 2:1 ratio of Zn to Al. This applicable technique provides a well-organized approach to generate exfoliated NCs with ideal structural properties, such as photostability and a gas barrier. Leroux and coworkers applied this method for the production of exfoliated PVP/Zn2Al-LDH NCs accompanied by alginate. Alginate as an effective separating agent prevents the accumulation of LDH plaques. The results from different analysis techniques indicated the improvement in ionic conductivity and dielectric properties of the obtained NCs (Leroux et al., 2012). Highly dispersed Ni NPs over carbon nanotubes (CNTs) were synthesized from NiAl-LDH/poly(acrylic acid) (PAA) functionalized CNTs (PAA-CNTs) hybrid for selective hydrogenation of o-chloronitrobenzene. The NiAl-LDH/PAA-CNTs NC was assembled by coprecipitation of Ni(NO3)2  6H2O and Al(NO3)2  9H2O as constructive metal salts of LDH with a 3:1 ratio. The supported Ni catalyst presented excellent catalytic activity in the selective hydrogenation of o-chloronitrobenzene to o-chloroaniline with a yield of 98.1% in 150 min (Wang et al., 2013).

23.3.1.2 In situ polymerization This solution-based process has been extensively utilized for fabrication of LDHbased polymer NCs. The procedure begins with the LDH dispersing in pristine monomer or solution monomer. Subsequently, in situ polymerization is carried out into the LDH layers using the proper initiator. Some polymer/LDH NCs have been successfully produced for different purposes by this technique (Zhu et al., 2016; Cui et al., 2012). The antimicrobial polyacrylonitrile/ZnAl-LDH NCs were synthesized by the in situ polymerization method (Barik et al., 2017). The thermal

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stability and antibacterial activity of the obtained NCs improved, which is presumably justified with a fine dispersion of LDH NPs and electrostatic contact between them and charged surface of bacterial cells. The multifunctional polydopamine (PDA)/LDH NCs were successfully synthesized by interlayer polymerization of dopamine in the gallery space of the CoAl-LDH and MgAl-LDH NPs (Nam et al., 2016). This synthetic strategy was used in order to obtain homogeneous and welldispersed LDHs within the PDA in resultant NCs, which ultimately leads to improvements in its catalytic activity and electrochemical properties. The PDA/CoAl-LDH and PDA/MgAl-LDH NCs exhibited high catalytic activity for reduction of p-nitrophenol into p-aminophenol by NaBH4 with a conversion efficiency greater than 97% and 87%, respectively. Also, the resultant NCs showed excellent capacitance value and structural sustainability without any noticeable drop during testing.

20.3.1.3 Solution blending This technique is used for straight intercalation of qualified polymers with suitable functional groups and various molecular weights into interlayered sections of LDHs. The intended LDH NPs are dispersed directly into the prepared polymer solution and then, the NCs are achieved by solvent evaporating. There are shortcomings that make it difficult to form uniform NCs. For example, incompatibility between LDH NPs and the polymer environment, narrow interlayer space, which reduces the intercalation of polymer chains, and aggregation of LDH NPs due to high charge density. Surface modification with suitable organic species is the most important proceeding that has been done to improve the intercalation conditions. Different compositions of pristine and modified LDH were incorporated within the various polymers. The proton conductivity of sulfonated poly(ether ether ketone) (SPEEK) was enhanced with the embedding diverse quantities of MgAl-LDH. Solution blending as a common technique was successfully applied for the preparation of these NCs. The results showed that the obtained SPEEK/MgAl-LDH NCs can be used as a good candidate for a polymer electrolyte membrane fuel cell (Kim et al., 2015). The new polyphenol oxidase immobilization hybrid based on Zn2Al-LDH/alginate was produced by a solution-blending process for fabricating phenol biosensors. This biosensor showed very sensitive performance for detection of phenol in water and also chloroform (Lopez et al., 2010).

20.3.1.4 Melt mixing Direct melt mixing is the typical traditional technique for the formation of PNCs with the excellent distribution of fillers on an industrial scale. In this process, the fluid polymers are diffused into the interlayer of LDH NPs under high local shear stress by heating them above the melting temperature or glass transition temperature of polymers. This convenient technology can manufacture the polymer/LDH NCs with uniform morphology without the need for complicated reactions. Melt mixing always entices the particular interest of industrialists and researchers due to advantages including availability, simplicity, solvent-free, and environmentally friendly. A brief content concerning the production of polymer/LDH NCs through melt

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Layered Double Hydroxide Polymer Nanocomposites

mixing is described in the following. The PS/NiAl-LDH NCs were prepared using a melt-mixing system by means of a twin-screw extruder. The significant intermolecular relations between the LDH NPs and neat PS improved the thermal decomposition, flexural strength, and tensile strength (Suresh et al., 2017). Donato and coworkers produced polypropylene (PP) NCs based on modified MgAl-LDH through melt blending (Donato et al., 2012). The results showed that this procedure can improve the properties of PP. The crystallinity, stiffness, and thermal degradation of PP were increased in the presence of modified LDH. These developments could be described by the role of modified LDH as a plasticizer and nucleating agent into the PP background. In another project, the properties of pristine ethylene vinyl acetate (EVA) copolymer and one filled with ZnAl-LDH were investigated. The EVA/LDH NCs were achieved by both melt mixing and solution intercalation methods with various quantities of LDHs. The amounts of loaded LDHs for the melt mixing were twice as high as the solution method, which can be attributed to the robust shear force interplay. The properties were completely dependent on the level of diffusion and LDH-loaded value. The obtained EVA/LDH NCs showed higher mechanical and thermal properties than pristine EVA and their values were ceaselessly enhanced with the proliferate LDH loading (Zhang et al., 2008). Also, the prepared polymer/LDH NCs by melt-mixing method can also have catalytic applications. For example, the modified LDH/high-density polyethylene (HDPE) NCs showed good antimicrobially properties over different bacteria (Kutlu et al., 2014). At first, the MgAl LDHs were modified with camphorsulfonic acid (CSA) and ciprofloxacin. The thermal stability of CSA was developed over 160 C under LDH shielding. Then, the functionalized LDHs were melt-compounded with HDPE. The antimicrobial testing showed that the prepared modified LDH/HDPE NCs have good susceptibility against the tested bacteria, while the pristine HDPE indicates no inhibitory effects. In addition to these general methods, layer-by-layer assembly (LBL) and reversible addition fragmentation chain transfer (RAFT) polymerization are also used commonly as fabrication methods of polymer/LDH NCs for catalytical applications. LBL is the wet chemical technique for the synthesis of ultrathin film onto a solid surface which has attracted enormous interest as a powerful methodology for producing photoactive and catalytic surfaces. Its advantages are the experimental simplicity and cheap price, ease in controlling the thickness of the layers, and possibility of using numerous materials as building units, containing metal oxide NPs, carbon-based nanomaterials, and polyelectrolytes. In this method, the films are prepared by the consecutive transfer of supplementary ingredients to a proper solid substrate through immersing, spraying, and spinning procedures. The adsorption process on the surface of substrates can be derived through electrostatic or nonelectrostatic interactions. This simple and flexible technique can be applied in the development of NC-based thin films (Nunes et al., 2017). RAFT polymerization is one such methodology which has been successfully employed for encapsulation of LDHs. RAFT as a kind of living radical polymerization is an ideal technique for fabrication of NCs with prechosen molecular weight and architecture. By this method, the LDH surface can be easily covered with

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various polymers that are susceptible to catalytic applications. The thiocarbonylthio compounds such as xanthates, dithioesters, and thiocarbamates were used as chain transfer agents in RAFT polymerization (O’Donnell, 2012; Perreira et al., 2017). Accordingly, the surfaces of LDHs should be modified initially with proper thiocarbonylthio compounds. In the next step, the intended polymer grows on LDH surfaces by providing reaction conditions such as monomer and initiator.

20.4 Applications of polymer/layered double hydroxide nanocomposites in catalysis As already mentioned, today catalytic processes are frequently applied in the majority of reactions, especially in industry. Many of these practical catalysts are inherently homogeneous, suffering from low chemical and thermal resistance. Moreover, their separation is difficult, costly, and may contaminate the final products and the environment. The use of supports for the preparation of heterogeneous catalysts is an ideal procedure to overcome the drawbacks. The supports not only facilitate the separation of the catalyst from the reaction mixture but also expand the catalytic activity. Therefore, discussion about the heterogeneous catalysts has become a hot subject in the chemical industry. Polymer NCs can be used as a standard support for the formation of catalysts (Pessoa and Maurya, 2017). The combination of polymers with catalyst active materials is a good method to develop their catalytic performances. This process, in addition to enhancing the properties of polymers in terms of mechanical, optical, electrical, thermal, and morphological properties, can promote the catalytic efficiency by increasing the active sites. Among the various materials, the ability of LDHs has stood the test of time in increasing the efficiency of polymers (Maheskumar et al., 2014). Therefore, many hybrids can be produced with the penetration of LDHs within the diverse polymers which have wide applications in different aspects (Mallakpour and Hatami, 2017b). Among the numerous applications of polymer/LDH NCs, their catalytic activity has been less evaluated, while their effectiveness has been reported in various fields such as the synthesis of chemical compounds, fuel cells, and sensors. The PVA/Au-LDH composite films were prepared by the bottom-up LBL assembly and successfully catalyzed the reduction of 4-nitrophenol (4-NP) by sodium borohydride (Shu et al., 2015). For this purpose, the prepared CoAl-LDH NPs were firstly modified with (3-aminopropyl)triethoxysilane to obtain monodispersed AuLDH hybrids. Then, the PVA was repeatedly coated on a glass substrate by spinning and exposed to a mixture of Au-LDH hybrids. The final hybrid films were achieved by transfer of the Au-LDH NPs to the coated PVA layers. The catalytic performance of the obtained hybrids was detected by UV
visible spectroscopy. Without the presence of synthesized hybrids, no changes were observed in reduction of 4-NP even after 3 days. As is clear in Fig. 20.5, with the progress of the reaction in the presence of PVA/Au-LDH composites, the absorption band related to 4-NP at 400 nm decreased expressively but the absorption band associated with

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Figure 20.5 (A) UV
vis spectra of the reduction of 4-NP in an aqueous solution recorded every 2 min using the Au NPs 2 LDH 2 PVA hybrid film as a catalyst. (B) Relationship between ln(Ct/C0) and the reaction time (t), wherein the ratios of the 4-NP concentration (Ct at time t) to its initial value C0 (t 5 0) were directly given by the relative intensity of the respective absorbance At/A0 and, therefore, the reduction process could be directly reflected by these absorption curves. Source: Adapted from Shu, Y., Yin, P., Liang, B., Wang, H., Guo, L., 2015. Artificial nacrelike gold nanoparticles
layered double hydroxide
poly (vinyl alcohol) hybrid film with multifunctional properties. Ind. Eng. Chem. Res. 54 (36), 8940
8946. With kind permission of ACS.

the produced 4-aminophenol (4-AP) at 295 nm was enhanced gradually. This means that 4-AP has been properly synthesized without any derivatives. Also, the kinetic experiment indicated that the reduction process is consistent with a pseudo-firstorder model. The efficiency of PVA/Au-LDH catalyst remained stable after recycling 10 times in the same conditions. Polymer-supported LDHs have been widely used in the synthesis of organic molecules with remarkable efficiency due to low cost, environmental compatibility, experimental simplicity, high stability, and simple separation. For example, the innovative PAA-grafted LDH hybrid as a high thermal stable catalyst was prepared by RAFT polymerization for promoting the synthesis of benzo[4,5]imidazo[1,2-α] pyrimidines (BIPs) (Reddy et al., 2017). Fig. 20.6 illustrates well the process of grafting PAA on MgAl-LDH, which was modified with S-(3-trimethoxysilyl) propyltrithiocarbonate (BTPT). For the synthesis of BIPs, the 1H-benzo[d]imidazole-2amine was coupled easily with diverse α,β-unsaturated carbonyl compounds with the help of effective PAA-LDH heterogeneous catalyst at 80 C (Fig. 20.7). In a catalyst-free reaction, the BIPs were slowly synthesized with low yield after 8 h but by putting the catalyst into the reaction, a superior yield (92%) was obtained within 20 min. The reaction conditions, such as the content of catalyst, solvent, time, and temperature, were optimized and the derived data are depicted in Table 20.1. Furthermore, the performance of PAA-LDH was compared with other catalysts. The results suggested that the highest yield was obtained by 10 mg of the newly fabricated catalyst under neat reaction at 80 C. According to the mechanism, the PAA-LDH catalyst with its acidic character conducted the formation of an imine intermediate at the initial step and in the final period produced the suitable products

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Figure 20.6 Synthesis of PAA-g-LDHs. Source: Adapted from Reddy, M.V., Reddy, G.C.S., Lien, N.T.K., Kim, D.W., Jeong, Y.T., 2017. An efficient and green synthesis of benzo [4, 5] imidazo [1, 2-a] pyrimidines using highly active and stable poly acrylic acid-supported layered double hydroxides. Tetrahedron 73 (10), 1317
1323. With kind permission of Elsevier.

Figure 20.7 Synthesis of benzo[4,5]imidazo[1,2-α]pyrimidines. Source: Adapted from Reddy, M.V., Reddy, G.C.S., Lien, N.T.K., Kim, D.W., Jeong, Y.T., 2017. An efficient and green synthesis of benzo [4, 5] imidazo [1, 2-a] pyrimidines using highly active and stable poly acrylic acid-supported layered double hydroxides. Tetrahedron 73 (10), 1317
1323. With kind permission of Elsevier.

by removing the H2 molecule. Mild conditions, low cost, high yield, short time, simplicity, and sequential reusability are the advantages of this procedure. In another similar research, the poly(oligoethylene glycol methacrylate)-gsupported CaAl-LDH (LDH-g-POEGMA) as a green catalyst was applied for the synthesis of chromene merged dihydroquinoline derivatives by a one-pot threecomponent condensation of aromatic amines, 4-hydroxy-2H-chromen-2-one, and different aldehydes through solvent-free conditions (Fig. 20.8) (Reddy et al., 2016). The RAFT polymerization was employed for the growing POEGMA on the surface of LDH-BTPT. The catalytic activity of LDH-g-POEGMA was compared with

Table 20.1 Optimization of reaction conditions for the synthesis of 3aa

Entry

Solvent

Catalyst

Temperature ( C)

Time (min)

Yieldb (%)

1 2c

Neat Neat

Neat PAA-g-LDHs (10 mg)

80 80

480 20

3 4c 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Neat Neat Neat Acetonitrile DMF THF Water Toluene Dioxane Benzene Neat Neat Neat Neat Neat Neat Neat Neat

PAA-g-LDHs (5 mg) PAA-g-LDHs (15 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA-g-LDHs (10 mg) PAA LDHs LDHs-BTPT PSPTSA PTSA Proline InCl3 FeCl3

80 80 100 80 120 80 100 100 80 80 80 80 80 80 80 80 80 80

20 65 20 55 85 85 110 75 70 125 80 60 60 60 120 86 110 90

25 92, 91, 90, 89 92 70 92 75 79 65 45 65 80 74 70 80 77 80 65 74 75 70

a

Reaction 1H-benzo[d]imidazol-2-amine (1, 1 mmol) and (E)-3-(4-isopropylphenyl)-1 phenylprop-2-en-1-one (2a, 1 mmol). Isolated yield. c Catalyst was reused four times. Source: Adapted from Reddy, M.V., Reddy, G.C.S., Lien, N.T.K., Kim, D.W., Jeong, Y.T., 2017. An efficient and green synthesis of benzo [4, 5] imidazo [1, 2-a] pyrimidines using highly active and stable poly acrylic acidsupported layered double hydroxides. Tetrahedron 73 (10), 1317
1323. With kind permission of Elsevier. b

Figure 20.8 Synthesis of chromene-incorporated dihydroquinoline derivatives (4a-z, 4a0 -d0 ). Source: Adapted from Reddy, M.V., Lien, N.T.K., Reddy, G.C.S., Lim, K.T., Jeong, Y.T., 2016. Polymer grafted layered double hydroxides (LDHs-g-POEGMA): a highly efficient reusable solid catalyst for the synthesis of chromene incorporated dihydroquinoline derivatives under solvent-free conditions. Green Chem. 18 (15), 4228
4239. With kind permission of RSC.

Layered double hydroxide polymer nanocomposites for catalysis

819

other existing catalysts for catalyzing the multicomponent mixture of 4-hydroxy2H-chromen-2-one (1), 4-methoxybenzenamine (2a), and 2-methylbenzaldehyde (3a) as a typical reaction. In addition, to achieve the optimal setting, the performance of LDH-g-POEGMA was considered in different conditions of temperature, time, solvent, and the amount of catalyst (Table 20.2). Based on overall results, the LDH-g-POEGMA showed the greatest potential as the best intended chromene-integrated dihydroquinoline with excellent yield and no Table 20.2 Optimization of reaction conditions for the synthesis of 4aa

Entry

Solvent

1 2

Neat Neat

3

Neat

c

Neat

5

Neat

6

Neat

7

Neat

8

Ethanol

9

THF

10

Toluene

11

DMF

12

DCM

13 14 15 16

Ethanol Ethanol DMF CH3CN

4

Catalyst (%)

Neat LDHs-g-POEGMA (2 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (8 mg) LDHs-g-POEGMA (3 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) LDHs-g-POEGMA (5 mg) PTSA (10 mol%) Cu(OTf)3 (10 mol%) Zn(OTf)3 (10 mol%) InCl3 (10 mol%)

Temperature ( C)

Yield (%)b

Time (min) 4a

5a

6a

RT RT

300 60


70

92 10





RT

60

73

10




60

10







60

10

95, 94, 92, 90, 89 95







60

25

83







80

15

95







60

45

65




20

60

60

55

30




60

65

53




41

60

48

69




22

60

60

55

40




80 80 120 80

30 300 240 240


30 35 29

85





52 55 51

(Continued)

820

Layered Double Hydroxide Polymer Nanocomposites

Table 20.2 (Continued) Entry

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Solvent

H2O Neat H2O DMF Ethanol CH3CN Water Neat Ethanol Neat CH3CN Ethanol Neat Neat Neat Neat Neat Neat

Catalyst (%)

FeCl3 (10 mol%) H3BO3 (10 mol%) Cu(OAc)2 (10 mol%) DBA (10 mol%) TMG (10 mol%) L-proline (10 mol%) L-proline (10 mol%) DBA (10 mol%) DBA (10 mol%) TMG (10 mol%) TMG (10 mol%) Et3N (10 mol%) PS/PTSA (10 mg) PS/ AlCl3 (10 mg) PS/ GaCl3 (10 mg) POEGMA (10 mg) LDH (10 mg) POEGMA 1 LDH (10 mg)

Temperature ( C) 70 60 100 120 80 80 100 60 80 60 80 80 60 60 60 60 60 60

Yield (%)b

Time (min) 180 300 180 240 300 240 300 240 300 255 240 270 90 88 100 120 55

4a

5a

6a

28 33 37 25 20 32 15 22 20 21 22 23 62 58 63 25 55 60




35 50 60
62 45 18 23 55 25 32 30 65
30

43 57 42 20 22
70
18 40 35 10



30


a

Reaction of 4-hydroxy-2H-chromen-2-one1 (1, 1 mmol), 4-methoxybenzenamine (2a, 1 mmol), and 2methylbenzaldehyde (3a, 1 mmol). Isolated yield. c Catalyst was reused five times. Adapted from Reddy, M.V., Lien, N.T.K., Reddy, G.C.S., Lim, K.T., Jeong, Y.T., 2016. Polymer grafted layered double hydroxides (LDHs-g-POEGMA): a highly efficient reusable solid catalyst for the synthesis of chromene incorporated dihydroquinoline derivatives under solvent-free conditions. Green Chem. 18 (15), 4228
4239. With kind permission of RSC. b

lateral product in a shorter reaction time. Also, catalyst stability, as an important factor for use in industrial applications, was considered on a model reaction (Table 20.2, entry 4). The results indicated that LDH-g-POEGMA can be reused for several sequential cycles without significantly reducing catalytic activity. The mechanism study revealed that the LDH-g-POEGMA with an active surface and inner core has an important role in the organization of enamine intermediate (Fig. 20.9). The LDH-g-POEGMA catalyzes the reactions through interactive, chelating, and binding with reacting component. Being environmentally friendly, with an easy work-up procedure, simplicity in catalyst separation from the reaction mixture, high product yields, and shorter reaction time are advantages of LDH-gPOEGMA as an inexpensive catalyst. In another study, the prepared LDHs-g-POEGMA was used as an efficient catalyst for synthesis of benzo[g]chromene-5,10-diones (4a-aa) by multicomponent reaction of a 2-hydroxy-1,4-naphthoquinone, different aldehydes, and (E)-N-methyl1-(methylthio)-2-nitroethenamine (Fig. 20.10) (Krishnammagari et al., 2017).

Layered double hydroxide polymer nanocomposites for catalysis

821

Figure 20.9 Schematic illustration of LDHs-g-POEGMA catalyzed synthesis of titled compounds (4a-z, 4a0 -d0 ). Source: Adapted from Reddy, M.V., Lien, N.T.K., Reddy, G.C.S., Lim, K.T., Jeong, Y.T., 2016. Polymer grafted layered double hydroxides (LDHs-g-POEGMA): a highly efficient reusable solid catalyst for the synthesis of chromene incorporated dihydroquinoline derivatives under solvent-free conditions. Green Chem. 18(15), 4228
4239. With kind permission of RSC.

To reach the optimum conditions in order to obtaining the highest yield, a model reaction was investigated under different conditions in terms of temperature, time, solvent, and catalyst consumption. The best result was achieved when the model

822

Layered Double Hydroxide Polymer Nanocomposites

Figure 20.10 Synthesis of benzo[g]chromene-5,10-diones catalyzed by LDHs-g-POEGMA in solvent-free conditions. Source: Adapted from Krishnammagari, S.K., Lee, S.M., Jeong, Y.T., 2017. Solvent-free synthesis of 4H-pyranonaphthoquinones using highly active and stable polymer-grafted layered double hydroxides (LDHs-g-POEGMA) as an efficient and reusable heterogeneous catalyst. Res. Chem. Intermed. 1
17. doi: 10.1007/s1116. With kind permission of Springer.

reaction was run with 10 mg of the catalyst at 80 C within 20 min under solventfree conditions. Also, in comparison with another applied catalysts, LDHs-gPOEGMA gives the highest product yields in the shortest time possible. Recycling studies revealed that the catalyst could be reused for four consecutive runs without loss of activity. According to mechanism sequences, the LDHs-g-POEGMA with appropriate functional groups activates the intermediates for subsequent reactions (Fig. 20.11). The alkaline direct ethanol fuel cells (DEFCs) due to high energy density, availability, and eco-friendly features are applied broadly in energy storage applications. Anion exchange membrane (AEM) has an important role in the performance of DEFCs. As a result, the design and assembly of AEMs with high heat resistance and high ion conductivity is particularly important for the development of fuel cells. The studies showed that PVA can be used as an AEM and its properties are improved by loading the qualified inorganic additives. Hence, the crosslinked PVA/ MgAl-LDH membranes were fabricated through a solution-casting system (Zeng et al., 2012). The extracted data from analyses indicated that among the different percentages of membranes, the PVA/LDH 20 wt.% showed low ethanol permeability and high ionic conductivity. The penetration of PVA within the interlayer galleries of LDHs makes a meandering route for transmission of ethanol and, naturally, permeability will be promoted with increasing values of LDHs. Considering the hydrophilic nature of the LDHs, the adsorbed water enhanced with increasing quantity of loaded LDHs, which will improve the ionic conductivity based on the Grotthuss mechanism. The cell performance of commercial A301 and PVA/LDH 20 wt.% membrane was compared in the same conditions at variable temperatures (Fig. 20.12). The graph clearly showed that synthesized membrane was best at both 60 C and 80 C. The A301 membrane is certainly destroyed at temperatures above 60 C, while the PVA/ LDH hybrid is completely stable and increases the power density. The smart polymers are determined to be stimuli-responsive materials that answer to slight external physical or chemical alterations such as pH, light, biological molecules, humidity, electric or magnetic field, and temperature. Their unique properties make them mainly suitable for drug delivery, bioseparation, and sensor applications. Unpleasant sensitivity, long response time, and insignificant stability restrict their applications (Ghizal et al., 2014; Aguilar et al., 2007). According to

Layered double hydroxide polymer nanocomposites for catalysis

823

Figure 20.11 Schematic mechanism for the catalytic activity of LDHs-g-POEGMA in the synthesis of title compounds (4a
aa). Source: Adapted from Krishnammagari, S.K., Lee, S.M., Jeong, Y.T., 2017. Solvent-free synthesis of 4H-pyranonaphthoquinones using highly active and stable polymer-grafted layered double hydroxides (LDHs-g-POEGMA) as an efficient and reusable heterogeneous catalyst. Res. Chem. Intermed. 1
17. doi: 10.1007/s1116. With kind permission of Springer.

recent studies, the incorporation of inorganic NPs within smart polymers is an ideal approach to the development of their properties like flexibility and reversibility. LDH NPs with special layered structures have been excellently applied in electrochemical, biology, and optical fields. These suggest that LDH NPs are a good candidate for combination with smart polymers in order to build electrochemical sensors. The temperature-responsive poly(N-isopropyl acrylamide) (PNIPAA)/ CoAl-LDH ultrathin films (UTFs) were employed as switchable electrocatalysts (Dou et al., 2012). The PNIPAA/LDH UTFs were produced by an LBL assembly procedure based on periodic deposition of PNIPAA and LDH NPs which exhibit the reversible on
off property by moderating the temperature between 20 C and 40 C. The XRD and scanning electron microscopy (SEM) analysis indicated that LDH nanoplates in the form of C were oriented on the substrate plane (Fig. 20.13). Also, the PNIPAA/LDH UTFs showed a monotonous layered arrangement with a thickness of about 132 nm on quartz substrate. The atomic force microscopy indicated a decline of about 43 nm in root-mean-square when the temperature increased

824

Layered Double Hydroxide Polymer Nanocomposites

Figure 20.12 Polarization and power density curves of AEM DEFC employing PVA/20LDH composite polymer membrane and commercial A301 at different temperatures. Closed symbols represent cell voltage, open symbols represent power density (anode: 3 M ethanol 1 1 M KOH, 1 mL/min; cathode: dry oxygen, 100 sccm). Source: Adapted from Zeng, L., Zhao, T.S., Li, Y.S., 2012. Synthesis and characterization of crosslinked poly (vinyl alcohol)/layered double hydroxide composite polymer membranes for alkaline direct ethanol fuel cells. Intern. J. Hydr. Energy 37 (23), 18425
18432. With kind permission of Elsevier.

from 20 C to 40 C, and then showed an increase in the previous value with cooling temperature to 20 C. Also, the ellipsometry evaluations indicated that the thickness of UTFs was approximately 136 nm at 20 C and then was reduced to almost 66 nm by increasing the temperature to 40 C. These processes were successfully repeated for several consecutives, indicating excellent reversibility for surface topography and thickness. The cyclic voltammetry and electrochemical impedance spectroscopy were applied to check the electrochemical on
off behavior of the PNIPAA/LDHmodified indium tin oxide (ITO) electrodes at 20 C and 40 C. The results revealed fine electrochemical on
off performance at both temperatures due to the fast
slow interfacial charge conduction brought by temperature-adjusted shape conversion of PNIPAA. Finally, the (PNIPAA/LDH)10/ITO electrodes demonstrated good temperature-responsive performance for electrocatalytic oxidation of glucose as a typical reaction (Fig. 20.14). The electrocatalytic current and sensitivity of electrodes were increased at 40 C, which is related to high electric communication between the shrunk polymer and LDH layers. The long-term service and nice stability of electrodes are attributed to well-organized assembly of PNIPAA into the qualified LDH.

Layered double hydroxide polymer nanocomposites for catalysis

825

Figure 20.13 (A) XRD pattern. (B, C) Top view of the SEM image with (B) low magnification and (C) high magnification. (D) Side view of the SEM image for the (LDH/ PNIPAA)10 UTF on an ITO substrate. Source: Adapted from Dou, Y., Han, J., Wang, T., Wei, M., Evans, D.G., Duan, X., 2012. Temperature-controlled electrochemical switch based on layered double hydroxide/poly (N-isopropylacrylamide) ultrathin films fabricated via layer-by-layer assembly. Langmuir 28 (25), 9535
9542. With kind permission of ACS.

Figure 20.14 (A) Current 2 time curves measured at 0.5 V for the (LDH/pNIPAM)10/ITO electrode with successive addition of glucose in 0.1 M NaOH at 20 C and 40 C. (B) Calibration curves at 20 C and 40 C. Source: Adapted from Dou, Y., Han, J., Wang, T., Wei, M., Evans, D.G., Duan, X., 2012. Temperature-controlled electrochemical switch based on layered double hydroxide/poly (N-isopropylacrylamide) ultrathin films fabricated via layer-by-layer assembly. Langmuir 28 (25), 9535
9542. With kind permission of ACS.

826

Layered Double Hydroxide Polymer Nanocomposites

Fuel cells are very hopeful options for clean energy conversion in the search for replacing the usual combustion-based technologies. The oxygen reduction reaction (ORR) is the most significant reaction for generating clean electrical energy in fuel cells. The development of stable, active, and inexpensive electrocatalysts for ORR in fuel cells is a substantial challenge. Recently, the LDHs due to the unique structure and versatile composition have shown significant potential in the field of electrocatalysis. LDHs with positive surface charges provide suitable conditions for oxygen uptake and subsequent ORR (Huo et al., 2014; Indra et al., 2014). Accordingly, in a research paper, the hybrids of polydopamine spheres (PDAS) with CoFe-LDHs were fabricated and their catalytic activity toward the ORR was examined (Zhang et al., 2015). The

Figure 20.15 SEM images of (A) PDAS and (B) CoFe-LDHs/PDAS. (C) TEM images of CoFe-LDHs/PDAS. (D) XRD patterns of (a) CoFe-LDHs/PDAS and (b) pure CoFe-LDHs. (E) SEM image of pure CoFe-LDHs. Source: Adapted from Zhang, X., Wang, Y., Dong, S., Li, M., 2015. Dual-site polydopamine spheres/CoFe layered double hydroxides for electrocatalytic oxygen reduction reaction. Electrochim. Acta 170, 248
255. With kind permission of Elsevier.

Layered double hydroxide polymer nanocomposites for catalysis

827

Figure 20.16 (A) CVs of (a) PDAS, (b) HT-PDAS, (c) CoFe-DHs, and (d) CoFe-LDHs/ PDAS in an O2- or N2-saturated 0.1 M KOH solution at a scan rate of 50 mV/s. (B) Linearsweep voltammetry (LSV) curves of PDAS, HT-PDAS, CoFe-LDHs, and CoFe-LDHs/PDAS in 0.1 M KOH solution at a scan rate of 10 mV/s at 1600 rpm. Source: Adapted from Zhang, X., Wang, Y., Dong, S., Li, M., 2015. Dual-site polydopamine spheres/CoFe layered double hydroxides for electrocatalytic oxygen reduction reaction. Electrochim. Acta 170, 248
255. With kind permission of Elsevier.

828

Layered Double Hydroxide Polymer Nanocomposites

Figure 20.17 Chronoamperometric responses of (a) CoFe-LDHs/PDAS and (b) 5% Pt/C in an O2-saturated 0.1 M KOH solution at
0.3 V. Source: Adapted from Zhang, X., Wang, Y., Dong, S., Li, M., 2015. Dual-site polydopamine spheres/CoFe layered double hydroxides for electrocatalytic oxygen reduction reaction. Electrochim. Acta 170, 248
255. With kind permission of Elsevier.

CoFe-LDHs/PDAS hybrids were synthesized by in situ growing CoFe-LDHs on the surface of PDAS. PDAS with nitrogen and oxygen functional groups provides ideal conditions for chelation of metal ions and subsequently construction of ORR catalyst. The SEM and TEM images and XRD analysis confirmed that the surface of PDAS has been covered by CoFe-LDHs (Fig. 20.15). The SEM images indicated that the smooth surface of PDAS has turned into a rough and dark surface after adsorption of the CoFe-LDHs. Furthermore, the TEM images confirmed the composite structure of CoFe-LDHs/PDAS hybrids and showed that the thickness of coated CoFe-LDHs is about 20 nm. The series peaks of LDHs are easily visible in the XRD analysis of prepared hybrid, which indicates the presence of CoFe-LDHs. The electrocatalytic activity of PDAS, CoFe-LDHs, CoFe-LDHs/PDAS hybrids, and PDAS treated hydrothermally (HT-PDAS) for fair comparison was evaluated by cyclic voltammetry (CV) (Fig. 20.16). The results indicated that among the samples, the CoFe-LDHs/PDAS hybrid showed the most positive onset potential and highest cathodic current. This may be related to the synergistic effect of PDAS and CoFe-LDHs. Linear-sweep voltammetry at rotating disk electrode was also studied for the assessment of ORR catalytic activity of PDAS, CoFe-LDHs, HT-PDAS, and CoFe-LDHs/PDAS. As shown in Fig. 20.16B, the CoFe-LDHs/PDAS hybrid showed the highest catalytic activity among the considered samples, which was absolutely compatible with the CV results. The durability of CoFe-LDHs/PDAS hybrid was compared with commercial 5% Pt/C ORR catalysts by chronoamperometry. As shown in Fig. 20.17, the current of CoFe-LDHs/PDAS hybrid is almost steady over 10,000 s of the test, while the current of Pt/C ORR catalysts is accompanied by a noticeable drop, indicating that the CoFe-LDHs/PDAS hybrid has a more favorable stability than 5% Pt/C. Due to the great importance of polymer/LDH NCs in improving catalytic activity in various fields, the design and construction of this type of catalyst, with different

Layered double hydroxide polymer nanocomposites for catalysis

829

types of materials, should be studied more and further researches need to be done in this area.

20.5 Conclusions Today, technology is aimed at catalysts, and LDHs with layered and flexible compositions can be used effectively in this direction. The high potential of LDH for surface adsorption of diverse metal ions, as well as the integration of functionalized anions between LDH layers, generates an active basic surface that makes it suitable for redox reactions. Characteristics like high adsorption capacity, high thermal stability, and anchoring effect, create promising LDHs for adsorption of CO, CO2, SO2, and NOx. The combination of polymers with LDHs is a good method in order to develop their catalytic performances. In general, the dispersion index of LDH in the polymer background directly affects the structure and properties of the resultant NCs. Hence, the development of methods for the preparation of consistent polymer/LDH NCs is essential. In situ LDH synthesis in the polymer solution and in situ polymerization are good policies for improving the interfacial relations between LDHs and host polymer. Solution blending is accompanied by defects such as aggregation and melt mixing which is introduced as a traditional technique for the formation of PNCs on an industrial scale. LDHs not only improve the performances of polymers by promoting mechanical, optical, electrical, and thermal properties but also enhance the catalytic efficiency by a synergistic effect and increase the active sites. The obtained results from applications of polymer/LDH hybrids in important fields, such as the synthesis of organic molecules, fuel cells, and sensors, indicate that they can be used as effective catalysts due to high chemical and thermal resistance and good interfacial relations. The mild conditions, low cost, high yield, short time, simplicity, and sequential reusability are advantages of polymer/LDH catalysts.

Acknowledgments The research group acknowledges the Research Affairs Division of Isfahan University of Technology (IUT), Isfahan, I. R. Iran. Thanks are also given to National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT), Isfahan, I. R. Iran.

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