Graphene oxide enhanced polyacrylamide-alginate aerogels catalysts

Graphene oxide enhanced polyacrylamide-alginate aerogels catalysts

Accepted Manuscript Title: Graphene oxide enhanced Polyacrylamide-Alginate aerogels catalysts Authors: Cong Shan, Lianxu Wang, Zhongxu Li, Xin Zhong, ...

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Accepted Manuscript Title: Graphene oxide enhanced Polyacrylamide-Alginate aerogels catalysts Authors: Cong Shan, Lianxu Wang, Zhongxu Li, Xin Zhong, Yaheng Hou, Long Zhang, Fengwei Shi PII: DOI: Reference:

S0144-8617(18)31088-9 https://doi.org/10.1016/j.carbpol.2018.09.024 CARP 14067

To appear in: Received date: Revised date: Accepted date:

7-6-2018 13-9-2018 13-9-2018

Please cite this article as: Shan C, Wang L, Li Z, Zhong X, Hou Y, Zhang L, Shi F, Graphene oxide enhanced Polyacrylamide-Alginate aerogels catalysts, Carbohydrate Polymers (2018), https://doi.org/10.1016/j.carbpol.2018.09.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphene oxide enhanced Polyacrylamide-Alginate aerogels catalysts

Cong Shana, Lianxu Wanga, Zhongxu Lia, Xin Zhonga, Yaheng Houa, Long Zhanga, Fengwei

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Shi*,a,b,c,

School of Chemical Engineering, Changchun University of Technology, Changchun 130012,

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P. R. China

Advanced Institute of Materials Science. Changchun University of Technology, Changchun

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130012, P. R. China c

author. E-mail address: [email protected] (F. Shi).

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Graphical abstract

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*Corresponding

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Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States

Highlightes:



Graphene oxide enhanced new kind of biomass aerogel catalysts;



Graphene oxide and polyacrylamide enhance the mechanical property of alginate 1

aerogel catalysts by 30 times; 

Graphene oxide also significantly increases the catalytic activity.

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ABSTRACT

Biomass aerogel is a promising catalyst and has attracted extensive attention. However, most

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of the biomass aerogels are fragile, which limits their practical application. Herein, we significantly enhance the mechanical property of biomass aerogel catalysts by 30 times

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through incorporating graphene oxide into polyacrylamide and Cu-cross-linked alginate

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formed supper-strong double network aerogels. In addition to enhance the mechanical

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property, the graphene oxide also significantly increases the catalytic activity. Graphene oxide

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enhancement for biomass aerogel catalyst provides a new method to develop next generation

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supper catalysts.

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Performance

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Keywords: Alginate, Mechanical Strength, Aerogels, Biomass Catalyst, High Catalytic

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1. Introduction Aerogels are advanced porous materials that have low weight, low density, and large surface area. Conventionally, aerogels are prepared from inorganic materials (silica, alumina, zirconia, or their oxides) (Gurav, Jung, Park, Kang, & Nadargi, 2010; Nguyen, Tang, Acierno,

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Michal, & MacLachlan, 2018; Benad et al., 2018), synthetic polymers (formaldehyde, polyuria, or polyimide) (Nicholas, Chakkaravarthy, Abhishek, & Chariklia, 2014; Pierre, &

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Pajonk, 2002; Qin et al., 2015) or carbon (Guo et al., 2018). Recently, biomass-derived polysaccharides aerogels, as a class of sustainable ingredients, have attracted great attention

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due to their potential to transform various industrial processes from petroleum-dependent into

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bio-economic. Technological advances in extraction and purification processes enable the

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obtaining of natural polysaccharides on a large-scale, rendering polysaccharides aerogels

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interesting for wide applications, such as in the pharmaceuticals field, magnetic

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nanomaterials, food materials and other functional materials (Gonçalves et al., 2016; Cicco et al., 2016; Kharissova, Rasika Dias, & Kharisov, 2015; Mikkonen, Parikka, Ghafar, &

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Tenkanen, 2013). In particular, polysaccharide aerogel catalysts and catalyst supports have

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been extensively researched in the past decade because of their stability in most organic solvents, diverse accessible surface functionalities, and ease to operate (Quignard, Valentinw, & Renzo, 2008). Chitosan, cellulose, alginate, and carrageenan aerogels are all reported as

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heterogeneous catalysts (Chtchigrovsky et al., 2009; Koga et al., 2012; Wang et al., 2017). However, in industry, besides catalytic performance, all kinds of catalysts should have excellent particle strength for storage, transport, and reaction. Compared with other kinds of aerogels, most of the biomass-derived polysaccharides aerogels have not extended their 3

further industrial and practical applications because of their poor mechanical properties, especially the resistance to compressive strain. Among the abundant polysaccharides, sodium alginate (SA), extracted from brown algae, has a number of merits such as hydrophilicity, solubility, and biocompatibility (Reakasame,

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& Boccaccini, 2018; Kinnaert, Daugaard, Nami, & Clausen, 2017; Dumitriu et al., 2014), which permits its wide applications (Vermonden, Censi, & Hennink, 2012; Hao et al., 2017;

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Chen, Chen, Zhang, Cai, & Li, 2017), especially for preparing catalysts. There are a number of free carboxyl and hydroxyl groups distributed along the alginate backbone, which endows

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the alginate with the excellent capacity to cross-link with divalent or trivalent metal ions to

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form stable hydrogels (Hecht, & Srebnik, 2016; Li, Illeperuma, Suo, & Vlassak, 2014;

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various forms to meet different purposes.

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Mallepally et al., 2013). In addition, the geometrical shape of the aerogels can be designed in

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In recent years, the alginate aerogels have been reported as desirable metal catalysts (Primo, Liebel, & Quignard, 2018; Horga, Renzo, & Quignard, 2007). However, the

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improvement of mechanical properties of alginate aerogels has highlighted a key challenge.

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Alginate aerogels are obtained from their hydrogels through a drying process and their mechanical performance has direct relation to their hydrogels. Therefore, the dominant problem is how to improve the mechanical properties of hydrogels. Recently, some reports

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design hybrid alginate hydrogels composed of two or three different polymers interpenetrating network for conquering this challenge (Darnell et al., 2013). Several polymers such as polyacrylamide, polyvinyl alcohol, and cellulose, have been reported to strengthen the SA hydrogel successfully (Choi et al., 2017; Majidnia, & Fulazzaky, 2016; Ma 4

et al., 2017). Furthermore, adding some inorganic reinforcements, such as graphene oxide (GO), to the network cross-linking agents can enhance the mechanical properties of the SA hydrogels and aerogels to a further extent (Zhuang, Yu, Ma, & Chen, 2017; Yu, Zhang, & Yang, 2017; Zhuang et al., 2016). GO contains both noncovalent and covalent functionalized

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groups, which can greatly improve the mechanical properties of the polymer hosts through molecular interactions with the polymer chains, resulting in comparatively better mechanical

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properties than the parent gels. Some reports have focused on the study of GO/PAAm/SA interpenetrating network hydrogels (Fan et al., 2013; Liu, Bastola, & Li, 2017; Zhang, Pang,

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& Qi, 2015), while studies on their hybrid aerogel and their application on catalysis is scarce.

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In this present work, GO was incorporated into PAAm and Cu-cross-linked SA hydrogels,

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then the hybrid aerogels were obtained by a freeze-drying method. GO, as the reinforcement,

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was supposed to strengthen the mechanical properties of the PAAm and SA double network

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aerogel effectively, in addition, the functional group in GO may improve the catalytic activity of the prepared catalyst successively. The GO/PAAm/SA aerogel as a biomass-based material

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is a more efficient alternative environmentally friendly catalyst for future industry use.

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2. Experimental Section

2.1. Reagents and materials Natural graphite, polyacrylamide (PAAm), and sodium alginate (SA) were purchased from

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Sigma-Aldrich. Concentrated sulphuric acid (H2SO4), phosphoric acid (H3PO4), potassium permanganate (KMnO4), N,N’-methylenebisacrylamide (MBA), hydrogen peroxide (H2O2, 30 wt%), and copper chloride dehydrate (CuCl2·2H2O) were obtained from Sinopharm Chemical Reagent Co.. Phenol was purchased from TianJing GuangFu Chemical Research 5

Institute. All the reactants in the experiments were of analytical grade and used without further purification. 2.2. Preparation of GO and purification GO was prepared by using the modified Hummers method (Hummers, & Offeman, 1958;

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Du et al., 2017) and dried for later use. In a typical synthesis, 3 g of natural graphite, 360 mL of H2SO4, and 90 mL H3PO4 were firstly mixed together in an ice bath. Then 18 g of KMnO4

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was slowly added. The dispersed suspension was transferred to a 35 oC water bath, followed with vigorous stirring for about 12 h. Finally, 400 mL of water and 3 mL of H 2O2 (30 wt%)

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were dropped into the solution, turning the colour from dark brown to yellow. The solid

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product in the suspension was separated and washed with deionized water using high-speed

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centrifugation at 8000 rpm for 4-5 times to remove small GO pieces and water-soluble

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impurities. Finally, the sediment was dried under vacuum at 60 oC to a constant weight and

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milled to an ideal particle size for later use.

2.3. Preparation of ternary composite aerogels

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The approach preparing the strengthened hydrogels was in a mixed solution containing

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PAAm, SA, and GO. MBA was chosen as a typical crosslinker for PAAm. In the first step, the given amount of dried GO was added into 30 mL of water and dispersed by ultrasonication for 30 minutes. Then, a set of mass ratios of PAAm/SA were added into the

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GO dispersed aqueous with stirring for 24 h to obtain a homogeneous solution at 25 oC. MBA was added into the solution which was kept stirring for 3 h under 0 oC. In the second step, the above uniform solution was injected into a cylindrical mold which was immersed into the CuCl2 solution. The solution was heated in a water-bath at 50 oC for 24 h to obtain 6

strengthened hydrogels. The obtained hydrogels were washed with distilled water to remove the non-cross-linked salt and dried to obtain aerogels through a SIM FD5505S freeze-drying apparatus. The aerogels of GO/PAAm/Cu-SA were labeled as GOz/PAAmy/Cux-S, where z =1, 2, 3,

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and 4 represents the grams of GO was 0.1, 0.15, 0.2 and 0.3 g, respectively, y = 1, 2, 3, and 4 represents the mass ratio of PAAm to SA was 1, 2, 3 and 4, and x = 1, 2, 3 and 4 represents

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the fed molar concentration of Cu2+ ions was 0.1, 0.2, 0.3 and 0.4 mol/L, respectively. 2.4. Characterization

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The chemical characterization of M/G molar ratio and molecular weight of SA were

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calculated by 1H NMR spectroscopy and Gel Permeation Chromatography (GPC). For NMR

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analysis, SA was dissolved in 1.0 mL D2O at neutral pH. 1H NMR spectra were recorded on a

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Bruker Avance-II 500 (Ultra shield) Spectrometer operating at 400 MHz at 80 oC. The M/G

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molar ratio was calculated following the ASTM F2259-03 standard (Figure S1). GPC (Agilent GPC Security 1200 system) measurements were carried out using 0.1 mol/L NaNO 3

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as eluent at a flow rate of 0.5 mL/min at 40 °C and pullulan P-50 standards were used to

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obtain calibration curves. Values of the specific refractive index increment (dn/dc) was

0.155. SA has the following characteristics: M/G ratio ≈ 1.0, molecular weight Mw = 240 ± 2.7 kg/mol, Mn = 198 ± 2.1 kg/mol.

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The surface morphologies of the prepared aerogels were characterized using a

field-emission scanning electron microscopy (SEM, JEOL, JSM-5600LV). The aerogels were fractured and coated with gold on the surface. Fourier transform infrared spectra (FT-IR) were recorded using a Nicolet IS10 IR 7

spectrophotometer operating at 4 cm-1 resolution across the 400-4000 cm-1 range. For FT-IR characterization, the samples were mixed with KBr powder. Then the mixture was ground in agate mortar evenly and a typical platelet for FT-IR was obtained in the mold. The contents of metal (wt. %) in the alginates were determined by a Leeman Prodigy Spec

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inductively coupled plasma atomic emission spectrometer (ICP-AES). The thermal stability of the aerogels was investigated using a thermogravimetric analysis

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(TGA) instrument (STA 6000, PerkinElmer) from 50 oC to 500 oC under a nitrogen atmosphere at a heating rate of 10 oC/min.

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Mercury intrusion porosimetry was obtained using a Quantachrome PoreMaster 60-GT

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instrument.

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Particle compressive strength was obtained using a DLⅢ Particle strength tester made by

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Dalian Penghui Science and Technology Development Co. Ltd., the compressive strength

2.5. Catalyst test

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was recorded at the moment of a sudden collapse of aerogels.

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The catalytic performance of the aerogels catalysts was evaluated through a phenol

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hydroxylation reaction. In a three-necked round bottom flask equipped with a reflux condenser, phenol (1.00 g, 11 mmol) and 0.05 g of catalyst were added in 30 mL of distilled water. Then, 20 mmol of H2O2 (30 wt%) was added dropwise for 20 min. After that, the

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mixed solution was stirred for 2 h at 70 °C. The product was collected and analyzed through a gas chromatography (Agilent GC6890) with an HP-5 column using a flame ionization detector (FID). 3. Results and Discussion 8

3.1. Characterization of GO/PAAm/Cu-S The hybrid double network hydrogels were developed by combining metal ions cross-linked alginate gel and a covalently cross-linked PAAm network, in addition, the introduction of GO can further improve the mechanical properties of the prepared aerogels

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(Figure S2). The 1st network was formed by PAAm through covalent bonds cross-linking, and the SA chains interpenetrated in the 1st network. Then, the PAAm/SA hydrogels were

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immersed in solutions with Cu2+ ions which served as cross-linker to obtain the PAAm and metal ion cross-linked alginate double network hydrogels. Scheme 1 illustrates how to obtain

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the toughened aerogels. The layered GO nanosheets were intercalated within the double

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networks of aerogels, and the functional groups of GO provided hydrogen bonding which

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could interact with the polymer chains. Therefore, the introduction of GO could enhance the

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mechanical properties of the prepared aerogels successfully (Jiao et al., 2016; Wan, Chen et

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al., 2014).

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In practical industry, the particle strength of a catalyst is an important factor for broadening its applications, so developing catalysts with outstanding mechanical properties is always an interesting issue. In our work, a compressive strength test was applied to provide the

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mechanical properties of the aerogels. As shown in Figure 1A, obviously, the particle compressive strengths of Cu-S catalysts were quite low, lower than 1.02 kg/cm2, which indicated that the cross-linked content of Cu ions had little influence on their particle compressive strength. Compared with Cu1-S, after introducing PAAm, the mechanical 9

strength of aerogels was improved. Keeping the concentration of Cu2+ at 0.1 mol/L, when the mass ratio of PAAm/SA increased from 1:1 to 4:1, the particle compressive strength of the aerogels increased from 7.23 to 9.06 kg/cm2. This is due to PAAm being able to form a more interpenetrating network structure to support the aerogels. In addition, after introducing GO,

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the particle compressive strength of the GOz/PAAm1/Cu1-S aerogels increased further, from 13.80 to 19.94 kg/cm2 with the mass of GO increasing from 0.1 g to 0.3 g. This trend is also

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verified by Figure 1B and Figure S3. Keeping the concentration of Cu2+ at the same (0.1 and 0.4 mol/L), both PAAm and GO played essential roles in enhancing the mechanical

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performance of the prepared material, and when their added amount increased, the prepared

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catalysts had better strength. The highest particle compressive strength of the

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GO4/PAAm4/Cu1-S aerogels could reach to 30.32 kg/cm2. It was found that only small

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amounts of GO could improve the strength of catalyst effectively.

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To illustrate the reason why GO can enhance the mechanical strength of the prepared and SEM images (Figure 3) were applied to illustrate the

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aerogels, FT-IR spectra (Figure 2)

functional groups and morphology change of the aerogels. Figure 2 illustrated the FT-IR spectra of the GO, SA, PAAm, and hybrid aerogels. The spectrum of GO presented typical

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absorption bands at 3387 cm-1 which was assigned to –OH stretching vibrations. The strong peak centered at 1730 cm-1 for C=O groups (–COOH), the C=C stretching mode of sp2 network peak at 1621 cm-1, the C–O–C stretching peak at 1224 cm-1, and the C–O (C–OH) stretching peak at 1049 cm-1 were the typical peaks of GO. (Fan et al., 2013; Liu, Bastola, & 10

Li, 2017) For the SA spectrum, the peak at 3421 cm-1 was attributed to –OH stretching vibration, the peaks near 1621 cm-1 and 1395 cm-1 were caused by asymmetric and symmetric stretching vibrations of COO- groups, respectively. In the spectrum of PAAm, the broad bands at 3421 and 3192 cm−1 were attributed to the N–H stretching vibrations. The

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absorbance peaks at 1655 cm−1 were ascribed to the C=O asymmetric stretching vibration. In the PAAm/Cu-S aerogel (Figure 2, from A to C), there were two types of cross-linked

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polymers: Cu-cross-linked alginate and covalently cross-linked PAAm. The asymmetric

stretching vibration bands of the C=O in SA and PAAm were located at about 1620-1655

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cm-1. While after cross-linking with Cu2+ ions, on the spectra of A to C, there were two peaks

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found at 1655 and 1586 cm-1. The red-shifting (from 1621 to 1586 cm-1) of the peak was

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resulted from the reason that the C=O bond in SA coordinated with the metal ions and led to

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the decrease of the stretching force constant of the carboxyl group. With the increasing of the

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content of metal (from 0.1 to 0.3 mol/L), the peak at 1586 cm-1 became obviously. The C=O bond in PAAm did not cross-link with metal cations, so the peak at about 1655 cm-1 did not

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change. Furthermore, the 1395 cm-1 peak was a result of the overlap of the C–O bond in the

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carboxyl group of SA and C–N bond in PAAm. Compared with the PAAm/Cu-S, after the introduction of GO, from D to G, the absorption of C=O stretching vibration at 1730 cm -1 disappeared, and the C=O asymmetric stretching vibration in both PAAm and Cu-S

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transferred (from 1655 cm-1 to 1650 cm-1 and from 1586 cm-1 to 1596 cm-1, respectively), which should be evidence to prove the formation of hydrogen bonding interactions between polymers and GO nanosheets (Liu et al., 2012).

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The network was obvious and the holes were smooth when the added amount of GO was only 0.1 g (Figure 3A). While increasing the GO amount, the holes of aerogels had filamentous structure with interconnecting pores inside and outside (Figure 3B-3D). It illustrated the layered GO participating in the formation of the network structure. The

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uniform macroporous structure was replaced by the combination of lamellar and network structures. GO retained the lamellar structure when it was inserted into the network of the

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aerogels, resulting in the improvement of mechanical performance of aerogels.

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The thermal stability of the prepared material was studied by TGA and presented in Figure

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4. Cu1–S showed two different weight loss regions: 130-200 °C which might be due to the

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breakup of segments of D-mannuronic acid and L-glucuronic acid, and 200-500 °C which

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was due to the decomposition of the SA polymer chains (Samanta, & Ray, 2014). PAAm was more thermally stable. There were also two weight loss regions: 230-370 °C was resulted

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from the dehydration and decomposition of the organic polymer and nitrate; 370-450 °C was

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due to the breakdown of the polymer main chain and release of carbon dioxide, water, and imides compounds (Huang, Yang, Gao, & Wang, 2012). The mass loss for GO from 150-410 °C could be ascribed to decomposition of oxygen functionality such as hydroxyl,

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epoxide, and carboxyl groups on the surface (Zhang, Pang, & Qi, 2015). It was found that both PAAm4/Cu4–S and GO4/PAAm4/Cu4–S had the similar weight loss trends with Cu4–S, but their decomposition temperature were higher than that of Cu4–S. Results illustrated that the thermal stability of the material was raised after incorporating PAAm and GO, indicating 12

that the mobility of polymer chains were suppressed by hydrogen bonding interactions with GO (Zhang, Pang, & Qi, 2015; Chen et al., 2015).

The property of porosity and high specific surface area of aerogels is also essential in

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catalytic reactions. In our previous work, it was found that the pore sizes of the prepared aerogels were mostly macroporous, so the mercury intrusion porosimetry method was applied

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to further illustrate the internal structure and the specific surface areas of the aerogel catalysts. The pore size distribution of Cu4-S and GO3/PAAmy/Cu4-S is displayed in Figure 5. For

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Cu4-S, the distribution of pore sizes varied widely from 25 to 200 µm, but the surface area

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was low, only 4.32 m2/g. Compared with Cu4-S, after introducing PAAm and GO, the pore

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size moved towards larger sizes, and the majority of pores were distributed from 50 to 150

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µm. As a kind of natural polymer hybrid material, the regularity of the pore size distribution

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needed further study, but the total specific surface areas of aerogels increased (from 8.73 to 14.98 m2/g) with the increase of the mass ratio of PAAm/SA (from 1:1 to 4:1). The increase

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of the surface areas might be caused by the second network formed by PAAm.

In catalytic reactions, an excellent catalyst should be equipped with a high number of

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active sites. ICP-AES was used to provide important information about the impact of PAAm and GO on Cu ion content of the prepared aerogels (Figure 6). In Cux-S, Cu ion content increased from 9.7 to 15.5% with the increase of CuCl2 from 0.1 to 0.4 moL/L. While in the PAAmy/Cu1-S, the Cu ion content decreased from 6.6 to 4.1% with the increasing of the mass 13

ratio of PAAm/SA from 1:1 to 4:1. The increased PAAm network might obstruct the contact between alginate and Cu2+, and reduce the cross-linked Cu ion content. In GOz/Cu1-S, the Cu ion content in catalysts changed slightly (from 9.5 to 9.2%) with the increasing of GO (from 0.1 to 0.3 g). In addition, the same trends were found in GO1/PAAmy/Cux–S (Figure 6B). As a

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result, the added dose of CuCl2 and PAAm had great effect on the cross-linked metal content, but GO had little influence. Although increasing the PAAm could reduce the Cu ion

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cross-linked number, it could improve the mechanical performance of the catalysts greatly.

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3.2. Catalytic Performance

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The GOz/PAAmy/Cu4–S aerogels were applied in phenol hydroxylation reaction with H2O2

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to study their catalytic performance (Figure 7, Table S1). SA and PAAm were unable to

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oxidize phenol to form hydroquinone and catechol effectively, the conversion of phenol was only 6.1% and 6.9%, respectively. GO was reported to have many functional groups and can

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disperse in water, so it can act as a metal-free catalyst (Navalon, Dhakshinamoorthy, Alvaro,

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& Garcia, 2014). The –COOH group on GO has intrinsic peroxidase catalytic activity and can catalyze the reduction of H2O2 to produce hydroxyl radical (HO•) (Scheme 2). GO was reported to have catalytic activity for phenol hydroxylation (Espinosa et al., 2015). An

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interesting result was discovered that GO has good catalytic activity with the conversion of phenol being 34.9% which is higher than reference Espinosa et al., 2015. The reason can be explained that the reaction temperature in our work was 70°C, under which phenol could dissolve in water easily and could react with the hydroxyl radical further to obtain more 14

oxidized products. Therefore, the conversions of phenol catalyzed by GO/PAAm/Cu–S were expected to be higher than that catalyzed by PAAm/Cu–S and some catalysts reported (Table S2). Obviously, the conversion of phenol increased with the increasing of Cu2+ ion amount

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which acted as active sites. Additionally, compared the GOz/PAAmy/Cu4–S catalysts, keeping the amount of Cu ion unchanged, the conversion of phenol increased with the increasing of

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GO and reduced with the increasing of PAAm. This is due to the fact that GO has catalytic

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performance and PAAm could affect the cross-linked metal amount.

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Conclusion

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In conclusion, a novel type of catalysts with outstanding mechanical and catalytic

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performance were prepared through introducing GO into the PAAm and Cu ion cross-linked alginate double network aerogel. Compared with the Cu ion cross-linked alginate catalysts,

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after incorporating GO, both the mechanical strength and catalytic performance of these

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aerogel catalysts were enhanced greatly. Although PAAm could reduce the Cu ion cross-linked content to some extent, the improvement of strength by PAAm was outstanding. Increasingly, GO played two important roles in the prepared material: GO could react with

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PAAm and alginate to increase the mechanical strength of the catalysts because of containing hydrogen bonding; in addition, GO could react with H2O2 and increase the catalytic activity of phenol hydroxylation because of containing –COOH group. Acknowledgments 15

This work was supported by the Natural Science Fund Council of China (21506015) and Fund of The Education Department of Jilin Province (JJKH20181009KJ).

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Figure captions

35

30

A

25

B

30

PAAMy/Cu1-S

GOz/PAAM3 GOz/PAAM2

GOz/PAAM4

2

Strength( kg/cm )

15 10 5 0

25 20

GOz/PAAM1

15 10 5 0

3 3 0.2

2 2 0.15

1 1 0.1

4 4 0.3

0.1 0.15 0.2 0.3

0.1 0.15 0.2 0.3

0.1 0.15 0.2 0.3

0.1 0.15 0.2 0.3

GO /PAAM /Cu -S 1 z y

Catalysts

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GOz/PAAM1/Cu1-S

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Strength(Kg/cm2)

Cux-S

A

1621 1395

SA 3421 PAAm

1655

M

A B

1224 1049

3421 3192

C

1586 1655

D E F

1650 1596

4000

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G

3500

3000

2500

2000

1500

GO

Transmittance(a.u.)

1730 1621 3387

ED

Transmittance(a.u.)

GO

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GOz/PAAmy/Cu1–S.

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Figure 1. Compressive strength of the prepared catalysts: (A) Cux–S, PAAmy/Cu1–S, GOz/ PAAm1/Cu1–S; (B)

1621

1621

PAAm 1655

C 1655 1586

F

1650 1596

1000 600 2000

-1

Wavenumber(cm )

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SA

1730

1500

1000

Wavenumber(cm-1)

Figure 2. FT-IR spectra of GO, PAAm, SA and hybrid aerogels. PAAm 1/Cu1-S (A), PAAm1/Cu2-S (B), PAAm1/Cu3-S

(C),

GO1/PAAm1/Cu1-S

(D),

GO2/PAAm1/Cu1-S

A

GO4/PAAm1/Cu1-S (G).

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(E),

GO3/PAAm1/Cu1-S

(F)

and

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Figure 3. SEM images of the aerogels, (A) GO1/PAAm4/Cu4-S, (B) GO2/PAAm4/Cu4-S, (C) GO3/PAAm4/Cu4-S

M

A

and (D) GO4/PAAm4/Cu4-S.

100

2

CC E A

4

1

60

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Weight / %

80

ED

5

40

1 Cu4-S

20

2 PAAm 3 GO 4 PAAm4/Cu4-S

3

5 GO4/PAAm4/Cu4-S 0 100

200

300

400

500

Temperature/C

Figure 4. TGA data for Cu4–S, PAAm, GO, PAAm4/Cu4–S and GO4/ PAAm4/Cu4–S at a constant heating rate of 10 °C min-1 under a flowing nitrogen atmosphere.

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Catalysts Cu4-S GO3/PAAm1/Cu4-S GO3/PAAm2/Cu4-S GO3/PAAm3/Cu4-S GO3/PAAm4/Cu4-S

dV/dlog (D)

GO3/PAAm3/Cu4-S

Total Surface Area(m2/g) 4.32 8.73 9.27 12.49 14.98

GO3/PAAm4/Cu4-S

Cu4-S 0

50

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GO3/PAAm2/Cu4-S

GO3/PAAm1/Cu4-S

100

150

Pore Diameter (m)

200

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Figure 5. Pore size distribution and specific surface areas of Cu4-S and

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A

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GO3/PAAmy/Cu4-S aerogels by mercury intrusion porosimetry.

Figure 6. The content of cross-linked Cu in material: (A) Cux–S, PAAmy/Cu1–S and GOz/Cu1–S; (B)

A

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GO1/PAAmy/Cux–S.

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GO4/PAAm1/Cu4-S GO3/PAAm1/Cu4-S GO2/PAAm1/Cu4-S GO1/PAAm1/Cu4-S GO4/PAAm2/Cu4-S GO3/PAAm2/Cu4-S GO2/PAAm2/Cu4-S GO1/PAAm2/Cu4-S GO4/PAAm3/Cu4-S GO3/PAAm3/Cu4-S GO2/PAAm3/Cu4-S GO1/PAAm3/Cu4-S

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GO4/PAAm4/Cu4-S GO3/PAAm4/Cu4-S GO2/PAAm4/Cu4-S GO1/PAAm4/Cu4-S GO

0

20

40

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PAAm SA

60

80

Conversion of Phenol /%

Figure 7. The catalytic performance of GO/PAAm/Cu-S catalysts.

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All reactions were performed under the following conditions: phenol 1.00 g, catalyst 50 mg, H2O2 2.0 mL,

A

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M

A

N

solvent (water) 30 mL, temperature 70°C, time 2 h.

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M

A

N

U

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Scheme captions

A

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Scheme 1. Structure and toughening suggested mechanism of GO/PAAm/Cu-S aerogels.

Scheme 2 Illustration of the phenol hydroxylation using GO/PAAm/Cu-S as catalyst.

25