Preparation and catalytic performance of alginate-based Schiff Base

Accepted Manuscript Title: Preparation and catalytic performance of Alginate-based Schiff Base Authors: Lianxu Wang, Yaheng Hou, Xin Zhong, Jianglei Hu, Fengwei Shi, Haoyu Mi PII: DOI: Reference:

S0144-8617(18)31508-X https://doi.org/10.1016/j.carbpol.2018.12.062 CARP 14416

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

2 November 2018 3 December 2018 19 December 2018

Please cite this article as: Wang L, Hou Y, Zhong X, Hu J, Shi F, Mi H, Preparation and catalytic performance of Alginate-based Schiff Base, Carbohydrate Polymers (2018), https://doi.org/10.1016/j.carbpol.2018.12.062 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.

Preparation and catalytic performance of Alginate-based Schiff Base Lianxu Wang, Yaheng Hou, Xin Zhong, Jianglei Hu, Fengwei Shi*, Haoyu Mi* Advanced Institute of Materials Science and School of Chemical Engineering

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Changchun University of Technology, Changchun 130012, P. R. China

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Corresponding author.

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Tel. or Fax: +86-431-85716785. E-mail address: [email protected] (F. Shi).

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Tel. or Fax: +86-431-85716785. E-mail address: [email protected] (H. Mi).

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Highlightes:

Novel alginate-based Schiff Base Cu (II) complex was prepared.

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The content of stablized metal of alginate-based Schiff Base was improved by

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

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1.3 times.

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Relation between degree of oxidation and stabilized metal content of alginate

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was illustrated.

Abstract

As a biomass polymer, alginate-based material has drawn considerable attention and 1

been applied in many fields. However, the research on alginate-based Schiff Base metal complex is scarce. Herein, a novel alginate-based Schiff Base Cu (II) complex was prepared via oxidation and imidization of alginate by periodate and organic amine, respectively. In additon, their catalytic performance in phenol hydroxylation reaction

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was investgated. Results illustrated that degree of oxidation had significant influence on metal loaded content. Compared with alginate, the alginate-based Schiff Base has

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more reactive groups, and can stablize more Cu cations which endowed their better

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catalytic activities.

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1. Introduction

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catalytic performance.

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Keywords: Alginate-based Schiff Base; Alginate dialdehyde; Biomass catalyst; High

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Alginates are anionic linear polysaccharides composed of 1,4-linked β-D-

mannuronic acid (M units) with 4C1 ring conformation and α-L-guluronic acid (G units)

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with 4C1 conformation, and have wide applications on drug delivery, tissue engineering, and sewage treatment, et al. (Ranu, & Chattopadhyay, 2009; Lückgen, Garske, Ellinghaus, Desai, Stafford, & Mooney, 2018; Srivastava, Yadav, & Samanta, 2015; Reakasame, & Boccaccini, 2018). With some divalent metal cations (Ca2+, Cu2+, Ni2+, et al.), alginates can form ionic bridges and ionic cross-linked hydrogels, which endows 2

alginates a favorable metal carrier and can be applied in many practical areas, such as wastewater treatment, toxic metal adsorption, and catalytic reactions (Quadrado, & Fajardo, 2017; Kühbeck, Saidulu, Reddy, & Díaz, 2012; Kühbeck, Dhar, Schön, Cativiela, Gotor-Fernández, & Díaz, 2013). In our early work, the well catalytic

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performance of metal cross-linked alginate (M-ALG) materials had been illustrated (Shi, Mu, Yu, Hu, & Zhang, 2014; Shi, Chen, Sun, Zhang, & Hu, 2012). However,

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because of the limitation of their structure, four blocks cross-link with one metal ion (Scheme 1). How to reconstruct the structure and improve the cross-linked metal

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content to enhance the catalytic activity is an interesting issue.

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There are plenty of free hydroxyl and carboxyl groups spreading along the alginate

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backbone, which are easy to be chemical functionalized (Pettignano et al., 2015;

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Dekamin, Ilkhanizadeh, Latifidoost, Daemi, Karimi, & Barikani, 2014; Dekamin,

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Azimoshan, & Ramezani, 2013). In addition, alginate could be oxidized by sodium periodate and form multiple functional aldehyde groups, which is usually named as

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alginate dialdehyde (ADA) (Hecht, & Srebnik, 2016; Zia, Zia, Zuber, Rehman, &

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Ahmad, 2015; Ali, Ganie, & Mazumdar, 2018). These multiple functional aldehyde groups are quite active and can react with amino groups to form Schiff Base (Cui et al., 2016; Meng et al., 2017). Many studies have illustrated that Schiff Bases are able to

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stabilize different metals in various oxidation states under mild reaction conditions and with high reaction rates, controlling the performance of metals in a large variety of useful applications, such as metal catalysts (Zhang, Xu, & Wong, 2017; Novozhilova et al., 2018; Zhao et al., 2018; Drury, & Mooney, 2003). Concerning metal catalysts, 3

the content of metal which plays as the catalytic center is one of the key issues to determine their catalytic performance. To our knowledge, although some reports have studied the oxidized alginate (Sakai, Yamaguchi, Takei, & Kawakami, 2008; Emami, Ehsani, Zandi, & Foudazi, 2018), the research on further preparing alginate-based

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Schiff Base metal complex is scarce. Furthermore, no study has reported about the relationship between degree of oxidation of alginate and stabilized metal content,

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neither the application of alginate-based Schiff Base metal complex in catalytic reaction. Therefore, in this present work, the alginate-based Schiff Base Cu (II) complex (Cu-

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ADA-N) was prepared. In addition, the content of loaded metal was studied and

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supposed to be improved in the alginate-based Schiff Base metal complex. The inner

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structure of the Cu-ADA-N material was evaluated through Fourier Transform Infrared

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Spectroscopy (FT-IR), Nuclear Magnetic Resonance (NMR), X-Ray Photoelectron

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Spectroscopy (XPS), atomic force microscope (AFM) and Scanning Electron Microscopy (SEM). The metal content was investigated by Inductively Coupled Plasma

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(ICP). Results indicated that, as illustrated in Scheme 1, at least four units were required

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to cross-link with one metal cation in M-ALG, but in M-ADA-N material, two units could stabilize one metal cation. So compared with the native alginate, the loaded metal content of alginate-based Schiff Base was improved approximate by 1.3 times, which

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endowed this functional material excellent catalytic performance.

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Scheme 1 Suggested schematic presentation of Cu-ALG and Cu-ADA-N.

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2. Experimental Section

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2.1. Materials

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Sodium alginate (ALG, viscosity: 1.05-1.15 Pa.s, pH: 6.8-8.0), ethylene glycol

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(CH2OH)2, ethanol (C2H5OH, 95%), copper chloride dihydrate (CuCl2·2H2O) and phenol were obtained from Tianjin Guangfu Fine Chemical Research Institute (China).

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Sodium periodate (NaIO4) was supplied by General-Reagent of Shanghai Titan

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Technology Co., Ltd.. Methylamine (CH3NH2) water solution (40%) and sodium chloride (NaCl) were obtained from Tianzheng Fine Chemical. Hydrogen Peroxide (H2O2, 30 wt%) was obtained from Sinopharm Chemical Reagent Co., Ltd.. All the

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other reagents used for analysis were of analytical grade and without further purification.

2.2. Preparation of Cu-ADA-N

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A series of mixture with different molar ratio of sodium periodate to sodium alginate were dissolved in a mixed solution of 100 ml of distilled water and 100 ml of ethanol, then the above solution was stirred at room temperature under dark for 24 h. The solution was neutralized by 20.0 mL of ethylene glycol to reduce the excess sodium

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periodate under dark for 2 h. After that, the product was precipitated by adding 10.0 g sodium chloride and 800 mL of ethanol. After filtering, the precipitate was dissolved in

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200 mL of deionized water and reprecipitated by the addition of 800 mL of ethanol.

This procedure was repeated three times. The product was lyophilized to obtain the

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alginate dialdehyde (ADA) (Xu et al., 2013; Gomez, Rinaudo, & Villar, 2007). The

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samples were labeled as ADA-X (X=20, 40, 60, 80, and 106 represents that the molar

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ratio of sodium periodate to sodium alginate monomer of 0.20, 0.40, 0.60, 0.80, and

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1.06, respectively.).

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The powder of 1.0 g ADA-X was dissolved in a mixed solution contained 30 mL of deionized water and 0.4 mL of methylamine water solution (40%), which was stirred at

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room temperature for 12 h. After filtered and dried at 40 oC for 24 h, the alginate-based

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Schiff Base (ADA-N) power was obtained. Then, 1.0 g ADA-N was dissolved in different concentration of copper chloride solution under magnetic stirring for 12 h. The reaction mixture was filtered and washed with deionized water until the filtrated was

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colorless. At room temperature, the product was dried for 3 days to obtain the alginatebased Schiff Base Cu complex (Cu-ADA-N-X, X=20, 40, 60, 80, and 106) (Scheme 2).

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2.3. Determination of degree of oxidation of ADA

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Scheme 2 Scheme for the preparation of Cu-ADA-N.

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Degree of oxidation (DO) of ADA was obtained by potentiometric titration of aldehyde groups through the hydroxylamine hydrochloride method applied automatic

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potentiometric titrator (LEICI ZDL-4A) (Xu, Li, Yu, Gu, & Zhang, 2012). The DO was

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calculated by Eq. (1). The analytical reactions were Eq. (2) and (3). DO = 𝑊

n(CHO)/2 𝑎𝑙𝑔𝑖𝑛𝑎𝑡𝑒 ⁄198.11

(1) (2)

HCl + NaOH → NaCl + H2O

(3)

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ADA-(CHO)n + n(H2N-OH·HCl) → ADA-(CH=N-OH)n + nH2O + nHCl

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2.4. Characterization

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H NMR spectroscopy and Gel Permeation Chromatography (GPC) were applied to

calculate the chemical characterization of M/G molar ratio and molecular weight of ALG. In NMR analysis, ALG and ADA was dissolved in 1.0 mL D2O at neutral pH. 1H 7

NMR spectra were recorded on a Bruker Avance-II 500 (Ultra shield) Spectrometer operating at 400 MHz at 80 oC. The M/G molar ratio was calculated following the ASTM F2259-03 standard (Figure S1). GPC was used to determine the molecular weight of ALG and ADA-X (Ding, Zhou, Zeng, Wang, Shi, 2017). GPC (Agilent GPC

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Security 1200 system) measurement was carried out using 0.1 mol/L NaNO3 as eluent at a flow rate of 0.5 mL/min at 40 °C and pullulan P-50 standards were used to obtain

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calibration curves. ALG has the following characteristics: M/G ratio ≈ 1.0, molecular

weight Mw = 24160 ± 2709 kg/mol.

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The fourier transform infrared spectra (FT-IR) of ALG, ADA, ADA-N and Cu-ADA-

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in the range of 400-4000 cm-1 with a resolution of 4 cm-1. The UV diffuse reflectance

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spectra were recorded on Agilent UV-Vis-NIR cary 5000. All spectra of the ALG, ADA,

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ADA-N and Cu-ADA-N were measured over the spectral range 200-1000 nm. The samples were placed between a quartz cell and a quartz plaque, and then put at the

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sample window of the integrating sphere.

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The contents of Cu (II) in the prepared material were determined by a Leeman Prodigy Spec inductively coupled plasma atomic emission spectroscopy (ICP-AES).

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All determinations were conducted at least in triplicate. The external surface morphology of ALG and ADA were examined using scanning

electron microscopy (SEM, JEOL JSM-5600LV). All the samples were fixed on the conductive adhesive, sputter coated with aurum. Meanwhile, the atomic force microscopy (AFM, SPA300HV) was used to evaluate the morphology change of ALG 8

and ADA. X-Ray Photoelectron Spectroscopy (XPS) was performed with Thermo ESCALAB 250 photoelectron spectrometer using a monochromatic Al Ka X-rays source at 150 W.

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High resolution spectra were acquired using a pass energy of 20 eV and a 0.1 eV step.

2.5. Catalyst test

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The catalytic performance of prepared Cu-ADA-N was evaluated through a phenol

hydroxylation reaction with H2O2 as an oxidation. In a three-necked round bottom flask

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equipped with a reflux condenser, 1.0 g (11 mmol) of phenol was dissolved in 30 mL

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of distilled water, and 0.05 g of catalyst was added. Then, 2.0 mL (20 mmol) of H2O2

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(30 wt%) was added for 20 min. The solution was stirred for 2 h at 70 °C. The products

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were analyzed by gas chromatography (Agilent GC6890) through an HP-5 column

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using a flame ionization detector (FID).

The characterization results of the concentrations of phenol, catechol (CAT), and

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hydroquinone (HQ) were evaluated using the equations given on reference Shi, Mu, Yu,

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Hu, & Zhang, 2014. The conversion of phenol is expressed by Xphenol: Xphenol (%) = 100 × (Cb, phenol – Ca, phenol) / Cb, phenol

where Cb, phenol and Ca, phenol are the molar concentrations of phenol before and after

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the reaction, respectively. The selectivity of the product is calculated under the equation: Selectivityp (%) = 100 × Cp / (Cb, phenol – Ca, phenol) where Cp is the molar concentration of the product and p = CAT and HQ. 3. Results and discussion 9

3.1. Characterization of ALG, ADA, ADA-N, and Cu-ADA-N The introduction of aldehydic groups onto alginate is the selected approach to activate the polysaccharide for chemical modification. The most outstanding advantage of this approach is that aldehydic groups can be transferred to imine groups via Schiff

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Base reaction, then obtain kinds of derivatives for different purposes (scheme 2). As described in scheme 1, after Schiff Base reaction, it is supposed that two blocks can

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stabilize one Cu ion in Cu-ADA-N, while four blocks are needed to cross-link with one

metal ion in Cu-ALG. Therefore, the alginate-based Schiff Base is supposed to stabilize

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more Cu ion.

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Synthesis of ADA was confirmed by 1H NMR as given in Fig. 1. 1H NMR spectrum

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of ALG exhibited peaks ranging from 3.6 to 4.9 ppm which belonged to protons of G

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and M units (Fig. 1A). After oxidization, in the 1H NMR spectrum of ADA, the H-1

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and H-5 signals changed their positions (Fig. 1B) (Tian et al., 2016). In addition, there were two new signals appeared at 5.3 and 5.6 ppm, which corresponded to a

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hemiacetalic proton formed from aldehydes and their neighbor hydroxyl groups

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(Scheme 2)(Gomez, Rinaudo, & Villar, 2007), and one new peak at about 9.5 ppm which was CHO signal. The above changes all confirmed the formation of ADA

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(Kholiya, Chaudhary, Vadodariya, & Meena, 2016).

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Hemiacetalic protons

Aldehyde

G-5 M-1

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G-1

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M-5

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B

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Figure 1. 1H NMR spectrum of ALG and ADA.

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Furthermore, the FT-IR and UV-vis spectra of ALG, ADA, ADA-N, and Cu-ADA-

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N were applied to compare the structure change (Fig. 2). The FT-IR spectrum of ALG presented the absorption bands of its characteristic structure, bands around 3440 cm-1

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(O–H stretching vibrations), 1617 cm-1 (C=O in the carboxyl group stretching vibrations), 1420 cm-1 (C–O in the carboxyl group stretching vibrations) and 1028 cm(C–OH stretching vibrations) (Fig. 2A). After reacted with sodium periodate, the

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spectrum of ADA showed a chemical structure difference. The ratio of hydroxyl stretching vibration peak centered at 3440 cm-1 increased which was attributed to the formation of more hydroxyl groups and the scission of glycosidic bond (Wasikiewicza, Yoshii, Nagasawa, Wach, & Mitomo, 2005). Some studies reported that the symmetric 11

vibrational band of ADA located at 1732 cm-1, but this band was weak, and was not detected due to hemiacetal formation of free aldehydes groups in some cases (Sarker et al. 2014). The new peak near 803 cm-1 was another proof that the formation of hemiacetal bonds between aldehydes and their neighbouring hydroxyl groups (Fan,

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Lewis, & Tapley, 2001; Lawrie et al., 2007). While after Schiff Base reaction, the peak at 1732 cm-1 reduced and difficult to be observed in ADA-N, which indicated that the

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C=O groups were replaced by other groups. The stretching vibration of the C=N band located at about 1630 cm-1 which was overlapped with the C=O in the carboxyl group

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and difficult to be indicated (Marin et al. 2015).

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To characterize the change of groups in a more precise way, the UV-vis spectra of

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the above four compounds were investigated (Fig. 2B). In the spectrum of ALG, the

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absorption peak at 204 nm belonged to -COOH group. In the spectrum of ADA-N, the

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absorption peak at 323 nm were belonged to the π→π* transition of C=N groups (Hosseiniet al. 2010). The addition new absorption peak at 750 nm in Cu-ADA-N was

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the d→d transition of Cu ion, which was a good evidence for the synthesis of Cu-ADA-

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N (Hu, Li, Li, Ma, & Guo, 2009).

B Cu-ADA-N

Absorbance

Transmittance(a.u.)

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A

ADA-N ADA 1732 803

ALG

1617 1420 1028

3440

4000

3500

3000

2500

2000

1500

1000

Cu-ADA-N

ALG

ADA-N

ADA 500

200

400

600

800

l/nm

Wavenumber(cm-1)

Figure 2. FT-IR spectra and UV-vis of ALG, ADA, ADA-N and Cu-ADA-N. 12

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XPS spectra was applied to identify the elements transfer in each step (Fig. 3). Carbon (1s, 278.4 eV) and oxygen (1s, 525.6 eV) were found in ALG and ADA (Jejurikar, Seow, Lawrie, Martin, Jayakrishnanc, & Grøndah, L. 2012). New N-1s photoelectron line appeared on the spectrum of ADA-N (Rella, Mazzotta, Caroli, Luca, Bucci, &

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Malitesta, 2018), and the Cu (2p3/2 and 2p 1/2) peaks were at 935.3 and 955.3 eV, separated by 20.0 eV, were found on the spectrum of Cu-ADA-N (Reddy,

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Balasubramanian, & Chennakesavulu, 2014), which provided precious evidence to

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A

N

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prove the formation of Cu-ADA-N.

Figure 3. XPS spectra of ALG, ADA, ADA-N and Cu-ADA-N.

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The vicinal hydroxyl groups of the monomeric units on ALG can be oxidized by

sodium periodate to form their dialdehyde derivatives, so the molar ratio of periodate

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to monomeric units plays key role on the degree of oxidation (DO) of ALG. The different DO and molecular weight (Mw) of ADA obtained at different molar ratio of NaIO4 to ALG were summarized at table 1. The DO of ADA increased with the increasing dosage of NaIO4, but all smaller than the theoretical values, which could be explained by that the formation of hemiacetal between aldehyde and hydroxyl group 13

prevented the further oxidation of hydroxyl group (Scheme 2). Compared with DO, the Mw showed different trends. In ethanol-water mixture, the Mw of ADA should be lower than that in the pure water system. Because in ethanol-water system, the oxidation of macromolecular polymers always be accompanied by the cleavage of the main chains,

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which was resulted by the hydroxyethyl radicals and hydroxide radicals produced by ethanol during oxidation (Balakrishnan, Lesieur, Labarre, & Jayakrishnan 2005).

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Therefore, when the DO was higher, the main chains were cleaved seriously, which lead to the reducation of Mw. Degree of oxidation and Molecular weight of ALG and ADA. Periodate equivalent

DO (%)

(%)a

40.0

ADA-60

60.0

ADA-80

80.0

ADA-106

106.0

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ADA-40

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a

20.0

2.416 e+4

12.3±0.2

4.596 e+3

32.4±0.2

1.959 e+4

54.1±0.3

5.287 e+3

69.6±0.4

4.800 e+3

90.4±0.3

4.142 e+3

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ALG ADA-20

Mw/(kg/mol)

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Samples

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Table 1

Periodate equivalent = M sodium periodate/M monomeric unit %

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To better understand the relationship between the structure of ADA and DO, SEM

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and AFM were applied to observe the morphology of ADA with different DO (Fig. 4 and Fig. S2). Without any treatment, the ALG presented big blocks and the shape was

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irregular (Fig. 4A and Fig. S2A). After oxidation, the morphology changed obviously. It could be found directly that the fractions of ADA became looser weave with DO increasing. When DO was 20% (Fig. 4B), ADA still presented as high polymer material which liked their parental ALG. However, when DO increased to 106% (Fig. 4F and Fig. S2B), the ADA sample appeared needle-like crystals, which indicated the decrease 14

of Mw. Therefore, the SEM and AFM images further demonstrated that DO has great

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influence on Mw, which was in agreement with the results of GPC.

Figure 4. SEM images, (A) ALG, (B) ADA-20, (C) ADA-40, (D) ADA-60, (E) ADA-80 and (F) ADA-106.

The contents of metal in Cu-ADA-N were determined by ICP-AES, and were

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compared with Cu-ALG (Fig. 5). The contents of Cu in both Cu-ALG and Cu-ADA-N increased with the increasing of concentration of CuCl2 (Fig. 5 B). The content of Cu increased from 5.5% to 18.1% and 9.2% to 23.8% in Cu-ALG and Cu-ADA-N-106, respectively, with the concentration of CuCl2 increased from 0.02 to 0.40 mol/L. 15

However, when the concentration of CuCl2 reached to 0.50 mol/L, the loaded content of Cu increased little. As we mentioned before (Shi, Chen, Sun, Zhang, & Hu, 2012), it might be resulted by reaching the saturation content of Cu in these polymers. While prepared with the same concentration of CuCl2, compared with Cu-ALG, the metal

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contents in Cu-ADA-N-106 were improved by 20% - 30 %. The reason could be illustrated by that in Cu-ADA-N, only two units could immobilize one metal ion, while

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four units were needed in ALG. In addition, there should be more than one style of

metal immobilization in Cu-ADA-N. Because not all the hydroxyl groups transferred

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to imine groups in Schiff Base reaction, and both imine and carboxyl groups could

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immobilize metal with hydroxyl groups (Scheme 2). Keeping the concentration of

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CuCl2 being 0.40 mol/L, DO had significant effect on the loaded content of Cu, which

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increased from 12.6% to 23.8% with the DO increasing from 20% to 106%,

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respectively (Fig. 5 A) . The result of ICP indicated that our strategy was feasible to

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25

B

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Metal content/%

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Metal content/%

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improve the loaded content of Cu.

20 18 16 14

Cu-ADA-N

20 Cu-ALG

15

10

A

12

20

40

60

80

5

106

DO/%

0.1

0.2

0.3

0.4

0.5

Concentration of CuCl2 /Mol/L

Figure 5. (A) Cu Contents of Cu-ADA-N at different theoretical DO (CuCl2 = 0.40 mol/L); (B) Cu Contents of Cu-ADA-N-106 and Cu-ALG at different molar concentration of CuCl 2.

3.2. Catalytic Performance 16

Schiff Base metal complex always have good catalytic performance and stabilization in organic reaction (Alshaheri, Tahir, Rahman, Ravoof, & Saleh, 2017; Shirase, Shinohara, Tsurugi, & Mashima, 2018). The catalytic performance of Cu-ADA-N and Cu-ALG were investigated in the hydroxylation of phenol reaction (Table S1). Results

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indicated that, in phenol hydroxylation reaction, the alginate-based Schiff Base Cu provided perfect catalytic activity which was better than Cu-ALG (Fig. 6). When the

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concentration of CuCl2 was 0.4 mol/L, the conversion of phenol increased from 41.72% to 75.41% with the DO of ADA increasing from 20% to 106%. However, the DO had

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little influence on selectivity, and the ratios of catechol (CAT) to hydroquinone (HQ)

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were all nearly 2:1. It was obvious that the catalytic activity had direct relation with the

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content of Cu which played as catalytic center (Fig. 6A).

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The Cu-ADA-N-106, which had excellent catalytic activity, was chosen to study the

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reusability of the prepared catalyst (Fig. 6B). The hydroxylation reaction was repeated five times under the same condition. After each run, the spent catalyst was recovered

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by filtration, washed thoroughly with water and ethanol, and dried. Compared with

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75.41% in the 1st run, the conversion of phenol was 70.95% at the 5th run. The conversion of phenol only reduced 5.9% after reacted 5 times, which illustrated the perfect catalytic stabilization of the prepared material. The reaction pathway is

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proposed to follow the Haber-Weis mechanism (Scheme 3) (Pachamuthu, Srinivasan, Maheswari,

Shanthi,

&

Ramanathan,

2010).

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2013;

Karakhanov

et

al.

100

80

40 30 20 20

10

60

Salectivity/%

60

70

CAT HQ

80

50

60

40 40

30 20

20 10

0

1st 60

80

2nd

3rd

0

5th

4th

106

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40

20

Coversation of Phenol/%

50

Coversation of Phenol/%

60

40

80

B 70

80

Selectivity/%

100

A

CAT HQ

Catalysts

Figure 6. Catalytic performance of Cu-ADA-N in the phenol hydroxylation reaction: (A) Catalytic

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A

N

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performance of Cu-ADA-N-X; (B) Catalytic stability of Cu-ADA-N-106.

4. Conclusion

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Scheme 3 Proposed reaction scheme for phenol hydroxylation under Cu-ADA-N.

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A series of alginate-based Schiff Base Cu (II) catalysts with perfect catalytic activity

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and stability were synthesized through oxidation and imidization. The degree of oxidation was the key issue in transferring the inner structure of the polymer and changing the loaded quantity of metal. The higher degree of oxidation was obtained,

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the more content of metal was loaded. Furthermore, compared with native alginate, the alginate-based Schiff Base could stabilize more Cu ions because of their imine groups, which endowed their better catalytic activities. Therefore, our strategy is a feasible method to enhance the catalytic performance of alginate-based material. 18

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

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A

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province science and technology development plan (20170520030JH).

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