nanofibrillated cellulose composite membrane loaded with Pd nanoparticles for dechlorination of dichlorobenzene

Journal Pre-proof Covalent organic framework/nanofibrillated cellulose composite membrane loaded with Pd nanoparticles for dechlorination of dichlorobenzene Xiaofang Wan, Xinying Wang, Guangxue Chen, Congbao Guo, Bowen Zhang PII:

S0254-0584(19)31384-7

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122574

Reference:

MAC 122574

To appear in:

Materials Chemistry and Physics

Received Date: 8 September 2019 Revised Date:

15 December 2019

Accepted Date: 23 December 2019

Please cite this article as: X. Wan, X. Wang, G. Chen, C. Guo, B. Zhang, Covalent organic framework/ nanofibrillated cellulose composite membrane loaded with Pd nanoparticles for dechlorination of dichlorobenzene, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/ j.matchemphys.2019.122574. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Covalent Organic Framework/Nanofibrillated Cellulose Composite Membrane Loaded with Pd nanoparticles for Dechlorination of Dichlorobenzene Xiaofang Wana, Xinying Wanga, Guangxue Chena,*, Congbao Guoa, Bowen Zhangb a

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,

Guangzhou, Guangdong, 510641, China b

School of Materials Science and Engineering, South China University of Technology, Guangzhou, Guangdong, 510641, China

ABSTRACT Covalent organic frameworks (COFs) have attracted much attention due to their high porosity and highly ordered crystalline structure. However, the insoluble and unprocessable features of bulk COF powder limit their applications. To overcome these limitations, COF-TpPa film was prepared based on amine-group functionalized nanofibrillated cellulose (NFC) by Schiff base reaction for the first time. The obtained COF-TpPa/NFC membrane had high specific surface area (BET, 357 m2/g) and uniform pore size of 2–20 nm with the thickness of 250 nm. Via a simple post-treatment,

a

Pd-containing

Pd@COF/NFC

composite

membrane,

was

accordingly synthesized, which showed excellent catalytic activity in catalyzing dechlorination of o-dichlorobenzene (o-DCB) with high activity (the apparent reaction rate was 0.0235 min−1).We expect that our approach will further boost research on designing and employing functional COF film for catalysis. 1

Keywords Covalent organic frameworks, nanofibrillated cellulose, membrane, dichlorobenzene, Pd nanoparticles 1. Introduction Recently, porous polymeric materials have attracted more and more attention due to their high surface areas [1, 2], consistent nanopores [3-5], and possibility of designing the network structure using a wide range of functional building blocks [6]. Among them, covalent organic frameworks (COFs) are a kind of organic porous polymer with periodicity and better crystallinity [7, 8], which are constructed via strong covalent bonds between light elements such as C, B, O, N, and Si [8-11]. Therefore, COFs have been used in gas adsorption, heterogeneous catalysis, photoconductivity, energy storage and drug delivery [12-17]. In particular, imine-based COFs are distinguished from many other COFs materials because of their excellent chemical and thermal stability, which do favor the development of such materials as catalyst carriers to burden noble metal [17-19]. For example, Au(0)@TpPa-1 catalyst has been successfully prepared by introducing Au nanoparticles into TpPa-1 powder, which exhibited high recyclability and superior reactivity for nitrophenol reduction [20].Wang’s group [21] synthesized imine-linked COF-LZU1 and treated it with Pd(OAc)2 at room temperature and the Pd(II)-containing COF catalyst was prepared, which showed excellent catalytic activity in catalyzing the Suzuki Miyaura coupling reaction. However, due to the

2

crystallization characteristics and rigid structure, most of the COFs currently reported are in the form of powder and it is hard to be processed and recycled, which limits the applications of COFs materials in catalytic fields [22, 23]. Thus, new strategies for the preparation of COFs into crystalline and ordered thin film are highly desired. The most common method for the preparation of COF thin films is top-down synthesis, through which the COF powder was directly exfoliated into a single atomic layer and further deposited to obtain a COF film. The whole process is simple and easy to operate. However, due to the weak van der Waals force on the stacking of atomic layers in the deposition process, the resulting COF films usually have defects such as low coverage, uneven thickness and low strength [22]. Another attractive method is bottom-up synthesis strategy, the key to which is to control the growth direction of the film, and sequentially avoid its irregularity and discontinuity caused by free dendritic growth process. The currently reported preparation methods of COF films based on bottom-up strategy included surface control synthesis, solvothermal synthesis, ultrasonic chemical synthesis, and microwave synthesis, etc [24-27]. These methods depended on the in-situ self-assembly growth of COF film on the substrate material, so the continuity, uniformity, thickness and film quality of the film are closely related to the properties of the substrate material [28]. Therefore, the selection of substrate material has great significance for optimizing the performance of COF membrane and accelerating its application. In recent years, research on nanofibrillated cellulose (NFC) and its derivatives

3

has become one of the most active research hotspots in the field of cellulose [29]. NFC is usually obtained by acid or alkali hydrolysis, enzymatic degradation or physical method, which can be uniformly dispersed in water to form a stable colloid [30-32]. NFC has the advantages of low density, large specific surface area, good mechanical properties, biodegradability and sustainable regeneration, etc [29]. In addition, it is easy to be chemically modified because of abundant hydroxyl on the surface and therefore has broad application prospects in various fields [33]. We have considered that a new type of flexible COF/NFC composite membrane can be prepared by modifying a nanocellulose film to enhance its compatibility with COF and simultaneously growing a layer of COF on its surface as a substrate. It is encouraging that studies in this regard are yet not to be carried out. In this paper, we chose a two-dimensional crystalline COF, TpPa-1, as the representative of the imine-linked COF because of its better chemical stability. The intramolecular hydrogen bond of TpPa-1 in the form of keto could significantly improve the stability of the material and also promote the adsorption of lone pair electrons to the metal catalyst. On the other hand, Pd is a well-studied catalytic material that can transfer the reactive hydrogen [H] for degradation of chlorinated organic compounds [34]. We prepared the funtionalized NFC firstly, and then COF/NFC membrane by in-situ self-assembly growth and subsequently decorated the membrane with Pd nanoparticles by a method of in-situ reduction. The substrate of NFC/COF membrane loaded with Pd nanoparticles was characterized by SEM-EDS

4

and XRD and XPS analysis. The as-synthesized Pd@COF/NFC hybrid membrane was finally used for the dichlorobenzene reduction. The high catalytic performance was showed, together with easy recyclability of this nanocatalyst and such an approach will boost the research on employing functional COF film for catalysis with high activity and better recyclability. 2. Experimental section 2.1. Materials Trifluoroacetic acid, 3-aminopropyltriethoxysilane (APTES), mesitylene and methanol were purchased from the Shanghai Aladdin Bio-Chem Technology Co., LTD

(Shanghai,

China).

Hexamethylenetetramine,

phloroglucinol,

p-phenylenediamine (Pa), and palladium (II) acetate were supplied by the Shanghai Macklin Biochemical Co., Ltd (Shanghai, China). The other reagents, such as 1,3,5-triformylphloroglucinol (Tp) and nanofibrillated cellulose (NFC).etc. will be used (Supporting Information). 2.2. Preparation and modification of NFC membrane The surface functionalized NFC membrane supports were obtained using 3-aminopropyltriethoxysilane (APTES) linkers and toluene solvent, as shown in Scheme 1. Aliquots of 10 g NFC solution (solid content = 0.47 wt%) were diluted in a 50 mL beaker with 20 mL of deionized water and sonicated for 15 to 20 min to disperse evenly. The dilution was dewatered by vacuum filtration to form a uniform and transparent film, which was compacted by a heavy object on a smooth glass and dried in an oven at 40 °C for 12 h. After weighed accurately, the absolute dry NFC membrane (with constant weight of 0.27 g) was soaked in a glass ampule with 5 mL 5

toluene solution, then the required amount of APTES (3.5 times mass of dry NFC membrane, i.e.1 mL) was added, which resulted in the maximum amino content (See supporting information). The resultant solution was sealed and ultrasonicated for 15 to 20 min, sequentially reacted under a nitrogen atmosphere at 80 °C for 3 h in a constant temperature water bath. After the reaction, the film was taken out and washed three times with anhydrous ethanol, and then drained in fuming cupboard to remove excess toluene remaining on the film to obtain a modified NFC membrane.

Scheme 1. Surface functionalization and modification of the NFC membrane using APTES in toluene

2.3. Fabrication of COF/NFC composite membrane 16 mg of p-phenylenediamine was milled into powder and added to a 20 mL glass ampule (o.d. × i.d. = 22 × 20 mm and length 18 cm). Then soaked in 1 mL of mesitylene for 15 to 20 minutes to make it evenly disperse, and 21mg of 1,3,5-triformylphloroglucinol was dissolved in 1 mL of 1,4-dioxane and sonicated for 10 min to ensure complete dissolution. Then the solution of TFP was transferred into the glass ampule and sonicated. Then, the APTES-modified NFC membrane was added to the mixture, and 0.6 mL of a 3 mol/L acetic acid solution was added dropwise as a catalyst. N2 was introduced into the glass ampule and the tube 6

immediately sealed, placed into a vacuum oven for reaction at 120

for 3 days. After

the reaction, the membrane was taken out and ultrasonically cleaned with acetone and 1,4-dioxane for 3 times, and then drained in a fume hood to obtain the COF/NFC composite film. Meanwhile, the NFC without modification was also used as the matrix, but the obtained membrane was failed to load COF because of the strong polarity of NFC.

Scheme 2. Schiff base reaction of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa) for the construction of the COF/NFC membrane. The upper and the lower structures are in the keto and enol forms respectively.

2.4. Preparation of Pd@COF/NFC composite membrane Dissolve 0.05 g of palladium acetate in 7 mL of methanol, then the COF/NFC composite membrane was added and permeated for 12 h under the condition of magnetic stirring. After the completion of the infiltration, 5 mL of 0.2 mol/L NaBH4 /methanol solution was added and reacted for 2 h. The resulting red-black film after 7

washing with methanol, the COF/NFC composite membrane doped with Pd metal, is named as Pd@COF/NFC composite membrane. 2.5. Instrumental Measurements Fourier transform infrared spectroscopy (FT-IR) spectra were recorded using Bruker Tensor 27 (Bruker Corporation, Billerica, MA) and scanned from 4000 to 400 cm−1. The samples were mixed with KBr powder at the weight ratio of 1:10. Thirty-two scans were taken for each sample with resolution of 2 cm−1. The morphology and structure observations were performed on a Zwiss field emission scanning electron microscope (Merlin, Zwiss, Germany) at an accelerating voltage of 10 kV. Energy dispersive X-ray spectroscope (EDS) was also used to obtain the information about elemental composition and distribution of the secondary metal (Pd) on the film surface. Powder X-ray diffraction (PXRD) data were obtained on a Bruker D8 Advance diffractometer using Cu Kα radiation at 40 kV and 40 mA with a scanning rate of 2 °/min and the range was from 2 to 60°. The XPS spectrum were recorded on Kratos Axis Ulra DLD spectrometer using monochrome Al Kα X-ray source with an energy of 1486.6 eV. The vacuum of the analysis room was 5×10−9 torr and the beam spot size were 700×300 µm. The BET surface area of COF/NFC composite membrane was tested with Quadra Sorb Station 2 (Quantachrome Instrument, Boynton Beach, FL) at 77±1 K using nitrogen as the adsorbate. The samples were pretreated in a vacuum oven (40 °C, −0.1

8

MPa) for at least 12 h to remove the residue moisture before determination. The N2-BET and porosity were recorded automatically by the instrument. 2.6. Catalytic Tests Pd@COF/NFC composite membrane was used to dechlorinate o-dichlorobenzene by catalytic reduction reaction. Add aqueous 15 mL of NaBH4 solution (0.36 mol/L) to the o-DCB solution (15 mL, 1 mmol/L) in a serum bottle. A few of Pd@COF/NFC composite membrane with the diameter of 1 cm were added under magnetic stirring of 250 rpm. Then, 2 mL solution was taken out every 15 min and filtered with a 200 nm filter. The concentration of o-DCB and its degradation products in obtained solution at different reaction time was determined by headspace gas chromatography method previously reported in the literature [35] on the HS-GC measurement system which was assembled by a TriPlus 300 automated headspace sampler (Thermo Fisher Scientific, Inc., Waltham, MA) and a series 7890A GC system (Agilent Technologies, Inc., San Diego, CA) equipped with a flame ionization detector (FID) and a DB-5 capillary column (J&W Scientific, Inc.) operating at a column temperature of 105 with a nitrogen carrier gas flow of 3.8 mL/min. For FID, the detector temperature was 250

and the hydrogen and air flow rates were 40 and 450 mL/min, respectively.

The headspace operating system conditions were as follows: vial pressurization time 15 s, sample loop fill time, 12 s, loop equilibration time 3 s.

3. Results and discussion

9

Fig. 1. The preparation process of COF/NFC composite membrane

3.1. Synthesis and Characterization of COF/NFC composite The synthesis of COF/NFC composite membrane was schematically illustrated in Fig. 1. The composite membrane loaded with crystalline COF TpPa-1 were synthesized under solvothermal condition via the dehydration reaction of aldehyde and amine in the presence of acetic acid as a catalyst. The FT-IR spectra were recorded to confirm the successful grafting of APTES and growth of COF thin film on the NFC substrate. As can be seen from Curve b in Fig. 2, the C–H stretching vibration peak appears at 1370 cm−1 after modification by the silane coupling agent (APTES), indicating that APTES has been successfully grafted on the NFC membrane, which could be proved by the elemental analysis results in Table S1 of SI. However, N-H shear vibration peak does not appear at around 1540 cm−1 because a small amount of amino group is grafted. The N–H (3191, 3301, and 3375 cm−1) and C–H

10

(2893 cm−1) stretching vibration peaks are not shown in the TpPa-1 and COF/NFC composite membrane compared with the FT-IR curves of the reactants Pa and Tp in Fig.2. It indicates that the reactants in the product have been consumed thoroughly [36].

Fig. 2. Comparison of the FT-IR spectra of (a) NFC, (b) NFC modified using APTES, (c) COF/NFC composite membrane, (d) TpPa-1, (e) Tp, (f) Pa.

Interestingly, TpPa-1 powder presents two merging peaks: (1605 cm−1) and (1587 cm−1) that may be attributed to the stretching characteristic peak C=O and C=C respectively. Such a particular peak indicates that TpPa-1 exists in keto form, which is transformed from enol form [37]. Compared with NFC substrate membrane，the decreased value of the C=O stretching frequency in the COF/ NFC composite membrane (1602 cm−1) is due to the interaction between strong intramolecular hydrogen bonding of NFC and delocalized conjugation of the double bond [38].

11

Fig. 3. (a, c) SEM images showing surface of NFC and COF/NFC membrane. (b) SEM images of TpPa-1 powder. (d, e) Surface morphology of COF/NFC membrane after sonication exfoliation with various scales. (f) The cross-sectional view of the COF/NFC membrane.

The morphology was recorded by SEM to investigate the formation of the COF/NFC membrane. The surface of the NFC film is smooth and uniform, except some fine particles and partial bumps on it, as shown in Fig. 3a. It’s due to uneven fiber dispersion during the dilution process and inhomogeneous heating during the drying process. Fig. 3b shows that TpPa-1 is a flower-like morphological crystal composed of large numbers of elongated petals with flaky nanostructures, which is similar to previous reports in the literature [37]. As shown in Fig. 3c, the TpPa-1 membrane grown on the APTES modified NFC matrix is continuous and compact without distinct defect or flaw. In addition, the granular and mountain-like

12

morphology on the surface of the COF/NFC membrane was observed, which indicated that the growth of TpPa-1 thin film occurred simultaneously in multiple regions and would eclipse when the crystallization areas met (Fig. 3d, e). SEM images of the cross-sectional view of the composite membrane also revealed the staggered fold between layers with a thickness of ca. 250 nm.

Fig. 4. (a) Nitrogen adsorption and desorption curves of COF/NFC composite membrane. (b) The pore size distribution of COF/NFC composite membrane

The specific surface area of the material was determined by measuring the amount of multi-layer adsorption of the COF/NFC composite membrane under different partial pressures of N2, as shown in Fig. 4 above. The porosity of TpPa-1 film grown on the surface of NFC membrane was evaluated by the N2 adsorption isotherm at 77 K. In this way, COF/NFC composite membrane presented a reversible type IV adsorption isotherm (Fig. 4a). The surface area of the composite membrane calculated using the Brunauer-Emmett-Teller (BET) model and the Langmuir model is 357 m2/g and 482 m2/g, respectively. It indicated that TpPa-1 still maintained a high porosity after film formation, which could compete with all composite membranes 13

including crosslinked Thin Film Composite membranes (TFC), graphene-based membranes and ceramic membranes reported in the literature [39-41]. Moreover, it could be seen that the pore size distribution of the composite membrane, with the range of 2–20 nm, was relatively uniform according to Fig. 4b, which demonstrated the film had prominent crystallinity and regularity [42]. By the way, the pore size most widely distributed is 3.75 nm.

Fig. 5. XRD patterns of the COF/NFC composite membrane, APTES-modified NFC membrane, and incidental TpPa-1 powder.

The XRD results were shown in Fig. 5, which demonstrated that the COF powder, collected from the same reaction tube in which the COF/NFC composite membrane was simultaneously formed, is the same as that reported in the literature [37]. The XRD curve was consistent with the AA eclipse model structure, which exhibited intense diffraction peaks at 4.7, 8.3, 11.1 and 27°, corresponding to (100), (110), (210)

14

and (001) lattice faces, respectively, matched well with those of TpPa-1 powder as reported previously. The results revealed the diffraction peaks at 14.9 and 22.3° that could be assigned to the starting NFC membrane. To further investigate the properties of the COF/NFC composite membrane, we performed two-dimensional synchrotron radiation X-ray diffraction surface scanning analysis on it (Fig. 5). The results indicated that the COF/NFC membrane exhibited strong diffraction fringes at 4.7°, i.e., the (100) crystal plane, while the other three peaks did not appear when compared with that of TpPa-1 powder. Combining the XRD results with the crystal structure characteristics of the two-dimensional material, we speculated that the molecular layer of the COF material was parallel to the NFC substrate, so the XRD pattern could only detect the (100) crystal plane of the material. Gou et al [23] also drew the similar conclusion when investigating the crystallinity of DAB-TFP membrane using XRD and grazing incidence diffraction (GID). The above results suggested the COF/NFC composite membrane grew orientabablly when it was formed and the finished product had a high degree of orientation [43, 44].

15

Fig. 6. TGA data of TpPa-1 powder, COF/NFC membrane, and NFC membrane under N2 atmosphere

The

as-synthesized

COF/NFC

membrane

were

characterized

by

thermogravimetric analysis (TGA) to investigate the thermostability, as illustrated in Fig. 6. There was a mass loss of about 13% when the temperature rose to 160

,

which was due to the free moisture and bound water between intramolecular and intermolecular nanofibrillated cellulose, as well as the residual solvent (viz., mesitylene, 1,4-dioxane). The temperature of initial thermal decomposition and maximum decomposition rate of COF/NFC membrane are about 287 and 340 respectively, both higher than the NFC substrate (254 and 312

,

), which is due to the

decomposition of carboxyl group in the C6-position of nanofibrillated cellulose and the β-1,4- glycosidic bond, respectively. Such a phenomenon means the COF/NFC

16

composite has greater thermal stability than pure NFC substrate due to the covalent bonds between NH2-group of NFC and formyl group of Tp, as well as carboxyl group of NFC and NH2 of p-phenylenediamine, which leads to the successful growth of TpPa-1 in the NFC membrane. However, we found the maximum mass loss rate of COF/NFC composite membrane (at 340 ℃) was higher than that of NFC substrate, which indicated both decomposition of glycosidic bond of NFC and part breakage of C=O, N-H and C=C bond of COF-TpPa. Moreover, the residual ratio of COF/NFC membrane at 600 °C is 23%, greater than that of NFC substrate (residual ratio of 15%). In summary, the COF/NFC composite membrane basically maintains the good thermal stability of COF-TpPa at high temperature, which plays a positive role in expanding the application range of COF materials.

Fig. 7. (a) XPS spectra of COF/NFC membrane and Pd@COF/NFC membrane, (b) EDS analysis results of Pd@COF/NFC membrane (Top) and Pd@COF powder (bottom).

3.2. Synthesis and characterization of Pd@COF/NFC composite membrane After metal-chelating with Pd, the XPS spectrum of Pd@COF/NFC composite membrane demonstrated a new characteristic peak of Pd3d at 338 eV (Fig. 7a), which 17

indicated the successful load of Pd nanoparticles onto COF/NFC composite membrane. The atom and weight percentages of palladium are estimated to be 1.79% and 12.62%, respectively, which was approximate to the results obtained by EDS (Fig. 7b). The EDS spectrum of Pd@COF powder was also recorded in order to further confirm the successful load of Pd under the premise that the characteristic peaks of Pd appeared in the same position.

Fig. 8. (a) Violet: typical time-dependent evolution of GC peak area of o-DCB showing the catalytic reduction of o-DCB by Pd@COF/NFC, orange: plot of o-DCB concentration over time, olive: kinetics of the reduction reaction of o-DCB to benzene. (b) Top: the morphological change of the composite membrane after cyclic catalysis, bottom: the relative activity of Pd@COF/NFC catalyst during 6 cycles of reaction

3.3. Catalytic properties of Pd@COF/NFC composite membrane The as-synthesized Pd@COF/NFC composite membrane was used for the dechlorination of o-DCB. The kinetic reaction rate of catalytic reaction and recycling efficiency of catalyst were evaluated by pseudo-first order kinetics and cyclic catalytic reaction, respectively. The results were shown in Fig. 8. When the reaction proceeded for 30 min at room temperature, the removal rate of o-DCB has reached 60%, and 18

reached nearly 90% at 105 min. Herein, a linear correlation of ln[o-DCB] versus time at any instant was obtained (Fig. 8a, inset: olive). The Pd@COF/NFC exhibited high activity with a rate constant of 0.0235 min−1, which is lower than that of the pure Pd@COF (0.0306 min−1). However, seeing that it is difficult to recycle Pd@COF nanoparticles from the benzene aqueous solution and reuse it for the next reduction reaction, Pd@COF/NFC shows special advantages. Additionally, the catalytic experiments were carried out on o-DCB by recycling the Pd@COF/NFC composite membrane for six times, and the relative activity reached 90% in the sixth cycle. It indicates that the Pd@COF/NFC composite membrane has excellent stability and recyclability, and can be used for industrial catalytic reduction of o-DCB. The structure stability of the Pd@COF/NFC catalyst after catalytic cycles was also furthermore verified by optical imaging of the catalyst recovered after six catalytic cycles. 4. Conclusions

COF-TpPa

film

was

prepared

based

on

amine-group

functionalized

nanofibrillated cellulose, which was characterized by FT-IR successfully. The obtained COF-TpPa/NFC membrane had the high specific surface area (BET, 357 m2/g) and uniform pore size of 3.75 nm, which favored the loading of Pd nanoparticles. The Pd@COF/NFC composite membrane was used for dechlorination of o-dichlorobenzene with high activity (the apparent reaction rate was 0.0235 min−1). Therefore, we believe that our findings open a novel and facile access to COF film as 19

a recyclable catalyst carrier and will bring COFs film into practical applications. Acknowledgements

The

authors

acknowledge

the

financial

support

from the National Key and Development Program of China (Grant No. 2018YFC190 2102) and State Key Laboratory of Pulp and Paper Engineering (Grant No 201832), Science and Technology Planning Project of Guangdong Province (Project No.2017B090901064).

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Design and Fabrication of Covalent Organic Framework/Nanofibrillated Cellulose Composite Membrane Loaded with Pd for Dechlorination of Dichlorobenzene Highlights 1. COF-TpPa/NFC membrane was fabricated based on functionalized nanofibrillated cellulose. 2.

Pd@COF/NFC composite membrane was synthesized by post treatment with palladium acetate and NaBH4.

3. Pd@COF/NFC composite membrane showed excellent catalytic activity during the dechlorination of o-dichlorobenzene.

The authors declared that they had no conflicts of interest to this work.