Nanoporous furfuryl-imine-chitosan fibers as a new pathway towards eco-materials for CO2 adsorption

European Polymer Journal 120 (2019) 109214

Contents lists available at ScienceDirect

European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Nanoporous furfuryl-imine-chitosan fibers as a new pathway towards ecomaterials for CO2 adsorption

T

Luminita Marina, Brindusa Dragoib, Niculae Olarua, Elena Perjua, Adina Coroabaa, Florica Dorofteia, Guido Scaviac, Silvia Destric, Stefania Zappiac, William Porzioc ”Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania Faculty of Chemistry, “Alexandru Ioan Cuza” University, Iasi, Romania c Instituto per lo Studio delle Macromolecole del C.N.R., Milano, Italy a

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Chitosan Fibers Furfural Imine CO2 adsorption

The goal of the paper was to design a new pathway towards eco-materials suitable for CO2 capture, by functionalization of nanoporous chitosan fibers with monoaldehydes via imination. To this end, nanoporous chitosan fibers were prepared by electrospinning and further reacted with furfural. Their morphology was investigated by SEM and AFM microscopies and revealed the formation of a network of nanoporous submicrometric fibers. The successful imination was proved by FTIR, XPS, and NMR spectroscopies. As a proof of concept, the CO2 adsorption isotherms displayed enhanced CO2 adsorption of the furfuryl-imine-chitosan fibers compared to pristine ones, indicating the imination of porous chitosan fibers as a feasible method to new eco-materials for CO2 adsorption.

1. Introduction Nowadays, the CO2 capture and sequestration represents one of the major challenges in the fight against the global warming. Therefore, efforts to address this issue are hugely increasing daily. To this end, a wide range of materials displaying different surface chemistry and porosity were proposed to solve this global problem with long-term impact on the environment. The modified carbons [1], graphene [2,3], lignin-based adsorbents [4], porous aromatic frameworks [5], zeolites [6], mesoporous silica [7], covalent organic frameworks [8], metal organic frameworks [9], and porous organic polymers [10] are the most used as promising materials for CO2 capture so far [11]. A great challenge for the improvement of the CO2 adsorption capacity is the chemical modification of the above materials, in particular polymers, with N-containing conjugated moieties [11–14]. To meet the environmental change requirements, the eco-materials are highly desirable, whose design and fabrication take into consideration their entire life cycle while maintaining a high performance. The European Commission stated that “nearly 80% of the product’s environmental impact can be improved through eco-design” [15]. Such an eco-design is expected to preserve the non-renewable resources, to prevent the pollution, and to be friendly for leaving beings. To fulfill these requirements, the eco-design of materials, e.g., carbonization of chitin, for CO2 capture and sequestration was pursued [16]. The idea of

taking chemical compounds from the nature and use them as precursors for the eco-designed materials aims not only to satisfy the relevant issues of green chemistry, but also the availability of the starting products, e.g., clays [17] or polysaccharides [18–22]. In this regard, the chitosan, a linear polysaccharide composed of randomly distributed β(1-4) linked -N-acetyl-2 amino-2-deoxy-D-glucose and 2-amino-2deoxy-D-glucose units, is a prototypal material due to its high availability and cost-effectiveness [23]. It is a biopolymer derived from the abundant renewable resources, such as crustaceans, insects, mushrooms, and seaweeds, which displays superior bioactive properties and it is safe for the consumers. In addition, it contains nitrogen heteroatoms, which endow it with adsorption properties, making chitosan one of the natural polymeric feedstock for CO2 capture. Moreover, due to its particular chemical composition, the chitosan is a versatile material, which allows further chemical modifications [24–26]. Our preliminary work in the field of chitosan modification revealed that the reversible imination is a flexible chemical pathway allowing to prepare valuable products [26–34]. In the light of our results alongside with the increasing need for the reduction of CO2 into the environment, we envisaged the design and preparation of chitosan-based eco-materials for CO2 capture and sequestration. To this aim, chitosan was prepared as electrospun fibers by a three-step procedure, as follows. First, the chitosan was mixed with polyethyleneglycol (PEG) to improve the chitosan ability to be electrospun. Secondly, the high water soluble

E-mail address: [email protected] (L. Marin). https://doi.org/10.1016/j.eurpolymj.2019.109214 Received 17 July 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 Available online 31 August 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

zirconium rotor: 80 µl volume, 4 mm in diameter and 18 mm high. The chemical shifts were recorded relative to glycine standard, previously acquired (C]O signal: 176.03 ppm, relative to tetramethylsilane reference). X-ray photoelectron spectra were collected on a Kratos Analytical Axis NOVA instrument using monochromatic Al Kα X-rays source (hν = 1486 eV), 20 mA current and 15 kV voltage (300 W), and base pressure of 10−8 to 10−9 Torr in the sample chamber. The incident monochromated X-ray beam was focused on a 0.7 × 0.3 mm2 area of the surface. The XPS survey spectra were collected in the range of 5–1200 eV with a resolution of 1 eV and a pass energy of 160 eV. The high resolution spectra for all the elements identified from the survey spectra were collected using a pass energy of 20 eV and a step size of 0.1 eV. The binding energy value was calibrated by the C1s peak (285 eV). The recorded spectra were always fitted using Gauss-Lorentz curves to determine more accurately the binding energy of the different element core levels. The curve deconvolution of the obtained XPS spectra was analyzed using the ESCApe software. Scanning electron microscopy (SEM) was performed with a field emission Scanning Electron Microscope SEM EDAX – Quanta 200, operated at an accelerating voltage of 20 keV. AFM measurements were performed on a commercial AFM NTMDT. The fibers were dispersed into ethanol and then drop-casted on the glass lamella. The images were taken in a tapping mode with typical resonance frequency of 240 kHz. Dry fibers have been supported on glass with a double tape. Gas adsorption measurements (CO2) were performed on a Micromeritics ASAP 2020 physisorption instrument at 295 K until an absolute pressure of about 457 mmHg (0.610 bar). The samples were degassed at 110 °C for 3 h before each analysis.

PEG was simply removed by washing. Thirdly, the obtained chitosan fibers were modified by imination with furfural, the oldest renewable reagent originating from a variety of agricultural byproducts [35]. As result, porous chitosan fibers displaying functional groups available for CO2 bonding (e.g., amine, amide, and imine) with increased area-tovolume ratio were obtained. This rational synthetic strategy provided a new material, which displays both interesting fiber morphology and good adsorption capacity for CO2. The enhanced adsorption capacity was explained in terms of nano-porosity and affinity of the CO2 for the Lewis basic sites, in particular the amine groups already present in chitosan and the imine bonds formed by their reaction with furfural. 2. Experimental section 2.1. Materials Chitosan (217.74 kDa, DA = 85%), furfural, polyethylene glycol (Mv = 1,000,000), acetic acid, and ethanol were purchased from Sigma-Aldrich. Ethanol was dried on molecular sieves prior usage. 2.2. Preparation of furfuryl-imine-chitosan fibers The furfuryl-imine-chitosan fibers were obtained by two steps, as follows. First, chitosan and poly(ethylene glycol) in a 7/3 weight ratio were dissolved in acetic acid 75% to obtain a viscous solution of 4.5% (g mL−1). The solution has been gently stirred overnight to assure a good homogenization and then it has been electrospun. The electrospinning process was conducted in air at room temperature using a voltage of 20 KV, a tip-to-collector distance of 12 cm, and a flow rate of the solution through the syringe of 0.75 mL h−1. In such conditions, fibers with a submicrometric diameter in the range 100–700 nm were obtained with a yield of ~50%. Further, the chitosan-PEG fibers were washed three times with 5% NaOH solution to remove the residual acetic acid and after that washed five times with distilled water to remove PEG. Further, the nanoporous fibers were allowed to react with furfural when the condensation of amine groups of chitosan with aldehyde groups of furfural took place in a heterogeneous system. To obtain fibers with different content of imine units, the imination has been performed in different conditions, as further described. (i) The lyophilized chitosan fibers were immersed in the liquid furfural at room temperature for 48 h. (ii) The lyophilized chitosan fibers were immersed in the furfural at 55 °C for 48 h. (iii) The wet chitosan fibers were immersed in the furfural at 55 °C for 48 h. At the end of each step, the solid was recovered by decantation and subjected to lyophilization. Finally, the unreacted furfural was removed by washing with dry ethanol. The obtained degrees of imination, as evaluated by CPMAS-NMR, were 11, 13, and 48%, respectively. The corresponding samples were denoted as CF1, CF2, and CF3, respectively.

3. Results and discussions 3.1. Preparation and characterization of the furfuryl-imine-chitosan fibers Furfuryl-imine-chitosan fibers (CF) were obtained by condensation reaction of the chitosan fibers (solid) with furfural (liquid) in a heterogeneous system. The chitosan fibers were obtained in two steps. In the first step, a mixture of chitosan – polyethylene glycol was subjected to electrospinning, and fibers with a submicrometric diameter in the range of 100–700 nm were obtained with a yield of around 50%. This sample was labeled as C-PEG. In the second one, the porous chitosan fibers were obtained by removing the acetic acid residue and PEG, after washing with NaOH solution and distilled water, respectively. This sample was labeled as C. The resulted fibers were further subjected to chemical modification with furfural to obtain the furfuryl-imine-chitosan, as illustrated in Scheme 1. As described in the experimental section, fibers with different degree of imination (noted CF1, CF2, and CF3) were obtained by varying the imination conditions, in particular, the reaction temperature and the state of the fibers, i.e., lyophilized vs wet. The successful imination of the chitosan fibers with furfural was firstly assessed by FTIR spectroscopy. Thus, the fibers based on the chitosan-PEG (C-PEG), chitosan (C), and furfuryl-imine-chitosan (CF1CF3), and pristine PEG were analyzed. The corresponding spectra are displayed in Fig. 1 and Fig. 1s in Supporting Information. As a first observation, it can be noticed that the spectra of C-PEG and C fibers are almost similar, only slight differences being displayed. They mainly consist of modifications of the intensity of the bands at 2918 and 2880 cm−1, which are characteristic to the vibrations of the CeH bonds, and of the bands around 1067 and 1027 cm−1, and 3200–3700 cm−1, typical for the vibration bands of the ether, hydroxyl, amine, as well as intra- and inter-molecular H-bonds [27,29,31]. As these kinds of bonds were present in both chitosan and PEG, the slight modifications of their vibration intensities were attributed to the removal of the PEG followed by the obtaining of the chitosan fibers.

2.3. Characterization of samples The fibers were lyophilized using a Labconco FreeZone Freeze Dry System equipment, for 24 h at −54 °C and 1.512 mbar, after the prior freezing in liquid nitrogen. ATR-FTIR spectra of the fibers were recorded on a FT-IR Bruker Vertex 70 Spectrofotometer. The spectra were processed using OPUS 6.5 software. The 13C cross-polarization magic-angle-spinning (CPMAS) spectra were recorded with an FT-NMR AvanceTM 500 (Bruker BioSpin S.r.l) with a superconducting ultrashield magnet of 11.7 Tesla operating at 125.76 MHz 13C frequency. The following conditions were applied for the cross-polarization (cp.av) experiments: repetition time (relaxation delay) = 2 s, 1H 90 pulse length = 4.2 µs, contact time = 1 ms, and spin rate at magic angle (MAS) = 12 kHz. The compounds were placed in a 2

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

Scheme 1. Schematic representation of the preparation of furfuryl-imine-chitosan fibers, highlighting the imination reaction.

imination [27,28,31,36]. This is interesting, because, reminding that the imination occurred in a heterogeneous medium created by the reaction of the liquid furfural with chitosan solid fibers, the freedom degree of the chitosan chains is expected to be physically restricted. The successful condensation of the furfural with chitosan fibers was further proved by XPS spectroscopy performed for CF2, C-PEG, and C reference samples (Fig. 2, Fig. 2s in Supporting Information). On the basis of the wide scan spectra, the percentage of carbon in the samples was calculated (Table 1, Fig. 2s in Supporting Information). Accordingly, the chitosan fibers (C) contain a lower C/O ratio (1.97) compared to the chitosan-PEG fibers (C-PEG, C/O = 2.37), confirming that PEG was effectively removed by washing with water (Table 1). The carbon contribution increased in the CF2 furfuryl-imine-chitosan compared to the C chitosan fibers, in agreement with the successful imination with furfural, which have a higher C to O ratio. The high resolution C1s, N1s, and O1s spectra provided the incontestable evidence for the imination process, based on the values of the binding energies. The first observation is related to the appearance of new binding energies in the C1s and N1s spectra of the CF2 sample, at 287.2 and 402.1 eV, ascribed to C]N and N]C bonds, respectively, in the newly formed imine groups (Fig. 2 and Table 1s in Supporting Information). Thus, while the high resolution C1s spectra of the C-PEG and C samples were resolved in four components attributed to CeC/CeH, CeO/CeN, CeNH2, and eC]O bonds, that of the CF2 was resolved in five components attributed to CeC/CeH, CeO/CeN, CeNH2, eC]O, and C]N bonds (Fig. 2 and Table 1s in Supporting Information). The decreased contribution of CeC and CeO simultaneously with the increasing of the CeNH2 percentage in the C sample compared to C-PEG one proved the removal of PEG from fibers. On the other hand, the higher percentage of the CeC in CF2 is in good agreement with the imination with furfural. Similarly, the N1s high resolution spectra offered complementary evidence of the occurrence of imination on the chitosan fibers. Accordingly, for the C-PEG and C samples, the corresponding spectra were resolved in two constituents assigned to CeNH2 and CeHNeC bonds, whereas that of the CF2 sample was resolved in three components originated from CeNH2, CeHNeC, and N]C bonds, respectively. Interesting enough, the high resolution O1s spectra for all three samples were resolved in only two components, arising from O]C and OeC bonds, although their contributions were different from a sample to another as a result of the physico-chemical processes to which they were subjected. For instance, the higher percentage of CeO bonds compared to O]C ones is in line with their preponderance in the

1.00

0.98

Transmittance

0.96

1649

0.94

0.92

2880 2918

0.90

1567

C-PEG C CF2

0.88

1643

0.86 4000

1067 1027 3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 1. Comparative FTIR spectra of the CF2 furfuryl-imine-chitosan and C and C-PEG reference fibers.

Important changes were observed in the FTIR spectrum of chitosan fibers reacted with furfural (Fig. 1 and Fig. 1s in Supporting Information). In the fingerprint domain, i.e., from 1500 to 1700 cm−1, it can be seen that the broad weak band at 1649 cm−1, characteristic to the vibration of the amide group in chitosan, was replaced by a sharp intense band shifted to 1643 cm−1. This band was previously attributed to the imine linkage in the imine-chitosan derivatives [27–34]. Therefore, the detection of this band in the furfuryl-imine-chitosan fibers investigated herein is a clear evidence of the imine bonds formation via reaction of eNH2 of the chitosan with eCHO groups of the furfural. Moreover, the band at 1567 cm−1 characteristic to the NeH bending of the free amine groups significantly diminished its intensity, in agreement with the partial conversion of amine groups into imine bonds (Fig. 1, Fig. 1s in Supporting Information). The remnant shoulder of this band proves that an amount of free NH2 groups was preserved in the CF1-CF3 fibers. Thus, the samples display both imine and amine groups, as active groups in CO2 adsorption, as designed in our research. Changes were also observed in the 3000–3700 cm−1 spectral domain, consisting of the shifting to higher wavenumbers, phenomenon usually associated with a modification of the H-bonds network caused by 3

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

300

295

290

285

C 1s - CF2

N 1s - CF2

O 1s - CF2

Background

Background

Background

C-C C-O/C-N C=N C-NH2 O-C=O Envelope

2HN-C N=C C-HN-C Envelope

O=C O-C Envelope

280

275

405

Binding Energy

402

399

C 1s - C-PEG Background C-C C-O/C-N C-NH2 O-C=O Envelope

300

295

290

285

295

290

280

285

393

540

536

532

528

275

405

402

399

524

Binding Energy N 1s - C-PEG Background 2HN-C HN-C Envelope

Binding Energy

300

396

Binding Energy

396

393

O 1s - C-PEG Background O-C O=C Envelope

540

Binding Energy

536

532

528

524

Binding Energy

C 1s - C

N 1s - C

O 1s - C

Background

Background

Background

C-C C-O/C-N C-NH2 O-C=O Envelope

2HN-C HN-C Envelope

O-C O=C Envelope

280

275

405

Binding Energy

402

399

Binding Energy

396

393

540

536

532

528

524

Binding Energy

Fig. 2. XPS deconvoluted high resolution spectra of C 1s (first column), N 1s (second column), and O 1s (third column) for CF2, C-PEG and C samples.

for the 10 nm depth, this result suggests a re-orientation of chitosan chains with the acetyl units to the surface after imination and thus, a highly nucleophilic active surface is generated by this rational modification of the chitosan. This also indicates a supramolecular organization of the furfuryl-imine-chitosan chains, which was driven by a preferential segregation of the furfuryl units. Such a hypothesis is supported by the FTIR data, which also indicated the spatial re-arrangements of the imine-chitosan chains. The 13C cross-polarization magic angle spinning (CP-MAS) NMR spectroscopy performed for the furfuryl-imine-chitosan samples confirmed the imination process. In addition, it allowed assessing the degree of conversion of the amino groups into imine units [26,37,38]. The 13 C solid state NMR spectra (Fig. 3s in Supporting Information) showed the characteristic chemical shifting of the imine carbon at 155 ppm, overlapped with the chemical shifting of the carbon atoms in furfuryl, i.e., those adjacent to the imine linkage (150 ppm) and oxygen (145 ppm), respectively. The other two carbon atoms in furfuryl gave overlapped bands at 114 and 118 ppm [39] while the carbon atoms from chitosan showed the characteristic bands between 20 and

Table 1 Elemental composition calculated from the wide scan XPS spectra. Sample code

CF2 C-PEG C

Atomic concentration (%)

Mass concentration (%)

C

N

O

C

N

O

65.91 68.71 61.30

3.39 2.32 7.66

30.70 28.97 31.04

59.51 62.46 54.93

3.57 2.46 8.01

36.92 35.08 37.06

sample, which is normally given by the higher number of CeO bonds in the glucose-based units of this versatile biopolymer. However, when comparing these two contributions from a sample to another, the ratio between O]C and OeC is slightly changed. Hence, it can be noted that the contribution of O]C after washing (C sample) is higher than in CPEG sample emphasizing the idea of PEG removal by water alongside with the availability of the O]C groups that were hidden by PEG. The number of accessible O]C groups is further increased in the CF2. Recalling that XPS is a surface analysis technique that gives information 4

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

a) C-PEG

b) C-PEG

c) C

d) C

e) CF2

f) CF2

Fig. 3. Representative SEM images of the studied fibers at the different modification steps (a, b) chitosan with PEG; (c, d) after PEG removal; and (e, f, g, h) after furfuryl functionalization.

of conversion of the amino into imine groups was obtained when the wet chitosan fibers (i.e., swollen fibers) were immersed in furfural at 55 °C for 48 h. For the lyophilized chitosan fibers, the maximum conversion degree was much lower (~12%) regardless the reaction temperature, i.e., room temperature or 55 °C.

105 ppm [26]. The carbon atom in the amide groups of chitosan displayed the chemical shifting at 174 ppm. Considering that the acetylation degree of chitosan is 15%, an estimation of the conversion of amine groups into imine units has been done from the integral ratio of the imine to amide bands (Iimine/Iamide × 15, where I represents the integral value). The conversion degree was 11, 13, and 48% for CF1, CF2, and CF3, respectively. These values show that the highest degree 5

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

g) CF3

h) CF3 Fig. 3. (continued)

Fig. 4. AFM morphology (height images) for C-PEG (a, d); C (b, e); CF2 (c, f).

entanglement towards a more compact material has been noticed. A deeper view of the furfuryl-imine-chitosan fibers was done by AFM on the dry samples supported on a double tape. Typical images are illustrated in Fig. 4. AFM morphology was investigated on chitosan materials at the different modification steps, i.e., chitosan with PEG (CPEG), after PEG removal (C), and after furfuryl functionalization (CF2). According to the images acquired on larger scanned areas, the morphology consists of fibers, single or aggregated, for all three cases (Fig. 4) with large size dispersity (Table 2s and Fig. 4s in Supporting Information). The C-PEG and C fibers (Fig. 4(a and b), respectively) appeared more entangled and aggregated compared to the CF2 fibers, which were isolated and displayed a random distribution (Fig. 4c). The images taken at high resolution on a fiber (Fig. 4d–f) revealed a flatter

3.2. Fibers morphology The morphology and size of the electrospun chitosan-based fibers were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM images of the C-PEG sample display long fibers of various sizes with an average diameter around 500 nm. They are entangled together, most probably due to the presence of residual acetic acid, which favored their sticking. Representative images taken at two magnifications, i.e., 5000× and 20000×, are shown in Fig. 3. Regarding the chitosan fibers obtained after PEG removal (C sample), no significant differences were observed. Apparently, neither for the furfuryl-imine-chitosan fibers with a lower degree of imination. However, for those with a higher imination degree (CF3), a strong 6

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

Fig. 5. AFM morphology (height images) of CF3 (a) 3 µm and (b) 1 µm.

-1

CO2 adsorbed (mmol.g )

3.3. CO2 adsorption

CF2 CF3 C

1.0 0.8

The potential of furfuryl-imine modified chitosan fibers for CO2 capture was further evaluated for the CO2 adsorption capacity. The CO2 adsorption isotherms measured at 0.610 bar and 295 K for furfurylimine modified chitosan fibers (CF2) and unmodified chitosan fibers (C) are displayed in Fig. 6. The amount of CO2 adsorbed at 0.610 bars and 295 K on the furfuryl-imine-chitosan fibers with 13 wt% imine groups (CF2) and chitosan fibers (C) was 0.978 and 0.438 mmol of CO2 per gram of solid, respectively. These data clearly indicated that the modification of the chitosan fibers with furfuryl-imine units did not led to an interesting fiber morphology only, but also significantly changed the adsorption properties of the resulted material, providing new valuable sites for an enhanced CO2 uptake. As discussed above, the spectroscopy data (NMR, FT-IR, and XPS) showed that the furfuryl-imine-chitosan fibers contain two different organic groups on the surface (i.e., amine and imine). Moreover, the results of XPS indicated the reorientation of the functional groups toward the fibers surface. It can be therefore asserted that the CO2 adsorption performance obtained for the furfuryl-imine-chitosan fibers can be mainly rationalized in terms of interactions between the CO2 and Lewis basic sites introduced by imine functionalization i.e., chemical adsorption, as discussed below. It is worth mentioning that there is a controversy in literature regarding the adsorption of CO2 on the amine/imine type bonds. It should be pointed out that due to its Lewis-acidic character and quadrupolar momentum, a low interest for the investigation of CO2 adsorption on nitrogen-doped materials was noticed so far [40]. On the other hand, a huge interest was paid for the investigation of CO2 retention on primary amines that provided interesting and acknowledged results [24,41–44]. However, recently, other groups brought to the attention of the scientific community the advantages offered by the C]N groups for enhancing the ability of the polymer based adsorbents to retain small molecules, such as H2, CO2, CH4, and C2H2, on their surface [45]. The results obtained by our investigation are in line with those reporting the improved CO2 uptake over an imine containing polymer. In the case of chemically adsorbed CO2 on amine, the generally accepted mechanism

0.6 0.4 0.2 0.0 0

100

200

300

400

500

Pressure (mmHg) Fig. 6. CO2 adsorption isotherms obtained at 295 K.

surface for C-PEG and C fibers (Fig. 4(d and e) respectively) with a more compact granular morphology, while for the CF2 (Fig. 4f) the surface was rougher and defect-rich, generating an intra-fiber nanoporous structure (see arrows in Fig. 4f). For the CF3 sample with a much higher degree of imination (48%), the visualization of a single chitosan fiber was difficult, although holes were visible on the surface (Fig. 5). In this case, the sample displayed a continuous surface, instead of a network of singular fibers. This was probably induced by the high density of the imine units on the fibers surface, which favored their aggregation into a more compact material. In the light of these results, it can be stated that a high degree of imination is possible due to the high concentration of available NH2 groups, but the resulted material does not seem to have practical applications as a result of the compacting, which hinders the accessibility to the nucleophilic Lewis bases, key sites during the adsorption phenomenon.

Table 2 Adsorption of CO2 on chitosan polymeric materials. Sample 1 2 3 4 5

CO2 adsorption/experimental conditions

Triphenyl amine porous chitosan derivative Imine-chitosan obtained from the reaction of glutaraldehyde functionality with 2-amino-2methyl-1-propanol Activated carbon-chitosan Polyethylenimine-functionalizedporous chitosan beads Nitrogen-doped activated carbons (prepared from chitosan)

7

−1

solid at 5 bars 0.85 mmol CO2 mmol 0.114 mmol (5.02 mg) CO2 g−1 at 328 K, atmospheric pressure (1.013 bars) 13.65 mmol g−1 at 298 K and 40 bars 2.3 mmol g−1 at 313 K and 15 kPa (0.15 bars) 1.6 mmol g−, at 298 K and 15 kPa (0.15 bars)

References [51] [52] [53] [46] [54]

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

involves the formation of carbamic acid/carbamate (RR′NH–CO2− + NH2-RR′ or) following a 1:1/1:2 reaction mechanism [46,47]. According to recently published neutron-scattering results, the interactions by which the CO2 is retained by the imine group in organic polymers are R = N(δ−)-C(δ+)O2 [48,49]. In the light of our results, as well as those already published and cited by this work, the sequestration of CO2 on imine modified polymers offers a promising new alternative to the amine-based organic polymer materials, as CO2 is expected to interact strongly with high polar imines [40,49,50]. To note that a higher amount of imine groups on the material, i.e., 48 wt% (CF3), manifested similar adsorption properties as pure chitosan fibers. Hence, 0.383 mmol g−1 was calculated for the CF3 sample while 0.438 mmol g−1 was obtained for the unmodified chitosan. The results discussed above for the CF2 sample (13 wt% imine groups) show that the imine groups are available to interact with the electron-deficient CO2 molecules and thus, to increase the adsorption capacity of the biofibers, which is in well agreement with the already mentioned results on imine containing organic polymers [40,49,50]. Yet, a high concentration of imine (e.g., 48 wt%) was unfavorable, extremely likely because it led to an almost compact structure hindering the accessibility to the amine/imine groups, and thus fewer active sites were available for adsorption. Table 2 summarizes the values of CO2 adsorption obtained on chitosan polymeric materials by other research groups. These literature reports were divided into two main categories, as follows. The first one gathered the data related to the CO2 adsorption on imine-modified chitosan (lines 1–2 in Table 2) while the second one was centered on other chemically modified chitosan (lines 3–5 in Table 2). To the best of our knowledge, there were only two reports on CO2 adsorption on imine containing chitosan [52,53]. However, it can be noticed that in both cases, the materials exhibit lower adsorption capacity in comparison with that manifested by the imine chitosan fibers reported in the present work. It is, however, interesting that in both cases, the authors compared the adsorption ability of the imine modified chitosan with unmodified chitosan, and acknowledged the key role played by C]N bonds in the enhanced CO2 uptake. Hence, these results reinforced the idea of high affinity of the imine bonds towards CO2 molecules. On the other hand, the CO2 adsorption on other chemically modified chitosan displayed similar or better adsorption properties, but the issues related to the materials and their cost of fabrication in the context of the use of renewable and cost-effective materials obtained in green conditions should be considered. Hence, the results obtained in this work for the furfuryl-imine modified chitosan are very encouraging and open up the possibility of using such materials at larger scale.

with pure chitosan. Hence, 0.978 mmol CO2 per gram of solid were adsorbed at 0.610 bars and 295 K for furfuryl-imine-chitosan fibers with 13 wt% imine, compared to 0.438 mmol per gram of solid measured for pristine chitosan fibers in similar conditions. The XPS and adsorption results indicated that CO2 was mainly adsorbed via chemical interactions with the organic groups (i.e., amine and imine) on the active surface of the fibers. These outcomes open up a new pathway towards sustainable materials with eco-design in view of their application as adsorbents for CO2 removal. Acknowledgements This work was supported by Romanian National Authority for Scientific Research MEN – UEFISCDI (grant number PN-III-P1-1.2PCCDI2017-0569, contract no. 10PCCDI/2018). The joint project between Italian CNR and the Romanian Academy is thanked for partial support. Thanks are due to Dr. Filippo Bossola of ISTM-CNR Milan for gasabsorption analyses and fruitful discussions. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.109214. References [1] E.S. Sanz-Pérez, C.R. Murdock, S.A. Didas, C.W. Jones, Direct capture of CO2 from ambient air, Chem. Rev. 116 (2016) 11840–11876, https://doi.org/10.1021/acs. chemrev.6b00173. [2] R. Rea, S. Ligi, C. Meganne, V. Morandi, M. Giacinti Baschetti, M.G. De Angelis, Permeability and selectivity of PPO/graphene composites as mixed matrix membranes for CO2 capture and gas separation, Polymers 10 (2018) 1–19, https://doi. org/10.3390/polym10020129. [3] A.B. Soliman, R.R. Haikal, Y.S. Hassana, M.H. Alkordi, The potential of a graphenesupported porous-organic polymer (POP) for CO2 electrocatalytic reduction, Chem. Commun. 52 (2016) 12032–12035, https://doi.org/10.1039/C6CC06773E. [4] N. Supanchaiyamat, K. Jetsrisuparb, J.T.N. Knijnenburg, D.C.W. Tsang, A.J. Hunt, Lignin materials for adsorption: current trend, perspectives and opportunities, Bioresource Technol. 272 (2019) 570–581, https://doi.org/10.1016/j.biortech. 2018.09.139. [5] U. Díaz, A. Corma, Ordered covalent organic frameworks, COFs and PAFs. From preparation to application, Coordin. Chem. Rev. 311 (2016) 85–124, https://doi. org/10.1016/j.ccr.2015.12.010. [6] R.V. Siriwardane, M. Shen, E.P. Fisher, Adsorption of CO2 on zeolites at moderate temperatures, Energ. Fuel. 19 (2005) 1153–1159, https://doi.org/10.1021/ ef040059h. [7] C. Gunathilake, M. Jaroniec, Mesoporous calcium oxide–silica and magnesium oxide–silica composites for CO2 capture at ambient and elevated temperatures, J. Mat. Chem. A 4 (2016) 10914–10924, https://doi.org/10.1039/C6TA03916B. [8] Y. Zeng, R. Zou, Y. Zhao, Covalent organic frameworks for CO2 capture, Adv. Mater. 28 (2016) 2855–2873, https://doi.org/10.1002/adma.201505004. [9] M.H. Yu, P. Zhang, R. Feng, Z.Q. Yao, Y.C. Yu, T.L. Hu, X.H. Bu, Construction of a multi-cage-based MOF with a unique network for efficient CO2 capture, ACS Appl. Mater. Inter. 9 (2017) 26177–26183, https://doi.org/10.1021/acsami.7b06491. [10] W. Wang, M. Zhouab, D. Yuan, Carbon dioxide capture in amorphous porous organic polymers, J. Mat. Chem. A 5 (2017) 1334–1347, https://doi.org/10.1039/ C6TA09234A. [11] G. Li, Q. Liu, B. Xia, J. Huang, S. Li, Y. Guan, H. Zhou, B. Liao, Z. Zhou, B. Liu, Synthesis of stable metal-containing porous organic polymers for gas storage, Eur. Polym. J. 91 (2017) 242–247, https://doi.org/10.1016/j.eurpolymj.2017.03.014. [12] Z.J. Li, S.Y. Ding, H.D. Xue, W. Cao, W. Wang, Synthesis of –C=N– linked covalent organic frameworks via the direct condensation of acetals and amines, Chem. Commun. 52 (2016) 7217–7220, https://doi.org/10.1039/C6CC00947F. [13] P. Puthiaraj, S.S. Kim, W.S. Ahn, Covalent triazine polymers using a cyanuric chloride precursor via Friedel-Crafts reaction for CO2 adsorption/separation, Chem. Eng. J. 283 (2016) 184–192, https://doi.org/10.1016/j.cej.2015.07.069. [14] J. Yu, L.H. Xie, J.R. Li, Y. Ma, J.M. Seminario, P.B. Balbuena, CO2 capture and separations using MOFs: computational and experimental studies, Chem. Rev. 117 (2017) 9674–9754, https://doi.org/10.1021/acs.chemrev.6b00626. [15] EU Science Hub. Sustainable Product Policy. https://ec.europa.eu/jrc/en/researchtopic/sustainable-product-policy. [16] R.S. Dassanayake, C. Gunathilake, N. Abidi, M. Jaroniec, Activated carbon derived from chitin aerogels: preparation and CO2 adsorption, Cellulose 25 (2018) 1911–1920, https://doi.org/10.1007/s10570-018-1660-3. [17] C. Yen-Hua, L. De-Long, CO2 capture by kaolinite and its adsorption mechanism, Appl. Clay Sci. 104 (2015) 221–228, https://doi.org/10.1016/j.clay.2014.11.036.

4. Conclusions In summary, new furfuryl-imine-chitosan fibers have been prepared with the aim to open-up a new pathway to develop innovative and effective eco-materials for the adsorption of CO2. The fibers having various degree of imination (i.e., 11, 13, 48 wt%) were successfully prepared by electrospinning method while furfural was used for chemical modification of the chitosan fibers, under different reaction conditions. The successful imination has been demonstrated by FTIR and XPS spectroscopies, which revealed the appearance of the characteristic vibrations and binding energy of imine group, respectively. Moreover, the NMR spectrum confirmed the imine formation by the characteristic chemical shifting of the corresponding carbon and allowed the evaluation of the imination degree. SEM images showed the formation of a network of submicrometric fibers of various sizes with an average diameter around 500 nm while AFM revealed their rough surface, i.e. their nanoporous characteristic. XPS results showed that the rational modification of chitosan can change the reorientation of the organic groups of polymeric fibers towards the surface providing valuable sites for adsorption phenomena. Indeed, the CO2 adsorption on furfuryl-imine-chitosan fibers generated interesting data in comparison 8

European Polymer Journal 120 (2019) 109214

L. Marin, et al.

saccharides, Greem Chem. 9 (2007) 342–350, https://doi.org/10.1039/B611568C. [36] L. Marin, A. Bejan, D. Ailincai, D. Belei, Poly(azomethine-phenothiazine)s with efficient emission in solid state, Eur. Polym. J. 95 (2017) 127–137, https://doi.org/ 10.1016/j.eurpolymj.2017.08.006. [37] C. King, R.S. Stein, J.L. Shamshina, R.D. Roge, Measuring the purity of chitin with a clean, quantitative solid-state, NMR method, ACS Sustain. Chem. Eng. 5 (2017) 8011–8016, https://doi.org/10.1021/acssuschemeng.7b01589. [38] L. Heux, J. Brugnerotto, J. Desbrieres, M.F. Versali, M. Rinaudo, Solid state NMR for determination of degree of acetylation of chitin and chitosan, Biomacromolecules 1 (2000) 746–751, https://doi.org/10.1021/bm000070y. [39] P. Bhaumik, P.L. Dhepe, Exceptionally high yields of furfural from assorted raw biomass over solid acids, RSC Adv. 4 (2014) 26215–26221, https://doi.org/10. 1039/C4RA04119D. [40] A. Alabadi, H.A. Abbood, Q. Li, N. Jing, B. Tan, Imine linked polymer based nitrogen-doped porous activated carbon for efficient and selective CO2 capture, Sci. Rep. 6 (2016) 38614, https://doi.org/10.1038/srep38614. [41] P.V. Kortunov, M. Siskin, L.S. Baugh, D.C. Calabro, In situ nuclear magnetic resonance mechanistic studies of carbon dioxide reactions with liquid amines in aqueous systems: new insights on carbon capture reaction pathways, Energy Fuels 299 (2015) 5919–5939. [42] A.F. Eftaiha, A.K. Qaroush, K.I. Assaf, F. Alsoubani, T.M. Pehl, C. Troll, M.I. ElBarghouthia, Bis-tris propane in DMSO as a wet scrubbing agent: carbamic acid as a sequestered CO2 species, New J. Chem. 41 (2017) 11941–11947. [43] J.C. Hicks, J.H. Drese, D.J. Fauth, M.L. Gray, G. Qi, C.W. Jones, Designing adsorbents for CO2 capture from flue gas-hyperbranched aminosilicas capable of capturing CO2 reversibly, J. Am. Chem. Soc. 130 (2008) 2902–2903, https://doi. org/10.1021/ja077795v. [44] A. Sayari, Y. Belmabkhout, Stabilization of amine-containing CO2 adsorbents: dramatic effect of water vapor, J. Am. Chem. Soc. 132 (2010) 6312–6314, https:// doi.org/10.1021/ja1013773. [45] Y. Zhu, H. Long, W. Zhang, Imine-linked porous polymer frameworks with high small gas (H2, CO2, CH4, C2H2) uptake and CO2/N2 selectivity, Chem. Mater. 25 (2013) 1630–1635, https://doi.org/10.1021/cm400019f. [46] J. Fujiki, K. Yogo, Carbon dioxide adsorption onto polyethylenimine-functionalized porous chitosan beads, Energ. Fuel 28 (2014) 6467–6474, https://doi.org/10.1021/ ef500975g. [47] N. Popp, T. Homburg, N. Stock, J. Senker, Porous imine-based networks with protonated imine linkages for carbon dioxide separation from mixtures with nitrogen and methane, J. Mat. Chem. A 3 (2015) 18492–18504, https://doi.org/10. 1039/C5TA02504D. [48] V.S. Pavan, K. Neti, X. Wu, S. Deng, L. Echegoyen, Selective CO2 capture in an imine linked porphyrin porous polymer, Polym. Chem. 4 (2013) 4566–5456, https://doi. org/10.1039/C3PY00798G. [49] V.S. Pavan, K. Neti, X. Wu, P. Peng, S. Deng, L. Echegoyen, Synthesis of a benzothiazole nanoporous polymer for selective CO2 adsorption, RSC Adv. 4 (2014) 669–9672, https://doi.org/10.1039/C3RA47587E. [50] G. Li, B. Zhang, J. Yan, Z. Wang, Directing effect of linking unit on building microporous architecture in tetraphenyladmantane-based poly(Schiff-base) networks, Chem. Commun. 50 (2014) 1897–1899, https://doi.org/10.1039/C3CC48593E. [51] S. Kumar, J.A. Silva, M.Y. Wani, C.M.F. Dias, A.J.F.N. Sobral, Studies of carbondioxide capture on porous chitosan derivative, J. Dispers. Sci. Technol. 37 (2016) 155–158, https://doi.org/10.1080/01932691.2015.1035388. [52] A. Valechha, J. Thote, N. Labhsetwar, S. Rayalu, Biopolymer based adsorbents for the post combustion CO2 capture, Int. J. Knowl. Eng. 3 (2012) 103–106 http:// www.bioinfo.in/contents.php?id=40. [53] M. Keramati, A.A. Ghoreyshi, Improving CO2 adsorption onto activated carbon through functionalization by chitosan and triethylenetetramine, Physica E 57 (2014) 161–168, https://doi.org/10.1016/j.physe.2013.10.024. [54] J. Fujiki, K. Yogo, The increased CO2 adsorption performance of chitosan-derived activated carbons with nitrogen-doping, Chem. Commun. 52 (2016) 186–189, https://doi.org/10.1039/C5CC06934C.

[18] A.K. Qaroush, H.S. Alshamaly, S.S. Alazzeh, R.H. Abeskhron, K.I. Assaf, A.F. Eftaiha, Inedible saccharides: a platform for CO2 capturing, Chem. Sci. 9 (2018) 1088–1100, https://doi.org/10.1039/C7SC04706A. [19] H. Xie, S. Zhang, S. Li, Chitin and chitosan dissolved in ionic liquids as reversible sorbents of CO2, Greem Chem. 8 (2006) 630–633, https://doi.org/10.1039/ B517297G. [20] A.F. Eftaiha, F. Alsoubani, K.I. Assaf, C. Troll, B. Rieger, A.H. Khaled, A.K. Qaroush, An investigation of carbon dioxide capture by chitin acetate/DMSO binary system, Carbohyd. Polym. 152 (2016) 163–169, https://doi.org/10.1016/j.carbpol.2016. 06.092. [21] T.S. Trung, W.W. Thein-Han, N.T. Qui, C.H. Ng, W.F. Stevens, Functional characteristics of shrimp chitosan and its membranes as affected by the degree of deacetylation, Bioresource Technol. 97 (2006) 659–663, https://doi.org/10.1016/j. biortech.2005.03.023. [22] A. Zubareva, B. Shagdarova, V. Varlamov, E. Kashirina, E. Svirshchevskaya, Penetration and toxicity of chitosan and its derivatives, Eur. Polym. J. 93 (2017) 743–749, https://doi.org/10.1016/j.eurpolymj.2017.04.021. [23] S.M. Rafigh, A. Heydarinasab, Mesoporous chitosan–SiO2 nanoparticles: synthesis, characterization, and CO2 adsorption capacity, ACS Sustain. Chem. Eng. 5 (2017) 10379–10386, https://doi.org/10.1021/acssuschemeng.7b02388. [24] A.K. Qaroush, K.I. Assaf, S.K. Bardaweel, A. Al-Khateeb, F. Alsoubani, E. Al-Ramahi, M. Masri, T. Brück, C. Troll, B. Rieger, A.F. Eftaiha, Chemisorption of CO2 by chitosan oligosaccharide/DMSO: organic carbamato–carbonato bond formation, Greem Chem. 19 (2017) 4305–4314, https://doi.org/10.1039/C7GC01830D. [25] H. Mittal, S.S. Ray, B.S. Kaith, J.K. Bhatia, Sukriti, J. Sharma, S.M. Alhassan, Recent progress in the structural modification of chitosan for applications in diversified biomedical fields, Eur. Polym. J. 109 (2018) 402–434, https://doi.org/10.1016/j. eurpolymj.2018.10.013. [26] L. Marin, B.C. Simionescu, M. Barboiu, Imino-chitosan biodynamers, Chem. Commun. 48 (2012) 8778–8780, https://doi.org/10.1039/C2CC34337A. [27] L. Marin, S. Morariu, M.C. Popescu, A. Nicolescu, C. Zgardan, B.C. Simionescu, M. Barboiu, Out-of-water constitutional self-organization of chitosan–cinnamaldehyde dynagels, Chem. A Eur. J. 20 (2014) 4814–4821, https://doi. org/10.1002/chem.201304714. [28] L. Marin, D. Ailincai, M. Mares, E. Paslaru, M. Cristea, V. Nica, B.C. Simionescu, Imino-chitosan biopolymeric films. Obtaining, self-assembling, surface and antimicrobial properties, Carbohydr. Polym. 117 (2015) 762–770, https://doi.org/10. 1016/j.carbpol.2014.10.050. [29] D. Ailincai, L. Marin, S. Morariu, M. Mares, A.C. Bostanaru, M. Pinteala, B.C. Simionescu, M. Barboiu, Dual crosslinked iminoboronate-chitosan hydrogels with strong antifungal activity against Candida planktonic yeasts and biofilms, Carbohyd. Polym. 152 (2016) 306–316, https://doi.org/10.1016/j.carbpol.2016. 07.007. [30] M. Iftime, S. Morariu, L. Marin, Salicyl-imine-chitosan hydrogels: supramolecular architecturing as a crosslinking method toward multifunctional hydrogels, Carbohyd. Polym. 165 (2017) 39–50, https://doi.org/10.1016/j.carbpol.2017.02. 027. [31] L. Marin, D. Ailincai, S. Morariu, L. Tartau-Mititelu, Development of biocompatible glycodynameric hydrogels joining two natural motifs by dynamic constitutional chemistry, Carbohyd. Polym. 170 (2017) 60–71, https://doi.org/10.1016/j. carbpol.2017.04.055. [32] M.M. Iftime, L. Marin, Chiral betulin-imino-chitosan hydrogels by dynamic covalent sonochemistry, Ultrason. Sonochem. 45 (2018) 238–247, https://doi.org/10.1016/ j.ultsonch.2018.03.022. [33] A. Bejan, D. Ailincai, B.C. Simionescu, L. Marin, Chitosan hydrogelation with a phenothiazine based aldehyde – toward highly luminescent biomaterials, Polym. Chem. 18 (2018) 2359–2369, https://doi.org/10.1039/C7PY01678F. [34] A.M. Olaru, L. Marin, S. Morariu, G. Pricope, M. Pinteala, L. Tartau-Mititelu, Biocompatible chitosan based hydrogels for potential application in local tumour therapy, Carbohyd. Polym. 179 (2018) 59–70, https://doi.org/10.1016/j.carbpol. 2017.09.066. [35] J.N. Chheda, Y. Román-Leshkova, J.A. Dumesic, Production of 5-hydroxymethylfurfural and furfural by dehydration of biomass-derived mono- and poly-

9