3.1 Material Characterization
The properties of the regenerated bead-shaped products prepared from cellulose direct dissolution (RC/D), cellulose carbamate solution (RC/C) and viscose solution (RC/V), are shown in Table 1. It could be observed that in all the bead types, the average diameter and the total porosity resulted similarly. The coefficients of variation (CV %) were below 10 % indicating a homogeneous diameter distribution in all cases.
Table 1
Characterization of the synthetized RC material.
Support
|
Dissolution method
|
Diameter (mm)
|
CV (%)
|
Total porosity (%)
|
RC/D
|
Direct
|
2.73 ± 0.03
|
5.57
|
93.80
|
RC/C
|
Carbamate
|
2.77 ± 0.02
|
3.87
|
92.70
|
RC/V
|
Viscose
|
2.57 ± 0.04
|
5.82
|
94.30
|
During the incorporation of copper nanoparticles into the supports, several reaction steps were observed. The different paths are shown in the Supplementary Data.
Table 2 shows the copper content in the prepared catalysts measured by TGA, and the copper addition yield referred to the initial concentration of the precursor solution (5 wt.%). The efficiency of Cu incorporation on RC/C and RC/V resulted higher when the reaction temperature was 25°C, when the temperature was 80°C, the yield decreased 60 % and 35 % respectively. However, the temperature did not greatly affect the incorporation of copper on RC/D beads.
The thermal degradation of the pure supports and catalysts were or was tested by TGA and DTG analysis (Fig. 1). The TGA thermograms showed the complete weight loss of the supports at 600°C, and no char residue was observed. However, in the presence of copper a residual weight was observed in all cases. This residual weight associated with copper oxide formed during the organic support combustion revealed the Cu content in the cellulosic matrix (Table 2). During the heating treatment in flowing air, three steps could be observed: (1) 50–100°C corresponding to the evaporation of adsorbed water, (2) 200–400°C because of the main degradation of cellulosic support and (3) 400–600°C final combustion of organic material (J. Wu, Zhao, Zhang, & Xu, 2012).
In all cases, the initial temperature of the second step decreased after the copper addition in the cellulosic matrix. On the other hand, the highest weight loss temperature decreased after the incorporation of the metal. The shifts of the degradation temperatures compared with that of bare beads depend on the support nature and on the Cu content. The higher Cu content, the lower temperature shifts (Table 2). As a result, the inclusion of the metallic NPs reduced the catalyst thermostability of the catalysts. That effect is lower on the samples prepared with the RC/D support.
Table 2
Thermogravimetric results.
Sample
|
Cu
(wt.%)1
|
Yield
(%)2
|
Tmax DTG
(°C)
|
ΔT
(°C)3
|
RC/D
|
-
|
-
|
334
|
-
|
Cu-25/D
|
0.80
|
16.0
|
331
|
3
|
Cu-80/D
|
0.92
|
18.4
|
332
|
2
|
RC/C
|
-
|
|
342
|
-
|
Cu-25/C
|
1.09
|
21.8
|
332
|
10
|
Cu-80/C
|
0.44
|
0.44
|
334
|
8
|
RC/V
|
-
|
-
|
329
|
-
|
Cu-25/V
|
1.40
|
1.40
|
319
|
10
|
Cu-80/V
|
0.69
|
0.96
|
324
|
5
|
(1) Catalyst copper content determined by TGA, (2) Copper addition yield related to the initial precursor concentration. (3) Difference between the observed maximum DTG temperature in the pure support and the corresponding Tmax observed in the catalyst.
The chemical state of the copper species supported on the surface of the RC beads was analysed by XPS. After scanning a wide spectra region, the Na 1s signal was slightly detected in the beads with lower Cu content (Cu-25/D and Cu-80/C), whereas in the samples with higher Cu content, Na signal was not observed. This is in agreement with Li et al. who reported that the Na 1s signal corresponding to Na-counterions on the structural carboxylates, disappeared after the CuNPs addition (Li, Messele, Boluk, & El-Din, 2019). This phenomenon could contribute to copper stabilization on the cellulosic matrix through coordination with -COO terminals, giving rise to more stable chemical links.
The Cu 2p XPS region presents two main peaks at around 932 eV and 952 eV, which correspond to Cu 2p3/2 and Cu 2p1/2 respectively. The shakeup located at 10 eV, which is higher than the Cu 2p3/2 signal, suggests the presence of Cu2+. In order to differentiate Cu0 entities from Cu+ species, X-ray excited Auger electron spectroscopy was measured. The Auger parameter is defined as the sum of the Cu 2p3/2 binding energy (BE) and the kinetic energy (KE) of the Auger Cu LMM signal (Devard, Brussino, Marchesini, & Ulla, 2019; Wagner & Joshi, 1988). The integration results are shown in Table 3, while Figure S2 in the Supplementary data displays the spectra used for such integrations and calculations. It could be seen that all spectra presented two components in the Cu 2p3/2 region. The lowest BEs 932.4 to 933.7 eV (Fig. S2. A) are related to both Cu0 and Cu+ species (Dai, Sun, Deng, Wu, & Sun, 2001; Ling, Li, & Wang, 2012), while the signals observed at 934.8 to 935.7 eV are assigned to Cu2+ entities linked to the support (Baoquan Jia & Li Cheng, Jinping Zhou, 2012). In all samples, the modified Auger parameter (α´) resulted between 1848.3 and 1848.9 eV, confirming the presence of Cu+ (Biesinger, 2017; Biesinger, Lau, Gerson, & Smart, 2010; Dai et al., 2001).
As a result, it could be concluded that there were no significant differences between the atomic ratio of Cu2+/ (Cu++Cu0) in the RC/D and RC/C samples. However, the Cu-80/V sample displayed the biggest quantity of the reduced component, showing the lowest atomic ratio. This observation suggests that some chemical groups which are present in the viscose support surface promote Cu0 formation.
Table 3
Sample
|
BE Cu 2p 3/2 (eV)
|
KE Cu LMM (eV)
|
α´* (eV)
|
Cu2+/ (Cu++Cu0)
|
Cu-80/D
|
933.2
935.5
|
915.1
|
1848.3
|
2.9
|
Cu-25/C
|
933.3
935.5
|
914.9
|
1848.2
|
2.4
|
Cu-80/C
|
933.7
935.7
|
915.2
|
1848.9
|
2.8
|
Cu-80/V
|
932.4
934.8
|
915.9
|
1848.3
|
1.6
|
*Auger parameter. |
Figure 2 shows FTIR normalized spectra of the regenerated cellulose (RC) supports and the corresponding catalysts. The spectra of all samples showed the characteristic bands of the cellulosic matrix. The bands at c.a. 3448 cm− 1 and c.a 2910 cm− 1 are assigned to O-H and C-H stretching vibrations respectively. While the signal at c.a. 1640 cm− 1 corresponds to the O-H bending due to the strong water molecules absorption in the cellulose structure. The C-H bending vibration and C-O stretching mode appear at c.a. 1371 cm− 1 and 1070 cm − 1 respectively (Teow, Kam, & Mohammad, 2018).
FTIR spectra of the supports prepared via viscose (RC/V) and direct dissolution (RC/D) presented identical bands. However, the cellulose carbamate beads (RC/C) showed a band at ca. 1723 cm− 1 which corresponds to the carbonyl stretching in the bases of urethanes formed during the synthesis of cellulose derivatization (Fu et al., 2015; Yin et al., 2007).
The intensity of the band at c.a. 1640 cm−1 remained stable after copper addition in all cases, regardless of the synthesis temperature. Nevertheless, some differences on the signals could be remarked when comparing the catalyst spectra with their corresponding bare supports. These differences suggest that copper entities were successfully joined on the cellulose chains. In the spectra of Cu-RC/V and Cu-RC/D catalysts, the band at c.a. 1723 cm−1 assigned to carbonyl groups added to the cellulose structure due to the presence of ascorbic acid. In all cases, the synthesis temperature increment led to 1723 cm−1 lower intensity signal, suggesting the loss of C=O terminals. The appearance of a slight peak at 612 cm−1 could also confirm the presence of copper nanoparticles on the cellulose matrix (Prenesti & Berto, 2002). After copper addition, the C-OH band decreased, in agreement with Dong et al. who proposed that these groups could immobilize the copper nanoparticles through electrostatic interactions (Dong & Hinestroza, 2009).
The optical images of supports and catalysts are shown in Figs. 3A and 3B. Figure 3C presents the copper impregnated beads after the cryo-congelation performed previously on the SEM observations. It could be seen that the beads preserved their shape and surface morphology. As a result, the morphological data obtained from SEM observations could be considered for the no cryo-treated beads.
SEM micrographs of the three types of regenerated cellulose and catalysts are shown in Figs. 4 and 5. RC/C support (Fig. 4A) showed a non-homogeneous pore structure with different shapes and sizes in a smooth surface. The RC/D sample (Fig. 4B) presented a surface with wavy cavities and big pores. In contrast, in the RC/V support (Fig. 4C) a highly porous smoothly wavy surface could be observed.
The presence of copper is evidenced in the surface of all the prepared supports (Fig. 5). The distribution of the copper particles was greatly influenced by the RC morphology; however, the metallic phase addition did not modify the initial support structure. It could be also suggested that the greater the material porosity, the more possibility of metallic entities deposition on the surface of the pores. The metal particles on the RC/C support appeared to be supported as platelet arrangement (Fig. 4). This conformation is present in all supports but, in the RC/C samples, in which the surface seems to be softer and more closed the particles are notably bigger (Sone, Diallo, Fuku, Gurib-Fakim, & Maaza, 2020). This could be observed by comparison of Fig. 5A to D. On the other hand, it could be seen that the higher the synthesis temperature the more aggregation of copper particles. The EDS results confirmed the presence of copper in all the prepared catalysts.
3.2 Catalytic tests
In order to prove the activity and efficiency for ECs elimination of the synthesized Cu/RC catalysts, the catalytic degradation of phenol with H2O2 was used as a reaction test.
Several supports were studied for the proposed catalytic reaction with copper as the active site (Table S1). Devard et al. reported the catalytic activity of Al2O3 supported catalysts containing 5 wt. % of copper as active phase (Devard et al., 2019). This catalyst presented 100 % phenol conversion after 10 min under reaction and high mineralization (TOC = 85%) at 70°C. Meanwhile, Lozano et al. (Lozano, Devard, Ulla, & Zamaro, 2020) studied Cu (5–10 wt.%) supported on MOF materials (Cu/UiO-66) under comparable reaction conditions. These authors also reported good results of these materials for the phenol CWPO reaction. On the other hand, Devard’s preliminary experiments (not reported, Table S1) showed that the synthesis of Cu/SiO2 and Cu/Al2O3 catalysts starting from pre-formed copper nanoparticles with controlled particle size (Gioria et al., 2019) with lower Cu loadings (lower than 1.0 wt.%) converted 100 % of phenol with a more than 50 % of mineralization (Table S1). As a result, cellulose was proposed as an interesting biodegradable low-cost organic material for the same reaction.
Firstly, in order to analyse the cellulosic support (RC/D, RC/C and RC/V) activity for phenol degradation, blank experiments were performed using the supports without metal active phase. The experiments were carried out under the same experimental conditions of temperature, time, and peroxide concentrations. As a result, phenol conversion was not detected.
Then the cellulosic supported catalysts were tested for the phenol CWPO reaction. Figure 6 shows the catalytic results of phenol conversion during 2 h reaction time. It could be observed that the catalysts prepared at 80°C presented more than 90% of phenol conversion. Whereas the catalyst synthesized at 25°C on RC/D and RC/V supports presented lower conversions. In contrast, the conversion of phenol with cellulose carbamate-based catalysts (Cu-RC/C) did not present differences in the catalytic activity (Fig. 6C).
The mineralization process (TOC) measured after 120 min reaction time (Fig. 6) suggested that the phenol was not totally degraded to CO2, resulting in a colourless organic by-product mixture. It could be observed, that: (i) in Cu-RC/D and Cu-RC/V solids, the higher phenol conversion the higher the mineralization process and (ii) that the catalysts prepared at 80°C showed similar TOC conversions and high phenol conversion after 120 min.
Clearly, it is not possible to establish a direct relationship between the solid catalytic performances with their total copper content. However, the XPS results could help drawing some relevant hypotheses. In fact, RC/D and RC/C catalysts contain a higher proportion of surface Cu2+ (Table 3), while the Cu2+/ (Cu++Cu0) ratio calculated in RC/V was lower than the other catalysts, suggesting a higher proportion of copper remained as reduced species.
It is well known that the organic contaminant present in the reaction medium could reduce the Cu+ 2 site deposited on the catalyst surface to Cu+ cation (Santos, Yustos, Quintanilla, Ruiz, & García-Ochoa, 2005). Then the obtained reduced species are again oxidized by the generated oxygen from H2O2, and then, the CE oxidation cycle restarts.
Catalyst morphology (surface area and porosity) plays an important role in catalytic activity together with the active site size and its distribution into the support. The combination of these catalyst properties could give rise to different availability and accessibility of the active sites into the catalytic matrix. As a result, it is difficult to establish a direct relationship between the catalytic results and the physicochemical characteristics of the catalysts due to the observed disparate morphological characteristics of the RC supports. Even so, to the best of our knowledge, the use of copper-based catalysts on this eco-friendly organic matrix has not been reported, at least for CEs degradation in water.
For this reaction, some advantages of the Cu-cellulosic beads could be remarked In comparison to Cu-based catalysts prepared on large specific area commonly used supports: (i) in the low specific area cellulosic material lower content of copper conduces to similar catalytic yield, (ii) the synthesis of cellulosic supports is more economic and simpler than the synthesis of other more complex materials, (ii) the protocol for the catalytic site incorporation on the cellulose matrix resulted facile and suitable, (iii) facile recovery of the catalyst from the reaction vessel.