Preparation of macromolecular NHPI catalyst
The synthesis of water-soluble macromolecular NHPI catalyst mainly includes two steps (Fig. 1): Firstly, the intermediate product of PEG-TAC is generated through the esterification reaction between the terminal hydroxyl group of PEG and the acid chloride group on the TAC. Then, hydroxylamine hydrochloride is added to make the PEG-TAC undergo hydroxylation reaction to obtain the macromolecular catalyst of PEG-NHPI.
Figure 2 shows the FT-IR spectra of the main raw material, intermediate product and final product for the synthesis of PEG4000-NHPI. It can be seen that TAC has the carbonyl peak of acid chloride at 1762 cm− 1, and PEG4000 has a characteristic peak of fatty ether at 1111 cm− 1, a C-H peak at 2883 cm− 1 and a hydroxyl peak at 3476 cm− 1. The above-mentioned characteristic peaks exist in the mixture of PEG4000 and TAC (PEG4000 + TAC). The carbonyl peak of the acid chloride at 1762 cm− 1 disappears in the spectrum of PEG4000-TAC, indicating the formation of the intermediate product. Compared with PEG4000-TAC, PEG4000-NHPI has characteristic absorption peaks of amide at 1543 cm− 1 and 1793 cm− 1, indicating that PEG4000-NHPI is successfully synthesized.
1H-NMR analysis of PEG4000-TAC and PEG4000-NHPI is carried out in Fig. 3. In the 1H-NMR spectrum of PEG4000-TAC, the characteristic shift peak of PEG main chain H appears at 3.5 ppm, and the characteristic shift of benzene ring H appears at 7.7–8.5 ppm. The main difference between PEG4000-NHPI and PEG4000-TAC is the conversion of anhydride groups to hydroxylamine groups. In the spectrum of PEG-NHPI, the characteristic shift peak of N-OH does appear at 11 ppm, further confirming the successful synthesis of PEG-NHPI.
GPC curves of PEG4000, PEG4000-TAC and PEG4000-NHPI shown in Fig. 4 are used to evaluate their hydrodynamic volumes. It can be seen that PEG4000-TAC has two peaks at the retention time below that of PEG4000, which may correspond to PEG4000-TAC with one terminal TAC group and two TAC groups, respectively. It shows that after TAC is bonded to PEG, the hydrodynamic volume increases accordingly. However, when the anhydride group of PEG-TAC is hydroxylated to give PEG-NHPI, the retention time is slightly increased. It may be due to that the hydrogen bonds formed between the hydroxylamine groups and the PEG backbone decrease its hydrodynamic volume slightly.
UV-vis light spectroscopy is used to determine the NHPI loading amount of PEG-NHPI using free NHPI as the standard substance and acetonitrile as the reference solution. Table 1 lists the NHPI loading amounts of water-soluble macromolecular NHPI catalysts with different molecular weights. For PEG-NHPI and mPEG-NHPI, both show a trend of increasing NHPI loading amount as the molecular weight of the carrier decreasing. When both the molecular weight and other reaction conditions are the same, mPEG containing one terminal hydroxyl group has a lower loading capacity comparing with PEG having two terminal hydroxyl groups.
Table 1
The NHPI loading amounts of water-soluble macromolecular NHPI catalysts with different molecular weights
Catalysts
|
OH:TAC
|
NHPI loading amount
(mmol/g)
|
PEG600-NHPI
|
1:1
|
0.93
|
PEG1000-NHPI
|
1:1
|
0.83
|
PEG2000-NHPI
|
1:1
|
0.43
|
PEG4000-NHPI
|
1:1
|
0.33
|
PEG6000-NHPI
|
1:1
|
0.25
|
PEG8000-NHPI
|
1:1
|
0.21
|
mPEG1000-NHPI
|
1:1
|
0.44
|
mPEG2000-NHPI
|
1:1
|
0.39
|
mPEG5000-NHPI
|
1:1
|
0.08
|
PEG-NHPI mediated oxidation of cellulose
It can be found from Table 2 that the carboxyl content of NHPI oxidized cellulose obtained in the acetonitrile/water medium is 1.04 mmol/g, equivalent to 71% of TEMPO oxidized cellulose. Because NHPI is insoluble in water, the carboxyl content of NHPI oxidized cellulose obtained in water (0.99 mmol/g) is slightly lower than that in the acetonitrile/water system. Interestingly, the DPv of NHPI oxidized cellulose is much higher than that of TEMPO oxidized cellulose, indicating that the degradation of cellulose for NHPI is much less than that of TEMPO. To clarify the relationship between carboxyl content and DPv of NHPI oxidized cellulose, samples with different carboxyl contents are prepared by changing the reaction time and the oxidant dosage. As shown in Fig. 5, the carboxyl content of NHPI oxidized cellulose is in inverse proportion to DPv, similar to that of TEMPO oxidized cellulose. However, when the carboxyl content is the same, the DPv of NHPI oxidized cellulose is much higher than that of TEMPO oxidized cellulose, further confirming the superiority of the NHPI system in inhibiting cellulose degradation.
The macromolecluar catalyst of PEG-NHPI has good water solubility, so it shows a better catalytic performance in water than in acetonitrile/water, which proves the hypothesis that loading NHPI on a water-soluble carrier is beneficial to improve its catalytic performance in water. Besides, the DPv of PEG-NHPI oxidized cellulose is also at a high level.
Table 2
Catalytic performances of PEG-NHPI, NHPI and TEMPO for oxidation of cellulose
Catalyst
|
Reaction medium
|
Reaction time (h)
|
Carboxyl content of oxidized cellulose (mmol/g)
|
DPv of oxidized cellulose
|
TEMPO
|
water
|
4
|
1.46
|
55
|
NHPI
|
acetonitrile/water
|
10
|
1.04
|
120
|
water
|
10
|
0.99
|
114
|
PEG4000-NHPI
|
acetonitrile/water
|
10
|
0.80
|
125
|
water
|
10
|
0.91
|
120
|
Effects of the molecular weight of PEG-NHPI on its catalytic oxidation performances have been studied (Table 3). Large molecular weight of PEG-NHPI can increase the steric hindrance and limit its mobility, thereby reducing its catalytic performance. When the molecular weight of PEG is 600, the prepared macromolecular catalyst can achieve an oxidation performance equivalent to that of free NHPI. In general, the DPv of PEG-NHPI oxidized cellulose is slightly higher than that of NHPI oxidized cellulose. By comparing the catalytic performances of PEG2000-NHPI and mPEG2000-NHPI, it can be concluded that PEG2000-NHPI gives a higher cellulose oxidation degree, indicating that high NHPI loading is beneficial to the improvement of the catalytic oxidation performance.
Table 3
Effects of the molecular weight of the macromolecular NHPI catalyst on its catalytic oxidation performances
Catalysts
|
The molecular weight of carrier
|
Carboxyl content of oxidized cellulose (mmol/g)
|
DPv of oxidized cellulose
|
PEG600-NHPI
|
600
|
0.99
|
131
|
PEG1000-NHPI
|
1000
|
0.96
|
127
|
PEG2000-NHPI
|
2000
|
0.94
|
126
|
PEG4000-NHPI
|
4000
|
0.91
|
119
|
PEG6000-NHPI
|
6000
|
0.84
|
121
|
PEG8000-NHPI
|
8000
|
0.75
|
124
|
mPEG1000-NHPI
|
1000
|
0.85
|
126
|
mPEG2000-NHPI
|
2000
|
0.84
|
120
|
mPEG5000-NHPI
|
5000
|
0.58
|
135
|
Figure 6 compares the carboxyl contents and DPv values of oxidized cellulose samples obtained with PEG4000-NHPI, NHPI, TEMPO and polymer-immobilized TEMPO as catalysts. For NHPI oxidized cellulose, the carboxyl content is 0.99 mmol/g (equivalent to 68% of TEMPO oxidized cellulose), and the DPv is as high as 120 (equivalent to 26% degradation rate), which is much higher than that of TEMPO oxidation cellulose (equivalent to 66% degradation rate). The carboxyl content of PEG4000-NHPI oxidized cellulose is 0.94 mmol/g, which reaches 64% of free TEMPO level and 95% of free NHPI level,respectively. The DPv of PEG4000-NHPI oxidized cellulose is 1.1 times higher than that of TEMPO oxidized cellulose, and the degradation degree is only 18%. Compared with the reported macromolecular TEMPO catalysts of P(AA-co-TA) (Liu et al. 2018a) and P(AM-co-VAm)-T (Liu et al. 2018b), the oxidation performance of PEG4000-NHPI is higher than P(AA-co-TA) and can reach 84% of P(AM-co-VAm)-T level. The cellulose degradation rates of these two macromolecular TEMPO catalysts (41% and 21%, respectively) are higher than that of PEG4000-NHPI (18%). These results show that loading water-insoluble NHPI on a water-soluble macromolecular carrier not only has a good oxidation performance but also significantly inhibits the degradation of cellulose.
Recycling of PEG-NHPI
Considering the good water solubility of PEG-NHPI, it should remain in the supernatant after the oxidation is completed. The aqueous supernatant also contains a certain amount of inorganic salts such as NaCl. Inorganic salts within a certain concentration range gave little affect on the oxidation of cellulose (Liu et al. 2018a, b). Therefore, the concentrated supernatant can be directly used for the next oxidation. After 6 cycles, the supernatant can be extracted with dichloromethane to obtain recovered PEG-NHPI (Fig. 7).
It can be seen from Fig. 8 that the carboxyl contents of the oxidized cellulose samples obtained both in water and in acetonitrile/water, can still be maintained at high levels after recycling the supernatant 5 times. After 3 recycles, the DPv of oxidized cellulose decreases slightly (Fig. 9). This may be due to that, the accumulation of inorganic salts increases the concentration of reactive ions in the system, which may cause a certain degree of cellulose degradation.
It can be seen from Fig. 10 that the characteristic peaks of the recovered catalyst and the fresh catalyst are the same, indicating that the structure of the recovered catalyst is not changed and can be reused (cycle 7 in Fig. 8 (left)). It can be seen that the carboxyl content of oxidized cellulose obtained in cycle 7 is similar to that of cycle 1. This shows that the recovered macromolecular catalyst by extraction still has a good catalytic effect.
Oxidized cellulose samples obtained by different catalytic systems are characterized by XRD. After the original XRD data is normalized using the software of OriginPro 8, the XRD patterns are plotted as shown in Fig. 11. It can be found that the diffraction angle 2θ peak positions of oxidized cellulose samples and raw cellulose are the same and correspond to the type I crystal structure, indicating that the oxidation modification does not change the crystal type of the cellulose.
As shown in Table 4, comparing with raw cellulose (Iam=0.175, I200 = 1.000, CrI = 82.5%), the crystallinity of NHPI oxidized cellulose is decreased. This may be due to that a part of cellulose in the crystalline area is oxidized to soluble oxidized cellulose, so the crystallinity of centrifuged water-insoluble oxidized cellulose is decreased. The crystallinity of PEG4000-NHPI oxidized cellulose is higher than that of NHPI oxidized cellulose. This may be due to that PEG4000-NHPI with large size is difficult to enter the crystalline region of cellulose, and can only oxidize the amorphous region and the surface of the crystalline region. The reason for the improved crystallinity of oxidized cellulose after recycling may be that the accumulation of residual oxidant in the system increases the oxidation degree of cellulose, increasing the amount of water-soluble oxidized cellulose.
Table 4
The crystallinity of NHPI oxidized cellulose and PEG4000-NHPI oxidized cellulose
Samples
|
acetonitrile/water system
|
water system
|
Iam
|
I200
|
CrI
|
Iam
|
I200
|
CrI
|
NHPI oxidized cellulose
|
0.321
|
1.000
|
67.9%
|
0.323
|
1.000
|
67.7%
|
PEG4000-NHPI oxidized cellulose for cycle 1
|
0.250
|
1.000
|
75.0%
|
0.226
|
1.000
|
77.4%
|
PEG4000-NHPI oxidized cellulose for cycle 6
|
0.183
|
1.000
|
81.7%
|
0.188
|
1.000
|
81.2%
|