Preparation and structural characterizations of pure Nanocellulose aerogels and Cu-BTC/Nanocellulose aerogel composites
Pure microcrystalline cellulose MCC was extracted from peanut shells using a combination of chemical treatments. The resulting microcrystalline cellulose fibers include amorphous and crystalline regions .
When MCC was treated with concentrated sulfuric acid, the acid diffused preferentially into the amorphous regions. The available glycosidic bonds were hydrolyzed, and as a result, individual crystallites transversely were released. The dispersion of the produced CNC in water was promoted by the charged sulfate esters groups onto the surface, which were generated by the reaction between hydroxyl groups of MCC and sulfuric acid . Although the resulting CNC could be turned into a gel by many physical methods, the next physical cross-linked aerogel would not have enough strength. Gelation of the CNC suspension was occurred by using MBA (N, N'-methylene bisacrylamide) as a linker without modifying CNCs. In this case, CNC particles play the role of Michael donors and MBA as the role of Michael acceptors. So, MBA attaches CNC particles forming a gel. This gel was aged to increase its mechanical strength by increasing the number of bonds formed between CNC and MBA particles. After freeze-drying, the obtained CNC aerogel could approach the high porosity and surface area of silica aerogels but is much less fragile.
On the other hand, the longer and more flexible nanofibrillated cellulose (NFC) was produced by the treatment of MCC suspension through a high-intensity ultrasonic method (HIUS purely mechanical process). Ultrasonic waves break the hydrogen bonds between MCC chains and lead to produce long neutral fibrillated cellulose chains with nanoscale widths and abundant hydroxyl groups on the surface of them. In this case, the resulting NFC has a gel-like state. Self-assembly without manual intervention spontaneously organized small building blocks into the nanostructure through interactions such as entanglements, intense Van der Waals forces, and hydrogen bonds. After solvent-exchange and freeze-drying, the NFC aerogel was obtained.
For fabrication of Cu-BTC/Nanocellulose aerogel composites, although a more uniform distribution of Cu-BTC crystals inside the composite can be achieved with the in situ technique when compared to the direct mixing method, Cu-BTC growth is not easily controlled. The concentration of precursors and reaction environments, such as temperature, are generally adjusted to control Cu-BTC growth inside the pores. Moreover, the polymer network has limited porosity for mass diffusion, and the sufficient exchange of metal ions for crystal growth takes a long time (> 2 days). Consequently, the direct mixing method may be a better choice , in which, Cu-BTC powder was added to aerogel precursors. According to the type of precursors, the resulting solution was changed into the gel with and without a linker. In this case, cellulosic gel clusters grew upon Cu-BTC crystals, and as a result, Cu-BTC crystals were entrapped on and inside the nanocellulose aerogel chunks. The resulting bonds between Cu-BTC crystals and nanocellulose aerogels are physical entanglement and hydrogen bonds, as well as van der Waals forces. It is important to note that all gel materials were aged before freeze-drying. During the aging period, the intermolecular bonds increase, and as a result, the mechanical strength of the subsequent aerogel material increases. A graphical representation of the fabrication of the composites and pure nanocellulose aerogels from peanut shells is described in scheme 1. The prepared materials were characterized and compared as follows. The resulting nanocellulose aerogels are white, while Cu-BTC/ Nanocellulose aerogel composites are sky blue, which is compatible with pure Cu-BTC color.
Figure 1a shows the powder X-ray diffraction (PXRD) of the prepared materials. As shown, the X-ray diffraction diagram of the MCC exhibited a strong peak at 2ϴ = 22.6º, and two overlapped weaker diffraction peaks at 2ϴ = 15.1º and 16.6º, besides a small diffraction peak at 34º, which are typical of cellulose I crystalline structure. The only difference between the spectrum of CNC aerogel and that of MCC is, the peak at 2ϴ = 22.6 splits into two weaker diffraction peaks at 2ϴ = 20.0 and 21.9, indicates that the crystal structure of native cellulose (cellulose І) partially converted to cellulose Ⅱ during hydrolysis of sulfuric acid. The peaks of NFC aerogel spectra became less intense, indicating the crystallinity of NFC aerogel has decreased, but the crystalline structure of native cellulose did not change. In Cu-BTC/ nanocellulose aerogel composites, in addition to CNC and NFC diffraction peaks, Cu-BTC diffraction peaks are present. As a result, Cu-BTC / CNC aerogel composite was prepared successfully by direct mixing without Cu-BTC crystallization disturbance.
Figure 1b illustrates the FTIR spectra of the prepared materials. In the MCC spectra, the specific peaks presented at 3354, 2893, 1639, 1434, 1168, 1050, and 893 cm− 1 are all the generic bands of cellulose molecules. The characteristic peaks of MCC were in good agreement with those reported elsewhere , indicating that microcrystalline cellulose has no impurity. No significant differences were observed in the spectrum NFC aerogel compared with that of MCC, indicating that the cellulose molecular structure did not change after HIUS treatment and freeze-drying.
Compared to MCC, the spectrum of CNC aerogel shows three new peaks at 810 cm− 1 attributed to symmetrical C-O-S vibration associated with the C-O-SO3 group, and at 1544 cm− 1 attributed to the bending vibration of N-H, as well as a characteristic peak at 1658 cm− 1, which could be attributed to the stretching vibration of C═O, overlapping with that adsorbing water. These results confirmed that the CNC particles chemically cross-linked with each other by the MBA linker.
The FTIR spectra of Cu-BTC shows the asymmetric stretching of the carboxylate group of H3BTC ligand appearing at 1631 cm− 1, and the symmetric stretching vibrations of it arise at 1573 cm− 1. Besides, several bonds located at 600–1300 cm− 1 attributed to the out-of-plane vibrations of BTC3− anions. The band at around 3687 cm− 1 and 2761 cm− 1 resulted from surface-adsorbed water. The results are consistent with those reported in the literature  . In the FTIR spectra of Cu-BTC/nanocellulose aerogel composites compared to that of pure nanocellulose aerogels, the characteristic peaks of Cu-BTC are appeared, indicating that Cu-BTC particles had successfully entrapped into the aerogel networks.
The morphology of the prepared materials was investigated using SEM microscopy. As shown in Fig. 2a, MCC contained large-sized fiber bundles composed of many microfibrils. These fiber clusters had an average length between 100–250 µm with an average width of 10 µm. Figures (2b and 2c) show the SEM images of Cu-BTC at various magnifications. These images show that Cu-BTC has an octagonal structure and a crystal size of 5µm. This structure is consistent with the Cu-BTC synthesized in the literature . As shown in Fig. 2d, the SEM image of CNC aerogels at a macro magnification (100) µm shows the formation of an interconnected porous sheet-like cellulose network. The appearance of the sheets is related to the physical constraint of the CNC particles between the growing ice crystals. In other words, the CNCs are assembled into sheets between the growing ice crystals, forming a hierarchical macro-porous aerogel with a pore size of (10–100) µm. The CNC aerogel shows smooth pore walls at a magnification greater than 5µm (Fig. 2e). By comparing the SEM images of the CNC aerogel (Fig. 2e), Cu-BTC (Fig. 2c), and Cu-BTC/CNC aerogel composite (Fig. 2f), a significant morphological change was observed, which might be due to the chemical cross-linking of aerogels by MBA. The MBA linkers with free electron pairs on nitrogen atoms change the morphology of Cu-BTC by coordinating open copper sites in Cu-BTC molecules and provided free space for CNC sheets to exfoliate. As seen in Fig. 2g, NFC aerogel shows a similar hierarchical porous structure to CNC aerogel structures. However, the macropores in NFC aerogels are larger than those in CNC aerogels. On the other hand, at a magnification greater (500 nm), as shown in Fig. 2h, the pore wall of NFC aerogel is not smooth but represents a lattice structure of nanofibers with macro-sized pores. That might because the adhesion of NFC fibers is higher compared to CNC crystals. With a large number of hydroxyl groups in NFC fibers, an intermolecular hydrogen-bond network forms. So NFC aerogel shrank, and consequently, the size of macropores increased. But, CNC particles, due to hemisulfate groups, have a negative charge. In Cu-BTC/NFC, the morphology of Cu-BTC did not change. As shown in Fig. 2i, several Cu-BTC particles appeared. Of course, because of the low loading of Cu-BTC 33 wt.%, Cu-BTC particles are buried inside the aerogel matrix and are not visible on its surface.
Although Cu-BTC has many applications, in this work, we have tested the performance of Cu-BTC/NFC aerogel composite in the field of water purification to ensure that Cu-BTC still retains their performance despite being trapped within the cellulose tissue. That is a promising field of application for this type of material because nanocellulose aerogels do not dissolve in water, and this aerogel can absorb more than 100 times its weight from water.
Among the many aqueous pollutants, we chose Congo Red dye to demonstrate the ability of Cu-BTC presented in the Cu-BTC/NFC aerogel composite to adsorption. The Congo Red adsorption performance of our composite was determined by UV-Vis spectrophotometer at the wavelength of 497nm.
After placing a small piece of Cu-BTC/NFC aerogel composite in the Congo Red solution, the color of the solution gradually faded to colorless, and its UV–Vis absorption maximum at 497 nm reduced significantly, as shown in Figures (3a and 3b). But the color of the aerogel piece changed from sky blue to red. That means Cu-BTC/NFC aerogel composite adsorbed Congo Red molecules.
To investigate the adsorption kinetics, the adsorption capacity at different times (qt mg/g) of this composite has been obtained for Congo Red solvents with the same initial concentration. The time-dependence curve of the UV-Vis adsorption at 497 nm is fitted to the pseudo-second-order kinetic model with kinetic parameters (adsorption rate k2 = 0.013 g/mg.h, adsorption capacity qe = 29.33 mg/g, and correlation coefficient R2 = 0.9493) (Fig. 3d and 3e).
Due to the large size of Congo Red molecules 21Å (more than the diameter of Cu-BTC pores 9Å), they cannot enter the Cu-BTC pores but, they were adsorbed on its surface.
To obtain the maximum Congo Red adsorption capacity by our composite, we investigate the adsorption isotherm at different initial concentrations at a constant temperature (25 ℃). As shown in Fig. 3c, the equilibrium adsorption data fitted well with a Langmuir model with a maximum adsorption capacity of up to 39 mg/g. We evaluated the affinity of this aerogel for Congo Red according to the equation of Kd = qe/ce, and we found its distribution coefficient that Kd = 3546 mL/g at equilibrium concentrations ce of 11 mg/L. The results show that Cu-BTC particles in the cellulose aerogel matrix retain their function.
CNC aerogel doesn’t show any adsorption capacity for CR, this might be due to the electrostatic repulsion between the negative charge of CNC aerogel in aqueous solution and the anionic nature of CR.
To investigate the removal of heavy metal ions, we specifically examined Cu-BTC/NFC aerogels composite and pure CNC aerogel for the elimination of potassium permanganate (KMnO4−). Although KMnO4− in low amounts is not toxic, and it is used excessively in water purification, however, new Canadian health research has shown that drinking water with large amounts of MnO4− can be a health hazard . It can also change the color and give an unpleasant taste of drinking water. It can also stain laundry. Therefore, it is necessary to remove permanganate from drinking water.
When a small piece of CNC aerogel dipped in a certain amount of aqueous solution containing potassium permanganate, the color of the solution immediately changed from purple to yellow and then to colorless with the appearance of a brown precipitate.
The color of the aerogel also turned brown. After separating the piece of aerogel and filtering the solution, ICP results show that, for example, in a solution of permanganate with an initial concentration of 50 ppm, the amount of Permanganate ions removed (MnO2) is 28 ppm, and there is still 22 ppm in the solution. Since the final solution is colorless, the manganese ions it contains are Mn2+. then to manganese ions (Mn2+). In contrast, methylol groups in nanocellulose aerogels were oxidized to aldehyde groups and then to carboxylates.
So, we believe that the nanocellulose aerogel substrate reduced permanganate ions through its methylol-reducing groups. In other words, the permanganate ions were reduced to manganese dioxide (brown precipitate) and
Therefore, we conclude from this experiment that CNC aerogels did not act as an adsorbent in this case, but as standing monolith solid reductant, which could perform a reducing function without creating any byproducts in solution. Therefore, it removes potassium permanganate by converting it to manganese dioxide precipitate.
Cu-BTC/NFC aerogel composite showed similar results but is slower. That because pure CNC aerogel was prepared under acidic conditions, so it has hemi-ester sulfate groups, which act as catalysts for this ox/red reaction.