Characterization of the synthesized bio-based catalyst
The FT-IR spectra of the synthesized materials were recorded to validate their structures and correlate it to their functional groups (Fig. 2A). The pieces of evidence for the Hal in the spectrum are seen at 580 cm− 1, which is attributed to the Al-O-Si vibration.50 The inner -OH stretching of Hal is represented by two adsorption bands at 3695 and 3622 cm− 1, whereas a Si–O stretching can be observed at 1035 cm− 1. New bands were found at 1483, 1672 and 2937 cm− 1 that correspond to Si–C stretching, C = C stretching and C–H stretching respectively in Hal-vinyl. In the spectrum of k-Cr, distinguishable bands appear at 3440, 2916, 1635, 1450, 1373, 1250, 1064 and 918 cm− 1, signifying OH stretching, vibrations of aliphatic CHs; C = O; sulfates-methylene group; sulfate esters; a 3,6-anhydrogalactose; as well as galactose-4-sulfates respectively 51. To sum up all three spectra, those of Hal, k-Cr and PAA, is featured in the FT-IR spectra of the catalysts with specific characteristic bands at 3440 cm− 1 (-OH), 3372 cm− 1 (-OH), 2900 cm− 1 (-CH2), 1737 cm− 1 (-C = O), 1108 cm− 1 (SO3− stretching), 1039 (O = S = O stretching in SO3H).47 Comparison of the FT-IR spectrum of Hal/K-Cr/PAA and Hal/PAA do not show any alteration, which can be due to the overlapping of the characteristic peaks of k-Cr and PAA.
In order to confirm the formation of Hal/k-Cr/PAA, XRD analysis was also conducted. Figure 2B demonstrates that both pristine Hal and the catalyst presented similar peaks at 2θ = 11.5°, 20.2°, 26.7°, 35.4°, 55.6°, and 62.1°; these are the characteristic peaks for Hal as seen in JCPDS no. 29-1487 52. Notably, in the k-Cr XRD pattern there was a broad peak of 2θ about 20°, indicating the amorphous structure of this compound 53. According to these results it is concluded that surface modification of Hal did not alter Hal structure 54. Notably, the expected peak of k-Cr in the XRD pattern of the catalysts overlapped with those of Hal.
To verify functionalization of Hal with k-Cr/PAA, TG curve of Hal/k-Cr/PAA was compared to that of Hal in Fig. 2C. It became clear that Hal exhibits higher thermal stability than Hal/k-Cr/PAA. It can be seen three weight losses experienced by Hal/k-Cr/PAA at 170, 340, and 540°C from water loss, decomposition of polymeric component (i.e., k-Cr and PAA), and deoxygenation of Hal, respectively 47,55, whereas Hal underwent only two weight losses from water loss and deoxygenation. This result established that the content of polymer on Hal is about 35 wt%.
The relative acidity of the materials can be found by obtaining the value of H° from Table 1. To do this, 4-nitroaniline (with a concentration of 5 mg/L and pKa equal to 0.99) and 5 mmol/L of samples were mixed in an aqueous solution and the highest UV absorbance was detected at λmax = 382 nm. According to a Hammett plot (Fig. 2D), the order of acidities of the samples was determined to be Hal/k-Cr/PAA (1:1) > reused Hal/k-Cr/PAA (1:1) > Hal/k-Cr/PAA (2.5:1) > Hal/PAA > k-Cr > Hal, which was verified with different H° values in Table 1.
Table 1
Values of Hammett function (Ho)
Entry | Catalyst | Amaxα | [I]% | [IH]% | Ho |
1 | Blank | 1.969 | 100 | - | - |
2 | k-Cr | 1.177 | 56.816 | 43.184 | 1.129 |
3 | Hal | 1.418 | 69.955 | 30.045 | 1.377 |
4 | Hal/PAA | 0.924 | 43.022 | 56.978 | 0.888 |
5 | Hal/k -Cr/PAA (1:1) | 0.598 | 25.267 | 74.733 | 0.539 |
6 | Hal/k -Cr/PAA (2.5:1) | 0.715 | 31.597 | 68.403 | 0.675 |
7 | Reused-Hal/k -Cr/PAA (1:1) | 0.668 | 29.076 | 70.924 | 0.623 |
Indicator: 4-nitroaniline. α |
The morphology of Hal/k-Cr/PAA was explored by recording SEM images. Figure 3 displays that Hal tubes were able to be observed in both SEM pictures of Hal/k-Cr/PAA. Moreover, EDX and Map examinations confirmed formation of the nanocomposite by stating that only C, O, N, Al, Si, and C are present, which proves the purity of the fabricated nanocomposite and suitable distribution of polymeric in Hal surface (Fig. 3).
Catalytic conversion of fructose to HMF
In order to optimize the reaction, multiple variables, such as reaction time, temperature, catalyst amount, and sugar type must be adjusted. The effect of reaction time on the HMF yield was first studied for fructose (200 mg) dehydration in the presence of Hal/k-Cr/PAA (1:1) catalyst (20 mg) in DMSO (2 mL) at 100°C as a model reaction. The results shown in Fig. 4A demonstrated that a yield of 97.9% was attained within 35 min, but lingering the reaction led to a significant decline in the HMF yield. This happened due to the propensity for HMF to produce humins alongside water-soluble polymers and formation of by-products, such as 2,5-dimethyl furfural and furfural as detected by GC-MS when kept at these temperatures for a longer duration. The evidence suggested that a reaction time of 35 min had to be maintained for ideal results. The relationship between the reaction temperature and conversion of fructose to HMF was found to be an important factor. To examine the effect of the temperature on fructose (200 mg) dehydration, experiment was conducted in the presence of Hal/k-Cr/PAA (1:1) catalyst (20 mg), in DMSO (2 mL) over 35 min (Fig. 4B). It was observed that changing the reaction temperature from 100 to 80°C caused HMF yield to decline from 97.9 to 73.9%. Though, further increasing of this parameter to 120°C did not further increase HMF yield (remaining at 97.9%), continued raising of temperature to 140°C reduced the yield down back to 81.2%. This can be explained by more extensive polymerization of HMF leading to the formation of insoluble humins and soluble polymers at high temperatures 44. Therefore, the best temperature for fructose dehydration reaction is found to be 100°C. In order to optimize the catalyst amount for achieving the highest HMF yield, various amounts of Hal/k-Cr/PAA (1:1) catalyst were used between 6 and 30 mg in 2 mL of DMSO being run for 35 min at 100°C. Results are illustrated in Fig. 4C and show that the yield of HMF increased from 81.8 to 97.9% with the increase in the availability and number of acid sites as catalyst loading changed from 6–20 mg. However, when increasing the amount even further, the reaction yield decreased again with a maximum yield of 97.9%. Thus, using 20 mg of the catalyst per 200 mg of fructose was enough to afford the highest HMF yield.
Furthermore, the conversion of different sugars was examined in the presence of Hal/k-Cr/PAA (1:1) catalyst (Fig. 4D). Amongst these sugars, the highest yield of HMF was produced from fructose. It is important to keep in mind that under the established optimal conditions, the glucose, sucrose, and galactose conversion to HMF involves two or three main steps hydrolysis, isomerization, and dehydration into HMF; as a consequence, their conversion to HMF is less efficient than fructose.
Figure 5 illustrates a rational route for fructose dehydration induced by DMSO with the aid of Hal/k-Cr/PAA catalyst. DMSO is a polar solvent consisting of a nucleophilic oxygen end and an electrophilic sulfur end. According to the proposed mechanism, the initiation stage involves the proton transfer from the solid Brønsted acid (Hal/k-Cr/PAA) to DMSO 56,57. Following that, the OH group attacks the electrophilic (S atom) center of protonated DMSO at the anomeric center of fructose. As a result, a covalent bond between the S and O atoms is formed. In the next stage, the proton is relocated from the hydroxyl group of the anomeric center to the oxygen atom in DMSO. After entering other DMSO molecules, followed by the elimination of three water molecules, 5-HMF is formed.
The catalytic performance of various control catalysts for dehydration of fructose to HMF under the optimum reaction conditions was compared in Fig. 6A. As illustrated, Hal/k-Cr/PAA (1:1) had the highest catalytic activity and led to the best HMF yield (97.9%), whereas Hal, k-Cr, k-Cr free catalyst (Hal/PAA), and the catalyst with lower content of k-Cr (Hal/k-Cr/PAA (2.5:1)) yielded lower HMF about 16.0%, 39.7%, 46.7% and 81.6%, respectively. Notably, the catalytic activity of, k-Cr is attributed to the presence of –SO3H functionalities in its backbone. Comparison of the catalytic activity of Hal and Hal/PAA indicates that conjugation of PAA, which has acidic moieties in its structure increases the acidity of Hal and consently the yield of HMF. Similarly, higher catalytic activity of Hal/k-Cr/PAA (1:1) compared to Hal/PAA approves the significant contribution of k-Cr to the catalytic activity. On the other hand, this can be due to the high acidity of k-Cr and possible synergism between the components. This issue is further confirmed through comparison of the activity of Hal/k-Cr/PAA (1:1) with Hal/k-Cr/PAA (2.5:1), which has higher content of k-Cr/PAA. The composite with lower content of the polymeric moiety was less effective, approving the role of the conjugated polymeric part in the catalytic activity.
The recyclability and stability of Hal/k-Cr/PAA (1:1) were investigated with regard to their significance from an economic, environmental, and industrial viewpoints. This catalyst was applied in the dehydration of fructose and recovered by centrifugation, water and ethanol rinsing and vacuum drying after each run. As a result, it was observed that the HMF production decreased by approximately 9% between the first and fourth stage runs due to the blockage of acid sites with byproducts, such as humin and soluble polymer (Fig. 6B). According to the Hammett plot, the active acid groups remained pretty constant after four runs, which were close to that of a fresh catalyst. Additionally, FT-IR (Fig. 2A-g) and XRD (Fig. 2B) analyses did not show any substantial alteration in the functional or crystals structure compared with a fresh sample. On account of these observations, it can be concluded that Hal/k-Cr/PAA (1:1) catalyst showed high recyclability and stability against several reaction cycles.
The catalytic performance of Hal/k-Cr/PAA (1:1) catalyst for HMF production from fructose was also compared with randomly selected catalysts. As demonstrated in Table 2, various catalysts ranging from dendrimers to magnetic catalysts have been reported so far. Notably, synthesis of Hal/k-Cr/PAA (1:1) catalyst is simpler compared to catalysts, such as dendrimer-based ones that are prepared through multi-step procedures. Moreover, use of bio-based and relatively cost-effective compounds for the synthesis of Hal/k-Cr/PAA (1:1) under aqueous condition make is an environmentally benign catalyst. On the other hand, high yield at relatively low temperature in short reaction time are the merits of Hal/k-Cr/PAA (1:1).
Table 2
Comparison of the catalytic performance of Hal/k-Cr/PAA (1:1) catalyst for HMF production from fructose substrate with reported solid acid catalysts.
Catalyst | Solvent | Temp. (o C) | Time (min) | HMF yield (%) | Ref. |
SO3H-dendrimer-SiO2@Fe3O4 | Solvent-free | 100 | 60 | 92 | 58 |
0.75 M-ZrO2/SO4 − 2 | DMSO | 120 | 60 | 85 | 25 |
N-MON-AS# | DMSO | 100 | 1200 | 91 | 59 |
Sulfonated amorphous carbon–silica | Deep eutectic solvent | 110 | 240 | 67 | 60 |
COP-SO3H/SB* | DMSO | 120 | 60 | 78 | 61 |
S-GCN | water | 200 | 300 | 43 | 62 |
APG-SO3H | Water | 160 | 60 | 67.9 | 31 |
SiNP-SO3H-C16@ | DMSO/water | 120 | 180 | 87 | 63 |
Fe3O4@SiO2-SO3H | DMSO | 100 | 120 | 93.1 | 64 |
SBA-15-SO3H | Ionic liquid (BmimCl) | 120 | 60 | 81 | 65 |
Hal/k-Cr/PAA | DMSO | 100 | 35 | 97.9 | This work |
# nanocatalytic systems based on microporous organic network (MON) bearing aliphatic sulfonic acids
* Triazine-based covalent organic polymer grown on mesoporous SBA-15
@ Functionalized silica nanoparticles