3.1 Characterization of the catalyst
Figure 1 shows the XRD patterns of the doped and undoped MMT catalysts. Montmorillonite has three main diffraction peaks at 2Θ = 9, 19 and 35 (Garade et al. 2010; Husin et al. 2015). The intense peak at 2Θ = 27 could be attributed to quartz. XRD analysis shows that the metals are well dispersed over the MMT clay and do not penetrate into the MMT skeleton. Similar XRD patterns were observed for all catalysts.
The N2 adsorption/desorption isotherms were used to analyze the porosity of the transition metal MMT catalysts. According to the IUPAC classification, all catalysts have the type IV with H3 hysteresis, indicating a typical mesoporous structure. All catalysts exhibited a maximum pore volume around 50Å diameter. The surface area, pore volume and average pore size of the samples are listed in Table 1. The BET surface area of montmorillonite was 230.4 m2/g. After the impregnation process, the surface area of the catalysts decreased due to the deposition of species, while the pore volume and pore diameter did not change significantly up to 10% Cr.
Table 1
Textural properties of MMT, Cr-MMT, Cu-MMT, Zn-MMT and Fe-MMT catalysts
|
Surface Area (m2/g)
|
Pore Volume (m3/g)
|
Average Pore size (Å)
|
MMT
|
230.4
|
0.2863
|
49.7
|
Cr-MMT
|
221.8
|
0.2821
|
50.9
|
Cu-MMT
|
214.0
|
0.2708
|
50.6
|
Zn-MMT
|
220.1
|
0.2784
|
50.6
|
Fe-MMT
|
223.3
|
0.2799
|
50.7
|
5Cr-MMT
|
214.4
|
0.2710
|
50.7
|
10Cr-MMT
|
221.8
|
0.2821
|
50.9
|
15Cr-MMT
|
204.6
|
0.2492
|
48.8
|
The NH3- TPD profiles of MMT, Cu-MMT, Cr-MMT, Zn-MMT and Fe-MMT catalysts are shown in Figure 2. In the area under the TPD curve, there were two distinct desorption peaks which divided them into two acidic regions, namely weak and strong acidity in the ranges of 100ºC to 200ºC and 350ºC to 600ºC, respectively (Putluru et al. 2016; Liu et al. 2013). The total acidity of the catalysts, as measured by the amount of ammonia desorbed, ranged from 0.249 to 0.460 mmol/g (Table 2). Compared to MMT, impregnation with metals decreased the total acidity of the catalysts (Table 2), which could be due to the metal species replacing or covering the acidic sites (He et al., 2019). However, the intensity of these two peaks increased with increasing Cr content, indicating that a larger number of acid sites were formed.
Table 2
Acid content and NH3-TPD peak temperatures of MMT catalysts
|
Weak acid
|
Strong acid
|
Total acid amount
(mmol/g)
|
Lewis/ Brønsted
|
Peak temperature (ºC)
|
Acid amount (mmol/g)
|
Peak temperature (ºC)
|
Acid amount (mmol/g)
|
MMT
|
157.2
|
0,001
|
536.2
|
0.459
|
0.460
|
0.82
|
Cr-MMT
|
155.6
|
0.014
|
571.8
|
0.299
|
0.313
|
1.08
|
Zn-MMT
|
189.1
|
0.026
|
543.3
|
0.380
|
0.386
|
0.89
|
Fe-MMT
|
182.2
|
0.014
|
556.7
|
0.309
|
0.323
|
0.87
|
Cu-MMT
|
194.6
|
0.012
|
550.5
|
0.304
|
0.316
|
0.87
|
5Cr/MMT
|
145.3
|
0.019
|
548.6
|
0.230
|
0.249
|
0.86
|
10Cr-MMT
|
155.6
|
0.014
|
571.8
|
0.299
|
0.313
|
1.08
|
15Cr-MMT
|
156.1
|
0.023
|
527.2
|
0.420
|
0.423
|
1.27
|
Brønsted and Lewis acid sites were evaluated by the pyridine adsorption method to determine the relationship between the acid type and the catalytic activity of the prepared catalyst (Yu et al. 2018). Brønsted acid sites are observed in the FTIR spectra with an absorption peak at 1540 cm-1, while the absorption peak of Lewis acid sites is found at 1450 cm−1 (Figure 3). The Lewis/Brønsted ratio (L/B) was calculated based on the area under the corresponding peaks (Cao et al. 2019) and is shown in Table 2. The L/B ratio was highest for the MMT catalyst modified with Cr among all metals.
3.2 Comparison of the performance of the catalysts under constant reaction conditions
In this work, the conversion of cellulose under hydrothermal conditions in the presence of MMT doped with transition metals as a solid acid catalyst was investigated. First, the performance of the different transition metal-MMT catalysts (Cr, Cu, Zn, Fe) was evaluated under constant conditions, at 180 ºC for 2 h in water as reaction medium. To understand the role of catalyst in the conversion of cellulose, an idle run (without catalyst) was carried out by treating cellulose with water.
The process of HMF synthesis from cellulose involves the steps of hydrolysis, isomerization and dehydration. During the process, levulinic acid and formic acid are formed from HMF. Further hydration of HMF to levulinic acid and polymerization of HMF by itself to form humins have been the bottlenecks in selective production of HMF from biomass conversion (He et al. 2019; Rout et al. 2016). The cellulose conversion results and yields of HMF, levulinic acid, and formic acid are shown in Figure 3. In the blank sample, the HMF yield was only 0.8%, with a low cellulose conversion of 8%. However, after addition of the catalysts, cellulose conversion of up to 15.4% was achieved, with the highest HMF yield (2.5%). The highest cellulose conversion and HMF yield was observed with Cr-MMT, followed by Zn-MMT, Cu-MMT and Fe-MMT. 5-HMF was further rehydrated to levulinic acid and formic acid. The maximum selectivity achieved using Cr loaded MMT catalyst as (16,5 %), followed by Zn-MMT (15.2 %), Cu-MMT (12.4 %), Fe-MMT (10.9 %) and blank (10.1%). These results were similar to the study reported by Zhou et al. (2016). They prepared metal loaded Bentonite catalyst with different type of metals and tested them for 5-HMF production from glucose. Among the metals studied, they found that Cr showed the highest catalytic activity (Zhou et al. 2016).
Both 5-HMF yield and selectivity was affected by Lewis/ Brønsted acid ratio. The highest HMF yield (2.56%) and selectivity (16.7%) was obtained with the catalyst (Cr-MMT) have lowest total acidity and highest Lewis/Brønsted acid ratio (Table 2). Generally, the isomerization of glucose into fructose is usually completed in an alkaline reaction system (Binder and Raines, 2009; Su et al. 2009), the dehydration of fructose to HMF requires an acidic environment (Nie et al., 2020).
Since Cr-MMT catalyst showed the highest cellulose conversion, 5-HMF yield and selectivity, the effects of various reaction parameters such as Cr loading, reaction time, reaction temperature and catalyst amount on the conversion of cellulose to 5-HMF were further investigated.
3.3 The effect of Cr loading.
The amount of catalyst plays an important role in the kinetics of production and needs to be optimized to achieve maximum selectivity for the desired product. Experiments were carried out varying the catalyst loading in the range of 5 to 10 wt% and the results are shown in Figure 4. All reactions were carried out at 180°C for 6 hours using a 1:2 catalyst:substrate ratio in water, and the results are shown in Figures 4 and 5. The cellulose conversion increased from 30.86–36.81% by increasing the amount of Cr from 0 to 10%. When the amount of Cr was further increased to 15%, the cellulose conversion decreased by 30.15%. The yield of 5-HMF production increased for all the synthesized catalysts with the increase in reaction time from 30 min to 360 min. The maximum 5-HMF yield (4.19%) was obtained at 360 min using 10Cr-MMT catalyst. However, when the amount of Cr was further increased from 10–15%, the 5-HMF yield decreased.
Although water used as the reaction medium in this study, the yield of 4.19% for 5-HMF is compatible with literature. For example, Kassaye and co-workers reported a 5.1% 5-HMF yield at 180ºC using sulfated zirconia catalyst and ionic liquid from cellulose (Kassaye et al., 2016).
Figure 4 shows the product distribution obtained as a result of 6 h catalytic cellulose conversion reaction. The increase of Cr content from 5 to 10% resulted in an increase of all products (glucose, 5-HMF, levulinic acid, formic acid). However, the yield of the products decreased slightly when Cr loading increased to 15%. A similar trend was also observed in cellulose conversion. This could be due to the decrease in surface area and pore volume of the catalyst when Cr loading was increased from 10–15%.
3.4 Effect of the reaction temperature and reaction time
Reaction temperature and reaction time are two important factors which greatly affect the dehydration of glucose. To evaluate how the reaction temperature affected the production of HMF, the treatments were performed keeping all other parameters constant. Temperature range of 160-220°C was tested to determine the optimal reaction temperature. As shown in Figure 5, the reaction temperature and reaction time had a great impact on the cellulose conversion as well as yield of the reaction products. Cellulose conversion increased from 17.85–87.32% sharply as the reaction temperature was raised from 160°C to 220°C for 6 h. 5-HMF yield increased from 0.65–4.41% with an increased temperature from 160oC to 200oC. However, further increase in the temperature to 220oC, the HMF yield and selectivity first increased to the maximum values of 10.88 % and 11.51%, respectively, and then decreased with an increase in reaction time (Figure 6). The reason for this phenomenon might be explained that the side reactions (i.e., rehydration) increased with the reaction time at high temperatures, which led to the generation of soluble polymers and insoluble humin (Nandiwale et al. 2014; Zi et al. 2015). At this temperature, longer reaction times resulted in the generation of more levulinic acid and formic acid. At low reaction temperatures (200°C), the HMF yield was as high as 6.74%, with a 4.95% levulinic acid yield and a 8.51% formic acid yield after 6 h.
3.5 Effect of initial cellulose loading
The ratio of cellulose to catalyst is a critical parameter for high cellulose conversion due to the solid-solid interaction between catalyst and cellulose. Adding an excess amount of catalyst to the cellulose conversion system can increase glucose conversion. There might be many reasons for these interactions. Chemical and van der wall interactions between the cellulose and the catalyst, the outer surface of the catalyst, and the morphology of the catalyst (Lanzafame et al. 2012). For the above reasons, we optimized the ratio of cellulose to catalyst. The effect of initial cellulose loading on 5-HMF production was studied by varying the amount of cellulose in the range of 1.25-5.0 g with 2.5 g of catalyst at a temperature of 200ºC for 6 hours. An increase in the initial amount of cellulose resulted in a decrease in the catalyst/substrate ratio (Figure 7). A lower initial amount of cellulose (1.25 g) increased the initial formation rate of 5-HMF at 120 minutes. After 2 hours, there was no significant change in 5-HMF yield with 2.5 g and 1.25 g of initial cellulose (Figure 8). However, there is a significant difference in 5-HMF yield when the initial amount of cellulose was increased to 5 g. Similar results were found by Su et al. 2018. When they decreased the ratio of catalyst to cellulose, their product yields decreased as much as our results, and they explained the reason for this situation as insufficient catalytically active sites formed between the solid acid catalyst and the substrate as a result of mass transfer resistance (Su et al., 2018). On the other hand, although a cellulose conversion was as high as 93.47%, only 30% of the reaction products was identified. These products in the solution could be soluble polymers and humic substances derived from the decomposition or self-polymerization of glucose (Qi et al., 2011).