The investigated parameters of this study are reaction temperature, biomass/solvent ratio, and PTSA concentration as shown in Table 4. During the experiments, samples were taken for analysis before the reactor reached the reaction temperature and during the reactions. The time the reactor reached the set temperature was accepted as the start of the reaction. The yields of different acids (LA, FA, and AA), 5-HMF, and monosaccharides (sucrose, glucose, (M + F + G + X + R), and arabinose) were obtained from the analysis of these samples.
Table 4
Variation of the acid and sugar yields with biomass/solvent ratio, reaction temperature, and catalyst concentration during the reaction time.
Biomass/solvent ratio, g/mL (at 200°C and 300 mM PTSA) | Yields, % | Time (min) |
0 | 10 | 20 | 30 | 50 | 70 | 90 | 110 |
0.04 | Total Acid | 20.1 | 23.4 | 27.1 | 26.5 | 30.0 | 28.9 | 28.3 | 29.0 |
Total Sugar | 33.4 | 28.4 | 23.6 | 16.5 | 19.6 | 16.2 | 12.8 | 11.3 |
Total | 53.5 | 51.8 | 50.7 | 43.0 | 49.6 | 45.1 | 41.2 | 40.3 |
0.06 | Total Acid | 13.9 | 16.8 | 19.2 | 20.3 | 19.8 | 22.6 | 22.0 | 20.7 |
Total Sugar | 25.9 | 21.6 | 22.5 | 12.8 | 12.3 | 11.5 | 10.8 | 9.4 |
Total | 39.8 | 38.3 | 41.7 | 33.1 | 32.0 | 34.1 | 32.8 | 30.1 |
0.08 | Total Acid | 14.8 | 17.9 | 19.8 | 19.7 | 19.9 | 19.6 | 19.5 | 20.5 |
Total Sugar | 25.9 | 24.4 | 18.7 | 18.2 | 14.3 | 12.5 | 9.6 | 8.8 |
Total | 40.7 | 42.3 | 38.5 | 38.0 | 34.2 | 32.1 | 29.1 | 29.3 |
Temperature, °C (at b/s:0.04 and 300 mM PTSA) | | |
180 | Total Acid | 0,0 | 0,0 | 12,1 | 21,9 | 26,8 | 24,5 | 29,8 | 23,9 | |
Total Sugar | 30.7 | 32.6 | 26.1 | 27.7 | 22.6 | 21.7 | 18.7 | 16.3 | |
Total | 30.7 | 32.6 | 38.2 | 49.5 | 49.4 | 46.1 | 48.5 | 40.2 | |
200 | Total Acid | 20.1 | 23.4 | 27.1 | 26.5 | 30.0 | 28.9 | 28.3 | 29.0 | |
Total Sugar | 33.4 | 28.4 | 23.6 | 16.5 | 19.6 | 16.2 | 12.8 | 11.3 | |
Total | 53.5 | 51.8 | 50.7 | 43.0 | 49.6 | 45.1 | 41.2 | 40.3 | |
220 | Total Acid | 17.1 | 19.1 | 21.4 | 24.9 | 30.0 | 26.6 | 26.8 | 23.6 | |
Total Sugar | 20.3 | 16.8 | 14.3 | 12.3 | 10.9 | 8.8 | 5.8 | 5.9 | |
Total | 37.4 | 35.9 | 35.7 | 37.2 | 40.8 | 35.4 | 32.6 | 29.5 | |
PTSA concentration, mM (at 200°C and b/s:0.04) | | |
100 | Total Acid | 13.5 | 15.3 | 18.3 | 16.1 | 17.6 | 19.7 | 20.5 | 22.3 |
Total Sugar | 35.6 | 33.6 | 27.6 | 24.4 | 22.8 | 16.7 | 16.6 | 15.3 |
Total | 49.0 | 48.9 | 45.9 | 40.6 | 40.4 | 36.5 | 37.1 | 37.6 |
300 | Total Acid | 20.1 | 23.4 | 27.1 | 26.5 | 30.0 | 28.9 | 28.3 | 29.0 |
Total Sugar | 33.4 | 28.4 | 23.6 | 16.5 | 19.6 | 16.2 | 12.8 | 11.3 |
Total | 53.5 | 51.8 | 50.7 | 43.0 | 49.6 | 45.1 | 41.2 | 40.3 |
600 | Total Acid | 22.1 | 22.8 | 25.9 | 28.4 | 36.2 | 34.8 | 33.4 | 28.1 |
Total Sugar | 27.1 | 23.7 | 20.1 | 15.7 | 12.3 | 9.4 | 8.9 | 2.9 |
Total | 49.3 | 46.5 | 46.1 | 44.1 | 48.5 | 44.2 | 42.3 | 31.0 |
3.1. Effects of Temperature and Reaction Time on Levulinic Acid and Other Product Yields
Three experiments were conducted at different temperatures (180, 200, and 220°C) using 300 mM PTSA as the catalyst. Moderate catalyst concentration was selected due to stable product yields throughout the reaction time. The biomass-to-solvent ratio was maintained at 1g/25mL (0.04) for all reactions. The catalyst type, concentration, and reaction duration were kept constant in this part of the experiments to observe the impact of temperature on the yields of the main product, LA, and by-products.
Cellulose decomposition in sub-critical water with an acidic catalyst initiates the isomerization of glucose to fructose. Firstly, fructose is transformed into 5-HMF as an intermediate compound [25], which subsequently decomposes to yield LA and FA, along with other by-products [26]. The C-based yields of LA and other by-products obtained at different temperatures are displayed in Fig. 3. The results showed similar trends for each temperature. At 180°C, a maximum LA yield of 22.6% (7.15 g/L) was achieved within a reaction time of 90 minutes. This temperature exhibited the slowest change in the yield among the tested conditions, indicating slower decomposition of the biomass. At 200°C, the LA yields displayed a continuous increase with increasing reaction time of up to 50 minutes. After reaching its maximum, the yield remained relatively constant and stabilized for the remainder of the reaction. Within the range of 50–110 minutes, the yield reached and maintained a maximum of approximately 23.0% (7.29 g/L). This temperature demonstrated slightly higher and more stable yields compared to 180°C. At 220°C, the yields continued to increase as the reaction progressed, but a decrease in the yield was observed towards the end of the reaction duration. The highest yield of 22.2% (7 g/L) was obtained within 50 minutes at this temperature. The trend observed here aligns with the expectation that higher temperatures lead to faster decomposition of biomass. As reported in the literature, LA yield increased at higher reaction temperatures in shorter reaction times [18, 27]. This indicates that at lower temperatures, a longer reaction time is required to achieve the desired end products. Based on these findings, 200°C was selected as the optimum temperature for LA production which is very close to 201.4°C found in the optimization session in Minitab represented in Fig. 9. This temperature offered both higher yields within reaction time intervals and greater stability compared to the other tested temperatures. The yields of LA were determined both as C-based and glucan weight-based and presented in Table 5 for simple comparison with literature findings. The highest C-based yield of 23.1% was obtained at 200°C, corresponding to 48.5% in weight-based yield, which indicates a successful decomposition of glucan in chicory into LA with PTSA as the catalyst.
Table 5
The carbon-based and weight-based yields of levulinic acid under different experimental conditions
Biomass/solvent ratio, g/mL (at 200°C and 300 mM PTSA) | Max LA Yield% (carbon-based) | Max LA Yield % (weight-based) | t max |
0.04 | 23.1 | 48.53 | 90 |
0.06 | 17.3 | 36.41 | 110 |
0.08 | 16.3 | 33.29 | 110 |
Temperature, °C (at b/s:0.04 and 300 mM PTSA) | | | |
180 | 22.60 | 47.58 | 90 |
200 | 23.10 | 48.53 | 90 |
220 | 22.20 | 46.60 | 50 |
PTSA concentration, mM (at 200°C and b/s:0.04) | | | |
100 | 16.4 | 34.47 | 110 |
300 | 23.1 | 48.53 | 90 |
600 | 25.8 | 54.25 | 50 |
As seen in the preheating stage of the reaction carried out at 180°C displayed in Fig. 3a., acid formation was not observed at temperatures lower than 180°C. Acids started to form after 10 minutes from the reaction start, and a simultaneous decrease in sugar levels began around the same point, down to 16.3% total sugar yield as seen in Table 4. Özşen confirmed the cellulose conversion is very low around 150°C and 5 bars and concluded that condition was not suitable breakdown the glycosidic bonds of cellulose [27]. After the start of the reaction at 200°C, acid yields increased continuously and stabilized towards the end of the experiment as displayed in Fig. 3b., whereas sugar yields decreased with time down to 11.3% total sugar yield. Identified compound formation was faster at 200°C compared to other reaction temperatures as seen in Table 4. At 220°C, sugars decomposed in the heating stage and converted rapidly down to 5.9% at the end of 110 min as shown in Fig. 3c. The yields of sugars decline with the rising acid concentration in the product, and this is caused by the reaction mechanism of cellulose breakdown in sub-critical water as expected and reported by other researchers [27, 28]. For the reaction at 220°C, total yields of sugars decreased over time and were lower than that of 200°C, while acid compounds reached a maximum at 50 min and then started to decrease. This can be due to the conversion of intermediate product 5-HMF to other compounds at elevated temperatures such as levoglucosane. As reported in the literature, subcritical decomposition of biomass dehydration results in furfural, 5-HMF, and levoglucosane formation [26]. These observations highlight the importance of selecting an appropriate reaction temperature to optimize the conversion of chicory biomass into the desired products. The acid yield results suggest that the reaction at 200°C offers a favorable balance between achieving high yields of target products and minimizing the formation of unwanted byproducts. The optimum temperature is highly affected by biomass, catalyst type, concentration, and other process conditions. Ya’aini et al. studied the conversion of empty fruit bunch and kenaf to LA over a new hybrid catalyst. Various runs were performed between 100 and 200°C temperatures in that study and they found 145.2°C as optimum [29]. Elevated temperatures and longer times may lead to undesired side reactions and humin formation as declared by Ya’aini et al.
The maximum total acid yields at 180, 200, and 220°C were quite similar, at 22.6%, 23.0%, and 22.2%, respectively, on a C-basis, while they were reached at different times. The highest yields of 5-HMF on a C-basis were determined as 8.7% at 200°C and 5.3% at the preheating stage in the earlier times of the reaction, then it continued to form and decomposed to LA and FA. Since the formation of 5-HMF is slower than its decomposition, the amounts of 5-HMF found in the reaction medium are low, as verified in the kinetic part of the study.
3.2. Effects of Biomass to Solvent Ratio on Levulinic Acid and Other Product Yields
The effect of biomass (b) to solvent (s) ratios using different amounts of biomass was investigated with b/s: 1g/25mL (0.04), 1.5g/25mL (0.06), and 2g/25mL (0.08) at 200°C and 300 mM PTSA was used as the catalyst concentration. The results revealed that more dilute solutions led to slightly higher yields of LA as seen in Fig. 4. The experiment with a biomass-to-solvent ratio of 0.04 demonstrated the highest yield of 23.0% (7.9 g/L). The maximum LA yields obtained for the 0.06 and 0.08 b/s ratios were 17.3 and 16.3% which correspond to 8.2 and 10 g/L concentrations, respectively.
Diluting the substrate solution can enhance the mass transfer of reactants and products in the reaction system. In some cases, when the feedstock has a high concentration, it may become difficult for the acid catalyst to effectively contact and interact with the biomass, leading to incomplete conversion. Dilution can help overcome this issue by increasing the contact area between the catalyst and the biomass, promoting better mixing, and ensuring more efficient mass transfer. As a result, the reaction may proceed more completely, leading to higher LA yields. Literature findings support this trend [30, 31]. In terms of weight-based yields of LA, they decreased sharply from 48.5 to 33.3% as the b/s ratio doubled. In the study of Su et al., cow dung is converted to LA with HCl at various operating conditions and substrate loading ratios. They performed the conversion with 0.1, 0.3, 0.5, and 1.0 g cow dung in 0.4 M HCl aqueous solution and found that the maximum yield of LA was achieved at low feed concentrations while slightly lower LA yields were observed with 0.5 and 1.0 g of substrate loading [30].
Increasing the biomass-to-solvent ratio typically leads to higher concentrations of biomass in the reaction mixture. This can result in higher yields of glucose/fructose since there are more biomass molecules available for hydrolysis. However, there is usually an upper limit to the concentration of biomass that can be effectively processed due to issues such as viscosity and heat transfer limitations.
Hu et al. investigated the inhibition of humin formation in the catalytic production of LA from cellulose with high substrate loading in the range of (2.0–50.0 wt.%). They observed lower LA yields at high substrate loads [32]. They reported that, for achieving high LA yields at high biomass concentrations, novel strategies are required to prevent the reactions of intermediate products and LA to humins, as well as to enhance the cellulose decomposition reaction.
The results in Table 4 provide a comprehensive overview of the total acid and sugar yields obtained from the acidic hydrolysis of chicory. The increase in the biomass-to-solvent ratio for the reaction at a temperature of 200°C, catalyzed by 300 mM PTSA, resulted in a decrease in total acid yields. This observation suggests that a lower biomass-to-solvent ratio, such as 0.04 g/mL, promotes the conversion of sugars to acids and proves to be more advantageous in obtaining higher yields of LA. The change in biomass/solvent ratio affects the total liquefaction of chicory which can be evaluated as the sum of the yields of all products identified as seen in Table 4. Total acid yields were higher with the lowest b/s ratio of 0.04 g/mL and reached 30.0% in 50 min reaction time, while they remained at lower levels with higher b/s ratios. The C-based total acid yields reached a maximum of 22.6% and 20.5% for 0.06 and 0.08 g/mL of b/s ratios, respectively. In terms of total sugar yields, similar findings were obtained for the effect of the b/s ratio. The C-based total sugar yield was found to be 33.4% with a biomass-to-solvent ratio of 0.04 g/mL, which was the highest, and it decreased to 25.9% with a higher concentration of biomass feedstock. As a result of these experiments, the most suitable conditions for the reaction were determined to be 200°C temperature and biomass to solvent ratio of 0.04 g/mL.
3.3. Effects of PTSA Concentration on Levulinic Acid and Other Product Yields
To examine the influence of PTSA concentration on LA yield, experimental runs were conducted using different concentrations of PTSA (100, 300, and 600 mM). The reaction temperature was maintained at 200°C, and the biomass/solvent ratio was 1/25 g/mL (0.04) for these reactions. At concentrations of 300 mM and 600 mM PTSA, the LA yield reached a maximum at 50 min, whereas with 100 mM PTSA it reached a maximum at the end of the reaction duration. While 100 mM catalyst concentration provided the lowest LA yield, 600 mM exhibited the highest yield. The maximum C-based LA yields were 16.4%, 23.1%, and 25.8% for 100, 300, and 600 mM PTSA, respectively as displayed in Fig. 5. The highest LA concentrations obtained with 100, 300, and 600 mM PTSA were 5.18, 7.29, and 8.15 g/L respectively. Higher concentrations of the acid catalysts provided the maximum yields in a shorter time while lower yields for LA were achieved with 100 mM PTSA towards the end of the reaction. These findings highlight the importance of PTSA concentration in the hydrothermal conversion process and suggest that an optimal PTSA concentration can enhance the production of LA within a short reaction duration.
Su et. al examined the influence of HCl concentration on the production of LA and represented the results for 0.1, 0.2, 0.4, and 0.6 M HCl [30]. Their findings are consistent with our study, and they concluded that maximum conversion succeeded with yields of LA as 309.7 and 338.9 g/kg at 0.4 and 0.6 M of HCl, respectively. Dussan et al. employed H2SO4 as a catalyst to produce LA from Miscanthus, they achieved significantly higher LA yields at 150°C and 0.5 M of H2SO4 than at 200°C and 0.1 M of H2SO4. High sulfuric acid concentrations enhanced the hydrolysis of cellulose, provided more glucose formation, and raised LA yields [33]. At higher catalyst concentrations and temperatures, corrosion occurs seriously and LA can be further converted to various byproducts (such as angelica lactone) [34]. In our previous study, safflower conversion into LA was investigated in the presence of aromatic sulfonic acid catalysts with 30, 100, and 300 mM concentration levels. The highest yield for LA was determined at 200°C, with 300 mM PTSA, and for a solvent/biomass ratio of 20 by weight [35].
The maximum total sugar yields varied significantly with the change in PTSA concentration as shown in Fig. 6. The total yield of sugars was achieved above 40% during the heating period (between 185°C-200°C) with 100 mM PTSA and 300 mM PTSA, and slightly lowered with 600 mM PTSA. A steady decrease is observed after the reaction begins for all concentrations of the catalyst, while higher PTSA concentrations result in a more rapid decrease in sugar yields that is confirmed with faster formation of the organic acid compounds. This suggests that the conversion of sugars to acids was promoted as the reaction progressed, leading to higher acid yields and lower sugar yields. At the end of the reaction catalyzed by the lowest concentration of PTSA, 15.3% of sugar compounds still existed in the reaction medium while at moderate and high PTSA concentrations most of the sugar converted to desired products and were found at lower yields of 11.3 and 2.9%, respectively. The total acid yields were lower for the reaction with 100 mM PTSA concentration, and it increased as the reaction progressed and reached up to 22.3% at 110 min as seen in Table 4.
The reactions catalyzed by higher concentrations of PTSA exhibited higher yields of acidic compounds and 5-HMF totally, in a shorter time. 30 and 36.2% total acid yield was achieved with 300 mM and 600 mM of PTSA, respectively. Although LA constitutes the majority of this yield, FA and AA were also formed in the highest amounts at elevated concentrations of catalysts. The amount of 5-HMF decreased over time as the concentration of catalyst increased as expected, which is due to high conversion to LA. The highest yield of 5-HMF was obtained with 100 mM PTSA at 10 min of reaction time at 6.4%. The highest total yields of the liquefaction products were achieved at 53.5%, with a moderate concentration of PTSA at the beginning of the reaction and it decreased over time, corresponding to a decrease in sugar yields.
A study on the conversion of poplar wood to LA was performed with PTSA as a catalyst. The results of this study showed that the usage of PTSA improved the LA yields to a level that was competitive with mineral acid catalysts. The research involved the conversion of the polysaccharide fraction of biomass to LA and a high yield of 54.9 mol% with an LA concentration of 17.4 g/L was obtained at the conditions of 160°C, 1.10 mol/L (1100 mM) PTSA, and 60 min. In our study, the highest yields of LA were found as 25.8 and 54.3% in terms of carbon and weight-based, respectively with 600 mM PTSA at 200°C and in 50 min as given in Table 5. This suggests that higher concentrations of this catalyst can favor the successful generation of levulinic acid and maximize the amount of LA in a shorter time [12].
3.5 GC-MS Analysis Results
Several samples were subjected to GC-MS analysis to identify the different compounds that have formed in the products. The GC-MS analysis results of samples obtained from the acidic hydrolysis of chicory at different experimental conditions are given in Table 6.
Table 6
GC-MS analysis results of sample obtained from acidic hydrolysis of chicory (T = 200°C, b/s:1g/25 mL)
Experimental Conditions | GC-MS Analysis Results | | |
| Compound | Area Percentage, % | Total Area Percentage, |
% |
100mM PTSA, t = 110min | Phenethylamine, p-alpha-dimethyl- | 0.94 | 97.91 |
Formic acid | 18.29 |
Acetic acid | 3.07 |
Furfural | 6.33 |
Oxime-, methoxy-phenyl- | 3.20 |
Pentanoic acid, 4-oxo- | 64.32 |
Levoglucosenone | 1.76 |
300mM PTSA, t = 50min | Formic acid | 6.50 | 91.65 |
Furfural | 3.29 |
Oxime-, methoxy-phenyl- | 23.82 |
Pentanoic acid, 4-oxo- | 60.63 |
Levoglucosenone | 2.62 |
Diethyl Phthalate | 1.29 |
600mM PTSA, t = 50min | Phenethylamine, p-methoxy-alpha-methyl- | 0.67 | 88,39 |
Formic acid | 10.28 |
Acetic acid | 1.60 |
Furfural | 2.67 |
Oxime-, methoxy-phenyl- | 24.70 |
Pentanoic acid, 4-oxo- | 47.94 |
1,2-Pentadiene | 0.53 |
GC-MS analysis confirmed the formation of compounds that are generally expected to form through acidic hydrolysis of biomass. The results of GC-MS show that pentanoic acid, 4-oxo-( LA) has the highest percentage in the aqueous product and by-products of 5-HMF, furfural, formic acid, and acetic acid exist in lower percentages. This fact satisfied that chicory was decomposed into valuable chemicals successfully. It can be verified that most of the compounds present in the products were identified and quantified by HPLC. In addition, several chemicals such as levoglucosenone, oxime-methoxy phenyl, diethyl phthalate, and phenethylamine,p-alpha-dimethyl were detected in the samples in lower percentages, which were not identified by HPLC. LA and furfural percentages decreased with increasing catalyst concentration as expected, also found via the response optimizer. The detected products, indicative of possible compounds resulting from the decomposition of chicory, can be attributed to the influence of an aromatic sulfonic acid catalyst on the decomposition process.
3.6 Optimization of the Operating Conditions for the PTSA Catalyzed System
The parameters were selected based on experimental results, which indicated that reaction temperature, time, and catalyst concentration influenced the LA yield. The biomass/solvent ratio was not considered a parameter, as the most dilute feed concentration was experimentally found to be the most advantageous, and increasing the ratio had no observed significant positive effect on LA yield.
The optimization process was performed for the PTSA-catalyzed system with independent variables and levels selected as follows:
Temperature
180, 200 and 220°C
PTSA concentration
100, 300, and 600 mM
Reaction time
0,10,20,30,50,70,90, and 110 minutes
The response variable was the LA yield.
The regression equation obtained for the PTSA-catalyzed system is as follows:
LA yield, % | = | − 451,8 + 1,032 time + 4,185 Temp + 0,0725 cat conc - 0,002075 time*time- 0,00973 Temp*Temp - 0,000073 cat conc*cat conc - 0,003410 time*Temp - 0,000051 time*cat conc |
(R-sq: 92.79%, R-sq(adj): 90.93%, R-sq(pred): 88.82%) |
The R-squared value is 92.79%, indicating a good fit of the model to the data.
The ANOVA test was conducted to determine the significance of the independent variables and the results are given in Table 7. For the PTSA-catalyzed reactions, P < 0.0001 for the independent variables which means that all the effects of these variables were of statistical significance. The P-value is higher in the time*cat conc interaction term which means that the term is not significant for this study. The Pareto chart in Fig. 7 provides a visual representation of the effects (%) of the independent variables and interactions on the response variable. This chart helps to identify the most influential factors and their interactions in the PTSA-catalyzed system. The results of the Pareto analysis revealed that time and catalyst concentration are the most significant factors affecting the yield of LA in the PTSA-catalyzed reactions. These two factors have the largest effects on the response variable.
Table 7
ANOVA results for the PTSA catalyzed reactions of chicory to levulinic acid
Source | DF | Adj SS | Adj MS | F-Value | P-Value |
Model | 8 | 1503,81 | 187,976 | 49,88 | 0,000 |
Linear | 3 | 926,61 | 308,869 | 81,96 | 0,000 |
time | 1 | 571,16 | 571,162 | 151,56 | 0,000 |
Temp | 1 | 66,00 | 65,997 | 17,51 | 0,000 |
cat conc | 1 | 319,19 | 319,191 | 84,70 | 0,000 |
Square | 3 | 317,74 | 105,914 | 28,10 | 0,000 |
time*time | 1 | 206,05 | 206,049 | 54,68 | 0,000 |
Temp*Temp | 1 | 80,85 | 80,851 | 21,45 | 0,000 |
cat conc*cat conc | 1 | 102,33 | 102,332 | 27,15 | 0,000 |
2-Way Interaction | 2 | 105,56 | 52,782 | 14,01 | 0,000 |
time*Temp | 1 | 101,85 | 101,849 | 27,03 | 0,000 |
time*cat conc | 1 | 3,71 | 3,714 | 0,99 | 0,329 |
Error | 31 | 116,83 | 3,769 | | |
Total | 39 | 1620,64 | | | |
To further understand the interactions among the independent variables and their influence on the yield, surface plots were generated. Figure 8 depicts these surface plots, providing visual representations of the relationships between temperature, PTSA concentration, reaction time, and the resulting LA yield. Figure 8a highlights the simultaneous effects of interaction between temperature and time on the yields of LA at hold values of 350 mM PTSA concentration. It shows that both terms are effective and have a peak on the response surface around the optimum point (around 200°C and towards the end of the reaction). Figure 8b represents the effects of catalyst concentration and temperature at hold values of 55 min reaction time. There is a maximum around high catalyst concentration and high temperatures at the mid-time range. Figure 8c visualizes the effect of catalyst concentration and time which are the most significant parameters at hold values of 200°C temperature. The highest yields are seen on the surface around 26% as found in the optimizer at around 80 min and with a moderate concentration of PTSA.
Based on the response optimization results, it was determined that the maximum yield of LA in the PTSA-catalyzed reactions can be achieved under the following optimized operating conditions: a catalyst concentration of 436.6 mM, a temperature of 201.4°C, and a reaction time of 77.8 minutes, as seen in Fig. 9. Under these optimized conditions, the LA yield (C-based) is estimated to reach a maximum value of 26.44%. This corresponds to an LA concentration of 8.3 g/L. These optimized conditions can be used as guidelines for conducting the PTSA-catalyzed reactions to maximize the production of LA.
3.7 Kinetic Modelling Results
Kinetic modeling was performed for the PTSA-catalyzed decomposition of chicory. Kinetic parameters for the reactions, performed at 200 and 220°C, with 100, 300, and 600 mM of PTSA and different biomass intakes were calculated. At the reaction temperature of 180°C, very low yields were obtained for the intermediate compound of 5-HMF and the main product LA, so it was not included in the kinetic modeling calculations.
The rate constants of the reactions are listed in Tables 8–10. Related kinetic parameters from the Arrhenius equation for this reaction are provided in Table 11. A comparison of experimental data and kinetic models is shown in Fig. 10. A good fit was observed between the experimental data and the kinetic model.
Table 8
Reaction rate constants of acidic hydrolysis of chicory catalyzed by 300 mM PTSA at different temperatures
T (°C) | Rate constant (min− 1) | Goodness of fit (R2 ) |
kGLN | kGLC | kGLC1 | kGLC2 | kHMF | kGLN/kGLC | kGLC1/kHMF | GLC | HMF | LA |
200 | 0.0163 | 0.0438 | 0.0059 | 0.038 | 0.0895 | 0.371 | 0.065 | 0.727 | 0.870 | 0.951 |
220 | 0.0348 | 0.1056 | 0.0126 | 0.093 | 0.1633 | 0.330 | 0.077 | 0.709 | 0.816 | 0.828 |
Table 9
Reaction rate constants of acidic hydrolysis of chicory at 200°C catalyzed by different concentrations of PTSA
C catalyst (mM) | Rate constant (min− 1) | Goodness of fit (R2 ) |
kGLN | kGLC | kGLC1 | kGLC2 | kHMF | kGLN/kGLC | kGLC1/kHMF | GLC | HMF | LA |
100 | 0.0055 | 0.0206 | 0.0045 | 0.0161 | 0.0407 | 0.266 | 0.110 | 0.846 | 0.821 | 0.985 |
300 | 0.0163 | 0.0438 | 0.0059 | 0.0379 | 0.0895 | 0.371 | 0.065 | 0.727 | 0.870 | 0.951 |
600 | 0.0207 | 0.0776 | 0.0089 | 0.0687 | 0.0917 | 0.266 | 0.097 | 0.878 | 0.884 | 0.900 |
Table 10
Reaction rate constants of acidic hydrolysis of chicory at 200°C catalyzed by 300 mM PTSA with different initial amounts of biomass
Biomass
(g)
|
Rate constant (min− 1)
|
Goodness of fit (R2)
|
kGLN
|
kGLC
|
kGLC1
|
kGLC2
|
kHMF
|
kGLN/kGLC
|
kGLC1/HMF
|
GLC
|
HMF
|
LA
|
1
|
0.0163
|
0.0438
|
0.0059
|
0.0379
|
0.0895
|
0.371
|
0.065
|
0.727
|
0.870
|
0.951
|
1.5
|
0.0156
|
0.0458
|
0.0051
|
0.0407
|
0.0910
|
0.341
|
0.056
|
0.719
|
0.899
|
0.977
|
2
|
0.0195
|
0.0480
|
0.0026
|
0.0453
|
0.0762
|
0.406
|
0.035
|
0.710
|
0.863
|
0.949
|
Table 11
Kinetic parameters for PTSA catalyzed hydrolysis of chicory
Rate constant | A (s− 1) | Ea (kJ/mol) | mi | R2 |
kGLN | 6.709×106 | 91.205 | 0.764 | 0.973 |
kGLC | 1.882×106 | 81.586 | 0.735 | 0.999 |
kGLC1 | 2.339×103 | 64.675 | 0.370 | 0.975 |
kGLC2 | 3.985×106 | 84.828 | 0.807 | 1.000 |
kHMF | 6.521×105 | 76.730 | 0.477 | 0.944 |
The study results reveal important insights into the kinetic behavior of the reactions involved in the catalytic biomass hydrolysis process. Among all the reaction rate constants, kGLC1 had the lowest values while kHMF had the highest values. This suggests that the consumption rate in the conversion reaction of 5-HMF into LA is faster than its formation rate in the reaction of glucose conversion into 5-HMF. This indicates that the reaction of LA formation from 5-HMF is not the rate-limiting step in the overall process. These findings are consistent with values reported in previous literature [6, 16–20].
The cellulose hydrolysis reaction and decomposition of glucose to humins had the highest activation energies which were 91.2 and 84.8 kJ/mol respectively. This implies that temperature has a significant effect on these reactions. The activation energy of 5-HMF conversion was 76.7 kJ/mol and lower than that of glucan and glucose conversions. The lowest activation energy belonged to the reaction leading to 5-HMF formation from glucose, which was 64.7 kJ/mol. Furthermore, the activation energy of the side reaction leading to humins was higher than 5-HMF formation from glucose and 5-HMF conversion into LA, suggesting that the formation of humins was harder than LA generation in this catalytic system.
A previous kinetic study on LA formation from poplar wood with PTSA as the catalyst reported activation energies of 122.9 kJ/mol for glucan hydrolysis, 111.9 kJ/mol for glucose conversion to 5-HMF and 120.5 kJ/mol for 5-HMF conversion to LA [12]. Another study utilizing sugarcane bagasse also reported activation energies in a similar range with H2SO4 as the catalyst [18]. The activation energies calculated in this study are slightly lower or within proximity. This can arise from the structural differences in the biomass. Activation energies of the reactions are in alignment with the literature and found in the same order of magnitude as Ea (kJ/mol) for glucan hydrolysis > Ea (kJ/mol) for 5-HMF conversion to LA > Ea (kJ/mol) for glucose conversion to 5-HMF [12, 19, 20].
The study results show that both kGLN and kGLC increase as the reaction temperature rises, indicating an overall acceleration of the reactions. However, the ratio of kGLN/kGLC decreases from 0.371 to 0.330 as the temperature changes from 200 to 220°C, which implies that the glucose conversion rate is higher and glucose concentration in the products would be lower at high reaction temperatures as seen in Table 8. Additionally, kHMF also increases with rising temperature significantly, indicating a faster conversion of 5-HMF into LA at higher temperatures. The ratio of kGLC1/kHMF also increases from 0.065 to 0.077 at higher temperatures, suggesting that the rate of 5-HMF formation increases in a greater ratio than its decomposition rate.
Increasing the catalyst concentration boosted all the reaction rates, yet the impact of temperature on the rate constants was more significant as presented in Table 9. The value of kGLC1/kHMF was lower for higher acid concentrations, indicating that the decomposition rate of 5-HMF increases faster than its formation rate. Higher acid concentrations also resulted in faster hydrolysis of cellulose and conversion of glucose.
The reaction rate constants do not exhibit a monotonic change with varying initial amounts of chicory, indicating that the rate constants are independent of the mass of biomass in the feed. High concentrations of biomass feed resulted in a small increase in rate constants of glucan conversion and humin formation while slight decreases were observed in the 5-HMF formation and decomposition reactions as shown in Table 10.
3.8 Carbon Efficiency of the Process
The efficiency of the process is provided in terms of the mass ratio of carbon content in the liquid and the solid products (coke) concerning the biomass carbon content. The carbon efficiencies of the liquid and solid products and the total conversion of the biomass are given in Table 12. The total organic carbon liquefaction efficiency (TOC-CLE) values give the percentage of the total organic carbon content in the aqueous products and it can be used as a measure of the efficiency of the process. The residue efficiency is determined by measuring the carbon content in the solid residues of the reactions. These values serve as an indicator of biomass conversion. The TOC-CLE values were within the range of 55–77%, while the residue efficiencies for all the reactions ranged from 3–15%. These values suggest a high degree of biomass conversion was achieved during the acidic hydrolysis process of chicory. The total conversion of the biomass was in the range of 70–85%. The remaining portion can be attributed to unreacted solids, other by-products, unidentified compounds, and loss.
Table 12
Carbon efficiencies of the liquid and solid products and the total conversion of biomass under different experimental conditions (C biomass = 40.78 wt. %)
Biomass/solvent ratio, g/mL (at 200°C and 300 mM PTSA) | TOC-CLE % | RE % | Conversion% |
0.04 | 76 | 8 | 84 |
0.06 | 63 | 12 | 75 |
0.08 | 55 | 15 | 70 |
Temperature, °C (at b/s:0.04 and 300 mM PTSA) | | | |
180 | 67 | 10 | 77 |
200 | 76 | 8 | 84 |
220 | 65 | 9 | 74 |
PTSA concentration, mM (at 200°C and b/s:0.04) | | | |
100 | 68 | 14 | 82 |
300 | 76 | 8 | 84 |
600 | 77 | 8 | 85 |