Article pubs.acs.org/EF
Effects of p‑Toluenesulfonic Acid in the Conversion of Glucose for Levulinic Acid and Sulfonated Carbon Production Shimin Kang,† Gang Zhang,† Xuesi Yang, Huibin Yin, Xiaobo Fu, Junxu Liao, Junling Tu, Xiangxuan Huang, Frank G. F. Qin, and Yongjun Xu* Guangdong Provincial Key Laboratory of Distributed Energy Systems, Dongguan University of Technology, Dongguan, 523808, China ABSTRACT: This work investigates the possibility that valuable levulinic acid and sulfonated carbon could be produced in a one-pot reaction. p-Toluenesulfonic acid is used as an acidic catalyst as well as a sulfonating agent in the reaction, and its main effects are determined, depending on the water dosage. Relatively low water dosage is conductive to sulfonated carbon formation with high H+ exchange capacity and −SO3H intensity, good carbon framework, improved thermal stability and surface hydrophobicity, and good catalytic activity. In contrast, low water dosage is detrimental to levulinic acid yield. Under optimal reaction conditions, the total carbon utilization ratio of levulinic acid and sulfonated carbon reaches 70%. The sulfonated carbon shows good catalytic activity in the esterification of levulinic acid with ethanol, resulting in a high conversion ratio (93%). Importantly, p-TSA is a strong acid with a pKa of −2.8, which could be a good catalyst for biomass hydrolysis.25 Thus, it seems that p-TSA may act as a sulfonating agent as well as an acidic catalyst under designed reaction conditions. In this work, p-TSA was used to catalytic conversion of glucose. The object was to investigate whether LA and sulfonated carbon (instead of humins) could be formed in a one-pot reaction. Importantly, water was determined to be an important parameter affecting reaction routes and product formation. The effects of water dosage on both LA yield and sulfonated carbon properties were studied and discussed.
1. INTRODUCTION Production of chemicals, fuels, and materials from renewable biomass has attracted much attention.1−3 Levulinic acid (LA), from hexoses (e.g., glucose) under acid catalysis, was considered as a chemical bridge connecting biomass and petroleum processing.4 LA is a sustainable platform molecule that can be upgraded to valuable chemicals and fuel additives.5,6 One example is that LA can be converted with alcohols, via acid catalysis, to produce low-molecular-weight esters (e.g., ethyl levulinate).7,8 The esters exhibit performance suitable for use as a gasoline additive at levels of 10 and 20 vol %.4 The routine conversion of glucose into LA involves consecutive reactions, including the dehydration of glucose into 5-hydroxylmethylfurfural (HMF) and rehydration of HMF to LA.6,9,10 Besides, a large amount of byproducts, especially insoluble humins, would be formed during the acid-catalyzed reactions.11,12 For a LA production process from cellulose hydrothermal conversion, the humins selectivity on a carbon basis reached 30%−60%.13 Humins are low-value-added byproducts, which decreases the utilization efficiency of cellulosic feed, and thereby decreases the economic viability of biorefineries. HMF is a primary intermediate that results in the formation of humins.14−16 Interestingly, it was reported that benzyl groups could be added to the humins during their formation.17,18 It seems that, in the presence of a special additive during hexoses conversion, targeted carbonaceous materials might be formed but not humins. This indicates the possibility that value-added carbonaceous materials could be produced during LA formation, which would bring additional benefit for the entire biorefineries. Sulfonated carbon catalyst becomes a research hot spot in recent years, which is widely used in the esterification, hydrolysis, and other organic synthesis.10,19−21 One way for the synthesis of sulfonated carbon is using organic carbon sources with suitable sulfonating agents. p-Toluenesulfonic acid (p-TSA) is a frequently used sulfonating agent,22 which can react with glucose for sulfonated carbon production.23,24 © XXXX American Chemical Society
2. MATERIALS AND METHODS 2.1. Materials. D-(+)-glucose (99%), levulinic acid (LA, 98%) and strong acid catalyst Amberlyst-36 were obtained from J&K Chemical (Beijing, China). Sulfuric acid (H2SO4, 98%) and p-toluenesulfonic acid hydrate (p-TSA·H2O, 98.5%) were obtained from Xiya Reagent (Chengdu, China). Fructose (analytical reagent) was obtained from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China). Zeolite catalyst HZSM-5 (Si/Al ratio = 38) was obtained from Tianjin Kaimeisite Technology Co., Ltd. (Tianjin, China). All of the water used in this work was deionized water and all reagents were used without any purification. 2.2. Reaction Conditions. All reactions were conducted in Teflon reactors with Teflon containers and stainless steel cylinders. The Teflon containers were sealed in stainless steel cylinders and heated in an air-circulated oven to a preset temperature. The reactor temperature reached the preset temperatures (±2 °C) with a heating time of ∼60 min. The holding time was started after 60 min of heating and recorded thereafter. After reaction, the reactors were quickly cooled under tap water. 2.3. Reactions for LA and Sulfonated Carbon Production. All the reactions were conducted at 170 °C with a holding time of 7 h to convert the glucose completely to LA and/or sulfonated carbon (SC). In a routine acid-catalyzed reaction, glucose (250 g/L) was used for Received: October 16, 2016 Revised: February 4, 2017
A
DOI: 10.1021/acs.energyfuels.6b02675 Energy Fuels XXXX, XXX, XXX−XXX
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The H+ exchange capacity was determined through neutralization titration.20,27 The titration was carried out as follows: 50 mg of solid sample and 30 mL of 0.03 mol L−1 NaOH solution were vibrated in a shaker for 1 h. The solids were filtered off and washed with 30 mL of water by three times. The combined filtrate (filtrate produced by the solids filtration and the 30 mL washing water) was added by using an excessive amount of HCl solution. After that, the acidic combined filtrate was back-titrated with 0.02 mol/L NaOH, using phenolphthalein as an indicator. All of the data about H+ exchange capacity, BET surface area, elemental analysis, and product yields were measured two or three times. These data were shown as averages, and the absolute errors were calculated based on the measured values and the average values. 2.6. Calculation. The yields of humins and SCs were calculated based on the initial weight of glucose. While the yields of LA and formic acid were calculated based on the initial molar content of glucose. For example, the yield of LA was calculated as follows:
LA production, with H2SO4 or p-TSA used as acid catalysts. After reaction, the solid residues and the liquid products were separated by vacuum filtration. The liquid products were diluted with water to 100 mL in volumetric flasks and kept for analysis. To test the effects of water dosage, glucose (or fructose) and p-TSA mass dosages were kept the same and mixed before reaction (see Table 1). After reaction, the solid and liquid products were separated
Table 1. Effects of Water Dosage on LA and SC Yields, and p-TSA Recovery in the Conversion of Glucose and Fructose at 170 °C for 7 h
No.
glucose (or fructose):pTSA:H2Oa (mass ratio)
SC name
1 2 3 4 5 6 7 8 9
100:100:10 100:100:45 100:100:70 100:100:120 100:100:170 100:100:270 100:100:10 100:100:70 100:100:270
SC-1 SC-2 SC-3 SC-4 SC-5 SC-6 SC-7 SC-8 SC-9
SC yield (wt %) 42.7 39.0 36.2 31.9 27.2 26.5 45.3 35.6 26.6
± ± ± ± ± ± ± ± ±
1.1 1.1 0.9 0.9 0.9 0.8 1.1 0.8 0.7
LA yield (mol %)
p-TSA recovery (%)
± ± ± ± ± ± ± ± ±
88 92 96 99 >99 >99 86 93 >99
12.6 20.3 28.1 36.5 40.8 52.8 12.3 27.8 51.7
0.7 0.6 0.6 0.5 0.5 0.5 0.8 0.6 0.5
yield of LA (%) =
number of moles of LA measured × 100 number of moles of initial glucose
The carbon utilization ratio for LA and SCs were calculated based on the total carbon of converted raw materials (the initial glucose and p-TSA minus the remaining glucose and p-TSA after reaction). For example, the carbon utilization ratio for LA was calculated as follows:
a Glucose was the raw material for Nos. 1−6, and fructose was the raw material for Nos. 7−9.
carbon utilization ratio (%) carbon weight of LA = × 100 total carbon weight of converted raw materials
by vacuum filtration. The solid products were washed repeatedly by deionized water until the filtrate reached a pH of 7.0. The obtained solid sample was dried at 110 °C for 12 h and then ground into a powder that was referenced as sulfonated carbon (SC). The SCs were labeled from SC-1 to SC-6, based on the water dosage in the reactions (see Table 1). 2.4. Catalyzed Esterification of LA. To test the catalytic activities of the SCs, esterification of LA with ethanol was conducted from 90 °C to 150 °C. The molar ratio of LA to ethanol was kept 1:10 for all the reactions. After reaction, the SCs and ethanol solution were separated by filtration. SC-3 was selected to test the catalyst stability in the cycle usage. After each reaction, the recovered SC-3 was washed and dried, and then added into fresh LA−ethanol solution for reuse reaction. The deactivated SC-3 was regenerated as follows:26 it was dipped into 10% H2SO4 solution with sonic oscillation for 6 h at 45 °C, and then washed with excessive water. Subsequently, the regenerated catalyst was dried and resupplied for a new LA esterification reaction to test if it had improved catalytic activity. 2.5. Analysis. The water diluted liquid solutions were analyzed with a high-performance liquid chromatography (HPLC) system that was equipped with a C18 column (Shimadzu, Japan). The column was kept at 30 °C and eluted with deionized water−methanol solution (0.1% phosphoric acid) at 0.6 mL/min. The volume ratio of methanol to water was 20:80. The C, H, S elemental compositions were analyzed by a Vario EL Elementar, and the O content was calculated from elemental balance. The LA esterification products in the ethanol solution were directly analyzed by gas chromatography−mass spectrometer (GCMS) with a Shimadzu QP 2010 Plus system. Xray diffraction (XRD) patterns were obtained using a Rigaku D/maxIIIA X-ray diffractometer. The functional groups were analyzed by Fourier transform infrared (FTIR) spectroscopy on a Tensor 27 (Bruker, Germany). Thermogravimetric (TG) analyses were conducted under N2 flow by a Model TG 209F3 instrument (Netzsch, Germany). A sample was placed in a sample pan and heated from room temperature to 700 °C with a heating rate of 10 °C min−1. The surface morphology of solid samples was studied using an environmental scanning electron microscopy (SEM) system (JEOL, Model JSM-6701F). The Brunauer−Emmett−Teller (BET) surface area of the solid samples was analyzed by a BET surface analyzer (JWGB Sci. & Tech. Co., Ltd., Beijing, China).
The catalytic activities in the esterification reactions were evaluated according to the LA conversion ratio. The LA conversion ratio was calculated as follows:
LA conversion ratio (%) moles of initial LA − moles of remained LA = × 100 moles of initial LA
3. RESULTS AND DISCUSSION 3.1. Effects of p-TSA in Diluted Solutions. To improve LA yield and inhibit the formation of humins, the conversion of biomass is usually kept at a relatively low initial biomass concentration ( SC-6. It is noted that the H+ exchange capacity (1.8−2.4 mmol/g) is much higher than the −SO3H contents for all of the SCs, and the extra H+ exchange value seems similar to the H+ exchange value of humins (Table 3). The extra H+ exchange value should be attributed to other acidic groups (e.g., carboxyl and hydroxyl groups) on SCs, in considering of the above FT-IR analysis. Therefore, low water dosage in the reaction is beneficial for both H+ exchange capacity and −SO3H contents on the SCs. 3.4.2. Surface Topography. The surface topography of the SCs was affected greatly by the water dosage. For the SC-6 formation, sulfonation should be somewhat inhibited, because of the relatively high water dosage. As shown in Figure 3A, the SC-6 seems like a solid accumulated by small particles, which is similar to humins obtained from routine acid-catalyzed reactions (Figure 3B). Differently, the SC-1 is a massive solid with a smooth surface (Figure 3C). Sulfonation should be a primary reaction during the formation of SC-1. Because of the medium water dosage for SC-3 formation, both sulfonation and routine acid-catalyzed reactions seem important. The SC-3 is a massive solid with wrinkles and small particles on its surface (Figure 3D). The massive solid structure indirectly means good binding strength of the SC constituents, which is important for the durability or stability of SCs as solid materials. Note that BET surface area of all the SCs and humins are low (in the range of 1−2 m2/g−1; see Table 3), which is consistent with the SEM results, since few micropores were found in these materials. 3.4.3. Carbon Framework. No matter how much water dosage was in the reactions, the crystal structure of glucose was destroyed. As shown in Figure 4, broad diffraction peaks are located between 2θ = 10° and 30° for all the SCs, assigned to the (002) planes of the carbon.37,38 These peaks are attributed to the randomly arranged amorphous carbon structures that contain low crystalline graphite content39,40 and indicates that SCs have become amorphous carbon materials.23,41 Note that the water dosage in the reaction also affected the carbon framework, since the SC-1 and SC-3 displayed a XRD pattern with higher intensity than that of SC-6. This means the SC-1 and SC-3 had a relatively better carbon framework than SC-6,23 although all the carbon frameworks were still amorphous and far away from graphitization. 3.4.4. Thermal Stability and Surface Hydrophobicity. The surface hydrophobicity and thermal stability are important indexes for using the SCs as catalyst or other materials. The TG curves of the three SCs are shown in Figure 5. The mass loss at temperatures below 150 °C was due to the loss of absorbed
Figure 1. Fourier transform infrared (FT-IR) spectra of humins, sulfonated carbons (SCs), and p-toluenesulfonic acid (p-TSA).
on humins are associated, because of the broad peaks with relatively strong intensity at ∼3420 cm−1. Since the raw material glucose is a multiple hydroxyl compound, the formation of hydroxyl groups in a free state indicates that most of the hydroxyl groups are lost. Compared with humins, SC-1 and SC-3 have less hydroxyl groups, because of the pTSA-involved reactions. The significant difference of the SCs to humins is that all SCs have −SO3H groups. The −SO3H group is shown by peaks at ∼1190 and 1130 cm−1 (asymmetric stretching vibration peak of OSO), 1040 cm−1 (symmetric stretching vibration peak of OSO), and 680 cm−1 (stretching vibration peak of C− S).33−35 The XPS analysis confirms the presence of −SO3H groups, based on a S peak at ∼167 eV (Figure 2). Generally, the S 2p photoelectron peak (163−168 eV) is of particular significance in the sulfonated materials and has been used to confirm the presence of −SO3H group.36 Unlike the humins, the presence of −SO3H group on SCs improves their performance as acidic solid materials.
Figure 2. X-ray photoelectron spectroscopy (XPS) analysis of sulfonated carbon. The sulfonated carbon (SC-3) was prepared with an initial mass ratio of glucose:p-toluenesulfonic acid (p-TSA):H2O equivalent to 100:100:70. D
DOI: 10.1021/acs.energyfuels.6b02675 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. Scanning electron microscopy (SEM) spectra of various sulfonated carbon (SC) catalysts and humins: (A) sulfonated carbon SC-6, (B) humins, (C) sulfonated carbon SC-1, and (D) sulfonated carbon SC-3. The sulfonated carbons (SC-6, SC-1, and SC-3) were prepared with initial glucose:p-toluenesulfonic acid (p-TSA):H2O mass ratios of 100:100:270, 100:100:10, and 100:100:70, respectively. Panel (E) shows an SEM micrograph of sulfonated carbon SC-3 reused three times.
Figure 4. X-ray diffraction (XRD) spectra of sulfonated carbons (SCs).
Figure 5. Thermogravimetric (TG) curves of sulfonated carbons (SCs).
hydrophobic SCs. The hydrophobic SCs with acidic −SO3H groups may be suitable esterification catalysts. Because water would contribute to a reverse hydrolysis in esterification reactions,43 the hydrophobic surface of catalyst was reported to be favorable,44 while hydrophilic surface was unfavorable, because of the absorption of water.
water, which could be used to estimate the surface hydrophobicity of the SCs.23,42 The weight loss of SC-1, SC-3, and SC-6 before reaching 150 °C was 3.1, 3.4, and 4.0 wt %, respectively. These low absorbed water contents indicated that the SCs had good surface hydrophobicity. Thus, low water dosage in the reaction was conductive to the production of E
DOI: 10.1021/acs.energyfuels.6b02675 Energy Fuels XXXX, XXX, XXX−XXX
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of SC-3 were obtained (Table 1). This means that 47.7%, 22.3%, and 30.0% of carbon in the converted raw materials were formed as the SC-3, LA, and other byproducts, respectively. Besides LA, SC-3 also seems to be a value-added product, which may be used as a solid acid catalyst. Therefore, the total carbon utilization ratio of valuable product (LA and SC-3) reached 70.0%. This carbon utilization ratio was much higher than that of only LA (41.5%) obtained from the routine acid-catalyzed reaction, as discussed above. 3.5. Influences of Process Parameters in the Esterification. SC-3 was used to test the effects of process parameters during the catalytic esterification reaction (see Figures 7 and 8). Within a limited range, the LA conversion
Importantly, the weight loss seemed insignificant when the temperature increased from 150 °C to 240 °C, indicating that these SCs were thermally stable at this temperature range. However, these SCs could not be used as a stable material above 240 °C, since weight loss became obvious. Besides, according to the TG remaining weight after heating to 700 °C, the thermal stability has a trend: SC-1 > SC-3 > SC-6. The thermal stability is mainly dependent on the binding strength of the SC constituents, including the carbon framework and −SO3H groups, thereby indicating that a low water dosage in the reaction promoted the chemical combination of p-STA with glucose or its reactive intermediates, and finally improved the thermal stability of the SCs. 3.4.5. Catalytic Activities. The catalytic activities of SCs formed with different water dosage were evaluated by esterification of LA with ethanol. GCMS analysis showed that ethyl levulinate was the product, and the LA conversion ratio was used to evaluate the degree of esterification. As shown in Figure 6, compared with the blank, all of the tested solid acids
Figure 7. Influence of sulfonated carbon dosage on levulinic acid esterification at 130 °C for 3 h. The sulfonated carbon (SC-3) was prepared with an initial mass ratio of glucose:p-toluenesulfonic acid (pTSA):H2O equivalent to 100:100:70.
Figure 6. Effects of various types of acid catalysts on levulinic acid esterification at 130 °C for 4 h with the addition of 50 g/L solid acid or 0.1 M H2SO4.
(HZSM-5, Amberlyst-36, and SCs) except the humins had obvious catalytic effects on the esterification reactions. The comparison of SCs and humins confirms that −SO3H groups were key catalytic activity sites. It was obvious that the SC-1, SC-2, and SC-3 had high LA conversion ratios, reaching ∼93%, which was comparable to the Amberlyst-36 and homogeneous acid H2SO4 in the experimental conditions. From SC-3 to SC-6, however, the catalytic activities gradually decreased. The LA conversion ratio catalyzed by SC-6 was even slightly lower than that of HZSM-5. The different catalytic activities could be explained by the different properties of the SCs, since the H+ exchange capacity, −SO3H group contents, and surface hydrophobicity of SC-3 were much better than those of SC6. On the other hand, BET surface area was not a main cause affecting the catalytic ability, since the BET surface areas for all the SCs were very low and almost the same (see Table 3). In summary, water dosage was a key factor affecting both the LA yield and SC properties. Relatively low water dosage was conductive to SC formation with high H+ exchange capacity and −SO3H group intensity, good carbon framework, improved thermal stability and surface hydrophobicity, and good catalytic activities. Considering the SC properties, and LA and SC yields, the process for the SC-3 formation from the glucose conversion seemed to be an optimal choice. Under this reaction condition, 96% of p-TSA was recovered, 28.1 mol % of LA and 36.2 wt %
Figure 8. Influence of holding time and reaction temperature on levulinic acid esterification with an initial sulfonated carbon dosage of 50 g/L. The sulfonated carbon (SC-3) was prepared with an initial mass ratio of glucose:p-toluenesulfonic acid (p-TSA):H2O equivalent to 100:100:70.
ratio increased with the extension of reaction time, the increase of temperature, and the increase in catalyst dosage. For example, at 150 °C, the LA conversion ratio reached 41% after the heating time (or 0 h of the holding time) and reached 92% after 1 h of holding time. In contrast to 150 °C, the conversion ratio at 90 °C reached only 37% after 1 h of holding time. However, the highest LA conversion ratio was maintained at ∼93%. Within the experimental conditions as shown in Figures 7 and 8, too much of an increase in SC-3 dosage or temperature, or too much extension of reaction time had little effect for LA conversion. Since the conversion of LA to ethyl F
DOI: 10.1021/acs.energyfuels.6b02675 Energy Fuels XXXX, XXX, XXX−XXX
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Technology Development Project of Dongguan (Nos. 2014108101036, 2014108101037), and Training Program of Innovation and Entrepreneurship for Undergraduates at Dongguan University of Technology (Nos. 201611819034 and 201611819097). Young Innovative Talents Program from Department of Education of Guangdong Province (No. 2015KQNCX163).
levulinate is reversible, the reaction may reach equilibrium at the LA conversion ratio of 93%. On the other hand, the catalyst was deactivated gradually during the reuse experiments. Under the reaction condition of 130 °C for 3 h with a catalyst dosage of 50 g/L, the LA conversion ratio decreased from 93.1% by fresh SC-3 to 56.8% by the three-times-reused SC-3. Similar to that of the fresh SC-3, the used SC-3 retained the massive solid structure (see Figures 3D and 3E). This indicates that the breakage or dissolution of SC-3 was not a problem that caused the deactivation. Since leaching and/or inhibition of the active catalytic sites via the formation of sulfonate esters are common problems in using sulfonated carbon catalysts,26,45 the leaching and inhibition of −SO3H groups on SCs were therefore investigated. As shown in Table 3, some of the S was lost in the reusing process, compared to the fresh SC-3 and reused SC-3. Thus, leaching of the −SO3H group should be an important reason for the deactivation. H2SO4 is an effective catalyst for ester hydrolysis in water solution.26 To determine if the −SO3H groups were inhibited by the formation of sulfonate esters, the reused SC-3 was dipped into 10% H2SO4 solution to hydrolyze the possible sulfonate esters. Compared with the reused SC-3, the hydrolysis-derived SC-3 maintained the same S content (see Table 3). However, the hydrolysis-derived SC-3 resulted in a high LA conversion ratio of 92.3%. On one hand, humins were treated under the same conditions (10% H2SO4 treatment), but this activity did not result in any improved catalytic activity. This indicates that the adsorption or loading of H2SO4 onto humins or SC-3 during the H2SO4 treatment could be eliminated, and, therefore, the recovered catalytic activity should mainly result from the release of −SO3H groups from the hydrolysis of sulfonate esters. On the other hand, the formation of sulfonate esters could be the other deactivation problem.
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4. CONCLUSIONS p-Toluenesulfonic acid (p-TSA) was used as acidic catalyst and sulfonating agent for the conversion of glucose in a one-pot reaction. The main effects of p-TSA were dependent on the water dosage, which is a key factor affecting both levulinic acid (LA) yield and sulfonated carbon (SC) properties. The combined production of SC-3 and LA seems to be an optimal choice, with the total carbon utilization ratio reaching 70.0%. The SC-3 exhibited good catalytic activity, compared with traditional solid acid catalysts in LA esterification. Moderate increases in temperature or catalyst dosage, or extension of reaction time, promoted the degree of esterification of LA. SC3 deactivation occurred in reuse reactions, which was mainly caused by the leaching and inhibition of −SO3H groups.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yongjun Xu: 0000-0002-2425-4035 Author Contributions †
These authors contributed equally to this paper.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21606045), Social Science and G
DOI: 10.1021/acs.energyfuels.6b02675 Energy Fuels XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.energyfuels.6b02675 Energy Fuels XXXX, XXX, XXX−XXX
