Abstract
A two-step strategy was demonstrated to synthesize porous polymeric solid acids with bifunctionality (chloride and sulfonic acid) to mimic cellulase for hydrolyzing cellulose. The solid acids were synthesized from aromatic monomers bearing chloride through Friedel-Crafts polymerization and then sulfonated with fuming sulfuric acid to introduce sulfonic acid. The chloride and sulfonic acid were expected to function as the cellulose-binding group (CBG) and the cellulose-hydrolytic group (CHG), respectively. It was found that the synthesized cellulase-mimetic solid acids were more effective in hydrolyzing microcrystalline cellulose (Avicel) than non-cellulase-mimetic solid acid (Amberlyst 15) and sulfuric acid at the same acid loading. Ball-milled Avicel could be hydrolyzed by up to 84.9 % by the cellulase-mimetic solid acids. The performance of the solid acids was supposedly attributed to the synergetic roles of the CBG and the CHG and the porous structure of the synthesized solid acids.
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Introduction
Glucose has been identified as an important platform chemical from lignocellulosic biomass for producing liquid fuels (e.g., bioethanol and biohydrocarbons) and value-added chemicals (e.g., sorbitol, 5-hydroxymethylfurfural, and levulinic acid) [1–3]. Effective hydrolysis of cellulose to glucose is the most crucial step when converting the biomass to fuels and chemicals through the sugar (glucose) platform. So far, cellulolytic enzymes, mineral acids, and solid acids have been used as the catalysts to hydrolyze cellulose to glucose [4–7]. Mineral acids such as sulfuric acid and cellulolytic enzymes both show exceptional catalytic activities, but they face the issues of separation and recycling for reuse. Traditional solid acids are readily recyclable, but they are usually not as effective as mineral acids and the enzymes in hydrolyzing cellulose because the solid nature of both solid acids and cellulose limits the interaction between the catalyst and the substrate.
To improve the performance of the traditional solid acids in cellulose hydrolysis, a new type of solid acid (cellulase-mimetic solid acid) has been proposed, which combines the advantages of both cellulolytic enzymes (high activity) and solid acids (ready recyclability) [8–15]. Functionally similar to cellulases, the cellulase-mimetic solid acids have bifunctional groups acting as the cellulose-hydrolytic group (CHG) responsible for cleaving β-1,4 glycosidic bond and the cellulose-binding group (CBG) for bringing or affiliating the CHG close to cellulose, respectively. Sulfonic acid is a strong acid and a good candidate of the CHG for the cellulase-mimetic solid acids. The functional groups able to bind or associate with the hydroxyl of cellulose through hydrogen or other bond could be used as the CBG of the cellulase-mimetic solid acids. Suganuma et al. found that the hydroxyl and carboxyl could function as CBG. They synthesized a cellulase-mimetic solid acid with sulfonic acid as the CHG and hydroxyl and carboxyl as the CBG [8]. This catalyst could hydrolyze cellulose to soluble saccharides (4 % glucose and 64 % oligomer). Li et al. also proved the CBG function of carboxyl [9]. As a matter of fact, the hydroxyl and carboxyl play important role in the cellulose-binding domain of cellulases [10].
Halides are strongly electronegative and usually considered as the hydrogen-bonding acceptors [11]. Therefore, halides could be a potential candidate of the CBG of cellulase-mimetic solid acid. Our previous study showed that the chloride on the surface of a strong cationic ion exchange resin could enhance the interaction between the resin and cellulose, and therefore, the resin was able to effectively hydrolyze cellulose in water [12]. Hu et al. also reported a solid acid bearing chloride and sulfonic acid for the hydrolysis of cellulose [13]. It was proposed that the hydrogen bonds between chloride and cellulose hydroxyl could improve the affinity between the solid acid and cellulose and thereby lower the activation energy for the solid acid to initiate the hydrolysis of cellulose [12, 13]. In another study, the ionic liquids with chloride were used as the CBG to fabricate cellulase-mimetic solid acids [14, 15].
However, most of the reported cellulase-mimetic solid acids were not as effective as cellulases in hydrolyzing cellulose. One of the reasons could be the low surface area (typically below 2 m2/g) of the solid acids, which limited accessible catalytic sites on catalyst surface [16]. In addition, the low density and/or unsatisfactory performance of the cellulose-binding group could be another possible contributor to the inefficiency of the solid acids. In the present study, a new strategy was developed to synthesize chloride-bearing cellulase-mimetic solid acids directly from aromatic monomers. First, porous chloride-bearing polymers were synthesized through the Friedel-Crafts polymerization, which is a proven method to synthesize porous polymers, from selected aromatic monomers bearing chloride. The resultant polymers were then sulfonated with fuming sulfuric acid to introduce sulfonic acid. It was expected that the chloride and the sulfonic acid in the synthesized polymeric solid acids functioned as the CBG and the CHG, respectively, while the porous structure ensured high surface area of the solid acids. In addition, different chloride-bearing aromatic monomers were selected to investigate the effect of chloride location and density and aromatic backbone structure on the performance of end solid acids.
Materials and Methods
Chemicals and Materials
Dimethyl sulfoxide, concentrated (98 %) sulfuric acid, and fuming sulfuric acid were bought from Fisher Scientific. Chlorobenzene, benzyl chloride, 2-chloroethyl benzene, 3-phenyl propyl chloride, 1-chloromethyl naphthalene, α,α-dichlorotoluene, anhydrous iron (III) chloride, 1,2-dichloroethane, formaldehyde dimethyl acetal, and Avicel cellulose were purchased from Alfa Aesar. All chemicals were used as received. Amberlyst 15 (AM) was bought from Mallinckrodt. Cellulase with activity of 70 filter paper unit (FPU)/g and β-glucosidase with activity of 250 cellobiohydrolase unit (CBU)/g were generously provided by Novozymes (Franklinton, NC).
Synthesis of Porous Polymer Bearing Chloride
Porous polymer bearing chloride was synthesized through the Friedel-Crafts “knitting” polymerization [17]. Typically, aromatic monomer with chloride (0.04 mol), as shown in Scheme 1, was dissolved in anhydrous 1,2-dichloroethane (30 mL) in a 250-mL flask attached with a condenser. Then, formaldehyde dimethyl acetal (0.12 mol) was added into the solution followed by anhydrous iron chloride (0.12 mol). The polymerization reaction was carried out at 45 °C for 5 h and then continued at 80 °C for 21 h under magnetic stirring. The formed brown precipitate was dispersed in methanol, separated by filtration, extensively washed with methanol, and then extracted in a Soxhlet with methanol for 24 h. The resultant porous polymer was dried under vacuum.
Sulfonation of the Porous Polymer Bearing Chloride
Synthesized porous polymer above was sulfonated with a mixture of concentrated (98 %) sulfuric acid and fuming sulfuric acid. Briefly, the porous polymer (8 g) was dispersed in the mixture of sulfuric acid (100 mL) and fuming sulfuric acid (20 mL) in a 250-mL flask fitted with a condenser. Then, the sulfonation reaction was carried out at 80 °C for 24 h under stirring. The sulfonated porous polymer was finally separated by centrifugation, thoroughly washed with deionized water until the pH of filtrate reached about 7, and then dried under vacuum.
Ball Milling of Cellulose (Avicel)
Ball milling was performed on a Retsch planetary PM-100 ball mill. In each batch, 5 g of Avicel cellulose was milled at room temperature for 4 h in a stainless steel chamber with ten ZrO2 balls with a procedure of each 10-min milling followed by a 10-min rest.
Hydrolysis of Cellulose with Solid Acid
Sulfonated porous polymer as solid acid (0.2 g) and Avicel (or ball-milled Avicel, 50 mg) were mixed with deionized water (2 mL) in a Teflon vial (6 mL) (Saville, USA). The hydrolysis experiment was carried out at 120 °C (oil bath) with a magnetic stirrer (1150 rpm). At the end of the hydrolysis, the mixture was filtrated, and glucose in the filtrate was quantified by high-performance ion chromatography (HPIC), as described below. All hydrolysis experiments were conducted in duplicates, and average result was reported.
Recycling of Solid Acid and Residual Avicel
Briefly, after a run of Avicel hydrolysis, the used solid acid and residual Avicel were recovered by vacuum filtration and thoroughly washed with water. Fresh Avicel was added to maintain the same amount of Avicel for the subsequent run at the beginning of the reaction.
Acidic Hydrolysis of Cellulose
Sulfuric acid (16 μL, equivalent to 0.6 mmol H+) and Avicel (or ball-milled Avicel, 50 mg) were mixed with deionized water (2 mL) in a Teflon vial (6 mL) (Saville, USA). The hydrolysis experiment was carried out at 120 °C with a magnetic stirrer (1150 rpm). At the end of the hydrolysis, the mixture was filtrated, and glucose in the filtrate was quantified by high-performance ion chromatography (HPIC), as described below. All hydrolysis experiments were conducted in duplicates, and average result was reported.
Enzymatic Hydrolysis of Cellulose
Enzymatic hydrolysis of Avicel was carried out at 50 °C and 2 % consistency in 40-mL sodium acetate buffer (50 mM and pH 4.8) with tetracycline chloride (2 mg) as antibiotics on a shaking incubator (Thermo Fisher Scientific, Model 4450, Waltham, MA) at 150 rpm. Cellulase and β-glucosidase loadings were 5 FPU/g cellulose and 10 CBU/g cellulose, respectively. Aliquots (0.5 mL) were taken at specified intervals for the glucose analysis by the HPIC, as described below. All hydrolysis experiments were conducted in duplicates, and average result was reported.
Characterization and Analysis
Surface area and pore size of sulfonated porous polymer were determined according to the nitrogen adsorption method (Brunauer-Emmett-Teller method) using an Autosorb-1 surface area analyzer (Quantachrome Instruments, Boynton Beach, FL). Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of sulfonated porous polymers were collected on a PerkinElmer Spectrum 100 Series FT-IR spectrophotometer with a universal ATR sampling accessory (PerkinElmer Life and Analytical Sciences, CT). Content of sulfonic acid was quantified with conductometric titration. Chloride in the sulfonated porous polymers was determined by elemental analysis with an inductively coupled plasma optical emission spectrometry (ICP-OES; Thermo Jarrell Ash IRIS Advantage). Glucose was quantified with a HPIC system (ICS-3000, Dionex, Sunnyvale, CA) equipped with integrated amperometric detector and Carbopac™ PA1 guard and analytic columns. Glucose yield (%) was calculated as a weight percentage of theoretically available glucose in the starting cellulose.
Results and Discussion
Synthesis of Bifunctional Porous Polymers as Solid Acids Bearing Sulfonic Acid and Chloride
As described in the experimental section, six porous polymers bearing chloride and sulfonic acid as cellulase-mimetic solid acids for cellulose hydrolysis were synthesized through the iron chloride-catalyzed Friedel-Crafts knitting polymerization of chloride-bearing aromatic monomers followed by the sulfonation [17]. In this method, formaldehyde dimethyl acetal was used as an external cross-linker for the polymerization of the monomers, and fuming sulfuric acid was employed to sulfonate the resultant porous polymers to introduce sulfonic acid. The selected chloride-containing aromatic monomers included chlorobenzene, benzyl chloride, 2-chloroethyl benzene, 3-phenyl propyl chloride, 1-chloromethyl naphthalene, and α,α-dichlorotoluene (Scheme 1). They have varying chloride number, diverse backbone aromatic structures (benzene, benzyl, and naphthalene), and different side chain length (−(CH2)n = 0, 1, 2, 3). The objective of selecting different monomers was to understand the effects of monomer structure on the performance of the end solid acids in cellulose hydrolysis. Since chloride could be involved in the internal cross-linking polymerization, certain amount of the chloride could be eliminated as hydrogen chloride during the polymerization [18]. This could affect the end chloride content of the solid acids.
The obtained porous polymers with chloride and sulfonic acid were characterized in terms of porous structure (surface area, pore volume, and pore diameter) and densities of sulfonic acid and chloride. Nitrogen adsorption/desorption isotherms of the porous polymers were collected at 77 K. Surface area (SABET), total volume of pore (Vtotal), average diameter of pore (DA), and densities of sulfonic acid (SO3H) and chloride (Cl) are summarized in Table 1. It turned out that all polymers had mesoporous structure, for the average diameter of the pores fell in the range of 2.46 to 7.00 nm. The pores were not big enough for Avicel particle (in the range of micrometers) to enter; however, the resultant water-soluble intermediates (oligomers of glucose) from cellulose hydrolysis would be able to enter the pores and be further hydrolyzed to glucose there, which might be a reason why the rate of the solid acid-catalyzed hydrolysis did not decline with time, as discussed below.
The polymers showed different porosities, although they were synthesized under similar conditions. For example, the polymers 3 and 4 had much lower surface area than others. This might result from the difference in the reactivity of the monomers toward the Friedel-Crafts polymerization, which thereby affected cross-linking density and porous structure of the polymers. In the Friedel-Crafts polymerization, the benzene rings of the monomers were cross-linked through–CH2– bridges, forming porous polymer network. The porosity and surface area of the resultant polymer were dependent on the cross-linking density, which was determined by the loading of the cross-linker (formaldehyde dimethyl acetal) and the reactivity of the monomer toward the cross-linking reaction. At the same polymerization conditions, the cross-linking density (surface area) was primarily dependent on the reactivity of the monomer. There were more available positions at naphthalene ring for cross-linking reaction (i.e., higher reactivity), and thereby, the solid acid 6 showed the highest surface area. Other aromatic monomers (1–5) have similarly available positions on benzene ring for cross-linking reaction; however, their polymers had very different surface areas. It was found that substituents (functional groups) on benzene ring had significant effect on the cross-linking reaction. For example, chlorobenzene had lower reactivity than benzene in the cross-linking reaction [17]. This could partially explain the diverse surface area of the synthesized solid acids. Monomer 3 has two chlorides that affect its reactivity toward cross-linking reaction, which might be a reason of the very low surface area of solid acid 3. However, monomer 4 was an exception. It has similar structure with monomers 2 and 5, but solid acid 4 had the lowest surface area.
The sulfonic acid density varied among the polymers from 0.45 to 1.35 mmol/g, which was presumably attributed to the reactivity of the polymers toward sulfonation reaction. Polymer structural properties, such as monomer structure, cross-linking density, porosity, surface area, number of unsubstituted positions on benzene ring, and the accessibility of the reactive positions, were expected to affect the reactivity of the polymer toward sulfonation. No clear correlations were found between the sulfonic acid density and these properties of the synthesized solid acids.
The density of chloride varied among the polymers in a wide range as well. For example, the polymers 3 (0.119 mmol/g) and 6 (0.429 mmol/g) had much higher densities of chloride than the polymers 1, 2, 4, and 5 (0.055–0.079 mmol/g). It was easy to understand that the polymer 3 had higher chloride because its monomer (α,α-dichlorotoluene) contains two chlorides. The high chloride content of the polymer 6, although its monomer (1-chloromethyl naphthalene) has the lowest chloride content, could be attributed to the limited chloride elimination during the polymerization. The chloride contents of the polymers (Table 1) are much lower than those of their monomers (8.89, 7.91, 12.4, 7.12, 6.47, and 5.67 mmol/g for monomers 1, 2, 3, 4, 5, and 6, respectively), suggesting that majority of the chloride was eliminated during the polymerization. The chloride elimination was apparently dependent on the monomer structure. It seems that the chloride linked to naphthalene was more tolerant of the elimination than that to benzene as mentioned above.
The presence of chloride and sulfonic acid in the synthesized solid acids was verified by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy (Fig. 1). The peak around 850 cm−1 was attributed to the stretching vibration of chloride. Two characteristic peaks of sulfonic acid (SO3H) were observed at 1035 cm−1 (SO3-H stretching) and 1365 cm−1 (O = S = O stretching), respectively.
Hydrolysis of Cellulose by the Bifunctional Porous Polymeric Solid Acids Bearing Sulfonic Acid and Chloride
Functionally, the synthesized bifunctional polymeric solid acids with sulfonic acid and chloride could be considered as cellulase mimetics for hydrolysis of cellulose, as sulfonic acid could act as the CHG to break down the β-1,4 glycosidic bonds, while chloride could serve as CBG to reach cellulose through hydrogen bonding [12, 13]. In addition, the porous structure of the solid acids was supposed to enhance the interaction between the catalysts and cellulose. Therefore, the cellulase-mimetic solid acids were expected to exhibit good performance in cellulose hydrolysis.
The time-dependent hydrolysis profiles of cellulose by the solid acids synthesized above are presented in Fig. 2. Amberlyst 15 (AM), cellulase, and sulfuric acid (SA) were also included as references. The synthesized solid acids were able to hydrolyze cellulose by 15–25 % at 12 h and 22–38 % at 24 h and further to 48–63 % glucose yield at 48 h, respectively. Because the glucose yield almost linearly increased with time, which was usually observed in acidic hydrolysis of cellulose [8, 12], it is reasonable to expect that extending the hydrolysis time beyond 48 h, the solid acids could lead to higher glucose yield.
As a reference, the non-cellulase-mimetic solid acid AM had 30-nm average pore diameter and 4.9 mmol/g sulfonic acid. The sulfonic acid density of AM was much higher than that of the synthesized solid acids (Table 1), but AM only hydrolyzed Avicel by 7.4 % and 7.7 % at 24 and 48 h, respectively, suggesting that the AM was not effective in hydrolyzing crystalline cellulose. In contrast, the synthesized solid acids were much more effective and had better performance in hydrolyzing Avicel than AM. This could be primarily attributed to the presence of chloride in the solid acids, which was believed to be able to bring the solid acids close to cellulose through hydrogen bond [12]. In addition, the chloride was probably able to form stronger hydrogen bonds with cellulose and thereby disrupt the original hydrogen bonds between cellulose molecules, which would help to swell the crystalline cellulose and improve the accessibility of the cellulose to the acids.
Hydrolysis of Avicel by sulfuric acid (SA) and cellulase was conducted with acid loading of 12 mmol proton (H+)/g cellulose and cellulase loading of 5 FPU/g cellulose, respectively. The hydrolysis of cellulose by the dilute sulfuric acid was carried out at 120 °C, while the enzymatic hydrolysis of cellulose was performed at 50 °C (the optimal application temperature of the enzyme). It was found that the dilute sulfuric acid only hydrolyzed Avicel by 14.0 % and 38.2 % at 24 and 48 h, respectively, which were significantly lower than those by the solid acids (Fig. 2), although the former had much higher proton loading than the latter (12 mmol/g cellulose vs. 1.8–5.4 mmol/g cellulose). The results suggested that the solid acids bearing chloride were more effective than dilute sulfuric acid in hydrolyzing cellulose at the same acid (proton) loading and temperature (120 °C). A reasonable explanation is that sulfuric acid is a homogenous acid and evenly distributed in the system, while the solid acid is a heterogeneous acid and acid groups are concentrated on the surface of the solid acid. Therefore, at the same acid loading, the solid acid provided a higher local acid concentration in the hydrolyzing zone of cellulose than sulfuric acid. In addition, sulfuric acid-catalyzed cellulose hydrolysis has higher activation energy [12] and thereby needs higher reaction temperature (typically 160–180 °C) than solid acid-catalyzed one. This probably was another reason why sulfuric acid had poor performance at 120 °C.
Cellulolytic enzymes achieved 39.9, 51.9, and 65.0 % glucose yields at 12, 24, and 48 h, respectively, at the investigated loadings of 5 FPU cellulase and 10 CBU β-glucosidase per gram cellulose, which were higher than the yields achieved by the solid acids. In addition, cellulases exhibited higher initial hydrolysis rate than the solid acids. As discussed above, the solid acids followed a linearly increasing hydrolysis profile, while cellulases quickly reached a rate plateau after initial fast hydrolysis stage. Therefore, if the hydrolyses were extended beyond 48 h, the solid acids, in particular 5 and 6, would be able to hydrolyze cellulose as effectively as cellulase and even better.
Effects of reaction temperature (100–140 °C) and catalyst loading (solid acid/cellulose, 0.5, 1, 2, 4, and 8 w/w) on the hydrolysis performance of solid acid (using catalyst 3 as an example) were also investigated, and the results are presented in Fig. 3. The results show that the solid acid had the temperature-dependent performance in cellulose hydrolysis. Higher temperature was favorable to the hydrolysis reaction of cellulose. Higher loadings of solid acids resulted in more cellulose hydrolyzed. It is apparent that the better hydrolysis performance resulted from more available catalytic sites (i.e., sulfonic acid groups).
Correlations between performance in cellulose hydrolysis and structural features (surface area, sulfonic acid, and chloride) of the solid acids were investigated, as shown in Fig. 4. Surface area was positively correlated to the performance of the solid acids. Larger surface area generally resulted in higher glucose yield. For example, the solid acids 3 and 4 that had lower surface area (14 and 13 m2/g, respectively) achieved lower glucose yields (45.3 and 50.8 %, respectively) than solid acid 6 (679 m2/g) did (63.2 %) at 48 h. A weak positive correlation, but not confidently conclusive, was observed between the glucose yield and sulfonic acid content (Fig. 4). In some data points, the solid acids with more sulfonic acid even had poor performance in hydrolysis cellulose. For example, solid acid 6 with 0.71 mmol/g sulfonic acid achieved better glucose yield than solids 2 and 5 with 1.35 and 1.09 mmol/g sulfonic acid, respectively. The results suggested that the performance of the solid acids were determined by multiple factors, and synergetic effects of the factors played more important roles. It seemed that the glucose yield was positively correlated with the chloride content (Fig. 4). For example, solid acid 6 had the highest density of chloride (0.429 mmol/g) and therefore achieved the highest glucose yield. However, since most of the data points from this study concentrated at the low chloride density end, further investigation (e.g., synthesizing more solid acids with diverse chloride density) is needed to verify the correlation. The observations above suggested that the performance of the cellulase-mimetic solid acids is affected by their structural features and synergetic effects of chloride (CBG), sulfonic acid (CHG), and surface area. In general, better performance could be expected from a solid acid with higher chloride, more sulfonic acid, and greater surface area.
It was not conclusive how the performance of end solid acids was affected by the structure of starting monomers. It seemed that the monomer with chloride directly attached on benzene ring did not give a good solid acid. The results of the solid acids 2, 3, 4, and 5 were not conclusive to tell the effect of side chain length (or the distance of chloride to benzene ring) on the performance of the resultant solid acids. Double-ring monomer (6) seemed to generate a more effective solid acid than single-ring monomers (1–5).
Recyclability of the synthesized solid acids was preliminarily evaluated using catalyst 3 as an example. After a run of Avicel hydrolysis, the used solid acid and residual Avicel were recovered by vacuum filtration and completely washed with water. The glucose in the hydrolysate was quantitated by high-performance ion chromatography (HPIC) to determine how much cellulose (Avicel) was hydrolyzed. Fresh Avicel was added for the subsequent run to maintain the same amount of Avicel at the beginning of the reaction. As shown in Fig. 5, the solid acid gradually lost its catalytic activity after the four runs of Avicel hydrolysis. Specifically, the solid acid achieved 50.8 % glucose yield in the first run, 32.8 % in the second run, 22.7 % in the third run, and only 17.2 % in the fourth run. Correspondingly, the density of sulfonic acid in the solid acid decreased from 0.74 to 0.16 mmol/g after four runs. Apparently, the leaching of sulfonic acid was responsible for the loss in catalytic activity. For comparison, the solid acid was treated in water at 120 °C for 8 days without cellulose. Similar leaching result was observed, and the level of sulfonic acid groups decreased from 0.74 to 0.25 mmol/g. The loss in catalytic activity caused by the leaching of sulfonic acid in water is a common issue of sulfonic acid-bearing solid acids, especially when used at elevated temperature [19–22].
To enhance the hydrolysis, Avicel was ball-milled. It was expected that the milling would reduce the crystallinity and particle size of Avicel and thereby improve the accessibility of Avicel to solid acids. The results are presented in Fig. 6. As expected, all tested solid acids achieved significantly higher glucose yields at 24 h from the ball-milled Avicel, compared to those from unmilled one. In particular, the solid acids 2 and 5 were able to hydrolyze the ball-milled Avicel by more than 80 % with 24 h. The improved hydrolysis yield of the ball-milled cellulose was attributed to the disruption of crystalline stucture of Avicel cellulose as well as substantial reduction of particle size. The ball milling improved the performance of dilute sulfuric acid as well to a 55.9 % glucose yield at 24 h. However, the improvement of Amberlyst 15 performance by the ball milling was not as significant as that of the celluase-mimetic solid acids. It hydrolyzed the ball-milled Avicel only by 19.5 % at 24 h.
Conclusions
A two-step strategy (Friedel-Crafts polymerization followed by sulfonation) was successfully developed to synthesize bifunctional porous polymers bearing sulfonic acid and chloride as cellulase-mimetic solid acids for cellulose hydrolysis. The results indicated that the introduction of cellulose-binding group chloride made the synthesized solid acids perform more effectively in hydrolyzing Avicel than non-cellulase-mimetic solid acid, such as Amberlyst 15. The results suggested that the synergetic effect of chloride as cellulose-binding group, sulfonic acid as cellulose-hydrolytic group, and porous structure of the solid acids was responsible for the performance of the synthesized cellulase-mimetic solid acids, but further investigation is necessary to verify the correlation and identify relative significance of the factors. The polymeric solid acids experienced significant leaching of sulfonic acid during cellulose hydrolysis and therefore resulted in significant loss in catalytic activity.
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This work was partially supported by NSF CAREER Award (CBET-0847049) to Dr. Pan.
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Yang, Q., Pan, X. Synthesis and Application of Bifunctional Porous Polymers Bearing Chloride and Sulfonic Acid as Cellulase-Mimetic Solid Acids for Cellulose Hydrolysis. Bioenerg. Res. 9, 578–586 (2016). https://doi.org/10.1007/s12155-015-9702-2
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DOI: https://doi.org/10.1007/s12155-015-9702-2