Direct Transformation of Cellulose to Gluconic Acid in a Concentrated

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Direct transformation of cellulose to gluconic acid in concentrated iron (III) chloride solution under mild conditions Hongdan Zhang, Ning Li, Xuejun Pan, Shubin Wu, and Jun Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00060 • Publication Date (Web): 05 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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ACS Sustainable Chemistry & Engineering

Direct transformation of cellulose to gluconic acid in concentrated iron (III) chloride solution under mild conditions

Hongdan Zhang,a,b,c Ning Li,b Xuejun Pan,b,* Shubin Wu,c and Jun Xiea

a

College of Materials and Energy, Key Laboratory of Energy Plants Resource and Utilization,

Ministry of Agriculture, Key Laboratory of Biomass Energy of Guangdong Regular Higher Education Institutions, South China Agricultural University, 483 Wushan Road, Guangzhou 510642, P.R. China b

Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry

Mall, Madison, WI 53706, USA c

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,

381 Wushan Road, Guangzhou 510640, P.R. China * Corresponding author: E-mail: [email protected]; Tel: +1 (608) 262-4951

ABSTRACT: A simple method was demonstrated for directly converting cellulose to gluconic acid in concentrated iron (III) chloride (FeCl3) solution without additional catalyst. It was found that the conversion of cellulose to gluconic acid in FeCl3 is a two-step process, fast cellulose hydrolysis to glucose followed by slow glucose oxidation to gluconic acid. It was confirmed that high-concentration FeCl3 (60%) is required to efficiently dissolve and subsequently hydrolyze cellulose. The sequential combination of 60% and 40% FeCl3 in two steps was optimal for converting cellulose to gluconic acid, and 40% FeCl3 in the second step could minimize the oxidative decomposition of gluconic acid. The results indicated that the maximum gluconic acid

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yield (50%) was achieved under the conditions of 10-minute hydrolysis in 60% FeCl3 followed by 110-min oxidation in 40% FeCl3 at 120 °C. The system presented in this study provided a new approach to produce gluconic acid directly from cellulose.

KEYWORDS: Cellulose, Iron (III) chloride, Hydrolysis, Oxidation, Gluconic acid

INTRODUCTION Lignocellulosic biomass is the most abundant and a renewable resource on earth, and its transformation into liquid fuels and chemicals that are currently from petroleum oil has attracted increasing attention.1,2 Cellulose, a polymer of glucose, counts for 35-45% of the biomass and has been the primary starting material for paper and fiber, cellulose derivatives, chemicals, and biofuels.3-7 Gluconic acid is an important platform chemical, which has a wide range of applications in pharmaceutical and food industries.8-11 Currently, gluconic acid is produced predominantly through fermentation of glucose by fungi A. niger in industry,11,12 although bacteria, yeast, and immobilized enzymes have been explored.11,1315

The A. niger fermentation is very efficient and selective process. For example, a recent

study by Lu et al. indicated that gluconic acid was produced by A. niger fermentation with a 96.5% theoretical yield.16 However, the major disadvantages of the fermentation process are long reaction time (15-24 h), restricted conditions (pH 6.0-6.5, 34 °C), and high operation cost (4 bar air pressure and stirring). In response to these issues of the fermentation processes, chemical methods have been explored recently for producing gluconic acid from glucose. For example, glucose was oxidized to gluconic acid in

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aqueous media by O2 or air with Au-based catalysts.17,18 In other studies, cellobiose and cellulose were used as feedstock for gluconic acid,19-23 but multi-functional supported gold catalysts were required. The supported noble-metal catalysts used in these studies performed well and gave good yield of gluconic acid (80-90%), but high cost and stability were the major issues. In addition, cellulose and cellobiose had to be hydrolyzed to glucose by acid catalysts before they could be oxidized to gluconic acid by oxidation catalysts. To our knowledge, no study has been reported to directly convert cellulose in a homogenous medium to gluconic acid. In our previous study, glucose was successfully oxidized into gluconic acid with high yield (~52%) by FeCl3 alone in water without any additional catalyst under mild condition (110 °C).24 As a follow-up, the aim of this study was to explore the possibility of directly converting cellulose to gluconic acid in concentrated FeCl3 solution without prior hydrolysis of cellulose to glucose, which would skip the energy- and cost-intensive step of cellulose hydrolysis to glucose and simplify the conversion of cellulose to gluconic acid. Our hypothesis was that since concentrated FeCl3 solution has both strong acidity and oxidizing power, the acidity could be used to hydrolyze cellulose into glucose, and the oxidizing power would oxidize the resultant glucose into gluconic acid. In the present study, cellobiose was first used as a model compound of cellulose to elucidate the conversion pathway of cellulose to gluconic acid and optimize process conditions, and then cellulose was directly used as feedstock to produce gluconic acid in the same system. The system was optimized for converting cellobiose and cellulose to gluconic acids under varying conditions, including FeCl3 concentrations (40 and 60%, w/w), temperatures

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(100-120 °C), and reaction times (0.5-5 h). The reaction pathway of cellulose to gluconic acid in concentrated FeCl3 solution was proposed.

MATERIALS AND METHODS Materials. Cellobiose (98%) and cellulose (Avicel PH-101) were purchased from Alfa Aesar (Ward Hill, MA) and Sigma-Aldrich Fluka (St. Louis, MO), respectively. D-Glucose (> 99%) was from Acros Organics (Morris Plains, NJ). Iron (III) chloride (98%) was from Alfa Aesar (Ward Hill, MA). Sodium gluconate (> 99%) was bought from TCI America (Portland, OR). Formic acid (99.9%) and acetic acid (99.9%) were purchased from Fisher Scientific (Pittsburgh, PA). All the chemicals were used as received. Reaction procedure. The conversion of cellobiose or cellulose to gluconic acid was carried out in a 40-mL glass vial with a magnetic stirrer. In brief, cellobiose or cellulose (0.25 g) and a certain volume of FeCl3 solution were loaded into the vial and heated up in a temperaturecontrolled oil bath. In case of two-step reaction, the reaction was first conducted in 60% (w/w) FeCl3 solution at a preset temperature for a preset period of time, and then the system was diluted to 40% (w/w) by adding water for the second step of the reaction. All the reactions were conducted under air and autogenous pressure. Since the boiling points of the FeCl3 solutions are higher than the reaction temperatures used in this study, there was no pressure buildup. This was why the reactions were conducted in glass vials. At the end of the reaction, the vial was removed from the oil bath and immediately put in an ice-water bath to stop the reaction. The mixture in the vial was filtered to remove solid residues. The filtrate was analyzed using High Performance Ion Chromatography (HPIC) and High Performance Liquid Chromatography (HPLC) to quantitate sugars (glucose and cellobiose) and products (gluconic, formic, and acetic acids),

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respectively, as described below. The solid residues (humins) were collected, washed with deionized water, and oven-dried at 105 °C for gravimetric quantitation. Analytical methods. Cellobiose and glucose were quantitated using a HPIC system (Dionex ICS-3000) with an integrated amperometric detector and a CarboPac PA1 column at 30 °C eluted with water/100 mM NaOH gradient at a flow rate of 0.7 mL/min, as described previously.25 Gluconic, formic, and acetic acids were analyzed using a HPLC system equipped with a Supelcogel C-610H column at 20 °C and a UV detector at 210 nm. Phosphoric acid (0.1%) was used as eluent at a flow rate of 0.6 mL/min.24 The analytes were quantitated against the calibration curves created using external standards. The feedstock conversion and product yield were calculated using the following equations 1 and 2. Moles of carbon in feedstock consumed ×100% Moles of carbon in feedstock input

(1)

Moles of carbon in product (glucose or organic acid) ×100% Moles of carbon in feedstock input

(2)

Feedstock conversion (%) = Product yield (%) =

RESULTS AND DISCUSSION Conversion of cellobiose to gluconic acid in FeCl3 solution. Cellobiose is the simplest and water-soluble model compound of cellulose. It was tested first to see whether it could be directly converted into gluconic acid in the proposed FeCl3 system. It was found in our previous study that 40% (w/w) was an appropriate concentration of FeCl3 solution for the glucose oxidation to gluconic acid,24 giving a glucose conversion of 83.8% and a gluconic acid yield of 48.7%. More diluted (40%) FeCl3 caused undesired oxidative degradation of

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the resultant gluconic acid. Therefore, this study started with 40% FeCl3 for the conversion of cellobiose to gluconic acid. As shown in Table 1, cellobiose was completely converted (consumed) in 40% FeCl3 at 110 °C within 2 h, yielding 39.6% gluconic acid, but the gluconic acid yield from cellobiose was lower than that (48.7%) from glucose under the same conditions.24 For comparison, 60% FeCl3 solution was tested as well. It turned out that 60% FeCl3 gave even lower gluconic acid yield (31.7%) and more humins (5.0%) than 40% FeCl3 under the same conditions (time and temperature). This was probably attributed to the strong acidity of the 60% FeCl3 solution that led to excessive degradation of glucose (dehydration to hydroxymethylfurfural (HMF)) and subsequent formation of humins. This was supported by the observations of low glucose yield (1.7%) and more humins (5.0 %) in the 60% FeCl3 system. Interestingly, when cellobiose was treated in 60% FeCl3 for 10 min and then in 40% FeCl3 for 110 min (total reaction time 2 h) at the same temperature (110 °C), sequentially, the gluconic acid yield was substantially elevated to 47.0%. This observation suggested that the conversion of cellobiose to gluconic acid seemed to be a two-step process, the hydrolysis of cellobiose to glucose followed by the oxidation of glucose to gluconic acid. Since 60% FeCl3 has stronger acidity and oxidizing power than 40% FeCl3 solution, it makes sense that 60% FeCl3 in the first step enhanced the hydrolysis of cellobiose to glucose, while 40% FeCl3 in the second step oxidized glucose to gluconic acid but avoided or reduced the excessive oxidative degradation of gluconic acid that 60% FeCl3 would cause. The results above are consistent with the two-step hypothesis that cellobiose was hydrolyzed to glucose first before being oxidized to gluconic acid. The results above indicated that the two-step reactions sequentially in 60% (for 10 min) and 40% (for 110 min) FeCl3 solutions could significantly improve the conversion yield of cellobiose

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to gluconic acid, compared to the single-step reaction in 40% or 60% FeCl3 alone. However, it was not clear how the hydrolysis time in 60% FeCl3 and the oxidation time in 40% FeCl3 affected the conversion of cellobiose to gluconic acid. Hence, varying hydrolysis times (5-30 min) in 60% FeCl3 were tested, while the total reaction time for the two steps in 60% and 40% FeCl3, respectively, was kept the same (120 min). As presented in Table 2, when the hydrolysis of cellobiose in the 60% FeCl3 solution was extended from 5 to 30 min, the gluconic acid yield first increased and then decreased, reaching the maximum yield of 47.0% at 10 min. These results suggested that 5-min hydrolysis in 60% FeCl3 seemed not enough for cellobiose to be hydrolyzed to glucose, but extending reaction (>10 min) resulted in undesired decomposition of glucose and consequent formation of humins (Table 2). Since the hydrolysis of cellobiose in 60% FeCl3 was a fast reaction, 10 min seemed to be sufficient for complete hydrolysis of cellobiose, which avoided the side reactions leading to undesirable glucose decomposition, caused by overlong reaction. It was observed that both temperature and reaction time were crucial variables in the conversion of cellobiose to gluconic acid. Elevating temperature could accelerate the conversion but inevitably promote the undesired side reactions as well. To understand the effects of temperature and time, the conversion of cellobiose in the two-step process (hydrolysis in 60% FeCl3 for 10 min and then oxidation in 40% FeCl3 for varying time) at the temperatures of 100, 110, and 120 °C was investigated, and the results are presented in Figs. 1A, 1B, and 1C, respectively. As shown in Fig. 1A, the reaction was slow at 100 °C. After 30 min (10 min in 60% FeCl3 and 20 min in 40% FeCl3), cellobiose was almost completely consumed/converted (99.4%, not shown in the figure), but 68.6% glucose was still retained in the system, and gluconic acid yield was negligible, indicating that the hydrolysis of cellobiose to glucose was fast, and the

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oxidation of glucose to gluconic acid was slow at 100 °C. As the reaction advanced to 5 h, glucose decreased gradually to 17.6%, and the yield of gluconic acid increased to 37.4%. These observations suggested that the cellobiose-to-gluconic acid conversion was indeed a two-step process in which glucose was the intermediate, and the hydrolysis of cellobiose to glucose was faster than the oxidation of glucose to gluconic acid. In other words, the oxidation of glucose to gluconic acid was the rate-control step. The formic acid yield increased with reaction time and reached 11.3% at 5 h due to the progressively oxidative degradation of gluconic acid.21 Similar to the observation in our previous study on glucose oxidation to gluconic acid,24 acetic acid was produced from the oxidation of cellobiose as well. Acetic acid yield reached 15.6% at 3 h and then dropped to 8.2% at 5 h. This was caused by the formation of Fe(OAc)2 that was insoluble in concentrated FeCl3 solution, which thereby reduced the concentration of acetic acid in the system, as observed and discussed previously.24 The formation of humins at 100 °C was insignificant because of the mild condition. It should be indicated that in addition to the major products (gluconic, acetic, and formic acids), small quantities of minor products including glyceric acid, glycolic acid, succinic acid, oxalic acid, lactic acid, and HMF among with some unidentified (but detected by HPLC) products were generated from cellobiose and cellulose. Because of their low-yield nature, they were not reported and discussed in detail in this study. When temperature was elevated to 110 °C (Fig. 1B), cellobiose was almost instantaneously hydrolyzed to glucose, and the resultant glucose was rapidly oxidized to gluconic acid. After 2-h reaction, about 47.0% of gluconic acid was yielded, and only 10.9% glucose was retained in the system, indicating that elevating temperature significantly accelerated both the hydrolysis and oxidation reactions. The highest yield of gluconic acid (48.6%) was observed at 4 h. Further extending the reaction to 5 h did not improve but hurt the yield due to the oxidative degradation

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of gluconic acid. The formation of formic acid and acetic acid from the oxidative decomposition of gluconic acid showed similar trends with those at 100 °C above (Fig. 1A), but more formic acid was formed, and acetic acid reached the maximum yield earlier. More humins were formed at elevated temperature. Further elevating temperature to 120 °C accelerated all the reactions (Fig. 1C). For example, only 28.4% glucose was retained in the system, and 26.1% gluconic acid was produced in the first 0.5 h, compared to those (68.6% and ~1%, respectively) at 100 °C (Fig. 1A). At 1 h, residual glucose dropped sharply to 6.7%, and the gluconic acid yield reached the maximum of 55.9%. The gluconic acid yield then decreased gradually to 42.9% when the reaction was extended to 5 h due to the oxidative decomposition of gluconic acid. The highest acetic acid yield (32.2%) was observed at 0.5 h, suggesting that higher reaction temperature favored the formation of acetic acid. The formic acid yield reached about 15.0% quickly. More humins were formed at the elevated reaction temperature. In summary, the results in Fig. 1 (A, B, and C) suggested that reaction temperature and time were two crucial factors affecting the conversion of cellobiose to gluconic acid and product selectivity, and high temperature and short reaction time favored the yield and selectivity of gluconic acid. The maximum yield of gluconic acid (55.9%) was observed at 120 °C and 1 h (10 min for hydrolysis in 60% FeCl3 and 50 min for oxidation in 40% FeCl3). High temperature accelerated both the hydrolysis of cellobiose to glucose and the oxidation of glucose to gluconic acid, but extending reaction at high temperature would cause excessive oxidative decomposition of gluconic acid, leading to side products. Direct transformation of cellulose to gluconic acid in FeCl3 solution. The results above clearly indicated that cellobiose as a model compound of cellulose could be efficiently converted

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to gluconic acid in FeCl3 solution via the two-step pathway of the cellobiose hydrolysis to glucose followed by the oxidation of glucose to gluconic acid. In the following part of this study, direct conversion of cellulose to gluconic acid in the FeCl3 solution was investigated. Different from cellobiose, cellulose was insoluble in 40% FeCl3. A large amount of cellulose was not converted and even not dissolved after 2-h reaction at 110 °C in 40% FeCl3. As shown in Table 1, only small portion of the cellulose was hydrolyzed into glucose, and the yields of gluconic acid and other organic acids were negligible. However, when the concentration of FeCl3 was increased to 60%, cellulose was readily dissolved and extensively converted at 110 °C, and gluconic acid yield increased to 33.3%. In addition, a significant amount of formic and acetic acids were produced as well, and a small amount of humins was formed. The solution became from dark brown of Fe (III) to the mixed dark brown and dark green Fe (II), indicating part of Fe (III) was reduced to Fe (II) when glucose was oxidized into gluconic acid and other products. These results suggested that high concentration of FeCl3 solution (60%) was needed to dissolve cellulose before being further hydrolyzed and oxidized to gluconic acid. It was believed that high-concentration Fe3+ and Cl- ions in the system, similar to those in a molten salt hydrate,26 were able to associate with the hydroxyl groups of cellulose and thereby interrupt the inter- and intra-cellulose hydrogen bonds, which therefore led to the swelling and dissolution of cellulose. The cellulose dissolution doubtlessly facilitated the cellulose hydrolysis to glucose. Similar to the results of cellobiose above, combination of 60% and 40% FeCl3 solutions favored the conversion of cellulose to gluconic acid. For example, sequential reactions in 60% (10 min) and 40% (110 min) FeCl3 at 110 °C yielded 44.4% gluconic acid (Table 1), which was much higher than that (33.3%) in 60% FeCl3 alone. As discussed above, 60% FeCl3 solution facilitated the dissolution and hydrolysis of cellulose, and 40% FeCl3 solution favored the

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oxidation of glucose to gluconic acid and meanwhile avoided or reduced undesired side reactions such as the excessive oxidative degradation of gluconic acid. To improve the yield of gluconic acid from cellulose, the process was optimized under varying conditions. The effects of reaction temperature (100, 110, and 120 °C), hydrolysis time in 60% FeCl3, and oxidation time in 40% FeCl3 on the conversion of cellulose to gluconic acid are presented in Fig. 1 (D, E, and F) and Table 2, respectively. The results (Table 2) indicated that within the investigated range of hydrolysis time in 60% FeCl3 from 5 to 30 min, 10 min hydrolysis in 60% FeCl3 solution gave the maximum gluconic acid yield of 44.4%, suggesting that 10 min seemed to be sufficient for cellulose dissolution and subsequent hydrolysis to glucose. As shown in Fig. 1D, when temperature was 100 °C, 55.7% of glucose was detected at 0.5 h in the system, which was lower than that observed above with cellobiose (68.6%, Fig. 1A), because the hydrolysis of cellulose to glucose was slower than that of cellobiose. Similar to the result of cellobiose, no gluconic acid was generated from cellulose at 0.5 h and 100 °C. When reaction time was extended to 5 h, glucose in the system was reduced to 14.4%, and the yield of gluconic acid increased to 42.8%. When the reaction temperature was elevated to 110 °C (Fig. 1E), the glucose yield from cellulose hydrolysis was lower than that at 100 °C at the same time point, suggesting that higher temperature enhanced the oxidation of glucose to gluconic acid, and thereby led to a faster reduction of glucose concentration in the system. Gluconic acid yield increased sharply from 2.0% to 44.4%, when the reaction advanced from 0.5 h to 2 h. The highest yield of gluconic acid (48.5%) was achieved at 4 h. When the temperature was further raised to 120 °C (Fig. 1F), only a small amount of glucose could be detected in the system, indicating that glucose was quickly oxidized as soon as it was generated from cellulose

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hydrolysis. Gluconic acid yield reached 50.0% at 2 h and then decreased gradually with time due to the oxidative degradation of gluconic acid. The results above indicated that the gluconic acid yield from cellulose was greatly dependent on reaction temperature and time, and the conversion of cellulose to gluconic acid was slower than that of cellobiose. At 120 °C and 2 h, gluconic acid reached the maximum yield of 50.0%, which was lower than that from cellobiose (55.9%). When the reaction temperature was further elevated and/or the reaction time was extended, gluconic acid yield dropped, because gluconic acid was further decomposed to organic acids. The present study successfully demonstrated that cellulose could be directly transformed to gluconic acid with a good yield (50%) in FeCl3 solution without additional catalyst. Compared with existing methods, the current method is potentially competitive in cost, reaction time, and product yield for gluconic acid production directly from cellulose. For example, the fermentation process using fungi or bacteria had high yield and high selectivity,27-29 but the process could only use glucose as substrate. Cellulose must be hydrolyzed to glucose using cellulase or acid catalysts before it can be fermented to gluconic acid. It is well known that energy- and costefficient hydrolysis of cellulose to glucose by either cellulases or acids has retained as an engineering challenge.30-32 It was reported that cellulose could be directly converted into gluconic acid using heterogeneous catalysts in the presence of O2, but harsh conditions were required. For example, An et al. transformed cellulose to gluconic acid using Au/CsxH3xPW12O40

system as catalyst, yielding 47-60% of gluconic acid after 11-h reaction at 145 °C

under 1.0 MPa O2.8,22 The activity of the catalyst decreased gradually during the repeated uses, because of the leaching of H+ from the catalyst.

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This study focused on demonstrating the direct conversion of cellulose into gluconic acid in FeCl3 solution. The separation of the products and the recycling of the FeCl3 solution were not addressed in the present study. Based on the nature of the products (organic acids) and the reaction medium (FeCl3), chromatographic and membrane technologies could separate the products from reaction medium. For example, chromatographic method has been proved to be an industrially feasible technology to separate salt from the products.33-35 After the separation of the products, the residual solution (a mixture of FeCl3 and FeCl2) can be oxidized (e.g. with oxygen) to convert Fe (II) to Fe (III). The regenerated FeCl3 solution may need concentration (e.g. by evaporation) to reach the required concentration (60%). In addition to cellobiose and cellulose, the conversion of real biomass (polar) in the FeCl3 system was also preliminarily tested (results are not reported here). As expected, due to the structure of the real biomass and co-existence of hemicelluloses and lignin, the dissolution, hydrolysis, and oxidation of poplar were slower than those of cellobiose and cellulose. Because hemicelluloses were simultaneously converted as well, the products were more complicated. It is anticipated that the presence of the lignin would make the product separation and FeCl3 recycling more challenging.

Conversion pathway of cellulose to gluconic acid in FeCl3 solution. Based on the observations and discussion above, the reaction pathways of cellulose to gluconic acid in concentrated FeCl3 solution are summarized below in Fig. 2. Cellulose was first dissolved and subsequently hydrolyzed into glucose in 60% FeCl3 solution because of the cellulose-swelling and dissolving capacity and acidity of the solution. Then, the intermediate glucose was oxidized to gluconic acid in 40% FeCl3 solution. The sequential combination of 60% and 40% FeCl3

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solutions ensured the sufficient and quick dissolution and hydrolysis of cellulose to intermediate glucose in the first step, and meanwhile minimized the excessive oxidative decomposition of product gluconic acid and other side reactions in the second step, because the former has the ability to dissolve cellulose, higher acidity to hydrolyze cellulose, and stronger oxidative power than the latter. Formic and acetic acids were the main co-products. In addition to the major products (gluconic, acetic, and formic acids), small quantities of glyceric acid, glycolic acid, succinic acid, oxalic acid, and lactic acid were observed among with some minor unidentified products. Similar products were reported in the previous studies on glucose oxidation to gluconic acid.19,21,23 Humins formed under severe conditions such as high temperature and extended reaction time due to the dehydration and condensation of the carbohydrates.

CONCLUSIONS In summary, direct oxidative conversion of cellulose to gluconic acid in concentrated FeCl3 solution was successfully demonstrated in this study. It was verified that the conversion of cellulose to gluconic acid in FeCl3 was a two-step process using cellobiose as model compound. Cellulose was first hydrolyzed into glucose, and the glucose was then oxidized into gluconic acid. The hydrolysis was faster than the oxidation. It was confirmed that 60% FeCl3 was essential for cellulose dissolution, and the combination of hydrolysis in 60% FeCl3 followed by oxidation in 40% FeCl3 was an efficient process to convert cellulose to gluconic acid. The maximum cellulose to gluconic acid yield (50.0%) was achieved under the conditions of 10minute hydrolysis in the 60% FeCl3 followed by oxidation in the 40% FeCl3 for 110 min at 120 °C. Formic and acetic acids were identified as major coproducts of the cellulose oxidation in FeCl3 with the yields of 5~10%, dependent on reaction conditions.

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ACKNOWLEDGEMENTS Authors thank China Scholarship Council (CSC) for supporting Hongdan Zhang to conduct this research at University of Wisconsin-Madison. This work was partially supported by the grants from NSF (CBET 1159561) and USDA McIntire Stennis (WIS01597) to Xuejun Pan and by the National Science and Technology Support Program (2015BAD15B03), the National Natural Science Foundation of China (21606091), and State Key Laboratory of Pulp and Paper Engineering (No. 201620).

Notes The authors declare no competing financial interest.

REFERENCES (1) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044-4098. (2) Corma, A.; Iborra, S.; Velty, A. Chemical routes for the transformation of biomass into chemicals. Chem. Rev. 2007, 107, 2411-2502. (3) Niu, M.; Hou, Y.; Ren, S.; Wang, W.; Zheng Q.; Wu, W. The relationship between oxidation and hydrolysis in the conversion of cellulose in NaVO3–H2SO4 aqueous solution with O2, Green Chem. 2015, 17, 335-342. (4) Fan, J. J.; De Bruyn, M.; Budarin, V. L.; Gronnow, M. J.; Shuttleworth, P. S.; Breeden, S.; Macquarrie, D. J.; Clark, J. H. Direct microwave-assisted hydrothermal depolymerization of cellulose. J Am. Chem. Soc. 2013, 135, 11728−11731.

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Table 1. Effect of FeCl3 concentration on the conversion of cellobiose and cellulose to gluconic acid Feedstock

Cellobiose

Cellulose

FeCl3 Concentration % (w/w)

Conversion %

Glucose

40 60 60 + 40a 40 60 60 + 40a

100 100 100 ND 100 100

11.4 1.7 10.9 3.2 4.3 12.2

Product yield (%) Gluconic Formic Acetic acid acid acid 39.6 31.7 47.0 0.5 33.3 44.4

a

12.7 12.8 13.0 1.8 10.7 10.8

11.1 7.6 9.2 0 4.1 6.4

Huminsb 1.4 5.0 3.2 ND 0.3 0.6

10 min in 60% FeCl3 and 110 min in 40% FeCl3, respectively. b mass yield based on starting feedstock. Other conditions: total reaction time 2 h, temperature 110 °C, and feedstock (cellobiose or cellulose) loading 5%. ND: not detected because cellulose was not completely soluble in 40% FeCl3 and the undissolved cellulose was not distinguishable from newly formed solid humins.

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Table 2. Effect of hydrolysis time in 60% FeCl3 on the conversion of cellobiose and cellulose to gluconic acid Feedstock

Cellobiose

Hydrolysis time in 60% FeCl3 (min) 5 10 15 20 30 5 10 15 20 30

Conversion (%)

Glucose

100 100 100 100 100 100 100 100 100 100

7.8 10.9 7.6 9.1 8.7

7.8 12.2

Product yield (%) Gluconic Formic Acetic acid acid acid 39.8 47.0 41.4 42.0 43.1 38.0 44.4 37.6 34.5 34.4

13.2 13.0 13.0 12.2 13.0 13.0 10.8 12.9 11.8 12.3

10.6 9.2 10.8 11.0 9.6 9.4 6.4 8.5 12.8 10.6

Huminsa 2.6 3.2 3.4 3.9 5.3 0.6 0.6 1.4 5.3 6.0

6.2 12.8 10.4 Reaction temperature: 110 °C; total reaction time (hydrolysis in 60% FeCl3 and oxidation in 40% FeCl3): 2 h; feedstock (cellobiose or cellulose) loading: 5%; a mass yield based on starting feedstock. Cellulose

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Fig. 1 Effect of reaction temperature and total reaction time in 60% FeCl3 and 40% FeCl3 on the conversion of cellobiose (A, B, and C) and cellulose (D, E, and F) to gluconic acid at 5% feedstock (cellobiose or cellulose) loading.

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Major pathway Disolution Hydrolysis

OH O O

O OH

OH

n

OH OH OH

60% FeCl3

OH OH

O OH

40% FeCl3

OH

OH

Gluconic acid

Isomerization

Minor pathways O

OH

OH

Glucose

Cellulose

OH

OH

Oxidation

O

OH

O OH

Dehydration

O

O

H

OH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


OH

HMF Condensation

+ OH

OH

Fructose

OH

O OH

O

+

O

Formic acid

OH OH

Succinic acid

Lactic acid

Decomposition O

Humins

OH HO

O

O OH

+

O

OH

Acetic acid

Glyceric acid

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OH OH

Oxalic acid

Fig. 2 Reaction pathways of cellulose to gluconic acid in FeCl3 solution.


O

+

OH OH

Glycolic acid


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Table of Contents (TOC):

Title: Direct transformation of cellulose to gluconic acid in concentrated iron (III) chloride solution under mild conditions Authors: Hongdan Zhang, Ning Li, Xuejun Pan, Shubin Wu, and Jun Xie Synopsis: Cellulose was directly converted to gluconic acid through hydrolysis and oxidation by FeCl3 in water.

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Direct Transformation of Cellulose to Gluconic Acid in a Concentrated

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