Hydrothermal CO2 reduction using biomass derivatives as reductants

Title: Hydrothermal CO2 reduction using biomass derivatives as reductants Autors: M. Andérez-Fernándeza, E. Pérezb, A. Martína, M.D. Bermejo *a Affiliations: aHigh Pressure Process Group, Department of Chemical Engineering and Environmental Technology, University of Valladolid (Spain) bTERMOCAL Research Group, Thermodynamics and Calibration, University of Valladolid (Spain) Hydrothermal CO2 reduction using biomass derivatives as reductants M. Andérez-Fernándeza, E. Pérez, A. Martína, M.D. Bermejo *a aHigh Pressure Process Group, Department of Chemical Engineering and Environmental Technology, University of Valladolid (Spain) bTERMOCALResearch Group, Thermodynamics and Calibration, University of Valladolid (Spain) *Email: [email protected]a.es Abstract A wide range of organic substances, potentially derived from biomass, were tested as reductants for CO2 (added in form of sodium bicarbonate) in hydrothermal media. The reactions were carried out in batch reactors at 300 ºC and 3h. All the substances reduced CO2 to formic acid in yields up to 6065%. For ethanol and ethylenglycol, additional conditions were tested to study the dependence of the reaction with time and temperature. These results agree to the mechanisms proposed in literature that suggested that reduction is carried out by a primary or a secondary alcohol. However, some substances not containing these groups gave significant yields to formic acid so new mechanisms were proposed to explain them. Out of all the compounds tested, glucose gave the highest yield to formic acid, probably due to its particular reaction pathways at the studied conditions. Keywords: NaHCO3; glucose; HTW; formic acid; hydrogenation 1. Introduction Carbon dioxide is one of the most concerning greenhouse gasses because of its increasing atmospheric level due to anthropogenic activity, e.g. fossil combustion and industrial processes. Different methods have been proposed to diminish the atmospheric CO2 by capturing at its source and storing it. As this technology develops, a great availability of CO2 is expected, then new opportunities arise to valorise it by transforming it into useful chemicals. This attractive approach is highly desirable since, apart from the economic benefits, it would bring a positive environmental impact because of the reincorporation of CO2 to the carbon cycle [1, 2]. Among the different processes for its conversion into chemicals, photochemical reduction, electrochemical reduction or hydrogenation of CO2 are the most promising strategies [3]. However, CO2 owns a great thermodynamic stability, which requires for its transformation high external energy substances, e. g. hydrogen, unsaturated compounds, organometallic compounds and small-membered ring compounds [4]. Another option for overcoming the high stability of CO2 is using reaction in hydrothermal media [4-7] i.e. using liquid water as solvent at high temperatures.. Another advantage of hydrothermal reduction of CO2 in hydrothermal media is avoiding the use of gaseous H2 as reductant. H2, is still a derivative of fossil fuels so the environmental benefit would be lost. Moreover, H2 utilization comprises safety issues due to its flammability and reactivity. The main product obtained from hydrothermal reduction of CO2 is formic acid [4-7], that is an important chemical feedstock with a wide range of industrial applications such as storing hydrogen and to release it to power fuel cells and obtain energy. [8-10] Industrial formic acid production by CO2 reduction would be an attractive approach to valorise this gas at the same time that its emissions are reduced. Zero-valent metals have proposed as reductants in hydrothermal media [7, 11-14]. However, despite of obtaining acceptable yields, the metals should be reduced again in order to recycle them. Therefore, there is an increasing interest in obtaining an alternative CO2 reductant. Biomass is a world-wide spread, sustainable and inexpensive feedstock and, sometimes, considered as a residue [15]. Due to the depletion of fossil fuel reserves and the environmental problems attached to their consumption, several technologies are being developed in order to obtain value-added chemicals and biofuels [15-17]. Biomass is mainly composed of cellulose, hemicellulose and lignin. These biopolymers can be isolated and depolymerized into its monomeric units such as monosaccharides and phenols. Hydrothermal routes have been intensively studied to process biomass due to the outstanding properties that hot compressed water exhibits such as lower dielectric constant and higher ion product than ambient liquid water [18-21]. Water can act as acidic and basic catalyst as well as environmentally benign solvent. Following this approach, biomass has been converted into a wide range of intermediates and/or valuable products, such as lactic acid, acetic acid, 5-hydrozymethylfurfural (5-HMF), phenol and vanillin, among others [22-27], and many of these reactions involve oxidations. Therefore, the combination of both hydrothermal processes, biomass conversion and CO2 reduction in one-pot reaction, would provide an attractive and sustainable approach for the valorisation of lignocellulosic residues and the decrease of CO2 atmospheric emissions by integrating this process in the main CO2 producers, such as power and industrial factories. Despite the undeniable advantages of this approach, there are not many reports in literature about it. Most of them are focused on reduction of bicarbonate with isopropanol and glycerol [4, 6, 28, 29]. According to these reports, primary or secondary -OH groups acts as CO2reductants and are oxidised to the corresponding aldehydes or ketones. However, additional mechanisms are present because the oxidation of glycerine yields lactic acid. Jin et al also demonstrated that glucose can act as reducing agent to convert bicarbonate as source of CO2 into formic acid [30] but the mechanisms could not be resolved. Su et al. [31] tested some other hydroxylic compounds at 240 ºC with heterogeneous Pd-based catalysts. They also obtained that along with CO2 reduction, primary alcohols are oxidised to the corresponding carboxylic acid, secondary alcohols to the ketone, tertiary alcohols did not react and polyols yielded lactic acid. However, there are still a lot of model compounds derived from lignocellulosic biomass that have not been tested. In this work, the ability of diverse compounds to reduce sodium bicarbonate (NaHCO3) as source of CO2 in hydrothermal medium is investigated. Particularly monosaccharides, disaccharides and lignin-derived phenols, but simple C2 and C3 alcohols and ketones are also tried to better understand the reactions pathways underlying. The main aim is to identify a potential compounds that makes possible to integrate both CO2 reduction and biomass conversion by hydrothermal processes. 2. Materials and methods 2.1. Materials NaHCO3 (100%) was acquired from COFARCAS (Spain). Glycerol (99.5%), npropanol (>99.7%), glyceraldehyde (90%), lactic acid (≥85%),pyruvaldehyde (40%), 5HMF (99%), furfural (99%), fructose (99%), D-(+)-glucose (100%),D-(+)-cellobiose (≥98%), resorcinol (99%), catechol (99%), guaiacol (>99%) and vanillin (99%) were purchased from Sigma Aldrich (Spain). Ethanol (EtOH, 99.5%), acetone (99.5%), isopropanol (iPrOH, 99.9%), sucrose (100%) and sulfuric acid (H2SO4, 96%) were obtained from Panreac (Spain), while propanaldehyde (99%) and phenol (99%) were acquired from Acros Organics. Ethylenglycol was purchased from Fluka. The reactants were used without further treatment of purification. 2.2. Experimental procedure Solutions in ultrapure (MilliQ) water of each organic compound were prepared with a concentration of 0.05M and a molar ratio of organic/NaHCO3 equal to 1:10 (NaHCO3 concentration equal to 0.50M).Experiments were carried out in batch reactors (length: 12 cm; o.d.: ½”, with 1 mm of thickness) made of SS 316 stainless steel with an internal volume of 15.6 mL. The NaHCO3 and reductant solutions were loaded in the reactor, filling the 50% of its total volume. The reactor was placed then in an electric oven previously heated to the desired reaction temperature300ºC. Pressure generated should be 85.9 bar taking as reference the properties of pure water.[32] After the required180 min reaction time, the reactor was rapidly quenched in a cold water/ethylene glycol bath and liquid samples were collected. In the cases of EtOH and ethylene glycol, different reaction times (30, 90 and 180 minutes) and reaction temperatures (250ºC and 300ºC) were tested, as well. Reactions were performed at least twice to assure reproducibility. In order to check the correct closure of the reactor and the no existence of leaks, the reactors were weighted before and after the reaction. 2.3. Product analysis After being filtered through a 0.45mm filter, liquid samples were analysed by HPLC (Waters, Alliance separation module e2695) using an Aminex 87H (Bio-Rad) column and two detectors: RI (Waters, 2414 module) and UV (210 nm, Waters, 2998 module). The mobile phase was 5 mM H2SO4 with a flow rate of 0.6 mL/min. The temperatures of the column and the detector were 60ºC and 30ºC, respectively. The yield to formic acid was calculated as shown below: 𝑌 𝐹𝐹 = 𝐶𝐹𝐹,𝑓 𝐶𝑜𝑜𝑜,𝑖 × 100 (1) Where CFA,f is the molar concentration of formic acid obtained at the end of the reaction and Corg,i is the initial molar concentration of the corresponding solution of the organic compound. 3. Results and discussion The potential of reduction of different organic substances derived from lignocellulosic biomass was tested. The reductants were classified in three categories: saccharides; phenolic derivatives, which are model compounds from lignin depolymerization and simple molecules such C2, C3 alcohols and aldehydes that may be obtained from hydrothermal decomposition of the former compounds [18, 21, 26]. The yields to formic acid obtained from the different solutions of organic compounds are shown in Figure 1. Figure 1. Yield to formic acid (%) from different organic compounds. In order to verify that formic acid production was whether due to the CO2 hydrothermal reduction, control reactions at 300ºC for 180 min were carried out without the addition of NaHCO3 but in presence of NaOH, as the former can partially decompose into the latter and this can catalyse formic acid formation from organic matter. In all these tests, NaOH 0.15 M was added. No formic acid or negligible amount was detected. Additional tests were carried out using only NaHCO3 and water at 300ºC during 180 min to check whether formic acid is produced without of an organic reductant. In these tests, again no formic acid was detected. Thus, formic acid was confirmed to be produced due to NaHCO3 reduction in hydrothermal processes at 300ºC. In most of the cases, oxidized by-products obtained from the starting organic molecule were identified. The presence of these molecules gives clues to determine possible reaction mechanisms. Table 1 gathers the main by-products obtained for the different reactions along with proposed reaction pathways. 0 10 20 30 40 50 60 70 80 Yield FA (%) C2 and C3 molecules Saccharides Phenolic derivates The differences observed can then be related to the fact that glucose splits into three C2 molecules (glycolaldehyde) whereas fructose splits into two C3 molecules (glyceraldehyde and pyruvaldehyde) [26, 35-38], or dehydrates to one 5-HMF molecule. It can be noticed that three times the yield obtained for glycolaldehyde is significantly higher than the sum of the yields for glyceraldehyde and pyruvaldehyde (Figure 1). More surprising are the yields to formic acid when using the disaccharides sucrose (glucose – fructose) and cellobiose (glucose – glucose), 60% and 35% respectively, since they readily hydrolyze in HTW to their constituent monosaccharides. The explanation may be a different thermal stability of either sucrose or cellobiose. In the case of saccharides derivatives furfural and 5-HMF, both compounds reached a yield to formic acid of 15%. The main by-product obtained from monosaccharides and disaccharides were acetic and lactic acids, in different proportions. That is in agreement to the mechanisms previously described [26, 35, 40]. However, the similarities in the composition of the by-products do not allow discerning differences in the reaction pathway. 3.3.- Lignin derivatives OCH 3 Guaiacol OH OH Vanillin OCH 3 O Phenolic derivates OH Catechol OH OH Resorcinol OH OH Phenol Figure 5. Molecular structure of phenolic derivatives. When using phenolic lignin model compounds, a wide range of yields to formic acid was obtained. The molecular structures of the different tested compounds are shown in Figure 5. The lowest yield was obtained from phenol, achieving only 2%. Higher yields to formic acid were obtained by using resorcinol and catechol solutions (19% and 9%, respectively). The highest yield to formic acid was 51% when using vanillin solution, followed by guaiacol (24%). Su et al. [31] also reported low reactivity for phenol, which is expectable since the reduction of CO2 is performed by primary or secondary -OH groups [4, 6]. However, the relatively high yields to formic acid achieved by the rest of the aromatic compounds tested are unexpected. Figure 6. Proposed mechanism for the reduction of bicarbonate to formate by catechol. By-products from resorcinol were propanaldehyde and/or acetone and acetic acid whereas that from catechol was lactic acid. These products can only be formed from a ring-opening mechanism. It should also be mentioned that the solutions obtained from aromatic compounds were reddish-brown coloured. In HTW, the aromatic rings can be oxidized to quinone groups, which are highly coloured and tend to undergo ring opening in water at high temperatures [41]. A proposed mechanism of CO2 reduction by catechol is depicted in Figure 6. Oxidation takes place through a cyclic transition state similar to that reported in the literature with the exception that the Hto be transferred is on the hydroxyl group at ortho position. The main by-products from guaiacol are methanol and catechol most likely produced from a hydrolysis of the methoxy group. However, yield to formic acid from guaiacol is higher than from catechol, indicating that methanol has also a reducing role, and it may produce extra amount of formic acid. On the other hand, oxidation of vanillin to vanillic acid can occur in the presence of oxidants. Additionally, at high temperatures (>280 ºC), the latter readily decarboxylates to guaiacol [41-43]. That is the reason why the same by-products are observed when starting from vanillin or guaiacol. However, yields to formic acid are higher than for the former, which suggests again that the oxidation of the carbonyl accompanies the reduction of CO2. That can be possible considering that vanillin can undergo a Cannizzaro disproportionation to vanillic acid and vanillic alcohol [44] and the latter reduces CO2. OH OH OMe Cannizzaro r. Vanillin CH 2 OH OH OMe COOH OH OMe + HCO 3 HCOO Vanillic alcohol Vanillic acid Decarboxylation OH OMe Guaiacol Hydrolysis OH OH Catechol HCO 3 - HCOO - O O Quinone Ring opening +MeOH HCO 3 - HCOO - CH 2 O Figure 7. Proposed sequential mechanism of HCO3reduction with vanillin, guaiacol and catechol. Figure 7 shows a proposed sequential mechanism of oxidation reaction. Each of these oxidations can be accompanied of a HCOOreduction, which explains that the YFA also follow a decreasing trend. Moreover, detected intermediate compounds (methanol and cathecol) agree with the proposed pathway. Código de campo cambiado 3.4.- Dependence of the reaction time and temperature The dependence of formic acid yield with reaction time and temperature for ethanol and ethylene glycol is shown in Figure 8. The evolution of the reaction is positive with time for every condition tested. At 300 ºC the reaction takes place mainly during the first 90 minutes while the rate decreases significantly at longer times. Temperature plays a more determinant role in the reaction rate as yields are much lower at 250 ºC. This behaviour has been observed in previous reports for CO2 reduction by glycerol [4] and isopropanol [6] indicating they proceed through similar reaction mechanisms. Reactions of these compounds at higher reaction temperature has not been tested since increasing the reaction temperature further than 300ºC would not provide better yields to formic acid. Previous reports showed the degradation of formic acid at higher temperature than 300ºC at short reaction times (<1m) [39] and, therefore, higher temperature would lead to a faster disappearance of it in the medium. Figure 8. Dependence of formic acid yield with time and temperature for the reduction of NaHCO3 using ethanol and ethylene glycol as reductants. (  ): Ethylene glycol, 300 ºC; (  ) Ethylene glycol, 250 ºC; (  ): Ethanol, 300 ºC; (  ): Ethanol, 250 ºC. 3.5.- Influence of NaOH concentration 4. Conclusions In this work, the hydrothermal reduction of NaHCO3 to formic acid was achieved by the conversion of different lignocellulosic biomass model compounds. Yields to formic acid up to 65% were obtained by using the different organic solutions as reductants after 180 min at 300ºC, being the yield to formic acid from glucose the highest yield achieved. Although the detailed reaction mechanism is still unknown and the operational parameters should be optimized, this study demonstrates the possibility to combine the CO2 hydrothermal reduction with the lignocellulosic biomass conversion to obtain value-added chemicals and energy from renewable sources. This work should provide interesting information to further integrate the CO2 reduction in biomass hydrothermal conversion at industrial scale, where glucose could be used as reductant due to the high yield to formic acid achieved and that can be selectively obtained from cellulose hydrolysis. Acknowledgments This project has been funded by MINECO through project ENE2014-53459-R. MAF acknowledges JCYL the predoctoral grant. EPV thanks JCyL for the postdoctoral fellowship. MDB thanks MINECO for Ramon y Cajal position. References [1] M. He, Y. Sun, B. Han, Green carbon science: scientific basis for integrating carbon resource processing, utilization, and reclycling, Angew. Chem. Int. Ed. 52 (2013) 2-16. [2] T. Sakakura, J.C. Choi, H. Yasuda, Transformation of carbon dioxide, Chem. Rev. 107 (2007) 2365-2387. [3] M. Mikkelsen, M. Jorgensen, F. C. Krebs, The teraton challenge. Areview of fixation and transformation of carbon dioxide, Energy Environ. Sci. 3 (2010) 43-81. [4] Z. Shen, Y. Zhang, F. Jin, The alcohol-mediated reduction of CO2 and NaHCO3 into formate: a hydrogen transfer reduction of NaHCO3 with glycerine under alkaline hydrothermal conditions, RSC Adv. 2 (2012) 797-801. [5] B. Wu, Y. Gao, F. Jin, J. Cao, Y. Du, Y. Zhang, Catalytic conversion of NaHCO3 into formic acid in mild hydrothermal conditions for CO2 utilization, Catalysis Today 148 (2009) 405-410. [6] Z. Shen, Y. Zhang, F. Jin, From NaHCO3 into formate and isopropanol into acetone: Hydrogen-transfer reduction of NaHCO3 with isopropanol in high-temperature water, Green Chem. 13 (2011) 820-823. [7] F. Jin, X. Zeng, J. Liu, L. Wang, H. Zhang, G. Yao, Z. Huo, Highly efficient and autocatalytic H2O dissociation for CO2 reduction into formic acid with zinc, Scientific Report 4 (2014) article number 4503. [8] A. Boddien, F. Gärtner, C. Federsel, P. Sponholz, D. Mellmann, R. Jackstell, H. Junge,M. Beller, CO2-"neutral" hydrogen storage based on bicarbonate and formates, Angew. Chem. Int. Ed. 50 (2011) 6411-6414. [9] S. Enthaler, J. Langermann, T. Schmidt, Carbon dioxide and formic acid - the couple for environmental-friendly hydrogen storage, Energy Environ. Sci. 3(2010) 1207-1217. [10] C. Federsel,R. Jackstell, M. Beller, State-of-art catalyst for hydrogenation of carbon dioxide, Angew. Chem. Int. Ed. 49 (2010) 6254-6257. [11] M. E. Berndt, D. Allen, W. E. Seyfried, Reduction of CO2 during sepertinization of olivine at 300ºC and 500 bar, Geology 24 (1996) 351-354. [12] H. Takahashi, T. Kori, T. Onoki, K. Tohji, N. Yamasaki, Hydrothermal processing of metal base compounds and carbon dioxide for the synthesis of organic compounds, J. Mater. Sci. 43 (2008) 2487-2491. [13] Y. Chen, Z. Jing, J. Miao, Y. Zhang, J. Fan, Reduction of CO2 with water splitting hydrogen under subcritical and supercritical hydrothermal conditions, International Journal of Hydrogen Energy 41 (2016) 9123-9127. [14] F. Jin, Y. Gao, Y. Jin, Y. Zhang, J. Cao, Z. Wei, R. L. Smith Jr., High-yield reduction of carbon dioxide into formic acid by zero-valent metal/metal oxide redox cycles, Energy Environ. Sci. 4 (2011), 881-884 [15] C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon, M. Poliakoff, Valorization of biomass: deriving more value from waste, Science 337 (2012) 695-699. [16] M. Stöcker, Biofuels and biomass-to-liquid fuels in the biorefinery: catalytic conversion of lignocellulosic biomass using porous materials, Angew. Chem. Int. Ed. 47 (2008) 9200-9211. [17] B. Kamm, Production of platform chemicals and synthesis gas from biomass, Angew. Chem. Int. Ed. 46 (2007) 5056-5058. [18] P. E. Savage, N. A. Rebacz, Water under extreme conditions for green chemistry, Handbook of Green Chemistry 5:11 (2010) 331-361. [19] N. Akiya, P. E. Savage, Roles of water for chemical reactions in high-temperature water, Chem. Rev. 102 (2002) 2725-2750. [20] A. Kruse, E. Dinjus, Hot compressed water as reaction medium and reactant. Properties and synthesis reactions, J. Supercrit. Fluids 39 (2007) 362-380. [21] D. A. Cantero, M. D. Bermejo, M. J. Cocero, Reaction engineering for process intensification of supercritical water biomass refining, J. Supercrit. Fluids 96 (2015) 2135. [22] P. C. Veggi, R. N. Cavalcanti, M. A. A. Miereles, Production of phenolic-rich extracts from Brazilian plants using supercritical and subcritical fluid extraction: experimental data and economic evaluation, J. of Food Engineering 131 (2014) 96-109. [23] J. M. Prado, L. A. Follegatti-Romero, T. Forster-Carneiro, M. A. Rostagno, F. Maugeri Filho, M. A. A. Miereles, Hydrolysis of sugarcane bagasse in subcritical water, J. Supercrit. Fluids 86 (2014) 15-22. [24] F. P. Cardenas-Tore, T. Forster-Carneiro, M. A. Rostagno, A. J. Petenate, F. Mauregi Filho, M. A. A. Miereles, Integrated supercritical fluid extraction and subcritical water hydrolysis for the recovery of bioactive compounds from pressed palm fiber, J. Supercrit. Fluids 93 (2014) 42-48. [25] F. Jin, X. Zeng, Z. Jing, H. Enomoto, A potentially useful technology by mimicking nature-rapid conversion of biomass and CO2 into chemical and fuels under hydrothermal conditions, Ind. Eng. Chem. Res. 51 (2012) 9921-9937. [26] D. A. Cantero, A. Álvarez, M. D. Bermejo, M. J. Cocero, Transformation of glucose into added value compounds in a hydrothermal reaction media, J. Supercrit. Fluids 98, 2015, 204-210. [27] S. Kang, X. Li, J. Fan, J. Chang, Hydrothermal conversion of lignin: a review, Renew. Sust. Energ. Rev. 27 (2013) 546-558. [28] Z. Shen, M. Gu, M. Zhang, W. Sang, X. Zhou, Y. Zhang, F. Jin, The mechanism for production of abiogenic formate from CO2 and lactate from glycerine: uncatalyzed transfer hydrogenation of CO2 with glycerine under alkaline hydrothermal conditions, RSC Adv. 4 (2014) 15256-15263. [29] Y. Wang, F. Wang, C. Li, F. Jin, Kinetics and mechanism of reduction of CO2 by glycerol under alkaline hydrothermal conditions, Int. J. Hydrogen Energy 46 (2016) 9128-9134. [30] F. Jin, Z. Huo, X. Zeng, H. Enomoto, Hydrothermal conversion of CO2 into valueadded products: a potential technology from improving global carbon clycle, ACS Advances in CO2 conversion and utilization 4 (2010), 31-53. [31] J. Su, L. Yang, X. Yang, M. Lu, B. Luo, H. Lin, Simultaneously converting carbonate/bicarbonate and biomass to value-added carboxylic acid salts by aqueousphase hydrogen transfer, ACS Sustainable Chem. Eng. 3 (2015) 195-203. [32] http://webbook.nist.gov/chemistry/ [33] H. Kishida, F. Jin, X. Yan, T. Moriyac, H. Enomoto, Formation of lactic acid from glycolaldehyde by alkaline hydrothermal reaction, Carbohyd. Res. 341 (2006) 26192623. [34] F. Jin, Z. Zhou, H. Kishida, H. Higashijima, H. Enomoto, Controlling hydrothermal reaction pathways to improve acetic acid production from carbohydrate biomass, Environ. Sci, Technol. 39 (2005) 1893-1902. [35] D. A. Cantero, A. Sánchez Tapia, M. D. Bermejo, M. J. Cocero, Pressure and temperature effect on cellulose hydrolysis in pressurized water, Chem. Eng. J. 276 (2015), 145-154. [36] T. M. Aida, K. Tajima, M. Watanabe, Y. Saito, K. Kuroda, T. Nonaka, H. Hattori, R. L. Smith Jr, K. Arai, Reactions of D-fructose in water at temperatures up to 400ºC and pressures up to 100 MPa, J. Supercrit. Fluids 42 (2007) 110-119. [37] M. Sasaki, K. Goto, K. Tajima, T. Adschiri, K. Arai, Rapid and selective retroaldol condensation of glucose to glycolaldehyde in supercritical water, Green Chem. 4 (2002) 285-287. [38] D. A. Cantero, L. Vaquerizo, C. Martínez, M. D. Bermejo, M. J. Cocero, Selective transformation of fructose and high fructose content biomass into lactic acid in supercritical water, Catal. Today 255 (2015) 80-86. [39] J.L. Yu, P.E. Savage. Decomposition of formic acid under hydrothermal conditions. Ind Eng Chem Res. 37 (1998) 2-10 [40]F. Jin, Z. Zhou, A. Kishita, H. Enomoto, Hydrothermal conversion of biomass into acetic acid, J. Mater. Sci. 41 (2006) 1495-1500. [410] E. Perez, J. Fraga-Dubreuil, E. García-Verdugo, P. A. Hamley, B. Thomas, D. Housley, W. Partenheimer, M. Poliakoff, Selective aerobic oxidation of para-xylene in suband supercritical water. Part 1. Comparison with ortho-xylene and the role of the catalyst, Green Chem. 13 (2011) 2389-2396. [4142] E. Perez, A. Davis, C. O. Tuck, M. Poliakoff, Quantitative analysis of products from lignin depolymerisation in high-temperature water, Bioresource. Technol. (submitted). [4243] G. González, J. Salvadó, D. Montané, Reactions of vanillic acid in suband supercritical water, J. Supercrit. Fluids 31 (2004) 57-66. [4344] I. A. Pearl, Reactions of vanillin and its derived compounds III. The Cannizzaro Reaction of vanillin, The Journal of organic chemistry 10 (1947) 79-48.