Investigation of biomass depolymerization by surface techniques † A. M. Ruppert, a * J. Grams, a M. Chełmicka, a T. Cacciaguerra b and D. Świerczyński b The application of time-of-flight (ToF)-SIMS to cellulose valorization is described. Cellulose samples subjected to ball milling or sulfuric acid impregnation, or combinations thereof, were subjected to hydrolysis. The material, which was impregnated in the last treatment step, no matter whether previously milled or not, exhibited the highest hydrolysis activity because of the highest accessibility of surface sulfonic groups. When milling followed impregnation, the activity was decreased because of possible encapsulation of sulfonic groups in the bulk. The ToF-SIMS analysis revealed that both the ball mill and the stainless steel reactor may be a source of chromium and iron impurities, which can decrease the hydrolysis yield. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: levulinic acid; ToF-SIMS; ball milling; lignocellulose; hydrolysis Introduction The increasing depletion of fossil fuel resources enforces the search for novel ways to obtain energy and chemicals. Among such possibilities, one of the most promising is the valorization of lignocellulosic biomass as the most abundant source of carbon on Earth, which does not compete with food production. [1] The US Department of Energy published a list of 15 target structures of high importance that could be produced from biorefinery carbohydrates, including lignocellulose. [2] It includes hydroxyme- thylfurfural (HMF) and its dehydration products – levulinic and formic acid (LA and FA) – primary biorefinery building blocks which can be applied as ideal platform chemicals for producing a number of bio-chemicals, including succinic acid, resins, poly- mers, herbicides, pharmaceuticals, solvents and biofuels. [3] Specific technical development of bio-based refinery is still in its infancy. According to predictions, however, as soon as this problem is overcome, production of chemicals from biomass could become profitable on industrial scale. [4] Depolymerization of lignocellulose to sugars, the most energy-comsuming step, is a major challenge. Cellulose is a linear polymer of glucose units linked by β-(l,4) glycosidic bonds. Hydrogen bonds within a cellulose microfibril determine the ‘straightness’ of a chain, rendering the structure either more ordered (crystalline) or disordered (amorphous). [5] The high number of hydrogen bonds in untreated crystalline cellulose is the reason of its recalcitrant nature, resulting in exceptionally low solubility and reactivity. Solubility enhancement can be approached via chemical, biological or physical treatment, or their combina- tion. [6] Biological pretreatment is associated with cellulose- depolymerizing enzymes but is considered too slow for industrial purposes. [7] In terms of chemical methods, acids, alkalis and ionic liquids have been reported to have significant effect on the native structure of lignocellulosic biomass. [8,9] Their application, however, is costly and environmentally harmful due to solvent recycling, wastewater treatment and reactor corrosion. [10] Among mechanical methods, milling techniques deserve special attention. Especially ball milling is capable of modifying the cellulose crystal structure. [11,12] Combinations of several methods can be applied as well, e.g. ball milling of cellulose combined with its impregnation with organic or mineral acid. It can be found in the literature that impregnation with acid in catalytic amounts followed by ball milling can fully convert cellulose within 2 h to water-soluble oligosaccharides, which can then be readily hydrolyzed in milder conditions. [13] This is a ‘greener’ way of cellulose depolymeriza- tion as it allows to avoid the direct use of mineral acids. The use of sulfuric acid in cellulose pretreatment before the reaction (not in the hydrolysis itself) serves the purpose of introducing sulfonic groups into the cellulose structure. It is expected that hydrolysis performance of such impregnated cellulose may be enhanced because of the ‘autocatalytic’ action of those strongly acidic groups. Sulfuric acid is used for impregnation in amounts, which theoretically would not remain in the reaction mixture and waste treatment problem should be avoided. Type of cellulose feedstock, products we want to obtain, as well as the overall economic and environmental assessment of the process, can all influence the choice of optimum pretreatment process. [14] Recently Weckhuysen et al. presented a novel elegant approach involving an application of ATR-IR spectroscopy to * Correspondence to: A.M. Ruppert, Institute of General and Ecological Chemistry, Faculty of Chemistry, Lodz University of Technology, ul. Zeromskiego 116, 90–924 Lodz, Poland. E-mail: agnieszka.ruppert@p.lodz.pl † Paper published as part of the ECASIA 2013 special issue. a Institute of General and Ecological Chemistry, Faculty of Chemistry, Lodz University of Technology, ul. Zeromskiego 116, 90-924 Lodz, Poland b Matériaux Avancés pour la Catalyse et la Santé, Institut Charles Gerhardt Mont- pellier-UMR 5253 CNRS-UMII-ENSCM-UMI, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France Surf. Interface Anal. 2014, 46, 832–836 Copyright © 2014 John Wiley & Sons, Ltd. ECASIA special issue paper Received: 23 September 2013 Revised: 9 December 2013 Accepted: 13 December 2013 Published online in Wiley Online Library: 21 January 2014 (wileyonlinelibrary.com) DOI 10.1002/sia.5374 832