Selective arabinose extraction from Pinus sp. sawdust by two-step soft acid hydrolysis
Graphical abstract
Introduction
Lignocellulosic biomass is considered nowadays as the most promising alternative to fossil resources, not only to produce fuels and energy but also to obtain many different chemical products (Chin et al., 2013, Ghaffar et al., 2015, Faba et al., 2015). For the last point, a correct isolation of the different lignocellulosic fractions – lignin (∼30%), cellulose (∼40%) and hemicellulose (∼30%) – is required (Menon and Rao, 2012). Lignin is a very complex aromatic fraction with high industrial value for the food and pharmaceutical industries. Cellulose is a crystalline homopolymer of glucose, typical raw material for the paper industry. Besides, new uses have been reported for these two fractions during the last few years, being considered the highest industrial valuable fractions of biomass (Gallezot, 2012, Ragauskas et al., 2014, Deuss et al., 2014). Hemicellulose is a non-crystalline heteropolymer of different sugars, being the glucose, xylose and the arabinose the most important ones (Saha, 2003). Arabinose is a five carbon sugar, one of the main monomers in the hemicellulose of softwoods. It can be used as a non-caloric sweetener, dietary fiber, in flavors and as an intermediate for the synthesis of different drugs, mainly related to human diabetes, bacteriological diagnostics, anti-virus, production of vitamin B2 (Aguedo et al., 2013, Jiang et al., 2015). Despite of these important uses, arabinose commercial production is still nowadays a complex and expensive process consisting of acid hydrolysis of Arabic gum or other raw materials followed by multiple procedures of purification (Cheng et al., 2011), so the development of an alternative and easier process (starting, for example, from a low-value raw material such as the hemicellulose) would be very interesting.
Nowadays, there are several alternatives proposed to separate cellulosic and hemicellulosic polymers from the lignin and obtain the corresponding monomers by a partial degradation of the polymeric structure. The acid hydrolysis using sulfuric acid is the most common one (Alonso et al., 2010, Mäki-Arvela et al., 2011). Other methods developed for this purpose are hot water and alkaline treatments, AFEX, steam explosion, pyrolysis, COSLIF and ionic liquids (Capolupo and Faraco, 2016). However, the reaction conditions are so harsh that these processes cannot be considered as green-processes and sugars are easily dehydrated and transformed to small molecular weight degradation products (Klinke et al., 2004). Concerning to the acid hydrolysis, most of the previous literature suggests the H2SO4 as catalyst for this reaction, concluding that the complete sugar hydrolysis (complete but not selective) is directly related to a good combination of temperature and acidity, in such a way that optimum results can be obtained after few minutes working at low temperature (<50 °C) using concentrated acid (30–70%) or using dilute acid (<1%) at high temperatures (>200 °C) (Rinaldi and Schüth, 2009, Kim and Mazza, 2008, Dong et al., 2016). Acid hydrolysis conditions could be modified, working at mild temperature with dilute acid and controlling the reaction time, in order to selectively hydrolyze the hemicellulose fraction without breaking the strong bonds between glucose monomers in the cellulose. This procedure balances the autohydrolysis and hydrolysis of biomass.
The particular properties of each wood can affect the results obtained with this pretreatment. In this work, sawdust of different pinewoods, all of them included in the Pinus sp. genus (mainly Pinus pinaster) is used as raw material. Pinus sp. is a softwood species, very common in woods of the north of Spain, with high value in the timber and wood industry. According to the literature, Pinus sp. is typically composed of 37.1–40% of cellulose, 28.3–28.5% of hemicellulose and 27.2–27.9% of lignin; with more than 10% of arabinose, being this percentage among the highest in woody species (Evtuguin and Neto, 2007, Mäki-Arvela et al., 2011). The manufacturing of Pinus sp. generates a high amount of sawdust, considered nowadays as a waste. The few references about the use of this material are focused in the pretreatment of this wood by the Organosolv process (Ballina-Casarrubias et al., 2015), using soft acid media at high temperatures (Rivas et al., 2016) or even microbiological systems (Barbosa et al., 1992) whereas, to the best of our knowledge, there are not systematic studies about the soft hydrolysis at low temperatures of this sawdust. Hardwood hemicellulose is generally easier to hydrolyze than softwood hemicellulose (Marzialetti et al., 2008), so different parameters must be optimized in order to obtain an effective hydrolysis of this type of sawdust.
The aim of this work is to identify the reaction conditions that allow obtaining the maximum hemicellulose extraction, with main focus on recovering the arabinose in the liquid phase without altering the cellulose fraction (different temperatures, acid concentration, and number of cycles). We propose the use of HCl as homogeneous catalyst because its ecotoxicity is considerably lower than the toxicity of H2SO4, with values of 25 and 10 mg/L, respectively (values published in their MSDS files).
Section snippets
Raw material
Pinus sawdust was supplied by a local sawmill (Maderas Prieto, Asturias, Spain). The sawdust was air-dried and maintained in sealed plastic bags. The particle size distribution of this sawdust was analyzed using a Mastersizer S long bench equipment (Malvern Instruments, Ltd.) obtaining a pseudo-Gaussian distribution with an average diameter of 629.7 ± 12 μm.
Reaction conditions
The hydrolysis was carried out in a 1 L double-walled, thermostatically temperature controlled, glass reactor. 50 g of wood sawdust was added to
Sawdust characterization
As it is detailed in Table 1, the pine sawdust presents the typical chemical composition of softwood, with high content of insoluble lignin (17.9%) and hexoses (55.9%). These results were obtained considering the dry wood, after evaporating the water (9.4% of moisture). The ash content is 1.1%. According to the previous equations, the monomeric sugar distribution corresponds to 40.2% and 31.4% of Mono-C and Mono-H, respectively. An individual analyses of the hemicellulose fraction conclude that
Conclusions
The diluted-acid treatment of pine sp. sawdust has shown to be effective for the selective extraction of arabinose monomers from the hemicellulose fraction, keeping intact the lignin and cellulose fractions. As it was expected, higher hemicellulose hydrolysis was reached at highest temperatures (80 °C) with 9.57 g/L of pentose monomers in the liquid phase after 4 h. However, these conditions are too aggressive and a relevant amount of glucose was also detected (almost 1 g/L), suggesting that the
Acknowledgments
The authors would like to acknowledge financial support for this work from the Government of the Principality of Asturias (Contract FC-15-GRUPIN14-078). Diego Garcés would also like to thank the Government of the Principality of Asturias for his Ph-D fellowship of the Severo Ochoa Program (BP14-110).
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