Lignin contributions to the nanoscale porosity of raw and treated lignocelluloses as observed by calorimetric thermoporometry
Introduction
Lignocellulosic biomass is a vast renewable resource made mainly from structural carbohydrates (cellulose and hemicelluloses) and a complex aromatic macromolecule (lignin). These components are entangled in a hierarchical structure so as to form cell walls, tissues and, ultimately, entire plants. Deconstruction of lignocellulosic biomass into simpler, reactive molecules (e.g., monomeric sugars) is a major goal toward biomass valorization (Lynd et al., 1999). However, biomass reactivity is often impaired by its compact structure that limits the transport of reactants, catalysts, and reaction products through the biomass porous structure. In particular, nanoscale pores within cell walls are thought to be critical for enzymatic catalysis, which requires accessibility for enzymes ∼5 nm in size (Arantes and Saddler, 2011, Ding et al., 2012, Grethlein, 1985).
Lignin is usually thought as a pore filler, occupying intercellular spaces (middle lamella) as well as nanoscale voids in-between the fibrillar cellulosic network from secondary cell walls (Ding et al., 2012, Donaldson, 2001). Although this seems to be an appropriate picture for raw biomass, chemical processing can remove, restructure, and relocate the lignin (Pu et al., 2015). A class of processes that include dilute acid, steam explosion, and hydrothermal is here termed acidic treatments, where the acidity may result from high-temperature water autoionization, release of biomass acetyl groups (Garrote et al., 1999), or acid addition. Acidic treatments were reported to phase-separate lignin with formation of globular lignin aggregates (Donaldson et al., 1988, Donohoe et al., 2008, Langan et al., 2014, Pingali et al., 2010). How such restructured and relocated lignins contribute to biomass nanoscale porosity is an open question that we want to address.
Nanoscale pores in water-saturated cellulosic samples can be probed by calorimetric thermoporometry. The technique – also named cryoporometry or thermoporosimetry – is based on (1) the temperature depression of ice–water transitions occurring under nanoscale confinement and (2) calorimetric detection of the phase transition (Brun et al., 1977, Landry, 2005, Petrov and Furó, 2009). Most applications of thermoporometry to cellulosics have been based on step-melting programs starting from ≈−30 °C for characterization of delignified samples (Fahlén and Salmén, 2005, Maloney, 1998, Park et al., 2006). Recently, we expanded the potential of the method, decreasing freezing temperature to −70 °C and introducing several corrections in the analysis of step-melting thermograms (Driemeier et al., 2012).
After this analytical development (Driemeier et al., 2012), calorimetric thermoporometry was established as routine analysis in CTBE/CNPEM. Since then, wide range of samples has been characterized, including delignified (Driemeier et al., 2012) as well as lignified samples such as raw sugarcane tissues (Maziero et al., 2013). Over a thousand analyses were recorded in the last four years and distinct signatures in thermoporometric profiles have been observed in raw, delignified, and acidic treated lignocelluloses. In this article, these distinct thermoporometric signatures are demonstrated. Beyond lignin as pore filler, our results reveal characteristic pores (∼<4 nm) that we attribute to be within the lignin aggregates left by acidic treatments.
Section snippets
Samples for this study
Six lignocellulosic feedstocks were analyzed specifically for this study: sugarcane rind, sugarcane pith, sisal fiber, coconut fiber, pine sawdust, and eucalyptus sawdust. Sugarcane stalks were provided by Usina Ipiranga de Áçucar e Álcool Ltda. Stalks were cut in disks whose center (pith) was drilled out, separating it from the periphery (rind). Pith was later pressed and washed to remove sugars, while rind was cut in pieces, washed, and extracted (in soxhlet system, 24 h in n-hexane plus 24 h
Results and discussion
Lignin contents of the samples analyzed for this study (Section 2.1) are reported in Table 1 for the raw, the hydrothermally treated, and the delignified states. Comparison among the raw samples shows significant variability in lignin contents. Hydrothermal treatments increase lignin contents (as percentage of treated solid mass) because hydrothermal conditions remove mainly hemicelluloses, with much less removal of lignin (Garrote et al., 1999, Driemeier et al., 2015). On the other hand, major
Acknowledgements
The authors acknowledge the CAPES-CNPEM doctorate scholarship and financial support from the CNPq and FAPESP grant 10/05523-3.
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