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Oliveira, M. M., Driemeier, C., and Curvelo, A. A. S. (2015). "Changes in nanoscale porosity by wet pressing pulps from sugarcane bagasse," BioRes. 10(4), 8518-8527.

Abstract

Nanoscale porosity is critical for cellulose reactivity and can be detrimentally affected by wet pressing. The present study evaluated how wet pressing reduced the nanoscale porosity of a set of pulps produced from sugarcane bagasse. The pulps were produced using hydrothermal treatments, followed by either 160 °C alkaline (sodium hydroxide) or 190 °C organosolv (ethanol-water) pulping. Pulping times (20, 40, 60, 80, and 100 min) and applied pressures in the pressing step (21, 43, 64, 85, and 107 MPa) were varied, and the resulting samples had their nanoscale porosity characterized using calorimetric thermoporometry. The lowest applied pressure (21 MPa) collapsed a considerable fraction of the nanoscale porosities. Otherwise, when additional pressure (up to 107 MPa) was applied, a much lower reduction in porosity was observed. The findings indicate that nanoscale porosity of pulps can be separated into compressible and incompressible components.



Full Article

Changes in Nanoscale Porosity by Wet Pressing Pulps from Sugarcane Bagasse

Marcelo M. Oliveira,a,b Carlos Driemeier,b and Antonio A. S. Curvelo b,c,*

Nanoscale porosity is critical for cellulose reactivity and can be detrimentally affected by wet pressing. The present study evaluated how wet pressing reduced the nanoscale porosity of a set of pulps produced from sugarcane bagasse. The pulps were produced using hydrothermal treatments, followed by either 160 °C alkaline (sodium hydroxide) or 190 °C organosolv (ethanol-water) pulping. Pulping times (20, 40, 60, 80, and 100 min) and applied pressures in the pressing step (21, 43, 64, 85, and 107 MPa) were varied, and the resulting samples had their nanoscale porosity characterized using calorimetric thermoporometry. The lowest applied pressure (21 MPa) collapsed a considerable fraction of the nanoscale porosities. Otherwise, when additional pressure (up to 107 MPa) was applied, a much lower reduction in porosity was observed. The findings indicate that nanoscale porosity of pulps can be separated into compressible and incompressible components.

Keywords: Biomass; Sugarcane bagasse; Lignocellulosic; Porosity; Thermoporometry; Wet pressing

Contact information: a: Escola de Engenharia de São Carlos (EESC), Universidade de São Paulo (USP), Caixa Postal:780, 13560-970, São Carlos, São Paulo, Brasil; b: Laboratório Nacional de Ciência e Tecnologia do Bioetanol (CTBE), Centro Nacional de Pesquisas em Energia e Materiais (CNPEM), Caixa Postal: 6192, 13083-970, Campinas, São Paulo, Brasil; c: Instituto de Química de São Carlos (IQSC), Universidade de São Paulo (USP), Caixa Postal: 780, 13560-970, São Carlos, São Paulo, Brasil;

* Corresponding author: aprigio@iqsc.usp.br

INTRODUCTION

Lignocellulosic biomass is a vast renewable feedstock for the production of materials, chemicals, and liquid biofuels. Sugarcane bagasse is one especially attractive type of lignocellulosic feedstock because it is available on-site as an abundant underutilized residue of sugar-ethanol mills (Cortez 2010). With such potential, intense research and development have been dedicated towards developing cost-effective processes to convert sugarcane bagasse into value-added products.

Chemical reactions for the deconstruction of lignocellulosic biomass depend on the transport of reactants, catalysts, and reaction products inside the porous structure of the biomass. Porosity includes the micrometric pores originating from the interior (lumen) of plant cells, as well as nanoscale pores within the cell walls (Stone and Scallan 1968; Fahlén and Salmén 2005; Maziero et al. 2013). Specific surface areas associated with nanometric pores are a few orders of magnitude greater than specific areas because of micrometric pores and external particle surfaces (Driemeier et al. 2011; Bragatto et al. 2012). Therefore, nanoscale porosity is often critical for biomass chemical reactivity. In particular, enzymatic saccharification of cellulose is critically affected by nanoscale porosity. This is because enzymes are approximately 5 nm in size, requiring pores larger than that for enzyme penetration into the walls, thus enabling extensive cellulose saccharification.

Lignocellulose nanoscale porosity is influenced by many factors. As a starting point, nanoscale porosity depends on plant tissue and its maturity (Maziero et al. 2013). Chemical treatments, such as pulping or hydrothermal treatments performed prior to enzymatic digestion, usually increase nanoscale porosity (Stone and Scallan 1968; Fahlén and Salmén 2005; Ishizawa et al. 2007), in addition to mechanical disintegration processes (Fahlén and Salmén 2005; Grönqvist et al. 2014; Wang et al. 2014). On the other hand, pressing and drying reduces nanoscale porosity (Maloney et al. 1997; Haggkvist et al. 1998; Park et al. 2006; Luo et al. 2011; Pönni et al. 2012).

The collapse of nanoscale pores because of drying implies that wet methods are required to characterize biomass nanoscale porosity in a state representative of aqueous processing. One such method is thermoporometry, which is based on the temperature depression of the ice-water phase transition occurring under nanometric confinement (Brun et al. 1977; Petrov and Furó 2009). The ice melting transition can be detected by nuclear magnetic resonance (Ishizawa et al. 2007; Petrov and Furó 2009), as well as by differential scanning calorimetry (Brun et al. 1977; Maloney and Paulapuro 1999; Park et al. 2006; Driemeier et al. 2012). Thermoporometry analysis of cellulosic biomass determines the pore size distributions in the 1 to 400 nm range.

This article investigates how cellulosic pulps, obtained from sugarcane bagasse, are modified in nanoscale porosity using the wet pressing technique. Wet pressing is potentially beneficial to biomass conversion economics because of the higher solid loading potential in the reactors. On the other hand, wet pressing is also potentially detrimental to processing because of the particulate compaction and loss of biomass porosity. By employing thermoporometry to characterize nanoscale porosity after different pulping and pressing conditions, this article contributes to the understanding of how wet pressing affects the nanoscale porosity of cellulosic pulps.

EXPERIMENTAL

Sugarcane Bagasse

Sugarcane bagasse was provided by Usina Ipiranga de Açúcar e Álcool Ltda, located in Descalvado, São Paulo, Brazil. Bagasse was washed in hot (70 °C) water in order to remove residual sucrose. Granulometric fractionation of bagasse was performed using a column of sieves of 16, 60, and 250 mesh (1.18; 0.250, and 0.063 mm, respectively). Fractions retained on the sieves of 16 and 60 mesh were rich in bagasse fibrous particles. These two fractions were mixed, homogenized, and then used in the following procedures.

Chemical Fractionation

Bagasse chemical fractionation was performed in two sequential steps. The first step was a hydrothermal treatment performed at 160 °C for 30 min with approximately 13 L of water and 1.3 kg (dry mass) of solid, in an electronic autoclave system (REGMED model AU/20). After the reaction, the solid fraction was washed with water. The hydrothermally-treated solid was subjected to a second chemical fractionation step of either organosolv or alkaline pulping. Both delignification reactions were performed with a solid-liquid ratio of 1:10 (w/v) in a 200 mL custom-built steel reactor, which was heated using a glycerin thermostatic bath with mechanical stirring and a digital temperature control. The reaction temperatures were set to 190 °C (organosolv) and 160 °C (alkaline). The organosolv liquid was a 50/50 (v:v), ethanol-water mixture. The alkaline liquid was an aqueous solution of 20% sodium hydroxide with 0.15% anthraquinone catalyst. Reaction times were 20, 40, 60, 80, and 100 min. Reactions began with the immersion of the reactors into the thermostatic bath (already at target temperature) and was stopped by immersion in an ice bath. After cooling, reactor contents were subjected to a defiber process with a solution of 1.0% sodium hydroxide to remove any absorbed lignin on the cellulosic fibers. Afterwards, the cellulosic pulps were washed with distilled water until the pH of the washing water remained unaltered.

Solubilization

Solubilization was defined as the percentage of dry mass removed from the solid into the solution during the delignification process. Solubilization was determined gravimetrically by measuring the initial (mi) and final (mf) masses of the solid submitted by delignification (Eq. 1). Determination of mi and mf accounted for sample moisture content, as determined by a thermobalance (TopRay series, Bel Engineering, Piracicaba, Brazil):

 (1)

Wet Pressing

Wet pressing was performed in a hydraulic press (Nowak, São Jose do Rio Preto, Brazil) with a nominal maximum load of 30 tons. A custom-built wet compression cell was made on polished 304 steel (Fig. 1).

Fig. 1. System used for wet pressing: a) Sketch of a wet pressing test, b) Images of the disassembled (left) and assembled (right) compression cell

Each pressing test employed 1.5 g (dry basis) of wet pulp (over 60% humidity) homogeneously distributed at the base of the cylindrical compression cell (54 mm of internal diameter). Wet pulp was then slowly compressed using a cylindrical piston of the same diameter. The load was maintained for 5 min after reaching the target load. Loads of 5, 10, 15, 20, and 25 tons were applied, corresponding to pressures of 21, 43, 64, 85, and 107 MPa, respectively. During compression, liquid was expelled through four radial holes of 5 mm diameter, located at the base and lateral sides of the cylindrical compression cell. Figure 1 shows a sketch of wet pressing and images of the compression cell. Pressed pulps formed tablets, which were stored at 5 °C in sealed containers prior to characterization of nanoscale porosity.

X-Ray Diffraction

X-ray diffraction was performed in transmission geometry with air-dried powdered specimens conditioned in capillary tubes. Two-dimensional diffraction patterns were recorded using a mar345 image plate, with calibration and intensity correction as detailed elsewhere (Driemeier and Calligaris 2011). This detection system was coupled to the MX1 beamline of the Brazilian Synchrotron Light Laboratory (Polikarpov et al. 1998), with X-ray wavelength set to 1.4535 Å. Note that this wavelength differs from the usual CuKα sources (1.54 Å), so that the diffraction peaks would appear in different scattering angles 2θ.

Thermoporometry

Nanoscale porosity was evaluated through thermoporometry using differential scanning calorimetry (DSC). Measurements were carried out in a TA Q200 instrument with RCS90 cooling system (TA Instruments, New Castle, USA). A steel punch (3 mm diameter) was used to collect samples from the compressed tablets. The wet samples were conditioned by immersion in deionized water for 24 h prior to insertion into the DSC Tzero® aluminum pans with hermetic lids.

As the first step of each DSC analysis, the sample was frozen at -70 °C. Then, the temperature was increased stepwise to 5 °C. Each heating step was comprised of a ramp followed by a stabilization isotherm. The calorimetric signal of ice melting below 0 °C was attributed to water confined in nanoscale pores, also known as freezing bound water (FBW). The temperature depression (ΔT) of ice melting was related to pore diameter (d) according to the Gibbs-Thomson equation, = 2 Kc T, where Kc = 19.8 nm K. The calorimetric signal was converted to FBW (in units of water g per dry matter g) as a cumulative distribution function of d, as detailed elsewhere (Driemeier et al. 2012).

Regression Analysis

The thermoporometry data of the pressed pulps was analyzed by linear regression. The FBW at a given pore diameter was modeled as the sum of a constant (C0), a term proportional to applied pressure (P), and another term proportional to solubilization (S), as follows (Eq. 2),

FBW = C+ (C× S) + (C× P) (2)

where, C0CS, and CP were determined by fitting the experimental data according to matrix algebra (Rencher 2003).

RESULTS AND DISCUSSION

The pulping processes solubilized variable fractions of the prehydrolyzed solid. For both the alkaline and organosolv delignification, the trend of solubilization increased with increasing treatment time. This trend was most likely caused by the progressive removal of biomass components, mainly lignin, as the treatment time was increased. An exception was the alkaline process at the longest treatment time (100 min), which exhibited a lower solubilization than the 80 min process. This exception was perhaps a result of lignin deposition on the surface of the material over a long treatment time. Table 1 shows the value of solubilization means ± the standard deviation of the duplicates.

Table 1. Percent Solubilization in Different Delignification Conditions

The unpressed pulps were characterized using X-ray diffraction. Organosolv pulps exhibited diffractograms characteristic of the native phase cellulose I, while alkaline pulps presented the characteristic features of cellulose II (Fig. 2). These findings indicate that the alkaline pulping conditions promoted cellulose mercerization, converting cellulose I into cellulose II. In addition to diffraction peaks from cellulose, minor and sharper peaks were observed at 2θ  27° (organosolv), as well as at 24° and 25° (alkaline). These minor peaks arose from unidentified crystalline contaminants.

Fig. 2. Diffractograms of pulps produced by alkaline and organossolv treatments (reaction time of 60 min), showing the typical patterns of cellulose II and cellulose I, respectively

The wet pressing of alkaline and organosolv pulps produced nonhomogeneous tablets, with the center of each tablet denser than its borders. This inhomogeneity resulted from the geometry of the pressing cell (Fig. 1). Figure 3 shows pulps before and after wet pressing. The alkaline pulps are whiter because more lignin was removed, as compared to the organosolv pulps. The size of the samples collected for thermoporometry is also indicated in the images below.

Fig. 3. Images taken of alkaline (a and b) and organosolv (c and d) pulps before and after wet pressing. The materials shown in the images were pulped for 20 min and pressed at 107 MPa. The circles in (d) have diameters of 3 cm and 3 mm, respectively, corresponing to the region considered for sampling and the actual size of one sample selected for thermoporometry analysis

The thermoporometry measurements of the pulps before and after wet pressing exhibited a reduction in nanoscale porosity at every applied pressure (Fig. 4).