Evaluation of agave bagasse recalcitrance using AFEX™, autohydrolysis, and ionic liquid pretreatments
Introduction
Agave bagasse (AGB) is a residual fiber left behind after the production of alcoholic beverages such as tequila or mezcal in Mexico, depending on the agave species used during the process. The annual agave consumption for tequila production in 2015 was around 8.09 × 105 tons. AGB represents usually ∼40% of the processed agave on a dry weight basis (Davis et al., 2011, CRT, 2015). AGB is a sustainable feedstock for producing biofuels comparable to other lignocellulosic biomass feedstocks (e.g. corn stover or switchgrass). To overcome biomass recalcitrance, a pretreatment step is mandatory, and an efficient one must be economically viable, minimize degradation of carbohydrates to inhibitors, and should not inhibit the subsequent downstream processing steps (saccharification and fermentation).
Many different biomass pretreatments are currently being developed and evaluated, including ammonia fiber expansion (AFEX), autohydrolysis (AH) and ionic liquid (IL) pretreatments (da Costa Sousa et al., 2009). AFEX is currently undergoing scale up for potential commercialization and has been successfully demonstrated at the 1 ton per day level in pilot plant. All though three pretreatment methods are capable of producing high sugar yields after enzymatic hydrolysis, several challenges remain before these processes could be commercially scaled up. These challenges include adequate chemical for pretreating biomass, and their recovery, energy requirements, feedstock handling issues, high water requirements and downstream processing problems.
Autohydrolysis biomass pretreatment uses only hot liquid water or saturated steam. This process has two primary effects. First is the auto ionization of water into acid hydronium ions (H3O+) and the second is the hydration of the acetyl groups in hemicellulose leading to formation of acetic acid. The hydrogen ions from acetic acid act as catalysts in the process. Then the hemicellulose is depolymerized and solubilized in the liquid phase and a small portion of the lignin is also dissolved. Lignin is relocated to the surface of the pretreated solids due to the operating conditions applied. The dissolved hemicellulose exists mostly in the form of xylose oligomers, and requires additional hemicellulases or dilute acid hydrolysis to be converted into fermentable sugars. Some biomass feedstocks such as corn stover have the buffering capability to maintain the pH of the mixture at around 4; this feature helps to carry out pretreatment under milder conditions (140–180 °C) (Ruiz et al., 2013).
AFEX pretreatment uses about one kg of anhydrous ammonia per kg of biomass at moderate temperatures (e.g. 90–100 °C) and high pressures (e.g. 250–300 psi) for approximately 30 min followed by release of pressure resulting in biomass disruption (Balan et al., 2009). About 97% of the ammonia can be recovered in the gas phase and recycled. AFEX greatly increases the biomass internal porosity by solubilizing some of the lignin, hemicellulose and relocating these components to the surface of the biomass. Ammonolysis and hydrolysis are two competing reactions that take place during AFEX pretreatment process. The acetyl, feruloyl, coumaryl ester linkages present in biomass are converted into the corresponding amides and acids (Chundawat et al., 2010).
Certain ILs such as 1-ethyl-3-methylimidazolium acetate or [C2C1Im][OAc] have demonstrated great potential as efficient solvents for biomass processing, due to complete plant cell wall structure dissolution. Disrupting the hydrogen bond network of cellulose causes decrystallization (from cellulose I to cellulose II), and delignification which makes the biomass structure amenable for downstream processing (Singh et al., 2009). Typical IL pretreatment conditions employ a temperature range of 100–160 °C and relatively short residence times (up to 3 h) and is carried out at atmospheric pressure which can be highly recovered as well as employing cost effective IL recycle methods that will reduce overall processing costs (Papa et al., 2015). IL pretreatment has been shown to be effective in efficiently processing a wide range of feedstocks (hardwood, softwood and grasses) at high solids loading levels and does not appear to require finely milled material to achieve high sugar yields (Cruz et al., 2013, Li et al., 2013).
To date, there has not been a side-by-side comparison of AGB sugar conversion using leading pretreatment processes such as AFEX, AH and IL technologies. However, independent bioconversion studies have been performed on AGB where various pretreatments have been used to reduce its recalcitrance for subsequent saccharification, including alkali, dilute acid, organosolv and ILs (Ávila-Lara et al., 2015, Caspeta et al., 2014, Perez-Pimienta et al., 2013). Moreover, different approaches for the production of biofuels such as methane, n-butanol or ethanol using AGB have been reported (Arreola-Vargas et al., 2015, Corbin et al., 2015, Mielenz et al., 2015). It is very difficult to make any meaningful comparison between these studies, since they used different agave species, different pretreatment conditions, the source and activities of enzymes are different.
The main objective of this work is to perform the first detailed qualitative and quantitative comparison of three pretreatments (AFEX, AH and IL) for AGB sugar conversion (Fig. S1). To enable this comparative study, a single source of AGB was used as a feedstock for all three pretreatments, and one source of enzymes was applied to the pretreated biomass. A 2D 13C–1H heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectroscopy is employed in this study to chemically characterize the different linkages present in the untreated and pretreated cell wall of AGB. Syringyl (S) and guaiacyl (G) lignin substructures ratios are determined by HSQC-NMR. A comparison of process flowsheet and mass balances for the three pretreatments is used based on glucan and xylan conversion from saccharification experiments and compositional analysis.
Section snippets
Sample preparation
Agave bagasse was donated by Tequila Corralejo based in Guanajuato, Mexico. This facility receives the central fruit (the stem or “pina”) from defoliated agave plants aged 7–8 years. The stems were cooked for 18 h in an autoclave, then milled and compressed to separate the syrup from wet bagasse. Samples of the wet bagasse were collected, washed thoroughly with distilled water and dried in a convection oven at 40 °C. The dried AGB was milled with a Thomas-Wiley Mini Mill fitted with a 20 mesh
Compositional analysis of untreated and pretreated agave bagasse samples
Table 1 summarizes the different pretreatment processes used to pretreat AGB and it composition in terms of major cell wall components before and after pretreatment. The composition of untreated AGB in dry basis was 31.2% glucan, 15.7% xylan, and 18.4% lignin, which is similar to other reported values in terms of xylan and lignin content (Ávila-Lara et al., 2015). However, the glucan content of untreated AGB is lower than that reported in other studies, namely 40% (Perez-Pimienta et al., 2013,
Conclusions
Compositional analysis of AFEX pretreated AGB did not show a significant differences when compared to untreated AGB. In contrast, about 62.4% of xylan was solubilized with AH pretreatment, and 25% of delignification occurred after IL pretreatment. All three pretreatments enhanced sugar production in enzymatic hydrolysis. Yields of glucose plus xylose were 42.5, 39.7 and 26.9 kg per 100 kg of untreated AGB in the major hydrolysate stream for AFEX, IL and AH, respectively. We believe that these
Acknowledgements
We gratefully acknowledge support for this research by the Office of Biological end Environmental research in the DOE Office of Science through the Joint BioEnergy Institute (JBEI Grant DE-AC02-05CH11231), National Science Foundation under Cooperative Agreement No. 1355438, internal funding from Universidad Autónoma de Nayarit (Autonomous University of Nayarit) and PROMEP project/103.5/13/6595 funded by the Secretary of Public Education of Mexico. We thank the Great Lakes Bioenergy Research
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