A review on the production and recovery of sugars from lignocellulosics for use in the synthesis of bioproducts

Highlights

• H₂SO₄ acidolysis and alkaline pretreatments are still the most used methods.

• Some eco-friendly pretreatments have been confined to small-scale operations.

• Most papers directly processed crude extracts from enzymatic hydrolysis slurries.

• Ethanol was the main sugar-derived product addressed by the studies we assessed.

• Production of XOS, xylitol, lactic acid, and lipids have gained commercial interest.

Abstract

In this study, concepts of systematic mapping (SM) were used to conduct a literature review covering the research on the production and recovery of sugars from lignocellulosics to synthesize bioproducts, mostly xylose-derived products. The SM approach constitutes a rigorous evidence review methodology used to catalog evidence, identifying knowledge gaps, unknown trends, and research clusters (Cook et al., 1997, James et al., 2016). Our results showed that some classical pretreatment methods (H₂SO₄ acidolysis and alkaline) are still among the most used methodologies for the initial processing of lignocellulosics. Some other technologies, such as liquid hot water and steam explosion, were used to minimize the production of inhibitors and waste generations related to the use of those classical pretreatments. Other eco-friendly strategies (ultrasonic, gamma irradiation, and ultra-high-pressure pretreatments) were rarely reported in our dataset and have been confined to small-scale operations thus far. The enzymatic hydrolysis stage was mainly conducted using commercial enzyme cocktails, which are more feasible and commonly used in large-scale processes than crude preparations. Physical separation systems (washing, drying, centrifugation, and macro filtration) were used to separate extracts and/or hydrolysates from unconverted biomass and then, in a few cases, purification methods (evaporation, chromatographic separation, crystallization, distillation, and precipitation) were used to separate the sugars from other components. Detoxification strategies were used in both cases. Ethanol was the main sugar-derived product addressed by the papers we assessed, followed by xylooligosaccharides, xylitol, lactic acid, and lipids. On the other hand, some products such as biogas, fatty acid methyl esters, diols, methane, and succinic acid, still constitute a minor fraction of the products targeted. The analysis of keywords revealed that many co-occurrences were found among most enzymatic activities and sugar-recovery methods, but a weak co-occurrence network was identified among pretreatment methods and sugar-derived products.

Introduction

In the upcoming decades, the replacement of fossil resources demands cost-effective and technically feasible solutions. Renewable carbon sources from lignocellulosic biomasses (LBs) are among the promising options (Canilha et al., 2012, Farmanbordar et al., 2020, Neumann et al., 2016, Santibáñez et al., 2020). LB is inexpensive, environmentally friendly, and one of the most abundant yet underutilized resources in the world (Anwar et al., 2014, Ravindran and Jaiswal, 2016, Tayyab et al., 2018). It mainly consists of cellulose (40–50% on average), hemicellulose (25–30%), lignin (15–20%), and traces of pectin, nitrogen, and inorganic compounds (Borges et al., 2014, Chen et al., 2017, Fonseca, 2015, Rojas et al., 2014). This biomass's high degree of polymerization makes its cell walls stable and difficult to degrade (Behera et al., 2014, Chaula et al., 2014). Thus, methods for converting plant material into monomeric sugars (mostly glucose from cellulose and xylose from xylan) are required for their further use as feedstocks in the production of a range of value-added products (Ravindran and Jaiswal, 2016). These procedures comprise pretreatment and enzymatic hydrolysis (EH) steps (Angarita et al., 2015, Batista et al., 2019; Burke et al., 2009a; Canilha et al., 2012; Delabona et al., 2013, Delabona et al., 2012; Furlan et al., 2012; Rodríguez-Zúñiga et al., 2015), which are generally undertaken in conjunction with downstream sugar-recovery to meet the target product attributes (Milessi-esteves et al., 2019).

Pretreatment is necessary to provide internal access to biomass by the enzymes that will be used in the hydrolysis stage, i.e., such process promotes enzyme and substrate interaction to stimulate the subsequent EH (Chandra et al., 2015, Michalska et al., 2012). Each pretreatment technology presents a different mechanism of action on the plant structure, inducing either physical or chemical modifications. Several approaches have been used for this process, such as autohydrolysis, acid hydrolysis, ammonia activation, kraft pulping, organic solvent pulping, hot water pretreatment, ammonia percolation, lime pretreatment, caustic solvent pulping, alkaline peroxide pretreatment, among other methods (Burke et al., 2009a, Burke, 2009b; Pourbafrani et al., 2014). The selection of a suitable pretreatment strategy depends on the LB source, economic viability, environmental sustainability of the process (minimizing chemical, heat, and power requirements) (Furlan et al., 2016, Kumar et al., 2009, Longati et al., 2018, Yu et al., 2016, Yu et al., 2015), formation of degradation byproducts, process yield, production scale, as well as configurations employed in the hydrolysis and further processing (Abo et al., 2019, Bhutto et al., 2017, Hendriks and Zeeman, 2009, Soltanian et al., 2020).

The first step in the EH process consists of breaking down cellulose and hemicellulose (containing xylan) into soluble oligosaccharides of glucose and xylose, respectively. Then, enzyme complexes are used to convert oligosaccharides to monomeric sugars suitable for fermentation or catalytic transformation to bioproducts (Burke et al., 2009b). This approach is designed to maximize enzyme performance, controlling the effect of inhibitors while optimizing hydrolysis rates and sugar yields (Burke et al., 2009a).

After obtaining monomeric sugars from lignocellulosics through an appropriate pretreatment method and EH, the material is recovered and purified to extract the desired sugar(s) (Kumar et al., 2017). Sugar recovery methods may include solid/liquid systems, chromatographic and membrane separations, ion exchange or crystallization, among others. These strategies can be applied to (1) separate the extracts from the non-treated biomass (physical separations); (2) recover the glucose and/or the xylose (the main monosaccharides resulting from the hydrolysis of the cellulosic and hemicellulosic fractions of the LB, respectively) (Poletto et al., 2020, Santibáñez et al., 2020) through purification approaches, aiming for their later use in the synthesis of a variety of products, including sugars, biofuels, and value-added chemicals. The use of the hexoses is widely known, but usage of pentose sugars is gaining more visibility in this realm as well, because of the opportunity to increase the sugar and process yields from lignocellulosics, since the hemicellulose stream was often underexploited after the LB hydrolysis (Geng et al., 2019, Vescovi et al., 2017). Nevertheless, like many other commercial processes, the overall development of these processes demands the tailoring of specific attributes of the sugar products required for different applications (Huang et al., 2010, Saini et al., 2020), considering throughput and purity targets and, when applicable, the scale-up of the process (Banerjee et al., 2012, Cheng et al., 2020, Koppram et al., 2013, Wang et al., 2016).

Here, we survey the peer-reviewed literature addressing the production of sugars from lignocellulosics, focusing on xylose use (with or without other sugars) as a substrate in the synthesis of biochemical products. Potential knowledge gaps and trends within this subject are also explored.