Lignin Fractionation Methods: Can Lignin Fractions Be Separated in a True Industrial Process?

Abstract

Lignin is among one of the largest natural resources in the world, with aromatic structures that can serve as a raw material in the synthesis of high value-added chemicals, the production of bioenergy, and the development of carbon-based products, among others. A better understanding of the chemical structure of lignin and its valorization methods is necessary to overcome current technological challenges for industrial scale applications. The main challenge found to provide the most abundant source of aromatics in the world is the fractionation of lignin. Current studies propose a way to fractionate lignin to obtain more homogeneous and less complex fractions, because of the fact that aromatic compounds are essential intermediates in the manufacture of polymers, and lignin is the main source of biologically aromatic-based substrates. Numerous review articles have addressed research aimed at understanding the structure of lignin, however, a limited focus has been given to its fractionation methods. This article aims to review papers published in recent years using different lignin fractionation techniques, focusing on the four main types of fractionations: in organic solvents, gel permeation chromatography in preparatory scale, ultrafiltration and precipitation by varying acid gradient. This a Review summarizes the most recent innovations in the area of biorefineries for the chemical valorization of lignin derived from various botanical sources through different fractionation methods.

Recovery of low molecular weight compounds from alkaline pretreatment liquor via membrane separations

“…25 However, to avoid excessive chemical demand, the process solvent needs to be recycled, for example by distillation, which leads to a high overall operating cost (∼$3 per kg of hydroxycinnamic acid). [26][27][28][29] Another commonly employed approach for fractionation of alkaline liquors is precipitation via an acid gradient. The advantage of precipitation is the ease of operation at the bench scale, although there are several disadvantages limiting feasibility such as a resulting low-yield of LMW lignin-related compounds, excessive chemical consumption, and colloid formation.…”

“…The advantage of precipitation is the ease of operation at the bench scale, although there are several disadvantages limiting feasibility such as a resulting low-yield of LMW lignin-related compounds, excessive chemical consumption, and colloid formation. 24,26 Lastly, the fractionation of lignin from alkaline liquors has been demonstrated by size exclusion chromatography; however, the process requires expensive high pressure equipment for continuous processing via simulated moving bed (SMB), and clarification of the APL is needed prior to processing to prevent stationary phase fouling. 26,30,31 Considering the above challenges, APL fractionation via membrane separations is a promising approach because membrane systems operate non-thermally, have a low energy demand, and are widely used industrially, thus offering a short developmental timeline via commercially available membranes and modules.…”

“…24,26 Lastly, the fractionation of lignin from alkaline liquors has been demonstrated by size exclusion chromatography; however, the process requires expensive high pressure equipment for continuous processing via simulated moving bed (SMB), and clarification of the APL is needed prior to processing to prevent stationary phase fouling. 26,30,31 Considering the above challenges, APL fractionation via membrane separations is a promising approach because membrane systems operate non-thermally, have a low energy demand, and are widely used industrially, thus offering a short developmental timeline via commercially available membranes and modules. 23,24,32 Membranes in the microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) range have been employed to recover defined molecular weight fractions from several lignin-rich streams.…”

Kraft Lignin with Improved Homogeneity Recovered Directly from Black Liquor and Its Application in Flexible Polyurethane Foams

“…17, 19 Although some of the fractionated lignin portions presented relatively narrowed molecular weight distributions, they entail either time- consuming operations or expensive devices, which limit their practical applications. 20 Thus, it is desirable to develop a simple and efficient technique to recover lignin with improved homogeneity directly from Kraft black liquor. Liquefaction is a facile thermochemical conversion technique that has been widely employed to convert biomass to chemicals and biofuels.…”

“…Currently, fractionation is the major approach to narrow down the molecular weight distribution of lignin, including sequential acid precipitation, solvent extraction, and ultrafiltration technologies. , These methods aim at separating different lignin fractions from one feedstock (lignin or black liquor). To achieve this goal, the first method requires multiple pH adjustments; the second method usually involves different solvents with variable components and ratios; the third method needs an ultrafiltration system integrated by a few membranes with specific cutoff values. , Although some of the fractionated lignin portions presented relatively narrowed molecular weight distributions, they entail either time-consuming operations or expensive devices, which limit their practical applications . Thus, it is desirable to develop a simple and efficient technique to recover lignin with improved homogeneity directly from Kraft black liquor.…”