Batch Microreactor Studies of Lignin Depolymerization by Bases. 2. Aqueous Solvents

Abstract

Biomass feedstocks contain roughly 15-30% lignin, a substance that can not be converted to fermentable sugars. Hence, most schemes for producing biofuels assume that the lignin coproduct will be utilized as boiler fuel. Yet, the chemical structure of lignin suggests that it will make an excellent high value fuel additive, if it can be broken down into smaller compounds. From Fiscal year 1997 through Fiscal year 2001, Sandia National Laboratories participated in a cooperative effort with the National Renewable Energy Laboratory and the University of Utah to develop and scale a base catalyzed depolymerization (BCD) process for lignin conversion. SNL's primary role in the effort was to perform kinetic studies, examine the reaction chemistry, and to develop alternate BCD catalyst systems. This report summarizes the work performed at Sandia during Fiscal Year 1999 through Fiscal Year 2001 with aqueous systems. Work with alcohol based systems is summarized in part 1 of this report.Our study of lignin depolymerization by aqueous NaOH showed that the primary factor governing the extent of lignin conversion is the NaOH:lignin ratio. NaOH concentration is at best a secondary issue. The maximum lignin conversion is achieved at NaOH:lignin mole ratios of 1.5 -2. This is consistent with acidic compounds in the depolymerized lignin neutralizing the base catalyst. The addition of CaO to NaOH improves the reaction kinetics, but not the degree of lignin conversion. The combination of Na 2 CO 3 and CaO offers a cost saving alternative to NaOH that performs identically to NaOH on a per Na basis. A process where CaO is regenerated from CaCO 3 could offer further advantages, as could recovering the Na as Na 2 CO 3 or NaHCO 3 by neutralization of the product solution with CO 2 . Model compound studies show that two types of reactions involving methoxy substituents on the aromatic ring occur: methyl group migration between phenolic groups (making and breaking ether bonds) and the loss of methyl/methoxy groups from the aromatic ring (destruction of ether linkages). The migration reactions are significantly faster than the demethylation reactions, but ultimately demethylation processes predominates.

Hydrothermal base catalysed treatment of Kraft Lignin for the preparation of a sustainable carbon fibre precursor

“…300°C and retention times of ca. 15 min are promising for altering technical lignin in the desired way (Beauchet et al, 2012;Erdocia et al, 2014;Katahira et al, 2016;Mahmood et al, 2013;Miller et al, 2002;Roberts et al, 2011;Schmiedl et al, 2012;Toledano et al, 2014). HBCD could be integrated into existing Kraft mills, since it employs existing technology and the chemicals required can be recovered in the mill's own recovery cycle.…”

Lignin Depolymerization into Aromatic Monomers over Solid Acid Catalysts

“…5 The composition of lignin varies considerably from plant to plant, particularly with regard to the type and quantity of linkages in the polymer and the number of methoxy groups present on the aromatic rings. Lignin can also be depolymerized under subcritical and supercritical conditions (> 290 o C, [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] to yield aromatic monomers and gases. Since, lignin in large quantities is produced as a waste in many industrial processes, such as pulp production (isolation of cellulose to make paper) and in the production of bioethanol from lignocelluloses, but can be used for multiple functions such as to make lignosulfonates, or can be burnt for generation of heat or even for land filling.…”

“…20 Supercritical water with p-cresol as a solvent is also used to treat organosolv lignin to yield phenols and gases. 18,[35][36] In this method, homogeneous bases like NaOH/KOH/CsOH etc. [22][23][24] Although these methods are capable of depolymerizing lignin, the major disadvantages of these methods are, use of high temperatures (290-400 o C), high pressures and risk of corrosion and loss of selectivity to aromatic monomers since these products at higher temperatures (>300 o C) generally undergo further reactions to yield gases, tar and char.…”

Forward osmosis :a new approach to water purification and desalination.

“…In an earlier project to develop a process for lignin depolymerization, the possibility of neutralizing and precipitating reaction products from a basic solution through the addition of CO 2 was considered [62]. It was thought that this approach might also be applicable to FO.…”