Lignin Depolymerization into Aromatic Monomers over Solid Acid Catalysts

Abstract

It is imperative to develop an efficient and environmentally benign pathway to valorize profusely available lignin, a component of non-edible lignocellulosic materials into value-added aromatic monomers, which can be used as fuel additives and platform chemicals. To convert lignin, earlier studies used mineral bases (NaOH, CsOH) or supported metal catalysts (Pt, Ru, Pd, Ni on C, SiO 2 , Al 2 O 3 etc.) under hydrogen atmosphere but these methods face several drawbacks such as corrosion, difficulty in catalyst recovery, sintering of metals, loss of activity etc. Here we show that under inert atmosphere various solid acid catalysts can efficiently convert six different types of lignins in to value-added aromatic monomers. Particularly, SiO 2 -Al 2 O 3 catalyst gave exceptionally high yields of ca. 60 % for organic solvent soluble extracted products with 95±10 % mass balance in the depolymerization of dealkaline lignin, bagasse lignin, ORG and EORG lignins at 250 o C within 30 min. GC, GC-MS, HPLC, LC-MS, GPC analysis of organic solvent soluble extracted products confirmed the formation of aromatic monomers with ca. 90 % selectivity. In the products, confirmation of retention of aromatic nature as present in lignin and appearance of several functional groups is done by FT-IR, 1 H and 13 C NMR studies. Further, isolation of major products by column chromatography was carried out to obtain aromatic monomers in pure form and their further characterization by NMR is presented. Detailed characterization of six different types of lignins obtained from various sources helped in substantiating the catalytic results obtained in these reactions. Meticulous study on fresh and spent catalysts revealed that the amorphous catalysts are preferred to obtain reproducible catalytic results. . Materials.Dealkaline lignin (TCI Chemicals, product no. L0045), Alkali lignin (Aldrich, product no. 370959), Organosolv lignin (Aldrich, product no. 371017) were purchased and used without any pre-treatment. ORG and EORG lignins are obtained from local industries. Bagasse lignin was isolated in the laboratory by organosolv method. Zeolites, H-USY (Si/Al=15), H-ZSM-5 (Si/Al= 11.5), H-MOR (Si/Al=10), H-BEA (Si/Al=19) were obtained from Zeolyst International. Prior to use, zeolites were calcined at 550 °C for 16 h in air flow. SiO 2 -Al 2 O 3 (Aldrich), K10 clay and Al pillared clay (Aldrich), niobium pentoxide (Spectrochem) were also purchased. Various aromatic monomers were purchased from Aldrich, Alfa Aesar, TCI chemicals and used as received. Solvents like methanol (99.9 %, LOBA), ethanol (99.7 %, LOBA), tetrahydrofuran (99.8 %, LOBA), ethyl acetate (99.9 %, LOBA), chloroform (99.8 %, LOBA), diethyl ether (99.5 %, LOBA), hexane (99 %, LOBA) and dichloro methane (99.8 %, LOBA) were purchased and used as received. NaCl (Merck), H 2 SO 4 (98.5-

High Catalytic Efficiency of Lignin Depolymerization over Low Pd-Zeolite Y Loading at Mild Temperature

“…In Deepa's report (Deepa and Dhepe, 2015), lignin conversion generated wide range of products from 10 to 25 min. However, in this work, only seven products were detected by GC-MS, indicating much higher selectivity of this catalyst.…”

“…Among various catalysts for lignin depolymerization, zeolites are widely accepted due to their low-cost, high surface area, and outstanding acid sites catalytic performance (Taarning et al, 2011;Ben and Ragauskas, 2012;Wang et al, 2017). External Brønsted acidity in meso-/microporous zeolites was reported to effectively affect the selectivity of de-alkylation and de-etherification reactions in parallel during lignin conversion (Taarning et al, 2011;Singh and Ekhe, 2014;Deepa and Dhepe, 2015). Deepa reported that aromatic monomers were the predominant products using zeolite as solid acid catalysts for lignin depolymerization at 250°C in H2O/CH3OH mixture (Deepa and Dhepe, 2015).…”

“…External Brønsted acidity in meso-/microporous zeolites was reported to effectively affect the selectivity of de-alkylation and de-etherification reactions in parallel during lignin conversion (Taarning et al, 2011;Singh and Ekhe, 2014;Deepa and Dhepe, 2015). Deepa reported that aromatic monomers were the predominant products using zeolite as solid acid catalysts for lignin depolymerization at 250°C in H2O/CH3OH mixture (Deepa and Dhepe, 2015).…”

Catalytic alcoholysis of alkaline extracted lignin for the production of aromatic esters over SO42−/ZrO2-ATP

“…4 As a natural amorphous three-dimensional polymer consisting mainly of methoxylated phenylpropane units, 2,5 lignin is linked to other main components, such as cellulose and hemicellulose, in a variety of ways, including hydrogen, ester, and covalent bonds. 6 Depolymerization of lignin is the key step towards making lignin cost-effective as a raw material, 7 but its highly random and branched three-dimensional polyphenolic structure leads to its resistance to depolymerization.…”

Mechanism of Alkaline Lignin Oxidation Using Laccase-methyl Syringate Mediator System

“…The rest of the available lignin is usually burned for providing the heat (Chapple et al, 2007) that does not satisfy the need for effective power generation. Different ways of lignin recycling by using catalysts and ionic liquids are reflected in numerous publications (Deepa & Dhepe, 2015., Nanayakkara et al, 2014 however these approaches are often expensive and do not meet the requirements for sustainable lignin recycling. One of the environmentally friendly approaches is to use naturally existing enzymes that are capable of degrading lignin and biomass (Christopher et al, 2014).…”