Common Pathways in Ethanolysis of Kraft Lignin to Platform Chemicals over Molybdenum-Based Catalysts

Abstract

One-pot complete catalytic ethanolysis of Kraft lignin into C6–C10 chemicals, that is, aliphatic alcohols, esters, phenols, benzyl alcohols, and arenes, is achieved with a batch reactor over a number of supported molybdenum-based catalysts at 553 K in pure ethanol under autogenous pressure of 10.6 MPa. Metallic molybdenum, its carbide, and nitride all show remarkable activity, with the carbide and metallic catalysts giving the higher overall yields: 1640 and 1390 mg/g lignin, respectively. The major phases composing the catalysts are well-preserved after the reaction; however, the detection of Mo­(V) species verifies the partial oxidation of molybdenum, which leads to the formation of the dissociative Mo species, such as molybdenum V ethoxide, in the fluid phase. Through the product analysis and catalyst characterization, the common route of lignin conversion to value added chemicals over the Mo-based catalyst is presented in detail. Kraft lignin is first fragmented into segments with m/z ∼ 700–1400 via a noncatalytic ethanolysis process. Meanwhile, the main active Mo­(V) species dissociate from the solid catalyst into the fluid due to the interaction of ethanol. Then mainly the dissociative species catalyze, with the participation of the radicals, the further degradation of the segments into small molecules.

Ethanolysis of Kraft lignin to platform chemicals on a MoC1-x/Cu-MgAlOz catalyst

“…For example, ethyl esters of carboxylic acid can be generated by the esterification reaction between ethanol and lignin-derived intermediates, such as hexenuronic acid formed in Kraft pulping [33]. Furthermore, the alcohols and esters formation were also reported in our previous work and the works done with woody biomass and a copper-doped Mg/Al mixed-oxide catalyst [24,31,34,35]. However, it's hard to determine how much of C6 alcohols and C8-C10 esters are produced from Kraft lignin.…”

“…The new catalyst gave a much higher overall yield than those reported results with MoC1-x and Cu-MgAlOx catalysts, which implies a synergistic effect between the two materials. Compared to Cu-MgAlOx, a heterogeneous catalyst, MoC1-x/Cu-MgAlOx can form molybdenum ethoxide (Mo(OEt)5) during the reaction in ethanol as we previously reported[35] , which is a homogeneous catalyst so that MoC1-x/Cu-MgAlOx functions as both homogeneous and heterogeneous catalysts simultaneously. Hence, more radicals and ions were formed from ethanol with the MoC1-x/Cu-MgAlOx catalyst than that with Cu-MgAlOx as the catalyst.…”

“…The yield of aromatics is higher than the amount of Klason lignin, 45.7%, which are caused by the alkylation of aromatics.Fig. 6(b)lists the yields of the compounds from ethanol self-reactions.The yields of acetic acid ethyl ester, butanol, ethyl butyrate, butyl acetate and isomerized octenoic acid ethyl ester tend to increase with the increase of temperature, which implies a higher concentration of ethanol derived radicals and ions facilitating the fragmentation of Kraft lignin[35]. That partially explains why the yield of aromatic compounds increases with the temperature.…”

Metal Carbides for Biomass Valorization

“…Kraft lignin is waste produced during the utilization of lignocellulosic biomass in paper production [67]. Lignin is a complex and irregular macromolecule, constituting of mainly hydroxyphenyl propane and aliphatic chains [5,68].…”

“…Ma et al reported on the transformation of Kraft lignin into valuable chemicals over a molybdenum carbide catalyst in supercritical ethanol [67]. The products of reaction were a mixture of aliphatic alcohols, esters, monophenols, benzyl alcohols and arenes, with ester the predominant product (yield of 947 mg/g of lignin).…”