Selective production of mono-aromatics from lignocellulose over Pd/C catalyst: the influence of acid co-catalysts

Abstract

The 'lignin-first' approach has recently gained attention as an alternative whole biomass pretreatment technology with improved yield and selectivity of aromatics compared with traditional upgrading processes using technical lignins. Metal triflates are effective co-catalysts that considerably speed up the removal of lignin fragments from the whole biomass. As their cost is too high in a scaled-up process, we explored here the use of HCl, HSO, HPO and CHCOOH as alternative acid co-catalysts for the tandem reductive fractionation process. HCl and HSO were found to show superior catalytic performance over HPO and CHCOOH in model compound studies that simulate lignin-carbohydrate linkages (phenyl glycoside, glyceryl trioleate) and lignin intralinkages (guaiacylglycerol-β-guaiacyl ether). HCl is a promising alternative to the metal triflates as a co-catalyst in the reductive fraction of woody biomass. Al(OTf) and HCl, respectively, afforded 46 wt% and 44 wt% lignin monomers from oak wood sawdust in tandem catalytic systems with Pd/C at 180 °C in 2 h. The retention of cellulose in the solid residue was similar.

Catalytic Scissoring of Lignin into Aryl Monomers

“…The monomer yield and distribution are profoundly affected by the catalyst type, biomass feedstock, solvent, and reaction conditions. Ni/C, TiN‐Ni, Ni/ZSM‐5, Ni/Al 2 O 3 , Ni‐W 2 C/C, Ru/β, Ru/C, and Pd/C are the most used RCF catalysts. Bimetallic catalytic systems have also been developed, and the synergistic effect improves the catalytic performance.…”

“…Abu‐Omar and co‐workers reported the fragmentation of various lignin samples with a bimetallic Pd/Zn catalyst system . The combination of acid and the supported metal catalyst could also enhance the monomer yields …”

Fractionation of Lignocellulosic Biomass over Core–Shell Ni@Al2O3 Catalysts with Formic Acid as a Cocatalyst and Hydrogen Source

“…As discussed in the role of Ni x –Al/AC and formic acid section, the role of formic acid in the RCF of wood is to act as an acid catalyst to enhance the separation of lignin from the wood matrix, and to act as a hydrogen source to stabilize LDPMs. In the early stages of the reaction, formic acid can act as an acid catalyst to enhance cleavage of ether and ester bonds between carbohydrates and lignin in the cell wall matrix, resulting in enhanced delignification . At later reaction times, formic acid is catalytically decomposed to produce H 2 .…”

“…In addition, the ethanol‐driven etherification of the α‐carbon of the lignin structure reduces the bond energy of β‐O‐4, which increases the degree of depolymerization of lignin . Formic acid has multiple roles in the RCF reaction sequence; in the fractionation stage, early in the RCF sequence, formic acid acts as an acid cocatalyst to enhance the separation of lignin, and later, during the stabilization of separated lignin, formic acid acts as a hydrogen source to enhance the yield of LDPMs . The composition of LDPMs (e.g., guaiacol and syringol substituted with propyl, propenyl, and propanol groups, Figure ) can be controlled to some extent by adjusting the process parameters (e.g., temperature, reaction time, quantity of formic acid, presence of external hydrogen, type of catalyst, catalyst/feed ratio).…”

Liquid phase pyrolysis of wheat straw and poplar in hexadecane solvent

“…3). It is known that mineral acids, such as sulfuric and hydrochloric acid can enhance the bio-oil production by fragmentation of lignin [31,32]. In present study heterogeneous (solid) acidic/basic catalysts were used.…”