Tandem Catalytic Depolymerization of Lignin by Water‐Tolerant Lewis Acids and Rhodium Complexes

Abstract

Lignin is an attractive renewable feedstock for aromatic bulk and fine chemicals production, provided that suitable depolymerization procedures are developed. Here, we describe a tandem catalysis strategy for ether linkage cleavage within lignin, involving ether hydrolysis by water‐tolerant Lewis acids followed by aldehyde decarbonylation by a Rh complex. In situ decarbonylation of the reactive aldehydes limits loss of monomers by recondensation, a major issue in acid‐catalyzed lignin depolymerization. Rate of hydrolysis and decarbonylation were matched using lignin model compounds, allowing the method to be successfully applied to softwood, hardwood, and herbaceous dioxasolv lignins, as well as poplar sawdust, to give the anticipated decarbonylation products and, rather surprisingly, 4‐(1‐propenyl)phenols. Promisingly, product selectivity can be tuned by variation of the Lewis‐acid strength and lignin source.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

“…[425] Ah omogeneous rhodium catalyst was selected for the decarbonylation step. [425] Them ethod (Scheme 14 g) was first validated with various model compounds to appropriately match the rate of the hydrolysis and decarbonylation steps,i ncluding the conversion of 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol to give guaiacol and 4-methylveratrole in 88 %a nd 51 %y ield, respectively.C leavage occurs via initial a-b dehydration, and subsequent hydrolysis of the resultant styryl ether, catalysed by the metal triflates.D ecarbonylation of the reactive aldehyde intermediate then affords am ethyl-substituted aromatic compound. Thed ehydration/hydrolysis of a-hydroxy-b-ethers has also previously been reported using abase [360] or methyldioxorhenium as catalyst.…”

“…Efficient depolymerisation of lignin by tandem hydrolysisdecarbonylation has been demonstrated with water-stable Lewis acids (such as scandium(III) or indium(III) triflate) rather than aB r ønsted acid, to catalyse the first ether hydrolysis step. [425] Ah omogeneous rhodium catalyst was selected for the decarbonylation step. [425] Them ethod (Scheme 14 g) was first validated with various model compounds to appropriately match the rate of the hydrolysis and decarbonylation steps,i ncluding the conversion of 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol to give guaiacol and 4-methylveratrole in 88 %a nd 51 %y ield, respectively.C leavage occurs via initial a-b dehydration, and subsequent hydrolysis of the resultant styryl ether, catalysed by the metal triflates.D ecarbonylation of the reactive aldehyde intermediate then affords am ethyl-substituted aromatic compound.…”

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

“…For instance, a two-step strategy has been developed for aspen lignin, consisting of selective oxidation of the secondary alcohol of b-O-4 linkages followed by a redox-neutral bond cleavage at 110 C. 5,15 A similar strategy was also applied to the depolymerization of a reactive dioxasolv birch lignin (an organosolv lignin extracted by 1,4-dioxane) at 80 C. 16 The use of a hydrosilane reductant and a Lewis acidic (B(C 6 F 5 ) 3 ) catalyst facilitated the conversion of extracted wood lignin into mono-aromatics with a yield of 7-24 wt% at room-temperature. 17 A combination of triic acid and a heterogeneous Ru or homogeneous Ir catalyst was also effective in depolymerizing dioxasolv lignin in 1,4-dioxane at 140 C. 18 A similar catalytic system involving the use of a metal triate and a homogeneous Rh catalyst was employed for the depolymerization of dioxansolv lignins in a 1,4dioxane/water solvent mixture at 175 C. 19 Two-step approaches consisting of a mild pretreatment of woody biomass followed by catalytic depolymerization have also been reported. 20,21 A suitable pretreatment method for the preparation of reactive lignin that preserves high ether linkage content is essential to these mild lignin depolymerization approaches.…”

Production of Jet Fuel‐Range Hydrocarbons from Hydrodeoxygenation of Lignin over Super Lewis Acid Combined with Metal Catalysts

“…The yield of other lignin depolymerization products, such as monomer and dimer aromatics, was also quite low (<10 wt %). The stability and reusability of metal triflates were proven very good . A slight efficiency loss in hydrocarbon production from lignin was found when the catalysts were reused, and this was probably due to the formation of some clusters in the Ru‐based catalyst.…”

“…The strong electron‐withdrawing − OTf group (CF 3 SO 3 − ) can make the metals in metal triflates very cationic. The cationic metals can selectively bond with electron‐rich atoms (e.g., oxygen atoms in β‐O‐4 and α‐O‐4 ether bonds) and promote the cleavage of related chemical bonds ,. Moreover, the phenolic hydroxyl groups in lignin can be exchanged with − OTf groups, facilitating the removal of oxygen on aromatic rings.…”