Formation and Fate of Carboxylic Acids in the Lignin-First Biorefining of Lignocellulose via H-Transfer Catalyzed by Raney Ni

Abstract

Lignin-first biorefining constitutes a new research field in which the overarching objective is the prevention of lignin recalcitrance while providing high-quality pulps. For this purpose, the solvent extraction of lignin is performed in the presence of a hydrogenation catalyst, employing H2 pressure or an H-donor solvent (e.g. 2-propanol), and thus leading to passivation of reactive lignin fragments via reductive processes. As a result, lignin-first biorefining methods generate high-quality pulps in addition to low-molecular-weight lignin streams with high molecular uniformity. Nonetheless, upon cooking lignocellulose in solvent mixtures containing water, other processes on the lignocellulosic matrix take place, releasing soluble intermediates. In fact, hemicellulose undergoes deacetylation, to a variable extent, releasing acetic acid into the liquor. Moreover, formic acid can also be formed as a degradation product of hemicellulose C6-sugars also released into the liquor. However, despite this general notion, the formation and fate of these carboxylic acids during the cooking of lignocellulosic substrates, and the effects these acids may have on hydrogenation catalyst performance remain poorly understood. In this report, we examine both the formation and subsequent fate of formic acid and acetic acid during lignocellulose deconstruction for both a lignin-first biorefining method (via H-transfer reactions in the presence of Raney Ni catalyst) and its equivalent Organosolv process with no added acid or hydrogenation catalyst. An apparent mechanism for the mitigation of formic acid formation in the presence of Raney Ni catalyst is outlined via the hydrogenation of sugars to sugar alcohols. Furthermore, the effects of the carboxylic acids on Raney Ni performance are assessed, using the transfer-hydrogenation of phenol to cyclohexanol/cyclohexanone as a model reaction, elucidating inhibition rates of the acids. Finally, we conclude with the implications of these results for the design of lignin-first biorefining processes. In a broader context, understanding of the formation and fate of carboxylic acids during CUB is crucial to producing high-quality pulps with high degrees of polymerization and high xylan contents. the higher pH of CUB liquors compared to those from the Organosolv process (∆pH = 1-2), suggesting that Raney Ni should be able to decompose acidic content in the liquor by reductive processes, (7),(13) mostly in the form of acetic acid and formic acid. (6),(7),(14)-(19) Acetic acid is produced via the hydrolysis of the acetyl groups (deacetylation) of hemicellulose. The concentration of acetic acid is especially significant in the hydrolysates of hardwoods as Oacetyl groups substitute a considerable number of the OH-groups of the xylose units. (18),(20),(21) Indeed, acetyl groups account for 3-5% of hardwood's composition, while this number is 1-2% in softwoods. (21) On the other hand, formyl groups rarely occur in woods. Typically, formyl contents do not exceed 0.04% of the wood composition. (21) It...

Lignin-First Biorefining of Lignocellulose: the Impact of Process Severity on the Uniformity of Lignin Oil Composition

“…The hydrogenation of hemicellulose sugars acts as an effective strategy to suppress the degradation of hemicellulose sugars, stopping the formation of formic acid. 22,24 The reduction in the content of formic acid in the liquor is conducive for both high xylan retention and the production of cellulosic pulps with a high degree of polymerization, as recently reported by us. 24 Demonstrated by several research groups, lignin-first biorefining produces four distinct categories of lignin platform chemicals, depending on the process conditions and catalyst employed, as summarized in Scheme 2.…”

“…22,24 The reduction in the content of formic acid in the liquor is conducive for both high xylan retention and the production of cellulosic pulps with a high degree of polymerization, as recently reported by us. 24 Demonstrated by several research groups, lignin-first biorefining produces four distinct categories of lignin platform chemicals, depending on the process conditions and catalyst employed, as summarized in Scheme 2. Notably, the degree of functionalization of the propyl side chain (R) of the lignin-derived phenolic products constitutes the feature distinguishing the catalytic systems already reported.…”

Reductive catalytic fractionation of pine wood: elucidating and quantifying the molecular structures in the lignin oil

“…[28][29][30][31][32] In a typical RCF process, biomass is contacted with a polar, protic solvent (e.g. MeOH, 4,29,[32][33][34][35][36][37][38][39][40][41] EtOH/H 2 O, [42][43][44] iPrOH/H 2 O [45][46][47] ), a heterogeneous redox catalyst (e.g. Pd/C, 34,35,42,[48][49][50] Ru/C, 33,51 Ni on various supports 29,31,32,37,45,52 ) and a hydrogen source at elevated temperatures (200-250 C).…”

A combination of experimental and computational methods to study the reactions during a Lignin-First approach

“…Formic acid was used as a hydrogen source (Fig. 8) since it can be formed from the biomass under the pulping conditions [29]. A primary isotope effect of 5 was observed, which support that the hydrogen transfer is slower than the desorption of product 7 from the Pd/C surface.…”