On the Reactivity of Dihydro‐p‐coumaryl Alcohol towards Reductive Processes Catalyzed by Raney Nickel

Abstract

There are severalestablished approaches for the reductive fractionation of lignocellulose (e.g.," catalytic upstream biorefining" and "lignin-first"approaches) that lead to alignin oil product that is composed primarily of dihydro-p-monolignols [e.g., 4-(3-hydroxypropyl)-2-methoxyphenol and 4-(3-hydroxypropyl)-2,6-dimethoxyphenol].A lthough effective catalytic methods have been developed to perform reductive or deoxygenative processeso nt he lignin oil, the influence of the 3-hydroxypropyls ubstituent on catalysta ctivity hasp reviously been overlooked. Herein, to better understand the reactivity of the depolymerized lignin oil obtained from catalytic upstream biorefiningp rocesses, dihydro-p-coumaryl alcohol was selected as am odel compound. Hydrogenation of this speciesi nt he presence of Raney Ni with molecular hydrogen led to ring saturation (100 %s electivity) in the absence of hydrodeoxygenation, whereas under hydrogen-transfer conditions with 2-propanol, hydrogenation occurred ( % 55 %s electivity) simultaneously with hydrodeoxygenation ( % 40 %s electivity). In ab roader context,t his study sheds light not only on the reactivity of dihydro-p-monolignols but also on the intricacies of the catalytic upstream biorefiningr eaction network in which these species are revealed to be key intermediates in the formation of lessfunctionalized p-alkylphenols.Undoubtedly,l ignin constitutest he primary renewable source of bulk and specialty aromatic chemicals.[1] Recently,s everal catalytic routes have been developed to convert lignocellulosic biomass into am ixture of compounds that show potential to replacep etroleum-basedr aw materials.[2] In this context,t he emerging field of "catalytic upstream biorefining" (CUB, sometimes referred to as "reductive fractionation" or the "ligninfirst" approach) has attracted increasinga ttention.[1a, 3] CUB encompasses processes for plant biomass deconstruction through the early-stage catalytic conversion of lignin (ECCL) by the action of ah ydrogenation catalyst.[1a] In our CUB process, on the basis of the ECCL by hydrogen-transfer( H-transfer) reactions catalyzed by Raney Ni, the lignin fractioni si solated as av iscous oil.[4] This lignin stream is rich in monophenolic species (< 65 %). Notably,t he type of functionalg roups on the propyls ide chain of the monophenolic compounds depends on both the selected catalysta nd the hydrogen source employed in the process. [4a, 5] Furthermore, the lignin oil also contains dimers and low-molecular-weight lignin oligomers (M w < 1000 Da) as minor products (> 35 %). Amid the monophenolic products obtained by H-transfer CUB in the presence of Raney Ni, two major products,d ihydro-p-sinapyl alcohol (1, Figure 1) and dihydro-p-coniferyl alcohol (2,F igure 1), are obtained from the biorefining of hardwoods. Understandably,t he CUB of (G-lignin-enriched) softwoods generates 2 as the major product.Products 1 and 2 incorporate three classes of oxygenated functional group, namely,the phenolic OH substituent, the methoxy group(s) ortho to the ph...

Hydrogenolysis of Lignin-Derived Aromatic Ethers over Heterogeneous Catalysts

“…However, care must be taken in drawing firm conclusions about such surface orientations since the solvent was switched from Me-THF to cyclohexane for these ATR-IR adsorption measurements, which were also conducted at lower temperature than full reactions (and in absence of i-PrOH or H 2 coreactants); it is unclear whether the orientation of these adsorbed ethers is temperature sensitive of influenced by solvent competitive adsorption. Indeed, ATR-IR flow studies suggest that alcohol solvents inhibit the catalytic transfer hydrogenation of phenol to cyclohexanol over RANEY Ni through competitive adsorption with 2-propanol (as H-donor); , primary alcohols and diols can displace 2-propanol from the catalyst surface, lowering CTH activity. Catalyst inhibition by alcohols released from holocellulose during biomass fractionation could limit the viability of CTH routes for lignin valorization.…”

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

“…Compounds 2a-2c derive from the loss of hydroxyl group in the side chain of 1, while 3 can be the product of the cleavage of the C β -C γ bond of the 4-(3-hydroxypropyl)phenols (1), resulting in the release of formaldehyde. 40 Table 3 compares the mole ratio 1a:1b:1c with the expected monomer composition and reveals information on the extent of demethoxylation.…”

Elucidating the reactivity of methoxyphenol positional isomers towards hydrogen-transfer reactions by ATR-IR spectroscopy of the liquid–solid interface of RANEY® Ni

“…5 The use of liquid organic H-donors instead of H2 enables a considerable degree of control over the reaction in terms of selectivity for given products. 5,13 In fact, organic Hdonors could transfer hydrogen through different mechanistic pathways to the substrate (e.g. hydride transfer or direct hydrogen transfer also known as Meerwein−Ponndorf−Verley mechanism) and compete with the reactants and intermediates The conversion of 2-methoxyphenol involves firstly the DMO of the phenolic substrate, which is followed by the HYD of the phenolic intermediate to the corresponding cyclohexanol.…”