Max-Planck-Institut für Kohlenforschung, 45470 Mülheim (Ruhr), Germany.
Faraday Discuss. 2017 Sep 21;202:403-413. doi: 10.1039/c7fd00069c.
In recent years, lignin valorization has gained upward momentum owing to advances in both plant bioengineering and catalytic processing of lignin. In this new horizon, catalysis is now applied to the 'pulping process' itself, creating efficient methods for lignocellulose fractionation or deconstruction (here referred to as Catalytic Upstream Biorefining or 'CUB'). These processes render, together with delignified pulps, lignin streams of low molecular weight (M) and low molecular diversity. Recently, we introduced a CUB process based on Early-stage Catalytic Conversion of Lignin (ECCL) through H-transfer reactions catalyzed by RANEY® Ni. This approach renders a lignin stream obtained as a viscous oil, comprising up to 60 wt% monophenolic compounds (M < 250 Da). The remaining oil fraction (40 wt%) is mainly composed of lignin oligomers, and as minor products, holocellulose-derived polyols and lignin-derived species of high M (0.25-2 kDa). Simultaneously, the process yields a holocellulose pulp with a low content of residual lignin (<5 wt%). Despite the efficiency of aqueous solutions of 2-propanol as a solvent for lignin fragments and an H-donor, there is scant information regarding the CUB process carried out in the presence of primary alcohols, which often inhibit the catalytic activity of RANEY® Ni, as revealed in model compound studies performed at low temperature. Considering the composition of the lignin oils obtained from CUB based on ECCL, the processes commonly render ortho-(di)methoxy-4-propylphenol derivatives with a varied degree of defunctionalization of the propyl side chain. In this contribution, we present the role of the alcohol solvent (methanol or 2-propanol) and Ni catalyst (Ni/C or RANEY® Ni) in control over selectivity of phenolic products. The current results indicate that solvent effects on the catalytic processes could hold the key for improving control over the degree of functionalization of the propyl side-chain in the lignin oil obtained from CUB, offering new avenues for lignin valorization at the extraction step.
近年来，由于植物生物工程和木质素催化加工技术的进步，木质素的增值利用得到了迅猛发展。在这个新领域中，催化现在已经应用到“制浆过程”本身，创造了用于木质纤维素分级或解构的有效方法（这里称为催化上游生物炼制或“CUB”）。这些过程与去木质素浆一起，产生低分子量（M）和低分子多样性的木质素流。最近，我们通过 RANEY® Ni 催化的 H 转移反应引入了一种基于木质素早期催化转化（ECCL）的 CUB 工艺。该方法产生一种粘性油形式的木质素流，其中包含高达 60wt%的单酚化合物（M<250Da）。剩余的油部分（40wt%）主要由木质素低聚物组成，作为次要产物，包含有源自全纤维素的多元醇和源自木质素的高 M 物种（0.25-2 kDa）。同时，该工艺产生一种残余木质素含量低（<5wt%）的全纤维素浆。尽管 2-丙醇的水溶液作为木质素片段和 H 供体的溶剂效率很高，但关于在伯醇存在下进行 CUB 工艺的信息很少，正如在低温下进行的模型化合物研究所示，伯醇常常会抑制 RANEY® Ni 的催化活性。考虑到从基于 ECCL 的 CUB 中获得的木质素油的组成，这些工艺通常会生成具有不同程度的丙基侧链去功能化的邻-（二）甲氧基-4-丙基苯酚衍生物。在本贡献中，我们介绍了醇溶剂（甲醇或 2-丙醇）和 Ni 催化剂（Ni/C 或 RANEY® Ni）在控制酚类产物选择性方面的作用。目前的结果表明，溶剂对催化过程的影响可能是控制从 CUB 获得的木质素油中丙基侧链的功能化程度的关键，为在提取步骤中提高木质素的增值利用提供了新途径。
