On the effect of methanol on the control over the defunctionalization of the propyl side chain of phenolics obtained from catalytic upstream biorefining
In the 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 (Mw) and low molecular diversity. Recently, we introduced a CUB process based on Early-stage Catalytic Conversion of Lignin (ECCL) through H-transfer reactions catalysed by Raney Ni. This approach renders a lignin stream obtained as a viscous oil, comprising up to 60 wt.% monophenolic compounds (Mw <250 Da). The remaining oil fraction (40%) is mainly composed of lignin oligomers, and as minor products, holocellulose-derived polyols and lignin-derived species of high Mw (0.25-2 kDa). Simultaneously, the process yields a holocellulose pulp with low content of residual lignin (<5%). Despite the efficiency of aqueous solutions of 2-propanol as a solvent for lignin fragments and an H-donor, there is not much 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 compounds studies carried out at low temperatures. Considering the composition of the lignin oils obtained from CUB based on ECCL, the processes commonly render ortho-(di)methoxy-4-propylphenol derivatives with varied degree of defunctionalization of the propyl side chain. In this communication, we will present the role of the alcohol solvent (methanol or 2-propanol) and Ni catalyst (Ni/C or Raney Ni) in the control over selectivity of phenolic products. The current results indicate that solvent effects on the catalytic processes could well hold the key to improve the control over the degree of defuncionalization of the propyl-side chain in the lignin oil obtained from CUB.