The products of base-catalyzed liquid-phase hydrolysis of lignin depend markedly on the operating conditions. By varying temperature, pressure, catalyst concentration, and residence time, the yield of monomers and oligomers from depolymerized lignin can be adjusted. It is shown that monomers of phenolic derivatives are the only primary products of base-catalyzed hydrolysis and that oligomers form as secondary products. Oligomerization and polymerization of these highly reactive products, however, limit the amount of obtainable product oil containing low-molecular-weight phenolic products. Therefore, inhibition of concurrent oligomerization and polymerization reactions during hydrothermal lignin depolymerization is important to enhance product yields. Applying boric acid as a capping agent to suppress addition and condensation reactions of initially formed products is presented as a successful approach in this direction. Combination of base-catalyzed lignin hydrolysis with addition of boric acid protecting agent shifts the product distribution to lower molecular weight compounds and increases product yields beyond 85%.

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Lignin constituting about 20 % of terrestrial biomass is a most interesting feedstock, as it contains a large fraction of phenolic motifs linked by ether bonds and would, therefore, offer an ideal starting point for the synthesis of functionalized phenols. It consists predominantly of alpha-and beta-O-4 ether bonds, which are easy to hydrolyze. Hydrolysis of the aryl-aryl ether bond, which represents 3-4 % of all lignin bonds, however, requires drastic conditions and much higher temperatures. Diphenyl ether (DPE) is, therefore, the most simple model compound to determine effective ways to selectively convert these more stable ether bonds in lignin. The drastic conditions may require using water above its supercritical point (at 374 8C and 22 10 6 Pa).Calculations show that the hydrolysis of DPE to phenol is thermodynamically not limited above 120 8C, when applying a pressure of 25 10 6 Pa and excess water (water to DPE ratio of 35:1). Literature reports only minor DPE conversions at reaction temperatures below 460 8C after 4 h reaction time in the absence of a catalyst. [1][2][3][4][5] Siskin et al. reported a 92 % conversion of DPE to phenol in 15 % phosphoric acid at 315 8C after 3 days. To speed up rates of conversion, they attempted converting DPE in supercritical water, 15 % aqueous formic acid, 15 % aqueous sodium formate, and, to investigate thermal cleavage, also in cyclohexane at 460 8C. In contrast to the initial experiment, at lower temperature and with phosphoric acid, DPE was shown to be unreactive, when reacted in these media and at these conditions, respectively. However, an attempt to catalytically convert DPE in a 15 % Na 2 CO 3 solution resulted in 33 % conversion to phenol. [2][3] Thus, a base-catalyzed mechanism was speculated to be operative under these conditions suggesting a nucleophilic attack of hydroxide anion on DPE with subsequent loss of phenolate. The conversion of DPE in supercritical water can proceed following two different reaction mechanisms (Scheme 1). [2][3] The ionic pathway, namely hydrolysis, leads to the formation of 2 mol phenol for 1 mol of converted DPE (Scheme 1 a). A radical mechanism, in which the ether bond is homolytically cleaved, generates a phenoxy and a phenyl radical (Scheme 1 b). These radical species can abstract hydrogen to form phenol and benzene (reaction b 1) or undergo recombination with themselves, DPE, phenol, or intermediates leading to dimers such as 4-hydroxy biphenyl (reaction b 2 and b 3) or even higher molecular compounds (e.g., phenoxy biphenyl).In an attempt to understand the potential of catalytic hydrolytic cleavage of the aryl-aryl ether bond and to find an appro-priate heterogeneous catalyst, we explored the role of ZrO 2 supported K 2 CO 3 , as well as unsupported alkali carbonates for the catalytic hydrolysis of diphenyl ether in sub-and supercritical water. The impact of solvent density on conversion and product selectivity is discussed in terms of ionic and radical reaction pathways. We showed that even at such drastic conditions, cata...