The Role of the Reactor Wall in Hydrothermal Biomass Conversions

Abstract

The processing of renewable feedstocks to platform chemicals and, to a lesser degree, fuels is a key part of sustainable development. In particular, the combination of lignocellulosic biomass with hydrothermal upgrading (HTU), using high temperature and pressure water (HTPW), is experiencing a renaissance. One of the many steps in this complicated process is the in-situ hydrogenation of intermediate compounds. As formic acid and related low-molecular-weight oxygenates are among the species generated, it is conceivable that they act as a hydrogen source. Such hydrogenations have been suggested to be catalyzed by water, by bases like NaOH, and/or to involve "reactive/nascent hydrogen". To achieve the temperatures and pressures required for HTU, it is necessary to conduct the reactions in high-pressure vessels. Metals are typical components of their walls and/or internal fittings. Here, using cyclohexanone as a model compound for more complex biomass-derived molecules, iron in the wall of high-pressure stainless steel reactors is shown to be responsible for the hydrogenation of ketones with low-molecular-weight oxygenates acting as a hydrogen source in combination with water.

Exploring the Myth of Nascent Hydrogen and its Implications for Biomass Conversions

“…However, during these reductions neither iron-carbonyl nor iron-hydride signals were detected (irrespective of whether sulfuric or formic acid was used). Nonetheless, as was shown previously, [18] the formation of catalytically inactive iron salts (formate or sulfate) occurred (see the Supporting Information for time-resolved IR spectra). When analyzing the gaseous product(s) formed (both in the presence and absence of cyclohexanone), only hydrogen could be detected.…”

“…It should be noted that the present study is the companion piece to its predecessor, [18] in which it was demonstrated that under the conditions used for hydrothermal upgrading (HTU) of lignocelluosic biomass, the walls of typical stainless steel reactors are able to effect ketone reductions in the presence of formic acid, itself a product typically formed under the HTU conditions.…”

“…[19] Use of either propionic acid or gaseous H 2 as the reductant under ambient conditions did not result in any conversion, which is consistent with the results that were observed under HTPW conditions in the presence of both SSS and Fe metal. [18] In considering the mechanism of the hydrogenation, there are two in-principle possibilities:…”

Unravelling Some of the Key Transformations in the Hydrothermal Liquefaction of Lignin

“…In these processes the stainless-steel reactor wall potentially acted as ac atalystf or the (de)hydrogenationsa nd hydrogenolyses. [23] Similarly,w hen vanillin was heated under neutral conditions only at race of guaiacol was generated. However,h eating under basic conditions generatedavariety of products,i nclud-ing guaiacol, catechol, and 4-methylguaiacol.…”

Promising Techniques for Depolymerization of Lignin into Value‐added Chemicals

“…On the other hand, high reactive oxygen species (ROS) can be generated by the Fenton‐like reaction between zero‐valent iron (ZVI) and O 2 or H 2 O 2 at room temperature . Besides, it is confirmed that the reactor wall of stainless steel has evident activity for catalytic hydrogenation using formic acid as hydrogen source . In a word, we speculate that the inconsistence in main products formed by this method is strongly affected by alkalinity of solution and reactor material.…”