Engineering the Reaction Environment: Hydropyrolysis of Lignin in Molten Salts with Catalytic Vapor-Phase Upgrading in a Tandem Micropyrolyzer

Abstract

The continuous exploration and subsequent optimization of new biomass conversion strategies to provide biobased chemicals and/or fuels remains key to build a biobased economy. In that regard, we herein explore the use of an eutectic mixture of chloride molten salts as a (catalytic) heat transfer medium to assess its influence on the pyrolysis of LignoBoost kraft lignin (LKL) toward (deoxygenated) depolymerization products (i.e., monoaromatics). The first goal of this study was to determine the advantages of incorporating chloride molten salts (ZnCl 2 /KCl/ NaCl in a molar ratio of 44.3:41.9:13.8 mol %) to LKL hydropyrolysis and identify the optimal process conditions for it. The studied process variables were pressure (0.4−3.0 MPa), biomass mass fraction in the biomass−salt mixture (9−24 wt %), and temperature (350−450 °C). For this first goal, benchmark studies of pure LKL hydropyrolysis were also considered. Second, this study set out to determine which of the constituting chloride salts within the salt mixture were the catalytic ones. Third, this research focused on the comparison of different scenarios for LKL high-pressure hydropyrolysis. These consisted of ex situ catalytic hydropyrolysis and molten salt hydropyrolysis with subsequent ex situ hydrotreatment. Four catalysts were used for these scenarios: HZSM-5, sulfided NiMo (NiMLo-S), Pt/C, and sulfided CoMo (CoMo-S). Especially in the latter case, the aim was to lower the oxygenated phenolics (methoxyphenols) and promote the production of aromatics (BTX and alkyphenols) and aliphatic hydrocarbons. Overall, high-pressure hydropyrolysis of LKL at 450 °C within molten salts shifted the product portfolio toward alkylphenols (rather than methoxyphenols without molten salt). The optimal conditions for LKL hydropyrolysis with molten salt were determined to be 3.0 MPa, 400−450 °C, and a biomass mass fraction in the biomass−salt mixture of 24 wt %. Under these conditions, 23.5 wt % mass yield in volatiles and 25.2 wt % carbon yield in volatiles were obtained. The addition of ex situ upgrading catalyst did not seem to upgrade the products more, than the salts already did, in terms of degree of deoxygenation. Moreover, this work provides insights into the formation of chloromethane and HCl from the interaction of the molten chloride salts with (chemical) water and/or with the produced organic volatiles upon pyrolysis.