Cleavage and hydrodeoxygenation (HDO) of C–O bonds relevant to lignin conversion using Pd/Zn synergistic catalysis

Abstract

The development of chemical methods for the direct catalytic conversion of biomass to high value organic molecules is an area of increasing interest. The plant matter component known as lignin is a polymer consisting of aromatic rings that could provide a means of obtaining aromatic materials currently derived solely from petroleum. This report describes a bimetallic Pd/C and Zn catalytic system that can perform selective hydrodeoxygenation (HDO) of monomeric lignin surrogates as well as successfully cleave the b-O-4 linkages found in dimeric lignin model complexes and synthetic lignin polymers with near quantitative conversions and yields between 80-90%. The reaction with lignin polymer was highly selective affording methoxy substituted propylphenol as the major product. These reactions were performed in a Parr reactor operating at relatively mild temperature (150 C) and pressure (20 bar H 2 ) using methanol as a solvent. Reaction products were characterized using high-pressure liquid chromatography coupled to a linear quadrupole ion trap mass spectrometer equipped with an electrospray ionization source using negative ion mode. Hydroxide ions were doped into the analyte solutions to encourage negative ion formation. This method ionizes all the mixture components to yield a single ion/analyte with no fragmentation. The catalyst is fully recyclable without the need for additional zinc. X-ray absorption spectroscopy (EXAFS) is consistent with Pd nanoparticles (4-5 nm) and no evidence of Pd-Zn alloy formation. A mechanistic hypothesis on the synergy between Pd and Zn is presented.

An Overview on Catalytic Hydrodeoxygenation of Pyrolysis Oil and Its Model Compounds

“…One reason for the relatively low hydrogenation rate of 5-5′ dimer might be caused by its intramolecular hydrogen bonds that could hinder the attack of the catalyst. Parsell et al used Zn/Pd/C catalyst for HDO of guaiacylglycerol-b-guaiacyl ether under relatively mild condition, 423 K and 20.7 bar hydrogen pressure, got complete conversion and yielded two main aromatic products [36]. The β-O-4 linkage was effectively cleaved to aromatic fragments, and its alcohol oxygen on alkyl chains was subsequently removed with loss of aromatic groups.…”

Hydrodeoxygenation of Lignin-Derived Phenols: From Fundamental Studies towards Industrial Applications

“…The same was valid for HDO of vanillin which maximally gave 65% selectivity towards methylcyclohexane at 300 °C under 50 bar over Ni/SiO2-ZrO2 catalyst (Table 1, entry 16). The main product from vanillyl alcohol HDO is 1,4-dimethoxyphenol using ZnCl2 as a catalyst in methanol (Table 1, entry 17) [21]. Transanthole contains both propyl and methoxy substituents and it is easy to deoxygenate (Table 1, entry 18).…”

Catalytic depolymerization of alkali lignin in subcritical water: influence of formic acid and Pd/C catalyst on the yields of liquid monomeric aromatic products

“…In this present study, alkali lignin has been subjected to subcritical water depolymerization reactions with the application of formic acid (FA) as capping agent and hydrogen source and 5 wt% palladium supported on carbon (Pd/C) as hydrodeoxygenation (HDO) catalyst at 265 °C, 6.5MPa. Pd/C has been used in literature for hydrodeoxygenation of bio-oils and oxygenates in alcohols [21] and a recent publications showed that Pd/C can be effective for deoxygenation of triglycerides in subcritical water with external hydrogen supply [22]. At first, the hydrothermal reaction of FA was investigated at the given reaction conditions to determine the influence of reaction time and the presence of Pd/C catalyst on its ultimate conversion according to the equation (1) [19];…”