Lignin-derived Model Compound Research Articles

Catalytic hydrogenation and hydrodeoxygenation of lignin-derived model compound eugenol over Ru/C: Intrinsic microkinetics and transport phenomena

Heterogeneous catalytic hydrotreatment is recognized as a key valorization route of bio-based oxygenates in a bio-refinery concept. Catalytic hydrodeoxygenation (HDO) and hydrogenation of eugenol, a lignin building block model component, was investigated in silico and experimentally in a three-phase slurry reactor over the ruthenium supported on carbon (Ru/C) and by using hexadecane (HD) solvent. The influence of operating process conditions on the activity and selectivity of the transformations was determined; specifically, the temperature (225–325°C), hydrogen gas pressure (4–7MPa), catalyst loading (0–0.35wt%), eugenol loading (0–5wt%), process gas (H2 and N2), stirring speed (100–1000min−1) were varied within the listed values. Based on the detailed chromatographic, spectroscopic, microscopic, temperature-programed desorption, reduction, oxidation or physisorption analysis of solid, liquid and gas phase, a rather complex reaction-pathway network for eugenol hydrotreatment was proposed. Experimental results revealed that allyl group and benzene ring saturation was favored in comparison to competitive deoxygenation reactions, namely dehydroxylation and demethoxylation of oxygen moieties on aromatic ring. The mechanism of ketone group formation has been clarified among various proposed pathways in the literature, as it most likely proceeds via keto-enol isomerization. Precursors responsible for cyclohexane-ring contraction to substituted 5-member ring compounds were also identified and mechanism and kinetics of transformation was proposed. Intrinsic microkinetic modelling was based on the proposed network of parallel and consecutive homogeneous and catalytic reactions, and accounted the gas–liquid and liquid–solid mass transfer, adsorption and desorption kinetics and thermodynamic equilibria. Model results indicate 36- and 42-times higher demethoxylation and dehydroxylation rate constants for aromatics in comparison to corresponding oxygen-containing cycloalkanes.

Hydrodeoxygenation of vanillin using palladium on mesoporous KIT-6 in vapour phase reactor

Mesoporous KIT-6 was synthesised by using hydrothermal method. Different weight percentage of 1, 3, 5 and 7wt.% of palladium (Pd) was impregnated on KIT-6 by wet impregnation method. All the palladium catalyst was characterized by XRD, ICP-OES, BET, TPR, XPS, FT-IR, DRS-UV and TGA techniques. The morphology of the catalyst was identified by FESEM with EDS and HRTEM analysis. A typical lignin derived model compound (vanillin) was tested for hydrodeoxygenation (HDO) reaction using Pd/KIT-6 catalyst in fixed bed reactor. The activity of the catalyst was evaluated for HDO of vanillin by optimizing temperature (100–300°C) and weight percentage of Pd loading onto KIT-6 (1, 3, 5 and 7wt.%). The conversion of vanillin, selectivity of vanillyl alcohol and p-cresol were confirmed by gas chromatography. The investigation of the study showed that 5wt.% of Pd/KIT-6 catalyst showed 98% conversion of vanillin and 94% selectivity of p-cresol at 300°C for 6h. The time studies were also conducted for all the catalyst in the temperature range of 100 to 300°C revealed that Pd/KIT-6 had higher conversion and maximum selectivity at 6h. The activity of Pd loaded catalyst was comparatively better up to 3 cycles. Selectivity of the vanillyl alcohol was preferably observed by Pd loading (1wt.%) at short time (1h) whereas p-cresol selectivity was observed at long time (6h).

Product tunable behavior of carbon nanotubes-supported Ni–Fe catalysts for guaiacol hydrodeoxygenation

Bimetallic Ni–Fe nanoparticles supported on carbon nanotubes (CNTs) are prepared and evaluated for the catalytic hydrodeoxygenation (HDO) of a lignin-derived model compound guaiacol. Appropriate combination of Ni and Fe affords high activity and significantly enhances selectivity to cyclohexane or phenol, whereas monometallic Ni and Fe catalysts display poor activities or selectivities. The product tunable behavior of guaiacol HDO is found to be dependent on Ni/Fe atomic ratios. Cyclohexane and phenol are the major products over Ni5–Fe1/CNT with Ni/Fe atomic ratio at 5/1 and Ni1–Fe5/CNT with Ni/Fe atomic ratio at 1/5, respectively. Characterization results confirm that Ni–Fe alloys are formed and elicit synergistic effects on the HDO performance. The selectivity-switchable performance of Ni–Fe/CNT can be assigned to the synergism between Ni domains, where H2 can be easily activated, and Fe domains, which exhibited strong oxophilicity. The bimetallic catalysts give an enhanced stability without significant sintering of metal nanoparticles, while the monometallic catalysts show obvious deactivation due to the agglomeration of metal nanoparticles. Further results reveal that the conversion of guaiacol depends on not only the chemical state but also the size of the metallic nanoparticles. The catalysts with appropriate Ni/Fe atomic ratio and smaller particle perform better hydrogenolysis of C–O bonds, resulting in high selectivity to cyclohexane or phenol.

Insights into the catalytic activity and surface modification of MoO3 during the hydrodeoxygenation of lignin-derived model compounds into aromatic hydrocarbons under low hydrogen pressures

MoO3 is an effective catalyst for the hydrodeoxygenation (HDO) of lignin-derived oxygenates to generate high yields of aromatic hydrocarbons without ring-saturated products.

Transetherification of guaiacol to o-ethoxyphenol with gamma Al2O3 as a catalyst in supercritical ethanol

The production of chemicals from lignin and lignin depolymerisation products is a promising alternative route to replace fossil fuels. Transetherification of guaiacol, a lignin derived model compound, to o-ethoxyphenol with γ-Al2O3 as the catalyst in supercritical ethanol has been investigated. The best reaction condition is 280 °C for 3 h, giving a yield of 42% with a selectivity of 86%. A reaction pathway is also proposed. Besides, this work provides an example for producing a longer chain ether from a short chain ether.

Advances in C–O Bond Transformations in Lignin-Derived Compounds for Biofuels Production

Renewable energy sources such as lignocellulosic biomass provide an attractive source of renewable carbon that can be sustainably converted into fuels and chemicals to allow for a reduction in the need for fossil fuels and a net reduction in the amount of CO2 emitted. This perspective highlights recent advances in catalyst development for the production of biofuels from lignin and lignin-derived model compounds. Recent reports have focused on the direct cleavage of C–O bonds through various catalytic processes including (i) zeolite upgrading, (ii) hydrodeoxygenation, (iii) multiple-step upgrading, and (iv) unsupported, organometallic catalysts for slurry processing. Results show progress toward the potential commercialization of biofuels technologies from lignocellulosic biomass resources.