Bifunctional metallic-acidic mechanisms of hydrodeoxygenation of eugenol as lignin model compound over supported Cu, Ni, Pd, Pt, Rh and Ru catalyst materials

Highlights

• High-throughput tests with coverage and micro-kinetics modelling in a slurry reactor.

• Hydrogenation and deoxygenation activity of Pt, Pd, Rh, Ru, Ni and Cu facets.

• Quantified activity contribution of C, Al₂O₃, SiO₂, SiO₂-Al₂O₃, TiO₂, HZSM-5.

• Pathway flux on metallic and acidic surface sites demonstrated with Sankey diagrams.

• Acidic support sites catalysed aliphatic C–O bond cleavage more than for aromatics.

Abstract

The hydrodeoxygenation (HDO) reaction kinetics of lignin monomer model compound eugenol was systematically investigated over various commercially available catalysts typically used for lignin valorisation by hydrotreatment. The role of noble metals Pt, Pd, Rh, Ru, and non-noble Ni and Cu on C was investigated in the previous studies, while the present work is focused on the support (Al₂O₃, SiO₂, SiO₂-Al₂O₃, TiO₂, HZSM-5) effect on the catalyst activity and selectivity, being estimated experimentally (at 225, 275, 325 °C and 5 MPa of initial hydrogen pressure) and in silico. The micro-kineticmodel took into consideration mass transfer resistances through convective films (gas bubbles and catalyst particles), reaction kinetics in bulk liquid, adsorption/desorption and surface reaction kinetics on metallic and acidic active sites, considering the occurrence of all reactions on both types of sites. The contribution of each support was quantitatively evaluated in terms of adsorption, desorption, and reaction rate constants and activation energies. SiO₂-Al₂O₃ and HZSM-5 showed superior contribution to the activity of the methoxyl and hydroxyl group removal from oxygenated aromatics and particularly saturated species. Except for the above-mentioned materials C_aromatic–O hydrogenolysis activity was not substantially affected, while the acidity of the support notably promoted C_aliphatic–O cleavage. Contribution to overall hydrogenation rate was practically negligible in presence of acidic sites. It has been also noted that the contribution of supports was not very prominent when very active noble metal phases were used (such as Ru), while it came to the fore in the case of almost inactive Cu, being responsible for formation of 4-propylphenol at 325 °C. Based on microkinetic modelling results, Sankey diagrams were formulated for the first time to represent pathway flux and graphically demonstrate the contribution of each catalytic reaction participating in the overall eugenol HDO mechanism over the used catalysts (and corresponding intermediates) within a complex reaction network, as well as the engagement of metallic and acidic sites.

Introduction

Non-edible lignocellulosic biomass has gained significant attention in recent decades as a green, renewable, and sustainable alternative to petroleum for chemicals and fuel production [1]. To increase the efficiency of bio-refineries and therefore profitability, streams of all three biomass components – cellulose, hemicellulose, and lignin – should find an economical path into value-added products. Although cellulose has been used for numerous industrial applications, creating viable value chains for the other two components seems inevitable if we are to achieve the goal of circular economy [2], [3].

Lignin is a largely available (approx. 60% more lignin is produced than is necessary to meet the internal energy needs of the pulp and paper industry) natural aromatic polymer and thus is a promising source of low molecular weight aromatic components [2], [3], [4]. Three monolignols constitute the lignin structure: p-coumaryl, coniferyl, and sinapyl alcohol. Their ratio in lignin and the amount of lignin itself differs from plant to plant. Monomeric units are interconnected via β-O-4 (predominant, accounting for roughly 50% of the bonds), α-O-4, 5-5, β-5 and the dibenzodioxocin linkage [3], [5], [6]. While the high content of oxygen would be an issue were lignin to be used as a fuel source, such high content can be beneficial for the production of platform chemicals. Lignin derivatives may find use in a broad application niche (e.g. automotive and food industry, packaging etc.) [7]. Valorisation by catalytic hydrotreatment has been considered the most promising route for lignin. It cleaves typical lignin bonds by hydrogen addition/substitution, yielding smaller fragments, hydrogenated, and deoxygenated products [8].

Catalytic hydrotreatment of lignin or lignin monomer model compounds has been extensively examined, whereas various catalytic formulations have been tested under a wide range of temperatures (150–350 °C) and pressures (0.1–10 MPa) [9], [10], [11]. Typically applied catalysts were Ni, Cu, Co, Mo, W, or their bimetallic formulations such as NiMo or CoMo, deposited on numerous neutral and acidic supports such as C, SiO₂, TiO₂, Al₂O₃, zeolites [3], [12]. However, perceived obstacles when using transition metals such as poisoning of the catalyst, sintering of the active metals, and coking give importance to precious metals (Pt, Pd, Rh, Ru) in which these issues are less pronounced [13]. The influence of the support nature has been also investigated when noble metals were applied, showing their important role in determining course of the HDO and product selectivity [14], [15].

We have systematically investigated the HDO reaction (which represents removal of oxygen containing groups by hydrogen substitution) mechanism of lignin monomer model compound eugenol over carbon-supported noble metals (Pt, Pd, Rh, Ru) in our previous works [16], [17], [18]. The role of metal and the influence of operating conditions have been experimentally and theoretically determined. Results indicated superior HDO activity for Ru (the lowest hydrogenation/deoxygenation ratio) that was found at the top of a volcano-like dependence of the reactivity upon adsorption energy determined by ab initio calculations and micro-kinetic modelling. Pt was shown as a not very promising HDO catalyst due to very strong interactions with adsorbates (eugenol, 2-methoxy-4-propylphenol), while Cu exhibited almost negligible activity caused by a very weak Cu-adsorbate interaction.

The present study has built upon the previous work by incorporating the effect of acidic support, besides the previously considered effect of only metallic ones, on the catalyst activity and selectivity. Thus the aim of the present study is to qualitatively and quantitatively estimate the catalytic contribution of the support acidic active sites to the overall catalyst activity which is, in this case, a result of both active sites (metallic and acidic support’s sites) activity. Al₂O₃-supported Pt, Pd, Rh, Ru, Ni, and Cu have been evaluated for the eugenol HDO. Ru supported on SiO₂, SiO₂-Al₂O₃, TiO₂, and HZSM-5 has been additionally applied in order to get deeper insights into support performance that influence the HDO course. In this way, upgrading of lignin-derived building blocks has been systematically and comprehensively investigated at fundamental level by means of micro-kinetics (and previously DFT) calculations providing a catalyst structure-activity relationship and kinetic data. The same computational treatment of ten catalysts in this study and six in the previous usually used for lignin HDO studies would provide more relevant and directly comparable kinetic parameters.