Hydrogenolysis of lignin over Ru-based catalysts: The role of the ruthenium in a lignin fragmentation process

Highlights

• Ru or/and Ni catalysts were prepared by coprecipitation or wet impregnation.

• Ru supported on alumina led to the highest degree of lignin depolymerisation.

• Presence of ruthenium led to dimers as dominant products.

• The interaction between ruthenium and nickel had an influence in the depolymerisation performance of lignin.

Abstract

The catalytic performances of two different classes of catalysts containing nickel or/and ruthenium as the active sites were studied in the depolymerisation of lignin isolated from Miscanthus × giganteus. The catalysts were prepared either by coprecipitation (ie, (RuNiMgAlO)x, (RuNiAlO)x, (NiAlO)x, (NiMgAlO)x) or by wet impregnation (ie, Ru/Al₂O₃) and characterized by nitrogen physisorption (BET), XRD, XPS, NH₃-TPD, Raman and H₂-TPR techniques. The experimental results indicate that the presence of ruthenium led to dimers as dominant products.

Introduction

Conversion of lignocellulosic materials to renewable fuels or chemicals has attracted important attention due to imminent depletion of fossil fuels. Lignin, a dominant part of lignocellulosic materials, is the world’s most abundant natural aromatic polymer having a three dimensional structure based on phenolic monomeric units (ie, sinapyl, coniferyl and p-coumaryl alcohols) which are inter-connected by several types of linkages such as β-O-4, β-5, 5-5, 4-O-5, β-1, and β-β bonds. Among these, β-O-4 linkage is the most abundant, representing almost 50% of the native material [1]. Moreover, the energy of β-O-4 linkage is lower compared to the 5-5 (biphenyl)-type or aromatic ring structures [2], a feature that promotes the depolymerisation of lignin to low molecular mass compounds. The interest for lignin depolymerisation is motivated by the possibility of obtaining important platform chemicals such as aromatic aldehydes, acids, alcohols and phenols or using lignin derivatives as flavors or chemical intermediates for pharmaceutical drugs [3]. However, the depolymerisation of lignin is not an easy task, its complex structure, the probability to form some condensed structures during the thermochemical processes, the poor product selectivity towards monomers and the existence of various linkage types between monomeric units being the most important challenges [4].

The conversion of lignin to monomers and dimers requires the COC bond cleavage [5]. Several methods such as pyrolysis [[6], [7]], hydrolysis [8], hydrogenolysis [[5], [9]] oxidation [[10], [11]] or hydrocracking [12] are reported in literature for this process. Among these, hydrogenolysis is considered as one of the most promising methods, the process leading to higher yields of monomers compared to other thermo-chemical methods [9]. However, selective hydrogenolysis of CO bonds is not a simple process since the average energy of this bond is higher than that of the other C-X single bonds (except CH and C-F bonds). As a result, the CO bond is difficult to break by most reagents and catalysts [13].

Noble metals as Pt, Rh and Ru supported on carbon carriers play an important role in the selective production of 4-ethylphenol and 4-ethylguaiacol by hydrogenolysis of lignin into ethanol/water solvent mixtures. The performance of Ru/C catalysts, the most efficient ones in this aspect, was influenced by the reaction temperature (an increase from 200 to 275 °C led to a yield increase for 4-ethylphenol from 0.79 to 3.1%). The obtained yields are comparable to those obtained via petrochemical routes [14].

The nature of the solvent is another parameter which can influence the distribution of products upon depolymerisation. Thus, using catalysts similar to the aforementioned ones, Yan et al. [5] reported a monomer yield of 46% (ie, guaiacylpropane, syringylpropane, guaiacylpropanol, syringylpropanol) at 200 °C in a dioxane/water solvent mixture. The importance of the solvent nature was also emphasized by Song et al. [15] upon lignosulfonate hydrogenolysis into phenols over Ni/AC. Therefore, the conversion could be increased from 2% to 68% by simply replacing the iso-propanol solvent with ethylene glycol.

The depolymerisation of lignin over Ni/AC led only to aromatic products, the hydrogenation of these compounds being carried out in the presence of Pd/AC (Activated Carbon). Nickel gives rise to better performance in terms of the aliphatic CO bond cleavage than the aromatic CO bonds hydrogenolysis [15]. Moreover, Ni/Al-SBA-15 was more efficient compared to systems relying on supports such as ZrO₂, Al₂O₃, Al₂O₃/KF or native SBA-15 [16].

Recently, Zhang et al. [17] reported NiM (M = Ru, Rh and Pd) catalysts as highly performing in the hydrogenolysis of CO bonds. The authors showed that the direct incorporation of 15% Ru led to an increased reduction rate of Ni as well as the formation of ultra-small bimetallic particles. Moreover, their catalytic activity was highly improved compared with NiRh and NiPd catalysts.

Porous metal oxides (PMOs) obtained by the calcination of the corresponding hydrotalcites, doped with metals such as Cu, La, Mn, Cr and Zn, were recently used for the depolymerisation of lignin to oligomer and monomer aromatics, in supercritical methanol [18]. The best results in the conversion of lignin to oligomer aromatics soluble in methanol were reported for Cu- and La-doped PMO (Cu₂₀La₂₀PMO), suggesting a synergetic effect of copper and lanthanum. On the other hand, Garron et al. [19] observed a similar synergetic effect of metals upon conversion of pine wood to low oxygenated fuel over Ru_x/Cu_y@CsWP and Ru_x/Cu_y@CsMoP catalysts. Moreover, it seems that ruthenium can partly substitute Mg²⁺ or Al³⁺ cations into octahedral layers of hydrotalcite [20], absolutely necessary in the complete conversion of benzene to cyclohexane under 60 atm H₂ at 120 °C.

Recently, also Kim et al. [21] have demonstrated the synergistic effect between metallic Pd and Fe in the selective cleavage of CO bond from benzyl-phenyl-ether (used as lignin model due to the presence of α-O-4) to aromatic compounds as toluene and phenol. The improvement of the catalytic performances in the cleavage of CO bond due to synergistic effects was obtained via incorporation of noble metals into Ni [17].

Based on the above state-of-the-art observations and with the aim to improve the selective depolymerisation of lignin, herein we report the preparation and characterization of mixed oxides such as (NiRuMgAlO)x, (NiRuAlO)x, (NiAlO)x, (NiMgAlO)x and Ru-based mesoporous alumina. The catalysts were tested in lignin depolymerisation and the synergetic effect between Ru and Ni in terms of their catalytic performance was demonstrated. Therefore, whereas the cleavage of CO bond of α-O-4 and β-O-4 linkage is catalyzed by Ni [22], the addition of Ru could improve the selective hydrogenolysis of lignin to monomers or dimers [23].