Hydrodeoxygenation of guaiacol to bio-hydrocarbons over Ni catalyst supported on activated coconut carbon in alkaline condition

Highlights

• The strong alkaline environment of black liquor was simulated successfully.

• Ni/AC catalysts were modified with different acids, and thus introduced functional acid groups.

• Acidic groups from HNO₃ promoted the C_(sp2)-O scission of guaiacol.

• Acidic groups reduced the Ni particle size, and thus promoted the HDO reactions.

Abstract

Pulping black liquor is a serious source of water pollution worldwide, which is very difficult to subsequently treat. Lignin is the main component of the black liquor and is the only natural aromatic recyclable resource, which is burnt as waste in the Kraft pulping industry. Designing an efficient lignin conversion catalyst that suitable for the strong alkaline environment of black liquor harbors tremendous carbon-neutral potential but still remains a scientific and technological challenge. In this work, the strong alkaline environment of black liquor was simulated, and low-cost pretreated Ni/AC catalysts were adopted to the hydrodeoxygenation (HDO) of a lignin model (guaiacol), and a breakthrough was achieved in the efficient transformation of lignin compound into hydrocarbons. By multitechniques, (e.g., FT-IR, XPS, Boehm titration, HRTEM), the relevance between introducing functional acid groups and the surface properties of catalysts was revealed. It is found that the increased acidic groups are responsible for the enhanced C_(sp2)-O cleavage reactions, while improving the dispersion of Ni NPs and thus enhance the HDO efficiency. It is worth noting that better selectivity of benzene and lower hydrogen consumption are achieved on Ni/NOAC during the HDO process. Furthermore, the reaction routes of guaiacol on catalysts with different functional acid groups were proposed. This work provides guidance for efficient utilization of lignin in waste black liquor, and is of great significance to the development of sustainable carbon-neutral biorefinery and environmental protection.

Introduction

Increasing concerns about global warming and the imminent depletion of fossil resources have recently drawn significant attention to lignocellulosic biomass [1,2], which is considered as a substitute and complement of fossil resources [[3], [4], [5]]. In addition, products from lignin upgrading can serve as green raw materials for production of fuels and chemicals, which is of great prospect for the development of sustainable carbon-neutral biorefinery [[6], [7], [8]].

Lignin upgrading involves two steps: (1) depolymerization of lignin polymer into monomers [[9], [10], [11]] and (2) transformation of monomers into target products [[12], [13], [14]]. For the second step, the hydrodeoxygenation (HDO) process is regarded as an effective and green technology in lignin upgrading, due to high carbon yields and the only by-product water [[15], [16], [17]]. Neutral reaction condition is mostly applied in the HDO of lignin compounds to give insights into the HDO of lignin in bio-oil [18,19] or wood [[20], [21], [22]]. In some cases, mineral acid is introduced into the reaction system to promote the deoxygenation reactions of oxygenated intermediate products [23]. However, the HDO reactions in alkaline condition should be considered as a valuable research field as well to investigate the HDO activity of lignin compounds in black liquor. Black liquor (PH∼14), which is formed in the pulp and papermaking industry by the digestion of wood, consists of water, lignin fragments (above 8 wt%), inorganic alkaline and salt (NaOH, Na₂S, etc.) and minor amounts of (hemi) cellulose, and each ton of pulp results in about 10 tons of black liquor during the Kraft pulping process [[24], [25], [26]]. However, in a typical industrial treatment, black liquor is burnt to recycle alkaline, during which lignin is burnt as waste, leading to severe lignin-resource waste [27]. Thus, the development of efficient catalysts and upgrading processes for transformation of lignin compounds in black liquor is urgently needed, which has great potential for the development of carbon-neutral biorefinery.

Although the utilization of lignin in black liquid has great potential for the catalytic depolymerization and HDO reactions, the direct utilization of black liquid as the raw material is hardly ever been reported in the existing references. Most researchers conduct the upgrading of Kraft lignin by a pre-acidizing process of black liquid followed by pre-depolymerization and HDO reactions of the obtained lignin solid [28,29], or by pyrolysis processes [30]. However, the introduction of large amounts of inorganic acid in the pre-acidizing process not only causes environmental pollution but also leads to resources waste of inorganic alkaline and acid. On the other hand, the pyrolysis processes lead to a serious coking of lignin compounds under high temperature due to the radical reactions of aromatics [31,32].

However, the direction use of black liquor as reactant in the depolymerization-HDO reactions of lignin compounds can avoid these drawbacks because of alkaline reservation in reactant and much lower reaction temperature than pyrolysis during the process. Moreover, the HDO of lignin with black liquor as reactant can not only produce valuable bio-hydrocarbons, but also recycle the alkaline resource by a simple separation method. This is because the depolymerization-HDO reactions of black liquor into hydrocarbons leads to a stratification of hydrocarbons and alkaline aqueous phase due to their different polarity, leaving alkaline, i.e. NaOH, in the aqueous phase, which can be recycled by evaporation or some other methods. Above all, the development of alkaline-resistant and highly efficient catalytic systems is of significant importance for the depolymerization-HDO process with black liquor as the reactant.

Activated carbons are frequently used as catalyst supports, because of their physical and chemical properties, such as the presence of oxygen-containing functional groups on the surface, the large micro or mesopore surface area, and their structural and morphological stability at high temperature and in various liquid media [[33], [34], [35]]. Thus, compared to acidic supports with poor water-resistance, such as Al₂O₃ and zeolites [[36], [37], [38], [39]], which is widely used in the HDO of lignin compounds [[40], [41], [42]], activated carbon is more suitable for the HDO of lignin compounds in alkaline solution.

Despite the wide application of carbon-supported catalysts in HDO reactions [17,43,44], no investigation has been devoted to the use of activated carbons supported Ni catalysts in the HDO of lignin compounds in alkaline condition. Moreover, the use of biomass derived carbon catalysts will be of great importance towards a sustainable future.

In this study, we examined three coconut-derive activated carbons (CAC) supported Ni catalysts, with or without acid pretreatment, for the HDO of guaiacol under alkaline condition to give a theoretical guidance for the development of catalysts for the HDO step of lignin monomer in depolymerization-HDO process of black liquor lignin. The reaction routes on three catalysts have been proposed to elucidate the effect of metal and acidic properties on the reaction routes. It is proved that the acid-pretreatment of coconut carbon can effectively enhance the content of acidic functional groups on catalyst surface and thus promotes the C_(sp2)-O cleavage of guaiacol, while the increased acidic functional groups improve the dispersion of Ni NPs and thus promotes the hydrodeoxygenation reactions of guaiacol on modified Ni/CAC. This is a powerful guidance for the development of effective methods for enhancing the HDO efficiency of lignin compounds in black liquor and facilitation of lignin valorization with high carbon-neutral value.