# Targeted Substituted-Phenol Production by Strategic Hydrogenolysis of Sugar-Cane Lignin
## 1 Introduction  
The interest in repurposing biomass waste has been growing over the years as its effective application can contribute to reduced reliance on fossil fuels [1]. In this context, the chemical structure of lignin is economically attractive in a biorefinery context [2] as it becomes a potential source of value-added molecules [3]. Lignin is an abundant and as yet not defined biopolymer. Its chemical structure varies depending upon the biomass feedstock and extraction method used in its separation from the cell wall network. Generally, three main monomers known as guiaacyl alcohol (G unit), syringyl alcohol (S unit) and \(p\)-couramyl alcohol (H unit) [4, 5] are considered building blocks of this complex molecule. These units are linked between each other by various linkages, including \(\beta\)-O-4, \(\alpha\)-O-4, \(\beta\)-\(\beta\), 5-5 and \(\beta\)-1 [6]. Lignin can be found in feedstocks such as sawdust, wood chips and sugar-cane bagasse [6, 7]. This last one is generated on a large scale (>200 MMT in Brazil) as residue in countries that are producers of bioethanol and sugar, like Brazil, India and China [8]. Most industries use this waste (0.3 T of wet bagasse per tonne sugar cane) in cyclic processes for energy generation [9], however, by using an appropriate pre-treatment it can be an abundant source of lignin (~22% of the dried bagasse). Several approaches have been used to extract lignin from sugar-cane bagasse [10, 11, 12, 13]. Alcohols are normally used associated with water in catalytic acidic mediums [14, 15]. For the valorization of lignin to fine chemicals, \(\beta\)-O-4 linkages have been shown to be important in depolymerisation [16] and manyresearchers have focused on optimizing extraction methods to enhance the quality of the obtained lignin [17]. A suitable metal catalyst with a lignin abundant in \(\beta\)-O-4 bonds could present a promising system for hydrogenolysis with selective production of alkyl-phenolic monomers [16; 18]. The methodology behind this is that alkyl-O-aryl bonds can be more readily cleaved compared to C-C bonds (\(\beta\)-\(\beta\), 5-5 and \(\beta\)-1) in lignin [19]. Selective production of monoaromatics is one of the main challenges regarding depolymerization, due to condensation of intermediates. In general, the lignin is thermally broken, followed by a range of catalytic reactions that lead to phenolics, while the product distribution depends on the catalysts used [18]. Metals such as platinum, rhodium, nickel and iron showed appreciable activity in breaking C-C and C-O-C bonds in model compounds [20; 21; 22]. However, when it comes to a real lignin stream, the challenge increases and obtaining selective outcomes becomes harder. Nevertheless, earlier investigations showed that monoaromatics yield can increase in strategic reactions like hydrogenolysis by using metal catalysts [18; 23]. One advantage when using acetone in reactions with lignin is this solvent's ability to dissolve the heavier fragments of lignin as previously reported in other studies [18]; thus, contact between lignin and a heterogenous catalyst can be enhanced. In addition, its commercial production from fermentation of agricultural products has been reported [24], this moves acetone from a petrochemical feedstock to one that is obtainable from renewable sources. In this work, our investigation focused on a residual-derived lignin with substantial \(\beta\)-O-4 bond content. In spite of considerable information about lignin hydrogenolysis using metal-alumina-based catalysts, there are only a few reports in the literature about compounds that are not generated in the thermolysis (or solvolysis) but exclusively in the catalytic run. Two noble metal catalysts (Pt/Al\({}_{2}\)O\({}_{3}\) and Rh/Al\({}_{2}\)O\({}_{3}\)) and two non-noble metals (Ni/Al\({}_{2}\)O\({}_{3}\) and Fe/Al\({}_{2}\)O\({}_{3}\)) were tested in sugar-cane lignin (SCL) depolymerization.