# Targeted Substituted-Phenol Production by Strategic Hydrogenolysis of Sugar-Cane Lignin
## 3 Results and Discussion
### Sugar-Cane Lignin (SCL) Characterization
#### 3.2.10 Catalytic Products  
Three molecules were detected in the catalytic experiments and identified as follows: 4-ethylphenol (2A), 4-methyl-2,6-dimethoxyphenol (9) and 4-propyl-2,6-dimethoxy-phenol (14). These were not simple thermal products but came from effective catalytic activity. Figure 5 presents the selectivity values according to the catalyst used. As regards product 9, there were similar yields for all. Alumina's acidic sites can catalyse hydrolysis and alkylation [45], thus, this product formation, first detected in reaction with only alumina (Figure 2), was associated with the Al\({}_{2}\)O\({}_{3}\) support with no relevant influence of the metals. Once in the presence of metals, major yield differences were observed between compounds 2A and 14. For 4-propyl-2,6-dimethoxyophenol (14), its generation may be in line with the ability of each catalyst to cleave \(\beta\)-O-4 bonds, as propyl side chain products are usually generated from this type of linkage rupture [49]. This ability increased as follows: Rh > Pt > Ni > Fe. Previously [27], temperature programmed reduction (TPR) analysis showed that uncondensed lignins degraded into smaller fragments in a range < 573 K. As confirmed by solvolysis, SCL fragmentation also started without the need of a catalyst. Considering SCL intermediates having either bonds (\(\beta\)-O-4) present in this mild temperature process (573 K), and a hydrogen rich environment with suitable catalyst's metal sites for its dissociative adsorption, a similar cleavage pathway to selective thioacidolysis [49] could occur. This could initiate with lignin fragmentation, followed by hydrogenolysis of \(C_{\text{alkyl}}\)-O bond of \(\beta\)-O-4 containing intermediates and stabilisation by the intake of a hydrogen atom at the unstable alkyl chain carbon, finally leading to the formation of a product such as 4-propyl-2,6-dimethoxyophenol. In this instance, Rh/Al\({}_{2}\)O\({}_{3}\) was the most efficient catalyst. Figure 4: Estimated yield in \(g\)/100g of phenolic compounds in solvolysis (reference), reactions in the presence of (**a**) Ni/Al\({}_{2}\)O\({}_{3}\) and (**b**) Fe/Al\({}_{2}\)O\({}_{3}\) catalysts. Standard deviation value: 0.1. Products identified by GC-MS: (1) 2-methoxyphenol, (2) 4-methyl-2-methoxyphenol, (2A) 4-ethylphenol, (3) 4-ethyl-2-methoxyphenol, (4) 4-propyl-2-methoxyphenol, (5) 1,2-dihydroxyoxygenase, (6A) 2,6-dimethoxyphenol, (7) 4-(3-hydroxypropyl)-2-methoxyphenol, (8) 4-(3-methoxypropyl)-2-methoxyphenol, (9) 4-methyl-2,6-dimethoxyphenol, (10) 4-(2-hydroxyethyl)-2,6-dimethoxyphenol, (11) 4-ethyl-2,6-dimethoxyphenol, (12) 4-propyl-2,6-dimethoxyphenol, (13) 4-(2-hydroxyethyl)-2-methoxyphenol, (14) 4-propyl-2,6-dimethoxyphenol, (15) 4-(1-hydroxy-2-methyl-pent-3-enyl)-2,6-dimethoxyphenol. Conditions: 573 K, 20 bang H\({}_{2}\), solvent, acetone/water 50:50 \(v\)/v. Regarding 4-ethylphenol (2A), while platinum and rhodium catalysts selectively produced -20% and 28%, nickel and iron gave \(\sim\)31% and \(\sim\)26%, respectively. It is important to note that nickel, a non-noble metal, had as good a catalytic effect as the noble metals. Two potential reaction pathways to generate 4-ethylphenol are suggested. First (Figure 6a), as the yield of 4-ethyl-2-methoxyphenol (3) dropped in the catalytic runs compared to solvolysis (Figure 3), 4-ethylphenol could come from simple catalytic demethoxylation of compound 3. Another route (Scheme 1B) could be related to the chemical structure of SCL, if a \(p\)-couamyl alcohol-type fragment was derived from SCL depolymerisation, (i) hydrogenolysis could occur in the presence of a heterogenous catalyst, followed by (ii) decarboxylation giving a 4-vinylphenol-type species as an intermediate, which by (iii) hydrogenation would result in 4-ethylphenol. The solvent mixture could also play a key role in the depolymerisation. Hydrolysis was likely to occur enhancing lignin cracking [50] and acetone can dissolve lignin heavier fragments [18].