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From Lignin to Chemicals: Hydrogenation of Lignin Models and Mechanistic Insights into Hydrodeoxygenation via Low Temperature C–O Bond Cleavage Charles E.J.J. Vriamont, Tianyi Chen, Charles ROMAIN, Paul Corbett, Phornphit Manageracharath, Janet Peet, Christopher M Conifer, Jason P. Hallett, and George J.P. Britovsek ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04714 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Catalysis

From Lignin to Chemicals: Hydrogenation of Lignin Models and Mechanistic Insights into Hydrodeoxygenation via Low Temperature C–O Bond Cleavage Charles E.J.J. Vriamont,a) Tianyi Chen,a) Charles Romain,a) Paul Corbett,b) Phornphit Manageracharath,a) Janet Peet,a) Christopher M. Conifer,a) Jason P. Hallettb) and George J. P. Britovseka)* a) Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AZ, UK b) Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK

Abstract The catalytic hydrogenation of a series of lignin model compounds, including anisole, guaiacol, 1,2-dimethoxy benzene, 4-propyl-2-methoxy-phenol and syringol has been investigated in detail, using a Ru/C catalyst in acetic acid as the solvent. Both hydrogenation of the aromatic unit and C–O bond cleavage are observed resulting in a mixture of cyclohexanes and cyclohexanols, together with cyclohexyl acetates due to esterification with the solvent. The effect on product composition of the reaction parameters temperature (80-140 ˚C), pressure (10-40 bar) and reaction time (0.5-4 h) has been evaluated in detail. The lignin model compound 4-propyl-2-methoxy-phenol was converted to 4-propyl cyclohexanol in 4 hours at 140 ˚C and 30 bar H2 pressure with 84 % conversion and 63 % selectivity. Mechanistic studies on the reactivity of reaction intermediates have shown that C–O bond cleavage under these relatively mild conditions does not involve a C–O bond hydrogenolysis reaction, but is due to elimination and hydrolysis reactions (or acetolysis in acetic acid solvent) of highly reactive cyclohexadiene- and cyclohexene-based enols, enol ethers and allyl ethers. Keyword: lignin, hydrogenation, hydrogenolysis, ruthenium, biomass, hydrodemethoxylation

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The development of new catalytic processes for the conversion of lignocellulosic biomass into biofuels and biochemicals is currently of tremendous interest.1-8 Lignin in particular, which constitutes approximately 30 wt% of all non-fossil organic carbon and typically 20-35 wt% of dry wood, has received increased attention recently as a potentially rich source of chemicals and materials.9-12 Rather than using lignin directly, the use of de-polymerised lignin, a so-called bio-oil, has become attractive as a starting material.13 While the composition of thermal or catalytic bio-oil varies somewhat depending on the source and process conditions, most common components include ortho-methoxylated para-alkyl or para-alkenyl phenols. With a view to applications of lignin at scale, we have set out to explore feasible and scalable routes from these bio-oil ingredients for sustainable polymer synthesis, in particular aromatic monomers such as terephthalic acid.14 Terephthalic acid is currently produced by air oxidation of petroleum-derived p-xylene in the Amoco process at 30 mtpa.15 Several routes from biomass to terephthalic acid have been reported recently,16 either starting from sugar-derived furfural or muconic acid,17-19 lignin-derived vanillic acid20 or isoprene obtained from natural rubber.21 terephthalic

Starting from lignin-derived phenols, we envisage a four-step sequence to acid

comprising

hydrogenation-carbonylation-dehydrogenation-oxidation

reactions, as shown in Scheme 1. The focus here is on the first step: the hydrogenation of lignin derived phenols to cyclohexanols.

Scheme 1 Investigations into hydrogenation of lignin-based chemicals started well over half a century ago,22 but recent years have witnessed a resurgence of interest in this topic.12,

23

Hydrogenations are generally carried out with hydrogen or alternatively via transfer hydrogenation using for example formic acid, isopropanol or an alkane as the hydrogen source.24-28 Our aim here has been to investigate the hydrogenation of phenolic substrates using hydrogen as shown in Eq. 1 and identify optimal reaction conditions for the formation of cyclohexanol derivatives through a deep understanding of the reaction mechanism.


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Cyclohexanol itself is an important chemical intermediate for the production of polymers such as polyesters and nylon.29

In addition to hydrogenation of the aromatic unit, C–O bond cleavage can occur during the hydrogenation of phenolic substrates, generally referred to as hydrodeoxygenation (HDO).3032

Indeed, cyclohexane is a common by-product in the hydrogenation of phenol and is

generally ascribed to hydrogenolysis, a C–O bond cleavage reaction.33 This C–O bond cleavage takes place even under mild conditions and can be facilitated by a metal catalyst, but a clear mechanistic understanding is currently lacking.

It should be noted that bond

dissociation energies (BDEs) of aromatic and aliphatic C–O bonds are approximately 468 and 385 kJ/mol, respectively,33 much stronger than a typical C–C bond (~350 kJ/mol). Consequently, direct aryl C–O bond hydrogenolysis only occurs under harsh conditions (>230˚C).28, 34-40 Any C–O bond cleavage observed under mild conditions ( 150 ˚C and P > 40 bar H2).59, 65-68 In our hydrogenation studies, acetic acid is both a solvent and a reagent to convert methoxy groups into methyl acetate. Methyl acetate could potentially be carbonylated to acetic acid (Eastman process),62 and thereby recovered so that both the aromatic units as well as the methoxy groups can be valorised.

Results and Discussion Hydrogenation reactions In order to investigate the hydrogenation of phenolic compounds, typically found in biooils, a series of lignin model compounds with various hydroxy, methoxy or alkyl substituents was chosen, as shown in Figure 1. Hydrogenation reactions of these model compounds have been carried out at elevated pressure in batch autoclaves, typically using 15 mmol substrate, 0.5 mol% Ru (Ru/C, 5 wt% Ru loading) as the catalyst and acetic acid (98 wt%) as the solvent. For comparison, methanol and isopropanol were also investigated as solvent, which gave similar results (see Figure S1a).

Surprisingly acetic anhydride as the solvent gave no


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conversion, the reasons for which are not understood at this stage, but suggests that protons, i.e. protic solvents, are important. Considering that any bio-oil or lignin will inevitably contain water, it was reassuring to find that addition of water up to 10 wt% did not affect the hydrogenation performance (see Figure S1b). Initial experiments on the hydrogenation of phenol at 120 ˚C and 30 bar H2 for 2 hours resulted in the formation of cyclohexanol and cyclohexyl acetate as the major products (91 mol%, ratio 23/68) together with 9 mol% cyclohexane. Importantly, when the product cyclohexanol or cyclohexyl acetate is used as substrate under the same hydrogenation conditions, there is no C–O bond cleavage to form cyclohexane. This indicates that cyclohexane must be formed during the phenol hydrogenation process.

Figure 1. Lignin model substrates used for hydrogenation studies. The hydrogenation of anisole (1) has been carried out over the temperature range from 80140 ˚C at 30 bar H2 pressure at four time intervals 30, 60, 120 and 240 min. (see Table 1). The mole balance in all cases is greater than 95% and a typical product distribution consists of methoxy cyclohexane (2), cyclohexanol (3), cyclohexyl acetate (3’) and cyclohexane (4) as shown in Eq. 2.

Table 1. Hydrogenation of anisole (1) with Ru/C in acetic acid Run

T ˚C

t h

conv. %

selectivity (mol%) 2 3 3’ 4

mole bal. %

1 2 3 4 5 6 7

80 80 80 80 100 100 100

0.5 1 2 4 0.5 1 2

23 49 75 85 51 88 100

63 62 59 58 53 55 53

96 95 98 98 97 97 100

16 21 22 20 20 18 17

0 0 0 1 2 3 5

21 17 19 21 25 24 25


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8 100 4 100 54 14 10 23 97 9 120 0.5 68 47 19 3 31 96 10 120 1 93 45 15 6 34 100 11 120 2 100 45 11 10 33 100 12 120 4 100 45 4 17 33 100 13 140 0.5 88 37 10 7 46 100 14 140 1 100 35 8 10 47 100 15 140 2 100 36 3 16 45 100 16 140 4 100 36 1 18 45 100 Conditions: anisole (1.63 mL; 15 mmol). Ru/C; solvent: acetic acid (10 mL), H2O (0.25 mL), hydrogen (30 bar).

The salient points from this study are an increase in selectivity for the de-oxygenated product cyclohexane (4) with temperature, relative to the oxygenated products (2, 3/3’), from 20 mol% at 80 ˚C to 47 mol% at 140 ˚C (see Figure 2). This indicates that C–O bond cleavage becomes more pronounced at higher temperature. Furthermore, at any given temperature, conversion increases with time, as expected (see Table 1 and Figure S9). However, once full conversion has been reached, the product composition remains essentially the same, apart from esterification of cyclohexanol to cyclohexyl acetate (see Table 1 and Figures S10-13). There is no further conversion of the oxygenated products (2, 3/3’) to cyclohexane (4), i.e. no further C–O bond cleavage takes place once all anisole has been converted. This indicates that the C– O bond cleavage must occur during the hydrogenation of anisole. A control experiment with the product methoxy cyclohexane (2) as the substrate under hydrogenation conditions at 140 ˚C for 2 hours showed no conversion, i.e. no C–O cleavage. 70

mol%

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60

2

50

3

40

3' 4

30 20 10 0 80

100 120 Temperature / ˚C


140

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Figure 2. Product composition for the hydrogenation of anisole (1) in mol% at different temperatures. Methoxy cyclohexane (2), cyclohexanol (3), cyclohexyl acetate (3’) and cyclohexane (4). Conditions: 2 hours, 30 bar H2, catalyst Ru/C; solvent: acetic acid (see Experimental Section for details).

Hydrogenation of guaiacol The hydrogenation of guaiacol (5) is more complicated due to the formation of cis and trans isomers of 2-methoxy cyclohexanol (6) and 2-methoxy cyclohexylacetate (6’), as well as the products methoxy cyclohexane (2), cyclohexanol (3), cyclohexyl acetate (3’) and cyclohexane (4) (Eq. 3). Conversion rates are considerably lower compared to anisole. After 4 hours at 80˚C, the conversion of guaiacol is only 24%, c.f. 85% for anisole, and full conversion is only reached after 4 hours at 140 ˚C (see Table 2). The additional OH group in guaiacol clearly deactivates the substrate towards hydrogenation. The mole balance is greater than 90 % in all cases.

At any given temperature, conversions increase with time, as expected. At lower temperatures and shorter reaction times, cis and trans 2-methoxy cyclohexanol (6) are the major products, followed by cyclohexanol/cyclohexyl acetate (3+3’). While the amount of 6 decreases with increase in temperature, the cis/trans ratio (c6/t6) appears essentially constant (see Figure S14). Methoxy cyclohexane (2) is hardly observed, which indicates that demethoxylation is much more facile than de-hydroxylation. This is an important observation as it indicates that de-methoxylation of lignin-derived aromatics with guaiacyl and syringyl units can generate cyclohexanol derivatives, as proposed in Eq. 1. The degree of hydro demethoxylation (HDMO) is expressed by the ratio of cyclohexanol/acetate to methoxycyclohexanol/acetate (3+3’)/(6+6’) (last column, Table 2). HDMO values are fairly constant over time but increase systematically with temperature from an average of 0.5 at 80 ˚C to 1.8 at 140 ˚C. Similar observations regarding the temperature dependence of HDMO values for guaiacol were observed by others.55, 58 Cyclohexane (4) is a minor product, whose selectivity


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increases with temperature from 7% at 80˚C to 14% at 140 ˚C after 4 hours. Similar to the case of anisole, the composition of the product spectrum does not change with conversion, except for the acetylation of cis/trans 2-methoxy cyclohexanol (6) and cyclohexanol (3), especially at higher temperature (see also Figure S17). Acetylated guaiacol is not observed, most likely because the acetylation reaction is relatively slow compared to hydrogenation. When acetylated guaiacol (2-methoxy phenyl acetate) is used as the substrate under the conditions of run 14 in Table 2, a different product distribution is observed (run 15). In addition to some guaiacol from ester hydrolysis, anisole and its hydrogenation products 2-4 are the major products (cf. run 15, Table 1). The lower conversion and different product distribution indicates the acetylated guaiacol is not an intermediate in the hydrogenation of guaiacol. Another important observation, there is no further C–O bond cleavage of any of the aliphatic cyclohexyl products once they have been formed. A separate hydrogenation experiment at 140 ˚C and 30 bar H2 for 2 hours, using trans-2-methoxycyclohexanol (t6) as the substrate, showed no C–O cleavage and only trans-2-methoxycyclohexyl acetate (t6’) as the product due to acetylation. Table 2. Hydrogenation results of guaiacol (5) with Ru/C. run

T ˚C

t h

conv. %

selectivity (mol%) c6 t6 c6’ t6’

2

3

3’

4

HDMO (3+3’)/(6+6’)

mole bal. %

1 80 1 13 51 15 1 1 0 25 1 5 0.4 99 2 80 2 20 49 13 4 1 2 17 7 7 0.4 96 3 80 4 24 43 12 3 1 2 25 7 7 0.6 98 4 100 0.5 15 42 10 2 2 0 31 7 6 0.7 98 5 100 1 18 39 9 3 1 0 33 8 7 0.8 99 6 100 2 29 37 10 4 2 1 28 10 7 0.7 99 7 100 4 43 31 8 6 2 0 30 14 8 0.9 91 8 120 0.5 28 30 11 3 1 1 35 8 9 1.0 94 9 120 1 39 31 9 4 2 0 30 15 9 1.0 96 10 120 2 51 27 9 6 3 2 25 20 10 1.0 99 11 120 4 88 21 6 10 4 2 15 27 15 1.0 100 12 140 0.5 38 21 8 4 2 0 33 22 10 1.7 94 13 140 1 62 17 6 6 3 0 25 29 12 1.7 95 14 140 2 86 14 5 9 5 0 15 40 13 1.7 97 15a) 140 2 65 1 0 2 1 13 4 10 33 3.5 88 16 140 4 100 8 3 12 6 1 6 50 14 2.0 96 Conditions: guaiacol (1.68 mL; 15 mmol). Ru/C (10 wt%); solvent: acetic acid (10 mL), H2O (0.25 mL), hydrogen (30 bar). a) Substrate: acetylated guaiacol (2-methoxy phenyl acetate; 15 mmol). Conditions: Ru/C (10 wt%); solvent: acetic acid (10 mL), H2O (0.25 mL), hydrogen (30 bar). Additional products: guaiacol (22 mol%) and anisole (13 mol%).


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In order to investigate the difference between the reactivity of OH versus OMe groups, we also investigated the hydrogenation of 1,2-dimethoxybenzene (7) (Eq. 4). Conversions and product selectivities are very similar to the those obtained with guaiacol (5) (see Table 3). The hydrogenation product cis/trans-1,2-dimethoxycyclohexane (8) is the major product with 67 % selectivity at lower temperature (80 ˚C) and the C–O cleavage product cyclohexane is the major product at high temperature (140 ˚C). Acetylations are faster at higher temperatures and hence cis/trans 2-methoxy cyclohexanol (6) is not observed, but the acetylated products (6’) appear in small amounts, even at short reaction times (run 6). For similar reasons, only acetylated 3’ is observed, but no cyclohexanol 3 and there is no formation of guaiacol (5) and thus no O–CH3 bond cleavage occurs.

Table 3. Hydrogenation results of 1,2-dimethoxybenzene (7) with Ru/C. run

P

T ˚C

t h

conv. %

c8

selectivity (mol%) t8 c6 t6 c6’

t6’

2

3

3’

4

mole bal. %

1 30 80 4 30 60 1 0 1 1 20 0 1 17 95 2 10 140 4 19 19 0 0 0 4 3 7 0 5 63 96 3 20 140 4 58 25 1 0 0 6 4 14 0 6 45 92 4 30 140 4 100 27 0 0 0 6 4 13 0 7 36 95 5 40 140 4 100 31 0 0 0 4 3 13 0 7 38 89 6 30 140 1 42 33 0 0 0 3 2 15 0 4 43 88 7 30 140 2 47 24 0 0 0 3 3 13 0 5 52 89 8 30 180 4 100 17 0 0 0 6 4 9 0 7 57 94 Conditions: 1,2-dimethoxy benzene (1.91 mL; 15 mmol). Ru/C (10 wt%); solvent: acetic acid (10 mL), H2O (0.25 mL), hydrogen.

Hydrogenation reactions were then carried out with substrates that more closely resemble the structural motifs found in lignin, such as 4-propyl-2-methoxy-phenol (9) and syringol (14). In view of the lower conversions obtained for the hydrogenation of guaiacol compared to anisole below 140 ˚C, all hydrogenations of 4-propyl-2-methoxy-phenol (9) were carried out at 140 ˚C up to 4 hours (see Table 4). Conversions were approximately halved compared to guaiacol under the same conditions, likely due to the presence of the propyl group. The propyl


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group also complicates the product spectrum which consists of more stereoisomers, i.e. four isomers for each of the tri-substituted products 10 and 10’, and two isomers for the disubstituted cyclohexanols 12 and 12’ (Eq. 5). These isomers could not be individually identified by GC and have been integrated together. Hydrogenation proceeds predominantly to 4-propyl-2-methoxycyclohexanol (10/10’) and 4-propyl-cyclohexanol derivatives (12/12’), with some propylcyclohexane (13) (11-12%). No detectable amount of 3-propyl-methoxycyclohexane (11) is obtained, indicating that de-methoxylation is again the preferred route, rather than de-hydroxylation. Conversion increases with time and the degree of demethoxylation HDMO = (12+12’)/(10+10’) remains constant at 2.4 at 140 ˚C. Overall, 84% conversion of 9 can be achieved after 4 hours at 140˚C, with a 63% selectivity to the target demethoxylated cyclohexanols 12/12’. These results are comparable to those reported under similar conditions in water as the solvent by Tomishige and Hensen.55, 57

Table 4. Hydrogenation results of 4-propyl-2-methoxy-phenol (9) with Ru/C. run

T ˚C

t min

conv. %

selecvtivity (mol%) 10 10’ 11 12 12’

HDMO 13

mole bal.

(12+12’)/(10+10’) %

1 140 30 20 22 3 0 44 20 11 2.6 93 2 140 60 31 22 5 0 35 27 11 2.3 96 3 140 120 56 18 9 0 23 37 11 2.2 100 4 140 240 84 13 12 0 14 49 12 2.5 100 Conditions: 4-propyl-2-methoxy-phenol (9) (2.40 mL; 15 mmol); catalyst: Ru/C (10 wt%); solvent: acetic acid (10 mL), H2O (0.25 mL), hydrogen 30 bar.

Syringol (14) hydrogenations have been carried out at temperatures ranging from 80 to 140 ˚C at 20-30 bar. A full quantitative analysis was hampered by the unavailability of any of the isomers of the hydrogenation product 1,3-dimethoxy-2-cyclohexanol (15) (Eq. 6). In addition to cyclohexane (4), the main other products that were detected by GC and quantified were the products of de-methoxylation: cis/trans-2-methoxy cyclohexyl acetate (6’) and cyclohexyl acetate (3’). 1,3-Dimethoxy cyclohexane (16) and methoxy cyclohexane (2) were not detected,


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indicating again that de-hydroxylation is not a significant pathway. The degree of demethoxylation increases with temperature and follows qualitatively the same general trends observed for guaiacol (5) and dimethoxy benzene (7).

Effect of pressure The effect of hydrogen pressure on conversion and selectivity was investigated for anisole (1) and 1,2-dimethoxybenzene (7). Hydrogenation reactions of anisole were carried out at 140 ˚C for 2 hours under standard conditions (see Figure 3). Conversions are generally low up to 20 bar and the main product is cyclohexane (4). At higher pressures, above 20 bar, the formation of methoxy cyclohexane (2) increases and cyclohexane formation decreases. Similar observations were made in the hydrogenation of 1,2-dimethoxybenzene (7), where cis/trans1,2-dimethoxycyclohexane (8) increases with pressure and cyclohexane (4) decreases (see Figure S16). These two sets of results show an important observation, that C–O bond cleavage becomes less prevalent at higher H2 pressure, which strongly supports the premise that this cleavage reaction does not involve H2 directly. The hydrogenation of the aromatic ring increases at higher pressure, and this reaction becomes more dominant at higher pressure. The C–O bond cleavage reaction must therefore involve a different mechanism, other than the commonly proposed hydrogenolysis pathway.


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120

Conv.

100

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

80

2

60

3'

40 20 0 10

20 Pressure (bars)

30

Figure 3. Effect of pressure on the hydrogenation of anisole. Conditions: 2 hours, 140 ˚C, catalyst: Ru/C; acetic acid.

Mechanistic studies The hydrogenation studies of aromatic phenols and phenolic ethers using Ru/C as the catalyst in acetic acid described in the previous section have shown that: 1) Additional substituents (methoxy, hydroxy, alkyl) slow down the rate of conversion, which could be due to an electronic effect from the electron donating substituents or due to steric effects. 2) Hydrogenation of phenols/phenolic ethers at temperatures between 80-140˚C and pressures up to 40 bar results in cyclohexanols and cyclohexyl ethers, together with cyclohexane due to a C–O bond cleavage reaction. 3) The formation of cyclohexanol/cyclohexyl ethers increases with temperature and pressure, whereas the formation of cyclohexane increases with temperature but decreases with pressure. 4) No C–O bond hydrogenolysis is observed in the hydrogenation of any of the cyclohexanol or cyclohexyl ether products. 5) De-methoxylation of phenols/phenolic ethers occurs in preference to de-hydroxylation. This C–O cleavage process increases with temperature, but is independent of time. Taken together, we propose that the hydrogenation of the aromatic lignin model substrates investigated here with metallic Ru-based catalysts proceeds in a stepwise fashion via cyclohexadiene and cyclohexene intermediates, in a similar fashion as for the hydrogenation


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of benzene.50 The partially hydrogenated di-ene and mono-ene intermediates are highly reactive and are generally not observed.

The key difference between benzene and the

oxygenated aromatics used here is that some of the intermediates are enols, enol ethers or allyl ethers, all with their particular reactivities. Enol ethers and allyl ethers can react with acetic acid to give cyclohexanone and allyl acetate intermediates. These intermediates in turn are further converted upon hydrogenation to cyclohexanol and cyclohexyl acetate products. As part of our mechanistic investigations we have synthesised the proposed intermediates by independent methods and investigated their reactivity under the reaction conditions used for the hydrogenation experiments.

The proposed reaction sequence for the hydrogenation of anisole 1 to the observed products 2-4 is shown in Scheme 2. Hydrogenation of the first double bond results in three possible cyclohexadiene isomers Ia-c, which are further hydrogenated to give three possible cyclohexene intermediates IIa-c, and eventually to methoxy cyclohexane 2. Two of the cyclohexadiene intermediates (Ia and Ib) are enol ethers, which are susceptible to acetolysis in acetic acid (vide infra). This results in the formation of cyclohexenone isomers IIIa and IIIb, respectively, which are hydrogenated to cyclohexanol (3) under the reaction conditions. The same applies to the enol ether IIa which after acetolysis converts to cyclohexanone and, after hydrogenation, to cyclohexanol (3). Intermediate 1c is unknown and is likely to undergo elimination of methanol and re-aromatise to form benzene VI. Benzene will be rapidly hydrogenated to cyclohexane (4) by Ru/C under the reaction conditions.69 Similar reaction sequences can be drawn for other model compounds, but their complexity due to the number of possible isomers increases dramatically, as shown in Scheme S1 for the conversion of guaiacol (5).


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Scheme 2 In order to investigate the reactivity of methoxy cyclohexadienes under the reaction conditions used for hydrogenation, a mixture of 1,3-, 1,5- and 1,4-methoxycyclohexadiene (Ia, Ib and Id) was used for our mechanistic studies (see Scheme 3). This mixture is available commercially (prepared via Birch reduction of anisole) with a composition of 63% Ia, 12% Ib, 23% Id and 2% anisole 1. The 1,3-isomer Ia is the major and most stable isomer in this mixture.70 This mixture was heated in d4-acetic acid at 120 ˚C and the reaction was followed by 1H NMR over time (Figure S3). All isomers are converted exclusively to 2-cyclohexenone IIIb, together with methyl acetate, with a half-life of approximately 2 hours under these conditions. These observations are in line with similar reactions of methoxy-cyclohexadiene Id in aqueous and acidic media,71-72 and with the known acid-catalysed rearrangement of IIIa to IIIb.73

Scheme 3 The enol ether 1-methoxy cyclohexene IIa, prepared from cyclohexane 1,1-dimethyl acetal,74 was converted in d4-acetic acid at 120 ˚C to cyclohexanone IV and methyl acetate with a half-life of approximately 1 hour (Eq. 7 in Scheme 4 and Figure S7). DFT calculations


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have shown that the formation of 1-acetoxy-1-methoxy cyclohexane as an intermediate is feasible, but further intra- or intermolecular attack by acetate is energetically unfavourable (see Figure S18). A more favourable pathway for the acetolysis reaction is the direct protonation (deuteration) of the enol ether at the C2 carbon to give a cationic intermediate as shown in Eq. 7, followed by nucleophilic attack by acetate at the methyl carbon to give d1-cyclohexanone d1-IV and methyl acetate (see Figure S19). This pathway also agrees with previous reports on the acid-catalysed hydrolysis of enol ethers in aqueous solution.75 Full deuteration of all hydrogens in the 2- and 6-positions to give cyclohexanone d4-IV proceeds at 120 ˚C in CD3COOD with a half-life of approximately 40 minutes (see Figures S7 and S8).71 Cyclohexanone (or its derivatives) were not observed for any of our substrates and reaction conditions, but have been seen by others in similar hydrogenation reactions.39, 49, 53 3-Methoxy cyclohexene IIb is an allylic ether with a different reactivity than the enol ether IIa and the reaction in d4-acetic acid at 120 ˚C leads to the formation of 3-acetoxy cyclohexene V and methyl acetate (Eq. 8). The half-life of this reaction is approximately 4 hours and proceeds most likely via nucleophilic attack of acetate at the C3 position and proton-assisted elimination of methanol, which undergoes esterification to methyl acetate.

4-Methoxy

cyclohexene IIc was also prepared and subjected to the same reaction conditions, but no reaction was observed in d4-acetic acid at 120 ˚C after 28 hours (Eq. 9). These observations regarding the reactivities of the intermediates support the mechanistic proposal outlined in Scheme 2.

Scheme 4


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The formation of benzene from cyclohexadiene intermediate 1c shown in Scheme 2 is likely to be very fast. At any given pressure and temperature, the rate of formation of cyclohexane (4) relative to the rate of formation of the final hydrogenation product methoxy cyclohexane (2) should be constant and therefore the ratio of 4/2 should be constant at a given temperature and time. When the 4/2 ratio’s obtained at four different time intervals (30, 60, 120 and 240 min.) are averaged and plotted as a function of temperature, a linear relationship can be observed in the case of anisole and, more convincingly, also for the ratio 4/(6+6’) in the analogous case of guaiacol (see Figure 4). The ratio of unwanted cyclohexane formation is clearly minimized at low temperature, but at the expense of lower conversion, which will need to be compensated by higher pressure.

Higher pressure will also increase the rate of

hydrogenation of the cyclohexadiene and cyclohexene intermediates to favour formation of methoxy cyclohexane (2) over cyclohexane (4). Cyclohexanol and cyclohexyl acetate are the other side products formed during the stepwise hydrogenation of anisole, and the ratio (3+3’)/2 follows a linear relationship (see Figure S14). Because all three ratio’s 4/2, 4/(6+6’) and also (3+3’)/2 show linear behaviour with temperature, the HDMO ratio (3+3’)/(6+6’) for guaiacol must follow a linear relationship with temperature, which is indeed observed (Figure S15).

1.60 1.40 4/2 (anisole)

1.20 1.00

ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4/(6+6') (guaiacol)

0.80 Linear (4/2 (anisole))

0.60 0.40

Linear (4/(6+6') (guaiacol))

0.20 0.00 60

80

100

120

140

160

Temperature / ˚C


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Figure 4. Average ratio between cyclohexane (4) and the hydrogenation products for anisole (2, methoxy cyclohexane) and for guaiacol (cis/trans methoxy cyclohexane (ct6) and methoxy cyclohexyl acetate (ct6’) as a function of temperature.

In conclusions, we have shown that the hydrogenation of phenols and phenolic ethers with Ru/C in acetic acid under mild reaction conditions results in the hydrogenation of the aromatic unit to give cyclohexyl ethers, together with cyclohexanol, cyclohexyl acetate and cyclohexane side-products. C–O bond cleavage does not proceed via a direct hydrogenolysis reaction of C– O bonds, but occurs due to side-reactions of the highly reactive intermediates formed during hydrogenation such as cyclohexadiene- and cyclohexene-based enols, enol ethers and allyl ethers. The lifetime of these intermediates, and therefore the formation of cyclohexane, can be minimised at low temperature and high hydrogen pressures. Increased hydrogen pressures ensures fast hydrogenation of the reactive intermediates and minimises side-product formation and rearrangements. Low temperatures will however lower the rate of conversion and a compromise between activity and selectivity will be required. Using the findings we have presented here, we are currently investigating the hydrogenation of lignin-derived bio-oils and how we can take advantage of the formation of the unstable cyclohexadiene and cyclohexene intermediates. We also continue our efforts on the process development for the conversion of lignin to terephthalic acid according to Scheme 1.

Experimental Section Hydrogenation experiments were carried out in a 100 mL Hastelloy steel PARR autoclave. Acetic acid (10 ml), deionised water (0.25 ml), the lignin model compound (15 mmol) and the Ru/C catalyst (5 wt% on carbon; 0.177 g, 8.76.10-5 mol Ru, 0.58 mol%) were introduced to the reactor. The autoclave was sealed and the system was purged 3 times with hydrogen, before introducing the desired pressure and then heated to the desired temperature in a pre-heated graphite bath. Once the desired internal reactor temperature had been reached, the mixture was allowed to stir for the desired duration (from 30 to 240 minutes). At the end of the reaction, the autoclave was cooled with an ice/water bath. Excess hydrogen and other volatiles were slowly vented in the fume hood. 1 ml of mesitylene, the internal standard, was added to the solution and stirred for a few minutes. The solution was filtered through a PVDF (0.22 ml pore


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size) membrane to remove the catalyst and an aliquot of the solution was dissolved in ethyl acetate for analysis by GC and/or GC-MS.

Supporting Information Experimental details, NMR spectra, additional graphs. Author Information Corresponding author: [email protected] Note: The authors declare no competing financial interest. Acknowledgements This work was supported by the EPSRC (Grant EP/K014676/1) and by The Coca Cola Company. We thank Dr. Rob Kriegel for valuable discussions.

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