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Selective conversion of lignin-derivable 4-alkylguaiacols to 4alkylcyclohexanols over noble and non-noble metal catalysts Wouter Schutyser, Gil Van den Bossche, Anton Raaffels, Sander Van den Bosch, Steven-Friso Koelewijn, Tom Renders, and Bert F. Sels ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01580 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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ACS Sustainable Chemistry & Engineering

Selective conversion of lignin-derivable 4-alkylguaiacols to 4alkylcyclohexanols over noble and non-noble metal catalysts Wouter Schutyser,a,b* Gil Van den Bossche,a Anton Raaffels,a Sander Van den Bosch,a StevenFriso Koelewijn,a Tom Rendersa and Bert F. Selsa* a

Center for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium. Email: [email protected]; [email protected]. b

National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States

ABSTRACT: Recent lignin-first catalytic lignocellulosic biorefineries produce large quantities of two potential platform chemicals, 4-n-propylguaiacol (PG) and 4-n-propylsyringol (PS). Since conversion into 4-n-propylcyclohexanol (PCol), a precursor for novel polymer building blocks, presents a promising valorization route, reductive demethoxylation of PG was examined here in the liquid-phase over three commercial hydrogenation catalysts, viz. 5 wt% Ru/C, 5 wt% Pd/C and 65 wt% Ni/SiO2-Al2O3, at elevated temperatures ranging from 200 to 300 °C under hydrogen atmosphere. Kinetic profiles suggest two parallel conversion pathways: Pathway I involves PG hydrogenation to 4-n-propyl-2-methoxycyclohexanol (PMCol), followed by its demethoxylation to PCol, while Pathway II constitutes PG hydrodemethoxylation to 4-npropylphenol (PPh), followed by its hydrogenation into PCol. Slowest step in the catalytic formation of PCol is the reductive methoxy removal from PMCol. PCol may under the reaction circumstances react further into hydrocarbons. Following criteria are therefore essential to reach a high PCol yield: i) Catalytic pathway II is preferred as this route does not involve stable intermediates; ii) Reactivity of PMCol should be higher than that of PCol, and iii) the overall carbon balance should be high. Both the catalyst type and the reaction conditions have a substantial impact on the PCol yield. Only the commercial Ni catalyst meets the three criteria, provided the reaction is performed at 250 °C in hexadecane. Additional advantages of this solvent choice are a high boiling point (low operational pressure in closed reactor systems), high solubility of PG and derived products, high thermal, reductive stability, and easy derivability form fatty biomass feedstock. This Ni catalyst also showed an excellent stability in recycling runs and is capable of converting highly concentrated (up to 20 wt%) PG in hexadecane. Ru and Pd on carbon showed a low PCol yield as they are not conform the three criteria. Low hydrogen pressure favors Pathway II, resulting in a very high PCol yield of 85% at 10 bars. Catalytic conversion of guaiacol, 4-methyl- and 4-ethylguaiacol in comparable circumstances showed similarly high yields of the corresponding cyclohexanols.

KEYWORDS: lignin, heterogeneous catalysis, upgrading, alkylguaiacols, alkylcyclohexanols INTRODUCTION Lignocellulose biomass is gaining increasing importance as a renewable resource for fuels, chemicals and materials.1-7 While processing of the carbohydrate fractions of the lignocellulose, viz. cellulose and hemicellulose, is well-developed,1, 6, 8 valorization of lignin, the third main constituent of lignocellulose, is still in its infancy.9-14 Currently, lignin is largely used as lowvalue energy source to fuel lignocellulose processing.11 Upgrading this phenolic biopolymer is however considered to be essential for the economic and environmental viability of future lignocellulose biorefineries.11, 15 Recent biomass valorization research is

therefore increasingly focusing on lignin as a renewable resource. Lignin is the largest renewable source of aromatics and due to its unique chemical structure, a large number of potential products can be envisioned.9-13 It is composed of methoxylated 4-propylphenol units, which are connected through various ether and C-C linkages. Lignin can either be used as such in material applications, e.g. in polymers, as dispersant, emulsifier, etc.,16-17 or it can be depolymerized into a family of aromatic monomers.9, 12-13 Depolymerization of lignin is challenging since lignin is very prone to condensation and degradation reactions,

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either during its isolation from the lignocellulose matrix or during depolymerization.11, 13, 18-19 Therefore, the yields of phenolic monomers from lignin are usually low.9, 13 In the last years, researchers have developed several elegant lignocellulose fractionation and lignin depolymerization methods.20-25 One very promising method is the ‘catalytic reductive fractionation’ or ‘lignin-first approach’, in which lignin extraction and catalytic depolymerization are performed in one process step.24, 26-34 This process disconnects lignin from the carbohydrates in the lignocellulose matrix and extracts the soluble lignin part, while immediately depolymerizing the lignin fragments through metal-catalyzed hydrogenolysis (of the ethers). Irreversible repolymerization with formation of stronger C-C bonds is largely suppressed by this reductive stabilization strategy, leading to high (close to theoretical) phenolic monomer yields (over 50 wt%). These phenolic monomers are typically methoxylated alkylphenols, viz. guaiacol and syringol, with an alkyl, alcohol or allyl substituent in para-position (see Figure 1).

Figure 1 Typical phenolic monomers obtained from catalytic reductive fractionation.

Although some lignin derived products are useful as such, vanillin being the most well-known example,35-36 the large scale implementation of other lignin-derived phenolic monomers, for instance as fuels or chemicals, requires further (bio)chemical upgrading. One promising route is microbial transformation, whether or not combined with a chemo-catalytic reaction step. In this way, lignin fragments were converted into adipic acid,37-38 polyhydroxyalkanoates,39-40 alkenoic acids39 and C9-14 hydrocarbons.40 The route that receives most attention is the catalytic hydrodeoxygenation (HDO) to alkanes and aromatics.41-43 These hydrocarbons can be used directly as liquid fuel or as feedstock for established petrochemical processes. One less investigated but also interesting route is the selective HDO to oxygenated products such as substituted phenols and cyclohexanols.44-53 Substituted phenols like 4-alkylphenols, though important chemicals in the current chemical industry,54 can easily be converted to phenol and the corresponding olefin.55 Cyclohexanol is an important precursor of polymer building blocks like caprolactam, caprolactone and adipic acid.56-57 The availability of alkyl-substituted cyclohexanols could offer opportunities for synthesizing alkylated variants of these polymer building blocks.44, 58 It can be intuitively understood that the presence of alkyl

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groups allows for tuning the polymer’s physical properties such as crystallinity, melting point, elasticity, etc. An important step in the conversion of lignin-derived phenolic monomers to phenols and cyclohexanols is the removal of the methoxy side-groups, also called hydrodemethoxylation (HDMO). The catalytic conversion of the lignin-derived methoxyphenols to phenols is usually performed under gas-phase conditions with a variety of catalysts.49-53 Only a few studies have investigated the on-purpose conversion of methoxyphenols to the corresponding cyclohexanols.44-47 This process involves a combination of hydrodemethoxylation (preferably yielding methanol)45, 47 and aromatic ring hydrogenation with retention of the hydroxyl group. Cyclohexanols are very often observed as intermediates in the complete HDO processes, but the obtained yields are generally low.41-43, 51-52, 59-62 Tomishige et al. studied the on-purpose production of cyclohexanol from guaiacol, and found that a combination of Ru/C and MgO in water at 160 °C enables a high cyclohexanol (Col) yield, up to 78%.45 The base co-catalyst enhances the HDMO step and prevents C-O hydrogenolysis of Col to cyclohexane. In a follow-up study, the authors reported on a bimetallic Ru-MnOx/C catalyst in water, reaching a Col yield of 81%.47 MnOx affects the product selectivity in a similar way as MgO. When using only Ru/C, the Col yield was considerably lower (66%), but still much higher compared to other noble metal catalysts such as Pt/C (45%), Rh/C (36%) and Pd/C (1%).45 Hensen et al. also compared Pt/C and Ru/C in the aqueous-phase conversion of guaiacol at 200 °C and obtained Col selectivities of 66-70% at about 80% conversion.61 Fu et al. reached very high 4-n-propylcyclohexanol (PCol) yields (up to 90%) in the conversion of eugenol, an unsaturated variant of 4-n-propylguaiacol (PG), over Ru supported on carbon nanofibers in a biphasic water : dodecane mixture at 160-180 °C.62 In another study, Fu et al. reported that Ru/ZrO2–La(OH)3, a metal-solid base bifunctional catalyst, is very selective in the aqueous-phase conversion of various 4-alkylguaiacols (RGs) and 4-alkylsyringols to the corresponding 4-alkycyclohexanols (RCols) at 200 °C, with RCol yields reaching over 87%.63 Next to noble metal-based catalysts, also Ni catalysts are reported for this transformation. Rinaldi et al. demonstrated that various guaiacols and syringols with a range of saturated and unsaturated side-chains can be selectively converted to the corresponding RCols over Raney Ni in 2-propanol at 80-120 °C, with the solvent acting as hydrogen source.46 In a previous study, we investigated the conversion of guaiacol to Col over Ni on various oxides in hexadecane at 300 °C, showing that the amphoteric oxides ZrO2 and CeO2 are most suitable supports, both yielding over 75% of Col.44 Next to guaiacol, various alkylated guaiacols were selectively converted to the corresponding RCol over Ni/CeO2. High Col and RCol yields, up to 83%, were achieved but only at high reaction temperature as a consequence of the high activation energy of the HDMO of the intermediate 4-alkyl-2-methoxycyclohexanol (RMCol). Furthermore, the conversion of real ligninderived PG to PCol over Ni/CeO2 and its further transformation to n-propylcyclohexanone (through

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dehydrogenation) and n-propyl-ɛ-caprolactone (through Baeyer-Villiger oxidation) was demonstrated.44 This contribution investigates the catalytic conversion of lignin-derived PG to PCol in presence of two commercial noble-metal catalysts, 5wt% Pd/C and 5wt% Pt/C, which are frequently reported in HDO studies, and one commercial base metal catalyst, viz. 65 wt% Ni/SiO2Al2O3. While these catalysts have usually been investigated in aqueous conditions, there catalytic performance was evaluated here in hexadecane.45, 59, 61-62 Hexadecane is a high-boiling inert solvent with high PG (and products) solubility (in contrast to water). Hexadecane and other long-chain alkanes may be readily available in a hydro-biorefinery from the vegetable oil fraction.5, 64 As it has a low vapor pressure, catalytic operation runs under mild pressures, while the products can be easily distilled.

recycling. The reaction time was increased to reach the required contact time.

Product analysis Quantitative analysis of the reaction samples was performed by GC-FID using an Agilent 6890 series GC equipped with a HP5 capillary column, a split injection system, and a flame ionization detector. The following operating conditions were used: injection temperature of 250 °C, column temperature program: 60 °C (5 min), 10 °C/min to 290 °C (10 min), detection temperature of 310 °C. Sensitivity factors of the reagents and products were obtained by calibration with commercial standards or by ECN-based calculations due to lack of commercial standards.65 Conversion, product yield, product selectivity and carbon balance are defined in equations 1-4:

EXPERIMENTAL Chemicals and materials All commercial chemicals were analytical reagents and were used without further purification. Hexadecane (99%), decane (99+%), tetrahydrofuran (99+%), guaiacol (98+%), cyclohexanol (99 %), cyclohexanone (99+%), cyclohexane (99+%), 4-methylguaiacol (98+%), 4ethylguaiacol (98+%), 4-n-propylguaiacol (99+%), 4-npropylcyclohexanone (99+%), N-Methyl-N(trimethylsilyl)-trifluoroacetamide, ruthenium on carbon (5 wt%), palladium on carbon (5 wt%), nickel on silica/alumina (~ 65 wt%) and ethylcyclohexane (99+%) were purchased from Sigma Aldrich. Pyridine (99+%) was purchased from Acros Organics. 4-Ethylcyclohexanol (97+%) and 4-n-propylcyclohexanol (98+%) were purchased from TCI chemicals. Ethanol (99+%) was purchased from Fischer Chemical Ltd. Catalytic reactions Catalytic reactions were performed in batch mode in a 50 mL Parr autoclave equipped with a mechanical stirrer, a liquid sampling tube, gas inlet and outlet tubes, a thermocouple, a rupture disc, and an electric heating jacket. The autoclave was loaded with substrate, catalyst, decane as an internal standard, and solvent. After flushing the reaction mixture with N2 (3 times), the temperature was raised to the reaction temperature and the autoclave was put under constant H2 pressure at the reaction temperature. The H2 pressure indicated in the text and in the captions of the figures and tables is the pressure at reaction temperature and represents the total pressure (sum of solvent and H2 partial pressure). The stirrer speed was set at 750 rpm. After the reaction, the autoclave was rapidly quenched to room temperature in a water bath. For the catalyst recycling experiments, the catalyst was separated by filtration, washed thoroughly with acetone and hexane, and dried overnight in an oven at 100 °C. A small loss of solid catalyst particles occurred during the washing and filtration steps, and approximately 90% of the catalyst weight was recovered after each catalyst

Conversion % =

n , − n × 100% 1 n,

Yield % =

n%&'( × 100% 2 n,

Selectivity % =

Carbon balance % =

Yield % 3 Conversion %

n + ∑ n%&'( × 100% 4 n,

In these equations, nsubstrate,0, nsubstrate and nproduct represent respectively the molar amount of substrate at the start of the reaction and the amount of substrate and product after reaction. Qualitative analysis of the reaction products was performed by GC-MS by using an Agilent 6890 series GC instrument equipped with a HP5MS capillary column and an Agilent 5973 series mass spectrometry detector. The same operating conditions were used as for the GC-FID. Silylation of the reaction mixture was performed by adding the reaction mixture (0.5 mL), pyridine (0.5 mL) and N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA; 0.25 mL) to a GC vial and putting the vial in an oven at 80 8C for 30 min. Identification and quantification of the silylated products was performed by using the same GC analysis as that described for the typical reactions. RESULTS & DISCUSSION Reaction mechanism of RG conversion As shown in our previous Ni/CeO2 study, conversion of RG to RCol proceeds through two competitive pathways (see Scheme 1), namely (i) hydrogenation (HYD) followed by HDMO, with RMCol as the intermediate product (Pathway I, indicated in blue), and (ii) HDMO of RG, followed by HYD, with RPh as the intermediate product (Pathway II, indicated in red).44 RCol can further react to hydrocarbons (CH) such as alkylated cyclohexane, cyclopentane or benzene (step indicated in green)



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through hydrogenolysis, dehydration, (de)hydrogenation, isomerisation and cracking reactions.41-44, 59-60, 66 In addition to RCol, little 4-alkylcyclohexanone is present in thermodynamic equilibrium. For the experiments performed in this work, the RCol to 4-alkylcyclohexanone ratio is typically between 7 to 10, depending on the reaction conditions. In the rest of the discussion, RCol represents the sum of RCol and 4-alkylcyclohexanone. Since RPh hydrogenation proceeds very fast (pathway II) and RMCol hydrodemethoxylation is slow (pathway I), it can be assumed that initial formation of RCol is an exclusive consequence of Pathway II. Summation of the initial RPh and RCol selectivity can therefore be used as a measure for the preference of Pathway II (see SPathway II, see eq. 5), whereas the initial selectivity for RMCol is a preference measure for Pathway I (SPathway I, see eq. 6). 45678 9 : 4;?,@A@@? 5

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not affected in the circumstances. This implies a catalyst with a high reaction rate for the RMCol-to-RCol conversion (rRMCol→RCol) compared to that of RCol-to-RH conversion (rRCol→CH). This corresponds to a high rRMCol→RCol to rRCol→CH ratio. Our previous study for instance proposed Ni/CeO2 and Ni/ZrO2, used at 300 °C in hexadecane for the selective guaiacol to Col conversion, since they exhibited a high rRMCol→RCol to rRCol→CH ratio.44 This contribution continues the selective catalytic HDMO investigation. It evaluates the catalytic performance of two supported noble metal and one base metal catalysts (Ru/C, Pd/C and Ni/SiO2-Al2O3) for their ability to form RCol from RG, with R varying from methyl (Me), ethyl (Et) to n-propyl (P) (as they are available through lignin processing).9, 12-13 A critical assessment of the proposed reaction network will be performed and discussed with regard to the proposed selectivity criteria, i.e. a high SPathway II and a high rRMCol→RCol to rRCol→CH ratio.

45678 99 : 4;56,@A@@? + 4;=>?,@A@@? 6 Initially, RG is thus competitively converted into RMCol (through Pathway I) and RPh (through Pathway II), the latter being immediately converted to RCol. Consecutive reactions such as HDMO of RMCol to RCol and HDO of RCol to CH occur somewhat later in the reaction (see scheme 1).

Scheme 1 Reaction mechanism for the conversion of 4alkylguaiacols to 4-alkycyclohexanols and strategies to reach a high 4-alkylcyclohexanol selectivity. Abbreviations: hydrogenation (HYD), hydrodemethoxylation (HDMO), 4alkylguaiacol (RG), 4-alkyl-2-methoxycyclohexanol (RMCol), 4-alkylphenol (RPh), 4-alkylcyclohexanol (RCol), hydrocarbons (CH) and selectivity (S).

In accordance to the scheme, the best strategy to reach a high RCol selectivity is perhaps a catalytic process that follows Pathway II (high SPathway II), since the intermediate product (RPh) is easily converted to RCol. This has been nicely demonstrated by Tomishige et al. for reactions in water at 160 °C with a combination of Ru/C and MgO45 or a bimetallic Ru-MnOx/C catalyst.47 Another motivating strategy is the design of catalysts with a high HDMO activity for RMCol conversion, provided RCol is

Kinetic profiles of PG conversion over Ru/C, Pd/C and Ni/SiO2-Al2O3 PG kinetic profiles over Ru/C, Pd/C and Ni/SiO2-Al2O3 were determined in hexadecane at three different temperatures (200, 250 and 300 °C). Conditions are specified in the experimental section and in the caption of the figures. As explained in the previous section, the initial product selectivity, i.e. the selectivity at low conversion (low contact time), provides information about the pathway preference, while the product distribution at long contact time (high conversion) reflects information about RMCol HDMO and RCol conversion to CH.

Figure 2 Comparison of product distribution at similar PG conversion at 200, 250 and 300 °C for Ru/C (a), Pd/C (b) and Ni/SiO2-Al2O3 (c). Reaction conditions: PG (0.332 g, 2 mmol), decane (0.7 mmol), catalyst (10/40 mg), hexadecane (20 mL), -2 -1 40 bar H2, contact time of 0.4-0.8·10 h gcat gPG for (a) and -2 -1 (c) and 6·10 h gcat gPG for (b).

For Ru/C, the initial product selectivity at the three temperatures is compared in Figure 2a. In agreement with the coloured arrows in Scheme 1, 4-n-propyl-2methoxyphenol (PMCol) is indicated in blue, PCol and PPh are indicated in shades of red, while oxygen-free


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hydrocarbons (CH) are marked in green. Figure 2a clearly shows that the preference for pathway II increases with temperature in disfavour of pathway I. A temperature increase thus promotes HDMO of PG over PG hydrogenation. A similar effect was reported by Tomishige et al. for the Ru/C-catalysed conversion of guaiacol in water.45 Irrespective of the temperature, presence of PPh is always low due to its rapid conversion to PCol. Only at 300°C, the combined initial PCol and PPh selectivity (47%) is higher than that of PMCol (42%), indicating that only at high temperature (≥ 300 °C), Pathway II is the dominant catalytic pathway to PCol.

Figure 3 Conversion and product distribution as a function of contact time for PG conversion over Ru/C at 200 (a), 250 (b) and 300 °C (c). If no carbon balance is indicated, it is close to 100% at all contact times. Reaction conditions: PG (0.332 g, 2 mmol), decane (0.7 mmol), 5 wt% Ru/C (10/40 mg), hexadecane (20 mL), 40 bar H2.

PG conversion and the product selectivity for Ru/C are plotted as a function of the catalyst contact time (as h gcat gPG-1) in Figure 3. Herein, each graph (a to c) represents a different reaction temperature. Clearly, the initially formed PMCol is converted to PCol with increasing contact time, and this conversion is faster at higher temperature. However, HDO of PCol to CH is also accelerated with increasing temperature, giving rise to a serious PCol selectivity decrease because of undesired CH formation (see for instance Figure 3c). Therefore, Ru/C fulfils the first criteria, that is the preferential occurrence of pathway II, but only at high reaction temperature, but

it fails to obey the second criteria as RCol-to-CH conversion is too dominant under these conditions. On top of this, we observed an increasing carbon deficiency (decreasing carbon balance, see Experimental Section for definition) with increasing temperature and contact time (see Figure 3a-c). This is likely due to the formation of gaseous products in agreement with earlier reports.67 Ru/C is thus not able to selectively convert PG to PCol in hexadecane, under the conditions used in this study. However, Tomishige et al. and Hensen et al. have indicated that in water at lower temperature (160-200 °C), Ru/C was capable of selectively converting guaiacol to Col with 70% Col selectivity at high conversion (75-100%).45, 61 Because of solubility issues, usage of water is likely not the preferred option to convert RG. In gas-phase catalysis, Ru/C shows a selective HDMO of guaiacol to phenol, but only very little hydrogenation of the aromatic ring occurs, perhaps a consequence of the low hydrogen pressure and high temperature usually applied.51-53 Anyway, these studies indicate that the reaction phase and choice of solvent (if in the condensed phase; here: water vs. hexadecane) play an essential role in the selectivity of the Ru/C-catalysed (R)G conversion process. The situation is different for reactions with Pd/C. During PG conversion over Pd/C at the various temperatures, the product distribution remains almost the same at all contact times (see Figure S1 in the Supporting Information). Figure 2b illustrates the product distribution at the different temperatures and at high contact time (here: at 6·10-2 h gcat gPG-1). At 200 and 250 °C, PMCol selectivity is near to 100 %, whereas at 300 °C, a small amount of PCol is observed corresponding to 10% selectivity. Pd/C thus follows almost exclusively Pathway I and it shows a remarkably low activity in the subsequent conversion of PMCol to PCol. This means that Pd/C is capable of hydrogenating the phenolic aromatic ring, but unable to perform HDMO of PG and PMCol. Therefore, Pd/C is not a catalyst of choice as it meets neither of the two selectivity criteria. Previous studies on guaiacol (and eugenol) support our observation as they also reported the selective formation of 2-methoxycyclohexanol (MCol) and PMCol in presence of Pd/C in decane,68 water45, 61-62 or a water : dodecane mixture62, unless long reaction times are applied.61 Catalysis in gas-phase with Pd/C, as with Ru/C, allows selective HDMO of guaiacol to phenol, but (apparently) not to cyclohexanol.51-52 Conversion of RG to RCol was not investigated in the gas phase over Pd/C, to the best of our knowledge. Figure 2c shows the initial product distribution for PG conversion in presence of Ni/SiO2-Al2O3 at the various temperatures. The time plots are presented in Figure 4a to c. This catalyst clearly favours Pathway II already at lower temperature (above 250°C), in contrast to Ru/C (see Figure 2a). Indeed, the combined initial PCol and PPh selectivity at 250 °C is already considerably higher than the initial PMCol selectivity, viz. 59 vs. 38%. Figure 4a to 4c shows a slow PMCol to PCol conversion at 200 °C, but the reaction proceeds considerably faster at 250 and 300 °C. Formation of CH becomes significant at higher temperature in presence of the Ni catalyst, resulting in a fast drop of the PCol selectivity at 300°C (Figure 4c).



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Detailed inspection of the catalytic results shows that Ni/SiO2-Al2O3 at 250 °C not only preferentially follows Pathway II, but also PMCol to PCol conversion can be accomplished without dominant CH formation (Figure 4b). On top of this, the carbon balance retains close to 100% at all contact times at this temperature. Therefore, due to the very high carbon balance and the two selectivity criteria being met, the PCol yield reaches up to 82% at a contact time of 12·10-2 h gcat gPG-1. In comparison to our previous catalytic study with Ni/CeO2,44 Ni/SiO2-Al2O3 enables a selective PG to PCol conversion at a considerably lower temperature, i.e. 250 vs. 300 °C. This may be somewhat surprising as that study promoted the amphoteric supports CeO2 and ZrO2 as most suitable supports, whereas Ni on an acidic support (like γ-Al2O3) was found unsuitable due to strong conversion of Col to CH.44 However, Ni/SiO2-Al2O3 contains much higher Ni loadings as to limit the impact of the support surface on the reaction mixture. The previous study used a very low Ni loading (3 wt%).44

Figure 4 Conversion and product distribution as a function of contact time for PG conversion over Ni/SiO2-Al2O3 at 200 (a), 250 (b) and 300 °C (c). If no carbon balance is indicated, it is close to 100% at all contact times. Reaction conditions: PG (0.332 g, 2 mmol), decane (0.7 mmol), 65 wt% Ni/SiO2Al2O3 (10/40 mg), hexadecane (20 mL), 40 bar H2. Table 1 Catalytic characteristics of Ru/C, Pd/C and Ni/SiO2Al2O3 with respect to the criteria for a selective RG to RCol conversion.

Criteria for a selective RG Ru/C to RCol conversion High SPathway II ± ± High rRMCol→RCol to rRCol→CH ratio High carbon balance a For reactions performed at 250 °C.

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Ni/SiO2-Al2O3a

-

+ +

+

+

To summarize the above observations, Table 1 summarizes the catalytic behaviour of the three catalysts with respect to the two selectivity criteria. Also a third criterion, namely a high carbon balance, is indicated. The results clearly show the inability of Pd/C and Ru/C to selectively convert PG to PCol, whereas the Ni catalyst is the catalyst of choice for further investigation as it obeys best the three selectivity criteria (at least at 250°C). Influence of H2 pressure on PG conversion over Ni/SiO2-Al2O3 Previous reactions were standardly performed under 40 bar H2 pressure, measured at reaction temperature. This section investigates the effect of the H2 pressure on the PG conversion and product selectivity over Ni/SiO2-Al2O3 at 250 °C. Figure 5 presents the conversion of PG and the product distribution for reactions at short contact time (here: at 0.8·10-2 h gcat gPG-1) under a hydrogen pressure ranging from 10 to 60 bar (at reaction temperature). Interestingly, though PG conversion proceeds slower at lower H2 pressures, the preference for Pathway II increases. This observation may be explained by the hydrogen consumption in the first reaction of the different pathways. A decrease in hydrogen pressure will have a stronger retarding effect on PG hydrogenation as 6 surface hydrogen atoms are required, whereas HDMO of PG consumes two surface hydrogen atoms. A similar hydrogen pressure effect on the reaction selectivity was reported by Tomishige et al. in the conversion of guaiacol over Ru/C in water.45 Reaction at low hydrogen pressure (10 bar) ultimately yields a maximum of 85% PCol at a contact time of 6·10-2 h gcat gPG-1 (right hand side in Figure 5), which is only half of the contact time required to reach the maximum PCol yield at 40 bar (Figure 4b).

Figure 5 Conversion and product distribution for PG -2 -1 conversion at 250 °C at various pressures at 0.8·10 h gcat gPG -1 -2 and at 10 bar H2 at 6·10 h gcat gPG . Reaction conditions: PG (0.332 g, 2 mmol), decane (0.7 mmol), 65 wt% Ni/SiO2-Al2O3 (40 mg), hexadecane (20 mL).


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So, although PG conversion proceeds slower at lower hydrogen pressure, the maximum PCol yield is obtained faster since less PMCol is produced that needs to be converted to PCol (due to the preference for Pathway II). To conclude, low hydrogen pressure affords conversion dominantly through Pathway II, resulting in a short contact time to ultimately reach a higher PCol yield. Solvent effects on PG conversion over Ni/SiO2-Al2O3 As the solvent is known to play an important role in hydrogenation, hydrogenolysis and hydrodeoxygenation reactions with oxygenated aromatics,69-74 conversion of PG over Ni/SiO2-Al2O3 was examined in different solvents. Choice of solvents was based on solubility of PG and its derived products. Water, though reported before, was therefore not selected. Next to the hydrophobic and high-boiling hexadecane, reactions were performed in low-boiling heptane, but also in polar protic (ethanol) and non-protic (THF) solvents. Obviously, in contrast to the three other solvents, use of hexadecane has the advantage of low partial pressure during catalytic operation, even at 300 °C. As a consequence of our experimental set-up, the hydrogen partial pressure is lower in reactions with the low-boiling solvents. Since the hydrogen pressure affects the PG conversion (see previous section), reactions in the various solvents is compared at relatively low temperature and high total pressure, i.e. 200 °C and 60 bar, in order to minimize the difference in partial hydrogen pressure. PG conversion in hexadecane is also indicated at 40 bar total pressure for comparison, as the partial hydrogen pressure in this reaction is suggested to be lower than in the reactions in heptane, ethanol and THF at 60 bar and 200 °C (see Supporting Information for explanation). PG conversion in the various solvents is presented in Figure 6a. The product distributions for the reactions in Figure 6a are shown in Figure S2 in the Supporting Information.

Figure 6 (a) PG conversion in various solvents vs. contact time for reaction at 200 °C and 60 bar H2. (b) Conversion and product distribution as a function of contact time for PG

conversion in THF at 250 °C and 60 bar H2. Reaction conditions: PG (0.332 g, 2 mmol), decane (0.7 mmol), 65 wt% Ni/SiO2-Al2O3 (10/40 mg), solvent (20 mL).

Figure 6a clearly illustrates that the PG conversion rate is highest in the apolar solvents hexadecane (both at 40 and 60 bar H2) and heptane and further decreases in the order THF > ethanol. As previously suggested by Rinaldi et al. in a HDO study of diphenyl ether,69 this solvent effect might be due to a higher tendency of the more polar solvent molecules to adsorb on the Ni surface, hindering chemisorption at the active Ni sites. In the conversion of phenol over Pd/C, Lou et al. reported a complete conversion in hexane, but not in ethanol or methanol.73 Since the conversion rate in the polar solvents is highest in THF, the PG conversion was examined in THF at the optimal temperature for PCol production in hexadecane, i.e. 250 °C. Figure 6b gives the product distribution as function of the contact time. In THF, the combined initial PCol and PPh selectivity of 77% is much higher than the initial PMCol selectivity of 21%, indicating a clear preference for Pathway II. The high SPathway II might be explained by a prohibited aromatic activation on the Ni surface (and thus a retarded PG to PMCol hydrogenation), as a result of competitive adsorption of solvents molecules on the metal surface. Although reaching a high SPathway II, conversion of PMCol to PCol is very slow in THF, likely due to competitive chemisorption, leading to considerable PCol to CH conversion. Reaction in THF ultimately led to 76% PCol yield at full PG conversion, which is considerably lower than that in hexadecane (85 and 82% at 10 and 40 bar, respectively). A remarkable difference in product distribution at low contact time between reaction in THF and hexadecane is the initial PPh selectivity. In THF, it can take up to 18% of the products, slowly decreasing at prolonged contact time, whereas it never exceeds 4% in hexadecane, even at very low hydrogen pressure, and rapidly drops with reaction progression. The slow hydrogenation of PPh in THF can also be explained by prohibited aromatic activation on the Ni surface. Rinaldi et al. also reported higher contents of unsaturated products such as phenol and benzene in the conversion of diphenyl ether in polar solvents, while only saturated products such as dicyclohexyl ether, cyclohexanol or cyclohexane were obtained in alkane solvents.69 Similarly, Bejblova et al. studied the conversion of benzophenone over Pd catalysts in hexane and methanol and they found ringhydrogenated products in hexane, but not in methanol.74 Conversion of PG was also performed over the Ni catalyst in heptane at 250 °C and 40 bar, similar to the reaction in hexadecane. The results are presented in Figure S3 in the Supporting Information. As in hexadecane, the reaction preferentially proceeds through Pathway II, while the relative conversion rates of PMCol vs PCol are in favour of high PCol yields. As such, a PCol yield of 82% PCol was obtained in these conditions at a contact time of 12·10-2 h gcat gPG-1.



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To conclude the solvent part, PG conversion and aromatic ring hydrogenation proceed faster in apolar solvents. In spite of the preference for Pathway II in THF, HDMO of PMCol to PCol is too slow, compared to formation of CH from PCol. Alkanes are thus the preferred solvents for synthesis of PCol from ligninderived PG. Next to its simple accessibility from fatty acids,5, 64 hexadecane has the additional advantage of being a high-boiler. As this keeps the operating pressure low, less constraint is put onto the reactor facility. As PG and its products have lower boiling points than hexadecane, one might expect easy separation through vaporization of only the product. Conversion of other RGs over Ni/SiO2-Al2O3 To broaden the substrate scope, reactions with other RGs (guaiacol, G, and other lignin-derivable RGs such as 4methyl- and 4-ethylguaiacol, MeG and EtG, respectively), were performed over Ni/SiO2-Al2O3 in hexadecane at 250 °C and 10 bar H2. Table 2 shows the product distribution at maximum RCol yield for the different RGs. RCol yields range from 82 to 85%, irrespective of the RG type. The main difference is seen in the contact time to reach the maximum yield. Reaction runs are somewhat faster with RGs containing longer alkyl chains. This is likely due to the higher preference of Pathway II with increasing alkyl chain length for steric reasons, in line with a previous study over Ni/CeO2.44 As less PMCol is thus produced, less time is required to convert this stable molecule to PCol. As indicated in our previous study,44 4-alkylated guaiacols can be converted to both 4- and 3-RCol. The 4and 3-RCol can only be distinguished during GC analysis after derivatisation, for instance via silylation. Therefore, a silylation treatment was applied here. All reactions show a very high 4-/3-RCol ratio, ranging from 20 to 25, indicating that only very little isomerization occurs under the reaction circumstances. Ni/SiO2-Al2O3 thus shows a high regioselectivity in the conversion of 4-RG to 4-RCol.

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investigated. The stability of the Ni/SiO2-Al2O3 catalyst in the conversion of PG was evaluated in several recycling experiments, without intermediate reactivation of the catalyst under H2. The results are presented in Figure 7a. For a constant contact time of 6·10-2 h gcat gPG-1, the PCol yield only slightly decreased from 85 to 78% after 4 runs (3 recycle runs), indicating a fair catalyst stability. Next, the conversion of a concentrated PG feed was examined. As the reaction was performed in hexadecane, reaction with highly concentrated PG, comprising 20 wt% (or 163 g/L) of PG, was possible. Prior experiments used 2 wt% (or 16.4 g/L) PG-feed. As expected, a larger hydrogen content is required to completely convert the concentrated PG solution. Where the shortness of hydrogen is evidenced under 10 bars of H2 by the main formation of PPh and some propylbenzene, reaction under 40 bars of hydrogen succeeded in realizing a high 83% PCol yield. As Figure 7b indicates a similar product distribution irrespective of PG concentration, Ni/SiO2Al2O3 may be considered as a stable catalyst capable of also converting concentrated PG feeds selectively to PCol.

Table 2 Product distribution for the conversion of guaiacol and various lignin-derivable 4-alkylguaiacols over Ni/SiO2a Al2O3 at maximum RCol yield. Yield (%) b

Entry Substrate

t (h)

RCol RMCol

CH

Others

1

G

0.62

82

8

8

2

2

MeG

0.57

3

EtG

0.53

84

7

8

1

83

9

6

2

4

PG

0.5

85

9

5

1

a

Reaction conditions: substrate (2 mmol), 65 wt% Ni/SiO2-Al2O3 (40 mg), decane (0.1 g), hexadecane (20 mL), 250 °C, 10 bar H2. b Substrate abbreviations: guaiacol (G), 4-methylguaiacol (MeG), 4-ethylguaiacol (EtG) and 4-n-propylguaicol (PG).

PG conversion over Ni/SiO2-Al2O3: Catalyst regeneration and conversion of a concentrated feed. In order to verify the Ni/SiO2-Al2O3-catalyzed RG conversion for large scale application, catalyst stability and the conversion of a concentrated feed were

Figure 7 (a) Product distribution for recycle runs of Ni/SiO2-2 Al2O3 in the conversion of PG at 250 °C, 10 bar H2 and 6·10 h -1 gcat gPG and (b) product distribution at maximum PCol yield at 250 °C for conversion of a 2 and 20 wt% PG feed over Ni/SiO2-Al2O3. In the recycle runs in (a), some catalyst was lost due to filtration and therefore the reaction time was increased to reach the required contact time. Reaction conditions for recycle runs (a): PG (0.332 g, 2 mmol), decane (0.7 mmol), 65 wt% Ni/SiO2-Al2O3 (40 mg for first run), hexadecane (20 mL). Reaction conditions for reaction with 2/20 wt% PG (b): PG (0.332/3.32 g, 2/20 mmol), decane (0.7/2.1 mmol), 65 wt% Ni/SiO2-Al2O3 (40/400 mg), -2 -1 hexadecane (20/17 mL), 10/40 bar H2, 6/4.5·10 h gcat gPG .

Despite the high PCol yields with Ni/SiO2-Al2O3, there remains a great challenge to control the product


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selectivity with regard to HDMO. Though methanol selectivity reaches initially up to 30% at 250 °C (for the reaction indicated in Fig. 5 at 10 bar and 0.8·10-2 h gcat gPG1 ), it quickly drops to nearly zero at longer contact time, methane being formed instead. Recuperation of the methoxy groups as methanol is not only interesting due to the higher value of this product, it also considerably reduces the hydrogen consumption, while methanol formation limits the operating pressure due to its lower partial pressure when compared to methane. High pressure build-up for the batch experiments was especially noticed during the reactions with high concentrated PG. The problem of pressure build-up due to methane formation can be circumvented by using a reactor set-up which enables continuous gas supply and removal, so that concentrated feeds, if not pure RG, can be converted under mild total operational pressure.

Next to PG, Ni/SiO2-Al2O3 can also selectively convert G, MeG and EtG to their corresponding cyclohexanols. Furthermore, the catalyst is stable in multiple recycle runs and is able to selectively convert a concentrated (20 wt%) PG feed. A disadvantage of the Ni/SiO2-Al2O3-catalyzed process is however that the HDMO reaction forms methane instead of methanol. Further catalytic research should therefore focus on developing a Ni catalyst that preserves methanol, similar to the Ru-based catalytic systems reported by Tomishige et al.45, 47 Another major challenge is the selective conversion of 4-n-alkylsyringols to RCols, which implies the removal of two methoxy side chains, since these compounds constitute the main phenolic monomers in the catalytic fractionation of hardwoods such as birch or poplar.26-27, 32-34

CONCLUSIONS

ASSOCIATED CONTENT †

A promising lignin valorization route is the conversion of lignin-derived phenolic monomers to alkylated cyclohexanols, which are precursors for novel polymer building blocks such as alkylated caprolactam, caprolactone and adipic acid. Here, the conversion of PG to PCol is examined over three commercial hydrogenation catalysts, viz. 5 wt% Ru/C, 5 wt% Pd/C and 65 wt% Ni/SiO2-Al2O3, in hexadecane at 200 to 300 °C. The conversion of PG to PCol proceeds through two competitive pathways, Pathway I and II, with the intermediate of the first pathway (PMCol) being much less reactive than the intermediate of the second pathway (PPh). PCol can further transform into undesirable CH. Three criteria were defined in order to realize a selective PCol production from PG: (i) a preference for Pathway II, since this circumvents formation of the stable intermediate PMCol, (ii) a high reactivity of PMCol compared to PCol, and (iii) a high carbon balance. Ru/C only partially meets these criteria, resulting in rather low PCHol selectivities, while Pd/C fails the first two, producing almost exclusively PMCol. Ni/SiO2-Al2O3 is the best catalyst in this study, meeting all three criteria provided the reaction is performed at 250 °C. At this temperature, a PCol yield above 80% is obtained. The maximum PCol yield increases with decreasing hydrogen pressure, due to an increasing selectivity for Pathway II, and under 10 bar H2, a PCol yield of 85% is obtained with the commercially available Ni/SiO2-Al2O3. When changing the solvent from hexadecane to the more polar THF or ethanol, the PG conversion rate drops considerably. Reaction in THF shows a high preference for Pathway II, but due to a comparable reactivity of PMCol and PCol, the PCol yield is limited to 76%. In heptane, a similar PG conversion rate and maximum PCol yield is obtained as in hexadecane. Hexadecane is however indicated as the most suitable solvent since as it has a negligible vapor pressure under reaction conditions and therefore makes it possible to perform the reaction at low operating pressure. It is also easily derivable from fatty biomass feedstock.64

Supporting information (ESI ). Detailed experimental procedures, This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work was performed in the framework of an IWTSBO project ARBOREF and an ICON project MAIA. W. S. thanks the internal funds of KU Leuven for a postdoctoral mandate (PDM). G. V. d. B. acknowledges the ICON project MAIA for a doctoral fellowship. S. V. d. B. acknowledges the Institute for the promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen) for a doctoral fellowship. S.-F.K. acknowledges funding through the IWT-SBO project ARBOREF. T.R. acknowledges the Research Foundation – Flanders (FWO) for a doctoral fellowship.

ABBREVIATIONS PG, 4-n-propylguaiacol; RG, 4-alkylguaiacol; EtG, 4ethylguaiacol; MeG, 4-methylguaiacol; G, guaiacol; PS, 4n-propylsyringol; PCol, 4-n-propylcyclohexanol; RCol, 4alkylcyclohexanol; Col, cyclohexanol; PMCol, 4-n-propyl2-methoxycyclohexanol; RMCol, 4-alkyl-2methoxycyclohexanol; MCol, 2-methoxycyclohexanol; PPh, 4-n-propylphenol; RPh, 4-alkylphenol; CH, hydrocarbons; HDO, hydrodeoxygenation; HYD, hydrogenation; HDMO, hydrodemethoxylation; S, selectivity; rA→B, reaction rate for the conversion of A to B; THF, tetrahydrofuran.

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Selective conversion of lignin-derivable 4-alkylguaiacols to 4-alkylcyclohexanols over noble and non-noble metal catalysts Wouter Schutyser, Gil Van den Bossche, Anton Raaffels, Sander Van den Bosch, Steven-Friso Koelewijn, Tom Renders and Bert F. Sels The conversion of lignin-derivable 4-alkylguaiacols to 4-alkylcyclohexanols, which are precursors for new polymer building blocks, is selective performed by a commercial nickel on silica-alumina catalyst.



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