Adsorption Isotherms of Lignin-Derived Compounds on a Palladium


Apr 1, 2019 - We have studied the interaction of lignin fragments obtained from catalytic fractionation with a heterogeneous palladium catalyst. By st...
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Adsorption isotherms of lignin derived compounds on a palladium catalyst Ivan Kumaniaev, and Joseph S. M. Samec Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06159 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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Adsorption isotherms of lignin derived compounds on a palladium catalyst Ivan Kumaniaev† and Joseph S. M. Samec*† †

Department of Organic Chemistry, Stockholm University, SE 106 91, Stockholm, Sweden

* E-mail: [email protected]

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ABSTRACT: We have studied the interaction of lignin fragments obtained from catalytic fractionation with a heterogeneous palladium catalyst. By studying the adsorption of verified substrate and product molecules on the palladium surface, understanding of what governs adsorption and desorption dynamics of both substrates and products has been obtained. In addition, we have studied the kinetic isotope effect of hydrogen transfer reactions occurring on the surface of the catalyst. These studies give insights into the thermodynamics of the process in which species from lignin-derived species adsorb to the catalyst surface, then are transformed by hydrogenation/hydrogenolysis reactions in a slow reaction step, and finally desorbed. It was found that the adsorption dynamics depended on degree of unsaturation as well as presence of methoxy groups on the aryl. Thereby, the adsorption is stronger for substrate molecules derived from lignin than reduced molecules obtained after the rate-determining transfer hydrogenation and hydrogenolysis transformations.

KEYWORDS: adsorption; lignin; biomass; palladium; catalysis

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INTRODUCTION Current bio-refineries yield a high quality pulp from a lignocellulosic feedstock.1 As the pulp is less than 50% of the mass balance and as low as 25% of the energy balance of the initial feedstock, these processes are very inefficient. On the other hand, the use of crude oil is associated with high costs, environmental concerns, political instability, and the depletion of ending resources. As the awareness of such factors has risen, the conversion of lignocellulosic feedstocks has received an increased attention especially during the last decade.2,3 Because the fundament of the current biorefinery is inefficient, methodologies to improve yield of what has previously been termed pulping, has been in focus.4 We prefer to use the term fractionation to broaden the bio-refinery concept from only pulp, i.e. cellulose, to include also lignin and hemicellulose. One such research direction is the Lignin-first approach, also called Catalytic Upgrading Biorefinery, Early Stage Catalytic Conversion of Lignin, or Catalytic fractionation.5,6 This approach comprises a tandem organosolv pulping and often a transition metal-catalysed (transfer) hydrogenolysis reaction to transfer lignin fragments.7 Using this approach, high yields of cellulose pulp together with water soluble hemicellulose and an oil phase consisting of monophenolic compounds and oligomers derived from lignin can be obtained.8,9,10,11,31 Very recently, Beckham-Roman12,13 and also our group14 developed a Lignin-First approach using a flow-through methodology in which the organosolv pulping and the (transfer) hydrogenolysis reactions were separated in time and space. This allowed to study both processes individually.

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extraction chamber "percolator"

O

O HO O OH

O

Transition metal catalyzed hydrogenations/depolymerizations

H

O O

oligomeric fragments are depolymerized

OH

OH O

O

Ar

cellulose OH cellulose stays in percolation chamber OH solvent flow

HO

O OH

Ar

Ar

Ar

H

H

OH

H

Ar

OH H

H Ar

H

H

OH

OH

OH

OH HO

hemicellulose hemicellulose extracted by the solvent

Ar

reactive monomeric species are reduced to less reactive ones

OH

lignin is partially depolymerized and extracted

O

O

Ar

OH lignin (-O-4-linkage)

Ar

OH H

OH O

O

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HO

O OH

analysis organosolv

OH H

O

O

O OH

OH

hemicellulose dehydrogenated to generate hydrogen

O OH

H

H

OH

H

analysis transfer hydrogenolys/ hydrogenation

Figure 1. Lignin First approach in a continuous flow enabling to study the effects on both organosolv pulping and transfer hydrogenolysis/hydrogenation reactions. Interestingly, it was found that the organosolv pulping alone depolymerized the lignin to a high extent, where we could obtain above 20% yield of monomeric species and this corresponds to 40–50% of the theoretical maximum yield. These species, liberated from the wood during the fractionation are highly reactive and repolymerize readily. Thus, weak C–O bonds in the lignin polymer are broken and strong C–C bonds form in the absence of a transition metal catalyst.15,16 In these studies, it was found that the role of the transition metal was to reduce the monophenolic compounds to less reactive species. In addition, it was found that the transition metal catalyst also cleave C–O bonds that are less available for the solvolysis reaction.17 An interesting observation made was that the organosolv fractionation was a fast process and that the transfer hydrogenolysis reaction was a slow process (Figure 2). While the organosolv pulping was

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finished within one hour, the transfer hydrogenolysis/hydrogenation reactions required more than three hours. Thereby, either the adsorption of the lignin fragments is a very fast process, the hydrogen transfer reactions are slow, and/or the desorption of the reduced species is a slow process. In this report, we study the interactions of verified lignin fragments and products with the heterogeneous catalyst in order to elucidate the adsorption/desorption dynamics as well as the kinetics of the hydrogen transfer reactions.

Figure 2. Kinetics of pulping and transfer hydrogenolysis in the flow-through system developed in our group previously14. The molar amount of lignin was measured by means of NMR and UV spectroscopy. Solvent: 2.8 g·L–1 H3PO4 in MeOH–H2O 7:3 v/v; temperature: 200°C. Adopted with permission from Green Chem., 2017, 19, 5767 © 2017 Royal Society of Chemistry.

RESULTS AND DISCUSSION A. Models of lignin selected in this study. Our approach was to use both molecules that have been isolated from the organosolv pulping step and products formed from the reduction step and to study the adsorption/desorption of these molecules to the palladium on carbon (compounds 1–

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7, Figure 3). Compound 1 represents a motif of a lignin oligomer in which the β-O-4’ bond is present. Compounds 2–3 are monomeric species observed in the organosolv pulping liquor.14,18 Lignin first approach is usually performed on hardwoods where the content of syringol units which form coniferyl alcohol (2) is 4–5 times higher than that of guaiacol units which form sinapyl alcohol (3). These compounds are known to be highly reactive by isomerization to form aldehydes that then can be alkylated by the aryl group of another molecule to form a new recalcitrant C–C bond. In the set-up used in the continuous flow, it was possible to cool down this stream rapidly before condensation reactions had occurred (Figure 1) and thus discover that pulping alone yield high amounts of monomeric species.14 However, when the stream was directed towards the second reactor with the palladium catalyst, these monomers underwent either hydrogenation or hydrogenolysis reactions to generate products 4–7, that then are desorbed from the palladium on carbon. This is in accordance with the variety of previously reported procedures for hydrogenation of double bonds in sinapyl and coniferyl alcohols.28,29 Reactive species from organosolv pulping OH

OMe

Identified products from transfer hydrogenolysis

MeO

OH

O

4

HO HO

1

OH

OMe OH

MeO

OH

5

HO

2

HO

OMe

OMe MeO OH

6

HO

3

HO

OMe

OMe

7

HO OMe

Figure 3. Lignin fragments and products studied

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B. Experimental design. To study the adsorption and desorption behavior of representative molecules from the organosolv pulping, we calculated the concentration of water, methanol, substrate or product and palladium on carbon at the point at which pulping can be considered finished to above 90% (Figure 2, third data point, 60 minutes). At this point, the molar ratio of water, methanol, palladium and lignin was 4∙103 : 4∙103 : 1 : 2, respectively. In reality, the oligomeric fraction will consist of different types of species (dimers to higher oligomers). To be able to calculate the thermodynamics of the adsorption/desorption of how the different molecules (Figure 3) adsorb to palladium, the concentrations of individual compounds (1–7) were varied. Adsorption of the lignin model compounds was studied (Figure 4) by mixing Pd/C, solvent and model compound in a test tube and vigorously stirring this mixture for 20 minutes. The mixture was then filtered into a test tube and extracted with DCM. The organic phase was dried, concentrated, and the amount of compound was analysed by 1H NMR using nitromethane as an internal standard. These data were then used to calculate the proportion of the compound that remained desorbed. 1) add Pd/C, substrate and solvent

3) filter

4) extract with DCM 5) concentrate 6) analyze by 1H NMR

2) stir for 20 min

Figure 4. General procedure to determine adsorption to palladium on carbon.

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C. Calculations. The adsorption (mmol of compound per gram of Pd/C) was determined as follows: 1

𝑎 = 𝑚(Pd/C) ∙

(

𝑚0(C) 𝑀𝑟(C)

𝑚(filtrate)

― 𝑚0(mixture) ∙ 𝑛(std) ∙ 𝑥

)

(Eq 1)

where 𝑀𝑟(C) (g·mol–1) is molar mass of the substrate, 𝑛(std) is molar amount of the added standard, 𝑥 is ratio of NMR signals of the standard and the compound. The dependence of adsorption on actual concentration is then modelled with Langmuir law:

𝑎

𝑎0𝐾𝑐

[mmolgramcompound Pd/C ] = 1 + 𝐾𝑐

(Eq 2)

where 𝑎0 is the maximum surface coverage which is achieved in case of very high concentrations, and 𝐾 is adsorption equilibrium constant: 𝜃

(Eq 3)

𝐾 = (1 ― 𝜃)𝑐 where 𝜃 = 𝑎/𝑎0 is the surface occupation degree. Langmuir’s isotherm is a basic model which does not take into account the possibility of

multilayer adsorption and the interactions of the adsorbed molecules between within a single layer. However, all advanced models require more than two parameters and therefore much higher accuracy of measurement.30 It is also more complicated to bind the variety of parameters with the features of molecular structure. For each compound, a C code was used to calculate values 𝑎0 and 𝐾 which correspond to the minimum standard deviation 𝜎min of the predicted adsorption versus experimental data. Intervals of 𝑎0 and 𝐾 were found for which 𝜎 ≤ 1.01𝜎min.

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D. Adsorption of models of lignin to palladium on carbon. Dimeric compound 1, showed the highest affinity towards palladium on carbon (Figure 5, Table 1). The adsorption constant K was determined to be 7.0∙103 L·mol–1. Monomer 2, represent a monomer that is formed in highest concentration during the organosolv pulping of hardwoods, i.e. birch. This compound also showed a high adsorption constant, K was calculated to be 6.6∙103 L·mol–1. This is not surprising taking into account that this molecule comprises both an electron rich aryl, a double bond and two hydroxyl groups that can bind to the catalyst surface. In addition to this, there are also two methoxy groups that can coordinate to the palladium either through oxygen or by donating electron density to the aryl. Surprisingly, corresponding guaiacol allylic alcohol 3 showed a significantly lower affinity towards the catalyst (K = 1.5∙103 L·mol–1). This could be explained by the lack of contribution of the second methoxy group on the aryl ring. Next, product molecules 4 or 5 formed after transfer hydrogenation reaction of the double bond were studied. Syringol propanol 4, formed from hydrogenation of compound 2, showed a reduced affinity towards the palladium on carbon (K = 3.4∙103 L·mol–1) as compared to compound 2 (K = 6.6∙103 L·mol–1). The lower affinity shows the importance of the double bond in the coordination to the palladium on carbon. For guaiacol derivative 5, a significantly lower affinity was observed (K = 0.4∙103 L·mol–1) than for the syringol analogue was observed. This supports the idea that methoxy groups coordinate relatively strongly to the palladium surface as was claimed previously.25 Finally, products 6-7 formed from a transfer hydrogenolysis of substrates 2 or 3, followed by a transfer hydrogenation was studied. Syringol propane 6 showed as expected a further reduction in affinity to palladium (K = 1.2∙103 L·mol–1), as compared to either 2 (K = 3.4∙103 L·mol–1) or 4 (K = 6.6∙103 L·mol–1). In the case of guaiacol propane 7 an unexpected increase was observed (K

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= 1.7∙103 L·mol–1) as compared to compound 5 (K = 0.4∙103 L·mol–1), and similar adsorption to substrate 3 (K = 1.5∙103 L·mol–1).

Figure 5. Adsorption plots of compounds 1–7 to palladium on carbon.

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These results show the factors that govern the adsorption between substrates and products to palladium on carbon. The strongest coordination was found for the dimeric substrate. As this substrate comprises a 1,3-diol and two electron-rich aryl groups this is not surprising. This is also consistent with our experimental results from both flow and batch experiments where near theoretical maximum yields of monophenolic compounds were achieved and only dimers and trimers were found as oligomers after transfer hydrogenation/hydrogenolysis reactions had occurred. No β-O-4’ interlinkage was observed in these oligomeric fractions after hydrogenolysis reactions. Thereby, oligomeric species adsorb strongly to the palladium on carbon and are transformed by cleavage of the β-O-4’ interlinkage. In the syringol series, there is a strong correlation between both number of alcohols and olefin on the molecule and the observed adsorption constant. Substrate 2 having two hydroxyl groups and one double bond present shows the highest adsorption (K = 6.6 ∙ 103 L·mol–1). When the olefin is reduced the adsorption constant drops (K = 3.4 ∙ 103 L·mol–1). When also the alkyl alcohol is on the propyl chain is removed the adsorption drops even further (K = 1.5 ∙ 103 L·mol– 1).

Thereby, both the effect of double bond and hydroxyl group coordination are important in the

adsorption to palladium on carbon. The adsorption of the mentioned structures was studied previously, and it was shown that allyl alcohol moiety coordinates through both double bond and hydroxy group.25,26,27,32 In the case of the guaiacol series, there is no correlation between number of hydroxyl groups or double bonds and adsorption. Instead, all species have a similar observed adsorption to palladium except diol 5. This result is intriguing and would need a complementary studies such as computational chemistry to fully understand. Noteworthy, it is well known that guaiacol derivatives undergo undesired condensation reactions much easier than syringol derivatives and

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this may explain the observed lower yield of monophenolic compounds such as 5 and 7 from catalytic fractionation. Up to now, the explanation for lower yields of monophenolics based on guaiacol containing biomass has exclusively been that the less substituted aryl ring can perform a nucleophilic attack on electrophilic intermediates formed during the reaction. Our results may supplement this explanation with that these molecules have less affinity to the catalyst and thereby remain in solution for a longer time where the condensation reactions occur. Table 1. Maximum adsorption and Adsorption constants for compounds 1–7 on palladium on carbon[a] Max. adsorption a0

Structural formula OH

[b] [c]

OMe O

HO

1

OH 2

HO

0.48–0.50

MeO

OH 3

MeO

4.6–8.8 (6.6)

0.05

1.2–2.2 (1.5)

0.16

2.9–4.0 (3.4)

0.08

1.27–1.46 (1.37)

OH OMe

0.04

0.28–0.30

0.94–0.97

4

HO

6.0–8.1 (7.0)

(0.29)

OMe

HO

Std. deviation 𝜎min [e]

(0.49)

OH

MeO

Adsorption constant K [b] [d]

(0.95)

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MeO

OH 5

HO

0.2–0.3 (0.3)

0.06

1.1–1.6 (1.2)

0.11

1.5–2.0 (1.7)

0.10

(1.70)

MeO

1.24–1.32

HO

6 OMe

(1.31)

1.07–1.11

MeO HO

1.65–1.76

7

(1.09)

[a] Mesured by mixing 0.1–20 mg of phenolic compound with 10 mg of 5% Pd/C in the organosolv pulping solvent (2.8 g·L–1 H3PO4 in MeOH–H2O 7:3 v/v, 1 mL total) (see Experimental section). [b] The interval corresponding to 𝜎 ≤ 1.01𝜎min. [c] Mmol of substrate per gram of Pd/C (best-fitting value). [d] 103 L·mol–1 (best-fitting value). [e] Min. std. deviation of the Langmuir curve versus the data, mmol·g–1

A linear correlation (r2 = 0.89) was observed between the maximum surface coverage 𝑎0 and the free energy of adsorption expressed as Δ𝐺ads = ―𝑅𝑇ln 𝐾 + const. This trend (Figure 6) may be explained as follows. It is reasonable to assume that 𝑎0 is in inverse ratio to the characteristic surface area occupied by a single molecule in the saturation state: 𝑎0 = 𝑢1/𝑠 ― 𝑢2, where 𝑢1 and 𝑢2 are parameters corresponding to the fact that 𝑎0 becomes zero under finite values of s (it is not necessary to have infinite 𝑠 to get 𝑎0 = 0). 𝑢1 is a proportionality coefficient which may be considered to be roughly equal to the overall active surface of the catalyst. The ratio 𝑢1/𝑢2 has the physical sense of the maximum surface area of a molecule which still can be adsorbed. If the

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majority of the surface is occupied by solvent or if the palladium is represented by small nanoparticles, the bonding of very large molecules is impossible. Then 𝑢2 ≫ 𝑎0 and 𝑠 ≈ 𝑢1(𝑢2 ― 𝑎0)/𝑢22, i.e. 𝑠 is tentatively a linear function of 𝑎0. On the other hand, 𝑠 is aligned with what can be called effective number of sites by which the adsorbed molecule is bounded to the surface. Therefore 𝑠 indeed may be linearly dependent on the free energy of bonding and so far as 𝑠(𝑎0) is linear, Δ𝐺(𝑎0) is also linear.

Figure 6. Linear correlation of maximum surface coverage a0 with ΔGads = –RT ln K + const. Each point corresponds to the molecule with the specified number. E. Kinetic isotope effect for palladium on carbon catalyzed transfer hydrogenation reactions. The adsorption study shed light on the thermodynamics of the interaction of intermediates from the organosolv fractionation and also products from reactions with the palladium on carbon. However, this study does not answer why the events on the palladium surface reactions are slow (Figure 2). Because the adsorption of the substrates is stronger than for

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the reduced products, we propose that the hydrogen transfer reactions on the catalyst is a slow process. To get a deeper understanding on what governs the reaction time in the Lignin-first approach, we supplemented the adsorption studies with a kinetic isotope effect study on the transfer hydrogenation reaction of isoeugenol (8) that also is formed as intermediate/product in the lignin first approach.7 We have previously showed that hemicellulose acts as a hydrogen donor in the hydrogen transfer reactions of lignin fragment. Moreover, hemicellulose is known to decompose to formic acid, and we therefore used formic acid as source of hydrogen.19,20,21 First, we attempted to use 2-propanol as a model for hemicellulose as it contains secondary alcohols that are present in hemicellulose. Interestingly, the hydrogen transfer reactions were very sluggish and low reproducibility was achieved in the current system using palladium as catalyst. This is in contrast to previously reported reactions performed with nickel where 2propanol operates as an efficient hydrogen donor.22,23 Thereby, transfer hydrogenation of 8 palladium on carbon using either HCOOH or DCOOD was performed instead of 2-propanol. The reactions were run in the solvent mixture which has been shown to be favorable for pulping, either acidic H2O–MeOH or acidic D2O–MeOD at 20C for 10–20 min (Scheme 1). For the pseudo-first order constants for hydrogenation and deuteration reactions, a kinetic isotope effect of kH/kD ~ 5 was observed. This corresponds to a primary kinetic isotope effect and shows that the hydrogen transfer is the rate-limiting step in the overall transformation. This demonstrates that the hydrogen transfer reactions on palladium are the slow processes in the catalytic fractionation of woody biomass (Figure 2).

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Scheme 1. Kinetic isotope effect for hydrogenation of lignin-derived monomer isoeugenol over Pd/C (two runs each isotope) 8

7-d

7

D

Pd/C, rt HO OMe

D2O–MeOD or H2O–MeOH 7:3 H3PO4 or D3PO4 (3 g·L–1) HCOOH or DCOOD Pseudo-first order rate constant

HO OMe

kH = (5.0 ± 0.3) · 10–3 min–1

o r

D

HO OMe

kD = (1.0 ± 0.4) · 10–3 min–1

Conclusions We have studied the adsorption isotherms of several confirmed lignin model compounds to palladium on carbon. The data is in accordance with previous observations made in the flowthrough system. The dimeric model compound, illustrating the native β-O-4’ bond, possesses the highest adsorption equilibrium constant (7.0 ∙ 103 L·mol–1). The monolignols which are also formed during organosolv pulping in the absence of catalyst demonstrate slightly lower but still significant values (6.6 ∙ 103 and 1.5 ∙ 103 L·mol–1). In the syringol series a strong correlation between number of alcohols and unsaturation on the molecule and adsorption to palladium on carbon was observed. This was not found in the case of the guaiacol series where overall weaker adsorption was observed. The data further show that the catalyst adsorbs the organosolv pulping products when the reaction is accomplished also in a dilute flow-through system. The products of transfer hydrogenation/hydrogenolysis show less affinity toward the catalyst surface (0.3–3.4 ∙

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103 L·mol–1). The maximum surface coverage values correlate with the free energy of adsorption, which supports the conclusion that the latter is correlated to the characteristic catalyst surface per each adsorbed molecule. The hydrogen transfer is the rate-limiting step of the catalytic fractionation and explain the overall reaction profile (Scheme 2).

Scheme 2. Mechanism of catalytic process of monolignol hydrogenation, exemplified as transformation of coniferyl alcohol (2) into 4-(3-hydroxypropyl)syringol (4) and 4propylsyringol (6).

Experimental section Model compounds 2 and 3 were purchased from Sigma Aldrich. Compound 4 was purchased from Tokyo Chemical Industry Company. Compound 5 was purchased from Acros. All other model compounds were synthesized according to the reported procedures14,24. Adsorption. Adsorption of the lignin model compounds was studied according to the following procedure. Pd/C (𝑚(Pd/C) = 10 mg) was placed into reaction vessel together with solvent (2.8 g·L–1 H3PO4 in MeOH–H2O 7:3 v/v, pH2, 1 mL total) and the studied compound ( ACS Paragon Plus Environment

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𝑚0(C) = 0.1–20 mg, dissolved in methanol). The mixture (mass 𝑚0(mixture)) was vigorously stirred for 20 minutes under argon, then filtered into a test tube. The filtrate was weighted (mass 𝑚(filtrate)). The filtrate was then diluted with H2O (3 mL) and DCM (3 mL). After extraction, the organic fraction was separated, dried with Na2SO4, and concentrated in vacuum. The product was dissolved in CDCl3 and subjected to NMR analysis using CH3NO2 as internal standard. The adsorption (mmol of compound per gram of Pd/C) was determined as described above. For each molecule, the measurements were repeated twice and the average values were used. Hydrogenation/deuteration. Isoeugenol (14 mg) was placed into a test tube together with solvent (3 g∙L–1 H3PO4/MeOH–H2O or D3PO4/MeOD–D2O, 1 mL) and 5% Pd/C (10 mg). HCOOH or DCOOD (0.1 mL) was added. The mixture is stirred under agron for 10 minutes, filtered, diluted with water and extracted with DCM. The organic fraction was concentrated, the residue was dissolved in CDCl3 and subjected to NMR analysis. Conversion of isoeugenol into (deuterated) 4-propylguaiacol was determined. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Numerical data for the adsorption curves, results of hydrogenation experiment AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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ORCID Joseph S. M. Samec: 0000-0001-8735-5397 Notes The authors declare no competing financial interest. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style). ACKNOWLEDGMENT We thank the Swedish Energy Agency for financial support. REFERENCES (1) Strassberger, Z.; Tanase, S.; Rothenberg, G. The pros and cons of lignin valorisation in an integrated biorefinery. RSC Adv. 2014, 4, 25310. (2) Goheen, D.W. Chemicals from wood and other biomass: Part II. The chemistry of conversion. J. Chem. Ed. 1981, 58, 544. (3) Ragauskas, A.J.; Williams, C. K.; Davison, B.H.; Britovsek, G.; Cairney, J.; Eckert, C.A.; Frederick, W.J.; Hallett, J.P.; Leak, D.J.; Liotta, C.L.; Mielenz, J.R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311, 484. (4) Sannigrahi, P.; Ragauskas, A.J. Fundamentals of Biomass Pretreatment by Fractionation, in Aqueous Pretreatment of Plant Biomass for Biological and Chemical Conversion to Fuels and Chemicals; Wiley: Chichester, 2013.

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