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Fragmentation of Lignin Samples with Commercial Pd/C under Ambient Pressure of Hydrogen Fang Gao, Jonathan D. Webb, Hagit Sorek, David E Wemmer, and John F. Hartwig ACS Catal., Just Accepted Manuscript • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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Fragmentation of Lignin Samples with Commercial Pd/C under Ambient Pressure of Hydrogen Fang Gao, Jonathan D. Webb, Hagit Sorek, David E. Wemmer and John F. Hartwig* Department of Chemistry, University of California, Berkeley, CA 94720 (USA). KEYWORDS: Heterogeneous Catalysis; Palladium on Carbon; Lignin; Miscanthus giganteus; β -O-4 Linkages ABSTRACT: We report the reagentless cleavage of prevalent β-O-4 linkages in lignin model compounds, as well as the cleavage of several types of organosolv lignins, catalyzed by commercially available Pd/C. Such lignin fragmentation occurred without added reagent if the indigenous double bonds were reduced first or it occurred under conditions in which just 1 atm of hydrogen was added to the system to reduce C=C bonds of the original lignin sample in situ prior to fragmentation. A detailed view of the sites of cleavage of lignin samples from various sources was gained by HSQC NMR experiments. Complex model compounds were prepared and shown to form simpler arenes and substituted phenols under catalytic conditions without added reagents. The hydrogen generated in situ from alcohol functionalities provides the reductant for concomitant hydrogenolysis of C–O bonds in beta aryl ethers. Decarbonylation of primary alcohols also occurred, and this process resulted in significant amounts of aromatic products containing substituents bearing one fewer carbon atoms than the original linkages in lignin. The fragmentations of synthetic lignin and several organosolv lignins derived from Miscanthus giganteus and pine tree were conducted. Because the lignins contain alkenes that accept the hydrogen, two procedures involving reduction of the alkenes prior to C–O bond cleavage were developed. The first procedure involves reduction of the alkenes, followed by catalytic cleavage of C–O bonds after saturation of the C–C bonds; a second involves cleavage of lignin samples in the presence of one atm of hydrogen to saturate the alkenes before cleavage in situ. These protocols convert solid lignin to monomeric phenolic compounds with 20 mol% catalyst or to an oil (with 5 mol% Pd/C loading) having favorable viscosity parameters upon blending with a renewable organic solvent.

Introduction Lignocellulosic biomass has the potential to contribute to 1 renewable energy and commodity chemicals. To extract greater value from proposed biorefineries, the conversion of lignin to 2 improved liquid fuels or to chemical feedstocks is necessary. The current strategy by which lignin is burned to provide power to a biorefinery that converts cellulose and hemicellulose to alcohols by biological fermentation is inefficient because the 3 oxygen-rich lignin contains a low fuel content and because more lignin is generated from biomass hydrolysis than can be 4 consumed within a biorefinery. Therefore, methods have been sought to transform lignin into value-added products, such as 5 6 liquid bio-oils, mixtures of saturated hydrocarbons, and 7 emulsifying agents. Chemical catalysis has been explored to produce well-defined aromatic compounds from lignin, taking advantage of the high 8 abundance of such units in the polymeric structure. Several systems produce monomeric arenes and phenols in substantial quantities. Liu and co-workers reported that a Ni/C catalyst degraded lignin samples derived from birch trees in supercritical a methanol to form good yields of alkyl-substituted phenols.8 Abu Omar and Agrawal reported the fragmentation of various lignin samples from poplar, eucalyptus, as well as genetically engineered plants, with a bimetallic Pd/Zn catalyst system; the hydrodeoxygenation reactions were performed under high pressure of hydrogen (up to 500 psi) to afford propyl-substituted d guaiacyl and syringyl monomers in high yields.8 A similar

bimetallic catalyst containing Pd and Cr developed by Zhang and co-workers exhibited moderate activity toward the fragmentation h of alkali lignin.8 In addition to these systems for reductive cleavage, Stahl and colleagues recently disclosed a two-step sequence comprising oxidation and elimination. A catalytic, aerobic oxidation of benzylic alcohols in lignin followed by cleavage of the resulting beta aryl ether bonds with excess formic acid and sodium formate furnished unique 1,2-diketone b derivatives from aspen lignin in 20% combined yield.8 Despite these advances, many challenges remain to valorize lignin in a practical fashion. We focus here on two critical limitations. First, the abovementioned protocols require the a presence of super-stoichiometric quantities of reagents,3 and the cost and quantity of reagents used for generating a fuel must be minimized. Second, lignin samples from the faster-growing grass designated for energy applications (e.g., miscanthus, switch grass, etc) have not been studied as extensively as lignin from trees 9 requiring multiple years of growth. Therefore, studies are needed that focus on cleaving major linkages of lignins from energy crops with limited amounts of additional reagents or no 10 added reagent at all. β-O-4 linkages are the most abundant connections in the polymeric structure of lignin and have been a principal target for 11 cleavage. β-O-4 linkages contain a secondary benzylic alcohol and a primary aliphatic alcohol that are suitable for following a “borrowing hydrogen” strategy to generate in situ the reductant for cleavage of C–O bonds. Bergman and Ellman initially

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developed a homogenous, phosphine-ligated Ru catalyst to cleave a beta aryl ether model compound and observed 12 formation of acetophenone and phenol in excellent yields. However, the same catalyst did not cleave a more complex substrate that more closely modeled the β-O-4 linkages in true lignin. As later demonstrated by James and co-workers, this Ru catalyst is rapidly deactivated by chelation of the dehydrogenated substrate, due to the proximity of the aliphatic and benzylic 13 alcohols (cf. Scheme 1). Rauchfuss et. al. built upon this concept and demonstrated that catalytic amounts of Pd/C cleave simple β-O-4 models 14a In addition, Samecs and without external reducing agents. coworkers showed that Pd/C in 1:1 EtOH:water media digested pine and birch sawdust to afford good yields of phenolic monomers featuring unsaturated double bonds on the propenyl b side chain.14 However, the same catalytic system yielded lower amounts of propenyl-based products from β-O-4 model compounds. Thus, a clear mechanistic scenario for the corresponding lignin degradation is lacking. We report the reagentless fragmentation of complex β-O-4 models and lignin itself with commercially available Pd/C. The reactions with model compounds form arenes and phenols in high yields. On the basis of the reactivity of these model compounds, conditions for depolymerization of organosolv lignins from Miscanthus giganteus were developed and occur without any reagent after reduction of alkene bonds with just one atmosphere of hydrogen. A set of phenols containing twocarbon substituents was identified as the major components of the product mixtures. Moreover, a detailed analysis of the sites of cleavage of the lignin was revealed by two-dimensional HSQC NMR spectroscopy. OH

Compound 4 is a plausible intermediate in the formation of 2 and 3, generated from dehydrogenation of 1 (Entry 3). Pd/Al2O3 was found to be slightly less active for this cleavage process than Pd/C, and Pd/BaSO4 was completely inactive (Entries 4-5, Table 1). Unlike the reactions of homogeneous ruthenium catalysts studied previously, those catalyzed by Ru on solid supports occurred to low conversion (Entries 6-7). Similarly, reactions catalyzed by 10 mol % heterogeneous Rh and Pt formed the cleaved products in low yield after 16 h (Entries 8-10, Table 1). Table 1. Evaluation of Heterogeneous Catalysts for Reagentless a Cleavage of Phenethoxyphenyl ether 1

O O

OH + 2

Ru-Xantphos ref. 12

1

3 O

O

Ru-Xantphos ref. 13

OH O

Ph

P

OC

Ru O O

OH 5

Pd/C this work

Ar

P

6

Efficiency? Product composition? Applicable to lignin?

Scheme 1. Protocols for cleaving β-O-4 model compounds without added reagents

Results and Discussions Identification of Reaction Conditions. To identify catalysts for the reagentless, redox-neutral cleavage of the β-O-4 linkages of lignin, we investigated reactions of the model compound 1 in the presence of a series of heterogeneous catalysts. As shown in Table 1, we initiated the study by assessing the activity of supported nickel species, which we showed to catalyze the low15 pressure hydrogenolysis of biaryl ethers. However, neither reactions catalyzed by Ni(0) particles embedded on activated carbon nor those catalyzed by Ni(0) particles on Al2O3/SiO2 afforded the cleavage products in more than 10% yields (Entries 1-2). In contrast, commercial Pd/C catalyzed the fragmentation of 1 within 2 h to furnish phenol and acetophenone in 94% and 93% yield, respectively, along with 4% of α-phenoxy ketone 4.

Fragmentation of Complex β-O-4-Model Compounds. To assess the reactivity of Pd/C for the analogous cleavage of alcohols that more closely resemble the β-O-4 linkages of lignin, we examined the cleavage of compound 5. This molecule mimics the β-O-4 linkage and contains the primary and secondary alcohols that can bind a catalyst in a chelating fashion. As shown in Scheme 2, the fragmentation of 5 in the presence of 5 mol % Pd/C furnished phenol in 88% yield, as well as two phenyl alkyl ketones (cf. 3 and 8) and ethylbenzene 7 in 95% combined yield. A temperature higher than that needed for the cleavage of 1 and a solvent mixture were required for full conversion of 5 (Eq. 1). To accommodate the high temperature, p-cymene was used as solvent, instead of m-xylene, to reduce the vapor pressure inside the glass reaction vessel; meanwhile, dioxane was added to help solubilize the more complex substrates. The aryl groups in β-O-4 linkages of lignin typically contain methoxy groups. Thus, we tested the cleavage of compound 9 (Eq. 2) containing 3,4-dimethoxyaryl groups. In this case, ethyl substituted arene 11 was obtained as the major alkylarene cleavage product in 65% yield, along with the accompanying ortho-hydroxy anisole 13 in 90% GC yield. Small quantities of unidentifiable, higher molecular weight products, which might be due to the polymerization of phenolic compounds, and a trace of propylarene 12 also formed. No ketone products were observed. Cleavage of compound 10 containing one free phenol, in addition to two methoxy groups, formed the ethyl-substituted phenol 14 as the major alkylarene component of the product mixture, along with 67% yield of 13. This reaction also generated


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a deoxygenated byproduct 15 in 15% yield (Eq. 3). In the latter two cases, however, 20 mol % catalyst loading was needed to achieve high conversions and the observed product yields. Nonetheless, Pd/C, which likely operates through a similar shuffling of hydrogen, delivered cleavage products in high combined yield with the diol substrates that did not undergo cleavage with the homogeneous Xantphos-Ru catalyst. The heterogeneous Pd/C, therefore, is less likely to be inhibited by chelation of the diols in more complex lignin models. Hence, the current system might be more suitable for reagentless cleavage of actual lignin samples.

Monitoring the fragmentation of 5 alone as a function of time by GC revealed further evidence that hydrogen is generated from the alcohol in the presence of catalytic amounts of Pd/C. Over the first 8 h of the reaction 90% conversion of 5 was observed (Figure 1); phenol 2 and acetophenone 3 formed steadily over time to reach 83% and 72% GC yields of the two products, respectively. In the first 30 min of the reaction of 5, we observed the accumulation of 4 to a maximum of 35% yield. The concentration of this product decreased to nearly zero after six hours. The rapid initial formation of β-aryloxy ketone 4 implies that dehydrogenative decarbonylation of 5 occurs rapidly in the presence of the Pd/C catalyst to remove the hydroxymethyl 16 group; the subsequent gradual decay of 4 suggests that this ketone serves as an intermediate en route to the formation of acetophenone 3. The production of the saturated and 17 unsaturated three-carbon intermediates 18 was much slower than the formation of 4. Intermediate 18 is derived from the hydrogenolysis of 5. The decrease in the amount of 18 over time corresponded to the increase in the amount of ethyl phenyl ketone 8. The fully deoxygenated ethylbenzene 7 also formed, indicating that more hydrogen is available than would arise from the two alcohols. As suggested in the study by Rauchfuss et. al., dioxane could serve as the source of hydrogen in the presence of Pd metal; hence, the same process might operate in our cases to a supplement hydrogen.14

Scheme 2. Pd/C-Catalyzed Fragmentation of more complex β-O4 model Compounds Experiments to Probe the Steps of the Reagentless Fragmentations: To assess whether the cleavage reactions catalyzed by Pd/C occur by a sequence of steps involving the dehydrogenation of alcohols to generate hydrogen, followed by hydrogenolysis of the C–O bonds, we conducted experiments with a mixture of ketone and alcohol. These experiments probe whether the hydrogen from an alcohol can be used to cleave the C–O bond of an α-phenoxy ketone. Ketone 16 and benzyl alcohol 1 in a 1:1 ratio were subjected to the hydrogenolysis conditions. Within the first 4 h of the reaction, almost complete consumption of both substrates was observed. As illustrated in Eq 4, the C–O bond of ketone 16 cleaved to form acetophenone 3 and ortho-hydroxy anisole 13 in high yields while the alcohol of 1 was converted to ketone 4. This experiment shows that the C–O bond of the ketone is more reactive toward hydrogenolysis than is the C–O bond of the alcohol and that the hydrogen for reduction of the C–O bond in 16 derives from the benzylic alcohol 1. This result indicates that the dehydrogenation and hydrogenolysis sequence is not necessarily intramolecular and that the hydrogen is transferred as either free dihydrogen or hydrogen in the Pd/C.

Figure 1. Time Profile of the Fragmentation of 5 under Conditions Described in Table 2; see SI for reaction details.


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Two-step Degradation of Acetonesolv Lignin: Having established conditions for the cleavage of compounds that model the β-O-4 linkages of lignin, we investigated the degradation of organosolv lignin samples isolated from the energy crop 18 Miscanthus giganteus. We studied the cleavage of acetonesolv, dioxanesolv, and ethanolsolv lignin samples. These lignin samples were obtained by HCl-catalyzed organosolvolysis (extraction of lignin into organic solvents from plant sources) with acetone, dioxane and ethanol, as described previously.17 These lignin samples readily dissolved in dioxane, they but precipitated from a mixture of dioxane and aromatic solvents. Therefore, we choose dioxane as the sole solvent in the following lignin cleavage. The changes to the lignin sample from heating in the presence of the Pd/C catalyst were assessed by GC/MS, GPC 19 and 2D-NMR spectroscopy.

1

pronounced changes in any specific H NMR signals in the C–O region. To address this competing hydrogenation, the alkenes within the lignin were reduced prior to conducting reagentless fragmentation. This reduction occurred readily with 1 atm of hydrogenation catalyzed by 1 mol % Pd/C, as determined by HSQC spectroscopy (for details, see SI). As depicted in Scheme 3, treatment of the freshly hydrogenated lignin with 20 mol % Pd/C without external reagents led to nearly complete disappearance of signals due to the β-O-4 linkages (95% reduction in the intensity of the blue-colored signal by HSQC, relative to the phenylcoumaran signal in green color). More than 85% weight of the starting lignin was recovered as a thick oil product after the fragmentation reaction. Analysis of the resulting sample by gel permeation chromatography (GPC) showed that the weight average molecular weight Mw decreased from 2131 for the initial lignin sample to 1036 for the material isolated from the reaction mixture (bottom chart, Scheme 3).

Scheme 3. Two-Step Fragmentation of Acetonsolv Lignin from Miscanthus giganteus Initial treatment of acetonesolv lignin with 20 mol % Pd/C at o 200 C for 24 h resulted in minimal change to the C–O region of the 2D HSQC NMR spectrum of the lignin before and after treatment with Pd/C. This information indicates a lack of cleavage of the β-O-4 linkages. Further examination of HSQC data showed that treatment of this lignin with Pd/C led to reduction of the conjugated olefins that are indigenous to these lignin polymers. We surmised that reduction of the alkenes by the hydrogen generated in situ occurs faster than hydrogenolysis and the generated enolate-bound palladium species re-enter the catalytic cycle less readily. Because there are multiple positions from which hydrogen can be generated, including the benzylic alcohols, the primary alcohols or dioxane, there are less

Scheme 4. One-Step Pd/C-catalyzed Fragmentation of Synthetic Lignin under Atmospheric Pressure of Hydrogen Degradation of Synthetic Lignin with 1 atm H2: Considering that alkenes will be present in most or all lignin, the inclusion of small amounts of hydrogen in the reaction vessel in which the fragmentation reaction is conducted would allow a one pot cleavage of these biopolymers to be conducted. This proposal was first tested on a synthetic lignin sample, prepared 20 as described by a method adapted from a previous report, through enzymatic, radical polymerization of coniferyl alcohol. As outlined in Scheme 4, the treatment of this polymer with 20


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mol % Pd/C and 1 atm H2 in dioxane resulted in complete cleavage of C–O bonds that link polymerized coniferyl units. This cleavage was evidenced by comparing HSQC spectra of the recovered material with those obtained from a control experiment involving heating of the lignin under 1 atm hydrogen in the absence of the Pd/C; for detailed peak assignments and cleavage analysis, please see the SI. GPC data showed that the molecular weight of the material was lower after the catalytic reaction (Mw = 1194) than at the beginning of the reaction (Mw = 6978), Scheme 4).

corresponded to the benzylic alcohols of the β-O-4 linkages almost completely disappeared in the bottom spectrum that depicted the catalytic reaction, whereas the same crosspeaks 1 13 between 4.9 ppm in the H NMR spectrum and 68 ppm in the C NMR spectrum (Aα, cf. Scheme 3) shown in the top spectrum of the control reaction (heating without catalyst) remained. Similarly, the Aβ signals from the secondary alkyl protons of β1 13 O-4 linkages (crosspeak between 4.4 ppm in H and 83 ppm in C in the top spectrum) were no longer present after heating with Pd/C.

Degradation of Organosolv Lignin Samples from M. giganteus and Pine Trees with 1 atm H2: Having shown that the protocol for cleavage with 1 atm H2 leads to cleavage of the synthetic lignin, we investigated cleavage of natural lignin samples under the same conditions. The cleavage of lignin from M. giganteus extracted with acetone was conducted in the presence 20 mol % Pd/C and 1 atm of H2. This reaction produced an oily material in 67% recovered yield. The molecular weight of this material is less than one third of that of the starting lignin (Entry 1, Table 2). Analysis of this material by HSQC NMR spectroscopy showed that the β-O-4 linkages were converted nearly quantitatively to alkyl chains. As shown in the second column of Figure 2, the Aα signals (labeled in blue color) that

Lignins solubilized by extraction with ethanol or dioxane also underwent cleavage. Heating of these lignin samples with Pd/C in dioxane solvent under 1 atm of hydrogen led to oily product mixtures having Mw values of 977 and 1236, respectively. The strength of the signal corresponding to the β-O-4 linkages (blue colored signals, the first and third columns of charts from the left of Figure 2) in the HSQC spectra of the products was much less intense than that of the reactant material. Although ethanol extraction of lignin leads to ethoxylation of the benzylic alcohols (cf. A’ vs A signals in the top spectrum of the third column of Figure 2), the reaction of the ethanolsolv lignin from M. giganteus occured in a fashion akin to that of lignin from M. giganteus extracted into acetone or dioxane.

Figure 2. HSQC Experiments of Fragmented Organosolv Lignins from Miscanthus giganteus and Pine Tree; see SI for reaction details. However, the pattern of cleavage from the Pd/C-catalyzed reaction of the lignin extracted with ethanol from pine trees was different from that resulting from cleavage of the ethanolsolv lignin from M. giganteus. In addition to cleaveage of the β-O-4 linkages, phenylcoumaran and resinol linkages were also removed. Like the sample from M. giganteus, the sample from pine underwent complete cleavage of β-O-4 linkages upon

heating with Pd/C and 1 atm of hydrogen, as determined by HSQC analysis (cf. the fourth column of charts from the left of Figure 2), and to a similar decrease in the average molecular weight, as determined by GPC (Entry 4, Table 2). However, HSQC analysis of the product showed that this lignin also underwent complete cleavage of phenylcoumaran (green signals) and resinol (purple signals) linkages as shown in the bottom spectrum of the fourth column (cf. Scheme 3 for linkage


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structures). Such differences in the degree of cleavage of these less reactive linkages in the ethanolsolv lignins from M. giganteus and pine can be attributed to the differences in their polymer structures and the higher ratio of phenylcoumaran and resinol linkages versus β-O-4 linkages in pine lignin than in Miscanthus lignin. We have further confirmed that degradation of these linkages occurs by examining hydrogenolysis of corresponding model compounds (see the SI for these data and the reaction details). Table 2. Fragmentation of Organosolv Lignins from Miscanthus a giganteus and Pine Tree with Low Pressure of Hydrogen

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to form oily materials almost quantitatively. The viscosities of these oil samples were high. However, the viscosity was reduced significantly by blending with γ-valerolactone. By adding 42-45 weight% of γ-valerolactone to the homogeneous light brown liquid mixtures, samples with viscosity values ranging from 12.122.4 cP were obtained. These values are comparable to those of light crude oil easily transported through pipelines. Table 3. Viscosity Measurement of Liquefied Lignin Samples a Blending with γ-Valerolactone

Conclusions

Hydrogenolysis of lignin at high pressures of hydrogen is known to generate volatile organic products. As summarized in Table 2, the hydrogenolysis with Pd/C under the low pressure of hydrogen in this work also generated volatile products. The reaction of Miscanthus lignins produced 12-15% combined yields of seven major products observed by GC/MS, which, in addition to the 67-79% oily residuals, demonstrate excellent recovered mass balance of starting lignin samples; therefore, no significant charring was observed under these conditions. These products contained mainly ethyl or propyl chains (Entries 1-3). High ratios of ethyl-substituted monomers are a unique feature of this 21 cleavage at low pressures and temperatures. The analogous degradation of lignins from pinewood furnished alkyl-substituted phenols in 9% yield. This yield is slightly lower than that from reactions of lignins derived from the grasses (Entry 4, Table 2). Viscosity Adjustment of Resulting Oily Materials: To probe possible applications that exploit the conversion of solid lignin to an oil under low pressures of H2, we analyzed the viscosity of the reaction product and the combination of the reaction product with small amounts of solvent derived from biomass. Increasing the mobility of solid lignins could enable more economical transportation of these materials, preferentially via pipelines, to locations where they could be further processed or used for energy. Table 3 shows the viscosity of the combination of the products of the lignin cleavage and the renewable solvent γ-valerolactone. The lignin samples were generated by heating with 5 mol % Pd/C

In summary, we have developed a reagentless fragmentation of complex β-O-4 model compounds catalyzed by commercially available Pd/C, furnishing decarbonylated products acetophenone and ethyl-substituted arenes and phenols containing one fewer carbon atoms in high yields. We also provide a detailed view by NMR spectroscopy into the cleavage of natural lignins under conditions with C=C bonds reduced by hydrogenation with 1 atm H2 prior to the reagentless cleavage of C–O bonds or under conditions in which both processes are conducted in the same vessel under 1 atm of hydrogen. The process occurs by the in situ generation of hydrogen by catalytic dehydrogenation of a secondary alcohol, followed by hydrogenolysis of the alkyl C–O bond. Due to the presence of olefins in natural lignin samples, this protocol leads to the cleavage of the C–O bonds of organosolv lignins by including 1 atm of added hydrogen to reduce the alkenes. Under these conditions, the molecular weights were decreased by roughly one half, as determined by GPC, the C–O bonds in the β-O-4 linkages were fully cleaved, as determined by HSQC NMR spectroscopy, and substantial yields of monomeric arenes and phenols were generated from natural lignins derived from both energy crops and pine trees. Moreover, the bio-oils generated from the cleavage exhibited low viscosity for pipeline transportation after blending with γ-valerolactone, a solvent derived from biomass.

ASSOCIATED CONTENT Supporting Information. Experimental details for all reactions and analytical details of all relevant materials. This material is free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

Author Contributions


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Funding Sources Energy and Biosciences Institute provided the research fund that supported this study.

ACKNOWLEDGMENT We gratefully acknowledged the financial support from Energy and Biosciences Institute. We thank Dr. Stefan Bauer for providing lignin samples and his assistance in GPC analysis. We also thank M. C. Chang group from UC Berkeley for providing synthetic lignin samples and the procedure for additional synthesis.

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ACS Catalysis

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[18] For the isolation method of lignin samples by organosovolysis from Miscanthus giganteus, see: S. Bauer, H. Sorek, V. D. Mitchell, A. B. Ibáñez, D. E. Wemmer J. Agric. Food Chem. 2012, 60, 8203−8212. [19] The HSQC spectrum of organosolv lignins derived from M. giganteus , as well as HSQC characterizations of degradation products from their oxidative cleavage, was reported in the degradation study by Toste and co-workers. For details, see ref. 10b. [20] The polymerization method was adapted from previous literature; see: Reale, S.; Attanasio, F.; Spreti, N.; De Angelis, F. Chem. Eur. J. 2010, 16, 6077−6087 and references cited therein. We thank the M.C. Chang group at U.C. Berkeley for providing these samples. [21] For one lignosulfonate degradation study, in which ethyl substituted phenols were obtained as minor components in the product mixutre, see: Q. Song, F. Wang, J. Xu Chem. Commun. 2012, 48, 7019−7021. [22] Our attempts to recover the Pd/C and reuse the catalyst were not successful. The solid material recovered by filtration and reactivation with H2 flow at 450 °C in a furnace was not active for cleavage of a sample of dioxane-solv lignin.

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