Selective Nickel-Catalyzed Conversion of Model andLignin-Derived Phenolic Compounds to Cyclohexanone-Based Polymer Building BlocksWouter Schutyser,[a] Sander Van den Bosch,[a] Jan Dijkmans,[a] Stuart Turner,[b]

Maria Meledina,[b] Gustaaf Van Tendeloo,[b] Damien P. Debecker,[c] and Bert F. Sels*[a]

Introduction

Lignocellulose, which is the most abundant biomass constitu-ent, represents a promising renewable resource for the pro-duction of biofuels, chemicals, and polymers.[1] It is composedof three major components: cellulose, hemicellulose, and ligni-n.[1a,c,d] Although the conversion of polysaccharides into renew-able chemicals has received much attention during the lastdecade,[1c–e, 2] lignin valorization technologies are substantiallyless developed today, mainly due to the inherent complexityand heterogeneity of the aromatic biopolymer.[3]

The structure of lignin may be regarded as an amorphousand highly branched polymer of phenyl propane units thatoriginate from three aromatic alcohols, namely, p-coumaryl,coniferyl, and sinapyl alcohol.[3b, 4] The building blocks aremainly connected through ether bonds, dominated by the b-O-4 linkage, next to C�C bonds.[3b, 4] As the largest renewablesource of aromatic and phenolic material with a unique chemi-cal structure, lignin is suggested to constitute a promisingfeedstock for a wide variety of bulk and fine chemicals, as wellas fuels.[3a–d, f, 5] Some interesting lignin applications are disper-sants, emulsifiers, and building blocks in resins and plastic pro-ducts,[3a, 6] and the production of fine chemicals, such as vanil-lin, is well established.[7] However, the development of valuable(platform) chemicals from lignin is likely to be one of thegreatest challenges in current biomass conversion.[3c, f, 6a] Theproduction of low-molecular-weight compounds from lignin isindeed a promising route for valorizing lignin and selective cat-alytic processes will play a key role in these conversionroutes.[3]

Selective (catalytic) depolymerization of lignin is challenging.The most important depolymerization methods are pyrolysis,hydrolysis, hydrogenolysis, liquid-phase reforming, chemicaloxidation, and gasification;[3b, d, 8] these usually yield moderateamounts of numerous monomeric compounds. Catalytic ornoncatalytic fast pyrolysis of lignocellulose or isolated lignin

Valorization of lignin is essential for the economics of futurelignocellulosic biorefineries. Lignin is converted into novelpolymer building blocks through four steps: catalytic hydro-processing of softwood to form 4-alkylguaiacols, their conver-sion into 4-alkylcyclohexanols, followed by dehydrogenation toform cyclohexanones, and Baeyer–Villiger oxidation to givecaprolactones. The formation of alkylated cyclohexanols is oneof the most difficult steps in the series. A liquid-phase processin the presence of nickel on CeO2 or ZrO2 catalysts is demon-strated herein to give the highest cyclohexanol yields. The cat-alytic reaction with 4-alkylguaiacols follows two parallel path-ways with comparable rates: 1) ring hydrogenation with theformation of the corresponding alkylated 2-methoxycyclohexa-nol, and 2) demethoxylation to form 4-alkylphenol. Althoughsubsequent phenol to cyclohexanol conversion is fast, the rateis limited for the removal of the methoxy group from 2-me-

thoxycyclohexanol. Overall, this last reaction is the rate-limitingstep and requires a sufficient temperature (>250 8C) to over-come the energy barrier. Substrate reactivity (with respect tothe type of alkyl chain) and details of the catalyst properties(nickel loading and nickel particle size) on the reaction ratesare reported in detail for the Ni/CeO2 catalyst. The best Ni/CeO2 catalyst reaches 4-alkylcyclohexanol yields over 80 %, iseven able to convert real softwood-derived guaiacol mixturesand can be reused in subsequent experiments. A proof of prin-ciple of the projected cascade conversion of lignocellulosefeedstock entirely into caprolactone is demonstrated by usingCu/ZrO2 for the dehydrogenation step to produce the resul-tant cyclohexanones (�80 %) and tin-containing beta zeoliteto form 4-alkyl-e-caprolactones in high yields, according toa Baeyer–Villiger-type oxidation with H2O2.

[a] W. Schutyser, S. Van den Bosch, J. Dijkmans, Prof. B. F. SelsCentre for Surface Chemistry and CatalysisKULeuven, Kasteelpark Arenberg 233001 Heverlee (Belgium)Fax: (+ 32) 16-321998E-mail : bert.sels@biw.kuleuven.be

[b] Dr. S. Turner, M. Meledina, Prof. G. Van TendelooElectron Microscopy for Materials Research (EMAT)University of Antwerp, Groenenborgerlaan 1712020 Antwerp (Belgium)

[c] Prof. D. P. DebeckerInstitute of Condensed Matter and NanoscienceMolecules, Solids and Reactivity (IMCN/MOST)Universit� catholique de LouvainCroix du Sud 2 box L7.05.171348 Louvain-La-Neuve (Belgium)

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201403375.

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products result in complex bio-oils with reasonable amountsof 4-alkylated and methoxylated phenols.[9] Base-catalyzedlignin depolymerization (BCD) also yields moderate amounts ofphenolics, mainly syringols, guaiacols, and catechols.[10] Verypromising routes for lignin depolymerization are reductivemethods such as hydrogenolysis and liquid-phase reforming.[11]

High yields of phenolic monomers, up to 55 %, have been re-ported.[11a–f]

Phenolic compounds derived from lignin possess uniquechemical structure features. The phenolic entity bears one ortwo methoxy groups, next to a 4-alkyl chain. Unfortunately,the use of such 4-alkyl guaiacols and syringols as novel inter-mediates for high-value target chemicals and polymer buildingblocks has thus far received little attention.[12] To date, themain research goals have focused on upgrading lignin-derivedcompounds to form hydrocarbon fuels or aromatics by com-plete removal of oxygen through hydrodeoxygenation(HDO).[13]

Encouraged by interest in bioderived chemicals, we investi-gated the conversion of a family of lignin-derived 4-alkylatedguaiacols, for example, derived from softwood, such as pinetrees, to a mixture of the corresponding cyclohexanols (CHols;Scheme 1). Oxidation,[14] dehydrogenation,[15] or transfer hydro-genation[16] routes are established to produce alkylated cyclo-hexanones (CHones), which are interesting biobased precur-sors of polymer building blocks, such as caprolactone, capro-lactam, or adipic acid, with an additional alkyl group (seeScheme 1).[17] Adipic acid is one of the most produced com-modity chemicals worldwide and the search for sustainablesynthesis routes was recently summarized by Van de Vyver andRoman-Leshkov.[18] Although not yet studied in detail, the pres-ence of alkyl groups in caprolactone, caprolactam, and adipic

acid monomers opens up new opportunities to tune the poly-mer’s physical properties, such as crystallinity, melting point,elasticity, and polarity.[19] For instance, the enantioselectiveBaeyer–Villiger oxidation of alkyl-substituted CHones to formenantiopure caprolactones has been demonstrated,[20] and theenantioselective ring-opening polymerization of a racemic mix-ture of alkylated caprolactones by enzymatic catalysis is intri-guing to synthesize high-value chiral polymers.[21]

One route to CHone production from phenol involves hydro-genation to form CHol, followed by dehydrogenation to giveCHone.[17] To avoid the thermodynamically undesirable secondstep, namely, dehydrogenation of CHol, direct selective hydro-genation of phenol to form CHone has been suggested as analternative. Excellent results for this reaction are reported withpalladium-based catalysts under low hydrogen pressure.[22]

A similar one-step conversion of guaiacols into the correspond-ing CHones would be highly desirable, but mild thermal condi-tions to selectively hydrogenate phenol to form CHone are notcompatible with the harsh conditions required to remove themethoxy groups in guaiacol. Herein, we therefore considera two-step process to be more realistic, in which the selectiveformation of alkylcyclohexanol is considered first, followed bya mild dehydrogenation to form the corresponding ketone.

Whereas dehydrogenation of alcohols to form ketones iswell established industrially, fast and selective removal of themethoxy substituent (demethoxylation) and hydrogenation ofthe aromatic ring, while retaining the hydroxyl and 4-alkylgroups of the resulting CHol, is challenging. Although the in-termediate formation of CHol is reported many times in HDOmechanistic studies of guaiacol,[13e, l, 23] Nakagawa et al. werethe first to demonstrate the deliberate production of CHolfrom guaiacol in the aqueous phase by using a combination of

Scheme 1. Conversion of lignin towards substituted polymer building blocks. The synthesis of branched caprolactones is anticipated in this contribution.

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Ru/C and MgO.[23a] The presence of base was essential to pro-mote the difficult demethoxylation step and to suppress theformation of cyclohexane by avoiding unselective C�O dissoci-ation by ruthenium metal.

We studied the selective formation of 4-alkylcyclohexanolsfrom several lignin-derived guaiacol monomers because theywill appear in a future lignocellulose biorefinery through soft-wood hydroprocessing (Scheme 1) with a liquid-phase catalyticprocess. An inert high-boiling alkane solvent (hexadecane) wasused because this solvent 1) shows good solubility for the 4-al-kylguaiacols and the corresponding CHols, 2) has a low vaporpressure for the reaction at relatively low (hydrogen) pressure,and 3) allows subsequent CHol dehydrogenation in the con-densed phase.

Additionally, rather than using noble metals, we purposelyaimed to find an appropriate, cheap, nickel-based catalyst.Industry is familiar with the use of supported nickel catalysts. Itis reported that the redox property of nickel, as well as the sta-bility of the CHols, is influenced significantly by the acid/baseproperties of the support.[23a, 24]

After selecting the best nickelcatalyst in terms of supportproperties, the reaction networkof the demethoxylation of 4-al-kylated guaiacols was investigat-ed to identify the most relevantreaction steps to determine theactivity and selectivity. Tempera-ture and nickel content wereverified in light of the productiv-ity of the catalytic reaction.Although guaiacol is initiallyused as a model substrate forsimplicity in terms of pathwaysand products, it will be shownthat 4-alkylguaiacols react simi-larly to guaiacol, and also signifi-cantly yield 4-alkylcyclohexanols.As a proof of concept, the best catalyst will be used to converta real 4-n-propylguaiacol-rich feedstock, which is obtained bysoftwood hydroprocessing. As a proof-of-principle, the thus-produced solution of 4-n-propylcyclohexanol is converted,with a traditional copper catalyst, into the corresponding cyclicketones, which will be further converted into the caprolac-tones in high yield in presence of a tin-containing beta zeolite.The sequence of reactions nicely demonstrates the feasibilityof using a multistep process to convert the lignocellulosicsinto polymer building blocks, as depicted in Scheme 1.

Results and Discussion

Screening of nickel catalysts for the conversion of guaiacolinto CHol in hexadecane

A range of nickel-supported oxide catalysts were tested in theconversion of guaiacol at 300 8C and 4 MPa H2 pressure in hex-adecane, in an effort to obtain a high yield of CHol. The cata-

lysts were synthesized by incipient wetness impregnation byusing nickel nitrate to obtain a metal loading of 3 wt %, fol-lowed by reduction under H2 flow at 500 8C. The exact nickelloadings, as measured by inductively coupled plasma atomicemission spectroscopy (ICP-AES), and the specific surface areasare indicated in Table S1 in the Supporting Information. Theexact nickel loadings vary from 2.5 to 3 wt %. All catalysts havea specific surface area of at least 40 m2 g�1. Oxide supportswith varying surface properties, namely, g-Al2O3, SiO2, Alfa ZrO2,TiO2 (P25), CeO2, and MgO, were selected to probe differentacid/base surface properties.

Plots of guaiacol conversion, product yield, and selectivityversus contact time are shown in Figures S1–S5 in the Support-ing Information for all catalysts, except for Ni/MgO. In additionto CHol, moderate amounts of CHone are also present in thereaction mixture in thermodynamic equilibrium with CHol. TheCHol to CHone molar ratio is typically between 8 and 10 for allcatalysts at maximum CHol yield (Table 1). In the rest of thediscussion, CHol represents the sum of CHol and CHone.

The catalytic reactions show the presence of many products.The origin of most of them have been reported in the vast lit-erature on HDO of guaiacol, albeit studied for a different prod-uct purpose (Scheme S1 in the Supporting Information).[13a–h,

l, 23a,c–h] Clearly, guaiacol is able to undergo many chemicaltransformations. In short, demethoxylation combined with ringhydrogenation ultimately leads to CHol, which is the productof interest in this study, but many other products may beformed as well. Methylcyclopentane, for instance, is derivedfrom acid-catalyzed cyclohexane isomerization[25] or from HDOof cyclopentanemethanol, which is a side product in the reac-tion.[13l] Cyclopentane originates through cracking of methylcy-clopentane or hydrodeoxygenation of cyclopentanol/one.Instead of demethoxylation, demethylation may also occur toyield catechol and 1,2-cyclohexanediol.[13e,f, 23a] Catechol and1,2-cyclohexanediol further react to form CHol, but they alsolead to cyclopentanol/one formation. Benzene is either formedthrough direct hydrogenolysis of phenol,[13h] but under the re-action conditions it is most likely to originate from dehydration

Table 1. Guaiacol conversion and product yield of nickel on oxide catalysts at maximum CHol yield.[a]

Entry Catalyst Contact time Conversion Yield[b] [%] CB[c]

[h gcat. gguaiacol�1] [%] CHol 2-MeOCHol cycloalkanes + aromatics dimers others [%]

1 Ni/Al2O3 0.033 72 14 11 29 1 10 932 Ni/SiO2 0.4 100 29 53 10 0 5 973 Ni/TiO2 0.2 86 40 5 25 1 12 964 Ni/ZrO2 0.2 100 75 5 11 2 4 975 Ni/CeO2 0.8 100 78 9 8 0 3 986 Ni/MgO 0.4 16 0 0 0 0 4 887[d] Ni/MgO 0.4 47 14 8 0 0 10 858[e] Ni/CeO2 0.4 100 81 2 9 3 3 989[e,f] Ni/CeO2 0.087 100 81 3 10 2 2 98

[a] Reaction conditions: guaiacol (0.5 g, 4 mmol), decane as internal standard (0.1 g, 0.7 mmol), hexadecane sol-vent (20 mL), 300 8C, 4 MPa H2. The nominal metal loading of the Ni/oxide catalysts is 3 wt % and the reductiontemperature is 500 8C. [b] 2-MeOCHol = 2-methoxycyclohexanol. [c] CB = carbon balance, see the ExperimentalSection for a definition. [d] Catalyst reduced at 625 8C. [e] Catalyst reduced at 300 8C. [f] 12 wt % Ni loading.

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of CHol to form cyclohexene, followed by dehydrogenation.[13g]

Methylcyclohexane and toluene emerge from acid-catalyzed al-kylation reactions with methanol or formaldehyde arising fromthe eliminated methoxy groups.[13h] The dimeric compoundsare suggested to be formed by alkylation reactions betweenphenol, CHol, and cyclohexene on acid sites,[26] or by aldol con-densation of CHone,[15a] followed by further hydrogenation andhydrodeoxygenation.

The formation of CHol from guaiacol proceeds through twocompetitive pathways: one involves guaiacol hydrogenation toform 2-MeOCHol, followed by demethoxylation to give CHol

(Scheme 2, Pathway I), and the other encompasses guaiacoldemethoxylation to give phenol, followed by its hydrogenationto form CHol (Pathway II).[13l, 23a, c, d] CHol may be converted fur-ther into cyclohexane through either acid-catalyzed dehydra-tion, followed by metal-catalyzed hydrogenation,[13e] or bydirect metal-catalyzed hydrogenolysis.

The catalytic results of a set of nickel catalysts are presentedin Table 1, entries 1–6. The maximum CHol yield increases inthe order Ni/MgO<Ni/g-Al2O3<Ni/SiO2<Ni/TiO2<Ni/ZrO2<

Ni/CeO2 (Table 1, entries 1–6). The acid/base property of sup-ported nickel is well known. Several spectroscopic and desorp-tion studies have demonstrated that Ni/g-Al2O3, Ni/TiO2 P25,Ni/ZrO2, and Ni/CeO2 possess acid sites of both medium andhigh strength, whereas Ni/SiO2 is only slightly acidic, and Ni/g-Al2O3, Ni/ZrO2, and Ni/CeO2 also exhibit basicity (amphotericoxides).[24, 27] Ni/MgO contains basic sites.[27c,d]

Plots of product selectivity versus contact time (Figures S1–S5 in the Supporting Information) show that the selectivity forCHol initially increases in the presence of Ni/g-Al2O3, Ni/SiO2,Ni/TiO2, Ni/ZrO2, and Ni/CeO2, but it decreases with contacttime at the expense of cycloalkane, aromatic, and dimer forma-tion. The formation of 2-MeOCHol is also observed, especiallyin the initial stage of the reaction, but the product disappearsafter full guaiacol conversion, which shows that demethoxyla-tion of 2-MeOCHol (step b in Pathway I, Scheme 2) is a slowstep. Pathway II, which proceeds via phenol, is also active, butphenol concentrations are always low and decrease with con-tinuing conversion. Thus, 2-MeOCHol and phenol are clearly

primary products in the reaction network, whereas CHol is es-sentially a secondary product, which is in agreement withScheme 2.

For Ni/g-Al2O3, degradation of CHol occurs considerably andshows rapid formation of cycloalkanes (Figure S1 b in the Sup-porting Information) ; the selectivity to CHol quickly decreaseswith increasing conversion, which ultimately leads to a lowmaximum CHol yield (Table 1, entry 1). For instance, after a con-tact time of 0.4 h gcat. gguaiacol

�1, cyclohexane, cyclopentane,methylcyclohexane, and methylcyclopentane yields of 70, 15,5, and 2 %, respectively, are obtained. The other products are

mainly cyclopentanemethanol (6 %) and 1,2-dime-thoxybenzene (2 %).

Ni/SiO2, which is a catalyst with milder Brønstedacidity, shows a high initial selectivity for 2-MeOCHol,but its conversion to CHol is slow relative to the fur-ther transformation of CHol into cycloalkanes (Fig-ure S2 b in the Supporting Information). Other mainproducts are cyclopentanemethanol (2 %) and me-thoxycyclohexane (2 %) (Table 1, entry 2).

Ni/TiO2 shows a high initial selectivity for CHol, butthe maximum CHol yield is limited to 40 % (seeTable 1, entry 3) due to a rapid formation of cycloal-kanes (Figure S3 b in the Supporting Information). Ni/TiO2 also shows intermediate formation of cyclopen-tanemethanol (5 %), 1,2-dimethoxybenzene (2 %), andanisol (2 %), but these compounds finally lead to theformation of cycloalkanes and aromatics with longercontact times. In our series, Ni/TiO2 was the only cata-

lyst that formed aromatics (mainly benzene) in significantamounts.

Ni/ZrO2 and Ni/CeO2 show the highest CHol yields of 75 and78 %, respectively (Table 1, entries 4 and 5). The plots showa constantly high CHol selectivity, which increases with conver-sion at the expense of 2-MeOCHol conversion (Figures S4 band S5 b in the Supporting Information). Ni/ZrO2 and Ni/CeO2

only show considerable formation of byproducts in terms ofcycloalkanes and dimerics at long reaction times; 2-cyclohexyl-cyclohexanol is the main dimer product.

The basic Ni/MgO catalyst was also tested (Table 1, entries 6and 7). It was recently demonstrated that HDO of guaiacolwith Ru/C in water gave a high CHol yield in the presence ofbase.[23a] The high selectivity was explained by the pronouncedpreference for Pathway II (Scheme 2, phenol route) and theprevention of CHol degradation to form cyclohexane. In ourhands, Ni on MgO showed a very low guaiacol conversion andthe main products were 1,2-dimethoxybenzene and catechol(Table 1, entry 6). The low activity of Ni/MgO is probably duea low reducibility of Ni on basic supports,[28] and therefore, theNi/MgO precursor was reduced at 625 8C instead of the stan-dard 500 8C in an additional experiment. More reduced Ni/MgO was indeed more active, showing guaiacol conversion of47 % (instead of 16 % for standard Ni/MgO). Despite this con-version increase, Ni/MgO was the least active Ni catalyst of thescreening test, and its mass balance showed a considerable de-ficiency in contrast to an almost closed balance for the othercatalysts. The main byproducts of Ni/MgO are 2-MeOCHol and

Scheme 2. Simplified reaction network for the conversion of alkylated guaiacols to thecorresponding CHols. The arrow width is an indication of the reaction rate.

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phenol (5 %), whereas no cycloalkanes were formed (Table 1,entry 7). The low activity of Ni/MgO in this study agrees witha recent study by Shen et al. , who found a similar low activityfor the hydrogenation of aromatics.[29]

To conclude on the support effects of guaiacol conversionover Ni, classic base supports such as MgO show low activity,whereas acid supports need to be avoided because of fastdegradation of CHol, leading to the formation of cycloalkanes.The use of Ni/ZrO2 and Ni/CeO2 leads to the highest CHol yieldand this is most likely to be due to their amphoteric proper-ties. Further (physicochemical) research efforts will be directedtowards refining the structure–property relationships that areat play in this reaction. Because CeO2 resulted in the highestCHol yield, this support was selected in the following series ofexperiments.

The Ni/CeO2 catalyst requires a long contact time to reachthe maximum CHol yield, that is, a value of 78 %, which corre-sponds to a space–time yield of 0.8 gCHol gcat.

�1 h�1. Two effectswere investigated to improve the catalytic activity: reductiontemperature prior to use and nickel loading. CeO2 is known tobe a reducible oxide.[30] Caballero et al. showed that, duringhigh-temperature reduction (�500 8C) of a Ni/CeO2 catalyst,partially reduced ceria moieties migrate onto Ni particles andblock active Ni sites.[31]

Such burial of Ni is avoided by reducing Ni on CeO2 at lowertemperature. By lowering the reduction temperature from thestandard 500 8C to 300 8C, significantly shorter contact timeswere required to reach the maximum CHol yield, thereby dou-bling the space–time yield to 1.6 gCHol gcat.

�1 h�1. Moreover, themaximum yield increased from 78 to 81 % (Table 1, entries 5and 8). Guaiacol conversion, product yield, and selectivityversus contact time are shown in Figure S6 in the SupportingInformation for Ni/CeO2 reduced at 300 8C. From this point on,the catalyst reduction temperature was set at 300 8C for all Ni/CeO2 catalysts further used in this work.

The stability of the 300 8C reduced Ni/CeO2 catalyst in guaia-col conversion was evaluated in recycling experiments, withoutintermediate reactivation of the catalyst under H2 (Figure S7 ain the Supporting Information). For constant contact times of0.4 h gcat. gguaiacol

�1, the CHol yield only slightly decreased from81 to 71 % after two cycles reusing the catalyst, which indicat-ed good material stability. Nickel leaching or sintering couldexplain the observed small loss in activity in the recycled runs.Therefore, these were investigated by ICP-AES analysis of thereaction solution to determine the amount of solubilizednickel, and hydrogen chemisorption of the spent catalyst.A nickel loss of merely 0.12 % (relative to the initial amount ofNi on the catalyst) and a small change in dispersion of 15.5 to14 % were found after one reaction, which suggested that theobserved loss in activity was probably not caused by changesin the nickel species of the catalyst. Thermogravimetric analysis(TGA) of the spent catalyst indicated the presence of a smallamount of carbon species (2 wt % of the spent catalyst), whichcould block the catalyst pores or nickel active sites; thus caus-ing the activity loss.

To investigate the effect of Ni metal loading on the conver-sion rate of guaiacol to CHol, Ni/CeO2 catalysts with Ni load-

ings ranging from 1 to 12 wt % were synthesized and tested inthe conversion of guaiacol at 250 8C. The amount of active Nisites and the Ni dispersions were determined by H2 chemisorp-tion. For all catalysts, the turnover frequency (TOF) of guaiacol,which is defined as the amount of guaiacol converted peractive nickel site per hour, was determined at low conversion(<50 %), and a clear increase was observed with increasing

nickel loading (Figure 1). The nickel sites thus become moreactive with a higher nickel loading, which might indicate a par-ticle size effect, since higher metal loadings typically result inlarger metal particles.[32] Therefore, the average nickel particlesizes were derived from the nickel dispersions by assumingcubic nickel crystals.[33] Figure S8 in the Supporting Informationshows the Ni dispersion and calculated average particle sizefor the different Ni/CeO2 catalysts. Higher nickel loadings giverise to systematically larger particles, with the average particlesize increasing from 2.5 to 20 nm for nickel loadings goingfrom 1 to 12 wt %.

Figure 1 shows the TOF as a function of the average nickelparticle size. Clearly, the TOF increases with increasing size,which suggests structure-dependent heterogeneous catalysisfor guaiacol conversion.[32, 34] Increasing the nickel loading isthus a suitable way to increase the catalyst activity in guaiacolconversion. With the 12 wt % Ni/CeO2 catalyst, the maximumCHol yield can be reached almost 5 times faster than that withthe 3 wt % Ni catalyst, without compromising the high CHolyield (Table 1, entries 8 and 9). The corresponding space–timeyield, that is, 7.5 gCHol gcat.

�1 h�1, is more than 9 times higherthan the space–time yield of the 3 wt % Ni catalyst reduced at500 8C (0.8 gCHol gcat.

�1 h�1). Similar to the 3 wt % Ni catalyst, the12 wt % Ni catalyst exhibits very low Ni leaching of only 0.13 %(relative to the initial amount of Ni on the catalyst).

The nickel phase of the catalysts was further investigated bypowder XRD, Z-contrast high-angle annular dark-field scanning

Figure 1. Plot of the TOFs of guaiacol, 4-n-propylguaiacol, 2-MeOCHol, andCHol at low conversion (<50 %) versus average nickel particle size for Ni/CeO2 catalysts with Ni loadings varying from 1 to 12 wt %. The nickel particlesizes are determined by H2 chemisorption. The TOFs of guaiacol and 4-n-propylguaiacol are assessed at 250 8C and the TOFs of 2-MeOCHol and CHolat 300 8C. Reaction conditions: substrate (4 mmol), decane (0.7 mmol), hexa-decane (20 mL), 4 MPa H2.

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transmission electron microscopy (HAADF-STEM) imaging,energy-dispersive X-ray (EDX) analysis, and electron energy-loss spectroscopy (EELS). XRD analysis of the Ni/CeO2 catalystsshows the presence of a relatively sharp (111) reflection of met-allic Ni, but also a broad reflection that corresponds to the(111) spacing of cubic NiO (Figure S9 in the Supporting Infor-mation). HAADF-STEM images of the 12 wt % Ni/CeO2 catalystprovide evidence for the size and crystal structure of the ceriananoparticles (Figure 2 c and d). Nickel is detected through useof both EDX (Figure 2 b) and EELS (Figure 2 g) elemental map-ping. A broad Ni(O) particle size distribution is apparent from

the maps. Due to the strong agglomeration of the ceria andNi(O) nanoparticles, determination of the degree of oxidationof the Ni(O) nanoparticles was carried out by using EELS, withthe Ni L2,3 edge as a fingerprint, and by comparing the finestructure to Ni and NiO references. Extracted EELS spectra dis-played in Figure 2 e provide evidence that the degree of oxida-tion of the nickel nanoparticles is dependent upon the nano-particle size. Indeed, the averaged Ni L2,3 EELS edge fine struc-ture acquired from a larger nanoparticle (�10 nm in diameter,blue arrow and spectrum; Figure 2 e and g) largely corre-sponds to the L2,3 fine structure reference for metallic Ni. Theaveraged Ni L2,3 fine structure from a small nanoparticle(�2 nm in diameter, green spectrum; Figure 2 e) matches wellwith the fine structure of the NiO reference.

A detailed study of a single, larger Ni(O) nanoparticle (Fig-ure S10 in the Supporting Information) shows that the surfaceof the larger nanoparticles is oxidized, whereas the core re-mains metallic. The EELS data presented above indicate thatthe smallest nanoparticles are completely oxidized. Althougha significant fraction of Ni is present in the oxidized state (asNiO), H2 temperature-programmed reduction (H2-TPR) of the12 wt % Ni/CeO2 catalyst (Figure S11 in the Supporting Infor-mation) demonstrates that reduction occurs below 200 8C,which indicates that Ni exists in a reduced form (metallic Ni)under the reaction conditions.

Kinetic aspects of guaiacol conversion over Ni/CeO2

Figure 3 shows the guaiacol conversion and product selectivityas a function of the catalyst contact time for the guaiacol reac-tion at 300 (Figure 3 a) and 250 8C (Figure 3 b). At 300 8C, themajor initial products are CHol and 2-MeOCHol. The selectivityto CHol increases to above 80 % at complete guaiacol conver-sion (after 0.4 h gcat. gguaiacol

�1) at the expense of 2-MeOCHol dis-appearance. Longer contact times led to a decreasing CHol se-lectivity with the formation of cycloalkanes and dimeric com-pounds (Figure 2 a).

The product evolution in time is somewhat different at250 8C. At the lower temperature, both CHol and 2-MeOCHolare formed concomitantly with a slight preference for theformer, and the two products remain stable under the reactionconditions up to 0.4 h gcat. gguaiacol

�1 (Figure 2 b). Yields of 58 and39 % are measured for CHol and 2-MeOCHol, respectively, atfull guaiacol conversion.

A search of the literature[13l, 23a,c,d] reveals two guaiacol con-version routes, Pathways I and II (Scheme 2), with 2-MeOCHoland phenol, respectively, being primary products and CHola secondary product. However, phenol was only observed inminute amounts during guaiacol conversion over Ni/CeO2.Therefore, to gain an insight into the reaction kinetics, individ-ual catalytic experiments with guaiacol, 2-MeOCHol, phenol,and CHol were carried out. Table 2 shows the correspondingTOFs and product selectivity (at low conversion) at 300 8C (en-tries 1 to 4). Interestingly, both 2-MeOCHol and phenol are se-lectively converted into CHol, but phenol is 60 times more re-active (Table 2, entries 2 and 3). The complete reactivity order

Figure 2. a) HAADF-STEM and b) corresponding EDX elemental map of the12 wt % Ni/CeO2 sample. The Ni(O) nanoparticles are homogeneously distrib-uted throughout the ceria material. c) and d) High-resolution HAADF-STEMimages showing the typical size of the ceria nanoparticles. The Fourier trans-form patterns provide evidence for the cubic fluorite crystal structure of theceria material. e) Averaged Ni L2,3 EELS spectra extracted from three differ-ently sized Ni(O) particles. f) Overview HAADF-STEM image. g) ElementalEELS map. The arrows indicate the Ni(O) nanoparticles from which the EELSdata in e) were extracted.

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was as follows: phenol>guaiacol>2-MeOCHol>CHol. This re-activity order suggests that guaiacol conversion preferably pro-ceeds through demethoxylation, followed by fast phenol hy-drogenation to CHol (Scheme 2, Pathway II), whereas guaiacolhydrogenation to 2-MeOCHol is only slightly slower, but thesubsequent formation of CHol is slow due to the rate-deter-mining demethoxylation of 2-MeOCHol (Scheme 2, Pathway I).From the large reactivity difference, it is fair to assume the ini-

tial molar ratio of CHol plus phenol to 2-MeOCHol is a measureof the preference for Pathway II (Scheme 2). A back-of-the-en-velope calculation reveals that 50 to 60 % of guaiacol conver-sion proceeds to CHol through intermediate phenol formation(Scheme 2, Pathway II), whereas 40 to 50 % follows the inter-mediate formation of 2-MeOCHol (Scheme 2, Pathway I). Thesubsequent conversion of 2-MeOCHol to CHol is possible, butthis reaction is slow and requires a high temperature (above250 8C; cf. Figure 3 a and b).

The results in Figure 4 a allow a comparison of the productselectivity for guaiacol conversion at 250 and 300 8C at lowconversion. Pathway II in Scheme 2 is preferred at the lowertemperature, but the preference is not exclusive to afforda high CHol yield. In other words, because Pathway I inScheme 2 (with intermediate formation of the unreactive 2-MeOCHol) remains a competitive route, high CHol yields fromguaiacol are only possible at a sufficiently high reaction tem-perature.

Conversion of CHol to cycloalkanes and dimers also happensover 3 wt % Ni/CeO2, but only after long reaction times (Fig-ure 3 a and Table 2, entry 4). Because this degradation reactionruns significantly slower than 2-MeOCHol demethoxylation(Table 2, entries 2 and 4), CHol selectivity above 80 % is possi-ble at full guaiacol conversion. As the apparent activation ener-gies of the conversion of both 2-MeOCHol (87 kJ mol�1) andCHol (88 kJ mol�1) in presence of 3 wt % Ni/CeO2 are very simi-lar, as calculated from the Arrhenius plots shown in Figure 4 b,an increase in reaction temperature will increase both reactionrates similarly, and thus, also the space–time yield, without af-fecting the maximum CHol selectivity (and yield).

The previous section showed that an increase in Ni loadingenhanced the catalyst activity in guaiacol to CHol conversion.The amount of nickel on the catalyst has no notable impact onthe selectivity for Pathways I or II in Scheme 2, since the initialproduct selectivity for guaiacol conversion is similar for the dif-

ferent catalysts (Figure S12 inthe Supporting Information).Furthermore, Figure 1 indicatesthat, similar to guaiacol, theTOFs of 2-MeOCHol and CHolare enhanced by increasing theNi loading and particle size. Inother words, the structure de-pendency of the reactions ratesholds for all nickel-catalyzed re-actions. Overall, reaction ofguaiacol to CHol is much fasterwith highly loaded nickel cata-lysts.

Conversion of lignin-derived phenolics

Because the major fraction of monomeric compounds ob-tained from lignin possess an alkyl chain, with almost exclu-sively para regioselectivity, guaiacols with a 4-alkyl chain ofvarying length and types, namely, -methyl, -ethyl, -n-propyl,and allyl substituents, are more relevant substrates. The reac-

Figure 3. Product distribution for the conversion of guaiacol over 3 wt % Ni/CeO2 at 300 (a) and 250 8C (b). Reaction conditions: guaiacol (0.5 g, 4 mmol),decane (0.7 mmol), hexadecane (20 mL), 4 MPa H2.

Table 2. TOF, conversion, and initial product selectivity for guaiacol, 2-MeOCHol, phenol, and CHol conversionover 3 wt % Ni/CeO2.[a]

Entry Substrate TOF[b] Contact time Conversion Selectivity[c] [%][h gcat. gsubstrate

�1] [%] CHol phenol 2-MeOCHol CH dimers CPol others

1 guaiacol 3262 0.0083 27 47 3 44 1 0 0 52 2-MeOCHol 286 0.17 50 82 0 / 8 0 6 13 phenol 19 084 0.0045 57 96 0 / 3 0 0 04 CHol 183 0.25 36 / 0 / 45 33 0 0

[a] Reaction conditions: substrate (4 mmol), decane (0.7 mmol), hexadecane (20 mL), 300 8C, 4 MPa H2. [b] TOFis the initial rate of substrate conversion normalized to the accessible Ni atoms [mol molNi(surf)

�1 h�1] . [c] Abbrevi-ations: CH = cycloalkanes, CPol = cyclopentanol and cyclopentanone.

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tivity (as indicated by the TOF values) decreases with increas-ing alkyl chain length (Figure 5).

Figure 5 presents the initial product selectivity for 4-alkyl-guaiacol conversion with 3 wt % Ni/CeO2 at 250 8C. The 4-alkyl-guaiacols react in the same way as guaiacol, namely, throughboth Pathways I and II in Scheme 2. Increasing the length of

the alkyl chain results in a clear preference for Pathway II, as in-dicated by the higher selectivity for alkylated CHol and corre-sponding phenol and the lower selectivity for 4-alkyl-2-MeO-CHol at 250 8C. Nevertheless, higher temperatures are again re-quired to convert the remaining 2-MeOCHols.

The initial product selectivity of 4-n-propylguaiacol conver-sion is therefore compared at 250 and 300 8C in Figure 4 a. Sim-ilar to guaiacol conversion, the selectivity for Pathway II inScheme 2 is somewhat higher at the lower temperature, butthe difference is again not sufficient to deliver a high yield of4-n-propylCHol at low temperature. As with guaiacol, a highertemperature (>250 8C) is required to overcome the energeticdemethoxylation of the corresponding 2-MeOCHol and toafford a high 4-n-propylCHol yield.

Table 3 shows the catalytic results for the conversion of a setof 4-substituted guaiacols over 3 wt % Ni/CeO2 at 300 8C. Yieldsto the corresponding 4-alkylCHol products of around 82 %were obtained for all substrates after 1 h of reaction (Table 3,entries 1–4). Similar to the conversion of guaiacol, highernickel loadings enhance the catalyst activity in the conversionof an alkylated guaiacol such as 4-n-propylguaiacol (seeFigure 1). The 12 wt % Ni/CeO2 catalyst reached an equallyhigh 4-n-propylCHol yield to that of the 3 wt % Ni catalyst, butin a much shorter reaction time (Table 3, entries 4 and 5), re-sulting in a considerably higher space–time yield (10.8 vs.2.3 g4-n-propylCHol gcat.

�1 h�1).Next to increasing the catalyst productivity or space–time

yield on catalyst basis, it is important to increase the produc-tivity of the reactor, that is, the amount of product formed perreactor volume per time (space–time yield on a reactor volumebasis). Therefore, the substrate concentration in the feedshould be as high as possible. To verify the efficiency of thecatalyst system with a more concentrated feed, the concentra-tion of 4-n-propylguaiacol in the reaction solution was in-creased from 4.1 to 20 wt % (Table 3, entries 5 and 6). Conver-sion of this feed over the 12 wt % Ni/CeO2 catalyst resulted ina 4-n-propylcyclohexanol yield of 81 % (Table 3, entry 6), whichis as high as that for a solution with the standard concentra-tion. The catalyst system is thus able to convert concentratedproduct feeds.

To further broaden the substrate scope of the catalystsystem, eugenol (or 4-allylguaiacol) was also tested, sinceguaiacol with an unsaturated side chain can also be obtainedfrom softwood lignin in high yield.[11e] A similar yield wasreached to that of 4-n-propylCHol; thus showing successful hy-drogenation of the allyl substituent next to demethoxylationand ring hydrogenation (Table 3, entry 7). To reach the highproduct yield from eugenol, it was essential to perform reactorheating under a hydrogen instead of a nitrogen atmospherebecause of the high reactivity of the isolated double bond forside reactions. Heating the reactor under a nitrogen atmos-phere resulted in a lower 76 % yield of 4-n-propylCHol.

Surprisingly, we also noted an isomerization activity of Ni/CeO2 in the conversion of 4-alkylguaiacols because both 3- and4-alkylCHol products were detected (see final column ofTable 3). Their detection was only evident by applying a silyla-tion treatment of the product mixture with 2,2,2-trifluoro-N-

Figure 5. TOF and initial product selectivity (at 20–30 % conversion) for 4-R-guaiacol conversion over 3 wt % Ni/CeO2 at 250 8C. Reaction conditions: sub-strate (4 mmol), decane (0.7 mmol), hexadecane (20 mL), 4 MPa H2.

Figure 4. a) TOF and initial product selectivity (at 20–30 % conversion) forguaiacol and 4-n-propylguaiacol conversion at 250 and 300 8C in the pres-ence of 3 wt % Ni/CeO2. b) Arrhenius plots (ln(TOF) vs. 1/T) for 2-MeOCHoland CHol conversion over 3 wt % Ni/CeO2. Reaction conditions: substrate(4 mmol), decane (0.7 mmol), hexadecane (20 mL), 4 MPa H2.

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methyl-N-trimethylsilylacetamide prior to GC analysis.[35] Thegas chromatograms of the silylated product mixtures of 4-eth-ylguaiacol and 4-n-propylguaiacol are illustrated in Figure S13 aand b, respectively, in the Supporting Information. The 3- and4-isomers are present as both cis and trans stereoisomers. Con-version of the isomers over Ni/CeO2 proceeds fairly regioselec-tively with a preference for 4-alkylCHol isomer formation; the4- to 3-isomer molar ratios range from 10 to 13 (Table 3, en-tries 1–6).

The regioselectivity of the reaction is useful to derive mecha-nistic information about the reaction network. It can, for in-stance, be used to provide hard evidence to exclude the domi-nant participation of demethylation (through catechol inter-mediates) rather than demethoxylation in the conversion ofguaiacols to the corresponding CHol products. For instance,the conversion of 4-ethylcatechol over Ni/CeO2 leads to anequimolar mixture of 4- and 3-ethylCHol, whereas this ratio isabout 11 for the conversion of 4-ethylguaiacol (Table 3, en-tries 3 and 8). Moreover, the conversion of 4-ethylcatechol alsoresults in high yields of 4-ethyl-1,2-cyclohexanediol (15 %) and3-ethylcyclopentanol/one (21 %), whereas these compoundswere only found in trace amounts in the reaction with 4-ethyl-guaiacol. The gas chromatogram of the silylated product mix-ture of 4-ethylcatechol is shown in Figure S13 c in the Support-ing Information.

The isomerization activity of Ni/CeO2 can be reduced signifi-cantly by performing the reactor heating under a hydrogen at-mosphere instead of nitrogen. In this way, reaction with 4-n-propylguaiacol led to a 4- to 3-n-propylCHol molar ratio of 22at full conversion, whereas the value of the ratio was 10 whenreactor heating was carried out under nitrogen (Table 3, en-tries 4 and 9). The high preference of 4-n-propylCHol in the re-action with eugenol is also likely to originate from the reduc-tive reaction atmosphere during reactor heating (Table 3,entry 7). More research is currently underway to explain theorigin of the peculiar isomerization activity of Ni/CeO2.

Conversion of a crude softwood-derived 4-n-propylguaiacol-rich feedstock

4-n-Propylguaiacol can be obtained in high yields by hydroge-nolysis of the lignin fraction in plant biomass.[11a–d] Hydrogenol-ysis of pine sawdust was carried out in methanol with rutheni-um on carbon to produce a lignin-derived oil. The oil was sub-sequently extracted with hexadecane to obtain a transparentsolution rich in 4-n-propylguaiacol (see Figure S14 in the Sup-porting Information), with the amount of 4-n-propylguaiacolcorresponding to about 12 wt % of the original lignin contentof the pine sawdust. The gas chromatogram of the hexade-cane fraction is shown in Figure 6 a. The gas chromatogram ofthe lignin oil is presented in Figure S15 in the Supporting In-formation. The monophenolic fraction in the lignin oil compris-es more than 80 % 4-n-propylguaiacol, with 3-guaiacylpropa-nol, 4-ethylguaiacol, and 4-n-propylphenol as the major by-products. The lignin-derived 4-n-propylguaiacol was subjectedto a catalytic reaction in the presence of 3 wt % Ni/CeO2 at300 8C, resulting in a n-propylCHol yield of 73 % (Table 3,entry 10). The corresponding gas chromatogram is indicated inFigure 6 b. GC analysis also showed the slight formation of eth-ylCHol, which was likely to be derived from 4-ethylguaiacol,also present in the original lignin oil. Conversion of the crudelignin-derived 4-n-propylguaiacol required a higher contacttime to reach full conversion, compared with that for pure 4-n-propylguaiacol ; this was likely to be due to the presence ofminor compounds with inhibiting properties.

4-AlkylCHol dehydrogenation to 4-alkylCHone

High yields of 4-alkylCHol can thus be realized from 4-alkyl-guaiacol derivatives and even from real lignin-derived 4-n-pro-pylguaiacol. The question remains whether such a mixture canbe dehydrogenated easily to 4-alkylCHone in accordance withthe proposal in Scheme 1. We therefore performed two catalyt-

Table 3. Conversion and product distribution of reactions with pure 4-alkylguaiacols, eugenol, and a 4-n-propylguaiacol fraction derived from pine saw-dust in the presence of Ni/CeO2

[a]

Entry Substrate Conversion Yield [%] CB 4-R/3-R[c]

[%] R[b]-CHol R[b]-2-MeOCHol cycloalkanes + aromatics others [%]

1 guaiacol 100 81 2 9 6 98 /2 4-methylguaiacol 100 82 1 10 4 97 113 4-ethylguaiacol 100 83 1 11 2 98 114 4-n-propylguaiacol 100 82 4 8 4 98 105[d] 4-n-propylguaiacol 100 82 2 12 3 99 136[e] 4-n-propylguaiacol 100 81 8 7 2 98 127[f] eugenol 100 83 5 7 3 97 188[g] 4-ethylcatechol 100 55 0 5 37 96 19[f] 4-n-propylguaiacol 100 83 2 11 1 97 2210[h] 4-n-propylguaiacol from pine sawdust 100 73 12 8 5 98 6

[a] Reaction conditions: substrate (4 mmol), 3 wt % Ni/CeO2 (0.2 g) decane (0.7 mmol), hexadecane (20 mL), 300 8C, 4 MPa H2, 1 h. [b] R = alkyl. [c] The selec-tivity ratio for 4- to 3-alkylcyclohexanol. [d] 12 wt % Ni/CeO2 (0.2 g), 13 min reaction time. [e] 12 wt % Ni/CeO2 (0.2 g), 24 mmol substrate (20 wt % substratein feed), 4.2 mmol decane, 6.5 MPa H2, 85 min reaction time. [f] 2 MPa H2 at room temperature, heated to 300 8C (�40 min, 3.6 MPa), 4 MPa H2, 40 min re-action time. [g] 2 mmol substrate. [h] The substrate is obtained by hexadecane extraction of pine lignin hydrogenolysis oil. Reaction conditions: hexade-cane extraction phase (20 mL; containing 0.7 mmol 4-n-propylguaiacol), decane (0.15 mmol), 3 wt % Ni/CeO2 (0.1 g), 2 MPa H2 at room temperature,heated to 300 8C (�40 min, 3.6 MPa), 4 MPa H2, 200 min reaction time.

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ic reactions: one with 4-ethylCHol and the other with 4-n-pro-pylCHol in the presence of Cu/ZrO2, which is a catalyst knownfor its dehydrogenation activity.[15b]

High yields of the corresponding ketones, namely, 78 % of 4-ethylCHone and 81 % of 4-n-propylCHone, were obtained at250 8C (Table 4, entries 1 and 2). The main side products are 4-

alkylphenol, dimer compounds,and cycloalkanes. Figure S16 aand b in the Supporting Infor-mation shows the product distri-butions for 4-ethylCHol and 4-n-propylCHol, respectively, duringthe dehydrogenation process.Also, real lignin-derived productmixtures of 4-alkylCHol, derivedfrom the corresponding 4-alkyl-guaiacols, could be convertedinto the ketones. To demonstratethe subsequent dehydrogena-tion step with Cu/ZrO2, thecrude reaction mixture (Table 3,entry 9) was dehydrogenated toa final yield of 81 % n-propyl-CHone relative to the amount ofn-propylCHol in the reactionmixture (Table 4, entry 3). Dehy-drogenation of the lignin-de-rived n-propylCHol-rich productmixture (Table 3, entry 10) wassuccessfully performed, yielding79 % n-propylCHone relative tothe amount of n-propylCHol inthe reaction mixture (Table 4,entry 4). The gas chromatogramof the raw product mixture isshown in Figure 6 c. This exam-ple nicely illustrates the feasibili-ty of the synthetic scheme pro-posed in Scheme 1.

Baeyer–Villiger oxidation of al-kylCHone to form branchedcaprolactones

Finally, this section provides evi-dence that alkylated e-caprolac-tone can be produced from thealkylCHone product mixture ob-tained from softwood lignin. Theconversion of CHone to e-capro-lactone proceeds throughBaeyer–Villiger oxidation; a reac-tion for which the Lewis acidictin-containing beta zeolite isknown to be a suitable cata-lyst.[36] First, the n-propylCHonemixture obtained from 4-n-pro-

pylguaiacol (Table 4, entry 3) was subjected to Baeyer–Villigeroxidation at 90 8C with a tin-containing beta zeolite catalystdeveloped in our laboratory[37] and H2O2 as the oxidizingagent. After 8 h, 89 % of n-propylCHone was converted witha selectivity of 92 % towards n-propyl-e-caprolactone. The n-propylCHone product obtained from the lignin-derived 4-n-

Figure 6. Gas chromatograms of a) the hexadecane-extracted phase of the lignin oil obtained by pine lignin hy-drogenolysis ; b) the product after 3 wt % Ni/CeO2-catalyzed conversion indicated in Table 3, entry 10; and c) thedehydrogenation product indicated in Table 4, entry 4.

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propylguaiacol (Table 4, entry 4) reached a conversion of 87 %after 8 h, with a selectivity of 85 % towards n-propyl-e-caprolac-tone. The gas chromatogram of the latter is indicated in Fig-ure S17 in the Supporting Information.

Conclusions

The potential of nickel supported on oxide catalysts in the se-lective conversion of a range of lignin-derived guaiacols intothe corresponding cyclohexanols in a liquid-phase alkane wasdemonstrated, and resulted in yields above 80 %.

The oxide support should be selected carefully becauseCHol was not stable in the presence of Ni on an acidic oxidesuch as g-Al2O3, and Ni on a base oxide such as MgO exhibitsvery low activity. Amphoteric supports such as ZrO2 and CeO2

were the most suitable supports. A detailed physicochemicalstudy of these catalysts will further elaborate understanding ofthe catalytic process.

The kinetics of Ni on CeO2 were studied in more detail withrespect to the catalyst properties. The overall reaction rate wasincreased by lowering the catalyst reduction temperature andby increasing the nickel loading. The average nickel particlesize increased with increasing nickel loading, and this resultedin increasing activity per accessible nickel site; this was indica-tive of structure-sensitive nickel catalysis. As such, high space–time yields up to 11 h�1 were obtained.

Guaiacol and alkylated guaiacol conversion to the corre-sponding CHols proceeded over Ni/CeO2 through two parallelpathways with similar rates; the resultant 2-MeOCHols and al-kylated phenols were the primary products. Both intermediateswere converted into the corresponding CHols; the phenolcompound was more reactive. Only with prolonged reactiontimes did the CHols further react to form cycloalkanes anddimer compounds.

Fast and selective hydrogenolysis of the methoxy group (de-methoxylation) was the major challenge to convert (alkylated)guaiacol. Two strategies were therefore suggested for highCHol yields from the alkylated guaiacols: 1) the utilization ofa catalyst that exhibited high selectivity for phenol Pathway II(Scheme 2), that is, a high tendency for guaiacol demethoxyla-tion compared with ring hydrogenation, as recently demon-strated with Ru on MgO,[23a] or 2) if both Pathways I and II(Scheme 2) were tolerated, the use of a catalyst (e.g. , Ni on

CeO2) with sufficient demethoxylation activity for 2-MeOCHol,without compromising CHol selectivity.

Ni/CeO2 was also able to convert a real lignin-derived feedrich in 4-n-propylguaiacol, obtained by hydroprocessing ofsoftwood, into 4-n-propylcyclohexanol. The CHol productcould effectively be dehydrogenated into the correspondingketone with a Cu/ZrO2 catalyst. Baeyer–Villiger oxidation of thecyclic ketone with H2O2 in the presence of tin-containing betazeolite yielded the desired 4-n-propyl-e-caprolactone.

Experimental Section

Chemicals and materials

All commercial chemicals were analytical reagents and were usedwithout further purification. Hexadecane (99 %), decane (99 + %),guaiacol (98 + %), CHol (99 %), CHone (99 + %), 4-propylcyclohexa-none (99 + %), cyclohexane (99 + %), 4-methylguaiacol (98 + %), 4-n-propylguaiacol (99 + %), 4-ethylcatechol (95 %), 1,2-cyclohexane-diol (98 %), N-Methyl-N-(trimethylsilyl)trifluoroacetamide, AerosilSiO2 (380 m2 g�1), Aeroxide TiO2 P25 (50 m2 g�1), Ru on carbon(5 wt %), methanol (99.9 + %), and ethylcyclohexane (99 + %) werepurchased from Sigma Aldrich. Phenol (99.5 + %), 4-ethylguaiacol(98 %), pyridine (99 + %), Cu(NO3)2·3 H2O, MgO (130 m2 g�1), andH2O2 (50 wt % in water, stabilized) were purchased from Acros Or-ganics. Ni(NO3)2·6 H2O, 2-MeOCHol (99 %), CeO2 (63 m2 g�1), andZrO2 (90 m2 g�1) were purchased from Alfa Aesar. 4-Ethylcyclohexa-nol (97 + %) and 4-n-propylcyclohexanol (98 + %) were purchasedfrom TCI chemicals. Puralox Al2O3 (150 m2 g�1) was purchased fromCondea Chemie. 1,4-Dioxane (99 + %) was purchased from Lab-Scan.

Catalyst synthesis

The nickel-supported catalysts were synthesized by incipient wet-ness impregnation of the oxide with an aqueous solution ofNi(NO3)2·6 H2O. After drying at 60 8C for 24 h, the samples were re-duced at 300 or 500 8C (heating rate 5 8C min�1) for 1 h undera flow of H2 (120 mL min�1 g�1). The Cu/ZrO2 catalyst was synthe-sized by incipient wetness impregnation of ZrO2 with an aqueoussolution of Cu(NO3)2·3 H2O. The sample was calcined at 500 8C(heating rate 5 8C min�1) for 1 h under a flow of O2

(120 mL min�1 g�1) and reduced at 500 8C (heating rate 5 8C min�1)for 1 h under a flow of H2 (120 mL min�1 g�1). The tin-containingbeta zeolite catalyst was synthesized as described by Dijkmanset al.[37]

Table 4. Cu/ZrO2-catalyzed dehydrogenation of 4-alkylcyclohexanols (4-R-) and the reaction products of the 3 wt % Ni/CeO2-catalyzed conversion of 4-n-propylguaiacol and 4-n-propylguaiacol from pine sawdust.[a]

Entry Substrate Product distribution [%]R[b]-CHone R[b]-CHol R[b]-phenol dimerics alkanes + aromatics

1 4-ethylcyclohexanol 78 9 2 9 12 4-n-propylcyclohexanol 81 11 1 5 13[c] product from 4-n-propylguaiacol conversion (Table 3, entry 9) 81 (67)[d] 11 (9) 2 (2) 3 (2) (11)4[e] product from pine lignin-derived 4-n-propylguaiacol conversion (Table 3, entry 10) 79 (58) 13 (10) 3 (2) 3 (2) (9)

[a] Reaction conditions: 4-alkylCHol (1 mmol), 3 wt % Cu/ZrO2 (0.025 g), decane (0.2 mmol), hexadecane (20 mL), 250 8C, 0.1 MPa N2, 15 min, 4 cycles. [b] R =

alkyl. [c] The product solution from Table 3, entry 9, was diluted four times with hexadecane. [d] The values in brackets represent the product distributionrelative to the amount of 4-n-propylguaiacol in the original product solution. [e] The product solution from Table 3, entry 10.

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Catalytic reactions

Catalytic reactions were performed in batch mode in a 50 mL Parrautoclave equipped with a mechanical stirrer, a liquid samplingtube, gas inlet and outlet tubes, a thermocouple, a rupture disc,and an electric heating jacket. The autoclave was loaded with sub-strate, typically guaiacol (0.5 g), catalyst (0.2 g), decane as an inter-nal standard (0.1 g), and hexadecane as the solvent (20 mL). Afterflushing the reaction mixture with N2 (3 times), the temperaturewas raised to typically 300 8C (�40 min heating time) and the au-toclave was put under constant H2 pressure of 4 MPa at the reac-tion temperature. The stirrer speed was set at 750 rpm. After thereaction, the autoclave was rapidly quenched to room temperaturein a water bath. If the autoclave was put under an initial H2 pres-sure of 2 MPa, the pressure increased up to 3.6 MPa at 300 8C andthe autoclave was subsequently put under constant H2 pressure of4 MPa at the reaction temperature. For the catalyst recycling ex-periments, the catalyst was separated by filtration, washed thor-oughly with acetone and hexane, and dried overnight in an ovenat 100 8C. A small loss of solid catalyst particles occurred duringthe washing and filtration steps, and approximately 90–95 % of thecatalyst weight was recovered after each catalyst recycling. In oneset of recycling experiments, only the recycled catalyst was usedand the reaction time was elongated to reach a contact time of0.4 h gcat. gguaiacol

�1. In a second set of recycling experiments, theamount of catalyst was kept constant and 5–10 % of fresh catalystwas added in each run. Pine sawdust hydrogenolysis was per-formed in a 100 mL Batch autoclave. The autoclave was loadedwith pine sawdust (4 g, 27.3 wt % Klason lignin), 5 wt % Ru/C(0.6 g), and methanol (40 mL). After flushing with N2, the reactorwas put under 3 MPa H2 and the temperature was raised to 250 8C(�25 min heating time). The stirrer speed was set at 750 rpm. Thereaction was performed at 250 8C for 3 h. After quenching the au-toclave to room temperature in a water bath, the reaction mixturewas collected and the autoclave was washed with methanol. Thewashing solution was combined with the reaction mixture and fil-tered over a glass filter to remove the catalyst and pine sawdustresidue. Methanol was removed from the liquid phase in a rotaryevaporator and the resulting oil phase was extracted with water(2 � 2 mL) to remove the sugar components present in the oil. Bymeans of this procedure, lignin oil (0.67 g) was obtained. 4-n-Pro-pylguaiacol was extracted from the lignin oil by adding hexade-cane (4 mL) and mixing this solution with a magnetic stirrer at80 8C for 30 min. Hexadecane was removed by decantation. Theprocedure was repeated four times. The hexadecane samples werecombined and this solution was used for further reaction. Dehydro-genation reactions were performed in a 50 mL batch autoclave byloading the substrate, typically 4-n-propylCHol (0.144 g), catalyst(0.025 g), decane (0.025 g), and hexadecane (20 mL) in the auto-clave, flushing the autoclave with N2, and raising the temperatureto 250 8C (�30 min heating time). After 15 min at 250 8C, the auto-clave was quenched to room temperature in a water bath and analiquot was removed from the reaction mixture for analysis. Subse-quently, the autoclave was sealed again and this reaction proce-dure was repeated three times. For dehydrogenation of the crudereaction mixture obtained after 4-n-propylguaiacol conversion, thereaction mixture (5 mL) was combined with hexadecane (15 mL).The Baeyer–Villiger oxidations were conducted in magneticallystirred glass reactor vials placed in a temperature-controlledcopper block at 90 8C. The dehydrogenation mixture (1.5 mL) wasmixed with 1,4-dioxane (6 mL) and tin-containing beta zeolite cata-lyst (25 mg) was added. A 50 wt % aqueous solution of H2O2 wasadded in a molar H2O2/ketone ratio of 2. To obtain the maximumproduct yield, an additional aliquot of the oxidant solution was

added after 4 h (bringing the H2O2/ketone ratio to 4). Ethylcyclo-hexane was used as an internal standard for chromatographic anal-ysis. Aliquots of the samples were taken at regular time intervalsthrough a rubber septum for analysis.

Product analysis

Quantitative analysis of the reaction samples was performed bygas–liquid chromatography by using two gas chromatographs:1) an Agilent 6890 series GC equipped with HP5 capillary column,an Agilent 7683 series autosampler, a split injection system, anda flame ionization detector; and 2) a Hewlett Packard 5890 GCequipped with a CPsil-5 column, a HP 6890 autosampler, a split in-jection system, and a flame ionization detector. The second gaschromatograph was used for the separation of benzene and cyclo-hexane. The following operation conditions were applied to bothGC instruments: injection temperature: 250 8C, column tempera-ture: 45 8C (6 min), 10 8C min�1 to 280 8C (5 min), detector tempera-ture (300 8C). Sensitivity factors of the reagents and products wereobtained by calibration with commercial standards or obtained byECN-based calculations[38] due to a lack of commercial standards.Conversion, product yield, product selectivity and carbon balance(CB) were defined by Equations (1)–(4), respectively:

Conversionð%Þ ¼ nsubstrate;0 � nsubstrate

nsubstrate;0� 100% ð1Þ

Yieldð%Þ ¼ a� nproduct

nsubstrate;0� 100% ð2Þ

Selectivityð%Þ ¼ Yieldð%ÞConversionð%Þ ð3Þ

CBð%Þ ¼ nsubstrate þP

nproduct

nsubstrate;0� 100% ð4Þ

in which nsubstrate,0, nsubstrate, and nproduct represented the amount(mol) of substrate at the start of the reaction, and the amount ofsubstrate and product after the reaction, respectively; a equaledone for all products, except the dimer compounds, for which itequaled two. The product distribution indicated in Table 4 was de-fined in the same way as that of the yield. Qualitative analysis ofthe reaction products was performed by GC-MS by using an Agi-lent 6890 series GC instrument equipped with a HP5MS capillarycolumn and an Agilent 5973 series mass spectrometry detector.The following operation conditions were applied to the GC-MS in-strument: 60 8C (5 min), 10 8C min�1 to 280 8C (5 min), detector tem-perature (300 8C). Silylation of the reaction mixture was performedby adding the reaction mixture (0.5 mL), pyridine (0.5 mL), and N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA; 0.25 mL) toa GC vial and putting the vial in an oven at 80 8C for 30 min. Iden-tification and quantification of the silylated products was per-formed by using the same GC analysis as that described for thetypical reactions.

Catalyst characterization

Quantitative elemental analysis of the Ni loading was performedby ICP-AES by using a Jobin–Yvon Ultima spectrometer. Therefore,the samples were degraded with aqua regia and HF. For the deter-mination of Ni leaching, the amount of solubilized Ni in the reac-tion solution was measured. Therefore, the reaction solution (1 mL)was extracted with a 2 % aqueous solution of HNO3 (3 � 3 mL),which was subsequently analyzed by ICP-AES. BET specific surface

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areas (SBET) of the catalysts were calculated from N2 physisorptiondata obtained at 77 K by using a Micromeritics Tristar 3000 appara-tus. Prior to the physisorption experiments, the samples were evac-uated at 473 K for at least 12 h. The amount of Ni surface atomswas determined by hydrogen chemisorption measurements. Thecatalyst was activated at 300 8C for 1 h in a flow of H2 and evacuat-ed at 300 8C for 1 h in vacuum (<5 mm Hg; 1 mm Hg = 133.3 Pa). Itwas then flushed for 1 h in a flow of He, cooled to 35 8C, and evac-uated again for 1 h (<5 mm Hg). H2 adsorption (chemisorption andphysisorption) was measured at 35 8C over the pressure rangefrom 0.5 to 415 mm Hg. Next, physisorbed H2 was removed by out-gassing the sample for 2 h at 35 8C (<5 mm Hg), and another iso-therm (physisorption) was measured. The difference between thetwo isotherms gave the H2-chemisorption isotherm. The concentra-tion of chemisorbed hydrogen on the metal was determined byextrapolating the differential isotherm to zero H2 pressure, and thisvalue was used to calculate the Ni dispersion. By assuming cubiccrystals, the average nickel crystallite size was calculated by usingEquation (5), wherein SNi was the Ni metal surface area in m2 g�1

and 1Ni was the metal density (8.908 g m�3).[33]

d ¼ 5=ðSNi1NiÞ ð5Þ

HAADF-STEM, STEM-EDX mapping, and EELS analyses were per-formed by using two FEI Titan “cubed” electron microscopes, bothoperated at 300 kV. The first was equipped with a Super-X highsolid-angle EDX detector, the second with a GIF Quantum spec-trometer and electron monochromator, set to provide a 250 meVenergy resolution for EELS. For imaging, the convergence semian-gle, a, was 22 mrad, whereas the HAADF-STEM collection semian-gle was 50 mrad. For EELS, the convergence semiangle was18 mrad and the collection semiangle was approximately100 mrad. The samples were prepared for TEM investigation bycrushing the powder with ethanol in a mortar and placing severaldrops of the suspension onto a holey carbon grid. Diffraction pat-terns were collected a Huber G670 Guinier diffractometer (CuKa1 ra-diation, curved Ge(111) monochromator, transmission mode, imageplate). H2-TPR was performed in a tubular reactor by using MS de-tection under the following conditions: sample weight 100 mg,heating rate 10 8C min�1, flow rate 12 mL min�1, and 5 vol % H2 inHe. TGA of the spent catalyst was performed with a TGA Q500 ana-lyzer from TGA Instruments. The sample was heated under O2 at3 8C min�1 to 60 8C, kept at this temperature for 30 min, heated at10 8C min�1 to 800 8C and kept at this temperature for 30 min. Therelative weight loss between 60 and 800 8C was used as a measureof the amount of carbon deposits on the catalyst.

Acknowledgements

This work was performed in the framework of an IAP-PAI networkfrom BELSPO (Federal Agency). W.S. and S.T. acknowledge fund-ing from the Research Foundation—Flanders (FWO). S.V.d.B. ac-knowledges the Institute for the Promotion of Innovation throughScience and Technology in Flanders (IWT-Vlaanderen) for a doctor-al fellowship. J.D. thanks Methusalem CASAS for funding. We aregrateful to I. Cuppens and G. G. Rangel for ICP-AES analysis, G.Vanbutsele for help with N2 physisorption analysis, W. Vermandelfor H2-TPR measurements, and Dr. M. Jacquemin for help with H2

chemisorption analysis.

Keywords: biomass · heterogeneous catalysis · lignin · nickel ·synthesis design

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Received: December 5, 2014

Published online on && &&, 0000

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Full Papers

FULL PAPERS

W. Schutyser, S. Van den Bosch,J. Dijkmans, S. Turner, M. Meledina,G. Van Tendeloo, D. P. Debecker, B. F. Sels*

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Selective Nickel-Catalyzed Conversionof Model and Lignin-Derived PhenolicCompounds to Cyclohexanone-BasedPolymer Building Blocks

In the Ni-ck of time : The four-step con-version of lignocellulose to caprolac-tones is demonstrated. One of themajor challenges is the selective remov-al of the methoxy group from theguaiacols. Ni/CeO2 is a suitable catalystfor this reaction, yielding considerableamounts of cyclohexanols, which areprone to undergo dehydrogenation andBaeyer–Villager oxidation.

ChemSusChem 0000, 00, 0 – 0 www.chemsuschem.org � 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim15 &

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