Selective nickel-catalyzed conversion of model and lignin-derived phenolic compounds to cyclohexanone-based polymer building blocks

DOI: 10.1002/cssc.201403375

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Selective Nickel-Catalyzed Conversion of Model and

Lignin-Derived Phenolic Compounds to Cyclohexanone-

Based Polymer Building Blocks

Wouter 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]

Valorization of lignin is essential for the economics of future

lignocellulosic biorefineries. Lignin is converted into novel

polymer building blocks through four steps: catalytic hydro-

processing of softwood to form 4-alkylguaiacols, their conver-

sion into 4-alkylcyclohexanols, followed by dehydrogenation to

form cyclohexanones, and Baeyer–Villiger oxidation to give

caprolactones. The formation of alkylated cyclohexanols is one

of the most difficult steps in the series. A liquid-phase process

in the presence of nickel on CeO₂or ZrO₂catalysts 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 the

formation of the corresponding alkylated 2-methoxycyclohexa-

nol, and 2) demethoxylation to form 4-alkylphenol. Although

subsequent phenol to cyclohexanol conversion is fast, the rate

is limited for the removal of the methoxy group from 2-me-

thoxycyclohexanol. Overall, this last reaction is the rate-limiting

step and requires a sufficient temperature (>2508C) to over-

come the energy barrier. Substrate reactivity (with respect to

the type of alkyl chain) and details of the catalyst properties

(nickel loading and nickel particle size) on the reaction rates

are reported in detail for the Ni/CeO₂catalyst. The best Ni/

CeO₂catalyst reaches 4-alkylcyclohexanol yields over 80%, is

even able to convert real softwood-derived guaiacol mixtures

and can be reused in subsequent experiments. A proof of prin-

ciple of the projected cascade conversion of lignocellulose

feedstock entirely into caprolactone is demonstrated by using

Cu/ZrO₂for the dehydrogenation step to produce the resul-

tant cyclohexanones (ꢁ80%) and tin-containing beta zeolite

to form 4-alkyl-e-caprolactones in high yields, according to

a Baeyer–Villiger-type oxidation with H₂O₂.

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 composed

of 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 last

decade,^[1c–e,2]lignin valorization technologies are substantially

less developed today, mainly due to the inherent complexity

and heterogeneity of the aromatic biopolymer.^[3]

The structure of lignin may be regarded as an amorphous

and highly branched polymer of phenyl propane units that

originate from three aromatic alcohols, namely, p-coumaryl,

coniferyl, and sinapyl alcohol.^[3b,4]The building blocks are

mainly connected through ether bonds, dominated by the b-

O-4 linkage, next to CꢀC bonds.^[3b,4]As the largest renewable

source of aromatic and phenolic material with a unique chemi-

cal structure, lignin is suggested to constitute a promising

feedstock for a wide variety of bulk and fine chemicals, as well

as 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 the

greatest challenges in current biomass conversion.^[3c,f,6a]The

production of low-molecular-weight compounds from lignin is

indeed a promising route for valorizing lignin and selective cat-

alytic processes will play a key role in these conversion

routes.^[3]

[a] W. Schutyser, S. Van den Bosch, J. Dijkmans, Prof. B. F. Sels

Centre for Surface Chemistry and Catalysis

KULeuven, Kasteelpark Arenberg 23

3001 Heverlee (Belgium)

Fax: (+32)16-321998

E-mail: bert.sels@biw.kuleuven.be

[b] Dr. S. Turner, M. Meledina, Prof. G. Van Tendeloo

Electron Microscopy for Materials Research (EMAT)

University of Antwerp, Groenenborgerlaan 171

2020 Antwerp (Belgium)

[c] Prof. D. P. Debecker

Selective (catalytic) depolymerization of lignin is challenging.

The most important depolymerization methods are pyrolysis,

hydrolysis, hydrogenolysis, liquid-phase reforming, chemical

oxidation, and gasification;^[3b,d,8]these usually yield moderate

amounts of numerous monomeric compounds. Catalytic or

noncatalytic fast pyrolysis of lignocellulose or isolated lignin

Institute of Condensed Matter and Nanoscience

Molecules, Solids and Reactivity (IMCN/MOST)

Universitꢀ catholique de Louvain

Croix du Sud 2 box L7.05.17

1348 Louvain-La-Neuve (Belgium)

Supporting Information for this article is available on the WWW under

http://dx.doi.org/10.1002/cssc.201403375.

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products result in complex bio-oils with reasonable amounts

of 4-alkylated and methoxylated phenols.^[9]Base-catalyzed

lignin depolymerization (BCD) also yields moderate amounts of

phenolics, mainly syringols, guaiacols, and catechols.^[10]Very

promising routes for lignin depolymerization are reductive

methods such as hydrogenolysis and liquid-phase reforming.^[11]

High yields of phenolic monomers, up to 55%, have been re-

ported.^[11a–f]

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 enantioselective

Baeyer–Villiger oxidation of alkyl-substituted CHones to form

enantiopure caprolactones has been demonstrated,^[20]and the

enantioselective ring-opening polymerization of a racemic mix-

ture of alkylated caprolactones by enzymatic catalysis is intri-

guing to synthesize high-value chiral polymers.^[21]

Phenolic compounds derived from lignin possess unique

chemical structure features. The phenolic entity bears one or

two 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 building

blocks has thus far received little attention.^[12]To date, the

main research goals have focused on upgrading lignin-derived

compounds to form hydrocarbon fuels or aromatics by com-

plete removal of oxygen through hydrodeoxygenation

(HDO).^[13]

One route to CHone production from phenol involves hydro-

genation to form CHol, followed by dehydrogenation to give

CHone.^[17]To avoid the thermodynamically undesirable second

step, namely, dehydrogenation of CHol, direct selective hydro-

genation of phenol to form CHone has been suggested as an

alternative. Excellent results for this reaction are reported with

palladium-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 not

compatible with the harsh conditions required to remove the

methoxy groups in guaiacol. Herein, we therefore consider

a two-step process to be more realistic, in which the selective

formation of alkylcyclohexanol is considered first, followed by

a mild dehydrogenation to form the corresponding ketone.

Whereas dehydrogenation of alcohols to form ketones is

well established industrially, fast and selective removal of the

methoxy substituent (demethoxylation) and hydrogenation of

the aromatic ring, while retaining the hydroxyl and 4-alkyl

groups of the resulting CHol, is challenging. Although the in-

termediate formation of CHol is reported many times in HDO

mechanistic studies of guaiacol,^[13e,l,23]Nakagawa et al. were

the first to demonstrate the deliberate production of CHol

from guaiacol in the aqueous phase by using a combination of

Encouraged by interest in bioderived chemicals, we investi-

gated the conversion of a family of lignin-derived 4-alkylated

guaiacols, for example, derived from softwood, such as pine

trees, 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 (see

Scheme 1).^[17]Adipic acid is one of the most produced com-

modity chemicals worldwide and the search for sustainable

synthesis routes was recently summarized by Van de Vyver and

Roman-Leshkov.^[18]Although not yet studied in detail, the pres-

ence of alkyl groups in caprolactone, caprolactam, and adipic

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 the

formation of cyclohexane by avoiding unselective CꢀO dissoci-

ation by ruthenium metal.

lysts were synthesized by incipient wetness impregnation by

using nickel nitrate to obtain a metal loading of 3 wt%, fol-

lowed by reduction under H₂flow at 5008C. The exact nickel

loadings, as measured by inductively coupled plasma atomic

emission spectroscopy (ICP-AES), and the specific surface areas

are indicated in Table S1 in the Supporting Information. The

exact nickel loadings vary from 2.5 to 3 wt%. All catalysts have

a specific surface area of at least 40 m²g^ꢀ1. Oxide supports

with varying surface properties, namely, g-Al₂O₃, SiO₂, Alfa ZrO₂,

TiO₂(P25), CeO₂, and MgO, were selected to probe different

acid/base surface properties.

We studied the selective formation of 4-alkylcyclohexanols

from several lignin-derived guaiacol monomers because they

will appear in a future lignocellulose biorefinery through soft-

wood hydroprocessing (Scheme 1) with a liquid-phase catalytic

process. An inert high-boiling alkane solvent (hexadecane) was

used because this solvent 1) shows good solubility for the 4-al-

kylguaiacols and the corresponding CHols, 2) has a low vapor

pressure for the reaction at relatively low (hydrogen) pressure,

and 3) allows subsequent CHol dehydrogenation in the con-

densed phase.

Plots of guaiacol conversion, product yield, and selectivity

versus contact time are shown in Figures S1–S5 in the Support-

ing Information for all catalysts, except for Ni/MgO. In addition

to CHol, moderate amounts of CHone are also present in the

reaction mixture in thermodynamic equilibrium with CHol. The

CHol to CHone molar ratio is typically between 8 and 10 for all

catalysts at maximum CHol yield (Table 1). In the rest of the

discussion, CHol represents the sum of CHol and CHone.

Additionally, rather than using noble metals, we purposely

aimed to find an appropriate, cheap, nickel-based catalyst.

Industry is familiar with the use of supported nickel catalysts. It

is reported that the redox property of nickel, as well as the sta-

bility of the CHols, is influenced significantly by the acid/base

properties of the support.^[23a,24]

After selecting the best nickel

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

catalyst in terms of support

properties, the reaction network

of the demethoxylation of 4-al-

kylated guaiacols was investigat-

ed to identify the most relevant

reaction steps to determine the

activity and selectivity. Tempera-

ture and nickel content were

verified in light of the productiv-

ity of the catalytic reaction.

Although guaiacol is initially

used as a model substrate for

simplicity in terms of pathways

and products, it will be shown

that 4-alkylguaiacols react simi-

larly to guaiacol, and also signifi-

cantly yield 4-alkylcyclohexanols.

Entry Catalyst Contact time

Conversion

] [%]

Yield^[b][%]

CB^[c]

ꢀ1

[hg_cat.g_guaiacol

CHol 2-MeOCHol cycloalkanes+aromatics dimers others [%]

1

Ni/Al₂O₃0.033

72

100

86

100

16

47

100

14

29

40

75

78

0

14

81

11

53

5

9

0

8

2

3

29

10

25

11

8

0

9

10

1

0

1

2

0

3

2

10

5

12

4

3

4

10

3

2

93

97

96

97

98

88

85

98

2

3

4

5

Ni/SiO₂0.4

Ni/TiO₂0.2

Ni/ZrO₂0.2

Ni/CeO₂0.8

Ni/MgO 0.4

Ni/CeO₂0.4

6

7^[d]

8^[e]

9^[e,f]Ni/CeO₂0.087

[a] Reaction conditions: guaiacol (0.5 g, 4 mmol), decane as internal standard (0.1 g, 0.7 mmol), hexadecane sol-

vent (20 mL), 3008C, 4 MPa H₂. The nominal metal loading of the Ni/oxide catalysts is 3 wt% and the reduction

temperature is 5008C. [b] 2-MeOCHol=2-methoxycyclohexanol. [c] CB=carbon balance, see the Experimental

Section for a definition. [d] Catalyst reduced at 6258C. [e] Catalyst reduced at 3008C. [f] 12 wt% Ni loading.

As a proof of concept, the best catalyst will be used to convert

a real 4-n-propylguaiacol-rich feedstock, which is obtained by

softwood hydroprocessing. As a proof-of-principle, the thus-

produced solution of 4-n-propylcyclohexanol is converted,

with a traditional copper catalyst, into the corresponding cyclic

ketones, 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 feasibility

of using a multistep process to convert the lignocellulosics

into polymer building blocks, as depicted in Scheme 1.

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 chemical

transformations. In short, demethoxylation combined with ring

hydrogenation ultimately leads to CHol, which is the product

of interest in this study, but many other products may be

formed as well. Methylcyclopentane, for instance, is derived

from acid-catalyzed cyclohexane isomerization^[25]or from HDO

of 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 to

yield catechol and 1,2-cyclohexanediol.^[13e,f,23a]Catechol and

1,2-cyclohexanediol further react to form CHol, but they also

lead to cyclopentanol/one formation. Benzene is either formed

through direct hydrogenolysis of phenol,^[13h]but under the re-

action conditions it is most likely to originate from dehydration

Results and Discussion

Screening of nickel catalysts for the conversion of guaiacol

into CHol in hexadecane

A range of nickel-supported oxide catalysts were tested in the

conversion of guaiacol at 3008C and 4 MPa H₂pressure in hex-

adecane, in an effort to obtain a high yield of CHol. The cata-

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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 from

the eliminated methoxy groups.^[13h]The dimeric compounds

are suggested to be formed by alkylation reactions between

phenol, CHol, and cyclohexene on acid sites,^[26]or by aldol con-

densation of CHone,^[15a]followed by further hydrogenation and

hydrodeoxygenation.

primary products in the reaction network, whereas CHol is es-

sentially a secondary product, which is in agreement with

Scheme 2.

For Ni/g-Al₂O₃, degradation of CHol occurs considerably and

shows rapid formation of cycloalkanes (Figure S1b in the Sup-

porting Information); the selectivity to CHol quickly decreases

with increasing conversion, which ultimately leads to a low

maximum CHol yield (Table 1, entry 1). For instance, after a con-

tact time of 0.4 hg_cat.g_guaiacol^ꢀ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%).

The formation of CHol from guaiacol proceeds through two

competitive pathways: one involves guaiacol hydrogenation to

form 2-MeOCHol, followed by demethoxylation to give CHol

Ni/SiO₂, which is a catalyst with milder Brønsted

acidity, 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 S2b in the Supporting Information). Other main

products are cyclopentanemethanol (2%) and me-

thoxycyclohexane (2%) (Table 1, entry 2).

Ni/TiO₂shows a high initial selectivity for CHol, but

the maximum CHol yield is limited to 40% (see

Table 1, entry 3) due to a rapid formation of cycloal-

kanes (Figure S3b in the Supporting Information). Ni/

TiO₂also shows intermediate formation of cyclopen-

Scheme 2. Simplified reaction network for the conversion of alkylated guaiacols to the

corresponding CHols. The arrow width is an indication of the reaction rate.

tanemethanol (5%), 1,2-dimethoxybenzene (2%), and

anisol (2%), but these compounds finally lead to the

formation of cycloalkanes and aromatics with longer

contact times. In our series, Ni/TiO₂was the only cata-

(Scheme 2, Pathway I), and the other encompasses guaiacol

demethoxylation to give phenol, followed by its hydrogenation

to 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 by

direct metal-catalyzed hydrogenolysis.

lyst that formed aromatics (mainly benzene) in significant

amounts.

Ni/ZrO₂and Ni/CeO₂show the highest CHol yields of 75 and

78%, respectively (Table 1, entries 4 and 5). The plots show

a constantly high CHol selectivity, which increases with conver-

sion at the expense of 2-MeOCHol conversion (Figures S4b

and S5b in the Supporting Information). Ni/ZrO₂and Ni/CeO₂

only show considerable formation of byproducts in terms of

cycloalkanes and dimerics at long reaction times; 2-cyclohexyl-

cyclohexanol is the main dimer product.

The catalytic results of a set of nickel catalysts are presented

in Table 1, entries 1–6. The maximum CHol yield increases in

the 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/TiO₂P25,

Ni/ZrO₂, and Ni/CeO₂possess acid sites of both medium and

high strength, whereas Ni/SiO₂is only slightly acidic, and Ni/g-

Al₂O₃, Ni/ZrO₂, and Ni/CeO₂also exhibit basicity (amphoteric

oxides).^[24,27]Ni/MgO contains basic sites.^[27c,d]

The basic Ni/MgO catalyst was also tested (Table 1, entries 6

and 7). It was recently demonstrated that HDO of guaiacol

with Ru/C in water gave a high CHol yield in the presence of

base.^[23a]The high selectivity was explained by the pronounced

preference for Pathway II (Scheme 2, phenol route) and the

prevention of CHol degradation to form cyclohexane. In our

hands, Ni on MgO showed a very low guaiacol conversion and

the main products were 1,2-dimethoxybenzene and catechol

(Table 1, entry 6). The low activity of Ni/MgO is probably due

a low reducibility of Ni on basic supports,^[28]and therefore, the

Ni/MgO precursor was reduced at 6258C instead of the stan-

dard 5008C in an additional experiment. More reduced Ni/

MgO was indeed more active, showing guaiacol conversion of

47% (instead of 16% for standard Ni/MgO). Despite this con-

version increase, Ni/MgO was the least active Ni catalyst of the

screening test, and its mass balance showed a considerable de-

ficiency in contrast to an almost closed balance for the other

catalysts. The main byproducts of Ni/MgO are 2-MeOCHol and

Plots of product selectivity versus contact time (Figures S1–

S5 in the Supporting Information) show that the selectivity for

CHol initially increases in the presence of Ni/g-Al₂O₃, Ni/SiO₂,

Ni/TiO₂, Ni/ZrO₂, and Ni/CeO₂, but it decreases with contact

time at the expense of cycloalkane, aromatic, and dimer forma-

tion. The formation of 2-MeOCHol is also observed, especially

in the initial stage of the reaction, but the product disappears

after full guaiacol conversion, which shows that demethoxyla-

tion of 2-MeOCHol (step b in Pathway I, Scheme 2) is a slow

step. Pathway II, which proceeds via phenol, is also active, but

phenol concentrations are always low and decrease with con-

tinuing conversion. Thus, 2-MeOCHol and phenol are clearly

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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 with

a recent study by Shen et al., who found a similar low activity

for the hydrogenation of aromatics.^[29]

ings ranging from 1 to 12 wt% were synthesized and tested in

the conversion of guaiacol at 2508C. The amount of active Ni

sites and the Ni dispersions were determined by H₂chemisorp-

tion. For all catalysts, the turnover frequency (TOF) of guaiacol,

which is defined as the amount of guaiacol converted per

active nickel site per hour, was determined at low conversion

(<50%), and a clear increase was observed with increasing

To conclude on the support effects of guaiacol conversion

over Ni, classic base supports such as MgO show low activity,

whereas acid supports need to be avoided because of fast

degradation of CHol, leading to the formation of cycloalkanes.

The use of Ni/ZrO₂and Ni/CeO₂leads to the highest CHol yield

and this is most likely to be due to their amphoteric proper-

ties. Further (physicochemical) research efforts will be directed

towards refining the structure–property relationships that are

at play in this reaction. Because CeO₂resulted in the highest

CHol yield, this support was selected in the following series of

experiments.

The Ni/CeO₂catalyst requires a long contact time to reach

the maximum CHol yield, that is, a value of 78%, which corre-

sponds to a space–time yield of 0.8 g_CHolg_cat.^ꢀ1h^ꢀ1. Two effects

were investigated to improve the catalytic activity: reduction

temperature prior to use and nickel loading. CeO₂is known to

be a reducible oxide.^[30]Caballero et al. showed that, during

high-temperature reduction (ꢂ5008C) of a Ni/CeO₂catalyst,

partially reduced ceria moieties migrate onto Ni particles and

block active Ni sites.^[31]

Figure 1. Plot of the TOFs of guaiacol, 4-n-propylguaiacol, 2-MeOCHol, and

CHol at low conversion (<50%) versus average nickel particle size for Ni/

CeO₂catalysts with Ni loadings varying from 1 to 12 wt%. The nickel particle

sizes are determined by H₂chemisorption. The TOFs of guaiacol and 4-n-

propylguaiacol are assessed at 2508C and the TOFs of 2-MeOCHol and CHol

at 3008C. Reaction conditions: substrate (4 mmol), decane (0.7 mmol), hexa-

decane (20 mL), 4 MPa H₂.

Such burial of Ni is avoided by reducing Ni on CeO₂at lower

temperature. By lowering the reduction temperature from the

standard 5008C to 3008C, significantly shorter contact times

were required to reach the maximum CHol yield, thereby dou-

bling the space–time yield to 1.6 g_CHolg_cat.^ꢀ1h^ꢀ1. Moreover, the

maximum yield increased from 78 to 81% (Table 1, entries 5

and 8). Guaiacol conversion, product yield, and selectivity

versus contact time are shown in Figure S6 in the Supporting

Information for Ni/CeO₂reduced at 3008C. From this point on,

the catalyst reduction temperature was set at 3008C for all Ni/

CeO₂catalysts further used in this work.

nickel loading (Figure 1). The nickel sites thus become more

active with a higher nickel loading, which might indicate a par-

ticle size effect, since higher metal loadings typically result in

larger metal particles.^[32]Therefore, the average nickel particle

sizes were derived from the nickel dispersions by assuming

cubic nickel crystals.^[33]Figure S8 in the Supporting Information

shows the Ni dispersion and calculated average particle size

for the different Ni/CeO₂catalysts. Higher nickel loadings give

rise to systematically larger particles, with the average particle

size increasing from 2.5 to 20 nm for nickel loadings going

from 1 to 12 wt%.

The stability of the 3008C reduced Ni/CeO₂catalyst in guaia-

col conversion was evaluated in recycling experiments, without

intermediate reactivation of the catalyst under H₂(Figure S7a

in the Supporting Information). For constant contact times of

0.4 hg_cat.g_guaiacol^ꢀ1, the CHol yield only slightly decreased from

81 to 71% after two cycles reusing the catalyst, which indicat-

ed good material stability. Nickel leaching or sintering could

explain the observed small loss in activity in the recycled runs.

Therefore, these were investigated by ICP-AES analysis of the

reaction solution to determine the amount of solubilized

nickel, and hydrogen chemisorption of the spent catalyst.

A nickel loss of merely 0.12% (relative to the initial amount of

Ni on the catalyst) and a small change in dispersion of 15.5 to

14% were found after one reaction, which suggested that the

observed loss in activity was probably not caused by changes

in the nickel species of the catalyst. Thermogravimetric analysis

(TGA) of the spent catalyst indicated the presence of a small

amount of carbon species (2 wt% of the spent catalyst), which

could block the catalyst pores or nickel active sites; thus caus-

ing the activity loss.

Figure 1 shows the TOF as a function of the average nickel

particle size. Clearly, the TOF increases with increasing size,

which suggests structure-dependent heterogeneous catalysis

for guaiacol conversion.^[32,34]Increasing the nickel loading is

thus a suitable way to increase the catalyst activity in guaiacol

conversion. With the 12 wt% Ni/CeO₂catalyst, the maximum

CHol yield can be reached almost 5 times faster than that with

the 3 wt% Ni catalyst, without compromising the high CHol

yield (Table 1, entries 8 and 9). The corresponding space–time

yield, that is, 7.5 g_CHolg_cat.^ꢀ1h^ꢀ1, is more than 9 times higher

than the space–time yield of the 3 wt% Ni catalyst reduced at

5008C (0.8 g_CHolg_cat.^ꢀ1h^ꢀ1). Similar to the 3 wt% Ni catalyst, the

12 wt% Ni catalyst exhibits very low Ni leaching of only 0.13%

(relative to the initial amount of Ni on the catalyst).

To investigate the effect of Ni metal loading on the conver-

sion rate of guaiacol to CHol, Ni/CeO₂catalysts with Ni load-

The nickel phase of the catalysts was further investigated by

powder XRD, Z-contrast high-angle annular dark-field scanning

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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/CeO₂catalysts

shows 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/CeO₂catalyst

provide evidence for the size and crystal structure of the ceria

nanoparticles (Figure 2c and d). Nickel is detected through use

of both EDX (Figure 2b) and EELS (Figure 2g) elemental map-

ping. A broad Ni(O) particle size distribution is apparent from

the maps. Due to the strong agglomeration of the ceria and

Ni(O) nanoparticles, determination of the degree of oxidation

of the Ni(O) nanoparticles was carried out by using EELS, with

the Ni L_2,3edge as a fingerprint, and by comparing the fine

structure to Ni and NiO references. Extracted EELS spectra dis-

played in Figure 2e provide evidence that the degree of oxida-

tion of the nickel nanoparticles is dependent upon the nano-

particle size. Indeed, the averaged Ni L_2,3EELS edge fine struc-

ture acquired from a larger nanoparticle (ꢁ10 nm in diameter,

blue arrow and spectrum; Figure 2e and g) largely corre-

sponds to the L_2,3fine structure reference for metallic Ni. The

averaged Ni L_2,3fine structure from a small nanoparticle

(ꢁ2 nm in diameter, green spectrum; Figure 2e) matches well

with 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 surface

of the larger nanoparticles is oxidized, whereas the core re-

mains metallic. The EELS data presented above indicate that

the smallest nanoparticles are completely oxidized. Although

a significant fraction of Ni is present in the oxidized state (as

NiO), H₂temperature-programmed reduction (H₂-TPR) of the

12 wt% Ni/CeO₂catalyst (Figure S11 in the Supporting Infor-

mation) demonstrates that reduction occurs below 2008C,

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 selectivity

as a function of the catalyst contact time for the guaiacol reac-

tion at 300 (Figure 3a) and 2508C (Figure 3b). At 3008C, the

major initial products are CHol and 2-MeOCHol. The selectivity

to CHol increases to above 80% at complete guaiacol conver-

sion (after 0.4 hg_cat.g_guaiacol^ꢀ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 2a).

The product evolution in time is somewhat different at

2508C. At the lower temperature, both CHol and 2-MeOCHol

are formed concomitantly with a slight preference for the

former, and the two products remain stable under the reaction

conditions up to 0.4 hg_cat.g_guaiacol^ꢀ1(Figure 2b). Yields of 58 and

39% are measured for CHol and 2-MeOCHol, respectively, at

full 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-MeOCHol

and phenol, respectively, being primary products and CHol

a secondary product. However, phenol was only observed in

minute 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 corresponding

TOFs and product selectivity (at low conversion) at 3008C (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 the

12 wt% Ni/CeO₂sample. The Ni(O) nanoparticles are homogeneously distrib-

uted throughout the ceria material. c) and d) High-resolution HAADF-STEM

images showing the typical size of the ceria nanoparticles. The Fourier trans-

form patterns provide evidence for the cubic fluorite crystal structure of the

ceria material. e) Averaged Ni L_2,3EELS spectra extracted from three differ-

ently sized Ni(O) particles. f) Overview HAADF-STEM image. g) Elemental

EELS map. The arrows indicate the Ni(O) nanoparticles from which the EELS

data in e) were extracted.

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tial molar ratio of CHol plus phenol to 2-MeOCHol is a measure

of 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). The

subsequent conversion of 2-MeOCHol to CHol is possible, but

this reaction is slow and requires a high temperature (above

2508C; cf. Figure 3a and b).

The results in Figure 4a allow a comparison of the product

selectivity for guaiacol conversion at 250 and 3008C at low

conversion. Pathway II in Scheme 2 is preferred at the lower

temperature, but the preference is not exclusive to afford

a high CHol yield. In other words, because Pathway I in

Scheme 2 (with intermediate formation of the unreactive 2-

MeOCHol) remains a competitive route, high CHol yields from

guaiacol are only possible at a sufficiently high reaction tem-

perature.

Conversion of CHol to cycloalkanes and dimers also happens

over 3 wt% Ni/CeO₂, but only after long reaction times (Fig-

ure 3a and Table 2, entry 4). Because this degradation reaction

runs 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 kJmol^ꢀ1) and

CHol (88 kJmol^ꢀ1) in presence of 3 wt% Ni/CeO₂are very simi-

lar, as calculated from the Arrhenius plots shown in Figure 4b,

an increase in reaction temperature will increase both reaction

rates 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 loading

enhanced the catalyst activity in guaiacol to CHol conversion.

The amount of nickel on the catalyst has no notable impact on

the selectivity for Pathways I or II in Scheme 2, since the initial

product selectivity for guaiacol conversion is similar for the dif-

ferent catalysts (Figure S12 in

Figure 3. Product distribution for the conversion of guaiacol over 3 wt% Ni/

CeO₂at 300 (a) and 2508C (b). Reaction conditions: guaiacol (0.5 g, 4 mmol),

decane (0.7 mmol), hexadecane (20 mL), 4 MPa H₂.

the Supporting Information).

Table 2. TOF, conversion, and initial product selectivity for guaiacol, 2-MeOCHol, phenol, and CHol conversion

Furthermore, Figure 1 indicates

that, similar to guaiacol, the

TOFs of 2-MeOCHol and CHol

are enhanced by increasing the

Ni loading and particle size. In

other words, the structure de-

pendency of the reactions rates

holds for all nickel-catalyzed re-

actions. Overall, reaction of

guaiacol to CHol is much faster

with highly loaded nickel cata-

lysts.

over 3 wt% Ni/CeO₂.^[a]

Entry Substrate

TOF^[b]Contact time

[hg_cat.g_substrate

Conversion

[%]

Selectivity^[c][%]

CHol phenol 2-MeOCHol CH dimers CPol others

ꢀ1

]

1

2

3

4

guaiacol

2-MeOCHol

phenol

3262 0.0083

286 0.17

19084 0.0045

183 0.25

27

50

57

36

47

82

96

/

3

0

44

/

1

8

3

0

6

0

5

1

0

CHol

/

45 33

[a] Reaction conditions: substrate (4 mmol), decane (0.7 mmol), hexadecane (20 mL), 3008C, 4 MPa H₂. [b] TOF

is the initial rate of substrate conversion normalized to the accessible Ni atoms [molmol_Ni(surf)^ꢀ1h^ꢀ1]. [c] Abbrevi-

ations: CH=cycloalkanes, CPol=cyclopentanol and cyclopentanone.

was as follows: phenol>guaiacol>2-MeOCHol>CHol. This re-

activity order suggests that guaiacol conversion preferably pro-

Conversion of lignin-derived phenolics

ceeds through demethoxylation, followed by fast phenol hy-

drogenation to CHol (Scheme 2, Pathway II), whereas guaiacol

hydrogenation to 2-MeOCHol is only slightly slower, but the

subsequent 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-

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 of

varying length and types, namely, -methyl, -ethyl, -n-propyl,

and allyl substituents, are more relevant substrates. The reac-

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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 2508C. 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 3008C in Figure 4a. Sim-

ilar to guaiacol conversion, the selectivity for Pathway II in

Scheme 2 is somewhat higher at the lower temperature, but

the difference is again not sufficient to deliver a high yield of

4-n-propylCHol at low temperature. As with guaiacol, a higher

temperature (>2508C) is required to overcome the energetic

demethoxylation of the corresponding 2-MeOCHol and to

afford a high 4-n-propylCHol yield.

Table 3 shows the catalytic results for the conversion of a set

of 4-substituted guaiacols over 3 wt% Ni/CeO₂at 3008C. Yields

to 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, higher

nickel loadings enhance the catalyst activity in the conversion

of an alkylated guaiacol such as 4-n-propylguaiacol (see

Figure 1). The 12 wt% Ni/CeO₂catalyst reached an equally

high 4-n-propylCHol yield to that of the 3 wt% Ni catalyst, but

in 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 g_{4-n-propylCHol}g_cat.^ꢀ1h^ꢀ1).

Figure 4. a) TOF and initial product selectivity (at 20–30% conversion) for

guaiacol and 4-n-propylguaiacol conversion at 250 and 3008C in the pres-

ence of 3 wt% Ni/CeO₂. b) Arrhenius plots (ln(TOF) vs. 1/T) for 2-MeOCHol

and CHol conversion over 3 wt% Ni/CeO₂. Reaction conditions: substrate

(4 mmol), decane (0.7 mmol), hexadecane (20 mL), 4 MPa H₂.

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 per

reactor volume per time (space–time yield on a reactor volume

basis). Therefore, the substrate concentration in the feed

should be as high as possible. To verify the efficiency of the

catalyst 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/CeO₂catalyst resulted in

a 4-n-propylcyclohexanol yield of 81% (Table 3, entry 6), which

is as high as that for a solution with the standard concentra-

tion. The catalyst system is thus able to convert concentrated

product feeds.

To further broaden the substrate scope of the catalyst

system, eugenol (or 4-allylguaiacol) was also tested, since

guaiacol with an unsaturated side chain can also be obtained

from softwood lignin in high yield.^[11e]A similar yield was

reached to that of 4-n-propylCHol; thus showing successful hy-

drogenation of the allyl substituent next to demethoxylation

and ring hydrogenation (Table 3, entry 7). To reach the high

product yield from eugenol, it was essential to perform reactor

heating under a hydrogen instead of a nitrogen atmosphere

because of the high reactivity of the isolated double bond for

side reactions. Heating the reactor under a nitrogen atmos-

phere resulted in a lower 76% yield of 4-n-propylCHol.

Figure 5. TOF and initial product selectivity (at 20–30% conversion) for 4-R-

guaiacol conversion over 3 wt% Ni/CeO₂at 2508C. Reaction conditions: sub-

strate (4 mmol), decane (0.7 mmol), hexadecane (20 mL), 4 MPa H₂.

tivity (as indicated by the TOF values) decreases with increas-

ing alkyl chain length (Figure 5).

Surprisingly, we also noted an isomerization activity of Ni/

CeO₂in the conversion of 4-alkylguaiacols because both 3- and

4-alkylCHol products were detected (see final column of

Table 3). Their detection was only evident by applying a silyla-

tion treatment of the product mixture with 2,2,2-trifluoro-N-

Figure 5 presents the initial product selectivity for 4-alkyl-

guaiacol conversion with 3 wt% Ni/CeO₂at 2508C. The 4-alkyl-

guaiacols react in the same way as guaiacol, namely, through

both Pathways I and II in Scheme 2. Increasing the length of

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Table 3. Conversion and product distribution of reactions with pure 4-alkylguaiacols, eugenol, and a 4-n-propylguaiacol fraction derived from pine saw-

[a]

dust in the presence of Ni/CeO₂

Entry

Substrate

Conversion

[%]

Yield [%]

cycloalkanes+aromatics

CB

[%]

4-R/3-R^[c]

R^[b]-CHol

R^[b]-2-MeOCHol

others

1

2

3

4

5^[d]

6^[e]

7^[f]

8^[g]

9^[f]

10^[h]

guaiacol

4-methylguaiacol

4-ethylguaiacol

4-n-propylguaiacol

eugenol

4-ethylcatechol

4-n-propylguaiacol

4-n-propylguaiacol from pine sawdust

100

81

82

83

82

81

83

55

83

73

2

1

4

2

8

5

0

2

9

10

11

8

12

7

5

11

8

6

4

2

4

3

2

3

37

1

5

98

97

98

99

98

97

96

97

98

/

11

10

13

12

18

1

22

6

12

[a] Reaction conditions: substrate (4 mmol), 3 wt% Ni/CeO₂(0.2 g) decane (0.7 mmol), hexadecane (20 mL), 3008C, 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% substrate

in feed), 4.2 mmol decane, 6.5 MPa H₂, 85 min reaction time. [f] 2 MPa H₂at room temperature, heated to 3008C (ꢁ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 H₂at room temperature,

heated to 3008C (ꢁ40 min, 3.6 MPa), 4 MPa H₂, 200 min reaction time.

methyl-N-trimethylsilylacetamide prior to GC analysis.^[35]The

gas chromatograms of the silylated product mixtures of 4-eth-

ylguaiacol and 4-n-propylguaiacol are illustrated in Figure S13a

and b, respectively, in the Supporting Information. The 3- and

4-isomers are present as both cis and trans stereoisomers. Con-

version of the isomers over Ni/CeO₂proceeds fairly regioselec-

tively with a preference for 4-alkylCHol isomer formation; the

4- to 3-isomer molar ratios range from 10 to 13 (Table 3, en-

tries 1–6).

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 transparent

solution rich in 4-n-propylguaiacol (see Figure S14 in the Sup-

porting Information), with the amount of 4-n-propylguaiacol

corresponding to about 12 wt% of the original lignin content

of the pine sawdust. The gas chromatogram of the hexade-

cane fraction is shown in Figure 6a. The gas chromatogram of

the 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 subjected

to a catalytic reaction in the presence of 3 wt% Ni/CeO₂at

3008C, resulting in a n-propylCHol yield of 73% (Table 3,

entry 10). The corresponding gas chromatogram is indicated in

Figure 6b. 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 crude

lignin-derived 4-n-propylguaiacol required a higher contact

time to reach full conversion, compared with that for pure 4-n-

propylguaiacol; this was likely to be due to the presence of

minor compounds with inhibiting properties.

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 of

guaiacols to the corresponding CHol products. For instance,

the conversion of 4-ethylcatechol over Ni/CeO₂leads to an

equimolar mixture of 4- and 3-ethylCHol, whereas this ratio is

about 11 for the conversion of 4-ethylguaiacol (Table 3, en-

tries 3 and 8). Moreover, the conversion of 4-ethylcatechol also

results in high yields of 4-ethyl-1,2-cyclohexanediol (15%) and

3-ethylcyclopentanol/one (21%), whereas these compounds

were 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 S13c in the Support-

ing Information.

The isomerization activity of Ni/CeO₂can 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 22

at full conversion, whereas the value of the ratio was 10 when

reactor 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 the

origin of the peculiar isomerization activity of Ni/CeO₂.

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 can

be dehydrogenated easily to 4-alkylCHone in accordance with

the proposal in Scheme 1. We therefore performed two catalyt-

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alkylphenol, dimer compounds,

and cycloalkanes. Figure S16a

and b in the Supporting Infor-

mation shows the product distri-

butions for 4-ethylCHol and 4-n-

propylCHol, respectively, during

the dehydrogenation process.

Also, real lignin-derived product

mixtures of 4-alkylCHol, derived

from the corresponding 4-alkyl-

guaiacols, could be converted

into the ketones. To demonstrate

the subsequent dehydrogena-

tion step with Cu/ZrO₂, the

crude reaction mixture (Table 3,

entry 9) was dehydrogenated to

a final yield of 81% n-propyl-

CHone relative to the amount of

n-propylCHol in the reaction

mixture (Table 4, entry 3). Dehy-

drogenation of the lignin-de-

rived n-propylCHol-rich product

mixture (Table 3, entry 10) was

successfully performed, yielding

79% n-propylCHone relative to

the amount of n-propylCHol in

the reaction mixture (Table 4,

entry 4). The gas chromatogram

of the raw product mixture is

shown in Figure 6c. 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 branched

caprolactones

Finally, this section provides evi-

dence that alkylated e-caprolac-

tone can be produced from the

alkylCHone product mixture ob-

tained from softwood lignin. The

conversion of CHone to e-capro-

lactone

proceeds

through

Baeyer–Villiger oxidation; a reac-

tion for which the Lewis acidic

tin-containing beta zeolite is

known to be a suitable cata-

lyst.^[36]First, the n-propylCHone

mixture obtained from 4-n-pro-

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) the

dehydrogenation product indicated in Table 4, entry 4.

ic reactions: one with 4-ethylCHol and the other with 4-n-pro-

pylCHol in the presence of Cu/ZrO₂, which is a catalyst known

for its dehydrogenation activity.^[15b]

pylguaiacol (Table 4, entry 3) was subjected to Baeyer–Villiger

oxidation at 908C with a tin-containing beta zeolite catalyst

developed in our laboratory^[37]and H₂O₂as the oxidizing

agent. After 8 h, 89% of n-propylCHone was converted with

a selectivity of 92% towards n-propyl-e-caprolactone. The n-

propylCHone product obtained from the lignin-derived 4-n-

High yields of the corresponding ketones, namely, 78% of 4-

ethylCHone and 81% of 4-n-propylCHone, were obtained at

2508C (Table 4, entries 1 and 2). The main side products are 4-

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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

1

2

4-n-propylcyclohexanol

product from 4-n-propylguaiacol conversion (Table 3, entry 9)

product from pine lignin-derived 4-n-propylguaiacol conversion (Table 3, entry 10) 79 (58)

81

11

11 (9)

13 (10) 3 (2)

1

2 (2)

5

3 (2)

1

(11)

(9)

3^[c]

4^[e]

81 (67)^[d]

[a] Reaction conditions: 4-alkylCHol (1 mmol), 3 wt% Cu/ZrO₂(0.025 g), decane (0.2 mmol), hexadecane (20 mL), 2508C, 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 distribution

relative to the amount of 4-n-propylguaiacol in the original product solution. [e] The product solution from Table 3, entry 10.

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.

CeO₂) with sufficient demethoxylation activity for 2-MeOCHol,

without compromising CHol selectivity.

Ni/CeO₂was also able to convert a real lignin-derived feed

rich in 4-n-propylguaiacol, obtained by hydroprocessing of

softwood, into 4-n-propylcyclohexanol. The CHol product

could effectively be dehydrogenated into the corresponding

ketone with a Cu/ZrO₂catalyst. Baeyer–Villiger oxidation of the

cyclic ketone with H₂O₂in the presence of tin-containing beta

zeolite yielded the desired 4-n-propyl-e-caprolactone.

Conclusions

The potential of nickel supported on oxide catalysts in the se-

lective conversion of a range of lignin-derived guaiacols into

the corresponding cyclohexanols in a liquid-phase alkane was

demonstrated, and resulted in yields above 80%.

Experimental Section

The oxide support should be selected carefully because

CHol was not stable in the presence of Ni on an acidic oxide

such as g-Al₂O₃, and Ni on a base oxide such as MgO exhibits

very low activity. Amphoteric supports such as ZrO₂and CeO₂

were the most suitable supports. A detailed physicochemical

study of these catalysts will further elaborate understanding of

the catalytic process.

Chemicals and materials

All commercial chemicals were analytical reagents and were used

without 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, Aerosil

SiO₂(380 m²g^ꢀ1), Aeroxide TiO₂P25 (50 m²g^ꢀ1), Ru on carbon

(5 wt%), methanol (99.9+%), and ethylcyclohexane (99+%) were

purchased from Sigma Aldrich. Phenol (99.5+%), 4-ethylguaiacol

(98%), pyridine (99+%), Cu(NO₃)₂·3H₂O, MgO (130 m²g^ꢀ1), and

H₂O₂(50 wt% in water, stabilized) were purchased from Acros Or-

ganics. Ni(NO₃)₂·6H₂O, 2-MeOCHol (99%), CeO₂(63 m²g^ꢀ1), and

ZrO₂(90 m²g^ꢀ1) were purchased from Alfa Aesar. 4-Ethylcyclohexa-

nol (97+%) and 4-n-propylcyclohexanol (98+%) were purchased

from TCI chemicals. Puralox Al₂O₃(150 m²g^ꢀ1) was purchased from

Condea Chemie. 1,4-Dioxane (99+%) was purchased from Lab-

Scan.

The kinetics of Ni on CeO₂were studied in more detail with

respect to the catalyst properties. The overall reaction rate was

increased by lowering the catalyst reduction temperature and

by increasing the nickel loading. The average nickel particle

size increased with increasing nickel loading, and this resulted

in 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^ꢀ1were obtained.

Guaiacol and alkylated guaiacol conversion to the corre-

sponding CHols proceeded over Ni/CeO₂through two parallel

pathways with similar rates; the resultant 2-MeOCHols and al-

kylated phenols were the primary products. Both intermediates

were converted into the corresponding CHols; the phenol

compound was more reactive. Only with prolonged reaction

times did the CHols further react to form cycloalkanes and

dimer compounds.

Catalyst synthesis

The nickel-supported catalysts were synthesized by incipient wet-

ness impregnation of the oxide with an aqueous solution of

Ni(NO₃)₂·6H₂O. After drying at 608C for 24 h, the samples were re-

duced at 300 or 5008C (heating rate 58Cmin^ꢀ1) for 1 h under

a flow of H₂(120 mLmin^ꢀ1g^ꢀ1). The Cu/ZrO₂catalyst was synthe-

sized by incipient wetness impregnation of ZrO₂with an aqueous

solution of Cu(NO₃)₂·3H₂O. The sample was calcined at 5008C

Fast and selective hydrogenolysis of the methoxy group (de-

methoxylation) was the major challenge to convert (alkylated)

guaiacol. Two strategies were therefore suggested for high

CHol yields from the alkylated guaiacols: 1) the utilization of

a 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

(heating rate 58Cmin^ꢀ1

)

for 1 h under

a

flow of O₂

(120 mLmin^ꢀ1g^ꢀ1) and reduced at 5008C (heating rate 58Cmin^ꢀ1

)

for 1 h under a flow of H₂(120 mLmin^ꢀ1g^ꢀ1). The tin-containing

beta zeolite catalyst was synthesized as described by Dijkmans

et al.^[37]

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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 intervals

through a rubber septum for analysis.

Catalytic reactions

Catalytic reactions were performed in batch mode in a 50 mL Parr

autoclave equipped with a mechanical stirrer, a liquid sampling

tube, gas inlet and outlet tubes, a thermocouple, a rupture disc,

and an electric heating jacket. The autoclave was loaded with 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). After

flushing the reaction mixture with N₂(3 times), the temperature

was raised to typically 3008C (ꢁ40 min heating time) and the au-

toclave was put under constant H₂pressure of 4 MPa at the reac-

tion temperature. The stirrer speed was set at 750 rpm. After the

reaction, the autoclave was rapidly quenched to room temperature

in a water bath. If the autoclave was put under an initial H₂pres-

sure of 2 MPa, the pressure increased up to 3.6 MPa at 3008C and

the autoclave was subsequently put under constant H₂pressure of

4 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 oven

at 1008C. A small loss of solid catalyst particles occurred during

the washing and filtration steps, and approximately 90–95% of the

catalyst weight was recovered after each catalyst recycling. In one

set of recycling experiments, only the recycled catalyst was used

and the reaction time was elongated to reach a contact time of

0.4 hg_cat.g_guaiacol^ꢀ1. In a second set of recycling experiments, the

amount of catalyst was kept constant and 5–10% of fresh catalyst

was added in each run. Pine sawdust hydrogenolysis was per-

formed in a 100 mL Batch autoclave. The autoclave was loaded

with 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 reactor

was put under 3 MPa H₂and the temperature was raised to 2508C

(ꢁ25 min heating time). The stirrer speed was set at 750 rpm. The

reaction was performed at 2508C for 3 h. After quenching the au-

toclave to room temperature in a water bath, the reaction mixture

was collected and the autoclave was washed with methanol. The

washing solution was combined with the reaction mixture and fil-

tered over a glass filter to remove the catalyst and pine sawdust

residue. Methanol was removed from the liquid phase in a rotary

evaporator and the resulting oil phase was extracted with water

(2ꢁ2 mL) to remove the sugar components present in the oil. By

means 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 at

808C for 30 min. Hexadecane was removed by decantation. The

procedure was repeated four times. The hexadecane samples were

combined and this solution was used for further reaction. Dehydro-

genation reactions were performed in a 50 mL batch autoclave by

loading 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 temperature

to 2508C (ꢁ30 min heating time). After 15 min at 2508C, the auto-

clave was quenched to room temperature in a water bath and an

aliquot 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 crude

reaction mixture obtained after 4-n-propylguaiacol conversion, the

reaction mixture (5 mL) was combined with hexadecane (15 mL).

The Baeyer–Villiger oxidations were conducted in magnetically

stirred glass reactor vials placed in a temperature-controlled

copper block at 908C. The dehydrogenation mixture (1.5 mL) was

mixed with 1,4-dioxane (6 mL) and tin-containing beta zeolite cata-

lyst (25 mg) was added. A 50 wt% aqueous solution of H₂O₂was

added in a molar H₂O₂/ketone ratio of 2. To obtain the maximum

product yield, an additional aliquot of the oxidant solution was

Product analysis

Quantitative analysis of the reaction samples was performed by

gas–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, and

a flame ionization detector; and 2) a Hewlett Packard 5890 GC

equipped with a CPsil-5 column, a HP 6890 autosampler, a split in-

jection system, and a flame ionization detector. The second gas

chromatograph was used for the separation of benzene and cyclo-

hexane. The following operation conditions were applied to both

GC instruments: injection temperature: 2508C, column tempera-

ture: 458C (6 min), 108Cmin^ꢀ1to 2808C (5 min), detector tempera-

ture (3008C). Sensitivity factors of the reagents and products were

obtained by calibration with commercial standards or obtained by

ECN-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:

n_substrate;0ꢀ n_substrate

Conversionð%Þ ¼

ꢃ 100%

ð1Þ

ð2Þ

ð3Þ

n_substrate;0

a ꢃ n_product

Yieldð%Þ ¼

ꢃ 100%

n_substrate;0

Yieldð%Þ

Selectivityð%Þ ¼

Conversionð%Þ

P

n_substrate

þ

n_product

n_substrate;0

ð4Þ

CBð%Þ ¼

ꢃ 100%

in which n_substrate,0, n_substrate, and n_productrepresented the amount

(mol) of substrate at the start of the reaction, and the amount of

substrate and product after the reaction, respectively; a equaled

one for all products, except the dimer compounds, for which it

equaled two. The product distribution indicated in Table 4 was de-

fined in the same way as that of the yield. Qualitative analysis of

the reaction products was performed by GC-MS by using an Agi-

lent 6890 series GC instrument equipped with a HP5MS capillary

column and an Agilent 5973 series mass spectrometry detector.

The following operation conditions were applied to the GC-MS in-

strument: 608C (5 min), 108Cmin^ꢀ1to 2808C (5 min), detector tem-

perature (3008C). Silylation of the reaction mixture was performed

by adding the reaction mixture (0.5 mL), pyridine (0.5 mL), and N-

methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA; 0.25 mL) to

a GC vial and putting the vial in an oven at 808C for 30 min. Iden-

tification and quantification of the silylated products was per-

formed by using the same GC analysis as that described for the

typical reactions.

Catalyst characterization

Quantitative elemental analysis of the Ni loading was performed

by 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(3ꢁ3 mL),

which was subsequently analyzed by ICP-AES. BET specific surface

&

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Full Papers

areas (S_BET) of the catalysts were calculated from N₂physisorption

data 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 atoms

was determined by hydrogen chemisorption measurements. The

catalyst was activated at 3008C for 1 h in a flow of H₂and evacuat-

ed at 3008C for 1 h in vacuum (<5 mm Hg; 1 mmHg=133.3 Pa). It

was then flushed for 1 h in a flow of He, cooled to 358C, and evac-

uated again for 1 h (<5 mmHg). H₂adsorption (chemisorption and

physisorption) was measured at 358C over the pressure range

from 0.5 to 415 mmHg. Next, physisorbed H₂was removed by out-

gassing the sample for 2 h at 358C (<5 mmHg), and another iso-

therm (physisorption) was measured. The difference between the

two isotherms gave the H₂-chemisorption isotherm. The concentra-

tion of chemisorbed hydrogen on the metal was determined by

extrapolating the differential isotherm to zero H₂pressure, and this

value was used to calculate the Ni dispersion. By assuming cubic

crystals, the average nickel crystallite size was calculated by using

Equation (5), wherein S_Niwas the Ni metal surface area in m²g^ꢀ1

and 1_Niwas the metal density (8.908 gm^ꢀ3).^[33]

Keywords: biomass · heterogeneous catalysis · lignin · nickel ·

synthesis design

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d ¼ 5=ðS_Ni1_NiÞ

ð5Þ

HAADF-STEM, STEM-EDX mapping, and EELS analyses were per-

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operated at 300 kV. The first was equipped with a Super-X high

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energy resolution for EELS. For imaging, the convergence semian-

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Received: December 5, 2014

Published online on && &&, 0000

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ChemSusChem 0000, 00, 0 – 0

www.chemsuschem.org

14

ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

FULL PAPERS

In the Ni-ck of time: The four-step con-

version of lignocellulose to caprolac-

tones is demonstrated. One of the

major challenges is the selective remov-

al of the methoxy group from the

guaiacols. Ni/CeO₂is a suitable catalyst

for this reaction, yielding considerable

amounts of cyclohexanols, which are

prone to undergo dehydrogenation and

Baeyer–Villager oxidation.

W. Schutyser, S. Van den Bosch,

J. Dijkmans, S. Turner, M. Meledina,

G. Van Tendeloo, D. P. Debecker, B. F. Sels*

&& – &&

Selective Nickel-Catalyzed Conversion

of Model and Lignin-Derived Phenolic

Compounds to Cyclohexanone-Based

Polymer Building Blocks

ChemSusChem 0000, 00, 0 – 0

www.chemsuschem.org

15

ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

These are not the final page numbers! ÞÞ

Article Doi

Selective nickel-catalyzed conversion of model and lignin-derived phenolic compounds to cyclohexanone-based polymer building blocks

DOI: 10.1002/cssc.201403375

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Full text of DOI:10.1002/cssc.201403375

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