Modifying MgO with Carbon for valorization of lignin to aromatics

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Modifying MgO with Carbon for valorization of lignin to aromatics Wei Lv, Yuting Zhu, Jing Liu, Chenguang Wang, Ying Xu, Qi Zhang, Guanyi Chen, and Longlong Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05237 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Sustainable Chemistry & Engineering

Modifying MgO with Carbon for valorization of lignin to aromatics

promising way to provide aromatic chemicals and recently has drawn great attention but still remains a challenge due to its complex linkages through relatively stable C-O and stubborn C-C bonds.4-5 The C-O bond takes up two-thirds of the all linkages and the β-O-4 ether bond accounts for

Wei Lv a, b, Yuting Zhu b, Jing Liu b, Chenguang Wang b, b

b

a*

Ying Xu , Qi Zhang , Guanyi Chen , Longlong Ma

＞50% of the monomer linkages in lignin. In general, C-O

a, b *

bonds are observably weaker and more unstable than C-C a

School of environmental science and engineering, Tianjin University,

bonds.6-8 Therefore, selective hydrolysis / hydrogenolysis

No.92 Weijin Rd, Nankai District, Tianjin 300000, P.R. (China)

cleavage of the C-O bond in β-O-4 linkage is a dominant

b

Guangzhou Institute of Energy Conversion, Chinese Academy of

target in lignin valorization to aromatics, which greatly

Sciences; CAS Key Laboratory of Renewable Energy; Guangdong

depends on the metal activity and the acid-base properties of catalytic metalsites.3-4, 9-10

Provincial Key Laboratory of New and Renewable Energy Research

Basic-catalyzed

and Development. No.2 nengyuan Rd, Tianhe District, Guangzhou

hydrolysis

of

lignin

is

one

exceptional route for the production of simple aromatic

510640 (China)

chemicals under mild conditions. The cleavage of ether Corresponding author: [email protected], [email protected]

linkages is a dominant reaction in alkaline delignification processes.11-12 The concentration and the nature of the base

We developed a Pd-doped catalyst that the support

are both of the most important factors for high selectivity

MgO was modified with carbon species via methanol

and yield of the products.12-14 Generally, the stronger base,

thermal annealing, catalyzing the cleavage of β-O-4

the higher conversion is given since the polarization of the

lignin-type dimers and actual lignin into aromatics.

base governs the kinetics and the mechanism of the

Methanol as a carbon precursor was processed to carbon

depolymerization reaction.2,

species that loaded metal palladium. The carbon species

base-catalyzed lignin hydrolysis reaction, in acid-

supported Pd on MgO surface changed the acid-base

catalyzed delignification of lignocellulose biomass, the

strength and affected the catalytic activity of C-O bond

hydrolytic cleavages of ether linkages also play a

hydrolysis / hydrogenolysis. The β-O-4 linkage in dimer

dominant role because of the stubborn linkages of lignin

was effectively fractured over Pd/MgO-C, and catalyst’s

units.2, 16 Further, in HDO case, the acid sites are required

strong acid facilitated aldehyde decarboxylation to alkyl

in the HDO mechanism.17

aromatics while relatively weak acid and strong base were

15

Like the behavior of the

Metal Pd possesses outstanding catalytic properties

highly selective for stable aromatics (such as 1-methoxy-

for valorization of lignin to aromatics, and by now,

4-propylbenzene and 1-methoxy-4-(proplenlyl) benzene). considerable attention still being paid to increasing the Allowing Pd/MgO-C-700 to be successfully applied to activity of Pd for lignin C-O bond cleavage via modulation

pine depolymerization, to give 24.6 wt% of aromatics and

of the electron structure, combine with other metals or/and

almost β-O-4 linkages were broken at 160 ℃ , and

acid/base regants.18 Lots of supports were used to tune the

surprisingly, 77.2% of 2-methoxy-4-(proplenlyl)phenols

reactivity via metal-support interaction.19 For instance,

was obtained. The reaction pathways for lignin

under mild hydrogenolysis conditions, the lignin from pine

valorization are hypothesized over Pd/MgO-C catalyst.

and olive tree was degraded into monomeric, dimeric and oligomers over Al-SBA-15 supported Ni, Pd, Pt, Ru, and

Introduction

Pd/C via selective cleavage of the aryl-O-aliphatic and Lignin is an important aromatic biopolymer with

aryl-O-aryl linkages. 20-21 Doping Pd with other atoms is

highly cross-linked polymer that comprise up to 30% of

another efficient method. Yan et al. prepared the bimetallic

the weight and more than 40% of the energy content of

PdNi catalyst for lignin model hydrogenolysis.

lignocellulosic biomass.

1-3

The valorization of lignin is a

Due to the extremely multifunctional groups of the

1

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Materials

lignin substrate, it is hard to valorized lignin by 1~2 kinds of reactions to realize. Integrated hydrogen-processing of

Ethyl

lignin and model compounds, including upgrading of pyrolysis

oils,

involves

hydrogenolysis,

(EtOAc,

99%),

methanol,

Fe(NO3)3.9H2O, citric acid, nano MgO (primary particles

HDO,

d ≈10~30nm, BET area 135 m2/g) and PdCl2 were

hydrogenation and transalkylation (hydroalkylation) etc.2, 7, 18

acetate

analytical grade and were provided from Shanghai

Thus, multifunctional catalyst is necessary to be

Aladdin biochemical technology co., LTD (Shanghai,

developed for addressing several types of reactions during

China) and used as received. The lignin linkage (β-O-4)

lignin valorization. Bifunctional catalysts containing both

models

metal and acid components, were employed to get rid of

of

3-Hydroxy-2-(2-methoxyphenoxy)-1-(4-

methoxyphenyl) propan-1-one (95 wt %) and 1-(4-

the deactivation problem caused by the conventional

hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)

sulfide-based HDO catalysts. Kou and Lercher et al.

propane-1, 3-diol (97 wt %) were purchased (from

reported a functional catalyst that combined Pd/C, Pt/C,

Shanghai baishun biotechnology co. LTD., China).

Ru/C, or Rh/C with phosphoric acid to catalyze the HDO

Preparation of MgO-C samples

of phenolic components into cycloalkanes and methanol.22 During the reaction, metal-catalyzed hydrogenation and

Carbon was grown on MgO that MgO supported Fe,

acid-catalyzed hydrolysis/dehydration were supposed to

impregnating appropriate amounts of ferric nitrate. A

couple together, which differs from the mechanism for

detailed description of this nanohybrids interface catalysts

22-23

A systematic kinetic study revealed

were produced as follows. For example, 0.364g

that the dual catalytic functions was indispensable and that

Fe(NO3)3.9H2O, 0.173g citric acid were dissolved in 3.0

the acid-catalyzed steps determined the overall HDO

mL deionized water. 1.0g nano MgO power was added

sulfide catalysts.

24

Further, Kou and Dyson substituted the Pd/C

slowly in mixed solution under magnetic stirring. The

and mineral acid by metal nanoparticles and Brönsted

milk-like light-orange slurry was aged at 80°C under

acidic IL, providing a more efficient and less energy-

vigorous stirring for 8 h. Then the little amount of water in

reaction;

Last but not a least,

the slurry was removed at 105 ℃ and was grinded to

etheric C-O bonds in lignin was clipped efficiently over C

power. The light-orange power was calcined at a heating

supported metal catalyst such as Ru, Ni, Pd, and Cu under

rate of 4°C/min from 30 to 450 °C and kept at 450 °C for

demanding upgrading process.

basic conditions.

23

25-26

2 h in static air. The calcined power was reduced in H2 at

Herein, we developed a multifunctional catalyst

500°C for 2h in the next step and then was heated in N2

(Pd/MgO-C) that MgO as support was modified by carbon

flow to 700°C for 1h. After that power was exposed to a

and palladium for conversion of β-O-4 lignin-type dimers

methanol vapor for 10 min at the given temperature (500,

(and actual lignin) into valuable aromatics. The acid-base

600, 700 and 800℃). Methanol was placed in Simax spiral

properties of catalyst was modulated via carbon

mouth gas washing bottle that heated in the 65℃ water

modification and palladium loading, while C is achieved

bath and methanol vapor was carried to the surface of

from methanol thermal annealing hydroxides. In this

power by N2 (80 mL/min). The samples were denoted by

strategy, the Palladium were incorporated on defect C

MgO-C-500, MgO-C-600, MgO-C-700 and MgO-C-800,

through processing C with a small calculated amount of

respectively.

HCl. Modulating the high temperature of methanol

Pd/MgO-C and Pd/MgO catalysts Preparation

thermal annealing to impact the carbon species to affect

To improve catalytic activity, palladium was

the acid-base strength of catalysts. Which was highly

incorporated

selective for the hydrolysis/hydrogenolysis of the lignin C-

impregnation using PdCl2 as a precursor to reach a final

O linkages and decarbonylation of aromatic aldehydes to

Pd loading of 5 % (wt.). Typically, PdCl2 was dissolved in

stable aromatics.

a mixture solvents (ethanol and a small stoichiometric of

Experimental section

HCl), and then MgO-C or nano-MgO power was added

onto

2


the

nanohybrids

by

wetness

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slowly with magnetic stirring. After drying overnight at

TPD) was carried out on a ChemStarTM. The sample (about

80°C and treating in H2 at 300°C for 2 h to decompose the

150 mg) was pretreated at 300 °C for 30 min in a flow of

metal precursor, the resulting catalysts were stored and

30 cc/min He. After pretreatment, the sample was cooled

used to catalytic reaction.

to 120°C and exposed to CO2 for 60 min. After sweeping

Catalyst Characterization

with He for 30 min to remove physisorbed CO2, the temperature was increased linearly at a rate of 10°C/min

Pd, Fe and Mg elemental content (wt.%) of the

to 800 °C in He. The amount of CO2 was quantified by

catalysts were measured by Inductively Coupled Plasma

pulse calibration. Temperature-programmed desorption of

Mass Spectrometry-Optical Emission Spectrometer (ICP-

NH3 (NH3-TPD) was carried out with the same scheduler

OES; Agilent 7900, Agilent Technologies, Santa Clara,

except the gas CO2 changed to 8% NH3/He mixed gas.

CA). C, H and N elemental content (wt.%) of the catalysts Typical process for β-O-4 linkage substrate and

were determined by an elemental analyzer (vario EL cube,

lignocellulose depolymerization

Elementar Analysensysteme GmbH, Hanau, Germany). And O content was calculated by mass difference. Power

Typically, 0.1mmol of the lignin model (3-Hydroxy-

X-ray diffraction (XRD)：Bruker Endeavor D4 with Cu

2-(2-methoxyphenoxy)-1-(4-methoxyphenyl)propan-1-

Kα radiation (40 kV and 30 mA) was used to analyze the

one (a1) or Guaiacylglycerol-beta-guaiacyl Ether (b1)), 5

catalysts. They were recorded with 0.0167°steps over the

mL of H2O and 20mL ethyl acetate (EtOAc) were put into

5 to 80°angular range. X-ray photoelectron spectrometer

50 ml autoclave (Hastelloy alloy, made by Anhui Kemi

(XPS): XPS spectra were performed on a Thermo Machinery Technology Co., Ltd.) equipped with a

ESCALAB 250Xi equipped with a monochromatic Al Kα

mechanical stirring, and then 30 mg of catalyst was added.

X-ray source and a delay-line detector. The spectra were

A total of 15 bar of H2 was charged. As for

obtained using the aluminum anode (Al Kα=1486.6 eV)

depolymerization of real lignin, 200 mg of pine as a

operating at 150W. The samples were dried in a vacuum at

substance and 100mg catalyst were placed into 50 ml

120 °C for 10 h and the charge neutralizer system was used

stainless autoclave. The depolymerization reaction was

for all of the analyses. The base pressure was 1 × 10 -8 Pa.

conducted at 160℃. All of degradation reactions were

High resolution spectra were recorded with 20 eV pass

heated to the given temperatures (100,120,140 and 160℃)

energy. The binding energy (BE) was calibrated to the C

and then kept for 4h with continuous stirring at 500 rpm.

1s signal (284.8 eV) as a reference. The curve fitting

After that, the reactor was cooled immediately in an iced-

procedure was conducted using an approximation based

water bath.

on a combination of the Gaussian and Lorentzian functions with subtraction of a Shirley-type background. Scanning

All of materials were took out form the reactor, the

electron microscope (SEM) images of the fresh was

solvents are still two phases and catalyst located at the

obtained on a Hitach S-4800 instrument (10 kV). Scanning

interface of two solvents. 4.0 mL of EtOAc phase product

transmission electron microscopy (STEM) was performed

and 1.0 mL H2O phase product were took and placed

using a JEM-2100F electron microscope operated at 200

together in a rotary evaporator, then removed the solvents

kV, equipped with a spherical-aberration (Cs) probe

and added 5.0 mL ethanol to dissolve the products.

corrector (CEOS GmbH) and a high-angle annular dark

Detailed products were detected by GC–MS (Agilent

field (HAADF) detector. A probe semi-angle of 25m rad

7890A-5975C) equipped with a Pxi-17Sil MS Cap.

and an inner collection semi-angle of the detector of 88m

Column (30m ×0.25 mm ×0.25μm) and compositions

rad were used. Compositional maps were obtained with

were identified according to the NIST MS library. The

energy-dispersive X-ray spectroscopy (EDX) using four

oven temperature was programmed as 40°C hold 5 min,

large-solid-angle symmetrical Si drift detectors. For EDX

and then ramped up to 300°C with 5.2 °C /min and hold

analysis, Pd K, Mg K, C K and O K peaks were used.

for another 4 min. The quantitative analysis of products

Temperature-programmed desorption of CO2 (CO2-

were analyzed by high performance liquid chromatograph

3



(HPLC Waters e2695) with the reverse phase C18 column.

pine valorization was measured by using a Bruker

The mobile phase was 5 mM sulfuric acid aqueous soluti

AvanceШ 400 MHz spectrometer. About 50mg non-

on with a flow rate of 0.5 ml/min and the column tempera

volatile fractions were dissolved in dimethylsulfoxide-d6

ture was maintained at 50 °C. In addition, the components

([D6] DMSO) (0.7 mL). For the HSQC analysis, the

of pine was measured according to NREL K-lignin

collecting and processing parameters were listed as

analysis (NREL LAP Determination of structural

follows: number of scans, 84; receiver gain, 203;

carbohydrates and lignin in biomass).

27

acquisition time, 0.2129/0.0636s; relaxation delay, 2.0 s;

Pd/MgO-C-700 catalyst recuperation and reuse：the

pulse width, 10s; spectrometer frequency, 400.15/100.61

first run condition: 100 mg of pine power, 100 mg

MHz; and spectral width, 4807.8/20124.9 Hz.28 The data

Pd/MgO-C-700 catalyst, 20 mL EtOAc and 5mL H2O, at

was processed with MestReNova software.

160℃ for 4h. In this work, EtOAc and H2O were applied

Results and discussion

to recover the used catalyst from the residue. The

Catalyst characterization

Pd/MgO-C-700 catalyst is relatively apolar and mainly To elucidate reasons for the catalytic activity of the

located in EtOAc phase. While the residue stayed at the

Pd/MgO-C samples in hydrolysis-hydrogenolysis lignin bottom of H2O phase. After EtOAc/H2O extraction, the model compound, we investigated the composites with a EtOAc phase containing part of the Pd/MgO-C-700 was series

removed and fresh EtOAc was added. This was repeated

of

characterization

techniques.

Before

characterizing, the carbon modified Pd/MgO-C catalysts until the EtOAc phase remained relatively clear. The were prepared from reduction under H2 at 300 °C. recoverable catalyst was used to verify stability and recyclability under the first run reaction conditions. Pd/MgO-C-500℃ Pd/MgO-C-600℃ Pd/MgO-C-700℃ Pd/MgO-C-800℃ MgO-C Pd/MgO

Feedstock conversion, monomer quantification and yield calculations

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 17

Model compound and pine conversion and monomer

*

* MgO

◆

C

▼Fe

★

Pd

O3

2

* *

★ ★

* *

★

◆

▼

aromatics yield were calculated by the weight comparison

▼ ▼

between the recovered and the feedstock as shown in Eq.

10

20

30

40 50 2θ (degree)

60

70

80

(1). The yield of any monomer aromatic and total monomer aromatics were calculated as shown in Eq. (2)

Fig.1 XRD patterns of fresh Pd/MgO-C catalysts prepared by

and (3), respectively. Conversion(%) =

different temperature

𝑀0 −𝑀𝑛 𝑀0

Fig.1 shows the XRD pattern of Pd/MgO-C catalyst

∙ 100% (1)

obtained by methanol thermal annealing at 500~800°C. 𝑌𝑖𝑒𝑙𝑑 𝐱 monomer aromatic (%) =

𝑀𝑥 /𝑊𝑥 𝑀0/𝑊0

𝑌𝑖𝑒𝑙𝑑total monomer aromatics (%) =

∙ 100% (2)

𝑀𝑡𝑜𝑡𝑎𝑙 𝑀𝑙𝑖𝑔𝑛𝑖𝑛

All Pd/MgO-C samples have the characteristic diffraction peaks of the C and MgO structure with Pd metal.29-30 For

∙ 100% (3)

instance, the diffraction peaks at 25.9°and 42.6°in the

In the equations, M0: the weight of feedstock; Mn: the

curves are attributed to the (002) and (100) plane of the

weight of unreacted feedstock; Mx: the weight of x product;

graphite-like structure of the C,30 respectively. This

Wx: the molecular weight of x product; W0: the molecular

structure of graphitic carbon were further confirmed in

weight of model compound. Mtotal: the weight of total

Raman and XPS analysis (Fig.2 and Fig.5). Other four

monomer aromatics; Mlignin: the weight of lignin in pine.

peaks at 42.88°, 62.30°, 74.69°and 78.63°is the (200), (220), (311) and (222) plane of the structure of MgO.

1

H-13C HSQC NMR analysis of non-volatile fraction

Another three peaks at 40.12°, 46.66° and 68.12° To investigate the degradation of β-O-4 linkages

correspond to the Pd (111), (200) and (220) planes in the

performance, NMR spectra of non-volatile fractions from

4


Page 5 of 17

Pd/MgO-C catalysts, respectively.

A

oxidation and the resources potential to Pd (111) crystal plane preferred orientation, that’s why Pd is more suitable for practical application of biomass depolymerization.

5

Adsorbed volume (cm3/g, STP)

To our knowledge, Pd (111) has more resistance to

500

5 600

40 700

2, 5 40

However, the peak at 56.40° （PDF#46-1211）was assigned

800

0.0

to a weak PdO diffraction peak in 500℃ and 600℃ pre-

0.2

0.4 0.6 Relative pressure (p/p0 )

0.8

1.0

0.8

treatment temperature cases. With the pre-treatment

500 600 700 800

B

temperature further increasing, the weak PdO2 diffraction

3

dV/dlog(D) Pore Volume (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peak at 54.55°(PDF#34-1101) was observed and the weak PdO peak at 56.40° further decreased (Fig.1). These changeable Pd species might be related to the effect of pretreatment temperature (Fig.2). Both PdO and PdO2 species

0.6

0.4

0.2

0.0 0

8

16

had little contribution to the cleavage of β-O-4, and the

24 32 Pore diameter (nm)

40

48

Fig. 2 (A) N2 adsorption-desorption isotherms and (B) Proe size

amount of Pd active sites decreased. distribution curves of Pd/MgO-C catalysts

Fe was used to facilitate surface carbon growth and

Fig.2 shows N2 adsorption–desorption isotherms (A)

was remained on the catalyst. The size of Fe species was

and pore size distributions (B) of four different Pd/MgO-

cling to change during the thermal annealing process

C catalysts. The catalysts textural properties are also

because of the evaporation of metal atoms at high

summarized in Table 1. The nitrogen sorption isotherms of

temperature and the dissolving of carbon species on the

the all of catalysts showed a typical IV isotherm with a

nanoparticles.31-32 The smaller Fe nanoparticles was too

hysteresis loop, indicating a mesoporous structure. 35 The

active and gave rise to much carbon dissolved at the

slow capillary condensation step in the range of P/P0 =

beginning, the excess carbon would form a continuous thin

0.4~1.0 might be attributed to the C mixture species. The

layer of graphite. While metal Fe nanoparticle with a

SBET, VP and DBJH values for the catalysts obviously

moderate and suitable size could nucleate growth to

increased from 66.39 m2g-1, 0.04 cm3g-1 and 5.81 nm to

33

carbon tube/rob on MgO. The larger metal Fe or metallic

134.06 m2g-1, 0.22 cm3g-1 and 6.58 nm when the pre-

oxide (Fe2O3 and Fe3O4) particles could not efficiently

treatment temperature increased from 500℃ to 800℃,

catalyze the decomposition of carbon stocks and cannot

indicating Fe metal species participated in and probably

supply enough carbon to nucleate the carbon tube, carbon

catalyzed the carbonization process36. The similar

stocks was in the form of amorphous carbon.32, 34 From the

phenomena were observed for Fe-N-C and Co-N-C

results of Fig.1, Fe species (Fe, Fe2O3 and Fe2O3) were

materials prepared 35, 37.

observed in catalysts. When the thermal annealing temperature were 500 ℃ and 600 ℃ , small metal Fe

Table 1 Textural properties of Pd/MgO-C on BET measurements

nanoparticles contributed to much graphite and carbon

Catalyst

SBET (m2g-1)

VP (cm3g-1)

DBJH (nm)

Pd/MgO-C-500

66.39

0.04

5.81

Pd/MgO-C-600

87.44

0.07

6.37

Pd/MgO-C-700

113.04

0.19

6.45

tube/rob formation (Fig.2 and Fig.5a~b). While more amorphous carbon, less graphite and carbon tube/rob were given because of the formation of larger metal Fe or metallic oxide (Fe2O3 and Fe3O4) particles when thermal annealing temperature increased to 700 ℃ and 800 ℃ (Fig.2 and Fig.5c~d).

5



Pd/MgO-C-800

134.06

0.22

basic sites in Pd/MgO-C catalysts. While all samples have

6.58

the broad desorption peak of the media basic site located Raman spectroscopy is a powerful tool for

between 300 and 680°C with the central of peak at about

identifying the nature of carbon species. The Raman

500°C.47, 49 The amount of total basic sites on these samples

spectra of the carbon from methanol thermal annealing are

allow to estimate the density of basic sites (Table 2). The

shown in Fig.2. Both peaks of D band (amorphous carbon)

density of basic sites was increased when the thermal

and G band (graphitic carbon) broaden with locating at 939~1490 cm-1 and 1490 ~1782 cm-1, respectively.

annealing temperature increased, which may be attributed to

38-39

the surface area of catalyst because the highest pre-treatment

The ID/IG value increased (from 1.7 to 7.0) with pyrolysis

temperature was in keep with the highest surface area of

temperature increasing, indicating that more structural

catalyst (Fig.2). Among all of the catalysts, Pd/MgO-C-800

defects and less graphitization degree were produced at higher

pyrolysis

temperatures.36,40

Moreover,

contains the largest amount of basic sites (2.77mmol/g).

the

A

-1

CO2 signal intensity (a.u.)

appearance of the 2D bands (2440~3270cm ) indicate three types of graphene, single-layer graphene (2620 cm1

), bilayer and few-layer graphene (2650 cm-1) and

graphitic graphene (2661 cm-1) in all of samples.38, 41-42

500℃

600℃

700℃

Therefore, the carbon species are in the form of rob-like 800℃

carbon, amorphous carbon and graphene, introducing

200

300

400 500 Temperature (℃ )

600

700

more defects that causes a change in vibration energy B

levels.43-44 The defects of carbon material is helpful to NH3 signal intensity (a.u.)

loading metal and metallic oxide,45 that’s why a large proportion of palladium species were observed on carbon species (Fig.6 b, c, e and f ). D

500℃

600℃ 700℃ 800℃

G

100

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 17

2D

800℃ (ID/IG=7.0) 700℃ (ID/IG=4.4)

400 500 Temperature (℃ )

600

700

catalysts after different temperature treatment

500℃ (ID/IG=1.7)

2000

300

Fig.4 CO2-TPD (A) and NH3-TPD (B) profiles of the Pd/MgO-C

600℃ (ID/IG=3.6)

1000

200

NH3-TPD are used to determine the density of acid

3000 -1

Raman shift (cm )

sites in Pd/MgO-C samples prepared at different thermal Fig.3 Raman spectra of Pd/MgO-C catalysts

annealing temperatures (Fig. 4B and Table 1). Commonly， the desorption peaks with TD at about ＞ 300°C,

The acid-base property of catalysts are reported to play crucial roles in promoting the activity of cleavage the

300~450°C and 450~650°C were denoted as the weak,

linkage β-O-4 with molecular hydrogen.2, 46 Here, the acid-

medium and strong acid site, respectively.50-51 It can be

base property of Pd/MgO-C catalysts were investigated by

seen from Fig.4B, all catalysts showed three desorption

TPD analyses. Fig.4 displays CO2-TPD and NH3-TPD profiles

peaks of a weak, a medium and a strong acid site. Except

of the catalysts. Generally, the desorption peak of the basic

for Pd/MgO-C-800, all samples possess two broader

sites are determined from CO2 desorption temperatures

desorption peaks in the range of 300~450 ℃ and

(TD), i.e., TD＜300℃, 300＜TD＜600℃ and TD＞600℃

450~650 ℃ . The desorption peaks of the acid sites

is assigned to weak basic site, media basic site and strong

decrease and shift to lower temperature with the thermal

basic site, respectively.

47-48

As shown in Fig. 4A, no

annealing temperature increasing. However, when the

desorption peak is found below 300°C, implying no weak

thermal annealing temperature was 800℃, the desorption

6


Page 7 of 17

eV, 337.2 eV, 338.42eV, and 340.0 eV (Fig.5B), these can

peak of the acid site shifted to higher temperature.

be assigned to Pd0, Pd2+, Pd4+, and Pd0, respectively.52-55 Table 2 Basic and acidic properties of Pd/MgO-C catalysts

As shown in Table 3, although the total Pd content of each

Amount of basic

Amount of acidic

catalyst was a little difference, changes occurred (except

sites (mmol g-1) a

sites (mmol g-1) b

for Pd2+ was a little difference) in the relative amounts of

Sample

Pd0, Pd2+ and Pd4+. For instance, the area ratio of Pd0 Pd/MgO-C-500

1.19

8.03

Pd/MgO-C-600

2.64

5.39

Pd/MgO-C-700

2.72

4.78

Pd/MgO-C-800

2.77

6.46

(Pd3d5+0 and Pd3d3+0) was increased when the pretreatment temperature increased, while the area ratio of Pd4+ was decreased when the pre-treatment temperature increased. There were mainly related to the catalyst surface area that affected by the pre-treatment temperature (Table 2). The higher surface area facilitates Pd species

The total concentration of base calculated based on CO2-TPD

dispersion, leading to more Pd4+ decreased and converted

calibration pulse. b The total concentration of acid calculated based on

to Pd0. However, no PdO species was observed on the

NH3-TPD calibration pulse.

profiles by XRD analysis (Fig.1), it may be attributed to

a

the low crystallinity of PdO species in XRD. Chemical states of surface atoms on the catalysts were

2

C-C SP bonding graphene

investigated by XPS. Fig. 5 shows the C1s and Pd3d spectra

3

C-C SP bonding graphene oxide

C-O-C

O-C=O Pd/MgO-C-500

Intensity

of Pd/MgO-C catalysts. The BE and the area ratio of the type of C1s and Pd3d in catalysts are summarized in Table 2. In the C1s spectrum (Fig. 5A), the wide peak ranging from 282 to 295 eV can be resolved into four individual component peaks at 284.0, 284.8, 286.0 and 288.5 eV, respectively,

Pd/MgO-C-600

Pd/MgO-C-700

Pd/MgO-C-800

39, 43

292

290

288

286 284 Binding energy (eV)

282

280

3

corresponding to the C–C SP bonding graphene oxide, C–C SP3 bonding graphene, C–O-C and O–C=O bonds,

respectively.

When

the

pre-treatment

Pd 3d5+4

Pd 3d5+2 Pd 3d5+0

Pd 3d3+0

temperature Pd/MgO-C-500

increased, the C content of Pd/MgO-C was decreased from Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19.49% to 12.68% (Table 5). The area ratio of C–C bond (SP3 bonding graphene oxide and SP3 bonding graphene)

Pd/MgO-C-600

Pd/MgO-C-700

was also decrease when pre-treatment temperature

Pd/MgO-C-800

increased. This result is also well consistent with the effect

344

342

340

338

336

334

332

330

Binding Energy (eV)

of pre-treatment temperature on the size of Fe specie

Fig.5 The XPS C1s and Pd3d spectra of the Pd/MgO-C catalysts

particles (the relatively lower pre-treatment temperature Table 3 The area ratio of the type of C1s and Pd3d in Pd/MgO-C

gave rise to smaller metal Fe nanoparticle that was

catalysts by XPS analysis

beneficial to graphene or/ and graphene oxide formation). On the other hand, the area ratio of O-C=O bond was

Binding energy

increased when the pre-treatment temperature increased

(eV) C1s or Pd3d

Area (%)

500℃

600℃

700℃

800℃

from 500℃ to 700℃, and then decreased to 9.79% at 800℃. Additionally, the area ratio of C-O-C bonds were

(O-C=O) 288.5

18.47

20.42

23.65

9.79

(C-O-C) 286.00

19.78

19.99

19.12

34.15

(sp3 C-C) 284.8

12.65

10.68

10.00

8.95

not big difference under 700℃, while reached to a high value of 34.15% at 800 ℃ . The reasons for these phenomena are unclear. Deconvolution of the Pd3d spectra suggests that there were four components peaks at Pd3d BE values of 335.13

7



(sp2 C-C) 284.0

49.09

48.91

47.23

47.11

(Pd0 ) 335.13

5.35

5.37

13.67

15.06

Page 8 of 17

decreased from 19.49% to 12.68% when the pre-treatment temperature increased from 500 ℃ to 800 ℃(Table 4). However, it is hard to identify the Pd and Fe nanoparticles

2+

(Pd ) 337.20

69.68

69.93

68.46

68.89

(Pd4+) 338.42

18.37

16.21

10.27

8.23

(Pd0 ) 340.00

6.60

8.50

7.60

7.82

in SEM image mainly due to their low weight metal addition (Table 4). The content of Fe and Pd were a little difference the theoretical additive value (~5.0 wt. %). Notably, the relatively higher C weight were given (19.8~25.86%) form the EDS analysis (Table 3). ICP-OES analysis of Pd/MgO-C samples was added (Table 4). Table 4 shows the content of C decreased from 19.49% to 12.68% when the pre-treatment temperature increased from 500℃ to 800℃. The content of Fe and Pd were a little difference the theoretical additive value (~5.0 wt. %).

Fig.6 SEM images (a) Pd/MgO-C-500, (b) Pd/MgO-C-600, (c) Pd/MgO-C-700, (d) Pd/MgO-C-800

SEM and TEM images were taken to investigate the morphology and structure of the materials. As shown in Fig.6, the magnified scanning electron microscopy (SEM) images show that these anomalous rod-like carbons were formed on the surface of MgO support (Fig.6a~c). The rod-like carbon probably assembled on Fe nanoparticles. The formation of rod-like carbon is mainly depended on

Fig.7 TEM image of Pd/MgO-C-700 catalyst (a~f), and element

the moderate metal Fe nanoparticle size, as Chenguang Lu

mapping (g~i)

et.al reported in their research. They found that graphite

Transmission electron microscopy (TEM; Fig.6a~f)

was given instead of rob-like carbon when metal Fe

and elemental mapping (Fig.6g~i) showed that carbon rob

particle very small. The small Fe particles were very active

(Fig.6b, be labeled with blue imaginary line), amorphous

and dissolved too much carbon covering the surface of Fe

carbon and graphite carbon (Fig.6a, c and d) were the form

nanoparticle, resulting in excess carbon formed a

of C species on the spherical MgO or covered/encased the

continuous thin layer of graphite. However, the large Fe

spherical MgO. (Fig. 6a~d and Fig.2). The Pd

34

nanoparticles

could

not

efficiently

catalyze

nanoparticles were loaded on carbon species. The mean Pd

the

particle size is about 1.7~22 nm (Fig.6f). Moreover,

decomposition of carbon stocks, and cannot supply

graphitic carbon with a spacing of 0.39 nm is observed on

enough carbon to nucleate robs. Only the moderate size of

MgO support (Fig.2 and Fig.6d).

Fe nanoparticle can ensure a suitable carbon supply for the nucleation and growth of robs.32,

34

Table 4 Element of Pd/MgO-C catalyst was detected by ICP-OES and

Thus, no rob-like

elemental analysis

carbon generation in Pd/MgO-C-800 material is attributed to the large Fe nanoparticles sintered/aggregated and a

Element

wt.%

portion of Fe2O3 and Fe3O4, which is in good consistent 500℃

600℃

700℃

800℃

Mg

32.06

33.69

34.53

37.58

Fe

4.78

4.26

4.35

4.27

with the diffraction peaks of Fe2O3 and Fe3O4 at 700℃ and 800℃ (Fig.1). Correspondingly, the content of C

8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pd

4.25

4.37

4.62

4.50

C

19.49

17.64

15.3

12.68

H

0.83

0.37

1.26

0.85

O

38.42

39.50

39.94

40.12

N

0.05

0.17

0

0

100 Pd/MgO-C-700

90 Pd/MgO-C-600

Monomer selectivity (%)

Page 9 of 17

Pd/MgO-C-800 Pd/MgO-C-500

80

70

60

Pd/MgO

50 MgO-C

40 50

The depolymerization activity of the catalyst Recently, the strategy of coupling reactions have

60

70 80 Conversion (%)

90

100

Fig.8. Catalytic results of Guaiacylglycerol-beta-guaiacyl Ether (a1)

been employed in the depolymerization of the primary β-

depolymerization. Reaction conditions: 30 mg of catalyst, 0.1m mol

O-4 lignin models (containing about 50% of all linkages)

of a1, 20 mL of EtOAc and 5 mL H2O, 15 bar of H2, 140°C, 1 h.

such as, hydrolysis / hydrodeoxygenation, hydrogenolysis, Here, we investigated the

With Pd/MgO-C catalysts, a further enhanced

depolymeirzation of model compound with hydrolysis /

conversion of a1 and monomer selectivity of product

hydrogenolysis /decarbonylation approaches (Fig.8 and

obtained. For instance, in Pd/MgO-C-600 case, 92.3% of

Table 5). Only 19.6% of a1 was converted and 12.5% of

conversion and 85.6% of monomer selectivity were given.

guaiacol (1) was given without a catalyst (Entry 1 in Table

Pd/MgO-C-700 was not only exhibited high activity for

5), implying that EtOAc/H2O combined with H2 showed a

breaking Cβ-O bond (conversion >99%) but providing a

weaker effect on breaking Cβ-O-4 bond through hydrolysis

good yield of monomer aromatic (>95%). Although,

or/and dehydration under 140℃.

Pd/MgO-C-500 and Pd/MgO-C-800 gave 86.8% and 90.4%

hydrogenation and so on.2,

56

of dimer compounds’ conversion, relative lower monomer When a1 was depolymerized over MgO-C (or

aromatic yields were given, owing to the formation of 8a

Pd/MgO) catalyst, the conversion of a1 reached 49.3% (or

(condensation product). Notably, the strength of catalyst

65.7%) with 41.8% (or 58.2%) of monomer selectivity.

acid/base properties played important roles for oligomer

These results confirmed that both MgO-C and Pd/MgO

depolymerization and monomers yield. Both of Pd/MgO-

catalysts were able to cleave the linkage of β-O-4 in a1. In

C-600 and Pd/MgO-C-700 possessed the close-up

Pd/MgO catalyst case, the monomer product of 1, 5a,

acid/base strength and contributed to a higher conversion

6a/7a, 3a and a little of a-2 dimer were obtained (Entry 9

and yield. On the other hand, the higher density of acid site

in Table 5). These were thereby inferred that a1 was

(and more strong acid site at 450~650℃) in Pd/MgO-C-

dehydrated to a-2 with the help of alkalinity and then the

500 and Pd/MgO-C-800 gave birth to a lower monomer

hydrolysis of a-2 was conducted. The product of 5a, 6a/7a,

yield (Table 2 and Table 5).

3a mainly derived hydrogenation and decarboxylation reactions over Pd active site. For MgO-C catalyst, only

The effect of acid/base activity of catalyst on

12.3% of 5a was given that could be lack of Pd active site

product distribution was assessed. As shown in Table 5,

for a-2 hydrogenolysis, while some part of a1 was

both in Pd/MgO-C-500 and Pd/MgO-C-800 catalyst cases,

hydrolyzed and then released a relative higher yield of

except, cleavage of β-O-4 model afforded the main

6a/7a through hydrogenolysis (Entry 10 in Table 5).

product guaiacol (1), as well as the relative higher yield of

Remarkably, MgO-C catalyst showed a relative weaker

4a (20.1% /18.3 %), 2a (8.5% / 10.6%) and 8a (12.0 % /

effect on a1 conversion and the cleavage of a-2 than

6.5 %). Nevertheless, only 4.8% and 2.9% of 4a were

Pd/MgO catalyst. Although both MgO-C and Pd/MgO

found in Pd/MgO-C-600 and Pd/MgO-C-700 cases,

catalysts were able to cleave the linkage of β-O-4 in a1,

respectively, and high yield of 1, 5a and 6a/7a were

the introduction of acidity on catalyst is essential for

afforded. They could be seen that the high density of acid

cleaving β-O-4 linkage under mild reaction condition.

site and more strong acid site were prone to obtain 4a and

9



Page 10 of 17

Table 5 Results of Pd/MgO-C catalytic cleavage of β-O-4 model compound a1 and main products observed [a]

Product yield (%) Entry

Catalyst

1

2a

3a

4a

5a

6a/7a

8a

1

Free

19.6

12.5

-

5.8

-

2.2

4.0

-

2

Pd/MgO-C-500

86.8

76.7

8.5

19.2

20.1

11.4

3.4

12.0

3

Pd/MgO-C-600

92.3

85.6

11.3

15.2

4.8

32.2

15.6

3.3

4

Pd/MgO-C-700

99.5

95.4

11.9

23.6

2.1

35.2

20.4

-

5

Pd/MgO-C-800

90.4

81.6

10.6

17.8

18.3

15.6

10.3

6.5

63.9

57.8

6.7

13.4

3.5

21.9

10.6

-

Pd/MgO-C-700

82.5

78.6

8.6

19.5

2.9

28.7

17.4

-

100

99.4

14.0

24.1

1.8

32.4

20.8

1.2

9a

Pd/MgO

65.7

58.2

-

7.4

-

44.8

4.6

(a-2) 4.5

10a

MgO-C-700

49.3

41.8

-

5.5

-

12.3

16.9

(a-2) 6.7

b

6

c

7

d

8

[a]

Conversion (%)

conditions: 30 mg of catalyst, 0.1 mmol of a1 model compound, 20 mL of EtOAc and 5 mL H2O, 15 bar of H2, 140°C, 1 h. The products were

determined by GC-MS with acetophenone as internal standard. [a] 140°C, [b] 100°C, [c] 120°C, [d] 160°C.

Table 6 The result of main products from the cleavage of b1 model compound over MgO-C catalysts [a]

Structure and name

Entry

1

2

3

4

5

6

7

8

Reaction

Free

Cat.-

Cat.-

Cat.-

Cat.-

condition

cat.

500

600

700

800

100℃

120℃

160℃

21.1

92.5

98.0

100

96.9

79.8

90.4

100

15.4

88.6

93.9

98.3

92.3

73.1

86.3

99.5

2.6

2.8

19.4

20.3

14.2

9.7

11.9

15.6

3.1

24.4

6.6

1.9

15.7

2.8

2.2

3.7

-

-

11.1

21.8

14.6

17.9

19.6

22.1

Conversion (%)

Yield (%)

10


Page 11 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[a]

-

12.2

15.3

11.5

13

11.5

13.7

12.0

10.7

10.3

13.1

9.4

15. 6

8.7

9.2

10.5

-

14.8

10.8

11.4

12.9

8.3

9.8

13.9

-

9

11.5

10.2

7.4

4.6

7.3

8.1

-

2.6

3.1

11.6

8.2

9.4

10.4

11.4

-

11.7

2.4

-

5.5

-

-

2.0

conditions: 30 mg of catalyst, 0.1 mmol of b1 model compound, 20 mL of EtOAc and 5 mL H2O, 15 bar of H2, 100~160°C, 1 h. The products were

determined by GC-MS and quantified by HPLC.

2a (derived from 4a decarbonylation reaction). Lower

kinetics and the mechanism of the depolymerization

density of acid site with more weak and media acid sites

reaction.2,

(Table 2 Pd/MgO-C-600 and Pd/MgO-C-700), as expected,

medium acid sites are lead to good yield of 5a, 3a and

resulted in a decrease of 4a with a concomitant drop in

6a/7a (Entry 3-5 in Table 5). These stable products are

condensation product (8a). The results are in good

likely to contribute to the high monomer aromatic yields.

15

It is important to note that weak or/and

consistent with the reports by and Bruijnincx et.al.56 Furthermore, another dimer b1 degradation was also Meanwhile, from the variation of 2a, 4a and 8a yields in conducted to assess the effect of catalyst acid/base Table 5, much lower yield of 2a than 4a (Entry 2 and 5 in properties on b1 conversion and product distribution. As Table 5, Scheme 1) demonstrated that the decarbonylation shown in Table 5. Without Pd/MgO-C catalyst, 21.1% of reaction cannot keep up with aldehyde formation.56-57 With b1 was converted with 15.4% of guaiacol (1) and 10.7% decarbonylation being too slow, larger molecular weight of 6b-1 (Entry 1 in Table 5), ascribing to the hydrolysis of compounds were formed, such as 8a (Scheme 1).2, 17, 56 b1 and b-2 was took off formaldehyde under the effect of Contrary to relative lower density of acid site catalyst with EtOAc/H2O combined with H2 at 140℃.With catalysts, weak or/and medium acid site (Entry 3 and 4 in Table 5), the model b1 also afforded excellent conversion and good the yield of 2a is higher than 4a with little trace of 8a. yield of guaiacol (1). The influences of acid/base density Marks et.al reported similar effect was found in the tandem on product of 4b, 2b, 7b, 7b-1 and 10b were similar to the catalytic system, where a weaker acid was employed to catalytic active effect on 4a, 2a, 5a and 8a. Namely, the remove reactive intermediates more effectively over Pd high density of basic site is attributed to from 4b, 2b and catalyst, giving rise to higher yields.58 Indeed, it is quite 10b, and relative low density of acid sites and high density necessary

that

the

kinetics

of

hydrolysis

and of basic sites were contributed to 7b-1, 7b and 5b.

decarbonylation are carefully matched. Obviously, the Furthermore, b1 was efficiently cleaved at 100℃ with high density of base site (Table 2) had effect on the higher conversion and yields of guaiacol (71.3%), 7b increase of 5a yield with little amount of 8a (Table 5). (17.9%), 3b (11.5%) and 5b (9.4%). And a higher Usually, the high content of alkaline gave higher conversion and monomer aromatic yield than that from a1 conversion since the polarization of the base governs the

11



Page 12 of 17

(Table 5 and Table 6). These results show that the β-O-4

hydrogenolysis), followed by acid-catalyzed hydrolysis

linkage in b1 is easier to fracture than that in a1 under the

step to 1 and 6a/7a (1 and 6b) and then rearrangement to

same reaction condition. They are associated with the

Hibbert’s Ketone 3a (3b).2 Another reaction pathway is

different between Cα-OH of β-O-4 alcohol and -Cα=O of

that a1 (b1) is dehydrated on Cα-OH by acid catalysis, as

β-O-4 ketone. The function group -Cα=O ketone in b1

well as the deprotonation of Cγ-OH for breaking off the

lowers the dissociative energy of Cβ-O-4 bond by 40~50

Cγ-carbon to a significant amount of a-2 (b-2) with

kJ·mol-1 than that Cα-OH of β-O-4 alcohol to the cleavage

formaldehyde. a-2 (b-2) goes on a slower acid-catalyzed

of Cβ-O-4 linkage in a1.

8, 56, 59-60

hydrolysis step to 1 and 4a (4b). With the help of strong acid and sufficiently metal Pd active, the decarbonylation

The effect of temperature on a1 (b1) conversion and reaction

is

carried

out

and

then

generates

the

the product yield were investigated over Pd/MgO-C-700 corresponding methyl-substituted product 2a (2b), whilst catalyst. The conversion of a1 (b1) and the yield of repolymerization takes place instead to give 8a (10b).7 monomer products increased when the temperature raised, Furthermore, an alternative pathway is also observed, the while the yield of 4a (4b) decreased, especially,， 8a (10a) enol ether a-2 (b-2) converted to 5a (5b) by appeared at 160℃ (Entry 4 and 6~8 in Table 5 and Table hydrogenation.61 Base for the cleavage of ether linkages is 6). Moreover, the increasing yield of 1, 5a (5b), 3a (3b) also kind of important. In the presence of base, the and 7b/7b-1 stable products were given when degradation structure of a1 involves a free phenolic OH functional temperature raised. Which were attributed to more intermediate

a-2

(b-2),

a-3

(b-3)

and

group in the para-position of the α-aryl ether group, which

(b-4) is meant to crack by converting phenolate unit into the

depolymerization to 1, 5a (5b), 7b/7b-1 and Hibbert’s

corresponding quinone methide intermediate (a-4). The ketones 3a (3b) (Scheme 1 and 2) when temperature

Cγ-carbon in a-4 is fractured to release formaldehyde and

elevated. The intermediates a-2, a-3, b-2, b-3 and b-4 a-2 via basic-catalyzed dealkylation reaction, which also hardly detected, demonstrating fast hydrolysis and contributes to 1 and 5a (or/and to 1, 4a and 2a) (Scheme hydrogenolysis were carried out. However, a high yield of

1). Model compound b1 is a β-Aryl ether that its Cα-

2a/2b was given at 160 ℃ due to the increased acid

position of the propane side chain possesses a free concentration

of

the

reaction

system

(the

acid alcoholic OH group and its phenolic OH group in the

concentration was increased owing to a portion of EtOAc paraposition is etherified. In the present of base, the hydrolyzed to acetic acid and ethanol at 160℃).The model

cleavage of β-O-4 linkage in b1 contains deprotonated

compound a1 (b1) depolymerization show that a1 (b1) hydroxyl groups in Cα or Cγ (b-4), which serve as compounds were almost converted with high yield of nucleophiles

in

replacing

the

neighboring

aroxy

monomer aromatics under 140℃ (Entry 4 in Table 5 and substituent by forming an oxirane ring (8b). Then 8b is Table 6). opened by addition of a hydroxide ion to form an The depolymerization mechanistic perceive provided

intermediate (9b) with glycol groups. Finally, the 9b was

by the model compounds are proposed in Scheme 1 and 2.

converted into 7b/7b-1 via dehydration and hydrogenation

Initial acid catalyzed dehydration of the Cα-OH in a1

reactions, as displayed in Scheme 2.

forms enol ethers a-3 (b1 converted to b-3 through

12


Page 13 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Proposed mechanism for the cleavage of β-O-4 model a1 and possible pathways to products

Scheme 2. Proposed mechanism for the cleavage of β-O-4 model b1 and possible pathways to products

Finally, we focused on pine power (real lignin)

(Fig.9), the monomer aromatics was obtained in

as a substrate. In this work, pine is consisted of

24.6 wt % yields (lignin based), and the selectivity

42.69% cellulose, 22.6% hemicellulose, 22.93%

of 2-methoxy-4-propylphenol was 77.2% (label 17

lignin, 0.59% ash, 4.89% moisture, and 4.90 %

in Fig.9). Other aromatics’ selectivity’s (label 10~19)

coloring matter. Pine was directly subjected to

are likely to depended on the effect of acid-base

depolymerization over Pd/MgO-C- 700 catalyst in

properties

EtOAc/H2O solvents under 160℃. Pine power was

hydrogenation process and decarbonylation. A large

pretty much dissolved in liquid phase. Form the

amount

result of liquid products GC-MS and GC analysis

of

of

MgO-C-700

products

from

on

hydrolysis,

cellulose

and

hemicellulose were also obtained, for instance,

13



hexane-2, 5-dione (label 9), ethane-1, 2-diol, butan-

Pd/MgOC-700 catalyst (Fig.12). During the first two runs,

1-ol, ethyl 2-hydroxypropanoate and glucose. It

stronger Pd and MgO diffraction peaks were observed than

should be pointed out that little amount of β-O-4

the previous run (run-1 and run-2 in Fig.12), implying Pd and

linkages was found on the spectrum of HSQC

MgO

species

were

aggregated

during

depolymerization process. Even worse, PdO and PdO2

(Fig.10, Aγ and Aβ). This indicates that MgO-C-700

species appeared after the third run, displaying some

catalyst possesses strong activity of fracturing β-O-

active sites of Pd/MgO-C-700 catalyst were oxide. 4 linkages in pine. Therefore, the aggregation of Pd species and MgO as well 1

2

O

19

4

O

3

9

O

O O

6

5

O

22

OH

O

23

O

10

11

OH

OH

OH

OH

deactivation

OH

13

OH

16

14

OH

OH

15

OH

decreasing

ability

of

pine

depolymerization.

OH

OH

O O

O

O

the

OH

O O OH

12

and

OH

O

21

O

O

O

O

O

OH

HO O

9 OH O

O

OH

O

OH 8 O

O

O O

O

as metal Pd oxidation were responsible for catalyst

O O

17

O

O

O

O

7 O

HO

20

OH

O

HO

OH HO

OH

O

O OH

17

O

O

18

OH

HO

O

11

6

10

5

2 3

4

internal standard

100

23 15 13 14 16

12

7

8

18

pine conversion monomer aromatics yield

22 19 20 21

80

6

9

12

15

18

21

24

Conversion and yield (%)

1

27

Retention time (min)

Fig.9 GC-MS spectrum of liquid product from pine valorization over Pd/MgO-C-700 catalyst.

60

40

20

Reaction conditions: 100mg of Pd/MgO-C-700, 200mg of Pine 0 run-1

material (about 40 mesh), EtOAc/H2O = 4:1, 15 bar of H2, 160°C, 4 h.

run-2 Run times

run-3

Fig.11 Pine conversion and monomer aromatics yield from the reactions over fresh and spent Pd/MgO-C-700 catalysts.

* MgO

*

◆

C

☆

▼Fe

O3

Fe3O4

2

PdO

@ PdO

2

★

Pd

* Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 17

fresh

◆

▼

★

*

★

* *

★

run-1

▼

run-2 ☆

run-3

Fig.10 The spectrum of HSQC of products (removed solvents)

10

from pine degradation over Pd/MgO-C-700 catalyst

20

30

40 50 2θ (degree)

@

60

70

80

Fig.12 XRD patterns of fresh and spent Pd/MgO-C-700 catalyst

In order to investigate catalyst stability and recyclability, the recycle experiments of pine valorization were

performed

over

Pd/MgO-C-700

and

Associated content

the

characterization of catalyst after every run reaction were analyzed by XRD analysis. From Fig.11, the types of

Author Information

products did not changed, however, the conversion of pine and the yield of monomer aromatics were obviously

Corresponding Author

decreased. After the third run, both the pine conversion

E-mail address: [email protected], [email protected]

and monomer aromatics yield were decreased from 95.7%

Tel.: +86-20-37029721; Fax: +86-20-87057673

and 24.6% to 68.2% and 11.3%, respectively. These undesired result of repeat reactions mainly ascribed to the

Notes

decreased of metal Pd and MgO activities with the

The authors declare no competing financial interest.

increasing times of Pd/MgO-C-700 catalyst reuse. The decreased of metal Pd and MgO activities were well

Acknowledgements

consistent with the characterization of the spent

This work was supported by NSFC (Natural Science

14


Page 15 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Foundation of China) project (No. 51606205, 51676191,

treated lignin for the production of green chemicals. Bioresource

51536009) and CAS Pioneer Hundred Talents Program.

Technol 2011, 102 (7), 4917-4920. 12. Yuan, Z. S.; Cheng, S. N.; Leitch, M.; Xu, C. B., Hydrolytic

Keywords:

C-O ester cleavage, β-O-4 lignin-type

degradation of alkaline lignin in hot-compressed water and ethanol.

dimers, depolymerization, C modified MgO, acid-base

Bioresource Technol 2010, 101 (23), 9308-9313.

property, palladium catalyst.

13. Roberts, V. M.; Stein, V.; Reiner, T.; Lemonidou, A.; Li, X. B.; Lercher,

J.

A.,

Towards

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Modifying MgO with Carbon for valorization of lignin to aromatics

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