Low temperature hydrodeoxygenation of guaiacol into cyclohexane over Ni/SiO2 catalyst combined with Hβ zeolite

Article abstract of DOI:10.1039/c8ra09972c

Hydrodeoxygenation (HDO) of guaiacol to cyclohexane, important for bio-oil upgrading, is usually performed at high reaction temperature (≥200 °C). In this work, low temperature transformation of guaiacol to cyclohexane was achieved at 140 °C over non-noble metal Ni/SiO₂ and various zeolites. Among zeolites tested (HUSY, HMOR, Hβ, HZSM-5, SAPO-34), Hβ zeolite exhibited superior catalytic activity due to its appropriate pore structure and acid strength. The open pore with three-dimensional structure of Hβ facilitates the diffusion of guaiacol and intermediates. Meanwhile, weak acid strength of Hβ efficiently reduces the competitive adsorption of guaiacol, and then promotes the dehydration of intermediate 2-methoxycyclohexanol. Moreover, the catalytic performance in guaiacol HDO to cyclohexane is also closely related to Si/Al ratio of Hβ. Owing to its moderate acid density, the maximum yield of cyclohexane reaches 91.7% on Hβ(Si/Al = 50) combined with Ni/SiO₂ at 140 °C, which is the lowest temperature ever reported over non-noble metal catalysts.

Full text of DOI:10.1039/c8ra09972c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Low temperature hydrodeoxygenation of guaiacol
into cyclohexane over Ni/SiO catalyst combined
2
Cite this: RSC Adv., 2019, 9, 3868
with Hb zeolite
a
b
b
b
bc
b
Xun Wang, Shanhui Zhu, * Sen Wang,
Yue He, Yang Liu, Jianguo Wang,
b
and Yongkang Lv*a
Weibin Fan
Hydrodeoxygenation (HDO) of guaiacol to cyclohexane, important for bio-oil upgrading, is usually
ꢀ
performed at high reaction temperature ($200 C). In this work, low temperature transformation of
ꢀ
guaiacol to cyclohexane was achieved at 140 C over non-noble metal Ni/SiO
2
and various zeolites.
Among zeolites tested (HUSY, HMOR, Hb, HZSM-5, SAPO-34), Hb zeolite exhibited superior catalytic
activity due to its appropriate pore structure and acid strength. The open pore with three-dimensional
structure of Hb facilitates the diffusion of guaiacol and intermediates. Meanwhile, weak acid strength of
Hb efficiently reduces the competitive adsorption of guaiacol, and then promotes the dehydration of
intermediate 2-methoxycyclohexanol. Moreover, the catalytic performance in guaiacol HDO to
Received 4th December 2018
Accepted 1st January 2019
cyclohexane is also closely related to Si/Al ratio of Hb. Owing to its moderate acid density, the maximum
DOI: 10.1039/c8ra09972c
ꢀ
yield of cyclohexane reaches 91.7% on Hb(Si/Al ¼ 50) combined with Ni/SiO
2
at 140 C, which is the
rsc.li/rsc-advances
lowest temperature ever reported over non-noble metal catalysts.
to cycloalkanes with high selectivity. The HDO of phenolic
monomers into alkanes involved metal-catalyzed hydrogena-
1
. Introduction
Bio-oil, produced by liquefaction or fast pyrolysis of biomass, is tion and acid-catalyzed hydrolysis and dehydration. However,
a promising liquid fuel and chemical raw material in light of the liquid acid catalyst is very corrosive. Later, various noble-metals
1–3
decreasing fossil fuels and increasing air pollution. However, supported on solid acid carriers were employed for the HDO of
8
bio-oil cannot be directly used as transportation fuel without guaiacol to cyclohexane. The group of Ha got cyclohexane at
ꢀ
any treatment due to its high oxygen content, which leads to good yield of 60% at 250 C over Ru/SiO
2
–Al
2
O
3
. In addition, Pt
4
9
high viscosity, chemical and thermal instability of fuels.
supported on various zeolites exhibited HDO performance of
Hydrodeoxygenation (HDO) is deemed as the most effective guaiacol to different degrees. Pt/Hb got the highest yield (about
method for bio-oil upgrading, which can reserve the caloric 45%) of cyclohexane at 250 C. Grunwaldt et al. achieved 93%
value protably. Owing to the complex composition of bio-oil, yield of cyclohexane from guaiacol over Pt/H-MFI-90 at
ꢀ
10
4–7
ꢀ
HDO of bio-oil's model compounds raises wide concern to moderate temperature of 180 C. Scheme 1 provides an overview
research on the mechanism of bio-oil upgrading. Guaiacol, at on the reaction pathways of guaiacol HDO in the presence of
2
good yield in bio-oil, contains two types of C–O bonds, sp Caryl– noble-metal catalysts. One pathway is deoxygenation as a rst
3
O and sp CH –O, which are prevalent in bio-oil. Due to its high step, and the other way is hydrogenation of aromatic ring
3
caloric value, catalytic HDO of guaiacol into cyclohexane,
a kind of fully deoxygenated product, is an excellent model
reaction for bio-oil upgrading.
Currently, a variety of catalysts have been developed for the
6
transformation of guaiacol to cyclohexane. In 2009, Kou rstly
reported a new catalytic route with a bifunctional combination
of Pd/C with H PO for converting phenolic bio-oil components
3
4
a
Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi
Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China.
E-mail: lykang@tyut.edu.cn
b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, PR China. E-mail: zhushanhui@sxicc.ac.cn
Scheme 1 Reaction pathway for the conversion of guaiacol over
different catalysts.
c
10,14
University of Chinese Academy of Sciences, Beijing 100049, PR China
3868 | RSC Adv., 2019, 9, 3868–3876
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
followed by acid-catalyzed dehydration. Due to the high MCH could be further converted into alkane by acid-catalyzed
hydrogenation activity of noble-metals, hydrogenation of deoxygenation, followed by successive hydrogenation.
aromatic ring to 2-methoxycyclohexanol (MCH) is preferred Compared with other solid acid catalysts, such as ion-
under mild condition. The selectivity towards various possible exchanged resin, oxides and supported heteropolyacids,
deoxygenation products (cyclohexanol, methoxycyclohexane, zeolites would be prospective choice due to their outstanding
cyclohexane, etc.) are closely related to the acidity of support thermal stability and tunable pore properties. Therefore,
materials. However, high price of noble-metals will greatly limit various kinds of zeolites combined with Ni/SiO
2
were utilized to
their further applications in industry. perform HDO of guaiacol to cyclohexane at very low reaction
Considering the industrial application, base metallic cata- temperature. In this contribution, the effects of porous struc-
lysts with earth-abundant elements are potential candidates. ture and acidic properties of zeolites were also explored on
Nickel-based catalysts, if designed elaborately, are promising catalytic performance.
for their high intrinsic ability of hydrogenation activity in the
11,12
HDO of guaiacol. Ma et al.
2 2
reported that Ni/SiO –ZrO
2
. Experimental
effectively converted guaiacol to cyclohexane at excellent yield of
ꢀ
2.1. Materials
9
6.8% at 300 C. Over carbon nanotubes supported Ni–Fe
13
catalysts, Yuan and coworkers gained 83.4% yield of cyclo- The following materials were provided by various suppliers:
hexane at 400 C. M. V. Bykova studied the Ni-based sol–gel guaiacol (Sinopharm Chemical Reagent Co., Ltd., China), dec-
ꢀ
14
catalysts for guaiacol HDO and got 63.9% yield of cyclohexane at alin (Tokyo Chemical Industry). Hb with different Si/Al ratios
ꢀ
3
20 C. Over Ni-based catalysts, guaiacol HDO is mainly pro- and HUSY were purchased from Catalyst Factory of Nankai
ceeded in two pathways as similar as noble-metal catalysts University (Tianjin, China). Hb with a Si/Al ratio of X was
(
Scheme 1), including hydrogenation of aromatic ring and denoted as Hb(X). These purchased zeolites were calcined at
ꢀ
direct hydrogenolysis of C–O bond, respectively. Under low 560 C for 4 h prior to use.
temperature, hydrogenation of aromatic ring is the main reac-
tion pathway, and Ni-based catalysts present low activity of
hydrogenation. High reaction temperature can promote the
activity of Ni-based catalysts on which guaiacol HDO mainly
proceeds in direct hydrogenolysis pathway. However, the
intermediate products of phenolic compounds in direct
hydrogenolysis pathway tend to cause signicant coke forma-
tion and catalyst deactivation at high temperature. Considering
the complex constituents of bio-oil, it would be highly desirable
to operate HDO of guaiacol under low temperature. There are
two approaches to decrease the reaction temperature for
guaiacol transformation to cyclohexane, namely, to enhance the
intrinsic activity of catalyst or to develop a novel pathway.
Herein, we report a new bifunctional catalyst of Ni/SiO2
physically mixed with zeolite to convert guaiacol to cyclohexane
2
.2. Catalyst preparation
Ni/SiO was synthesized by AEH method according to our
previous work. ZSM-5 (Si/Al ¼ 30), SAPO-34, and mordenite (Si/
2
15
19,20
Al ¼ 10) were synthesized by a hydrothermal method.
a typical synthesis of ZSM-5, adequate amounts of sodium
aluminate (NaAlO ), tetrapropylammonium hydroxide
TPAOH), sodium hydroxide (NaOH), silica sol (JN-40) and water
H O) were mixed in a plastic vessel and then aged for 4 h under
In
2
(
(
2
vigorous stirring at room temperature. The molar composition
2 3
of the reactants in the solution was 1SiO : 0.0167Al O -
:
in a Teon-lined stainless steel autoclave at 170 C for 48 h. The
synthesized solid product was centrifuged and washed with
water for several times. Then the solid product was dried at
2
0.15TPAOH : 30H
2
O. Crystallization of zeolite was carried out
ꢀ
15
at low temperature. In our recent work, a conned nano Ni/
SiO catalyst prepared by ammonia evaporation hydrothermal
AEH) method has been obtained, in which high-dispersed Ni
nanoparticles were entrapped in brous nickel phyllosilicate
Fig. 1).
Owing to its unique structure, Ni/SiO2 gave complete
ꢀ
ꢀ
100 C overnight and calcined at 550 C for 6 h. Finally, the
2
obtained Na-form zeolite was changed into H-form by ion-
exchange with NH NO solution at 80 C for 5 h, and then
calcined at 560 C for 6 h.
(
ꢀ
4
3
ꢀ
(
ꢀ
2.3. Catalytic test
conversion of guaiacol and 95.5% yield of MCH at 140 C.
16–18
ꢀ
Prior to the reaction, Ni/SiO was prereduced at 600 C for 2 h in
2

Similar to deoxygenation of polyols to liquid hydrocarbons,
ꢁ
1
ow of 10 v% H /Ar gas (60 mL min ). The catalytic perfor-
2
mance of Ni/SiO with different zeolites in the conversion of
2
guaiacol was tested in a 50 mL stainless steel autoclave lined
with Teon. Typically, 0.10 g guaiacol, 20 mL decalin, 50 mg Ni/
SiO and a certain amount of zeolite were loaded into the
2
autoclave. Then, the autoclave was lled with 3.0 MPa
ꢀ
hydrogen. The reaction mixture maintained at 140 C for
a certain time.
The components of liquid products were identied by GC-
MS (6890N, Agilent, USA). Quantitative analysis of the liquid
products was measured with ethyl benzoate as an internal
standard by gas chromatography (GC-2014C, SHIMADZU)
2
Fig. 1 TEM image of reduced Ni/SiO prepared by ammonia evapo-
ration hydrothermal method.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3868–3876 | 3869

View Article Online
RSC Advances
Paper
ꢀ
equipped with a ame ionization detector (FID) and a DB-WAX under owing Ar. Then the sample was heated to 600 C at a rate
ꢀ
ꢁ1
capillary column. Carbon balance was checked in the range of of 10 C min and the desorbed NH was recorded by TCD.
3
95–99%. The conversion of guaiacol and product yield were
calculated according to the following equations:
2.5. Density functional theory (DFT) calculation of
molecular kinetic diameters
conversionð%Þ
DFT calculation was performed on Gaussian 09.D01 package.
Molecule geometries were optimized by using the uB97X-D
hybrid functional and the 6-311+G(2df, 2p) basis set. Molec-
moles of guaiacol ðinitialÞ ꢁ moles of guaiacolðfinalÞ
¼
moles of guaiacolðinitialÞ
ular kinetic diameters (d ) were dened as the internuclear
s
distance (d ) between the two nuclei that intersected the surface
of the smallest possible cylinder plus an estimate of the van der
i
moles of product i
yieldð%Þ ¼
moles of guaiacolðinitialÞ
˚
˚
Waals radii of the hydrogen (1.200 A) or oxygen (1.520 A) atoms
involved, according to the Lennard-Jones model (d /21/6),
s
¼ d
i
22
23
24
as reported by Huber, Ding, Gao and co-workers.
2.4. Catalyst characterization
2.6. FTIR study of guaiacol adsorption on zeolite
The X-ray diffraction (XRD) patterns of different zeolites were FTIR study of guaiacol adsorption on zeolite was performed on
performed on Rigaku MiniFlex II desktop X-ray diffractometer a Bruker TENSOR 27 FT-IR spectrometer. Firstly, the powder of
ꢀ
ꢁ1
with Cu Ka radiation. The scanning speed was set as 4 min
.
zeolite was pressed into a self-supporting wafer, and then was
The actual Si/Al ratio of zeolite was measured by ICP optical placed into the IR cell. Prior to measurement, the sample was
ꢀ
emission spectroscopy (Optima 2100DV, PerkinElmer).
pretreated at 350 C for 2 h in vacuum. Subsequently, guaiacol
N
2
adsorption–desorption isotherms was performed on was introduced by dropping 20 mL guaiacol on the wafer. In
Micromeritics TriStar 3000 instrument. Before measurement, order to remove excessive guaiacol, the cell was evacuated at
ꢀ
ꢀ
350 C until the spectra became constant. Finally, the IR spec-
0
.10 g sample was degassed under vacuum at 300 C for 8 h.
The different types of acid sites in zeolite were determined by trum of surface species was recorded.
pyridine adsorption of Fourier transform infrared spectra (Py-
IR) on a Bruker Tensor 27 FT-IR spectrometer. Zeolite sample
was rstly pressed into self-supported wafer. Prior to measure-
3
. Results and discussion
ꢀ
3.1. Catalyst characterization
ment, the wafer was pretreated in vacuum at 350 C for 2 h. The
ꢀ
IR spectrum was recorded at 150 C as reference. When the In this work, a series of zeolites combined with Ni/SiO
2
were
ꢀ
temperature fell to 35 C, the sample was exposed to pyridine used as bifunctional catalysts for one-pot HDO of guaiacol to
vapor for 30 min. The Py-IR spectrum was then recorded at cyclohexane through multistep reactions consisting of hydro-
ꢀ
1
50 C aer vacuumizing for 60 min. The concentration of genation and dehydration. Thus, a crucial factor that affects this
acidic sites was calculated by the procedures reported by reaction rate is the diffusion of hydrogenation reaction product
2
1
25
Madeira et al.
to the acid sites of zeolite, which is closely related to the
26
Temperature-programmed desorption of ammonia (NH3- porous structure of zeolite. According to previous work, the
TPD) was performed in an Auto Chem II 2920 chemisorption pore diameters of zeolites decline as HUSY > HMOR > Hb >
ꢀ
analyzer. Typically, 0.10 g sample was pretreated at 550 C for HZSM-5 > SAPO-34 (Table 1). As shown in Table 1, the pore
ꢁ
1
˚
˚
2
h in owing Ar (30 mL min ). Once the temperature cooled diameters of HUSY, HMOR and Hb zeolites are 7.4 A, 7.0 A and
ꢀ
˚
down to 50 C, NH
3
was introduced into the system for 30 min. 6.7 A, respectively, which are larger than the kinetic diameters
Subsequently, physisorbed NH
ꢀ
27
˚
was removed at 120 C for 2 h of guaiacol (6.68 A˚ ) and MCH (6.55 ꢂ 6.67 A) calculated by
3
Table 1 Physicochemical properties of zeolites
Acid sites
ꢁ
1
(mmol g
)
2
ꢁ1
3
ꢁ1
Zeolites
Pore diameter ( A˚ )
Channel structure
S
BET (m g
)
V
p,t (cm g
)
BAS
LAS
Si/Al ratio
SAPO-34
HZSM-5
Hb(12.5)
HMOR
3.8 ꢂ 3.8
5.3 ꢂ 5.6
6.6 ꢂ 6.7
6.5 ꢂ 7.0
3D, 8-ring
3D, 10-ring
3D, 12-ring
1D, 12-ring
1D, 8-ring
3D, 12-ring
3D, 12-ring
3D, 12-ring
3D, 12-ring
562
375
528
452
0.31
0.34
0.39
0.25
0
0
0.1
26.3
10.8
9.5
0.35
0.35
0.13
0.08
0.31
0.02
5
.7 ꢂ 2.6
HUSY
7.4 ꢂ 7.4
6.6 ꢂ 6.7
6.6 ꢂ 6.7
6.6 ꢂ 6.7
516
545
495
577
0.29
0.52
0.30
0.34
0.92
0.23
0.21
0.04
0.28
0.25
0.12
0.01
3.3
16.0
49.0
Hb(20)
Hb(50)
Hb(250)
229.6
3870 | RSC Adv., 2019, 9, 3868–3876
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
3
Fig. 2 Space-filling model of (A) cis-2-methoxycyclohexanol and (B) Fig. 5 NH -TPD profiles of various zeolites.
trans-2-methoxycyclohexanol.
zeolite with different Si/Al ratios was also measured by Py-IR.
With an increase in the Si/Al molar ratio, the amounts of BAS
and LAS decrease evidently (Fig. 4B). According to quantitative
analysis, the ratio of BAS to LAS increases from 0.9 to 4.0 with an
increase of Si/Al molar ratio from 12.5 to 250, indicating that
BAS turns into primary acid species on Hb zeolite with high Si/
30
3
Al molar ratio. According to NH -TPD proles (Fig. 5), all
zeolites present two ammonia desorption peaks, one in low-
temperature region and the other in high-temperature region,
corresponding to weak acid sites and strong acid sites, respec-
tively. Of the examined catalysts, HMOR has the strongest acid
Fig. 3 XRD patterns of (A) zeolites with different structures (B) Hb with
different Si/Al ratios.
ꢀ
sites, in which the high-temperature peak is centered at 500 C
in NH -TPD prole. Hb(12.5) has large number of weak acid
sites and small amount of medium-strong acid sites, and its
acid strength is lower than that of other zeolites.
3
DFT theory (Fig. 2). In contrast, HZSM-5 and SAPO-34 have
˚
˚
smaller average pore sizes (namely, 5.6 A and 3.8 A) than the
kinetic diameters of guaiacol and MCH.
The physicochemical properties of zeolites were character-
ized by BET, XRD, Py-IR and NH -TPD. As shown in Table 1, all
3
3.2. FTIR study of guaiacol adsorption on zeolite
of zeolites have high surface area and pore volume. Among The adsorption states of guaiacol over HUSY, HMOR and Hb
2
them, the BET surface area of SAPO-34 is the highest (562 m
g
were investigated by FT-IR (Fig. 6). As reference, FTIR spectrum
ꢁ1
), while Hb(20) displays the maximum pore volume. The of pure guaiacol was also measured, and the assignments of
XRD patterns in Fig. 3 illustrate that all the as-synthesized characteristic vibration peaks are listed in Table 2. Aer intro-
zeolites are well crystallized with the characteristic peaks duction guaiacol to zeolite, the positions of guaiacol charac-
according to JADE 5.0. For Hb, the crystallinity of zeolite teristic peaks shi compared with that of pure guaiacol,
becomes strong with an increase of Si/Al ratio.
indicating that guaiacol virtually chemisorbs on zeolite. Espe-
The Py-IR proles and acid amounts of zeolites are depicted cially, the g(C]Cring) peak of pure guaiacol at 1502 cm shis
ꢁ
1
in Fig. 4 and Table 1, respectively. As displayed in Fig. 4A, two to low wavenumber, suggesting the presence of electrodonation
ꢁ
1
31
pyridine adsorption peaks are centered at 1450 cm
and effect of zeolite's LAS on the aromatic ring of guaiacol.
540 cm , corresponding to Lewis acid sites (LAS) and Brons- However, the adsorption strength of guaiacol on various zeolites
ted acid sites (BAS), respectively.
ꢁ
1
1
28,29
The peak area represents is signicantly different, which can be reected by the degree of
the density of acid sites. Obviously, the acid density of HUSY is red shi about g(C]Cring) vibration peak (Fig. 6A). Notably, the
ꢁ
1
highest among tested zeolites. However, no peak of pyridine characteristic peak of g(C]Cring) shis down to 1487 cm and
ꢁ
1
adsorption was detected on SAPO-34 owing to its smaller pore 1490 cm aer guaiacol adsorption on HMOR and HUSY,
size than molecular diameter of pyridine. The acidity of Hb respectively. This indicates the strong interaction between
ꢀ
Fig. 4 FTIR profile of pyridine adsorbed on (A) zeolites with different Fig. 6 IR spectra of guaiacol adsorption at 350 C on (A) zeolites with
pore structures and (B) Hb with different Si/Al ratios.
different structures and (B) Hb with different Si/Al ratios.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3868–3876 | 3871

View Article Online
RSC Advances
Table
Paper
35
2
IR characteristic peaks of pure guaiacol and their percentage of the total acid sites. In particular, no consecutive
a
assignments
conversion of MCH into cyclohexane occurs on SAPO-34,
because it possesses no acid sites on the external surface of
zeolite. For zeolites that have large pore size with 12-membered
Assignment
Guaiacol
g(C]Cring
d(CH
d(OH)
)
1616, 1597, 1502 rings (i.e., Hb, HMOR and HUSY), guaiacol and reaction inter-
3
)
1444–1470
1364
1260
1225
mediates can diffuse into the internal porous structure of
zeolites. The dehydration of MCH can proceed not only on the
external acid sites, but also in the internal porous structure of
zeolites, in which acid sites account for the majority of the total
g(C–OH)
g(C–OCH
3
)
a
The assignment is based on our experiment about IR spectrum of pure
35
acid sites.
15
guaiacol on KBr.
However, though HUSY and HMOR possess large pore size,
low yields of cyclohexane were obtained (Table 3, entry 5 and 6).
On the one hand, the poor catalytic performance may be
assigned to the inherent pore structure of zeolites. HUSY is
a three-dimensional channel structure containing supercage,
and HMOR is a one-dimensional pore structure containing 8-
ring channel. This makes it difficult for the diffusion of reactant
and product, resulting in lower yield of cyclohexane than that
over Hb with three-dimensional porous structure without excess
space (supercage) (Table 3, entry 4). On the other hand, the low
guaiacol and strong acid sites of zeolites, which were veried by
NH -TPD (Fig. 5). Compared with HMOR and HUSY, the g(C]
ring) peak of guaiacol shis downward to 1498 cm aer
adsorption on Hb, implying the relatively weak interaction
between guaiacol and Hb. Fig. 6B shows the adsorption states of
guaiacol on Hb with various Si/Al ratios. Obviously, the
adsorption strength of guaiacol has less relationship to the Si/Al
ratio of zeolite. However, the adsorption capacity of guaiacol on
Hb gradually reduces with increasing of Si/Al ratio owing to
decreasing acid density (Table 1).
3
ꢁ
1
C
18
yields of cyclohexane on HUSY and HMOR are closely related to
their strong acid strength. As can be seen from Fig. 6A, the
adsorption strength of guaiacol on HUSY and HMOR is higher
than that of Hb due to their strong acid sites. Thus, guaiacol
HDO conversion of guaiacol strongly adsorbed on the acid sites of HUSY and HMOR plugs
3
.3. Effect of zeolite structure on guaiacol HDO
6,32,33
According to previous works,
into cyclohexane in HYD pathway contains multistep reactions the pore of zeolite, and then reduces the available acid sites for
consisting of metal-catalyzed hydrogenation and acid-catalyzed MCH dehydration.
15
dehydration. In our recent work, low-temperature trans-
formation of guaiacol to MCH has been achieved in high yield highest reaction rate in the dehydration of MCH. Firstly, the
95.5%) over Ni/SiO . MCH could be converted into cyclohexane excellent performance may be ascribed to its proper pore size
Among zeolites with large size pore, Hb(12.5) exhibited the
(
2
by dehydration–hydrogenation over zeolite mixed with Ni/SiO2. and three-dimensional porous structure with no excess space
Thus, the reaction rate of dehydration could be affected by (supercage). The open three-dimensional pore structure is
various parameters of zeolite including pore structure, acidity, favorable for the rapid diffusion of MCH to the internal acid
as well as the diffusion rates of reactant and product inside the sites of Hb(12.5), leading to high reaction rate. Meanwhile, the
zeolite pores. To gain insight into the effect of zeolite's pore super catalytic performance is partially assigned to its weak acid
structure on the product distribution, a series of typical zeolites strength. As shown in Fig. 6A, the adsorption strength of
were deliberately chosen as solid acid catalysts to explore their guaiacol on Hb(12.5) is lowest among zeolites with large pore
catalytic performance in HDO of guaiacol.
size. Guaiacol could keep self balance of adsorption–desorption
According to previous results, BAS is signicantly more in pore of zeolite, which can not block the channel of zeolite.
34
active than LAS for alcohols dehydration. To eliminate the MCH can effectively diffuse to the acid sites in the hole of zeolite
effect of acid density, the catalytic performance of zeolites with for dehydration. Thus, in comparison with other zeolites
different channels was compared under the same content of investigated, Hb is the optimum one for guaiacol HDO due to its
BAS, as shown in Table 3. It is evident that the catalytic suitable acid strength and pore structure with three-
performance of Ni/SiO
2
mixed with zeolite strongly depends on dimensional (3D) system.
the structure of zeolite. Among them, Hb(12.5) exhibited
uniquely active and excellent yield of cyclohexane (55.6%) with
3
.4. Effect of Si/Al ratio on guaiacol HDO over zeolite
>
99% conversion of guaiacol, followed by an intermediate
production level of cyclohexane on HMOR (16.8%) and H-ZSM5 MCH, produced from guaiacol hydrogenation on Ni/SiO
50.5%). SAPO-34 and HUSY showed much lower yield of a kind of alcohol, which could be transformed into alkane over
cyclohexane (0.6% and 1.6%, respectively), in contrast to other bifunctional catalyst. That is coupling dehydration on acid sites
zeolites. of zeolite and hydrogenation on metal active sites. The dehy-
2
, is
(
Compared with the maximum pore size of zeolite (Table 1) dration of alcohol mainly proceeds via parallel reaction mech-
and kinetic diameter of MCH, it can be seen that MCH molec- anism: intramolecular dehydration to alkene on BAS and
ular is too large to enter the internal porous structure of SAPO- intermolecular dehydration to ether depending on BAS and
36,37
34 and HZSM-5, and then can not access the acid sites within LAS.
Hence, the product distribution of guaiacol HDO is
zeolite pores. The dehydration of MCH mainly proceeds on the related to the properties of acid sites (acid density and acid type)
external acid sites, which only constitutes a very small in zeolite, affected by Si/Al ratio over zeolite.
3872 | RSC Adv., 2019, 9, 3868–3876
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
a
Table 3 Hydrodeoxygenation of guaiacol over different catalysts
Yield (%)
b
Entry
Catalyst
Zeolite dosage (mg)
BAS (mmol)
LAS (mmol)
Conv. (%)
Cyclohexane
Cyclohexanol
MCH
1
2
3
4
5
6
Ni/SiO
Ni/SiO
Ni/SiO
Ni/SiO
Ni/SiO
Ni/SiO
2
2
2
2
2
2
0
50
50
50
134.6
19.0
—
0
17.5
17.5
17.5
17.5
—
0
4.0
15.5
2.7
5.3
>99
0.2
0.2
50.5
55.6
16.8
1.6
4.0
4.8
0.4
0.8
8.9
3.6
95.5
87.3
40.7
29.7
67.4
88.0
+ SAPO-34
+ HZSM-5
+ Hb(12.5)
+ HMOR
+ HUSY
98.2
98.9
>99
93.8
95.1
a
ꢀ
b
Reaction conditions: guaiacol (100 mg), zeolite, Ni/SiO
2
(50 mg), decalin (20 mL), 3 MPa H
2
, 140 C, 5 h. MCH: 2-methoxycyclohexanol.
a
2
Table 4 Hydrodeoxygenation of guaiacol over Ni/SiO combined with Hb zeolite at different Si/Al ratio
Acid sites (mmol)
Yield (%)
b
Entry
Catalyst
Total
BAS
LAS
Conv. (%)
MCH
Cyclohexane
1
2
3
4
Ni/SiO
Ni/SiO
Ni/SiO
Ni/SiO
2
2
2
2
+ Hb(12.5)
+ Hb(20)
+ Hb(50)
+ Hb(250)
33.0
25.0
15.0
2.3
17.5
12.0
9.5
15.5
13.0
5.5
72.0
77.6
97.4
99.4
41.1
31.8
5.4
15.6
29.2
79.6
8.5
1.8
0.5
82.6
a
ꢀ
b
Reaction conditions: guaiacol (0.10 g), Ni/SiO (50 mg), Hb (50 mg), decalin (20 mL), 140 C, 3 MPa H , 2 h. MCH: 2-methoxycyclohexanol.
2
2
Table
4 shows the catalytic performance of Ni/SiO2 cyclohexane yield (Table 4, entry 1). This may be due to the
combined with Hb zeolite (different Si/Al ratio) on guaiacol competitive adsorption of guaiacol on acid sites of zeolite. As
HDO. Obviously, Si/Al ratio of Hb has a signicant effect on shown in Table 5 (entry 4 and 7), addition of guaiacol reduces
guaiacol conversion and yield of cyclohexane. In the case of the reaction rate of MCH dehydration on zeolite. Fig. 6B shows
Hb(12.5) (Table 4, entry 1), guaiacol conversion and cyclohexane the adsorption capacities of guaiacol on Hb with different Si/Al
yield were 72.0% and 15.6%, respectively. The yield of cyclo- ratios. Obviously, the adsorption amount of guaiacol varies
hexane reached its maximum at Si/Al ¼ 50, being 79.6%. conversely with the Si/Al ratio of Hb. The large amount of
Further increasing of Si/Al ratio to 250 declined the yield of guaiacol adsorbed on Hb(12.5) denitely inhibits the conver-
cyclohexane. An excessively low or high Si/Al ratio is unsuitable sion of MCH on zeolite. Moreover, Hb with low Si/Al ratio tends
for the production of cyclohexane.
to deactivate due to its fast coke formation on excessive acid
As shown in Scheme 2, guaiacol is rst converted into MCH sites, which was conrmed by TG result of used catalysts
on Ni/SiO
2
and then deoxygenated on acid sites of Hb zeolite. In (Fig. 7). As shown in Fig. 7, when the temperature is lower than
ꢀ
cascade reaction, the consecutive transformation of MCH on 100 C, catalyst has little weight loss on the TG curve arising
acid sites of zeolite should be benet for accelerating guaiacol from evaporation of water. During 100–500 C, there is large
ꢀ
conversion. However, there is no positive correlation between weight loss on the TG curve, due to the oxidation of coke on the
guaiacol conversion and the concentration of acid sites over Hb catalyst. According to quantitative analysis, the ratio of coke
with different Si/Al ratios. Despite possessing highest acid deposited on catalysts are 10.14% (Hb(12.5)), 9.22% (Hb(20)),
density, Hb(12.5) exhibited low conversion of guaiacol and 8.32% (Hb(50)) and 8.29% (Hb(250)), respectively.
However, Hb(250) with high Si/Al ratio is also not good
choice for guaiacol HDO to cyclohexane. As described in
Section 3.5, acid-catalyzed dehydration of MCH on Hb
zeolite is the rate-determining step in this tandem reaction.
For Hb with high Si/Al ratio, insufficient acid sites cannot
effectively promote the conversion of MCH, and then result
in enhanced yield of MCH. Competitive adsorption of
guaiacol and MCH conversion on acid sites of zeolite are
conict with each other, and the key is to balance both in
this reaction system. As a compromise, Hb zeolite with Si/Al
molar ratio of 50, possessing moderate acid density, gives
the highest yield of cyclohexane in the reaction of guaiacol
Scheme 2 Proposed reaction pathway for guaiacol conversion over
Ni/SiO
2
combined with Hb.
HDO.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3868–3876 | 3873

View Article Online
RSC Advances
Paper
a
Table 5 Catalytic performance of different catalysts for the conversion of different substrates
Entry
Substrate
Catalyst
Time
Conv. (%)
>99
Main product
1
2
3
4
5
6
7
Guaiacol
Guaiacol
MCH
MCH
Cyclohexanol
Cyclohexanol
Guaiacol + MCH
Ni/SiO
Hb(50)
2
5 h
5 h
5 h
10 min
5 min
5 min
10 min
MCH
Coke
—
Cyclohexanone
—
Cyclohexene
Cyclohexanone
Ni/SiO
2
—
72.9
—
90.9
Hb(50)
Ni/SiO
Hb(50)
Hb(50)
2
MCH (12%)
a
ꢀ
Reaction conditions: substrate, 80.6 mmol; catalyst, 50.0 mg; decalin, 20 mL; 140 C, 3 MPa H
2
.
3
.5. Reaction pathway of guaiacol HDO over bifunctional
results, the reaction pathway of guaiacol conversion is
summarized in Scheme 2. The initial product of MCH from
catalytic system
guaiacol hydrogenation on Ni/SiO is converted into cyclohex-
anone by dealcoholization on zeolite and subsequently hydro-
2
Fig. 8A plots the conversion of guaiacol versus reaction time at
ꢀ
140 C over Ni/SiO
2
physically mixed with Hb(50). The reaction
genation to cyclohexanol on Ni/SiO . Then, intermediate
2
progress was accompanied by a decrease of guaiacol (reactant)
and increase of cyclohexane (end-product). MCH and cyclo-
hexanol are the main intermediate products in HDO progress of
guaiacol. Further conversion of MCH is the rate-determining
step in this cascade reaction. As displayed in Fig. 8B, the
guaiacol conversion and MCH yield increased with increasing
cyclohexanol is quickly transformed into cyclohexane in dehy-
dration–hydrogenation pathway.
However, olen species was not typically observed during the
2
whole reaction process of guaiacol conversion on Ni/SiO mixed
with Hb(50). This suggests that the reaction rate of olen
hydrogenation on Ni/SiO is faster than that of alcohols dehy-
2
reaction time from 1 h to 5 h over Ni/SiO . However, further
2
dration on Hb. Compared with guaiacol reaction over only Ni/
conversion of MCH to cyclohexane did not occur even for long
reaction time of 5 h. It is obvious that the acid sites of zeolite are
the indispensable active sites for deoxygenation. Meanwhile,
the addition of zeolite greatly accelerated the conversion of
guaiacol and promoted HDO process. The synergistic effect of
active metal sites and acid sites are critical to guaiacol HDO
SiO , the fast conversion of MCH on Hb(50) promotes the
2
degradation rate of guaiacol according to the law of chemical
equilibrium. We could further accelerate the reaction rate by
modulating the mass of Hb(50).
2
Obviously, over Ni/SiO physically mixed with Hb(50), the
38,39
high-efficiency transformation of guaiacol to cyclohexane is
achieved at very low reaction temperature. As shown in Table 6,
guaiacol HDO over representative catalysts carried out at high
reaction.
To probe the network of guaiacol conversion in this system,
many evaluations were conducted by using different reactants
ꢀ
reaction temperatures, which were higher than 240 C for non-
(
guaiacol, MCH or cyclohexanol) over selected catalyst (Ni/SiO
or Hb(50)) (Table 5). As shown in Table 5, the initial reactant of
guaiacol could be converted into MCH on Ni/SiO with high
selectivity, while there was only coke produced only on Hb(50)
Table 5, entry 1 and 2). Thus, we can infer that guaiacol is rst
2
precious Ni-based catalysts. In contrast, in our catalytic system,
the transformation of guaiacol to cyclohexane proceeded at very
2
ꢀ
low temperature of 140 C, and the maximum yield of cyclo-
hexane was up to 91.7% for 2.5 h. The realization of guaiacol
HDO with high cyclohexane yield under low temperature is
(
transformed into MCH on Ni/SiO . According to entry 3 and 4 in
2
ascribed to the perfect combination of Ni/SiO with Hb(50).
2
Table 5, the intermediate product MCH was degraded rapidly
into cyclohexanone on Hb(50), and no reaction happened on Ni/
On the one hand, guaiacol can be highly selective hydroge-
ꢀ
15
nated into MCH on Ni/SiO at low temperature (140 C). In this
2
SiO
2
. Similar to MCH, the dehydration of cyclohexanol into
(Table 5,
work, HDO of guaiacol to cyclohexane mainly proceeded in
hydrogenation–dehydration pathway (Scheme 1). However,
oxygen removal of guaiacol on previous catalysts was realized by
cyclohexene takes place on Hb rather than Ni/SiO
2
entry 5 and 6). Hb provides the active sites for conversion of
MCH and cyclohexanol. Based on the above experimental
Fig. 8 The product distribution trends of guaiacol HDO with reaction
time over (A) Hb mixed with Ni/SiO
2
2
and (B) Ni/SiO . Reaction condi-
Fig. 7 TG curves of used catalysts containing Ni/SiO
different Si/Al ratio.
2
and Hb with tions: guaiacol (0.10 g), Hb(50) (50 mg), Ni/SiO
2
(50 mg), decalin (20
ꢀ
mL), 140 C, 3 MPa H
2
.
3874 | RSC Adv., 2019, 9, 3868–3876
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Table 6 Catalytic behavior of representative catalysts for the transformation of guaiacol to cyclohexane
ꢀ
a
Entry
Catalyst
T ( C)
PH2 (MPa)
Conv. (%)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Pt/zeolite
Pt/Al-SBA-15
Pd/C + zeolite
250
220
275
250
250
300
180
400
240
250
300
300
300
140
4
3
1.5
5
4
7
5
3
5
5
>90
100
100
100
100
100
100
96.8
100
90
45.3
80
0.3
75.7
60
88
9
40
33
6
8
41
10
13
42
43
12
11
Pd/C + H
Ru/SiO –Al
Pd/WO /g-Al
3
PO
4
2
2
O
3
x
2
O
3
Pt/H-MFI-90
Nix-Fey/CNT
Ni/MCM-41 + HZSM5
NiMo/SBA-15
93
80.7
84.1
56
96.8
86.4
16
0
1
2
3
4
Ni/SiO
Ni/TiO
Sulded Ni/W/TiO
Ni/SiO + Hb(50)
2
–ZrO
2
5
4
7
3
100
100
100
>99
2
–ZrO
2
2
44
2
91.7
This work
a
Yield: yield of cyclohexane.
hydrogenation–deoxygenation in pathway
deoxygenation in pathway 2, simultaneously (Scheme 1).
1
and direct-
Conflicts of interest
Compared with pathway 1, guaiacol hydrogenolysis to phenolic There are no conicts to declare.
compounds in pathway 2 needs to cleave C–O bond whose bond
energy is higher than that of C]Cring. Transformation of
guaiacol into cyclohexane in pathway 1 can realize at lower
reaction temperature than that in pathway 2.
Acknowledgements
On the other hand, Hb(50) is the optimum zeolite for MCH This work is nancially supported by National Natural Science
dehydration due to its three-dimensional pore structure with Foundation of China (21878321, 51778397, 21776196), Natural
1
2-membered ring and suitable acid properties. For large pores Science Foundation of Shanxi Province (2016021033,
of 12-membered ring channels in Hb, both internal and external 201601D011078), Science Foundation for Youth Scholars of
acid sites can contribute to the dehydration pathway, realizing State Key Laboratory of Coal Conversion (2016BWZ002), and
high conversion of MCH. Suitable acid properties can effectively Youth Innovation Promotion Association CAS (2015140).
balance the competitive adsorption of guaiacol and conversion
of MCH on zeolite, and then obtain high yield of cyclohexane.
References
1
2
M. Stocker, Angew. Chem., Int. Ed., 2008, 47, 9200–9211.
D. M. Alonso, J. Q. Bond and J. A. Dumesic, Green Chem.,
2010, 12, 1493–1513.
4
. Conclusions
A new and efficient bifunctional catalyst has been developed for
guaiacol HDO to cyclohexane at low temperature. By this cata-
lytic system, 2-methoxycyclohexanol (MCH) produced from
guaiacol hydrogenation can be easily converted into cyclo-
hexane by multistep reactions of dehydration–hydrogenation
3 D. Mohan, C. U. Pittman Jr and P. H. Steele, Energy Fuels,
2006, 20, 848–889.
4 E. Furimsky, Appl. Catal., A, 2000, 199, 147–190.
5 P. M. Mortensen, J. D. Grunwaldt, P. A. Jensen,
K. G. Knudsen and A. D. Jensen, Appl. Catal., A, 2011, 407,
1–19.
2
on zeolite and Ni/SiO in sequence. Typically, guaiacol was
successfully converted into cyclohexane with 91.7% yield at
ꢀ
140 C over Ni/SiO
2
combined with Hb(50). The superior cata-
6 C. Zhao, Y. Kou, A. A. Lemonidou, X. Li and J. A. Lercher,
Angew. Chem., Int. Ed., 2009, 48, 3987–3990.
lytic performance is ascribed to unique pore structure and
appropriate acid properties of Hb(50). Compared with other
zeolites (HUSY, HMOR, SAPO34 and HZSM-5), Hb with three-
dimensional large pore structure is benet for the diffusion of
reactant and products. Meanwhile, weak acid property of Hb
reduces the adsorption strength of guaiacol, and thus decreases
7 D. C. Elliott, Energy Fuels, 2007, 21, 1792–1815.
8 C. R. Lee, J. S. Yoon, Y.-W. Suh, J.-W. Choi, J.-M. Ha, D. J. Suh
and Y.-K. Park, Catal. Commun., 2012, 17, 54–58.
9 E. H. Lee, R. Park, H. Kim, S. H. Park, S.-C. Jung, J.-K. Jeon,
S. C. Kim and Y.-K. Park, J. Ind. Eng. Chem., 2016, 37, 18–21.
the toxic effect of guaiacol on acid sites. Moreover, proper acid 10 M. Hellinger, H. W. P. Carvalho, S. Baier, D. Wang, W. Kleist
density of Hb(50) can effectively balance the competitive
and J. D. Grunwaldt, Appl. Catal., A, 2015, 490, 181–192.
adsorption of guaiacol and conversion of alcohols on acid sites, 11 X. Zhang, J. Long, W. Kong, Q. Zhang, L. Chen, T. Wang,
enabling to achieve high yield of cyclohexane. Expectedly, such L. Ma and Y. Li, Energy Fuels, 2014, 28, 2562–2570.
a low-cost and efficient bifunctional catalytic system will hold 12 X. Zhang, Q. Zhang, L. Chen, Y. Xu, T. Wang and L. Ma, Chin.
potential for the hydrodeoxygenation of bio-oil.
J. Catal., 2014, 35, 302–309.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3868–3876 | 3875

View Article Online
RSC Advances
Paper
1
3 H. Fang, J. Zheng, X. Luo, J. Du, A. Roldan, S. Leoni and 29 G. Poncelet and M. L. Dubru, J. Catal., 1978, 52, 321–331.
Y. Yuan, Appl. Catal., A, 2017, 529, 20–31. 30 C. A. Emeis, J. Catal., 1993, 141, 347–354.
4 M. V. Bykova, D. Y. Ermakov, V. V. Kaichev, 31 J. Chang, T. Danuthai, S. Dewiyanti, C. Wang and A. Borgna,
1
O. A. Bulavchenko, A. A. Saraev, M. Y. Lebedev and
V. A. Yakovlev, Appl. Catal., B, 2012, 113, 296–307.
5 X. Wang, S. Zhu, S. Wang, J. Wang, W. Fan and Y. Lv, Appl.
Catal., A, 2018, 568, 231–241.
ChemCatChem, 2013, 5, 3041–3049.
32 M. Hellinger, S. Baier, P. M. Mortensen, W. Kleist,
A. D. Jensen and J.-D. Grunwaldt, Catalysts, 2015, 5, 1152–
1166.
1
1
1
1
1
2
2
2
6 S. Zhu, Y. Zhu, S. Hao, H. Zheng, T. Mo and Y. Li, Green 33 H. Shafaghat, P. S. Rezaei and W. M. A. W. Daud, RSC Adv.,
Chem., 2012, 14, 2607–2616. 2015, 5, 33990–33998.
7 S. Raju, M.-E. Moret and R. J. M. Klein Gebbink, ACS Catal., 34 C. D. Baertsch, K. T. Komala, Y.-H. Chua and E. Iglesia, J.
015, 5, 281–300. Catal., 2002, 205, 44–57.
8 H. Kobayashi, H. Yokoyama, B. Feng and A. Fukuoka, Green 35 T. Liang, J. Chen, Z. Qin, J. Li, P. Wang, S. Wang, G. Wang,
2
Chem., 2015, 17, 2732–2735.
9 S. Zhu, Y. Xue, J. Guo, Y. Cen, J. Wang and W. Fan, ACS
Catal., 2016, 6, 2035–2042.
0 L. Chen, R.-W. Wang, S. Ding, B.-B. Liu, H. Xia, Z.-T. Zhang 37 W. Song, Y. Liu, E. Barath, L. L. Wang, C. Zhao, D. Mei and
and S.-L. Qiu, Chem. J. Chin. Univ., 2010, 31, 1693–1696. J. A. Lercher, ACS Catal., 2016, 6, 878–889.
1 F. F. Madeira, K. Ben Tayeb, L. Pinard, H. Vezin, S. Maury 38 R. Shu, Y. Xu, L. Ma, Q. Zhang, P. Chen and T. Wang, Catal.
and N. Cadran, Appl. Catal., A, 2012, 443, 171–180. Commun., 2017, 91, 1–5.
2 J. Jae, G. A. Tompsett, A. J. Foster, K. D. Hammond, 39 W. Song, Y. Liu, E. Barath, C. Zhao and J. A. Lercher, Green
M. Dong, W. Fan and J. Wang, ACS Catal., 2016, 6, 7311–
7325.
36 H. Knozinger, Angew. Chem., Int. Ed., 1968, 7, 791–805.
S. M. Auerbach, R. F. Lobo and G. W. Huber, J. Catal.,
011, 279, 257–268.
3 J. Gu, Z. Zhang, P. Hu, L. Ding, N. Xue, L. Peng, X. Guo,
M. Lin and W. Ding, ACS Catal., 2015, 5, 6893–6901.
4 Y. Liu and Z. Gao, Acta Pet. Sin., Pet. Process. Sect., 1996, 12,
Chem., 2015, 17, 1204–1218.
2
40 M. S. Jang, R.-s. Park, I.-G. Lee, J. M. Kwak, Y.-K. Park and
C. H. Ko, Res. Chem. Intermed., 2016, 42, 3–17.
41 Y.-K. Hong, D.-W. Lee, H.-J. Eom and K.-Y. Lee, Appl. Catal.,
B, 2014, 150, 438–445.
2
2
2
2
2
2
3
5–40.
42 S. Qiu, Y. Xu, Y. Weng, L. Ma and T. Wang, Catalysts, 2016, 6,
134–149.
43 B. M. Q. Phan, Q. L. M. Ha, N. P. Le, P. T. Ngo, T. H. Nguyen,
T. T. Dang, L. H. Nguyen, D. A. Nguyen and L. C. Luu, Catal.
Lett., 2015, 145, 662–667.
5 J. Bao, J. He, Y. Zhang, Y. Yoneyama and N. Tsubaki, Angew.
Chem., 2008, 120, 359–362.
6 M. D. Foster, I. Rivin, M. M. J. Treacy and O. Delgado
Friedrichs, Microporous Mesoporous Mater., 2006, 90, 32–38.
ˇ ´
7 J. Hor ´a ˇc ek, G. St'avova, V. Kelbichova and D. Kubicka, Catal. 44 Y.-K. Hong, D.-W. Lee, H.-J. Eom and K.-Y. Lee, J. Mol. Catal.
Today, 2013, 204, 38–45.
´
´
ˇ
A: Chem., 2014, 392, 241–246.
8 A. Garc ´ı a-Trenco and A. Mart ´ı nez, Appl. Catal., A, 2012, 411–
412, 170–179.
3876 | RSC Adv., 2019, 9, 3868–3876
This journal is © The Royal Society of Chemistry 2019