One-pot catalytic conversion of methanol to C6-C21 hydrocarbons over bi-functional MFe2O4 (M = Ni, Zn, Mn, Co) catalysts

Article abstract of DOI:10.1039/c4ra14509g

We report a novel catalytic conversion of methanol to C6-C21 hydrocarbons over the bi-functional MFe₂O₄ (M = Ni, Zn, Mn, Co) catalysts prepared by a solvothermal method. The process consists of two steps: (i) the catalytic reforming of methanol to H₂ and CO, and (ii) the subsequent conversion of the syngas to hydrocarbons via Fischer-Tropsch synthesis (FTS). For comparison purposes, two other series of catalysts including MO (M = Ni, Zn, Mn, Co, Fe), and M_0.5Co_0.5Fe₂O₄ (M = Ni, Zn, Mn) catalysts were also prepared and tested. All the catalysts were characterized by X-ray diffraction, nitrogen adsorption, transmission electron microscopy, scanning electron microscopy, and H₂-temperature programmed reduction. Among these catalysts, ZnFe₂O₄ exhibited the highest activity with a high methanol conversion of 26% and a high selectivity to C6-C21 hydrocarbons above 94% at 300 °C. Moreover, this ZnFe₂O₄ catalyst was still stable and reusable after 4 runs under the reaction conditions. This work demonstrates a possibility to directly convert methanol to liquid fuels.

Full text of DOI:10.1039/c4ra14509g

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One-pot catalytic conversion of methanol to
C6–C21 hydrocarbons over bi-functional MFe2O4
(M ¼ Ni, Zn, Mn, Co) catalysts†
Cite this: RSC Adv., 2015, 5, 13374
b
b
a
Huilong Lai,ab Zailei Zhang, Fangna Gu, Zhengming Yi, Ziyi Zhongc
*
*
b
*
and Fabing Su
We report a novel catalytic conversion of methanol to C6–C21 hydrocarbons over the bi-functional
MFe2O4 (M ¼ Ni, Zn, Mn, Co) catalysts prepared by a solvothermal method. The process consists of two
steps: (i) the catalytic reforming of methanol to H2 and CO, and (ii) the subsequent conversion of the
syngas to hydrocarbons via Fischer–Tropsch synthesis (FTS). For comparison purposes, two other series
of catalysts including MO (M ¼ Ni, Zn, Mn, Co, Fe), and M0.5Co0.5Fe2O4 (M ¼ Ni, Zn, Mn) catalysts were
also prepared and tested. All the catalysts were characterized by X-ray diffraction, nitrogen adsorption,
transmission electron microscopy, scanning electron microscopy, and H2-temperature programmed
Received 14th November 2014
Accepted 19th January 2015
reduction. Among these catalysts, ZnFe2O4 exhibited the highest activity with
a high methanol
conversion of 26% and a high selectivity to C6–C21 hydrocarbons above 94% at 300 ꢀC. Moreover, this
ZnFe2O4 catalyst was still stable and reusable after 4 runs under the reaction conditions. This work
demonstrates a possibility to directly convert methanol to liquid fuels.
DOI: 10.1039/c4ra14509g
www.rsc.org/advances
In the past decades, the catalysts based on group VIII metals
such as Fe, Co, Ru, Rh and Ni were extensively investigated for
FTS.11–15 In order to improve the catalytic performance,
1. Introduction
With the increasing depletion of petroleum resources and
growing concerns over global climate change, production of
fuels from renewable and low-cost resources such as bio-
methanol has attracted tremendous attention because of the
high sustainability and environmental benignity of this
approach.1 Although methanol itself is a potential motor fuel or
can be blended with gasoline, large investment would be
required to overcome the technical problems connected with
the direct use of methanol as a fuel. Therefore, recently some
researchers have paid much attention to catalytic reforming of
methanol to syngas, which can be further converted to various
fuels and fuel additives through FTS.2 The reported catalysts for
methanol reforming are mainly copper-based and group VIII
metal-based catalysts.3–6 However, the copper-based catalysts
suffer from some problems such as pyrophoric phenomenon
and easy deactivation caused by thermal sintering.7,8 Compared
with these Cu-based catalysts, it seems group VIII catalysts have
better resistance to sintering and long-term stability.9,10
promoted catalysts, e.g., K promoted Fe catalyst,16 and bime-
tallic catalysts with generated synergistic geometric and elec-
tronic effects, e.g., cobalt ruthenium catalysts17 were tested. In
general, the catalytic activity of VIII metals for FTS is in the
order of Ru > Fe > Ni > Co > Ph > Pd > Pt.18 Although Ru catalysts
show the highest yield for high-carbon hydrocarbons and low
selectivity for methane, the high price and limited resource of
Ru restricts their application. Hence, most reported catalysts
are still Fe-, Ni- and Co-based catalysts. Even though oil
companies around the world have developed their patented
FTS technology and processes, which usually start from coal
gasication, desulfurization and decarbonization to syngas,
followed with conversion of the syngas to hydrocarbons.19,20
This process is very mature but quite complicated. We had
wondered for a long time whether the methanol reforming
reaction and the FTS of syngas could be integrated into a
one-pot process to produce hydrocarbons using bi-functional
catalysts.
Herein, we report a novel one-pot catalytic conversion of
methanol to C6–C21 hydrocarbons over Fe-based spinel
MFe2O4 (M ¼ Ni, Zn, Mn, Co) catalysts synthesized by a simple
solvothermal method. It is found that in the conversion of
methanol, the bi-functional Fe-based spinel catalysts rstly
catalyze methanol reforming to generate H2 and CO, and then
convert these gas products to C6–C21 hydrocarbons. Among
these catalysts, ZnFe2O4 shows the best catalytic properties and
aSchool of Chemical Engineering, Xiangtan University, Xiangtan 411105, Hunan,
China. E-mail: YIZM@xtu.edu.cn
bState Key Laboratory of Multiphase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail: fngu@
ipe.ac.cn; su@ipe.ac.cn; Fax: +86-10-82544851; Tel: +86-10-82544850
cInstitute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island,
Singapore 627833, Singapore
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra14509g
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good stability. The major liquid products containing C6–C21
hydrocarbons produced by this method can be used as diesel
and jet fuel.
2.3. Catalyst evaluation
In a typical run, 0.1–0.9 g catalyst and 30 mL methanol (Tianjin
Xihua Special Type Reagent Factory, China) were put into a
100 mL stainless-steel autoclave reactor (Parr Instrument
Company). Aer purged the reactor with nitrogen, the reaction
was carried out at a given temperature between 240–340 ꢀC for a
certain time (10–240 min) under a continuous stirring at
400 rpm. Aer the reaction, the reactor was cooled down to
room temperature. The gas products were collected by a gas bag
and analyzed by a Micro GC (3000A; Agilent Technologies)
equipped with a thermal conductivity detector (TCD). The
liquid products were separated from the catalyst by ltration
and analyzed by gas chromatography/mass spectrometry
(GCMS-QP2010 Ultra, Shimadzu) with a 30 m ꢂ 0.25 mm, and
0.25 mm lm thickness RX-5ms column. The compound was
determined by the computer soware from the characterized
GC/MS graphs. The residue methanol in the liquid products
were analyzed by GC (9790; Zhejiang Fuli Analytical Instrument
Co., Ltd) equipped with a hydrogen ame ionization detector
(FID). The liquid products over different catalysts were classi-
ed into ve groups, namely hydrocarbons, alcohols, methyl
esters, ketones, and others. The GC peak area and peak area%
of each product were employed to compare the relative content
among the detected products.22,23 Each group liquid products
were analyzed at least three times and the average values of peak
area and peak area% were calculated for discussion.24 Methanol
conversion was calculated using the following formulas:
2. Experimental section
2.1. Catalyst preparation
All the chemicals of analytical grade were purchased from
Sinopharm Chemical Reagent Co. Ltd., China, and used
without further treatment. All the catalysts were prepared by a
solvothermal method, and the used reactant, mineralizer and
solvent and their amounts are listed in Table S1.† In a typical
synthesis, Zn(CH3COO)2$4H2O (1.0 mmol), FeCl3$6H2O
(2.0 mmol), and CH3COONa$3H2O (18 mmol) were dissolved in
a solution of HOCH2CH2OH (40.0 mL) and deionized water
(40.0 mL) to form a homogeneous slurry by stirring, which was
subsequently sealed in a stainless-steel autoclave and heated at
200 C for 48 h to obtain the desired products.21 The resulting
ꢀ
precipitate was collected by centrifugation, washed with
distilled water and absolute ethanol, and nally dried in
vacuum at 80 ꢀC for 24 h. The synthesized samples was denoted
as MFe2O4 (M ¼ Ni, Zn, Mn, Co). Likewise, single metal oxide of
MO (M ¼ Ni, Zn, Mn, Co, Fe) catalysts and ternary metal oxides
of M0.5Co0.5Fe2O4 (M ¼ Ni, Zn, Mn) catalysts were prepared by
the same method but with addition of only one metal salt or
three metal salts.
2.2. Characterization
Massmethanol ꢁ Massresidue
Methanol conversion ¼
ꢂ 100% (1)
Massmethanol
The X-ray powder diffraction patterns of the samples were
obtained with a PRO (PANalytical) diffractometer operated at
40 kV and 40 mA by using Cu Ka radiation (l ¼ 1.5418 A). The
where Massmethanol is the mass of methanol used, Massresidue is
the mass of residual methanol aer reaction.
The stability of catalytic activity was tested at 300 ꢀC and
120 min for 4 runs. The catalyst aer reaction was washed with
deionized water and ethanol several times for next run.
˚
crystal size was calculated using the Debye–Scherrer equation.
The microscopic feature of the samples was characterized by
eld-emission scanning electron microscopy (SEM) with an
energy dispersive X-ray spectrometer (EDX) (JSM-7001F, JEOL,
Tokyo, Japan) and transmission electron microscopy (TEM)
(JEM-2010F, JEOL, Tokyo, Japan). Adsorption–desorption
isotherms were measured using N2 at ꢁ196 ꢀC with NOVA 3200e
(Quantachrome, USA) nitrogen adsorption apparatus. Before
3. Results and discussion
3.1. Characterization
measurement, the sample was degassed at 200 ꢀC for 5 h under Fig. 1a and b show the XRD patterns of the fresh MO (M ¼ Zn,
vacuum. The specic surface area was determined according to Co, Fe, Ni, Mn) and MFe2O4 (M ¼ Ni, Zn, Mn, Co) catalysts. The
the Brunauer–Emmett–Teller (BET) method in the relative typical diffraction peaks of MO and MFe2O4 match well with
pressure range of 0.05–0.2. Temperature-programmed reduc- their corresponding PDF les. Fig. 1c shows the XRD patterns of
tion with H2 (H2-TPR) was carried out on an automated chem-
M
0.5Co0.5Fe2O4 (M
¼
Ni, Zn, Mn) catalysts before the
isorption analyzer (chem-BET pulsar TPR/TPD, Quantachrome). reaction. The typical diffraction peaks of Ni0.5Co0.5Fe2O4,25
Prior to the measurement, 0.1 g of catalyst placed in a quartz Zn0.5Co0.5Fe2O4 (ref. 26) and Mn0.5Co0.5Fe2O4 (ref. 27) appear in
ꢀ
U-tube was pretreated in an Ar stream at 300 C for 1 h. Then, the XRD patterns of Ni0.5Co0.5Fe2O4, Zn0.5Co0.5Fe2O4 and
the sample was cooled to room temperature, followed by heat- Mn0.5Co0.5Fe2O4 catalysts, respectively, indicating the success-
ing to 1000 ꢀC at 10 ꢀC minꢁ1 under a binary gas (10.0 vol% H2/ ful synthesis of MFe2O4 (M
¼
Mn, Ni, Zn, Co) and
Ar) with a gas ow of 30 mL minꢁ1. The surface chemical
M
0.5Co0.5Fe2O4 (M ¼ Mn, Ni, Zn) with the spinel structure by the
composition of the samples was determined by X-ray photo- solvothermal method. The crystal size was calculated by
electron spectroscopy (XPS) on a VG ESCALAB 250 spectrometer Scherrer equation and summarized in Table 1. The particle
(Thermo Electron, U.K.), using a nonmonochromatized Al Ka sizes of the MO catalysts are varied in a wide range. Among
X-raysource (1486 eV). The binding energies were calibrated them, NiO and Mn3O4 have a similar size of ca. 42 nm, while
using C 1s peak of contaminant carbon (BE ¼ 284.6 eV) as an Fe3O4 and ZnO have a size distribution of 93–102 nm, and the
internal standard.
size of Co3O4 is about 82.8 nm. Comparing to the MO (M ¼ Zn,
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Co, Fe, Ni, Mn) catalysts, both MFe2O4 (M ¼ Ni, Zn, Mn, Co) and increase in the intensity peak of FeC2. These results suggest that
0.5Co0.5Fe2O4 (M ¼ Ni, Zn, Mn) have much smaller particle the reaction conditions and the catalyst amount greatly affect
sizes (10–30 nm). The H2-TPR curves of the MFe2O4 (M ¼ Zn, the state of the components of the catalysts.
M
Mn, Co) catalysts (Fig. 1d) show that the MnFe2O4 (M ¼ Zn, Mn,
Table 1 compiles the surface area of all the catalysts. The
Co) catalysts were not reduced below 340 C under the H2-TPR surface areas of the MO catalysts are 0.1–5.7 m2 gꢁ1, except that
ꢀ
test condition. The XPS spectra of ZnFe2O4 catalyst reduced at of Mn3O4 is 45.2 m2 gꢁ1. Compared with the MO catalysts, the
340 ꢀC with H2 are given in Fig. S1.† The signals at 1044.7 and MFe2O4 and M0.5Co0.5Fe2O4 catalysts have larger surface areas,
1021.4 eV can be attributed to Zn 2p1/2 and Zn 2p3/2 of Zn2+
.
varying from 21.1 to 76.5 m2 gꢁ1
.
28
Additionally, the signals at around 711.6 and 725.5 eV are from
Fig. S3† and 2a display the SEM images of the MO catalysts.
Fe3+ at octahedral sites.29 These results conrm that ZnFe2O4 As we can see, the MO catalysts show different morphologies,
cannot be reduced at 340 ꢀC under H2-TPR condition. Fig. S2a† such as sphere-like Fe3O4 and Mn3O4, akiness Co3O4 and NiO,
shows the XRD patterns of the MFe2O4 and Mn0.5Co0.5Fe2O4 rod-shaped ZnO. Fig. 2b–e display the SEM images of the
catalysts aer reaction. For all the used catalysts, some new MFe2O4 catalysts. All the MFe2O4 catalysts exhibit spherical
diffraction peaks appear at 26.6 and 32.5ꢀ, attributing to carbon morphology, and the sizes of them are smaller than that of
(03-065-6212) and FeCO3 (00-003-46) respectively, and a group of Fe3O4, implying adding second metal salt favors forming
diffraction peaks at 36.9ꢀ, 32.9ꢀ, corresponding to FeC2 (03- smaller particles, which is consistent with the above XRD
065-1456). The Fe3+ can be reduced to Fe2+ by the CO produced results. The SEM images of M0.5Co0.5Fe2O4 show that these
by methanol reforming.30 Carbon is formed through the samples consist of nanoparticles with a size of 20–30 nm
disproportionation reaction of CO. It is possible that CO2 (Fig. 2f–h), similar to that of the MFe2O4 catalysts.
formed by the water–gas shi reaction plays a key role in the
Fig. 3a shows a representative transmission electron
formation of FeCO3. These results indicate a small amount of microscopy (TEM) image of the ZnFe2O4 catalyst. It reveals that
Fe-based spinel catalysts can be reduced during methanol the nanoparticles sizes of this catalyst are about 30 nm. The
conversion reaction. Moreover, with the increase of reaction HRTEM image of these nanoparticles (Fig. 3b) shows that the
temperature from 220 to 340 ꢀC, the peak intensity of FeC2 lattice fringe spacing is about 0.4847 nm, corresponding to the
increases and that of the carbon decreases, indicating that the interplanar distance of (110) planes in ZnFe2O4. The corre-
reduction of Fe3+ is promoted at high temperature under the sponding elemental mappings for the MFe2O4 (M ¼ Zn, Mn, Co)
reaction conditions (Fig. S2b†). The peak intensity of carbon catalysts are displayed in Fig. 3c and S4 and S5.† As can be seen,
increases with the reaction time from 10 to 60 min, and then all the elements are evenly distributed within the whole
decreases with prolonging reaction time. With the increase of samples. The molar ratios of each element in the nal samples
reaction time from 10 to 240 min, the peak intensity of FeC2 are approximately equal to the initial ratios of the metal
shows almost no changes (Fig. S2c†). Additionally, when the precursors added in the preparation process (Table S1†). The
catalyst amount is changed from 0.1 to 0.9 g, there is a gradual SEM-EDX spectrum of ZnFe2O4 nanoparticles is given in Fig. 3d,
which demonstrates the presence of Zn, Fe, and O elements
with an approximate atomic ratio of 1 : 2 : 4.
3.2. Catalytic conversion of methanol over the MO, MFe2O4
and M0.5Co0.5Fe2O4 catalysts
The catalytic conversion results of methanol over all the cata-
lysts are summarized in Table 2. The blank test (without any
Table 1 Physicochemical properties of all the catalysts
Sample
SBETa (m2 gꢁ1
)
Crystal sizeb (nm)
NiO
ZnO
Mn3O4
Co3O4
Fe3O4
NiFe2O4
ZnFe2O4
MnFe2O4
CoFe2O4
Ni0.5Co0.5Fe2O4
Zn0.5Co0.5Fe2O4
Mn0.5Co0.5Fe2O4
4.5
0.5
45.2
0.1
42.9
102.1
41.9
82.8
92.8
11.6
29.4
19.2
11.9
14.7
18.4
21.6
5.7
76.5
21.1
40.1
63.1
69.8
66.3
49.2
a
Surface area, derived from BET equation. b Crystal size, derived from
Fig. 1 XRD patterns of all the catalysts (a–c), and H2-TPR curves of
XRD by Debye–Scherrer equation.
ZnFe2O4, MnFe2O4, and CoFe2O4 catalysts (d).
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catalyst) gives about 8.9% conversion of methanol with a little
amount of gas product. Aer added with the MO (M ¼ Zn, Co,
Fe, Ni, Mn) catalysts, the conversion of methanol is increased,
which reaches 35.9% over the Co3O4 catalyst, implying that the
methanol reforming reaction is catalyzed. Meanwhile, MFe2O4
and M0.5Co0.5Fe2O4 exhibit higher conversion of methanol
(above 22%) compared with most of the MO catalysts except for
the Co3O4 and NiO catalysts. The yields of gas products over NiO
and Co3O4 are similar and highest among all the MO catalysts,
and that over Fe3O4 is second highest. Among all the MFe2O4
catalysts, NiFe2O4, ZnFe2O4 and MnFe2O4 catalysts exhibit
higher yield of gas products, whereas the CoFe2O4 catalyst yields
less gas product than the Fe3O4 catalyst. It may be because the
addition of the second metal element promotes methanol
reforming reaction. Additionally, the larger specic surface
areas of the formed NiFe2O4, ZnFe2O4 and MnFe2O4 catalysts
may enable them to expose more active sites for methanol
reforming reaction. However, the addition of Co element is in
favor of methanation reaction, which is a gas volume dimin-
ishing reaction. Compared to the MFe2O4 catalysts, the yields of
the gas products over the M0.5Co0.5Fe2O4 catalysts are lower,
probably due to the fact that the Co element is in favor of
methanation reaction.
Fig. 4a shows the distribution of the gas products obtained
over all the catalysts. The products of methanol conversion
reaction vary with the catalysts used. In general, the gas prod-
ucts mainly contain H2, CO, CO2, CH4 and small portion of
gaseous hydrocarbons such as C2H4, C2H6, C3H6, and C3H8. For
both ZnO and Mn3O4, the main gas products are H2 and CO,
which are similar to the case without adding catalyst, however,
the amounts of the obtained gas product over these two cata-
lysts are obviously larger than that obtained in the blank
experiment (Table 2), indicating that ZnO and Mn3O4 catalysts
can promote the methanol reforming reaction (eqn (2)).31,32
Meanwhile, the NiO and Co3O4 catalysts are observed to
promote production of CH4 and CO2, suggesting the occurrence
of both the water–gas shi (eqn (3)) and the methanation
Fig. 2 SEM images of the catalysts: (a) Fe3O4, (b) NiFe2O4, (c) ZnFe2O4,
(d) MnFe2O4, (e) CoFe2O4, (f) Ni0.5Co0.5Fe2O4, (g) Zn0.5Co0.5Fe2O4, and
(h) Mn0.5Co0.5Fe2O4.
Table 2 Conversion of methanol and the total gas product mass over
MO, MFe2O4 and M0.5Co0.5Fe2O4 catalysts
Catalyst
Conversiona (%)
Total gas mass (g)
Without catalyst
NiO
ZnO
Mn3O4
Co3O4
Fe3O4
NiFe2O4
ZnFe2O4
MnFe2O4
CoFe2O4
8.9
1.8
6.1
2.9
2.8
6.1
4.7
6.5
5.6
4.8
4.2
4.6
4.0
4.8
26.6
12.0
11.3
35.9
19.4
29.7
26.2
22.1
24.2
24.8
22.8
27.3
Ni0.5Co0.5Fe2O4
Zn0.5Co0.5Fe2O4
Mn0.5Co0.5Fe2O4
Fig. 3 TEM image of ZnFe2O4 (a), HRTEM image of ZnFe2O4 (b), SEM
image and elemental mapping images of O, Zn and Fe for ZnFe2O4
nanoparticles (c), and SEM-EDX spectrum (d).
a
Reaction conditions: 0.5 g catalyst, 30 mL methanol, 180 min, 280 ꢀC.
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Fig. 4 Distribution of gas (a) and liquid (b) products over all the catalysts. (Reaction conditions: 0.5 g catalyst, 30 mL methanol, 180 min, 280 ꢀC).
reactions (eqn (4)–(6)). Moreover, the product distribution over catalyze methanol reforming reaction but also facilitates the
the Fe3O4 catalyst is similar to that obtained in the blank methanation reaction and water–gas shi reaction. The gas
experiment, just with lower H2 and CO but higher CH4 and CO2 product compositions over ZnFe2O4 and MnFe2O4 catalysts are
yields. This indicates that adding Fe3O4 catalyst not only similar to that over Fe3O4 catalyst, and in contrast, the NiFe2O4
Scheme 1 Proposed reaction pathways for conversion of methanol over iron-based catalysts.
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Table 3 Effect of reaction conditions on the conversion of methanol over ZnFe2O4 catalyst
Entry
Reaction temperature (ꢀC)
Reaction time (min)
Catalyst amount (g)
Conversion (%)
Total gas mass (g)
1
2
3
4
5
6
7
8
240
260
280
300
320
340
300
300
300
300
300
300
300
300
300
300
300
180
180
180
180
180
180
10
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
0.3
0.9
0.5
0.5
0.5
17.0
22.5
26.2
26.7
27.9
27.8
19.2
21.8
23.8
26.1
27.4
19.0
23.7
27.7
25.7
26.0
25.3
4.1
5.1
5.6
5.7
5.8
5.8
4.7
5.0
5.3
5.6
5.6
5.0
5.4
5.9
5.6
5.5
5.5
30
60
9
10
11
12
13
14
15
16
17
120
240
120
120
120
120
120
120
and CoFe2O4 catalysts promote the production of CH4. the catalysts. These hydrocarbons may stem from the pyrolysis
Compared with that of the MFe2O4 catalysts (except NiFe2O4), of higher hydrocarbon or the Fischer–Tropsch synthesis.
the yields of H2 and CO are decreased, but that of CH4 is
Fig. 4b presents the liquid product distribution in the
increased over the M0.5Co0.5Fe2O4 catalysts, indicating that methanol conversion over all the catalysts. The main liquid
adding the third metal element Co promotes the methanation products are a mixture of different compounds, such as C6–C21
reaction. Furthermore, a small amount of gaseous hydrocar- hydrocarbons, alcohols, esters and ketones. There is nearly no
bons such as C2H4, C2H6, C3H6, and C3H8 are formed over all liquid product in the absence of any catalyst (blank test in
Fig. 5 Effect of reaction temperature on the distribution of gas (a) and liquid (b) products over ZnFe2O4 catalyst. (Reaction conditions: 0.5 g
catalyst, 30 mL methanol, 180 min, 240–340 ꢀC).
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Fig. 6 Effect of reaction time on the distribution of gas (a) and liquid (b) products over ZnFe2O4 catalyst. (Reaction conditions: 0.5 g catalyst, 30
mL methanol, 300 ꢀC, 1–240 min.).
Fig. 4b). Likewise, the yields of liquid products over most of the results and product distributions. The formation of gas prod-
MO catalysts are also very low except for the Co3O4 and Fe3O4 ucts from CH3OH may involves the reaction of methanol
catalysts. Especially, Fe3O4 catalyst shows relatively high selec- reforming (eqn (2))31 to produce CO and H2, and the water–gas
tivity for C6–C21 hydrocarbons. Compared with that of Fe3O4, shi reaction in which CO reacts with water to form CO2 and H2
the yields of liquid products over ZnFe2O4, MnFe2O4 and (eqn (3)),33,34 as well as the methanation reaction (eqn (4)–(6)):35
CoFe2O4 catalysts are increased generally and the major liquid
CH3OH % 2H2 + CO
CO + H2O % CO2 + H2
CO + 3H2 % CH4 + H2O
2CO + 2H2 % CH4 + CO2
CO2 + 4H2 % CH4 + 2H2O
(2)
(3)
(4)
(5)
(6)
products are C6–C21 hydrocarbons (Table S2†). In contrast, the
yields of liquid products over NiFe2O4 catalyst are lower than
that over Fe3O4, which may be because the addition of Ni
promotes the methanation reaction and most of the syngas is
converted to CH4 (Fig. 4a). Additionally, compared to the
MFe2O4 (M ¼ Ni, Zn, Mn) catalysts, the addition of the third
metal Co does not further increases the yields of the liquid
products. In particular, Ni0.5Co0.5Fe2O4 shows a very low yield of
C6–C21 hydrocarbons (Table S3† and Fig. 4b). This is because
Ni and Co species can signicantly promote syngas conversion
to methanation, which is a competitive reaction to FTS. Among
In methanol reforming reaction process, methanol is rapidly
all these catalysts, ZnFe2O4 catalyst exhibits the highest yield of decomposed into CO and H2. In this step, different catalysts
C6–C21 hydrocarbons. Based on the results of peak area (%) in result in different conversions and gas compositions. As we can
Table S2 and S3,† the relative content of C6–C21 hydrocarbons see from Table 2 and Fig. 4, the MFe2O4 and M0.5Co0.5Fe2O4
over ZnFe2O4 is also the highest in the liquid products.
catalysts generate higher conversations and more gas products
as compared with Fe3O4 catalyst, and the Ni, Co and Fe con-
taining catalysts are more favorable to producing CH4. Addi-
tionally, the Fe-containing catalysts generate more CO2 product
as compared with the others. Therefore, we can conclude that
the catalysts containing Zn, Ni, Mn, Co and Fe element can
promote methanol reforming reaction,24,31,32,36–38 while the
3.3. Reaction pathways analysis
The mechanism for methanol conversion to hydrocarbons over
all the catalysts is complicated. As shown in Scheme 1, we
propose the reaction pathways involved in the conversion of
methanol over all the catalyst based on the above catalytic
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Fig. 7 Effect of catalyst amount on the distribution of gas (a) and liquid (b) products over ZnFe2O4 catalyst. (Reaction conditions: 0.1–0.9 g
catalyst, 30 mL methanol, 120 min, 300 ꢀC).
catalysts containing Ni, Co and Fe element can promote the dehydrogenation of intermediate-1 to hydrocarbons. Moreover,
subsequent methanation reaction,39–41 and the Fe-containing the addition of ZnFe2O4, MnFe2O4 and CoFe2O4 catalysts not
catalysts promote water–gas shi reaction.42 Subsequently, the only promoted the hydrogenation,43–45 but also promoted
hydrogen in the gas products of methanol reforming can be dehydration reaction,46–48 both of which are favorable for the
converted to hydrocarbons via Fischer–Tropsch synthesis (FTS). production of hydrocarbons, and thus adding these three
The main reactions during FTS can be expressed in eqn (7)–(10): catalysts can increase the yield of hydrocarbons. However, the
Mn0.5Co0.5Fe2O4 and Zn0.5Co0.5Fe2O4 catalysts can decrease the
(2n + 1)H2 + nCO / CnH2n+2 + nH2O
(7)
yield of hydrocarbons as compared with the MFe2O4 catalyst.
This is because Co species can signicantly promote syngas
conversion to methanation, which is a competitive reaction to
FTS. On the other hand, the intermediate-1 also can be con-
verted to oxygenated compounds directly, such as aldehydes,
alcohols, which can be explained by the CO insertion mecha-
nism. Additionally, the ketones can be formed through two
ways. One is that alcohols react with intermediate-1 to form
ketones (see red solid line of Scheme 1), and the other is that
aldehydes are converted to carboxylic acid via cannizzaro reac-
tion, and then the formed carboxylic acid reacts with alcohols to
form ketones (see green solid lines of Scheme 1).49 As we
investigated, ZnO showed a signicant activity in hydrogenation
of ethylene, and also was effective to catalyze the dehydration of
glycerol to yield acetol.43,46 Likewise, in our reaction system,
ZnFe2O4 is identied as the most effective catalyst for conver-
sion of methanol to C6–C21 hydrocarbons, due to its catalytic
capability for the hydrogenation and dehydration reactions.
Therefore, it can conclude that catalyst compositions greatly
2nH2 + nCO / CnH2n + nH2O
(8)
nCO + 2nH2 / CnH2n+1OH + (n ꢁ 1)H2O
(n + 1)CO + (2n + 1)H2 / CnH2n+1CHO + nH2O
(9)
(10)
In the FTS process, rstly, CO and H2 are converted to
intermediate-1 through hydrogenation and dehydration reac-
tion. Subsequently, the intermediate-1 is further converted via a
series of successive or parallel steps. On the one hand, the
intermediate-1 can be converted to intermediate-2 through
hydrogenation and dehydration reaction,18 and then the
intermediate-2 is further converted to alkanes and olens by
hydrogenation or dehydrogenation. In the process of CO and H2
conversion to hydrocarbons, only Co3O4 and Fe3O4 among the
MO catalysts show the high catalytic activity with a high selec-
tivity towards C6–C21 hydrocarbons. The reason may be that
Co3O4 and Fe3O4 catalyst accelerate the hydrogenation and
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Fig. 8 Distribution of gas (a) and liquid (b) products over 4 circles over ZnFe2O4 catalyst. (Reaction conditions: 0.5 g catalyst, 30 mL methanol,
120 min, 300 ꢀC).
affect the yield and distribution of both gas and liquid products. CH4. The yield of CO2 and CH4 is highly dependent on the
ZnFe2O4 exhibits the best catalytic activity among all the increase of the reaction temperature, indicating a relative
examined catalysts. The following study was therefore focusing higher temperature is in favor of the water–gas shi and
on the conversion of methanol over ZnFe2O4.
methanation reaction in the examined temperature window.
When the temperature is above 300 ꢀC, the yield of CO2 is
almost not changed, while that of H2 is decreased gradually,
and those of ethane and propane are increased more obviously
due to FTS. Fig. 5b shows the distribution of liquid products at
different reaction temperatures. It reveals that the yield of
C6–C21 hydrocarbons is increased with the change of the
reaction temperature from 240 to 300 ꢀC, but the increment
becomes very small at higher reaction temperatures. Addition-
ally, when the temperature is above 300 ꢀC, more ketone is
formed. This can be attributed to the occurrence of the
competitive reactions, which are promoted at high temperature
leading to the formation of byproducts and more gas prod-
ucts.50 Based on the measured peak areas (%) in Table S4,† the
main liquid products over ZnFe2O4 catalyst are C6–C21 hydro-
carbons at 240–340 ꢀC, including straight chain hydrocarbon
and their derivatives. The relative content of C6–C21 hydrocar-
bons is as high as 100% when the reaction temperature is below
3.4. Methanol conversion over ZnFe2O4 catalyst
3.4.1. Effect of reaction temperature. Table 3 (entries 1–6)
shows the effect of the reaction temperature on the conversion
of methanol and the gas products. When the reaction temper-
ꢀ
ature is 220 C (not shown here), negligible amount of the gas
products is collected, indicating that the methanol reforming
reaction is still not initiated at this temperature. With increase
of the reaction temperature from 240 to 340 ꢀC, both the
methanol conversion and the gas products are increased grad-
ually, and then maintained at a certain value. These results
suggest higher reaction temperature favors the production of
ꢀ
gas products in the temperature range from 220 to 320 C.
Fig. 5a shows the inuence of the reaction temperature on
the gas product distribution in methanol conversion over the
ZnFe2O4 catalyst. Around 240 ꢀC, H2 and CO are detected as the
main products, because the methanol reforming reaction is
likely to occur at this temperature. Further increasing the
reaction temperature leads to the decrease of CO and H2 yield
but also simultaneously to the rising up both the CO2 and CH4
yield, because of the occurrence of methanation reaction and
water–gas shi reaction which convert CO and H2 to CO2 and
ꢀ
260 C. It is noteworthy that with the increase of the reaction
temperature, the relative content of low carbon number
hydrocarbons become higher, and simultaneously, the relative
content of C14–C21 hydrocarbons becomes lower (entries 37–45
in Table S4†), suggesting that the high carbon number
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hydrocarbons are decomposed to the ones with lower carbon however, it declines to 91% as the catalyst amount is increased
numbers at higher reaction temperatures.51 By comprehensively to 0.9 g. The reason should be that although the increased
considering both the conversion of methanol and the yield of amount of catalyst can provide more active sites for the meth-
C6–C21 hydrocarbons, the optimum reaction temperature is anol reforming and subsequent FTS reactions, the excess cata-
ꢀ
determined to be ca. 300 C.
lyst further promotes decomposition of hydrocarbons to by-
3.4.2. Effect of reaction time. As shown in Table 3, the products and gas products.53 Considering both the conversion
conversion of methanol reaches 19.2% when the reaction time of methanol and the yield of C6–C21 hydrocarbons, the opti-
is 10 minutes (entry 7 in Table 3), suggesting that the reforming mized catalyst amount is 0.5 g.
reaction of methanol to CO and H2 did occur under this reac-
3.4.4. Catalytic stability of the ZnFe2O4 catalyst. Fig. 8
tion condition. As the reaction time is extended from 10 to shows the catalytic activity of ZnFe2O4 catalyst for the methanol
120 min, the methanol conversion and the total amount of gas conversion undergoing four runs. It reveals that the distribution
products (entries 4, 7–11 in Table 3) are increased gradually. of gas products changes somewhat in these four circles, while
However, these values are only slightly increased when the that of the liquid products is not changed obviously in the next
reaction time is further extended to 240 min. Fig. 6a shows that three circles. In the next three stability tests, a little amount of
the distribution of gas products is obviously changed with catalyst is inevitably lost. The distribution of C6–C21 hydro-
reaction time, indicating that reaction time has much effect on carbons in liquid products is also similar in the four circles
gas product distribution. With the reaction time increasing (Table S7†). These results indicate that ZnFe2O4 catalyst is
from 10 to 240 min, the relative amounts of H2 and CO in gas relatively stable and reusable under the reaction condition used
products become lower, while those of CH4 and CO2 become in this work.
higher. This may be because the active sites of methanation are
formed simultaneously with the reaction under the reaction
conditions.52 The effect of reaction time on the distribution of
4. Conclusions
liquid products is presented in Fig. 6b. It can be seen that the In summary, it has demonstrated that methanol can be effi-
yields of both the total liquid products and the C6–C21 hydro- ciently converted to C6–C21 hydrocarbons over the bi-
carbons become higher with the reaction time increasing from functional Fe-based spinel catalysts synthesized by a facile
10 to 120 min, indicating in the initial stage of the reaction, the solvothermal method, in which the Fe-based spinel catalysts
FT synthetic is the dominant reaction. When the reaction time play dual roles: (i) catalyzing methanol reforming to generate H2
is longer than 120 min, there is little change for the peak area and CO, and then (ii) converting these gas products to hydro-
values of C6–C21 hydrocarbons, suggesting that it has reached carbons through Fischer–Tropsch reaction. The MnFe2O4,
equilibrium between the FT synthetic and decomposition ZnFe2O4 catalysts enhance the production of C6–C21 hydro-
reactions of hydrocarbons. Table S5† demonstrates that as the carbons while NiFe2O4 and CoFe2O4 catalysts promote the
reaction time is above 180 min, the relative content of low formation of more gas products such as CH4 and CO2. It is
carbon number hydrocarbons becomes higher, while that of found that all the used MO catalysts promote methanol
C14–C21 hydrocarbons becomes lower (entries 33–43 in Table reforming reaction except that Co3O4 and Fe3O4 favor for FTS.
S5†), conrming that these hydrocarbons are decomposed into Comparing with the Fe-based spinel catalysts, further doping of
hydrocarbons with lower carbon numbers at longer reaction Co to the spinel structure does not improve the catalytic activity.
time. Accordingly, based on the conversion of methanol and the On the contrary, Ni0.5Co0.5Fe2O4 has lower catalytic activity as
yield of C6–C21 hydrocarbons, the optimized reaction time is compared with the Fe3O4 catalyst, because Co species can
120 min.
signicantly promote syngas conversion to methanation, which
3.4.3. Effect of catalyst amount. As shown in Table 3 is a competitive reaction to FTS. Among these catalysts, ZnFe2O4
(entries 10, 12–14), both the conversion of methanol and total catalyst exhibits the highest catalytic activity and yield towards
amount of the gas product are increased with the addition of C6–C21 hydrocarbons. The stability test shows that the ZnFe2O4
more catalyst but not in a proportional way. Fig. 7a shows the catalyst is stable and reusable under the reaction conditions.
effect of catalyst amount on the distribution of gas products. It This work provides a new promising route to convert abundant
reveals that the volume fraction of CH4 is increased but that of renewable methanol to liquid fuels by one-pot reaction. Further
H2 and CO decreased drastically with the addition of more work in selection of supports for the ZnFe2O4 active component
catalyst, which is attributed to the fact that with addition of for further increasing the yield and selectivity towards the liquid
more catalyst, it will provide more active sites for methanation products is undergoing.
reaction, which will consume more H2 and CO. Fig. 7b presents
the distribution of the liquid products against the catalyst
amount. When the catalyst amount is changed from 0.1 to 0.5 g,
Acknowledgements
there is a gradual increase in the yield of C6–C21 hydrocarbons. This work was supported by the Major State Basic Research
With further addition of the catalyst, the yield of C6–C21 Development Program of China (973 Project) (nos
hydrocarbons becomes a little higher and more by-products 2011CB200906 and 2014CB744306), National Natural Science
such as ketones and alcohols are formed. As shown in Table Foundation of China (no 21161140329), National High Tech-
S6,† the relative content of C6–C21 hydrocarbons keeps at about nology Research and Development Program of China (863) (no
97% when the catalyst content is maintained below 0.5 g; SS2012AA021402), and “Strategic Priority Research Program” of
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the Chinese Academy of Sciences (nos XDA07010100 and 27 H. M. I. Abdallah, T. Moyo and J. Z. Msomi, J. Phys.: Conf.
XDA07010200).
Ser., 2010, 217, 012141.
28 B. H. Davis, Catal. Today, 2009, 141, 25–33.
29 S. Bera, A. A. M. Prince, S. Velmurugan, P. S. Raghavan,
R. Gopalan, G. Panerslvam and S. V. Narasihan, J. Mater.
Sci., 2001, 36, 5379–384.
30 Z. Xing, Z. C. Ju, J. Yang, H. Y. Xu and Y. T. Qian, Nano Res.,
2012, 5, 477–485.
References
1 C. N. Hamelinck and A. P. Faaij, J. Power Sources, 2002, 111,
1–22.
2 Z. X. Wang, T. Dong, L. X. Yuan, T. Kan, X. F. Zhu,
Y. Torimoto, M. Sadakata and Q. X. Li, Energy Fuels, 2007,
21, 2421–2432.
´
31 D. V. Cesar, R. F. Robertson and N. S. Resende, Catal. Today,
2008, 133–135, 136–141.
3 H. Kobayashi, N. Takezawa and C. Minochi, J. Catal., 1981,
69, 487–494.
32 R. O. Idem and N. N. Bakhshi, Chem. Eng. Sci., 1996, 51,
3697–3708.
4 H. Agaras, G. Cerrella and M. Laborde, Appl. Catal., 1988, 45,
53–60.
5 C. Fukuhara, Y. Kamata and A. Igarashi, Appl. Catal., A, 2007,
330, 108–116.
6 N. Iwasa, T. Mayanagi, N. Ogawa, K. Sakata and N. Takezawa,
Catal. Lett., 1998, 54, 119–123.
7 A. M. Karim, T. Conant and A. K. Datye, Phys. Chem. Chem.
Phys., 2008, 10, 5584–5590.
33 J. Amelse, L. Schwartz and J. Butt, J. Catal., 1981, 72, 95–110.
34 G. Chinchen, R. Logan and M. Spencer, Appl. Catal., 1984,
12, 89–96.
35 M. Araki and V. Ponec, J. Catal., 1976, 44, 439–448.
36 M. Mrad, D. Hammoud, C. Gennequin, A. Aboukais and
E. Abi-Aad, Appl. Catal., A, 2014, 471, 84–90.
37 N. Laosiripojana and S. Assabumrungrat, J. Power Sources,
2007, 163, 943–951.
8 C. Z. Yao, L. C. Wang, Y. M. Liu, G. S. Wu, Y. Cao, W. L. Dai,
H. Y. He and K. N. Fan, Appl. Catal., A, 2006, 297, 151–158.
9 D. R. Palo, R. A. Dagle and J. D. Holladay, Chem. Rev., 2007,
107, 3992–4021.
`
38 E. Manova, T. Tsoncheva, C. Estournes, D. Paneva,
K. Tenchev, I. Mitov and L. Petrov, Appl. Catal., A, 2006,
300, 170–180.
39 R. Hayes, W. Thomas and K. Hayes, J. Catal., 1985, 92, 312–
326.
40 P. K. Agrawal, J. R. Katzer and W. H. Manogue, J. Catal., 1981,
69, 312–326.
41 A. L. Kustov, A. M. Frey, K. E. Larsen, T. Johannessen,
J. K. Norskov and C. H. Christensen, Appl. Catal., A, 2007,
320, 98–104.
42 D. S. Newsome, Catal. Rev.: Sci. Eng., 1980, 21, 275–318.
43 A. Dent and R. Kokes, J. Phys. Chem., 1969, 73, 3781–3790.
44 H. Itoh, S. Nagano, K. Takeda and E. Kikuchi, Appl. Catal., A,
1993, 96, 125–134.
45 D. Schanke, S. Vada, E. Blekkan, A. Hilmen, A. Hoff and
A. Holmen, J. Catal., 1995, 156, 85–95.
46 M. Balaraju, V. Rekha, P. S. Prasad, R. Prasad and
N. Lingaiah, Catal. Lett., 2008, 126, 119–124.
47 S. Hocevar, J. Batista and V. Kaucic, J. Catal., 1993, 139, 351–
361.
48 P. J. Skrdla and R. T. Robertson, J. Mol. Catal. A: Chem., 2003,
194, 255–265.
´
˜
10 S. Sa, H. Silva, L. Brandao, J. M. Sousa and A. Mendes, Appl.
Catal., B, 2010, 99, 43–57.
11 A. Y. Khodakov, W. Chu and P. Fongarland, Chem. Rev.,
2007, 107, 1692–1744.
12 J. M. Gonzalez Carballo, E. Finocchio, S. Garcia-Rodriguez,
M. Ojeda, J. L. Garcia Fierro and S. Rojas, Catal. Today,
2013, 214, 2–11.
13 E. Iglesia, S. L. Soled and R. A. Fiato, J. Catal., 1992, 137, 212–
224.
14 R. A. Dictor and A. T. Bell, J. Catal., 1986, 97, 121–136.
15 C. K. Rofer-DePoorter, Chem. Rev., 1981, 81, 447–474.
16 S. Li, S. Krishnamoorthy, A. Li, G. D. Meitzner and E. Iglesia,
J. Catal., 2002, 206, 202–217.
17 E. Iglesia, S. L. Soled, R. A. Fiato and G. H. Via, J. Catal., 1993,
143, 345–368.
18 H. Schulz, Appl. Catal., A, 1999, 186, 3–12.
19 M. E. Dry, Appl. Catal., A, 2004, 276, 1–3.
20 B. Jager, P. Van Berge and A. P. Steynberg, Stud. Surf. Sci.
Catal., 2001, 136, 63–68.
21 Z. L. Zhang, Q. Q. Tan, Y. F. Chen, J. Yang and F. B. Su,
J. Mater. Chem. A, 2014, 2, 5041–5050.
22 C. Q. Dong, Z. F. Zhang, Q. Lu and Y. P. Yang, Energy Convers.
Manage., 2012, 57, 49–59.
49 E. de Smit and B. M. Weckhuysen, Chem. Soc. Rev., 2008, 37,
2758–2781.
50 A. A. Mirzaei, B. Shirzadi, H. Atashi and M. Mansouri, J. Ind.
Eng. Chem., 2012, 18, 1515–1521.
51 J. S. Kim, S. Lee, S. B. Lee, M. J. Choi and K. W. Lee, Catal.
Today, 2006, 115, 228–234.
52 C. W. Hu, J. Yao, H. Q. Yang, Y. Chen and A. M. Tian, J.
Catal., 1997, 166, 1–7.
23 A. Pattiya, J. O. Titiloye and A. V. Bridgwater, J. Anal. Appl.
Pyrolysis, 2008, 81, 72–79.
24 Y. H. Wu, F. N. Gu, G. W. Xu, Z. Y. Zhong and F. B. Su,
Bioresour. Technol., 2013, 137, 311–317.
25 K. Maaz, S. Karim, K. J. Lee, M. H. Jung and G. H. Kim, Mater.
Chem. Phys., 2012, 133, 1006–1010.
53 M. A. Dasari, P. P. Kiatsimkul, W. R. Sutterlin and
G. J. Suppes, Appl. Catal., A, 2005, 281, 225–231.
26 K. Verma, A. Kumar and D. Varshney, J. Alloys Compd., 2012,
526, 91–97.
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