Direct and Highly Selective Conversion of Synthesis Gas into Lower Olefins: Design of a Bifunctional Catalyst Combining Methanol Synthesis and Carbon-Carbon Coupling

Article abstract of DOI:10.1002/anie.201601208

The direct synthesis of lower (C₂ to C₄) olefins, key building-block chemicals, from syngas (H₂ /CO), which can be derived from various nonpetroleum carbon resources, is highly attractive, but the selectivity for lower olefins is low because of the limitation of the Anderson-Schulz-Flory distribution. We report that the coupling of methanol-synthesis and methanol-to-olefins reactions with a bifunctional catalyst can realize the direct conversion of syngas to lower olefins with exceptionally high selectivity. We demonstrate that the choice of two active components and the integration manner of the components are crucial to lower olefin selectivity. The combination of a Zr-Zn binary oxide, which alone shows higher selectivity for methanol and dimethyl ether even at 673K, and SAPO-34 with decreased acidity offers around 70 % selectivity for C₂-C₄ olefins at about 10 % CO conversion. The micro- to nanoscale proximity of the components favors the lower olefin selectivity. It's a gas: Syngas can be directly converted into C₂-C₄ olefins with a selectivity of 74 % at 673K over a bifunctional catalyst composed of Zr-Zn oxide for methanol synthesis and SAPO-34 for selective C-C coupling. The control of the hydrogenation ability of the two components and their integration manner play crucial roles in obtaining high C₂-C₄ olefin selectivity.

Full text of DOI:10.1002/anie.201601208

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International Edition: DOI: 10.1002/anie.201601208
German Edition: DOI: 10.1002/ange.201601208
Heterogeneous Catalysis Very Important Paper
Direct and Highly Selective Conversion of Synthesis Gas into Lower
Olefins: Design of a Bifunctional Catalyst Combining Methanol
Synthesis and Carbon–Carbon Coupling
+
+
+
Kang Cheng , Bang Gu , Xiaoliang Liu , Jincan Kang, Qinghong Zhang,* and Ye Wang*
Abstract: The direct synthesis of lower (C to C ) olefins, key
ucts generally follow a statistical distribution known as the
2
4
[
1]
building-block chemicals, from syngas (H /CO), which can be
Anderson–Schulz–Flory (ASF) distribution. According to
this distribution, the maximum C –C (including paraffins and
2
derived from various nonpetroleum carbon resources, is highly
attractive, but the selectivity for lower olefins is low because of
the limitation of the Anderson–Schulz–Flory distribution. We
report that the coupling of methanol-synthesis and methanol-
to-olefins reactions with a bifunctional catalyst can realize the
direct conversion of syngas to lower olefins with exceptionally
high selectivity. We demonstrate that the choice of two active
components and the integration manner of the components are
crucial to lower olefin selectivity. The combination of a Zr–Zn
binary oxide, which alone shows higher selectivity for meth-
anol and dimethyl ether even at 673 K, and SAPO-34 with
2
4
olefins) selectivity is limited to 58%. Recently, a C –C olefin
2
4
selectivity of approximately 60% was achieved through
careful design of catalysts containing S- and Na-modified Fe
[
2]
nanoparticles dispersed on weakly interactive supports.
Lower olefins can also be produced from syngas by two
consecutive processes, namely methanol synthesis and meth-
[
6]
anol to olefins (MTO). Compared to the two-process route,
the direct route would be more energy- and cost-efficient. A
direct route that employs bifunctional catalysts capable of
catalyzing the CO hydrogenation to heavier hydrocarbons
and the selective CÀC cleavage of heavier hydrocarbons has
decreased acidity offers around 70% selectivity for C –C4
2
olefins at about 10% CO conversion. The micro- to nanoscale
proximity of the components favors the lower olefin selectivity.
been reported to be effective for the production of gasoline or
[7]
diesel fuel. We expect that the development of a bifunctional
catalyst containing active components for the conversion of
CO to C intermediates (e.g., CH OH or CH O) and the
L
ower olefins are key building-block chemicals. Currently,
1
3
3
lower olefins are produced primarily by thermal cracking of
naphtha. The growing demand for lower olefins and the
depletion of oil reserves have stimulated the development of
processes for lower olefin synthesis from alternative feed-
selective CÀC coupling would be a promising strategy to
realize the selective conversion of syngas to C –C olefins
2
4
(Figure 1). Carrying out CÀC coupling and CO activation on
[1]
stocks such as coal, natural gas (also shale gas), and biomass.
Synthesis gas (syngas, CO/H ) is a key platform for the
2
transformation of nonpetroleum carbon resources. The syn-
thesis of lower olefins from syngas is a promising route.
Many studies have been devoted to the hydrogenation of
CO to lower olefins by Fischer–Tropsch (FT) synthesis over
[
1–5]
Fe catalysts.
However, the selectivity for C –C olefins is
2 4
Figure 1. Reaction coupling for the direct synthesis of lower olefins
from syngas by the integration of active components for CO activation
and C C coupling. (a)=adsorbed.
limited. Typically, FT synthesis proceeds by the dissociation of
CO, formation of CH (x = 1–3), coupling of CH to C H ,
x
x
n
m
À
and the hydrogenation or dehydrogenation of C H to
n
m
[
1]
paraffin or olefin products. The coupling of CH is uncon-
x
trollable over metal catalysts (Fe, Co, and Ru) capable of
catalyzing CO dissociation, and thus the hydrocarbon prod-
different sites may allow the coupling to proceed in a con-
trolled manner, thus breaking the ASF distribution. Some
studies have combined methanol synthesis and methanol-to-
hydrocarbon catalysts in one reactor, but the major products
[
+]
[+]
[+]
[
*] Dr. K. Cheng, B. Gu, X. Liu, Dr. J. Kang, Prof. Dr. Q. Zhang,
Prof. Dr. Y. Wang
[
8]
are C –C paraffins. The direct synthesis of C –C olefins
2
4
2
4
State Key Laboratory of Physical Chemistry of Solid Surfaces
Collaborative Innovation Center of Chemistry for Energy Materials
National Engineering Laboratory for Green Chemical Productions of
Alcohols, Ethers and Esters
College of Chemistry and Chemical Engineering, Xiamen University
Xiamen 361005 (China)
from syngas with high selectivity (> 60%) is a big challenge.
Herein, we report a successful design of bifunctional catalysts
with exceptionally high selectivity for C –C4 olefins by
2
integrating catalyst components that are active in methanol
synthesis and in CÀC coupling. We demonstrate that the
choice of the components and their integration are crucial to
the product selectivity.
E-mail: zhangqh@xmu.edu.cn
wangye@xmu.edu.cn
+
[
We chose molecular sieve SAPO-34, an excellent MTO
] These authors contributed equally to this work.
[
6]
catalyst, as the CÀC coupling component. The optimized
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
temperature for SAPO-34-catalyzed C –C olefin synthesis is
2
4
[
6]
1
002/anie.201601208.
673–723 K. However, methanol synthesis at such high
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temperatures is thermodynamically restrained (See Figure S1
in the Supporting Information). Our calculations show that
the coupling with the MTO reaction, which is thermodynami-
cally more feasible at higher temperatures, can drive the
conversion of syngas and the formation of lower olefins is
thermodynamically feasible at 673 K (Figure S1). We first
used ball-milling to combine a typical methanol synthesis
catalyst, namely Cu–Zn–Al mixed oxide, with SAPO-34 for
syngas conversion. The Cu–Zn–Al catalyst alone exhibited
high selectivity for the production of methanol at 523 K, but
C olefin selectivity reached 63% with CO conversion of
4
9.5% at 673 K over the Zr–Zn/SAPO-34 catalyst with a Zr/Zn
molar ratio of 2:1. Unlike the Cu–Zn–Al catalyst, methanol
and DME remained as the major products over the Zr–Zn
catalyst over a wide temperature range (Figure S2). The CO
conversion was lower over the Zr–Zn catalyst, but the
coupling with SAPO-34 significantly accelerated the conver-
sion at 673 K because of the thermodynamic driving force.
We performed further studies to understand the unique
catalytic functions of the Zr–Zn-based catalysts. Use of ZrO2
the major product shifted to CH as the temperature rose to
alone provided CH OH with a high selectivity, but CO
4
3
6
73 K (Table 1), thus indicating that the hydrogenolysis of the
conversion was low (Table 1). The ZrO /SAPO-34 composite
2
exhibited a C –C olefin selectivity of 90% at CO conversion
2
4
of 1.0%. The high olefin selectivity is likely due to the weak
hydrogenation ability of ZrO . Actually, ZrO is a unique
Table 1: Hydrogenation of CO over methanol synthesis catalysts and
2
2
bifunctional catalysts composed of methanol synthesis catalyst and
[
a]
catalyst in CO activation with surface oxygen vacancies, and
SAPO-34.
[9]
forms surface methoxide via formate in the presence of H2.
[
b]
Catalyst
CO
conv.
Selectivity for
[
c]
However, the ability of ZrO to dissociate H is low, thus
2 2
resulting in its low activity even in combination with SAPO-
34. The presence of a second component that can accelerate
CO hydrogenation [%]
=
0
[
%]
CH4 C2–4
C2–4
C5+ CH OH
3
(DME)
[
d]
dissociative H adsorption would favor the CO conversion
2
Cu–Zn–Al
Cu–Zn–Al
6.3
5.7
4.8
10
0.3
86
0.1
9.3
0.1
19
1.1
6.0
4.6
4.0
7.9
4.2
35
0.3
0.1
0
1.2
0.1
9.3
4.6
63
11
90
2.6
69
5.8
37
1.9
9.1
0.1
86
0.1
3.3
2.4
29
2.8
5.5
0.5
25
5.2
48
1.0 96(0.8)
over ZrO . ZnO is well known to function in the heterolytic
2
0
0
3.8(0.8)
8.0(92)
[
d]
dissociation of H , forming hydrogen species capable of
Cu–Zn–Al/SAPO-34
Cu–Zn–Al/SAPO-34
Zr–Zn(2:1)
Zr–Zn(2:1)
Zr–Zn(2:1)/SAPO-34
Zr–Zn(2:1)/SAPO-34
ZrO2
ZrO /SAPO-34
Zr–Zn(4:1)
Zr–Zn(4:1)/SAPO-34
Zr–Zn(1:1)
Zr–Zn(1:1)/SAPO-34
ZnO
2
[10]
3.7 0(0)
participating in hydrogenation reactions.
The results in
[
d]
0.2
1.1
0.1
9.5
0.3
1.0
0.8
6.8
1.4
7.5
1.8
3.3
0.8 98(0.7)
3.4 25(40)
2.0 29(61)
2.2 0(0)
1.0 80(0.8)
1.1 0(0)
1.2 54(33)
2.1 0(0)
3.1 19(31)
3.2 0(0)
Table 1 suggest that ZnO activates H in our system, thus
2
enhancing the hydrogenation of CO over the Zr–Zn/SAPO-
[
d]
3
4 catalyst. The catalyst with a Zr/Zn molar ratio of 2:1
exhibited the highest activity for C –C olefin formation. A
2
4
higher Zn/Zr ratio led to higher CH selectivity and C –C
4
4
2
2
paraffin/olefin ratio because of the higher hydrogenation
ability. Characterization by using XRD and TEM confirmed
that the Zr–Zn binary oxide was composed of nanosized ZrO2
and ZnO particles (Figures S3 and S4).
Our results suggest that the CO hydrogenation active
component significantly affects the product selectivity in the
subsequent conversion of methanol or the methoxide inter-
mediate. We performed methanol conversion in the presence
11
84
43
4.4
8.1
6.5
49
0.5 4.7(0.2)
ZnO/SAPO-34
0
0(0)
[
3
a] Reaction conditions: catalyst 0.50 g; H /CO=2:1; 1 MPa;
0 mLmin ; time on stream 30 h; 673 K. [b] Cu/Zn/Al=6:3:1 (molar
ratio); the Zr/Zn molar ratio is shown in the parentheses; the
composites were prepared by ball-milling for 24 h. [c] Selectivity was
calculated on a molar carbon basis for CO hydrogenation; the formation
2
À1
of H at 673 K to gain more insights. The methanol conversion
2
was greater than 98% over the catalysts displayed in Figure 2.
C –C4 olefins and paraffins were formed as the major
2
=
0
of CO by the WGS reaction was shown in Table S1; C2–4 , C2–4 , and C5+
2
denote C –C olefins, C –C paraffins, and hydrocarbons with carbon
numbers greater than 5, respectively. [d] 523 K.
products over SAPO-34, which had been ball-milled for
2
4
2
4
2
4 h for a better comparison and is denoted as SAPO-34(24h).
The presence of ZrO did not significantly affect the product
2
CÀO bond occurred at such a high temperature. Methanol
selectivity. The increase in the Zn/Zr ratio in Zr–Zn/SAPO-34
composites gradually increased CH selectivity. The C –C
was mostly converted to dimethyl ether (DME) over the Cu–
Zn–Al/SAPO-34 composite at 523 K, thus suggesting that
SAPO-34 catalyzed the intermolecular dehydration of meth-
anol under such conditions. At 673 K, the CÀC coupling
4
2
4
4
olefin/paraffin ratio decreased at the same time. Only C –C
2
paraffins were formed over the Cu–Zn–Al/SAPO-34 catalyst.
These trends are similar to those observed for syngas
conversion (Table 1) and can explain why the Zr–Zn/
SAPO-34 is unique for C –C olefin formation. Thus, the
products, C –C paraffins, were formed along with CH over
2
4
4
this catalyst, but the C –C olefin selectivity was very low. CO
2
4
2
2
4
was also formed with a selectivity of around 45% over the
Cu–Zn–Al or Cu–Zn–Al/SAPO-34 catalyst at 673 K because
of the water–gas-shift (WGS) reaction (Table S1). Similar
results were observed when SAPO-34 was integrated with
several other methanol synthesis catalysts such as Pd–Zn and
choice of a CO hydrogenation active component with
appropriate hydrogenation ability is quite important for the
selective formation of C –C olefins.
2
4
It is noteworthy that the C –C olefin/paraffin ratios for
2
4
some catalysts were lower for methanol conversion under H2
(Figure 2) than those for syngas conversion (Table 1) and
[8]
Cr–Zn (Table S2) that have been previously reported.
[
6]
Distinct from these results, we discovered that a bifunctional
catalyst composed of Zr–Zn binary oxide and SAPO-34 is
unique for the formation of C –C olefins (Table 1). The C –
those reported for the MTO reaction. We clarified that this
behavior arises from the effect of the presence of H or the
2
difference in H pressure. Under the N atmosphere typically
2
4
2
2
2
4
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Figure 2. Influence of the CO hydrogenation active component on the
conversion of methanol under H . All the catalysts were prepared by
2
ball-milling for 24 h. Reaction conditions: catalyst 0.50 g, 673 K; H
2
À1
À1
1
.0 MPa, 30 mLmin ; liquid methanol 0.010 mLmin ; time on
stream 200 min.
used for the MTO reaction, the C –C4 olefin selectivity
2
reached around 80% over SAPO-34(24h) (Figure S5). The
increase in H pressure resulted in a decreased selectivity for
2
C –C olefins and increased selectivity for C –C paraffins
2
4
2
4
because of the hydrogenation over the Brønsted acid sites of
Figure 3. Effect of Brønsted acidity of SAPO-34 on catalytic behavior.
[11]
a) Methanol conversion over SAPO-34 with different acidities under
reaction conditions shown in Figure 2. b) Syngas conversion over
composite catalysts prepared by simple mixing in an agate mortar of
Zr–Zn (Zr/Zn=2:1) oxide and SAPO-34 with different acidities under
reaction conditions given in Table 1.
SAPO-34.
For the same reason, the increase in syngas
pressure from 1.0 to 2.0 MPa decreased the C –C olefin/
2
4
paraffin ratio in syngas conversion over the Zr–Zn/SAPO-34
catalyst (Table S3).
Because the hydrogenation of olefins can occur on the
Brønsted acid sites, the acidity of SAPO-34 may also affect
the product selectivity. We found that the ball-milling
decreased the crystal size (Figure S6), the crystallinity (Fig-
ure S7a), and the specific surface area (Table S4) of SAPO-
samples with different densities of Brønsted acid sites (Fig-
ure 3b). A C –C olefin selectivity of 74% was obtained at
2
4
CO conversion of 11% over the Zr–Zn/SAPO-34(Si0.048)-M
(M = mortar-mixing) catalyst with a density of Brønsted acid
3
3
4. The coordination environments of Si, Al, and P in SAPO-
4 were also altered at the same time (Figure S8). It is known
À1
sites of 0.21 mmolg .
that the Brønsted acidity of SAPO-34 arises from the
The integration manner of the active components is also
crucial for the conversion of syngas to lower olefins. The use
of a dual-bed configuration with SAPO-34(24h) downstream
from the Zr–Zn oxide provided a very low CO conversion
(Figure 4a). This result suggests that the thermodynamic
driving force does not arise from separating the two active
components. The CH selectivity remained high, but C –C
olefins and paraffins were formed because of the catalytic
function of SAPO-34 in the downstream bed. The mixing of
the two components significantly increased CO conversion
[
6]
substitution of Si for P or Al in the framework. Thus, the
change in the coordination environment may lead to the
change in Brønsted acidity. Our NH3 temperature-pro-
grammed desorption (NH -TPD) studies showed that the
3
density of Brønsted acid sites decreased with an increase in
the ball-milling time (Figure S9a and Table S4). We also
studied a series of SAPO-34 samples with different Si
contents [x = Si/(Si + Al + P)] (Table S4), which were
denoted as SAPO-34(Six) and had good crystallinity (Fig-
ure S7b), and thus different densities of Brønsted acid sites
4
2
4
and decreased CH selectivity (Figure 4b–d). The proximity
4
(
Figure S9b and Table S4). The results for methanol con-
of the two components increased as the integration manner
changed from granule-stacking to ball-milling (Figure 4b to
4d), as demonstrated by the electron microscopy results
(Figures S10–S13). For the typical Zr–Zn/SAPO-34 sample
with the two components ball-milled together for 24 h
(Figure 4d), the smaller Zr–Zn particles (10–40 nm) were
tightly attached on SAPO-34 particles with sizes of 200–
500 nm (Figures S12 and S13). On the other hand, the contact
between the SAPO-34 particles with sizes of 200–500 nm and
version under H with these two series of SAPO-34 samples
demonstrate that the density of Brønsted acid sites is a key
factor that determines the C –C olefin/paraffin ratio (Fig-
ure 3a). A larger density of Brønsted acid sites resulted in
2
2
4
a lower olefin/paraffin ratio or lower C –C olefin selectivity.
2
4
Similar trends were observed for syngas conversion over
bifunctional catalysts prepared by a simple mixing in an agate
mortar of the Zr–Zn (Zn/Zr= 1:2) oxide and SAPO-34
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selectivity. The proximity of the two components also plays
a key role in the direct conversion of syngas to lower olefins.
Experimental Section
The Zr–Zn binary oxide was prepared by a coprecipitation method.
The Zr–Zn/SAPO-34 composites with a mass ratio of the two
components of 1:2 were typically prepared by ball-milling for 24 h.
The conversion of syngas was performed in a fixed-bed reactor using
syngas (H /CO = 2:1) with a pressure of 1.0 MPa at 673 K. The
2
conversion of methanol was performed in the same reactor under H2
(1.0 MPa). See the Supporting Information for experimental details.
Acknowledgements
This work was supported by the National Basic Research
Program of China (2013CB933102), the Natural Science
Foundation of China (91545203, 21433008 and 21503174), and
the China Postdoctoral Science Foundation (2015M571967).
Figure 4. Effect of integration manner on catalytic behaviors of the
composite catalysts containing Zr–Zn (Zr/Zn=2:1) oxide and SAPO-
3
4(24 h). a) Dual-bed configuration. b) Stacking of granules with sizes
Keywords: bifunctional catalysts · CÀC coupling ·
of 250–600 mm. c) Simple mixing of the two components in an agate
mortar. d) Ball-milling of two components together for 24 h. o/p ratio
denotes the C –C olefin/paraffin ratio. See Table 1 for reaction
heterogeneous catalysis · synthesis gas · zeolites
2
4
How to cite: Angew. Chem. Int. Ed. 2016, 55, 4725–4728
Angew. Chem. 2016, 128, 4803–4806
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SAPO-34 and Zr–Zn particles in an agate mortar (Figure 4c).
The sample represented in Figure 4b was composed of
micrometer-sized particles stacked together (Figure S10).
CO conversion of around 10% was obtained over the
catalysts with nanoscale dimensions (Figure 4c,d). The selec-
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[12]
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[
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[
direct synthesis of lower olefins from syngas with excellent
selectivity. The C –C olefin selectivity can reach 74% with
[
[
2
4
a CO conversion of 11% at 673 K, thus breaking the ASF
distribution. We have demonstrated that the control of the
hydrogenation ability of the two components in the bifunc-
2
015, 528, 245 – 248.
tional catalyst is crucial to obtaining high C –C4 olefin
2
Received: February 2, 2016
Published online: March 9, 2016
4
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