Selective synthesis of gasoline from syngas in near-critical phase

Article abstract of DOI:10.1016/j.cattod.2013.10.037

The conversion of syngas (CO + H₂) to hydrocarbons in gasoline fraction over a hybrid catalyst composed of Cu-ZnO and Pd/ZSM-5 in a near-critical n-hexane solvent was investigated. Methanol was synthesized from syngas over Cu-ZnO; subsequently, was converted to hydrocarbons through the formation of dimethyl ether (DME) over Pd/ZSM-5. The yield of hydrocarbons increased from ca. 29% to ca. 54% with increasing the partial pressure of n-hexane from 0 MPa to 1.5 MPa. By contrast, the yields of CO₂ and DME decreased with increasing the partial pressure of n-hexane. A decrease in the particle size of ZSM-5 as well as an increase in a Pd loading led to the selective production of hydrocarbons in the gasoline fraction. The hybrid catalyst containing 0.5 wt% Pd/ZSM-5 with ca. 100 nm in size exhibited 51% gasoline fraction yield with 75% CO conversion. The catalyst stability was also improved by increasing the Pd loading during the reaction.

Full text of DOI:10.1016/j.cattod.2013.10.037

Catalysis Today 228 (2014) 167–174
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Selective synthesis of gasoline from syngas in near-critical phase
Ting Maa, Hiroyuki Imaia,∗, Yoshifumi Suehirob, Chun Chena, Toshiyuki Kimuraa,
Sachio Asaokaa, Xiaohong Lia
a Faculty of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu, Kitakyushu, Fukuoka 808-0135, Japan
b Strategic Planning Division, General Coordination Department, Japan Oil, Gas and Metals National Corporation (JOGMEC), 2-10-1 Toranomon, Minato-ku,
Tokyo 105-0001, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The conversion of syngas (CO + H2) to hydrocarbons in gasoline fraction over a hybrid catalyst composed
of Cu-ZnO and Pd/ZSM-5 in a near-critical n-hexane solvent was investigated. Methanol was synthe-
sized from syngas over Cu-ZnO; subsequently, was converted to hydrocarbons through the formation of
dimethyl ether (DME) over Pd/ZSM-5. The yield of hydrocarbons increased from ca. 29% to ca. 54% with
increasing the partial pressure of n-hexane from 0 MPa to 1.5 MPa. By contrast, the yields of CO2 and DME
decreased with increasing the partial pressure of n-hexane. A decrease in the particle size of ZSM-5 as well
as an increase in a Pd loading led to the selective production of hydrocarbons in the gasoline fraction. The
hybrid catalyst containing 0.5 wt% Pd/ZSM-5 with ca. 100 nm in size exhibited 51% gasoline fraction yield
with 75% CO conversion. The catalyst stability was also improved by increasing the Pd loading during the
reaction.
Received 26 June 2013
Received in revised form
27 September 2013
Accepted 9 October 2013
Available online 8 November 2013
Keywords:
Near-critical phase
Hybrid catalyst
Syngas to hydrocarbons
Pd/ZSM-5
© 2013 Elsevier B.V. All rights reserved.
Gasoline fraction
Methanol to hydrocarbons
1. Introduction
By employing hybrid catalysts composed of a methanol synthe-
sis catalyst, e.g. Pd/SiO2, Cu-ZnO and Cr-ZnO, and a zeolite catalyst,
Because of a drastic increase in worldwide energy consumption,
the development of hydrocarbon production processes with carbon
resources alternative to crude oil has been highly desirable. Syngas
(CO + H2), which can be produced from a variety of carbon resources
such as natural gas, biomass, and coal, has been focused on as a raw
material for the production of fuels and chemicals. Syngas can be
converted directly to hydrocarbons through the Fischer-Tropsch
synthesis (FTS) [1–7]. Another way of uses of syngas is the produc-
tion of methanol. Methanol can be utilized for the production of
hydrocarbons through the methanol-to-hydrocarbons (MTH) reac-
tion using zeolites or zeolite-type catalysts. In the MTH reaction,
hydrocarbon distribution is strongly dependent on the micropore
size of zeolites, and the production of heavy hydrocarbons such as
diesel fraction hydrocarbons are strictly inhibited [8–12]. Thus, a
process of the methanol synthesis from syngas following the MTH
reaction would be effective on the selective production of light
hydrocarbons with the efficient utilization of the alternative carbon
resources.
e.g. ultra-stable Y zeolite (USY) and beta zeolite, the direct synthe-
sis of light hydrocarbons in liquefied petroleum gas (LPG) fraction
from syngas has been developed [13–17]. In these LPG synthesis
processes, syngas can be primarily converted to methanol over the
metal-based catalyst. Subsequent conversion of methanol to hydro-
carbons readily takes place over the zeolite which neighbors the
metal-based catalyst in the hybrid catalyst. The rapid consump-
tion of methanol gives the methanol synthesis from syngas an
advantage over the reverse reaction of methanol to syngas over
the metal-based catalysts, leading to higher yield of hydrocarbons
than the calculated value based on the thermodynamic equilibrium
[13].
ZSM-5, a MFI-type aluminosilicate material, is well known as
a zeolite catalyst having the pore size and acid properties suit-
able for the production of gasoline-range hydrocarbons in the MTH
reaction [8–10]. Considering the selective production of specified
fuel hydrocarbons, the employment of a hybrid catalyst contain-
ing ZSM-5 would be suitable for the hydrocarbon production from
syngas. Furthermore, since olefins are primarily produced in the
MTH reaction, metal catalysts with high hydrogenation ability are
required for obtaining paraffins as a final product. It was reported
that Pd supported on zeolite efficiently converts olefins to paraf-
fins during the conversion of syngas to hydrocarbons [15]. Thus,
∗
Corresponding author. Tel.: +81 93 695 3733; fax: +81 93 695 3398.
E-mail address: h-imai@kitakyu-u.ac.jp (H. Imai).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.10.037

168
T. Ma et al. / Catalysis Today 228 (2014) 167–174
Pd/ZSM-5 is expected to efficiently produce paraffins in gasoline
fraction from methanol that is generated from syngas.
were obtained on an S-5200 microscope (Hitachi) operating at
10–15 kV.
Reaction processes using solid catalysts in near- and super-
critical solvents have received considerable attention in the fields
of organic synthesis [18–21] and biomass conversion [22–26] in
the recent decades because a reaction rate can be significantly
improved in the solvents. Supercritical hydrocarbon fluids have
been applied to the FTS to increase the hydrocarbon yield and to
suppress the generation of undesired products due to the efficient
extraction of products from catalysts [27–31]. In a manner similar
to the FTS reaction in the supercritical phase, the LPG synthesis from
syngas using a hybrid catalyst in a near-critical solvent of n-hexane
has been studied [32]. Efficient removal of heat generated during
the reaction by the near-critical fluid leads to improvements in the
stability of the catalyst as well as the selectivity for light hydrocar-
bons. In this paper, the term “near-critical” is used to connote the
region close to the critical temperature and pressure of n-hexane.
In this study we investigated the catalytic activity of a hybrid
catalyst composed of Cu-ZnO and Pd/ZSM-5 in a near-critical n-
hexane solvent in the conversion of syngas to hydrocarbons via
methanol. During the experimental trials, the reaction in the near-
critical phase was compared with those in conventional phases to
clarify the effect of the near-critical solvent on the catalytic activity
and product distribution. We also investigated the effects of the
acid amount and particle size of ZSM-5 as well as Pd loading on the
hydrocarbon yield.
Temperature programmed ammonia desorption (NH3-TPD)
profiles were recorded on BELCAT-B (BEL Japan). The sample was
pretreated under a He flow at 723 K for 1 h, and then cooled down
to 373 K. Ammonia was allowed to make contact with the sample
at 373 K for 1 h. Subsequently, the sample was evacuated to remove
weakly adsorbed ammonia at 373 K for 30 min. Finally, the sample
was heated from 373 K to 883 K at a raising rate of 10 K/min in a He
flow. The desorbed ammonia was monitored on a TCD.
Temperature programmed hydrogen desorption (H2-TPD) pro-
files were recorded on BELCAT-B (BEL Japan). The sample was
pretreated under a He flow at 473 K for 5 h, and then reduced under
10 vol% H2 balanced by He at 573 K for 10 h. After cooling down to
323 K, a mixed gas composed of 10 vol% H2 and He balance flowed
into the sample for 1 h. Finally, the sample was heated from 323 K to
923 K at raising rate of 10 K/min in a He flow. A mass spectrometer
was used to monitor the desorbed hydrogen (m/e = 2).
2.3. Catalytic reaction test
A pressurized flow type of reaction apparatus with a fixed-bed
reactor was used for this study. The experimental set-up scheme is
shown in Fig. 1. The apparatus was equipped with an electronic
temperature controller for a furnace, a vaporizer of a solvent, a
stainless tubular reactor with an inner diameter of 6 mm, thermal
mass flow controllers for gas flows and a back-pressure regulator. A
solvent was pumped into the reactor by a high-pressure pump. 1 g
of a hybrid catalyst was loaded in the reactor, and inert glass sand
was placed above and below the catalyst. The length of the catalyst
bed was about 6.0–6.5 cm. The catalyst was reduced in a flow of
a mixture of 5% hydrogen and 95% nitrogen with 100 mL min−1 at
573 K for 3 h. After the reduction of the catalyst, the catalyst was
cooled down to 473 K. Syngas (60% H2, 32% CO, 5% CO2, and 3%
Ar) and n-hexane as a solvent were introduced into the catalyst to
make the total pressure inside reach to 4.0 MPa in a He flow, and
then the catalyst was heated up to 553 K. The partial pressure of
syngas, Psyngas, of 2.5 MPa was retained, and the partial pressure of
n-hexane, Pn-hexane, was varied from 0 MPa to 1.5 MPa. The catalyst
2. Experimental
2.1. Catalyst preparation
A hybrid catalyst was prepared by physically mixing the
355–710 ␮m pellets of a Cu-ZnO methanol synthesis catalyst of
0.5 g with those of a Pd/ZSM-5 catalyst of 0.5 g. Cu-ZnO was a com-
mercial catalyst (MK-121, TOPSØE). Pd/ZSM-5 was prepared by
impregnation method with a 4.557 wt% Pd(NH3)2(NO3)2 aqueous
solution and commercial ZSM-5 with the SiO2/Al2O3 molar ratio of
+
23 (CBV2314, Zeolyst) or 80 (CBV8014, Zeolyst). Commercial NH4
-
type ZSM-5 was calcined at 823 K for 3 h to become proton-type
ZSM-5. Proton-type ZSM-5 was immersed in the Pd(NH3)2(NO3)2
aqueous solution with a supported Pd weight at room temper-
ature overnight. The resultant was evaporated at 333 K, dried at
393 K for 3 h, and calcined at 823 K for 3 h. In addition, a MFI zeo-
lite was hydrothermally synthesized by using an aluminosilicate
gel with the SiO2/Al2O3 ratio of 23 and tetrapropylammonium
hydroxide (TPAOH) as a structure-directing agent (SDA) at 443 K for
24 h, according to the previous report [33]. The obtained Na+-type
MFI zeolite was transformed to a proton-type MFI zeolite by ion-
exchange treatment with 2.2 M NH4NO3 aqueous solution followed
by calcination at 823 K for 3 h.
weight to the flow rate ratio (W/F-syngas) was 9.7 g-cat h/mol-syngas
.
CO, CO2 and CH4 of the reaction products were analyzed with an
on-line gas chromatograph (Shimadzu GC-8A) equipped with a
thermal conductivity detector (TCD) and a packed column of acti-
vated charcoal. The light hydrocarbon products were analyzed with
another on-line gas chromatograph (Shimadzu GC-2014) equipped
2.2. Characterization
The structure of the catalysts was examined by X-ray diffrac-
tion (XRD, Rigaku XRD-DSC-XII). The diffractometer was operated
at 40 kV and 20 mA using Cu-K␣ radiation source. XRD patterns
were recorded at 6 degree/min over the angular range of 5–50◦.
The SiO2/Al2O3 ratios of the samples were determined by X-ray
fluorescence analysis (XRF, Rigaku ZSX101E). The BET surface area
and micropore volume were estimated from nitrogen adsorption
isotherms at 77 K with a Micromeritics ASAP 2010 instrument. Prior
to the analyses, the sample was treated at 573 K for 3 h under nitro-
gen flow in order to remove adsorbed compounds. External surface
area (SEXT) was estimated by the t-plot method. Field-emission
scanning electron microscopic (FE-SEM) images of the catalysts
Fig. 1. Scheme of experimental set-up.

T. Ma et al. / Catalysis Today 228 (2014) 167–174
169
with a flame ionization detector (FID) and a capillary column
of Porapak-Q. The products liquefied by condensation at room
temperature were analyzed with an off-line gas chromatograph
(Shimadzu GC-2014) equipped with an FID detector and a cap-
illary column of TC-1. For the analyses of the liquefied products,
decahydronaphthalene (C10H18) was used as an internal standard.
2.4. Calculation method
The conversion of CO was defined as
ꢀ
ꢁ
COout (mol)
CO in feed (mol)
CO conversion (%) = 1 −
× 100
The yield of C-containing products was defined based on the
atomic carbon. The yield of CO2 was defined as
CO2 (mol)
CO in feef (mol)
CO2 yield (C-%) =
× 100
The yield of DME was defined as
DME (mol) × 2
DME yield (C-%) =
× 100
Fig. 2. Product yield resulting from conversion of syngas to hydrocarbons over
hybrid catalyst composed of Cu-ZnO and 0.5 wt% Pd/ZSM-5 as a function of par-
tial pressure of n-hexane. Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g Cu-ZnO
and 0.5 g Pd/ZSM-5); temp., 553 K; Ptotal = 4.0 MPa (balanced by He); Psyngas = 2.5 MPa
or 1.0 MPa; W/F = 9.7 g-cat h/mol-syngas; H2/CO = 1.9.
CO in feed (mol)
The yield of hydrocarbon was defined as
Cn (mol) × n
Cn yield (C-%) =
× 100
CO in feed (mol)
study. Hydrocarbon yield was drastically increased by co-feeding
n-hexane with syngas to the catalyst, and increased from ca. 29%
to ca. 54% with the increase in the partial pressure of n-hexane
from 0 to 1.5 MPa. Hydrocarbon distributions at 6.5 h after the reac-
tion started are shown in Fig. 3. By employing the near-critical
fluid (Pn-hexane = 1.5 MPa), hydrocarbons with carbon numbers of
5–9 were efficiently produced, while the selectivities of ethane
and propane decreased. In contrast to the hydrocarbon yield, DME
yield was decreased from 33% to 0.8% by the employment of the
near-critical solvent. In order to compare the catalytic properties
of the hybrid catalyst in the near-critical fluid with those in a
supercritical fluid, the partial pressure of n-hexane was increased
to 3.0 MPa, while the partial pressure of syngas was decreased to
1.0 MPa to keep the total pressure at 4.0 MPa. Surprisingly, DME and
CO2 were little detected, and hydrocarbons were dominantly pro-
duced although the CO conversion decreased due to the decrease in
the partial pressure of syngas. The employment of n-hexane as the
near-critical solvent improves the heat transfer during the reaction,
leading to the increase in the LPG yield through the conversion of
methanol [32]. It is suggested that the introduction of n-hexane
as the near-critical solvent improved the conversion of DME to
hydrocarbons due to the good heat transfer in the near-critical
phase.
3. Results and discussion
3.1. Conversion of syngas to hydrocarbons over hybrid catalyst in
near-critical phase
The catalytic activity of a hybrid catalyst composed of Cu-ZnO
and Pd/ZSM-5 in a near-critical n-hexane solvent was investigated
in the conversion of syngas to hydrocarbons via methanol. 0.5 wt%
Pd was supported on ZSM-5 with the SiO2/Al2O3 ratio of 23. Fig. 2
shows product yields resulting from the syngas conversion over the
hybrid catalyst. The partial pressure of n-hexane was varied from
0 MPa to 1.5 MPa with the syngas partial pressure of 2.5 MPa con-
stant. The critical pressure and critical temperature of n-hexane
are 507.5 K and 3.01 MPa, respectively. The total pressure in the
reaction system of 4.0 MPa was kept with He as a carrier gas dur-
ing the reaction. CO conversion decreased from ca. 90% to ca. 74%
as the partial pressure of n-hexane increased from 0 to 1.5 MPa.
The employment of supercritical fluids resulted in decreasing CO
conversion in the FT synthesis due to lower diffusion efficiency
of syngas compared with that in gas phase [13,30,34]. It is indi-
cated that the near-critical fluid also suppressed the diffusion of
syngas during the reaction to decrease the CO conversion in this
Fig. 3. Hydrocarbon distributions resulting from conversion of syngas to hydrocarbons over hybrid catalyst composed of Cu-ZnO and 0.5 wt% Pd/ZSM-5. Reaction conditions:
cat., 1.0 g hybrid catalyst (0.5 g Cu-ZnO and 0.5 g Pd/ZSM-5); temp., 553 K; Ptotal = 4.0 MPa (balanced by He); Psyngas = 2.5 MPa; W/F = 9.7 g-cat h/mol-syngas; H2/CO = 1.9.

170
T. Ma et al. / Catalysis Today 228 (2014) 167–174
The yield of CO2 decreased with increasing the partial pressure
of n-hexane. CO2 is generated through the water-gas shift (WGS)
reaction of CO with H2O over Cu-ZnO. Supercritical fluids in the
FT processes extracts H2O generated during the reaction from the
catalyst, while the extraction capability of the fluids to H2O is pro-
portional to the partial pressure of the fluids due to low affinity
of H2O to organic solvents [29,35]. It is thus suggested that the
near-critical fluid helped the emission of H2O from the surfaces of
the catalyst, leading to the suppression of the WGS reaction over
Cu-ZnO. Considering the use of high-pressure equipment (super-
critical system), it generally costs much to set up apparatuses which
can be safely operated under sever conditions. Thus, processes
with the near-critical n-hexane fluid are expected to be economical
due to easier handling of apparatuses for the selective production
of gasoline-range hydrocarbons from syngas via methanol using
hybrid catalysts.
Fig. 5. NH3-TPD profiles of 0.5 wt% Pd/ZSM-5 samples with different SiO2/Al2O3
3.2. Effect of acid amount of ZSM-5 on catalytic activity
ratios.
Product distribution in the MTH reaction is strongly dependent
on physicochemical properties of zeolites including acid amount,
pore size and crystal size. In order to investigate the effect of the
acid amount of ZSM-5 in the hybrid catalyst on the catalytic prop-
erties, the syngas conversion to hydrocarbons was conducted over
hybrid catalysts containing ZSM-5 with different SiO2/Al2O3 ratios.
The results obtained in the reaction are shown in Fig. 4. The CO
conversion increased with increasing the Al content of ZSM-5. In
addition, the increase in the Al content of ZSM-5 increased the yield
of the hydrocarbons, while the yield of DME drastically decreased.
The trends resulting from the reaction are consistent with those
obtained using beta zeolite in the LPG synthesis [32]. Fig. 5 shows
NH3-TPD profiles of the 0.5 wt% Pd/ZSM-5 catalysts. The peaks
which appeared at higher temperature (573–873 K) correspond to
NH3 desorption from catalytically active acid sites [36,37]. The acid
amount of ZSM-5 was strongly dependent on the Al content of ZSM-
5, and the acid amounts estimated from the NH3-TPD profiles were
0.67 mmol g−1 and 0.26 mmol g−1 for 0.5 wt% Pd/ZSM-5 with the
SiO2/Al2O3 ratio of 23 and that with the SiO2/Al2O3 ratio of 80,
respectively. The conversion of methanol or DME to hydrocarbons
should depend on the amount of acid sites of zeolite catalysts. The
large amount of the acid sites is probably required for the complete
conversion of methanol to hydrocarbons via DME at low tempera-
ture below 573 K, compared with the MTH reaction carried out at
temperatures of 623–723 K. Therefore, ZSM-5 with the higher Al
content was advantageous to yielding hydrocarbons from syngas
via methanol or DME.
The generation of CO2 was mitigated during the reaction when
the Al content of ZSM-5 in the hybrid catalyst decreased (Fig. 4).
The smaller amount of H2O could be generated over ZSM-5 with the
SiO2/Al2O3 ratio of 80 than over ZSM-5 with the SiO2/Al2O3 ratio
of 23 due to the insufficient conversion of DME to hydrocarbons,
resulting in the suppression of the CO2 generation through the WGS
reaction.
3.3. Effect of particle size of ZSM-5 on catalytic activity
The effect of the particle size of Pd/ZSM-5 on the catalytic
activity was investigated using ZSM-5 with different particle sizes.
Fig. 6 shows XRD patterns and SEM images of commercial ZSM-5
(CBV2314, Zeolyst) and hydrothermally synthesized ZSM-5 using
tetraorthosilicate as a silica source and TPAOH as an SDA. Both
samples exhibited the typical patterns assigned to the MFI struc-
ture (Fig. 6A). The FE-SEM images revealed that commercial ZSM-5
(ZSM-5-CM) was composed of cuboid blocks with an average size of
ca. 600 nm, while hydrothermally synthesized ZSM-5 (ZSM-5-HT)
was composed of cubic crystallites about 100 nm in size (Fig. 6B).
The physicochemical properties of both the ZSM-5 samples are
summarized in Table 1. The SiO2/Al2O3 ratio of 32 for ZSM-5-HT
was similar to that of 23 for ZSM-5-CM. The surface area (SBET
)
of ZSM-5-HT (373 m2 g−1) was also close to that of ZSM-5-CM
(363 m2 g−1). By contrast, the external surface area (SEXT) of ZSM-5
increased due to a decrease in the particle size.
Fig. 7 shows the product yields of ZSM-5-CM and ZSM-5-HT in
the syngas conversion to hydrocarbons via methanol. The CO con-
version was independent of the particle size of the ZSM-5 sample.
In addition, the yields of CO2, DME and the hydrocarbons gen-
erated over the hybrid catalyst containing ZSM-5-CM were very
similar to those of the hybrid catalyst containing ZSM-5-HT. These
results indicate that the particle size of the zeolite influences lit-
tle the catalytic activity of the zeolite for the methanol conversion
to hydrocarbons. By contrast, the product distribution, especially
the hydrocarbon distribution, was strongly dependent on the par-
ticle size of ZSM-5. Fig. 8 shows hydrocarbon distributions over
Fig. 4. Product yield resulting from conversion of syngas to hydrocarbons
over hybrid catalysts composed of Cu-ZnO and 0.5 wt% Pd/ZSM-5 with dif-
ferent SiO2/Al2O3 ratios. Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g
Cu-ZnO and 0.5 g Pd/ZSM-5); temp., 553 K; Psyngas = 2.5 MPa; Pn-hexane = 1.5 MPa;
W/F = 9.7 g-cat h/mol-syngas; H2/CO = 1.9.

T. Ma et al. / Catalysis Today 228 (2014) 167–174
171
Fig. 6. XRD patterns (A) and SEM images (B) of commercial ZSM-5 (CBV2314) (ZSM-5-CM) and hydrothermally synthesized ZSM-5 (ZSM-5-HT).
Table 1
Physicochemical properties of commercial and hydrothermal synthesized ZSM-5 catalysts.
a
c
d
e
Sample
SiO2/Al2O3
Sizeb (nm)
SBET (m2 g−1
)
SEXT (m2 g−1
)
Vmicro (cm3 g−1
)
ZSM-5-CM
ZSM-5-HT
ZSM-5-CM
23
32
80
600
100
–
363
373
440
54
115
183
0.13
0.11
0.13
a
Estimated from XRF measurement.
Average particle size estimated from FE-SEM observations.
SBET, surface area estimated by N2 adsorption method.
SEXT, external surface area estimated by N2 adsorption method.
Vmicro, micropore volume estimated by N2 adsorption method.
b
c
d
e
the hybrid catalysts with ZSM-5-CM or ZSM-5-HT at 6.5 h of the
reaction time. The selectivity of hydrocarbon fractions (C5–C9)
associated with gasoline increased from 44% to 65% with decreasing
the particle size of ZSM-5, while those of methan, ethane and
propane decreased with decreasing the particle size. In the MTH
reaction, the crystal size of zeolites does not significantly influ-
ence the selectivity of various hydrocarbon products. However, a
decrease in the crystal size retards the deposition of carbonaceous
species in the pores and cavities of zeolites due to the high mass
transfer of reactants as well as products, leading to the improve-
ment of the catalyst life [33]. On the other hand, it was reported
that the carbonaceous species formed in the cavities of the zeolite
induced an effect of transition-state shape selectivity due to the
reduction of the pore and cavity spaces, promoting the formation
of smaller olefins [38,39]. ZSM-5-HT composed of smaller parti-
cles than those of ZSM-5-CM exhibited the selective production
of the hydrocarbons in the gasoline fraction. In ZSM-5-HT, hydro-
carbons generated in the pores of the zeolite could more readily
transfer out of the pores during the reaction due to the shorter
pore channels in comparison with ZSM-5-CM. It is assumed that
the further oligomeration of the products in the pores could be
suppressed in ZSM-5-HT, resulting in keeping large cavity spaces.
Thus, the efficient transfer of larger hydrocarbons than the LPG and
dry gas fractions to out of the pores could take place in ZSM-5-
HT. On the other hand, in the cavities of ZSM-5-CM, the transfer
of the large hydrocarbons was suppressed by the carbonaceous
species deposited, promoting the formation of the small hydrocar-
bons through the cracking of the large hydrocarbons.
3.4. Effect of mixing way of Cu-ZnO with Pd/ZSM-5 on catalytic
properties
In order to investigate the effects of the catalyst preparation
method on the catalytic properties, the hybrid catalysts were
prepared by two different methods: powder mixing and granule
mixing. In the powder mixing, after Cu-ZnO of 0.5 g and Pd/ZSM-5
of 0.5 g were physically mixed with a mortar, the mixed pow-
der was pelletized, crushed, and then sieved to particles with
355–710 ␮m. In order to distinguish the physical mixing method of
each pelletized particle of Cu-ZnO and Pd/ZSM-5 described in the
experimental section from the powder mixing, the usual prepa-
ration method is denoted as granule mixing in Fig. 9. The hybrid
catalyst prepared by the granule mixing exhibited a slightly higher
CO conversion, higher CO2 yield and lower DME yield than that
Fig. 7. Product yield resulting from conversion of syngas to hydrocarbons over
hybrid catalysts composed of Cu-ZnO and 0.5 wt% Pd/ZSM-5 with different particle
sizes. Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g Cu-ZnO and 0.5 g Pd/ZSM-
5); temp., 553 K; Psyngas = 2.5 MPa;
Pn-hexane = 1.5 MPa; W/F = 9.7 g-cat h/mol-syngas;
H2/CO = 1.9.

172
T. Ma et al. / Catalysis Today 228 (2014) 167–174
Fig. 8. Hydrocarbon distributions resulting from conversion of syngas to hydrocarbons over hybrid catalyst composed of Cu-ZnO and 0.5 wt% Pd/ZSM-5 with different
particle sizes. Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g Cu-ZnO and 0.5 g Pd/ZSM-5); temp., 553 K; Psyngas = 2.5 MPa; Pn-hexane = 1.5 MPa; W/F = 9.7 g-cat h/mol-syngas
;
H2/CO = 1.9.
prepared by the powder mixing in the conversion of syngas to
hydrocarbons in the near-critical n-hexane fluid (Fig. 9). In addition,
the catalyst prepared by the granule mixing gave the higher yield
(ca. 33%) of gasoline fraction hydrocarbons (C5–C9) than the cata-
lyst prepared by the powder mixing. On the catalyst prepared by
the powder mixing, hydrogen dissociated on Cu-ZnO could readily
transfer to the neighboring Pd/ZSM-5, resulting in the formation of
CH4 through the hydrogenation of CO and CO2 on the Pd particles.
Yoshie et al. reported that a hybrid catalyst prepared by the granule
mixing showed a high selectivity and activity in the hydrocarbon
synthesis from CO2 [40].
The adsorption of H2 on ZSM-5 was almost negligible as a result of
the observation of no peak derived from desorbed H2 molecules. By
contrast, on the Pd/ZSM-5 samples, a broadened peak was observed
in the range from 673 K to 773 K, indicating that H2 molecules were
adsorbed on only the Pd sites. The amount of the adsorbed H2
was estimated from the profiles to be 0.017 mmol g−1 for 0.3 wt%
Pd/ZSM-5 and 0.018 mmol g−1 for 0.5 wt% Pd/ZSM-5, respectively.
Thus, ZSM-5 with the increased Pd loading is capable of preventing
the oligomerization of coke precursors through the hydrogenation
of the coke precursors to paraffins.
In order to clarify the effects of Pd on the catalytic activity in
the syngas conversion, the reaction was carried out employing var-
ious Pd loadings on ZSM-5 prepared by the hydrothermal treatment
in the hybrid catalyst. The results are shown in Figs. 11 and 12.
The hybrid catalyst containing ZSM-5 exhibited 82% CO conver-
sion at 0.5 h after the reaction started. The CO conversion gradually
decreased after the reaction started until the conversion reached to
ca. 65% at 4 h of the reaction time. By employing the hybrid catalyst
composed of Cu-ZnO and Pd supported on ZSM-5, the deactivation
became slow, although the initial CO conversion was almost the
same (ca. 80%) regardless of the Pd loadings (Fig. 11). The effect
of the Pd loadings on the CO conversion was consistent with that
obtained in the LPG synthesis over hybrid catalysts composed of
Cu-ZnO and Pd/beta [16]. In addition, the hybrid catalysts contain-
ing Pd/ZSM-5 exhibited the higher CO conversion at 6.5 h than that
with ZSM-5, while the CO conversion at 6.5 h of 0.3 wt% Pd/ZSM-5
3.5. Effect of palladium loading on ZSM-5 on catalytic properties
Pd on the zeolite plays roles in the efficient hydrogenation of
olefins to paraffins in the syngas conversion to hydrocarbons [15].
Since olefins are primarily produced in the methanol conversion to
hydrocarbons, Pd is required for the synthesis of saturated hydro-
carbons through the hydrogenation. Fig. 10 shows H2-TPD profiles
of the 0.3 and 0.5 wt% Pd/ZSM-5 samples and the ZSM-5 sample.
Fig. 9. Influence of mixing way of Cu-ZnO with Pd/ZSM-5 on conversion of syngas
to hydrocarbons in near-critical phase over hybrid catalysts composed of Cu-
ZnO and 0.5 wt% Pd/ZSM-5. Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g
Cu-ZnO and 0.5 g Pd/ZSM-5,); temp., 553 K; Psyngas = 2.5 MPa; Pn-hexane = 1.5 MPa;
W/F = 9.7 g-cat h/mol-syngas; H2/CO = 1.9.
Fig. 10. H2-TPD profiles of Pd/ZSM-5-HT samples with different Pd loadings.

T. Ma et al. / Catalysis Today 228 (2014) 167–174
173
Fig. 11. CO conversion over hybrid catalysts composed of Cu-ZnO and Pd/ZSM-5-
HT with different Pd loadings. Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g
Cu-ZnO and 0.5 g Pd/ZSM-5); temp., 553 K; Psyngas = 2.5 MPa; Pn-hexane = 1.5 MPa;
W/F = 9.7 g-cat h/mol-syngas; H2/CO = 1.9.
Fig. 12. Product yield resulting from conversion of syngas to hydrocarbons over
hybrid catalysts composed of Cu-ZnO and Pd/ZSM-5-HT with different Pd loadings.
Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g Cu-ZnO and 0.5 g Pd/ZSM-
5); temp., 553 K; Psyngas = 2.5 MPa;
Pn-hexane = 1.5 MPa; W/F = 9.7 g-cat h/mol-syngas;
H2/CO = 1.9.
was similar to that of 0.5 wt% Pd/ZSM-5. In the MTH reaction, rapid
deactivation occurs due to the deposition of carbonaceous species
resulting from the oligomerization of primarily formed unsaturated
products on the surface or inside pores of the zeolite [41]. The
hydrogenation of the unsaturated products to saturated hydrocar-
bons over Pd/ZSM-5 would retard the oligomerization to suppress
the deactivation.
Fig. 12 shows the product yields of the hybrid catalysts con-
taining ZSM-5, 0.3 wt% Pd/ZSM-5 and 0.5 wt% Pd/ZSM-5. The yield
of the hydrocarbons (gasoline, LPG and dry gas) was increased
by employing Pd/ZSM-5 as a component of the hybrid catalyst.
Moreover, the yield of the hydrocarbons in the gasoline fraction
increased from 25% to 33% with the increase in the Pd loadings from
0 wt% to 0.5 wt%, while the yield of the dry gas fractions decreased
with increasing the Pd loadings. The increase in the Pd loadings
would improve the mass transfer of the products during the reac-
tion because the hydrogenation over Pd leads to suppressing the
blocking of the pores and cavities of the zeolite by the carbona-
ceous species. Therefore, the transfer of the gasoline fractions out
of the zeolite intracrystallites was promoted with increasing the
Pd loadings; simultaneously, the hydrocracking of the products to
small fractions was suppressed. The decrease in the yield of the dry
gas fractions with increasing the Pd loading also indicates that the
direct hydrogenation of CO to methane was not accelerated even
when Pd was employed as a hydrogenation catalyst. In addition,
the CO2 yield was almost the same regardless of the Pd loadings,
although the larger amount of H2O was generated over Pd/ZSM-5
compared with ZSM-5 alone as a result of the methanol conversion
to hydrocarbons. These results indicate that Pd hardly contributed
to the WGS reaction during the syngas conversion to hydrocarbons.
3.6. Catalytic stability of hybrid catalyst in near-critical and gas
phases
As shown in Fig. 13, in the conversion of syngas to hydrocar-
bons in the near-critical phase of n-hexane, the hybrid catalyst
consisting of Cu-ZnO and 0.5 wt% Pd/ZSM-5 exhibited the higher CO
conversion with the lower selectivity of CO2 in comparison with the
reaction in the gas phase under the conditions at 553 K and 4.0 MPa.
Fig. 13. Conversion of syngas to hydrocarbons over the hybrid catalyst consisting of Cu-ZnO and 0.5 wt% Pd/ZSM-5. Reaction conditions: cat., 1.0 g hybrid catalyst (0.5 g Cu-ZnO
and 0.5 g Pd/ZSM-5); temp., 553 K; W/F = 9.7 g-cat h/mol-syngas; H2/CO = 1.9; Psyngas/Pn-hexane = 2.5 MPa/0 MPa (gas phase); Psyngas/Pn-hexane = 2.5 MPa/1.5 MPa (near-critical phase).

174
T. Ma et al. / Catalysis Today 228 (2014) 167–174
The CO conversion of ca. 65% was kept until 30 h of the reaction in
the near-critical phase. Similar to the reaction in the near-critical
phase, in the gas phase, the CO conversion was kept with a con-
stant value of 58% during the reaction although the CO2 selectivity
slightly increased at the initial stage of the reaction.
The deactivation of the catalyst can be mitigated through bet-
ter heat and mass transfer, leading to prolonging the longevity of
catalyst. Owing to the advantages mentioned above, the reaction
system with a hybrid catalyst consisting of Cu-ZnO and Pd/ZSM-5
in a near-critical phase is promising for commercial application of
the synthesis of gasoline fraction hydrocarbons from syngas.
[5] L. Shi, Y. Jin, C. Xing, C. Zeng, T. Kawabata, K. Imai, K. Matsuda, Y. Tan, N. Tsubaki,
Appl. Catal. A: Gen. 435-436 (2012) 217–224.
[6] B. Todic, T. Bhatelia, G.F. Froment, W. Ma, G. Jacobs, B.H. Davis, D.B. Bukur, Ind.
Eng. Chem. Res. 52 (2013) 669–679.
[7] N.E. Tsakoumis, R. Dehghan, R.E. Johnsen, A. Voronov, W. van Beek, J.C. Walms-
ley, Ø. Borg, E. Rytter, D. Chen, M. Rønning, A. Holmen, Catal. Today 205 (2013)
86–93.
[8] M. Stöcker, Microporous Mesoporous Mater. 29 (1999) 3–48.
[9] M. Stöcker, Microporous Mesoporous Mater. 82 (2005) 257–292.
[10] T. Mokrani, M. Scurrell, Catal. Rev. 51 (2009) 1–145.
[11] B. Vora, J.Q. Chen, A. Bozzano, B. Glover, P. Barger, Catal. Today 141 (2009)
77–83.
[12] J.F. Haw, W. Song, D.M. Marcus, J.B. Nicholas, Acc. Chem. Res. 36 (2003) 317–
326.
[13] Q. Zhang, X. Li, K. Asami, S. Asaoka, K. Fujimoto, Fuel Process. Technol. 85 (2004)
1139–1150.
[14] K. Asami, Q. Zhang, X. Li, S. Asaoka, K. Fujimoto, Catal. Today 106 (2005)
247–251.
4. Conclusions
[15] Q. Ge, X. Li, H. Kaneko, K. Fujimoto, J. Mol. Catal. A 278 (2007) 215–219.
[16] Q. Ge, Y. Lian, X. Yuan, X. Li, K. Fujimoto, Catal. Commun. 9 (2008) 256–261.
[17] X. Ma, Q. Ge, J. Ma, H. Xu, Fuel Process. Technol. 109 (2013) 1–6.
[18] M.G. Hiltzler, M. Poliakoff, Chem. Commun. (1997) 1667–1668.
[19] A. Baiker, Chem. Rev. 99 (1999) 453–473.
[20] P.G. Jessop, W. Leitner (Eds.), Chemical Synthesis Using Supercritical Fluids,
Wiley, New York, 1999.
[21] C.V. Rode, U.D. Joshi, O. Sato, M. Shirai, Chem. Commun. 9 (2003) 1960–1961.
[22] Y. Matsumura, T. Minowa, B. Potic, S.R.A. Kersten, W. Prins, W.P.M. van Swaaij, B.
van de Beld, D.C. Elliott, G.G. Neuenschwander, A. Kruse, M.J. Antal Jr., Biomass
Bioenergy 29 (2005) 269–292.
The catalytic performances of hybrid catalysts composed of Cu-
ZnO and Pd/ZSM-5 in the syngas conversion to hydrocarbons via
methanol in a near-critical n-hexane solvent have been studied.
The near-critical fluid was effective in the selective production of
hydrocarbons in gasoline fraction during the syngas conversion
over the hybrid catalyst. In addition, the employment of the near-
critical solvent led to depressing the formation of CO2 during the
reaction.
The characteristics of Pd/ZSM-5 influenced the synthesis of the
hydrocarbons in the syngas conversion. An increase in the acid
amount of ZSM-5 was effective on the generation of hydrocarbons
in the syngas conversion to hydrocarbons at lower temperature
than those applied in the MTH reaction. Hydrocarbon distribution
was strongly dependent on the particle size of ZSM-5 and Pd load-
ing. A decrease in the particle size of ZSM-5 and an increase in the
Pd loading improved the yield of hydrocarbons in gasoline frac-
tion with high catalytic stability. It is likely that the improvement
of the mass transfer due to the decrease of the particle size and
the hydrogenation of unsaturated hydrocarbons over Pd suppress
the deposition of carbonaceous species. Subsequently, the pore and
cavity spaces in the zeolite can be retained, leading to the smooth
mass transfer of large hydrocarbons out of the intracrystallites.
Therefore, Pd/ZSM-5 composed of nano-sized particles is effective
on the selective production of gasoline fractions from syngas via
methanol with the near-critical fluid.
[23] A.A. Peterson, F. Vogel, R.P. Lachance, M. Fröling, M.J. Antal Jr., J.W. Tester,
Energy Environ. Sci. 1 (2008) 32–65.
[24] D.C. Elliott, Biofuels Bioprod. Biorefining 2 (2008) 254–265.
[25] A. Yamaguchi, N. Hiyoshi, O. Sato, M. Shirai, Green Chem. 13 (2011) 873–881.
ˇ
[26] E. Markocˇicˇ, B. Kramberger, J.G. Van Bennekom, H. Jan Heeres, J. Vos, Z. Knez,
Renew. Sustain. Energy Rev. 23 (2013) 40–48.
[27] N. Tsubaki, K. Yoshii, K. Fujimoto, J. Catal. 207 (2002) 371–375.
[28] W. Linghu, X. Li, K. Asami, K. Fujimoto, Fuel Process. Technol. 85 (2004)
1121–1138.
[29] R.M. Malek Abbaslou, J.S. Soltan Mohammadzadeh, A.K. Dalai, Fuel Process.
Technol. 90 (2009) 849–856.
[30] A.K. Mogalicherla, N.O. Elbashir, Energy Fuels 25 (2011) 878–889.
[31] E.E. Elmalik, E. Tora, M. El-Halwagi, N.O. Elbashir, Fuel Process. Technol. 92
(2011) 1525–1530.
[32] Q. Zhang, P. Liu, Y. Fujiyama, C. Chen, X.H. Li, Appl. Catal. A: Gen. 401 (2011)
147–152.
[33] H. Mochizuki, T. Yokoi, H. Imai, R. Watanabe, S. Namba, J.N. Kondo, T. Tatsumi,
Microporous Mesoporous Mater. 145 (2011) 165–171.
[34] A.K. Mogalicherla, E.E. Elmalik, N.O. Elbashir, Chem. Eng. Process.: Process
Intensification 62 (2012) 59–68.
[35] K. Yokota, Y. Hanaoka, K. Fujimoto, Fuel 70 (1991) 989–994.
[36] M. Niwa, N. Katada, Catal. Surv. Jpn. 1 (1997) 215–226.
[37] K. Suzuki, Y. Aoyagi, N. Katada, M. Choi, R. Ryoo, M. Niwa, Catal. Today 132
(2008) 38–45.
References
[38] D. Chen, K. Moljord, T. Fuglerud, A. Holmen, Microporous Mesoporous Mater.
29 (1999) 191–203.
[1] R.B. Anderson, Fischer–Tropsch Synthesis, Academic Press, New York, 1984.
[2] M.E. Dry, Appl. Catal. A: Gen. 276 (2004) 1–3.
[3] B.H. Davis, Ind. Eng. Chem. Res. 46 (2007) 8938–8945.
[4] H. Schulz, in: B.H. Davis, M.L. Occelli (Eds.), Fischer–Tropsch Synthesis and
Hydroformylation on Cobalt Catalysts, CRC Press/Taylor and Francis Group,
Boca Raton/London/New York, 2010, pp. 165–183.
[39] D. Chen, K. Moljord, A. Holmen, Microporous Mesoporous Mater. 164 (2012)
239–250.
[40] S. Yoshie, F. Masahiro, K. Roger, A. Hisanori, Q. Xu, Stud. Surf. Sci. Catal. 114
(1998) 327–332.
[41] H. Schulz, Catal. Today 154 (2010) 183–194.