Y. Hou, et al.
Molecular Catalysis 485 (2020) 110824
restrict the final products, intermediates, and even transition states to
be no larger than the size of the zeolite’s cavities or channels [12,20].
These features limit chain growth, resulting in the production of more
light hydrocarbons [20]. Furthermore, acid sites present in zeolites
render it reactive towards cracking of long-chain hydrocarbons, which
will also contribute to the formation of light hydrocarbons [21,22].
However, the confinement and cracking effects of zeolites should be
precisely controlled, because if the zeolite pore channels are too small
or strongly acidic, over-cracking might occur. This effect results in high
desorption.
The
surface
area
was
evaluated
by
the
Brunauer–Emmett–Teller (BET) equation, whereas the microporous
surface area was calculated by the t-plot method. The pore volume of
micropores and mesopores were calculated by the t-plot method and
Barrett–Joyner–Halenda (BJH) method, respectively. The pore size
distribution was calculated by density functional theory (DFT) method.
The X-ray diffraction (XRD) patterns were recorded by a Rigaku D/
MAX2550VB X-ray diffractometer. The Mössbauer spectroscopy (MES)
were recorded in an MR-351 constant-acceleration Mössbauer spec-
trometer (FAST, Germany). A scanning electron microscope (SEM,
Quanta 200, FEI) was used to characterize the size and morphology of
different zeolites. Raman spectroscopy with laser excitation at 532 nm
were conducted on a DXR2xi (Thermo, America) apparatus at room
4
selectivity for undesirable products (CH and C2-4 alkanes). In view of
the confinement and acidic cracking features of zeolites, the combina-
tion of zeolites with conventional FTS catalysts, which are rich in long-
chain olefins, might achieve high selectivity for lower olefins and high
utilization efficiency for carbon. Compared with other methods of
combining FTS active metals supported on zeolites, or by physical
mixing of FTS catalyst and zeolites, the dual-bed configuration is ad-
vantageous for withdrawing reaction heat and regenerating the catalyst
3
temperature. Temperature-programmed desorption of ammonia (NH -
TPD) was conducted on an AutoChem II 2920 instrument
(Micromeritics, USA) equipped with a mass spectrometry detector. The
samples were pretreated in He at 400 °C for 1 h, then cooled to 100 °C
[
23]. However, the development of an efficient dual-bed catalyst for
3
and saturated with NH , followed by purging with He at 100 °C for
FTO would still be desirable.
30 min and heating to 600 °C in He flow at a rate of 10 °C/min.
Herein, we constructed a dual-bed configuration catalyst with zeo-
lites downstream from the FTS catalyst FeZnNa, which is reported to be
rich in heavier olefins [19] for the FTO process. By exploring different
zeolites with hydrocracking function (HY [24], NaY, ZSM-5 [25],
SAPO-34 [26], Hβ [27], Liβ, Naβ, Kβ, and Rbβ) and optimizing the
process parameters in detail (integration manners and mass ratios of the
two active components, reaction temperature, pressure, space velocity,
2.3. Catalyst performance
The catalytic performance was in a fixed-bed reactor with a stain-
less-steel reaction tube with an inner diameter of 12 mm. The FeZnNa
and zeolites at a mass ratio of 1:1, unless otherwise stated, were filled
into the isothermal region of the reaction tube and completely sepa-
rated by silica wool. Before the FTO reaction, the samples were acti-
vated by H2 at a flow rate of 60 mL/min at 360 °C, at atmospheric
pressure for 4 h. Then, the reaction systems were operated under the
desired reaction conditions. The reaction products were analyzed by an
on-line gas chromatograph (Agilent GC 6820) equipped with a 5A
molecular sieve column connected to a TCD detector and an Al O
and ratio of H /CO), the dual-bed catalyst FeZnNa/Naβ achieved se-
2
=
lectivity as high as 50.5 % C2-4 and CO conversion up to 92.9 %. The
catalyst also exhibited outstanding long-term stability and ran stably for
100 h without notable deactivation.
2. Experimental
2
3
capillary column and two Propake Q columns connected to a FID de-
2.1. Catalyst preparation
tector. CH was used as a reference bridge between TCD and FID. An ice
4
trap was used to capture the oil phase and water phase. The oil phase
The FeZnNa catalyst was prepared by co-precipitation. Briefly, 1 M
products were analyzed by another gas chromatograph (Agilent GC
6820) equipped with an HP-5 column connected to the FID detector.
Aromatics in the oil phase were determined from the gas chromato-
graphy-mass spectrum (GC–MS, GCMS-QP2010, SHIMADZU, Japan).
The CO conversion, CO2 selectivity, hydrocarbons (C H ) selectivity
FeSO
4
3 2
and 1 M Zn(NO ) solutions were mixed in a volume ratio of 1:1.
A 2M Na
2
CO solution was used as a precipitator to co-precipitate the
3
former mixture solution. The temperature and pH value were controlled
to be approximately 80 °C and 9.0, respectively. After the precipitation,
the precipitant was aged at 80°C for 5 h, followed by washing and fil-
tration. The precursor was dried at 60 °C overnight and calcined at
n
m
=
excluding CO , and space time yield (STY) of C
2
2-4
were calculated
with the following formulae:
4
00 °C for 4 h in a muffle furnace.
Hβ, HY, and SAPO-34 are commercial zeolites purchased from
CO conversion = (COin − COout)/COin × 100 %,
CO selectivity = CO2 out/(COin − COout) × 100 %,
selectivity = NCnHm/(COin − COout − CO2 out) × 100 %,
(1)
(2)
Nankai University Catalyst Co., Ltd. The NaY and Naβ were derived
from a post-treatment of the HY and Hβ zeolite, respectively. In a ty-
pical treatment, 1.85 g of HY or Hβ zeolite was added to 50 mL of NaOH
solution (0.25 M) and stirred vigorously. The mixtures were then
transferred to an 80-mL Teflon-sealed autoclave and treated at 150 °C
for 21 h. The treated zeolites were recovered by washing, filtering, and
drying at 120 °C overnight and then calcined in a muffle at 550 °C for
2
C
n
H
m
(3)
=
=
STY of C2-4 = GHSV × CO conversion × C2-4 selectivity × (100
%
−CO selectivity)/(MFe × 3600), (4)
2
where COin and COout represent the moles of CO at the inlet and outlet,
respectively; CO2 out represents moles of CO at the outlet; GHSV re-
5
h. The synthesis of ZSM-5 was performed according to the method
2
reported by Zhou et al. [28]. Different alkalis-exchanged β zeolites (i.e.,
Liβ, Kβ and Rbβ) were prepared by ion exchange. Briefly, 1.0 g Naβ was
exchanged with a 100 mL aqueous solution of alkali nitrate (0.2 M) at
presents gas hourly space velocity; MFe represents mass of Fe in FeZnNa
(g).
8
1
0 °C for 12 h. The exchanged β zeolites were washed, filtered, dried at
20 °C overnight and calcined at 550 °C for 6 h. The obtained zeolites
3. Results and discussion
were denoted as Liβ, Kβ, and Rbβ.
3.1. Characterization
2.2. Catalyst characterizations
The textural properties of the different zeolites used in this work
2
were determined from N adsorption-desorption analysis. The surface
The elemental components were detected using an inductively
area, pore volume and average pore diameter are summarized in
Table 1. The ZSM-5 and HY zeolites had the largest surface area, up to
650 m /g. Relatively small surface areas were detected for SAPO-34
coupled plasma optical emission spectroscopy (ICP, Optima2100DV,
PerkinElmer) or an XRF-1800 spectrometer with Rh radiation at
working conditions of 60 KV and 95 mA. A micromeritics ASAP 2020 M
2
and Hβ. After post-treatment with NaOH, the surface area decreased
2
2
2
system was applied to analyze the textural properties via N
2
adsorption-
from 655.2 m /g of HY to 460.5 m /g of NaY, and from 368.8 m /g of
2