A comparative study of PdZSM-5, Pdβ, and PdY in hybrid catalysts for syngas to hydrocarbon conversion

Article abstract of DOI:10.1039/c4cy01494d

The catalytic conversion of syngas into hydrocarbons over hybrid catalysts consisting of a methanol synthesis catalyst and Pd modified zeolites (PdZSM-5, Pdβ, and PdY) was investigated. The results indicate that the intermediate dehydration step of DME in the syngas to hydrocarbon process mainly occurs on the Bronsted acid sites of the Pd zeolite. The large pores and cavities of Pdβ and PdY promote the formation of C₄₊ hydrocarbons, which are mainly gasoline-type branched hydrocarbons. Moreover, the large pores and cavities provide enough space for the formation of higher aromatics which are the precursor of coke. The blockage of micropores of the Pd zeolites by hydrocarbon-type coke results in the deactivation of the hybrid catalysts.

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ARTICLE TYPE
A comparative study of PdZSMꢀ5, Pdβ, and PdY in hybrid catalyst for
syngas to hydrocarbons
Chun Wanga,b, Xiangang Maa , Qingjie Ge a,* and Hengyong Xu a,*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Catalytic conversion of syngas into hydrocarbons over hybrid catalyst consisting of methanol synthesis catalyst and Pd modified zeolite
(PdZSMꢀ5, Pdβ, and PdY) was investigated. The results indicate that the intermediate dehydration step of DME in syngas to hydrocarbon
process mainly happens on the Brönsted acid sites of the Pd zeolite. The large pores and cavities of Pdβ and PdY promote the formation
of C4+ hydrocarbons, which are mainly gasolineꢀtype branched hydrocarbons. Moreover, the large pores and cavities provide enough
10 space for the formation of higher aromatics which are the precursor of coke. The blockage of micropores in Pdꢀzeolite by hydrocarbonꢀ
type coke results in the deactivation of hybrid catalysts.
45 combination could break the thermodynamic equilibrium
limitation of methanol synthesis due to the in situ removal of
Introduction
methanol. In addition, the modification of Pd in Pd modified
It is wellꢀknown that petrochemicals are mainly produced from
zeolite was found to promote the stability of the hybrid catalyst as
oil refineries. With the depletion of petroleum resources,
well as the formation of saturated hydrocarbons8, 18. Herein, we
15 synthesis of petrochemicals from other resources becomes more
50 have carried out a comparative study around syngas conversion
attractive. Syngas can be produced from various nonꢀpetroleum
over hybrid catalysts containing different types of Pd modified
resources such as natural gas, biomass and other carbonaceous
zeolites: PdZSMꢀ5, Pdβ and PdY, with similar SiO2/Al2O3 ratios.
materials including coal, recycled plastics and municipal wastes1,
All these zeolites were modified with Pd by ionꢀexchange
2. Thus, developing a syngas to hydrocarbon process can
20 effectively reduce the consumption of petroleum resources.
methods and then physically mixed with CuꢀZnOꢀAl2O3
55 methanol synthesis catalyst to form a hybrid catalyst. Catalyst
There are two typical routes for producing hydrocarbons
coke analysis was investigated by TPOꢀMS and N2ꢀPhysical
from syngas: FischerꢀTropsch synthesis (FTS)3ꢀ5 and syngasꢀtoꢀ
adsorption techniques.
hydrocarbons via methanol/dimethyl ether (DME)6ꢀ8. The FTS
process usually gives a wide carbon number distribution of
Experimental
25 hydrocarbons due to the limitation of AndersonꢀSchulzꢀFlory
(ASF) polymerization kinetics9. Many efforts have been made to
1. Catalyst preparation
increase the selectivity to fractions within a certain range by
60 Parent HZSMꢀ5(SiO2:Al2O3=23), Hβ (SiO2:Al2O3=25) and HY
designing novel catalysts, such as coreꢀshell structured catalyst10
(SiO2:Al2O3=30) were purchased from Zeolyst International.
and mesoporous zeolite supported catalyst11, 12. However, it was
CuOꢀZnOꢀAl2O3 catalyst (CZA) powder and γꢀAl2O3 (197.4
30 reported that the formed methane (CH4) from secondary cracking
m2/g) powder were mixed uniformly at a weight ratio of 9:5 to
was relatively high. Compared with the FTS process, the syngasꢀ
form DME synthesis catalyst. The DME synthesis catalyst was
toꢀhydrocarbons via methanol/DME process could selectively
65 crushed into 20ꢀ40 mesh and denoted as CZAA. The prepared
achieve hydrocarbons with different cuts while keeping the
CZAA showed excellent performance for DME synthesis in our
methane selectivity low8, 13, 14. The process had been realized in
previous work8.
35 producing liquefied petroleum gas (LPG, C3 and C4 alkanes)14ꢀ16,
Pd
modified
HZSMꢀ5(SiO2:Al2O3=23),
Hβ
aromatic hydrocarbon mixtures17 and gasoline fractions8, 18. The
hybrid catalyst usually consists of two components: a methanol
synthesis catalyst and a zeolite or metal modified zeolite. Syngas
was first converted into methanol over a methanol synthesis
40 catalyst, and then to hydrocarbons over a zeolite or metal
modified zeolite. For example, syngas could be selectively
converted into LPG over a hybrid catalyst composed of CuꢀZnOꢀ
Al2O3 methanol synthesis catalyst (CZA) and PdY14 while the
major product became gasoline over CZA/PdZSMꢀ518. Such
(SiO2:Al2O3=25), and HY (SiO2:Al2O3=30) were prepared by
70 ionꢀexchange methods while the theoretical loading of Pd in Pd
modified zeolite was 0.5 wt%. After washing with deionized
water, the Pd modified zeolite catalysts were dried at 120oC for 4
hrs and calcined at 550oC for 4 hrs. Finally, the Pd modified
zeolite catalysts were crushed into particles of 20ꢀ40 mesh and
75 denoted as PdZSMꢀ5, Pdβ, and PdY, respectively. CuꢀZnOꢀAl2O3
catalyst (CZA) and Pd modified zeolite were physically mixed
together at a weight ratio of 1:1 to form the hybrid catalyst. The
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hybrid catalysts were denoted as CZA/PdZSMꢀ5, CZA/Pdβ, 55 are as follows.
CZA/PdY and CZA/γꢀAl2O3, respectively.
CO + 2H2 ↔ CH3OH
2CH3OH ↔ CH3OCH3 + H2O
nCH3OCH3 ↔ CnH2n + nH2O
(1)
(2)
(3)
2. Catalyst test
mCnH2n+pH2 ↔ aCxH2x + bCyH2y+2 + cCzH2yꢀ6
60 CO + H2O ↔ CO2 + H2
Overall reaction of Equations (1), (2) and (5):
3CO + 3H2 ↔ CH3OCH3 + CO2
(4)
(5)
A pressurized flow type reaction apparatus with a fixed bed
reactor was used for this study. The fixed bed reaction system
consisted of a 12 mm inner diameter tubular reactor and an
electronic temperature controller for the furnace. The reaction
apparatus was equipped with four gas mass flowmeters for
controlling gas flow rate and a backꢀpressure valve for regulating
10 the pressure of system. Before the reaction, the hybrid catalyst
was activated at 250oC for 4 hrs in pure hydrogen flow. Then
synthesis gas was introduced into the reaction system. The hybrid
catalysts were evaluated under the condition of 260oC, 2 MPa,
and 1875 ml.hꢀ1.gꢀ1. CO, CO2, CH4 and N2 were analyzed by a
15 GC equipped with a thermal conductivity detector (TCD), and
organic compounds were analyzed by another GC equipped with
a flame ionization detector (FID).
5
(6)
Overall reaction of Equations (1)ꢀ(5):
3nCO + (3nꢀp)H2 ↔ (1ꢀm)CnH2n + aCxH2x + bCyH2y+2 + cCzH2yꢀ6
65 + 3n/2CO2 (7)
Figure 1 shows the relationship of CO conversion with time
on stream (TOS) over different hybrid catalysts. It is observed
that the CO conversion over hybrid catalysts is far higher than
that over CZA catalyst. This phenomenon is due to the inꢀsitu
70 removal of methanol from methanol/DME dehydration
component, which breaks the thermodynamic equilibrium
limitation of the methanol synthesis process (Equation 1)19, 20
.
Compared with the other three hybrid catalysts, the CO
conversion over CZA/Pdβ was sharply decreased within the
75 primary 60 minutes while the CO conversion over CZA was still
stable under the same reaction conditions. The above results
indicate that the deactivation of Pdβ leads to the quick
deactivation of CZA/Pdβ.
3. Catalyst characterization
NH3ꢀTPD Temperatureꢀprogrammed desorption of NH3 (NH3ꢀ
20 TPD) was conducted on an Autochem 2910 instrument
(Micromeritics). A 100 mg sample was preꢀtreated at 500°C for 1
hr and then heated from 100°C to 700°C at a constant rate of
10°C/min after saturation adsorption of NH3 at 100°C.
PyꢀIR A pyridine adsorption Fourierꢀtransform Infrared (PyꢀIR)
25 spectrum was recorded by a Bruker IF113V FTIR spectrometer to
study the type of acidity. The IR spectrometer was equipped with
an inꢀsitu cell containing CaF2 windows. In PyꢀIR experiment,
the sample disc was preꢀtreated under vacuum (1×10ꢀ3 Pa) at
450°C for 1 hr and then cooled down to room temperature.
30 Subsequently, pyridine was introduced into IR cell at room
temperature for 5 minutes to ensure that all acid sites were
covered. The sample was heated to 200°C and 350°C respectively
at a rate of 10°C/min and the IR spectra was collected at each
temperature.
35 TPOꢀMS Temperatureꢀprogrammed oxidation (TPO) was carried
out using a quadrupole mass spectrometer OmnistarTM GSD 301
(Pfeiffer Vacuum) for signal collection. A 40 mg sample was
heated from ambient temperature to 900°C with a heating rate of
10°C/min under a flow of 5%O2/95%Ar.
40 N2ꢀPhysical Absorption N2 adsorption isotherms were measured
by QuadraSorb SI4 instrument at ꢀ196°C, Before the N2
absorption, all the samples were pretreated at 300°C for 5 hrs
under vacuum. The total surface area was determined by the BET
method while the total pore volume was calculated by the amount
45 of N2 adsorbed at p/p0=0.98. The tꢀplot method was used for the
calculation of micropores volume and surface area.
Figure 1. TOS data of CO conversion over different hybrid
80 catalyst. Reaction condition: T=260oC, P=2 MPa, H2/CO=2,
GHSV=1875 ml.gꢀ1.hꢀ1.
Table 1 shows the product distribution of syngas conversion
over different hybrid catalysts. It is shown that the major product
in CZA catalyst was MeOH. Byproducts like hydrocarbons were
85 also formed under the reaction condition. Methanol synthesis
process was usually operated at an elevated pressure of 5ꢀ10 MPa
and temperature of 200ꢀ300°C21. High reaction temperature and
low pressure would lead to a low CO conversion as well as
promoting the formation of byproducts. The major product of
90 syngas conversion over CZA/γꢀAl2O3 became DME and few
hydrocarbons were detected in the product. Additionally, plenty
of CO2 was produced. CuꢀZnO based catalyst had great
performance for the water gas shift (WGS) reaction (Equation
5)22. Methanol formed from CZA catalyst could be dehydrated to
Result and discussion
1. Syngas conversion over different hybrid catalysts
Syngas to hydrocarbon process over a hybrid catalyst of CZA/Pdꢀ
50 zeolite mainly involves the following reactions: (1) the
hydrogenation of CO to methanol, (2) the dehydration of
methanol to DME, (3) the further dehydration of DME to light
olefins, (4) light olefins to final hydrocarbon products, and (5)
waterꢀgas shift (WGS) reaction. The relevant reaction equations
2
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DME on the weak acid sites of γꢀAl2O3 (Equation 2)23. The
Table 2. PyꢀIR data of PdZSMꢀ5, Pdβ, PdY and γꢀAl2O3.
formed water from the methanol dehydration takes part in the
WGS reaction and produces much CO2. More CO2 was produced
over hybrid catalysts of CZA/Pdꢀzeolite compared with CZA/γꢀ
Al2O3. As we discussed above, water was one of the reactants in
the WGS reaction. Compared with syngas conversion over
CZA/γꢀAl2O3, extra water was formed from the further
dehydration of DME to hydrocarbons (Equation 3). The extra
formed water further promoted the WGS reaction and thus
Acidity(mmol pyridine/g catalyst
)
Catalyst
Brönsted acidity(B) Lewis acidity(L)
B/L
200oC
350oC
200oC 350oC 200oC 350oC
5
PdZSMꢀ5 0.262
Pdβ
PdY
γꢀAl2O3 ꢀꢀꢀ
0.223
0.058
0.125
0.075
0.149
0.040
0.103
0.060
0.062
4.5
1.1
2.1
0.0
5.6
0.8
2.0
0.0
0.136
0.155
0.081
0.118
ꢀꢀꢀ
10 produced more CO2. The experimental results were also
consistent with the overall reaction of syngas to DME process
(Equation 4) and syngas to hydrocarbon process (Equation 6).
Table 1 shows that little DME and no methanol were detected
over the hybrid catalyst of CZA/Pdꢀzeolite. This was in stark
15 contrast to the product distribution over CZA/γꢀAl2O3. To further
study this phenomenon, we loaded DME synthesis catalyst
(CZAA) into the 1st reactor and γꢀAl2O3 into the 2nd reactor. Thus
the feedstock to the 2nd reactor consisted of DME, MeOH, CO2,
unreacted syngas and few hydrocarbons. The major product was
20 still the mixture of DME and methanol (see Table 1). This result
showed that γꢀAl2O3 had great performance for the dehydration of
methanol to DME but had little performance for the dehydration
of DME to hydrocarbons, which indicated that the active sites for
methanol and DME dehydration were different. We think that this
25 difference is related to the surface acidity of the dehydration
components.
Note: The amount of L and B acidity was calculated from the
50 peak areas at 1446 cmꢀ1 and 1548 cmꢀ1. The band at 1548 cmꢀ1
corresponds to Brönsted acidity and the band at 1446 cmꢀ1 to
Lewis acidity.
The hydrocarbon distribution was also different over hybrid
catalysts of CZA/Pdꢀzeolite. As shown in Table 3, a lot of C4+
55 hydrocarbons were produced over hybrid catalysts of CZA/PdY
and CZA/Pdβ. Over 89% of the C4 fractions were isobutane. In
contrast, plenty of C2ꢀC3 paraffins (50%) were produced over
CZA/PdZSMꢀ5. Less C2ꢀC3 paraffins were found in the
hydrocarbon product over CZA/PdY and CZA/Pdβ. This
60 phenomenon was related to the acidity strength and the topology
of the Pd modified zeolite. From literature, it was reported that
the cracking activity of the catalyst was strongly dependent on the
density and strength of the Brönsted acid sites24. The strong
Brönsted acid promoted the cracking of higher hydrocarbons to
65 lower hydrocarbons. The acid strength and density of different
dehydration components were characterized by NH3ꢀTPD
technique. The NH3 desorption peaks in the temperature ranges of
180ꢀ240oC, 260ꢀ320oC and 450ꢀ600oC are the chemical
desorption of NH3 from weak, medium and strong acid sites,
70 respectively25. The peak areas are proportional to the number of
the acid sites over the catalyst. As shown in Figure 2, NH3ꢀTPD
spectra mainly shows one peak before 200oC and one peak above
320oC, which is corresponding to the weak acid sites and strong
acid sites of catalyst, respectively. PdZSMꢀ5 shows the strongest
75 and has the most acid sites among the dehydration catalysts while
γꢀAl2O3 shows the weakest and least ones. From Table 2 we
know that the strong acid sites in PdZSMꢀ5 were mainly Brönsted
acid sites. The result indicates that the high content of C2ꢀC3
paraffins over CZA/PdZSMꢀ5 may come from the cracking of
80 higher hydrocarbons on strong Brönsted acid sites of PdZSMꢀ5.
Table 1. Product distribution of syngas conversion over different
hybrid catalysts.
Catalyst bed
Product selectivity, %
CO2 DME MeOH EtOH HCs
CZA
CZA/ γꢀAl2O3
CZA/PdZSMꢀ5
CZA/Pdβ
CZA/PdY
9.9 0.7
31.8 64.1
49.2 1.0
47.1 0.8
48.5 1.0
32.6 62.4
83.0
3.8
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
2.2
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
4.2
0.3
49.9
52.1
50.4
0.7
a
b
CZAA+γꢀAl2O3
4.4
Reaction condition: T=260oC, P=2 MPa, GHSV=1875 ml.hꢀ1.gꢀ1,
30 TOS=60 min. a. CZA and γꢀAl2O3 were homogenously mixed
with granule type. b. CZAA was loaded in the 1st reactor,
T=260oC; γꢀAl2O3 was loaded in the 2nd reactor, T=260oC, P=2
MPa, H2/CO=2, GHSV=1875 ml.hꢀ1.gꢀ1, TOS=60 min.
Table 3. Effect of zeolite topology on hydrocarbon distribution.
PyꢀIR data in Table 2 show that only Lewis acid sites
35 existed in γꢀAl2O3 while Pd modified zeolites had both Brönsted
acid sites and Lewis acid sites. The above experimental results
suggested that the dehydration of methanol to DME mainly
happens on the weak Lewis acid sites while the dehydration of
DME to hydrocarbons on the Brönsted acid sites. DME might be
40 one of the intermediate products in the syngas to hydrocarbon
process under the existence of Lewis acid sites. Next, DME was
further dehydrated to hydrocarbons on neighbouring Brönsted
acid sites of Pd modified zeolites.
Product distribution in
Catalyst bed
IsoꢀC4/C4,%
Hydrocarbon, %
C3 C4
CZA/PdZSMꢀ5 2.4 20.8 29.2 16.5
CZA/Pdβ 0.5 2.8 9.6 46.6
CZA/PdY 0.9 10.4 11.8 54.5
C1 C2
C5ꢀC12
31.0
40.5
65.4
90.2
89.1
22.3
Reaction condition: T=260oC, P=2 MPa, H2/CO=2, GHSV=1875
ml.hꢀ1.gꢀ1, TOS=60 min.
85
45
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could provide more space for chain growth reactions like
oligomerization. DME was dehydrated to lower olefins and
converted into higher hydrocarbons such as C5+ olefins, aliphatic
25 hydrocarbons, naphthenes and aromatics by oligomerization,
cyclization and aromatization on PdY and Pdβ. However, the
detected C5ꢀC12 hydrocarbons over the CZA/PdY catalyst were
the least among the three hybrid catalysts. A similar phenomenon
was also found by Fujimoto et al6. The large pore in HY was
30 considered to enhance the mass transfer of both reactants and
products, which would shorten the residence time of lower olefins
in the channels of HY and weaken the oligomerization reactions.
Note that hydrocarbons in C5ꢀC12 over the above three hybrid
catalysts are mainly gasolineꢀtype branched paraffins (see Figure
35 4).
Table 4. The structure parameters of zeolite HZSMꢀ5, Hβ and
HY.
Zeolite Framework Ring size Pore
type type (Tꢀatoms) size/nm system
HZSMꢀ5 MFI
Channel Cavity
Figure 2. NH3ꢀTPD spectrum of different catalyst PdZSMꢀ5, Pdβ,
PdY and γꢀAl2O3
size/ nm
10, 6, 5, 4 0.53×0.56 3 D
12, 6, 5, 4 0.67×0.66 3D
none
~1nm
Hβ
HY
BEA
FAU
Figure 3 shows the carbon number distribution of C5ꢀC12
hydrocarbons over the three different hybrid catalysts. Few C11ꢀ
C12 hydrocarbons were detected over CZA/PdZSMꢀ5 and
CZA/PdY. Meanwhile, about 26% C11ꢀC12 hydrocarbons were
detected over the CZA/Pdꢀβ catalyst. It is noteworthy that almost
all the C11ꢀC12 hydrocarbons were hexamethylbenzene. This
phenomenon has close correlation with the topology of different
12, 6, 4
0.74×0.74 3D
~1.3nm
5
Note: Data are from IZA.
10 Pd zeolites.
Figure 4. Compositions in C5ꢀC12 fractions. Note: nꢀ: nꢀparaffin;
40 isoꢀ: isoꢀparaffin; oleꢀ: olefins; Naꢀnaphthene; Arꢀ aromatics.
Reaction conditions: 260oC, 2 MPa, H2/CO=2, GHSV=1875
ml.hꢀ1.gꢀ1.
Figure 3. Percentage of different fractions in C5ꢀC12 hydrocarbon.
Reaction conditions: 260oC, 2 MPa, H2/CO=2, GHSV=1875
ml.hꢀ1.gꢀ1. Note: about 100% C11ꢀC12 hydrocarbons over
CZA/Pdβ catalyst is hexamethylbenzene.
2. Catalyst deactivation analysis
Catalyst deactivation by coke is one of the main problems in the
45 field of catalytic reactions26. M. Guisnet et al thought that the
formation of coke had close correlation with the topology of the
zeolite27ꢀ29: (i) pore size (ii) existence or nonꢀexistence of cavities
and (iii) the diffusion of the reactants and/or the products in
zeolite. Additionally, the density of Brönsted acid and Lewis acid
50 also influences coke formation over zeolite catalyst. From the
discussion of 3.1, we know that the deactivation of Pdβ leads to
the quick deactivation of the hybrid catalyst of CZA/Pdβ. To
understand the reason for the deactivation of the hybrid catalysts
15
Table 4 gives detailed information about the topology of
zeolite HZSMꢀ5, Hβ and HY. Both Hβ and HY possess a cavity
with different sizes compared with HZSMꢀ5. The pore size order
of the three zeolites is as follows: HZSMꢀ5<Hβ<HY. Compared
with PdZSMꢀ5, it was much easier for produced methanol and
20 DME to enter the channel of PdY and Pdβ due to their larger
channel openings. The large pore and cavity of Pdβ and PdY
4
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for syngas conversion, TPOꢀMS and N2 physical adsorption
techniques had been carried out.
literature, we know that the kinetic diameter of
hexamethylbenzene is approximately 0.72 nm30, which is bigger
45 than the pore size of Pdβ. The formation of hexamethylbenzene
should happen in the cavity of Pdβ. As the chemical bond in
zeolite framework is elastic and vibrational31, the
hexamethylbenzene could also diffuse out from the large pores of
Pdβ. That’s why we detected hexamethylbenzene over CZA/Pdβ.
50 The cavity size of Pdβ (~ 1.0 nm) is smaller than the cavity size
of PdY (~1.3 nm). The maximal size of hydrocarbons formed in
the cavities of PdY should be larger than that formed in the
cavities of Pdβ. Thus, the formed hydrocarbons in the cavities of
PdY might be higher aromatics with 2ꢀring and 3ꢀring (Polycyclic
55 aromatic hydrocarbons, PAHs). The formed higher aromatics in
cavities were trapped into the pores of zeolites which become the
precursors of coke. For PdZSMꢀ5, the chain growth reactions
were limited for its narrow pore size. The formation of higher
aromatics was restrained in PdZSMꢀ5. However, there were also
60 carbonaceous compounds deposited over the external surface of
PdZSMꢀ5. In general, the blockage of micropores by
hydrocarbonꢀtype coke caused the deactivation of hybrid
catalysts.
Table 5a Textural properties of Pd zeolite and spent Pd zeolite
S External
Vpore
Vmicro
Smicro
SBET
Sample
(cm3. gꢀ1)a (cm3. gꢀ1)b (m2 .gꢀ1)b (m2. gꢀ1)c （m2. gꢀ1)c
PdZSMꢀ5 0.215
PdZSMꢀ5ꢀS 0.199
0.133
0.107
0.140
0.090
0.276
0.204
326.9
265.0
350.9
220.0
610.3
491.5
378.3
308.0
529.2
395.1
709.1
590.6
51.4
43.0
178.3
175.0
98.9
99.1
Pdβ
0.555
0.489
0.478
0.388
PdβꢀS
PdY
PdYꢀS
a. Measured Vads at p/p0 = 0.98.
b. tꢀplot method.
c. BET method.
“S” means “spent”
5
Table 5b The reduction rate of physical properties in spent Pd
modified zeolite
Sample
dVpore
/
dVmicro
/
dSmicro
/
dSBET
/
dSExternal/
Vpore,% Vmicro,% Smicro,% SBET %
,
SExternal, %
PdZSMꢀ5ꢀS 7.4
19.5
35.7
26.1
18.9
37.3
19.5
18.6
25.3
16.7
16.3
1.9
~0
PdβꢀS
PdYꢀS
11.9
18.8
10 Note: dX/X=(X0ꢀXi)/X0, which means the reduction rate of the
samples’ physical properties. X0 means the property value of
fresh catalyst; Xi means the property value of spent catalyst.
As shown in Figure 5, obvious CO2 and H2O signals were
detected by mass spectrometer (MS) in the three spent Pd
15 modified zeolites (PdZSMꢀ5ꢀS, PdβꢀS and PdYꢀS), especially in
PdβꢀS and PdYꢀS. Only the weakest H2O and CO2 signals were
detected over PdZSMꢀ5ꢀS which was consistent with good
stability of CZA/PdZSMꢀ5. TPOꢀMS data shows three CO2 peaks
for PdZSMꢀ5ꢀS while two peaks for both PdβꢀS and PdYꢀS. N2
20 physical adsorption data (see Table 5a) show that the total pore
volume of the three spent Pdꢀzeolites is decreased. The order of
reduced volume is PdZSMꢀ5ꢀS < PdβꢀS < PdYꢀS which is in line
with the results of TPOꢀMS. Table 5b shows that the reduction
rate order of external surface is PdYꢀS < PdβꢀS < PdZSMꢀ5ꢀS. In
25 contrast, the reduction rate order of volume is PdZSMꢀ5ꢀS < Pdβꢀ
S < PdYꢀS. The results of TPOꢀMS and N2 physical adsorption
techniques indicate that external coke was formed over PdZSMꢀ5,
while higher hydrocarbons were formed within the micropores of
Pdβ and PdY during syngas conversion reactions. These trapped
30 higher hydrocarbons became carbonaceous compounds and
blocked the pores, especially micropores, of Pdꢀzeolites. The first
peak at about 300oC belongs to the calcinations of external coke
while peaks greater than 400oC belong to the calcinations of
internal coke. Table 3 shows that both Hβ and HY contain a 12ꢀ
35 ring opening channel system and a cavity while HZSMꢀ5
contains a 10ꢀring opening channel system but no cavity. The
pore sizes of the three Pdꢀzeolites are 0.53×0.56 nm for PdZSMꢀ5,
0.67×0.66 nm for Pdβ and 0.74×0.74 nm for PdY, respectively.
The large cavity of PdY (1.3 nm) and Pdβ (1.0 nm) can provide a
40 much bigger space for the following chain growth reactions to
form higher hydrocarbons like polymethylbenzene. Many
hexamethylbenzene were detected over CZA/Pdβ. From the
65 Figure 5. TPOꢀMS spectrum of spent PdZSMꢀ5ꢀS, PdβꢀS and
PdYꢀS zeolite.
Conclusion
Synthesis gas could be converted into different hydrocarbon
fractions over hybrid catalyst composed of CuꢀZnOꢀAl2O3
70 methanol synthesis catalyst and Pd modified zeolite. The results
indicate that the intermediate step of DME dehydration to
hydrocarbons mainly happened on the Brönsted acid sites while
methanol could be effectively converted into DME on the weak
Lewis acid sites. Additionally, hydrocarbon distribution and
75 carbonaceous compound formation had a close relationship with
the topology and the acidity of Pd modified zeolite. More C4+
hydrocarbons were formed over hybrid catalyst of CZA/Pdꢀβ and
CZA/PdꢀY due to their large pore and cavities. However, the
cavity also provided enough space for the formation of higher
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DOI: 10.1039/C4CY01494D
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