Hydrogenation of CO2to LPG over CuZnZr/MeSAPO-34 catalysts

Article abstract of DOI:10.1039/d0nj00907e

The utilization of CO₂to synthesize environmentally benign liquid fuels offers a solution to replacing depleting petroleum resources. Herein, a ternary CuZnZr (CZZ) metal oxide catalyst and a SAPO-34 zeolite were synthesized by co-precipitation and hydrothermal synthesis, respectively. Different metals were impregnated into the latter to obtain MeSAPO-34 (Me = Mn, Zn and Zr). A granule mixture of CZZ and MeSAPO-34 components (CZZ/MeSAPO-34 catalyst) was then effectively utilized in a tandem catalytic process for one-step CO₂hydrogenation to liquefied petroleum gas (LPG). The CZZ/MeSAPO-34 catalysts were characterized by using XRD, H₂-TPR, BET, SEM-EDS and NH₃-TPD techniques. SEM-EDS and XRD results indicated that an appropriate amount of Zr metal loading induced minimum zeolite framework collapse compared to a similar amount of Mn and Zn, which was more favorable for higher activity. In addition, NH₃-TPD results revealed that the acidity of SAPO-34 could be altered after impregnation with different metals in different quantities. Tuning the acid density and strength, together with adjusting the CZZ to MeSAPO-34 weight ratio, had a collectively critical effect on LPG selectivity. An effective hydrogenation microenvironment which favors lower alkane formation (C₃-C₄) was enhanced after the acidity of the molecular sieve was tuned. LPG selectivity could reach 86% over the CZZ/5% ZrSAPO-34 catalyst at 2 MPa, 350 °C, a W/F ratio of 6, a H₂/CO₂ratio of 3 and a weight ratio of 1.

Full text of DOI:10.1039/d0nj00907e

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Hydrogenation of CO2 to LPG over CuZnZr/MeSAPO-34
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Catalyst
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Mingliang Tonga+, Emmerson Hondoa+, Linet Gapu Chizemaa, Ce Dua,c, Qingxiang
Mab, Shuting Moa, Chengxue Lua, Peng Lua,b*, Noritatsu Tsubaki d*
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a Zhejiang Provincial Key Lab for Chem. & Bio. Processing Technology of Farm
Product, School of Biological and Chemical Engineering, Zhejiang University of
Science and Technology, Hangzhou, 310023, PR China
b State Key Laboratory Cultivation Base of Natural Gas Conversion, Ningxia
University, Yinchuan 750021, PR China
c Department of Applied Chemistry, School of Engineering, University of Toyama,
Gofuku 3190, Toyama 930-8555, Japan
+These authors have equal contribution
*Corresponding authors
Email: lvpeng0830@zust.edu.cn (Peng Lu)
Email: tsubaki@eng.u-toyama.ac.jp (Noritatsu Tsubaki)

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Abstract
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The utilization of CO2 to synthesize environmentally begnin liquid fuels offers a
solution to replacing depleting petroleum resources. Herein, a ternary CuZnZr (CZZ)
metal oxide catalyst and a SAPO-34 zeolite was synthesized by co-precipitation and
hydrothermal synthesis, respectively. Different metals were impregnated into the latter
to obtain MeSAPO-34 (Me= Mn, Zn and Zr). A granule mixture of CZZ and
MeSAPO-34 components (CZZ/MeSAPO-34 catalyst) was then effectively utilized in
a tandem catalytic process for one-step CO2 hydrogenation to liquefied petroleum gas
(LPG). The CZZ/MeSAPO-34 catalysts were characterized by XRD, H2-TPR, BET,
SEM-EDS and NH3-TPD techniques. SEM-EDS and XRD results indicated that an
appropriate amount of Zr metal loading induced minimum zeolite framework collapse
compared to a similar amount of Mn and Zn, which was more favorable for higher
activity. In addition, NH3-TPD results revealed that the acidity of SAPO-34 could be
altered after impregnation with different metals in varied quantities. Tuning the acid
density and strength, together with adjusting the CZZ to MeSAPO-34 weight ratio, had
a collectively critical effect on LPG selectivity. An effective hydrogenation
microenvironment which favors lower alkane formation (C3-C4) was enhanced after the
acidity of molecular sieve was tuned. LPG selectivity could reach 86% over
CZZ/5%ZrSAPO-34 catalyst at 2 MPa, 350 oC, W/F of 6, H2/CO2 of 3 and weight ratio
of 1.
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Keywords: CO2; CZZ; Hydrogenation; LPG; MeSAPO-34
1. Introduction
Global energy crisis escalated by gradual crude oil depletion and increase in
population has driven current research towards synthesis of high value hydrocarbon
fuels from cheap and abundant one-carbon (C1) feed stocks such as CO2 and CO. The
accumulation of anthropogenic CO2 in the atmosphere has also given rise to serious
environmental concerns such as climate changes. To date, viable strategies namely
carbon capture and utilization (CCU) have been developed to minimize direct CO2
emission into the atmosphere. There are quite a number of existing technologies in

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CCU, involving the conversion of CO2 to C1 and short-chain products such as CH4,
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HCOOH and CH3OH.[1-3]
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Catalytic CO2 hydrogenation to liquid petroleum gas (LPG) also provides a
sustainable route for production of clean fuel and diminution of carbon emissions.
Liquefied petroleum gas is a mixture of propane and butane which is mostly obtained
from crude oil refining and natural gas. LPG is comparable to petrol or diesel in terms
of engine performance and maintenance.[4, 5] It has environmentally benign
characteristics, thus, it can be conveniently used in the household and industry as a
substitute for other petroleum-based fuels. Additionally, LPG can be easily converted,
stored and transported as a liquid phase. Therefore, it is a worthy target product for
CO2 conversion.
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There are two major catalytic approaches to transformation of CO2 to LPG. The
first route involves the traditional Fischer Tropsch synthesis (FTS) route which is
combined with the reverse water–gas shift (RWGS) reaction (RWGS-FT) using
modified FT catalysts.[6, 7] RWGS reaction produces CO from CO2 hydrogenation
(eqn 1) which then proceeds via FT route (eqn 2) to produce light olefins and
paraffins (eqn 3). FTS is a process that produces hydrocarbons through syngas
(CO/H2) transformation over FT catalysts, namely Fe, Co, Ru and Ni. However, the
selectivity of any particular hydrocarbon species is restricted by Anderson-
Schulz-Flory (ASF) law of distribution, unless the catalysts are modified. Another
inherent drawback of this pathway is the thermodynamic feasibility of the two
reactions at different temperature ranges. RWGS reaction is endothermic; therefore, it
is thermodynamically favored at higher temperatures, whereas FTS reaction is
exothermic and requires a lower temperature.
H2 + CO2 → CO + H2O
(1)
(2)
(3)
2nH2 + nCO → CnH2n + nH2O
(2n + 1)H2 + nCO → CnH2n+2 + nH2O
The second route, a catalytic approach different from FTS, involves CO2
hydrogenation to light alkanes over a hybrid catalyst consisting of a methanol

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synthesis catalyst and a zeolite.[8] The light alkanes can be specifically synthesized
over bifunctional catalysts via reaction intermediates such as methanol and dimethyl
ether followed by chain growth into C3 and C4 hydrocarbons.
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Previous reports have concurred that metallic Cu facilitates H2 dissociation
reactions and provides hydrogen resource [9-12], and also Zr-modified Cu/Zn
catalysts (e.g. CZZ) have commendable activity and good stability for methanol
synthesis via CO2 hydrogenation.[9, 13] It has been realized that Zr improves surface
basicity, which could promote CO2 adsorption [14] and the Cu/ZrO2 synergy
establishes Cu–ZrO2 interfacial sites [9], which may improve the dissociation of H2
and spillover of atomic hydrogen.[15] In addition, it has also been reported that an
intrinsic interaction between ZnO and CuO species can enhance particle dispersion,
where ZnO particles function as spacers amongst Cu particles hindering them from
sintering.[16] ZnO can be viewed as a structure directing support which bridles
morphology, activity and dispersion of metal particles.[10, 17] Wang et al., [9]
regarded the ZnO-ZrO2 interface as the active sites for CO2 adsorption and conversion,
whereas metallic Cu facilitated dissociative chemisorption of H2 according to density
functional theory (DFT) calculations and in-situ diffuse reflectance infrared fourier
transform spectroscopy (DRIFTS) measurements.
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It is well acknowledged that light olefins can be synthesized from methanol or
dimethyl ether (DME) over SAPO-34 zeolite owing to its shape selectiveness (small
eight-ring pore size=0.38 nm). Generally, in order to extend zeolite catalysts life time
and improve stability, modification with metals has been employed.[18-20] Recently,
studies have been conducted where cerium, zirconium [20], and zinc [21] were used
to tune some SAPO-34 properties. Apart from stability enhancement, the metal
incorporation also modified the zeolite acid density for higher product selectivity.
In this work, the granule mixed combination of a ternary metallic CZZ catalyst
and modified zeolite catalyst (5%ZrSAPO-34) were found suitable for the series
reaction of converting CO2 to LPG. Critical reaction intermediates such as light
olefins could undergo secondary hydrogenation due to enhanced hydrogen transfer
reactions, resulting in an escalated LPG formation. The results revealed that the

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multifunctional catalyst has remarkable activity and selectivity for C3-C4-rich
hydrocarbons. The process of hydrocarbon formation and the product distribution
have been discussed in detail in this work.
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2. Experimental Section
2.1 Catalyst Preparation
CuOZnOZrO2 catalyst
Ternary oxide catalyst with Cu/Zn/Zr molar ratio of 5:3:1 was prepared by
traditional co-precipitation method. Firstly, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O and
Zr(NO3)4·5H2O were dissolved in deionized water to make a mixed salt solution
(solution A) with total metal ion concentration of 1 mol/L. Solution A was then stirred
o
for 1 h at 60 C. An excess amount of Na2CO3 was dissolved in deionized water to
make solution B, which was added dropwise to solution A until the pH value reached
8. During precipitation, the mixture was kept under vigorous stirring at 80 oC, for 2 h,
then subsequently aged at room temperature. Hereafter, the obtained precipitate was
separated by centrifugation and washed for at least 5 times with deionized water.
Drying process was done at 120 oC for 3 h followed by calcination in air at 500 oC for
5 h. The fresh CZZ catalyst was granulated into pellet size of 20-40 mesh.
MeSAPO-34 catalyst
SAPO-34 molecular sieve was prepared following the hydrothermal synthesis
technique.[21] Modified SAPO-34 was then prepared by ultrasonic-impregnation
method. The as-synthesized SAPO-34 (2 g) was weighed into an evaporating dish and
a certain amount of either Zn(NO3)2·6H2O, Zr(NO3)4·5H2O, or Mn(NO3)2 solution
was loaded into the SAPO-34 under ultrasonic agitation for 30 min. The modified
zeolite, identified as Me/SAPO-34 (Me=Zn, Zr, Mn) was placed in a vacuum chamber
for 10 mins before drying in air at 60 oC, followed by calcination at 500 oC for 4 h in
air. Me/SAPO-34 catalysts were also separately granulated into 20-40 mesh size prior
mixing with CZZ catalyst in varying weight ratios. The most suitable metal additive
for zeolite modification (denoted as xMeSAPO-34 where x=1wt%, 3wt%, 5wt%,
7wt% and 10wt%) was further varied to investigate the most appropriate amount. The

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preparation steps are summarized in Scheme 1.
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Scheme 1. Schematic representation of the CZZ/MeSAPO-34 catalyst preparation
process.
2.2 Catalysts characterization
Temperature programmed reduction (H2-TPR) and NH3 temperature programmed
desorption (NH3-TPD) were performed on a BELCAT-B3 (BEL, Japan) apparatus
furnished with a thermal conductivity detector (TCD).
N2 physisorption was carried out on an automatic gas adsorption system
(Micromeritics, ASAP 2460). Specific surface area and pore sizes of the catalysts
were analyzed by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
methods respectively.
X-ray diffraction (XRD) patterns were determined with RINT 2400
diffractometer using Cu Kα radiation source (λ=1.54 Å, scanning rate of 4o min-1 in
the range of 5o-80o, at 40 kV and 40 mA.).
The surface morphology of all catalysts was obtained by a scanning electron
microscope (SEM, JEOL JSM6360LV). The elemental content of the various catalysts
was determined by the energy dispersive spectrometer (EDS, JED 2300) connected to

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the SEM apparatus.
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2.3 Catalytic activity testing
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LPG synthesis reaction was performed in a continuous flow fixed-bed stainless
steel tubular reactor (i.d: 6.8 mm). 1 g of catalyst, mixed with a certain amount of
inert SiO2 to prevent hot spot generation, was loaded by granular stacking into the
reactor and held in position by quartz wool. All samples were reduced in pure H2 for 4
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h (280 C, GHSV=4800 h-1 at atmospheric pressure) prior reaction. Synthesis gas
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(H2/CO2=3) was used as feed gas under reaction conditions of 350 C，GHSV=4200
h-1 and 2 MPa. Feed gas flow was controlled by a mass flow controller. Catalytic
performance was discussed after 360 mins of reaction. The gaseous products were
analyzed online by a gas chromatograph consisting of a flame-ionization (FID)
detector equipped with an Agilent HP-Plot/Q column. The gaseous effluent (CO, CO2
and CH4) was quantitatively analyzed using an online gas chromatograph comprising
a thermal conductivity detector (TCD) furnished with a TDX-01 column.
Calculations for hydrocarbons distribution was on a molar carbon basis. CO2
conversion and product selectivity were calculated by the following formulae:
Fin
C
co2, in  FoutC
xco2% 
co2, out 100%
FinCco2, in
n
i
C
i, out
si
% 
100%
n
i
C
i, out

where xco2%: CO2 conversion, Fin: inlet feed gas flow, Cco2,in: inlet CO2 molar
content, Fout: outlet gas flow, Cco2,out: outlet CO2 molar content, si%: selectivity of i
component, ni: number of carbon atoms of i component in product and Ci,out
:
component i molar content of gas outlet.

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3. Results and Discussion
3.1 XRD Patterns
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Fig. 1 a. XRD patterns of the fresh CZZ, SAPO-34 and 5%MeSAPO-34 samples.
Fig. 1 shows characteristic XRD diffractions for Cu/Zn/Zr and MeSAPO-34
catalysts. For CZZ, the peaks for CuO appeared at 2θ=35o, 38.6o, 48.7o, 61o, and 68o,
and ZnO at 2θ= 34.5o, 36.2o, 56.5o and 62.9o, as presented in Fig. 1 a. There were no
peaks assignable to ZrO2, possibly because it existed in a microcrystalline or
amorphous state in the ternary oxide catalyst. The hydrothermally synthesized
SAPO-34 zeolite exhibited diffraction peaks at 2θ=9.5o, 12.8o, 16.2o, 20.8o and 31.2o
[22-24] corresponding to JCPDS number 01-087-1527, demonstrating that the
characteristic chabazite structure was successfully constructed. The distinctive zeolite
peaks were inherited into MeSAPO-34 (Me= Zn, Zr, Mn) though with slightly weaker
intensities, marginally broadened, they indicate an efficacious impregnation procedure
of the different metal ions into SAPO-34 microstructure. Fig. 1 b shows that the
intensity of SAPO-34 characteristic peaks decreased with increase in Zr metal content.
Recent reports have indicated that addition of high amounts Zr into the molecular
sieve results in deposition of its oxides in the channels or crystal outer surface,

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leading to a smooth zeolite crystal surface, hence reduced crystallinity. [20, 25] At
10wt% Zr content (10%ZrSAPO-34), characteristic zeolite peak intensities were
lowest, which further reveals that high zirconia loading amount reduces crystallinity
of SAPO-34.
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Fig. 1 b. XRD patterns of the fresh xZrSAPO-34 samples
3.2 NH3-TPD
As displayed in Fig. 2, an almost flat NH3 desorption profile was observed for
CZZ catalyst. The remaining catalysts, denoted as MeSAPO-34 showed desorption
peaks with a similar trend, derived from pristine SAPO-34, each with two peaks
attributed to weak (hydroxyl groups bound to defect Si
̶ OH, Al ̶ OH and P ̶ OH sites)
and strong (hydroxyl groups bound to Si OH Al sites) acid sites.[25, 26] There was a
̶
̶
slight backward peak shift to a lower temperature observed for 5%MnSAPO-34 and a
near-complete degeneration of the strong acid sites for 5%ZnSAPO-34, both
attributed to a change in ionic composition of the zeolite after impregnation.
Furthermore, weak and strong acid site peaks with closer resemblance to pristine
zeolite, but with slightly lower intensities and marginally shifted to lower
temperatures, were observed on 5%ZrSAPO-34.

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In addition, Table 1 shows the total amount of acid sites in the order of
SAPO-34 > 5%MnSAPO-34 > 5%ZnSAPO-34 > 5%ZrSAPO-34, indicating that Zr
ions had the ability not only to easily substitute H+ ions, but to also quickly orientate
to the zeolite ionic structure. Table 1 also reveals a sharp decline of weak acid sites in
5%ZrSAPO-34 (0.36 mmol·g-1). This could have been because of external surface
deposition of ZrO2 on SAPO-34, covering some of the external acid sites of the
zeolite. Because of this phenomenon, it could be deduced that incorporation of an
appropriate amount zirconium ions in the molecular sieve ionic environment, yielded
a better interaction and controlled effect on the zeolite acidity, thereby making this
metal more favorable for enhancing catalytic stability and performance. Acidity
analysis of different Zr compositions in SAPO-34 was displayed in Fig. S1 and Table
S1.
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Fig. 2. NH3-TPD profiles of the fresh CZZ, SAPO-34 and 5%MeSAPO-34 catalysts

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Table 1. Acidity properties of as-synthesized catalysts
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Weak acid
Strong acid
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Total amount
(mmol·g-1)
Catalysts
Temperature Acid amount Temperature
Acid amount
(mmol·g-1)
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(oC)
(mmol·g-1)
(oC)
CZZ
-
-
-
-
-
SAPO-34
177
186
176
0.68
0.62
0.36
389
400
368
0.51
0.03
0.18
1.19
0.65
0.54
5%ZnSAPO-34
5%ZrSAPO-34
5%MnSAPO-34
175
0.59
340
0.26
0.85
3.3 H2-TPR
As shown in Fig. 3, H2-TPR analysis was used to investigate the hydrogen
consumption of a series of catalysts. Both ZnO and ZrO2 are not reducible within the
region experimented, then it becomes obvious that all the reduction peaks are
associated to CuO specie [11, 27]. The peaks for CZZ/SAPO-34 exhibited a
somewhat subtle shift towards higher reduction temperatures. The same observation
was realized for CZZ/xMeSAPO-34, which could suggest that the mixture of the two
components assumed a mutual interaction which in turn slightly enhanced the surface
adsorption of H2, resulting in the slightly higher reduction temperature (Fig. 3, S2). In
o
essence, the reduction region (200-300 C) already ascribed to the reduction of CuO
species in all catalysts, was maintained.

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Fig. 3. H2-TPR profiles of the catalysts
3.4 N2 adsorption and desorption
In Fig. 4, SAPO-34 and MeSAPO-34 displayed similar N2 adsorption-desorption
isotherms, with strong adsorption observed at relatively low pressures, denoting
mostly microporous structures. A series of xZrSAPO-34 (x=1, 3, 7, 10wt%) samples
also displayed similar isotherms as shown in Fig. S3. However, CZZ catalyst
exhibited type IV isotherms with an H3 hysteresis loop (IUPAC catalogue), occurring
from P/P0=0.4 to 1.0, which indicates that its pore structure was mainly mesoporous.
Additionally, textural properties of the as-prepared catalysts are shown in Table 2 and
Table S2. SAPO-34 possessed the largest surface area (420 m2/g). On incorporating
Mn, Zn or Zr (MeSAPO-34), relative surface areas and pore volumes were
significantly reduced. This could be ascribed to the clogging effect of some metal
particles on the surface or on the small zeolite pores during the impregnation
procedure. The result confirms a possible proposed reason why the relative
crystallinity was reduced on modified zeolite structures from the corresponding XRD
profiles.

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Fig. 4. N2 adsorption-desorption isotherms of the catalysts
Table 2. Textural properties of the catalysts
Catalysts
BET (m2g-1)
Total pore volume (cm-3g-1)
41
0.136
0.252
0.125
0.137
0.131
CZZ
420
248
222
238
SAPO-34
5%ZnSAPO-34
5%ZrSAPO-34
5%MnSAPO-34
3.5 Morphology of catalysts
Fig. 5 shows SEM images of the synthesized CZZ, SAPO-34 and
5%MeSAPO-34 catalysts. CZZ catalyst revealed irregular shaped particles of various
sizes. As expected, the as-synthesized SAPO-34 zeolite exhibited fairly uniform sized,
cubic shaped granules related to its characteristic CHA structure. SAPO-34
impregnated with up to 5wt% Zr had a smooth surface and maintained the firm
characteristic shape indicating that the structure of the zeolite remained intact after

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modification. Upon further increase in Zr loading, slight alterations in particle sizes
and small particle fragments were observed on the surface of the catalyst (Fig. S4).
This could be attributed to the incomplete incorporation of Zr component into zeolite
structure which was consistent with the intensity reduction in the XRD analysis (Fig.
1b). Furthermore, EDS analysis (Fig. 5 f) proved that the morphology of the final
product relied on the successful impregnation of metal into the zeolite surface
structure. Quantitative analysis of 5%ZrSAPO-34 SEM images was conducted using
ImageJ software and Fig. S5 represents the particle size distribution histogram. The
average particle size of the cubic crystals used for the for MTO step in this
investigation was approximated to 1.46 µm. Small particle size distribution is an
essential attribute for improving the synthesis of desired products with highest
selectivity.
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Al: 49 %
Cu: 54 %
(a)
(b)
Si: 11 %
P: 40 %
Zn: 32 %
Zr: 14 %
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Al: 46 %
Al: 45 %
Si: 10 %
P: 39 %
(c)
(d)
Si: 11 %
P: 38 %
Al: 47 %
Si: 11 %
P: 37 %
Zr: 5%
(e)
Fig. 5. SEM images (a) CZZ; (b) SAPO-34; (c) 5%MnSAPO-34; (d) 5%ZnSAPO-34;
(e) 5%ZrSAPO-34 and (f) EDX analysis of 5%ZrSAPO-34

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3.6 Catalytic activity
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The CZZ/xMeSAPO-34 catalysts, arranged using the granule stacking method
were evaluated for the direct synthesis of LPG from CO2 hydrogenation and the results
are shown in Table 3. Pure Cu-Zn-Zr catalyst and CZZ/SAPO-34 catalyst were also
tested under the same reaction conditions as reference catalysts. Synthesis of
hydrocarbons from CO2 hydrogenation over a multifunctional catalyst comprising a
methanol synthesis catalyst and modified zeolite generally involves a series of
reactions: (1) CO2 hydrogenation to methanol, (2) methanol dehydration to dimethyl
ether (DME) and further dehydration (of generated DME) to olefins over zeolite, (3)
olefins hydrogenation to hydrocarbons and (4) reversed water gas shift (RWGS)
reaction.[14] The site for methanol synthesis and RWGS reaction (CO2 + H2 → CO +
H2O) was the pure Cu-Zn-Zr catalyst, with CH4 and CO being the major products as
clearly shown in Table 3. These two reactions compete, and a considerably high
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reaction temperature of 350 C was used as an optimizing adjustment in attaining a
better synergy between the two catalysts. CO which mainly dominates the product is
recycled to ensure highest productivity. CH4 selectivity was very high (89.1%) over
the ternary oxide, which might have resulted from the direct methanol hydrogenation
on metallic sites of CZZ.[28]
Generally, the CO2 conversion for all catalysts was around 25%. The introduction
of a zeolite significantly reduced both methane and CO selectivity. It is highly likely
that the synergetic effect established between the hybrid catalysts created a forward
thermodynamic driving force which may have depressed the unfavorable RWGS
reaction, hence the drop in CO selectivity as observed. Typically, the methanol formed
on CZZ catalyst undergoes dehydrogenation to DME and then to hydrocarbons over
the modified zeolite, thus molecular sieve catalyst acidity and shape selectivity play a
key role in product yield in MTH reaction.
Usually, increasing the strong acid sites of zeolites would improve hydrogen
transfer reactions consequently enhancing the transformation of olefins to paraffins.
However, the small pore sizes of the SAPO-34 zeolite, to some extent, retards the
hydrogen transfer reaction.[29] In this investigation, modification preferably reduced

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the acid density of SAPO-34 zeolite which considerably restrained its secondary
hydrogenation reaction ability and enhanced the selectivity of olefins. Thus, it is likely
that the atomic hydrogen required for LPG formation may have proceeded from the
H-spillover of adsorbed hydrogen on Cu under the H2-rich condition.
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A large surface area of SAPO-34 exposed the highest amount of active phase
(strong acid sites) and enhanced mass transfer hence a better catalytic activity
compared to 5%ZnSAPO-34 and 5%MnSAPO-34. We believe the introduction of Mn
and Zn into the zeolite had a somewhat collapsing effect on the overall catalyst
framework (Fig. 5) and ionic orientation. LPG selectivity for CZZ/5%ZnSAPO-34
(63.7%) and CZZ/5%MnSAPO-34 (68.1%), was lower probably because both
catalysts mixtures contained insufficient acid sites necessary for complete conversion
of intermediates to desired product (see acid distribution in NH3-TPD profile), hence
the observed residual DME detected in products.
Table 3. Effect of metal loading on multifunctional catalyst composed of CZZ oxides
and 5%MeSAPO-34 for CO2 hydrogenation to LPG
33
Catalyst
CO2 conversion (%)
Selectivity (%)
Hydrocarbons distribution (%)
34
35
36
37
38
39
CO
HC
CH4
DME/
C2
C3-C4
C5+
(CH3OH)
39.3
41.4
21.7
22.0
31.6
CZZ
27.9
24.0
60.7
58.6
78.3
78.0
68.4
89.1
22.8
27.1
33.7
9.7
(1.7)
1.5
3.6
0.6
0.0
2.0
4.2
5.6
0.0
0.6
0.0
0.0
0.0
40
41
CZZ/SAPO34
75.1
68.1
63.7
86.1
42
43
44
CZZ/5%MnSAPO-34 24.8
4.8
45
46
CZZ/5%ZnSAPO-34 25.2
0.6
0.0
o
Reaction conditions: pressure = 2.0 MPa, reaction temperature =350 C, GHSV=4200
0
0
h-1, C3 -C4 paraffins, TOS=360 mins, ratio of CZZ to zeolite = 1, *all hydrocarbon
selectivities were calculated CO free.
CZZ/5%ZrSAPO-34 presented the highest C3-C4 paraffins selectivity of 86.1%.

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The superior performance has been attributed to the availability of an appropriate
amount of strong acid sites favorable for LPG selectivity, tuned by metal impregnation.
This means there was neither too much acid density (which could cause hydrocarbon
cracking or massive coke deposition leading to severe mass transfer limitations), nor
too less secondary hydrogenation, which favored target product selectivity.
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Furthermore, CZZ/5%ZrSAPO-34 induced minimum zeolite framework collapse
as
a
result of dealumination compared to CZZ/5%MnSAPO-34 and
CZZ/5%ZnSAPO-34 catalysts, as indicated by XRD results and SEM images. Rather,
an appropriate amount of Zr (≤ 5wt%) is believed to have the ability to facilitate
zeolite framework completion [20], maybe because of better interaction capabilities
(Fig. 5 e). Such an amount of Zr possibly enhances the ionic microenvironment of the
molecular sieve and has a limited collapsing effect. Therefore, there is an appropriate
density of acid sites for a more favorable and escalated selective formation of LPG via
light olefins transformation.
3.7 Effect of Zr content in zeolite on catalytic performance
Fig. 6. Effect of zirconia content in SAPO-34 catalysts on hydrocarbon distribution.
o
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h-1, C3 -C4 paraffins, TOS=360 mins, ratio of CZZ to xZrSAPO-34 = 1. *all
0
0
5
6
hydrocarbon selectivities were calculated CO free.
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It has been established that during CO2 conversion to hydrocarbons, it is essential
to control the hydrogenation ability and acidity of the zeolite catalysts to attain high
selectivity of olefins which will then be hydrogenated to paraffins. As already stated,
the introduction of a certain amount of metal-additive had some effect on the
crystallinity of SAPO-34 zeolite (Fig. 1 a) as well as a large influence on its surface
acidity (Fig. 2, Table 1).
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Upon increasing ZrO2 content from 1 to 3wt%, the CZZ/5%ZrSAPO-34 catalyst
exhibited reduced CH4 and light olefins selectivity whereas paraffins selectivity
increased as shown in Fig. 6. The catalyst displayed a highest CO2 conversion of
25.7% on further increasing Zr content to 5wt%. A significant reduction in methane
selectivity (9.7%) and a complete disappearance of intermediate products selectivity,
was observed. On the other hand, the selectivity of C3-C4 paraffins increased to 86.1%.
The CZZ/5%ZrSAPO-34 catalyst possibly provided a sufficient supply of methanol
and a hydrogen-rich environment where the synthesized C2–C4 olefins unavoidably
encountered hydrogen transfer reactions on the metallic sites to generate LPG.
Therefore, an increased LPG synthesis was a result of a synergetic parallel reaction
phenomenon, where CO2 conversion to intermediates could be accelerated by
intermediates conversion to LPG.
The activity results are in good agreement with, and well supported by the
characterization techniques. The NH3-TPD results indicated that with an increase in Zr
content on zeolite catalyst from 1 to 5wt%, the acidity of SAPO-34 was linearly
reduced. It has been established that decreasing the acid density of SAPO-34 catalysts
improves the selectivity of propylene and higher olefins, formed via the alkene-based
reaction route.[30,31]. With further zirconium loading (up to 10wt%), methane
selectivity increased (see Table S3), which could be ascribed to DME decomposition
over the extra framework ZrO2 deposited on the outer surface of the zeolite crystals
(see Fig. 1b, S4). A decrease in LPG selectivity to 71.6% could be attributed to a

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change in the zeolite ionic structure responsible for light olefins synthesis, hence the
increase in DME in the products. Furthermore, the diffusion of larger C5+ molecules
through the narrow pores of the molecular sieves was considerably restricted.
Consequently, the C5+ hydrocarbons selectivity was negligible (<1%) for CZZ/SAPO
composite and 0% for modified CZZ/5%ZrSAPO-34 catalyst. The latter besides a
remarkable show of stability (Fig. S6), displayed high selectivity toward C3-C4
hydrocarbons because of the suitable acid sites and effective shape selectivity.
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3.8 Effect of metal oxide to modified zeolite weight ratio on catalytic performance
Fig. 7. Effect of metal oxide-to-modified zeolite weight ratio.
o
Reaction conditions: pressure = 2.0 MPa, reaction temperature =350 C, GHSV=4200
h-1, C3 -C4 paraffins, TOS=360 mins, *all hydrocarbon selectivities were calculated
0
0
CO free

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To further examine the roles played by each component of the composite catalyst,
the weight ratio of the metal oxide to molecular sieve was varied. As given in Fig. 7,
the steady increase in metal oxide to zeolite (unmodified) ratio from 0.5 to 2, resulted
in the increase in CH4 production to 30.5% and a decrease in the selectivity of LPG to
65.1%. This observation was attributed to the strong hydrogenation ability of the
excess CZZ particles under high reaction temperature. CH4 production was well below
15% for the CZZ/5%ZrSAPO-34, implying that reducing the acid strength of
molecular sieve somewhat has some effect on the hydrogenation ability of the CZZ
catalyst component. Evidently, the acidic properties of the zeolite exhibited a huge
influence on LPG synthesis.
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With the increase in weight ratio of CZZ to 5%ZrSAPO-34 from 0.5 to 1, C3 -C4
paraffins selectivity increased tremendously (Table S4). At ratio=1, a proportionally
abundant amount of methanol was produced which could occupy a large number of
acid sites on 5%ZrSAPO-34 to produce a huge quantity of olefins for LPG formation.
The observation was associated with the synergetic mechanism of two active sites
which facilitated sequential reactions, accelerating transformation of intermediates into
product and raw materials into intermediates. However, as the weight ratio of CZZ to
5%ZrSAPO-34 zeolite was further increased, CO2 conversion and LPG selectivity
decreased. An increase in DME in the products distribution was observed, as a short
supply of 5%ZrSAPO-34 zeolite could not effectively convert intermediates to LPG in
time, possibly due to insufficient reaction sites, thus CO2 conversion was also reduced.
Therefore, the most suitable metal to zeolite weight ratio of 1 was realized for optimal
product generation for composite CZZ/5%ZrSAPO-34 catalyst.
4. Conclusions
A composite ternary catalyst comprising CZZ and a modified SAPO-34 molecular
sieve were prepared by granule mixing and employed to facilitate LPG direct synthesis
from CO2 hydrogenation. The influence of acid density and strength on product
selectivity was probed by modifying the zeolite acid properties, using the facile, low
cost ultrasonic-impregnation method. Characterization results clearly indicated that

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impregnation of the same amount of different metals induced a weak and strong acid
reduction effect, but to different degrees. It was found that the CZZ/5%ZrSAPO-34
catalyst exhibited higher performance for LPG synthesis compared to the unmodified
CZZ/SAPO-34 catalyst. The CZZ/5%ZrSAPO-34 catalyst did not only reduce the CH4
selectivity from 22.8% to 9.7%, but also completely consumed generated
intermediates (DME) for increased selectivity toward desired product. CO2 conversion
and LPG selectivity for CZZ/5%ZrSAPO-34 catalyst were highest, 25.7% and 86.1%
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respectively, at reaction temperature of 350 C, pressure 2.0 MPa, flow GHSV=4200
h-1 and an optimized weight ratio CZZ:5%ZrSAPO-34=1. The excellent catalytic
performance of CZZ/5%ZrSAPO-34 was mainly attributed to the appropriate acid
density and a synergetic relationship established between the metal and modified
zeolite components which favored formation of C3-C4 olefins that were hydrogenated
to LPG. Additionally, this simple and scalable catalyst design inspires the development
strategy of new catalysts with better and enhanced product selectivity for improved
utilization of CO2 as a carbon resource.
Acknowledgements
The research work has been funded by the Zhejiang Province Natural Science
Foundation (LY19B060001, LY17B060002), Youth Foundation of ZUST
(2019QN23) and Foundation of State Key Laboratory of High-efficiency Utilization of
Coal and Green Chemical Engineering (2019-KF-21).
Conflict of Interest
The authors have no conflicts to declare.

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Graphical Abstract
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A synergetic relationship established between the metal and modified zeolite components
favored higher selectivity for target product (LPG).